Superbulky penta-aryl cyclopentadienyl ligands in lanthanide chemistry van Velzen, Nicolaas Johannes Cornelis

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1 University of Groningen Superbulky penta-aryl cyclopentadienyl ligands in lanthanide chemistry van Velzen, Nicolaas Johannes Cornelis IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2017 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): van Velzen, N. J. C. (2017). Superbulky penta-aryl cyclopentadienyl ligands in lanthanide chemistry. [Groningen]: Rijksuniversiteit Groningen. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Chapter 6 Reactivity studies on Cp BIG 2Sm complexes 6.1 Introduction Inorganic divalent lanthanide compounds have been known for more than a century and their history, synthesis and properties have been described in section 1.3. On the contrary, soluble organometallic divalent lanthanide species have only been known since the late 1960s and the reactivity and physical properties of such species are markedly different from the inorganic ones. A clear example of this difference is Sm II I 2 (THF) 2 compared to Cp* 2 Sm II. Cp* 2 Sm II reduces N 2 to form (Cp* 2 Sm III ) + 2 (N 2 2 ), see Eq. 6.1, 1 while Sm II I 2 (THF) 2 is stable under nitrogen atmosphere. Instead, Sm II I 2 (THF) 2 is soluble in THF, has a reduction potential of V vs. NHE, 2 and has been extensively studied in the last decades as a single-electron transfer reducing agent. 3 Some of its reactivity is described in more detail in section The current chapter focuses on reactivity of organometallic divalent lanthanide complexes and Cp BIG 2Sm II in particular. Eq. 6.1 Single-electron transfer reductions of many small molecules (e.g. CO 2, P 4 ) and organic species (e.g. aromatics, ketones, nitriles) with divalent lanthanide complexes have been published in literature. Among the divalent lanthanides, samarium is of particular interest due to its reduction potential (II/III couple E 1/2 = 1.55 V vs. NHE) 4 and relative accessibility of divalent samarium complexes compared to more reducing Ln II ions (e.g. thulium with a II/III couple E 1/2 = 2.27 V vs. NHE). 4 For example, [Cp* 2 Sm II (THF) 2 ] 5 is stable at room temperature in solution, whereas Cp* 2 Tm II (THF) n cannot be isolated 6 189

3 from the reaction of Tm II I 2 with Cp*K in THF. The oxidation product Cp* 2 Tm III I is isolated instead. Another intriguing example of unprecedented reactivity with divalent lanthanides has been published by Evans et al. in In an attempt to isolate [K(2.2.2-cryptand)][(indenyl) 3 Y II ], (indenyl) 3 Y III was allowed to react with excess KC 8 in the presence of cryptand which afforded reaction mixtures were Y II EPR signals were not observed and crystallization attempts were unsuccessful. In the presence of an external NdFeB magnet, the complex [K(2.2.2-cryptand)] 2 [(C 9 H 7 ) 2 Dy II (μ-η 5 :η 1 -C 9 H 6 )] 2 could be crystallized from the analogous reaction with (indenyl) 3 Dy III, see Figure External magnetic fields have been shown to improve crystallization of compounds including proteins, organic species, inorganic materials and polymers This reaction is the first example of C H bond activation of the indenyl mono-anion (C 9 H 7 ) 1 in Dy and Y complexes, and formation of the indenyl dianion (C 9 H 6 ) 2. Figure 6.1. X-ray structure of the anion of [K(2.2.2-cryptand)] 2 [(C 9 H 7 ) 2 Dy(μ η 5 :η 1 -C 9 H 6 )] 2. For clarity, hydrogen atoms have been omitted. 7 Recently, Evans et al. published the structural and spectroscopic comparison of a series of Cp 3 Ln III and [K(2.2.2-cryptand)][Cp 3 Ln II ] (Cp = C 5 H 4 SiMe 3 ) complexes. 12 These are 190

4 the first examples in which also non-classical Ln II metals (all except for classical Eu, Yb, Sm, Tm) are isolated in the oxidation state +II. In these X-ray structures, there is a clear trend in the Ln (Cp ring centroid) distances with larger distances observed for the four classical +II lanthanides (Eu, Yb, Sm, Tm) and much smaller distances observed for the other lanthanides. The experimental differences of the Ln (Cp ring centroid) distances between [Cp 3 Ln III ] and [Cp 3 Ln II ] 1 (in Å) are 0.156, 0.143, and for Eu, Yb, Sm and Tm, respectively. For the classical Eu, Yb, Sm and Tm complexes, the significant Ln (Cp ring centroid) differences are in line with expectations, since the ionic radii of 4f n (Ln III ) and 4f n+1 (Ln II ) configurations show large differences and these orbitals have little interaction with ligand orbitals. 12 Smaller differences of Å are observed for La, Nd, Ce, Pr, Dy, Gd, Tb, Ho, Y, Er and Lu. X-ray and spectroscopic data on [K(2.2.2-cryptand)][Cp 3 Ln II ] (Ln = Nd, Dy) contradict most literature data. Ln (Cp ring centroid) differences for these complexes are Nd: and Dy: Å. In general, larger Shannon radii are observed for eight-coordinate Dy II (0.163) and Nd II (0.181 Å) compared to Dy III and Nd III, respectively. 13 UV-vis spectra of [K(2.2.2-cryptand)][Cp 3 Ln] (Dy and Nd) show extinction coefficients of M 1 cm 1, and the solutions are visibly more intensely coloured than the analogous Eu, Yb, Sm and Tm solutions (ε 900 M 1 cm 1 ). The absorption coefficients for Eu, Yb, Sm and Tm are in agreement with weak Laporte-forbidden f f transitions. DFT calculations suggest a mixed 4f/5d ground-state configuration for [Cp 3 Nd] 1 and [Cp 3 Dy] 1 and f d transitions in these complexes agree well with their UV-vis spectra. Traditionally, Dy II and Nd II have a 4f n+1 configuration. However, [Cp 3 Nd] 1 and [Cp 3 Dy] 1 demonstrate that the ground state of lanthanide ions can change with the ligand arrangement and that also low-lying d-orbitals can play a role. The reason for d-orbital participation is the ligand-field effect for a trigonal coordination that significantly lowers its d 2 z -orbital energy. In addition, the three Cp ligands stabilize the entire series of Ln II ions, which is demonstrated by crystallization of all [K(2.2.2-cryptand)][Cp 3 Ln II ] complexes, except promethium which, on account of its radioactivity, has not been probed. 6.2 Cp* vs. Cp BIG and Sterically Induced Reduction (SIR) Complexes with the very bulky penta-arylcyclopentadiene ligands described in Chapter 2 show large differences in reactivity compared to those with 191

5 pentaalkylcyclopentadiene ligands. One example is the reactivity with Sm III species. The reaction of KCp* with Sm III Br 3 in the presence of benzyl potassium does not yield the expected Cp*Sm III Bn 2 (Bn = CH 2 C 6 H 5 ) complex, but the polymeric product [Cp* 2 Sm III (Bn) 2 -K(THF) 2 ] is formed instead, see Eq In contrast, Harder et al. attempted the synthesis of half-sandwich complex Cp BIG-n-Bu5 Sm III (DMA) 2 (Cp BIG-n-Bu5 = (4-n-BuC 6 H 4 ) 5 Cp; DMA = 2-Me 2 N-C 6 H 4 CH 2 ) from the reaction of DMA 3 Sm III and one equivalent of Cp BIG-n-Bu5 H. 15 Despite the Sm III /Sm II couple of E 1/2 = 1.55 V vs. NHE, 4 they did not obtain the expected product, Cp BIG-n-Bu5 Sm III (DMA) 2, but instead obtained the spontaneously reduced sandwich complex Cp BIG-n-Bu5 2Sm II in 36 % yield, see Eq The differences between Cp* and Cp BIG have been discussed in more detail in section 1.2. Eq. 6.2 Eq. 6.3 The discovery of the formation of Cp BIG-n-Bu5 2Sm II from a Sm III complex is the first example of the isolation of a Sm II complex from a spontaneously reduced Sm III species, found in literature. Evans et al. have shown that isolated reaction products from reactions of Cp* 3 Sm III with organic substrates are in most cases identical to analogous reactions with Cp* 2 Sm II. 16 These identical reaction products suggest that Cp* 3 Sm III can react as a Sm II species where the reducing electron comes from the ligand rather than from the metal (vide infra), although a Sm II complex was not isolated from any of these reaction mixtures. In addition, Cp* 2 Sm II can reduce PhNNPh by one electron or by two electrons, depending on the stoichiometry between Cp* 2 Sm II and PhNNPh, whereas Cp* 3 Sm III reduces PhNNPh by one electron only This difference suggests that Cp* 2 Sm II is not an intermediate in Cp* 3 Sm III reductions. 192

6 The mother liquor of the reaction of DMA 3 Sm III with Cp BIG-n-Bu5 H contained DMA-DMA; a product that was formed during coupling of two [2-Me 2 N-benzyl] radicals during the reaction. 15 A possible mechanism for the formation of the Sm II sandwich that explains the observed coupling side-product is shown in Eq A double acid-base reaction of DMA 3 Sm III with two equivalents of Cp BIG-n-Bu5 H gives species Cp BIG-n-Bu5 2Sm III (DMA) and two equivalents of DMA-H. Probably, steric overload in this species is the driving force for the formation of a DMA radical and the concomitant reduction to the Sm II sandwich complex. Subseqently, the DMA radical dimerizes to DMA-DMA. Another possible driving force for the spontaneous reduction to Cp BIG-n-Bu5 2Sm II is the remarkable stability of Cp BIG metallocenes. This stability is based on attractive Cp BIG Cp BIG interactions and discussed in detail in section The synthesis of the sterically crowded complex Cp* 3 Sm III was first described in 1991 as starting from Cp* 2 Sm II and C 8 H 8 (COT) (which yields Cp* 3 Sm III and Cp*Sm III COT). 22 Despite the steric crowding, Cp* 3 Sm III is highly reactive and can react in polymerization, insertion, ring-opening and reduction reactions. 19 This type of reduction has been named Sterically Induced Reduction (SIR) and occurs in sterically crowded molecules only. 23,24 For example, Cp* 3 Sm III quickly reacts with CO in an insertion reaction which yields the unusual non-classical carbocation Cp* 2 Sm III (O 2 C 7 Me 5 ), see Figure Cp* 3 Sm III reacts with benzonitrile and in the reaction product, one benzonitrile inserts in the Sm Cp* bond. 19 This reaction product suggests that, at least during the reaction, one of the Cp* ligands in Cp* 3 Sm III coordinates in an η 1 -fashion, or that an equilibrium between Cp* 3 Sm III and Cp* 2 (η 1 -Cp*)Sm III exists. 193

7 Figure 6.2. X-ray structure of Cp* 2 Sm III (O 2 C 7 Me 5 ). 25 Similar reductive reactivity to these Cp* 3 Sm III -examples is observed for Cp* 2 Sm II. 16 For example, both Cp* 3 Sm III and Cp* 2 Sm II react with Ph 3 P=S in THF with formation of (Cp* 2 Sm III ) 2 (µ-s) and a 2-electron reduction of Ph 3 P=S to S Since Sm IV is not a feasible oxidation state, the electron in the reduction with Cp* 3 Sm III must come from the ligand. A Cp* anion can dissociate from Cp* 3 Sm III, to yield [Cp* 2 Sm III+ ][Cp* ]. The Cp* anion can reduce the substrate. The resulting Cp* radical dimerizes to Cp*-Cp*, which is can be isolated from reaction mixtures of reductions with Cp* 3 Sm III, see Figure 6.3. Other evidence that the electron for substrate reduction does not come from the metal, but rather from the ligand is the observed difference in reactivity between Cp* 2 Sm II and Cp* 3 Sm III with some substrates. Anthracene and pyrene are reduced by Cp* 2 Sm II, but not by Cp* 3 Sm III. 19 Cp* 2 Sm II can reduce azobenzene by one or two electrons, depending on the stoichiometry between Cp* 2 Sm II and azobenzene used in the reaction, whereas Cp* 3 Sm III reduces azobenzene by only one electron. 19 These differences clearly indicate that the reduction potentials of Cp* 2 Sm II and Cp* 3 Sm III are different. The reduction potential of Cp* 3 Sm III implies that other Cp* 3 Ln III complexes could also have viable reductive properties. In 2005, Evans et al. showed that this is indeed the 194

8 case; in agreement with expectations, complexes of smaller metals are more crowded and have a larger redox potential. 26 Figure 6.3. Sterically Induced Reduction of Cp* 3 Sm compared to metal reduction. 16 Surprisingly, Cp* 3 Sm III can be isolated in 84 % yield from the oxidation of Cp* 2 Sm II with Cp*-Cp*. 26 This observation suggests that Cp* 2 Sm II and Cp* 3 Sm III form an equilibrium with Cp*-Cp*, which is on the side of Cp* 3 Sm III. The basicity of the benzyl anion is higher than that of to the Cp* anion, making Cp*-Cp* easier to reduce than PhCH 2 CH 2 Ph. Another bulky ligand that has been used to stabilize low-oxidation state lanthanide complexes is the 2,5-di-tert-butyl-3,4-dimethylphospholyl ligand t-bu 2 Me 2 C 4 P (dtp). Nief et al. synthesized the homoleptic (dtp) 2 Sm II and (dtp) 2 Tm II complexes in In 2014, a publication with a detailed study of the single electron transfer (SET) reactivity of (dtp) 2 Tm II with bipyridine (bipy) and tetramethylbiphosphine (tmbp) was published, see Figure Addition of bipy or tmbp to base-free (dtp) 2 Tm II leads to formation of (dtp) 2 Tm III L (L = bipy, tmbp), in which an electron transfers to bipy or tmbp to yield (dtp) 2 Tm III+ L -type complexes. This electron is still available for further reactivity, and addition of bipy to dtp 2 Tm III+ L leads to formation of (dtp)tm III (bipy) 2 and 0.5 equiv. of the oxidized dtp-dtp dimer. 195

9 Figure 6.4. Single electron transfer in phospholyl thulium complexes

10 6.3 Spontaneous reduction of europium(iii) complexes Europium has the lowest reduction potential of all Ln III ions (II/III couple E 1/2 = 0.35 V vs. NHE) 4 and Eu III can even be reduced by BH 4 under normal conditions. In the reaction of Eu III Cl 3 with LiBH 4, Eu II Cl 2 precipitated with the formation of 0.5 equiv. of B 2 H The authors speculated that initially Eu III (BH 4 ) 2 Cl is formed, and that it is reduced to Eu II Cl 2 with concomitant formation of LiCl, 0.5 equiv. of B 2 H 6 and 0.5 equiv. of H 2. Spontaneous reduction of Eu III to Eu II can be relatively easily accomplished. It was first published in 1980 where addition of Cp*Na to Eu III Cl 3 in refluxing THF and subsequent crystallization from diethyl ether gave Cp* 2 Eu II (THF)(Et 2 O), see Eq A similar reaction has been published in 2000, where [1,2,4-t-Bu 3 C 5 H 2 ]Na was allowed to react with Eu III Cl 3 in THF to yield the solvent-free sandwich complex (1,2,4-t-Bu 3 C 5 H 2 ) 2 Eu II, see Eq Also observed in this reaction is the formation of 0.5 equiv. of Cp -Cp coupling product (t-bu 3 C 5 H 2 ) 2. This coupling product suggests the formation of a Cp radical during the reaction, which dimerizes to the observed Cp -Cp product. Eq. 6.4 Eq. 6.5 Several other examples of spontaneous reduction of Eu III to Eu II have been published in the literature. Attempted synthesis of a tris-β-diketiminate Eu III species led to oxidative coupling of β-diketiminates and the formation of L 2,6-Me2 2Eu III (THF); L 2,6-Me2 = [{N(2,6-Me 2 C 6 H 3 )-C(Me)} 2 CH]. 32 The oxidized β-diketiminate-β-diketiminate coupling product was formed in the reaction, see Eq

11 Eq Spontaneous reduction of ytterbium(iii) complexes Due to its higher reduction potential, Yb III is harder to reduce than Eu III. The analogous reaction of 3 equivalents of [DIPP-nacnac]Na with Yb III Cl 3 did not give the reduced product. Instead, (L 2,6-Me2 ) 1 (L 2,6-Me2 ) 2 Yb III was isolated in 41 % crystalline yield, see Eq In this reaction, one β-diketiminato ligand is eliminated and one of the methyl groups of one ligand is deprotonated to make this ligand dianionic. 198

12 Eq. 6.7 Nevertheless, spontaneous reduction has been observed in some Yb III complexes. The reaction of two equivalents of a substituted lithium indene with Yb III and Eu III amides afforded the bis(indenyl )Yb II and bis(indenyl )Eu II complexes (indenyl = Me 2 NCH 2 CH 2 C 9 H 7 ) in ca. 70 % yield, see Eq The authors proposed that initially (indenyl ) 2 Ln III N(SiMe 3 ) 2 (Ln = Eu, Yb) was formed. This species is then reduced to (indenyl ) 2 Ln II via homolysis of the Ln N bond and formation of a N(SiMe 3 ) 2 radical, which subsequentely dimerized to the observed (Me 3 Si) 2 N N(SiMe 3 ) 2 side-product. Eq. 6.8 A second example comes from a paper which was published in The reaction of Eu III Cl 3 or Yb III Cl 3 with sodium N,N-dimethylaminodiboranate in THF gave the spontaneously reduced complexes (H 3 BNMe 2 BH 3 )Yb II (THF) 2 and (H 3 BNMe 2 BH 3 )Eu II (THF) 2, 35 under mild conditions at room temperature, see Eq

13 Eq. 6.9 Samarium has the second-highest reduction potential (II/III couple E 1/2 = 1.55 V vs. NHE) 4 of the four classical lanthanides (i.e. Eu, Yb, Sm, Tm). Spontaneous reduction and formation of Cp BIG-n-Bu5 2Sm II has also been published. 15 It was therefore decided to limit the reactivity studies in this chapter to samarium; thulium was not studied because (Ar 5 Cp) 2 Tm II species are not known in literature, europium and ytterbium complexes were not studied because of their relatively lower II/III reduction potentials of 0.35 V vs. NHE and 1.15 V vs. NHE, respectively (compared to Sm) Goals The first goal of the current study was to synthesize Cp BIG 2Sm II metallocenes with a moderate solubility in aromatic solvents that crystallize readily on a large scale in good yields. The second goal was to study the reactivity of these samarocenes with small molecules and simple organic substrates to gain more insight into the reactivity and stability of such sandwich complexes and their isolable reaction products. 6.6 Results and discussion As a result of the unusual properties of the penta-arylated cyclopentadienyl ligands and their complexes as described in more detail in Chapter 2, the reactivity of Cp BIG 2Sm II towards various small molecules and organic substrates was studied. The properties of the analogous sandwich complex Cp* 2 Sm II had already been studied and published in great detail and will be taken as a benchmark for comparison with the properties of sandwich complex Cp BIG 2Sm II, as will be discussed below. The synthesis of sandwich complex Cp BIG 2Sm II is described in section and its crystal structure in section Sections through contain reactivity of Cp BIG 2Sm II with various small molecules and organic substrates, including analysis of obtained crystal structures. 200

14 6.6.1 Synthesis of Cp BIG-Et5 2Sm II A synthetic pathway to a moderately-soluble Cp BIG H ligand and its corresponding sandwich complex Cp BIG 2Sm II on a large scale in good yields is necessary for in-depth reactivity studies. For solubility reasons, the Cp BIG-n-Bu5 2Sm II (Cp BIG-n-Bu5 = (4-n-BuC 6 H 4 ) 5 Cp) complex as first published by Harder et al. contains an n-butyl tail on the para position of each phenyl ring. Since the solubility of Cp BIG-n-Bu5 2Sm II is very high, even in aliphatic solvents such as pentane, crystallization of products is often complicated and yields are low. In addition, the many flexible n-bu groups often lead to severe disorder in crystal structures. Moreover, the ligand synthesis requires a column chromatography purification step, which is more elaborate to scale-up than a simple crystallization of the ligand. A Cp BIG ligand with a single n-butyl substituent on only one of the five phenyl rings is described in chapter 2 and appeared to yield complexes that are difficult to crystallize. It was therefore decided to reduce the length of all five alkyl tails and thereby also reduce the solubility of the complexes. The synthesis of this truncated Cp BIG-Et5 ligand, which has an ethyl substituent on the paraposition of each phenyl ring, is described in detail in Chapter 2. Summarizing, Pd-catalyzed cross coupling of 1-bromo-4-ethylbenzene with Cp 2 Zr IV Cl 2 on a 20 gram scale afforded the desired product Cp BIG-Et5 H in 74 % yield. Synthesis of Cp BIG-n-Bu5 2Sm II via reduction of (DMA) 3 Sm III gives low yield of the sandwich complex. 15 Fortunately, a simple protonation of (DMAT) 2 Sm II (THF) 2 (DMAT = o-nme 2 CHSiMe 3 C 6 H 4 ) with two equivalents of Cp BIG-Et5 H in toluene on a 5 gram scale afforded Cp BIG-Et5 2Sm II in 66 % yield as a dark-red crystalline material after crystallization from toluene/pentane at 30 C X-ray structure of Cp BIG-Et5 2Sm II The X-ray structure of Cp BIG-Et5 2Sm II is shown in Figure 6.5 and from a structural point of view, it is similar to the published X-ray structure of Cp BIG-n-Bu5 2Sm II. 15 Cp BIG-Et5 2Sm II crystallizes in the monoclinic space group P2 1 /n with an inversion center located on the samarium atom. Cp BIG-n-Bu5 2Sm II crystallizes in the triclinic space group P1 with an inversion center located on the samarium atom. The samarium atom in both complexes is slightly disordered within a plane parallel to the Cp rings. 201

15 Figure 6.5. Top and side-view of the X-ray structure of Cp BIG-Et5 2Sm II. For clarity, the ethyl substituents, hydrogen atoms and disordered aryl ring carbon atoms have been omitted. The molecule is crystallographically centrosymmetric and the Sm atom is disordered (a model with 4 Sm positions was found satisfactorily) Reaction of Cp BIG-Et5 2Sm II with dinitrogen Evans et al. published the unsolvated complex Cp* 2 Sm II, which forms a bent sandwich complex, and allowed it to react with N 2 in the solid state as well as in solution. 1 The 2 dimeric complex that is formed in this reaction contains a side-on coordinated N 2 moiety that bridges between two Cp* 2 Sm III units. Such dinitrogen reduction has also been described by Guan et al. who added dinitrogen gas to a THF solution of a (calix-tetrapyrrole)sm II complex and characterized two different species using single crystal X-ray diffraction, of which one is dimeric and contained a bridging N 2 2 ion between the two samarium atoms. 36 In the current study, Cp BIG-Et5 2Sm II was put under 50 bar of N 2 pressure at 50 C, in the solid state as well as in benzene or toluene solutions. After removal of N 2, no reactions were observed. Since the reaction of Cp* 2 Sm II with N 2 is reversible, 1 a sample of Cp BIG-Et5 2Sm II maintaining N 2 pressure was studied as well. Subsequent comparison with samples handled under argon atmosphere showed identical NMR spectra. A J. Young valve NMR tube that contained a solution of Cp BIG-Et5 2Sm II in benzene-d 6 was exposed to 4 bar of N 2 and subsequently heated to 110 C. No differences in its NMR spectrum or the intensities of its absorptions were observed. These observations strongly suggest that Cp BIG-Et5 2Sm II does not reduce dinitrogen. 202

16 6.6.4 Reaction of Cp BIG-Et5 2Sm II with CO In 1985, Evans et al. published that addition of 6.2 bar of CO to a room temperature solution of Cp* 2 Sm II (THF) 2 in THF resulted in a colour change from red-purple to darkbrown over a 3-day period, with formation of a ketenecarboxylate Sm III bimetallic complex, see Figure However, Selg et al. failed to reproduce this experiment. 38 Their IR experiments with Cp* 2 Sm II (THF) 2 under CO pressure showed only an absorption band for unbound CO when % pure CO was used. The authors did see an instant colour change to green-blue and a slow colour change to brown when they bubbled % CO through a THF-solution of Cp* 2 Sm II (THF) 2. They did not manage to crystallize a pure product from this reaction mixture and they concluded that the formation of this ketenecarboxylate complex is strongly dependent on the presence of impurities. Figure 6.6. Reduction of carbon monoxide by Cp* 2 Sm. 37 In the current study, a J. Young valve NMR tube with a benzene-d 6 solution of Cp BIG-Et5 2Sm II was exposed to 4 bar of % carbon monoxide. No changes in its 1 H NMR spectra or signal intensities were observed after the sample was kept at 110 C for 24 h. Cp BIG-Et 2Sm II does not react with CO, even under these forcing conditions. 203

17 6.6.5 Reaction of Cp BIG-Et5 2Sm II with CO 2 The reaction of Cp* 2 Sm II (THF) 2 with CO 2 in toluene or hexane at room temperature or at 78 C, as described by Evans et al., yields mixtures for which four to fifteen signals in the C 5 Me 5 region of the crude 1 H NMR spectra were observed. 39 However, in THF at room temperature a clean reaction is observed. The oxalate complex (Cp* 2 Sm III ) 2 (µ-η 2 :η 2 -O 2 CCO 2 ) was isolated in high yield, see Eq The X-ray structure of this complex is of poor quality, though, and a related samarium oxalate with a higher-quality X-ray structure was published in 2003, see Eq In a second paper from 2012, DFT calculations and reaction mechanisms for the oxidative reactions of CO 2 were published. 41 An example from 2006 shows a Sm II porphyrinogen complex which is oxidized to its carbonate-bridged bimetallic Sm III complex, see Eq Eq Eq

18 Eq In the current study, a J. Young valve NMR tube with a benzene-d 6 solution of Cp BIG-Et5 2Sm II was exposed to 4 bar of carbon dioxide. No changes in its 1 H NMR spectra or signal intensities were observed after the sample was kept at 110 C for 24 h, which means that complex Cp BIG-Et5 2Sm II does not react with CO 2, even under these forcing conditions Reaction of Cp BIG-Et5 2Sm II with stilbene and styrene Addition of excess cis-stilbene to Cp* 2 Sm II (THF) 2 results in catalytic isomerization to trans-stilbene. 43 Up to 30 equivalents per samarium can be isomerized within 2 h. Solvent-free Cp* 2 Sm II isomerizes cis-stilbene much faster. At least 70 equivalents of cis-stilbene can be isomerized by Cp* 2 Sm II within 5 minutes. Reaction of solvent-free Cp* 2 Sm II with 0.5 equiv. of either cis-stilbene or trans-stilbene yields the bimetallic complex (Cp* 2 Sm III ) 2 (PhCHCHPh), see Eq In the current study, to a J. Young valve NMR tube with a benzene-d 6 solution of Cp BIG-Et5 2Sm II was added 1 equivalent of trans-stilbene. No changes to its 1 H NMR spectra or signal intensities were observed after the sample was kept at 110 C for 24 h, which means that Cp BIG-Et5 2Sm II does not react with trans-stilbene, even under forcing conditions. 205

19 Eq Styrene reacts instantaneously with Cp* 2 Sm II to form the red-maroon complex [(Cp* 2 Sm III ) 2 (µ-η 2 :η 4 -PhCHCHPh)], see Eq Addition of THF to this complex gives free styrene and Cp* 2 Sm II (THF) 2. In the current study, experiments in which a Cp BIG-Et5 2Sm II solution was heated in the presence of styrene, showed no change in the intensity of the signals of the complex; only styrene olefinic signals disappeared. This disappearance is consistent with the well-known thermally induced polymerization of styrene. 44 Cp BIG-Et5 2Sm II, however, remains unreacted and is inert towards styrene. Eq Reaction of Cp BIG-Et5 2Sm II with ethylene According to Evans et al., ethylene is polymerized by Cp* 2 Sm II. 45 In the current study, the reactivity of Cp BIG-Et5 2Sm II with ethylene was explored. A J. Young valve NMR tube with a benzene-d 6 solution of this complex was exposed to 1 bar of ethylene. No changes to its 1 H NMR spectra or signal intensities were observed after the sample was kept at 110 C for several days, which means that complex Cp BIG-Et5 2Sm II does not react with ethylene, even under forcing conditions. 206

20 6.6.8 Reaction of Cp BIG-Et5 2Sm II with naphthalene, anthracene, phenazine and other cyclic aromatics Evans et al. have extensively studied the reactivity of Cp* 2 Sm II with other polycyclic aromatic hydrocarbons and related nitrogen containing compounds. 43,46 They isolated a series of complexes with reduced aromatic ligands, the structures of these complexes gave insight into the reducing properties of Cp* 2 Sm II. For example, anthracene (see Eq. 6.15) and pyrene (see Eq. 6.16) both react fast with Cp* 2 Sm II in a 1/2 (aromatic/complex) ratio to form stable complexes of the type (Cp* 2 Sm III ) 2 (aromatic). The second reduction potential of anthracene, which is E 1/2 = 2.20 V vs. NHE, suggests that Cp* 2 Sm II reduces aromatics with a double reduction potential of up to 2.20 V. 46,47 In the case of phenazine, which has a first reduction potential of V vs. NHE, 48 the reduced complex (Cp* 2 Sm III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ) is formed in quantitative yield, see Eq

21 Eq Eq Eq

22 In the current study, similar reactivity of Cp BIG-Et5 2Sm II was studied. A J. Young valve NMR tube that contained a solution of Cp BIG-Et5 2Sm II with 1 equivalent of phenazine in toluene was kept at 110 C for 24 h. No changes in NMR spectra or its signal intensities were observed. Complex Cp BIG-Et5 2Sm II does not react with phenazine, even under forcing conditions. Phenazine has a lower reduction potential than naphthalene, anthracene and similar polycyclic aromatics and it is therefore easier to reduce. Since no reaction between Cp BIG-Et5 2Sm II and phenazine was observed, the aromatics naphthalene, anthracene and pyrene also will not be reduced by Cp BIG-Et5 2Sm II and hence were not probed for their reactivity with Cp BIG-Et5 2Sm II Reaction of Cp BIG-Et5 2Sm II with pyridine A paper published by Evans et al. in 2012 showed that both Cp* 2 Sm II (THF) 2 and tmp 2 Sm II (Tmp = Me 4 C 4 P) reacted fast with pyridine, but gave different products. 49 Complex tmp 2 Sm II formed the black Sm II pyridine adduct, see Eq Treatment of Cp* 2 Sm II (THF) 2 in diethyl ether with a slight excess of pyridine gave red (Cp* 2 Sm III C 5 H 5 N) 2 (µ-nc 5 H 5 -C 5 H 5 N), see Eq Eq Eq In the current study, reactivity of Cp BIG-Et5 2Sm II with pyridine was studied. To a J. Young valve NMR tube containing a benzene-d 6 solution of Cp BIG-Et5 2Sm II, two drops of pyridine were added. No changes to its 1 H NMR spectra or signal intensities were observed after the sample was kept at 60 C for 24 h, which means that complex Cp BIG-Et5 2Sm II does not react with pyridine, even under forcing conditions. 209

23 Reaction of Cp BIG-Et5 2Sm II with pyrazine Pyrazine or 1,4-diazabenzene has a reduction potential of E 1/2 = 1.57 V vs. a mercury electrode. 50 It is easier to reduce than pyridine which has a reduction potential of E 1/2 = 2.15 V vs. a mercury electrode. 50 A reaction of Cp 3 Yb III with pyrazine in benzene was published in 1977, which yields the dinuclear complex (Cp 3 Yb III ) 2 (µ-nc 4 H 4 N), see Eq Here, only the adduct is formed since the authors are starting from an Yb III complex which cannot be oxidized further. No literature examples of a lanthanide complex with a reduced pyrazine were found in literature. In 2008, Carlson et al. published several (Cp* 2 Yb III ) 2 (bridge)-type complexes, but in this paper larger heterocyclic bridges containing five to seven linked aromatic rings were used. 52 Based on spectroscopic and magnetic data, Carlson et al. concluded that these complexes consist of two Yb III centers and a dianionic bridge. Eq The lower reduction potential of pyrazine compared to pyridine and the finding that pyridine does not react with Cp BIG-Et5 2Sm II, justifies the study of the reactivity between Cp BIG-Et5 2Sm II and pyrazine, which was executed accordingly. Addition of excess pyrazine to a Cp BIG-Et5 2Sm II solution in benzene-d 6 did not show any changes to its 1 H NMR spectra or signal intensities, not even after keeping the solution at 80 C for 18 h, which means that complex Cp BIG-Et5 2Sm II does not react with pyrazine under these conditions Reaction of Cp BIG-Et5 2Sm II with isocyanates In literature, the reaction of phenyl isocyanate with (ArO) 2 Sm II (THF) 3 (ArO = 2,6-di-tert-butyl-4-methylphenoxide) 53 yields the reduction coupling product [(ArO) 2 Sm III (DME)] 2 [µ-η 4 -(PhN)OCCO(NPh)] This reaction is shown in Eq

24 Complex (MeC 5 H 4 ) 2 Sm II (THF) 2 reacts with PhNCO to yield [(MeC 5 H 4 ) 2 (THF)Sm III ] 2 [μ-η 4 -(PhN)OCCO(NPh)], see Eq ,56 Single electron reduction is also observed in the reductive coupling of two molecules of phenyl isocyanate by [Me 2 NCH 2 CH 2 N(CH 2-2-OC 6 H 2-3,5-t-Bu 2 ) 2 ] 2 Yb II (THF) 2, see Eq Eq Eq Eq In the current study, experiments in which ca. 1.2 equivalents of phenyl isocyanate were added to a Cp BIG-Et5 2Sm II solution in benzene-d 6 resulted in ca. 40 % conversion after the sample was kept at 60 C for 1 week. The 1 H NMR spectrum of the reaction mixture showed a large number of overlapping signals in the CH 3 - and CH 2 -regions of 211

25 the Cp BIG-Et5 ligand, indicating complex mixtures of multiple reaction products. Any attempts to isolate well-defined products by crystallization failed. The reaction of Cp BIG-Et5 2Sm II with phenyl isocyanate was therefore not further studied Reaction of Cp BIG-Et5 2Sm II with NO, Me 3 NO and epoxybutane Well-defined reactivity of lanthanide complexes with (small) oxygen-containing substrates is often challenging. Cp* 2 Sm II reacts with NO, to yield a mixture of several products, of which the oxide species (Cp* 2 Sm III ) 2 (µ-o) could be isolated in 29 % yield, see Eq This oxide is a common product from reactions of Cp* 2 Sm II with oxygen-containing molecules. It could also be isolated from analogous reactions with N 2 O, 1,2-epoxybutane and pyridine-n-oxide. 57 It was thought that (Cp* 2 Sm III ) 2 (µ-o) would be too sterically crowded to coordinate a molecule of THF. Nevertheless, the THF complex (Cp* 2 Sm III THF) 2 (µ-o) was isolated as a side-product in 21 % yield from the reaction between Cp* 2 Sm II (THF) 2 and 1,2-epoxybutane. 59 Attempts to crystallize the product were unsuccessful. However, a crystal structure of the reaction product of (indenyl) 2 Sm II (THF) n (n = 1 3, depending on isolation conditions) with N 2 O could be characterized with X-ray diffraction, see Figure The poor yields of isolated products and the NMR spectra from the reactions of Cp* 2 Sm II and (indenyl) 2 Sm II (THF) n with oxygen-containing reagents suggest that other samarium oxide species are formed in these reactions, but these side-products have not been identified yet. Eq

26 Figure 6.7. Crystal structure of the product from the reaction of (indenyl) 2 Sm II (THF) n (n = 1 3, depending on isolation conditions) with N 2 O. 60 In the current study, analogous reactions of Cp BIG-Et5 2Sm II with NO and N 2 O were studied. In calibrated gas bulb addition experiments, full consumption of the starting material was only observed after addition of ca. 2.5 equivalents of NO. This resulted in an instant colour change from brick-red to deep blue. Subsequent removal of all the volatiles in vacuo gave a dark-blue sticky solid which was washed with pentane to obtain a violet-blue solution and a brick-red powder. The violet-blue colour suggests the formation of the Cp BIG-Et5 radical species. 61 Indeed, formation of the radical is confirmed by EPR spectroscopy, see Figure 2.9. Unfortunately, multiple attempts to crystallize the brick-red powder from the NO reaction were all unsuccessful and the identity and structure of the material could not be resolved. The 1 H NMR spectrum of the crude reaction mixture of Cp BIG-Et5 2Sm II with 2 equivalents of Me 3 NO, showed multiple signals that could be assigned to a Cp BIG-Et5 ligand, suggesting formation of a mixture of products. Repeated attempts to isolate a welldefined product from the reaction mixture were unsuccessful. Isolation of a welldefined product from the reaction of Cp BIG-Et5 2Sm II with O 2 was also unsuccessful. However, addition of phenazine to this O 2 reaction allowed trapping of a Sm III peroxo complex, which was isolated and characterized, see section Unfortunately, addition of Me 3 NO to a solution of Cp BIG-Et5 2Sm II in the presence of phenazine did not yield any well-defined product either. Apparently, the reaction of Cp BIG-Et5 2Sm II with 213

27 Me 3 NO is not selective and separating complex mixtures of multiple organometallic species appears to be a significant challenge. Similar issues were encountered in the reaction of Cp BIG-Et5 2Sm II with 1,2-epoxybutane and isolation of a single reaction product was unsuccessful. The reactions of Cp BIG-Et5 2Sm II with Me 3 NO and with 1,2- epoxybutane were therefore not further studied. Overall, it is clear from the current experiments that well-defined reactivity with (small) oxygen-containing substrates and lanthanide complexes is more the exception than the rule Reaction of Cp BIG-Et5 2Sm II with (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl is a well-known stable radical species and reagent in organic chemistry. In an attempt to synthesize and isolate a stable oxoproduct, 1 equiv. of TEMPO was added to a toluene solution of Cp BIG-Et5 2Sm II. After stirring overnight at room temperature, and cooling to 30 C, a small amount of yellow crystals was obtained from the cold reaction mixture. Single crystal X-ray diffraction showed that the dimeric complex (ONC 5 H 6 Me 4 ) 2 Sm III (µ-onc 5 H 6 Me 4 ) was formed. This species has also been isolated by Evans et al. in 68 % yield from the reaction of Cp* 3 Sm III with TEMPO in a 1/3 mol ratio, see Eq In conclusion, since only 1 equivalent of TEMPO already results in formation of (TEMPO) 3 Sm III, a Cp BIG-Et5 2Sm III TEMPO species is probably not stable towards further reactivity and replacement of the Cp BIG-Et5 ligand. 214

28 Eq Reaction of Cp BIG-Et5 2Sm II with white phosphorus (P 4 ) Cyclic phosphorus compounds have been intensely studied in the last decades and multiple reviews have been published Elemental phosphorus can exist as four common allotropes, of which white phosphorus (P 4 ) is the most reactive one that is stable at room temperature if kept in deoxygenated water, see Figure 6.8. It spontaneously ignites in air and is soluble in aromatic solvents. Diphosphorus (P 2 ) is only stable above 800 C in the gas phase 66 or in solution stabilized by transition metals such as Nb 67 or Ni. 68 Red, violet and black phosphorus have a polymeric structure, do not ignite in air at room temperature and are insoluble in organic solvents. Figure 6.8. Allotropes of phosphorus, from left to right, top to bottom: white, red, violet and black phosphorus. 215

29 Several polyphosphide aromatic rings are known in the literature. These structures include P 2 4, P 5 ions 69,70 and the P 4 6 unit in M 4 P 6 (M = K 71, Rb 72, Cs 73 ). Prominent polyphosphide clusters include [P 3 7 ] 74 (see Figure 6.9a) and [P 3 11 ] 70 (see Figure 6.9b). a b Figure 6.9. Structures of P 3 7 (left) and P 3 11 (right) clusters. 70,74 In 2009, the reaction of Cp* 2 Sm II in toluene with P 4 vapour was published. 75 The reaction product contains four samarium atoms and a formally P 4 8 cluster, see Figure 6.10 for its X-ray structure. Dimerization of P 4 to P 8 is enthalpically disfavored. 76 Roesky et al. suggest that the formation of the P 8 moiety is induced by electron transfer from the Sm II center to a P 4 molecule. 75 Figure Reaction product of Cp* 2 Sm with P 4. De methyl groups on the Cp rings have been omitted for clarity

30 In order to probe the reactivity of Cp BIG-Et5 2Sm II, in the current study, a P 4 solution in benzene-d 6 was added to a Cp BIG-Et5 2Sm II solution in benzene-d 6. Subsequently, the reaction mixture was kept at 80 C for 18 h. The 1 H NMR spectrum showed no changes and the 31 P spectrum showed only one signal at δ ppm that can be assigned to P 4. It must therefore be concluded that Cp BIG-Et5 2Sm II does not react with P 4 under these conditions Reaction of Cp BIG-Et5 2Sm II with sulfur (S 8 ) 2 Evans et al. have explored the accessibility of a bimetallic compound with a bridging S 3 unit. The yttrium complex [{(Me 3 Si) 2 N} 2 Y III (THF)] 2 (μ-η 2 :η 2 -N 2 ) was allowed to react with S 8 in THF, and the S 2 3 bridging dimer was obtained, see Eq Eq In the current study, Cp BIG-Et5 2Sm II was also allowed to react with S 8 to compare its reactivity with the yttrium complex published by Evans et al. Addition of 0.5 equiv. of S 8 to a Cp BIG-Et5 2Sm II solution in benzene-d 6 did not result in any changes in its 1 H NMR spectra, after several days at room temperature. The mixture was therefore kept at 80 C for 18 h. The 1 H NMR spectrum showed a large number of overlapping signals in the CH 3, CH 2 and aromatic regions, indicating a mixture of multiple species. The nonselective reaction of Cp BIG-Et5 2Sm II with S 8 was therefore not further studied Reaction of Cp BIG-Et5 2Sm II with diphenyl disulfide In the current study, Cp BIG-Et5 2Sm II slowly reacts with diphenyl disulfide. Full conversion requires 2 equivalents of PhSSPh and heating to 110 C for several days. 1 H NMR spectra of the crude reaction mixtures showed three triplets between δ ppm in a 1/0.6/0.4 ratio, suggesting multiple species are formed in the reaction. Unfortunately, repeated attempts to isolate a pure product via crystallization 217

31 experiments were unsuccessful. Reactions and crystallization experiments in which Cp BIG-i-Pr5 2Sm II (Cp BIG-i-Pr5 = (4-i-Pr 5 C 6 H 4 ) 5 Cp) was used instead, were also unsuccessful. In order to study the reactivity of Cp BIG-Et5 2Sm II, substituted diaryl disulfides were obtained in a single-step reaction from S 2 Cl 2 and an alkyl-substituted benzene. The reactions of S 2 Cl 2 with mesitylene, 1,3,5-tri-iso-propylbenzene and 1-tert-butyl-3,5-dimethylbenzene in acetic acid afforded the corresponding diaryl disulfides in good yields. Unfortunately, repeated attempts to isolate a reaction product from the reaction of Cp BIG-Et5 2Sm II with these disulfides were also unsuccessful. On one occasion, a mixture of a small amount of crystalline material in a viscous oil was obtained. A crystal from the material was isolated and its X-ray structure was determined as [Cp BIG-Et5 Sm III (μ-sph) 2 ] 2, see Figure Four phenylsulfide anions (from reductive S S bond cleavage in PhSSPh) bridge between two Sm III centers of which one of the two Cp BIG-Et5 ligands dissociated Reaction of Cp BIG-Et5 2Sm II with dioxygen Selective reactivity with highly reactive and/or oxophilic organometallic complexes and strong oxidizing agents such as O 2 can be challenging. Therefore, literature examples of well-defined products from reactions of Ln II complexes with O 2 are scarce. The reaction of Cp* 2 Sm II with O 2 has not been published. The hydrotris-(3,5-dimethylpyrazolyl)borate (Tp Me2 ) and hydrotris-(pyrazolyl)borate (Tp) complexes Tp 2 Ln II and Tp Me2 2Ln II (Ln = Sm, Yb) were allowed to react with O 2 in toluene at 78 C to yield monomeric superoxide complexes Tp Me2 2Ln III (η 2 -O 2 ) and Tp 2 Ln III (η 2 -O 2 ), see Eq ,79 In these superoxide complexes, O 1 2 is side-on coordinated to the Ln III center. In contrast, the dimeric peroxo complex (Tp Me2 2Sm III ) 2 -(µ-η 2 :η 2 -O 2 ), where O 2 2 is bridging between two Ln III centers, can be prepared from Tp Me2 2Sm II and pyridine-n-oxide

32 Eq In an attempt to isolate a well-defined Sm III species from the reaction of Cp BIG-Et5 2Sm II with O 2, this reaction was studied. Full conversion was observed minutes after addition of 2 equivalents of O 2 to a degassed benzene or toluene solution of Cp BIG-Et5 2Sm II at ambient temperatures. However, after repeated attempts no reaction products could be isolated or characterized. In one experiment, phenazine was also present in the reaction mixture. This experiment yielded red needles, for which a crystal structure was measured. Although crude NMR spectra suggested a single major product, the crystallization was not reproducible and a hundred milligrams of analytically pure material for NMR experiments and elemental analysis could not be obtained. In an attempt to obtain a larger amount of analytically pure material of this reaction product, Cp BIG-i-Pr5 2Sm II, was used instead of Cp BIG-Et5 2Sm II. Addition of 1.5 equivalents of O 2 to a degassed benzene or toluene solution of Cp BIG-i-Pr5 2Sm II in the presence of 2 equivalents of phenazine gave an instant colour change from intense red to deep blue. The observed deep blue colour of the reaction mixture is nearly identical to the colour of the NO reaction mixture. This colour suggests formation of Cp BIG-i-Pr5 radical. 61 After ca. 5 minutes, the reaction mixture was concentrated to dryness and the solids were washed with hexane. A blue solution was obtained which yielded intense red crystals upon cooling to 30 C. Although single crystal X-ray diffraction confirmed the formation of the ligand radical, Cp BIG-i-Pr5, the structure is highly disordered and was not further refined. 219

33 Pentane vapour diffusion into a toluene solution of the remaining brick-red solids afforded dark-red prism-shaped crystals. X-ray crystallography showed the formation of the bimetallic complex [Cp BIG-i-Pr5 Sm III (η 1 -C 12 H 8 N 2 )] 2 [µ-η 2 :η 2 -O 2 ] 2, abbreviated as [Cp BIG-i-Pr5 Sm III phz] 2 O 4. The crude reaction product could be obtained in ca. 70 % yield after washing the crude material with pentane to remove the Cp BIG-i-Pr5 radical. It is ca. 90 % pure, as indicated by 1 H NMR spectroscopy. Slow condensation of pentane into a toluene solution of this crude product via vapour diffusion yielded single crystals suitable for X-ray diffraction in 25 % yield. NMR data and good yield of the crude product, both suggest a relatively selective oxidation of Cp BIG-i-Pr5 2Sm II to a single major product X-ray structure analysis of complex (Cp BIG-i-Pr5 Sm III phz) 2 O 4 Complex (Cp BIG-i-Pr5 Sm III phz) 2 O 4 crystallized as dark-red needles in the triclinic space group P1 with two independent molecules in the asymmetric unit. In Figure 6.11 the structure is presented of one of the two independent molecules in the asymmetric unit. There is a center of inversion located between the two samarium atoms. The O O bond lengths in (Cp BIG-i-Pr5 Sm III phz) 2 O 4 are 1.517(4) and 1.509(4) Å. The only monomeric lanthanide superoxide complexes found in literature are Tp Me2 2Sm III (η 2 -O 2 ), Tp 2 Sm III (η 2 -O 2 ) and Tp 2 Yb III (η 2 -O 2 ), see Eq ,79 The O O bond length in complex Tp Me2 2Sm III (η 2 -O 2 ) is 1.319(5) Å. Unfortunately there are no crystal structures of the other monomeric complexes available. The observed superoxide O O bond in Tp Me2 2Sm III (η 2 -O 2 ) length is significantly shorter than the observed O O bond lengths in (Cp BIG-i-Pr5 Sm III phz) 2 O 4. This difference is a strong indication that the O 2 units in (Cp BIG-i-Pr5 Sm III phz) 2 O 4 are not O 1 2. The O O bond lengths observed in lanthanide peroxo (O 2 2 ) complexes are in the order of Å. For example, cerium peroxide, which contains the Ce IV (O 2 ) 2 Ce IV unit, has been known since In 1976 and in 1990, the crystal structures of some complexes that contain this (O 2 ) 2 core, including [{(CO 2 3 ) 4 Ce IV } 2 (µ-η 2 :η 2 -O 2 ) 2 ] 4 [C(NH 2 ) 3 ] + H 4 2 O and K 2 Na 2 [{(edta)ce IV } 2 (µ-η 2 :η 2 -O 2 ) 2 ] 13H 2 O (edta = ethylenediaminetetraacetic acid) were published, respectively. 81,82 The O O bond lengths are 1.455(12) Å in the first structure and 1.488(5) Å in the second 220

34 structure. In 2002, the complexes Ln III 4(O 2 ) 2 Cl 8 (py) 10 Py (Ln = Sm, Eu, Gd; py = pyridine) were published. 83 The observed O O bond lengths in these clusters are Sm: 1.538(3) Å; Eu: 1.522(4) Å and Gd: 1.538(5) Å. The observed O O bond lengths of 1.517(4) and 1.509(4) Å in (Cp BIG-i-Pr5 Sm III phz) 2 O 4 are in good agreement with the typical O O bond lengths published for lanthanide peroxo clusters. Based on the crystal structure data, (Cp BIG-i-Pr5 Sm III phz) 2 O 4 is most likely a peroxo complex that contains two O 2 2 units Figure X-ray structure of (Cp BIG-i-Pr5 Sm III phz) 2 O 4. For clarity, hydrogen atoms and iso-propyl groups have been omitted. Bond lengths in the central ring in phenazine in (Cp BIG-i-Pr5 Sm III phz) 2 O 4 were compared with (Cp* 2 Sm III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ) and unbound neutral phenazine. 46,84 The bond lengths in the phenazine ligands of the samarium complexes (Cp BIG-i-Pr5 Sm III phz) 2 O 4 and (Cp* 2 Sm III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ) and of unbound neutral phenazine are shown in Figure In (Cp* 2 Sm III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ), the bridging phenazine is doubly reduced. The extra electrons in the anti-bonding π* orbitals result in an increase in bond lengths 221

35 within the phenazine moiety, which is reflected in the increased C N bond lengths in the phenazine unit in the X-ray structure of (Cp* 2 Sm III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ). In (Cp BIG-i-Pr5 Sm III phz) 2 O 4 however, the C N bond lengths in the phenazine unit are very similar to the bond lengths observed in unbound neutral phenazine, which presents strong proof that the phenazine coordinates to the samarium center as a neutral ligand. Hence, a composition of the complex analogous to (Cp BIG-Et5 ) 1 Sm III (phz 1 )(O 1 2 ) can be excluded. Figure Bond distances (Å) in the central ring of the phenazine ligand in (Cp BIG-i-Pr5 Sm III phz) 2 O 4 (left), in unbound neutral phenazine (middle) and in (Cp* 2 Sm) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ) (right). No lanthanide complexes with a neutral phenazine or with an η 1 -coordinating phenazine where found in literature and (Cp BIG-i-Pr5 Sm III phz) 2 O 4 is the first example of a lanthanide complex which bears a neutral phenazine as well as the first example of a lanthanide complex which bears an η 1 -coordinating phenazine. Several lanthanide complexes with a phenazine ligand have been published. Samarium complex (Cp* 2 Sm III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ) with a bridging dianionic phenazine was published in 1994, see Eq This complex can also be synthesized from Cp* 3 Sm III and phenazine. 26 In this sterically induced reduction, the one-electron oxidation of the Cp* ligand with formation of the radical coupling product (C 5 Me 5 ) 2 is the driving force of this reaction. Evans et al. published the double reduction of phenazine with Cp* 3 La III to yield both (Cp* 2 La III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ), analogous to the Sm III complex shown in Eq. 6.15, and the oxidation coupling product (C 5 Me 5 ) In 2009, the double reduction of phenazine with complexes [(C 5 Me 4 H) 2 La III (THF) 2 ] 2 (µ-η 2 :η 2 -N 2 ) and [(C 5 Me 4 H) 2 Lu III ] 2 (µ-η 2 :η 2 -N 2 ) was published. 85 In these reactions, the complexes (Cp* 2 Ln III ) 2 (µ-η 3 :η 3 -C 12 H 8 N 2 ) (Ln = La, Lu), analogous to Eq. 6.15, were formed. Apparently, (L x M) 2 (C 12 H 8 N 2 ) type complexes (L x = one or more 222

36 ligands; M = lanthanide or actinide) are stable products since a room temperature reaction of [(C 8 H 8 )Cp*U IV ] 2 (µ-η 3 :η 3 -C 8 H 8 ) with phenazine yields [(C 8 H 8 )Cp*U IV ] 2 (µ-η 1 :η 1 -C 12 H 8 N 2 ), a product with a bridging dianionic phenazine, see Figure The structure of this uranium complex is remarkable with respect to the phenazine that is η 1 -coordinating. In the analogous lanthanide complexes, the lanthanide center coordinates in a side-on azide allyl fashion. 26,85 Figure Crystal structure of the reaction product of [(C 8 H 8 )Cp*U IV ] 2 (µ-η 3 :η 3 -C 8 H 8 ) with phenazine. 86 The Sm O distances in (Cp BIG-i-Pr5 Sm III phz) 2 O 4 are 2.338(3) and 2.273(3) Å, which indicate asymmetrical bridging of the peroxo units between the Sm III centers, presumably because of steric strain of the coordinated phenazine. The longer Sm O bond is closest to phenazine. This is in the same range, but slightly shorter than the average Sm O distance of 2.378(2) Å in the published Sm III 4 (O 2 ) 2 Cl 8 (Py) 10 Py cluster. 83 The Sm O distances in monomeric hydrotris(3,5-dimethylpyrazolyl)borate (Tp Me2 ) complex Tp Me2 2Ln III (η 2 -O 2 ) are 2.329(3) and 2.321(3) Å, which indicates symmetrical bonding of O 2 to Sm III. 78 These distances are comparable to those observed in (Cp BIG-i-Pr5 Sm III phz) 2 O 4. The Sm (Cp ring centroid) distance in Evans complex (Cp* 2 Sm III ) 2 (µ-o) is 2.469(19) Å, 57 and in Cp BIG-Et5 2Sm II this distance is (3) Å. The same distances in complex (Cp BIG-i-Pr5 Sm III phz) 2 O 4 are significantly longer than in (Cp* 2 Sm III ) 2 (µ-o) and are (19) and (19) Å, which makes them nearly identical to those in Cp BIG-Et5 2Sm II. 223

37 Table 6.1. Cp (M ring centroid) distances (Å). Complex Cp (M ring centroid) (Cp BIG-i-Pr5 Sm III phz) 2 O 4 [a] (19) (19) Cp BIG-Et5 2Sm II (3) Cp BIG-Et5 2Sm III [(ArCO) 2 ] [b] (10) (10) [Cp BIG-Et5 Sm III (SPh) 2 ] (1) [a] Values for the two residues in the asymmetric unit. [b] Values for the two different Sm (Cp BIG-Et5 ring centroid) distances Reactivity of Cp BIG-Et5 2Sm II with carbonyl compounds Single-electron reduction of ketones yields ketyls. This reaction was published in the early 20 th century with alkali metals as the reducing agent This discovery led to new reactivity with many reducing agents and carbonyl species. For example, in the McMurry coupling, Ti III Cl 3 /LiAlH 4 can reductively couple carbonyl compounds to alkenes. 90 Another well-established reaction is the Pinacol coupling, 91 where a Mg 0 /Mg II I 2 mixture (that presumably yields highly reducing Mg I I) couples two ketones to a 1,2-diol. 92 This reaction can also be catalyzed by low-valence cerium 93 and samarium. 94 Carbonyl compounds with divalent organometallic lanthanide complexes exhibit a different reactivity. Bulky hydrotris(pyrazol-1-yl)borate (Tp R2 ) sandwich complexes have been published for Eu II (R = Me, 95 i-pr 96 ), Yb II (R = Me, 97,98 i-pr 96 ), Sm II (R = Me, 97,99, i-pr 100 ) and Tm II (R = i-pr 100 ), see Figure X-ray crystallography shows that these Tp R2 2Ln II (Ln = Yb, Sm, Tm; R = Me, i-pr) complexes adopt a bent-sandwich complextype structure with B Ln N angles of

38 Figure Synthesis of several bulky hydrotris(pyrazol-1-yl)borate (Tp R2 ) complexes These sandwich complexes can stabilize ligands with a radical anion character and allow the isolation of these reactive species. For example, addition of phenantrenoquinone to Tp Me2 2Sm II resulted in formation of dark-red Tp Me2 Sm III (η 2 -O 2 C 14 H 8 ), see Figure 6.16a. 101 The radical anion nature of the ketone ligands is demonstrated by the strongly shifted 1 H signals of the ligands in their NMR spectra, and by the intense colours of the complexes. 101 Moreover, the carbon oxygen bond lengths in these complexes are significantly longer compared to the unbound neutral ligand. 101 For example, the C=O bond in unbound neutral fluorenone is 1.220(4) Å compared to 1.313(8) Å in Tp Me2 2Sm III OC 13 H 8, synthesized from Tp Me2 2Sm II and fluorenone, see Figure 6.16b. 101 Evans et al. allowed Cp* 2 Y III BPh 4 to react with fluorenone and obtained diamagnetic [(Cp* 2 Y III (OC 13 H 8 )(THF) x ][BPh 4 ]. 104 In this complex, fluorenone is a neutral adduct and not a ketyl radical. The 1 H NMR and EPR spectra from the crude reaction mixture suggest formation of a second, paramagnetic product with the composition Cp* 2 Y III (OC 13 H 8 ) n (THF) x (n = 1 2). Evans et al. suggested that this product was formed after single-electron reduction of fluorenone and that this electron comes from BPh 4, as evidenced by the presence of BPh 3 and Ph 2 in the reaction mixture. Hou et al. 225

39 allowed Cp* 2 Ln II (Ln = Sm, Yb) to react with fluorenone in THF. The reaction goes to completion within 20 min. at room temperature and they isolated the ketyl complexes Cp* 2 Ln III (THF)(OC 13 H 8 ) in 71 % (Sm) and 85 % (Yb) yield, respectively. 102 The crystal structure of Cp* 2 Sm III (THF)(OC 13 H 8 ) is shown in Figure Unlike lanthanide Cp* and Tp R2 ketyl species, Cp BIG lanthanide ketyl complexes are not known in literature. Since Cp BIG-Et5 2Sm II does not react with carbon monoxide or carbon dioxide under forcing conditions, the reactivity of this sandwich complex with a series of organic carbonyl substrates was explored. In the current study, fluorenone quickly reacts at room temperature with Cp BIG-Et5 2Sm II to form extremely thin microcrystalline plates. This material is insoluble in common organic solvents and repeated attempts to identify its structure by X-ray crystallography were unsuccessful. The reaction of Cp BIG-Et5 2Sm II with fluorenone was therefore not further studied. Figure Crystal structure of Cp* 2 Sm III (THF)(OC 13 H 8 ). 102 For clarity, hydrogen atoms have been omitted. Tp Me2 2Sm II quickly reacts with benzophenone to yield the ketyl complex Tp Me2 2Sm III OCPh 2, see Figure 6.16c. 101,103 The Y III complex [Cp* 2 Y III (OCPh 2 ) 2 ][BPh 4 ] was prepared from Cp* 2 Y III BPh 4 and benzophenone, see Eq Evans et al. also prepared the analogous Sm complex [Cp* 2 Sm III (OCPh 2 ) 2 ][BPh 4 ] from Cp* 3 Sm III and benzophenone. 104 In these reactions, benzophenone is not reduced, and forms a 226

40 neutral adduct in the complexes [Cp* 2 Ln III (OCPh 2 ) 2 ][BPh 4 ] (Ln = Y, Sm). Remarkably, in the current study, benzophenone does not react with a solution of Cp BIG-Et5 2Sm II in benzene-d 6, even at 100 C. Eq Addition of 2,6-di-tert-butylbenzoquinone to Tp Me2 2Sm II results in formation of monomeric Tp Me2 2Sm III (η 1 -OC 6 H 2 t-bu 2 O), see Figure 6.16d. 97 Bending and twisting of the two Tp Me2 ligands in Tm Me2 2Sm II allows the bulky nature of 2,6-di-tert-butylbenzoquinone to slip in between the two ligands and coordinate to the metal center with one of the two oxygen atoms. In the product Tp Me2 2Sm III (OC 6 H 2 t-bu 2 O), Figure 6.16d, the two Tp Me2 ligands coordinate in an η 3 -fashion to the Sm III center. 97 The steric bulk of the two ligands stabilize the radical anion of the ligand. Lopes et al. 101 claim that the less-bulky 1,4-benzoquinone is not sterically hindered enough to prevent dimer formation with this ligand, and that both the monometallic and bimetallic complexes can be prepared. Bimetallic complex (Tp Me2 2Sm III ) 2 (µ-oc 6 H 4 O) with a double reduced 1,4-benzoquinone ligand crystallizes as yellow crystals, see Figure 6.16e. In contrast, the monomeric complex Tp Me2 2Sm III (OC 6 H 2 t-bu 2 O), see Figure 6.16d, prepared from the more bulky 2,6-di-tert-butylbenzoquinone is intense red, as expected for a radical anion. 97 Addition of 1 equiv. of 1,4-benzoquinone to Tp Me2 2Sm II, followed by crystallization from toluene or THF always results in formation of yellow dimer (Tp Me2 2Sm III ) 2 (µ-oc 6 H 4 O), see Figure 6.16e. 101 Based on the colour change, the authors claim that initially the monometallic complex Tp Me2 2Sm III (OC 6 H 4 O) analogous to the more bulky Tp Me2 2Sm III (OC 6 H 2 t-bu 2 O), see Figure 6.16d, is formed. However, crystallization always led to formation of crystals of the dimeric compound (Tp Me2 2Sm III ) 2 (µ-oc 6 H 4 O), see Figure 6.16e. In addition, the reaction of two equivalents of Tp Me2 2Sm II with 227

41 1,4-benzoquinone results in quantitative formation of the dimeric complex (Tp Me2 2Sm III ) 2 (µ-oc 6 H 4 O). Benzoquinone instantly reacts at room temperature with Tp Me2 2Sm II to the bimetallic complex (Tp Me2 2Sm III ) 2 (µ-oc 6 H 4 O). 103 Benzoquinone slowly reacts at 50 C with Cp BIG-Et5 2Sm II and in this reaction too, extremely thin, insoluble plates were formed. Due to poor crystal quality, X-ray diffraction was unsuccessful in characterization of the structure of this benzoquinone reaction product. Jing et al. showed that aromatic aldehydes can be efficiently converted into benzils (1,2-diketones) in a coupling reaction in water, catalyzed by N,N-dioctylbenzimidazolium bromide with Fe III Cl 3 as the oxidant. 105 Therefore, experiments were conducted to study the reactivity of Cp BIG-Et5 2Sm II with the 1,2-diketone (ArCO) 2, Ar = 4-i-Pr-C 6 H 4 (cuminil). Cuminil was synthesized from 4-iso-propylbenzaldehyde analogous to the coupling reaction published by Jing et al. Crystallization from hot ethanol at 5 C yielded large, bright-yellow crystals, which were analyzed by X-ray diffraction. The X-ray structure is shown in Figure The compound crystallizes in the monoclinic space group C2/c. After addition of 1 equivalent of cuminil to a benzene-d 6 solution of Cp BIG-Et5 2Sm II, complete conversion of this sandwich complex at ambient temperatures was observed. The 1 H NMR spectrum of the crude reaction mixture suggested clean conversion to a single product. This product proved to be highly soluble in aliphatic solvents and could be crystallized as intense-red blocks from pentane. A block-shaped crystal was selected for X-ray crystallography and the crystal structure of Cp BIG-Et5 2Sm III [(ArCO) 2 ] (Ar = 4-i-Pr-C 6 H 4 ) is shown in Figure

42 a b c d e Figure Structures of Tp Me2 Sm III (η 2 -O 2 C 14 H 8 ) (a), Tp Me2 2Sm III (OC 13 H 8 ) (b), Tp Me2 2Sm III (OCPh 2 ) (c), Tp Me2 2Sm III (OC 6 H 2 t-bu 2 O) (d) and (Tp Me2 2Sm III ) 2 (µ-oc 6 H 4 O) (e). 96,97 229

43 Figure Structure of 1,2-bis(4-isopropylphenyl)ethane-1,2-dione (cuminil). For clarity, hydrogen atoms have been omitted in the crystal structure. Figure X-ray structure of Cp BIG-Et5 2Sm III [(ArCO) 2 ]. For clarity, hydrogen atoms and ethyl- and iso-propyl groups have been omitted. In the product, the (ArCO) 2 ligand is reduced to a radical anion where it coordinates in an η 2 -fashion to form a five-membered ring that consists of two carbon atoms, two oxygen atoms and the samarium center. The carbon oxygen bond lengths in unbound neutral vs. coordinated reduced cuminil are (14) vs (3) and 1.287(3) Å, respectively. Surprisingly, the (ArCO) 2 ligand is slipped in between the two sterically demanding Cp BIG-Et5 ligands. In the literature there is only one example of an Ar 5 Cp sandwich complex where an extra ligand coordinates to the metal center. This complex 230

44 is the tungsten species (Ph 5 Cp) 2 W IV O, in which two Ph 5 Cp ligands and an oxygen atom coordinate to the metal center, see Figure In this tungsten complex there is significant steric strain. The two Cp rings are no longer parallel with respect to each other, coordinate in a slightly deviated η 5 -fashion with W C bond distances ranging from 2.322(6) 2.646(6) Å (averaging 2.469(6) Å), and form an angle of 24.5(4) to allow coordination of the oxygen atom. Moreover, the three C 6 H 5 rings closest to the oxygen atom bent away from the metal center by 2.0(5) 13.6(5). This published bending is very similar to that observed in the X-ray structure of Cp BIG-Et5 2Sm III [(ArCO) 2 ]. The aryl rings closest to the (ArCO) 2 ligand bent away from the samarium center by 3.04(18) 11.23(18). The two Cp rings in Cp BIG-Et5 2Sm III [(ArCO) 2 ] from an angle of 31.35(13) with respect to each other. Figure X-ray structure of (Ph 5 C 5 ) 2 W IV O. For clarity, hydrogen atoms have been omitted. Bending angles ( ) of the Cp Ph bond with respect to the Cp plane are shown. All angles indicate phenyl rings that bent away from the metal center

45 Figure X-ray structure of Cp BIG-Et5 2Sm III [(ArCO) 2 ]. For clarity, hydrogen atoms, ethyl groups and iso-propyl groups have been omitted. Bending angles ( ) of the Cp (ipso-car) bond with respect to the Cp plane are shown. All angles indicate phenyl rings that bent away from the metal center. The top views of (Ph 5 Cp) 2 W IV O and of Cp BIG-Et5 2Sm III [(ArCO) 2 ] are shown for comparison in Figure 6.19 and Figure The orientation of the phenyl rings of Ph 5 Cp close to oxygen and of Cp BIG-Et5 close to the (ArCO) 2 ligand in (Ph 5 Cp) 2 W IV O and Cp BIG-Et5 2Sm III [(ArCO) 2 ], respectively, is similar in both crystal structures. The (ArCO) 2 and oxygen ligands push the closest phenyl rings away from the metal center by 8.89(18) 11.23(18) (Sm) and by 3.6(5) 13.6(5) (W). The dramatic difference in reactivity between Cp* 2 Sm II and Cp BIG-Et5 2Sm II can be explained by the difference in steric hindrance. Cp* 2 Sm II has a bent structure, which renders the metal center easily accessible and highly reactive. On the contrary, the metal center of Cp BIG-Et5 2Sm II is highly shielded by the ten phenyl groups, due to the attractive interactions between the phenyl groups of the two ligands (vide infra). This shielding prevents interactions of the substrate with the metal center. 232

46 C H C interactions observed in the X-ray structures of Cp BIG-Et5 2Sm II and oxidation products In this section the C H C interactions are discussed for a number of the Ar 5 Cp complexes discussed before in this chapter. The synthesis of Cp BIG-Et5 2Sm II is discussed in section Cp BIG-Et5 2Sm II is thermally very stable and a number of small reactive molecules do not react with Cp BIG-Et5 2Sm II, even under forcing conditions (vide supra). Toluene solutions of Cp BIG-Et5 2Sm II are stable up to 130 C. This stability is in apparent contradiction with the structure of the sandwich complex, as steric repulsion between the two Cp BIG-Et5 ligands and decreased complex stability would be expected, based on the bulky phenyl groups. Ruspic et al. analyzed the X-ray structures of several Cp BIG-n-Bu5 2M II (M = Sm, Yb, Ca) complexes and found that the M C distances in these sandwich complexes are remarkably short when compared to those in other mono- Ar 5 Cp complexes. 15 In addition, short distances between the ortho H of a Ph group in one Cp BIG ligand and the ortho C of a Ph group in the other Cp BIG ligand were observed. These contacts are shorter than the sum of the van der Waals radii for C+H (2.90 Å). For Cp BIG-n-Bu5 2Sm II, Harder et al. reported (ortho-h) (ortho-c) distances between the two Cp BIG-n-Bu5 ligands of Å with an average distance of 2.67 Å and an angle of (average: 154 ). 15 In a paper published in 1989, the concept of a strong C H M interaction is discussed. 107 The authors note that neutron diffraction locates the position of the hydrogen nucleus whereas X-ray diffraction locates the electron cloud. 107,108 Due to various reasons, accurate determination of the position of the electron cloud is not always possible in X-ray diffraction. Since there is little variation in C H bond lengths for sp, sp 2 and sp 3 bonds, it is common practice to place hydrogen atoms in idealized positions rather than refining them. These calculated positions correspond best to the highest density position of the electron cloud and the resulting bond lengths are considerably shorter compared to bond lengths obtained from neutron diffraction data, in which case the nucleus is observed. For the analysis of C H C interactions in Cp BIG-n-Bu5 2M II complexes, Ruspic et al. have set the arene sp 2 C H bond lengths to 1.08 Å, a realistic C H distance for neutron diffraction studies, rather than 0.95 Å. 15 To analyze C H C interactions in the Cp BIG-Et5 complexes from the current study, the arene sp 2 C H bond lengths were also set to 1.08 Å. The observed C H C(π) 233

47 interactions contribute to the remarkable stability of Cp BIG-Et5 2Sm II. As expected, Cp BIG-n-Bu5 2Sm II and Cp BIG-Et5 2Sm II are very similar. The strongest C H C distances and angles observed in Cp BIG-Et5 2Sm II are in the range of Å and The average observed C H C distance in Cp BIG-Et5 2Sm II is 2.73 Å for the ortho-h ortho-c and for the C H C angle. A visual representation is shown in Figure Figure C H C interactions observed in Cp BIG-Et5 2Sm II. For clarity, selected hydrogen atoms and ethyl groups have been omitted. The synthesis of (Cp BIG-i-Pr5 Sm III phz) 2 O 4 is described in section Short C H C distances with an average of 2.62 Å (138 ) between ortho-c H protons of the Cp BIG-i-Pr5 ligand and carbon atoms of the phenazine can be observed. In addition, remarkably short C H O distances (average 2.38 Å, 143 ) between ortho-c H Cp BIG-i-Pr5 protons and the oxygen atoms and between phenazine C H and the oxygen atoms can be observed. Given that the sum of the Van der Waals radii for H and O is 2.72 Å, these should be considered as stabilizing attractive interactions. The strongest C H C 234

48 distances and angles observed in (Cp BIG-i-Pr5 Sm III phz) 2 O 4 are in the range of Å and A visual representation with lines is shown in Figure Figure C H C and C H O interactions observed in [Cp BIG-i-Pr5 Sm III phz] 2 O 4. For clarity, selected hydrogen atoms and iso-propyl groups have been omitted. The synthesis of complex [Cp BIG-Et5 Sm III (SPh) 2 ] 2 is described in section Although a larger batch of analytically pure material of [Cp BIG-Et5 Sm III (SPh) 2 ] 2 was not obtained, the C H C interactions in its X-ray structure can still be discussed. There are fewer attractive C H C interactions in [Cp BIG-Et5 Sm III (SPh) 2 ] 2 than in Cp BIG-Et5 2Sm II and in peroxo complex (Cp BIG-i-Pr5 Sm III phz) 2 O 4. There are no C H C interactions between phenyl rings of the two Cp BIG ligands in the complex, which is also reflected in the bending angles of the phenyl rings with respect to the Cp plane, see Table 6.2. Three of the five phenyl rings bent away from the samarium by 10 to 4.0. There are, however, attractive interactions between Cp BIG-Et5 phenyl rings and 235

49 the PhS phenyl ring with average distance and angles of 2.68 Å and 147. The strongest C H C distances and angles observed in [Cp BIG-Et5 Sm III (SPh) 2 ] 2 are in the range of Å and A visual representation with lines is shown in Figure Figure C H C interactions observed in [Cp BIG-Et5 Sm III (SPh) 2 ] 2. For clarity, selected hydrogen atoms and ethyl groups have been omitted. The synthesis of Cp BIG-Et5 2Sm III [(ArCO) 2 ] is described in section In this complex, strong interactions between ortho-c H Cp BIG-Et5 protons and the cuminil oxygen atoms are observed, see Figure The average angle is 139 and the average CH O distance is 2.30 Å, which is 0.42 Å shorter than the sum of the Van der Waals radii. The (ArCO) 2 ligand, which is located between the two Cp BIG ligands introduces significant steric strain in Cp BIG-Et5 2Sm III (ArCO) 2. The two Cp s in Cp BIG-Et5 2Sm III (ArCO) 2 are no longer parallel to each other and form an angle of 31.35(13). As a result, the phenyl rings located on the opposite side of the (ArCO) 2 ligand strongly bent away from the Cp plane by 11.12(15) (ring C131-C136), 3.04(18) (ring C141-C146) and 7.35(18) (ring C231-C236). 236

50 As expected, the shortest C H C distances (averaging 2.56 Å) and C H C angles (averaging 149 ) are observed between phenyl rings located opposite the (ArCO) 2 ligand, where steric strain forces these phenyl rings closer together. Short C H C distances are observed between (ArCO) 2 and Cp BIG-Et5 phenyl rings located close to the (ArCO) 2 ligand. The average distance and angle for these interactions is 2.67 Å and 156. The strongest C H C distances and angles observed in Cp BIG-Et5 Sm III (ArCO) 2 are in the range of Å and and the strongest C H O contacts are in the range of Å and A visual representation with lines is shown in Figure A comparison of angles ( ) of the Cp C ipso (Ar) bond with the Cp plane for Cp BIG-Et5 2Sm II, [Cp BIG-i-Pr5 Sm III phz] 2 O 4, Cp BIG-Et5 2Sm III [(ArCO) 2 ] and [Cp BIG-Et5 Sm III (SPh) 2 ] 2 is shown in Table 6.2. Figure C H C and C H O interactions observed in Cp BIG-Et5 2Sm III [(ArCO) 2 ]. For clarity, selected hydrogen atoms, ethyl- and iso-propyl groups have been omitted. 237