1. Insertion reactions of Ga(DDP) into metal-halide and metal methyl bonds.

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1 This study mainly focuses on the synthesis and characterisation of new transition metal complexes and compounds of the sterically encumbered aluminium- and gallium-heterocycles E I (DDP) in the low oxidation state +I. Within the limits of this study, new monomeric and unsaturated transition metal compounds as well as complexes with higher nuclearity (two and more metal centers in the cluster core) are synthesized and characterised in more detail, thus authenticating the mentioned conjecture of E I (DDP) being suitable ligands in organometallic chemistry due to their increased steric demand (cone angle 147 vs. 112 ) as well as their different electronic properties compared to ECp*. In this respect, the formation of -E bonds was achieved by using classical synthetic strategies of organometallic chemistry like inserting E I (DDP) into metal-halide and metal-methyl bonds or via substitution of easily replaceable ligands. 1. Insertion reactions of (DDP) into metal-halide and metal methyl bonds. Generally, the -center of the (DDP) ligand becomes more electrophilic on coordination to a metal center. This fact is reflected in a decrease of the - bond lengths observed in all complexes reported so far compared to those in free (DDP) (2.05 Å). If a halide or methyl group is present at the metal center, the electrophilic -center competes with the metal for the electrons of the group and, with only one exception, yields the full migration of the halide or methyl group to the gallium center to give new complexes with a coordinated {(DDP)} motif. The known complexes of this type are summarized in Table 43. Additionally, besides single-insertion, also double insertions into - bonds are possible, if two or more ligands are coordinated at the metal center, as seen in the formation of the compounds 2, 5, and 6. However, based on the results of this study, these double insertion reactions seem to depend on two different properties of the metal centers used; the size of the metal atoms (i.e. the covalent radius) and their coordination number. Thus, comparably small metal centers (e.g. Si, covalent radius of 117 pm) with a high coordination number of 4 only allow mono-insertions (compound 7) according to structure a in Scheme 50, whereas for bigger metal atoms like Sn (covalent radius: 140 pm), a double insertion is observed (structure b, represented by compound 6,). Consequently, if the metal atom has an average size (e.g., Zn, covalent radius 125 pm) and a low coordination number (coord. number: 2), a double insertion is preferred (structure c; compound 2), whereas an increase in coordination number 157

2 complicates (coord. number: 3, structure d and e; compounds 4 and 5) or completely prevents a double insertion (coord. number: 4, structure a, compound 3). a + 4 c (DDP) d b e Scheme 50: structural motives of the products of insertion reactions into - bonds ( = CH 3, chloride, = 2,6- i Pr 2 -C 6 H 3 ), depending on the metal size and its coord. number. In contrast to the previously reported insertion chemistry of ECp* compounds (E = Al,, In), that either show reduction of the metal (e.g. 2+ to 0 ) with formation of Cp*E 2, Cp* transfer and coordination of E 2 to the metal center or a halide atom bridging two or three coordinated Cp* units in the complexes (Chapter II), the coordinated {(DDP)} moiety is less acidic compared to its counterpart {Cp*} and thus remains coordinated at the metal center. This lower acidity is also reflected in the comparably long -Cl bond lengths (2.247 Å to Å in the complexes 1, 3 and 6) compared to those in free [(DDP)Cl 2 ] (2.222 Å). 158

3 Table 43: known compounds synthesized via insertion reactions into - bonds. Compound / E it. [(COE)(η 6 -benzene)h{(ddp)cl}] (1) 1 / 1 This study [(Ph 3 P) 2 h{(ddp)}(μ-cl] 1 / 1 [{(DDP)e} 2 Zn] (2) 1 / 2 This study [{(DDP)Cl}ZnCl(THF) 2 ] (3) 1 / 1 This study [{(DDP)e}e 2 ] (4) 1 / 1 This study [{(DDP)e} 2 e] (5) 1 / 2 This study [e 2 Sn{Cl(DDP)} 2 ] (6) 1 / 2 This study [Cl 3 Si{Cl(DDP)}] (7) 1 / 1 This study [(DDP)( t Bu)(C)] (8) 1 / 1 This study [(DDP)( t Bu)(Cl)] (9) 1 / 1 This study [(Ph 3 P)Au{(DDP)Cl}] 1 / 1 [{(DDP)}Au{(DDP)Cl}] 1 / 2 [(Ph 3 P)Au{(DDP)e}] 1 / 1 [81] [80] [80] [80] 2. Cationic complexes featuring -(DDP) bonds. The rather long -Cl bond lengths (vide supra), accompanied with the observed fast chlorine exchange in 3 also indicate a comparably weak coordination of the chlorides to the gallium center, thus allows its abstraction by a[bar F ] to give cationic complexes of the type [ n {(DDP)} m ] + ( = additional ligand). These cationic complexes, exemplified shown by the isolation of the linear symmetric cationic complex [{(DDP)} 2 Au][BAr F ] (10) from the reaction of [{(DDP)}Au{Cl(DDP)}] with a[bar F ] in flourobenzene, exhibit an rather strong electrophilicity of the coordinated -center in the cation, that becomes visible in the axially coordination of a THF molecule to each gallium center in the isolated complexes [{(DDP). THF} 2 Au][BAr F ] (10. 2THF) and [{THF. (DDP)}Zn(THF)(μ-Cl)] 2 [BAr F 2] (11) (Chapter III.2.). These results point to the general possibility to synthesize cationic transition metal-(ddp) complexes by abstraction of the respective chlorides in [ n {Cl(DDP)} m ] compounds by a[bar F ], which is generally accompanied by the increase in electrophilicity of the coordinated -centers. 159

4 3. onomeric complexes featuring -(DDP) bonds. The substitution of labile olefin ligands is another and very common way to establish T-E I bonds. In this work, a variety of different monomeric transition metal complexes with either one or a maximum of two E(DDP) moieties coordinating the metal center are obtained depending on the transition metal used. Table 44 gives an overview of the known monomeric complexes synthesized via substitution of labile ligands by E(DDP), which are so far restricted to the d 10 metals i, Pd and Pt. Table 44: monomeric complexes synthesized via substitution of labile ligands by E(DDP). Compound / E it. [{(DDP)} 2 i(1,3-cod)] 1 / 2 This study [{(DDP)}i(cdt)] (12) 1 / 1 This study [{(DDP)}i(C 2 H 4 ) 2 ] (13) 1 / 1 This study [{(DDP)}i(styrene) 2 ] (14) 1 / 1 This study [{(DDP)}i(dvds)] (15) 1 / 1 This study [{(DDP)Al}Pd(dvds)] (16) 1 / 1 This study [{(DDP)}Pd(dvds)] 1 / 1 [{(DDP)} 2 Pt(1,3-cod)] 1 / 2 [159] and [160] [159] and [160] trans-[{(ddp)} 2 Pt(H) 2 ] (17) 1 / 2 This study cis-[{(ddp)} 2 Pt(H)(SiEt 3 )] (18) 1 / 2 This study Whereas the olefins of the ewis-acidic i(0) center in i(cdt) can not be replaced by the σ-donor ligand (DDP) and instead leads to the formation of the 18VE ewis acid base adduct [(cdt)i{(ddp)}] (12) in nearly quantitative yields, the substitution of labile olefins in other d 10 -metal olefin complexes like i(c 2 H 4 ) 3, Pd 2 (dvds) 3 or Pt(cod) 2 is readily observed. However, the coordinated cdt-ligand in 12 can easily be replaced by other olefins like ethylene, styrene or dvds giving the compounds [{(DDP)}i(C 2 H 4 ) 2 ] (13), [{(DDP)}i(styrene) 2 ] (14) and [{(DDP)}i(dvds)] (15), respectively. 160

5 As a result of this study, three general structural motives of possible (i.e. isolable) products of these substitution reactions can be identified, which are depicted in Scheme 51. It seems, that the nature of the metal center (i.e. its size, reflected by its covalent radius) and the co-ligands present in the reaction mixture are relevant factors for the observed structures of the products in such reactions. f g = small metal, = non-chelating olefin + 3 (DDP) + 2 = d 10 metal = 2 or olefin 2 = chelating olefin + i(cdt) + (cod) 2 i 12 ewis-acidic metal h = big metal Scheme 51: structural motives of the products of substitution reactions of olefins at d 10 metal centers ( = 2,6- i Pr 2 -C 6 H 3 ). Thus, due to its tremendous steric demand, a coordination of more than one (DDP) ligand to the rather small i 0 center (covalent radius: 115 pm) similar to structure h in Scheme 51 is not possible, leading to decomposition of the formed complex, as already discussed for [{(DDP)} 2 i(1,3-cod)] (Chapter III ). Instead, only one olefin is substituted and stable complexes of the type [{(DDP)}i 2 ] ( = non-chelating olefins, structure f, Scheme 51) are synthesizable. However, increasing the size of the metal center, i.e. when going from i 0 to Pt 0 (covalent radius: 129 pm), the complex [{(DDP)} 2 Pt(1,3-cod)] [159, 160] is isolable. Obviously, the Pt-center is large enough to allow coordination of two (DDP) ligands, giving a structure similar to h. 161

6 Whereas the covalent radii of Pt and Pd are almost identical (129 pm vs. 128 pm), one might suggest, that the substitution of olefins in a Pd 0 complex like Pd 2 (dvds) 3 lead to the coordination of two (DDP) ligands and thus, a structure similar to h for the reaction product. The synthesis of the complexes [{(DDP)}i(dvds)] (15) and [{(DDP)E}Pd(dvds)] (E =, Al(16)) however indicate, that also the co-ligands are crucial for the outcome of these substitution reactions. Accordingly, if a chelating olefin like dvds is present in the reaction mixture, the size of the metal center becomes neglible and only compounds of the type [{(DDP)}] ( = chelating olefin, structure g) are obtained. The chelating effect of the co-ligand dvds might be responsible for the stability of those complexes. Additionally, the elongation of the C=C bond distances in all reported complexes indicate, that (DDP) is a rather strong σ-donor ligand and thus comparable to electron rich phosphines or - heterocyclic carbenes, HCs. These results show, that, in contrast to the well examined ECp* ligands, the low-valent, bulky group 13 bis-imidinates E I (DDP) exhibit a great potential in terms of synthesis and stabilisation of unsaturated transition metal fragments. Further studies will now mainly focus on a detailed examination of the properties of these ligands to control the reactivity of small, electron rich and coordinatively unsaturated complexes of the type [(E I ) n ] towards typical building blocks in organic chemistry (like olefins, H 2, C-, Si- and in general element- bond activation reactions) and a comprehensive comparison with well established ancilliary ligands like phosphins or -heterocyclic carbenes HCs. In this respect, going from compounds containing d 10 metal centers like i(0), Pd(0) or Pt(0) to d 8 -complexes of e.g. h(i), Ir(I), u(0), Pd(II) or Pt(II) seems to be very promising. Thus, related compounds with phosphines or HCs have shown to be very reactive in bond activation reactions. As seen in previous studies, the six-membered group 13 bis-imidinates E(DDP) also exhibit a comparable reactivity towards such transition metal fragments. Based on -spectroscopic studies, it is suggested, that e.g. the reaction of [(Ph 3 P) 3 ucl 3 ] with (DDP) yields in a C-H bond activation and the formation of one or more orthometallated species, although these compounds could not be crystallographically investigated in more detail so far. The catalytic hydrogenation of cyclooctadiene at the Pt(II) complex [{(DDP)} 2 Pt(H) 2 ] (17) (chapter III.3.3.) clearly points in this direction. In order to have a catalytic activation of a strong element- bond, both thermodynamic, as well as kinetic premises must be fulfilled. For a thermodynamic preferred process, new strong 162

7 bonds must be established, if strong bonds are catalytically broken during the activation reaction. From a kinetically point of view, the relevant intermediates must be reactive enough to activate such strong bonds. In this respect, the prevention of the possible alkyl- or aryl group transfer in such activation reactions from the transition metal in [(E I ) n ( )] to the group 13 metal center to give [(E I ) n-1 (E )] is of general importance. Otherwise, the formation of the thermodynamically preferred E- bond is in conflict with a catalytic bond activation. One possibility to overcome this problem is the use of ligand systems, which are designed in such a way, that these alkyl transfer reactions are electronically unfavourable or sterically impossible. The specific features of the six-membered heterocycles E I (DDP) make them advantageous in comparison to their ECp* congeners. Besides a sterically blocking, these ligands also exhibit a sp 2 hybridized E I -system, in which no energetically favourable, free and electrophilic orbital is available at the group 13 metal center. Thus, together with the comparably less reductive properties, a catalytic course of the reaction may be possible. The nicely reversible oxidative addition of HSiEt 3 to the unsaturated fragment [{(DDP)} 2 Pt], as well as the obviously catalytic H/D exchange reaction with the solvent C 6 D 6, which is suggested to proceed via a C-D activation of C 6 D 6 and the formation of a Pt(IV) intermediate (chapter III.3.3.), support, that these ligands possess a notably potential for bond activation reactions, which might stimulate further studies. 4. complexes with higher nuclearities featuring -(DDP) bonds. ulti-atom, naked metal clusters are increasingly the subject of investigation by complex physical and quantumchemical methods, for example, in nanotechnology because of their special properties in the transitional region between molecular and solid-state chemistry. [262] Also, metalloid cluster compounds can be seen as intermediates on the way to the corresponding element. [255] In this respect, the synthesis of mixed-metal homoleptic and heteroleptic, E I substituted complexes and clusters of the general type [ a E b (E I ) c ] m (with a,b,c,m = 0, 1, 2, ; = different metals; E = different group 13 metals) is a synthetic challenge also in organometallic chemistry. The general possibility of the E I ligands ( = Cp*, alkyls ) to stabilize large clusters of this type was disclosed in 2004, when Schnöckel and co-workers reported on the synthesis of the large Al 50 cluster [Al 38 (AlCp*) 12 ], which is stabilized solely by AlCp* ligands. [263] The synthetic approach of Schnöckel et al. is the controlled disproportionation reaction of metastable Al(I) and (I) halide solutions and the 163

8 concomitant substitution of the halides by appropriate ligands to trap the resulting clusters. However, by doing so, this modus operandi is somehow limited to pure main group metal clusters. An alternative way comes from the idea, to use small transition metal complexes of the group 13 organyls ECp* as building blocks for the systematic synthesis of larger cluster compounds. This type of synthesis was recently embarked by our group and yielded in the controlled synthesis of homoleptic clusters of the type [ 2 (E I Cp*) 5 ]. [54] In contrast to that, the high steric demand of the DDP moiety prevents the formation of homoleptic /(DDP) complexes or clusters, thus leading to heteroleptic dinuclear complexes of the type [ 2 {(DDP)} 2 n ], with one or more additional ligands coordinating the metal center (chapter III.4.). In these reactions, the choice of the transition metal olefin precursor as well as the stoichiometry of the reactants is most often crucial for the formation of the new compounds, which are summarized in Table 45. Table 45: dinuclear compounds with coordinated E(DDP). Compounds / E it. [{(C 2 H 4 ) 2 i} 2 (μ 2 -(DDP)] (19) 2 / 1 This study [{(C 2 H 4 )i} 2 (μ 2 -(DDP))(μ 2 -C 2 H 4 )] (20) 2 / 1 This study [(cod)i 2 {μ 2 -(DDP)}{μ 2 -(PhC CPh)}(PhC CPh)] (21) 2 / 1 This study [{(DDP)}Pd(CO)] 2 1 / 1 [159] and [160] [Pd{μ 2 -(DDP)}( t BuC)] 2 (22) 2 / 1 This study [{Pd(dvds)} 2 {μ 2 -Al(DDP)}] (23) 1 / 1 This study [Pd 2 (Cp*) 2 (μ 2 -Cp*) 2 {μ 2 -Al(DDP)}] (24) 2 / 4 / 1 This study [{(DDP)}Pt(CO)] 2 1 / 1 [159] and [160] [Pt{μ 2 -(DDP)}( t BuC)] 2 (25) 1 / 1 This study [Pt{μ 2 -(DDP)}(H) 2 ] 2 (26) 1 / 1 This study Based on the results of this study, four general structural motives can be identified for the dinuclear transition metal-e(ddp) complexes, which are shown in Scheme 52. Whereas in all dinuclear examples synthesized in this study the E I (DDP) ligand is found in a bridging position between the two transition metal centers, it is most likely, that the nature of the coligand affects the structure of the formed dinuclear complexes containing a E I (DDP) moiety, i.e. the arrangement of the additional ligands surrounding the cluster core. Thus, as represented by the compounds 22 and 25 and their related CO complexes [{(DDP)}(CO)] 2 ( = Pd, Pt), the use of strong π-acceptors like CO or isonitrils yields 164

9 in the formation of a dinuclear compound with two E(DDP) ligands, which, due to their strong σ-donor capabilities, are bridging the two metal centers, whereas the π-acceptor ligands are terminally coordinated to the metals (structure i). 1 Al j = strong σ-donors E I (DDP) 1 = strong π-acceptors, 2e or 4e π-donors 2 = 1 or chelating olefins k = strong π-acceptors, weak σ-donors i Al l chelating olefins Scheme 52: structural motives for dimeric complexes featuring -E I (DDP) bonds. In contrast to that, the use of strong (however sterically less demanding) σ-donors (e.g. Cp*) gives a 2 {μ 2 -E(DDP)}-core, which is additionally coordinated by two terminal and two bridging donors, giving the structural analogue to [ 2 (ECp*) 5 ] ( = Pd, Pt) (structure j, complex 24). As seen in the formation of the complexes 19, 20 and 21, the use of π-donor ligands (like olefins or acetylenes) gives dinuclear complexes with one bridging and two (20) or three terminal (19) π-donors, which could also be chelating olefins (21) (structure k). Finally, if only chelating olefins are present in the reaction mixture, the formation of a complex with two chelated metals bridged by a E(DDP) moiety (structure l), comparable to 23, can be expected. 165

10 Additionally, the difference between Al(I) and (I) ligands, which was reported in both theoretical [49] as well as experimental manner for the E I Cp* compounds [45, 55], is also reflected in the bulky bis-imidinates E(DDP). Whereas the monomeric complex [(dvds)pd{(ddp)}] is the exclusive product in the reaction of [Pd 2 (dvds) 3 ] with (DDP), the 1:1 reaction with Al(DDP) gives the dimeric complex [Pd 2 (dvds) 2 (μ 2 -Al(DDP)] (23), with the Al(DDP) again being located in a bridging position between the two Pd(0) centers. The monomeric compound [(dvds)pd{al(ddp)}] (16) can only be synthesized by using an excess (2.5 eq.) of Al(DDP) in this reaction. This observed preference of Al(DDP) to be coordinated in bridging mode is also observed for the AlCp* congener, as the previously described substitution of the bridging Cp* units in [Pt 2 (Cp*) 5 ] by AlCp* to give [Pt 2 (µ 2 [45, 55] -AlCp*) 3 (Cp*) 2 ] reveals. According to this, comparably stronger σ-donor abilities towards transition metal centers can be suggested for Al(DDP) than for (DDP). However, the general problem of a limitation in the cluster size by the steric bulk of the ligands, as already discussed in a comprehensive study for ECp* by Tobias Steinke in 2005, [264] is also obvious from the results of this study. Besides one exception, only compounds with two transition metals in the cluster core are obtained, in which the E I (DDP) ligand is always located in a bridging position rather than showing a terminal coordination mode. Further supported by the fact, that in all of these dimeric complexes only small coligands (linear carbonyls or isocyanides, H-atoms) are coordinated in terminal position, which is presumably because of steric bulk of the bridging E(DDP) ligands, a building block synthesis of larger clusters [ a {E(DDP)} b ] (with a > 3) seems to be hopeless. In consideration of this, a decrease in steric demande, e.g. by substituting the 2,6- diisopropylphenyl groups of the DDP-ligand with smaller groups like phenyl- (PPP) or t Bugroups (TTP), might help to overcome this problem. However, such compounds of the lowvalent group 13 metals Al(I) or (I) are unknown so far. evertheless, taking into account, that the naked + ion [Cp* 2 ][BAr F ] or related [][Otf] are nowadays quite well accessible and could serve as + sources, [112, 126] the synthesis of the respective (PPP) or (TTP) compounds is only a question of time and will, most likely, open the door for a more facile synthesis of new and larger cluster compounds by this building block approach. 166

11 Another route for the synthesis of larger cluster compounds, as already known and commonly applied for the tetrel atoms, is the reductive coupling reaction of a or 3 precursor ( = e.g. tetrel atom) with an adequate reducing agent (e.g. alkaline earth metals like KC 8 ), in which the naked metal atoms are inserted by adding a (II)-halide, that is completely reduced and inserted into the cluster core. In the case of the group 14 elements, bulky ligands are used for the kinetic stabilizing of the cluster core. [255] Both required properties, the reducing power as well as the stabilizing capability, are combined in the group 13 heterocycles E(DDP), thus allowing the synthesis of the largest pure tin cluster [Sn 17 {Cl(DDP)} 4 ] (28) reported so far. This result points to a general possibility to use the specific features of this ligands, most often described as being exotic, for the synthesis and stabilization of large cluster compounds. A more detailed analysis of the formation process itself, as well as optimization of the reaction conditions and the identification of suitable metal (also transition metal) precursors might be a suitable way to synthesize large clusters and thus get a deeper understanding of the development of the bonding situation on the way to the metallic state. [255] 167