Organometallic Chemistry and Homogeneous Catalysis

Transcription

1 Organometallic Chemistry and Homogeneous Catalysis Dr. Alexey Zazybin Lecture N1 Kashiwa Campus, October 9, 2009 What compounds we can call organometallic compounds? Compounds containing direct metal-carbon bonds: e e e W e e e OC CO Fe CO CO CO Fe PPh 3 Cl Cl Ru CF 2 OC PPh 3 RO Ph C W OR OR eli PhgBr

2 Organometallic vs. etal-organic 2 Organometallic compounds contain -C bonds etal-organic compound e.g. Ti(Oe) 4 Coordination compounds e.g. [Co(NH 3 ) 6 ] 3+

3 Which of the following compounds is (are) organometallic compound(s)? 3 a) OCH 3 Ti CH 3 O OCH 3 OCH 3 b) NH 3 2+ Cu H 3 N NH 3 NH 3 - Cl c) CH Cl Pt 2 d) CH 2 Cl O O Pt O O Ph Li e P CO OC CO e) e Li Co C O e Li f) OC Co Co CO O C Co CO Li e OC OC P Ph

4 Why organometallic chemistry? 4 a) From practical point of view: OC are related to many industrial processes, especially catalytic processes, e.g. In production of fine chemicals In production of chemicals in large-scale b) Organometallic chemistry is related to material science, e.g. some organometallic compounds can be used to prepare solid materials: Organometallic Polymers, e.g. PBu 3 Pt C C C C PBu 3 n PBu 3 Pt C C C C PBu 3 n

5 Why organometallic chemistry? 5 c) OC are precursors to: - films for coating (η 3 -C 3 H 5 ) 2 Pd Pd film CH 3 C C-Au-C NCH 3 Au film -nanoparticles (appliations in electronic, magnetic, or optical devices or in sensors) d) ain use in organic synthesis and (homogeneous) catalysis. Using metals, you can make complicated organic structures that would be hard to make otherwise. This is because, compared to "standard organic chemistry", metals display new and unusual reaction types.

6 History 1) Zeise s Salt synthesized in 1827 K[Pt(C 2 H 4 )Cl 3 ] H 2 O Confirmed to have H 2 C=CH 2 as a ligand in 1868 Structure not fully known until ) Ni(CO) 4 synthesized in ) Grignard Reagents (XgR) synthesized about 1900 Accidentally produced while trying to make other compounds

7 4) Ferrocene synthesized in 1951 a)odern Organometallic Chemistry begins with this discovery b)any new ligands and reactions produced ever since 7 5) Organometallic Chemistry has really been around for millions of years a)naturally occurring Cobalimins contain Co C bonds b)vitamin B 12

8 Nobel-Prize Winners related to the area: 8 K. Ziegler, G. Natta (1963) Ziegler-Natta catalyst: polymerization catalyst E. O. Fisher, G. Wilkinson (1973) Organometallic sandwich compounds

9 Nobel-Prize Winners related to the area: 9 K. B. Sharpless, R. Noyori, W. S. Knowles (2001) Asymmetric catalysis Y. Chauvin, R. H. Grubbs, R. R. Schrock (2005) Y. Chauvin, R. H. Grubbs, R. R. Schrock (2005) etathesis method in organic synthesis

10 Structure of organometallic compounds 0 L 1 L 2 L 3 L 4 L 6 L 5 at least one ligand is attached to metal through C-atom Properties I. Nature of II. Nature of L II.a. Ligands with C-atom attached to (CH 3, aryls, alkenes, arenes, cyclopentadienyls) II.b. Ligands with other atoms attached to metal (H, halogens, CO, phosphines)

11 I. Nature of ain and Transition etals 1 Early iddle Late

12 Early Transition etals 2 Groups 3,4 Strongly electrophilic and oxophilic Few redox reactions (exception: Ti) Nearly always < 18e Polar and very reactive -C bonds (to alkyl and aryl) Few d-electrons: - preference for "hard" σ-donors - weak complexation of π-acceptors

13 Early Transition etals 3 Groups 3,4 Typical catalysis: Polymerization e e e etc

14 "iddle" Transition etals 4 Groups 5-7 any accessible oxidation states ostly 18e Ligands strongly bound Strong, not very reactive -C bonds Preference for σ-donor/π-acceptor combinations (CO!)

15 "iddle" Transition etals 5 Groups 5-7 Typical catalysis: Alkene and alkyne metathesis CH 2 CH 2 CH 2 CH 2 CH CH 2 2 CH 2 CH 2 CH 2

16 Late Transition etals 6 Groups 8 (and 9) any accessible oxidation states ostly 18e or 16e 16e common for square-planar complexes Easy ligand association/dissociation Weak, not very reactive -C bonds Even weaker, reactive -O/-N bonds Preference for σ-donor/weak π-acceptor ligands (phosphines)

17 Late Transition etals 7 Groups 8 (and 9) Typical catalysis: Hydroformylation H H CO CO O O O H O H H H 2 H 2

18 Transition etals 8 1 st row nd 2 row rd 3 row

19 9 1 st row: often unpaired electrons different spin states (HS/LS) accessible highest oxidation states not very stable 2 nd /3 rd row: nearly always "closed shell" highest oxidation states fairly stable 2 nd row often more reactive than 3 rd

20 II.a. Ligands with C-atom attached to metal Alkyl ligands pure alkyls: alkyls with other functional groups: 0 CH 3 methyl CH 3 CH 2 ethyl CF 3 trifluoromethyl CH 3 CH 2 CH 2 propyl H 3 C H 3 C CH isopropyl H 3 C CH 3 Si CH 2 neosilyl H 3 C CH 3 C tert-butyl HC 3 CH 3 C CH 2 neopentyl CH 3 CH 3 CH 3 H 2 C benzyl norbornyl adamantyl CH 3 C CH 2 neophyl C 2.5 Nature of -C bond: Electronegativity so, polarization is δ+ -C δ CH 3

21 II.a. Ligands with C-atom attached to metal Alkyl ligands 1 Synthesis of etal Alkyls 1) Alkylation with a nucleophilic organometallic species 2) Nucleophilic addition to an electrophilic alkyl 3) Oxidative Addition 4) Insertion

22 II.a. Ligands with C-atom attached to metal Alkyl ligands 2 Internal stability of metal-alkyl compounds: β-hydride elimination ß-Hydride elimination occurs for nearly all transition metal-alkyls that meet the following requirements: 1. The β-carbon must have hydrogen substituents Do not have β-hydrogens: CH 3 CF 3 H 2 C CH 3 H 3 C C CH 2 CH 3 CH 3 CH 3 H 3 C Si CH 2 C CH 2 CH 3 CH 3

23 II.a. Ligands with C-atom attached to metal Alkyl ligands 3 ß-Hydride elimination occurs for nearly all transition metal-alkyls that meet the following requirements: 2. The -C-C-H unit can adopt a roughly syn-coplanar conformation that brings the ß-hydrogen close to the metal center. Can not adobe syn-coplanar conformation: 3. There is a vacant coordination site (orbital) cis to the alkyl substituent

24 II.a. Ligands with C-atom attached to metal Alkyl ligands 4 Stability of metal-alkyl compounds towards water, oxygen, etc. ain group and early transition metal alkyls are very sensitive to water and oxygen. Late transition metal alkyls are more stable to water and oxygen. Both main group and transition metal alkyls are unstable towards halogenes and solutions of halogenes: R + I 2 I + R I Agostic interactions The word agostic is derived from the Greek word for "to hold on to oneself". It most commonly refers to a C-H bond on a ligand that undergoes an interaction with the metal complex.

25 Agostic interaction is a term for the interaction of a coordinately-unsaturated Transition metal with a C-H bond, when the two electrons involved in the C-H bond enter the empty d-orbital of a transition metal, resulting in a two electron three center bond. 5 II.a. Ligands with C-atom attached to metal Aryl ligands Aryl ligands are relatively strong anionic two electron donors, essentially just like alkyls. Since they cannot easily β-hydride eliminate (formation of the benzyne intermediate is typically too unstable), metal aryl complexes are usually relatively stable compared to alkyls with β-hydrogens. But stable is a relative term since transition metal aryl complexes are also quite airsensitive and reactive.

26 6 Compare stability of alkyl and aryl ligands: Rare example of benzyne complex 1) etal complex Dissociation energy, kcal/mol CH 3 -n(co) 5 36 C 6 H 5 -n(co) ) TiPh 4 and Tie 4 TiPh 4 decomposes only above 0 C, whereas Tie 4 decomposes at 40 C.

27 It is possible to obtain quite stable aryl complexes if the β-helimination reaction is blocked by replacing both ortho H β-atoms of the phenyl ligands by substituents. This condition is perfectly achieved in the mesityl ligand: 7 Aryls do have the potential for both π-donation and π-backbonding through the filled aryl π-orbitals and empty π* antibonding orbitals. This can provide additional stability to a metal complex, depending on whether the metal needs additional electrons from the ligand or wants to dump excess electron density onto the ligand.

28 Allyl ligands 8 1 η 3 η Allyl ligands can act both as 1 η ligand (similar to any other alkyl) and 3 η ligand (3-electron ligand). Usually 2 resonance forms can be drawn as hybrid: Cyclopentadienyl ligands KC 5 H 5, 1901; Fe(C 5 H 5 ) 2, iller, 1951 H 3 C CH 3 6estrong donor H 3 C CH 3 6e- stronger donor bulky ligand H 3 C Cp Cp*

29 Cyclopentadienyl ligand that normally coordinates in an 5 η mode as a 6e - donor, but it can adopt 3 η- and 1 η-coordination modes: 9 H η 5 η 3 η 1 Different types of coordination can be seen within one range of complexes: OC CO Pe CO Pe 3 e 3 3 P Re e 3 P Re Re CO CO e 3 P CO CO η 5 η 3 η 1 CO CO

30 Cp-variants: 0 2- fulvalenediyl (2-) indenyl (-) fluorenyl (-) o 2.45Å 7edonor azulene Behrens, Angew. Chem., 1987 In compare to Cp, indenyl can more easy switch between 5 η and 3 η coordination mode. Indenyl is also a better π- acceptor than Cp

31 Complexes of the cyclopentadienyl ligand fall into 4 classes: 1 L n X L n Half-sandwich Scorpionate Sandwich Bent-metallocenes Reactivity of ferrocene complex: