Transition metal carbonyl clusters incorporating silicon

Abstract

This thesis reports the reactions of silicon hydrides with the following metal carbonyls; Fe(CO)₅, Fe₂(CO)₉, Co₂(CO)₈ and Co₄(CO)₁₂. The silicon hydrides investigated include SiH₄, Si₂H₆, PhSiH₃, BrSiH₃, ISiH₃, H₃SiOSiH₃ and H₃SiSSiH₃. The reaction of Si₂H₆ with Co₂(CO)₈ at low reactant ratios (1 : 2.5) gave the cluster H₂Si₃Co₆(CO)₂₀ (3.7). This is a rare example of a silicon-transition metal cluster with Si-H bonds. The structure can be regarded as Si[Co₂(CO)₆(μ-CO)]₂ with both bridging carbonyls replaced by μ-Si(H)Co(CO)₄ groups. Further infrared evidence for the previously postulated cluster Si₂Co₆(CO)₂₀ was also obtained. [Figure 3.7] In attempts to crystallise an unknown product from the reaction of SiH₄ with Fe(CO)₅ and Co₂(CO)₈ using THF, ionic products were formed. [FeCo₃(CO)₁₂]⁻ was isolated with [Fe(THF)₄(H₂O)₂]²⁺ as the counter ion. The structure adopted by [FeCo₃(CO)₁₂]⁻ is very similar to that found for the neutral carbonyl, (Co₄CO)₁₂. [Cp(CO)₂FeGe]₂Fe₃(CO)₉ (3.19) was prepared for NMR and crystallographic studies for comparison with the previously reported silicon and tin analogues. The reaction of PhSiH₃ with Co₄(CO)₁₂ resulted in the formation of the square-bipyramidal cluster (PhSi)₂Co₄(CO)₁₁ (4.6) in high yield. This is compared with the wide range of known (RP)₂M₄Lₓ (R = phenyl or p-tolyl) clusters. [Figure 3.19] [Figure 4.6] Initial investigation of BrSiH₃ with Co₄(CO)₁₂ tentatively identified a similar cluster (BrSi)₂Co₄CO)₁₁, which retains a functional halogen group. The reaction of BrSiH₃ or ISiH₃ with Co₂(CO)₈ shows evidence for the formation of the orange clusters (CO)₄CoXSiCo₂(CO)₇ (X = Br, I). These clusters also retain the halogen group via which further substitution could occur. The reaction of H₃SiOSiH₃ with Co₂(CO)₈ proved to be very unpredictable. Tentatively identified products include two clusters which retain the Si-O-Si linkage, O{[Si[Co(CO)₄]Co₂(CO)₇}₂ and O[SiCo₃(CO)₉]₂. Reaction of the sulphur analogue H₃SiSSiH₃ with Co₂(CO)₈ forms the unique cluster SSi₂Co₄(CO)₁₄ (5.9). This cluster consists of a Co₂(CO)₆ unit doubly bridged by SiCo(CO)₄ groups. Both silicon atoms are further linked together with a sulphur atom giving rise to a very acute Si-S-Si angle of 70.3(1)°. The reaction of the analogous hydride H₃GeSGeH₃, with Co₂(CO)₈ shows no formation of a similar product. At higher temperatures the reaction of H₃SiSSiH₃ with Co₂(CO)₈ forms the previously reported cluster S₂Co₄(CO)₁₀ (5.16) which consists of a rectangular cobalt plane with sulphur atoms quadruply bridging both faces. An accurate structural determination of this cluster allows for a detailed discussion. [Figure 5.9] [Figure 5.16] The reactions of various silicon chlorides (SiCl₄, Si₂Cl₆ and Cl₃SiOSiCl₃) with [Co(CO)₄]⁻ were investigated. With SiCl₄, initial formation of the siloxy cluster Cl₃Si-O-CCo₃(CO)₉ was observed and further substitution resulted in the formation of (CO)₄Co(Cl)₂SiOCCo₃(CO)₉ (6.4, characterised crystallographically). In attempts to further substitute the remaining chlorine atoms the carbido cluster [Co₆C(CO)₁₅]²⁻ (6.9) was formed and the structure of the [Et₄N]⁺ salt was determined. [Figure 6.9] [Figure 6.4]