Balancing Adduct Formation and Ligand Coupling with the Bulky Allyl Complexes [1,3-(SiMe3)2C3H3]2M (M = Fe, Co, Ni)

Abstract

Passage of CO at atmospheric pressure through solutions of A′2M (M = Fe, Co, Ni; A′ = [1,3-(SiMe3)2C3H3]−) in hexanes produces the corresponding allyl complexes A′2Fe(CO)2, A′Co(CO)3, and A′2Ni(CO), respectively. Although the iron and nickel species can be isolated as pure liquids, the cobalt complex is accompanied by the coupling product 1,3,4,6-tetrakis(trimethylsilyl)-1,5-hexadiene. A′Co(CO)3 was independently prepared from the reaction of Co2(CO)8, A′Br, and PhCH2N(C2H5)3+Cl– in aqueous base. The IR stretching frequencies of A′2Fe(CO)2 and A′Co(CO)3 are lower than those in the unsubstituted analogues, indicating that the trimethylsilated allyl ligand is a better electron donor than the parent version. Density functional theory calculations were performed on various conformations of the complexes, which reproduced the frequency-lowering effect of the trimethylsilyl groups. They also indicate that the thermodynamics of the formation of A′2Ni(CO) and the unknown (C3H5)2Ni(CO) are similar, suggesting that...