t-Butyl and Trimethylsilyl Substituents in Nickel Allyl Complexes: Similar but Not the Same

Metal complexes with t-Bu-substituted allyl ligands are relatively rare, especially compared to their conceptually similar trimethylsilyl-substituted analogs. The scarcity partially stems from the few general synthetic entry points for the t-Bu versions. This situation was studied through a modified synthesis for the allyl ligand itself and by forming several mono(allyl)nickel derivatives. After 2,2,6,6-tetramethyl-4-hepten-3-one was converted to the related 5-bromo-2,2,6,6-tetramethylhept-3-ene (A^2tBr), a mixture of Ni(COD)₂ and A^2tBr in the presence of a neutral donor ligand such as MeCN was found to produce the dark red dimeric π-allyl complex [{A^2tNiBr}₂]. Both NMR and X-ray crystallographic data confirmed that the t-Bu substituents are in a syn, syn-conformation, like that in the previously described [{A'NiBr}₂] (A' = 1,3-(TMS)₂C₃H₃) complex. [{A^2tNiBr}₂] will form adducts with neutral donors such as PPh₃ and IMes (IMes = 1,3-dimesitylimidazol-2-ylidene), but the resulting [A^2tNi(PPh₃)Br] complex is not as stable as its trimethylsilyl analog. The [A^2tNi(IMes)Br] complex crystallizes from hexanes as a monomer, with an η³-coordinated [A^2t] ligand, and in contrast to the starting arrangement in [{A^2tNiBr}₂], the t-Bu groups on the A^2t ligand are in a syn, anti-relationship. This structure is paralleled in the trimethylsilyl analog [A'Ni(IMes)Br]. DFT calculations were used to compare the structures of t-Bu- and related trimethylsilyl-substituted complexes.