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

Metal complexes with -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 -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 (ABr), a mixture of Ni(COD) and ABr in the presence of a neutral donor ligand such as MeCN was found to produce the dark red dimeric π-allyl complex [{ANiBr}]. Both NMR and X-ray crystallographic data confirmed that the -Bu substituents are in a , -conformation, like that in the previously described [{A'NiBr}] (A' = 1,3-(TMS)CH) complex. [{ANiBr}] will form adducts with neutral donors such as PPh and IMes (IMes = 1,3-dimesitylimidazol-2-ylidene), but the resulting [ANi(PPh)Br] complex is not as stable as its trimethylsilyl analog. The [ANi(IMes)Br] complex crystallizes from hexanes as a monomer, with an η-coordinated [A] ligand, and in contrast to the starting arrangement in [{ANiBr}], the -Bu groups on the A ligand are in a , relationship. This structure is paralleled in the trimethylsilyl analog [A'Ni(IMes)Br]. DFT calculations were used to compare the structures of -Bu- and related trimethylsilyl-substituted complexes.