β-phosphinoethylboranes as ambiphilic ligands in nickel-methyl complexes

The ambiphilic beta-phosphinoethylboranes Ph2PCH2CH2BR2 (BR2 = BCy2 (1a), BBN (1b)), which feature the ethano spacer CH2CH2 between the Lewis acidic boryl and Lewis basic phosphino groups, were synthesized in nearly quantitative yields via the hydroboration of vinyldiphenylphosphine. Compounds 1a,b were fully characterized by elemental analysis and by NMR and IR spectroscopy. X-ray crystallographic studies of compound 1b revealed infinite helical chains of the molecules connected through P center dot center dot center dot B donor-acceptor interactions. The ability of these ambiphilic ligands to concurrently act as donors and acceptors was highlighted by their reactions with (dmpe)NiMe2. The zwitterionic complexes (dmpe)NiMe(Ph2PCH2CH2BCy2Me) (2a) and (dmpe)NiMe(Ph2PCH2CH2[BBN]Me) (2b) were generated via the abstraction of one of the methyl groups, forming a borate, and intramolecular coordination of the phosphine moiety to the resulting cationic metal center. Compound 2b was characterized by X-ray crystallography. Furthermore, B(C6F5)(3) abstracts the methyl group of a coordinated borate ligand to generate a free, three-coordinate borane center in [(dmpe)NiMe(la)](+)[MeB(C6F5)(3)](-) (3).