Inductive modulation of tris(phosphinomethyl)phenylborate donation at group VI metals: Via borate phenyl substituent modification

A series of zerovalent group VI metal complexes of tris(diisopropylphosphinomethyl)phenylborate ([PhB(CH₂PⁱPr₂)₃]^-, PhBP^iPr₃), including [PPN][M(CO)₃(PhBP^iPr₃)] (M = Cr, Mo, W) and the first bimetallics in which PhBP^iPr₃ serves as a bridging ligand via binding M(CO)₃ units at the three phosphorus atoms and the borate phenyl substituent, have been synthesized and fully characterized. Two new tris(phosphinomethyl)borates featuring 3,5-dimethylphenyl and 3,5-bis(trifluoromethyl)phenyl borate substituents were prepared as crystallographically characterized thallium salts, and metallated giving their inaugural transition metal complexes [PPN][M(CO)₃(((3,5-Me)C₆H₃)BP^Ph₃)] and [PPN][M(CO)₃(((3,5-CF₃)C₆H₃)BP^Ph₃)]. A comparative ν(CO) infrared spectroscopic analysis and examination of half wave potentials assessed by cyclic voltammetry supports a ligand donor ranking of Tp > PhBP^iPr₃ ≥ Cp > PhBP^Ph₃ > triphos. For these anionic complexes, in which a lower electrostatic contribution to zerovalent metal-PhBP^R₃ binding is likely operative relative to that present in the zwitterionic complexes most commonly prepared with tris(phosphinomethyl)borates, PhBP^R₃ ligands do not function as strongly donating scorpionates. Nevertheless, PhBP^Ph₃ is a substantially stronger donor than triphos towards zerovalent M(CO)₃; the half wave potentials of [Et₄N][M(CO)₃(PhBP^Ph₃)] are ∼340 mV lower than those of M(CO)₃(triphos). The potentials of the ((3,5-Me)C₆H₃)BP^Ph₃ group VI metal tricarbonyl anions are more negative than those of the corresponding ((3,5-CF₃)C₆H₃)BP^Ph₃ group VI metal tricarbonyl anions by ∼50 mV, suggesting a modest, yet rational, tuning of PhBP^Ph₃ donation via inductive modulation of the borate anion charge.