Group VI metal complexes of tris(diphenylphosphinomethyl)phenylborate: Modulation of ligand donation via coordination of M(CO)₃ units at the borate phenyl substituent

A series of d⁶ metal complexes of tris(diphenylphosphinomethyl)phenylborate ([PhB(CH₂PPh₂)₃]^-, PhBP₃), including [Et₄N][M(CO)₃(PhBP₃)] (M = Cr, Mo, W), inaugural group VI metal tris(phosphino)borate complexes, and zwitterionic Mn(CO)₃(PhBP₃) have been synthesized and fully characterized. An analysis of IR ν(CO) data for [Et₄N][M(CO)₃(PhBP₃)] indicates that PhBP₃ is significantly less strongly donating than Tp towards zerovalent M(CO)₃ fragments; PhBP₃ does not function as a strongly donating scorpionate in this system as its does towards cationic metal fragments suggesting that PhBP₃ may not function as an effective surrogate of hydrotris(1-pyrazolyl)borate towards zerovalent metals. While the metal centers of [Et₄N][M(CO)₃(PhBP₃)] are very likely still more electron-rich than those of M(CO)₃(triphos), the anions of [Et₄N][M(CO)₃(PhBP₃)] do not provide robust oxidative addition products analogous to those of M(CO)₃(triphos). A new bi-functional role for PhBP₃ was investigated via the synthesis of seven structurally characterized bimetallics in which zerovalent M(CO)₃ and monovalent [Mn(CO)₃]⁺ fragments bind the three phosphine atoms and the borate phenyl substituent. IR ν(CO) data support modest attenuation of PhBP₃ donor ability at phosphorus upon η⁶-phenyl substituent binding, representing a new inductive strategy for tuning tris(phosphino)phenylborate donation at the κ³-phosphine-bound metal fragment.