N-Heterocyclic Carbene Analogues of Nucleophilic Phosphinidene Transition Metal Complexes

Chloride abstraction from the complexes (η⁶-p-cymene){(IDipp)P}MCl] ( 2 a , M=Ru; 2 b , M=Os) and (η⁵-C₅Me₅){(IDipp)P}IrCl] ( 3 b , IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with sodium tetrakis3,5-bis(trifluoromethyl)phenyl]borate (NaBAr^F) in the presence of trimethylphosphine (PMe₃), 1,3,4,5-tetramethylimidazolin-2-ylidene (^MeIMe) or carbon monoxide (CO) afforded the complexes (η⁶-p-cymene){(IDipp)P}M(PMe₃)]BAr^F] ( 4 a , M=Ru; 4 b , M=Os), (η⁶-p-cymene){(IDipp)P}Os(^MeIMe)]BAr^F] ( 5 ) and (η⁵-C₅Me₅){(IDipp)P}IrL]BAr^F] ( 6 , L=PMe₃; 7 , L=^MeIMe; 8 , L=CO). These cationic N-heterocyclic carbene-phosphinidene complexes feature very similar structural and spectroscopic properties as prototypic nucleophilic arylphosphinidene complexes such as low-field ³¹P NMR resonances and short metal-phosphorus double bonds. Density functional theory (DFT) calculations reveal that the metal-phosphorus bond can be described in terms of an interaction between a triplet (IDipp)P]⁺ cation and a triplet metal complex fragment ligand with highly covalent σ- and π-contributions. Crystals of the C?H activated complex 9 were isolated from solutions containing the PMe₃ complex, and its formation can be rationalized by PMe₃ dissociation and formation of a putative 16-electron intermediate (η⁵-C₅Me₅)Ir{P(IDipp)}I]BAr^F], which undergoes C?H activation at one of the Dipp isopropyl groups and addition along the iridium-phosphorus bond to afford an unusual η³-benzyl coordination mode.