Phosphorus-substituted carbene complexes: Chelates, pincers and spirocycles

The syntheses, structural characterizations and reactivity patterns of main group and late transition metal carbene complexes of the bis(phosphoranimino)methandiide, [C(Ph2Pdouble bond; length as m-dashNSiMe3)2]2−, and the carbodiphosphorane, Ph3Pdouble bond; length as m-dashCdouble bond; length as m-dashPPh3, are described and compared to previously reviewed early transition metal analogues. Bimetallic spirocyclic aluminum complexes of the former ligand are accessed by spontaneous double deprotonation of the central carbon atom of the parent, CH2(Ph2Pdouble bond; length as m-dashNSiMe3)2, by two equivalents of AlMe3, whereas the synthesis of platinum complexes requires the intermediacy of the tetralithium dimer, [Li2C(Ph2Pdouble bond; length as m-dashNSiMe3)2]2, and elimination of LiCl from a metal chloride precursor. In contrast to the early transition metal analogues, which are N,C,N-pincer, Schrock-type alkylidenes, the C,N-chelated platinum complexes are more akin to Fischer carbenes, and their chemistry is dominated by the nucleophilicity of free nitrogen atom and insertions into labile N–Si bonds. Chelated and pincer carbene complexes of rhodium result from single and double orthometallations, respectively, of the phenyl rings in Ph3Pdouble bond; length as m-dashCdouble bond; length as m-dashPPh3; the latter compounds represent a wholly new class of C,C,C-pincer complexes. Electronic structure calculations show that the metal–carbon interaction in these compounds may be described as a dative, two-electron, C → M σ-bond. The free bis(phosphoranimino)methandiide and carbodiphosphorane ligands, while not having formal six valence electron resonance forms, may be thought of as having “pull–pull” Fischer carbene character, but the metal to which they become coordinated ultimately dictates their chemistry.