Early transition metal and lanthanide bis(iminophosphorano)methandiide complexes; ‘pincer’ and bridging bis(phosphorus) metal carbenes

The spontaneous double deprotonation of bis(iminophosphorano)methane ligands with alkyl or bis(trimethylsilyl)amido metal precursors yields ‘pincer’ carbenes of Group 4 and lanthanide metals. The generality of the system is extended with the dilithiumbis(iminophosphorano)methandiide salt which is obtained by lithiating bis(trimethylsilylimino(diphenylphosphorano))methane with two moles of lithium alkyl. This dilithium salt reacts with metal halides with metathetical elimination of LiCl to access the same types of pincer complexes. Herein the synthesis of a complete triad of Group 4 ‘pincer’ complexes is reviewed and their reaction chemistry demonstrating (a) nucleophilic alkylation, (b) the Lewis Acidity of the metal, (c) 1,2 addition across the MC carbene bond and (d) [2+2] cycloaddition across this MC bond is surveyed. The structural data for the ‘pincer’ complexes suggests strongly that there is a MC carbene double bond. The addition reactions also support this interpretation because a MC bond persists after the addition. The reactivity pattern also strongly suggests that the chemistry of this carbene center is similar to the metal alkylidene complexes, and not the Fischer heteroatom carbene complexes. The chemistry extends to many other elements. The extension to Sm as a representative lanthanide is described. This species adopts a structure very similar to the Group 4 ‘pincers’. Chromium(II) gave a novel bridging carbene dimer instead of the monomeric pincer. This bridging structure is developing as another structural motif in this system.