Phospha-Fischer Carbenes: Synthesis, Structure, Bonding, and Reactions of Pd(0)- and Pt(0)-Phosphenium Complexes

The analogy between cationic group 10 metal-phosphenium complexes and Fischer carbenes has been formalized through structural and reactivity studies and by energy decomposition analysis (EDA) of the M-P bond. The studied compounds were the three-coordinate, 16-electron species [(NHPMes)M(PPh3)(2)]OTf (M = Pt (1) and Pd (2); [NHPMes](+) is the N-heterocyclic phosphenium (NHP) cation, [PN(2,4,6-Me-3-C6H2)CH2CH2N(2,4,6-Me-3-C6H2)](+), OTf = trifluoromethanesulfonate); these were made by reaction of [NHPMes]OTf with M(PPh3)(4). The metal-phosphenium bond in both compounds was dominated by metal-to-ligand pi-donation. This differed from the M-C bonds in the analogous N-heterocyclic carbene (NHC) complexes, (NHCMes)M(PPh3)(2) (M = Pt (6), Pd (7)), which were instead predominantly sigma-type. Structural determination of I by X-ray crystallography revealed the shortest yet reported Pt-P bond of 2.107(3) angstrom, consistent with significant double-bond character, and trigonal planar geometries at both the P-atom within the [NHPMes](+) ligand (Sigma(angles) = 359.99 degrees) and at the Pt-atom (Sigma(angles) = 360.00 degrees), which indicated that I was better described as a Pt(0)-phosphenium rather than as a Pt(II)-phosphide. Reactions of I and 2 with excess PMe3 cleanly gave the four-coordinate species [(NHPMes)M(PMe3)(3)]OTf (M = Pt (3) and Pd (4)), while reaction of 1 with bis(diphenylphosphino)ethane (dppe) gave [(NHPMes)Pt(dppe-kappa P-2)(dppe-kappa P)]OTf (5). Hydrolysis of these complexes resulted in metal hydrides and oxidation of the NHP to phosphine oxide via intial nucleophilic attack of water at the P-atom in the coordinated [NHPMes](+) ligand, which was calculated to bear a significantly positive charge in 1.