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

Abstract

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, [tiebar above startPN(2,4,6-Me3-C6H2)CH2CH2tiebar above endN(2,4,6-Me3-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 π-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 σ-type. Structural determination of 1 by X-ray crystallography revealed the shortest yet reported Pt−P bond of 2.107(3) Å, consistent with significant double-bond character, and trigonal planar geometries at both the P-atom within the [NHPMes]+ ligand (∑(angles) = 359.99°) and at the Pt-atom (∑(angles) = 360.00°), which indicated that 1 was better described as a Pt(0)−phosphenium rather than as a Pt(II)−phosphide. Reactions of 1 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-κ2P)(dppe-κP)]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.