N-Heterocyclic Carbene-Phosphinidene and Carbene-Phosphinidenide Transition Metal Complexes

Half-sandwich complexes of the N-heterocyclic carbene-phosphinidene adduct [(IPr)PH] ( 1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by its reaction with dimeric complexes of the type [LMCl 2 ] 2, which afforded the three-legged piano-stool complexes [LMCl 2 {HP(IPr)}] ( 9a / 9b: M = Ru/Os, L = η 6 - p -cymene; 10a / 10b: M = Rh/Ir, L = η 5 -C 5 Me 5 ). Their conversion into the corresponding carbene-phosphinidenide complexes [LMCl{P(IPr)}] ( 11a / 11b: M = Ru/Os; 12a / 12b: M = Rh/Ir) with a two-legged piano stool geometry was studied by NMR spectroscopy in the presence of the strong base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Alternatively, the complexes 11 and 12 were isolated in high yields from the reactions of the carbene-phosphinidene adduct [(IPr)PTMS] ( 2 ) with [LMCl 2 ] 2, whereby formation of the metal-phosphorus bonds was accompanied by elimination of trimethylsilyl chloride (Me 3 SiCl). Theoretical calculations reveal a strong polarization of the phosphorus ligands upon metal complexation, which can be ascribed to the ability of the imidazole moiety to effectively stabilize a positive charge. Dehydrohalogenation of complexes 9 / 10 to 11 / 12 affords a significant increase of the metal-phosphorus bond order, with the carbene-phosphinidenide ligand acting as a strong 2σ,2π-electron donor.