Bulky N-Phosphinomethyl-Functionalized N-Heterocyclic Carbene Chelate Ligands: Synthesis, Molecular Geometry, Electronic Structure, and Their Ruthenium Alkylidene Complexes

A new, extremely bulky, and electron-rich N-phosphinomethyl-functionalized N-heterocyclic carbene ligand, 5a (tBuNHCPtBu), and a somewhat less bulky congener, 5b (MesNHCPtBu), forming five-membered chelate rings with metal centers, have been synthesized in four steps starting from the easily accessible di-tert-butyl(hydroxymethyl)phosphine oxide (1). 5a was isolated and fully characterized by spectroscopic methods including UV-photoelectron spectroscopy and X-ray diffraction. The reaction of 5a with [Ru(COD)Cl2]n under hydrogen pressure or with [Ru(p-cymene)Cl2]2 led to the formation of the unsaturated dinuclear complex [Ru(tBuNHCPtBu)(μ-Cl)(Cl)]2 (6), which serves as a precursor for a series of ruthenium carbene complexes (7a–f) using substituted phenyldiazomethanes (p-X-C6H4(CH)N2; X = H (a), Br (b), CF3 (c), NO2, (d), CH3 (e)) and trimethylsilyldiazomethane (f). Treatment of 6 with phosphine or pyridine ligands led to the formation of the mononuclear adducts, [Ru(tBuNHCPtBu)(Cl2)(PR3)] (R = Me (8), Ph ...