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 ((NHCPtBu)-N-tBu), and a somewhat less bulky congener, 5b ((NHCPtBu)-N-Mes), 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)Cl-2](n) under hydrogen pressure or with [Ru(p-cymene)Cl-2](2) led to the formation of the unsaturated dinuclear complex [Ru((NHCPtBu)-N-tBu)(mu-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)N-2; 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((NHCPtBu)-N-tBu)(Cl-2)(PR3)] (R = Me (8), Ph (9), Cy (10)) and [Ru((NHCPtBu)-N-tBu)(Cl-2)(py)(n)] (n = 1 (11), 2 (12); if (py)2 = bipy (13)), which were synthesized in order to find alternative precursor complexes because the dimer 6 showed very low solubility in most organic solvents. Complex 7a was obtained analytically pure on a different route via transmetalation from a silver complex bearing (NHCPtBu)-N-tBu (15) to the first-generation Grubbs catalyst as the ruthenium precursor. Complexes 7a-c and 7e were characterized by X-ray diffraction analysis, revealing a geometry that can be viewed as both a distorted square pyramid and a distorted trigonal bipyramid with the two chloro ligands in a cis configuration. The steric bulk, especially of Sa with its N-tBu moiety, stabilizes 16 VE Ru complexes. In contrast to ligand Sa, the somewhat less bulky (NHCPtBu)-N-Mes ligand 5b has allowed its direct metalation with two ruthenium alkylidene precursors, affording the two new carbene complexes 17 and 18.