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 ( t Bu NHCP t Bu ), and a somewhat less bulky congener, 5b ( Mes NHCP t Bu ), 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( t Bu NHCP t Bu )(μ-Cl)(Cl)] 2 ( 6 ), which serves as a precursor for a series of ruthenium carbene complexes ( 7a – f ) using substituted phenyldiazomethanes ( p -X-C 6 H 4 (CH)N 2; X = H ( a ), Br ( b ), CF 3 ( c ), NO 2, ( d ), CH 3 ( e )) and trimethylsilyldiazomethane ( f ). Treatment of 6 with phosphine or pyridine ligands led to the formation of the mononuclear adducts, [Ru( t Bu NHCP t Bu )(Cl 2 )(PR 3 )] (R = Me ( 8 ), Ph ( 9 ), Cy ( 10 )) and [Ru( t Bu NHCP t Bu )(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 t Bu NHCP t Bu ( 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 5a with its N - t Bu moiety, stabilizes 16 VE Ru complexes. In contrast to ligand 5a, the somewhat less bulky Mes NHCP t Bu ligand 5b has allowed its direct metalation with two ruthenium alkylidene precursors, affording the two new carbene complexes 17 and 18 .