No abstract

A new, extremely bulky, and electron-rich N-phosphinomethyl-functionalized N-heterocyclic carbene ligand, 5a ( tBuNHCP tBu), and a somewhat less bulky congener, 5b (MesNHCP tBu), 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( tBuNHCP tBu)(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( tBuNHCP tBu)(Cl2)(PR3)] (R = Me (8), Ph (9), Cy (10)) and [Ru( tBuNHCP tBu)(Cl2)(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 tBuNHCP 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 5a with its N-tBu moiety, stabilizes 16 VE Ru complexes. In contrast to ligand 5a, the somewhat less bulky MesNHCP tBu ligand 5b has allowed its direct metalation with two ruthenium alkylidene precursors, affording the two new carbene complexes 17 and 18.

The cis-[RuCl2(PPh3)2(N−N)] (N−N = bipy (1), Me-bipy (2), phen (3), and bathophen (4)) complexes were used to synthesize five new electron-rich phosphine-containing complexes cis-[RuCl2(dcype)(N−N)] (N−N = bipy (1a), Me-bipy (2a), phen (3a), and bathophen (4a)) and cis-[RuCl2(PEt3)2(bipy)] (1c) by phosphine exchange. These complexes were obtained and characterized by NMR (31P{1H}, 1H), cyclic voltammetry, and elemental analysis. Electrochemical studies of these complexes reveal a single reduction process (RuIII/RuII). These complexes are more easily oxidized than their analogues cis-[RuCl2(dppb)(N−N)]. The reactivity of complexes cis-[RuCl2(dcype)(N−N)] with carbon monoxide was tested, and dissociation of one chloride was observed, leading to the formation of four new cationic species with general formula [RuCl(CO)(dcype)(N−N)](PF6) (bipy (1b), Me-bipy (2b), phen (3b), and bathophen (4b)). The complexes described here and elsewhere with general formulas cis-[RuCl2(P−P)(N−N)], [RuCl(CO)(dcype)(N−N)](PF6), and cis-[RuCl2(P)2(N−N)] were used as precatalysts in the transfer hydrogenation of functionalized aryl-ketones, and most of them were active. X-ray structures of cis-[RuCl2(PEt3)2(bipy)] (1c) and [RuCl(CO)(dcype)(bipy)](PF6) (1b) will be presented.