Bulky, electron-rich N-phosphanyl-substituted N-heterocyclic carbenes (NHCP ligands: EtCNP 3, MesCNP 5a, DippCNP 5b) are potential ligands for strained, very small bite angle four-membered transition metal chelate complexes with (κ-C,κ-P) metal coordination. Various compounds of this type have been synthesized and were fully characterized inter alia by X-ray crystallography. Reactions of isolated NHCP species 3, 5a, and 5b with appropriate metal precursors furnished a series of (κ-C,κ-P)-coordinated four-membered NHCP chelate complexes for the metals of the nickel triad and for ruthenium. The cis-dimethyl Pd(II) complex (MesCNP-κC,κ-P)PdMe2 (8) reacted at room temperature with acceptor-substituted alkenes such as fumaronitrile and maleic anhydride to yield the corresponding η2-olefin complexes 9 and 10 with reductive elimination of ethane. Palladacyclopentadiene complex 11 was formed by addition of two equivalents of dimethyl acetylene dicarboxylate to complex 8 via alkyne–alkyne coupling.

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.