HETEROCYCLIC CARBENES .4. METALS COMPLEXE S WITH HETEROCYCLIC CARBENELIGANDS - SYNTHESIS, STRUCTURE AND STRUCTURAL DYNAMICS

Group-VI transition metal complexes of heterocyclic carbene ligands ar e obtained in 41-45% yields through reaction of azolium precursor salt s with carbonyl metallates. Dicarbene complexes of the type cis-M(CO)( 4) (L^L) and fac-M(CO)(3)(L)(3) (M = Cr, Mo, W) are obtained by optima l choice of the heterocyclic azolinylidene ligands (I = imidazolinylid ene, I^I = imidazolinylidene-methylene-imidazolinylidene, and I^BI = i midazolinylidene-methylene-benzimidazolinylidene) with respect to thei r donor and acceptor properties. These complexes reflect the electroni c properties of their carbene ligands. The tungsten complex cis-W(CO)( 4)(I^BI), exhibiting a mixed-carbene chelate ligand, and the molybdenu m complex, representative of the first example of a tris(carbene) comp lex of the type fac-Mo(CO)(3)(I)(3), were characterized by means of si ngle crystal X-ray diffraction studies. Substituted complexes of the t ype fac-Mo(CO)(3)(L^L)L' (L' = pyridin, P(C6H5)(3), P(OCH3)(3), (t)BuN C, I) were synthesized and structurally characterized for the example of fac-M(CO)(3)(L^L)[P(C6H5)(3)]. Low-temperature NMR-spectroscopy rev eals a dynamic behavior for certain chelating dicarbene ligands, which process implies a butterfly-type conformational movement.