HETEROCYCLIC CARBENES .4. METALS COMPLEXES WITH HETEROCYCLIC CARBENE LIGANDS - SYNTHESIS, STRUCTURE AND STRUCTURAL DYNAMICS

Group-VI transition metal complexes of heterocyclic carbene ligands are obtained in 41-45% yields through reaction of azolium precursor salts 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 optimal choice of the heterocyclic azolinylidene ligands (I = imidazolinylidene, I^I = imidazolinylidene-methylene-imidazolinylidene, and I^BI = imidazolinylidene-methylene-benzimidazolinylidene) with respect to their donor and acceptor properties. These complexes reflect the electronic properties of their carbene ligands. The tungsten complex cis-W(CO)(4)(I^BI), exhibiting a mixed-carbene chelate ligand, and the molybdenum complex, representative of the first example of a tris(carbene) complex of the type fac-Mo(CO)(3)(I)(3), were characterized by means of single crystal X-ray diffraction studies. Substituted complexes of the type fac-Mo(CO)(3)(L^L)L' (L' = pyridin, P(C6H5)(3), P(OCH3)(3), (t)BuNC, I) were synthesized and structurally characterized for the example of fac-M(CO)(3)(L^L)[P(C6H5)(3)]. Low-temperature NMR-spectroscopy reveals a dynamic behavior for certain chelating dicarbene ligands, which process implies a butterfly-type conformational movement.