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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 = mimidazolinylidene , I ∧ I = imidazolinylidene-methylene-imidazolinylidene , and I ∧ I = 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(C 6 H 5 ) 3 , P(OCH 3 ) 3 , t BuNC, I ) were synthesized and structurally characterized for 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. |
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