Dinuclear niobium(III) and tantalum(III) complexes with thioether and selenoether ligands [{M^IIIX₂(L)}₂(μ-X) ₂(μ-L)] (M = Nb, Ta; X = Cl, Br; L = R₂S, R ₂Se): Syntheses, structures, and the optimal conditions and the mechanism of the catalysis for regioselective cyclotrimerization of alkynes

A series of dinuclear niobium(III) and tantalum(III) halide complexes with chalcogen donors [{M^IIIX₂(L)}₂(μ-X) ₂(μ-L)] (M = Nb; X = Cl, Br; L = R₂S, R₂Se) (M = Ta; X = Cl, Br; L = R₂S) have been prepared by the reaction of dimetal(V) decahalide (M₂X₁₀) with magnesium and L. The structures of five of those new complexes were determined by X-ray crystallography to have a M-M double bond. It is evidenced that the solvent and the temperature play important roles in achieving high yield and regioselectivity for cyclotrimerization of alkynes. The reaction of the niobium(III) chloride complexes (L = dimethyl sulfide (Me₂S), tetrahydrothiophene (C₄H₈S, THT, thiolane)) with phenylacetylene at room temperature in toluene gave both head-to-tail cycloadded trimers of alkyne, 1,3,5-(Ph)₃-2,4,6-(H)₃-benzene and head-to-head cycloadded 1,2,4-(Ph)₃-3,5,6-(H)₃-benzene, in high yields. The smaller the thioether ligands, the higher the catalytic activity. The niobium(III) chloride complexes with selenoether (L = dimethyl selenide (Me₂Se), tetrahydroselenophene (C₄H ₈Se, THSe, selenolane) have higher rates for the reaction with alkynes, but low activity for the cyclotrimerization. Tantalum(III) chloride complexes (L = Me₂S, THT, tetrahydrothiopyran (C₅H ₁₀S, THTP, thiane)) have lower catalytic activities than the niobium(III) ones, because the Ta-S(μ-L) bond lengths are shorter than those of niobium analogs. The stability of the precursor complexes toward the first oxidative addition depends on the M-S(μ-L) bond strength, and controls the concentration of catalytic active species. The niobium(III) bromide complexes (L = Me₂S, THT) and the tantalum(III) bromide one (L = Me₂S) react with alkynes to give head-to-tail compounds regioselectively, but are less catalytically active than those of chloride complexes.