SATURATED AND UNSATURATED (HYDROTRIS(3,5-DIMETHYLPYRAZOLYL)BORATO)TUNGSTEN(II) CARBENE COMPLEXES

Reaction of [Tp'(CO)2W(PhC2H)][BF4](Tp' = hydrotris(3,5-dimethylpyrazolyl)borato) with PhC2H or reaction of Tp'(CO)IW(PhC2R) (R = H, Me) with PhC2H/AgBF4 generates cationic bisalkyne complexes [Tp'(CO)W(PhC2H)(PhC2R)][BF4]. NMR and crystal data suggest that the two alkyne ligands donate a total of six electrons to the metal with phenyl groups distal to each other. Crystals of the bis(phenylacetylene) complex belong to the monoclinic space group P2(1)/n, Z = 4, with unit cell dimensions of a = 14.868 (3), b = 13.085 (4), c = 19.835 (3) angstrom, beta = 109.22 (1)-degrees. Refinement of 442 variables over 3155 reflections led to R = 5.2% with R(W) = 7.1%. Deprotonation of the cationic bis(phenylacetylene) complex yields an alkyne acetylide product, Tp'(CO)(PhC2H)W(C = CPh), which exists as two isomers in solution. Protonation of the acetylide compound with aqueous HCl regenerates the bisalkyne cation. Hydride addition to the bis(phenylacetylene) complex forms an alkyne sigma-vinyl complex, Tp(CO)(PhC2H)W(eta-1-CPh = CH2), which also exists as two isomers. Protonation of the eta-1-vinyl complex at C-beta results in the formation of an 18-electron alkyne carbene cation, [Tp'(CO)(PhC2H)W = C(Me)Ph][BF4]. Protonation of the eta-2-vinyl complexes Tp(CO)2W(eta-2-CPh = CHR) (R = Me, H) at C-beta of the eta-2-vinyl ligand forms beta-agostic carbenes [Tp'(CO)2W = C(R)Ph][BF4] (R = Me, Et). The methyl derivative crystallized in the monoclinic space group P2(1)/n with unit cell dimensions of a = 12.87 (1), b = 12.376 (8), c = 20.93 (3) angstrom; beta = 103.34 (7)-degrees, with Z = 4. Refinement of 425 variables over 3200 reflections led to R = 7.0% and R(W) = 8.7%. The W-C (methyl) distance is 2.49 angstrom and the W-C-alpha-C-beta angle is 91-degrees. Rotation of the methyl group which provides the C-H agostic bond is fast for the methyl derivative. Variable temperature NMR experiments on the ethyl derivative show a barrier of 11.7 kcal/mol for interconversion of agostic enantiomers.