Kinetische und mechanistische untersuchungen von übergangsmetallkomplex-reaktionen : VI. Kinetik und mechanismus der einschiebung von dimethylcyanamid in die metall—carbenkohlenstoff-bindung von pentacarbonyl(arylphenylcarben)wolfram

Abstract

Pentacarbonyl(arylphenylcarbene)tungsten complexes, (CO) 5W[C( p-C 6H 4R)C 6H 5] (Ia, R = OCH 3; Ib, R = CH 3; Ic, R = H; Id, R = Br; Ie, R = CF 3) react with dimethylcyanamide (II) via insertion of the CN group into the metalcarbene bond. The formation of pentacarbonyl[dimethylamino(imino)carbene]tungsten(0) (IIIa–IIIe) follows a second-order rate law: d[III]/d t = k[I][II]. Replacement of R = H by electron-withdrawing substituents (Br, CF 3) results in an increase, by electron-donating groups (CH 3, OCH 3) in a decrease of the reaction rate. The rate constants correlate well with Hammett's σ-constants. The activation enthalpies Δ H ≠ are low (37.3–41.6 kJ mol −1), the activation entropies Δ S ≠ strongly negative (−119 to −133 J mol −1 K −1). The results are discussed on the basis of an associative stepwise mechanism with a nucleophilic attack of the CN group of II at the carbene carbon in the first reaction step.