'1,3-dimetallabenzene' derivatives of niobium and tantalum

The alkylidyne bridged compounds [(cb)(2)M(mu-CSiMe3)(2)M(cb)(2)] (1a, M = Nb; 1b, M = Ta; cb = carbazole) react thermally with one equivalent of alkynes EtC=CEt and Me3SiC=CH to produce new organometallic derivatives 2 and 3, respectively. The molecular structures of 2 and 3 are shown to consist of non-planar six-membered di-metallacycles originating from insertion of an alkyne unit into one of the alkylidyne bridges of 1. Treatment of 1b with 3,5-di-tert-butyl-2,6-diphenylphenol (ArOH) leads to the mono-phenoxide [(ArO)(cb)Ta(mu-CSiMe3)(2)Ta(cb)(2)] 4. Treatment of 4 with Me3SiC=CH leads to the corresponding metallacycle [(ArO)(cb)Ta(mu-CSiMe3){mu-C(SiMe3)}Ta(cb)(2)] 5. Structural parameters for 2b, 3a, 3b and 5 obtained by X-ray diffraction show delocalization is present with four equivalent M-C and two equivalent C-C distances. The non-planar structure is best described as a twist of the planar [M(mu-C)-M] and [C-C-C] units within the ring. The non-planar structure of 2a and 2b is maintained in solution as evidenced by diastereotopic methylene protons for the CH2CH3 substituents. At elevated temperatures coalescence to a simple A(2)B(3) pattern is observed. From the coalescence temperatures of 328 K for 2b an activation barrier of 16.1(5) kcal mol(-1) can be estimated for the adoption of a planar structure. The insertion of Me3SiC=CH produces a 2.4,6-substitution pattern for 3a, 3b and 5 with the 5-H (beta to two metal centers) being downfield shifted to delta 8.43 (3a), 8.64 (3b) and 9.03 ppm (5). (C) 1998 Elsevier Science Ltd. All rights reserved.