(HYDROTRIS(3,5-DIMETHYLPYRAZOLYL)BORATO)NIOBIUM ALKYNE COMPLEXES - DICHLORO AND DIALKYL DERIVATIVES AND FORMATION OF A BUTADIENYL COMPLEX WITH AN ALPHA-AGOSTIC INTERACTION

A high-yield one-pot synthesis of TpNbCl2(alkyne) is described [Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; alkyne = PhC = CCH3 (1a), PhC = CCH2CH3 (1b), PhC = CCH2CH2-CH3 (1c), CH3C = CCH3 (1d), PhC = CPh ( 1e)]. It involves treatment of NbCl3(DME) (DME = 1,2-dimethoxyethane) with first an alkyne and then with KTp. The dimethyl derivatives Tp*N b(CH3)(2)(alkyne) (alkyne = PhC = CCH3 (2a), PhC = CCH2CH3 (2b), PhC = CCH2CH2CH3 (2c)) and the dibenzyl complex TpNb(CH(2)Ph)(2)(PhC = CCH 3) (3a) have been obtained from the appropriate dichloro complex and 2 equiv of either methyllithium or benzylmagnesium chloride, respective ly. These 16-electron niobium(III) d(2) complexes, 1a-e, 2a-c, and 3a, have been characterized spectroscopically. In all cases the alkyne is shown to occupy the molecular mirror plane. The barrier to alkyne rot ation is high and could be measured only in the case of Id (68 kJ mol( -1) at 358 K). On the basis of qualitative arguments, the structure of these complexes is basically governed by steric interactions. A compa rison with the isoelectronic cyclopentadienyl derivatives is provided. When la is treated with 2 equiv of ethylmagnesium chloride, the forma l dehydrogenation of an ethyl group results in the alkyne-coupled prod uct TpNb(CH2CH3)[C(Ph)C(CH3)CHCH2] (4a), which has been spectroscopic ally characterized. H-1 and C-13 NMR data suggest that an a-agostic in teraction is present in the ethyl group in this complex.