Niobium complexes butadiene

Diaza-l,3-butadienes niobium complexes, 5, 89 with platinum(II), 8, 463... 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

Complexes XVII are labile, in particular in halogenated solvents, and must be prepared at —10 to — 40°C . Butadiene, isoprene, cyclohexadiene, etc., may be used. In contrast, the niobium hydride XVIH reacts smoothly with butadiene to give XK ... 

Mashima K, Matsuo Y, Tani K. Unique complexation of 1,4-diaza-l,3-butadiene ligand on half-metallocene fragments of niobium and tantalum. OrganometaUics 1999 18 1471-1481. 

On the other hand, efficient catalytic dimerization of simple alkenes can be usually achieved by early transition metal alkene or diene complexes. For example the niobium-butadiene complexes 44 and 45 also showed good catalytic activity for dimerization of isoprene into a mixture of the head-to-tail and head-to-head dimers 46 and 47 (Scheme 20). The former catalyst gave products in 85 15 ratio and the latter one gave rise to 70 30 ratio [26]. 

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