The reaction of (butadiene)tantalocene cation with bulky carbonyl derivatives

The (s-trans-eta(4)-butadiene)tantalocene cation (1, with CH3B(C6F5)(3)(-) anion) adds one molar equivalent of adamantanone to yield the metallacyclic seven-membered ring product 2a. The X-ray crystal structure analysis of th e tetrahydrotantalaoxepine cation 2a reveals a non-planar envelope shaped s tructure of the metallacyclic framework. In solution a conformational equil ibration of this framework is observed to take place on the NMR time scale with a Gibbs-activation energy of Delta G(not equal) (260 K) = 12.5 +/- 0.3 kcal mol(-1). Treatment of(eta(4)-C4H6)TaCp2+ (1) with 1-cyano-adamantane at 60 degrees C in bromobenzene gave the analogously structured 2,2-bis(eta (5)-cyclopentadienyl)-3,6-dihydro-7-(1-adamantyl)-2-tantala-2H-azepine cati on complex (3a) (topomerization barrier in solution: Delta G(not equal) (25 6 K) = 12.1 +/- 0.3 kcal mol(-1)). (C) 1999 Published by Elsevier Science S .A. All rights reserved.