MULTIPLE BONDS BETWEEN MAIN-GROUP ELEMENTS AND TRANSITION-METALS .157. NEUTRAL AND CATIONIC ANSA-METALLOCENES OF NIOBIUM(V) AND TANTALUM(V)- SYNTHESIS, STRUCTURES AND STEREOCHEMICAL NONRIGIDITY

The metal amide route has been exploited to synthesize eta(5):eta(1)- and eta(5):eta(5) ansa-metallocenes of pentavalent niobium and tantalu m. dimethylamido)(2,6-diisopropylphenylimido)-niobium 2a and -tantalum (V) 2b were found to be appropriate starting compounds to cleanly reac t with CH-acidic ligand precursors such as cyclopentadiene and 2,2-bis (cyclopentadienyl)propane. In the latter case, the novel ansa-metalloc enes of formula (CH3)(2)-C5H4]M[=N(2,6-(Pr2C6H3)-Pr-i)][N(CH3)(2)] (4a , M = Nb; 4b, M = Ta) result in high yields. While the hydrocarbon lig and adopts an eta(1):eta(5)-coordination to the metals in the solid st ate (X-ray diffraction studies), a rapid eta(1):eta(5)-eta(5):eta(1) e quilibration occurs in solution. Complexes 4a,b react promptly with (C H3)(3)SiCl, thus affording the ansa-metallocenes [C5H4-C(CH3)(2)-C5H4] M[=N(2,6-(Pr2C6H3)-Pr-i)]Cl (5a, M = Nh; 5b, M = Ta) quantitatively. T he crystal structures of the niobium(V) and tantalum(V) chloro derivat ives 5a,b reveal a 'conventional' eta(5):eta(5)-coordination for the c helating bis(cyclopentadienyl) ligand. Protonolysis of 4a,b with [NHMe 3][B(C6H5)(4)] afforded the first cationic ansa-metallocenes ]M[=N(2,6 -(Pr2C6H3)-Pr-i)][NH(CH3)(4)][B(C6H5)(4)] (6a, M = Nh; 6b, M = Ta) in high yields. The crystal structure of 6a shows an eta(5):eta(5)-coordi nation mode for the bis(cyclopentadienyl) ligand. The (1)J(C,H) coupli ng constants of the bis(cyclopentadienyl) ligand of the ansa-metalloce nes 4a,b, 5a,b and 6a,b are related to its hapticity, which is a resul t of the electronic properties of the metal center. (C) 1997 Elsevier Science S.A.