Multiple bonds between main-group elements and transition metals: Part 157: Neutral and cationic ansa-metallocenes of niobium( V) and tantalum( V) : Synthesis, structures and stereochemical non-rigidity

Article abstract of DOI:10.1016/S0022-328X(97)00121-6

The metal amide route has been exploited to synthesize η⁵:η¹-and η⁵:η⁵ ansa-metallocenes of pentavalent niobium and tantalum. Tris(dimethylamido)(2,6-diisopropylphenylimido)-niobium(V) 2a and -tantalum(V) 2b were found to be appropriate starting compounds to cleanly react with CH-acidic ligand precursors such as cyclopentadiene and 2,2-bis(cyclopentadienyl)propane. In the latter case, the novel ansa-metallocenes of formula [C₅H₄-C(CH₃)₂-C₅H₄]M[=N(2,6-ⁱPr₂C₆H₃)][N(CH₃)₂] (4a, M = Nb; 4b, M = Ta) result in high yields. While the hydrocarbon ligand adopts an η¹:η⁵-coordination to the metals in the solid state (X-ray diffraction studies), a rapid η¹:η⁵-η⁵:η¹ equilibration occurs in solution. Complexes 4a,b react promptly with (CH³)³SiCl, thus affording the ansa-metallocenes [C₅H₄-C(CH₃)₂-C₅H₄]M[=N(2,6-ⁱPr₂C₆H₃)]Cl (5a, M = Nb; 5b, M = Ta) quantitatively. The crystal structures of the niobium(V) and tantalum(V) chloro derivatives 5a,b reveal a 'conventional' η⁵:η⁵-coordination for the chelating bis(cyclopentadienyl) ligand. Protonolysis of 4a,b with [NHMe₃][B(C₆H₅)₄] afforded the first cationic ansa-metallocenes [C₅H₄-C(CH₃)₂-C₅H₄]M[=N(2,6-ⁱPr₂C₆H₃)][NH(CH₃)₂][B(C₆H₅)₄] (6a, M = Nb; 6b, M = Ta) in high yields. The crystal structure of 6a shows an η⁵:η⁵-coordination mode for the bis(cyclopentadienyl) ligand. The ¹J(C,H) coupling constants of the bis(cyclopentadienyl) ligand of the ansa-metallocenes 4a,b, 5a,b and 6a,b are related to its hapticity, which is a result of the electronic properties of the metal center.