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

Wolfgang A. Herrmann, Walter Baratta, Eberhardt Herdtweck

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Abstract

The metal amide route has been exploited to synthesize η51-and η55 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 η15-coordination to the metals in the solid state (X-ray diffraction studies), a rapid η1551 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' η55-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 η55-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.

Original languageEnglish
Pages (from-to)445-460
Number of pages16
JournalJournal of Organometallic Chemistry
Volume541
Issue number1-2
DOIs
StatePublished - 15 Aug 1997
Externally publishedYes

Keywords

  • Multiple bonds
  • Niobium (V)
  • Tantalum (V)

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