New Group 5 and 6 transition metal imido complexes with monodeprotonated triazacyclononane ligands

Abstract

The 1,4-dialkyl-1,4,7-triazacyclononanes HR₂[9]aneN₃ (R = Me or Prⁱ) were smoothly deprotonated with n-butyllithium to give corresponding lithiated amides {Li[R₂[9]aneN₃]}_x1, 2 which are in turn useful precursors to new transition metal complexes. Reaction of 1 or 2 with the mono(imido) or bis(imido) complexes [M(NBu^t)Cl₃(py)₂] (M = Nb or Ta) or [Mo₂(NC₆H₃R′-2,6)₂Cl₂(dme)] gave [M(NBu^t)(R₂[9]aneN₃)Cl₂] (M = Nb, R = Me 3 or Prⁱ4; M = Ta, R = Me 5 or Prⁱ6) or [Mo(NC₆H₃R′₂-2,6)₂(R₂[9]aneN₃)Cl] (R = Me, R′ = Prⁱ8; R = Prⁱ, R′ = H 9, Me 10 or Prⁱ11). The compounds 3–6 are relatives of the d⁰ metallocene dichlorides [M(η-C₅R₅)₂Cl₂] (M = Group 4 metal). The titanium imido analogue [Ti(NBu^t)(HPrⁱ₂[9]aneN₃)Cl₂] 7 has been prepared for purposes of comparison and exists as a mixture of isomers in solution. The crystal structure of the bis(imido) compound 11 has been determined and shows that the co-ordination of the amido N-donor of Prⁱ₂[9]aneN₃ is not greatly distorted from planarity. Examination of the molecular structure suggests that in this compound the Prⁱ₂[9]aneN₃ ligand acts effectively as a 5-electron donor with the molybdenum d_π-acceptor orbitals dominated by the strongly π-donating arylimido ligands. Extrapolation to the mono(imido) complexes [M(NBu^t)(R₂[9]aneN₃)Cl₂] 3–6, in which the amido N-donor of Prⁱ₂[9]aneN₃ lies trans to one of the Cl ligands and cis to NBu^t, suggests that the amido N-donor should be able at least partly to π-donate to the metal centres. Hence in 3–6 the Prⁱ₂[9]aneN₃ ligand can in principle act as up to a 7-electron donor.