Cycloaddition reactions of titanium and zirconium imido, oxo and hydrazido complexes supported by tetraaza macrocyclic ligands‡

Abstract

The tetraaza macrocycle-supported titanium and zirconium imido complexes [Ti(NR)(Me_ntaa)] [R = Bu^t, Ph, Tol or 4-C₆H₄NO₂; n = 4 or 8 where H₂Me_ntaa = tetra- or octa-methyldibenzotetraaza[14]annulene, respectively] and [Zr(NC₆H₃Prⁱ₂-2,6)(py)(Me₄taa)] react with isocyanates or carbon dioxide to form cycloaddition products generally of the type [M{N(R)C(O)E}(Me_ntaa)] (M = Ti or Zr, R = aryl or tert-butyl, E = O, NBu^t or N–aryl). The complex [Zr{N(C₆H₃Prⁱ₂-2,6)C(O)N(Bu^t)}(Me₄taa)] is also formed by reaction of the bis(arylimide) [Zr(NHC₆H₃Prⁱ-2,6)₂(Me₄taa)] with Bu^tNCO. The crystal structures of [Ti{N(Tol)C(O)O}(Me₄taa)] (Tol = p-tolyl) and [Zr{N(C₆H₃Prⁱ₂-2,6)C(O)N(Bu^t)}(Me₄taa)] are described. The tert-butyl imido complexes [Ti(NBu^t)(Me_ntaa)] (n = 4 or 8) react with Ph₂NNH₂ to give the corresponding terminal N,N-diphenylhydrazido derivatives [Ti(NNPh₂)(Me_ntaa)] and these yield the cycloaddition products [Ti{N(NPh₂)C(O)O}(Me_ntaa)] or [Ti{N(NPh₂)C(O)N(Tol)}(Me₄taa)] with CO₂ or TolNCO, respectively. The related titanium oxo complexes [Ti(O)(Me_ntaa)] (n = 4 or 8) react with p-tolyl isocyanate to form the N,O-carbamate products [Ti{N(Tol)C(O)O}(Me_ntaa)], and with ditolylcarbodiimide to form the N,N-ureate derivative [Ti{N(Ph)C(O)N(Ph)}(Me_ntaa)] in which complete rupture of the TiO linkage has occurred. Reaction of the N-phenyl-N-tolyl (asymmetric) ureate [Ti{N(Ph)C(O)N(Tol)}(Me₄taa)] with an excess of PhNCO gave quantitative conversion to the N,N-symmetric product [Ti{N(Ph)C(O)N(Ph)}(Me₄taa)] and TolNCO. Crossover NMR tube experiments suggest that this reaction occurs via an associative mechanism.