A New Tantalum Dinitrogen Complex and a Parahydrogen-Induced Polarization Study of Its Reaction with Hydrogen

Reduction of Cp*2TaCl2 with sodium amalgam in THF under a nitrogen atmosphere results in the formation of the
novel complex (Cp*2TaCl)2(μ-N2). This dinuclear complex containing a μ-η1:η1 dinitrogen bridge has been
characterized by NMR and X-ray crystallography. The complex possesses a C2-symmetric structure with each Ta
bound to diastereotopic Cp* rings and chloride in addition to the μ-N2 bridge. The Ta−N and N−N distances of
1.885(10) and 1.23(1) Å, respectively, suggest modest reduction of the dinitrogen moiety. The two Cp* resonances
on each Ta center remain inequivalent in solution, even up to 80 °C. Addition of hydrogen results in the formation
of two isomers of the dihydride complex Cp*2TaH2Cl. Under parahydrogen, polarized resonances are observed for
the unsymmetrical isomer with adjacent hydrides as the product of H2 oxidative addition. The symmetric isomer of
Cp*2TaH2Cl also forms, most likely by isomerization of the unsymmetrical kinetic isomer. The reactivity of (Cp*2TaCl)2(μ-N2) was compared to that of the related monomer, Cp*2TaCl(THF). The THF adduct yields the same
hydrogen addition products, but the reaction is much more facile than for the nitrogen dimer, indicative of the
structural integrity of the μ-N2 complex.