Ditantalum Dinitrogen Complex: Reaction of H2 Molecule with End-on-Bridged [TaIV]2(μ-η1:η1-N2) and Bis(μ-nitrido) [TaV]2(μ-N)2 Complexes

To elucidate (i) the physicochemical properties of the {(eta(5)-C5Me5) [Ta-IV] (i-Pr)C(Me)N(i-Pr)}(2)(mu-eta(1):eta(1)-N-2), I, [Ta-IV](2)(mu-eta(1):eta(1)-N-2), and {(eta(5)-C5Me5)[Ta-V](i-Pr)C(Me)N(i-Pr)}(2)(mu-N)(2), II, [Ta-V](2)(mu-N)(2), complexes; (ii) the mechanism of the I -> II isomerization; and (iii) the reaction mechanism of these complexes with an H-2 molecule, we launched density functional (B3LYP) studies of model systems 1, 2, and 3 where the C5Me5 and (i-Pr)C(Me)N(i-Pr) ligands of I (or II) were replaced by C5H5 and HC(NCH3)(2), respectively. These calculations show that the lower-lying electronic states of 1, [Ta-IV](2)(mu-eta(1):eta(1)-N-2), are nearly degenerate open-shell singlet and triplet states with two unpaired electrons located on the Ta centers. This finding is in reasonable agreement with experiments [J. Am Chem. Soc. 2007, 129, 9284-9285] showing easy accessibility of paramagnetic and diamagnetic states of I. The ground electronic state of the bis(mu-nitrido) complex 2, [Ta-V](2)(mu-N)(2), is a closed-shell singlet state in agreement with the experimentally reported diamagnetic feature of II. The 1-to-2 rearrangement is a multistep and highly exothermic process. It occurs with a maximum of 28.7 kcal/mol free energy barrier required for the (mu-eta(1):eta(1)-N-2) -> (mu-eta(2):eta(2)-N-2) transformation step. Reaction of 1 with H-2 leading to the 1,4-addition product 3 proceeds with a maximum of 24.2 kcal/mol free energy barrier associated by the (mu-eta(1):eta(1)-N-2) -> (mu-eta(2):eta(1)-N-2) isomerization step. The overall reaction 1 + H-2 -> 3 is exothermic by 20.0 kcal/mol. Thus, the addition of H-2 to 1 is kinetically and thermodynamically feasible and proceeds via the rate-determining (mu-eta(1):eta(1)-N-2) -> (mu-eta(2):eta(1)-N-2) isomerization step. The bis(mu-nitrido) complex 2, [Ta-V](2)(mu-N)(2), does not react with H-2 because of the large energy barrier (49.5 kcal/mol) and high endothermicity of the reaction. This conclusion is also in excellent agreement with the experimental observation [J. Am Chem. Soc. 2007, 129, 9284-9285].