Call the binuclear dinitrogen complex [P2N2]Zr(μ-η2-N2)Zr[P2N2] activate more than one hydrogen molecule?: A theoretical study

The reaction mechanisms of model complexes[p(2)n(2)]Zr(mu-eta(2)-NNH)(mu-H)Zr[p(2)n(2)], B1 (A7), [p(2)n(2)](H)Zr(mu-eta(2)-NNH)Zr[p(2)n(2)], B11(A3), [p(2)n(2)]Zr(mu-eta(2)-cis-HNNH)(mu-H)Zr(H)[p(2)n(2)], C1(B3), [p(2)n(2)](H)Zr(mu-eta(2)-cis-HNNH)Zr(H)[p(2)n(2)], C4 (B13), [p(2)n(2)](H)Zr(mu-eta(2)-trans-HNNH)Zr (H)[p(2)n(2)], C7 (B21), where [p(2)n(2)] = [(PH3)(NH2)], with molecular hydrogen have been studied using density functional theory and compared with those for [p(2)n(2)]Zr(mu-eta(2)-N-2)Zr[p(2)n(2)], A1. The addition of a H-2 molecule to B1 (A7) (i.e., the addition of the second H-2 to Al) takes place with a 19.5 kcal/mol barrier, which is about 2 kcal/mol smaller than that for the first Ha addition A1 + H-2 --> A7 reaction. From B3, product of B1 +/- H-2, the process proceeds via either channel I.a, the reverse reaction B3 -> B1 + H-2, or/and channel I.b, the dihydrogen elimination B3 --> [p(2)n(2)]Zr(mu-eta(2)-cis-HNNH)Zr[p(2)n(2)] (A15) + H-2, with barriers of 11.7 and 21.5 kcal/mol, respectively. Since addition of the first H-2 to A1 is known to occur at laboratory conditions, one predicts that the addition of the H-2 to B1 (A7) will also be feasible under proper experimental conditions. Once A15 is' produced, reaction leads to [p(2)n(2)]Zr(mu-NH)(mu-NH2)(mu-H)Zr[p(2)n(2)] (B8) via formation of [p(2)n(2)]Zr(mu-NH)(2)Zr[p(2)n(2)], A17, which was kinetically unreachable by A1 + H-2 because of a very high barrier separating it from A7. Addition of Ha to the intermediate complex B11 (A3) leads to B13, where the N-H bonds are located cis to each other. Subsequently, B13 most likely rearranges to complex B3 and follows the reactions of B3. Addition of the third H-2 molecule to A1 is found to be kinetically less favorable than the first,two.