Can 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(μ-η 2 -NNH)(μ-H)Zr[p 2 n 2 ], B1 (A7), [p 2 n 2 ](H)Zr(μ-η 2 -NNH)Zr[p 2 n 2 ], B11 (A3), [p 2 n 2 ]Zr(μ-η 2 - cis -HNNH)(μ-H)Zr(H)[p 2 n 2 ], C1 (B3), [p 2 n 2 ](H)Zr(μ-η 2 - cis -HNNH)Zr(H)[p 2 n 2 ], C4 (B13), [p 2 n 2 ](H)Zr(μ-η 2 - trans -HNNH)Zr (H)[p 2 n 2 ], C7 (B21), where [p 2 n 2 ] = [(PH 3 )(NH 2 )], with molecular hydrogen have been studied using density functional theory and compared with those for [p 2 n 2 ]Zr(μ-η 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 A1 ) takes place with a 19.5 kcal/mol barrier, which is about 2 kcal/mol smaller than that for the first H 2 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(μ-η 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(μ-NH)(μ-NH 2 )(μ-H)Zr[p 2 n 2 ] ( B8 ) via formation of [p 2 n 2 ]Zr(μ-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 H 2 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.