THE ACTIVATION AND ELIMINATION OF H-2 BY ZR COMPLEXES

An ab initio analysis of the reaction of molecular hydrogen with the Zr-imido complex (NH2)2Zr = NH is reported. Several interesting points are noted. The calculated stretching frequency of the Zr = N bond in (NH2)2Zr = NH is 860 cm-1 when properly scaled to account for electron correlation effects. The value supports the assignment of an infrared (IR) band at 865 cm-1 to the Zr = N stretch of a tetrahydrofuran (THF) adduct of the putative reactive intermediate (NHSi')2Zr = NSi'. Although a weakly bound, H-2 complex is found, the interaction is small (1.3 kcal mol-1) compared with experimentally characterized H-2 complexes. Variations in bond lengths and intrinsic stretching frequencies demonstrate that pi-bonding for amido (NH2) ligands can be substantial in a coordinatively saturated complex. H-2 activation by the bis(amido)imido reactive intermediate is calculated to be significantly more favorable by the 1,2-addition of H-2 across the Zr = N bond to form the tris(amido)hydride than a sigma-bond metathesis pathway. The transition state (TS) for the addition of H-2 across the Zr = N bond of the bis(amido)imido complex is 9.8 kcal mol-1 above the charge transfer (CT) complex, (NH2)2Zr = NH.H2 at the MP2 level; the reverse process, extrusion of H-2 from the tris(amido)hydride has a 28.2 kcal mol-1 barrier. The geometry of the four-center TS is of interest, deviating markedly from that of a square and being more "kite" shaped (i.e., one obtuse and three acute angles). Mulliken Bond Overlap Populations suggest that there is some interaction between the Zr and the H atom being transferred (H(t)) in the various TSs studied. It is suggested that the interaction plays an important role in the ability of the Zr-imido complexes, and indeed other high-valent, multiply bonded complexes, to activate X = H (X = H, C, Si, N, and H) bonds. The design of materials and catalysis precursors which enhance the metal-hydrogen interaction in the TS could conceivably lead to lower chemical vapor deposition (CVD) processing temperatures and higher catalytic activities.