SMALL-MOLECULE ELIMINATION FROM GROUP-IVB (TI, ZR, HF) AMIDO COMPLEXES

An ab initio quantum chemical analysis of HX (X = H, CH3, Cl, NH2, SiH3) elimination by group IVB (Ti, Zr, Hf) amidos (H2(X)M-NH2 --> H2M=NH + HX), of interest in the context of CVD precursor design, is reported. Several deductions may be drawn from the calculations. First, in the transition state (TS) for HX elimination, electropositive and electroneutral X give rise to metal-transannular hydrogen (H(t)) distances only slightly longer than normal metal-terminal hydride bonds lengths, while electronegative X groups yield substantially longer MH(t) distances. Second, the HX elimination barrier (DELTAH(double dagger)elim) is lower when HX is polarized H(delta-)-X(delta+) (X = SiH3) or nonpolar (X = H). Third, a plot of calculated DELTAH(double dagger)elim versus MH(t) distances in the TS for a given metal shows good correlation between low HX elimination barriers and short MH(t) distance in the TS. Fourth, analysis of the electronic structure along the intrinsic reaction coordinate (IRC) supports the importance of N-H...M agostic interactions preceding N-H scission. Fifth, the IRC shows the MH(t) distance decreasing as H(t) is transferred from N to X, reaching a minimum when the transfer is roughly half complete, and then increasing once more as HX is eliminated. These results point to the leaving group (X) playing a crucial role in tuning the bonding and energetics of the TS, and thus the rate of HX elimination. The results lead to the conclusion that materials precursors designed to enhance MH(t) interaction, through the intermediacy of X, in the TS and along the reaction coordinate will lead to lower activation barriers to XH elimination.