Theoretical Study on σ-Bond Activation of (HO)2B−XH3 by M(PH3)2 (X = C, Si, Ge, or Sn; M = Pd or Pt). Noteworthy Contribution of the Boryl pπ Orbital to M−Boryl Bonding and Activation of the B−X σ-Bond

Abstract

Oxidative addition of (HO)2B−XH3 to M(PH3)2 (X = C, Si, Ge, or Sn; M = Pd or Pt) was theoretically investigated with MP2-MP4(SDQ) and CCSD(T) methods. (HO)2B−XH3 easily undergoes oxidative addition to Pt(PH3)2 with a moderate activation energy for X = C and either a very small barrier or no barrier for X = Ge, Si, and Sn. Also, (HO)2B−SiH3, (HO)2B−GeH3, and (HO)2B−SnH3 undergo oxidative addition to Pd(PH3)2 with either a very small barrier or no barrier. Only the oxidative addition of (HO)2B−CH3 to Pd(PH3)2 cannot take place, but the reductive elimination of (HO)2B−CH3 from Pd(CH3)[B(OH)2](PH3)2 occurs with no barrier. The transition states (TS) of these oxidative additions are nonplanar except for the nearly planar TS of the oxidative addition of (HO)2B−CH3 to Pt(PH3)2. This TS structure is very sensitive to steric and electronic factors; for instance, the TS becomes nonplanar by substituting PH2(C2H5) for PH3, to decrease the steric repulsion between (HO)2B−CH3 and PH2(C2H5). A noteworthy feature of these reactions is that the TS is much stabilized by the charge-transfer interaction between M d and B(OH)2 pπ orbitals, which is the main reason for the high reactivity of (HO)2B−XH3 in the oxidative addition reaction. Pt−B(OH)2 and Pd−B(OH)2 bonds are much stronger than Pt−XH3 and Pd−XH3 bonds, respectively. This is because the M−B(OH)2 bond is stabilized by the π-back-donating interaction between the empty pπ orbital of B(OH)2 and the doubly occupied dπ orbital of Pt and Pd. Also, it should be noted that the trans influence of the boryl group is stronger than the very strong trans influence of silyl group.