Spin-crossing in heterogeneous catalysis by atomically dispersed transition metals

Isolated, high-spin tetrahedral Co(II) atoms in the weak field of amorphous SiO2 are active and selective for light alkane dehydrogenation. To improve metal utilization and exploit the theoretical efficiencies of paramagnetic metal centers, it is critical to acquire molecular level understanding of the structures and mechanisms at the surface of the support. We employ electronic structure calculations and microkinetic simulations. We find that the C-H activation is protolytic but not rate controlling. The β-H elimination involves two spin-crossing events, one preceding the elimination over a low-spin planar transition state and another following it. By calculating the spin-orbit coupling elements, we estimate crossing coefficients close to unity and infer that the β-H elimination proceeds adiabatically and is rate limiting. We further show that these findings extend to dimeric Co(II) sites and that, remarkably, the spin-crossing mechanism is energetically more efficient than a cooperative mechanism involving the two neighboring Co sites even at high temperatures.