(109h) Uncovering the Electronic Origin of the Selectivity Reversal for Cleavage of C?O Bonds between Transition Metals and Transition Metal Phosphides

2-methyltetrahydrofuran (2-MTHF) cleaves favorably at the tertiary carbon over Ni₂P to produce 1-pentanol. Contrary to pure transition metals, which are selective to the secondary carbon . The electronic origin of this reversal in selectivity, while not understood, remains important for the rational design of catalysts with further improved selectivity.

In present work, we examined simpler but quantum chemically analogous reaction, cleavage of the C-O bond in methanol. Specifically, we examine whether C-O bond cleavage occurs through the H₂COH or HCOH intermediate, which corresponds to the tertiary and secondary carbons respectively for 2-MTHF.We find that the barrier for C-O cleavage in HCOH is lower on transition metals, while the barrier for C-O cleavage in H₂COH is lower on metal phosphides. Also, the difference in the barriers between the two transition states correlates with the following reaction energies:

CH₃(t) => CH₂(t) + ½ H₂

CH₂(b) => CH(b) + ½ H₂

due the transition state for C-O cleavage in H₂COH resembles a CH₂* fragment on an atop site while the transition state for C-O cleavage in HCOH resembles an CH* fragment in a bridge site. Thus, the preference for one specific pathway depends on CH_x fragment stability in the transition state by the metal surface. In particular, the CH₂* fragment is stabilized about the same on metals and metal phosphides, while the CH* fragment is stabilized strongly on metals comparatively.

To explain the above, we apply the constrained-orbital density functional theory (CO-DFT) method to decompose the electronic changes that occur during the C-O cleavage step. CO-DFT enables the direct manipulation of certain orbital interactions in a DFT calculation. We “turned off” the π interactions between the CH_x intermediates and the surface, showing that the stronger binding of CH* on metals is due to an enhanced π interaction on the former surfaces.