(5d) Identifying Effects of Phosphorous in Transition Metal Phosphides for Selective Hydrogenolysis of Hindered C–O Bonds

Contrasts between hindered and unhindered C–O activations within biomass derived molecules can be investigated using 2-methyltetrahydrofuran (MTHF) as a model compound. MTHF contains a furan ring whose O atom has neighboring secondary (²C) and tertiary (³C) carbons. Experiments and DFT suggest that adding P to Ni results in a shift in selectivity towards a preference for ³C–O activation over the undesired ²C–O reaction. Activation enthalpy (ΔΔH) and free energy (ΔΔG) differences (ΔG_2C–O – ΔG_3C–O) were calculated to predict selectivity with the Ni(111) surface having lower ΔΔH and ΔΔG values (−48 and −70 kJ mol⁻¹, respectively) than Ni₂P(001) (34 and 0 kJ mol⁻¹), suggesting that selectivity increases with P content [1]. Recent work attempted to understand the reason why P causes this shift in selectivity by investigating electronic effects of P addition through other transition metal phosphides isostructural to Ni₂P(001). Pd₂P, Rh₂P Fe₂P, Ru₂P and Co₂P (in addition to Ni₂P) were all investigated and a nearly ubiquitous shift in selectivity was observed with all phosphide materials except Ru₂P showing greater ΔΔH and ΔΔG values than their pure metal counterparts (Fig. 1) [2]. In the present contribution, we continue this work by investigating the effects of geometry (crystal structure) on selectivity and P content for a different series of metal phosphides. We contrast Ni₂P in the Pnma spacegroup to Ni₂P in the original P62m spacegroup, and find that, at least between these two spacegroups, there is an almost negligible shift in selectivity. For Pd, we contrast Pd, Pd₆P, Pd₃P, and Pd₂P, and show a monotonic increase in ΔΔH and ΔΔG values (consistent with greater ³C–O activation selectivity) like that observed for Ni phosphides. Overall, we see that P incorporation increases selectivity towards ³C–O activation for all metals except Ru.

References

[1] ACS Catal. 2018, 8, 7141–7157. [2] J. Catal. 2023, 421, 403–418.