Fast Proton Transfer and Hydrogen Evolution Reactivity Mediated by [Co₁₃C₂(CO)₂₄]^4–

Abstract

A common approach to speeding up proton transfer (PT) by molecular catalysts is manipulation of the secondary coordination sphere with proton relays and these enhance overall reaction rates by orders of magnitude. In contrast, heterogeneous electrocatalysts have band structures that promote facile PT concerted with electron transfer (ET), known as the Volmer mechanism. Here, we show that [Co13C2(CO)24]4–, containing multiple Co–Co bonds to statistically enhance observed rates of PT, promotes PT on the order of 2.3 × 109 M–1 s–1 which suggests a diffusion-limited rate. The fast ET and PT chemistry is attributed to the delocalized electronic structure of [Co13C2(CO)24]4–. Electrochemical characterization of [Co13C2(CO)24]4– in the presence and absence of protons reveals ET kinetics and diffusion behavior similar to other small clusters such as nanomaterials and fullerenes.

Cobalt Carbonyl Clusters Enable Independent Control of Two Proton Transfer Rates in the Mechanism for Hydrogen Evolution

“…The fast rate for PT1 is also consistent with our previous report of fast PT by 2 4À where we reported that k PT1 = 2.3 × 10 9 M À 1 s À 1 , with an overpotential of 760 mV. [19] Both 1 3À and 2 4À have many CoÀ Co bonds making up their surface and these serve as many sites for PT which statistically enhances the rate of PT1 just like a proton relays in a molecular catalyst enhance PT rates. Similarly, heterogeneous catalysts routinely display fast PT via the Volmer mechanism.…”

“…It is known that a linear correlation exists between ν CO and z/M, where z is the cluster charge, and M is the number of cobalt atoms; and this relationship holds true over cobalt carbide carbonyl clusters with 2-13 cobalt atoms and charge À 1 to À 4 (Figure 1 bottom). [19,21] We generated 1 3À in situ using one equivalent of cobaltocene as a reductant in acetonitrile. The IR spectrum displayed absorption bands at 1974 and 1768 cm À 1 which are lower in energy, as expected for a more electron rich cluster.…”

“…Large metal carbonyl clusters (MCCs) provide an alternative over proton relays for statistically enhancing PT rates for the HER. [19] This rate for HER is limited only by diffusion of protons in solution and showed that large metal carbonyl clusters (MCCs) provide an alternative over proton relays for statistically enhancing PT rates for the HER. A question remained from that work: is the statistical enhancement of the PT rate limited to PT1?…”

Reactions of [Fe6C(CO)14(S)]2–: Cluster Growth, Redox, Sulfiding

“…The catalytic properties of carbide clusters for hydrogen evolution has been reported. [19] Prospectus. The convertibility of [Fe 6 C(CO) 14 S] 2into clusters that more closely resemble FeMoco remains an intriguing but challenging opportunity.…”

“…Fe-C1: Fe6-C1, 1.866(6); Fe1-C1, 1.879(6); Fe2-C1, 1.896(6); Fe3-C1, 1.909(6); Fe4-C1, 1.909(6); Fe5-C1, 1.907(6). Fe-S1: Fe7-S1, 2.1612(19); Fe6-S1, 2.174(2); Fe3-S1, 2.1998 (19). -1 , similar to that of [5] 2-) is quite distinct from that of the precursor.…”