The perspective of integrating molecular catalysts for hydrogen evolution into operating devices requires the benchmarking of their activity preferentially in aqueous media. Within a series of cobalt complexes assessed in that way, cobalt diimine–dioxime derivatives were shown to be the most active catalysts with onset overpotential for proton reduction as low as 260 mV in phosphate buffer (pH = 2.2) (McCrory et al. J. Am. Chem. Soc. 2012, 134, 3164–3170). Combining a set of analytical techniques (electrochemistry, gas chromatography, SEM, and XPS), we demonstrate here that the electrochemical wave previously assigned to H2 evolution catalyzed by the molecular complex actually corresponds to low levels of catalytic hydrogen production (≤27% faradaic yield). Instead, we assign this wave to the reductive degradation of the molecular complex and to the formation of a nanoparticulate deposit at the electrode. Actually, this coating is responsible for the high faradaic yields for hydrogen evolution observed at more cathodic potentials. The catalytic nanoparticulate material is metastable and readily redissolves, so that rinse-test experiments were insufficient here to rule out the formation of solid-state materials. This point accounts for the previous misidentification of the active species in H2 evolution mediated by a cobalt diimine–dioxime complex in aqueous phosphate buffer (pH = 2.2). Our finding, exemplified on a cobalt complex, may be extended to other molecular systems and suggests that the routine use of rinse-test experiments may not be sufficient to ascertain the molecular nature of active water-splitting catalytic species.

The missing link: Ferrocene and porphyrin monolayers are tethered on silicon surfaces with short (see picture, left) or long (right) linkers. Electron transfer to the silicon substrate is faster for monolayers with a short linker.Ferrocene and porphyrin derivatives are anchored on Si(100) surfaces through either a short two-carbon or a long 11-carbon linker. The two tether lengths are obtained by using two different grafting procedures: a single-step hydrosilylation is used for the short linker, whereas for the long linker a multistep process involving a 1,3-dipolar cycloaddition is conducted, which affords ferrocene-triazole-(CH(2))(11)-Si or Zn(porphyrin)-triazole-(CH(2))(11)-Si links to the surface. The modified surfaces are characterized by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Cyclic voltammetry experiments show that the redox activity of the tethered ferrocene or porphyrin is maintained for both linker types. Microelectrode capacitor devices incorporating these modified Si(100) surfaces are designed, and their capacitance-voltage (C-V) and conductance-voltage (G-V) profiles are investigated. Capacitance and conductance peaks are observed, which indicates efficient charge transfer between the redox-active monolayers and the electrode surface. Slower electron transfer between the ferrocene or porphyrin monolayer and the electrode surface is observed for the longer linker, which suggests that by adjusting the linker length, the electrical properties of the device, such as charging and discharging kinetics and retention time, could be tuned.