Reports: UNI551724-UNI5: Surface Immobilization of Cobalt-Diglyoxime Complexes: The Effects of Interfacial Chemistry on Electrocatalytic Activity in Aqueous Media

Michael S. Hambourger, PhD, Appalachian State University

Cobalt bis(glyoxime) complexes, a.k.a. cobaloximes, are promising hydrogen production catalysts that may find use in electrochemical or photochemical energy conversion systems. We are pursuing cobaloxime surface immobilization to stabilize the compound and test the electrocatalyticactivity in an aqueous environment. During the first-year funding period, support from the Petroleum Research Fund (PRF) has provided for materials and supplies, as well as enabling five undergraduate researchers to participate in this work through funded positions. This support has been essential to our recent progress in the laboratory. Surface immobilization was undertaken to circumvent the acid instability and low solubility of cobaloximes in water. Our initial approach involved ligation of the cobalt ion by a surface-grafted pyridine group. Phenyl cobaloxime, Co(dpgBF2)2, and 4-(pyridin-4-yl)phenyl diazonium were prepared by literature methods. Electroreduction of the diazonium salt created a pyridyl film at the electrode surface (as evidence by redox inhibition using ferricyanide as the probe species). Treatment of these electrodes with cobaloxime resulted in cobalt-based redox signals that were retained after rinsing the electrode. As shown in Figure 1, this redox signal was stable when monitored in dichloromethane. However, when introduced to an aqueous solution, the CoII/Isignal was quickly attenuated, suggesting loss of cobaloxime from the electrode surface. This unexpected finding led us to investigate the stability of the cobalt-pyridine bond.

Figure 1: Cyclic voltammograms (100 mV/s) of a Co(dpgBF2)2 / pyridine modified glassy carbon electrode recorded in (a) dichloromethane (0.1 M TBAPF6) or (b) water (0.1 M sodium phosphate, pH 7.0). The cobalt signal was stable in dichloromethane but not in water.

For this purpose, Co(dpgBF2)2 was treated with ten equivalents of pyridine to yield the pyridyl adduct Co(dpgBF2)2(py). UV-vis spectroscopy of purified cobaloximes revealed a spectral shift upon pyridine ligation. This signature allowed us to monitor the cobalt-pyridine bond under various solvent conditions. Absorption spectra revealed that the axial pyridyl ligand was stable in dichloromethane but not in acetonitrile (Figure 2). Similarly, titration of Co(dpgBF2)2(py) with trifluoroaceticacid in dichloromethane induced loss of the axial pyridine. Comparable results were obtained from CHN analysis (data not shown), indicating that the pyridyl ligand was displaced under acidic conditions.

Figure 2: UV-vis spectra of (a) Co(dpgBF2)2 titrated with pyridine in dichloromethane and (b) purified Co(dpgBF2)2 and Co(dpgBF2)2(py) in dichloromethane and acetonitrile. UV-vis spectra of Co(dpgBF2)2(py) titrated with trifluoroacetic acid in (c) dichloromethane and (d) acetonitrile. The axial pyridyl ligand was displaced in the presence of acid or a coordinating solvent.

To further explore the possibility of ligand exchange in response to the solvent conditions, we began work with a porphyrin – cobaloxime system. The well known 5,10,15,20-tetramesitylporphyrin (TMP) was prepared, along with 5,10,15-trimesityl-20-(4-pyridyl)porphyrin (MPTMP). Steady-state fluorescence was used to monitor quenching of the porphyrin excited-singlet state upon the addition of Co(dpgBF2)2. Given the short lifetime (~10 ns) of this excited state, quenching was only expected in the case of a ground-state porphyrin-cobaloxime complex. As shown in Figure 3, upon the addition of Co(dpgBF2)2, quenching was only observed for MPTMP in dichloromethane. No quenching was observed for TMP or for MPTMP in acetonitrile solution. These results provide further evidence that a weakly coordinating solvent (acetonitrile) was sufficient to disrupt the cobalt-pyridine bond.

Figure 3: A Stern-Volmer plot showing quenching of the porphyrin singlet-excited state as a function of Co(dpgBF2)2 concentration for MPTMP and TMP in dichloromethane and acetonitrile. Quenching was only observed for the pyridyl porphyrin in dichloromethane.

Given that axial pyridine ligation appears unstable, we began to explore alternate immobilization methods. At present we are using hydrophobic interactions to retain cobaloximes at the electrode surface. Hydrophobic films are prepared by electroreduction of (1,1'-biphenyl)-4-diazonium or 4-hexyl-benzenediazonium. Incubation of modified surfaces with cobaloxime results in cobalt-based redox signals that are stable in water. Given the availability of protons under aqueous conditions, a catalytic wave is observed at sufficiently negative potentials, corresponding to hydrogen production. Figure 4 shows a pH series for Co(dpgBF2)2 immobilized in a biphenyl layer at a carbon electrode surface. Ongoing work seeks to optimize the diazoniumdeposition conditions to maximize the catalytic activity and stability of immobilized cobaloximes. Simultaneously, we are preparing a series of cobaloximes intended to tune the redox properties of the compound. Catalytic hydrogen production is thought to proceed following protonation of a low-valent state of the metal. Under aqueous conditions, turnover of Co(dpgBF2)2 only occurs at low pH.To extend the catalytic activity to higher pH, electron-donating groups are being introduced on the glyoxime ligand. Preliminary results indicate that such modifications increase electron density on the cobalt ion, favoring hydrogen production under more basic conditions.

Figure 4: Catalytic hydrogen production (reported as the current at -1.0 V vs Ag/AgCl divided by the peak current of the Co2+/1+ couple) for Co(dpgBF2)2 immobilized in a biphenyl film under aqueous conditions. Cyclic voltammograms (100 mV/s) were recorded across a range of pH values in a universal buffer system. The data show that Co(dpgBF2)2 is only active at low pH. Work is ongoing to develop cobaloxime derivatives suited for operation in neutral water.

The support of the Petroleum Research Fund has had a substantial, positive impact on the output of my laboratory. During the current funding period, undergraduate students working on this project have made four poster presentations at national conferences and four presentations at regional meetings. This project has been the subject of one undergraduate honors thesis and one senior research project. We are preparing to publish our results related to stability of the cobalt-pyridine bond. Results from our PRF funded research were used in a successful NSF-MRI application (award number 1228904, $273,500). The availability of PRF funding has allowed undergraduates to devote greater time to research, including a full time summer position, while increasing the students' own valuation of their research efforts. PRF funding has allowed for greater productivity from the laboratory, benefitting both undergraduate education and my own research agenda.