Book Description

As the incorporation of more renewable electricity into the power-grid leads to surplus generation, methods to utilize electricity to drive chemical reactions are becoming more relevant. Immobilizaton of molecular electrocatalysts combines the mechanistic understanding of molecular catalysts with the advantages of heterogeneous catalysts. Electrocatalysts of the type Ni(P2N2)2 are well-understood molecular catalysts that can achieve enzyme-like activity for hydrogen evolution and oxidation in solution. This extraordinary performance is attributed to their unique structure with proton shuttles in the second coordination sphere. Previously this amine substituent was used for surface attachment to immobilize this catalyst onto electrodes. However, the mobility of this substituent is crucial to the activity of the catalyst. We evaluated possible synthetic pathways to incorporate surface attachable functionality on the phosphine substituent of these ligands. Due to the high reactivity of the phosphines involved in the synthesis, incorporation of surface attachable groups through established synthetic protocols was found to be not feasible. A synthesis based on post-synthetic modification of P[superscript ArBr]2N[superscript Ph]2 was identified as the best way to incorporate attachable surface groups. This strategy was subsequently utilized to synthesize complexes of the type Ni(P2N2)2 with unprecedented, highly functionalized, surface attachable phosphine substituents. Phosphonate modified ligands and their corresponding nickel complexes were isolated and characterized. Subsequent deprotection of the phosphonic ester derivatives provided the first Ni(P2N2)2 catalyst that can be covalently attached via pendent phosphonate groups to an electrode without the involvement of hte important pendent amine groups. Mesoporous TiO2 electrodes were surface modified by attachment of the new phosphonate functionalized complexes, and these provided electrocatalytic materials that proved to be competent and stable for sustained hydrogen evolution in aqueous solution at mild pH and low over potential. We directly compared the new ligand to a previously reported complex that utilized the amine moiety for surface attachment. Using HER as the benchmark reaction, the P-attached catalyst showed a marginally (9-14%) higher turnover frequency than its N-attached counterpart. Finally, we report the synthesis of three new iridium piano-stool complexes that are immobilized on gold surfaces through thiol groups. We characterized these molecules using surface-sensitive IR spectroscopy. Further studies with these molecules are geared towards promoting the non-faradaic electrochemical tuning of catalysts using interfacial electric fields.