Nickel is a Different Pickle: Trends in Water Oxidation Catalysis for Molecular Nickel Complexes

Abstract

The development of novel water oxidation catalysts is important in the context of renewable fuels production. Ligand design is one of the key tools to improve the activity and stability of molecular catalysts. The establishment of ligand design rules can facilitate the development of improved molecular catalysts. In this paper it is shown that chemical oxidants can be used to probe oxygen evolution activity for nickel‐based systems, and trends are reported that can improve future ligand design. Interestingly, different ligand effects were observed in comparison to other first‐row transition metal complexes. For example, nickel complexes with secondary amine donors were more active than with tertiary amine donors, which is the opposite for iron complexes. The incorporation of imine donor groups in a cyclam ligand resulted in the fastest and most durable nickel catalyst of our series, achieving oxygen evolution turnover numbers up to 380 and turnover frequencies up to 68 min−1 in a pH 5.0 acetate buffer using Oxone as oxidant. Initial kinetic experiments with this catalyst revealed a first order in chemical oxidant and a half order in catalyst. This implies a rate‐determining oxidation step from a dimeric species that needs to break up to generate the active catalyst. These findings lay the foundation for the rational design of molecular nickel catalysts for water oxidation and highlight that catalyst design rules are not generally applicable for different metals.

Homogeneous Water Oxidation Catalyzed by First‐Row Transition Metal Complexes: Unveiling the Relationship between Turnover Frequency and Reaction Overpotential

“…Reek and co‐workers examined a series of Ni complexes with different ligands, showing that rational ligand design may vary across different transition metals [160] and thereby demonstrating the need for a comprehensive understanding of the effects of same‐class ligands on the TOF and η of first‐row transition‐metal‐based WOCs. Therefore, different first‐row transition metal complexes ligated by a family of TAMLs were chosen to investigate the log(TOF)‐ η relationship (Figure 11a; Fe‐6 , Fe‐7 , Co‐13 , Ni‐8 , Ni‐9 , and Cu‐8 to Cu‐11 ).…”

“…In addition to their readily accessible redox states, Ni complexes are prospective alternatives to the rare and costly Ru and Ir complexes as homogeneous WOCs. [158][159][160] The structures of the reported Ni-based WOCs are presented in Figure 7a.…”

“…[166,167] Reek and co-workers investigated a cyclam-type ligand system with a pyridine-based scaffold (Ni-11 and Ni-13 to Ni-15). [160] Among these pyridineligated complexes, Ni-13 with secondary amine donor groups showed the highest activity. Wang and Bruner developed a bioinspired Ni-phenolate complex (Ni-16) to electrochemically catalyze water oxidation at pH 7.…”

“…Lu and co‐workers reported two Ni complexes based on tetradentate aminopyridine ligands ( Ni‐11 and Ni‐12 ) and featuring two cis labile sites capable of chelating two labile aqua ligands, showing that these complexes homogeneously electrocatalyzed water oxidation [166,167] . Reek and co‐workers investigated a cyclam‐type ligand system with a pyridine‐based scaffold ( Ni‐11 and Ni‐13 to Ni‐15 ) [160] . Among these pyridine‐ligated complexes, Ni‐13 with secondary amine donor groups showed the highest activity.…”

“…The interest in elucidating the involved mechanisms has reinvigorated the study of well‐defined Ni‐coordination complexes as WOCs. In addition to their readily accessible redox states, Ni complexes are prospective alternatives to the rare and costly Ru and Ir complexes as homogeneous WOCs [158–160] . The structures of the reported Ni‐based WOCs are presented in Figure 7a.…”