Expanding the Horizon of Bio-Inspired Catalyst Design with Tactical Incorporation of Drug Molecules.

The development of potent H2 production catalysts is a key aspect in our journey toward the establishment of a sustainable carbon-neutral power infrastructure. Hydrogenase enzymes provide the blueprint for designing efficient catalysts by the rational combination of central metal core and protein scaffold-based outer coordination sphere (OCS). Traditionally, a biomimetic catalyst is crafted by including natural amino acids as OCS features around a synthetic metal motif to functionally imitate the metalloenzyme activity. Here, we have pursued an unconventional approach and implanted two distinct drug molecules (isoniazid and nicotine hydrazide) at the axial position of a cobalt core to create a new genre of synthetic catalysts. The resultant cobalt complexes are active for both electrocatalytic and photocatalytic H2 production in near-neutral water, where they significantly enhance the catalytic performance of the unfunctionalized parent cobalt complex. The drug molecules showcased a dual effect as they influence the catalytic HER by improving the surrounding proton relay along and exerting subtle electronic effects. The isoniazid-ligated catalyst C1 outperformed the nicotine hydrazide-bound complex C2, as it produced H2 from water (pH 6.0) at a rate of 3960 s-1 while exhibiting Faradaic efficiency of about 90 %. This strategy opens up newer avenues of bio-inspired catalyst design beyond amino acid-based OCS features.

Redox-Induced Structural Switching through Sporadic Pyridine-Bridged CoIICoII Dimer and Electrocatalytic Proton Reduction.

The homodinuclear CoII helicate complex [CoII(DQPD)]2 (1) was prepared by treating [Co(H2O)6](ClO4)2 with the deprotonated form of the ligand N2,N6-bis(quinolin-8-yl)pyridine-2,6-dicarboxamide (DQPDH2). Complex 1 represents a discrete homodinuclear helicate complex with two CoII centers having a distorted-octahedral geometry through an unprecedented pyridine bridge. Complex 1, upon treatment with H2O2, undergoes oxidation at one of the CoII centers followed by a structural deformation to generate the mixed-valence complex [CoIIICoII(DQPD)2](ClO4) (2·ClO4). In complex 2, the bridging through the central pyridine collapses along with the formation of Co(III) octahedral and Co(II) tetrahedral environments. Complexes 1 and 2 interconvert to one another. The effective magnetic moments for complexes 1 and 2 are respectively 5.88 and 4.30 µB. Complexes 1 and 2 have been employed for electrocatalytic proton reduction using AcOH as the proton source in 95/5 (v/v) DMF/H2O. A TOF of 30000 mmol of H2 h-1 (mol of 1)-1 at a potential of -1.7 V vs SCE was achieved. A resting-state analysis has been carried out to support the mechanism for the catalytic proton reduction.

Proton reduction by a nickel complex with an internal quinoline moiety for proton relay.

The complex Ni(DQPD) (where DQPD = deprotonated N(2),N(6)-di(quinolin-8-yl)pyridine-2,6-dicarboxamide (DQPDH2)) behaves as a visible light driven active catalyst to reduce protons from water when employed with the photosensitizer fluorescein (Fl) and triethylamine (TEA) as the sacrificial electron donor. The photocatalytic system shows very high activity, attaining 2160 turnovers and an initial turnover rate of 0.032 s(-1) with respect to the catalyst. The proposed electrocatalytic mechanism is of the CECE type (C is a chemical step protonation and E is the electrochemical step reduction), where the Ni(DQPD) catalyst undergoes rapid protonation at the non-coordinating nitrogen atom of the quinoline before undergoing reduction. The location of the pendant base is a key factor such that the N-H resulting from the protonation of the non-coordinating nitrogen atom of the quinoline is properly located to interact with the Ni-H hydride leading to heterocoupling between protons and hydrides. Theoretical calculations for the catalytic system were carried out using the density functional level of theory (DFT) and are consistent with a mechanism for catalysis in a polypyridine nickel system. This is the first report of a polypyridine based nickel catalyst where the pendant base is responsible for the internal proton relay towards the metal center through the heterocoupling between protons and hydrides to generate hydrogen.

Improving the synthetic H2 production catalyst design strategy with the neurotransmitter dopamine.

The strategic incorporation of the neurotransmitter dopamine around a cobaloxime core resulted in excellent electrocatalytic (rate 8400 s-1) and photocatalytic H2 production under neutral aqueous conditions. The influence of the synthetic outer coordination sphere features continues even with a phenylene-diimino-dioxime motif-coordinated cobalt core.

A homogeneous cobalt complex mediated electro and photocatalytic O2/H2O interconversion in neutral water.

The O2/H2O redox couple is vital in various renewable energy conversion strategies. This work delves into the Co(L-histidine)2 complex, a functional mimic of oxygen-carrying metalloproteins, and its electrochemical behavior driving the bidirectional oxygen reduction (ORR) and oxygen evolution (OER) activity in neutral water. This complex electrocatalyzes O2 via two distinct pathways: a two-electron O2/H2O2 reduction (catalytic rate = 250 s-1) and a four-electron O2 to H2O production (catalytic rate = 66 s-1). The formation of the key trans-µ-1,2-Co(III)-peroxo intermediate expedites this process. Additionally, this complex effectively oxidizes water to O2 (catalytic rate = 15606 s-1) at anodic potentials via a Co(IV)-oxo species. Additionally, this complex executes the ORR and OER under photocatalytic conditions in neutral water in the presence of appropriate photosensitizer (Eosin-Y) and redox mediators (triethanolamine/ORR and Na2S2O8/OER) at an appreciable rate. These results highlight one of the early examples of both electro- and photoactive bidirectional ORR/OER catalysts operational in neutral water.

Exploring the Cobalt-Histidine Complex for Wide-Ranging Colorimetric O2 Detection.

Developing a robust, cost-effective, and user-friendly sensor for monitoring molecular oxygen (O2) ranging from a minute to a medically relevant level (85-100%) in a stream of flowing breathable gas is vital in various industrial domains. Here, we report an innovative application of the cobalt(l-histidine)2 complex, a bioinspired model of O2-carrying metalloproteins, for rapid and reliable sensing of O2 from 0 to 100% saturation levels under realistic conditions. We have established two distinct colorimetric O2 detection techniques, which can be executed with the use of a common smartphone camera and readily available color-detecting software. A series of spectroscopic experiments were performed to demonstrate the molecular-level alteration in cobalt(l-histidine)2 following its exposure to oxygen, leading to an exclusive pink-to-brown color change. Therefore, this study establishes a template for designing bioinspired molecular complexes for O2 sensing, leading to practical and straightforward solutions. This metal-amino acid complex's broad-spectrum sensing of O2 has widened the scope of bioinspired model complexes for divergent applications in industrial sectors.