Square-Planar Nickel Bis(phosphinopyridyl) Complexes for Long-Lived Photocatalytic Hydrogen Evolution

Abstract

Phosphinopyridyl ligands are used to synthesize a class of Ni(II) bis(chelate) complexes, which have been comprehensively characterized in both solid and solution phases. The structures display a square-planar configuration within the primary coordination sphere, with axially positioned labile binding sites. Their electrochemical data reveal two redox couples during the reduction process, suggesting the possibility of accessing two-electron reduction states. Significantly, these complexes serve as robust catalysts for homogeneous photocatalytic H₂ evolution. In a system utilizing an organic photosensitizer and a sacrificial electron donor, an optimal turnover number of 27,100 is achieved in an alcohol-containing aqueous solution. A series of photophysical and electrochemical measurements were conducted to elucidate the reaction mechanism of photocatalytic hydrogen generation. Density function theory calculations propose a catalytic pathway involving two successive one-electron reduction steps, followed by two proton discharges. The sustained photocatalytic activity of these complexes stems from their distinct ligand system, which includes phosphine and pyridine donors that aid in stabilizing the low oxidation states of the Ni center.

Scheme 1. Ni Complexes for Photocatalytic H₂ Evolution Reported in the Literature and This Work

Figure 1

ORTEP diagrams of (A) [1][(ClO₄)₂], (B) [2][(ClO₄)₂], (C) [3][(ClO₄)₂] and (D) [4][(ClO₄)₂]·2.5CH₃CN with a 35% probability. The anions ClO₄^–, H atoms and solvated molecules are omitted for clarity. The nickel ion is located in a crystallographic inversion center in [3][(ClO₄)₂]. Two Ni complexes are contained in an asymmetry unit with similar structural parameters in [4][(ClO₄)₂]·2.5CH₃CN. Only one is shown, while the other is omitted.

Figure 2

Variable–temperature ¹H NMR spectra of complexes 1, 2 and 4 at 3.5 to 6.3 ppm.

Figure 3

Cyclic voltammograms of 1–4 (A–D) in CH₃CN. Conditions: NBu₄PF₆, Pt, and Ag/AgNO₃ as the supporting electrolyte, working electrode, and reference electrode, respectively. Scan rate: 100 mV/s.

Figure 4

Photocatalytic hydrogen production from systems containing 1–4 (4.4 μM) as catalysts, Fl (18.6 mM) and TEOA (0.42 M). Reactions were carried out in MeOH/H₂O (1:1) (for 1 and 2) and EtOH/H₂O (1:1) (for 3 and 4) at pH 10.45 at room temperature upon irradiation (λ = 415–420 nm, 300 mW, LED).

Figure 5

(A) Proposed photocatalytic mechanism for hydrogen evolution and (B) energy levels of relevant species in the photocatalytic reaction.

Scheme 2. Free Energy Profile for the HER of 1 in an Aqueous Medium, Calculated at U = −1.53 V vs Fc⁺/Fc and pH 10.45