Nickel Pyridinethiolate Complexes as Catalysts for the Light-Driven Production of Hydrogen from Aqueous Solutions in Noble-Metal-Free Systems

Abstract

A series of mononuclear nickel(II) thiolate complexes (Et4N)Ni(X-pyS)3 (Et4N = tetraethylammonium; X = 5-H (1a), 5-Cl (1b), 5-CF3 (1c), 6-CH3 (1d); pyS = pyridine-2-thiolate), Ni(pySH)4(NO3)2 (2), (Et4N)Ni(4,6-Y2-pymS)3 (Y = H (3a), CH3 (3b); pymS = pyrimidine-2-thiolate), and Ni(4,4'-Z-2,2'-bpy)(pyS)2 (Z = H (4a), CH3 (4b), OCH3 (4c); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that 2 is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H2O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex 4c achieves over 7300 turnovers of H2 in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter's greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni-H(-)/N-H(+) intermediate that forms the H-H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H2.

Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes

“…In this study, all of the dyads and PtN 2 S 2 complexes, as well as Bodipy dye 6, were attached to PtTiO 2 in the manner described above and then examined for lightdriven H 2 generation in pH 4 water with ascorbic acid (AA) as the sacrificial electron donor under 530 nm irradiation. For each sample, system pressure was monitored in real time, and at the end of each irradiation period, gas from the headspace of each sample was examined for H 2 content quantitatively, as previously described (26,27,66,67). In Fig.…”

Photogeneration of hydrogen from water using CdSe nanocrystals demonstrating the importance of surface exchange

“…In this system, the excited dye Fl* is quenched reductively by TEA, after which electron transfer occurs from the reduced dye to the Ni catalyst. Other derivatives have recently been reported for this catalyst that functions by dechelation and pyridine protonation (90). When [Ni(pyS) 3 ]…”

Light to Hydrogen: Photocatalytic Hydrogen Generation from Water with Molecularly-Defined Iron Complexes

“…This is motivated by several advantages of such catalysts due to lower costs and higher abundance compared to noble metals. Exemplarily, significant efforts have been reported applying cobalt and nickel complexes as proton reduction catalysts (WRC) (Although the term "water reduction catalyst" as well as the related abbreviation WRC is commonly used in relevant literature it should be named proton reduction catalyst in the strict sense) [30][31][32][33][34][35][36][37][38][39][40]. For example, various groups investigated cobaloxime-based catalysts [25,31].…”

“…The DuBois system showed excellent activity with a TOF up to 100,000 s −1 in the electrocatalytic hydrogen evolution reaction (HER) [41,42]. However, in photocatalytic hydrogen generation, this system achieved only a TON Ni of 2700 over 150 h with Ru photosensitizer A and ascorbic acid (SR) [37], while applying thiolate nickel complexes improved the TON Ni up to 7300 after 30 h with fluorescein, a xanthene-type organic dye, as the photosensitizer and TEA as SR [40]. A stability of more than 100 h was obtained using this system with TEOA as the electron donor [39].…”

“…For this purpose, especially ruthenium complexes have played a key role since the 1970s [68][69][70][71][72][73][74], later followed by various iridium [75,76], platinum [77][78][79] and rhenium [80][81][82][83][84][85] complexes. In contrast, more abundant metals or even metal-free photocatalytic systems were reported: examples include, e.g., iron [86], zinc [24,25,[87][88][89] and magnesium-based [24,[90][91][92][93][94] photosensitizers, CdTe [95], CdSe [39] or carbon [96] quantum dots or organic dyes [38][39][40][97][98][99][100][101][102][103][104] together with either cobalt or nickel catalysts. Mostly, the reported activities and stabilities were still low.…”