Synthesis and Photophysical Study of a [NiFe] Hydrogenase Biomimetic Compound Covalently Linked to a Re-diimine Photosensitizer

Abstract

The synthesis, photophysics, and photochemistry of a linked dyad ([Re]-[NiFe2]) containing an analogue ([NiFe2]) of the active site of [NiFe] hydrogenase, covalently bound to a Re-diimine photosensitizer ([Re]), are described. Following excitation, the mechanisms of electron transfer involving the [Re] and [NiFe2] centers and the resulting decomposition were investigated. Excitation of the [Re] center results in the population of a diimine-based metal-to-ligand charge transfer excited state. Reductive quenching by NEt3 produces the radically reduced form of [Re], [Re]− (kq = 1.4 ± 0.1 × 107 M–1 s–1). Once formed, [Re]− reduces the [NiFe2] center to [NiFe2]−, and this reduction was followed using time-resolved infrared spectroscopy. The concentration dependence of the electron transfer rate constants suggests that both inter- and intramolecular electron transfer pathways are involved, and the rate constants for these processes have been estimated (kinter = 5.9 ± 0.7 × 108 M–1 s–1, kintra = 1.5 ± 0.1 × 105 s–1). For the analogous bimolecular system, only intermolecular electron transfer could be observed (kinter = 3.8 ± 0.5 × 109 M–1 s–1). Fourier transform infrared spectroscopic studies confirms that decomposition of the dyad occurs upon prolonged photolysis, and this appears to be a major factor for the low activity of the system toward H2 production in acidic conditions.

Identifying a Real Catalyst of [NiFe]‐Hydrogenase Mimic for Exceptional H2 Photogeneration

“…To our delight, a photocatalytic system of the designed heterobimetallic mimic 1 with tris(2‐phenylpyridyl) iridium (Ir(ppy) 3 ) as a photosensitizer, 1,3‐dimethyl‐2‐phenol‐2,3‐dihydro‐1 H ‐benzo[d]imidazole (BIH) as a sacrificial electron donor and triethylamine hydrochloride (Et 3 NHCl) as a proton source, was found to produce H 2 . Under the optimal irradiation conditions (Supporting Information, Figures S6, S7, S22), mimic 1 showed proton reduction in both organic solvent and a mixture of organic solvent and water . In the absence of BIH, photosensitizer, Et 3 NHCl, or visible light, the amount of H 2 produced much lower or below the detection limit, suggesting that all components in this system were necessary.…”

O2 sensitivity and H2 production activity of hydrogenases—A review

“…In this regard, hydrogenases have inspired various research groups. The exploration on the O 2 ‐tolerant mechanism, in particular, has inspired the design and synthesis of biomimetic artificial catalysts, with the hope of enabling more stable and efficient production or oxidation of H 2 (Brazzolotto et al, ; Ghosh et al, ; Ogo, ; Summers et al, ). Another original, indirect application of the hydrogenase is to couple its enzymatic activity with other biochemical reactions.…”

Probing the use of long lived intra-ligand π–π* excited states for photocatalytic systems: A study of the photophysics and photochemistry of [ReCl(CO)3(dppz-(CH3)2)]

“…These systems employ [ReCl(CO) 3 hydrogenases. [14,15] In our previous studies we noted that the catalytic efficacy of these systems is compromised by the decomposition of the photosensitiser during turnover and the catalyst decomposition on photolysis, complicating the mechanistic study of [ 3 ; i-Pr-PyCa = pyridine-2-carbaldehyde N-isopropylimine; dapa = 2,6-diacetylpyridine bis(anil); dpp = 2,3-di(2-pyridylpyrazine); abpy = azo-2,2'-bipyridine) have been studied following their one-electron reduction. [16] The stability of the Re-X bond appears to be related to the ability of the complex to 6 accommodate the radical electron in the π*-LUMO (lowest unoccupied molecular orbital) localised on the diimine unit; the stability of the Re-X bond follows the order bpy < i-PrPyCa < dapa < dpp < abpy.…”