Photoproduction of dihydrogen (H2) by a low molecular weight analogue of the active site of [NiFe] hydrogenase has been investigated by reduction of the [NiFe2] cluster, 1, by a photosensitier PS (PS = [ReCl(CO)3(bpy)] or [Ru(bpy)3][PF6]2). Reductive quenching of the (3)MLCT excited state of the photosensitizer by NEt3 or N(CH2CH2OH)3 (TEOA) generates PS(•-), and subsequent intermolecular electron transfer to 1 produces the reduced anionic form of 1. Time-resolved infrared spectroscopy (TRIR) has been used to probe the intermediates throughout the reduction of 1 and subsequent photocatalytic H2 production from [HTEOA][BF4], which was monitored by gas chromatography. Two structural isomers of the reduced form of 1 (1a(•-) and 1b(•-)) were detected by Fourier transform infrared spectroscopy (FTIR) in both CH3CN and DMF (dimethylformamide), while only 1a(•-) was detected in CH2Cl2. Structures for these intermediates are proposed from the results of density functional theory calculations and FTIR spectroscopy. 1a(•-) is assigned to a similar structure to 1 with six terminal carbonyl ligands, while calculations suggest that in 1b(•-) two of the carbonyl groups bridge the Fe centers, consistent with the peak observed at 1714 cm(-1) in the FTIR spectrum for 1b(•-) in CH3CN, assigned to a ν(CO) stretching vibration. Formation of 1a(•-) and 1b(•-) and production of H2 was studied in CH3CN, DMF, and CH2Cl2. Although the more catalytically active species (1a(•-) or 1b(•-)) could not be determined, photocatalysis was observed only in CH3CN and DMF.

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.

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This article reviews the synthesis and chemistry of complexes that are of relevance to the structure and catalysis observed at the active sites of [NiFe], [FeFe] and [Fe] hydrogenase enzymes. Both structural and functional modelling of these biosites has been accomplished using ligand design to assemble low molecular weight metal complexes incorporating polythiolate coordination at the metal centre, coupled to appropriate amine, phosphine, cyanide and CO co-ligands. Exchange of ligands and the use of alternative metal centres, particularly Ru and Co, are discussed, and recent developments leading to the incorporation of proton relays and photoactive moieties are highlighted as strategies to generate photocatalysts for hydrogen production and oxidation for use in fuel cells.

Polyurethane (PUR) shoe soles from collections held by the Alfred Gillett Trust, stored for up to 50 years, were analysed for the presence of 4,4′-diaminodiphenylmethane (4,4′-MDA)-a substance of very high concern in Europe and classified as a carcinogen in USA. A review of the literature revealed no papers on long term room-temperature hydrolysis of urethane or urea linkages leading to the spontaneous formation of 4,4′-MDA in polyester or polyether polyurethanes made from 4,4′-diisocyanatodiphenylmethane (MDI). However, evidence emerged of its potential formation and a possible handling hazard was consequently identified in the heritage collection. By chemical analysis, shortcomings in the current literature could be addressed. Calibration of liquid chromatography-mass spectrometry equipment showed analysis of 4,4′-MDA was possible down to 1 ppm with an error of 2.5 ppm. No 4,4′-MDA was found in the PUR analyte solutions at a concentration > 1 ppm. Under these experimental parameters the samples were shown to comply with the industrially accepted CertiPUR 2017 standard for commercial slab-stock foams. Furthermore, no 4,4′-MDA was found in solution after an 'accelerated anaerobic hydrolysis test' on the sole materials. This test was designed and developed to assess the likelihood of future formation of the aromatic amine via a hydrolysis-only mechanism/s. Finally, 4,4′-MDA itself was heated in air at 70 °C under 'humid' conditions to examine its stability. In this experiment the 4,4′-MDA altered in appearance and was reduced to ca 30% of its original weight. Subject to more work, it is conceivable 4,4′-MDA could be formed by hydrolysis, but degraded over time, and not detected in these tests.