Photo- and electrocatalytic H2 production by new first-row transition-metal complexes based on an aminopyridine pentadentate ligand

Abstract

The synthesis and characterisation of the pentadentate ligand 1,4-di(picolyl)-7-(p-toluenesulfonyl)-1,4,7-triazacyclononane (Py 2 Tstacn) and their metal complexes of general formula [M(CF3SO3)(Py2 Tstacn)][CF 3SO3], (M=Fe (1Fe), Co (1Co) and Ni (1Ni)) are reported. Complex 1Co presents excellent H 2 photoproduction catalytic activity when using [Ir(ppy) 2(bpy)]PF6 (PSIr) as photosensitiser (PS) and Et3N as electron donor, but 1Ni and 1Fe result in a low activity and a complete lack of it, respectively. On the other hand, all three complexes have excellent electrocatalytic proton reduction activity in acetonitrile, when using trifluoroacetic acid (TFA) as a proton source with moderate overpotentials for 1Co (0.59 V vs. SCE) and 1Ni (0.56 V vs. SCE) and higher for 1Fe (0.87 V vs. SCE). Under conditions of CH3CN/H2O/Et3N (3:7:0.2), 1 Co (5 μM), with PSIr (100 μM) and irradiating at 447 nm gives a turnover number (TON) of 690 (nH2/n1Co) and initial turnover frequency (TOF) (TON×t-1) of 703 h-1 for H 2 production. It should be noted that 1Co retains 25% of the catalytic activity for photoproduction of H2 in the presence of O2. The inexistence of a lag time for H2 evolution and the absence of nanoparticles during the first 30 min of the reaction suggest that the main catalytic activity observed is derived from a molecular system. Kinetic studies show that the reaction is -0.7 order in catalyst, and time-dependent diffraction light scattering (DLS) experiments indicate formation of metal aggregates and then nanoparticles, leading to catalyst deactivation. By a combination of experimental and computational studies we found that the lack of activity in photochemical water reduction by 1Fe can be attributed to the 1Fe II/I redox couple, which is significantly lower than the PSIr III/II, while for 1Ni the pK a value (-0.4) is too small in comparison with the pH (11.9) imposed by the use of Et3N as electron donor. Cobalt is king! The Fe, Co, and Ni complexes based on the same ligand have shown excellent electrocatalytic H2 activity (see figure). While only the Co complex has excellent H2 photoproduction catalytic activity, when using [Ir(ppy) 2(bpy)]PF6 (PSIr) as photosensitiser and Et3N as electron donor. Analytic and kinetic studies show that the catalytic activity is molecular. The lack of photochemical activity of the Fe and Ni complexes is ascribed to their redox chemistry and the pKa, respectively