Photo‐ and Electrocatalytic H 2 Production by New First‐Row Transition‐Metal Complexes Based on an Aminopyridine Pentadentate Ligand

The synthesis and characterisation of the pentadentate ligand 1,4‐di(picolyl)‐7‐( p ‐toluenesulfonyl)‐1,4,7‐triazacyclononane (Py 2 Ts tacn) and their metal complexes of general formula [M(CF 3 SO 3 )(Py 2 Ts tacn)][CF 3 SO 3 ], (M=Fe ( 1 Fe ), Co ( 1 Co ) and Ni ( 1 Ni )) are reported. Complex 1 Co presents excellent H 2 photoproduction catalytic activity when using [Ir(ppy) 2 (bpy)]PF 6 ( PS Ir ) as photosensitiser (PS) and Et 3 N as electron donor, but 1 Ni and 1 Fe 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 1 Co (0.59 V vs. SCE) and 1 Ni (0.56 V vs. SCE) and higher for 1 Fe (0.87 V vs. SCE). Under conditions of CH 3 CN/H 2 O/Et 3 N (3:7:0.2), 1 Co (5 μ M ), with PS Ir (100 μ M ) and irradiating at 447 nm gives a turnover number (TON) of 690 ( n   H   2/ n   1   Co) and initial turnover frequency (TOF) (TON×t −1 ) of 703 h −1 for H 2 production. It should be noted that 1 Co retains 25 % of the catalytic activity for photoproduction of H 2 in the presence of O 2 . The inexistence of a lag time for H 2 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 1 Fe can be attributed to the 1 Fe II/I redox couple, which is significantly lower than the PS Ir III/II , while for 1 Ni the p K a value (−0.4) is too small in comparison with the pH (11.9) imposed by the use of Et 3 N as electron donor.