Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (qpy) Complexes: Evidence for Ruthenium(III) qpy‐N,N′′′‐dioxide as the Real Catalysts

Abstract

Polypyridyl and related ligands have been widely used for the development of water oxidation catalysts. Supposedly these ligands are oxidation-resistant and can stabilize high-oxidation-state intermediates. In this work a series of ruthenium(II) complexes [Ru(qpy)(L) 2 ] 2+

A broad view on the complexity involved in water oxidation catalysis based on Ru–bpn complexes

“…In both cases, the Ru center adopts a slightly distorted octahedral geometry with the bpn ligand occupying the equatorial positions and the pic and aqua occupying the axial ones. Bonding distances and angles are unremarkable for Ru(II) polypyridyl type of complexes 35,37,[49][50][51] except perhaps for the N-Ru-N angles between the outer pyridyl groups of phenanthroline moieties that turn out to be 122.5 o and 125.3 o for 1Cl2 and 4(CF3SO3)2, respectively, and that reflect the geometrical distortion imposed by the bpn ligand.…”

“…Equally important is to uncover the reaction pathways that either block catalysis or lead to decomposition processes. 33 Recently a family of Ru complexes [Ru II (N4)(pic)2)] 2+ , containing tetradentate equatorial N4 ligands (Chart I) and picoline as the axial ligand, has been described for N4 = 2,2'-(1,10-phenanthroline-2,9-diyl)bis(pyridine) (pbp) [34][35][36] and 2,2':6',2'':6'',2'''-quaterpyridine (qpy), 37 respectively. These [Ru II (N4)(pic)2)] 2+ complexes in the presence of an oxidant, undergo ligand oxidation at the two external pyridyl groups forming the corresponding N4-O2 di-N-oxide ligands, 2,2'-(1,10-phenanthroline-2,9diyl)bis(pyridine-1-oxide) (bpb-O2) and [2,2':6',2'':6'',2'''-quaterpyridine] 1,1'''-dioxide (qpy-O2).…”

A Ruthenium(II) Water Oxidation Catalyst Containing a pH-Responsive Ligand Framework

“… 30 , 31 To reveal whether, also for Ru(dppip-NO 2 ), such a species might represent an intermediate of the catalytic cycle, chemical oxidation of Ru(dppip-NO 2 ) using (NH 4 ) 2 [Ce(NO 3 ) 6 ] (CAN) according to a previous literature report was performed. 31 Mass spectrometry revealed that, by increasing the amount of added CAN, the signal assigned to Ru(dppip-NO 2 ) decreased and new peaks that can be assigned to [M-2PF 6 – + O] 2+ and [M-2PF 6 – + 2O] 2+ increased in intensity (see Figures S16–S19 ). Interestingly, by increasing the amount of added CAN from 8 to 12 equiv, peaks that were assigned to [M-2PF 6 – + O] 2+ -derived species vanished and those of the [M-2PF 6 – + 2O] 2+ species increased.…”

“…Nevertheless, it has also been suggested that, prior to active oxygen evolution, an oxygen atom transfer from Ru to the N-atoms of the outer pyridines may take place. This leads to the formation of a di-oxo species as the active catalyst displaying an O,N,N,O tetradentate ligand sphere. , To reveal whether, also for Ru­(dppip-NO 2 ), such a species might represent an intermediate of the catalytic cycle, chemical oxidation of Ru­(dppip-NO 2 ) using (NH 4 ) 2 [Ce­(NO 3 ) 6 ] (CAN) according to a previous literature report was performed . Mass spectrometry revealed that, by increasing the amount of added CAN, the signal assigned to Ru­(dppip-NO 2 ) decreased and new peaks that can be assigned to [M-2PF 6 – + O] 2+ and [M-2PF 6 – + 2O] 2+ increased in intensity (see Figures S16–S19).…”

Strategic factors to design the next generation of molecular water oxidation catalysts: Lesson learned from ruthenium complexes

“…(7a) and (ii) [Ru II (qpy)(pic)2)] 2+ (where qpy is 2,2':6',2'':6'',2'''-quaterpyridine) (10). [55] These complexes in the presence of an chemical oxidant, undergo ligand oxidation at the two external pyridyl groups forming the corresponding N2-O2 quatrodentate ligand ((i) 2,2'-(1,10-phenanthroline-2,9diyl)bis(pyridine 1-oxide) (pbp-O2) and (ii) [2,2':6',2'':6'',2'''-quaterpyridine] 1,1'''-dioxide (qpy-O2))…”