Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

Abstract

In this work we describe the synthesis, structural characterization and redox properties of two Ru complexes containing the dianionic potentially pentadentate [2,2':6',2''-terpyridine]-6,6''-dicarboxylate (tda 2-) ligand that coordinates Ru at the equatorial plane and with additional pyridine or dmso acting as monondentate ligand in the axial positions: [Ru II (tda--N 3 O)(py)(dmso)], 1 II and [Ru III (tda--N 3 O 2 )(py)(H2O) ax ] + , 2 III (H2O) + . Complex 1 II has been characterized by single crystal XRD in the solid state and in solution by NMR spectroscopy. The redox properties of 1 II and 2 III (H2O) + have been thoroughly investigated by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex 2 II (H2O) displays poor catalytic activity with regard to the oxidation of water to dioxygen and its properties have been analyzed based on foot of the wave analysis (FOWA) and catalytic Tafel plots. The activity of 2 II (H2O) has been compared with related water oxidation catalysts (WOCs) previously described in the literature. Despite its moderate activity, 2 II (H2O) constitutes the cornerstone that has triggered the rationalization of the different factors that govern overpotentials as well as efficiencies in molecular water oxidation catalysts (WOCs). The present work uncovers the interplay between different parameters namely, coordination number, number of anionic groups bonded to the first coordination sphere of the metal center, water oxidation catalysis overpotential, pKa and hydrogen bonding and the performance of a given WOC. It thus establishes the basic principles for the design of efficient WOCs operating at low overpotentials.

Catalytic Oxidation of Water to Dioxygen by Mononuclear Ru Complexes Bearing a 2,6‐Pyridinedicarboxylato Ligand

“…Here, the III/II redox potential is significantly higher than for ( 1‐O ) as in the present complex the pdc 2− ligand acts in a κ‐N 1 O 1 mode and, thus, only one of the two anionic charges is directly felt by the Ru center. In sharp contrast, the V/IV redox potentials are similar which is attributed to the cancelling effect of CN7 versus CN6, 1 versus 2 anionic charges, a phenomenon that has been previously described for related Ru–aquo complexes …”

“…Ruthenium complexes have also been crucial to understand the mechanistic pathways responsible for the O−O bond formation, and how the coordination sphere around the metal center influence these pathways. Other factors such as the p K a of the ruthenium–aquo (Ru‐OH 2 ) intermediate species have also been shown to strongly impact the performance of the water oxidation catalysts . All these insights are of paramount importance because they have allowed the rational design of catalysts that can perform as fast as 7900 s −1 at neutral pH …”

Synthesis, Structure, and Redox Properties of a trans-Diaqua Ru Complex That Reaches Seven-Coordination at High Oxidation States

“…Thus the dicarboxylato ligand tda 2-, renders the Ru(IV) species at least 10 million times more basic than the bpy ligand. 25 The similarity is a consequence of the fact that the replacement of one of the aqua ligands by a pyridyl maintains almost unchanged the potentials and the pKa at oxidation state II, III and IV (see Table 1 for the values and Figure S15 for the Finally, it is interesting to notice that the VI/V couple for 3 VI (O)2 is well above the thermodynamic potential for the oxidation of water to dioxygen and thus could potentially act as a water oxidation catalyst. Unfortunately, decomposition reactions possibly involving polymeric oxobridged type of species such as the ones described for 10 IV (OH) 3+ (see Figure S16), can compete with the catalytic reaction.…”

“…17,18 For the particular case of the water oxidation, the best catalysts described so far involve the formation of seven coordinate complexes at high oxidations states, stabilized by sigma donor groups such as carboxylates. 19,20,21,22,23,24 A good example of this family of catalysts is the [Ru 25 Complex 4 II (50 mg, 0.1 mmol) was dissolved in a mixture of water (2.5 mL) and pyridine (7.5 mL). The solution was stirred for 2 days at room temperature.…”

Universal scaling relations for the rational design of molecular water oxidation catalysts with near-zero overpotential

“…In the following, however, we demonstrate that this unsuspected behavior is caused by an oversimplification of the reaction mechanism, typically assumed in most theoretical studies of heterogeneous OER catalysts. Notably, some metals, such as Ru, Mn, or Fe, have been experimentally proven to undergo an additional one-electron transfer (ET) before O-O bond formation [52][53][54][55] . Bearing this in mind, we computed (whenever possible) the Gibbs energies for the ET from the M(IV)-oxo to M(V)-oxo species with M = Ru, Mn, Fe, hereafter referred to as O(IV) Ã and O(V) Ã , respectively (see Supplementary Tables 2 and 3 for details).…”