Can the Disproportion of Oxidation State III Be Favored in Ru^II−OH₂/Ru^IVO Systems?

Abstract

Three new Ru-aqua complexes containing a mixed carbene and pyridylic ligands with general formulas [Ru(CNC)(bpy)(H2O)](PF6)2 (1) (CNC is 2,6-bis(butylimidazol-2-ylidene)pyridine; bpy is 2,2'-bipyridine) and cis-/trans-[Ru(CNC)(nBu-CN)(H2O)](PF6)2 (cis-2 and trans-2) (nBu-CN is 2-(butylimidazol-2-ylidene)pyridine) have been prepared and structurally characterized both in the solid state (monocrystal X-ray diffraction analysis for 1 and for the related complex trans-[Ru(Br)(CNC)(nBu-CN)](PF6)) and in solution (for all of them) through NMR. The electrochemical properties of these three Ru-aqua complexes have been investigated by cyclic voltammetry, differential pulse voltammetry and Coulombimetric techniques. It is found that, for complex 1 at pH 7, the difference between the IV/III and the III/II redox couples (DeltaE1/2) is 50 mV, which is the smallest ever reported for this type of complex. On the other hand, for complexes cis-2 and trans-2, the oxidation state III is unstable with respect to disproportionation to II and IV. The reactivity of their Ru=O species has been tested toward cis-beta-methylstyrene oxidation, and it has been compared to [Ru(O)(trpy)(bpy)]2+. An inverse correlation between the degree of cis/trans-epoxide isomerization and DeltaE1/2 is found. In particular, for complexes cis-2 and trans-2, which have a DeltaE1/2 < 0, the epoxidation is highly stereoselective, yielding only cis-epoxide.

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

“…An interesting feature of the Pourbaix diagram for 2 IV (OH) + is the pH 10-14 zone, that has been built by extrapolating the values of the V/IV couple at this pH and by assuming that the redox potential for the Ru V =O/Ru IV =O couple for [Ru IV (bda)(Me-py)2(O) eq ], 3 IV (O) (1.12 V), 15 is the same as for 2 IV (O) (Me-py is 4-methylpyridine). This assumption is based on Lever's approach 22,23,24 where it is shown that the Ru III /Ru II couple for a set of complexes can be calculated based on additive contribution of their respective ligands bonded to the first coordination sphere. In the present case, complexes 2 IV (O) and 3 IV (O) can be considered as "pseudo isomers" since in both cases the first coordination sphere is occupied by 4 pyridyl groups, two carboxylato and one terminal oxo ligand (see drawn structures Chart 1).…”

Role of ligands in catalytic water oxidation by mononuclear ruthenium complexes

“…The σ-donor/π-acceptor properties of the tri-and bidentate ligands bound in mononuclear Ru WOCs represented by complexes 8-10 (Chart 4) play important roles in stabilizing higher metal oxidation states [38,39] and further facilitating water oxidation e.g., by lowering redox potentials, increasing the thermodynamic driving force or decreasing energy barriers [32]. Therefore, smart modification or variation of ligands in WOCs is one of the major challenges in order to further improve the catalytic activity towards water oxidation.…”

“…They revealed an interesting linear correlation between the number of carbene ligands in the complexes and ΔE 1/2 where each carbene unit causes a 100 mV decrease in the ΔE 1/2 value. The parameter ΔE 1/2 is defined as the difference between E 1/2 (Ru IV/III ) and E 1/2 (Ru III/II ) involved in the two PCET processes and the exact values of these two pH-dependent redox potentials can be extracted from Pourbaix diagrams [39]. [29].…”

Red Phosphorescence in RuII Complexes of a Tridentate N‐Heterocyclic Carbene Ligand Incorporating Tetrahydropyrimidine

“…The conformation of the saturated ring thus does not seem to have any noticeable influence on the structure, as found in other coordination complexes incorporating (CH 2 ) 3 -bridged donor atoms. [46,47] [39,[41][42][43][44][45] and are at the short end of the distances observed in Ru carbene complexes, in general (1.91-2.20 Å, av. 2.06(5) Å].…”