Aqueous Hydricity of Late Metal Catalysts as a Continuum Tuned by Ligands and the Medium

Abstract

Aqueous hydride transfer is a fundamental step in emerging alternative energy transformations such as H2 evolution and CO2 reduction. "Hydricity," the hydride donor ability of a species, is a key metric for understanding transition metal hydride reactivity, but comprehensive studies of aqueous hydricity are scarce. An extensive and self-consistent aqueous hydricity scale is constructed for a family of Ru and Ir hydrides that are key intermediates in aqueous catalysis. A reference hydricity is determined using redox potentiometry and spectrophotometric titration for a particularly water-soluble species. Then, relative hydricity values for a range of species are measured using hydride transfer equilibria, taking advantage of expedient new synthetic procedures for Ru and Ir hydrides. This large collection of hydricity values provides the most comprehensive picture so far of how ligands impact hydricity in water. Strikingly, we also find that hydricity can be viewed as a continuum in water: the free energy of hydride transfer changes with pH, buffer composition, and salts present in solution.

Figure 1

Scheme illustrating the hydricity of reference complex [Cp*Ir(bpy-COO)(H)]⁻ (2H) and thermochemical cycles that establish aqueous hydricity of Ir and Ru hydrides.

Figure 2

(A) Spectral changes of a pH 14 solution of [Cp*Ir(bpy-COO) (OH)]⁻ (2OH) as the solution potential is decreased by electrolysis to form [Cp*Ir(bpy-COO)]^2– (2). (B) Absorbance at 620 nm stepping in the negative potential direction (red dots), the positive potential direction (blue dots), and the fit to the Nernst equation (dot-dashed line) giving E°′ = −0.60 V. The lack of hysteresis indicates that equilibrium was established. (C) Absorbance at 570 nm of a pH titration of [Cp*Ir(bpy-COO)(H)]⁻ (2H) forming 2 (red dots) and the fit to the Henderson–Hasselbalch equation (dot-dashed line) giving pK_a = 12.4.

Scheme 1

Figure 3

(A) Summary of thermochemical values of [Cp*Ir(bpy-COO)(H)]⁻ (2H). Free energies (kcal·mol^–1) and reduction potentials (V vs NHE) are cited at the standard state of pH 0, 1 M reagents, and 1 atm gases, except for ΔG_H^–(P_i) that refers to pH 7. (B) Summary of the pH dependence of ΔG_H^–(Y) with the H₂O/H₂ and CO₂/HCO₂^– couples.

Figure 4

Relative hydricity values of Ir and Ru complexes (blue). The equilibria used to determine hydricity are represented by blue arrows.

Figure 5

Aqueous hydricity scale of the complexes we report along with those previously reported in the literature. Y represents the incoming ligand such that the top scale shows ΔG°_H^–(Cl) and the bottom scale shows ΔG°_H^–(OH₂). TSPP = tetra(p-sulfonatophenyl)porphyrin; TMPS = tetrakis(3,5-disulfonatomesityl)porphyrin; tpy = terpyridine; DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane.^,,,

Figure 6

Correlation between σ_p^– and ΔG°_H^–(Cl).