Strong Ligand Stabilization Based on π-Extension in a Series of Ruthenium Terpyridine Water Oxidation Catalysts

Abstract

The substitution behavior of the monodentate Cl ligand of a series of ruthenium(II) terpyridine complexes (terpyridine (tpy)=2,2':6',2''-terpyridine) has been investigated. ¹ H NMR kinetic experiments of the dissociation of the chloro ligand in D₂ O for the complexes [Ru(tpy)(bpy)Cl]Cl (1, bpy=2,2'-bipyridine) and [Ru(tpy)(dppz)Cl]Cl (2, dppz=dipyrido[3,2-a:2',3'-c]phenazine) as well as the binuclear complex [Ru(bpy)₂ (tpphz)Ru(tpy)Cl]Cl₃ (3 b, tpphz=tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine) were conducted, showing increased stability of the chloride ligand for compounds 2 and 3 due to the extended π-system. Compounds 1-5 (4=[Ru(tbbpy)₂ (tpphz)Ru(tpy)Cl](PF₆ )₃ , 5=[Ru(bpy)₂ (tpphz)Ru(tpy)(C₃ H₈ OS)/(H₂ O)](PF₆ )₃ , tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine) are tested for their ability to run water oxidation catalysis (WOC) using cerium(IV) as sacrificial oxidant. The WOC experiments suggest that the stability of monodentate (chloride) ligand strongly correlates to catalytic performance, which follows the trend 1>2>5≥3>4. This is also substantiated by quantum chemical calculations, which indicate a stronger binding for the chloride ligand based on the extended π-systems in compounds 2 and 3. Additionally, a theoretical model of the mechanism of the oxygen evolution of compounds 1 and 2 is presented; this suggests no differences in the elementary steps of the catalytic cycle within the bpy to the dppz complex, thus suggesting that differences in the catalytic performance are indeed based on ligand stability. Due to the presence of a photosensitizer and a catalytic unit, binuclear complexes 3 and 4 were tested for photocatalytic water oxidation. The bridging ligand architecture, however, inhibits the effective electron-transfer cascade that would allow photocatalysis to run efficiently. The findings of this study can elucidate critical factors in catalyst design.

Keywords: DFT calculations; homogenous catalysis; ligand; ruthenium-terpyridine complex exchange; water oxidation.

Figure 1

Complexes investigated in this study.

Figure 2

¹H NMR spectra of the protons, which will be shifted strongly upon chloride substitution (signals in close proximity to the monodentate ligand, i. e., H^a,H^c; Figure 1) in the aromatic region of compound 1–4 in an [D₆]acetone/[D₄]methanol mixture (9 : 1, v/v) at 25 °C. Note that the spectra for 3  a and 3  b are identical.

Figure 3

Normalized (to maximum absorbance of 1 at 478 nm) UV‐vis absorption spectra of compound 1 (black), 2 (red), 3 (blue), 4 (green).

Figure 4

¹H NMR spectra of compound 1 in D₂O at 25 °C, top: freshly prepared solution, bottom: solution after 24 h.

Figure 5

Peak area of H^a vs. time for the RuCl (red) and the Ru(D₂O) (blue) species derived from ¹H NMR spectra of compound 1, 2 and 3  b taken after different times in D₂O at 25 °C.

Figure 6

Catalytic TONs using cerium(IV)ammonium nitrate as sacrificial oxidant in water (1: black, 2: red, 3  b: orange, 4: green, 5: purple). Conditions: 1 and 2: cat: 100 μM, CAN: 0.5 M, H₂O; 3, 4 and 5: cat: 70 μM, CAN: 0.35 M, H₂O. (The best result for each catalyst is shown, errors are given in Table 2)

Figure 7

General catalytic cycle of the water oxidation reaction in mononuclear Ruthenium complexes.

Figure 8

Reaction mechanism and computed relative Gibbs free energies (ΔG ${}_{298,\mathrm{H}{}_2\mathrm{O}}$ [kcal/mol]) for the a) water nucleophilic attack (WNA) and b) O₂‐release events with corresponding reactants (R), associated reactants (AR), transition states (TS), associated products (AP) and product (P) for complex 1 (black) and 2 (red). In (a), the energy of the product (P) is not shown because the sum of the energies of separated products corresponds to the unphysical result: the product is energetically higher than the corresponding transition states. The latter is due to the insufficient stabilization of the “naked” (without explicit solvent molecules) hydronium H₃O⁺. The sum of the reactant energies is taken as a reference (0.0 kcal/mol). Level of theory: UPBE0‐D3BJ‐CPCM(water)/def2‐TZVP//UPBE0‐D3BJ‐CPCM(water)/def2‐SVP.

Figure 9

Reaction mechanism and computed relative Gibbs free energies (ΔG ${}_{298,\mathrm{H}{}_2\mathrm{O}}$ [kcal/mol]) for the ligand‐exchange step, with corresponding reactants (R), associated reactants (AR), transition states (TS), associated products (AP), and product (P) steps of complexes 1 (black), 2 (red) and 3 (orange). The sum of the reactant energies is taken as a reference (0.0 kcal/mol). Level of theory: UPBE0‐D3BJ‐CPCM(water)/def2‐TZVP//UPBE0‐D3BJ‐CPCM(water)/def2‐SVP.