Influence of chloro substituent on photoisomerization, redox reactions and water oxidation catalysis of mononuclear ruthenium complexes

Abstract

distal-[Ru(Cl-tpy)(pynp)Cl] + (d-2Cl) (Cl-tpy = 4'-chloro-2,2';6',2"-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), and distaland proximal-[Ru(Cl-tpy)(pynp)OH 2 ] 2+ (d-and p-2H 2 O) complexes are newly synthesized and characterized to compare structures and physicochemical properties with a 2,2';6',2"-terpyridine (tpy) ligand derivatives of distal-[Ru(tpy)(pynp)Cl] + (d-1Cl), distal-and proximal-[Ru(tpy)(pynp)OH 2 ] 2+ (d-and p-1H 2 O). The equilibrium turned out to be involved in the aquation reaction of d-2Cl to d-2H 2 O in contrast to observed irreversible aquation reaction of d-1Cl under the same conditions. The kinetic analysis showed that the aquation reaction of d-2Cl is slightly slower than that of d-1Cl. The stoichiometric photoisomerization of d-2H 2 O to p-2H 2 O occurs by visible light irradiation as it is for d-1H 2 O, and Φ (2.1%) at 520 nm for photoisomerization of d-2H 2 O was higher than that (1.5%) observed for d-1H 2 O. d-2H 2 O undergoes the two-step reaction involving the successive one-proton-coupled one-electron reactions of the Ru II-OH 2 /Ru III-OH and Ru III-OH/Ru IV =O redox couples, whereas p-2H 2 O undergoes the one-step reaction involving the two-proton-coupled two-electron reaction of the Ru II-OH 2 /Ru IV =O redox couple. These redox potentials of d-and p-2H 2 O are higher than those for d-and p-1H 2 O at pH = 7.0 by 10 ~ 50 mV due to the electron-withdrawing chloro substitution. The turnover frequency (k O2 = 6.3 × 10-3 s-1) of d-2H 2 O for water oxidation was higher than that (3.9 × 10-4 s-1) of p-2H 2 O by a factor of 16. k O2 for d-2H 2 O was also 1.6 times higher than that (3.8 × 10-3 s-1) for d-1H 2 O, whereas k O2 for p-2H 2 O was 1.2 times lower than that (4.8 × 10-4 s-1) for p-1H 2 O.

Visible‐Light‐Induced Morphological Changes of Giant Vesicles by Photoisomerization of a Ruthenium Aqua Complex

“…We recently reported irreversible [35][36][37][38] and reversible photoisomerizations [39] of mononuclear ruthenium aqua complexes havinga na symmetric bidentate ligand (Scheme 1). The reversibilityo ft he reactioni sc ontrolled by intramolecular hydrogen bondingb etween the aqua ligand and the pendant moiety of the bidentate ligand.…”

Mechanisms of Photoisomerization and Water Oxidation Catalysis of Ruthenium(II) Aquo Complexes

“…When an aqueous solution of d-Cl1 was irradiated with visible light (λ > 420 nm, 180 mWcm À2 ), the stoichiometric photoisomerization of d-Cl1 to p-Cl1 was observed as it is for d-1 (Figure 1). The rate constant for photoisomerization of d-Cl1 was estimated to be (k d-p ) obs = 4.9 x 10 À3 s À1 [35], which is higher than that ((k d-p ) obs = 4.1 x 10 À3 s À1 ) [22,34] observed for d-1 under the same conditions (Table 1). Additionally, the Φ value of d-Cl1 (2.1%) is higher than that (Φ =1.5%) of d- 1. Several groups reported that some polypyridyl complexes undergo photosubstitution reactions via the triplet metal centered ( 3 MC) state from the 3 MLCT excited state [25][26][27].…”

“…The chemical water oxidation catalyzed by d-1Cl and p-1Cl was investigated under the same conditions as the dÀ/p-1 isomer system to understand the effect of 4 0 -chloro-substitusion on tpy ligand on the catalytic activity of the distalÀ/proximal-isomer complexes for water oxidation [35]. v O2 increased linearly with respect to the complex amount for d-1Cl and p-1Cl, as is the case of the dÀ/p-1 isomer system (Figure 18).…”

“…We presented irreversible and stoichiometric photoisomerization of distal-[Ru (tpy)(pynp)OH 2 ] 2+ (d-1) (tpy = 2,2 0 ;6 0 ,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8naphthyridine) to proximal-[Ru(tpy)(pynp)OH 2 ] 2+ (p-1) as shown in Figure 1 [21][22][23], which had not been characterized previously for polypyridyl ruthenium (II) aquo complexes although various photochemical reactions of the ruthenium(II) complexes have been reported [24][25][26][27][28][29][30][31][32][33]. We have attempted to provide insight into the mechanism of the photoisomerization of polypyridyl ruthenium(II) aquo complexes and distinguish between the distalÀ/proximal-isomers in their physicochemical properties and functions [21][22][23][34][35][36]. We have also developed new synthetic strategy to form dinuclear ruthenium(II) complexes utilizing the photoisomerization [37,38].…”

“…We have also developed new synthetic strategy to form dinuclear ruthenium(II) complexes utilizing the photoisomerization [37,38]. Moreover, polypyridyl ruthenium(II) aquo complexes have been intensively studied as active WOCs [21,22,34,35,37,39,40] which are indispensable for artificial photosynthesis. The catalytic aspect and mechanism of water oxidation by the distal-/proximal-isomers of polypyridyl ruthenium(II) aquo complexes have been investigated to provide the guided thought to develop more efficient molecular catalysts for water oxidation.…”