Photocatalytic hydrogen evolution using a Ru(ii)-bound heteroaromatic ligand as a reactive site.

A RuII complex, [RuII(tpphz)(bpy)2]2+ (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [RuII(tpphz-HH)(bpy)2]2+ (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (3MLCT*) of 1 is firstly formed upon photoirradiation and the 3MLCT* state is reductively quenched with an electron donor to afford [RuII(tpphz˙-)(bpy)2]+, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its 3MLCT* state to react with a proton and aldehydes. In photocatalytic H2 evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

Complete photochromic structural changes in ruthenium(II)-diimine complexes, based on control of the excited states by metalation.

The thermal and photochemical reactions of a newly synthesized complex, [Ru(II)(TPA)(tpphz)](2+) (1; TPA=tris(2-pyridylmethyl)amine, tpphz=tetrapyrido[3,2-a:2',3'-c:3'',2''-h: 2''',3'''-j]phenazine), and its derivatives have been investigated. Heating a solution of complex 1 (closed form) and its derivatives in MeCN caused the partial dissociation of one pyridylmethyl moiety of the TPA ligand and the resulting vacant site on the Ru(II) center was occupied by a molecule of MeCN from the solvent to give a dissociated complex, [Ru(II)(η(3)-TPA)(tpphz)(MeCN)](2+) (1', open form), and its derivatives, respectively, in quantitative yields. The thermal dissociation reactions were investigated on the basis of kinetics analysis, which indicated that the reactions proceeded through a seven-coordinate transition state. Although the backwards reaction was induced by photoirradiation of the MLCT absorption bands, the photoreaction of complex 1' reached a photostationary state between complexes 1 and 1' and, hence, the recovery of complex 1 from complex 1' was 67%. Upon protonation of complex 1 at the vacant site of the tpphz ligand, the efficiency of the photoinduced recovery of complex 1+H(+) from complex 1'+H(+) improved to 83%. In contrast, dinuclear µ-tpphz complexes 2 and 3, which contained the Ru(II)(TPA)(tpphz) unit and either a Ru(II)(bpy)2 or Pd(II)Cl2 moiety on the other coordination edge of the tpphz ligand, exhibited 100% photoconversion from their open forms into their closed forms (2'→2 and 3'→3). These results are the first examples of the complete photochromic structural change of a transition-metal complex, as represented by complete interconversion between its open and closed forms. Scrutinization by performing optical and electrochemical measurements allowed us to propose a rationale for how metal coordination at the vacant site of the tpphz ligand improves the efficiency of photoconversion from the open form into the closed form. It is essential to lower the energy level of the triplet metal-to-ligand charge-transfer excited state ((3)MLCT*) of the closed form relative to that of the triplet metal-centered excited state ((3)MC*) by metal coordination. This energy-level manipulation hinders the transition from the (3)MLCT* state into the (3)MC* state in the closed form to block the partial photodissociation of the TPA ligand.

Mechanistic insights into photochromic behavior of a ruthenium(II)-pterin complex.

The pterin-coordinated ruthenium complex, [Ru(II) (dmdmp)(tpa)](+) (1) (Hdmdmp=N,N-dimethyl-6,7-dimethylpterin, tpa=tris(2-pyridylmethyl)amine), undergoes photochromic isomerization efficiently. The isomeric complex (2) was fully characterized to reveal an apparent 180° pseudorotation of the pterin ligand. Photoirradiation to the solution of 1 in acetone with incident light at 460 nm resulted in dissociation of one pyridylmethyl arm of the tpa ligand from the Ru(II) center to give an intermediate complex, [Ru(dmdmp)(tpa)(acetone)](2+) (I), accompanied by structural change and the coordination of a solvent molecule to occupy the vacant site. The quantum yield (ϕ) of this photoreaction was determined to be 0.87 %. The subsequent thermal process from intermediate I affords an isomeric complex 2, as a result of the rotation of the dmdmp(2-) ligand and the recoordination of the pyridyl group through structural change. The thermal process obeyed first-order kinetics, and the rate constant at 298 K was determined to be 5.83×10(-5) s(-1). The activation parameters were determined to be ΔH(≠) =81.8 kJ mol(-1) and ΔS(≠) =-49.8 J mol(-1) K(-1). The negative ΔS(≠) value indicates that this reaction involves a seven-coordinate complex in the transition state (i.e., an interchange associative mechanism). The most unique point of this reaction is that the recoordination of the photodissociated pyridylmethyl group occurs only from the direction to give isomer 2, without going back to starting complex 1, and thus the reaction proceeds with 100 % conversion efficiency. Upon heating a solution of 2 in acetonitrile, isomer 2 turned back into starting complex 1. The backward reaction is highly dependent on the solvent: isomer 2 is quite stable and hard to return to 1 in acetone; however, 2 was converted to 1 smoothly by heating in acetonitrile. The activation parameters for the first-order process in acetonitrile were determined to be ΔH(≠) =59.2 kJ mol(-1) and ΔS(≠) =-147.4 kJ mol(-1) K(-1). The largely negative ΔS(≠) value suggests the involvement of a seven-coordinate species with the strongly coordinated acetonitrile molecule in the transition state. Thus, the strength of the coordination of the solvent molecule to the Ru(II) center is a determinant factor in the photoisomerization of the Ru(II)-pterin complex.