Soushi Miyazaki

Proton-coupled electron transfer of ruthenium(III)-pterin complexes: a mechanistic insight.

Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK(a) and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru(II)(Hdmp)(TPA)](ClO(4))(2) (1), [Ru(II)(Hdmdmp)(TPA)](ClO(4))(2) (2), [Ru(II)(dmp(-))(TPA)]ClO(4) (3), and [Ru(II)(dmdmp(-))(TPA)]ClO(4) (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA = tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru(III)(dmp(-))(TPA)](2+), was determined to be 89 kcal mol(-1), which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol(-1)) to [Ru(III)(dmp(-))(TPA)](2+), the activation parameters were determined to be DeltaH(double dagger) = 1.6 +/- 0.2 kcal mol(-1) and DeltaS(double dagger) = -36 +/- 2 cal K(-1) mol(-1). This small activation enthalpy suggests a hydrogen-bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.

Photochemical and Thermal Isomerization of a Ruthenium(II)−Alloxazine Complex Involving an Unusual Coordination Mode

A Ru(II) complex having a flavin analogue as a ligand in a unusual coordination mode exhibits a photochemical and thermal isomerization; the bistability of the complex is attained by chelate effect and intramolecular CH···O interaction.

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.

Proton Shift upon One‐Electron Reduction in Ruthenium(II)‐Coordinated Pterins

Pterins are ubiquitous heteroaromatic coenzymes that are involved in many biological redox reactions in the vicinity of various metal ions. The redox processes of pterins proceed through proton-coupled electron transfer (PCET) involving the pyrazine moiety, in which up to four protons and electrons are manipulated in a concerted manner. Such processes can convert fully oxidized biopterin into fully reduced 5,6,7,8tetrahydrobiopterin. In the course of the redox processes of pterins, as shown in Scheme 1, a 5,8-dihydropterin is formed as a two-electronreduced species of the pterin. In contrast, the two-electron oxidation of a 5,6,7,8-tetrahydrobiopterin gives a quinonoid 6,7-dihydropterin, which contains a C=N bond involving the carbon atom at the 2-position. Both dihydropterins undergo thermal rearrangement to form a 7,8-dihydropterin as a thermodynamic sink. Fully oxidized pterins are known to release a proton from the nitrogen atom at the 3-position or the oxygen atom at the 4-position to coordinate to metal ions in a deprotonated imidate form (Scheme 2). Subsequent protonation gives a neutral pterin ligand, which undergoes reduction. Recently, we reported the one-electron reduction of monoprotonated pterins coordinated to a ruthenium(II)–tris(2-pyridylmethyl)amine (tpa) unit. The reduction gives ruthenium-bound monohydropterin radicals in which an unpaired electron is delocalized over the PCET region of the pyrazine moiety. Herein, we report the unprecedented observation of a proton shift from the nitrogen atom at the 1-position of the pyrimidinone moiety to the nitrogen atom at the 8-position of the pyrazine moiety upon the one-electron reduction of novel monoprotonated pterins in ruthenium(II) complexes (Scheme 2). [Ru(Hdmp)(tpa)](ClO4)2 (1; Hdmp = 6,7-dimethylpterin) and [Ru(Hdmdmp)(tpa)](ClO4)2 (2 ; Hdmdmp = N,Ndimethyl-6,7-dimethylpterin) were prepared through protonation of the corresponding precursor complexes [Ru(dmp)(tpa)](ClO4) (3) [9b] and [Ru(dmdmp)(tpa)](ClO4) (4), [9, 10] respectively, which have deprotonated, anionic pterin ligands, by adding 1 equivalent of HClO4 in CH3CN. Vapor diffusion of diethyl ether into the CH3CN solutions of 1 and 2 gave single crystals suitable for X-ray crystallography. ORTEP drawings of 1 and 2 are shown in Figure 1 (see also Figure S1 in the Supporting Information). As a common feature of 1 and 2, one of the perchlorate anions forms a hydrogen bond with an NH group at the 1position of the pterin ligand, as indicated by the close contact between one oxygen atom and N7 (Figure 1). In the crystal of 1, one of the perchlorate anions forms two hydrogen bonds to the neutral Hdmp ligand, one to the amino group at the 2position (O···N 2.93(1)–3.046(9) ) and one to the NH group at the 1-position (O···N 2.84(1)–2.88(1) ). This result clearly indicates that the proton is attached to the nitrogen atom at the 1-position in 1. In the crystal of 2, the NH group at the 1-position of the neutral Hdmdmp ligand forms a hydrogen bond with a perchlorate anion (O3···N7 2.94(1) ) or a water molecule of crystallization (O···N 2.78(1) ), confirming the protonation of the nitrogen atom at the 1position in 2. These results indicate that the first protonation occurs at the nitrogen atom at the 1-position of the coordinated pterins, rather than the nitrogen atom at the 8position. Prior to this work, it was thought that the first site of Scheme 1. PCET processes of pterins.

A tetranuclear iridium(III) complex with a flavin analogue as a bridging ligand in different coordination modes and exchangeable anion encapsulation in a supramolecular cage

A novel tetranuclear Ir(iii) complex involving unprecedented coordination modes of alloxazine formed a closed pi-space by intermolecular hydrogen bonding and the counter anions encapsulated in the space could be exchanged via self-assembly.

Ruthenium(II)-Pyridylamine Complexes Having Functional Groups via Amide Linkages

Functionalization of metal complexes by introduction of functional groups has been recognized to be important toward the development of further functionality of metal complexes, including ion sensing, molecular recognition, and selective catalysis. Convergence of functional groups into certain direction and appropriate spatial arrangement can be achieved by coordination of metal ions to ligands with those groups to perform novel functions that cannot be achieved by organic ligand molecules for themselves. This strategy allows us to access multifunctional molecules more easily than that with well-designed organic molecules in terms of synthetic availability. Ruthenium complexes bearing chelating pyridylamine ligands are robust enough to hold those ligands in the coordination spheres for the convergence of functional groups attached to the ligands and to maintain their appropriate spatial geometry. We have used tris(2pyridylmethyl)amine (TPA) and its derivatives which coordinate to the ruthenium ion as tetradentate ligands. Introduction of functional groups to the 6-position of pyridine rings of TPA can provide additional functionality for ruthenium-TPA complexes (Figure 1). The concept, i.e., the introduction of amide groups at the 6-positions of pyridine rings in TPA, has been originally introduced by Masuda and coworkers to construct a hydrophobic and sterically protected environment in copper complexes by using pivaloylamide groups (Harata et al., 1994, 1995, 1998; Wada et al., 1998). They have succeeded in a number of important metal complexes in bioinorganic chemistry. Inspired by their works, we have developed our concept to functionalize ruthenium-TPA complexes by introducing various functional groups via amide linkages. In our case, the ruthenium complexes bearing trisubstituted TPA is not suitable for functionalization due to its large steric crowding. Therefore we have applied bisamide and monoamide-TPA as ligands. In this chapter, we will present an overview of a chemistry of ruthenium complexes bearing bisamide-TPA and monoamide-TPA as ligands and their characteristics.