Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and protoncoupled electron transfer (PCET) processes. Thermodynamic parameters including pK a , bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru II (Hdmp) (Hdmp = 6, Hdmdmp = N,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 ΔH ‡ = 1.6 ± 0.2 kcal mol −1 and ΔS ‡ = −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.kojima@chem.tsukuba.ac.jp (T. K.), mayer@chem.washington.edu (J. M. M.) and fukuzumi@chem.eng.osaka-u.ac.jp. † Osaka University ‡ Present address: Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennou-dai, Tsukuba, Ibaraki 305-8571, Japan. § University of Washington Supporting Information Available: UV-vis spectra in the spectroscopic titrations to determine the pK a values, the ESR spectrum of 2,4,6-tri-tert-butyl phenoxyl radical obtained by the hydrogen atom transfer reaction from 2,4,6-tri-tert-butyl phenol to [Ru III (dmp − ) (TPA)] 2+ , the mass spectrum of the reaction product of 4-nitrophenol with [Ru III (dmp − )(TPA)] 2+ . This material is available free of charge via the Internet at

Ruthenium(II)-TPA-diimine complexes, [Ru(TPA)(diimine)]2+ (TPA=tris(2-pyridylmethyl)amine; diimine=2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpm), 1,10-phenanthroline (phen)) were synthesized and characterized by spectroscopic and crystallographic methods. Their crystal structures demonstrate severe steric hindrance between the TPA and diimine ligands. They exhibit drastic structural changes on heating and photoirradiation at their MLCT bands, which involve partial dissociation of the tetradentate TPA ligand to exhibit a facially tridentate mode accompanied by structural change and solvent coordination to give [Ru(TPA)(diimine)(solvent)]2+ (solvent=acetonitrile, pyridine). The incoming solvent molecules are required to have pi-acceptor character, since sigma-donating solvent molecules do not coordinate. The thermal process is irreversible dissociation to give the solvent-bound complexes, which takes place by an interchange associative mechanism with large negative activation entropies. The photochemical process is a reversible reaction reaching a photostationary state, probably by a dissociative mechanism involving a five-coordinate intermediate to afford the same product as obtained in the thermal reaction. Quantum yields of the forward reactions to give dissociated products were lower than those of the backward reactions to recover the starting complexes. In the photochemical process, the conversions of the forward and backward reactions depend on the absorption coefficients of the starting materials and those of the products at certain wavelength, as well as the quantum yields of those reactions. The reversibility of the motions can be regulated by heating and by photoirradiation at certain wavelength for the recovery process. In the bpm system, we could achieve about 90 % recovery in thermal/photochemical structural interconversion.

[Structure: see text] Photocatalytic carbon-carbon bond formation of 9,10-dimethylanthracene (DMA) in chloroform occurs efficiently via the electron-transfer oxidation of DMA with the photoinduced electron-transfer state of 9-mesityl-10-methylacridinium ion (Acr+-Mes), followed by deprotonation from the methyl group of DMA radical cation and the radical coupling reaction between anthracenylmethyl radicals to produce dimethyllepidopterene.

Iron(III) spin-crossover compounds with H3-OMe-salRen ligands [Fe(3-OMe-salRen)2]ClO4 (1–5) were prepared and characterized by single-crystal X-ray diffraction, Mössbauer spectra, magnetic susceptibilities, and electronic spectra, where 3-OMe-salRen is a tridentate ligand derived from 3-methoxysalicylaldehyde and N-R-ethylenediamine (R = H, Me, Et, Pr, and Bu for 1, 2, 3, 4, and 5). The structures of compounds 1, 2, 3, and 5 at 90 and 298 K, and that of compound 4 at 298 K were determined. Compounds 1, 2, 3, and 5 exhibited a spin transition depending on temperature; the transition temperatures for 1, 2, 3, and 5 were >400, 360, 196, and 223 K, respectively. Compound 4 was in the high-spin state in the temperature range of 5 to 400 K. σ–π or π–π interactions exist in compounds 1–5, and the structures of the compounds were made clear in both the high-spin and low-spin states. Compound 3 exhibited a spin transition with thermal hysteresis.