From the reaction of in situ generated 1,2-di(4-tert-butylphenyl)ethylene-1,2-dithiol, 2LH2, and Na[AuCl4].2H2O in 1,4-dioxane, green brown crystals of diamagnetic [N(n-Bu)4][AuIII(2L)2] (1) were obtained. As shown by cyclic voltammetry, 1 is a member of an electron-transfer series comprising the dianion [AuII(2L)2]2-, the monoanion [AuIII(2L)2]-, the neutral species [AuIII(2L*)(2L)]0 <--> [AuIII(2L)(2L*)]0, and the monocation [AuIII(2L*)2]+. (2L*)1- represents the pi radical anion (Srad = 1/2) of the one-electron oxidized closed-shell dianion (2L)2-. Oxidation of 1 in CH2Cl2 with ferrocenium hexafluorophosphate affords green, paramagnetic microcrystals of [AuIII(2L*)(2L)] <--> [AuIII(2L)(2L*)] (2) (S = 1/2). Complexes 1 and 2 have been characterized by X-ray crystallography. Both species possess square-planar monoanions and neutral molecules, respectively. From the oxidation reaction of 1 or [N(n-Bu)4][AuIII(3L)2] with 2-3 equiv of [NO]BF4 in CH2Cl2, a green solution of [AuIII(2L*)2]+ and green microcrystals of [AuIII(3L*)2]BF4 (3) were obtained, respectively; (3L)2- represents the dianion 1,2-di(4-diphenyl)ethylene-1,2-dithiolate, and (3L*)1- is its pi radical monoanion. The electronic structures of this series of gold species have been elucidated by UV-vis, EPR spectroscopies, and DFT calculations. It is shown computationally by density functional theoretical (DFT) methods that the electronic structure of [AuIII(1L*)2]+ is best described as a singlet diradical (St = 0); the ligand mixed valency in the neutral species 2 is of class (III) (delocalized); the monoanion in 1 contains a AuIII ion and two closed-shell dianionic ligands; and the corresponding dianions [Au(L)2]2- are best described as an intermediate AuII/AuIII species with a metal-ligand delocalized SOMO (25% Au 5d, 75% 3p of four S atoms). (1L)2- is the dianion 1,2-di(phenyl)ethylene-1,2-dithiolate, and (1L*)1- is the pi radical monoanion. The neutral species [PdII(2L*)2] (4) has also been synthesized and characterized by X-ray crystallography. Its electronic structure is the same as described for [AuIII(1L*)2]+ (singlet diradical), whereas that of the monoanion [PdII(2L*)(2L)]- <--> [Pd(2L)(2L*)]- corresponds to that of the neutral gold complex 2. Anodic oxidation of the analogous monoanion [AuIII(mnt)2]-, where mnt = maleonitriledithiolate, gave the neutral complex [Au(mnt)(mnt*)] (E1/2 = 0.91 V vs Fc+/Fc). The optical and EPR spectroscopies of [Au(mnt)(mnt*)] were consistent with those observed for the corresponding di(tert-butylphenyl)ethylenedithiolate complex 2.

Two mononuclear Cu II complexes with tetrapeptides incorporating a L-2,3-diaminopropionic acid (dap) branching unit are reported to undergo PCET and catalyse water oxidation. C-terminal His extension of dap (L = 2GH) instead of Gly (L = 3G) lowers the pK a for Cu III H À2 L (9.36 vs. 9.98) and improves the TOF at pH 11 (53 vs. 24 s À1 ).Reactions requiring the synchronous transfer of multiple protons and electrons become energetically viable under mild conditions through the catalytic promotion of proton-coupled electron transfer (PCET) mechanisms that help circumventing high-energy intermediates. 1 Splitting water into its elements, which attracts growing attention as a prospective renewable tool to generate H 2 as an energy carrier, 2-4 ranks among reactions where PCET is of critical importance. Water oxidation catalysts (WOCs) can improve the efficiency of the oxidative half-reaction: 2H 2 O -O 2 + 4H + + 4e À , which has long been considered the bottleneck of the water splitting process. Bioinspired, homogeneous WOCs (Fe, 5 Co, 6 Ru, 7 or Ir 8 ), although inherently less robust than heterogeneous catalysts, 9 represent a meaningful source of mechanistic insight into the multiple proton and electron-transfer events associated with O 2 formation. A growing number of studies conclude that PCET helps in stabilising high-valent MQO or M-O intermediates by preventing charge accumulation upon oxidation and, as a consequence these intermediates can complete the O-O bond formation step. 10 Cu has rich oxygen chemistry, 11 yet, homogeneous WOCs involving this metal appeared only recently, when surprisingly robust Cu II complexes with 2,2 0 -bipyridine (bpy, TOF B 100 s À1 at pH 13) 12 and subsequently, with triglycylglycine (GGGG, or H-Gly-Gly-Gly-Gly-OH, TOF = 33 s À1

Binuclear metal-metal bonded complexes such as [Rh 2 -(OAc) 4 ] (Scheme 1, A) are celebrated because of their unmatched ability to catalyze reactions that directly functionalize CÀH bonds.[1] These metal-metal bonded catalysts function by assisting the transfer of a carbene or nitrene group (CR 2 or NR, respectively) to an organic substrate. [1] The key intermediates in these C À H activation reactions are proposed to have structures such as B (Scheme 1) that feature both a metal-metal bond and a metal-ligand multiple bond. Despite many years of mechanistic study on these reactions, no multiply bonded species, such as B, has, to our knowledge, ever been isolated and characterized.[2] In order to synthesize an M À M = E metal-metal/metal-ligand multiply bonded system, we chose to target a RuÀRuN nitrido complex (Scheme 1, C) for which no structural precedents exist. This species could be synthesized from thermal or photolytic decomposition of the appropriate RuÀRuÀN 3 azido precursor.[3] This experimental strategy is attractive because synthetically useful [Ru 2 (L) 4 X] compounds (L = ligand) are well known [4] and also because Ru is known to stabilize mononuclear nitrido Ru VI complexes [5] that form a useful comparison to C.We used the previously reported azido compound [Ru 2 -(dPhf) 4 N 3 ] (2, dPhf = N,N'-diphenylformamidinate) [6] as a precursor for the photoreaction (Scheme 2), since it has intense absorption bands at 520 nm and 660 nm.[6] The nitrido complex, [Ru 2 (dPhf) 4 N] (1) was detected in positive-ion mode MALDI-TOF experiments on 2. At low laser power, only one set of peaks with the characteristic distribution of natural Ru isotopes can be seen, centered at 984.1, which is assigned as [2ÀN 3 ] + whereas, at higher laser power, a new set of peaks is detected at 14 mass units higher (998.1), that is ascribed unequivocally to 1 + , as is further supported by the calculated isotopic pattern. [7] Owing to its quartet ground state, the X-band EPR spectrum of 2 at 8 K features a distinct S = 3/2 signal at g eff = 4.22. Photolysis of 2 at À40 8C produces a new sharp axial S = 1/2 EPR signal, described below, that is unmistakably due to a highly reactive intermediate species because it decays rapidly at À40 8C.Photolysis of frozen samples of 2 in CH 2 Cl 2 at 77 K allowed us to maximize the yield of this new species, and to observe an accompanying color change from purple to pink upon its formation (UV/Vis spectroscopic data are given in the Supporting Information). The new axial EPR signal (Figure 1) clearly indicates an S = 1/2 species that we suggest is due to the photooxidation product [Ru 2 (dPhf) 4 N] (1), because photooxidation formally removes two electrons from the {Ru 2 } moiety, leaving only one unpaired electron located in an Ru 2 d* orbital (see below). Simulation of the EPR data yielded g tensor components, g ? = 2.189 and g k = 1.900, consistent with axial molecular symmetry. The g k value < 2.00 suggests spin-orbit coupling with vacant d-orbital-based molecular orbitals, that is, a higher ...

Mononuclear manganese(II) and iron(III) flavonolates were synthesized as synthetic enzyme-substrate complexes, and their oxygenation reactions as biomimetic functional models with relevance to flavonol 2,4-dioxygenases are briefly described.

The complex [Fe(indH)(solvent)3](ClO4)2 (1) has been isolated from the reaction of equimolar amounts of 1,3-bis(2′-pyridylimino)isoindoline (indH) and Fe(ClO4)2 in acetonitrile and characterized by X-ray crystallography and several spectroscopic techniques. It is a suitable catalyst for the oxidation of thioanisoles and benzyl alcohols with H2O2 as the oxidant. Hammett correlations and kinetic isotope effect experiments support the involvement of an electrophilic metal-based oxidant. A metastable green species (2) is observed when 1 is reacted with H2O2 at −40 °C, which has been characterized to have a FeIII(μ-O)(μ-O2)FeIII core on the basis of UV-Vis, electron paramagnetic resonance, resonance Raman, and X-ray absorption spectroscopic data.

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