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Oxygenation of [Cu2(UN-O(-))(DMF)](2+) (1), a structurally characterized dicopper Robin-Day class I mixed-valent Cu(II)Cu(I) complex, with UN-O(-) as a binucleating ligand and where dimethylformamide (DMF) binds to the Cu(II) ion, leads to a superoxo-dicopper(II) species [Cu(II)2(UN-O(-))(O2(•-))](2+) (2). The formation kinetics provide that kon = 9 × 10(-2) M(-1) s(-1) (-80 °C), ΔH(‡) = 31.1 kJ mol(-1) and ΔS(‡) = -99.4 J K(-1) mol(-1) (from -60 to -90 °C data). Complex 2 can be reversibly reduced to the peroxide species [Cu(II)2(UN-O(-))(O2(2-))](+) (3), using varying outer-sphere ferrocene or ferrocenium redox reagents. A Nernstian analysis could be performed by utilizing a monodiphenylamine substituted ferrocenium salt to oxidize 3, leading to an equilibrium mixture with Ket = 5.3 (-80 °C); a standard reduction potential for the superoxo-peroxo pair is calculated to be E° = +130 mV vs SCE. A literature survey shows that this value falls into the range of biologically relevant redox reagents, e.g., cytochrome c and an organic solvent solubilized ascorbate anion. Using mixed-isotope resonance Raman (rRaman) spectroscopic characterization, accompanied by DFT calculations, it is shown that the superoxo complex consists of a mixture of μ-1,2- (2(1,2)) and μ-1,1- (2(1,1)) isomers, which are in rapid equilibrium. The electron transfer process involves only the μ-1,2-superoxo complex [Cu(II)2(UN-O(-))(μ-1,2-O2(•-))](2+) (2(1,2)) and μ-1,2-peroxo structures [Cu(II)2(UN-O(-))(O2(2-))](+) (3) having a small bond reorganization energy of 0.4 eV (λin). A stopped-flow kinetic study results reveal an outer-sphere electron transfer process with a total reorganization energy (λ) of 1.1 eV between 2(1,2) and 3 calculated in the context of Marcus theory.

This study details the electronic structure of the heme–peroxo–copper adduct {[(F8)Fe(DCHIm)]-O2-[Cu(AN)]}+ (LS(AN)) in which O2(2–) bridges the metals in a μ-1,2 or “end-on” configuration. LS(AN) is generated by addition of coordinating base to the parent complex {[(F8)Fe]-O2-[Cu(AN)]}+ (HS(AN)) in which the O2(2–) bridges the metals in an μ-η2:η2 or “side-on” mode. In addition to the structural change of the O2(2–) bridging geometry, coordination of the base changes the spin state of the heme fragment (from S = 5/2 in HS(AN) to S = 1/2 in LS(AN)) that results in an antiferromagnetically coupled diamagnetic ground state in LS(AN). The strong ligand field of the porphyrin modulates the high-spin to low-spin effect on Fe–peroxo bonding relative to nonheme complexes, which is important in the O–O bond cleavage process. On the basis of DFT calculations, the ground state of LS(AN) is dependent on the Fe–O–O–Cu dihedral angle, wherein acute angles (<~150°) yield an antiferromagnetically coupled electronic structure while more obtuse angles yield a ferromagnetic ground state. LS(AN) is diamagnetic and thus has an antiferromagnetically coupled ground state with a calculated Fe–O–O–Cu dihedral angle of 137°. The nature of the bonding in LS(AN) and the frontier molecular orbitals which lead to this magneto-structural correlation provide insight into possible spin topology contributions to O–O bond cleavage by cytochrome c oxidase.

To better understand the effect of thioether coordination in copper-O(2) chemistry, the tetradentate N(3)S ligand L(ASM) (2-(methylthio)-N,N-bis((pyridin-2-yl)methyl)benzenamine) and related alkylether ligand L(EOE) (2-ethoxy-N,N-bis((pyridin-2-yl)methyl)ethanamine) have been studied. The corresponding copper(I) complexes, [(L(ASM))Cu(I)](+) (1a) and [(L(EOE))Cu(I)](+) (3a), were studied as were the related compound [(L(ESE))Cu(I)](+) (2a, L(ESE) = (2-ethylthio-N,N-bis((pyridin-2-yl)methyl)ethanamine). The X-ray structure of 1a and its solution conductivity reveal a monomeric molecular structure possessing thioether coordination which persists in solution. In contrast, the C-O stretching frequencies of the derivative Cu(I)-CO complexes reveal that for these complexes, the modulated ligand arms, whether arylthioether, alkylthioether, or ether, are not coordinated to the cuprous ion. Electrochemical data for 1a and 2a in CH(3)CN and N,N-dimethylformamide (DMF) show the thioanisole moiety to be a poor electron donor compared to alkylthioether (1a is ∼200 mV more positive than 2a). The structures of [(L(ASM))Cu(II)(CH(3)OH)](2+) (1c) and [(L(ESE))Cu(II)(CH(3)OH)](2+) (2c) have also been obtained and indicate nearly identical copper coordination environments. Oxygenation of 1a at reduced temperature gives a characteristic deep blue intermediate [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) with absorption features at 442 (1,500 M(-1) cm(-1)), 530 (8,600 M(-1) cm(-1)), and 605 nm (10,400 M(-1) cm(-1)); these values compare well to the ligand-to-metal charge-transfer (LMCT) transitions previously reported for [{(L(ESE))Cu(II)}(2)(O(2)(2-))](2+) (2b(P)). Resonance Raman data for [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) support the formation of μ-1,2-peroxo species ν(O-O) = 828 cm(-1)(Δ((18)O(2)) = 48), ν(sym)(Cu-O) = 547 cm(-1) (Δ((18)O(2)) = 23), and ν(asym)(Cu-O) = 497 cm(-1) (Δ((18)O(2)) = 22) and suggest the L(ASM) ligand is a poorer electron donor to copper than is L(ESE). In contrast, the oxygenation of [(L(EOE))Cu(I)](+) (3a), possessing an ether donor as an analogue of the thioether in L(ESE), led to the formation of a bis(μ-oxo) species [{(L(EOE))Cu(III)}(2)(O(2-))(2)](2+) (3b(O); 380 nm, ε ∼ 10,000 M(-1) cm(-1)). This result provides further support for the sulfur influence in 1b(P) and 2b(P), in particular coordination of the sulfur to the Cu. Thermal decomposition of 1b(P) is accompanied by ligand sulfoxidation. The structure of [{(L(EOE))Cu(II)(Cl)}(2)](+) (3c) generated from the reductive dehalogenation of organic chlorides suggests that the ether moiety is weakly bound to the cupric ion. A detailed discussion of the spectroscopic and structural characteristics of 1b(P), 2b(P), and 3b(O) is presented.

A powerful means of enhancing our understanding of the structures and functions of enzymes that contain nickel-sulfur bonds, such as Ni superoxide dismutase, acetyl-coenzyme A synthase/carbon monoxide dehydrogenase, [NiFe] hydrogenase, and methyl-CoM reductase, involves the investigation of model compounds with similar structural and/or electronic properties. In this study, we have characterized a trans-mu-1,2-disulfido-bridged dinickel(II) species, [{(tmc)Ni}(2)(S(2))](2+) (1, tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) by using electronic absorption, magnetic circular dichroism (MCD), and resonance Raman (rR) spectroscopic techniques, as well as density functional theory (DFT) and time-dependent DFT computational methods. Our computational results, validated on the basis of the experimental MCD data and previously reported (1)H NMR spectra, reveal that 1 is best described as containing two antiferromagnetically coupled high-spin Ni(II) centers. A normal coordinate analysis of the rR vibrational data was performed to quantify the core bond strengths, yielding force constants of k(Ni-S) = 2.69 mdyn/A and k(S-S) = 2.40 mdyn/A. These values provide a useful basis for a comparison of metal-S/O bonding in 1 and related Ni(2)(O(2)), Cu(2)(O(2)), and Cu(2)(S(2)) dimers. In both the disulfido and the peroxo species, the lower effective nuclear charge of Ni(II) as compared to Cu(II) results in a decreased covalency, and thus relatively weaker metal-S/O bonding interactions in the Ni(2) dimers than in the Cu(2) complexes.