Transition-metal peroxos have been implicated as key intermediates in a variety of critical biological processes involving O2. Due to their highly reactive nature, very few metal-peroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a binuclear Mn-peroxo, either as a precursor to O2, or derived from O2, in both photosynthetic H2O oxidation and DNA biosynthesis, arguably two of the most fundamental processes of life. Neither of these biological intermediates has been observed. Herein we describe the dioxygen chemistry of coordinatively unsaturated [MnII(SMe2N4(6-MeDPEN))] +(1), and the characterization of intermediates formed en route to a binuclear mono-oxo bridged Mn(III) product {[MnIII(SMe2N4(6-MeDPEN)]2-(μ-O)}2+ (2), the oxo atom of which is derived from 18O2. At low-temperatures, a dioxygen intermediate, [Mn(SMe2N4(6-MeDPEN))(O2)]+ (4), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (k1(−10 °C)= 3780±180M−1s−1, ΔH1‡ = 26.4±1.7 kJ mol−1, ΔS1‡ = − 75.6±6.8 J mol−1K−1), and then convert more slowly (k2(−10 °C)= 417±3.2 M−1s−1, ΔH2‡ = 47.1±1.4 kJ mol−1, ΔS2‡ = − 15.0±5.7 J mol−1K−1) to a species 3 with isotopically sensitive stretches at νo-o (Δ18O) = 819(47) cm−1, kO–O= 3.02 mdyn/Å, and νMn-O(Δ18O) = 611(25) cm−1 consistent with a peroxo. Intermediate 3 releases approximately 0.5 equiv of H2O2 per Mn ion upon protonation, and the rate of conversion of 4 to 3 is dependent on [Mn(II)] concentration, consistent with the formation of a binuclear Mn-peroxo. This was verified by X-ray crystallography, where the peroxo of {[MnIII(SMe2N4(6-Me-DPEN)]2(trans–μ–1,2–O2)}2+ (3) is shown to be bridging between two Mn(III) ions in an end-on trans-μ-1,2-fashion. This represents the first characterized example of a binuclear Mn(III)-peroxo, and a rare case in which more than one intermediate is observed en route to a binuclear μ–oxo bridged product derived from O2. Vibrational and metrical parameters for binuclear Mn-peroxo 3 are compared with those of related binuclear Fe- and Cu-peroxo compounds.

From the reaction mixture of [M(II)(bpy)Cl(2)], the ligand 2-anilino-4,6-di-tert-butylphenol, H[L(AP)], and 2 equiv of a base (NaOCH(3)) in CH(3)CN under anaerobic conditions were obtained the blue-green neutral complexes [M(II)(L(AP)-H)(bpy)] (M = Pd (1), Pt (2)). (L(AP)-H)(2)(-) represents the o-amidophenolato dianion, (L(AP))(1)(-) is the o-aminophenolate(1-), (L(ISQ))(1)(-) is its one-electron-oxidized, pi-radical o-iminobenzosemiquinonate(1-), and (L(IBQ))(0) is the neutral quinone. Complexes 1 and 2 can be oxidized by ferrocenium hexafluorophosphate, yielding the paramagnetic salts [M(II)(L(ISQ))(bpy)]PF(6) (S = (1)/(2)) (M = Pd (1a), Pt (2a)). The reaction of PtCl(2), 2 equiv of H[L(AP)], and 4 equiv of base in CH(3)CN in the presence of air yields diamagnetic [Pt(L(ISQ))(2)] (3), which is shown to possess an electronic structure that is best described as a singlet diradical. Complexes 1, 1a, 2, 2a, and 3 have been structurally characterized by X-ray crystallography at 100 K. It is clearly established that O,N-coordinated (L(AP)-H)(2)(-) ligands have a distinctly different structure than the corresponding O,N-coordinated (L(ISQ))(1)(-) radicals. It is therefore possible to unambiguously assign the protonation and oxidation level of o-aminophenol derived ligands in coordination compounds. All complexes have been investigated by cyclic voltammetry, spectroelectrochemistry, EPR, and UV-vis spectroscopy. Complexes 1 and 2 can be reversibly oxidized to the [M(II)(L(ISQ))(bpy)](+) and [M(II)(L(IBQ))(pby)](2+) mono- and dications, respectively, and reduced to the [M(L(AP)-H)(bpy(*))](-) anion, where (bpy(*))(1)(-) is the radical anion of 2,2'-bipyridine. Complex 3 exhibits four reversible one-electron-transfer waves (two oxidations and two reductions) which are all shown to be ligand centered. The EPR spectra of the one-electron-reduced species [Pt(L(AP)-H)(L(ISQ))](-) (S = (1)/(2)) and of the one-electron-oxidized species [Pt(L(ISQ))(L(IBQ))](+) (S = (1)/(2)) in CH(2)Cl(2) solutions have been recorded. To gain a better understanding of the electronic structure of 3 and its monooxidized and reduced forms, relativistic DFT calculations have been carried out. Magnetic coupling parameters and hyperfine couplings were calculated and found to be in very good agreement with experiment. It is shown that both the one-electron oxidation and reduction of 3 are ligand centered. A simple MO model is developed in order to understand the EPR properties of the monocation and monoanion of 3.

We report the isolation, characterization, and reactions of the unsaturated complex L tBu Co (L tBu = bulky β-diketiminate ligand). The unusual slipped kN,η 6 -arene binding mode in L tBu Co interconverts rapidly and reversibly with the traditional k 2 N,N 0 ligation mode upon binding of Lewis bases, making it a "masked" two-coordinate complex. The mechanism of this isomerization is demonstrated using kinetic studies. L tBu Co is a stable yet reactive synthon for low-coordinate cobalt(I) complexes and is capable of cleaving the CÀF bond in fluorobenzene.H emilabile ligands, which contain a Lewis basic moiety that can reversibly dissociate from a metal, create transient coordinative unsaturation that can be used for bond activation and catalysis. 1 Notable examples include catalysts for olefin metathesis and cross-coupling reactions. 2 In this communication, we report that a bulky β-diketiminate ligand on cobalt(I) undergoes a novel isomerization that allows it to behave as a hemilabile ligand. Though β-diketiminates are represented in thousands of metal complexes, 3 this is the first example of this new binding mode. We show that the ligand isomerization is rapidly reversible and that it provides a "masked two-coordinate" cobalt(I) center for ligand binding and for activation of a strong CÀF bond.The addition of tetrahydrofuran (THF) to a solution of L tBu CoNNCoL tBu [L tBu = 2,2,6,6-tetramethyl-3,5-bis(2,4,6-triisopropylphenylimido)hept-4-yl] 4 in C 6 D 6 gives an immediate color change from brown to dark-green. This color change corresponds to the appearance of a set of signals in the 1 H NMR spectrum for a new species, L tBu Co(THF) (1), in quantitative yield. On a preparative scale, 1 can be produced from the reaction of L tBu CoCl with KC 8 in THF under Ar, which gives 1 as a dark-green crystalline solid in 64% yield. Complex 1 (Figure 1 left) has pseudo-C 2 symmetry in the solid state. The threecoordinate cobalt atom is planar, as the sum of the NÀCoÀN and NÀCoÀO bond angles is 359.7(2)°. The THF ligand is bent slightly toward one aryl arm of the β-diketiminate ligand, with NÀCoÀO angles of 135. Dissolution of 1 in C 6 D 6 under Ar gives a dark-orange solution whose 1 H NMR spectrum shows peaks from 1 plus a number of additional resonances. Evaporation of the volatile materials and redissolution in fresh C 6 D 6 leads to an 1 H NMR spectrum that contains very little 1. The displacement of THF with aromatic solvents can be used for the synthesis of the unknown species on a gram scale. Thus, 1 was dissolved in toluene, and two cycles of solvent removal and addition of more toluene removed all of the THF. Crystallization from a concentrated pentane solution at À45°C under Ar gave L tBu Co (2) in 72% yield as brown crystals.The molecular structure of 2 (Figure 1 right) shows a cobalt atom and a single β-diketiminate ligand with no additional donors. In contrast to the typical k 2 N,N 0 binding mode for L tBu , the β-diketiminate ligand in 2 is bound to cobalt in a kN,η 6 -arene mode in which the co...

Bis(1,1-diphenylhydrazido(1-))ruthenium(IV) porphyrins, [Ru(IV)(Por)(NHNPh2)2] (Por = TPP, TTP, 4-Cl-TPP, 4-MeO-TPP), were prepared in approximately 60% yields through the reaction of dioxoruthenium(VI) porphyrins, [Ru(VI)(Por)O2], with 1,1-diphenylhydrazine in ethanol. This new type of ruthenium complex has been characterized by 1H NMR, IR, UV-vis, and FABMS with elemental analysis. The crystal structure of [Ru(IV)(TTP)(NHNPh2)2], which reveals an eta1-coordination mode for both hydrazido axial ligands, has been determined. The average Ru-NHNPh2 distance and Ru-N-N angle were found to be 1.911(3) A and 141.1(3) degrees, respectively. The porphyrin ring exhibits a ruffling distortion that is unprecedentedly large for ruthenium complexes with simple porphyrinato ligands (such as TTP). This is probably due to the steric effect of the axial hydrazido(1-) ligands.

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The benzanellated analogues (NEt(4))(2)[Fe(2)S(2)(indolate)(4)] (2) and (NEt(4))(2)[Fe(2)S(2)(carbazolate)(4)] (3) of the previously reported parent (NEt(4))(2)[Fe(2)S(2)(pyrrolate)(4)] cluster (1) were synthesized and characterized spectroscopically. In contrast to 1 and 3, compound 2 can be applied as a versatile precursor in ligand exchange reactions with various thiophenols affording the thiophenolate-coordinate [2Fe-2S] clusters. Heteroaromatic thiols and chelating biphenols are suitable substrates in this conversion as well, providing a convenient access to a variety of new [2Fe-2S] ferredoxin analogues and related complexes. Several new S- and O-coordinate [2Fe-2S] clusters have been prepared and fully characterized, including five X-ray crystal structures.

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A peroxide dianion (O2(2-)) can be isolated within the cavity of hexacarboxamide cryptand, [(O2)⊂mBDCA-5t-H6](2-), stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O2)⊂mBDCA-5t-H6](2-) with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler-Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O2(2-) occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O2(2-) within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal-air batteries and the use of [(O2)⊂mBDCA-5t-H6](2-) as a chemical reagent.