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A detailed spectroscopic and quantum chemical analysis is presented to elucidate the electronic structures of the octahedral complexes [Fe(Et(2)dtc)(3-n)(mnt)(n)](n-) (1-4, n=3, 2, 1, 0) and their one-electron oxidized analogues [Fe(Et(2)dtc)(3-n)(mnt)(n)]((n-1)-) (1(ox)-4(ox)); (mnt)(2-) represents maleonitriledithiolate(2-) and (Et(2)dtc)(1-) is the diethyldithiocarbamato(1-) ligand. By using X-ray crystallography, Mossbauer spectroscopy, and Fe and S K-edge X-ray absorption spectroscopy (XAS) it is convincingly shown that, in contrast to our previous studies on [Fe(cyclam)(mnt)](1+) (cyclam=1,4,8,11-tetraazacyclotetradecane), the oxidation of 1-4 is metal-centered yielding the genuine Fe(IV) complexes 1(ox)-4(ox). For the latter complexes, a spin ground state of S=1 has been established by magnetic susceptibility measurements, which indicates a low-spin d(4) configuration. DFT calculations at the B3LYP level support this electronic structure and exclude the presence of a ligand pi radical coordinated to an intermediate-spin ferric ion. Mossbauer parameters and XAS spectra have been calculated to calibrate our computational results against the experiment. Finally, a simple ligand-field approach is presented to correlate the structural features obtained from X-ray crystallography (100 K) with the spectroscopic data.

Sco is a mononuclear red copper protein involved in the assembly of cytochrome c oxidase. It is spectroscopically similar to red copper nitrosocyanin, but unlike the latter, which has one copper cysteine thiolate, the former has two. In addition to the two cysteine ligands (C45 and C49), the wild-type (WT) protein from Bacillus subtilis (hereafter named BSco) has a histidine (H135) and an unknown endogenous protein oxygen ligand in a distorted tetragonal array. We have compared the properties of the WT protein to variants in which each of the two coordinating Cys residues has been individually mutated to Ala, using UV/visible, Cu and S K-edge X-ray absorption, electron paramagnetic resonance, and resonance Raman spectroscopies. Unlike the Cu(II) form of native Sco, the Cu(II) complexes of the Cys variants are unstable. The copper center of C49A undergoes autoreduction to the Cu(I) form, which is shown by extended X-ray absorption fine structure to be composed of a novel two-coordinate center with one Cys and one His ligand. C45A rearranges to a new stable Cu(II) species coordinated by C49, H135 and a second His ligand recruited from a previously uncoordinated protein side chain. The different chemistry exhibited by the Cys variants can be rationalized by whether a stable Cu(I) species can be formed by autoredox chemistry. For C49A, the remaining Cys and His residues are trans, which facilitates the formation of the highly stable two-coordinate Cu(I) species, while for C45A such a configuration cannot be attained. Resonance Raman spectroscopy of the WT protein indicates a net weak Cu-S bond strength at approximately 2.24 A corresponding to the two thiolate-copper bonds, whereas the single variant C45A shows a moderately strong Cu-S bond at approximately 2.16 A. S K-edge data give a total covalency of 28% for both Cu-S bonds in the WT protein. These data suggest an average covalency per Cu-S bond lower than that observed for nitrosocyanin and close to that expected for type-2 Cu(II)-thiolate systems. The data are discussed relative to the unique Cu-S characteristics of cupredoxins, from which it is concluded that Sco does not contain highly covalent Cu-S bonds of the type expected for long-range electron-transfer reactivity.

The synthesis, characterization and biological properties of a novel platinum compound, (H 2 Std)PtCl 4 (H 2 O) x (1, where x= 1-4; H 2 Std stands for streptidine, the cationic sugar component of streptomycin), are reported in the solid state and in solution. Reaction of streptidine sulfate ((H 2 Std)SO 4 ) with K 2 PtCl 4 , yields the sparingly soluble hydrated compound (H 2 Std)PtCl 4· Structural information has been attained by a variety of spectroscopic techniques, as well as by using Pt L 3 - and CI K-edge X-ray absorption spectroscopy (XAS). The analyses of the data were completed by performing extended X-ray structural characterization of the free ligand as streptidine sulfate, which was found crucial to interpret the nature and structural arrangement of complex 1. In addition, this work also presents the biological activity of complex 1, i.e. cytotoxic assays, antimicrobial studies, DNA titration and fluorescence in vitro experiments. The studies presented in this paper contribute to our understanding of the synergy between platinum and pharmaceutically relevant compounds.

Many proteins contain copper in a range of coordination environments, where it has various biological roles, such as transferring electrons or activating dioxygen. These copper sites can be classified by their function or spectroscopic properties. Those with a single copper atom are either type 1, with an intense absorption band near 600 nm, or type 2, with weak absorption in the visible region. We have built a novel copper(ii) binding site within structurally modified Pseudomonas aeruginosa azurins that does not resemble either existing type, which we therefore call 'type zero'. X-ray crystallographic analysis shows that these sites adopt distorted tetrahedral geometries, with an unusually short Cu–O (G45 carbonyl) bond. Relatively weak absorption near 800 nm and narrow parallel hyperfine splittings in electron paramagnetic resonance spectra are the spectroscopic signatures of type zero copper. Cyclic voltammetric experiments demonstrate that the electron transfer reactivities of type-zero azurins are enhanced relative to that of the corresponding type 2 (C112D) protein.

The reaction of ReCl(5) with 3 equiv of a benzene-1,2-dithiolate derivative in CH(3)CN produced, after the addition of [C(8)H(16)N]Br ([C(8)H(16)N](+) is 5-azonia-spiro[4,4]nonane), brownish-green crystals of [C(8)H(16)N][Re(tms)(3)] (1c) and [C(8)H(16)N][Re(Cl(2)-bdt)(3)] (2c), where (tms)(2-) represents 3,6-bis(trimethylsilyl)benzene-1,2-dithiolate and (Cl(2)-bdt)(2-) is 3,6-dichlorobenzene-1,2-dithiolate. Chemical reduction of [Re(bdt)(3)] (3b) with n-butyllithium in the presence of PPh(4)Br yielded [PPh(4)][Re(bdt)(3)] (3c), where (bdt)(2-) is benzene-1,2-dithiolate. The three monoanionic complexes possess a diamagnetic ground state (Re(V), d(2), S = 0). The crystal structures of 1c x 2 CH(3)CN and 2c x C(3)H(6)O have been determined by X-ray crystallography. The electrochemistry establishes that the complexes are members of electron transfer series involving a monocation [Re(V)(L(*))(2)(L)](+) (S = 0(?)), a neutral [Re(V)(L(*))(L)(2)](0) (S = 1/2), a monoanion [Re(V)(L)(3)](1-) (S = 0), a dianion [Re(IV)(L)(3)](2-) (S = 1/2), and a trianion [Re(III)(L)(3)](3-) (S = 1(?)). The unique X-band EPR spectrum of the neutral species clearly describes a diamagnetic Re(V) d(2) central ion with the unpaired electron located in a purely ligand-centered molecular orbital, whereas it is metal-centered in the dianionic form: a Re(IV) d(3) ion with three dithiolate(2-) ligands. S K-edge and Re L-edge X-ray absorption spectroscopy confirms these assignments and furthermore shows that the monoanion has a Re(V) central ion with three dianionic ligands. The geometrical and electronic structures of all members of the electron transfer series have been calculated by density functional theoretical methods, and the S K-pre-edge spectra have been simulated and assigned using a time-dependent DFT protocol.

The reaction of [RuIII(cyclam)Cl2]Cl with 2 equiv of sodium N,N-diethyldithiocarbamate in methanol afforded [RuII(cyclam)(Et2dtc)](BPh4) (1(BPh4)). The same reaction with only 1 equiv of toluene-3,...

The reaction of cis-[Fe(III)(cyclam)Cl(2)]Cl with 1 equiv of sodium N-diethyldithiocarbamate, toluene-3,4-dithiolate, and maleonitriledithiolate in methanol in the presence of triethylamine afforded the cations [Fe(III)(cyclam)(Et(2)dtc)](2+) (1), [Fe(III)(cyclam)(tdt)](+) (2), and [Fe(III)(cyclam)(mnt)](+) (3), which were isolated as triflate, hexafluorophosphate, and tetrafluoroborate salt, respectively, using sodium triflate, potassium hexafluorophosphate, or sodium tetrafluoroborate as the source for the counteranion. Complexes 1, 2, and 3 possess an S = (1)/(2) ground state (low-spin ferric d(5)). These salts were characterized by X-ray crystallography, UV-vis, Mössbauer, and electron paramagnetic resonance spectroscopies. Cyclic voltammetry revealed that 2 and 3 are reversibly one-electron-reduced, generating neutral 2(red) and 3(red), respectively, and one-electron-oxidized, generating dicationic 2(ox) and 3(ox), respectively. Fe and S K-edge X-ray absorption spectroscopy (XAS) revealed that 2 (S = (1)/(2)) and 2(ox) (S = 0) possess a low-spin ferric ion. Complexes 2 and 3 are S,S'-coordinated to a closed-shell dithiolate(2-) ligand, whereas 2(ox) and 3(ox) consist of a low-spin ferric ion antiferromagnetically coupled to a dithiolate(1-) pi radical ligand. They are singlet diradicals [Fe(III)(cyclam)(dithiolate(*))](2+). The analysis of the sulfur K pre-edge transitions reveals significant multiplet effects in the spectra of 2 and 2(ox), which provide rare experimental evidence for a singlet diradical description for 2(ox). Mössbauer spectroscopy on frozen solutions of 2(red) clearly show the presence of a high-spin ferrous ion (S = 2). The experimentally established electronic structures of the three members of the electron transfer series [Fe(cyclam)(dithiolate)](2+,+,0) have been verified by broken symmetry density functional theoretical calculations, which have been calibrated against the experiment by calculating XAS and Mössbauer spectra.

From the reaction mixture of 3,6-dichlorobenzene-1,2-dithiol, H(2)(Cl(2)-bdt), [CrCl(3)(thf)(3)], and NEt(3) in tetrahydrofuran (thf) in the presence of air, dark green crystals of [N(n-Bu)(4)](2)[Cr(Cl(2)-bdt)(3)] (S = 1) (1) were isolated upon addition of [N(n-Bu)(4)]Br. Oxidation of the AsPh(4)(+) salt of 1 with [Fc]PF(6) yielded microcrystals of [AsPh(4)][Cr(Cl(2)-bdt)(3)] (S = (1)/(2)) (2) whereas the reduction of 1 with sodium amalgam produced light green crystals of [N-(n-Bu)(4)](3)[Cr(Cl(2)-bdt)(3)].thf (S = (3)/(2)) (3). The corresponding maleonitriledithiolato complexes [PPh(4)](2)[Cr(mnt)(3)] (S = 1) (4) and [PPh(4)](3)[Cr(mnt)(3)] (S = (3)/(2)) (5) have been synthesized. Isoelectronic manganese complexes of 3 and 5, namely, [NEt(4)](2)[Mn(Cl(2)-bdt)(3)] (S = (3)/(2)) (6) and [PPh(4)](2)[Mn(mnt)(3)] (S = (3)/(2)) (7), have also been prepared. Complexes 1, 6, and 7 have been characterized by single crystal X-ray crystallography. Complexes 1-7 have been electrochemically studied and their UV-vis and electron paramagnetic resonance spectra (EPR) have been recorded; magnetic properties have been elucidated by temperature-dependent susceptibility measurements. It is shown by chromium K-edge and sulfur K-edge X-ray absorption spectroscopy (XAS) that the oxidation state of the central Cr ion in each compound is the same (+III, d(3)) and that all one-electron redox processes are ligand-based, involving one, two, or three ligand pi radical monoanions. Complexes 6 and 7 possess a Mn(IV) ion with three dianionic ligands. The results have been corroborated by broken symmetry (BS) density functional theoretical (DFT) calculations by using the B3LYP functional. Time-dependent DFT calculations have been performed to calculate the metal and sulfur K-pre-edges. It is suggested that the neutral complexes [Cr(dithiolene)(3)](0) S = 0 possess octahedral rather than trigonal prismatic CrS(6) polyhedra. Three ligand pi radicals (S(rad) = (1)/(2)) couple antiferromagnetically to the central Cr(III) ion (d(3)) yielding the observed diamagnetic ground state. It is established that the four members of the [Cr(dithiolene)(3)](z) (z = 0, 1-, 2-, 3-) electron transfer series are related by ligand-based one-electron transfer processes; for each of the four members it is shown that they contain a central Cr(III) (d(3)) ion, and the CrS(6) polyhedron is a (distorted) octahedron.

An investigation of the electronic interplay between ligand radical(s) and a high-valent metal center in the three-member electron-transfer series shown in the picture reveals that, upon oxidation and removal of both ligand radicals, the chromium center becomes reduced from Cr{sup IV} to Cr{sup III} with concomitant formation of an imidyl radical (NR{center_dot}){sup -}.

The ligand L (2-) (H 2L = N, N'-dimethyl- N, N'-bis(3,5-di- t-butyl-2-hydroxybenzyl)-1,2-diaminoethane) has been employed for the synthesis of two mononuclear Fe (III) complexes, namely, [LFe(eta (2)-NO 3)] and [LFeCl]. L (2-) is comprised of four strongly electron-donating groups (two tert-amines and two phenolates) that increase the electron density at the coordinated ferric ions. This property should facilitate oxidation of the complexes, that is, stabilization of the oxidized species. The molecular structures in the solid state have been established by X-ray diffraction studies. [LFeCl] is five-coordinate in a square-pyramidal coordination environment with the ligand adopting a trans-conformation, while [LFe(eta (2)-NO 3)] is six-coordinate in a distorted octahedral environment with the ligand in a beta-cis conformation. The electronic structures have been studied using magnetization, EPR, Mossbauer (with and without applied field), UV-vis-NIR, and X-ray absorption spectroscopies, which demonstrate highly anisotropic covalency from the strong sigma- and pi-donating phenolates. This analysis is supported by DFT calculations on [LFeCl]. The variations of the well-understood spectroscopic data in the solid state to the spectroscopic data in solution have been used to obtain insight in the molecular structure of the two complexes in solution. While the molecular structures of the solid states are retained in solutions of nonpolar aprotic solvents, there is, however, one common molecular structure in all protic polar solvents. The analysis of the LMCT transitions and the rhombicity E/ D clearly establish that both compounds exhibit a beta-cis conformation in these protic polar solvents. These two open coordination sites, cis to each other, allow access for two potential ligands in close proximity. Electrochemical analysis establishes two reversible oxidation waves for [LFeCl] at +0.55 V and +0.93 V vs Fc (+)/Fc and one reversible oxidation wave at +0.59 V with an irreversible oxidation at +1.07 V vs Fc (+)/Fc for [LFe(eta (2)-NO 3)]. The one- and the two-electron oxidations of [LFeCl] by chronoamperometry have been followed spectroscopically. The increase of a strong band centered at 420 nm indicates the formulation of [LFeCl] (+) as a Fe (III) monophenoxyl radical complex and of [LFeCl] (2+) as a Fe (III) bisphenoxyl radical complex. These studies imply that the ligand L (2-) is capable of providing a flexible coordination geometry with two binding sites for substrates and the allocation of two oxidation equivalents on the ligand.

The Cl K-edge spectra of a series of metal tetrachloride complexes have been simulated using a simple TD-DFT based protocol. The influence of the chosen density functional, relativistics, basis set, and solvation has been systematically investigated. The results show that good agreement between the calculated spectra and the relative experimental energies and intensities can be achieved by using the BP86 functional with standard polarized triple-ζ basis sets (such as the TZVP basis of Ahlrichs’s and co-workers). Calculated energies and intensities are improved by using the more flexible CP(PPP) basis set on the metal atom. The inclusion of scalar-relativistic effects or solvation does not yield any significant improvement in the results. The subtleties arising from the incorporation of electric quadrupole and magnetic dipole intensity contributions into the calculations are discussed in some detail, and the contributions are shown to be negligible (∼1%) at the Cl K-edge.

From the reaction mixture of 3,6-di-tert-butylcatechol, H2[3,6Lcat], [CrCl3(thf)3], and NEt3 in CH3CN in the presence of air, the neutral complex [CrIII(3,6L•sq)3] (S = 0) (1) was isolated. Reduction of 1 with [Co(Cp)2] in CH2Cl2 yielded microcrystals of [Co(Cp)2][CrIII(3,6L•sq)2(3,6Lcat)] (S = 1/2) (2) where (3,6L•sq)1- is the π-radical monoanionic o-semiquinonate of the catecholate dianion (3,6Lcat)2-. Electrochemistry demonstrated that both species are members of the electron-transfer series [Cr(3,6LO,O)]z (z = 0, 1−, 2−, 3−). The corresponding tris(benzo-1,2-dithiolato)chromium complex [N(n-Bu)4][CrIII(3,5L•S,S)2(3,5LS,S)] (S = 1/2) (3) has also been isolated; (3,5LS,S)2- represents the closed-shell dianion 3,5-di-tert-butylbenzene-1,2-dithiolate(2−), and (3,5L•S,S)1- is its monoanionic π radical. Complex 3 is a member of the electron-transfer series [Cr(3,5LS,S)3]z (z = 0, 1−, 2−, 3−). It is shown by Cr K-edge and S K-edge X-ray absorption, UV−vis, and EPR spectroscopies, as well as X-ray crystallogr...

The reaction of 3 equiv of the ligand 2-mercapto-3,5-di-tert-butylaniline, H{sub 2}[L{sub N,S}], or 3,5-di-tert-butyl-1,2-benzenedithiol, H{sub 2}[L{sub S,S}], with 1 equiv of [MoO{sub 2}(acac){sub 2}] or WCl{sub 6} (acac = acetonylacetate(1-)) in methanol or CCl{sub 4} afforded the diamagnetic neutral complexes [MoV(L{sub N,S}){sub 2}(L{sup {center_dot}}{sub N,S})]{sup 0} (1), [MoV(L{sub S,S}){sub 2}(L{sup {center_dot}}{sub S,S})] (2), and [W{sup V}(L{sub S,S}){sub 2}(L{sup {center_dot}}{sub S,S})] (3), where (L{sup {center_dot}}{sub N,S}){sup -} and (L{sup {center_dot}}{sub S,S}){sup -} represent monoanionic {pi}-radical ligands (S{sub rad} = 1/2), which are the one-electron oxidized forms of the corresponding closed-shell dianions (L{sub N,S}){sup 2-} and (L{sub S,S}){sup 2-}. Complexes 1-3 are trigonal-prismatic members of the electron-transfer series [ML{sub 3}]{sup z} (z = 0, 1-, 2-). Reaction of 2 and 3 with [N(n-Bu){sub 4}](SH) in CH{sub 2}Cl{sub 2} under anaerobic conditions afforded paramagnetic crystalline [N(n-Bu){sub 4}][Mo{sup V}(L{sub S,S}){sub 3}] (4) and [N(n-Bu){sub 4}][W{sup V}(L{sub S,S}){sub 3}] (5). Complexes 1-5 have been characterized by X-ray crystallography. S K-edge X-ray absorption and infrared spectroscopy prove that a {pi}-radical ligand (L{sup {center_dot}}{sub S,S}){sup -} is present in neutral 2 and 3, whereas the monoanions [M{sup V}(L{sub S,S}){sub 3}]{sup -} contain only closed-shell dianionic ligands. These neutral species have previously been incorrectly described as [M{sup VI}(L){submore » 3}]{sup 0} complexes with a Mo{sup VI} or W{sup VI} (d{sup 0}) central metal ion; they are, in fact M{sup V} (d{sup 1}) (M = Mo, W) species: [Mo{sup V}(L{sub S,S}){sub 2}(L{sup {center_dot}}{sub S,S})] and [W{sup V}(L{sub S,S}){sub 2}(L{sup {center_dot}}{sub S,S})] with a diamagnetic ground state S{sub t} = 0, which is generated by intramolecular, antiferromagnetic coupling between the M{sup V} (d{sup 1}) central ion (S{sub M} = 1/2) and a ligand radical (L{sup {center_dot}}{sub S,S}){sup -} (S{sub rad} = 1/2).« less

The preparation and structural characterization of the neutral, square planar complexes [PtII(tbpy)(A)] (1), [PtII(tbpy)(B)] (2), and [PtII(PPh3)2(B)] (3) have been accomplished, where (tbpy) = 4,4‘-di-tert-butylpyridine, (A)2- = 3,6-bis(trimethylsilyl)-1,2-benzenedithiolate(2−), and (B)2- = 1,2-bis(4-tert-butylphenyl)ethylene-1,2-dithiolate(2−) and (A•)1- and (B•)1- represent the corresponding monoanionic radicals. Electrochemical and chemical one-electron oxidation of 1 and 2 in CH2Cl2 solution affords the monomeric monocations [PtII(tbpy)(A•)]+ (1a) and [PtII(tbpy)(B•)]+ (2a), both of which possess an S = 1/2 ground state. The corresponding spin doublet monocationic dimers [PtII2(tbpy)2(A)(A•)]+ (1b) and [PtII2(tbpy)2(B)(B•)]+ (2b) were electrochemically generated in solution (50% oxidation) and identified by X-band EPR spectroscopy. Complete one-electron oxidation of 1 and 2 yielded the diamagnetic dimers [PtII2(tbpy)2(A•)2]2+ (1c) and [PtII2(tbpy)2(B•)2]2+ (2c) which are in equilibrium with the corre...

A combination of Cu L-edge and S K-edge X-ray absorption data and density functional theory (DFT) calculations has been correlated with 33S electron paramagnetic resonance superhyperfine results to obtain the dipole integral (Is) for the S 1s--3p transition for the dithiolene ligand maleonitriledithiolate (MNT) in (TBA)2[Cu(MNT)2] (TBA= tetra-n-butylammonium). The results have been combined with the Is of sulfide derived from XPS studies to experimentally obtain a relation between the S 1s--4p transition energy (which reflects the charge on the S atom, QSmol) and the dipole integral over a large range of QSmol. The results show that, for high charges on S, Is can vary from the previously reported Is values, calculated using data over a limited range of QSmol. A combination of S K-edge and Cu K- and L-edge X-ray absorption data and DFT calculations has been used to investigate the one-electron oxidation of [Cu(MNT)2]2- and [Ni(MNT)2]2-. The conversion of [Cu(MNT)2]2- to [Cu(MNT)2]- results in a large change in the charge on the Cu atom in the molecule (QCumol) and is consistent with a metal-based oxidation. This is accompanied by extensive charge donation from the ligands to compensate the high charge on the Cu in [Cu(MNT)2]- based on the increased S K-edge and decreased Cu L-edge intensity, respectively. In contrast, the oxidation of [Ni(MNT)2]2- to [Ni(MNT)2]- results in a small change in QNimol, indicating a ligand-based oxidation consistent with oxidation of a molecular orbital, psiSOMO (singly occupied molecular orbital), with predominant ligand character.

The reactions of manganese(III) porphyrin complexes with terminal oxidants, such as m-chloroperbenzoic acid, iodosylarenes, and H(2)O(2), produced high-valent manganese(V)-oxo porphyrins in the presence of base in organic solvents at room temperature. The manganese(V)-oxo porphyrins have been characterized with various spectroscopic techniques, including UV-vis, EPR, 1H and 19F NMR, resonance Raman, and X-ray absorption spectroscopy. The combined spectroscopic results indicate that the manganese(V)-oxo porphyrins are diamagnetic low-spin (S = 0) species with a longer, weaker Mn-O bond than in previously reported Mn(V)-oxo complexes of non-porphyrin ligands. This is indicative of double-bond character between the manganese(V) ion and the oxygen atom and may be attributed to the presence of a trans axial ligand. The [(Porp)Mn(V)=O](+) species are stable in the presence of base at room temperature. The stability of the intermediates is dependent on base concentration. In the absence of base, (Porp)Mn(IV)=O is generated instead of the [(Porp)Mn(V)=O](+) species. The stability of the [(Porp)Mn(V)=O](+) species also depends on the electronic nature of the porphyrin ligands: [(Porp)Mn(V)=O](+) complexes bearing electron-deficient porphyrin ligands are more stable than those bearing electron-rich porphyrins. Reactivity studies of manganese(V)-oxo porphyrins revealed that the intermediates are capable of oxygenating PPh(3) and thioanisoles, but not olefins and alkanes at room temperature. These results indicate that the oxidizing power of [(Porp)Mn(V)=O](+) is low in the presence of base. However, when the [(Porp)Mn(V)=O](+) complexes were associated with iodosylbenzene in the presence of olefins and alkanes, high yields of oxygenated products were obtained in the catalytic olefin epoxidation and alkane hydroxylation reactions. Mechanistic aspects, such as oxygen exchange between [(Porp)Mn(V)=16O](+) and H(2)(18)O, are also discussed.

Sulfur K-edge X-ray absorption spectroscopy (XAS) was used to characterize the approximately 0.1% sulfur found both in native reticulated vitreous carbon (RVC) foam and in RVC oxidatively modified using 0.2 M KMnO4 in 2 M H2SO4. Sulfur valences and functional groups were assessed using K-edge XAS spectral curve-fitting and employing explicit sulfur compounds as models. For native RVC, these were episulfide (approximately 3%), thianthrene (approximately 9%), disulfide (approximately 10%), sulfenate ester (approximately 12%), benzothiophene (approximately 24%), N,N'-thiobisphthalimide (approximately 30%), alkyl sulfonate (approximately 1.2%), alkyl sulfate monoester (approximately 6%), and sulfate dianion (approximately 6%). Permanganate oxidation of RVC diminished sulfenic sulfur to approximately 9%, thianthrenic sulfur to approximately 7%, and sulfate dianion to approximately 1% but increased sulfate monoester to approximately 12%, and newly produced sulfone (approximately 2%) and sulfate diester (approximately 5%). A simple thermodynamic model was derived that allows proportionate functional group comparisons despite differing (approximately +/-15%) total sulfur contents between RVC batches. The limits of accuracy in the XAS curve-fitting analysis are discussed in terms of microenvironments and extended structures in RVC carbon that cannot be exactly modeled by small molecules. Sulfate esters cover approximately 0.15% of the RVC surface, increasing to approximately 0.51% following permanganate/sulfuric acid treatment. The detection of episulfide directly corroborates a proposed mechanism for the migration of elemental sulfur through carbon.

Ti-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) provides a means for generating Ti(III) complexes by homolysis of the Ti-O bond. It has been determined that bis-Cp-Ti-TEMPO complexes readily undergo homolytic cleavage while the mono-Cp-Ti-TEMPO complexes do not. Here Ti K- and Cl K-edge XAS are applied to directly determine the oxidation state of TiCl3TEMPO, TiCpCl2TEMPO, and TiCp2ClTEMPO, with reference to Ti(III) and Ti(IV) complexes of known oxidation state. The Ti K-edge data show that Ti(III) complexes exhibit a pre-edge feature approximately 1 eV lower than any of the Ti(IV) complexes; while the Cl K-edges show that Ti(III) complexes have a Cl K- pre-edge feature to approximately 1 eV higher energy than any of the Ti(IV) complexes. Taken together, the Ti and Cl K-edge data indicate that the Ti-TEMPO complexes are best described as Ti(IV)-TEMPO anions (rather than Ti(III)-nitroxyl radicals). In addition, the Cl K-edges indicate that replacement of Cl by Cp weakens the bonding with the remaining ligands, with the Cl 3p covalency decreasing from 25% to 21% to 17% on going from TiCl3TEMPO to TiCpCl2TEMPO to TiCp2ClTEMPO. DFT calculations also show that the electronic structures of the Ti-TEMPO complexes are modulated by the replacement of chloride by Cp. The effect of the Cp on the ancillary ligation is one factor that contributes to facile Ti-O bond homolysis in TiCp2ClTEMPO. However, the results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.

A variety of spectroscopic techniques have been applied to characterize and compare two model complexes with relevance to red and blue copper centers in metalloproteins, [Cu(SMeIm)(HB(3,5-iPr 2 pz) 3 )] ( 1 ) and [Cu(SCPh 3 )(HB(3,5-iPr 2 pz) 3 )] ( 2 ), which are five- and four-coordinate, respectively. The key spectral differences in low temperature absorption and magnetic circular dichroism (MCD) include an increase in the Sσ→Cu charge transfer (CT) band intensity at 370 nm and a decrease in the absorption intensity at ∼570 nm for the Sπ→Cu CT. The energies of the d→d transitions in 1 are increased relative to 2 reflecting a more tetragonal geometry and a stronger ligand field. S K-edge X-ray absorption spectroscopy (XAS) measurements demonstrate a less covalent thiolate Cu interaction in the HOMO of 1 (15% Sp) compared to 2 (52% Sp). XAS at the Cu L-edge indicates that the Cu d-character in the HOMO of 1 has increased relative to that of 2 . The electronic perturbation resulting from the increased coordination number has been evaluated. The thiolate rotates in the NNS plane resulting in increased σ overlap with the Cu. Additionally, the Cu–S bond length increases. The associated reduced covalency of the thiolate can contribute to the function of perturbed blue copper sites in proteins.

Spectroscopic methods covering many energy regions together provide complementary insight into metalloenzyme active sites. These methods probe geometric and electronic structure and define these contributions to reactivity. Two recent advances--determination of the polarizations of electronic transitions in solution using magnetic circular dichroism, electron paramagnetic resonance and quantum chemistry, and experimental estimation of covalency using metal L-edges and ligand K-edges--are particularly important.

A tris(pyrazolyl)hydroborate triphenylmethylthiolate Cu(II) model complex (1) that reproduces structural and spectroscopic features of active sites of blue Cu proteins is characterized using low-temperature absorption, magnetic circular dichroism (MCD), X-ray absorption (XAS), and resonance Raman (rR) spectroscopies combined with DFT calculations to define its electronic structure. The electronic structure of 1 is further related to the oxidized Cu site in plastocyanin. The key spectral differences relative to plastocyanin include an increase in the intensity of the S pπ → Cu CT band and a decrease in the absorption intensity at ∼450 nm. The energies of d → d transitions in 1 decrease relative to plastocyanin, which reflects the more tetrahedral geometry of 1. S K-edge XAS measurements demonstrate a more covalent thiolate interaction in the HOMO of 1 (52% S p) than in plastocyanin (38% S p). The effects of the high thiolate covalency on the absorption and Raman spectral features for 1 are evaluated. Addit...

To evaluate the importance of the axial ligand in blue Cu centers, the electronic structure of a three-coordinate model compound LCuSCPh3 (2, L = β-diketiminate ligand) is defined using low-temperature absorption, magnetic circular dichroism (MCD), X-ray absorption spectroscopy (XAS), and resonance Raman (rR) profiles coupled with density functional calculations. Using these excited-state spectroscopic methods the electronic structure of 2 is compared to that of a four-coordinate blue Cu model compound LCuSCPh3 (1, L = tris(pyrazolyl)hydroborate ligand) and the three-coordinate blue Cu center in fungal laccase. The spectral features of 2 are substantially altered from those of 1 and reflect a trans influence of the β-diketiminate ligand that involves a strong interaction with the Cu dx2-y2 orbital, concomitant with a decreased S pπ interaction with the Cu dx2-y2 orbital. The lack of an axial ligand coupled with the influence of this equatorial donor leads to many of the perturbed spectral features of 2 re...

Scanning transmission X-ray microscopy (STXM) has emerged as an important technique for chemical imaging and spectromicroscopy on the

The eighth Structural Molecular Biology Summer School was held at SSRL from September 8–11, 2009. The school focused on three synchrotron based techniques: macromolecular crystallography, small angle X-ray scattering, and X-ray absorption spectroscopy, and the application of these techniques to biological problems. This year's Summer School was attended by 24 students and was led by a team of 18 tutors (who are internationally recognized experts in their field). It consisted of two days of lectures, followed by a day and a half of rotating practical sessions and a final afternoon comprising a tour of the SSRL facility and a wrap-up question and answer session.

Copper Proteins with Type 1 Sites is a general review that focuses on single-domain proteins that bind ‘blue’ or ‘type 1’ copper. Larger, multidomain proteins that contain type 1 copper in addition to other types of copper centers are not discussed at any length in this article. Blue copper proteins are so named because in solution they are a brilliant blue/azure color when the copper ion is in its Cu(II) oxidation state. The bright blue color comes from a charge transfer (CT) between Cu(II) and the sulfur (thiolate) of a ligating cysteine residue. Because of this intense color, type 1 copper proteins were among the first to be isolated because they could be tracked easily during purification processes. This, together with their unique spectroscopic properties that have no parallel in small-molecule copper complexes, has made them the subject of intense study over the last 35 years. This article focuses on three main aspects of type 1 copper proteins: (1) their occurrence, distribution, and classification based on analyses of genomic and expressed sequence tag data; (2) their three-dimensional structures as determined by the well-established tools of single-crystal X-ray diffraction and nuclear magnetic resonance (NMR); and (3) their electronic, spectroscopic, and electron-transfer properties. Keywords: type 1 copper; blue copper; X-ray crystallography; entatic state; electron paramagnetic resonance; resonance Raman; EXAFS/XAS; electron transfer; cupredoxins; phytocyanins

A detailed spectroscopic and quantum chemical analysis is presented to elucidate the electronic structures of the octahedral complexes [Fe(Et(2)dtc)(3-n)(mnt)(n)](n-) (1-4, n=3, 2, 1, 0) and their one-electron oxidized analogues [Fe(Et(2)dtc)(3-n)(mnt)(n)]((n-1)-) (1(ox)-4(ox)); (mnt)(2-) represents maleonitriledithiolate(2-) and (Et(2)dtc)(1-) is the diethyldithiocarbamato(1-) ligand. By using X-ray crystallography, Mössbauer spectroscopy, and Fe and S K-edge X-ray absorption spectroscopy (XAS) it is convincingly shown that, in contrast to our previous studies on [Fe(cyclam)(mnt)](1+) (cyclam=1,4,8,11-tetraazacyclotetradecane), the oxidation of 1-4 is metal-centered yielding the genuine Fe(IV) complexes 1(ox)-4(ox). For the latter complexes, a spin ground state of S=1 has been established by magnetic susceptibility measurements, which indicates a low-spin d(4) configuration. DFT calculations at the B3LYP level support this electronic structure and exclude the presence of a ligand pi radical coordinated to an intermediate-spin ferric ion. Mössbauer parameters and XAS spectra have been calculated to calibrate our computational results against the experiment. Finally, a simple ligand-field approach is presented to correlate the structural features obtained from X-ray crystallography (100 K) with the spectroscopic data.

Most bacteria and fungi produce low-molecular-weight iron chelators called siderophores. Although different siderophore structures have been characterized, the iron-binding moieties often contain catecholate or hydroxamate groups. Siderophores function because of their extraordinarily high stability constants (K(STAB) = 10(30)-10(49)) and selectivity for Fe(III), yet the origin of these high stability constants has been difficult to quantify experimentally. Herein, we utilize Fe L-edge X-ray absorption spectroscopy to determine the differential orbital covalency (i.e., the differences in the mixing of the metal d-orbitals with ligand valence orbitals) of a series of siderophore model compounds. The results enable evaluation of the electronic structure contributions to their high stability constants in terms of sigma- and pi-donor covalent bonding, ionic bonding, and solvent effects. The results indicate substantial differences in the covalent contributions to stability constants of hydroxamate and catecholate complexes and show that increased sigma as well as pi bonding contributes to the high stability constants of catecholate complexes.

Sulfur K-edge X-ray absorption spectroscopy has been proven to be a powerful tool for investigating the electronic structures of sulfur-containing coordination complexes. The full information content of the spectra can be developed through a combination of experiment and time-dependent density functional theory (TD-DFT). In this work, the necessary calibration is carried out for a range of contemporary functionals (BP86, PBE, OLYP, OPBE, B3LYP, PBE0, TPSSh) in a scalar relativistic (0(th) order regular approximation, ZORA) DFT framework. It is shown that with recently developed segmented all-electron scalar relativistic (SARC) basis sets one obtains results that are as good as with large, uncontracted basis sets. The errors in the calibrated transition energies are on the order of 0.1 eV. The error in calibrated intensities is slightly larger, but the calculations are still in excellent agreement with experiment. The behavior of full TD-DFT linear response versus the Tamm-Dancoff approximation has been evaluated with the result that two methods are almost indistinguishable. The inclusion of relativistic effects barely changes the results for first row transition metal complexes, however, the contributions become visible for second-row transition metals and reach a maximum (of an approximately 10% change in the calibration parameters) for third row transition metal species. The protocol developed here is approximately 10 times more efficient than the previously employed protocol, which was based on large, uncontracted basis sets. The calibration strategy followed here may be readily extended to other edges.

The reaction of [Ru(III)(cyclam)Cl(2)]Cl with 2 equiv of sodium N,N-diethyldithiocarbamate in methanol afforded [Ru(II)(cyclam)(Et(2)dtc)](BPh(4)) (1(BPh(4))). The same reaction with only 1 equiv of toluene-3,4-dithiol and a base yielded [Ru(cyclam)(tdt)](PF(6)) (2(PF(6))) which was oxidized by 1 equiv of ferrocenium hexafluorophosphate generating [Ru(cyclam)(tdt)](PF(6))(2) (2(ox)(PF(6))(2)). The crystal structures of 1 and 2 have been determined by X-ray crystallography at 100 K. The electronic structures of diamagnetic 1(BPh(4)), paramagnetic 2(PF(6)) (S = (1)/(2)), and diamagnetic 2(ox)(PF(6))(2) have been studied by magnetochemistry, spectroelectrochemistry, electron paramagnetic resonance spectroscopy, and (1)H NMR spectroscopy. X-ray absorption spectroscopy on Ru K- and L-edges as well as S K-edges has been employed. Finally, the molecular and electronic structures of all three species have been calculated by using density functional theory (B3LYP). It is shown that 2(ox) comprises an electronic structure which is best described by three resonance structures {[Ru(II)(cyclam)(tdt(0))](2+)--[Ru(III)(cyclam)(tdt(*))](2+)--[Ru(IV)(cyclam)(tdt(2-))](2+)} with a closed-shell singlet ground state. This is in stark contrast to the isoelectronic iron species, namely, [Fe(III)(cyclam)(tdt(*))](2+) which is a singlet diradical with a low-spin ferric ion coupled intramolecularly antiferromagnetically to a ligand pi radical monoanion (tdt(*))(-).

Despite a wealth of studies exemplifying the utility of the 2–5 keV X-ray range in speciation and electronic structure elucidation, the exploitation of this energy regime for the study of photochemical processes has not been forthcoming. Herein, a new endstation set-up for in situ photochemical soft X-ray spectroscopy in the 2–5 keV energy region at the Stanford Synchrotron Radiation Lightsource is described for continuous photolysis under anaerobic conditions at both cryogenic and ambient temperatures. Representative examples of this approach are used to demonstrate the potential information content in several fields of study, including organometallic chemistry, biochemistry and materials chemistry.

An N for Ir: The synthesis and X-ray crystal structure of a late-transition-metal complex with a terminal nitrido ligand and its hydrogenation to the related amido complex are reported (see scheme).

9 pages; International audience; Reaction of 2,4-(2,20-dipyridylamino)-6-(2-pyridylhydrazino)-1,3,5-triazine, abbreviated dppht (5), with copper(II) chloride in methanol yields a mixed-valent CuII 4CuI compound (6), also involving the presence of a radical anion. The single-crystal structure determination for [Cu5(5M)(5MR)Cl8] (6) reveals that the original dppht ligand (5) has been monochlorinated at one of its pyridine rings and oxidized at its hydrazyl moiety, generating the ligand 2,4-(2,20-dipyridylamino)-6-(2-(5- chloropyridyl)azo)-1,3,5-triazine (5M). Moreover, the molecular structure of 6 indicates that one of the two coordinated ligands 5M is in a radical-anion state, symbolized as 5MR, characterized by a typical N-N bond length of about 1.33A° for a one-electron reduced azo group. The nature of this unique [CuII 4CuI(radical)] assembly is corroborated by X-ray absorption spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements. Density-functional theory calculations are consistent with the unprecedented structural features and support the spectroscopic data.

This Communication addresses a long-standing problem: the exact vibrational assignments of the low-energy modes of the Fe-N-O subunit in six-coordinate ferrous heme nitrosyl model complexes. This problem is addressed using nuclear resonance vibrational spectroscopy (NRVS) coupled to (15)N(18)O isotope labeling and detailed simulations of the obtained data. Two isotope-sensitive features are identified at 437 and 563 cm(-1). Normal coordinate analysis shows that the 437 cm(-1) mode corresponds to the Fe-NO stretch, whereas the 563 cm(-1) band is identified with the Fe-N-O bend. The relative NRVS intensities of these features determine the degree of vibrational mixing between the stretch and the bend. The implications of these results are discussed with respect to the trans effect of imidazole on the bound NO. In addition, a comparison to myoglobin-NO (Mb-NO) is made to determine the effect of the Mb active site pocket on the bound NO.

This paper investigates the interaction between five-coordinate ferric hemes with bound axial imidazole ligands and nitric oxide (NO). The corresponding model complex, [Fe(TPP)(MI)(NO)](BF4) (MI = 1-methylimidazole), is studied using vibrational spectroscopy coupled to normal coordinate analysis and density functional theory (DFT) calculations. In particular, nuclear resonance vibrational spectroscopy is used to identify the Fe-N(O) stretching vibration. The results reveal the usual Fe(II)-NO(+) ground state for this complex, which is characterized by strong Fe-NO and N-O bonds, with Fe-NO and N-O force constants of 3.92 and 15.18 mdyn/A, respectively. This is related to two strong pi back-bonds between Fe(II) and NO(+). The alternative ground state, low-spin Fe(III)-NO(radical) (S = 0), is then investigated. DFT calculations show that this state exists as a stable minimum at a surprisingly low energy of only approximately 1-3 kcal/mol above the Fe(II)-NO(+) ground state. In addition, the Fe(II)-NO(+) potential energy surface (PES) crosses the low-spin Fe(III)-NO(radical) energy surface at a very small elongation (only 0.05-0.1 A) of the Fe-NO bond from the equilibrium distance. This implies that ferric heme nitrosyls with the latter ground state might exist, particularly with axial thiolate (cysteinate) coordination as observed in P450-type enzymes. Importantly, the low-spin Fe(III)-NO(radical) state has very different properties than the Fe(II)-NO(+) state. Specifically, the Fe-NO and N-O bonds are distinctively weaker, showing Fe-NO and N-O force constants of only 2.26 and 13.72 mdyn/A, respectively. The PES calculations further reveal that the thermodynamic weakness of the Fe-NO bond in ferric heme nitrosyls is an intrinsic feature that relates to the properties of the high-spin Fe(III)-NO(radical) (S = 2) state that appears at low energy and is dissociative with respect to the Fe-NO bond. Altogether, release of NO from a six-coordinate ferric heme nitrosyl requires the system to pass through at least three different electronic states, a process that is remarkably complex and also unprecedented for transition-metal nitrosyls. These findings have implications not only for heme nitrosyls but also for group-8 transition-metal(III) nitrosyls in general.

Recent advances in synthetic chemistry have led to the discovery of “superoxidized” iron centers with valencies Fe(V) and Fe(VI) [K. Meyer et al., J. Am. Chem. Soc., 1999, 121, 4859–4876; J. F. Berry et al., Science, 2006, 312, 1937–1941; F. T. de Oliveira et al., Science, 2007, 315, 835–838.]. Furthermore, in recent years a number of high-valent Fe(IV) species have been found as reaction intermediates in metalloenzymes and have also been characterized in model systems [C. Krebs et al., Acc. Chem. Res., 2007, 40, 484–492; L. Que, Jr, Acc. Chem. Res., 2007, 40, 493–500.]. These species are almost invariably stabilized by a highly basic ligand Xn− which is either O2− or N3−. The differences in structure and bonding between oxo- and nitrido species as a function of oxidation state and their consequences on the observable spectroscopic properties have never been carefully assessed. Hence, fundamental differences between high-valent iron complexes having either FeO or FeN multiple bonds have been probed computationally in this work in a series of hypothetical trans-[FeO(NH3)4OH]+/2+/3+ (1–3) and trans-[FeN(NH3)4OH]0/+/2+ (4–6) complexes. All computational properties are permeated by the intrinsically more covalent character of the FeN multiple bond as compared to the FeO bond. This difference is likely due to differences in Z* between N and O that allow for better orbital overlap to occur in the case of the FeN multiple bond. Spin-state energetics were addressed using elaborate multireference ab initio computations that show that all species 1–6 have an intrinsic preference for the low-spin state, except in the case of 1 in which S = 1 and S = 2 states are very close in energy. In addition to Mossbauer parameters, g-tensors, zero-field splitting and iron hyperfine couplings, X-ray absorption Fe K pre-edge spectra have been simulated using time-dependent DFT methods for the first time for a series of compounds spanning the high-valent states +4, +5, and +6 for iron. A remarkably good correlation of these simulated pre-edge features with experimental data on isolated high-valent intermediates has been found, allowing us to assign the main pre-edge features to excitations into the empty Fedz2 orbital, which is able to mix with Fe 4pz, allowing an efficient mechanism for the intensification of pre-edge features.

Recent advances in synthetic chemistry have led to the discovery of "superoxidized" iron centers with valencies Fe(v) and Fe(vi) [K. Meyer et al., J. Am. Chem. Soc., 1999, 121, 4859-4876; J. F. Berry et al., Science, 2006, 312, 1937-1941; F. T. de Oliveira et al., Science, 2007, 315, 835-838.]. Furthermore, in recent years a number of high-valent Fe(iv) species have been found as reaction intermediates in metalloenzymes and have also been characterized in model systems [C. Krebs et al., Acc. Chem. Res., 2007, 40, 484-492; L. Que, Jr, Acc. Chem. Res., 2007, 40, 493-500.]. These species are almost invariably stabilized by a highly basic ligand X(n-) which is either O(2-) or N(3-). The differences in structure and bonding between oxo- and nitrido species as a function of oxidation state and their consequences on the observable spectroscopic properties have never been carefully assessed. Hence, fundamental differences between high-valent iron complexes having either Fe=O or Fe=N multiple bonds have been probed computationally in this work in a series of hypothetical trans-[FeO(NH(3))(4)OH](+/2+/3+) (1-3) and trans-[FeN(NH(3))(4)OH](0/+/2+) (4-6) complexes. All computational properties are permeated by the intrinsically more covalent character of the Fe=N multiple bond as compared to the Fe=O bond. This difference is likely due to differences in Z* between N and O that allow for better orbital overlap to occur in the case of the Fe=N multiple bond. Spin-state energetics were addressed using elaborate multireference ab initio computations that show that all species 1-6 have an intrinsic preference for the low-spin state, except in the case of 1 in which S=1 and S=2 states are very close in energy. In addition to Mössbauer parameters, g-tensors, zero-field splitting and iron hyperfine couplings, X-ray absorption Fe K pre-edge spectra have been simulated using time-dependent DFT methods for the first time for a series of compounds spanning the high-valent states +4, +5, and +6 for iron. A remarkably good correlation of these simulated pre-edge features with experimental data on isolated high-valent intermediates has been found, allowing us to assign the main pre-edge features to excitations into the empty Fe d(z(2)) orbital, which is able to mix with Fe 4p(z), allowing an efficient mechanism for the intensification of pre-edge features.

From the reaction mixture of 3,6-di-tert-butylcatechol, H2[3,6L(cat)], [CrCl3(thf)3], and NEt3 in CH3CN in the presence of air, the neutral complex [CrIII(3,6L*(sq))3] (S = 0) (1) was isolated. Reduction of 1 with [Co(Cp)2] in CH2Cl2 yielded microcrystals of [Co(Cp)2][CrIII(3,6L*(sq))2(3,6L(cat))] (S = 1/2) (2) where (3,6L*(sq)(1-) is the pi-radical monoanionic o-semiquinonate of the catecholate dianion (3,6Lcat)(2-). Electrochemistry demonstrated that both species are members of the electron-transfer series [Cr(3,6LO,O)]z (z = 0, 1-, 2-, 3-). The corresponding tris(benzo-1,2-dithiolato)chromium complex [N(n-Bu)4][CrIII(3,5L*S,S)2(3,5LS,S)] (S = 1/2) (3) has also been isolated; (3,5LS,S)(2-) represents the closed-shell dianion 3,5-di-tert-butylbenzene-1,2-dithiolate(2-), and (3,5L*S,S)(1-) is its monoanionic pi radical. Complex 3 is a member of the electron-transfer series [Cr(3,5L(S,S))3]z (z = 0, 1-, 2-, 3-). It is shown by Cr K-edge and S K-edge X-ray absorption, UV-vis, and EPR spectroscopies, as well as X-ray crystallography, of 1 and 3 that the oxidation state of the central Cr ion in each member of both electron-transfer series remains the same (+III) and that all redox processes are ligand-based. These experimental results have been corroborated by broken symmetry density functional theoretical calculations by using the B3LYP functional.

The characterization of high-valent iron species is of interest due to their relevance to biological reaction mechanisms. Recently, we have synthesized and characterized an [Fe(V)-nitrido-cyclam-acetato]+ complex, which has been characterized by Mossbauer, magnetic susceptibility data, and XAS spectroscopies combined with DFT calculations (Aliaga-Alcade, N.; DeBeer George, S.; Bill, E.; Wieghardt, K.; Neese, F. Angew. Chem., Int. Ed. 2005, 44, 2908-2912). The results of this study indicated that the [Fe(V)-nitrido-cyclam-acetato]+ complex is an unusual d3 system with a nearly orbitally degenerate S=1/2 ground state. Although the calculations predicted fairly different Fe-N stretching frequencies for the S=1/2 and the competing S=3/2 ground states, a direct experimental determination of this important fingerprint quantity was missing. Here we apply synchrotron-based nuclear resonance vibrational scattering (NRVS) to characterize the Fe-N stretching frequency of an Fe(V)-nitrido complex and its Fe(III)-azide precursor. The NRVS data show a new isolated band at 864 cm(-1) in the Fe(V)-nitrido complex that is absent in the precursor. The NRVS spectra are fit and simulated using a DFT approach, and the new feature is unambiguously assigned to a Fe(V)-N stretch. The calculated Fe-N stretching frequency is too high by approximately 75 cm(-1). Anharmonic contributions to the Fe-N stretching frequency have been evaluated and have been found to be small (-5.5 cm(-1)). The NRVS data provided a unique opportunity to obtain this vibrational information, which had eluded characterization by more traditional vibrational spectroscopies.

High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3d(z(2))-4p(z) mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3d(xz,yz) transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.

The preparation and structural characterization of the neutral, square planar complexes [PtII(tbpy)(A)] (1), [PtII(tbpy)(B)] (2), and [PtII(PPh3)2(B)] (3) have been accomplished, where (tbpy) = 4,4'-di-tert-butylpyridine, (A)2- = 3,6-bis(trimethylsilyl)-1,2-benzenedithiolate(2-), and (B)2- = 1,2-bis(4-tert-butylphenyl)ethylene-1,2-dithiolate(2-) and (A*)1- and (B*)1- represent the corresponding monoanionic radicals. Electrochemical and chemical one-electron oxidation of 1 and 2 in CH2Cl2 solution affords the monomeric monocations [PtII(tbpy)(A*)]+ (1a) and [PtII(tbpy)(B*)]+ (2a), both of which possess an S = 1/2 ground state. The corresponding spin doublet monocationic dimers [PtII2(tbpy)2(A)(A*)]+ (1b) and [PtII2(tbpy)2(B)(B*)]+ (2b) were electrochemically generated in solution (50% oxidation) and identified by X-band EPR spectroscopy. Complete one-electron oxidation of 1 and 2 yielded the diamagnetic dimers [PtII2(tbpy)2(A*)2]2+ (1c) and [PtII2(tbpy)2(B*)2]2+ (2c) which are in equilibrium with the corresponding paramagnetic monomers 1a and 2a in solution. The crystal structure of [PtII2(tbpy)2(B*)2](PF6)2.3CH2Cl2 (2c) revealed a centrosymmetric, lateral dimer whose bridging part is a PtII2(mu2-S)2 rhomb; the metal ions possess a square based pyramidal geometry. Solid-state sulfur K-edge X-ray absorption spectra of 1, 2, 2a, 2c, and [PtII(B*)2]0 (4) have been recorded, which clearly show that a sulfur-centered radical (B*)1- is present in 2a, 2c, and 4. The absence of ligand-based radicals in 1 and 2 is also clearly established. One-electron oxidation of [Pt(PPh3)2(B)] (3) afforded only the spin doublet species [PtII(PPh3)2(B*)]+ (3a); no dimer formation was detected. Synthesis and crystal structure of square planar [PtII(B*)2]0.thf are also reported.

A combination of spectroscopy and density functional theory (DFT) calculations has been used to evaluate the pH effect at the CuZ site in Pseudomonas nautica (Pn) nitrous oxide reductase (N2OR) and Achromobacter cycloclastes (Ac) N2OR and its relevance to catalysis. Absorption, magnetic circular dichroism, and electron paramagnetic resonance with sulfur K-edge X-ray absorption spectra of the enzymes at high and low pH show minor changes. However, resonance Raman (rR) spectroscopy of PnN2OR at high pH shows that the 415 cm-1 Cu-S vibration (observed at low pH) shifts to higher frequency, loses intensity, and obtains a 9 cm-1 18O shift, implying significant Cu-O character, demonstrating the presence of a OH- ligand at the CuICuIV edge. From DFT calculations, protonation of either the OH- to H2O or the mu4-S2- to mu4-SH- would produce large spectral changes which are not observed. Alternatively, DFT calculations including a lysine residue at an H-bonding distance from the CuICuIV edge ligand show that the position of the OH- ligand depends on the protonation state of the lysine. This would change the coupling of the Cu-(OH) stretch with the Cu-S stretch, as observed in the rR spectrum. Thus, the observed pH effect (pKa approximately 9.2) likely reflects protonation equilibrium of the lysine residue, which would both raise E degrees and provide a proton for lowering the barrier for the N-O cleavage and for reduction of the [Cu4S(im)7OH]2+ to the fully reduced 4CuI active form for turnover.

The electronic structures of [M(L(Bu))(2)](-) (L(Bu)=3,5-di-tert-butyl-1,2-benzenedithiol; M=Ni, Pd, Pt, Cu, Co, Au) complexes and their electrochemically generated oxidized and reduced forms have been investigated by using sulfur K-edge as well as metal K- and L-edge X-ray absorption spectroscopy. The electronic structure content of the sulfur K-edge spectra was determined through detailed comparison of experimental and theoretically calculated spectra. The calculations were based on a new simplified scheme based on quasi-relativistic time-dependent density functional theory (TD-DFT) and proved to be successful in the interpretation of the experimental data. It is shown that dithiolene ligands act as noninnocent ligands that are readily oxidized to the dithiosemiquinonate(-) forms. The extent of electron transfer strongly depends on the effective nuclear charge of the central metal, which in turn is influenced by its formal oxidation state, its position in the periodic table, and scalar relativistic effects for the heavier metals. Thus, the complexes [M(L(Bu))(2)](-) (M=Ni, Pd, Pt) and [Au(L(Bu))(2)] are best described as delocalized class III mixed-valence ligand radicals bound to low-spin d(8) central metal ions while [M(L(Bu))(2)](-) (M=Cu, Au) and [M(L(Bu))(2)](2-) (M=Ni, Pd, Pt) contain completely reduced dithiolato(2-) ligands. The case of [Co(L(Bu))(2)](-) remains ambiguous. On the methodological side, the calculation led to the new result that the transition dipole moment integral is noticeably different for S(1s)-->valence-pi versus S(1s)-->valence-sigma transitions, which is explained on the basis of the differences in radial distortion that accompany chemical bond formation. This is of importance in determining experimental covalencies for complexes with highly covalent metal-sulfur bonds from ligand K-edge absorption spectroscopy.

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO 4 2– . We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. Mössbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an Fe N triple bond of 1.57 angstroms and a singlet \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{d}_{xy}^{2}\) \end{document} ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.

Two oxo-bis(benzene-1,2-dithiolato)chromate(V) complexes, namely, [CrO(L(Bu))2]1- and [CrO(L(Me))2]1-, have been synthesized and studied by UV-vis, EPR, magnetic circular dichroism (MCD), and X-ray absorption spectroscopy and by X-ray crystallography; their electro- and magnetochemistries are reported. H2L(Bu) represents the pro-ligand 3,5-di-tert-butylbenzene-1,2-dithiol, and H2L(Me) is the corresponding 4-methyl-benzene-1,2-dithiol. A structural feature of interest for both the complexes is the folding of the dithiolate ligands about the S-S vector providing Cs symmetry to the complexes. Geometry optimizations using all-electron density functional theory with scalar relativistic corrections at the second-order Douglas-Kroll-Hess (DKH2) and zeroth-order regular approximation (ZORA) levels result in excellent agreement with the experimentally determined structures and electronic and S K-edge X-ray absorption spectra. From DFT calculations, the Cs instead of C2v symmetry for the complexes is attributed to the strong S(3p) --Cr(3d(x2-y2)) pi-donation in Cs geometry providing additional stability to the complexes.