Sebastian A. Stoian

EPR Evidence for Co(IV) Species Produced During Water Oxidation at Neutral pH

Thin-film water oxidation catalysts (Co-Pi) prepared by electrodeposition from phosphate electrolyte and Co(NO(3))(2) have been characterized by electron paramagnetic resonance (EPR) spectroscopy. Co-Pi catalyst films exhibit EPR signals corresponding to populations of both Co(II) and Co(IV). As the deposition voltage is increased into the region where water oxidation prevails, the population of Co(IV) rises and the population of Co(II) decreases. The changes in the redox speciation of the film can also be induced, in part, by prolonged water oxidation catalysis in the absence of additional catalyst deposition. These results provide spectroscopic evidence for the formation of Co(IV) species during water oxidation catalysis at neutral pH.

Hangman Corroles: Efficient Synthesis and Oxygen Reaction Chemistry

The construction of a new class of compounds--the hangman corroles--is provided efficiently by the modification of macrocyclic forming reactions from bilanes. Hangman cobalt corroles are furnished in good yields from a one-pot condensation of dipyrromethane with the aldehyde of a xanthene spacer followed by metal insertion using microwave irradiation. In high oxidation states, X-band EPR spectra and DFT calculations of cobalt corrole axially ligated by chloride are consistent with the description of a Co(III) center residing in the one-electron oxidized corrole macrocycle. These high oxidation states are likely accessed in the activation of O-O bonds. Along these lines, we show that the proton-donating group of the hangman platform works in concert with the redox properties of the corrole to enhance the catalytic activity of O-O bond activation. The hangman corroles show enhanced activity for the selective reduction of oxygen to water as compared to their unmodified counterparts. The oxygen adduct, prior to oxygen reduction, is characterized by EPR and absorption spectroscopy.

Ligand Reactivity in Diarylamido/Bis(Phosphine) PNP Complexes of Mn(CO)3 and Re(CO)3

The syntheses of meridionally ligated tricarbonyl complexes (PNP)Mn(CO)(3) and (PNP)Re(CO)(3) are described (PNP = [2-P(CHMe(2))(2)-4-MeC(6)H(3)](2)N(-)). Cyclic voltammograms show reversible one-electron redox couples for both parent compounds (-0.34 V vs Cp(2)Fe(0/+) for (PNP)Mn(CO)(3), -0.25 V vs Cp(2)Fe(0/+) for (PNP)Re(CO)(3)), and chemical oxidation with AgOTf results in formation of the corresponding paramagnetic triflate salts [(PNP)Mn(CO)(3)]OTf and [(PNP)Re(CO)(3)]OTf. Electron paramagnetic resonance spectra and computational results indicate that this event is primarily ligand centered; allylation of the organic ligand moiety of [(PNP)Mn(CO)(3)]OTf with allyltributylstannane is consistent with this assignment. The oxidation (PNP)Mn(CO)(3) to [(PNP)Mn(CO)(3)]OTf results in a shift in average CO stretching frequency of 30 cm(-1); protonation of (PNP)Mn(CO)(3) with TfOH to form [(PNHP)Mn(CO)(3)]OTf results in a similar magnitude shift.

Mössbauer, Electron Paramagnetic Resonance, and Magnetic Susceptibility Studies on Members of a New Family of Cyano-Bridged 3d-4f Complexes. Demonstration of Anisotropic Exchange in a Fe―Gd Complex

The synthesis and crystallographic characterization of a new family of M(mu-CN)Ln complexes are reported. Two structural series have been prepared by reacting in water rare earth nitrates (Ln(III) = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho) with K(3)[M(CN)(6)] (M(III) = Fe, Co) in the presence of hexamethylenetetramine (hmt). The first series consists of six isomorphous heterobinuclear complexes, [(CN)(5)M-CN-Ln(H(2)O)(8)].2hmt ([FeLa] 1, [FePr] 2, [FeNd] 3, [FeSm] 4, [FeEu] 5, [FeGd] 6), while the second series consists of four isostructural ionic complexes, [M(CN)(6)][Ln(H(2)O)(8)].hmt ([FeDy] 7, [FeHo] 8, [CoEu] 9, [CoGd] 10). The hexamethylenetetramine molecules contribute to the stabilization of the crystals by participating in an extended network of hydrogen bond interactions. In both series the aqua ligands are hydrogen bonded to the nitrogen atoms from both the terminal CN(-) groups and the hmt molecules. The [FeGd] complex has been analyzed with (57)Fe Mossbauer spectroscopy and magnetic susceptibility measurements. We have also analyzed the [FeLa] complex, in which the paramagnetic Gd(III) is replaced by diamagnetic La(III), with (57)Fe Mossbauer spectroscopy, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements, to obtain information about the low-spin Fe(III) site that is not accessible in the presence of a paramagnetic ion at the complementary site. For the same reason, the [CoGd] complex, containing diamagnetic Co(III), was studied with EPR and magnetic susceptibility measurements, which confirmed the S = 7/2 spin of Gd(III). Prior knowledge about the paramagnetic sites in [FeGd] allows a detailed analysis of the exchange interactions between them. In particular, the question of whether the exchange interaction in [FeGd] is isotropic or anisotropic has been addressed. Standard variable-temperature magnetic susceptibility measurements provide only the value for a linear combination of J(x), J(y), and J(z) but contain no information about the values of the individual exchange parameters J(x), J(y), and J(z). In contrast, the spin-Hamiltonian analysis of the variable-field, variable-temperature Mossbauer spectra reveals an exquisite sensitivity on the anisotropic exchange parameters. Analysis of these dependencies in conjunction with adopting the g-values obtained for [FeLa], yielded the values J(x) = +0.11 cm(-1), J(y) = +0.33 cm(-1), and J(z) = +1.20 cm(-1) (S(1).J.S(2) convention). The consistency of these results with magnetic susceptibility data is analyzed. The exchange anisotropy is rooted in the spatial anisotropy of the low-spin Fe(III) ion. The condition for anisotropic exchange is the presence of low-lying orbital excited states at the ferric site that (i) effectively interact through spin-orbit coupling with the orbital ground state and (ii) have an exchange parameter with the Gd site with a value different from that for the ground state. Density functional theory (DFT) calculations, without spin-orbit coupling, reveal that the unpaired electron of the t(2g)(5) ground configuration of the Fe(III) ion occupies the xy orbital, that is, the orbital along the plane perpendicular to the Fe...Gd vector. The exchange-coupling constants for this orbital, j(xy), and for the other t(2g) orbitals, j(yz) and j(xz), have been determined using a theoretical model that relates them to the anisotropic exchange parameters and the g-values of Fe(III). The resulting values, j(yz) = -5.7 cm(-1), j(xz) = -4.9 cm(-1), and j(xy) = +0.3 cm(-1) are quite different. The origin of the difference is briefly discussed.

Xanthene-modified and hangman iron corroles.

Iron corroles modified with a xanthene scaffold are delivered from easily available starting materials in abbreviated reaction times. These new iron corroles have been spectroscopically examined with particular emphasis on defining the oxidation state of the metal center. Investigation of their electronic structure using (57)Fe Mössbauer spectroscopy in conjunction with density functional theory (DFT) calculations reveals the non-innocence of the corrole ligand. Although these iron corroles contain a formal Fe(IV) center, the deprotonated corrole macrocycle ligand is one electron oxidized. The electronic ground state of these complexes is best described as an intermediate spin S = 3/2 Fe(III) site strongly antiferromagnetically coupled to the S = 1/2 of the monoradical dianion corrole [Fe(III)Cl-corrole(+•)]. We show here that iron corroles as well as xanthene-modified and hangman xanthene iron corroles are redox active and catalyze the disproportionation of hydrogen peroxide via the catalase reaction, and that this activity scales with the oxidation potential. The meso position of corrole macrocycle is susceptible toward nucleophilic attack during catalase turnover. The reactivity of peroxide within the hangman cleft reported here adds to the emerging theme that corroles are good at catalyzing two-electron activation of the oxygen-oxygen bond in a variety of substrates.

NO Disproportionation at a Mononuclear Site-Isolated Fe2+ Center in Fe2+-MOF-5

The weak-field ligand environments at the metal nodes of metal-organic frameworks (MOFs) mimic the electronic environment of metalloenzyme active sites, but little is known about the reactivity of MOF nodes toward small molecules of biological relevance. Here, we report that the ferrous ions in Fe(2+)-exchanged MOF-5 disproportionate nitric oxide to produce nitrous oxide and a ferric nitrito complex. Although mechanistic studies of N-N bond forming transformations often invoke a hyponitrite species, as in nitric oxide reductase and NOx reduction catalysis, little is known about this intermediate in its monoanionic state. Together with the first report of N-N coupling between NO molecules in a MOF, we present evidence for a species that is consistent with a ferric hyponitrite radical, whose isolation is enabled by the spatial constraints of the MOF matrix.

A High-Spin Organometallic Fe−S Compound: Structural and Mössbauer Spectroscopic Studies of [Phenyltris((tert-butylthio)methyl)borate]Fe(Me)

The synthesis and structure of the pseudotetrahedral, sulfur-rich, high-spin organoiron(II) [phenyltris((tert-butylthio)methyl)borate]Fe(Me), [PhTt(tBu)]Fe(Me), 1, are reported. Low-temperature Mössbauer spectroscopic studies reveal an isomer shift of delta = 0.60(3) mm/s and DeltaE(Q) = 0.00(1) mm/s and an S = 2 ground multiplet with a negative zero-field splitting, D = -33(3) cm(-1), E/D approximately = 0.01. The small separation of the ground doublet, Delta approximately = 0.01 cm(-1), allows for observation of X-band EPR signals at g(eff) approximately = 10 (g(z) = 2.6, g(x,y) = 2.00). The relatively large negative zero-field splitting and a highly anisotropic magnetic hyperfine tensor, containing a large orbital z component, {-10(4), -10(4), +33.8(2) MHz}, are concordant with the presence of unquenched orbital angular momentum. Density functional theory (DFT) calculations predict that the lowest-lying orbitals have predominantly d(xy)- and d(x(2)-y(2))-like character, separated by an energy gap small enough to allow mixing through spin-orbit coupling, to generate a negative zero-field splitting, consistent with the experimental observations. The experimental and DFT-calculated isomer shifts are in good agreement (delta(calcd) = 0.5 mm/s). The unusual (for a high-spin ferrous site) null electric field gradients can be qualitatively explained in the frame of the spin-orbit coupling mixing. The very small Fermi contact component of the magnetic hyperfine tensor (A(FC)(exp) = -9 MHz) is not well described by the DFT approach (A(FC)(calcd) = +2 MHz). To our knowledge, this is the first study of a sulfur-coordinated high-spin organoiron(II) complex.

Spectroscopic and theoretical investigation of a complex with an [O═Fe(IV)-O-Fe(IV)═O] core related to methane monooxygenase intermediate Q.

Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=Fe(IV)-O-Fe(IV)-OH core that has two ferromagnetically coupled Sloc = 1 sites. Addition of base to 2 at -85 °C elicits its conjugate base 6 with a novel O═Fe(IV)-O-Fe(IV)═O core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (Sloc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral O═Fe-Fe═O angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled Sloc = 1 and Sloc = 2 sites to give total spin St = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O═Fe-Fe═O angle of 6b is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear Fe(IV)═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.

Synthesis and Characterization of a Stable High-Valent Cobalt Carbene Complex

The formally Co(IV) carbene Co(OR)2(═CPh2) is formed upon the reaction of diphenyldiazomethane with the cobalt bis(alkoxide) precursor Co(OR)2(THF)2. Structural, spectroscopic, and theoretical studies demonstrate that Co(OR)2(═CPh2) has significant high-valent Co(IV)═CPh2 character with non-negligible spin density on the carbene moiety.

Dinuclear Metallacycles with Single M–X–M Bridges (X = Cl–, Br–; M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)): Strong Antiferromagnetic Superexchange Interactions

A series of monochloride-bridged, dinuclear metallacycles of the general formula [M2(μ-Cl)(μ-L)2](ClO4)3 have been prepared using the third-generation, ditopic bis(pyrazolyl)methane ligands L = m-bis[bis(1-pyrazolyl)methyl]benzene (Lm), M = Cu(II), Zn(II), and L = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*), M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II). These complexes were synthesized from the direct reactions of M(ClO4)2·6H2O, MCl2, and the ligand, Lm or Lm*, in the appropriate stoichiometric amounts. Three analogous complexes of the formula [M2(μ-Cl)(μ-L)2](BF4)3, L = Lm, M = Cu(II), and L = Lm*, M = Co(II), Cu(II), were prepared from the reaction of [M2(μ-F)(μ-L)2](BF4)3 and (CH3)3SiCl. The bromide-bridged complex [Cu2(μ-Br)(μ-Lm*)2](ClO4)3 was prepared by the first method. Three acyclic complexes, [Co2(μ-Lm)μ-Cl4], [Co2(μ-Lm*)Cl4], and [Co2(μ-Lm*)Br4], were also prepared. The structures of all [M2(μ-X)(μ-L)2]3+ (X = Cl-, Br-) complexes have two ditopic bis(pyrazolyl)methane ligands bridging two metals in a metallacyclic arrangement. The fifth coordination site of the distorted trigonal bipyramidal metal centers is filled by a bridging halide ligand that has an unusual linear or nearly linear M-X-M angle. The NMR spectra of [Zn2(μ-Cl)(μ-Lm*)2](ClO4)3 and especially [Cd2(μ-Cl)(μ-Lm*)2](ClO4)3 demonstrate that the metallacycle structure is maintained in solution. Solid state magnetic susceptibility data for the copper(II) compounds show very strong antiferromagnetic exchange interactions, with -J values of 536 cm-1 for [Cu2(μ-Cl)(μ-Lm)2](ClO4)3·xCH3CN, 720 cm-1 for [Cu2(μ-Cl)(μ-Lm*)2](ClO4)3, and 945 cm-1 for [Cu2(μ-Br)(μ-Lm*)2](ClO4)3·2CH3CN. Smaller but still substantial antiferromagnetic interactions are observed with other first row transition metals, with -J values of 98 cm-1 for [Ni2(μ-Cl)(μ-Lm*)2](ClO4)3, 55 cm-1 for [Co2(μ-Cl)(μ-Lm*)2](ClO4)3, and 34 cm-1 for [Fe2(μ-Cl)(μ-Lm*)2](ClO4)3. EPR spectra of [Cu2(μ-Cl)(μ-Lm*)2](BF4)3 confirm the dz2 ground state of copper(II). In addition, the sign of the zero-field splitting parameter D was determined to be positive for [Cu2(μ-F)(μ-Lm*)2](BF4)3. Electronic spectra of the copper(II) complexes as well as Mössbauer spectra of the iron(II) complexes were also studied in relation with the EPR spectra and magnetic properties, respectively. Density functional theory calculations were performed using ORCA, and exchange integral values were obtained that parallel but are slightly higher than the experimental values by about 30%.