High-spin Ground State Research Articles

Magnetic and electronic properties of Sn1–xCrxO2 diluted alloys

a b s t r a c t We present the results of first-principles electronic structure calculations of Sn0.96Cr0.04O2 alloys, simu- lating chromium in rutile tin dioxide as an impurity in a diluted magnetic semiconductor configuration. A magnetic bistability has been observed, with the occurrence of a low-spin state with magnetic moment m = 0� B in addition to the high-spin ground state with m = 2� B. When an oxygen vacancy is included, as one of the nearest-neighbors to Cr impurity, a remarkable change is observed in the magnetic metastabil- ity. The energy barrier for the crossover from the low-spin to high-spin is lowered by 75%. These findings suggest that these materials may be used in applications which require different magnetized states. © 2011 Elsevier B.V. All rights reserved.

Rational design of single-molecule magnets: a supramolecular approach

Since the discovery that Mn(12)OAc acts as a single-molecule magnet (SMM), an increasing number of transition metal complexes have been demonstrated to behave as SMMs. The signature of a SMM is a slow relaxation of the magnetization at low temperatures accompanied by a magnetic hysteresis. The origin of SMM behaviour is the existence of an appreciable thermal barrier U for spin-reversal called magnetic anisotropy barrier which is related to the combination of a large total spin ground state (S(t)) and an easy-axis magnetic anisotropy. The extensive research on Mn(12)OAc and other SMMs has established more prerequisites for a rational development of new SMMs besides the high-spin ground state and the magnetic anisotropy: the symmetry should be at least C(3) to minimize the quantum tunneling of the magnetization through the anisotropy barrier but lower than cubic to avoid the cancellation of the local anisotropies upon projection onto the spin ground state. Based on these prerequisites, we have designed the ligand triplesalen which combines the phloroglucinol bridging unit for high spin ground states by the spin-polarization mechanism with a salen-like ligand environment for single-site magnetic anisotropies by a strong tetragonal ligand field. The C(3) symmetric, trinuclear complexes of the triplesalen ligand (talen(t-Bu(2)))(6-) exhibit a strong ligand folding resulting in an overall bowl-shaped molecular structure. This ligand folding preorganizes the axial coordination sites of the metal salen subunits for the complementary binding of three facial nitrogen atoms of a hexacyanometallate unit. This leads to a high driving force for the formation of heptanuclear complexes [M(t)(6)M(c)](n+) by the assembly of three molecular building blocks. Attractive van der Waals interactions of the tert-butyl phenyl units of two triplesalen trinuclear building blocks increase the driving force. In this respect, we have been able to synthesize the isostructural series [Mn(III)(6)Cr(III)](3+), [Mn(III)(6)Fe(III)](3+), and [Mn(III)(6)Co(III)](3+) with [Mn(III)(6)Cr(III)](3+) being a SMM. A detailed analysis and comparison of the magnetic properties of the three heptanuclear complexes and the tetranuclear half-unit [Mn(III)(3)Cr(III)](3+) provides significant insight for further optimization of the SMM properties. The modular assembly of the heptanuclear complexes from three molecular building blocks allows the fine-tuning of the molecular and steric properties of each building block without losing the driving force for the formation of the heptanuclear complexes. This possibility of rational improvements of our isostructural series is the main advantage of our supramolecular approach.

Halide Ligated Iron Porphines: A DFT+U and UB3LYP Study

We apply the density functional theory + U (DFT+U) and unrestricted hybrid functional DFT-UB3LYP methods to study the electronic structure and magnetic properties of two prototypical iron porphines: iron(III) porphine chloride (FePCl) and difluoro iron(III-IV) porphine. Plain DFT within the generalized gradient approximation (GGA) implementation fails in describing the correct high-spin ground state of these porphine molecules, whereas DFT+U and UB3LYP provide an improved description. For a range of U values (4-8 eV), we compare the results of the DFT+U approach to those obtained previously with the hybrid functional (B3LYP) and with the CASPT2 approach. The DFT+U and UB3LYP methods successfully predict the molecular high spin (S = 5/2) ground state of FePCl, and also provide the nontrivial S = 3 high spin ground state for FePF(2). For the latter six-coordinated Fe porphine, our DFT+U calculations show that the S = 2, S = 5/2, and S = 3 states are energetically very close together (differences of 30 meV). Nonetheless, S = 3 is obtained as the ground state of the whole molecule, in accordance with the spin expected from the electron count. Our DFT+U calculations show furthermore that the Fe 3d occupancy is similar for FePF(2) and FePCl, i.e., DFT+U does not support Fe(IV) for FePF(2), but rather an Fe(III) porphyrin π-cation radical species, with an Fe high spin S(Fe) = 5/2, and an additional S = 1/2 stemming from spin density distributed over the porphine ring. This observation is also supported by our UB3LYP calculations.

Search for ferromagnetism in doped semiconductors in the absence of transition metal ions

In contrast to semiconductors doped with transition metal magnetic elements (e.g., ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$), which become ferromagnetic at temperatures below $\ensuremath{\sim}{10}^{2}\text{ }\text{K}$, semiconductors doped with nonmagnetic ions (e.g., silicon doped with phosphorous) have not shown evidence of ferromagnetism down to millikelvin temperatures. This is despite the fact that for low densities the system is expected to be well modeled by the Hubbard model, which is predicted to have a ferromagnetic ground state at $T=0$ on two-dimensional (2D) or three-dimensional bipartite lattices in the limit of strong correlation near half-filling. We examine the impurity band formed by hydrogenic centers in semiconductors at low densities, and show that it is described by a generalized Hubbard model which has, in addition to strong electron-electron interaction and disorder, an intrinsic electron-hole asymmetry. With the help of mean-field methods as well as exact diagonalization of clusters around half filling, we can establish the existence of a ferromagnetic ground state, at least on the nanoscale, which is more robust than that found in the standard Hubbard model. This ferromagnetism is most clearly seen in a regime inaccessible to bulk systems but attainable in quantum dots and 2D heterostructures. If observed, this would be the first experimental realization of a system exhibiting Nagaoka ferromagnetism. We present extensive numerical results for small systems that demonstrate the occurrence of high-spin ground states in both periodic and positionally disordered 2D systems. We examine how properties of real doped semiconductors, such as positional disorder and electron-hole asymmetry, affect the ground state spin of small 2D systems, and use the results to infer properties at longer length scales.

Oligo‐m‐phenyleneoxalamide Copper(II) Mesocates as Electro‐Switchable Ferromagnetic Metal–Organic Wires

Double-stranded copper(II) string complexes of varying nuclearity, from di- to tetranuclear species, have been prepared by the Cu(II)-mediated self-assembly of a novel family of linear homo- and heteropolytopic ligands that contain two outer oxamato and either zero (1 b), one (2 b), or two (3 b) inner oxamidato donor groups separated by rigid 2-methyl-1,3-phenylene spacers. The X-ray crystal structures of these Cu(II) (n) complexes (n=2 (1 d), 3 (2 d), and 4 (3 d)) show a linear array of metal atoms with an overall twisted coordination geometry for both the outer CuN(2)O(2) and inner CuN(4) chromophores. Two such nonplanar all-syn bridging ligands 1 b-3 b in an anti arrangement clamp around the metal centers with alternating M and P helical chiralities to afford an overall double meso-helicate-type architecture for 1 d-3 d. Variable-temperature (2.0-300 K) magnetic susceptibility and variable-field (0-5.0 T) magnetization measurements for 1 d-3 d show the occurrence of S=nS(Cu) (n=2-4) high-spin ground states that arise from the moderate ferromagnetic coupling between the unpaired electrons of the linearly disposed Cu(II) ions (S(Cu)=1/2) through the two anti m-phenylenediamidate-type bridges (J values in the range of +15.0 to 16.8 cm(-1)). Density functional theory (DFT) calculations for 1 d-3 d evidence a sign alternation of the spin density in the meta-substituted phenylene spacers in agreement with a spin polarization exchange mechanism along the linear metal array with overall intermetallic distances between terminal metal centers in the range of 0.7-2.2 nm. Cyclic voltammetry (CV) and rotating-disk electrode (RDE) electrochemical measurements for 1 d-3 d show several reversible or quasireversible one- or two-electron steps that involve the consecutive metal-centered oxidation of the inner and outer Cu(II) ions (S(Cu)=1/2) to diamagnetic Cu(III) ones (S(Cu)=0) at relatively low formal potentials (E values in the range of +0.14 to 0.25 V and of +0.43 to 0.67 V vs. SCE, respectively). Further developments may be envisaged for this family of oligo-m-phenyleneoxalamide copper(II) double mesocates as electroswitchable ferromagnetic 'metal-organic wires' (MOWs) on the basis of their unique ferromagnetic and multicenter redox behaviors.

Unusual Electronic Effects Imparted by Bridging Dinitrogen: an Experimental and Theoretical Investigation

We describe the preparation, structural and magnetic characterizations, and electronic structure calculations for a redox-related family of dinitrogen-bridged chromium acetylide complexes containing the [RC(2)Cr(μ-N(2))CrC(2)R](n+) (R = Ph-, (i)Pr(3)Si-; n = 0, 1, 2) backbone: [(dmpe)(4)Cr(2)(C(2)Ph)(2)(μ-N(2))] (1), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(μ-N(2))] (2), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(μ-N(2))]BAr(F)(4) (3), and [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(μ-N(2))](BAr(F)(4))(2) (4). Compounds 3 and 4 are synthesized via chemical oxidation of 2 with [Cp(2)Co](+) and [Cp*(2)Fe](+), respectively. X-ray structural analyses show that the alteration of the formal Cr oxidation states does not appreciably change the Cr-N-N-Cr skeletal structures. Magnetic data collected for 2 and 4 are consistent with high-spin triplet and quintet ground states, respectively. The mixed-valent complex 3 exhibits temperature dependent magnetic behavior consistent with a quartet ⇌ doublet two-center spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in 2 and 4 suggest that the high-spin states arise from singly occupied orthogonal π* orbitals coupled with a variable occupation of dδ orbitals. Significant N-N and Cr-N π-bonding pins the occupation of the π manifold, leading to variable occupation of the dδ space. In contrast, mixed-valent 3 is not well described by a B3LYP hybrid density functional model. A [9,11] CAS-SORCI study on a simplified model of 3 reproduces the observed Hund's rule violation for the S = 1/2 ground state and places the lowest quartet 1.45 kcal/mol above the doublet ground state.

Geometric Control of Ground State Multiplicity in a Copper(I) Bis(verdazyl) Complex

A copper(I) complex of a 3-(6'-isopropylpyridyl)-substituted verdazyl was synthesized and characterized by X-ray crystallography and magnetic susceptibility. The complex crystallizes in the monoclinic space group C2/c with cell dimensions a = 22.544 A, b = 11.576 A, c = 17.157 A, β = 123.907°, V = 3716.2 A(3). The coordination geometry at copper is distorted tetrahedral, with the two ligand planes separated by 75°. Magnetic susceptibility measurements indicate that the ground state of the diradical is a triplet at this geometry. Fitting to a simple Heisenberg Hamiltonian (H = -JS(1)·S(2)) gave J = 47(1) cm(-1). The triplet ground state results from exchange mediated by the copper ion; in particular, the direction of the distortion from tetrahedral geometry appears to be essential to maintain the high-spin ground state.

Electronic structure, spin state, and magnetism of the square-lattice Mott insulatorLa2Co2Se2O3from first principles

Electronic and magnetic structures of the recently synthesized cobalt oxyselenide ${\text{La}}_{2}{\text{Co}}_{2}{\text{Se}}_{2}{\text{O}}_{3}$ (structurally similar to the superconducting iron pnictides) are studied through density-functional calculations. The obtained results show that this material is a Mott insulator and it has a very stable ${\text{Co}}^{2+}$ high-spin ground state with a ${t}_{2g}$-like orbital ordering, which is substantiated by the calculated crystal-field excitation energies. The square lattice of the ${\text{Co}}^{2+}$ spins is found to have a strong antiferromagnetic (a weak ferromagnetic) coupling for the second-nearest neighbors via O $({\text{Se}}_{2})$ and an intermediate antiferromagnetic one for the first-nearest neighbors with the strength ratio about 10:1:3. The present results account for the available experimental data and the prediction of a planar frustrated $(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic structure would motivate a new experiment.

Two-dimensionally extended organic high-spin poly(aminium cationic radical)s and their magnetic force microscopic images

Purely organic high-spin poly(aminium cationic radical)s of two-dimensionally extended soluble aromatic polyamines P1 and P2 were prepared by the palladium-catalyzed polycondensation of two multifunctional monomers, followed by chemical or electrochemical oxidations. The chemical structures of the precursor polyamines were fully characterized by 1H- and 13C nuclear magnetic resonance (NMR), infrared and MALDI-TOF mass spectra (MS). The aminium radical sites of N,N,N′,N′-tetraaryl-p-phenylenediamines were connected through the m-phenylene ferromagnetic coupler, which satisfies the non-Kekulé-type molecular design. Oxidation of radical sites was facilitated by electron-donating methoxy substituents. The first oxidation potential (Eo′(1)) of P2 with methoxy substituents, evaluated from the cyclic voltammograms in CH2Cl2, was 0.69 V (vs Ag/Ag+), whereas the Eo′(1) of P1 without electron-donating substituents was 0.78 V. This result was further verified by spectroelectrochemistry measurements. The electrochemically oxidized P2 displayed a more intense low-energy band at ca. 800 nm ascribed to aminium cationic radicals and bications. Chemical oxidation of P1 and P2 with NOPF6 solubilized with 18-crown-6 in CH2Cl2 gave the corresponding poly(aminium cationic radical)s. The high-spin ground states were confirmed by the temperature dependence of forbidden electron spin resonance (ESR) transition peak intensities, and the average spin quantum number (S) determined by the magnetic curves reached 8.4/2 at low temperatures. Remarkably, when a small amount of trifluoroacetic acid was added, the half-life of polyradicals reached ca. 1 week at room temperature (RT). This result prompted us to observe the single polymer-based molecular and magnetic images by atomic force microscopy and magnetic force microscopy, respectively. The precursor polyamine P1 showed globular-shaped polymer particles with an average vertical distance of 1.43 nm. After chemical oxidation, the vertical distance of P1 definitely decreased, reflecting the improved p-type character of the nitrogen atoms. Moreover, the detection of weak MFM images supported the presence of magnetic spins in the single polymers.

Nickel(II) Complexes of tpa Ligands with 6-Phenyl Substituents (Phntpa). Structure and H2O2-Reactivity

Abstract Nickel(II) complexes supported by a series of tpa {tris[(pyridin-2-yl)methyl]amine} ligand derivatives containing different numbers of phenyl substituent on the 6-position of pyridine donor group (Phntpa, n = 0–3) have been prepared and structurally characterized. Ligand tpa and its derivatives with one or two 6-Ph substituent(s) (Ph0tpa, Ph1tpa, and Ph2tpa) afforded nickel(II) complexes 1[0], 1[1], and 1[2] with a distorted octahedral geometry, whereas the ligand having three 6-Ph groups (Ph3tpa) gave nickel(II) complex 1[3] with a five-coordinate trigonal bipyramidal structure. All the complexes exhibited a high spin ground state (S = 1). Reactivity of the nickel(II) complexes 1[0] and 1[1] toward H2O2 was very poor, whereas 1[2] and 1[3] readily gave bis(μ-oxo)dinickel(III) complexes 2[2] and 2[3], respectively, in the reaction with H2O2 at a low temperature. The bis(μ-oxo)dinickel(III) complexes 2[2] and 2[3] gradually decomposed to cause an aromatic hydroxylation reaction of one of the 6-Ph groups of the supporting ligands. The ligand substituent effects on the formation and decomposition processes of the bis(μ-oxo)dinickel(III) complexes are discussed based on the structures and the redox potentials of the starting nickel(II) complexes.

Cobalt(II) “Scorpionate” Complexes as Models for Cobalt-Substituted Zinc Enzymes: Electronic Structure Investigation by High-Frequency and -Field Electron Paramagnetic Resonance Spectroscopy

A series of complexes of formula Tp(R,R')CoL, where Tp(R,R'-) = hydrotris(3-R,5-R'-pyrazol-1-yl)borate ("scorpionate") anion (R = tert-butyl, R' = H, Me, 2'-thienyl (Tn), L = Cl(-), NCS(-), NCO(-), N(3)(-)), has been characterized by electronic absorption spectroscopy in the visible and near-infrared (near-IR) region and by high-frequency and -field electron paramagnetic resonance (HFEPR). Reported here are also crystal structures of seven members of the series that have not been reported previously: R' = H, L = NCO(-), N(3)(-); R' = Me, L = Cl(-), NCS(-), NCO(-), N(3)(-); R' = Tn, L = Cl(-), NCS(-). These include a structure for Tp(t-Bu,Me)CoCl different from that previously reported. All of the investigated complexes contain a four-coordinate cobalt(II) ion (3d(7)) with approximate C(3v) point group symmetry about the metal ion and exhibit an S = (3)/(2) high-spin ground state. The use of HFEPR allows extraction of the full set of intrinsic S = (3)/(2) spin Hamiltonian parameters (D, E, and g values). The axial zero-field splitting parameter, D, for all investigated Tp(R,R')CoL complexes is always positive, a fact not easily determined by other methods. However, the magnitude of this parameter varies widely: 2.4 cm(-1) <or= D <or= 12.7 cm(-1), indicating the extreme sensitivity of this parameter to environment. The spin Hamiltonian parameters are combined with estimates of 3d energy levels based on the visible-near-IR spectra to yield ligand-field parameters for these complexes following the angular overlap model (AOM). This description of electronic structure and bonding in pseudotetrahedral cobalt(II) complexes can enhance the understanding of similar sites in metalloproteins, specifically cobalt-substituted zinc enzymes.

High-spin ground states via electron delocalization in mixed-valence imidazolate-bridged divanadium complexes

The field of molecular magnetism has grown tremendously since the discovery of single-molecule magnets, but it remains centred around the superexchange mechanism. The possibility of instead using a double-exchange mechanism (based on electron delocalization rather than Heisenberg exchange through a non-magnetic bridge) presents a tantalizing prospect for synthesizing molecules with high-spin ground states that are well isolated in energy. We now demonstrate that magnetic double exchange can be sustained by simple imidazolate bridging ligands, known to be well suited for the construction of coordination clusters and solids. A series of mixed-valence molecules of the type [(PY5Me(2))V(II)(micro-L(br)) V(III)(PY5Me(2))](4+) were synthesized and their electron delocalization probed through cyclic voltammetry and spectroelectrochemistry. Magnetic susceptibility data reveal a well-isolated S = 5/2 ground state arising from double exchange for [(PY5Me(2))(2)V(2)(micro-5,6-dimethylbenzimidazolate)](4+). Combined modelling of the magnetic data and spectral analysis leads to an estimate of the double-exchange parameter of B = 220 cm(-1) when vibronic coupling is taken into account.

Assembly of two ferrous coordination polymers with triazole derivative: Syntheses, structures and magnetic properties

Abstract The reactions of Fe(II) salts with 1,4-bis(1,2,4-triazole-1-ylmethyl-benzene) (btx) give two new Fe(II) coordination polymers {[Fe(btx)3](ClO4)2}n (1) and {[Fe(btx)(SO4)(H2O)2]}n (2), which have been characterized by element analysis, PXRD, single-crystal X-ray diffraction and magnetic susceptibility. 1 shows infinite plait-like chain structure, while 2 exhibits as 2D grid. Magnetic studies indicate that the Fe(II) ions in the two coordination polymers retain in high-spin ground state upon cooling from 300 to 2 K, showing antiferromagnetic interactions.

Theoretical predictions of a highly reactive non-heme Fe(iv)O complex with a high-spin ground state

Computations show that a non-heme iron(iv)-oxo complex with a trigonal bipyramidal structure, [(Me(6)TREN)Fe(IV)=O](2+), has a quintet ground state and a low activation barrier for H-abstraction from cyclohexane, whereas its ruthenium analogue, [(Me(6)TREN)Ru(IV)=O](2+), has a triplet ground state and a high H-abstraction barrier.

Introduction of ester and amido functions in tetrairon(iii) single-molecule magnets: synthesis and physical characterization

Tetrairon(III) complexes with a propeller-like structure derived from [Fe(4)(OMe)(6)(dpm)(6)] (1) (Hdpm = 2,2,6,6-tetramethylheptane-3,5-dione) are providing a growing class of Single Molecule Magnets (SMMs) displaying unprecedented synthetic flexibility and ease of functionalization. Herein we report the synthesis, crystal structures and magnetic properties of two novel tetrairon(III) SMMs, [Fe(4)(esterC5)(2)(dpm)(6)] (2) and [Fe(4)(amideC5)(2)(dpm)(6)].Et(2)O.4MeOH (3.Et(2)O.4MeOH), in which functionalization of the cluster core is achieved using ester and amido linkages, respectively. To this aim, two new tripodal ligands were prepared by acylation of pentaerythritol (2,2-bis(hydroxymethyl)propane-1,3-diol) and TRIS (2-amino-2-(hydroxymethyl)propane-1,3-diol), namely H(3)esterC5 = RC(O)OCH(2)C(CH(2)OH)(3) and H(3)amideC5 = RC(O)NHC(CH(2)OH)(3) with R = n-butyl. The compounds were structurally investigated by single-crystal XRD, which demonstrated coordination of the tripodal ligands to the cluster core. The products display SMM behavior with anisotropy barriers U(eff)/k(B) congruent with 11 K due to a high-spin (S = 5) ground state and an easy axis anisotropy, described by D = -0.421 cm(-1) in 2 and -0.414 cm(-1) in 3.Et(2)O.4MeOH. The departure of U(eff) from the total splitting of the S = 5 ground multiplet, U/k(B) congruent with 15 K, has to be ascribed to the sizeable rhombic anisotropy that characterizes the two compounds (E = 0.021 cm(-1) in 2 and 0.019 cm(-1) in 3.Et(2)O.4MeOH), as confirmed by master matrix calculations of the temperature-dependent relaxation time.

Ferromagnetic manganese “cubes”: from PSII to single-molecule magnets

The reaction of Mn(O₂CMe)₂·2H₂O with Me-saoH₂ (Me-saoH₂ = 2-hydroxyphenylethanone oxime) in MeCN forms the complex [Mn(III)₄(Me-sao)₄(Me-saoH)₄] (1) in good yields. Replacing Me-saoH₂ with Naphth-saoH₂ (Naphth-saoH₂ = 2-hydroxy-1-napthaldoxime) in the presence of CH₃ONa forms the complex [Mn(III)₄(Naphth-sao)₄(Naphth-saoH)₄] (2) in low yields, while the reaction between Mn(ClO₄)₂·6H₂O, Et-saoH₂ (Et-saoH₂= 2-hydroxypropiophenone oxime) and NBu₄OH in MeCN gives the complex [Mn(III)₄(Et-sao)₄(Et-saoH)₄] (3) in moderate yields. All three tetrametallic cages exclusively contain Mn(III) centres arranged in a "cube"-like topology, in which the metal centres are connected by -N-O(oximate) groups. The magnetic properties of 1-3 are near identical, revealing the presence of only ferromagnetic interactions between the metal ions leading to high-spin ground states of S = 8. The complexes display frequency dependent out-of-phase signals in ac susceptibility studies and, in the case of 1 single-molecule magnetism has been observed by means of single-crystal hysteresis loop measurements.

Electronic Structure of an Iron-Porphyrin−Nitrene Complex

Middle and late transition metal imido complexes (which may also be viewed as metal-nitrene adducts) are rather rare, especially for square-pyramidal and octahedral coordination geometries. However, an iron(II) porphyrin aminonitrene adduct, denoted here as Fe(Por)(NN), has been known for almost a quarter of a century. Unlike the corresponding S = 1 oxene and S = 0 carbene adducts, Fe(Por)(NN) exhibits an S = 2 ground state. DFT calculations reported herein provide a molecular orbital description of this unusual species as well as a rationale for its S = 2 ground state. The electronic configuration of Fe(Por)(NN) may be described as d(pi)(2)d(xy)(1)d(z(2))(1)d(x(2)-y(2))(1)d(pi')(1), where the z direction corresponds to the Fe-NN axis. The stability and double occupancy of one of the d(pi) orbitals may be attributed to a pi-backbonding interaction with the N-N pi* orbital. The weak sigma-donor ability of the aminonitrene ligand results in a relatively low-energy d(z(2)) orbital and an overall d orbital splitting pattern that engenders a high-spin ground state.

Density Functional Theory Applied to a Difference in Pathways Taken by the Enzymes Cytochrome P450 and Superoxide Reductase: Spin States of Ferric Hydroperoxo Intermediates and Hydrogen Bonds from Water

Cytochrome P450 monooxygenase and superoxide reductase (SOR) have the same first atom coordination shell at their iron active sites: an Fe[N(4)S] center in a square-pyramidal geometry with the sixth coordinate site open for the catalytic reaction. Furthermore, both pass through ferric hydroperoxo intermediates. Despite these similarities, the next step in their catalytic cycle is very different: distal oxygen protonation and O-O cleavage (P450) versus proximal oxygen protonation and H(2)O(2) release (SOR). One of the factors leading to this difference is the spin state of the intermediates. Density functional theory (DFT) applied to models for the ferric hydroperoxo, (SCH(3))(L)Fe(III)-OOH (L = porphyrin for P450 and four imidazoles for SOR), gives different ground spin states; the P450 model with the porphyrin, which contrains the Fe-N distances, prefers a low-spin ground state, whereas the SOR model with four histidines, in which Fe-N bonds are extendable, prefers a high-spin ground state. Their ground spin states lead to geometric and electronic structures that assist in (1) the protonation on distal oxygen for P450, which leads to O-O bond cleavage and formation of the oxo-ferryl, (SCH(3))(L)Fe(IV) horizontal lineO (Cpd I), and H(2)O, and (2) the protonation on proximal oxygen for SOR, which leads to the formation of the ferric hydrogen peroxide, (SCH(3))(L)Fe(III)-HOOH, intermediate before the Fe-O bond cleavage and H(2)O(2) production. Specifically, the quartet ground state of the water-bound oxo-ferryl, (SCH(3))(L)Fe(IV) horizontal lineO...H(2)O, is more stable than the sextet ground state of (SCH(3))(L)Fe(III)-HOOH by -14.29 kcal/mol for the P450 model. Another important factor is the differences in the location of the active site: P450's active site is embedded within the enzyme, whereas SOR's active site is exposed to the aqueous environment. In the latter location, water molecules can freely form hydrogen bonds with both proximal and distal oxygen to stabilize the (SCH(3))(L)Fe(III)-HOOH intermediate. When two explicit water molecules are included in the model, the sextet ground state of (SCH(3))(L)Fe(III)-HOOH...2H(2)O is more stable than the quartet ground state of (SCH(3))(L)Fe(IV) horizontal lineO...3H(2)O by -2.14 kcal/mol for the SOR model. Our calculations show that both the spin state, which is controlled by the differences between four N donors in porphyrin versus those in imidazoles, and the degree of solvent exposure of the active sites play important roles in the fate of the (SCH(3))(L)Fe(III)-OOH intermediate, leading to O-O cleavage in one situation (P450) and hydrogen peroxide production in the other (SOR).

Hetero-Metallic {3d-4f-5d} Complexes: Preparation and Magnetic Behavior of Trinuclear [(LMe2Ni−Ln){W(CN)8}] Compounds (Ln = Gd, Tb, Dy, Ho, Er, Y; LMe2 = Schiff base) and Variable SMM Characteristics for the Tb Derivative

Assembling bimetallic {Ni-Ln}(3+) units and {W(CN)(8)}(3-) is shown to be an efficient route toward heteronuclear {3d-4f-5d} compounds. The reaction of either the binuclear [{L(Me2)Ni(H(2)O)(2)}{Ln(NO(3))(3)}] complexes or their mononuclear components [L(Me2)Ni] and Ln(NO(3))(3) with (HNBu(3))(3){W(CN)(8)} in dmf followed by diffusion of tetrahydrofuran yielded the trinuclear [{L(Me2)NiLn}{W(CN)(8)}] compounds 1 (Ln = Y), 2a,b (Gd), 3a,b (Tb), 4 (Dy), 5 (Ho), and 6 (Er) as crystalline materials. All of the derivatives possess the trinuclear core resulting from the linkage of the {W(CN)(8)} to the Ni center of the {Ni-Ln} unit. Differences are found in the solvent molecules acting as ligands and/or in the lattice depending on the crystallization conditions. For all the compounds ferromagnetic {Ni-W} and {Ni-Ln} (Ln = Gd, Tb, Dy, and Er} interactions are operative resulting in high spin ground states. Parameterization of the magnetic behaviors for the Y and Gd derivatives confirmed the strong cyano-mediated {Ni-W} interaction (J(NiW) = 27.1 and 28.5 cm(-1)) compared to the {Ni-Gd} interaction (J(NiGd) = 2.17 cm(-1)). The characteristic features for slow relaxation of the magnetization are observed for two Tb derivatives, but these are modulated by the crystal phase. Analysis of the frequency dependence of the alternating current susceptibility data yielded U(eff)/k(B) = 15.3 K and tau(0) = 4.5 x 10(-7) s for one derivative whereas no maxima of chi(M)'' appear above 2 K for the second one.

Ni(II)/H2O2 Reactivity in Bis[(pyridin-2-yl)methyl]amine Tridentate Ligand System. Aromatic Hydroxylation Reaction by Bis(μ-oxo)dinickel(III) Complex

The nickel(II) complexes 1(X) supported by bis[(pyridin-2-yl)methyl]benzylamine tridentate ligands carrying m-substituted phenyl groups (X = OMe, Me, H, Cl, NO(2)) at the 6-position of pyridine donor groups (L(X), N,N-bis[(6-m-substituted-phenylpyridin-2-yl)methyl]benzylamine) have been synthesized and characterized. The X-ray crystallographic analyses have revealed that [Ni(II)(L(H))(CH(3)CN)(H(2)O)](ClO(4))(2) (1(H)), [Ni(II)(L(OMe))(CH(3)CN)(MeOH)](ClO(4))(2) (1(OMe)), [Ni(II)(L(Me))(CH(3)CN)(H(2)O)](ClO(4))(2) (1(Me)), and [Ni(II)(L(Cl))(CH(3)CN)(H(2)O)](ClO(4))(2) (1(Cl)) have a five-coordinate square pyramidal geometry, whereas [Ni(II)(L(NO(2)))(CH(3)CN)(2)(H(2)O)](ClO(4))(2) (1(NO(2))) exhibits a six-coordinate octahedral geometry having an additional CH(3)CN co-ligand. (1)H NMR spectra of the nickel(II) complexes 1(X) in CD(3)CN have indicated that all the complexes have a high spin ground state. The nickel(II) complexes 1(X) react with hydrogen peroxide (H(2)O(2)) in acetone to give bis(mu-oxo)dinickel(III) complexes 2(X) exhibiting a characteristic UV-vis absorption band at approximately 420 nm. In the case of 2(H), a resonance Raman band ascribable to a Ni(2)O(2) core vibration was observed at 611 cm(-1) that shifted to 586 cm(-1) upon H(2)(18)O(2). The bis(mu-oxo)dinickel(III) intermediates 2(X) undergo an efficient aromatic ligand hydroxylation reaction, producing a mononuclear nickel(II)-phenolate complexes 4(X) via a putative intermediate (mu-phenoxo)(mu-hydroxo)dinickel(II) (3(X)). The kinetic studies on the aromatic ligand hydroxylation process including m-substituent effects (Hammett analysis) and kinetic deuterium isotope effects (KIE) have indicated that the reaction of 2(X) to 3(X) involves an electrophilic aromatic substitution mechanism, where C-O bond formation and C-H bond cleavage occur in a concerted manner. Intermediate 3(H) was detected by ESI-MS during the course of the reaction, and the final product 4(H) was characterized by elemental analysis, ESI-MS, and X-ray crystallographic analysis.