Quintet Spin States Research Articles

Chemical reactions of ultracold alkali-metal dimers in the lowest-energy3Σstate

We show that the interaction of polar alkali-metal dimers in the quintet spin state leads to the formation of a deeply bound reaction complex. The reaction complex can decompose adiabatically into homonuclear alkali-metal dimers (for all molecules except KRb) and into alkali-metal trimers (for all molecules). We show that there are no barriers for these chemical reactions. This means that all alkali-metal dimers in the ${a}^{3}{\ensuremath{\Sigma}}^{+}$ state are chemically unstable at ultracold temperature, and the use of an optical lattice to segregate the molecules and suppress losses may be necessary. In addition, we calculate the minimum-energy path for the chemical reactions of alkali-metal hydrides. We find that the reaction of two molecules is accelerated by a strong attraction between the alkali-metal atoms, leading to a barrierless process that produces hydrogen atoms with large kinetic energy. We discuss the unique features of the chemical reactions of ultracold alkali-metal dimers in the ${a}^{3}{\ensuremath{\Sigma}}^{+}$ electronic state.

Transition metal decorated borazine complex for hydrogen storage and unfavourable consequence of spin shift for hydrogen storage on Ti-decorated borazine: A DFT study

Density functional theory calculations have been carried out using hybrid B3PW91-functional for investigating hydrogen storage over Ti, Cr, and Fe doped borazine. The natural bond order and partial density of states analysis executed over ground (triplet) and excited (quintet) spin state of Ti–B3N3H6 revealed that the extent of hybridization between 4s and 3d-orbitals of Ti is significantly different in the two spin states. Presence of borazine results in the reduction of transition energy from triplet to quintet state of Ti and hence lead to easy accessibility to higher spin state. The binding energy for single hydrogen molecule in the quintet state of Ti–B3N3H6 is found to be less as compared to triplet state of the same. The overall hydrogen storage capability of Ti–B3N3H6 in quintet state is estimated to be half as compared to its triplet state and is thus unfavourable for hydrogen storage. Thermo-chemistry calculations recommend Ti-decorated over triplet state borazine complex as attractive candidate for hydrogen storage up to 400 K. The nature of interaction of hydrogen with Ti–B3N3H6 has also been investigated using many-body interaction energy calculations.

Ab Initio Extension of the AMOEBA Polarizable Force Field to Fe(2.).

We extend the AMOEBA polarizable molecular mechanics force field to the Fe(2+) cation in its singlet, triplet, and quintet spin states. Required parameters are obtained either directly from first principles calculations or optimized so as to reproduce corresponding interaction energy components in a hexaaquo environment derived from quantum mechanical energy decomposition analyses. We assess the importance of the damping of point-dipole polarization at short distance as well as the influence of charge-transfer for metal-water interactions in hydrated Fe(2+); this analysis informs the selection of model systems employed for parametrization. We validate our final Fe(2+) model through comparison of molecular dynamics (MD) simulations to available experimental data for aqueous ferrous ion in its quintet electronic ground state.

Binuclear pentalene manganese carbonyl complexes: conventional trans and unconventional cis structures

The binuclear pentalene manganese carbonyl trans-(η5,η5-C8H6)Mn2(CO)6 has been synthesized by the reaction of the pentalene dianion with a manganese carbonyl halide. This species as well as its decarbonylation products (C8H6)Mn2(CO) n (n = 5, 4, 3) have been investigated by density functional theory. In all of these optimized structures both pentalene rings are bonded to manganese atoms as pentahapto ligands. The lowest energy (C8H6)Mn2(CO)6 structure is the experimentally observed singlet trans structure but a closely related cis structure lies ∼7 kcal/mol higher. The lowest energy (C8H6)Mn2(CO)5 structure is a singlet structure having a four-electron donor bridging CO group and a Mn–Mn single bond of length ∼2.9 Å. The lowest energy (C8H6)Mn2(CO)4 structure is a singlet structure having all terminal CO groups and a short Mn ≡ Mn triple bond of length ∼2.3 Å. For (C8H6)Mn2(CO)3 low energy structures are found with either triplet or quintet spin states. Low energy structures with pentalene rings only partially bonded to manganese atoms were found for the carbonyl-rich species (C8H6)Mn2(CO) n (n = 7, 8). However, the latter species are predicted not to be viable with respect to carbonyl loss to give the stable (C8H6)Mn2(CO)6.

Regioselectivity of substrate hydroxylation versus halogenation by a nonheme iron(IV)–oxo complex: possibility of rearrangement pathways

Several nonheme iron enzymes and biomimetic model complexes catalyze a substrate halogenation reaction. Recent computational studies (Borowski et al. J Am Chem Soc 132:12887-12898, 2010) on α-ketoglutarate dependent halogenase proposed an initial isomerization reaction that is important to give halogenated products. We present here a series of density functional theory calculations on a biomimetic model complex-[Fe(IV)(O)(TPA)Cl](+), where TPA is tris(2-pyridylmethyl)amine-and investigate the mechanisms of substrate halogenation versus hydroxylation using the reactant and its isomer where the oxo and chloro groups have changed positions. We show here that the reactions occur on a dominant quintet spin state surface, although the reactants are in a triplet state. Despite the fact that the reactants can exist in two stable isomers with the oxo group either trans or cis to the axial ligand, they react differently with substrates, where one gives dominant hydroxylation and the other gives dominant chlorination of substrates. The ligand in the cis position of the oxo group is found to be active in the reaction mechanism and donated to the substrate during the reaction. A detailed thermochemical analysis of possible reaction mechanisms reveals that the strengths of the Fe-OH and Fe-Cl bonds in the radical intermediates are the key reasons for this regioselectivity switch of hydroxylation over halogenation. This study highlights the differences between enzymatic and biomimetic halogenases, where the former only react after an essential isomerization step, which is not necessary in model complexes.

The Structural Features of the Hydrated Ferrous Ion Clusters: [Fe(H2O) n ]2+ (n = 1–19)

The low-lying structures of the hydrated ferrous ion clusters [Fe(H2O) n ]2+ (n = 1–19) were extensively searched at the level of the density functional theory. The results show that the first hydration shell consists of six water molecules, and the second hydration shell contains seven water molecules. Furthermore, it is found that all the lowest-energy states of [Fe(H2O) n ]2+ (n = 1–19) clusters are spin quintet states. These lowest-energy states keep well even at finite temperatures. The analyses of the successive water binding energy and natural charges population on ferrous ion clearly show that the influence of ferrous ion on the surrounding water molecules goes beyond the second hydration shell.

Mechanism of S-Oxygenation by a Cysteine Dioxygenase Model Complex

In this work, we present the first computational study on a biomimetic cysteine dioxygenase model complex, [Fe(II)(LN(3)S)](+), in which LN(3)S is a tetradentate ligand with a bis(imino)pyridyl scaffold and a pendant arylthiolate group. The reaction mechanism of sulfur dioxygenation with O(2) was examined by density functional theory (DFT) methods and compared with results obtained for cysteine dioxygenase. The reaction proceeds via multistate reactivity patterns on competing singlet, triplet, and quintet spin state surfaces. The reaction mechanism is analogous to that found for cysteine dioxygenase enzymes (Kumar, D.; Thiel, W.; de Visser, S. P. J. Am. Chem. Soc. 2011, 133, 3869-3882); hence, the computations indicate that this complex can closely mimic the enzymatic process. The catalytic mechanism starts from an iron(III)-superoxo complex and the attack of the terminal oxygen atom of the superoxo group on the sulfur atom of the ligand. Subsequently, the dioxygen bond breaks to form an iron(IV)-oxo complex with a bound sulfenato group. After reorganization, the second oxygen atom is transferred to the substrate to give a sulfinic acid product. An alternative mechanism involving the direct attack of dioxygen on the sulfur, without involving any iron-oxygen intermediates, was also examined. Importantly, a significant energetic preference for dioxygen coordinating to the iron center prior to attack at sulfur was discovered and serves to elucidate the function of the metal ion in the reaction process. The computational results are in good agreement with experimental observations, and the differences and similarities of the biomimetic complex and the enzymatic cysteine dioxygenase center are highlighted.

Binuclear heptalene iron carbonyl complexes: Comparison with related azulene and pentalene complexes

Reactions of heptalene and its 1,6-dimethyl derivatives with iron carbonyls have been reported to give structurally characterized cis and trans stereoisomers of the bis(tetrahapto)diene complex (η4,η4-C12H10)Fe2(CO)6 in which the uncomplexed CC double bonds form a transoid 1,3-diene unit including the C–C double bond shared by the two heptalene rings. These two structures were found by density functional theory to have essentially identical energies and to lie in energy below any other of the four possible (η4,η4-C12H10)Fe2(CO)6 isomers. The lowest energy structure for C12H10Fe2(CO)5 has a carbonyl-bridged Fe–Fe formal single bond of length ∼2.79 Å. The heptalene ligand in this structure is bonded to one iron atom as a tetrahapto cisoid 1,3-diene unit and to the other iron atom as a tetrahapto Y-shaped trimethylenemethane unit leaving an uncomplexed cisoid 1,3-diene unit. Four higher energy C12H10Fe2(CO)5 structures are found without iron–iron bonds and with one heptalene ring bonded to an iron atom as a hexahapto ligand and to the other iron atom as a tetrahapto ligand leaving one uncomplexed CC bond. In contrast to the related experimentally known (pentalene)Fe2(CO)5 and (azulene)Fe2(CO)5, these (heptalene)Fe2(CO)5 derivatives are predicted to have only marginal stability with respect to disproportionation into (heptalene)Fe2(CO)6 + (heptalene)Fe2(CO)4. This may account for the failure to isolate (heptalene)Fe2(CO)5 from reactions of heptalene with iron carbonyls. The low energy structures for the unsaturated (heptalene)Fe2(CO)n (n = 4, 3, 2) provide examples of triplet and even quintet spin states (the latter for n = 2) as well as variable hapticities of the heptalene rings ranging from trihapto to heptahapto.

Low-lying electronic states of the ferrous high-spin ( S = 2) heme in deoxy-Mb and deoxy-Hb studied by highly-sensitive multi-frequency EPR

The low-lying electronic states of the ferrous high-spin heme in deoxy-myoglobin (deoxy-Mb) and deoxy-hemoglobin (deoxy-Hb) were probed by multi-frequency electron paramagnetic resonance (MFEPR) spectroscopy. An unexpected broad EPR signal was measured at the zero magnetic field using cavity resonators at 34–122 GHz that could not be simulated using any parameter sets for the S = 2 spin Hamiltonian assuming spin quintet states in the 5 B 2 ground state. Furthermore, we have observed novel, broad EPR signals measured at 70–220 GHz and 1.5 K using a single pass transmission probe. These signals are attributed to the ferrous high-spin heme in deoxy-Mb and deoxy-Hb. The resonant peaks shifted to a higher magnetic field with increasing frequency. The energy level separation between the ground singlet and the first excited state at the zero magnetic field was directly estimated to be 3.5 cm − 1 for deoxy-Hb. For deoxy-Mb, the first two excited singlet states are separated by 3.3 cm − 1 and 6.5 cm − 1 , respectively, from the ground state. The energy gap at the zero magnetic field is directly derived from our MFEPR for deoxy-Mb and deoxy-Hb and strongly supports the theoretical analyses based on the Mössbauer and magnetic circular dichroism experiments.

Theoretical Investigation of a Titanium–Aniline Complex with and without an Alkyl Chain

Density functional theory computations are employed to investigate the geometric, electronic, and vibrational properties of a neutral Ti(aniline) experimentally soft-landed onto an alkanethiol self-assembled monolayer (CH-SAM) matrix. Five optimized structures of a bare Ti(aniline) are obtained: Three of the five optimized structures show Ti–phenyl ring bonds with singlet, triplet, and quintet spin states and the other two show Ti–N bonds with triplet and quintet states. For their calculated IR spectra in the range 0–2000 cm–1, the peak position and intensity of the NH2 scissor mode (∼1600 cm–1) are insensitive to the Ti binding and spin states, whereas those of the NH2–C6H5 intergroup stretching (∼1200 cm–1) and NH2 inversion (500–1000 cm–1) modes are sensitive to these factors. To study Ti(aniline) in a CH-SAM matrix, geometric structures of a modeled system of Ti(aniline)–C3H8 are optimized and their infrared reflection absorption spectra are directly calculated by projecting the normal mode derivatives of the dipole moment of the system onto the surface normal. C3H8 stabilizes Ti(aniline) by about 8–10 kcal/mol but causes little change in the IR spectrum in the range 1300–1700 cm–1, except for the quintet state. From a comparison of the predicted IRAS spectra with the experiment, Ti(aniline) in CH-SAM is concluded to be in a triplet state with a Ti-phenyl ring bond.

Triple-Decker-Sandwich versus Rice-Ball Structures for Tris(benzene)dimetal Derivatives of the First-Row Transition Metals

Compounds of the type M(2)Bz(3) (Bz = benzene, C(6)H(6)) have been of interest since the related triple-decker mesitylenechromium sandwich (1,3,5-Me(3)C(6)H(3))(3)Cr(2) has been synthesized and characterized structurally by X-ray crystallography. Theoretical studies predict the lowest-energy M(2)Bz(3) structures of the early transition metals Ti, V, and Cr to be the triple-decker sandwiches trans-Bz(2)M(2)(η(6),η(6)-μ-C(6)H(6)) having quintet, triplet, and singlet spin states, respectively. In these structures, the central benzene ring functions as a hexahapto ligand to each metal atom. The singlet rice-ball cis-Bz(2)M(2)(μ-C(6)H(6)) structures with a 2.64-Å Mn═Mn double bond or a 2.81-Å Fe-Fe single bond are preferred for the central transition metals Mn and Fe. Singlet triple-decker-sandwich structures trans-Bz(2)M(2)(μ-C(6)H(6)) return as the lowest-energy structures for the late transition metals Co and Ni but with the central benzene ring only partially bonded to each metal atom. Thus, the lowest-energy cobalt derivative has a trans-Bz(2)Co(2)(η(3),η(3)-μ-C(6)H(6)) structure in which the central benzene ring acts as a trihapto ligand to each metal atom. Similarly, the lowest-energy nickel derivative has a trans-Bz(2)Ni(2)(η(2),η(2)-μ-C(6)H(6)) structure in which the central benzene ring acts as a dihapto ligand to each metal atom, leaving an uncomplexed C═C double bond. The metal-metal bond orders in the singlet "rice-ball" structures cis-Bz(2)M(2)(μ-C(6)H(6)) (M = Mn, Fe) and the hapticities of the central benzene rings in the singlet late-transition-metal triple-decker-sandwich structures trans-Bz(2)M(2)(μ-C(6)H(6)) (M = Co, Ni) are governed by the desirability for the metal atoms to attain the favored 18-electron configuration.

About the Barriers to Reaction of CCl4 with HFeOH and FeCl2

The reactions of zerovalent iron with water and carbon tetrachloride are of interest for environmental remediation of contaminated water and soil. Atom-dropping experiments have shown that the reactions of iron atoms with water and CCl(4) may produce HFeOH and FeCl(2), respectively, but these compounds are themselves unreactive toward CCl(4) at the low temperatures under which the atom-dropping experiments were performed. We report a modeling study of these reactions using density functional theory, ab initio Hartree-Fock and couple-cluster theory, and principles of Marcus-Hush theory to characterize the underlying intrinsic barriers and rationalize the experimental results. Electron-correlated CCSD(T) calculations (at B3LYP/TZVP optimized structures) show that the transition state for Cl atom transfer from CCl(4) to HFeOH arises from crossing of electronic states in which the configuration of Fe changes from a quintet high spin state in the Fe(II) reactant to a sextet high spin state in the Fe(III) products. The crossing point is 23.8 kcal/mol above a long-range precursor complex that is 2.1 kcal/mol more stable than the separated reactants. The electronic structure changes in these Cl atom transfer reactions involve unpairing of d electrons in Fe(II) and their recoupling with Cl-C σ bond electrons. These processes can be conveniently described by invoking the self-exchange reactions HFeOH/HFeClOH, FeCl(2)/FeCl(3), and CCl(4)/(•)CCl(3) for which we determined the energy barriers to be 15.5, 13.1, 18.6 kcal/mol, respectively. For the cross reaction FeCl(2)/CCl(4), we estimated a barrier of 16.6 kcal/mol relative to the separated reactants and 21.1 kcal/mol from the precursor complex. The magnitudes of the reaction barriers are consistent with reports of the absence of products in the atom-dropping experiments.

Theoretical study of the mechanism of oxoiron(IV) formation from H2O2 and a nonheme iron(II) complex: O-O cleavage involving proton-coupled electron transfer.

It has recently been shown that the nonheme oxoiron(IV) species supported by the 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane ligand (TMC) can be generated in near-quantitative yield by reacting [Fe(II)(TMC)(OTf)(2)] with a stoichiometric amount of H(2)O(2) in CH(3)CN in the presence of 2,6-lutidine (Li, F.; England, J.; Que, L., Jr. J. Am. Chem. Soc. 2010, 132, 2134-2135). This finding has major implications for O-O bond cleavage events in both Fenton chemistry and nonheme iron enzymes. To understand the mechanism of this process, especially the intimate details of the O-O bond cleavage step, a series of density functional theory (DFT) calculations and analyses have been carried out. Two distinct reaction paths (A and B) were identified. Path A consists of two principal steps: (1) coordination of H(2)O(2) to Fe(II) and (2) a combination of partial homolytic O-O bond cleavage and proton-coupled electron transfer (PCET). The latter combination renders the rate-limiting O-O cleavage effectively a heterolytic process. Path B proceeds via a simultaneous homolytic O-O bond cleavage of H(2)O(2) and Fe-O bond formation. This is followed by H abstraction from the resultant Fe(III)-OH species by an •OH radical. Calculations suggest that path B is plausible in the absence of base. However, once 2,6-lutidine is added to the reacting system, the reaction barrier is lowered and more importantly the mechanistic path switches to path A, where 2,6-lutidine plays an essential role as an acid-base catalyst in a manner similar to how the distal histidine or glutamate residue assists in compound I formation in heme peroxidases. The reaction was found to proceed predominantly on the quintet spin state surface, and a transition to the triplet state, the experimentally known ground state for the TMC-oxoiron(IV) species, occurs in the last stage of the oxoiron(IV) formation process.

Highly Unsaturated Binuclear Butadiene Iron Carbonyls: Quintet Spin States, Perpendicular Structures, Agostic Hydrogen Atoms, and Iron-Iron Multiple Bonds

The highly unsaturated binuclear butadiene iron carbonyls (C4H6)2Fe2(CO)n (n = 2, 1) have been examined using density functional theory. For (C4H6)2Fe2(CO)n (n = 2, 1), both coaxial and perpendicular structures are found. The global minima of (C4H6)2Fe2(CO)n (n = 2, 1) are the perpendicular structures 2Q-1 and 1Q-1, respectively, with 17- and 15-electron configurations for the iron atoms leading to quintet spin states. The Fe=Fe distance of 2.361 Å (M06-L) in the (C4H6)2Fe2(CO)2 structure 2Q-1 suggests a formal double bond. The Fe≡Fe bond distance in the (C4H6)2Fe2(CO) structure 1Q-1 is even shorter at 2.273 Å (M06-L), suggesting a triple bond. Higher energy (C4H6)2Fe2(CO)n (n = 2, 1) structures include structures in which a bridging butadiene ligand is bonded to one of the iron atoms as a tetrahapto ligand and to the other iron atom through two agostic hydrogen atoms from the end CH2 groups. Singlet (C4H6)2Fe2(CO) structures with formal Fe–Fe quadruple bonds of lengths ∼2.05 Å were also found but at very high energies (∼47 kcal/mol) relative to the global minimum.

Exchange-Enhanced H-Abstraction Reactivity of High-Valent Nonheme Iron(IV)-Oxo from Coupled Cluster and Density Functional Theories

This letter reports a computational study of methane hydrogen abstraction (HAT) process by several nonheme high-valent iron(IV)-oxo model oxidants, in the quintet and triplet spin states, employing coupled-cluster (RCCSD(T)) and density functional theory (DFT) methods. The exchange-enhanced HAT reactivity of the quintet state found previously by B3LYP calculations is confirmed in this work by means of the RCCSD(T) calculations. In addition, we benchmark herein 29 functionals against the RCCSD(T) results. It is found that B3LYP gives very good triplet-state transition state (TS) geometries, while its quintet-state TS is early relative to RCCSD(T). TPSSh and M06L perform well in TS geometry optimization. However, all 29 tested DFT methods underestimate the quintet-state HAT barriers, and most of them underestimate the triplet-state HAT barriers as well. Although B3LYP, B3LYP-D, and TPSSh underestimate both quintet- and triplet-state barriers, their barriers, and quintet-triplet differences thereof, in a series of reactions correlate extremely well with the corresponding RCCSD(T) quantities.

DFT study of a model system for the dealkylation step catalyzed by AlkB

E. coli AlkB is a DNA repair enzyme that catalyzes the de-methylation of DNA by means of a non-heme iron and alpha-keto glutarate as a co-factor. The proposed reaction mechanism can be separated in four stages. The first stage involves the binding of the co-factor and molecular oxygen to the Fe in the active site. This is followed by the formation of a ferryl intermediate in a high-spin state, along with CO(2) and succinate. Subsequently, the O atom on the Fe center is reoriented. The last stage comprises the oxidative de-methylation of the base to produce the native DNA base and formaldehyde. This stage also includes the rate limiting step in the reaction. Here, the last stage of the proposed reaction mechanism of AlkB has been studied for a model of the active site with DFT methods. Minimum structures have been calculated for all intermediates along the path in triplet and quintet spin states. Our results point to the quintet states as more stable, in agreement with previously reported calculations. Potential energy barriers have been obtained for all the steps along this last stage in the quintet state. In the first step the oxygen bound to the Fe center of the ferryl intermediate abstracts a hydrogen atom from the methyl moiety. This first step corresponds to the rate limiting step in the reaction. The calculated barrier for this step is 26.7 kcal/mol. The subsequent steps are highly exoergic. This energetic picture is in qualitative agreement with previously reported results. The calculated energy difference between the ferryl intermediate and the final product is -75.7 kcal/mol for a model with succinate in the active site and -49.3 kcal/mol for a model where the succinate is replaced by water. Our calculated mechanism is slightly different than the previously reported one. These results suggest the possibility of more than one mechanism. This is currently under investigation by ab initio QM/MM methods.

Unsaturation and Variable Hapticity in Binuclear Azulene Iron Carbonyl Complexes

The bicyclic hydrocarbon azulene has long been known to form a variety of metal carbonyl derivatives with diverse metals. In this connection the binuclear azulene iron carbonyls C10H8Fe2(CO)n (n = 6, 5, 4, 3, 2, 1) have been investigated herein. For C10H8Fe2(CO)6 the singlet cis and trans structures with the two iron atoms on the same side or opposite sites of the azulene ring system were found to be of comparable energies. The lowest energy structure for C10H8Fe2(CO)5 is the experimentally known singlet cis-η5,η3-C10H8Fe2(CO)5 structure with the C5 ring bonded to the Fe(CO)2 group as a pentahapto ligand and the C7 ring bonded to the Fe(CO)3 group as a trihapto ligand and with a formal Fe−Fe single bond length of 2.83 ± 0.05 Å. This structure is thermodynamically stable with respect to disproportionation into C10H8Fe2(CO)6 + C10H8Fe2(CO)4 by 31 ± 3 kcal/mol and resistant toward CO dissociation by 45 ± 3 kcal/mol, consistent with the fact that it is readily obtained in the laboratory by simple thermal reactions of azulene with iron carbonyls. The lowest energy C10H8Fe2(CO)4 structure is also a singlet cis-η5,η3-C10H8Fe2(CO)4 structure. This compound is predicted to have either a four-electron donor bridging η2-μ-CO group and a formal Fe−Fe single bond of length 2.73 Å by B3LYP or, more likely, all two-electron donor carbonyl groups and a formal Fe═Fe double bond of length 2.48 Å by BP86. The lowest energy structures of the more highly unsaturated C10H8Fe2(CO)n (n = 3, 2, 1) are triplet (n = 3) or quintet (n = 2, 1) spin states with both iron atoms on the same side of the azulene system. In these systems all 10 carbon atoms of the azulene system are within bonding distance of an iron atom with a short enough iron−iron distance to suggest multiple bonding. The lowest energy C10H8Fe2(CO)n (n = 4, 3) structures are predicted to be either thermodynamically unstable or marginally stable with respect to disproportionation into C10H8Fe2(CO)n+1 + C10H8Fe2(CO)n−1. This again is consistent with the experimental observation of cis-C10H8Fe2(CO)5 as the major product from the thermal reaction of azulene with iron carbonyls.

Inverted Sandwich Type Dinuclear Chromium(I) Complex and Its Analogues of Scandium(I), Titanium(I), Vanadium(I), Manganese(I), and Iron(I): Theoretical Study of Electronic Structure and Bonding Nature

Inverted sandwich type chromium(I) complexes, (mu-eta(6):eta(6)-C(6)H(5)CH(3))[Cr(DDP)](2) (DDPH = 2-(4-{(2,6-diisopropylphenyl)imino}pent-2-ene) and (mu-eta(6):eta(6)-C(6)H(5)CH(3))[V(DDP)](2), synthesized by Tsai et al., and (mu-eta(6):eta(6)-C(6)H(6))[Cr(DDP)](2) synthesized by Monillas et al., were theoretically investigated with MRMP2 and DFT methods, where model compounds (mu-eta(6):eta(6)-C(6)H(6))[Cr(AIP)](2) (AIPH = (Z)-1-amino-3-imino-prop-1-ene) were mainly employed. Both computational methods succeeded in reproducing the experimental facts that the chromium and vanadium complexes take surprisingly high spin states, septet and quintet spin states, respectively. MO diagrams of these complexes present a clear understanding of the reasons why they take such high spin states. We also calculated their analogues, (mu-eta(6):eta(6)-C(6)H(6))[M(DDP)](2) (M = Sc, Ti, Mn, or Fe). The spin multiplicities of the Sc and Ti complexes were calculated to be singlet and triplet, respectively, by the DFT(B3LYP) method. Those of Mn and Fe complexes were calculated to be quintet and triplet, respectively, by the DFT(B3LYP) method, but nonet and singlet, respectively, by the MRMP2 method, suggesting that the DFT method cannot be applied to these complexes. The MRMP2 calculations indicate that the spin multiplicity increases upon going to Mn from Sc and reaches the maximum, nonet spin state, at Mn, and then suddenly decreases to singlet at Fe. This interesting change in spin multiplicity is discussed in terms of occupation of MOs.

High-Field EPR and Magnetic Susceptibility Studies on Binuclear and Tetranuclear Copper Trifluoroacetate Complexes. X-ray Structure Determination of Three Tetranuclear Quinoline Adducts of Copper(II) Trifluoroacetate

Magnetic properties and high-field EPR spectra of three previously unknown tetranuclear quinoline adducts of copper(II) trifluoroacetate were studied, and their X-ray structures were determined. Two green complexes containing a mu(4)-oxo bridge, Cu(4)O(CF(3)COO)(6)(quin)(4) x (C(6)H(5)CH(3))(0.6) (orthorhombic, Pbca, a = 15.278(3), b = 23.227(5), c = 34.895(7) A) and Cu(4)O(CF(3)COO)(6)(quin)(4) x (C(6)H(6))(0.8) (monoclinic, P2/c, a = 21.933(4), b = 11.176(2), c = 23.927(5) A, beta = 97.41(3) degrees) are ferromagnetic, whereas the blue complex [Cu(4)(OH)(2)(CF(3)COO)(8)(quin)(2)](2-)(quinH(+))(2) (monoclinic, a = 21.933(4), b = 11.176(2), c = 23.927(5) A, beta = 97.41(3) degrees), formed in humid air from the solid dimeric [Cu(CF(3)COO)(2) x (quin)](2), is antiferromagnetic, as is the tetranuclear blue product formed in humid air of the mu(4)-oxo tetramers. High-field EPR spectra allowed determination of the spin Hamiltonian parameters for the spin quintet state (S = 2) in the ferromagnetic complexes, which facilitated accurate interpretation of their magnetic susceptibility data. "Broken symmetry" DFT calculations were performed to estimate the exchange integrals in all three tetranuclear complexes, showing surprisingly good agreement with experimental results. Negative sign of the zero-field splitting parameter D in two binuclear complexes, [Cu(CF(3)COO)(2) x CH(3)CN](2) and [Cu(CF(3)COO)(2) x (quin)](2), was found from single-crystal high-field EPR spectra, confirming recent results for nonhalogenated dimeric copper carboxylates.

Why Do Cysteine Dioxygenase Enzymes Contain a 3-His Ligand Motif Rather than a 2His/1Asp Motif Like Most Nonheme Dioxygenases?

Density functional theory calculations on the oxygen activation process in cysteine dioxygenase (CDO) and three active site mutants whereby one histidine group is replaced by a carboxylic acid group are reported. The calculations predict an oxygen activation mechanism that starts from an Fe(III)-O-O(*) complex that has close lying singlet, triplet, and quintet spin states. A subsequent spin state crossing to the quintet spin state surfaces leads to formation of a ring-structure whereby an O-S bond is formed. This weakens the central O-O bond, which is subsequently broken to give sulfoxide and an iron-oxo complex. The second oxygen atom is transferred to the substrate after a rotation of the sulfoxide group. A series of calculations were performed on cysteine dioxygenase mutants with a 2His/1Asp motif rather than a 3His motif. These calculations focused on the differences in catalytic and electronic properties of nonheme iron systems with a 3His ligand system versus a 2His/1Asp motif, such as taurine/alpha-ketoglutarate dioxygenase (TauD), and predict why CDO has a 3His ligand system while TauD and other dioxygenases share a 2His/1Asp motif. One mutant (H86D) had the ligand trans to the dioxygen group replaced by acetate, while in another set of calculations the ligand trans to the sulfur group of cysteinate was replaced by acetate (H88D). The calculations show that the ligands influence the spin state ordering of the dioxygen bound complexes considerably and in particular stabilize the quintet spin state more so that the oxygen activation step should encounter a lower energetic cost in the mutants as compared to WT. Despite this, the mutant structures require higher O-O bond breaking energies. Moreover, the mutants create more stable iron-oxo complexes than the WT, but the second oxygen atom transfer to the substrate is accomplished with much higher reaction barriers than the WT system. In particular, a ligand trans to the sulfur atom of cysteine that pushes electrons to the iron will weaken the Fe-S bond and lead to dissociation of this bond in an earlier step in the catalytic cycle than the WT structure. On the other hand, replacement of the ligand trans to the dioxygen moiety has minor effects on cysteinate binding but enhances the barriers for the second oxygen transfer process. These studies have given insight into why cysteine dioxygenase enzymes contain a 3His ligand motif rather than 2His/1Asp and show that the ligand system is essential for optimal dioxygenation activity of the substrate. In particular, CDO mutants with a 2His/1Asp motif may give sulfoxides as byproduct due to incomplete dioxygenation processes.