Systematic theoretical study of the zero-field splitting in coordination complexes of Mn(III). Density functional theory versus multireference wave function approaches

Origin of the Zero-Field Splitting in Mononuclear Octahedral Mn(IV) Complexes: A Combined Experimental and Theoretical Investigation

The aim of this work was to determine and understand the origin of the electronic properties of Mn(IV) complexes, especially the zero-field splitting (ZFS), through a combined experimental and theoretical investigation on five well-characterized mononuclear octahedral Mn(IV) compounds, with various coordination spheres (N6, N3O3, N2O4 in both trans (trans-N2O4) and cis configurations (cis-N2O4) and O4S2). High-frequency and -field EPR (HFEPR) spectroscopy has been applied to determine the ZFS parameters of two of these compounds, MnL(trans-N2O4) and MnL(O4S2). While at X-band EPR, the axial-component of the ZFS tensor, D, was estimated to be +0.47 cm(-1) for MnL(O4S2), and a D-value of +2.289(5) cm(-1) was determined by HFEPR, which is the largest D-magnitude ever measured for a Mn(IV) complex. A moderate D value of -0.997(6) cm(-1) has been found for MnL(trans-N2O4). Quantum chemical calculations based on two theoretical frameworks (the Density Functional Theory based on a coupled perturbed approach (CP-DFT) and the hybrid Ligand-Field DFT (LF-DFT)) have been performed to define appropriate methodologies to calculate the ZFS tensor for Mn(IV) centers, to predict the orientation of the magnetic axes with respect to the molecular ones, and to define and quantify the physical origin of the different contributions to the ZFS. Except in the case of MnL(trans-N2O4), the experimental and calculated D values are in good agreement, and the sign of D is well predicted, LF-DFT being more satisfactory than CP-DFT. The calculations performed on MnL(cis-N2O4) are consistent with the orientation of the principal anisotropic axis determined by single-crystal EPR, validating the calculated ZFS tensor orientation. The different contributions to D were analyzed demonstrating that the d-d transitions mainly govern D in Mn(IV) ion. However, a deep analysis evidences that many factors enter into the game, explaining why no obvious magnetostructural correlations can be drawn in this series of Mn(IV) complexes.

Determination and prediction of the magnetic anisotropy of Mn ions

EPR spectroscopy combined with quantum chemistry for the investigation of the magnetic anisotropy of Mn^II, Mn^III and Mn^IV.

A Mn(iii) single ion magnet with tridentate Schiff-base ligands

Single ion magnet behaviour is reported for a mononuclear Mn(iii) ion with tridentate Schiff-base ligands revealing that the size of the axial anisotropy is the highest reported to date.

Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)-Porphyrin Complex

We report on a novel manganese(III)-porphyrin complex with the formula [Mn(III) (TPP)(3,5-Me2 pyNO)2 ]ClO4 ⋅CH3 CN (2; 3,5-Me2 pyNO=3,5-dimethylpyridine N-oxide, H2 TPP=5,10,15,20-tetraphenylporphyrin), in which the Mn(III) ion is six-coordinate with two monodentate 3,5-Me2 pyNO molecules and a tetradentate TPP ligand to build a tetragonally elongated octahedral geometry. The environment in 2 is responsible for the large and negative axial zero-field splitting (D=-3.8 cm(-1) ), low rhombicity (E/|D|=0.04) of the high-spin Mn(III) ion, and, ultimately, for the observation of slow magnetic-relaxation effects (Ea =15.5 cm(-1) at H=1000 G) in this rare example of a manganese-based single-ion magnet (SIM). Structural, magnetic, and electronic characterizations were carried out by means of single-crystal diffraction studies, variable-temperature direct- and alternating-current measurements and high-frequency and -field EPR spectroscopic analysis followed by quantum-chemical calculations. Slow magnetic-relaxation effects were also observed in the already known analogous compound [Mn(III) (TPP)Cl] (1; Ea =10.5 cm(-1) at H=1000 G). The results obtained for 1 and 2 are compared and discussed herein.

Origin of the relaxation barriers in a family of MReIV(CN)2 single-chain magnets (M = MnII, NiII, and CoII): a theoretical investigation

Theoretical exploration of the Δ_ξ and Δ_A in terms of D, J, and S in three MRe^IV(CN)₂ single-chain magnets.

Calcium and heterometallic manganese–calcium complexes supported by tripodal pyridine-carboxylate ligands: structural, EPR and theoretical investigations

Rare examples of heteronuclear μ-carboxylato bridged Mn–Ca complexes are reported.

A first-principles approach to the calculation of the on-site zero-field splitting in polynuclear transition metal complexes

The interpretation of electron paramagnetic resonance spectra of polynuclear transition metal complexes in terms of individual contributions from each paramagnetic center can be greatly facilitated by the availability of theoretical methods that enable the reliable prediction of local spectroscopic parameters. In this work we report an approach that enables the application of multireference ab initio methods for the calculation of local zero field splitting tensors, one of the leading terms in the spin Hamiltonian for exchange-coupled systems of high nuclearity. The method referred to as local complete active space configuration interaction (L-CASCI) represents a multireference calculation with an active space composed of local orbitals of the center of interest. By successive permutation of the active space to include the localized orbitals corresponding to a particular center of the complex, all on-site parameters can be easily obtained at a high-level of theory with a corresponding low computational cost. Benchmark calculations on synthetic complexes confirm the validity of the approach. As an example of the applicability of the L-CASCI method to large systems, we determine the local anisotropy of the Mn(III) ion of the tetranuclear manganese cluster of photosystem II in both structural forms of its S2 state.

Magnetic anisotropy of mononuclear Ni(II) complexes: on the importance of structural diversity and the structural distortions

Mononuclear Ni(II) complexes are particularly attractive in the area of single-molecule magnets as the axial zero-field splitting (D) for the Ni(II) complexes is in the range of -200 to +200 cm(-1) . Despite this advantage, very little is known on the origin of anisotropy across various coordination ligands, coordination numbers, and particularly what factors influence the D parameter in these complexes. To answer some of these questions, herein we have undertaken a detailed study of a series of mononuclear Ni(II) complexes with ab initio calculations. Our results demonstrate that three prominent spin-conserved low-lying d-d transitions contribute significantly to the D value. Variation in the sign and the magnitude of D values are found to correlate to the specific structural distortions. Apart from the metal-ligand bond lengths, two different parameters, namely, Δα and Δβ, which are correlated to the cis angles present in the coordination environment, are found to significantly influence the axial D values. Developed magneto-structural D correlations suggest that the D values can be enhanced significantly by fine tuning the structural distortion in the coordination environment. Calculations performed on a series of Ni(II) models with coordination numbers two to six unfold an interesting observation-the D parameter increases significantly upon a reduction in coordination number compared with a reference octahedral coordination. Besides, if high symmetry is maintained, even larger coordination numbers yield large D values.

Assigning the EPR fine structure parameters of the Mn(II) centers in Bacillus subtilis oxalate decarboxylase by site-directed mutagenesis and DFT/MM calculations

Oxalate decarboxylase (OxDC) catalyzes the Mn-dependent conversion of the oxalate monoanion into CO2 and formate. EPR-based strategies for investigating the catalytic mechanism of decarboxylation are complicated by the difficulty of assigning the signals associated with the two Mn(II) centers located in the N- and C-terminal cupin domains of the enzyme. We now report a mutational strategy that has established the assignment of EPR fine structure parameters to each of these Mn(II) centers at pH 8.5. These experimental findings are also used to assess the performance of a multistep strategy for calculating the zero-field splitting parameters of protein-bound Mn(II) ions. Despite the known sensitivity of calculated D and E values to the computational approach, we demonstrate that good estimates of these parameters can be obtained using cluster models taken from carefully optimized DFT/MM structures. Overall, our results provide new insights into the strengths and limitations of theoretical methods for understanding electronic properties of protein-bound Mn(II) ions, thereby setting the stage for future EPR studies on the electronic properties of the Mn(II) centers in OxDC and site-specific variants.

Can anisotropic exchange be reliably calculated using density functional methods? A case study on trinuclear Mn(III)-M(III)-Mn(III) (M=Fe, Ru, and Os) cyanometalate single-molecule magnets

Density functional studies have been performed on a set of trinuclear single-molecule magnets (SMMs) of general formula [{Mn2(5-Br salen)2(MeOH)2}M(CN)6](NEt4) (M=Fe(III) (1), Ru(III) (2) and Os(III) (3); 5-Brsalen=N,N'-ethylenebis(5-bromosalicylidene)iminato anion). We have computed the orbital-dependent exchange interaction for all three complexes for the first time using DFT and complete active space self-consistent field (CASSCF) methods. DFT calculations yield the anisotropic exchange as J(ξξ)=3.5 cm(-1) for 1; J(ξξ)=12.1 cm(-1), J(ζζ)=-6.9 cm(-1) and J(ηη)=-14 cm(-1) for 2; and J(ξξ)=23.7 cm(-1) and J(ζζ) =-11.1 cm(-1) for 3. The computed values are in agreement with the experimental report, and this suggests that the established methodology can be used to compute the anisotropic exchange in larger clusters. Our calculations reiterate the fact that the exchange is described by a three-axis anisotropic exchange for complexes 2 and 3 as evidenced by the experiments. A stronger exchange coupling as we move down the periodic table from 3d to 5d is reproduced by our calculations, and the origin of this enhancement in the exchange interaction has been probed by using molecular orbital analysis. The electronic origin of different types of exchange observed in this series is found to be related to the energy difference between possible degenerate pairs and the nature of orbital interactions. By computing the exchange interaction, the single-ion anisotropy of Mn(III) and zero-field splitting of the S=9/2 ground state of complexes 1-3 using CASSCF and/or DFT methods, we have attempted to shed light on the issue of anisotropic exchange and the barrier height for the magnetisation reversal in SMMs. Comprehensive magneto-structural correlations have been developed to offer clues on how to further enhance the barrier height in this class of SMMs.

High-affinity manganese coordination by human calprotectin is calcium-dependent and requires the histidine-rich site formed at the dimer interface

Calprotectin (CP) is a transition metal-chelating antimicrobial protein of the calcium-binding S100 family that is produced and released by neutrophils. It inhibits the growth of various pathogenic microorganisms by sequestering the transition metal ions manganese and zinc. In this work, we investigate the manganese-binding properties of CP. We demonstrate that the unusual His(4) motif (site 2) formed at the S100A8/S100A9 dimer interface is the site of high-affinity Mn(II) coordination. We identify a low-temperature Mn(II) spectroscopic signal for this site consistent with an octahedral Mn(II) coordination sphere with simulated zero-field splitting parameters D = 270 MHz and E/D = 0.30 (E = 81 MHz). This analysis, combined with studies of mutant proteins, suggests that four histidine residues (H17 and H27 of S100A8; H91 and H95 of S100A9) coordinate Mn(II) in addition to two as-yet unidentified ligands. The His(3)Asp motif (site 1), which is also formed at the S100A8/S100A9 dimer interface, does not provide a high-affinity Mn(II) binding site. Calcium binding to the EF-hand domains of CP increases the Mn(II) affinity of the His(4) site from the low-micromolar to the mid-nanomolar range. Metal-ion selectivity studies demonstrate that CP prefers to coordinate Zn(II) over Mn(II). Nevertheless, the specificity of Mn(II) for the His(4) site provides CP with the propensity to form mixed Zn:Mn:CP complexes where one Zn(II) ion occupies site 1 and one Mn(II) ion occupies site 2. These studies support the notion that CP responds to physiological calcium ion gradients to become a high-affinity transition metal ion chelator in the extracellular space where it inhibits microbial growth.

Azurin as a protein scaffold for a low-coordinate nonheme iron site with a small-molecule binding pocket

The apoprotein of Pseudomonas aeruginosa azurin binds iron(II) to give a 1:1 complex, which has been characterized by electronic absorption, Mössbauer, and NMR spectroscopies, as well as X-ray crystallography and quantum-chemical computations. Despite potential competition by water and other coordinating residues, iron(II) binds tightly to the low-coordinate site. The iron(II) complex does not react with chemical redox agents to undergo oxidation or reduction. Spectroscopically calibrated quantum-chemical computations show that the complex has high-spin iron(II) in a pseudotetrahedral coordination environment, which features interactions with side chains of two histidines and a cysteine as well as the C═O of Gly45. In the (5)A(1) ground state, the d(z(2)) orbital is doubly occupied. Mutation of Met121 to Ala leaves the metal site in a similar environment but creates a pocket for reversible binding of small anions to the iron(II) center. Specifically, azide forms a high-spin iron(II) complex and cyanide forms a low-spin iron(II) complex.

Origin of the magnetic anisotropy in heptacoordinate Ni(II) and Co(II) complexes

The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni(II) and Co(II) complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters D of [Ni(H(2)DAPBH)(H(2)O)(2)](NO(3))(2)⋅2 H(2)O (1) and [Co(H(2)DAPBH)(H(2)O)(NO(3))](NO(3)) [2; H(2)DAPBH = 2,6-diacetylpyridine bis- (benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni(II) complex indicates stabilization of the highest M(S) value of the S = 1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co(II) indicates stabilization of the M(S) = ±1/2 sublevels of the S = 3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni(II) complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d(xy) and d(x(2)-y(2)) orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co(II), all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.

Investigating magnetostructural correlations in the pseudooctahedral trans-[Ni(II){(OPPh2)(EPPh2)N}2(sol)2] complexes (E = S, Se; sol = DMF, THF) by magnetometry, HFEPR, and ab initio quantum chemistry

In this work, magnetometry and high-frequency and -field electron paramagnetic resonance spectroscopy (HFEPR) have been employed in order to determine the spin Hamiltonian (SH) parameters of the non-Kramers, S = 1, pseudooctahedral trans-[Ni(II){(OPPh(2))(EPPh(2))N}(2)(sol)(2)] (E = S, Se; sol = DMF, THF) complexes. X-ray crystallographic studies on these compounds revealed a highly anisotropic NiO(4)E(2) coordination environment, as well as subtle structural differences, owing to the nature of the Ni(II)-coordinated solvent molecule or ligand E atoms. The effects of these structural characteristics on the magnetic properties of the complexes were investigated. The accurately HFEPR-determined SH zero-field-splitting (zfs) D and E parameters, along with the structural data, provided the basis for a systematic density functional theory (DFT) and multiconfigurational ab initio computational analysis, aimed at further elucidating the electronic structure of the complexes. DFT methods yielded only qualitatively useful data. However, already entry level ab initio methods yielded good results for the investigated magnetic properties, provided that the property calculations are taken beyond a second-order treatment of the spin-orbit coupling (SOC) interaction. This was achieved by quasi-degenerate perturbation theory, in conjunction with state-averaged complete active space self-consistent-field calculations. The accuracy in the calculated D parameters improves upon recovering dynamic correlation with multiconfigurational ab initio methods, such as the second-order N-electron valence perturbation theory NEVPT2, the difference dedicated configuration interaction, and the spectroscopy-oriented configuration interaction. The calculations showed that the magnitude of D (∼3-7 cm(-1)) in these complexes is mainly dominated by multiple SOC contributions, the origin of which was analyzed in detail. In addition, the observed largely rhombic regime (E/D = 0.16-0.33) is attributed to the highly distorted metal coordination sphere. Of special importance is the insight by this work on the zfs effects of Se coordination to Ni(II). Overall, a combined experimental and theoretical methodology is provided, as a means to probe the electronic structure of octahedral Ni(II) complexes.

How Do Heavier Halide Ligands Affect the Signs and Magnitudes of the Zero-Field Splittings in Halogenonickel(II) Scorpionate Complexes? A Theoretical Investigation Coupled to Ligand-Field Analysis

This work presents a detailed analysis of the physical origin of the zero-field splittings (ZFSs) in a series of high-spin (S = 1) nickel(II) scorpionate complexes Tp*NiX (Tp* = hydrotris(3,5-dimethylpyrazole)borate, X = Cl, Br, I) using quantum chemical approaches. High-frequency and -field electron paramagnetic resonance studies have shown that the complexes with heavier halide ligands (Br, I) have greater magnitudes but opposite signs of the ZFSs compared with the chloro congener (Desrochers, P. J.; Telser, J.; Zvyagin, S. A.; Ozarowski, A.; Krzystek, J.; Vicic, D. A. Inorg. Chem.2006, 45, 8930-8941). To rationalize the experimental findings, quantum chemical calculations of the ZFSs in this Ni(II) halide series have been conducted. The computed ZFS using wave-function-based ab initio methods (state-averaged CASSCF, NEVPT2, and SORCI) are in good agreement with the experiment. For comparison, density functional theory was only marginally successful. The ligand-field analysis demonstrates that the signs and magnitudes of the ZFSs are subtly determined by the trade-off between the negative contributions from the (1,3)A1(1e→2e) transitions relative to the positive contributions from the remaining d-d excited states. The term from (1,3)A1(1e→2e) stems from the structural feature that the metal center displaces out of the equatorial plane, and gains the importance when heavier halide ligand is involved.

Substrate and metal control of barrier heights for oxo transfer to Mo and W bis-dithiolene sites

Reaction coordinates for oxo transfer from the substrates Me(3)NO, Me(2)SO, and Me(3)PO to the biologically relevant Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)(2)](-) where mdt = 1,2-dimethyl-ethene-1,2-dithiolate(2-), and from Me(2)SO to the analogous W(IV) complex, have been calculated using density functional theory. In each case, the reaction first proceeds through a transition state (TS1) to an intermediate with substrate weakly bound, followed by a second transition state (TS2) around which breaking of the substrate X-O bond begins. By analyzing the energetic contributions to each barrier, it is shown that the nature of the substrate and metal determines which transition state controls the rate-determining step of the reaction.