Intermediate Spin Ground State Research Articles

Light-Induced Spin Crossover in an Intermediate-Spin Penta-Coordinated Iron(III) Complex.

(PMe3)2FeCl3 is an Fe(III) complex that exists in the intermediate-spin ground state in a distorted trigonal bipyramidal geometry. An electronic state with high-spin configuration lies in close vicinity to the ground state, making it a potential spin crossover candidate. A mechanistic account of the spin crossover from the lowest quartet state (Q0) to the lowest sextet state (S1) of this complex is provided by exploring both thermal and light-induced pathways. The presence of a large barrier between the two spin states suggests a possible thermal spin crossover at a rather high temperature. The light-induced spin crossover is investigated by employing complete active space self-consistent field calculations together with dynamic correlation and spin-orbit coupling for the lowest seven quartet and lowest five sextet states. The system in the Q0 state upon light absorption is excited to the optically bright Q4 LMCT state. By following minimum energy pathways along the electronic states, two light-induced pathways for spin crossover are identified. From the Q4 state, the system can photo-regenerate the ground intermediate-spin state (Q0) through an internal conversion of Q4/Q3 followed by Q3/S1 and S1/Q0 intersystem crossings. In an alternate route, through Q4/S2 intersystem crossing followed by S2/S1 internal conversion, the system can complete the spin crossover from the Q0 to S1 state.

Ab initio investigation of magnetic anisotropy in intermediate spin iron(iii) complexes.

Mononuclear Fe(iii) complexes commonly exist in high-spin or low-spin states, whereas their occurrence in the intermediate-spin state (S = 3/2) is scarce. The magnetic anisotropy in two trigonal-bipyramidal mononuclear Fe(iii) complexes, (1) and (2), in their intermediate-spin ground state has been examined by ab initio electronic structure calculations. The calculations successfully reproduce the experimental magnetic anisotropic barrier, U eff in 1 (81 cm-1) and 2 (42 cm-1), which is shown to arise due to thermally assisted quantum tunneling of magnetization from the second Kramer's doublets. The magnetic anisotropy in both the complexes is found to be significantly influenced by the axial ligands, while the equatorial ligands have negligible contribution. The large reduction in U eff of 2 has been shown to arise due to the phenyl groups, which results in the lifting of orbital degeneracy of e″ and e' frontier orbitals and leads to a net quenching of the orbital angular momentum of the metal center causing a diminished spin-orbit splitting in 2. While the crystal structure of 2 shows two phenyl rings out of plane to each other, the present study discovered another stable conformation of 2, where the two phenyl rings are in the same plane (2a). Unlike 2, the planarity of the two phenyl rings in 2a restores the degeneracy of the frontier orbitals, thereby increasing the spin-orbit splitting and a consequent rise in U eff from 42 to 80 cm-1 in 2a.

A Strongly Spin-Frustrated Fe(III) 7 Complex with a Canted Intermediate Spin Ground State of S=7/2 or 9/2.

A disk-shaped [Fe(III) 7 (Cl)(MeOH)6 (μ3 -O)3 (μ-OMe)6 (PhCO2 )6 ]Cl2 complex with C3 symmetry has been synthesised and characterised. The central tetrahedral Fe(III) is 0.733 Å above the almost co-planar Fe(III) 6 wheel, to which it is connected through three μ3 -oxide bridges. For this iron-oxo core, the magnetic susceptibility analysis proposed a Heisenberg-Dirac-van Vleck (HDvV) mechanism that leads to an intermediate spin ground state of S=7/2 or 9/2. Within either of these ground state manifolds it is reasonable to expect spin frustration effects. The (57) Fe Mössbauer (MS) analysis verifies that the central Fe(III) ion easily aligns its magnetic moment antiparallel to the externally applied field direction, whereas the other six peripheral Fe(III) ions keep their moments almost perpendicular to the field at stronger fields. This unusual canted spin structure reflects spin frustration. The small linewidths in the magnetic Mössbauer spectra of polycrystalline samples clearly suggest an isotropic exchange mechanism for realisation of this peculiar spin topology.

Interaction Modes and Absolute Affinities of α‐Amino Acids for Mn2+: A Comprehensive Picture

Manganese is involved as a cofactor in the activation of numerous enzymes as well as the oxygen-evolving complex of photosystem II. Full understanding of the role played by the Mn(2+) ion requires detailed knowledge of the interaction modes and energies of manganese with its various environments, a knowledge that is far from complete. To bring detailed insight into the local interactions of Mn in metallopeptides and proteins, theoretical studies employing first-principles quantum mechanical calculations are carried out on [Mn-amino acid](2+) complexes involving all 20 natural α-amino acids (AAs). Detailed investigation of [Mn-serine](2+), [Mn-cysteine](2+), [Mn-phenylalanine](2+), [Mn-tyrosine](2+), and [Mn-tryptophan](2+) indicates that with an electron-rich side chain, the most stable species involves interaction of Mn(2+) with carbonyl oxygen, amino nitrogen, and an electron-rich section of the side chain of the AA in its canonical form. This is in sharp contrast with aliphatic side chains for which a salt bridge is formed. For aromatic AAs, complexation to manganese leads to partial oxidation as well as aromaticity reduction. Despite multisite binding, AAs do not generate strong enough ligand fields to switch the metal to a low- or even intermediate-spin ground state. The affinities of Mn(2+) for all AAs are reported at the B3LYP and CCSD(T) levels of theory, thereby providing the first complete series of affinities for a divalent metal ion. The trends are compared with those of other cations for which affinities of all AAs have been previously obtained.

Novel in situ manganese-promoted double-aldol addition

A novel in situ Mnn+-promoted double-aldol reaction is reported. Single crystal X-ray measurements confirm the addition of acetone to two o-vanillin molecules in an original in situα,α double aldol reaction promoted by Mn ions in the presence of base. The newly formed ligands coordinate to four MnIII ions forming a defect-dicubane core structure (1) bridged exclusively by oxygen-based ligands. Other 3d metals were employed under the same reaction conditions, however no aldol addition occurred and tetranuclear cubane-like structures formed using CoII (2) and NiII (3) ions. Magnetic measurements were carried out on all complexes using SQUID magnetometry. Dominant ferromagnetic interactions were observed for complexes 1 and 3 with J=1.8cm−1, J′=−2.5cm−1, g=1.95 for 1 and J=3.1cm−1, g=2.17 for 3 while complex 2 exhibited antiferromagnetic exchange interactions. Notably, complex 1 was shown to exhibit spin frustration rarely seen in {MnIII}4 systems resulting in an intermediate spin ground state of ST=6.

Ab initiocalculated XANES and XMCD spectra of Fe(II) phthalocyanine

We report a density-functional calculation that predicts for iron(II) phthalocyanine (FePc) an unusual orbitally degenerate intermediate-spin ground state ${^{3}E}_{g}$. The calculated $\text{Fe}\text{ }{L}_{2,3}$ near-edge x-ray absorption spectrum agrees reasonably with experiment, whereas the presence of an unquenched orbital moment is corroborated by a recent discovery in $\ensuremath{\alpha}\text{-FePc}$ of a very large hyperfine field of positive sign [Filoti et al., Phys. Rev. B 74, 134420 (2006)]. It is suggested that the orbital moment of FePc should be observed more directly in x-ray magnetic circular dichroism experiments.

Intermediate spin ground state of an isosceles triangular [MnII3] complex

[{Mn(tmeda)}(3){GeSe(3)(OMe)}(2)] (), synthesized by the reaction of [K(4)(H(2)O)(3)][Ge(2)Se(6)] with MnCl(2).4H(2)O in MeOH-tmeda (tmeda = N,N,N',N'-tetramethyl-1,2-diaminoethane), exhibits an isosceles triangular arrangement of Mn(2+) ions that are antiferromagnetically coupled with an S = 3/2 ground state.

Iron Chelates of 2,2′‐Bidipyrrin: Stable Analogues of the Labile Iron Bilins

A unique series of halogenidoiron(III) complexes of the open-chain tetrapyrrolic ligand 2,2'-bidipyrrin (bpd) ([FeX(bdp)] X=F, Cl, Br, I) was prepared from simple pyrrolic and bipyrrolic precursors and iron chloride by a one-pot condensation/metalation strategy, followed by salt metathesis with CsF, LiBr, or NaI. Crystallographic analysis revealed that in all cases the 2,2'-bidipyrrin ligand is forced to reside in a helical conformation when bound to the iron atom. Whereas the extremely sensitive fluorido derivative was isolated as a CsF adduct and forms 1D polymeric chains in the solid state, the more stable chlorido, bromido, and iodido derivatives crystallize as discrete monomeric molecules with a distorted pentacoordinate iron(III) ion in an intermediate spin ground state. Magnetic susceptibility measurements and Mössbauer data of the compounds are in agreement with this interpretation. In solution, however, all the compounds are pentacoordinate with the iron atom in the high-spin (S=5/2) state and dynamic with respect to helix inversion. In the presence of air, the iron chelates react stepwise with the nucleophiles methanol and imidazolate at the tetrapyrrole terminal alpha,omega-positions, presumably through the hexacoordinate species [Fe(bdp)(MeOH)2]+ and [Fe(im)2-(bdp)](-), respectively. The successive increase of strain at these positions results in increasingly labile intermediates that spontaneously release the iron ion from the mono- or disubstituted tetrapyrrole ligands.

Syntheses, Structures and Magnetic Properties of Two Mixed‐Valent Disc‐like Hepta‐nuclear Compounds of [FeIIFeIII6(tea)6](ClO4)2 and [MnII3MnIII4(nmdea)6(N3)6]·CH3OH (tea = N(CH2CH2O)33−, nmdea = CH3N(CH2CH2O)22−)

AbstractTwo mixed‐valent disc‐like hepta‐nuclear compounds of [FeIIFeIII6(tea)6](ClO4)2 (1Fe, tea = N(CH2CH2O)33−) and [MnII3MnIII4(nmdea)6(N3)6]·CH3OH (2Mn, nmdea = CH3N(CH2CH2O)22−) have been synthesized by the reaction of Fe(ClO4)2·6H2O with triethanolamine (H3tea) for the former and reaction of Mn(ClO4)2·6H2O with diethanolamine (H2nmdea) and NaN3 for the later, respectively. 1Fe has the cationic cluster with a planar [FeIIFeIII6] core consisting of one central FeII and six rim FeIII atoms in hexagonal arrangement. The Fe ions are linked by the oxo‐bridges from the alcohol arms in the manner of edge‐sharing of their coordination octahedra. 2Mn is a neutral cluster with a [MnII3MnIII4] core possessing one central MnII atom surrounded by six rim Mn ions, two MnII and four MnIII. The structure is similar to 1Fe but involves six terminal azido ligands, each coordinate one rim Mn ion. 1Fe showed dominant antiferromagnetic interaction within the cluster and long‐range ordering at 2.7 K. The cluster probably has a ground state of low spin of S = 5/2 or 4/2. The long‐range ordering is weak ferromagnetic, showing small hysteresis with a remnant magnetization of 0.3 Nβ and a coercive field of 40 Oe. Moreover, the isofield of lines 1Fe are far from superposition, indicating the presence of significant zero–field splitting. Ferromagnetic interactions are dominant in 2Mn. An intermediate spin ground state 25/2 is observed at low field. In high field of 50 kOe, the energetically lowest state is given by the ms = 31/2 component of the S = 31/2 multiplet due to the Zeeman effect. Despite of the large ground state, no single‐molecule magnet behavior was found above 2 K.

Electronic Structure of Some Substituted Iron(II) Porphyrins. Are They Intermediate or High Spin?

The electronic structure of some substituted, four-coordinate iron(II) porphyrins has been investigated with DFT methods. These systems include iron tetraphenylporphine (FeTPP), iron octamethyltetrabenzporphine (FeOTBP), iron tetra(alpha,alpha,alpha,alpha-orthopivalamide)phenylporphine (FeTpivPP, also called "picket fence" porphyrin), halogenated iron porphyrins (FeTPPXn, X=F, Cl; n=20, 28), and iron octaethylporphine (FeOEP). A number of density functionals were used in the calculations. Different from the popular, intermediate-spin FeTPP, the ground states of FeOTBP, FeTPPCl28, and FeTPPF20betaCl8 are predicted to be high spin. The calculated result for FeOTBP is in agreement with the early experimental measurement, thereby changing the previous conclusion drawn from the calculations with only the BP functional (J. Chem. Phys. 2002, 116, 3635). But FeTpivPP might have an intermediate-spin ground state, a conclusion that is different from the "experimental" one. With a notably expanded Fe-N bond length, FeOEP might exist as an admixed-spin (S=1, 2) state. We also calculated the electron affinities (EAs) for the various iron porphyrins and compared them to experiment. On the basis of the calculated trends in the EAs and in the orbital energies, the experimental EAs for FeTpivPP, FeTPPF20, and FeTPPCl28 may be too small by 0.4-0.5 eV.

Microscopic model for superexchange interactions and photomagnetism in binuclear transition metal complexes

Recent experiments show that the superexchange interaction in molecular clusters containing transition metal ions A = NiII and B = WV, NbIV or MoV in some cases is antiferromagnetic, contrary to the conventional superexchange rules. To understand this anomaly, we develop a quantum many-body model Hamiltonian and solve it exactly using a valence bond (VB) approach. We identify the various model parameters which control the ground state spin in different clusters of the A-B system. We present quantum phase diagrams that delineate the high and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region of intermediate spin ground state in the parameter space, in clusters of larger size. The spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The above microscopic model is generic and can also be employed to explain photomagnetism in the MoCu6 system. We solve the model for MoCu6 and find that ground state is degenerate and is spanned by the S = 0, 1, 2 and 3 manifolds with doubly occupied Mo site corresponding to Mo(IV) and singly occupied Cu sites corresponding to Cu(II) configurations. In each of these spin spaces, we observe that there exist charge-transfer (CT) states at ≈3 eV above the ground state which are dipole coupled to the ground state. The transition dipole in the S = 3 manifold is the largest for the CT excitations. Coupled with the fact that the density of states of the S = 3 manifold is sparse, compared to other spin manifolds, we expect that the S = 3 CT excited state to be long-lived, thereby explaining the experimentally observed photomagnetism in the MoCu6 system.

Effects of Peripheral Substituents and Axial Ligands on the Electronic Structure and Properties of Iron Phthalocyanine

The effects of peripheral substituents and axial ligands on the electronic structure and properties of iron phthalocyanine, H(16)PcFe, have been investigated using a DFT method. Substitution by electron-withdrawing fluorinated groups alters the ground state of H(16)PcFe and gives rise to large changes in the ionization potentials and electron affinity. For the six-coordinate adducts with acetone, H(2)O, and pyridine, the axial coordination of two weak-field ligands leads to an intermediate-spin ground state, while the strong-field ligands make the system diamagnetic. The electronic configuration of a ligated iron phthalocyanine is determined mainly by the axial ligand-field strength but can also be affected by peripheral substituents. Axial ligands also exert an effect on ionization potentials and electron affinity and can, as observed experimentally, even change the site of oxidation/reduction.

Four-coordinate, planar RuII. A triplet state as a response to a 14-valence electron configuration.

The ligand (tBu2PCH2SiMe2)2N1- (PNP) in [PNP]RuCl leads to an intermediate spin ground state, S = 1, which has been characterized by NMR and X-ray diffraction as having a planar structure. This spin state is attributed in part to N --> Ru pi donation. DFT calculations confirm that the singlet state lies higher in energy and is nonplanar. The molecule is converted to a diamagnetic product by addition of 2 mol of PhCN. The half-filled orbitals of the S = 1 state are suggested to be the reason agostic interactions do not compensate for the 14-valence electron count.

MO-Theoretical Studies on a Model Complex for Deoxymyoglobin

Abstract The origin of the displacement of the Fe atom in deoxymyoglobin with respect to the porphyrin plane in the high-spin state is examined by a qualitative molecular orbital (MO) analysis on the extended Hückel level. We find that attachment of a fifth ligand (imidazole in our model complex) to Fe(II)porphyrin favors the out-of-plane shift due to a strengthening of the bonding interaction between Fe and the nitrogen of the imidazole ligand. This results in a high-spin (5 = 2) ground state with Fe shifted out-of-plane for the five-coordinate complex instead of an intermediate spin ground state (5 = 1) with Fe lying in the plane for four-coordinate Fe(II)porphyrin. The relative energies of the different spin states as a function of the distance between Fe and the porphyrin plane are evaluated using an ROHF (restricted open shell Hartree-Fock) version of an INDO (intermediate neglect of differential overlap) method. We observe a level crossing between high-spin and intermediate spin states whereas the low-spin (5 = 0) state remains always higher in energy.

Possibility for an intermediate-spin ground state in the charge-transfer material SrCoO3.

We discuss the possibility of an intermediate-spin ground state for a ${\mathit{d}}^{5}$ (${\mathit{d}}^{6}$) system. The intermediate-spin state is stabilized by the relative stability of the ligand hole state that it hybridizes with. Using atomic multiplet calculations we showed that an intermediate-spin ground state is possible for ${\mathrm{Co}}^{4+}$ (${\mathit{d}}^{5}$) when the ${\mathit{d}}^{6}$L state dominates the ground state. From a comparison of the experimental Co 2p x-ray absorption spectroscopy spectrum with the calculated one we assume an intermediate-spin ground state for ${\mathrm{SrCoO}}_{3}$. The intermediate-spin ground state is a highly symmetrical state with high-spin Co ${\mathit{d}}^{6}$ ions on each site. Each oxygen then contributes 1/3 hole which is antiferromagnetically coupled to both neighboring Co ions. In this way the itinerant oxygen holes couple the high-spin Co ${\mathit{d}}^{6}$ ions ferromagnetically. With this model of oxygen holes that introduce ferromagnetic correlations we can also explain the spin-glass behavior for slightly doped ${\mathrm{LaCoO}}_{3}$.

Ground-state variability in .mu.3-oxide trinuclear mixed-valence manganese complexes: spin frustration

A reexamination of bulk magnetic susceptibility data supplemented by variable-field magnetization and EPR studies definitively established the ground state of the mixed-valence complex [Mn II Mn III 2 O(O 2 CCH 3 ) 6 (py) 3 ](py) (1) as having S=3/2, in contrast to the S=1/2 ground state reported previously. This represents a rare example of a triangular μ 3 -oxide metal complex charaterized as having an intermediate spin ground state (i.e., ground-state spin value other than the minimum available to the system)

High nuclearity molecular species exhibiting spin frustration: fusion of two Mn III 4O2 butterfly complexes to yield an intermediate spin ground state Mn III 7O4 complex

The preparation and characterization of (NEt4)[Mn7O4(OAc)10(dbm)4]·3CH2Cl2·2C6H14(1·3CH2Cl2·2C6H14)(H dbm = dibenzoylmethane)are reported; the MnIII7O4 core of 1 consists of two Mn4O2 butterfly units fused together by sharing of one wing-tip manganese atom and it is shown using magnetization data that complex 1 has either an S= 3 or S= 4 ground state.

Near-infrared magnetic circular dichroism of cytochrome c'

The near-infrared magnetic circular dichroism (MCD) of Rhodospirillum rubrum, Chromatium vinosum, and Rhodopseudomonas palustris cytochromes c' are reported. The spectra of the reduced protein are very similar to those of deoxymyoglobin. The spectra of the oxidized proteins in the pD range 1-13 can be analyzed on the basis of four species A, B, C, and D. The existence of nine species, reported in a recent electron paramagnetic resonance study, is not substantiated. The MCD spectra support the assignment of B as high spin and C and D as low spin. The MCD of species A is close to that of high-spin proteins and does not support the recently proposed assignment of a mixed high- and intermediate-spin ground state for this species. The energies of the near-IR electronic transitions of all four oxidized species point to axial ligation via oxygen, assuming histidine to be the opposite axial ligand. Unfortunately, insufficient model compounds with ligation by carboxyl or hydroxyl moieties exist to enable more precise assignments.