On the general theory of magnetic susceptibilities of polynuclear transition-metal compounds

Strengths of covalent bonds in LnO2 determined from O K-edge XANES spectra using a Hubbard model

In LnO2 (Ln = Ce, Pr, and Tb), the amount of Ln 4f mixing with O 2p orbitals was determined by O K-edge X-ray absorption near edge (XANES) spectroscopy and was similar to the amount of mixing between the Ln 5d and O 2p orbitals. This similarity was unexpected since the 4f orbitals are generally perceived to be "core-like" and can only weakly stabilize ligand orbitals through covalent interactions. While the degree of orbital mixing seems incompatible with this view, orbital mixing alone does not determine the degree of stabilization provided by a covalent interaction. We used a Hubbard model to determine this stabilization from the energies of the O 2p to 4f, 5d(eg), and 5d(t2g) excited charge-transfer states and the amount of excited state character mixed into the ground state, which was determined using Ln L3-edge and O K-edge XANES spectroscopy. The largest amount of stabilization due to mixing between the Ln 4f and O 2p orbitals was 1.6(1) eV in CeO2. While this energy is substantial, the stabilization provided by mixing between the Ln 5d and O 2p orbitals was an order of magnitude greater consistent with the perception that covalent bonding in the lanthanides is largely driven by the 5d orbitals rather than the 4f orbitals.

Synthesis, Structural Versatility, Magnetic Properties, and I- Adsorption in a Series of Cobalt(II) Metal-Organic Frameworks with a Charge-Neutral Aliphatic (O,O)-Donor Bridge

Four new metal-organic frameworks based on cobalt(II) salts and 1,4-diazabicyclo[2.2.2]octane N,N'-dioxide (odabco) were obtained. Their crystallographic formulae are [Co3(odabco)2(OAc)6] (1, OAc- = acetate), [Co(H2O)2(HCOO)2]·odabco (2), [Co2(H2O)(NO3)(odabco)5](NO3)3·3.65H2O (3), and [Co2(DMF)2(odabco)4](NO3)4·3H2O (4; DMF = N,N-dimethylformamide). Crystal structures of 1-4 were determined by single-crystal X-ray crystallography. Coordination polymer 1 comprises binuclear and mononuclear metal-acetate blocks alternating within uncharged one-dimensional chains, in which odabco acts as a bridging ligand. A layered Co(II) formate 2 contains odabco only as guest molecules located in the interlayer space. Layered compound 3 and three-dimensional 4 have cationic coordination frameworks with 26% and 34% specific void volumes, respectively, unveiling high structural diversity of Co(II)-odabco MOFs based on quite a rare aliphatic moiety. Magnetization measurements were performed for 1, 3, and 4 and the obtained data were interpreted on the basis of their crystal structures. A strong (J/kB~100 K) antiferromagnetic coupling was found within binuclear metal blocks in 1. Ion exchange experiments revealed a considerable iodide uptake by 3 resulting in an up to 75% guest nitrate substitution within the voids of a coordination framework, found by capillary zone electrophoresis data and confirmed by single-crystal XRD. A preservation of 3 crystallinity during the exchange allowed for the guest I- positions within a new adduct with the formula [Co2(H2O)(NO3)(odabco)5]I2(NO3)·1.85H2O (3-I) to be successfully determined and the odabco aliphatic core to be revealed as a main adsorption center for quite large and easily polarizable iodide anions. In summary, this work presents a comprehensive study for a series of 1,4-diazabicyclo[2.2.2]octane N,N'-dioxide-based MOFs of cobalt(II) within the framework of magnetic properties and reports the first example of anion exchange in odabco-based coordination networks, supported by direct X-ray structural data. The reported results unveil promising applications of such frameworks bearing ligands with an aliphatic core in the diverse structural design of selective adsorbents and other types of functional materials.

Experimental evaluation of the stabilization of the COT orbitals by 4f orbitals in COT2Ce using a Hubbard model

Significant orbital mixing is rare in lanthanide complexes because of the limited radial extent of the 4f orbitals, which results in a generally small stabilization due to 4f orbital interactions. Nevertheless, even a small amount of additional stabilization could enhance lanthanide separations. One lanthanide complex in which orbital mixing has been extensively studied both experimentally and computationally is cerocene, COT₂Ce, where COT is cyclooctatetraene. This compound has a singlet ground state with a low-lying, triplet excited state. Previous fluorescence studies on trimethylsilyl-substituted cerocenes indicate the triplet state is 0.4 eV higher in energy than the singlet state. In addition, computational studies predict that the triplet is 0.3 to 1 eV higher in energy than the singlet. The synthesis of highly pure COT₂Ce by Walter and Andersen allowed its physical properties to be accurately measured. Using these measurements, we evaluate the stabilization of the 4f orbitals using two, independent approaches. A Hubbard model is used to evaluate the stabilization of the ground state due to orbital mixing. This stabilization, which is also the singlet-triplet gap, is -0.29 eV using this model. This gap was also from the temperature independent paramagnetism of COT₂Ce, which yielded a value of -0.32 eV.

Structural, Spectroscopic, and Theoretical Investigation of a T-Shaped [Fe3(μ3-O)] Cluster

The synthesis, X-ray crystal and electronic structures of [Fe₃(μ₃-O)(mpmae)₂(OAc)₂ Cl₃], 1, where mpmae-H = 2-(N-methyl-N-((pyridine-2-yl)methyl)amino)ethanol, are described. This cluster comprises three high-spin ferric ions and exhibits a T-shaped site topology. Variable-frequency electron paramagnetic resonance measurements performed on single crystals of 1 demonstrate a total spin S_T = 5/2 ground state, characterized by a small, negative, and nearly axial zero-field splitting tensor D = -0.49 cm^-1, E/D ≈ 0.055. Analysis of magnetic susceptibility, magnetization, and magneto-structural correlations further corroborate the presence of a sextet ground-spin state. The observed ground state originates from the strong anti-ferromagnetic interaction of two iron(III) spins, with J = 115(5) cm^-1, that, in turn, are only weakly coupled to the spin of the third site, with j = 7(1) cm^-1. These exchange interactions lead to a ground state with magnetic properties that are essentially entirely determined by the weakly coupled site. The contributions of the individual spins to the total ground state of the cluster were monitored using variable-field ⁵⁷Fe Mössbauer spectroscopy. Field-dependent spectra reveal that, while one of the iron sites exhibits a large negative internal field, typical of ferric ions, the other two sites exhibit small, but not null, negative and positive internal fields. A theoretical analysis reveals that these small internal fields originate from the mixing of the lowest S_T = 5/2 excited state into the ground state which, in turn, is induced by a minute structural distortion.

Decamethylytterbocene complexes of bipyridines and diazabutadienes: multiconfigurational ground states and open-shell singlet formation

Partial ytterbium f-orbital occupancy (i.e., intermediate valence) and open-shell singlet formation are established for a variety of bipyridine and diazabutadiene adducts with decamethylytterbocene, (C(5)Me(5))(2)Yb, abbreviated as Cp*(2)Yb. Data used to support this claim include ytterbium valence measurements using Yb L(III)-edge X-ray absorption near-edge structure spectroscopy, magnetic susceptibility, and complete active space self-consistent field (CASSCF) multiconfigurational calculations, as well as structural measurements compared to density functional theory calculations. The CASSCF calculations indicate that the intermediate valence is the result of a multiconfigurational ground-state wave function that has both an open-shell singlet f(13)(pi*)(1), where pi* is the lowest unoccupied molecular orbital of the bipyridine or diazabutadiene ligands, and a closed-shell singlet f(14) component. A number of other competing theories for the unusual magnetism in these materials are ruled out by the lack of temperature dependence of the measured intermediate valence. These results have implications for understanding chemical bonding not only in organolanthanide complexes but also for f-element chemistry in general, as well as understanding magnetic interactions in nanoparticles and devices.

Isotropic and Antisymmetric Double-Exchange, Zero-Field, Zeeman, and Hyperfine Splittings in Trinuclear Valence-Delocalized [Cu37+] Clusters

Valence delocalization in the [Cu3(7+)] trimer is considered in the model of the double-exchange coupling, in which full delocalization corresponds to the migration of the single d(x2-y2) hole and relatively strong isotropic double-exchange coupling. Strong double exchange results in the pairing of the individual spins in the delocalized trimer even at room temperature. The model explains the delocalized singlet 1A1 ground state in the planar Cu3(mu3-O) core by strong double exchange with positive double-exchange parameter t(0), whereas the delocalized triplet ground state of the [Cu3(7+)] trimer, which was observed in the Cu3(mu3-S)2 cluster, may be explained by the double exchange with relatively weak positive t(0): 0 < t(0) < 2J (degenerate 3E ground state) or negative t(0) (triplet 3A2 ground state). An analysis of the splitting of the delocalized degenerate 3E term requires inclusion of the antisymmetric double-exchange interaction, which takes into account the spin-orbit coupling in the double-exchange model. The cluster parameter KZ of the antisymmetric double-exchange coupling is proportional to t(0) and anisotropy of the g factor Deltag(parallel)[Cu(II)], KZ << t(0). Antisymmetric double exchange is relatively large in the [Cu3(7+)] cluster with the d(x2-y2) magnetic orbitals lying in the Cu3 plane [Cu3(mu3-O) core], whereas for the d(x2-y2) magnetic orbitals lying in the plane perpendicular to Cu3, antisymmetric double-exchange coupling is weak [Cu3(mu3-S)2 cluster]. The antisymmetric double-exchange coupling results in the linear zero-field splitting DeltaK = 2[equation: see text]KZ (approximately t(0)) of the delocalized degenerate 3E term that leads to strong anisotropy of the Zeeman splittings in the external magnetic field and a complex electron paramagnetic resonance (EPR) spectrum. The delocalized model of hyperfine interaction explains the hyperfine structure [10 hyperfine lines with the relative intensities 1:3:6:10:12:12:10:6:3:1 and the interval a/3] of the EPR transitions in the triplet states that was observed in the EPR spectra of the Cu3(mu3-S)2 cluster.

Hyperfine Splittings in Spin-Frustrated Trinuclear Cu3 Clusters

The hyperfine structures of the EPR spectra of the spin-frustrated and distorted Cu(II) trimers were calculated in the spin-coupling model. The correlations between the hyperfine structures of the EPR spectra and geometry of the Cu(3) clusters (equilateral, isosceles, and scalene triangles) were found. For the EPR spectrum of the spin-frustrated ground state 2(S = 1/2) of an equilateral triangle Cu(3) cluster (J(12) = J(13) = J(23) = J), the calculated hyperfine structure represents the complicated spectrum of the 24 hyperfine lines, of total length 5a, where a is the hyperfine constant of the mononuclear Cu center. For an isosceles Cu(3) cluster (J(12) not equal J(13) = J(23)), the hyperfine splittings of the EPR spectra of the two split S = 1/2 levels with intermediate spins S(12) = 0 and S(12) = 1 are essentially different. The EPR signal of the |(S(12) = 0)S = 1/2> level is characterized by the four equally spaced hyperfine lines (interval A = a) with the same relative spectral amplitudes 16:16:16:16 and total length 3a. For the |(S(12) = 1)S = 1/2> level, the calculated hyperfine structure represents the spectrum of the 16 hyperfine lines with equal spacing (interval A' = a/3), the spectral intensity distribution 1:1:3:3:5:5:7:7:7:7:5:5:3:3:1:1 and total length 5a. These hyperfine spectra differ from the hyperfine structure (10 lines with interval a/3) of the EPR signals of the excited S = 3/2 level of the Cu(3) cluster. The quartet hyperfine structure, characteristic of a single Cu(2+) nucleus, which was observed experimentally for the doublet ground state of the spin-frustrated Cu(3)(II) clusters, corresponds to the hyperfine structure of the EPR signal of the |(S(12) = 0)S = 1/2> level. This hyperfine structure is evidence of the lowering of the Cu(3) cluster symmetry from trigonal to orthorhombic and the small splitting of the spin-frustrated 2(S = 1/2) ground state.

Cyclic trinuclear and chain of cyclic trinuclear copper(II) complexes containing a pyramidal Cu(3)O(H) core. Crystal structures and magnetic properties of [Cu(3)(mu(3)-OH)(aaat)(3)(H(2)O)(3)](NO(3))(2).H(2)O [aaat = 3-acetylamino-5-amino-1,2,4-triazolate] and ([Cu(3)(mu(3)-OH)(aat)(3)(mu(3)-SO(4))].6H(2)O)(n) [aat = 3-acetylamino-1,2,4-triazolate]: new cases of spin-frustrated systems

New copper(II) complexes of the cyclic trinuclear type with 1,2,4-triazole ligands, [Cu(3)(mu(3)-OH)(aaat)(3)(H(2)O)(3)](NO(3))(2).H(2)O [Haaat = 3-acetylamino-5-amino-1,2,4-triazole] (1) and ([Cu(3)(mu(3)-OH)(aat)(3)(mu(3)-SO(4))].6H(2)O)(n) [Haat = 3-acetylamino-1,2,4-triazole] (2), have been prepared and characterized by X-ray crystallography and magnetic measurements. Compound 1, the first reported with the ligand (H)aaat, consists of discrete trinuclear cations, associated NO(3)(-) anions and lattice water molecules. Compound 2 consists of unusual chains of trinuclear units with a tridentate sulfato group linking the trimeric units and water molecules stabilizing the crystal lattice. In both complexes, 1 and 2, the trinuclear [Cu(3)(OH)L(3)] unit contains a pyramidal Cu(3)-mu(3)OH core, and an almost flat Cu(3)N(6) ring formed by the N,N-bridging triazolato groups. The Cu...Cu' intratrimeric distances are 3.35-3.37-3.39 A in 1 and 3.34-3.34-3.36 A in 2. The copper atoms are five-coordinated with a distorted square-pyramidal geometry. Magnetic measurements have been performed in the 1.9-300 K temperature range. In the high-temperature region (T > 90 K), experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -J(S(1)S(2) + S(2)S(3) + S(1)S(3)), with J = -194.6 cm(-1) and g = 2.08 for 1, and J = -185.1 cm(-1) and g = 2.10 for 2. The magnitude of the antiferromagnetic exchange in both complexes is discussed on the basis of their structural features by comparison with reported N,N-pheripherically bridged trinuclear systems. In order to fit the experimental magnetic data at low temperature, an antisymmetric exchange term, H(AS) = G(S(1)xS(2) + S(2)xS(3) + S(1)xS(3)), had to be introduced, with G = 27.8 (1) and 31.0 (2) cm(-1). Crystal data: C(12)H(27)Cu(3)N(17)O(14) (1) (MW = 824.13) crystallizes in the triclinic space group, P(-)1, Z = 2, with the cell dimensions a = 8.852(2) A, b = 11.491(3) A, c = 15.404(3) A, alpha = 70.43(3) degrees, beta = 75.11(2) degrees, gamma = 88.43(2) degrees, and V = 1423.8(5) A(3), D(calcd) = 1.922 g cm(-)(3); the final agreement values were R1 = 0.0822 and wR2 = 0.2300 for 4989 unique reflections. C(12)H(28)Cu(3)N(12)O(14)S (2) (MW = 787.14) crystallizes in the triclinic space group, P(-)1, Z = 2, with the cell dimensions a = 7.146(6) A, b = 14.26(1) A, c = 15.35(2) A, alpha = 109.0(9) degrees, beta = 93.6(9) degrees, gamma = 99.5(7) degrees, and V = 1448(2) A(3), D(calcd) = 1.806 g cm(-3); the final agreement values were R1 = 0.0628 and wR2 = 0.1571 for 3997 "observed" reflections.

Competing interactions in the tetranuclear spin cluster [Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O. An inelastic neutron scattering and magnetic study

The properties of the spin state manifold of the tetranuclear cluster Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O (bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine) are investigated by combining magnetic susceptibility and magnetization measurements with an inelastic neutron scattering (INS) study on an undeuterated sample of Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O. The temperature dependence of the magnetic susceptibility indicates an S = (1)/(2) ground state, which requires antiferromagnetic interactions both between Cr(3+) and Ni(2+) ions and among the Cr(3+) ions. INS reveals potential single-ion anisotropies to be negligibly small and enables an accurate determination of the exchange parameters. The best fit to the experimental energy level diagram is obtained by an isotropic spin Hamiltonian H = J(CrNi)(S(1)().S(4)() + S(2)().S(4)() + S(3)().S(4)()) + J(CrCr)(S(1)().S(2)() + S(1)().S(3)() + S(2)().S(3)()) with J(CrNi) = 1.47 cm(-)(1) and J(CrCr) = 1.25 cm(-)(1). With this model, the experimental intensities of the observed INS transitions as well as the temperature dependence of the magnetic data are reproduced. The resulting overall antiferromagnetic exchange is rationalized in terms of orbital exchange pathways and compared to the situation in oxalato-bridged clusters.

Is ferromagnetism an intrinsic property of the CuII/GdIII couple? 2. Structures and magnetic properties of novel trinuclear complexes with mu-phenolato-mu-oximato (Cu-Ln-Cu) cores (Ln = La, Ce, Gd)

The present paper is devoted to the study of original trinuclear (CuII, LnIII, CuII) complexes (Ln = La, Ce, Gd). They derive from the polydentate ligands H2Li (i = 1, 3, 4) represented in Figure 1. The crystal and molecular structures of two complexes have been determined at room temperature. The (Cu, Gd, Cu) complex of H2L1 1Gd and the (Cu, Ce, Cu) complex of H2L3 3Ce crystallize in the triclinic space group P1 (no. 2) with the following cell parameters: a = 14.005(2) A, b = 14.7581(13) A, c = 11.3549(13) A, alpha = 96.273(9) degrees, beta = 97.648(11) degrees, gamma = 72.946(9) degrees, V = 2217.7(4) A3, and Z = 2 for 1Gd and a = 11.226(2) A, b = 16.927(3) A, c = 11.010(2) A, alpha = 108.67(2) degrees, beta = 110.48(1) degrees, gamma = 92.35(2) degrees, V = 1828.7(5) A3, and Z = 2 for 3Ce. Regarding possible supports for magnetic interactions, it may be noted that, in both complexes, each of the main bridging pathways between the equatorial positions of a copper(II) ion and the related lanthanide ion is double and not symmetrical. It involves a phenolato oxygen atom and an oximato nitrogen-oxygen pair of atoms. The resulting Cu(O,N-O)Gd networks are not planar, but 3Ce displays much larger deviations than does 1Gd. Determination of the thermal dependence of chi M (molar susceptibility) and the field variations of M (magnetization) show that in 3Gd and 4Gd the Cu-Gd interactions are antiferromagnetic while more "usual" ferromagnetic interactions are observed for 1Gd. The possibility of a relationship between structural and magnetic parameters is considered.

Investigation of exchange couplings in [Fe3S4]+ clusters by electron spin-lattice relaxation

We have studied four proteins containing oxidized 3Fe clusters ([Fe3S4]+, S=1/2, composed of three, antiferromagnetically coupled high-spin ferric ions) by continuous wave (CW) and pulsed EPR techniques: Azotobacter vinelandii ferredoxin I, Desulfovibrio gigas ferredoxin II, and the 3Fe forms of Pyrococcus furiosus ferredoxin and aconitase. The 35 GHz (Q-band) CW EPR signals are simulated to yield experimental g tensors, which either had not been reported, or had been reported only at X-band microwave frequency. Pulsed X- and Q-band EPR techniques are used to determine electron spin-lattice (T1, longitudinal) relaxation times at several positions on the samples' EPR envelope over the temperature range 2-4.2 K. The T1, values vary sharply across the EPR envelope, a reflection of the fact that the envelope results from a distribution in cluster properties, as seen earlier as a distribution in g3 values and in 57 Fe hyperfine interactions, as detected by electron nuclear double resonance spectroscopy. The temperature dependence of 1/T1 is analyzed in terms of the Orbach mechanism, with relaxation dominated by resonant two-phonon transitions to a doublet excited state at approximately 20 cm(-1) above the doublet ground state for all four of these 3Fe proteins. The experimental EPR data are combined with previously reported 57Fe hyperfine data to determine electronic spin exchange-coupling within the clusters, following the model of Kent et al. Their model defines the coupling parameters as follows: J13=J, J12=J(1+epsilon'), J23=J(1+epsilon), where Jij is the isotropic exchange coupling between ferric ions i and j, and epsilon' and epsilon' are measures of coupling inequivalence. We have extended their theory to include the effects of epsilon' not equal to 0 and thus derived an exact expression for the energy of the doublet excited state for any epsilon, epsilon'. This excited state energy corresponds roughly to epsilonJ and is in the range 5-10 cm(-1) for each of these four 3Fe proteins. This magnitude of the product epsilonJ, determined by our time-domain relaxation studies in the temperature range 2-4 K, is the same as that obtained from three other distinct types of study: CW EPR studies of spin relaxation in the range 5.5-50 K, NMR studies in the range 293-303 K, and static susceptibility measurements in the range 1.8-200 K. We suggest that an apparent disagreement as to the individual values of J and epsilon be resolved in favor of the values obtained by susceptibility and NMR (J > or approximately 200 cm(-1) and epsilon> or =0.02 cm(-1)). as opposed to a smaller J and larger r as suggested in CW EPR studies. However, we note that this resolution casts doubt on the accepted theoretical model for describing the distribution in magnetic properties of 3Fe clusters.

Electron Exchange and the Photophysics of Metal-Quinone Complexes. 1. Synthesis and Spectroscopy of Chromium-Quinone Dyads

The synthesis, structural and spectroscopic characterization of monosemiquinone and monocatechol complexes of chromium(III) are described. Compounds of the general form [Cr(N(4))Q](n+), where N(4) represents a tetradentate or bis-bidentate nitrogenous ligand or ligands and Q represents a reduced form of an orthoquinone, have been prepared by two different routes from Cr(III) and Cr(II) starting materials. The complex [Cr(tren)(3,6-DTBSQ)](PF(6))(2), where tren is tris(2-aminoethyl)amine and 3,6-DTBSQ is 3,6-di-tert-butylorthosemiquinone, crystallizes in the monoclinic space group P2(1)/c with a = 11.9560(2) Å, b = 17.0715(4) Å, c = 17.1805(4) Å, beta = 90.167(1) degrees, V = 3506.6(1) Å(3), Z = 4, with R = 0.056 and R(w) = 0.070. Alternating C-C bond distances within the quinoidal ligand confirm its semiquinone character. Variable temperature magnetic susceptibility data collected on solid samples of both [Cr(tren)(3,6-DTBSQ)](PF(6))(2) and [Cr(tren)(3,6-DTBCat)](PF(6)) in the range 5-350 K exhibit temperature-independent values of 2.85 +/- 0.1 &mgr;(B) and 3.85 +/- 0.1 &mgr;(B), respectively. These data are consistent with a simple Cr(III)-catechol formulation (S = (3)/(2)) in the case of [Cr(tren)(3,6-DTBCat)](PF(6)) and strong antiferromagnetic coupling (|J| > 350 cm(-)(1)) between the Cr(III) and the semiquinone radical in [Cr(tren)(3,6-DTBSQ)](PF(6))(2). The absorption spectrum of the semiquinone complex exhibits a number of sharp, relatively intense transitions in fluid solution. Group theoretical arguments coupled with a qualitative ligand-field analysis including the effects of Heisenberg spin exchange suggest that several of the observed transitions are a consequence of exchange interactions in both the ground- and excited-state manifolds of the compound. The effect of electron exchange on excited-state dynamics has also been probed through static emission as well as time-resolved emission and absorption spectroscopies. It is suggested that the introduction of exchange coupling and subsequent change in the molecule's electronic structure may contribute to an increase of nearly 4 orders of magnitude in the rate of radiative decay (k(r)), and a factor of ca. 10(7) in the rate of nonradiative decay (k(nr)).