Structure and magnetism of [M3](6/7+) metal chain complexes from density functional theory: analysis for copper and predictions for silver

The ground-state electronic structure of the trinuclear complex Cu3(dpa)4Cl2 (1), where dpa is the anion of di(2-pyridyl)amine, has been investigated within the framework of density functional theory (DFT) and compared with that obtained for other known M3(dpa)4Cl2 complexes (M = Cr, Co, Ni) and for the still hypothetical Ag3(dpa)4Cl2 compound. Both coinage metal compounds display three singly occupied x2-y2-like (delta) orbitals oriented toward the nitrogen environment of each metal atom, generating antibonding M-(N4) interactions. All other metal orbital combinations are doubly occupied, resulting in no delocalized metal-metal bonding. This is at variance with the other known symmetric M3(dpa)4Cl2 complexes of the first transition series, which all display some delocalized bonding through the metal backbone, with formal bond multiplicity decreasing in the order Cr > Co > Ni. An antiferromagnetic coupling develops between the singly occupied MOs via a superexchange mechanism involving the bridging dpa ligands. This magnetic interaction can be considered as an extension to the three aligned Cu(II) atoms of the well-documented exchange coupling observed in carboxylato-bridged dinuclear copper compounds. Broken-symmetry calculations with approximate spin projection adequately reproduce the coupling constant observed for 1. Oxidation of 1 removes an electron from the magnetic orbital located on the central Cu atom and its ligand environment; 1+ displays a much weaker antiferromagnetic interaction coupling the terminal Cu-N4 moieties via four ligand pathways converging through the x2-y2 orbital of the central metal. The silver homologues of 1 and 1+ display similar electronic ground states, but the calculated magnetic couplings are stronger by factors of about 3 and 4, respectively, resulting from a better overlap between the metal centers and their equatorial ligand environment within the magnetic orbitals.     

A theoretical analysis of the explanation of the significant differences in antiferromagnetic interactions between homologous μ-alkoxo and acetate bridged dicopper(II) complexes: ab initio and semi-empirical calculations

A magnetostructural classification of dimmers, containing the Cu (μ-alkoxo) Cu core, based on data obtained from X-ray diffraction analysis reported in the literature has been performed. In these complexes, the local geometry around the copper ions is generally a square planar and each copper ion is surrounded by one N atom and three O atoms. The influence of the overlap interactions between the bridging ligands and the metal (Cu) d orbitals on the super-exchange coupling constant has been studied by means of ab initio Restricted Hatree–Fock molecular orbital calculations. The interaction between the magnetic d orbitals and highest occupied molecular orbitals of the acetate oxygens has been investigated in homologous μ-acetato-bridged dicopper(II) complexes which have significantly different −2J values (the energy separation between the spin-triplet and spin-singlet states). In order to determine the nature of the fronter orbitals, Extended Hückel molecular Orbital calculations are also reported. Ab initio restricted Hartree–Fock calculations have shown that the acetato bridge and the alkoxide bridge contribute to the magnetic interaction countercomplementarily to reduce antiferromagnetic interaction.     

Theoretical studies of magnetic interactions in Mn(II)(hfac)(2)[di(4-pyridyl)phenylcarbene] and Cu(II)(hfac)(2)[di(4-pyridyl)phenylcarbene].

Bis(hexafluoroacetylacetonato(hfac))manganese(II) coordinated with di(4-pyridyl)phenylcarbene, Mn(II)(hfac)(2)[di(4-pyridyl)phenylcarbene] (1a) and its copper analogue Cu(II)(hfac)(2)[di(4-pyridyl)phenylcarbene] (2a) have attracted great interest from the viewpoint of photoinduced magnetism. The complexes 1a and 2a are regarded as the new d-pi-p conjugated systems containing transition metal ion and carbene as spin sources. The magnetic measurements demonstrated antiferromagnetic and ferromagnetic effective exchange interactions for 1a and 2a, respectively. Here, we have performed UHF and UHF plus DFT hybrid calculations (UB3LYP) to elucidate the nature of the through-bond effective exchange interaction between Mn(II) (or Cu(II)) ion and triplet carbene sites in 1a (or 2a) and their model complexes. The natural orbital analysis of the UHF and UB3LYP solutions and CASCI calculations for the simplest models of 1a and 2a are performed to elucidate relative contributions of spin polarization (SP) and spin delocalization (SD) (or superexchange (SE)) interactions for determination of the sign of J(ab) values. Mn(II) carbene complex 1a shows an antiferromagnetic interaction because of the pi-type antiferromagnetic SE effect and the pi-type SP effect, while the positive J(ab) value for Cu(II) carbene complex 2a can be explained by the fact that ferromagnetic SE and SP interactions due to orbital orthogonality are more effective than the sigma-type antiferromagnetic SE interaction. The ligand coordination effects of both 4-pyridylcarbene and hfac play crucial roles for determination of the J(ab) values, but the ligand coordination effect of hfac is more important for the active control of charge or spin density distributions than that of 4-pyridylcarbene. The spin alignment mechanisms of 1a and 2a are indeed consistent with SE plus SP rule, which is confirmed with the shape and symmetry of natural orbitals, together with charge and spin density distributions.     

Ferro- and ferrimagnetic chains of hin-bridged copper(II) and manganese(II) and hnn-bridged manganese(II) complexes (hin = 4,4,5,5-tetramethylimidazolin-1-oxyl; hnn = 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide)

We have exploited potential utility of 4,4,5,5-tetramethylimidazolin-1-oxyl (hin) and 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide (hnn) as mu-1,4 and mu-1,5 bridging ligands, respectively, carrying an unpaired electron in development of metal-radical hybrid magnets. X-ray diffraction measurements of [Cu(hfac)(2)hin] (1), [Mn(hfac)(2)hin] (2), and [Mn(hfac)(2)hnn] (3) revealed one-dimensional metal-radical alternating chain structures, where hfac denotes 1,1,1,5,5,5-hexafluoropentane-2,4-dionate. Magnetic measurements of 1 indicate the presence of intrachain ferromagnetic coupling between copper and radical spins. The magnetic exchange parameter was estimated as 2J/k = 56.8 K based on an S = 1/2 equally spaced ferromagnetic chain model (H = -2J summation operator S(i).S(i+1)). This ferromagnetic interaction can be explained in terms of the axial coordination of the hin nitrogen or oxygen to Cu(II). The chi(m)T value of 2 and 3 increased on cooling, and the magnetic data could be analyzed by Seiden's ferrimagnetic chain model, giving 2J/k = -325 and -740 K, respectively. The antiferromagnetic interaction of 2 and 3 can be attributed to orbital overlap between the manganese and the oxygen or nitrogen magnetic orbitals. The exchange interactions between Cu-hin and Mn-hnn are larger than those of typical Cu- and Mn-nitronyl nitroxide complexes, indicating that the choice of small ligands is a promising strategy to bestow strong exchange interaction. Compound 3 became a ferrimagnet below 4.4 K, owing to ferromagnetic coupling among the ferrimagnetic chains.     

The ligand field as a pseudo-potential

The introduction of a pseudo-potential in crystal field theory is shown to lead to an expression for the orbital splittings which depend upon the squares of the group overlap integrals between the metal and ligand orbitals. A formula is derived whereby the group overlap integral can be directly expressed in terms of the diatomic sigma- and pi-integrals.     

Density functional studies on dinuclear {Ni(II)Gd(III)} and trinuclear {Ni(II)Gd(III)Ni(II)} complexes: magnetic exchange and magneto-structural maps

Theoretical calculations using density functional methods have been performed on two dinuclear {Ni(II)-Gd(III)} and two trinuclear {Ni(II)-Gd(III)-Ni(II)} complexes having two and three μ-OR (R = alkyl or aromatic groups) bridging groups. The different magnetic behaviour, having moderately strong ferromagnetic coupling for complexes having two μ-OR groups and weak ferromagnetic coupling for complexes having three μ-OR groups, observed experimentally is very well reproduced by the calculations. Additionally, computation of overlap integrals MO and NBO analysis reveals a clear increase in antiferromagnetic contribution to the net exchange for three μ-OR bridged {Ni-Gd} dimers and also provides several important clues regarding the mechanism of magnetic coupling. Besides, MO and NBO analysis discloses the role of the empty 5d orbitals of the Gd(III) ion on the mechanism of magnetic coupling. Magneto-structural correlations for Ni-O-Gd bond angles, Ni-O and Gd-O bond distances, and the Ni-O-Gd-O dihedral angle have been developed and compared with the published experimental {Ni-Gd} structures and their J values indicate that the Ni-O-Gd bond angles play a prominent role in these types of complexes. The computation has then been extended to two trinuclear {Ni(II)-Gd(III)-Ni(II)} complexes and here both the {Ni-Gd} and the {Ni-Ni} interactions have been computed. Our calculations reveal that, for both structures studied, the two {NiGd} interactions are ferromagnetic and are similar in strength. The {Ni-Ni} interaction is antiferromagnetic in nature and our study reveals that its inclusion in fitting the magnetic data is necessary to obtain a reliable set of spin Hamiltonian parameters. Extensive magneto-structural correlations have been developed for the trinuclear complexes and the observed J trend for the trinuclear complex is similar to that of the dinuclear {Ni-Gd} complex. In addition to the structural parameters discussed above, for trinuclear complexes the twist angle between the two Ni-O-Gd planes is also an important parameter which influences the J values.     

Insights into the control of magnetic coupling in the Mn4(III) complex: from ferromagnetic to antiferromagnetic

Magnetic coupling interactions of a Mn(III)(4) system are investigated by calculations based on density functional theory combined with a broken-symmetry approach (DFT-BS). Three different interactions including ferromagnetic and antiferromagnetic coupling are concomitant in this complex. This magnetic phenomenon of the complex is due to the different bridging angles between the Mn(III) centers in the three different models and the orbital complementarity of the μ-pzbg and μ-OCH(3) bridging ligands, which is proven by the analyses of the molecular orbitals. According to the analyses of the magneto-structural correlation, it is revealed that the magnetic coupling interaction switches from ferromagnetic to antiferromagnetic at the point of the bridging angle Mn-(μ-OCH(3))-Mn = 99°, which is equal to the value in the origin crystal. Significant correlation between the magnetic properties and the component of the d orbitals in these systems shows that the larger contribution of the d(z(2)) orbital corresponds to the larger ferromagnetic coupling interaction. These results should provide a means to control the magnetic coupling of the polynuclear Mn systems, which is instructive for the design of new molecular magnetic materials.     

Magnetic exchange interactions and magneto-structural correlations in heterobridged μ-phenoxo-μ(1,1)-azide dinickel(II) compounds: a combined experimental and theoretical exploration

This investigation presents the syntheses, crystal structures, magnetic properties, and density functional theoretical modeling of magnetic behavior of two heterobridged μ-phenoxo-μ(1,1)-azido dinickel(II) compounds [Ni(II)(2)(L(1))(2)(μ(1,1)-N(3))(N(3))(H(2)O)]·CH(3)CH(2)OH (1) and [Ni(II)(2)(L(2))(2)(μ(1,1)-N(3))(CH(3)CN)(H(2)O)](ClO(4))·H(2)O·CH(3)CN (2), where HL(1) and HL(2) are the [1+1] condensation products of 3-methoxysalicylaldehyde and 1-(2-aminoethyl)-piperidine (for HL(1))/4-(2-aminoethyl)-morpholine (for HL(2)), along with density functional theoretical magneto-structural correlations of μ-phenoxo-μ(1,1)-azido dinickel(II) systems. Compounds 1 and 2 crystallize in orthorhombic (space group Pbca) and monoclinic (space group P2(1)/c) systems, respectively. The coordination environments of both metal centers are distorted octahedral. The variable-temperature (2-300 K) magnetic susceptibilities at 0.7 T of both compounds have been measured. The interaction between the metal centers is moderately ferromagnetic; J = 16.6 cm(-1), g = 2.2, and D = -7.3 cm(-1) for 1 and J = 16.92 cm(-1), g = 2.2, and D(Ni1) = D(Ni2) = -6.41 cm(-1) for 2. Broken symmetry density functional calculations of exchange interaction have been performed on complexes 1 and 2 and provide a good numerical estimate of J values (15.8 cm(-1) for 1 and 15.35 cm(-1) for 2) compared to experiments. The role of Ni-N bond length asymmetry on the magnetic coupling has been noted by comparing the structures and J values of complexes 1 and 2 together with previously published dimers 3 (Eur. J. Inorg. Chem. 2009, 4982), 4 (Inorg. Chem. 2004, 43, 2427), and 5 (Dalton Trans. 2008, 6539). Our extensive DFT calculations reveal an important clue to the mechanism of coupling where the orientation of the magnetic orbitals seems to differ with asymmetry in the Ni-N bond lengths. This difference in orientation leads to a large change in the overlap integral between the magnetic orbitals and thus the magnetic coupling. DFT calculations have also been extended to develop several magneto-structural correlations in this type of complexes and the correlation aim to focus on the asymmetry of the Ni-N bond lengths reveal that the asymmetry plays a proactive role in governing the magnitude of the coupling. From a completely symmetric Ni-N bond length, two behaviors have been noted: with a decrease in bond length there is an increase in the ferromagnetic coupling, while an increase in the bond lengths leads to a decrease in ferromagnetic interaction. The later correlation is supported by experiments. The magnetic properties of 1, 2, and three previously reported related compounds have been discussed in light of the structural parameters and also in light of the theoretical correlations determined here.     

The dilemma of CrIIINiII exchange interactions: ferromagnetism versus antiferromagnetism

The sign of the exchange interaction in dinuclear Cr(III)Ni(II) complexes was analyzed using theoretical methods based on density functional theory. This approach allowed us to reproduce the experimental J values correctly. In addition, the Kahn-Briat model, which uses the square of the sum of the overlaps between the magnetic orbitals to correlate with the exchange coupling constant, provided a reasonable correlation between the different types of Cr(III)Ni(II) complexes when using biorthogonalized orbitals. We also examined the exchange interactions in two polynuclear Cr(III)Ni(II) complexes: a Cr(7)Ni ring and an S-shaped Cr(12)Ni(3) complex. We concluded that both systems exhibit antiferromagentic interactions, and that the Cr(III)···Ni(II) interactions are similar in value to the C(III)···Cr(III) exchange couplings.     

Isotropic non-Heisenberg behavior in M3(dpa)4Cl2 extended metal atom chains

Isotropic deviations to the standard Heisenberg Hamiltonian have been extracted for a series of trinuclear extended metal atom chain complexes, namely, [Ni(3)(dpa)(4)Cl(2)], and the hypothetical [NiPdNi(dpa)(4)Cl(2)] and [Pd(3)(dpa)(4)Cl(2)], following a scheme recently proposed by Labe?guerie and co-workers (J. Chem. Phys 2008, 129, 154110) within the density functional theory framework. Energy calculations of broken symmetry monodeterminantal solutions of intermediate M(s,tot.) values can provide an estimate of the magnitude of the biquadratic exchange interaction (λ) that accounts for these deviations in systems with S = 1 magnetic sites. With the B3LYP functional, we obtain λ = 1.37, 13.8, and 498 cm(-1) for the three molecules, respectively, meaning that a simple Heisenberg Hamiltonian is enough for describing the magnetic behavior of the Ni(3) complex but definitely not for Pd(3). In the latter case, the origin of such extreme deviation arises from (i) an energetically affordable local non-Hund state (small intrasite exchange integral, K ～ 1960 cm(-1)) and (ii) a very effective overlap between Pd-4d orbitals and a large J. Furthermore, this procedure enables us to determine the relative weights of the two types of magnetic interactions, σ- and δ-like, that contribute to the total magnetic exchange (J = J(σ) + J(δ)). In all of the systems, J is governed by the σ interaction by 95-98%.     

Superexchange in magnetic insulators: an interpretation of the metal-metal charge transfer energy in the Anderson theory

The superexchange interactions in four three-center model systems A-L-B, for A and B being paramagnetic centers and L a diamagnetic bridging ligand, are analyzed by valence bond configuration interaction models in combination with fourth-order perturbation theory. We analyze the four distinct cases where a bridging ligand orbital simultaneously interacts with half-filled orbitals localized on A and B (case i), a half-filled orbital localized on A and an empty orbital localized on B (case ii), a full orbital localized on A and a half-filled orbital localized on B (case iii), and finally a full orbital localized on A and an empty orbital localized on B (case iv). In all four cases we compare our new results using localized orbitals with the equivalent results obtained using the Anderson ansatz of delocalized (magnetic) orbitals. The effective metal-to-metal electron transfer energy Ueff in the old formalism with delocalized orbitals is expressed in terms of the metal-to-metal electron transfer energy U and the ligand-to-metal electron transfer energy delta using localized orbitals. We find that the old formalism containing only Ueff is in general not sufficient. For cases i and ii we show that Ueff can be regarded as an effective U strongly reduced with respect to the free ion as a result of hybridization effects, whereas the same reduction of U for the cases iii and iv is not possible. The relevance and applicability of our theoretical results is demonstrated on magnetochemical data from the literature.     

Electronic structure of the [MNH2]+ (M = Sc-Cu) complexes

B3LYP geometry optimizations for the [MNH2]+ complexes of the first-row transition metal cations (Sc+-Cu+) were performed. Without any exception the ground states of these unsaturated amide complexes were calculated to possess planar geometries. CASPT2 binding energies that were corrected for zero-point energies and including relativistic effects show a qualitative trend across the series that closely resembles the experimental observations. The electronic structures for the complexes of the early and middle transition metal cations (Sc+-Co+) differ from the electronic structures derived for the complexes of the late transition metal cations (Ni+ and Cu+). For the former complexes the relative higher position of the 3d orbitals above the singly occupied 2p(pi) HOMO of the uncoordinated NH2 induces an electron transfer from the 3d shell to 2p(pi). The stabilization of the 3d orbitals from the left to the right along the first-row transition metal series causes these orbitals to become situated below the HOMO of the NH2 ligand for Ni+ and Cu+, preventing a transfer from occurring in the [MNH2]+ complexes of these metal cations. Analysis of the low-lying states of the amide complexes revealed a rather unique characteristic of their electronic structures that was found across the entire series. Rather exceptionally for the whole of chemistry, pi-type interactions were calculated to be stronger than the corresponding sigma-type interactions. The origin of this extraordinary behavior can be ascribed to the low-lying sp2 lone pair orbital of the NH2 ligand with respect to the 3d level.     

Antiferromagnetic versus ferromagnetic exchange interactions in bis(μ-O(oximate))dinickel(II) units for a series of closely related cube shaped carboxamideoximate-bridged Ni(4) complexes. A combined experimental and theoretical magneto-structural study

The syntheses, crystal structures, and the experimental and theoretical magnetochemical characterization for three tetrametallic Ni(II) clusters, namely, [Ni(4)(L)(4)(Cl)(2)(MeOH)(2)](ClO(4))(2)·4MeOH (1), [Ni(4)(L)(4)(N(3))(2)(MeOH)(2)](ClO(4))(2)·2MeOH (2), and [Ni(4)(L1)(4)(pyz)(2)(PhCOO)(2)(MeOH)(2)](ClO(4))(2)·7MeOH (3) (where HL and HL1 represent bipyridine-2-carboxamideoxime and pyrimidine-2-carboxamideoxime, respectively) are reported. Within the Ni(4)(2+) units of these compounds, distorted octahedral Ni(II) ions are bridged by carboxamideoximato ligands to adopt a distorted tetrahedral disposition. The Ni(4)(2+) unit, of C(2) symmetry, can also be viewed as a cube with single [O-atom] and double [NO oxime] bridging groups as atom edges, which define two almost square-planar Ni(O)(2)Ni rings and four irregular hexagonal Ni(NO)(2)Ni rings. To analyze the magnetic properties of 1-3, we have considered the simplest two-J model, where J(1) = J(2) (exchange interactions between the Ni(II) ions belonging to the Ni(O)(2)Ni square rings) and J(a) = J(b) = J(c) = J(d) (exchange interactions between the Ni(II) ions belonging to the Ni-(NO)(2)Ni hexagonal rings) with the Hamiltonian H = -J(1)(S(1)S(2) + S(3)S(4)) - J(a)(S(1)S(3) + S(1)S(4) + S(2)S(3) + S(2)S(4)). The J(1) and J(a) values derived from the fitting of the experimental susceptibility data are -5.8 cm(-1) and -22.1 cm(-1) for 1; -2.4 cm(-1) and -22.8 cm(-1) for 2, and +15.6 cm(-1) and -10.8 cm(-1) for 3. The magneto-structural results and density-functional theory (DFT) calculations demonstrate that the exchange interactions inside the Ni(μ-O)(2)Ni square rings depend on the Ni-O-Ni bridging angle (θ) and the out-of-plane angle of the NO oximate bridging group with respect to the Ni(O)(2)Ni plane (τ), whereas the interactions propagated through the Ni-N-O(Ni)-Ni exchange pathways defining the side of the hexagonal rings depend on the Ni-N-O-Ni torsion angle (α). In both cases, theoretical magneto-structural correlations were obtained, which allow the prediction of the angle for which ferromagnetic interactions are expected. For compound 3, the existence of the axial magnetic exchange pathway through the syn-syn benzoate bridge may also contribute (in addition to the θ and τ angles) to the observed F interaction in this compound through orbital countercomplementarity, which has been supported by DFT calculations. Finally, DFT calculations clearly show that the antiferromagnetic exchange increases when the dihedral angle between the O-Ni-O planes of the Ni(μ-O)(2)Ni square ring, β, increases.     

金属串配合物[Ni3(L)4(NCS)2](L = dpa-, mpta-, mdpa-, mppa-)结构和磁性的理论研究

应用密度泛函理论BP86方法结合自然键轨道分析方法对具有分子导线潜在应用前景的金属串配合物[Ni₃(L)₄(NCS)₂](L = dpa^- (1), mpta^- (2), mdpa^- (3), mppa^- (4))进行研究,分析了桥联配体L对Ni―Ni相互作用和磁耦合性质的影响.结果得到: (1)配合物的基态均是对应于五重态(HS)的反铁磁(AF)单重态, HS的能量和结构与AF态相近, Ni₃⁶⁺链形成了三中心四电子σ键(σ₂σ_nb¹σ^*1). (2) dpa^-引入甲基成为mdpa^-,对Ni―Ni、Ni―N距离影响不大; 3H-吡咯环和噻唑环取代吡啶环后, N1―N2、Ni―Ni距离增大, Ni2―N2键长缩短,但噻唑环的影响较小;故Ni―Ni相互作用强度为1 ≈ 3 > 2 > 4. (3)预测了3和4的J_ab值为-103和-88 cm^-1,随Ni―Ni相互作用增强磁耦合效应增大. Ni―Ni相互作用越大,通过Ni₃⁶⁺链σ型轨道的直接磁耦合越强; Ni2―N2键越强,通过涉及桥联配体的间接磁耦合越强,直接磁耦合比间接磁耦合更强.     

Metal-sulfur valence orbital interaction energies in metal-dithiolene complexes: determination of charge and overlap interaction energies by comparison of core and valence ionization energy shifts

The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal-dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp(2)M(bdt) (Cp = η(5)-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato). Comparison of the gas-phase core and valence ionization energy shifts provides a unique quantitative energy measure of valence orbital overlap interactions between the metal and the sulfur orbitals that is separated from the effects of charge redistribution. The results explain the large amount of sulfur character in the redox-active orbitals and the 'leveling' of oxidation state energies in metal-dithiolene systems. The experimentally determined orbital interaction energies reveal a previously unidentified overlap interaction of the predominantly sulfur HOMO of the bdt ligand with filled π orbitals of the Cp ligands, suggesting that direct dithiolene interactions with other ligands bound to the metal could be significant for other metal-dithiolene systems in chemistry and biology.     

混合桥基双核配合物中的轨道互补与反互补效应

在混合桥基的双核体系中,金属中心的磁轨道的线形组合可与相同对称性的桥基最高占据轨道(HOMO'S)相互作用.如果两种桥联配体稳定同一磁轨道组合,则称它们以互补方式起作用;反铁磁偶合就会被加强.反之,如果桥联配体稳定不同的磁轨道组合,则称它们以反互补方式起作用;这将减小反铁磁偶合.本文就互补和反互补效应以及相关的磁学知识做一简要概述.     

Copper(II) complexes with new polypodal ligands presenting axial-equatorial phenoxo bridges {2-[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol, 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methyl-6-(methylthio)phenol}: examples of ferromagnetically coupled bi- and trinuclear copper(II) complexes

Two new ligands, 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol (HL) and 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methyl-6-(methylthio)phenol (HSL), were synthesized and were used to prepare the trinuclear copper(II) complex {[CuSL(Cl)]2Cu}(PF6)2.H2O (1) and the corresponding binuclear complexes [Cu2(SL)2](PF6)2 (2) and [Cu2L2](PF6)2 (3). The crystal structure of 1 shows two different coordination environments: two square base pyramidal centers (Cu1 and Cu1a, related by a C2 axes), acting as ligands of a distorted square planar copper center (Cu2) by means of the sulfur atom of the SCH3 substituent and the bridging phenoxo oxygen atom of the ligand (Cu2-S = 2.294 A). Compounds 2 and 3 show two equivalent distorted square base pyramidal copper(II) centers, bridged in an axial-equatorial fashion by two phenoxo groups, thus defining an asymmetric Cu2O2 core. A long copper-sulfur distance measured in 2 (2.9261(18) A) suggests a weak bonding interaction. This interaction induces a torsion angle between the methylthio group and the phenoxo plane resulting in a dihedral angle of 41.4(5) degrees. A still larger distortion is observed in 1 with a dihedral angle of 74.0(6) degrees. DFT calculations for 1 gave a ferromagnetic exchange between first neighbors interaction, the calculated J value for this interaction being +11.7 cm-1. In addition, an antiferromagnetic exchange for 1 was obtained for the second neighbor interaction with a J value of -0.05 cm-1. The Bleaney-Bowers equation was used to fit the experimental magnetic susceptibility data for 2 and 3; the best fit was obtained with J values of +3.4 and -16.7 cm-1, respectively. DFT calculations for 2 and 3 confirm the nature and the values of the J constants obtained by the fit of the experimental data. ESR and magnetic studies on the reported compounds show a weak exchange interaction between the copper(II) centers. The low values obtained for the coupling constants can be explained in terms of a poor overlap between the magnetic orbitals, due to the axial-equatorial phenoxo bridging mode observed in these complexes.     

Broken-symmetry density functional theory investigation on bis-nitronyl nitroxide diradicals: influence of length and aromaticity of couplers

A series of nitronyl nitroxide (NN) diradicals with linear conjugated couplers and another series with aromatic couplers have been investigated by the broken-symmetry (BS) DFT approach. The overlap integral between the magnetically active orbitals in the BS state has been explicitly computed and used for the evaluation of the magnetic exchange coupling constant (J). The calculated J values are in very good agreement with the observed values in the literature. The magnitude of J depends on the length of the coupler as well as the conformation of the radical units. The aromaticity of the spacer decreases the strength of the exchange coupling constant. The SOMO-SOMO energy splitting analysis, where SOMO stands for the singly occupied molecular orbital, and the calculation of electron paramagnetic resonance (EPR) parameters have also been carried out. The computed hyperfine coupling constants support the intramolecular magnetic interactions. The nature of magnetic exchange coupling constant can also be predicted from the shape of the SOMOs as well as the spin alternation rule in the unrestricted Hartree-Fock (UHF) treatment. It is found that pi-conjugation along with the spin-polarization plays the major role in controlling the magnitude and sign of the coupling constant.     

Magnetic studies of a syn-anti triatomic carboxylate-bridging chainlike copper(II) complex exhibiting ferromagnetic exchange

The magnetic properties of triatomic syn-anti carboxylate bridging copper(II) complex, {[Cu(2,2'-bipydine)(maleate)].2H2O}infinity (complex 1), were investigated experimentally and theoretically, suggesting weak ferromagnetic intrachain interaction. The magnetic data were analyzed and interpreted in terms of Heisenberg chain model corrected by a mean molecular field. Fitting parameters obtained for J, g, and zJ' are 3.14 cm(-1), 2.08, and -0.13, respectively. Density functional theory with generalized gradient approximation was applied to calculate the electronic structure and spin distribution of the present complex. The structural and electronic factors controlling the magnetic interactions were also determined. Ferromagnetic intrachain interactions through triatomic syn-anti carboxylate bridge result from nonplanarity of the bridging network, the exchange pathway involving both the sigma and pi orbitals of the carboxylate bridge and the spin delocalization of each magnetic orbital on the atoms of the carboxylate bridge from the copper(II) centers.