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.     

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.     

Toward the control of the magnetic anisotropy of Fe(II) cubes: a DFT study

We present the results of our all-electron density-functional calculations on the magnetic anisotropy of the [Fe4(sap)4(MeOH)4] and [Fe4(sae)4(MeOH)4] polynuclear complexes. Our calculations, which predict that only the second complex is a single-molecule magnet (with a magnetic anisotropy energy barrier of 5.6 K), are in qualitative agreement with the experimental data. The analysis of the projected anisotropies of each Fe(II) ion, together with a study of the variation of the D value as a function of several geometrical parameters, allows us to qualitatively understand the different magnetic behaviors of both complexes. In addition to this, we also present a simple rule based on the analysis of the molecular orbitals of the system that allows us to predict how to enhance (by a factor of 6, approximately) the magnetic anisotropy barrier of these systems. Specifically, we will show that, for high-spin Fe(II) ions, the local easy axis of magnetization is perpendicular to the plane defined by the Fe(II)-d orbital which is doubly occupied. If similar rules were found for other metal ions, rational synthetic strategies to control magnetic anisotropy could be established.     

Theoretical study of exchange coupling in 3d-Gd complexes: large magnetocaloric effect systems

Polynuclear 3d transition metal-Gd complexes are good candidates to present large magnetocaloric effect. This effect is favored by the presence of weak ferromagnetic exchange interactions that have been investigated using methods based on Density Functional Theory. The first part of the study is devoted to dinuclear complexes, focusing on the nature and mechanism of such exchange interactions. The presence of two bridging ligands is found more favorable for ferromagnetic coupling than a triple-bridged assembly, especially for complexes with small M-O···O-Gd hinge angles. Our results show the crucial role of the Gd 5d orbitals in the exchange interaction while the 6s orbital seems to have a negligible participation. The analysis of the atomic and orbital spin populations reveals that the presence of spin density in the Gd 5d orbital is mainly due to a spin polarization effect, while a delocalization mechanism from the 3d orbitals of the transition metal can be ruled out. We propose a numerical DFT approach using pseudopotentials to calculate the exchange coupling constants in four polynuclear first-row transition metal-Gd complexes. Despite the complexity of the studied systems, the numerical approach gives coupling constants in excellent agreement with the available experimental data and, in conjunction with exact diagonalization methods (or Monte Carlo simulations), it makes it possible to obtain theoretical estimates of the entropy change due to the magnetization/demagnetization process of the molecule.     

Molecular metamagnet [Ni(4ImNNH)(2)(NO(3))(2)] (4ImNNH = 4-imidazolyl nitronyl nitroxide) and the related compounds showing supramolecular H-bonding interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)(2)X(2)], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO(3); M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P2(1)/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/k(B) = +85 K, where the spin Hamiltonian is defined as H = -2J(S(1).S(2) + S(2).S(3)) based on the molecular structures determined as the linear radical-metal-radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical pi and metal dsigma orbitals. Compounds 1-3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)(2)X(2)] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical-metal-radical system. The crystal structures determined here on 1-4, [Mn(4ImNNH)(2)Cl(2)], and [Cu(4ImNNH)(2)Br(2)] always have intermolecular hydrogen bonds of H(imidazole).X(axial ligand)-M, where X = NO(3), Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling.     

Experimental evidence and DFT studies of next-nearest-neighbor magnetic interactions through diamagnetic 3d and 4d ions

The copper template effect allows the preparation of tridentate ligands that chelate copper ions, leaving unoccupied the fourth basal coordination position and at least one axial position of the copper coordination polyhedron. Two such cationic complexes, [LCu](+) and [L(1)Cu](+) (L(-) = 2-{(E)-[(2-aminoethyl)imino]methyl}phenoxo] and L(1-) = 2-{(E)-[(2-aminopropyl)imino]methyl}phenoxo), react with diamagnetic polycyanometalate tectons such as Ni(CN)(4)(2-) or Ag(CN)(2)(-) to yield different neutral 1D complexes. In {[(LCu)(2)Ni(CN)(4)]}(n) (1) the four cyano nitrogen atoms are involved in coordination with copper ions in such a manner that each copper atom is pentacoordinated and linked to two cyano functions that occupy axial and equatorial coordination positions. Two L(1)Cu(+) cationic entities are linked, through their equatorial plane, to two trans cyano groups of the Ni(CN)(4)(2-) tecton in complex [(L(1)Cu)(2)Ni(CN)(4)] (2), the two uncoordinated cyano groups being involved in hydrogen bonds. 2 is a racemate, a S stereoisomer being associated with a R one in each [(L(1)Cu)(2)Ni(CN)(4)] unit. Zigzag Cu-Ag chains are present in [(LCu)Ag(CN)(2)] (3), where the copper centers are pentacoordinated and connected to the cyano groups in an alternate axial-equatorial coordination scheme. A bidimensional structure is developed by interchain argentophilic interactions. In complex 4, {(L(1)CuMeOH)(L(1)Cu)[Ag(CN)(2)](2)}, two L(1)Cu units are connected by a NC-Ag-CN bridge in an equatorial position. These resulting units exhibit argentophilic interactions with [Ag(CN(2))](-) entities that are monocoordinated in the equatorial position to the next unit, ultimately leading to a chain. Weak Cu-Cu magnetic interactions are detected in the four compounds, antiferromagnetic in the case of equatorial-equatorial copper interactions, ferromagnetic for orthogonal interacting copper orbitals (axial-equatorial interactions), while axial-axial bridges are characterized by an absence of interaction. The presence of weak ferromagnetic interactions through large NC-Ni-CN or NC-Ag-CN bridges (Cu···Cu distances larger than 10 ?) furnishes experimental evidence for the existence of next-nearest-neighbor interactions through diamagnetic centers. DFT calculations do confirm the existence of these magnetic transmission pathways through the diamagnetic metal bridge.     

Magnetic Coupling Constants of Self‐Assembled CuII [3×3] Grids: Alternative Spin Model from Theoretical Calculations

This paper reports a theoretical analysis of the electronic structure and magnetic properties of a ferromagnetic Cu^II [3×3] grid. A two‐step strategy, combining calculations on the whole grid and on binuclear fragments, has been employed to evaluate all the magnetic interactions in the grid. The calculations confirm an S=7/2 ground state, which is in accordance with the magnetisation versus field curve and the thermal dependence of the magnetic moment data. Only the first‐neighbour coupling terms present non‐negligible amplitudes, all of them in agreement with the structure and arrangement of the Cu 3d magnetic orbitals. The results indicate that the dominant interaction in the system is the antiferromagnetic coupling between the ring and the central Cu sites (J₃=J₄≈?31 cm^?1). In the ring two different interactions can be distinguished, J₁=4.6 cm^?1 and J₂=?0.1 cm^?1, in contrast to the single J model employed in the magnetic data fit. The calculated J values have been used to determine the energy level distribution of the Heisenberg magnetic states. The effective magnetic moment versus temperature plot resulting from this ab initio energy profile is in good agreement with the experimental curve and the fitting obtained with the simplified spin model, despite the differences between these two spin models. This study underlines the role that the theoretical evaluations of the coupling constants can play on the rationalisation of the magnetic properties of these complex polynuclear systems.     

DFT description of the electronic structure and spectromagnetic properties of strongly correlated electronic systems: NiII, CuII and ZnII o-dioxolene complexes

The spectroscopic and magnetic properties of dioxolene complexes of zinc, copper and nickel were studied by DFT calculations on model complexes of formulas [(NH(3))(4)M(II)(SQ)](+) (M=Zn, Ni; SQ=semiquinonato) and [(NH(3))(2)Cu(II)(SQ)](+). Standard approaches such as time-dependent DFT (TDDFT), the Slater transition state (STS), and broken symmetry (BS) were found to be unable to completely account for the physical properties of the systems, and complete active space-configuration interaction (CAS-CI) calculations based on the Kohn-Sham (KS) orbitals was applied. The CAS-CI energies, properly corrected with multireference perturbation theory (MR-PT), were found to be in good agreement with experimental data. We present here a calculation protocol that has a low CPU cost/accuracy ratio and seems to be very promising for interpreting the properties of strongly correlated electronic systems in complexes of real chemical size.     

A ferromagnetically coupled CrCu3 tetramer and GdCu4 pentamer with a [15]N4 macrocylic ligand incorporating an oxamido bridge

The synthesis and structural and magnetic properties of heteropolynuclear complexes [(L(3)Cu)(3)Cr](CH(3)CN)(3)(ClO(4))(3) (2) and [(L(3)Cu)(4)Gd.H(2)O](CH(3)OH)(H(2)O)(ClO(4))(3) (3) (H(2)L(3) ligand is 2,3-dioxo-5,6:14,15-dibenzo-1,4,8,12-tetraazacyclo-pentadeca-7,12-diene) and their precursor L(3)Cu (1) are presented. Complex 2 crystallizes in space group P2(1)/n with cell parameters a = 20.828(6) A, b = 18.321(5) A, c = 7.578(5) A, alpha = 90 degrees, beta = 91.990(8) degrees, gamma = 90 degrees, and Z = 4. The Cr(III) center is coordinated by six oxygen atoms from three Cu(II) precursors. The Cr-O bonds range over 1.948-1.982 A. The coordination environments of all the terminal Cu(II) ions change in comparison with their Cu(II) precursor. The ferromagnetic coupling (J = 16.48(1) cm(-)(1)) observed for 2 can be rationalized by symmetry considerations. For any pair of interacting magnetic orbitals, strict orthogonality is obeyed and the interaction is ferromagnetic. Complex 3 crystallizes in space group P1 with cell parameters a = 14.805(4) A, b = 16.882(5) A, c = 17.877(5) A, alpha = 75.403(5) degrees, beta = 83.317(6) degrees, gamma = 70.600(5) degrees, and Z = 2. The central Gd(III) assumes an 8 + 1 coordination environment, namely eight oxygen atoms from four Cu(II) precursors and one oxygen atom from H(2)O. The fit of the experimental data gives J = 0.27(2) cm(-)(1), g(Gd) = 1.98(1), and g(Cu) = 2.05(1). This small and positive J value shows weak ferromagnetic interaction between metal ions.     

On the Controversial Fitting of Susceptibility Curves of Ferromagnetic CuII Cubanes: Insights from Theoretical Calculations

This paper reports a theoretical analysis of the electronic structure and magnetic properties of a tetranuclear Cu^II complex, [Cu₄(HL)₄], which has a 4+2 cubane‐like structure (H₃L=N,N′‐(2‐hydroxypropane‐1,3‐diyl)bis(acetylacetoneimine)). These theoretical calculations indicate a quintet (S=2) ground state; the energy‐level distribution of the magnetic states confirm Heisenberg behaviour and correspond to an S₄ spin–spin interaction model. The dominant interaction is the ferromagnetic coupling between the pseudo‐dimeric units (J₁=22.2 cm^?1), whilst a weak and ferromagnetic interaction is found within the pseudo‐dimeric units (J₂=1.4 cm^?1). The amplitude and sign of these interactions are consistent with the structure and arrangement of the magnetic Cu 3d orbitals; they accurately simulate the thermal dependence of magnetic susceptibility, but do not agree with the reported J values (J₁=38.4 cm^?1, J₂=?18.0 cm^?1) that result from the experimental fitting. This result is not an isolated case; many other polynuclear systems, in particular 4+2 Cu^II cubanes, have been reported in which the fitted magnetic terms are not consistent with the geometrical features of the system. In this context, theoretical evaluation can be considered as a valuable tool in the interpretation of the macroscopic behaviour, thus providing clues for a rational and directed design of new materials with specific properties.     

Does the Sign of the Cu–Gd Magnetic Interaction Depend on the Number of Atoms in the Bridge?

Several theoretical investigations with CASSCF methods confirm that the magnetic behavior of Cu–Gd complexes can only be reproduced if the 5d Gd orbitals are included in the active space. These orbitals, expected to be unoccupied, do present a low spin density, which is mainly due to a spin polarization effect. This theory is strengthened by the experimental results reported herein. We demonstrate that Cu–Gd complexes characterized by Cu–Gd interactions through single‐oxygen and three‐atom bridges consisting of oxygen, carbon, and nitrogen atoms, present weak ferromagnetic exchange interactions, whereas complexes with bridges made of two atoms, such as the nitrogen–oxygen oximato bridge, are subject to weak antiferromagnetic exchange interactions. Therefore, a bridge with an odd number of atoms induces a weak ferromagnetic exchange interaction, whereas a bridge with an even number of atoms supports a weak antiferromagnetic exchange interaction, as observed in pure organic compounds and also, as in this case, in metal–organic compounds with an active spin polarization effect.     

Is [Cu3(L2)Cl2]2+ a molecule with geometrically spin frustration? DFT study on magnetic competing interaction in hetero-bridged tricopper(II) complex

The theoretical calculations on magnetic exchange interaction of the hetero-bridged tricopper(II) complex [Cu₃(L²)Cl₂]²⁺ and a related binuclear copper(II) model are carried out by using the density functional theory combined with the broken-symmetry approach. Meanwhile, one strategy computationally, so called isolated magnetic pair approach, is suggested to explore the spin frustration from geometry topology in poly-nuclear magnetic systems. It is found that the ferromagnetic coupling (J₂>0) of Cu1–Cu3 pair bridged by double μ–Cl ligands, in nature, is intrinsic, not resulted from geometrically spin frustration in the hetero-bridged tricopper(II) studied. However, in the whole molecule exist two competing contributions of antiferromagnetic and ferromagnetic coupling, and the antiferromagnetic coupling (J₁) from Cu1–(μ–OR)–Cu2 and Cu2–(μ–OR)–Cu3 pairs dominates the magnetic behavior of the whole molecular system. On the other hand, the variation of J₁/J₂ ratio affects significantly on magnetic properties of the system. The calculated effective magnetic moment μ_eff of 2.26μ_B at the OPerdew functional level is compared to experimentally observation of 2.70μ_B in the solution. The briefly analysis of molecular magnetic orbitals demonstrated that the two local magnetic orbitals on Cu1 and Cu3 ions are orthogonal each other, and primarily responsible for the intrinsic ferromagnetic coupling between Cu1–(μ–Cl)₂–Cu3. By comparison of spin population distributions for the Cu-triad and Cu-dimer the validity of the isolated magnetic pair approach is confirmed.     

Anion influence on the structure and magnetic properties of a series of multidimensional pyrimidine-2-carboxylato-bridged copper(II) complexes

Seven new polynuclear copper(II) complexes of formula [Cu(mu-pymca)2] (1) (pymca(-) = pyrimidine-2-carboxylato), [Cu(mu-pymca)Br] (2), [Cu(mu-pymca)Cl] (3), [Cu(mu-pymca)(SCN)(H2O)] x 4 H2O (4), [Cu(mu-pymca)N3] (5), [Cu2(mu1,5-dca)2(pymca)2] (6) (dca = dicyanamide), and K{[mu-Au(CN)2]2[(Cu(NH3)2)2(mu-pymca)]}[Au(CN)2]2 (7) have been synthesized by reactions of K-pymca with copper(II) ions in the presence of different counteranions. Compound 1 is a linear neutral chain with a carboxylato bridging ligand in a syn-anti coordination mode, whereas complexes 2 and 3 consist of cationic linear chains with cis and trans bis(chelating) pymca bridging ligands. Complex 4 adopts a helical pymca-bridged chain structure. In complex 5, zigzag pymca-bridged chains are connected by double end-on azide bridging ligands to afford a unique honeycomb layer structure. Complex 6 is a centrosymmetric dinuclear system with double mu 1,5-dicyanamide bridging ligands and pymca end-cap ligands. Complex 7 is made of pymca-bridged dinuclear [Cu(NH3)2(mu-pymca)Cu(NH3)2](3+) units connected by [Au(CN)2](-) anions to four other dinuclear units, giving rise to cationic (4,4) rectangular nets, which are linked by aurophilic interactions to afford a singular 3D network. Variable-temperature magnetic susceptibility measurements show that complex 1 exhibits a very weak antiferromagnetic coupling through the syn-anti (equatorial-axial) carboxylate bridge (J = -0.57 cm(-1)), whereas complexes 2-4 and 7 exhibit weak to strong antiferromagnetic couplings through the bis(chelating) pymca bridging ligand J = -17.5-276.1 cm(-1)). Quantum Monte Carlo methods have been used to analyze the experimental magnetic data for 5, leading to an antiferromagnetic coupling (J = -34 cm(-1)) through the pymca ligand and to a ferromagnetic coupling (J = 71 cm(-1)) through the azide bridging ligands. Complex 6 exhibits a very weak antiferromagnetic coupling through the dicyanamide bridging ligands (J = -5.1 cm(-1)). The magnitudes of the magnetic couplings in complexes 2-5 have been explained on the basis of the overlapping between magnetic orbitals and DFT theoretical calculations.     

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.     

Structures with tunable strong ferromagnetic coupling: from unordered (1D) to ordered (Discrete)

The X-ray crystal structures, magnetic susceptibilities from 2 to 300 K, and theoretical analyses of the magnetism for 1D and trinuclear azido Cu(II) carboxylate complexes [Cu(1.5)(hnta)(N(3))(2)(H(2)O)](n) (1) and [Cu(3)(hnta)(4)(N(3))(2)(H(2)O)(3)] (2), respectively, where hnta is 6-hydroxynicotinate, are described. Although both exhibit strong ferromagnetic coupling, discrete complex 2 exhibits long-range ferromagnetic ordering, while the very similar 1D system 1 does not. Density functional calculations provided accurate J values and allowed rationalization of the ferromagnetic coupling in terms of the magnetic orbitals and spin densities.     

An orthogonal ferromagnetically coupled tetracopper(II) 2 x 2 homoleptic grid supported by micro-O4 bridges and its DFT study

A pyrazole based ditopic ligand (PzOAP), prepared by the reaction between 5-methylpyrazole-3-carbohydrazide and methyl ester of imino picolinic acid, reacts with Cu(NO3)2.6H2O to form a self-assembled, ferromagnetically coupled, alkoxide bridged tetranuclear homoleptic Cu(II) square grid-complex [Cu4(PzOAP)4(NO3)2] (NO3)2.4H2O (1) with a central Cu4[micro-O4] core, involving four ligand molecules. In the Cu4[micro-O4] core, out of four copper centers, two copper centers are penta-coordinated and the remaining two are hexa-coordinated. In each case of hexa-coordination, the sixth position is occupied by the nitrate ion. The complex 1 has been characterized structurally and magnetically. Although Cu-O-Cu bridge angles are too large (138-141 degrees) and Cu-Cu distances are short (4.043-4.131 A), suitable for propagation of expected antiferromagnetic exchange interactions within the grid, yet intramolecular ferromagnetic exchange (J = 5.38 cm(-1)) is present with S = 4/2 magnetic ground state. This ferromagnetic interaction is quite obvious from the bridging connections (d(x2-y2)) lying almost orthogonally between the metal centers. The exchange pathways parameters have been evaluated from density functional calculations.     

Crystal refinement and magnetic structure of KNi4(PO4)3: a novel example of three interacting magnetic sub-lattices

KNi(4)(PO(4))(3) has been synthesised following a method previously reported by some of us and studied on the basis of magnetization and neutron powder diffraction (NPD) data. Magnetization measurements suggest the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interactions: magnetization versus magnetic field curves present a remanent magnetization of around 2.15 micro(B) at T=2 K. The magnetic structure of the KNi(4)(PO(4))(3) has been determined at low temperature from the NPD data. These measurements show that there are three magnetic sub-lattices of Ni(2+) ions, which interact through common oxygen or phosphate groups, giving rise to FM and AFM couplings. The resulting interactions are FM in nature. Such a complex behaviour could provide an interesting model to analyse magnetic interactions in more condensed systems, such in mixed metal oxides.     

A new fragment-based approach for calculating electronic excitation energies of large systems

We present a new fragment-based scheme to calculate the excited states of large systems without necessity of a Hartree-Fock (HF) solution of the whole system. This method is based on the implementation of the renormalized excitonic method [M. A. Hajj et al., Phys. Rev. B 72, 224412 (2005)] at ab initio level, which assumes that the excitation of the whole system can be expressed by a linear combination of various local excitations. We decomposed the whole system into several blocks and then constructed the effective Hamiltonians for the intra- and inter-block interactions with block canonical molecular orbitals instead of widely used localized molecular orbitals. Accordingly, we avoided the prerequisite HF solution and the localization procedure of the molecular orbitals in the popular local correlation methods. Test calculations were implemented for hydrogen molecule chains at the full configuration interaction, symmetry adapted cluster/symmetry adapted cluster configuration interaction, HF/configuration interaction singles (CIS) levels and more realistic polyene systems at the HF/CIS level. The calculated vertical excitation energies for lowest excited states are in reasonable accordance with those determined by the calculations of the whole systems with traditional methods, showing that our new fragment-based method can give good estimates for low-lying energy spectra of both weak and moderate interaction systems with economic computational costs.