Intramolecular "Aryl-metal chelate ring" pi,pi-interactions as structural evidence for metalloaromaticity in (aromatic alpha,alpha'-diimine)-copper(II) chelates: molecular and crystal structure of aqua(1,10-phenanthroline)(2-benzylmalonato)copper(II) three-hydrate

By reaction of Cu(2)CO(3)(OH)(2), 2-benzylmalonic acid (H(2)Bzmal), and 1,10-phenanthroline (phen), [Cu(Bzmal)(phen)(H(2)O)] x 3H(2)O (compound 1) has been obtained and characterized by thermal, spectral, magnetic, and X-ray diffraction methods. The molecular structure of 1 is remarkably similar to that of [Cu(Bzmal)(bipy)(H(2)O)] x 2H(2)O (compound 2, bipy = 2,2'-bipyridine). In both complexes, the aryl(Bzmal) ring produces an unexpected pi,pi-stacking interaction with the Cu(II)-(aromatic alpha,alpha'-diimine) chelate ring, at an average distance d(pi)(-)(pi) of 3.40 A, involving roughly parallel and smoothly slipped rings. This insight is discussed as new structural evidence for metalloaromaticity of Cu(II)-(aromatic alpha,alpha'-diimine) chelate rings. Interestingly, 1 recognizes itself by a weak intermolecular pi,pi-stacking interaction between aryl(Bzmal) ligands to give pairs of complex molecules. In contrast, there is an intermolecular pyridyl-pyridyl pi,pi-stacking interaction also forming pairs of complex molecules in 2.     

Coordination complexes of 2-(4-quinolyl)nitronyl nitroxide with M(hfac)(2) [M = Mn(II), Co(II), and Cu(II)]: syntheses, crystal structures, and magnetic characterization

Three new complexes of the formula M(2)L(2) derived from 2-(4-quinolyl)nitronyl nitroxide (4-QNNN) and M(hfac)(2) [M = Mn(II), Co(II), and Cu(II)], (4-QNNN)(2).[Mn(hfac)(2)](2) (1), (4-QNNN)(2).[Co(hfac)(2)](2).2H(2)O (2), and (4-QNNN)(2).Cu(hfac)(2).Cu'(hfac)(2) (3), were synthesized and characterized structurally as well as magnetically. Complexes 1 and 2 are four-spin complexes with quadrangle geometry, in which both the nitrogen atoms of quinoline rings and oxygen atoms of nitronyl nitroxides are involved in the formation of coordination bonds. For complex 3, however, the nitrogen atoms of quinoline rings are coordinated with Cu(II) ion to afford a three-spin complex, which is further linked to another molecule of Cu(hfac)(2) (referred to as Cu'(hfac)(2)) to form a 1D alternating chain. The magnetic behaviors of the three complexes were investigated. For complex 1, as the nitronyl nitroxides and Mn(II) ions are strongly antiferromagnetically coupled, consequently its temperature dependence of magnetic susceptibility was fitted to the model of spin-dimer with S = 2, yielding the intradimer magnetic exchange constant of J = -0.82 cm(-1). For complex 2, the temperature dependence of the magnetic susceptibility in the T > 50 K region was simulated with the model of two-spin unit with S(1) = 3/2 and S(2) = 1/2, leading to J = -321.9 cm(-1) for the magnetic interaction due to Co(II).O coordination bonding, D = -16.3 cm(-1) (the zero-field splitting parameter), g = 2.26, and zJ = -3.8 cm(-1) for the magnetic interactions between Co(II) ions and nitronyl nitroxides through quinoline rings and those between nitronyl nitroxides due to the short O.O short contacts. The temperature dependence of magnetic susceptibility of 3 was approximately fitted to a model described previously affording J(1) = -6.52 cm(-1) and J(2) = 3.64 cm(-1) for the magnetic interaction between nitronyl nitroxides and Cu(II) ions through the quinoline unit via spin polarization mechanism and the weak O.Cu coordination bonding, respectively.     

Structures and Stabilities of Ternary Copper(II) Complexes with 3,5-Diiodo-L-tyrosinate. Weak Interactions Involving Iodo Groups

Structures and stabilities of the ternary copper(II) complexes Cu(DA)(AA), where AA refers to 3,5-diiodo-L-tyrosinate (I(2)tyr) or L-tyrosinate (Tyr) and DA refers to 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), 2-(aminomethyl)pyridine (ampy), histamine (hista), or ethylenediamine (en), have been investigated by potentiometric, spectroscopic, and X-ray diffraction methods. The stability constants have been determined by potentiometric titrations at 25 degrees C and ionic strength I = 0.1 M (KNO(3)). The equilibrium constants K for a hypothetical equilibrium, Cu(DA)(Ala) + Cu(en)(AA) Cu(DA)(AA) + Cu(en)(Ala) where Ala refers to L-alanine, have been calculated from the determined overall stability constants of the ternary complexes for estimating the stability enhancement due to the stacking interaction between the aromatic rings in Cu(DA)(AA). Large positive log K values have been obtained for the Cu(DA)(I(2)tyrOH) and Cu(DA)(I(2)tyrO(-)) systems (DA = phen or bpy, OH and O(-) refer to the protonated and deprotonated forms of the phenol moiety, respectively), indicating that the complexes are stabilized by effective stacking. Differences between the log K values for Cu(DA)(I(2)tyr) and Cu(DA)(Tyr) systems indicate that the iodine substituents greatly contribute to the stability enhancement. A distinct circular dichroism (CD) magnitude anomaly was also observed for the systems with large log K value, supporting the existence of the stacking interaction in Cu(DA)(AA). Two complexes, [Cu(bpy)(I(2)tyrO(-))(H(2)O)].2H(2)O (1) and [Cu(bpy)(I(2)tyrOH)(NO(3))].CH(3)OH (2), have been isolated as crystals and analyzed by the X-ray diffraction method. Both 1 and 2 crystallized in the orthorhombic space group P2(1)2(1)2(1) with four molecules in a unit cell of dimensions a = 9.2339(4), b = 16.9230(8), and c = 14.8584(5) ? for complex 1, and a = 11.2240(8), b = 11.715(1), and c = 17.966(2) ? for complex 2. The central Cu(II) ion for both complexes has a similar distorted five-coordinate square-pyramidal geometry with the equatorial positions occupied by the two nitrogen atoms of bpy and the nitrogen and oxygen atoms of I(2)tyr, and the apical position is occupied by a water molecule (for 1) or a nitrate ion (for 2). The opposite site to the axial water or nitrate oxygen atom is intramolecularly occupied by the side chain aromatic ring, which is approximately parallel to the copper coordination plane with the average spacing of 3.31 or 3.30 ? for complex 1 or 2, respectively, directly exhibiting the effective stacking interaction between the aromatic rings in the solid state. Distances between the iodine and one of the pyridine rings of bpy (3.79 ? for 1 and 3.56 ? for 2) are shorter than the van der Waals distance (3.85 ?), implying that the iodine substituent may be involved in a weak bonding interaction with the pyridine ring. Effects of the iodine substituents on the stacking interactions between the diiodophenol side ring and the coordinated aromatic diamine and their possible biological relevance have been discussed.     

Kinetic resolution of rac-phenylalanine by stereoselective complexation to a chiral cobalt complex through pi-pi stacking interaction

A cobalt(III) complex with chiral ligand, H2cpel (N-carboxymethyl-N-pyridylethyl-l-leucine), was prepared for chiral recognition of amino acids. Through the competitive coordination of racemic phenylalanine to the chiral cobalt complex, [Co(cpel)(CO(3))](-) (1), enantioselective recognition was achieved on the ternary complex, which was determined on the basis of HPLC analysis with a chiral column. The formation rate for the [Co(cpel)(l-phe)] complex (2) was 6-times superior to that of [Co(cpel)(d-phe)] (3). The preferential formation of 2 might be illustrated by the interligand pi-pi stacking interaction. Crystal structural analysis for 2 and 3 revealed that aromatic rings, pyridine ring of CPEL and phenylalanine sidechain, in 2 were very close each other but those in 3 were far apart. Such interligand aromatic interaction in 2 was also examined by the use of (1)H NMR spectra.     

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

Large amplitude motions (LAMs) form a fundamental phenomenon that demands the development of specific theoretical and Hamiltonian models. In recent years, along with the strong progress in instrumental techniques on high-resolution microwave spectroscopy and computational capacity in quantum chemistry, studies on LAMs have become very diverse. Larger and more complex molecular systems have been taken under investigation, ranging from series of heteroaromatic molecules from five- and six-membered rings to polycyclic-aromatic-hydrocarbon derivatives. Such systems are ideally suited to create families of molecules in which the positions and the number of LAMs can be varied, while the heteroatoms often provide a sufficient dipole moment to the systems to warrant the observation of their rotational spectra. This review will summarize three types of LAMs: internal rotation, inversion tunneling, and ring puckering, which are frequently observed in aromatic five-membered rings such as furan, thiophene, pyrrole, thiazole, and oxazole derivatives, in aromatic six-membered rings such as benzene, pyridine, and pyrimidine derivatives, and larger combined rings such as naphthalene, indole, and indan derivatives. For each molecular class, we will present the representatives and summarize the recent insights on the molecular structure and internal dynamics and how they help to advance the field of quantum mechanics.     

Molecular assembly directed by metal-aromatic interactions: control of the aggregation and photophysical properties of Zn-salen complexes by aromatic mercuration

There is widespread interest in non-covalent bonding and weak interactions, such as electrostatic interactions, hydrogen bonding, solvophobic/hydrophobic interactions, metal-metal interactions, and π-π stacking, to tune the molecular assembly of planar π-conjugated organic and inorganic molecules. Inspired by the roles of metal-aromatic interaction in biological systems, such as in ion channels and metalloproteins, herein, we report the first example of the use of Hg(2+) -aromatic interactions to selectively control the assembly and disassembly of zinc-salen complexes in aqueous media; moreover, this process exhibited significant "turn on" fluorescent properties. UV/Vis and fluorescence spectroscopic analysis of the titration of Hg(2+) ions versus complex ZnL(1) revealed that the higher binding affinity of Hg(2+) ions (compared to 13 other metal ions) was ascribed to specific interactions between the Hg(2+) ions and the phenyl rings of ZnL(1); this result was also confirmed by (1)H NMR spectroscopy and HRMS (ESI). Further evidence for this type of interaction was obtained from the reaction of small-molecule analogue L(1) with Hg(2+) ions, which demonstrates the proximity of the N-alkyl group to the aromatic protons during Hg(2+)-ion binding, which led to the consequential H/D exchange reaction with D(2) O. DFT modeling of such interactions between the Hg(2+) ions and the phenyl rings afforded calculated distances between the C and Hg atoms (2.29 ?) that were indicative of C-Hg bond-formation, under the direction of the N atom of the morpholine ring. The unusual coordination of Hg(2+) ions to the phenyl ring of the metallosalen complexes not only strengthened the binding ability but also increased the steric effect to promote the disassembly of ZnL(1) in aqueous media.     

Synthesis, characterisation and magnetic properties of octahedral chromium(III) compounds with six C-donor ligands

The homoleptic, square pyramidal organochromium(III) compound [NBu(4)](2)[Cr(C(6)F(5))(5)] (1) reacts with excess organic isocyanides, CNR [R = (t)Bu, 2,6-dimethylphenyl (Xy)], under dissociation of the apical C(6)F(5) ligand to give the more saturated, singly charged complexes [NBu(4)][trans-Cr(C(6)F(5))(4)(CNR)(2)] [R = (t)Bu (2), Xy (3)], containing six monodentate C-donor ligands. These compounds exhibit an axially distorted octahedral structure (single-crystal X-ray diffraction) with the four C(6)F(5) groups defining the equatorial positions and the CNR ligands occupying the axial ones. Compounds 2 and 3 both behave as spin quartet species (S = 3/2) at microscopic level (EPR spectroscopy), their macroscopic magnetic properties depending upon the nature of the terminal R group, as established by magnetisation measurements. When the R substituent is the saturated alkyl group (t)Bu, the compound (2) behaves as a simple paramagnet, with no magnetic interaction between individual Cr(III) centres along the whole temperature range measured (1.8-265 K). By contrast, a weak antiferromagnetic interaction is detected for compound 3 at low temperature with T(N) = 0.19(1) K. Since the closest intermetallic distances are similar in the crystals of 2·CH(2)Cl(2) and 3·1.75CH(2)Cl(2) (ca. 1.1 nm), we conclude that the insaturation of the aromatic Xy group together with the extended intermolecular π-π stacking interactions between Xy rings observed in the crystal lattice of 3·1.75CH(2)Cl(2) (centroid-to-centroid distance: 0.35 nm) favour magnetic interaction between the individual magnetic centres.     

Sandwich complexes based on the "all-metal" Al4 2- aromatic ring

We report on novel sandwichlike structures [Al(4)MAl(4)](q-) (q=0-2 and M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W) based on the recently synthesized all-metal aromatic Al(4)(2-) square ring. The sandwichlike structures have two aromatic tetraaluminum square rings which trap a transition-metal cation from either the first, second, or third row. The stability of the anionic sandwichlike complexes towards electron detachment is discussed, and addition of alkali cations is found to stabilize the 2- charged complexes, preventing spontaneous electron detachment. Once the sandwichlike complexes are formed, the Al(4)(2-) square properties remain nearly unchanged; this fact strongly supports the hypothesis that in these complexes the Al(4)(2-) square rings remain aromatic.     

Ca(Ⅱ)-NTP-HRB混配物的稳定性和配体堆积作用研究

The stability constants of the ternary mixed-ligand complexes of Ca(NTP)(HRB) n-(n=2 or 3) have been determined by potentiometric pH titration in aqueous solution (I=0.1mol/L,KNO3,25℃),Where NTP refers to adenosine 5′-triphorphat e (ATP) and uridine 5′-triphoophate (UTP);HRB,1,10-phenanthroline (Phen),2,2 ′-bipyridyl (Bpy),and L-tryptophan (Trp).The stability difference between the ternany complexes and the binary complexes has been compared and discussed.The increased stability in ternany mixed-ligand complexes was attributed to the coo perative effect of πA-πB and the intramolcular ligand-ligand aromatic st acking interaction.The extent of the stacking interaction in the ternary calcium compl exes was calculated,which is in accordance with the size of the aromatic rings f orming the stacks.     

Metal ligand aromatic cation-pi interactions in metalloproteins: ligands coordinated to metal interact with aromatic residues

Cation-pi interactions between aromatic residues and cationic amino groups in side chains and have been recognized as noncovalent bonding interactions relevant for molecular recognition and for stabilization and definition of the native structure of proteins. We propose a novel type of cation-pi interaction in metalloproteins; namely interaction between ligands coordinated to a metal cation--which gain positive charge from the metal--and aromatic groups in amino acid side chains. Investigation of crystal structures of metalloproteins in the Protein Data Bank (PDB) has revealed that there exist quite a number of metalloproteins in which aromatic rings of phenylalanine, tyrosine, and tryptophan are situated close to a metal center interacting with coordinated ligands. Among these ligands are amino acids such as asparagine, aspartate, glutamate, histidine, and threonine, but also water and substrates like ethanol. These interactions play a role in the stability and conformation of metalloproteins, and in some cases may also be directly involved in the mechanism of enzymatic reactions, which occur at the metal center. For the enzyme superoxide dismutase, we used quantum chemical computation to calculate that Trp163 has an interaction energy of 10.09 kcal mol(-1) with the ligands coordinated to iron.     

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.     

Molecular and electronic structure, magnetotropicity and absorption spectra of benzene-trinuclear copper(I) and silver(I) trihalide columnar binary stacks

The molecular and electronic structures, stabilities, bonding features, magnetotropicity and absorption spectra of benzene-trinuclear Cu(I) and Ag(I) trihalide columnar binary stacks with the general formula [c-M(3)(μ(2)-X)(3)](n)(C(6)H(6))(m) (M = Cu, Ag; X = halide; n, m ≤ 2) have been investigated by means of electronic structure calculation methods. The interaction of c-M(3)(μ(2)-X)(3) clusters with one and two benzene molecules yields 1:1 and 1:2 binary stacks, while benzene sandwiched 2:1 stacks are formed upon interaction of two c-M(3)(μ(2)-X)(3) clusters with one benzene molecule. In all binary stacks the plane of the alternating c-M(3)(μ(2)-X)(3) and benzene components adopts an almost parallel orientation. The separation distance between the centroids of the benzene and the proximal c-M(3)(μ(2)-X)(3) metallic cluster found in the range 2.97-3.33 ? at the B97D/Def2-TZVP level is indicative of a π···π stacking interaction mode, for the centroid separation distance is very close to the sum of the van der Waals radii of Cu···C (3.10 ?) and Ag···C (3.44 ?). Energy decomposition analysis (EDA) at the SSB-D/TZP level revealed that the dominant term in the c-M(3)(μ(2)-X)(3)···C(6)H(6) interaction arises from dispersion and electrostatic forces while the covalent interactions are predicted to be negligible. On the other hand, charge decomposition analysis (CDA) illustrated very small charge transfer from C(6)H(6) toward the c-M(3)(μ(2)-X)(3) clusters, thus reflecting weak π-base/π-acid interactions which are further corroborated by the respective electrostatic potentials and the fact that the total dipole moment vector points to the center of the metallic ring of the c-M(3)(μ(2)-X)(3) cluster. The absorption spectra of all aromatic columnar binary stacks simulated by means of TD-DFT calculations showed strong absorptions in the UV region. The main features of the simulated absorption spectra are thoroughly analyzed, and assignments of the contributing electronic transitions are given. The magnetotropicity of the binary stacks evaluated by the NICS(zz)-scan curves indicated an enhancement of the diatropicity of the inorganic ring upon interaction with the aromatic benzene molecule. Noteworthy is the slight enhancement of the diatropicity of the benzene ring, particularly in the region between the interacting rings, probably due to the superposition (coupling) of the diamagnetic ring currents of the interacting aromatic ring systems.     

Tuning of magnetic properties of polynuclear lanthanide(III)-octacyanotungstate(V) systems: determination of ligand-field parameters and exchange interaction

The self-assembly reaction between trivalent lanthanide ions, 2,2':6',2' '-terpyridine (terpy) ligand, and octacyanotungstate(V) leads to the formation of two series of isomorphous cyano-bridged compounds: (i) one-dimensional (1-D) chains [Ln(terpy)(DMF)(4)][W(CN)(8)].6H(2)O.C(2)H(5)OH (Ln = Ce-Dy) and (ii) dinuclear molecules [Ln(terpy)(DMF)(2)(H(2)O)(2)][W(CN)(8)].3H(2)O (Ln = Ho, Er, Yb) and the ionic [Tm(III)(terpy)(DMF)(2)(H(2)O)(3)][WV(CN)(8)].4H(2)O.DMF (DMF = N,N-dimethylformamide) system. The crystal structures of 1-D chains consist of alternating {[W(CN)(8)]} and {[Ln(terpy)]} building blocks. The neighboring chains are weakly linked through the pi-pi stacking interactions of the aromatic rings, leading to two-dimensional supramolecular layers. The dinuclear species are weakly linked through the hydrogen bonds between H2O molecules and terminal cyano ligands resulting in a columnlike arrangement of dimers. Taking into account the ligand-field splitting and the exchange interaction, we have estimated the magnetic couplings between the Ln(III) and WV centers in a series of polycrystalline 1-D chains and in dimeric systems. The corresponding exchange constants have been shown to change the sign along the series of chains. The coupling is antiferromagnetic for 1 (J = -0.24 cm(-1)) and 2 (J = -0.07 cm(-1)), whereas 3 (J = +0.47 cm(-1)), 7 (J = +0.28 cm(-1)), and 8 (J = +0.23 cm(-1)) have ferromagnetic character. In the case of dimeric systems, the coupling constants seem to be independent of the lanthanide center. The splitting structures of the ground-state multiplets of the Ln(III) centers have been shown to explain the temperature dependences of the magnetic susceptibilities.     

MP2 study of cation-(pi)n-pi interactions (n = 1-4)

Ab initio calculations at the MP2(full)/6-31++G**, RI-MP2(full)/6-31++G**, and RI-MP2(full)/6-311++G(2d,2p) levels of theory demonstrate important synergic effects between two noncovalent interactions that involve aromatic rings, that is, cation-pi and pi-pi interactions. The presence of a cation interacting with the pi cloud of an aromatic ring favors the face-to-face stacking interaction with additional aromatic rings. This effect is extended in the space up to five stacked aromatic rings.     

Growth format, electronic architecture, magnetic, and optical properties of aromatic cyclo-Cu3Au3 homotops

Bimetallic Cu(3)Au(3) clusters have been investigated using electronic structure calculation techniques (DFT) to understand their electronic, magnetic, and optical properties as well as the geometrical structures. The most stable homotop is the planar cyclo-[Cu(3)(micro-Au)(3)] form consisting of a triangular positively charged Cu(3) structural core with negatively charged Au atoms occupying exposed positions. This structure is characterized by the maximum number of heterobonds and peripheral positions of Au atoms. Possible growth formats of the cyclo-[Cu(3)(micro-Au)(3)] homotops have been explored following both the edge-capping and the stepwise metal atom substitution mechanism. The bonding pattern along with the density of states (DOS) plots of the cyclo-[Cu(3)(micro-Au)(3)] homotop are thoroughly analyzed and compared with those of the pure cyclo-[Cu(3)(micro-Cu)(3)] and cyclo-[Au(3)(micro-Au)(3)] clusters. Particular attention was paid on the stability of these bimetallic clusters in relation with the ring-shaped electron density distribution (aromaticity). It was found that all 3-membered metal rings exhibit significant aromatic character, which was verified by a number of established criteria of aromaticity, such as structural, energetic, magnetic (NICS profiles), and out-of-plane ring deformability criteria. The NICS (1) values correlate well with the out-of-plane ring deformation energy. Finally, a comprehensive analysis of the optical spectra of the CuAu, Cu(2), and Au(2) diatomics and the cyclo-[Cu(3)(micro-Au)(3)], cyclo-[Cu(3)(micro-Cu)(3)], and cyclo-[Au(3)(micro-Au)(3)] clusters placed the electronic assignments of the optical transitions on a firm footing.     

Analysis and interpretation of metal-radical coupling in a series of square planar nickel complexes: correlated Ab initio and density functional investigation of [Ni(L(ISQ))(2)] (L(ISQ)=3,5-di-tert-butyl-o-diiminobenzosemiquinonate(1-))

This paper reports a detailed theoretical study of the interaction between a central low-spin d(8) nickel ion and two N,N-coordinating diiminobenzosemiquinonate(1-) ligands in a square planar arrangement. Such complexes have recently attracted much attention due to their unusual bonding patterns, structures, optical, and magnetic properties. Geometry optimizations using various levels of density functional theory (DFT) result in excellent agreement with the experimentally determined structure and in particular reproduce the quinoidal distortions in the aromatic rings well. A detailed analysis of the orbital structure reveals that the complex features essentially two strongly interacting ligand radicals which interact with each other via an efficient superexchange mechanism that is mediated by a back-bonding interaction to the central metal. An analysis of the broken symmetry DFT wave function is presented and a new index for the diradical character is proposed which shows that [Ni(L(ISQ))(2)] has a diradical character of approximately 77%. These results are in full agreement with elaborate multireference post-Hartree-Fock ab initio calculations for [Ni(L(ISQ))(2)] using the difference dedicated configuration interaction (DDCI) method as well as second-order multireference M?ller-Plesset (MR-MP2) theory, which give diradical characters of 65-80%. On the basis of these calculations our best estimate for the singlet-triplet gap in this system is 3096 cm(-)(1). This very large value results from an efficient mixing of the ionic configurations into the mainly singlet diradical ground state which is feasible because the semiquinonate SOMOs are delocalized and, therefore, have moderate on-site Coulomb repulsion parameters. As pointed out in the discussion, this represents an interesting difference to the case of magnetically interacting transition metal ions which typically show much smaller magnetic exchange couplings.     

Ln(terpy)]3+ (Ln = Sm, Gd) entity forms isolated magnetic chains with [W(CN)8]3-

The reaction between Ln(NO3)3*xH2O, Cs3[W(V)(CN)8]*H2O and 2,2':6',2'-terpyridine (terpy) leads to the original isomorphous cyano-bridged [Ln(III)(terpy)(DMF)4][W(V)(CN)8] *6H2O [Ln = Gd (1), Sm (2)] 1-D chains. The crystal structures of {Ln(III)W(V)} chains and consist of alternating {[W(CN)8]} and {[Ln(terpy)]} building blocks. The neighbouring 1-D chains are weakly linked through pi-pi stacking interactions of the aromatic rings leading to 2-D supramolecular layers. The layers are linked through hydrogen bonds between H2O molecules and terminal cyano ligands. Magnetic studies revealed a weak antiferromagnetic coupling (J = -2.3(2) K) within the {Gd(III)W(V)} chains in . The positive effective coupling constant J = +2.0(5) K between the total angular momentum of the Sm(III) centre and the spin of the W(v) ion is equivalent to an antiferromagnetic character of the spin coupling between both centres in the {Sm(III)W(V)} chains of 2. The magnetic measurements suggest that they display an isolated magnetic chain behaviour.     

Heterometallic 20-membered {Fe16Ln4} (Ln = Sm, Eu, Gd, Tb, Dy, Ho) metallo-ring aggregates

The reaction of triethanolamine (teaH(3)) with [Fe(III)(3)O(O(2)CCH(3))(6)(H(2)O)(3)]Cl·6H(2)O and Ln(NO(3))(3)·6H(2)O in acetonitrile yields [Fe(16)Ln(4)(tea)(8)(teaH)(12)(μ-O(2)CCH(3))(8)](NO(3))(4)·16H(2)O·xMeCN (Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6); x = 10 or 11). These 20-membered metallo-ring complexes are the largest such single-stranded oxygen-bridged rings so far reported. The structure is stabilised by two of the acetate ligands, which form anti,anti-bridges across the centre of the ring, pinching the ring and giving it rigidity. The magnetic properties are dominated by the antiferromagnetic couplings between the Fe(III) centres. Although the Fe(2) and Fe(6) sub-chains within the ring are fully spin-compensated at low temperatures with S(subchain) = 0, coupling between the Gd(III) cations and the Fe(III) centres at the ends of the sub-chains (in 3) results in a pinning of the lanthanide spins. The (57)Fe M?ssbauer spectra of 3 and 5 obtained at low temperatures are consistent with the presence of Fe(III) intracluster strong antiferromagnetic coupling. The applied field spectrum for 3 reveals no magnetic hyperfine interaction apart from that of the nucleus with the applied field, while the one for 5 is a superposition of three subspectra which show contributions from each of the peripheral as well as from the central iron sites.     

Synthesis, crystal structure, and luminescence of a new complex [Co(6,6′-Bpbc)(Phen)(H2O)] · 2EtOH

A novel complex constructed with [Co(6,6′-Bpbc)(Phen)(H₂O)] · 2EtOH (6,6′-Bpbc = 2,2′-bipyridine-6,6′-dicarboxylic acid, Phen = 1,10-phenanthroline) has been successfully synthesized and characterized by X-ray diffraction, and elementary analysis. The photoluminescence properties of this complex were also studied. In the crystal, the cobalt(II) ion adopts the formation of a heptacoordination environment, and the structure units aggregate together to give birth to the infinite 1D chains, 2D-networks, and 3D-frameworks through either hydrogen bonding or π···π-stacking interaction between the aromatic rings.