Lanthanide-based coordination polymers assembled from derivatives of 3,5-dihydroxy benzoates: syntheses, crystal structures, and photophysical properties

Two new aromatic carboxylic acids, namely, 3,5-bis(benzyloxy)benzoic acid (HL1) and 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2), have been prepared by replacing the hydroxyl hydrogens of 3,5-dihydroxy benzoic acid with benzyl and pyridyl moieties, respectively. The anions derived from HL1 and HL2 have been used for the support of a series of lanthanide coordination compounds [Eu(3+) = 1-2; Tb(3+) = 3-4; Gd(3+) = 5-6]. The new lanthanide complexes have been characterized on the basis of a variety of spectroscopic techniques in conjunction with an assessment of their photophysical properties. Lanthanide complexes 2, 4, and 6, which were synthesized from 3,5-bis(pyridine-2-ylmethoxy)benzoic acid, were structurally authenticated by single-crystal X-ray diffraction. All three complexes were found to exist as infinite one-dimensional (1-D) coordination polymers with the general formula {[Ln(L2)(3)(H(2)O)(2)]·xH(2)O}(n). Scrutiny of the packing diagrams for 2, 4, and 6 revealed the existence of interesting two-dimensional molecular arrays held together by intermolecular hydrogen-bonding interactions. Furthermore, the coordinated benzoate ligands serve as efficient light harvesting chromophores. In the cases of 1-4, the lowest energy maxima fall in the range 280-340 nm [molar absorption coefficient (ε) = (0.39-1.01) × 10(4) M(-1) cm(-1)]. Moreover, the Tb(3+) complexes 3 and 4 exhibit bright green luminescence efficiencies in the solid state (Φ(overall) = 60% for 3; 27% for 4) and possess longer excited state lifetimes than the other complexes (τ = 1.16 ms for 3; 1.38 ms for 4). In contrast to the foregoing, the Eu(3+) complexes 1 and 2 feature poor luminescence efficiencies.     

Three-dimensional metal phosphonodicarboxylates with GIS-zeolite topology: syntheses, structures and magnetic studies

Compounds [M(2)(4-pi)(H(2)O)(4)]·1.5H(2)O [M(II) = Mn(1), Co(2), Ni(3)] and a heterometallic analogue [Co(1.14)Ni(0.86)(4-pi)(H(2)O)(4)]·1.5H(2)O (4) have been synthesized utilizing a rigid 4-phosphonoisophthalic acid (4-piH(4)). Compounds 1-4 are isostructural, confirmed by their powder XRD measurements. Single crystal structural determinations of 1 and 2 reveal that these compounds display a novel pillared layered open framework structure with zeolite gismondine (GIS) topology. Within the layer, edge-sharing {MO(6)} octahedra are found, which are connected by carboxylate and phosphonate groups along the c- and b-axis, respectively. Dominant antiferromagnetic interactions are propagated between the magnetic centers in 1, 2 and 4. While in 3, ferromagnetic exchange couplings are observed.     

Three silver(I) coordination polymers assembled from 2-sulfoterephthalic acid and N-donor ligands: syntheses, crystal structures, and luminescence properties

The reactions of silver nitrate with 2-sulfoisophthalic acid (H₃stp) in the presence of N-donor ligands produced three coordination polymers; [Ag₃(stp)(pyz)_0.5]_n (1), {[Ag₄(dpp)₄]·2(Hstp)·9H₂O}_n (2), and {[Ag(bpe)]₂[Ag₂(bpe)₂]₂·2(stp)·19H₂O}_n (3) [pyz = pyrazine, bpp = 1,2-bis(4-pyridyl)propane, bpe = 1,2-di(4-pyridyl)-ethylene]. The complexes have been characterized by single-crystal X-ray diffraction, physico-chemical, and spectroscopic methods. Single-crystal X-ray diffraction reveals that complex 1 is a 2D silver carboxylate-sulfonate layered structure, in which the 2D layers are further linked by the N-donor atoms of pyz ligands into a 3D supramolecular structure. Complex 2 is an infinite 1D chain arrangement with the [Ag₂(dpp)₂]²⁺ unit in which weak Ag···Ag or Ag···O interactions extend the chains into 2D structures. Complex 3 has a 3D supramolecular structure constructed by hydrogen bonding, π–π stacking, and Ag···O interactions to link the ligands, metal atoms, and water molecules together. The luminescence properties of the complexes were investigated.     

Three-dimensional 3d-4f polymers containing heterometallic rings: syntheses, structures, and magnetic properties

Two novel three-dimensional (3D) 3d-4f mixed complexes [Ln(H(2)O)(4)][Ni(2)TTHA(SCN)(2)].H(3)O+ [Ln = Pr (1), Ce (2); H(6)TTHA = triethylenetetraaminehexaacetic acid], based on the building blocks of [Ni(2)TTHA(SCN)(2)](4-), were synthesized and characterized by X-ray crystal diffraction and magnetic properties. The single-crystal structures show that these complexes form a 3D framework, comprised of an unusual infinite one-dimensional chain based on heterometallic Ln2Ni2 rings. The temperature-dependent magnetic susceptibilities were analyzed by an approximate model, leading to g = 2.06. Delta = 2.83, zJ' = -0.6 cm-1 for complex 1 and g = 2.07, Delta = 1.00, zJ' = -0.5 cm(-1) for complex 2.     

Anisotropic two-dimensional sheets assembled from rod-shaped metal complexes

The facile formation of anisotropic two-dimensional sheets with different sizes, ranging from nanometre-scale to micrometre-scale, was achieved by the assembly of rod-shaped palladium complexes.     

Arylthiazylamides: syntheses, structures, and bonding properties

Air-sensitive, thermally unstable tris(dimethylamino)sulfonium (TAS) salts (3) of the title anions [ArNSN]- have been prepared from corresponding sulfurdiimides Ar-N=S=N-SiMe3 (2) by Si-N bond cleavage with [(Me2N)3S]-[Me3SiF2]- (TASF). They are characterized by low-temperature X-ray crystallography as Z isomers. Because of the very short terminal S-N distance (144.2 (3h)-147.9 (3i)pm) and the relatively long internal S-N distance (158.3 (3i)-160.3 (3c) pm) the [ArNSN]- ions should be regarded as thiazylamides 1b, rare species containing a S triple bond N triple bond. A bonding model is developed and the experimental results are compared with those of restricted Hartree-Fock (RHF), density functional theory (DFT), and M?ller-Plesset second-order (MP2) calculations.     

Lithium fluoroarylamidinates: syntheses, structures, and reactions

Lithium fluoroarylamidinates [(Ar(F)C(NSiMe(3))(2)Li)(n).xD] (Ar(F) = 4-CF(3)C(6)H(4), n = 2, D = OEt(2), x = 1 (2a); n = 1, D = TMEDA, x = 1 (4a); Ar(F) = 2-FC(6)H(4), n = 2, D = OEt(2), x = 1 (2b); Ar(F) = 4-FC(6)H(4), n = 2, D = OEt(2), x = 2 (2c); Ar(F) = 2,6-F(2)C(6)H(3), n = 2, D = OEt(2), x = 1 (2d); n = 2, D = 2,6-F(2)C(6)H(3)CN, x = 2 (3d); Ar(F) = C(6)F(5), n= 2, D = OEt(2), x = 1 (2e), n = 1, D = TMEDA, x = 1 (4e); n = 1, x = 2, D = OEt(2) (5e); D = THF (6e)) were prepared by the well-known method from LiN(SiMe(3))(2) and the corresponding nitrile in diethyl ether or by addition of the appropriate donor D to the respective diethyl ether complexes. Depending on the substituents at the aryl group and on the donors D, three different types of structures were confirmed by X-ray crystallography. Hydrolysis of 2e gave C(6)F(5)C(NSiMe(3))N(H)SiMe(3) (7e) and C(6)F(5)C(NH)N(H)SiMe(3) (8e). The lithium fluoroarylamidinates 2a-2d react with Me(3)SiCl to give the corresponding tris(trimethylsilyl)fluoroarylamidines Ar(F)C(NSiMe(3))N(SiMe(3))(2) (9a-9d). Attempts to prepare C(6)F(5)C(NSiMe(3))N(SiMe(3))(2) from 2e and Me(3)SiCl failed; however, the unprecedented cage [[C(6)F(5)C(NSiMe(3))(2)Li](4)LiF] (10e) in which a fluoride center is surrounded by a distorted trigonal bipyramid of five Li atoms was obtained from this reaction.     

d(10) Metal complexes assembled from isomeric benzenedicarboxylates and 3-(2-pyridyl)pyrazole showing 1D chain structures: syntheses, structures and luminescent properties

Seven new d10 metal coordination polymers with isomeric benzenedicarboxylates and 3-(2-pyridyl)pyrazole ligands, [Zn2 L2(1,2-BDC)(H2O)]n ( 1), {[Cd2(H L)2(1,2-BDC)2] x H2O}n ( 2), [Cd(H L)(1,2-BDC)(H2O)]n (3), [Zn(H L)(1,3-BDC)(H2O) x 3H2O]n ( 4), [Cd2 L2(1,3-BDC)(H2O)]n (5), [Zn(H L)2(1,4-BDC)]n ( 6) and [Cd(H L)2(1,4-BDC)]n (7) (BDC = benzenedicarboxylate, H L = 3-(2-pyridyl)pyrazole), have been synthesized and structurally characterized by elemental analysis, IR and X-ray diffraction. Single-crystal X-ray analyses reveal that each complex takes a different one-dimensional (1D) chain structure. In 1-7, the BDCs act as bridging ligands, exhibiting rich coordination modes to link metal ions. The three BDC isomers exhibit different coordination modes: micro(1)-eta(1):eta(1)/micro(3)-eta(2):eta(1), micro(3)-eta(1):eta(2)/micro(3)-eta(2):eta(1), micro(2)-eta(1):eta(1)/micro(1)-eta(1):eta(0) and micro(1)-eta(1):eta(1)/micro(1)-eta(1):eta(0) for 1,2-BDC, micro(1)-eta(1):eta(1)/micro(1)-eta(1):eta(0) and micro(1)-eta(1):eta(0)/micro(2)-eta(2):eta(1) for 1,3-BDC, and micro(1)-eta(1):eta(0)/micro(1)-eta(0):eta(1), micro(1)-eta(1):eta(0)/micro(1)-eta(1):eta(0) and micro(1)-eta(1):eta(1)/micro(1)-eta(1):eta(1) for 1,4-BDC, respectively. In these complexes, H acts as a simple bidentate chelate ligand (in 2, 3, 4, 6 and 7), similar to 2,2'-bipyridine, or as a tridentate chelate-bridging ligand (in 1 and 5) via deprotonation of the pyrazolyl NH group and coordination of the pyrazolyl N atom to a second metal ion. The structural differences indicate that the backbone of such dicarboxylate ligands plays an important role in governing the structures of such metal-organic coordination architectures, and the chelating bipyridyl-like ligand H leads to the formation of these coordination polymers with one-dimensional structures by occupying the coordination sites of metal ions. Moreover, the photoluminescent properties of complexes were also studied in the solid-state at room temperature.     

Novel one-dimensional polymers generated from p-Ferrocenylbenzoate: syntheses, structures, and magnetic properties

Treatment of p-ferrocenylbenzoate [p-HOOCH4C6Fc, Fc = (eta5-C5H5)Fe(eta5-C5H5)] with Mn(OAc)2 x 2H2O or Cd(OAc)2 x 2H2O afforded one-dimensional linear chain polymer [[Mn(OOCH4C6Fc)2(mu2-OH2)(H2O)2](H2O)]n (1), double-bridge polymer [Mn(mu2-OOCH4C6Fc)2(phen)]n (phen = phenanthroline) (2), and ladderlike framework [[Cd(mu2-OOCH4C6Fc)(eta2-OOCH4C6Fc)(bbp)](CH3OH)]n (bbp = 4,4'-trimethylene-dipyridine) (3). The solution-state cyclic voltammograms indicate that the half-wave potentials of the ferrocenyl moieties in these polymers are all shifted to positive potential compared to that of sodium p-ferrocenylbenzoate. Both 1 and 2 behave as 1D Heisenberg Mn(II) chains with weak intrachain antiferromagnetic interactions between the local high-spin Mn(II) ions, and the exchange coupling parameters J (-5.20 and -3.25 cm(-1) for 1 and 2, respectively) are larger than those of most of the reported di-Mn(II) complexes that contain mu2-aqua and mu2-carboxylato units and one-dimensional Mn(II) carboxylic polymers.     

Phytoalexins from the crucifer rutabaga: structures, syntheses, biosyntheses, and antifungal activity

Phytoalexins are inducible chemical defenses produced de novo by plants in response to diverse forms of stress, including microbial attack. Our search for phytoalexins from economically important crucifers lead us to examine rutabaga tubers (Brassica napus L. ssp. rapifera). Three new phytoalexins, named isalexin (9), brassicanate A (10), and rutalexin (11), were isolated together with five known phytoalexins, brassinin (4), 1-methoxybrassinin (5), spirobrassinin (13), brassicanal A (14), and brassilexin (15). The chemical structures of the new phytoalexins were proven by syntheses, and their biological activity against four plant pathogens were determined. Biosynthetic studies using tetra- and pentadeuterated precursors established that indolyl-3-acetaldoxime (22) and brassinin (4) are precursors of brassicanate A (10) and rutalexin (11) and that cyclobrassinin (23) is a biosynthetic precursor of rutalexin (11), whereas tryptamine (24) is not a precursor of rutabaga phytoalexins.     

α-Methyl-benzylcalcium complexes: syntheses, structures and reactivity

α-Methyl-benzylcalcium complexes were prepared analogue to α-Me₃Si-benzylcalcium complexes for which procedures were reported earlier. The crystal structures of homoleptic bis(2-Me₂N-α-Me-benzyl)calcium·(THF)₂ and heteroleptic (9-Me₃Si-fluorenyl)(2-Me₂N-α-Me-benzyl)calcium·(THF) were determined. For both compounds only one of the two diastereomers crystallized. Barriers for inversion of the chiral benzylic carbon were estimated by variable temperature NMR spectroscopy. The α-methyl-benzylcalcium compounds are less stable and show a higher reactivity and faster initiation of styrene polymerization than the analogue α-Me₃Si-benzylcalcium complexes. Intramolecular CH activation in a heteroleptic α-methyl-benzylcalcium complex was observed and the product, a calcium complex with a dianionic alkyl/fluorenyl ansa-ligand, was characterized by crystal structure determination.     

Dihydrooxazolones and dihydroimidazolones derived from acylglycines: syntheses,molecular structures and supramolecular assembly

Syntheses and structures are described for some alkylidene‐substituted dihydrooxazolones and dihydroimidazoles derived from simple acylglycines. A second, triclinic, polymorph of 4‐benzylidene‐2‐(4‐methylphenyl)‐1,3‐oxazol‐5(4H)‐one, C₁₇H₁₃NO₂, (I), has been identified and the structure of 2‐methyl‐4‐[(thiophen‐2‐yl)methylidene]‐1,3‐oxazol‐5(4H)‐one, C₉H₇NO₂S, (II), has been rerefined taking into account the orientational disorder of the thienyl group in each of the two independent molecules. The reactions of phenylhydrazine with 2‐phenyl‐4‐[(thiophen‐2‐yl)methylidene]‐1,3‐oxazol‐5(4H)‐one or 2‐(4‐methylphenyl)‐4‐[(thiophen‐2‐yl)methylidene]‐1,3‐oxazol‐5(4H)‐one yield, respectively, 3‐anilino‐2‐phenyl‐5‐[(thiophen‐2‐yl)methylidene]‐3,5‐dihydro‐4H‐imidazol‐4‐one, C₁₀H₁₅N₃OS, (III), and 3‐anilino‐2‐(4‐methylphenyl)‐5‐[(thiophen‐2‐yl)methylidene]‐3,5‐dihydro‐4H‐imidazol‐4‐one, C₂₁H₁₇N₃OS, (IV), which both exhibit orientational disorder in their thienyl groups. The reactions of 2‐phenyl‐4‐[(thiophen‐2‐yl)methylidene]‐1,3‐oxazol‐5(4H)‐one with hydrazine hydrate or with water yield, respectively, N‐[3‐hydrazinyl‐3‐oxo‐1‐(thiophen‐2‐yl)prop‐1‐en‐2‐yl]benzamide and 2‐(benzoylamino)‐3‐(thiophen‐2‐yl)prop‐2‐enoic acid, which in turn react, respectively, with thiophene‐2‐carbaldehyde to form 2‐phenyl‐5‐[(thiophen‐2‐yl)methylidene]‐3‐{[(E)‐(thiophen‐2‐yl)methylidene]amino}‐3,5‐dihydro‐4H‐imidazol‐4‐one, C₁₉H₁₃N₃OS₂, (V), which exhibits orientational disorder in only one of its thienyl groups, and with methanol to give methyl (2Z)‐2‐(benzoylamino)‐3‐(thiophen‐2‐yl)prop‐2‐enoate, C₁₅H₁₃NO₃S, (VI). There are no direction‐specific intermolecular interactions in the crystal structure of the triclinic polymorph of (I), but the molecules of (II) are linked by two independent C—H...O hydrogen bonds to form C₂²(14) chains. Compounds (III) and (IV) both form centrosymmetric R₂²(10) dimers built from N—H...O hydrogen bonds, while compound (V) forms a centrosymmetric R₂²(10) dimer built from C—H...O hydrogen bonds. In the structure of compound (VI), a combination of N—H...O and C—H...π(arene) hydrogen bonds links the molecules into sheets. Comparisons are made with some similar compounds.     

Two binuclear tetracarboxylate-bridged complexes: syntheses,structures, and properties

Cu₂(C₉N₅H₉)₂(C₆H₅CO₂)₄ (1) and Zn₂(C₉N₅H₉)₂(C₁₀H₇CH₂CO₂)₄ (2) have been synthesized by hydrothermal methods with 2,4-diamine-6-phenyl-1,3,5-triazine (phdat) and aromatic carboxylic acid (benzoic acid (ba) or naphthylacetic acid (naa)) as ligands and characterized by single-crystal X-ray diffractions, elemental analyses, infrared spectra, magnetism, fluorescence spectra, and thermogravimetric analyses. In 1 and 2, the two metals are bridged by four carboxylates in paddle-wheel-shaped binuclear [M₂(CO₂)₄] units (M=Cu (1) and Zn (2)) and coordinated by one nitrogen from phdat, forming five-coordinate centers. The temperature-dependent magnetic susceptibility of 1 was obtained from 300 to 2?K, showing an anti-ferromagnetic interaction between Cu(II)'s. Compound 2 exhibits solid state fluorescence at 404?nm upon excitation at 304?nm.     

Cobalt 2-pyrazinephosphonates: syntheses,structures, and magnetic properties

Reactions of Co(ClO₄)₂·6H₂O with 2-pyrazinephosphonic acid (2-paPO₃H₂) under two different hydrothermal conditions gave two new cobalt 2-pyrazinephosphonates, Co(2-paPO₃H)₂ (1) and Co(2-paPO₃)(H₂O) (2). Compound 1 was obtained at 160 °C, while 2 was obtained at 180 °C. In 1, two Co(II) units are bridged by two O–P–O’s into a dimer and the dimers are linked by pyrazinyl groups into a 1-D double chain. In 2, the {CoO₄N₂} and {CPO₃} polyhedra are interconnected via edge and corner-sharing into a metal phosphonate layer, and the layers are pillared by pyrazinyl groups into a 3-D network. Variable-temperature magnetic susceptibility studies reveal that both complexes have weak antiferromagnetic coupling between Co(II) ions.     

3-D Hydrogen-bonded networks of metal complexes with chelidamic acid and 1,10-phenanthroline: syntheses,structures, and optical properties

The title complexes, (C₁₂H₈N₂) ? [La(C₇H₃NO₅)(C₇H₄NO₅) ? 3H₂O] ? 1.75H₂O (1), (C₁₂H₈N₂) ? [Pr(C₇H₃NO₅)(C₇H₄NO₅) ? 3H₂O] ? 2H₂O (2), (C₁₂H₈N₂)[Nd(C₇H₃NO₅)(C₇H₄NO₅) ? 3H₂O] ? 2.25H₂O (3), and (C₁₂H₈N₂) ? [Fe(C₇H₃NO₅)(C₇H₄NO₅)] ? 2H₂O (4), were synthesized and characterized by single-crystal X-ray diffraction. The crystal structures of 1–3 reveal that they are isomorphous, among which the metal atoms are all nine-coordinate with distorted tricapped trigonal prismatic coordination geometries. The Fe is six-coordinate with a distorted octahedron by two chelidamic acid ligands in 4. Complexes 1–4 are formed into 3-D networks by H-bonds and π–π stacking interactions. The fluorescence spectra of 1–3 were investigated and all exhibit strong luminescence.     

Fluoroaryl-substituted aminoalane dimers: syntheses and structures

Six dimeric aminoalanes of formula [Me(2)Al-mu-N(H)Ar(F)](2)(Ar(F)= 4-C(6)H(4)F (1), 2-C(6)H(4)F (2), 3,5-C(6)H(3)F(2)(3), 2,3,4,5-C(6)HF(4)(4), 2,3,5,6-C(6)HF(4)(5) and C(6)F(5)(6)) have been prepared by treatment of the appropriate fluoroaniline with AlMe(3) in toluene solution at 25 degrees C. The structures of 1-6 were determined by X-ray crystallography.     

Three transition-metal polymers from imidazole dicarboxylates-bearing methoxyphenyl groups: syntheses,crystal structures and properties

Three coordination polymers, namely, {[Cu₂(HMOPhIDC)(4,4′-bipy)]}_n (H₃MOPhIDC = 2-(3-methoxyphenyl)-1H-imidazole-4,5-dicarboxylic acid) (1), [Co(HDMOPhIDC)(phen)]_n (H₃DMOPhIDC = 2-(3,4-dimethoxyphenyl)-1H-imidazole-4,5-dicarboxylic acid) (2) and [Ni₂(HDMOPhIDC)₂(H₂O)₄]_n (3) have been prepared under hydrothermal condition and characterised by elemental analyses, infrared spectroscopy and single-crystal X-ray diffraction. Each of the polymers 1–3 is a 1D column-like structure and displays a 3D supramolecular network via the π…π stacking or hydrogen bond interactions. Furthermore, fluorescence and UV–vis spectroscopic properties of the polymers have been studied.     

Substituted thienyl stibines and bismuthines: syntheses,structures and cytotoxicity

New stibine and bismuthine substituted thienyl ring compounds, i.e. tris(3‐methyl‐2‐thienyl)stibine (1), tris(3‐methyl‐2‐thienyl)bismuthine (2), tris(3‐thienyl)stibine (3), tris(3‐thienyl)bismuthine (4) and tris(5‐chloro‐2‐thienyl)stibine (5), have been synthesized and characterized by IR, mass, ¹H, ¹³C, COSY, and HETCOR NMR spectroscopy. The metal centres in all compounds are pyramidal, and molecules in the stibine compound (1) and bismuthine compound (2) associate via Sb···S or Bi···S interactions to form supramolecular chains. The cytotoxicity of compounds 1 and 5 was determined. For compound 5, 85% of carcinogenic cell growth inhibition (U, K and H) was observed. Compound 1 shows a significant selectivity (>80%) for carcinogenic cell growth (K and U) inhibition. Both the compounds are highly toxic for the growth of normal lymphocytes with ～95% lethality. Compound 1 is approximately 20 times more toxic than 5 against Artemia salina. Copyright © 2005 John Wiley & Sons, Ltd.     

Anion induced diversification from heptanuclear to tetranuclear clusters: syntheses, structures and magnetic properties

Three new polynuclear complexes, [Co(7)(bm)(12)]·(ClO(4))(2)·13H(2)O (1), [Co(4)(bm)(4)Cl(4)(C(3)H(7)OH)(4)] (2), and [Co(4)(bm)(4)(μ-HCO(2))(2)(μ(2)-HCO(2))(2)(C(3)H(7)OH)(2)] (3) (Hbm = (1H-Benzimidazol)-methanol), have been synthesized and characterized by elemental analysis, IR, powder X-ray diffraction and X-ray single-crystal diffraction. Compound 1 features a centrosymmetric wheel-like heptanuclear Co(II) cluster. Compound 2 having a I4(1)/a space group exhibits a tetranuclear Co(II) cluster with a cubane topology in which the central Co(II) ion and oxygen atoms from bm occupy the alternate vertices of the cube. However, compound 3 has a tetranuclear Co(II) cluster with a C2/c space group different from that of compound 2. These results show that the geometries and sizes of the corresponding anions as well as their coordinating and hydrogen-bonding properties are essential in determining the final structures of the assemblies. Magnetic properties of 1-3 in the 2-300 K have also been discussed. The {Co(7)} (1) and {Co(4)} (2) cores display dominant ferromagnetic interactions while the {Co(4)} (3) core displays dominant anti-ferromagnetic interactions.