Octanuclear zinc(II) and cobalt(II) clusters produced by cooperative tetrameric assembling of oxime chelate ligands

We have synthesized an octanuclear zinc(II) cluster [L4Zn8(H2O)3] by the complexation of 3-hydroxysalamo (H4L) with zinc(II) acetate. The complex crystallizes in the triclinic system, space group P, with unit cell parameters a = 18.233(10) A, b = 20.518(11) A, c = 21.366(11) A, alpha = 98.7557(2) degrees, beta = 99.191(11) degrees, gamma = 108.309(10) degrees, and Z = 4. The crystallographic analysis revealed the S4 symmetrical assembling of four ligands and that the tetrameric complex has three water molecules in an unsymmetrical fashion. Spectroscopic analysis of the complex strongly suggests that the octanuclear cluster also exists in solution and maintains a conformation similar to that in the crystal structure, although exchange of the coordinating water molecules presumably takes place. In addition, the formation process of the octanuclear complex is highly cooperative. A high coordinating ability of the [(salamo)Zn] unit as well as the catecholato2- moieties probably stabilizes the octanuclear assembly and makes the complexation process cooperative. The corresponding octanuclear cobalt(II) cluster [L4Co8(EtOH)3] was prepared in a similar manner. Complex [L4Co8(H2O)2X] (X = H2O or EtOH) was obtained by the recrystallization from chloroform/hexane. The complex crystallizes in the triclinic system, space group P, with unit cell parameters a = 15.2359(10) A, b = 16.9625(12) A, c = 18.9325(13) A, alpha = 101.9710(10) degrees, beta = 105.5410(10) degrees, gamma = 97.1290(10) degrees, and Z = 2. Temperature dependence of magnetic susceptibility showed a continuous decrease in the chi(M)T value with decreasing temperature, suggesting antiferromagnetic interaction among cobalt(II) ions. The magnetic susceptibility above 40 K obeys the Curie-Weiss law with a Weiss constant theta of -39 K and a Curie constant C of 19.7 cm(3) K mol(-1).     

Low-temperature field dependence of the magnetic moment of ferromagnetic tetranuclear nickel(II) clusters

We examine the characteristics of the low-temperature behavior of the magnetic moment of tetranuclear methoxo complexes of nickel(II) of the type [Ni(OCH₃L· (CH₃OH)]₄ (L is the residue of 2,4-dinitrophenol (I), 2,4,6-trichlorophenol (II) and acetylacetone (III)), having the cubane structure. Within the model taking into account the isotropic Heisenberg interaction (J₁₂=J₂₃=J₃₄=J₄₁=J, J₁₃=J₂₄=J_iso), the local anisotropy (parameters D and g_i), intercluster exchange (parameter J_ic), we could interpret both the field dependence (=0.4-0.8 T) and the temperature dependence of the magnetic moment. We have established that the nature of the ligand L exerts a substantial effect on the magnetic proper ties of the considered compounds.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 339–344, May–June, 1989.     

The first azide(mu1,1)-bridged binuclear cobalt(II)-imino nitroxide complex with ferromagnetic behavior

The first azide(mu1,1)-bridged binuclear cobalt(II) complex with a chelated imino nitroxide radical, [Co2(immepy)2(N3)(4)].2EtOH, was structurally and magnetically characterized, where immepy = 4,4,5,5-tetramethyl-2-(6'-methyl-2'-pyridyl) imidazoline-1-oxyl. Five nitrogen atoms complete the coordination sphere of the Co(II) ion, showing a distorted trigonal bipyramid geometry. Two N(3)(-) anions act as bridges between cobalt ions in the mu1,1 coordination mode, resulting in a binuclear structure with an inversion center. Magnetic studies show that ferromagnetic couplings occurred between the adjacent cobalt(II) ions through N3(-)(mu1,1)) bridges, and antiferromagnetic couplings between the cobalt(II) ions and organic radicals.     

Magnetic polyoxometalates: anisotropic antiferro- and ferromagnetic exchange interactions in the pentameric cobalt(II) cluster

The ground-state properties of the pentameric Co(II) cluster [Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)](12-) were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na(12)[Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)].46D(2)O. The encapsulated magnetic Co(5) unit consists of three octahedral and two tetrahedral oxo-coordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co(5) spin cluster is anisotropic and is expressed as H = -2 summation operator(i= x,y,z)J(1)(i)[S(1)(i)S(2)(i) + S(2)(i)S(3)(i)] + J(2)(i)[S(1)(i)S(5)(i) + S(2)(i)S(5)(i) + S(2)(i)S(6)(i) + S(3)(i)S(6)(i)], where J(1)(i) are the components of the exchange interaction between the octahedral Co(II) ions and J(2)(i) are the components of the exchange interaction between the octahedral and tetrahedral Co(II) ions (see Figure 1d). The study of the exchange interactions in the two structurally related polyoxoanions [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10)(-) and [Co(3)W(H(2)O)(2)(ZnW(9)O(34))(2)](12)(-) allowed an independent determination of the ferromagnetic exchange parameters J(1)(x) = 0.70 meV, J(1)(y) = 0.43 meV, and J(1)(z) = 1.51 meV (set a) and J(1)(x) = 1.16 meV, J(1)(y) = 1.16 meV and J(1)(z) = 1.73 meV (set b), respectively. Our analysis proved to be much more sensitive to the size and anisotropy of the antiferromagnetic exchange interaction J(2). We demonstrate that this exchange interaction exhibits a rhombic anisotropy with exchange parameters J(2)(x) = -1.24 meV, J(2)(y) = -0.53 meV, and J(2)(z) = -1.44 meV (set a) or J(1)(x) = -1.19 meV, J(1)(y) = -0.53 meV, and J(1)(z) = -1.44 meV (set b). The two parameter sets reproduce in a satisfactory manner the susceptibility, magnetization, and INS properties of the title compound.     

Diazabutadiene cobalt(II) complexes

Summary Studies on the chelates of cobalt(II) with the bidentate ligands 1,4-diphenyl(2,3-dimethyl-1,4-diazabutadiene) (PMB) and 1,4-di(p-methoxyphenyl)-2,3-dimethyl-1,4-diazabutadiene (MPMB) have been carried out. On the basis of elemental analyses, the complexes are [Co(PMB)Cl₂], [Co(PMB)₂(C1O₄)₂], [Co(MPMB)Cl₂] and [Co(MPMB)₂(ClO₄)₂].Both ligands are bidentatevia nitrogen atoms in all the complexes. The magnetic susceptibility and i.r. and u.v.-visible spectra are reported and discussed. The chloro-compounds involving two chlorine ligands and, in the perchlorate compounds, the ClO ₄ ^– groups are bound to the cobalt(II) centre.     

Spectral and magnetic studies on normal cobalt(II) planar and cobalt(III) octahedral, spin-crossover cobalt(III) octahedral and planar-tetrahedral cobalt(II) carbodithioates

Five cobalt(II) complexes, a normal complex Co(4-PPipzcdt)₂ (4-PPipzcdt = 4-phenylpiperazine-1-carbodithioate), and four zwitterionic complexes, Co(4-PPipzcdtH)₂X₂ and Co(4-MPipzcdtH)₂X₂ (X = Cl, Br; 4-PPipzcdtH = 4-phenylpiperazine-1-carbodithioic acid, 4-MPipzcdtH = 4-methylpiperazine-1-carbodithioic acid), have been synthesized. Normal cobalt(III) complexes of the type Co(4-MPipzcdt)₃ and Co₂ {2-MPipz(cdt)₂}₃ (2-MPipz(cdt)₂ = 2-methylpiperazine-1,4-dicarbodithioate) and two zwitterionic cobalt(III) complexes of the type Co(4-MPipzcdtH)₃X₃ (X = Cl, Br) have also been obtained. In addition to the room temperature IR and electronic spectra and magnetic susceptibility studies, all the complexes, except the normal Co(4-MPipzcdt)₃ and Co(4-PPipzcdt)₂ and zwitterionic Co(4-MPipzcdtH)₃Cl₃, have been investigated by variable-temperature magnetic susceptibility measurements. The results of the variable-temperature magnetic susceptibility studies suggest that two cobalt(II) carbodithioates exhibit a square planar-tetrahedral equilibrium, while two cobalt(III) octahedral carbodithioates show a spin-crossover phenomenon.     

Reactions of 2-pyridylphenylacetonitrile with copper(II) chloride,copper(II) bromide,cobalt(II) chloride and cobalt(II) bromide

Reaction of 2-pyridylphenylacetonitrile with copper(II) chloride and copper(II) bromide in dry ethanol gives the hitherto unreported compound 1,2-di(cyano,phenyl,2′-pyridyl)ethane. The cobalt halides react with 2-pyridylphenylacetonitrile to form 1:1 complexes containing O-ethyl-2-phenyl-2-(2′-pyridyl)acetimidate as ligand. Removal of the ligand by the action of dry liquid ammonia on the complexes provides a better route to the pure imidate than the well known Pinner method.     

Nickel(II) and cobalt(II) complexes of rhodanine

Summary Nickel(II) and cobalt(II) complexes of rhodanine (Hrd) were prepared from the metal chloride or acetate and the ligand. With an excess of NH3, the octahedral [Ni(NH₃)₆](Rd)₂ and [Co(NH₃)₅Rd]Rd complexes are ob-tained; use of only two NH₃ equivalents per metal ion yields the Ni(Rd)₂ sd HRd · NH₃ and [Co(Rd)₂ ] · 1.5 H₂O complexes, the first with tetragonally distorted hexacoordination and the second with polymeric octahedral coordination. By using two equivalents of NaOH per metal ion, the binuclear [Ni(Rd)₂][Ni(Rd)₂ · (HRd)₂] · 2 H₂O complex is formed having one diamagnetic planar and one high spin octahedral chromophore. Rhodanine is coordinated through the thiocarbonylic sulphur in the neutral form and through the thiocarbonylic sulphur and the deprotanated nitrogen atoms in the rhodanidato anionic form.     

Magnetization of ferromagnetic clusters

The magnetization and deflection profiles of magnetic clusters in a Stern-Gerlach magnet are calculated for conditions under which the magnetic moment is fixed in the intrinsic frame of the cluster, and the clusters enter the magnetic field adiabatically. The predicted magnetization is monotonic as a function of the ratio of magnetic energy ₀ B to the rotational thermal energyk _BT. In low field the average magnetization is 2/3 of the Langevin function. The high-field moment approaches saturation asymptotically asB ^–1/2 instead of theB ^–1 dependence in the Langevin function     

Symmetry and ferromagnetic clusters

Isomers of pure Fe₁₃ and icosahedral Fe₁₂X clusters are studied using the all-electron linear-combination-of-Gaussian-type-orbital (LCGTO) local-density-functional (LDF) methods that allow the spin and geometry of the cluster to be determined self-consistently. The Fe₁₃ ground state is icosahedral. The icosahedral cluster also has the greatest magnetic moment because of increased symmetry-required orbital degeneracy for electrons of different spins. The central atom of the icosahedral iron cluster has been varied to optimize the spin of the cluster keeping the oribital contribution to the magnetic moment quenched. Varying the central atom under this constraint can alter the magnetic moment by more than 20%. Similar studies have begun on 55-atom icosahedral iron clusters.     

N-acetyl-DL-leucinate cobalt(II), nickel(II) and zinc(II) complexes

Summary Complexes of the type M(AcLeu)₂ · B₂ (M = Co^II, Ni^II or Zn^II; B = H₂O, py, 3-pic, 4-pic; AcLeu =N-acetyl-DL-leucinate ion) and M(AcLeu)₂ B (M = Co^II or Zn^II and B = o-phen) were prepared and investigated by means of magnetic and spectroscopic measurements. The i.r. spectra of all the complexes are consistent with bidentate coordination of the amino acid to the metal ion. The room temperature solid state electronic spectra indicate that the symmetry of this species is closer toD _4h and that MO₆ and MO₄N₂ chromophores are present in the M(AcLeu)₂ · 2 H₂O and M(AcLeu)₂B_n · x H₂O (B = py, 3-pic, 4-pic, n=2 and x=0 for M = Ni^II; B = o-phen, n=1 and x=0 for M = Co^II; B = py, 3-pic, 4-pic, n=1 and x=1 for M = Co^II) complexes, respectively. By comparing the Dq values of the amino acid and those of other N-substituted amino acids previously studied, a spectrochemical series of the the cobalt(II) and nickel(II) complexes is proposed. The¹ H n.m.r. spectra of the zinc(II) complexes confirm the proposed stereochemistry.     

Radiolysis of aqueous solutions of cobalt(II) and cobalt(III)

Radiolysis of aqueous solution of di and trivalent cobalt with 1:2 (bis) carboxymethylaminodiethyltetraacetic acid (EGTA) was investigated, both in absence and in presence of oxygen. A radiolytic mechanism has been proposed. It has been shown that the degradation at the ligand of the chelate is due to OH only.     

Cymantrenecarboxylate complexes of nickel(II) and cobalt(II)

Reactions of cymantrenecarboxylic acid (CO)₃MnC₅H₄COOH (CymCOOH) with Ni(II) and Co(II) pivalates in boiling THF followed by extraction of the products with diethyl ether or benzene and treatment with triphenylphosphine gave the binuclear complexes LM(CymCOO)₄ML (M = Ni (I) and Co (II); L = PPh₃). Treatment of the benzene extract of the intermediate cobalt cymantrenecarboxylate with 2,6-lutidine (L’) yielded the trinuclear complex L’Co(CymCOO)₃Co(CymCOO)₃CoL’ (III). Complex I is antiferromagnetic; μ_eff decreases from 3.7 to 0.9 μ_B in a temperature range from 300 to 2 K. Structures I-III were identified using X-ray diffraction. The frameworks of complexes I and II are like Chinese lanterns, having four carboxylate bridges and axial ligands L (Ni-P, 2.358(1) Å; Co-P, 2.412(2) Å). The metal atoms are not bonded to each other (Ni…Ni, 2.7583(9) Å; Co…Co, 2808 (2) Å). In complex III, either terminal Co atom is coordinated to one ligand L’ (Co-N, 2.059(2) Å). The Co atoms form a linear chain showing no M-M bonds (Co…Co, 3.346(1) Å), in which either terminal Co atom is linked with the central Co atom by three carboxylate bridges (on average, Co_centr-O, 2.164 Å; CO_term-O, 2.094 Å). In one of three carboxylate groups, only one carboxylate O atom serves as a bridge, while the other is bonded to the terminal Co atom only (Co_term-O, 2.094 and 2.389 Å); so this carboxylate group is a bridging and chelating ligand.     

Organic amines reduce trans-dichlorotetrakis(pyridine)cobalt(III) chloride to cobalt(II)

Summary The reaction of dichlorotetrakis(pyridine)cobalt(III) chloride. [CoCl₂(Py)₄]Cl, with alkyl- or arylamines in EtOH or i-PrOH yielded [CoCl₂(Py)₂] in all cases. This reduction of Co^III to Co^II takes place only in the presence of the amines. [CoCl₂(Py)₂] in EtOH is oxidized by Cl₂ gas and in the presence of pyridine gives [CoCl₂(Py)₄] ⁺, while in pyridine alone [CoCl₂(Py)₄] is formed.