Antonio C. Fabretti

Structure and Magnetic Properties of a Dodecanuclear Twisted‐Ring Iron(III) Cluster

An unprecedented nonplanar structure characterizes the complex [Fe(OCH3 )2 (dbm)]12 (on the left in the picture), which contains the largest cyclic ferric cluster yet reported with chemically equivalent bridging units. It is made up of twelve high-spin, antiferromagnetically coupled iron(III) centers and neatly reacts with NaI or LiI templates in organic solution to give hexairon(III) coronates (right). Fe=•, O=○, NaI or LiI=• Hdbm=dibenzoylmethane.

Exchange coupling in dinuclear copper(II) complexes with oxalato, oxamidato and oxamato ligands

We have synthesized a series of dinuclear copper-(II) complexes in which the metal ions are bridged by oxamidato, oxamato and oxalato ligands, with the purpose of studying the influence of the ligands on the extent of the antiferromagnetic coupling between the two metal ions. Magnetic susceptibility measurements between 77 and 300 K showed that the singlet-triplet splitting ranges from 330 cm−1 to 550 cm−1; J is found to increase in the order oxalato s oxamato s oxamidato.

Manganese(iii) Formate: A Three-Dimensional Framework That Traps Carbon Dioxide Molecules**

Carbon dioxide, formic acid, and water molecules are trapped in the crystal lattice of manganese(III) formate (see 1), which was obtained by reducing permanganate with formic acid. Each CO2 guest molecule exhibits four C-H⋅⋅⋅O-C-O interactions with the three-dimensional host framework of Mn(HCOO)3 units. Compound 1 undergoes an antiferromagnetic phase transition at 27 K.

The syntheses and properties of cobalt(II), nickel(II) and copper(II) complexes with some heterocyclic dithiocarbamates

Abstract New cobalt(II), Nickel(II) and copper(II) dithiocarbamato complexes of the type M(Rdtc) 2 (Rdtc = 4-phenylpiperidinedithiocarbamate and N-phenylpiperazinedithiocarbamate) have been prepared and characterized through elemental analyses, conductivity measurements, spectral (electronic and IR) studies, magnetic moment measurements at different temperatures, e.p.r. techniques and thermal analyses (TG and DTG). The dithioligands exhibit bidentate behaviour in all the complexes. The magnetic moments studies suggest that there is no significant interaction between copper ions, and the e.p.r. data provide parameters typical of sulphur coordination in planar CuS 4 chromophores.

Molecular structure and magnetic properties of copper(II), manganese(II) and iron(II) croconate tri-hydrate

The compounds MIIC5O5·3H2O with M=Cu (I), Mn (II), Fe (III), were prepared and characterized by means of structural and magnetic measurements. The crystals are orthorhombic, space group Pbca, with a=11.770(2), b=8.085(2), c=15.571(2) A for I, a=12.063(3), b=8.213(2), c=15.652(3) A. for II, a=11.999(2), b=8.158(1), c=15.536(4) A for III; Z=8. The previously reported structures of I and II, refined in this work, and the novel structure of III were solved by direct methods and least-squares refinement of structural parameters led to conventional R factors of 0.029, 0.025 and 0.025 for I, II and III, respectively. The overall structure is similar for the three compounds and consists of infinite one-dimensional chains of metal ions bridged by croconate ligands acting in a monodentate and bidentate fashion. The magnetic properties of I, II and III are reported and discussed on the basis of intrachain and interchain exchange couplings and, for III, of single-ion axial zero-field splitting contributions. Weak antiferromagnetic intrachain interactions via the croconate dianion (J=0.99(3), 0.22(1) and ⩽0.54 cm−1 for I, II and III, respectively) appear to be responsible of most of the observed magnetic behavior.

Single‐Ion versus Dipolar Origin of the Magnetic Anisotropy in Iron(III)‐Oxo Clusters: A Case Study

A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.

New perspectives in phosphonodithioate coordination chemistry. Synthesis and X-ray crystal structure of trans-bis-[O-ethyl-(4-methoxyphenyl)phosphonodithioato] nickel(II)

Abstract Attempts to optimise the synthesis of a new class of nickel(II) dithiolene complexes starting from 1,3-dialkylimidazolidine-2-thione-4,5-dione (1) and Lawessons reagent (2) have afforded a new and easy one-step synthesis of phosphonodithioate nickel(II) complexes. This consists of the direct reaction between NiCl2 and Lawessons reagent in the appropriate alcohol R′OH (R=Me, Et, i-Pr, Bu, Bz) as solvent, to give the bis-[O-alkyl/aryl-(4-methoxyphenyl)phosphonodithioato] nickel(II) complexes (5a–e) with high yields (64–91%). Among them, trans-bis-[O-ethyl-(4-methoxyphenyl)phosphonodithioato] nickel(II) (5b) was characterised by means of X-ray diffraction measurements on a single crystal. The compound crystallises in the triclinic, P 1 space group with a=6.4840(7), b=7.6032(9), c=13.055(2) A, α=99.481(9), β=99.43(1), γ=104.487(9)° and Z=1. The compound is centrosymmetric and exhibits discrete monomeric units with the ligand acting as S,S-bidentate. To explain the reactivity of Lawessons reagent, the formation of 4-methoxyphenyldithiophosphorane (6) as intermediate has been suggested. The reaction was also tested for Pd(II) and Pt(II), but in these cases the yields were not as satisfactory as for Ni(II).

Spectroscopic, magnetic and thermogravimetric studies of piperazine-bis-(dithiocarbamate) complexes

Abstract A new 1,4-piperazine-bis-dithiocarbamate sodium salt and its metal complexes have been prepared and characterized by elemental analyses, conductivity measurements, magnetic susceptibilities, thermal analyses (TG and DTG) and spectroscopic (i.r., electronic and EPR) techniques. The ligand is tetrasulphur coordinated to the metals in polymeric structures. The magnetic measurements are indicative of one unpaired electron in the copper(II) complex; a μ value typical of planar low-spin complexes has been obtained in the case of the cobalt(II) derivative, while a distortion from the planarity is suggested in the case of the nickel(II) complex. Tentative structures of the complexes are proposed on the basis of the obtained results.

Crystal structures, magnetic and non-linear optical properties of methoxyphenyl nitronyl–nitroxide radicals

The three isomers (para, meta, and ortho, hereafter denoted P, M and O) of 2-(methoxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide have been synthesized and their crystal structures solved. The lattice parameters and space groups are: P, a= 9.997(2)A, b= 19.347(3)A, c= 9.322(4)A, β= 128.67(2)°, Cc, Z= 4; M, a= 6.391(1)A, b= 11.230(1)A, c= 10.228(1)A, β= 107.78(1)°, P21, Z= 2; O, a= 12.203(6)A, b= 15.677(6)A, c= 14.670(3)A, Pbca, Z= 8. The exchange couplings between the radical molecules are deduced from an analysis of the linewidths of the electron paramagnetic resonance (EPR) spectra at X- and Q-band, and are found to be |J|= 0.2 cm–1 for P, |J|= 1.6 cm–1 for M and |J|= 0.35 cm–1 for O. The properties of M and P as frequency-doubling materials are briefly investigated on the basis of second-harmonic generation (SHG) experiments on powders: both are found to be non-phase-matchable materials.

Disorder effects in Mn12–acetate at 83 K

The structure of hexadeca-mu-acetato-tetraaquadodeca-mu(3)-oxo-dodecamanganese bis(acetic acid) tetrahydrate, [Mn(12)O(12)(CH(3)COO)(16)(H(2)O)(4)] x 2CH(3)COOH x 4H(2)O, known as Mn(12)-acetate, has been determined at 83 (2) K by X-ray diffraction methods. The fourfold (S(4)) molecular symmetry is disrupted by a strong hydrogen-bonding interaction with the disordered acetic acid molecule of solvation, which displaces one of the acetate ligands in the cluster. Up to six Mn(12) isomers are potentially present in the crystal lattice, which differ in the number and arrangement of hydrogen-bonded acetic acid molecules. These results considerably improve the structural information available on this molecular nanomagnet, which was first synthesized and characterized by Lis [Acta Cryst. (1980), B36, 2042-2046].