Francisco Lloret

Low-spin six-co-ordinate cobalt(II) complexes. A solution study of tris(violurato)cobaltate(II) ions

The formation of complexes of CoII with violuric acid (1H,3H-pyrimidine-2,4,5,6-tetrone 5-oxime), H3vi, and its monomethyl (H2mvi) and dimethyl (Hdmvi) derivatives in dimethyl sulphoxide–water (80 : 20) has been investigated. Potentiometric, calorimetric, magnetic, and e.s.r. measurements clearly evidence that [Co(H2vi)3]–, [Co(Hmvi)3]–, and [Co(dmvi)3]– are low-spin octahedral complexes. The high spin–low spin change occurs upon binding of the third ligand.

Spin canting in Re(IV) complexes: magnetic properties of [ReX4(bpym)] ( X = Cl and Br; bpym = 2,2′-bipyrimidine)

The mononuclear complexes [ReCl4(bpym)] (1) and [ReBr4(bpym)] (2) (bpym = 2,2′-bipyrimidine) are weak ferromagnets. Magnetic ordering occurs below 7.0 (1) and 20.0 K (2) and good hysteresis loops are observed for the two compounds at 2.0 K. A spin-canting phenomenon, i.e., a non-strict linearity of the individual spins aligned in an anti-parallel way by intermolecular antiferromagnetic coupling occurring in many Re(IV) complexes, accounts for these magnetic features which are unusual in molecular solids such as 1 and 2.

Catecholato complexes of o-phenylenebis(salicylideneiminato)iron(III) and meso-tetra(parasulphonatephenyl)porphyrinatoferrate(III). A comment on the structure of the active site in catechol-1,2-dioxygenase

Abstract Formation of catecholato complexes of Fe(saloph) + and Fe(TPPS) 3− in solution is studied. Fe(saloph)(cat) − contains a cat 2− bidentate ligand. Its formation in solution competes efficiently with the hydrolysis and dimerization of Fe(saloph) + to give Fe 2 (saloph) 2 O. This behaviour shows that the planar saloph 2− ligand, as the analogous salen 2− , is easily distorted, and is not as rigid as generally considered. Iron(III) porphyrin Fe(TPPS) 3− with catechol gives the complex [Fe(TPPS)(Hcat)] 4− . Deprotonation of the unidentate Hcat − ligand cannot be studied because the smaller stability of the complex, and the dimerization of the metalloporphyrin dominates in basic medium. The strong tendency of the cat 2− anion to be coordinated to Fe(III) in chelate form only can be sterically hindered. On the basis of these results the suggested structure of the active site of catechol-1,2-dioxygenase is discussed.

Solution chemistry of NN′-ethylenebis(salicylideneiminato)iron(III). Part 1. Deprotonation equilibria and reversible decomposition in acid medium of NN′- ethylenebis(salicylideneimine). Stability constant of NN′- ethylenebis(salicylideneiminato)iron(III)

NN′-Ethylenebis(salicylideneimine), H2 salen, behaves as a weak diprotic acid in dimethyl sulphoxide (dmso)–H2O (80 : 20 w/w) solution. The values of the overall association constants are βj1=(1.63 ± 0.05)× 1012 dm3 mol–1 and βj2=(2.04 ± 0.02)× 1023 dm6 mol–2. H2 salen undergoes slow hydrolytic decomposition in acid medium [equations (i) and (ii)](Hsal = salicylaldehyde, en = ethylenediamine); βh1=(2.60 ± 0.05)× 104 and βh2=(1.60 ± 0.02)× 107. H2 salen + H+ [graphic omitted] Hsal + HOC6H4CHN(CH2)2NH3+(i), H2 salen + 2 H+ [graphic omitted] 2 Hsal + H2en2+(ii) The favourable thermodynamics for the hydrolysis are provided by the protonation of en. In neutral or basic media the hydrolysis is not spontaneous in spite of the large water content of the solvent. Solutions of [{Fe(salen)Cl}2] in dmso–H2O contain the cation [Fe(salen)]+. The chloride is completely displaced from the co-ordination sphere of FeIII by a solvent molecule, and [Fe(salen)]+ decomposes, in acid medium, very slowly [equation (iii)]. [Fe(salen)]++ 4 H+⇄ Fe3++ 2 Hsal + H2en2+(iii) The study of this equilibrium allows the stability constant of the complex, Fe3++ salen2–⇄[Fe(salen)]+, β=(7.1 ± 0.1)× 1025 dm3 mol–1 to be obtained. This is the first reported stability constant of a metal complex of H2 salen. All equilibrium constants were determined at 25 °C and 0.1 mol dm–3 KClO4 in dmso–H2O (80 : 20 w/w).

Solution chemistry of NN′-ethylenebis(salicylidenimeinato)iron(III). Part 2. Formation equilibria of µ-oxo-bis[NN′-thylenebis(salicylideneiminato)iron(III)] and catecholato[NN′-ethylenebis(salicylideneiminato)]ferrate(III)

The complex [Fe(salen)]+[H2salen =NN′-ethylenebis(salicylideneimine)] can dimerize in dimethyl sulphoxide–H2O (80 : 20 w/w) solution according to the following equation 2 [Fe(salen)]++ H2O [graphic omitted] [{Fe(salen)}2O]+ 2 H+. Although the µ-oxo-complex has been known for some time, this equilibrium has not been studied up to now; the equilibrium constant,KD=(5.00 ± 0.05)× 10–13 mol dm–3(0.1 mol dm–3 KClO4, 25 °C). [Fe(salen)]+ interacts in solution with catechol (H2cat) to give [Fe(salen)(cat)]–. We have determined the complex formation constant, [Fe(salen)]++ cat2– [graphic omitted] [Fe(salen)(cat)]–, K=(4.79 ± 0.02)× 1014 dm3 mol–1(0.1 mol dm–3 KClO4, 25 °C). In this complex, cat2– is co-ordinated as a bidentate ligand. This fact implies a marked structural change in the salen2– ligand, from the normal planar configuration to a very distorted one. There is no kinetic hindrance to this change, as shown by the fast formation equilibrium, but the distorted anion causes reduced stability compared to the planar anion.

Effects of electron donating/withdrawing groups in the 5-substituted-2-hydroxybenzaldehyde on the synthesis of neutral cubanes with a NiII4O4 core: synthesis, crystal structures and magnetic properties

Two tetranuclear cubane-like nickel(II) complexes of formula [Ni4(CH3O)4(L1)4(CH3OH)4] (1) and [Ni4(CH3O)4(L2)4(CH3OH)4] (2) (HL1 = 2-hydroxybenzaldehyde and HL2 = 2-hydroxy-5-methylbenzaldehyde) have been prepared by the reaction of NiCl2·6H2O with methanolic solutions of HL1 and HL2 in the presence of triethylamine at room temperature. Complexes 1 and 2 have been characterized by elemental analyses, IR spectra and single-crystal X-ray diffraction. The X-ray crystal structure analysis revealed that 1 and 2 have a cubane-type structural topology with four Ni(II) ions and four methoxo-oxygen atoms regularly alternating at the corners. Each metal ion in 1 and 2 is six-coordinate in a distorted octahedral geometry defined by three bridging methoxo-oxygens, one oxygen atom from a methanol molecule and two oxygen atoms from the bidentate L1 (1)/L2 (2) ligands. The magnetic properties of 1 and 2 were investigated in the temperature range 1.9–300 K. They are indicative of an overall ferromagnetic behaviour, the decrease in the very low temperature region being due to zero-field splitting and intermolecular interactions. A detailed analysis of the magnetic data of 1 and 2 assuming an idealized D2d symmetry for the Ni4O4 unit shows the occurrence of a low-lying spin nonet and two opposite magnetic interactions, one being ferromagnetic [Ja = +12.6 (1) and +10.5 cm−1 (2)] and the other antiferromagnetic [Jb = −4.10 (1) and −2.10 cm−1 (2)]. These values fit well the linear correlation between the exchange coupling parameter (J) and the Ni–O–Ni angle (α) which was established for Ni4O4 cubane motifs in previous works.

A novel molecular based ferromagnet ordering at 14 K

The compound MnCu(obbz)·H2O [obbz = oxamido bis(benzoato)], obtained by reaction of the Mn2+ ion with the copper(II)‘brick’[Cu(obbz)]2–, orders ferromagnetically at Tc= 14 K; below Tc it exhibits a hysteresis loop characteristic of a soft ferromagnet.