Structure and chromotropism properties of an unsymmetrical alkoxo-bridged nickel(II) cubane-like complex

A new tetranuclear cubane-like complex, [Ni₄(L)₄Cl₄(H₂O)₃(EtOH)]·(H₂O), has been synthesized from the reaction of a metal salt with the bidentate ligand 2-hydroxymethylpyridine (LH), and its crystal structure, spectroscopic and chromotropic properties have been studied. The complex has a [Ni₄O₄] core comprising a distorted cubane arrangement, of which four nickel(II) ions were bridged by μ₃-alkoxo moieties. Each nickel(II) was coordinated to three μ₃-alkoxo oxygens, one pyridine nitrogen and one chloride. The peripheral ligation was completed by an oxygen atom of water or ethanol ligand, which participated in intramolecular hydrogen bonding. Chromotropism properties of the complex including solvato-, thermo-, and ionochromism were investigated. The complex displayed strong ionochromism and shows high-sensitive and selective activity toward CN^? and SCN^? anions in the presence of other halides and pseudo-halides. The solvatochromic property of the complex was analyzed by a multi-parametric equation using SPSS/PC software. The stepwise multiple linear regression method demonstrated that the acceptor power of the solvent plays the most important role in the observed negative solvatochromism of the compound. It shows reversible thermochromism in solution due to loss of the weakly coordinated water and ethanol from the nickel(II) units.     

Synthesis and evaluation of a high relaxivity manganese(II)-based MRI contrast agent

The manganese(II) ion has many favorable properties that lead to its potential use as an MRI contrast agent: high spin number, long electronic relaxation time, labile water exchange. The present work describes the design, synthesis, and evaluation of a novel Mn(II) complex (MnL1) based on EDTA and also contains a moiety that noncovalently binds the complex to serum albumin, the same moiety used in the gadolinium based contrast agent MS-325. Ultrafiltration albumin binding measurements (0.1 mM, pH 7.4, 37 degrees C) indicated that the complex binds well to plasma proteins (rabbit: 96 +/- 2% bound, human: 93 +/- 2% bound), and most likely to serum albumin (rabbit: 89 +/- 2% bound, human 98 +/- 2% bound). Observed relaxivities (+/- 5%) of the complex were measured (20 MHz, 37 degrees C, 0.1 mM, pH 7.4) in HEPES buffer (r(1) = 5.8 mM(-)(1) s(-)(1)), rabbit plasma (r(1) = 51 mM(-)(1) s(-)(1)), human plasma (r(1) = 46 mM(-)(1) s(-)(1)), 4.5% rabbit serum albumin (r(1) = 47 mM(-)(1) s(-)(1)), and 4.5% human serum albumin (r(1) = 48 mM(-)(1) s(-)(1)). The water exchange rate was near optimal for an MRI contrast agent (k(298) = 2.3 +/- 0.9 x 10(8) s(-)(1)). Variable temperature NMRD profiles indicated that the high relaxivity was due to slow tumbling of the albumin-bound complex and fast exchange of the inner sphere water. The concept of a high relaxivity Mn(II)-based contrast agent was validated by imaging at 1.5 T. In a rabbit model of carotid artery injury, MnL1 clearly delineated both arteries and veins while also distinguishing between healthy tissue and regions of vessel damage.     

Synthesis and studies of a trinuclear Mn(II) carboxylate complex

We report a carboxylate triangle consisting of three manganese(II) centres which is made from manganese(II) carbonate and pivalic acid. The magnetic exchange within the triangle is extremely weak, and antiferromagnetic. Several models have been used to fit the magnetic data, and the best fit uses two weak antiferromagnetic coupling constants of J(1)=-0.588 cm(-1) and J(2)=-0.855 cm(-1). Exchange interactions between the metal centres has been calculated using DFT adopting all the three possible Heisenberg models for a trinuclear system and the results are compared with experimental values. Spin density distribution is used to analyse the nature of the coupling between the metal centres. EPR spectroscopy has been used to explore the nature of the ground state. Recrystallisation of the trinuclear compound from MeCN gives a polymer, while oxidation in air leads to a known compound--an edge-sharing bitetrahedral (MnIII2MnII4) cage.     

Crystal structure of a Mn(II) complex with isatin-hydrazone-S-benzyldithiocarbazate

The reaction of Mn(OAc)₂·4H₂O with isatin-hydrazone-S-benzyldithiocarbazate (HIsa-SBn) results in the formation of a bis-ligand complex [Mn(Isa-SBn)₂]·2DMF. Its single crystal is obtained and the structure is determined by X-ray diffraction. It belongs to a monoclinic space group C2/c with the cell parameters: a = 23.290(4) Å, b = 11.4980(18) Å, c = 18.483(5) Å, V = 4087.6(14) ų. The Mn(II) atom is six-coordinated with two amide O atoms, two azomethine N atoms, and two thiolate S atoms, resulting in a distorted octahedral geometry.     

Homogeneous and heterogeneous olefin epoxidation catalyzed by a binuclear Mn(II)Mn(III) complex

The synthesis and characterization of a binuclear carboxylated bridged manganese complex containing the heptadentate ligand N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-2-ol-1,3-propanediamine (H₃bbppnol) is reported. This complex was characterized by elemental analysis; infrared, electronic (UV–vis) and EPR spectroscopy; and conductivity measurements. The complex was immobilized on silica by either adsorption or entrapment via a sol–gel route. The obtained solids were characterized by thermogravimetric analyses (TG and DSC), UV–vis and infrared spectroscopy, and X-ray diffraction. The catalytic performance of the binuclear manganese complex in epoxidation reactions was evaluated for both homogeneous and heterogeneous systems. The catalytic investigation revealed that the complex performs well as an epoxidation catalyst for the substrates cyclohexene (26–39%) and cyclooctene (29–74%). The solids containing the immobilized complex can be recovered from the reaction medium and reused, maintaining good catalytic activity.     

Novel heteroditopic chelate for self-assembled gadolinium(III) complex with high relaxivity

[Fe(tpy-DTTA)(2)Gd(2)] is a self-assembled trinuclear complex based on a novel ligand in which a terpyridine and a poly(amino carboxylate) moiety are connected; it has a well-defined topology with favourable features to attain high relaxivities, i.e. a rigid Fe(II)(tpy)(2) core, reduced flexibility at the periphery thanks to a short linker, and efficient separation of the two Gd(III) centres.     

Synthesis and Crystal Structure of an Mn(II) Complex with Benzoylacetone

The complex (benzoato)benzoylacetonato(bipyridine)Mn(II) has been prepared and its crystal structure determined by X-ray diffraction methods. Benzoic acid, benzoylacetone (bzac) and 2,2'-bipyridine all chelate to Mn(II) to form a six coordinate complex. As bond angles around the Mn(II) atom greatly deviate from those expected for an octahedron, the coordination geometry may be described as distorted pyramidal with a bidentate carboxyl group occupying the apex of the pyramid. Although the Mn atom deviates by 0.550 Å from the enol ring plane of bzac, Mn-O distances [2.105(2) and 2.098(2) Å] are normal. This suggests the existence of electrostatic interactions between Mn(II) and the bzac ligand.     

Two new lead(II) nitrite complexes,crystal structure of [Pb(phen)2(NO2)2] an eight-coordinate lead(II) complex

Two new lead(II) complexes containing nitrite, [Pb(L)₂(NO₂)₂], L?=?1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy), have been synthesized and characterized. The crystal structure of [Pb(phen)₂(NO₂)₂] shows monomeric units. The coordination number is eight (four from “phen” ligands and four nitrite anions), weak interaction of lead(II) with oxygen atoms of adjacent molecules produce dimer units in the solid state. The arrangement of ligands exhibits a coordination hole around the lead(II), occupied possibly by a stereoactive lone pair of electrons on lead(II), and the coordination around lead is hemidirected. There is a π–π stacking interaction between the parallel aromatic rings that may help to increase the “gap” around lead(II).     

Preparation and properties of a monomeric Mn(IV)-oxo complex

Manganese-oxo complexes have long been investigated because of their proposed roles in biological and chemical catalysis. However, there are few examples of monomeric complexes with terminal oxo ligands, especially those with oxomanganese(IV) units. A oxomanganese(IV) complex has been prepared from [MnIIIH3buea(O)]2- ([H3buea]3-, tris[(N'-tert-butylureaylato)-N-ethylene]aminato), a monomeric MnIII-O complex in which the oxo ligand arises from cleavage of dioxygen. Treating [MnIIIH3buea(O)]2- with [Cp2Fe]BF4 in either DMF at -45 degrees C or DMSO at room temperature produces [MnIVH3buea(O)]-: lambdamax = 635 nm; nu(Mn-16O) = 737 cm-1; nu(Mn-18O) = 709 cm-1; g = 5.15, 2.44, 1.63, D = 3.0 cm-1, E/D = 0.26, aMn = 66 G (A = 190 MHz). These spectroscopic properties support the assignment of a mononuclear MnIV-oxo complex with an S = 3/2 ground state. Density functional theory supports this assignment and the Jahn-Teller distortion around the high-spin MnIV center that would alter the molecular structure of [MnIVH3buea(O)]- from trigonal symmetry (as indicated by the highly rhombic EPR signal). [MnIVH3buea(O)]- is relatively unstable in DMSO, converting to [MnIIIH3buea(OH)]- via a proposed X-H bond cleavage. [MnIVH3buea(O)]- reacts with 1,2-diphenylhydrazine to from azobenzene (95% yield) and [MnIIIH3buea(OH)]-. The MnIV-oxo does not react with triphenyl- or tricyclohexylphosphine. However, O-atom transfer is observed with methyldiphenylphosphine and dimethylphenylphosphine, producing the corresponding phosphine oxides. These results illustrate the diverse reactivity of the MnIV-oxo unit.     

Bromo-substituted Mn(II) and Mn(III)-tetraarylporphyrins: synthesis and properties

Abstract

Mono-, tetra-, and octa-bromo substituted Mn(II)- and Mn(III)-tetraarylporphyrins were synthesized by reactions of manganese(II) chloride with corresponding porphyrin ligands or their Cd(II)-complexes in DMF. With the use of the metal exchange reaction, the time of the Mn-porphyrins formation is significantly reduced with increase in yield of final products in comparison with the complexation reaction. Mn(III)-tetraarylporphyrins reduce to the Mn(II)-porphyrins in DMF in the presence of NaOH and in pure DMF. The obtained compounds were identified using UV–vis and ¹H NMR spectroscopy, mass-spectrometry, and elemental analysis.     

Structure and magnetic properties of a mixed‐ligand copper(II) complex

The hydrothermal synthesis of the novel complex poly[[μ₂‐N¹,N⁴‐bis(pyridin‐3‐yl)naphthalene‐1,4‐dicarboxamide‐κ²N³:N^3′](μ₄‐phthalato‐κ⁴O¹:O¹:O^1′:O^2′)copper(II)], [Cu(C₈H₄O₄)(C₂₂H₁₆N₄O₂)]_n, is described. With the phthalate ligand connecting neighbouring Cu^II cations, an infinite one‐dimensional chain is formed. Adjacent one‐dimensional chains are connected by the dicarboxamide ligand, forming an intriguing two‐dimensional framework. The magnetic properties and thermal stability of this complex are also described.     

Molecular cube of Re(II) and Mn(II) that exhibits single-molecule magnetism

A pseudocubic, paramagnetic cluster of ReII and MnII, [{MnCl}4{Re(triphos)(CN)3}4], has been prepared, and its magnetic properties have been investigated. Antiferromagnetic coupling is observed between the "S = 1/2" ReII and S = 5/2 MnII centers resulting in an effective S = 8 ground state. AC susceptibility studies reveal that the molecule is a single-molecule magnet with an effective barrier for magnetization reversal of Ueff = 8.8 cm-1.     

Synthesis, structure and magnetic properties of the first Mn(II)-Gd(III) heteronuclear complex

The synthesis and crystal structure of a novel pentanuclear Mn(II)-Gd(III) complex is reported; the magnetic measurement conclusively demonstrated anti-ferromagnetic behaviour between Mn(II) and Gd(III) for this complex.     

Structure and magnetic properties of a new dinuclear nickel(II) complex with a dinucleating hexaazamacrocycle

The crystal structure and magnetic properties of a new dinuclear nickel(II) complex [LNi2(AcO)4]·14H2O, where L = 3,6,9,17,20,23-hexaazatricyclo [23.3.1.111,13] triaconta-1(29),11(30),12,14,25(26),27-hexene, has been studied. X-ray structure analysis shows that the compound consists of a discrete [LNi2(AcO)4] complex and 14 lattice water molecules. Each Ni atom is six coordinated by three N atoms from the macrocycle and three O atoms from the two coordinated acetates; two nickel atoms in each macrocycle are at the distance of 7.028Å. The result of the magnetic measurement indicates that the zero-field splitting constant of nickel(II) centres |D| = 1.87 cm–1.     

Preparation and properties of a monomeric high-spin Mn(V)-oxo complex

Oxomanganese(V) species have been implicated in a variety of biological and synthetic processes, including their role as a key reactive center within the oxygen-evolving complex in photosynthesis. Nearly all mononuclear Mn(V)-oxo complexes have tetragonal symmetry, producing low-spin species. A new high-spin Mn(V)-oxo complex that was prepared from a well-characterized oxomanganese(III) complex having trigonal symmetry is now reported. Oxidation experiments with [FeCp(2)](+) were monitored with optical and electron paramagnetic resonance (EPR) spectroscopies and support a high-spin oxomanganese(V) complex formulation. The parallel-mode EPR spectrum has a distinctive S = 1 signal at g = 4.01 with a six-line hyperfine pattern having A(z) = 113 MHz. The presence of an oxo ligand was supported by resonance Raman spectroscopy, which revealed O-isotope-sensitive peaks at 737 and 754 cm(-1) assigned as a Fermi doublet centered at 746 cm(-1)(Δ(18)O = 31 cm(-1)). Mn Kβ X-ray emission spectra showed Kβ' and Kβ(1,3) bands at 6475.92 and 6490.50 eV, respectively, which are characteristic of a high-spin Mn(V) center.