"Switching on" the properties of single-molecule magnetism in triangular manganese(III) complexes

The reaction between oxide-centered, triangular [MnIII3O(O2CR)6(py)3](ClO4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [MnIII3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [MnIII3(mu3-O)]7+ triangular core, but with each Mn2 edge now bridged by an eta1:eta1:mu-RCO2- and an eta1:eta1:eta1:mu-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(mu3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying approximately 0.3 A above the Mn3 plane. This structural distortion leads to ferromagnetic exchange interactions within the molecule and a resulting S = 6 ground state. Fits of dc magnetization data for 4-6 collected in the 1.8-10.0 K and 10-70 kG ranges confirmed S = 6 ground states, and gave the following D and g values: -0.34 cm(-1) and 1.92 for 4, -0.34 cm(-1) and 1.93 for 5, and -0.35 cm(-1) and 1.99 for 6, where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4-6 all exhibit frequency-dependent out-of-phase (chi" M) ac susceptibility signals suggesting them possibly to be single-molecule magnets (SMMs). Relaxation rate vs T data for complex 4 down to 1.8 K obtained from the chi" M vs T studies were supplemented with rate vs T data measured to 0.04 K via magnetization vs time decay studies, and these were used to construct Arrhenius plots from which was obtained the effective barrier to relaxation (Ueff) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4.3CH2Cl2 displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below approximately 0.3 K, indicating only ground-state QTM between the lowest-lying Ms = +/-6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4.3CH2Cl2 have provided precise spin Hamiltonian parameters, giving D = -0.3 cm(-1), B40 = -3 x 10(-5) cm(-1), and g = 2.00. The spectra also suggest a significant transverse anisotropy of E > or = 0.015 cm(-1). The combined work demonstrates the feasibility that structural distortions of a magnetic core imposed by peripheral ligands can "switch on" the properties of an SMM.

Non-steroidal antiinflammatory drug-copper(II) complexes: structure and biological perspectives

Copper(II) complexes with the non-steroidal antiinflammatory drug mefenamic acid in the presence of aqua or nitrogen donor heterocyclic ligands (2,2'-bipyridine, 1,10-phenanthroline, 2,2'-bipyridylamine or pyridine) have been synthesized and characterized. The crystal structures of [(2,2'-bipyridine)bis(mefenamato)copper(II)], 2, [(2,2'-bipyridylamine)bis(mefenamato)copper(II)], 4, and [bis(pyridine)bis(methanol)bis(mefenamato)copper(II)], 5, have been determined by X-ray crystallography. UV study of the interaction of the complexes with calf-thymus DNA (CT DNA) has shown that the complexes can bind to CT DNA and [bis(aqua)tetrakis(mefenamato)dicopper(II)] exhibits the highest binding constant to CT DNA. The cyclic voltammograms of the complexes in the presence of CT DNA solution have shown that the complexes can bind to CT DNA by the intercalative binding mode verified also by DNA solution viscosity measurements. Competitive studies with ethidium bromide (EB) indicate that the complexes can displace the DNA-bound EB suggesting strong competition with EB. Mefenamic acid and its complexes exhibit good binding propensity to human or bovine serum albumin protein having relatively high binding constant values. All the compounds have been tested for their antioxidant and free radical scavenging activity as well as for their in vitro inhibitory activity against soybean lipoxygenase showing significant activity.

Initial Example of a Triangular Single-Molecule Magnet from Ligand-Induced Structural Distortion of a [MnIII3O]7+ Complex

The reaction of [Mn3O(O2CR)6(py)3](ClO4) (R = Me, Et) with methyl 2-pyridyl ketone oxime (mpkoH) in a 1:3 molar ratio in MeOH/MeCN leads to [Mn3O(O2CR)3(mpko)3](ClO4) in 80-90% isolated yield. Ferromagnetic exchange interactions between the three MnIII ions in the nonplanar [MnIII3O]7+ triangular core lead to a spin ground state of S = 6; single-crystal studies reveal the temperature and sweep rate dependent hysteresis loops expected for a single-molecule magnet.

Manganese citrate chemistry: syntheses, spectroscopic studies, and structural characterizations of novel mononuclear, water-soluble manganese citrate complexes

The first two mononuclear manganese citrate complexes, (NH4)4[MnII(C6H5O7)2] (1) and (NH4)5[MnIII(C6H4O7)2].2H2O (2) were synthesized in aqueous solutions near physiological pH values. They were isolated in their pure crystalline forms and characterized by elemental analyses and spectroscopic techniques, including UV/visible, electron paramagnetic resonance, Fourier transformed infrared, and magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.777(1) A, b = 13.656(3) A, c = 9.162(2) A, beta = 113.62(2) degrees, V = 1006.2(6) A3, and Z = 2. Compound 2 crystallizes in the triclinic space group P1, with a = 9.606(3) A, b = 9.914(3) A, c = 7.247(3) A, alpha = 91.05(1) degrees, beta = 105.60(1) degrees, gamma = 119.16(1) degrees, V = 571.3(3) A3, and Z = 1. The X-ray crystal structures of 1 and 2 revealed that, in both cases, the manganese ion is six-coordinate and is bound by two citrate ligands in a distorted octahedral fashion. In the case of complex 1, the citrate ion binds to Mn2+ as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound 2, the citrate ligand coordinates to Mn3+ as a fully deprotonated entity. Compound 2 contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn(3+)-citrate assembly. There are significant contributions to the stabilities of the assembled lattices in 1 and 2 arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn2+ and Mn3+ species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5-300 K. Complexes 1 and 2 present clear cases of mononuclear manganese citrate species relevant to manganese speciation in biological media and potentially related to the beneficial as well as toxic effects of manganese on humans.

Anti-inflammatory drugs interacting with Zn(II), Cd(II) and Pt(II) metal ions

Complexes of Zn(II), Cd(II) and Pt(II) metal ions with the anti-inflammatory drugs, 1-methyl-5-(p-toluoyl)-1H-pyrrole-2-acetic acid (Tolmetin), alpha-methyl-4-(2-methylpropyl)benzeneacetic acid (Ibuprofen), 6-methoxy-alpha-methylnaphthalene-2-acetic acid (Naproxen) and 1-(4-chlorobenzoyl)-5-methoxy-2-methyl-1H-indole-3-acetic acid (indomethacin) have been synthesized and characterized. In the structurally characterized Cd(naproxen)2 complex the anti-inflammatory drugs acts as bidentate chelate ligand coordinatively bound to metal ions through the deprotonated carboxylate group. Crystal data for 1: [C32H26O8Cd], orthorhombic, space group P22(1)2(1), a = 5.693(2) (A), b = 8.760(3) (A), c = 30.74(1) (A), V = 1533(1) A3, Z = 2. Antibacterial and growth inhibitory activity is higher than that of the parent ligands or the platinum(II) diamine compounds.

Vanadium(IV)-citrate complex interconversions in aqueous solutions. A pH-dependent synthetic, structural, spectroscopic, and magnetic study

Citrate is abundantly encountered in biological fluids as a natural metal ion chelator. Vanadium participates in biological processes as a catalyst in the active sites of metalloenzymes, as a metabolic regulator, as a mitogenic activator, and as an insulin-mimicking agent. Thus, vanadium chemistry with natural chelators, such as citrate, may have immediate implications on its role in a cellular milieu, and its action as a biological agent. In an effort to comprehend the aqueous chemistry of one of vanadium's oxidation states, namely, V(IV), implicated in its biological activity, reactions of VCl(3) and citric acid were pursued in water and led to V(IV)-citrate complexes, the nature and properties of which depend strongly on the solution pH. Analytical, FT-IR, UV/vis, EPR, and magnetic susceptibility data supported the formulation of X(4)[[VO(H(-1)Cit)](2)] x nH(2)O (H(-1)Cit = C(6)H(4)O(7)(4-); X = K(+), n = 6 (1); X = Na(+), n = 12 (2); X = NH(4)(+), n = 2 (3)) (pH approximately 8) and X(3)[[V(2)O(2)(H(-1)Cit)(Cit)]] x nH(2)O (X = K(+), n = 7 (4)) (pH approximately 5). Complex 2 crystallizes in space group P2(1)/c, a = 11.3335(9) A, b = 15.788(1) A, c = 8.6960(6) A, beta = 104.874(3) degrees, V = 1503.8(2), Z = 2. Complex 3 crystallizes in space group P one macro, a = 9.405(1) A, b = 10.007(1) A, c = 13.983(2) A, alpha = 76.358(4) degrees, beta = 84.056(4) degrees, gamma = 66.102(4) degrees, V = 1169.2(3), Z = 2. Complex 4 crystallizes in space group P2(1)nb, a = 9.679(4) A, b = 19.618(8) A, c = 28.30(1) A, V = 5374.0(4), Z = 8. The X-ray structures of 1-4 are V(2)O(2) dimers, with the citrate displaying varying coordination numbers and modes. 1 exhibits a small ferromagnetic interaction, whereas 4 exhibits an antiferromagnetic interaction between the V(IV) ions. 1 and 4 interconvert with pH, thus rendering the pH a determining factor promoting variable structural, electronic, and magnetic properties in V(IV)-citrate species. The observed aqueous behavior of 1-4 is consistent with past solution speciation studies, and contributes to the understanding of significant aspects of the biologically relevant vanadium(IV)-citrate chemistry.

Octanuclearity and tetradecanuclearity in manganese chemistry: an octanuclear manganese(II)/(III) complex featuring the novel [Mn8(mu4-O)2(mu3-OH)2]14+ core and [Mn10(II)Mn4(III)O4(O2CMe)20[(2-py)2C(OH)O]4] (2-py = 2-pyridyl)

Reactions of Mn sources with di-2-pyridyl ketone, (2-py)2CO, and phenyl 2-pyridyl ketone oxime, (ph)(2-py)CNOH, give the novel clusters [Mn10(II)Mn4(III)O4(O2CMe)20[(2-py)2C(OH)O]4] 1 and [Mn4(II)Mn4(III)O2(OH)2(O2CPh)10[(ph)(2-py)CNO]4] 2, respectively, which possess low-spin ground states; the observed tetradecanuclearity in 1 is extremely rare in 3d-metal chemistry, while the core of 2 has a unique topology consisting of two linked [Mn2(II)Mn2(III)O(OH)] units.

Synthesis, pH-dependent structural characterization, and solution behavior of aqueous aluminum and gallium citrate complexes

Reactions of Al(III) and Ga(III) with citric acid in aqueous solutions, yielded the complexes (NH(4))(5)[M(C(6)H(4)O(7))(2)].2H(2)O (M(III) = Al (1), Ga (2)) at alkaline pH, and the complexes (Cat)(4)[M(C(6)H(5)O(7))(C(6)H(4)O(7))].nH(2)O (M(III) = Al (3), Ga (4), Cat. = NH(4)(+), n = 3; M(III) = Al (5), Ga (6), Cat. = K(+), n = 4) at acidic pH. All compounds were characterized by spectroscopic (FT-IR, (1)H, (13)C, and (27)Al NMR, (13)C-MAS NMR) and X-ray techniques. Complex 1 crystallizes in space group P1, with a = 9.638(5) A, b = 9.715(5) A, c = 7.237(4) A, alpha = 90.96(1) degrees, beta = 105.72(1) degrees, gamma = 119.74(1) degrees, V = 557.1(3) A(3), and Z = 1. Complex 2 crystallizes in space group P1, with a = 9.659(6) A, b = 9.762(7) A, c = 7.258(5) A, alpha = 90.95(2) degrees, beta = 105.86(2) degrees, gamma = 119.28(1) degrees, V = 564.9(7) A(3), and Z = 1. Complex 3 crystallizes in space group I2/a, with a = 19.347(3) A, b = 9.857(1) A, c = 23.412(4) A, beta = 100.549(5) degrees, V = 4389(1) A(3), and Z = 8. Complex 4 crystallizes in space group I2/a, with a = 19.275(1) A, b = 9.9697(6) A, c = 23.476(1) A, beta = 100.694(2) degrees, V = 4432.8(5) A(3), and Z = 8. Complex 5 crystallizes in space group P1, with a = 7.316(1) A, b = 9.454(2) A, c = 9.569(2) A, alpha = 64.218(4) degrees, beta = 69.872(3) degrees, gamma = 69.985(4) degrees, V = 544.9(2) A(3), and Z = 1. Complex 6 crystallizes in space group P1, with a = 7.3242(2) A, b = 9.4363(5) A, c = 9.6435(5) A, alpha = 63.751(2) degrees, beta = 70.091(2) degrees, gamma = 69.941(2) degrees, V = 547.22(4) A(3), and Z = 1. The crystal structures of 1-6 reveal mononuclear octahedral complexes of Al(III) (or Ga(III)) bound to two citrates. Solution NMR, on both 4- and 5- species, reveals rapid intramolecular exchange of the bound and unbound terminal carboxylates. Upon dissolution in water, the complexes, through a complicated reaction cascade, transform to oligonuclear 1:1 species that, in agreement with previous studies, represent the thermodynamically stable state in solution. The data provide, for the first time, structural details of low MW, mononuclear complexes of Al(III) (or Ga(III)) with citrate that are dictated, among other factors, by pH. The properties of 1-6 may provide clues relevant to their biological association with humans.

Nickel-quinolones interaction. Part 1 - Nickel(II) complexes with the antibacterial drug sparfloxacin: structure and biological properties

The mononuclear nickel(II) complexes with the third-generation quinolone antibacterial agent sparfloxacin in the absence or presence of nitrogen donor heterocyclic ligands (1,10-phenanthroline or 2,2'-bipyridine) have been synthesized and characterized. The experimental data suggest that sparfloxacin acts as deprotonated bidentate ligand coordinated to Ni(II) ion through the ketone and carboxylato oxygens. The crystal structure of (1,10-phenanthroline)bis(sparfloxacinato) nickel(II), 2 has been determined by X-ray crystallography. The cyclic voltammograms of the complexes recorded in dmso solution and in 1/2 dmso/buffer (containing 150 mM NaCl and 15 mM trisodium citrate at pH 7.0) solution have shown that in the presence of CT DNA they can bind to CT DNA by the intercalative binding mode. UV study of the interaction of the complexes with calf-thymus DNA (CT DNA) has shown that the complexes can bind to CT DNA and 2 exhibits the highest binding constant to CT DNA. Competitive study with ethidium bromide (EB) has shown that the complexes can displace the DNA-bound EB indicating that they bind to DNA in strong competition with EB for the intercalative binding site. The antimicrobial activity of the complexes has been tested on three different microorganisms and has revealed that the inhibition provided by the complexes is slightly decreased in comparison to free sparfloxacin. The complexes exhibit good binding propensity to human and bovine serum albumin proteins having relatively high binding constant values.

Manganese(II/II/II) and Manganese(III/II/III) Trinuclear Compounds. Structure and Solid and Solution Behavior

Two mixed-valence Mn(III)Mn(II) complexes and a homo-valence Mn(II) trinuclear manganese complex of stoichiometry Mn(III)Mn(II)Mn(III)(5-Cl-Hsaladhp)(2)(AcO)(4)(MeOH)(2).4CH(3)OH (1a), Mn(III)Mn(II)Mn(III) (Hsaladhp)(2)(AcO)(2)(5-Cl-Sal)(2)(thf)(2) (3a) and Mn(II)Mn(II)Mn(II) (AcO)(6)(pybim)(2) (1b) where H(3)saladhp is a tridentate Schiff base ligand and pybim a neutral bidentate donor ligand, have been structurally characterized by using X-ray crystallography. The structurally characterized mixed-valence complexes have strictly 180 degrees Mn(III)-Mn(II)-Mn(III) angles as required by crystallographic inversion symmetry. The complexes are valence trapped with two terminal Mn(III) ions showing Jahn-Teller distortion along the acetate or salicylate-Mn(III)-X axis. The Mn.Mn separation is 3.511 Å and 3.507 Å respectively. The mixed-valence complexes have S = (3)/(2) ground state and the homovalence complex S = (5)/(2), with small antiferromagnetic exchange J couplings, -5.6 and -1.8 cm(-1), respectively, while the powder ESR spectra at 4 K show a broad low field signal with g approximately 4.3 for Mn(III)Mn(II)Mn(III) and a broad temperature-dependent signal at g = 2 for Mn(II)Mn(II)Mn(II). Crystal data for 1a: [C(36)H(60)O(20)N(2)Cl(2)Mn(3)], triclinic, space group P&onemacr;, a = 9.272(7) Å, b = 11.046(8) Å, c = 12.635(9) Å, alpha = 76.78(2) degrees, beta = 81.84(2) degrees, gamma = 85.90(2) degrees, Z = 1. Crystal data for 3a: [C(48)H(56)O(18)N(2)Cl(2)Mn(3)], monoclinic, space group P2(1)/n, a = 8.776(3) Å, b = 22.182(7) Å, c = 13.575(4) Å, beta = 94.44(1) degrees, Z = 2. Crystal data for 1b: [C(36)H(36)O(12)N(6)Mn(3)], triclinic, space group P&onemacr;, a = 13.345(6) Å, b = 8.514(4) Å, c = 9.494(4) Å, alpha = 75.48(1) degrees, beta = 75.83(1) degrees, gamma = 76.42(1) degrees, Z = 1.

Correlations of synthetic, spectroscopic, structural, and speciation studies in the biologically relevant cobalt(II)-citrate system: the tale of the first aqueous dinuclear cobalt(II)-citrate complex

Synthetic efforts targeting soluble species of Co(II) with the low molecular mass physiological ligand citric acid led to the isolation of the first dinuclear complex [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-), at pH approximately 5, in the form of its K+ (1) and Na+ (2) salts. Both 1 and 2 were characterized analytically, spectroscopically (FT-IR, UV/visible, EPR), and magnetically. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 10.348(5) A, b = 11.578(6) A, c = 12.138(6) A, beta = 112.62(2) degrees, V = 1342(1) A(3), and Z = 2. Complex 2 crystallizes in the monoclinic space group P2(1)/c, with a = 9.234(4) A, b = 11.913(4) A, c = 11.728(6) A, beta = 99.93(2) degrees, V = 1271(1) A(3), and Z = 2. X-ray crystallography on 1 and 2 reveals the presence of two Co(II) ions, in a dinuclear assembly, octahedrally coordinated by two citrate ligands in a tridentate fashion. The octahedral environment around each Co(II) is complemented by another singly bonded citrate belonging to the adjacent Co(II) unit and two water molecules. Magnetic susceptibility and EPR studies on 1, in the solid state, corroborate the X-ray results, indicating a weak interaction between the two Co(II) ions. Moreover, EPR and UV/visible studies in solution suggest that 1 does not retain its dimeric structure, yielding a mononuclear octahedral Co(II)-citrate species. Detailed speciation studies suggest the presence of a number of species including the mononuclear complex [Co(C(6)H(5)O(7))](-), optimally present around pH approximately 5. In consonance with EPR and UV/visible spectroscopy, [Co(C(6)H(5)O(7))](-) is likely the scaffolding unit on the basis of which the dimer [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-) is isolated from aqueous solutions. Collectively, this comprehensive study offers significant structural insight into the Co(II)-citrate speciation and the elucidation of the role of Co(II) in biological fluids.