Theory of enhanced magnetic anisotropy induced by Pt adatoms on two-dimensional Co clusters supported on a Pt(111) substrate

Calculations are reported of the magnetic anisotropy energy of two-dimensional (2D) Co nanostructures on a Pt(111) substrate. The perpendicular magnetic anisotropy (PMA) of the 2D Co clusters strongly depends on their size and shape, and rapidly decreases with increasing cluster size. The PMA calculated is in reasonable agreement with experimental results. The sensitivity of the results to the Co-Pt spacing at the interface is also investigated and, in particular, for a complete Co monolayer we note that the value of the spacing at the interface determines whether PMA or in-plane anisotropy occurs. We find that the PMA can be greatly enhanced by the addition of Pt adatoms to the top surface of the 2D Co clusters. A single Pt atom can induce in excess of 5 meV to the anisotropy energy of a cluster. In the absence of the Pt adatoms the PMA of the Co clusters falls below 1 meV/Co atom for clusters of about 10 atoms whereas, with Pt atoms added to the surface of the clusters, a PMA of 1 meV/Co atom can be maintained for clusters as large as about 40 atoms. The effect of placing Os atoms on the top of the Co clusters is also considered. The addition of 5d atoms and clusters on the top of ferromagnetic nanoparticles may provide an approach to tune the magnetic anisotropy and moment separately.

Ab initio and tight-binding calculations of noncollinear magnetism in manganese clusters

Combining ab initio and tight-binding calculations, we have studied the noncollinear magnetism in manganese clusters. The oscillations in the per-atom moments observed experimentally are reproduced theoretically. The tendency of antiferromagnetic coupling between near neighbors leads to noncollinear coupling between atoms within the clusters. For clusters containing 12, 13, 15, 19, and 23 atoms, the geometrical structures were optimized from ab initio calculations with collinear coupled spin moments among different atomic sites. For larger clusters such as Mn-36 and Mn-55, the geometries are taken as portions of an fcc structure. Although the local atomic moments have high values close to 4 mu(B), the net moments lie in the range of 0.4-1.2 mu(B)/atom. Taking the noncollinear coupling into account brings the calculated magnetic moments much closer to the experimental results.

Dispersal of NK homeobox gene clusters in amphioxus and humans

The Drosophila melanogaster genome has six physically clustered NK-related homeobox genes in just 180 kb. Here we show that the NK homeobox gene cluster was an ancient feature of bilaterian animal genomes, but has been secondarily split in chordate ancestry. The NK homeobox gene clusters of amphioxus and vertebrates are each split and dispersed at two equivalent intergenic positions. From the ancestral NK gene cluster, only the Tlx-Lbx and NK3-NK4 linkages have been retained in chordates. This evolutionary pattern is in marked contrast to the Hox and ParaHox gene clusters, which are compact in amphioxus and vertebrates, but have been disrupted in Drosophila.

1-(2 '-pyridylazo)-2-naphtholate complexes of ruthenium: synthesis, characterization, and DNA binding properties

Reaction of 1-(2'-pyridylazo)-2 -naphthol (Hpan) with [Ru(dmso)(4)Cl-2] (dmso=dimethylsulfoxide), [Ru(trpy)Cl-3] (trpy=2,2',2 ''-terpyridine), [Ru(bpy)Cl-3] (bpy=2,2'-bipyridine) and [Ru(PPh3)(3)Cl-2] in refluxing ethanol in the presence of a base (NEt3) affords, respectively, the [Ru(pan)(2)], [Ru(trpy)(pan)](+) (isolated as perchlorate salt), [Ru(bpy)(pan)Cl] and [Ru(PPh3)(2)(pan)Cl] complexes. Structures of these four complexes have been determined by X-ray crystallography. in each of these complexes, the pan ligand is coordinated to the metal center as a monoanionic tridentate N,N,O-donor. Reaction of the [Ru(bpy)(pan)Cl] complex with pyridine (py) and 4-picoline (pic) in the presence of silver ion has yielded the [Ru(bpy)(pan)(py)](+) and [Ru(bpy)(pan)(pic)](+) complexes (isolated as perchlorate salts), respectively. All the complexes are diamagnetic (low-spin d(6), S = 0) and show characteristic H-1 NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ru(II)-Ru(III) oxidation on the positive side of SCE. Except in the [Ru(pan)(2)] complex, a second oxidative response has been observed in the other five complexes. Reductions of the coordinated ligands have also been observed on the negative side of SCE. The [Ru(trpy)(pan)]ClO4, [Ru(bpy)(pan)(py)]ClO4 and [Ru(bpy) (pan)(pic)]ClO4 complexes have been observed to bind to DNA, but they have not been able to cleave super-coiled DNA on UV irradiation. (c) 2008 Elsevier Ltd. All rights reserved.

Stabilization of a helical water chain in a metal-organic host of a trinuclear Schiff base complex

Three heterometallic trinuclear Schiff base complexes, [{GuL(1)(H2O)}(2)Ni(CN)(4)]center dot 4H(2)O (1), [{CuL2(H2O)}(2)Ni(CN)(4)] (2), and [{CuL3(H2O)}(2)Ni(CN)(4)] (3) (HL1 = 7-amino-4-methyl-5-azahept-3-en-2-one, HL2 = 7-methylamino-4-methyl-5-azahept-3-en-2-one, and HL3 = 7-dimethylamino-4-methyl-5-azahept-3-en-2-one), were synthesized. All three complexes were characterized by elemental analysis, IR and UV spectroscopies, and thermal analysis. Two of them (1 and 3) were also characterized by single crystal X-ray crystallography. Complex 1 forms a hydrogen-bonded one-dimensional metal-organic framework that stabilizes a helical water chain into its cavity, but when any of the amine hydrogen atoms of the Schiff base are replaced by methyl groups, as in L 2 and L 3, the water chain, vanishes, showing explicitly the importance of the host-guest H-bonding interactions for the stabilization of a water cluster.

Ruthenium (III) complexes of tetradentate NSNO pyridylthioazophenolates: Synthesis, spectral studies, crystal structure and redox properties

A family of ruthenium (III) complexes of tetradentate monobasic NSNO donor chelators (HL) have been synthesized and isolated in their pure form. On chromatographic separation, trans-dichloro and cis-dichloro ruthenium (111) complexes of pyridylthioazophenolates are eluted using 19:1 and 7:3 (v/v) DCM-MeOH mixtures, respectively. Both cis and trans isomers of the dark brown colored ruthenium (111) complexes, having the general formula of [Ru(L)Cl-2], have been characterized by elemental analyses, spectroscopic and other physico-chemical tools. The magnetic moments of both the cis- and trans-[Ru(L)Cl-2] complexes are in the range of 1.71-1.79 BM. One of the complexes, trans-[Ru(L1)Cl-2] (2a), has been subjected to single-crystal X-ray analysis which confirms that the chlorines are in mutually trans positions in the molecule. The EPR spectra of the cis-[Ru(L)Cl-2] complexes (1) in DMF are consistent with the fact that the complexes are low-spin octahedral with one unpaired electron having three different g values (g(x) not equal g(y) not equal g(z)) complexes are monomeric with an octahedral coordination sphere. The electrochemical studies of [Ru(L)Cl,] in DMF show a quasi-reversible voltammogram. The reduction potentials for the cis-isomers are comparatively lower than those of the corresponding trans isomers. On reaction with the bidentate bipyridyl ligand in the presence of AgNO3, the cis-[Ru(L)Cl-2] complexes (1) produce a series of complexes with the general formula [Ru(L)(bpy)(2)](PF6)(2) (3). which have also been characterized by elemental analyses, spectroscopic and other physico-chemical tools. (c) 2006 Elsevier Ltd. All rights reserved.

Use of HNO3 as the source of NO to prepare a nitric oxide complex of ruthenium

Reaction of cis-Ru(bisox)(2)Cl-2, where bisox is 4,4,4',4'-tetramethyl-2,2'-bisoxazoline, with HNO3 in 1 : 4 molar proportion in boiling water under N-2 atmosphere and subsequent addition of an excess of NaClO4 center dot H2O yields [Ru(bisox)(HL)(NO)](ClO4)(NO3) (1). HL is a hydrolysed form of bisox where one of the oxazoline rings opens up. X-Ray crystallography shows that 1 contains an octahedral RuN5O core. HL binds the metal through an imino N, an amide N and an alcoholic O atom. Reaction of cis-Ru(bisox)(2)Cl-2 with an excess of NaNO2 in water gives cis-Ru(bisox)(2)(NO2)(2) (2). On acidification by HClO4 in methanol, 2 is smoothly converted to cis-[Ru(bisox)(2)(NO2)(NO)](ClO4)(2) (3) due to equilibrium (1).

A deliberate synthesis of a mononuclear trans-dimethoxide complex of ruthenium(III). X-ray crystal structure, electrochemistry and electron paramagnetic resonance

Using bis(3,5-dimethylpyrazol-1-yl) methane as the bidentate N donor ligand L, the yellow compound trans-[(RuL2)-L-III(OMe)(2)]ClO4 center dot CH2Cl2 is synthesized. It is a rare example of a mononuclear dialkoxo complex of Ru(III). It shows a quasireversible Ru(II/III) couple at -0.65 V versus NHE in acetonitrile at a Pt electrode. Its magnetic moment at room temperature corresponds to one unpaired electron. It displays a rhombic EPR spectrum in acetone at 77 K with g = 2.219, 2.062 and 1.855. (C) 2009 Elsevier B. V. All rights reserved.

Ruthenium complexes of C,C '-bis(ethynyl)carboranes: an investigation of electronic interactions mediated by spherical pseudo-aromatic spacers

The complexes [Ru(1-C=C-1,10-C2B8H9)(dppe)Cp*] (3a), [Ru(1-C C-1,12-C2B10H11)(dppe)-Cp*] (3b), [{Ru(dppe)Cp*}(2){mu-1,10-(C C)(2)-1,10-C2B8H8}] (4a) and [{Ru(dppe)Cp*}(2){mu-1,12-(C C)2- 1,12-C2B10-H-10}] (4b), which form a representative series of mono- and bimetallic acetylide complexes featuring 10- and 12-vertex carboranes embedded within the dethynyl bridging ligand, have been prepared and structurally characterized. In addition, these compounds have been examined spectroscopically (UV-is-NIR, IR) in all accessible redox states. The significant separation of the two, one-electron anodic waves observed in the cyclic voltammograms of the bimetallic complexes 4a and 4b is largely independent of the nature of the electrolyte and is attributed to stabilization of the intermediate redox products [4a](+) and [4b](+) through interactions between the metal centers across a distance of ca. 12.5 angstrom. The mono-oxidized bimetallic complexes (4a](+) and [4b](+) exhibit spectroscopic properties consistent with a description of these species in terms of valence-localized (class II) mixed-valence compounds, including a unique low-energy electronic absorption band, attributed to an, IVCT-type transition that tails into the IR region. DFT calculations with model systems [4a-H](+) and [4b-H](+) featuring simplified ligand sets reproduce the observed spectroscopic data and localized electronic structures for the mixed-valence cations [4a](+) and [4b](+).

Synthesis, structure, and electrochemical properties of a family of 2-(arylazo)phenolate complexes of ruthenium with unusual C-C coupling and N=Ncleavage

Reaction of 2-(4'-R-phenylazo)-4-methylphenols (R = OCH3, CH3, H, Cl, and NO2) with [Ru(dmso)(4)Cl-2] affords a family of five ruthenium(III) complexes, containing a 2-(arylazo)phenolate ligand forming a six-membered chelate ring and a tetradentate ligand formed from two 2-(arylazo) phenols via an unusual C-C coupling linki.ng the two ortho carbons of the phenyl rings in the arylazo fragment. A similar reaction with 2-(2'-methylphenylazo)-4-methylphenol with [Ru(dmso)(4)Cl-2] has afforded a similar complex, in which one 2-(2'-methylphenylazo)-4-methylphenolate ligand is coordinated forming a six-membered chelate ring, and the other two ligands have undergone the C-C coupling reaction, and the coupled species is coordinated as a tetradentate ligand forming a five-membered N,O-chelate ring, a nine-membered N,N-chelate ring, and another five-membered chelate ring. Reaction of 2-(2',6'-dimethylphenylazo)-4-methylphenol with [Ru(dmso)(4)Cl-2] has afforded a complex in which two 2-(2',6'-dimethylphenylazo)-4-methylphenols are coordinated as bidentate N,O-donors forming five- and six-membered chelate rings, while the third one has undergone cleavage across the N=N bond, and the phenolate fragment, thus generated, remains coordinated to the metal center in the iminosemiquinonate form. Structures of four selected complexes have been determined by X-ray crystallography. The first six complexes are one-electron paramagnetic and show rhombic ESR spectra. The last complex is diamagnetic and shows characteristic H-1 NMR signals. All the complexes show intense charge-transfer transitions in the visible region and a Ru(III)-Ru(IV) oxidation on the positive side of SCE and a Ru(III)-Ru(II) reduction on the negative side.

A novel trinuclear nickel(II) complex of an unsymmetrical tetradentate ligand involving bridging oxime and acetylacetone functions

A novel trinuclear nickel(II) complex, [Ni-3(L)(2)(H2O)(2)](ClO4)(2), where L is a bridging unsymmetrical tetradentate ligand, involving o-phenylenediamine, diacetyl monoxime and acetylacetone (H2L = 4-[2-(3-hydroxy-1-methyl-but-2-enylideneamino)-phenylimino]-pentan-2- one oxime) has been synthesized and characterized structurally. In the complex, an octahedral Ni( II) centre is held in the middle by two square planar units with the aid of oxime and ketonic bridges. (c) 2007 Elsevier B. V. All rights reserved.

Tuning of redox potential and visible absorption band of ruthenium(II) complexes of (benzimidazolyl) derivatives: synthesis, characterization, spectroscopic and redox properties, X-ray structures and DFT calculations

Six ruthenium(II) complexes have been prepared using the tridentate ligands 2,6-bis(benzimidazolyl) pyridine and bis(2-benzimidazolyl methyl) amine and having 2,2'-bipyridine, 2,2':6',2 ''-terpyridine, PPh3, MeCN and chloride as coligands. The crystal structures of three of the complexes trans-[Ru(bbpH(2))(PPh3)(2)(CH3CN)I(ClO4)(2) center dot 2H(2)O (2), [Ru(bbpH(2))(bpy)Cl]ClO4 (3) and [Ru(bbpH(2))(terpy)](ClO4)(2) (4) are also reported. The complexes show visible region absorption at 402-517 nm, indicating that it is possible to tune the visible region absorption by varying the ancillary ligand. Luminescence behavior of the complexes has been studied both at RT and at liquid nitrogen temperature (LNT). Luminescence of the complexes is found to be insensitive to the presence of dioxygen. Two of the complexes [Ru(bbpH(2))(bpy)Cl]ClO4 (3) and [Ru(bbpH(2))(terpy]ClO4)(2) (4) show RT emission in the NIR region, having lifetime, quantum yield and radiative constant values suitable for their application as NIR emitter in the solid state devices. The DFT calculations on these two complexes indicate that the metal t(2g) electrons are appreciably delocalized over the ligand backbone. (C) 2006 Elsevier B.V. All rights reserved.

Mononuclear and binuclear ruthenium(II) complexes with 4-(phenyl) thiosemicarbazone of benzaldehyde: a discussion on the relative stabilities of the four-membered and five-membered chelate rings formed by the ligand

Three new ruthenium complexes of the formulae cis-[Ru(PPh3)(2)(BzTscbz)(2)] (1a), [Ru-2(PPh3)(2)(BzTscbz)(4)] (1b) and [Ru(PPh3)(2)(BzTscHbz)(2)](ClO4)(2) (2) [BzTscHbz = 4-(phenyl) thiosemicarbazone of benzaldehyde] have been synthesized and characterized by various physicochemical methods including X-ray structure determinations for 1a and 1b. The relative stabilities of the four-membered versus five-membered chelate rings formed by the deprotonated ligand BzTscbz are discussed on the basis of the experimental results and some semi-empirical as well as DFT calculations. (c) 2005 Elsevier Ltd. All rights reserved.

Synthesis, spectroscopic and redox properties of some ruthenium(II) thiosemicarbazone complexes: structural description of four of these complexes

Sixteen neutral mixed ligand thiosemicarbazone complexes of ruthenium having general formula [Ru(PPh3)(2)L-2], where LH = 1-(arylidine)4-aryl thiosemicarbazones, have been synthesized and characterized. All complexes are diamagnetic and hence ruthenium is in the +2 oxidation state (low-spin d(6), S = 0). The complexes show several intense peaks in the visible region due to allowed metal to ligand charge transfer transitions. The structures of four of the complexes have been determined by single-crystal X-ray diffraction and they show that thiosemicarbazone ligands coordinate to the ruthenium center through the hydrazinic nitrogen and sulfur forming four-membered chelate rings with ruthenium in N2S2P2 coordination environment. In dichloromethane solution, the complexes show two quasi-reversible oxidative responses corresponding to loss of electron from HOMO and HOMO - 1. The E-0 values of the above two oxidations shows good linear relationship with Hammett substituents constant (sigma) as well as with the HOMO energy of the molecules calculated by the EHMO method. A DFT calculation on one representative complex suggests that there is appreciable contribution of the sulfur p-orbitals to the HOMO and HOMO - 1. Thus, assignment of the oxidation state of the metal in such complexes must be made with caution. (c) 2005 Elsevier B.V. All rights reserved.