Employment of 2,6-diacetylpyridine dioxime as a new route to high nuclearity metal clusters: Mn6 and Mn8 complexes

The employment of the anion of 2,6-diacetylpyridine dioxime (dapdoH2) as a pentadentate chelate in transition metal cluster chemistry is reported. The syntheses, crystal structures, and magnetochemical characterization are described for [Mn6O2(OMe)2(dapdo)2(dapdoH)4](ClO4)2 (1), [Mn6O2(OMe)2(dapdo)2(dapdoH)4][Ca(NO3)4] (2), and [Mn8O4(OH)4(OMe)2(N3)2(dapdo)2(dapdoH)2(H2O)2] (3). The reaction of [Mn3O(O2CMe)6(py)3](ClO4) with 3 equiv of dapdoH2 (with or without 2 equiv of NEt3) in MeOH gave 1. The same cation, but with a [Ca(NO3)4]2- anion, was found in complex 2, which was obtained from the reaction in MeOH between Mn(NO3)2, Ca(NO3)2, and dapdoH2 in the presence of NEt3. In contrast, addition of NaN3 to several reactions comprising MnCl2, dapdoH2, and NEt3 in MeOH gave the octanuclear complex 3. Complexes 1-3 all possess rare topologies and are mixed-valence: 2MnII, 4MnIII for 1 and 2, and 2MnII, 6MnIII for 3. The core of the cation of 1 and 2 consists of two edge-sharing Mn4 tetrahedra at the center of each of which is a micro4-O2- ion. Peripheral ligation is provided by two micro-OMe-, four micro-dapdoH-, and two micro3-dapdo2- groups. The core of 3 consists of two [MnIIMnIII3(micro3-O)2]7+ "butterfly" units linked together by one of the micro3-O2- ions, which thus becomes micro4. Peripheral ligation is provided by four micro-OMe-, two micro-OH-, two micro-dapdoH-, and two micro4-dapdo2- groups. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-3 in the 5.0-300 K range; the data for 1 and 2 are identical. Fitting of the obtained magnetization versus field (H) and temperature (T) data by matrix diagonalization and including only axial anisotropy (zero-field splitting, D) established that 1 possesses an S=5 ground state with D=-0.24 cm(-1). For 3, low-lying excited states precluded obtaining a good fit from the magnetization data, and the ground state was instead determined from the ac data, which indicated an S=1 ground state for 3. The combined work demonstrates the ligating flexibility of pyridyl-dioxime chelates and their usefulness in the synthesis of new polynuclear Mnx clusters without requiring the co-presence of carboxylate ligands.     

New routes to high nuclearity clusters: fluoride-based octametallic and tridecametallic clusters of manganese

The reaction of MnF(3) with 5,6-dimethylbenzotriazole (Me(2)BTAH) gives the [Mn(III)(8)] complex [Mn(8)O(4)(OMe)(2)(Me(2)BTA)(6)F(8)(Me(2)BTAH)(MeOH)(8)] and the [Mn(IV)(3)Mn(III)(10)] complex [Mn(13)O(12)(Me(2)BTA)(12)F(6)(MeOH)(10)(H(2)O)(2)]. The octametallic species is an "intermediate" in the formation of the tridecametallic cluster.     

A high nuclearity mixed valence {Mn32} complex

A mixed-valence {Mn(32)} complex, containing Mn(II), Mn(III) and Mn(IV) ions, with a probable S = 5 ground state, utilizes pivalate as a capping ligand and a variety of oxo-type bridging ligands. AC susceptibility data on fresh samples suggest that the Mn(32) complex is one of the largest nuclearity single molecule magnets found to date.     

ESR investigation of paramagnetic carbonyl-metal clusters of high nuclearity

An ESR study has been made of the high nuclearity paramagnetic metal cluster anions [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3-, [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- and [Co₆(CO)₈(μ₂-CO)₆C]^-. The assignment of the HOMO is based on a mixed valence model which relates the g tensor components of cluster systems to those of an appropriate conventional paramagnetic center. With this model the HOMOs of [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3- and of [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- are found to be mainly comprised of metal d_z² atomic orbitals, while for [Co₆(CO)₈(μ₂-CO)₆C]^- a large overlap between d atomic orbitals and ligand orbitals is suggested. The occupation of the valence molecular orbitals deduced from the ESR data is consistent with the variations in MM bond distance observed by X-ray analysis.     

Molecular orbital analysis of the bonding in high nuclearity gold cluster compounds

Extended Hückel molecular orbital calculations on high nuclearity gold clusters of the general type [Au(AuPH₃)_n]^x+ have demonstrated that they can be classified into two broad topological classes according to the three-dimensional disposition of the peripheral gold atoms. If they lie approximately on a sphere they are characterised by a total of 12n + 18 valence electrons, but if they adopt a toroidal or eliptical arrangement the total electron count is 12n + 16. The computed energy differences between alternative polyhedral geometries is generally small and accounts for the stereochemical non-rigidity of the gold cluster compounds in solution. Detailed aspects of the structures of the high nuclearity gold cluster compounds have been interpreted in terms of molecular orbital calculations on clusters derived from the centred chair [Au₇(PH₃)₆]⁺ by edge- and face-capping with Au(PH₃)⁺ fragments.     

MVPACK: A package to calculate energy levels and magnetic properties of high nuclearity mixed valence clusters

We present a FORTRAN code based on a new powerful and efficient computational approach to solve the double exchange problem for high‐nuclearity MV clusters containing arbitrary number of localized spins and itinerant electrons. We also report some examples in order to show the possibilities of the program. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Synthesis and characterization of high nuclearity iron(III) phosphonate molecular clusters

Three new phosphonic acid ligands (4- (t)butylphenyl phosphonic acid, 3,5-dimethylphenyl phosphonic acid, and diphenylmethylphosphonic acid) have been synthesized and employed in search of high molecularity iron(III) clusters. The cluster compounds are characterized by single crystal X-ray diffraction and magnetic measurements. The solvothermal reaction of FeCl 3.6H 2O with diphenylacetic acid and p- (t)butylphenyl phosphonic acid resulted in an unprecedented dodecanuclear cluster [Fe 12(mu 2-O) 4(mu 3-O) 4(O 2CCHPh 2) 14(4- (t)buPhPO 3H) 6]( 1) having a double butterfly like core structure. [Fe 12(mu 2-O) 4(mu 3-O) 4(O 2CPh) 14(C 10H 17PO 3H) 6]( 2), another dodecanuclear cluster having core structure similar to 1, has been synthesized in a reaction between [Fe 3O(O 2CPh) 6(H 2O) 3]Cl and camphylphosphonic acid in the presence of triethylamine at ambient condition. 3,5-Dimethylphenyl phosphonic acid on reacting solvothermally with an oxo-centered iron triangle [Fe 3O(O 2CCMe 3) 6(H 2O) 3]Cl gives a nonanuclear cluster [Fe 9(mu 3-O) 4(O 3PPh(Me) 2) 3(O 2CCMe 3) 13]( 3) having icosahedral type core structure where three positions of the iron atoms have been replaced by phosphorus. Another nonanuclear [Fe 9(O) 3(OH) 3(O 3PCHPh 2) 6(O 2CCMe 3) 6(H 2O) 9] ( 4), having a distorted cylindrical core structure, has been synthesized in a similar solvothermal reaction between [Fe 3O(O 2CCMe 3) 6(H 2O) 3]Cl and biphenylmethyl phosphonic acid. All compounds are characterized by IR spectra, elemental analysis, as well as single crystal X-ray analysis. Magnetic measurements for all the compounds reveal that there are antiferromagnetic interactions between the metal centers.     

键连接性指数的建构及其在有机体系中的应用研究

以化学键为基础建构了键连接性指数及分子键连接性指数，该指数同时考虑顶 点原子的化学特征及键的性质．对于任意化学键，键连接性指数Si＝1+△I/R·[（ Z1-h1)m2/n1+(Z2-h2)m1/n2]，分子键连接性指数S为∑i=1^m√Si或∑i=1^m1/√ Si，其中，Z1，Z2为化学键键连原子的价电子数；n1，n2成键原子的价层最高主量 子数；m1，m2为成键原子的氧化数；h1，h2为与成键原子相连的氢原子个数；△I 为成键原子的电负性差(△I≥0)；R为化学键的相对键长．与以顶点为基础的价连 接性指数不同，该指数不仅能方便而有效地应用于饱和碳氢体系亦能有效地应用于 含多重键的不饱和体系及含杂原子的有机体系．研究了饱和碳氢体系标准生成焓， 不饱和碳氢体系和酮、醚、酯体系在水中的溶解度和辛醇/水分配系数，卤代甲烷 体系的标准生成焓，卤代苯体系辛醇/水分配系数，均取得比较满意的结果。     

A method for topological analysis of rod packings

A method for analysis of geometrical and topological properties of crystal structures, which contain one-periodic (chain-like) structural units, is proposed and implemented as a computer procedure into the program package ToposPro. The method is based on representation of the set of parallel chains as two kinds of two-periodic nets: rod net, which characterizes the way of linking the chains with each other, and rod packing net, which describes the method of spatial arrangement of the chains irrespective of their interconnections. The geometrical (Domains method) and topological criteria are formulated to construct rod nets and rod packing nets from the initial crystallographic data. It is shown that rod packing net formalizes earlier classifications of rod packings in crystals. Examples are presented, which demonstrate the applications of the method to one-periodic coordination polymers, metal–organic frameworks, and hydrogen-bonded molecular crystals.     

A simple high pH liquid chromatography-tandem mass spectrometry method for basic compounds: application to ephedrines in doping control analysis

Solvent systems for use with LC-MS often result in a compromise between chromatographic performance and mass spectrometric detection, exemplified here by a LC-MS/MS method development for the analysis of ephedrines in doping control. Ephedrines, frequently found in therapeutic and nutritional preparations, are among the most commonly administered doping agents in competitive sport. Improved separation of these hydrophilic, basic compounds, some of which are diastereoisomers, is achieved in reversed-phase LC by the use of a high pH mobile phase in order to suppress analyte ionisation, and thus alter their polarity, resulting in reduced peak tailing and enhanced retention. However, when coupled to an ESI-MS detector, this eluent composition generated a non-linear and poorly reproducible signal. APCI yielded greater stability and reproducibility and is here presented as an ion source for the analysis of basic compounds under conditions that suppress their ionisation. Errors as large as 49.3% were observed with ESI, compared with 15.4% generated using APCI, for pseudoephedrine over the calibration range (25-400 μg/mL) in urine with a simple dilution and injection of samples. These data highlight the importance of suitable MS conditions for stable performance, necessary for accurate quantification, without undue compromise to the LC separation.     

The classification of tensor surface harmonic functions for clusters and coordination compounds

A simple method for enumerating theL and functions for polyhedral cluster and coordination molecules, within Stone's tensor surface harmonic methodology, is described. The nature of theL orbitals which are generated depends on the polyhedral topology and in particular the number of layers of vertices and the number of vertices within each layer. The functions are enumerated from theL 's by a number of spherical harmonic multiplication rules.     

Assembly of heterometallic clusters and coordination polymers by combining Mo-S-based clusters with Mn2+

Addition of [Mo(V)2O2S2(edt)2]2- (edt =1,2-ethanedithiolate) to acetonitrile and/or methanol solutions of MnII containing bipyridines [4,4'-trimethylenedipyridine (TDP), 4,4'-bipyridine (4,4'-bpy), 2,2'-bipyridine (2,2'-bpy)] or 15-crown-5 produces three new heterometallic cluster coordination polymers, [Mn2[Mo2O2S2(edt)2]2(TDP)3(CH3OH)2(NCMe)2].3CH3OH.0.25MeCN (1), [Mn(TDP)2(H2O)2]2+[Mn[Mo2O2S2(edt)2)2(TDP)2]]2-.6CH3OH (2), [Mn[Mo2O2S2(edt)2](TDP)2(CH3OH)(H2O)].CH3OH (3), and three new multinuclear clusters, [Mn[Mo2O2S2(edt)2](4,4'-bpy)(CH3OH)4].0.5(4,4'-bpy) (4), [Mn[Mo2O2S2(edt)2](2,2'-bpy)2].2CH3OH (5), and (NEt4)2[Mn(15-crown-5)[Mo2O2S2(edt)2]2] (6). All compounds were characterized by X-ray crystallography. The coordination mode of Mn in these compounds depends on the ligands and the crystallization conditions. Compound 2 readily converts to 1 or 3 depending on the reaction and solvent conditions. Compounds 1 and 2 were analyzed using thermogravimetric analysis combined with mass spectroscopy (TG-MS) in the temperature range 25-500 degrees C. The room-temperature magnetic moments for compounds 1-6 were determined.     

A topological method for global optimization of clusters: application to (TiO2)n (n = 1-6)

A new topological method is presented to generate the isomer structures of compound clusters with well defined covalent bonds. This method, combined with density functional theory, has been used to perform global optimization of (TiO(2))(n) (n = 1-6) clusters. Our comprehensive search not only reproduces all of the known lowest-energy structures reported in previous works but also reveals some new low-energy structures. Some energetically unfavorable motifs that induce energy penalties are obtained and discussed. Based on the ground state structures of the anionic (TiO(2))(n). clusters, the electron affinities and photoelectron spectra are simulated and compared with available experimental data.     

Improved liquid-liquid-liquid microextraction method and its application to analysis of four phenolic compounds in water samples

An improved liquid-liquid-liquid microextraction (LLLME) technique has been put forward based on the principle of single drop LLLME. In the technique, a vial insert was firstly utilized as acceptor phase container. Because the diameter of the bottom of the vial insert was small, the contact area between the acceptor phase and the vial insert was bigger than that between microsyringe and microdrop of acceptor phase in single drop LLLME, and the stability of microdrop was increased markedly. More acceptor phase could be held in the improved method than that in single drop LLLME, and the sensitivity of the method was increased. The sample vial and vial insert were horizontally placed so that the density of organic solvent has little effect on the selection of organic solvents. Aqueous ammonia and toluene were selected as the acceptor phase and the organic phase, respectively. The improved method was successfully applied to determine four phenolic compounds in real aqueous samples. Good recoveries that ranged from 82.2% to 117.2% were obtained. The intra-day and inter-day reproducibilities (RSD) were under 4.8% and 6.8%, respectively. The extraction efficiency of the improved method was 11-47 times higher than that of single drop LLLME method. The improved LLLME method is economical, rapid, simple, efficient, low organic solvent consumption and no cross-containment. This method is very suitable for the extraction of ionizable and chargeable analyte in complex environmental or biological samples.     

General molecular mechanics method for transition metal carboxylates and its application to the multiple coordination modes in mono- and dinuclear Mn(II) complexes

A general molecular mechanics method is presented for modeling the symmetric bidentate, asymmetric bidentate, and bridging modes of metal-carboxylates with a single parameter set by using a double-minimum M-O-C angle-bending potential. The method is implemented within the Molecular Operating Environment (MOE) with parameters based on the Merck molecular force field although, with suitable modifications, other MM packages and force fields could easily be used. Parameters for high-spin d (5) manganese(II) bound to carboxylate and water plus amine, pyridyl, imidazolyl, and pyrazolyl donors are developed based on 26 mononuclear and 29 dinuclear crystallographically characterized complexes. The average rmsd for Mn-L distances is 0.08 A, which is comparable to the experimental uncertainty required to cover multiple binding modes, and the average rmsd in heavy atom positions is around 0.5 A. In all cases, whatever binding mode is reported is also computed to be a stable local minimum. In addition, the structure-based parametrization implicitly captures the energetics and gives the same relative energies of symmetric and asymmetric coordination modes as density functional theory calculations in model and "real" complexes. Molecular dynamics simulations show that carboxylate rotation is favored over "flipping" while a stochastic search algorithm is described for randomly searching conformational space. The model reproduces Mn-Mn distances in dinuclear systems especially accurately, and this feature is employed to illustrate how MM calculations on models for the dimanganese active site of methionine aminopeptidase can help determine some of the details which may be missing from the experimental structure.     

Novel method for geometry optimization of molecular clusters: application to benzene clusters

A heuristic and unbiased method for searching optimal geometries of clusters of nonspherical molecules was constructed from the algorithm recently proposed for Lennard-Jones atomic clusters. In the method, global minima are searched by using three operators, interior, surface, and orientation operators. The first operator gives a perturbation on a cluster configuration by moving molecules near the center of mass of a cluster, and the second one modifies a cluster configuration by moving molecules to the most stable positions on the surface of a cluster. The moved molecules are selected by employing a contribution of the molecules to the potential energy of a cluster. The third operator randomly changes the orientations of all molecules. The proposed method was applied to benzene clusters. It was possible to find new global minima for (C6H6)11, (C6H6)14, and (C6H6)15. Global minima for (C6H6)16 to (C6H6)30 are first reported in this article.     

Functionalization of SWNTs to facilitate the coordination of metal ions, compounds and clusters

The carboxylate residues of the open ends of aryl-tert-butyl and arylsulfonic acid side-walled functionalized single walled carbon nanotubes (SWNTs) have been investigated for the complexation conditions of the iron-molybdenum cluster [H(x)PMo(12)O(40)CH(4)Mo(72)Fe(30)(O(2)CMe)(15)O(254)(H(2)O)(98)] ("FeMoC"). A range of alternative donor groups for the attachment of FeMoC have been investigated for piranha etched SWNTs, dodecyl side-walled functionalized SWNTs (DD-SWNTs) and ultra-short SWNTs (US-SWNTs), including include pyridines, thiols and phosphines, using coupling reactions to either the carboxylate or hydroxide residues of the SWNTs' open ends. The functionalized SWNTs have been characterized by XPS, uptake of Fe(3+) and, where appropriate, MAS (31)P NMR. The efficacy of binding is dependent on the presence and identity of the ligand moiety. TEM and AFM of the SWNT-FeMoC conjugates show the presence of a 2-3 nm spherical feature on the tip of individual SWNTs.     

An organometallic approach to the synthesis of high nuclearity Mo-Fe-S clusters as potential models for the iron-molybdenum cofactor of nitrogenase

Reaction of the [Fe₂S₂(CO)₆]^2– dianion with molybdenum reagents produces a number of high-nuclearity Mo-Fe-S carbonyl clusters with Fe/Mo ratios 5, as well as a variety of new Fe-S carbonyl clusters. The former are particularly relevant as models or precursors to models for the iron-molybdenum cofactor [FeMo-cofactor] of nitrogenase. General strategies for the synthesis of FeMo-cofactor models are briefly reviewed, and the structures of clusters produced in the [Fe₂S₂(CO)₆]^2–/Mo systems examined to date are described.     

A new internal reference method for activation analysis and its application

A new internal reference method for activation analysis has been developed. The method can be used effectively for special samples in which suitable elements as internal standards are absent and the self-shielding effect can be neglected. In this method, W_b g of element B as an internal reference is added to the sample which contains W_a g of element A to be determined, whereas the comparative standard is prepared by mixing only the element A and B in a known concentration ratio of W _a ^* /W_b. When the sample and comparative standard are irradiated by particles with the same energy distribution, even though both are irradiated separately by particles with different flux, W_a can be determined easily by the following equation. $$W_a = \left( {{{A_R } \mathord{\left/ {\vphantom {{A_R } {A_R^* }}} \right. \kern-\nulldelimiterspace} {A_R^* }}} \right)W_a^* $$ where A_g and A_R are count ratios between gamma-rays emitted by two radioactive nuclides produced from elements A and B in the sample and comparative standard, respectively. The usefulness of the present method was examined through the determination of Ti, Cr, Ni and Zr in several commercial aluminium alloys by means of photon activation, and the accuracy and precision of the method were verified.     

Time autocorrelation function analysis of master equation and its application to atomic clusters

We derive the energy fluctuation Delta(2)E, and the time autocorrelation kappa(tau) and its Fourier transformation--the fluctuation spectra S(omega)--of the master-equation transition matrix. The contribution from each eigenmode of the transition matrix to these fluctuation quantities reveals the relevant importance of the individual mode in the relaxation processes. The time scales associated with these relaxation processes are determined by the corresponding eigenvalues. Unlike traditional time evolution analysis, the autocorrelation function and fluctuation spectra analysis does not involve an arbitrary initial population. It is also more suitable for analyzing the underlying dynamic, kinetic behavior near the equilibrium and the behavior of the long-time-scale rare events. We utilize our technique to analyze the solid-liquid phase coexistence of the 13-atom Morse cluster and the fcc-to-icosahedral structure transition of the 38-atom Lennard-Jones cluster. For the processes studied, the fluctuation spectra from the master equation simplify the analysis of the transition matrix, and the important relaxation modes are easily extracted.