Pyrophosphate-mediated magnetic interactions in Cu(II) coordination complexes

The reaction in water of Cu(NO(3))(2)·2.5H(2)O with 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), or 1,10-phenanthroline-5-amine (phenam), and sodium pyrophosphate (Na(4)P(2)O(7)), at various pHs, afforded three new copper(II)-pyrophosphate complexes, namely, {[Cu(bipy)(cis-H(2)P(2)O(7))](2)}·3H(2)O (1a), {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(ClO(4))(2)·4H(2)O (2), and {[Cu(2)(phenam)(2)(P(2)O(7))](2)·25H(2)O}(n) (3). A solvent free crystalline phase of 1a was also isolated with formula {[Cu(bipy)(trans-H(2)P(2)O(7))](2)} (1b), which can be regarded as a pseudo-polymorph of 1a. Single crystal X-ray analyses revealed these compounds to have uncommon molecular architectures, with 3 being an unprecedented pyrophosphate-containing two-dimensional (2D) polymer. Compounds 1a/1b and 2 are discrete di- and tetra-nuclear complexes, respectively. The cationic {[Cu(phen)(H(2)O)](4)(HP(2)O(7))(2)}(2+) unit in 2 presents a unique quasi-flat structure, held together by solely in-plane pyrophosphate bridging modes (short O(eq)-P-O(eq) and long O(eq)-P-O-P-O(eq) pathways), a coordination arrangement also not previously reported. A different tetranuclear copper(II)-pyrophosphate arrangement is found in 3, with two classically bridged dimers (O(eq)-P-O(eq) pathway) joined together by auxiliary equatorial-axial μ-O pyrophosphate bridges. Here, the bidimensionality is reached through bridging phenam ligands, which provide further inter-"tetramer" metal-metal connections [(N,N')(eq)-(N')(ax) pathway], leading to the formation of an expanded covalent network based on the [Cu(2)(phenam)(2)(P(2)O(7))](2) moiety. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 2 and 3 revealed net antiferromagnetic coupling between metal centers with J(2a) = -7.9(2) cm(-1), J(2b) = -46.9(3) cm(-1), J(2c) = 0 cm(-1) in 2 (H = -J(2a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(2b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(2c)S(Cu(2))·S(Cu(2a))), and J(3a) = -87.9(2) cm(-1), J(3b) = -5(1) cm(-1) and J(3c) = +5(3) cm(-1) in 3 (H = -J(3a)[S(Cu(1))·S(Cu(2)) + S(Cu(1a))·S(Cu(2a))] - J(3b)[S(Cu(1))·S(Cu(2a)) + S(Cu(1a))·S(Cu(2))] - J(3c)S(Cu(2))·S(Cu(2a))). For 1a, a net ferromagnetic coupling is observed with J(1a) = +0.86(1) cm(-1) (H = -J S(A)·S(B) + S(A)·D· S(B) + βH (g(A)S(A) + g(B)S(B)). This is the first example of ferromagnetic coupling in pyrophosphate-complexes reported to date. A structure-function correlation study focusing on magnetic exchange across the observed diverse pyrophosphate-bridges is described with density functional theory (DFT) calculations included to support the stated observations.     

Unexpected catalytic reactions of silyl-protected enol diazoacetates with nitrile oxides that form 5-arylaminofuran-2(3H)-one-4-carboxylates

Silyl-protected enol diazoacetates undergo dirhodium(II)-catalyzed reactions with nitrile oxides to form acid-labile ketenimines via dipolar cycloaddition of nitrile oxides to a donor/acceptor cyclopropene and Lossen rearrangement of the dipolar adduct; acid catalysis converts the ketenimine to the furan product.     

Synthesis, structure and thermal analysis of the gallium complex of 1,4,7,10-tetraazacyclo-dodecane-N,N′,N″,N-tetraacetic acid (DOTA)

As part of our efforts to synthesize gallium bioconjugates based on folic acid and thiamin we have utilized 1,4,7,10-tetraazacyclo-dodecane-N,N′,N″,N-tetraacetic acid (DOTA) as the chelating ligand for gallium(III). The reaction of gallium chloride with DOTA at room temperature in aqueous solution affords the complex [Ga(HDOTA) · 5.5H₂O] (1), which is characterized by single-crystal X-ray spectroscopy, electrospray mass spectrometry and thermal gravimetric analysis. Gallium displays 6-coordinate distorted octahedral geometry within a puckered macrocyclic DOTA framework. The equatorial plane comprises two nitrogens of the DOTA ring and two oxygens from two of the four pendant carboxylic acid groups. The axial positions are comprised of the remaining two transannular nitrogens of the DOTA ring.     

Hydrogen-bond tuning of ferromagnetic interactions: synthesis, structure and magnetic properties of polynuclear copper(II) complexes incorporating p-block oxo-anions

The reaction of copper(II) hydroxide with 2,2'-bipyridine (bipy) (1 : 1) in alkaline aqueous solution (pH 14) at room temperature affords the alternating carbonate/hydroxo-bridged copper(II) polymeric chain compound {[Cu3(bipy)3(mu-OH)2(mu-CO3)2].11H2O}n, 1, as determined by single-crystal X-ray diffraction. The structure of 1 is built up from two similar centro-symmetric dinuclear [(bipy)Cu(mu-OH)]2 cores which link together via bridging carbonate groups to mononuclear [(bipy)Cu] fragments to form the chain. Interdigitation of adjacent chains through pi-pi interactions, which involve each bipy ligand, forms sheets that are separated by the water molecules of crystallisation. Variable-temperature magnetic susceptibility measurements have shown that 1 behaves as an isolated spin doublet with two non-interacting spin triplets with the magnetic coupling through the bis-mu-hydroxo bridges being strongly ferromagnetic in nature. The hydrothermal reaction of copper(II) hydroxide, bipy and ammonium hydrogenphosphate (pH 8) yields a dinuclear copper(II) complex of formula {[Cu2(bipy)2(mu-OH)2(HPO4)(H2O)].4H2O}, 2, as determined by single-crystal X-ray diffraction. The structure of consists of an asymmetric dinuclear bis-mu-hydroxo copper(II) core with a monodentate hydrogenphosphate dianion coordinating to one copper(II) atom (above) and a water molecule to the other (below). Intra- and inter-molecular hydrogen bond interactions involving the hydrogenphosphate, water molecules and bis-mu-hydroxo groups link adjacent dinuclear fragments into chains, which interdigitate to form sheets that are separated by the water molecules of crystallisation. The investigation of the magnetic properties of 2 showed that the strength of the ferromagnetic interaction through the bis-mu-hydroxo bridges is influenced by the significant out-of-plane displacement of the hydrogen atom of the hydroxo groups, brought about through hydrogen bonding to the hydrogenphosphate ligand, and yielded the strongest ferromagnetic coupling yet reported for the bis-mu-hydroxo copper(II) core.     

Perspective on dirhodium carboxamidates as catalysts

Dirhodium compounds are emerging as highly efficient catalysts for diverse reactions, and those with carboxamidate ligands have the broadest applications. The unique features of these compounds are their structural rigidity, ease of ligand exchange, open diaxial sites for coordination with Lewis bases, and their low oxidation potential. As consequences of this, dirhodium carboxamidates are efficient and effective catalysts for metal carbene reactions, Lewis acid-catalyzed processes, and chemical oxidations. With chiral carboxamidate ligands these dirhodium compounds show exceptional enantiocontrol for intramolecular cyclopropanation and carbon-hydrogen insertion reactions of diazoacetates, and they are also highly efficient and selective for hetero-Diels-Alder reactions. Their limitations lie in their moderate reactivities for metal carbene generation and Lewis acid catalysis and in the cost of the precious metal rhodium.     

A Brownian dynamics-finite element method for simulating DNA electrophoresis in nonhomogeneous electric fields

The objective of this work is to develop a numerical method to simulate DNA electrophoresis in complicated geometries. The proposed numerical scheme is composed of three parts: (1) a bead-spring Brownian dynamics (BD) simulation, (2) an iterative solver-enhanced finite element method (FEM) for the electric field, and (3) the connection algorithm between FEM and BD. A target-induced searching algorithm is developed to quickly address the electric field in the complex geometry which is discretized into unstructured finite element meshes. We also develop a method to use the hard-sphere interaction algorithm proposed by Heyes and Melrose [J. Non-Newtonian Fluid Mech. 46, 1 (1993)] in FEM. To verify the accuracy of our numerical schemes, our method is applied to the problem of lambda-DNA deformation around an isolated cylindrical obstacle for which the analytical solution of the electric field is available and experimental data exist. We compare our schemes with an analytical approach and there is a good agreement between the two. We expect that the present numerical method will be useful for the design of future microfluidic devices to stretch and/or separate DNA.     

Electronic structure of C60 on Au(887)

We present an analysis of the electronic structure of C60 adsorbed on a vicinal Au(111) surface at different fullerene coverages using photoemission, x-ray absorption, and scanning tunneling microscopy/spectroscopy (STS). STS provides a straightforward determination of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels with respect to the Fermi energy. At C60 coverages of 0.5 and 1 ML a 2.7 eV wide HOMO-LUMO gap is found. The near-edge x-ray absorption fine structure (NEXAFS) spectrum for the 0.5 ML C60 nanomesh structure displays a significant intensity at the low energy side of the LUMO exciton peak, which is explained as due to absorption into HOMO-LUMO gap states localized at individual C60 cluster edges. From 0.5 to 1 ML we observe a rigid shift of the HOMO-LUMO peaks in the STS spectra and an almost complete quenching of the gap states feature in NEXAFS.     

Gel-induced selective crystallization of polymorphs

Although nanoporous materials have been explored for controlling crystallization of polymorphs in recent years, polymorphism in confined environments is still poorly understood, particularly from a kinetic perspective, and the role of the local structure of the substrate has largely been neglected. Herein, we report the use of a novel material, polymer microgels with tunable microstructure, for controlling polymorph crystallization from solution and for investigating systematically the effects of nanoconfinement and interfacial interactions on polymorphic outcomes. We show that the polymer microgels can improve polymorph selectivity significantly. The polymorphic outcomes correlate strongly with the gel-induced nucleation kinetics and are very sensitive to both the polymer microstructure and the chemical composition. Further mechanistic investigations suggest that the nucleation-templating effect and the spatial confinement imposed by the polymer network may be central to achieving polymorph selectivity. We demonstrate polymer microgels as promising materials for controlling crystal polymorphism. Moreover, our results help advance the fundamental understanding of polymorph crystallization at complex interfaces, particularly in confined environments.     

Graphical Frobenius representations

A Frobenius group is a transitive permutation group that is not regular and such that only the identity fixes more than one point. A graphical Frobenius representation (GFR) of a Frobenius group G is a graph whose automorphism group, as a group of permutations of the vertex set, is isomorphic to G. The problem of classifying which Frobenius groups admit a GFR is a natural extension of the classification of groups that have a graphical regular representation (GRR), which occupied many authors from 1958 through 1982. In this paper, we review for graph theorists some standard and deep results about finite Frobenius groups, determine classes of finite Frobenius groups and individual groups that do and do not admit GFRs, and classify those Frobenius groups of order at most 300 having a GFR. Because a Frobenius group, as opposed to a regular permutation group, has a highly restricted structure, the GFR problem emerges as algebraically more complex than the GRR problem. This paper concludes with some further questions and a strong conjecture.     

Effective methods for the syntheses of 2-pyrazolines and pyrazoles from diazocarbonyl compounds

Dipolar addition of diazocarbonyl compounds to α,β-unsaturated esters and nitriles in the presence of pyridine results in the production of 2-pyrazolines in nearly quantitative yields. In one case, reaction of α-diazoacetophenone with acrylonitrile, Michael addition of the 2-pyrazoline to the conjugated olefin is observed, but this process is minimized by limiting the amount of pyridine employed. 5-Cyano-2-pyrazolines are converted to derivative pyrazoles through alkoxide promoted hydrogen cyanide elimination. With vinyl sulfone, α-diazoacetophenone undergoes sequential dipolar addition, Michael addition, and vinyl sulfite elimination under exceptionally mild conditions to form the derivative β-(1-pyrazolyl)ethyl vinyl sulfone in good yield.