Boraguanidinates: synthesis, X-ray structures, and reactions of {Li2[(i)Pr2NB(NDipp)2]}2 with p-block and group 12 element halides

The new Dipp-substituted boranes (Dipp = 2,6-diisopropylphenyl) (i)Pr2NB(NHDipp)2 (5), BrB(NHDipp)2 (7b), and B(NHDipp)3 (8) have been prepared in high yields and characterized in solution by multinuclear NMR spectroscopy ((1)H, (11)B and (13)C) and in the solid state by X-ray crystallography. Reaction of 5 with 2 equiv of (n)BuLi in n-hexane produces {Li2[(i)Pr2NB(NDipp)2]}2 (6) the first example of a stable dilithiated boraguanidinate. The unsolvated complex 6 has a dimeric structure in the solid state. A survey of the reactions of 6 with p-block and Group 12 element halides revealed various types of reactivity including (a) disproportionation (InCl), (b) reduction to the metal (PbCl2, CdCl2, TeBr4, TeI4, and TlCl), and (c) simple metathesis (GeCl2, MgCl2, and ZnCl2). The metathetical products were characterized by multinuclear NMR spectra but, in contrast to the dilithiated complex 6, they readily decompose in non-coordinating solvents to form the diprotonated compound 5.     

The single molecular precursor approach to metal telluride thin films: imino-bis(diisopropylphosphine tellurides) as examples

Interest in metal telluride thin films as components in electronic devices has grown recently. This tutorial review describes the use of single-source precursors for the preparation of metal telluride materials by aerosol-assisted chemical vapour deposition (AACVD) and acquaints the reader with the basic techniques of materials characterization. The challenges in the design and synthesis of suitable precursors are discussed, focusing on metal complexes of the recently-developed imino-bis(diisopropylphosphine telluride) ligand. The generation of thin films and nanoplates of CdTe, Sb(2)Te(3) and In(2)Te(3) from these precursors are used as illustrative examples.     

Experimental and Theoretical Investigations of Structural Isomers of Dichalcogenoimidodiphosphinate Dimers: Dichalcogenides or Spirocyclic Contact Ion Pairs?

A synthetic protocol for the tert-butyl-substituted dichalcogenoimidodiphosphinates [Na(tmeda){(EPtBu(2))(2)N}] (3 a, E=S; 3 b, E=Se; 3 c, E=Te) has been developed. The one-electron oxidation of the sodium complexes [Na(tmeda){(EPR(2))(2)N}] with iodine produces a series of neutral dimers (EPR(2)NPR(2)E--)(2) (4 b, E=Se, R=iPr; 4 c, E=Te, R=iPr; 5 a, E=S, R=tBu; 5 b, E=Se, R=tBu; 5 c, E=Te, R=tBu). Attempts to prepare 4 a (E=S, R=iPr) in a similar manner produced a mixture including HN(SPiPr(2)). Compounds 4 b, 4 c and 5 a-c were characterised by multinuclear NMR spectra and by X-ray crystallography, which revealed two alternative structures for these dimeric molecules. The derivatives 4 b, 4 c, 5 a and 5 b exhibit acyclic structures with a central chalcogen-chalcogen linkage that is elongated by approximately 2 % (E=S), 6 % (E=Se) and 8 % (E=Te) compared to typical single-bond values. By contrast, 5 c adopts an unique spirocyclic contact ion-pair structure in which a [(TePtBu(2))(2)N](-) ion is Te,Te' chelated to an incipient [(TePtBu(2))(2)N](+) cyclic ion. DFT calculations of the relative energies of the two structural isomers indicate a trend towards increasing stability for the contact ion pair relative to the corresponding dichalcogenide on going from S to Se to Te for both the isopropyl and tert-butyl series. The two-electron oxidation of [Na(tmeda){(EPtBu(2))(2)N}] (E=S, Se, Te) with iodine produced the salts [(EPtBu(2))(2)N](+)X(-) (7 a, E=S, X=I(3); 7 b, E=Se, X=I; 7 c, E=Te, X=I), which were characterised by X-ray crystallography. Compound 7 a exists as a monomeric, ion-separated complex with [d(S--S)=2.084(2) A]; 7 b and 7 c are dimeric [d(Se--Se)=2.502(1) A; d(Te--Te)=2.884(1) A].     

New bonding modes for boraamidinate ligands in heavy group 15 complexes: fluxional behavior of the 1:2 complexes, LiM[PhB(NtBu)2]2 (M = As, Sb, Bi)

The reactions of MCl3 with Li2[PhB(NtBu)2] in 1:1, 1:1.5, and 1:2 molar ratios in diethyl ether produced the monoboraamidinates ClM[PhB(NtBu)2] (1a, M = As; 1b, M = Sb; 1c, M = Bi), the novel 2:3 boraamidinate complexes [PhB(NtBu)2]M-micro-N(tBu)B(Ph)N(tBu)M[PhB(NtBu)2] (2b, M = Sb; 2c, M = Bi), and the bisboraamidinates LiM[PhB(NtBu)2]2 (3a, 3a.OEt2, M = As; 3b, M = Sb; 3c.OEt2, M = Bi), respectively. The 2:3 complexes 2b and 2c were also observed in the reactions carried out in a 1:2 molar ratio at room temperature. All complexes have been characterized by multinuclear NMR spectroscopy (1H, 7Li, 11B, and 13C) and by single-crystal X-ray structural determinations. The molecular units of the mono-boraamidinates 1a-c are isostructural, but their crystal packing is distinct as a result of stronger intermolecular close contacts going from 1a to 1c. In the novel 2:3 bam complexes 2b and 2c, each metal center is N,N'-chelated by a bam ligand and these two [M(bam)]+ units are bridged by the third [bam]2- ligand. The structures of the unsolvated bis-boraaminidate complexes 3a and 3b consist of [Li(bam)]- and [M(bam)]+ monomeric units linked by Li-N and M-N bonds to give a tricyclic structure. Solvation of the Li+ ion by diethyl ether results in a bicyclic structure composed of four-membered BN2As and six-membered BN3AsLi rings in 3a.OEt2. In contrast, the analogous bismuth complex 3c.OEt2 exhibits a tetracyclic structure. Variable-temperature NMR studies reveal that the nature of the fluxional behavior of 3a-c in solution is dependent on the group 15 center.     

Identification of the radical anions of C2N4S2 and P2N4S2 rings by in situ EPR spectroelectrochemistry and DFT calculations

The previously unknown radical anions of unsaturated E2N4S2 ring systems (E=RC, R2NC, R2P) can be generated voltammetrically by the one-electron reduction of the neutral species and, despite half-lives on the order of a few seconds, have been unambiguously characterized by electron paramagnetic resonance (EPR) spectroelectrochemistry using a highly sensitive in situ electrolysis cell. Cyclic voltammetric studies using a glassy-carbon working electrode in CH3CN and CH2Cl2 with [nBu4N][PF6] as the supporting electrolyte gave reversible formal potentials for the [E2N4S2]0/- process in the range of -1.25 to -1.77 V and irreversible peak potentials for oxidation in the range of 0.66 to 1.60 V (vs the Fc+/0 couple; Fc=ferrocene). Reduction of the neutral compound undergoes an electrochemically reversible one-electron transfer, followed by the decay of the anion to an unknown species via a first-order (chemical) reaction pathway. The values of the first-order rate constant, kf, for the decay of all the radical anions in CH2Cl2 have been estimated from the decay of the EPR signals for (X-C6H4CN2S)2*-, where X=4-OCH3 (kf=0.04 s(-1)), 4-CH3 (kf=0.02 s(-1)), 4-H (kf=0.08 s(-1)), 4-Cl (kf=0.05 s(-1)), 4-CF3 (kf=0.05 s(-1)), or 3-CF3 (kf=0.07 s(-1)), and for [(CH3)3CCN2S]2*- (kf=0.02 s(-1)), [(CH3)2NCN2S]2*- (kf=0.05 s(-1)), and [(C6H5)2PN2S]2*- (kf=0.7 s(-1)). Values of kf for X=4-H and for [(CH3)2NCN2S]2*- were also determined from the cyclic voltammetric responses (in CH2Cl2) and were both found to be 0.05 s(-1). Possible pathways for the first-order anion decomposition that are consistent with the experimental observations are discussed. Density functional theory calculations at the UB3LYP/6-31G(d) level of theory predict the structures of the radical anions as either planar (D2h) or folded (C2v) species; the calculated hyperfine coupling constants are in excellent agreement with experimental results. Linear correlations were observed between the voltammetrically determined potentials and both the orbital energies and Hammett coefficients for the neutral aryl-substituted rings.     

The microscopic visualisation of the sonocrystallisation of ice using a novel ultrasonic cold stage

This work reports dynamic video images of the influence of ultrasonic cavitation on the sonocrystallisation of ice at a microscopic level. This has been achieved through the construction of a unique ultrasonic system for an optical microscope. The system consists of (1). an ultrasonic cold stage, (2). a temperature control system, and (3). a microscope and imaging setup. This allows the temperature of a sample to be systematically controlled while it is subjected to simultaneous excitation with alternating pressures in the ultrasonic frequency range. Both the amplitude of excitation and the frequency can be varied. Experiments on ice crystals in pure water and sucrose solutions were conducted. Three distinct phenomena were observed. Firstly, there is a tendency for cavitation bubbles to form at the grain boundaries between ice crystals. Secondly, there is a progressive melting of ice by cavitation bubbles which appear to eat their way into the ice phase. Thirdly, the dendritic ice structures may fragment under the influence of ultrasound, thus increasing the number of nuclei which may subsequently grow (secondary nucleation). These observations form the basis of a significantly enhanced understanding and exploitation of the sonocrystallisation of ice.     

Modeling loop backbone flexibility in receptor‐ligand docking simulations

The relevance of receptor conformational change during ligand binding is well documented for many pharmaceutically relevant receptors, but is still not fully accounted for in in silico docking methods. While there has been significant progress in treatment of receptor side chain flexibility sampling of backbone flexibility remains challenging because the conformational space expands dramatically and the scoring function must balance protein–protein and protein–ligand contributions. Here, we investigate an efficient multistage backbone reconstruction algorithm for large loop regions in the receptor and demonstrate that treatment of backbone receptor flexibility significantly improves binding mode prediction starting from apo structures and in cross docking simulations. For three different kinase receptors in which large flexible loops reconstruct upon ligand binding, we demonstrate that treatment of backbone flexibility results in accurate models of the complexes in simulations starting from the apo structure. At the example of the DFG‐motif in the p38 kinase, we also show how loop reconstruction can be used to model allosteric binding. Our approach thus paves the way to treat the complex process of receptor reconstruction upon ligand binding in docking simulations and may help to design new ligands with high specificity by exploitation of allosteric mechanisms. © 2012 Wiley Periodicals, Inc.