Showing papers by "James Alexis Platts published in 2011"

First experimental characterization of a non-nuclear attractor in a dimeric magnesium(I) compound.

Abstract: High-resolution X-ray diffraction data, coupled with theoretical calculations, are used to demonstrate the presence of a non-nuclear local maximum in the electron density of a dimeric Mg(I) molecule. This is the first time such a non-nuclear attractor (NNA) has been observed in a stable molecular species. Multipole modeling of the Mg(I) centers requires use of expansion/contraction (κ) coefficients taken from density functional theory (DFT), since accurate scattering factors for Mg(I) are not available. The model developed accurately accounts for the electron density in the Mg−Mg region and is in excellent agreement with directly calculated DFT data. Within the quantum theory of atoms in molecules (QTAIM), this molecule is not bound by a Mg−Mg bond but rather by two Mg−“pseudo-atom” bonds. The NNA is associated with a large region of negative Laplacian in the Mg−Mg internuclear region and arises from the overlap of 3s orbitals in this long, nonpolar “bond”. The pseudoatomic basin associated with the NNA contains 0.8 electrons, which are highly delocalized and hence weakly bound. Possible implications of this unusual electronic structure for the chemistry of such molecules, including their use as excellent reducing agents, are discussed.

Molecular and statistical modeling of reduction peak potential and lipophilicity of platinum(IV) complexes

Abstract: We report the results of the quantitative structure–property relationship analysis of 31 Pt(IV) complexes, for three of which the synthesis is reported for the first time. The X-ray structural analysis of one complex of the series was performed to demonstrate that the PM6 semiempirical method satisfactorily reproduces key features of the geometry of the complexes investigated. Molecular properties extracted from such calculations were then used to construct models of experimental data such as electrochemical peak potentials (evaluated by cyclic voltammetry) and the octanol–water partition coefficient (evaluated by a reversed-phase high performance liquid chromatography method), which are key aspects in the design of such Pt(IV) complexes as potential anticancer prodrugs. Statistically accurate models for both properties were found using combinations of surface areas, orbital energies, dipole moments, and atomic partial charges. These models could form the basis of virtual screening of potential drug molecules, allowing the prediction of properties, closely related to the antiproliferative activity of Pt(IV) complexes, directly from calculated data.

Experimental charge density analysis of a gallium(I) N-heterocyclic carbene analogue.

Abstract: The experimental electron density of the only known example of a four-membered Ga(I) N-heterocyclic carbene analogue has been determined by multipole modeling of 90 K X-ray diffraction data and compared to theoretical data. In order to obtain a satisfactory model, it is necessary to modify the radial dependency of the core electrons of Ga using two separate scaling parameters for s,p- and d-electrons. Evidence for significant lone-pair density on Ga is found in the electron density and derived properties despite the partial positive charge of this atom. Static deformation density and molecular electrostatic potential clearly show a directional lone pair on Ga, whereas the Laplacian of the total electron density does not; this feature is, however, present in the Laplacian of the valence-only density. The Ga center also acts as an acceptor in four intramolecular C–H···Ga contacts, whose nature is probed by density properties. Substantial covalent character is apparent in the Ga–N bonds, but no sign of donation from filled N p-orbitals to empty Ga p-orbitals is found, whereas π-delocalization over the organic ligand is evident. This study highlights the utility of experimental charge density analysis as a technique to investigate the unusual bonding and electronic characteristics of low oxidation state/low coordinate p-block complexes.

Novel C–H⋯C contacts involving 3,5-dimethylpyrazole ligands in a tetracoordinate Co(II) complex

Abstract: A violet-blue cobalt(II) complex [Co(4-nbz)2(DMP)2] (1), where 4-nbz = 4-nitrobenzoate and DMP = 3,5-dimethylpyrazole, has been prepared at room temperature. Crystallographic studies on 1·0.5H2O reveal that the molecules of 1 are linked by a variety of non-covalent bonds including a novel type of C–H⋯C contact forming, with assistance from N–H⋯O, C–H⋯O and C–H⋯π interactions, an intricate 3-D supramolecular network. Theoretical calculations suggest that the observed C–H⋯C interactions are energetically quite significant.

On the Roles of Protic Solvents in Imidazolidinone‐Catalyzed Transformations

Abstract: Step by step: The effect of protic solvents on the rate and stereochemical outcome of the imidazolidinone-catalyzed Diels–Alder cycloaddition was rationalized. The following picture emerges: the solvent accelerates iminium ion formation (step 1), the Diels–Alder cycloaddition is reversible (step 2), and the solvent intercepts the iminium ion adduct (step 3

Density functional theory studies of interactions of ruthenium-arene complexes with base pair steps.

Abstract: Density functional theory (DFT) calculations have been performed to determine the strength and geometry of intermolecular interactions of “piano-stool” ruthenium arene complexes, which show potential as anticancer treatments. Model complexes with methane and benzene indicate that the coordinated arene has C–H···π acceptor ability similar to that of free benzene, whereas this arene acts as a much stronger C–H donor or partner in π-stacking than free benzene. The source of these enhanced interactions is identified as a combination of electrostatic and dispersion effects. Complexes of Ru-arene complexes with base-pair step fragments of DNA, in which the arene has the potential to act as an intercalator, have also been investigated. Binding energies are found to be sensitive to the size and nature of the arene, with larger and more flexible arenes having stronger binding. π-stacking and C–H···π interactions between arene and DNA bases and hydrogen bonds from coordinated N–H to DNA oxygen atoms, as well as cov...

Wood Protection Properties of Quaternary Ammonium Spiroborate Esters Derived from Alkyl Tartrates

Abstract: Four new tetra-n-butylammonium spiroborates derived from dimethyl, diethyl, di-iso-propyl and di-n-butyl esters of l-tartaric acid have been prepared and their potential as environmentally benign wood protectants investigated. These compounds showed good activity in a no-choice cellulose paper bioassay with Coptotermes acinaciformis termites. The hydrolytic stability of these spiroborates is not high relative to other spiroborates, yet the tetra-n-butylammonium spiroborate derived from di-iso-propyl-l-tartrate showed remarkable permanence in timber in a demanding leaching test.

Metal complexes of a structurally embellished phosphinane ligand: an assessment of stereoelectronic effects

Abstract: The stereoelectronic features of the pentacyclic phosphane (1S,4R,4aS,5aR,6R,9S,9aS,10aR)-4,6,11,11,12,12-hexamethyl-10-phenyldodecahydro-1,4:6,9-dimethanophenoxaphosphinine (phenop) have been explored through a range of empirical methods including single-crystal X-ray structure determinations of the sulfide derivative phenopS (1), the selenide phenopSe (2), [Fe(CO)4(κ1-phenop)] (3), [W(CO)5(κ1-phenop)] (4) and trans-[Rh(κ1-phenop)(CO)Cl] (5). Cone angles derived from the structural data range from 164–203° with the smaller values being observed for the compounds possessing a phenyl group that is orthogonal to the P–Z bond and the larger values for the compounds expressing a parallel phenyl ring orientation. The cone angle data suggest a moderately bulky phosphane comparable, in steric terms, to PCy3. This is further borne out on inspection of the M–P bond lengths which tend towards the longer end of the known scale. Some flexibility is observed in the central ring which approximates to a boat conformation at one extreme and an envelope at the other depending on the nature of the P-substituent. The electronic properties of κ1-phenop have been assessed using a combination of infrared and NMR spectroscopy. The absolute value of the one-bond coupling constants 1JP-Se and 1JP-Rh are very close to those reported for PPh3, suggesting a close analogy between κ1-phenop and the well known triphenylphosphane. In addition, relevant υ(CO) stretches in the IR spectra of the metal carbonyl complexes also closely mimic those for the analogous complexes containing PPh3. These conclusions are supported by molecular electrostatic potential calculations at the DFT level which place phenop close to PPh3 in terms of lone pair availability

Studies on log Po/w of quinoxaline di-N-oxides: a comparison of RP-HPLC experimental and predictive approaches.

Abstract: As reported in our previous papers, a series of quinoxaline-2-carboxamide 1,4-di-N-oxide derivatives were synthesized and studied as anti-tuberculosis agents. Here, the capability of the shake-flask method was studied and the retention time (expressed as log K) of 20 compounds were determined by RP-HPLC analysis. We found that the prediction of log P by the RP-HPLC analysis can result in a high accuracy and can replace the shake-flask method avoiding the experimental problems presented by quinoxaline di-N-oxides. The studied compounds were subjected to the ALOGPS module with the aim of comparing experimental log Po/w values and predicted data. Moreover, a preliminary in silico screening of the QSAR relationship was made confirming the influence of reduction peak potential, lipophilicity, H-bond donor capacity and molecular dimension descriptors on anti-tuberculosis activity.