No abstract

A neutron diffraction study of the complex RuCl(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (1) defines the precise nature of the delta agostic interactions between the unsaturated metal center and two o-methyl groups of the xylyl substituents. The CH(3) carbon atoms lie in the RuP(2) equatorial plane with Ru...C distances of 2.637(7) and 2.668(6) A, whereas four short Ru.H distances (from 2.113(11) to 2.507(11) A) indicate that each methyl group interacts with two C-H bonds. A survey of the X-ray structures with beta, gamma, delta, and epsilon M...H(3)C-C moieties (no neutron data have been previously reported) shows a linear correlation between the angle M.C-C and the torsion of the methyl group about the C-C bond. Thus, the agostic interactions span the range between the classical (M...eta(2)-HC) and the nonclassical (M...eta(3)-H(2)C) types. A solution study of 1 shows intramolecular rearrangement of each xylyl substituent that equilibrates the environments of its two ortho CH(3) groups. Activation parameters, evaluated from the analysis of (1)H NMR line shape as a function of temperature, are Delta H(++) = 9.6 +/- 0.2 kcal mol(-1) with Delta S(++) = -15.4 +/- 0.7 eu (CDCl(3)). The related 14-electron complexes RuX(2)[PPh(2)(2,6-Me(2)C(6)H(3))](2) (X = I, 2; NCO, 3), prepared from 1 and NaX, show a similar dynamic process in solution, with the iodo derivative displaying the most hindered rotation of the xylyl group. A DFT optimization of the complex RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))](2) (1a) reproduces well the nonclassical Ru...eta(3)-H(2)C agostic mode, whereas the classical Ru...eta(2)-HC one corresponds to a transition state 1b, destabilized by 3.4 kcal mol(-1). A similar barrier (ca. 3.8 kcal mol(-1)) is calculated for the xylyl rotation in the further simplified model RuCl(2)[PH(2)(2,6-Me(2)C(6)H(3))][PH(2)CH[double bond]CHCH(3)] (1c), the absence of bulky phenyl substituents being largely responsible for the difference with respect to the experimental value. Finally, the MO analysis addresses the intrinsic stability of the 14-electron complex RuCl(2)(PH(3))(2) and, in agostic complexes, accounts for the different interactions between the methyl group and the metal atom in relation to the length of their interconnecting chain.

The linear tetraphosphine 1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (tetraphos-1, P4) was used as its rac and meso isomers for the synthesis of both molecularly defined and in situ formed Fe­(II) complexes. These were used as precatalysts for sodium bicarbonate hydrogenation to formate and formic acid dehydrogenation to hydrogen and carbon dioxide with moderate to good activities in comparison to those for literature systems based on Fe. Mechanistic details of the reaction pathways were obtained by NMR and HPNMR experiments, highlighting the role of the Fe­(II) monohydrido complex [FeH­(rac-P4)]+ as a key intermediate. X-ray crystal structures of different complexes bearing rac-P4 were also obtained and are described herein.

The stability of phosphorene is shown to be degraded by water. However, the presence of a small amount of water allows the synthesis of high‐quality material in liquid exfoliation of black phosphorus using dimethylsulfoxide as solvent. A phosphorus/water molar ratio between 1.5 and 14 maximizes the quality of the phosphorene flakes and their stability.

Pyrrolic and imino (3) or amino (4) H-bonding ligands were incorporated into a benzene-based tripodal scaffold to develop a new generation of receptors for molecular recognition of carbohydrates. Receptors 3 and 4 effectively bound a set of octylglycosides of biologically relevant monosaccharides, including glucose (Glc), galactose (Gal), mannose (Man), and N-acetyl-glucosamine (GlcNAc), showing micromolar affinities in CDCl3 and millimolar affinities in CD3CN by NMR titrations. Both receptors selectively recognized Glc among the investigated monosaccharides, with 3 generally less effective than 4 but showing selectivities for the all-equatorial beta-glycosides of Glc and GlcNAc among the largest reported for H-bonding synthetic receptors. Selectivities in CDCl3 spanned a range of nearly 250-fold for 3 and over 30-fold for 4. Affinities and selectivities were univocally assessed through the BC50 descriptor, for which a generalized treatment is described that extends the scope of the descriptor to include any two-reagent host-guest system featuring any number of binding constants. ITC titrations of betaGlc in acetonitrile evidenced, for both receptors, a strong enthalpic contribution to the binding interaction, suggesting multiple H bonding. Selectivity trends toward alphaGlc and betaGlc analogous to those obtained in solution were also observed in the gas phase for 3 and 4 by collision-induced dissociation experiments. From comparison with appropriate reference compounds, a substantial contribution to carbohydrate binding emerged for both the imino/amino and the pyrrolic H-bonding groups but not for the amidic group. This previously undocumented behavior, supported by crystallographic evidence, has been discussed in terms of geometric, functional, and coordinative complementarity between H-bonding groups and glycosidic hydroxyls and opens the way to a new designer strategy of H-bonding receptors for carbohydrates.

The position of the sulfur atom in the thienyl group of 6-(thienyl)-2-(imino)pyridine ligands strongly affects the catalytic activity of the corresponding tetrahedral high-spin dihalide CoII complexes in the oligomerization of ethylene to R-olefins upon activation with methylaluminoxane (MAO). Complexes with the sulfur atom in the 3-position of the thienyl ring catalyze the selective conversion of ethylene to 1-butene, while catalysts containing thien-2-yl groups give C4-C14 R-olefins. In situ EPR experiments showed the occurrence of a spin state changeover with the formation of low-spin CoII species upon activation of the catalyst precursors by MAO. DFT calculations suggest that only thien-2-yl rings allow for the coordination of the sulfur atom to the cobalt center in the MAO-activated systems

Metal-organic frameworks (MOFs) have gained widespread attention due to their modular construction that allows the tuning of their properties. Within this vast class of compounds, metal carboxylates containing tri- and...

This paper investigates geometric and electronic features of linear I 3 − and I 4 2− anions, as building blocks of larger polyiodides. Most experimental structures are quasi D ∞h , although one lateral linkage is occasionally elongated with I•••I separations approaching those of I•••I−R − species, typical of halogen bonding (HalB). Hirshfeld surfaces from crystal data highlight solid state effects depending on the distribution of the counterions around I 3 − or I 4 2− units. Corresponding experimental asymmetries have been mimicked with density functional theory calculations through different surroundings of positive point charges. The consequent deformations are interpreted in terms of the s/p rehybridizations occurring at the central I atom(s) of the populated frontier σ* wave functions. The origin is a charge-induced variation of the orbital energies at lateral iodides (electronegativity), hence by their the donor power in a nucleophilic attack. The calculations also provide energy information on I 2 + I − or I 2 + 2I − additions, and, in solvent, the intrinsic energy stability of I 4 2− is for the first time validated. In the absence of positive charge perturbations, the 1− charge of a remote iodide polarizes I 3 − and promotes incipient electrostatic attraction, which is quickly accompanied by electron transfer with a generalized σ delocalization throughout I 4 2− . Implicit orbital overlap supports a covalent picture, or better to say hypervalency, given the electron richness of the central atoms. Molecular electrostatic potential (MEP) surfaces are expected to show σ holes in support of the purely electrostatic HalB model, typically proposed for I•••I−R − systems. However, the computed surfaces show little evidence of σ holes in the equilibrium adducts I 3 − , I 4 2− and I•••I−R − suggesting that HalB cannot be purely electrostatic.