Principles of weakly ordered domains in intermetallics: the cooperative effects of atomic packing and electronics in Fe2Al5.

Many complex intermetallic structures feature a curious juxtaposition of domains with strict 3D periodicity and regions of much weaker order or incommensurability. This article explores the basic principles leading to such arrangements through an investigation of the weakly ordered channels of Fe2Al5. It starts by experimentally confirming the earlier crystallographic model of the high-temperature form, in which nearly continuous columns of electron density corresponding to disordered Al atoms emerge. Then electronic structure calculations on ordered models are used to determine the factors leading to the formation of these columns. These calculations reveal electronic pseudogaps near 16 electrons/Fe atom, an electron concentration close to the Al-rich side of the phase's homogeneity range. Through a reversed approximation Molecular Orbital (raMO) analysis, these pseudogaps are correlated with the filling of 18-electron configurations on the Fe atoms with the support of isolobal σ Fe-Fe bonds. The resulting preference for 16 electrons/Fe requires a fractional number of Al atoms in the Fe2Al5 unit cell. Density functional theory-chemical pressure (DFT-CP) analysis is then applied to investigate how this nonstoichiometry is accommodated. The CP schemes reveal strong quadrupolar distributions on the Al atoms of the channels, suggestive of soft atomic motions along the undulating electron density observed in the Fourier map that allow the Al positions to shift easily in response to compositional changes. Such a combination of preferred electron counts tied to stoichiometry and continuous paths of CP quadrupoles could provide predictive indicators for the emergence of channels of disordered or incommensurately spaced atoms in intermetallic structures.

Deciphering composition and connectivity of a natural product with the assistance of MS and 2D NMR.

A complementary application of three analytical techniques, viz. multidimensional nuclear magnetic resonance spectroscopy (NMR), mass spectrometry (MS), and single-crystal X-ray diffractometry was required to identify and refine two natural products isolated from Millettia versicolor and solvent of crystallization. The two compounds, namely 3-(2H-1,3-benzodioxol-5-yl)-6-methoxy-8,8-dimethyl-4H,8H-pyrano[2,3-h]chromen-4-one, or durmillone, (I), and (2E)-1-(4-{[(2E)-3,7-dimethylocta-2,6-dien-1-yl]oxy}-2-hydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one, (II), could not be separated by routine column chromatography and cocrystallized in a 2:1 ratio with 0.13 molecules of ethanol solvent. Compound (II) and ethanol could not be initially identified by single-crystal X-ray analysis due to complex disorder in the aliphatic chain region of (II). Mass spectrometry ensured that (II) represented only one species disordered over several positions in the solid state, rather than several species cohabitating on the same crystallographic site. The atomic identification and connectivity in (II) were established by several 2D (two-dimensional) NMR techniques, which in turn relied on a knowledge of its exact mass. The derived connectivity was then used in the single-crystal analysis to model the disorder of the aliphatic chain in (II) over three positions and allowed identification of a partially occupied ethanol solvent molecule that was disordered over an inversion center. The disordered moieties were refined with restraints and constraints.

Crystal structure of {(R)-N2-[(benzo[h]quinolin-2-yl)meth-yl]-N2'-[(benzo[h]quinolin-2-yl)methyl-idene]-1,1'-binaphthyl-2,2'-di-amine-κ4N,N',N'',N'''}(trifluoromethane-sulfonato-κO)zinc(II)} trifluoromethane-sulfonate di-chloro-methane 1.5-solvate.

The zinc(II) atom in the title compound, [Zn(C48H31N4)(CF3SO3)](CF3SO3)·1.5CH2Cl2, adopts a distorted five-coordinate square-pyramidal geometry. It is coordinated by one tri-fluoro-methane-sulfonate ligand and four N atoms of the N2-[(benzo[h]quinolin-2-yl)meth-yl]-N2'-[(benzo[h]quinolin-2-yl)methyl-idene]-1,1'-binaphthyl-2,2'-di-amine ligand. The complex is present as a single-stranded P-helimer monohelical structure incorporating π-π and/or σ-π inter-actions. One of the imine bonds present in the original ligand framework is reduced, leading to variations in bond lengths and torsion angles for each side of the ligand motif. The imine-bond reduction also affects the bond lengths involving the metal atom with the N-donor atoms located on the imine bond. There are two mol-ecules of the complex in the asymmetric unit. One of the mol-ecules exhibits positional disorder within the coordinating tri-fluoro-methane-sulfonate ion making the mol-ecules symmetric-ally non-equivalent.

Crystal structure of ({(1R,2R)-N,N'-bis-[(quino-lin-2-yl)methyl]cyclo-hexane-1,2-di-amine}-chlorido-iron(III))-µ-oxido-[tri-chlorido-ferrate(III)] chloro-form monosolvate.

The first FeIII atom in the solvated title compound, [Fe2Cl4O(C26H28N4)]·CHCl3, adopts a distorted six-coordinate octa-hedral geometry. It is coordinated by one chloride ligand, four N atoms from the (1R,2R)-N,N'-bis-[(quinolin-2-yl)methyl]cyclo-hexane-1,2-di-amine ligand, and a bridging oxido ligand attached to the second FeIII atom, which is also bonded to three chloride ions. A very weak intra-molecular N-H⋯Cl hydrogen bond occurs. In the crystal, the coordination complexes stack in columns, and a grouping of six such columns create channels, which are populated by disordered chloro-form solvent mol-ecules. Although the Fe-Cl bond lengths for the two metal atoms are comparable to the mean Fe-Cl bond lengths as derived from the Cambridge Structural Database, the Fe-O bond lengths are notably shorter. The solvent chloro-form mol-ecule exhibits 'flip' disorder of the C-H moiety in a 0.544 (3):0.456 (3) ratio. The only directional inter-action noted is a weak C-H⋯Cl hydrogen bond.

Synthesis, Characterization, and Variable-Temperature NMR Studies of Silver(I) Complexes for Selective Nitrene Transfer.

An array of silver complexes supported by nitrogen-donor ligands catalyze the transformation of C═C and C-H bonds to valuable C-N bonds via nitrene transfer. The ability to achieve high chemoselectivity and site selectivity in an amination event requires an understanding of both the solid- and solution-state behavior of these catalysts. X-ray structural characterizations were helpful in determining ligand features that promote the formation of monomeric versus dimeric complexes. Variable-temperature 1H and DOSY NMR experiments were especially useful for understanding how the ligand identity influences the nuclearity, coordination number, and fluxional behavior of silver(I) complexes in solution. These insights are valuable for developing improved ligand designs.

18-Electron Resonance Structures in the BCC Transition Metals and Their CsCl-type Derivatives.

Bonding in elemental metals and simple alloys has long been thought of as involving intense delocalization, with little connection to the localized bonds of covalent systems. In this Article, we show that the bonding in body-centered cubic (bcc) structures of the group 6 transition metals can in fact be represented, via the concepts of the 18-n rule and isolobal bonding, in terms of two balanced resonance structures. We begin with a reversed approximation Molecular Orbital (raMO) analysis of elemental Mo in its bcc structure. The raMO analysis indicates that, despite the low electron count (six valence electrons per Mo atom), nine electron pairs can be associated with any given Mo atom, corresponding to a filled 18-electron configuration. Six of these electron pairs take part in isolobal bonds along the second-nearest neighbor contacts, with the remaining three (based on the t2g d orbitals) interacting almost exclusively with first-nearest neighbors. In this way, each primitive cubic network defined by the second-nearest neighbor contacts comprises an 18-n electron system with n = 6, which essentially describes the full electronic structure of the phase. Of course, either of the two interpenetrating primitive cubic frameworks of the bcc structure can act as a basis for this discussion, leading us to write two resonance structures with equal weights for bcc-Mo. The electronic structures of CsCl-type variants with the same electron count can then be interpreted in terms of changing the relative weights of these two resonance structures, as is qualitatively confirmed with raMO analysis. This combination of raMO analysis with the resonance concept offers an avenue to extend the 18-n rule into other transition metal-rich structures.

Toward Design Principles for Diffusionless Transformations: The Frustrated Formation of Co-Co Bonds in a Low-Temperature Polymorph of GdCoSi2.

Diffusionless (or displacive) phase transitions allow inorganic materials to show exquisite responsiveness to external stimuli, as is illustrated vividly by the superelasticity, shape memory, and magnetocaloric effects exhibited by martensitic materials. In this Article, we present a new diffusionless transition in the compound GdCoSi2, whose origin in frustrated bonding points toward generalizable design principles for these transformations. We first describe the synthesis of GdCoSi2 and the determination of its structure using single crystal X-ray diffraction. While previous studies based on powder X-ray diffraction assigned this compound to the simple CeNi1-xSi2 structure type (space group Cmcm), our structure solution reveals a superstructure variant (space group Pbcm) in which the Co sublattice is distorted to create zigzag chains of Co atoms. DFT-calibrated Hückel calculations, coupled with a reversed approximation Molecular Orbital (raMO) analysis, trace this superstructure to the use of Co-Co isolobal bonds to complete filled 18 electron configurations on the Co atoms, in accordance with the 18-n rule. The formation of these Co-Co bonds is partially impeded, however, by a small degree of electron transfer from Si-based electronic states to those with Co-Co σ* character. The incomplete success of Co-Co bond creation suggests that these interactions are relatively weak, opening the possibility of them being overcome by thermal energy at elevated temperatures. In fact, high-temperature powder and single crystal X-ray diffraction data, as well as differential scanning calorimetry, indicate that a reversible Pbcm to Cmcm transition occurs at about 380 K. This transition is diffusionless, and the available data point toward it being first-order. We expect that similar cases of frustrated interactions could be staged in other rare earth-transition metal-main group phases, providing a potentially rich source of compounds exhibiting diffusionless transformations and the unique properties these transitions mediate.

Solubilities and Glass Formation in Aqueous Solutions of the Sodium Salts of Malonic Acid With and Without Ammonium Sulfate.

The solubility of sodium hydrogen malonate and sodium malonate in water both with and without ammonium sulfate present has been studied using differential scanning calorimetry and infrared spectroscopy. The crystals that form from sodium hydrogen malonate/water solutions were determined to be sodium hydrogen malonate monohydrate by single-crystal X-ray diffractometry. The crystals formed in sodium malonate/water solutions were determined to be sodium malonate monohydrate, a compound whose structure had not been previously known. When ammonium sulfate is added to these respective aqueous systems, the precipitation solids contain sodium sulfate decahydrate under low to moderate ammonium concentrations and lecontite (NaNH4SO4·2H2O) under high ammonium concentrations, which can be found under dry atmospheric conditions. Thus, it appears the presence of malonate and hydrogen malonate ions does not significantly affect the precipitation of inorganic salts in these systems. The glass transition temperatures of all solutions were also determined, and it was observed that the addition of ammonium sulfate slightly lowers the glass transition temperature in these solutions.