Heterobimetallic [NiFe] Complexes Containing Mixed CO/CN- Ligands: Analogs of the Active Site of the [NiFe] Hydrogenases.

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging, and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN- ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni( N2 S2)Fe(CO)2(CN)2], [Ni( S4)Fe(CO)2(CN)2], and [Ni( N2 S3)Fe(CO)2(CN)2] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)2(CN)2} unit. X-ray crystallographic studies on [Ni( N2 S3)Fe(CO)2(CN)2], supported by DFT calculations, are consistent with a solid-state structure containing distinct molecules in the singlet ( S = 0) and triplet ( S = 1) states. Each cluster exhibits irreversible reduction processes between -1.45 and -1.67 V vs Fc+/Fc and [Ni( N2 S3)Fe(CO)2(CN)2] possesses a reversible oxidation process at 0.17 V vs Fc+/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a NiIIIFeII formulation for [Ni( N2 S3)Fe(CO)2(CN)2]+. The singly occupied molecular orbital (SOMO) in [Ni( N2 S3)Fe(CO)2(CN)2]+ is based on Ni 3dz2 and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a NiIIIFeII formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni( N2 S3)Fe(CO)2(CN)2] and its [Ni( N2 S3)] precursor, together with calculations on the oxidized [Ni( N2 S3)Fe(CO)2(CN)2]+ and [Ni( N2 S3)]+ forms suggests that the binding of the {Fe(CO)(CN)2} unit to the {Ni(CysS)4} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

QTAIM Assessment of the Intra- and Intermolecular Bonding in a Bis(nitramido-oxadiazolate) Energetic Ionic Salt at 20 K.

Accurate experimental determination of the electron density distribution for the energetic ionic salt bis(ammonium) 2,2'-dinitramido-5,5'-bis(1-oxa-3,4-diazolate) dihydrate (1) is obtained from multipole modeling of single-crystal X-ray diffraction data collected at 20 K. The intra- and intermolecular bonding is assessed in terms of the quantum theory of atoms in molecules (QTAIM) with a view to better understanding the physicochemical properties in relation to chemical bonding. Topological analysis reveals stronger bonding for the N-NO2 bond relative to energetic nitramines RDX and HMX and the indication of a trend between this and impact sensitivity of nitro-containing energetic materials is noted. The intermolecular bonding of 1 is dominated by classical H-bonds but includes multiple π-bonding interactions and interactions between H-bond donor and acceptor atoms where bond paths are deflected by H atoms. There also exists a weak O···O interaction between end-on nitro groups, as well as an intramolecular ring-forming 1,5-type interaction. An anharmonic description of thermal motion was required to obtain the best fitting model, despite the low temperature of the study. The experimental study was complemented by periodic boundary DFT calculations at the experimental geometry as well as gas phase calculations on the isolated dianion.

Experimental Charge-Density Study of the Intra- and Intermolecular Bonding in TKX-50.

The intra- and intermolecular bonding in the known phase of dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate, TKX-50, has been analyzed on the basis of the experimentally determined charge density distribution from high-resolution X-ray diffraction data obtained at 20 K. This was compared to the charge density obtained from DFT calculations with periodic boundary conditions using both direct calculations and derived structure factors. Results of topological analysis of the electron density corroborate that TKX-50 is best described as a layered structure linked primarily by a number of hydrogen bonds as well as by a variety of other interactions. Additional bonding interactions were identified, including a pair of equivalent 1,5-type intramolecular closed-shell interactions in the dianion. Refinement of anharmonic motion was shown to be essential for obtaining an adequate model, despite the low temperature of the study. Although generally unusual, the implementation of anharmonic refinement provided a significant improvement compared to harmonic refinement of both traditional and split-core multipole models.

Crystal structure of {2,2'-[ethyl-enebis(nitrilo-methanylyl-idene)]diphenolato-κ(4) O,N,N',O'}oxidovanadium(IV) methanol monosolvate.

Two independent mol-ecules of the title solvated complex, [V(C16H14N2O2)O]·CH3OH, also known as [N,N'-bis-(salicyl-idene)ethyl-enedi-amine]-oxidovanadium(IV) or vanadyl salen, crystallize in the asymmetric unit. Each disordered methanol solvent mol-ecule [occupancy ratios 0.678 (4):0.322 (4) and 0.750 (5):0.250 (5)] is linked to a [N,N'-bis-(salicyl-idene)ethyl-enedi-amine]-oxidovanadium(IV) mol-ecule by an O-H⋯O hydrogen bond and to others by C-H⋯O hydrogen bonds. The resulting extended structure consists of a bilayer of mol-ecules parallel to the ab plane. Despite the fact that solvates are common in complexes derived from substituted analogues of the N,N'-bis-(salicyl-idene)ethyl-enedi-amine ligand, the title solvate is the first one of [N,N'-bis-(salicyl-idene)ethyl-enedi-amine]-oxidovanadium(IV) to be structurally characterized. The two vanadyl species have very similar inter-nal geometries, which are best characterized as distorted square-based pyramidal with the vanadium atom displaced from the N2O2 basal plane by 0.5966 (9) Šin the direction of the doubly-bonded oxide ligand.

Polymorphism and Structural Variety in Sn(II) Carboxylate Coordination Polymers Revealed from Structure Solution of Microcrystals.

The crystal structures of four coordination polymers constructed from Sn(II) and polydentate carboxylate ligands are reported. All are prepared under hydrothermal conditions in KOH or LiOH solutions (either water or methanol-water) at 130-180 °C and crystallize as small crystals, microns or less in size. Single-crystal structure solution and refinement are performed using synchrotron X-ray diffraction for two materials and using 3D electron diffraction (3DED) for the others. Sn2 (1,3,5-BTC)(OH), where 1,3,5-BTC is benzene-1,3,5-tricarboxylate, is a new polymorph of this composition and has a three-dimensionally connected structure with potential for porosity. Sn(H-1,3,5-BTC) retains a partially protonated ligand and has a 1D chain structure bound by hydrogen bonding via ─COOH groups. Sn(H-1,2,4-BTC) contains an isomeric ligand, benzene-1,2,4-tricarboxylate, and contains inorganic chains in a layered structure held by hydrogen bonding. Sn2 (DOBDC), where DOBDC is 2,5-dioxido-benzene-1,4-dicarboxylate, is a new polymorph for this composition and has a three-dimensionally connected structure where both carboxylate and oxido groups bind to the tin centers to create a dense network with dimers of tin. In all materials, the Sn centers are found in highly asymmetric coordination, as expected for Sn(II). For all materials phase purity of the bulk is confirmed using powder X-ray diffraction, thermogravimetric analysis, and infrared spectroscopy.

Pronounced interplay between intrinsic phase-coexistence and octahedral tilt magnitude in hole-doped lanthanum cuprates.

Definitive understanding of superconductivity and its interplay with structural symmetry in the hole-doped lanthanum cuprates remains elusive. The suppression of superconductivity around 1/8th doping maintains particular focus, often attributed to charge-density waves (CDWs) ordering in the low-temperature tetragonal (LTT) phase. Central to many investigations into this interplay is the thesis that La1.875Ba0.125CuO4 and particularly La1.675Eu0.2Sr0.125CuO4 present model systems of purely LTT structure at low temperature. However, combining single-crystal and high-resolution powder X-ray diffraction, we find these to exhibit significant, intrinsic coexistence of LTT and low-temperature orthorhombic domains, typically associated with superconductivity, even at 10 K. Our two-phase models reveal substantially greater tilting of CuO6 octahedra in the LTT phase, markedly buckling the CuO2 planes. This would couple significantly to band narrowing, potentially indicating a picture of electronically driven phase segregation, reminiscent of optimally doped manganites. These results call for reassessment of many experiments seeking to elucidate structural and electronic interplay at 1/8 doping.

High-pressure studies of three polymorphs of a palladium(II) oxathioether macrocyclic complex.

The three reported phases of the mononuclear macrocyclic Pd(II) complex [PdCl2([9]aneS2O)] [(1); [9]aneS2O = 1-oxa-4,7-dithiacyclononane] were each studied up to pressures exceeding 9 GPa using high-pressure single-crystal X-ray diffraction. The α- and γ-phases both exhibit smooth compression of the unit-cell parameters with third-order Birch-Murnaghan bulk moduli of 14.4 (8) and 7.6 (6) GPa, respectively. Between 6.81 and 6.87 GPa ß-[PdCl2([9]aneS2O)] was found to undergo a reversible transition to a phase denoted as ß' and characterized by a tripling of the unit-cell volume. Across the phase transition, rearrangement of the conformation of the bound macrocycle at two of the resulting three unique sites gives rise to an extensively disordered structure.

Epitaxial Retrieval of a Disappearing Polymorph.

Recrystallization of [PdCl2([9]aneS2O)] ([9]aneS2O = 1-oxa-4,7-dithiacyclononane), 1, and [PtCl2([9]aneS2O)], 2, by diffusion of Et2O vapor into solutions of the complexes in MeNO2 yielded three phases of 1 and two phases of 2. The known phase of 1, herein designated α-1, was obtained under ambient conditions. A second phase, designated ß-1, was initially also obtained by this method; however, following the advent of a third phase, γ-1, all subsequent efforts over a period of a year to crystallize ß-1 yielded either γ-1, obtained by carrying out the recrystallization at elevated temperature, or α-1, commonly found throughout the study. This persistent absence of a phase which could initially be crystallized with ease led us to the conclusion that ß-1 was an example of a "disappearing polymorph". The first phase obtained of 2, designated α-2, was obtained by recrystallization under ambient conditions and is isomorphous and isostructural with α-1. The second phase ß-2 was obtained by slight elevation of the recrystallization temperature and was found to be isomorphous and isostructural with ß-1. Subsequently, ß-2 was used to seed the growth of the disappearing polymorph ß-1. No third phase of 2 (γ-2) has been isolated thus far.

High-pressure studies of palladium and platinum thioether macrocyclic dihalide complexes.

The mononuclear macrocyclic Pd(II) complex cis-[PdCl2([9]aneS3)] ([9]aneS3 = 1,4,7-trithiacyclo-nonane) converts at 44 kbar into an intensely coloured chain polymer exhibiting distorted octahedral coordination at the metal centre and an unprecedented [1233] conformation for the thioether ligand. The evolution of an intramolecular axial sulfur-metal interaction and an intermolecular equatorial sulfur-metal interaction is central to these changes. High-pressure crystallographic experiments have also been undertaken on the related complexes [PtCl2([9]aneS3)], [PdBr2([9]aneS3)], [PtBr2([9]aneS3)], [PdI2([9]aneS3)] and [PtI2([9]aneS3)] in order to establish the effects of changing the halide ligands and the metal centre on the behaviour of these complexes under pressure. While all complexes undergo contraction of the various interaction distances with increasing pressure, only [PdCl2([9]aneS3)] undergoes a phase change. Pressure-induced I...I interactions were observed for [PdI2([9]aneS3)] and [PtI2([9]aneS3)] at 19 kbar, but the corresponding Br...Br interactions in [PdBr2([9]aneS3)] and [PtBr2([9]aneS3)] only become significant at much higher pressure (58 kbar). Accompanying density functional theory (DFT) calculations have yielded interaction energies and bond orders for the sulfur-metal interactions.