Diastereomeric dinickel(II) complexes with non-innocent bis(octaazamacrocyclic) ligands: isomerization, spectroelectrochemistry, DFT calculations and use in catalytic oxidation of cyclohexane

Diastereomeric dinickel(II) complexes with bis-octaazamacrocyclic 15-membered ligands [Ni(L1-3-L1-3)Ni] (4-6) have been prepared by oxidative dehydrogenation of nickel(II) complexes NiL1-3 (1-3) derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide. The compounds were characterized by elemental analysis, ESI mass spectrometry, and IR, UV-vis, 1H NMR, and 13C NMR spectroscopy. Single crystal X-ray diffraction (SC-XRD) confirmed the isolation of the anti and syn isomers of bis-octaazamacrocyclic dinickel(II) complexes 4a and 4s, the syn-configuration of 5s and the anti-configuration of the dinickel(II) complex 6a. Dimerization of prochiral nickel(II) complexes 1-3 generates two chiral centers at the bridging carbon atoms. The anti-complexes were isolated as meso-isomers (4a and 6a) and the syn-compounds as racemic mixtures of R,R/S,S-enantiomers (4s and 5s). The syn-anti isomerization (epimerization) of the isolated complexes in chloroform was disclosed. The isomerization kinetics of 5a was monitored at five different temperatures ranging from 20 °C to 50 °C by 1H NMR spectroscopy indicating the clean conversion of 5a into 5s. The activation barrier determined from the temperature dependence of the rate constants via the Eyring equation was found to be ΔH‡ = 114 ± 1 kJ mol-1 with activation entropy ΔS‡ = 13 ± 3 J K-1 mol-1. The complexes contain two low-spin nickel(II) ions in a square-planar coordination environment. The electrochemical behavior of 4a, 4s, 5s and 6a and the electronic structure of the oxidized species were studied by UV-vis-NIR-spectroelectrochemistry (SEC) and DFT calculations indicating the redox non-innocent behavior of the complexes. The dinickel(II) complexes 4a, 4s, 5s and 6a/6s were investigated as catalysts for microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide to produce a mixture of cyclohexanone and cyclohexanol (KA oil). The best value for KA oil yield (16%) was obtained with a mixture of 6a/6s after 2 h of microwave irradiation at 100 °C.

Thermal expansion properties of organic crystals: a CSD study

The thermal expansion properties of crystalline organic compounds are investigated by data mining of the Cambridge Structural Database (CSD). The mean volumetric thermal expansion coefficient is 168.8 × 10-6 K-1 and the mean uniaxial thermal expansion coefficient is 71.4 × 10-6 K-1, based on 745 and 1129 different observations, respectively. Normal and anomalous coefficients can be identified using these values and the associated standard deviations. The anisotropy of the thermal expansion is also evaluated and found to have a very broad distribution. 4719 different structures, comprising 4093 different molecular compounds and 626 additional polymorphs have been analyzed on their thermal expansion properties. Approximately 34% of these structures may have at least one orthogonal axis with negative thermal expansion, much more than generally believed. Moreover 127 structures have been identified which could have negative volumetric thermal expansion. Experimental validation using a robust protocol with data collected at more than 2 different temperatures is required to validate these cases.

Coordination versus hydrogen bonds in the structures of different tris(pyridin-2-yl)phosphoric triamide derivatives

The crystalline forms of tris(pyridin-2-yl)phosphoric triamide, OP[NH-2Py]3 were investigated using single crystal X-ray diffraction, both as a metal complex [Co{(O)P[NH-2Py]2[NH-2PyH]}2]Cl3, compound 1, and as the purely organic pseudopolymorphs, the pure/anhydrate OP[NH-2Py]3, 2, monohydrate 2OP[NH-2Py]3·H2O, 3 and solvate 2OP[NH-2Py]3·2DMF·H2O, 4, respectively. An improved model of the metal organic framework (MOF) structure of Cu(ii)O6 {[Cu(OCHO)3][(CH3)2NH2]} n , 5 is also reported. Compound 1 is the first example of a discrete chelate phosphoric triamide (PT) complex with an [N]3P(O)-based backbone which in the PT compound acts as a flexible tridentate ligand. The structures 1, 3 and 4 crystallize in the monoclinic space group P21/n, while the anhydrous form 2 crystallizes in the R3̄ trigonal space group. 5 which was obtained as a byproduct of the synthesis process of coordination compounds of OP[NH-2Py]3, crystallizes in the monoclinic space group I2/c. Hydrogen bonding pattern analysis for the different solid state forms of OP[NH-2Py]3 (2-4) shows a 2D sheet of the N-H⋯N linked molecules for 2, and a 1D chain and a ten-membered ring motif, both formed via the hydrogen bonds N-H⋯O, N-H⋯N and O-H⋯O, for 3 and 4, respectively. Of note is the unusual absence of the expected hydrogen bond interaction N-H⋯O[double bond, length as m-dash]P of PT compounds and also a lack of any significant π interactions in structure 2. The role of the solvent/hydrate and substituent (aminopyridine) in the formation of the expected hydrogen bond interactions in the studied solid state forms is also investigated. For these structures, crystal packing analysis using 3D Hirshfeld surface (HS), 2D fingerprint plot (FP) and enrichment ratio (E) calculations confirm a competition between the pyridinyl nitrogen and phosphoryl oxygen atoms as H-bond acceptors to construct the N-H⋯N or N-H⋯O contacts. Moreover, the significant differences between the anhydrous form of OP[NH-2Py]3, 2, and the monohydrate and solvate forms, 3 and 4, can be correlated to the pseudo-infinite (in 2) versus finite values (in 3 and 4) for the upper values of d e and d i on the related full FPs.

Cyanido-bridged {FeIIILnIII} heterobimetallic chains assembled through the [FeIII{HB(pz)3}(CN)3]− complex as metalloligand: synthesis, crystal structure and magnetic properties

The first isostructural series of cyanido-bridged 3d–4f chains showing a unique zig-zag motif have been obtained by using the heteroleptic [Fe^III{HB(pz)₃}(CN)₃]⁻ complex as a metalloligand towards the preformed [Ln^III(bpdo)(NO₃)₂(H₂O)]⁺ species.

A positive to negative uniaxial thermal expansion crossover in an organic benzothienobenzothiophene structure

Compound 6,6'-([1]benzothieno[3,2-b][1]benzothiophene-2,7-diyl)bis(butan-1-ol) (BTBT-C4OH) displays a continuous type 0 first-order isosymmetric phase transition at 200 K which is accompanied by a continuous change of the thermal expansion along the b axis from positive to negative. The equivalent isotropic atomic displacement parameters for all non-hydrogen atoms as well as all the eigenvalues of the anisotropic atomic displacement tensor show discontinuous behavior at the phase transition. The eigenvalues of the translational tensor in a rigid-body description of the molecule are all discontinuous at the phase transition, but the librational eigenvalues are discontinuous only in their temperature derivative. BTBT-C4OH displays a similar type of quasi-supercritical phase transition as bis(hydroxyhexyl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-C6OH), despite the difference in molecular packing and the very large difference in thermal expansion magnitudes.

Evaluation of N–H···O hydrogen bond interactions in two new phosphoric triamides with a P(O)[NHCH(CH3)2]2 segment by means of topological (AIM) calculations, Hirshfeld surface analysis and 3D energy framework approach

Two new phosphoric triamides having a common part XP(O)[NHCH(CH3)2]2, with X =[2,3,6-F3–C6H2C(O)NH] (1) and [C6H11(CH3)N] (2), were prepared and characterized by spectroscopic techniques (FT-IR and 1H-, 13C-, 31P-NMR) and single crystal X-ray diffraction. The asymmetric unit of 1 is composed of one molecule, whereas for 2 it consists of six symmetry independent molecules. In all molecules, the P atoms are in a distorted tetrahedral environment of one oxygen and three nitrogen atoms. The latter have mainly sp 2 character and a nearly planar environment. The crystal structures are stabilized via N–H · · · O hydrogen bond interactions, forming a linear arrangement for 1 and three independent parallel linear chains for 2, along the b and a axis, respectively. The intermolecular interactions in the molecular packing were analyzed using the Hirshfeld surface methodology, two-dimensional (2D) fingerprint plots and enrichment ratios (E). The prevalent interactions revealed by Hirshfeld surfaces are O · · · H type interactions for both structures 1 and 2, additionally C · · · O for 1 and H · · · H interactions for 2. The most favored contacts responsible for the molecular packing are C · · · F, N · · · H and O · · · H for 1 confirmed by E values greater than 1.30, whereas for 2, O · · · H and N · · · H intermolecular interactions with E values about 1.04 representing the favored contacts. Thus, the N–H · · · O hydrogen bond interactions are the dominant interactions in both compounds. For more details, a topological AIM analysis of N–H · · · O hydrogen bond interactions was performed: NCP–H · · · O=C hydrogen bond (the NCP is referred to the nitrogen atom within the C(O)NHP(O) segment) interactions in 1 are stronger than N–H · · · O=P interactions in both 1 and 2. Furthermore, a 3D topology of the molecular packing via the energy framework approach showed that the N–H · · · O hydrogen bond interactions in C(O)NHP(O)-based phosphoric triamide are predominantly electrostatic based, while they are electrostatic-dispersion based for other phosphoric triamides with a [N]P(O)[NH]2 skeleton.

Pyrene-box capsules for adaptive encapsulation and structure determination of unstable or non-crystalline guest molecules

“Pyrene-box” cages easily crystallize from aqueous solutions and readily encapsulate compounds of biological interest.