bis-Nitrile and bis-Dialkylcyanamide Platinum(II) Complexes as Efficient Catalysts for Hydrosilylation Cross-Linking of Siloxane Polymers

cis- and trans-Isomers of the platinum(II) nitrile complexes [PtCl2(NCR)2] (R = NMe2, N(C5H10), Ph, CH2Ph) were examined as catalysts for hydrosilylation cross-linking of vinyl-terminated polydimethylsiloxane and trimethylsilyl-terminated poly(dimethylsiloxane-co-ethylhydrosiloxane) producing high quality silicone rubbers. Among the tested platinum species the cis-complexes are much more active catalysts than their trans-congeners and for all studied platinum complexes cis-[PtCl2(NCCH2Ph)2] exhibits the best catalytic activity (room temperature, c = 1.0 10 4 mol/L, pot-life 60 min, curing 6 h). Although cis-[PtCl2(NCCH2Ph)2] is less active than the widely used Karstedt’s catalyst, its application for the cross-linking can be performed not only at room temperature (c = 1.0 10 4 mol/L), but also, more efficiently, at 80 C (c = 1.0 10 4–1.0 10 5 mol/L) and it prevents adherence of the formed silicone rubbers to equipment. The usage of the cis- and trans-[PtCl2(NCR)2] complexes as the hydrosilylation catalysts do not require any inhibitors and, moreover, the complexes and their mixtures with vinyl- and trimethylsilyl terminated polysiloxanes are shelf-stable in air. Tested catalysts do not form colloid platinum particles after the cross-linking.This project was supported by Federal Target Program (grant 14.576.21.0028). Andrey V. Vlasov and Vadim Yu. Kukushkin are much obliged to Saint Petersburg State University for a postdoctoral fellowship (12.50.1188.2014) and research grant (12.38.225.2014), correspondingly. The authors also express their gratitude to the Center of Thermal Analysis and Calorimetry (Saint Petersburg State University) for physicochemical measurements

Zwitterionic iodonium species afford halogen bond-based porous organic frameworks

Porous architectures characterized by parallel channels arranged in honeycomb or rectangular patterns are identified in two polymorphic crystals of a zwitterionic 4-(aryliodonio)-benzenesulfonate. The channels are filled with disordered water molecules which can be reversibly removed on heating. Consistent with the remarkable strength and directionality of the halogen bonds (XBs) driving the crystal packing formation, the porous structure is stable and fully preserved on almost quantitative removal and readsorption of water. The porous systems described here are the first reported cases of one-component 3D organic frameworks whose assembly is driven by XB only (XOFs). These systems are a proof of concept for the ability of zwitterionic aryliodonium tectons in affording robust one-component 3D XOFs. The high directionality and strength of the XBs formed by these zwitterions and the geometrical constraints resulting from the tendency of their hypervalent iodine atoms to act as bidentate XB donors might be key factors in determining this ability

The Distance between Minima of Electron Density and Electrostatic Potential as a Measure of Halogen Bond Strength

In this study, we present results of a detailed topological analysis of electron density (ED) of 145 halogen-bonded complexes formed by various fluorine-, chlorine-, bromine-, and iodine-containing compounds with trimethylphosphine oxide, Me3PO. To characterize the halogen bond (XB) strength, we used the complexation enthalpy, the interatomic distance between oxygen and halogen, as well as the typical set of electron density properties at the bond critical points calculated at B3LYP/jorge-ATZP level of theory. We show for the first time that it is possible to predict the XB strength based on the distance between the minima of ED and molecular electrostatic potential (ESP) along the XB path. The gap between ED and ESP minima exponentially depends on local electronic kinetic energy density at the bond critical point and tends to be a common limiting value for the strongest halogen bond

New Crystal Forms for Biologically Active Compounds. Part 2: Anastrozole as N-Substituted 1,2,4-Triazole in Halogen Bonding and Lp-π Interactions with 1,4-Diiodotetrafluorobenzene

For an active pharmaceutical ingredient, it is important to stabilize its specific crystal polymorph. If the potential interconversion of various polymorphs is not carefully controlled, it may lead to deterioration of the drug’s physicochemical profile and, ultimately, its therapeutic efficacy. The desired polymorph stabilization can be achieved via co-crystallization with appropriate crystallophoric excipients. In this work, we identified an opportunity for co-crystallization of anastrozole (ASZ), a well-known aromatase inhibitor useful in second-line therapy of estrogen-dependent breast cancer, with a classical XB donor, 1,2,4,5-tetrafluoro-3,6-diiodobenzene (1,4-FIB). In the X-ray structures of ASZ·1.5 (1,4-FIB) co-crystal, different non-covalent interactions involving hydrogen and halogen atoms were detected and studied by quantum chemical calculations and QTAIM analysis at the ωB97XD/DZP-DKH level of theory

New Crystal Forms for Biologically Active Compounds. Part 1: Noncovalent Interactions in Adducts of Nevirapine with XB Donors

Stabilization of specific crystal polymorphs of an active pharmaceutical ingredient is crucial for preventing uncontrollable interconversion of various crystalline forms, which affects physicochemical properties as well as physiological activity. Co-crystallization with various excipients is an emerging productive way of achieving such stabilization in the solid state. In this work, we identified an opportunity for co-crystallization of antiviral drug nevirapine (NVP) with a classical XB donor, 1,2,4,5-tetrafluoro-3,6-diiodobenzene (1,4-FIB), as well as 1,3-diiodobenzene (1,3-DIB), which has been seldom employed as an XB donor to date. In the X-ray structures of NVP·1,4-FIB and NVP·1,3-DIB co-crystals, different hydrogen and halogen bonding modes were detected and further investigated via DFT calculations as well as topological analysis of the electron density distribution within the framework of the QTAIM method at the M06/DZP-DKH level of theory. Estimated energies of these supramolecular contacts vary from 0.6 to 5.7 kcal/mol

Stacking Interactions: A Supramolecular Approach to Upgrade Weak Halogen Bond Donors

The co-crystallization of tetracyanobenzene (TCB) with haloarenes ArX provided six new co-crystals TCB center dot ArX (ArX=PhCl, PhBr, 4-MeC6H4Cl, 4-MeC6H4Br, 4-MeOC6H4Cl, 1,2-Br2C6H4) which were studied by X-ray diffraction. In these systems, the strong collective effect of pi center dot center dot center dot pi stacking interactions and lone pair-(X)center dot center dot center dot pi-hole-(C) bondings between TCB and ArX promote the strength of X center dot center dot center dot N-cyano halogen bonding (HaB). Theoretical studies showed that the stacking interactions affect the o-hole depth of the haloarenes, thus significantly boosting their ability to function as HaB donors. According to the molecular electrostatic potential calculations, the sigma- hole-(Cl) value (1.5 kcal/mol) in the haloarene 4-MeOC6H4Cl (featuring an electron-rich arene moiety and exhibiting very poor sigma-hole-(CI) ability) increases significantly in the stacked trimer (TCB)(2)center dot 4-MeOC6H4Cl (12.5 kcal/mol). Theoretical DFT calculations demonstrate the dramatic increase of X center dot center dot center dot N-cyano HaB strength for stacked trimers in comparison with parent unstacked haloarenes

Studies of Nature of Uncommon Bifurcated I–I···(I–M) Metal-Involving Noncovalent Interaction in Palladium(II) and Platinum(II) Isocyanide Cocrystals

Two isostructural trans-[MI2(CNXyl)2]·I2 (M = Pd or Pt; CNXyl = 2,6-dimethylphenyl isocyanide) metallopolymeric cocrystals containing uncommon bifurcated iodine···(metal–iodide) contact were obtained. In addition to classical halogen bonding, single-crystal X-ray diffraction analysis revealed a rare type of metal-involved stabilizing contact in both cocrystals. The nature of the noncovalent contact was studied computationally (via DFT, electrostatic surface potential, electron localization function, quantum theory of atoms in molecules, and noncovalent interactions plot methods). Studies confirmed that the I···I halogen bond is the strongest noncovalent interaction in the systems, followed by weaker I···M interaction. The electrophilic and nucleophilic nature of atoms participating in I···M interaction was studied with ED/ESP minima analysis. In trans-[PtI2(CNXyl)2]·I2 cocrystal, Pt atoms act as weak nucleophiles in I···Pt interaction. In the case of trans-[PdI2(CNXyl)2]·I2 cocrystal, electrophilic/nucleophilic roles of Pd and I are not clear, and thus the quasimetallophilic nature of the I···Pd interaction was suggested.peerReviewe

Structure-Directing Interplay between Tetrel and Halogen Bonding in Co-Crystal of Lead(II) Diethyldithiocarbamate with Tetraiodoethylene

The co-crystallization of the lead(II) complex [Pb(S2CNEt2)2] with tetraiodoethylene (C2I4) gave the co-crystal, [Pb(S2CNEt2)2]∙½C2I4, whose X-ray structure exhibits only a small change of the crystal parameters than those in the parent [Pb(S2CNEt2)2]. The supramolecular organization of the co-crystal is largely determined by an interplay between Pb⋯S tetrel bonding (TeB) and I⋯S halogen bonding (HaB) with comparable contributions from these non-covalent contacts; the TeBs observed in the parent complex, [Pb(S2CNEt2)2], remain unchanged in the co-crystal. An analysis of the theoretical calculation data, performed for the crystal and cluster models of [Pb(S2CNEt2)2]∙½C2I4, revealed the non-covalent nature of the Pb⋯S TeB (−5.41 and −7.78 kcal/mol) and I⋯S HaB (−7.26 and −11.37 kcal/mol) interactions and indicate that in the co-crystal these non-covalent forces are similar in energy

Diaryliodonium Tetracyanidometallates Self-Assemble into Halogen-Bonded Square-Like Arrays

Two diphenyliodonium tetracyanidometallates, [Ph2I](2)[M(CN)(4)] (M = Ni and Pd), were prepared through anion metathesis. Their X-ray structural analyses show that the structure-defining contact for both crystals is the charge-assisted I center dot center dot center dot N halogen bond (HaB) formed between the I atom of the iodonium cations and the N atoms of the CN- ligands. These HaBs assemble the bidentate and 90 degrees-orienting HaB donor Ph2I+ and the tetradentate, square planar, and 90/180 degrees-orienting HaB acceptors [M(CN)(4)](2-) into supramolecular rectangles, which further assemble into infinite chains by sharing the vertexes occupied by the [M(CN)(4)](2-) anions. The noncovalent nature of these contacts was confirmed by density functional theory calculations (M06/def2-TZVP) followed by combined topological analysis of the electron density distribution in the quantum theory of the atoms-in-molecules approach and noncovalent interaction analysis. The philicities of the HaB partners were further verified by the analysis of electron localization function projections, electron density/electrostatic potential profiles along the I center dot center dot center dot N bond paths, natural bond orbital analysis, and the natural population analysis or atoms-in-molecules charge sums in model systems