Various Approaches to Studying the Phase Transition in an Octamethylcyclotetrasiloxane Crystal: From X-ray Structural Analysis to Metadynamics

The structure, thermodynamic parameters, and the character of thermal motion in octamethylcyclotetrasiloxane (D4) were investigated using the combination of experimental (single-crystal X-ray diffraction, thermochemistry) and theoretical (density functional theory calculations, ab initio molecular dynamics and metadynamics) methods. Single crystals of D4 were grown in a glass capillary in situ and the structures of high- (238–270 K) and low-temperature (100–230 K) phases were studied in detail. In the temperature interval 230–238 K, a phase transition with rather low enthalpy (−1.04(7) kcal/mol) was detected. It was found that phase transition is accompanied by change of conformation of cyclosiloxane moiety from boat-saddle (cradle) to chair. According to PBE0/6-311G(d,p) calculation of isolated D4, such conformation changes are characterized by a low barrier (0.07 kcal/mol). The character of molecular thermal motion and the path of phase transition were established with combination of periodic DFT calculations, including molecular dynamics and metadynamics. The effect of crystal field led to an increase in the calculated phase transition barrier (4.27 kcal/mol from low- to high-temperature phase and 3.20 kcal/mol in opposite direction)

Synthesis of CF₃-Containing Spiro-[Indene-Proline] Derivatives via Rh(III)-Catalyzed C-H Activation/Annulation

An efficient method of accessing new CF3-containing spiro-[indene-proline] derivatives has been developed based on a Cp*Rh(III)-catalyzed tandem C-H activation/[3+2]–annulation reaction of 5-aryl-2-(trifluoromethyl)-3,4-dihydro-2H-pyrrole-2-carboxylates with alkynes. An important feature of this spiro annulation process is the feasibility of dehydroproline moiety to act as a directing group in the selective activation of the aromatic C-H bond

Mononuclear Copper(I) 3-(2-pyridyl)pyrazole Complexes: The Crucial Role of Phosphine on Photoluminescence

A series of emissive Cu(I) cationic complexes with 3-(2-pyridyl)-5-phenyl-pyrazole and various phosphines: dppbz (1), Xantphos (2), DPEPhos (3), PPh3 (4), and BINAP (5) were designed and characterized. Complexes obtained exhibit bright yellow-green emission (ca. 520–650 nm) in the solid state with a wide range of QYs (1–78%) and lifetimes (19–119 µs) at 298 K. The photoluminescence efficiency dramatically depends on the phosphine ligand type. The theoretical calculations of buried volumes and excited states explained the emission behavior for 1–5 as well as their lifetimes. The bulky and rigid phosphines promote emission efficiency through the stabilization of singlet and triplet excited states

Expanding the Family of Octahedral Chiral-at-Metal Cobalt(III) Catalysts by Introducing Tertiary Amine Moiety into the Ligand

Chiral metal-templated complexes are attractive catalysts for organic synthetic transformations. Herein, we introduce a novel chiral cobalt(III)-templated complex based on chiral trans-3,4-diamino-1-benzylpyrrolidine and 3,5-di-tert-butyl-salicylaldehyde which features both hydrogen bond donor and Brønsted base functionalities. The obtained complexes were fully characterized by 1H, 13C NMR, IR-, UV-vis, CD-spectroscopy and by a single X-ray diffraction analysis. It was shown that chlorine anion is connected with amino groups of the complex via a hydrogen bonding. DFT calculations of charges and molecular electrostatic potential of the cobalt(III) complex showed that the basicity of the complex is certainly diminished as compared with the routine tertiary amines but the acidity of the conjugated acid of the complex should be increased. Thus, the catalytic potential of the complex may be much greater as a chiral acid than a chiral base. We believe that this work opens a new way in chiral bifunctional catalyst design

Tetranuclear Copper(I) and Silver(I) Pyrazolate Adducts with 1,1′-Dimethyl-2,2’-bibenzimidazole: Influence of Structure on Photophysics

A reaction of a cyclic trinuclear copper(I) or silver(I) pyrazolate complex ([MPz]3, M = Cu, Ag) with 1,1′-dimethyl-2,2’-bibenzimidazole (L) leads to the formation of tetranuclear adducts decorated by one or two molecules of a diimine ligand, depending on the amount of the ligand added (0.75 or 1.5 equivalents). The coordination of two L molecules stabilizes the formation of a practically idealized tetrahedral four-metal core in the case of a copper-containing complex and a distorted tetrahedron in the case of a Ag analog. In contrast, complexes containing one molecule of diimine possess two types of metals, two- and three-coordinated, forming the significantly distorted central M4 cores. The diimine ligands are twisted in these complexes with dihedral angles of ca. 50–60°. A TD-DFT analysis demonstrated the preference of a triplet state for the twisted 1,1′-dimethyl-2,2’-bibenzimidazole and a singlet state for the planar geometry. All obtained complexes demonstrated, in a solution, the blue fluorescence of the ligand-centered (LC) nature typical for free diimine. In contrast, a temperature decrease to 77 K stabilized the structure close to that observed in the solid state and activated the triplet states, leading to green phosphorescence at ca. 500 nm. The silver-containing complex Ag4Pz4L exhibited dual emission from both the singlet and triplet states, even at room temperature

Coordination Chemistry of Mercury-Containing Anticrowns. Complexation of Perfluoro‑<i>o</i>,<i>o</i>′‑biphenylenemercury with <i>o</i>‑Xylene and Acetonitrile and the First X‑ray Diffraction Evidence for Its Trimeric Structure

The paper reports the first X-ray diffraction data evidencing the cyclic trimeric structure of the earlier synthesized octafluoro-<i>o</i>,<i>o</i>′-biphenylenemercury (<b>8</b>), being of considerable interest as a potential anticrown. The conclusion on the trimeric (<i>o</i>,<i>o</i>′-C₆F₄C₆F₄Hg)₃ structure of this mercuracycle is based on an X-ray structural analysis of its <i>o</i>-xylene and acetonitrile complexes {[(<i>o</i>,<i>o</i>′-C₆F₄C₆F₄Hg)₃]­(<i>o</i>-Me₂C₆H₄)₂} (<b>9</b>) and {[(<i>o</i>,<i>o</i>′-C₆F₄C₆F₄Hg)₃]­(MeCN)₃} (<b>10</b>), which were obtained from <b>8</b> in an analytically pure state and fully characterized. Complex <b>9</b> contains two <i>o</i>-xylene species per one molecule of <b>8</b> and forms in the crystal infinite chains consisting of alternating mercuramacrocycle units and bridging <i>o</i>-xylene ligands. One more <i>o</i>-xylene molecule in each macrocyclic fragment of the chain serves as a terminal ligand. Both bridging and terminal molecules of <i>o</i>-xylene are coordinated in all cases with only one Hg site of the corresponding mercuracycle. The back transformation of complex <b>9</b> into <b>8</b> and <i>o</i>-xylene occurs on its heating in a vacuum at 100–120 °C for 2 h. In contrast to <b>9</b>, complex <b>10</b>, containing three acetonitrile ligands per one molecule of <b>8</b>, has a discrete structure in the crystal. In this complex, two of three acetonitrile species are bonded to one and the same Hg center of <b>8</b>, whereas the third MeCN species is coordinated with the other Hg atom of the mercuramacrocycle

Distorted <i>commo</i>-Cobaltacarboranes Based on the 5,6-Dicarba-<i>nido</i>-decaborane(12): The First Bimetal Cobalt–Copper Zwitterion-Containing Cluster with Four (B–H)₄···Cu Bonds Not Showing Fluxional Behavior in Solution

Treatment of a recently reported complex [Ph₄P]­[<i>closo,nido-</i>CoH­(2,4-C₂B₈H₁₀)­(7,8-C₂B₈H₁₁)] (<b>1</b>) either by H₂O₂ in acetone or NaH in THF leads to the loss of both the bridging and terminal hydrides yielding the diamagnetic salt of an anionic <i>commo</i>-cobaltacarborane [Ph₄P]­[Co­(2,4-<i>isonido</i>-C₂B₈H₁₀)₂] (<b>2</b>) with the {CoC₂B₈}-cluster units adopting a distorted skeletal geometry of the <i>isonido</i>-type. The anionic <i>commo</i> complex <b>2</b> reacts with in situ generated cationic [CuPPh₃]⁺ species to give stable copper–cobalt zwitterion [Ph₃PCu]­[Co­(2,4-<i>isonido</i>-C₂B₈H₁₀)₂] (<b>3</b>) with four two-electron, three-center (B–H)₄···Cu bonds, and exhibits no fluxional behavior in solution. Complex <b>3</b>, at the same time, in CH₂Cl₂ in the presence of 2-fold excess of PPh₃ readily converts to a new anionic species [(Ph₃P)₃Cu]­[Co­(2,4-<i>isonido</i>-C₂B₈H₁₀)₂] (<b>4</b>) which retains initial <i>isonido</i> geometry. All newly obtained diamagnetic <i>commo</i> complexes were characterized by a combination of analytical and multinuclear NMR spectroscopic data and by single-crystal X-ray diffraction studies of complexes <b>2</b> and <b>3</b>

Coordination Chemistry of Anticrowns. Synthesis and Structures of Double-Decker Sandwich Complexes of the Three-Mercury Anticrown (<i>o</i>‑C₆F₄Hg)₃ with Halide Anions Containing and Not Containing Coordinated Dibromomethane Molecules

The interaction of the three-mercury anticrown (<i>o</i>-C₆F₄Hg)₃ (<b>1</b>) with [PPh₄]­[BF₄] in methanol at room temperature leads to fluoride anion transfer from BF₄^– to <b>1</b> with the formation of a fluoride complex, [PPh₄]­{[(<i>o</i>-C₆F₄Hg)₃]₂F}, having a double-decker sandwich structure. The fluoride ion in this unique adduct is disposed between the mutually parallel planes of the central nine-membered rings of the anticrown units and cooperatively coordinated by all six Hg sites. The iodide anion also forms a double-decker sandwich in the interaction with <b>1</b>, but this sandwich, [PPh₄]­{[(<i>o</i>-C₆F₄Hg)₃]₂I}, has a wedge-shaped geometry. The reaction of <b>1</b> with [^<i>n</i>Bu₄N]Cl in dibromomethane at −15 °C affords a complex, [^<i>n</i>Bu₄N]­{[(<i>o</i>-C₆F₄Hg)₃]₂Cl­(CH₂Br₂)₂}, containing one chloride anion and two coordinated CH₂Br₂ species per two molecules of <b>1</b>. A similar bromide complex of <b>1</b>, containing two coordinated CH₂Br₂ moieties, has also been synthesized and structurally characterized. Both compounds represent wedge-shaped double-decker sandwiches wherein the halide anion is simultaneously bonded to all Hg centers of the anticrown molecules. The dibromomethane species in the isolated adducts are also arranged in the space between the mercuramacrocycles. One of these species is coordinated by each of its bromine atoms to a single Hg site of the adjacent macrocycle while the other interacts by only one bromine atom with a Hg center of the neighboring molecule of <b>1</b>

Coordination Chemistry of Anticrowns. Synthesis and Structures of Double-Decker Sandwich Complexes of the Three-Mercury Anticrown (<i>o</i>‑C₆F₄Hg)₃ with Halide Anions Containing and Not Containing Coordinated Dibromomethane Molecules

The interaction of the three-mercury anticrown (<i>o</i>-C₆F₄Hg)₃ (<b>1</b>) with [PPh₄]­[BF₄] in methanol at room temperature leads to fluoride anion transfer from BF₄^– to <b>1</b> with the formation of a fluoride complex, [PPh₄]­{[(<i>o</i>-C₆F₄Hg)₃]₂F}, having a double-decker sandwich structure. The fluoride ion in this unique adduct is disposed between the mutually parallel planes of the central nine-membered rings of the anticrown units and cooperatively coordinated by all six Hg sites. The iodide anion also forms a double-decker sandwich in the interaction with <b>1</b>, but this sandwich, [PPh₄]­{[(<i>o</i>-C₆F₄Hg)₃]₂I}, has a wedge-shaped geometry. The reaction of <b>1</b> with [^<i>n</i>Bu₄N]Cl in dibromomethane at −15 °C affords a complex, [^<i>n</i>Bu₄N]­{[(<i>o</i>-C₆F₄Hg)₃]₂Cl­(CH₂Br₂)₂}, containing one chloride anion and two coordinated CH₂Br₂ species per two molecules of <b>1</b>. A similar bromide complex of <b>1</b>, containing two coordinated CH₂Br₂ moieties, has also been synthesized and structurally characterized. Both compounds represent wedge-shaped double-decker sandwiches wherein the halide anion is simultaneously bonded to all Hg centers of the anticrown molecules. The dibromomethane species in the isolated adducts are also arranged in the space between the mercuramacrocycles. One of these species is coordinated by each of its bromine atoms to a single Hg site of the adjacent macrocycle while the other interacts by only one bromine atom with a Hg center of the neighboring molecule of <b>1</b>