Ultra-fast rotors for molecular machines and functional materials via halogen bonding: crystals of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane with distinct gigahertz rotation at two sites

As a point of entry to investigate the potential of halogen-bonding interactions in the construction of functional materials and crystalline molecular machines, samples of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane (BIBCO) were synthesized and crystallized. Knowing that halogen-bonding interactions are common between electron-rich acetylenic carbons and electron-deficient iodines, it was expected that the BIBCO rotors would be an ideal platform to investigate the formation of a crystalline array of molecular rotors. Variable temperature single crystal X-ray crystallography established the presence of a halogen-bonded network, characterized by lamellarly ordered layers of crystallographically unique BIBCO rotors, which undergo a reversible monoclinic-to-triclinic phase transition at 110 K. In order to elucidate the rotational frequencies and the activation parameters of the BIBCO molecular rotors, variable-temperature (1)H wide-line and (13)C cross-polarization/magic-angle spinning solid-state NMR experiments were performed at temperatures between 27 and 290 K. Analysis of the (1)H spin-lattice relaxation and second moment as a function of temperature revealed two dynamic processes simultaneously present over the entire temperature range studied, with temperature-dependent rotational rates of k(rot) = 5.21 × 10(10) s(-1)·exp(-1.48 kcal·mol(-1)/RT) and k(rot) = 8.00 × 10(10) s(-1)·exp(-2.75 kcal·mol(-1)/RT). Impressively, these correspond to room temperature rotational rates of 4.3 and 0.8 GHz, respectively. Notably, the high-temperature plastic crystalline phase I of bicyclo[2.2.2]octane has a reported activation energy of 1.84 kcal·mol(-1) for rotation about the 1,4 axis, which is 24% larger than E(a) = 1.48 kcal·mol(-1) for the same rotational motion of the fastest BIBCO rotor; yet, the BIBCO rotor has three fewer degrees of translational freedom and two fewer degrees of rotational freedom! Even more so, these rates represent some of the fastest engineered molecular machines, to date. The results of this study highlight the potential of halogen bonding as a valuable construction tool for the design and the synthesis of amphidynamic artificial molecular machines and suggest the potential of modulating properties that depend on the dielectric behavior of crystalline media.     

Stereochemically inactive lone pairs in phosphorus(III) compounds: the characterisation of some derivatives with the 2,5-(CF3)2C6H3 (Ar) substituent and their complexation behaviour towards Pt(II) species

Some new phosphorus(III) derivatives Ar(2)PX (X = Br, Cl, F or H), ArPX(2) (X = Br or Cl), Ar(3)P and Ar(t)BuPCl, with the 2,5-bis(trifluoromethyl)phenyl (Ar) substituent on phosphorus, have been prepared, and characterised by (31)P and (19)F NMR solution-state spectroscopy. The complexing ability of Ar(2)PX, Ar(3)P and Ar(t)BuPCl towards the dimeric platinum(II) complexes [PtY(μ-Y)(PEt(3))](2) (Y = Cl or Br, the latter for X = Br only) has also been investigated. Single-crystal X-ray diffraction studies at low temperature have been carried out for Ar(3)P, Ar(2)PCl and the hydrolysis or oxidation products Ar(2)P(H)OH and Ar(2)P(O)OH. The structures of Ar(3)P and Ar(2)PCl are particularly interesting as in each compound the geometry around P is approximately octahedral. In Ar(3)P there are three short contacts to fluorine as well as the three bonded C atoms for both of the independent molecules in the unit cell. For Ar(2)PCl there are two short P-F contacts, and the octahedron is completed by a weak P-P interaction to a neighbouring molecule. In both instances the lone pair on the P(III) centre appears to be stereochemically inactive, and does not play a significant role in the structure.     

Synthesis of N,N-dimethyl(diethyl)-7-aryl-5-methyl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-carboxamides

Russian Journal of Organic Chemistry -     

Synthesis of 2‐(Selenophen‐2‐yl)pyrroles and Their Electropolymerization to Electrochromic Nanofilms

Bridging pyrrole and selenophene chemistries : Molecular assemblies have been developed that allow scrutiny of the electronic communication between pyrrole and selenophene nuclei. Divergent syntheses of 2‐(selenophen‐2‐yl)pyrroles and their N‐vinyl derivatives from available 2‐acylselenophenes and acetylenes in a one‐pot procedure have been devised (see scheme), which provide access to these exotic heterocyclic ensembles.

     

Synthesis of N,6-diaryl-4-methyl-2-thioxo-1,2,3,6-tetrahydropyrimidine-5-carboxamides

Russian Journal of Organic Chemistry -     

The evolution of roughness of supersmooth surfaces by ion-beam etching

The behavior of roughness on surfaces of Cr/Sc multilayer structure, crystalline silicon [100], and fused quartz upon ion beam etching in the region of middle and high frequencies of the spatial spectrum (10⁻²–10² μm⁻¹) is studied. The possibility of keeping surface roughness at level σ ∼ 0.3 nm upon etching by Ar atoms/ions to depths of up to 10 μm is demonstrated.     

New layer megaborate Sm3[B13O22(OH)3](OH) · 3H2O and its place in the structural systematics

Crystals of a new aqueous rare earth borate Sm₃[B₁₃O₂₂(OH)₃](OH) · 3H₂O, space group P2/c, are obtained under hydrothermal conditions. The structure is determined by the heavy-atom method without preliminary knowledge of the chemical formula. The anionic radical is a boron-oxygen sheet in which two corrugated layers are related by centers of inversion. An independent layer is akin to pentaborate layers; it differs from the layer in (Nd_0.925Na_0.075)Nd[B₉O₁₅(OH)₂]Cl_0.85 · 2.65H₂O by an additional branch in the form of a 4: (3[2?? + 1T] + 1??) group. The intersheet space and large holes of the sheet accommodate Sm atoms, (OH) groups, and water molecules. The new Sm-borate and the related Nd-borate are polyborates (megaborates) with complex anionic radicals. In the Sm-borate, the new two-dimensional [B₁₃O₂₂(OH)₃]_??? complex anionic radical of the 13{(4: [2T + 2??])_?? + (5: [3T + 2??] + 4: (3[2?? + 1T] +1??))_??}_??? formula is built of three different blocks, unlike the [B₉O₁₅(OH)₂]_??? radical of the 9{(4: [2T + 2??])_?? + (5: [3T + 2??])_??}_??? formula in the Nd-borate, which consists of two blocks. The rule of the inverse relationship between the polymerization degrees of the boron-oxygen radical and rare earth polyhedra holds for both borates.     

Design and evaluation of a crystalline hybrid of molecular conductors and molecular rotors

Combining recent concepts from the fields of molecular conductivity and molecular machinery we set out to design a crystalline molecular conductor that also possesses a molecular rotor. We report on the structures, electronic and physical properties, and dynamics of two solids with a common 1,4-bis(carboxyethynyl)bicyclo[2.2.2]octane (BABCO) functional rotor. One, [nBu(4)N(+)](2)[BABCO][BABCO(-)](2), is a colorless insulator where the dicarboxylic acid cocrystallizes with two of its monoanionic conjugated bases. The other is self-assembled by electrocrystallization in the form of black, shiny needles, with highly conducting molecular slabs of (EDT-TTF-CONH(2))(2)(+) (EDT-TTF = ethylenedithiotetrathiafulvalene) and anionic [BABCO(-)] rotors. Using variable-temperature (5-300 K) proton spin-lattice relaxation, (1)H T(1)(-1), we were able to assign two types of Brownian rotators in [nBu(4)N(+)](2)[BABCO][BABCO(-)](2). We showed that neutral BABCO groups have a rotational frequency of 120 GHz at 300 K with a rotational barrier of 2.03 kcal mol(-1). Rotors on the BABCO(-) sites experience stochastic 32 GHz jumps at the same temperature over a rotational barrier of 2.72 kcal mol(-1). In contrast, the BABCO(-) rotors within the highly conducting crystals of (EDT-TTF-CONH(2))(2)(+)[BABCO(-)] are essentially "braked" at room temperature. Notably, these crystals possess a conductivity of 5 S cm(-1) at 1 bar, which increases rapidly with pressure up to 50 S cm(-1) at 11.5 kbar. Two regimes with different activation energies E(a) for the resistivity (180 K above 50 and 400 K below) are observed at ambient pressure; a metallic state is stabilized at ca. 8 kbar, and an insulating ground state remains below 50 K at all pressures. We discuss two likely channels by which the motion of the rotors might become slowed down in the highly conducting solid. One is defined as a low-velocity viscous regime inherent to a noncovalent, physical coupling induced by the cooperativity between five C(sp3)-H···O hydrogen bonds engaging any rotor and five BABCO units in its environment. The rotational barrier calculated with the effect of this set of hydrogen bonds amounts to 7.3 kcal mol(-1). Another is quantum dissipation, a phenomenon addressing the difference of dynamics of the rotors in the two solids with different electrical properties, by which the large number of degrees of freedom of the low dimensional electron gas may serve as a bath for the dissipation of the energy of the rotor motion, the two systems being coupled by the Coulomb interaction between the charges of the rotors (local moments and induced dipoles) and the charges of the carriers.