Mechanically compliant single crystals of a stable organic radical.

Mechanically compliant organic crystals are the foundation of the development of future flexible, light-weight single-crystal electronics, and this requires reversibly deformable crystalline organic materials with permanent magnetism. Here, we report and characterize the first instance of a plastically bendable single crystal of a permanent organic radical, 4-(4'-cyano-2',3',4',5'-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl. The weak interactions between the radicals render single crystals of the ß phase of this material exceedingly soft, and the S-N interactions facilitate plastic bending. EPR imaging of a bent single crystal reveals the effect of deformation on the three-dimensional spin density of the crystal. The unusual mechanical compliance of this material opens prospects for exploration into flexible crystals of other stable organic radicals towards the development of flexible light-weight organic magnetoresistance devices based on weak, non-hydrogen-bonded interactions in molecular crystals.

Physics in Africa.

Africa tends to be isolated and sometimes forgotten when it comes to scientific research and in particular physics. But the region has great potential, being home to the youngest population in the world, and despite a variety of issues, there has been steady, albeit uneven progress towards establishing a scientific infrastructure. Ten African researchers discuss the diverse challenges and opportunities faced by physicists across the continent.

A Multistimulus Responsive Porous Coordination Polymer: Temperature-Mediated Control of Solid-State [2+2] Cycloaddition.

Either of two different isomeric products can be selectively obtained by photochemical [2+2] cycloaddition of 1,4-bis[2-(4-pyridyl)ethenyl]-benzene within a porous coordination polymer (PCP). The cycloaddition product obtained depends on the temperature at which irradiation occurs. A rare temperature-induced phase transition alters the conformation of the photoactive ligand in the PCP, and thereby the positions at which cycloaddition occurs. We show that the multistimulus responsive nature of this particular PCP is due to the flexibility of the ligands employed.

Solvent-Mediated Synthesis of Cyclobutane Isomers in a Photoactive Cadmium(II) Porous Coordination Polymer.

Photochemical [2+2] cycloaddition of 1,4-bis[2-(4-pyridyl)ethenyl]benzene, carried out in a CdII porous coordination polymer (PCP), produces different isomeric products depending on the guest solvent present in the PCP during irradiation. The nature of the included guest influences the conformation of the ligand, and thus the outcome of the cycloaddition reaction. We demonstrate controlled production of the two isomers from the same PCP by simple exchange of solvent.

Cobalt Porphyrin-Thiazyl Radical Coordination Polymers: Toward Metal-Organic Electronics.

Herein we delineate an unusual one-dimensional coordination polymer (CP), 3, prepared from S = 1/2 Co(TPP), 1 (TPP = 5,10,15,20-tetraphenylporphyrin dianion), and S = 1/2 4-(4'-pyridyl)-1,2,3,5-dithiadiazolyl (py-DTDA) radical, 2. The atypically long S-S distance for CP 3 (2.12 Å) reflects fractional electron transfer from the formally Co(II) ion into the antibonding π-SOMO of the metal-bound py-DTDA bridging ligand. The bonding in solid CP 3 involves noninteger redox states in a resonance hybrid repeat unit best formulated as [Co(TPP)]0.5+ hemication (Co2.5+) bound to a dithiadiazolide hemianion (py-DTDA0.5-). DFT calculations confirm the metal to ligand charge transfer (MLCT) character of the low-lying electronic states (641, 732, and 735 nm) observed for CP 3 and show that oligomer chains of length ≥14 repeat units tend toward a band structure with a limiting band gap energy of 0.669(6) eV. In dichloromethane, the reaction between radicals 1 and 2 involves coordination of the Co(II) ion by a py-DTDA ring sulfur atom, orbitally favored spin-pairing, and the formation of the thermodynamically favored diamagnetic five-coordinate S-bound adduct, Co(TPP)(S-py-DTDA), 3a. Polymerization and crystallization of 3a affords diamagnetic CP 3. Dissolution of CP 3 in DMSO favors Co-S bond heterolysis, yielding the diamagnetic six-coordinate purple N-bound CoIII(TPP)(N-py-DTDA-)(O═SMe2) complex (λmax, 436 nm). However, monomerization of CP 3 in dry 1,2-dichloroethane affords bright green diamagnetic CoIII(TPP)(N-py-DTDA-), 3b, with multiple MLCT bands in the 800-1100 nm NIR region and a red-shifted Soret band (λmax, 443 nm). Implications for the use of CP 3 in electronic devices are discussed based on its density of states.

Inclusion of a dithiadiazolyl radical in a seemingly non-porous solid.

Inclusion of the dithiadiazolyl radical PhCNSSN˙ into the dynamically porous metallocycle [Cu2(L1)2Cl4], where L1 is the bidentate ligand 1,3-bis(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene, has been achieved by gas phase diffusion. Single crystal X-ray diffraction, powder X-ray diffraction, UV-visible spectroscopy, EPR and SQUID magnetometry studies confirm inclusion of the radical into this seemingly non-porous material, and illustrate the antiferromagnetic coupling between the paramagnetic host and guest species. The radical guest is readily released by heating or by the addition of solvent (CH2Cl2).

Interconversion between different stoichiometric forms of a three-component crystal via liquid-assisted grinding.

A second polymorphic form of a 1 : 2 : 3 pamoate : DABCO : water salt has been obtained by liquid-assisted grinding, and it is shown that interconversion between this salt and a salt with 1 : 1 : 2 stoichiometry is facile via liquid-assisted grinding with additional amounts of pamoic acid (to generate the 1 : 1 : 2 salt) or DABCO (to generate the 1 : 2 : 3 salt).

Porous salts based on the pamoate ion.

A number of porous salts based on the pamoate anion in combination with lutidinium or picolinium cations have been characterised. One salt undergoes single-crystal to single-crystal solvent exchange, while another isostructural series of salts contain channels that can be partly evacuated without collapse of the framework.

Inclusion of thiazyl radicals in porous crystalline materials.

The incorporation of benzodithiazolyl (BDTA) and methylbenzodithiazolyl (MBDTA) radicals into porous hybrid frameworks via gas phase diffusion revealed that inclusion appeared selective for the MIL53(Al) framework against a range of other potential hosts. Both PXRD and EPR studies are consistent with retention of a π*-π* dimer motif for BDTA in MIL53(Al)@BDTA whereas MBDTA in MIL53(Al)@MBDTA appears to be monomeric. The guests are readily released by the addition of solvent (CH(2)Cl(2)).

Search for a predicted hydrogen bonding motif--a multidisciplinary investigation into the polymorphism of 3-azabicyclo[3.3.1]nonane-2,4-dione.

The predictions of the crystal structure of 3-azabicyclo[3.3.1]nonane-2,4-dione submitted in the 2001 international blind test of crystal structure prediction (CSP2001) led to the conclusion that crystal structures containing an alternative hydrogen bonded dimer motif were energetically competitive with the known catemer-based structure. Here we report an extensive search for a dimer-based crystal structure. Using an automated polymorph screen a new catemer-based metastable polymorph (form 2) and two new catemer-based solvates were found, and concurrent thermal studies reproduced form 2 and identified a plastic phase (form 3), whose powder X-ray diffraction pattern was consistent with the cubic space group I23 (a = 7.5856(1) A). Computational studies on the monomer showed that the imide N-H was a weak hydrogen bond donor, rationalizing the occurrence of the plastic phase which involved the breaking of all hydrogen bonds, and modeling of small clusters showed that dimers could easily reorganize to give the catemer. FTIR spectra confirmed the weakness of the hydrogen bond, with the solute showing no self-assembly in solution. It is concluded that the weakness of the N-H donor, coupled with the globular shape of the molecule, allows unusually facile transformation between alternative hydrogen bonding motifs during aggregation and nucleation.

Screening for crystalline salts via mechanochemistry.

Neat grinding and solvent-drop grinding methods are found to be effective screening tools for indicating the potential for crystalline salt formation involving a given acid-base pair, as demonstrated with two model pharmaceuticals.

Crystal structures and magnetic properties of a sterically encumbered dithiadiazolyl radical, 2,4,6-(F3C)3C6H2CNSSN..

The sterically protected dithiadiazolyl radical (F3C)3C6H2CNSSN. (1) crystallises in two polymorphs: 1alpha, comprised of monomeric units and 1beta, containing a mixture of both pi*-pi* dimers and S = (1/2) monomers; whilst both polymorphs exhibit similar structure-directing motifs, the variation in packing leads to different magnetic behaviour.

Occurrence of pharmaceutically acceptable anions and cations in the Cambridge Structural Database.

The occurrence of a number of pharmaceutically acceptable counterions in the Cambridge Structural Database (CSD) has been investigated. The results have been compared to the occurrence of the same counterions in a list of known pharmaceutical salts. Chloride salts are by far the highest occurring in both groups. The occurrence of hydrates in the structures of salts of pharmaceutically acceptable counterions in the CSD has also been investigated. It was found that salts of these counterions show an increased tendency to hydrate when compared to the database average. The CSD was also searched for co-crystals of the list of pharmaceutically acceptable acids and bases corresponding to the list of counterions used for the salt investigation.

Small molecule fixation by a dithiadiazolyl radical: X-ray crystal structures of (CF3C6H3FCNSSN)2 and (CF3C6H3FCNSSN)2 x G (G = N2, Ar, CO2 and SO2).

The title dithiadiazolyl radical, F3CC6H3FCNSSN (1) forms inclusion structures with small molecules (such as N2, CO2, Ar and SO2) when sublimed under partial atmospheres of the guest.