Joel Bernstein (1941-2019).

Joel Bernstein, former Barry and Carol Kaye Professor of Applied Science at Ben-Gurion University, passed away on January 2, 2019. His legacy to the scientific community is a body of work on organic solid-state chemistry, chemical crystallography, polymorphism, hydrogen bonding, graph sets, structure-property relations, organic conducting materials, and pharmaceuticals.

Kinetics of solid state photodimerization of 1,4-dimethyl-2-pyridinone in its molecular compound.

The [4 + 4] photodimerization of 1,4-dimethyl-2-pyridinone (A) in its molecular compound with 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) was investigated by irradiation of a single crystal of the compound. The conversion to the product in a single-crystal to single-crystal transformation enabled the determination of its crystal structure after each exposure cycle of 10-20 min with a pulsed 355 nm laser light. The unit cell changes as a function of the conversion were monitored and showed monotonic changes. The kinetics of the reaction was studied at two different temperatures (230 and 280 K) and revealed a sigmoidal behavior that could be explained by the JMAK model for crystal growth with a mechanism that is intermediate between random distribution of products in the crystal and the existence of growing nuclei. The Arrhenius plot provided calculated activation energy of 32.5 kJ/mol.

Structural comparisons between methylated and unmethylated nitrophenyl lophines.

The lophine derivative 2-(2-nitrophenyl)-4,5-diphenyl-1H-imidazole, C21H15N3O2, (I), crystallized from ethanol as a solvent-free crystal and from acetonitrile as the monosolvate, C21H15N3O2.C2H3N, (II). Crystallization of 2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole from methanol yielded the methanol monosolvate, C21H15N3O2.CH4O, (III). Three lophine derivatives of methylated imidazole, namely, 1-methyl-2-(2-nitrophenyl)-4,5-diphenyl-1H-imidazole methanol solvate, C22H17N3O2.CH4O, (IV), 1-methyl-2-(3-nitrophenyl)-4,5-diphenyl-1H-imidazole, C22H17N3O2, (V), and 1-methyl-2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole, C22H17N3O2, (VI), were recrystallized from methanol, acetonitrile and ethanol, respectively, but only (IV) produced a solvate. Compounds (III) and (IV) each crystallize with two independent molecules in the asymmetric unit. Five imidazole molecules in the six crystals differ in their molecular conformations by rotation of the aromatic rings with respect to the central imidazole ring. In the absence of a methyl group on the imidazole [compounds (I)-(III)], the rotation angles are not strongly affected by the position of the nitro group [44.8 (2) and 45.5 (1) degrees in (I) and (II), respectively, and 15.7 (2) and 31.5 (1) degrees in the two molecules of (III)]. However, the rotation angle is strongly affected by the presence of a methyl group on the imidazole [compounds (IV)-(VI)], and the position of the nitro group (ortho, meta or para) on a neighbouring benzene ring; values of the rotation angle range from 26.0 (1) [in (VI)] to 85.2 (1) degrees [in (IV)]. This group repulsion also affects the outer N-C-N bond angle. The packing of the molecules in (I), (II) and (III) is determined by hydrogen bonding. In (I) and (II), molecules form extended chains through N-H...N hydrogen bonds [with an N...N distance of 2.944 (5) A in (I) and 2.920 (3) A in (II)], while in (III) the chain is formed with a methanol solvent molecule as the mediator between two imidazole rings, with O...N distances of 2.788 (4)-2.819 (4) A. In the absence of the imidazole N-H H-atom donor, the packing of molecules (IV)-(VI) is determined by weaker intermolecular interactions. The methanol solvent molecule in (IV) is hydrogen bonded to imidazole [O...N = 2.823 (4) A] but has no effect on the packing of molecules in the unit cell.

Structural effects on the solid-state photodimerization of 2-pyridone derivatives in inclusion compounds.

The structures of six crystalline inclusion compounds between various host molecules and three guest molecules based on the 2-pyridone skeleton are described. The six compounds are 1,1'-biphenyl-2,2'-dicarboxylic acid-2-pyridone (1/2), C(14)H(10)O(4).2C(5)H(5)NO, (I-a), 1,1'-biphenyl-2,2'-dicarboxylic acid-4-methyl-2-pyridone (1/2), C(14)H(10)O(4).2C(6)H(7)NO, (I-c), 1,1'-biphenyl-2,2'-dicarboxylic acid-6-methyl-2-pyridone (1/2), C(14)H(10)O(4).2C(6)H(7)NO, (I-d), 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol-1-methyl-2-pyridone (1/2), C(30)H(22)O(2).2C(6)H(7)NO, (II-b), 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol-4-methy-2-pyridone (1/2), C(30)H(22)O(2).2C(6)H(7)NO, (II-c), and 4,4',4''-(ethane-1,1,1-triyl)triphenol-6-methyl-2-pyridone-water (1/3/1), C(20)H(18)O(3).3C(6)H(7)NO.H(2)O, (III-d). In two of the compounds, (I-a) and (I-d), the host molecules lie about crystallographic twofold axes. In two other compounds, (II-b) and (II-c), the host molecules lie across inversion centers. In all cases, the guest molecules are hydrogen bonded to the host molecules through O-H...O=C hydrogen bonds [the range of O...O distances is 2.543 (2)-2.843 (2) A. The pyridone moieties form dimers through N-H...O=C hydrogen bonds in five of the compounds [the range of N...O distances is 2.763 (2)-2.968 (2) A]. In four compounds, (I-a), (I-c), (I-d) and (II-c), the molecules are arranged in extended zigzag chains formed via host-guest hydrogen bonding. In five of the compounds, the guest molecules are arranged in parallel pairs on top of each other, related by inversion centers. However, none of these compounds underwent photodimerization in the solid state upon irradiation. In one of the crystalline compounds, (III-d), the guest molecules are arranged in stacks with one disordered molecule. The unsuccessful dimerization is attributed to the large interatomic distances between the potentially reactive atoms [the range of distances is 4.027 (4)-4.865 (4) A] and to the bad overlap, expressed by the lateral shift between the orbitals of these atoms [the range of the shifts from perfect overlap is 1.727 (4)-3.324 (4) A]. The bad overlap and large distances between potentially photoreactive atoms are attributed to the hydrogen-bonding schemes, because the interactions involved in hydrogen bonding are stronger than those in pi-pi interactions.

Lophine (2,4,5-triphenyl-1H-imidazole).

The title compound, C(21)H(16)N(2), has been known since 1877. Although the crystal structure of 36 derivatives of lophine are known, the structure of parent compound has remained unknown until now. The three phenyl rings bonded to the imidazole core are not coplanar with the latter, with dihedral angles of 21.4 (3), 24.7 (3), and 39.0 (3)°, respectively, between the phenyl ring planes in the 2-, 4- and 5-positions of the imidazole ring. The mol-ecules are packed in layers running perpendicular to the b axis. Although there are acceptor and donor atoms for hydrogen bonds, no such inter-actions are detected in the crystal in contrast to other lophine derivatives.

Photochemistry of 1-isopropylcycloalkyl aryl ketones: ring size effects, medium effects, and asymmetric induction.

[reaction: see text] The n = 0, 1, and 2 ketones shown above undergo Yang photocyclization in solution, but only the n = 1 analogues react this way in the solid state. Based on X-ray crystallography, these differences in reactivity are attributed to an unusually large distance for 1,4-hydroxybiradical cyclization in the solid state for the n = 0 and 2 ring systems, which leads to predominant reverse hydrogen transfer (rht). Enantiomeric excesses of up to 99% can be achieved in the case of the n = 1 system through the use of the solid-state ionic chiral auxiliary method of asymmetric synthesis.

Solvates of zafirlukast.

The crystal structures of three solvates of zafirlukast [systematic name: cyclopentyl N-{1-methyl-3-[2-methyl-4-(o-tolylsulfonylaminocarbonyl)benzyl]-1H-indol-5-yl}carbamate], viz. the monohydrate, C(31)H(33)N(3)O(6)S.H(2)O, (I), the methanol solvate, C(31)H(33)N(3)O(6)S.CH(3)OH, (II), and the ethanol solvate, C(31)H(33)N(3)O(6)S.C(2)H(5)OH, (III), have been determined by single-crystal X-ray diffraction analysis. All three compounds crystallize in the monoclinic crystal system. Zafirlukast adopts a similar Z-shaped conformation in all three solvates. The methanol and ethanol solvates are isostructural. The packing of the zafirlukast molecules in all three crystal structures is similar and is expressed by hydrogen-bonded molecules that are related by translation, along (101) in (I) and along the b axis in (II) and (III). The methanol and ethanol solvent molecules are hydrogen bonded to two molecules of zafirlukast. The water molecule, on the other hand, acts as a connector via hydrogen bonds between three molecules of zafirlukast. The solvent molecules are not released at temperatures below the melting points of the solvates.

Different packing in three polymorphs of 2,4,6-trimethoxy-1,3,5-triazine.

2,4,6-trimethoxy-1,3,5-triazine was found to exhibit three different polymorphs. The alpha-polymorph undergoes reversible phase transformation to the beta-polymorph at 340 K with an enthalpy of 3.9 kJ mol(-1). The heat of fusion of the beta-polymorph at 404 K is 18.1 kJ mol(-1). The low-temperature phase (alpha-polymorph) crystallizes in the orthorhombic space group Pnma. The high-temperature phase (beta-polymorph) can be obtained from the melt and crystallizes in the monoclinic space group P2(1). The gamma-polymorph is obtained by crystallization from a 1:1 mixture of methanol and methylenechloride from hydrolyzed 2,4-dimethoxy-6-oxybenzophenone-1,3,5-triazine. The gamma-polymorph melts at 409 K with an enthalpy of 11.4 kJ mol(-1). The gamma-polymorph crystallizes in the trigonal space group R3c. The molecules occupy a crystallographic threefold axis. Molecules of 2,4,6-trimethoxy-1,3,5-triazine are planar in all three polymorphs. The major difference between the three polymorphs is the mode of packing of the molecules in the crystal.

Two hydrates of 6-methoxypurine.

6-Methoxypurine crystallizes from N,N-methylformamide as the hemihydrate, C(6)H(6)N(4)O.0.5H(2)O, and from water as the trihydrate, C(6)H(6)N(4)O.3H(2)O. Both forms crystallize in the triclinic crystal system. Upon heating the trihydrate, molecules of water are liberated successively; the hemihydrate is formed at 383 K. In the hemihydrate, the H atom on the imidazole N atom is disordered between the two N atoms. The water molecule in the hemihydrate and the H atoms of a water molecule in the trihydrate are also disordered. In the hemihydrate, the organic moieties are connected by N-H...N hydrogen bonds, while they are connected via water molecules in the trihydrate.

Two dimorphs of 5-methylsulfanyl-1H-tetrazole.

5-Methylsulfanyl-1H-tetrazole, C(2)H(4)N(4)S, crystallizes in dimorphic forms; the alpha-form crystallizes at room temperature in the monoclinic crystal system, space group P2(1)/m, and the beta-form crystallizes by sublimation at 423 K in the orthorhombic crystal system, space group Pbcm. In both forms, the molecules occupy crystallographic mirror planes and are connected to one another via N-H...N hydrogen bonds, the amino H atoms being disordered. The two forms differ from one another in their packing; there are polar layers in the alpha-form and non-polar layers in the beta-form.

Inclusion compounds of 2,5-diphenylhydroquinone.

The crystal structures of three 1:2 inclusion compounds that consist of host molecule 2,5-diphenylhydroquinone (C(18)H(14)O(2)) and the guest molecules 2-pyridone (C(5)H(5)NO), 1,3-diphenyl-2-propen-1-one (chalcone, C(15)H(12)O) and 1-(4-methoxyphenyl)-3-phenyl-2-propen-1-one (4'-methoxychalcone, C(16)H(14)O(2)) were determined in order to study the ability of guest molecules in inclusion compounds to undergo photoreaction. All of the crystals were found to be photoresistant. The three inclusion compounds crystallize in triclinic space group P1. In each case, the host/guest ratio is 1:2, with the host molecules occupying crystallographic centers of symmetry and the guest molecules occupying general positions. The guest molecules in each of the inclusion compounds are linked to the host molecules by hydrogen bonds. In the inclusion compound where the guest molecule is pyridone, the host molecule is disordered so that the hydroxy groups are distributed between two different sites, with occupancies of 0.738 (3) and 0.262 (3). The pyridone molecules form dimers via N-H...O hydrogen bonds.

2-Chloro-4,6-dimethoxy-1,3,5-triazine.

The molecules of 2-chloro-4,6-dimethoxy-1,3,5-triazine, C(5)H(6)ClN(3)O(2), lie on a crystallographic mirror plane. There is a close contact of 3.180 (3) A between one of the methyl C atoms and the N atom of a neighboring molecule. Differential scanning calorimetry measurements show that methyl rearrangement does not take place in the solid state, despite the close proximity of the methyl group to the N atom.

Derivatives of 2,4-dimethoxy-1,3,5-triazine.

The crystal structure of three derivatives of 2,4-dimethoxy-1,3,5-triazine are described. In 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)morpholine, C(9)H(14)N(4)O(3), the morpholine moiety adopts a chair conformation, and the dimethoxytriazine molecule adopts a butterfly conformation with respect to the two methoxy groups. In 3-(4,6-dimethoxy-1,3,5-triazin-2-yloxy)-2-methylphenol, C(12)H(13)N(3)O(4), the dimethoxytriazine moiety adopts a propeller conformation with respect to the methoxy groups, and the molecules form dimers held together by O-H.N hydrogen bonds. 4-(4,6-Dimethoxy-1,3,5-triazin-2-yloxy)phenyl phenyl ketone, C(18)H(15)N(3)O(4), crystallizes with two crystallographically independent molecules in the asymmetric unit. The two molecules adopt different conformations with respect to the geometric relations between the phenyl ketone and triazine moieties.

A conformationally flexible tetrahydroxycalix[4]arene adopting the unusual 1,3-alternate conformation.

The use of a chiral solvating agent enabled the determination of the NMR-silent ring-inversion process of the ketocalixarene 3. Spectroscopic and crystal data indicate that 3 adopts the unusual 1,3-alternate conformation.

Structure and magnetic properties of tetraarylporphinatomagnesium(II) electron transfer salts of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, TCNQF4.

The crystal structures of the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, TCNQF(4), electron transfer salts of meso-tetraphenylporphinatomagnesium(II), [MgTPP][TCNQF(4)].PhMe and [MgTPP][TCNQF(4)].3(1,2-C(6)H(4)Cl(2)), and meso-tetrakis(3,4,5-trimethoxyphenyl)porphinatomagnesium(II), [MgT(3,4,5-OMe)PP][TCNQF(4)].3(1,2-C(6)H(4)Cl(2)), provide the first structurally characterized examples of 1-D metal-radical chains involving [Mg(II)(porphyrin(*))](+). These salts possess [TCNQF(4)](*-) stabilized by trans-mu-coordination to Mg(II) and exhibit nu(CN) at 2199 and 2177, 2212 and 2187, and 2194 and 2172 cm(-1), respectively. The [TCNQF(4)](*-) species is planar and bridges two cations with MgN distances of 2.266(16), 2.221(2), and 2.276(3) A, respectively, which are shorter than the MnN 2.321(3) A distance observed for [MnT(3,4,5-OMe)PP][TCNQF(4)].3(1,2-C(6)H(4)Cl(2)). The room-temperature effective moments for [MgTPP][TCNQF(4)].xS (S = PhMe and 1,2-C(6)H(4)Cl(2)) and [MgTPP][C(4)(CN)(6)].PhMe are consistent with the calculated spin only value of 2.45 micro(B) with weak antiferromagnetic coupling (J(intra)/k(B) approximately -2.9 K; H = -2JSa.Sb) for these [TCNQF(4)](*-) salts and for this [C(4)(CN)(6)](*-) salt (J(intra)/k(B) approximately -0.8 K) on the basis of fits to several models. The coupling is significantly reduced with respect to that of the Mn analogues due to lack of spin on the metal site for [Mg(II)(por(*))](+). The antiferromagnetic coupling is enhanced for [MgT(3,4,5-OMe)PP][TCNQF(4)] with respect to [MgTPP][TCNQF(4)] as [TCNQF(4)](*-) gets closer to the [Mg(II)(por(*))](+) plane, which leads to greater interactions and coupling.

1-Methyl-5,6-diphenylpyrazine-2(1H)-thione.

The title compound, C(17)H(14)N(2)S, crystallizes in a triclinic unit cell, with two crystallographically independent molecules in the asymmetric unit. The two independent molecules pack in the same sense and form segregated layers along the c axis. The crystal is light-stable and no dimers are formed under irradiation. The intermolecular distances between the potential reactive centers (the C-3 and C-5 ring positions) are 4.093 (4) and 5.643 (4) A for molecule A, and 4.081 (4) and 5.614 (4) A for molecule B.

1-Methyl-5,6,7,8-tetrahydroquinoxalin-2(1H)-one.

The title compound, C(9)H(12)N(2)O, crystallizes in a triclinic unit cell, with two crystallographically independent molecules in the asymmetric unit. The two independent molecules adopt different modes of packing. One type of molecule is arranged in infinite columns, while the other type packs as dimers, forming spacers between the parallel columns. Each type of molecule is arranged in pairs related by inversion centers. The distances between potential reaction centers are 3.395 (2), 3.457 (2) and 3.522 (2) A. As a result of the symmetry of the pairs and the close distances between the potential photoreactive centers, it is expected that the dimer will be the anti-trans isomer.

Dispiro[2H-benzimidazole-2,1'-cyclohexane-4',2''-[2H]benzimidazole] 1-oxide and dispiro[2H-benzimidazole-2,1'-cyclohexane-4',2''-[2H]benzimidazole] 1,1''-dioxide.

Oxidation of tetrahydrodispirobenzimidazole by m-chloroperbenzoic acid did not produce dispiro-2H-benzimidazole, which is the product obtained by oxidation with MnO(2). Instead, a mixture of two compounds was identified, namely dispiro[2H-benzimidazole-2,1'-cyclohexane-4',2''-[2H]benzimidazole] 1-oxide, C(18)H(16)N(4)O, (III), and dispiro[2H-benzimidazole-2,1'-cyclohexane-4',2''-[2H]benzimidazole] 1,1''-dioxide, C(18)H(16)N(4)O(2), (IV). In (III), the molecules are disordered about a twofold rotation axis and have 2/m site symmetry. In (IV), the crystals are triclinic and the molecules occupy crystallographic inversion centers. Although the two compounds are very similar and are arranged in layers, they adopt completely different packing modes within the layers, viz. herring-bone in (III) and parallel molecules in (IV). The molecules within the layers are held together by C-H...O and C-H...N hydrogen bonds.