No abstract

The first crystal structures of a dinuclear iron(II) complex with three N1,N2-1,2,4-triazole bridges in the high-spin and low-spin states are reported. Its sharp spin transition, which was probed using X-ray, calorimetric, magnetic, and (57)Fe Mossbauer analyses, is also delineated in the crystalline state by variable-temperature fluorimetry for the first time.

An asymmetrically substituted fluorescent difluoroboron dipyrromethene (BODIPY) dye, with a phenylamino group at the 3-position of the BODIPY chromophore, has been synthesized by nucleophilic substitution of 3,5-dichloro-8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. The solvent-dependent spectroscopic and photophysical properties have been investigated by means of UV-vis spectrophotometry and steady-state and time-resolved fluorometry and reflect the large effect of the anilino substituent on the fluorescence characteristics. The compound has a low fluorescence quantum yield in all but the apolar solvents cyclohexane, toluene, and chloroform. Its emission maxima in a series of solvents from cyclohexane to methanol are red-shifted by approximately 50 nm in comparison to classic BODIPY derivatives. Its oxidation potential in dichloromethane is at ca. 1.14 V versus Ag/AgCl. The absorption bandwidths and Stokes shifts are much larger than those of typical, symmetric difluoroboron dipyrromethene dyes. The values of the fluorescence rate constant are in the (1.4-1.7) x 10(8) s(-1) range and do not vary much between the solvents studied. X-ray diffraction analysis shows that the BODIPY core is planar. Molecular dynamics simulations show that there is no clear indication for aggregates in solution.

Co-crystal screening was applied under the assumption that two molecules having relatively similar chemical structures are likely to form co-crystals with identical coformers, in an attempt to improve co-crystal screening efficiency. Piracetam and Levetiracetam were used as model compounds. Both molecules are racetam compounds and have a relatively similar molecular structure. Eleven co-crystals of Piracetam have been described in the literature using ten different acids. These ten acids were selected as potential coformer candidates for the preparation of Levetiracetam cocrystals. Four co-crystals of Levetiracetam were successfully identified by solvent drop and neat grinding: Levetiracetam− D-tartaric acid 1:1 (LDTA), Levetiracetam−R/S-mandelic acid 1:1 (L(RS)MA), Levetiracetam−S-mandelic acid 1:1 (LSMA), and Levetiracetam−2,4-dihyroxybenzoic acid 1:1 (L2,4DHBA). The overall success rate of 40% shows the usefulness of the presented approach. Structural investigation shows the increased success rate to most likely be due to the proficiency of two similar molecules to share the same driving force for assembling multicomponent systems with similar coformers.

A cocrystal screening of a series of chiral target compounds was performed in order to investigate the propensity for two optically active compounds to cocrystallize in an enantiospecific manner. Thirteen novel cocrystal systems were identified, out of which 11 are enantiospecific and two present a diastereomeric cocrystal pair, yielding a total of 15 novel cocrystals. Six of these are structurally characterized in this study. A meticulous search in the Cambridge Structural Database (CSD) has allowed expanding this study. The results led us to the conclusion that enantiospecific cocrystallization seems to be the common rule of thumb, as over 85% of cocrystal systems behave enantiospecifically. Directionality of the hydrogen bonding motifs is likely responsible for the cocrystals’ predilection toward enantiospecificity, while salts are mainly stabilized by less directional electrostatic interactions, leading to the formation of diastereomeric pairs.

Metal–organic frameworks (MOFs) have emerged as an important, yet highly challenging class of electrochemical energy storage materials. The chemical principles for electroactive MOFs remain, however, poorly explored because precise chemical and structural control is mandatory. For instance, no anionic MOF with a lithium cation reservoir and reversible redox (like a conventional Li-ion cathode) has been synthesized to date. Herein, we report on electrically conducting Li-ion MOF cathodes with the generic formula Li2-M-DOBDC (wherein M = Mg2+ or Mn2+; DOBDC4– = 2,5-dioxido-1,4-benzenedicarboxylate), by rational control of the ligand to transition metal stoichiometry and secondary building unit (SBU) topology in the archetypal CPO-27. The accurate chemical and structural changes not only enable reversible redox but also induce a million-fold electrical conductivity increase by virtue of efficient electronic self-exchange facilitated by mix-in redox: 10–7 S/cm for Li2-Mn-DOBDC vs 10–13 S/cm for the isoreticular H2-Mn-DOBDC and Li2-Mg-DOBDC, or the Mn-CPO-27 compositional analogues. This particular SBU topology also considerably augments the redox potential of the DOBDC4– linker (from 2.4 V up to 3.2 V, vs Li+/Li0), a highly practical feature for Li-ion battery assembly and energy evaluation. As a particular cathode material, Li2-Mn-DOBDC displays an average discharge potential of 3.2 V vs Li+/Li0, demonstrates excellent capacity retention over 100 cycles, while also handling fast cycling rates, inherent to the intrinsic electronic conductivity. The Li2-M-DOBDC material validates the concept of reversible redox activity and electronic conductivity in MOFs by accommodating the ligand’s noncoordinating redox center through composition and SBU design.

Six conformationally restricted BODIPY dyes with fused carbocycles were synthesized to study the effect of conformational mobility on their visible electronic absorption and fluorescence properties. The symmetrically disubstituted compounds (2, 6) have bathochromically shifted absorption and fluorescence spectral maxima compared to those of the respective asymmetrically monosubstituted dyes (1, 5). Fusion of conjugation extending rings to the α,β-positions of the BODIPY core is an especially effective method for the construction of boron dipyrromethene dyes absorbing and emitting at longer wavelengths. The fluorescence quantum yields Φ of dyes 1-6 are high (0.7 ≤ Φ ≤ 1.0). The experimental results are backed up by quantum chemical calculations of the lowest electronic excitations in 1, 2, 5, 6, and corresponding dyes of related chemical structure but without conformational restriction. The effect of the molecular structure on the visible absorption and fluorescence emission properties of 1-6 has been examined as a function of solvent by means of the recent, generalized treatment of the solvent effect, proposed by Catalán (J. Phys. Chem. B 2009, 113, 5951-5960). Solvent polarizability is the primary factor responsible for the small solvent-dependent shifts of the visible absorption and fluorescence emission bands of these dyes.

Processes leading to enantiomerically pure compounds are of utmost importance, in particular for the pharmaceutical industry. Starting from a racemic mixture, crystallization‐induced diastereomeric transformation allows in theory for 100 % transformation of the desired enantiomer. However, this method has the inherent limiting requirement for the organic compound to form a salt. Herein, this limitation is lifted by introducing cocrystallization in the context of thermodynamic deracemization, with the process applied to a model chiral fungicide. We report a new general single thermodynamic deracemization process based on cocrystallization for the deracemization of (R,S)‐4,4‐dimethyl‐1‐(4‐fluorophenyl)‐2‐(1H‐1,2,4‐triazol‐1‐yl)pentan‐3‐one. This study demonstrates the feasibility of this novel approach and paves the way to further development of such processes.