A comparative study of two polymorphs of L‐aspartic acid hydrochloride

Two polymorphs of L‐aspartic acid hydrochloride, C₄H₈NO₄⁺·Cl⁻, were obtained from the same aqueous solution. Their crystal structures have been determined from single‐crystal data collected at 100 K. The crystal structures revealed three‐ and two‐dimensional hydrogen‐bonding networks for the triclinic and orthorhombic polymorphs, respectively. The cations and anions are connected to one another via N—H...Cl and O—H...Cl interactions and form alternating cation–anion layer‐like structures. The two polymorphs share common structural features; however, the conformations of the L‐aspartate cations and the crystal packings are different. Furthermore, the molecular packing of the orthorhombic polymorph contains more interesting interactions which seems to be a favourable factor for more efficient charge transfer within the crystal.     

Unprecedented Reverse Volume Expansion in Spin-Transition Crystals

The current craze for research around the spin crossover phenomenon can be justified to some extent by the mechanical properties due to the decrease of volume associated with the transition of the metal ion from the HS state to the LS state. As demonstrated here, the molecular complex [Fe(PM-pBrA)₂(NCS)₂] exhibits, on the contrary, an increase of the unit-cell volume from HS to LS. This counter-intuitive and unprecedented behavior that concerns both the thermal and the photoexcited spin conversions is revealed by a combination of single-crystal and powder X-ray diffraction complemented by magnetic measurements. Interestingly, this abnormal volume change appears concomitant with the wide rotation of a phenyl ring which induces a drastic modification, though reversible, of the structural packing within the crystal. In addition, the light-induced HS state obtained through the Light-Induced Excited Spin-State Trapping shows a remarkably high relaxation temperature, namely T(LIESST), of 109 K, one of the highest so far reported. The above set of quite unusual characteristics opens up new fields of possibilities within the development of spin crossover materials.     

Photoinduced Mo−CN Bond Breakage in Octacyanomolybdate Leading to Spin Triplet Trapping

The photoinduced properties of the octacoordinated complex K₄Mo^IV(CN)₈⋅2 H₂O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10 K after blue light irradiation reveals an heptacoordinated Mo(CN)₇ species originating from the light-induced cleavage of one Mo−CN bond, concomitant with the photoinduced formation of a paramagnetic signal. When this complex is heated to 70 K, it returns to its original diamagnetic ground state, demonstrating full reversibility. The photomagnetic properties show a partial conversion into a triplet state possessing significant magnetic anisotropy, which is in agreement with theoretical studies. Inspired by these results, we isolated the new compound [K(crypt-222)]₃[Mo^IV(CN)₇]⋅3 CH₃CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.     

Properties of metastable linkage NO isomers in Na2[Fe(CN)5NO]·2H2O incorporated in mesopores of silica xerogels

We study the properties of photoinduced metal-nitrosyl linkage isomers in sodium nitroprusside (SNP) as a function of particle size. By embedding the molecular complex at various concentrations into mesopores of silica xerogels the size of the particles can be adjusted. The ground state is characterized by X-ray diffraction, absorption and infrared spectroscopy. The physical properties of the photoswitched molecules were analysed by steady-state low-temperature absorption, infrared spectroscopy and by nanosecond transient absorption spectroscopy. The electronic structure as well as the activation energies of the metastable linkage isomers are independent of the particle size down to single isolated molecules, indicating that the SNP complexes are quasi-free inside the pores of the gel.     

Light‐Induced Bistability in the 2 D Coordination Network {[Fe(bbtr)3][BF4]2}∞: Wavelength‐Selective Addressing of Molecular Spin States

Whereas the neat polymeric Fe^II compound {[Fe(bbtr)₃][ClO₄]₂}_∞ (bbtr=1,4‐di(1,2,3‐triazol‐1‐yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)₃][BF₄]₂}_∞, Fe^II stays in the high‐spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent, and wavelength‐selective photoswitching between the low‐spin and high‐spin manifolds. This compound thus shows true light‐induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time, and intensity. Crystallographic studies reveal a photoinduced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean‐field approximation.     

The experimental library multipolar atom model refinement of l‐aspartic acid

The crystal structure of l ‐aspartic acid, C₄H₇NO₄, has been determined using two types of refinement, viz. the standard independent atom model (IAM) and the experimental library multipolar atom model (ELMAM). The ELMAM refinement shows a good improvement of the statistical indices compared with the IAM model, notably in terms of thermal displacement parameters and bond distances involving H atoms.     

Photoinduced Mo−CN Bond Breakage in Octacyanomolybdate Leading to Spin Triplet Trapping

The photoinduced properties of the octacoordinated complex K₄Mo^IV(CN)₈?2 H₂O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10 K after blue light irradiation reveals an heptacoordinated Mo(CN)₇ species originating from the light‐induced cleavage of one Mo?CN bond, concomitant with the photoinduced formation of a paramagnetic signal. When this complex is heated to 70 K, it returns to its original diamagnetic ground state, demonstrating full reversibility. The photomagnetic properties show a partial conversion into a triplet state possessing significant magnetic anisotropy, which is in agreement with theoretical studies. Inspired by these results, we isolated the new compound [K(crypt‐222)]₃[Mo^IV(CN)₇]?3 CH₃CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.     

Can a formally zwitterionic rhodium(I) complex emulate the charge density of a cationic rhodium(I) complex? A combined synchrotron X-ray and theoretical charge-density study

The molecular electron densities of structurally related cationic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indene)Rh(COD)](CF(3)SO(3)), [1c](CF(3)SO(3))) and formally zwitterionic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indenide)Rh(COD)], 1z) complexes were accurately determined using synchrotron bright-source X-ray radiation at 30 K followed by multipolar refinement (COD = η(4)-1,5-cyclooctadiene). The densities were also obtained from density functional theory calculations with a large, locally dense basis set. A 28-electron ([Ar]3d(10)) core of the Rh atom was modeled by an effective core potential to obtain a density that was then augmented with relativistic cores according to the Keith-Frisch approximation. Calculations were performed at the experimental geometry and after vacuum-phase geometry optimization starting from the experimental geometry. Experimental and calculated geometries and electron-density distributions show that the electron density and electronic structure in the region of the Rh center are not significantly altered by protonation of the aromatic ring and that formal removal of CF(3)SO(3)H from [1c](CF(3)SO(3)) affords a complex 1z possessing substantial zwitterionic character (with a charge separation of ca. 0.9 electronic charge) featuring a negatively charged aromatic indenide framework. Further, the molecular electrostatic potentials of 1c and 1z exhibit similar topography around the metal, despite being drastically different in the vicinity of the indene or indenide portion of the cation (1c) and zwitterion (1z), respectively. Collectively, these observations obtained from high-level experimental and theoretical electron-density analysis confirm, for the first time, that appropriately designed zwitterionic complexes can effectively emulate the charge distribution found within ubiquitous cationic platinum-group metal catalyst complexes, in keeping with recent catalytic investigations.     

The structural properties of a noncentrosymmetric polymorph of 4‐aminobenzoic acid

The crystal structure of a polymorph of 4‐aminobenzoic acid (PABA), C₇H₇NO₂, at 100 K is noncentrosymmetric, as opposed to centrosymmetric in the structures of the other known polymorphs. The two crystallographically independent PABA molecules form pseudocentrosymmetric O—H...O hydrogen‐bonded dimers that are further linked by N—H...O hydrogen bonds into a three‐dimensional network. The benzene rings stack in the b direction. The CO₂ moieties are bent out slightly from the benzene ring plane.