C-halogen...pi dimer and C-H...pi interactions in 1-(2-bromo-4,5-dimethoxybenzyl)-2-butyl-4-chloro-1H-imidazole-5-carbaldehyde and 2-butyl-4-chloro-1-(6-methyl-1,3-benzodioxol-5-ylmethyl)-1H-imidazole-5-carbaldehyde

Valence tautomerism in a cobalt-dioxolene complex containing an imidazolic ancillary ligand

This work reports the synthesis, structural, spectroscopic and magnetic investigation of two complexes, [Co(bmimapy)(3,5-DTBCat)]PF₆·H₂O (1) and [Co(bmimapy)(TCCat)]PF₆·H₂O (2), where bmimapy is an imidazolic tetradentate ancillary ligand and 3,5-DTBCat and TCCat are the 3,5-di-tert-butyl-catecholate and tetrachlorocatecholate anions, respectively. Their structures have been elucidated using single crystal X-ray diffraction, showing a pseudo-octahedral cobalt ion bound to a chelating dioxolene ligand and the ancillary bmimapy ligand in a folded conformation. Magnetometry displayed an entropy-driven, incomplete, Valence Tautomeric (VT) process for 1 in the 300-380 K temperature range, while 2 displayed a temperature independent, diamagnetic low-spin cobalt(iii)-catecholate charge distribution. This behaviour was interpreted on the basis of the cyclic voltammetric analysis, allowing the estimation of the free energy difference associated with the VT interconversion of +8 and +96 kJ mol^-1 for 1 and 2, respectively. A DFT analysis of this free energy difference highlighted the ability of the methyl-imidazole pendant arm of bmimapy favouring the onset of the VT phenomenon. This work introduces the imidazolic bmimapy ligand to the scientific community working in the field of valence tautomerism, increasing the library of ancillary ligands to prepare temperature switchable molecular magnetic materials.

(7E)-5-Benzyl-7-(2-chlorobenzylidene)-3-(2-chlorophenyl)-2-phenyl-3,3a,4,5,6,7-hexahydro-2H-pyrazolo[4,3-c]pyridine

In the title 2H-pyrazolo­[4,3-c]pyridine derivative, C₃₂H₂₇Cl₂N₃, the dihydro­pyrazole ring adopts an envelope conformation and the piperidine fused ring a twisted-chair conformation. Two short intra­molecular C—H⋯Cl contacts are observed. The crystal packing is characterized by dimeric C—Cl⋯π inter­actions involving the 5-benzyl ring, with Cl⋯centroid and closest atomic Cl⋯π distances of 3.778 (2) and 3.366 (4) Å, respectively.

Structural plasticity of an aminoacyl-tRNA synthetase active site

Recently, tRNA aminoacyl-tRNA synthetase pairs have been evolved that allow one to genetically encode a large array of unnatural amino acids in both prokaryotic and eukaryotic organisms. We have determined the crystal structures of two substrate-bound Methanococcus jannaschii tyrosyl aminoacyl-tRNA synthetases that charge the unnatural amino acids p-bromophenylalanine and 3-(2-naphthyl)alanine (NpAla). A comparison of these structures with the substrate-bound WT synthetase, as well as a mutant synthetase that charges p-acetylphenylalanine, shows that altered specificity is due to both side-chain and backbone rearrangements within the active site that modify hydrogen bonds and packing interactions with substrate, as well as disrupt the alpha8-helix, which spans the WT active site. The high degree of structural plasticity that is observed in these aminoacyl-tRNA synthetases is rarely found in other mutant enzymes with altered specificities and provides an explanation for the surprising adaptability of the genetic code to novel amino acids.