Copper(II)/di-2-pyridyl ketone chemistry: A triangular cluster displaying antisymmetric exchange versus an 1D coordination polymer

Shared Hydrogen Atom Location and Chemical Composition in Picolinic Acid and Pyridoxal Hydrochloride Derivatives Determined by X-ray Crystallography

Three new single-crystal structures were isolated for picolinic acid (2), the trifluoroacetate salt of picolinic acid (1), and pyridoxal hydrochloride (3). These compounds displayed unconventional crystallographic features that must be considered when structural refinements are carried out. Thus, the generated Fourier differences map obtained with the diffraction data collected at 100 K was crucial to visualize electron densities, which were balanced by either one hydrogen atom or a hydrogen atom with an occupancy factor of 1/2 located between either two carboxylate moieties, two phenolic oxygen atoms, or two pyridinic nitrogen atoms. Moreover, NMR studies were conducted to analyze the bulk chemical composition of single crystals of 2-pyridinecarboxylic acid obtained from the gem-diol/hemiacetal forms and the polymerization products after the treatment of 2-pyridinecarboxaldehyde with TFA:H₂O (1) or a diluted Cu(NO₃)₂ solution (2). The quantitative yield of the pyridoxal hydrochloride crystalline material (3) obtained from a diluted CuCl₂ solution was exhaustively characterized by solid-state NMR methods. These methods allowed the resolution of the signals corresponding to the protons of the hydroxyl moiety of the intramolecular hemiacetal group and the phenolic hydrogen. Theoretical calculations using DFT methods were done to complement the atomic location of the hydrogen atoms obtained from the X-ray analysis.

Paramagnetic solid-state NMR assignment and novel chemical conversion of the aldehyde group to dihydrogen ortho ester and hemiacetal moieties in copper(ii)- and cobalt(ii)-pyridinecarboxaldehyde complexes

The complex chemical functionalization of aldehyde moieties in Cu(ii)- and Co(ii)-pyridinecarboxaldehyde complexes was studied. X-ray studies demonstrated that the aldehyde group (RCHO) of the four pyridine molecules is converted to dihydrogen ortho ester (RC(OCH₃)(OH)₂) and hemiacetal (RCH(OH)(OCH₃)) moieties in both 4-pyridinecarboxaldehyde copper and cobalt complexes. In contrast, the aldehyde group is retained when the 3-pyridinecarboxaldehyde ligand is complexed with cobalt. In the different copper complexes, similar paramagnetic ¹H resonance lines were obtained in the solid state; however, the connectivity with the carbon structure and the ¹H vicinities were done with 2D ¹H–¹³C HETCOR, ¹H–¹H SQ/DQ and proton spin diffusion (PSD) experiments. The strong paramagnetic effect exerted by the cobalt center prevented the observation of ¹³C NMR signals and chemical information could only be obtained from X-ray experiments. 2D PSD experiments in the solid state were useful for the proton assignments in both Cu(ii) complexes. The combination of X-ray crystallography experiments with DFT calculations together with the experimental results obtained from EPR and solid-state NMR allowed the assignment of NMR signals in pyridinecarboxaldehyde ligands coordinated with copper ions. In cases where the crystallographic information was not available, as in the case of the 3-pyridinecarboxaldehyde Cu(ii) complex, the combination of these techniques allowed not only the assignment of NMR signals but also the study of the functionalization of the substituent group.

The complex chemical functionalization of the aldehyde group was elucidated in copper and cobalt complexes for 4- and 3-pyridinecarboxaldehyde ligands.

Crystal structures of two CuII compounds: catena-poly[[chlorido-copper(II)]-μ-N-[eth-oxy(pyridin-2-yl)methyl-idene]-N'-[oxido(pyridin-3-yl)methyl-idene]hydrazine-κ4 N,N',O:N''] and di-μ-chlorido-1:4κ2 Cl:Cl-2:3κ2 Cl:Cl-di-chlorido-2κCl,4κCl-bis[μ3-eth-oxy(pyridin-2-yl)methano-lato-1:2:3κ3 O:N,O:O;1:3:4κ3 O:O:N,O]bis-[μ2-eth-oxy(pyridin-2-yl)methano-lato-1:2κ3 N,O:O;3:4κ3 N,O:O]tetracopper(II)

Two CuII complexes [Cu(C14H13N4O2)Cl] n , I, and [Cu4(C8H10NO2)4Cl4] n , II, have been synthesized. In the structure of the mononuclear complex I, each ligand is coordinated to two metal centers. The basal plane around the CuII cation is formed by one chloride anion, one oxygen atom, one imino and one pyridine nitro-gen atom. The apical position of the distorted square-pyramidal geometry is occupied by a pyridine nitro-gen atom from a neighbouring unit, leading to infinite one-dimensional polymeric chains along the b-axis direction. Each chain is connected to adjacent chains by inter-molecular C-H⋯O and C-H⋯Cl inter-actions, leading to a three-dimensional network structure. The tetra-nuclear complex II lies about a crystallographic inversion centre and has one core in which two CuII metal centers are mutually inter-connected via two enolato oxygen atoms while the other two CuII cations are linked by a chloride anion and an enolato oxygen. An open-cube structure is generated in which the two open-cube units, with seven vertices each, share a side composed of two CuII ions bridged by two enolato oxygen atoms acting in a μ3-mode. The CuII atoms in each of the two CuO3NCl units are connected by one μ2-O and two μ3-O atoms from deprotonated hydroxyl groups and one chloride anion to the three other CuII centres. Each of the penta-coordinated CuII cations has a distorted NO3Cl square-pyramidal environment. The CuII atoms in each of the two CuO2NCl2 units are connected by μ2-O and μ3-O atoms from deprotonated alcohol hy-droxy groups and one chloride anion to two other CuII ions. Each of the penta-coordinated CuII cations has a distorted NO2Cl2 square-pyramidal environment. In the crystal, a series of intra-molecular C-H⋯O and C-H⋯Cl hydrogen bonds are observed in each tetra-nuclear monomeric unit, which is connected to four tetra-nuclear monomeric units by inter-molecular C-H⋯O hydrogen bonds, thus forming a planar two-dimensional structure in the (01) plane.

Generation and Stability of the gem-Diol Forms in Imidazole Derivatives Containing Carbonyl Groups. Solid-State NMR and Single-Crystal X-ray Diffraction Studies

The stability of gem-diol forms in imidazolecarboxaldehyde isomers was studied by solid-state nuclear magnetic resonance (ss-NMR) combined with single-crystal X-ray diffraction studies. These methodologies also allowed determining the factors governing the occurrence of such rare functionalization in carbonyl moieties. Results indicated that the position of the carbonyl group is the main factor that governs the generation of geminal diols, having a clear and direct effect on hydration, since, under the same experimental conditions, only 36% of 5-imidazolecarboxaldehydes and 5% of 4-imidazolecarboxaldehydes were hydrated, as compared to 2-imidazolecarboxaldehydes, with which a 100% hydration was achieved. Not only did trifluoroacetic acid favor the addition of water to the carbonyl group but also it allowed obtaining single crystals. Single crystals of the gem-diol and the hemiacetal forms 2-imidazolecarboxaldehyde and N-methyl-2-imidazolecarboxaldehyde, respectively, were isolated and studied through ¹H ss-NMR. Mass spectrometry and solution-state NMR experiments were also performed to study the hydration process.

Probing the electronic structure of a copper(ii) complex by CW- and pulse-EPR spectroscopy

The electronic structure of a mononuclear octahedral copper(ii) complex has been studied using CW EPR spectroscopy and other advanced methods including Davies ENDOR and HYSCORE (¹H and ¹³C) spectroscopy.

A unique copper(ii)-assisted transformation of acetylacetone dioxime in acetone that leads to one-dimensional, quinoxaline-bridged coordination polymers

A novel Cu^II-assisted transformation of acetylacetone dioxime to coordinated quinoxaline has been discovered.

Interesting copper(ii)-assisted transformations of 2-acetylpyridine and 2-benzoylpyridine

Novel Cu^II-assisted transformations have been observed in the reactions of copper(ii) sources and 2-pyridyl ketones.

Synthesis, Structures, and Single-Molecule Magnet Behaviour of High-Nuclearity Nickel(II) Dicubane-Type Complexes with Pyridyl-Alcohol Ligands

The condensation reaction between two chemically different tetranuclear cubane-like clusters [Ni(μ₃ -Cl)(Cl)(HL)]₄ (1; HL=2-methyl-1-(pyridin-2-yl)propan-2-ol) and [Ni(μ₃ -OH)(Cl)(HL)]₄ (2) provides new access to higher nuclearity complexes, as shown with the synthesis of the octanuclear complex [Ni₈ (μ₃ -Cl)₃ (μ₃ -OH)₅ (μ-Cl)₂ Cl₆ (HL)₇ ] (3). In one instance, we also isolated the heptanuclear complex [Ni₇ (μ₃ -Cl)₂ (μ₃ -OH)₆ Cl₄ (H₂ O)₂ (HL)₆ ]Cl₂ (4). Magnetic properties measured by superconducting quantum interference device (SQUID) susceptometry evidenced S=8 and S=5 ground states for complexes 3 and 4, respectively. In particular, alternating current (ac) susceptometry evidenced slow relaxation of the magnetisation for complex 3, and suggests a similar behaviour at sub-kelvin temperatures for complex 4.