1D Hydrogen-bonded Chains Research Articles

Exploration of one-dimensional hydrogen bonding organic framework (1D-HOF) and charge transfer dynamics in N’-benzylbenzohydrazide: A comprehensive structural and quantum computational investigation

This study presents a comprehensive structural analysis of N’-benzylbenzohydrazide (NBBH) using experimental and computational techniques, shedding light on its non-covalent interactions and charge transfer dynamics. Single crystal X-ray diffraction studies confirm the three-dimensional molecular structure with high precision, revealing an orthorhombic crystal system and non-planarity of the NBBH molecule. Supramolecular interactions, including hydrogen bonds and short intermolecular interactions, are investigated, forming the basis for the assembly of molecular entities into extensive frameworks. Hirshfeld surface analysis provides insights into the intermolecular interactions in the crystal system, with the visualization of hydrogen bonds and short contacts. Energy framework analysis further explores pairwise interaction energies, emphasizing the role of dispersive forces in stabilizing the crystal lattice. Density functional theory studies, including molecular orbital analysis and global chemical reactivity descriptors, offer a deeper understanding of the electronic structure, redox potential, and charge transfer properties of NBBH. Natural bond orbital analysis reveals the charge transfer mechanism within the 1D Hydrogen-Bonded Organic chain, highlighting hyperconjugative interactions contributing to the stabilization of the molecule. Non-covalent interaction analysis using the Reduced Density Gradient method visualizes intra and intermolecular interactions, providing valuable insights into the formation and stability of the 1D network chain in the crystal structure of NBBH. Reorganization energy is discussed in the context of electron and hole transfer processes, emphasizing its importance in understanding the charge transfer dynamics within the molecular framework. The observed inverse relationship between reorganization energy and charge mobility contributes a key insight into the stability and conductivity of the material. The electron charge density distribution analysis, provides a detailed spatial distribution of electrons within the NBBH molecule. The findings have implications for understanding the molecular properties and potential applications of NBBH in various fields.

Proton Conductive Mononuclear Hydrogen-Bonded Cobalt(II) Spin Crossover Complex

The development of molecule-based switchable multifunctional materials remains an important challenge in molecular science. In particular, achieving high proton conductivities in spin crossover (SCO) compounds is of significant interest as the spin state can be controlled via proton transfer induced by an electric field, which leads to magnetoelectric effects and molecular spintronics. However, to the best of our knowledge, no studies have reported on proton-conductive SCO complexes. Thus, herein, we demonstrated the high proton conductivity of a mononuclear hydrogen-bonded Co(II) SCO compound, [Co(COOH-terpy)2](ClO4)2·4H2O (1·4H2O; COOH-terpy = 4′-carboxyl-2,2′:6′,2″-terpyridine). The [Co(COOH-terpy)2] 2+ cation assembled in the solid state via hydrogen-bonding between lattice water molecules and formed a 1D hydrogen-bonded chain. Each 1D chain was linked via CH−π interactions and C–C interactions and produced a complicated 3D supramolecular framework. Magnetic susceptibility and alternative current impedance measurements for 1·4H2O revealed incomplete spin transition behaviors and high-proton-conduction behaviors in identical temperature regions. 1·4H2O exhibited a proton conductivity of 1.68 × 10–3 S cm–1 at 353 K and 98% relative humidity. Notably, this is the first example of a proton-conductive mononuclear SCO compound having the hydrogen-bonding networks. Thus, the results of this study can be used to develop new design guidelines for molecular proton conductors, further leading to the development of multifunctional molecular switching materials.

Exceptionally high temperature spin crossover in amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine iron(ii) complex revealed by variable temperature Raman spectroscopy and single crystal X-ray diffraction.

The synthesis of a novel amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH2) is described. The complex salts [Fe(bppCONH2)2](BF4)2 and [Fe(bppCONH2)2](ClO4)2 were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH2)2](BF4)2 indicate a spin-crossover (SCO) transition with T↑ at 481 K and T↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T↑ at 459 K and T↓ at 445 K with a 14 K hysteresis. For the BF4- salt analysis of low and high-spin state crystal structures at 101, 290 and 500 K, suggest stabilisation of the low spin state due to the formation of 1D hydrogen-bonded cationic chains. Variable temperature Raman studies of the BF4 salt support the presence of a high temperature SCO. It is speculated that the presence of hysteresis may be attributed to differences in the inter-molecular hydrogen bonding in the low spin and high spin states.

Two Hg(II) complexes of 4-pyridinecarbacylamidophosphates: Synthesis, crystal structures, theoretical studies and in vitro antibacterial evaluation

Two new Hg(II) complexes of 4-pyridinecarbacylamidophosphate ligands, HgCl2(L1)2C1 (L1 = 4-NC5H4C(O)NHP(O)[NHC(CH3)3]2); [Hg(L2)Cl3]−[(L2)H]+C2 (L2 = 4-NC5H4C(O)NHP(O)[NHC5H9]2) were synthesized and characterized on the basis of elemental analysis, IR, UV–Vis, and multinuclear NMR spectroscopy. Molecular structures of C1 and C2 were determined by X-ray crystallography and revealed that coordination of ligands to Hg(II) has occurred from the nitrogen atom of the pyridine ring in neutral (C1) and ionic (C2) metal complexes. Coordination of L1 and L2 from Npyridine, as a monodentate ligand, resulted in infinite 1D and 2D hydrogen-bonded polymeric chains for C1 and C2, respectively. Mass spectral data further support the structure of the compounds and XRD indicates the crystalline state of metal complexes. The intermolecular hydrogen bonding interactions which connected the chains into 3D frameworks in both complexes were supported by Hirshfeld surface analysis and fingerprint plots. Also, natural bond orbital analysis divulges that the metal–ligand interaction in C1 and C2 is the LMCT. By using parameters derived from the quantum theory of atoms in molecules, the nature of Hg-ligand interaction is found to be mainly electrostatic with a small amount of covalent character. Hg(II) complexes and the corresponding ligands were also screened for their antibacterial activity against the selected Gram-positive and Gram-negative bacteria. Results indicated that C2 possess strong cytotoxic activity against the tested bacteria with MIC values ranging from 2.79 to 22.32 µg/ml. The activity of compounds was interpreted by their calculated descriptors, density, log P, and the lower unoccupied molecular orbital.

New Copper(II) Coordination Compounds Assembled from Multifunctional Pyridine-Carboxylate Blocks: Synthesis, Structures, and Catalytic Activity in Cycloalkane Oxidation.

Two new copper(II) coordination compounds, namely a 1D coordination polymer [Cu(µ-cpna)(phen)(H2O)]n (1) and a discrete tetracopper(II) derivative [Cu(phen)2(H2O)]2[Cu2(µ-Hdppa)2(Hdppa)2] (2), were hydrothermally synthesized from copper(II) chloride as a metal source, 5-(4-carboxyphenoxy)nicotinic acid (H2cpna) or 5-(3,4-dicarboxylphenyl)picolinic acid (H3dppa) as a principal building block, and 1,10-phenanthroline (phen) as a crystallization mediator. Compounds 1 and 2 were isolated as air-stable microcrystalline solids and fully characterized by elemental and thermogravimetric analyses, IR spectroscopy, powder and single-crystal X-ray diffraction. In the solid state, the structure of 1 discloses the linear interdigitated 1D coordination polymer chains with the 2C1 topology. The crystal structure of an ionic derivative 2 shows that the mono- and dicopper(II) units are extended into the intricate 1D hydrogen-bonded chains with the SP 1-periodic net (4,4)(0,2) topology. Thermal stability and catalytic properties of 1 and 2 were also investigated. In fact, both Cu derivatives act as efficient homogeneous catalysts (catalyst precursors) for the mild oxidation of cycloalkanes by hydrogen peroxide to give the corresponding alcohols and ketones; the substrate scope and the effects of type and amount of acid promoter as well as bond-, regio-, and stereo-selectivity features were investigated.

Two seven-coordinate Mn(II) complexes derived from pentadentate ligands: Structural characterization and investigation of photophysical properties

ABSTRACT Two mononuclear Mn(II) complexes derived from pentadentate ligands were prepared and characterized. Molecular structures of the complexes were determined by X-ray diffraction studies. Mn(II) ions in both structures are seven-coordinate with approximate pentagonal bipyramidal geometry where the pyridinediimine unit and phenolic or alcoholic oxygen atoms (N3O2) of the ligand are located at the pentagonal plane and two exogenous ligands at the axial positions. In the structure of [Mn(HL2)Cl(CH3OH)]·2(CH3OH), methanol solvates link complex molecules into a 1D hydrogen-bonded chain. Emission spectra of the complexes show only one emission band in the 595–625 nm range upon excitation in the 585–615 nm range.

1D hydrogen-bonded infinite chains from tetraaza macrocycle nickel(II) complexes and ligands

The reaction of square planar complex [Ni(L2)]Cl2·2H2O (L2=3,14-diethyl-2,6,13,17-tetraazatricyclo[14,4,01.18,07.12]docosane) with ligands KNCS and H2cpdc generates 1D hydrogen-bonded infinite chains [Ni(L2)(NCS)2] (1) and [Ni(L2)(H-cpdc)2] (2) (H2cpdc=cyclopropanedicarboxylic acid). These compounds have been characterized by X-ray crystallography, spectroscopic, cyclic voltammetry and thermogravimetry. The crystal structures of compounds 1 and 2 show that each nickel(II) centre has an elongated distorted octahedral geometry with the axial ligands. Electronic spectra and redox potentials of the complexes 1 and 2 exhibit a high-spin octahedral environment, which is reflected by the nature of the axial ligands. The TGA behaviors of two compounds 1 and 2 are also significantly affected by the nature of the axial ligands.

Structural Tuning of Anion-Templated Motifs with External Stimuli through Crystal-to-Crystal Transformation.

Protonation of trans-1,2-bis(4-pyridyl)ethylene (4,4'-bpe) with dilute sulfuric acid (33 %) afforded a protonated adduct [{4,4'-bpe⋅2 H+ }2 {HSO4 }-2 {SO4 }-2 {H2 O}2 ] (1). The neighboring olefinic bond in 1 is in a suitable range (3.931-4.064 Å) to undergo a photochemical [2+2] cycloaddition reaction. Upon irradiation with UV light (365 nm), 1 undergoes a molecular sliding involving the 4,4'-bpe⋅2 H+ units, affording 2, stabilized through OSO4 ⋅⋅⋅π interactions. Heating 1 to 50° C leads to a 3D hydrogen-bonded organic framework (HOF) (3). This process occurs through thermal dissociation of the bisulfate anion. Diffusion of iodine through the crystal lattice of 1 and 3 enables the reduction of sulfate to bisulfate, affording a 1D hydrogen-bonded chain (4). Solid-state 13 C CPMAS NMR, IR, DSC, and powder XRD studies further support stimuli-responsive structural tuning through crystal-to-crystal transformation. All these conversions occur with significant translational and rotational movements along with a series of bond-breaking and bond-forming processes.

N-ferrocenyl isonicotinamide, a redox-active 1D hydrogen-bonded chain structure capable of supporting a mixed-valent redox state

N-ferrocenyl isonicotinamide was prepared in 28% isolated yield by the acetylation of aminoferrocence with isonicotinoyl chloride in THF and 61% isolated yield by HOBt mediated DCC coupling in CH2Cl2 and characterized by NMR, IR, ESI-MS, and UV-vis spectroscopy. The structure of Nferrocenyl isonicotinamide, C16H14FeN2O, at 100 K exhibits orthorhombic (Pbca) symmetry. The N-ferrocenyl isonicotinamide's amide group is twisted by 31.7° with respect to the py group and 9.7° relative to the attached Cp ring, in contrast to the planar structure observed for the isomeric (4- Pyridylaminocarbonyl)ferrocene. Adjacent isonicotinamide groups form 1D hydrogen bonded chains through the amide CO and NH groups; this H-bonding network is oriented along the [010] direction. Solid state cyclic voltammetry indicates that this hydrogen bonding network is capable of supporting a class III mixed valent redox state. In contrast, CH2Cl2 and MeCN solutions of the complex exhibit a Fc+/0 redox potential of 420-430 mV vs. Ag/AgCl.

Chiral resolution of l- and d-alanine and a racemic macrocyclic nickel(II) complex: synthesis and crystal structures

The reactions of a racemic four-coordinate Ni(II) complex [Ni(rac-L)](ClO4)2 with l- and d-alanine in acetonitrile/water gave two six-coordinate enantiomers formulated as [Ni(RR-L)(l-Ala)](ClO4)·2CH3CN (1) and [Ni(SS-L)(d-Ala)](ClO4) (2) (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-tetradecane, Ala− = alanine anion), respectively. Evaporation from the remaining solutions gave two four-coordinate enantiomers characterized as [Ni(SS-L)](ClO4)2 (S-3) and [Ni(RR-L)](ClO4)2 (R-3), respectively. Single-crystal X-ray diffraction analyses of complexes 1 and 2 revealed that the Ni(II) atom has a distorted octahedral coordination geometry, being coordinated by four nitrogen atoms of L in a folded configuration, plus one carboxylate oxygen atom and one nitrogen atom of l- or d-Ala− in mutually cis-positions. Complexes 1 and 2 are supramolecular stereoisomers, constructed via hydrogen bonding between [Ni(RR-L)(l-Ala)]+ or [Ni(SS-L)(d-Ala)]+ monomers to form 1D hydrogen-bonded zigzag chains. The homochiral natures of complexes 1 and 2 have been confirmed by CD spectroscopy.

Self-Assembly of a 1D Hydrogen-Bonded Polymer from a Hexamethyltetraaza Macrocyclic Nickel(II) Complex and Isophthalic Acid

The compound [Ni(L)(isoph)2][Ni(L)]·8H2O (1; L = C-meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; H2-isoph = isophthalic acid) has been synthesized and structurally characterized. Complex 1 exhibits a geometrically symmetric core with a {4/6} coordination number set. The coordination environment around the Ni(1) ion is a distorted octahedron, while the geometry around the four-coordinate Ni(2) is depicted as square planar in 1D hydrogen-bonded infinite chain. The compound crystallizes in the triclinic system P-1 with a = 8.602(2), b = 10.684(7), c = 16.550(3) Å, α = 91.04(4), β = 94.09(2), γ = 111.09(4)°, V = 1413.9(10) Å3, Z = 1. The cyclic voltammogram of 1 undergoes one-electron wave corresponding to NiII/NiI process. The electronic spectra, electrochemical and TGA behavior of the complex are significantly affected by the nature of the hexamethyltetraaza macrocycle and the axial isoph2− ligand.

Supramolecular modulation of spin crossover profile in manganese(III)

Six salts of mononuclear manganese(III) complexes with Schiff-base ligands L3–L5 are reported. Solvated complexes with L3, [MnL3]ClO4·0.5MeOH, 3a, and [MnL3]NO3·EtOH, 3b, are low spin (LS) at 80 K and exhibit a gradual and incomplete spin crossover (SCO) on warming to 281 K. Complexes with L4, [MnL4]ClO4, 4a, and [MnL4]NO3, 4b, are predominantly high spin (HS) at RT and undergo gradual and incomplete SCO on cooling. Complexes [MnL5]NO3, 5b, and [MnL5]PF6, 5c, are fully HS over the measured temperature range. Both salts with the dihalogenated ligand L3, 3a and 3b, crystallise with alcohol solvent molecules and exhibit discrete cation–counterion hydrogen bonding and a hydrogen-bonded macrocyclic anion–cation–solvent dimer, respectively. Complex 4a contains two unique complex sites, each with internal rotation symmetry, which assemble into 1D hydrogen-bonded chains at 100 K, which persist on warming to RT. Complex 4b also contains two unique cation sites, without internal symmetry, one of which shows a discrete cation–anion hydrogen bond similar to that of 3a, which dissociates on warming. Complex 5c forms a weak hydrogen bond between complex and counterion, whereas 5b does not participate in any hydrogen bonding.

Magnetic Tuning of All-Organic Binary Alloys between Two Stable Radicals

Mixtures of 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) and 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (BImNN) crystallize as solid solutions (alloys) across a wide range of binary compositions. (F4BImNN)(x)(BImNN)((1-x)) with x < 0.8 gives orthorhombic unit cells, while x ≥ 0.9 gives monoclinic unit cells. In all crystalline samples, the dominant intermolecular packing is controlled by one-dimensional (1D) hydrogen-bonded chains that lead to quasi-1D ferromagnetic behavior. Magnetic analysis over 0.4-300 K indicates ordering with strong 1D ferromagnetic exchange along the chains (J/k = 12-22 K). Interchain exchange is estimated to be 33- to 150-fold weaker, based on antiferromagnetic ordered phase formation below Néel temperatures in the 0.4-1.2 K range for the various compositions. The ordering temperatures of the orthorhombic samples increase linearly as (1 - x) increases from 0.25 to 1.00. The variation is attributed to increased interchain distance corresponding to decreased interchain exchange, when more F4BImNN is added into the orthorhombic lattice. The monoclinic samples are not part of the same trend, due to the different interchain arrangement associated with the phase change.

Lattice Effects on the Spin‐Crossover Profile of a Mononuclear Manganese(III) Cation

Six solvated salts of a mononuclear manganese(III) complex with a chelating hexadentate Schiff base ligand are reported. One member of the series, [MnL]PF(6)⋅0.5 CH(3)OH (1), shows a rare low-spin (LS) electronic configuration between 10-300 K. The remaining five salts, [MnL]NO(3)⋅C(2)H(5)OH(2), [MnL]BF(4)⋅C(2)H(5)OH(3), [MnL]CF(3)SO(3)⋅C(2)H(5)OH (4), [MnL]ClO(4)⋅C(2)H(5)OH (5) and [MnL]ClO(4)⋅0.5 CH(3)CN (6), all show gradual incomplete spin-crossover (SCO) behaviour. The structures of all were determined at 100 K, and also at 293 K in the case of 3-6. The LS salt [MnL]PF(6)⋅0.5 CH(3)OH is the only member of the series that does not exhibit strong hydrogen bonding. At 100 K two of the four SCO complexes (2 and 4) assemble into 1D hydrogen-bonded chains, which weaken or rupture on warming. The remaining SCO complexes 3, 5 and 6 do not form 1D hydrogen-bonded chains, but instead exhibit discrete hydrogen bonding between cation/counterion, cation/solvent or counterion/solvent and show no significant change on warming.

Mechanochemical synthesis of pyrazine:dicarboxylic acid cocrystals and a study of dissociation by quantitative phase analysis

Cocrystals of pyrazine with dicarboxylic acids were synthesised by mechanochemical methods. Crystal structure determination was performed by single crystal and powder X-ray diffraction (PXRD). The cocrystals consist of infinite 1D hydrogen-bonded chains exploiting the familiar pyridine-carboxylic acid synthon. The kinetics of dissociation of the pyrazine:phthalic acid cocrystal was studied at non-ambient conditions using PXRD and quantitative Rietveld phase analysis. We show that the dissociation process is both temperature and humidity dependent providing useful information about a possible dissociation mechanism.

Binuclear vanadium(V) complexes of bis(aryl)adipohydrazone: synthesis, spectroscopic studies, crystal structure and catalytic activity

Three new binuclear vanadium(V) complexes of bis(aryl)adipohydrazones (H4L1 = bis((2-hydroxynaphthalen-1-yl)methylene)adipohydrazide, H4L2 = bis(5-bromo-2-hydroxybenzylidene)adipohydrazide, and H4L3 = bis(2-hydroxy-3-methoxybenzylidene)adipohydrazide) were synthesized by direct reaction of [VO(acac)2] with the hydrazone ligands. The ligands and complexes were characterized by FT–IR, UV–Vis, and NMR spectroscopic methods. The crystal structures of the complexes of L1 and L3 were determined by X-ray analyses. The solid-state structure of the complex of L1 features a 1D hydrogen-bonded chain from N⋯H–O hydrogen bonding. The catalytic activities of these complexes have been tested in the oxidation of various hydrocarbons using H2O2 as the terminal oxidant. Generally, good to excellent conversions have been obtained.

Synthesis, Crystal Structures and Fluorescence Property of Bis(benzimidazol-2-yl)methane Dichloride

The title compound, a salt of C15H13N4 2+2Cl−, was synthesized and determined by X-ray crystallography. It crystallizes in the orthorhombic system, space group Pbcn, with lattice parameters a = 9.973(2) A, b = 9.5012(19) A, c = 15.574(3) A, V = 1475.8(5) A3, Z = 4, C15H13Cl2N4, Mr = 320.19, Dc = 1.441 g/cm3. In the crystal structure, the Cl− anion interact with the same benzimidzaole through two kinds of hydrogen bonds forming a 1D hydrogen-bonded chain, two 1D hydrogen-bonded chain cross with each other and form 2D layer structure. The EA, UV, IR and TG-DTG were studied and the possible structures of the title compound were speculated. Moreover, the fluorescence of the title compound was studied. The results reveal that it can emit purple fluorescence in DMF solvent. A salt of C15H13N4 2+2Cl− was synthesized and determined by X-ray crystallography. In the crystal structure, the Cl− anion interact with the same benzimidzaole through two kinds of hydrogen bonds forming a 1D hydrogen-bonded chain, two 1D hydrogen-bonded chain cross with each other and form 2D layer structure. The title compound can emit purple fluorescence in DMF solvent.

One-Dimensional Hydrogen-Bonded Infinite Chain from Nickel(II) Tetraaza Macrocyclic Complex and 1,2-Cyclopentanedicarboxylate Ligand

The reaction of [Ni(L)]Cl2·2H2O (L = 3,14-dimethyl-2,6,13,17-tetraazatricyclo [14,4,01.18,07.12]docosane) with trans-1,2-cyclopentanedicarboxylic acid (H2-cpdc) yields a 1D hydrogen-bonded infinite chain with formula [Ni(L)(H-cpdc−)2] (1). This complex has been characterized by X-ray crystallography, spectroscopy and cyclic voltammetry. The crystal structure of 1 exhibits a distorted octahedral geometry about Ni atom with four nitrogen atoms of the macrocycle and two oxygen atoms of the H-cpdc− ligand at the axial position. Compound 1 crystallizes in the monoclinic system P21/c with a = 8.7429(17), b = 10.488(2), c = 18.929(4) Å, β = 91.82(2), V = 1734.8(6) Å3, Z = 2. Electronic spectrum of 1 reveals a high-spin octahedral environment. Cyclic voltammetry of 1 undergoes two waves of a one-electron transfer corresponding to NiII/NiIII and NiII/NiI processes.

Photochemical control of dielectric properties based on intermolecular proton transfer in a hydrogen-bonded diarylethene crystal

The photochromic reaction of a diarylethene derivative having imidazoline rings reversibly modulates the dielectric properties of the crystal, which is based on the intermolecular proton transfer in one-dimensional (1D) hydrogen-bonded chains.

Concentration dependent tautomerism in green [Cu(HL1)(L2)] and brown [Cu(L1)(HL2)] with H2L1 = (E)-N′-(2-hydroxy-3-methoxybenzylidene)benzoylhydrazone and HL2 = pyridine-4-carboxylic (isonicotinic) acid

The in situ formed hydrazone Schiff base ligand (E)-N'-(2-hydroxy-3-methoxybenzylidene)benzoylhydrazone (H₂L¹) reacts with copper(II) acetate in ethanol in the presence of pyridine-4-carboxylic acid (isonicotinic acid, HL²) to green-[Cu(HL¹)(L²)]·H₂O·C₂H₅OH (1) and brown-[Cu(L¹)(HL²)] (2) complexes which crystallize as concomitant tautomers where either the mono-anion (HL¹)⁻ or di-anion (L¹)²⁻ of the Schiff base and simultaneously the pyridine-carboxylate (L²)⁻ or the acid (HL²) (both through the pyridine nitrogen atom) function as ligands. The square-planar molecular copper(II) complexes differ in only a localized proton position either on the amide nitrogen of the hydrazone Schiff base in 1 or on the carboxyl group of the isonicotin ligand in 2. The proportion of the tautomeric forms in the crystalline solid-state can be controlled over a wide range from 1:2 = 95 : 5 to ∼2 : 98 by increasing the solution concentration. UV/Vis spectral studies show both tautomers to be kinetically stable (inert), that is, with no apparent tautomerization, in acetonitrile solution. The UB3LYP/6-31+G* level optimized structures of the two complexes are in close agreement with experimental findings. The solid-state structures feature 1D hydrogen-bonded chain from charge-assisted O((-))H-N and O-H((-))N hydrogen bonding in 1 and 2, respectively. In 1 pyridine-4-carboxylate also assumes a metal-bridging action by coordinating a weakly bound carboxylate group as a fifth ligand to a Cu axial site. Neighboring chains in 1 and 2 are connected by strong π-stacking interactions involving also the five- and six-membered, presumably metalloaromatic Cu-chelate rings.