Crystal Structures and Luminescence of Two Cadmium‐Carboxylate Cluster‐based Compounds with Mixed Ligands

Reactions of Cd(NO₃)₂ · 4H₂O with 2‐quinolinecarboxylic acid (H‐QLC) in the presence of 1,4‐benzenedicarboxylic acid (H₂‐BDC) or 1,3,5‐benzenetricarboxylic acid (H‐BTC) in DMF/H₂O solvent afforded two compounds, namely, [Cd(QLC)(BDC)_1/2(H₂O)]_n ( 1 ) and [Cd(QLC)(BTC)_1/3]_n ( 2 ). Both compounds are two‐dimensional (2D) frameworks but feature different cadmium‐carboxylate clusters as a result of the presence of the polycarboxylate ligands with different geometries and coordination preference. The dinuclear Cd₂(QLC)₂ units in 1 are bridged by the pairs of bridging water ligands to give a one‐dimensional (1D) chain, which is further linked by the second ligand of BDC^2– to form a 2D structure. Compound 2 is constructed from unique hexanuclear macrometallacyclic Cd₆(QLC)₆ clusters, which are linked by the surrounding BTC^3– ligands to generate a 2D structure. Photoluminescence studies showed both compounds exhibit ligand‐centered luminescent emissions with emission maxima at 405 and 401 nm, respectively.     

Structure and magnetic properties of a copper(II) compound with syn–anti carboxylato- and linear Cu–Cl–Cu chloro-bridges

The copper(II) polymer Cu(2-qic)Cl (2-qic = quinoline-2-carboxylate) was synthesized and then characterized by X-ray crystal structure determination, spectroscopic and magnetic studies. The crystal structure consists of copper(II) ions with two different chromophores: four-coordinated in a square-planar geometry and five-coordinated in an environment between square-pyramidal and trigonal-bipyramidal. The copper ions are bridged sequentially through the carboxylate groups in a syn–anti conformation, forming an infinite one-dimensional zigzag chain with two alternating non-equivalent copper(II) chromophores. The chloride atom acts as a single chloro-bridge link to adjacent chains, forming a ribbon type structure (1D). The variable-temperature (1.8–300 K) magnetic susceptibility data of the complex were interpreted with the dimer law using the molecular field approximation. The results obtained indicate a very weak ferromagnetic (J₂ = 0.37 cm⁻¹) interchain interaction through the syn–anti carboxylate bridge. A relatively strong antiferromagnetic interaction, transmitted through the chloro-bridge with an exchange coupling of J₁ = −57.0 cm⁻¹, dominates the magnetic properties of this complex. The magnitude and the nature of the exchange coupling are explained on the basis of the structural results.     

Co(II)-promoted transformation of diethyl(quinolin-2-ylmethyl)phosphonate to quinoline-2-carboxylate (2-qca): Synthetic,structural and magnetic studies of [Co(2-qca)2(EtOH)2]

The interaction of the diethyl(quinolin-2-ylmethyl)phosphonate (2-qmpe) ligand with CoCl₂ · 2H₂O unexpectedly leads to the formation of a compound with the formula [Co(2-qca)₂(EtOH)₂] (2-qca = quinoline-2-carboxylate). This compound is a product of the oxidative cleavage of the C–P bond in 2-qmpe and the formation of the 2-qca ligand. The title compound was characterized by infrared, ligand field, EPR spectroscopy and low temperature magnetic (1.8–300 K) studies. Particularly, the crystal and molecular structures were determined by the X-ray diffraction. The CoN₂O₄ chromophore shows an elongated octahedron geometry, resulting from the two didentate N,O-bonded chelate ligands and two ethanol molecules – quinolil nitrogen atoms are located in axial positions and oxygen atoms are positioned in the basal plane. The crystal packing is due to hydrogen bonds and π–π stacking interactions, which give rise to a three-dimensional (3D) polymeric network. The magnetic properties reflect the molecular character of the compound, with a very weak magnetic exchange interaction. The moments are enhanced due to an important orbital contribution via spin–orbit coupling.