The Impact of Supramolecular Forces on the Magnetic and Optical Properties of Bis(2-amino-6-bromopyridinium) Tetrachloridocuprate (C5H6BrN2)2[CuCl4]

Abstract

The organic/inorganic hybrid compound bis(2-amino-6-bromopyridinium) tetrachloridocuprate(II) (HABPy)2[CuCl4] was synthesized in crystalline form in a 77% yield from aqueous HCl solutions containing Cu(OAc)2 and 2-amino-6-bromopyridine (ABPy). Single-crystal X-ray diffraction analysis revealed that the compound crystallizes in the monoclinic, centrosymmetric space group C2/c. The Cu atom shows a distorted tetrahedral coordination geometry with a τ4 value of 0.69 (τ4 = 1 for a perfect tetrahedron). The structure consists of organic (HABPy)+ cation layers at z = 0 and z = ½, alternating with inorganic [CuCl4]2− dianion layers at z = ¼ and z = ¾. These layers, parallel to the (001) plane, are interconnected by a plethora of supramolecular forces such as N–H···Cl hydrogen bonds, forming a three-dimensional network. Powder X-ray diffraction confirmed the purity of the synthesized bulk material. Fourier-transform infrared (FT-IR) spectroscopy and Raman spectroscopy support the protonation of the pyridine N atom. Hirshfeld surface analysis allowed us to further study the supramolecular forces in the crystal structure. The material shows purely paramagnetic behavior according to S = ½ with an effective magnetic moment µeff of 1.85 µB and a g factor of 2.14, in keeping with magnetically isolated [CuCl4]2− dianions. UV-vis diffuse reflectance spectroscopy of the orange-red material showed a tiny band at 314 nm and an intense band peaking at 622 nm. The optical gap was found to be 2.25 eV. The photoluminescence spectrum shows a partially structured band with maxima at 416 and 436 nm when irradiating at 370 nm, the wavelength of the maximum band found in the excitation spectrum.