Complexes with lignin model compound vanillic acid. Two different carboxylate ligands in the same dinuclear tetracarboxylate complex [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2]

Two copper(II) coordination compounds with vanillic acid C8H8O4 (1), namely [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2] (2) and [Cu2(C8H7O4)4(H2O)2] (3), were synthesized and characterized. Single crystals of 1–3 were obtained and their crystal structures determined. The structure of 2 shows dinuclear cage structure of copper acetate hydrate type, however with two different carboxylates, acetates and vanillic acid anions, respectively. Both bridging anions are in pairs in trans orientation. Methanol molecules are apically coordinated (Cu–O7 2.160(2) Å), fulfilling square-pyramidal coordination sphere around both copper ions. The compound 2 decomposes outside mother-liquid (yielding [Cu2(C8H7O4)2(O2CCH3)2(H2O)2] (2a)) with the removal of methanol, but without significant change of the dicopper tetracarboxylate cage structure, as noticed by μeff 1.48 BM for 2a. Similar was found also in the X-band EPR spectra with three signals Hz1, H⊥2 and Hz2 in the region from 0 to 600 mT. The structure of free vanillic acid 1 is composed of dimeric units of two molecules, connected by two parallel hydrogen bonds between carboxylate group of each other (O1–H⋯O2 2.642(3) Å), while the structure of 3 is of [Cu2(O2CCH3)4(H2O)2] type. Interestingly, an additional signal in the EPR spectra of 3 is found at 80 mT (H⊥1) at 298 and at 116 K, next to three signals Hz1, H⊥2 and Hz2.

Graphical abstractTwo different carboxylate anions were found in the dinuclear copper(II) tetracarboxylate [Cu2(C8H7O4)2(O2CCH3)2(CH3OH)2] (heterocarboxylate). The compound decomposes outside mother-liquid, by replacement of methanol with water molecules. EPR spectra and μeff < 1.73 BM confirm the dinuclear cage structure of the decomposed complex. The compound is compared to a related homocarboxylate complex [Cu2(C8H7O3)4(H2O)2]. In the EPR spectra of this complex, six signals of the triplet state S = 1 (Hz1, H⊥1 (Hx1, Hy1), H⊥2 (Hx2, Hy2), Hz2) are observed.Figure optionsDownload full-size imageDownload as PowerPoint slide