CHEMISTRY JOURNAL OF MOLDOVA. General, Industrial and Ecological Chemistry. 2023, 18(2), 53-60 ISSN (p) 1857-1727 ISSN (e) 2345-1688 http://cjm.ichem.md https://doi.org/10.19261/cjm.2023.1074 © Chemistry Journal of Moldova CC-BY 4.0 License CRYSTAL STRUCTURE AND NMR SPECTROSCOPIC CHARACTERIZATION OF 1,5-BIS(2-HYDROXY-3- METHOXYBENZYLIDENE)CARBONOHYDRAZIDE Natalia Talmaci a , Diana Dragancea a,b* , Elena Gorincioi a , Pavlina Bourosh c , Victor Kravtsov c a Institute of Chemistry, Moldova State University, 3, Academiei str., Chisinau MD 2028, Republic of Moldova b “C. D. Nenitzescu” Institute of Organic and Supramolecular Chemistry of the Romanian Academy, 202B, Splaiul Independentei str., Bucharest 060023, Romania c Institute of Applied Physics, Moldova State University, 5, Academiei str., Chisinau MD 2028, Republic of Moldova * e-mail: ddragancea@gmail.com Abstract. The solid-state structure of a symmetrical carbohydrazone, namely 1,5-bis(2-hydroxy-3- methoxybenzylidene)carbonohydrazide was determined by X-ray single crystal diffraction method. Compound 1 crystallizes in the monoclinic space group P2 1 /n with unit cell parameters a= 10.1198(6), b= 22.7847(11), c= 15.1738(10) Å, β= 100.458(6)°, Z= 4, V= 3440.6(3) Å 3 , R 1 = 0.0540. Crystal structure of 1 is defined by two crystallographic independent molecules, which are bonded via N–H···O hydrogen bond. The organic molecules are as keto tautomers with respect to t he carbamide fragment, and adopt the anti conformation. 1D and 2D NMR experiments have argued on the presence of the title compound in DMSO-d 6 solution mostly as keto tautomer in syn conformation, and enol-imino form when considering o-vanillin residue. Keywords: carbohydrazide, o-vanillin, syn-anti isomer, X-ray diffraction, NMR spectroscopy. Received: 22 May 2023/ Revised final: 25 August 2023/ Accepted: 29 August 2023 Introduction Due to its two terminal amino groups, carbohydrazide can serve as a precursor for the synthesis of ditopic polydentate Schiff base ligands through an easy condensation reaction with appropriate aldehydes/ketones, containing pyridine, carboxylate, oxime functional groups. Endowed by conformational and tautomeric flexibility, these ligand systems displayed rich coordination flexibility towards both d- and f-block ion metals [1-7]. Via the condensation reaction of carbohydrazide with 2-hydroxybenzaldehyde results 1,5-bis(salicylidene)carbohydrazide, a chelating agent that has been reported to generate a number of coordination compounds, ranging from mono- to polynuclear ones, with both discrete and polymeric structures [8-16]. 1,5-Bis(2-hydroxy-3-methoxybenzylidene) carbonohydrazide (1), formed by the reaction between 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) and carbohydrazide, looks as a promising ligand due to several potential N,O-coordination sites, as it contains additional donor atoms placed in suitable positions, the fact that could lead to increasing of its topicity. Indeed, it proved its proficiency in construction of dinuclear and nonanuclear dysprosium clusters [17,18]. The ligand also imposed asymmetrical coordination with ONO and ONN chelating compartments generating discrete and polymeric ensembles based on asymmetrical dinuclear Mo(VI) building blocks [19]. Depending on the reaction conditions, this compound may adopt different geometric configurations syn or anti, related to rotation around simple C-N bond in the amide C(O)-NH fragment (Scheme 1). The aim of this study was to prove the formation of a distinct new solid form of a known compound, 1,5-bis(2-hydroxy-3- methoxybenzylidene)carbonohydrazide (1), and to follow the supramolecular organization within the crystal by means of intermolecular weak bonds. Herein, the molecular and crystal structures of 1 are presented, that showed a solvatomorphus with a conformation different from the one reported in the literature, which crystallizes as a syn conformer [20]. The relative stabilities of the obtained crystalline form and its conversion were investigated through a number of solution NMR experiments and the obtained results were compared with the available NMR data for syn-1,5-bis(2-hydroxy-3-methoxybenzylidene) carbonohydrazide and its congeners. 53