Abstract
High-yield synthesis of the initial cyanoxime N-piperidine-2-cyano-2-oximino-acetamide, HPiPCO (later as 1) and its Ni-complexes of [Ni(PiPCO)2]·H2O (2), [Ni3(PiPCO)6·(CH3CN)n] (3) and [Ni4(OH)2(NO3)2(PiPCO)4(H2O)6] ·2H2O (4) composition are presented. The cyanoxime represents weak organic acid that forms yellow anion 1− upon deprotonation which readily reacts with Ni(II) ions in aqueous solutions forming Werner-type complexes. The initial cyanoxime was studied using common spectroscopic techniques including variable temperature 13C{1H} NMR. All obtained compounds were characterized by thermal analysis. Crystal structures were determined for 1, 3 and 4. In this work we report the first structural characterization of transition metals complexes in a system containing this particular cyanoxime ligand. The starting ligand 1 was found to exist in crystal state as a mixture of two diastereomers: commonly observed trans-anti (33%) isomer and rare, but dominant in this case, trans-syn (67%) isomer. This cyanoxime crystallized in different from previously reported structure of this compound in non-centrosymmetric space group P212121, and represents another polymorph of HPiPCO. However, in complexes 3 and 4, the cyanoxime adopts only cis-anti geometry which implies segregation between the two isomers during complex formation selecting the most sterically unstrained, favorable and stable geometry of the anion in metal complexes for the metal chelation. These complexes represent polynuclear compounds containing three Ni(II) centers in 3 and four metal centers in 4. In two metal complexes that are structurally characterized, the anion acts simultaneously as a chelating and bridging acidoligand that forms five-membered rings with N,O donor atoms that extends to other metal ion via O-atom of coordinated oxime group of the cyanoxime. Despite bridging function of cyanoximes, all metal complexes represent molecular compounds, not coordination polymers.
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