Journal of Molecular Structure 1258 (2022) 132646 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstr Structural characterization of products in the Ni(II) – 2-oximino-2-cyan-N-piperidineacetamide (HPiPCO) system  Adedamola A. Opalade a , Oleksandr Hietsoi b , Nikolay Gerasimchuk a,∗ a Department of Chemistry, Temple Hall 456, Missouri State University, Springfield, MO 65897, United States b Department of Chemistry, Middle Tennessee State University, Davis Science Building, Office 113, Murfreesboro, TN 37132, United States a r t i c l e i n f o Article history: Received 4 December 2021 Revised 14 February 2022 Accepted 15 February 2022 Available online 18 February 2022 Keywords: Synthesis Cyanoximes Ni(II) complexes X-ray analysis Thermal analysis a b s t r a c t 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 ]·H 2 O (2), [Ni 3 (PiPCO) 6 ·(CH 3 CN) n ] (3) and [Ni 4 (OH) 2 (NO 3 ) 2 (PiPCO) 4 (H 2 O) 6 ] ·2H 2 O (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 13 C{ 1 H} 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 P2 1 2 1 2 1 , 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. © 2022 Elsevier B.V. All rights reserved. 1. Introduction Divalent metals ions of the Ni-triade have a long history of forming a variety of complexes [1] with coordination numbers ranging from 4 to 8, with the most common shape being square- planar. The nd 8 electronic configuration is highly favored for the low-spin square planar arrangement, although tetrahedral, penta- coordinate, and octahedral complexes for Ni are well known as well [2]. Inorganic, organometallic and Werner-type coordination chemistry of Nickel is well developed with chelating ligands and particularly with oximes-based small molecules, with the most fa- mous being magenta-colored Ni(II) dimethylglyoximate, Ni(DMG) 2 , that was historically used in analytical chemistry for quantitative  Dedicated to Dr. Tatyana Shunyakova on occasion of her 50th birthday. ∗ Corresponding author. E-mail addresses: NNGerasimchuk@MissouriState.edu, nick.gerasimchuk@gmail.com (N. Gerasimchuk). determination of this metal [3]. Not much is known, however, about complexes of Ni(II) with cyanoximes. The latter represent a new subclass of oximes that have received intense investigations in recent years because of their biological activity [4–7] and am- polydentate ligands’ properties [8–10]. There are 48 cyanoxime lig- ands that are currently known and being studied with only a small group acting as chelating agents in complexes with transition met- als [11–13]. (SI 1: Electronic Supporting Information section). Recently, we reported several Pt-based complexes with biden- tate chelating cyanoximes [14,15] which formed 1D mixed va- lence ‘poker chips’ stacks that showed semiconductivity and an in- tense emission in the NIR region beyond 1000 nm [16]. The ob- served strong NIR emission [14,15], plus a strong propensity for the formation of square-planar columnar oximes-based Ni(II) com- plexes [17–21], inspired further investigation of cyanoximes-based complexes of nickel(II) as a significantly less expensive and bio- logically by far more friendly [22] alternative to the other two metals of the group X triade. In our previous study [23,24] we https://doi.org/10.1016/j.molstruc.2022.132646 0022-2860/© 2022 Elsevier B.V. All rights reserved.