Crystal structure of trans-di-aqua-(1,4,8,11-tetra-aza-undeca-ne)nickel(II) bis-(pyridine-2,6-di-carboxyl-ato)nickel(II)

Abstract

The asymmetric unit of the title compound, trans-di-aqua-(1,4,8,11-tetra-aza-undecane-κ⁴ N ¹,N ⁴,N ⁸,N ¹¹)nickel(II) bis-(pyridine-2,6-di-carboxyl-ato-κ³ O ²,N,O ⁶)nickel(II) {[Ni(L)(H₂O)₂][Ni(pdc)₂] where L = 1,4,8,11-tetra-aza-undecane (C₇H₂₀N₄) and pdc = the dianion of pyridine-2,6-di-carb-oxy-lic acid (C₇H₃NO₄ ^2-)} consists of an [Ni(L)(H₂O)₂]²⁺ complex cation and a [Ni(pdc)₂]^2- anion. The metal ion in the cation is coordinated by the four N atoms of the tetra-amine ligand and the mutually trans O atoms of the water mol-ecules in a tetra-gonally elongated octa-hedral geometry with the average equatorial Ni-N bond length slightly shorter than the average axial Ni-O bond [2.087 (4) versus 2.128 (4) Å]. The ligand L adopts its energetically favored conformation with five-membered and six-membered chelate rings in gauche and chair conformations, respectively. In the complex anion, the Ni^II ion is coordinated by the two tridentate pdc^2- ligands via their carboxyl-ate and nitro-gen atom donors in a distorted octa-hedral trans-NiO₄N₂ geometry with nearly orthogonal orientation of the planes defining the carboxyl-ate rings and the average Ni-N bond length [1.965 (4) Å] shorter than the average Ni-O bond distance [2.113 (7) Å]. In the crystal, the NH donor groups of the tetra-amine, the carb-oxy-lic groups of the pdc^2- anion and the coordinated water mol-ecules are involved in numerous N-H⋯O and O-H⋯O hydrogen bonds, leading to electroneutral sheets oriented parallel to the (001) plane.

Keywords: crystal structure; cyclam; hydrogen bonds; nickel; pyridine-2,6-di­carboxyl­ate.