Crystal structure of trans-(1,8-dibutyl-1,3,6,8,10,13-hexaazacyclotetradecane-κ4 N 3,N 6,N 10,N 13)bis(5-methyltetrazolato-κN)nickel(II) from synchrotron data

The NiII ion in the title compound shows a slightly distorted octahedral coordination geometry with four N atoms of the azamacrocylic ligand and two N atoms of the 5-methyl-1H-tetrazolate ions. In the crystal, molecules are connected by an N—H⋯N hydrogen bond, forming a supramolecular sheet structure.

The structure of the title compound, [Ni(C 2 H 3 N 4 ) 2 (C 16 H 38 N 6 )], has been characterized from synchrotron radiation. The asymmetric unit consists of one half of the Ni II complex molecule, which is related to the other half-molecule by an inversion center. The Ni II ion is coordinated by four secondary N atoms of the macrocyclic ligand in a square-planar fashion in the equatorial plane and by two N atoms of the 5-methyltetrazolate anions in axial positions, resulting in a tetragonally distorted octahedral geometry. The average equatorial Ni-N bond length [2.060 (8) Å ] is shorter than the axial Ni-N bond length [2.2183 (11) Å ]. An intramolecular N-HÁ Á ÁN hydrogen bond between the secondary amine N atom of the macrocyclic ligand and the non-coordinating N atom of the 5methyltetrazolate ion stabilizes the molecular structure. Moreover, an intermolecular N-HÁ Á ÁN hydrogen bond between the macrocyclic ligand and 5methyltetrazolate group gives rise to a supramolecular sheet structure parallel to the bc plane.

Chemical context
Coordination compounds with macrocyclic ligands have been studied widely in chemistry, metalloenzymes and materials science (Lehn, 1995). In particular, Ni II macrocyclic complexes having vacant sites in the axial positions are good building blocks for assembling supramolecular frameworks (Min & Suh, 2001), with potential applications in gas adsorption/ desorption (Lee & Suh, 2004), carbon dioxide reduction (Froehlich & Kubiak, 2012) and chiral separation (Ryoo et al., 2010). For example, Ni II complexes with tetra-azamacrocyclic ligands have been studied as catalysts for water oxidation at neutral pH (Zhang et al., 2014) and their magnetic properties have been investigated with various auxiliary anionic moieties such as azide, dicyanamide and ferricyanide (Yuan et al., 2011). Moreover, tetrazole derivatives are versatile anions which can easily bridge to transition metal ions, thus allowing the assembly of multi-dimensional compounds (Zhao et al., 2008).

Structural commentary
In the title compound, the coordination environment around the Ni II ion, in which the Ni II ion lies on an inversion center, has a tetragonally distorted octahedral geometry. The Ni II ion is bonded to four secondary N atoms of the azamacrocyclic ligand in a square-planar fashion in the equatorial plane, and to two N atoms from the 5-methyltetrazolate anions at the axial positions, as shown in Fig. 1. The average Ni-N eq bond length and the Ni-N ax length are 2.060 (8) and 2.2183 (11) Å , respectively. The axial bond lengths are much longer than the equatorial bond lengths, which can be attributed to a rather large Jahn-Teller distortion of the Ni II ion and/or ring contraction of the azamacrocyclic ligand (Halcrow, 2013). The six-membered chelate rings adopt chair conformations and the five-membered chelate rings assume gauche conformations (Min & Suh, 2001). The N-N bond lengths in the 5-methyltetrazolate ion range from 1.3182 (15) to 1.3543 (16) Å , indicating that the tetrazolate ring is fully delocalized. An intramolecular N-HÁ Á ÁN hydrogen bond between the secondary amine group of the macrocyclic ligand and the N atom of the 5-methyltetrazolate ion stabilizes the molecular structure ( Fig. 1 and Table 1).

Supramolecular features
The packing in the structure involves an intermolecular N-HÁ Á ÁN hydrogen bond between the secondary amine group of the macrocyclic ligand and the non-coordinating N atom of the 5-methyltetrazolate ion (Table 1), which forms a rigid supramolecular sheet structure parallel to the bc plane (Fig. 2).

Database survey
A search of the Cambridge Structural Database (Version 5.35, May 2014 with 3 updates; Groom & Allen, 2014) indicated that 71 Ni II azamacrocyclic complexes with alkyl pendant groups have been reported. These complexes with various alkyl pendant groups were investigated as good building blocks for supramolecular chemistry and also studied for their magnetic properties and gas sorption abilities due to the anions such as cyanido groups and carboxylic acid derivatives (Hyun et al., 2013;Shen et al., 2012). No corresponding Ni II azamacrocyclic complex with a butyl pendant group and tetrazole derivatives has been reported, and the title compound was newly synthesized for this research.

Synthesis and crystallization
The title compound (I) was prepared as follows. The starting complex, [Ni(C 16 H 38 N 6 )(ClO 4 ) 2 ], was prepared by a slight modification of the reported method (Jung et al., 1989). To an MeCN (10 mL) solution of [Ni(C 16  View of the molecular structure of the title compound, showing the atomlabelling scheme, with displacement ellipsoids drawn at the 50% probability level. H atoms bonded to C atoms have been omitted for clarity. Intramolecular N-HÁ Á ÁN hydrogen bonds are shown as green dashed lines. [Symmetry code: (i) Àx + 1 2 , Ày + 3 2 , Àz + 1.] Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
View of the crystal packing of the title compound, with N-HÁ Á ÁN hydrogen bonds drawn as green (intramolecular)