Crystal structure of trans-diaqua(1,4,8,11-tetraazaundecane)nickel(II) bis(pyridine-2,6-dicarboxylato)nickel(II)

The coordination polyhedra of the nickel(II) ions of the title compound in the complex cation and the anion, viz., trans-NiN4O2 and trans-NiO4N2, are distorted octahedra. In the crystal, the donor groups of the tetraamine and the coordinated water molecules and the carboxylate groups of the pyridine-2,6-dicarboxylate anions are involved in numerous N—H⋯O and O—H⋯O hydrogen bonds, thereby forming sheets of ions lying parallel to the (001) plane.

The asymmetric unit of the title compound, trans-diaqua (1,4,8,11-tetraazaundecane-4 N 1 ,N 4 ,N 8 ,N 11 )nickel(II) bis(pyridine-2,6-dicarboxylato-3 O 2 ,N,O 6 )nickel(II) {[Ni(L)(H 2 O) 2 ] [Ni(pdc) 2 ] where L = 1, 4,8,11-tetraazaundecane (C 7 H 20 N 4 ) and pdc = the dianion of pyridine-2,6-dicarboxylic acid (C 7 H 3 NO 4 2À )} consists of an [Ni(L)(H 2 O) 2 ] 2+ complex cation and a [Ni(pdc) 2 ] 2anion. The metal ion in the cation is coordinated by the four N atoms of the tetraamine ligand and the mutually trans O atoms of the water molecules in a tetragonally elongated octahedral geometry with the average equatorial Ni-N bond length slightly shorter than the average axial  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 2ligands via their carboxylate and nitrogen atom donors in a distorted octahedral trans-NiO 4 N 2 geometry with nearly orthogonal orientation of the planes defining the carboxylate 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 tetraamine, the carboxylic groups of the pdc 2anion and the coordinated water molecules are involved in numerous N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds, leading to electroneutral sheets oriented parallel to the (001) plane.
Multidentate aromatic carboxylates are known as the most common linkers in MOFs (Rao et al., 2004). Although the bridging properties of one of the simplest representative of this class of compounds, 1,3-benzenedicarboxylate, with macrocyclic nickel(II) cations are well studied (see, for example, Tsymbal et al., 2021), coordination polymers based on its structural analogue, pyridine-2,6-dicarboxylate (C 7 H 3 NO 4 2-; pdc 2-), are confined to a sole example (Choi et al., 2003). Interestingly, an attempt to prepare a coordination polymer containing the [Ni(cyclam)] 2+ cation with pdc 2led to the ionic product [Ni(cyclam) [Ni(pdc) 2 ]Á2.5H 2 O due to sequestering of the metal ion from the cavity of the macrocycle by this chelating ligand (Park et al., 2007).
As part of our research on MOFs formed by nickel(II) tetraaza cations and aromatic carboxylates, we report here the synthesis and crystal structure of the product of the reaction of [Ni(L)] 2+ with pdc 2-, namely [trans-diaqua(1,4,8,11-tetraazaundecane-k 4 [Ni(pdc) 2 ], I. Similar to the reaction of pyridine-2,6-dicarboxylate with the [Ni(cyclam)] 2+ cation, the formation of the title compound is explained by the sequestering of the metal ion from the starting cation with the formation of the [Ni(pdc) 2 ] 2anion. Additionally, to the best of our knowledge, the structure of the [trans-diaqua(1,4,8,11-tetraazaundecane)nickel(II)] moiety has not previously been reported in the literature.

Structural commentary
The molecular structure of the title compound I is shown in Fig. 1. Atom Ni1 is coordinated by the two tridentate pdc 2ligands via their carboxylate and nitrogen donors, resulting in the formation of the [Ni(pdc) 2 ] 2divalent anion, which is charge-balanced by the [Ni(L)(H 2 O) 2 ] 2+ divalent cation formed by atom Ni2.
The coordination polyhedron of Ni1 II in the complex anion ion can be described as a tetragonally compressed trans-NiO 4 N 2 octahedron with the Ni-N bond lengths [average value 1.965 (4) Å ] shorter than the Ni-O ones [average value 2.113 (7) Å ] (Table 1). Another source of distortion is the alternating displacement (by ca 0.43 Å ) of the coordinated oxygen atoms of deprotonated carboxylic groups from the mean equatorial plane formed by the four oxygen atoms. The values of the bite angles in the five-membered chelate rings in the complex anion are very similar (Table 1). The pdc 2carboxylate rings are oriented nearly orthogonally with an angle of 81.5 (3) between their mean planes.
The Ni2 II ion in the complex cation is coordinated by the four N atoms of the ligand L and the mutually trans O atoms of the water molecules in a tetragonally elongated trans-NiN 4 O 2 octahedral geometry with the average equatorial Ni-N bond length slightly shorter than the average axial Ni-O bond [2.087 (4) and 2.128 (4) Å , respectively (Table 1)]. The ligand L in I adopts its energetically favored conformation with the five-membered and six-membered chelate rings in gauche and chair conformations, respectively, which resemble 1176 Andriichuk et al. [Ni(C 7  View of the molecular structure of I, showing the partial atom-labeling scheme, with displacement ellipsoids drawn at the 40% probability level. C-bound H atoms are omitted for clarity. Hydrogen-bonding interactions are shown as dotted lines. Table 1 Selected geometric parameters (Å , ).

Supramolecular features
The crystals of I are composed of [Ni(L)(H 2 O) 2 ] 2+ complex cations and [Ni(pdc) 2 ] 2anions connected by numerous hydrogen bonds (Table 2). Each ion is surrounded by four counter-ions (Figs. 2 and 3); the cation acts as the hydrogenbond donor due to the presence of the N-H fragments of amino groups and the O-H groups of coordinated water molecules, while the anion displays proton-acceptor properties because of the availability of the carboxylic groups. These aggregates are further arranged into two-dimensional sheets oriented parallel to the (001) plane (Fig. 4). There are no hydrogen-bonding contacts between the sheets, and the three- Nearest surroundings of the cation in I formed by hydrogen bonding (dotted lines). [Symmetry codes: Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x À 1; y; z; (ii) Àx þ 2; y þ 1 2 ; Àz þ 3 2 ; (iii) Àx þ 1; y þ 1 2 ; Àz þ 3 2 .

Synthesis and crystallization
All chemicals and solvents used in this work were purchased from Sigma-Aldrich and used without further purification. The complex [Ni(L)](ClO 4 ) 2 was prepared by mixing equimolar amount of L and nickel perchlorate hexahydrate in ethanol. The title compound I was prepared as follows. A solution of [Ni(L)](ClO 4 ) 2 (11 mg, 0.026 mmol) in 1 ml of DMF was added to 0.4 ml of an aqueous solution of Na 2 (pdc) (2.7 mg, 0.013 mmol). Blue crystals formed in a day, which were filtered off, washed with diethyl ether and dried in air. Yield: 1.3 mg (15.5%). Analysis calculated for C 21 H 30 N 6 Ni 2 O 10 : C 39.17, H 4.66, N 13.06%. Found: C 39.04, H 5.0, N 13.21%. Single crystals of I suitable for X-ray diffraction analysis were selected from the sample resulting from the synthesis.
Safety note: Perchlorate salts of metal complexes are potentially explosive and should be handled with care.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms in I were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.95 (ring H atoms) or 0.99 Å (aliphatic H atoms), N-H distances of 0.91 (primary amino groups) or 1.00 Å (secondary aminogroups) with U iso (H) values of 1.2U eq of the parent atoms. Water H atoms were positioned geometrically (O-H = 0.71-0.85 Å ) and refined as riding with U iso (H) = 1.5U eq (O). Table 3 Experimental details.

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Crystal data Chemical formula [Ni(C 7

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.