Crystal structure of a nickel compound comprising two nickel(II) complexes with different ligand environments: [Ni(tren)(H2O)2][Ni(H2O)6](SO4)2

The title compound, [Ni(tren)(H2O)2][Ni(H2O)6](SO4)2, consists of two octahedral nickel complexes within the same unit cell. It co-precipitates with the starting material, [Ni(H2O)6](SO4). The crystals of the title compound are purple, different from the [Ni(H2O)6](SO4) crystals, which are turquoise.

The title compound, diaqua[tris(2-aminoethyl)amine]nickel(II) hexaaquanickel(II) bis(sulfate), [Ni(C 6  , which are turquoise. The reaction was performed both in a 1:1 and 1:2 metal-ligand molar ratio, always yielding the co-precipitation of the two types of crystals. The asymmetric unit of the title compound, which crystallizes in the space group Pnma, consists of two half Ni II complexes and a sulfate counteranion. The mononuclear cationic complex [Ni(tren)

Chemical context
Tris(2-aminoethyl)amine (tren) has been used extensively as an ancillary tripodal ligand for capping transition metals to form mononuclear and polynuclear complexes. The tren ligand has the capacity to chelate metal ions through its central tertiary amine and through its three terminal amine groups in a spider-like conformation, leaving one or two positions available for additional ligand coordination (Marzotto et al., 1993;Albertin et al., 1975;Blackman, 2005;Brines et al., 2007). Metal complexes with a variety of ligands in which also tren is coordinating to the metal center have been proposed for applications in catalysis (Ruffin et al., 2017), sensors, and as precursors of bioinorganic reactions (Sakai et al., 1996). For instance, Ni(tren) complexes have been proposed for applications in biological systems (Salam & Aoki, 2001) or as a model to study enantioselective synthesis or asymmetric catalysis (Rao et al., 2009), and as coordination polymers in magnetism, electrical conductivity and ion exchange (Park et al., 2001;Tanase et al., 1996).

Structural commentary
The Ni2 ion of the mononuclear complex [Ni(H 2 O) 6 ] 2+ also shows an octahedral geometry. In the asymmetric unit, the atom Ni2 sits on an inversion center on a screw axis along the b-axis direction. The Ni2-O water bond lengths with O3, O4 and O5 range between 2.051 (1) and 2.074 (1) Å , respectively, with angles of 180 due to symmetry.

Figure 1
View of the molecular structure of the title compound with displacement ellipsoids drawn at the 20% probability level and labeling scheme for the symmetry-independent atoms. The CH 2 hydrogen atoms have been omitted for clarity. The symmetry operations generating the equivalent atoms are 1 À x, 1 À y, 2 À z and x, 1

Synthesis and crystallization
The synthesis of the title compound is summarized in the reaction scheme shown in Fig. 3. NiSO 4 Á6H 2 O and tris(2aminoethyl)amine (tren) were used without further purification. A methanolic solution of NiSO 4 Á6H 2 O (0.0265 g, 0.1 mmol) was added slowly to a tren (0.0146 g, 0.1 mmol) solution (4 mL MeOH) at room temperature. The resulting solution was stirred for two h and it changed color from light green to purple. The solution was then filtered through celite and evaporated under reduced pressure. Single crystals of the title compound were obtained by vapor diffusion of methanol into 2-propanol. In the crystallization process, two types of crystal were formed: the starting reagent hexahydrate nickel (II) complex (turquoise crystals) and the nickel(II) tren complex (purple crystals, Fig. 4). The reaction was performed both in a 1:1 and 1:2 metal-ligand molar ratio, always yielding the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were included in geometrically calculated positions for the alkyl and amine groups using a riding model: C-H = 0.97 Å and N-H = 0.89 Å with U iso (H) =1.2U eq (C, N). The hydrogen atoms of the water molecules were located from the difference-Fourier map; they were refined freely in the case of O1 and O2, with a DFIX of 0.85 (2)    Computer programs: CrysAlis PRO (Rigaku OD, 2015), olex2.solve (Bourhis et al., 2015), SHELXL2016 (Sheldrick, 2015) and OLEX2 (Dolomanov et al., 2009