Crystal structure of catena-poly[(μ₂-1,4-bis(1H-benzimidazol-1-yl)-2-butene-κ²N:N′)-(μ₂-homophthalate-κ⁴O,O′:O″,O‴)nickel(II)] dihydrate, C₂₇H₂₆NiN₄O₆

1 Source of material

With NaOH aqueous solution (0.5 M, 0.1 mL) added for deprotonation of homophthalic acid, a mixture consisting of 1,4-bis(1H-benzimidazol-1-yl)-2-butene (bbib, 28.8 mg, 0.1 mmol), homophthalic acid (H₂ hmph, 9.0 mg, 0.1 mmol), Ni(OAc)₂.4 H₂O (49.8 mg, 0.2 mmol) was dissolved in a mixed solvent of H₂O (5 mL) and N,N-dimethylformamide (1 mL). This solution was transferred into a 23 ml Teflon lined stainless steel reactor, which then was placed in a oven and heated to 393 K under autogenous pressure for 72 h. Once the reaction time elapsed, the reactor was removed from the oven and allowed to cool naturally to room temperature. As a result, green block crystals of the title compound were obtained.

2 Experimental details

A suitable crystal was selected elaborately and mounted on a EosS2 diffractometer. Using Olex2, ² the structure was solved with the ShelXT structure solution program and refined with the ShelXL refinement package. ^{3 , 4} All hydrogen atoms were modeled at their calculated positions and included in the refinement via the riding model. The U_iso of the H-atoms were constrained to 1.2 times U_eq of their bonding carbon atoms and 1.5 times U_eq of their bonding oxygen atoms for the hydrogen atoms in water molecules.

3 Discussion

As we all known, organic ligands with suitable functional moieties play a vital role in the design, self-assemblies and properties of coordination polymers (CPs). ^{5 , 6} Especially, multi-carboxylate ligands and N-heterocyclic ligands with outstanding coordination abilities have been widely used to construct the expected CPs. ^{7 , 8 , 9} The semi-rigid bis(benzimidazole) ligands with butenyl spacer, such as 1,4-bis(benzimidazol-1-yl)-2-butylene, 1,4-bis(2-methylbenzimidazol-1-yl)-2-butylene and 1,4-bis(5,6-dimethylbenzimidazol-1-yl)-2-butylene, are N-heterocyclic derivatives containing two imidazole rings, two benzene rings and one flexible butenyl group. The conformations of these bis(benzimidazole) ligands maybe bend or distort partly to meet the coordination requirements, owing to the free rotation possibility of benzimidazole rings around butenyl group. ^{10 , 11} Meanwhile, imidazole ring can act as donor to form hydrogen bond interaction through the sp³ hybridized nitrogen atom. In addition, there is a large conjugated π system among the imidazole ring and the benzene ring, benefitting the π-π stacking interaction in the self-assembly process. So, these semi-rigid bis(benzimidazole) ligands have been employed in the preparation of mixed-ligand CPs, with the potential application in catalysis, luminescent sensing, biomedicine and so on. ^{12 , 13 , 14 , 15 , 16}

The titled complex is a neutral polymer with double-strand chain structure. As shown in the figure, the asymmetric unit contains one crystallographically independent Ni(II) cation, one completely deprotonated hmph dianion, one bbib ligand, one uncoordinated water molecule and two uncoordinated hemi-water molecules. Each Ni1 atom is six-coordinated, exhibiting a weakly distorted octahedral configuration by four oxygen atoms (O1, O2, O3#1, O4#1) from two symmetry-related hmph dianions, and two nitrogen atoms (N1, N4#1) from two symmetry-related bbib ligands (symmetry codes: #1: 2-x, 1-y, 1-z). Within the [NiO₄N₂] polyhedron, the basal plane is defined by O1, O2, O4#1 and N1 atoms, while the apical sites are occupied by O3#1 and N4#1 atoms. Four Ni–O distances range from 2.0796(19) Å to 2.192(2) Å. The Ni–N distances are 2.038(2) Å and 2.047(2) Å, respectively.

Two carboxylate groups of hmph dianion both adopt chelating bidentate coordination mode. As a result, neighbor [NiO₄N₂] octahedra are spaced by two μ₂-hmph dianions to produce one dinuclear species with the Ni⋯Ni seperation of 5.7595(7) Å. These dinuclear subunits are then propagated through μ₂-bibb ligands to generate a double-strand chain along the a direction.

There are abundant hydrogen-bonding interactions among two carboxylate groups of hmph dianions and three uncoordinated water molecules. And neighbor double-strand chain motifs are further induced to a two dimensional layer lying the ab plane by these hydrogen bonds. Additionally, intra-layer off-set π-π stacking interactions are observed between the benzene rings and imidazole rings of bibb molecules, characterized by a centroid-centroid distance of 3.7860(1) Å. Detailed calculations reveal that the centroid to face and slippage distances are 3.4114 Å and 1.64 Å, respectively.