Synthesis and Structure of (Bis(1-propylbenzimidazol-2-yl)-trans-1,2-cyclohexane)copper(II) Dichloride and the Structure of a Novel, Unexpected By-Product

Abstract

Two structures containing the novel ligand bis(1-propylbenzimidazol-2-yl)-trans-1,2-cyclohexane, L, are reported. The first structure, (bis(1-propylbenzimidazol-2-yl)-trans-1,2-cyclohexane)copper(II) dichloride, Cu(II)(L)Cl₂ 1, crystallized in the monoclinic space group P 2₁/n with a = 10.992(4) Å, b = 13.576(5) Å, c = 18.047(7) Å, β = 106.909(8) and V = 2576.7(16) ų and Z = 4. Complex 1 exhibits a D _2d-flattened tetrahedral Cu(II)N₂Cl₂ coordination geometry, for which the four-coordinate geometry index τ₄ of 0.59 lies almost midway between the ideal geometry values of zero (square-planar) and 1 (tetrahedral). The second and unexpected structure, (bis(1-hydro-3-propylbenzimidazol-1-ium)-trans-1,2-cyclohexane)(μ⁴-oxo-hexa-(μ²-chloro)-tetra(chloro-copper(II)) di(nitromethane) solvate, 2[H₂L]²⁺[Cu₄OCl₁₀]⁴⁻·(CH₃NO₂)₂, 2, consists of three dimensionally hydrogen bonded bis(benzimidazolium) dications and tetra-copper(II)-oxo-chloride tetra-anions, interspersed with nitromethane solvate molecules. A balanced equation for the formation of the byproduct suggests that it forms via an acid–base reaction in the presence of water in the reaction mixture. The complex crystallized in the triclinic space group P −1 with a = 13.696(3) Å, b = 13.819(3) Å, c = 16.797(4) Å, α = 89.480(4), β = 87.308(4)°, γ = 89.434(4) and V = 3175.3(12) Å³ and Z = 2. Ligand L contains two stereogenic centers, each of the same handedness, and, since the space groups determined for structures 1 and 2 are centrosymmetric, both structures are racemates.

Graphical Abstract

Both title complexes contain the ligand bis(1-propylbenzimidazol-2-yl)-trans-1,2-cyclohexane. The first structure corresponds to the target complex with D _2d-flattened tetrahedral Cu(II)N₂Cl₂ coordination. For this structure, the four-coordinate geometry index τ₄ of 0.59 lies almost midway between the ideal geometry values of zero (square-planar) and 1 (tetrahedral). The second and unexpected structure is a minor by-product which contains two hydrogen bonded diprotonated ligand dictations and a copper-oxo tetra-anion interspersed with nitromethane. A balanced equation for the formation of the byproduct suggests that it forms via an acid–base reaction in the presence of water in the reaction mixture.