[μ-N,N,N′,N′-Tetrakis(2-pyridylmethyl)butane-1,4-diamine]bis[dichloridocopper(II)] trihydrate

The title dinuclear copper complex, [Cu2Cl4(C28H32N6)]·3H2O, is located on a crystallographic inversion center. The unique CuII ion is coordinated in a slightly distorted square-pyramidal environment in which the N atoms of the dipicolylamine group and a chloride ligand form the basal plane. The apical position is occupied by a second chloride atom. While the Cu—N distances of the pyridine N atoms are the same within expermental error, the Cu—N distance to the tertiary N atom is slightly elongated. The apical Cu—Cl distance is elongated due to typical Jahn–Teller distortion. One of the water O atoms was refined as disordered over two sites with occupancies 0.734 (17):0.266 (17) and another with half occupancy. H atoms for the disordered solvent atoms were not included in the refinement.


Comment
The described ligand has been used as starting material for hydrothermal synthesis of metal-organic transition metal/molybdateoxide frameworks in the principal author's laboratory (Bartholomä, unpublished results). The dipicolylamine moiety has originally been developed in our laboratory as metal chelating entity for binding of the M(CO) 3 core (M = Re, 99m Tc) for radiopharmaceutical purposes. However, a different coordination mode has been observed for the M(CO) 3 core in which the dipicolylamine metal chelate is coordinated in a facial manner (Bartholomä, 2009).
The title complex was prepared as part of a series with different cadmium and copper salts to study the coordination properties of the ligand with these metals without the interaction of metaloxide clusters (Bartholomä, 2010a,b). The use of copper bromide as metal salt gave a structurally comparable complex with a square pyramidal coordination sphere of both copper atoms (Bartholomä, 2010c). The Cu-N py distances were determined to 2.015 (6) Å and 2.019 (5) Å, and the Cu-N tert distance is 2.053 (5) Å. The extension of the spacer between the two dipicolylamine moieties in the case of N 1 ,N 1 ,N 5 ,N 5 -tetrakis(pyridin-2-ylmethyl)pentane-1,5-diamine with copper chloride also resulted in a structurally similar complex with Cu-N py distances of 1.986 (4) Å and 1.996 (4) Å, and a Cu-N tert distance of 2.077 (4) Å (Bartholomä et al., 2010d).
An amount of 1.00 g (11.34 mmol) 1,4-diaminobutane was dissolved in 30 ml anhydrous dichloroethane under an inert atmosphere (argon) followed by the addition of 4.55 ml (47.65 mmol) pyridine-2-carboxaldehyde. The mixture was stirred for 15 min at r.t. and then cooled with an ice bath prior to the portionwise addition of 14.43 g (68.06 mmol) sodium triacetoxyborohydride (gas evolution, exothermic reaction). The reaction was stirred overnight allowing the temperature slowly to rise to room temperature. The reaction was quenched by the dropwise addition of saturated sodium bicarbonate solution and stirring was continued until the gas evolution ceased.
The mixture was separated and the organic layer was further washed with saturated sodium bicarbonate solution, water and brine. The organic phase was dried with anhydrous sodium sulfate, filtered and the solvent removed under reduced pressure.
The crude reaction mixture was then purified by silica gel column chromatography starting with chloroform and increasing gradient to chloroform:methanol 10:1 (v/v). Yield: 4.02 g (78%

sup-2
Synthesis of metal complex. To 2 ml of an aqueous solution of copper chloride, two equivalents (50 mg, 0.11 mmol) of N 1 ,N 1 ,N 4 ,N 4 -tetrakis(pyridin-2-ylmethyl)butane-1,4-diamine in 2 ml methanol were added followed by the addition of 2 ml N,N-dimethylformamide. Single crystals were obtained after a week by slow evaporation of the solvents at room temperature.

Refinement
All the C-H atoms were placed in idealized positions and refined in a riding-model approximation with C-H aryl = 0.95, C-H methyl = 0.98 and C-H methylene = 0.99Å and U iso (H) = 1.5U eq (C methyl ) and 1.2U eq (C methylene/aryl ). The water H atoms were not included in the refinement.
Figures Fig. 1. The molecular structure of the title complex. The displacement ellipsoids are drawn at 50% probability level. Solvent water and hydrogen atoms are omitted for clarity. Unlabeled atoms are related by the symmetry code (-x + 1, -y + 1, -z + 1).