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

The title dinuclear copper complex, [Cu2Br4(C28H32N6)], is located on an inversion center. The unique CuII ion is in a slightly distorted square-pyramidal environment in which the N atoms of a dipicolylamine group and a bromide ligand form the basal plane. The apical site is occupied by a second Br atom. While the Cu—N distances involving the pyridine N atoms are the same within experimental error, the Cu—N distance involving the tertiary N atom is slightly elongated. Due to the typical Jahn–Teller distortion of copper(II) complexes, the apical Cu—Br distance is elongated.

The title dinuclear copper complex, [Cu 2 Br 4 (C 28 H 32 N 6 )], is located on an inversion center. The unique Cu II ion is in a slightly distorted square-pyramidal environment in which the N atoms of a dipicolylamine group and a bromide ligand form the basal plane. The apical site is occupied by a second Br atom. While the Cu-N distances involving the pyridine N atoms are the same within experimental error, the Cu-N distance involving the tertiary N atom is slightly elongated. Due to the typical Jahn-Teller distortion of copper(II) complexes, the apical Cu-Br distance is elongated.

Comment
The described ligand N 1 ,N 1 ,N 4 ,N 4 -tetrakis(pyridin-2-ylmethyl)butane-1,4-diamine 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synthesis of metal-organic transition metal/molybdateoxide frameworks in the principal author's laboratory (Bartholomä, unpublished results).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 used in our laboratory as metal chelating entity for binding of the M(CO) 3 core (M = Re, 99m Tc) for radiopharmaceutical purposes. 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,c). The crystalline sample obtained with copper chloride as metal source and N 1 ,N 1 ,N 5 ,N 5 -tetrakis(pyridin-2-ylmethyl)pentane-1,5-diamine gave a structurally similar compound with a distorted square pyramidal coordination geometry of the copper atoms as observed for the described complex. The Cu-N py distances of 1.986 (4) Å and 1.996 (4), and a Cu-N tert distance of 2.076 (4) Å (Bartholomä, 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

sup-2
Synthesis of metal complex. To 2 ml of an aqueous solution of copper bromide, 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 H atoms were placed in idealized positions and refined in a riding-model approximation with C-H aryl = 0.95 Å, C-H methyl = 0.98Å and C-H methylene = 0.99Å and U iso (H) = 1.5U eq (C methyl ) and 1.2U eq (C methylene/aryl ). Fig. 1. The crystal structure of the title complex. The displacement ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Unlabeled atoms are related by the symmetry code (-x + 1, -y + 1, -z + 1).

[µ-N,N,N',N'-Tetrakis(2-pyridylmethyl)butane-1,4-diamine]bis[dibromidocopper(II)]
Crystal data [ Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.