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

The title dinuclear cadmium complex, [Cd2(NO3)4(C28H32N6)], is located on an inversion center. The unique CdII ion displays a 5 + 2 coordination. A distorted square-pyramidal geometry is formed by the dipicolylamine group of the ligand via the N atoms in a meridional fashion and two O atoms of the nitrate ligands with short Cd—O distances. The coordination is completed by two loosely bound O atoms of the nitrate ligands.

The title dinuclear cadmium complex, [Cd 2 (NO 3 ) 4 -(C 28 H 32 N 6 )], is located on an inversion center. The unique Cd II ion displays a 5 + 2 coordination. A distorted squarepyramidal geometry is formed by the dipicolylamine group of the ligand via the N atoms in a meridional fashion and two O atoms of the nitrate ligands with short Cd-O distances. The coordination is completed by two loosely bound O atoms of the nitrate ligands.

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 in the hydrothermal synthesis of metal-organic transition metal/molybdateoxide frameworks in the principal author's laboratory (Bartholomä, unpublished results). 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, 2010b,c,d).
Another crystalline species of a monomeric dinuclear cadmium complex using the corresponding acetate salt as metal source has been reported by our group (Bartholomä, 2010a). In the cadmium acetate structure, the Cd-N distances of the pyridine N atoms are slightly longer [2.313 (3) Å and 2.379 (3) Å] whereas the distance of the tertiary nitrogen atom is with marginally shorter [2.405 (3)] Å when compared to the distances of the title complex.
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).
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 cadmium nitrate, 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 using the riding model approximation with C-H aryl = 0.95 Å, C-H methyl = 0.98Å and C-H methylene = 0.99Å and with 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, -y + 2, -z + 1).