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

The title dinuclear complex, [Cd2(CH3CO2)4(C28H32N6)]·9H2O, is located on a crystallographic inversion center. The unique CdII ion displays a 5 + 2 coordination. A distorted square-pyramidal geometry is formed by the dipicolylamine unit of the ligand via the N atoms in a meridional fashion and two O atoms of the acetate ligands with short Cd—O distances. The coordination is completed by two loosely bound O atoms of the acetate ligands. The Cd—N distances involving the pyridine N atoms differ slightly from each other and the Cd—N distance involving the tertiary N atom is the longest. In the crystal structure, complex molecules and solvent water molecules are connected into a three-dimensional network via intermolecular O—H⋯O hydrogen bonds. One of the water molecules lies on a twofold rotation axis.


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.
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. However, a different coordination mode has been observed for the M(CO) 3 core in which the dipicolylamine metal chelate is bound 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, 2010b,c,d). We have reported another crystal structure of a molecular dinuclear cadmium complex using the corresponding nitrate salt as metal source (Bartholomä, 2010a). In the cadmium nitrate structure, the Cd-N distances involving the pyridine N atoms [2.250 (2) Å and 2.251 (2) Å] are slightly shorter whereas the Cd-N distance involving the tertiary nitrogen atom [2.427 (2) Å] is marginally longer when compared to the related distances in the title compound. Experimental N 1 ,N 1 ,N 4 ,N 4 -tetrakis(pyridin-2-ylmethyl)butane-1,4-diamine. 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 acetate, 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 H atoms were placed in idealized positions and refined using a riding-model approximation with C-H(aryl) = 0.95Å, 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 ). Water hydrogen atoms were located in a difference Fourier map and refined freely.