Crystal structure of tetra(3-carboxyl-2,2,5,5-tetramethylpyrrolidin-1-oxyl) bis(dimethyl fumarate) bicopper(II) complex, C21H37CuN3O7

Abstract

Source of material 4-oxo-2,2,6,6-tetramethylpiperdin-1-oxyl I can be conveniently obtained by procedures given in the literature [1].The reaction of I with concentrated hydrochloric acid gave the isolable intermediate l-hydroxy-4-oxo-2,2,6,6-tetramethylpiperidine hydrochloride II which then was brominated to the 3-bromo-l-hydroxy-4-oxo-2,2,6,6-tetramethylpiperidine derivative which, in turn, was converted in situ to the corresponding radical III by oxidation using sodium nitrite. Compound III (25 g, 0.1 mmol) was added, portionwise, to a stirred solution of sodium hydroxide (8 g, 0.2mol) in water (150 mL). The reaction mixture was stirred for 3 h at about 20°C, acidified with 6 mol/L aqueous HC1 solution to pH 2, and quickly extracted with chloroform (3´80 mL). The organic layer was dried with anhydrous magnesium sulfate. After removal of the solvent in a rotating evaporator, the residue was recrystallized from a mixture of chloroform and hexane (3/1, v/v) to give 14.8 g (79.5%) of the yellow product IV.A solution of HCTemPO (372 mg, 2 mmol) in water (10 mL) was dropped slowly into CuO (80 mg, 1 mmol) in water (5 mL). DiscussionThe study of transition metal complexes with four carboxylate bridges (so-called "paddle-wheel") was stimulated by the antiferromagnetic properties of binuclear cupric acetate hydrate due to spin-spin interaction through the bridging carboxylate groups without Cu-Cu bonding (Cu-Cu 2.64 Å) [2,3]. The analogous carboxylate complexes of other 3d-metals were prepared lately with using of steric effects of axial ligands: alphasubstituted pyridines (2-picoline, 2,6-lutidine, acridine or N,Ndimethylformamide) or cyclopentadienyl groups [4][5][6][7]. [7]. The coordination of each Cu(II) ion is a distorted tetragonal pyramid. The Addison's criterion, which is 1 for an ideal trigonal bipyramid and 0 for an ideal tetragonal pyramid, respectively [9], has a value of 0.016 for the title moiety. The longer (Cu1-O7 DMF = 2.125(2) Å) distance is viewed at the apical position. The Cu1 atom deviates by 0.2049 Å from the basal plane formed by the O1, O2, O3 and O4 atoms. A DMF molecule is bound trans to the Cu1-Cu1A vector on each of the Cu atoms, and a further two DMF molecules are loosely held in the lattice. These lattice DMF sites were highly mobile, and their sites were only partially occupied. During the data collection, some reductions in the intensities of the standard reflections were noted, and these are consistent with slow loss of these molecules of crystallization. The fused and conjugated five-membered rings are close to being planar (deviations less than 0.2339 Å), and the molecule makes no close contacts with other molecules in the structure. The two copper atoms in each dimeric unit are separated by a distance of