Zn(II) Curcuminate Complexes with 2,2′-bipyridine and Carboxylates

Two novel zinc(II) compounds with curcuminate (abbreviated as cur(−)), [Zn(CH(3)COO)(cur)(bpy)](1)·CH(3)OH·2H(2)O (bpy = 2,2′-bipyridine) and [Zn(PhCOO)(cur)(bpy)] (2)·CH(3)OH, have been synthesized and characterized. Their composition has been determined by single-crystal X-ray structure analysis. Complexes 1 and 2 are similar: in both a five-fold coordination environment of zinc(II) consists of a monodentate carboxylate, a chelating bidentate 2,2′-bipyridine, and curcuminate, which is bound via a deprotonated 1,3-dione moiety. In 1, 2,2′-bipyridine nitrogen atoms and curcuminate oxygen atoms form the base of a square pyramid, whereas the acetate oxygen occupies its apex. The O(3)N(2) donor set in 2 defines a polyhedron which more closely resembles a trigonal bipyramid. The packing in the crystal lattices of both compounds is governed by hydrogen-bonds. Complexes 1 and 2 display higher stability than curcumin in buffered media at pH = 7.0, however, the degradation of coordinated cur(−) is comparable to that of yellow pigment curcumin (curH) when the pH is raised to 7.2. Both complexes 1 and 2 in DMSO exhibit fluorescence with Stokes shifts of 5367 and 4634 cm(−1), respectively.