Robust Packing Patterns and Luminescence Quenching in Mononuclear [Cu(II)(phen)₂] Sulfates

Abstract

Three mixed-ligand Cu(II) complexes with compositions [Cu(phen) 2 (SO 4 )]·CH 3 OH (1), [Cu(phen) 2 (SO 4 )]-(H 2 O) 2 (dmf) (2), and [Cu(phen) 2 H 2 O](SO 4 )(H 2 O) 4 (3), where phen = 1,10-phenanthroline and dmf = N,N′-dimethylformamide, were prepared and studied. These compounds belong to the landscape of the mononuclear Cu(phen) 2 sulfates, and the solvated complexes undergo frequent anion/water exchange at the metal center in aqueous solutions. Complexes are similar by the metal trigonal bipyramidal coordination geometry but differ by the mode of enclathration and number of protic and aprotic solvent guest molecules being accommodated in the crystal lattice. Crystal packing in 1−3 is determined by the robust supramolecular patterns that consist of stacking interactions between the planar extended phen fragments. These are observed in all three solids regardless of the interplay of other noncovalent interactions, including rather strong hydrogen bonds. The dual luminescence is detected at 580 and 470 nm for both crystals of phen and 3. Detailed analysis of singlet and triplet excitations in phen and 3 is performed by time-dependent density functional methods. Fluorescence is predicted with a low quantum yield at 386 nm, and dual phosphorescence from n−π* and π−π* triplet states is predicted at 523 and 496 nm. Emission quenching was demonstrated for 3 and explained by nonradiative decay involving supramolecular stacking and low-lying metal-centered states. ■ INTRODUCTIONFor decades Cu(II) coordination compounds have been attractive targets for magneto-and biochemistry. 1,2 Engineering of metal−organic materials with specific properties using a molecular building blocks approach is possible only by understanding the interplay of different interactions involved in self-assembly processes. From a crystal engineering perspective, one of the advantages of using transition metal ions is that the shape of the main building block can be controlled by way of organic ligand-bound metal-containing modules in directions dictated by the coordination geometry of the metal center and by careful choice of the ligands. 3−8 Design strategies employing simultaneously coordination bonds, hydrogen bonds, and π−π stacking interactions in crystal engineering are mostly not well documented so far. 9 We are involved in engineering, structural studies, and evaluation of properties of low-dimensional clusters and coordination polymers that include the Cu(II)−phen building block and reveal the contribution of stacking interactions in the crystal packing. 10,11 The necessity of careful examination and disclosure of the robust recurring patterns in such lowdimensional solids is dictated by their wide exploitation as medicinal forms with obvious antitumor efficacy. It has been reported that, in particular, phen derivatives [Cu-(CH 3 COO) 2 (phen)] and [Cu(sal)(phen)] demonstrate approximately seven times higher activity than cisplatin against HepG2, A-498, and A-549 cancer cells. Among the factors that influence the cytotoxic activity, the...

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