Synthesis and structure of unusually stable linear copper(I) complexes with blue fluorescence

A new fluorescent ligand 2-(4-aminophthalimidoethyl)pyridine (L-2) has been synthesized. Solid state structures of L-2 and 2-(4'-aminophthalimidomethyl)pyridine (L-1) have been determined by X-ray crystallography. While L-1 crystallizes in the non-centrosymmetric orthorhombic space group Fdd2, L-2 crystallizes in the monoclinic space group C2/c. The molecular packing in both systems is dominated by intermolecular N-H center dot center dot center dot O and N-H center dot center dot center dot N hydrogen bonds and relative bending of the planes defined by the pyridine and phthalimide moieties. An unusually stable two-coordinate complex of copper(I) with L-2, Cu(L-2)(2), has been synthesized upon in situ reduction of copper(II) at room temperature. This complex, which has been characterized by X-ray crystallography, crystallizes in the centrosymmetric space group C2/c with a linear N-Cu-N geometry. The Cu-N bond distance (1.89 angstrom) in Cu(L-2)(2) is found to be similar to that in Cu(L-1)(2). Both the complexes display interesting structural feature, arising from weak Cu center dot center dot center dot O interactions (3.12 angstrom in Cu(L-1)(2) and 2.71 angstrom in Cu(L-2)(2)) between the copper and oxygen atoms of the ClO4- counteranion, leading to a linear network with alternate ClO4- and Cu+ ions. A 2-D network determined by intermolecular N-H center dot center dot center dot O hydrogen bonding is also observed. These complexes are found to be highly resistant toward oxidation both in the solid state and in solution. The new complex Cu(L-2)(2), in which the metal center is more separated from the 4-aminophthalimide moieties, exhibits lesser stability than Cu(L-1)(2). Close contact between copper and one of the imide oxygen atoms of the ligand framework was detected as 3.03 angstrom in Cu(L-1)(2) and 4.0 angstrom in Cu(L-2)(2), indicating a relatively weaker electronic interaction in the later complex. Thus, the exceptional stability of these complexes is attributed to the electronic environment provided by the 4-aminophthalimide and perchlorate moieties on the metal center. (C) 2006 Elsevier Ltd. All rights reserved.