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

Abstract

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⋯O and N–H⋯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 Å) 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⋯O interactions (3.12 Å in Cu(L 1 ) 2 and 2.71 Å in Cu(L 2 ) 2 ) between the copper and oxygen atoms of the ClO 4 - counteranion, leading to a linear network with alternate ClO 4 - and Cu + ions. A 2-D network determined by intermolecular N–H⋯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 Å in Cu(L 1 ) 2 and 4.0 Å 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.