Naphthalimide-NHC complexes : synthesis and properties in catalytic, biological and photophysical applications

Abstract

The following review provides an account of all 82 N-heterocyclic carbene metal complexes containing a naphthalimide-NHC moiety, incorporating either an imidazolylidene, imidazolinylidene or benzimidazolylidene ligand, reported in literature. Only three of these are biscarbene complexes. Naphthalimide-NHC complexes constitute a very successful class of compounds that has been applied across many fields, thus providing a focal point of coinciding interest that may appeal to a wide audience and incite future work. Herein, the complexes are divided among those with the naphthalimide moiety tethered via a (benz)imidazol(in)ylidene N-atom and those where it is bonded to the NHC backbone, since the electronic properties and conformations of the two classes vary considerably. These complexes are interesting target molecules that rely on common to more advanced synthetic methods for preparation, including post-synthetic modification in some instances. Due to their bulkiness, these complexes may also exhibit notable conformations in the solid state, with π-π stacking interactions contributing to their fascinating properties. Catalytic applications for this class of complexes are limited in literature and include only the Larock heteroannulation reaction and Suzuki-Miyaura coupling reaction. Pd(II) complexes were primarily studied as catalysts for these reactions. Au(I), Cu(I), Ag(I), Rh(I) and Ru(II) complexes were applied in a biological context, with bis-naphthalimide Ag(I) and bis-NHC Au(I) complexes showing the greatest activity against cancerous colorectal (HT-29), breast (MCF-7) or lung (A549) cells. The naphthalene rings of the naphthalimide moiety in the complexes were responsible for π-π stacking interactions and binding of complexes to DNA through intercalation. Rh(I), Ru(II) and Ag(I) complexes successfully inhibited gram positive bacterial strains. In addition, Au(I), Cu(I), Ag(I), Rh(I), Ir(I) and Ir(III) complexes exhibited photoluminescent properties, emitting in red, blue-green or blue regions with either phosphorescence or fluorescence lifetimes. The unique properties of naphthalimide-NHC complexes allowed for the theranostic application in selected anticancer studies, which has been illustrated by imaging of cytotoxic luminescent complexes. Integration of the photophysical properties with catalytic processes, in photocatalysis or in additional biological applications e.g. photodynamic therapy, is envisaged for these naphthalimide-containing complexes in future endeavours.