Photophysical and catalytic properties of naphthalimide-tethered N-heterocyclic carbene complexes

Abstract

Naphthalimide moieties were integrated with N-heterocyclic carbene metal chemistry to yield eleven novel complexes that contained Rh(III), Ru(II) or Ir(III/I) metal centres. Normal NHC complexes were bonded in a monodentate (C1-C4) or bidentate (C5-C9) fashion (via N- or Catom(s)) whilst bidentate abnormal complexes (C10-C12) contained picolyl N-tethers that bonded to the metal via the N-atom. The first naphthalimide-NHC complexes that contain Rh(III) and Ir(I) centres and the first abnormal NHC complexes of this type are reported here. A new method for the synthesis of imidazolium salts with an isopropyl group bonded at the C2-position is presented. Blocking of the C2-position enabled abnormal ligation of the NHC ligands to the metal centres. The complexes were synthesised by silver transmetallation reactions. Cu-NHCs of L1-L3 were assessed but found to be unfeasible as alternative transmetallating agents. Complexation of the respective carbene ligands to metallic copper was incomplete and the obtained Cu-NHCs decomposed. The Ag-NHCs were more stable and could be characterised with SCXRD. Ag-3 was highly emissive in the solid-state with a maximum emission intensity of 21.0 x 10^4 CPS in the blue region (480 nm). Literature naphthalimide-NHC complexes were comprehensively reviewed and it was shown that these compounds have been studied either for their catalytic, anticancer or photophysical properties. All three of these properties have been investigated for the complexes of this study. The solid-state fluorescence emission intensities of the complexes were quenched relative to precursor imidazolium salts (L1-L3 and L4b) and intermediate Ag(I) complexes (Ag-1-Ag-3) mainly due to the heavy-atom effect. The imidazolium salts (L1-L3 and L4b) and organic precursors (1, 4-6) fluoresced in the blue-cyan range. The fluorescence emission intensities of compounds 4, L1-L3 and L4b were quenched compared to 1, 5a, 5b and 6. Compound 1 was highly emissive with a maximum emission intensity of 14.4 x 10^4 CPS at 461-464 nm. Crystal structure data evidenced the presence of intermolecular hydrogen bonding in 4 which accounted for the double bands in its emission spectrum. The two emission bands originated from the respective monomer and intermolecular dimer. For ten of the novel complexes of this study, crystal structures were obtained. In total, 23 novel crystal structures of organic and inorganic compounds, including a rare Cu(II) complex (Cu-2d), were obtained. The solid-state emission intensities of bidentate complexes (C7, C8) with picolyl N-tethers were enhanced relative to complexes with benzyl N-tethers (C1, C2) even though quenching of the emission intensities was overwhelming. In general, C1, C2, C7 and C8 showed appreciable activity against alveolar carcinoma (A549), triple-negative breast carcinoma (BT-20 and MDA-MB-231), hepatocarcinoma (HepG2), breast adenocarcinoma (MCF-7) and tamoxifen-resistant breast carcinoma variant of MCF-7 (MCF-7/TAMR1). C1 was however not selective relative to non-tumorigenic cells. C2 had the lowest IC50 value (13.99 (3.32)) against MDA-MB-231 and was selective towards cancer cells. This result correlates with a literature report where C2 was studied for its anticancer properties against colon (HT-29) and breast (MCF-7) cancer cells. The complexes were active in the catalytic transfer hydrogenation reaction of acetophenone to 1 phenylethanol. The abnormal complexes (C10-C12) showed superior activity compared to normal NHC analogues (C7-C9). The Ir(I) complex C4 was highly active with a TOFin value of 2240 h^-1 at a catalyst loading of 0.1 mol %. The reduction of aromatic ketones and aldehydes with various donor- and acceptor-groups bonded at para-positions proceeded with modest to good conversions (TOFin varied between 13 and 53 h^-1) when 0.1 mol % of C4 was used as catalyst.