C(naphthyl)-H bond activation by rhodium: isolation, characterization and TD-DFT study of the cyclometallates

The C1(naphthyl)-H, C2(naphthyl)-H, C3(naphthyl)-H and C8(naphthyl)-H bonds of the naphthyl group present in a group of naphthylazo-29-hydroxyarenes (H2L) have been activated by [Rh(PPh3)(3)Cl] in a toluene medium. Here the cyclometallation is accompanied by metal centered oxidation [Rh(I)-> Rh(III)]. All the resulting cyclometallates [Rh(PPh3)(2)(L)Cl] (2-5) have been isolated in a pure form. The characterization of the cyclometallates [Rh(PPh3()2)(L) Cl] have been done on the basis of spectral (IR, UV-vis, and FAB mass) data. The structures of the representative cyclometallates 2a, 3a, 4a, 4b and 5b have been determined by X-ray diffraction. In all the cyclometallates, rhodium(III) is coordinated to naphthylazo-2'-hydroxyarenes via terdentate C(naphthyl), N(diazene), O(phenolato/naphtholato) donor centers & one chloride ion in a plane along with two axial trans PPh3 molecules. Intermolecular association in the solid state is observed due to C-H center dot center dot center dot pi and pi center dot center dot center dot pi interactions. Compounds show an oxidative response within 0.93 to 1.11 V (vs. SCE) and a reductive response at similar to -1.0 V (vs. SCE). Both the responses are based on the coordinated diazene function and are irreversible in nature, indicating limited stability of the oxidized and reduced species. The electronic structures of selected cyclometallates have been calculated using a TD-DFT model and the simulated spectra are consistent with the observed spectra of those cyclometallates.