Syntheses and structural characterization of mononuclear Rh-Cp* and Ir-Cp* complexes with η-phenanthrene, η-pyrene and η-triphenylene

Four novel mononuclear Rh-Cp* and Ir-Cp* complexes with polycyclic aromatic hydrocarbons (PAHs), [M(Cp*)(η⁶-PAHs)](BF₄)₂ (M = Rh and Ir; Cp* = η⁵-C₅Me₅; PAHs = phenanthrene (phn), pyrene (pyr) and triphenylene (triph)), were prepared by the reactions of the intermediate [M(Cp*)(Me₂CO)₃]²⁺ with appreciable PAHs. Their structures were characterized by a single crystal X-ray analysis, ¹H, ¹³C {¹H} NMR and 2D NMR techniques. The X-ray crystallographic studies showed that the [M(Cp*)]²⁺ fragment is η⁶-coordinated to one terminal benzene ring in each PAH. In particular, it is interesting to note that the partial π/π/π/π interaction was formed in the Ir-pyr complex [Ir(Cp*)(η⁶-pyr)](BF₄)₂. The 1D and 2D NMR studies described that the Rh-Cp* and Ir-Cp* complexes with PAHs gave unique ¹H and ¹³C {¹H} NMR spectra with positive coordination shifts (Δδ(¹H, ¹³C)) in (CD₃)₂CO at 23 °C, which are likely induced by the local effect and the non-local effect on the coordination of the [M(Cp*)]²⁺ fragment to PAHs. The decreasing of the coupling constants (³J_H-H) in the η⁶-coordinated benzene ring is also induced, with no changes in the uncoordinated benzene rings. The time-course of ¹H NMR spectra showed that Rh-Cp* and Ir-Cp* complexes with PAHs are partially dissociated to [M(Cp*)(Me₂CO)₃]²⁺ and metal-free PAHs in (CD₃)₂CO at 23 °C. It was demonstrated that their stabilities are in the order of Ir-triph, Ir-phn, Ir-pyr and Rh-triph complexes in (CD₃)₂CO.