Synthesis, Spectroscopic and Computational Studies of Rhodium(III) Complexes Bearing N-Heterocyclic Carbene-Based C^{^}N^{^}C Pincer Ligand and Bipyridine/Terpyridine

Rhodium(III) complexes containing tridentate N-heterocyclic carbene (NHC)-based pincer ligand in the form of [Rh(C^{^}N^{^}C)(N^{^}N)X]²⁺ and [Rh(C^{^}N^{^}C)(tpy)]³⁺ (C^{^}N^{^}C = (2,6-bis(1-butylimidazol-2-ylidene)pyridine (C^{1^}N^{^}C¹) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C^{2^}N^{^}C²); N^{^}N = bipyridine-type aromatic diimines; tpy = 2,2':6',2''-terpyridine; X = Cl or Br) were prepared. Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations revealed the lowest-energy electronic transition associated with these complexes are different from their Ru(II), Os(II) and Ir(III) analogues. The Rh(III) complexes exhibit luminescent behavior at 77 K upon photoexcitation. The impact of the C^{^}N^{^}C pincer ligands on the photophysics of the Rh(III) complexes was investigated.