Stable mononuclear rhodium(II) polypyridyl complexes: synthesis, spectroscopic and structural characterisation

Abstract

The reaction of RhCl₃·3H₂O and 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol–water (1∶1) resulted in the hydrolysis of tptz to bis(2-pyridylcarbonyl)amide anion (bpca) and afforded a mixture of Rh^III and Rh^II complexes which were separated and characterised as [Rh^III(bpca)₂][PF₆] and [Rh^II(bpca)₂]·3H₂O 1. Similarly the reaction of tptz and Rh(tpy)Cl₃ (tpy = 2,2′∶6′,2″-terpyridyl) yielded mixed ligand complexes [Rh^III(bpca)(tpy)]²⁺ and [Rh^Ii(bpca)(tpy)]Cl·8H₂O 2. The molecular structures of complexes 1 and 2 have been established by single-crystal X-ray analysis. In complex 1 two bpca moieties are co-ordinated to Rh^II with nitrogen as donor atoms in a mutually perpendicular fashion. In complex 2 bpca and tpy are bound to the metal ion in a similar fashion to that found in 1. The axial Rh–N bond distances in 1 and 2 are significantly shorter compared to equatorial Rh–N bond distances, indicating an axially compressed octahedral geometry of the metal ion. Complexes 1 and 2 exhibit absorption bands in the 545–600 nm region whereas their Rh^III analogues do not show any band in this region. Electrochemical studies of 1 and 2 revealed a metal based reduction (Rh^II→Rh^I) at –1.13 and –0.72 V, respectively, followed by two ligand-based redox couples. EPR studies of 1 and 2 in acetonitrile at 77 K show g_|| > g_⊥ ≈ g_e indicating a d_{x² – y²} ground state and a compressed octahedral geometry for the metal ion, consistent with the crystal structures.