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 RhIII and RhII complexes which were separated and characterised as [RhIII(bpca)2][PF6] and [RhII(bpca)2]·3H2O 1. Similarly the reaction of tptz and Rh(tpy)Cl3 (tpy = 2,2′∶6′,2″-terpyridyl) yielded mixed ligand complexes [RhIII(bpca)(tpy)]2+ and [RhIi(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 RhII 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 RhIII analogues do not show any band in this region. Electrochemical studies of 1 and 2 revealed a metal based reduction (RhII→RhI) 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⊥ ≈ ge indicating a dx2 – y2 ground state and a compressed octahedral geometry for the metal ion, consistent with the crystal structures.