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

The reaction of RhCl3. 3H(2)O 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)]. 3H(2)O 1. Similarly the reaction of tptz and Rh(tpy)Cl-3 (tpy=2,2':6',2 "- terpyridyl) yielded mixed ligand complexes [Rh-III(bpca)(tpy)](2+) and [Rh-Ii(bpca)(tpy)]Cl . 8H(2)O 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(parallel to) > g(perpendicular to) approximate to g(e)indicating a d(x2-y2) ground state and a compressed octahedral geometry for the metal ion, consistent with the crystal structures.