Predominance of electron-withdrawing effect over angular strain in the metal-promoted hydrolysis of 2,4,6-tris(2-pyridyl)-1,3,5-triazine

Abstract

The reaction of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) and RhCl ₃ ·3H ₂ O in refluxing ethanol–water (1:1) resulted in the hydrolysis of tptz to bis(2-pyridylcarbonyl)amide anion (bpca) and afforded a complex of composition [Rh(bpca) ₂ ][PF ₆ ] 1. However, hydrolysis of tptz did not occur when it was treated with RuCl ₃ ·3H ₂ O under similar condition, yielding instead the complex [Ru(tptz) ₂ ][PF ₆ ] ₂ ·H ₂ O 2. The molecular structures of 1 and 2 have been established by single-crystal X-ray analysis. In complex 1 the bis(2-pyridylcarbonyl)amido moiety functions as a tridentate ligand with nitrogen donor atoms and is bound to Rh ^III in a mutually perpendicular fashion forming a distorted-octahedral geometry around the metal ion. In complex 2 two tridentate tptz are co-ordinated to Ru ^II in a similar manner as found for 1. From a comparison of bond lengths and angles in the co-ordination spheres of 1 and 2 it is suggested that the electron-withdrawing effect (L→M) of the metal ion is the predominant factor, rather than angular strain at the carbonyl carbon atoms, responsible for hydrolysis of tptz. Electrochemical studies of 1 revealed a metal-based two-electron couple (Rh ^III –Rh ^I ) at -1.15 V and two ligand-based redox couples at -1.44 and -1.84 V. In the case of 2 the Ru ^II –Ru ^III couple appears at +1.77 V and the ligand-based reduction at -0.62 and -0.80 V which are significantly positively shifted compared to those of free tptz.