Unusual stabilities of 6,6′-bis(aminomethyl)-2,2′-bipyridyl chelates of transition-metal ions and crystal structures of the ligand and its copper(II) and nickel(II) complexes

Abstract

An improved four-step synthesis of the tetradentate compound 6,6′-bis(aminomethyl)-2,2′-bipyridyl (badp) has been developed and the crystal structure of its dihydrochloride monohydrate determined: space group C2/c, a= 11.991(2), b= 5.703(1), c= 22.109(3)Å, β= 97.17(1)° and Z= 4. The complexes of copper(II) and nickel(II) with badp have also been synthesized and structurally characterized. The metal ion in [Cu(badp)(H₂O)]Br₂ is five-co-ordinated and the complex crystallizes in the monoclinic P2₁/c space group with a= 11.508(3), b= 9.584(3), c= 14.362(3)Å, β= 107.93(2)° and Z= 4. The complex [Ni(badp)(H₂O)Br]Br contains six-co-ordinated Ni^II and crystallizes in the monoclinic P2₁/c space group with a= 11.387(3), b= 9.417(2), c= 14.317(5)Å, β= 106.21(2)° and Z= 4. The stepwise stability constants of the 1 : 1 ligand to metal complexes of badp with divalent transition-metal ions have been determined in KCl as supporting electrolyte (0.100 mol dm^–3) at 25.0 °C: log K_ML=[ML]/[M][L] for the Fe^II, Co^II, Ni^II, Cu^II and Zn^II complexes is 8.94, 10.16, 11.87, 15.05 and 10.02 respectively. Iron(II) and cobalt(II) also formed 2:1 complexes with badp. The unusually low stabilities of the transition-metal complexes are considered due to the low basicity of the ligand and to the unfavourable spacing of its donor groups as indicated by the structures of the complexes of Ni^II and Cu^II.