Low symmetry pyrazole-based tripodal tetraamineligands: metal complexes and ligand decomposition reactions

Abstract

The new low symmetry pyrazole-based tripodal tetraamine ligands 2-(1H-pyrazol-1-yl)-N,N-bis(1H-pyrazol-1-ylmethyl)ethanamine (bmpz) and 2-(1H-pyrazol-1-yl)-N-[2-(1H-pyrazol-1-yl)ethyl]-N-(1H-pyrazol-1-ylmethyl)ethanamine (bepz) have been prepared and characterised, as have metal complexes containing these ligands. X-ray crystal structures of [Co(bmpz)Cl]₂[CoCl₄]·H₂O, [Co(bmpz)MeCN](ClO₄)₂·0.13H₂O, [Zn(bmpz)MeCN](ClO₄)₂·0.15H₂O, [Zn(bepz)OH₂](ClO₄)₂·0.5H₂O and [(Co(bepz)Cl)₂]Cl₂·6H₂O confirm coordination of the intact tripodal ligands to the metal ions through all four N atoms. However, attempts to make Cu²⁺ complexes containing bmpz and bepz gave, respectively, [Cu(7)Cl₂]·0.2H₂O and [Cu(8)Cl₂] (7 = 1-(1H-pyrazol-1-yl)-N-(1H-pyrazol-1-ylmethyl)ethanamine, 8 = 2-(1H-pyrazol-1-yl)-N-[2-(1H-pyrazol-1-yl)ethyl]ethanamine), complexes containing the tridentate ligands 7 and 8 which are formed by loss of a pyrazolylmethyl arm from the appropriate tripodal ligand. This decomposition reaction occurs in protic solvents both in the presence and absence of metal ions, and is ascribed to the presence of an aminal functionality in the tripodal ligands. A possible mechanism for the decomposition, based on NMR and ESMS data, is suggested.