Steric effects caused by N-alkylation of the tripodal chelatorN,N′,N″-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane (tachpyr): structural and electronic properties of the Mn(ii), Co(ii), Ni(ii), Cu(ii) and Zn(ii) complexes

Abstract

The effects of steric hindrance on the complexation of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) by chelators based on cis,cis-1,3,5-triaminocyclohexane (tach) have been studied. The chelators studied are the 2-methylpyridyl pendant-arm derivatives of tach, ‘tachpyr’ (N,N′,N″-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane), and the N-alkylated analogs ‘(N–Me)₃tachpyr’ (N,N′,N″-trimethyl-N,N′,N″-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane) and ‘(N–Et)₃tachpyr’ (N,N′,N″-triethyl-N,N′,N″-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane). Hexacoordinate complexes, [M^IIL]X₂ resulted from reaction of the appropriate metal ion salts with L in alcoholic medium (M = Mn, Co, Ni, Cu, Zn; L = tachpyr, (N–Me)₃tachpyr, (N–Et)₃tachpyr; X = ClO₄⁻, NO₃⁻, Cl⁻). The Mn(II) and Co(II) complexes have high-spin electron configurations, based on solution magnetic susceptibility, electronic spectral and/or X-ray crystallographic studies. However, solution visible-near IR electronic spectra and single-crystal X-ray crystallography demonstrate weakened bonding in the complexes of (N–R)₃tachpyr relative to tachpyr, due to steric effects of methyl or ethyl groups on coordinated tach amines. Structures of [Zn(tachpyr)]²⁺ and [Ni(tachpyr)]²⁺ indicate a clear preference of tachpyr and derivatives for octahedral geometry, while [Cu((N–Me)₃tachpyr)]²⁺ exhibits a classic Jahn–Teller tetragonal distortion. The complex [Mn(tachpyr)](ClO₄)₂ demonstrates a striking influence of the metal ion on the coordination geometry, with a trigonal-prismatic coordination of Mn(II) and considerable distortion of the tach-amino donor groups. The coordination geometry and distortions are attributed to the large size of high-spin d⁵ Mn(II) and the absence of ligand-field stabilization energy, coupled with the observed preference of tachpyr derivatives for octahedral geometry. The [M^II(tachpyr)]²⁺ complexes (M = Co, Ni, Cu and Zn) are inert in aqueous pH 5.5 media. However, all metal complexes of N-alkylated tachpyr, as well as [Mn(tachpyr)](ClO₄)₂, dissociate metal ion in aqueous pH 5.5 medium, consistent with their steric hindrance and distortions respectively.