Synthesis, crystal structures and ab initio studies of some heptaaza manganese(II) macrocyclic Schiff-base complexes with two 2-aminoethyl pendant arms

Abstract

Two new heptaaza Mn(II) macrocyclic bis(pendant donor) Schiff-base complexes, [MnL 6] 2+ and [MnL 7] 2+, have been prepared via Mn(II) templated [1 + 1] cyclocondensation of 2,6-diacetylpyridine with an appropriate branched hexaamine, and characterized by X-ray crystallography and spectroscopic methods. The ligands are 16- and 17-membered pentaaza macrocycles with two 2-aminoethyl pendant arms [L 6 = 2,14-dimethyl-6,10-bis(2-aminoethyl)-3,6,10,13,19-pentaazabicyclo[13.3.1]nonadeca-1(19),2,13,15,17-pentaene and L 7 = 2,15-dimethyl-6,11-bis(2-aminoethyl)-3,6,11,14,20-pentaazabicyclo[14.3.1]eicosa-1(20),2,14,16,18-pentaene]. The crystal structures of [MnL 7] 2+ and another related complex, [MnL 5] 2+ [L 5 = 2,13-dimethyl-6,9-bis(2-aminoethyl)-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene] were determined from X-ray diffraction data and the structure of each complex indicates that the geometry of the coordination sphere is that of a slightly distorted pentagonal bipyramid with the Mn(II) ion located within a pentaaza macrocycle and two pendant amines coordinating on opposite sides. The increased macrocycle ring size in [MnL 7] 2+ introduces an unusual seven-membered chelate ring and leads to long Mn–N bond lengths within the macrocycle, but to shorter pendant amine–Mn(II) bond lengths. Ab initio HF-MO calculations produce structures for the two complexes which are in close agreement with experimental findings and predict a similar structure for [MnL 6] 2+, but with the longest pendant amine–Mn(II) bond lengths of the three structures.