Through-bond exchange coupling and triplet excitons in a dinuclear copper(II) macrocyclic complex

The reaction of the bicyclic cryptand 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosan-5,6,14,15,22,23-hexone (1) with copper(II) leads to the dinuclear complex {1,10-bis(2-aminoethyl)-1,4,7,10,13,16-hexaazacyclooctadecane-5,6,14,15-tetronato(4-)}dicopper(II) (2). 2 crystallizes in the triclinic system (space group <P(1)over bar>, No. 2), with cell parameters a = 6.751(1) Angstrom, b = 10.153(1) Angstrom, c = 10.748(2) Angstrom, alpha = 71.83(1)degrees, beta = 89.79(1)degrees, gamma = 73.56(1)degrees, and Z = 1. The structure is made up of centrosymmetric neutral dinuclear copper(II) units stacking along the a axis. Each copper(II) ion assumed a distorted-octahedral environment. The basal positions are occupied by four nitrogen atoms, two from deprotonated amide groups and two from the tertiary and primary amines; the apical positions are occupied by one amido nitrogen atom from the same dimer and one amido oxygen atom from a neighboring unit within the stack. The analysis of variable-temperature magnetic susceptibility data reveals the occurrence of a significant intramolecular antiferromagnetic interaction (J = -47 cm(-1)) in 2. The analysis of the exchange pathways by extended Huckel calculations shows a strong contribution of the N-C-C-N sigma bonds to the coupling between the two copper(II) ions. The triplet exciton type EPR spectrum of 2 allowed the evaluation of the interdimer interaction (J' approximate to 10(-3) cm(-1)), a possible description of 2 as a magnetic alternating chain being ruled out.