Structure and magnetic properties of tri- and heca-nuclear hydroxo-bridged copper(II) complexes formed by a trimacrocyclic derivative of 1,4,7-triazacyclononane

摘要

Two hydroxo-bridged Cu(II) complexes of the trimacrocyclic ligand, 1,3,5-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene (L~(mes)), have been prepared and characterized. The trinuclear complex, [Cu_3L~(mes)(#mu#-OH)_2(H_2O_2)](ClO_4)_4·3.2H_2O (1), formed when the pH of an aqueous solution of [Cu_3L~(mes)(H_2O)_6](ClO_4)_6·6H_2O, was adjusted to ca. 6. X-Ray structural analysis confirmed the presence of a binuclear [Cu_2(#m#-OH)_2]~(2+) core and an isolated Cu(II) centre. The "roof-shaped" [Cu_2(#mu#-OH)_2]~(2+) core has a dihedral angle (#delta#) of 152 deg between the CuO_2 planes and exhibits a relatively short Cu…Cu distance of 2.9041(8) A. An increase in pH to 9.5 generates the hexanuclear complex, [Cu_6(L~(mes)_2(#mu#-OH_6](ClO_4)_6·2H_2O (2), by linking two [Cu_3L~(mes)(#mu#-OH)_2]~(4+) trinuclear units via two #mu#-hydroxo bridges. The structure of 2 features three [Cu_2(#mu#-OH)_2]~(2+) units, two with bent geometries, similar to that observed in 1 [#delta# = 153 deg,Cu…Cu =2.8757(8) A], and one with planar geometry [Cu…Cu = 2.961(1) A]. A variable temperature magnetic susceptibility study on 1 has identified an S=1/2 ground state, consistent with a system composed of an antiferromagnetically coupled Cu(II) pair (J = -24 cm~(-1)) and a magnetically isolated Cu(II) centre. Variable temperature Q-band EPR spectra confirmed this interpretation. Comparisons to the Q-band EPR spectra of the previously reported trinuclear complex, {[Cu_3L~(mes)(#mu#-OH)(#mu#_3-HPO_4)(H_2O)](PF_6)_3·3H_2O}_n, were made, the latter also shows a S = 1/2 ground state but with the unpaired electron delocalised between two Cu(II) ions. The susceptibility data for 2 were interpreted in terms of the presence of three independent Cu(II) binuclear units with seak antiferromagnetic coupling observed in both the bent and planar [Cu_2(#mu#-OH)_2]~(2+) cores (J = -61 and -29 cm~(-1), respectively).