6, 13-Diamino-6,13-dimethyl-1,4,8,11-tetra-azacyclotetradecane, L7, a new, potentially sexidentate polyamine ligand. Variable co-ordination to cobalt (III) and crystal structure of the complex[CO(L7)]Cl2[ClO4]

Abstract

The facile synthesis of the hydrochloride salt of anti-6,13-diamion-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane, a new type of potentially sexidentate ligand, is described. The ligand can act as a quadridentate macrocyclic ligand to cobalt(III) with both amine pendants unco-ordinated, or as a quinque- or sexi-dentate one with one or two amine pendants co-ordinated axially, respectively. All three types of cobalt(III) complexes have been prepared and characterized spectroscopically. The sexidentate form crystallizes in the space group C2/c, with a= 15.616(4), b= 7.497(1), c= 17.922 Å, β= 108.63(1)°, and Z= 4, and displays the shortest Co–N bond lengths reported for a precisely determined cobalt(III) hexa-amine structure. The macrocyclic Co–N distances are 1.937(2)Å whereas the pendant Co–N distances are similar at 1.946(2)Å. The short Co–N bond lengths are reflected in physical properties, with maxima in the electronic spectra shifted toward higher energy and the cobalt(III)–cobalt(II) redox couple shifted to a more negative potential compared with other CoN₆ analogues. The cobalt(III)–cobalt(II) self-exchange rate for the sexidentate complex, determined via a cross reaction with Zn^I, is estimated as ≈ 900 dm³ mol^–1 s^–1, ca. 100-fold faster than the value for [Co(L¹)]³⁺(L′= 3,6,10,13,16,19-hexa-azabicyclo[6.6.6]-eicosane), and nearly 10⁸- fold faster than the value for [Co(en)₃]³⁺(en = ethane-1,2-diamine).