Bowl adamanzanes—bicyclic tetraamines: syntheses and crystal structures of complexes with cobalt(iii) and chelating coordinated oxo-anions

Abstract

Seven cobalt(III) complexes of the macrobicyclic tetraamine ligand [2⁴.3¹]adamanzane ([2⁴.3¹]adz) are reported along with the crystal structure of six of these complexes. The solid state and solution structures are discussed, and a detailed assignment of the NMR spectra of the sulfato complex is provided. Four of the seven complexes contain a chelate coordinating oxo-anion (sulfate, formiate, nitrate, carbonate). Equilibration of these species with the corresponding diaqua complex is generally slow. The rates of equilibration in 5 mol dm⁻³ perchloric acid at 25 °C have been measured, yielding half lives of 20 min, 10 min and 3 h for the sulfato, formiato and carbonato species respectively. The corresponding reaction for the nitrato complex occurs with a half life of less than 3 min. The concentration acid dissociation constant for the Co([2⁴.3¹]adz)(HCO₃)²⁺ ion has been measured to K_a = 0.33 mol dm⁻³ [25 °C, I = 2 mol dm⁻³] and K_a = 0.15 mol dm⁻³ [25 °C, I = 5 mol dm⁻³]. The propensity for coordination of sulfate was found to be large enough for a quantitative conversion of the carbonato complex to the sulfato complex to occur in 3 mol dm⁻³ triflic acid containing a small sulfate contamination. On this basis the decarboxylation in 5 mol dm⁻³ triflic acid of the corresponding cobalt(III) carbonato complex of the larger macrobicyclic tetraamine ligand [3⁵]adz was reinvestigated and found to lead to the sulfato complex as well. The difference in exchange rate of the oxo-anion ligands for the cobalt(III) complexes of the two adamanzane ligands is discussed and attributed to fundamental differences in the molecular structure where an inverted configuration of the secondary non-bridged amine groups is seen for the complexes of the larger [3⁵]adz ligand. The high affinity for chelating coordination of oxo-anions for these two cobalt(III)–adamanzane-moieties is rationalised on basis of the N–Co–N angles. N–Co–N angles are compared for a series of adamanzane complexes, and the structural consequences are discussed.