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

Seven cobalt(III) complexes of the macrobicyclic tetraamine ligand [2(4).3(1)]adamanzane ([2(4).3(1)]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 degrees 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([ 2(4).3(1)] adz)( HCO3)(2+) ion has been measured to K-a = 0.33 mol dm(-3) [ 25 degrees C, I = 2 mol dm(-3)] and K-a = 0.15 mol dm(-3) [ 25 degrees 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 [ 3(5)] 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 con. guration of the secondary non-bridged amine groups is seen for the complexes of the larger [ 3(5)] adz ligand. The high af. nity 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.