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 [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)(HCO(3))(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 configuration of the secondary non-bridged amine groups is seen for the complexes of the larger [3(5)]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.

Incorporation of Cobalt‐Cyclen Complexes into Templated Nanogels Results in Enhanced Activity

“…The nanogel imprinted with the carbonate P1 , shows even higher catalytic efficiency and turnover, despite the carbonate being considerably smaller than the cognate template and containing a pseudo‐octahedral active site. Initially this result appears to be rather counter intuitive and surprising, however, analysis of the crystal structure of an analogue of [Co( 15 b )(CO 3 )]Cl reveals the O‐Co‐O angle to be rather strained (O(1)‐Co(1)‐O(2)=68.54(9)°), which is very similar to that seen in other cobalt(III) complexes containing bidentate carbonate ligands in the Cambridge Crystallographic Database . In contrast, the equivalent angle in cobalt(III) phosphate chelates is considerably larger at 76° …”

“…Initially this result appears to be rather counter intuitive and surprising,h owever,a nalysis of the crystal structure of an analogue of [Co(15 b)(CO 3 )]Cl re- (9)8), [44] which is very similart ot hat seen in other cobalt(III)c omplexes containing bidentate carbonate ligands in the Cambridge Crystallographic Database. [45] In contrast, the equivalent angle in cobalt(III) phosphate chelates is considerably larger at 768. [46] This suggests that imprinting with carbonate produces an active site in the polymer, whichf urther enhances catalytic activity.Acombination of factors is likely to contribute to these results.…”

Macrocyclic coordination chemistry

“…63 The X-ray crystal structures of six cobalt(III) complexes of bowl adamanzane ligands were characterised. 64 The affinities and exchange rates were investigated for a series of oxo-anions showing slow exchange rates and a high propensity to bind to such anions. The structural data could be used to account for differences in exchange rates between different ring size adamanzane complexes.…”

“…A cross bridged cyclam macrocyclic ligand with pendant propionoate arms L 87 has been developed as a chelator for the radioisotope 64 Cu showing similarly high stability to decomplexation under acidic conditions to the acetate analogue previously studied. 124 However, this copper(II) complex is significantly easier to reduce and shows poorer bio-clearance properties, suggesting the reduction potential is particularly important to the in vivo profile.…”

“…124 However, this copper(II) complex is significantly easier to reduce and shows poorer bio-clearance properties, suggesting the reduction potential is particularly important to the in vivo profile. A detailed study was carried out by the same group to probe the in vivo properties of 64 Cu labelled cross bridged cyclam compounds with one amide and one acetate arm L 88 . 125 These compounds are models of peptide conjugates and appear to show approporiate stability for use as targeted PET imaging agents.…”