Abstract
A family of novel tripodal, tetraamine (N4) ligands incorporating three heterocyclic donors and inequivalent donor arms has been synthesized and characterized. All of the ligands incorporate at least one pyrazolyl or 3,5-(dimethyl)pyrazolyl donor, and many include either a pyridyl, imidazolyl, or 1,2,3-triazolyl ring. Additionally, these ligands have been complexed with cobalt, copper or zinc and crystallographically characterized.
Ligands incorporating the pyrazolylmethylamino moiety were found to be unstable in the presence of protic solvents, regardless of the alkylation pattern of the pyrazolyl donor, and in solution they are found to be in equilibrium with their N-dealkylated counterparts. On the basis of crystallographic, mass spectral and 1H NMR studies, an acid-promoted mechanism for this transformation is suggested. Despite observation of N-dealkylation, complexes of the intact tripodal ligands can be synthesized with kinetically inert metal ion (i.e cobalt(III)), and the intact ligands are stabilized on metal coordination.
Mixed pyrazolyl-1,2,3-triazolyl ligands have been readily synthesized using the functional group tolerant copper-catalyzed azide-alkyne cycloaddition (CuAAC). These ligands have been coordinated to a cobalt(III) carbonato fragment. Evidence for the formation of ligands with one, two or three N4 binding pockets, and their mono-, bis- and tris-cobalt complexes, is presented. CuAAC also allows for synthesis of ligands with different types of binding pockets, followed by sequential metallation. Synthesis of a ligand with an N4 binding pocket and a pyridyl-1,2,3-triazole binding pocket is described, and the cobalt(III) carbonato complex of this complex has been synthesized and crystallographically characterized. Coordination of silver(I) is observed, and a cobalt-silver complex can be isolated and characterized; however, data collected are ambiguous on the full structure of the mixed metallic complex.
The acid hydrolysis of cobalt(III) carbonato complexes of the N4 ligands described in this thesis is studied in detail, and the factors impacting this reaction are studied using a variety of techniques, including solution-phase infrared spectroscopy, 59Co NMR spectroscopy, UV-visible spectroscopy, and cyclic voltammetry. It has been previously observed that electronic differences between carbonato ligands in various cobalt(III) carbonato complexes are minimal, and further evidence for this observation is presented. Data agree with previous suggestions that steric bulk inhibits the rate of hydrolysis, suggesting an associative rate-limiting mechanistic step. This is rationalized by noting the bidentate nature of the carbonato ligand, and an addition to the currently accepted mechanism of reaction is suggested.