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Walker AN, Ayala MA, Mondal S, Bergagnini MC, Bui PJD, Chidester SN, Doeden CI, Esjornson L, Sweany BR, Garcia L, Krause JA, Oliver AG, Prior TJ, Hubin TJ. A Bridge too Far? Comparison of Transition Metal Complexes of Dibenzyltetraazamacrocycles with and without Ethylene Cross-Bridges: X-ray Crystal Structures, Kinetic Stability, and Electronic Properties. Molecules 2023; 28:molecules28020895. [PMID: 36677952 PMCID: PMC9863842 DOI: 10.3390/molecules28020895] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/06/2022] [Revised: 01/09/2023] [Accepted: 01/10/2023] [Indexed: 01/18/2023]
Abstract
Tetraazamacrocycles, cyclic molecules with four nitrogen atoms, have long been known to produce highly stable transition metal complexes. Cross-bridging such molecules with two-carbon chains has been shown to enhance the stability of these complexes even further. This provides enough stability to use the resulting compounds in applications as diverse and demanding as aqueous, green oxidation catalysis all the way to drug molecules injected into humans. Although the stability of these compounds is believed to result from the increased rigidity and topological complexity imparted by the cross-bridge, there is insufficient experimental data to exclude other causes. In this study, standard organic and inorganic synthetic methods were used to produce unbridged dibenzyl tetraazamacrocycle complexes of Co, Ni, Cu, and Zn that are analogues of known cross-bridged tetraazamacrocycles and their transition metal complexes to allow direct comparison of molecules that are identical except for the cross-bridge. The syntheses of the known tetraazamacrocycles and the new transition metal complexes were successful with high yields and purity. Initial chemical characterization of the complexes was conducted by UV-Visible spectroscopy, while cyclic voltammetry showed more marked differences in electronic properties from bridged versions. Direct comparison studies of the unbridged and bridged compounds' kinetic stabilities, as demonstrated by decomposition using high acid concentration and elevated temperature, showed that the cyclen-based complex stability did not benefit from cross-bridging. This is likely due to poor complementarity with the Cu2+ ion while cyclam-based complexes benefited greatly. We conclude that ligand-metal complementarity must be maintained in order for the topological and rigidity constraints imparted by the cross-bridge to contribute significantly to complex robustness.
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Affiliation(s)
- Ashlie N. Walker
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Megan A. Ayala
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Somrita Mondal
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Mackenzie C. Bergagnini
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Phuong John D. Bui
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Stephanie N. Chidester
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Chad I. Doeden
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Louise Esjornson
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Brian R. Sweany
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Leslie Garcia
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
| | - Jeanette A. Krause
- Department of Chemistry, University of Cincinnati, Cincinnati, OH 45220, USA
| | - Allen G. Oliver
- Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Timothy J. Prior
- Department of Chemistry, School of Natural Sciences, University of Hull, Kingston Upon Hull HU6 7RX, UK
- Correspondence: (T.J.P.); (T.J.H.)
| | - Timothy J. Hubin
- Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA
- Correspondence: (T.J.P.); (T.J.H.)
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