Designing Novel 1,2,3- and 1,2,4-Triazoles for Coordination Complexes and Frameworks

Triazoles are a versatile ligand that forms a variety of compounds and extended materials within the field of coordination chemistry. The reaction of triazoles with metals leads to the synthesis of homoleptic complexes, coordination polymers and metal-organic frameworks (MOFs). These triazole compounds and materials have been employed for a variety of applications in chemistry ranging from nanomaterials to anion recognition.

The synthesis of tris(1,2,3-triazolyl)borate completed and complemented the initial research began by Trofimenko in 1966. This ligand was used in metal reactions and found to form both homoleptic complexes and coordination polymers. The connectivity of the tris(1,2,3-triazolyl)borate varied dependent on the transition metal employed.

The success of incorporating a triazole onto a borate ligand led to further work utilizing novel triazole derivatives. By combining a borane with a 1,2,3-triazole, a borate-nitrenium that has an overall neutral charge can be synthesized. Nitrenium ligands are the nitrogen analog of Arduengo’s N-heterocyclic carbene and have only recently been explored. Although initial attempts to synthesize a metal complex with the designed monodentate neutral nitrenium ligands were unsuccessful, the realization of this potential complex would influence our understanding of transition metal bonds.

Metal-organic frameworks were initially synthesized with carboxylates as the organic linkers connected to the metal centers of the scaffold. Recently, triazoles and tetrazoles have been utilized as an alternative for the formation of porous MOFs. The gate-opening effect of some breathing MOFs has been successfully shown with the inclusion of a bis-triazole linker with a rotating central moiety. By extending the central moiety from a phenyl ring to a biphenyl unit on the bis-triazole linker, we were able to demonstrate three separate states of rotation (open/open, open/closed, closed/closed) within three separate MOFs. These separate states were observed through single crystal X-ray diffraction and were the first example of an isostructurally designed family of gate-based breathing MOFs.

Spin-crossover has been shown in iron MOFs by incorporating triazoles and tetrazoles within the organic linker. A family of mixed triazole/tetrazole as well as bis-tetrazole ligands were synthesized and tested with metal salts. A coordination polymer was synthesized with these ligands.