Exploring the Structural, Magnetic, Optical, and Catalytic Properties of High-Spin Materials

This thesis explores the design, synthesis, and characterization of lanthanide-based coordination frameworks (MOFs) and heterometallic 3d/4f complexes, with a focus on tunable magnetic behavior, photophysical properties, and catalytic performance. Four complementary research directions were pursued, each addressing key challenges in molecular magnetism, luminescence and sustainable catalysis. The first project examines a family of isomorphous Dy(III), Gd(III), and Tb(III)-MOFs constructed from 2',5'-bis(methoxymethyl)-[1,1':4',1''-terphenyl]-4,4'' dicarboxylic acid (H2L2.1). Systematic post-synthetic solvent exchange led to tunable structural perturbations that profoundly affected the magnetic relaxation dynamics and photophysical behavior of the Dy-MOF. Alternating current (ac) magnetic studies of the Dy-MOF series revealed distinct relaxation pathways linked to subtle crystallographic changes. A second ligand, 2',5' bis(ethoxymethyl)-[1,1':4',1''-terphenyl]-4,4''-dicarboxylic acid (H2L2.2), demonstrated how minimal chemical modifications to the linker can afford new MOFs with conserved coordination geometries and consistent magnetic and luminescent properties across the lanthanide series. These results underscore the sensitivity of magnetic relaxation and energy transfer processes to both the framework structure and local symmetry. In the second project, three novel, isostructural Dy-, Gd- and Tb-MOFs were synthesized from 4,4′-(1,2,4,5-tetrazine-3,6-diyl)dibenzoic acid (H₂TZDC). ac magnetic studies revealed the MOFs exhibit complex temperature-dependent slow magnetic relaxation, attributed to combined quantum tunneling (QTM), Orbach, and Raman mechanisms. Notably, the Gd-based system displays rare frequency-dependent relaxation, rationalized by a small axial anisotropy in the crystal field. Photoluminescence studies on this series of MOFs reveal inefficient ligand-to-metal energy transfer, attributed to mismatched electronic energies. In the third study a series of tetranuclear {Cu₄Ln₄} clusters assembled from the rac-mpzmH ligand are reported. Magnetostructural studies reveal that weak Cu–Ln antiferromagnetic coupling, strong Cu–Cu ferromagnetic interactions, and variations in the coordination geometries collectively govern the magnetic behaviour. Field-induced slow relaxation was observed in the {Cu4Dy4} cluster, while the Tb and Er analogues exhibited active QTM, effectively suppressing relaxation over the energy barrier even in the presence of an external magnetic field. The photoluminescence was largely quenched by the Cu(II) centers, although trace Ln-centered emission was observed in the {Cu4Tb4} cluster. These results highlight the interplay between nuclearity, bridging geometry, and electronic structure in determining the magnetic and optical properties. Project 4 focuses on heterometallic 3d/4f complexes of the form [TM₂Ln(TX-TACN)]⁺ (TM(II) = Cu and Zn; Ln(III) = Gd, Dy, Tb, Eu and Ho); and TX TACNH3 = N,N′,N″-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) as catalysts for the cycloaddition of CO₂ with epoxides. X-ray diffraction studies revealed that all the complexes comprise of a linear TM–Ln–TM motif with a subtle variation in the coordination geometry dependent upon the 3d ion. Catalytic studies showed that the Zn(II)-based complexes significantly outperform their Cu(II) counterparts, achieving high turnover numbers and frequencies for propylene oxide conversion. Selectivity studies using cyclohexene oxide demonstrate stereochemical control and unique reactivity pathways. The catalytic performance varies with the 4f metal and counterion, demonstrating the modularity and adaptability of these systems for green chemistry applications.

Name thesis:degree_name

Ph.D. Chemistry

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Faculty of Mathematics and Science

Department dc:contributor.department

Department of Chemistry

Grantor dc:publisher

Brock University

Year dc:date.issued

2025

Author dc:creator

• Bakhshi, Hamed

Advisor dc:contributor.advisor

• Pilkington, Melanie

• NATURAL SCIENCES::Chemistry::Inorganic chemistry::Coordination chemistry
• Single-Molecule Magnets
• Photoluminescence Spectroscopy
• Metal-Organic Frameworks
• Catalysis

Language dc:language.iso

eng

Handle dc:identifier.uri

https://hdl.handle.net/10464/19455

OAI identifier oai:identifier

oai:brocku.scholaris.ca:10464/19455