Design, syntheses, and magnetic properties of Dy(III)-based single-molecule magnets and isotopologues

Single-molecule magnets (SMMs) have a wide range of potential applications in molecular spintronics, high-density information storage materials, and quantum computing. Due to the inherent strong spin-orbital coupling effect and, consequently, the very high magnetic anisotropy of the ${}^6$H${}_{15/2}$ ground state, Dy(Ⅲ) ions are currently among the best candidates for SMMs. This thesis focuses on constructing Dy(Ⅲ) single molecular magnets based on different $\beta$-diketonate and additional auxiliary ligands. The main work is be divided into the following three parts: 1. In Chapter 2, six mononuclear Dy(Ⅲ) complexes are described based on four different $\beta$-diketonate ligands, tmhd, BTFA, NTFA, and hfac, as well as two auxiliary ligands, POPh${}_3$ and 2,5-tpy. The synthesized complexes are [Dy(thmd)${}_3$(POPh${}_3$)] (2-1), [Dy(thmd)${}_3$(2,5-tpy)] (2-2), [Dy(BTFA)${}_3$(2,5-tpy)] (2-3), [Dy(NTFA)${}_3$(2,5-tpy)] (2-4), [Dy(NTFA)${}_3$(2,5-tpy)$\cdot$C${}_2$H${}_5$OH] (2-5), and [Dy(hfac)${}_3$(2,5-tpy)(H${}_2$O)$\cdot$(2,5-tpy)] (2-6). The Dy(Ⅲ) ion in complex 2-1 is placed in a N${}_2$O${}_5$ coordination environment. The overall geometry is best described as a capped octahedron with C${}_{3v}$ ligand field symmetry around the Dy(III). In complexes 2-2, 2-3, 2-4, and 2-5, the Dy(III) ions are placed in a N${}_2$O${}_6$ coordination environment. The geometry of the complexes is best described as square antiprism with D${}_{4d}$ symmetry. The coordination geometry of complex 2-6 is the spherical capped square antiprism with C${}_{4v}$ symmetry; the Dy(III) ion is placed in the N${}_2$O${}_7$ coordination environment. Magnetic investigations evidence that all the complexes are SMMs with energy barriers (U${}_{eff}$) of 35.5 K (2-1, H${}_{dc}$ = 1200 Oe), 36.7 K (2-2, H${}_{dc}$ = 800 Oe), 16.6 K (2-3, H${}_{dc}$ = 0 Oe), 75.5 K (2-4, H${}_{dc}$ = 0 Oe), 63.9 K (2-5, H${}_{dc}$ = 0 Oe), 5.3 K (2-6, H${}_{dc}$ = 800 Oe). The unique relaxation properties exhibited by the complexes are a result of the varying coordination environment surrounding the Dy(Ⅲ) ions. This work demonstrates that the SMM properties can be modulated by subtle changes in the coordination environment resulting from changes in the substituents of $\beta$-diketonate ligands. 2. In Chapter 3, seven dinuclear Dy(Ⅲ) complexes are successfully synthesized using isotopically enriched ${}^{163}$Dy(Ⅲ), ${}^{164}$Dy(Ⅲ) and commercial Dy(Ⅲ) with three distinct sets of bridging ligands, 2,2':5',5'':2'',2'''-Quaterpyridine (bisbpy), 1,6,7,12-Tetraazaperylene (TAPE), 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) as well as three $\beta$-diketonates as co-ligands, tmhd, BTFA, and NTFA. In case of the bptz ligand we observed an unexpected ring opening reaction forming N'-[(E)-pyrazin-2-yl)methylidene]pyrazine-2-carbohydrazonate (PHZP). The following are the structural formula of the complexes: [(Dy(tmhd)${}_3$)${}_2$(bisbpy) CH${}_2$Cl${}_2$] (3-1), [${}^{163}$Dy${}_2$(tmhd)${}_6$(TAPE)] (3-2), [${}^{164}$Dy${}_2$(tmhd)${}_6$(TAPE)] (3-3), [Dy${}_2$(BTFA)${}_6$(TAPE)] (3- 4) [Dy${}_2$(NTFA)${}_6$(TAPE)] (3-5), [(${}^{163}$Dy(BTFA)${}_2$)${}_2$(PHZP)${}_2$] (3-6) and [(${}^{164}$Dy(BTFA)${}_2$)${}_2$(PHZP)${}_2$] (3-7). For complexes 3-1 ‒ 3-5, all the Dy(III) ions are placed in the N${}_2$O${}_6$ coordination environment, the Dy(III) ions of complex 3-1 are both with D${}_{4d}$ symmetry, while the geometries of all the Dy(III) ions are best described as with D${}_{2d}$ symmetry for complexes 3-2, 3-3, and 3-5, and D${}_{4d}$ symmetry for complex 3- 4. In complexes 3-6 and 3-7, the Dy1 ions are in a spherical tricapped trigonal prism D${}_{3h}$ symmetry. For Dy2 in 3-6 and 3-7, the geometry is better described as a Muffin architecture with Cs symmetry. All complexes are zero field SMMs - U${}_{eff}$ values of 30.8 (3-1), 80.6 (3-2), 73.3 (3-3), 47.0 (3-4), 24.5 K (3-5), 50.7 (3-6), and 57.1 K (3-7) are obtained. Comparing complexes 3-2-3-5, the lowest energy barrier is observed in complex 3-5 due to the presence of electron-donating groups in the $\beta$-diketonate co-ligands. This indicates that it is possible to tune the magnetic properties of dinuclear Dy(Ⅲ) complexes by changing the substituents on the $\beta$-diketonate ligands. The results of the μ-SQUID experiments, at low temperatures, between 3-2${}^{(I=5/2)}$, 3-3${}^{(I=0)}$ and 3-6${}^{(I=5/2)}$, 3-7${}^{(I=0)}$ showed that the nuclear spin plays a crucial part in the relaxation process, and the Dy-Dy distances impact the strength of the intramolecular exchange coupling. 3. Chapter 4, two trinuclear Dy(Ⅲ) complexes-[Dy${}_3$(tmhd)${}_9$(TBB)] (4-1) and [Dy${}_3$(tmhd)${}_9$(TBPB)$\cdot$CH${}_2$Cl${}_2$] (4-2)-are successfully synthesized using two different bridging ligands, 1,3,5-tri(2,2' -bipyridin-5-yl)benzene (TBB) and 1,3,5-tris{[2,2'- bipyridin-5-ylethynyl]phenyl}benzene (TBPB) and the same $\beta$-diketonate ligand, tmhd, namely. In both complexes, the Dy(Ⅲ) ions are in square-antiprismatic N${}_2$O${}_6$ coordination environment with D${}_{4d}$ axial symmetry. The complexes are zero field SMMs with U${}_{eff}$ = 75.6 K (4-1) and 92.5 K (4-2), as inferred from the AC susceptibility measurements. The determined Ueff values for the complexes are nominal despite the axially symmetric ligand field around the Dy(III) ions. The different natures of the N-donor ligands induce changes in the coordination microenvironment, giving rise to distinct dynamic magnetic behaviors.