Magnetic Direct Exchange and Heavy Atom Effects on Slow Magnetic Relaxation in Transition Metal Single-Molecule Magnets

This dissertation describes the design, synthesis, and characterization of a wide array of transition metal complexes for application in single-molecule magnets. Magnetic relaxation in paramagnetic metal complexes proceed through several processes. This work investigates how synthetic chemistry tools can lead to a better understanding of and control over these relaxation processes, with the goal to slow down all possible relaxation pathways so that single-molecule magnets can retain their magnetization as long as possible. Chapter One provides a fundamental background of single-molecule magnet behavior in transition metal complexes and delineates important criteria to consider in designing single-molecule magnets. Two major strategies are explored. First, large spin clusters with strong magnetic exchange are explored in Chapters Two, Three, and Four with the goal to minimize through-barrier relaxation processes. Second, the effect of heavy atoms are investigated in Chapters Five and Six, with the aim to understand spin-vibronic coupling which undermines the spin-reversal barrier./nChapter Two describes the synthesis and characterization of semiquinone-bridged dinuclear transition metal complexes of iron(II), cobalt(II), and nickel(II). In this system, tris(2-dimethylaminoethyl)amine (Me6tren) ligand scaffold enforces trigonal pyramidal geometry on the metal centers, engendering large magnetic anisotropy. This work also showcases the first magnetometry measurement of a semiquinone radical, which subsequently shows exceptionally strong magnetic direct exchange. This results in the first example of a thermally isolated large spin ground state in a multinuclear Ni complex realized in [(Me6tren)2Ni2(µ2-C6H4O2)]3+. Slow magnetic relaxation is observed in [(Me6tren)2Co2(µ2-C6H4O2)]3+ and [(Me6tren)2Ni2(µ2-C6H4O2)]3+ with an evident Orbach relaxation barrier of 22 and 46 cm–1, respectively. Through-barrier relaxation processes are partially suppressed as a result of magnetic coupling in this system./nChapter Three describes the work on tetranuclear metal clusters [M4(NPtBu3)4]+/0 (M = Co, Ni, Cu; tBu = tert-butyl) featuring low-coordinate metal centers engaged in direct metal–metal orbital overlap. These clusters show thermally isolated large spin ground states as a result of extremely strong ferromagnetic direct exchange from magnetic orbital direct overlap and delocalized, itinerant electrons. This exchange mechanism is analogous to magnetic interactions in ferromagnetic metals. Unusually large magnetic anisotropy is observed, which is attributed to the low-coordinate environment around the cobalt and nickel centers. The [Ni4(NPtBu3)4]+ complex exhibits the first example of zero-field slow magnetic relaxation in easy-plane molecular magnets, and slow magnetic relaxation under a small applied field solely follows Orbach process. Magnetic characterization of the copper analog [Cu4(NPtBu3)4]+ reveals that the alternative spin-vibronic relaxation is sufficiently slow, thus enabling the observation of the sole Orbach barrier in [Ni4(NPtBu3)4]+ with a spin reversal barrier of 16 cm–1. The [Co4(NPtBu3)4]+ complex exhibits a thermally isolated S = 9/2 ground state and a large magnetic anisotropy D = –12.34 cm–1. The molecule exhibits a spin reversal barrier of 87 cm–1, the largest value reported for transition metal clusters./nChapter Four describes the design, synthesis, and characterization of a series of dinuclear trigonal nickel paddlewheel complexes, Ni2DArF3 (DArF– = N,N′-diarylformamidinate). This work is motivated by the need to rationally control magnetic anisotropy in a system featuring direct metal–metal orbital overlap. The synthesis of the trigonal nickel paddlewheel complexes are described, showing the first examples of high spin nickel paddlewheel complexes. Due to the direct metal orbital overlap, the compounds exhibit thermally isolated S = 3/2 ground state. By changing the aryl substituents of the ligands, the trigonal symmetry around the metal centers can be tuned. Consequently, magnetic anisotropy of the complexes can be adjusted from D = –13 to –29 cm–1. Spin reversal barriers of 26 to 55 cm–1 are observed, with partially suppressed through-barrier relaxations./nChapter Five describes a mononuclear triad M(CNDipp)6 (M = V, Nb, Ta; Dipp = 2,6-diisopropylphenyl) as an experimental validation of the newly proposed spin-vibronic relaxation model. These low-spin S = 1/2 isocyanide complexes exhibit slow magnetic relaxation via spin-vibronic coupling. Analysis of relaxation dynamics in this series indicates that spin-orbit coupling of the metal center facilitates spin-vibronic relaxation, as is evident by the observation that the spin relaxation rate of the tantalum complex is the fastest, followed by niobium, and vanadium complexes, respectively./nChapter Six describes the study of heavy ligand effect on slow magnetic relaxation in two-coordinate nickel(I) complexes, (IPr)NiE(SiMe3)3 (IPr = IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazoline-2-ylidene; E = C, Si, Ge, Sn). As in previous chapter, these nickel complexes feature S = 1/2 ground state and slow magnetic relaxation arising from spin-vibronic coupling. However, partially unquenched orbital angular momentum due to the linear geometry significantly adds anisotropy to the system. The alkyl complex (IPr)NiC(SiMe3)3 exhibits notable quantum tunneling of magnetization at low temperature, and Raman relaxation at higher temperature. Upon moving to heavier ligands in (IPr)NiE(SiMe3)3 (E = Si, Ge, Sn), the complexes show significantly slower quantum tunneling and approximately five times slower Raman relaxation. This work provides an experimental evidence of the effect of heavy ligand on slow magnetic relaxation, a much-debated topic at the current time.

Advisor: Long, Jeffrey R