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Synthesis and Magnetic Properties of Functionalized Tris(Pyrazolyl)Borate Complexes of Iron and Cobalt.

Record Type:	Language materials, manuscript : Monograph/item
Title/Author:	Synthesis and Magnetic Properties of Functionalized Tris(Pyrazolyl)Borate Complexes of Iron and Cobalt./
Author:	Ma, Chenyang.
Description:	1 online resource (624 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 84-07, Section: B.
Contained By:	Dissertations Abstracts International84-07B.
Subject:	Chemistry. -
Online resource:	click for full text (PQDT)
ISBN:	9798368435121

Synthesis and Magnetic Properties of Functionalized Tris(Pyrazolyl)Borate Complexes of Iron and Cobalt.
Ma, Chenyang.

Synthesis and Magnetic Properties of Functionalized Tris(Pyrazolyl)Borate Complexes of Iron and Cobalt. - 1 online resource (624 pages)

Source: Dissertations Abstracts International, Volume: 84-07, Section: B.

Thesis (Ph.D.)--The George Washington University, 2023.

Includes bibliographical references

Spin crossover (SCO) complexes and single molecule magnets (SMMs) are promising candidate for spin-based electronic devices. However, it is a challenge to retain these properties after integrating these molecules into devices. Therefore, we target the synthesis of functionalized di(tris(pyrazolyl)borate) complexes of iron and cobalt (II) ([(Tp)2M] M = Fe, Co) complexes, as the two tridentate ligands form a rigid structure with robust magnetic properties. In the case of iron complexes, while prior studies of the functionalization of those derivatives have focused on the electronic and steric effect of alkyl and -CF3 groups in position 3, a pyrazole exchange reaction between nitro- or iodo-substituted pyrazole and either e or its iron complex allows the regioselective installation of nitro or iodo substituents in positions 3, 4 and 5 of [(Tp)2Fe] complexes. The degree of substitution can be varied from 1 to 4 functionalized pyrazoles per complex. The amine functionalized analogues are accessed by reduction of the nitro analogues under hydrogen transfer conditions. With the exception of [(3-NO2Tp)2Fe] and [((3-NO2)2Tp)2Fe] substituted complexes, all derivatives display spin crossover properties in the solid state and in solution, with transition temperatures ranging from 180 to 380 K and showing different degree of abruptness, but no hysteresis. The Slichter-Drickamer model was used to extract empirical thermodynamic transition parameters for solid state samples, while a simple equilibrium model was used for solution studies. The results allow a systematic investigation of the influence of stoichiometry, position, and electronic nature of the substituent on the magnetic properties of the complexes. Overall, steric effects dominate for substitution in position 3, but electronic effects are significant for the other positions, a trend that is also reflected in the Fe(II)/Fe(III) redox potentials. The enthalpies of spin transition were found to be remarkably consistent between solution and solid state, while the entropies of spin transitions are higher in solution, leading to lowered transition temperature. Cobalt(II) analogues were also synthesized and their field induced single-molecule magnetic properties were investigated. Despite modifications of the substituents on the pyrazole ring, these high spin CoII complexes show very similar optical properties. Temperature-dependent magnetic susceptibility data from direct current (dc) measurements confirm that these molecules have easy or Z-axis anisotropy. Alternating current (ac) measurements were performed at different field strengths, fitting high temperature data points from Arrhenius plots to Orbach relaxation process yields the effective energy barrier, Ueff, which for these complexes ranges from 6 to 62 K. Interestingly, iron and cobalt complexes bearing 2-4 substituents across the two ligands display an equilibrium between a cis and a trans isomers in solution. Cis/trans ratios were measured by NMR spectroscopy in dichloromethane and acetonitrile solvents. The isomerization is mainly governed by steric effects. In the case of cobalt, the polarity of the solvent also affects the isomerization. These easy sublimable complexes are promising compounds for the fabrication of film devices towards spintronic applications. Complexes with reactive amino and iodo substitutents are also ideal for covalent binding to substrates such as functionalized single-wall carbon nanotubes and will allow future investigations of spin transition and spin reversal at the single molecule level.

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2024

Mode of access: World Wide Web

ISBN: 9798368435121Subjects--Topical Terms:

593913
Chemistry.
Subjects--Index Terms:

Single-molecule magnetsIndex Terms--Genre/Form:

554714
Electronic books.

Synthesis and Magnetic Properties of Functionalized Tris(Pyrazolyl)Borate Complexes of Iron and Cobalt.
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520 $a Spin crossover (SCO) complexes and single molecule magnets (SMMs) are promising candidate for spin-based electronic devices. However, it is a challenge to retain these properties after integrating these molecules into devices. Therefore, we target the synthesis of functionalized di(tris(pyrazolyl)borate) complexes of iron and cobalt (II) ([(Tp)2M] M = Fe, Co) complexes, as the two tridentate ligands form a rigid structure with robust magnetic properties. In the case of iron complexes, while prior studies of the functionalization of those derivatives have focused on the electronic and steric effect of alkyl and -CF3 groups in position 3, a pyrazole exchange reaction between nitro- or iodo-substituted pyrazole and either e or its iron complex allows the regioselective installation of nitro or iodo substituents in positions 3, 4 and 5 of [(Tp)2Fe] complexes. The degree of substitution can be varied from 1 to 4 functionalized pyrazoles per complex. The amine functionalized analogues are accessed by reduction of the nitro analogues under hydrogen transfer conditions. With the exception of [(3-NO2Tp)2Fe] and [((3-NO2)2Tp)2Fe] substituted complexes, all derivatives display spin crossover properties in the solid state and in solution, with transition temperatures ranging from 180 to 380 K and showing different degree of abruptness, but no hysteresis. The Slichter-Drickamer model was used to extract empirical thermodynamic transition parameters for solid state samples, while a simple equilibrium model was used for solution studies. The results allow a systematic investigation of the influence of stoichiometry, position, and electronic nature of the substituent on the magnetic properties of the complexes. Overall, steric effects dominate for substitution in position 3, but electronic effects are significant for the other positions, a trend that is also reflected in the Fe(II)/Fe(III) redox potentials. The enthalpies of spin transition were found to be remarkably consistent between solution and solid state, while the entropies of spin transitions are higher in solution, leading to lowered transition temperature. Cobalt(II) analogues were also synthesized and their field induced single-molecule magnetic properties were investigated. Despite modifications of the substituents on the pyrazole ring, these high spin CoII complexes show very similar optical properties. Temperature-dependent magnetic susceptibility data from direct current (dc) measurements confirm that these molecules have easy or Z-axis anisotropy. Alternating current (ac) measurements were performed at different field strengths, fitting high temperature data points from Arrhenius plots to Orbach relaxation process yields the effective energy barrier, Ueff, which for these complexes ranges from 6 to 62 K. Interestingly, iron and cobalt complexes bearing 2-4 substituents across the two ligands display an equilibrium between a cis and a trans isomers in solution. Cis/trans ratios were measured by NMR spectroscopy in dichloromethane and acetonitrile solvents. The isomerization is mainly governed by steric effects. In the case of cobalt, the polarity of the solvent also affects the isomerization. These easy sublimable complexes are promising compounds for the fabrication of film devices towards spintronic applications. Complexes with reactive amino and iodo substitutents are also ideal for covalent binding to substrates such as functionalized single-wall carbon nanotubes and will allow future investigations of spin transition and spin reversal at the single molecule level.
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