Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

Liu, Jingjing; Reta, Daniel; Cleghorn, Jake A.; Yeoh, Yu Xuan; Ortu, Fabrizio; Goodwin, Conrad A. P.; Chilton, Nicholas F.; Mills, David P.

doi:10.1002/chem.201901167

Cited by 30 publications

(27 citation statements)

References 28 publications

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“…The unique coordination environment provided by rigid η 5 -cyclopentadienyl ligands has led to remarkable magnetic properties in lanthanide complexes; − ,− this can be attributed to the constrained number of metal–ligand vibrational modes that limit the possible pathways for fast, through-barrier magnetic relaxation. , Through-barrier processes, such as quantum tunneling and Raman relaxation, tend to dominate magnetic relaxation in actinide-based systems, − although it was recently predicted that the cation [Cp ttt 2 U] + should be capable of relaxing via an Orbach (overbarrier) process . Furthermore, the increased spin–orbit coupling of the 5f orbitals imparts the actinides with greater magnetic anisotropy than the lanthanides, which can, in principle, lead to larger barriers to magnetic relaxation. − , However, the Cp iPr5 analogue of the uranium metallocenium cation, [(Cp iPr5 ) 2 U][B(C 6 F 5 ) 4 ], and its iodide adduct, (Cp iPr5 ) 2 UI, were recently shown to exhibit relatively fast, field-induced slow magnetic relaxation via a Raman process .…”

Section: Resultsmentioning

confidence: 62%

Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes

et al. 2019

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Addition of the potassium salt of the bulky tetra-(isopropyl)cyclopentadienyl (Cp iPr4 ) ligand to UI 3 (1,4-dioxane) 1.5 results in the formation of the bent metallocene uranium(III) complex (Cp iPr4 ) 2 UI (1), which is then used to obtain the uranium(IV) and uraniumas mononuclear, donor-free complexes, for X − = F − , Cl − , Br − , and I − . Interestingly, reaction of 1 with chloride and cyanide salts of alkali metal ions leads to isolation of the chloride-and cyanide-bridged coordination solids [(Cp iPr4 ) 2 U(μ-Cl) 2 Cs] n (4-Cl) and [(Cp iPr4 ) 2 U(μ-CN) 2 Na(OEt 2 ) 2 ] n (4-CN). Abstraction of the iodide ligand from 1 further enables isolation of the "base-free" metallocenium cation salt [(Cp iPr4 ) 2 U][B(C 6 F 5 ) 4 ] (5) and its DME adduct [(Cp iPr4 ) 2 U(DME)][B(C 6 F 5 ) 4 ] (5-DME). Solid-state structures of all of the compounds, determined by X-ray crystallography, facilitate a detailed analysis of the effect of changing oxidation state or halide ligand on the molecular structure. NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV−visible spectroscopy studies of 2-X and 3-X further reveal that the difluoride species in both series exhibit properties that differ significantly from trends observed among the other dihalides, such as a substantial negative shift in the potential of the [(Cp iPr4 ) 2 UX 2 ] uranium(III/IV) redox couple. Magnetic characterization of 1 and 5 reveals that both compounds exhibit slow magnetic relaxation of molecular origin under applied magnetic fields; this process is dominated by a Raman relaxation mechanism.

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Section: Resultsmentioning

confidence: 62%

Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes

et al. 2019

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“…The global yield of 1 is 26% over three reaction steps based on DyI 3 . The triethylammonium reagent was selected as it provides an entropic driving force with dual amine and alkene elimination during the reaction, and the [Al{OC(CF 3 ) 3 } 4 ] − anion is more weakly coordinating than the [B(C 6 F 5 ) 4 ] − anion, which has been used for the synthesis of all Ln metallocenium cations to date. ,, The direct reaction of [Dy(Dtp) 2 (I)] with [H(SiEt 3 ) 2 ][B(C 6 F 5 ) 4 ] gave an intractable mixture of products. 1 H, 13 C, and 31 P NMR spectra of a sample of 1 in d 5 -chlorobenzene were uninformative due to paramagnetism, but the [Al{OC(CF 3 ) 3 } 4 ] − anion was detected by 19 F NMR spectroscopy (δ F : −90.50 ppm; v 1/2 = 300 Hz); the presence of paramagnetic [Dy(Dtp) 2 ] + cations has broadened this signal and shifted it considerably from the [NEt 3 H][Al{OC(CF 3 ) 3 } 4 ] precursor (δ F : −75.70 ppm, d 2 -DCM).…”

Section: Resultsmentioning

confidence: 99%

Bis-Monophospholyl Dysprosium Cation Showing Magnetic Hysteresis at 48 K

et al. 2019

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Single-molecule magnets (SMMs) have potential applications in high-density data storage, but magnetic relaxation times at elevated temperatures must be increased to make them practically useful. Bis-cyclopentadienyl lanthanide sandwich complexes have emerged as the leading candidates for SMMs that show magnetic memory at liquid nitrogen temperatures, but the relaxation mechanisms mediated by aromatic C5 rings have not been fully established. Here we synthesize a bis-monophospholyl dysprosium SMM [Dy(Dtp)2][Al{OC(CF3)3}4] (1, Dtp = {P(CtBuCMe)2}) by the treatment of in-situ-prepared “[Dy(Dtp)2(C3H5)]” with [HNEt3][Al{OC(CF3)3}4]. SQUID magnetometry reveals that 1 has an effective barrier to magnetization reversal of 1760 K (1223 cm–1) and magnetic hysteresis up to 48 K. Ab initio calculation of the spin dynamics reveals that transitions out of the ground state are slower in 1 than in the first reported dysprosocenium SMM, [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2 tBu3-1,2,4); however, relaxation is faster in 1 overall due to the compression of electronic energies and to vibrational modes being brought on-resonance by the chemical and structural changes introduced by the bis-Dtp framework. With the preparation and analysis of 1, we are thus able to further refine our understanding of relaxation processes operating in bis-C5/C4P sandwich lanthanide SMMs, which is the necessary first step toward rationally achieving higher magnetic blocking temperatures in these systems in the future.

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“…When the ligand field splitting is comparable to the splitting between spin–orbit multiplets, J is no longer a good quantum number, and the coupling scheme breaks down, for example, for Sm(III), leading to the phenomenon of “ J mixing”, commonly invoked to explain unusual oscillator strengths and odd transitions in polarized emission spectra. − Despite this, many other spectral phenomena defy explanation, and “ J mixing” is often cited as a “catch-all”, highlighting limitations in current understanding. ,, …”

Section: Electronic Structure Introductionmentioning

confidence: 99%

How the Ligand Field in Lanthanide Coordination Complexes Determines Magnetic Susceptibility Anisotropy, Paramagnetic NMR Shift, and Relaxation Behavior

et al. 2020

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Conspectus Complexes of lanthanide(III) ions are being actively studied because of their unique ground and excited state properties and the associated optical and magnetic behavior. In particular, they are used as emissive probes in optical spectroscopy and microscopy and as contrast agents in magnetic resonance imaging (MRI). However, the design of new complexes with specific optical and magnetic properties requires a thorough understanding of the correlation between molecular structure and electric and magnetic susceptibilities, as well as their anisotropies. The traditional Judd–Ofelt–Mason theory has failed to offer useful guidelines for systematic design of emissive lanthanide optical probes. Similarly, Bleaney’s theory of magnetic anisotropy and its modifications fail to provide accurate detail that permits new paramagnetic shift reagents to be designed rather than discovered. A key determinant of optical and magnetic behavior in f-element compounds is the ligand field, often considered as an electrostatic field at the lanthanide created by the ligands. The resulting energy level splitting is a sensitive function of several factors: the nature and polarizability of the whole ligand and its donor atoms; the geometric details of the coordination polyhedron; the presence and extent of solvent interactions; specific hydrogen bonding effects on donor atoms and the degree of supramolecular order in the system. The relative importance of these factors can vary widely for different lanthanide ions and ligands. For nuclear magnetic properties, it is both the ligand field splitting and the magnetic susceptibility tensor, notably its anisotropy, that determine paramagnetic shifts and nuclear relaxation enhancement. We review the factors that control the ligand field in lanthanide complexes and link these to aspects of their utility in magnetic resonance and optical emission spectroscopy and imaging. We examine recent progress in this area particularly in the theory of paramagnetic chemical shift and relaxation enhancement, where some long-neglected effects of zero-field splitting, magnetic susceptibility anisotropy, and spatial distribution of lanthanide tags have been accommodated in an elegant way.

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Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

Abstract: As the dysprosocenium complex [Dy(Cp ttt ) 2 ][B(C 6 F 5 ) 4 ] (Cp ttt =C 5 H 2 t Bu 3 ‐1,2,4, 1‐Dy ) exhibits magnetic hysteresis at 60 K, similar lanthanide (Ln) complexes have been targeted to provide insights into this remarkable property. We recently reported homolog… Show more

Cited by 30 publications

References 28 publications

Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes

Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes

Bis-Monophospholyl Dysprosium Cation Showing Magnetic Hysteresis at 48 K

How the Ligand Field in Lanthanide Coordination Complexes Determines Magnetic Susceptibility Anisotropy, Paramagnetic NMR Shift, and Relaxation Behavior

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