Conrad A. P. Goodwin

Molecular magnetic hysteresis at 60 kelvin in dysprosocenium

Lanthanides have been investigated extensively for potential applications in quantum information processing and high-density data storage at the molecular and atomic scale. Experimental achievements include reading and manipulating single nuclear spins, exploiting atomic clock transitions for robust qubits and, most recently, magnetic data storage in single atoms. Single-molecule magnets exhibit magnetic hysteresis of molecular origin—a magnetic memory effect and a prerequisite of data storage—and so far lanthanide examples have exhibited this phenomenon at the highest temperatures. However, in the nearly 25 years since the discovery of single-molecule magnets, hysteresis temperatures have increased from 4 kelvin to only about 14 kelvin using a consistent magnetic field sweep rate of about 20 oersted per second, although higher temperatures have been achieved by using very fast sweep rates (for example, 30 kelvin with 200 oersted per second). Here we report a hexa-tert-butyldysprosocenium complex—[Dy(Cpttt)2][B(C6F5)4], with Cpttt = {C5H2tBu3-1,2,4} and tBu = C(CH3)3—which exhibits magnetic hysteresis at temperatures of up to 60 kelvin at a sweep rate of 22 oersted per second. We observe a clear change in the relaxation dynamics at this temperature, which persists in magnetically diluted samples, suggesting that the origin of the hysteresis is the localized metal–ligand vibrational modes that are unique to dysprosocenium. Ab initio calculations of spin dynamics demonstrate that magnetic relaxation at high temperatures is due to local molecular vibrations. These results indicate that, with judicious molecular design, magnetic data storage in single molecules at temperatures above liquid nitrogen should be possible.

[UIII{N(SiMe2tBu)2}3]: A Structurally Authenticated Trigonal Planar Actinide Complex

We report the synthesis and characterization of the uranium(III) triamide complex [UIII(N**)3] [1, N**=N(SiMe2tBu)2−]. Surprisingly, complex 1 exhibits a trigonal planar geometry in the solid state, which is unprecedented for three-coordinate actinide complexes that have exclusively adopted trigonal pyramidal geometries to date. The characterization data for [UIII(N**)3] were compared with the prototypical trigonal pyramidal uranium(III) triamide complex [UIII(N“)3] (N”=N(SiMe3)2−), and taken together with theoretical calculations it was concluded that pyramidalization results in net stabilization for [UIII(N“)3], but this can be overcome with very sterically demanding ligands, such as N**. The planarity of 1 leads to favorable magnetic dynamics, which may be considered in the future design of UIII single-molecule magnets.

Investigation into the Effects of a Trigonal Planar Ligand Field on the Electronic Properties of Lanthanide(II) Tris(silylamide) Complexes (Ln = Sm, Eu, Tm, Yb)

In recent work we have reported the synthesis and physical properties of near-linear Ln(II) (Ln = lanthanide) complexes utilizing the bulky bis(silylamide) {N(SiiPr3)2}. Herein, we synthesize trigonal-planar Ln(II) complexes by employing a smaller bis(silylamide), {N(SitBuMe2)2} (N**), to study the effects of this relatively rare Ln geometry/oxidation state combination on the magnetic and optical properties of complexes. We show that the charge-separated trigonal-planar Ln(II) complexes [K(2.2.2-cryptand)][Ln(N**)3] (Ln = Sm (1), Eu (2), Tm (3), Yb (4)) can be prepared by the reaction of 1.5 equiv of [{K(N**)}2] with LnI2THF2 (Ln = Sm, Yb) or LnI2 (Ln = Eu, Tm) and 1 equiv of 2.2.2-cryptand in Et2O. Complex 3 is the first structurally characterized trigonal-planar Tm(II) complex. In the absence of 2.2.2-cryptand, [K(DME)3][Sm(N**)3] (5) and [Ln(N**)2(μ-N**)K(toluene)2] (Ln = Sm (6), Eu (7)) were isolated in the presence of DME (dimethoxyethane) or toluene, respectively. The 1:1 reaction of [{K(N**)}2] with LnI2THF2 (Ln = Sm, Yb) in THF gave the four-coordinate pseudo-tetrahedral Lewis base adducts [Ln(N**)2(THF)2] (Ln = Sm (8), Yb (9)) and the cyclometalated complex [Yb(N**){N(SitBuMe2)(SitBuMeCH2)}(THF)] (10). Complexes 1-10 have been characterized as appropriate by single-crystal XRD, magnetic measurements, multinuclear NMR, EPR, and electronic spectroscopy, together with CASSCF-SO and DFT calculations. The physical properties of 1-4 are compared and contrasted with those of closely related near-linear Ln(II) bis(silylamide) complexes.

Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations

The origin of 60 K magnetic hysteresis in the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2tBu3-1,2,4, 1-Dy) remains mysterious, thus we envisaged that analysis of a series of [Ln(Cpttt)2]+ (Ln = lanthanide) cations could shed light on these properties. Herein we report the synthesis and physical characterization of a family of isolated [Ln(Cpttt)2]+ cations (1-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu), synthesized by halide abstraction of [Ln(Cpttt)2(Cl)] (2-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu). Complexes within the two families 1-Ln and 2-Ln are isostructural and display pseudo-linear and pseudo-trigonal crystal fields, respectively. This results in archetypal electronic structures, determined with CASSCF-SO calculations and confirmed with SQUID magnetometry and EPR spectroscopy, showing easy-axis or easy-plane magnetic anisotropy depending on the choice of Ln ion. Study of their magnetic relaxation dynamics reveals that 1-Ho also exhibits an anomalously low Raman exponent similar to 1-Dy, both being distinct from the larger and more regular Raman exponents for 2-Dy, 2-Er, and 2-Yb. This suggests that low Raman exponents arise from the unique spin-phonon coupling of isolated [Ln(Cpttt)2]+ cations. Crucially, this highlights a direct connection between ligand coordination modes and spin-phonon coupling, and therefore we propose that the exclusive presence of multihapto ligands in 1-Dy is the origin of its remarkable magnetic properties. Controlling the spin-phonon coupling through ligand design thus appears vital for realizing the next generation of high-temperature single-molecule magnets.

Silylamides: towards a half-century of stabilising remarkable f-element chemistry

This review highlights some of the outstanding contributions to f-element chemistry that have been supported by monodentate silylamide ligands over the past 45 years, since Bradley reported the first 3-coordinate lanthanide (Ln) complexes, [Ln(N″)3] (N″={N(SiMe3)2}) in 1972. As well as providing a historical perspective, this review presents a plethora of monodentate silylamide f-element complexes with significant geometries, bonding motifs, physical properties and reactivity profiles.

CCDC 1844352: Experimental Crystal Structure Determination

Related Article: Conrad A. P. Goodwin, Daniel Reta, Fabrizio Ortu, Jingjing Liu, Nicholas F. Chilton, David P. Mills|2018|Chem.Commun.|||doi:10.1039/C8CC05261A

CCDC 1844354: Experimental Crystal Structure Determination

Related Article: Conrad A. P. Goodwin, Daniel Reta, Fabrizio Ortu, Jingjing Liu, Nicholas F. Chilton, David P. Mills|2018|Chem.Commun.|54|9182|doi:10.1039/C8CC05261A

CCDC 1844350: Experimental Crystal Structure Determination

Related Article: Conrad A. P. Goodwin, Daniel Reta, Fabrizio Ortu, Jingjing Liu, Nicholas F. Chilton, David P. Mills|2018|Chem.Commun.|54|9182|doi:10.1039/C8CC05261A

Structural Characterization of Lithium and Sodium Bulky Bis(silyl)amide Complexes

Alkali metal amides are vital reagents in synthetic chemistry and the bis(silyl)amide {N(SiMe3)2} (N′′) is one of the most widely-utilized examples. Given that N′′ has provided landmark complexes, we have investigated synthetic routes to lithium and sodium bis(silyl)amides with increased steric bulk to analyse the effects of R-group substitution on structural features. To perform this study, the bulky bis(silyl)amines {HN(SitBuMe2)(SiMe3)}, {HN(SiiPr3)(SiMe3)}, {HN(SitBuMe2)2}, {HN(SiiPr3)(SitBuMe2)} and {HN(SiiPr3)2} (1) were prepared by literature procedures as colourless oils; on one occasion crystals of 1 were obtained. These were treated separately with nBuLi to afford the respective lithium bis(silyl)amides [Li{μ-N(SitBuMe2)(SiMe3)}]2 (2), [Li{μ-N(SiiPr3)(SiMe3)}]2 (3), [Li{N(SitBuMe2)2}{μ-N(SitBuMe2)2}Li(THF)] (4), [Li{N(SiiPr3)(SitBuMe2)}(DME)] (6) and [Li{N(SiiPr3)2}(THF)] (7) following workup and recrystallization. On one occasion during the synthesis of 4 several crystals of the ‘ate’ complex [Li2{μ-N(SitBuMe2)2}(μ-nBu)]2 (5) formed and a trace amount of [Li{N(SiiPr3)2}(THF)2] (8) was identified during the recrystallization of 7. The reaction of {HN(SitBuMe2)2} with NaH in the presence of 2 mol % of NaOtBu gave crystals of [Na{μ-N(SitBuMe2)2}(THF)]2 (9-THF), whilst [Na{N(SiiPr3)2}(C7H8)] (10) was prepared by deprotonation of 1 with nBuNa. The solid-state structures of 1–10 were determined by single crystal X-ray crystallography, whilst 2–4, 7, 9 and 10 were additionally characterized by NMR and FTIR spectroscopy and elemental microanalysis.