Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations

Abstract

The origin of 60 K magnetic hysteresis in the dysprosocenium complex [Dy(Cp)][B(CF)] (Cp = CHBu-1,2,4, 1-Dy) remains mysterious, thus we envisaged that analysis of a series of [Ln(Cp)] (Ln = lanthanide) cations could shed light on these properties. Herein we report the synthesis and physical characterization of a family of isolated [Ln(Cp)] cations (1-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu), synthesized by halide abstraction of [Ln(Cp)(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(Cp)] 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.

Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

“…However, for diamagnetic 1‐La and 2‐La these could be interpreted: for 1‐La the expected three Cp ttt ring carbon signals were identified at 135.96, 147.78 and 149.70 ppm, whereas six resonances for the t Bu groups were seen between 29.86 and 32.82 ppm. These are assigned as three quaternary and three methyl signals; multiple signals in this region were previously observed for 1‐Lu . The [B(C 6 F 5 ) 4 ] − anions in 1‐Ln were observed by 11 B{ 1 H} and 19 F{ 1 H} NMR spectroscopy (see Supporting Information).…”

“…According to previously published synthetic methods for the synthesis of 1‐Ln (Ln=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y), [Ln(Cp ttt ) 2 {(C 6 F 5 ‐κ 1 ‐ F )B(C 6 F 5 ) 3 }] ( 1‐Ln ; Ln=La, Ce, Pr, Nd) and [Sm(Cp ttt ) 2 ][B(C 6 F 5 ) 4 ] ( 1‐Sm ) were all synthesised by the reactions of [Ln(Cp ttt ) 2 (Cl)] ( 2‐Ln ; Ln=La, Ce, Pr, Nd, Sm) with [H(SiEt 3 ) 2 ][B(C 6 F 5 ) 4 ] in benzene (Scheme ). [H(SiEt 3 ) 2 ][B(C 6 F 5 ) 4 ] was selected as a reagent for Ln metallocenium cation formation as: (i) it is soluble in benzene, hence strongly coordinating O‐donor solvents can be avoided; (ii) there is a significant enthalpic effect due to formation of a strong Si−Cl bond, and (iii) there is an additional entropic effect arising from two reactant species giving three product species.…”

“…State‐of‐the‐art calculations of the spin–phonon coupling in 1 ‐ Dy show that magnetic relaxation in the Orbach (over‐barrier) regime occurs due to localised molecular vibrations, and suggests that the high magnetic hysteresis temperature arises from the constrained metal–ligand vibrational modes intrinsic to the bis ‐η 5 ‐Cp ttt coordination geometry . We have subsequently shown that isolated [Ln(Cp ttt ) 2 ] + cations exhibit anomalously low Raman exponents, distinct from analogues with equatorial ligands, suggesting the unique vibrational modes of Cp ttt are important across a wide temperature range …”

“…Although isolated d‐block metallocenium cations have been known since ferrocenium, [Fe(Cp) 2 ] + , was isolated in 1952 and Ln metallocenium cations, [Ln(Cp) 2 ] + , were first predicted in 1956 by Birmingham and Wilkinson, it was only recently with the report of 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 ), and heavy Ln metallocenium homologues [Ln(Cp ttt ) 2 ][B(C 6 F 5 ) 4 ] ( 1 ‐ Ln , Ln=Y, Gd, Tb, Ho, Er, Tm, Yb, Lu), that structurally authenticated Ln metallocenium complexes with no equatorial interactions were achieved. The paucity of isolated Ln metallocenium cations prior to 2017 can be attributed to the propensity for large electropositive Ln III cations to maximise their coordination numbers, coupled with their preference for hard donor ligands not being well‐satisfied by soft Cp R coordination .…”

“…We previously reported that the bulky bis ‐Cp ttt framework in combination with the weakly coordinating anion [B(C 6 F 5 ) 4 ] − provide isolated Ln metallocenium cations for smaller, heavier Ln . These studies showed that [H(SiEt 3 ) 2 ][B(C 6 F 5 ) 4 ] is effective at abstracting either fluoride or chloride from [Ln(Cp ttt ) 2 (X)] precursors to yield 1 ‐ Ln .…”