Terbocenium: completing a heavy lanthanide metallocenium cation family with an alternative anion abstraction strategy

Abstract

A series of heavy lanthanide metallocenium cations [Ln(Cpttt)2]+ (Ln = Y, Gd, Dy, Ho, Er, Tm, Yb, Lu; Cpttt = C5H2tBu3-1,2,4) were recently prepared by halide abstraction of [Ln(Cpttt)2(Cl)], but curiously the Tb analogues remained elusive. Here we report an alternative anion abstraction strategy to access [Tb(Cpttt)2]+, completing the heavy [Ln(Cpttt)2]+ family.

Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

“…In the 19 F{ 1 H} NMR spectrum of 1‐Sm , the para ‐F signal is a triplet from coupling to two adjacent meta ‐F atoms ( J FF =20.4 Hz), but this is a singlet for 1‐La , presumably due to broadening by quadrupolar 139 La ( I =7/2). The 11 B and 19 F NMR spectra of [Sc(Cp*) 2 {(C 6 F 5 ‐κ 2 ‐ F )B(C 6 F 5 ) 3 }] and [Ln(Cp*) 2 {(μ‐C 6 F 5 ‐κ 1 ‐ F ) 2 B(C 6 F 5 ) 2 }] 2 (Ln=Pr, Nd) have not been reported to the best of our knowledge, but all NMR spectra reported herein are similar to those previously discussed for the heavy 1‐Ln (Ln=Gd–Lu) . Together, these data indicate that the [B(C 6 F 5 ) 4 ] − anions in light 1‐Ln are only weakly bound in solution, and dissociate to give isolated [Ln(Cp ttt ) 2 ] + cations, as evidenced by the symmetry of 19 F NMR spectra .…”

“…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 .…”