Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

Abstract As the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt=C5H2 tBu3‐1,2,4, 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 homologous [Ln(Cpttt)2][B(C6F5)4] (1‐Ln) for all the heavier Ln from Gd–Lu; herein, we extend this motif to the early Ln. We find, for the largest LnIII cations, that contact ion pairs [Ln(Cpttt)2{(C6F5‐κ1‐F)B(C6F5)3}] (1‐Ln; La–Nd) are isolated from reactions of parent [Ln(Cpttt)2(Cl)] (2‐Ln) with [H(SiEt3)2][B(C6F5)4], where the anion binds weakly to the equatorial sites of [Ln(Cpttt)2]+ through a single fluorine atom in the solid state. For smaller SmIII, [Sm(Cpttt)2][B(C6F5)4] (1‐Sm) is isolated, which like heavier 1‐Ln does not exhibit equatorial anion interactions, but the EuIII analogue 1‐Eu could not be synthesised due to the facile reduction of EuIII precursors to EuII products. Thus with the exception of Eu and radioactive Pm this work constitutes a structurally similar family of Ln metallocenium complexes, over 50 years after the [M(Cp)2]+ series was isolated for the 3d metals.

Storage at -25 °C afforded 1-Ce as yellow crystals (0.137 g, 21%). Anal. Calcd (%) for (0.455 g, 0.5 mmol) and [Nd(Cp ttt )2(Cl)] (0.323 g, 0.5 mmol) at room temperature to give a greenish blue reaction mixture. The mixture was stirred for 16 hours, forming a green precipitate. The volatiles were removed under vacuum to give a green powder, which was washed with hexane (10 mL) and benzene (10 mL). The crude material was dissolved in DCM (2 mL) at -78 °C, and layered with hexane (2 mL give a dark brown reaction mixture. The mixture was stirred for 16 hours, forming a dark brown precipitate. The volatiles were removed under vacuum to give a reddish brown powder, which was washed with hexane (15 mL) and benzene (15 mL). The crude material was dissolved in hot toluene (5 mL  [La(Cp ttt )2(Cl)] (2-La). THF (30 mL) was added to a pre-cooled (−78 °C) ampoule containing LaCl3 (0.491 g, 2 mmol) and KCp ttt (1.089 g, 4 mmol). The reaction mixture was allowed to reflux for 48 hours. The solvent was removed in vacuo and toluene (30 mL) was added. The reaction mixture was allowed to reflux for 48 hours. The resultant white suspension was allowed to settle for 3 hours and filtered. The pale yellow solution was concentrated to 2 mL and stored at 8 °C to afford 2-La as colourless crystals (0.395 g, 31%). Anal. Calcd [Ce(Cp ttt )2(Cl)] (2-Ce). THF (30 mL) was added to a pre-cooled (−78 °C) ampoule containing CeCl3 (0.493 g, 2 mmol) and KCp ttt (1.089 g, 4 mmol). The reaction mixture was allowed to reflux for 48 hours. The solvent was removed in vacuo and toluene (30 mL) was added. The reaction mixture was allowed to reflux for 48 hours. The resultant suspension was allowed to settle for 3 hours and filtered. The orange solution was concentrated to 2 mL and stored at 8 °C to afford 2-Ce as orange crystals (0.651g, 51%). Anal. Calcd (%) for  [Nd(Cp ttt )2(Cl)] (2-Nd). THF (30 mL) was added to a pre-cooled (-78 °C) ampoule containing NdCl3 (0.501 g, 2 mmol) and KCp ttt (1.089 g, 4 mmol). The reaction mixture was allowed to reflux for 48 hours. The light blue solvent was removed in vacuo and toluene (30 mL) was added. The reaction mixture was allowed to reflux for 48 hours. The resultant suspension was allowed to settle for 3 hours and filtered. The green solution S6 was concentrated to 2 mL and stored at 8 °C to afford 2-Nd as blue crystals (0.593 g, 46%). Anal. Calcd (%)  [Sm(Cp ttt )2(Cl)] (2-Sm). THF (30 mL) was added to a pre-cooled (-78 °C) ampoule containing SmCl3 (0.513 g, 2 mmol) and KCp ttt (1.089 g, 4 mmol). The reaction mixture was allowed to reflux for 24 hours. The solvent was removed in vacuo and toluene (30 mL) was added. The reaction mixture was allowed to reflux for 24 hours. The resultant suspension was allowed to settle for 3 hours and filtered. The orange solution was concentrated to 2.5 mL and stored at 8 °C to afford yellow crystals (0.656 g, 50%). Anal. Calcd (%) for correction with a beam profile was applied. [1] The structures were solved using SHELXS; [2] the datasets were refined by full-matrix least-squares on all unique F 2 values, [3] with anisotropic displacement parameters for all non-hydrogen atoms, and with constrained riding hydrogen geometries; Uiso(H) was set at 1.2 (1.5 for methyl groups) times Ueq of the parent atom. The largest features in final difference syntheses were close to heavy atoms and were of no chemical significance. CrysAlisPro [1] was used for control and integration, and SHELX [2,3] was employed through OLEX2 [4] for structure solution and refinement. ORTEP-3 [5] and POV-Ray [6] were employed for molecular graphics. CCDC 1867384-1867395 contain the supplementary crystal data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.                                                             constants. [7] Figure S120. Fitting of relaxation rates for 2-Ce. Table S11. Best fit parameters to the relaxation rates for 2-Ce.   Figure S122. Fitting of relaxation rates for 1-Nd in a 0.1 T DC field. Table S13. Best fit parameters to the relaxation rates for 1-Nd in a 0.1 T DC field.   Figure S125. Fitting of relaxation rates for 2-Nd in a 0.1 T DC field. Table S15. Best fit parameters to the relaxation rates for 2-Nd in a 0.1 T DC field.

Model (s) (cm -1 ) (s -1 K -n )
S86 10. EPR spectra X-band spectra were recorded with a Bruker EMX spectrometer fitted with a Super High Q X-band resonator.
Polycrystalline samples of the Kramers ions (Ce, Nd and Sm) were sealed in quartz Q-band EPR tubes under an inert atmosphere; we crushed all samples as best we could without decomposing the sample, but we note that some effects due to polycrystallinity remain in the spectra below. We observed field-induced orientation effects in some of the samples, and therefore we have immobilized these with eicosane (following a similar procedure to that described for the SQUID samples) to prevent orientation effects. The presence of a very sharp resonance at g = 2.00 is attributed to an impurity in the quartz EPR tubes, and serves as an internal reference for comparing relative intensities. EPR spectra are simulated with PHI [8] using an isotropic frequency-space Gaussian linewidth in all cases.

CASSCF-SO electronic structure
We used MOLCAS 8.0 [9] to perform CASSCF-SO calculations of the 1-Ln and 2-Ln complexes in order to determine their electronic structures. We employed the molecular geometries of the metal-containing molecules from single crystal XRD structure with no optimization, taking the largest disorder component only.
If the [B(C6F5)4]counter ion was in contact with the cation, it was also included in the calculations. Basis sets from ANO-RCC library [10,11] were employed with VTZP quality for Ln atoms, VDZP quality for the cyclopentadienyl C atoms and the equatorial chloride atom ( Table S16. Most of the resulting states were mixed together by spin-orbit coupling (Table S16), and these spin-orbit wavefunctions were decomposed into a CF Hamiltonian (below), and the magnetic susceptibility calculated (see Magnetism and EPR spectroscopy) using SINGLE_ANISO. [12] Figure S131. Representation of the ground state g3 direction in the molecular frame for 2-Ce. tBu groups and H-atoms omitted for clarity.           Figure S137. Representation of g3 direction in the molecular frame for 2-Nd. tBu groups and H-atoms omitted for clarity. Figure S138. Representation of g3 direction of the most excited Kramers doublet in the molecular frame for 2-Nd. tBu groups and H-atoms omitted for clarity. S100 Table S28. Electronic structure of 1-Sm calculated with CASSCF-SO at the solid-state geometry in zerofield, quantised along the g3 direction of the ground doublet.  Figure S139. Representation of g3 direction in the molecular frame for 1-Sm. tBu groups and H-atoms omitted for clarity. S101  Figure S140. Representation of g3 direction in the molecular frame for 2-Sm. tBu groups and H-atoms omitted for clarity.