Triangular Monometallic Cyanide Cluster Entrapped in Carbon Cage with Geometry-Dependent Molecular Magnetism

Abstract

Clusterfullerenes are capable of entrapping a variety of metal clusters within carbon cage, for which the entrapped metal cluster generally keeps its geometric structure (e.g., bond distance and angle) upon changing the isomeric structure of fullerene cage, and whether the properties of the entrapped metal cluster is geometry-dependent remains unclear. Herein we report an unusual triangular monometallic cluster entrapped in fullerene cage by isolating several novel terbium cyanide clusterfullerenes (TbNC@C₈₂) with different cage isomeric structures. Upon varying the isomeric structure of C₈₂ cage from C₂(5) to C_s(6) and to C_2v(9), the entrapped triangular TbNC cluster exhibits significant distortions as evidenced by the changes of Tb-C(N) and C-N bond distances and variation of the Tb-C(N)-N(C) angle by up to 20°, revealing that the geometric structure of the entrapped triangular TbNC cluster is variable. All three TbNC@C₈₂ molecules are found to be single-ion magnets, and the change of the geometric structure of TbNC cluster directly leads to the alternation of the magnetic relaxation time of the corresponding TbNC@C₈₂ clusterfullerene.

Figure 1

Negative-ion LD-TOF mass spectrum of the purified TbNC@C₈₂ (I−III). Insets: measured and calculated isotopic distributions of TbNC@C₈₂.

Figure 2

Single crystal X-ray structures of TbNC@C₈₂ (C_s(6), C_2v(9)). The structures of TbNC@C_s(6)-C₈₂·Ni^II(OEP)·2(C₆H₆) (a) and TbNC@C_2v(9)-C₈₂·Ni^II(OEP)·2(C₆H₆) (c) are shown with only the predominant Tb (Tb1) positions, and solvent benzene molecules and hydrogen atoms are omitted for clarity. The structures of TbNC@C_s(6)-C₈₂ and TbNC@C_2v(9)-C₈₂ with the major TbNC clusters are illustrated in (b) and (d), respectively.

Figure 3

Comparison of the geometric structures of the TbNC clusters within TbNC@C₈₂ (C₂(5), C_s(6), C_2v(9)). The structures of the major TbNC clusters within C₂(5)-C₈₂ (a, ref 36), C_s(6)-C₈₂ (b) and C_2v(9)-C₈₂ (c) with X-ray determined bond lengths and bond angles are shown.

Figure 4

UV−vis−NIR spectra and photographs of TbNC@C₈₂ (C₂(5), C_s(6), C_2v(9)) dissolved in CS₂. Insets: Photographs of the corresponding solutions in toluene.

Figure 5

Cyclic voltammograms of TbNC@C₂(5)-C₈₂ (a), TbNC@C_s(6)-C₈₂ (b), and TbNC@C_2v(9)-C₈₂ (c) measured in o-DCB solution. Ferrocene (Fc) was added as the internal standard, TBAPF₆ as supporting electrolyte, scan rate: 100 mV·s⁻¹. The half-wave potentials (E_1/2) of each redox step are marked with a solid dot to aid comparison. The asterisk labels the oxidation peak of ferrocene.

Figure 6

Magnetization of TbNC@C₂(5)-C₈₂ (a), TbNC@C_s(6)-C₈₂ (b) and TbNC@C_2v(9)-C₈₂ (c) versus the applied field temperature quotient x. The color codes of the different temperatures are indicated. The magnetization curves scale with the applied field temperature quotient x = μ₀H/T.

Figure 7

Imaginary part of AC susceptibility measured at different temperatures for TbNC@C₂(5)-C₈₂ (a), TbNC@C_s(6)-C₈₂ (b), and TbNC@C_2v(9)-C₈₂ (c). μ₀H = B0 + B1 × sin(ωt), B0 = 200 mT, B1 = 0.25 mT.

Figure 8

Magnetic relaxation times (τ) of TbNC@C₈₂ (C₂(5), C_s(6), C_2v(9)) as a function of the distance between the Tb ion and the closest C/N atom of the NC unit. Data at 1.8 K (circles) indicate a clear decrease of the magnetization relaxation time with the increase of Tb−N(C) distance, while at 6.0 K (squares) the trend is much weaker, presumably due to thermal smearing. The dashed lines are exponential trends for TbNC@C₈₂ (C₂(5), C_s(6), C_2v(9)), with the linear correlation coefficient between log(τ) and the Tb−C(N) distance being −0.99 ± 0.17 and −0.69 ± 0.72 for the line at 1.8 and 6.0 K, respecitvely. Only the cage isomeric structures of TbNC@C₈₂ are given in the labels for clarity.