Heterospin frustration in a metal-fullerene-bonded semiconductive antiferromagnet

Yongbing Shen,Mengxing Cui,Akihiro Otsuka,Masahiro Yamashita,Kunihisa Sugimoto,Norihisa Hoshino,Hideyuki Kawasoko,Tomoyuki Akutagawa,Takao Tsumuraya,Takefumi Yoshida,Eunsang Kwon,Yasuhiko Kasama,Kazuhiko Kawachi,Shinya Takaishi,Tomoteru Fukumura

doi:10.1038/s41467-022-28134-w

Abstract

Lithium-ion-encapsulated fullerenes (Li+@C60) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal–fullerene-bonded framework {[Cu4(Li@C60)(L)(py)4](NTf2)(hexane)}n (1) (L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf2− = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu2(L)(py)4 and lithium-ion-encapsulated fullerene salt (Li+@C60)(NTf2−). Electron donor Cu2(L)(py)2 bonds to acceptor Li+@C60 via eight Cu‒C bonds. Cu–C bond formation stems from spontaneous charge transfer (CT) between Cu2(L)(py)4 and (Li+@C60)(NTf2−) by removing the two-terminal py molecules, yielding triplet ground state [Cu2(L)(py)2]+(Li+@C60•−), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li+@C60•− radicals (S = ½) and Cu2+ ions (S = ½) interact antiferromagnetically in triangular spin lattices in the absence of long-range magnetic ordering to 1.8 K. The low-temperature heat capacity indicated that compound 1 is a potential candidate for an S = ½ quantum spin liquid (QSL).

Highlights

We find that the dxz orbitals of the Cu ions in Cu2(L)(py)[4] are delocalised and strongly coupled with the N(pz) orbitals of the ligand, thereby yielding delocalised electrons in the highest occupied molecular orbital (HOMO)
One Li+@C60 cage coordinates with four Cu ions from four Cu2(L)(py)[2] molecules (Fig. 1a), and the remaining four Cu ions coordinate with the Li+@C60 cage to afford infinite 1D ladder-like structures along the b-axis (Fig. 2a)
Each Cu ion exhibits an equivalent of five coordination numbers in a distorted trigonal bipyramid geometry (Supporting Information Fig. 1)

Summary

Introduction

It is noteworthy that the HOMO of Cu2(L)(py)[4] has the equivalent energy level of the LUMO of Li+@C60, thereby facilitating CT interactions in the ground state. We find that the dxz orbitals of the Cu ions in Cu2(L)(py)[4] are delocalised and strongly coupled with the N(pz) orbitals (πelectrons) of the ligand, thereby yielding delocalised electrons in the HOMO.

Results

Conclusion