Heterospin Frustration in a Metal-Fullerene-bonded Semiconductive Antiferromagnet

Lithium-ion-encapsulated fullerenes (Li + @C 60 ) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal–fullerene-bonded framework {[Cu 4 (Li@C 60 )(L)(py) 4 ](NTf 2 )(hexane)} n ( 1 ) ( L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf 2 − = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu 2 (L)(py) 4 and lithium-ion-encapsulated fullerene salt (Li + @C 60 )(NTf 2 − ). Electron donor Cu 2 (L)(py) 2 bonds to acceptor Li + @C 60 via eight Cu‒C bonds. Cu–C bond formation stems from spontaneous charge transfer (CT) between Cu 2 (L)(py) 4 and (Li + @C 60 )(NTf 2 − ) by removing the two-terminal py molecules, yielding triplet ground state [Cu 2 (L)(py) 2 ] + (Li + @C 60 •− ), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li + @C 60 •− radicals ( S = ½) and Cu 2+ 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).