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).