CaY@C<sub>2<i>n</i></sub>: Exploring Molecular\nQubits with Ca–Y Metal–Metal Bonds

Metal–metal bonding is crucial in chemistry for\nadvancing\nour understanding of the fundamental aspects of chemical bonds. Metal–metal\nbonds based on alkaline-earth (Ae) elements, especially the heavier\nAe elements (Ca, Sr, and Ba), are rarely reported due to their high\nelectropositivity. Herein, we report two heteronuclear di-EMFs CaY@<i>C</i><sub><i>s</i></sub>(6)-C<sub>82</sub> and CaY@<i>C</i><sub>2<i>v</i></sub>(5)-C<sub>80</sub>, which\ncontain unprecedented single-electron Ca–Y metal–metal\nbonds. These compounds were characterized by single-crystal X-ray\ncrystallography, electron paramagnetic resonance (EPR) spectroscopy,\nand DFT calculations. The crystallographic study of CaY@<i>C</i><sub><i>s</i></sub>(6)-C<sub>82</sub> shows that Ca and\nY are successfully encapsulated into the carbon cage with a Ca–Y\ndistance of 3.691 Å. The CW-EPR study of both CaY@<i>C</i><sub><i>s</i></sub>(6)-C<sub>82</sub> and CaY@<i>C</i><sub>2<i>v</i></sub>(5)-C<sub>80</sub> exhibits a doublet,\nsuggesting the presence of an unpaired electron located between Ca\nand Y. The combined experimental and theoretical results confirm the\npresence of a Ca–Y single-electron metal–metal bond\nwith substantial covalent interaction, attributed to significant overlap\nbetween the 4s4p orbitals of Ca and the 5s5p4d orbitals of Y. Furthermore,\npulse EPR spectroscopy was used to investigate the quantum coherence\nof the electron spin within this bond. The unpaired electron, characterized\nby its s orbital nature, is effectively protected by the carbon cage,\nresulting in efficient suppression of both spin–lattice relaxation\nand decoherence. CaY@<i>C</i><sub><i>s</i></sub>(6)-C<sub>82</sub> behaves as an electron spin qubit, displaying\na maximum decoherence time of 7.74 μs at 40 K. This study reveals\nan unprecedented Ae–rare-earth metal–metal bond stabilized\nby the fullerene cages and elucidates the molecular qubit properties\nstemming from their unique bonding character, highlighting their potential\nin quantum information processing applications.