Steering Single-Electron Metal–Metal Bonds and Hyperfine Coupling between a Transition Metal-Lanthanide Heteronuclear Bimetal Confined in Carbon Cages

Abstract

Metal complexes bearing single-electron metal-metal bonds (SEMBs) exhibit unusual electronic structures evoking strong magnetic coupling, and such bonds can be stabilized in the form of dimetallofullerenes (di-EMFs) in which two metals are confined in a carbon cage. Up to now, only a few di-EMFs containing SEMBs are reported, which are all based on a high-symmetry icosahedral (<i>I</i>_<i>h</i>) C₈₀ cage embedding homonuclear rare-earth bimetals, and a chemical modification of the <i>I</i>_<i>h</i>-C₈₀ cage is required to stabilize the SEMB. Herein, by introducing 3d-block transition metal titanium (Ti) along with 4f-block lanthanum (La) into the carbon cage, we synthesized the first crystallographically characterized SEMB-containing 3d-4f heteronuclear di-EMFs based on pristine fullerene cages. Four novel La-Ti heteronuclear di-EMFs were isolated, namely, LaTi@<i>D</i>_3<i>h</i>(5)-C₇₈, LaTi@<i>I</i>_<i>h</i>(7)-C₈₀, LaTi@<i>D</i>_5<i>h</i>(6)-C₈₀, and LaTi@<i>C</i>_2<i>v</i>(9)-C₈₂, and their molecular structures were unambiguously determined by single-crystal X-ray diffraction. Upon increasing the cage size from C₇₈ to C₈₂, the La-Ti distance decreases from 4.31 to 3.97 Å, affording fine-tuning of the metal-metal bonding and hyperfine coupling, as evidenced by an electron spin resonance (ESR) spectroscopic study. Density functional theory (DFT) calculations confirm the existence of SEMB in all four LaTi@C_2<i>n</i> di-EMFs, and the accumulation of electron density between La and Ti atoms shifts gradually from the proximity of the Ti atom inside C₇₈ to the center of the LaTi bimetal inside C₈₂ due to the decrease of the La-Ti distance. The electronic properties of LaTi@C_2<i>n</i> heteronuclear dimetallofullerenes differ apparently from their homonuclear La₂@C_2<i>n</i> counterparts, revealing the peculiarity of heteronuclear dimetallofullerenes with the involvement of 3d-block transition metal Ti.