Crystallographic characterization of Y2C2n (2n = 82, 88–94): direct Y–Y bonding and cage-dependent cluster evolution

Abstract

Direct yttrium–yttrium bonding has been a long-sought puzzle in organometallic chemistry to understand the catalytic processes that involve yttrium. Herein, we report the first crystallographic authentication of direct Y–Y bonding inside the hollow cavity of fullerene cages by forming endohedral metallofullerenes (EMFs). Based on an efficient separation/purification process, which involves Lewis-acid treatment and HPLC separation, we have obtained sufficient amounts of a series of Y₂C_2n (2n = 82, 88–94) isomers for systematic studies. The unambiguous single-crystal X-ray diffraction (XRD) crystallographic results show that two of them are di-EMFs, namely Y₂@C_s(6)-C₈₂ and Y₂@C_3v(8)-C₈₂, in which the long-sought Y–Y single bond between the two divalent yttrium ions is experimentally confirmed for the first time. In contrast, all the other EMFs with relatively large cages are carbide cluster metallofullerenes (CCMFs), namely, Y₂C₂@C_s(15)-C₈₆, Y₂C₂@C₁(26)-C₈₈, Y₂C₂@C₂(41)-C₉₀ and Y₂C₂@C₂(61)-C₉₂. Consistently, our computational results prove that these experimentally obtained EMFs are all abundant at the high temperatures for fullerene formation (∼1500–3000 K) due to the strong coordination ability of yttrium ions, which enables the formation of either direct Y–Y bonds (for Y₂@C_s(6)-C₈₂ and Y₂@C_3v(8)-C₈₂) or the inclusion of a C₂-unit (in Y₂C₂@C_s(15)-C₈₆, Y₂C₂@C₁(26)-C₈₈, Y₂C₂@C₂(41)-C₉₀ and Y₂C₂@C₂(61)-C₉₂). Our results suggest that metal atoms such as yttrium tend to adopt a low valence state during the arc-discharge process because of the presence of the highly reductive carbon plasma in the chamber, enabling the formation of an Y₂ dimer with direct Y–Y bonding in small cages like C₈₂.