On the viability of small endohedral hydrocarbon cage complexes

X@C 4H4, X@C8H8, X@C8H 14, X@C10H16, X@C12H12, and X@C16H16

Damian Moran, H. Lee Woodcock, Zhongfang Chen, Henry F. Schaefer, Paul V R Schleyer*

*Corresponding author for this work

Research output: Contribution to journalArticle

39 Citations (Scopus)

Abstract

Small hydrocarbon complexes (X@cage) incorporating cage-centered endohedral atoms and ions (X = H+, H, He, Ne, Ar, Li0,+, Be0,+,2+, Na0,+, Mg0,+,2+) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C 4H4, Td) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C 8H8, Oh) and bicyclo[2.2.2]octane (C 8H14, D3h) minima are limited to encapsulating species smaller than Ne and Na+. Despite its intermediate size, adamantane (C10H16, Td) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li0,+, Be0,+,2+, Na0,+, and Mg2+. In contrast, the truncated tetrahedrane (C12H12, Td) encapsulates fewer species, while the D4d symmetric C 16H16 hydrocarbon cage (see Table of Contents graphic) encapsulates all but the larger Be, Mg, and Mg+ species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H antibonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (E endo to the sum of their isolated components (Einc = Eendo -Ecage - Ex) and to their exohedral isomer energies (Eisom = Eendo - Eexo). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e., Eisom is always exothermic), Be2+@C 10H16 (Td; -235.5 kcal/mol), Li+@C 12H12 (Td; 50.2 kcal/mol), Be 2+@C12H12 (Td; -181.2 kcal/mol), Mg2+@C12H12 (Td; -45.0 kcal/mol), Li+@C16H16 (D4d; 13.3 kcal/mol), Be+@C16H16 (C4v; (C4v; 31.8 kcal/mol), Be2+@C16H16 (D4d; -239.2 kcal/mol), and Mg2+@C16H16 (D 4d; -37.7 kcal/mol) are relatively stable as compared to experimentally known He@C20H20 (lh), which has an Einc = 37.9 kcal/mol and Eisom = -35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets.

Original languageEnglish
Pages (from-to)11442-11451
Number of pages10
JournalJournal of the American Chemical Society
Volume125
Issue number37
DOIs
Publication statusPublished - 17 Sep 2003
Externally publishedYes

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