Volatility studies on gallium chalcogenide cubanes: Thermal analysis and determination of sublimation enthalpies

Volatility trends are established for a series of organometallic molecular solids with a cubane geometry and the general form [(R)Ga(mu(3)-E)](4), where R = Me(3)C ((t)Bu), EtMe(2)C, Et(2)-MeC, or Et(3)C and E = S, Se, or Te. While the temperature of volatilization, T-20 defined as 20% mass loss from thermogravimetric analysis, was found to generally increase in a linear fashion with respect to increasing molecular mass, perturbations were observed that can be attributed to intermolecular ligand interactions. Sublimation enthalpies (Delta H-sub) for each cubane were calculated from thermogravimetric data, which show that the dependence of Delta H-sub on the degree of branching of the alkyl ligand appears to be more substantial than molecular mass effects alone. The C-H ... H-C van der Waals interactions between alkyl substituents are estimated to account for more than 60% of the Delta H-sub in the organometallic cubane system [(R)Ga(mu(3)-E)](4). Calculations based on the number of hydrogen atoms present in each alkyl yields an approximate value of ca. 4 kJ mol-l for each C-H H-C interactions. Using the TGA sublimation data, vapor pressures may be calculated for each of the cubane compounds over a wide range of temperatures. All new cubanes have been characterized by MS, NMR, IR, and TG/DTA. The molecular structures of [(Et(3)C)(2)Ga(mu-Cl)](2), [(Et(3)C)Ga-(mu(3)-S)](4), and [(Et(3)C)Ga(mu(3)-Se)](4) have been determined by X-ray crystallography.