Volatility Studies on Gallium Chalcogenide Cubanes: Thermal Analysis and Determination of Sublimation Enthalpies

Abstract

Volatility trends are established for a series of organometallic molecular solids with a cubane geometry and the general form [(R)Ga(μ3-E)]4, where R = Me3C (tBu), EtMe2C, Et2-MeC, or Et3C and E = S, Se, or Te. While the temperature of volatilization, T20 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 (ΔHsub) for each cubane were calculated from thermogravimetric data, which show that the dependence of ΔHsub 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 ΔHsub in the organometallic cubane system [(R)Ga(μ3-E)]4. Calculations based on the number of hydrogen atoms present in each alkyl yields an approximate value of ca. 4 kJ mol-1 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 [(Et3C)2Ga(μ-Cl]2, [(Et3C)Ga-(μ3-S)]4, and [(Et3C)Ga(μ3-Se)]4 have been determined by X-ray crystallography.