Cubane, Cuneane, and Their Carboxylates: A Calorimetric, Crystallographic, Calculational, and Conceptual Coinvestigation

This study is a multinational, multidisciplinary contribution to the thermochemistry of dimethyl1,4-cubanedicarboxylate and the corresponding isomeric, cuneane derivative and provides both
structural and thermochemical information regarding the rearrangement of dimethyl 1,4-cubanedicarboxylate to dimethyl 2,6-cuneanedicarboxylate. The enthalpies of formation in the
condensed phase at T = 298.15 K of dimethyl 1,4-cubanedicarboxylate (dimethyl pentacyclo[4.2.0.0.2,50.3,804,7]octane-1,4-dicarboxylate) and dimethyl 2,6-cuneanedicarboxylate (dimethyl pentacyclo[3.3.0.0.2,40.3,706,8]octane-2,6-dicarboxylate) have been determined by combustion calorimetry,
ΔfH°m(cr)/kJ·mol-1 = −232.62 ± 5.84 and −413.02 ± 5.16, respectively. The enthalpies of sublimation
have been evaluated by combining vaporization enthalpies evaluated by correlation-gas chromatography and fusion enthalpies measured by differential scanning calorimetry and adjusted to T =
298.15 K, ΔcrgHm(298.15 K)/kJ·mol-1 = 117.2 ± 3.9 and 106.8 ± 3.0, respectively. Combination of
these two enthalpies resulted in ΔfH°m(g., 298.15 K)/kJ·mol-1 of −115.4 ± 7.0 for dimethyl 1,4-cubanedicarboxylate and −306.2 ± 6.0 for dimethyl 2,6-cuneanedicarboxylate. These measurements,
accompanied by quantum chemical calculations, resulted in values of ΔfHm(g, 298.15 K) = 613.0 ±
9.5 kJ·mol-1 for cubane and 436.4 ± 8.8 kJ·mol-1 for cuneane. From these enthalpies of formation,
strain enthalpies of 681.0 ± 9.8 and 504.4 ± 9.1 kJ·mol-1 were calculated for cubane and cuneane
by means of isodesmic reactions, respectively. Crystals of dimethyl 2,6-cuneanedicarboxylate are
disordered; the substitution pattern and structure have been confirmed by determination of the
X-ray crystal structure of the corresponding diacid.