Rotational barriers in substituted (cycloheptatriene)Cr(CO)3 complexes

Article abstract of DOI:10.1021/om00125a004

The barriers of rotation about the cycloheptatriene-Cr axis have been determined for six (7-substituted cycloheptatriene)Cr(CO)₃ complexes. The barriers were determined from a complete line-shape analysis of the ¹³C spectra for the carbonyl region as a function of temperature. The values of ΔH? for the cycloheptatriene (CHT), 7-exo-MeCHT, 7-endo-MeOCHT, and 7,7-di-MeOCHT complexes are essentially identical (9.9 ± 0.2,10.4 ± 0.5, 9.9 ± 0.3, and 9.9 ± 0.4 kcal/mol, respectively) and the values of ΔS? cluster around zero. On the other hand, ΔH? values for 7-exo-CNCHTCr(CO)₃ (8.9 ± 0.2 kcal/mol) and 7-exo-t-BuCHTCr(CO)₃ (10.9 ± 0.3 kcal/mol) are, respectively, slightly lower and higher than these values. This appears to be consistent with arguments concerning the electronic origin of these rotational barriers, namely, that the magnitude of the barrier is related to the cycloheptatriene-norcaradiene equilibrium. Estimates of the rotational barriers in three (1,6-methanoannulene)Cr(CO)₃ complexes are also consistent with this theory. Finally the barrier in (tropone)Cr(CO)₃ was found to be very small (～6 kcal/mol); extended Hu?ckel molecular orbital calculations predict a barrier of 5.8 kcal/mol. A rationale why the barrier in this complex is much lower than the other cycloheptatriene complexes is given.