DYNAMIC NMR-STUDIES OF THE RESTRICTED ROTATION OF THIOPHENES (TH) ANDSELENOPHENES (SELN) IN THE CR(CO)3(ETA-5-TH) AND CR(CO)3(ETA-5-SELN) COMPLEXES

The reaction of Cr(CO)6 with thiophenes (Th) and selenophenes (Seln) p roduced Cr(CO)3-(eta5-Th) (Th = T, 2-MeT, 2-EtT, 3-MeT, 2,5-Me2T, and Me4T) and Cr(CO)3(eta5-Seln) (Seln = Sel, 2-MeSel, and 2,5-Me2Sel). Va riable-temperature C-13 NMR spectra of these complexes in dimethyl eth er indicated that there is restricted rotation about the Cr-Th and -Se ln bonds. Band shape analyses of the C-13 NMR spectra using DNMR5 gave activation parameters for ring rotation. Activation enthalpies (DELTA H(double dagger), kcal/mol) for these complexes increase in the follow ing order: T (6.2), 3-MeT (6.5), 2-EtT (6.9), 2-MeT (7.1), Me4T (7.4), 2,5-Me2T (7.7), Sel (7.8), 2-MeSel (8.4), and 2,5-Me2Sel (9.0). In ge neral, alkyl substitution increases DELTAH(double dagger) and a-substi tution leads to a larger DELTAH(double dagger) than beta-substitution. Substitution of the S heteroatom by Se also increases DELTAH(double d agger). These results are discussed in terms of the relative electron densities on the heteroatom and the diene fragment, which were estimat ed using extended Huckel molecular orbital (EHMO) calculations. The st ructure of Cr(CO)3(eta5-2,5-Me2T) was established by a single crystal X-ray diffraction study. The rotational barriers for these complexes a re larger than those of Cr(CO)3(eta6-arene) complexes presumably due t o the strong interaction between the chromium and the heteroatom.