EXPERIMENTAL INVESTIGATION ON EDGE INVERSION AT TRIVALENT BISMUTH AND ANTIMONY - GREAT ACCELERATION BY INTRAMOLECULAR AND INTERMOLECULAR NUCLEOPHILIC COORDINATION

The activation free energy of inversion at the bismuth atom of the trivalent organobismuth compounds {2-C6H4C(CF3)(2)O}Bi(4-CH3C6H4) (4a), {2-C6H4C(CF3)(2)O}Bi(2-C(6)H(4)CH(2)NMe(2)) (10), and {2-C6H4C(CF3)(2)O}Bi(2-C(6)H(4)CMe(2)OMe) (11) in DMSO-d(6) was measured by use of variable temperature F-19 NMR and found to be 21.2 (175 degrees C), 15.4 (55 degrees C), and 18.6 (125 degrees C) kcal mol(-1), respectively. The corresponding energy of 10 in pyridine decreased to 14.6 (40 degrees C), whereas that in 2,6-lutidine was 20.6 (170 degrees C) kcal mol(-1). These results imply that intramolecular coordination of the methoxy and the dimethylamino groups and also intermolecular coordination of the pyridine solvent greatly stabilize the transition state of inversion at the bismuth atom. This phenomenon can only be rationalized by the edge inversion mechanism. Inversion at the bismuth atom of {2-C6H4C(CF3)(2)O}Bi{4-(R)-2,6-C6H2(CH(2)NMe(2))(2)} (21, R = t-Bu; 22, R = H) bearing two potential intramolecularly coordinating groups was so fast that the barrier could not even be measured at -90 degrees C. The barrier of the corresponding antimony compound {2-C6H4C(CF3)(2)O}Sb{2,6-C6H3(CH(2)NMe(2))(2)} (23) in CD2Cl2 was found to be 9.5 kcal mol(-1) at -70 degrees C. The M-N dissociation in the NMe(2) group was determined to be 12.3 kcal mol(-1) for 22 (M = Bi, T-c = -5 degrees C in toluene-d(8)) and 12.0 kcal mol(-1) for 23 (M = Sb, T-c = -19.2 degrees C in CD2Cl2). Thus, inversion for the bismuth (22) and the antimony atom (23) can be regarded as taking place without dissociation of the Bi-N (or Sb-N) bonds, that is, via a 12-M-5 (M = Sb and Bi) transition state. X-ray crystallographic analyses of 10, 22, and 23 are presented which showed the presence of intramolecular coordination of N and O to the central atom (Bi or Sb).