Molybdenum Complexes with Sterically Demanding Cycloheptatrienyl Ligands

Sterically demanding tropylium salts of the type (1,3,5-C7H4R3)BF4 (3a, R = tert-butyl;
3b, R = isopropyl) and (C7H6R)BF4 (3c, R = tert-butyl; 3d, R = SiMe3) react with [(η6-p-xylene)Mo(CO)3] by arene exchange and Mo(CO)3 group transfer leading to cationic
η7-cycloheptatrienyl complexes 4a−4d in excellent yields. 4a further reacts with alkali halides
to give [(η7-1,3,5-C7H4R3)Mo(CO)2X] (5a, R = tert-butyl, X = Cl; 6a, R = tert-butyl, X = I).
The X-ray crystal structures of 1,3,5-tri-tert-butyltropylium tetrafluoroborate (3a) and of
[(η7-1,3,5-C7H4R3)Mo(CO)3]BF4 (4a, R = tert-butyl) are reported.