HIGH-PRESSURE NMR KINETICS .55. MECHANISMS OF AXIAL AND EQUATORIAL DIMETHYL-SULFOXIDE EXCHANGE ON [TIO(DMSO)5]2+ - A VARIABLE-TEMPERATURE AND VARIABLE-PRESSURE H-1-NMR STUDY

The low-temperature 400-MHz H-1 NMR spectrum of [TiO(e-DMSO)4(a-DMSO)](CF3SO3)2 in nitromethane diluent containing excess free dimethyl sulfoxide (DMSO) reveals the signals of equatorial, axial, and free DMSO. Variable-temperature and -pressure line shape measurements were analyzed in terms of two exchange processes, a fast exchange between the a-DMSO (k(ax)) and the bulk and a slower exchange between the e-DMSO (k(eq)) and the coalesced a-DMSO and f-DMSO: k(ax)298 (k(eq)298) = 6100 +/- 800 (161 +/- 4) s-1, DELTAH(double dagger) = 59.8 +/- 2 (57.5 +/- 1) kJ mol-1, DELTAS(double dagger) = +28.3 +/- 6 (-9.9 +/- 3) J K-1 mol-1, and DELTAV(double dagger) = +1.6 +/- 0.1 (+4.8 +/- 0.1) cm3 mol-1. A D mechanism is suggested for the fast exchange of the loosely bonded a-DMSO with the bulk. It is proposed that the second process, on the other hand, takes place through migration of the a-DMSO molecule to the equatorial plane and concerted loss of an e-DMSO molecule. This leads to an expanded transition state in which the breaking of the Ti(IV)-e-DMSO bond is predominant, as suggested by the positive volume of activation.