HIGH-PRESSURE NMR INVESTIGATION OF HYDROGEN-ATOM TRANSFER AND RELATED DYNAMIC PROCESSES IN OXO CATALYSIS

The cobalt center in H Co(CO)(4) exchanges with those in Co-2(CO)(8) through a facile hydride ligand transfer reaction which has been studied by Co-59 NMR line-shape analysis over the temperature range of 80 to 200 degrees C and total system pressures up to 370 atm in supercritical carbon dioxide. The lifetime of the cobalt center in HCo(CO)(4) varies from 2 ms at 80 degrees C to 10 mu s at 200 degrees C, exhibiting an activation energy of 15.3 +/- 0.4 kcal/mol. The hydride ligand transfer process is highly specific for the HCo(CO)(4) and Co-2(CO)(8) complexe(s). Thus, neither Co-4(CO)(12) nor MnCo(CO)(9) exhibit measurable chemical exchange line broadening in the Co-59 NMR spectra within solutions where the resonances for HCo(CO)(4) and Co-2(CO)(8) coalesce. In addition, the full peak widths at half-height (W-1/2) for the hydride, dihydrogen, and water resonances vary by less than 3 Hz in the H-1 NMR spectra, while the line widths (W-1/2) for the HCo(CO)(4) and Co-2(CO)(8) resonances broaden by more than 15 000 Hz in the Co-59 NMR spectra. A similar hydride ligand transfer reaction exchanges the hydride moieties in HCo(CO)(4) and HMn(CO)(5). This latter heterometallic hydride ligand transfer reaction has been investigated by H-1 NMR line-width analysis over the temperature range of 110 to 190 degrees C at two initial carbon monoxide concentrations, 1.39 and 4.13 M. The lifetime of the hydride moiety on the manganese center in the heterometallic hydride ligand transfer reaction between HCo(CO)(4) and HMn(CO)(5) is independent of the carbon monoxide pressure and exhibits an activation energy of 19 +/- 1 kcal/mol. The Mn-55 NMR spectra indicate no measurable exchange (less than 30 transfers per second) between the manganese centers in HMn(CO)(5), MnCo(CO)(9), and Mn-2(CO)(10) under the same reaction conditions, where the hydride moieties in HMn(CO)(5) and HCo(CO)(4) are undergoing facile exchange (greater than 10(4) transfers per second) as evident in the H-1 NMR spectra. This lack of measurable exchange between the manganese centers in HMn(CO)(5), MnCo(CO)(9), and Mn-2-(CO)(10) is inconsistent with an oxidative addition reaction mechanism for the heterometallic hydride ligand transfer reaction. Alternatively, the kinetics of these hydride ligand transfer reactions are interpreted in terms of a hydrogen atom transfer reaction mechanism involving Co-.(CO)(4) and .Mn(CO)(5) radicals. Thus, the degenerate hydrogen atom transfer reaction between HCo(CO)(4) and Co-.(CO)(4) proceeds with activation parameters of Delta H-double dagger = 5.5 +/- 0.6 kcal/mol and Delta S-double dagger = -16 +/- 1 cal/(K.mol), while the endothermic hydrogen atom transfer from manganese in HMn(CO)(5) to cobalt in Co-.(CO)(4) exhibits an activation enthalpy of 10 +/- 1 kal/mol. In addition, the kinetics for the ligand exchange reaction between the coordinated carbonyl groups in Co-2(CO)(8) and free carbon monoxide has been studied in mesitylene solvent by C-13 NMR line-shape analysis over the temperature range of 100 to 180 degrees C under 8.2 M of carbon monoxide. In this temperature range, the free carbon monoxide ligand exhibits a strongly temperature-dependent chemical shift in the presence of Co-2(5CO)(8). This temperature-dependent (CO)-C-13 chemical shift is interpreted as a contact chemical shift due to facile ligand exchange with Co-.(CO)(4) radicals. The resultant analysis of the contact chemical shift data yields a calculated Co-Co bond dissociation enthalpy (BDE) of 19 +/- 2 kcal/mol. The magnetic susceptibility of a solution of Co-2(CO)(8) in carbon monoxide has been studied by H-1 NMR spectroscopy over the temperature range of 120 to 225 degrees C, consistently yielding an enthalpy and entropy for Co-CO bond homolysis of Delta H degrees = 19 +/- 2 kcal/mol and Delta S degrees = 29 +/- 4 cal/(K.mol). This Co-Co BDE when used in combination with the enthalpy of hydrogenation for Co-2(CO)(8) yields a Co-H BDE in HCo(CO)(4) of 59 +/- 1 kcal/mol. Consistently, the activation enthalpy for hydrogen atom transfer in the HCo(CO)(4)/Co-.(CO)(4) system is between 5 and 10% of the measured Co-H bond enthalpy (Delta H-double dagger/BDE = 0.093) in agreement with the theory for atom transfer reactions developed by Marcus.