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

The cobalt center in H Co(CO)(4) exchanges with those in Co-2(CO)(8) t hrough a facile hydride ligand transfer reaction which has been studie d 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 supercritic al carbon dioxide. The lifetime of the cobalt center in HCo(CO)(4) var ies 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 tran sfer process is highly specific for the HCo(CO)(4) and Co-2(CO)(8) com plexe(s). Thus, neither Co-4(CO)(12) nor MnCo(CO)(9) exhibit measurabl e chemical exchange line broadening in the Co-59 NMR spectra within so lutions 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 hydri de, 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 sp ectra. A similar hydride ligand transfer reaction exchanges the hydrid e moieties in HCo(CO)(4) and HMn(CO)(5). This latter heterometallic hy dride ligand transfer reaction has been investigated by H-1 NMR line-w idth analysis over the temperature range of 110 to 190 degrees C at tw o initial carbon monoxide concentrations, 1.39 and 4.13 M. The lifetim e 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 measur able exchange (less than 30 transfers per second) between the manganes e 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 pe r second) as evident in the H-1 NMR spectra. This lack of measurable e xchange between the manganese centers in HMn(CO)(5), MnCo(CO)(9), and Mn-2-(CO)(10) is inconsistent with an oxidative addition reaction mech anism for the heterometallic hydride ligand transfer reaction. Alterna tively, the kinetics of these hydride ligand transfer reactions are in terpreted in terms of a hydrogen atom transfer reaction mechanism invo lving Co-.(CO)(4) and .Mn(CO)(5) radicals. Thus, the degenerate hydrog en 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 endo thermic 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 c oordinated carbonyl groups in Co-2(CO)(8) and free carbon monoxide has been studied in mesitylene solvent by C-13 NMR line-shape analysis ov er 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 presen ce of Co-2(5CO)(8). This temperature-dependent (CO)-C-13 chemical shif t is interpreted as a contact chemical shift due to facile ligand exch ange with Co-.(CO)(4) radicals. The resultant analysis of the contact chemical shift data yields a calculated Co-Co bond dissociation enthal py (BDE) of 19 +/- 2 kcal/mol. The magnetic susceptibility of a soluti on of Co-2(CO)(8) in carbon monoxide has been studied by H-1 NMR spect roscopy over the temperature range of 120 to 225 degrees C, consistent ly 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.mo l). This Co-Co BDE when used in combination with the enthalpy of hydro genation for Co-2(CO)(8) yields a Co-H BDE in HCo(CO)(4) of 59 +/- 1 k cal/mol. Consistently, the activation enthalpy for hydrogen atom trans fer in the HCo(CO)(4)/Co-.(CO)(4) system is between 5 and 10% of the m easured Co-H bond enthalpy (Delta H-double dagger/BDE = 0.093) in agre ement with the theory for atom transfer reactions developed by Marcus.