Carl D. Hoff

The thermodynamic driving force for CH activation at iridium

Abstract This paper discusses the relationship between the intermolecular oxidative addition reaction of carbon-hydrogen bonds in organic molecules to transition metal centres, and the dissociation energies of the CH, MH and MR bonds that undergo changes during this process. Earlier studies of transition metal MH and MR bond energies are reviewed, followed by a summary of relative and absolute bond energies measured more recently for the (η 5 -C 5 Me 5 )(PMe 3 )Ir(X)(Y) system. The MH and MR energies are unusually large in this system compared with most others that are presently known; an important exception are those in the thorium series, where intermolecular CH activation is also observed. The IrC and IrH bond energy values are utilized in discussing the propensity of iridium for intermolecular CH insertion, and in predicting thermochemistries for its RH insertion and M(H)(R) reductive elimination reactions. Finally, the physical basis for the strong metal-carbon and -hydrogen bonds in the iridium system is discussed.

The heats of hydrogenation of the metal-metal bonded complexes [M(CO)3C5H5]2 (M = Cr, Mo, W)

Abstract Direct measurement of the enthalpy of decomposition of HCr(CO) 3 C 5 H 5 to [Cr(CO) 3 C 5 H 5 ] 2 and H 2 was made by differential scanning calorimetry. The heat of hydrogenation of 1,3-cyclohexadiene by HM(CO) 3 C 5 H 5 for M = Cr, Mo, and W was measured by solution calorimetry. The enthalpies of iodination of [M(CO) 3 C 5 H 5 ] 2 and HM(CO) 3 C 5 H 5 were measured for M = Mo and W. These data have been used to calculate the heats of hydrogenation for each of the metal—metal bonded dimers, [M(CO) 3 C 5 H 5 ] 2 (M = Cr, Mo, and W). C 5 H 5 (CO) 3 M-M(CO) 3 C 5 H 5 (s) + H 2 (g) → 2HM(CO) 3 C 5 H 5 (s) Addition of hydrogen has been found to be exothermic for M = Cr, W (−3.3 kcal/mol and −1.5 kcal/mole, respectively) but endothermic for M = Mo (+6.3 kcal/mol). These results are consistent with the trend of increasing MH bond strengths upon descending Group VI. Addition of H 2 to [Cr(CO) 3 C 5 H 5 ] 2 is favored by the unusually weak chromium—chromium bond.

Thermodynamic, Kinetic, and Computational Study of Heavier Chalcogen (S, Se, and Te) Terminal Multiple Bonds to Molybdenum, Carbon, and Phosphorus

Enthalpies of chalcogen atom transfer to Mo(N[t-Bu]Ar)3, where Ar = 3,5-C6H3Me2, and to IPr (defined as bis-(2,6-isopropylphenyl)imidazol-2-ylidene) have been measured by solution calorimetry leading to bond energy estimates (kcal/mol) for EMo(N[t-Bu]Ar)3 (E = S, 115; Se, 87; Te, 64) and EIPr (E = S, 102; Se, 77; Te, 53). The enthalpy of S-atom transfer to PMo(N[ t-Bu]Ar) 3 generating SPMo(N[t-Bu]Ar)3 has been measured, yielding a value of only 78 kcal/mol. The kinetics of combination of Mo(N[t-Bu]Ar)3 with SMo(N[t-Bu]Ar)3 yielding (mu-S)[Mo(N[t-Bu]Ar)3]2 have been studied, and yield activation parameters Delta H (double dagger) = 4.7 +/- 1 kcal/mol and Delta S (double dagger) = -33 +/- 5 eu. Equilibrium studies for the same reaction yielded thermochemical parameters Delta H degrees = -18.6 +/- 3.2 kcal/mol and Delta S degrees = -56.2 +/- 10.5 eu. The large negative entropy of formation of (mu-S)[Mo(N[t-Bu]Ar)3]2 is interpreted in terms of the crowded molecular structure of this complex as revealed by X-ray crystallography. The crystal structure of Te-atom transfer agent TePCy3 is also reported. Quantum chemical calculations were used to make bond energy predictions as well as to probe terminal chalcogen bonding in terms of an energy partitioning analysis.

High Quantum Yield Molecular Bromine Photoelimination from Mononuclear Platinum(IV) Complexes

Pt(IV) complexes trans-Pt(PEt3)2(R)(Br)3 (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe3 analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br2. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination.

The heats of reaction of phosphines and phosphites with toluene-molybdenum tricarbonyl. Importance of both steric and electronic factors in determining the MoPR3 bond strength

Abstract The heats of reaction of tolueneMo(CO) 3 with a series of phosphines and phosphites have been measured by solution calorimetry. The order of stability toward formation of fac -(PR 3 ) 3 Mo(CO) 3 in THF solution is: P(OCH 3 ) 3 s > PMe 3 > P n Bu 3 > PMe 2 Ph > PEt 3 > triphos > P(OPh) 3 > PMePh 2 > PPh 3 > PCl 3 and spans a range of 25 kcal/mol reflecting individual bond strength differences up to 8 kcal/mol. The bulky phosphines PCy 3 and P t Bu 3 react with tolueneMo(CO) 3 in THF, but 30–40 kcal/mol less heat is evolved in these reactions than with the other phosphines and phosphites. The coordinately unsaturated five-coordinate complexes (PR 3 ) 2 Mo(CO) 3 are proposed as the reaction products. The importance of both steric and electronic factors in the MoP bond is discussed.

Thermochemistry of molybdenum tricarbonyl complexes of arenes and cyclic polyolefins

The heats of reaction of arene, cycloheptatriene, and cyclooctatetraene complexes of molybdenum tricarbonyl with pyridine yielding (py)3Mo(CO)3 have been measured by solution calorimetry. Reaction of toluene molybdenum tricarbonyl with cyclopentadiene yielding HMo(CO)3C5H5 and with tetrahydrofuran yielding (THF)3Mo(CO)3 have also been studied thermochemically. These measurements yield accurate values for the enthalpy of arene exchange in methylene chloride solution for a number of organomolybdenum complexes. The order of stability: benzene < toluene < cyclooctatetraene < mesitylene < hexamethylbenzene < cycloheptatriene < (tris)-tetrahydrofuran < η5-cyclopentadienylhydrido < (tris)-pyridine spans a range of 31 kcal/mol. The enthalpy of isomerization of cycloheptatriene to toluene is reduced by 7.1 ± 1.2 kcal/mol upon coordination to molybdenum tricarbonyl, indicative of a loss of “resonance” energy for the complexed arene. The MoH bond strength in HMo(CO)3C5H5 is estimated as 66 ± 8 kcal/mol. The importance of entropic factors in arene exchange is discussed.

Synthesis and thermochemistry of HMo(CO)3C5Me5; Comparison of cyclopentadienyl and pentamethylcyclopentadienyl ligands

Toluenemolybdenum tricarbonyl reacts quantitatively with pentamethylcyclopentadiene in THF at room temperature yielding HMo(CO)3C5Me5. The heat given off in this reaction has been measured by solution calorimetry and indicates there is little difference in the MoC5H5 and MoC5Me5 bond strengths. Thermochemical measurements of the reaction of HMo(CO)3C5R5 (R = H, CH3) with pyridine yielding (py)3Mo(CO)3 confirm this result. In THF solution, HMo(CO)3C5Me5 is a weaker acid than HMo(CO)3C5H5, ΔpKa ⩾ 3. The heat of hydrogenation of (Mo(CO)3C5R5)2 to yield two mol of HMo(CO)3C5R5 is more favorable by about 2 kcal/mol for R = CH3 compared to R = H, probably due to steric repulsion in (Mo(CO)3C5Me5)2.

Thermodynamic and kinetic studies of H atom transfer from HMo(CO)3(eta(5)-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: direct insertion of benzonitrile into the Mo-H bond of HMo(N[t-Bu]Ar)3 forming (Ph(H)C=N)Mo(N[t-Bu]Ar)3.

Synthetic studies are reported that show that the reaction of either H2SnR2 (R = Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp = eta(5)-C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar = 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct (PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)C=N)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of 1-H and 2 in toluene solution has been measured by solution calorimetry (DeltaH = -13.1 +/- 0.7 kcal mol(-1)) and used to estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol(-1). The enthalpy of reaction of 1-H and 2-NCPh in toluene solution was determined calorimetrically as DeltaH = -35.1 +/- 2.1 kcal mol(-1). This value combined with the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (1(2)) gives an estimated value of 90 kcal mol(-1) for the BDE of the ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrile insertion into 2-H would be exothermic by approximately 36 kcal mol(-1), and this reaction was observed experimentally. Stopped flow kinetic studies of the rapid reaction of 1-H with 2-NCPh yielded DeltaH(double dagger) = 11.9 +/- 0.4 kcal mol(-1), DeltaS(double dagger) = -2.7 +/- 1.2 cal K(-1) mol(-1). Corresponding studies with DMo(CO)3(Cp) (1-D) showed a normal kinetic isotope effect with kH/kD approximately 1.6, DeltaH(double dagger) = 13.1 +/- 0.4 kcal mol(-1) and DeltaS(double dagger) = 1.1 +/- 1.6 cal K(-1) mol(-1). Spectroscopic studies of the much slower reaction of 1-H and 2 yielding 2-H and 1/2 1(2) showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 1(2). The presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant for H atom transfer from 1-H has been measured: 0.09 +/- 0.01 M(-1) s(-1) at 1.3 degrees C and 0.26 +/- 0.04 M(-1) s(-1) at 17 degrees C. Study of the rate of reaction of 1-D yielded kH/kD = 1.00 +/- 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N)3Mo...HMo(CO)3(Cp) is rate limiting.

THE RATE AND MECHANISM OF OXIDATIVE ADDITION OF H2 TO THE CR(CO)3C5ME5 RADICAL-GENERATION OF A MODEL FOR REACTION OF H2 WITH THE CO(CO)4 RADICAL

Abstract The rate of reaction of hydrogen with the 17 e − metal centered radical Cr(CO) 3 C 5 Me 5 obeys the third-order rate law d[P]/d t = k obs [ Cr] 2 [H 2 ] in toluene solution. In the temperature range 20–60°C, k obs =330±30 M −2 s −1 , Δ H ≠ =0±1 kcal mol −1 , Δ S ≠ =−47±3 cal mol −1 deg −1 . The rate of oxidative addition is not inhibited by added pressure of CO. The rate of binding of D 2 is slower than that of H 2 : k (H 2 )/ k (D 2 )=1.18. These results are combined with earlier work to generate a complete reaction profile for hydrogenation of the metal–metal bonded dimer [Cr(CO) 3 C 5 Me 5 ] 2 +H 2 →2H–Cr(CO) 3 C 5 Me 5 . A similar reaction profile for Co 2 (CO) 8 +H 2 →2H–Co(CO) 4 under high pressures of CO is constructed based on literature data and estimated activation parameters for reaction of the Co(CO) 4 radical with hydrogen.

Thermodynamic and Kinetic Study of Cleavage of the N–O Bond of N-Oxides by a Vanadium(III) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2, 1) from compounds containing N-O bonds with a range of BDEs spanning nearly 100 kcal mol(-1): PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N2O (62) > MesCNO (53) > N2O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis(diisopropyl)phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N2O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N2O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to 1 show that the V-O bond in O≡V(N[t-Bu]Ar)3 is strong (BDE = 154 ± 3 kcal mol(-1)) compared with all the N-O bonds cleaved. In contrast, measurement of the N-O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol(-1)) and that dissociation of dbabhNO to anthracene, N2, and a (3)O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N2O and MesCNO to the bulky complex 1 show a faster rate than in the case of free N2O or MesCNO despite increased steric hindrance of the adducts.