Thermodynamic, Kinetic, Structural, and Computational Studies of the Ph₃Sn–H, Ph₃Sn–SnPh₃, and Ph₃Sn–Cr(CO)₃C₅Me₅ Bond Dissociation Enthalpies

The kinetics of the reaction of Ph₃SnH with excess •Cr­(CO)₃C₅Me₅ = •<b>Cr</b>, producing H<b>Cr</b> and Ph₃Sn–<b>Cr</b>, was studied in toluene solution under 2–3 atm CO pressure in the temperature range of 17–43.5 °C. It was found to obey the rate equation <i>d</i>[Ph₃Sn–<b>Cr</b>]/<i>d</i>t = <i>k</i>[Ph₃SnH]­[•<b>Cr</b>] and exhibit a normal kinetic isotope effect (<i>k</i>_H/<i>k</i>_D = 1.12 ± 0.04). Variable-temperature studies yielded Δ<i>H</i>^‡ = 15.7 ± 1.5 kcal/mol and Δ<i>S</i>^‡ = −11 ± 5 cal/(mol·K) for the reaction. These data are interpreted in terms of a two-step mechanism involving a thermodynamically uphill hydrogen atom transfer (HAT) producing Ph₃Sn• and H<b>Cr</b>, followed by rapid trapping of Ph₃Sn• by excess •<b>Cr</b> to produce Ph₃Sn–<b>Cr</b>. Assuming an overbarrier of 2 ± 1 kcal/mol in the HAT step leads to a derived value of 76.0 ± 3.0 kcal/mol for the Ph₃Sn–H bond dissociation enthalpy (BDE) in toluene solution. The reaction enthalpy of Ph₃SnH with excess •<b>Cr</b> was measured by reaction calorimetry in toluene solution, and a value of the Sn–Cr BDE in Ph₃Sn-<b>Cr</b> of 50.4 ± 3.5 kcal/mol was derived. Qualitative studies of the reactions of other R₃SnH compounds with •<b>Cr</b> are described for R = ⁿBu, ^tBu, and Cy. The dehydrogenation reaction of 2Ph₃SnH → H₂ + Ph₃SnSnPh₃ was found to be rapid and quantitative in the presence of catalytic amounts of the complex Pd­(IPr)­(P­(<i>p</i>-tolyl)₃). The thermochemistry of this process was also studied in toluene solution using varying amounts of the Pd(0) catalyst. The value of Δ<i>H</i> = −15.8 ± 2.2 kcal/mol yields a value of the Sn–Sn BDE in Ph₃SnSnPh₃ of 63.8 ± 3.7 kcal/mol. Computational studies of the Sn–H, Sn–Sn, and Sn–Cr BDEs are in good agreement with experimental data and provide additional insight into factors controlling reactivity in these systems. The structures of Ph₃Sn–<b>Cr</b> and Cy₃Sn–<b>Cr</b> were determined by X-ray crystallography and are reported. Mechanistic aspects of oxidative addition reactions in this system are discussed

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

Pt­(IV) complexes <i>trans</i>-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

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

Pt­(IV) complexes <i>trans</i>-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

Two-Step Binding of O₂ to a Vanadium(III) Trisanilide Complex To Form a Non-Vanadyl Vanadium(V) Peroxo Complex

Treatment of V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>1</b>) (Ar = 3,5-Me₂C₆H₃) with O₂ was shown by stopped-flow kinetic studies to result in the rapid formation of (η¹-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>2</b>) (Δ<i>H</i>^⧧ = 3.3 ± 0.2 kcal/mol and Δ<i>S</i>^⧧ = −22 ± 1 cal mol^–1 K^–1), which subsequently isomerizes to (η²-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>3</b>) (Δ<i>H</i>^⧧ = 10.3 ± 0.9 kcal/mol and Δ<i>S</i>^⧧ = −6 ± 4 cal mol^–1 K^–1). The enthalpy of binding of O₂ to form <b>3</b> is −75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of <b>3</b> and <b>1</b> to form 2 equiv of OV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>4</b>) occurs by initial isomerization of <b>3</b> to <b>2</b>. The results of computational studies of this rearrangement (Δ<i>H</i> = 4.2 kcal/mol; Δ<i>H</i>^⧧ = 16 kcal/mol) are in accord with experimental data (Δ<i>H</i> = 4 ± 3 kcal/mol; Δ<i>H</i>^⧧ = 14 ± 3 kcal/mol). With the aim of suppressing the formation of <b>4</b>, the reaction of O₂ with <b>1</b> in the presence of ^<i>t</i>BuCN was studied. At −45 °C, the principal products of this reaction are <b>3</b> and ^<i>t</i>BuC­(O)­NV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>5</b>), in which the bound nitrile has been oxidized. Crystal structures of <b>3</b> and <b>5</b> are reported

Role of Axial Base Coordination in Isonitrile Binding and Chalcogen Atom Transfer to Vanadium(III) Complexes

The enthalpy of oxygen atom transfer (OAT) to V­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>, forming OV­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>–O, and the enthalpies of sulfur atom transfer (SAT) to <b>1</b> and V­(N­[<i>t</i>-Bu]­Ar)₃, <b>2</b> (Ar = 3,5-C₆H₃Me₂), forming the corresponding sulfides SV­[(Me₃SiNCH₂CH₂)₃N], <b>1</b>–S, and SV­(N­[<i>t</i>-Bu]­Ar)₃, <b>2</b>–S, have been measured by solution calorimetry in toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene) and Ph₃SbS as chalcogen atom transfer reagents. The V–O BDE in <b>1</b>–O is 6.3 ± 3.2 kcal·mol^–1 lower than the previously reported value for <b>2</b>–O and the V–S BDE in <b>1</b>–S is 3.3 ± 3.1 kcal·mol^–1 lower than that in <b>2</b>–S. These differences are attributed primarily to a weakening of the V–N_axial bond present in complexes of <b>1</b> upon oxidation. The rate of reaction of <b>1</b> with dbabhNO has been studied by low temperature stopped-flow kinetics. Rate constants for OAT are over 20 times greater than those reported for <b>2</b>. Adamantyl isonitrile (AdNC) binds rapidly and quantitatively to both <b>1</b> and <b>2</b> forming high spin adducts of V­(III). The enthalpies of ligand addition to <b>1</b> and <b>2</b> in toluene solution are −19.9 ± 0.6 and −17.1 ± 0.7 kcal·mol^–1, respectively. The more exothermic ligand addition to <b>1</b> as compared to <b>2</b> is opposite to what was observed for OAT and SAT. This is attributed to less weakening of the V–N_axial bond in ligand binding as opposed to chalcogen atom transfer and is in keeping with structural data and computations. The structures of <b>1</b>, <b>1</b>–O, <b>1</b>–S, <b>1</b>–CNAd, and <b>2</b>–CNAd have been determined by X-ray crystallography and are reported

Two-Step Binding of O₂ to a Vanadium(III) Trisanilide Complex To Form a Non-Vanadyl Vanadium(V) Peroxo Complex

Treatment of V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>1</b>) (Ar = 3,5-Me₂C₆H₃) with O₂ was shown by stopped-flow kinetic studies to result in the rapid formation of (η¹-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>2</b>) (Δ<i>H</i>^⧧ = 3.3 ± 0.2 kcal/mol and Δ<i>S</i>^⧧ = −22 ± 1 cal mol^–1 K^–1), which subsequently isomerizes to (η²-O₂)­V­(N­[^<i>t</i>Bu]­Ar)₃ (<b>3</b>) (Δ<i>H</i>^⧧ = 10.3 ± 0.9 kcal/mol and Δ<i>S</i>^⧧ = −6 ± 4 cal mol^–1 K^–1). The enthalpy of binding of O₂ to form <b>3</b> is −75.0 ± 2.0 kcal/mol, as measured by solution calorimetry. The reaction of <b>3</b> and <b>1</b> to form 2 equiv of OV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>4</b>) occurs by initial isomerization of <b>3</b> to <b>2</b>. The results of computational studies of this rearrangement (Δ<i>H</i> = 4.2 kcal/mol; Δ<i>H</i>^⧧ = 16 kcal/mol) are in accord with experimental data (Δ<i>H</i> = 4 ± 3 kcal/mol; Δ<i>H</i>^⧧ = 14 ± 3 kcal/mol). With the aim of suppressing the formation of <b>4</b>, the reaction of O₂ with <b>1</b> in the presence of ^<i>t</i>BuCN was studied. At −45 °C, the principal products of this reaction are <b>3</b> and ^<i>t</i>BuC­(O)­NV­(N­[^<i>t</i>Bu]­Ar)₃ (<b>5</b>), in which the bound nitrile has been oxidized. Crystal structures of <b>3</b> and <b>5</b> are reported

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[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) 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 <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-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 ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts

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[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) 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 <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-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 ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts

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[<i>t</i>-Bu]­Ar)₃ (Ar = 3,5-C₆H₃Me₂, <b>1</b>) 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 <b>1</b> show that the V–O bond in OV­(N­[<i>t</i>-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 ³O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N₂O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N₂O or MesCNO despite increased steric hindrance of the adducts

Ligand-Directed Reactivity in Dioxygen and Water Binding to <i>cis</i>-[Pd(NHC)₂(η²‑O₂)]

Reaction of [Pd­(IPr)₂] (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) and O₂ leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex <i>cis</i>-[Pd­(IPr)₂(η²-O₂)]. At temperatures ≳ −40 °C, <i>cis</i>-[Pd­(IPr)₂(η²-O₂)] adds a second O₂ to form <i>trans</i>-[Pd­(IPr)₂(η¹-O₂)₂]. Squid magnetometry and EPR studies yield data that are consistent with a singlet diradical ground state with a thermally accessible triplet state for this unique bis-superoxide complex. In addition to reaction with O₂, <i>cis</i>-[Pd­(IPr)₂(η²-O₂)] reacts at low temperature with H₂O in methanol/ether solution to form <i>trans</i>-[Pd­(IPr)₂(OH)­(OOH)]. The crystal structure of <i>trans</i>-[Pd­(IPr)₂(OOH)­(OH)] is reported. Neither reaction with O₂ nor reaction with H₂O occurs under comparable conditions for <i>cis</i>-[Pd­(IMes)₂(η²-O₂)] (IMes = 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene). The increased reactivity of <i>cis</i>-[Pd­(IPr)₂(η²-O₂)] is attributed to the enthalpy of binding of O₂ to [Pd­(IPr)₂] (−14.5 ± 1.0 kcal/mol) that is approximately one-half that of [Pd­(IMes)₂] (−27.9 ± 1.5 kcal/mol). Computational studies identify the cause as interligand repulsion forcing a wider C–Pd–C angle and tilting of the NHC plane in <i>cis</i>-[Pd­(IPr)₂(η²-O₂)]. Arene–arene interactions are more favorable and serve to further stabilize <i>cis</i>-[Pd­(IMes)₂(η²-O₂)]. Inclusion of dispersion effects in DFT calculations leads to improved agreement between experimental and computational enthalpies of O₂ binding. A complete reaction diagram is constructed for formation of <i>trans</i>-[Pd­(IPr)₂(η¹-O₂)₂] and leads to the conclusion that kinetic factors inhibit formation of <i>trans</i>-[Pd­(IMes)₂(η¹-O₂)₂] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate <i>trans</i>-[Pd­(NHC)₂(η¹-O₂)] for either NHC = IMes or IPr is attributed to dynamic effects. A partial potential energy diagram for initial binding of O₂ is constructed. A range of low-energy pathways at different angles of approach are present and blur the distinction between pure “side-on” or “end-on” trajectories for oxygen binding