Activation of C−H / H−H Bonds by Rhodium(II) Porphyrin Bimetalloradicals

Reactivity, kinetic, and thermodynamic studies are reported for reactions of a rhodium(II) bimetalloradical with H2, and with the methyl C−H bonds for a series of substrates CH3R (R = H, CH3, OH, C6H5) using a m-xylyl diether tethered diporphyrin ligand. Bimolecular substrate reactions involving the intramolecular use of two metalloradical centers and preorganization of the four-centered transition state (M•···X···Y•···M) result in large rate enhancements as compared to termolecular reactions of monometalloradicals. Activation parameters and deuterium kinetic isotope effects for the substrate reactions are reported. The C−H bond reactions become less thermodynamically favorable as the substrate steric requirements increase, and the activation free energy (ΔG⧧) decreases regularly as ΔG° becomes more favorable. An absolute Rh−H bond dissociation enthalpy of 61.1 ± 0.4 kcal mol-1 is directly determined, and the derived Rh−CH2R BDE values increase regularly with the increase in the C−H BDE

Superoxo, Peroxo, and Hydroperoxo Complexes Formed from Reactions of Rhodium Porphyrins with Dioxygen: Thermodynamics and Kinetics

Rhodium(II) porphyrin complexes react with dioxygen to form terminal superoxo and bridged μ-peroxo complexes. Equilibrium constants for dioxygen complex formation with rhodium(II) tetramesitylporphyrin ((TMP)Rh•) and a m-xylyl-tethered dirhodium(II) diporphyrin complex (•Rh(m-xylyl)Rh•) are reported. (TMP)Rh−H reacts with oxygen to form a transient hydroperoxy complex ((TMP)Rh−OOH), which reacts on to form the rhodium(II) complex ((TMP)Rh•) and water. Kinetic studies for reactions of (TMP)Rh−H with O2 suggest a near concerted addition of dioxygen to the (TMP)Rh−H unit. Reactivity studies for mixtures of H2/O2 and CH4/O2 with the dirhodium(II) complex (•Rh(m-xylyl)Rh•) are reported

Formation and Reactivity of a Porphyrin Iridium Hydride in Water: Acid Dissociation Constants and Equilibrium Thermodynamics Relevant to Ir–H, Ir–OH, and Ir–CH₂– Bond Dissociation Energetics

Aqueous solutions of group nine metal(III) (M = Co, Rh, Ir) complexes of tetra(3,5-disulfonatomesityl)porphyrin [(TMPS)M^III] form an equilibrium distribution of aquo and hydroxo complexes ([(TMPS)M^III(D₂O)_2–<i>n</i>(OD)_<i>n</i>]^{(7+<i>n</i>)–}). Evaluation of acid dissociation constants for coordinated water show that the extent of proton dissociation from water increases regularly on moving down the group from cobalt to iridium, which is consistent with the expected order of increasing metal–ligand bond strengths. Aqueous (D₂O) solutions of [(TMPS)Ir^III(D₂O)₂]^7– react with dihydrogen to form an iridium hydride complex ([(TMPS)Ir–D(D₂O)]^8–) with an acid dissociation constant of 1.8(0.5) × 10^–12 (298 K), which is much smaller than the Rh–D derivative (4.3 (0.4) × 10^–8), reflecting a stronger Ir–D bond. The iridium hydride complex adds with ethene and acetaldehyde to form organometallic derivatives [(TMPS)Ir–CH₂CH₂D(D₂O)]^8– and [(TMPS)Ir–CH(OD)CH₃(D₂O)]^8–. Only a six-coordinate carbonyl complex [(TMPS)Ir–D(CO)]^8– is observed for reaction of the Ir–D with CO (<i>P</i>_CO = 0.2–2.0 atm), which contrasts with the (TMPS)Rh–D analog which reacts with CO to produce an equilibrium with a rhodium formyl complex ([(TMPS)Rh–CDO(D₂O)]^8–). Reactivity studies and equilibrium thermodynamic measurements were used to discuss the relative M–X bond energetics (M = Rh, Ir; X = H, OH, and CH₂−) and the thermodynamically favorable oxidative addition of water with the (TMPS)Ir(II) derivatives

Dimerization of the Octaethylporphyrin π Cation Radical Complex of Cobalt(II): Thermodynamic, Kinetic, and Spectroscopic Studies

Dimerization of the Octaethylporphyrin π Cation Radical Complex of Cobalt(II):  Thermodynamic, Kinetic, and Spectroscopic Studie

Comparative Studies of Preferential Binding of Group Nine Metalloporphyrins (M = Co, Rh, Ir) with Methoxide/Methanol in Competition with Hydroxide/Water in Aqueous Solution

Aqueous solutions of iridium(III) tetra-(p-sulfonatophenyl)porphyrin [(TSPP)IrIII] form a hydrogen ion dependent equilibrium distribution of bisaquo ([(TSPP)IrIII(OD2)2]3−), monoaquo/monohydroxo ([(TSPP)IrIII(OD2)(OD)]4−) and bishydroxo ([(TSPP)IrIII(OD)2]5−) complexes. Comparison of acid dissociation constants of group nine ([(TSPP)MIII(OD2)2]3−) (M = Co, Rh, Ir) complexes show that the extent of proton dissociation in water increases regularly on moving down the group from cobalt to iridium consistent with increasing metal ligand bond strength. Addition of small quantities of methanol to aqueous solutions of [(TSPP)IrIII] results in the formation of methanol and methoxide complexes in equilibria with aquo and hydroxo complexes that are observed by 1H NMR. Direct quantitative evaluation of competitive equilibria of [(TSPP)IrIII] complexes reveals a remarkable thermodynamic preference for methanol binding over that of water (ΔG° (298 K) = −5.2 kcal mol−1) and methoxide binding over that of hydroxide (ΔG° (298 K) = −6.1 kcal mol−1) in aqueous media. A comparison of equilibrium thermodynamic values for displacement of hydroxide by methoxide for group nine (TSPP)MIII (M = Co, Rh, Ir) complexes in aqueous media are also reported

Hydrogen and Methanol Exchange Processes for (TMP)Rh-OCH₃(CH₃OH) in Binary Solutions of Methanol and Benzene

Tetramesityl porphinato rhodium(III) methoxide ((TMP)Rh-OCH3) binds with methanol in benzene to form a 1:1 methanol complex ((TMP)Rh-OCH3(CH3OH)) (1). Dynamic processes are observed to occur for the rhodium(III) methoxide methanol complex (1) that involve both hydrogen and methanol exchange. Hydrogen exchange between coordinated methanol and methoxide through methanol in solution results in an interchange of the environments for the non-equivalent porphyrin faces that contain methoxide and methanol ligands. Interchange of the environments of the coordinated methanol and methoxide sites in 1 produces interchange of the inequivalent mesityl o-CH3 groups, but methanol ligand exchange occurs on one face of the porphyrin and the mesityl o-CH3 groups remain inequivalent. Rate constants for dynamic processes are evaluated by full line shape analysis for the 1H NMR of the mesityl o-CH3 and high field methyl resonances of coordinated methanol and methoxide groups in 1. The rate constant for interchange of the inequivalent porphyrin faces is associated with hydrogen exchange between 1 and methanol in solution and is observed to increase regularly with the increase in the mole fraction of methanol. The rate constant for methanol ligand exchange between 1 and the solution varies with the solution composition and fluctuates in a manner that parallels the change in the activation energy for methanol diffusion which is a consequence of solution non-ideality from hydrogen bonded clusters

Bimetallo-Radical Carbon−Hydrogen Bond Activation of Methanol and Methane

Carbon−hydrogen bond cleavage reactions of CH3OH and CH4 by a dirhodium(II) diporphyrin complex with a m-xylyl tether (·Rh(m-xylyl)Rh·(1)) are reported. Kinetic-mechanistic studies show that the substrate reactions are bimolecular and occur through the use of two Rh(II) centers in the molecular unit of 1. Second-order rate constants (T = 296 K) for the reactions of 1 with methanol (k(CH3OH) = 1.45 × 10-2 M-1 s-1) and methane (k(CH4) = 0.105 M-1 s-1) show a clear kinetic preference for the methane activation process. The methanol and methane reactions with 1 have large kinetic isotope effects (k(CH3OH)/k(CD3OD) = 9.7 ± 0.8, k(CH4)/k(CD4) = 10.8 ± 1.0, T = 296 K), consistent with a rate-limiting step of C−H bond homolysis through a linear transition state. Activation parameters for reaction of 1 with methanol (ΔH⧧ = 15.6 ± 1.0 kcal mol-1; ΔS⧧ = −14 ± 5 cal K-1 mol-1) and methane (ΔH⧧ = 9.8 ± 0.5 kcal mol-1; ΔS⧧ = −30 ± 3 cal K-1 mol-1) are reported

Comparison of Rh−OCH₃ and Rh−CH₂OH Bond Dissociation Energetics from Methanol C−H and O−H Bond Reactions with Rhodium(II) Porphyrins

Reaction of methanol in toluene with tetramesityl rhodium(II) porphyrin ((TMP)RhII•) produces a 1H NMR-observable equilibrium with rhodium methoxide ((TMP)Rh−OCH3(CH3OH)) and rhodium hydride ((TMP)Rh−H) complexes. Equilibrium concentrations for each of these species, obtained from integration of 1H NMR spectra, were used in determining the equilibrium constant, K(298 K) = [Rh−OCH3(CH3OH)][Rh−H]/[RhII•]2[CH3OH]2 = 3.0(0.3), and free energy change, ΔG0(298 K) = −0.65(0.5) kcal mol−1, for the reaction. Equilibrium thermodynamic measurements in CD2Cl2 give ΔG0(298 K) = −5.5(0.2) kcal mol−1 for association of methanol with (TMP)Rh−OCH3 to form the six-coordinate 18-electron complex (TMP)Rh−OCH3(CH3OH). Equilibrium measurements in conjunction with (TMP)Rh−H and CH3O−H bond energetics are used to evaluate the (TMP)Rh−OCH3 bond dissociation free energy (Rh−OCH3 BDFE(298 K) = 38 (1.3) kcal mol−1), which is 15 kcal mol−1smaller than the Rh−H BDFE and approximately equal to the Rh−CH2OH BDFE

Regioselectivity and Equilibrium Thermodynamics for Addition of Rh−OH to Olefins in Water

Rhodium(III) tetra(p-sulfonato phenyl) porphyrin ((TSPP)Rh) aquo and hydroxo complexes react with a series of olefins in water to form β-hydroxyalkyl complexes. Addition reactions of (TSPP)Rh−OH to unactivated terminal alkenes invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh−CH2CH(OH)R complexes. Acrylic and styrenic olefins initially react to place rhodium on the terminal carbon to form Rh−CH2CH(OH)X as the kinetically preferred isomer but subsequently proceed to an equilibrium distribution of regioisomers where Rh−CH(CH2OH)X is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound ([(TSPP)RhIII(H2O)2]-3) in water with a series of terminal olefins that form β-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh−OH unit to olefins. The ΔG° for reactions of the Rh−OH unit with olefins in water is approximately 3 kcal mol-1 less favorable than the comparable Rh−H reactions in water. Comparisons of the regioisomers and thermodynamics for addition reactions of olefins with Rh−H and Rh−OH units in water are presented and discussed

Iridium Porphyrins in CD₃OD: Reduction of Ir(III), CD₃–OD Bond Cleavage, Ir–D Acid Dissociation and Alkene Reactions

Methanol solutions of iridium­(III) tetra­(<i>p</i>-sulfonatophenyl)­porphyrin [(TSPP)­Ir^III] form an equilibrium distribution of methanol and methoxide complexes ([(TSPP)­Ir^III(CD₃OD)_{(2–<i>n</i>)}(OCD₃)_n]^{(3+<i>n</i>)–}). Reaction of [(TSPP)­Ir^III with dihydrogen (D₂) in methanol produces an iridium hydride [(TSPP)­Ir^III–D­(CD₃OD)]^4– in equilibrium with an iridium­(I) complex ([(TSPP)­Ir^I(CD₃OD)]^5–). The acid dissociation constant of the iridium hydride (Ir–D) in methanol at 298 K is 3.5 × 10^–12. The iridium­(I) complex ([(TSPP)­Ir^I(CD₃OD)]^5–) catalyzes reaction of [(TSPP)­Ir^III–D­(CD₃OD)]^4– with CD₃–OD to produce an iridium methyl complex [(TSPP)­Ir^III–CD₃(CD₃OD)]^4– and D₂O. Reactions of the iridium hydride with ethene and propene produce iridium alkyl complexes, but the Ir–D complex fails to give observable addition with acetaldehyde and carbon monoxide in methanol. Reaction of the iridium hydride with propene forms both the isopropyl and propyl complexes with free energy changes (Δ<i>G</i>° 298 K) of −1.3 and −0.4 kcal mol^–1 respectively. Equilibrium thermodynamics and reactivity studies are used in discussing relative Ir–D, Ir–OCD₃ and Ir–CD₂- bond energetics in methanol