Elon A. Ison

Mechanistic Investigations of the Iridium(III)-Catalyzed Aerobic Oxidation of Primary and Secondary Alcohols

The commercially available catalysts [(Cp*IrCl2)2] is employed with O2 as the terminal oxidant in the presence of catalytic amounts of Et3N for the aerobic oxidation of primary and secondary alcohols. A new mechanism for the Ir-catalyzed aerobic oxidation is also presented that suggests that the transition metal maintains its +3 oxidation state throughout the entire catalytic cycle.

The Electronic Nature of Terminal Oxo Ligands in Transition-Metal Complexes: Ambiphilic Reactivity of Oxorhenium Species

The synthesis of the Lewis acid-base adducts of B(C6F5)3 and BF3 with [DAAmRe(O)(X)] DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5 (X = Me, 1, COCH3, 2, Cl, 3) as well as their diamidopyridine (DAP) (DAP=(2,6-bis((mesitylamino)methyl)pyridine) analogues, [DAPRe(O)(X)] (X = Me, 4, Cl, 5, I, 6, and COCH3,7), are described. In these complexes the terminal oxo ligands act as nucleophiles. In addition we also show that stoichiometric reactions between 3 and triarylphosphine (PAr3) result in the formation of triarylphosphine oxide (OPAr3). The electronic dependence of this reaction was studied by comparing the rates of oxygen atom transfer for various para-substituted triaryl phosphines in the presence of CO. From these experiments a reaction constant ρ = -0.29 was obtained from the Hammett plot. This suggests that the oxygen atom transfer reaction is consistent with nucleophilic attack of phosphorus on an electrophilic metal oxo. To the best of our knowledge, these are the first examples of mono-oxo d(2) metal complexes in which the oxo ligand exhibits ambiphilic reactivity.

Oxyfunctionalization with Cp*IrIII(NHC)(Me)(Cl) with O2: Identification of a Rare Bimetallic IrIV μ-Oxo Intermediate

Methanol formation from [Cp*Ir(III)(NHC)Me(CD2Cl2)](+) occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)2(μ-O)][(BAr(F)4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an Ir(IV)-O-Ir(IV) core in which two d(5) Ir(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the μ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the μ-oxo core (Ir-O-Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir-O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium-methyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BAr(F)4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BAr(F)4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.

Mechanism for the Activation of Carbon Monoxide via Oxorhenium Complexes

Activation of CO by the rhenium(V) oxo complex [(DAAm)Re(O)(CH(3))] (1) [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C(6)F(5), Mes] resulted in the isolation of the rhenium(III) acetate complex [(DAAm)Re(O(2)CCH(3))(CO)] (3). The mechanistic details of this reaction were explored experimentally. The novel oxorhenium(V) acyl intermediate [(DAAm)Re(O)(C(O)CH(3))] (2) was isolated, and its reactivity with CO was investigated. An unprecedented mechanism is proposed: CO is activated by the metal oxo complex 1 and inserted into the rhenium-methyl bond to yield acyl complex 2, after which subsequent migration of the acyl ligand to the metal oxo ligand yields acetate complex 3. X-ray crystal structures of 2 and 3 are reported.

Synthesis of Oxorhenium(V) Complexes with Diamido Amine Ancillary Ligands and Their Role in Oxygen Atom Transfer Catalysis

The detailed syntheses of complexes of the form [Re(O)(X)(RNCH(2)CH(2))(2)N(Me)] (X = Me, Cl, I, R = mesityl, C(6)F(5)), 1-3, incorporating diamidoamine ancillary ligands are described. X-ray crystal structures for the complexes [Re(O)(Me)((C(6)F(5))NCH(2)CH(2))(2)N(Me)], 1a, [Re(O)(I)((C(6)F(5))N CH(2)CH(2))(2)N(Me)], 3a, and [Re(O)(I)((Mes)NCH(2))(2)N(Me)], 3b, are reported. The geometry about the metal center in 1a is best described as a severely distorted square pyramid with the oxo ligand in the apical position. In contrast, the geometry about the metal center in 3a is best described as a severely distorted trigonal bipyramid, with the iodo ligand occupying the apical position and the diamido nitrogens and the oxo ligand occupying the equatorial plane. The catalytic activities of these complexes for oxygen atom transfer, OAT, from pyridine-N-oxides, PyO, to PPh(3) were also examined. The reactions exhibited a clear dependence on the diamido ligand substituent and the X ligand (Me, I, Cl) attached to the metal, with the combined effect that electron-withdrawing substituents on the diamido ligand and poor sigma donors directly attached to the metal center increases the rate of catalytic activity. The kinetics of OAT from pyridine-N-oxides to Re were also investigated. The reactions displayed clean first order kinetics in Re and saturation kinetics for the dependence on PyO. Changing the PyO substrate had no effect on the saturation value, k(sat), suggesting that the OAT reaction in these five-coordinate complexes appears to be governed by isomerization of the starting complex. Attempts to isolate a postulated Re(VII) intermediate were not successful because of hydrolytic degradation. The product of hydrolytic degradation [((C(6)F(5))N(H)CH(2)CH(2)))(2)NH(Me)][X], (X = ReO(4)(-), or I(-)), 4 can be isolated, and its X-ray crystal structure is reported. Although the Re(VII) intermediate could not be isolated, its activity in OAT reactions was probed by competition experiments with PPh(3) and four para-substituted triarylphosphines (p-X-Ph)(3)P (X = OMe, Me, Cl, CF(3)). These experiments led to a Hammett that yielded a reaction constant of rho = -0.30 +/- 0.01. This data suggests a positive charge buildup on phosphorus for the OAT reaction and is consistent with the nucleophilic attack of phosphorus on an electrophilic metal oxo.

Transition-Metal Oxos as the Lewis Basic Component of Frustrated Lewis Pairs

The reaction of oxorhenium complexes that incorporate diamidopyridine (DAP) ligands with B(C6F5)3 results in the formation of classical Lewis acid-base adducts. The adducts effectively catalyze the hydrogenation of a variety of unactivated olefins at 100 °C. Control reactions with these complexes or B(C6F5)3 alone did not yield any hydrogenated products under these conditions. Mechanistic studies suggest a frustrated Lewis pair is generated between the oxorhenium DAP complexes and B(C6F5)3, which is effective at olefin hydrogenation. Thus, we demonstrate for the first time that the incorporation of a transition-metal oxo in a frustrated Lewis pair can have a synergistic effect and results in enhanced catalytic activity.