The mechanism of the Pd-catalyzed diamination and carboamination of alkenes promoted by N-fluorobenzenesulfonimide (NFBS) was investigated. Stereochemical labeling experiments established that the diamination reaction proceeds via overall syn addition of the two nitrogen groups, whereas carboamination is the result of an anti addition of arene and nitrogen to the alkene. The intermediate Pd-alkyl complex arising from aminopalladation was observed, and an X-ray crystal structure of its 2,2'-bipyridine (bipy) complex was obtained, revealing strong chelation of the amide protecting group to palladium. Aminopalladation was shown to be an anti-selective process in both the presence and the absence of added ligands, proceeding via external attack of the nitrogen on a Pd-coordinated alkene. The intermediate Pd-alkyl complex was converted to diamination product upon exposure to NFBS with inversion of configuration via oxidative addition followed by dissociation of the benzenesulfonimide anion and S(N)2 displacement of the Pd-C bond. Conversely, arylation of the Pd-alkyl complex proceeds via retention of stereochemistry, consistent with C-H activation of the arene at the Pd(IV) center. A small intermolecular isotope effect (k(H)/k(D) = 1.1) and a large intramolecular isotope effect (k(H)/k(D) = 4) were measured for this process, indicating that C-H activation occurs via a poorly selective product-determining coordination of the arene followed by a highly selective C-H activation. Competition between arenes reveals an unusual reactivity order of toluene > benzene > bromobenzene > anisole.

Ruthenium bis(β-diketonato) complexes have been prepared at both the Ru II and Ru III oxidation levels and with protonated and deprotonated pyridine-imidazole ligands. Ru II (acac) 2 (py-imH) (1), [Ru III (acac) 2 (py-imH)]OTf (2), Ru III (acac) 2 (py-im) (3), Ru II (hfac) 2 (py-imH) (4), and [DBU-H] [Ru II (hfac) 2 (py-im)] (5) have been fully characterized, including X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH = 2-(2′-pyridyl) imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2, cyclic voltammetry in MeCN shows the Ru III/II reduction potential (E 1/2 ) to be −0.64 V vs. Cp 2 Fe +/0 . E 1/2 for the deprotonated imidazolate complex 3 (−1.00 V) is 0.36 V more negative. The Ru II bis-hfac analogs 4 and 5 show the same ΔE 1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 V and −0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pK a values of 22.1 and 19.3 in MeCN for 1 and 4. From the E 1/2 and pK a values, the bond dissociation free energies (BDFEs) of the N-H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol −1 in MeCN -a remarkable difference of 17.6 kcal mol −1 for such structurally similar compounds. Consistent with these values, there is facile net hydrogen atom transfer from 1 to TEMPO • (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The ΔG° for this reaction is −4.5 kcal mol −1 . Complex 4 is not oxidized by TEMPO • (ΔG° = +13.1 kcal mol −1 ), but in the reverse direction TEMPO-H readily reduces in situ generated Ru III (hfac) 2 (py-im) (6). A Ru II -imidazoline analog of 1, Ru II (acac) 2 (py-imnH) (7), reacts with 3 equiv of TEMPO • to give the imidazolate complex 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.

New iridium complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP), have been prepared and used in the study of hydrocarbon C-H bond activation. Intermolecular oxidative addition of a benzene C-H bond was directly observed with [(PONOP)Ir(I)(cyclooctene)][PF(6)] at ambient temperature, resulting in a cationic five-coordinate iridium(III) phenyl hydride product. Protonation of the (PONOP)Ir(I) methyl complex yielded the corresponding iridium(III) methyl hydride cation, a rare five-coordinate, 16-valence electron transition metal alkyl hydride species which was characterized by X-ray diffraction. Kinetic studies of C-H bond coupling and reductive elimination reactions from the five-coordinate complexes have been carried out. Exchange NMR spectroscopy measurements established a barrier of 17.8(4) kcal/mol (22 degrees C) for H-C(aryl) bond coupling in the iridium(III) phenyl hydride cation and of 9.3(4) kcal/mol (-105 degrees C) for the analogous H-C(alkyl) coupling in the iridium(III) methyl hydride cation. The origin of the higher barrier of H-C(aryl) relative to H-C(alkyl) bond coupling is proposed to be influenced by a hindered rotation about the Ir-C(aryl) bond, a result of the sterically demanding PONOP ligand.

Kinetic studies aimed at determining the most probable mechanism for the proton-dependent [FeII(SMe2N4(tren))]+ (1) promoted reduction of superoxide via a thiolate-ligated hydroperoxo intermediate [FeIII(SMe2N4(tren))(OOH)]+ (2) are described. Rate laws are derived for three proposed mechanisms, and it is shown that they should conceivably be distinguishable by kinetics. For weak proton donors with pKa(HA) >pKa(HO2) rates are shown to correlate with proton donor pKa, and display first-order dependence on iron, and half-order dependence on superoxide and proton donor HA. Proton donors acidic enough to convert O2− to HO2 (in tetrahydrofuran, THF), that is, those with pKa(HA) < pKa(HO2), are shown to display first-order dependence on both superoxide and iron, and rates which are independent of proton donor concentration. Relative pKa values were determined in THF by measuring equilibrium ion pair acidity constants using established methods. Rates of hydroperoxo 2 formation displays no apparent deuterium isotope effect, and bases, such as methoxide, are shown to inhibit the formation of 2. Rate constants for p-substituted phenols are shown to correlate linearly with the Hammett substituent constants σ−. Activation parameters ((ΔH‡ = 2.8 kcal/mol, ΔS‡ = −31 eu) are shown to be consistent with a low-barrier associative mechanism that does not involve extensive bond cleavage. Together, these data are shown to be most consistent with a mechanism involving the addition of HO2 to 1 with concomitant oxidation of the metal ion, and reduction of superoxide (an “oxidative addition” of sorts), in the rate-determining step. Activation parameters for MeOH- (ΔH‡ = 13.2 kcal/mol and ΔS‡ = −24.3 eu), and acetic acid- (ΔH‡ = 8.3 kcal/mol and ΔS‡ = −34 eu) promoted release of H2O2 to afford solvent-bound [FeIII(SMe2N4(tren))(OMe)]+ (3) and [FeIII(SMe2N4(tren))(O(H)Me)]+ (4), respectively, are shown to be more consistent with a reaction involving rate-limiting protonation of an Fe(III)–OOH, than with one involving rate-limiting O–O bond cleavage. The observed deuterium isotope effect (kH/kD = 3.1) is also consistent with this mechanism.