Hydrogen atom transfer (HAT, eq 1) is an elementary chemical transformation that results in the net transfer of both a proton and an electron. 1,2 MetalÀoxo complexes are widely used to abstract hydrogen atoms from organic compounds through HAT, which leads to the metal hydroxide (Scheme 1). 3 In oxidations by cytochrome P450, the mechanism is generally accepted to be HAT to an ironÀoxo species (FedO) followed by radical rebound. 4 Other enzymatic systems such as soluble methane monooxygenase, 5 ribonucleotide reductases and other B 12 -dependent enzymes, 6 lipoxygenases, 7 isopenicillin-N synthase, 8 and TauD 9 also utilize mechanisms with key HAT steps.Considerable effort has been devoted to synthesizing and studying biomimetic oxoiron complexes 10 in order to help elucidate the enzymatic mechanisms and to develop homogeneous iron-based oxidation catalysts. Studies on heme ironÀoxo complexes in the 1980s by Balch, La Mar, and Groves pioneered this field. 11 More recently, Que, Nam, and co-workers have reported isolable non-heme oxoiron(IV) complexes that react with hydrocarbons via HAT. 12 Reactions proceeding by HAT mechanisms have also been studied for terminal oxo complexes of Mn, Ru, Cr, and V. 13À19 In general, the selectivity of nonenzymatic HAT reactions is thermodynamically controlled, and reaction rates follow a linear correlation with the bond dissociation enthalpy (BDE) of the XÀH bond being broken (the BellÀEvansÀPolanyi relation), 1,20 i.e., homolytically weaker substrate bonds react more rapidly.Imido (NR 2À ) ligands are isoelectronic to oxo (O 2À ) ligands (Scheme 1), and imido complexes are often proposed as intermediates in hydrocarbon amination mechanisms in which the imido species performs the cleavage of the CÀH bond that precedes CÀN bond formation. Imido complexes are also more versatile than oxo complexes, because there is an opportunity to tune the steric and electronic properties of the complex by changing the nitrogen substituent. However, the HAT reactivity of imido complexes (MdNR) has not been investigated in as much detail as that of their oxo counterparts. There has been a recent renaissance of activity in the synthesis of imido complexes of the late transition metals (groups 8À11), 21 and this activity has resulted in the isolation of late transition metal complexes Received: January 18, 2011 ABSTRACT: In the literature, ironÀoxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. IronÀimido complexes have been isolated more recently, and the community needs experimental evaluations of the mechanism of HAT from late-metal imido species. We report a mechanistic study of HAT by an isolable iron(III) imido complex, L Me FeNAd (L Me = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded by binding of tert-butylpyridine ( t Bupy) to form a reactive fourcoordinate intermediate L Me Fe(NAd)( t Bupy), as shown by equilibrium and kinetic studies. In the HAT step, very la...

The widely used C-H functionalization strategies and some complexities in the Pd-catalyzed chemical transformations were analyzed. It was emphasized that in the course of catalysis various Pd-intermediates (including nano-scale Pd-clusters) could act as active catalysts. However, both identification of these catalytically active species and determination of factors controlling the overall catalytic process require more comprehensive and multi-disciplinary approaches. Recent joint computational and experimental approaches were instrumental in: (1) demonstrating that the addition of Pd(OAc)2 as a catalyst precursor to RSeH and RSH reagents forms the [Pd(SeR)2]n and [Pd(SR)2]n clusters, respectively, which show an unprecedented ability for selective synthesis of Markovnikov-type products starting with a mixture of reagents RSH/RSeH and acetylenic hydrocarbons; (2) predicting a valid mechanism of the amino acid ligand-assisted Pd(II)-catalyzed C-H activation that is shown to proceed via the formation of the catalytically active Pd(II) intermediate with a bidentately coordinated dianionic amino acid ligand; (3) demonstrating that the amino acid ligand plays crucial roles in the ligand-assisted Pd(II)-catalyzed C-H activation by acting as: (a) a weakly coordinating ligand to stabilize the desirable Pd(II)-precatalyst, (b) a soft proton donor and a bidentately coordinated dianionic ligand in the catalytically active Pd(II) intermediate, and (c) a proton acceptor accelerating the C-H deprotonation via the CMD mechanism; and (4) revealing the roles of the CsF base (and "cesium effect") in the Pd(0)/PCy3-catalyzed intermolecular arylation of the terminal β-C(sp(3))-H bond of aryl amide and predicting the unprecedented "Cs2-I-F cluster" assisted mechanism for this reaction.

A DFT study was performed to understand the role of cooperativity between iron-β-diketiminate fragments and potassium promoters in N2 activation. Sequential addition of iron fragments to N2 reveals that a minimum of three iron centers interact with N2 in order to break the triple bond. The potassium promoter stabilizes the N3− ligand formed upon N2 scission, thus making the activated iron-nitride complex more energetically accessible. Reduction of the complex and stabilization of N3− by K+ have similar impact on the energetics in the gas phase. However, upon inclusion of continuum THF solvent effects, coordination of K+ has a reduced influence upon the overall energetics of dinitrogen fixation; thus, reduction of the trimetallic Fe complex becomes more impactful than coordination of K+ vis-à-vis N2 activation upon the inclusion of solvent effects.

Metal-mediated formation of C−O bonds is an important transformation that can occur by a variety of mechanisms. Recent studies suggest that oxygen-atom insertion into metal−hydrocarbyl bonds in a reaction that resembles the Baeyer−Villiger transformation is a viable process. In an effort to identify promising new systems, this study is designed to assess the impact of metal identity on such O-atom insertions for the reaction [(bpy

Key mechanistic features of the [Cp*MCl 2 ] 2 (M = Ir, Rh, Co; all are in group 9) catalyzed C−H amination of benzamides with organic azides were investigated with a strong emphasis on the metal effects on the reaction mechanism, revealing that the Rh-and Ir-catalyzed reactions follow a similar reaction profile, albeit with different individual kinetic and thermodynamic parameters. The observation that the Irbased system was much superior in terms of the rates and efficiency in comparison to Rh was attributed to the intrinsically strong relativistic ef fects in iridium. While a cobalt system [Cp*Co III ] showed little catalytic activity for most azides examined, plausible [(BA)(Cp*)CoNR] + intermediates of these reactions were characterized as a "Co(III)-nitrenoid radical" species with a weak ("one electron−two center type") Co−NPh bond. Its Rh and Ir analogues are characterized as diamagnetic metal nitrenoids with a strong MNR double bond. The provided experimental and computational investigations indicate that the rate-limiting step of the reaction resides in the final stage (protodemetalation) that takes place via a concerted metalation− deprotonation (CMD) mechanism. While experimental measurements of thermodynamic parameters were in good agreement with DFT calculations, theoretical predictions on the electronic nature of key intermediates and energy barriers were successfully used to rationalize the experimentally observed reactivity pattern.

A Hammett analysis of platinum-mediated oxy-insertion into Pt–aryl bonds is performed using DFT calculations. Modeled transformations involve the conversion of cationic PtII-aryl complexes [(Xbpy)Pt(R)(OY)]+ (R = p-X-C6H4; Y = 4-X-pyridine; Xbpy = 4,4′-X-bpy; X = NO2, H, NMe2) to the corresponding [(Xbpy)Pt(OR)]+ complexes via an organometallic Baeyer–Villiger (BV) pathway. Computational modeling predicts that incorporation of an electron-deficient NO2 group at the 4-position of pyridine-N-oxide lowers the activation barrier to the organometallic BV transformation. In contrast, computational studies reveal that increasing the donor ability of the migrating aryl group, by placement of NMe2 at the para position, lowers the activation barrier to the oxy-insertion step. The impact on the calculated activation barrier is greater for variation of the R group than for modification of Y of the oxygen delivery reagent. For the p-NO2/p-NMe2-substituted aryl migrating groups (R), the ΔΔG ‡ for X = NMe2 versus X = NO2 is 12 kcal/mol, which is three times larger than that calculated for the changes that occur upon substitution of NO2 and NMe2 groups (ΔΔG ‡ ≈ 4 kcal/mol) at the 4-position of the pyridine group. For these PtII complexes with bipyridine (bpy) supporting ligands, the influence of modification of the bpy ligand is calculated to be minimal with ΔΔG ‡ ≈ 0.4 kcal/mol for the oxy-insertion of bpy ligands substituted at the 4/4′ positions with NMe2 and NO2 groups. Overall, the predicted activation barriers for oxy-insertion (from the YO adducts [(Xbpy)Pt(R)(OY)]+) are large and in most cases are >40 kcal/mol, although some calculated ΔG ‡'s are as low as 32 kcal/mol.

Mechanistic details pertaining to the Pd(0)/PCy3-catalyzed intermolecular arylation of a terminal β-C(sp(3))-H bond aryl amide substrate (SM = EtCONH-Ar, where Ar = C6H5, C6F5 and CONH-Ar is a directing group (DG)) in the presence of CsF base were elucidated. Key mechanistic features of this reaction are (1) oxidative addition of the aryl halide PhI to Pd(0)/PCy3, (2) deprotonation of SM by CsF to form DG' = [EtCON-Ar]Cs(+) for subsequent coordination to intermediate I-Pd(II)(PCy3)Ph (the substantially lower pKa of the EtCONHC6F5 in comparison to EtCONHC6H5 is instrumental for the presence of a larger population of the reactive deprotonated amides for Ar = C6F5), (3) "Cs2-I-F" cluster formation upon external (the second) CsF molecule approach to the active site of the I-Pd(II)(PCy3)Ph(DG') intermediate, (4) "Cs2-I-F cluster" assisted β-C(sp(3))-H bond activation via a concerted metalation-deprotonation (CMD) mechanism, and (5) reprotonation of the amide directing group to facilitate the C(sp(3))-Ph reductive elimination. The energy barriers, ΔG(‡) (ΔG(‡disp), associated with the "Cs2-I-F cluster" mediated β-C(sp(3))-H bond activation transition state are 6.5 (8.7) and 10.2 (12.9) kcal/mol when DG = CONHC6H5, CONHC6F5, respectively. It was shown that (a) the PCy3 ligand only semidissociates upon β-C(sp(3))-H bond cleavage and (b) the I-to-F substitution in I-[Pd(II)](Ph)(PCy3)(DG') is a facile process that makes the "direct-halide" assisted β-C(sp(3))-H bond activation relatively less energy demanding and opens the possibility for a competing Ph-F bond formation reaction. It was shown that the "direct-I" assisted C-H bond activation TS, which associates with a relatively large energy barrier, is an H-atom insertion transition state into the Pd-I bond, while the "direct-F" assisted C-H bond activation TS, which occurs with a relatively low energy barrier (but still is much larger than that required for the "Cs2-I-F cluster" assisted pathway), is a direct proton abstraction transition state.

Transition-metal-mediated oxy insertion into metal–carbon bonds is useful for the development of catalytic cycles for selective hydrocarbon oxidation. However, there are few bona fide examples of net oxy insertion with transition-metal complexes. An extremely rare example of a 3d metal mediating oxy insertion into metal–carbon bonds is a series of NiII alkyl complexes reacting with nitrous oxide (N2O) reported by Hillhouse and co-workers; however, the mechanism was never fully elucidated. A computational study has been performed on bipyridyl nickel metallacycles that form nickel alkoxides upon reaction with N2O to attain insight into future catalyst design for oxygen atom transfer reactions. Two possible mechanisms are explored. Of the two pathways, the computations suggest that the preferred mechanism proceeds through a Ni–oxyl intermediate followed by alkyl migration of the nickel–carbon bond to form an alkoxide. Oxyl formation was found to be the rate-determining step, with a free energy barrier of 29.4 kcal/mol for bpyNiII(cyclo-(CH2)4). Complexes that contain sp2-hybridized molecules at the β-carbon site within the metallacycle ring do not undergo oxy insertion due to elevated barriers. While exploring insertion with another oxidant, namely pyridine N-oxide, we found that N2O is critical for net oxy insertion with this complex due to the substantial thermodynamic advantage of N2 expulsion. Reaction with pyridine N-oxide necessitated expulsion of a “worse” leaving group, resulting in much higher barriers (ΔG ⧧ = 49.7 kcal/mol) for the oxyl formation step.