Catalytic Tuning of a Phosphinoethane Ligand for Enhanced C−H Activation

A Dicopper Nitrenoid by Oxidation of a CuICuI Core: Synthesis, Electronic Structure, and Reactivity

A dicopper nitrenoid complex was prepared by formal oxidative addition of the nitrenoid fragment to a dicopper(I) center by reaction with the iminoiodinane PhINTs (Ts = tosylate). This nitrenoid complex, (DPFN)Cu₂(μ-NTs)[NTf₂]₂ (DPFN = 2,7-bis(fluorodi(2-pyridyl)methyl)-1,8-naphthyridine), is a powerful H atom abstractor that reacts with a range of strong C-H bonds to form a mixed-valence Cu(I)/Cu(II) μ-NHTs amido complex in the first example of a clean H atom transfer to a dicopper nitrenoid core. In line with this reactivity, DFT calculations reveal that the nitrenoid is best described as an iminyl (NR radical anion) complex. The nitrenoid was trapped by the addition of water to form a mixed-donor hydroxo/amido dicopper(II) complex, which was independently obtained by reaction of a Cu₂(μ-OH)₂ complex with an amine through a protonolysis pathway. This mixed-donor complex is an analogue for the proposed intermediate in copper-catalyzed Chan-Evans-Lam coupling, which proceeds via C-X (X = N or O) bond formation. Treatment of the dicopper(II) mixed donor complex with MgPh₂(THF)₂ resulted in generation of a mixture that includes both phenol and a previously reported dicopper(I) bridging phenyl complex, illustrating that both reduction of dicopper(II) to dicopper(I) and concomitant C-X bond formation are feasible.

5d Metal(IV) Imide Complexes. The Impact (or Lack Thereof) of d-Orbital Occupation on Methane Activation and Functionalization

This research evaluates 5d metal imide complexes, (OH)₂M═NMe (M = W, Re, or Os), and their reactions with methane to form dimethylamine. Each is calculated to follow a consistent reaction pathway regardless of the metal's d-orbital occupation, whereby the methane C-H bond undergoes oxidative addition (OA) to the metal and then the methyl migrates from the metal to the nitrogen to form an amide. Finally, hydrogen migrates to the nitrogen before dissociating to form amine products. While homolytic M-imide, M-amide M-H, and M-CH₃ bond dissociation free energies (BDFEs) were analyzed, the BDFEs of neither hexavalent nor tetravalent metal moieties reflect the oxidative addition kinetics. Instead, a strain theory approach, supported by electron density analysis for the OA transition state, is found to be explanatory. Notably, the rate-determining step, the hydrogen migration transition state, has a consistent jump in free energy versus the preceding intermediate, (OH)₂M^IV(H)-N(CH₃)Me, for all metals evaluated. Thus, the height of the RDS is largely reflective of the stability of the M^IV-amide intermediate, suggesting a strategy for viable catalysis.

Activation of CS2 by a “masked” terminal nickel sulfide

Activation of carbon disulfide (CS₂) by “masked” terminal nickel sulfide, [K(18-crown-6)][(L^tBu)Ni(S)], gives a trithiocarbonate complex. This result confirms the nucleophilicity of the sulfide ligand and expands the scope of reactivity for late metal sulfides.

Synthesis of a "Masked" Terminal Nickel(II) Sulfide by Reductive Deprotection and its Reaction with Nitrous Oxide

The addition of 1 equiv of KSCPh3 to [L(R)NiCl] (L(R) = {(2,6-iPr2C6H3)NC(R)}2CH; R = Me, tBu) in C6H6 results in the formation of [L(R)Ni(SCPh3)] (1: R = Me; 2: R = tBu) in good yields. Subsequent reduction of 1 and 2 with 2 equiv of KC8 in cold (-25 °C) Et2O in the presence of 2 equiv of 18-crown-6 results in the formation of "masked" terminal Ni(II) sulfides, [K(18-crown-6)][L(R)Ni(S)] (3: R = Me; 4: R = tBu), also in good yields. An X-ray crystallographic analysis of these complexes suggests that they feature partial multiple-bond character in their Ni-S linkages. Addition of N2O to a toluene solution of 4 provides [K(18-crown-6)][L(tBu)Ni(SN=NO)], which features the first example of a thiohyponitrite (κ(2)-[SN=NO](2-)) ligand.

Evidence of two-state reactivity in alkane hydroxylation by Lewis-acid bound copper-nitrene complexes

The behavior of the Lewis-acid adducts of two copper-nitrene [Cu(NR)](+) complexes in nitrene-transfer and H-atom abstraction reactions have been demonstrated to depend on the nature of the nitrene substituents. Two-state reactivity, in which a singlet ground state and a nearby triplet excited-state both contribute, provides a useful model for interpreting reactivity trends of the two compounds.

Carbon-hydrogen bond activation, C-N bond coupling, and cycloaddition reactivity of a three-coordinate nickel complex featuring a terminal imido ligand

The three-coordinate imidos (dtbpe)Ni═NR (dtbpe = (t)Bu2PCH2CH2P(t)Bu2, R = 2,6-(i)Pr2C6H3, 2,4,6-Me3C6H2 (Mes), and 1-adamantyl (Ad)), which contain a legitimate Ni-N double bond as well as basic imido nitrogen based on theoretical analysis, readily deprotonate HC≡CPh to form the amide acetylide species (dtbpe)Ni{NH(Ar)}(C≡CPh). In the case of R = 2,6-(i)Pr2C6H3, reductive carbonylation results in formation of the (dtbpe)Ni(CO)2 along with the N-C coupled product keteneimine PhCH═C═N(2,6- (i)Pr2C6H3). Given the ability of the Ni═N bond to have biradical character as suggested by theoretical analysis, H atom abstraction can also occur in (dtbpe)Ni═N{2,6-(i)Pr2C6H3} when this species is treated with HSn((n)Bu)3. Likewise, the microscopic reverse reaction--conversion of the Ni(I) anilide (dtbpe)Ni{NH(2,6-(i)Pr2C6H3)} to the imido (dtbpe)Ni═N{2,6-(i)Pr2C6H3}--is promoted when using the radical Mes*O(•) (Mes* = 2,4,6-(t)Bu3C6H2). Reactivity studies involving the imido complexes, in particular (dtbpe)Ni═N{2,6-(i)Pr2C6H3}, are also reported with small, unsaturated molecules such as diphenylketene, benzylisocyanate, benzaldehyde, and carbon dioxide, including the formation of C-N and N-N bonds by coupling reactions. In addition to NMR spectroscopic data and combustion analysis, we also report structural studies for all the cycloaddition reactions involving the imido (dtbpe)Ni═N{2,6-(i)Pr2C6H3}.

Bis(sulfonylimide)ruthenium(VI) porphyrins: X-ray crystal structure and mechanism of C-H bond amination by density functional theory calculations

The X-ray crystal structure of [Ru(VI) (NMs)2 (tmp)] (Ms=SO2 - p-MeOC6 H4 ; tmp=5,10,15,20-tetramesitylporphyrinato(2-)), a metal sulfonylimide complex that can undergo alkene aziridination and C-H bond amination reactions, shows a Ru=N distance of 1.79(3) Å and Ru-N-S angle of 162.5(3)°. Density functional theory (DFT) calculations on the electronic structures of [Ru(VI) (NMs)2 (tmp)] and model complex [Ru(VI) (NMs)2 (por(0) )] (por(0) =unsubstituted porphyrinato(2-)) using the M06L functional gave results in agreement with experimental observations. For the amination of ethylbenzene by the singlet ground state of [Ru(VI) (NMs)2 (por(0))], DFT calculations using the M06L functional revealed an effectively concerted pathway involving rate-limiting hydrogen atom abstraction without a distinct radical rebound step. The substituent effect on the amination reactivity of ethylbenzene by [Ru(VI) (NX)2 (por(0) )] (X=SO2 -p-YC6 H4 with Y=MeO, Me, H, Cl, NO2 ) was examined. Electron-withdrawing Y groups lower the energy of the LUMOs of [Ru(VI) (NX)2 (por(0))], thus facilitating their interaction with the low-lying HOMO of the ethylbenzene C-H bond and hence increasing the reactivity of [Ru(VI) (NX)2 (por(0) )]. DFT calculations on the amination/aziridination reactions of [Ru(VI) (NSO2 C6 H5 )2 (por(0) )] with pent-4-enal, an aldehyde substrate bearing acyl, homoallylic, and allylic C-H bonds and a C=C bond, revealed a lower reaction barrier for the amination of the acyl C-H bond than for both the amination of the other C-H bonds and aziridination of the C=C bond in this substrate.

The radical mechanism of cobalt(II) porphyrin-catalyzed olefin aziridination and the importance of cooperative H-bonding

The mechanism of cobalt(II) porphyrin-mediated aziridination of styrene with PhSO(2)N(3) was studied by means of DFT calculations. The computations clearly indicate the involvement of a cobalt 'nitrene radical' intermediate in the Co(II)(por)-catalyzed alkene aziridination. The addition of styrene to this species proceeds in a stepwise fashion via radical addition of the 'nitrene radical'C to the C=C double bond of styrene to form a γ-alkyl radical intermediate D. The thus formed tri-radical species D easily collapses in an almost barrierless ring closure reaction (TS3) to form the aziridine, thereby regenerating the cobalt(II) porphyrin catalyst. The radical addition of the 'nitrene radical'C to the olefin (TS2) proceeds with a comparable barrier as its formation (TS1), thus providing a good explanation for the first order kinetics in both substrates and the catalyst observed experimentally. Formation of C is clearly accelerated by stabilization of C and TS1 via hydrogen bonding between the S=O and N-H units. The computed radical-type mechanism agrees well with all available mechanistic and kinetic information. The computed free energy profile readily explains the superior performance of the Co(II)(porAmide) system with H-bond donor functionalities over the non-functionalized Co(TPP).

A computational study of metal-mediated decomposition of nitrene transfer reagents

Metal-mediated decomposition to form nitrene complexes is investigated by using DFT for prototypical organic azides and iodonium imides used in organic synthesis. Each system exhibited exothermic pathways via formation of cyclic intermediates, which decompose to yield LNi = NX + Y (L = bis-phosphine, NX = nitrene, Y = N2 or IPh). Also, the typical heterotransfer reagents used in organic synthesis show a greater tendency toward triplet nitrene complexes and hence the potential for metal-free reactivity than aliphatic and aromatic substituted versions.