Copper-Catalyzed C(sp3 )-H Amidation: Sterically Driven Primary and Secondary C-H Site-Selectivity.

Undirected C(sp3 )-H functionalization reactions often follow site-selectivity patterns that mirror the corresponding C-H bond dissociation energies (BDEs). This often results in the functionalization of weaker tertiary C-H bonds in the presence of stronger secondary and primary bonds. An important, contemporary challenge is the development of catalyst systems capable of selectively functionalizing stronger primary and secondary C-H bonds over tertiary and benzylic C-H sites. Herein, we report a Cu catalyst that exhibits a high degree of primary and secondary over tertiary C-H bond selectivity in the amidation of linear and cyclic hydrocarbons with aroyl azides ArC(O)N3 . Mechanistic and DFT studies indicate that C-H amidation involves H-atom abstraction from R-H substrates by nitrene intermediates [Cu](κ2 -N,O-NC(O)Ar) to provide carbon-based radicals R. and copper(II)amide intermediates [CuII ]-NHC(O)Ar that subsequently capture radicals R. to form products R-NHC(O)Ar. These studies reveal important catalyst features required to achieve primary and secondary C-H amidation selectivity in the absence of directing groups.

New water oxidation chemistry of a seven-coordinate ruthenium complex with a tetradentate polypyridyl ligand.

The mononuclear ruthenium(II) complex [Ru](2+) (Ru = Ru(dpp)(pic)2, where dpp is the tetradentate 2,9-dipyrid-2'-yl-1,10-phenanthroline ligand and pic is 4-picoline) reported by Thummel's group (Inorg. Chem. 2008, 47, 1835-1848) that contains no water molecule in its primary coordination shell is evaluated as a catalyst for water oxidation in artificial photosynthesis. A detailed theoretical characterization of the energetics, thermochemistry, and spectroscopic properties of intermediates allowed us to interpret new electrochemical and spectroscopic experimental data, and propose a mechanism for the water oxidation process that involves an unprecedented sequence of seven-coordinate ruthenium complexes as intermediates. This analysis provides insights into a mechanism that generates four electrons and four protons in the solution and a gas-phase oxygen molecule at different pH values. On the basis of the calculations and corroborated substantially by experiments, the catalytic cycle goes through [(2)Ru(III)](3+) and [(2)Ru(V)(O)](3+) to [(1)Ru(IV)(OOH)](3+) then [(2)Ru(III)(···(3)O2)](3+) at pH 0, and through [(3)Ru(IV)(O)](2+), [(2)Ru(V)(O)](3+), and [(1)Ru(IV)(OO)](2+) at pH 9 before reaching the same [(2)Ru(III)(···(3)O2)](3+) species, from which the liberation of the weakly bound O2 might require an additional oxidation to form [(3)Ru(IV)(O)](2+) to initiate further cycles involving all seven-coordinate species.

Mechanistic insights into C-H amination via dicopper nitrenes.

We examine important reactivity pathways relevant to stoichiometric and catalytic C-H amination via isolable ß-diketiminato dicopper alkylnitrene intermediates {[Cl2NN]Cu}2(µ-NR). Kinetic studies involving the stoichiometric amination of ethylbenzene by {[Cl2NN]Cu}2(µ-N(t)Bu) (3) demonstrate that the terminal nitrene [Cl2NN]Cu═N(t)Bu is the active intermediate in C-H amination. Initial rates exhibit saturation behavior at high ethylbenzene loadings and an inverse dependence on the copper species [Cl2NN]Cu, both consistent with dissociation of a [Cl2NN]Cu fragment from 3 prior to C-H amination. C-H amination experiments employing 1,4-dimethylcyclohexane and benzylic radical clock substrate support a stepwise H-atom abstraction/radical rebound pathway. Dicopper nitrenes [Cu]2(µ-NCHRR') derived from 1° and 2° alkylazides are unstable toward tautomerization to copper(I) imine complexes [Cu](HN═CRR'), rendering 1° and 2° alkylnitrene complexes unsuitable for C-H amination.

Cp*Co(III) catalysts with proton-responsive ligands for carbon dioxide hydrogenation in aqueous media.

New water-soluble pentamethylcyclopentadienyl cobalt(III) complexes with proton-responsive 4,4'- and 6,6'-dihydroxy-2,2'-bipyridine (4DHBP and 6DHBP, respectively) ligands have been prepared and were characterized by X-ray crystallography, UV-vis and NMR spectroscopy, and mass spectrometry. These cobalt(III) complexes with proton-responsive ligands predominantly exist in their deprotonated [Cp*Co(DHBP-2H(+))(OH2)] forms with stronger electron-donating properties in neutral and basic solutions, and are active catalysts for CO2 hydrogenation in aqueous bicarbonate media at moderate temperature under a total 4-5 MPa (CO2:H2 1:1) pressure. The cobalt complexes containing 4DHBP ligands ([1-OH2](2+) and [1-Cl](+), where 1 = Cp*Co(4DHBP)) display better thermal stability and exhibit notable catalytic activity for CO2 hydrogenation to formate in contrast to the catalytically inactive nonsubstituted bpy analogues [3-OH2](2+) (3 = Cp*Co(bpy)). While the catalyst Cp*Ir(6DHBP)(OH2)(2+) in which the pendent oxyanion lowers the barrier for H2 heterolysis via proton transfer through a hydrogen-bonding network involving a water molecule is remarkably effective (ACS Catal. 2013, 3, 856-860), cobalt complexes containing 6DHBP ligands ([2-OH2](2+) and [2-Cl](+), 2 = Cp*Co(6DHBP)) exhibit lower TOF and TON for CO2 hydrogenation than those with 4DHBP. The low activity is attributed to thermal instability during the hydrogenation of CO2 as corroborated by DFT calculations.

Water oxidation with mononuclear ruthenium(II) polypyridine complexes involving a direct Ru(IV)═O pathway in neutral and alkaline media.

The catalytic water oxidation mechanism proposed for many single-site ruthenium complexes proceeds via the nucleophilic attack of a water molecule on the Ru(V)═O species. In contrast, Ru(II) complexes containing 4-t-butyl-2,6-di-1',8'-(naphthyrid-2'-yl)-pyridine (and its bisbenzo-derivative), an equatorial water, and two axial 4-picolines follow the thermodynamically more favorable "direct pathway" via [Ru(IV)═O](2+), which avoids the higher oxidation state [Ru(V)═O](3+) in neutral and basic media. Our experimental and theoretical results that focus on the pH-dependent onset catalytic potentials indicative of a PCET driven low-energy pathway for the formation of products with an O-O bond (such as [Ru(III)-OOH](2+) and [Ru(IV)-OO](2+)) at an applied potential below the Ru(V)═O/Ru(IV)═O couple clearly support such a mechanism. However, in the cases of [Ru(tpy)(bpy)(OH2)](2+) and [Ru(tpy)(bpm)(OH2)](2+), the formation of the Ru(V)═O species appears to be required before O-O bond formation. The complexes under discussion provide a unique functional model for water oxidation that proceeds by four consecutive PCET steps in neutral and alkaline media.

Preparation of aluminum(III) (bis(amido)pyridine)(thiolate) complexes: unexpected transmetalation mediated by LiAlH(4).

Treatment of an unsymmetrical bis(imino)pyridyl-thiolate zinc(II) complex [Zn(II)(LN(3)S)(OTf)] (1) with LiAlH(4) results in the double reduction of the two imino groups in the ligand backbone, and at the same time causes a rare transmetalation reaction to occur. The products formed in this reaction are two novel aluminium(III) bis(amido)pyridyl-thiolate complexes [(R,S/S,R-[Al(III)(LH(2)N(3)S)(THF)] (2a) and [(R,R/S,S-[Al(III)(LH(2)N(3)S)(THF)] (2b), which are diastereomers of each other. These complexes have been characterized by single-crystal X-ray diffraction and (1)H NMR spectroscopy. Single crystal X-ray structure analysis shows that the Al(III) ion is bound in an almost idealized square pyramidal geometry in 2a, while being held in a more distorted square pyramidal geometry in 2b. The major difference between 2a and 2b arises in the orientation of the terminal methyl groups of the ligand backbone in relation to the Al(III)N(3)S plane. These two complexes are crystallized at different temperatures (room temperature vs -35 °C), allowing for their separate isolation. Structural analysis shows that these complexes are reduced by the formal addition of one hydride ion to each imino group, resulting in a deprotonated bis(amido)pyridyl-thiolate ligand. A detailed analysis of metrical parameters rules out the possibility of pure one- or two-electron reduction of the π-conjugated bis(imino)pyridine framework. (1)H NMR spectra reveal a rich pattern in solution indicating that the solution state structures for 2a and 2b match those observed in the solid-state crystal structures, and reveal that both complexes are severely conformationally restricted. Direct organic synthetic methods failed to produce the reduced bis(amino)pyridyl-thiol ligand in pure form, but during the course of these efforts an unusual unsymmetrical aminopyridyl ketone, 1-(6-(1-(2,6-diisopropylphenylamino)ethyl)pyridin-2-yl)ethanone was synthesized in good yield and can be used as a possible precursor for further ligand development.

O2 activation by bis(imino)pyridine iron(II)-thiolate complexes.

The new iron(II)-thiolate complexes [((iPr)BIP)Fe(II)(SPh)(Cl)] (1) and [((iPr)BIP)Fe(II)(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO(2)H at a (His)(3)Fe(II) center. Reaction of 1 and 2 with O(2) leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O(2), the spectroscopic and reactivity data, including (18)O isotope studies, are consistent with an assignment of an iron(IV)-oxo complex, [((iPr)BIP)Fe(IV)(O)(Cl)](+) (3), as the product of oxygenation. In contrast, 2 + O(2) results in direct S-oxygenation to give a sulfonato product, PhSO(3)(-). The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O(2) activation. The thiolate ligands in 1 and 2 are essential for O(2) reactivity and exhibit an important influence over the Fe(III)/Fe(II) redox potential.

Plasma-Initiated Graft Polymerization of Acrylic Acid onto Fluorine-Doped Tin Oxide as a Platform for Immobilization of Water-Oxidation Catalysts.

The discovery of new and versatile strategies for the immobilization of molecular water-oxidation catalysts (WOCs) is crucial for developing clean energy conversion devices [e.g., (photo)electrocatalytic cells for water splitting]. The traditional approach for surface attachment to transparent conductive oxides [e.g., fluorine doped tin oxide (FTO)] is via synthetic modification of the ligand architecture to incorporate functional groups such as carboxylic acids (-COOH) or phosphonates (-PO3H2) prior to immobilization. However, challenges arising from desorption and the cumbersome derivatizations steps have limited the scope and applications of surface-bound WOCs. Herein, we report the successful immobilization of underivatized Ru(II)-based WOCs (Ru-Cat1 = [Ru(tpy) (bpy) (H2O)]2+ (tpy = 2,2':6'2″-terpyridine and bpy = 2,2'-bipyridine) and Ru-Cat2 = [Ru(Mebimpy) (bpy) (H2O)]2+ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) pyridine)) and the Ru(II) polypyridyl chromophore Ru-C3 = [Ru(bpy)3]2+ onto a FTO plasma-grafted poly(acrylic acid) surface (PAA|FTO). Various characterization techniques such as attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry measurements provide evidence for the plasma-induced grafted PAA|FTO film and immobilization. Surface stability and electrocatalytic properties of these new hybrid composite films upon cycling were investigated at different pH values. Immobilized Ru-Cat1 and Ru-Cat2 onto PAA|FTO displayed pH-dependent (RuIII/RuII) couples and onset potentials indicative of PCET (proton-coupled electron transfer) reactions. Based on cyclic voltammetry results and spectroscopic monitoring, the immobilized WOCs Ru-Cat1 and Ru-Cat2 exhibited a higher surface stability in neutral aqueous solutions relative to Ru-C3 upon electrochemical oxidation. We attribute the surface PCET and stability to the presence of a water ligand in the coordination sphere of immobilized Ru-Cat1 and Ru-Cat2 which can H-bond with negatively charged carboxylate groups of the cross-linked PAA brushes. Our findings demonstrate that the plasma-grafted polymeric network onto FTO offers a versatile platform to directly anchor unmodified homogeneous WOCs or chromophores for potential applications in solar-to-fuel energy conversion.

Transient terminal Cu-nitrene intermediates from discrete dicopper nitrenes.

Reaction of the copper(I) beta-diketiminate {[Me3NN]Cu}2(mu-toluene) with the aryl azide N3Ar (Ar = 3,5-Me2C6H3) in toluene results in immediate effervescence and formation of the dicopper nitrene {[Me3NN]Cu}2(mu-NAr) (2) in 77% yield. The X-ray structure of 2 shows nearly symmetric bonding of the nitrene to two Cu centers separated by 2.911(1) A with Cu-N distances of 1.794(5) and 1.808(5) A along with a Cu-N-Cu angle of 107.8(2) degrees . This structure is conceptually related to the dicopper carbenes {[MexNN]Cu}2(mu-CPh2) (x = 2 or 3) (Dai, X.; Warren J. Am. Chem. Soc. 2004, 126, 10085. Badiei, Y. M.; Warren J. Organomet. Chem. 2005, 690, 5989.) which exhibit shorter Cu-Cu distances (2.4635(7) or 2.485(1) A) and acute Cu-C-Cu angles (79.51(14) or 80.1(2) degrees ). Addition of the Cu(I) anilidoimine {[Me2AI]Cu}2 (prepared from CuOtBu and the aniline-imine H[Me2AI] in 77% yield) to a benzene-d6 solution of 2 results in the formation of two new anilidoimine complexes {[Me2AI]Cu(mu- NAr)Cu[Me3NN] (5) and {[Me2AI]Cu}2(mu-NAr) (6) as well as [Me3NN]Cu(benzene) over 3 h. These observations are consistent with the slow dissociation of a [Me3NN]Cu fragment from 2 to generate the transient terminal nitrenes [Me3NN]Cu=NAr and [Me2AI]Cu=NAr quickly trapped by the [Me2AI]Cu fragment to form the new unsymmetrical and symmetrical dicopper nitrenes 5 and 6. Preliminary reactivity studies indicate electrophilic reactivity at the nitrene moiety. Dicopper nitrene 2 reacts with 10 equiv PMe3 and CNtBu to give ArN=PMe3 and ArN=C=NtBu in 94% and 92% yields, respectively, with concomitant formation of [Me3NN]Cu(L) (L = PMe3 and CNtBu). Reaction between 2 and 2 equiv PMe3 allows for observation of the structurally characterized Cu(I) phosphaimide [Me3NN]Cu(ArN=PMe3) (7).