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Molecular catalysts for ammonia oxidation to dinitrogen represent enabling components to utilize ammonia as a fuel and/or source of hydrogen. Ammonia oxidation requires not only the breaking of multiple strong N-H bonds, but also controlled N-N bond formation. We report a novel β-diketiminato copper complex [iPr2NNF6]Cu-NH3 ([Cu(I)]-NH3 (2)) as a robust electrocatalyst for NH3 oxidation in acetonitrile under homogeneous conditions. Complex 2 operates at a moderate overpotential (700 mV) with a TOFmax = 940 h-1 as determined from CV data in 1.3 M NH3 MeCN solvent. Prolonged (>5 h) controlled potential electrolysis (CPE) reveals the stability and robustness of the catalyst under electrocatalytic conditions. Detailed mechanistic investigations indicate that electrochemical oxidation of [Cu(I)]-NH3 forms {[Cu(II)]-NH3}+ (4) which undergoes deprotonation by excess NH3 to form reactive copper(II)-amide [Cu(II)]-NH2 (6) unstable towards N-N bond formation to give the dinuclear hydrazine complex [Cu(I)]2(mu-N2H4). Electrochemical studies reveal that the bisammine complex [Cu(I)](NH3)2 (7) forms at high ammonia concentration as part of the {[Cu(II)](NH3)2}+/[Cu(I)](NH3)2 redox couple that is electrocatalytically inactive. DFT analysis reveals a much higher thermodynamic barrier for deprotonation of {[Cu(II)](NH3)2}+ (8) by NH3 to give the four-coordinate copper(II) amide [Cu(II)](NH2)(NH3) (9) (dG = 31.7 kcal/mol) as compared to deprotonation of the three coordinate {[Cu(II)]-NH3}+ by NH3 to provide the reactive three coordinate parent amide [Cu(II)]-NH2 (dG = 18.1 kcal/mol) susceptible to N-N coupling to form [Cu(I)]2(mu-N2H4) (dG = -11.8 kcal/mol).

Copper nitrite reductases (CuNIRs) convert NO

NO and H(2)S serve as signaling molecules in biology with intertwined reactivity. HSNO and HSSNO with their conjugate bases (−)SNO and (−)SSNO form in the reaction of H(2)S with NO as well as S-nitrosothiols (RSNO) and nitrite (NO(2)(−)) that serve as NO reservoirs. While the elusive nature of HSNO and HSSNO renders their study challenging, their conjugate bases form isolable zinc complexes (Ph,Me)TpZn(SNO) and (Ph,Me)TpZn(SSNO) supported by tris(pyrazolyl)borate ligands. Reaction of Na(15-C-5)SSNO with (Ph,Me)TpZn(ClO(4)) provides (Ph,Me)TpZn(SSNO) that undergoes S-atom removal by PEt(3) to give (Ph,Me)TpZn(SNO) and S=PEt(3). Unexpectedly stable at room temperature, these Zn-SNO and Zn-SSNO complexes release NO upon heating. (Ph,Me)TpZn(SNO) and (Ph,Me)TpZn(SSNO) quickly react with acidic thiols such as C(6)F(5)SH to form N(2)O and NO, respectively. Increasing the thiol basicity in p-substituted aromatic thiols (4-X)ArSH in the reaction with (Ph,Me)TpZn(SNO) turns on competing S-nitrosation to form (Ph,Me)TpZn-SH and RSNO, the latter a known precursor for NO.

Nitric oxide (NO) is a key signaling molecule in health and disease. While nitrite acts as a reservoir of NO activity, mechanisms for NO release require further understanding. A series of electronically varied β-diketiminatocopper(II) nitrite complexes [Cu(II)](κ(2)-O(2)N) react with a range of electronically tuned triarylphosphines PAr(Z)(3) that release NO with the formation of O=PAr(Z)(3). Second-order rate constants are largest for electron-poor copper(II) nitrite and electron-rich phosphine pairs. Computational analysis reveals a transition-state structure energetically matched with experimentally determined activation barriers. The production of NO follows a pathway that involves nitrite isomerization at Cu(II) from κ(2)-O(2)N to κ(1)-NO(2) followed by O-atom transfer (OAT) to form O=PAr(Z)(3) and [Cu(I)]-NO that releases NO upon PAr(Z)(3) binding at Cu(I) to form [Cu(I)]-PAr(Z)(3). These findings illustrate important mechanistic considerations involved in NO formation from nitrite via OAT.

Vanadium alkylidenes can be highly active initiators for ring-opening metathesis polymerization of cyclic olefins; however, attempts to expand their use to cross metathesis have been unsuccessful d...

Copper(II) alkynyl species are proposed as key intermediates in numerous Cu-catalyzed C-C coupling reactions. Supported by a β-diketiminate ligand, the three-coordinate copper(II) alkynyl [CuII]-C≡CAr (Ar = 2,6-Cl2C6H3) forms upon reaction of the alkyne H-C≡CAr with the copper(II) tert-butoxide complex [CuII]-OtBu. In solution, this [CuII]-C≡CAr species cleanly transforms to the Glaser coupling product ArC≡C-C≡CAr and [CuI](solvent). Addition of nucleophiles R'C≡C-Li (R' = aryl, silyl) and Ph-Li to [CuII]-C≡CAr affords the corresponding Csp-Csp and Csp-Csp2 coupled products RC≡C-C≡CAr and Ph-C≡CAr with concomitant generation of [CuI](solvent) and {[CuI]-C≡CAr}-, respectively. Supported by density functional theory (DFT) calculations, redox disproportionation forms [CuIII](C≡CAr)(R) species that reductively eliminate R-C≡CAr products. [CuII]-C≡CAr also captures the trityl radical Ph3C· to give Ph3C-C≡CAr. Radical capture represents the key Csp-Csp3 bond-forming step in the copper-catalyzed C-H functionalization of benzylic substrates R-H with alkynes H-C≡CR' (R' = (hetero)aryl, silyl) that provide Csp-Csp3 coupled products R-C≡CR via radical relay with tBuOOtBu as oxidant.

The dinuclear β-diketiminato NiII tert-butoxide {[Me3NN]Ni}2(μ-OtBu)2 (2), synthesized from [Me3NN]Ni(2,4-lutidine) (1) and di-tert-butylperoxide, is a versatile precursor for the synthesis of a se...

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron-transfer, redox-innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C(6)F(5))(3) coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C(6)F(5))(3) coordination increases their reduction potential by over 1 V into the physiologically accessible + 0.1 V vs. NHE. Outer-sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans-[LA-O-N=N-O-LA](2−)[LA = B(C(6)F(5))(3)], which releases N(2)O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C(6)F(5))(3)] radical anion, which is susceptible to N–N coupling prior to loss of RSSR.

In this issue of Chem, Liu, Li, and co-workers report the synergistic use of copper and photoredox catalysis to reveal a general platform to form C(sp3)–S bonds via redox-active esters of abundantly available aliphatic carboxylic acids with organosulfinates under mild conditions.

Reduction of nitrite anions (NO

Commercialy available benzphenone imine (HN=CPh2) reacts with b-diketiminato copper(II) tert-butoxide complexes [CuII]-OtBu to form isolable copper(II) ketimides [CuII]-N=CPh2. Structural characterization of the three coordinate copper(II) ketimide [Me3NN]Cu-N=CPh2 reveals a short Cu-Nketimide distance (1.700(2) Å) with a nearly linear Cu-N-C linkage (178.9(2)°). Copper(II) ketimides [CuII]-N=CPh2 readily capture alkyl radicals R• (PhCH(•)Me and Cy•) to form the corresponding R-N=CPh2 products that competes with N-N coupling of copper(II) ketimides [CuII]-N=CPh2 to form the azine Ph2C=N-N=CPh2. Copper(II) ketimides [CuII]-N=CAr2 serve as intermediates in catalytic sp3 C-H amination of substrates R-H with ketimines HN=CAr2 and tBuOOtBu as oxidant to form N-alkyl ketimines R-N=CAr2. This protocol enables the use of unactivated sp3 C-H bonds to give R-N=CAr2 products easily converted to primary amines R-NH2 via simple acidic deprotection.

The life-sustaining reduction of N2 to NH3 is thermoneutral yet kinetically challenged by high energy intermediates such as N2H2. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [xHetTpCu]2(𝜇-N2H2) complexes that exhibit H-bonding between pendant aromatic N-heterocycles (XHet) such as pyridine and a bridging trans-N2H2 ligand at copper(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [pyMeTpCu]2(𝜇-N2H2) while low temperature 1H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the xHet pendant negligibly influences the electronic structure of xHetTpCuI centers in xHetTpCu(CNAr2,6-Me2) complexes that lack H-bonding as judged by nearly indistinguishable n(CN) frequencies (2113 - 2117 cm-1). Nonetheless, H-bonding in the corresponding [xHetTpCu]2(𝜇-N2H2) complexes results in marked changes in n(NN) (1398 - 1419 cm-1) revealed through rRaman studies. Due to the closely matched N-H BDE’s of N2H2 and the neutral pyH0 cation radical, the aromatic N-heterocylic pendants may encourage partial H-atom transfer (HAT) from N2H2 to xHet through redox non-innocent H-bonding in [xHetTpCu]2(𝜇-N2H2). DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated N2H2 to the xHet pendants to generate tautomeric [xHetHTpCu]2(𝜇-N2) complexes, identifying concerted dual HAT as a thermodynamically favorable pathway for N2 / N2H2 interconversion.

Commercially available benzophenone imine (HN Created by potrace 1.16, written by Peter Selinger 2001-2019 CPh2) reacts with β-diketiminato copper(ii) tert-butoxide complexes [CuII]–OtBu to form isolable copper(ii) ketimides [CuII]–NCPh2. Structural characterization of the three coordinate copper(ii) ketimide [Me3NN]Cu–NCPh2 reveals a short Cu-Nketimide distance (1.700(2) Å) with a nearly linear Cu–N–C linkage (178.9(2)°). Copper(ii) ketimides [CuII]–NCPh2 readily capture alkyl radicals R˙ (PhCH(˙)Me and Cy˙) to form the corresponding R–NCPh2 products in a process that competes with N–N coupling of copper(ii) ketimides [CuII]–NCPh2 to form the azine Ph2CN–NCPh2. Copper(ii) ketimides [CuII]–NCAr2 serve as intermediates in catalytic sp3 C–H amination of substrates R–H with ketimines HNCAr2 and tBuOOtBu as oxidant to form N-alkyl ketimines R–NCAr2. This protocol enables the use of unactivated sp3 C–H bonds to give R–NCAr2 products easily converted to primary amines R–NH2via simple acidic deprotection., Commercially available benzophenone imine (HNCPh2) reacts with β-diketiminato copper(ii) tert-butoxide complexes [CuII]–OtBu to form isolable copper(ii) ketimides [CuII]–NCPh2 that serve as intermediates in catalytic sp3 C−H amination via radical relay.

A series of β-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

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.

Copper(II) alkynyl species are proposed as key intermediates in numerous Cu−catalysed C−C coupling reactions. Supported by a β−diketiminate ligand, the three coordinate copper(II) alkynyl [CuII]−C≡CAr (Ar = 2,6−Cl2C6H3) forms upon reaction of the alkyne H−C≡CAr with the copper(II) tert−butoxide complex [CuII]−OtBu. In solution, this [CuII]−C≡CAr species cleanly transforms the to the Glaser coupling product ArC≡C−C≡CAr and [CuI](solvent). Addition of nucleophiles R′C≡CLi (R′ = aryl, silyl) and Ph–Li to [CuII]−C≡CAr affords the corresponding Csp−Csp and Csp−Csp2coupled products RC≡C−C≡CAr and Ph–C≡CAr with concomitant generation of [CuI](solvent) and {[CuI]−C≡CAr}−. Supported by DFT calculations, redox disproportionation forms [CuIII](C≡CAr)(R) species that reductively eliminate R−C≡CAr products. [CuII]−C≡CAr also captures the trityl radical Ph3C• to give Ph3C−C≡CAr. Radical capture represents the key Csp−Csp3 bond forming step in the copper catalysed C-H functionalization of benzylic substrates R−H with alkynes H−C≡CR′ (R′ = (hetero)aryl, silyl) that provide Csp−Csp3 coupled products R−C≡CR via radical relay with tBuOOtBu as oxidant.

Linkage isomers of reduced metal-nitrosyl complexes serve as key species in nitric oxide (NO) reduction at monometallic sites to produce nitrous oxide (N2O), a potent greenhouse gas. While factors leading to extremely rare side-on nitrosyls are unclear, we describe a pair of nickel-nitrosyl linkage isomers through controlled tuning of noncovalent interactions between the nitrosyl ligands and differently encapsulated potassium cations. Furthermore, these reduced metal-nitrosyl species with N-centered spin density undergo radical coupling with free NO and provide a N-N coupled cis-hyponitrite intermediate whose protonation triggers the release of N2O. This report outlines a stepwise molecular mechanism of NO reduction to form N2O at a mononuclear metal site that provides insight into the related biological reduction of NO to N2O.

We report herein three new modes of reactivity between arylazides N3Ar with a bulky copper(I) β-diketiminate. Addition of N3ArX3 (ArX3=2,4,6-X3C6H2; X=Cl or Me) to [iPr2NN]Cu(NCMe) results in triazenido complexes from azide attack on the β-diketiminato backbone. Reaction of [iPr2NN]Cu(NCMe) with bulkier azides N3Ar leads to terminal nitrenes [iPr2NN]Cu]=NAr that dimerize via formation of a C−C bond at the arylnitrene p-position to give the dicopper(II) diketimide 4 (Ar=2,6-iPr2C6H3) or undergo nitrile insertion to give diazametallocyclobutene 8 (Ar=4-Ph-2,6-iPr2C6H2). Importantly, reactivity studies reveal both 4 and 8 to be “masked” forms of the terminal nitrenes [iPr2NN]Cu=NAr that undergo nitrene group transfer to PMe3, tBuNC, and even into a benzylic sp3 C−H bond of ethylbenzene.

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology and are responsible for a myriad of physiological responses. While copper enzymes promote NO release from RSNOs by serving as Lewis acids capable of intramolecular electron-transfer, redox innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C6F5)3 coordinates to the RSNO oxygen atom in adducts RSNO-B(C6F5)3, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials (-1.0 to -1.1 vs. NHE), B(C6F5)3 coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer-sphere chemical reduction results in formation of the Lewis acid stabilized hyponitrite dianion trans-[LA–O–N=N–O–LA]2– (LA = B(C6F5)3) that releases N2O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C6F5)3]•/- radical-anion susceptible to N-N coupling prior to loss of RSSR.

α-substituted ketones are important chemical targets as synthetic intermediates as well as functionalities in in natural products and pharmaceuticals. We report the sp3 C-H α-acetylation of sp3 C-H substrates R-H with arylmethyl ketones ArC(O)Me to provide α-alkylated ketones ArC(O)CH2R at RT with tBuOOtBu as oxidant via copper(I) β-diketiminato catalysts. Proceeding via alkyl radicals R•, this method enables α-substitution with bulky substituents without competing elimination that occurs in more traditional alkylation reactions between enolates and alkyl electrophiles. DFT studies suggest the intermediacy of copper(II) enolates [CuII](CH2C(O)Ar) that capture alkyl radicals R• to give R-CH2C(O)Ar under competing dimerization of the copper(II) enolate to give the 1,4-diketone ArC(O)CH2CH2C(O)Ar.

Nitrite is involved in a plethora of biological phenomena that includes tyrosine nitration associated with neurodegenerative disorders and gastric phenol metabolism. Reaction of the b-diketiminato model complex [Cl2NNF6]Cu(k2-O2N) with phenols outlines the coupled generation of NO with phenol oxidation by nitrite at copper(II). Kinetic studies support nucleophilic attack of the hydroxyl group of phenols ArOH on the bound nitrite in [CuII](k2-O2N) to give the copper(II) hydroxide [CuII]-OH along with the O-nitrosated phenol ArONO that ultimately leads to the corresponding biphenol or o-nitrophenol. The especially electron-rich antioxidant a-tocapherol (vitamin E) quickly generates NO upon interaction with [CuII](k2-O2N). X-ray analysis of the oxidation products of the a-tocapherol analogue PMC reveal formation of an elusive O-quinone methide bound to [CuI], revealing two electron oxidation of PMC by [CuII](k2-O2N). These studies illustrate anaerobic pathways that generate NO from nitrite at copper(II) sites that result in phenol oxidation.

Recognizing the potential of ammonia to serve as a carbon-free fuel, we describe an electrocatalytic system for the oxidation of ammonia based on ferrocene (Cp2Fe), an inexpensive, robust catalyst utilizing Earth-abundant iron. Ferrocenium (Cp2Fe+), the 1-electron oxidized form of ferrocene, cleanly oxidizes ammonia to generate nitrogen gas (N2) and protons captured by excess ammonia as NH4+ with electrons reducing ferrocenium to ferrocene. This process occurs under electrocatalytic conditions to generate N2 with sustained current. Simple modification of ferrocene through sulfonation allows for solubility in liquid ammonia to enable electrocatalysis in highly concentrated, energy dense solutions of ammonia. Kinetic and computational analysis provides mechanistic insight into the oxidation of ammonia by ferrocenium.

The dinuclear b-diketiminato NiIItert-butoxide {[Me3NN]Ni}2(μ-OtBu)2 (2), synthesized from [Me3NN]Ni(2,4-lutidine) (1) and di-tert-butylperoxide, is a versatile precursor for the synthesis of a series of NiIIcomplexes [Me3NN]Ni-FG to illustrate C-C, C-N, and C-O bond formation at NiII via radicals. {[Me3NN]Ni}2(μ-OtBu)2 reacts with nitromethane, alkyl and aryl amines, acetophenone, benzamide, ammonia and phenols to deliver corresponding mono- or dinuclear [Me3NN]Ni-FG species (FG = O2NCH2, R-NH, ArNH, PhC(O)NH, PhC(O)CH2, NH2and OAr). Many of these NiII complexes are capable of capturing the benzylic radical PhCH(•)CH3 to deliver corresponding PhCH(FG)CH3 products featuring C-C, C-N or C-O bonds. DFT studies shed light on the mechanism of these transformations and suggest two competing pathways that depend on the nature of the functional groups. These radical capture reactions at [NiII]-FG complexes outline key C-C, C-N, and C-O bond forming steps and suggest new families of nickel radical relay catalysts.

A modular synthesis provides access to a series of new tris(pyrazolyl)borate ligands XpyMeTpK that possess a single functionalized pendant pyridyl (py) or pyrimidyl (pyd) arm designed to engage in tunable intramolecular H-bonding to metal–bound functionalities. To illustrate such H-bonding interactions, a series of [XpyMeTpCu]2(𝜇–OH)2(6a–6e) complexes were synthesized from the corresponding XpyMeTpCu–OAc (5a–5e) complexes. Single crystal X-ray structures of three new dinuclear [XpyMeTpCu]2(𝜇–OH)2complexes reveal H-bonding between the pendant heterocycle and bridging hydroxide ligands while the donor arm engages the copper center in an unusual monomeric DMAPMeTpCu–OH complex. Vibrational studies (IR) of each bridging hydroxide complex reveal reduced 𝜈OH frequencies that tracks with the H-bond accepting ability of the pendant arm.

A variety of acyl protected phenols AcOAr participate in sp3 C-H etherification of substrates R-H to give alkyl aryl ethers R-OAr employing tBuOOtBu as oxidant with copper(I) β-diketiminato catalysts [CuI]. Although 1°, 2°, and 3° C-H bonds may be functionalized, selectivity studies reveal a preference for the construction of hindered, 3° C-OAr bonds. Mechanistic studies indicate that β-diketiminato copper(II) phenolates [CuII]-OAr play a key role in this C-O bond forming reaction, formed via transesterification of AcOAr with [CuII]-OtBu intermediates generated upon reaction of [CuI] with tBuOOtBu.

The Lewis acidic β-styryl-B(C6F5)2 reagent 6a undergoes a clean 1,1-carboboration reaction with 1-(PMes2)-2-cyclohexenyl acetylene 9 at 60 °C to give the vicinal P/B-substituted conjugated triene product 10a. At 80 °C this undergoes a stereoselective thermally induced disrotatory electrocyclic ring closure to give the cyclohexadiene-derived P/B system 11. Subsequent TEMPO oxidation gave the substituted phenylene-bridged P/B product 12. Both 11 and 12 are active phosphane/borane frustrated Lewis pairs (FLPs). The FLP 11 reacts in a typical way with phenylacetylene to give the phosphonium/alkynylborate product 13. Compound 12 cleaves dihydrogen at near ambient conditions to give the respective phosphonium/hydridoborate zwitterion 14. Both the FLPs 11 and 12 cooperatively add P/B to the nitrogen atom of nitric oxide (NO) within minutes at room temperature to give the persistent P/B FLPNO• radicals 19 and 21, respectively (both characterized by X-ray diffraction and by EPR spectroscopy). The FLPs 11 and 12 ar...

Nitric oxide (NO) participates in numerous biological processes, such as signalling in the respiratory system and vasodilation in the cardiovascular system. Many metal-mediated processes involve direct reaction of NO to form a metal-nitrosyl (M-NO), as occurs at the Fe(2+) centres of soluble guanylate cyclase or cytochrome c oxidase. However, some copper electron-transfer proteins that bear a type 1 Cu site (His2Cu-Cys) reversibly bind NO by an unknown motif. Here, we use model complexes of type 1 Cu sites based on tris(pyrazolyl)borate copper thiolates [Cu(II)]-SR to unravel the factors involved in NO reactivity. Addition of NO provides the fully characterized S-nitrosothiol adduct [Cu(I)](κ(1)-N(O)SR), which reversibly loses NO on purging with an inert gas. Computational analysis outlines a low-barrier pathway for the capture and release of NO. These findings suggest a new motif for reversible binding of NO at bioinorganic metal centres that can interconvert NO and RSNO molecular signals at copper sites.

Linkage isomers of reduced metal-nitrosyl complexes serve as key species in nitric oxide (NO) reduction at monometallic sites to produce nitrous oxide (N(2)O), a potent greenhouse gas. While factors leading to extremely rare side-on nitrosyls are unclear, we describe a pair of nickel-nitrosyl linkage isomers through controlled tuning of noncovalent interactions between the nitrosyl ligands and differently encapsulated potassium cations. Furthermore, these reduced metal-nitrosyl species with N-centered spin density undergo radical coupling with free NO and provide a N–N coupled cis-hyponitrite intermediate whose protonation triggers the release of N(2)O. This report outlines a stepwise molecular mechanism of NO reduction to form N(2)O at a mononuclear metal site that provides insight into the related biological reduction of NO to N(2)O.

Undirected C(sp

The unsaturated vicinal phosphane/borane frustrated Lewis pairs (P/B FLPs) Ph2PC(p-tolyl) = C(R)B(C6F5)2 (R = C6F5 or CH3) react with tert-butyl isocyanide to give an equilibrium mixture of the starting materials, the borane/isonitrile adduct and the P/B addition product to the isonitrile carbon atom. The cooperative 1,1-adducts were exclusively formed upon treatment of these FLPs with n-butyl isocyanide. The saturated vicinal FLP Mes2PCH2CH2B(C6F5)2 reacts analogously with carbon monoxide yielding the respective five-membered zwitterionic carbonyl heterocycle. Similarly cooperative CO addition was observed starting from a norbornane based vicinal P/B FLP. These saturated vicinal P/B FLPs (and several related systems) undergo analogous cooperative 1,1-addition reactions to nitric oxide (NO) to give the corresponding persistent five-membered heterocyclic FLP-NO nitroxide radicals. They were characterized spectroscopically and by X-ray diffraction and shown to undergo typical N-oxyl radical reactions. Some vicinal P/B FLPs also undergo 1,1-addition reactions to mesityl azide. Anomalous Staudinger reactions were observed to take place upon subsequent thermolysis or (in one case) photolysis.

The 1-trimethylsilylethynyl-2-diarylphosphanylethynylbenzene substrates 7a–7c, with the diarylphosphanyl groups = PPh2, P(o-tolyl)2, or P(mesityl)2, were reacted with the strong boron Lewis acid B(C6F5)3 at ambient conditions. The starting materials 7a and 7b, featuring rather small nucleophilic phosphanyl substituents, underwent selective 1,1-carboboration at the C≡C-PAr2 moiety, forming rather unreactive unsaturated vicinal P/B Lewis pairs 8a and 8b that show a relatively strong internal P···B Lewis base/Lewis acid interaction. In contrast, the PMes2 system 7c reacted with B(C6F5)3 by a reaction sequence which was initiated by 1,1-carboboration and involved tandem C6F5 and mesityl group migration to yield the unsaturated annulated P/B Lewis pair 9, featuring an electron-withdrawing C6F5 group at phosphorus. Compound 9 underwent cooperative P/B addition to the nitrogen atom of nitric oxide to give the persistent FLPNO• nitroxide radical 12. Radical 12 was characterized by electron spin resonance spectros...

Three [Me2NN]Cu(η2-L2) complexes (Me2NN = HC[C(Me)NAr]2; L2 = PhNO (2), (3), PhCH 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CH2 (4); Ar = 2,6-Me2-C6H3; ArF = 3,5-(CF3)2-C6H3) have been studied by Cu K-edge X-ray absorption spectroscopy, as well as single- and multi-reference computational methods (DFT, TD-DFT, CASSCF, MRCI, and OVB). The study was extended to a range of both known and theoretical compounds bearing 2p-element donors as a means of deriving a consistent view of how the pre-edge transition energy responds in systems with significant ground state covalency. The ground state electronic structures of many of the compounds under investigation were found to be strongly influenced by correlation effects, resulting in ground state descriptions with majority contributions from a configuration comprised of a Cu(ii) metal center anti-ferromagentically coupled to radical anion O2, PhNO, and ligands. In contrast, the styrene complex 4, which displays a Cu K pre-edge transition despite its formal d10 electron configuration, exhibits what can best be described as a Cu(i):(styrene)0 ground state with strong π-backbonding. The Cu K pre-edge features for these complexes increase in energy from 1 to 4, a trend that was tracked to the percent Cu(ii)-character in the ground state. The unexpected shift to higher pre-edge transition energies with decreasing charge on copper (QCu) contributed to an assignment of the pre-edge features for these species as arising from metal-to-ligand charge transfer instead of the traditional Cu1s → Cu3d designation.

Copper(II) aryl species are proposed key intermediates in Cu-catalyzed cross-coupling reactions. Novel three-coordinate copper(II) aryls [CuII]-C6F5 supported by ancillary β-diketiminate ligands form in reactions between copper(II) alkoxides [CuII]-OtBu and B(C6F5)3. Crystallographic, spectroscopic, and DFT studies reveal geometric and electronic structures of these Cu(II) organometallic complexes. Reaction of [CuII]-C6F5 with the free radical NO(g) results in C-N bond formation to give [Cu](η2-ONC6F5). Remarkably, addition of the phenolate anion PhO- to [CuII]-C6F5 directly affords diaryl ether PhO-C6F5 with concomitant generation of the copper(I) species [CuI](solvent) and {[CuI]-C6F5}-. Experimental and computational analysis supports redox disproportionation between [CuII]-C6F5 and {[CuII](C6F5)(OPh)}- to give {[CuI]-C6F5}- and [CuIII](C6F5)(OPh) unstable toward reductive elimination to [CuI](solvent) and PhO-C6F5.

The intramolecular frustrated phosphane/borane Lewis pair Mes2PCH2CH2B(C6F5)2 (3) reacts with acetyl chloride to give a 1 : 1 mixture of the FLP-ketene adduct, 6a, and the FLP-HCl addition product, 7. Similarly, the acyl chlorides phenylacetyl chloride and isobutyryl chloride react with 3 to give the respective substituted FLP ketene adducts, 6b and 6c, with HCl elimination. A series of aroyl chlorides reacts with 3 by formation of the respective aroyl-phosphonium/chloroborate zwitterions, 11a–d. The hetero-aromatic furoyl chloride reacts with 3 in a more complicated, but mechanistically probably closely related pathway, to form the 2 : 1 reaction product, 14. Most of the products were characterized by X-ray diffraction.

We report a series of novel β-diketiminato copper(II) anilides [Cl2NN]Cu-NHAr that participate in C-H amination. Reaction of H2NAr (Ar = 2,4,6-Cl3C6H2 (ArCl3), 3,5-(CF3)2C6H3 (ArF6), or 2-py) with the copper(II) t-butoxide complex [Cl2NN]Cu-tOBu yields the corresponding copper(II) anilides [Cl2NN]Cu-NHAr. X-ray diffraction of these species reveal three different bonding modes for the anilido moiety: κ1-N in the trigonal [Cl2NN]Cu-NHArCl3 to dinuclear bridging in {[Cl2NN]Cu}2(μ-NHArF6)2 and κ2-N,N in the square planar [Cl2NN]Cu(κ2-NH-2-py). Magnetic data reveal a weak antiferromagnetic interaction through a π-stacking arrangement of [Cl2NN]Cu-NHArCl3; solution EPR data are consistent with monomeric species. Reaction of [Cl2NN]Cu-NHAr with hydrocarbons R-H (R-H = ethylbenzene and cyclohexane) reveals inefficient stoichiometric C-H amination with these copper(II) anilides. More rapid C-H amination takes place, however, when tBuOOtBu is used, which allows for HAA of R-H to occur from the tBuO• radical generat...

The vicinal frustrated Lewis pair (FLP) mes2P-CH2CH2-B(C6F5)2 (3) reacts with phenyl(trimethylsilyl)acetylene by 1,1-carboboration to give the extended C3-bridged FLP 6 featuring a substituted vinylborane subunit. The FLP 6 actively cleaves dihydrogen. The FLP 3 also undergoes a 1,1-carboboration reaction with diphenylphosphino(trimethylsilyl)acetylene to give the P/B/P FLP 11 that features a central unsaturated four-membered heterocyclic P/B FLP and a pendant CH2CH2-Pmes2 functional group. Compound 11 reacts with nitric oxide (NO) by oxidation of the pendant Pmes2 unit to the P(O)mes2 phosphine oxide and N,N-addition of the P/B FLP unit to NO to yield the persistent P/B/PO FLPNO aminoxyl radical 14. This reaction is initiated by P(O)mes2 formation and opening of the central Ph2P···B(C6F5)2 linkage triggered by the pendant CH2CH2-P(O)mes2 group.

Addition of nitrosobenzene to β-diketiminato and tris(pyrazolyl)borate copper complexes [[Me2NNF6]Cu(NCMe)] (2) and [iPr2TpCu(NCMe)] (3) gives the corresponding nitrosobenzene adducts [[Me2NNF6]Cu(η2-ONPh)] (5) and [iPr2TpCu{κ1-N(O)Ph}] (6). Backbonding to the nitrosoarene is strongest via the η2-ON binding mode, which is sterically inaccessible for the tris(pyrazolyl)borate complexes. The strength of the interaction is judged by the N–O bond length, the νNO stretching frequencies observed for 5 and 6, as well as by the DFT LUMO energies of 5-η2-ON, 5-κ1-N, 6-κ1-N, which range from highest to lowest. Nitrosoarene complexes 5 and 6 are also produced upon reaction of [[Me2NNF6]Cu(NCMe)] (2) and [iPr2TpCu(NCMe)] (3) with 2 equiv. N-phenylhydroxylamine (PhNHOH), though a considerable amount of the azoxybenzene PhN=N(O)Ph is observed along with 6. The addition of slight excess of NO to 5 and 6 results in the formation of copper(II) diazeniumdiolates [[Me2NNF6]Cu(κ2-O2N2Ph)] (7) and [iPr2TpCu(κ2-O2N2Ph)] (8), which exhibit square-planar and square-pyramidal geometries, respectively. These findings give insight into PhNO and PhHNOH reactivity at copper(I) and foreshadow possible transformations of considerably more reactive nitroxyl (HNO) and hydroxylamine (H2NOH) analogues.

We examine important reactivity pathways relevant to stoichiometric and catalytic C-H amination via isolable β-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.

The β-diketiminato nickel(I) species [Me3NN]Ni(2-picoline) (1) serves as an efficient catalyst for carbodiimide (RN═C═NR′) formation in the reactions of a range of organoazides N3R with isocyanides R′NC. [Me3NN]Ni(CNR)2 (R = tBu, Ar (Ar = 2,6-Me2C6H3)) species provide carbodiimides RN═C═NAr′ upon reaction with Ar′N3 (Ar′ = 3,5-Me2C6H3). Nitrene transfer takes place via the intermediacy of nickel imides. Reaction of [MexNN]Ni(2-picoline) (x = 2 or 3) with Ar′N3 gives the new dinickel imides {[MexNN]Ni}2(μ-NAr′) (4 (x = 3) and 5 (x = 2)) as deep purple, diamagnetic substances. The X-ray structure of {[Me2NN]Ni}2(μ-NAr′) (5) features short Ni–Nimide distances of 1.747(2) and 1.755(2) A along with a short Ni–Ni distance of 2.7210(3) A. These dinickel imides 4 and 5 react stoichiometrically with tBuNC to provide the corresponding carbodiimides tBuN═C═NAr′ in good yield. Azide transfer takes place upon reaction of 1 with TMS-N3 to give the square planar nickel(II) azide [Me3NN]Ni(N3)(2-picoline) (7). Stoichiome...

The dimesitylphosphinocyclopentene/HB(C6 F5 )2 -derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane-borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO2 . It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C6 F5 )2 ] to generate a FLP-η2 -formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.

Nitric oxide is a vital signaling molecule that controls blood flow and oxygenation and nitrite serves as an important reservoir for nitric oxide in biology. While copper containing enzymes are known to reduce nitrite to nitric oxide, herein we report a new pathway to release nitric oxide via oxygen atom transfer from nitrite at a copper(ii) site.

A variety of acyl protected phenols AcOAr participate in sp