Triflate Complexes Research Articles

(2-Fluoroallyl)palladium complexes as intermediates in Pd-catalyzed Tsuji-Trost 2-fluoroallylations: Synthesis and reactivity

The first examples of π-(2-fluoroallyl)palladium complexes were synthesized, isolated and characterized including chloride dimers, neutral chloride or cationic triflate complexes bearing PPh3 or SPhos as the ligands. Preliminary reactivity patterns indicating strong dependence of the chemoselectivity on “hardness” of the nucleophile and the ligand type were studied.

A Stiboranyl Platinum Triflate Complex as an Electrophilic Catalyst

With the view of developing electrophilic late-transition-metal catalysts, we have now synthesized [(o-(Ph2P)C6H4)2Sb(OTf)2]Pt(OTf) (2) and [(o-(iPr2P)C6H4)2Sb(OTf)2]Pt(OTf) (4) by treatment of the corresponding trichlorides ([(o-(R2P)C6H4)2SbCl2]PtCl (R = Ph, iPr)) with 3 equiv of AgOTf. The crystal structures of 2 and 4 confirmed that the chloride ligands have been fully substituted by more labile triflate ligands. Despite structural similarities in the dinuclear cores of 2 and 4, only 2 acts as a potent carbophilic catalyst in enyne cyclization reactions. The high activity of 2 is also reflected by its ability to promote the addition of pyrrole and thiophene derivatives to alkynes. Structural and computational analyses suggest that the superior reactivity of 2 results from both favorable steric and electronic effects. Finally, a comparison of 2 with the previously reported self-activating catalyst [(o-(Ph2P)C6H4)2Sb(OTf)2]PtCl underscores the benefits of triflate for chloride substitution.

Triazacyclohexane chromium triflate complexes as precursors for the catalytic selective olefin trimerisation and its investigation by mass spectrometry

A novel set of linear α-olefin trimerisation pre-catalysts is presented. The R3TACCr(OTf)3 (R3TAC = 1,3,5-trialkyl-1,3,5-triazacyclohexane) complexes produce an active system after facile activation with less than 10 equivalents of trialkylaluminium. Isomerisation was observed in many cases, which is proposed to occur via the π-allyl complex mechanism, though this was minimised by optimisation of the reaction conditions. The pre-catalysts can be synthesised from R3TACCrCl3 in neat TfOH or by addition of R3TAC to Cr(OTf)3 or better Cr(OTf)3(THF)3, the synthesis and structure of which is presented here. The use of this highly defined system allowed the identification of 2-methyl-1-hexene as a side product of activation with AlMe3, in agreement with the proposed metallacyclic mechanism. Isomer distribution of the trimer product is similar to R3TACCrCl3/MAO activated systems and depends mostly on the ligand substituent R. ESI mass spectrometry of an ortho-difluorobenzene solution of the activated catalyst was analysed at different stages of reaction. A series of signals was observed that matched those expected for cationic chromium species predicted by the metallacyclic mechanism. In particular, [R3TACCrMe2]+ was observed to form immediately after alkylation with AlMe3, while [R3TACCr(olefin)n]+ (n = 0, 1, 2, 3, 4 and olefin = 1-hexene or 1-octene) formed after addition of 1-hexene. Absence of any detected tetramer and observation of [R3TACCr(olefin)4]+ leads to the conclusion that a metallacycloheptane-olefin complex may lead to β-H abstraction rather than insertion and may even be required to avoid formation of [R3TACCr(olefin)1]+ which is observed in only very small amounts indicating a low stability. Chromium(I) complexes [R3TACCr(arene)]+ with the arene solvent can also be observed and their signal intensity indicate a relative stability of o-C6H4F2 < cymene < benzene < PhF, toluene < p-xylene.

Perfluoropyridine as an Efficient, Tunable Scaffold for Bis(pyrazol‐1‐yl)pyridine Copper Complexes

This report details the syntheses of five new 2,6‐bis(pyrazol‐1‐yl)pyridine ligands carried out by a unique strategy of utilizing pentafluoropyridine for the pyridine core. Formation of each ligand is accomplished under mild conditions utilizing carbonate bases. Each ligand reacts readily with copper(II) triflate to form the corresponding salt in high yield. Ligands and complexes were characterized by a combination of multi‐nuclear NMR, FTIR, UV/Vis, and single‐crystal X‐ray diffraction. Copper(II) triflate complexes demonstrating additional coordination modes of this multi‐pyrazole ligand system were also obtained. Despite the electron withdrawing nature of fluorine, the coordination parameters of the new complexes are comparable to previous pyrazole‐based complexes. This synthetic methodology provides efficient entry to a previously difficult to access ligand system.

Effect of the Counterion on Circularly Polarized Luminescence of Europium(III) and Samarium(III) Complexes.

Each enantiopure europium(III) and samarium(III) nitrate and triflate complex of the ligand L, with L = N,N′-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine ([LnL(tta)2]·NO3 and [LnL(tta)2(H2O)]·CF3SO3, where tta = 2-thenoyltrifluoroacetylacetonate) has been synthesized and characterized from a spectroscopic point of view, using a chiroptical technique such as electronic circular dichroism (ECD) and circularly polarized luminescence (CPL). In all cases, both ligands are capable of sensitizing the luminescence of both metal ions upon absorption of light around 280 and 350 nm. Despite small differences in the total luminescence (TL) and ECD spectra, the CPL activity of the complexes is strongly influenced by a concurrent effect of the solvent and counterion. This particularly applies to europium(III) complexes where the CPL spectra in acetonitrile can be described as a weighed linear combination of the CPL spectra in dichloromethane and methanol, which show nearly opposite signatures when their ligand stereochemistries are the same. This phenomenon could be related to the presence of equilibria interconverting solvated, anion-coordinated complexes and isomers differing by the relative orientation of the tta ligands. The difference between some bond lengths (M–N bonds, in particular) in the different species could be at the basis of such an unusual CPL activity.

Tuning the copper(II)/copper(I) redox potential for more robust copper-catalyzed C-N bond forming reactions.

Complexes of copper and 1,10-phenanthroline have been utilized for organic transformations over the last 50 years. In many cases these systems are impacted by reaction conditions and perform best under an inert atmosphere. Here we explore the role the 1,10-phenanthroline ligand plays on the electronic structure and redox properties of copper coordination complexes, and what benefit related ligands may provide to enhance copper-based coupling reactions. Copper(II) triflate complexes bearing 1,10-phenanthroline (phen), ([Cu(phen)2(OTf)]OTf, 1) and oxidized derivatives of phen including [Cu(edhp)2](OTf)2 (2), [Cu(pdo)2](OTf)2 (3), [Cu(dafo)2](OTf)2 (4) were prepared and characterized. X-ray crystallographic data show these related ligands subtly impacted the coordination geometry of the copper(II) ion. Complexes 1-3 had only incremental changes to the redox properties of the copper ions, complex 4 showed a drastically different redox potential affording a remarkably air stable copper(I) complex. These complexes 1-4 were then used to catalyze the C-N bond forming cross coupling between imidazole and various boronic acid substrates, where the increased stability of the copper(I) species in complex 4 appears to better support these CEL cross couplings.

Preparation and Characterization of the Triflate Complex [Cp*(dppe)FeOSO2CF3]: A Convenient Access to Labile Five‐ and Six‐Coordinate Iron(I) and Iron(II) Complexes

Treatment of the iron hydride [Cp*(dppe)FeH] (1) with methyl triflate (CH3OSO2CF3) yielded the iron triflate adduct [Cp*(dppe)FeOSO2CF3] (4, 85 %). In the solid state, the triflate is coordinated at the iron center as shown by XRD (dFe–O = 2.118(4) Å) and IR spectroscopy (νSO = 1305 cm–1). In solution, 4 is in equilibrium with the 16‐electron species [Cp*(dppe)Fe]OSO2CF3 (5(OSO2CF3)), 4/5(OSO2CF3) = 2:1). The CV of 4 displays two waves (E1 = –0.74 V, E2 = 0.24 V vs. SCE) assigned to the [Cp*(dppe)Fe(I)]/[Cp*(dppe)Fe(II)]+ and [Cp*(dppe)Fe(II)]+/[Cp*(dppe)Fe(III)]2+ redox couples. Reduction of 4 with Cp2Co provided the complex [Cp*(dppe)Fe(I)] (6, 95 %) and oxidation of 6 with [Cp2Fe]PF6 gave [Cp*(dppe)Fe]PF6 (5(PF6), 98 %). XRD established the pseudo‐trigonal bipyramidal geometry for the five‐coordinated cation 5+. The reactivity of 5(PF6) and 6 toward small molecules (CH2Cl2, H2O, CO, H2, N2) is reported.

Characterization of Terminal Iron(III)-Oxo and Iron(III)-Hydroxo Complexes Derived from O2 Activation.

O2 activation at nonheme iron centers is a common motif in biological systems. While synthetic models have provided numerous insights into the reactivity of high-valent iron-oxo complexes related to biological processes, the majority of these complexes are synthesized using alternative oxidants. This report describes O2 activation by an iron(II)-triflate complex of the imino-functionalized tris(pyrrol-2-ylmethyl)amine ligand framework, H3[N(piCy)3]. Initial reaction conditions result in the formation of a mixture of oxidation products including terminal iron(III)-oxo and iron(III)-hydroxo complexes. The relevance of these species to the O2 activation process is demonstrated through reactivity studies and electrochemical analysis of the iron(III)-oxo complex.

Regioselective Ring-Opening of Glycidol to Monoalkyl Glyceryl Ethers Promoted by an [OSSO]-FeIII Triflate Complex.

A FeIII -triflate complex, bearing a bis-thioether-di-phenolate [OSSO]-type ligand, was discovered to promote the ring-opening of glycidol with alcohols under mild reaction conditions (0.05 mol % catalyst and 80 °C). The reaction proceeded with high activity (initial turnover frequency of 1680 h-1 for EtOH) and selectivity (>95 %) toward the formation of twelve monoalkyl glyceryl ethers (MAGEs) in a regioselective fashion (84-96 % yield of the non-symmetric regioisomer). This synthetic approach allows the conversion of a glycerol-derived platform molecule (i.e., glycidol) to high-value-added products by using an Earth-crust abundant metal-based catalyst.

Mechanism of Olefin Metathesis with Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

A series of neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and monotriflate monoalkoxide complexes as well as cationic molybdenum imido alkylidene triflate complexes have been subjected to NMR spectroscopic, X-ray crystallographic, and reaction kinetic measurements in order to gain a comprehensive understanding about the underlying mechanism in olefin metathesis of this new type of catalysts. On the basis of experimental evidence and on DFT calculations (BP86/def2-TZVP/D3/cosmo) for the entire mechanism, olefinic substrates coordinate trans to the NHC of neutral 16-electron complexes via an associative mechanism, followed by dissociation of an anionic ligand (e.g., triflate) and formation of an intermediary molybdacyclobutane trans to the NHC. Formation of a cationic complex is crucial in order to become olefin metathesis active. Variations in the NHC, the imido, the alkoxide, and the noncoordinating anion revealed their influence on reactivity. The reaction of neutral 16-electron complexes with 2-methoxystyrene is faster for catalysts bearing one triflate and one fluorinated alkoxide than for catalysts bearing two triflate ligands. This is also reflected by the Gibbs free energy values for the transition states, ΔG‡303, which are significantly lower for catalysts bearing only one triflate than for the corresponding bistriflate complexes. Reaction of a solvent-stabilized cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) monotriflate complex with 2-methoxystyrene proceeded via an associative mechanism too. Reaction rates of both solvent-free and solvent-stabilized cationic Mo imido alkylidene NHC catalysts with 2-methoxystyrene are controlled by the cross-metathesis step but not by adduct formation.

Reduction of Dinitrogen to Ammonia and Hydrazine on Low-Valent Ruthenium Complexes.

The ruthenium(0) dinitrogen complexes [Ru(N2)(PP3R)] [PP3R = P(CH2CH2PR2)3; R = iPr or Cy] react with triflic acid and other strong acids to afford mixtures of ammonia and hydrazine. In this reaction, Ru(0) is oxidized to Ru(II), and depending on the solvent, Ru(II) benzene or triflate complexes are isolated and characterized from the reactions with triflic acid as the final metal-containing products from the reaction. The Ru(II) products are isolated and reduced back to Ru(0) dinitrogen complexes providing a cycle for the reduction of coordinated dinitrogen.

Palladium Acyclic Diaminocarbene (ADC) Triflate Complexes as Effective Precatalysts for the Hiyama Alkynylation/Cyclization Reaction Yielding Benzofuran Compounds: Probing the Influence of the Triflate Co‐Ligand in the One‐Pot Tandem Reaction

AbstractPalladium acyclic diaminocarbene (ADC) complexes of the type cis‐[(R1NH)(R2)methylidene]Pd(OCOCF3)2(CNR1) [R1 = 2,4,6‐(CH3)3C6H2: R2 = NC4H8 (1); NC5H10 (2)] have been synthesized and structurally characterized. These 1 and 2 complexes are effective precatalysts for the one‐pot tandem Hiyama alkynylation/cyclization reaction producing benzofuran compounds and, quite interestingly, higher yields of ca. 33−52 % were observed for the aryl triethoxysilylalkyne substrates as compared to that of ca. 15−21 % for the aliphatic triethoxysilylalkyne substrates. The 1 and 2 complexes were conveniently prepared by the salt metathesis reaction of the chloro derivatives with AgOCOCF3 in excellent yields (ca. 84−94 %) under ambient conditions.

Neutral and cationic phosphine and arsine complexes of tin(iv) halides: synthesis, properties, structures and anion influence.

The reaction of trans-[SnCl4(PR3)2] (R = Me or Et) with trimethylsilyltriflate (TMSOTf) in CH2Cl2 solution substitutes one chloride to form [SnCl3(PR3)2(OTf)]; addition of excess TMSOTf does not substitute further chlorides. The complexes have been fully characterised by microanalysis, IR and multinuclear NMR (1H, 13C{1H}, 19F{1H}, 31P{1H}, 119Sn) spectroscopy. The crystal structure of [SnCl3(PMe3)2(OTf)] revealed mer-chlorines and trans-phosphines. In contrast, trans-[SnBr4(PR3)2], [SnCl4{Et2P(CH2)2PEt2}], [SnCl4{o-C6H4(PMe2)2}] and [SnCl4{o-C6H4(AsMe2)2}] did not react with TMSOTf in CH2Cl2 solution even after 3 days. The arsine complexes, [SnX4(AsEt3)2] (X = Cl, Br), were confirmed as trans-isomers by similar spectroscopic and structural studies, while attempts to isolate [SnI4(AsEt3)2] were unsuccessful and reaction of SnX4 with SbR3 (R = Et, iPr) resulted in reduction to SnX2 and formation of R3SbX2. trans-[SnCl4(AsEt3)2] is converted by TMSOTf into [SnCl3(AsEt3)2(OTf)], whose X-ray structure reveals the same geometry found in the phosphine analogues, with the triflate coordinated. The salts, [SnCl3(PEt3)2][AlCl4] and [SnCl2(PEt3)2][AlCl4]2 were made by treatment of [SnCl4(PEt3)2] with one and two mol. equivalents, respectively, of AlCl3 in anhydrous CH2Cl2, whereas reaction of [SnCl4(AsEt3)2] with AlCl3 produced a mixture including Et3AsCl2 and [Et3AsCl][Sn(AsEt3)Cl5] (the latter identified crystallographically). In contrast, using Na[BArF] (BArF = [B{3,5-(CF3)2C6H3}4]-) produced [SnCl3(PEt3)2][BArF] and also allowed clean isolation of the arsine analogue, [SnCl3(AsEt3)2][BArF]. [SnCl4{o-C6H4(PMe2)2}] also reacts with AlCl3 in CH2Cl2 to form [SnCl3{o-C6H4(PMe2)2}][AlCl4] and [SnCl2{o-C6H4(PMe2)2}][AlCl4]2. Multinuclear NMR spectroscopy on the [AlCl4]- salts show that δ31P and δ119Sn move progressively to high frequency on conversion from the neutral complex to the mono- and the di-cations, whilst 1J(119Sn-31P) follow the trend: [SnCl3{o-C6H4(PMe2)2}]+ > [SnCl4{o-C6H4(PMe2)2}] > [SnCl2{o-C6H4(PMe2)2}]2+. DFT studies on selected complexes show only small changes in ligand geometries and bond lengths between the halide and triflate complexes, consistent with the X-ray crystallographic data reported and the HOMO and LUMO energies are relatively unperturbed upon the introduction of (coordinated) triflate, whereas the energies of both are ca. 4 eV lower in the cationic species and reveal significant hybridisation across the pnictine ligands.

Di- and trivalent chromium bis(pyrazol-3-yl)pyridine pincer complexes with good leaving groups

The consequences of installing two versions of a potentially redox active pincer ligand on Cr(III) and Cr(II) are described, and reveal influence of the Jahn-Teller effect. The ligand involves two pyrazole rings attached by pyrazole carbons to the ortho positions of pyridine, thus creating two 1,4-diazabutadiene moieties. In one version, each pyrazole carries an NH proton (H2L), while a comparison version (LMe) involves methyl in place of the two weakly acidic protons, to identify the impact of hydrogen bonding. Complexes synthesized include (H2L)CrX3, where XCl, triflate and nitrate, (LMe)CrCl3 and (LMe)Cr(OTf)3, together with the divalent metal complexes (H2L)CrCl2, (LMe)CrCl2 and (LMe)Cr(OTf)2. Cyclic voltammetry establishes the relationship between the tri- and divalent triflate complexes of LMe, and DFT calculations indicate further reduction of the latter occurs at the LMe ligand. Nitrate is shown to be a stronger ligand than triflate to Cr(III) here. NaBArF24 in THF removes only one chloride from (H2L)CrCl2, to give square pyramidal [(H2L)CrCl(THF)]+.

Visible light mediated aerobic photocatalytic activation of C[sbnd]H bond by riboflavin tetraacetate and N-hydroxysuccinimide

A green and highly efficient cooperative photocatalytic system which consists of riboflavin tetraacetate (RFT), N-hydroxysuccinimide (NHS) and FeCl3·6H2O has been developed for CH bond oxygenation with visible light under mild reaction conditions. Compared to the previous procedures about CH bond activation by using flavins as photocatalyst, we use cheap ferric chloride and NHS instead of expensive scandium triflate or biomimetic non-heme iron complex, further expand the substrate scopes. In addition, a possible radical mechanism for the CH bond oxygenation in this photocatalytic system is proposed.

Sequential Electrophilic Substitution Reactions of Tungsten-Coordinated Phosphenium Ions and Phosphine Triflates

ion of chloride from [W(CO)5{PPhCl2}] with AgOSO2CF3 leads to the phosphine triflate complex [W(CO)5{PPhCl(OSO2CF3)}] which undergoes electrophilic substitution reactions with N,N-diethylaniline, anisole, N,N-dimethyl-p-toluidine, toluene, biphenyl, naphthalene, 2,7,9,9-tetramethyl xanthene, and allyltrimethylsilane to form the chlorophosphine complexes [W(CO)5{PPhClR}], where R = p-diethylanilinyl, p-anisyl, 2-(N,N-dimethyl-4-methylphenyl), p-tolyl, p-phenylphenyl, 1-naphthyl, 4-(2,7,9,9-tetramethylxanthyl), and allyl. ion of the second chloride with AgOSO2CF3 leads, in most cases, to the respective phosphine triflates [W(CO)5{PPhR(OSO2CF3)}], which react with ferrocene to form the ferrocenyl phosphine complexes [W(CO)5{PPhR(C10H9Fe)}]. The W(CO)5 unit can be removed via photolysis in the presence of bis(diphenylphosphino)ethane to form metal-free phosphines.

Mono- and dinuclear CuII complexes of the benzyldipicolylamine (BDPA) ligand: crystal structure, synthesis and characterization.

The crystal structures of mono- and dinuclear CuII trifluoromethanesulfonate (triflate) complexes with benzyldipicolylamine (BDPA) are described. From equimolar amounts of Cu(triflate)2 and BDPA, a water-bound CuII mononuclear complex, aqua(benzyldipicolylamine-κ3N,N',N'')bis(trifluoromethanesulfonato-κO)copper(II) tetrahydrofuran monosolvate, [Cu(CF3SO3)2(C19H19N3)(H2O)]·C4H8O, (I), and a triflate-bridged CuII dinuclear complex, bis(μ-trifluoromethanesulfonato-κ2O:O')bis[(benzyldipicolylamine-κ3N,N',N'')(trifluoromethanesulfonato-κO)copper(II)], [Cu2(CF3SO3)4(C19H19N3)2], were synthesized. The presence of residual moisture in the reaction medium afforded water-bound complex (I), whereas dinuclear complex (II) was synthesized from an anhydrous reaction medium. Single-crystal X-ray structure analysis reveals that the CuII centres adopt slightly distorted octahedral geometries in both complexes. The metal-bound water molecule in (I) is involved in intermolecular O-H...O hydrogen bonds with triflate ligands and tetrahydrofuran solvent molecules. In (II), weak intermolecular C-H...F(triflate) and C-H...O(triflate) hydrogen bonds stabilize the crystal lattice. Complexes (I) and (II) were also characterized fully using FT-IR and UV-Vis spectroscopy, cyclic voltammetry and elemental analysis.

Iminopyridine ligand complexes of group 14 dihalides and ditriflates – neutral chelates and ion pair formation

Abstract Reaction of the chelating imino-pyridine ligand SIMPY, (SIMPY=2-(DippN=CH)-C5H4N), Dipp=2,6- i Pr2-C6H3, with germanium(II) and tin(II) halides provides the respective neutral complexes [SIMPY·EX2] (EX2: E=Ge, X=Cl, Br; E=Sn, X=Cl, Br, I). The method is readily extendable to give the tin(II) triflate complex [SIMPY·Sn(OTf)2] (OTf, triflate=CF3SO3 −). In the solid state, the neutral compounds [SIMPY·EX2] exist as monomers, in which the four-coordinate tetrel atoms feature a slightly distorted disphenoidal geometry around germanium and tin. Reaction of the tridentate imino-pyridine ligand DIMPY, (DIMPY=2,6-(DippN=CH)2-C5H3N) with Sn(OTf)2 provided access to a neutral tin(II) complex. Similar to the previously reported reactions leading to the germanium and tin chloride complexes [DIMPY·SnCl]+[SnCl3]−, and [Me2DIMPY·EX]+[EX3]− (Me2DIMPY=2,6-(DippN=C(Me))2-C5H3N, E=Ge, Sn; X=Cl), the reactions of DIMPY with GeX2·dioxane (X=Cl, Br) and SnX2 (X=Br, I) yielded Ge(II) and Sn(II) based ion pairs [DIMPY·EX]+[EX3]− (E=Ge, X=Cl, Br; E=Sn, X=Br, I) as a consequence of spontaneous dissociation of the group 14 dihalides. The tetrel atoms in the cationic parts in [DIMPY·EX]+[EX3]− are four-coordinate as one halide substituent is replaced by the coordination of a second imino donor group from the ligand. The anionic fragments adopt a pyramidally, tri-coordinate geometry. In contrast, the DIMPY tin(II) ditriflate complex crystallizes with two independent, neutral molecules per asymmetric unit, in which one of the tin centers is five- coordinate by interaction with three donor sites of the chelating bis(imino)pyridine ligand and two additional contacts towards the oxygen atoms of the triflate counter-anions. In the second crystallographically independent complex the tin atom is six-coordinate with a slightly distorted octahedral geometry via interaction with THF as an additional donor molecule. All compounds reported were studied by means of multinuclear NMR spectroscopy. In addition, the solid state structures of the complexes [SIMPY·EX2] (EX2: E=Ge, X=Cl, Br; E=Sn, X=Cl, Br, I), the ion pairs [DIMPY·EX]+[EX3]− (E=Ge, X=Cl; E=Sn, X=Br) and the tin(II) ditriflate [DIMPY·Sn(OTf)2] were authenticated by means of single-crystal X-ray diffraction analyses. Moreover, [DIMPY·Sn(OTf)2] was investigated by 119Sn Mössbauer spectroscopy.

Three-Fold-Symmetric Selenium-Donor Metallaboratranes of Cobalt and Nickel

A novel selenium-containing pyridazinyl-based soft scorpionate ligand (KTnse) was synthesized. It reacts with CoCl2 and NiCl2, yielding the first metallaboratrane complexes with selenium in their donor positions. Further substitution with Ag(OTf) or NaN3 allows isolation of the respective triflate or azide complexes. Reaction with Ag(OTf) leads in the case of nickel to a dinuclear, dicationic complex with a short Ni-Ni distance, while cobalt gave a mononuclear cationic species. Substitution of the chloride by azide yields with both metals the respective azide complexes. All compounds were characterized via single-crystal X-ray diffraction analysis. Density functional theory calculations on the chloride species point to oxidized cobalt(III) and nickel(III) centers.

Hydrogen-Atom Transfer Oxidation with H2O2 Catalyzed by [FeII(1,2-bis(2,2'-bipyridyl-6-yl)ethane(H2O)2]2+: Likely Involvement of a (μ-Hydroxo)(μ-1,2-peroxo)diiron(III) Intermediate.

The iron(II) triflate complex (1) of 1,2-bis(2,2'-bipyridyl-6-yl)ethane, with two bipyridine moieties connected by an ethane bridge, was prepared. Addition of aqueous 30% H2O2 to an acetonitrile solution of 1 yielded 2, a green compound with λmax=710 nm. Moessbauer measurements on 2 showed a doublet with an isomer shift (δ) of 0.35 mm/s and a quadrupole splitting (ΔEQ) of 0.86 mm/s, indicative of an antiferromagnetically coupled diferric complex. Resonance Raman spectra showed peaks at 883, 556 and 451 cm-1 that downshifted to 832, 540 and 441 cm-1 when 1 was treated with H218O2. All the spectroscopic data support the initial formation of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that oxidizes carbon-hydrogen bonds. At 0°C 2 reacted with cyclohexene to yield allylic oxidation products but not epoxide. Weak benzylic C-H bonds of alkylarenes were also oxidized. A plot of the logarithms of the second order rate constants versus the bond dissociation energies of the cleaved C-H bond showed an excellent linear correlation. Along with the observation that oxidation of the probe substrate 2,2-dimethyl-1-phenylpropan-1-ol yielded the corresponding ketone but no benzaldehyde, and the kinetic isotope effect, kH/kD , of 2.8 found for the oxidation of xanthene, the results support the hypothesis for a metal-based H-atom abstraction mechanism. Complex 2 is a rare example of a (μ-hydroxo)(μ-1,2-peroxo)diiron(III) complex that can elicit the oxidation of carbon-hydrogen bonds.