Rapid access to polycyclic N-heteroarenes from unactivated, simple azines via a base-promoted Minisci-type annulation.

Conventional synthetic methods to yield polycyclic heteroarenes have largely relied on metal-mediated arylation reactions requiring pre-functionalised substrates. However, the functionalisation of unactivated azines has been restricted because of their intrinsic low reactivity. Herein, we report a transition-metal-free, radical relay π-extension approach to produce N-doped polycyclic aromatic compounds directly from simple azines and cyclic iodonium salts. Mechanistic and electron paramagnetic resonance studies provide evidence for the in situ generation of organic electron donors, while chemical trapping and electrochemical experiments implicate an iodanyl radical intermediate serving as a formal biaryl radical equivalent. This intermediate, formed by one-electron reduction of the cyclic iodonium salt, acts as the key intermediate driving the Minisci-type arylation reaction. The synthetic utility of this radical-based annulative π-extension method is highlighted by the preparation of an N-doped heptacyclic nanographene fragment through fourfold C-H arylation.

Annulative π-Extension of Unactivated Benzene Derivatives through Nondirected C-H Arylation.

Annulative π-extension chemistry provides a concise synthetic route to polycyclic arenes. Herein, we disclose a nondirected annulation approach of unactivated simple arenes. The palladium-catalyzed 2-fold C-H arylation event facilitates tandem C-C linkage relays to furnish fully benzenoid triphenylene frameworks using cyclic diaryliodonium salts. The inseparable regioisomeric mixture of 1- and 2-methyltriphenylenes is identified by the combined analysis of ion mobility-mass spectrometry, gas-phase infrared spectroscopy, and molecular simulation studies.

Chemoselective Trifluoroethylation Reactions of Quinazolinones and Identification of Photostability.

Herein, we report chemoselective trifluoroethylation routes of unmasked 2-arylquinazolin-4(3 H)-ones using mesityl(2,2,2-trifluoroethyl)iodonium triflate at room temperature. Homologous C-, O-, and N-functionalized subclasses are accessed in a straightforward manner with a wide substrate scope. These chemoselective branching events are driven by Pd-catalyzed ortho-selective C-H activation at the pendant aryl ring and base-promoted reactivity modulation of the amide group, leveraging the intrinsic directing capability and competing pronucleophilicity of the quinazolin-4(3 H)-one framework. Furthermore, outstanding photostability of the quinazolin-4(3 H)-one family associated with nonradiative decay is presented.

Organic Radical-Linked Covalent Triazine Framework with Paramagnetic Behavior.

The production of multifunctional pure organic materials that combine different sizes of pores and a large number of electron spins is highly desirable due to their potential applications as polarizers for dynamic nuclear polarization-nuclear magnetic resonance and as catalysts and magnetic separation media. Here, we report a polychlorotriphenylmethyl radical-linked covalent triazine framework (PTMR-CTF). Two different sizes of micropores were established by N2 sorption and the presence of unpaired electrons (carbon radicals) by electron spin resonance and superconducting quantum interference device-vibrating sample magnetometer analyses. Magnetization measurements demonstrate that this material exhibits spin-half paramagnetism with a spin concentration of ∼2.63 × 1023 spins/mol. We also determined the microscopic origin of the magnetic moments in PTMR-CTF by investigating its spin density and electronic structure using density functional theory calculations.

Nickel-Catalyzed Azide-Alkyne Cycloaddition To Access 1,5-Disubstituted 1,2,3-Triazoles in Air and Water.

Transition-metal-catalyzed or metal-free azide-alkyne cycloadditions are methods to access 1,4- or 1,5-disubstituted 1,2,3-triazoles. Although the copper-catalyzed cycloaddition to access 1,4-disubstituted products has been applied to biomolecular reaction systems, the azide-alkyne cycloaddition to access the complementary 1,5-regioisomers under aqueous and ambient conditions remains a challenge due to limited substrate scope or moisture-/air-sensitive catalysts. Herein, we report a method to access 1,5-disubstituted 1,2,3-triazoles using a Cp2Ni/Xantphos catalytic system. The reaction proceeds both in water and organic solvents at room temperature. This protocol is simple and scalable with a broad substrate scope including both aliphatic and aromatic substrates. Moreover, triazoles attached with carbohydrates or amino acids are prepared via this cycloaddition.

Formation and reactivity of an (alkene)peroxoiridium(III) intermediate supported by an amidinato ligand.

An Ir(I) complex of an acetamidinato ligand was synthesized by reaction of N,N'-diphenylacetamidine, PhN[double bond, length as m-dash]C(Me)NHPh, with either MeLi and [{Ir(cod)}2(µ-Cl)2] or [{Ir(cod)}2(µ-OMe)2] and was characterized by X-ray crystallography as a mononuclear complex, [Ir{PhNC(Me)NPh}(cod)] (1; where cod = 1,5-cyclooctadiene). Reaction of 1 with CO afforded a dinuclear carbonyl complex, [{Ir(CO)2}2{µ-PhNC(Me)NPh-κN:κN'}2] (2), as indicated by EI mass spectrometry and solution- and solid-state IR spectroscopy [νCO (n-pentane) = 2067, 2034 and 1992 cm(-1)]. Activation of O2 by 1 in solution at 20 °C was irreversible and produced an (alkene)peroxoiridium(iii) intermediate, [Ir{PhNC(Me)NPh}(cod)(O2)] (3), which was characterized by one- and two-dimensional NMR techniques and IR spectroscopy (for 3, νOO = 860 cm(-1); for 3-(18)O2, νOO = 807 cm(-1)). Complex 3 oxidized PPh3 to OPPh3, and its decay in the absence of added substrates followed by reaction with cod yielded 4-cycloocten-1-one and a minor amount of 1. In comparison with the results for the previously reported guanidinato complex [Ir{PhNC(NMe2)NPh}(cod)(O2)] (4), the formation of 3 and its reaction with PPh3 are significantly faster, indicating considerable ligand effects in these reactions.

Guanidinato complexes of iridium: ligand-donor strength, O2 reactivity, and (alkene)peroxoiridium(III) intermediates.

A series of seven [Ir{ArNC(NR2)NAr}(cod)] complexes (1a-1g; where R = Me or Et; Ar = Ph, 4-MeC6H4, 4-MeOC6H4, 2,6-Me2C6H3, or 2,6-(i)Pr2C6H3; and cod = 1,5-cyclooctadiene) were synthesized by two different methods from the neutral guanidines, ArN═C(NR2)NHAr, using either MeLi and [{Ir(cod)}2(µ-Cl)2] or [{Ir(cod)}2(µ-OMe)2]. Reaction of 1a-1g with CO produced the corresponding [Ir{ArNC(NR2)NAr}(CO)2] complexes (2a-2g), which were characterized by NMR and solution- and solid-state IR spectroscopy. Complexes 1b (R = Et, Ar = Ph), 1d (R = Et, Ar = 4-MeC6H4), 1f (R = Me, Ar = 2,6-Me2C6H3), and 2b (R = Et, Ar = Ph) were characterized by X-ray crystallography as mononuclear complexes with a guanidinato-κ(2)N,N' ligand and a cod or two CO ligands coordinated to the Ir center in a distorted square-planar environment. On the basis of the CO stretching frequencies of 2a-2g [avg. νCO (n-pentane) = 2016-2019 cm(-1)] and the alkene (13)C chemical shifts of 1a-1g [δ((13)CC═C) = 58.7-61.0 ppm], the donor strength of the guanidinato ligands was evaluated and compared to that of related monoanionic ligands. Reaction of 1a-1g in solution with O2 at 20 °C afforded (alkene)peroxoiridium(III) intermediates, [Ir{ArNC(NR2)NAr}(cod)(O2)] (3). The steric properties of the supporting ligand play a decisive role in O2 binding in that complexes without ortho substituents react largely irreversibly with O2 (1a-1e; where Ar = Ph, 4-MeC6H4 or 4-MeOC6H4), whereas complexes with ortho substituents exhibit fully reversible O2 binding (1f and 1g; where Ar = 2,6-Me2C6H3 or 2,6-(i)Pr2C6H3). Complexes 3a-3f were characterized by (1)H NMR and IR spectroscopy (νOO = 857-872 cm(-1)). Decay of the new intermediates and subsequent reaction with cod produced 4-cycloocten-1-one and the respective Ir(I) precursor.

Evidence for a reactive (alkene)peroxoiridium(III) intermediate in the oxidation of an alkene complex with O2.

An (alkene)peroxoiridium(III) complex, [Ir(L)(cod)(O(2))] [where LH = PhN=C(NMe(2))NHPh and cod = 1,5-cyclooctadiene], was identified as an intermediate in the reaction of the Ir(I) precursor [Ir(L)(cod)] with O(2) and characterized by spectroscopic methods. Decay of the intermediate and further reaction with 1,5-cyclooctadiene produced 4-cycloocten-1-one.

Reaction of an oxoiron(IV) complex with nitrogen monoxide: oxygen atom or oxide(•1-) ion transfer?

Reaction of [FeO(tmc)(OAc)](+) with the free radical nitrogen monoxide afforded a mixture of two Fe(II) complexes, [Fe(tmc)(OAc)](+) and [Fe(tmc)(ONO)](+) (where tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and AcO(-) = acetate anion). The amount of nitrite produced in this reaction (ca. 1 equiv with respect to Fe) was determined by ESI mass spectrometry after addition of (15)N-enriched NaNO(2). In contrast to oxygen atom transfer to PPh(3), the NO reaction of [FeO(tmc)(OAc)](+) proceeds through an Fe(III) intermediate that was identified by UV-vis-NIR spectroscopy and ESI mass spectrometry and whose decay is dependent on the concentration of methanol. The observations are consistent with a mechanism involving oxide(•1-) ion transfer from [FeO(tmc)(OAc)](+) to NO to form an Fe(III) complex and NO(2)(-), followed by reduction of the Fe(III) complex. Competitive binding of AcO(-) and NO(2)(-) to Fe(II) then leads to an equilibrium mixture of two Fe(II)(tmc) complexes. Evidence for the incorporation of oxygen from the oxoiron(IV) complex into NO(2)(-) was obtained from an (18)O-labeling experiment. The reported reaction serves as a synthetic example of the NO reactivity of biological oxoiron(IV) species, which has been proposed to have physiological functions such as inhibition of oxidative damage, enhancement of peroxidase activity, and NO scavenging.

Reaction of a redox-active ligand complex of nickel with dioxygen probes ligand-radical character.

Bis(imino)pyridine complex [Ni{2,6-(ArN=CMe)(2)C(5)H(3)N}Cl] (where Ar = 2,6-(i)Pr(2)C(6)H(3)) was synthesized by reduction of the corresponding dichloride complex and characterized as a ligand-radical complex of Ni(II). Reaction of this complex with O(2) caused intraligand C-C bond cleavage to afford the Ni complex of the new iminoethylpyridylcarboxamidato ligand, which also was isolated as the corresponding carboxamide, 6-(ArN=CMe)C(5)H(3)N-2-C(O)NHAr. This reaction serves as an example of small-molecule activation effected directly at the redox-active bis(imino)pyridine ligand without an overall oxidation state change at the Ni center.

Stabilization of iridium(IV) by monoanionic dialkyldiarylguanidinato ligands.

Electron-rich tris(guanidinato) complexes of Ir(III), [Ir{ArNC(NR(2))NAr}(3)] (where R = Me or Et; Ar = Ph or 4-MeC(6)H(4)), were synthesized from the respective [Ir{ArNC(NR(2))NAr}(C(8)H(14))(2)] precursors (C(8)H(14) = cis-cyclooctene), are air-sensitive, and can be electrochemically oxidized in two one-electron transfer steps. The first electron transfer is reversible and occurs at much lower potentials than typical for Ir(III). Chemical oxidation by [FeCp(2)]PF(6) afforded isolable, paramagnetic Ir(IV) compounds, [Ir{ArNC(NR(2))NAr}(3)]PF(6), which were characterized by analytical and spectroscopic methods and a single-crystal structure determination, demonstrating that Ir(IV) is accessible in a nitrogen-donor ligand environment.

Synthesis, characterization, and O2 reactivity of iridium(I) complexes supported by guanidinato ligands.

Mononuclear [Ir{ArNC(NR(2))NAr}(C(8)H(12))] complexes (where R = Me or Et; Ar = Ph, 4-MeC(6)H(4), or 2,6-Me(2)C(6)H(3); and C(8)H(12) = 1,5-cyclooctadiene) were synthesized from the neutral N,N-dialkyl-N',N''-diarylguanidines via deprotonation and transmetalation. As confirmed by single-crystal structure determinations, the guanidinato(1-) ligands coordinate the low-valent d(8) Ir(I) center in an N,N'-chelating binding mode, and the (13)C NMR chemical shifts of the alkene carbon atoms establish that these ligands function as stronger donors than related monoanionic, bidentate nitrogen-based ligands. In the reactions of the complexes with O(2), the observed reactivity trends correlate with the electronic and steric influences of the substituents of the guanidinato ligands.

Axial ligand effects on the geometric and electronic structures of nonheme oxoiron(IV) complexes.

A series of complexes [Fe(IV)(O)(TMC)(X)](+) (where X = OH(-), CF3CO2(-), N3(-), NCS(-), NCO(-), and CN(-)) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex [Fe(IV)(O)(TMC)(NCMe)](2+) (TMC = tetramethylcyclam) with the appropriate NR4X salts. Because of the topology of the TMC macrocycle, the [Fe(IV)(O)(TMC)(X)](+) series represents an extensive collection of S = 1 oxoiron(IV) complexes that only differ with respect to the ligand trans to the oxo unit. Electronic absorption, Fe K-edge X-ray absorption, resonance Raman, and Mossbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 Fe(IV)=O unit, namely, (i) the near-IR absorption properties, (ii) X-ray absorption pre-edge intensities, and (iii) quadrupole splitting parameters, are strongly dependent on the identity of the trans ligand. However, on the basis of extended X-ray absorption fine structure data, most [Fe(IV)(O)(TMC)(X)](+) species have Fe=O bond lengths similar to that of [Fe(IV)(O)(TMC)(NCMe)](2+) (1.66 +/- 0.02 A). The mechanisms by which the trans ligands perturb the Fe(IV)=O unit were probed using density functional theory (DFT) computations, yielding geometric and electronic structures in good agreement with our experimental data. These calculations revealed that the trans ligands modulate the energies of the Fe=O sigma- and pi-antibonding molecular orbitals, causing the observed spectroscopic changes. Time-dependent DFT methods were used to aid in the assignment of the intense near-UV absorption bands found for the oxoiron(IV) complexes with trans N3(-), NCS(-), and NCO(-) ligands as X(-)-to-Fe(IV)=O charge-transfer transitions, thereby rationalizing the resonance enhancement of the nu(Fe=O) mode upon excitation of these chromophores.

Spectroscopic and quantum chemical studies on low-spin FeIV=O complexes: Fe-O bonding and its contributions to reactivity.

High-valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes and have been structurally defined in model systems. Variable-temperature magnetic circular dichroism (VT-MCD) spectroscopy has been used to evaluate the electronic structures and in particular the Fe-O bonds of three FeIV=O (S = 1) model complexes, [FeIV(O)(TMC)(NCMe)]2+, [FeIV(O)(TMC)(OC(O)CF3)]+, and [FeIV(O)(N4Py)]2+. These complexes are characterized by their strong and covalent Fe-O pi-bonds. The MCD spectra show a vibronic progression in the nonbonding --> pi* excited state, providing the Fe-O stretching frequency and the Fe-O bond length in this excited state and quantifying the pi-contribution to the total Fe-O bond. Correlation of these experimental data to reactivity shows that the [FeIV(O)(N4Py)]2+ complex, with the highest reactivity toward hydrogen-atom abstraction among the three, has the strongest Fe-O pi-bond. Density functional calculations were correlated to the data and support the experimental analysis. The strength and covalency of the Fe-O pi-bond result in high oxygen character in the important frontier molecular orbitals (FMOs) for this reaction, the unoccupied beta-spin d(xz/yz) orbitals, that activates these for electrophilic attack. An extension to biologically relevant FeIV=O (S = 2) enzyme intermediates shows that these can perform electrophilic attack reactions along the same mechanistic pathway (pi-FMO pathway) with similar reactivity but also have an additional reaction channel involving the unoccupied alpha-spin d(z2) orbital (sigma-FMO pathway). These studies experimentally probe the FMOs involved in the reactivity of FeIV=O (S = 1) model complexes resulting in a detailed understanding of the Fe-O bond and its contributions to reactivity.

X-ray absorption spectroscopic studies of high-spin nonheme (alkylperoxo)iron(III) intermediates.

The reactions of iron(II) complexes [Fe(T(pt-Bu,i-Pr))(OH)] (1a, Tp(t-Bu,i-Pr) = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate), [Fe(6-Me2BPMCN)(OTf)2] (1b, 6-Me2BPMCN = N,N'-bis((2-methylpyridin-6-yl)methyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane), and [Fe(L8Py2)(OTf)](OTf) (1c, L8Py2 = 1,5-bis(pyridin-2-ylmethyl)-1,5-diazacyclooctane) with tert-BuOOH give rise to high-spin FeIII-OOR complexes. X-ray absorption spectra (XAS) of these high-spin species show characteristic features, distinct from those of low-spin Fe-OOR complexes (Rohde, J.-U.; et al. J. Am. Chem. Soc. 2004, 126, 16750-16761). These include (1) an intense 1s --> 3d preedge feature, with an area around 20 units, (2) an edge energy, ranging from 7122 to 7126 eV, that is affected by the coordination environment, and (3) a 1.86-1.96 A Fe-OOR bond, compared to the 1.78 A Fe-OOR bond in low-spin complexes. These unique features likely arise from a flexible first coordination sphere in those complexes. The difference in Fe-OOR bond length may rationalize differences in reactivity between low-spin and high-spin FeIII-OOR species.

XAS characterization of a nitridoiron(IV) complex with a very short Fe-N bond.

X-ray absorption spectroscopy has been used to characterize the novel nitridoiron(IV) units in two [PhBPR3]Fe(N) complexes (R=iPr and CyCH2) and obtain direct spectroscopic evidence for a very short Fe-N distance. The distance of 1.51-1.55 A reflects the presence of an FeN triple bond in accord with the observed FeN vibration observed for one of these species (nuFeN=1034 cm(-1)). This highly covalent bonding interaction results in the appearance of an unusually intense pre-edge peak, whose estimated area of 100(20) units is much larger than those of the related tetrahedral complexes with FeI-N2-FeI, FeII-NPh2, and FeIIINAd motifs, and those of recently described six-coordinate FeVN and FeVIN complexes. The observation that the FeIV-N distances of two [PhBPR3]Fe(N) complexes are shorter than the FeIV-O bond lengths of oxoiron(IV) complexes may be rationalized on the basis of the greater pi basicity of the nitrido ligand than the oxo ligand and a lower metal coordination number for the Fe(N) complex.

Nonheme oxoiron(IV) complexes of tris(2-pyridylmethyl)amine with cis-monoanionic ligands.

Treatment of [Fe(IV)(O)(TPA)(NCMe)](CF3SO3)2 [TPA, N,N,N-tris(2-pyridylmethyl)amine] with 3 equiv of NR4X (X = CF3CO2, Cl, or Br) in MeCN at -40 degrees C affords a series of metastable [Fe(IV)(O)(TPA)(X)]+ complexes. Some characteristic features of the S = 1 oxoiron(IV) unit are quite insensitive to the ligand substitution in the equatorial plane, namely, the Fe-O distances (1.65-1.66 A), the energy ( approximately 7114.5 eV) and intensity [25(2) units] of the 1s-to-3d transition in the X-ray absorption spectra, and the Mössbauer isomer shifts (0.01-0.06 mm.s(-1)) and quadrupole splittings (0.92-0.95 mm.s(-1)). The coordination of the anionic X ligand, however, is evidenced by red shifts of the characteristic near-IR ligand-field bands (720-800 nm) and spectroscopic observation of the bound anion by (19)F NMR for X = CF3CO2 and by EXAFS analysis for X = Cl (r(Fe-Cl) = 2.29 A) and Br (r(Fe-Br) = 2.43 A). Density functional theory calculations yield Mössbauer parameters and bond lengths in good agreement with the experimental data and produce excited-state energies that follow the trend observed in the ligand-field bands. Despite mitigating the high effective charge of the iron(IV) center, the substitution of the MeCN ligand with monoanionic ligands X- decreases the thermal stability of [Fe(IV)(O)(TPA)]2+ complexes. These anion-substituted complexes model the cis-X-Fe(IV)=O units proposed in the mechanisms of oxygen-activating nonheme iron enzymes.

Self-hydroxylation of perbenzoic acids at a nonheme iron(II) center.

Treatment of mononuclear nonheme iron(II) complexes bearing two cis-labile sites with perbenzoic acids results in the self-hydroxylation of the aromatic ring to form the corresponding iron(III)-salicylate complexes through an intramolecular oxo-transfer process.