Synthetic, spectroscopic, and DFT studies of iron complexes with iminobenzo(semi)quinone ligands: implications for o-aminophenol dioxygenases

Observation of oxygenated intermediates in functional mimics of aminophenol dioxygenase

Aminophenol dioxygenases (APDO) are mononuclear nonheme iron enzymes that utilize dioxygen (O2) to catalyze the conversion of o-aminophenols to 2-picolinic acid derivatives in metabolic pathways. This study describes the synthesis and O2 reactivity of two synthetic models of substrate-bound APDO: [FeII(TpMe2)(tBu2APH)] (1) and [FeII(TpMe2)(tBuAPH)] (2), where TpMe2 = hydrotris(3,5-dimethylpyrazole-1-yl)borate, tBu2APH = 4,6-di-tert-butyl-2-aminophenolate, and tBuAPH2 = 4-tert-butyl-2-aminophenolate. Both Fe(II) complexes behave as functional APDO mimics, as exposure to O2 results in oxidative CC bond cleavage of the o-aminophenolate ligand. The ring-cleaved products undergo spontaneous cyclization to give substituted 2-picolinic acids, as verified by 1H NMR spectroscopy, mass spectrometry, and X-ray crystallography. Reaction of the APDO models with O2 at low temperature reveals multiple intermediates, which were probed with UV-vis absorption, electron paramagnetic resonance (EPR), Mössbauer (MB), and resonance Raman (rRaman) spectroscopies. The most stable intermediate at -70 °C in THF exhibits multiple isotopically-sensitive features in rRaman samples prepared with 16O2 and 18O2, confirming incorporation of O2-derived atom(s) into its molecular structure. Insights into the geometric structures, electronic properties, and spectroscopic features of the observed intermediates were obtained from density functional theory (DFT) calculations. Although functional APDO models have been previously reported, this is the first time that an oxygenated ligand-based radical has been detected and spectroscopically characterized in the ring-cleaving mechanism of a relevant synthetic system.

Reversible Dioxygen Binding to Co(II) Complexes with Noninnocent Ligands

A series of mononuclear Co(II) complexes with noninnocent (redox-active) ligands are prepared that exhibit metal-ligand cooperativity during the reversible binding of O2. The complexes have the general formula, [CoII(LS,N)(TpR2)] (R = Me, Ph), where LS,N is a bidentate o-aminothiophenolate and TpR2 is a hydrotris(pyrazol-1-yl)borate scorpionate with R-substituents at the 3- and 5-positions. Exposure to O2 at room temperature results in one-electron oxidation and deprotonation of LS,N. The oxidized derivatives possess substantial "singlet diradical" character arising from antiferromagnetic coupling between an iminothiosemiquinonate (ITSQ•-) ligand radical and a low-spin Co(II) ion. The [CoII(TpMe2)(X2ITSQ)] complexes, where X = H or tBu, coordinate O2 reversibly at reduced temperatures to provide Co/O2 adducts. The O2 binding reactions closely resemble those previously reported by our group (Kumar et al., J. Am. Chem. Soc. 2019,141, 10984-10987) for the related complexes [CoII(TpMe2)(tBu2SQ)] and [CoII(TpMe2)(tBu2ISQ)], where tBu2(I)SQ represents 4,6-di-tert-butyl-(2-imino)semiquinonate radicals. In each case, the oxygenation reaction proceeds via the addition of O2 to both the cobalt ion and the ligand radical, generating metallocyclic cobalt(III)-alkylperoxo structures. Thermodynamic measurements elucidate the relationship between O2 affinity and redox potentials of the (imino)(thio)semiquinonate radicals, as well as energetic differences between these reactions and conventional metal-based oxygenations. The results highlight the utility and versatility of noninnocent ligands in the design of O2-absorbing compounds.

Aerobic Oxidation Reactivity of Well-Defined Cobalt(II) and Cobalt(III) Aminophenol Complexes

This paper describes the synthesis and reactivity studies of three cobalt complexes bearing aminophenol-derived ligands without nitrogen substitution: Co^II(^tBu2APH)₂(^tBu2AP)₂ (1), Co^III₂(^tBu2APH)₂(^tBu2AP)₂(μ-^tBu2BAP)₂ (2), and Co^III(^tBu2AP)₃ (3), where ^tBu2APH = 2-amino-4,6-di-tert-butylphenol, ^tBu2AP = 2-amino-4,6-di-tert-butylphenolate, and μ-^tBu2BAP = bridging 2-amido-4,6-di-tert-butylphenolate. Stoichiometric reactivity studies of these well-defined complexes demonstrate the catalytic competency of both cobalt(II) and cobalt(III) complexes in the aerobic oxidative cyclization of ^tBu2APH with tert-butylisonitrile. Reactions with O₂ reveal the aerobic oxidation of the cobalt(II) complex 1 to generate the cobalt(III) species 2 and 3. UV-visible time-course studies and electron paramagnetic resonance spectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent the first example of well-defined cobalt aminophenol complexes that participate in catalytic aerobic oxidation reactions and highlight a key role for a ligand radical in the oxidation sequence.

Tuning the stereoelectronic factors of iron(II)-2-aminophenolate complexes for the reaction with dioxygen: oxygenolytic C-C bond cleavage vs. oxidation of complex

Oxidative C-C bond cleavage of 2-aminophenols mediated by transition metals and dioxygen is a topic of great interest. While the oxygenolytic C-C bond cleavage reaction relies on the inherent redox non-innocent property of 2-aminophenols, the metal complexes of 2-aminophenolates often undergo 1e-/2e- oxidation events (metal or ligand oxidation), instead of the direct addition of O2 for subsequent C-C bond cleavage. In this work, we report the isolation, characterization and dioxygen reactivity of a series of ternary iron(ii)-2-aminophenolate complexes [(TpPh,Me)FeII(X)], where X = 2-amino-4-tert-butylphenolate (4-tBu-HAP) (1); X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-tBu-HAP) (2); X = 2-amino-4-nitrophenolate (4-NO2-HAP)(3); and X = 2-anilino-4,6-di-tert-butylphenolate (NH-Ph-4,6-di-tBu-HAP) (4) supported by a facial tridentate nitrogen donor ligand (TpPh,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate). Another facial N3 ligand (TpPh2 = hydrotris(3,5-diphenyl-pyrazol-1-yl)borate) has been used to isolate an iron(ii)-2-anilino-4,6-di-tert-butylphenolate complex (5) for comparison. Both [(TpPh,Me)FeII(4-tBu-HAP)] (1) and [(TpPh,Me)FeII(4,6-di-tBu-HAP)] (2) undergo regioselective oxidative aromatic ring fission reaction of the coordinated 2-aminophenols to the corresponding 2-picolinic acids in the reaction with dioxygen. In contrast, complex [(TpPh,Me)FeII(4-NO2-HAP)] (3) displays metal based oxidation to form an iron(iii)-2-amidophenolate complex. Complexes [(TpPh,Me)FeII(NH-Ph-4,6-di-tBu-HAP)] (4) and [(TpPh2)FeII(NH-Ph-4,6-di-tBu-HAP)] (5) react with dioxygen to undergo 2e- oxidation with the formation of the corresponding iron(iii)-2-iminobenzosemiquinonato radical species implicating the importance of the -NH2 group in directing the C-C bond cleavage reactivity of 2-aminophenols. The systematic study presented in this work unravels the effect of the electronic and structural properties of the redox non-innocent 2-aminophenolate ring and the supporting ligand on the C-C bond cleavage reactivity vs. the metal/ligand oxidation of the complexes. The study further reveals that proper modulation of the stereoelectronic factors enables us to design a well synchronised proton transfer (PT) and dioxygen binding events for complexes 1 and 2 that mimic the structure and function of the nonheme enzyme 2-aminophenol-1,6-dioxygenase (APD).

Probing the electronic structure of [Ru(L1)2]Z (z = 0, 1+ and 2+) (H2L1: a tridentate 2-aminophenol derivative) complexes in three ligand redox levels

Aerobic reaction between [RuII(DMSO)4Cl2], a redox-active 2-aminophenol-based ligand (H2L1: 2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) and Et3N in MeOH under refluxing conditions afforded a purple complex [Ru(L1)2] (S = 0). Structural analysis reveals that the tridentate ligand coordinates in a mer conformation providing a distorted octahedral RuN2O2S2 coordination. Cyclic voltammetry on 1 in CH2Cl2 reveals the accessability of the monocation, dication and monoanion forms. Reddish purple monocation [Ru(L1)2](PF6)·CH2Cl2 ([1OX1](PF6)·CH2Cl2; S = 1/2) and green dication [Ru(L1)2](BF4)2·H2O ([1OX2](BF4)2·H2O; S = 0) have been isolated through the chemical oxidation of 1 in CH2Cl2 by [FeIII(η5-C5H5)2](PF6) and AgBF4, respectively. A structural analysis of the single crystals of the monocation and the dication with the compositions [1OX1](PF6)·CH2Cl2·H2O (2) and [1OX2](BF4)2·1.7H2O (3), respectively, has been done. Metrical (metal-ligand and ligand backbone) parameters, values of metrical oxidation states of coordinated ligands, 1H NMR spectra of 1 and [1OX2](BF4)2·H2O, EPR spectra of [1OX1](PF6)·CH2Cl2, X-ray photoelectron and UV-VIS-NIR spectra of 1-3, spin population analysis from broken-symmetry (BS) density functional theory (DFT) calculations and quasi-restricted orbital (QRO) analysis have allowed us to assign the electronic structure of the complexes. The complexes exhibit highly covalent metal-ligand interactions. The electronic states of 1, [1OX1]1+ and [1OX2]2+ are best described as [RuII{(LISQ)˙-}2] ↔ [RuIII{(LAP)2-}{(LISQ)˙-}] (S = 0), [RuIII{(LISQ)˙-}2]1+ (S = 1/2) and [RuII{(LIBQ)0}2]2+ ↔ [RuIII{(LISQ)˙-}{(LIBQ)0}]2+ (S = 0), respectively. Notably, all redox processes are ligand-centred. Absorption spectral properties have been rationalized based on time-dependent (TD)-DFT calculations. For 1, the appearance of an IVCT band at 1100 nm supports its Class II-III (borderline) ligand-based mixed-valence character.

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures

The purpose of this Viewpoint is to provide a broad-ranging update of advances in the coordination chemistry of redox-active (noninnocent) 2-aminophenolates, with emphasis on two ligand backbone structural parameters, the average of C-O and C-N (C-O/N) bond distances and Δa values, signifying the degree of bond-length alternation in the six-membered ring, in order to identify the redox level of the coordinated ligands. In the absence of magnetic, spectroscopic, and redox results, it has been established that it is possible to assign the electronic ground state of a coordination complex of 2-aminophenolates with consideration of charge, metal-ligand bond distances, and two very promising ligand backbone structural parameters. From a closer look at the sensitive ligand backbone metrical parameters of a diversified group of about 120 transition-metal complexes, a few very useful generalizations have been made.

Molecular Insights into the Ligand-Based Six-Proton- and Six-Electron-Transfer Processes Between Tris-ortho-Phenylenediamines and Tris-ortho-Benzoquinodiimines

The global demand for energy and the concerns over climate issues renders the development of alternative renewable energy sources such as hydrogen (H₂ ) important. A high-spin (hs) Fe^II complex with o-phenylenediamine (opda) ligands, [Fe^II (opda)₃ ]²⁺ (hs-[6R]²⁺ ), was reported showing photochemical H₂ evolution. In addition, a low-spin (ls) [Fe^II (bqdi)₃ ]²⁺ (bqdi: o-benzoquinodiimine) (ls-[0R]²⁺ ) formation by O₂ oxidation of hs-[6R]²⁺ , accompanied by ligand-based six-proton and six-electron transfer, revealed the potential of the complex with redox-active ligands as a novel multiple-proton and -electron storage material, albeit that the mechanism has not yet been understood. This paper reports that the oxidized ls-[0R][PF₆ ]₂ can be reduced by hydrazine giving ls-[Fe^II (opda)(bqdi)₂ ][PF₆ ]₂ (ls-[2R][PF₆ ]₂ ) and ls-[Fe^II (opda)₂ (bqdi)][PF₆ ]₂ (ls-[4R][PF₆ ]₂ ) with localized ligand-based proton-coupled mixed-valence (LPMV) states. The first isolation and characterization of the key intermediates with LPMV states offer unprecedented molecular insights into the design of photoresponsive molecule-based hydrogen-storage materials.

DFT-Assisted Spectroscopic Studies on the Coordination of Small Ligands to Palladium: From Isolated Ions to Nanoparticles

A combination of experimental spectroscopies (UV-vis and Fourier-transform infrared) and computational modeling was used to investigate the coordination of small ligands (aminopropanol and propanediol) to Pd species during the metal nanoparticle formation process. Differences emerged between O- (propanediol) and N-containing (aminopropanol) ligands. In particular, a strong interaction between the NH amino group and Pd²⁺ ions could be inferred on the basis of spectroscopic evidences, which was corroborated by theoretical simulations, which confirmed the preferential coordination of aminopropanol through the NH group. This interaction seems to potentially cause the aminopropanol ligand to control the particle shape through a selective blocking of Pd(100) facets, which promote the growth on the Pd(111) facets.

Iron-Catalyzed Reductive Coupling of Alkyl Iodides with Alkynes To Yield cis-Olefins: Mechanistic Insights from Computation

In a recent study, a new procedure for Z-selective olefin synthesis by reductive coupling of alkyl iodides with terminal alkynes in the presence of iron salts is described. This transformation is representative of many newly developed synthetic routes through the involvement of multiple species and phases, which makes mechanistic insight hard to obtain. Here, we report computational work aimed at exploring the possible reaction pathways. DFT calculations lead to two suggested routes, one involving C-I reduction by metallic zinc and radical addition to the alkyne and the other involving addition of two reduced iron species to the alkyne bond followed by reductive elimination. Comparison to experimental results as well as kinetic modeling is used to discuss the likelihood of these and related mechanisms.

Cobalt Superoxo and Alkylperoxo Complexes Derived from Reaction of Ring-Cleaving Dioxygenase Models with O2

The syntheses and O₂ reactivities of active-site models of cobalt-substituted ring-cleaving dioxygenases are presented. The pentacoordinate cobalt(II)-aminophenolate complex, [Co(Tp^Me2)(^tBu2APH)], gives rise to two distinct dioxygen adducts at reduced temperatures. The first is a paramagnetic (S = 1/2) cobalt(III)-superoxo species that was characterized with spectroscopic and computational techniques. The identity of the second Co/O₂ adduct was elucidated by X-ray crystallography, which revealed an unprecedented cobalt(III)-alkylperoxo structure generated by O₂ addition to the metal ion and ligand. These results provide synthetic precedents for proposed intermediates in the catalytic cycles of O₂-activating cobalt enzymes.

Six-coordinate [CoIII(L)2]z (z = 1-, 0, 1+) complexes of an azo-appended o-aminophenolate in amidate(2-) and iminosemiquinonate π-radical (1-) redox-levels: the existence of valence-tautomerism

Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = -0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50-300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1-3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233-313 K) 1H NMR spectral features of 1-3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2-}{(LISQ)}˙-] or [CoIII{(L1)2}˙3-] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN (3) [(LAP)2- and (LISQ)˙- represent the redox-level of coordinated ligands o-amidophenolate(2-) ion and o-iminobenzosemiquinonate(1-) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1-3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1-3 from experimental data. The origins of the observed UV-VIS-NIR absorptions for 1-3 have been assigned, based on time-dependent (TD)-DFT calculations.

Dioxygen Reactivity of an Iron Complex of 2-Aminophenol-Appended Ligand: Crystallographic Evidence of the Aromatic Ring Cleavage Product of the 2-Aminophenol Unit

2-Aminophenol appended pentadentate ligand H₂Gan^AP was synthesized by mixing equimolar amounts of 2-[bis(2-pyridylmethyl)aminomethyl]aniline (A) and 3,5-di-tert-butyl catechol in hexane in the presence of Et₃N under air. The ligand reacted with Fe(ClO₄)₂·6H₂O or Fe(ClO₄)₃·6H₂O in the presence of tetrabutylammonium perchlorate, and Et₃N under air and provided a μ₂ oxo-bridged dinuclear iron complex (1). X-ray single-crystal analysis of complex 1 revealed the presence of a furan derivative, resulting from the oxidative aromatic C-C bond cleavage product of 2-aminophenol derivative, in the coordination sphere of each iron center. Mechanistic investigation for the formation of complex 1 established that in the absence of molecular oxygen no oxidation of the appended 2-amidophenolate unit took place. An iron(III)-amidophenolate complex, formed initially, further reacted with molecular oxygen and caused oxidative aromatic C-C bond cleavage via a putative alkylperoxo species.

Oxygen activation by mononuclear Mn, Co, and Ni centers in biology and synthetic complexes

The active sites of metalloenzymes that catalyze O₂-dependent reactions generally contain iron or copper ions. However, several enzymes are capable of activating O₂ at manganese or nickel centers instead, and a handful of dioxygenases exhibit activity when substituted with cobalt. This minireview summarizes the catalytic properties of oxygenases and oxidases with mononuclear Mn, Co, or Ni active sites, including oxalate-degrading oxidases, catechol dioxygenases, and quercetin dioxygenase. In addition, recent developments in the O₂ reactivity of synthetic Mn, Co, or Ni complexes are described, with an emphasis on the nature of reactive intermediates featuring superoxo-, peroxo-, or oxo-ligands. Collectively, the biochemical and synthetic studies discussed herein reveal the possibilities and limitations of O₂ activation at these three "overlooked" metals.

Localized Mixed-Valence and Redox Activity within a Triazole-Bridged Dinucleating Ligand upon Coordination to Palladium

The new dinucleating redox-active ligand (L^H4 ), bearing two redox-active NNO-binding pockets linked by a 1,2,3-triazole unit, is synthetically readily accessible. Coordination to two equivalents of Pd^II resulted in the formation of paramagnetic (S=1/2 ) dinuclear Pd complexes with a κ² -N,N'-bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin-Day Class II mixed-valence within the redox-active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one-electron oxidation allowed for isolation of the corresponding cationic ligand-based diradical species. SQUID (super-conducting quantum interference device) measurements of these compounds revealed weak anti-ferromagnetic spin coupling between the two ligand-centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox-active ligand framework thus allows for efficient electronic communication between the two binding pockets.

Dehydrogenation of anhydrous methanol at room temperature by o-aminophenol-based photocatalysts

Dehydrogenation of anhydrous methanol is of great importance, given its ubiquity as an intermediate for the production of a large number of industrial chemicals. Since dehydrogenation of methanol is an endothermic reaction, heterogeneous or homogeneous precious-metal-based catalysts and high temperatures are usually required for this reaction to proceed. Here we report the photochemical dehydrogenation of anhydrous methanol at room temperature catalysed by o-aminophenol (apH2), o-aminophenolate (apH(-)) and the non-precious metal complex trans-[Fe(II)(apH)2(MeOH)2]. Under excitation at 289±10 nm and in the absence of additional photosensitizers, these photocatalysts generate hydrogen and formaldehyde from anhydrous methanol with external quantum yields of 2.9±0.15%, 3.7±0.19% and 4.8±0.24%, respectively, which are the highest values reported so far to the best of our knowledge. Mechanistic investigations reveal that the photo-induced formation of hydrogen radicals triggers the reaction.

Nickel(II) Complex of a Hexadentate Ligand with Two o-Iminosemiquinonato(1-) π-Radical Units and Its Monocation and Dication

Aerobic reaction of a hexadentate redox-active o-aminophenol-based ligand, H4L(3) = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino(diphenyldithio)-ethane, in CH3OH with Ni(II)(O2CCH3)2·4H2O and Et3N afforded isolation of a reddish-brown crystalline solid [Ni(L(3))] 1. Cyclic voltammetry (CV) experiment exhibits two oxidative responses at E1/2 = 0.09 and 0.53 V vs SCE (saturated calomel electrode). Chemical oxidation of 1 in air by [Fe(III)(η(5)-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of one-electron oxidized species [1](1+) as purple [1][PF6]·CH2Cl2 and two-electron oxidized species [1](2+) as dark purple [1][BF4]2·CH2Cl2, respectively. X-ray crystallographic analysis at 100(2) K unambiguously established that the ligand is present in [Ni(II){(L(ISQ)O,N)(•-)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}] 1, [Ni(II){(L(IBQ)O,N)(0)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}][PF6]·CH2Cl2, and [Ni(II){(L(IBQ)O,N)(0)}{(L(IBQ)O,N)(0)}{(LS,S)(0)}][BF4]2·CH2Cl2, as monoanionic o-iminosemiquinonate(1-) π-radical (Srad = 1/2) (L(ISQ))(•-) and neutral o-iminoquinone (L(IBQ))(0) redox-levels. Complexes 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 possess an S = 2, 3/2, and 1 ground-state, respectively, established by temperature-dependent (2-300 K) magnetic behavior of 1 and [1][PF6]·CH2Cl2, and a μeff value of [1][BF4]2·CH2Cl2 at 300 K. Both 1 and [1][PF6]·CH2Cl2 exhibit ferromagnetic exchange-coupling between the two electrons of Ni(II) and two/one ligand π-radicals, respectively. The redox processes are shown to be ligand-based. Spectroscopic and redox properties, and density functional theory (DFT) calculations at the CAM-B3LYP-level of theory adequately describe the electronic structure of 1, [1](1+), and [1](2+). The observed UV-vis-NIR absorptions for 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 have been assigned, based on time-dependent (TD)-DFT calculations.

Substrate-dependent aromatic ring fission of catechol and 2-aminophenol with O2 catalyzed by a nonheme iron complex of a tripodal N4 ligand

The catalytic reactivity of an iron(ii) complex [(TPA)Fe(II)(CH3CN)2](2+) (1) (TPA = tris(2-pyridylmethyl)amine) towards oxygenative aromatic C-C bond cleavage of catechol and 2-aminophenol is presented. Complex 1 exhibits catalytic and regioselective C-C bond cleavage of 3,5-di-tert-butylcatechol (H2DBC) to form intradiol products, whereas it catalyzes extradiol-type C-C bond cleavage of 2-amino-4,6-di-tert-butylphenol (H2AP). The catalytic reactions are found to be pH-dependent and the complex exhibits maximum turnovers at pH 5 in acetonitrile-phthalate buffer. An iron(iii)-catecholate complex [(TPA)Fe(III)(DBC)](+) (2) is formed in the ring cleavage of catechol. In the extradiol-type cleavage of H2AP, an iron(iii)-2-iminobenzosemiquinonate complex [(TPA)Fe(III)(ISQ)](2+) (3) (ISQ = 4,6-di-tert-butyl-2-iminobenzosemiquinonate radical anion) is observed in the reaction pathway. This work shows the importance of the nature of 'redox non-innocent' substrates in governing the mode of ring fission reactivity.

Radical non-radical states of the [Ru(PIQ)] core in complexes (PIQ = 9,10-phenanthreneiminoquinone)

9,10-Phenanthreneiminosemiquinonate anion radical (PIQ˙(-)) complexes of ruthenium of types trans-[Ru(II)(PIQ˙(-))(PPh3)2(CO)Cl] () and trans-[Ru(III)(PIQ˙(-))(PPh3)2Cl2] () are reported. Reactions of and with I2 afford trans-[Ru(III)(PIQ˙(-))(PPh3)2(CO)Cl](+)I3(-)·½CH2Cl2 ((+)I3(-)·½CH2Cl2) and trans-[Ru(PIQ˙(-))2(PPh3)2(μ-Cl)3](+)I3(-)·¼I2·¼toluene) ((+)I3(-)·¼I2·¼toluene), while the reaction of with Br2 yields a 9,10-phenanthreneiminoquinone (PIQ) complex of the type mer-[Ru(III)(PIQ)(PPh3)Br3]·½CH2Cl2 (·½CH2Cl2). In comparison, the reaction of trans-[Ru(III)(PQ˙(-))(PPh3)2Cl2] (PQ), a 9,10-phenanthrenequinone (PQ) analogue of affords only trans-[Ru(III)(PQ)(PPh3)2Cl2](+)Br3(-) ((+)Br3(-)). Considering the X-ray bond parameters, EPR spectra and the atomic spin densities obtained from the density functional theory (DFT) calculations, is defined as a PIQ˙(-) (average C-O/N and C-C lengths, 1.280(2) and 1.453(3) Å) complex of ruthenium(ii) while is a neutral PIQ (average C-O, C-N, C-C and C-O/N lengths, 1.248(7), 1.284(7), 1.485(8) and 1.266(7) Å) complex of the ruthenium(iii) ion. The single crystal X-ray bond parameters proposed that (+)I3(-) (average C-O/N and C-C lengths, 1.294(8) and 1.449(9) Å) and (average C-O/N and C-C lengths, 1.289(2) and 1.447(4) Å) are PIQ˙(-) complexes of ruthenium(iii), while the (+) ion (average C-O/N and C-C lengths, 1.288 ± 0.004 and 1.450 ± 0.017 Å) is a co-facial bi-octahedral complex of ruthenium(iii). In contrast, the (+) ion is a PQ complex of the ruthenium(iii) ion. EPR spectra and the calculated atomic spin densities authenticated that the (+) ion obtained after constant potential coulometric oxidation of is a PIQ complex of ruthenium(iii), while the (-) ion is a hybrid state of [Ru(II)(PIQ˙(-))] and [Ru(III)(PIQ(2-))] states. It is observed that the PIQ˙(-) state in which spin is more localized on the nitrogen (∼38% in and ∼35% in (-) ion) is stable and the coordination of the PIQ(2-) state is not observed in this study. Redox activities, UV-vis/NIR absorption spectra and their origins and the spectroelectrochemical measurements for → (+), → (-) and (+) → (2+) conversions are analyzed.

Structural and spectroscopic characterization of iron(II), cobalt(II), and nickel(II) ortho-dihalophenolate complexes: insights into metal-halogen secondary bonding

Metal complexes incorporating the tris(3,5-diphenylpyrazolyl)borate ligand (Tp(Ph2)) and ortho-dihalophenolates were synthesized and characterized in order to explore metal-halogen secondary bonding in biorelevant model complexes. The complexes Tp(Ph2)ML were synthesized and structurally characterized, where M was Fe(II), Co(II), or Ni(II) and L was either 2,6-dichloro- or 2,6-dibromophenolate. All six complexes exhibited metal-halogen secondary bonds in the solid state, with distances ranging from 2.56 Å for the Tp(Ph2)Ni(2,6-dichlorophenolate) complex to 2.88 Å for the Tp(Ph2)Fe(2,6-dibromophenolate) complex. Variable temperature NMR spectra of the Tp(Ph2)Co(2,6-dichlorophenolate) and Tp(Ph2)Ni(2,6-dichlorophenolate) complexes showed that rotation of the phenolate, which requires loss of the secondary bond, has an activation barrier of ~30 and ~37 kJ/mol, respectively. Density functional theory calculations support the presence of a barrier for disruption of the metal-halogen interaction during rotation of the phenolate. On the other hand, calculations using the spectroscopically calibrated angular overlap method suggest essentially no contribution of the halogen to the ligand-field splitting. Overall, these results provide the first quantitative measure of the strength of a metal-halogen secondary bond and demonstrate that it is a weak noncovalent interaction comparable in strength to a hydrogen bond. These results provide insight into the origin of the specificity of the enzyme 2,6-dichlorohydroquinone 1,2-dioxygenase (PcpA), which is specific for ortho-dihalohydroquinone substrates and phenol inhibitors.

Dioxygen reactivity of biomimetic Fe(II) complexes with noninnocent catecholate, o-aminophenolate, and o-phenylenediamine ligands

This study describes the O2 reactivity of a series of high-spin mononuclear Fe(II) complexes each containing the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate ((tBu)CatH(-)), 4,6-di-tert-butyl-2-aminophenolate ((tBu2)APH(-)), or 4-tert-butyl-1,2-phenylenediamine ((tBu)PDA). The preparation and X-ray structural characterization of [Fe(2+)((Ph2)TIP)((tBu)CatH)]OTf, [3]OTf and [Fe(2+)((Ph2)TIP)((tBu)PDA)](OTf)2, [4](OTf)2 are described here, whereas [Fe(2+)((Ph2)TIP)((tBu2)APH)]OTf, [2]OTf was reported in our previous paper [Bittner et al., Chem.-Eur. J. 2013, 19, 9686-9698]. These complexes mimic the substrate-bound active sites of nonheme iron dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is oxidized in the presence of O2, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, Mössbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O2 to yield the ferric-catecholate species [Fe(3+)((Ph2)TIP)((tBu)Cat)](+) (3(ox)), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex [4](2+) experiences a two-electron (2e(-)), ligand-based oxidation to give [Fe(2+)((Ph2)TIP)((tBu)DIBQ)](2+) (4(ox)), where DIBQ is o-diiminobenzoquinone. The reaction of [2](+) with O2 is also a 2e(-) process, yet in this case both the Fe center and (tBu2)AP ligand are oxidized; the resulting complex (2(ox)) is best described as [Fe(3+)((Ph2)TIP)((tBu2)ISQ)](+), where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from [Fe(3+)(L(2-))](+) (3(ox)) to [Fe(3+)(L(•-))](2+) (2(ox)) to [Fe(2+)(L(0))](2+) (4(ox)). Notably, the O2 reaction rates vary by a factor of 10(5) across the series, following the order [3](+) > [2](+) > [4](2+), even though the complexes have similar structures and Fe(3+/2+) redox potentials. To account for the kinetic data, we examined the relative abilities of the title complexes to bind O2 and participate in H-atom transfer reactions. We conclude that the trend in O2 reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.