Tripodal Tetradentate Ligands Research Articles

Iron(III) Complexes of Tripodal Monophenolate Ligands as Models for Non-Heme Catechol Dioxygenase Enzymes: Correlation of Dioxygenase Activity with Ligand Stereoelectronic Properties

The iron(III) complexes [Fe(L)Cl(2)] 1-6 of the tripodal monophenolate ligands N,N-bis(2-pyridylmethyl)-N'-(2-hydroxybenzyl)amine H(L1), [(1-methylimidazol-2-ylmethyl)-(pyrid-2-ylmethyl)aminomethyl]-phenol H(L2), 2,4-dimethyl-6-[(1-methylimidazol-2-ylmethyl)(pyrid-2-ylmethyl)aminomethyl]phenol H(L3), N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxybenzyl)ethylenediamine H(L4), N,N-dimethyl-N'-(1-methylimidazol-2-ylmethyl)-N'-(2-hydroxybenzyl)ethylenediamine H(L5), and N,N-dimethyl-N'-(1-methylimidazol-2-ylmethyl)-N'-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine H(L6) have been isolated and studied as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The complexes have been characterized using elemental analysis, electrospray ionization mass spectrometry, and absorption spectral and electrochemical methods. The single crystal X-ray structures of [Fe(L3)Cl(2)] 3 and [Fe(L6)Cl(2)] 6 have been successfully determined, and the rhombically distorted octahedral coordination geometry around iron(III) in them are constituted by the phenolate oxygen and pyridyl/-NMe(2), and N-methylimidazolyl and tertiary amine nitrogens of the tripodal tetradentate ligands H(L3)/H(L6) and two cis-coordinated chloride ions. The sterically demanding -NMe(2) group as in 6 imposes an Fe-O-C bond angle (139.8 degrees) and Fe-O bond length (1.852 A), which are very close to those (Fe-O-C, 133, 148 degrees; Fe-O(tyrosinate), 1.81, 1.91 A) of 3,4-PCD enzymes. The Fe-O-C bond angle observed for 6 is higher than that for 3 (125.1 degrees), and the Fe-O(phenolate) bond distance in 6 is shorter than that in 3 (1.905 A). In methanol solution all the complexes exhibit two phenolate-to-Fe(III) ligand-to-metal charge transfer (LMCT) bands in the ranges 536-622 and 329-339 nm. Further, when 3,5-di-tert-butylcatechol (H(2)DBC) pretreated with two moles of Et(3)N is added to 1-6, two new intense DBC(2-)-to-iron(III) LMCT bands (466-489, 676-758 nm) are observed, which are similar to those observed for 3,4-PCD enzyme-substrate complex. All the complexes elicit oxidative intradiol cleavage of H(2)DBC in the presence of O(2). Interestingly, among the present complexes, 3 containing coordinated N-methylimidazolyl nitrogen shows the highest rate of intradiol cleavage, which correlates with the highest energy of DBC(2-)-to-iron(III) LMCT band and the most negative DBSQ/DBC(2-) redox potential. Also, the catecholate adducts of complexes 4 and 5, both containing a -NMe(2) donor group, react faster and produce higher amounts of intradiol cleavage products (4: 55.3; 5, 50.6%) than the analogous complexes 1 (43.2%) and 2 (32.7%), both containing a pyridyl nitrogen donor, which is consistent with the more negative DBSQ/DBC(2-) redox potentials for 4 and 5. The increase in rate of catechol dioxygenation with increase in the DBC(2-)-to-iron(III) LMCT band energy and decrease in DBSQ/DBC(2-) redox potential is illustrated by invoking a facile alpha-electron transfer from iron(III) to catecholate-bound molecular oxygen in the substrate activation mechanism proposed for the intradiol-cleaving catechol dioxygenases. Also, when the substituents on the phenolate arm are varied to tune the Lewis acidity of iron(III) center, the reaction rate decreases with decrease in Lewis acidity and, interestingly, extradiol cleavage is also observed when the Lewis acidity is decreased further by incorporating a 3,5-dimethylphenolate arm as in 6.

Synthesis and Characterization of Tripodal Tetradentate Ligand Type NS3 and its Complexes with Re(V), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II)

This work represents the preparation of the starting material, 3-chloro-2-oxo-1,4-dithiacyclohexane (S) using a new method. This material was reacted with, 4-phenylthiosemicarbazide to give (H3NS3) as a tetradentate ligand H3L. New complex of rhenium (V) with this ligand of the formula [ReO(L)] was prepared. New complexes of the general formula [M(HL)] of this ligand when reacted with some metal ions where: M = Ni(II), Cu(II), Cd(II), Zn(II), Hg(II) have been reported. The ligand and the complexes were characterized by infrared, ultraviolet–visible, mass, 1H nuclear magnetic resonance and atomic absorption spectroscopic techniques and by (HPLC), elemental analysis, and electrical conductivity. The proposed structure for H3L with Re (V) is square pyramidal, while Ni(II) complex was square planar geometry, and with the rest of metal ions are distorted tetrahedral.

Synthesis and Characterization of Tripodal Tetradentate Ligand Type NS3 and its Complexes with Re(V), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II)

This work represents the preparation of the starting material, 3-chloro-2-oxo-1,4-dithiacyclohexane (S) using a new method. This material was reacted with, 4-phenylthiosemicarbazide to give (H3NS3) as a tetradentate ligand H3L. New complex of rhenium (V) with this ligand of the formula [ReO(L)] was prepared. New complexes of the general formula [M(HL)] of this ligand when reacted with some metal ions where: M = Ni(II), Cu(II), Cd(II), Zn(II), Hg(II) have been reported. The ligand and the complexes were characterized by infrared, ultraviolet–visible, mass, 1H nuclear magnetic resonance and atomic absorption spectroscopic techniques and by (HPLC), elemental analysis, and electrical conductivity. The proposed structure for H3L with Re (V) is square pyramidal, while Ni(II) complex was square planar geometry, and with the rest of metal ions are distorted tetrahedral.

Towards robust alkane oxidation catalysts: electronic variations in non-heme iron(ii) complexes and their effect in catalytic alkane oxidation

A series of non-heme iron(ii) bis(triflate) complexes containing linear and tripodal tetradentate ligands has been prepared. Electron withdrawing and electron donating substituents in the para position of the pyridine ligands as well as the effect of pyrazine versus pyridine and sulfur or oxygen donors instead of nitrogen donors have been investigated. The electronic effects induced by these substituents influence the strength of the ligand field. UV-vis spectroscopy and magnetic susceptibility studies have been used to quantify these effects and VT (1)H and (19)F NMR spectroscopy as well as X-ray diffraction have been used to elucidate structural and geometrical aspects of these complexes. The catalytic properties of the iron(ii) complexes as catalysts for the oxidation of cyclohexane with hydrogen peroxide have been evaluated. In the strongly oxidising environment required to oxidise alkanes, catalyst stability determines the overall catalytic efficiency of a given catalyst, which can be related to the ligand field strength and the basicity of the ligand and its propensity to undergo oxidation.

Zinc and Cobalt Complexes Derived from Tetradentate Tripodal Ligands with N2O2 Donorset as Model Compounds for Phosphatases

AbstractStarting from the tripodal tetradentate ligands ‐(3,5‐dibromo‐2‐hydroxybenzyl)(2‐hydroxybenzyl)(2‐pyridyl)methylamine (H2L1), (3,5‐dibromo‐2‐hydroxybenzyl)(2‐hydroxy‐5‐nitrobenzyl)(2‐pyridyl)methylamine (H2L2), and (3,5‐dichloro2‐hydroxybenzyl)(2‐hydroxy‐5‐nitrobenzyl)(2‐pyridyl)methylamine (H2L3) the new isostructural dinuclear zinc compounds [Zn2(L1)2]·N(CH2CH3)3 (1), [Zn2(L2)2]·2CH3OH (2) and [Zn2(L3)2]·C4H10O (3) were synthesized. Due to their enzyme‐like trigonal bipyramidal N2O3 coordination environment of the zinc ions and the similar Zn···Zn distances the complexes can be considered to be structural models for the active sites in phospholipase C and nuclease P1. With H2L3 also the dinuclear complex [Co2(L2)2(CH3OH)]·2CH3OH·0.5C4H10O (4) could be prepared.The new compounds were isolated and characterized by single crystal X‐ray crystallography as well as infrared spectroscopy. The cobalt compound 4 was additionally characterized by UV‐Vis spectroscopy and magnetic measurements. 1 crystallizes in the monoclinic space group P21/n with a = 11.2814(2), b = 28.6154(2), c = 13.1866(3) Å, β = 96.995(1)°, V = 4225.2(2) Å3, Z = 4. 2 and 3 are monoclinic, space group C2/c with a = 23.084(5), b = 9.232(2), c = 21.849(4) Å, &β; = 96.83(3)°, V = 4623(2) Å3, Z = 4, and a = 22.7834(3), b = 9.2463(1), c = 21.6351(3) Å, &β; = 97.592(1)°, V = 4517.7(2) Å3, Z = 4, respectively. 4 crystallizes in the monoclinic space group I2/a with a = 22.4680(4), b = 20.5517(4), c = 22.8910(6) Å, &β; = 111.938(1)°, V = 9804.7(4) Å3, Z = 8. 4 shows an effective magnetic moment of 6.72 μB at 300 K which clearly indicates the presence of two cobalt(II) high spin ions with Curie‐Weiss behaviour above 80 K. At lower temperatures a decrease of the effective magnetic moment was observed.

Syntheses, crystal structures and magnetic properties of three new binuclear Ni(II) complexes derived from tripodal tetradentate (N 4) ligands

Three new binuclear Ni(II) complexes [{Ni(L 22py)Cl} 2](ClO 4) 2 ( 1), [{Ni(L 23py)Cl} 2](ClO 4) 2 ( 2), and [{Ni(L 33py)Cl} 2](ClO 4) 2 ( 3), {L 22py = N-(2-pyridylmethyl)- N-(2-aminoethyl)-1,2-diaminoethane, L 23py = N-(2-pyridylmethyl)- N-(2-aminoethyl)-1,3-diaminopropane, L 33py = N-(2-pyridylmethyl)- N-(3-aminopropyl)-1,3-diaminopropane} have been synthesized. Single crystal X-ray structure analysis showed that in each complex two distorted octahedral Ni(II) ions are bridged asymmetrically by a pair of chloride anions. Variable temperature magnetic susceptibility measurements of 1 and 3 revealed dominant ferromagnetic exchange interactions.

Copper Dioxygen Adducts: Formation of Bis(μ-oxo)dicopper(III) versus (μ-1,2)Peroxodicopper(II) Complexes with Small Changes in One Pyridyl-Ligand Substituent

The preference for the formation of a particular Cu 2O 2 isomer coming from (ligand)-Cu (I)/O 2 reactivity can be regulated with the steric demands of a TMPA (tris(2-pyridylmethyl)amine) derived ligand possessing 6-pyridyl substituents on one of the three donor groups of the tripodal tetradentate ligand. When this substituent is an -XHR group (X = N or C) the traditional Cu (I)/O 2 adduct forms a (mu-1,2)peroxodicopper(II) species ( A). However, when the substituent is the slightly bulkier XR 2 moiety {aryl or NR 2 (R not equal H)}, a bis(mu-oxo)dicopper(III) structure ( C) is favored. The reactivity of one of the bis(mu-oxo)dicopper(III) species, [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) (6tbp = (6- (t)Bu-phenyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine), was probed, and for the first time, exogenous toluene or ethylbenzene hydrocarbon oxygenation reactions were observed. Typical monooxygenase chemistry occurred: the benzaldehyde product includes an 18-O atom for toluene/ 7- (1) (8)O 2 reactivity, and a H-atom abstraction by 7-O 2 is apparent from study of its reactions with ArOH substrates, as well as the determination of k H/ k D approximately 7 in the toluene oxygenation (i.e., PhCH 3 vs PhCD 3 substrates). Proposed courses of reaction are presented, including the possible involvement of PhCH 2OO (*) and its subsequent reaction with copper(I) complex, the latter derived from dynamic solution behavior of 7-O 2 . External TMPA ligand exchange for copper in 7-O 2 and O-O bond (re)formation chemistry, along with the ability to protonate 7-O 2 and release of H 2O 2 indicate the presence of an equilibrium between [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) and a (mu-1,2)peroxodicopper(II) form.

The Synthesis, Structures and Characterisation of New Mixed‐Ligand Manganese and Iron Complexes with Tripodal, Tetradentate Ligands

AbstractThe preparation of new manganese and iron complexes with the general formula [M(tripod)(anion)] is described, where M = FeIII or MnIII, “tripod” is a dianionic tetradentate tripodal ligand and the anion is a chelating β‐diketonate, 8‐oxyquinoline or acetate. The synthesis of this type of complexes was found to be straightforward, which allows for the preparation of a large variety of such coordination compounds. The complexes are characterised by X‐ray crystallography, infrared spectroscopy, UV/Vis spectroscopy, cyclic voltammetry and elemental analysis. A correlation between the ligand sets and the electron density at the metal centre in the complexes is proposed, based on the UV/Vis data and the CV measurements. The tripodal ligands are significant π‐donor ligands, and electron‐withdrawing or electron‐donating substituents on the phenolate arms were found to have a large influence on both the position of the d–d transitions in the UV/Vis spectra and the peak potentials in the CV measurements. The “secondary” β‐diketonate or acetate ligand does not have such a large effect on the electron density of the metal centre.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Rhenium and technetium complexes bearing quinazoline derivatives: progress towards a 99mTc biomarker for EGFR-TK imaging

The quinazoline derivatives (3-chloro-4-fluorophenyl)quinazoline-4,6-diamine (2) and (3-bromophenyl)quinazoline-4,6-diamine (3) were labelled with (99m)Tc using the "4 + 1" mixed-ligand system [Tc(NS3)(CN-R)] and the tricarbonyl moiety fac-[Tc(CO)3]+. In the "4 + 1" approach the technetium(iii) is stabilized by a monodentate isocyanide bearing a quinazoline fragment (L1,L2 ) and by the tetradentate tripodal ligand tris(2-mercaptoethyl)-amine (NS3). In the "4 + 1" approach, 99mTc-labelling was performed in a two-step procedure, the complexes [Tc(NS3)(L1)] (7a) and [Tc(NS3)(L2)] (8a) being obtained in about 50-70% yield. In the tricarbonyl approach, the fac-[Tc(CO)3]+ unit is anchored by two different monoanionic chelators bearing the quinazoline derivatives (3-chloro-4-fluorophenyl)quinazoline-4,6-diamine (2) and (3-bromophenyl)quinazoline-4,6-diamine (3). Both chelators have a N2O donor atom set, but one contains a pyrazolyl ring (L5H) and the other contains a pyridine unit (L6H). In both cases the conjugation of the quinazoline to the chelator was done through the secondary amine of the potentially tridentate and monoanionic chelators, the corresponding 99mTc-complexes (10a, 11a) being obtained in quantitative yield. The identities of the 99mTc-labelled quinazolines (7a, 8a, 10a, 11a) were confirmed by comparison with the HPLC profiles of the analogous Re compounds (7, 8, 10, 11). All these Re complexes were characterized by NMR and IR spectroscopy, elemental analysis and in some cases by MS and X-ray diffraction analysis. In vitro studies indicate that the quinazoline fragments, after conjugation to the cyano group (L1, L2) or to the pyrazolyl containing chelator (L5H), as well as the corresponding Re complexes (7, 8, 10) inhibit significantly the EGFR autophosphorylation and also inhibit A431 cell growth. These two effects were also found for the pyridine-containing chelator (L6H) and corresponding Re complex (11), although to a lesser extent.

Synthesis and structure of iron(iii) diamine-bis(phenolate) complexes

The structures and properties of six new iron(iii) diamine-bis(phenolate) complexes are reported. Reaction of anhydrous FeX(3) salts (where X = Cl or Br) with the diprotonated tripodal tetradentate ligands 2-pyridylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), H(2)[L(1)], and N,N-dimethyl-N',N'-bis(2-methylene-4-methyl-6-tert-butylphenol)ethylenediamine, H(2)[L(2)], produces the trigonal bipyramidal iron(iii) complexes, [L(1)]FeCl , [L(1)]FeBr , [L(2)]FeCl and [L(2)]FeBr . Reaction of FeX(3) with the related linear tetradentate ligand N,N'-bis(4,6-tert-butyl-2-methylphenol)-N,N'-bismethyl-1,2-diaminoethane, H(2)[L(3)], generates square pyramidal iron(iii) complexes, [L(3)]FeCl and [L(3)]FeBr . Complexes have been characterized using electronic absorption spectroscopy and magnetometry. Single crystal X-ray molecular structures have been determined for complexes 1, 3, 5 and 6.

Origin of Compressed Jahn−Teller Octahedra in Sterically Strained Manganese(III) Complexes

A method is presented that enables the bond length changes resulting from the Jahn-Teller interaction to be predicted from a knowledge of the angular geometry of the complex and the identity of the ligators. The calculation of the energy minimum within the subspace of the Jahn-Teller active e skeletal mode proceeds by diagonalization of the potential energy component of the cubic Ee Jahn-Teller problem, incorporating low-symmetry distortions by way of angular overlap model calculations. The theory is applied to a series of manganese(III) complexes formed with tetradentate tripodal ligands that largely dictate the angular coordinates. A good account of the contrasting molecular structures is obtained by allowing the sigma-bonding strength to vary according to expectations based upon the spectrochemical series.

Spectroscopic and Theoretical Study of a Mononuclear Manganese(III) Complex Exhibiting a Tetragonally Compressed Geometry

Inelastic neutron scattering and high-field electron paramagnetic resonance data are presented for [Mn(bpia)(OAc)(OCH(3))](PF(6)), where bpia is bis(picolyl)(N-methylimidazole-2-yl)amine. Modeling of the data to the conventional fourth-order spin-Hamiltonian yielded the following parameters: D = 3.526(3) cm(-1), E = 0.588(6) cm(-1), B(0)(4) = -0.00084(7) cm(-1), B(2)( 4)= -0.002(2) cm(-1), (4)(4) = -0.0082(5) cm(-1), g(x) = 1.98(1), g(y) = 1.952(6), and g(z) = 1.978(5). The complex is of particular interest given the biochemical activity and the unusual stereochemistry distinguished by a rare example of a tetragonally compressed octahedron and a pronounced angular distortion imposed by the tetradentate tripodal bpia ligand. Ligand field, density functional theory, and complete active space self-consistent field ab initio calculations are presented that aim to relate the spectroscopic data to the molecular geometry.

Carbon Monoxide Coordination and Reversible Photodissociation in Copper(I) Pyridylalkylamine Compounds

Systematic studies of CO coordination and photodissociation have been carried out for a series of copper(I) carbonyl compounds possessing tripodal tetradentate ligands, [CuI(L)(CO)]B(C6F5)4 (L = Me2N-TMPA (1Me2N), MeO-TMPA (1MeO), H-TMPA (1H), PMEA (2pmea), PMAP (2pmap), BQPA (3bqpa). Detailed structural, electrochemical, and infrared characterization has been accomplished. In addition, various experimental techniques were utilized to determine equilibrium binding constants (KCO), association (kCO), and dissociation (k-CO) rate constants, as well as the thermodynamic (DeltaH degrees , DeltaS degrees ) and activation parameters (DeltaH, DeltaS) that regulate these processes. With increased ligand-electron-donating ability, greater pi back-bonding results in stronger Cu-CO bonds, leading to KCO values on the order 1Me2N-CO > 1MeO-CO > 1H-CO. With systematic synthetic expansion of the five-membered chelate rings like 1R to six-membered chelate rings like 2R, the stability of the CO adduct decreases, 1H-CO > 2pmea-CO > 2pmap-CO. The CO-binding properties of 3bqpa did not follow trends observed for the other compounds, presumably because of its bulkier ligand framework. Through solid- and solution-state analyses, we concluded that the photolabile carbonyl species in solution possess a tridentate coordination mode, forming strictly five-membered chelate rings to the copper ion with one dangling arm of the tripodal ligand. Carbon monoxide reversibly photodissociated from complexes 1Me2N-CO, 1MeO-CO, 1H-CO, and 3bqpa-CO in coordinating (CH3CN) and weakly coordinating (THF) solvent but not from 2pmea-CO and 2pmap-CO. Comparisons to O2-binding data available for these copper complexes as well as to small molecule (O2, CO, NO) reactions with hemes and copper proteins are discussed.

Synthesis and characterization of a bis-μ,η 1-carboxylate-bridged dinuclear manganese(II) complex containing a tetradentate tripodal ligand, N-(benzimidazol-2-ylmethyl)iminodiacetic acid

The dinuclear Mn(II) complex, [Mn 2(Hbida) 2(H 2O) 2], was prepared using a tetradentate tripodal ligand, N-(benzimidazol-2-ylmethyl)iminodiacetic acid (H 3bida) which has two carboxylate and one benzimidazole groups. The manganese ions are doubly bridged using μ,η 1-bridging monodentate carboxylate oxygen atoms. The Mn–Mn bond distance of 3.446 Å in the complex is comparable to that observed in the active site of the native manganese arginase enzyme (3.30 Å). The geometry of the complex with four carboxylates in two different types of binding modes, non-bridging monodentate and μ,η 1-type bridging monodentate, mimics the active site structure of manganese arginase. The magnetic properties of the complex show a coupling constant of J = −0.471(1) cm −1, which is consistent with weakly coupled antiferromagnetic Mn II ( S = 5/2) centers. The cyclic voltammograms of the Mn(II) complex in DMF show irreversible oxidation occurring around 570 mV (versus Ag/AgCl).

Mimicking the photosynthetic system with strong hydrogen bonds to promote proton electron concerted reactions

A Copper(2+) complex with a Cu(II)-C bond containing sp(3) configuration was used to investigate the role of strong hydrogen bonds in proton coupled electron transfer (PCET) reactions. The only example of a Cu(II)-C system realized so far is that using tris-(pyridylthio)methyl (tptm) as a tetradentate tripodal ligand. Using this ligand, [CuF(tptm)] and [Cu(tptm)(OH)] have been prepared. The former complex forms supra-molecular arrays of layers of the complex between which hydroquinone is intercalated in the crystalline phase. This hydroquinone intercalation crystal was prepared via the photochemical conversion of quinone during the crystallization process. This conversion reaction probably involves a proton coupled electron transfer process. The nuclear magnetic resonance spectroscopic analysis of the reaction mixture shows the presence of Cu(III) during the conversion reaction. These results strongly suggest the presence of the molecular aggregate of the [CuF(tptm)] complex, water and quinone in the solution phase during the quinone to hydroquinone conversion. The presence of this type of aggregate requires a strong hydrogen bond between the [CuF(tptm)] complex and water. The presence of this particular hydrogen bond is a unique character of such a complex that has the Cu(II)-C bond. This complex is used as a model for photosynthetic water splitting since the photoconversion of quinone to hydroquinone also involves the production of oxygen from water.

Ligand Effects on Dioxygen Activation by Copper and Nickel Complexes: Reactivity and Intermediates

Copper and nickel complexes having various active-oxygen species M n -O 2 ( n = 1 or 2), such as trans-(micro-1,2-peroxo)Cu (II) 2, bis(micro-oxo)M (III) 2, bis(micro-superoxo)Ni (II) 2, and ligand-based alkylperoxo-M (II) n , can be produced by a series of tetradentate tripodal ligands (TMPA analogues) containing sterically demanding 6-methyl substituent(s) on the pyridyl group(s), where TMPA = tris(2-pyridylmethyl)amine. Roles of the methyl substituent(s) for the formation of the active-oxygen species and their oxidation reactivities are reported.

Aryl Hydroxylation from a Mononuclear Copper-Hydroperoxo Species

Mononuclear CuII−-OOH entities or derived species have been seriously considered as important in chemical or biochemical oxidations. Yet, synthetic chemistry investigations have thus far revealed they have very limited substrate reactivity. Here, a significant aryl hydroxylation reaction occurs from a hydroperoxocopper(II) complex possessing a tripodal tetradentate ligand with appended aryl substituent. A phenolate-Cu(II) complex is detected following a proposed O−O cleavage event, that is suggested on the basis of precedent from non-heme iron chemistry. A bis-μ-oxo-dicopper(III) complex as active oxygenating agent is ruled out.

Copper(II)−Hydroperoxo Complex Induced Oxidative N-Dealkylation Chemistry

A significant oxidative N-demethylation reaction occurs from a hydroperoxo−copper(II) species when formed within a tripodal tetradentate ligand framework possessing a pendant dimethylamine substrate. This mimics the monoxygenase activity occurring in the copper enzyme PHM. Observation of a product-based alkoxide−Cu intermediate and determination of a reaction kinetic isotope effect (kH/kD(intra) ∼ 2.3) by studying the ligand−N(CH3)(CD3) substrate provide further insights. The mononuclear CuII(-OOH) entity or species derived from this can promote biomimetic reactivity and thus requires further attention in biological or synthetic mechanistic studies.

Mononuclear non-heme iron(III) complexes as functional models for catechol dioxygenases

This article provides an overview of our work on mononuclear iron(III) complexes of phenolate and non-phenolate ligands as structural and functional models for the intradiol-cleaving non-heme catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) enzymes. All the complexes are cis-facially coordinated to iron(III) with a distorted octahedral geometry. The iron(III) complexes of linear tridentate 3N ligands and tetradentate tripodal phenolate ligands possess octahedral geometries with cis-coordination positions available for bidentate coordination of catechols. In two of these complexes with sterically demanding –NMe2 pendant, the Fe–O–C bond angle is around 135.7°, which is close to those (Fe–O–C, 133°, 148°) in 3,4-PCD enzyme. Also, interestingly, one of the bis-phenolate complexes displays trigonal bipyramidal coordination geometry as in the enzymes. The efficiency of the complexes to catalyze the intradiol-cleavage of 3,5-di-tert-butylcatechol (H2DBC) could be illustrated not only on the basis of Lewis acidity of the iron(III) center alone, but also by assuming that product release is the rate-determining phase of the catalytic reaction.

A 1:1 Copper−Dioxygen Adduct is an End-on Bound Superoxo Copper(II) Complex which Undergoes Oxygenation Reactions with Phenols

Employing a strongly electron-donating tripodal tetradentate ligand along with a reaction strategy designed to suppress binuclear peroxo complex formation, an end-on bound superoxo−copper(II) complex [CuII(NMe2−TMPA)(O2-)]+ (1) has been generated in solution [UV−vis (THF, −85 °C): λmax = 418 nm (ε, 4300 M-1 cm-1), 615 nm (ε, 1100), 767 nm (ε, 840)]. Resonance Raman spectroscopy employing isotopically substituted dioxygen (including mixed isotope 16/18O2) proves the end-on superoxo CuII(O2-) structural formulation, ν(O−O) = 1121 cm-1; ν(Cu−O) = 472 cm-1. The first demonstration of CuII(O2-) oxidative reactivity with exogenous substrates, likely involving H-atom abstraction chemistry, comes with the finding that 1 effects the oxygenation and hydroperoxylation of substituted phenols.