Biswarup Chakraborty

Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen: Mechanistic Investigations on the Oxidative C–C Bond Cleavage of Substituted 2-Aminophenols

The isolation and characterization of a series of iron(II)-2-aminophenolate complexes [(6-Me3-TPA)Fe(II)(X)](+) (X = 2-amino-4-nitrophenolate (4-NO2-HAP), 1; X = 2-aminophenolate (2-HAP), 2; X = 2-amino-3-methylphenolate (3-Me-HAP), 3; X = 2-amino-4-methylphenolate (4-Me-HAP), 4; X = 2-amino-5-methylphenolate (5-Me-HAP), 5; X = 2-amino-4-tert-butylphenolate (4-(t)Bu-HAP), 6 and X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-(t)Bu-HAP), 7) and an iron(III)-2-amidophenolate complex [(6-Me3-TPA)Fe(III)(4,6-di-(t)Bu-AP)](+) (7(Ox)) supported by a tripodal nitrogen ligand (6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine) are reported. Substituted 2-aminophenols were used to prepare the biomimetic iron(II) complexes to understand the effect of electronic and structural properties of aminophenolate rings on the dioxygen reactivity and on the selectivity of C-C bond cleavage reactions. Crystal structures of the cationic parts of 5·ClO4 and 7·BPh4 show six-coordinate iron(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While 1·BPh4 does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of 2·ClO4 with dioxygen affords 2-amino-3H-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes 3·BPh4, 4·BPh4, 5·ClO4 and 6·ClO4 react with O2 to exhibit C-C bond cleavage of the bound aminophenolates. Complexes 7·ClO4 and 7(Ox)·BPh4 produce a mixture of 4,6-di-tert-butyl-2H-pyran-2-imine and 4,6-di-tert-butyl-2-picolinic acid. Labeling experiments with (18)O2 show the incorporation of one oxygen atom from dioxygen into the cleavage products. The reactivity (and stability) of the intermediate, which directs the course of aromatic ring cleavage reaction, is found to be dependent on the nature of ring substituent. The presence of two tert-butyl groups on the aminophenolate ring in 7·ClO4 makes the complex slow to cleave the C-C bond of 4,6-di-(t)Bu-HAP, whereas 4·BPh4 containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me3-TPA)Fe(III)(4-(t)Bu-AP)](+) (6(Ox)) to gain a mechanistic insight into the regioselective C-C bond cleavage reaction. On the basis of the experimental and computational studies, an iron(II)-2-iminobenzosemiquinonate intermediate is proposed to react with dioxygen resulting in the oxidative C-C bond cleavage of the coordinated 2-aminophenolates.

Aromatic Ring Cleavage of 2‐Amino‐4‐tert‐butylphenol by a Nonheme Iron(II) Complex: Functional Model of 2‐Aminophenol Dioxygenases

A large variety of dioxygenases, which catalyze the ring cleavage of aromatic compounds, are found in aerobic bacteria. Catechol-cleaving dioxygenases are well-studied examples in this class of enzymes. 4–8] Pseudomonas Pseudoalcaligenes, which can be found in nitrobenzene-contaminated soil and groundwater, is involved in the catabolism of nitrophenol; the catabolic pathway proceeds through the reduction of 2-nitrophenol to 2-aminophenol. Oxidative C C bond cleavage of 2-aminophenol then forms 2-aminomuconic acid semialdehyde, which spontaneously loses a water molecule to form 2-picolinic acid. In addition, other nitroaromatic compounds are degraded by some bacteria in a similar pathway. Furthermore, the metabolism of one of the essential amino acids, tryptophan, proceeds through the formation of 3-hydroxyanthranilate followed by C3 C4 bond cleavage to form quinolinic acid via 2-amino-3-carboxymuconic acid semialdehyde. 2-Aminophenol-1,6-dioxygenase (APD), isolated and purified from Pseudomonas Pseudoalcaligenes, is responsible for the C C bond cleavage of 2-aminophenols under aerobic conditions (Scheme 1). The degradation of 3hydroxyanthranilate to quinolinate is catalyzed by 3-hydroxyanthranilate-3,4-dioxygenase (HAD) in the presence of dioxygen (Scheme 1). Structural studies show that the active site of HAD contains an iron(II) center that is coordinated by the “2-His-1-Glu facial triad”. Both APD and HAD belong to the class of nonheme iron(II) enzymes and share functional similarity with extradiol-cleaving catechol dioxygenases. A mechanism, similar to that of extradiol-cleaving catechol dioxygenases, has been proposed for aromatic C C bond cleavage of 2-aminophenols (Scheme 1). Despite the existence of a large number of iron complexes with the coordinated 2-aminophenolates in different redox states, thus far there is no example of a biomimetic iron(II) complex that exhibits C C bond cleavage activity of 2-aminophenolate in the presence of dioxygen. Herein, we report the synthesis, characterization, and dioxygen reactivity of a nonheme iron(II) complex, [(6-Me3-TPA)Fe (4-tBuHAP)](ClO4) (1·ClO4), in which 6-Me3-TPA = tris(6-methyl2-pyridylmethyl)amine and 4-tBu-HAP = monoanionic 2amino-4-tert-butylphenolate. The oxidative C C bond cleavage of 2-amino-4-tert-butylphenolate on complex 1 mimicking the function of APD and HAD is discussed. The iron(II)–2-aminophenolate complex (1) was isolated from the reaction of 6-Me3-TPA ligand, iron(II) perchlorate, and 2-amino-4-tert-butylphenol (4-tBu-H2AP) in the presence of one equivalent of triethylamine in methanol. The yellow solution of complex 1 in acetonitrile displays an intense charge-transfer (CT) band at 404 nm. H NMR spectrum of the complex in CDCl3 exhibits paramagnetically shifted proton resonances in the region of 40 ppm to 60 ppm (see Figure S1 in the Supporting Information). The NMR data along with the magnetic moment of 5.1 mB at room temperature are indicative of the high-spin nature of the iron(II) complex. Complex 1 was further characterized by singlecrystal X-ray diffraction. Unfortunately, all attempts to grow single crystals of 1·ClO4 were unsuccessful. However, X-rayquality single crystals of 1·BPh4 (see Experimental Section in the Supporting Information) were isolated from a dichloromethane/methanol/diethyl ether solvent mixture at 273 K. X-ray crystal structure of the complex cation shows a sixcoordinate iron center ligated by four nitrogen donors from the tetradentate ligand, and one nitrogen and one oxygen donor from the aminophenol ligand (Figure 1). The Fe–Npy distances are in a range of 2.186(3) to 2.305(3) , similar to those reported for other high-spin iron(II) complexes of the tetradentate 6-Me3-TPA ligand. [31,32] The aminophenol ligand binds to the metal center through N5 and O1 with Fe1–N5 and Fe1–O1 distances of 2.282(3) and 1.934(3) , respectively (see Table S1 in the Supporting Information). A short Fe1–O1 distance implies a monoanionic binding of the aminophenolate (4-tBu-HAP). Recently, Fiedler and co-workers have reported an iron(II)–aminophenolate complex, [(Tp)FeScheme 1. Reaction catalyzed by APD and HAD.

Role of α-hydroxycarboxylic acids in the construction of supramolecular assemblies of nickel(II) complexes with nitrogen donor coligands

The topology of the supramolecular self assembly structures of six nickel(II) complexes containing nitrogen donor coligands is described, and the structural features induced by the deprotonated α-hydroxy acids (mandelic and benzilic) used in the complex synthesis are compared. In all the complexes, namely [(6-Me3-TPA)Ni(MA)]B(Ph)4 (1), [(6-Me3-TPA)Ni(R(−)-MA)]B(Ph)4 (2R), [(6-Me3-TPA)Ni(BA)]B(Ph)4 (3), [(AP)2Ni(MA)2] (4), [(AP)2Ni(S(+)-MA)2] (5S) and [(AP)2Ni(BA)2] (6) (where 6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine, AP = 2-aminopyridine, MA = racemic mandelate, R(−) and S(+) are enantiomeric mandelates, BA = benzilate anion), the metal ion exhibits a distorted octahedral environment with mandelate or benzilate anions acting as chelating agents through the hydroxyl group and a carboxylate oxygen. The differences in the crystal structures of 2R and 5S (containing enantiomerically pure mandelate anions) are compared with those of 1 and 4, obtained with a racemic mixture of the deprotonated acid. All the nickel(II) complexes form infinite one-dimensional chains built up by hydrogen bonds. The mononuclear building blocks, 1–3, are assembled head-on, with a very similar topology, through H-bond interactions between the hydroxycarboxylate groups of adjacent units, while in neutral complexes 4–6 the cooperative presence of solvent molecules leads to different connections between the complexes. In addition, intramolecular π–π stacking interactions between pyridine rings of the ligand and the phenyl of mandelate or benzilate anions are observed in 1, 3 and 6.

Reductive Activation of O2 by Non-Heme Iron(II) Benzilate Complexes of N4 Ligands: Effect of Ligand Topology on the Reactivity of O2-Derived Oxidant

A series of iron(II) benzilate complexes (1-7) with general formula [(L)FeII(benzilate)]+ have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N4 ligands (tris(2-pyridylmethyl)amine (TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me3-TPA in 2), N1,N1-dimethyl-N2,N2-bis(2-pyridylmethyl)ethane-1,2-diamine (iso-BPMEN in 3), N1,N1-dimethyl-N2,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me2-iso-BPMEN in 4), and tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two linear tetradentate amine ligands (N1,N2-dimethyl-N1,N2-bis(2-pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N1,N2-dimethyl-N1,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me2-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1-3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron-oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4-6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O2-derived iron-oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II) hydroperoxides are energetically favorable for the complexes of 6-Me3-TPA, 6-Me2-iso-BPMEN, BPMEN, and 6-Me2-BPMEN ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.

Mimicking the Aromatic-Ring-Cleavage Activity of Gentisate-1,2-Dioxygenase by a Nonheme Iron Complex

Gentisate-1,2-dioxygenase (GDO), a nonheme iron enzyme in the cupin superfamily, catalyzes the cleavage of the aromatic-ring of 2,5-dihydroxybenzoic acid (gentisic acid) to form maleylpyruvic acid in the microbial aerobic degradation of aromatic compounds. To develop a functional model of GDO, we have isolated a nonheme iron(II) complex, [(TpPh2 )FeII (DHN-H)] (TpPh2 =hydrotris(3,5-diphenylpyrazole-1-yl)borate, DHN-H=1,4-dihydroxy-2-naphthoate). In the reaction with O2 , the biomimetic complex oxidatively cleaves the aromatic ring of the coordinated substrate with the incorporation of both the oxygen atoms from molecular oxygen into the cleavage product. The presence of para-hydroxy group on the substrate plays a crucial role in directing the aromatic-ring cleaving reaction.

Synthesis, characterization, and crystal structures of cobalt(II), copper(II), and zinc(II) complexes of a bidentate iminophenol

A bidentate iminophenol (HL = 2-((4-methoxyphenylimino)methyl)-4,6-di-tert-butylphenol derived from condensation of 4-methoxyaniline and 3,5-di-tert-butyl-2-hydroxybenzaldehyde) was mixed with divalent metal salts to form the corresponding mononuclear metal complexes [MII(L)2] (M = Co (1), Cu (2), and Zn (3)). The complexes are characterized by different spectroscopic and analytical tools. X-ray crystal structures of the complexes revealed homoleptic mononuclear complexes with MN2O2 coordination. The cobalt(II) (1) and zinc(II) (3) complexes display a pseudo-tetrahedral coordination geometry, whereas the copper(II) complex (2) exhibits a distorted square-planar coordination. The zinc(II) complex (3) emits at 460 nm with a twofold enhancement of emission with respect to the free iminophenol.