Synthesis and characterization of a tripodal amide ligand and its binding with anions of different dimensionality

Synthesis and crystal structure of a tren-based amide, L1,N,N‘,N‘‘-tris[(2-amino-ethyl)-3-nitro-benzamide] is reported. The crystallographic results show intramolecular hydrogen bonding and aromatic π···π stacking among tripodal arms which prevent opening of the receptor cavity. Intermolecular hydrogen bonding in L1 generates the sheetlike network in the solid state. The structural aspects of binding halides (1 and 2), nitrate (3), perchlorate (4), and hexafluorosilicate (5) with the protonated L1 are examined crystallographically. Protonation at the apical nitrogen of L1 in the presence of anions shows a structural transformation from sheet to bilayer network. Anion binding with multiple receptor units is observed via amide N−H···anion and aryl C−H···anion hydrogen-bonding interactions in all the complexes. The aryl group having nitro functionality that contributes to anion binding in complexes 1−5 through CH···anion interactions (either para or meta to nitro C−H) is noteworthy. These studies also show higher anion coordination of chloride (8) and hexafluorosilicate (14) with L1H+

A new tripodal iron(III) monophenolate complex: effects of ligand basicity, steric hindrance, and solvent on regioselective extradiol cleavage

The new iron(III) complex [Fe(L3)Cl2], where H(L3) is the tripodal monophenolate ligand N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine, has been isolated and studied as a structural and functional model for catechol dioxygenase enzymes. The complex possesses a distorted octahedral iron(III) coordination geometry constituted by the phenolate oxygen, pyridine nitrogen and two amine nitrogens of the tetradentate ligand, and two cis-coordinated chloride ions. The Fe-O-C bond angle (134.0° ) and Fe-O bond length (1.889 Å) are very close to those (Fe-O-C, 133° and 148° , Fe-O(tyrosinate), 1.81 and 1.91 Å) of protocatechuate 3,4-dioxygenase enzymes. When the complex is treated with AgNO3, the ligand-to-metal charge transfer (LMCT) band around 650 nm (ε , 2390 M-1 cm-1) is red shifted to 665 nm with an increase in absorptivity (ε , 2630 M-1 cm-1) and the FeIII/FeII redox couple is shifted to a slightly more positive potential (-0.329 to -0.276 V), suggesting an increase in the Lewis acidity of the iron(III) center upon the removal of coordinated chloride ions. Furthermore, when 3,5-di-tert-butylcatechol (H2DBC) pretreated with 2 mol of Et3N is added to the complex [Fe(L3)Cl2] treated with 2 equiv of AgNO3, two intense catecholate-to-iron(III) LMCT bands (719 nm, ε , 3150 M-1 cm-1; 494 nm, ε , 3510 M-1 cm-1) are observed. Similar observations are made when H2DBC pretreated with 2 mol of piperidine is added to [Fe(L3)Cl2], suggesting the formation of [Fe(L3)(DBC)] with bidentate coordination of DBC2-. On the other hand, when H2DBC pretreated with 2 mol of Et3N is added to [Fe(L3)Cl2], only one catecholate-to-iron(III) LMCT band (617 nm; ε , 4380 M-1 cm-1) is observed, revealing the formation of [Fe(L3)(HDBC)(Cl)] involving monodentate coordination of the catecholate. The appearance of the DBSQ/H2DBC couple for [Fe(L3)(DBC)] at a potential (-0.083 V) more positive than that (-0.125 V) for [Fe(L3)(HDBC)(Cl)] reveals that chelated DBC2- in the former is stabilized toward oxidation more than the coordinated HDBC-. It is remarkable that the complex [Fe(L3)(HDBC)(Cl)] undergoes slow selective extradiol cleavage (17.3%) of H2DBC in the presence of O2, unlike the iron(III)-phenolate complexes known to yield only intradiol products. It is probable that the weakly coordinated (2.310 Å) -NMe2 group rather than chloride in the substrate-bound complex is displaced, facilitating O2 attack on the iron(III) center and, hence, the extradiol cleavage. In contrast, when the cleavage reaction was performed in the presence of a stronger base-like piperidine before and after the removal of the coordinated chloride ions, a faster intradiol cleavage was favored over extradiol cleavage, suggesting the importance of the bidentate coordination of the catecholate substrate in facilitating intradiol cleavage. Also, intradiol cleavage is favored in dimethylformamide and acetonitrile solvents, with enhanced intradiol cleavage yields of 94 and 40%, respectively

Highly selective hydroxylation of alkanes catalyzed by (μ -oxo)bis(μ -carboxylato)-bridged diiron(III) complexes: involvement of mononuclear iron(III) species in catalysis

A few diiron(III) complexes [Fe2(O)(OAc)2(L1)2](ClO4)21, [Fe2(O)(OBz)2(L1)2](ClO4)22, [Fe2(O)(OAc)2(L2)2](ClO4)23 and [Fe2(O)(OBz)2(L2)2](ClO4)24, where L1 = N,N-bis(pyrid-2-ylmethyl)-iso-butylamine, L2 = N,N-bis(pyrid-2-ylmethyl)benzylamine, AcO = acetate and BzO = benzoate, have been isolated and characterized by means of elemental analysis and spectral and electrochemical methods. The molecular structures of the complexes 2 and 4 have been determined by single-crystal X-ray diffraction analysis and they possess a distorted bioctahedral geometry in which each iron atom is coordinated to the oxygen atom of the μ-oxo bridge, two oxygen atoms of the μ-benzoato bridges and three nitrogen atoms of L1 and L2 ligands capping the two ends of the diiron(III) cluster. The ESI-MS spectral data of the complexes reveal that the complexes remain intact in dichloromethane (DCM) solution. Upon adding one equivalent of Et3N to a mixture of one equivalent of the diiron(III) complexes and excess of m-chloroperbenzoic acid (m-CPBA) in DCM, an intense absorption band (λ max, 670-700 nm) appears, which corresponds to the species [Fe2(O)(OAc)(m-CPBA)(L)2]2+ (ESI-MS, m/z 466) suggested as the intermediate involved in the oxygenation reactions. All the present complexes show efficient alkane hydroxylation with 300-400 turn over numbers and good selectivities for cyclohexane (A/K, 10-14) and adamantane (3° /2° , 9-11). Interestingly, the formation of monoiron(III) species has been discerned in the alkane hydroxylation reactions beyond ~50 turnovers. The mononuclear 1 : 1 iron(III) complexes of L1 and L2 ligands generated in situ are also found to catalyze the oxygenation reactions with high selectivity and efficiency for cyclohexane (A/K, 10-14). Upon their reaction with m-CPBA in DCM, a characteristic absorption band ( λmax, 600 nm, ε max, 355 M-1 cm-1) appears and decays at room temperature. This spectral feature is consistent with the mononuclear high-valent iron-oxo species suggested as an intermediate in the oxygenation reactions

Synthesis, structure, spectra and reactivity of iron(III) complexes of imidazole and pyrazole containing ligands as functional models for catechol dioxygenases

A series of new 1:1 iron(III) complexes of the type [Fe(L)Cl3], where L is a tridentate 3N donor ligand, has been isolated and studied as functional models for catechol dioxygenases. The ligands (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethyl-amine (L1), N,N-dimethyl-N'-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L2) and N-(1-methyl-1H-imidazol-2-ylmethyl)-N'-phenylethane-1,2-diamine (L3) are linear while the ligands tris(1-pyrazolyl)methane (L4), tris(3,5-dimethyl-1-pyrazolyl)methane (L5) and tris(3-iso-propylpyrazolyl)methane (L6) are tripodal ones. All the complexes have been characterized by spectral and electrochemical methods. The X-ray crystal structure of the dinuclear catecholate adduct [Fe(L2)(TCC)]2O, where TCC2- is a tetrachlorocatecholate dianion, has been successfully determined. In this complex both the iron(III) atoms are bridged by a μ-oxo group and each iron(III) center possesses a distorted octahedral coordination geometry in which the ligand L2 is facially coordinated and the remaining coordination sites are occupied by the TCC2- dianion. Spectral studies suggest that addition of a base like Et3N induces the mononuclear complex species [Fe(L2)(TCC)Cl] to dimerize forming a μ-oxo-bridged complex. The spectral and electrochemical properties of the catecholate adducts of the complexes generated in situ reveal that a systematic variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(III) center and hence the interaction of the complexes with simple and substituted catechols. The 3,5-di-tert-butylcatecholate (DBC2-) adducts of the type [Fe(L)(DBC)Cl], where L is a linear tridentate ligand (L1-L3), undergo mainly oxidative intradiol cleavage of the catechol in the presence of dioxygen. Also, the extradiol-to-intradiol product selectivity (E:I) is enhanced upon removal of the coordinated chloride ion in these adducts to obtain [Fe(L)(DBC)(Sol)]+ and upon incorporating coordinated N-methylimidazolyl nitrogen in them. In contrast to the iron(III) complexes of imidazole-based ligands, those of the tripodal pyrazole-based ligands L4-L6 yield major amounts of the oxidized product benzoquinone and small amounts of both intra- and extradiol products. One of the pyrazole arms coordinated in the equatorial plane of these sterically constrained complexes is substituted by a solvent molecule upon adduct formation with DBC2-, which encourages molecular oxygen to attack this site leading to benzoquinone formation. The DBSQ/DBC2- redox potentials of both the imidazole- and pyrazole-based complexes fall in the narrow range of -0.186 to -0.214 V supporting this proposal

Iron(III) complexes of tridentate N<SUB>3</SUB> ligands as models for catechol dioxygenases: stereoelectronic effects of pyrazole coordination

The iron(III) complexes of the tridentate N3 ligands pyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L1), 3,5-dimethylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L2), 3-iso-propylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L3) and (1-methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethylamine (L4) have been isolated and studied as functional models for catechol dioxygenases. They have been characterized by elemental analysis and spectral and electrochemical methods. The X-ray crystal structure of the complex [Fe(L1)Cl3] 1 has been successfully determined. The complex possesses a distorted octahedral coordination geometry in which the tridentate ligand facially engages iron(III) and the Cl- ions occupy the remaining coordination sites. The Fe-Npz bond distance (2.126(5) &#197;) is shorter than the Fe-Npy bond (2.199(5) &#197;). The systematic variation in the ligand donor substituent significantly influences the Lewis acidity of the iron(III) center and hence the interaction of the present complexes with a series of catechols. The catecholate adducts [Fe(L)(DBC)Cl], where H2DBC = 3,5-di-tert-butylcatechol, have been generated in situ and their spectral and redox properties and dioxygenase activities have been studied in N,N-dimethylformamide solution. The adducts [Fe(L)(DBC)Cl] undergo cleavage of DBC2- in the presence of dioxygen to afford major amounts of intradiol and smaller amounts extradiol cleavage products. In dichloromethane solution the [Fe(L)(DBC)Cl] adducts afford higher amounts of extradiol products (64.1-22.2%; extradiol-to-intradiol product selectivity E/I, 2.6:1-4.5:1) than in DMF (2.5-6.6%; E/I, 0.1:1-0.4:1). The results are in line with the recent understanding of the function of intra- and extradiol-cleaving catechol dioxygenases

Olefin aziridination by copper(II) complexes: effect of redox potential on catalytic activity

A series of new copper(II) complexes of four sterically hindering linear tridentate 3N ligands N'-ethyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L1), N'-benzyl-N'-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L2), N'-benzyl-N'-(6-methylpyrid-2-yl-methyl)-N,N-dimethylethylenediamine (L3) and N'-benzyl-N'-(quinol-2-ylmethyl)-N,N-dimethylethylenediamine (L4) have been isolated and examined as catalysts for olefin aziridination. The complexes [Cu(L1)Cl2]&#183;CH3OH 1, [Cu(L2)Cl2]&#183;CH3OH 2, [Cu(L3)Cl2]&#183;0.5 H2O 3 and [Cu(L4)Cl2] 4 have been structurally characterized by X-ray crystallography. In all of them copper(II) adopts a slightly distorted square pyramidal geometry as inferred from the values of trigonality index (t) for them (&#915; : 1, 0.02; 2, 0.01; 3, 0.07; 4, 0.01). Electronic and EPR spectral studies reveal that the complexes retain square-based geometry in solution also. The complexes undergo quasireversible Cu(II)/Cu(I) redox behavior (E&#189;, -0.272 - -0.454 V) in acetonitrile solution. The ability of the complexes to mediate nitrene transfer from PhINTs and chloramine-T trihydrate to olefins to form N-tosylaziridines has been studied. The complexes 3 and 4 catalyze the aziridination of styrene very slowly yielding above 80% of the desired product. They also catalyze the aziridination of the less reactive olefins like cyclooctene and n-hexene but with lower yields (30-50%). In contrast to these two complexes, 1 and 2 fail to catalyze the aziridination of olefins in the presence of both the nitrene sources. All these observations have been rationalized based on the Cu(II)/Cu(I) redox potentials of the catalysts

Novel square pyramidal iron(III) complexes of linear tetradentate bis(phenolate) ligands as structural and reactive models for intradiol-cleaving 3,4-PCD enzymes: quinone formation vs. intradiol cleavage

The iron(III) complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxy-4-methyl-benzyl)-1,4-diazepane H2(L1), 1,4-bis(2-hydroxy-4-nitrobenzyl)-1,4-diazepane H2(L2), 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane H2(L3) and 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane H2(L4) have been isolated and studied as structural and functional models for 3,4-PCD enzymes. The complexes [Fe(L1)Cl] 1, [Fe(L2)(H2O)Cl] 2, [Fe(L3)Cl] 3 and [Fe(L4)Cl] 4 have been characterized using ESI-MS, elemental analysis, and absorption spectral and electrochemical methods. The single crystal X-ray structure of 3 contains the FeN2O2Cl chromophore with a novel square pyramidal ( &#964; , 0.20) coordination geometry. The Fe-O-C bond angle (135.5 &#176; ) and Fe-O bond length (1.855 &#197;) are very close to the Fe-O-C bond angles (133, 148 &#176; ) and Fe-O(tyrosinate) bond distances (1.81, 1.91 &#197;) in 3,4-PCD enzyme. All the complexes exhibit two intense absorption bands in the ranges 335-383 and 493-541 nm, which are assigned respectively to phenolate (p&#960; ) &#8594; Fe(III) (d&#963; &#8727; ) and phenolate (p&#960; ) &#8594; Fe(III) (d&#960;&#8727; ) LMCT transitions. The Fe(III)/Fe(II) redox potentials of 1, 3 and 4 (E1/2, -0.882- -1.010 V) are more negative than that of 2 (E1/2, -0.577 V) due to the presence of two electron-withdrawing p-nitrophenolate moieties in the latter enhancing the Lewis acidity of the iron(III) center. Upon adding H2DBC pretreated with two equivalents of Et3N to the iron(III) complexes, two catecholate-to-iron(III) LMCT bands (656, &#949; , 1030; 515 nm, &#949; , 1330 M-1 cm-1) are observed for 2; however, interestingly, an intense catecholate-to-iron(III) LMCT band (530-541 nm) is observed for 1, 3 and 4 apart from a high intensity band in the range 451-462 nm. The adducts [Fe(L)(DBC)]- generated from 1-4in situ in DMF/Et3N solution react with dioxygen to afford almost exclusively the simple two-electron oxidation product 3,5-di-tert-butylbenzoquinone (DBQ), which is discerned from the appearance and increase in intensity of the electronic spectral band around 400 nm, and smaller amounts of cleavage products. Interestingly, in DMF/piperidine the amount of quinone product decreases and those of the cleavage products increase illustrating that the stronger base piperidine enhances the concentration of the catecholate adduct. The rates of both dioxygenation and quinone formation observed in DMF/Et3N solution vary in the order 1 &gt; 3 &gt; 4 &lt; 2 suggesting that the ligand steric hindrance to molecular oxygen attack, the Lewis acidity of the iron(III) center and the ability of the complexes to rearrange the Fe-O phenolate bonds to accommodate the catecholate substrate dictate the extent of interaction of the complexes with substrate and hence determine the rates of reactions. This is in line with the observation of DBSQ/H2DBC reduction wave for the adduct [Fe(L2)(DBC)]- at a potential (E1/2: -0.285 V) more positive than those for the adducts of 1, 3 and 4 (E1/2: -0.522 to -0.645 V)

Construction of heterocycles via 1,4-dipolar cycloaddition of quinoline-DMAD zwitterion with various dipolarophiles

Quinoline forms 1,4-zwitterion with dimethyl acetylenedicarboxylate, which is trapped by various dipolarophiles to yield a variety of pyridoquinoline and oxazinoquinoline derivatives

Hexabromide salt of a tiny octaazacryptand as a receptor for encapsulation of lower homolog halides: structural evidence on halide selectivity inside the tiny cage

Tiny azacryptand 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (L) upon reaction with 48% hydrobromic acid (containing &#60;0.05% chloride contamination) forms hexabromide salt (1). Single crystal X-ray crystallographic investigation of the hexaprotonated bromide (1) shows no guest encapsulation inside the tiny cage. This bromide salt 1 with an empty proton cage has been utilized as the receptor for encapsulation of chloride (2) and fluoride (3). Crystallographic results of mixed chloride/bromide (2) and fluoride/bromide (3) complexes of L are examined, which show monotopic recognition of chloride in the case of 2 and fluoride in the case of 3 inside the proton cage with five bromide and three water molecules outside the cavity. Single crystals obtained from an experiment on mixed anionic system (chloride and fluoride), 1 shows selective encapsulation of fluoride, which supports the formation of complex 3 and crystals obtained upon treatment of 2 with tetrabutyl ammonium fluoride also yields complex 3. In a separate reaction between L and 49% hydrobromic acid containing higher chloride contamination (&#60;0.2%) forms chloride/bromide salt (2). 1H NMR studies of 1 with sodium chloride and fluoride support the encapsulation of the respective anions inside the proton cage

A hybrid water–chloride structure with discrete undecameric water moieties self-assembled in a heptaprotonated octaamino cryptand

Odd water clusters: An unprecedented propeller-shaped undecameric water motif is stabilized within the lattice of the heptahydrochloride of a homoditopic octaamino cryptand with C<SUB>3</SUB> symmetry. The discrete water clusters are oriented symmetrically around the cryptand moiety and interact with encapsulated chloride inside the cavity (see structure; Cl green, O red, N blue)