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)Cl(2)], 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 degrees) and Fe-O bond length (1.889 Angstrom) are very close to those (Fe-O-C, 133 degrees and 148 degrees, Fe-O(tyrosinate), 1.81 and 1.91 Angstrom) of protocatechuate 3,4-dioxygenase enzymes. When the complex is treated with AgNO(3), the ligand-to-metal charge transfer (LMCT) band around 650 nm (epsilon, 2390 M(-1) cm(-1)) is red shifted to 665 nm with an increase in absorptivity (epsilon, 2630 M(-1) cm(-1)) and the Fe(III)/Fe(II) 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 (H(2)DBC) pretreated with 2 mol of Et(3)N is added to the complex [Fe(L3)Cl(2)] treated with 2 equiv of AgNO(3), two intense catecholate-to-iron(III) LMCT bands (719 nm, epsilon, 3150 M(-1) cm(-1); 494 nm, epsilon, 3510 M(-1) cm(-1)) are observed. Similar observations are made when H(2)DBC pretreated with 2 mol of piperidine is added to [Fe(L3)Cl(2)], suggesting the formation of [Fe(L3)(DBC)] with bidentate coordination of DBC(2-). On the other hand, when H(2)DBC pretreated with 2 mol of Et(3)N is added to [Fe(L3)Cl(2)], only one catecholate-to-iron(III) LMCT band (617 nm; epsilon, 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/H(2)DBC 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 DBC(2-) 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 H(2)DBC in the presence of O(2), unlike the iron(III)-phenolate complexes known to yield only intradiol products. It is probable that the weakly coordinated (2.310 Angstrom) -NMe(2) group rather than chloride in the substrate-bound complex is displaced, facilitating O(2) 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.     

Biomimetic iron(III) complexes of N3O and N3O2 donor ligands: protonation of coordinated ethanolate donor enhances dioxygenase activity

A series of iron(III) complexes 1-4 of the tripodal tetradentate ligands N,N-bis(pyrid-2-ylmethyl)-N-(2-hydroxyethyl)amine H(L1), N,N-bis(pyrid-2-ylmethyl)-N-(2-hydroxy- propyl)amine H(L2), N,N-bis(pyrid-2-ylmethyl)-N-ethoxyethanolamine H(L3), and N-((pyrid-2-ylmethyl)(1-methylimidazol-2-ylmethyl))-N-(2-hydroxyethyl)amine H(L4), have been isolated, characterized and studied as functional models for intradiol-cleaving catechol dioxygenases. In the X-ray crystal structure of [Fe(L1)Cl(2)] 1, the tertiary amine nitrogen and two pyridine nitrogen atoms of H(L1) are coordinated meridionally to iron(III) and the deprotonated ethanolate oxygen is coordinated axially. In contrast, [Fe(HL3)Cl(3)] 3 contains the tertiary amine nitrogen and two pyridine nitrogen atoms coordinated facially to iron(III) with the ligand ethoxyethanol moiety remaining uncoordinated. The X-ray structure of the bis(μ-alkoxo) dimer [{Fe(L5)Cl}(2)](ClO(4))(2)5, where HL is the tetradentate N(3)O donor ligand N,N-bis(1-methylimidazol-2-ylmethyl)-N-(2-hydroxyethyl)amine H(L5), contains the ethanolate oxygen donors coordinated to iron(III). Interestingly, the [Fe(HL)(DBC)](+) and [Fe(HL3)(HDBC)X] adducts, generated by adding ～1 equivalent of piperidine to solutions containing equimolar quantities of iron(III) complexes 1-5 and H(2)DBC (3,5-di-tert-butylcatechol), display two DBC(2-)→ iron(III) LMCT bands (λ(max): 1, 577, 905; 2, 575,915; 3, 586, 920; 4, 563, 870; 5, 557, 856 nm; Δλ(max), 299-340 nm); however, the bands are blue-shifted (λ(max): 1, 443, 700; 2, 425, 702; 3, 424, 684; 4, 431, 687; 5, 434, 685 nm; Δλ(max), 251-277 nm) on adding 1 more equivalent of piperidine to form the adducts [Fe(L)(DBC)] and [Fe(HL3)(HDBC)X]. Electronic spectral and pH-metric titration studies in methanol disclose that the ligand in [Fe(HL)(DBC)](+) is protonated. The [Fe(L)(DBC)] adducts of iron(III) complexes of bis(pyridyl)-based ligands (1,2) afford higher amounts of intradiol-cleavage products, whereas those of mono/bis(imidazole)-based ligands (4,5) yield mainly the auto-oxidation product benzoquinone. It is remarkable that the adducts [Fe(HL)(DBC)](+)/[Fe(HL3)(DBC)X] exhibit higher rates of oxygenation affording larger amounts of intradiol-cleavage products and lower amounts of benzoquinone.     

Mononuclear iron(III) complexes of 3N ligands in organized assemblies: spectral and redox properties and attainment of regioselective extradiol dioxygenase activity

Four octahedral iron(III) complexes of the type [Fe(L)Cl(3)], where L is a tridentate 3N ligand like N,N-bis(pyrid-2-ylmethyl)amine (bpa, L1), N,N-bis(benzimidazol-2-ylmethyl)amine (bba, L2), 1,4,7-triazacyclononane (tacn, L3) and 2,2';6',2'-terpyridine (terpy, L4), have been isolated and their catechol dioxygenase activity investigated in dichloromethane, water and different aqueous micellar media. The positions of both the catecholato-to-iron(III) LMCT bands observed for the DBC(2-) (H(2)DBC = 3,5-di-tert-butylcatechol) adducts reveal that the adducts are present as cationic [Fe(L)(DBC)(H(2)O)](+) species, which interact strongly with anionic SDS micelles and dock themselves on the anionic micellar surface, and that they exist in the aqueous phase in CTAB and TX 100 micelles. The Fe(III)/Fe(II) redox potentials of the complexes throw light on the Lewis acidity of the iron(III) center as modified by the ligand donor atoms and hence the interaction of the complexes with different micelles. The DBSQ/DBC(2-) redox potentials in SDS micellar media are more positive than those in aqueous solution confirming the presence of the aqua species [Fe(L)(DBC)(H(2)O)](+). The DBC(2-) adducts of the iron(III) complexes of bpa, bba and tacn ligands, all with facial coordination, elicit extradiol (E) cleavage to different extents while the adduct of the terpy complex with meridional coordination of the ligand shows always intradiol (I) cleavage. It is remarkable that the bpa complex shows the highest yield of extradiol product and high product selectivity in aqueous SDS solution (E, 84.0%; E/I, 61.0?:?1) and in SDS?:?n-hexane reverse micellar medium (E, 93.7%) illustrating that a vacant or solvent coordinated site is essential for observing extradiol cleavage. Interestingly, the rates of dioxygenase reactions in aqueous and aqueous micellar solutions are significantly higher than those in non-aqueous solvents. Also, they diminish in the order, SDS > TX-100 > CTAB, illustrating the facile substitution of coordinated water molecule by molecular oxygen in [Fe(L)(DBC)(H(2)O)](+) bound to anionic SDS micelles.     

Iron(III) complexes of tridentate 3N ligands as functional models for catechol dioxygenases: the role of ligand N-alkyl substitution and solvent on reaction rate and product selectivity

A series of iron(III) complexes of the type [Fe(L)Cl3], where L is the variously N-alkyl-substituted bis(pyrid-2-ylmethyl)amine ligand such as bis(pyrid-2-ylmethyl)amine (L1), N,N-bis(pyrid-2-ylmethyl)methylamine (L2), N,N-bis(pyrid-2-ylmethyl)-n-propylamine (L3), N,N-bis(pyrid-2-ylmethyl)-iso-butylamine (L4), N,N-bis(pyrid-2-ylmethyl)-iso-propylamine (L5), N,N-bis(pyrid-2-ylmethyl)cyclohexylamine (L6), and N,N-bis(pyrid-2-ylmethyl)-tert-butylamine (L7), have been isolated and characterized by elemental analysis and spectral and electrochemical methods. The crystal structures of the complexes [Fe(L2)Cl3] 2, [Fe(L3)Cl3] 3, and the complex-substrate adduct [Fe(L5)(TCC)(NO3)] 5a, where TCC2- is the tetrachlorocatecholate dianion, have been determined by single-crystal X-ray crystallography. The complexes [Fe(L2)Cl3] 2 and [Fe(L3)Cl3] 3 possess a distorted octahedral geometry, in which the linear tridentate 3N ligands are cis-facially coordinated to the iron(III) center, and three chloride ions occupy the remaining coordination sites. The replacement of the N-methyl group in 2 by N-n-propyl group as in 3 leads to the formation of the Fe-Npy bonds and also the Fe-Cl bonds located trans to them of different lengths. The catecholate adduct 5a also possesses a distorted octahedral geometry, in which the ligand is cis-facially coordinated to iron(III) center, TCC2- is asymmetrically chelated trans to the two pyridyl moieties of the ligand, and one of the oxygen atoms of the nitrate ion occupies the sixth coordination site. All of the present complexes have been interacted with simple and substituted catechols. The catecholate adducts [Fe(L)(DBC)Cl] and [Fe(L)(DBC)(Sol)]+, where H2DBC is 3,5-di-tert-butylcatechol and Sol=H2O/CH3CN, have been generated in situ, and their spectral and redox properties and dioxygenase activities have been studied in dimethylformamide and dichloromethane solutions. All of the complexes catalyze the cleavage of H2DBC using molecular oxygen to afford both intra- and extradiol cleavage products. The formation of extradiol cleavage products is facilitated by cis-facial coordination of the 3N ligands and availability of vacant coordination site on iron(III) center for dioxygen binding. It is remarkable that the nature of the N-alkyl substituent in 3N ligands controls the regioselectivity of cleavage, with the n-propyl, iso-butyl, iso-propyl, and cyclohexyl groups enhancing the yield of extradiol products (46-68%) in dichloromethane. The rate of oxygenation depends upon the solvent and the Lewis acidity of iron(III) center as modified by the sterically demanding N-alkyl groups-length and degree of substitution. The plot of log (kO2) versus energy of the low-energy DBC2--to-iron(III) LMCT band is linear, demonstrating the importance of the Lewis acidity of the iron(III) center in dictating the rate of the dioxygenase reaction.     

Dioxygen Reactivity of Biomimetic Iron-Catecholate and Iron-o-Aminophenolate Complexes of a Tris(2-pyridylthio)methanido Ligand: Aromatic CC Bond Cleavage of Catecholate versus o-Iminobenzosemiquinonate Radical Formation

An iron(III)-catecholate complex [L(1) Fe(III) (DBC)] (2) and an iron(II)-o-aminophenolate complex [L(1) Fe(II) (HAP)] (3; where L(1) =tris(2-pyridylthio)methanido anion, DBC=dianionic 3,5-di-tert-butylcatecholate, and HAP=monoanionic 4,6-di-tert-butyl-2-aminophenolate) have been synthesised from an iron(II)-acetonitrile complex [L(1) Fe(II) (CH(3) CN)(2) ](ClO(4) ) (1). Complex 2 reacts with dioxygen to oxidatively cleave the aromatic C?C bond of DBC giving rise to selective extradiol cleavage products. Controlled chemical or electrochemical oxidation of 2, on the other hand, forms an iron(III)-semiquinone radical complex [L(1) Fe(III) (SQ)](PF(6) ) (2(ox) -PF(6) ; SQ=3,5-di-tert-butylsemiquinonate). The iron(II)-o-aminophenolate complex (3) reacts with dioxygen to afford an iron(III)-o-iminosemiquinonato radical complex [L(1) Fe(III) (ISQ)](ClO(4) ) (3(ox) -ClO(4) ; ISQ=4,6-di-tert-butyl-o-iminobenzosemiquinonato radical) via an iron(III)-o-amidophenolate intermediate species. Structural characterisations of 1, 2, 2(ox) and 3(ox) reveal the presence of a strong iron?carbon bonding interaction in all the complexes. The bond parameters of 2(ox) and 3(ox) clearly establish the radical nature of catecholate- and o-aminophenolate-derived ligand, respectively. The effect of iron?carbon bonding interaction on the dioxygen reactivity of biomimetic iron-catecholate and iron-o-aminophenolate complexes is discussed.     

The effect of ligand charge on the coordination geometry of an Fe(III)ion: five- and six-coordinate Fe(III) complexes of tris(2-benzimidazolylmethyl)amine

By using the tripodal tetradentate ligand tris(2-benzimidazolylmethyl)amine (H(3)ntb), which can have several charge states depending on the number of secondary amine protons, mononuclear octahedral and dinuclear trigonal bipyramidal Fe(III) complexes were prepared. The reaction of mononuclear octahedral [Fe(III)(H(3)ntb)Cl(2)]ClO(4), 1, with 3 equiv of sec-butylamine in methanol led to the formation of mononuclear cis-dimethoxo octahedral Fe(III)(H(2)ntb)(OMe)(2), 2. One equivalent of the sec-butylamine was used to generate the monoanionic H(2)ntb(-) ligand where one of the three amines in the benzimidazolyl groups was deprotonated. The remaining 2 equiv were used to generate two methoxides that were coordinated to the octahedral Fe(III) ion in a cis fashion as demonstrated by the chlorides in 1. Reaction of 1 with excess (7 equiv) sec-butylamine generated the doubly deprotonated dianionic Hntb(2-) that stabilized the dinuclear mu-oxo Fe(III)(2)O(Hntb)(2), 3, adopting a five-coordinate trigonal bipyramidal geometry. The magnetic data for 3 are consistent with the antiferromagnetically coupled Fe(III) (S = 5/2) sites with the coupling constant J = -127 cm(-1).     

Modeling the 2-His-1-carboxylate facial triad: iron-catecholato complexes as structural and functional models of the extradiol cleaving dioxygenases

Mononuclear iron(II)- and iron(III)-catecholato complexes with three members of a new 3,3-bis(1-alkylimidazol-2-yl)propionate ligand family have been synthesized as models of the active sites of the extradiol cleaving catechol dioxygenases. These enzymes are part of the superfamily of dioxygen-activating mononuclear non-heme iron enzymes that feature the so-called 2-His-1-carboxylate facial triad. The tridentate, tripodal, and monoanionic ligands used in this study include the biologically relevant carboxylate and imidazole donor groups. The structure of the mononuclear iron(III)-tetrachlorocatecholato complex [Fe(L3)(tcc)(H2O)] was determined by single-crystal X-ray diffraction, which shows a facial N,N,O capping mode of the ligand. For the first time, a mononuclear iron complex has been synthesized, which is facially capped by a ligand offering a tridentate Nim,Nim,Ocarb donor set, identical to the endogenous ligands of the 2-His-1-carboxylate facial triad. The iron complexes are five-coordinate in noncoordinating media, and the vacant coordination site is accessible for Lewis bases, e.g., pyridine, or small molecules such as dioxygen. The iron(II)-catecholato complexes react with dioxygen in two steps. In the first reaction the iron(II)-catecholato complexes rapidly convert to the corresponding iron(III) complexes, which then, in a second slow reaction, exhibit both oxidative cleavage and auto-oxidation of the substrate. Extradiol and intradiol cleavage are observed in noncoordinating solvents. The addition of a proton donor results in an increase in extradiol cleavage. The complexes add a new example to the small group of synthetic iron complexes capable of eliciting extradiol-type cleavage and provide more insight into the factors determining the regioselectivity of the enzymes.     

Iron(III) complexes with a tripodal N3O ligand containing an internal base as a model for catechol intradiol-cleaving dioxygenases

A bis(mu-alkoxo)-bridged dinuclear iron(III) complex [Fe(L)(NO3)]2(NO3)2 [1; HL = N,N-bis(2-pyridylmethyl)-N-(2-hydroxyethyl)amine] of the tripodal N3O ligand was prepared as a biomimetic model for the intradiol-cleaving dioxygenase enzymes. The reaction of 1 and catechol in the presence of excess triethylamine gave the catecholate (CAT) chelate bis(mu-alkoxo)-bridged dinuclear iron(III) complex [Fe(L)(CAT)]2 (2). The molecular structures of complexes 1 and 2 were determined by X-ray crystallography. Diiron complexes 1 and 2 contain the same bis(mu-alkoxo)diiron diamond core. All heteroatoms (N3O) of the ligand are coordinated to the iron center in complex 1 with two pyridine nitrogen atoms on the axial bonds, while one of the pyridyl arms of the ligand is left uncoordinated in complex 2. The interaction of the diiron complex 1 and 3,5-di-tert-butylcatechol (H2DBC) was investigated by electronic and mass spectroscopy. Complex 1 displays the intradiol-cleaving dioxygenase activity, and the coordinate ethoxyl arm of the ligand is capable of accepting the proton from catechol, which mimics the function of Tyr447 in the protocatechuate 3,4-dioxygenase as an internal base. The spectrophotometric titration experiment indicates the relatively low demand of the external base (0.8 equiv based on Fe(3+)) for attaining the highest dioxygenase activity of complex 1. The reaction rate of the reactive intermediate [Fe(HL)(DBC)]+ with dioxygen is 0.38 M(-1) s(-1) determined by kinetic studies.     

(Alpha-diimine)chromium complexes: molecular and electronic structures; a combined experimental and density functional theoretical study

Dark brown crystals of [Cr( (1)L) 2] ( 1) were obtained from the reaction of [Cr (III)(acac) 3] (acac (-) = 2,4-pentanedionate) with 2 equiv of 2-methyl-1,4-bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene ( (1)L) and 3 equiv of sodium in tetrahydrofuran (thf) under an Ar atmosphere. Complex 1 possesses an S = 1 ground state, which is attained via intramolecular antiferromagnetic coupling between a high-spin Cr (II) ion ( S Cr = 2) and two anionic alpha-diiminato(1-) ligand pi radicals ( (1)L (*)) (1-). The molecular structure of 1 exhibits a distorted tetrahedral, nearly square-planar geometry. The average C-N imine bond length at 1.346 A is characteristic for the pi radical anion ( (1)L (*)) (1-), and therefore, the electronic structure of 1 is best described as [Cr (II)( (1)L (*)) 2]. This has been confirmed by broken symmetry density functional theoretical calculations BS(4,2) (DFT) at the B3LYP level. The reaction of [Cr (III)(acac) 3] with 1 equiv of 2,3-dimethyl-1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene ( (2)L) and 1 equiv of Na in thf under Ar yields red-brown crystals of [Cr (III)( (2)L (*))(acac) 2] ( 2) ( S = 1). The oxidation of 2 with 1 equiv of Fc(PF 6) (Fc (+) = ferrocenium) in CH 2Cl 2 affords crystals of [Cr (III)( (2)L (ox))(acac) 2](PF 6) ( 3) ( S = (3)/ 2). The crystal structure determinations of 2 and 3 revealed that 2 contains a neutral, octahedral Cr (III) species [Cr (III)( (2)L (*))(acac) 2], whereas in 3 the ligand is oxidized, yielding an octahedral monocation [Cr (III)( (2)L (ox))(acac) 2] (+). These electronic structures have been confirmed by DFT calculations.     

Syntheses, structures and properties of a series of non-heme alkoxide-Fe(III) complexes of a benzimidazolyl-rich ligand as models for lipoxygenase

The treatment of Fe(ClO(4))(2)·6H(2)O or Fe(ClO(4))(3)·9H(2)O with a benzimidazolyl-rich ligand, N,N,N',N'-tetrakis[(1-methyl-2-benzimidazolyl)methyl]-1,2-ethanediamine (medtb) in alcohol/MeCN gives a mononuclear ferrous complex, [Fe(II)(medtb)](ClO(4))(2)·?CH(3)CN·?CH(3)OH (1), and four non-heme alkoxide-iron(III) complexes, [Fe(III)(OMe)(medtb)](ClO(4))(2)·H(2)O (2, alcohol = MeOH), [Fe(III)(OEt)(Hmedtb)](ClO(4))(3)·CH(3)CN (3, alcohol = EtOH), [Fe(III)(O(n)Pr)(Hmedtb)](ClO(4))(3)·(n)PrOH·2CH(3)CN (4, alcohol = n-PrOH), and [Fe(III)(O(n)Bu)(Hmedtb)](ClO(4))(3)·3CH(3)CN·H(2)O (5, alcohol = n-BuOH), respectively. The alkoxide-iron(III) complexes all show 1) a Fe(III)-OR center (R = Me, 2; Et, 3; (n)Pr, 4; (n)Bu, 5) with the Fe-O bond distances in the range of 1.781-1.816 ?, and 2) a yellow color and an intense electronic transition around 370 nm. The alkoxide-iron(III) complexes can be reduced by organic compounds with a cis,cis-1,4-diene moiety via the hydrogen atom abstraction reaction.     

Coordination versatility of tridentate pyridyl aroylhydrazones towards iron: tracking down the elusive aroylhydrazono-based ferric spin-crossover molecular materials

The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL(1)) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL(2)) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe(II)(HL)(2)](ClO(4))(2), [Fe(II)L(2)] and [Fe(III)L(2)]ClO(4) (HL = HL(1) or HL(2)) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe(III)(L(1))(2)]ClO(4) is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe(III)(L(1))(2)]ClO(4) and [Fe(III)(L(2))(2)]ClO(4) are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the (6)A(1)?(2)T(2) transition curve is sigmoidal with T(?)～280 K and the SCO is virtually complete. As regards [Fe(III)(L(1))(2)]ClO(4), M?ssbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.     

Reactivity of a (mu-oxo)(mu-hydroxo)diiron(III) diamond core with water, urea, substituted ureas, and acetamide

A series of iron(III) complexes of the tetradentate ligand BPMEN (N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) were prepared and structurally characterized. Complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)](ClO(4))(3) (1) contains a (mu-oxo)(mu-hydroxo)diiron(III) diamond core. Complex [Fe(BPMEN)(urea)(OEt)](ClO(4))(2) (2) is a rare example of a mononuclear non-heme iron(III) alkoxide complex. Complexes [Fe(2)(mu-O)(mu-OC(NH(2))NH)(BPMEN)(2)](ClO(4))(3) (3) and [Fe(2)(mu-O)(mu-OC(NHMe)NH)(BPMEN)(2)](ClO(4))(3) (4) feature N,O-bridging deprotonated urea ligands. The kinetics and equilibrium of the reactions of 1 with ligands L (L = water, urea, 1-methylurea, 1,1-dimethylurea, 1,3-dimethylurea, 1,1,3,3-tetramethylurea, and acetamide) in acetonitrile solutions were studied by stopped-flow UV-vis spectrophotometry, NMR, and mass spectrometry. All these ligands react with 1 in a rapid equilibrium, opening the four-membered Fe(III)(mu-O)(mu-OH)Fe(III) core and forming intermediates with a (HO)Fe(III)(mu-O)Fe(III)(L) core. The entropy and enthalpy for urea binding through oxygen are DeltaH degrees = -25 kJ mol(-1) and DeltaS degrees = -53.4 J mol(-1) K(-1) with an equilibrium constant of K(1) = 37 L mol(-1) at 25 degrees C. Addition of methyl groups on one of the urea nitrogen did not affect this reaction, but the addition of methyl groups on both nitrogens considerably decreased the value of K(1). An opening of the hydroxo bridge in the diamond core complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)] is a rapid associative process, with activation enthalpy of about 60 kJ mol(-1) and activation entropies ranging from -25 to -43 J mol(-1) K(-1). For the incoming ligands with the -CONH(2) functionality (urea, 1-methylurea, 1,1-dimethylurea, and acetamide), a second, slow step occurs, leading to the formation of stable N,O-coordinated amidate diiron(III) species such as 3 and 4. The rate of this ring-closure reaction is controlled by the steric bulk of the incoming ligand and by the acidity of the amide group.     

Proton control of oxidation and spin state in a series of iron tripodal imidazole complexes

Reaction of iron salts with three tripodal imidazole ligands, H(3)(1), H(3)(2), H(3)(3), formed from the condensation of tris(2-aminoethyl)amine (tren) with 3 equiv of an imidazole carboxaldehyde yielded eight new cationic iron(III) and iron(II), [FeH(3)L](3+or2+), and neutral iron(III), FeL, complexes. All complexes were characterized by EA(CHN), IR, UV, M?ssbauer, mass spectral techniques and cyclic voltammetry. Structures of three of the complexes, Fe(2).3H(2)O (C(18)H(27)FeN(10)O(3), a = b = c = 20.2707(5), cubic, I3d, Z = 16), Fe(3).4.5H(2)O (C(18)H(30)FeN(10)O(4.5), a = 20.9986(10), b = 11.7098(5), c = 19.9405(9), beta = 109.141(1), monoclinic, P2(1)/c), Z = 8), and [FeH(3)(3)](ClO(4))(2).H(2)O (C(18)H(26)Cl(2)FeN(10)O(9), a = 9.4848(4), b = 23.2354(9), c = 12.2048(5), beta = 111.147(1) degrees, monoclinic, P2(1)/n, Z = 4) were determined at 100 K. The structures are similar to one another and feature an octahedral iron with facial coordination of imidazoles and imine nitrogen atoms. The iron(III) complexes of the deprotonated ligands, Fe(1), Fe(2), and Fe(3), are low-spin while the protonated iron(III) cationic complexes, [FeH(3)(1)](ClO(4))(3) and [FeH(3)(2)](ClO(4))(3), are high-spin and spin-crossover, respectively. The iron(II) cationic complexes, [FeH(3)(1)]S(4)O(6), [FeH(3)(2)](ClO(4))(2), [FeH(3)(3)](ClO(4))(2), and [FeH(3)(3)][B(C(6)H(5))(4)](2) exhibit spin-crossover behavior. Cyclic voltammetric measurements on the series of complexes show that complete deprotonation of the ligands produces a negative shift in the Fe(III)/Fe(II) reduction potential of 981 mV on average. Deprotonation in air of either cationic iron(II) or iron(III) complexes, [FeH(3)L](3+or2+), yields the neutral iron(III) complex, FeL. The process is reversible for Fe(3), where protonation of Fe(3) yields [FeH(3)(3)](2+).     

Ligand-directed molecular architectures: self-assembly of two-dimensional rectangular metallacycles and three-dimensional trigonal or tetragonal prisms

Three angular ditopic ligands (1,3-bis(benzimidazol-1-ylmethyl)-4,6-dimethylbenzene L(1), 1,3-bis(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene L(2), and 1,4-bis(benzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene L(3)) and one tripodal ligand 1,3,5-tris(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene L(4) have been prepared. Reaction of these shape-specific designed ligands with different metal salts affords a series of discrete molecular architectures: [Ag(2)L(1)(2)](BF(4))(2) 1, [Ag(2)L(2)(2)](CF(3)SO(3))(2) 2, [CF(3)SO(3)(-) subset Ag(2)L(3)(2)]CF(3)SO(3) 3, [CF(3)SO(3)(-) subset Ag(2)L(3)(3)]CF(3)SO(3) 4, [ClO(4)(-) subset Cu(2)L(2)(4)](ClO(4))(3) 5, [4H(2)O subset Ni(2)L(2)(4)Cl(4)].6H(2)O 6, [BF(4)(-) subset Ag(3)L(4)(2)](BF(4))(2) 7, [ClO(4)(-) subset Ag(3)L(4)(2)](ClO(4))(2) 8, and [CuI(3)(2-) subset Cu(3)L(4)(2)](2)[Cu(2)I(4)] 9. The compounds were characterized by elemental analysis, ESI-MS, IR, and NMR spectroscopy, and X-ray crystallography. 1 is a dinuclear metallacycle with 2-fold rotational symmetry in which two syn-conformational L(1) ligands are connected by two linearly coordinated Ag(+) ions. 2 and 3 are structurally related, consisting of rectangular structures assembled from two linearly coordinated Ag(+) ions and two L(2) or L(3) ligands. The structure of 4 is a trigonal prismatic box consisting of two Ag(+) ions in trigonal planar coordination linked by three L(3) ligands, while the structures of 5 and 6 are tetragonal prismatic cages constructed by two square planar Cu(2+) or Ni(2+) ions linked by four L(2) ligands. The topologies of 7-9 are similar to that of 4; however, these three structures are assembled from three linearly coordinated Ag(+) or Cu(+) ions and two tripodal ligands, representing an alternative strategy to assembling a trigonal prism. (1)H NMR and ESI-MS were utilized to elucidate the solution structures of these macrocycles.     

Synthesis, crystal structures, and magnetic properties of a new family of heterometallic cyanide-bridged Fe(III)2M(II)2 (M=Mn, Ni, and Co) square complexes

New heterobimetallic tetranuclear complexes of formula [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Mn(II)(bpy)(2)](2)(ClO(4))(2)·CH(3)CN (1), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2a), [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2b), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3a), and [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3b), [HB(pz)(3)(-) = hydrotris(1-pyrazolyl)borate, B(Pz)(4)(-) = tetrakis(1-pyrazolyl)borate, dmphen = 2,9-dimethyl-1,10-phenanthroline, bpy = 2,2'-bipyridine] have been synthesized and structurally and magnetically characterized. Complexes 1-3b have been prepared by following a rational route based on the self-assembly of the tricyanometalate precursor fac-[Fe(III)(L)(CN)(3)](-) (L = tridentate anionic ligand) and cationic preformed complexes [M(II)(L')(2)(H(2)O)(2)](2+) (L' = bidentate α-diimine type ligand), this last species having four blocked coordination sites and two labile ones located in cis positions. The structures of 1-3b consist of cationic tetranuclear Fe(III)(2)M(II)(2) square complexes [M = Mn (1), Ni (2a and 2b), Co (3a and 3b)] where corners are defined by the metal ions and the edges by the Fe-CN-M units. The charge is balanced by free perchlorate anions. The [Fe(L)(CN)(3)](-) complex in 1-3b acts as a ligand through two cyanide groups toward two divalent metal complexes. The magnetic properties of 1-3b have been investigated in the temperature range 2-300 K. A moderately strong antiferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Mn(II) (S = 5/2) ions has been found for 1 leading to an S = 4 ground state (J(1) = -6.2 and J(2) = -2.7 cm(-1)), whereas a moderately strong ferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Ni(II) (S = 1) and Co(II) (S = 3/2) ions has been found for complexes 2a-3b with S = 3 (2a and 2b) and S = 4 (3a and 3b) ground spin states [J(1) = +21.4 cm(-1) and J(2) = +19.4 cm(-1) (2a); J(1) = +17.0 cm(-1) and J(2) = +12.5 cm(-1) (2b); J(1) = +5.4 cm(-1) and J(2) = +11.1 cm(-1) (3a); J(1) = +8.1 cm(-1) and J(2) = +11.0 cm(-1) (3b)] [the exchange Hamiltonian being of the type H? = -J(S?(i)·S?(j))]. Density functional theory (DFT) calculations have been used to substantiate the nature and magnitude of the exchange magnetic coupling observed in 1-3b and also to analyze the dependence of the exchange magnetic coupling on the structural parameters of the Fe-C-N-M skeleton.     

Models for extradiol cleaving catechol dioxygenases: syntheses, structures, and reactivities of iron(II)-monoanionic catecholate complexes

Crystallographic and spectroscopic studies of extradiol cleaving catechol dioxygenases indicate that the enzyme-substrate complexes have both an iron(II) center and a monoanionic catecholate. Herein we report a series of iron(II)-monoanionic catecholate complexes, [(L)Fe(II)(catH)](X) (1a, L = 6-Me(3)-TPA (tris(6-methyl-2-pyridylmethyl)amine), catH = CatH (1,2-catecholate monoanion); 1b, L = 6-Me(3)-TPA, catH = DBCH (3,5-di-tert-butyl-1,2-catecholate monoanion); 1c, L = 6-Me(2)-bpmcn (N,N'-dimethyl-N,N'-bis(6-methyl-2-pyridylmethyl)-trans-1,2-diaminocyclohexane), catH = CatH; 1d, L = 6-Me(2)-bpmcn, catH = DBCH), that model such enzyme complexes. The crystal structure of [(6-Me(2)-bpmcn)Fe(II)(DBCH)](+) (1d) shows that the DBCH ligand binds to the iron asymmetrically as previously reported for 1b, with two distinct Fe-O bonds of 1.943(1) and 2.344(1) A. Complexes 1 react with O(2) or NO to afford blue-purple iron(III)-catecholate dianion complexes, [(L)Fe(III)(cat)](+) (2). Interestingly, crystallographically characterized 2d, isolated from either reaction, has the N-methyl groups in a syn configuration, in contrast to the anti configuration of the precursor complex, so epimerization of the bound ligand must occur in the course of isolating 2d. This notion is supported by the fact that the UV-vis and EPR properties of in situ generated 2d(anti) differ from those of isolated 2d(syn). While the conversion of 1 to 2 in the presence of O(2) occurs without an obvious intermediate, that in the presence of NO proceeds via a metastable S = (3)/(2) [(L)Fe(catH)(NO)](+) adduct 3, which can only be observed spectroscopically but not isolated. Intermediates 3a and 3b subsequently disproportionate to afford two distinct complexes, [(6-Me(3)-TPA)Fe(III)(cat)](+) (2a and 2b) and [(6-Me(3)-TPA)Fe(NO)(2)](+) (4) in comparable yield, while 3d converts to 2d in 90% yield. Complexes 2b and anti-2d react further with O(2) over a 24 h period and afford a high yield of cleavage products. Product analysis shows that the products mainly derive from intradiol cleavage but with a small extent of extradiol cleavage (89:3% for 2b and 78:12% for anti-2d). The small amounts of the extradiol cleavage products observed may be due to the dissociation of an alpha-methyl substituted pyridyl arm, generating a complex with a tridentate ligand. Surprisingly, syn-2d does not react with O(2) over the course of 4 days. These results suggest that there are a number of factors that influence the mode and rate of cleavage of catechols coordinated to iron centers.     

Spectrophotometric studies on ion-pair extraction equilibria of the iron(ii) and iron(III) complexes with 4-(2-pyridylazo)resorcinol

The ion-pair extraction equilibria of the iron(II) and iron(III) chelates of 4-(2-pyridylazo)resorcinol (PAR, H(2)L) are described. The anionic chelates were extracted into chloroform with benzyldimethyltetradecylammonium chloride (QC1) as counter-ion. The extraction constants were estimated to be K(ex1)(Fe(II)) = [Q{Fe(II)(HL)L}](0)/[Q(+)][{Fe(II)(HL)L}(-)] = 10(8.59 +/- 0.11), K(ex2)(Fe(II)) = [Q(2){Fe(II)L(2)}](o)/ [Q(+)](2)[{Fe(II)L(2)}(2-)] = 10(12.17 +/- 0.10) and K(ex1)(Fe(III)) = [Q{Fe((III))L(2)}](o)/(Q(+)][{Fe(III)L(2)}(-)] = 10(6.78 +/- 0.15) at I = 0.10 and 20 degrees , where [ ](o) is concentration in the chloroform phase. Aggregation of Q{Fe(III)L(2)} in chloroform was observed and the dimerization constant (K(d) = [Q(2){Fe(III)L(2)}(2)](o)/[Q{Fe(III)L(2)}](o)(2)) was evaluated as log K(d) = 4.3 +/- 0.3 at 20 degrees . The neutral chelates of {Fe(II)(HL)(2)} and {Fe(III)(HL)L}, and the ion-pair of the cationic chelate, {Fe(III)(HL)(2)}ClO(4), were also extracted into chloroform or nitrobenzene. The relationship between the forms and extraction properties of the iron(II) and iron(III) PAR chelates are discussed in connection with those of the nickel(II) and cobalt(III) complexes. Correlation between the extraction equilibrium data and the elution behaviour of some PAR chelates in ion-pair reversed-phase partition chromatography is also discussed.     

Redox-noninnocence of the S,S'-coordinated ligands in bis(benzene-1,2-dithiolato)iron complexes

The electronic structures of complexes of iron containing two S,S'-coordinated benzene-1,2-dithiolate, (L)(2)(-), or 3,5-di-tert-butyl-1,2-benzenedithiolate, (L(Bu))(2)(-), ligands have been elucidated in depth by electronic absorption, infrared, X-band EPR, and Mossbauer spectroscopies. It is conclusively shown that, in contrast to earlier reports, high-valent iron(IV) (d(4), S = 1) is not accessible in this chemistry. Instead, the S,S'-coordinated radical monoanions (L(*))(1)(-) and/or (L(Bu)(*))(1)(-) prevail. Thus, five-coordinate [Fe(L)(2)(PMe(3))] has an electronic structure which is best described as [Fe(III)(L)(L(*))(PMe(3))] where the observed triplet ground state of the molecule is attained via intramolecular, strong antiferromagnetic spin coupling between an intermediate spin ferric ion (S(Fe) = (3)/(2)) and a ligand radical (L(*))(1)(-) (S(rad) = (1)/(2)). The following complexes containing only benzene-1,2-dithiolate(2-) ligands have been synthesized, and their electronic structures have been studied in detail: [NH(C(2)H(5))(3)](2)[Fe(II)(L)(2)] (1), [N(n-Bu)(4)](2)[Fe(III)(2)(L)(4)] (2), [N(n-Bu)(4)](2)[Fe(III)(2)(L(Bu))(4)] (3); [P(CH(3))Ph(3)][Fe(III)(L)(2)(t-Bu-py)] (4) where t-Bu-py is 4-tert-butylpyridine. Complexes containing an Fe(III)(L(*))(L)- or Fe(III)(L(Bu))(L(Bu)(*))- moiety are [N(n-Bu)(4)][Fe(III)(2)(L(Bu))(3)(L(Bu)(*))] (3(ox)()), [Fe(III)(L)(L(*))(t-Bu-py)] (4(ox)()), [Fe(III)(L(Bu))(L(Bu)(*))(PMe(3))] (7), [Fe(III)(L(Bu))(L(Bu)(*))(PMe(3))(2)] (8), and [Fe(III)(L(Bu))(L(Bu)(*))(PPr(3))] (9), where Pr represents the n-propyl substituent. Complexes 2, 3(ox)(), 4, [Fe(III)(L)(L(*))(PMe(3))(2)] (6), and 9 have been structurally characterized by X-ray crystallography.     

Iron(III) complexes of sterically hindered tetradentate monophenolate ligands as functional models for catechol 1,2-dioxygenases: the role of ligand stereoelectronic properties

The iron(III) complexes of the monophenolate ligands 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol [H(L1)], N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl)ethylenediamine [H(L2)], N,N-dimethyl-N'-(6-methyl-pyrid-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl)ethylenediamine [H(L3)], and N,N-dimethyl-N'-(1-methylimidazole-2-ylmethyl)-N'-(2-hydroxy-4-nitrobenzyl)ethylenediamine [H(L4)] have been obtained and studied as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The complexes [Fe(L1)Cl(2)].CH(3)CN (1), [Fe(L2)Cl(2)] (2), [Fe(L3)Cl(2)] (3), and [Fe(L4)Cl(2)] (4) have been characterized using absorption spectral and electrochemical methods. The single crystal X-ray crystal structures of 1 and 2 have been successfully determined. Both the complexes possess a rhombically distorted octahedral coordination geometry for the FeN(3)OCl(2) chromophore. In 2, the phenolate oxygen, the pyridine nitrogen, an amine nitrogen, and a chloride ion are located on the corners of a square plane with the nitrogen atom of a -NMe(2) group and the other chloride ion occupying the axial positions. In 1, also the equatorial plane is constituted by the phenolate oxygen, the pyridine nitrogen, an amine nitrogen atom, and a chloride ion; however, the axial positions are occupied by the second pyridine nitrogen and the second chloride ion. Interestingly, the Fe-O-C angle of 136.1 degrees observed for 2 is higher than that (128.5 degrees ) in 1; however, the Fe-O(phenolate) bond distances in both the complexes are the same (1.929 A). This illustrates the importance of the nearby sterically demanding coordinated -NMe(2) group and implies similar stereochemical constraints from the other ligated amino acid moieties in the 3,4-PCD enzymes, the enzyme activity of which is traced to the difference in the equatorial and axial Fe-O(tyrosinate) bonds (Fe-O-C, 133 degrees, 148 degrees ). The nature of heterocyclic rings of the ligands and the methyl substituents on them regulates the electronic spectral features, Fe(III)/Fe(II) redox potentials, and catechol cleavage activity of the complexes. Upon interacting the complexes with catecholate anions, two catecholate to iron(III) charge transfer bands appear, and the low energy band is similar to that of catechol dioxygenase-substrate complex. Complexes 1 and 3 fail to catalyze the oxidative intradiol cleavage of 3,5-di-tert-butylcatechol (H(2)DBC). However, interestingly, the replacement of pyridine pendant in 1 by the -NMe(2) group to obtain 2 restores the dioxygenase activity, which is consistent with its higher Fe-O-C bond angle. Remarkably, the more basic N-methylimidazole ring in 4 facilitates the rate-determining product releasing phase of the catalytic reaction, leading to enhancement in reaction rate and efficient conversion (77.1%) of the substrate to intradiol cleavage products as well. All these observations provide support to the novel substrate activation mechanism proposed for the intradiol-cleavage pathway.     

On, off and intermediate coordination of a bridgehead triarylamine donor in tripodal complexes: towards the tuning of coordinative bond distance

A series of first row transition metal complexes of the tripodal ligand 2,2',2"-nitrilotribenzoic acid H3L has been prepared and characterised by X-ray crystallography: Mononuclear [M(L)]- species [Cu(H2O)4]3[Cu(L)(H2O)]6.25H2O (2), [Co(H2O)6][Co(L)(H2O)].8H2O (4), [Zn(H2O)6][Zn(L)(H2O)].8H2O (5) and a neutral [M(L)] complex [Fe(III)2(L)(H2O)3].5H2O (8) are formed as well as dimeric [M(L)]2 2- species (HNEt3)2[Cu(L)]2.2CH3CN (1), (HNEt3)3[Ni(L)]2(ClO4).H2O (3), (HNEt3)2[Fe(II)(L)]2.2CH3CN (6) and (HNEt3)2[Fe(III)2(L)2(mu-O)](7). The complexes display a unique variation in the M-N distance (2.09 A for Cu(II) to 3.29 A for Fe(III)) to the bridgehead triphenylamine donor and are classified into compounds with "On","Off" and "Intermediate" N-coordination. The trigonal-bipyramidal coordination polyhedron changes towards tetrahedral in the intermediate and octahedral in the Off-state. The M-N distance of individual complexes is reversibly tuned by external chemical input such as changes of metal ion oxidation state (Fe(II)/Fe(III)) or variation of the axial coligand as a consequence of solvent or pH variation. Possible reasons for the exceptional tolerance of the M-N bond to distance variations are discussed under consideration of gas phase DFT calculations of [Zn(L)]-.