Correlation of Spin States and Spin Delocalization with the Dioxygen Reactivity of Catecholatoiron(III) Complexes

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS-LS], [LS-HS], and [HS-HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{FeIII(tpa)}2spiro](PF6)2 (1), [{FeIII(tpa)}2Br4spiro](PF6)2 (2), and [{FeIII(tpa)}2thea](PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol, Br4spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS-HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS-HS] state. In solution, all complexes undergo SCO from [HS-HS] at room temperature, via [LS-HS] to mixtures including [LS-LS] at 77 K, with the extent of SCO increasing in the order 1 < 2 < 3. Gas phase density functional theory calculations suggest a [LS-LS] ground state for all complexes, with the [LS-HS] and [HS-HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4- < Br4spiro4- < thea4-, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS-HS] state in relation to the midpoint energy between [LS-LS] and [HS-HS]. The relative stability of the [LS-HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 < 2 < 3. The bromo substituents of Br4spiro4- increase the ligand field strength relative to spiro4-, while the stronger ligand field provided by thea4- arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states.

Spin transitions in ferric catecholate complexes mediated by outer-sphere counteranions

A family of ionic ferric catecholate complexes 1-4 bearing a disubstituted 3,6-di-tert-butyl-catecholate ligand (3,6-DBCatH₂) and tetradentate tris(2-pyridylmethyl)amine (TPA) was prepared and its spin transitions were investigated. Variation of the outer-sphere counteranions (PF₆, BPh₄, ClO₄, BF₄) is accompanied by changes in the magnetic behavior of the compounds under consideration. The crystal structures of complexes 1, 3 and 4 were determined by single crystal X-ray diffraction analysis at 100 K and 293 K. The complexes were characterized by the occurrence of a thermally induced spin-crossover process in the solid state with different degrees of completeness, which was confirmed by the comprehensive spectroscopic investigation (EPR, magnetic susceptibility, Mössbauer, and XAS) of the isolated compounds. Complex 4 containing BF₄ anions was found to demonstrate valence tautomeric transition along with spin-crossover. This finding makes compound 4 the first salt-like mononuclear ferric catecholate complex exhibiting valence tautomerism.

Nitro-Group Functionalization of Dopamine and its Contribution to the Viscoelastic Properties of Catechol-Containing Nanocomposite Hydrogels

Linear polyacrylamide (PAAm) is modified with dopamine or nitrodopamine (PAAm-D and PAAm-ND, respectively) to evaluate the effect of nitro-group modification on the interfacial binding properties of polymer-bound catechol. Nanocomposite hydrogels are prepared by mixing PAAm-based polymers with Laponite and the viscoelastic properties of these materials are determined using oscillatory rheometry. The incorporation of a small amount of catechol (≈0.1 wt% in swollen hydrogel) drastically increases the shear moduli by 1-2 orders of magnitude over those of the catechol-free control. Additionally, PAAm-ND exhibits higher shear moduli values than PAAm-D across the whole pH range tested (pH 3.0-9.0). Based on the calculated effective crosslinking density, effective functionality, and molecular weight between crosslinks, nitro-group functionalization of dopamine results in a polymer network with increased crosslinking density and crosslinking points with higher functionality. Nitro-functionalization enhances the interfacial binding property of dopamine and increases its resistant to oxidation, which results in nanocomposite hydrogels with enhanced stiffness and a viscous dissipation property.

Sustainable production of dimethyl adipate by non-heme iron(iii) catalysed oxidative cleavage of catechol

Selective catechol cleavage by a non-heme iron(iii) complex followed by hydrogenation and transesterifaction yields dimethyl adipate in a green and sustainable manner.

Experimental and computational evidence for the mechanism of intradiol catechol dioxygenation by non-heme iron(III) complexes

Catechol intradiol dioxygenation is a unique reaction catalyzed by iron-dependent enzymes and non-heme iron(III) complexes. The mechanism by which these systems activate dioxygen in this important metabolic process remains controversial. Using a combination of kinetic measurements and computational modelling of multiple iron(III) catecholato complexes, we have elucidated the catechol cleavage mechanism and show that oxygen binds the iron center by partial dissociation of the substrate from the iron complex. The iron(III) superoxide complex that is formed subsequently attacks the carbon atom of the substrate by a rate-determining C-O bond formation step.

Iron(III) complexes with meridional ligands as functional models of intradiol-cleaving catechol dioxygenases

Six dichloroiron(III) complexes of 1,3-bis(2'-arylimino)isoindoline (BAIH) with various N-donor aryl groups have been characterized by spectroscopy (infrared, UV-vis), electrochemistry (cyclic voltammetry), microanalysis, and in two cases X-ray crystallography. The structurally characterized Fe(III)Cl(2)(L(n)) complexes (n = 3, L(3) = 1,3-bis(2'-thiazolylimino)isoindoline and n = 5, L(5) = 1,3-bis(4-methyl-2'-piridylimino)isoindoline) are five-coordinate, trigonal bipyramidal with the isoindoline ligands occupying the two axial and one equatorial positions meridionally. These compounds served as precursors for catechol dioxygenase models that were formed in solution upon addition of 3,5-di-tert-butylcatechol (H(2)DBC) and excess triethylamine. These adducts react with dioxygen in N,N-dimethylformamide, and the analysis of the products by chromatography and mass spectrometry showed high intradiol over extradiol selectivity (the intradiol/extradiol product ratios varied between 46.5 and 6.5). Kinetic measurements were performed by following the change in the intensity of the catecholate to iron ligand-to-metal charge transfer (LMCT) band, the energy of which is influenced by the isoindolinate-ligand (827-960 nm). In combination with electrochemical investigations the kinetic studies revealed an inverse trend between reaction rates and oxidation potentials associated with the coordinated DBC(2-). On the basis of these results, a substrate activation mechanism is suggested for this system in which the geometry of the peroxide-bridged intermediate may be of key importance in regioselectivity.

π Back-bonding of iron(II) complexes supported by tris(pyrid-2-ylmethyl)amine and its nitro-substituted derivatives

The electronic and geometric structures of a series of iron(II) complexes supported by tetradentate tris(pyrid-2-ylmethyl)amine-type ligands with different numbers of 4-nitropyridine groups, [(PyCH(2))(3-n)(4-NO(2)PyCH(2))(n)N] (n = 0-3), were examined by X-ray absorption fine-structure and variable-temperature (1)H NMR spectroscopies and theoretical calculations to reveal how the low-spin state is stabilized through π back-bonding interactions between iron(II) and 4-nitropyridine donor group(s).

Octahedral monodithiolene complexes of iron: characterization of S,S'-coordinated dithiolate(1-) pi radical monoanions: a spectroscopic and density functional theoretical investigation

The reaction of cis-[Fe(III)(cyclam)Cl(2)]Cl with 1 equiv of sodium N-diethyldithiocarbamate, toluene-3,4-dithiolate, and maleonitriledithiolate in methanol in the presence of triethylamine afforded the cations [Fe(III)(cyclam)(Et(2)dtc)](2+) (1), [Fe(III)(cyclam)(tdt)](+) (2), and [Fe(III)(cyclam)(mnt)](+) (3), which were isolated as triflate, hexafluorophosphate, and tetrafluoroborate salt, respectively, using sodium triflate, potassium hexafluorophosphate, or sodium tetrafluoroborate as the source for the counteranion. Complexes 1, 2, and 3 possess an S = (1)/(2) ground state (low-spin ferric d(5)). These salts were characterized by X-ray crystallography, UV-vis, Mössbauer, and electron paramagnetic resonance spectroscopies. Cyclic voltammetry revealed that 2 and 3 are reversibly one-electron-reduced, generating neutral 2(red) and 3(red), respectively, and one-electron-oxidized, generating dicationic 2(ox) and 3(ox), respectively. Fe and S K-edge X-ray absorption spectroscopy (XAS) revealed that 2 (S = (1)/(2)) and 2(ox) (S = 0) possess a low-spin ferric ion. Complexes 2 and 3 are S,S'-coordinated to a closed-shell dithiolate(2-) ligand, whereas 2(ox) and 3(ox) consist of a low-spin ferric ion antiferromagnetically coupled to a dithiolate(1-) pi radical ligand. They are singlet diradicals [Fe(III)(cyclam)(dithiolate(*))](2+). The analysis of the sulfur K pre-edge transitions reveals significant multiplet effects in the spectra of 2 and 2(ox), which provide rare experimental evidence for a singlet diradical description for 2(ox). Mössbauer spectroscopy on frozen solutions of 2(red) clearly show the presence of a high-spin ferrous ion (S = 2). The experimentally established electronic structures of the three members of the electron transfer series [Fe(cyclam)(dithiolate)](2+,+,0) have been verified by broken symmetry density functional theoretical calculations, which have been calibrated against the experiment by calculating XAS and Mössbauer spectra.

Theoretical study of dioxygen binding process in iron(III) catechol dioxygenase: "oxygen activation" vs "substrate activation"

Dioxygen binding process of nonheme iron(III) center in intradiol catechol dioxygenase was investigated with CASSCF/CASPT2 method to incorporate multiconfigurational character participating in Fe-O(2) interaction. In this process, two alternative mechanisms were proposed: one is called "oxygen activation" and the other is called "substrate activation". Our CASSCF/CASPT2-calculated results support the oxygen activation. Potential energy curves and electronic structure evaluated with SA(state-averaged)-CASSCF/CASPT2 method indicate that the charge transfer directly occurs from the catecholate moiety to the dioxygen moiety in the O(2) binding process, to produce eta(1)-end-on type iron(III)-superoxo complex. This is the key step of the dioxygen activation. Interestingly, the iron center always keeps high spin d(5) character during the O(2) binding process, indicating the iron(III) center does not receive charge transfer from the catecholate moiety. However, this does not mean that the iron(III) center is not necessary to the dioxygen activation. The important role which the iron(III) center plays in catechol dioxygenase is to adjust the energy level of O(2) to induce the charge transfer from the catecholate moiety to the dioxygen moiety. Besides the eta(1)-end-on iron(III)-superoxo complex, eta(2)-side-on type iron(III)-superoxo complex is also optimized. This species is more stable than the eta(1)-end-on type iron(III)-superoxo complex, suggesting that this is considered as a stable isomer in the early stage of the catalytic cycle.

Iron(III)-catecholato complexes as structural and functional models of the intradiol-cleaving catechol dioxygenases

The structural and spectroscopic characterization of mononuclear iron(III)-catecholato complexes of ligand L4 (methyl bis(1-methylimidazol-2-yl)(2-hydroxyphenyl)methyl ether, HL4) are described, which closely mimic the enzyme-substrate complex of the intradiol-cleaving catechol dioxygenases. The tridentate, tripodal monoanionic ligand framework of L4 incorporates one phenolato and two imidazole donor groups and thus well reproduces the His2Tyr endogenous donor set. In fact, regarding the structural features of [FeIII(L4)(tcc)(H2O)] (5.H2O, tcc = tetrachlorocatechol) in the solid state, the complex constitutes the closest structural model reported to date. The iron(III)-catecholato complexes mimic both the structural features of the active site and its spectroscopic characteristics. As part of its spectroscopic characterization, the electron paramagnetic resonance (EPR) spectra were successfully simulated using a simple model that accounts for D strain. The simulation procedure showed that the observed g = 4.3 line is an intrinsic part of the EPR envelope of the studied complexes and should not necessarily be attributed to a highly rhombic impurity. [FeIII(L4)(dtbc)(H2O)] (dtbc = 3,5-di-tert-butylcatechol) was studied with respect to its dioxygen reactivity, and oxidative cleavage of the substrate was observed. Intradiol- and extradiol-type cleavage products were found in roughly equal amounts. This shows that an accurate structural model of the first-coordination sphere of the active site is not sufficient for obtaining regioselectivity.

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.

Trigonal-Bipyramidal Geometry Induced by an External Water Ligand in a Sterically Hindered Iron Salen Complex, Related to the Active Site of Protocatechuate 3,4-Dioxygenase

A unique distorted trigonal-bipyramidal geometry observed for the non-heme iron center in protocatechuate 3,4-dioxygenase (3,4-PCD) was carefully examined utilizing a sterically hindered iron salen complex, which well reproduces the endogenous His2Tyr2 donor set with water as an external ligand. X-ray crystal structures of a series of iron model complexes containing bis(3,5-dimesitylsalicylidene)-1,2-dimesitylethylenediamine indicate that a distorted trigonal-bipyramidal geometry is achieved upon binding of water as an external ligand. The extent of a structural change of the iron center from a preferred square-pyramidal to a distorted trigonal-bipyramidal geometry varies with the external ligand that is bound in the order Cl << EtO < H2O, which is consistent with the spectrochemical series. The distortion in the model system is not due to steric repulsions but electronic interactions between the external ligand and the iron center, as evidenced from the X-ray crystal structures of another series of iron model complexes with a less-hindered bis(3-xylylsalicylidene)-1,2-dimesitylethylenediamine ligand, as well as by density functional theory calculations. Further spectroscopic investigations indicate that a unique distorted trigonal-bipyramidal geometry is indeed maintained even in solution. The present model study provides a new viewpoint that a unique distorted trigonal-bipyramidal iron site might not be preorganized by a 3,4-PCD protein but could be electronically induced upon the binding of an external hydroxide ligand to the iron(III) center. The structural change induced by the external water ligand is also discussed in relation to the reaction mechanism of 3,4-PCD.

Photoexcitation and Relaxation Dynamics of Catecholato–Iron(III) Spin-Crossover Complexes

The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF(6) (-), BPh(4) (-); R-Cat=catecholate dianion substituted by R=NO(2), Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below approximately 40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, k(HL)(T-->0), vary in the range from 0.58 to 8.84 s(-1). These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry.