Emile L. Bominaar

A Synthetic High‐Spin Oxoiron(IV) Complex: Generation, Spectroscopic Characterization, and Reactivity

High versus low: The high-yield generation of a synthetic high-spin oxoiron(IV) complex, [Fe(IV)(O)(TMG(3)tren)](2+) (see picture, TMG(3)tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine), has been achieved by using the very bulky tetradentate TMG(3)tren ligand, in order to both sterically protect the oxoiron(IV) moiety and enforce a trigonal bipyramidal geometry at the iron center, for which an S = 2 ground state is favored.

A thiolate-ligated nonheme oxoiron(IV) complex relevant to cytochrome P450.

Thiolate-ligated oxoiron(IV) centers are postulated to be the key oxidants in the catalytic cycles of oxygen-activating cytochrome P450 and related enzymes. Despite considerable synthetic efforts, chemists have not succeeded in preparing an appropriate model complex. Here we report the synthesis and spectroscopic characterization of [FeIV(O)(TMCS)]+ where TMCS is a pentadentate ligand that provides a square pyramidal N4(SR)apical, where SR is thiolate, ligand environment about the iron center, which is similar to that of cytochrome P450. The rigidity of the ligand framework stabilizes the thiolate in an oxidizing environment. Reactivity studies suggest that thiolate coordination favors hydrogen-atom abstraction chemistry over oxygen-atom transfer pathways in the presence of reducing substrates.

Iron-containing proteins and related analogs — complementary Mössbauer, EPR and magnetic susceptibility studies

The three methods covered by this review — Mossbauer spectroscopy, electron paramagnetic resonance and magnetic susceptibility-provide a powerful set of tools for detailed studies of electronic structure and molecular magnetism of iron-containing proteins and related analogs. The interpretation of measured data is based on the spin-Hamiltonian concept which is described in detail. A short introduction to the principles of the three methods as well as a basic description of the spectrometers is given. The major part of the review deals with applications, e.g. to mononuclear iron complexes and to spin-coupled iron complexes. Wherever possible, the complementarity in applying the three methods is described.

Trapping and spectroscopic characterization of an FeIII-superoxo intermediate from a nonheme mononuclear iron-containing enzyme

intermediates are well known in heme enzymes, but none have been characterized in the nonheme mononuclear FeII enzyme family. Many steps in the O2 activation and reaction cycle of FeII-containing homoprotocatechuate 2,3-dioxygenase are made detectable by using the alternative substrate 4-nitrocatechol (4NC) and mutation of the active site His200 to Asn (H200N). Here, the first intermediate (Int-1) observed after adding O2 to the H200N-4NC complex is trapped and characterized using EPR and Mössbauer (MB) spectroscopies. Int-1 is a high-spin (S1 = 5/2) FeIII antiferromagnetically (AF) coupled to an S2 = 1/2 radical (J ≈ 6 cm-1 in ). It exhibits parallel-mode EPR signals at g = 8.17 from the S = 2 multiplet, and g = 8.8 and 11.6 from the S = 3 multiplet. These signals are broadened significantly by hyperfine interactions (A17O ≈ 180 MHz). Thus, Int-1 is an AF-coupled species. The experimental observations are supported by density functional theory calculations that show nearly complete transfer of spin density to the bound O2. Int-1 decays to form a second intermediate (Int-2). MB spectra show that it is also an AF-coupled FeIII-radical complex. Int-2 exhibits an EPR signal at g = 8.05 arising from an S = 2 state. The signal is only slightly broadened by (< 3% spin delocalization), suggesting that Int-2 is a peroxo-FeIII-4NC semiquinone radical species. Our results demonstrate facile electron transfer between FeII, O2, and the organic ligand, thereby supporting the proposed wild-type enzyme mechanism.

Mössbauer and electron paramagnetic resonance study of the double‐exchange and Heisenberg‐exchange interactions in a novel binuclear Fe(II/III) delocalized‐valence compound

In this paper we present the characterization by UV‐VIS, Mossbauer, and EPR spectroscopy of [L2Fe2(μ‐OH)3](ClO4)2⋅2CH3OH⋅2H2O, with L=N,N’,N‘‐trimethyl‐1,4,7‐triazacyclononane, a novel dimeric iron compound, which is shown to possess a central exchange‐coupled delocalized‐valence Fe(II/III) pair. Complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mossbauer spectroscopy. Mossbauer, EPR, and magnetic susceptibility data imply a system spin St =9/2 for the ground state. This finding is explained as being a consequence of the double‐exchange interaction which is generated by the delocalized electron. Experimental values obtained from UV‐VIS, Mossbauer, and EPR spectroscopy are for the double‐exchange parameter B=1300 cm−1, the g factors gx,y =2.04 and gz =2.3, the parameters for zero‐field splitting D=4 cm−1 and E≊0 cm−1, and for the hyperfine parameters ΔEQ =−2.14 mm s−1, Ax,y =−21.2 T, Az =−27 T, and δ=0.74 mm s−1. Fr...

A More Reactive Trigonal Bipyramidal High-Spin Oxoiron(IV) Complex with a cis-Labile Site

The trigonal-bipyramidal high-spin (S = 2) oxoiron(IV) complex [Fe(IV)(O)(TMG(2)dien)(CH(3)CN)](2+) (7) was synthesized and spectroscopically characterized. Substitution of the CH(3)CN ligand by anions, demonstrated here for X = N(3)(-) and Cl(-), yielded additional S = 2 oxoiron(IV) complexes of general formulation [Fe(IV)(O)(TMG(2)dien)(X)](+) (7-X). The reduced steric bulk of 7 relative to the published S = 2 complex [Fe(IV)(O)(TMG(3)tren)](2+) (2) was reflected by enhanced rates of intermolecular substrate oxidation.

(TAML)FeIV═O Complex in Aqueous Solution: Synthesis and Spectroscopic and Computational Characterization

Recently, we reported the characterization of the S = (1)/ 2 complex [Fe (V)(O)B*] (-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe (IV)(O)B*] (2-) ( 2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe (III)(H 2O)B*] (-) ( 1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe (IV)-O-Fe (IV)B*] (2-) ( 3), with a p K a near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with Delta E Q = 3.95(3) mm/s and delta = -0.19(2) mm/s, parameters consistent with a S = 1 Fe (IV) state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm (-1) together with the magnetic hyperfine tensor A/ g nbeta n = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) A, a distance consistent with a Fe (IV)O bond. DFT calculations for [Fe (IV)(O)B*] (2-) reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent-water molecules. It is shown, using DFT, that the (57)Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.

Shape‐Selective Interception by Hydrocarbons of the O2‐Derived Oxidant of a Biomimetic Nonheme Iron Complex

Picky ferryl: The complex [Fe(Tp(Ph(2)))(BF)] (Tp(Ph(2)) = hydrotris(3,5-diphenylpyrazolyl)borate; BF = benzoylformate) reacts with O(2) to generate an oxidant (see picture; O red, pink; Fe yellow; N blue; C gray; H white) that oxidizes added hydrocarbons shape-selectively. Discrimination derives from a cleft formed by two phenyl groups of the Tp(Ph(2)) ligand, favoring oblate spheroidal substrates.

Structure and electronic properties of (N,N′-bis(4-methyl-6-tert-butyl-2-methyl-phenolato)-N,N′-bismethyl-1,2-diaminoethaneFeIII (DBSQ). Spectroelectrochemical study of the red-ox properties. Relevance to intradiol catechol dioxygenases

Abstract The species LFe III Cl ( 1 ) was synthesized where L 2− is the dianion N , N ′-bis(4-methyl-6-tert-butyl-2-methyl-phenolato)- N , N ′-bismethyl1,2-diaminoethane. It crystallizes in the triclinic space group P -1 with a = 14.704(6), b = 17.421(7), c = 17.328(8) A , α = 89.45(8) , β = 129.76(9), γ = 102.71(9)°, V = 3277(2) A 3 and Z = 2 . The molecule has approximately a square pyramidal structure. In the presence of DBCH 2 (3,5-di-tert-butylcatechol), this complex gives LFe III (DBSQ) ( 2 ), a stable Fe(III)-semiquinonato complex (DBSQ − stands for the 3,5-di-tert-butyl- o -benzosemiquinone monoanion). It crystallizes in the monoclinic space group C 2/ c with a = 36.24(2), b = 10.438(5), c = 23.928(12) A , β = 115.31(5)°, V = 8183(7) A 3 and Z = 8 . This X-ray study allows the structure of the DBSQ − monoanion complexed to Fe(III) to be compared with that of the DBC 2− dianion complexed to Fe(III) as found in several complexes already described (see for instance H.G. Jang, D.D. Cox and L. Que, Jr., J. Am. Chem. Soc., 113 (1991) 9200–9204). A clear alternation is found in CC bond lengths in the DBSQ − monoanion. Magnetic coupling between the S Fe = 5 2 electronic spin on Fe(III) and the S R = 1 2 electronic spin on the DBSQ − anion radical has been deduced from the behavior of the magnetic susceptibility as a function of temperature. It has been found antiferromagnetic with J = −206 cm −1 (with the notation H = − JS Fc S R ). This is a weaker coupling than that found in other analogous complexes. Mossbauer spectroscopy confirms the S = 2 nature of the ground state. Analysis of the 57 Fe hyperfine coupling parameters prove that the earlier description of the electronic structure of 2 is correct. It is possible to reduce 2 to get [LFe III (DBC)] − ( E 0 = −0.3 V/SCE in AcN). This species has UV-Vis and EPR properties typical of this type of complex. In the absence of protons, [LFe III (DBC)] − is stable under pure O 2 but the addition of protons results in its oxidation to form 2 . The insensitivity of [LFe III (DBC)] − to O 2 is compatible with the idea that the system has to go through an Fe(II) (DBSQ) form by internal electron transfer to allow the attack of the aromatic ring by O 2 (see L. Que and R.Y.N. Ho, Chem. Rev., 96 (1996) 2607–2624, for a review). Observation of the lowest energy LMCT band in [LFe III (DBC)] − at 620 nm (2.00 eV, AcN) suggests that the Fe(II) (DBSQ) form is indeed at high energy ( ΔG 0 ) above the catechol form. This contributes from Marcus theory to a high activation energy. In intradiol dioxygenases the decoordination of one of the tyrosine ligands (D.H. Ohlendorf, A.M. Orville and J.D. Lipscomb, J. Mol. Biol., 244 (1994) 586–608) is certainly important for the deprotonation of the catechol substrate but could also accelerate the formation of the postulated Fe(II)-semiquinonato intermediate. The stability of 2 under O 2 also demonstrates the importance of the Fe(II) oxidation state in order to lead to a peroxo form of the oxygen adduct. The insensitivity of [LFe III (DBC)] − to O 2 argues against the view of the dioxygenase activity as the result of an electrophilic attack of the catecholato by O 2 in model complexes.

One-electron oxidation of an oxoiron(IV) complex to form an [O═FeV═NR]+ center

Oxoiron(V) species are postulated to be involved in the mechanisms of the arene cis-dihydroxylating Rieske dioxygenases and of bioinspired nonheme iron catalysts for alkane hydroxylation, olefin cis-dihydroxylation, and water oxidation. In an effort to obtain a synthetic oxoiron(V) complex, we report herein the one-electron oxidation of the S = 1 complex [FeIV(O)(TMC)(NCCH3)]2+ (1, where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable complex 2 at -44 °C, which has been characterized by UV-visible, resonance Raman, Mössbauer, and EPR methods. The defining spectroscopic characteristic of 2 is the unusual x/y anisotropy observed for the 57Fe and 17O A tensors associated with the high-valent Fe═O unit and for the 14N A tensor of a ligand derived from acetonitrile. As shown by detailed density functional theory (DFT) calculations, the unusual x/y anisotropy observed can only arise from an iron center with substantially different spin populations in the dxz and dyz orbitals, which cannot correspond to an FeIV═O unit but is fully consistent with an FeV center, like that found for [FeV(O)(TAML)]- (where TAML is tetraamido macrocyclic ligand), the only well-characterized oxoiron(V) complex reported. Mass spectral analysis shows that the generation of 2 entails the addition of an oxygen atom to 1 and the loss of one positive charge. Taken together, the spectroscopic data and DFT calculations support the formulation of 2 as an iron(V) complex having axial oxo and acetylimido ligands, namely [FeV(O)(TMC)(NC(O)CH3)]+.