Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions (original) (raw)
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Inorganic chemistry, 2015
We report the photochemical generation and study of a family of water-soluble iron(IV)-oxo complexes supported by pentapyridine PY5Me2-X ligands (PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine; X = CF3, H, Me, or NMe2), in which the oxidative reactivity of these ferryl species correlates with the electronic properties of the axial pyridine ligand. Synthesis of a systematic series of [Fe(II)(L)(PY5Me2-X)](2+) complexes, where L = CH3CN or H2O, and characterizations by several methods, including X-ray crystallography, cyclic voltammetry, and Mössbauer spectroscopy, show that increasing the electron-donating ability of the axial pyridine ligand tracks with less positive Fe(III)/Fe(II) reduction potentials and quadrupole splitting parameters. The Fe(II) precursors are readily oxidized to their Fe(IV)-oxo counterparts using either chemical outer-sphere oxidants such as CAN (ceric ammonium nitrate) or flash-quench photochemical oxidation with [Ru(bpy)3](2+) as a photosensitizer and K2S...
Inorganic Chemistry, 2017
A series of iron(II) benzilate complexes (1−7) with general formula [(L)Fe II (benzilate)] + have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N 4 ligands (tris(2-pyridylmethyl)amine (TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me 3-TPA in 2), N 1 ,N 1-dimethyl-N 2 ,N 2-bis(2-pyridylmethyl)ethane-1,2-diamine (iso-BPMEN in 3), N 1 ,N 1-dimethyl-N 2 ,N 2-bis(6-methyl-2pyridylmethyl)ethane-1,2-diamine (6-Me 2-iso-BPMEN in 4), and tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two linear tetradentate amine ligands (N 1 ,N 2-dimethyl-N 1 ,N 2-bis(2pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N 1 ,N 2dimethyl-N 1 ,N 2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me 2-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1−3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron−oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4−6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O 2-derived iron− oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II) hydroperoxides are energetically favorable for the complexes of 6-Me 3-TPA, 6-Me 2-iso-BPMEN, BPMEN, and 6-Me 2-BPMEN ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.
Inorganic Chemistry, 2005
Acyclic pyridine-2-carboxamide-and thioether-containing hexadentate ligand 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane (H 2 bpctb), in its deprotonated form, has afforded purple low-spin (S) 0) iron(II) complex [Fe-(bpctb)] (1). A new ligand, the pyrazine derivative of H 2 bpctb, 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4dithiobutane (H 2 bpzctb), has been synthesized which has furnished the isolation of purple iron(II) complex [Fe(bpzctb)]‚ CH 2 Cl 2 (4) (S) 0). Chemical oxidation of 1 by [(η 5-C 5 H 5) 2 Fe][PF 6 ] or [Ce(NO 3) 6 ][NH 4 ] 2 led to the isolation of low-spin (S) 1/2) green Fe(III) complexes [Fe(bpctb)][PF 6 ] (2) or [Fe(bpctb)][NO 3 ]‚H 2 O (3), and oxidation of 4 by [Ce(NO 3) 6 ][NH 4 ] 2 afforded [Fe(bpzctb)][NO 3 ]‚H 2 O (5) (S) 1/2). X-ray crystal structures of 1 and 4 revealed that (i) in each case the ligand coordinates in a hexadentate mode and (ii) bpzctb 2binds more strongly than bpctb(2-), affording distorted octahedral M II N 2 (pyridine/pyrazine)N′ 2 (amide)S 2 (thioether) coordination. To the best of our knowledge, 1 and 4 are the first examples of six-coordinate low-spin Fe(II) complexes of deprotonated pyridine/ pyrazine amide ligands having appended thioether functionality. The Fe(III) complexes display rhombic EPR spectra. Each complex exhibits in CH 2 Cl 2 /MeCN a reversible to quasireversible cyclic voltammetric response, corresponding to the Fe III −Fe II redox process. The E 1/2 value of 4 is more anodic by ∼0.2 V than that of 1, attesting that compared to pyridine, pyrazine is a better stabilizer of iron(II). Moreover, the E 1/2 value of 1 is significantly higher (∼1.5 V) than that reported for six-coordinate Fe(II)/Fe(III) complexes of the tridentate pyridine-2-carboxamide ligand incorporating thiolate donor site.
Inorganica Chimica Acta, 2007
Hydrogen bonding networks proximal to metal centers are emerging as a viable means for controlling secondary coordination spheres. This has led to the regulation of reactivity and isolation of complexes with new structural motifs. We have used the tridenate ligand bis[(N 0-tert-butylureido)-N-ethyl]-N-methylaminato ([H 2 1] 2À) that contains two hydrogen bond donors to examine the oxidation of the Fe II-acetate complex, [Fe II H 2 1(g 2-OAc)] À with dioxygen, amine N-oxides, and xylyl azide. A complex with Fe III-O-Fe III core results from the oxidation with dioxygen and amine N-oxides, in which the oxo ligand is involved in hydrogen bonding to the [H 2 1] 2À ligand. A distinctly different hydrogen bonding network was found in Fe III dimer isolated from the reaction with the xylyl azide: a rare Fe III-N(R)-Fe III core was observed that does not have hydrogen bonds to the bridging nitrogen atom. The intramolecular H-bond networks within these dimers appear to adjust to the presence of the bridging species and rearrange to its size and electron density.
European Journal of Inorganic Chemistry, 2003
The ligand N,N′‐bis(6‐methyl‐2‐pyridylmethyl)‐N,N′‐bis(2‐pyridylmethyl)ethane‐1,2‐diamine (L622M) has allowed to prepare the new FeII complex [(L622M)FeCl2] (1), and to compare its structural and spectroscopic characteristics with [(L622M)FeCl]PF6 (2). The molecular structure of 1, resolved by X‐ray diffraction, exhibits the ligand tetracoordinated with the two non‐methylated pyridine rings coordinated. As shown by UV/Vis spectroscopy and cyclic voltammetry, upon dissolution in methanol or acetonitrile, one chloride ion is released and replaced by a pyridine group. Therefore, complexes 1 and 2 adopt identical structures in solution, i.e. [(L622M)FeCl+. However, upon oxidation complex 1 gives several ferric complexes with the ligand L622M pentacoordinated, or tetracoordinated with two chloride ions. This peculiar behaviour is due to the presence of chloride ions in solution in the case of 1. Upon reaction with H2O2 in methanol, complex 2 leads to the formation of low‐spin [(L622M)FeI...
Proton Control of Oxidation and Spin State in a Series of Iron Tripodal Imidazole Complexes
Inorganic Chemistry, 2004
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, Mössbauer, 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, I43d, 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), β= 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), β= 111.147(1)°, 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 spincrossover, 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+ .
Inorganic Chemistry, 2009
In a previous paper (Jensen et al., J. Am. Chem. Soc. 2005, 127, 10512), we reported the synthesis of the turquoise-colored intermediate [Fe IV (β-BPMCN)(OO t Bu)(OH)] 2+ (Tq; BPMCN = N,N′-bis (2-pyridylmethyl)-N,N′-dimethyl-trans-1,2-diaminocyclohexane). The structure of Tq is unprecedented, as it represents the only synthetic example to date of a non-heme Fe IV complex with both alkylperoxo and hydroxide ligands. Given the significance of similar high-valent Fe intermediates in the mechanisms of oxygenase enzymes, we have explored the reactivity of Tq at −70 °C, a temperature at which it is stable, and found that it is capable of activating weak X-H bonds (X = C, O) with bond dissociation energies ≤~80 kcal/mol. The Fe IV-OH unit of Tq, and not the alkylperoxo moiety, performs the initial H-atom abstraction. However at −45 °C, Tq decays at a rate that is independent of substrate identity and concentration, forming a species capable of oxidizing substrates with stronger C-H bonds. Parallel reactivity studies were also conducted with the related oxoiron(IV) complexes [Fe IV (β-BPMCN)(O)(X)] 2+ (3-X; X = pyridine or nitrile), thereby permitting a direct comparison of the reactivity of Fe IV centers with oxo and hydroxide ligands. We found that the H-atom abstracting ability of the Fe IV =O species greatly exceeds that of the Fe IV-OH species, generally by greater than 100-fold. Examination of the electronic structures of Tq and 3-X with density functional theory (DFT) provides a rationale for their differing reactivities.