Electronic Structures of the [Fe(N₂)(SiP^iPr₃)]^+1/0/–1 Electron Transfer Series: A Counterintuitive Correlation between Isomer Shifts and Oxidation States

The electronic structure analysis of the low-spin iron­(II/I/0) complexes [Fe­(N₂)­(SiP^iPr₃)]^+/0/– (SiP^iPr₃ = [Si­(<i>o</i>-C₆H₄PⁱPr₂)₃]⁻) recently published by J. Peters et al. (<i>Nature Chem.</i> <b>2010</b>, <i>2</i>, 558–565) reveals that the redox processes stringing this electron transfer series are best viewed as metal-centered reductions, i.e. Fe^IIN₂⁰ → Fe^IN₂⁰ → Fe⁰N₂⁰. Superficially, the interpretation seems to be incompatible with the Mössbauer measurement, because the observed isomer shifts are more negative for the lower oxidation states, whereas typically iron-based reduction tends to increase the isomer shift. To rationalize the experimental findings, we analyzed the contributions from the 1s to 4s orbitals to the charge density at the Mössbauer nucleus and found that the positive correlation between the isomer shift and the oxidation state results from an unusual shrinking of the Fe–N₂ bond upon reduction due to enhanced N₂ to Fe π-backbonding. The other effects of reduction arising from shielding of the nuclear potential, decreasing covalency, and changes in the 4s population would induce the usual negative correlation. The structure distortion dictates the radial distribution of the 4s orbital and the charge density at the nucleus such that a virtually linear relationship between the isomer shift and the Fe–N₂ distance could be identified for this series

Group 6 Complexes with Iron and Zinc Heterometals: Understanding the Structural, Spectroscopic, and Electrochemical Properties of a Complete Series of MM···M′ Compounds

Binuclear quadruply bonded complexes Cr2(dpa)4 (1, dpa = 2,2′-dipyridylamide), Mo2(dpa)4 (2), and W2(dpa)4 (3) react with anhydrous FeCl2, yielding heterometallic compounds CrCrFe(dpa)4Cl2 (4), MoMoFe(dpa)4Cl2 (5), and WWFe(dpa)4Cl2 (6). These molecules are structurally similar, having a linear MM···Fe chain that is axially capped by chloride ions and is equatorially supported by the helically twisted dpa ligands. A structurally related zinc analog, CrCrZn(dpa)4Cl2 (7), can be prepared upon metalation of 1 with ZnCl2. This reaction also persistently produces a 2:1 adduct of ZnCl2 with 1, [Cr2(dpa)4](ZnCl2)2 (8), which is in equilibrium with 7 and has the two zinc ions bound externally to the Cr2 core and axial bridging chloro ligands attached to each Cr ion. The sole isolable product of the addition of ZnCl2 to 3 is a 1:1 adduct, [W2(dpa)4]ZnCl2 (9). The structurally related chain complexes 4, 5, 6, and 7 are characterized by X-ray crystallography, UV–vis spectroscopy, cyclic voltammetry, and 57Fe Mössbauer spectroscopy for the iron complexes in order to gain insights into the nature of heterometallic interactions, electronic excited states, and redox properties of these compounds, which have implications for all other MM···M′ molecules. Additionally, NMR spectroscopy has been used to gain insight into the mechanism of the metalation of 1 by Zn(II)

Group 6 Complexes with Iron and Zinc Heterometals: Understanding the Structural, Spectroscopic, and Electrochemical Properties of a Complete Series of MM···M′ Compounds

Binuclear quadruply bonded complexes Cr2(dpa)4 (1, dpa = 2,2′-dipyridylamide), Mo2(dpa)4 (2), and W2(dpa)4 (3) react with anhydrous FeCl2, yielding heterometallic compounds CrCrFe(dpa)4Cl2 (4), MoMoFe(dpa)4Cl2 (5), and WWFe(dpa)4Cl2 (6). These molecules are structurally similar, having a linear MM···Fe chain that is axially capped by chloride ions and is equatorially supported by the helically twisted dpa ligands. A structurally related zinc analog, CrCrZn(dpa)4Cl2 (7), can be prepared upon metalation of 1 with ZnCl2. This reaction also persistently produces a 2:1 adduct of ZnCl2 with 1, [Cr2(dpa)4](ZnCl2)2 (8), which is in equilibrium with 7 and has the two zinc ions bound externally to the Cr2 core and axial bridging chloro ligands attached to each Cr ion. The sole isolable product of the addition of ZnCl2 to 3 is a 1:1 adduct, [W2(dpa)4]ZnCl2 (9). The structurally related chain complexes 4, 5, 6, and 7 are characterized by X-ray crystallography, UV–vis spectroscopy, cyclic voltammetry, and 57Fe Mössbauer spectroscopy for the iron complexes in order to gain insights into the nature of heterometallic interactions, electronic excited states, and redox properties of these compounds, which have implications for all other MM···M′ molecules. Additionally, NMR spectroscopy has been used to gain insight into the mechanism of the metalation of 1 by Zn(II)

Single- and Double-Cubane Clusters in the Multiple Oxidation States [VFe₃S₄]^3+,2+,1+

A new series of cubane-type [VFe3S4]z clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz3)VFe3S4Cl3]2- (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz3)VFe3S4Cl3]1- (3, z = 3+). Reaction of 2 with Et3P results in chloride substitution to give [(HBpz3)VFe3S4(PEt3)3]1+ (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz3)2V2Fe6S8(PEt3)4] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz3)2V2Fe6S8Cl4]4- (6, z = 1+, 1+). X-ray structures of (Me4N)[3], [4](PF6), 5, and (Et4N)4[6]·2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4] and contains two VFe3S4 cubanes connected across edges by a Fe2S2 rhomb in which the bridging Fe-S distances are shorter than intracubane Fe−S distances. Mössbauer (2−5), magnetic (2−5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of 57Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 → 3 + e- results in a change in electron density localized largely or completely on the Fe3 subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V3+Fe3+Fe2+2S4]2+ (Fe2.33+) for 1, 2, and 4 and [V3+Fe3+2Fe2+S4]3+ (Fe2.67+) for 3 and [V2Fe6S8(SEt)9]3+. Conclusion i applies to every MFe3S4 cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster

<i>S,O</i> or <i>S,N</i> Coordination? Unraveling the Coordination Modes of Arenesulfonylthiourea Ligands

Two arenesulfonylthioureas were prepared and reacted with a variety of transition-metal salts, including Ni­(OAc)2, Co­(OAc)2, Cu­(OAc)2, CuCl2, AgNO3, and Zn­(OAc)2. In all cases, complexes containing the arenesulfonylthioureas acting as monoanionic [S,N]− chelating ligands were isolated and characterized by X-ray diffraction, NMR spectroscopy, and magnetic measurements. Copper­(I) and silver­(I) complexes exist as hexanuclear [M6L6] clusters, which are luminescent in the solid state and solution

Molecular and Electronic Structures of Iron Complexes Containing N,S-Coordinated, Open-Shell <i>o</i>-Iminothionebenzosemiquinonate(1−) π Radicals

The reaction of the dinuclear species (μ-NH,NH)[FeIII(LIP)(LAP)]2 dissolved in CH2Cl2 with dioxygen affords black microcrystals of diamagnetic (μ-S,S)[FeIII(LIP)(LISQ)]2·n-hexane (6) upon the addition of n-hexane, where (LIP)2- represents the dianion of 4,6-di-tert-butyl-2-aminothiophenol, (LAP)- is the corresponding monoanion, and (LISQ)- is the corresponding o-iminothionebenzosemiquinonate(1−) π radical monoanion; similarly, the dianion (‘H2N2S2‘)2- is derived from 1,2-ethanediamine-N,N‘-bis(2-benzenethiol), and (‘N2S2•‘)3- is its monoradical trianion. The above reaction in a CH2Cl2/CH3OH (1:1) mixture yields the diamagnetic isomer (μ-NH,NH)[FeIII(LIP)(LISQ)]2·5CH3OH (7), whereas air oxidation of (μ-S,S)[FeII(‘H2N2S2‘)]2 in CH3CN yields diamagnetic (μ-S,S)[FeIII(‘N2S2•‘)]2 (8). Complexes 6 and 8 were shown to undergo addition reactions with phosphines, phosphites, or cyanide affording the following complexes:  trans-[FeII(LISQ)2(P(OPh)3)] (9; St = 0) and [N(n-Bu)4][FeII(LISQ)2(CN)] (St = 0). Oxidation of 6 in CH2Cl2 with iodine, bromine, and chlorine respectively yields black microcrystals of [FeIII(LISQ)2X] (X = I, Br, or Cl) with St = 1/2. The structures of complexes 6−9 have been determined by X-ray crystallography at 100 K. The oxidation level of the ligands and iron ions in all complexes has been unequivocally established, as indicated by crystallography; electron paramagnetic resonance, UV−vis, and Mössbauer spectroscopies; and magnetic-susceptibility measurements. The N,S-coordinated o-iminothionebenzosemiquinonate(1−) π radicals have been identified in all new complexes. The electronic structures of the new complexes have been determined, and it is shown that no evidence for iron oxidation states >III is found in this chemistry

<i>N,N</i>-Coordinated π Radical Anions of <i>S</i>-Methyl-1-phenyl-isothiosemicarbazide in Two Five-Coordinate Ferric Complexes [Fe^III(L^Me•)₂X] (X = CH₃S^-, Cl^-)

The reaction between [FeIII(dmf)6](ClO4)3 and the ligand S-methyl-1-phenyl-isothiosemicarbazide, H2[LMe], and triethylamine (1:3:6) in methanol under an argon blanketing atmosphere at elevated temperatures (reflux) yields a purple solution from which upon cooling to 20 °C dark green crystals of [FeIII(LMe•)2(SCH3)] (1) were obtained in 15% yield. From a similar reaction mixture using FeCl3 as starting material in the solvent acetone under anaerobic conditions at −80 °C, dark green crystals of [FeIII(LMe•)2Cl] (2) were obtained in 21% yield. The structures of complexes 1 and 2 have been determined by single-crystal X-ray crystallography at 100 K. Both complexes are five-coordinate square base pyramidal ferric species containing two N,N-coordinated, monoanionic π radicals, (LMe•)1-, of the parent S-methyl-1-phenyl-isothiosemicarbazide(2−) dianion in the basal positions whereas the axial position is occupied by methylthiolate in 1 and chloride in 2, respectively. The electronic structure of both species has been elucidated by their electronic spectra, magnetic properties, and X-band EPR and Mössbauer spectra. Both possess an St = 1/2 ground state which is attained via an antiferromagnetic coupling between the spins of an intermediate spin ferric ion (SFe = 3/2) and two ligand π radical anions (Srad = 1/2)

Intramolecular Spin Interactions in Bis(phenoxyl)metal Complexes of Zinc(II) and Copper(II)

The pendent arm macrocyclic ligand 1-ethyl-4,7-bis(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, H2L, forms stable complexes in methanol with zinc(II) and copper(II) ions:  [ZnII(L)]·H2O (1); [CuII(L)]·0.5 CH2Cl2 (2); [CuII(LH)](ClO4) (3). The crystal structures of 1 and 2 have been determined by X-ray crystallography:  1 crystallizes in the orthorhombic space group Pbca with a = 21.100(4) Å, b = 10.267(2) Å, c = 28.896(6) Å, V = 6260(2) Å3, Z = 8; 2 crystallizes in the monoclinic space group C2/c with a = 14.447(2) Å, b = 25.522(4) Å, c = 17.296(3) Å, V = 6300(2) Å3, Z = 8. In CH2Cl2 solution complexes 1 and 2 can electrochemically be reversibly oxidized by two successive one-electron processes generating the stable phenoxyl mono- ([1]•+; [2]•+) and diradicals ([1]2•2+, [2]2•2+). In contrast, 3 containing a coordinated phenol and one phenolate can only be oxidized to the monoradical [3]•2+. The electronic structure of these mono- and diradicals have been established by UV/vis and EPR spectroscopy in fluid and/or frozen solution. All oxidations are ligand-centered generating coordinated phenoxyl radicals. In [1]2•2+ the two unpaired electrons interact with each other via exchange and weak dipolar couplings of the order of −3 and 10-2 cm-1, respectively. The monoradicals [2]•+ and [3]•2+ are nearly EPR-silent; an St = 1 excited state for both species is barely observable due to large zero-field splitting. In [1]•+ the phenoxyl radical electron is localized on one phenyl ring whereas for [2]•+ some degree of delocalization over both phenyl rings may be present. The diradical [1]2•2+ possesses a diamagnetic whereas [2]2•2+ has an St = 3/2 ground state

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, (LBu)2-, ligands have been elucidated in depth by electronic absorption, infrared, X-band EPR, and Mössbauer spectroscopies. It is conclusively shown that, in contrast to earlier reports, high-valent iron(IV) (d4, S = 1) is not accessible in this chemistry. Instead, the S,S‘-coordinated radical monoanions (L•)1- and/or (LBu•)1- prevail. Thus, five-coordinate [Fe(L)2(PMe3)] has an electronic structure which is best described as [FeIII(L)(L•)(PMe3)] where the observed triplet ground state of the molecule is attained via intramolecular, strong antiferromagnetic spin coupling between an intermediate spin ferric ion (SFe = 3/2) and a ligand radical (L•)1- (Srad = 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(C2H5)3]2[FeII(L)2] (1), [N(n-Bu)4]2[FeIII2(L)4] (2), [N(n-Bu)4]2[FeIII2(LBu)4] (3); [P(CH3)Ph3][FeIII(L)2(t-Bu-py)] (4) where t-Bu-py is 4-tert-butylpyridine. Complexes containing an FeIII(L•)(L)- or FeIII(LBu)(LBu•)- moiety are [N(n-Bu)4][FeIII2(LBu)3(LBu•)] (3ox), [FeIII(L)(L•)(t-Bu-py)] (4ox), [FeIII(LBu)(LBu•)(PMe3)] (7), [FeIII(LBu)(LBu•)(PMe3)2] (8), and [FeIII(LBu)(LBu•)(PPr3)] (9), where Pr represents the n-propyl substituent. Complexes 2, 3ox, 4, [FeIII(L)(L•)(PMe3)2] (6), and 9 have been structurally characterized by X-ray crystallography

S = ³/₂ ⇋ S = ¹/₂ Spin Crossover Behavior in Five-Coordinate Halido- and Pseudohalido-bis(<i>o</i>-iminobenzosemiquinonato)iron(III) Complexes

Five-coordinate halido- and pseudohalido-bis(o-iminobenzosemiquinonato)iron(III) complexes [FeIIIX(LISQ)2] (X = Cl- (1), Br- (2a, 2b), I- (3), N3- (4), and NCS- (5)) have been synthesized where (LISQ)1•- represents the π radical anion N-phenyl-o-imino(4,6-di-tert-butyl)benzosemiquinonate(1−). The molecular structures of the two polymorphs 2a and 2b have been determined at 100, 220, and 295 K, respectively, by single crystal X-ray crystallography. Variable temperature magnetic susceptibility data reveal the following electronic ground states, St:  For 1, it is 3/2. Polymorph 2a contains a 1:1 mixture of 3/2 and 1/2 forms in the range 4.2 to ∼150 K; above 150 K the latter form undergoes a spin crossover 1/2 → 3/2. Polymorph 2b contains only the St = 3/2 form (4−300 K). Complex 3 contains the St = 1/2 form in the range 4−130 K, but above 130 K, a spin crossover to the 3/2 form is observed which is confirmed by three crystal structure determinations at 100, 220, and 295 K. Complex 4 possesses an St = 1/2 ground state at 80 K and undergoes a spin crossover at higher temperatures. Complex 5 has a temperature-independent St = 3/2 ground state. All crystal structures of 1, 2a, 2b, 3, 4, and 5, regardless at which temperature the data sets have been measured, show that two o-iminobenzosemiquinonate(1−) π radical anions are N,O-coordinated in all of these neutral iron complexes. The Fe−N and Fe−O bond distances are longer in the St = 3/2 and shorter in the St = 1/2 forms. The St = 3/2 ground state is attained via intramolecular antiferromagnetic coupling between a high spin ferric ion (SFe = 5/2) and two ligand π radicals whereas the St = 1/2 form is generated from exchange coupling between an intermediate spin ferric ion (SFe = 3/2) and two ligand radicals