Valence Trapping Research Articles

Ligand Field Strength Mediates Electron Delocalization in Octahedral [((H)L)2Fe6(L')m](n+) Clusters.

To assess the impact of terminal ligand binding on a variety of cluster properties (redox delocalization, ground-state stabilization, and breadth of redox state accessibility), we prepared three electron-transfer series based on the hexanuclear iron cluster [((H)L)2Fe6(L')m](n+) in which the terminal ligand field strength was modulated from weak to strong (L' = DMF, MeCN, CN). The extent of intracore M-M interactions is gauged by M-M distances, spin ground state persistence, and preference for mixed-valence states as determined by electrochemical comproportionation constants. Coordination of DMF to the [((H)L)2Fe6] core leads to weaker Fe-Fe interactions, as manifested by the observation of ground states populated only at lower temperatures (<100 K) and by the greater evidence of valence trapping within the mixed-valence states. Comproportionation constants determined electrochemically (Kc = 10(4)-10(8)) indicate that the redox series exhibits electronic delocalization (class II-III), yet no intervalence charge transfer (IVCT) bands are observable in the near-IR spectra. Ligation of the stronger σ donor acetonitrile results in stabilization of spin ground states to higher temperatures (∼300 K) and a high degree of valence delocalization (Kc = 10(2)-10(8)) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (Kc = 10(12)-10(20)) and spin ground state population beyond room temperature. Across the series, at a given formal oxidation level, the capping ligand on the hexairon cluster dictates the overall properties of the aggregate, modulating the redox delocalization and the persistence of the intracore coupling of the metal sites.

Effect of pressure on a mixed valence FeiiFeiii2complex

Intramolecular electron transfer (ET) in mixed valence (MV) oxo-centered [FeiiFeiii2O(carboxylate)6(ligand)3]·solvent complexes is highly dependent on temperature, on the nature of the ligands, and on the presence of crystal solvent molecules [1]. Whereas the effects of temperature, crystal solvent, and ligand variation on the details of the ET have been explored thoroughly, the effect of pressure is less well described [2]. The effect of pressure on the ET in MV Fe3O(cyanoacetate)6(water)3has been investigated with single crystal X-ray diffraction and Mössbauer spectroscopy. Previous multi-temperature studies have shown that at room temperature the ET between the three Fe sites is fast and the observed structure of the Fe3core is a perfectly equilateral triangle [3]. Cooling the complex below 130 K induces a phase transition as the ET slows down. Below 120 K the Fe3core is distorted due to the localization of the itinerant electron on one of the three Fe sites in the triangle (the complex is then in the valence trapped state). The valence trapping is complete within a temperature interval of just 10 K. The abruptness of the transition has been attributed to the extended hydrogen bond network involving water ligands and cyano groups, promoting intermolecular cooperative effects. The high-pressure X-ray diffraction data show that there is a 900flip of half the cyano groups at 3.5 GPa, which dramatically changes the hydrogen bond network. At a slightly higher pressure, a phase transition is found to occur. The five single crystals investigated all broke into minor fragments at the transition; however triclinic unit cells, similar to the low temperature unit cell, could be indexed from selected spots. Additional evidence that the complex is valence trapped comes from high pressure Mössbauer spectra measured above the phase transition (4 GPa). The relationship between valence trapping and the structural changes will in this work be highlighted using void space and Hirshfeld surface analysis.

Extending Metal‐to‐Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location

In an effort to develop robust molecular sensitizers for solar fuel production, the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compound in this context, was examined. Experimental and computational techniques including femtosecond (fs) transient absorption spectroscopy have been used to study the cobalt-containing Keggin POMs, [Co(II) W12 O40 ](6-) (1 a), [Co(III) W12 O40 ](5-) (2 a), [SiCo(II) (H2 O)W11 O39 ](6-) (3 a), and [SiCo(III) (H2 O)W11 O39 ](5-) (4 a), finding the longest lived charge transfer excited state so far observed in a POM and elucidating the electronic structures and excited-state dynamics of these compounds at an unprecedented level. All species exhibit a bi-exponential decay in which early dynamic processes with time constants in the fs domain yield longer lived excited states which decay with time constants in the ps to ns domain. The initially formed states of 1 a and 3 a are considered to result from metal-to-polyoxometalate charge transfer (MPCT) from Co(II) to W, while the longer-lived excited state of 1 a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1 a) is far longer-lived (τ=420 ps in H2 O; τ=1700 ps in MeCN) than that of 3 a (τ=1.3 ps), in which the single Co(II) atom is located in a pseudo-octahedral addendum site. Short-lived states are observed for the two Co(III) -containing heteropolyanions 2 a (τ=4.4 ps) and 4 a (τ=6.3 ps) and assigned solely to O→Co(III) charge transfer. The dramatically extended lifetime for 1 a versus 3 a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.

Homo- and Heterometal Complexes of Oxido–Metal Ions with a Triangular [V(V)O–MO–V(V)O] [M = V(IV) and Re(V)] Core: Reporting Mixed-Oxidation Oxido–Vanadium(V/IV/V) Compounds with Valence Trapped Structures

A new family of trinuclear homo- and heterometal complexes with a triangular [V(V)O-MO-V(V)O] (M = V(IV), 1 and 2; Re(V), 3] all-oxido-metal core have been synthesized following a single-pot protocol using compartmental Schiff-base ligands, N,N'-bis(3-hydroxysalicylidene)-diiminoalkanes/arene (H4L(1)-H4L(3)). The upper compartment of these ligands with N2O2 donor combination (Salen-type) contains either a V(IV) or a Re(V) center, while the lower compartment with O4 donor set accommodates two V(V) centers, stabilized by a terminal and a couple of bridging methoxido ligands. The compounds have been characterized by single-crystal X-ray diffraction analyses, which reveal octahedral geometry for all three metal centers in 1-3. Compound 1 crystallizes in a monoclinic space group P2(1)/c, while both 2 and 3 have more symmetric structures with orthorhombic space group Pnma that renders the vanadium(V) centers in these compounds exactly identical. In DMF solution, compound 1 displays an 8-line EPR at room temperature with <g> and <A> values of 1.972 and 86.61 × 10(-4) cm(-1), respectively. High-resolution X-ray photoelectron spectrum (XPS) of this compound shows a couple of bands at 515.14 and 522.14 eV due to vanadium 2p(3/2) and 2p(1/2) electrons in the oxidation states +5 and +4, respectively. All of these, together with bond valence sum (BVS) calculation, confirm the trapped-valence nature of mixed-oxidation in compounds 1 and 2. Electrochemically, compound 1 undergoes two one-electron oxidations at E(1/2) = 0.52 and 0.83 V vs Ag/AgCl reference. While the former is due to a metal-based V(IV/V) oxidation, the latter one at higher potential is most likely due to a ligand-based process involving one of the catecholate centers. A larger cavity size in the upper compartment of the ligand H4L(3) is spacious enough to accommodate Re(V) with larger size to generate a rare type of all-oxido heterotrimetallic compound (3) as established by X-ray crystallography.

Pressure versus Temperature Effects on Intramolecular Electron Transfer in Mixed‐Valence Complexes

Mixed-valence trinuclear carboxylates, [M(3)O(O(2)CR)(6)L(3)] (M = metal, L = terminal ligand), have small differences in potential energy between the configurations M(II)M(III)M(III)⇔M(III)M(II)M(III)⇔M(III)M(III)M(II), which means that small external changes can have large structural effects, owing to the differences in coordination geometry between M(2+) and M(3+) sites (e.g., about 0.2 Å for Fe-O bond lengths). It is well-established that the electron transfer (ET) between the metal sites in these mixed-valence molecules is strongly dependent on temperature and on the specific crystal environment; however, herein, for the first time, we examine the effect of pressure on the electron transfer. Based on single-crystal X-ray diffraction data that were measured at 15, 90, 100, 110, 130, 160, and 298 K on three different crystals, we first unexpectedly found that our batch of Fe(3)O (O(2)CC(CH(3))(3))(6)(C(5)H(5)N)(3) (1) exhibited a different temperature dependence of the ET process than previous studies of compound 1 have shown. We observed a phase transition at around 130 K that was related to complete valence trapping and Hirshfeld surface analysis revealed that this phase transition was governed by a subtle competition between C-H⋅⋅⋅π and π⋅⋅⋅π intermolecular interactions. Subsequent high-pressure single-crystal X-ray diffraction at pressures of 0.15, 0.35, 0.45, 0.74, and 0.96 GPa revealed that it was not possible to trigger the phase transition (i.e., valence trapping) by a reduction of the unit-cell volume, owing to this external pressure. We conclude that modulation of the ET process requires anisotropic changes in the intermolecular interactions, which occur when various directional chemical bonds are affected differently by changes in temperature, but not by the application of pressure.

Coloring ionic trapping states in WO 3 and Nb 2O 5 electrochromic materials

Complex electro-optical analysis is a very useful approach to separate different kinetic processes that occur during ionic insertion reactions in electrochromic oxide materials. In this paper, we use this type of combined technique to investigate ionic and optical changes in different oxide host systems, i.e., in two oxide hosts, specifically WO 3 and Nb 2O 5. A comparison of their electro-optical responses revealed the presence of an ionic trapping contribution to the kinetics of the coloring sites, which was named here as coloring ionic trapping state. As expected, this coloring trapping process is slower in Nb 2O 5 since the reduction potential of Nb 2O 5 is more negative (more energy is needed for a higher degree of coloration). A phenomenological solid-state model that encompasses homogeneous charge transfer and valence trapping was proposed to explain the coloring ionic trapping process. Basically the model is able to explain how ionic dynamics at low frequency region, i.e., the slower kinetic step, controls the coloring kinetics, i.e., how it is capable to regulate the coloring rates. Optical transient analyses demonstrated the possibility of the presence of more than one coloring ionic trap, indicating the complexity of the processes involved in coloration phenomenon in metal oxide host systems.

Examination of the Mixed‐Valence State of the Doubly Boron‐Bridged Diferrocene Cation [(FeCp)2{μ‐C10H6(BPh)2}]+

A series of mixed-valent (MV) complexes [(FeCp)2(mu-C10H6(BPh)2)]+X ([1+]X; X=I 5, PF6, SbF6, B(C6F5)4) were prepared by oxidation of diboradiferrocene [(FeCp)2(mu-C10H6(BPh)2)] (1) with I 2, AgPF6, and AgSbF6, respectively, and through anion exchange of the I 5(-) salt with [Li(Et2O)x][B(C6F5)4] in the case of X=B(C6F5)4. The MV state of the cation was investigated in solution by multinuclear NMR spectroscopy, CV, and UV/Vis-NIR absorption spectroscopy, and in the solid state by IR spectroscopy, single-crystal X-ray crystallography, and Mössbauer spectroscopy. The cyclic voltammogram of 1 shows two distinct redox waves with a large redox splitting of Delta E=510 mV in CH2Cl2 and the NIR spectrum for the mono-oxidized species displays an intervalence charge-transfer band at around 1500 to 1700 nm depending on the specific counterion present. The X-ray crystal structures of [1+]X show inversion-symmetric cations with X=I 5 and B(C6F5)4 and unsymmetric valence-trapped structures composed of one ferrocene and one ferrocenium moiety with X=PF6 and SbF6. Mössbauer data for X=PF6 are consistent with valence trapping at all temperatures between 90 and 343 K. In comparison, fast electron transfer is evident on the Mössbauer timescale for X=I 5 and temperature-dependent behavior is observed for X=B(C6F5)4. The anion dependence of the X-ray structural and Mössbauer data is discussed in the context of crystal symmetry and the possibility of static and dynamic disorder effects is considered.

Quantifying the effects of the self-interaction error in DFT: When do the delocalized states appear?

The self-interaction error in density-functional theory leads to artificial stabilization of delocalized states, most evident in systems with an odd number of electrons. Clear examples are dissociations of carbocation radicals that often give delocalized states at long distances and large errors in computed binding energies. On the other hand, many mixed-valence transition-metal dimers known to exhibit valence trapping are correctly predicted to be localized. To understand the effects of the self-interaction error on these different systems, energy differences between delocalized and localized states are calculated with B3LYP. In the dissociation of radicals into symmetric fragments at infinite distance, this energy difference equals the error of the density-functional treatment. The energy difference decreases with increasing size of the system, from 55 kcal/mol in H2(+) to 15 kcal/mol for C12H26(+). Solvent corrections stabilize the localized state and result in smaller errors. Most reactions are asymmetric and this decreases the effect of the self-interaction error. In many systems, delocalization will not occur if the cost to move the electron from one fragment to the other is 70-80 kcal/mol (3.0-3.5 eV). This estimate refers to a situation where the distance between the fragments is infinite. The limit decreases with decreasing fragment distance. B3LYP calculations on the ferromagnetic state of a Mn(III,IV) dimer predict that the correct localized state is 22 kcal/mol more stable than the incorrect delocalized state. At short metal-metal distances the effect of the self-interaction error is predicted to be small. However, as the distance between the two manganese centers is increased to 7 A, the dimer starts to delocalize and the energy artificially decreases. In the dissociation limit, the error is 10 kcal/mol. This is interpreted as an artifact originating from the self-interaction error. Delocalization is not encountered in many systems due to relatively short metal-metal distances and asymmetric ligand environments. However, some charge-transfer complexes cannot be properly calculated and delocalized states may become a problem in large models of enzyme systems with multiple transition-metal complexes.

Paramagnetic NMR Spectroscopy and Density Functional Calculations in the Analysis of the Geometric and Electronic Structures of Iron−Sulfur Proteins

Paramagnetic NMR spectroscopy has been underutilized in the study of metalloproteins. One difficulty of the technique is that paramagnetic relaxation broadens signals from nuclei near paramagnetic centers. In systems with low electronic relaxation rates, this makes such signals difficult to observe or impossible to assign by traditional methods. We show how the challenges of detecting and assigning signals from nuclei near the metal center can be overcome through the combination of uniform and selective 2H, 13C, and 15N isotopic labeling with NMR experiments that utilize direct one-dimensional (2H, 13C, and 15N) and two-dimensional (13C-X) detection. We have developed methods for calculating NMR chemical shifts and relaxation rates by density functional theory (DFT) approaches. We use the correspondence between experimental NMR parameters and those calculated from structural models of iron-sulfur clusters derived from X-ray crystallography to validate the computational approach and to investigate how structural differences are manifested in these values. We have applied this strategy to three iron-sulfur proteins: Clostridium pasteurianum rubredoxin, Anabaena [2Fe-2S] ferredoxin, and human [2Fe-2S] ferredoxin. Provided that an accurate structural model of the iron-sulfur cluster and surrounding residues is available from diffraction data, our results show that DFT calculations can return NMR observables with excellent accuracy. This suggests that it might be possible to use calculations to refine structures or to generate structural models of active sites when crystal structures are unavailable. The approach has yielded insights into the electronic structures of these iron-sulfur proteins. In rubredoxin, the results show that substantial unpaired electron spin is delocalized across NH...S hydrogen bonds and that the reduction potential can be changed by 77 mV simply by altering the strength of one of these hydrogen bonds. In reduced [2Fe-2S] ferredoxins, hyperfine shift data have provided quantitative information on the degree of valence trapping. The approach described here for iron-sulfur proteins offers new avenues for detailed studies of these and other metalloprotein systems.

Density functional study of valence trapping in a mixed-valent dimanganese complex.

Density functional calculations are reported for the linked system [(Fulv)[Mn(CO)(2)](2)(mu-dppm)] in the neutral and singly and doubly charged states. The neutral system and dication are characterized by well-localized valence states involving two Mn(II) and Mn(I) centers, respectively. At the BP86/AE1 level, the electronic structure of the monocation is that of a delocalized mixed-valence species. At the B3LYP/AE1 level, the spin density of the unpaired electron is largely localized on one Mn atom, in accord with the formulation of a trapped-valence ground state (Atwood, C. G.; Geiger, W. E. J. Am. Chem. Soc. 2000, 122, 5477). The qualitative IR pattern of the CO stretching vibrations is well reproduced at the B3LYP level, supporting the empirical charge distribution parameter employed for the interpretation of the IR spectra.

From Valence Trapped to Valence Delocalized by Bridge State Modification in Bis(triarylamine) Radical Cations: Evaluation of Coupling Matrix Elements in a Three-Level System

This paper presents an analysis of the visible/near-infrared (vis/NIR) spectra of four bis(triarylamine) radical cation mixed valence systems with varying bridge units in the framework of the generalized Mulliken−Hush theory. We outline how to apply a three-level model by using both computational AM1-CI derived as well as experimental transition moments and energies in order to extract electronic coupling matrix elements. The most important outcome is that the much simpler two-level model is a good approximation only if the adiabatic dipole moment difference between the terminal states is large compared to the transition moments associated with the bridge state. This implies that the two-level model is only applicable to mixed valence compounds in the Robin−Day class II with strongly localized redox states if qualitative correct values are desired. We demonstrate that both the spectral features and the potential energy surface of the mixed valence compounds can solely be tuned by bridge state modification...

Theoretical analysis of the [Mn2(μ-oxo)2(μ-carboxylato)2]+core

The first example of a dinuclear manganese complex containing two oxo and two carboxylate bridges, [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ (where bpy = 2,2′-bipyridine, and ArTolCO2− = 2,6-di(p–tolyl)benzoate), was reported recently (J. Am. Chem. Soc. 2003, 125, 13010). X-ray crystallographic analysis performed on this complex reveals a trapped mix-valence species as evidenced, for example, by very different metal–ligand bond distances at the MnIII and MnIV centers. The fact that there are rather bulky bridging carboxylate ligands present in this recently reported dinuclear species raises the question as to whether they affect the extent of valence trapping and the metrical parameters in general. Specifically, it was thought that intramolecular nonbonded contacts could play an important role. In the work reported here, density functional theory calculations were used to address this issue. Structural parameters obtained from calculations on a model compound bearing sterically small bridging carboxylates, [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+, are in good agreement with the experimentally determined single crystal X-ray structure. Thus, the sterically large carboxylate bridges in [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ appear not to have a significant effect on the metal–ligand bond distances and angles. There is calculated to be minimal Mn⋯Mn bonding despite contraction of the Mn⋯Mn distance relative to related complexes. In addition to calculations on the mixed-valence MnIIIMnIV complex, various electronic configurations of the corresponding MnIIIMnIII and MnIVMnIV complexes are explored. Although our calculations support assignment of [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+ as a valence-trapped MnIIIMnIV configuration involving high-spin MnIII, a delocalized configuration arising from low-spin MnIII is calculated to lie very close in energy. The energetic proximity of the delocalized configuration is attributed to an effective crossed-exchange mechanism, which permits mixing of an eg-based orbital (nominally on high-spin MnIII) with a vacant t2g-based orbital (nominally on MnIV).

Electronic Structure of Linear Thiophenolate-Bridged Heteronuclear Complexes [LFeMFeL]n+(M = Cr, Co, Fe;n= 1−3): A Combination of Kinetic Exchange Interaction and Electron Delocalization

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n)(+) with M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1). The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2). In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3). The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4). The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5). In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6). Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound.

Valence Delocalization despite Weak Metal−Metal Coupling in a Bis(organoosmium(III,II)) Complex with a Pyrazine Bridge

Carbonyl vibrational spectroelectrochemistry of {(μ-pz)[Os(PiPr3)2(CO)(H)Cl]2}0/+ in dichloromethane reveals valence delocalization of the mixed-valent state despite relatively weak metal−metal coupling, as evident from the comproportionation constant Kc = 104.3 and the intervalence charge-transfer band at 1705 nm (ε = 1250 M-1 cm-1, Δν1/2 = 3700 cm-1). The rather low charge and nonpolar medium, i.e., the absence of valence trapping by counterions or solvent molecules, favor this particular situation.

Dinuclear iron(III)–metal(II) complexes as structural core models for purple acid phosphatases †

A series of mixed-valent iron and mixed-metal FeIII–MII (M = Zn, Cu, Ni or Co) complexes of the phenolate-hinged dinucleating ligand 2,6-bis{[bis(2-pyridylmethyl)amino]methyl}-4-tert-butylphenolato(1–), bpbp– have been prepared and characterized. Both exogenous bidentate bridging groups and different terminal ligands bound to each different metal ion at the exogenous site were identified. The structure of the mixed-valence complex [(bpbp)Fe2(F)2(H2O)2][BF4]2 confirms that it is a rare example of a dimetallic complex of a single-atom hinged acyclic dinucleating ligand with a ‘non-bridged’ arrangement at the exogenous bridging site. Mössbauer spectroscopy indicates valence trapping in this complex with the parameters, ΔEQ 3.242 mm s–1, δ 1.169 mm s–1 and ΔEQ 0.221 mm s–1, δ 0.460 mm s–1, respectively for the high spin Fe2+ and high spin Fe3+ ions. Crystals of [(bpbp)Fe2(F)2(H2O)2][BF4]2·4H2O are triclinic, space group P (no. 2), with a = 12.695(1), b = 19.197(2), c = 10.202(1) Å, α = 102.95(1), β = 97.61(1), γ = 93.76(1)°, Z = 2. The structure was refined to R = 0.1009 on F using 4338 reflections with I > 2σ(I) (wR2 on all data and F 2 = 0.3522). The FeII and FeIII atoms are bridged asymmetrically by the phenolic oxygen atom of bpbp– with FeII–O 2.175(6) Å and FeIII–O 2.033(6) Å with a FeIII · · · FeII distance of 3.726(2) Å. The two terminal fluoride ions are bound to the FeIII atom and strongly hydrogen bonded to two water molecules bound to the adjacent FeII atom. This complex may model the mode in which fluoride ions bind to the active site of the purple acid phosphatases (PAPs) thereby inhibiting the activity of these enzymes. Tetrahedral oxo anions are known also to inhibit PAPs and to mimic this inhibition a FeIII–ZnII complex incorporating molybdate bridging groups was prepared. Crystals of [(bpbp)FeZn(MoO4)2]·C3H7OH· 2H2O are monoclinic, space group P21/n with a = 11.773(13), b = 21.394(7), c = 17.001(11) Å and β = 90.98(7)°, Z = 4. The structure was refined to R = 0.0434 on F using 3758 reflections with I > 2σ(I) (wR2 on all data and F 2 = 0.1339). The FeIII · · · ZnII distance is 3.819(4) Å. A series of acetate-bridged complexes were prepared by the novel method of diffusing ethyl acetate or isopropyl acetate into mixtures of Hbpbp and iron perchlorate in the presence and absence of a second type of metal ion. The acetate bridging groups are the result of the hydrolysis of the alkyl acetate. These complexes have the general formulation [(bpbp)FeM(CH3CO2)2][ClO4]2. Crystals of [(bpbp)FeCu(CH3CO2)2][ClO4]2·0.5CH3OH are monoclinic, space group P21/n

A pentaammineruthenium(III) dimer with the novel bridging ligand 4,4′-dicyanamidobiphenyl dianion

The novel ligand 4,4′-dicyanamidobiphenyl dianion (bp2−) has been synthesized and characterized by 13C NMR spectroscopy, cyclic voltammetry, and crystallography. The crystal structure of [Ph4As]2[bp]•H2O showed that bp2− is approximately planar with a dihedral angle of 8.2° between phenyl ring planes and the cyanamide groups in an anti conformation. The water of crystallization is asymmetrically hydrogen bonded between cyanamide groups of adjacent bp2− ions. The crystal data for C62H48N4As2 + H2O are monoclinic crystal system and space group P21/c with a = 12.998(5) Å, b = 13.465(4) Å, c = 28.703(13) Å, β = 98.94(3)°, V = 4963(3) Å3 and Z = 4. The structure was refined by using 4555 reflections with I &gt; 2.5σ(I) to an R factor of 0.058. The complex, [{(NH3)5Ru)2(μ-bp)][X]4, where X = tosylate or PF6− ions, was also synthesized and characterized by 1H NMR spectroscopy, cyclic voltammetry, spectroelectrochemistry, and temperature-dependent magnetic susceptibility measurements. From cyclic voltammetry measurements, the comproportionation constants to form the mixed-valence complex [{(NH3)5Ru)2(μ-bp)]3+ were estimated to be 4.1, 16, and 22 in water, acetonitrile, and nitromethane, respectively. The trend and magnitude of Kc suggests solvent valence trapping of a weakly coupled Class II ion. The MMCT band of the mixed-valence complex had to be deconvoluted from the low-energy LMCT band and had the following properties in acetonitrile, νmax = 8400 cm−1, εmax = 3300 M−1 cm−1, and Δν1/2 = 3300 cm−1. The weak superexchange mediating properties of bp2− compared to 1,4-dicyanamidobenzene dianion were suggested to arise from the larger barrier to the formation of the radical anion bp−. Key words: cyanamido, mixed valence, superexchange, ruthenium.

Selectivity and compositional dependence of response of gas-sensitive resistors part 4.—Properties of some rutile solid solution compounds

Solid solutions based on the rutile structure offer the possibility of independently varying the cation composition and dopant density in the lattice and hence exploring the effects that these have upon the electrical response to reactive gases. Thus, solid solutions of CrNbO4(p-type response) with Nb-doped TiO2 and FeNbO4(n-type response) have been investigated. The resistivities and activation energies increased in the order FeNbO4 CrNbO4, FeNbO4 and this was postulated to be due either to Nb5+ surface segregation or to the narrowing of the conduction bandwidth leading to valence trapping of electrons. An enhanced gas response at T <300 °C was found for the compounds after they had been heated to ca. 700 °C in air. This was attributed to the desorption of surface hydroxy, carbonyl and lattice oxygen species to produce a vacancy around which the surface reconstructed. It is proposed that ionosorption of oxygen at this site produced a species able to react with CO and produce a resistance change at low temperatures.

Structure and mixed-valence states of the trinuclear iron(II,III,III) pivalate complex-pivalic acid adduct, [Fe 3O(O 2CCMe 3) 6(Me 3CCO 2H) 3] (Me = −CH 3)

The structure of the mixed-valence trinuclear iron pivalate complex-pivalic acid adduct, [Fe 3O(O 2CCMe 3) 6(Me 3CCO 2H 3 (Me = −CH 3), was characterized by single crystal X-ray diffraction. The adduct crystallizes in the orthorhombic system, space group Cmcm, with a = 18.437(1), b = 20.339(2), c = 17.171(1) A ̊ , V = 6456(1) A ̊ 3 and Z = 4 , without solvate molecules of crystallization. The adduct is first characterized to be a mixed-valence Fe 3O compound with carboxylic acid coordinating to the iron atom. The acid is coordinated through the carbonyl O atom trans to the μ 3-oxo atom, disordered for crystallographic mirror symmetry. Although there are two crystallography non-equivalent iron sites, no significant difference is seen between their coordination environments at room temperature. The complex shows a valence-trapped state below ∼ 100 K and a valence-detrapped state above ∼ 230 K up to room temperature on the 57Fe Mössbauer time-scale (∼10 −7 − 10 −8 s), whereas it shows an evidence of valence trapping on the IR time-scale (∼ 10 −12 − 10 −13 s) at room temperature. The temperature-dependent valence transformation is found to culminate with a first-order phase transition at ∼ 230 K.

Mössbauer study and molecular orbital calculations on some bimetallic derivatives of ferrocene and ferricinium

We have studied by absorption Mossbauer spectroscopy, some bridged derivatives of biferrocene, unoxidized (FeII– FeII) and mono-oxidized (FeII – FeIII). The mixed-valence species exhibits a quasi-delocalized behavior at the Mossbauer timescale (∼ 10−8 s), with a splitting of the lines which is interpreted in terms of a partial valence trapping. This in turn reveals intramolecular properties at variance from those of the unbridged cation. SCC-Xα MO calculations have been performed; they correctly reproduce the measured quadruple splittings and provide evidence that the HOMO of the mixed-valence complex is of\(d_{xy} ,d_{x^2 - y^2 } \) type. Some consequences on the mixed-valence properties are discussed.

Electron transfer in iron(II)iron(III) model complexes of iron-oxo proteins

The nature of valence trapping is studied for a series of six μ-phenoxo-bis(μ-carboxylate)-bridged Fe II Fe III complexes. The septadentate ligand bimp - binds the two metal ions and provides a phenoxide bridge as well as four imidazole moieties. The X-ray structure of [Fe II Fe III (bimp)(μ-O 2 CPh) 2 ](BPh 4 ) 2 . 3 / 2 CH 3 CN at 295 K is reported