Intervalence Charge Transfer Band Research Articles

Charge and Spin Delocalization in Mixed-Valent Vinylruthenium–Triarylamine-Conjugates with Planarized Triarylamines

We report on three new vinylruthenium–triarylamine (TA) conjugates with planarized TA substituents. The neutral, mono-, and dioxidized forms of the complexes were scrutinized by UV/vis/NIR, IR, and EPR spectroscopies as well as by quantum chemical calculations. By analyzing the intervalence charge-transfer (IVCT) transitions of the mixed-valent radical cations, we obtain information on the extent of electronic coupling between the chemically different redox sites. Completely delocalized Ru-MeTA+ shows a vibrationally structured, non-solvatochromic IVCT band. Decreased blueshifts of the indicative Ru(CO) stretching vibration during the first oxidation and larger A(14N) EPR hyperfine splitting constants in concert with smaller g-values indicate enhanced TA contributions to the singly occupied molecular orbital of the other two complexes Ru-TOTA•+ and Ru-DOTA•+ and hence less symmetrical charge and spin density distributions. This is also reflected by the solvatochromism and the asymmetric shape of the IVCT band with a smaller bandwidth at the low-energy side. Temperature-dependent UV/vis/NIR spectroscopy of mixed-valent Ru-TOTA•+ and Ru-DOTA•+ revealed that the band skewing is due to the vibrational coupling of the IVCT transition to symmetrical vibrations, placing these radical cations near or at the class II/III borderline according to Robin and Day.

Exciton Coupling in Redox-Active Salen based Self-Assembled Metallacycles.

The incorporation of a redox-active nickel salen complex into supramolecular structures was explored via coordination-driven self-assembly with homobimetallic ruthenium complexes (bridged by oxalato or 5,8-dihydroxy-1,4-naphthoquinato ligands). The self-assembly resulted in the formation of a discrete rectangle using the oxalato complex and either a rectangle or a catenane employing the larger naphthoquinonato complex. The formation of the interlocked self-assembly was determined to be solvent and concentration dependent. The electronic structure and stability of the oxidized metallacycles was probed using electrochemical experiments, UV-Vis-NIR absorption, EPR spectroscopy and DFT calculations, confirming ligand radical formation. Exciton coupling of the intense near-infrared (NIR) ligand radical intervalence charge transfer (IVCT) bands provided further confirmation of the geometric and electronic structures in solution.

Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-).

The Cs8AuIII4MIIIX23 (M = In3+, Sb3+, Bi3+; X = Cl-, Br-, I-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au3+ to Au+ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs8Au4InCl23 from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

Evaluation of Through‐Space Electronic Coupling in the Cofacially Aligned π‐Stacked Organic Mixed‐Valence System

A new π‐stacked compound, 1,8‐bis(2′,5′‐dimethoxy‐4′‐methyl‐[1,1′‐biphenyl]‐4‐yl)naphthalene (1) has been synthesized and characterized as a precursor molecule of monocationic mixed‐valence system (MVS). Cyclic and differential pulse voltammograms reveal that the ΔE°ox value between two sequential oxidation potentials is 163 mV. The three‐dimensional geometries of 1, 1•+, and 12+ were obtained via using density functional theory (DFT) calculations with a range‐separated hybrid functional and dispersion corrections (LC‐wPBE‐D3). Two dimethoxytoluene redox centers were aligned in parallel with van der Waals interplanar distances (3.50–3.70 Å). The electronic coupling in this MVS was evaluated by two different approaches: (1) Mulliken–Hush analysis of intervalence charge transfer (IVCT) band obtained from the spectroelectrochemical method, and (2) partial charge distribution analysis of quinonoidal distortion of D•+/D centers obtained from theoretical DFT calculations. The H12 were 2055 and 1903 cm−1 by (1) and (2), respectively. This work suggests that the pure through‐space electronic coupling can be large when two aromatic redox centers are properly aligned.

Delocalization tunable by ligand substitution in [L2Al] n- complexes highlights a mechanism for strong electronic coupling.

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I2P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I2P−)(I2P2−)Al and [(I2P2−)(I2P3−)Al]2− show intervalence charge transfer (IVCT) transitions in the regions 6850–7740 and 7410–9780 cm−1, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I2P2−)(I2P3−)Al]2− complexes containing –PhNMe2 or –PhF5 substituents. Minor localization of charge in [(I2P2−)(I2P3−)Al]2− was observed when –PhOMe substituents are included.

Time-Resolved Spectroscopy of Photoinduced Electron Transfer in Dinuclear and Tetranuclear Fe/Co Prussian Blue Analogues.

The dynamics of the photodriven charge transfer-induced spin transition (CTIST) in two Fe/Co Prussian Blue Analogues (PBAs) are revealed by femtosecond IR and UV/vis pump-probe spectroscopy. Depending on temperature, the known tetranuclear square-type complex [Co2Fe2(CN)6(tp*)2(4,4'-dtbbpy)4](PF6)2 (1) exists in two electronic states. In acetonitrile solution, at <240 K, the low temperature (LT) phase is prevalent consisting of low-spin Fe(II) and low-spin Co(III), [FeIILSCoIIILS]2. Temperature rise is the reason behind thermally-induced CTIST toward the high temperature (HT) phase consisting of low-spin Fe(III) and high-spin Co(II), [FeIIILSCoIIHS]2, being prevalent at >300 K. Photoexcitation into the intervalence charge transfer (IVCT) band of the LT phase at 800 nm induces electron transfer in one Fe-Co edge of PBA 1 and produces a [FeIIILSCoIILS] intermediate which by spin-crossover (SCO) is stabilized within 400 fs to a long-lived (>1 ns) [FeIIILSCoIIHS] species. In contrast, IVCT excitation of the HT phase at 400 nm generates a [FeIILSCoIIIHS] species with a lifetime of 3.6 ps. Subsequent back-electron transfer populates the vibrationally hot ground state, which thermalizes within 8 ps. The newly synthesized dinuclear PBA, [CoFe(CN)3(tp*)(pz*4Lut)]ClO4 (2), provides a benchmark of the HT phase of 1, i.e., [FeIIILSCoIIHS], as verified by variable temperature magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. The photoinduced charge transfer dynamics of PBA 2 indeed are almost identical to that of the HT phase of PBA 1 with a lifetime of the excited [FeIILSCoIIIHS] species of 3.8 ps.

Redox-Induced Hydrogen Bond Reorientation Mimicking Electronic Coupling in Mixed-Valent Diruthenium and Macrocyclic Tetraruthenium Complexes.

We present the coordination-driven self-assembly of three tetranuclear metallacycles containing intracyclic NH2, OH, or OMe functionalities through the combination of various isophthalic acid building blocks with a divinylphenylene diruthenium complex. All new complexes of this study were characterized by means of nuclear magnetic resonance spectroscopy, ultrahigh-resolution ESI mass spectrometry, cyclic and square wave voltammetry and, in two cases, X-ray diffraction. The hydroxy functionalized macrocycle 4-BOH and the corresponding half-cycle 2-OH stand out, as their intracyclic OH···O hydrogen bonds stabilize their mixed-valent one- (2-OH, 4-BOH) and three-electron-oxidized states (4-BOH). Despite sizable redox splittings between all one-electron waves, the mixed-valent monocations and trications do not exhibit any intervalence charge-transfer band, assignable to through-bond electronic coupling, but nevertheless display distinct IR band shifts of their charge-sensitive Ru(CO) tags. We ascribe these seemingly contradicting observations to a redox-induced shuffling of the OH···O hydrogen bond(s) to the remaining, more electron-rich, reduced redox site.

Synthesis, Characterization, and Electrochemistry of Diferrocenyl β-Diketones, -Diketonates, and Pyrazoles.

The synthesis of FcC(O)CH(R)C(O)Fc (Fc = Fe(η5-C5H4)(η5-C5H5); R = H, 5; nBu, 7; CH2CH2(OCH2CH2)2OMe, 9), [M(κ2O,O′-FcC(O)CHC(O)Fc)n] (M = Ti, n = 3, 10; M = Fe, n = 3, 11; M = BF2, n = 1, 12), and 1-R′-3,5-Fc2-cC3HN2 (R′ = H, 13; Me, 14; Ph, 15) is discussed. The solid-state structures of 5, 7, 9, 12, 13, 15, and 16 ([TiCl2(κ2O,O′-PhC(O)CHC(O)Ph)2]) show that 7 and 9 exist in their β-diketo form. Compound 13 crystallizes as a tetramer based on a hydrogen bond pattern, including one central water molecule. The electrochemical behavior of 5–7 and 9–16 was studied by cyclic and square-wave voltammetry, showing that the ferrocenyls can separately be oxidized reversibly between −50 and 750 mV (5–7, 9, 12–15: two Fc-related events; 10, 11: six events, being partially superimposed). For complex 10, Ti-centered reversible redox processes appear at −985 (TiII/TiIII) and −520 mV (TiIII/TiIV). Spectro-electrochemical UV-Vis/NIR measurements were carried out on 5, 6, and 12, whereby only 12 showed an IVCT (intervalence charge-transfer) band of considerable strength (νmax = 6250 cm−1, Δν½ = 4725 cm−1, εmax = 240 L·mol−1·cm−1), due to the rigid C3O2B cycle, enlarging the coupling strength between the Fc groups.

Effects of oxidation on pyroxene visible-near infrared and mid-infrared spectra

Pyroxene spectral features in the visible near-infrared (VNIR) and mid-infrared (MIR) wavelengths are affected by oxidation resulting from traditional metamorphic processes as well as impact metamorphism. The observed effects are due to modifications in the crystal arising from changes in crystallization temperature or pressure or from substituting Fe3+ for Fe2+. Highly oxidized pyroxenes from terrestrial mantle xenoliths and shock experiments indicate that the spectral effects of oxidation are greater in clinopyroxene than orthopyroxene because clinopyroxene can accommodate more Fe3+ structurally. Changes in clinopyroxene VNIR related to increasing oxidation include a shift in the 0.8 μm absorption band to shorter wavelengths and a strengthening of the Fe2+↔Fe3+ intervalence charge transfer (IVCT) band, which reduces the band depth of the 1.0 μm feature by ~20%. Although shocked clinopyroxenes are oxidized to similar levels to that seen in the mantle xenoliths, the effects of shock overprint those of oxidation in the VNIR. These include a decrease of ~76% intensity of the 2.35 μm feature and a decrease of ~70% intensity of the 1.0 μm feature. In the MIR, the effects of oxidation and shock are minimal, resulting in a 5% overall decrease in band depth. These shifts and changes can be interpreted as a result of changes in the polyhedra surrounding the Fe cations which reduce crystal field splitting and the order of the crystal structure. Determination of planetary surface composition through VNIR remote sensing methods requires careful consideration of potential changes induced via shock and/or oxidation processes.

Unified Model for Photophysical and Electro-Optical Properties of Green Fluorescent Proteins

Green fluorescent proteins (GFPs) have become indispensable imaging and optogenetic tools. Their absorption and emission properties can be optimized for specific applications. Currently, no unified framework exists to comprehensively describe these photophysical properties, namely the absorption maxima, emission maxima, Stokes shifts, vibronic progressions, extinction coefficients, Stark tuning rates, and spontaneous emission rates, especially one that includes the effects of the protein environment. In this work, we study the correlations among these properties from systematically tuned GFP environmental mutants and chromophore variants. Correlation plots reveal monotonic trends, suggesting that all these properties are governed by one underlying factor dependent on the chromophore's environment. By treating the anionic GFP chromophore as a mixed-valence compound existing as a superposition of two resonance forms, we argue that this underlying factor is defined as the difference in energy between the two forms, or the driving force, which is tuned by the environment. We then introduce a Marcus-Hush model with the bond length alternation vibrational mode, treating the GFP absorption band as an intervalence charge transfer band. This model explains all of the observed strong correlations among photophysical properties; related subtopics are extensively discussed in the Supporting Information. Finally, we demonstrate the model's predictive power by utilizing the additivity of the driving force. The model described here elucidates the role of the protein environment in modulating the photophysical properties of the chromophore, providing insights and limitations for designing new GFPs with desired phenotypes. We argue that this model should also be generally applicable to both biological and nonbiological polymethine dyes.

Distance Dependence of Electronic Coupling in Rigid, Cofacially Compressed, π-Stacked Organic Mixed-Valence Systems.

A series of new π-stacked compounds, 1,8-bis(2',5'-dimethoxybenzene-1'-yl)naphthalene (1), 1,4-bis(8'-(2″,5″-dimethoxybenzene-1″-yl)naphthalen-1'-yl)benzene (2), and 1,8-bis(4'-(8″-(2‴,5‴-dimethoxybenzene-1‴-yl)naphthalen-1″-yl)benzene-1'-yl)naphthalene (3), have been synthesized and characterized herein as precursor molecules of monocationic mixed-valence systems (MVSs). The three-dimensional geometries of these compounds were determined by X-ray crystallography. A near-orthogonal alignment of the naphthalene pillaring motif to the dimethoxybenzene redox center, or the phenylene spacer, imposes cofacial alignment of these units in a juxtaposed manner with sub-van der Waals interplanar distances. Cyclic and differential pulse voltammograms reveal that the ΔE values between two sequential oxidation potentials are 0.30, 0.11, and 0.10 V for 1, 2, and 3, respectively. MVSs derived from these compounds are recognized as class II according to the Robin and Day classification. The decay parameter β, which describes the distance dependence of the squared electronic coupling in the three mixed-valence systems, was experimentally determined via Mulliken-Hush analysis of the intervalence charge transfer band (β = 0.37 Å-1) and theoretically assessed from charge-resonance contributions derived from DFT computations (β = 0.37 Å-1). These values are extraordinarily mild, indicating that the electronic interaction between redox centers in the longitudinal direction may be comparable to that in the transverse direction, if the MVS system is appropriately designed.

Zwitterionic Mixed Valence: Internalizing Counteranions into a Biferrocenium Framework toward Molecular Expression of Half-Cells in Quantum Cellular Automata.

Realization of molecular quantum cellular automata (QCA), a promising architecture for molecular computing through current-free processes, requires improved understanding and application of mixed-valence (MV) molecules. In this report, we present an electrostatic approach to creating MV subspecies through internalizing opposite charges in close proximity to MV ionic moieties. This approach is demonstrated by unsymmetrically attaching a charge-responsive boron substituent to a well-known organometallic MV complex, biferrocenium. Guest anions (CN- and F- ) bind to the Lewis acidic boron center, leading to unusual blue-shifts of the intervalence charge-transfer (IVCT) bands. To the best of our knowledge, this is the first reported example of a zwitterionic MV series in which the degree of positive charge delocalization can be varied by changing the bound anions, and serves to clarify the interplay between IVCT parameters. The key underlying factor is the variable zero-level energy difference in the MV states. This work provides new insight into imbuing MV molecules with external charge-responsiveness, a prerequisite of molecular QCA techniques.

Unified Model for Photophysical and Electro-Optical Properties of Green Fluorescent Proteins.

Green fluorescent proteins (GFPs) have become indispensable imaging and optogenetic tools. Their absorption and emission properties can be optimized for specific applications. Currently, no unified framework exists to comprehensively describe these photophysical properties, namely the absorption maxima, emission maxima, Stokes shifts, vibronic progressions, extinction coefficients, Stark tuning rates, and spontaneous emission rates, especially one that includes the effects of the protein environment. In this work, we study the correlations among these properties from systematically tuned GFP environmental mutants and chromophore variants. Correlation plots reveal monotonic trends, suggesting that all these properties are governed by one underlying factor dependent on the chromophore's environment. By treating the anionic GFP chromophore as a mixed-valence compound existing as a superposition of two resonance forms, we argue that this underlying factor is defined as the difference in energy between the two forms, or the driving force, which is tuned by the environment. We then introduce a Marcus-Hush model with the bond length alternation vibrational mode, treating the GFP absorption band as an intervalence charge transfer band. This model explains all of the observed strong correlations among photophysical properties; related subtopics are extensively discussed in the Supporting Information. Finally, we demonstrate the model's predictive power by utilizing the additivity of the driving force. The model described here elucidates the role of the protein environment in modulating the photophysical properties of the chromophore, providing insights and limitations for designing new GFPs with desired phenotypes. We argue that this model should also be generally applicable to both biological and nonbiological polymethine dyes.

Organic-Inorganic Hybrid Gold Halide Perovskites: Structural Diversity through Cation Size.

Crystal structures of a series of organic-inorganic hybrid gold iodide perovskites, formulated as A2 [AuI I2 ][AuIII I4 ] [A=methylammonium (MA) (1) and formamidinium (FA) (2)], A'2 [I3 ]1-x [AuI I2 ]x [AuIII I4 ] [A'=imidazolium (IMD) (3), guanidinium (GUA) (4), dimethylammonium (DMA) (5), pyridinium (PY) (6), and piperizinium (PIP) (7)], systematically changed depending on the cation size. In addition, triiodide (I3 - ) ions were partly incorporated into the AuI2 - sites of 3-7, whereas they were not incorporated into those of 1 and 2. Such a difference comes from the size of the organic cation. Optical absorption spectra showed characteristic intervalence charge-transfer bands from AuI to AuIII species, and the optical band gap increased as the size of the cation became larger.

The nephelauxetic effect on the electron binding energy in the 4fq ground state of lanthanides in compounds

In the construction of a vacuum referred binding energy (VRBE) diagram with the lanthanide 4fq ground states, always a compound independent variation with the number q= 1 to 14 is assumed. Experimental data from thermo-luminescence, intervalence charge transfer bands, and thermo-bleaching studies provide first indications that a minor compound dependence does exist. To explain its origin we will first apply Jørgensen spin pairing theory to reproduce the VRBE in the ground states of the free di- and trivalent lanthanide ions which is equivalent to the 3rd and 4th ionization potentials of the lanthanide atoms. By combining experimental data and calculated trends therein, the relevant Racah E1, Racah E3, and spin orbit coupling ζff parameters for all di-, tri-, and tetravalent free ion lanthanides are derived. Using that as input for the spin pairing theory, the characteristic zigzag shapes in VRBE as function of q, as derived from ionization potentials, are nicely reproduced. Because of the nephelauxetic effect the parameter values are lowered when lanthanides are in compounds. How that reduction affects the VRBE curves will be treated in this work.

A Spectroscopic and Computationally Minimal Approach to the Analysis of Charge-Transfer Processes in Conformationally Fluxional Mixed-Valence and Heterobimetallic Complexes.

Class II mixed-valence bimetallic complexes {[Cp'(PP)M]C≡C-C≡N[M'(PP)'Cp']}2+ (M, M'=Ru, Fe; PP=dppe, (PPh3 )2 ; Cp'=Cp*, Cp) exist as conformational ensembles in fluid solution, with a population of structures ranging from cis- to trans-like geometries. Each conformer gives rise to its own series of low-energy intervalence charge-transfer (IVCT) and local d-d transitions, which overlap in the NIR region, giving complex band envelopes in the NIR absorption spectrum, which prevent any meaningful attempt at analysis of the band shape. However, DFT and time-dependent (TD)DFT calculations with dispersion-corrected global-hybrid (BLYP35-D3) or local hybrid (lh-SsirPW92-D3) functionals on a small number of optimised structures chosen to sample the ground state potential energy hypersurfaces of each of these complexes has proven sufficient to explain the major features of the electronic spectra. Although modest in terms of computational expense, this approach provides a more accurate description of the underlying molecular electronic structure than would be possible through analysis of the IVCT band by using the static point-charge model of Marcus-Hush theory and derivatives, or TDDFT calculations from a single (global) minimum energy geometry.

Combined Microspectroscopic Characterization of a Red-Colored Granite Rock Sample.

Combined microspectroscopic mapping have been conducted on a red-colored Tenzan granite sample by using an original visible-fluorescence-Raman microspectrometer together with a low vacuum scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) without coating. Visible darkfield reflectance spectra were converted to L*a*b* color values and Kubelka-Munk (KM) spectra. Large a* value (red) positions correspond to large band areas at 500-560 nm, possibly due to hematite-like iron oxide, while large b* value (yellow) positions to large band areas at 450-500 nm, due to epidote-like mineral. Scanning electron microscopy-energy dispersive spectrometry analyses indicated that the reddish parts are Na and K-feldspars with low Fe contents (<0.5 wt%). Raman microspectroscopy could not detect hematite-like minerals. Since some hematite-like minerals were only identified by transmission electron microscope, they are considered to be submicron microcrystals disseminated in feldspar matrices. The KM spectra for prehnite-like minerals show a weak broad band around 430 nm due possibly to a ligand field band of Fe3+ without clear Fe2+-Fe3+ inter-valence charge transfer (IVCT) bands around 720 nm. Therefore, Fe in prehnite is not considered to be present as hematite-like iron oxide, but can be mainly present as Fe3+ replacing Al3+ in the crystal structure. Since determination of physicochemical states of Fe such as valence and coordination states (Fe2+ or Fe3+, oxide or in crystal lattice, etc.) and their distributions are extremely difficult, especially in complex colored materials such as rocks, the combined microspectroscopic methods are useful for their nondestructive characterization.

Diverse π-π stacking motifs modulate electrical conductivity in tetrathiafulvalene-based metal-organic frameworks.

We report three electrically conductive metal-organic frameworks (MOFs) based on a tetrathiafulvalene linker and La3+. Depending on the solvent ratios and temperatures used in their solvothermal synthesis, these MOFs crystallize with different topologies containing distinct π-π stacking sequences of the ligand. Notably, their transport properties correlate rationally with the stacking motifs: longer S···S contact distances between adjacent ligands coincide with lower electrical conductivities and higher activation energies. Diffuse reflectance spectroscopic measurements reveal ligand-based intervalence charge transfer bands in each phase, implicating charge delocalization among mixed-valent tetrathiafulvalene units as the dominant mode of transport. Overall, these frameworks demonstrate how tuning the intermolecular interactions in MOFs serves as a route towards controlling their physical properties.

Controlling Nuclearity and Stereochemistry in Vanadyl(V) and Mixed Valent VIV/VV Complexes of Oxido‐Pincer Pyridine‐2,6‐dimethanol Ligands

The coordination chemistry of three oxido‐pincer ligands 2,6‐(HOCR2)2(pyridine) (H2L) based on 2,6‐pyridinedimethanol [R = H (H2pydim), Me (H2pydip), Ph (H2pyphen)] towards vanadium(V) was explored. Reaction of NH4VO3 with the protoligands H2L gave the dinuclear complexes [(L)OV(μ‐O)VO(L)]. Mononuclear anionic species [VO2(L)]–, which were isolated as alkaline metal salts were obtained from reactions of [VO(acac)2] (acac– = acetylacetonate) and H2L under basic conditions and addition of HCl to these species allowed to isolate the unprecedented oxido chlorido complexes [VOCl(L)] for pydip and pyphen. Cyclic voltammograms of the dinuclear [V2O3(L)2] and mononuclear [VOCl(L)] complexes show reversible VV/VIV reduction waves, while corresponding waves of the anionic [VO2(L)]– are completely irreversible. The mixed‐valent VIV/VV species [V2O3(L)2]·– were characterized by EPR and UV/Vis spectroelectrochemistry revealing a delocalized system with a 15 line EPR spectrum and an intervalence charge transfer (IVCT) band for the bulky pyphen ligand but localized radicals in case of the pydim and pydip derivatives (8 line EPR, no IVCT). DFT calculated structures of the three derivatives show an V–O–V arrangement for [V2O3(pyphen)2]·– of about 145° ideally suited for delocalization, whereas for [V2O3(pydip)2]·– an angle of 128° was found.

Thermally Persistent High-Spin Ground States in Octahedral Iron Clusters.

Chemical oxidation and reduction of the all-ferrous (HL)2Fe6 in THF affords isostructural, coordinatively unsaturated clusters of the type [(HL)2Fe6] n: [(HL)2Fe6][BArF24] (1, n = +1; where [BArF24]- = tetrakis[(3,5-trifluoromethyl)phenyl]borate), [Bu4N][(HL)2Fe6] (2a, n = -1), [P][(HL)2Fe6] (2b, n = -1; where [P]+ = tributyl(1,3-dioxolan-2-ylmethyl)phosphonium), and [Bu4N]2[(HL)2Fe6] (3, n = -2). Each member of the redox-transfer series was characterized by zero-field 57Fe Mössbauer spectroscopy, near-infrared spectroscopy, single-crystal X-ray crystallography, and magnetometry. Redox-directed trends are observed when comparing the structural metrics within the [Fe6] core. The metal octahedron [Fe6] decreases marginally in volume as the molecular reduction state increases as gauged by the Fe-Feavg distance varying from 2.608(11) Å ( n = +1) to 2.573(3) ( n = -2). In contrast, the mean Fe-N distances and ∠Fe-N-Fe angles correlate linearly with the [Fe6] oxidation level, or alternatively, the changes observed within the local Fe-N4 coordination planes vary linearly with the aggregate spin ground state. In general, as the spin ground state ( S) increases, the Fe-N(H)avg distances also increase. The structural metric perturbations within the [Fe6] core and measured spin ground states were rationalized extending the previously proposed molecular orbital diagram derived for (HL)2Fe6. Chemical reduction of the (HL)2Fe6 cluster results in an abrupt increase in spin ground state from S = 6 for the all-ferrous cluster, to S = 19/2 in the monoanionic 2b and S = 11 for the dianionic 3. The observation of asymmetric intervalence charge transfer bands in 3 provides further evidence of the fully delocalized ground state observed by 57Fe Mössbauer spectroscopy for all species examined (1-3). For each of the clusters examined within the electron-transfer series, the observed spin ground states thermally persist to 300 K. In particular, the S = 11 in dianionic 3 and S = 19/2 in the monoanionic 2b represent the highest spin ground states isolated up to room temperature known to date. The increase in spin ground state results from population of the antibonding orbital band comprised of the Fe-N σ* interactions. As such, the thermally persistent ground states arise from population of the resultant single spin manifolds in accordance with Hund's rules. The large spin ground states, indicative of strong ferromagnetic electronic alignment of the valence electrons, result from strong direct exchange electronic coupling mediated by Fe-Fe orbital overlap within the [Fe6] cores, equivalent to a strong double exchange magnetic coupling B for 3 that was calculated to be 309 cm-1.