Migration of Condensed Aromatic Hydrocarbons During Alkyne-Vinylidene Rearrangements.

Diarylacetylenes ArCºCAr featuring condensed aromatic hydrocarbon fragments (Ar) such as naphthalene, anthracene, phenanthrene and pyrene were converted into vinylidene ligands by 1,2-migration reactions within the coordination sphere of half-sandwich complexes [MII(dppe)Cp]+ (MII = RuII, FeII). Comparison of the extent of conversion of the alkyne substrates to the vinylidene complexes [Ru{=C=CAr2}(dppe)Cp]+ with those obtained from acetylenes functionalized by smaller groups (H, CH3, Ph) show that the molecular volume (VM) of the migrating group and relief of steric congestion plays a role during the rearrangement process. Conversely, the H-atoms from the larger condensed ring aryl groups that are in close proximity to the migrating sites also have a significant influence on the efficacy and extent of the reaction by restricting access of the alkyne to the metal center, resulting in a less effective migration reaction. This combination of competing steric factors (acceleration due to relief of steric congestion and restricted access of the alkyne moiety to the reaction site) is exemplified by the facile migration of 1-pyryl entities and the low yields of vinylidene products formed from 1,2-bis(9-anthryl)acetylene.

Excited State Mixed-Valence Complexes: From the Special Pair to the Creutz-Taube Ion and Beyond.

Compounds and complexes with mixed-valence electronic ground states, such as the Creutz-Taube ion, have proven to be excellent vehicles through which to study intramolecular electron-transfer processes. In a recent contribution by Cadranel and co-workers, time-resolved pump-probe spectroscopy reveals photo-induced metal-to-bridge charge transfer within the homovalent analogue of the Creutz-Taube ion, [{(NH3 )5 Ru}(µ-pz){Ru(NH3 )5 }]4+ , giving rise to two closely lying excited states with mixed-valence character, one with a shorter lifetime (τ=136 ps) and weakly-coupled (Robin-Day Class II) character, the other a longer-lived (τ=2.8 ns) configurational isomer with more delocalized electronic structure. Electron transfer reactions from the longer-lived species demonstrate analogies with the photo-induced reactions of the photosynthetic special pair, suggesting this state as a reference system for excited state mixed-valency, and a framework from which to explore photocatalytic reactions.

The Use of Bridging Ligand Substituents to Bias the Population of Localized and Delocalized Mixed-Valence Conformers in Solution.

The electronic structure and associated spectroscopic properties of ligand-bridged, bimetallic 'mixed-valence' complexes of the general form {M}(µ-B){M+ } are dictated by the electronic couplings, and hence orbital overlaps, between the metal centers mediated by the bridge. In the case of complexes such as [{Cp*(dppe)Ru}(µ-C≡CC6 H4 C≡C){Ru(dppe)Cp*}]+ , the low barrier to rotation of the half-sandwich metal fragments and the arylene bridge around the acetylene moieties results in population of many energy minima across the conformational energy landscape. Since orbital overlap is also sensitive to the particular mutual orientations of the metal fragment(s) and arylene bridge through a Karplus-like relationship, the different members of the population range exemplify electronic structures ranging from strongly localized (weakly coupled Robin-Day Class II) to completely delocalized (Robin-Day Class III). Here, we use electronic structure calculations with the hybrid density functional BLYP35-D3 and a continuum solvent model in combination with UV-vis-NIR and IR spectroelectrochemical studies to show that the conformational population in complexes [{Cp*(dppe)Ru}(µ-C≡CArC≡C){Ru(dppe)Cp*]+ , and hence the dominant electronic structure, can be biased through the steric and electronic properties of the diethynylarylene (Ar) moiety (Ar=1,4-C6 H4 , 1,4-C6 F4 , 1,4-C6 H2 -2,5-Me2 , 1,4-C6 H2 -2,5-(CF3 )2 , 1,4-C6 H2 -2,5-i Pr2 ).

Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism.

Valence tautomerism (VT) and spin crossover (SCO) are promising avenues for developing a range of molecular materials for sensing, memory, and optoelectronic applications. However, these phenomena arise only when specific metal-ligand combinations are employed. The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene (Ar-BIAN) ligands, which can exist as neutral Ar-BIAN0 (L0), monoanionic radical Ar-BIAN•- (L•-), and dianionic Ar-BIAN2- (L2-) forms, has potential to afford both VT and SCO. Aiming to develop a new family of switchable molecules, we systematically explored a dual-tuning approach by varying the redox state and aryl substituents in a series of homoleptic [Co(Ar-BIAN)3]n+ complexes (Ar = Ph, n = 2 (12+), 1 (1+), 0 (1); Ar = 3,5-CF3-Ph, n = 0 (2); Ar = 4-MeO-Ph, n = 2 (32+), 0 (3)). As a prelude to synthetic and experimental studies, density functional theory (DFT) calculations were used to explore the structure and relative energies of the different electronic forms of each complex, comprising different cobalt oxidation and spin states and different ligand oxidation states. Except for compound 3, DFT identified a HS-CoII-L0 containing ground state for all complexes, precluding thermally induced SCO or VT. For 3, calculations suggested a possible thermally accessible LS-CoIII-(L•-)3 ⇌ HS-CoII-(L•-)2(L0) VT interconversion. Experimentally, structural and magnetic data reveal a HS-CoII-L0 containing ground state for all six compounds in the solid state, including 3, discounting thermally induced VT or SCO. In solution, electrochemical and spectroscopic analysis also indicate that all compounds exist as the HS-CoII-L0-containing electromer at 298 K. Intervalence charge transfer (IVCT) bands observed for neutral 1, 2, and 3 at room temperature suggest the mixed-valence HS-CoII-(L•-)2(L0) charge distribution. However, cooling 3 to 243 K in acetonitrile uniquely affords a substantial reduction in the intensity of this IVCT band, consistent with thermally induced VT interconversion to the LS-CoIII-(L•-)3 ground state as predicted by DFT calculations. This study emphasizes the utility of computationally guided molecular design for complicated systems with redox activity at the metal and multiple ligands, thus opening new avenues for tuning electronic structure and developing new families of switchable molecules.

Redox-Addressable Single-Molecule Junctions Incorporating a Persistent Organic Radical.

Integrating radical (open-shell) species into non-cryogenic nanodevices is key to unlocking the potential of molecular electronics. While many efforts have been devoted to this issue, in the absence of a chemical/electrochemical potential the open-shell character is generally lost in contact with the metallic electrodes. Herein, single-molecule devices incorporating a 6-oxo-verdazyl persistent radical have been fabricated using break-junction techniques. The open-shell character is retained at room temperature, and electrochemical gating permits in situ reduction to a closed-shell anionic state in a single-molecule transistor configuration. Furthermore, electronically driven rectification arises from bias-dependent alignment of the open-shell resonances. The integration of radical character, transistor-like switching, and rectification in a single molecular component paves the way to further studies of the electronic, magnetic, and thermoelectric properties of open-shell species.

Quantum Interference in Mixed-Valence Complexes: Tuning Electronic Coupling Through Substituent Effects.

Whilst 2- or 5-OMe groups on the bridging phenylene ring in [{Cp*(dppe)RuC≡C}2 (µ-1,3-C6 H4 )]+ have little influence on the electronic structure of this weakly coupled mixed-valence complex, a 4-OMe substituent enhances ground state electron delocalization, and increases the intensity of the IVCT transition. Vibrational frequency and TDDFT calculations (LH20t-D3(BJ), def2-SVP, COSMO (CH2 Cl2 )) on ([{Cp*(dppe)RuC≡C}2 (µ-1,3-C6 H3 -n-OMe)]+ (n=2, 4, 5) models are in excellent agreement with the experimental results. The stronger ground state coupling is attributed to the change in composition of the ß-HOSO brought about by the 4-OMe group, which is ortho or para to each of the metal fragments. The intensity of the IVCT transition increases with the greater overlap of the ß-HOSO and ß-LUSO, whilst the relative phases of the ß-HOSO and ß-LUSO in the 4-OMe substituted complex are consistent with predictions of constructive quantum interference from molecular circuit rules.

Molecular Structure-(Thermo)electric Property Relationships in Single-Molecule Junctions and Comparisons with Single- and Multiple-Parameter Models.

The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO-LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed "single-parameter" models of junction conductance. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer-Büttiker model.

Rip It off: Nitro to Nitroso Reduction by Iron Half-Sandwich Complexes.

Activation of [FeCl(dppe)Cp] (1) by chloride abstraction with Na[BArX4] (X = F, [B(3,5-(CF3)2-C6H3)4]; X = Cl, [B(3,5-Cl2-C6H3)4]) permits reactions with a range of nitro aromatics, RC6H4NO2 (R = halogen, Me, OMe, NO2 or NMe2), to give the cationic iron nitroso complexes [Fe{N(O)-C6H4R}(dppe)Cp][BArX4]) ([3][BArX4]). Similar reactions of 1 and Na[BArX4] with [Fe(NCC6H4NO2)(dppe)Cp][BArX4] gave bimetallic [{Fe(dppe)Cp}2{µ-N≡CC6H4N(O)}][BArF4]2. However, reactions of 1 and Na[BArX4] with 4-nitrophenol gave the first example of the bench-stable iron half-sandwich phenolate complex [Fe(OC6H4NO2)(dppe)Cp]+ rather than NO2 activation. The formation of complexes [3]+ likely proceeds via the unusual blue bimetallic species [{Fe(dppe)Cp}2{µ,κ2O,O'-O2NAr}]2+. This compound undergoes N-O bond cleavage, resulting in [3]+ and a FeIV═O species, which reacts via an internal C-H activation of the dppe ligand to give [FeIII(κ3O,P,P'-P(2-O-C6H4)(Ph)-C2H4-PPh2)Cp]+. Complexes [3]+ are stable under ambient conditions, are readily purified by column chromatography and can be isolated in up to 50% yield, considering that 0.5 equiv of 1 is required as the oxygen acceptor.

Further Evidence for 'Extended' Cumulene Complexes: Derivatives from Reactions with Halide Anions and Water.

Reactions of [Ru{C=C(H)-1,4-C6 H4 C≡CH}(PPh3 )2 Cp]BF4 ([1 a]BF4 ) with hydrohalic acids, HX, results in the formation of [Ru{C≡C-1,4-C6 H4 -C(X)=CH2 }(PPh3 )2 Cp] [X=Cl (2 a-Cl), Br (2 a-Br)], arising from facile Markovnikov addition of halide anions to the putative quinoidal cumulene cation [Ru(=C=C=C6 H4 =C=CH2 )(PPh3 )2 Cp]+ . Similarly, [M{C=C(H)-1,4-C6 H4 -C≡CH}(LL)Cp ]BF4 [M(LL)Cp'=Ru(PPh3 )2 Cp ([1 a]BF4 ); Ru(dppe)Cp* ([1 b]BF4 ); Fe(dppe)Cp ([1 c]BF4 ); Fe(dppe)Cp* ([1 d]BF4 )] react with H+ /H2 O to give the acyl-functionalised phenylacetylide complexes [M{C≡C-1,4-C6 H4 -C(=O)CH3 }(LL)Cp'] (3 a-d) after workup. The Markovnikov addition of the nucleophile to the remote alkyne in the cations [1 a-d]+ is difficult to rationalise from the vinylidene form of the precursor and is much more satisfactorily explained from initial isomerisation to the quinoidal cumulene complexes [M(=C=C=C6 H4 =C=CH2 )(LL)Cp']+ prior to attack at the more exposed, remote quaternary carbon. Thus, whilst representative acetylide complexes [Ru(C≡C-1,4-C6 H4 -C≡CH)(PPh3 )2 Cp] (4 a) and [Ru(C≡C-1,4-C6 H4 -C≡CH)(dppe)Cp*] (4 b) reacted with the relatively small electrophiles [CN]+ and [C7 H7 ]+ at the ß-carbon to give the expected vinylidene complexes, the bulky trityl ([CPh3 ]+ ) electrophile reacted with [M(C≡C-1,4-C6 H4 -C≡CH)(LL)Cp'] [M(LL)Cp'=Ru(PPh3 )2 Cp (4 a); Ru(dppe)Cp* (4 b); Fe(dppe)Cp (4 c); Fe(dppe)Cp* (4 d)] at the more exposed remote end of the carbon-rich ligand to give the putative quinoidal cumulene complexes [M{C=C=C6 H4 =C=C(H)CPh3 }(LL)Cp']+ , which were isolated as the water adducts [M{C≡C-1,4-C6 H4 -C(=O)CH2 CPh3 }(LL)Cp'] (6 a-d). Evincing the scope of the formation of such extended cumulenes from ethynyl-substituted arylvinylene precursors, the rather reactive half-sandwich (5-ethynyl-2-thienyl)vinylidene complexes [M{C=C(H)-2,5-c C4 H2 S-C≡CH}(LL)Cp']BF4 ([7 a-d]BF4 add water readily to give [M{C≡C-2,5-c C4 H2 S-C(=O)CH3 }(LL)Cp'] (8 a-d)].

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.

From ferrocene to fluorine-containing penta-substituted derivatives and all points in-between; or, how to increase the available chemical space.

In spite of the growing interest in fluorine-containing compounds, and the improvements in materials, optical and biological properties that can arise from substitution of a phenyl ring by ferrocene within a molecular scaffold, synthetic strategies that allow the efficient preparation of fluoroferrocene derivatives are scarce. Following conversion of ferrocene to fluoroferrocene, we have developed routes to fluorine-containing di-, tri-, tetra- and penta-substituted ferrocene derivatives to extend the available chemical space. Our approach is based on the identification of suitable reagents and conditions to achieve fluorine-directed deprotometalation, and exploitation of the halogen 'dance' rearrangement in the ferrocene series.

Turning the Tap: Conformational Control of Quantum Interference to Modulate Single-Molecule Conductance.

Together with the more intuitive and commonly recognized conductance mechanisms of charge-hopping and tunneling, quantum-interference (QI) phenomena have been identified as important factors affecting charge transport through molecules. Consequently, establishing simple and flexible molecular-design strategies to understand, control, and exploit QI in molecular junctions poses an exciting challenge. Here we demonstrate that destructive quantum interference (DQI) in meta-substituted phenylene ethylene-type oligomers (m-OPE) can be tuned by changing the position and conformation of methoxy (OMe) substituents at the central phenylene ring. These substituents play the role of molecular-scale taps, which can be switched on or off to control the current flow through a molecule. Our experimental results conclusively verify recently postulated magic-ratio and orbital-product rules, and highlight a novel chemical design strategy for tuning and gating DQI features to create single-molecule devices with desirable electronic functions.

Chiral NH-Controlled Supramolecular Metallacycles.

Chiral NH functionalities-based discrimination is a key feature of Nature's chemical armory, yet selective binding of biologically active molecules in synthetic systems with high enantioselectivity poses significant challenges. Here we report the assembly of three chiral fluorescent Zn6L6 metallacycles from pyridyl-functionalized Zn(salalen) or Zn(salen) complexes. Each of these metallacycles has a nanoscale hydrophobic cavity decorated with six, three, or zero chiral NH functionalities and packs into a three-dimensional supramolecular porous framework. The binding affinity and enantioselectivity of the metallacycles toward α-hydroxycarboxylic acids, amino acids, small molecule pharamaceuticals (l-dopa, d-penicillamine), and chiral amines increase with the number of chiral NH moieties in the cyclic structure. From single-crystal X-ray diffraction, molecular simulations, and quantum chemical calculations, the chiral recognition and discrimination are attributed to the specific binding of enantiomers in the chiral pockets of the metallacycles. The parent metallacycles are fluorescent with the intensity of emission being linearly related to the enantiomeric composition of the chiral biorelevant guests, which allow them to be utilized in chiral sensing. The fact that manipulation of chiral NH functionalities in metallacycles can control the enantiorecognition of biomolecular complexes would facilitate the design of more effective supramolecular assemblies for enantioselective processes.

All-Carbon Electrode Molecular Electronic Devices Based on Langmuir-Blodgett Monolayers.

Nascent molecular electronic devices, based on monolayer Langmuir-Blodgett films sandwiched between two carbonaceous electrodes, have been prepared. Tightly packed monolayers of 4-((4-((4-ethynylphenyl)ethynyl)phenyl)ethynyl)benzoic acid are deposited onto a highly oriented pyrolytic graphite electrode. An amorphous carbon top contact electrode is formed on top of the monolayer from a naphthalene precursor using the focused electron beam induced deposition technique. This allows the deposition of a carbon top-contact electrode with well-defined shape, thickness, and precise positioning on the film with nm resolution. These results represent a substantial step toward the realization of integrated molecular electronic devices based on monolayers and carbon electrodes.

Single-Molecule Conductance Studies of Organometallic Complexes Bearing 3-Thienyl Contacting Groups.

The compounds and complexes 1,4-C6 H4 (C≡C-cyclo-3-C4 H3 S)2 (2), trans-[Pt(C≡C-cyclo-3-C4 H3 S)2 (PEt3 )2 ] (3), trans-[Ru(C≡C-cyclo-3-C4 H3 S)2 (dppe)2 ] (4; dppe=1,2-bis(diphenylphosphino)ethane) and trans-[Ru(C≡C-cyclo-3-C4 H3 S)2 {P(OEt)3 }4 ] (5) featuring the 3-thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3-5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break-junction (STM-BJ) and STM-I(s) methods (measuring the tunnelling current (I) as a function of distance (s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM-I(s) methods, and a higher feature by the STM-BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi-parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non-resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2-5.

Rational Control of Conformational Distributions and Mixed-Valence Characteristics in Diruthenium Complexes.

The electronic characteristics of mixed-valence complexes are often inferred from the shape of the inter-valence charge transfer (IVCT) band, which usually falls in the near infrared (NIR) region, and relationships derived from Marcus-Hush theory. These analyses typically assume one single, dominant molecular conformation. The NIR spectra of the prototypical delocalised (Class III Robin-Day mixed-valence) complexes [{Ru(pp)Cp'}2 (µ-C≡C-C≡C)]+ ([1]+ : Cp'=Cp, pp=(PPh3 )2 ; [2]+ : Cp'=Cp, pp=dppe; [3]+ : Cp'=Cp*, pp=dppe) feature a 'two-band' pattern, which complicates band-shape analysis using these traditional methods. In the past, the appearance of sub-bands within or near the IVCT transition has been attributed to vibronic effects or localised d-d transitions. Quantum-chemical modelling of a series of rotational conformers of [1]+ -[3]+ reveals the two components that contribute to the NIR absorption band envelope to be a π-π* transition and an MLCT transition. The MLCT components only gain appreciable intensity when the orientation of the half-sandwich ruthenium ligand spheres deviates from idealised cis (Ω P-Ru-Ru-P=0°) or trans (Ω P-Ru-Ru-P=180°) conformations. The increased steric demand of the supporting ligands, together with some underlying inter-phosphine ligand T-shaped CH⋅⋅⋅π stacking interactions across the series [1]+ to [2]+ to [3]+ results in local minima biased towards such non-idealised conformations of the metal-ligand fragments (Ω P-Ru-Ru-P=33-153°). Experimentally, this is indicated by appearance of multiple bands within the IR ν˜ (C≡C) band envelopes and increasing intensity of the higher-energy MLCT transition(s) relative to the π-π* transition across the series, and the appearance of a pronounced 'two-band' pattern in the experimental NIR absorption envelopes. These conformational effects and the methods of analysis presented here, which combine analysis of IR and NIR spectra with quantum-chemical calculations on a range of energetically similar conformational minima, are expected to be quite general for mixed-valence systems.

Effects of Electrode-Molecule Binding and Junction Geometry on the Single-Molecule Conductance of bis-2,2':6',2″-Terpyridine-based Complexes.

The single molecule conductances of a series of bis-2,2':6',2″-terpyridine complexes featuring Ru(II), Fe(II), and Co(II) metal ions and trimethylsilylethynyl (Me3SiC≡C-) or thiomethyl (MeS-) surface contact groups have been determined. In the absence of electrochemical gating, these complexes behave as tunneling barriers, with conductance properties determined more by the strength of the electrode-molecule contact and the structure of the "linker" than the nature of the metal-ion or redox properties of the complex.

Electron Delocalization in Reduced Forms of 2-(BMes2)pyrene and 2,7-Bis(BMes2)pyrene.

Reduction of 2-(BMes2)pyrene (B1) and 2,7-bis(BMes2)pyrene (B2) gives rise to anions with extensive delocalization over the pyrenylene bridge and between the boron centers at the 2- and 2,7-positions, the typically unconjugated sites in the pyrene framework. One-electron reduction of B2 gives a radical anion with a centrosymmetric semiquinoidal structure, while two-electron reduction produces a quinoidal singlet dianion with biradicaloid character and a relatively large S0-T1 gap. These results have been confirmed by cyclic voltammetry, X-ray crystallography, DFT/CASSCF calculations, NMR, EPR, and UV-vis-NIR spectroscopy.

Electrochemical Single-Molecule Transistors with Optimized Gate Coupling.

Electrochemical gating at the single molecule level of viologen molecular bridges in ionic liquids is examined. Contrary to previous data recorded in aqueous electrolytes, a clear and sharp peak in the single molecule conductance versus electrochemical potential data is obtained in ionic liquids. These data are rationalized in terms of a two-step electrochemical model for charge transport across the redox bridge. In this model the gate coupling in the ionic liquid is found to be fully effective with a modeled gate coupling parameter, ξ, of unity. This compares to a much lower gate coupling parameter of 0.2 for the equivalent aqueous gating system. This study shows that ionic liquids are far more effective media for gating the conductance of single molecules than either solid-state three-terminal platforms created using nanolithography, or aqueous media.

Influence of P-Bonded Bulky Substituents on Electronic Interactions in Ferrocenyl-Substituted Phospholes.

2,5-Diferrocenyl-1-Ar-1H-phospholes 3 a-e (Ar=phenyl (a), ferrocenyl (b), mesityl (c), 2,4,6-triphenylphenyl (d), and 2,4,6-tri-tert-butylphenyl (e)) have been prepared by reactions of ArPH2 (1 a-e) with 1,4-diferrocenyl butadiyne. Compounds 3 b-e have been structurally characterized by single-crystal XRD analysis. Application of the sterically demanding 2,4,6-tri-tert-butylphenyl group led to an increased flattening of the pyramidal phosphorus environment. The ferrocenyl units could be oxidized separately, with redox separations of 265 (3 b), 295 (3 c), 340 (3 d), and 315 mV (3 e) in [NnBu4 ][B(C6 F5 )4]; these values indicate substantial thermodynamic stability of the mixed-valence radical cations. Monocationic [3 b](+)-[3 e](+) show intervalence charge-transfer absorptions between 4650 and 5050 cm(-1) of moderate intensity and half-height bandwidth. Compounds 3 c-e with bulky, electron-rich substituents reveal a significant increase in electronic interactions compared with less demanding groups in 3 a and 3 b.