Extremely Electron-Poor Bis(diarylmethylium)-Substituted Ferrocenes and the First Peroxoferrocenophane

We have prepared and studied extremely electron-poor, deeply colored dicationic 1,1 -bis(diarylmethylium)-substituted ferrocenes [(η-C5H4-CAr2)2Fe] with various aryl substituents as their [B{C6H3(CF3)2-3,5}4] salts. Due to the strong acceptor substitution, the redox potential for the ferrocene-based oxidation of the anisylor 2-methylanisyl-substituted congeners 1b2+ and 1c2+ is close to or even surpasses that of the second oxidation of parent ferrocene, i.e. the Cp2Fe couple. The strongly Lewis-acidic character of these com-


Introduction
Triarylmethylium (tritylium) substituted ferrocenes constitute the most prominent examples of metal-organic tritylium dyes and were studied early on for their electronic structures. Particular focus was on the ability of the iron nucleus of the ferrocene scaffold to stabilize the positively charged, adjacent methylium center by σor π-interactions as expressed by the resonance forms IV and V in Scheme 1. [1] Such kind of interactions were initially inferred by the 20.7°bending of the CPh 2 + plane toward the Fe 2+ ion in the crystallographically determined structure of Fc-CPh 2 + [2] (Fc = ferrocenyl, (η 5 -C 5 H 5 )Fe(η 5 -C 5 H 4 )) and by quantum chemical calculations, [3] but later refuted by the notion of a small energy barrier for rotation around the Cp-C + Ar 2 bond [4] and the generally large dipole moments of ferrocenyl carbenium ions. [1f] The electron-donating ferrocenyl substituent and the strong dipolar character of such compounds usually lead to intense charge-transfer absorptions in the visible regime of the electronic spectrum (Vis), which are typical of donor-substituted tritylium dyes. Vinylogous or alkynylogous expansion of the ferrocenyl arm of such dyes shifts the corresponding chargetransfer band further red and increases its oscillator strength. [5] In ferrocenyl-tritylium dyes Fc-C + Ar 2 , the CT transitions from the carbocyclic substituents to the methylium center are augmented by an additional, weaker CT band (the HOMOǞ LUMO transition) concomitant with the direct transfer of electron density from the Fe 2+ ion to the methylium acceptor. In agreement with this assignment, this band is bleached on oxidation, whereby the ferrocene donor is changed into an electronpoor ferrocenium ion. The same behavior is observed on reduction of the tritylium entity to a trityl radical, which essentially removes its electron-accepting capabilities. Neutral ferrocenyl(diaryl)methyl radicals FcC • Ar 2 are prone to dimerize, and an authentic hexaarylethane structure was proposed for these dimers. [6] Quite surprisingly, no 1,1Ј-disubstituted, dicationic bis(diarylmethylium)-substituted ferrocenes seem to be known to date. Only the 1,1Ј-bis(isopropylium)-derivative [(η 5 -C 5 H 4 CMe 2 ) 2 Fe] 2+ derivative was generated by either protonation and dehydration of the corresponding diol in FSO 3 H/SbF 5 [7] or protonation of the bis(isopropenyl) derivative with FSO 3 H in liquid SO 2 and characterized by NMR spectroscopy, [8] but found to persist only at temperatures below -30°C. Herein we present the first three representatives of such complexes, bearing aryl substituents of differing electron-donating capabilities. We have probed their electronic properties by means of electrochemistry and UV/Vis/NIR as well as EPR spectroscopic studies on the dications and their reduced and oxidized forms. We also report a unique ansa-peroxoferrocenophane, which is formed by the selective reaction of the monoreduced radical cation of the bis(anisyl) derivative with molecular oxygen.

Synthesis and Spectroscopic Characterization
The target 1,1Ј-bis(triarylmethylium)-substituted ferrocenes 1a 2+ to 1c 2+ were prepared in a two-step procedure analogous to AA'BB' spin system for the para-substituted arene substituents at δ = 7.57 and 6.69 ppm, and of the N-methyl groups at δ = 3.15 ppm in addition to the proton resonances of the BArF 24 anion in their correct integration ratios (see Figure S3, Supporting Information). All these resonances are shifted to lower field when compared to the bis(carbinol) precursor 1a-(OH) 2 . Other characteristic assets are the resonance signal of the carbenium centers at δ = 181.1 ppm and the resonances at δ = 127.8 ppm and 84.2 ppm for the adjacent carbon atoms at the aryl and the Cp substituents (see Figures S4, Supporting Information and the Experimental Section). These signals likewise experience substantial low-field shifts with respect to the corresponding carbinol precursor. As expected, the magnitude of this shift is largest for the newly formed carbenium centers, where it amounts to more than 100 ppm. The methylium resonance signal appears nevertheless at a slightly higher field when compared to δ = 188.0 ppm for its monotritylium congener 2a + (Scheme 2). [6] Of the remaining two bis(tritylium)fer- rocenes, 1b 2+ provides rather well-resolved resonances in the 1 H NMR spectrum at lower concentration levels, which become paramagnetically broadened at higher concentrations (see Figure S6, Supporting Information). This also precluded us from recording meaningful 13 C NMR spectra.
In keeping with the presence of small amounts of a paramagnetic component, dicationic 1b 2+ shows indeed an isotropic, unstructured EPR signal in fluid solution at a g value of 2.0086. This signal was initially of only weak intensity at room temperature, but gradually intensified on warming the solution to 60°C (Figure 1, left) and remained at a rather constant intensity level on cooling back to room temperature. Further cooling to temperatures of -20°C or below, however, induces a gradual decrease of the signal intensity until the initial level is reached (Figure 1, right). The entire process can be cycled several times. Such reversible intensity changes combined with a hysteretic behavior point to the existence of a thermally accessible paramagnetic state along with a substantial activation barrier connecting this state to the diamagnetic ground state.
A likely explanation for this unusual behavior is thermal equilibration between the native 1,1'-bis(diarylmethylium)ferrocene form (η 5 -Ar 2 C + -C 5 H 4 )Fe II (η 5 -C 5 H 4 -C + Ar 2 ) and its (η 5 -Ar 2 C + -C 5 H 4 )Fe III+ (η 5 -C 5 H 4 -C • Ar 2 ) valence tautomer, where the positive charge has shifted from a triarylmethylium site to the ferrocene nucleus (Scheme 3, top). This renders one of the former triarylmethylium centers a trityl-type radical. As ferrocenium ions are EPR inactive under these conditions, only the trityl component of this biradical is detected by EPR spectroscopy. This valence tautomer was computationally modeled as the triplet state of 1b 2+ . Our quantum chemical calculations produced indeed substantial spin densities on the ferrocene nucleus and the methyl(ium) carbon atoms (Figure 2, left). Complex 1b 2+ thus complements VecianaЈs ferrocenyl-perchlorotriphenylmethyl radicals, which were found to equilibrate with their zwitterionic ferrocenium-perchlorotriphenylmethanide isomers. [13] These two kinds of systems are compared in Scheme 3. They mainly differ in the identity and charge state of the triarylmethyl(ium)-based acceptor units, i.e. cationic Ar 2 C + in 1b 2+ vs. neutral -C 6 Cl 4 -• C(C 6 Cl 5 ) 2 in VecianaЈs systems. Figure 2. DFT-calculated spin densities for 1b 2+ in its triplet state (left), monoreduced 1b •+ (middle) and neutral 1b (right).

ARTICLE
Neither meaningful NMR nor EPR spectra were, however, obtained for complex 1c 2+ , such that its formation was only verified by UV/Vis spectroscopy. As expected from their vivid colors, all three complexes present intense electronic transitions in their Vis spectra. Their electronic absorption spectra are compared in Figure 3. Like in the solid state, solutions of the complexes in CH 2 Cl 2 are intensely blue (1a 2+ ) or purplish red (1b,c 2+ ). The electronic spectrum of 1a 2+ is dominated by a strong band (ε = 85000 m -1 ·cm -1 ) at 653 nm, which, in analogy to its monotritylium analog 2a + (Scheme 2) is caused by charge-transfer (CT) from the electron-rich 4-dimethylaminophenyl substituents to the carbenium acceptor (the so-called x-band). As a token of the increased acceptor strength owing to the presence of two tritylium-type acceptors, this band is red-shifted by 1130 cm -1 (i.e. from 608 nm) when compared to the monotritylium-substituted counterpart 2a + . The expected CT transition originating from the substituted Cp ring as the donor (y-band) is presumably associated with the weaker feature at 530 nm, which is present in the spectra of all three complexes. No FeǞtritylium CT band (the so-called y M -band) is, however, observed; it is likely hidden underneath the intense main band of 1a 2+ . show a y M band at 799 nm (ε = 9500 m -1 ·cm -1 ) or at 765 nm (ε = 9900 m -1 ·cm -1 , Figure 3). According to our TD-DFT calculations, this absorption is associated with charge-transfer from mainly the Fe d x 2 -y 2 orbital to the empty p-orbitals at the methylium carbon atoms. A second feature with a similar character, but differing in the identity of the Fe d donor orbital (d z 2), is observed as a shoulder on the high-energy side of the first y M band at ca. 630 nm for 1b 2+ or at ca. 690 nm for 1c 2+ . The main absorption feature of 1b 2+ is an intense (ε = 65000 m -1 ·cm -1 ), structured peak with a main maximum at 525 nm and separate shoulders at 485 nm and 457 nm. According to our TD-DFT calculations and in agreement with the behavior of other triarylmethylium dyes, these bands are assigned as the combined x-and y-bands with concomitant CT from the donor-substituted aryl rings or the attached Cp ligand to the methylium centers. [14] Figure 4 shows the calculated spectrum and the charge-density differences associated with the individual excitations of complex 1b 2+ . The Vis spectrum of 1c 2+ differs from that of 1b 2+ mainly in that the intensity of particularly the x,y bands at 513 nm is greatly diminished. This is a likely result of a larger torsion of the sterically more hindered 2-methyl-4-methoxyphenyl rings with respect to the plane of the methylium acceptors.
Complexes 1b 2+ and 1c 2+ show a remarkable solvatochromic behavior. While giving purplish red solutions in CH 2 Cl 2 and nitromethane, their solutions in N-or O-donor solvents like acetone, methanol, THF, Et 2 O, CH 3 CN, or pyridine assume a grass-green to orange-yellow color. The observable color impressions and absorption spectra of equally concentrated solutions in various solvents are collected in Figure 5; for details to the band positions and absorptivities see Table S1 (Supporting Information). Of note are the greatly diminished absorptivities of all CT bands in any donor solvent. This indicates strong interactions between Lewis-basic solvent molecules and the Lewis-acidic methylium centers of the solute, which reduce the electron-accepting capabilities of the latter. The decrease of the band intensities consequently follows, at least on a qualitative level, the Gutmann donor number DN. The latter is defined as the negative value of ΔH 0 in kcal·mol -1 for adduct formation between an electron pair donor with SbCl 5 in a highly diluted solution of 1,2-dichloroethane. [15] The stronger quenching of the bands in 1c 2+ when compared to 1b 2+ is consistent with an increased Lewis-acidity of the methylium centers in the former complex as a result of a lower  Me 2 NC 6 H 4 substituents, shows a strongly attenuated solvatochromism, changing its color from deep blue in CH 2 Cl 2 or nitromethane to dark green in THF or MeOH. Figure 6. Left: Cyclic voltammograms of complexes 1a 2+ to 1c 2+ in CH 2 Cl 2 /NBu 4 BAR F24 (0.02 m) at room temperature at a scan rate v = 100 mV·s -1 . Right: Square wave voltammograms of complexes 1b 2+ and 1c 2+ in SO 2(l) at -20°C with 0.02 m NBu 4 BAR F24 . The peak at 0 mV is due to the internal Cp 2 Fe 0/+ redox standard.

Electrochemical Studies
The redox properties of the bis(diarylmethylium)-substituted ferrocenes were probed by cyclic and square wave voltammetry. The results of this study are shown in Figure 6; pertinent data are compiled in Table 1. All complexes show two consecutive one-electron reductions for the stepwise transformations of the cationic triarylmethylium to neutral triarylmethyl substituents, i.e. the CpAr 2 C +/• couples ( Figure 6, left). The splitting of half-wave potentials ΔE 1/2 amounts to 405 mV for 1a 2+ and increases to 670 mV in 1c 2+ and to 740 mV in 1b 2+ . In every instance, the first reduction wave appears anodic of the first reduction of the corresponding monosubstituted diarylmethylium ferrocenes 2a + to 2c + (Table 1), which is a clear token of the decreased electron density of the tritylium acceptors. The magnitude of the shift between the first reduction of complexes 1a 2+ to 1c 2+ and those of their corresponding counterparts 2a + to 2c + increases with decreasing electron-donating capabilities of the aryl substituents. It thus becomes larger the more the effect of changing an electron-donating neutral ferrocenyl entity in complexes 2a-2c to an electronaccepting, cationic (η 5 -C 5 H 4 )Fe(η 5 -C 4 H 4 -CAr 2 + ) substituent in complexes 1a-1c is felt by the methylium acceptors and is not compensated by the other aryl substituents. One should note here that, in 1a 2+ and 2a + , the 4-Me 2 NC 6 H 4 substituents are stronger donors than ferrocenyl, while ferrocenyl is the Table 1. Electrochemical data a) b) for all complexes.

Complex
Reductions Oxidation strongest donor in complexes 2b + and 2c + . Again, a larger torsion of the 2-methyl-4-methoxyphenyl substituents of 1c 2+ as compared to the anisyl residues of 1b 2+ is held responsible for the finding that 1c 2+ is considerably easier to reduce than 1b 2+ despite the nominally stronger donor properties of the aryl substituents in 1c 2+ . This effect is even amplified with respect to the corresponding monosubstituted analogs 2b + and 2c + (Table 1).

Spectroscopic Investigations on Some Oxidized and Reduced Forms
The three reversible redox processes of 1a 2+ provided us with the opportunity to probe for the spectral changes in the Vis/NIR spectra concomitant with these transformations. The results of this study are depicted in Figure 7. On one-electron oxidation, the prominent Vis band of 1a 2+ at 653 nm is bleached and gives way to less intense bands peaking at 790 nm, 543 nm, and 450 nm. The lower-energy bands are very likely associated with CT from the C 6 H 4 NMe 2 donors to the methylium and ferrocenium acceptor entities. The red-shift of the 790 nm band with respect to that in 1a 2+ is in line with an inner array of three interconnected strong electron acceptors after ferrocene oxidation.
The two consecutive one-electron reductions likewise bleach the prominent x-band of 1a 2+ to ultimately leave two bands at 540 nm and 439 nm. By inference from their monotriarylmethylium-substituted congeners, for which a similar behavior was observed, these bands are assigned as a mixed transition within the ferrocene nucleus and a 1,1Ј-ferrocenedi-Z. Anorg. Allg. Chem. 2020, 712-725 www.zaac.wiley-vch.de ylǞCAr 2 • CT transition (λ = 540 nm) and as πǞπ* transitions within the trityl chromophore (λ = 439 nm). [6] Of note is the absence of an electronic transition at low energy specific for any radical cation with one neutral Ar 2 C • and one cationic Ar 2 C + substituent attached to the same ferrocene-1,1Ј-diyl scaffold (see also Figures S9 and S10, Supporting Information for the results of such studies on complexes 1b,c 2+ ). In view of the substantial half-wave potential splitting for the individual reductions one would expect such a charge transfer absorption between differently charged pendants, may it occur through space or via the ferrocene coupling unit. Figure S11 (Supporting Information) displays a possible structure, which would allow for CT through space. Electronic transitions of this kind are well-known for π-stacked mixed-valent donor/acceptor dyads comprising of the reduced and the oxidized forms of a planar, π-conjugated electrophore (so-called pimers). [19] Only during the reduction of 1a 2+ a suspicious feature was observed as a weak band (ε ≈ 1000 m -1 ·cm -1 ). This band was, however, found to persist during the second reduction and is hence not specific to 1a •+ .
The reduced forms of complexes 1a 2+ and 1b 2+ were also investigated by EPR spectroscopy. Singly reduced radical cations 1a •+ and 1b •+ were generated by reacting the dicationic precursors with slightly less than 1 equiv. of cobaltocene in order to avoid overreduction. EPR spectra of these samples ARTICLE were measured in CH 2 Cl 2 solutions in the temperature range of 20°C to -40°C or of 20°C to -60°C, respectively. As shown in Figure 8, both paramagnetic species display an isotropic signal without any resolved hyperfine splitting at g values of 2.0115 (1a •+ ) or 2.0301 (1b •+ ). Both values are slightly larger than that of ca. 2.003 expected for ordinary trityl radicals, [20] but similar to those of e.g. the triferrocenylmethyl, phenyldiferrocenylmethyl, the ferrocenyl-ruthenocenylmethyl, or the ferrocenyl-diruthenocenylmethyl radicals. [21] DFT-calculated spin densities (Figure 2, middle) show indeed large contributions of the ferrocene-1,1'-diyl entity to the singly occupied molecular orbital (SOMO) of 1b •+ . During these studies we also noted an odd temperature-dependence for both radical cations. Thus the intensity of the EPR signal of 1a •+ steadily and reversibly decreases upon cooling. This is just the opposite of the normal Curie behavior as expressed by Equation (1), where g denotes the isotropic g value of the compound, μ B is BohrЈs magneton, B 0 the magnetic field strength and k B is the BoltzmannЈs constant. According to this equation, the population of the thermally excited state (decreasing value of ΔN) is expected to diminish with decreasing T, and this, in turn, should lead to larger signal intensities for samples with equal spin concentrations as T is lowered. Such odd behavior was already observed for reduced samples of triarylmethylium-substituted ferrocenes 2a + to 2c + and traced to the formation of hexaarylethane-type dimers. [6] The same obviously applies here. We however note that the reduction in signal intensity upon cooling is less pronounced for cationic 1a •+ as compared to 2a • , which is reasonably expected as a consequence of electrostatic repulsion between the positively charged CAr 2 + pendants of such a dimer. This holds to an even larger degree for radical cation 1b •+ , whose signal intensity initially increases on lowering T, complying with the normal Curie-behavior, but then decreases upon cooling to even lower temperatures. We thus conclude that dimerization only commences to a noticeable degree at temperatures of or below -20°C. This can be seen as a token of a lesser delocalization of the positive charge at the remaining tritylium site onto the aryl substituents, which is in full agreement with our electrochemistry data.
Fully reduced 1a, generated by treating 1a 2+ with an excess over 2 equiv. of cobaltocene, proved unfortunately too reactive to be reliably characterized by EPR spectroscopy. In the case of 1b, however, an indicative EPR signal was obtained and its T dependence was likewise studied. The results are shown in Figure 9. Diradical 1b produces two different isotropic signals at g values of 2.0283 and of 2.0144. The former value falls close to that of 2.0301 observed for 1b •+ . On progressive cooling to lower T, this signal gradually decreases in intensity, while that of the signal at higher field first increases and then stays rather constant. We tentatively assign the signal at g = 2.0283 to the dissociated diradical and the one at g = 2.0144 to a diradical dimer or higher oligomers. We also note that, in contrast to cationic 1b •+ , but in agreement with neutral 2b • , dimerization already sets in at or slightly below room temperature as shown by the slight intensity decrease of the low-field EPR resonance in the T interval of 20°C to 0°C.

Reactivity Studies of 1b: Formation and Characterization of a Unique Peroxo-[4]ferrocenophane
During attempts to reduce 1b 2+ to its radical cation on a preparative scale with Cp* 2 Fe (E 1/2 = -550 mV) as a selective reductant and without protection from the atmosphere, we observed a fading of the solution color from deep red to greenish yellow. Extraction of the solid obtained after solvent evaporation with n-hexane provided a dark yellow solution and a green, insoluble residue, which was not characterized further.

ARTICLE
The 1 H NMR spectrum of the product obtained from the soluble fraction after solvent removal consists of four separate AB doublets at δ = 7.86, 7.35, 6.90, and 6.52 ppm for the paradisubstituted anisyl rings, integrating as 4 protons each, 4 broad singlet resonances at δ = 5.11, 4.21, 4.03, and 3.91 ppm for the Cp protons with an integral of 2 H each, and two singlets, each accounting for 6 H, for the methoxy protons at δ = 3.33 and 3.22 ppm, along with minor impurities ( Figure S12, Supporting Information). This pattern of resonance signals indicates the formation of a new 1,1Ј-disubstituted ferrocene derivative with C 2 symmetry. A pure product was obtained, when 2 equiv. of cobaltocene were added to a CH 2 Cl 2 solution of 1b 2+ inside a glovebox and stirred for some min before the reaction vessel was taken out of the glove box and left stirring open to the air overnight. Workup as before gave the pure compound in quantitative yield after evaporation of the solvent. The ESI mass spectrum of this product showed the molecular ion peak at m/z = 699.1925, which matches with an adduct of 1b with one molecule of oxygen and one proton ( Figure S14, Supporting Information). Crystallization of this compound by slow evaporation of a saturated solution in CH 2 Cl 2 afforded yellow blocky crystals that proved suitable for X-ray structure determination. As shown in Figure 10, the newly formed product is a neutral peroxo- [4]ferrocenophane 1b-O 2 (only one of the two enantiomers of the racemic pair in the crystal is shown; for details of the data collection and refinement, the cell parameters and the bond lengths and bond angles see Tables S2 to S4, Supporting Information).  [4]ferrocenophane structure. This is mirrored by differences in Fe-C Cp bond lengths, which range from 2.0229(18) to 2.0356 (19) Å for the C atoms at or in the immediate vicinity of the hinge to 2.067(2) and 2.0667 (19) Å at the open side.
Like their monotritylium-substituted counterparts 2a • -2c • (Scheme 2), reduced 1a-c •+/0 tend to dimerize/oligomerize via their triarylmethyl centers with concomitant formation of C-C bonds. This tendency is attenuated for the radical cations, which is likely due to electrostatic repulsion between the remaining triarylmethylium centers. Direduced, anisyl-substituted 1b was found to be reactive towards molecular oxygen and to form a novel peroxo-(bisdiarylmethyl) [4]ferrocenophane with a -Ar 2 C-O-O-CAr 2 -linkage between the two Cp Z. Anorg. Allg. Chem. 2020, 712-725 www.zaac.wiley-vch.de decks. As it was shown by X-ray crystallography, the fouratom linker induces an only moderate ring strain as revealed by a 7.2°tilt between the Cp rings.
Another intriguing finding of this study is that anisyl-substituted 1b 2+ exists as a mixture of a diamagnetic and a paramagnetic form. These two isomers interconvert via a sizeable energy barrier, which gives rise to hysteresis. The paramagnetic form is obviously the result of an electron transfer from the ferrocene nucleus to one of the triarylmethylium centers. Such behavior of 1b 2+ is reminiscent of a magnetochemical switch, where, by action of an external trigger, a diamagnetic state can be altered into a diradical state with two unpaired spins. Further work in our laboratories is directed to exploring this prospect further.

Experimental Section
General Methods: All manipulations were carried out at room temperature in a nitrogen atmosphere using standard Schlenk techniques, unless stated otherwise. Solvents were dried and distilled by standard procedures and degassed by saturation with nitrogen prior to use. Ferrocene-1,1Ј-dicarboxylic acid was prepared according to a literature procedure [31] and converted to its dimethyl ester following the procedure procured for ferrocene carboxylic acid. [32] 1 H NMR (400 MHz), 13 C{ 1 H} NMR (101 MHz) and 31 P{ 1 H} NMR (162 MHz) spectra of the compounds were measured on a Bruker Avance III 400 spectrometer at room temperature in the indicated deuterated solvent. The spectra were referenced to the signal of residual protonated solvent ( 1 H) or the solvent signal ( 13 C). UV/Vis/NIR spectra were recorded on a TIDAS fiber optic diode array spectrometer (combined MCS UV/ NIR and PGS NIR instrumentation) from j&m in HELLMA quartz cuvettes with 0.1 cm optical path lengths.
All electrochemical experiments were executed in a custom-built cylindrical vacuum-tight one-compartment cell. A spiral-shaped Pt wire and a Ag wire as the counter and pseudoreference electrodes were sealed into glass capillaries and fixed by Quickfit screws via standard joints. A platinum electrode was introduced as the working electrode through the top port via a Teflon screw cap with a suitable fitting. It was polished with first 1 μm and then 0.25 μm diamond paste before measurements. The cell was attached to a conventional Schlenk line via a side ARTICLE arm equipped with a Teflon screw valve, allowing experiments to be performed under an argon atmosphere with approximately 5 mL of analyte solution. NBu 4 + [B{C 6 H 3 (CF 3 ) 2 } 4 ] -(0.02 m) was used as the supporting electrolyte. Referencing was done with addition of an appropriate amount of decamethylferrocene (Cp* 2 Fe, E 1/2 = -550 mV with respect to Cp 2 Fe) as an internal standard to the analyte solution after all data of interest had been acquired. Representative sets of scans were repeated with the added standard. Electrochemical data were acquired with a computer controlled BASi CV50 potentiostat.
The optically transparent thin-layer electrochemical (OTTLE) cell was also custom-built according to the design of Hartl et al. [33] It consists of a Pt working and counter electrode and a thin silver wire as a pseudoreference electrode sandwiched between two CaF 2 windows of a conventional liquid IR cell. The working electrode is positioned in the center of the spectrometer beam. Electron paramagnetic resonance (EPR) studies were performed on a MiniScope MS 400 Table-top Xband spectrometer from Magnettech. X-ray diffraction analysis was performed at 100 K on a STOE IPDS-II diffractometer equipped with a graphite-monochromated radiation source (λ = 0.71073 Å) and an image plate detection system. A yellow, blocky crystal of 1b-O 2 obtained from slow evaporation of a solution of this complex in CH 2 Cl 2 was mounted on a fine glass fiber with silicon grease. The selection, integration, and averaging procedure of the measured reflection intensities, the determination of the unit cell dimensions and a least-squares fit of the 2θ values as well as data reduction, LP-correction and space group determination were performed using the X-Area software package delivered with the diffractometer. A semiempirical absorption correction was performed. [34] The structure was solved by the heavy-atom method. Structure solution was completed with difference Fourier syntheses and full-matrix leastsquares refinements using SHELX-2017 [35] and OLEX2, [36] minimizing ω(F o 2 -F c 2 ) 2 . The weighted R factor (wR 2 ) and the goodness of the fit GOOF are based on F 2 . All non-hydrogen atoms were refined with anisotropic displacement parameters, while hydrogen atoms were introduced in a riding model.
Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository number CCDC-1971352 (Fax: +44-1223-336-033; E-Mail: deposit@ ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
The ground state electronic structures of the full complexes were calculated by density functional theory (DFT) methods using the GAUSSIAN 09 program package. [37] Geometry optimizations were performed without any symmetry constraints. Open shell systems were calculated by the unrestricted Kohn-Sham approach (UKS). Geometry optimization followed by vibrational analysis was performed in solvent media. Solvent effects were described by the polarizable continuum model (PCM) with standard parameters for 1,2-dichloroethane. [38] For Fe, the ten-electron quasirelativistic effective core potential (ECP) MDF10 was used, [39] and 6-31G(d) polarized double-ξ basis sets [40] were employed together with the Perdew, Burke, Ernzerhof exchange and correlation functional (PBE0). [41] The GaussSum program package was used to analyze the results, [42] while the visualization of the results was performed with the Avogadro program package. [43] Graphical representations of molecular orbitals were generated with the help of GNU Parallel [44] and plotted using the vmd program package [45] in combination with POV-Ray. [

Synthesis and Characterization of 1b-O 2 :
Inside a glove box 1b 2+ ·2BAr F24 -(100 mg, 0.042 mmol, 1 equiv.) was dissolved in 3 mL of degassed CH 2 Cl 2 and 16 mg (0.084 mmol, 2 equiv.) of cobaltocene were added. The mixture was stirred for 10 min, then removed from the glove box, exposed to the air and stirred at room temperature overnight. The solvent was evaporated under vacuum and the residue was extracted with n-hexane.

Synthesis and Characterization of 1b-HOH:
Inside a glove box 1b 2+ ·2BAr F24 -(100 mg, 0.042 mmol, 1 equiv.) was dissolved in 3 mL of degassed CH 2 Cl 2 . 16 mg (0.084 mmol, 2 equiv.) of cobaltocene and 0.76 μL of water were added and the mixture was allowed to stir at room temperature for 2 h. During this time the solution turned green. The solvents were removed under reduced pressure and the green solid was extracted with n-hexane. Solvent was stripped off the filtered extract and the solid residue was purified by solvent chromatography (silica, PE/EA 5/1). The first fraction contained small quantities of 6,6-diansiylfluorene. The second, orange band provided complex 1b-HOH after solvent evaporation and washing with cold npentane. 1  Supporting Information (see footnote on the first page of this article): NMR spectra of the complexes, UV/Vis/NIR spectra of the reduced forms of complexes 1b 2+ and 1c 2+ , ESI-MS of complexes 1b-O 2 and 1b-HOH, Tables with UV/Vis data in various solvents and details of the crystallographic structure determination as well as the bond lengths and bond angles.