phenanthrene Ferrocenyl Complexes - American Chemical Society

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Charge Transfer Properties in Cyclopenta[l]phenanthrene Ferrocenyl Complexes Alessandro Donoli,† Annalisa Bisello,† Roberta Cardena,† Cristina Prinzivalli,† Marco Crisma,‡ and Saverio Santi*,† †

Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy Institute of Biomolecular Chemistry, Padova Unit, CNR, via Marzolo 1, 35131 Padova, Italy

‡

S Supporting Information *

ABSTRACT: The new complexes (2-ferrocenyl)cyclopenta[l]phenanthrene and (2-ferrocenyl)(η5-cyclopenta[l]phenanthrenyl)FeCp have been prepared and the charge transfer properties of their monocationic derivatives investigated. The cations were generated by chemical oxidation using ferrocenium(BF4) or acetylferrocenium(BF4) as the oxidative agent and monitored in the visible, IR, and near-IR regions. The electrochemistry of the two complexes and, for comparison, of the previously reported (η5-cyclopenta[l]phenanthrenyl)FeCp was analyzed. The charge transfer bands in the near-IR spectral region of the monocations are rationalized in the framework of Marcus−Hush theory. In particular, the monometallic (2ferrocenyl)cyclopenta[l]phenanthrene displays a single oxidation wave at a potential very close to that of (η5cyclopenta[l]phenanthrenyl)FeCp and its monocations exhibits a ligand-to-metal charge transfer band in the vis−near-IR region. The unsymmetrical diiron species (2-ferrocenyl)(η5-cyclopenta[l]phenanthrenyl)FeCp undergoes two consecutive and well-resolved one-electron oxidations producing, at the first oxidation step, a mixed-valence monocation which displays an intervalence charge transfer band in the vis−near-IR region.

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INTRODUCTION Among metallocene-based metallorganic frameworks having a conjugated system and displaying multielectron redox chemistry,1 ferrocenyl and multiferrocenyl compounds have been suggested as potential candidates for molecular electronics, due to their well-defined and robust redox properties.2,3 Molecules containing at least two redox-active metal centers bound by a πconjugated organic framework offer the possibility to generate mixed-valence states by oxidizing one of the metals, giving rise to species which possess the transition metals in different oxidation states.4 Mixed valency has attracted interest in recent decades, as the redox-active species may be suitable for the design of novel electroactive materials5 such as molecular wires, conducting polymers,6 and nanometer-scale molecular components with specific redox, optoelectronic, magnetic, or conductive properties which are now fundamental requirements of modern technology.7 Moreover, mixed-valence species can be used as model systems for the investigation of intramolecular electron transfer.8 Multi(ferrocenyl) compounds with metal−metal coupling are of particular interest, owing to their multiredox, magnetic coupling, and unpaired electron density migration properties.9 Such switchable arrays have been intensely studied because conjugated organic chains containing FeII/FeIII couples of the ferrocenyl group can be potentially used in molecular electronics, quantum cellular automata, optoelectronic materials, and biochemistry for application in redox or photonic devices.10 The mixed-valence states having at least two ferrocenyl groups in different oxidation states are excellent © 2014 American Chemical Society

benchmarks for the investigation of intramolecular charge (electron) transfer,11 as the formation of mixed-valence intermediates is often responsible for the aforementioned properties in multi(ferrocenyl) complexes. The factors affecting the formation, stability, and nature of mixed-valence states in multi(ferrocenyl) derivatives were thoroughly examined.9−12 In particular, it was found that the electron transfer mechanism in mixed-valence intermediates is mostly based on Marcus−Hush theory.13 Electrochemistry and optical spectroscopy of compounds containing multiple ferrocenyl groups are among the most important techniques which provide insight into the nature of the mixed-valence species, the thermodynamic stability of the redox intermediates, and the magnitude of metal−metal electronic interactions. Unambiguously, electrochemistry gives information on the thermodynamic stability of the redox intermediates13 and, with some caution, on the interactions between the different metal atoms.14 Moreover, the characteristics of the spacer connecting two or more redox units play a relevant role in governing the ΔE separation between consecutive electron transfer steps and in affording distinguishable redox waves. If the mixed-valence intermediate is detectable, optical spectroscopy represents a powerful probe for evaluating the magnitude of the metal−metal interaction in mixed-valence compounds involving the analysis of the intervalence charge Received: October 2, 2013 Published: February 26, 2014 1135

dx.doi.org/10.1021/om4009668 | Organometallics 2014, 33, 1135−1143

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Scheme 1. Synthesis of (2-ferrocenyl)cyclopenta[l]phenanthrene (1a), (η5-cyclopenta[l]phenanthrenyl)FeCp (1b), and (2ferrocenyl)(η5-cyclopenta[l]phenanthrenyl)FeCp (2)

Figure 1. Molecular structure obtained by X-ray diffraction of 1a, with atom numbering. Anisotropic displacement ellipsoids for the non-H atoms are drawn at the 30% probability level. H atoms are depicted as spheres of arbitrary radii.

transfer (IVCT) absorption bands in the near-IR region.13 In addition, in oligomers and polymers in which the energy gap between the metal group and the organic bridge is small, a ligand-to-metal charge transfer (LMCT) band appears in the near-IR region.15 Since the pioneering studies on biferrocene ion,16 electronic coupling and electron transfer rates of mixed-valence biferrocene monocation derivatives have attracted continuing attention and have been investigated by means of different synthetic modifications17 and physicochemical techniques.18 Herein, we describe the synthesis and structural characterization of the new biferrocene-like complex (2-ferrocenyl)(η5cyclopenta[l]phenanthrenyl)FeCp, in which one ferrocenyl has been replaced by the (η5-cyclopenta[l]phenanthrenyl)FeCp moiety. For comparison, (2-ferrocenyl)cyclopenta[l]phenanthrene and (η 5 -cyclopenta[l]phenanthrenyl)FeCp monometallic complexes were prepared. The charge transfer

properties of their monocations, generated by chemical oxidation using ferrocenium(BF4) or acetylferrocenium(BF4) and monitored by vis−near-IR−mid-IR spectroscopy, were also investigated.

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RESULTS AND DISCUSSION Synthesis. The preparation of (2-ferrocenyl)cyclopenta[l]phenanthrene (1a) was achieved following the approach initially described by Plenio19 by the reaction of 1,3dihydrocyclopenta[l]phenanthren-2-one20 with ferrocenyllithium, generated by the reaction of ferrocene with t-BuLi in THF (Scheme 1). The synthesis of the bimetallic (2-ferrocenyl)(η5-cyclopenta[l]phenanthrenyl)FeCp compound (2) was achieved through a sequence which proceeds through an initial step providing the η 1 -[Fe(CO) 2 Cp] adduct, a revised approach 21 to the 1136


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preparation of CpFe(CO)2(η1-indenyl), initially described by Cotton and co-workers,22 followed by η1 → η5 conversion with concurrent elimination of two molecules of CO (Scheme 1). The same procedure, starting from cyclopenta[l]phenanthrene,23 was adopted for the preparation of the reference complex (η5-cyclopenta[l]phenanthrenyl)FeCp24 (1b). Molecular Structures. The molecular structure of 1a, as determined by single crystal X-ray diffraction, is illustrated in Figure 1 with atom numbering. The values of the C1−C2 and C1−C5 bond lengths are 1.457(5) and 1.483(4) Å, respectively, whereas the C2−C3 and C3−C4 bonds (1.387(5) and 1.476(5) Å, respectively) show a much more pronounced difference in length. These findings strongly support the sp3 hybridization of C1. In addition, two electron density peaks appropriately positioned for a CH2 group were located on a difference Fourier map at the level of C1, while for C3 only one H atom lying in the ring plane could be found. As for the geometry of the ferrocenyl (Fc) unit, the Fe−C bond lengths range from 2.020(3) to 2.042(4) Å, the shortest being to the C23 and C24 atoms of the distal Cp ring. The Fe−C5H4 and Fe−C5H5 perpendicular distances are 1.643 and 1.647 Å, respectively. The two cyclopentadienyl rings are coplanar to each other (the angle between normals to their average planes is 2.1(3)°), and they are found in an eclipsed disposition, the values of the inter-ring H−C−C−H twist angles not exceeding 3.7°. The angle between normals to the cyclopentaphenanthrene and the proximal Cp ring is 8.5(2)°. The values of the torsion angles about the C2−C18 bond, namely C1−C2−C18−C19 6.1(6)° and C3−C2−C18−-C22 8.7(6)°, indicate that the Cp ring is laterally tilted relative to the polycyclic system. This finding seems to be related to packing effects rather than to unfavorable intramolecular contacts. In the crystals of 1b, the asymmetric unit is composed of four independent molecules. The molecular structure of molecule A with atom numbering is depicted in Figure 2. Atoms in molecules B−D are numbered in the same order as in molecule A. The arrangement of the four independent molecules in the asymmetric unit is shown in Figure 3. Bond lengths and bond angles are in general agreement with literature data. The apparent shortening of the distal C−C

Figure 3. The four independent molecules (A−D) in the asymmetric unit of 1b. H atoms are omitted for clarity.

bonds of the phenanthrene moieties might be ascribed to librational effects. The Fe−C bond lengths range from 2.019(7) to 2.063(5) Å, the longest being to the C4 and C5 atoms in each molecule. The Fe−C5H3 and Fe−C5H5 perpendicular distances, being in the ranges 1.642−1.651 and 1.643−1.653 Å, respectively, do not differ significantly within each Fc moiety. In each of the Fc units, the two cyclopentadienyl rings are coplanar with each other (the angle between normals to their average planes is within 1.6(3)°), and they are found in the eclipsed disposition, the values of the average inter-ring H−C− C−H twist angles not exceeding 3.3°. Within the asymmetric unit of 1b, the cyclopenta[l]phenanthrene rings of molecules A and C are nearly perpendicular to each other, the angle between normals to their average plane being 89.4°. A similar perpendicular arrangement is found for molecules B and D (angle between normals 89.2°). Relative to molecule A, the normals to the average plane of molecules B and D are at 36.2 and 79.0°, respectively, whereas the angle between normals to the average plane of molecules B and C is 80.1°. Electrochemistry. Cyclic voltammograms (CVs) of 1a,b, and 2 were recorded under argon in CH2Cl2/0.1 M nBu4NPF6 (Table 1, Figure 4). In the range from 0 to 1 V vs SCE, all of the complexes show oxidation waves which consistently met the chemical reversibility criteria in the scan rate range of 0.1− 50 V s−1, as they all showed cathodic/anodic peak current ratios of ia/ic = 1 and E1pa − E1pa/2 values around 80 mV. Monometallic ferrocenyl complexes 1a,b exhibited a single reversible oxidation wave at anodic peak potential, Epa, of 0.49 and 0.46 V vs SCE, respectively (Table 1, Figure 4a,b). For both 1a and 1b a second oxidation wave is observed at Epa of 1.35 and 1.72 V vs SCE and is assigned to cyclopenta[l]phenanthrene and cyclopenta[l]phenanthrenyl oxidation, respectively. In the case of 1a it is chemically irreversible but Epa reaches a constant value by increasing the scan rate and becomes partially chemically reversible (see the Supporting Information, Figure S5 and S6). Bimetallic 2 showed two consecutive oxidation processes, 2/ 2+ and 2+/22+, at Epa values of 0.40 and 0.70 V vs SCE, respectively (Table 1, Figure 4c). The first oxidation wave is fully reversible (chemically and electrochemically) and occurs at an Epa value lower than that of ferrocene (0.40 vs 0.50 V). Similarly to what we found for (2-ferrocenyl)(η5-indenyl)FeCp (3) (Scheme 2),12d the first wave is assigned to the oxidation of

Figure 2. Molecular structure obtained by X-ray diffraction of one of the four independent molecules (A) of 1b with atom numbering. Anisotropic displacement ellipsoids for the non-H atoms are drawn at the 30% probability level. H atoms are depicted as spheres of arbitrary radii. 1137


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Table 1. Cyclic Voltammetric Dataa complex

E11/2

E1pa

E1pa − E1pa/2

E21/2

E2pa

E2pa − E2pa/2

ΔEb

ferrocene 1a 1b 2 3 4 5

0.46 0.44 0.42 0.37 0.25 0.30 0.42

0.50 0.48 0.46 0.40 0.28 0.33 0.46

0.078 0.076 0.078 0.069 0.062 0.059 0.078

1.32d 1.69 0.67 0.63 0.68

1.35d 1.72 0.70 0.66 0.71

0.075d 0.075 0.069 0.059 0.059

0.87 0.30 0.38 0.38

Kcc

1.4 × 105 3.4 × 106 3.4 × 106

Conditions: solvent CH2Cl2; potential in volts vs SCE at a 0.5 mm diameter gold-disk electrode,; T = 20 °C; potential scan rate 0.5 V s−1; supporting electrolyte 0.1 M [nBu4N][PF6]. bΔE = E21/2 − E11/2. cKc = exp(FΔE/RT). dScan rate 30 V s−1.

a

communication by means of ΔE1/2 values must be handled cautiously.14 More reliable information can be obtained by the analysis of near-IR spectra. Optical Spectroscopy. Stable solutions of 1a+, 1b+, 2+, and 2+ 2 were generated by chemical oxidation with ferrocenium(BF4) or acetylferrocenium(BF4) in CH2Cl2 containing 0.1 M [nBu4N][PF6] in order to realize the most similar medium conditions of electrochemical measurements. The obtained solutions were analyzed at room temperature by optical spectroscopy, and the charge transfer properties of the monooxidized species were probed in the near-IR region. Oxidation of the yellow CH2Cl2 solution of 1a by addition of 1 equiv of acetylferrocenium(BF4) afforded a deep red solution and the appearance of an intense band at 8062 cm −1 corresponding to the selective formation of cation 1a+ (Figure 5a and Table 2) and due to a cyclopenta[l]phenanthrene to Fc+ electron transfer. Similar bands were observed for other conjugated cationic ferrocenyl aryl complexes and assigned to aryl to iron ligand-to-metal charge transfer (LMCT) transitions.15,25 The classical electron transfer Hush model, developed for interpretation of the intervalence charge transfer (IT) bands of bimetallic26 molecules, were also extended to the LMCT and MLCT transfer processes to get insight into the delocalization in monometallic metal-based conjugated systems.27 We previously reported for a family of substituted (ferrocenyl)indenes28 that the energy and the intensity of the band can be predicted for the oxidation potential of indene relative to that of ferrocenyl (ΔE), according to the classical electron transfer model of Hush, which relates the absorption maximum (ν̃max = ΔE + ΔE′ + λ) of the LMCT transition to the difference in the electrochemical potentials between the donor and acceptor metal groups, in which ΔE is the oxidation potential difference between the organic moiety (donor) and the ferrocenyl(1+) (acceptor), ΔE′ the difference between the FeII/III oxidation potential with the oxidized indene and the measured FeII/III potential, and λ the nuclear reorganization energy. In particular, the degree of redox matching of the πconjugated aryl hydrocarbon and ferrocene oxidation poten-

Figure 4. Oxidative CVs in CH2Cl2/0.1 M [nBu4N][PF6] of 1a,b and 2. Conditions: scan rate v = 0.5 V s−1; gold-disk electrode (diameter 0.5 mm); T = 20 °C. Dotted lines indicate the E1/2 values.

the (η5-cyclopenta[l]phenanthrenyl)FeCp moiety. The second oxidation wave is fully reversible as well and occurs at potential far higher than that of ferrocene. Thus, the electrochemical characteristics allow determination for the peak separation between the two waves (ΔE1/2) and the related value of the equilibrium constant Kc (Table 2) for the comproportionation reaction (eq 1). The estimated value of Kc [Fe II−Fe*II ] + [Fe III−Fe*III ]2 + = 2[Fe II−Fe*III ]+

(1)

is comparable with those found for 3 and biferrocene (4) (Scheme 2) and is indicative of the thermodynamic stability14e of the cationic species 2+, the mixed-valence characteristics of which will be verified below. The ΔE1/2 value is much larger than the difference of the oxidation potentials between ferrocene and 1b but, due to the short proximity of the metal atoms in 2+, a large part of this redox splitting may be caused by electrostatic interactions and the assessment of the degree of metal−metal electronic

Scheme 2. Reference Compounds (2-ferrocenyl)(η5-indenyl)FeCp (3), Biferrocene (4), and (2-Ferrocenyl)indene (5)

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Table 2. Near-IR Data in CH2Cl2 +

1a 1b+ 2+ 22+

ν̃max (cm−1)

type

εmax (M−1 cm−1)

(Δν̃1/2)obsd (cm−1)

8084 12420 4830 13280, 11260

LMCT LMCT IVCT

2120 1482 230 360, 150

3110, 1658 1426, 2876 2740 2790, 1520

(Δν̃1/2)Husha (cm−1)

3015

Γb

0.09

f

d

(cm−1)

0.026 0.017 0.003

(Δν̃1/2)Hush (cm−1) = [16RT ln 2ν̃max − E0]1/2) calculated for the deconvoluted low-energy Gaussian component. T = 20 °C. bΓ = 1 − (Δν̃1/2)obsd/ (Δν̃1/2)Hush.13h c[nBu4N][PF6] as supporting electrolyte. dOscillator strength of LMCT band calculated from the sum of fitted n Gaussians: f = (4.6 × 10−9)∑n(εmaxΔν̃1/2n).27b a

tials, which is a prerequisite for the best efficiency of the donor−acceptor interaction, is increased by electron-donating groups, as suggested by the decrease in energy and the increase in intensity of the LMCT band: for instance, with methylation of the indene. Interestingly, the ν̃max value of 1a+ (8084 cm−1) vs ΔE (0.87 V) nicely fits the linear correlation previously found for the series of (ferrocenyl)indenes (Figure 6a),28 suggesting that the fusion of two benzene rings to indene in the cyclopenta[l]phenanthrene is an alternative and efficient structural modification of aryl hydrocarbons in order to realize the desirable matching of the redox potentials of the π-conjugated backbone and the pendant metal group as closely as possible. For a Gaussian-shaped peak the oscillator strength ( f), which represents a measure of the donor−acceptor interaction, is related to εmax and Δν̃1/2 represents the half-bandwidth (see Table 2).13a,f Gaussian deconvolution of the spectrum of 1a+ in Figure 5b allows for the determination of the spectral parameters of the LMCT band and of the value of the oscillator strength, f = 0.026 cm−1. The latter is higher (0.015 cm−1)28 than that found for the cation of (2-ferrocenyl)indene (5) (Scheme 2), indicating that the cyclopenta[l]phenanthrene increases the efficiency of ligand-to-metal charge transfer with respect to indene. In addition to ν̃max, the f value of 1a+ vs E0 nicely fits the linear correlation found for the series of (ferrocenyl)indenes (Figure 6b).28 Similar correlations have been observed for other LMCT transitions in Fc-conjugated cations15a,25a,c and for IVCT transitions in ferrocenyl-based heterobimetallic12d and, more recently, homobimetallic25e cations. Oxidation of the yellow CH2Cl2 solutions of 1b by addition of 1 equiv of acetylferrocenium(BF4) afforded a deep red solution and the appearance of an intense band at wavenumbers (12420 cm−1) much higher than that of 1a+, corresponding to the selective formation of cation 1b+ (Figure

Figure 5. UV−vis−near-IR spectra in CH2Cl2/0.1 M [nBu4N][PF6] solution at 20 °C: (a) 1a (black line) and 1a+ (red line); (b) Gaussian deconvolution (open circles) of the low-energy band of 1a+ (red line); (c) 1b (black line) and 1b+ (red line); (d) Gaussian deconvolution (open circles) of the low-energy band of 1b+ (red line); (e) 2 (black line), 2+ (red line), and 22+ (blue line); (f) Gaussian deconvolution (open circles) of the low-energy bands of 2+ (red line) and of 22+ (blue line).

Figure 6. Optical energy (νmax) and oscillator strength ( f) of near-IR transition vs oxidation potential difference (ΔE) for 1a+ and a series of methylated (ferrocenyl)indenes.33 1139


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5c,d and Table 2). This blue shift suggests that the η5 coordination of the cyclopenta[l]phenanthrenyl by the FeCp moiety strongly reduces the π-electron delocalization of the aromatic hydrocarbon and, consequently, the energy of the LMCT transition increases. Addition of 1 equiv of ferrocenium(BF4) to a solution of 2 initially afforded a green solution and the appearance of a lowenergy absorption band (Figure 5e,f and Table 2) corresponding to the formation of the monocation 2+ (ν̃max 4830) and assigned, on the basis of the electrochemical data and similarly to what was found for (2-ferrocenyl)(η5-indenyl)FeCp,12d to a Fc-to-(η5-cyclopenta[l]phenanthrenyl)FeCp+ electron transfer. Further addition of 1 equiv of acetylferrocenium(BF4) to the solution caused the quantitative disappearance of the band attributed to 2+ and the concomitant appearance of a new band at wavenumbers near those found for 1b+ (ν̃max 13280−11260 cm−1; Figure 5c,f and Table 2), which can be confidently assigned to the fully oxidized species 22+ (see also Figure S7 in the Supporting Information). In addition to the absorptions described above, the spectrum of the Fe(II)−Fe(III) mixed-valence species 2+ displays the tail of a very low intensity and weak band around ν̃max 3800 cm−1 (Figure 5f). This band can be assigned to transitions between the Kramer’s doublets, also called a dπ → dπ or interconfigurational (IC) transition.13e It is a forbidden ligand field (LF) transition which gains intensity through spin−orbit coupling and metal−ligand mixing. Distinct IC bands are usually not observed for complexes of transition metals of the first series unless there is a large electronic asymmetry in the ligand field.13e Spin−orbit coupling is low for Fe(III) (400−500 cm−1) and has the effect of shifting the IC bands into the IR region and decreasing their absorptivity.13g Gaussian deconvolution of the spectra in Figure 5f allowed us to determine the spectral parameters of the low-energy bands of cations 2+ (Table 2) and to analyze them by using the classical electron transfer Hush model. The narrowness of these bands is stressed by the comparison of the experimental and calculated half-bandwidths, (Δν̃1/2)Hush (cm−1) = [16RT ln 2ν̃1/2 − E0]1/2),13a where E0 is the redox asymmetry: i.e., the energy difference between the two states FeIIIFe*II and FeIIFe*III.29 The magnitude of Γ = 1 − (Δν̃1/2)obsd/ (Δν̃1/2)Hush,13h a criterion proposed to classify the mixedvalence species, is consistent with a weakly coupled (class II) cation and is identical with that found for the cation of biferrocene 4+ (0.09).16c

and the intensity of the band can be predicted for the oxidation potential of cyclopenta[l]phenanthrene relative to that of the ferrocenyl group, ΔE, that is the degree of redox matching of the π-conjugated aryl hydrocarbon and ferrocene oxidation potentials, which is a prerequisite for the best efficiency of the donor−acceptor interaction. On the basis of the oscillator strength values, f, it appears that cyclopenta[l]phenanthrene increases the efficiency of ligand-to-metal charge transfer with respect to indene. On the other hand, oxidation of 2 generates an IVCT absorption in the near-IR region, which is due to a Fc-to-(η5cyclopenta[l]phenanthrenyl)FeCp+ electron transfer. Hush analysis of the IVCT band indicated that 2+ is a class II mixed-valence system. The narrowness of this band was highlighted by comparing the experimental and calculated half-bandwidths. The Γ value of 2+ is identical with that found for the cation of biferrocene 4+, indicating that the fusion of two benzene rings to indenyl does not modify the magnitude of the electronic coupling. In summary, in the monoiron cation a ligand-to-metal charge transfer (LMCT) f rom cyclopenta[l]phenanthrene to ferrocenyl(1+) occurs, which is more efficient than that occurring in the correlated cation of (2-ferrocenyl)indene. Conversely, in the diiron cation a metal-to-metal charge transfer (IVCT) f rom ferrocenyl to the (η5-cyclopenta[l]phenanthrenyl)FeCp+ moiety takes place.

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EXPERIMENTAL SECTION

General Methods. All reactions and complex manipulations were performed under an oxygen- and moisture-free atmosphere utilizing standard Schlenk techniques or in a Mecaplex glovebox. Solvents were dried by reflux over the appropriate drying agent and distilled under a stream of argon. Ferrocene and n-butyllithium solution were SigmaAldrich products. Ferrocenium(BF4) and acetylferrocenium(BF4),30 1,3-dihydrocyclopenta[l]phenanthren-2-one, 20 cyclopenta[l]phenanthrene,23 and (η5-cyclopenta[l]phenanthrenyl)FeCp24 were synthesized according to published procedures. Microanalyses were performed at the Dipartimento di Scienze Chimiche, Università di Padova. Crystals of (η5-cyclopenta[l]phenanthrenyl)FeCp and (2-ferrocenyl)cyclopenta[l]phenanthrene were grown from a dichloromethane/n-pentane 1/9 mixture. X-ray diffraction data were collected with an Oxford Diffraction Gemini E four-circle kappa diffractometer equipped with a 92 mm EOS CCD detector, using graphite-monochromated Mo Kα radiation (λ = 0.71069 Å) and the ω oscillation (1.0°) scan technique. For (η5cyclopenta[l]phenanthrenyl)FeCp a total of 834 frames were collected in the 2.17−28.88° θ range, whereas for (2-ferrocenyl)cyclopenta[l]phenanthrene 264 frames were collected in the 2.21−31.93° θ range. Data collection and reduction were performed with the CrysAlisPro software (version 1.171.33.52; Oxford Diffraction). For processing, both data sets were cut at θ = 26.37°, as the crystals, owing to their small thickness, did not diffract significantly above 0.80 Å resolution. Overall, for (η5-cyclopenta[l]phenanthrenyl)FeCp, 42216 reflections were integrated, 12714 of which are independent (Rint = 0.0530). Similarly, for (2-ferrocenyl)cyclopenta[l]phenanthrene, 7534 reflections were integrated, 3801 of which are independent (Rint = 0.0515). A semiempirical absorption correction based on the multiscan technique using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, was applied to each data set. Both structures were solved by ab initio procedures of the SIR 2002 program.31 Four independent molecules characterize the asymmetric unit of (η5-cyclopenta[l]phenanthrenyl)FeCp in space group P1̅. The search for an alternative cell setting in higher-symmetry space groups was fruitless. The refinements were carried out by full-matrix least squares on F2, using all data, by application of the SHELXL-97 program,32 with anisotropic displacement parameters for all of the non-H atoms. H atoms in (η5-cyclopenta[l]phenanthrenyl)FeCp were

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CONCLUSIONS The synthesis of the monometallic complex 1a was obtained by the reaction of cyclopenta[l]phenanthren-2-one with ferrocenyllithium, generated by the reaction of ferrocene with t-BuLi. The preparation of the bimetallic complex 2 was achieved through a sequence which proceeds through an initial step providing an η1-[Fe(CO)2Cp] adduct followed by η1 → η5 conversion with concurrent elimination of two molecules of CO. The charge transfer cations of these ferrocenyl complexes were generated by chemical oxidation using ferrocenium(BF4) or acetylferrocenium(BF4) as the oxidant and monitored in the visible, IR, and near-IR spectral regions. The charge transfer bands in the near-IR spectra were rationalized in the framework of Marcus−Hush theory. The oxidation of the ferrocenyl group of 1a generates a LMCT absorption in the near-IR region, which is due to a cyclopenta[l]phenanthrene-to-Fc+ electron transfer. The energy 1140


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(s, 5H, Cp), 4.44 (t, 1H, H2, J = 2.5 Hz), 5.37 (d, 2H, H1, H3, J = 2.5 Hz), 7.49 (m, 4H, H5, H6, H9, H10), 8.04 (m, 2H, H4, H11), 8.48 (m, 2H, H7, H8). 13C NMR (100.61 MHz, acetone-d6): δ 63.22 (C1, C3), 69.87 (C2), 70.36 (CCp), 81.79 (C3a, C11b), 124.24 (C7, C8), 124.63 (C4, C11), 126.38 and 128.07 (C5, C6, C9, C10), 130.51 (C3b, C11a), 135.18 (C7a, C7b). HRMS (ESI+): m/z calcd for C22H16Fe (M+), 336.0601; found, 336.0538. Anal. Calcd for C22H16Fe: C, 78.59; H, 4.80. Found: C, 78.88; H, 4.82. Preparation of (2-ferrocenyl)(η5-cyclopenta[l]phenanthrenyl)FeCp (2). A solution of n-butyllithium (0.94 mL, 1.6 M in hexanes, 1.5 mmol) was added dropwise under argon to a well-stirred solution of 2-ferrocenyl-1H-cyclopenta[l]phenanthrene (400 mg, 1.0 mmol) in dry THF (10 mL) at −10 °C. The reaction mixture was stirred at −10 °C for 1 h, followed by cooling to −78 °C for 10 min, and then solid [Fe(CO)2(Cp)I] (456 mg, 1.5 mmol) was added. The mixture was stirred overnight, the temperature was raised to room temperature, and the solvent was evaporated to yield a deep red solid. Purification by preparative TLC on silica gel (hexane/ CH 2 Cl 2 , 4/1) gave recovered (2-ferrocenyl)cyclopenta[l]phenanthrene (180 mg) and 2 as an orange solid. Yield: 52 mg, 10%. 1H NMR (400.13 MHz, acetone-d6): δ 3.47 (s, 5H, Cp′), 4.02 (s, 5H, Cp), 4.26 (m, 2H, Hb, Hb′), 4.67 (m, 2H, Ha, Ha′), 5.72 (s, 2H, H1, H3), 7.49 (m, 2H, H5, H10), 7.53 (m, 2H, H6, H9), 8.11 (m, 1H, H11), 8.12 (m, 1H, H4), 8.49 (m, 2H, H7, H8). 13C NMR (100.61 MHz, acetone-d6): δ 61.63 (C1, C3), 67.05 (Ca, Ca′), 68.42 (Cb, Cb′), 69.67 (CCp), 71.41 (CCp′), 81.68 (C3a, C11b), 84.15 (Cj), 86.62 (C2), 124.24 (C7, C8), 124.60 (C4, C11), 126.23 (C5, C10), 128.00 (C6, C9), 130.61 (C3b, C11a), 135.48 (C7a, C7b). HRMS (ESI+): m/z calcd for C32H24Fe2 (M+), 520.0577; found, 520.0555 Anal. Calcd for C32H24Fe2: C, 73.88; H, 4.65. Found: C, 74.02; H, 4.66.

calculated at idealized positions and refined using a riding model. The positions of the H atoms in (2-ferrocenyl)cyclopenta[l]phenanthrene were in part recovered from a difference Fourier map (including those on C1) and in part calculated. Subsequently, all H atoms were refined using a riding model. Crystal data and structure refinement parameters for 1a,b are reported in the Supporting Information (Table S1). CCDC-951720 and 951721 contain supplementary crystallographic data for this paper. These data can be obtained from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. HRMS spectra were obtained using an ESI-TOF Mariner 5220 (Applied Biosystem) mass spectrometer with direct injection of the sample and collecting data in the positive mode. 1 H and 13C NMR spectra were obtained on a Bruker Avance DRX spectrometer operating at 400.13 and 100.61 MHz, respectively (T = 298 K). The assignments of the proton resonances were performed by standard chemical shift correlation and 2D (NOESY and COSY) experiments. The 13C resonances were assigned through 2Dheterocorrelated COSY experiments (HMQC33 for the H-bonded carbon atoms, HMBC33 for the quaternary carbons). CV experiments were performed in an airtight three-electrode cell connected to a vacuum/argon line. The reference electrode was a SCE (Tacussel ECS C10) separated from the solution by a bridge compartment filled with the same solvent/supporting electrolyte solution used in the cell. The counter electrode was a platinum spiral with ca. 1 cm2 apparent surface area. The working electrodes were disks obtained from a cross section of gold wires of different diameters (0.5, 0.125, and 0.025 mm) sealed in glass. Between successive CV scans the working electrodes were polished on alumina according to standard procedures and sonicated before use. An EG&G PAR-175 signal generator was used. The currents and potentials were recorded on a Lecroy 9310L oscilloscope. The potentiostat was home built with a positive feedback loop for compensation of the ohmic drop.34 The measurements were conducted in an airtight three-electrode cell, the same used for the CV experiments. UV−vis and near-IR absorption spectra were recorded with a Varian Cary 5 spectrophotometer. Preparation of (2-ferrocenyl)cyclopenta[l]phenanthrene (1a). A solution of tert-butyllithium (6.33 mL, 1.7 M in pentane, 10.8 mmol) was added dropwise under argon to a well-stirred solution of ferrocene (2.000 g, 10.8 mmol) in dry THF (10 mL) kept at 0 °C. The mixture was stirred for 1 h, and then 1,3-dihydrocyclopenta[l]phenanthren-2-one (2.500 g, 10.8 mmol) in dry THF (10 mL) was added within 5 min. After the mixture was warmed to room temperature and stirred overnight, it was hydrolyzed with 5% aqueous HCl (2 × 10 mL) and washed with water (2 × 10 mL). The solvent was evaporated to yield a red solid. Purification of the crude product on a silica gel column (hexane/CH2Cl2, 20/1) gave 1a as an orange solid. Yield: 0.648 g, 15%. 1H NMR (400.13 MHz, acetone-d6): δ 4.15 (s, 5H, Cp), 4.22 (m, 2H, H1), 4.39 (m, 2H, Hb, Hb′), 4.80 (m, 2H, Ha, Ha′), 7.57 (m, 1H, H8), 7.61 (m, 1H, H3), 7.63 (m, 1H, H10), 7.67 (m, 1H, H7), 7.68 (m, 1H, H5), 8.08 (m, 1H, H11), 8.28 (m, 1H, H4), 8.81 (m, 1H, H9), 8.84 (m, 1H, H6). 13C NMR (100.61 MHz, acetone-d6): δ 40.62 (C1), 67.08 (Ca, Ca′), 69.62 (Cb, Cb′), 70.08 (CCp), 81.92 (Cj), 122.15 (C3), 123.98 (C6), 124.16 (C9), 124.51 (C11), 125.16 (C4), 125.43 (C8), 126.71 (C7), 126.81 (C5), 127.50 (C10), 128.02 (C7b), 129.13 (C7a), 130.18 (C11a), 131.72 (C3b), 137.26 (C11b), 141.29 (C3a), 148.27 (C2). HRMS (ESI+): m/z calcd for C27H20Fe (M+), 400.0914; found, 400.0854. Anal. Calcd for C27H20Fe: C, 81.01; H, 5.04. Found: C, 81.30; H, 5.02. Preparation of (η5-cyclopenta[l]phenanthrenyl)FeCp (1b). A solution of n-butyllithium (0.70 mL, 1.6 M in hexanes, 1.1 mmol) was added dropwise under argon to a well-stirred solution containing 200 mg of cyclopenta[l]phenanthrene (0.93 mmol) in dry THF (20 mL) at −78 °C within 5 min to form a red mixture, which was stirred for 1 h; [Fe(CO)2(Cp)I] (280 mg, 0.92 mmol) in dry THF (10 mL) was then added dropwise. The mixture was stirred overnight and the temperature raised to room temperature; the solvent was then evaporated to yield a deep red solid. Purification of the crude product on a silica gel column (hexane/CH2Cl2, 20/1) gave 1b as an orange solid. Yield: 47 mg, 15%. 1H NMR (400.13 MHz, acetone-d6): δ 3.63

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ASSOCIATED CONTENT

S Supporting Information *

CIF files and a table giving crystal data and structure refinement parameters for 1a,b and figures giving characterization data for new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Progetto di Ateneo 2008 of University of Padova (CPDA089018/08) for financial support of this work. We wish to gratefully acknowledge Dr. Barbara Biondi for ESI-MS analysis and Prof. A. Dolmella (Department of Pharmaceutical Sciences, University of Padova) for granting access to the Gemini diffractometer and for his help with X-ray data collection and processing.

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REFERENCES

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