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Unsymmetrically Substituted 1,1′-Biferrocenylenes Maintain Class III Mixed-Valence Character Rochus Breuer and Michael Schmittel* Center of Micro- and Nanochemistry and Engineering, Organische Chemie I, Universität Siegen, Adolf-Reichwein-Straße 2, D-57068 Siegen, Germany S Supporting Information *

ABSTRACT: UV−vis−near-IR electronic absorption spectra of unsymmetrically substituted derivatives of 1,1′-biferrocenylene (bis(fulvalene)diiron = BFD) have been systematically evaluated upon oxidation using spectroelectrochemical techniques. Upon reversible electrochemical oxidation to the BFD+ monocation and BFD2+ dication state, the characteristic intervalence chargeresonance (IVCR) transitions of a class III mixed-valence system in solution state was confirmed. The class III mixed-valence character of the parent BFD+ system was not altered upon unsymmetric substitution. Peak-fit analysis and TD-DFT calculations were utilized to assign the observed IVCR transitions.

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INTRODUCTION Due to the occurrence of two fundamentally different mechanisms of electron transfer (ET) in chemistry1 and biology,2 a distinction between inner- and outer-sphere ET has been made. Inner-sphere ET was first recognized in the famous Creutz−Taube complex3 [(NH3)5RuNC4H4NRu(NH3)5]5+, a seminal discovery that has paved the way to the identification and theoretical description of mixed-valence (MV) systems.4 In contrast to organic MV systems,1b bimetallic MV systems provide a well-defined distance between the redox centers and thus have served as ideal model compounds to study innersphere ET processes.1f,j,5 In recent years, ferrocene (Fc)-based MV compounds have found increasing utility in various fields of material science due to the robust electrochemical reversibility of the FeII/FeIII redox couple, with promising applications available in the wide fields of sensor technology6 and molecular electronics.7 Many applications, however, require invariably defined and highly stable MV sytems. Because many of the putative MV systems have to be further functionalized for applications, the effect of substitution on the MV character is of fundamental importance. In this regard, the singly bridged biferrocene8 FcII−FcII (1; Scheme 1) represents one of the most extensively investigated bimetallic metallocenes, with a large diversity of symmetrically9 and unsymmetrically10 substituted derivatives. Smooth transitions between charge-localized and -delocalized MV states have been observed, depending on the temperature and degree of substitution. For instance, the singly bridged biferrocene monocation (FcII−FcIII) is defined as an unsteady MV system fluctuating over a wide range of ET rates near the class II−class III borderline of the Robin−Day classification.11 Irregularities of that kind may entail crucial drawbacks in the quest for © 2013 American Chemical Society

Scheme 1. Mixed-Valence Metallocene Monocations

consistent, robust, and unperturbed MV properties that are indispensable for reliable technological applications. In this paper, we report about the MV class III character of monosubstitited derivatives of 1,1′-biferrocenylene12 (2), a doubly bridged metallocene (FcIIFcII) that is also called bis(fulvalene)diiron (BFD). In contrast to the abundant information on ferrocenes and biferrocenes, the knowledge of BFD and its derivatives is rather limited, although the BFD0/ BFD+ redox couple exhibits a more than 1000-fold higher stability in plain aqueous media in comparison to that of the Fc0/Fc+ redox standard. Furthermore, BFD-terminated selfassembled monolayers have shown their utility as redoxSpecial Issue: Ferrocene - Beauty and Function Received: June 4, 2013 Published: September 5, 2013 5980

dx.doi.org/10.1021/om400502e | Organometallics 2013, 32, 5980−5987

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the solubility of α isomers in organic solvents proved to be superior to that of the corresponding β isomers. Spectroelectrochemistry of BFD Derivatives 3a,b− 11a,b. UV−vis−near-IR absorption spectra of the alkanoyland aroyl-BFDs 3a,b−7a,b and of the corresponding alkyl- and arylmethyl-substituted BFDs 8a,b−11a,b were recorded in dichloromethane. Spectra of both the monocation and dication of alkanoyl-BFDs 4a,b are shown in Figure 1. Upon anodic

switchable surfaces under strongly corrosive conditions13 and for detection of heparin.14 Introduced in 1969, many different techniques, such as 57Fe Mössbauer,15 ESR,15e,f,16 photoelectron,15f,17 and UV−vis− near-IR spectroscopy,15c,e,f,16b,18 as well as magnetic susceptibility15a,e,f,16a,19 and electrochemistry,16a,b,20 have uniformly shown the stable and temperature-independent MV class III character of the parent BFD+. Over a wide range of experimental time scales the positive charge has been proven to be equally delocalized over both redox centers of the BFD+ core (FcII1/2FcII1/2). To the best of our knowledge, BFD+ represents the solitary example of a biferrocenyl metallocene21 deploying a steadfast MV class III character at all tested exterior conditions. To date, only one publication has reported on experimental results with regard to the electronic structure of a substituted BFD+ cation, actually 3b+.22 Unfortunately, the latter results were based on an incorrect structural assignment of constitutional isomers. Instead of the claimed isomer 3acetyl-BFD (3b), in fact 2-acetyl-BFD (3a) was investigated, as demonstrated later by 2D-NMR techniques.23 Obviously, even small changes caused by substituents11b and chirality11a,24 may lead to drastic differences in electron transfer rates of diiron MV compounds. Hence, we decided to elucidate the implications of unsymmetric substitution at BFD by performing spectroelectrochemical measurements on a series of monosubstituted BFD derivatives and their oxidized congeners.

Figure 1. UV−vis−near-IR spectra of 4a (neutral, red −··−; monocation, red ; dication, red - - -) and 4b (neutral, black −··−; monocation, black ; dication, black - - -), both in dichloromethane at 298 K.

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RESULTS AND DISCUSSION Synthesis and Characterization of BFD Derivatives 3a,b−11a,b. The preparation of the acyl-BFD derivatives 3a,b−7a,b and the respective alkyl and benzylmethyl derivatives 8a,b−11a,b have been published recently.23 Biferrocene 1 was synthesized according to a literature procedure.25 Monosubstitution at BFD (Scheme 2) leads to the constitutional α isomers 3a−11a (substitution at the 2-

generation of the monocation the characteristic intervalence charge resonance (IVCR)26,27 transitions18c,e of an MV class III system appear in the region between 4000 and 12000 cm−1 (Table 1). The absorption maxima for 4a,b+ are located at 7400 cm−1 (ε = 1900 M−1 cm−1) and 6900 cm−1 (ε = 2400 M−1 cm−1), respectively. The differences in band energy and intensity between both isomeric acyl derivatives 4a,b+ are obvious. In contrast, the corresponding alkyl derivatives 9a+ (6300 cm−1, ε = 2100 M−1 cm−1) and 9b+ (6600 cm−1, ε = 2300 M−1 cm−1) exhibit IVCR bands very similar in band shape and position to those of the parent BFD+PF6− (Figure 2). The corresponding results of the acetyl-substituted 3a,b+ and ethylfunctionalized 8a,b+ as reported in Table 1 confirm the above observations. IVCR bands of BFD+ apparently exhibit their spectral position and shape almost invariant of alkyl vs H exchange in positions 2 and 3. In contrast, in acyl derivatives an influence of the adjacent carbonyl group on the electronic structure of the BFD+ cation is clearly visible and even the substitution pattern matters. More insight will be provided below using deconvolution peak-fit analysis of the different IVCR bands. For a complete picture, the corresponding dications 4a,b2+, 8a,b2+, and 9a,b2+ have also been prepared by anodic oxidation. As expected for a prototypical BFD2+ dication, the IVCR bands vanish completely and thus the mixed-valence class III character is annihilated. Simultaneously, a new and intense band at 21100−21300 cm−1 (ε = 4500−4900 M−1 cm−1) emerges (Table 1). In the case of acetyl substitution, the investigation of 3a,b2+ failed due to their low solubility in CH2Cl2. The insensitivity of IVCR transitions to solvent exchange is a generally accepted criterion for identification of mixed-valence class III systems.1f,28 Hence, the solvatochromic behavior of alkanoyl- and alkyl-BFDs 4a+ and 9a+ was probed by their UV−

Scheme 2. Investigated BFD Derivatives 3a,b−11a,b

position) and β isomers 3b−11b (substitution at the 3position). The clear-cut identification of their constitution was achieved by applying 1H−1H-ROESY and 1H−1H-COSY NMR techniques as described recently.23 It is worth mentioning that 5981


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Table 1. UV−Vis−Near-IR Absorption Bands of 3a,b−11a,b and BFD+PF6− in Dichloromethane (DCM) and Propylene Carbonate (PC), As Measured in a Transparent Gold-Minigrid Thin-Layer Cell UV−vis−near-IR absorption/cm−1 (ε /M−1 cm−1) neutral entry BFD PF6− BFD+PF6−

DCM

monocation PC

+

3a 3b 4a 4a (in PC) 4b 5a 5b 6a 6b 7a 7b 8a 8b 9a 9a (in PC) 9b 10a 10b 11a 11b

(in PC) 21186 (548) 21186 (547) 21186 (547) 21186 (547) 21186 (542) 21368 (620) 20833 (928) 21277 (787) 20833 (1148) 20492 (727) 20576 (792) 21645 (291) 21645 (302) 21645 (296) 21645 (296) 21645 (298) 21645 (304) 21552 (314) 21739 (308) 21645 (320)

21186 (528) 21186 (518) 21186 (539) 21186 (539) 21186 (521) 21186 (645) 20833 (921) 20747 (815) 20833 (1149) 20492 (755) 20576 (784) 21645 (283) 21645 (291) 21645 (291) 21645 (291) 21645 (285) 21739 (292) 21645 (302) 21739 (297) 21645 (311)

IVCR 6500 6600 7400 6900 7400 7500 6900 6700 6900 7000 7000 7300 6700 6300 6600 6300 6400 6600 6300 6500 6300 6400

band II

(2100) (2000) (2000) (2300) (1900) (1900) (2400) (2000) (2300) (2000) (2300) (2000) (2200) (2200) (2400) (2100) (1900) (2300) (2100) (2300) (2000) (2300)

16700 16700 17100 16500 17100 17100 16300 16800 16400 16900 16400 17000 16700 16600 16600 16600 16700 16600 16700 16700 16700 16700

(400) (400) (450) (450) (450) (450) (450) (450) (500) (500) (500) (500) (550) (450) (500) (450) (400) (450) (450) (500) (450) (450)

band III 21400 21400 21400 21100 21400 21400 21100 21400 21200 21400 21400 21200 21100 21300 21300 21300 21300 21300 21300 21300 21300 21300

(1100) (1100) (1300) (1200) (1200) (1200) (1200) (1300) (1300) (1500) (1400) (1400) (1400) (1200) (1300) (1200) (1100) (1300) (1250) (1300) (1300) (1300)

LMCT 30100 (5500) 30300 (5300) 30500 (5000) 30000 (6200) 30500 (4900) 30700 (4600) 30000 (6300) shoulder (no peak) shoulder (no peak) shoulder (no peak) shoulder (no peak) 30700 (6200) 30500 (7500) 30100 (6000) 30000 (6300) 30200 (5700) 30300 (5300) 30000 (6300) 30300 (6100) 30100 (6900) 30300 (5700) 30100 (6300)

dication

insoluble insoluble 21100 (4500) 21100 (4900) 21400 (4300) insoluble insoluble insoluble 21400 (4700) 21100 (4700) 21300 (4500) 21300 (4600) 21300 (4500) 21500 (4000) 21300 (4600) 21400 (4700) 21300 (5500) insoluble insoluble

Figure 3. UV−vis−near-IR spectra of 4a+ in CH2Cl2 () and propylene carbonate (red - - -) at 298 K.

Figure 2. UV−vis−near-IR spectra of 9a (neutral, red −··−; monocation, red ; dication, red - - -) and 9b (neutral, black −··−; monocation, black ; dication, black - - -), both in dichloromethane at 298 K.

thus further endorse the MV class III character of substituted BFDs. Analogous investigations on 3-substituted BFD derivatives 3b−11b failed in the spectroelectrochemical setup due to insufficient solubility in propylene carbonate. Alternatively, the BFD monocations were generated in a 1 cm cuvette by titrating 0.5 mM solutions of the respective BFD derivatives with aliquots (1−2 μL) of a 0.1−0.2 M triethyloxonium hexachloroantimonate solution ([O(C2H5)3][SbCl6]) and monitored by UV−vis−near-IR.30 It was found that the insensitivity toward solvent exchange of the β-BFD derivatives is similar to that of the respective α isomers, as shown in Table 2. Similarly, for the parent BFD+, the maxima of the IVCR transitions were determined at 6500 cm−1 in CH2Cl2 (ε = 2100 M−1 cm−1) and at 6600 cm−1 in propylene carbonate (ε = 2000

vis−near-IR spectra in CH2Cl2 and propylene carbonate (PC), depicted in Figures 3 and 4. The polarity parameters of the selected solvents (ETN = 0.309 (CH2Cl2) and 0.491 (propylene carbonate) are sufficiently different (e.g., ETN = 0.460 for acetonitrile) to impose detectable solvatochromic effects on the metallocene cation.29 IVCR transition energies and band intensities of 4a+ are found at 7200 cm−1 (ε = 1700 M−1 cm−1) in CH2Cl2 and at 7500 cm−1 (ε = 1900 M−1 cm−1) in propylene carbonate. The respective IVCR bands of 9a+ are located at 6200 cm−1 (ε = 1900 M−1 cm−1) in CH2Cl2 and 6300 cm−1 (ε = 1900 M−1 cm−1) in propylene carbonate (Table 2). Such small variations of the IVCR bands in different solvents suggest the absence of solvatochromic behavior and 5982


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Figure 5. Peak-fit analysis of IVCR band of α-acetyl-BFD+ (3a+) as measured in CH2Cl2 (black ), fitted curve (red - - -), and deconvoluted peaks (green −·−).

Figure 4. UV−vis−near-IR spectra of 9a+ in CH2Cl2 () and propylene carbonate (red - - -) at 298 K.

Table 2. IVCR Absorption Band Maxima of Monocations of 3a,b−11a,b in Dichloromethane (DCM) and Propylene Carbonate (PC), Generated in a Cuvette (d = 1 cm) by Titration with Triethyloxonium Hexachloroantimonate, [O(C2H5)3][SbCl6] UV−vis−near-IR absorption/cm−1 (ε/M−1 cm−1) of monocation entry 3a 3b 4a 4b 5a 5b 6a 6b 7a 7b 8a 8b 9a 9b 10a 10b 11a 11b

DCM 7200 6700 7200 6600 6400 6500 6800 6700 7200 6400 6200 6500 6200 6500 6200 6400 6200 6400

(1700) (1900) (1700) (2000) (1700) (2000) (1700) (2000) (1700) (1900) (1900) (2100) (1900) (2100) (1900) (2100) (2000) (2200)

PC 7600 7100 7500 7100 7000 7000 7300 7200 7400 6900 6300 6600 6300 6600 6400 6600 6400 6600

(1900) (2000) (1900) (2100) (1800) (2100) (1900) (2100) (1900) (2000) (1900) (2100) (1900) (2100) (1900) (2100) (2000) (2200)

Figure 6. Peak-fit analysis of IVCR band of β-acetyl-BFD+ (3b+) as measured in CH2Cl2 (black ), fitted curve (red - - -), and deconvoluted peaks (green −·−).

cm−1 (ε = 2300 M−1 cm−1) are shifted to higher energy in comparison to that of the parent BFD+PF6− at 6500 cm−1 (ε = 2100 M−1 cm−1) (Table 1). A peak-fit deconvolution analysis of BFD+PF6− based on Gauss functions (Supporting Information) allowed us to identify two intense transitions at 6200 cm−1 (ε = 1900 M−1 cm−1) and 8500 cm−1 (ε = 1400 M−1 cm−1). In contrast, the corresponding deconvolution of the IVCRs of both isomers of acetyl-BFD+ indicated only one major transition. In the case of 3b+ the most intense absorption was found at 7400 cm−1 (ε = 2200 M−1 cm−1) along with two minor transitions at 5200 cm−1 (ε = 450 M−1 cm−1) and 6100 cm−1 (ε = 500 M−1 cm−1). In the case of 3a+ the major band located at 7410 cm−1 (ε = 2000 M−1 cm−1) completely determines the IVCR maximum due to rather minor absorptions at 4900 cm−1 (ε = 200 M−1 cm−1) and 5600 cm−1 (ε = 300 M−1 cm−1). Obviously, the IVCRs of both constitutional isomers have in common one major transition around 7400 cm−1 and their experimentally observed maxima of IVCRs are shifted to lower energy depending on the intensity of some minor transitions between 5000 and 6000 cm−1. Similar observations were made in case of the trimethylbenzoyl-substituted 7a,b+, shown in the Supporting Information. Apparently, the 2- vs 3-position of the carbonyl group does not exert any influence on the major transition, only

M−1 cm−1), as shown in Figure S2 (Supporting Information). These minor differences of only 2−5% in band energy and band intensity confirm the expected MV class III character of BFD+ monocations. In contrast, the unsubstituted biferrocene cation 1+ exhibits IVCT bands at 5000 cm−1 (ε = 919 M−1 cm−1) in CH2Cl2 and 5570 cm−1 (ε = 650 M−1 cm−1) in propylene carbonate.29 Such notable variations of 11% in band energy and 41% in band intensity as seen for 1+ clearly exemplify the solvatochromic dependence of IVCT transitions in mixed-valence class II cations, in contrast to the solventindependent IVCR transitions of pure class III systems such as BFD+. Deconvolution Peak-Fit Analysis of IVCR Transitions. The IVCR transitions of 2-acetyl-BFD+ (3a+) and 3-acetylBFD+ (3b+) are depicted in Figures 5 and 6. The band maxima of 3a+ at 7400 cm−1 (ε = 2000 M−1 cm−1) and 3b+ at 6900 5983


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on the intensity of the minor transitions. A carbonyl group at the 2-position of BFD seems to weaken the intensity of the deconvoluted minor transitions more than in the corresponding 3-position. Upon replacing the carbonyl group by a methylene unit, the shape and position of the resulting IVCRs of alkyl and arylmethyl BFD+ monocations 8a,b+−11a,b+ are almost identical with those of unsubstituted BFD+. In 2-ethyl-BFD+ (8a+) the experimental IVCR maximum returns to 6300 cm−1 (ε = 2200 M−1 cm−1), while deconvolution identifies two major contributors at 5800 cm−1 (ε = 1600 M−1 cm−1) and 8000 cm−1 (ε = 1700 M−1 cm−1), respectively. In 3-ethyl-BFD+ (8b+), the experimental IVCR maximum is positioned at 6600 cm−1 (ε = 2400 M−1 cm−1) and the respective deconvolution furnishes two intense absorptions at 6000 cm−1 (ε = 1700 M−1 cm−1) and 8200 cm−1 (ε = 1800 M−1 cm−1), similar to the situation of the corresponding 2-isomer 8a+. The experimental IVCR energy gap between 2- and 3-isomers in case of alkyl substituents (8a,b, 9a,b) is reduced to 300 cm−1 and even down to 200−100 cm−1 for the benzylmethyl derivatives 10a,b and 11a,b. Interestingly, the IVCR band with the higher energy is now associated with 3-substituted BDF+. This finding is in line with electrochemical investigations on the respective derivatives, where electron-withdrawing and -donating effects of substituents attached at the 3-position exerted a larger influence on the redox potential of BFD than in the 2-position.23 In the absence of the carbonyl dipole, electronic interactions of the substituents keep dominating the influence on the IVCR of the BFD+ core. DFT Computations. In order to evaluate the peak-fit deconvolution results of the IVCR transitions, TD-DFT calculations were performed on acetyl- and ethyl-substituted BFDs 8a,b+ and 3a,b+ (Figures 7 and 8, respectively).

Figure 8. Calculated excitation energies of BFD+, 2-acetyl-BFD (3a+), and 3-acetyl-BFD (3b+) within the regime of the IVCR band.

the LANL2DZ basis set for BFD+ derivatives substituted at the 2- and 3-positions. The best agreement of calculated and measured IVCRs was found for 2-ethyl- and 3-ethyl-BFD+. In both optimized structures, the Fe−Fe distance was calculated to be 3.785 Å (2-ethyl) and 3.780 Å (3-ethyl), thus being similar to that in unsubstituted BFD+ (3.783 Å). Unaffected by the structural asymmetry in 8a,b+ the Fe−Cpcentroid distance of 1.69 Å was found to be identical within the substituted and unsubstituted ferrocene subunits (cf. BFD+, 1.67 Å). Regarding the fulvalene units, the distortion of the dihedral angle was less than 1° in 8b+ but increased to 11 and 5° for the substituted and unsubstituted fulvalene units in 8a+, respectively, due to steric repulsion. Within the regime of the IVCR band, the strongest transition was split into two contributions at 6563 cm−1 ( f = 0.0087) and 6942 cm−1 (f = 0.0100) for 2-ethylBFD+ (8a+) and 6894 cm−1 (f = 0.0042) and 7168 cm−1 (f = 0.0170) for 3-ethyl-BFD+ (8b+). Interestingly, the added oscillator strength of both contributors reaches almost the same level as that found for the strongest transition in unsubstituted BFD+ (f = 0.0206). In the deconvolution, only single transitions at 5800 and 6000 cm−1 were detected for 8a,b+. A second intense transition was calculated for 2-ethylBFD+ at 9560 cm−1 ( f = 0.0089) and for 3-ethyl-BFD+ at 9744 cm−1 (f = 0.0098), both at positions similar to those found for unsubstituted BFD+ (9741 cm−1; f = 0.0104). The shift to higher energy of 3-ethyl-BFD+ transitions in comparison to 2ethyl-BFD+ confirms the analogous deconvolution results of the respective isomers (8200 vs 8000 cm−1). Concerning acetyl-BFD+ (3a,b+), it was difficult to fully reconcile the TD-DFT calculations with the experimental IVCR absorptions. Similar to case for the ethyl derivatives 8a,b+, the Fe−Cpcentroid distance in the optimized structures of 3a,b+ remained almost unaffected by the structural asymmetry. It was calculated to be 1.69 and 1.70 Å within the unsubstituted and substituted ferrocene units, respectively. Equally, the dihedral angle was distorted less than 1° within the substituted and unsubstituted fulvalene system, as detailed in the Supporting Information. The optimized structure of 3-acetylBFD+ (3b+) showed an Fe−Fe distance of 3.770 Å with the carbonyl group in a coplanar conformation to the fulvalene unit. The analogous coplanar conformation in 2-acetyl-BFD increased the Fe−Fe distance to 3.804 Å, which is significantly longer than in unsubstituted BFD+ (3.783 Å). In an alternative minimum structure the carbonyl group was shifted out of

Figure 7. Calculated excitation energies of BFD+, 2-ethyl-BFD (8a+), and 3-ethyl-BFD (8b+) within the regime of the IVCR band.

Respective TD-DFT calculations on neutral BFD (2) and unsubstituted BFD+ (2+) have been performed in detail by Bally and Tuczek.18e Accordingly, the two intense transitions at 7125 cm−1 (f = 0.0206; 12Ag state) and 9732 cm−1 ( f = 0.0103; 22Ag state) contribute to the experimental IVCR band of unsubstituted BFD+. The relevant MOs for excitation were identified to be metal-centered. Furthermore, the Fe−Fe distance in BFD+ (3.783 Å) was shorter than in neutral BFD (4.001 Å). We were able to fully confirm these computations and subsequently utilized the same BP86 functional along with 5984


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in a previous publication.23 The atom-numbering system used for the BFD system was proposed by Goldberg and Matteson.33 Instrumental Methods. For all cyclic voltammetry measurements (EG&G 2273 Potentiostat/Galvanostat), a conventional threeelectrode setup was used (1 mm platinum-disk working electrode, a platinum-wire counter electrode, and a silver wire as pseudoreference electrode). Tetra-n-butylammonium hexafluorophosphate (TBA+PF6−) served as the supporting electrolyte, and decamethylferrocene (E1/2 = −0.54 V vs Fc in CH2Cl2) was used as the internal standard. Spectroelectrochemical measurements were performed on a Perkin-Elmer Lambda 750 UV−vis−near-IR spectrometer. Spectroelectrochemistry was carried out in transmission mode by using a thinlayer cell that has been already described in detail,34 with a transparent gold minigrid serving as working electrode. Molar extinction coefficients ε (M−1 cm−1) of the neutral BFD derivatives 3a,b− 11a,b and of the BFD+PF6− salt in dichloromethane and propylene carbonate (PC) were determined in a 1 cm cuvette and were averaged over three measurements, leading to an accuracy of at least ±0.7%. TD-DFT calculations were performed with the Gaussian 03 program package,35 using the BP86 functional in conjunction with LANL2DZ effective core potentials. Structures were fully optimized before being subjected to single-point TD-DFT excited-state calculations.

coplanarity and the Fe−Fe distance was settled at 3.778 Å. The Fe−Fe distance in 2-acetyl-BFD+ is thus larger than in the corresponding 3-isomer, in full agreement with deconvolution results, where the observed IVCR shifts to higher energies with acyl substituents in position 2. As reported earlier, the strong electronic coupling between the two iron atoms is a decisive factor in the barrierless intervalence charge resonance transfer.15e,22 Moreover, the unpaired electron is thought to be located in an orbital composed equally of both ferrocene units.18e As confirmed by the TD-DFT calculations, the SOMO involved in the IVCR transitions is metal centered for 1+, 3a,b+, and 8a,b+ (see the Supporting Information). The IR-allowed transitions closely correspond in shape and symmetry to those of the parent BFD+.18e Exemplified by the acetyl-substituted derivatives 3a,b+, the main transitions in the IVCR regime have been assigned to 6b3g → SOMO 9b2u, 10ag → SOMO and 9ag → SOMO,31 analogous to those in the unsubstituted BFD+. The respective MOs of 3a,b+ are illustrated in Table S4 (Supporting Information). From this point of view it is understandable that the observed IVCR band will respond more sensitively to conformational effects, which influence the effective Fe−Fe distance, than to the electron-withdrawing effects of the carbonyl group. It is noteworthy to reconsider the comproportionation constants (Kc) determined previously from cyclic voltammetry in light of the measured IVCR bands of 3a,b and 7a,b.23 The large Kc value of 7a (Kc = 1.1 × 1012) indicates an increased mixed-valence class III character in comparison to 7b (Kc = 1.1 × 1011), and the difference in IVCR band energies between the trimethylbenzoyl-substituted BFDs 7a,b of 600 cm−1 confirms such a tendency. On the other hand, a similar energy gap was observed from the IVCRs of acetyl-functionalized 3a+ (7400 cm−1) and 3b+ (6900 cm−1) but was not reflected in the same manner by Kc values, which were almost identical in 3a (Kc = 1.5 × 1011) and 3b (Kc = 2.3 × 1011). This discrepancy once more exemplifies that caution is necessary in drawing conclusions about the MV character just on the basis of Kc values computed from cyclic voltammetry data.32

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* Supporting Information Figures and tables giving details of the TD-DFT calculations, additional peak-fit analysis of IVCR bands of BFD+PF6− (2+), 7a,b, and 8a,b, and UV−vis−near-IR spectra of 5a,b−7a,b, 10a,b, and 11a,b and of the solvatochromic behavior of BFD+PF6− (2+). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.S.: e-mail, [email protected]; fax, (+49) 271 740 3270. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft for financial support (FOR 516) and Dr. Gilbert Nöll for help with his homemade spectroelectrochemical setup.

CONCLUSIONS We have investigated the intervalence charge resonance (IVCR) transitions of unsymmetrically substituted 1,1′biferrocenylene (BFD) by means of spectroelectrochemistry. BFD has been functionalized in positions 2 and 3 with acyl (3a,b−7a,b), alkyl (8a,b and 9a,b), and arylmethyl (10a,b and 11a,b) substituents. Peak-fit deconvolution analysis supported by TD-DFT calculations allowed us to assess the influence of substituents on the energy shift of the respective IVCR bands. Overall, the shift was more pronounced in the case of 2substitution at the BFD core. Electron-withdrawing effects of substituents seem to play a minor role. As expected for a MV class III cation, the IVCT transitions of all monosubstituted derivatives do not exhibit significant solvatochromic shifts. In conclusion, the MV class III character of BFDs seems to be robust even with unsymmetric substitution. With regard to the potential application of BFD in materials chemistry, any monofunctionalization thus seems possible without hampering the MV class III behavior at the stage of the BFD + monocations.

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ASSOCIATED CONTENT

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REFERENCES

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EXPERIMENTAL SECTION

General Considerations. The preparation of 1,1′-biferrocenylene (BFD, 2), BFD+PF6−, and derivatives 3a,b−11a,b has been described 5985


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Unsymmetrically Substituted 1,1 ... - ACS Publications

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