Observation of the Strong Electronic Coupling in Near-Infrared

Article pubs.acs.org/IC

Observation of the Strong Electronic Coupling in Near-InfraredAbsorbing Tetraferrocene aza-Dipyrromethene and aza-BODIPY with Direct Ferrocene−α- and Ferrocene−β-Pyrrole Bonds: Toward Molecular Machinery with Four-Bit Information Storage Capacity Yuriy V. Zatsikha,†,‡ Cole D. Holstrom,† Kullapa Chanawanno,§ Allen J. Osinski,∥ Christopher J. Ziegler,*,∥ and Victor N. Nemykin*,†,‡ †

Department Department § Department ∥ Department ‡

of of of of

Chemistry & Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, United States Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Chemistry, University of Akron, Akron, Ohio 44325, United States

S Supporting Information *

ABSTRACT: The 1,3,7,9-tetraferrocenylazadipyrromethene (3) and the corresponding 1,3,5,7-tetraferrocene aza-BODIPY (4) were prepared via three and four synthetic steps, respectively, starting from ferrocenecarbaldehyde using the chalcone-type synthetic methodology. The novel tetra-iron compounds have ferrocene groups directly attached to both the α- and the βpyrrolic positions, and the shortest Fe−Fe distance determined by X-ray crystallography for 3 was found to be ∼6.98 Å. These new compounds were characterized by UV−vis, nuclear magnetic resonance, and high-resolution electrospray ionization mass spectrometry methods, while metal−metal couplings in these systems were probed by electro- and spectroelectrochemistry, chemical oxidations, and Mössbauer spectroscopy. Electrochemical data are suggestive of the well-separated stepwise oxidations of all four ferrocene groups in 3 and 4, while spectroelectrochemical and chemical oxidation experiments allowed for characterization of the mixed-valence forms in the target compounds. Intervalence charge-transfer band analyses indicate that the mixed-valence [3]+ and [4]+ complexes belong to the weakly coupled class II systems in the Robin−Day classification. This interpretation was further supported by Mössbauer spectroscopy in which two individual doublets for Fe(II) and Fe(III) centers were observed in room-temperature experiments for the mixed-valence [3]n+ and [4]n+ species (n = 1−3). The electronic structure, redox properties, and UV−vis spectra of new systems were correlated with Density Functional Theory (DFT) and time-dependent DFT calculations (TDDFT), which are suggestive of a ferrocene-centered highest occupied molecular orbital and chromophore-centered lowest unoccupied molecular orbital in 3 and 4 as well as predominant spin localization at the ferrocene fragment attached to the α-pyrrolic positions in [3]+ and [4]+.

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INTRODUCTION Ferrocenyl containing systems1 have been intensively studied over the past few decades because their prominent electrontransfer properties can be potentially used in molecular electronics,2 random-access molecular modules,3 redox-switchable fluorescence markers,4 and light-harvesting applications.5 The fundamental aspects of electron-transfer processes in poly(ferrocenyl)-containing platforms in which ferrocenyl (Fc) groups are directly interconnected via a metal ion or simple aromatic systems are now well-understood.7 In addition, several research groups have made significant progress toward understanding the long-range metal−metal coupling in Fccontaining porphyrins and their analogues, in which Fc fragments are directly connected to the macrocyclic core.7−10 More recently, Fc-containing boron-dipyrromethenes (BODIPYs) and aza-BODIPYs have gained substantial attention because of their interesting optical and redox properties.11,12 © XXXX American Chemical Society

Several Fc−(aza)-BODIPY and Fc−(aza)dipyrromethene donor−acceptor systems with Fc groups linked to the core via a spacer located at the α-, β-pyrrolic, or meso- position have been published during recent years.13 We also recently characterized the first diferrocenyl-containing aza-BODIPY and azadipyrromethene systems that have direct ferrocene−αpyrrole bonds.14 It was shown that these compounds have strong metal−metal coupling and two well-separated, Fccentered oxidation processes. As it was pointed out by Lindsay, Bocian, and co-workers, at least 150 mV of separation between reversible redox processes is required for implementation of the molecular systems for two- and four-bit information storage.15 Earlier-reported diferrocenyl-containing aza-BODIPY and azadipyrromethene systems with direct ferrocene-α-pyrrole Received: November 22, 2016

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DOI: 10.1021/acs.inorgchem.6b02806 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

and [4]2+ and by prolonged refluxing of chloroform suspensions in the case of cations [3]3+, [3]4+, [4]3+, and [4]4+. Computational Details. All computations were performed using Gaussian 09 software running under a Windows or UNIX OS.17 Molecular orbital contributions were compiled from single-point calculations using the QMForge program.18 In all calculations, hybrid B3LYP19 exchange correlation functional was used for all calculations. In all calculations, Wachter’s full-electron basis set for iron20 and 6311G(d) basis set for all other atoms21 were employed. Solvent effects were modeled using the polarizable continuum model (PCM) approach using DCM as a solvent.22 In all time-dependent density functional theory (TDDFT) calculations, the lowest 100 excited states were calculated to cover experimentally observed transitions in the UV−vis−near-infrared (NIR) region. X-ray Crystallography. X-ray intensity data were measured on a Bruker CCD-based diffractometer with dual Cu−Mo ImuS microfocus optics (Cu Kα radiation, λ = 1.54178 Å). Crystals were mounted on a cryoloop using Paratone oil and placed under a steam of nitrogen vapor at 100 K (Oxford Cryosystems). The detector was placed at a distance of 5.00 cm from the crystal. The data were corrected for absorption with the SADABS program. The structures were refined using the Bruker SHELXTL Software Package (Version 6.1)23 and were solved using direct methods until the final anisotropic full-matrix, least-squares refinement of F2 converged. Hydrogen atoms were assigned ideal positions and refined isotropically as riding atoms (see the CIF in the Supporting Information for details; CCDC reference number for 3: 1443408). These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre ([email protected]). Synthetic Work. 2(E)-1,3-Diferrocenylprop-2-en-1-one (1). The synthesis of compound 1 was carried out by the modified method of a previously reported procedure.24 To a solution of acetylferrocene (6.86 mmol, 1.56 g) and ferrocenecarboxaldehyde (7.55 mmol, 1.61 g) in 15 mL of dry DMF, sodium hydride (6.86 mmol, 164 mg) was added. The solution was stirred for 20 min at room temperature. Next, 10 mL of water was added, and the desired product was isolated by filtration as a red solid (yield: 77%, 2.23 g). All spectroscopic properties were identical to the previously reported characteristics.24 1,3-Diferrocenyl-4-nitrobutan-1-one (2). To the solution of compound 1 (5.26 mmol, 2.23 g) in 20 mL of nitromethane, DBU (5.26 mmol, 790 mg) was added. The solution was refluxed for 40 min. After cooling to room temperature, 10 mL of water was added, and then the desired product was precipitated from the resulting mixture by addition of methanol. The product was isolated by filtration as a yellow powder (yield: 2.10 g, 82%). 1H NMR (500 MHz, CDCl3, δ): 4.82−4.78 (m, 4H, α-Cp+CH2NO2), 4.52 (s, 2H, α-Cp), 4.17− 4.09 (m, 14H, β-Cp+CpH), 3.92−3.87 (m, 1H, CH), 3.19−3.18 (m, 2H, −COCH2−); 13C NMR (CDCl3, 125 MHz, δ): 201.5, 88.4, 80.1, 78.7, 72.6, 70.0, 69.4, 68.9, 68.3, 68.1, 67.7, 66.8, 42.9, 33.8; HRMS (ESI positive) [M + H]+ calcd for C24H23NO3Fe2, 485.0368; found, 485.0391. [5,3-Diferrocenyl-1H-pyrrol-2-yl]-[5,3-diferrocenyl-pyrrol-2-ylidene] Amine (3). To a solution of compound 2 (4.1 mmol, 2g) in 20 mL of n-BuOH, ammonium acetate was added (0.91 mol, 22g). The resulting mixture was refluxed for 12 h under an argon atmosphere. After cooling to room temperature, the resultant precipitate was filtered, and the product was purified by flash column chromatography on neutral alumina using toluene as the solvent. Yield 19% (340 mg). 1 H NMR (500 MHz, CDCl3, δ): 6.57 (s, 2H, β-pyrr), 5.22 (t, J = 1.6 Hz, 4H, α-Cp1), 4.85 (t, J = 1.6 Hz, 4H, α-Cp2), 4.58 (t, J = 1.6 Hz, 4H, β-Cp1), 4.50 (t, J = 1.6 Hz, 4H, β-Cp2), 4.22 (s, 10H, CpH1), 4.18 (s, 10H, CpH2); 13C NMR (CDCl3, 125 MHz, δ): 129.2, 128.4, 125.4, 112.6, 71.0, 70.4, 70.3, 69.7, 69.7, 67.6; HRMS (ESI positive) [M + H]+ calcd for C48H39N3Fe4, 882.0658; found, 882.0632. BF2 Chelate of [5,3-Diferrocenyl-1H-pyrrol-2-yl]-[5,3-diferrocenylpyrrol-2-ylidene] Amine (4). To a solution of compound 4 (200 mg, 0.277 mmol) in 10 mL of toluene, 2 mL of DIEA was added under an argon atmosphere. After 20 min, 2 mL of BF3·Et2O was then added. The resulting mixture was stirred overnight. The desired product was precipitated with methanol and filtered off as a green solid (yield: 81%,

bonds satisfy requirements for two-bit information storage because their ferrocene-centered first and second oxidation waves are separated by 340−460 mV.14 In this report, we discuss two new members of the Fc-containing aza-BODIPY and azadipyrromethene family, in which four Fc groups are directly attached to α- and β-pyrrolic positions, which support strong electronic coupling between ferrocene groups and thus have potential for four-bit information storage (Scheme 1). Scheme 1. Preparation of 3 and 4a

a

Reagents and conditions: (i) FcCOCH3, NaH−DMF, room temperature, 20 min. Yield: 77%. (ii) CH3NO2, DBU, 40 min. Yield: 82%. (iii) NH4OAc, n-BuOH, heat, 12 h. Yield: 19%. (iv) BF3· Et2O DIPEA−toluene, heat, 12 h. Yield: 81%.

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EXPERIMENTAL SECTION

Materials. All commercial reagents were ACS grade and were used without further purification. Neutral alumina and silica gel were purchased from Sorbent Technologies. The tetrabutylammonium tetrakis(pentafluorophenyl)borate (TFAB) electrolyte was prepared according to literature procedures.16 All synthetic procedures were performed under a dry argon atmosphere. Instrumentation. UV−vis data were obtained on Jasco V-670 spectrophotometer. Electrochemical measurements were conducted using a CH-620 electrochemical analyzer utilizing a three-electrode scheme with platinum working, auxiliary, and Ag/AgCl pseudoreference electrodes in a 0.05 M solution of tetrabutylammonium tetrakis(pentafluorophenyl)borate (TFAB) in DCM or 0.1 M solution of tetrabuylammonium perchlorate (TBAP) in DCM with redox potentials corrected using an internal standard, decamethylferrocene, FcH*, in all cases. The redox potentials were then corrected to ferrocene using appropriate oxidation potentials for FcH*/FcH*+ versus FcH/FcH+ in the DCM−0.05 M TFAB or DCM−0.1 M TBAP system. Estimated accuracy for measured potentials was ±5 mV in all cases. Spectroelectrochemical data were collected using a homemade 1 mm cell equipped with a platinum mesh working electrode in 0.15 M solution of TFAB in DCM. NMR spectra were recorded on a Varian INOVA instrument with a 500 MHz frequency for protons and 125 MHz frequency for carbons. Chemical shifts are reported in parts per million (ppm) and referenced to tetramethylsilane (Si(CH3)4) as an internal standard. High-resolution ESI mass spectra were recorded for compounds 2−4 using a Bruker micrOTOFQIII in THF solutions. All Mössbauer spectra were collected at the room temperature using SEE instrument with 50mCi 57Co source. All isomer shifts are given relative to α-Fe at room temperature. Stepwise oxidations of 3 and 4 were carried out by addition of DDQ in chloroform solution at room temperature for cations [3]+, [3]2+, [4]+, B


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Inorganic Chemistry 174 mg). 1H NMR (500 MHz, CDCl3, δ): 6.67 (s, 2H, β-pyrr), 5.33 (s, 4H, α-Cp1), 5.23 (s, 4H, α-Cp2), 4.74 (s, 4H, β-Cp1), 4.62 (s, 4H, β-Cp2), 4.23 (s, 10H, CpH1), 4.20 (s, 10H, α-Cp2); 13C NMR (CDCl3, 125 MHz, δ): 132.5, 131.0, 128.9, 115.5, 74.6, 72.9, 71.2, 70.8, 70.7, 69.3, 68.3; 19F NMR (CDCl3, δ): 136.1 (q, J = 136.1 Hz); HRMS (ESI positive) [M + H]+ calcd for C48H38N3Fe4B1F2, 929.0471; found, 929.0480.

Similar to the previously reported diferrocene aza-BODIPY system with direct Fc-to-α-pyrrolic bonds,14 compounds 3 and 4 have two intense bands in the Vis-NIR region with a relatively narrow higher energy band and broader, more-intense lowerenergy bands (Figure 2). These bands in aza-BODIPY 4 (599

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RESULTS AND DISCUSSION Synthesis, Spectroscopy, and X-ray Crystallography. Similar to the previously reported14 diferrocenyl-containing aza-BODIPY, diferrocenyl-containing chalcone 1, prepared by an aldol condensation between ferrocenecarbaldehyde and acetylferrocene, was the initial precursor to 3. This chalcone 1 was further modified via reaction with nitromethane to afford intermediate 2. Compound 2 was reacted with ammonium acetate to produce the tetraferrocenyl-containing azadipyrromethene 3. Finally, aza-BODIPY 4 was prepared by the reaction of 3 and BF3·Et2O under basic conditions (Scheme 1). Structures of the target compounds 3 and 4 were confirmed by the 1H and 13C NMR, high-resolution ESI mass spectrometry (Figures S1−S4), and UV−vis spectroscopy. The X-ray crystal structure of 3 is shown in Figure 1 and correlates well with the previously published X-ray structures of

Figure 2. Experimental and PCM−TDDFT-predicted UV−vis−NIR spectra of 3 and 4 in DCM.

and 837 nm) were detected at considerably lower energies compared to those in 3 (535 and 697 nm) and are very close to the energies of similar bands observed in the diferrocene analogues of 3 and 4.14 Steady-state fluorescence spectra of 3 and 4 show full quenching in both compounds and agree well with the usual quenching mechanism in Fc-containing complexes, which implies electron-transfer from the Fe(II) center of ferrocene to the photoexcited aza-BODIPY or azadipyrromethene fragment.26 Mössbauer spectra of 3 and 4 consist of a single doublet that is characteristic of the low-spin Fe(II) ferrocene centers. Redox Properties. The redox properties of compounds 3 and 4 were investigated using electrochemical CV and DPV methods in DCM/0.05 M TFAB and DCM/0.1 M TBAP systems (Figures 3, S5, and Table S1). In the case of electrochemical experiments in DCM/0.05 M TFAB system, four well-separated reversible oxidation waves associated with individual Fc groups were observed for both complexes, which is indicative of their electronic coupling.27 The differences between the first and the second oxidations in DCM/0.05 M TFAB are 280 and 160 mV for 3 and 4, respectively, which is significantly smaller than that for diferrocene analogues of 3

Figure 1. X-ray crystal structure of 3 with 35% thermal ellipsoids. All hydrogen atoms except NH are omitted for clarity.

diferrocenyl azadipyrromethene and aza-BODIPY analogues.14,25 In particular, the ferrocene substituents at the αand β-pyrrolic positions in 3 are not coplanar (∼11.1−24.1°) with the azadipyrromethene plane. A pair of Fc substituents at the α-pyrrolic positions and two Fc groups at the β-pyrrolic positions were found in an anti- conformation. A pair of Fc substituents located at the α- and β-pyrrolic positions of the same pyrrole heterocycle were found in a syn conformation. The crystallographic Fe−Fe distances in complex 3 are ∼6.98 Å for Fc substituents at α-and β-pyrrolic positions in the same pyrrole heterocycle, 8.107(8) Å for adjacent Fc groups at different pyrrole heterocycles, and 10.832(9) Å for opposite Fc groups at different pyrrole heterocycles. Thus, one might expect that the geometric arrangement of the Fc substituents in 3 facilitates electronic communication between all four iron centers in this compound. C


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Inorganic Chemistry

Figure 3. Room-temperature CV and DPV data on 3 (top) and 4 (bottom) in DCM/0.05 M TFAB system.

Figure 4. Room-temperature spectroelectrochemical oxidation of 3 to [3]+ (A), [3]2+ (B), [3]3+ (C), and [3]4+ (D) in DCM/0.15 M TFAB system.

(340 mV) and 4 (460 mV).14 Nevertheless, these values are indicative of a quite high (4.65 × 104 for 3 and 5.08 × 102 for 4) electrochemical comproportionation constant Kc. The total span for Fc groups oxidation potentials for 3 and 4 are 530 and 780 mV, respectively, and similar spans were observed in the case of tetra(ferrocenyl)-containing pyrroles, thiophenes, and furans.28 The separation between each individual oxidation wave in complex 3 (280, 100, and 150 mV for [3]+ → [3]2+, [3]2+ → [3]3+, and [3]3+ → [3]4+ processes, respectively) is not sufficient for potential application of this compound in molecular-based four-bit information storage modules. However, because of its symmetric nature and easiness of preparation, as well as the large separation between individual oxidation waves (160, 260, and 360 mV for for [4]+ → [4]2+, [4]2+ → [4]3+, and [4]3+ → [4]4+ processes, respectively), complex 4 can be viewed as a good candidate for application in a molecular-based four bit information memory module. No oxidations of the aza-BODIPY or azadipyrromethene core were detected in 3 and 4 within the electrochemical window, while a quasi-reversible reduction was observed at −1.31 V for azaBODIPY 4. Similar to the electrochemical data observed in DCM/0.05 M TFAB system, oxidation waves for 3 and 4 in “standard” DCM/0.1 M TBAP solution are also well-separated although (because of higher degree of ion pairing) to a lesser extent (Figure S5). The spectroscopic signatures of the redox-active [3]n+ and [4]n+ species in solution were obtained from spectroelectrochemical and chemical oxidation experiments (Figures 4, 5, S6−S9). On the basis of the previous work of the Lang group, our group, and other groups on poly(ferrocenyl)-containing heterocycles,6,14,28,29 one might expect the appearance of a broad and intense intervalence charge-transfer (IVCT) band in the NIR region if the first oxidation is centered at the Fc group connected to the α-pyrrolic position, while relatively narrow and less-intense IVCT band is expected if the first oxidation localized at the Fc group connected to the β-pyrrolic position. In the case of complexes 3 and 4, spectroelectrochemical experiments reveal four transformations upon stepwise change of the oxidation potential (Figure 4 and 5). During the stepwise oxidation of 4 to [4]+ and [4]2+, the two most intense bands at 600 and 841 nm reduce in intensities and undergo a low-energy

Figure 5. Room-temperature spectroelectrochemical oxidation of 4 to [4]+ (A), [4]2+ (B), [4]3+ (C), and [4]4+ (D) in DCM/0.15 M TFAB system.

shift to 616 and 912 nm, respectively (Figure 5A,B), while two new broad absorption bands at ∼1150 and ∼2600 nm appear in the spectra of [4]+ and [4]2+. The broad NIR band centered at ∼2600 nm is quite typical for the mixed-valence α-ferrocenyl pyrroles6,28,29 and aza-BODIPY.14 During the third oxidation to [4]3+, IVCT band at ∼2600 nm partially lost its intensity, and a new broad band at ∼1525 nm appears in the spectrum (Figure 5C). A similar decrease of the low-energy IVCT band and growth of the high-energy NIR band was observed in tetraferrocene furans28a and pyrrole.29b In addition, the NIR bands of [4]2+ at 912 and ∼1150 nm and the band at 616 nm undergo high-energy shifts to 830, ∼ 1050, and 600 nm, respectively, upon formation of [4]3+. Finally, during the fourth oxidation transformation, all NIR bands disappear from the spectrum, which is indicative of formation of the [4]4+ complex (Figure 5D). In addition, the appearance of two new bands at 674 and 776 nm was also observed during formation of [4]4+. Reduction of [4]4+ under spectroelectrochemical conditions D


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Inorganic Chemistry results in the formation of starting complex 4 (Figure S9). We also found that the mixed-valence [4]+ − [4]3+ can be formed upon chemical oxidation of 4 with Fe(ClO4)3 or “magic blue” oxidants (Figures S6 and S7). In both cases, spectroscopic UV−vis-NIR signatures of the mixed-valence species are very close to those observed during spectroelectrochemical experiments. Very similar behavior was also observed for 3 → [3]+ → [3]2+ → [3]3+ → [3]4+ transformation under spectroelectrochemical or chemical oxidation conditions (Figures 4 and S8). In particular, growth of the NIR IVCT band at ∼2600 nm upon formation of [3]+ and [3]2+ as well as formation of the higher-energy NIR band during [3]2+ → [3]3+ transformation was clearly documented upon stepwise oxidation of 3 under spectroelectrochemical and chemical oxidation conditions. Because the NIR band in the mixed-valence complexes [3]+ and [4]+ is very broad, we used band deconvolution analysis to elucidate the IVCT transitions in these compounds (Figures 6

Figure 7. Mössbauer spectra of 4 (A), [4]+ (B), [4]2+ (C), and [4]3+ (D).

of DDQ in a suspension of [4]3+. This protocol results in a Mössbauer spectrum of [4]4+ in which no Fe(II) doublet was observed. However, the fwhm value for a doublet in [4]4+ is quite large, which can be attributed to the presence of several Fe(III) sites in slightly nonequivalent environment.32 Although experimental Mössbauer spectrum of [4]4+ can be easily deconvoluted with two doublets, such deconvolution will be quite subjective, and thus, we prefer to keep a single Mössbauer doublet for [4]4+. The solubility of [3]n+ complexes in DCM is significantly lower compared to respective complexes [4]n+. Because of this, and because we would like to compare Mössbauer spectra of [3]n+ and [4]n+ prepared under identical conditions, the reaction time for preparation of [3]n+ (n= 2−4) species was much longer than for the corresponding [4]n+ compounds. Thus, it is not surprising that only [3]+ species had a narrow Mössbauer doublet associated with the Fe(III) sites, while significant broadening was observed for Mössbauer doublets associated with the Fe(III) centers in [3]n+ (n = 2− 4) compounds (Figure 8). DFT and TDDFT Calculations. Relative energies of the individual atropisomers of the neutral 3 and 4 were estimated by DFT calculations, which indicated that the energy differences for atropisomers of 3 and 4 are quite small (up to 0.6 and 0.3 kcal/mol, respectively). As a result, DFT and TDDFT calculations indicate that the electronic structures and the vertical excitation energies for all studied atropisomers of 3 and 4 are very close to each other. DFT-predicted electronic structures and TDDFT-predicted vertical excitation energies for complexes 3 and 4 correlate very well with the experimental results (Figures 2, 9, and S12−S15). The DFT-predicted HOMO−HOMO-23 MOs in complexes 3 and 4 are predominantly Fc-centered although the aza-BODIPY or azadipyrromethene π-system contributes ∼42−47% to the HOMO and also has a significant contribution to HOMO-8− HOMO-10 MOs. The predominant Fc-centered character of the HOMO is in agreement with electrochemical and spectroelectrochemical data, which indicate that the first oxidation process is localized at the Fc group. The contribution from the Fc groups at α-pyrrolic positions into HOMO is the largest, which indicates that the first oxidation in 3 and 4 likely is centered on one of these fragments. The LUMO in 3 and 4 is well-separated in energy from the LUMO+1 and is an chromophore core-centered π-orbital. Similar to other aza-

Figure 6. IVCT band deconvolution analysis for [3]+ (top) and [4]+ (bottom) generated under spectroelectrochemical oxidation in the DCM/0.15 M TFAB system.

and S10). The IVCT band parameters in complexes [3]+ and [4]+ were found in the typical range for weakly coupled class II (in the Robin-Day classification)30 mixed-valence compounds (Table S2). Because of the broadness of the NIR absorption in [3]+ and [4]+, a large potential uncertainty in the band deconvolution analysis is expected, and thus, IVCT band parameters should be treated with some caution. Nevertheless, deconvolution analysis data correlate well with Mössbauer spectroscopy on [3]n+ and [4]n+ species (Figures 7, 8, and S11 and Table S3). The observation of two clear doublets for the Fe(II) and Fe(III) centers in the mixed-valence species is indicative of the slow (on the Mössbauer time scale, ∼ 10−7 s) electron-transfer rates and support their class II assignment. Moreover, in the case of [4]n+, a quadrupole splitting of the doublet associated with Fe(II) center(s) slightly reduces, which is indicative of somewhat stronger electronic coupling in [4]n+ species compared to [3]n+.31 In general, the fwhm values of lines for the Mössbauer doublet, which corresponds to Fe(III) center(s) in [4]n+ (n = 1−3) are very close to those observed for Fe(II) center(s) (Figure 7). During the oxidation of [4]3+ to [4]4+, we had to overcome poor solubility of [4]3+ in DCM solvent and thus used prolonged (∼48 h) refluxing of solution E


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Inorganic Chemistry

Figure 8. Mössbauer spectra of 3 (A), [3]+ (B), [3]2+ (C), and [3]3+ (D).

the HOMO-8 to the LUMO (Tables S4 and S5). Both the HOMO and the HOMO-8 have substantial contribution from the aza-BODIPY and azadipyrromethene π-system (Figures S12 and S13) and thus assume a significant π−π* character in the excited states 1 and 9. In contrast, the predominant MLCT excited states in 3 and 4, which originate from almost pure Fccentered HOMO-1−HOMO-7 to LUMO, have low TDDFTpredicted intensity. In general, TDDFT calculated UV−vis-NIR spectra of 3 and 4 are in very good agreement with corresponding experimental data in both the predicted excited-state energies and intensities (Figure 2). DFT calculations on [3]+ and [4]+ mixed-valence cations are indicative of the predominant spin-localization at the Fc fragment connected to the α-pyrrolic position (Figure 10) supporting the tentative assignment on the basis of the IVCT band profile. In agreement with spin-localization results, the βLUMO in [3]+ and [4]+ is dominated by contribution from one of the Fc fragments connected to the α-pyrrolic position (Figures 11 and S16). DFT-predicted spin-polarization for the SOMO in [3]+ and [4]+ is quite large (α-HOMO-4−β-LUMO and α-HOMO-3−β-LUMO, respectively). Again, both αLUMO and β-LUMO in [3]+ and [4]+ have significant contribution from the meso-nitrogen atoms, while α- and βpyrrolic carbons contribute into both α-HOMO and β-HOMO.

Figure 9. DFT−PCM-predicted frontier orbital energies for the most stable atropisomers of complexes 3 and 4 with images of the frontier MOs.

BODIPY compounds,32 the LUMO in 3 and 4 has significant contribution from the meso-nitrogen atom, while contribution from the α- and β-pyrrole carbon atoms of aza-BODIPY core dominate for the HOMO. The DFT-predicted electronic structure of 3 and 4 should facilitate numerous low-energy Fcto-aromatic core MLCT transitions. The two most-intense bands predicted by TDDFT calculations in the Vis-NIR region for 3 and 4 originate from the excited states 1 and 9, which are dominated by two singleelectron excitations from the HOMO to the LUMO and from

Figure 10. DFT−PCM-predicted spin densities for cations [4]+ (left) and [3]+ (right). All hydrogen atoms are omitted for clarity. F


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Inorganic Chemistry

each other as evident from electrochemical, spectroelectrochemical and chemical oxidation data. The spectroscopic signatures of the mixed-valence [3]+ and [4]+ species were obtained and analyzed on the basis of Hush theory. IVCT band analysis indicates that the mixed-valence [3]+ and [4]+ complexes belong to the class II compounds in Robin−Day classification. This assignment correlates well with Mössbauer data, in which two individual doublets were observed for the mixed-valence [3]n+ and [4]n+ (n = 1−3) species. The DFT and TDDFT predicted electronic structures and the vertical excitation energies in complexes [3]n+ and [4]n+ (n = 0 or 1) are in good agreement with the experimental data, indicating that the first oxidation is localized on the Fc fragment attached to the α-pyrrolic positions.

Figure 11. DFT−PCM-predicted frontier orbital energies for cations [3]+ and [4]+.

TDDFT calculations on [3]+ and [4]+ (Figures 12 and S17) correlate well with experimental data (only the first 80 excited

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02806. 1 H and 13C NMR, CV, and DPV spectra; UV−vis oxidative titration results; reduction of [4]4+, IVCT band deconvolution; Mössbauer spectra; DFT−PCM predicted spectra and compositions; redox properties; IVCT band and Mössbauer parameters; and TDDFT predicted characteristics. (PDF) Additional crystallographic data for target compounds. (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.J.Z.). *E-mail: [email protected] (V.N.N.). ORCID

Christopher J. Ziegler: 0000-0002-0142-5161 Victor N. Nemykin: 0000-0003-4345-0848 Notes

The authors declare no competing financial interest.

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Figure 12. Experimental (top) and TDDFT-predicted (bottom) NIR spectra of the mixed-valence [3]+ and [4]+ in DCM.

ACKNOWLEDGMENTS Generous support by the National Science Foundation (grant nos. CHE-1464711, NSF MRI-1420373, and MRI-0922366), the Minnesota Supercomputing Institute, and the University of Manitoba and Canada WestGrid Supercomputing Facility to V.N.N. is greatly appreciated.

states were calculated by taking into consideration size of the molecules and unrestricted nature of the wave functions). TDDFT predicts four excited states with low intensities between 1200 and 3500 nm for both [3]+ and [4]+. In both cases, excited state 2 originates from two major single electron excitations (β-HOMO → β-LUMO and α-HOMO → αLUMO) and can be assigned as the IVCT transition. The energies of the TDDFT-predicted IVCT transition (5190 cm−1 for [3]+ and 5230 cm−1 for [4]+) correlate well with the estimated by band deconvolution analysis energies of IVCT band in [3]+ (4800 cm−1) and [4]+ (4350 cm−1). In addition, TDDFT predicts a large number (>70) of excited states between 500 and 1200 nm, which again correlate well with the experimental data on [3]+ and [4]+. The most intense bands in this spectral range are associated with MLCT character.

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CONCLUSIONS In conclusion, we have prepared and investigated the first azadipyrromethene and aza-BODIPY compounds with four Fc groups directly linked to both α- and β-pyrrolic positions. The Fc substituents in these complexes are electronically coupled to G


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