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Tuning Electron-Transfer Properties in 5,10,15,20-Tetra(1′hexanoylferrocenyl)porphyrins as Prospective Systems for Quantum Cellular Automata and Platforms for Four-Bit Information Storage Nathan R. Erickson,† Cole D. Holstrom,† Hannah M. Rhoda,† Gregory T. Rohde,† Yuriy V. Zatsikha,†,¶ Pierluca Galloni,‡ and Victor N. Nemykin*,†,¶ †

Department of Chemistry & Biochemistry, University of Minnesota Duluth, Duluth, Minnesota 55812, United States Department of Chemistry, University of Manitoba, Winnipeg, MB R3T 2N2, Canada ‡ Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma “Tor Vergata”, via Della Ricerca Scientifica, 00133 Rome, Italy ¶

S Supporting Information *

ABSTRACT: Metal-free (1) and zinc (2) 5,10,15,20-tetra(1′hexanoylferrocenyl)porphyrins were prepared using an acidcatalyzed tetramerization reaction between pyrrole and 1′-(1hexanoyl)ferrocencarboxaldehyde. New organometallic compounds were characterized by combination of 1H, 13C, and variable-temperature NMR, UV−vis, magnetic circular dichroism, and high-resolution electrospray ionization mass spectrometry methods. The redox properties of 1 and 2 were probed by electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation approaches coupled with UV−vis−near-IR and Mössbauer spectroscopy. Electrochemical data recorded in the dichloromethane/TBA[B(C6F5)4] system (TBA[B(C6F5)4] is a weakly coordinating tetrabutylammonium tetrakis(pentafluorophenyl)borate electrolyte) are suggestive of “1e− + 1e− + 2e−” oxidation sequence for four ferrocene groups in 1 and 2, which followed by oxidation process centered at the porphyrin core. The separation between all ferrocene-centered oxidation electrochemical waves is very large (510−660 mV). The nature of mixed-valence [1]n+ and [2]n+ (n = 1 or 2) complexes was probed by the spectroelectrochemical and chemical oxidation methods. Analysis of the intervalence charge-transfer band in [1]+ and [2]+ is suggestive of the Class II (in Robin−Day classification) behavior of all mixed-valence species, which correlate well with Mössbauer data. Density functional theory−polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were applied to correlate redox and optical properties of organometallic complexes 1 and 2 with their electronic structures.

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INTRODUCTION

charged cells can be further used to construct molecular wired and logic gates (Figure 1B,C).6,7 Ferrocene and its derivatives are well-known as extremely robust redox-active mediators, which can be used in variety of applications that require reliable electron-transfer processes. Not surprisingly, a variety of di- and tetra(ferrocenyl)containing compounds were investigated as perspective materials for QCA applications.8−10 During the last several decades, it was realized that changing the ion-pairing ability of the electrolyte and the polarity of the solvent could easily affect the separation between ferrocene-centered oxidation waves observed in electrochemical experiments.11−13 Such electrochemical behavior suggests the strong influence of Coulombic effects on the degree of interaction between ferrocene-centered sites, which is important for QCA application of these

Molecular platforms with identical, electronically coupled, redox-active substituents connected via a symmetric conducting core and capable of charge localization upon oxidation or reduction of the redox sites are of great interest as logic elements for Quantum-dot Cellular Automata (QCA) systems.1−4 QCA operation requires the formation of a specifically configured multiple-charged “cell”. Such a cell usually consists of a symmetric conductive platform with four redox-active sites, which can be transformed into a two electrons/two holes configuration capable of exchange around the cell by 180° rotation (Figure 1A). The Coulombic repulsion in these mixed-valence configurations would result in an “opposite” arrangement of the electrons/holes, and two different charge states would represent a classic “0” and “1” in a binary system (Figure 1A).5 In addition, intercellular Coulombic repulsions between localized charges in doubly © XXXX American Chemical Society

Received: February 13, 2017

A

DOI: 10.1021/acs.inorgchem.7b00397 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

centered oxidation states would encode a single bit of information (Figure 2). At least 150 mV separation between these oxidation processes was suggested as a minimum requirement for accurate write/read cycles in prospective systems.15a During the past decade, we and other research groups have studied electron-transfer properties of a large number of tetra(ferrocenyl)-containing porphyrins and their analogues as prospective platforms for a variety of applications.26−28 It was found that in the case of tetra(ferrocenyl)-containing porphyrins, even in the presence of the electrolytes with low ion-pairing ability, a good separation between the first and the second ferrocene-centered oxidation waves can be achieved, while separation between the second and third oxidation processes is rather small. One of the interesting observations found by Burrell and co-workers as well as our group was that increasing the rotational barrier for ferrocene groups results in a larger separation and better resolution for redox waves in electrochemical experiments.29−32 Following this observation, we investigated a slightly more sterically crowded (yet very synthetically inexpensive) symmetric metal-free (1) and zinc (2) 5,10,15,20-tetra(1′-hexanoylferrocenyl)porphyrin (Scheme 1). From the discussion presented below, it turns out that both of these compounds are very good candidates for both QCA and four-bit information storage applications.

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EXPERIMENTAL SECTION

Reagents and Materials. Solvents were purified using standard approaches: toluene was dried over sodium metal, tetrahydrofuran (THF) was dried over sodium−potassium alloy, and hexane and dichloromethane (DCM) were dried over calcium hydride. Silica gel (60 Å, 60−100 μm) was purchased from Dynamic Adsorbents Inc. Ferrocene carboxaldehyde, anhydrous AlCl3, hexanoyl chloride, pyrrole, and trifluoroacetic acid were purchased from commercial sources and used without further purification except pyrrole, which was freshly distilled prior to reactions. DFT-PCM and TDDFT-PCM Calculations. In all calculations, the alkyl chains in porphyrins 1 and 2 were truncated to the methyl group. All geometries were optimized using a hybrid TPSSh exchangecorrelation functional and C2 point group.33 Energy minima in optimized geometries were confirmed by the frequency calculations (absence of the imaginary frequencies). Solvent effects were calculated using the polarized continuum model (PCM).34 In all calculations, DCM was used as the solvent. In PCM time-dependent density

Figure 1. QCA-based binary logic cell (A); QCA-based molecular wire (B); QCA-based logic gate (C).

platforms, as they can utilize charge-storage and geometrical switching functions. In addition to QCA devices in which data are stored by the charge-specific configuration of a multiple-charged cell, ferrocenyl-containing systems were considered as perspective platforms capable of one-, two-, or four-bit binary code storage using well-resolved single-charge states.14−25 In the case of tetra(ferrocenyl)-containing systems, each of the ferrocene-

Figure 2. Idealized representation of prospective molecule for four-bit information storage through stepwise ferrocene oxidations. B


Article

Inorganic Chemistry Scheme 1. Synthetic Pathways for Preparation of Tetrahexanoylferrocene Porphyrins 1 and 2

functional theory (TDDFT) calculations, the first 80 states were considered. In all calculations, all atoms were modeled using 631G(d)35 basis set. Gaussian 09 software was used in all calculations.36 QMForge program was used for molecular orbital analysis.37 The first 80 excited states were considered in TDDFT calculations. Spectroscopy and Electrochemistry. A Jasco V-670 spectrophotometer was used to collect UV−vis data. OLIS DCM-17 instrument was used for magnetic circular dichroism (MCD) experiments. Electrochemical cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were conducted using a CH620 Instruments electrochemical analyzer utilizing a three-electrode scheme with platinum working, auxiliary, and Ag/AgCl pseudoreference electrodes. DCM was used as a solvent along with 0.05 M solution of tetrabutylammonium tetrakis(pentafluorophenyl)borate (TBA[B(C6F5)4]) as electrolyte to minimize ion-pairing effect in CV and DPV experiments. In all cases, experimental redox potentials are recorded versus a decamethylferrocene (Cp*2Fe) as an internal standard. Spectroelectrochemical experiments were conducted in DCM/0.15 M TBA[B(C6F5)4] system using a custom-made 1 mm cell and platinum mesh working electrode. NMR spectra were recorded on a Bruker Avance 300 MHz. Chemical shifts are reported in parts per million (ppm) and are referenced to tetramethylsilane (TMS; Si(CH3)4) as an internal standard. In all cases, final assignments of 1H and 13C signals were made using COSY spectra. High-resolution electrospray ionization (ESI) mass spectra were collected using Bruker QTof-III instrument in THF solutions. Mössbauer spectra were recorded using a SEE Co. model W302 Resonant Gamma-ray Spectrometer in constant acceleration mode. The Source was 57Co in a rhodium matrix with an initial activity of 50 mCi. The isomer shifts are referenced against α-Fe at 298 K. Fits for the Mössbauer spectra were completed with WMOSS Mossbauer fitting software.38 All oxidized samples were prepared by oxidation of porphyrins 1 and 2 with 1.2, 2.5, and 10 equiv of 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) in DCM either at room temperature or at reflux conditions. Synthesis. Synthesis of 1′-(1-Hexanoyl)ferrocencarboxaldehyde. The mixture of AlCl3 (1.12 g, 8.4 mmol) and ferrocencarboxaldehyde (1.5 g, 7 mmol) in dry DCM (45 mL) was stirred at 0 °C under argon atmosphere in a Schlenk flask. To this solution, the mixture of hexanoyl chloride (1.96 mL, 15 mmol) and AlCl3 (2.24 g, 17 mmol) in DCM (55 mL) was slowly added dropwise over a 20 min period at 0 °C. The reaction mixture was allowed to warm to room temperature and stirred under argon atmosphere for 5 h. The reaction was then quenched with ice, and the organic layer was washed with water (2 × 50 mL) and brine (50 mL), dried (Na2SO4), and concentrated in vacuo. The crude product was chromatographed on silica gel using a hexane−AcOEt (3:2 v/v) mixture as the eluent (66% yield). 1H NMR (CDCl3, 300 MHz, 25 °C): δ 0.92(t, 3JHH = 6.9 Hz, 3H, CH3), 1.37− 1.34 (m, 4H, CH2 + CH2), 1.72−1.66 (m, 2H, CH2), 2.64 (t, 3JHH = 6.9 Hz, 2H, COCH2), 4.55 (t, 3JHH = 1.5 Hz, 2H, β′-Cp), 4.59 (t, 3JHH

= 1.5 Hz, 2H, α′-Cp), 4.78 (t, 3JHH = 1.5 Hz, 2H, β-Cp), 4.85 (t, 3JHH = 1.5 Hz, 2H, α-Cp), 9.92 (s, 1H, CHO). Synthesis of Metal-Free 5,10,15,20-Tetra(1′-hexanoylferrocenyl)porphyrin (1). 1′-(1-Hexanoyl)ferrocencarboxaldehyde (700 mg, 2.2 mmol) and distilled pyrrole (150 μL, 2.2 mmol) were dissolved in 200 mL of anhydrous CH2Cl2 under argon atmosphere. After the mixture was protected from light, trifluoroacetic acid (250 μL, 3.25 mmol) was added dropwise to the reaction solution. The solution was kept stirring at room temperature for 2 h, and then p-chloranil (810 mg, 3.3 mmol) was added to the reaction mixture. After the resulting solution was stirred at room temperature for 4 h, the solvent was evaporated under reduced pressure. The crude compound was purified by column chromatography on silica gel using CH2Cl2 as the eluent to give 240 mg (29%) of 1 isolated as a green solid. UV−vis: nm (ε × 10−3 M−1 cm−1) 720 (9.4), 648 (13.7), ca. 479 (33.3), 435 (84.2). 1H NMR (CDCl3, 300 MHz, 25 °C): δ −0.63 (s, NH), 0.69 (t, 3JHH = 6.9 Hz, 12H, CH3), 1.07−0.98 (m, 16H, CH2 + CH2), 1.54−1.44 (m, 8H, CH2), 2.43 (t, 3JHH = 6.9 Hz, 8H, CH3) 4.10 (s, 8H, β′-Cp), 4.71 (s, 8H, α′-Cp), 4.80 (s, 8H, β-Cp), 5.37 (s, 8H, α-Cp), 9.59 (s, 8H, βpyrrole); 13CNMR (CDCl3, 75 MHz, TMS, 25 °C): δ 204.55 (C1), 130.1 (α-Pyrr), 128.95 (β-Pyrr), 116.09 (Cmeso), 90.59 (Cipso-Cp), 79.63 (Cipso′-Cp), 77.95 (α-Cp), 75.87 (α′-Cp), 71.43 (β-Cp), 71.17 (β′-Cp), 39.93 (C2), 31.52 (C3), 24.11 (C4), 22.41 (C5), 13.90 (C6); high-resolution mass spectrometry (HRMS; ESI positive) calcd for C84H86N4O4Fe4 [M + H]+: 1439.4132, found 1439.4109. Anal. Calcd for C84H86Fe4N4O4 C: 70.11; H: 6.02; N: 3.89; found C: 69.36; H: 6.12; N: 3.70%. Synthesis of Zinc 5,10,15,20-Tetra(1′-hexanoylferrocenyl)porphyrin (2). Metal-free 5,10,15,20-tetra(1′-hexanoylferrocenyl)porphyrin 1 (44 mg, 0.03 mmol) was combined with zinc acetate (55.3 mg, 0.3 mmol) in toluene and refluxed for 2 h while protected from light. The solvent was subsequently evaporated under reduced pressure, and a product was purified using alumina column with CH2Cl2 as eluent. Sample was recrystallized using toluene/hexane mixture (1:10) to give 22 mg of pure 2 (48% yield). UV−vis: nm (ε × 10−3 M−1 cm−1) 667 (21.1), 612 (9.4), ca. 465 (67.6), 437 (93.3). 1H NMR (CDCl3, 300 MHz, 25 °C): δ 0.58 (t, 3JHH = 7.0 Hz, 12H, CH3), 0.95−0.86 (m, 16H, CH2 + CH2), 1.41−1.31 (m, 8H, CH2), 2.31 (t, 3 JHH = 7.0 Hz, 8H, CH2), 4.23 (s, 8H, β′-Cp),4.68 (s, 8H, α′-Cp), 4.80 (s, 8H, β-Cp), 5.40 (s, 8H, α-Cp), 9.81 (s, 8H, β-pyrrole); 13C NMR (CDCl3, 75 MHz, TMS, 25 °C): δ 204.58 (C1), 149.09 (α-Pyrr), 131.96 (β-Pyrr), 116.97 (Cmeso), 91.68 (Cipso-Cp), 79.60 (Cipso′-Cp), 78.31 (α-Cp), 75.75 (α′-Cp), 71.43 (β-Cp), 71.01 (β′-Cp), 39.81 (C2), 31.50 (C3), 24.05 (C4), 22.35 (C5), 13.81 (C6); HRMS (ESI positive) calcd for C84H84N4O4Fe4Zn [M]+: 1502.3195, found 1502.3193. Anal. Calcd for C84H84Fe4N4O4Zn C: 67.16; H: 5.64; N: 3.73; found C: 66.74; H: 5.82; N: 3.81%. C


Article

Inorganic Chemistry

Table 2. Estimated Hab and α2 Values for Mixed-Valence States of Ferrocenyl-Containing Porphyrins Generated under Chemical and Electrochemical Conditions complex

[1]+ a

[1]+ b

[2]+ a

[2]+ b

ν, cm−1 ν1/2, cm−1 ε, M−1 cm−1 Hab, cm−1 ν1/2(theor), cm−1 RMM, Å Γ

10 775 1744 16 125 1157c 4989 9.755c 0.650

11 649 1677 24 642 1458c 5187 9.755c 0.676

10 739 1630 12 836 996c 4980 9.755c 0.673

11 627 1483 13 683 1021c 5182 9.755c 0.714

a

Spectroelectrochemical oxidation. bChemical oxidation. cMinimum Fe−Fe distances predicted by DFT calculations.

Figure 3. Room-temperature UV−vis and MCD data for porphyrins 1 (a) and 2 (b) in DCM.

Figure 4. CV (red line) and DPV (blue line) of porphyrins 1 and 2 obtained in DCM/0.05 M TBA[B(C6F5)4] system. * denotes Cp*2Fe as internal standard.

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Figure 5. Transformation of 1 into [1]+ (A); [1]+ into [1]2+ (B); and [1]2+ to [1]4+ (C) under spectroelectrochemical conditions in the DCM/0.15 M TBA[B(C6F5)4] system. Recorded using an optically transparent thin-layer electrode under bulk electrolysis conditions in spectroelectrochemical cell.

RESULTS AND DISCUSSION Synthesis and Characterization. The preparation of 1′(1-hexanoyl)ferrocenecarboxaldehyde was achieved by a standard acylation reaction using AlCl3 as a Lewis acid catalyst.39 We found that use of the standard Lindsey method with BF3 as the

Table 1. Redox Properties of Tetraferrocenyl Porphyrins in DCM/0.05M TBA[B(C6F5)4] System at Room Temperature Determined Using CV and DPV Dataa porphyrin 1a 2 H2TFcPb ZnTFcPc H2Fc3FcCOMePd H2Fc3FcCO(CH2)5BrPd a

Ox5

Ox4

Ox3

Ox2

Ox1

Red1

Red2

0.61 0.58 0.24 0.17 0.21 0.20

0.25 0.27 0.15 0.09 0.15 0.12

0.10 −0.08 −0.07 −0.14 −0.07 −0.07

−1.73 −2.26 −1.78 −2.03 −1.84 −1.82

−2.07

1.25 1.09

0.86 1.15 0.34 0.25 0.46 0.44

−2.06 −2.10 −2.03

All potentials are versus FcH/FcH+ couple. bReference10d. cReference27c. dReference39a. D


Article

Inorganic Chemistry

Figure 8. Mössbauer spectra of 1, [1]+, and [1]2+ (left); and 2, [2]+, and [2]2+ (right). DDQ was used as an oxidant.

Figure 6. Transformation of 2 into [2]+ (A); [2]+ into [2]2+ (B); and [2]2+ to [2]4+ (C) under spectroelectrochemical conditions in the DCM/0.15 M TBA[B(C6F5)4] system. Recorded using an optically transparent thin-layer electrode under bulk electrolysis conditions in spectroelectrochemical cell.

Figure 9. DFT-PCM predicted frontier orbital energies for the most stable atropisomers of complexes 1 (left) and 2 (right) with pictorial representation of the selected MOs.

Table 3. Room-Temperature Mössbauer Parameters for [1]n+ and [2]n+ Species (n = 0−4) doublet 1

doublet 2

compound

IS, mm/s

QS, mm/s

1 [1]+ [1]2+ [1]4+ 2 [2]+ [2]2+ [2]4+

0.45 0.44 0.44 0.41 0.44 0.44 0.44 0.41

2.23 2.17 2.15 0.72 2.26 2.18 2.20 0.66

IS, mm/s

QS, mm/s

0.37 0.39

0.69 0.66

0.41 0.41

0.70 0.75

acid-catalyzed tetramerization reaction followed by oxidation of the resulting porphyrinogen with p-chloranil was adopted for preparation of the metal-free porphyrin 1 in 29% yield. The zinc insertion reaction was conducted at the standard conditions and resulted in zinc complex 2 in 48% yield. Both porphyrins are soluble in a variety of organic solvents and stable in a solid state in ambient atmosphere. The structures of the target porphyrins were confirmed by 1H, 13C, and COSY NMR,

Figure 7. NIR band deconvolution analysis for [1]+ (left) and [2]+ (right) generated under spectroelectrochemical conditions.

catalyst in preparation of the metal-free porphyrin 1 results in a very low yield of the desired product. Thus, a trifluoroacetic E


Article

Inorganic Chemistry

Table 4. Compositions of the Frontier MOs of Organometallic Porphyrins 1 and 2 Predicted by DFT-PCM Calculationsa contribution, % E, eV

symm

nitrogen

porphyrin

−5.347 −5.261 −5.256 −5.256 −5.244 −5.232 −5.215 −5.204 -4.708 -2.813 −2.775 −1.737

a b a b a b b a a b b a

1.19 0.14 0.2 0.12 0.1 0.14 0.23 0.17 14.56 10.53 10.1 0.56

39.63 2.49 2.74 2.63 4.38 2.24 2.43 10.78 33.85 68.86 70.78 36.4

−5.268 −5.234 −5.232 −5.231 −5.198 −5.197 −5.191 −5.164 −4.653 −2.684 −2.678 −1.689

a b a b a b b a a b b a

0.49 0.12 0.19 0.17 0.03 0.22 0.19 0.55 13.91 10.26 10.29 0.47

19.33 2.32 2.75 2.89 2 2.69 2.25 42.96 35.96 68.73 68.78 30.8

Cp

Fe

RCO−

33.48 53.93 52.69 53.33 55.77 56.83 56.21 51.17 30.9 13.02 12.76 16.41

25.23 42.22 43.17 42.69 38.96 40.58 40.76 37.18 20.34 6.63 5.55 24.56

0.48 1.21 1.2 1.22 0.79 0.21 0.36 0.69 0.36 0.96 0.81 22.08

47.34 54.25 52.39 53.1 58 55.67 57.2 31.44 29.04 13.28 13.16 16.89

31.72 42.04 43.49 42.66 39.67 41.26 39.96 24.17 18.97 6.43 6.48 26.07

1.05 1.26 1.17 1.19 0.3 0.15 0.4 0.85 0.36 1.05 1.05 25.76

Zn

complex 1

complex 2

a

0.07 0 0 0 0 0 0 0.03 1.76 0.25 0.25 0.02

HOMO and LUMO are shown in bold.

corresponding spectra of H2TFcP and ZnTFcP compounds, respectively.40,41 In case of the metal-free porphyrin 1, the UV− vis spectrum is dominated by an intense Soret band at 435 nm, a shoulder at 475−485 nm, and two Q-bands of lower intensity detected at 648 and 720 nm. In the MCD spectrum of 1, centered at 439 nm, a Faraday pseudo A-term is associated with the Soret band, and two Q-bands are associated with negative and positive Faraday B-terms (665 and 725 nm), having close to the absorption-bands energies. Finally, associated with a broad shoulder in absorption spectrum, a Faraday pseudo Aterm was observed at 478 nm in the MCD spectrum of 1. The UV−vis spectrum of zinc porphyrin 2 has a much broader and more intense shoulder observed close to the Soret band. Increase in the effective symmetry of zinc porphyrin 2 can be clearly seen from its MCD spectrum, as it is dominated by three Faraday A-terms centered at 669, 433, and 493 nm and associated with the Q-band at 667 nm, Soret band at 437 nm, and a broad shoulder at 491 nm. Redox Properties. As it was mentioned in the Introduction section, QCA application of the porphyrins 1 and 2 requires the formation of a stable dication state,10 while four-bit information storage in 1 and 2 can only be achieved if four oxidation processes in these porphyrins could be clearly separated by a minimum of 150 mV.15a Thus, to evaluate the applicability of porphyrins 1 and 2 for QCA and four-bit information storage, their redox properties were studied using electrochemical CV and DPV methods. To minimize ion-pairing processes between solute and electrolyte and to improve the resolution between ferrocene-centered oxidation processes, electrochemical data were collected using DCM as a relatively low-polarity solvent and a weakly coordinating TBA[B(C6F5)4] electrolyte.11

UV−vis, and MCD spectroscopy, as well as high-resolution ESI mass spectrometry and elemental analyses. 1H NMR spectra of porphyrins 1 and 2 are typical for ferrocene-containing porphyrin signals with β-pyrrolic protons, four signals of protons associated with disubstituted ferrocene fragments, and five multiplets associated with the side-chain protons in alkyl groups (Supporting Information Figures S1 and S2). In addition, metal-free porphyrin 1 has a NH protons signal at −0.63 ppm, which is similar to that observed in H2TFcP (−0.5 ppm; TFcP = 5,10,15,20-tetraferroceneporphyrin dianion)40 and indicative of nonplanarity of their porphyrin cores. The presence of the side-chain carbonyl carbon atom in porphyrins 1 and 2 was confirmed by the 13C NMR spectroscopy (Supporting Information Figures S3 and S4), while their composition was supported by HRMS (Supporting Information Figure S5). An influence of the midsize acyl substituent on the rotational barrier of ferrocene group was studied by the variable-temperature NMR spectroscopy. In case of the metalfree porphyrin 1, the first coalescence point for the β-pyrrolic protons was observed at ca. −50 °C, which is a slightly higher temperature compared to H2TFcP analogue (ca. −60 °C),40 suggesting a rather minor, although clear influence of the side chain on the dynamic processes in 1 (Supporting Information Figure S6). The observation of the dynamic processes in zinc porphyrin 2 was hindered by a fast exchange of the axially coordinated THF solvent molecule at high temperatures and slow exchange of the axial ligand at low temperatures. UV−vis and MCD spectra of porphyrins 1 and 2 are shown in Figure 3 with numerical peak positions and intensities provided in the Experimental Section. The spectra of porphyrins 1 and 2 are close to the earlier published F


Article

Inorganic Chemistry

In the case of the symmetric and asymmetric tetra(ferrocenyl)-containing porphyrins studied so far, a very similar picture for the oxidation of ferrocene groups was observed in the DCM/0.05 M TBA[B(C6F5)4] system; that is, all four stepwise single-electron oxidation waves (“1e− + 1e− + 1e− + 1e−” sequence) can be clearly seen in CV and DPV experiments with the separation between the first and the second and third and fourth waves being larger than the separation between the second and the third oxidation waves.10d,27a A close proximity between the second and the third oxidation potentials hinders a potential application of these porphyrins in QCA. In contrast to the earlier observations, the quasi-reversible oxidation processes in symmetric porphyrins 1 and 2 (in the same DCM/0.05 M TBA[B(C6F5)4] system) reveal only three oxidation waves for ferrocene oxidation in 1e− + 1e− + 2e− sequence followed by a quasi-reversible single-electron oxidation of the porphyrin core (Figure 4). The 1e− + 1e− + 2e− sequence is unprecedented for tetra(ferrocenyl)-containing porphyrins. Moreover, the first (1e−), second (1e−), third (2e−), and fourth (1e−) oxidation processes in 1 and 2 are well-separated (150−570 mV) from each other, which makes porphyrins 1 and 2 good candidates for both QCA and four-bit information-storage applications. As expected, the introduction of an electron-withdrawing substituent into the ferrocene core results in oxidation potentials in 1 and 2 to more positive values. In addition, two (1) or one (2) reversible porphyrin-centered one-electron reductions were observed between −1.73 and −2.26 V in CV and DPV experiments. The electrochemical comproportionation constants (Kc)42 for the mixed-valence porphyrins [1]n+ and [2]n+ (n = 1 or 2) can be calculated based on the following equilibrium eqs (Table 2): [1or2]0 + [1or2]2 + ⇄ 2[1or2]+

(1)

[1or2]1 + + [1or2]3 + /4 + ⇄ 2[1or2]2 +

(2)

Calculated on the basis of the CV and DPV data, comproportionation constants for the formation of mixedvalence [1 or 2]+ (343 and 824 000, respectively) and [1 or 2]2+ (1 216 000 and 174 000, respectively) are quite high and indicative of the stability of these species under electrochemical and spectroelectrochemical conditions. It is well-recognized, however, that the electrochemical comproportionation constants should be treated with caution, as they will heavily depend on the solvent/electrolyte combination.42 Another complication in interpretation of the electrochemical results can come from the Columbic repulsion between ferrocene fragment in [1 or 2]n+ species, as it clearly might affect stability of the different atropisomers in solution. Formation of an intense intervalence charge-transfer (IVCT) band in the near-infrared (NIR) region is usually considered as a sign of electronic communication between identical substituents connected via a linking group. Thus, the spectroelectrochemical oxidation of the neutral porphyrins 1 and 2 in the DCM/0.15 M TBA[B(C6F5)4] system was conducted to identify spectroscopic signatures of the mixedvalence [1 or 2]+ and [1 or 2]2+ species and to support an assignment of oxidation processes observed in CV and DPV experiments. In both porphyrins 1 and 2, three clear and reversible oxidative transformations were found during spectroelectrochemical experiments (Figures 5 and 6), which correlate well with the electrochemical data. During the first oxidation process, which was assigned to the [1 or 2]0→[1 or

Figure 10. DFT-PCM predicted pseudo Gouterman’s MOs of porphyrins 1 (top) and 2 (bottom).

Figure 11. Experimental (DCM) and PCM-TDDFT predicted UV− vis spectra of 1 (left) and 2 (right).

Collected in DCM/0.05 M TBA[B(C6F5)4] system CV and DPV data for porphyrins 1 and 2 are presented in Figure 4 with corresponding redox potentials shown in Table 1. G


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Inorganic Chemistry 2]+ transformation, the most intense Soret band at ∼435 nm in 1 and 2 decreases in intensity, while a new red-shifted band at 460 nm for both (1) and (2) appears in the spectrum. In addition, intensities of transitions in the Q-band region decrease, and an intense, very characteristic IVCT transition appears in the NIR region of the spectra at 924 nm of [1]+ or at 896 nm of [2]+ (Figures 5A and 6A). Position and intensity of the IVCT band in [1]+ and [2]+ are close to those earlier observed for the mixed-valence [H2TFcP]+ and [ZnTFcP]+ complexes,10d,27d which further confirm this transformation to be formation of a mixed-valence [1 or 2]+ species. The second oxidation process results in the significant reduction of the Soret-type band intensity and the low-energy shift (1159 nm for [1]2+ and 1215 nm for [2]2+) of the IVCT band in the NIR region of the spectra (Figures 5B and 6B). Taking into consideration the very large difference in potentials between the first, second, and third oxidation waves observed in CV and DPV experiments, a second transformation observed in spectroelectrochemical experiments could be assigned with a confidence to [1 or 2]+→[1 or 2]2+ process. Observed IVCT band energy in [1 or 2]2+ is similar to earlier observed IVCT in [H2TFcP]2+ and [ZnTFcP]2+ mixed-valence complexes.10d,27 Finally, during a third oxidation, the Soret-type band reduced in intensity, and the IVCT band disappears in the NIR region suggesting transformation of [1 or 2]2+ to fully oxidized [1 or 2]4+ species (Figures 5C and 6C). Spectroscopic UV−vis−NIR signatures of the mixed-valence [1 or 2]+/2+ compounds were further supported by chemical oxidation of porphyrins 1 and 2. Indeed, during the chemical oxidation of 1 and 2, spectra very similar to spectroelectrochemical transformations were observed (Supporting Information Figures S7 and S8). In particular, the Soret band first undergoes a red shift and then loses its intensity. The IVCT appears in the NIR region during the first oxidation, undergoes a low-energy shift during second oxidation, and disappears during the third, two-electron oxidation process. To get an additional insight into nature of the mixed-valence [1 or 2]+ species, we conducted band deconvolution analysis of the corresponding NIR regions of the spectra generated under spectroelectrochemical and chemical oxidation conditions. Hush formalism43 was then used to analyze IVCT band parameters (Table 2). The values of key IVCT band parameters, that is, the electronic coupling matrix element (Hab) and the degree of delocalization (α2), were estimated using standard equations with the Fe−Fe distances estimated from the DFT-predicted geometries of neutral porphyrins 1 and 2. Hab = 2.05 × 10−2[(νmaxεmax Δν1/2)1/2 /rab]

(3)

α 2 = 4.24 × 10−4[(εmax Δ1/2)/(rab2νmax )]

(4)

Robin−Day classification)44 mixed-valence compounds (Table 2). Class II (in Robin−Day formalism) behavior of the mixedvalence [1 or 2]+ species correlates well with the reported earlier behavior of the mixed-valence [H2TFcP] + and [MTFcP]+ complexes. As usual, parameters listed in Table 2 should be treated with a grain of salt, because (i) a relatively broad NIR spectral envelope for [1 or 2]+ species adds uncertainty factor to band deconvolution procedure, and (ii) it is hard to predict an average Fe−Fe distance in the mixedvalence species, as the ferrocene groups have relatively low rotational barriers in porphyrins 1 and 2. Nevertheless, data shown in Table 2 adds support to the understanding of the behavior of the mixed-valence [1 or 2]+ species.43 Mössbauer spectroscopy was employed to provide further evidence into the exact nature of the stepwise oxidations. The isomer shifts and quadrupole splitting shown in the initial spectra for both 1 and 2 are close to those of parent ferrocene (room-temperature isomer shift of 0.44 mm s−1, and quadrupole splitting of 2.37 mm s−1)45 indicating the initial presence of only one type of low-spin, ferrous center. Additions of an oxidant (DDQ) result in the formation of a new doublet that grows in intensity as the initial neutral compound is further oxidized (Figures 8 and 9 and Table 3). The small decrease of quadrupole splitting of the remaining Mössbauer doublet for Fe(II) centers is indicative of interaction between Fe(II) and Fe(III) ions, while a relatively broad Mössbauer doublet for Fe(III) centers reflects the influence of the crystal packing with counterion(s) in a solid state. The simultaneous presence of two Mössbauer doublets for the mixed-valence [1 or 2]n+ (n = 1 or 2) species is indicative of the Class II behavior and suggestive of the relatively slow (in Mössbauer time scale) electron transfer rates between Fe(II) and Fe(III) centers.10 DFT and TDDFT Calculations. To correlate spectroscopy and redox properties of the porphyrins 1 and 2 with their electronic structures, DFT-PCM and TDDFT-PCM calculations were conducted on both systems. The DFT-PCM optimized geometries of porphyrins 1 and 2 support their nonplanar nature and indicate that the most stable conformation in both compounds should have α,β,α,β arrangement of the ferrocene substituents with respect to the porphyrin core. DFT-PCM predicted molecular orbital compositions of the frontier orbitals is listed in Table 4; energy diagram is shown in Figure 9, while representative images of the frontier molecular orbitals and the molecular orbital energy diagram are shown in Figure 10. In agreement with the previous DFT calculations on tetra(ferrocenyl)containing porphyrins, the lowest unoccupied molecular orbital (LUMO) and LUMO+1 were predicted to be π* orbitals, predominantly centered at the porphyrin core with their shapes resembling classic Gouterman’s46 “eg” pair (in D4h point group notation). The LUMO and LUMO+1 are well-separated in energy from the other unoccupied MOs (Figure 9). The DFTPCM predicted highest occupied molecular orbital (HOMO) in porphyrins 1 and 2 resembles Gouterman’s “a2u”orbital (in D4h point group notation) and consists of slightly larger contributions from the ferrocene fragments complimented by the contribution from the porphyrin cores’ nitrogen and mesocarbon atoms. The HOMO in porphyrins 1 and 2 is wellseparated in energy (∼0.5 eV) from the other occupied orbitals, and the HOMO in zinc complex 2 is slightly higher in energy compared to the HOMO in metal-free 1, which correlate well with experimental electrochemical data. DFT-PCM predicts

The energy of the IVCT at band maximum (in cm−1) is νmax, Δν1/2 is the width at the band maximum (in cm−1), εmax is the molar extinction coefficient of the IVCT, and rab is the distance between redox centers (in Å). Since the NIR band in the mixed-valence [1]+ and [2]+ complexes is quite broad, band deconvolution analysis was used to analyze IVCT transitions in these mixed-valence compounds obtained by the chemical oxidation or under spectroelectrochemical conditions (Figure 7). The IVCT band parameters in complexes [1]+ and [2]+ were found in the typical range for the coupled class II (in H


Article

Inorganic Chemistry that the HOMO−1 to HOMO−9 region in 1 consists of very closely spaced (∼0.2 eV) orbitals (Figure 9). Although all of them are dominated by contributions from ferrocene fragments, HOMO−8 in 1 and HOMO−1 in 2 resemble classic Gouterman’s a1u orbital (in D4h point group notation) and have a large contribution from the porphyrin carbon atoms located at α- and β-pyrrolic positions (Figure 10). TDDFT-PCM predicted UV−vis spectra of porphyrins 1 and 2 are shown in Figure 11 (the energy-scale data are presented in Supporting Information Figure S9) and are in a good agreement with the experimental data. TDDFT-PCM predicts that the Q-band region in UV−vis spectra of porphyrins 1 and 2 should be dominated by the HOMO→LUMO, LUMO+1 single-electron transitions (excited states 1 and 2). In addition, the metal-to-ligand charge-transfer (MLCT) transitions would be responsible for the intense shoulders between 440 and 520 nm observed in the UV−vis spectra of porphyrins 1 and 2. TDDFT-PCM also predicts that the most intense bands in the Soret band region should have a predominant π−π* character with significant contributions from HOMO, HOMO−8 (1) or HOMO, HOMO−1 (2) to LUMO, LUMO+1 and the other porphyrin-centered single-electron excitations (Figure 12, Supporting Information Figure S9).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pierluca Galloni: 0000-0002-0941-1354 Victor N. Nemykin: 0000-0003-4345-0848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support from the NSF, CHE-1464711, MRI1420373, Minnesota Supercomputing Institute, Univ. of Manitoba, CFI, NSERC, and WestGrid supercomputing facility to V.N. is greatly appreciated.

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REFERENCES

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CONCLUSIONS Metal-free (1) and zinc (2) 5,10,15,20-tetra(1′hexanoylferrocenyl)porphyrins have been investigated by a variety of spectroscopic and computational methods. It has been found that unlike all other tetra(ferrocenyl)-containing porphyrins in a DCM/0.05 M TBA[B(C6F5)4] system, the ferrocene groups oxidized in a 1e− + 1e− + 2e− sequence and all three oxidation waves were very well (150−350 mV) separated from each other. A large separation of the second oxidation wave is indicative of a stable mixed-valence [1 or 2]2+ state, which implies that porphyrins 1 and 2 could be very good candidates for QCA application. The mixed-valence [1 or 2]+ species were characterized by their UV−vis−NIR spectra obtained under spectroelectrochemical and chemical oxidation conditions. Band deconvolution analysis was conducted for the NIR part of the UV−vis−NIR and Mössbauer spectra of mixedvalence [1 or 2]+ compounds. It was found that all mixedvalence species belong to the Class II in Robin−Day classification. A presence of the additional reversible, porphyrin-centered oxidation (which is also well-separated from the other oxidation waves) allows us to propose porphyrins 1 and 2 as prospective platforms for four-bit information storage. Theoretical DFT-PCM and TDDFT-PCM calculations correlate well with experimental observations and confirm the nature of redox-active orbitals in 1 and 2 as well as provide insight into the character of the excited states in these compounds.

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porphyrins 1 and 2 in inverse centimeters scale. Reduction of [1 or 2]n+ under bulk electrolysis conditions. Band deconvolution analysis for [1 or 2]+ species (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00397. Characterization data for porphyrins 1 and 2. Coordinates for DFT-PCM optimized structures of 1 and 2. Predicted by TDDFT-PCM energies and expansion coefficients for porphyrins 1 and 2. Predicted by TDDFT-PCM and experimental and UV−vis spectra of I


Article

Inorganic Chemistry

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