Synthesis and Properties of Subphthalocyanine–Tetracyanobutadiene

Jan 31, 2018 - A series of boron subphthalocyanine–tetracyanobutadiene–ferrocene (SubPc–TCBD–Fc) triads was synthesized by subjecting SubPcs w...
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Cite This: J. Org. Chem. 2018, 83, 2227−2234

Synthesis and Properties of Subphthalocyanine− Tetracyanobutadiene−Ferrocene Triads Alberto Viñas Muñoz, Henrik Gotfredsen, Martyn Jevric, Anders Kadziola, Ole Hammerich, and Mogens Brøndsted Nielsen* Department of Chemistry, Center for Exploitation of Solar Energy, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark S Supporting Information *

ABSTRACT: A series of boron subphthalocyanine−tetracyanobutadiene−ferrocene (SubPc−TCBD−Fc) triads was synthesized by subjecting SubPcs with a ferrocenylethynyl substituent at either the axial or peripheral position to a [2 + 2] cycloaddition reaction with tetracyanoethylene followed by retroelectrocyclization. The ferrocenylethynyl unit was introduced at the axial position (at the boron atom) by a simple aluminum chloride-mediated alkynylation reaction, while functionalization at the SubPc periphery was accomplished by a Sonogashira coupling reaction. The conversion of one alkyne unit into a TCBD unit in combination with the location of the resulting TCBD−Fc moiety was found to have a strong influence on the optical and redox properties, which is ascribed to very different ground-state interactions between the individual donor/acceptor systems. The first electrochemical oxidation could thus be anodically shifted by as much as 0.4 V from the strongest donor molecule (with most unperturbed ferrocene character) to the poorest donor molecule (with strongly perturbed ferrocene character). Six redox states could be reached reversibly for the SubPc−TCBD−Fc triads, −3, −2, −1, 0, + 1, + 2, and for one compound the formation of a tetraanion persistent at the time scale of slow scan voltammetry was observed.

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INTRODUCTION Molecules that form long-lived and highly energetic chargeseparated states are key in solar organic devices to efficiently convert sunlight into electricity.1 Such states rely on suitable arrangement of chromophore and electron donor/acceptor units. Porphyrins and phthalocyanines are some of the most studied chromophores,2 but in recent years, other macrocyclic structures such as the contracted boron subphthalocyanines (SubPcs; Figure 1) have attracted interest.3 SubPcs are chromophores that exhibit good photophysical properties and possess a nonplanar geometry despite being 14-π aromatic systems, which facilitates their purification by chromatography (diminished ability to aggregate). The possibility of function-

alization at both axial and peripheral positions bestows SubPc with great synthetic flexibility and tunability of photophysical and electrochemical properties. Several boron subphthalocyanine−ferrocene (SubPc−Fc) dyads have been reported in the past where photoinduced electron transfer has been found to occur.4 Interest in ferrocenyl derivatives arises from their thermal and photochemical stability, together with their high electrochemical stability. We became interested to further explore SubPc−Fc compounds by bridging the units by alkyne spacers. Acetylenic scaffolding has been shown to be a versatile approach to easily introduce additional functionality in SubPcs: it enables the functionalization at the ring periphery by Pdcatalyzed cross-couplings (using a halide-substituted SubPc as substrate)5 or at the axial position by introducing an alkyne unit there.4b,6 We reasoned that SubPc−CC−Fc compounds could serve as precursors for introducing the 1,1,4,4tetracyanobuta-1,3-diene (TCBD; Figure 1) electron acceptor in the spacer between SubPc and Fc, generating SubPc− TCBD−Fc triads. Thus, the thermal [2 + 2] cycloaddition− retroelectrocyclization reaction between activated alkynes and

Figure 1. Molecular building blocks used throughout this work.

Received: December 11, 2017 Published: January 31, 2018

© 2018 American Chemical Society

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DOI: 10.1021/acs.joc.7b03122 J. Org. Chem. 2018, 83, 2227−2234

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The Journal of Organic Chemistry

to prepare the SubPc−Fc dyad 1a from the readily available10 trimethylsilylethynylferrocene. By employing the same conditions, the method is here extended to allow incorporation of a butadiynylferrocene at the axial position from the known trimethylsilyl-protected butadiyne substrate,11 providing dyad 1b as shown in Scheme 1. We have performed the synthesis of 1a at two different concentrations of SubPc-Cl (0.003 M, yield of 1a: 48%; 0.004 M, yield of 1a: 66%). In both cases the conversion seemed to be spot-to-spot on TLC, so the small variation in yields is rather due to various losses during column chromatographic workup. The experimental section provides a synthesis of 1a at dilute conditions and one of 1b under more concentrated conditions. Both dyads 1a and 1b were subsequently subjected to a cycloaddition−retroelectrocyclization with TCNE to furnish the SubPc−TCBD−Fc triads 2a and 2b, respectively. The yield of 2a was significantly lower than that of 2b, which likely is a result of steric constraints. A peripheral SubPc−TCBD−Fc triad was prepared from the monoiodo-SubPc precursor 3 in two high-yielding steps (Scheme 2). First, a Sonogashira cross-coupling reaction with ethynylferrocene12 afforded conjugate 4, which was then treated with TCNE to provide triad 5 as an inseparable mixture of diastereoisomers on account of point chirality at boron and axial chirality (atropisomerism) at TCBD (vide infra).13 Each of the two diastereoisomers shown in Scheme 2 (where the configuration of the TCBD unit is changed by rotation around the single bond of the butadiene unit) is formed as a racemic mixture. The starting material 3 was prepared in two steps according to a standard route9b from a cyclization reaction between 4-iodophthalonitrile and an excess of phthalonitrile in the presence of BCl3, followed by substitution of the axial chloride of the resulting SubPc by treatment with tert-butylphenol (see Experimental Section); the overall yield of 3 was rather low (13%). Molecular Structures. Single crystals of 1b (grown from chloroform-d/heptane) as well as of 2a and 2b (grown from dichloromethane/heptane) were subjected to X-ray crystallographic analysis (while 1a was studied previously6c). Molecular structures are shown in Figure 2, while packing diagrams are shown in the Supporting Information (SI) (1b and 2a were obtained as solvates). Compounds 1b and 2b pack in a concave−concave head-to-head arrangement as often observed for peripherally unsubstituted boron SubPcs.14 Compound 2a packs in the less common cv-l (concave to ligand) geometry, with the normal component of the unsubstituted cyclopentadienyl ring in ferrocene pointing toward the concave part of the subphthalocyanine ring. Compounds 2a and 2b crystallize as racemates of the Ra and Sa atropisomers, as it has been recently shown for structurally analogous SubPc−TCBD−aniline derivatives.7j The butadiene backbone in the TCBD unit of 2b presents a torsional angle of 114° (“torsional TCBD angle”), which is found within the usual range,7e while that of 2a is smaller (values of 82° and 89° from two forms in crystal). The B−C bond distance for the axial substituent in 2b is 1.61 and 1.63 Å for 2a. The distance between the plane defined by the three N atoms directly attached to boron and boron (“B−N-plane distance”) is 0.61 Å in 2b and slightly smaller in 2a (0.59 Å), which is also found within the usual values.3 The so-called “bowl depth” is often expressed by the distance between the plane formed by the most external carbon atoms in the benzene ring of the isoindole units and boron and falls within the range of previously

the electron-poor tetracyanoethylene (TCNE) offers a powerful tool for introducing the TCBD moiety and can be considered as a click-type reaction.7 Indeed, this reaction has been shown to proceed with ethynylferrocenes,7b and, in addition, Torres and co-workers7j have recently shown that this reaction can be conveniently used to convert SubPcs with an anilinoethynyl substituent at the axial boron into SubPc− TCBD−aniline triads. Along the same line, Diederich, Armaroli, and co-workers8 have linked anilino-substituted TCBD acceptors to a zinc porphyrin in photoactive multicomponent systems for which a long-lived charge-separated state was achieved by photoinduced electron transfer. Here we show how the TCBD−Fc units can be introduced either at the axial or peripheral position of the SubPc. The exact location is found to have strong implications for both the optical and redox properties.

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RESULTS AND DISCUSSION Synthesis. Axial SubPc−TCBD−Fc triads were prepared in two steps from the known9 SubPc-Cl (Scheme 1). Recently, we found that TMS-protected acetylenes react smoothly with SubPc-Cl in the presence of excess AlCl3 to afford axially alkynylated SubPcs.6c The method was successfully employed Scheme 1. Synthesis of Axial SubPc−TCBD−Fc Triads 2a and 2ba

a

o-DCB = o-dichlorobenzene. 2228

DOI: 10.1021/acs.joc.7b03122 J. Org. Chem. 2018, 83, 2227−2234

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reported values with 2.56 Å for 2b and 2.44 Å for 2a, indicating a more concave surface for the former. The structures of the six SubPc−Fc and SubPc−TCBD−Fc compounds were also subjected to a calculational study (B3LYP/cc-pVDZ). The optimized geometries of 4 and 5 are shown in Figure 3, while those of 1a, 1b, 2a, and 2b are shown

Scheme 2. Synthesis of Peripheral SubPc−TCBD−Fc Triad 5a

Figure 3. Calculated geometries of compounds 4 (left) and 5 (right) (B3LYP/cc-pVDZ).

in SI (Figure S41), and a comparison of structural data obtained by X-ray crystallography and computations is shown in Table 1. The calculated “B−N-plane distances” and “bowl depths” are very similar for all six compounds, and the “B−Nplane distances” agree well with those obtained from X-ray analysis. Instead, the “bowl shapes” obtained from X-ray analysis are a bit flattened relative to the optimized structures (i.e., smaller “bowl depths”), and the calculated “torsional TCBD angle” of 2b is significantly smaller than that obtained from X-ray analysis, while relatively close to that calculated for 2a. These deviations are likely due to crystal packing effects. NMR Spectroscopy. Incorporation of the TCBD moiety is accompanied by significant changes of the NMR features for 2a and 2b in comparison to their alkynyl precursors. This is very evident from the 1H NMR signals of the protons in the substituted cyclopentadienyl (Cp) ring, which are all inequivalent in 2a and 2b, giving in total four signals, due to the chiral axis of the vicinally located TCBD unit (see SI, Figures S5 and S7). The effect appears more dramatic for 2a with its substituted-Cp protons spread over a wide region of 2.6 ppm (δH from 2.75 to 5.30 ppm) while for 2b these protons are narrowly grouped together within 0.4 ppm (δH from 4.43 to 4.82 ppm). However, this is most likely a consequence of the interfering shielding and deshielding effects of the aromatic SubPc core being most predominant in system 2a with a closer proximity of the Cp protons to the 14-π electron system. From the combination of two chiral elements (axial and point

a

Two diastereoisomers of 5 are shown with different configurations of the TCBD unit (a total of four stereoisomers results from the synthesis; i.e., two racemic pairs).

Figure 2. Molecular structures of 1b (left; CCDC 1589509), 2a (center; CCDC 1589510), and 2b (right; CCDC 1589511) obtained by X-ray crystallographic analysis (solvent molecules were removed for clarity). Ellipsoids presented at 50% probability level. 2229

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The Journal of Organic Chemistry Table 1. Comparison of Structural Data from X-ray Analysis and Computational Chemistry (B3LYP/cc-pVDZ) 1a torsional TCBD angle (deg) B−C bond length (Å) B−O bond length (Å) B−N-plane distancec (Å) bowl depthd (Å)

1b

2a

2b

X-raya

calcd

X-rayb

calcd

X-rayb

calcd

X-rayb

calcd

4 calcd

5 calcd

− 1.59 − 0.64 2.59

− 1.57 − 0.65 2.70

− 1.58 − 0.64 2.61

− 1.58 − 0.64 2.70

82, 89 1.63, 1.64 − 0.59, 0.60 2.43, 2.44

86 1.66 − 0.62 2.72

114 1.61 − 0.61 2.56

78 1.59 − 0.62 2.67

− − 1.44 0.63 2.67

91 − 1.44 0.63 2.68

a From ref 6c. bThis work. cDistance between the boron atom and the plane defined by the three nitrogen atoms attached to boron. dDistance between the boron atom and the plane defined by the six most external carbon atoms in the benzene rings of the isoindole units.

agreement with the little effect that axial substitution is known to have on the electronic properties of SubPc chromophores.15 Interestingly, however, this is not the case for the axially substituted TCBD conjugate 2a, where the Qband is shifted by 10 nm and the Soret band by 5 nm. Compound 2b shows insignificant shifts for the Q and the Soret band, consistently with the longer distance separating the boron atom and the TCBD unit due to the ethynyl spacer. Aside from the red-shifted absorptions, another interesting feature arises for compound 2a at λmax 408 nm in benzonitrile, while absent in toluene and anisole. A similar absorption band was observed by Torres and co-workers7j for a triad having an N,N-dimethylaniline donor instead of ferrocene, and it was ascribed to a charge-transfer absorption as a result of groundstate interactions between the donor and the TCBD acceptor. Compound 1a exhibits an emission maximum at 590 nm in toluene (see SI, Figure S32), corresponding to a Stokes shift of 19 nm. The spectra of the peripheral analogues 4 and 5 indicate markedly different electronic properties from those of the axial conjugates (Figure 4). Covalently linking the alkynyl-ferrocene at the SubPc periphery (SubPc−Fc 4) results in a broadening of the Q-band accompanied by a decrease in the extinction coefficients of both the Q and the Soret band. For 4, an emission maximum of 590 nm in toluene was observed (Figure S33), similar to that of 1a. The incorporation of the TCBD unit (SubPc−TCBD−Fc 5) has a huge effect on the electronic properties of the system, with an absorption spectrum that features two bands at the sides of the position where the Qband of SubPc was found for 4. To rule out intermolecular interactions, spectra at different concentrations were recorded (see SI, Figure S35); no deviation from Lambert−Beer’s law was observed. Increasing the solvent polarity reveals that for 5 the longest-wavelength absorption is hypsochromically shifted, with a λmax of 606 (benzonitrile), 615 (anisole), and 612 (toluene) nm (Figure 5). An additional set of measurements was carried out in three other solvents, and the trend was once more reproduced, with absorption maxima of 609 (dichloromethane), 596 (acetonitrile), and 595 (acetone) nm (spectra included in SI). The hypsochromic shift upon increasing solvent polarity can be rationalized in terms of a more polar ground state than excited state, and the charge-transfer transition seems to be related to a SubPc−TCBD interaction. Electrochemistry. The electrochemical behavior of compounds 1a, 1b, 2a, 2b, 4, and 5 was studied by cyclic voltammetry in CH2Cl2 (0.1 M Bu4NPF6) at a voltage scan rate of 0.1 V s−1. Figure 6 includes the resulting voltammograms for 1a, 1b, and 4 in which Fc is attached to CC units, and Figure 7 includes those for 2a, 2b, and 5 in which Fc is attached to a TCBD unit. In the following, the formal potentials, E°′, are reported for reversible one-electron transfers with reverse

chirality), compound 5 was obtained as a mixture of two diastereoisomers in a ratio of 55:45 (determined by integration of 1H NMR signals). Optical Properties. Figure 4 shows the absorption spectra of the axially substituted compounds 1a, 2a, and 2b in

Figure 4. Absorption spectra of 1a (orange line), 2a (blue line), 2b (black line), 4 (red line), and 5 (green line) in benzonitrile.

benzonitrile as well as of the peripherally substituted compounds 4 and 5, while Table 2 lists the absorption maxima. For dyad 1a, the two characteristic SubPc bands at λmax 305 nm (Soret band) and 571 nm (Q-band) are only slightly red-shifted with respect to the parent SubPc, in good Table 2. UV-Vis Absorption Maxima of Compounds in Various Solvents

a

compound

solvent

1a 1a 1b 2a 2a 2a 2b 2b 4 4 5 5 5

PhCN PhMe PhMe PhCN PhMe PhOMe PhCN PhOMe PhCN PhMe PhCN PhMe PhOMe

λmax (nm) (ε (104 M−1 cm−1)) 571 568 569 581 583 583 573 575 580 569 606 612 615

(8.07), (9.01), (9.87), (3.48), (5.15), (6.33), (3.69), (4.69), (6.16), (7.16), (3.24), (3.38), (3.32),

534sh,a 305 (6.09) 526sh, 305 (4.68) 527sh, 307 (5.50) 533sh, 408 (1.87), 310 (5.30) 535sh, 315 (5.30) 535sh, 316 (6.68) 534sh, 305 (6.09) 530sh, 309 (5.04) 521sh, 309 (3.70) 520sh, 307 (4.12) 548 (3.12), 315 (4.23) 547 (3.23), 312 (4.25) 547 (3.23), 312 (4.25)

sh = shoulder. 2230

DOI: 10.1021/acs.joc.7b03122 J. Org. Chem. 2018, 83, 2227−2234

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Figure 5. Absorption spectra of 5 in benzonitrile (black line), anisole (red line), and toluene (blue line).

Figure 7. Cyclic voltammograms of 2b (top), 5 (middle), and 2a (bottom) recorded in CH2Cl2 (0.1 M Bu4NPF6) at a glassy carbon working electrode (d = 3 mm) and a voltage scan rate of 0.1 V s−1.

Table 3. Formal Potentials, E°′, or Peak Potentials, Ep, for the Electron Transfer Reactions Observed by Cyclic Voltammetrya,b compd

EoFc,ox ′

EoSubPc,ox

EoTCBD,red1 ′

EoTCBD,red2 ′

EoSubPc,red1 ′

EpSubPc,red2

1a 1b 2a 2b 4 5

0.12 0.18 0.47 0.49d 0.13 0.49

0.57 0.56 0.76 0.58e 0.58 0.65

− − −1.29 −0.85 − −0.89

− − −1.43 −1.14 − −1.23

−1.57 −1.52 −1.97 −1.62 −1.53 −1.67

−2.11 −2.10 −c −2.22f −2.11 −2.36

a

In V vs Fc/Fc+. bThe subscripts Fc, SubPc, and TCBD indicate the part of the molecule associated with the observed electron transfer reaction (see the text). cClose to the background; not shown in Figure 6. dEstimated from the potential for the peak observed during the backward scan by addition of 40 mV (= half the value of the peak separation typically observed for this series of compounds). eEstimated from the potential for the oxidation peak by subtraction of 40 mV (= half the value of the peak separation typically observed for this series of compounds). fQuasireversible electron transfer.

Figure 6. Cyclic voltammograms of 1a (top), 1b (middle), and 4 (bottom) recorded in CH2Cl2 (0.1 M Bu4NPF6) at a glassy carbon working electrode (d = 3 mm) and a voltage scan rate of 0.1 V s−1.

molecules. It is worth mentioning also that the computed geometries of the TCBD−Fc units are essentially the same in 2a, 2b, and 5 (see the SI). Thus, the difference in electrochemical behavior reported below for these three compounds appears not to be caused by differences in the TCBD−Fc geometry. The voltammograms for 1a, 1b, and 4 are similar as seen in Figure 6. During the scan in the positive direction, the first oneelectron process observed is that corresponding to the reversible oxidation of the Fc part to Fc+ with the E°′ values being 0.12 V (1a), 0.18 V (1b), and 0.13 V (4). These are a little higher than that for unsubstituted Fc that by definition has an E°′ value of 0 V and illustrates the effect of the slightly

current being clearly observed during the backward scan. When follow-up reactions are so fast that reverse currents are absent, and the potentials for that reason do not necessarily have a thermodynamic significance, the peak potentials, Ep, are reported instead. All potentials are given in V vs Fc/Fc+ and summarized in Table 3. As it appears in the discussion to follow, the electronic interactions between the Fc, SubPc, and TCBD units in the six compounds are not negligible; this is true in particular for 2a in which TCBD is attached to the SubPc boron atom. Still it makes sense and is convenient to discuss the observed electron transfer reactions as being associated with specific parts of the 2231

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The Journal of Organic Chemistry electron withdrawing CC and CC−CC groups. In comparison, the E°′ value observed for the one-electron oxidation of a model compound, Fc-CC-TMS, is 0.13 V (see SI, Figure S40). The second one-electron transfer corresponds to the oxidation of the SubPc part of the molecules, the E°′ values being 0.57 V (1a), 0.56 (1b), and 0.58 V (4). During the scan in the negative direction, the reversible one-electron reduction of the SubPc part of the molecules to the corresponding radical anions are observed at −1.57 V (1a), −1.52 (1b), and −1.53 V (4), which is followed by further reduction to reactive dianions at Ep close to −2.11 V. Signals corresponding to reduction of the CC−Fc unit were not observed in agreement with the voltammogram for Fc-CCTMS (see SI, Figure S43) that shows that the CC−Fc unit is not reducible within the applied potential range. A number of peaks corresponding to the oxidation of anions resulting from the SubPc dianion follow-up reactions are observed in the voltage range −0.5 to −1.25 V during the backward scan. Altogether the electrochemical behavior of these three compounds compares favorably with that reported for related SubPc derivatives.5c,16 The voltammograms for 2a, 2b, and 5 in which the Fc unit is attached to the TCBD part of the molecules appear quite different as seen in Figure 7. For all three compounds, it is seen that the reversible one-electron oxidation of the Fc unit is more difficult than observed for 1a, 1b, and 4 and now takes place at potentials close to 0.5 V owing to the strongly electronwithdrawing properties of the TCBD unit. A similar effect of TCBD on the Fc oxidation potential has been reported for other TCBD−Fc derivatives.17,18 The potentials for oxidation of the SubPc units are less affected by the presence of TCBD, and for 2b and 5 this means that the oxidation of the SubPc unit takes place in the region where Fc is now oxidized, resulting in two closely spaced (5) or nearly merged (2b) oxidation peaks. In contrast, the oxidation of the SubPc unit in 2a is more difficult owing to the fact that the electronwithdrawing TCBD in this compound is directly attached to the SubPc boron atom. Oxidation takes place with an E°′ value of 0.76 V, and the two oxidation peaks are now well separated. The reduction of 2b and 5 illustrates clearly the presence of the easily reduced TCBD unit. During the negatively going voltage scan, the first two reversible one-electron reduction peaks observed are caused by the successive formation of the TCBD radical anion and dianion with the E°′ values being −0.85 V and −1.14 V (2b) and −0.89 V and −1.23 V (5). Further reversible one-electron reduction of the SubPc unit is observed at the E°′ values −1.62 V (2b) and −1.67 V (5) followed by the formation of the SubPc dianions at Ep = −2.22 V (2b) and −2.36 V (5). It is of interest to notice that for 2b the tetraanion resulting from these altogether four one-electron processes is indeed persistent at the time scale of slow scan voltammetry as evidenced by the observation of oxidation current for this species during the backward scan. The effect of having a TCBD unit directly attached to the SubPc boron atom is seen also for the reduction of 2a. The boron atom that formally carries a negative charge causes the TCBD in 2a to be more difficult to reduce than observed for the TCBD in 2b and 5, the two E°′ values being −1.29 V and −1.43 V, and the presence of the TCBD dianion resulting from these first two electron transfers causes the reduction of the SubPc unit to be more difficult than observed for 2b and 5, the E°′ value now being −1.97 V. Further reduction of the SubPc unit in 2a to the dianion stage takes place close to the background (not shown).

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CONCLUSION

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EXPERIMENTAL SECTION

The [2 + 2] cycloaddition−retroelectrocyclization reaction is a convenient tool for the preparation of redox-active SubPc− TCBD−Fc triads along axial or peripheral directions. Changing the alkyne unit in the precursor SubPc−Fc dyad for a TCBD unit has important consequences for the optical and redox properties as has the exact position of the TCBD−Fc moiety, placed at either an axial or peripheral position of the SubPc. The potentials of reductions and oxidations can thus be finely tuned as can the longest-wavelength absorption maxima due to various degrees of donor−acceptor ground-state interactions. The large shift in energy of the CT absorption (>200 nm) band for compounds 2a and 5 and the opposite solvatochromic behavior of these suggest distinct donor/acceptor interactions between the different units. This could indicate a change in the donor/acceptor directionality within the molecule. Several redox states could be reached reversibly for the SubPc− TCBD−Fc triads, corresponding to stepwise one-electron reductions to the trianion and stepwise one-electron oxidations to the dication. For one of the axially functionalized triads, even the tetraanion could be observed in a quasireversible reduction of the trianion. The simple and atom-economically synthetic routes to these rather complicated compounds open a range of possibilities in the design of future redox-active dyes.

General Methods. All reagents and solvents were obtained from commercial suppliers and used as received unless otherwise stated. SubPc-Cl,9 ethynylferrocene,12 trimethylsilylethynylferrocene,11 trimethylsilylbuta-1,3-diynylferrocene,12 and 4-iodophthalonitrile19 were prepared according to literature procedures. THF and dioxane were obtained by distillation from a Na/benzophenone couple. Pyridine was obtained from storage over KOH. Purification by column chromatography was carried out on silica gel (SiO2, 60 Å, 40−63 μm). Thin-layer chromatography (TLC) was carried out using commercially available aluminum sheets precoated with silica gel with fluorescence indicator and visualized under UV light at 254 or 360 nm. 1H, 13C, and 11B NMR spectra were recorded on a 500 MHz instrument at 500, 126, and 160 MHz, respectively. Chemical shift values are quoted in ppm and coupling constants (J) in hertz (Hz). 1H and 13C NMR spectra are referenced against the residual solvent peak (CDCl3 δH = 7.26 ppm, δC = 77.16 ppm). 11B NMR spectra are referenced against an external standard of BF3 diethyl etherate (BF3·(OC2H5)2; δB = 0 ppm). HRMS MALDI spectra were recorded on an FT-ICR instrument equipped with a 7T magnet (prior to the experiments, the instrument was calibrated using NaTFA cluster ions). UV−vis absorption measurements were performed in a 1 cm path-length cuvette, and the neat solvent was used as baseline. All melting points are uncorrected. SubPc−Fc Dyad 1a. To a stirred suspension of SubPc-Cl (80 mg, 0.19 mmol) and trimethylsilylethynylferrocene (108 mg, 0.38 mmol) in o-DCB (65 mL) was added AlCl3 (122 mg, 0.91 mmol), and the mixture was stirred overnight (ca. 16 h) at rt. Pyridine (0.15 mL) was added, and the mixture was filtered through a short plug of neutral alumina using toluene as eluent. The filtrate was concentrated under reduced pressure and subjected to flash column chromatography (SiO2, 30% EtOAc/heptane) to afford 1a as bright golden crystals (71 mg, 66%). Rf = 0.41 (30% EtOAc/heptane). Mp > 230 °C. 1H NMR (500 MHz, CDCl3) δ 8.87 (dd, J = 5.9, 3.1 Hz, 6H), 7.89 (dd, J = 5.9, 3.1 Hz, 6H), 3.83−3.81 (m, 2H), 3.79 (s, 5H), 3.77−3.76 (m, 2H) ppm. 13C NMR (126 MHz, CDCl3) δ 150.5, 131.0, 129.7, 122.2, 71.2, 69.8, 68.2, 64.5 ppm (two signals missing). 11B NMR (160 MHz, CDCl3) δ −21.5 ppm. HRMS (MALDI+): m/z [M + H]+ calcd for [C36H22BFeN6]+ 605.1340, found 605.1343. SubPc−Fc Dyad 1b. To a stirred suspension of SubPc-Cl (180 mg, 0.42 mmol) and ((trimethylsilyl)buta-1,3-diynyl)ferrocene (135 mg, 0.44 mmol) in o-DCB (5 mL) was added AlCl3 (279 mg, 2.1 2232

DOI: 10.1021/acs.joc.7b03122 J. Org. Chem. 2018, 83, 2227−2234

Article

The Journal of Organic Chemistry

2 h at rt, poured into water (100 mL), and extracted with CH2Cl2 (3 × 30 mL). The combined organic extracts were dried over MgSO4, filtered, and concentrated under reduced pressure. Purification by flash column chromatography (SiO2, gradient elution: 2.5−5% EtOAc/ CH2Cl2) followed by size-exclusion chromatography (Biobeads SX-3, CH2Cl2) afforded 4 (101 mg, 90%) as a golden-brown solid. Rf (2.5% EtOAc/CH2Cl2) = 0.82. Mp 201−204 °C. 1H NMR (500 MHz, CDCl3) δ 8.99−8.96 (m, 1H), 8.87−8.81 (m, 4H), 8.77 (dd, J = 8.2, 0.6 Hz, 1H), 7.96 (dd, J = 8.2, 1.4 Hz, 1H), 7.92−7.88 (m, 4H), 6.76 (d, J = 8.8 Hz, 2H), 5.32 (d, J = 8.8 Hz, 2H), 4.62−4.57 (m, 2H), 4.33−4.31 (m, 2H), 4.30 (s, 5H), 1.08 (s, 9H) ppm. 13C NMR (126 MHz, CDCl3) δ 152.1, 151.9, 151.8, 151.6, 151.1, 150.9, 150.2, 143.7, 134.6, 132.5, 131.24, 131.19, 131.1, 130.1, 130.04, 130.02, 129.99, 129.3, 128.8, 125.83, 125.75, 125.3, 122.39, 122.38, 122.36, 122.3, 122.1, 117.8, 91.9, 86.2, 71.8, 70.3, 69.4, 64.8, 34.0, 31.5 ppm. 11B NMR (160 MHz, CDCl3) δ −15.1 ppm. HRMS (MALDI+): m/z [M + H]+ calcd for [C46H34BFeN6O]+ 753.2231, found 753.2227. SubPc−TCBD−Fc Dyad 5. Tetracyanoethylene (43 mg, 0.33 mmol) was added to a stirred solution of 4 (50 mg, 0.066 mmol) in oDCB (3 mL), and the reaction mixture was sonicated overnight. Purification by flash column chromatography (SiO2, 40% EtOAc/ heptane) afforded 5 (53 mg, 91%) as a purple blue solid as a mixture of two atropisomers A (major) and B (minor), evident from both 1H NMR and 13C NMR spectroscopies. The ratio between atropisomers A and B was estimated by 1H NMR integration as 55:45. Rf (40% EtOAc/heptane) = 0.43. Mp > 230 °C. In the following, each number of protons corresponds to the number of protons in the structure(s) that gives rise to the resonance(s) rather than to the exact integral ratios; that is, the numbers do not take the actual ratio between isomers into account: 1H NMR (500 MHz, CDCl3) δ 9.16 (d, J = 1.7 Hz, 1H, isomer B), 9.14 (d, J = 1.7 Hz, 1H, isomer A), 8.96 (d, J = 8.5 Hz, 2H, isomer A+B), 8.87−8.76 (m, 8H, isomer A+B), 8.20 (dd, J = 8.5, 1.7 Hz, 1H, isomer A), 8.17 (dd, J = 8.5, 1.7 Hz, 1H, isomer B), 7.98−7.90 (m, 8H, isomer A+B), 6.77 (d, J = 8.7 Hz, 2H, isomer B), 6.75 (d, J = 8.7 Hz, 2H, isomer A), 5.33−5.28 (m, 5H, isomer A+B), 5.11 (broad s, 1H, isomer B), 5.04−5.00 (m, 1H, isomer A), 4.95 (broad s, 1H, isomer B), 4.92 (broad s, 1H, isomer B), 4.92−4.90 (m, 1H, isomer A), 4.90−4.88 (m, 1H, isomer A), 4.88−4.85 (m, 1H, isomer A), 4.60 (s, 5H, isomer A), 4.51 (s, 5H, isomer B), 1.08 (s, 9H, isomer B), 1.07 (s, 9H, isomer A) ppm. 13C NMR (126 MHz, CDCl3) δ 172.9, 172.7, 166.4, 166.3, 154.7, 154.6, 154.22, 154.16, 152.61, 152.56, 151.9, 151.8, 149.71, 149.68, 149.1, 149.0, 148.1, 148.0, 143.99, 143.95, 132.4, 132.3, 131.6, 131.3, 131.2, 131.01, 131.00, 130.96, 130.95, 130.94, 130.90, 130.88, 130.85, 130.61, 130.57, 130.46, 130.45, 129.98, 129.95, 128.0, 127.7, 125.82, 125.80, 124.0, 123.49, 123.46, 122.72, 122.70, 122.6, 122.4, 117.6, 113.77, 113.75, 113.1, 113.0, 111.9, 111.8, 111.60, 111.58, 86.81, 86.78, 79.2, 79.0, 75.9, 75.7, 75.4, 75.32, 75.30, 75.25, 72.8, 72.7, 72.6, 72.40, 72.38, 72.1, 33.9, 33.8, 31.3, 31.3 ppm (mixture of diastereoisomers, nine signals missing presumably due to overlap). 11B NMR (160 MHz, CDCl3) δ −15.0 ppm. HRMS (MALDI+): m/z [M + H]+ calcd for [C52H34BFeN10O]+ 881.2354, found 881.2346. Optical Measurements. UV−vis absorption measurements were performed in a 1 cm path-length cuvette, and the neat solvent was used as baseline. Emission and excitation spectra were recorded for samples with absorbances below 0.1 absorbance units. Electrochemical Measurements. Cyclic voltammetry was carried out at room temperature in CH2Cl2 containing Bu4NPF6 (0.1 M) as the supporting electrolyte using an Autolab PGSTAT12 instrument driven by the Nova 1.11 software. The working electrode was a circular glassy carbon disk (d = 3 mm), the counter electrode was a thin platinum wire, and the reference electrode was a silver wire immersed in the solvent-supporting electrolyte mixture and physically separated from the solution containing the substrate by a ceramic frit. The potential of the reference electrode was determined vs the ferrocene/ ferrocenium (Fc/Fc+) redox system in separate experiments. The voltage sweep rate was 0.1 V s−1. iR-Compensation was used in all experiments. Solutions were purged with argon saturated with CH2Cl2 for at least 10 min before the measurements were made after which a stream of argon was maintained over the solutions. The formal

mmol), and the mixture was stirred overnight (ca. 16 h) at rt. Pyridine (0.15 mL) was added, and the mixture was filtered through a short plug of neutral alumina using toluene as eluent. The resulting filtrate was concentrated under reduced pressure and subjected to flash column chromatography (SiO2, 30% EtOAc/heptane) to afford 2b as bright golden crystals (42 mg, 40%). Rf = 0.38 (30% EtOAc/heptane). Mp > 230 °C. 1H NMR (500 MHz, CDCl3) δ 8.86 (dd, J = 5.9, 3.1 Hz, 6H), 7.90 (dd, J = 5.9, 3.1 Hz, 6H), 4.18−4.15 (m, 2H), 4.06− 4.03 (m, 2H), 4.01 (s, 5H) ppm. 13C NMR (126 MHz, CDCl3) δ 150.4, 131.0, 129.9, 122.3, 76.0, 72.2, 70.1, 69.9, 69.2, 63.3, 62.6 ppm (1 signal missing). 11B NMR (160 MHz, CDCl3) δ −21.7 ppm. HRMS (MALDI+): m/z [M + H]+ calcd for [C38H22BFeN6]+ 629.1343, found 629.1325. SubPc−TCBD−Fc Dyad 2a. A mixture of 1a (56.5 mg, 0.09 mmol) and TCNE (59 mg, 0.46 mmol) in o-DCB (5 mL) was stirred for 2 days. The reaction mixture was subjected to flash column chromatography (SiO2, 10% EtOAc/toluene), and the isolated purple solid was recrystallized from CH2Cl2/heptane. The crystals were washed with heptane to afford 2a (22 mg, 33%) as golden crystals. Rf = 0.33 (10% EtOAc/toluene). Mp > 230 °C. 1H NMR (500 MHz, CDCl3) δ 8.88−8.75 (m, 6H), 7.98−7.89 (m, 6H), 5.30−5.26 (m, 1H), 4.64−4.61 (m, 1H), 4.32−4.28 (m, 1H), 4.09 (s, 5H), 2.78−2.73 (m, 1H) ppm. 13C NMR (126 MHz, CDCl3) δ 174.1, 151.4, 151.2, 130.9, 130.8, 130.63, 130.61, 122.6, 122.5, 113.1, 111.6, 110.4, 109.6, 89.4, 75.2, 75.0, 73.3, 73.1, 72.3, 68.0 ppm (two signals missing). 11B NMR (160 MHz, CDCl3) δ −17.5 ppm. HRMS (MALDI+): m/z [M + H]+ calcd for [C42H22BFeN10]+ 733.1466, found 733.1458. SubPc−TCBD−Fc Dyad 2b. A mixture of 1b (35 mg, 0.06 mmol) and TCNE (71 mg, 0.55 mmol) in o-DCB (6 mL) was stirred for 2 days. The reaction mixture was subjected to flash column chromatography (SiO2, 10% EtOAc/toluene), and the isolated purple solid was recrystallized from CH2Cl2/heptane. The crystals were washed with heptane to afford 2b (35 mg, 83%) as golden crystals. Rf = 0.38 (30% EtOAc/toluene). Mp > 230 °C. 1H NMR (500 MHz, CDCl3) δ 8.87 (dd, J = 6.0, 3.0 Hz, 6H), 7.93 (dd, J = 6.0, 3.0 Hz, 6H), 4.83−4.81 (m, 1H), 4.81−4.79 (m, 1H), 4.78−4.75 (m, 1H), 4.46− 4.41 (m, 1H), 3.99 (s, 5H) ppm. 13C NMR (126 MHz, CDCl3) δ 167.1, 150.7, 131.0, 130.3, 122.5, 113.7, 112.3, 110.1, 109.9, 95.2, 76.3, 76.0, 75.7, 72.9, 72.6, 72.5, 70.3 ppm (three signals missing). 11B NMR (160 MHz, CDCl3) δ −21.0 ppm. HRMS (MALDI+): m/z [M + H]+ calcd for [C44H22BFeN10]+ 757.1466, found 757.1431. SubPc 3. A solution of BCl3 (110 mL, 111 mmol, 1.0 M in hexanes) was added to a stirred solution of 4-iodophthalonitrile (1.56 g, 6.14 mmol) and phthalonitrile (3.93 g, 30.7 mmol) in o-DCB (200 mL), and the resulting suspension was heated to reflux point (ca. 68 °C) for 30 min. The hexanes were distilled off, and the reaction mixture was once more heated to reflux point (180 °C) for 1.5 h. Residual BCl3 was removed by a stream of N2 while the reaction mixture was allowed to cool to rt and then concentrated under reduced pressure. The dry purple solids were transferred to a reaction flask charged with tert-butylphenol (5.54 g, 36.9 mmol), and toluene (20 mL) was added. The reaction mixture was stirred at reflux for 24 h, and the toluene and remaining tert-butylphenol were then boiled off under a stream of N2. Purification by flash column chromatography (SiO2, 5% EtOAc/toluene) gave 3 (550 mg, 13%) as a golden-brown solid. Rf (5% EtOAc/toluene) = 0.35. Mp 166−168 °C. 1H NMR (500 MHz, CDCl3) δ 9.21 (dd, J = 1.5, 0.6 Hz, 1H), 8.94−8.75 (m, 4H), 8.56 (dd, J = 8.3, 0.6 Hz, 1H), 8.17 (dd, J = 8.3, 1.5 Hz, 1H), 8.01− 7.83 (m, 4H), 6.75 (d, J = 8.8 Hz, 2H), 5.29 (d, J = 8.8 Hz, 2H), 1.07 (s, 9H) ppm. 13C NMR (126 MHz, CDCl3) δ 152.6, 152.4, 151.9, 151.7, 150.6, 150.1, 149.4, 143.9, 138.3, 132.3, 131.4, 131.33, 131.30, 131.2, 131.1, 130.3, 130.2, 130.1, 129.8, 125.9, 123.5, 122.48, 122.46, 122.40, 122.36, 117.8, 95.6, 34.0, 31.5 ppm (one signal missing). 11B NMR (160 MHz, CDCl3) δ −15.1 ppm. HRMS (MALDI+): m/z [M + H]+ calcd for [C34H25BIN6O]+ 671.1222, found 671.1221. SubPc−Fc Dyad 4. A degassed solution of Et3N/toluene 1:3 (4 mL) was added to an argon-purged flask charged with 3 (100 mg, 0.15 mmol), ethynylferrocene (41 mg, 0.19 mmol), Pd2dba3 (34 mg, 0.037 mmol, 25 mol %), AsPh3 (91 mg, 0.23 mmol, 200 mol %), and CuI (5.7 mg, 0.030 mmol, 20 mol %). The reaction mixture was stirred for 2233

DOI: 10.1021/acs.joc.7b03122 J. Org. Chem. 2018, 83, 2227−2234

Article

The Journal of Organic Chemistry potentials for reversible one-electron transfers were determined as the average of the peak potentials for reduction and oxidation. Computations. The computations were carried out using computers hosted by the High Performance Computing Center at the University of Copenhagen. The Gaussian G09 suite of programs (Rev. E.01)20 was used throughout. The initial structures were generated by GaussView 5.0. True minima resulted in all cases as evidenced by the absence of imaginary frequencies.

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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.joc.7b03122. CIF data for 2a (CIF) CIF data for 2b (CIF) CIF data for 1b (CIF) NMR spectra, UV−vis absorption and emission spectra, electrochemical data, X-ray and computational data (optimized geometries and coordinates) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ole Hammerich: 0000-0002-2080-1206 Mogens Brøndsted Nielsen: 0000-0001-8377-0788 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS University of Copenhagen is acknowledged for financial support. REFERENCES

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DOI: 10.1021/acs.joc.7b03122 J. Org. Chem. 2018, 83, 2227−2234