helicene-Based Complexes of Ferrocene and - American Chemical

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Tetrathia[7]helicene-Based Complexes of Ferrocene and (η5Cyclohexadienyl)tricarbonylmanganese: Synthesis and Electrochemical Studies Françoise Rose-Munch,*,† Ming Li,† Eric Rose,† Jean Claude Daran,‡ Alberto Bossi,§ Emanuela Licandro,*,§ and Patrizia Romana Mussini∥ †

UPMC Univ Paris 6, IPCM UMR CNRS 7201, Equipe Chimie Organique et Organométallique, Case 181, 4 place Jussieu, F-75252 Paris Cedex 05, France ‡ Laboratoire de Chimie de Coordination, 205, route de Narbonne, 31077 Toulouse Cedex 04, France § Dipartimento di Chimica Organica e Industriale and ∥Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, via Venezian 21, I-20133 Milan, Italy S Supporting Information *

ABSTRACT: Tetrathia[7]helicene ([7]TH)-based complexes substituted at the thienyl ring ends by a ferrocenyl group (Fc) or by a (η5-cyclohexadienyl)Mn(CO)3 derivative have been prepared by Sonogashira coupling reactions starting from the mono- or diiodo [7]TH compounds. The molecular structure of one of the diferrocenyl [7]TH complexes was established by X-ray analysis. Electrochemical investigation on the Fc-[7]TH systems show that the Fc groups are significantly electron poorer with respect to Fc (ΔE° ≈ 0.15 V), due to the effective conjugation of the Fc redox moiety with the triple bond + helicene system, as also confirmed by spectroscopic data. Potential cycling around the second oxidation peak, assigned to the thiahelicene moiety, affords fast, regular growth of electrodeposited conducting films, provided that one terminal α-thiophene position be available for coupling; on the other hand, long alkyl chains hamper film formation. The conducting films feature a broad oxidation wave resulting from the merging of several redox peaks, having its onset at the Fc oxidation. Since conducting films obtained by electrooligomerization of parent tetrathiahelicene have their onset potentials 0.45 V more positive than the Fc redox sites in this studied Fc-[7]TH conjugates, the above continuity could point to some coupling between Fc redox centers and conjugated π systems, favored by solid-state stacking.

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INTRODUCTION The development of optical telecommunications and information technologies, where the light is the carrier of the information, is pushing research more and more toward the development of efficient systems that manipulate and modulate an incoming optical signal. Research focused on the development of new organic and organometallic materials with significant nonlinear optical (NLO) properties has been an area of considerable attention, due to the relevance of these systems in optoelectronic technologies.1 Materials useful for NLO applications require the absence of centrosymmetry not only at the molecular level (such as in the so-called “firstgeneration” linear “push−pull” molecules) but also at the supramolecular level. The current research trend focuses on molecules intrinsically yielding noncentrosymmetric aggregates, i.e. two- and three-dimensionally branched structures (the socalled “second-generation” chromophores), and, even better, structures including one or more stereogenic centers (the “third-generation” chromophores).2 Helicenes are an extremely attractive class of conjugated molecules currently being investigated for optoelectronic applications.3 Helicenes combine the electronic properties afforded by their conjugated system with the chiroptical © 2011 American Chemical Society

properties provided by their peculiar helixlike structure, resulting from the ortho annulation of a series of aromatic (and/or heteroaromatic) rings. Thiahelicenes, consisting of alternating benzene and thiophene rings, are particularly important, having much higher effective conjugation, at a constant number of condensed rings, than both their allthiophene and all-benzene counterparts. For all these reasons thiahelicenes are appealing candidates for chiral NLO materials, since the whole molecule is intrinsically asymmetric, therefore providing an ideal material to induce asymmetric aggregation. Recently some of us have reported the first systematic electrochemical investigation of this attractive molecule family.4 Tetrathia[7]helicenes ([7]TH) feature mild first oxidation potentials and extreme first reduction peak potentials, which is in line with the p-type semiconductor character typical of unsubstituted oligothiophene systems. With free terminal αthiophene positions the shape of the first oxidation peak accounts for a fast irreversible chemical reaction following the first electron transfer, with an irregular return peak indicating Received: June 30, 2011 Published: December 16, 2011 92

dx.doi.org/10.1021/om200571a | Organometallics 2012, 31, 92−104

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RESULTS AND DISCUSSION Syntheses and Structural Studies. In the course of studies concerning the structure and the reactivity of neutral (η6-arene)Cr(CO)3,9 cationic (η6-arene)Mn(CO)3,10 and cationic (η6-arene)Fe(C5H5)11 derivatives it has been reported that a nucleophile can easily add to the coordinated arene ring, affording (η5-cyclohexadienyl)[M] ([M] = Cr(CO)3−, Mn(CO)3, Fe(C5H5)) complexes. In the case of the Mn series, these η5 complexes are very stable and easily functionalizable either by a lithiation/electrophilic quench sequence12 or by palladium cross-coupling reactions.13 The latter reactions were developed by us as an efficient method of coupling Fc- or (η5cyclohexadienyl)Mn(CO)3-based complexes to thienyl,14 benzodithiophenyl,15 or binaphthyl16 derivatives using a triplebond spacer. The starting organometallic complexes were ethynyl Fc or alkyne-substituted (η5-cyclohexadienyl)Mn(CO)3 complexes. Taking advantage of these previous results, we undertook the synthesis of [7]THs substituted with iron or manganese complexes, selecting, as starting materials, the helicene scaffolds 1a,b17 and the organometallic candidates 218 and 316 (Figure 1).

the formation of an electrochemically active layer on the electrode surface. These features are consistent with the typical oligothiophene radical cation electropolymerization mechanism. The same investigation also provides important rationalization criteria concerning the effect of three key parameters: (i) the size of the π system, (ii) effects of substitution, and (iii) oligomerization ability. The substitution effect (ii) and electrooligomerization ability (iii) are briefly summarized below, in order to better understand the detailed electrochemical study reported in the present paper. (ii). Modulation of the Electronic Distribution of the [7]THs by the Nature and Position of Substituents. A huge electron attractor inductive effect is observed with perfluorinated alkyl chains on the terminal 2,13-positions (which inter alia undergo multielectronic cleavage at the first reduction peak potential), while the effects of weakly electron donating alkyl and trialkylsilyl groups (effective tools to improve solubility and processability) appear less conspicuous and are partially or totally shadowed by steric hindrance effects; in any case, such combined inductive + steric effects appear, as expected, to be clearer when substitution concerns the terminal, potentially reacting 2,13-positions rather than the distant 7,8positions. (iii). Electro-Oligomerization Ability. Fast, regular electrodimerization occurs on [7]THs having a single free terminal position per molecule, whereas electrooligomerization occurs on [7]THs having two free terminal positions per molecule; however, oligomerizations appear increasingly hampered on inserting increasingly bulkier alkyl substituents on the 7,8positions, which can be interpreted by taking into account both increased solubility (preventing accumulation of coupling products on the electrode surface) and steric hindrance (decreasing the probability of radical cation effective encounters). The search for new methods in the preparation of [7]THs and their functionalization has been recently intensively expanded, allowing the synthesis of original derivatives substituted by formyl, silyl, ester, and ketone groups.5 To the best of our knowledge, the synthesis of organometallic derivatives of tetrathia[7]helicenes has been only recently reported for some ruthenium(II) and iron(II) complexes carrying a helicene as ancillary ligand, as well as cyclometalated Pt(II) systems.6 Organometallic chromophores have been shown to be potential interesting candidates for electro-optical and nonlinear optical (NLO) purposes.1 Indeed, when they present a donor−π system−acceptor (push−pull) structure they offer a large diversity of tunable electronic behavior, depending on the size of the π system and the nature and the redox properties of the transition-metal atoms.7,8 In this context, the preparation of mono- and bimetallic π-conjugated complexes with a chiral heteroaromatic helix as spacer may be deemed as an exciting challenge in order to study the physicochemical and electrochemical properties of such original push−pull systems. Herein, we report the preparation of tetrathia[7]helicenes substituted, at the thiophene end rings, by Fc- and (η5cyclohexadienyl)Mn(CO)3-based complexes using Sonogashira coupling reactions and the optoelectrochemical studies of the corresponding mono- and bimetallic Fc complexes.

Figure 1. Compounds used as starting materials.

In order to couple the organometallic moieties 2 and 3 with the [7]THs 1a,b, the proper iodo-[7]THs have been synthesized (Scheme 1). Synthesis of Fc-Substituted Complexes. The monoiodation of helicene 1a was revealed to be difficult, and the reaction of 1.5 equiv of n-BuLi, followed by quenching with iodine, provided a mixture of di- and monoiodo derivatives in a 5/2 ratio (Scheme 1). Pure 2-iodo helicene 4 was obtained after three recrystallizations in n-hexane in 41% yield. When 1,2diiodoethane or diiodotetrafluoroethane was used as the iodation reagent, no reaction occurred and 1a was totally recovered. An even lower yield was achieved starting from 1b, and by the same experimental procedure, monoiodo derivative 6 was isolated in 12% yield. In contrast, diiodo derivatives 519 and 7 were formed in almost quantitative yields after treating helicenes 1a,b with 3 equiv of n-BuLi followed by the addition of iodine at low temperature. For the preparation of the monometallic complexes, ethynylferrocene 218 reacted with the monoiodo compounds 4 and 6, using typical Sonogashira reaction conditions, and complexes 8 and 10 were obtained in 75% and 88% yields, respectively (Scheme 1). Starting from 5 and 7, dimetallic complexes 9 and 11 were also formed in good to excellent yields (76 and 93% yields, respectively) using the same procedure. Dark red monocrystals of the dinuclear complex 9, suitable for X-ray analysis, were grown in a dichloromethane/n-hexane mixture.20 An ORTEP view is given in Figure 2. The structural parameters were compared with those of the tetrathia[7] helicene 1a and with the already reported 2,13-bis93


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Scheme 1. Synthesis of Mono- and Diferrocenyl Complexes 8−11 by Sonogashira Coupling

Table 2. Mean Deviations (Å) of the Ring Atoms from the Least-Squares Planes of the Rings for Compounds 9, silyl[7]TH, and 1a A A′ B B′ C C′ D

9

silyl[7]TH

1a

0.009(1) 0.009(1) 0.030(2) 0.030(2) 0.048(1) 0.048(1) 0.069(2)

0.015(3) 0.013(4) 0.034(4) 0.036(4) 0.040(4) 0.036(4) 0.058(5)

0.013(3) 0.013(3) 0.030(3) 0.030(3) 0.035(3) 0.035(3) 0.055(3)

Scheme 2. Nomenclature of Fused Rings for Helicene 9

Figure 2. ORTEP view of complex 9. Ellipsoids are drawn at the 30% probability level.

Table 3. Selected Dihedral Angles (deg) between the LeastSquares Planes of the Rings for Compounds 9, silyl[7]TH, and 1a

(triisopropylsilyl)tetrathia[7]helicene19 (silyl[7]TH) as an example of tetrathia[7]helicene sterically encumbered at the C2 and C13 positions (Table 1). Table 1. Selected Bond Lengths (Å) for Compound 9 Compared with Those of silyl[7]TH and 1a C2−C8 C8−C9 C9−C10 C4−C5 C7−C7i C3b-C5b C6b-C6bi

9

silyl[7]TH

1a

1.440(3) 1.187(3) 1.434(3) 1.360(4) 1.382(6) 1.424(3) 1.428(5)

1.367(5) 1.359(7) 1.417(5) 1.419(5)

1.376(7) 1.356(7) 1.426(5) 1.443(6)

A−B A′−B′ B−C B′−C′ C−D C′−D′ A−A′

9

Silyl[7]TH

1a

7.66(11) 7.66(11) 10.79(12) 10.79(12) 12.59(12) 12.59(12) 56.26(8)

10.7(1) 11.7(1) 11.1(1) 12.5(1) 11.9(1) 10.5(1) 59.1(1)

8.8(2) 8.8(2) 8.6(1) 8.6(1) 11.3(1) 11.3(1) 48.6(1)

The triple-bond C8−C9 distance (1.187(3) Å) as well as the adjacent C2−C8 and C9−C10 bond distances (1.440(3) and 1.434(3) Å, respectively) are in the range typical of an arylsubstituted ethyne and are similar to those of the benzodithiophene ethynyl-Fc derivative reported by us.15 The IR stretching frequencies of the CC bond in the ethynyl-Fc derivatives 8 (2218 cm−1), 9 (2206 cm−1), 10 (2208 cm−1), and 11 (2204 cm−1) are somewhat lower compared to the frequency of a prototypical bisalkyl ethyne (e.g., 2-butyne, 2240 cm−1) and of a diphenylacetylene (2220 cm−1); this would point to a slightly less strong triple bond due to its conjugation to the helicene and the ferrocene units. Similar IR frequencies were observed in a benzodithiophene ethynyl-Fc previously mentioned.15 In order to test the possibility of further functionalization of the thiophenyl ring at the C13 carbon atom of the monometallic

The C−C outer core bonds of 9 are shorter and the C−C inner core bonds are longer than the expected 1.39 Å, as already observed in the unsubstituted helicene 1a and in several carbo- and heterohelicenes.21 All seven fused rings deviate from planarity, as evidenced in Table 2, and there is an increase in the deformation on going from the external A and A′ rings to the central D ring (see Scheme 2 for the nomenclature of fused rings). One of the most striking features of complex 9 structure is the very large dihedral angle of 56.26(8)°, which is notably greater than that observed in the unsubstituted helicene 1a (48.6(1)°) but very close to that of the silyl-disubstituted species (59.1(1)°)19 (Table 3). No intermolecular interaction through π−π stacking is observed. 94


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Scheme 3. Functionalization of Complex 8

complex 8, we attempted a lithiation/formylation sequence. Complex 8 reacted with 3 equiv of n-BuLi at low temperature and then with an excess of DMF, which gave the monoformyl complex 12 in 55% yield together with the unexpected new complex 13, obtained in 13% yield. As inferred from 1H NMR studies, this last compound appeared to be a dialdehyde bearing one formyl group at position 13, as in compound 12, and an additional formyl substituent on one of the three benzene rings (Scheme 3). Thus, while the 1H NMR spectrum of 12 exhibits, as expected, one singlet at 9.38 ppm for the C13 formyl group proton, the 1H NMR of 13 shows two singlets at 9.35 (formyl group at C13) and 10.41 ppm. The ArH proton signals appear as one multiplet at 8.34 ppm integrating for four protons and one deshielded singlet at 8.88 ppm, which could be attributed to a proton β to the second formyl group. Thus, it seems reasonable to assume that the second formyl group is a substituent on one of the three six-membered rings but it is difficult to establish the right position. No byproduct due to the reactivity of the ferrocenyl group, particularly inert toward the experimental conditions used, was observed during the reaction. The reactivity of the aldehyde group in 12 was tested in a typical reaction. Thus, the monoformyl complex 12 easily reacted with benzenesulfonylamide in the presence of trifluoroacetic anhydride22 to give the imine complex 14 in 66% yield, in addition to 12% of the starting material (Scheme 3). In order to study the influence of the presence of a sterically demanding group close to the ferrocenyl entity, complex 8 was reacted with the Co2(CO)8 dimer. The trimetallic complex 15 was obtained in 50% yield as a dark green solid where the triple bond is coordinated by the Co2(CO)6 entity (Scheme 4). This

through a triple bond. We chose the Mn complex 3 as the organometallic candidate and monoiodo (4) and diiodo helicenes (5) as the organic counterparts. The catalytic system used for the Sonogashira coupling reaction was Pd2dba3/AsPh3 in a mixture of triethylamine and THF as solvent, which has been proven to be the most efficient to prepare alkynesubstituted (η 5 -cyclohexadienyl)Mn(CO) 3 complexes. 13a Under these conditions, the mono- and dimetallic complexes 16 and 17 were isolated in 65 and 70% yields, respectively (Scheme 5). Scheme 5. Synthesis of Mono- and Dimanganese Complexes 16 and 17 by Sonogashira Coupling

The 1H NMR spectrum of complex 16 shows the presence of two diastereoisomers due to the helicoidal chirality of the helicene scaffold associated with the planar chirality of the Mn complex and exhibits the typical fingerprints of the η5cyclohexadienyl moiety: 2.23, m, 2 H6′ exo; 2.71, m, 2 H6′ endo; 2.95, m, 2 H5′; 4.97, d, J = 6 Hz, H2′; 5.05, d, J = 6 Hz, H2′; 5.64, dd, J = 6 and 2 Hz, H3′; 5.67, dd, J = 6 and 2 Hz, H3′. In the case of dimetallic complex 17, the 1H NMR spectrum appears more complicated because of the presence of four diastereoisomers; nevertheless, the η5-cyclohexadienyl fingerprint is again readily observable with its specific signals. Optical and Electrochemical Studies. The normalized UV−vis absorption spectra of the thiahelicene complexes 8− 10, together with those of their parent compounds 1a,b, are reported in Figure 3, the selected absorbance data being collected in Table 4. All helicenes show two distinct absorption regions; the first set is located between 350 and 420 nm and consists of two vibronic transitions and an unresolved shoulder at shorter wavelength. The second set of transitions is located between 230 and 340 nm. On comparison of the parent helicene 1a with the corresponding mono- and bis-functionalized ferrocenyl derivatives, the long-wavelength transitions undergo a regular bathochromic shift of ca. 15 nm upon each substitution (1a, λmax 387 nm; 8, λmax 401 nm; 10, λmax 415 nm), resulting from the combined effects of the introduction of a triple-bond spacer on the [7]TH moiety as well as its conjugation with the Fc unit. An analogous trend is observed on comparison of 1b and 10

Scheme 4. Reactivity of Complex 8 toward Co2(CO)8

compound is rather unstable in air, and unfortunately, all attempts to obtain crystals in order to study its X-ray structure were unsuccessful. Synthesis of (η5-cyclohexadienyl)Mn(CO)3-Substituted Complexes. Encouraged by the above results, we extended our study to manganese complexes using an efficient method that we previously developed to link the (η5-cyclohexadienyl)Mn(CO)3 moiety to thienyl14 and benzothiophenyl moieties15 95


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Figure 3. Optical absorption in DCM of the ferrocene complexes 8−10 and their parent compounds 1a,b.

particular, this implies that conjugation to the thiahelicene via the triple bond results in an electron poorer Fc group with respect to that of a free group. Actually, it is reasonable that ferrocene oxidation, being centered on the metal core, can be hampered by conjugation of the surrounding cyclopentadienyl groups to the triple-bond linker and the thiahelicene system, since it implies partial delocalization of the electron density far from the metal center. The subsequent chemically irreversible oxidation peak with its delayed irregular return peak resembles 1a in both potential and shape; it clearly belongs to the tetrathiahelicene moiety and should be interpreted in the same mechanistic terms as for 1a, the availability of one free α-thiophene position still affording radical cation coupling, although of course only dimerization can be expected in this case. This peak shifts slightly to less positive potentials (easier oxidation) upon substitution with Fc. The first reduction peak belongs to the [7]TH moiety and appears as a shoulder shortly before the background in the working CH2Cl2 solvent (as in the cited systematic study about tetrathiahelicenes4); however, it is easily perceivable (and can be further enhanced by background subtraction and/or differentation) so that its potential can be estimated with an uncertainty of a few hundredths of volts and its trend with helicene substitution is quite clear. In particular, the peak regularly shifts to less negative potentials (easier reduction) with an increasing number of Fc groups. Note that increasing effective conjugation should favor both [7]TH reduction and oxidation, resulting in comparable shifts of the former toward less negative potentials and of the latter toward less positive potentials; moreover, electron donation from the Fc groups through the triple-bond linker should make the [7]TH moiety slightly electron richer, thus further promoting oxidation. However, the shift of the first [7]TH oxidation peak is much smaller than that of the first reduction. This can be explained in terms of a repulsive interaction between the incipient radical cation forming on the thiahelicene moiety at the second oxidation peak and the positive charge formed on the ferrocene core at the first oxidation peak. Therefore, all of the above three features, i.e. the remarkable positive shift of the Fc oxidation potential, the positive shift of the first [7]TH reduction potential, and the lower than expected negative shift of the first [7]TH oxidation potential, point to some communication between the Fc groups and the [7]TH moiety through the triple-bond linker, which is fully consistency with the UV−vis evidence (small but significant and regular anticipation of the π−π* bands with increasing number of ferrocene substituents) and with the IR evidence

Table 4. Selected Absorbance Data for Several Thiahelicenesa compd

λonset/ nm

λmax/ nm

Eg,onset/ eV

Eg,max/ eV

λonset Fc/ nm

Eg,onset Fc/ eV

1a 1b 8 9 10

407 406 427 436 433

386 390 399 414 403

3.05 3.06 2.91 2.85 2.87

3.21 3.18 3.11 3.00 3.08

548 548 538

2.27 2.27 2.31

a Onset and maximum absorption wavelengths, λonset and λmax, are reported for both the [7]TH moiety transitions and for the ferrocenyl unit transition; the corresponding energy gaps are reported as Eg,onset, Eg,max, and Eg,onset Fc.

(λmax 391 and 404 nm, respectively). The n-propyl chains in positions 7 and 8 on the [7]TH central benzene ring cause only a 3 nm red shift of the alkyl derivatives (e.g. 8 and 9). Whereas the helicene π−π* transitions are slightly but significantly influenced by the introduction of the ferrocenyl unit, the broad, featureless transition extending in the visible region between 420 and 550 nm present in complexes 8−10 can be ascribed to the largely localized 3d−3d* transition of the iron atom in the Fc unit.23 The band matches well the absorption spectra of ferrocene as reported in the literature (superimposed in the inset of Figure 3a). The CV patterns in CH2Cl2 solvent of thiahelicenes 8−10 and, for sake of comparison, of the parent thiahelicene 1a4 are contrasted in Figure 4, their key parameters being collected in Table 5. Unlike 1a, thiahelicene 8 only features one free terminal α-thiophene position, the other one being linked to a Fc group via a triple bond. As a consequence, a new oxidation peak, electrochemically and chemically reversible, corresponding to the reversible ferrocene/ferrocenium couple, appears well before the first oxidation peak of the tetrathiahelicene [7]TH moiety (Eonset [7]TH − Ep,Fc ≈ 0.55 V). It is worth noting that the half-wave peak potential (E1/2, corresponding to the formal potential, i.e. to the standard redox potential neglecting activity coefficients) is more positive by +0.14 V for the Fc couple conjugated to the tetrathiaelicene moiety than for the free Fc+/Fc couple in the same solvent (corresponding, of course, to 0.00 V on this potential scale). This feature is confirmed by the Fc group peak potentials in the cases of 9 and 10, being practically coincident with the case for 8. Since the electron transfer is electrochemically reversible (i.e., very fast) in both the free and conjugated Fc cases, the positive potential shift upon conjugation should be explained by thermodynamic rather than kinetic considerations; in 96


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(slightly lower frequency with respect to unconjugated triplebond cases). This finding is also fully consistent with a former systematic investigation on conjugated (ferrocenylethynyl)oligothiophenes,24 where conjugation was confirmed by nearIR data; it is worthwhile noting that in such case the displacement of the ferrocenyl standard potential toward more positive values upon conjugation to the π-conjugated system via the triple bond was less conspicuous by far than in our present case (+0.05 V rather than +0.15 V). An alternative (or concurrent) justification for the experimental evidence about communication between the metal complex and the πconjugated system could be sought in 3D intramolecular interactions; however, our structural data reported above apparently do not support such an assumption. Thiahelicene 10, only differing from 8 by the two propyl chains at the 7,8-positions, gives nearly the same CV pattern. A mild, asymmetric electron-donating effect of the propyl chains can be again perceived in the negative shift of the first reduction potential (the oxidation peak potential remaining, again, practically constant). In the case of thiahelicene 9, both terminal α-thiophene positions are conjugated to a Fc group through a triple bond; accordingly, the reversible ferrocene peak in the CV pattern doubles its height while maintaining the features of a monoelectronic peak; this indicates that the two Fc groups conjugated to the thiahelicene moiety via the triple-bond linkers are equivalent and reciprocally noninteracting; this is by no means surprising, since they are at the opposite ends of a long π-conjugated system having a large torsion angle. The same happened, for instance, in the cited study on (ferrocenylethynyl)oligothiophenes, which was justified by comparison with the case of all-trans compounds Fc(C C)nFc, in which no Fc peak separation is observable for n > 3, and of FcCHCH−C6H5−CHCHFc, where no Fc peak separation is observable (however, communication along the π system is present in both cases).24 Since no free α-thiophene position is available in 9, the first oxidation peak referring to the tetrathiahelicene moiety loses its irregular return peak, as no radical cation coupling can take place. From either the maxima or the onsets of the oxidation and reduction peak potentials the energies of the molecule HOMO and LUMO can be estimated by eqs 1 and 2, where Ea and Ec are the peak or onset potentials corresponding to the first oxidation and reduction, respectively.25,26

Figure 4. CV patterns recorded at a GC electrode, in CH2Cl2 + 0.1 M TBAP solutions at a scan rate of 0.2 V s−1, for tetrathiahelicenes 8−10, together with parent tetrathiahelicene 1a, normalized against scan rate and concentration.

EHOMO (eV) ≈ − 1e × [(Ea (V vs Fc+|Fc) + 4.8 (V Fc+|Fc vs zero)]

(1)

Table 5. Selected CV Data (Scan Rate 0.2 V s−1) for Several Thiahelicenesa first [7]TH redn peak

1a 1b 8 9 10

Eonset [7]TH/V (Fc+| Fc)

Ep/V (Fc+| Fc)

−2.43 −2.46 −2.34 −2.26 −2.37

−2.57 −2.64 −2.42 −2.38 −2.54

reversible Fc oxidn peak Ep/V (Fc+| Fc)

0.172 0.170 0.168

Ep − Ep/2/ V

0.061 0.057 0.060

E1/2/V (Fc+| Fc)

0.140 0.132 0.131

first [7]TH oxidn peak Eonset/V (Fc+| Fc)

Eonset [7]TH/V (Fc+|Fc)

Ep/V (Fc+| Fc)

Ep − Ep/2/ V

0.050 0.056 0.048

0.73 0.73 0.71 0.71 0.72

0.860 0.860 0.836 0.845 0.858

0.052 0.078 0.043 0.071 0.064

a

Onset potentials Eonset, peak potentials Ep, and half-wave potentials E1/2 are all referenced to the ferrocenium|ferrocene intersolvental reference couple. Half-peak widths (Ep − Ep/2) and electrochemical HOMO−LUMO energy gaps are obtained from the first oxidation and first reduction peak potentials, Eg,max, or from the first oxidation and first reduction onset potentials, Eg,onset. [7]TH = tetrathiahelicene. 97


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2.59 2.55 2.71 2.39 2.32 2.42 −4.97 −4.97 −4.97 −4.85 −4.86 −4.85

Eg,max Fc−[7]TH/eV Eg,max [7]TH−[7]TH/eV Eg,onset Fc−[7]TH/eV

3.43 3.50 3.26 3.23 3.40 3.16 3.19 3.05 2.97 3.09

Eg,onset [7]TH−[7]TH/eV ELUMO,[7]TH max/eV

−2.23 −2.16 −2.38 −2.42 −2.26 −2.37 −2.34 −2.46 −2.54 −2.43

ELUMO,[7]TH onset/eV EHOMO,Fc max/eV

−5.66 −5.66 −5.64 −5.65 −5.66

EHOMO,Fc onset/eV EHOMO,[7]TH max/eV

Such EHOMO and ELUMO values, together with the derived HOMO−LUMO energy gaps Eg, are reported in Table 6, and they are quite similar to the spectroscopic values, calculated following the same two criteria, i.e. from the maximum or onset absorption wavelengths (Table 4), although the electrochemical and spectroscopic approaches correspond to two different processes, i.e. electron transfer between molecule and electrode in the first case, and electron transition between different energy states in the second case. In the cases of 8−10 HOMO levels can be calculated both from the first tetrathiahelicene oxidation peak and from the Fc redox peak (i.e., the first oxidation peak of the molecule on the whole). The HOMO−LUMO energy gaps calculated in the first case (labeled as [7]TH−[7]TH) nearly coincide with the spectroscopic HOMO−LUMO gaps calculated from the first well-defined absorption peak (Figure 3 and Tables 4 and 6); those calculated in the second case (labeled as Fc−[7]TH) resemble the spectroscopic energy gaps calculated from the first onset of the whole absorption pattern, starting much before the first well-defined absorption peak. Since the first [7]TH oxidation peaks in the monomer CV patterns correspond to radical cation formation, possibly leading to coupling and formation of conducting films, we performed several oligomerization experiments, by repeated potential cycling around such first oxidation peak (Figure 5) followed by testing the deposited electroactive film in a monomer-free solution (Figure 6). In the case of parent compound 1a fast and regular oligomerization is achieved, as showed by the regular increase of the CV pattern upon potential cycling (only early cycles are reported in Figure 5, but the film can regularly grow for many more cycles; consider for example the 60th cycle reported in Figure 6). The resulting conducting film is more easily oxidized than the corresponding monomer, pointing to a more extended and/or efficient π system. This feature can be explained in terms of the increased number of conjugated thiophene rings and/or in terms of a solid-state effect (in particular, π-stacking interactions). The electroactive film is rather stable upon washing and testing in a monomer-free solution (Figure 6), provided that CV scans and the charging/uncharging (“doping/ undoping”) process be restricted to the oxidation region, while it appears to collapse upon injection of a negative charge. The case of 1b, not reported in Figure 5, shows a dramatic decrease in film deposition aptitude, in spite of the CV monomer pattern being very similar to that of 1a; this has been ascribed to the effect of the propyl chains, possibly hampering the formation of a regular conducting layer on the electrode surface and/or improving the oligomer solubility. In the case of 8 no oligomerization at all is achieved, as expected, upon cycling around the very first oxidation peak in the molecule: i.e. that corresponding to the Fc group. Instead, very fast and regular deposition of a conducting layer, possibly consisting of dimers, since only one α-thiophene position is available, is obtained by including the first [7]TH oxidation peak in the potential scan cycles (Figure 5). Investigating the electrochemical properties of the resulting conducting film is attractive, since it potentially combines the features of a redox polymer (localized redox sites on a polymer scaffold, charge transport taking place by redox exchanges

−5.53 −5.53 −5.51 −5.51 −5.52

(2)

EHOMO,[7]TH onset/eV

+ 4.8 (V Fc+|Fc vs zero)]

1a 1b 8 9 10

ELUMO (eV) ≈ − 1e × [(Ec (V vs Fc+|Fc)

compd

Table 6. HOMO and LUMO Energies and HOMO−LUMO Energy Gaps Calculated from the Electrochemical Data in Table 2, Considering Either the First Tetrathiahelicene [7]TH Oxidation Peak or the Fc Oxidation Peak for HOMO Calculation and Applying either the Maximum or the Onset Criterion

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Organometallics

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Figure 6. Comparison between the CV features of the conducting oligomer films obtained by electrooxidation of tetrathiahelicenes 1a (top) and 8 (bottom), in CH2Cl2 + 0.1 M TBAP at a scan rate of 0.2 V s−1: CV patterns of the monomers (thin lines a); last polymerization cycles, 60th for 1a and 54th for 8 (thick lines b); first and second complete CV scans of the two oligomer films in monomer-free solution (thick line c and thin line d, respectively).

Figure 5. Oligomerization attempts by CV oxidative cycles performed on tetrathiahelicenes 8−10, plus parent tetrathiahelicene 1a as a reference, at 0.000 75 M concentration in CH2Cl2 + 0.1 M TBAP and a scan rate of 0.2 V s−1: thick line, first cycle; thin lines, selected subsequent cycles (reported as Roman numerals).

This anodic event can hardly be ascribed to increased conjugation in the π system, comparing the onset potential of the conducting film obtained by oligomerization of parent thiahelicene 1a, considering that 8 can only dimerize having only one free α-thiophene position (unless β-thiophene positions could be involved in coupling processes, which appears unlikely). Therefore the redox features of the conducting film point to some coupling in charge transfer between the Fc sites and the triple-bond/tetrathiahelicene sites within the solid film, possibly favored by solid-state stacking; in other words, the conducting material as a whole seems to benefit from the very low oxidation potential of its Fc sites, providing a low-energy onset for the whole material oxidation and, therefore, a very low HOMO level with respect to the [7]TH HOMO in the monomer case. Also in the case of 8, as in the 1a case, the conducting film is sufficiently stable upon washing and testing in monomer-free solution, while restricting the potential cycling and doping/ undoping processes to the oxidative region. In contrast, again as in the case of 1a, the film hardly bears the injection of a

between adjacent redox sites) and of a conducting polymer based on an extended π-conjugated system. In particular, it is interesting to see whether the features of the two systems are simply additive or are also coupled. In this context it is remarkable that while in the monomer case the redox peak of the Fc moiety appears to be independent and is at potentials quite less positive than that of the [7]TH moiety (ΔE ≈ 0.45 V), the CV pattern of the conducting film features a broad oxidation wave resulting from the merging of several redox peaks, having its onset at the ferrocene oxidation and no solution of continuity with the (far more positive) potential range for the oxidation of polythiahelicene films. This is remarkable, since even in the cited literature case,24 in which conjugation was present, the peak related to the ferrocenyl groups and those related to the conjugated system were neatly separated. In our present case the reversible redox system perceivable at Ep,a ≈ 0.41 V (Fc+|Fc) is particularly intriguing. 99


Organometallics

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H and 13C NMR spectra were recorded on a Bruker AC 400 MHz or DRX 500 MHz spectrometer. Infrared spectra were measured on a Perkin-Elmer 1420 spectrometer. Crystallographic data were collected on a Oxford Diffraction XCALIBUR diffractometer. High-resolution mass spectra analyses were performed by the Groupe de Spectrométrie de Masse (UMR 7201, UPMC). Ethynylferrocene 3,18 diiodo derivative 5,19 and ethynyl-(η5-4methoxylcyclohexadienyl)Mn(CO)3 316 were prepared according to the literature. The cyclovoltammetric studies of thiahelicenes 8−10, like those of parent thiahelicenes 1a,b, have been performed at scan rates typically ranging 0.05−2 V s−1 in CH2Cl2 solutions 0.000 25, 0.0005, and 0.000 75 M in each substrate, deareated by N 2 bubbling, with tetrabutylammonium perchlorate (TBAP, 0.1 M) as the supporting electrolyte, at 298 K.27 The ohmic drop has been compensated by the positive feedback technique.28 The experiments were carried out using an AUTOLAB PGSTAT potentiostat (EcoChemie, The Netherlands) run by a PC with GPES software. The working electrode was glassy carbon (GC) (AMEL, diameter 1.5 mm), cleaned by diamond powder (Aldrich, diameter 1 μm) on a wet cloth (STRUERS DP-NAP), the counter electrode was a platinum wire, and the reference electrode was an aqueous saturated calomel electrode (SCE), having in our working solvent (CH2Cl2) a difference of −0.495 V vs the Fc+|Fc couple (the intersolvental redox potential reference currently recommended by IUPAC)25 and 0.052 V vs the Me10Fc+|Me10Fc couple (an improved intersolvental reference under investigation).26 Electrochemical oligomerization has been carried out on the above GC electrodes by repeated cycling around the first oxidation peak at a scan rate of 0.2 V s−1, after which the electrochemical activity of the resulting conducting film was tested by extracting the electrode from the working solution, washing it with CH2Cl2, and inserting it into a new, monomer-free solution. Purification of Tetrathia[7]helicene 1a. Helicene (153 mg) was dissolved in dry THF (10 mL). The solution was cooled to −78 °C, and n-butyllithium (0.95 mL, 1.6 M in n-hexane) was introduced dropwise. After the mixture was stirred for 30 min at this temperature, H2O (1 mL) was added. The reaction mixture was warmed to room temperature. A solution of saturated NH4Cl (20 mL) was added, and the organic layer was separated. The inorganic phase was extracted with diethyl ether (20 mL × 3). Combined ether solutions were washed with brine (20 mL × 2) and dried over anhydrous MgSO4. After removal of solvent under reduced pressure, the residue was purified by flash chromatography on silica gel (petroleum ether/ CH2Cl2, 8/1) to afford compound 1a (113 mg) as a light yellow powder. 2-Iodotetrathia[7]helicene 4. A solution of helicene 1a (120 mg, 0.30 mmol) in THF (10 mL) was cooled to −78 °C and treated with n-BuLi (1.6 M in n-hexane, 0.24 mL, 0.39 mmol). The reaction mixture was stirred for an additional 40 min, then a solution of I2 (122 mg, 0.48 mmol) in THF (6 mL) was added, and the mixture was stirred for another 30 min at −78 °C and then warmed to room temperature. A solution of saturated Na2S2O3 (20 mL) was added to quench the reaction, and the organic layer was separated. The inorganic phase was extracted with diethyl ether (20 mL × 2). The combined ether solution was washed with brine (20 mL) and H2O (20 mL) and dried over anhydrous MgSO4. After removal of solvent under reduced pressure, the residue was purified by flash chromatography on silica gel (petroleum ether/CH2Cl2, 10/1) to afford a mixture of mono- and diiodo derivatives. After three successive recrystallizations in n-hexane, pure 2-iodotetrathia[7]helicene 4 (65 mg, 41% yield) was obtained as a yellow-green powder.

negative charge: the first reduction step takes place at a considerably less negative potential with respect to the monomer case, again confirming the higher conjugation efficiency of the solid material but resulting in immediate collapse of the conducting layer on the electrode (compare curves c and d for the case of 8 in Figure 6). Once more the presence of two propyl chains at the 7,8positions drastically hampers the formation of the conducting layer, as is evident on comparing the cases of 8 and 10 in Figure 5, although some tendency for the deposition of an electroactive film can be perceived also in this case (possibly a better performance might be achieved by increasing the monomer concentration). In contrast, as expected, no conducting film is obtained in the case of 9.

■

CONCLUSION In this paper, we have described the synthesis, via Sonogashira coupling, of tetrathia[7]helicenes substituted at one of the thiophene rings by a Fc (8 and 10) or a (η5-cyclohexadienyl)Mn(CO)3 derivative (16). The second thiophene end ring could be easily functionalized on one side by another iron or manganese entity (9, 11, or 17) and on the other side by an aldehyde (12) or an imine group (14). The X-ray structure of the diferrocenyl complex 9 exhibits a large dihedral angle between the two terminal thiophene rings of 56.2°, which stresses that the helix pitch is sensitive to the steric hindrance of the substituents of these rings. The detailed electrochemical investigation performed on the new compounds shows that the Fc groups are conjugated to the triple bond and the thiahelicene π system. Localization of the first oxidation on the Fc moieties rather than on the thiahelicene system implies a much higher HOMO and a much narrower HOMO−LUMO gap. Provided that one of the tetrathiahelicene terminals is free, fast and regular deposition of an electroactive conducting film is achieved by potential cycling around the second oxidation peak (the one assigned to the thiahelicene moiety); the process is hampered by long alkyl chains, which can be explained in terms of both increased solubility and/or steric hindrance. While in the monomer case the redox peak of the Fc moiety appears to be well separated from the thiahelicene peak, the conducting film features a broad oxidation wave resulting from the merging of several redox peaks, having its onset at the Fc oxidation and no solution of continuity with the much more positive potential range for thiahelicene oxidation. Since conducting films obtained by electrooligomerization of parent [7]TH have their onset potentials 0.45 V more positive than the Fc redox sites in the conjugates, the above continuity could point to some coupling between Fc redox centers and conjugated π systems, favored by solid-state stacking.

■

EXPERIMENTAL SECTION

General Procedures. All reactions were performed in oven-dried glassware under a dry nitrogen or argon atmosphere using Schlenk tube techniques. Reagent grade CH2Cl2 was dried over LiAlH4 and distilled, and THF was dried over benzophenone ketyl and distilled. NEt3 was distilled with KOH before use. DMF was distilled from 4 Å molecular sieves before use. TFAA was distilled under a dry N2 atmosphere. Reagents were used as received without further purification. Analytical thin-layer chromatography was performed on Merck silica gel 60 F 254 plates (0.25 mm). Flash column chromatography was carried out using a forced flow of the indicated solvent on Roth Kieselgel 60 (0.02−0.045 mm). 100


Organometallics

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Yield: 41%. 1H NMR (500 MHz, CDCl3): δ 6.73 (d, 1H, J = 5 Hz, H or H14), 6.95 (s, 1H, H1), 7.05 (d, 1H, J = 5 Hz, H14 or H13), 7.92 (s, 2H, Ar H), 8.03 (m, 3H, Ar H), 8.12 (d, 1H, J = 8 Hz, Ar H). 13C NMR (125 MHz, CDCl3): δ 118.6, 118.7, 119.8, 120.3, 120.7, 121.6, 124.7 (C13 or C14), 124.9 (C14 or C13), 129.4, 129.7, 129.9, 130.5, 132.5, 136.0, 136.1 (C1), 136.8, 136.9, 137.0, 137.1, 137.6, 137.7, 141.4 ppm. IR (neat): ν 2924, 1725 cm−1. HRMS (ESI): 527.862 63 (M+), 550.852 40 (M+ + Na), calcd for C22H9IS4 527.863 18. 2-Iodo-7,8-di-n-propyltetrathia[7]helicene 6. The same procedure as for 4 with 1.4 equiv of n-BuLi was applied to 1b for the preparation of 6 (flash chromatography: pure pentane). 13

or C13), 128.3, 129.9 (C1), 130.1, 130.7, 130.9, 136.5, 137.3, 137.4, 137.5, 137.6, 137.7, 138.2, 138.3 ppm. IR (neat): ν 2954, 2923, 2361, 2342, 2218 cm−1. UV/vis (CH2Cl2): λ (ε) 399 (13 000), 325 (19 100), 275 nm (25 900). HRMS (ESI): 609.963 59 (M+), 632.953 36 (M+ + Na), calcd for C34H18FeS4 609.9640. Anal. Calcd for C34H18FeS4: C, 66.88; H, 2.97. Found: C, 67.03; H, 3.09. 2,13-Diferrocenylethynyltetrathia[7]helicene 9. The same procedure as for 8 was applied to 5 for the preparation of 9 in the presence of Pd(PPh3)2Cl2 and CuI (10% mol) (flash chromatography: petroleum ether/Et2O, 25/1).

Yield: 12%. 1H NMR (500 MHz, CDCl3): δ 1.16 (t, 6H, J = 6 Hz, CH3CH2CH2−), 1.88 (m, 4H, CH3CH2CH2−), 3.13 (m, 4H, CH3CH2CH2−), 6.72 (dd, 1H, J = 5 Hz, J = 1 Hz, H14), 6.94 (d, 1H, J = 1 Hz, H1), 7.02 (d, 1H, J = 5 Hz, H13), 7.87 (d, 1 H, J = 8 Hz, Ar H), 7.91 (d, 1H, J = 8 Hz, Ar H), 7.98 (d, 1H, J = 8 Hz, Ar H). 13C NMR (125 MHz, CDCl 3 ): δ 14.7 (CH 3 CH 2 CH 2 −), 23.3 (CH3CH2CH2−), 34.4 (CH3CH2CH2−), 75.4, 118.6, 118.7, 119.3, 121.0, 124.4 (C13), 124.7 (C14), 127.9, 128.1, 130.0, 131.1, 132.2, 132.5, 135.8, 136.0, 136.1 (C1), 136.3, 136.7, 136.9, 139.7, 139.8, 141.2 ppm. IR (neat): ν 2957, 2924, 1561 cm−1. HRMS (ESI): 611.956 53 (M+), calcd for C28H21IS4 611.957 08. 2,13-Diiodo-7,8-di-n-propyltetrathia[7]helicene 7. The same procedure as for 4, but with 5.0 equiv of n-BuLi, was applied to 1b for the preparation of 7 (flash chromatography: petroleum ether/CH2Cl2, 10/1).

Yield: 76%. 1H NMR (500 MHz, d8-THF): δ 4.41 (br, 7H, Fc H), 4.58 (s, 1H, Fc H), 4.64 (s, 1H, Fc H), 7.03 (s, 1H, H1), 8.27 (dd, 2H, J = 24 Hz, J = 8 Hz, H4 and H5), 8.34 (s, 1H, H7). 13C NMR (125 MHz, d8-THF): δ 65.4 (Fc C), 70.3 (Fc C), 71.2 (Fc C), 72.5 (Fc C), 72.6 (Fc C), 79.7 (Cethynyl), 95.7 (Cethynyl), 120.8 (C4 or C5), 121.9 (C7), 122.1 (C5 or C4), 124.1, 130.4 (C1), 130.8, 131.3, 136.8, 138.4, 138.7, 139.2 ppm. IR (neat): ν 2955, 2923, 2361, 2341, 2206 cm−1. HRMS (ESI): 840.951 05 (M+ + Na), 856.925 01 (M+ + K), calcd for C46H26Fe2S4 817.961 61. Anal. Calcd for C46H26Fe2S4: C, 66.88; H, 2.97. Found: C, 67.03; H, 3.09. 2-Ferrocenylethynyl-7,8-di-n-propyltetrathia[7]helicene 10. The same procedure as for 8 was applied to 6 for the preparation of 10 in the presence of Pd(PPh3)2Cl2 and CuI (5% mol) (flash chromatography: petroleum ether/EtOAc, 80/1).

Yield: 99%. 1H NMR (400 MHz, CDCl3): δ 1.17 (t, 6H, J = 7 Hz, CH3CH2CH2−), 1.87 (m, 4H, CH3CH2CH2−), 3.12 (m, 4H, CH3CH2CH2−), 6.91 (s, 2H, H1 and H14), 7.91 (dd, 4H, J = 9 Hz, J = 8 Hz, H4, H5, H10, and H11). 13C NMR (100 MHz, CDCl3): δ 14.7 (CH3CH2CH2−), 23.2 (CH3CH2CH2−), 34.4 (CH3CH2CH2−), 75.7, 118.7 (C4 or C5), 119.5 (C5 or C4), 127.7, 129.7, 132.5, 135.5 (C1), 136.3, 136.4, 139.8, 141.6 ppm. IR (neat): ν 2952, 2923, 1726 cm−1. HRMS (ESI): 737.853 07 (M+), 760.842 83 (M+ + Na), calcd for C28H20I2S4 737.853 73. 2-Ferrocenylethynyltetrathia[7]helicene 8. Pd(PPh3)2Cl2 (4.3 mg, 0.006 mmol), CuI (1.2 mg, 0.06 mmol), and helicene 4 (65 mg, 0.123 mmol) were added successively to ethynylferrocene 3 (34 mg, 0.160 mmol) in a mixture of anhydrous degassed triethylamine (10 mL) and freshly distilled THF (8 mL). The resulting mixture was heated to reflux for 2 h and cooled to room temperature, and 20 mL of diethyl ether was added. After filtration on Celite and removal of the solvent under vacuum, the residue was purified by flash chromatography on silica gel (flash chromatography: petroleum ether/CH2Cl2/ EtOAc, 250/20/2) to afford 2-(ferrocenylethynyl)tetrathia[7]helicene 8 in 75% yield as a red powder. Yield: 75%. 1H NMR (500 MHz, d8-THF): δ 4.25−4.28 (br, 7H, Fc H), 4.40 (br, 1H, Fc H), 4.47 (s, 1H, Fc H), 6.81 (d, J = 6 Hz, H13 or H14), 6.87 (s, 1H, H1), 7.26 (d, 1H, J = 6 Hz, H14 or H13), 8.05 (d, 1H, J = 8 Hz, Ar H), 8.13 (t, 2H, J = 8 Hz, Ar H), 8.19−8.21 (m, 3H, Ar H). 13C NMR (125 MHz, d8-THF): δ 64.5 (Fc C), 69.4 (Fc C), 70.3 (Fc C), 71.6 (Fc C), 71.7 (Fc C), 78.6 (Cethynyl), 94.5 (Cethynyl), 119.0, 120.0, 120.8, 121.0, 121.1, 121.9, 122.7, 124.9 (C13 or C14), 125.5 (C14

Yield: 88%. 1H NMR (500 MHz, d8-THF): δ 1.05 (t, 6H, J = 5 Hz, CH3CH2CH2−), 1.79 (m, 4H, CH3CH2CH2−), 3.06 (m, 4H, CH3CH2CH2−), 4.09 (m, 7H, Fc H), 4.24 (s, 1H, Fc H), 4.31(s, 1H, Fc H), 6.64 (d, 1H, J = 5 Hz, H13 or H14), 6.70 (s, 1H, H1), 7.07 (d, 1H, J = 5 Hz, H13 or H14), 7.85 (d, 1H, J = 8 Hz, Ar H), 7.94 (t, 2H, J = 8 Hz, Ar H), 8.00 (d, 1H, J = 8 Hz, Ar H). 13C NMR (125 MHz, d8-THF): δ 14.3 (CH3CH2CH2−), 23.6 (CH3CH2CH2−), 34.5 (CH3CH2CH2−), 64.6 (Fc C), 69.3 (Fc C), 70.3 (Fc C), 71.6 (Fc C), 71.7 (Fc C), 78.3 (Cethynyl), 94.3 (Cethynyl), 118.9 (Ar C), 119.8 (Ar C), 120.6 (Ar C), 121.5 (Ar C), 122.5, 124.7 (C13 or C14), 125.3 (C13 or C14), 128.5, 129.8 (C1), 131.3, 131.5, 132.4, 132.6, 135.8, 136.3, 136.6, 136.9, 137.3, 137.5, 137.6, 140.1, 140.2 ppm. IR (neat): ν 2960, 2926, 2360, 2341, 2208 cm−1. UV/vis (CH2Cl2): λ (ε) 403 (17 500), 385 (15 400), 329 (22 800), 296 (21 900), 276 nm (25 200). HRMS (ESI): 717.047 28 (M+ + Na), 733.021 22 (M+ + K), calcd for 101


Organometallics

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C35H18FeOS4 637.958 98. Anal. Calcd for C35H18FeOS4: C, 65.83; H, 2.84. Found: C, 65.69; H, 2.69.

C40H30FeS4 694.057 97. Anal. Calcd for C40H30FeS4: C, 69.15; H, 4.35. Found: C, 69.02; H, 4.24. 2,13-Diferrocenylethynyl-7,8-di-n-propyltetrathia[7]helicene 11. The same procedure as for 8 was applied to 7 for the preparation of 11 in the presence of Pd(PPh3)2Cl2 and CuI (10% mol) (flash chromatography: petroleum ether/EtOAc, 80/1 to 50/1).

13-Ferrocenylethynyltetrathia[7]helicenedicarbaldehyde 13. A more polar product from the mixture above was separated with

the eluent petroleum ether/CH2Cl2/EtOAc (200/90/10), and 13 was obtained. Yield: 13%. 1H NMR (500 MHz, d8-THF): δ 4.27 (s, 5H, Fc H), 4.29 (m, 2H, Fc H), 4.44 (t, 1H, J = 1 Hz, Fc H), 4.51 (t, 1H, J = 1 Hz, Fc H), 6.86 (s, 1H, H1), 7.36 (s, 1H, H14), 8.34 (m, 4H, H4, H5, H10, and H11), 8.88 (s, 1H, H7 or H8), 9.38 (s, 1H, thiophene CHO), 10.41 (s, 1H. Ar CHO). 13C NMR (125 MHz, d8-THF): δ 64.3 (Fc C), 69.49 (Fc C), 69.53 (Fc C), 70.4 (Fc C), 71.77 (Fc C), 71.79 (Fc C), 78.4 (Cethynyl), 96.2 (Cethynyl), 121.7, 122.7, 122.8, 123.1, 127.1, 127.2 (C7 or C8), 128.1 (C1), 129.2, 129.7, 129.9, 130.1, 131.8, 133.5, 134.2 (C14), 135.4, 136.6, 137.4, 138.5, 139.3, 140.8, 141.2, 142.7, 147.3, 183.6 (thiophene CHO), 190.2 (Ar CHO) ppm. IR (neat): ν 2924, 2202, 1744, 1674 cm−1. UV/vis (CH2Cl2): λ (ε) 435 (13 300), 354 (15 400), 274 nm (25 200). HRMS (ESI): 665.953 42 (M+), 688.943 19 (M+ + Na), calcd for C36H18FeO2S4 665.953 90. 13-Ferrocenylethynyltetrathia[7]helicene-2-carbaldehyde N-Benzenesulfonyl Imine 14. To a stirred mixture of aldehyde 12 (15 mg, 0.024 mmol) and benzenesulfonamide (4 mg, 0.026 mmol) in methylene chloride (3 mL) was added trifluoroacetic anhydride (6 mg, 0.027 mmol), and the solution was heated to reflux for 66 h. The reaction mixture was poured into cold water (5 mL) and extracted with CH2Cl2 (10 mL × 2). The organic solution was dried with MgSO4 and evaporated to afford the crude product. Flash chromatography on silica gel (petroleum ether/EtOAc, 70/10 to 30/10) gave pure imine 14 as a red solid.

Yield: 93%. 1H NMR (500 MHz, d8-THF): δ 1.06 (t, 6H, J = 7 Hz, CH3CH2CH2−), 1.79 (m, 4H, CH3CH2CH2−), 3.10 (m, 4H, CH3CH2CH2−), 4.13 (m, 14H, Fc H), 4.30 (t, 2H, J = 1 Hz, Fc H), 4.36 (t, 2H, J = 1 Hz, Fc H), 6.72 (s, 2H, H1 and H14), 7.93 (d, 2H, J = 8 Hz, Ar H), 8.00 (d, 2H, J = 8 Hz, Ar H). 13C NMR (125 MHz, d8-THF): δ 15.0 (CH3CH2CH2−), 24.3 (CH3CH2CH2−), 35.1 (CH3CH2CH2−), 65.1 (Fc C), 70.0 (Fc C), 71.0 (Fc C), 72.2 (Fc C), 72.3 (Fc C), 79.5 (Cethynyl), 95.3 (Cethynyl), 120.5 (C5 or C4), 121.5 (C4 or C5), 123.6, 128.8, 130.0 (C1), 131.6, 133.3, 136.2, 137.6, 138.0, 140.8 ppm. IR (neat): ν 2958, 2927, 2360, 2359, 2204 cm−1. UV/vis (CH2Cl2): λ (ε) 414 (12 100), 397 (11 500), 338 (20 800), 313 (21 000), 274 nm (23 000). HRMS (ESI): 902.054 57 (M+), 925.044 25 (M+ + Na), calcd for C52H38Fe2S4 902.055 51. Anal. Calcd for C52H38Fe2S4: C, 69.18; H, 4.24. Found: C, 69.05; H, 4.13. 13-Ferrocenylethynyltetrathia[7]helicene-2-carbaldehyde 12. A solution of compound 8 (100 mg, 0.16 mmol) in THF (8 mL) was cooled to −78 °C and treated with n-BuLi (1.6 M in n-hexane, 0.31 mL, 0.49 mmol). The reaction mixture was stirred for 30 min at −78 °C and another 10 min at −55 °C. Then the reaction mixture was cooled to −78 °C again, freshly distilled DMF (14 mg, 2.46 mmol) was added, and this mixture was stirred for 30 min at this low temperature and warmed to room temperature. A solution of saturated NH4Cl (20 mL) was added to quench the reaction, and the organic layer was separated. The inorganic phase was extracted with diethyl ether (20 mL × 2). The combined ether solution was washed with brine (30 mL) and H2O (30 mL) and dried over anhydrous MgSO4. After removal of solvent under reduced pressure, the residue was purified by flash chromatography on silica gel (petroleum ether/ CH2Cl2/EtOAc, 250/90/10) to afford aldehyde 12 in 50% yield as a red solid.

Yield: 66%. 1H NMR (500 MHz, d8-THF): δ 4.20 (s, 5H, Fc H), 4.26 (br, 2H, Fc H), 4.38 (br, 1H, Fc H), 4.41 (br, 1H, Fc H), 6.74 (s, 1H, H1), 7.53 (s, 1H, H14), 7.63 (m, 2H, −SO2Ar H), 7.71 (m, 1H, −SO2Ar H), 7.95 (d, 2H, J = 8 Hz, −SO2Ar H), 8.18 (dd, 2H, J = 18 Hz, J = 8 Hz, Ar H), 8.25 (d, 1H, J = 8 Hz, Ar H), 8.26 (s, 2H, H7 and H8), 8.32 (d, 1H, J = 8 Hz, Ar H), 8.69 (s, 1H, −CHN−). 13C NMR (125 MHz, d8-THF): δ 64.3 (Fc C), 69.38 (Fc C), 69.40 (Fc C), 70.3 (Fc C), 71.65 (Fc C), 71.67 (Fc C), 78.3 (Cethynyl), 95.2 (Cethynyl), 120.1, 121.2, 121.6, 121.7, 121.9, 123.3, 123.6, 128.1 (−SO2Ar C), 129.39 (−SO2Ar C), 129.44, 129.87 (C1), 129.90, 129.92, 131.6, 133.4 (−SO2Ar C), 135.3, 135.5, 137.1, 137.4, 137.8 (C14), 138.1, 138.6, 139.0, 139.8, 141.7, 147.3, 164.0 (−CHN−) ppm. IR (neat): ν 2956, 2924, 2359, 2200, 1571, 1321 cm−1. UV/vis (CH2Cl2): λ (ε) 425 (11 700), 356 (17 700), 313 (20 900), 274 (26 000), 252 (10 500) nm. HRMS (ESI): 776.967 71 (M+), 799.957 48 (M+ + Na), calcd for C41H23FeNO2S5 776.968 17. 2-Ferrocenylethynyltetrathia[7]helicene Cobalt Carbonyl Complex 15. A solution of Co2(CO)8 (28 mg, 0.082 mmol) in THF (10 mL) was added dropwise to a solution of compound 8 (50 mg, 0.082 mmol) in THF (5 mL). The resulting mixture was stirred at

Yield: 50%. 1H NMR (500 MHz, d8-THF): δ 4.24 (m, 7H, Fc H), 4.38 (t, 1H, J = 1 Hz, Fc H), 4.46 (t, 1H, J = 1 Hz, Fc H), 6.75 (s, 1H, H1), 7.40 (s, 1H, H14), 8.09 (d, 1H, J = 8 Hz, Ar H), 8.15 (d, 1H, J = 8 Hz, Ar H), 8.23 (s, 2H, H7 and H8), 8.23 (d, 1H, J = 8 Hz, Ar H), 8.29 (d, 1H, J = 8 Hz, Ar H), 9.38 (s, 1H, −CHO). 13C NMR (125 MHz, d8-THF): δ 64.3 (Fc C), 69.4 (Fc C), 69.5 (Fc C), 70.4 (Fc C), 71.7 (Fc C), 78.3 (Cethynyl), 95.2 (Cethynyl), 120.1, 121.3, 121.5, 121.6, 122.3, 123.1, 123.4, 129.5, 129.8 (C1), 129.9, 130.0, 132.0, 134.7 (C14), 135.4, 135.5, 137.4, 138.1, 138.2, 138.6, 138.9, 140.7, 142.4, 183.6 (−CHO) ppm. IR (neat): ν 2957, 2925, 2204, 1725 cm−1. UV/vis (CH2Cl2): λ (ε) 412 (18 300), 344 (27 600), 307 (29 200), 275 nm (29 800). HRMS (ESI): 637.958 36 (M+), 660.948 27 (M+ + Na), calcd for 102


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room temperature for 2.5 h and filtered with Celite. After removal of the solvent, the residue was purified by flash chromatography on silica gel (petroleum ether/CH2Cl2, 200/10) to afford cobalt complex 15 as a dark green solid.

4H, H6′ exo), 2.64 (s, 6H, CH3), 2.65 (s, 6H, CH3), 4.54 (d, 1H, J = 6 Hz, H3′), 4.58 (d, 1H, J = 6 Hz, H3′), 4.73 (d, 1H, J = 6 Hz, H3′), 4.82 (d, 1H, J = 6 Hz, H3′), 5.00−5.04 (m, 2H, H2′), 5.08 (dd, 1H, J = 6 Hz, J = 3 Hz, H2′), 5.12 (dd, 1H, J = 6 Hz, J = 3 Hz, H2′), 6.94−6.97 (m, 4H, H1), 7.44−7.48 (m, 4H, Ar H), 7.52−7.57 (m, 4H, Ar H), 7.64− 7.71 (m, 4H, Ar H). 13C NMR (100 MHz, C6D6): δ 30.2 (C6′), 30.5 (C6′), 31.5 (C6′), 31.7 (C6′), 36.5 (C5′), 36.6 (C5′), 36.8 (C5′), 46.1 (C1′), 46.2 (C1′), 46.8 (C1′), 46.9 (C1′), 53.9 (OCH3), 68.2 (C2′), 68.5 (C2′), 79.86 (Cethynyl), 79.91 (Cethynyl), 80.2 (Cethynyl), 80.3 (Cethynyl), 96.01 (Cethynyl), 96.05 (Cethynyl), 96.13 (Cethynyl), 96.16 (Cethynyl), 96.73 (C3′), 96.78 (C3′), 97.58 (C3′), 97.65 (C3′), 119.8, 119.9, 120.7, 121.2, 121.3, 122.0, 122.2, 128.7, 130.2 (C1), 130.4 (C1), 130.5 (C1), 133.5, 136.0, 137.6, 137.87, 137.93, 138.1, 143.3 (C4′), 143.4 (C4′). IR (neat): ν 2922, 2195, 2012, 1924 cm−1. UV/vis (CH2Cl2): λ (ε) 422 (41 300), 370 (43 400), 336 (56 400), 321 (52 000), 276 nm (53 500). HRMS (ESI): 964.900 72 (M+ + Na), calcd for C46H24Mn2O8S4 941.911 49.

Yield: 50%. 1H NMR (500 MHz, d8-THF): δ 3.47 (br, 1H, Fc H), 3.84 (s, 5H, Fc H), 4.29 (br, 1H, Fc H), 4.33 (br, 1H, Fc H), 4.42 (br, 1H, Fc H), 7.55 (s, 1H, H1), 7.60 (br, 1H, H13 or H14), 7.73 (br, 1H, H13 or H14), 8.21 (m, 6H, Ar H). 13C NMR (125 MHz, d8-THF): δ 65.8 (Fc C), 69.4 (Fc C), 69.8 (Fc C), 69.9 (Fc C), 70.1 (Fc C), 70.7 (Fc C), 84.1 (Cethynyl), 93.1 (Cethynyl), 119.5, 120.0, 120.8, 121.3, 121.7, 122.4, 124.9 (C1), 129.0 (C13 or C14), 130.1, 130.3, 131.1 (C13 or C14), 131.2, 131.3, 133.0, 136.2, 137.4, 137.9, 138.0, 138.5, 138.6, 138.7, 143.9, 196.0 (Co-CO) ppm. IR (neat): ν 2957, 2925, 2084, 2049, 2020 cm−1. UV/vis (CH2Cl2): λ (ε) 399 (13 200), 379 (12 900), 281 (30 700) nm. HRMS (ESI): 895.799 50 (M+), calcd for C40H18Co2FeO6S4 895.799 96. 2-[(η5(1′-5′)-4′-Methoxycyclohexa-2′,4′-dienyl)tricarbonylmanganeseethynyl]tetrathia[7]helicene 16. Pd2(dba)3 (2.7 mg, 0.003 mmol), AsPh3 (3.2 mg, 0.010 mmol), and 2-iodo helicene 4 (14 mg, 0.026 mmol) were added successively to Mn complex 3 (11 mg, 0.040 mmol) in a mixture of anhydrous degassed triethylamine (10 mL) and freshly distilled THF (5 mL). The solution was stirred for 2.5 h at 40 °C and then filtered with Celite. Then 20 mL of water was added to the filtrate, the two phases were separated, and the aqueous phase was extracted with Et2O (20 mL × 3). The combined organic layers were washed with a saturated ammonium chloride solution (30 mL), dried over MgSO4, and evaporated under reduced pressure. The residue was then purified by flash chromatography on silica gel (petroleum ether/EtOAc 12/1) to afford compound 16 as a yellow powder.

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ASSOCIATED CONTENT

S Supporting Information *

A CIF file and tables giving crystal data for complex 9. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *F.R.-M.: e-mail, [email protected]; fax, 00 33 (0)1 44 27 55 04.

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ACKNOWLEDGMENTS F.R.-M. and E.R. thank the CNRS for a grant to M.L. E.L. acknowledges the MIUR and the University of Milan, project PRIN 2007 (prot. 2007XFA27F_004), for financial support.

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REFERENCES

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Yield: 65%. 1H NMR (400 MHz, C6D6; two diastereoisomers): δ 2.23 (m, 2H, 2 H6′ exo), 2.71 (m, 2H, 2 H6′ endo), 2.95 (m, 2H, 2 H5′), 3.46 (s, 6H, 2 OCH3), 4.97 (d, 1H, J = 6 Hz, H2′), 5.05 (d, 1H, J = 6 Hz, H2′), 5.64 (dd, 1H, J = 6 Hz, J = 2 Hz, H3′), 5.67 (dd, 1H, J = 6 Hz, J = 2 Hz, H3′), 6.91 (m, 4H, 2 H1, H13, H14), 6.95 (m, 2H, H13, H14), 7.78 (m, 4H, Ar H), 7.85 (m, 4H, Ar H), 7.96 (m, 8H, Ar H). 13 C NMR (100 MHz, C6D6): δ 30.2 (C6′), 30.3 (C6′), 36.6 (C5′), 36.7 (C5′), 45.8 (C1′), 53.9 (OCH3), 68.4 (C2′), 68.5 (C2′), 79.7 (Cethynyl), 96.0 (Cethynyl), 97.1 (C3′), 97.4 (1 C3′), 118.7, 118.8, 120.0, 120.5, 120.8, 120.9, 121.6, 122.07, 122.11, 125.1, 125.2, 125.3, 128.7, 130.3, 130.4, 130.9, 131.0, 131.1, 136.2, 136.5, 137.2, 137.4, 137.5, 138.09, 138.12, 143.3 (C4′) ppm. IR (neat): ν 2956, 2922, 2195, 2012, 1926 cm−1. UV/vis (CH2Cl2): λ (ε) 403 (15 800), 356 (12 600), 326 (18 600), 276 nm (22 900). HRMS (ESI): 672.945 33 (M+ + 1), 694.928 24 (M+ + Na), calcd for C46H24Mn2O8S4 671.939 01. 2,13-Bis[(η 5 (1′-5′)-4′-methoxycyclohexa-2′,4′-dienyl)tricarbonylmanganeseethylnyl]tetrathia[7]helicene 18. The same procedure as for 16 was applied for the preparation of 18 (petroleum ether/EtOAc 10/1 to 4/1). Yield: 70%. 1H NMR (400 MHz, C6D6; four diastereoisomers): δ 1.88−2.04 (m, 4H, H6′ endo), 2.38−2.45 (m, 4H, H5′), 2.47−2.56 (m, 103


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(20) Crystal data for 9 (CCDC no. 827074): C23H12FeS2, Mr = 408.30, monoclinic, a = 16.8995(4) Å, b = 9.7167(3) Å, c = 20.6494(5) Å, V = 3390.74(16) Å3, T = 180 K, space group I2/a, Z = 8, μ(Mo Kα) = 0.114 mm−1, Dexptl= 1.600 Mg/m3, 16 757 reflections measured, 4955 unique (Rint = 0.036), which were used in all calculations. Structure solution using SIR97.20a Refinement by least squares using SHELXL-97.20b The final R and Rw(F2) values were 0.0367 and 0.1004, respectively (I > 2σ(I)). Drawing of the molecule by ORTEP.20c (a) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (b) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (c) Farrugia, L. J. ORTEP-3 for Windows. J. Appl. Crystallogr. 1997, 30, 565. (21) Lee, Y. K.; Lee, C. G.; Kim, J. N. Tetrahedron Lett. 2003, 44, 1231 and references therein.. (22) Nakagawa, H.; Obata, A.; Yamada, K.-I.; Kawazura, H. J. Chem. Soc., Perkin Trans. 2 1985, 1899. (23) (a) Armstrong, A. T.; Smith, F.; Elder, E.; McGlynn, S. P. J. Chem. Phys. 1967, 46 (11), 4321. (b) Du, H.; Fuh, R. A.; Li, J.; Corkan, A.; Lindsey, J. S. PhotochemCAD: A computer-aided design and research tool in photochemistry. Photochem. Photobiol. 1998, 68, 141−142. (c) Bozak, R. E. Photochemistry in the metallocenes. In Advances in Photochemistry; Pitts, J. N., Jr., Hammond, G. S., Noyes, W. A., Jr., Eds.; Wiley: New York, 1971; Vol. 8, pp 227−244. (24) Zhu, Y.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122, 10121− 10125. (25) (a) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 561, 461. (b) Gritzner, G. Pure Appl. Chem. 1990, 62, 1839. (26) (a) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713. (b) Ruiz, J.; Astruc, D. C. R. Acad. Sci., Ser. IIc: Chim. 1998, 1, 21. (27) TBAP: contact with combustible material may cause fire. Heating may cause an explosion. Labeling according to Regulation (EC) No. 1272/2008 [CLP]. Hazard statement(s): H272, H315, H319, H335. Precautionary statement(s): P220, P261, P305 + P351 + P338. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications; Wiley: New York, 2002; pp 648−650.

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