Article pubs.acs.org/Organometallics
Synthesis, Structure, and Spectroelectrochemistry of Ferrocenyl− Meldrum’s Acid Donor−Acceptor Systems Konrad Kowalski,*,† Łukasz Szczupak,† Joanna Skiba,† Obadah S. Abdel-Rahman,‡ Rainer F. Winter,*,‡ Rafał Czerwieniec,§ and Bruno Therrien⊥ †
Faculty of Chemistry, Department of Organic Chemistry, University of Łódź, Tamka 12, PL-91403 Łódź, Poland Fachbereich Chemie, Universität Konstanz, Universitätsstraße 10, D-78453 Konstanz, Germany § Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstraße 31, D-93040 Regensburg, Germany ⊥ Institute of Chemistry, University of Neuchâtel, Avenue de Bellevaux 51, CH-2000 Neuchatel, Switzerland ‡
S Supporting Information *
ABSTRACT: The synthesis of two new donor−acceptor ferrocenyl derivatives with Meldrum’s acid based nonplanar acceptor substituents is presented. Both compounds are obtained in high yields in a simple reaction protocol under mild conditions using either 1-acetyl- or 1,1′-diacetylferrocene and Meldrum’s acid. Both products have been characterized spectroscopically, by single-crystal X-ray structure analysis, by electrochemical and UV/vis/IR spectroelectrochemical measurements, and by (TD)-DFT calculations. The spectroelectrochemical measurements disclose that the 2,2-dimethyl-1,3-dioxane-4,6-dione moiety is a moderately strong electron acceptor.
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INTRODUCTION Due to its high stability, well-developed synthetic chemistry, reversible electrochemistry, and commercial availability, ferrocene has evolved into an important and versatile functional building block in many areas of chemistry. Ferrocene derivatives have thus entered many different fields of research such as catalysis,1 biology,2−4 and materials science. Within the last context, ferrocenyl derivatives have been utilized in the preparation of polymers,5,6 liquid crystals,7,8 sensors,9 and optoelectronic10 and magnetic materials11 and have been used in the study of “electronic communication” phenomena.12 The attachment of electron-withdrawing substituents to cyclopentadienyl ligands, either by direct means or via a πconjugated linker, results in donor−acceptor molecules. Monofunctional (systems A and A′) and bifunctional modes (systems B and B′) of substitution have been realized and investigated (Figure 1). Acceptor-substituted ferrocenes are commonly described by the two mesomeric structures I and II (Chart 1), with the zwitterionic charge-separated η6-fulvene structure II only being favored for exceptionally strong acceptors and in dipolestabilizing polar media.13−15 The impetus for the development of donor−acceptor ferrocenyl derivatives arises from their possible applications in molecular electronics16 and optoelectronics.10,17−24 In particular, such derivatives are usually highly colored owing to the presence of two characteristic, solvatochromic intramolecular charge-transfer (CT) absorption bands which are both red© XXXX American Chemical Society
Figure 1. General structures of monofunctional (A) and bifunctional (B) donor−acceptor ferrocenes.
shifted when the conjugation length and the acceptor strength increases.13,14,25−27 On excitation into these bands, electron density is shifted from the electron-rich ferrocene nucleus to the appended acceptor(s). Both bands receive some contribution from Fe-to-acceptor and d → d transitions and hence Special Issue: Organometallic Electrochemistry Received: December 10, 2013
A
dx.doi.org/10.1021/om401191q | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
reagent (Scheme 1).41 The six-membered ring of 1 adopts a rigid boat conformation42 which is stabilized by an intra-
Chart 1. Mesomeric Structures of Acceptor-Substituted Ferrocenes
Scheme 1. Synthesis of Ferrocenes 4 and 5
assume a mixed character.25,28 This makes donor−acceptor ferrocenes interesting NLO chromophores for second-harmonic generation15,26−32 with the added prospect of NLO tuning on ferrocene oxidation.33−36 For this purpose, compounds with planar acceptor groups have been investigated to a great extent. Apart from inducing sizable charge transfer in the excited state, acceptor substituents also decrease the electron density on the Fe nucleus, thus lowering the energy of the metal-based HOMO. These ground-state effects are conveniently probed by voltammetric techniques.27As expected, the potential of the iron-centered oxidation in acceptor-substituted ferrocenes is shifted anodically in comparison to that for the parent ferrocene or ferrocenes lacking an acceptor moiety. In contrast to the aforementioned planar donor−acceptor ferrocenes, relatively little is known about compounds in which ferrocenyl donors are conjugated to a nonplanar acceptor. Such systems might exhibit some advantages over their counterparts with planar conjugation such as improved solubility, diminished tendency toward aggregation by π-stacking, and the formation of amorphous phases instead of crystals. Diederich and coworkers reported on a series of nonplanar donor−acceptor ferrocenyl tetracyanoquinodimethanes (TCNQs),37 while the group of Shoji recently investigated a series of (ferrocenylethynyl)azulenes.38 Significant hypsochromic shifts of the CT band in the ferrocenyl dicyanoquinodimethanes have been observed as a consequence of substituent-enforced deconjugation between the dicyanovinyl acceptor halves.37 This deconjugation elevates the LUMO rather than the HOMO orbitals, as the ferrocene-centered oxidation occurs at almost identical potentials, independent of the steric demand of the acceptor group. Nonplanar (twisted) donor−acceptor ferrocenes with 1,3-diethyl-2-thiobarbituric acid acceptors have also been investigated as nonlinear optical materials by Marder and co-workers.13,22 The measurements comprised electric field induced second-harmonic generation (EFISH) and revealed large first molecular hyperpolarizabilities. In contrast to the great deal of attention devoted to 2,2dimethyl-1,3-dioxane-4,6-dione (1; known also under the trade name Meldrum’s acid)39−41 in synthetic organic chemistry, reports of its utilization as an acceptor group are scarce.41 We herein report the synthesis, (spectro)electrochemistry, DFT calculations and single-crystal X-ray structures of two novel donor−acceptor ferrocenes bearing nonplanar 2,2-dimethyl1,3-dioxane-4,6-dione-derived acceptor groups.
molecular H···H interaction involving a hydrogen atom of the CH2 group and one hydrogen atom of the axial methyl group.43 The methylene protons of Meldrum’s acid are remarkably acidic. The pKa value of 1 is in the range 4.83−4.97 in water44 and is around 7.2 in DMSO.45 Due to these acidic properties Meldrum’s acid can be easily functionalized and its derivatives have attracted considerable attention as reagents and intermediates in organic chemistry.46−50 The synthesis of ferrocenes 4 and 5 is outlined in Scheme 1; it took advantage of the aforementioned reactivity of the methylene protons in 1. We note here that Knoevenagel condensation has already been employed in the preparation of acceptor-substituted ferrocenes, as exemplified by the reactions of nonamethylferrocenecarboxaldehyde with malononitrile, indane-1,3-dione, and other electron-poor heterocycles with active methylene functions51 or that of ethenylthiophenylcarbaldehyde-appended ferrocenes with 2-(3,5,5-trimethylcyclohex-2-enylidene)malononitrile. 36 1-Acetylferrocene (2) or 1,1′-diacetylferrocene (3) was condensed with 1 under mild conditions52 to afford the desired products 4 and 5 in 70% and 90% chemical yields, respectively. Compounds 4 and 5 are air-stable, violet solids and were characterized by standard spectroscopic methods including 1H NMR, 13C NMR, and IR spectroscopy, MS, and elemental analysis. The 1H NMR and 13C NMR spectra of both products are provided in Figures S1−S6 of the Supporting Information. All analytical data confirm the proposed structures. Single-Crystal X-ray Structure Analysis of 4 and 5. Crystals of 4 and 5 suitable for X-ray diffraction study were grown from layered chloroform−hexane mixtures. The molecular structures of complexes 4 and 5 are displayed in Figures 2 and 3, respectively, whereas Table 1 contains selected structural parameters for 5. Data pertaining to the data collection and structure refinement as well as the parametric data of the unit cells are found in Table S3 of the Supporting Information. Despite the poor quality of the crystals obtained for 4 (see the Experimental Section), the molecular structure of the monofunctionalized derivative was confirmed. Complex 4 crystallizes in the triclinic space group P1̅ with two independent molecules per unit cell (see Figure 2). In both molecules the ferrocene moiety is found in an eclipsed conformation, while the six-membered Meldrum’s acid ring adopts a boat conformation. The eclipsed conformation is almost perfect, all C−Cpcentroid−Cpcentroid−C torsion angles being less than 10°. The C11−C13 and C31−C33 bond lengths of 1.36(1) and 1.37(1) Å, respectively, are clearly in the range of a CC
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RESULTS AND DISCUSSION Synthesis. Our synthetic approaches to the target compounds 1-[(2,2-dimethyl-1,3-dioxane-4,6-dione-5methylene)methyl]ferrocene (4) and 1,1′-bis[(2,2-dimethyl1,3-dioxane-4,6-dione-5-methylene)methyl]ferrocene (5) employ 2,2-dimethyl-1,3-dioxane-4,6-dione (1) as a common B
dx.doi.org/10.1021/om401191q | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
Figure 2. ORTEP drawing of 4, showing the two independent molecules per asymmetric unit. Ellipsoids are drawn at the 50% probability level.
bifurcated hydrogen bridge to one C−H proton of each of two different molecules with O···H contacts in the range 2.543− 2.655 Å, which is 0.177−0.065 Å smaller than the sum of the van der Waals radii. For each of the independent molecules a one-dimensional, infinite chain of dimers of dimers results, which runs along the a axis of the unit cell. The ferrocenyl units of the chains formed by molecules 1 and 2 are tilted by 32.7° with respect to each other. Individual chains are connected via weaker hydrogen bonds of 2.709 or 2.625 Å involving the other carbonyl O atom O2 or O5. A packing diagram of complex 4 is shown as Figure 11 of the Supporting Information. The data obtained for complex 5 are much better, and accordingly, the structural parameters of complex 5 can be discussed in greater detail (Table 1) and compared with those of other ferrocenyl derivatives. As in 4, the ferrocene moiety in 5 adopts an eclipsed conformation (C−Cpcentroid−Cpcentroid−C torsion angles
