Hybrid materials comprising ferrocene and diaminoborane moieties: linear concatenation versus macrocyclization

Abstract

Combination of borane and diaminoferrocene monomers by Si/B exchange condensation reactions afforded either diazabora-[3]ferrocenophanes or, via stepwise processes, larger macrocycles and a series of linear oligomers. Additional incorporation of p-phenylene moieties in the backbone yielded alternating concatenation.

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Metallopolymers comprising a polyferrocene backbone with bridging heteroatoms have evolved into a versatile class of inorganic–organic hybrid materials with unique properties and functions imparted by the special characteristics of ferrocene as well as the respective linking moieties.¹ Assuredly the most prominent representatives thereof are poly(ferrocenylsilane)s (A, Fig. 1), which have been applied, for instance, as redox-active stimuli-responsive gels, high refractive index materials and polymeric precursors to nanostructured solid-state materials.¹ In recent times, polyferrocenes with boron atoms in bridging positions have emerged as desired targets.^2–5 Wagner and colleagues accomplished the synthesis of poly(ferrocenyl borane)s B, showing strong electronic coupling of the ferrocene moieties through the boron centres.² The synthesis of B-amino-substituted poly(ferrocenyl borane)s C by ring-opening polymerisation was first pursued by Manners and Braunschweig.³ Müller and co-workers achieved to obtain soluble derivatives thereof by introducing solubility-enhancing side groups (R′′).⁴ The π-interaction over the boron centres is in C significantly reduced due to the cross-conjugated B=N double bond.

Fig. 1 Polyferrocenes A–C (R, R′, R′′ = organic substituents), macrocycle D having pendant ferrocenyl (Fc) groups, and BN-containing hybrid polymers E–G.

In recent years, we have been pursuing the development of novel polymers, oligomers and macrocycles, such as D–F, comprising B=N bonds as part of a π-conjugated backbone.^6–10 For example, E can be regarded as a BN analogue of poly(p-phenylene vinylene) (PPV),^6a,c wherein the linear C=C units are replaced by isoelectronic and isosteric B=N couples.^11–13 Similarly, we reported BN congeners of poly(thiophene vinylene) (PTV)^6b,c and polyacetylene.⁷ Polymer F features electron-rich diaminoborane moieties in the main chain,⁸ which can be regarded as neutral isoelectronic equivalents of an allyl anion moiety.¹⁴ The bulky Tip (triisopropylphenyl) or Mes (mesityl) groups in E–G have the function of kinetically stabilising the boron centres by steric protection.¹⁵ An attempt to incorporate ferrocene (Fc) as side groups into BN-PPV-type structures yielded the hybrid macrocycle D.⁹ We recently reported the unprecedented B=N-bridged polyferrocene G and some monodisperse oligomers of that type.¹⁶ These species showed moderate electronic communication between the ferrocenes along the polymer chain.

We now combined ferrocene with electron-releasing diaminoborane moieties. Herein, we report the synthesis and characterisation of oligomers with linearly concatenated ferrocene and NBN units, as well as macrocycles of different ring sizes, and a polycondensate composed of diaminoborane-bridged alternating ferrocene and phenylene groups in the main chain.

With the aim of synthesising a polymer composed of 1,1′-linked ferrocene and diaminoborane moieties in the main chain, we reacted 1,1′-[N,N′-bis(trimethylsilyl)diamino]ferrocene 1 in DCM in 1 : 1 ratio with aryl(dibromo)boranes ArBBr₂ (Ar = Ph, Mes and Tip) at 25 °C (Scheme 1). This afforded Si/B exchange with condensation of Me₃SiBr; however, instead of the desired polymer, the respective 1,3,2-diazabora-[3]ferrocenophane 2^Ar was selectively formed in each case. We characterised compounds 2^Ar by multinuclear NMR spectroscopy and mass spectrometry. Additionally, we accomplished to determine the molecular structures of 2^Ph and 2^Mes by single-crystal X-ray diffraction (SXRD; Fig. 2). The ¹H NMR spectra of 2^Ar showed sharp signals consistent with their symmetrical molecular structure. Other derivatives of the substance class of 1,3,2-diazabora-[3]ferrocenophanes have been previously described by Wrackmeyer and Herberhold and co-workers.^17,18

Scheme 1 Synthesis of [3]ferrocenophanes 2^Ar and [3.3]ferrocenophanes 4^Ar (the latter formed in a mixture with larger cyclic products 5^Ar).

Fig. 2 Molecular structures of 2^Ph, 2^Mes, 7^Mes and 4^Tip in the solid state (ellipsoids are shown at the 50% probability level; ellipsoids of the peripheral groups and all hydrogen atoms except for N bounded Hs are omitted for clarity).

Different from the phenyl group in 2^Ph, the bulky Mes and Tip groups provide sufficient kinetic stabilisation to make ferrocenophanes 2^Mes and 2^Tip stable towards air and moisture. Therefore, we decided to restrict our subsequent investigations to the use of these two substituents.

To suppress the formation of small ferrocenophanes 2, and with a view to favour the synthesis of either linearly catenated species or larger macrocycles, we reacted 1 first with a slight excess of ArBCl₂ (Ar = Mes, Tip; ∼3 equiv.) to give bis(aminoborane)s 3^Ar (Scheme 1). After isolating 3^Ar by precipitation in n-pentane, we reacted them in 1 : 1 ratio with another equivalent of 1. Analysis of the crude products by mass spectrometry suggested the formation of a mixture of larger macrocycles 4^Ar and 5^Ar (n = 3 and 4) in both cases. For the Tip derivative we accomplished to separate the products of larger ring sizes and isolated the previously unknown 1,3,2-diazabora-[3.3]ferrocenophane 4^Tip in 24% yield. We characterized 4^Tip by multinuclear NMR spectroscopy, mass spectrometry, and SXRD (Fig. 2).

Next, we aimed at preparing well-defined oligomers 7^Ar and 9^Ar that feature linearly concatenated ferrocene and diaminoborane moieties in alternating fashion (Scheme 2). We synthesised 7^Ar by the reaction of two equivalents of N-ferrocenyl-N-trimethylsilyl amine 6 with the respective borane ArBBr₂ (Ar = Mes, Tip). To obtain the oligomer 9^Ar, containing three ferrocene and two diaminoborane units, we reacted 6 first with the respective aryl(dichloro)borane to give the intermediates 8^Ar, which we isolated in good yields (80% for 8^Mes; 74% for 8^Tip). These species show a typical ¹¹B NMR shift of δ ≈ 39 ppm, as comparable B–Cl substituted aminoboranes.^14a Compounds 8^Ar were subsequently reacted with 1 in 2 : 1 ratio. We thus obtained oligomers 7^Ar and 9^Ar in moderate to good yields (79% for 7^Mes, 65% for 7^Tip, 62% for 9^Mes and 36% for 9^Tip). All isolated compounds were stable towards air and moisture. Their identities were unambiguously ascertained by multinuclear NMR spectroscopy and mass spectrometry. In addition, we accomplished to grow single crystals of 7^Mes and analyse them by XRD (Fig. 2).

Scheme 2 Synthesis of linear oligomers 7^Ar and 9^Ar (for 1 see Scheme 1).

The structural parameters of 2^Ph and 2^Mes in the solid state agree well with those of previously reported related compounds such as the N-SiMe₃-/B-Ph-substituted 1,3,2-diazabora-[3]ferrocenophane.¹⁷ The N–B–N bond angles of 2^Ph [123.8(2)°] and 2^Mes [124.6(3)°] are close to that of the N-TMS substituted derivative [126.0(4)°],¹⁷ while the B–N bond lengths of 2^Ph [1.416(3) Å] and 2^Mes [1.417(4)/1.419(4) Å] are slightly shorter than those of the latter [1.432(6)/1.465(6) Å].¹⁷

Compound 7^Mes shows E,Z-configuration at the partial B–N double bonds in the solid state, as observed in other structurally characterized B,N,N′-trisubstituted diaminoboranes.^8,14 The B–N bond lengths (1.421(3) Å and 1.424(3) Å) and angles [N–B–N, 118.07(17)°] are in the same range with those of the phenyl-substituted diaminoborane PhN(H)B(Mes)N(H)Ph, previously reported by us [(B–N, 1.418(5) and 1.428(4) Å); N–B–N, 119.2(3)°].⁸ [3.3]Ferrocenophane 4^Tip shows E,Z-configuration at the NBN moiety as well, which results in the anti-conformation for this bridged ferrocenophane system.¹⁹ The B–N bond lengths are 1.407(3) and 1.423(3) Å, thus, in the same range with those of 2^Ph, 2^Mes and 7^Mes. The boron and nitrogen centres of 4^Tip are all trigonal-planar coordinated (Σ_< ≈ 360° each).

As our above-described attempts to prepare a polymer composed of alternating ferrocene and diaminoborane units were unsuccessful, we aimed at the synthesis of an alternating copolymer 10 (Scheme 3) featuring diaminoborane-linked ferrocene and p-phenylene moieties in the main chain. We chose mesityl side groups on boron, as this substituent was already sufficient to effectively stabilize 2^Mes, 4^Mes, 7^Mes and 9^Mes. For the polymerisation, we performed a polycondensation reaction between 3^Mes and N,N′-bis(trimethylsilyl)-p-phenylenediamine (1 : 1) in DCM. After the mixture had been stirred for 24 h at 25 °C, we added (p-^tBu)C₆H₄NHTMS to terminate eventually remaining reactive B–Cl end groups. We purified the product by precipitation in cold n-pentane. Additionally, we prepared compound 11 as a molecular model system for 10. Analysis of 10 by gel permeation chromatography (GPC) suggested a relatively low number-average molecular weight of M_n = 1.9 kDa (PDI = 1.66), corresponding to an average degree of polymerization (DP_n) of 7 (see ESI,† Fig. S60). Line broadening effects in the ¹H NMR spectrum of 10 point to simultaneous dynamic interconversion of the E- and the Z-configuration at both B–N bonds, as we observed previously for polymers and oligomers of type F (Fig. 1),⁸ and similar to the situation found in oligoferrocenes connected by phosphaalkene units.²⁰

Scheme 3 Polycondensation to 10 and synthesis of model compound 11.

We investigated the electrochemical properties of all isolated final products by cyclic voltammetric (CV) measurements in DCM (Fig. 3). The voltammograms of [3]ferrocenophanes 2^Ar showed each a reversible redox wave at E_1/2 ≈ 80 mV, assigned to oxidation of their ferrocene moiety²¹ (see ESI,† Fig. S61). Compound 4^Tip exhibited two reversible oxidation events at E_1/2 = −270 and −55 mV, with a separation of ΔE_1/2 = −215 mV (Fig. 3), according to consecutive oxidation of the two ferrocene units in the molecule.

Fig. 3 Cyclic voltammograms of 4^Tip, 7^Ar, 9^Ar, 10 and 11 in DCM containing 0.1 M [n-Bu₄N][PF₆] with a scan rate of 250 mV s⁻¹. Referenced against [Fc]^0/+.

The linear species 7^Mes and 7^Tip exhibited two reversible oxidation events as well, at E_1/2 = −253 and −112 mV (7^Mes) and −236 and −107 mV (7^Tip), assigned to consecutive oxidation of both ferrocenyl groups. The two waves have a small separation of ΔE_1/2 = −141 mV (7^Mes) and −129 mV (7^Tip). The oligomers with three ferrocene and two diaminoborane units 9^Ar gave rise to two reversible oxidation events at E_1/2 = −420 and −162 mV (9^Mes) and −376 and −165 mV (9^Tip), with a separation of ΔE_1/2 = −258 mV (9^Mes) and −211 mV (9^Tip), respectively. Here, the central diamino ferrocene unit, which is the most electron-rich one, is oxidized first. Subsequently, the terminal ferrocene units are oxidized simultaneously in a two-electron process. Overall, these results suggest a certain but low degree of communication over the NBN units in these species. The mixed species 10 and 11 showed each one reversible oxidation event at E_1/2 = −531 (10) and −245 mV (11), respectively.

In conclusion, linkage of ferrocene units via diaminoborane moieties yielded either macrocyclic or linearly concatenated compounds. While Si/B exchange condensation reactions between N,N′-bissilyl-diaminoferrocene 1 and dibromoboranes gave exclusively 1,3,2-diazabora-[3]ferrocenophanes 2, stepwise procedures afforded mixtures of larger macrocycles 4 and 5, of which we succeeded in isolating the unprecedented diazabora-[3.3]ferrocenophane 4^Tip. We accomplished to prepare linear oligomers 7 and 9 that feature two and three ferrocene units bridged by one and two diaminoborane moieties, respectively. These species show weak electronic coupling between the ferrocene units over NBN. Additional incorporation of rigid p-phenylene building blocks afforded the alternating cooligomer 10. Our future studies aim at the development of further novel BCN hybrid materials and exploring their full potential.

Financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 466754611, Research Grant HE 6171/6-2, 401739196, and Heisenberg Grant HE 6171/9-1, 468457264 – is gratefully acknowledged.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 2^Ph, 2^Mes, 4^Tip and 7^Mes has been deposited at the CCDC under 2374123–2374126 and can be obtained from https://www.ccdc.cam.ac.uk.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, X-ray crystallographic and characterisation data. CCDC 2374123–2374126. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc03813d

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