Imperial College London · Web viewThe synthesis of monocyanoferrocene (FcCN) was first reported in 1957,37 only five years after the reported synthesis of ferrocene itself.38 The

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Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

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Cyanoferrocenes as redox-active metalloligands for coordination-driven self-assemblyLuke A. Wilkinson,a Thomas T. C. Yue,a Emma Massey,a Andrew J. P. Whitea and Nicholas J. Long*a

Ferrocene-based Lewis bases have found utility as metalloligands in a wide variety of applications. The coordination chemistry of cyanoferrocenes however, is underexplored. Herein, we describe a new synthetic protocol for the generation of cyanoferrocenes. The coordination chemistry of these metalloligands to [Cu(NCMe)4][PF6], [(PPh3)2Cu(NCMe)2][PF6] and [(dppf)Cu(NCMe)2][PF6] salts have been explored, providing crystallographic evidence of cluster and polymeric forms of 1,1’- and 1,2-dicyanoferrocene complexes. The stability of the complexes and ligand dissociation were found to be strongly solvent-dependent.

IntroductionMetalloligands are Lewis basic molecules containing a metal fragment. They are important in current research as they present a facile method for incorporating highly desirable properties into larger assemblies, or discrete heterometallic complexes. With judicious choice of metalloligand, functional molecules can be developed toward applications in a number of areas such as magnetism,1–3 catalysis,4–8 optics,9,10 electronics,11–13 and in generating large coordination networks such as molecular helicates,14 metal-organic polyhedra (MOPs), or metal-organic frameworks (MOFs).15–18

Ferrocene derivatives are well-known for their robust chemical, optical and redox properties and compounds bearing donor substituents have been employed as metalloligands in numerous instances.19–21 Perhaps the most well-known example of a ferrocene-based metalloligand is 1,1’-bis-(diphenylphosphino)ferrocene (dppf) which is often used as a ligand in catalysis5,8,22–24 or as a chelating ligand for the development of novel coordination compounds.25,26 Based on the success and versatility of dppf, a number of analogues have been synthesized.27 A particularly relevant analogue, showcased by the Štěpnička group, is 1-diphenylphosphino-1’-cyanoferrocene which has been shown to coordinate to a number of copper(I)28 and silver(I)29 salts in a varied array of coordination geometries. When coordinated to gold(I) centres,

dimeric or polymeric forms were obtained (depending on the nature of the counteranion) which could be disrupted on addition of donor species (Cl-, tetrahydrothiophene) to generate coordinatively-unsaturated Au(I) complexes that were shown to be active annulation catalysts.30 The nitrile component in the above example is key to the activity of the catalyst. Nitriles are traditionally weak donors in transition metal chemistry, able to form stable metal-nitrogen dative bonds but easily cleaved or substituted with the addition of a stronger donor.31 Organonitriles are regularly used as “placeholder” ligands in the construction of supramolecular structures32–34 and metalloligands with nitrile (or M-CN) based donor groups are also routinely employed in the construction of complex assemblies for a range of applications.10,35,36 In contrast, the coordination chemistry of cyanoferrocenes is poorly explored which is perhaps surprising when considering the advantages that an air-stable, hemilabile, redox-active metalloligand could provide.

The synthesis of monocyanoferrocene (FcCN) was first reported in 1957,37 only five years after the reported synthesis of ferrocene itself.38 The synthesis of 1,1’-dicyanoferrocene (1,1’-Fc(CN)2) has only been reported rarely,39,40 and the 1,2- analogue (1,2-Fc(CN)2) has been reported just once.41 Furthermore, reports of the coordination chemistry of cyanoferrocenes are also rare. FcCN has been shown to coordinate Ru,33 Pt,42 and Cu43 as has 1,1’-Fc(CN)2

40,42,44,45

however, there are currently no examples of 1,2-Fc(CN)2

coordination complexes. Herein, we report a new and high yielding procedure for

the conversion of ferrocenecarboxaldehydes into their corresponding cyanoferrocenes. We then demonstrate the coordination behaviour of these underexplored ligands to

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1

a. Molecular Sciences Research Hub, Imperial College London, White City,b. London W12 0BZ, UK. E-mail: [email protected] Supplementary Information (ESI) available: full synthetic experimental, and crystallographic, IR spectral and electrochemical data, See DOI: 10.1039/x0xx00000x.



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copper(I) centres, exploring their lability toward other solvents and examine their electrochemical properties.

Results and discussionSynthesis of cyanoferrocenes

There are multiple-known pathways towards the synthesis of organo-nitriles and the starting point is often the corresponding aldehyde. The first report on the synthesis of cyanoferrocene converted ferrocenecarboxaldehyde to the desired nitrile in a multi-step synthesis via an intermediate oxime.37 This procedure has been optimized into a one-pot process in the subsequent years (figure 1, top),46 and remains the most popular method. The only known synthesis of 1,2-Fc(CN)2 requires a two-step process (figure 1, middle).41 Our improved protocol (figure 1, bottom), inspired by the work of the Fang group,47 is quick, uses easily accessible reagents (e.g. NH3 and I2) and occurs with yields which are comparable to, if not greater than the current standards.

The syntheses of formylferrocenes are well-established procedures that have been reported before.37,48,49 However, in our hands, purification beyond standard column chromatography has been required to access the aldehydes in high yields. Our observations and comments on their purification are provided in the experimental section.

Our optimization procedure for the conversion of formylferrocenes into cyanoferrocenes was undertaken using 1,1’-ferrocenedicarboxaldehyde. It was quickly discovered that the reaction occurred more readily if I2 and 1,1’-Fc(CHO)2 were pre-dissolved in a small amount of THF before adding to the ammonia water, and the reaction vessel was sealed to prevent the escape of gaseous NH3.

By monitoring the reaction by 1H NMR spectroscopy it was observed that the reaction reached completion within 10 minutes (figure S1). Varying the I2 concentration showed 2 equivalents to be optimal (1 equivalent per aldehyde) as more equivalents of I2 did not appear to significantly affect the reaction rate or completion (figure S2). Thus, an optimum procedure involves the addition of a THF solution of Fc(CHO)n

and nI2 to ammonia water, stirring for 10 minutes in a sealed vessel and finally quenching with aqueous sodium thiosulfate solution. Yields of around 90 % (Fc(CN) 87%, 1,1’-Fc(CN)2 97% and 1,2-Fc(CN)2 87%) can be routinely reached and the reaction can easily be scaled to gram quantities if required.

Figure 1: A comparison of current synthetic pathways to cyanoferrocenes. Top = ref 45, Middle = ref 40

Coordination chemistry of cyanoferrocenes

The coordination chemistry of cyanoferrocenes has been rarely explored but the Lang43,50 and Štěpnička28 groups have both reported examples wherein monocyanoferrocenes have coordinated to copper(I). Our efforts investigate the coordination behaviour of dicyanoferrocenes to copper(I) salts.

Coordination chemistry of 1,2-Fc(CN)2. Upon stirring a 1:1 ratio of [Cu(MeCN)4][PF6] and 1,2-Fc(CN)2 in CH2Cl2, a bright orange solid immediately precipitated. This powder proved to be too insoluble for initial analysis in solution, with only sparing solubility in DMSO. ATR-IR of this solid displayed two sharp, medium-intensity signals at 2239 and 2250 cm-1. The free ligands FcCN, 1,1’-Fc(CN)2 and 1,2-Fc(CN)2 each display a similarly sharp vCN resonance at 2225 cm-1 and departure from this suggests coordination. While the positive shift of the vCN stretch is somewhat counterintuitive (based on a back-bonding description of M-nitrile bonds), many examples report an increase in frequency upon coordination.31 The main rationale for this focuses on an increasing strength of the C-N σ bond upon coordination.

Due to the poor solubility of the powder, structural characterisation was not possible. Though the insolubility of the complex indicated the formation of a polymeric species, it was not possible to determine this with the powder alone. To overcome this, a solution containing an excess (10 equiv.) of 1,2-Fc(CN)2 in Et2O was layered onto a THF solution of [Cu(MeCN)4]PF6. The resulting orange/brown crystals (figure 2, complex 1) were found to consist of a 1D coordination polymer wherein each tetrahedral copper(I) centre is bound to four separate 1,2-Fc(CN)2 ligands, which themselves bridge two copper(I) ions. A single THF molecule and a PF6 counter-anion are associated with each Cu(I) ion and are situated within the natural clefts formed by the structure.

To generate a soluble analogue of 1, a copper complex bearing lipophilic triphenylphosphine ligands was employed. [(PPh3)2Cu(NCMe)2][PF6] was generated in situ (by the reaction between [Cu(NCMe)4][PF6] and two equivalents of PPh3) and was added to a stirring solution of 1,2-Fc(CN)2 to yield an orange

2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx



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Figure 2: The crystal structure of a 1D polymer (1) wherein each tetrahedral Cu(I) ion is ligated by four separate Fc(CN)2

ligands. THF solvate, PF6 counter-anion and H atoms are omitted for clarity.

Figure 3: The ferrocene region of the 1H NMR (400 MHz) spectrum (CD2Cl2) for free 1,2-Fc(CN)2 (top) and (2) (bottom).

precipitate (scheme 1, top). Vibrational spectroscopy displayed two vCN resonances at 2230 and 2240 cm-1 respectively, implying coordination of the nitriles. Gratifyingly, complex 2 was soluble in dichloromethane so it was possible to obtain a 1H NMR spectrum (figure 3). A clear shift is observed for all ferrocene-related peaks, but most notably for the protons on the substituted Cp ring. A shift to higher ppm is indicative of a decrease in electron density and consistent with electron donation onto the copper. The 31P{1H} NMR spectrum shows the expected septet for the PF6

- anion at -143 ppm, and a slightly broadened signal at 0.91 ppm, indicative of a coordinated triphenylphosphine moiety. Mass spectral analysis by either ESI or MALDI-TOF did not show the expected molecular ion peaks or any easily assignable species in the spectrum. This is likely due to the labile nature of the nitrile ligands and the high likelihood of any complexes dissociating and/or rearranging in the spectrometer. Crystals of suitable quality for X-ray diffraction studies were grown either by layering a THF solution of the complex with hexane or by slow evaporation of a chloroform solution. Both methods yielded plate-like crystals

Figure 4: The molecular structure of [(PPh3)2Cu][μ-1,2-Fc(CN)2]2[PF6]2.THF (2) grown from THF/hexane layer. H atoms, PF6 anion and THF solvate are omitted for clarity

Scheme 1: The synthetic routes to [(PPh3)2Cu]2[μ-1,2-Fc(CN)2]2[PF6]2 (2) and [(dppf)Cu]2[μ-1,2-Fc(CN)2]2[PF6]2 (3)

with identical unit cells. The resulting molecular structure (complex 2, figure 4) clearly shows two tetrahedral Cu(I) ions, each ligated by two triphenylphosphines and bridged by two 1,2-Fc(CN)2 units, generating a rhombus-like ring with the Cu ions and substituted Cp rings in the same plane. The ferrocene moieties point in different directions being related by the inversion centre at the middle of the [Cu]2[μ-1,2-Fc(CN)2]2 ring. Encouraged by this result, the reaction was repeated, employing 1 equivalent of dppf in place of the triphenyl phosphine. Our motivation was the construction of a cluster (scheme 1, bottom), with multiple redox-active ferrocene components akin to previously reported examples.51 The synthesis proceeded as before: a CH2Cl2 solution of 1,2-Fc(CN)2

was added to a THF solution of [(dppf)Cu(NCMe)2][PF6] (generated in situ from [Cu(NCMe)4][PF6] and dppf). Precipitation quickly occurred in the form of a bright orange powder (3). The IR spectrum shows two vCN stretches at 2231 and 2243 cm-1. The precipitate was not soluble in either dichloromethane or chloroform but a 1H NMR spectrum could be obtained in acetone-d6. A 1:1 ratio of 1,2-Fc(CN)2 and dppf was observed and there was a small but significant shift in the signals of 1,2-Fc(CN)2 vs the unbound ligand, implying some degree of coordination. We propose the structure shown





above in 3 and the main evidence for this comes from the 1H NMR spectrum. If the species was polymeric in nature ( i.e. bridging, rather than chelating dppf ligands) the copper centre would likely be coordinatively unsaturated as in 4 (vide infra) and this would lead to a lower integral of -PPh2 protons in the 1H NMR spectrum. High resolution ES-TOF MS analysis of the orange precipitate gave a signal at 459.2337 m/z, which corresponds to [(dppf)Cu(1,2-Fc(CN)2)]+; half the value of our proposed structure, [(dppf)Cu]2[μ-1,2-Fc(CN)2)2]2+ (3). Elemental analyses are also consistent with the structure of 3.

In an attempt to obtain crystals, a THF solution of [(dppf)Cu(NCMe)2][PF6] was layered with an Et2O solution of Table 1: DFT frequency calculations (calc) of 1,2-Fc(CN)2 and the model compound [(PH3)2Cu(μ-1,2-Fc(CN)2)2]2+ (2’) (Δv = vCNasym - vCNsym) compared to experimental values (exp). Optimisation and frequency analysis achieved using the B3LYP functional and the 6-311+G (d, p) basis set.

CompoundvCNsym

(cm-1)vCNasym

(cm-1)Δv

(cm-1)Calc. 1,2-Fc(CN)2 2345 2346 1Exp. 1,2-Fc(CN)2 2225 2225 0Calc. 2’ 2325 2330 5Exp. 2 2230 2239 9

1,2-Fc(CN)2 (10 equiv.). Instead, crystals were formed of the pentametallic compound [{(dppfκ2-P,P’)Cu}2(μ-dppf)][PF6]2

(complex 4, figure S8). Copper(I) dppf complexes are by no means new,25,52–54

however, this particular complex has not been reported previously. We were able to characterise this compound via 1H NMR spectroscopy and elemental analysis.

Computational rationale for vibration spectra of 1,2-Fc(CN)2

and its complexes. For 1,2-Fc(CN)2, two vCN modes are expected corresponding to a symmetric and asymmetric CN stretch (vCNsym and vCNasym respectively). Only a single resonance is observed in the IR spectrum of the free ligand, whereas two resonances are observed in each spectrum of complexes 1-3. DFT calculations were performed to analyse the vibrational frequencies of 1,2-Fc(CN)2 and the model complex [(PH3)2Cu(μ-1,2-Fc(CN)2)2]2+ (2’) (PPh3 converted to PH3

to reduce computational time) in the gas phase (table 1). The results revealed that the frequencies of the expected symmetric and asymmetric vCN resonances for 1,2-Fc(CN)2

differed (Δv) by only a single wavenumber; thus clarifying the observation of a single peak in the measured spectrum of 1,2-Fc(CN)2. Furthermore, in complex 2’, Δv was more pronounced (5 cm-1).

Coordination chemistry of 1,1’-Fc(CN)2. The Lang group has previously published evidence of the coordination chemistry of 1,1’-Fc(CN)2 to copper in the form of a trigonal paddlewheel-type complex wherein two copper(I) ions were bridged by three 1,1’-Fc(CN)2 ligands.44 As such, we decided to investigate reactions of 1,1’-Fc(CN)2 with [(PPh3)2Cu(NCMe)2][PF6]. Using the same procedures as described above for the 1,2-Fc(CN)2

Scheme 2: Synthetic route to [(PPh3)2Cu(μ-1,1’-Fc(CN)2)]∞[PF6]∞

(5) and dppf complex (6 - predicted structure).

Figure 5: The crystal structure of the coordination polymer [(PPh3)2Cu(μ-1,1’-Fc(CN)2)]∞[PF6]∞.CH2Cl2 (5) grown from vapour diffusion of Et2O into dichloromethane at room temperature. H atoms, and DCM solvate are omitted for clarity.

complexes, additional 1,1’-Fc(CN)2 copper(I) complexes were synthesised (scheme 2). By combining 1,1’-Fc(CN)2 and [(PPh3)2Cu(NCMe)2][PF6], an orange precipitate was formed with an ATR-IR spectrum displaying a single vCN stretch at 2243 cm-1, thereby confirming coordination of the nitrile. In the 1H NMR spectrum (CDCl3), the signals for the ferrocene backbone shifted to a higher ppm relative to the free ligand, which is also consistent with coordination. Crystals of a suitable quality for X-ray diffraction studies were grown by vapour diffusion of Et2O onto a CH2Cl2 solution at room temperature. The resulting molecular structure (complex 5) is displayed in figure 5 and is shown to exist as a polymer, wherein the nitriles on the ferrocene occupy positions approximately 180 oC to each other and bridge two Cu(I) ions. Each Cu(I) ion is ligated by two PPh3 units and exists in the expected tetrahedral coordination environment. The polymeric structure of the crystal is somewhat surprising given the solubility of the powder. We considered that the crystalline polymer could be insoluble but the crystals themselves readily dissolved in CH2Cl2.

Encouraged by the solubility of the triphenylphosphine analogue we again looked towards dppf as a substitute for the two PPh3 ligands. Using the same synthetic procedure as above (scheme 2), an orange powder was obtained (6) with vCN = 2240 cm-1 in the vibration spectrum. The shift in the vCN stretch versus the free ligand indicated that a cyanoferrocene complex had indeed formed in preference to a multimetallic


%T




dppf copper complex as in 4. However, unlike 5, complex 6 is insoluble in CH2Cl2, so 1H NMR spectroscopy data were obtained in acetone- d6. The slight shift of the 1,1’-Fc(CN)2

signals in 6 versus the unbound ligand was inconclusive evidence of coordination in solution but the ratio of dppf to 1,1’-Fc(CN)2 signals suggests a structure similar to 5. As with 3, mass spectrometry gave a signal at 853.0363 m/z which corresponds to [(dppf)Cu(1,1’-Fc(CN)2]n

n+.

Lability of cyanoferrocenes

As mentioned previously, nitrile ligands are often considered to be weak ligands, relatively labile and displaced easily by stronger donors such as phosphines, or with an excess of coordinating solvent molecules. To demonstrate this, a sample

Figure 6: The vCN region of the IR spectrum of complex 2 before (red trace) and after (subsequent traces) repeated exposure to acetone. The dashed line is free 1,1-Fc(CN)2

of complex 2 was dissolved in acetone, stirred for 1 minute, evacuated to dryness and the ATR-IR spectrum was measured again. Initially, very little change was observed, beyond a coalescence of the two vCN stretches, but on subsequent iterations, a broad peak centred around 2196 cm-1 grew in (figure 6). Interestingly, this broad peak does not correspond purely to the free ligand, and acetone does not seem to be fully displacing the ligand either as the carbonyl signal for acetone (1723 cm -1) is absent in some spectra (likely due to more efficient evacuation in those cases). It is clear however, that 1,2-Fc(CN)2 is at least partially dissociating from the copper, forming complexes with a range of different coordinating environments, and demonstrating the lability of these ligands. Similar behaviour was observed in the IR spectrum for 5 (figure S23). The 1H and 31P{1H} NMR spectra were collected in CD2Cl2 (2) or CDCl3 (5) after the final IR measurement was taken. There was a small, observable peak-shift of the ferrocene signals in the 1H NMR spectrum of 2, which indicates a change in coordination environment of 1,2-Fc(CN)2, but there was no observable change in the 31P{1H} spectrum, illustrating the integrity of the Cu-P bond. There was no change in any of the spectra for 5 which was surprising, considering the clear differences in the IR

Figure 7: Cyclic voltammetry (100 mV/s) of 1,2-Fc(CN)2

(dashed) and 2 (solid) vs the Fc/Fc+ couple in CH2Cl2 (0.1 M NBu4PF6) with a glassy carbon electrode.spectra. The solubility of polymer 5 suggests that it may exist as a dynamic coordination polymer in solution. 1H DOSY experiments were collected for 2 and 5 (in CD2Cl2 and CDCl3

respectively) and hydrodynamic volumes were calculated for each. While 5 has a greater volume than 2 (583 Å3 vs 293 Å3

respectively) and is consistent with a polymer vs a cluster, the absolute values are not indicative of macromolecular structures in solution as they are smaller than discrete paddlewheel units reported by Patmore et. al. (ca. 1600 Å3).55 Rather, they likely describe discrete molecular units in solution, at least, on the timescale of NMR spectroscopy.

Electrochemistry

The electrochemistry of complexes 2 and 5 was interrogated and compared to the free ligands. Both complexes display similar redox profiles, so a representative cyclic voltammogram (CV) of 2 and 1,2-Fc(CN)2 is shown in figure 7 (for 5, see figure S25). The CV of 1,2- and 1,1’-Fc(CN)2 are reported in figures S21 and S24 and show a single one-electron (∆E = 73 mV) oxidation at E1/2 = 0.767 and 0.763 V (vs Fc/Fc+) respectively. While the 1,1’-Fc(CN)2 is fully reversible (ipa/ipc = 0.96 and ipa

proportional to scan rate) under our conditions, the 1,2-Fc(CN)2 analogue becomes less reversible at lower scan rates, indicative of a ErCi mechanism. Similar poor reversibility is observed for complexes 2 and 5 with an obvious, but less pronounced increase in reversibility at higher scan rates. In this case, it is possible that the poor reversibility is a result of dissociation of the oxidised cyanoferrocene ligand. In the CV of complexes 2 and 5, there was only a slight shift in potential with respect to the free ligands and this is consistent with the [Cu(FcCN)4]+ example reported by Lang.43 Additionally, there was no evidence (in the CV or differential pulse voltammogram, figures S27 and S30) of communication between Fc(CN)2 ligands on the copper.

Conclusions and SummaryThe work herein presents a new, efficient synthetic procedure for the synthesis of cyanoferrocenes and has explored their coordination behaviour to copper centres. The crystal structures obtained showcase a variety of coordination modes of the ligands. As expected, they have been shown to be


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relatively labile ligands, and yet, they can also support robust polymeric structures (1). Electrochemical investigations of 2 and 5 show no communication between Fc(CN)2 ligands (consistent with previously reported examples) however, the redox behaviour of the cyanoferrocene ligands, coupled with their labile nature is an interesting combination which could lead to potential applications as electron shuttles in redox-controlled catalysis. We are currently exploring the coordination of cyanoferrocenes to other metal centres.

Conflicts of interestThe authors declare no conflicts of interest

AcknowledgementsWe gratefully acknowledge Dr Pete Haycock for running the DOSY NMR spectra and we thank the EPSRC funding (grant number EP/N032977/1). We also thank Dr. Michael Inkpen and Dr. Ian Butler for helpful discussions.

DedicationThis paper is dedicated to Prof. Dr. Dietmar Stalke on the occasion of his 60th birthday.

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Imperial College London · Web viewThe synthesis of monocyanoferrocene (FcCN) was first reported in 1957,37 only five years after the reported synthesis of ferrocene itself.38 The