1,1 ’ -Ferrocenylene-Bridged Bis(N-Heterocyclic Olefin) Derivatives

The 1,1 ’ -ferrocenylene (fc)-bridged bis(N-heterocyclic olefin) compounds (IMes = CH) 2 fc ( 1 , IMes = 1,3-dimesitylimidazolin-2-ylidene) and (IPr = CH) 2 fc ( 2 , IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were synthesised from (ICH 2 ) 2 fc ( 3 ) and the respective N-heterocyclic carbene IMes or IPr. Ligand substitution reactions of 1 and 2 with [BH 3 (THF)] afforded the complexes [ 1 (BH 3 ) 2 ] and [ 2 (BH 3 ) 2 ] as mixtures of the rac - and meso -diastereomers. The new ferrocene derivatives 1 – 3 and the complexes meso -[ 1 (BH 3 ) 2 ], rac -[ 1 (BH 3 ) 2 ] and rac - [ 2 (BH 3 ) 2 ] were structurally characterised by single-crystal X-ray diffraction. 1 and 2 represent a new class of ferrocene-based, and hence redox-active, bidentate ligands. The presence of three redox-active moieties, viz. the ferrocene unit and the two NHO units, is reflected by three consecutive oxidations according to an electrochemical investigation exemplarily performed with 2 by cyclic voltammetry.


Synthetic work
The synthesis of the new NHOs 1 and 2 (Scheme 1) was inspired by the method reported by Rivard for the preparation of IPr=CH 2 from IPr (2 equivalents) and MeI. [18]Surprisingly, 1,1'di(iodomethyl)ferrocene ( 3), which we utilised as starting material, had not been described before.This compound was easily obtained in excellent yield from the long known chloro homologue (ClCH 2 ) 2 fc [19] by a Finkelstein reaction.Its structure was determined by single-crystal X-ray diffraction (XRD, vide infra).IMes and IPr were generated in situ from the respective imidazolium chloride and KOtBu in THF and subsequently reacted with (ICH 2 ) 2 fc at ambient temperature.Compounds 1 and 2 were isolated in high yields (81 and 95 %, respectively) as orange solids and were structurally characterised by XRD (vide infra).According to Rivard, a drawback of his method mentioned above is the occurrence of variable amounts of residual NHC in the product. [8]In our case, this problem could be avoided by using slightly less than the stoichiometrically required 4 equivalents of the NHC.16d,20] Due to the presence of two dissimilar bonding partners (H and cyclopentadienyl) at the NHC=C atoms of 1 and 2, the NHC units exhibit two sets of signals for their backbone and respective Nsubstituents in the 1 H and 13 C NMR spectra.
The capability of 1 and 2 to act as bidentate ligands was exemplarily tested with [BH 3 (THF)].The corresponding ligand substitution reactions with two equivalents of this reagent cleanly afforded the expected complexes [1(BH 3 ) 2 ] and [2(BH 3 ) 2 ] in good yields (70 and 80 %, respectively; Scheme 1).Their structures were determined by XRD (vide infra).The terminal C atom of the exocyclic double bond is tricoordinate in a free NHO, but becomes tetracoordinate upon complex formation with a Lewis acid (LA).For NHOs of the type NHC=CHR, complexation leads to the presence of four different substituents in a distorted pseudotetrahedral arrangement at this particular C atom, which thus constitutes a centre of chirality in [NHCÀ CHRÀ LA].Two equivalent C atoms of this type are present in 1 and 2, which gives rise to the formation of rac-and meso-diastereomers upon complexation.A 1 H NMR spectroscopic analysis of [1(BH 3 ) 2 ] and [2(BH 3 ) 2 ] reveals two distinct sets of signals in each case, indicating the formation of mixtures of both diastereomers.The rac/meso ratio thus determined for crude [1(BH 3 ) 2 ] and [2(BH 3 ) 2 ] is ca.2/1 and 5/4, respectively.The diastereomers exhibit significantly different solubilities in both cases.Due to its higher solubility, the meso-diastereomer could be extracted from crude [1(BH 3 ) 2 ] and [2(BH 3 ) 2 ] with benzene and dichloromethane (DCM), respectively, which allowed the characterisation of this diastereomer in almost pure form by NMR spectroscopy in solution.
The molecules of 1 and 2 are both centrosymmetric, showing an antiperiplanar staggered conformation of the cyclopentadienyl rings with diametrically opposed substituents.The lengths of their ylidic CÀ C double bonds are very similar, viz.1.343(5) and 1.356(3) Å for 1 and 2, respectively.These values lie in the middle of the range (1.32-1.38Å) found for the previously reported NHOs in Table 1.The NHO π-systems of 1 and 2 are not coplanar with the cyclopentadienyl rings.The angles between the cyclopentadienyl ring plane and the plane formed by the cyclopentadienyl C ipso atom and the two C atoms of the ylidic unit attached to it are 41.1°and 37.9°for 1 and 2, respectively.Comparable dihedral angles τ are formed by the best planes of the five-membered NHC ring and the corresponding cyclopentadienyl ring (Table 1).Similar deviations from coplanarity can be noted for the benzylidene-containing NHOs listed in Table 1.The N atoms are in an essentially trigonal planar bonding environment in all cases (sum of angles close to 360°).The largest, but still rather small, deviations from planarity are shown by the N atoms of one of the four independent molecules of SIPr=CH 2 (sum of angles 353.1°and   354.1°) and by an N atom of Me IMe=CHPh (sum of angles 354.1°).In the latter case, the slight pyramidalisation of the N atom has been ascribed to steric repulsion between the phenyl and an N-methyl group. [27] structural investigation of [1(BH 3 ) 2 ] by XRD was possible for both diastereomers.Pertinent metric parameters are collected in Table 1.The molecular structure of rac-[1(BH 3 ) 2 ] (obtained as a racemic compound) is shown in Figure 5. Unfortunately, meso-[1(BH 3 ) 2 ] afforded crystals of poor quality, which affected the XRD result (see Figure S1 in the Supporting Information).The connectivities could be established unambiguously.However, a discussion of bond parameters is not meaningful.In the case of [2(BH 3 ) 2 ], crystals suitable for XRD were obtained for the rac-diastereomer only (Figure 6, Table 1).Although the first NHOÀ BH 3 complex was published already in 1993, [6a] only a single example has been structurally characterised so far, viz.[SIPrÀ CH 2 À BH 3 ]. [29]Data for this compound as well as for the related complexes [IPrÀ CH 2 À BH 2 NMe 2 BH 3 ], [30] [ Me IMeÀ CH 2 À B(C 6 F 5 ) 3 ] and [ Me IMeÀ CHPhÀ B(C 6 F 5 ) 3 ] [27] are included in Table 1 for comparison.For the NHOs contained in Table 1 borane coordination induces an elongation of the exocyclic CÀ C bond of slightly more than 0.10 Å and a shortening of the bond lengths in the N 2 C unit between ca.0.03 and 0.05 Å, in line with a more pronounced ylidic character of the coordinated NHO. [1]

Electrochemistry
The new bis(NHO) compounds 1 and 2 each contain three redox-active moieties, viz. the ferrocene unit and the two formally equivalent NHO units.Their redox behaviour was exemplarily studied for 2 by cyclic voltammetry in DCM (Figure 7) and THF solutions (see Figures S2-S4 in the Supporting Information).Potential data are collected in Table 2.
Compound 2 undergoes a first oxidation at a rather negative half-wave potential of À 1.03 V vs. the ferrocenium/ ferrocene couple (DCM, 0.1 m nBu 4 N[PF 6 ] supporting electro- [a] Dihedral angle between the best planes of a cyclopentadienyl or phenyl ring and the NHC ring connected to it.[b] Two independent molecules.[c] Four independent molecules.lyte).17a] Compound 2 exhibits a chemically reversible second oxidation at a half-wave potential of À 0.44 V.This value may be compared with the anodic peak potential E p,ox � À 0.1 V vs. SCE (corresponding to ca.À 0.6 V on the ferrocenium/ferrocene scale) [31] reported recently for the irreversible oxidation of the NHO 1tert-butyl-3-methyl-2-methyleneimidazolidine in DMF solution. [32]A third, and irreversible, oxidation is observed for 2 at a peak potential E p,ox = 0.36 V. We ascribe the chemically reversible first and second oxidation observed for 2 to a ferrocene-centred and an NHO-centred one-electron process, respectively.The second NHO moiety is plausibly involved in the irreversible third oxidation.A more detailed study will be performed to shed more light on this redox behaviour and the electronic structure of the cationic species 2 n + (n = 1-3) involved.

Conclusion
The new bis(NHO) compounds (IMes=CH) 2 fc ( 1    presence of three redox-active moieties, viz. the ferrocene unit and the two NHO units, is reflected by three consecutive oxidations according to an electrochemical investigation exemplarily performed with 2 by cyclic voltammetry.The electronic structure of the oxidised species (in particular that of the dication formed in the second oxidation step) will be addressed in future work.The capability of 1 and 2 for the coordination of Lewis acidic centres was demonstrated by ligand substitution reactions with [BH 3 (THF)], which furnished the complexes [1(BH 3 ) 2 ] and [2(BH 3 ) 2 ].A study addressing the ability of such bidentate C,C-ligands for chelate formation, analogous to DPPF and its many relatives, is presently underway.We will also investigate the deprotonation of (NHC=CH) 2 fc to afford [(NHC=C)fc] 2À , because N-heterocyclic vinyl groups have started to emerge as powerful tools for the stabilisation of electrondeficient low-coordinate centres. [33]perimental Section General considerations: All reactions involving air-sensitive compounds were performed in an inert atmosphere (argon or dinitrogen) by using standard Schlenk techniques or a conventional glovebox.Starting materials were procured from standard commercial sources and used as received.1,1'-Di(hydroxymethyl)ferrocene, [34] 1,1'-di(chloromethyl)ferrocene [35] and the imidazolium chlorides IMesHCl and IPrHCl [36] were synthesised by adapted versions of the published procedures.NMR spectra were recorded at ambient temperature with Varian NMRS-500 and MR-400 spectrometers operating at 500 and 400 MHz, respectively, for 1 H. Elemental analyses were carried out with a HEKAtech Euro EA-CHNS elemental analyser at the Institute of Chemistry, University of Kassel, Germany.Cyclic voltammetry was performed using a threeelectrode cell with a platinum working electrode, a silver counter electrode and a silver pseudo-reference electrode.All measurements were performed in a nitrogen-filled glovebox (MBraun/ GB2202-C-VAC) in dried and degassed HPLC grade THF or DCM with 0.1 m nBu 4 N[PF 6 ] as supporting electrolyte and an analyte concentration of 1 mm.All potentials were referenced versus the ferrocenium/ferrocene couple.For this purpose ferrocene was added to the analyte solution after all data of interest had been acquired.The electrochemical cell was connected to a WaveDriver 20 Bipotentiostat (Pine Research) and the electrochemical data were recorded and processed with AfterMath (Ver.1.5.9807,Pine Research).The data were subsequently exported and plotted with QtiPlot (Ver.0.9.8.0).

Synthesis of 1,1'-Di(iodomethyl)ferrocene (3):
Sodium iodide (1.06 g, 7.1 mmol) was added to a solution of 1,1'-di(chloromethyl)ferrocene (1.00 g, 3.5 mmol) in acetone (100 mL).The mixture was stirred for 2 h.Insoluble material was removed by filtration through a short pad of celite, which was subsequently washed with acetone (2 × 5 mL).The combined solutions were reduced to dryness under vacuum.The orange residue was extracted with hot n-hexane (100 mL).Insoluble components were removed from the extract by filtration through a short pad of celite, which was subsequently washed with hot n-hexane (3 × 10 mL).The solutions were combined.Volatile components were removed under vacuum, leaving the product as a bright orange microcrystalline solid.Further purification to obtain an analytical sample and crystals suitable for XRD was achieved by recrystallization from n-hexane or diethyl ether.Yield Synthesis of (IMes=CH) 2 fc (1): 1,1'-Di(iodomethyl)ferrocene (500 mg, 1.07 mmol) was added to a stirred solution of IMes, which had been freshly prepared from IMesHCl (1.409 g, 4.13 mmol) and KOtBu (463 mg, 4.13 mmol) in THF (15 mL).The mixture was stirred for 18 h.Volatile components were removed under vacuum.The residue was extracted with a toluene (50 mL).Insoluble components were removed from the extract by filtration through a short pad of celite, which was subsequently washed with toluene (2 × 15 mL).The solutions were combined.Volatile components were removed under vacuum, leaving the product as an orange microcrystalline solid, which was washed with diethyl ether (3 × 3 mL) and dried under vacuum.Further purification to obtain an analytical sample and crystals suitable for XRD was achieved by recrystallization from n-hexane.

X-ray Crystallography:
For each data collection a single crystal was mounted on a micro-mount at 100(2) K and all geometric and intensity data were taken from this sample.Data collections were carried out using MoK α radiation (λ = 0.71073 Å) on a Stoe IPDS2 diffractometer equipped with a 2-circle goniometer and an area detector in the case of 1 and 3, whereas CuK α radiation (λ = 1.54186Å) and a Stoe StadiVari diffractometer equipped with a 4circle goniometer and a DECTRIS Pilatus 200 K detector was used in all other cases.The data sets were corrected for absorption, Lorentz and polarisation effects.The structures were solved by direct methods (SHELXT) and refined using alternating cycles of leastsquares refinements against F 2 (SHELXL2014/7). [37]C-bonded H atoms were included in the models in calculated positions, heteroatom-bonded H atoms have been found in the difference Fourier lists.All H atoms were treated with the 1.2 fold or 1.5 fold isotropic displacement parameter of their bonding partner.Experimental details for each diffraction experiment are given in Table S1 in the Supporting Information.

Figure 3 .
Figure 3. Molecular structure of 1 in the crystal (ORTEP with 30 % probability ellipsoids, N-substituents drawn in wireframe representation and H atoms except IMesCH omitted for clarity).

Figure 4 .
Figure 4. Molecular structure of 2 in the crystal (ORTEP with 30 % probability ellipsoids, N-substituents drawn in wireframe representation and H atoms except IPrCH omitted for clarity).

Table 1 .
Pertinent metric parameters for the bis(NHO) compounds of this study.Data for closely related compounds are included for comparison.
(see FigureS11in the Supporting Information).The mixture of diastereomers was triturated with benzene (3 × 3 mL) to extract the meso-diasteromer and filtered.The filtrates were combined and volatile components removed under vacuum, affording essentially pure meso-[1(BH 3 ) 2 ].Yield 37 mg.Single crystals of meso-[1(BH 3 ) 2 ] were obtained from a DCM solution which was layered which a small amount of n-hexane.Single crystals of rac-[1(BH 3 ) 2 ] were obtained by dissolving the residue from the extraction with DCM in benzene and 2115939 (for rac-[1(BH 3 ) 2 ]), 2115940 (for meso-[1(BH 3 ) 2 ]), and 2115941 (for rac-[2(BH 3 ) 2 ]) contain the supplementary crystallographic data for this paper.These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.Supporting Information (see footnote on the first page of this article): Crystallographic data, plots of cyclic voltammograms and NMR spectra.