Fluoride, bifluoride and trifluoromethyl complexes of iridium(I) and rhodium(I)

Abstract

Herein we report robust methods for the preparation and full characterisation of a range of Ir(I) and Rh(I) fluoride and bifluoride complexes using N-heterocyclic carbenes (NHCs) as ancillary ligands. The processes that link the fluoride and the bifluoride species are investigated and reports of the first Ir–bifluoride and Ir(I)–NHC and Rh(I)–NHC trifluoromethyl complexes are revealed.

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Fluorinated organometallic complexes are highly interesting species. Fluorides are known to confer unique properties to the metal and their coordination can lead to fascinating reactivity. Nevertheless, metal fluorides remain the least understood member of the metal-halide family.¹

Additionally, the catalytic preparation of fluorinated organic compounds is an area of great importance, particularly for the radio therapeutic, pharmaceutical and agrochemical sectors.² The isolation of fluorinated organometallic complexes permits us to study the behaviour of potential catalysts and catalytic intermediates under homogeneous conditions.³ This knowledge is of paramount importance in designing new catalytic cycles and improving existing fluorination methods.^2b,4

The interaction between a hard base such as fluoride and a low-oxidation state metal (behaving as a soft Lewis acid) should lead to weak metal–fluoride bonds.^1a,5 Stabilisation of M–F bonds and isolation of LTM–fluorides has mostly been reported using phosphine, N-donor or σ-aryl ancillary ligands.^1,6 Some of the most extensively investigated fluoride complexes are analogues of Vaska's complex ([IrCl(CO)(PPh₃)₂]) and related rhodium complexes.^1a,6c,7 In contrast, fluoride complexes bearing olefinic ligands remain scarce,⁸ and even more scarce are those containing N-heterocyclic carbenes (NHCs).^6a,9 NHC ligands have featured extensively in the literature as electron-rich donor ligands, and have proven particularly useful in stabilising hard/soft metal–ligand mismatches such as metal hydroxides.¹⁰

Bifluoride complexes [M(FHF)] are very interesting, and since the first report by Coulson¹¹ on Pt, several examples of metal bifluorides have been reported, including Mn,¹² W,¹³ Pd,¹⁴ Ru,¹⁵ Mo,¹⁶ Ni,¹⁷ and Rh,^7a,8b,18 mostly bearing phosphine or amine based supporting ligands. The only examples of LTM–HF₂ complexes bearing NHC ligands are those reported by the groups of Leyssens on Cu;¹⁹ and by Whittlesey on Rh (bearing ring expanded six-membered NHC ligands).^18c,20 Furthermore, reports of LTM(NHC) complexes bearing trifluoromethyl (CF₃) groups are exclusive to Cu.²¹ In order to build on the reactivity of Ir(I) and Rh(I) compounds, we aimed to establish robust methods for the preparation of fluorinated Ir(I)- and Rh(I)-complexes bearing F, HF₂ and CF₃ moieties.

Based on previous success in stabilising M–OH bonds on Rh(I) and Ir(I), our strategy was to use the [M(cod)(NHC)(X)] scaffold (M = Rh, Ir; cod = 1,5-cyclooctadiene; X = Cl, OH) to gain access to M(I)–F bonds.¹⁰ Gratifyingly, a transmetallation/halide substitution reaction between [Ir(cod)(NHC)Cl] and AgF^6b delivered [Ir(cod)(NHC)F] complexes 1–4 as yellow solids in excellent yields (Scheme 1). Multinuclear NMR spectroscopy and elemental analysis were used to unambiguously confirm the identity of the products with the ¹⁹F NMR signals corresponding to the Ir(I)–F moiety resonating between −221 and −227 ppm (see ESI†).

Scheme 1 Preparation of Ir(I) and Rh(I)–fluoride complexes.

It was clear that the three species were intrinsically linked and we were able to convert the fluoride complex to either the Ir(I)–OH (via reaction with CsOH or KOH), or to the Ir(I)–Cl (via reaction with NaCl). However, when we attempted to extend the methodology to rhodium, the reactions were at best very slow (65% conversion after 60 h for [Rh(cod)(IPr)Cl]). Treating either of the metal chlorides or hydroxides with fluoride salts such as KF or CsF was also unsuccessful and a new approach had to be devised. To our delight, the Rh(I)–OH reacted with cheap and readily available KHF₂ to deliver the desired Rh(I)–F in high yield in just 5 h. This method proved general for both metals (Scheme 1) and analysis by ¹⁹F NMR indicated that Rh(I)–F signals resonated at a higher field than the analogous Ir(I)–F complexes (δ −258, br (5); δ −253, d, ¹J_RhF = 77 Hz (6)).

X-ray analysis of complexes 1, 3, 5 and 6 (Fig. 1 and Fig. S56 and S57, ESI†) indicated 16e⁻, square planar geometries. Compared to the parent complexes, the M(I)–F bonds (1.991–2.032 Å for Ir and 2.016–2.092 Å for Rh) are shorter than M(I)–Cl bonds (2.334–2.382 Å)²² and closer in magnitude to the corresponding M(I)–hydroxides (2.014–2.038 Å).^10,23

Fig. 1 Thermal ellipsoid representations (50%) of 1 and 5. H atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (°), numbers in parentheses are for second independent molecule: 1: Ir(1)–C(1) 2.014(16), [2.029(17)]; Ir(1)–F(1) 2.023(9), [2.002(11)]; C(1)–Ir(1)–F(1): 87.5(5), [88.2(6)]. 5: Rh(1)–C(1) 2.022(3); Rh(1)–F(1) 2.018(2); C(1)–Rh(1)–F(1) 88.20(11).

As a third method, we examined the reaction between the Rh(I) and Ir(I)–hydroxides and Et₃N·3HF. Ir(I)–OH and Rh(I)–OH complexes were reacted with Et₃N·3HF (0.33 equiv.) in benzene to successfully deliver the Rh(I) and Ir(I)–fluorides in good to excellent yields (Scheme 2).

Scheme 2 Reaction of Rh(I)- and Ir(I)- hydroxides with Et₃N·3HF.

As described by Riant and Leyssens,^19b changing the solvent to THF and increasing the relative concentration of Et₃N·3HF (0.66 equiv.) afforded the expected bifluoride species, [Ir(cod)(IPr)(HF₂)] (7) and [Rh(cod)(IPr)(HF₂)] (8) from the corresponding M–hydroxides (Scheme 2). NMR spectroscopic analyses of these new complexes showed subtle differences to the analogous fluoride complexes (1 and 5). At room temperature the ¹H NMR spectrum for each compound contained a low-field broad doublet (δ 11.4, ¹J_FH = 395 Hz (7); δ 12.3, ¹J_FH = 350 (8)). Upon cooling (200 K, CD₂Cl₂) the signals partially resolved into doublets of doublets in each case with a spin–spin coupling of 395 and 40 Hz for 7 and 350 and 25 Hz for 8 (see ESI†). These ¹H NMR signals were assigned to the acidic proton of the bifluoride moiety; with a strong bond to one fluoride and a weaker bond to a second fluoride. The assignment was confirmed by the identification of two fluoride groups by ¹⁹F NMR analysis of 7 and 8.

In each case, a ¹⁹F NMR signal was observed in the same region as that found for 1 and 6, corresponding to the metal-bound (proximal) fluoride. In addition, a doublet resonated further downfield with a spin–spin coupling constant of 395 Hz on 7 and 350 Hz on 8. These signals were assigned to the distal fluoride, showing a strong bond to the acidic proton. At 200 K, all ¹⁹F NMR signals resolved into distinct doublets of doublets, e.g. for 7: δ −178 (dd, ¹J_HF = 395, ²J_FF = 120, Ir–F⋯HF), −239 (dd, ²J_FF = 120, ¹J_HF = 40, Ir–F⋯HF, see ESI†). Based on these results, we proposed the composition of 7 and 8 as [Ir(cod)(IPr)(FHF)] and [Rh(cod)(IPr)(FHF)], respectively (Scheme 2) with the HF₂ moieties comprising a strong bond between the metal and the proximal fluoride, and a relatively weak hydrogen bond to the HF moiety (i.e. [M]–F⋯HF as opposed to [M]⋯F–HF), in agreement with literature reports.^{15b,18a,b,19b,20}

X-ray crystallographic analysis of 7 and 8 (Fig. 2) showed square planar complexes, similar to the mono-fluorides. The X-ray structure for 7 shows a significantly elongated M–F bond. This weakening of the M–F bond is a consequence of the hydrogen bond to HF.^15b The F–F separation on 7 is in the range of 2.35–2.39 Å and 2.32–2.33 Å for complex 8. These values are considerably less than the sum of the van der Waals radii of two F atoms (2.94 Å)²⁴ and are indicative of strong hydrogen bonds to the two fluorides, in agreement with reports for other metal bifluorides including those of Mo and Rh.^{13,14c,15b,c,16–18,19b,20}

Fig. 2 Thermal ellipsoid representations of 7, 8 and 9 showing 50% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (°) 7: Ir(1)–C(1) 2.033(4), [2.049(4)]; Ir(1)–F(1) 2.057(3), [2.062(2)]; C(1)–Ir(1)–F(1) 88.92(13), [92.59(12)]. 8: Rh(1)–C(1) 2.047(5), [2.035(6)]; Rh(1)–F(1) 2.089(3), [2.069(3)]; C(1)–Rh(1)–F(1) 93.09(16), [89.14(18)]. 9: Ir(1)–C(1) 2.045(6), [2.053(6)]; Ir(1)–F(1) 2.064(5), [2.082(4)]; C(1)–Ir(1)–F(1) 91.0(2), [87.5(2)].

The unsymmetrical nature of the bifluoride ligand is further supported by FTIR analyses of 7 and 8, which showed two broad bands each (7: ν = 2623 and 1807 cm⁻¹; 8: ν = 2530 and 1890 cm⁻¹) corresponding to the H–F modes of the bifluoride (absent from FTIR of 1 or 6, see ESI†).^14b,16,20 Compared to the IR bands expected for free HF (ν = 3896 and 3895 cm⁻¹ in an Ar matrix),²⁵ it is clear that there is weakened hydrogen bonding within the HF moiety as a result of the H-bond to the proximal fluoride, causing a shift to lower wavenumber.

Compared to symmetrical inorganic bifluoride salts, for example Cs⁺, K⁺ or Na⁺ HF₂⁻ (1372–1284 cm⁻¹),²⁶ the IR bands of 7 and 8 are considerably higher in wavenumber, indicating that the HF₂ moiety in these complexes is highly unsymmetrical, with a stronger hydrogen bond to one fluorine atom than the other.

In addition to 7 and 8, an alkyl-substituted NHC complex of Ir(I)–(HF₂) was prepared in the form of [Ir(cod)(IⁱPr)(HF₂)] (9) (Scheme 2). The less sterically encumbered 9 exhibited different structural characteristics to 7 and 8, both in the solid-state and in solution. For example, at room temperature the ¹H NMR signal corresponding to the acidic proton resonates as a broad unresolved peak, as do both signals on the ¹⁹F NMR spectrum (see ESI†). Upon cooling to 200 K, the signals resolved into doublets of doublets in the same way as 7 and 8.

From the X-ray structure for 9 (Fig. 2), it is apparent that while the Ir–F bond is comparable in length to 7, the F–F separation (2.457(6), [2.587(6)] Å) is considerably larger, even compared to the values for most metal bifluorides reported to date.^{8b,13–16,18a–c,19b,20} On closer inspection of the X-ray structures of 7 and 8, it is clear that F(1) and the isopropyl proton on each side of the IPr ligand are in very close proximity to each other (ca. 2.33–2.49 Å in 7; 2.32–2.43 Å in 8). This is considerably less than the sum of the van der Waals radii for H and F (2.67 Å)²⁴ Such a close contact is not possible in 9 and the added flexibility may account for the fluxional behaviour in solution that is associated with 9 (this close contact is not observed on the monofluoride complexes).

Next we studied the link between the fluoride and bifluoride complexes. The ease with which the bifluoride can be generated from the monofluoride and the lability of the HF molecule is especially important for catalytic fluorination and fluorination/ring-opening processes,²⁷ where catalysts, particularly metal fluorides are coupled with an HF source. In order to examine the persistence of the bifluoride moiety in the presence of a base, both the Rh(I) and Ir(I)–bifluorides (7 and 8) were reacted with a stoichiometric amount of KOH. In both cases, full conversion to the corresponding M–F (1 and 6) was seen within 2 h. Interestingly, the addition of a second equivalent of KOH yielded the corresponding M(I)–OH; whilst addition of Et₃N·3HF (0.33 equiv.) to 1 or 6 regenerated M–HF₂ (7 and 8). Hence, the three species are inherently linked (Scheme 3) and in the presence of an excess of Et₃N·3HF, a metal fluoride or hydroxide will more than likely deliver the metal bifluoride.

Scheme 3 Processes linking M(I)–OH, M(I)–F and M(I)–HF₂.

Having established reliable routes to mono and bifluoride complexes of Ir(I) and Rh(I), our final goal was to access the first examples of Ir(I)– and Rh(I)–NHC trifluoromethyl complexes. Our initial attempts were conducted using Ruppert–Prakash's reagent, trifluoromethyl(trimethyl)silane (TMSCF₃). Unfortunately all attempts to transfer the CF₃ moiety from Si to Ir or Rh failed. The only complex that reacted with TMSCF₃ was [Ir(cod)(IⁱPr)(OH)], delivering [Ir(cod)(IⁱPr)(OSiMe₃)] (10) (96%).^10b Reactions between TMSCF₃ and the other Ir(I)– or Rh(I)–hydroxides, –chlorides, or –bifluorides were all unsuccessful and clearly a new approach was necessary. Roper has previously prepared Ir(I)–CF₃ complexes from [Ir(CO)₂(PPh₃)₂H] using Hg(CF₃)₂ ²⁸ and similar strategies have been employed on other metals using Cd(CF₃)₂. In an attempt to avoid such toxic reagents, we considered the use of Ag to transfer the CF₃. Hence AgCF₃ was prepared in situ from AgF and TMSCF₃ at −40 °C²⁹ and reacted with [Ir(cod)(IⁱPr)Cl] to give Ir(I)–CF₃ (11) as a bright red solid (Scheme 4). All attempts at isolating the Ir–IPr and Rh–IPr analogues using this procedure failed. In order to rationalise these results we studied the structure of the isolated compound, 11. Hence, single crystals of 11 were grown and analysed by X-ray diffraction (Fig. 3).

Scheme 4 Preparation of [M(cod)(NHC)(CF₃)] complexes (11–13).

Fig. 3 Thermal ellipsoid representations of 11 and 13 showing 50% thermal ellipsoid probability. H atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (°): 11: Ir(1)–C(1) 2.065(8); Ir(1)–C(7) 2.086(8); C(7)–F(1) 1.357(10); C(1)–Ir(1)–C(7) 88.3(3). 13: Rh(1)–C(2) 2.042(5); Rh(1)–C(18) 2.067(5); C(18)–F(1) 1.366(6); C(8)–Rh(1)–C(29) 87.3(2).

In the solid-state it was apparent that two of the fluorine atoms were in very close proximity to a methine proton on each side of the NHC ligand (ca. 2.45 Å). This distance is considerably less than the sum of the Van der Waals radii of H and F (2.67 Å)²⁴ and may indicate an interaction between the two groups. Furthermore, in the ¹³C{¹H} NMR spectrum of 11, a spin–spin coupling interaction causes one of the methyl signals to split into a quartet (δ 22.8 ppm, J_FC = 1 Hz). Since the methyl group is separated from the fluorine atoms by six bonds, the interaction must be a “through-space” coupling.³⁰ This evidence indicates that there are considerable steric factors to take into account and that the presence of bulky NHC ligand substituents such as diisopropyl(phenyl) might hinder the coordination of a CF₃ group to the metal centre.

To test this hypothesis, we investigated the reaction between [M(cod)(ICy)Cl] and AgCF₃, which delivered [M(cod)(ICy)(CF₃)] 12 and 13 (Scheme 4). The same structural features were evident on the ¹³C{¹H} NMR spectrum, showing a spin–spin coupling interaction between a methylene on the cyclohexyl ring and the fluoride groups of the CF₃ on 12. Furthermore, an X-ray crystal structure of 13 (Fig. 3) indicated a similar close proximity between the fluorides and a proton on each of the cyclohexyl rings (2.51–2.52 Å). Hence, this synthetic method is somewhat limited by the steric bulk of the NHC ligands and therefore, independent of the metal used. These structural investigations have given us further insights into the stability of this scaffold and the viability of the method.

In conclusion, we have demonstrated three methods for accessing Ir(I)–NHC fluoride compounds, two of which were effective on rhodium. The first examples of Ir(I)–bifluorides are reported in high yields, via a general method. Access to Ir(I) and Rh(I)–F and –HF₂ complexes enabled examination of the link between the two species. This knowledge could prove paramount in developing new fluorination strategies since the bifluoride can be quantitatively generated from the monofluoride in solution and vice versa. A new method was also established to access the first examples of Ir(I)–NHC trifluoromethyl complexes. This method was also applied to access the first 5-membered NHC-based Rh(I)–CF₃ complex. Consequently, we have assembled a toolbox for the preparation of fluorinated complexes to take advantage of the reactivity of already successful Ir(I)– and Rh(I)–NHC scaffolds. This will enable us to pursue fluorination strategies and may well provide active catalytic species or catalytic intermediates in the future.

The ERC (Advanced Researcher Award FUNCAT to SPN), the EPSRC and Sasol (Stipend to BJT) are gratefully acknowledged for support. Umicore AG are thanked for their gift of materials. We thank Prof. Andrew Smith for access to FTIR spectrometry. Thank you to Dr A. Collado and S. R. Patrick for helpful discussions. SPN is a Royal Society Wolfson Award holder.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthetic procedures and full characterisation details for new complexes. CCDC 1000750 (1), 1000751 (3), 1000752 (5), 1001354 (6), 1000753 (7), 1000754 (8), 1000755 (9), 1000756 (11), 1000757 (13). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc07772e

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