Fluorocarbyne complexes via electrophilic fluorination of carbido ligands

Abstract

The new fluorocarbynes [M(≡CF)(CO)₂(Tp*)] (M = Mo, W; Tp* = tris(dimethylpyrazolyl)borate) arise from electrophilic fluorination of the lithiocarbynes [M(≡CLi)(CO)₂(Tp*)] with FN(SO₂Ph)₂. The reactions of [W(≡CF)(CO)₂(Tp*)] with [AuCl(SMe₂)] and PhICl₂ afford the first μ₂-fluorocarbyne complex [WAu(μ-CF)Cl(CO)₂(Tp*)] and the first high oxidation state fluorocarbyne [W(≡CF)Cl₂(Tp*)], respectively.

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Introduction

Isolable terminal fluorocarbyne complexes remain exceedingly rare,¹ being limited to those reported by Hughes² and Ozerov³ (Scheme 1). In both cases, access exploits the enhanced nucleofugacity of fluorine bound to a carbon α-to a transition metal.⁴

Scheme 1 Synthesis of fluorocarbyne complexes via fluoride abstraction from CF₄, trifluoromethyl and difluorocarbene ligands. (i) Ar_(s), 8K.¹ (ii) 2KC₈, –2F⁻, –CO.² (iii) [H(SiEt₃)₂][HCB₁₁Cl₁₁], –Et₃SiF.³

Beyond the addition of cobalt carbonyl to [Mo(≡CF)(CO)₂(η⁵-C₅Me₅)] to afford a μ₃-fluorocarbyne capped cluster [MoCo₂(μ₃-CF)(CO)₈(η⁵-C₅Me₅)],^2a no subsequent reactivity of fluorocarbyne ligands has been reported. This is in marked contrast to the heavier halocarbynes [M(≡CX)(CO)₂(Tp*)] (M = Mo, W; X = Cl, Br; Tp* = hydrotris(dimethylpyrazolyl)borate),⁵ the synthetic utility of which has been convincingly demonstrated.^6–9 This utility centres on nucleophilic substitution of the halogen that may be spontaneous^6–9 or palladium-mediated,¹⁰ the latter via the intermediacy of μ-carbido complexes [MPd(μ-C)Br(CO)₂(PPh₃)₂(Tp*)].¹¹ Alternatively, lithium/halogen exchange with ⁿBuLi affords, in situ, the lithiocarbynes [M(≡CLi)(CO)₂(Tp*)] (M = Mo, W) that allow the introduction of carbyne substituents in electrophilic form.^9,12 This latter approach has allowed the synthesis of carbyne ligands that bear substituents based on boron and all the elements of groups 14–16 as well as a range of transition metals.^11,12

Missing from the otherwise complete series of molybdenum and tungsten halocarbyne complexes are the lightest members [M(≡CF)(CO)₂(Tp*)] (M = W 1a, Mo 2a) that might be seen as analogues of Hughes' fluorocarbynes [M(≡CF)(CO)₂(η⁵-C₅R₅)]. These are not available via Lalor's original halocarbyne synthesis due to a range of very specific factors peculiar to that system,^5b thereby preventing a complete comparative analysis beyond computational interrogation of the hypothetical complexes [Mo(≡CX)(CO)₂(Tp)] (X = F, Cl, Br, I; Tp = hydrotris(pyrazolyl)borate).¹³ Herein, we now describe (i) the synthesis of the low-valent fluorocarbyne complexes 1a and 2avia a novel strategy; (ii) the reaction of 1a to afford the first example of a μ₂-fluorocarbyne complex 3; (iii) oxidation to afford the first isolable¹ high-valent fluorocarbyne complex [W(≡CF)Cl₂(Tp*)](4).

Results and discussion

In the absence of the requisite but yet to be described complexes [M(CF₃)(CO)₃(Tp*)], Hughes' fluorocarbyne synthetic strategy via 2-electron reduction of trifluoromethyl complexes (Scheme 1) is not applicable. Although thermodynamically feasible, bromide substitution of, e.g., [W(≡CBr)(CO)₂(Tp*)] (1c) by fluoride ([ⁿBu₄N]F or AgF) fails under the conditions we have so far explored. We therefore turned to an alternative strategy employing the lithiocarbyne [W(≡CLi)(CO)₂(Tp*)] originally described by Templeton,¹⁴ but now more conveniently accessible via a simple lithium/halogen exchange reaction of 1c with ⁿBuLi (THF, −78 °C, Scheme 2).^10a,12 Templeton has previously accessed the iodocarbyne [W(≡C–I)(CO)₂(Tp*)] (1d)^5bvia iodination of [W(≡CLi)(CO)₂(Tp*)].¹⁴ We therefore considered whether such a strategy might also afford fluorocarbyne complexes.

Scheme 2 Synthesis of fluorocarbyne complexes via electrophilic fluorination of lithiocarbynes. (i) ⁿBuLi, –ⁿBuBr. (ii) F–N(SO₂Ph)₂. (iii) [AuCl(SMe₂)]. (iv) PhICl₂.

Treating the lithiocarbyne [W(≡CLi)(CO)₂(Tp*)], generated in situ, with an electrophilic source of fluorine, viz. FN(SO₂Ph)₂,¹⁵ affords the new fluorocarbyne complex [W(≡CF)(CO)₂(Tp*)] (1a, 59%). Similar treatment of [Mo(≡CBr)(CO)₂(Tp*)] (2c) affords the molybdenum analogue [Mo(≡CF)(CO)₂(Tp*)] (2a, 77%, Scheme 2).

In addition to spectroscopic and electrochemical data (Table 1) 1a was characterised crystallographically to confirm connectivity, however detailed geometric analysis is precluded by positional disorder of isosteric CO and CF ligands (one contributor shown in Fig. 1 inset), as also recognised for Hughes' complexes [M(≡CF)(CO)₂(η⁵-C₅R₅)] (M/R = Mo/Me, W/Me, W/H).² Accordingly, and not surprisingly, crystals of 1a were found to be essentially isomorphous with those of the radical [W(CO)₃(Tp*)].¹⁶ Crystals of the molybdenum complex 2a, however, led to a precise structural model free of disorder, geometric data for which indicate that the CF ligand exerts a pronounced trans influence on the unique pyrazolyl donor that is reproduced in the computationally optimised geometries of 1a, 2a and the simpler model complex [W(≡CF)(CO)₂(Tp)] (see ESI†). It is a useful feature of the Tp* ligand that in contrast to cyclopentadienyls for which analogies are entertained, the more clearly octahedral geometry allows both structural and computational interrogation of bonding and implications, e.g., the trans influence. Comparative data for F–C(sp) bonds are surprisingly limited to Hughes' (in all but one case disordered) fluorocarbynes^2,3 and the unique fluoroethynyl complex [Ru(C≡CF)(dppe)(η⁵-C₅Me₅)] (C–F = 1.324(4) Å).^15a Amongst the spectroscopic data of interest, the carbyne gives rise to a doublet resonance in the ¹³C{¹H} NMR spectrum at δ_C = 200.8 showing coupling to both ¹⁹F (¹J_CF = 528 Hz) and ¹⁸³W (¹J_CW = 274.6 Hz) nuclei, the latter being somewhat larger than typically found for other carbyne complexes of pseudo-octahedral tungsten.^5c The ¹⁹F{¹H} NMR spectrum comprises a single resonance (δ_F = 45.5) straddled by a doublet due to the ¹⁸³W isotopomer (²J_WF = 111.1 Hz).

Table 1 Data for the complexes [M(≡CR)(CO)₂(Tp*)] (M = W, Mo)

M	R	ν CO cm^−1	k CO N cm^−1	δ WC ppm	^1 J WC Hz	E ^0 V
a Measured in CH2Cl2. b k CK = Cotton–Kraihanzel CO force constant. c Measured in CDCl3. d Relative to ferrocene (E1/2 = 0.460 V cf. Ag/Ag^+ = 0), measured in CH2Cl2 with 1.0 M [N^nBu4][PF6] at 100 mV s^−1; see ESI for cyclic and square-wave voltammograms. n.r. = not reported.
W	F (1a)	1988, 1888	15.15	200.8	275	0.795
W	Cl (1b)	1991, 1902	15.29	205.7	259	0.830
W	Br (1c)	1994, 1905	15.33	198.0	254	0.820
W	I (1d)^14	1992, 1907	15.33	183.2	n.r.	—
Mo	F (2a)	2002, 1910	15.44	193.5	—	0.865
Mo	Cl (2b)	2005, 1921	15.54	208.7	—	0.825
Mo	Br (2c)	2008, 1924	15.59	202.5	—	0.830
Mo	I (2d)^5b	2009, 1927	15.62	n.r.	—	—
W	H^14	1986, 1891	15.16	280.6	192	n.r.
W	CH3 (ref. 7d and 17)	1968, 1867	14.86	289.3	n.r.	+0.510
W	Ph^18	1969, 1876	14.91	277.9	187	n.r.
W	C≡C^tBu^10c,19	1977, 1886	15.05	250.6	199	+0.761

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Fig. 1 Molecular structure of [Mo(≡CF)(CO)₂(Tp*)] (2a) in a crystal (50% displacement ellipsoids, pyrazolyl groups simplified). Selected bond lengths (Å) and angles (°): Mo1–N1 2.247(5), Mo1–N3 2.221(5), Mo1–N5 2.210(5), Mo1–C1 1.886(7), Mo1–C2 1.938(8), Mo1–C3 1.959(7), F1–C1 1.214(8), F1–C1–Mo1 168.4(7), C1–Mo1–N3 100.3(3), C1–Mo1–N5 98.2(3). Inset = less precise structural model for 1a (positional disorder of CF and CO ligands, one contributor shown).

With the complete series of halocarbynes now in hand, a host of physico-chemical data is available to benchmark the nature of the fluorocarbyne ligand against other halocarbynes and more conventional carbyne ligands in the complexes [W(≡CR)(CO)₂(Tp*)] (R = F, Cl, Br, I, H,¹⁴ CH₃,¹⁷ Ph,¹⁸ C≡CPh,¹⁹Table 1) and to validate recent predictions based on the model complexes [Mo(≡CX)(CO)₂(Tp)] (X = F, Cl, Br, I).¹³

Descending group 17 for the halocarbyne series, the metal centre becomes marginally more electron-poor and less π-basic (increase in ν_CO and k_CO). In the absence of 1a it might seem that the carbyne ¹³C resonance moves to higher frequency down group 17 showing a normal halogen dependence arising from relativistic spin–orbit coupling.²⁰ The non-conforming position of 1a in the series therefore most likely reflects a competing inverse halogen effect arising from paramagnetic shielding due to magnetic coupling of occupied and unoccupied orbitals, e.g., σ*(C–F) and π*(WC). The oxidation potential is remarkably insensitive to the nature of the halogen, as might be expected from computational studies on the hypothetical series [Mo(≡CX)(CO)₂(Tp)] that show the energy of the metal-based HOMO (orthogonal to the C–X vector) to be essentially in variant.

Whilst μ₃-fluorocarbyne complexes are well-known^2a,21 there are no previous examples of doubly bridging fluorocarbyne ligands. The addition of the ‘AuCl’ fragment to terminal carbyne complexes has on numerous occasions been shown to afford heterobimetallic μ₂-carbynes²² including the recent isolation of bromo- and chlorocarbyne examples.¹³ Accordingly, the reaction of 1a with [AuCl(SMe₂)] was investigated and found to afford the μ₂-fluorocarbyne complex [WAu(μ-CF)Cl(CO)₂(Tp*)] (3). The resonance for the fluorocarbyne carbon appears at δ_C = 236.6, to higher frequency of the precursor and displays a reduced coupling to tungsten-183 (¹J_CW = 143.8 Hz) and fluorine-19 nuclei (¹J_CF = 454 Hz), consistent with a decrease in s-character for the sp²vs. sp carbon hybridisation and the increase in W–C bond length (1.876(6) Å). The molecular structure (Fig. 2) reveals that the fluorocarbyne ligand adopts an unsymmetrical semi-bridging mode (W1–C1–F1 = 148.7(6)°), as is commonly encountered for carbyne (and carbonyl) ligands bridging between tungsten and d¹⁰ metal centres. Therein, the carbyne may be viewed as a Z-type σ-acceptor ligand, i.e., donation from gold(I) to a π-hole on the carbonyl ligand. Similar geometric features are reproduced upon geometry optimisation (ωB97X-D/6-31G*/LANL2Dζ, see ESI†) of the model complex [WAu(μ-CF)Cl(CO)₂(Tp)] (3′: W=C = 1.893 Å; Au–C = 2.042 Å, W–Au = 2.829 Å, W–C–Au = 91.8° W–C–F = 149.7°) for which Löwdin bond orders (WC = 1.80, AuC = 0.72) suggest that a dimetallacyclopropene description is useful.

Fig. 2 Molecular structure of [WAu(μ-CF)Cl(CO)₂(Tp*)] (3) in a crystal (50% displacement ellipsoids, pyrazolyl groups simplified). Selected bond lengths (Å) and angles (°): Au1–W1 2.7974(4), Au1–Cl1 2.268(2), Au1–C1 2.007(8), W1–C1 1.876(6), F1–C1 1.318(7), Cl1–Au1–W1 150.49(6), C1–Au1–W1 42.08(18), C1–Au1–Cl1 167.42(19), W1–C1–Au1 92.1(3), F1–C1–Au1 119.2(5), F1–C1–W1 148.7(6).

The complex 3 is unstable in solution, gradually depositing elemental gold (accompanied by some reformation of 1a) to provide a new purple species identified as the high-valent fluorocarbyne complex [W(≡CF)Cl₂(Tp*)] (4). Once recognised as such, complex 4 could be more expediently and directly prepared from 1a and iodobenzene dichloride. Key spectroscopic features of note for 4 include a resonance for the carbyne carbon (δ_C = 224.9) that is moved to higher frequency of that for 1a, with a similar ¹J_CF coupling (512.8 Hz) but augmented ¹J_WC coupling (316 Hz). The ²J_WF coupling (105.1 Hz) observed in the ¹⁹F{¹H} NMR spectrum is similar to that observed for 1a. Taken together, these suggest retention of octahedral geometry at tungsten and sp-hybridisation at the carbyne carbon. Oxidative interconversion of ‘Fischer’ and ‘Schrock’ classes of carbyne is well established²³ but in this case is gratifying in that 4 represents a connection between the exotic matrix-isolated ‘Schrock-type’ carbynes X₃M≡CF (X = F, Cl)¹ and tractable low-valent compounds such as 1a, 2a and Hughes' [M(≡CF)(CO)₂(η⁵-C₅R₅)] (M = Cr, Mo, W; R = H, Me). For simplicity, the frontier molecular orbitals of interest for the model complexes [W(≡CF)(CO)₂(Tp)] (1a′) and [W(≡CF)Cl₂(Tp)] (4′) are shown in Fig. 3 (those for the full and sterically congested molecules 1a and 4 are provided in the ESI† alongside those for the matrix molecules X₃W≡CF, X = F, Cl, calculated at the same level of theory). In the case of 1a, the HOMO is primarily associated with metal carbonyl back-donation (‘d_xy’ taking ‘z’ as the W⋯F vector), explaining why the oxidation potentials (Table 1) are rather insensitive to changes in the carbyne substituent. The HOMO − 1 and HOMO − 2 comprise the two orthogonal π-components of the W≡C multiple bond for 1a′. For 4′, the manifold of frontier orbitals is rather similar with the notable exception that the ‘d_xy’ orbital is now vacant and available for nucleophilic attack. This leaves the near degenerate pair of WC π-bonding orbitals as the highest in energy, as is also the case for the Schrock-type molecules X₃W≡CF. Notably, for 1a′, while the LUMO has both metal and carbyne–carbon character, the real complex 1a is not especially susceptible to hydrolysis (cf. difluorocarbene complexes), easily surviving chromatographic purification without any identifiable formation of the anticipated hydrolysis product [WH(CO)₃(Tp*)]. The situation for the high-valent 4′ is intriguing in that, by analogy with chloro- and bromo-carbynes, nucleophilic substitution of the fluoride would seem plausible. Furthermore, oxidation state has been shown to profoundly enhance the susceptibility of difluorocarbene ligands towards nucleophilic attack, as demonstrated for the zero- and divalent ruthenium complexes [Ru⁰(=CF₂)(CO)₂(PPh₃)₂]²⁴cf. [Ru^IICl₂(=CF₂)(CO)(PPh₃)₂], respectively.²⁵ High-valent Schrock-type carbyne complexes of the form [W(≡CR)X₂(Tp′)] (X = Cl, Br; Tp′ = Tp, Tp*)^23c,26 are generally prone to hydrolysis via nucleophilic attack (H₂O/HO⁻) at the metal to provide [W(=CHR)Cl(=O)(Tp′)]. It is therefore noteworthy and somewhat surprising that complex 4 is comparatively robust, air stable and not readily hydrolysed.

Fig. 3 Frontier orbitals (ωB97X-D/6-31G*/LANL2Dζ/gas phase) of interest for the complexes [W(≡CF)(CO)₂(L)] and [W(≡CF)Cl₂(L)] (L = Tp, red, Tp* blue).

In conclusion, electrophilic fluorination of carbido ligands provides a viable route to fluorocarbyne complexes, building upon previous examples of the iodination of terminal carbido ligands^14,27 and the chlorination or bromination of bridging μ₂-carbido complexes.²⁸ The ‘Fischer-type’ fluorocarbyne [W(≡CF)(CO)₂(Tp*)] may be oxidised to a rare example^23d of a high oxidation state halocarbyne [W(≡CF)Cl₂(Tp*)] that is best viewed as a ‘Schrock-type’ carbyne.

Experimental

General experimental details and instrumentation, synthetic methods, spectroscopic data, selected spectra, Cartesian coordinates and computational details are provided in the accompanying ESI.†

Data availability

The datasets supporting this article have been uploaded as part of the ESI.† Crystallographic data have been deposited at the CCDC under 2226627 and 2226628 and can be obtained from http://ccdc.cam.ac.uk.

Author contributions

RAM was responsible for the design and execution of the experimental research, the acquisition and critical analysis of the characterising data. AFH was responsible for funding acquisition, project conceptualisation and administration, validation and compilation of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the Australian Research Council (DP190100723 and DP200101222) for funding.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, spectroscopic, computational and crystallographic data. CCDC 2226627 and 2226628. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc00261f

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