Ortho-F,F-DPEphos: Synthesis and Coordination Chemistry in Rhodium and Gold Complexes, and Comparison with DPEphos

Graphical Abstract

A ortho-fluorine substituted DPEphos ligand has been synthesized: o-F,F-DPEphos. The influence of the ortho-substitution is highlighted by the comparison of the aurophilic interactions in Au₂Cl₂(o-R,R-DPEphos) and the hydrogenation product from [Rh(o-R,R-DPEhphos)(NBD)][BAr^F₄] complexes (R=H or F). An unexpected dicationic dimer of o-H,H-DPEphos with an agostic C−H group is revealed for the latter.

Abstract

The synthesis of a new ortho-fluorine substituted diphosphine ligand based upon the DPEphos ligand is reported: o-F,F-DPEphos (DPEphos=bis(2-diphenylphosphinophenyl)ether). The corresponding bimetallic gold complex Au₂Cl₂(o-F,F-DPEphos) and Schrock-Osborn type Rh(I) complex [Rh(o-F,F-DPEphos)(NBD)][BAr^F₄] have been synthesized (Ar^F=3,5-(CF₃)₂-C₆H₃). The solid-state and solution phase NMR data are examined in comparison with the o-H,H-DPEphos equivalents, and demonstrate the influence of ortho-fluorine substitution through the control of aurophilic and Rh⋅⋅⋅X interactions (X=H or F). Characterization of the organometallic products upon hydrogenation of the Schrock-Osborn complexes in 1,2-F₂C₆H₄ revealed a Rh dimeric complex [Rh(H)(fac-κ³-P,O,P-μ-CH-DPEphos)]₂[BAr^F₄]₂ with a bridging C−H agostic interaction for the parent DPEphos. The substituted variant formed a Rh(III) monomer [Rh(mer-κ³-P,O,P-o-F,F-DPEphos)H₂][BAr^F₄], highlighting the structural consequences of ortho-fluorine substitution.

Introduction

The manipulation of the steric and electronic properties of phosphine ligands is a well-known method for tuning reactivity and selectivity in transition-metal mediated catalysis.^{1, 2} Ortho-substitution at phosphorus aryl-substituents is one method to do this, and there are a number of examples of how ortho-functionalization can affect the coordination geometry,³ reactivity^4-6 and selectivity of homogeneous catalysts.^7-10 Ortho-fluorine substitution, however, is much rarer.^{11, 12} Ortho-aryl fluorine substitution has been used to increase the rate of biphenyl reduction in Pt(Ph)₂(o-F-diphosphine) complexes,¹³ and shown to increase catalyst productivity in Cr-catalyzed ethylene oligomerisation.^{14, 15} DPEphos (DPEphos=bis(2-diphenylphosphinophenyl)ether, named o-H,H-DPEphos here for clarity, Figure 1A), is an example of a POP-type diphosphine ligand that can be readily adapted by changing the phosphorus substituents.^{16, 17} Initially synthesized for use as a κ²-P,P wide bite-angle diphosphine ligand for selective hydroformylation catalysis,¹⁸ o-H,H-DPEphos has since been shown to coordinate to a transition metal atom in a variety of geometries, including: μ²,κ¹,κ¹-P,P, κ²-P,P and κ³-P,O,P (Figure 1B).¹⁹ This coordination flexibility, facilitated by the hemilabile¹⁹ nature of the oxygen, has been demonstrated in a range of homogeneous catalytic processes. For example the Pd(o-H,H-DPEphos)-catalyzed Suzuki cross-coupling to form sterically hindered biaryls²⁰ and aryl-phosphonates,²¹ or amination of allylic alcohols using a Pt(o-H,H-DPEphos)Cl₂ precatalyst.²² The cationic [Rh(o-H,H-DPEphos)]⁺ system has been employed to catalyze alkene and alkyne intermolecular hydroacylation^23-25 and amine-borane dehydropolymerization.^{26, 27} Transitioning between the possible coordination modes has been shown to be important for catalytic activity in some cases.^{23, 25, 28} Examples of aryl modification in DPEphos are rare, and catalytic applications, thus far, are limited.^{16, 29-33} Most recently, rhodium Schrock-Osborn type^{34, 35} precatalysts containing ortho-aryl-substituted DPEphos ligands have been reported, in which steric bulk was varied [Rh(R-DPEphos)(NBD)][BAr^F₄] (R=H, Me, OMe, ⁱPr; NBD=norbornadiene and Ar^F=3,5-(CF₃)₂-C₆H₃).¹⁷ Changes in the steric properties of the ortho-substituent influenced the coordination geometry and further reactivity of the precatalyst upon hydrogenation of the norbornadiene fragment. For example, the identification of Rh⋅⋅⋅H−C anagostic³⁶ interactions in the NBD complexes and ligand C−H activation in the (R=ⁱPr) when activated using dihydrogen. In light of these studies, and wanting to explore the wider scope of ortho-aryl substitution in DPEphos ligands, we now report the synthesis of ortho-F,F-DPEphos (Figure 1C), in which the ortho-phenyl hydrogen atoms in parent DPEphos have been replaced with fluorine, and preliminary coordination chemistry studies with Au and Rh-centers. The influence of this ortho-fluorine substitution switches off solid-state aurophilic interactions in the bimetallic Au(I) complex Au₂Cl₂(o-F,F-DPEphos) compared to the o-H,H-DPEphos equivalent;³⁷ while in the Schrock-Osborn type precatalyst,^{34, 35} [Rh(o-F,F-DPEphos)(NBD)][BAr^F₄], ortho-fluorine substitution stops the formation of hydride-bridged dimers upon hydrogenation, for which – unexpected – supporting ortho-C−H phenyl agostic interactions occur with parent DPEphos.

Figure 1

A) o-H,H-DPEphos and ortho-substituted DPEphos. B) Some examples of the coordination modes of DPEphos in metal-DPEphos complexes. C) The work reported in this article – a comparison of the rhodium and gold coordination chemistry of o-H,H-DPEphos and o-F,F-DPEphos.

Results and Discussion

Synthesis of o-F,F-DPEphos, 1-F,F

The di-ortho-fluorine analogue of DPEphos, o-F,F-DPEphos (1-F,F) was synthesized via addition of Li[2,6-F₂C₆H₃] to 2,2’-(PCl₂)₂Ph₂O (Scheme 1), as previously reported for the synthesis of other substituted DPEphos ligands.¹⁷ Aqueous workup afforded an analytically pure product as a white microcrystalline solid on recrystallization, in moderate (30 %) yield, which was fully characterized by multinuclear NMR spectroscopy (CD₂Cl₂). One environment is observed in both the ³¹P{¹H} (δ −62.5) and ¹⁹F{¹H} (δ −100.7) NMR spectra, each displaying J(PF) coupling (40 Hz), to give a quintet and doublet respectively. The ³¹P signal is shifted 45.9 ppm upfield compared to the parent DPEphos (δ −16.6). A similar shift is observed between PPh₃ (δ −4.7)³⁸ and PPh(2,6-F₂C₆H₄)₂ (δ −50.6).¹² The single ¹⁹F environment observed suggests free rotation of the aryl group³⁹ as well as ether backbone flexibility that makes all of the substituted aryl groups equivalent.

Scheme 1

Synthesis of ortho-F,F-DPEphos, 1-F,F.

Synthesis, characterization and comparison of Au₂Cl₂(o-R,R-DPEphos) complexes

With the new ligand in hand its coordination chemistry with the {AuCl} fragment was investigated. Gold complexes with ortho-substituted aryl phosphines are well established in catalysis,^{4, 5} but ortho-substituted DPEphos variants are, to our knowledge, unknown. The corresponding Au₂Cl₂(o-F,F-DPEphos) complex, 2-F,F, was synthesized using a similar method as reported for parent o-H,H-DPEphos,³⁷ 2-H,H, by addition of two equivalents of Au(THT)Cl (THT=tetrahydrothiophene) to 1-F,F in CH₂Cl₂ solution (Scheme 2). 2-F,F was isolated as a white crystalline solid in a moderate yield (52 %). Figure 2 shows the solid-state structure of 2-F,F, which shows a μ²,κ¹,κ¹-o-F,F-DPEphos ligand bridging two linear AuCl groups [Cl−Au−P; 175.24(5)° and 177.09(6)°]. The Au atoms are orientated in a trans arrangement, facilitated by a twist of the ligand backbone by 73° (arene-arene plane angle), imposing non-crystallographic, C₂ symmetry. There are no aurophilic interactions⁴⁰ [Au⋅⋅⋅Au=6.1792(6) Å].

Scheme 2

Formation of Au₂Cl₂(o-F,F-DPEphos), 2-F,F.

Figure 2

Crystallographically determined structure of 2-F,F. Ellipsoids shown at the 50 % probability level. Hydrogen atoms are omitted and the terminal aryl groups are modelled in stick form for clarity. Selected bond lengths (Å) and angles (°): P1−Au1, 2.225(1); Au1−Cl1, 2.276(1); P2−Au2, 2.230(1); Au2-Cl2, 2.278(1); Au1−Au2, 6.1792(6); Cl1−Au1−P1, 177.09(6); Cl2−Au2−P2, 175.24(5); C18-O1-C19, 117.8(3).

This structure is in contrast with the previously reported solid-state structure of 2-H,H, which has a much closer Au⋅⋅⋅Au contact [3.0116(4) Å], typical for an aurophilic interaction, i. e. 2.5–3.5 Å.⁴⁰ Each Au center in 2-F,F shows a number of relatively close Au⋅⋅⋅F distances (∼3.0 Å), which sit at the threshold defined for significant M⋅⋅⋅F interactions.⁴¹ The C₂ symmetry observed in the solid-state is retained in the solution state. In the ³¹P{¹H} NMR spectrum (CD₂Cl₂, 295 K) a single ³¹P signal is observed at δ −23.1 as a sharp 1 : 2 : 3 : 2 : 1 apparent quintet, coupling to four equivalent ¹⁹F nuclei [J(PF)=25 Hz]. This is a large upfield shift compared to 2-H,H (δ 21.6).³⁷ Two equal intensity ¹⁹F environments are observed at δ −97.3 and δ −97.9 in the ¹⁹F{¹H} NMR spectrum as doublets of triplets, both coupling to one ³¹P [J(PF)=25 Hz] and two ¹⁹F nuclei, J(FF) (7 Hz) – the latter confirmed by a ¹⁹F-COSY experiment. These are not particularly shifted from free ligand (δ −100.7). A total of eight aromatic environments were observed for 2-F,F in the ¹H NMR spectrum, assigned to the diphenylether backbone and two sets of meta-H and para-H environments. These data suggest a rapid rotation of the 2,6-F₂C₆H₃ groups around the P−C bond, but that inversion of the diphenyl backbone does not occur, or is very slow on the NMR timescale, leading to two different aryl groups. We suggest the ¹⁹F-¹⁹F coupling observed is due to through space coupling,^{42, 43} for which a dependence on the F⋅⋅⋅F distance has been investigated computationally by Mallory and co-workers.⁴⁴ A F⋅⋅⋅F distance of 3.0 Å would be expected to give a J(FF) of ∼7 Hz.⁴⁵ This is consistent with that measured for the closest F⋅⋅⋅F distance (F5⋅⋅⋅F7) on adjacent aryl groups of 2.894(6) Å in 2-F,F, and the observed coupling constant. These interactions would be time-averaged in solution leading to the observed symmetry in solution.

Synthesis, characterization and structural behavior of [Rh(o-F,F-DPEphos)(NBD)][BAr^F₄], 3-F,F

[Rh(o-F,F-DPEphos)(NBD)][BAr^F₄], 3-F,F, was synthesized via addition of 1-F,F to [Rh(NBD)₂][BAr^F₄] in a 1,2-F₂C₆H₄ solution using a similar synthetic procedure to that previously used for the synthesis of [Rh(o-H,ⁱPr-DPEphos)(NBD)][BAr^F₄] (Scheme 3).¹⁷ An analytically pure orange microcrystalline powder was obtained in a good yield (77 %). A small number of crystals suitable for analysis by x-ray diffraction were obtained from slow diffusion of pentane into a THF solution of 3-F,F. The resulting solid-state structure of 3-F,F, Figure 3, shows the diphosphine bound in a cis-κ²-P,P arrangement, with no oxygen coordination [Rh⋅⋅⋅O 3.480(2) Å]. With the η²,η² bound NBD ligand there is an overall pseudo-square planar geometry for the cation. Rh−P bond distances of 2.3180(6) Å and 2.3757(6) Å are typical of such Rh(I) complexes.¹⁷ The ether-oxygen atom sits just off the square plane, in an envelope-like⁴⁶ conformation, as previously observed in [Rh(o-H,H-DPEphos)(NBD)][BAr^F₄] (3-H,H).¹⁷ While this gives the complex overall crystallographic C₁ symmetry, in solution a low energy ring inversion of the diphenyl ether backbone would lead to the observed time-averaged C₂ symmetry. Relatively close F⋅⋅⋅F distances are present [F2⋅⋅⋅F3=2.820(0) Å, F6⋅⋅⋅F7=2.859(2) Å]. In the room temperature ³¹P NMR spectrum of 3-F,F, a single doublet was observed at δ −17.3 [J(RhP)=162 Hz] with no observable coupling to fluorine, unlike for 2-F,F. Very broad signals centered at δ −90.9, −97.7 and −102.5, in the ratio 1 : 2 : 1, were observed in the ¹⁹F NMR spectrum, in addition to the signal due to [BAr^F₄]⁻. In the ¹H NMR spectrum, three NBD environments observed in a 4 : 2 : 2 ratio suggests time averaged C_2V symmetry in solution. On cooling to 183 K in CD₂Cl₂ the integral 4H alkene signal resolves into two separate, integral 2H, signals. The ³¹P NMR spectrum is essentially unchanged on cooling apart from the doublet being broadened (fwhm=275 Hz) that may suggest unresolved coupling to ¹⁹F. In the ¹⁹F NMR spectrum at 183 K, four ¹⁹F signals were observed, in a 2 : 2 : 2 : 2 ratio at δ −90.8, −97.5, −98.1 and −103.5. Only one of these (δ −98.1) is resolved into a doublet [J(PF)=38 Hz] which is consistent with the broadening of the ³¹P NMR signal at this temperature. Similar behavior with regard to selective J(PF) coupling has been reported in trans-PtCl₂(PEt₃)(P(2,6-F₂C₆H₃)₃, in which only one pair of ¹⁹F nuclei couple to ³¹P, J(PF)=30 Hz.¹² No ¹⁰³Rh-¹⁹F coupling is observed in 3-F,F. Warming to 318 K, results in a single very broad signal being observed in the ¹⁹F{¹H} NMR spectrum, centered at δ −98.1. The aromatic signals in the ¹H NMR spectrum are overlapping, even at 183 K, and therefore not helpful for structural elucidation. Combined, these data suggest a low energy ring flipping process combined with restricted rotation of the fluorinated aryl groups^{12, 13} are occurring, giving the complex overall C₂ symmetry at low temperature, with four distinct ¹⁹F environments. We suggest this backbone flipping attenuates through space F⋅⋅⋅F couplings, despite the reasonably close contacts being observed in the solid-state. This is in contrast to 2-F,F where F⋅⋅⋅F coupling is observed but inversion of the diphenyl ether unit does not occur. Ring-flipping processes have been reported previously in POP-type ligands,^{47, 48} and most relevantly, a similar dynamic process was reported in 3-H,H.¹⁷ Interestingly, in the solid-state structure of 3-F,F, there is a fluorine atom located in the apical position of the pseudo square planar Rh(I) center, with relatively close Rh⋅⋅⋅F distance: F1⋅⋅⋅Rh1=2.876(2), Figure 4. Similar close metal-fluorine contacts [M⋅⋅⋅F=3.0997(8), 3.074(1) Å] were reported by Togni et al. for [MCl(COD)(diphenyl(5,6,7,8-tetrafluoronapthalen-1yl)] (M=Rh or Ir, COD=cyclooctadiene), for which ¹⁹F-³¹P coupling constants of 62 and 75 Hz (Ir and Rh respectively) were also measured. For these complexes it is also interesting to note that the fluorine atom in close contact with the metal is shifted ∼20 ppm downfield compared to the other signals in the ¹⁹F NMR spectrum.⁴⁹ In solution for 3-F,F these interactions are likely to be time averaged, between F1⋅⋅⋅Rh1 and F6⋅⋅⋅Rh1, given the C₂ symmetry and restricted rotation observed. No J(RhF) was observed, although this is likely to be small (<10 Hz).⁴⁹ Geometrically related anagostic^{36, 50} Rh⋅⋅⋅H−C interactions are observed in complex 3-H,H (Figure 4),¹⁷ expressed by downfield chemical shifts in the ¹H NMR spectrum for the C−H groups involved. Such interactions are geometrically enforced, with the spatial orientation of the C−H bonds perpendicular to the Rh-square plane leading to ring-current induced chemical shift changes. While we have been unable to definitively assign a particular ¹⁹F chemical shift to the close Rh⋅⋅⋅F interactions in 3-F,F it is tempting to suggest that the signal at δ −98.1 that shows the coupling to ³¹P [J(PF)=30 Hz] is due to the F1/F6 pair via coupling through the Rh-center. However, this signal is not particularly shifted from free ligand (δ −100.7), whereas there is a signal upfield shifted by ∼10 ppm at δ −90.8. More detailed computational studies are needed to reconcile these chemical shift differences with the observed structure.

Scheme 3

Preparation of [Rh(o-F,F-DPEphos)(NBD)][BAr^F₄] 3-F,F.

Figure 3

Crystallographically determined structure of 3-F,F. Ellipsoids shown at the 50 % probability level. Hydrogen atoms and [BAr^F₄]⁻ anion omitted and substituted aryl groups in stick form for clarity. Selected bond lengths (Å) and angles (°): Rh1-P1, 2.3180(6); Rh1-P2, (2.3757(6); Rh1-O1, 3.480(2); Rh1-F1, 2.876(2); Rh1-F6, 3.147(2); P1-Rh1-P2, 101.48(2); C18-O1-C19, 116.8(2).

Figure 4

Truncated solid-state structure of 3-F,F and diagramed structure of the cationic portion of 3-H,H¹⁷ highlighting the close Rh⋅⋅⋅F and Rh⋅⋅⋅H contacts respectively. Ellipsoids at the 50 % probability level. [BAr^F₄]⁻, hydrogen atoms and aryl groups not involved in Rh⋅⋅⋅F contacts are removed for clarity.

Hydrogenation products of 3-H,H and 3-F,F

Schrock-Osborn [Rh(chelating-phosphine)(NBD)]⁺ systems are popular precatalysts for a variety of transformations, for example in olefin hydrogenation and hydroacylation reactions.^{1, 35} They are typically activated by hydrogenation of the diene moiety to form Rh(I) solvated^{25, 51} or Rh(III) dihydride containing complexes.^{52, 53} Hydrogenation of [Rh(o-H,H-DPEphos)(NBD)][BAr^F₄] in the presence of coordinating solvents, such as o-xylene, acetone and C₆H₅F have previously been shown to form Rh(I) solvated complexes: [Rh(o-H,H-DPEphos)(η⁶-o-xylene)][BAr^F₄],²⁸ [Rh(o-H,H-DPEphos)(acetone)₂][BAr^F₄],^{24, 25} and [Rh(o-H,H-DPEphos)(η⁶-C₆H₅F)][BAr^F₄]²⁶ respectively, the last two characterised in-situ. With the bulkier, ortho-methyl substituted DPEphos ligand a dihydride Rh(III) complex forms, [Rh(o-Me-DPEphos)H₂(acetone)][BAr^F₄], whereas with ortho-ⁱPr a C−H activated product results.¹⁷ Here, we contrast the activation products of 3-H,H and 3-F,F after treatment with H₂ in the less coordinating^{54, 55} solvent, 1,2-F₂C₆H₄ (Scheme 4). An atmosphere of 1 bar H₂ was applied to a 1,2-F₂C₆H₄ solution of 3-H,H and a color change from orange to dark red was observed after several minutes.

Scheme 4

Formation of 4-H,H and 4-F,F from the hydrogenation of 3-H,H and 3-F,F in 1,2-F₂C₆H₄.

After 20 minutes, in-situ ³¹P{¹H} NMR spectroscopy indicated that all of the 3-H,H had reacted, and two new ³¹P signals were observed (δ 42.9 and δ 36.2) alongside free norbornane in the ¹H NMR spectrum. The resulting organometallic product, 4-H,H, was isolated as a purple solid and characterized using NMR spectroscopy and ESI-MS. Unfortunately, crystals suitable for x-ray diffraction could not be obtained, despite repeated attempts. In the ³¹P NMR spectrum of this isolated product, two broad signals at δ 43.3 and δ 36.5 are observed as a doublet [J(RhP)=174 Hz] and a doublet of doublets [J(RhP)=154 Hz, J(PH)=74 Hz] respectively at a very similar chemical shift to the in situ prepared complex. The large measured J(PH) coupling suggests a trans-PH arrangement.⁵⁶ Upon ¹H-decoupling, the signal at δ 43.3 sharpens to a doublet of doublets [J(RhP)=174 Hz, J(PP)=26 Hz]. The signal at δ 36.5 resolves into a complex second order multiplet on ¹H decoupling.⁵⁷ These signals were shown to couple to one another via ³¹P-³¹P-COSY NMR experiments. In the ¹H NMR spectrum of 4-H,H, two hydride signals at δ −11.13 and δ −15.59 are observed as complex multiplets (Figure 5), integrating in the ratio 1 : 1 : 2 relative to the [BAr^F₄]⁻ anion. These data are suggestive of a dimer with bridging hydrides, i. e. [Rh₂(DPEphos)₂(μ-H)₂][BAr^F₄]₂. Spin simulation of a AA'MM'NN'XX’ coupling systems (Figure 5) for the hydride resonances revealed the hydride signal at δ −11.13 is coupling to two of each of the following: Rh [J(RhH)=20 Hz], trans-P [J(PH)=74 Hz], cis-P [J(PH)=17 Hz] and very small cis-hydride/hydride coupling [J(HH) ≈2 Hz]. The signal at δ −15.59 also couples to two Rh, and two sets of two ³¹P nuclei [J(RhH)=34 Hz], cis-P [J(PH)=5 Hz], cis-P [J(PH)=9 Hz] and a corresponding small hydride/hydride coupling [J(HH) ≈2 Hz]. In decoupling ³¹P both hydride signals collapse to triplets [J(RhH)=20 Hz and 34 Hz respectively]. These data suggest both hydrides are bridging two Rh atoms, with one trans to a ³¹P atom.

Figure 5

The experimental and spin-simulated AA'MM'NN'XX’ coupling pattern in the hydride signal of 4-H,H (600 MHz, 298 K, CD₂Cl₂).

An upfield shifted aromatic signal is also observed at δ 5.15 of relative integral 2H. This signal has doublet of doublet multiplicity [J(HH)=8 Hz and J(PH)=4 Hz], which collapses to a doublet [J(HH)=8 Hz] upon ³¹P-decoupling, and is shifted 2.1 ppm upfield compared to the ortho-phenyl signal in free o-H,H-DPEphos in CD₂Cl₂.¹⁷ A similar upfield shifted signal (δ 3.94, integral 2H) was reported in the crystallographically characterized dimer [Rh₂(σ,μ-CH-κ²-P,P-o-H,H-DPEphos)₂(σ,μ-H₂B)₂NHMe)][Al(OC(CF₃)₃)₄],²⁷ in which an ortho-phenyl C−H bond of DPEphos coordinates via an agostic bond to the neighboring Rh-center.²⁷ When 4-H,H was analyzed by anerobic ESI-MS⁵⁸ only [Rh(DPEphos)]⁺ (m/z=641.2, calc. 641.1) and [Rh(o-H,H-DPEphos)]₂²⁺ (641.2 m/z, isotopologues increasing in m/z=0.5) were observed due to facile H₂ loss and dimer fragmentation. In contrast, when D₂ was used to activate 3-H,H, forming [Rh(o-H,H-DPEphos)(μ-D)]₂[BAr^F₄]₂ d₂-4-H,H the deuteride [Rh(o-H,H-DPEphos)(μ-D)]₂²⁺ was also observed in the ESI-MS spectrum (m/z=643.2, calc. 643.1). This is likely a consequence of D₂ being lost less easily than H₂ under ESI-MS conditions.^{59, 60} Combined, these NMR and mass spectrometric data allow a dicationic dimeric structure to be proposed for 4-H,H, which has two different bridging hydrides and a bridging σ-CH agostic interaction: [Rh(H)(fac-κ³-P,O,P-μ-CH-DPEphos)]₂[BAr^F₄]₂ (Scheme 4). Without a solid-state structure we cannot determine if the ether-linkage in the DPEphos backbone is coordinated or not. Both motifs are known for cis-PP-DPEphos complexes.¹⁹ We favor a κ³-P,O,P coordination mode with a Rh−Rh bond on the basis of a 34 cluster valence electron count for the dimer.⁶¹ Related dimeric complexes have previously been reported in [Rh₂(o-H,H-DPEphos)₂(μ-H)(μ-(H₂B=NHMe)][BAr^F₄],²⁷ Rh₂(H)(μ-H)₃(o-H,H-DPEphos)₂,^{26, 27} and [Ir(κ³-P,O,P-Xantphos)(H)(μ-H)]₂[BAr^F₄]₂ (Xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene).⁶²

When the same H₂ activation process was employed with 3-F,F a monomeric complex is formed. Addition of H₂ to 3-F,F in 1,2-F₂C₆H₄ solvent resulted in a color change from orange to yellow, rather than the dark red of 4-H,H. The resulting complex was characterized by in-situ NMR spectroscopy. In contrast to 4-H,H a single ³¹P environment was observed in the ³¹P{¹H} spectrum at δ −6.7 [J(RhP)=130 Hz], with a reduced magnitude J(RhP) than measured in 3-H,H [J(RhP)=162 Hz] suggestive of a Rh(III) complex with a trans-P,P arrangement.¹⁹ A single hydride resonance, of relative integral 2H, was observed at δ −20.00 as a doublet of triplets [J(RhH)=34 Hz, J(PH)=17] that collapses to a doublet upon ³¹P-decoupling. A single environment is observed in the ¹⁹F NMR spectrum, very close to free ligand, δ −100.9, with no coupling to ³¹P seen. These data allow us to assign this complex as five-coordinate [Rh^III(mer-κ³-P,O,P-o-F,F-DPEphos)H₂][BAr^F₄], 4-F,F (Scheme 4). Degassing the sample of 4-F,F resulted in decomposition to multiple products, likely due to loss of H₂, and therefore 4-F,F could not be isolated. Consistent with facile H₂ loss, ESI-MS analysis showed a single species at m/z=785.0, corresponding to [Rh(o-F,F-DPEphos)]⁺ (calc. m/z=785.0). Closely related complexes to 4-F,F have been reported [Rh(mer-κ³-P,O,P-Xantphos)H₂][BAr^F₄]^{53, 63} and [Rh(o-Me-DPEphos)H₂(acetone)][BAr^F₄].^{17, 64} We currently can only speculate on the mechanism of formation of dimeric 4-H,H, and thus why ortho-F substitution provides a different, monomeric product for 4-F,F. However that 4-H,H is suggested to have a bridging Rh⋅⋅⋅H−C agostic interaction, and that equivalent M⋅⋅⋅F−C interactions are less common,⁴⁹ dimer formation through partial H₂ loss may be stabilized with the parent DPEphos ligand whereas no such stabilization is possible for 4-F,F and decomposition occurs.

Conclusion

We have highlighted the impact of ortho-fluorine substitution in organometallic chemistry through the control of aurophilic interactions in Au₂Cl₂(o-R,R-DPEphos) complexes (R=H or F) and the different hydrogenation products of common Schrock-Osborn precatalysts. In the process, we have characterized an unexpected dimeric complex with the parent o-H,H-DPEphos ligand which has a bridging C−H⋅⋅⋅Rh agostic interaction.

Experimental Section

All experiments were performed under an atmosphere of argon, using standard Schlenk techniques on a dual vacuum/inlet manifold unless specified otherwise. Glassware was dried in an oven at 140 °C overnight or flame dried under vacuum prior to use. Pentane, hexane, THF, diethyl ether and CH₂Cl₂ were dried using an Mbraun SPS-800 solvent purification system and degassed by three freeze-pump-thaw cycles. 1,2-F₂C₆H₄ was stirred over Al₂O₃ for two hours and then CaH₂ overnight before vacuum transfer and subsequent degassing by three freeze-pump-thaw cycles. Dichloromethane-D₂ (CD₂Cl₂) was dried overnight with CaH₂ before vacuum transfer and subsequent degassing by three freeze-pump-thaw cycles and storage over 3 Å molecular sieves. [Rh(NBD)₂][BAr^F₄] was prepared via the literature procedure.⁶⁵ All other reagents, were purchased from commercial vendors and used as received. NMR data was collected on either a Bruker 500 MHz AVC or Bruker AVIII 600 MHz widebore spectrometer. Residual protio solvent resonances were used as a reference for ¹H NMR spectra.³¹P and ¹¹B NMR spectra were referenced externally to 85 % H₃PO₄ and F₃B⋅OEt₂ respectively. All chemical shifts (δ) are quoted in ppm and coupling constants in Hz. Aerobic electrospray ionization mass spectrometry (ESI-MS) was carried out using a Bruker compact® time of flight mass spectrometer by Mr. Karl Heaton at the University of York for compounds 1-F,F, 2-F,F and 3-F,F. Air sensitive mass spectrometry, using a bespoke N₂ filled glovebox connected to a Bruker ESI-ion trap spectrometer⁵⁸ was used for analysis of 4-H,H and 4-F,F. Elemental analyses were conducted by Dr. Graeme McAllister at the University of York.

o-F,F-DPEphos 1-F,F: Synthesis of 2,2’-(PCl₂)₂Ph₂O (Scheme 1) was done in accordance with literature procedures.^{16, 30} In a separate J. Young's ampoule, N,N,N’,N’-tetramethylenediamine (0.42 ml, 2.8 mmol, 4.3 equiv.) was added dropwise to ⁿBuLi (1.07 ml, 2.4 M solution in hexanes, 2.6 mmol, 4 equiv.) at 0 °C. The mixture was cooled to −78 °C and THF (2 ml) followed by 2,6-difluorobromobenzene (0.29 ml, 2.6 mmol, 4 equivalents) were added dropwise and the mixture was left to stir for 1 hour at −78 °C. The previously prepared 2,2’-(PCl₂)₂Ph₂O (200 mg, 0.54 mmol, 1 equiv.) in 4 ml of THF was cooled to −78 °C and the aryl lithium was added dropwise, keeping both solutions at −78 °C. The resulting orange mixture was allowed to warm to room temperature, then stirred for 30 minutes and checked by ¹⁹F and ³¹P NMR spectroscopy for completion. The mixture was then cooled to 0 °C, quenched with methanol (1 ml) and saturated ammonium chloride solution (10 ml). The now air stable product was extracted with ethyl acetate (3×10 ml), dried over Na₂SO₄, filtered, and washed with cold pentane yielding the product as a white solid. Recrystallisation with pentane and CH₂Cl₂ yielded 1-F,F as a white microcrystalline solid (109 mg, 30 %): ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 295 K): δ −62.5 (quint, J_FP = 40 Hz). ¹⁹F NMR (471 MHz, CD₂Cl₂, 295 K): δ −100.7 (d, J_PF=40 Hz). ¹H NMR (500 MHz, CD₂Cl₂, 295 K): δ 7.33 (tt, J=8 Hz and 6 Hz, 4H, Ar), 7.26 (m, J=8 and 2 Hz, 2H, Ar), 7.06 (dd, J=7 and J_PH=4 Hz, 2H, Ar), 7.00 (dd, J=8 and 7 Hz, 2H, Ar), 6.82 (dt, J=8 and 2 Hz, 8H, meta-H on substituted phenyl), 6.74 (dd, J_HH=8 and J_PH=5 Hz, 2H, ortho-H on backbone). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂, 295 K): δ 165.6–163.6 (dt, J_CF=250 and J_CP=9 Hz, Ar), 158.9 (s, Ar), 158.5 (s, Ar), 132.2 (s, Ar), 132.0 (t, J=11 Hz), 130.4 (d, J=1 Hz Ar), 123.5 (s, Ar), 117.4 (s, Ar), 111.6 (d, J=5 Hz, Ar), 111.4 (dd, J=5 Hz, J=1 Hz, Ar). ESI-MS (CH₂Cl₂): m/z [M+H]⁺ 683.0942 (calc. 683.0934) with the correct isotope pattern. Elemental analysis found (calc. for C₃₆H₂₀F₈OP₂): C 63.38 (63.35) H 3.19 (2.95).

Au₂Cl₂(o-F,F-DPEphos) 2-F,F: A solution of 1-F,F (1.05 equiv.) in CH₂Cl₂ (2 ml) was added dropwise to a CH₂Cl₂ (2 ml) solution of Au(THT)Cl (2 equiv.),⁶⁶ and left to stir for one hour, leaving a colorless solution. The solvent was mostly removed in vacuo before pentane (10 ml) was added, precipitating out a white solid. The precipitate was the filtered and washed with further pentane (4×5 ml) and dried under Schlenk line vacuum overnight (<1×10⁻¹ mbar) leaving a white powder (both white powders) that was transferred into an argon glovebox for storage. Colorless crystals suitable for x-ray diffraction were obtained by slow diffusion of pentane into a CH₂Cl₂ solution of 2-F,F (26 mg, 52 %): ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 295 K): δ −23.1 (app. quint., J_FP=25 Hz). ¹⁹F NMR (471 MHz, CD₂Cl₂, 295 K): δ −97.3 (dt, J_PF=25 Hz, J_FF=7 Hz), −97.9 (dt, J_PF=25 Hz, J_FF=7 Hz). ¹H NMR (500 MHz, CD₂Cl₂, 295 K): δ 7.60 (m, 2H, Ar), 7.56 (dd, J=7 Hz, 2H, Ar), 7.44 (m, 2H, Ar), 7.27 (dd, J=7 Hz, 2H, Ar), 7.22 (m, 2H, Ar), 7.06-6.99 (m, 6H {2+4 coincidence}, Ar), 6.82 (ddd, J=9 and 4 Hz, 4H, Ar). ESI-MS (CH₂Cl₂): m/z [M−Cl]⁺ 1110.9852 (calc. 1110.9881) with correct the isotope pattern. Multiple samples were submitted for elemental analysis, but no results were within 0.4 % of the theoretical percentage mass by weight for carbon or hydrogen. Persistent pentane may be the cause of the inconsistent elemental analysis (see ¹H NMR spectrum in the ESI).

[Rh(o-F,F-DPEphos)(NBD)][BAr^F₄] 3-F,F: 1,2-F₂C₆H₄ (10 ml) was added to [Rh(NBD)₂][BAr^F₄] (63 mg, 0.055 mmol, 1 equiv.)^{65, 67} and 1-F,F (38 mg, 0.055 mmol, 1 equiv.) in a J. Young's Ampoule to form an orange solution. The mixture was stirred for two hours at room temperature. The solvent was removed in vacuo to leave a purple oil which was triturated with pentane to give a red solid. The solid was filtered and washed with further pentane (3×5 ml) and dried under Schlenk line vacuum (<1×10⁻¹ mbar) overnight, leaving 3-F,F as an orange microcrystalline powder (73 mg, 77 %,): ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 295 K): δ −17.4 (d, J_RhP=163 Hz). ¹H NMR (500 MHz, CD₂Cl₂, 295 K): δ 7.72 (s, 8H, o-CH, BAr^F₄), 7.55 (s, 4H, p-CH, BAr^F₄), 7.51 (br m, 4H, Ar), 7.40 (m, J=9 Hz and 2 Hz, 4H, Ar), 7.13–6.83 (complex multiplet, 12H, Ar), 4.16 (s, 4H, sp²-CH NBD), 3.80 (s, 2H, sp³-CH NBD), 1.44 (s, 2H, CH₂ NBD). ¹H NMR (500 MHz, CD₂Cl₂, 318 K) selected data: δ 7.73 (s, 8H, o-CH, BAr^F₄), 7.55 (s, 4H, p-CH, BAr^F₄), 4.18 (s, 4H, sp²-CH NBD), 3.81 (s, 2H, sp³-CH NBD), 1.45 (s, 2H, CH₂ NBD). ¹H NMR (500 MHz, CD₂Cl₂, 183 K): δ 7.72 (s, 8H, o-CH, BAr^F₄), 7.57 (m, 4H, Ar), 7.51 (s, 4H, p-CH, BAr^F₄), 7.37 (m, 4H, Ar), 7.11 (t, J=9 Hz, 2H, Ar), 7.07–6.93 (br m, 8H, Ar), 6.49 (t, J=9 Hz, 2H, Ar), 4.23 (s, 2H, sp²-CH NBD), 3.87 (s, 2H, sp²-CH NBD), 3.74 (s, 2H, sp³-CH NBD), 1.35 (s, 2H, CH₂ NBD). ¹⁹F NMR (471 MHz, CD₂Cl₂, 295 K): δ −62.9 (s, 24F, BAr^F₄), −88.9 to −92.5 (br s, 2F), −95.2 to −100.2 (br s, 4F), −100.6 to 104.5 (br s, 2F). ¹⁹F NMR (471 MHz, CD₂Cl₂, 183 K) selected data: δ −90.8 (s, 2F), −97.9 (s, 2F), −98.1 (br d, J=38 Hz, 2F), −103.5 (s, 2F). ¹⁹F NMR (471 MHz, CD₂Cl₂, 318 K) selected data: δ −98.1 (br s, 8F). ESI-MS (1,2-F₂C₆H₄): m/z [M]⁺ 877.0557 (calc. 877.0543) with correct isotope pattern. Elemental analysis found (calc. for C₇₅H₄₀BF₃₂OP₂Rh): C 51.74 (51.75) H 2.40 (2.32).

[Rh(o-H,H-DPEphos)(μ-H)₂]₂[BAr^F₄] 4-H,H: A sample of [Rh(o-H,H-DPEphos)(NBD)][BAr^F₄] 2-H,H (40 mg, 0.025 mmol) was dissolved in 1–2-F₂C₆H₄ (0.5 ml) in a high pressure J. Youngs NMR tube. The atmosphere was removed by three successive freeze-pump-thaws and the NMR tube was placed under an atmosphere of 15 PSI H₂ and successively rotated for ten minutes which formed a dark red solution. The 1–2-F₂C₆H₄ solution was transferred to a Youngs Flask equipped with a stirrer bar. Pentane (5 ml) was added to the solution with vigorous stirring and a purple solid precipitated. The solvent was removed by filter cannula and the purple solid was washed with pentane twice more (5 ml) before drying under Schlenk line vacuum (<1×10⁻¹ mbar) for two hours (30 mg, 0.010 mmol, 80 %): ³¹P{¹H} NMR (243 MHz, CD₂Cl₂, 298 K): δ 43.3 (dd, J_RhP=174 Hz and J_PP=26 Hz), 36.5 (complex 2^nd order multiplet). ³¹P NMR (243 MHz, CD₂Cl₂, 298 K): δ 43.3 (br d, J_RhP=174 Hz), 36.5 (br dd, J_RhP=154 Hz, J_PH=74 Hz). ¹H NMR (600 MHz, CD₂Cl₂, 298 K) δ 7.23 (s, 16H, ortho-H BAr^F₄), 7.60–7.50 (m, 6H, Ar), 7.55 (s, 8H, para-H BAr^F₄), 7.47 (m, 3H, Ar) 7.48 7.44–7.37 (m, 6H, Ar), 7.34–7.20 (m, 14H, Ar), 7.14 (dd, J=6 Hz, 3H, Ar), 7.12–7.04 (m, 5H, Ar), 6.97 (dd, J=8 and 12 Hz, 4H, Ar), 6.84–6.78 (m, 6H, Ar), 6.68 (dd, J=7 Hz, 2H, Ar), 5.15 (dd, J_PH=4 and J_HH=8 Hz, 2H, CH σ-agostic), −11.13 (complex multiplet, J_PH(trans)=74 Hz, J_PH(cis)=17 Hz, J_RhH=20 Hz, 1H, bridging hydride trans to ³¹P) and −15.59 (complex multiplet, J_RhH=34 Hz, J_PH(cis)=5 Hz, J_PH(cis)=9 Hz, 1H, bridging hydride trans to CH σ-agostic). ¹H NMR (600 MHz, CD₂Cl₂, 298 K) selected data: δ −11.13 (triplet, J_RhH=20 Hz, 1H, bridging hydride trans to ³¹P), −15.59 (triplet, J_RhH=34 Hz, 1H, bridging hydride trans to CH σ-agostic).

In-situ characterization of [Rh(o-F,F-DPEphos)(H)₂][BAr^F₄] 4-F,F: A sample of [Rh(o-F,F-DPEphos)(NBD)][BAr^F₄] 3-F,F (40 mg, 0.023 mmol) was dissolved in 1–2-F₂C₆H₄ (0.5 ml) in a high pressure J. Youngs NMR tube. The atmosphere was removed by three successive freeze-pump-thaws and the NMR tube was placed under an atmosphere of 15 PSI H₂ and successively rotated for ten minutes which formed a yellow solution. 4-F,F was characterised in-situ by multinuclear NMR spectroscopy: ³¹P{¹H} NMR (243 MHz, 1,2-F₂C₆H₄, 298 K): δ −6.7 (d, J_RhP=130 Hz). ¹⁹F NMR (565 MHz, 1,2-F₂C₆H₄, 298 K): −100.9 (s, 2,6-F₂C₆H₃). ¹H NMR (600 MHz, 1,2-F₂C₆H₄, 298 K) selected data: δ −20.0 (dt, J_PH=17 Hz, J_RhH=34 Hz, 2H, Rh−H). ¹H NMR (600 MHz, 1,2-F₂C₆H₄, 298 K) selected data: δ −20.00 (d, J_RhH=34 Hz). Removal of the hydrogen atmosphere resulted in decomposition of 4-F,F.

Deposition Numbers 2160287 (for 2-F,F) and 2160288 (for 3-F,F) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Center and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Acknowledgements

The EPSRC for funding (ASW: EP/M024210, JJR and TMB PhD studentships through the Doctoral Training Partnership).

Conflict of interest

The authors declare no conflict of interest.

Open Research

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.