A CH₂Cl₂ complex of a [Rh(pincer)]⁺ cation

Abstract

The CH₂Cl₂ complex [Rh(^tBuPONOP)(κ¹-ClCH₂Cl)][BAr^F₄] is reported, that also acts as a useful synthon for other complexes such as N₂, CO and H₂ adducts; while the analogous PNP complex undergoes C–Cl activation.

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Coordinatively and electronically unsaturated transition-metal pincer complexes, [M(pincer)], are key intermediates in alkane dehydrogenation processes,¹ as well as other catalytic transformations.² They have also played a major role in the elucidation of fundamental bond transformations, such as C–H, C–C and C–X breaking and making.³ Recently, Brookhart and co-workers reported the synthesis of transition-metal methane and ethane sigma complexes, by a low temperature (ca. −110 °C to −150 °C) protonation of the corresponding Rh(^tBuPONOP)R precursors using [H(OEt₂)₂][BAr^F₄] in CDF₂Cl–CH₂Cl₂ solvent to give [Rh(^tBuPONOP)(H-R)][BAr^F₄] [^tBuPONOP = 2,6-(^tBu₂PO)₂C₅H₃N; R = Me, Et; Ar^F = 3,5-(CF₃)₂C₆H₃], Scheme 1.⁴ Such complexes are key, but transient, intermediates in C–H bond activation processes. On warming above −87 °C (R = Me) or −130 °C (R = Et) they lose alkane and generate complexes tentatively characterised in situ on the basis of ³¹P NMR spectroscopy as [Rh(^tBuPONOP)(solv)][BAr^F₄] (solv = CDF₂Cl or CD₂Cl₂). These solvent adducts remain to be definitively characterised. They are particularly interesting given their role in alkane coordination chemistry, and more generally as latent-low coordinate intermediates in catalytic processes.

Scheme 1 Formation of a sigma alkane complex and decomposition to give tentatively characterised solvent complexes (Brookhart and co-workers). [BAr^F₄]⁻ anions are not shown.⁴

We now report the full characterisation of the CH₂Cl₂ adduct accessed via a different, halide abstraction, route including a single crystal X-ray diffraction study and its onward reactivity. We also demonstrate that changing the pincer ligand to the more electron donating ^tBuPNP [2,6-(^tBu₂PCH₂)₂C₅H₃N] results in C–Cl bond activation of the solvent molecule.

Addition of Na[BAr^F₄] to a CH₂Cl₂ solution of Rh(^tBuPONOP)Cl, 1,^4a results in the formation of orange [Rh(^tBuPONOP)(κ¹-ClCH₂Cl)][BAr^F₄], 2 (Scheme 2). Filtration and removal of the solvent affords 2 in good isolated yield as a powder. Complex 2 can be recrystallised from CH₂Cl₂–pentane under an Ar atmosphere to give crystals suitable for an X-ray diffraction study. Under these conditions, orange 2 crystallises alongside the dinitrogen adduct, [Rh(^tBuPONOP)(κ¹-N₂)][BAr^F₄], 3, in an approximate 1 : 1 ratio (as measured by ³¹P NMR spectroscopy, vide infra). Single crystals of 2 suitable for an X-ray diffraction study were obtained by mechanical separation from orange/brown 3.‡ Presumably the exogenous N₂ comes from trace (1–2 ppm) levels of N₂ present in the argon, as has been noted previously,⁵ and is driven by relative solubilities of 2 and 3; as in neat CD₂Cl₂ under the same Ar atmosphere 2 does not go onto to form 3 to the detection limit of ³¹P{¹H} NMR spectroscopy. The solid-state structure (Fig. 1A) shows a pseudo square planar cationic [Rh(^tBuPONOP)]⁺ centre coordinated in the fourth position by a CH₂Cl₂ molecule. The Rh–Cl1 distance [2.350(2) Å] is significantly shorter than reported for related [RhCp*(PMe₃)(Ph)(κ¹-ClCH₂Cl)][BAr^F₄],⁶ 2.512(2) Å, and [RhCp*(PMe₃)(Me)(κ¹-ClCH₂Cl)][BAr^F₄], 2.488(1) Å Cp* = η⁵−C₅Me₅).⁷ Complex 2 adds to the relatively small number of CH₂Cl₂ complexes that have been crystallographically characterised, and in particular CH₂Cl₂ adducts of pincer, or closely related, complexes.⁸

Scheme 2 Synthesis of complex 2. [BAr^F₄]⁻ anion is not shown.

Fig. 1 Solid-state structures of: (A) Complex 2; (B) Complex 3; (C) Complex 5. Displacement ellipsoids are shown at the 50% probability level, hydrogen atoms and the [BAr^F₄]⁻ anions are not shown. Selected bond lengths (Å) and angles (°): (2) Rh1–Cl1, 2.350(2); Rh1–N1, 2.011(4); Rh1–P1, 2.272(1); Rh1–P2, 2.285(1); Cl1–C22, 1.710(8); Cl2–C22, 1.758(7); Cl1–C22–Cl2, 114.3(4); N1–Rh1–Cl1, 169.65(11). (3) Rh1–N1, 2.018(3); Rh1–N2, 1.967(3); Rh1–P1, 2.2745(8); Rh1–P2, 2.2724(8); N2–N3, 1.063(5); Rh1–N2–N3, 179.3(4); N1–Rh1–N2, 179.37(13). (5) Rh1–Cl1, 2.311(2); Rh1–N1, 2.066(6); Rh1–P1, 2.335(2); Rh1–P2, 2.339(2); Rh1–C8, 2.196(15); C8–Cl2, 1.79(2); Rh1–C8–Cl2, 112.5(9). Complex 5 co-crystallises with [Rh(^tBuPNP)(H)Cl][BAr^F₄], 6, at the same lattice position in a 50 : 50 ratio.‡

Although the short Rh–Cl distance might suggest a stronger interaction in 2, in solution (vide infra) rapid exchange between solvent and bound CH₂Cl₂ occurs. The two C–Cl distances in the bound solvent molecule are similar, 1.710(8) [C22–Cl1] and 1.758(7) [C22–Cl2] Å, although the distal C–Cl bond is the slightly longer of the two. This is in contrast to other reported CH₂Cl₂ complexes in which the bound C–Cl bond is longer.^8,9 We suggest that the slight lengthening of C22–Cl2 may be due to a number of weak C–H⋯Cl hydrogen bonds between proximal ^tBu groups and Cl2.¹⁰

Complex 2 is stable in the solid-state under an Ar atmosphere, and in solution (CD₂Cl₂) for at least 1 week. In the ³¹P{¹H} NMR spectrum (CD₂Cl₂) a single resonance is observed at δ 204.5 [J(RhP) 136 Hz]. These data are identical to those previously reported by Brookhart and co-workers for the complex tentatively characterised as [Rh(^tBuPONOP)(CH₂Cl₂)][BAr^F₄], i.e.2. The ^tBu groups are observed as a single environment in the ¹H NMR spectrum. The bound CH₂Cl₂ ligand is not observed, even at −80 °C in the ¹³C{¹H} NMR spectrum, presumably as it is undergoing fast exchange with the solvent.¹¹ The electrospray ionisation mass spectrum of 2 using N₂ as a desorption gas showed only 3 as the molecular ion.

Complex 2 is a useful synthon for the preparation of other pincer complexes (Scheme 3). Addition of H₂ to a CD₂Cl₂ solution of 2 forms the previously reported dihydrogen complex [Rh(^tBuPONOP)(η²-H₂)][BAr^F₄]¹² [δ(¹H) −8.27, lit. −8.26]. Addition of N₂ forms the new complex [Rh(^tBuPONOP)(κ¹-N₂)][BAr^F₄], 3, for which a solid-state structure is shown in Fig. 1B. This demonstrates an end-on bound, monomeric, N₂ adduct [N–N, 1.063(5); Rh–N2, 1.967(3) Å]. The ³¹P{¹H} NMR spectrum displays a single environment at δ 211.0 [J(RhP) 132 Hz], while in the IR spectrum the N–N stretch is observed at 2201.9 cm⁻¹. The N–N bond length is very similar (albeit a little shorter) than that in free N₂ [1.09 Å], suggesting only a small degree of activation. Complex 3 can also be compared with previously reported [Rh(^tBuPNP)(κ¹-N₂)][OTf] which shows a slightly longer N–N bond, a shorter Rh–N bond and a more red-shifted N–N stretch: 1.116(4), 1.898(3) Å, and 2153 cm⁻¹ respectively; suggesting greater N₂ activation for this more electron rich pincer ligand.¹³ This greater metal-based basicity in the ^tBuPNP complexes is reflected in the CO stretching frequencies of the corresponding CO-adducts: [Rh(^tBuPONOP)(CO)][BAr^F₄], 4 [2020 cm⁻¹] and [Rh(^tBuPNP)(CO)][BAr^F₄] [1982 cm⁻¹].¹⁴ Complex 4 was prepared by adding CO to a CH₂Cl₂ solution of 2, further demonstrating the utility of complex 2 in synthesis.

Scheme 3 Reactivity of complex 2. CH₂Cl₂ solvent. [BAr^F₄]⁻ anions are not shown.

The difference in electron-donating power of the ^tBuPONOP versus^tBuPNP ligands can also been shown by the attempted synthesis of the CH₂Cl₂ adduct of the {Rh(^tBuPNP)}⁺ fragment, analogous to complex 2. Rather than simple coordination, this resulted in a number of products as measured by ³¹P{¹H} NMR spectroscopy. Analysis of single crystals suitable for an X-ray diffraction study, obtained from recrystallisation of the reaction mixture, demonstrated co-crystallisation of two complexes [Rh(^tBuPNP)(CH₂Cl)Cl][BAr^F₄], 5, and [Rh(^tBuPNP)(H)Cl][BAr^F₄], 6, in an approximate 50 : 50 ratio (Scheme 4); for which the solid-state structure of 5 is shown in Fig. 1C. Because of this co-crystallisation the metrical data associated with 5 should be treated with caution. The ¹H NMR spectrum of these crystals showed a broad hydride signal at δ −15.48 (relative integral relative to [BAr^F₄] of ∼0.5 H) which is assigned to 6. Given the number of products formed we are reluctant to speculate on mechanism of formation of 6, but protonation of 5 by trace acid arising from other decomposition pathways could form 6. Addition of H₂ to this mixture of 5 and 6 in CD₂Cl₂ afforded mixture of products, from which [Rh(^tBuPNP)(η²-H₂)][BAr^F₄] could be identified as the major species present.¹⁶

Scheme 4 Reactivity of Rh(^tBuPNP)Cl¹⁵ with Na[BAr^F₄]. CH₂Cl₂ solvent. [BAr^F₄]⁻ anions are not shown.

Conclusions

The CH₂Cl₂ complex [Rh(^tBuPONOP)(κ¹-ClCH₂Cl)][BAr^F₄] has been isolated, confirming its formation in the decomposition of the corresponding alkane adduct at low temperature, itself formed from protonation of an alkyl precursor.⁴ Synthesis has been achieved by an alternative halide-abstraction route in CH₂Cl₂ solvent, starting from a readily available chloride precursor. This complex, with its weakly bound CH₂Cl₂ ligand, also acts as a useful synthon for other complexes such as N₂, CO and H₂ adducts. The corresponding PNP ligand complex undergoes C–Cl activation to form a mixture of products, highlighting the difference in electron donating properties of these two ligands.

Acknowledgements

The EPSRC for funding (EP/K035908/1) and Dr Adrian Chaplin for the initial synthesis of complex 5.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Full experimental, characterisation and X-ray crystallography details. CCDC 1044741, 1044743, 1044744 and 1044745. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt00481k

‡ Crystal data: (2) RhP₂O₂NCl₂C₂₂H₄₁·C₃₂H₁₂BF₂₄, Monoclinic (C2/c), a = 16.9996(5) Å, b = 18.1716(4) Å, c = 39.8254(10) Å, α = γ = 90°, β = 96.458(2)°, volume = 12 224.4(5) ų, Z = 8, λ = 0.71073 Å, T = 150(2) K, μ = 0.53 mm⁻¹, 16 021 independent reflections [R(int) = 0.029], R₁ = 0.0814, wR₂ = 0.1692 [I > 2σ(I)]. CCDC: 1044744; (3): RhP₂O₂N₃C₂₁H₃₉·C₃₂H₁₂BF₂₄, Monoclinic (C2/c), a = 16.8578(4) Å, b = 18.1533(3) Å, c = 39.7792(7) Å, α = γ = 90°, β = 95.9972(17)°, volume = 12 106.8(4) ų, Z = 8, λ = 1.54180 Å, T = 150(2) K, μ = 3.83 mm⁻¹, 12 215 independent reflections [R(int) = 0.031], R₁ = 0.0483, wR₂ = 0.1183 [I > 2σ(I)]. CCDC: 1044745; (5/6) RhP₂NCl₂C₂₄H₄₅·C₃₂H₁₂BF₂₄ : RhP₂NClC₂₃H₄₄·C₃₂H₁₂BF₂₄, Monoclinic (P2₁/c), a = 13.8327(2) Å, b = 23.4907(3) Å, c = 20.1051(2) Å, α = γ = 90°, β = 97.5982(11)°, volume = 6475.59(4) ų, Z = 2, λ = 1.54180 Å, T = 150(2) K, μ = 4.12 mm⁻¹, 12 878 independent reflections [R(int) = 0.029], R₁ = 0.1064, wR₂ = 0.2958 [I > 2σ(I)]. CCDC: 1044741.

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