Gold(I) dimers with hemilabile azophosphine ligands in catalysis

Abstract

The first example of azophosphines acting as a bridging ligand between two metal centres is reported. The hemilability of the azophosphine ligand renders the gold(I) dimer formation reversible, so that the complex is not a catalytic dead-end. The catalytic ability of the gold(I) azophosphine complexes is demonstrated through a series of cyclisation reactions that form C–C and C–O bonds.

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Introduction

Gold(I) catalysis has emerged as a versatile tool in modern synthetic chemistry, offering unique reactivity patterns that complement traditional transition metal catalysts.¹ The soft Lewis acidic nature of gold(I) complexes enables the efficient activation of π-systems such as alkynes, allenes, and alkenes, facilitating a wide array of transformations under mild conditions with high selectivity.² Alongside catalytic applications, Au(I) complexes of the type LAuX have applications in medicine,³ as therapeutic and imaging agents,⁴ and materials science.⁵

Active [L–Au]⁺ cations are commonly generated in situ from L–Au–Cl precursors using AgX salts, however this approach can introduce problems from incomplete abstraction of chloride, light and moisture sensitivity, and the fact that the Ag(I) cation is not catalytically innocent.⁶ As such, avoiding in situ catalyst activation is potentially advantageous and at minimum simplifies the experimental regime.⁷ Although the LAuX motif only requires a monodentate ligand, bidentate ligands have found significant utility in gold chemistry, including bridging ligands for the formation of dimeric (dinuclear) gold complexes,⁸ and in stabilising Au(III) centres for catalysis.⁹ In catalysis, while the use of bidentate ligands that facilitate the irreversible formation of gold dimers from catalytically active [L–Au]⁺ species would be deleterious, if such dimers can dissociate to an active monomeric form in solution then such species can be considered as stable pre-catalysts.

1,3-P,N ligands contain coordinating phosphorus and nitrogen centres linked by a single atom, typically carbon, and have many attractive properties as hybrid ligands, including cooperative effects, hemilability, and substrate activation for catalysis.¹⁰ They present an interesting motif for gold(I) chemistry, and several dimeric complexes of the form [{Au(μP,N-L)}₂] featuring head-to-tail 1,3-P,N ligands have been crystallographically characterised (Scheme 1A),¹¹ as have a range of analogous complexes with P,N ligands featuring longer linkers between the P and N atoms.¹² The 1,3-P,N ligands shown in Scheme 1A all feature the binding nitrogen atom in a heterocycle. A is a prototypical pyridyl phosphine,^11a and only the coordination chemistry was probed. B is an imidazolyl phosphine, and the corresponding dimer displayed solid-state luminescence, and two different emission spectra observed depending on preparation conditions.^11b The tris(imidazolyl) phosphine C forms the dimer with only one of the imidazolyl arms, and the complex showed promising anti-cancer properties,^11c while the complex with benzimidazolyl phosphine ligand D displayed luminescent properties.^11d E and F are phosphines bearing an anionic indole substituent which led to neutral dimers, but no further reactivity for these complexes was explored.^11e

Scheme 1 A) Crystallographically characterised [{Au(μP,N-L)}₂] complexes, where L is a 1,3-P,N ligand A–F. (B) General structure for azophosphine, and previous coordination modes observed in ruthenium complexes. (C) This work.

Azophosphines (ArN = N–PR₂; Scheme 1B) are heavier analogues of triazenes, and have recently emerged as precursors in the formation of phosphorus- and nitrogen-containing heterocycles.¹³ We have also shown that azophosphines are a new class of 1,3-P,N ligand in which the central atom is a nitrogen.¹⁴ A range of monodentate (κ¹P) and bidentate (κ²P,N) ruthenium azophosphine complexes have been prepared (Scheme 1B), and the electronegative central N atom makes the ligand a relatively poor donor so the hemilabile ligands could promote transfer hydrogenation catalysis.^14b To date the coordination chemistry of azophosphines has not been studied with metals other than ruthenium, and the potential utility of azophosphines as bridging ligands between multiple metal centres is unknown.

In this study we show that azophosphines can act as κ¹P ligands towards a gold(I) centre, and for the first time as μP,N ligands between metal centres (Scheme 1C). The reversible nature of the dimer formation is probed, and we demonstrate that both the monomeric and dimeric complexes are catalytically competent in a range of cyclisation reactions that form C–O or C–C bonds.

Results and discussion

Synthesis and characterisation of gold(I) azophosphine complexes

Azophosphine (p-OMe)C₆H₄N₂P(^tBu)₂ (1) was prepared from a diazonium salt and a secondary phosphine borane adduct according to our previously published method.^14b A dichloromethane solution of 1 was added to a stirring dichloromethane solution of (Me₂S)AuCl, resulting in the immediate formation of a red solution (Scheme 2). ³¹P NMR spectroscopy showed quantitative conversion of the free azophosphine 1 at 111.7 ppm to a new species at 124.6 ppm. A similar downfield shift was previously observed in κ¹P coordination of azophosphines to ruthenium centres.¹⁴ Removal of volatiles in vacuo isolated a vibrant red powder in 86% isolated yield. Red blocks of 2 suitable for single-crystal X-ray diffraction (Fig. 1) were grown by slow evaporation of a saturated dichloromethane solution of the product at room temperature, and showed the expected composition Au(κ¹P-1)Cl (2) with an almost linear P–Au–Cl bond angle of 178.03(3)°.¹⁵ Monomer 2 exhibits no intermolecular Au⋯Au interactions, with the closest separation being over 7 Å in the extended crystal structure.

Scheme 2 Synthesis of gold azophosphine complexes 2 and 3 (with isolated yield).

Fig. 1 Single crystal structure of Au(κ¹P-1)Cl (2). Thermal ellipsoids drawn at the 50% probability level. Selected bond distances (Å) and bond angles (°): Au1–P1 2.2354(10), Au1–Cl1 2.2970(9), P1–N1 1.728(3), N1–N2 1.261(4); P1–N1–N2 114.1(3), P1–Au1–Cl1 178.03(3).

Reaction of complex 2 with one equivalent of AgSbF₆ led to formation of an orange solution and an orange precipitate; extraction of the product in excess dichloromethane yielded an orange solution and an off-white solid (AgCl) that could be separated by filtration. ³¹P NMR spectroscopy of this dichloromethane solution revealed clean formation of a new resonance at 108.1 ppm. Removal of volatiles in vacuo afforded an orange solid in 83% isolated yield, and single crystals (orange blocks) could be grown by slow evaporation of a saturated dichloromethane solution. Single crystal X-ray diffraction studies confirmed the head-to-tail bridging nature of azophosphine 1 in the dimeric complex [{Au(μP,N-1)}₂][SbF₆]₂ (3) (Fig. 2).¹⁵ Complex 3 crystallised in the monoclinic space group P2₁/c with the Au–Au vector residing on a crystallographic inversion centre, rendering only half of the complex crystallographically unique.

Fig. 2 Single crystal structure of [{Au(μP,N-1)}₂]²⁺, the cationic component of 3. The two SbF₆− counterions have been removed for clarity. Thermal ellipsoids drawn at the 50% probability level. Selected bond distances (Å) and bond angles (°): Au1–P1′ 2.2513(15), Au1–N2 2.123(5), P1–N1 1.741(5), N1–N2 1.276(7), Au–Au 2.7782(5); P1–N1–N2 121.8(4), P1′–Au1–N2 176.50(14).

Key bond metric data for complex 3 compared to the other crystallographically authenticated [{Au(μP,N-L)}₂] dimers featuring 1,3-P,N ligands A–F are tabulated in Table 1.¹¹ All the previous P,N ligands in Table 1 feature a bridging carbon between the phosphorus and nitrogen centres, and the nitrogen is found within a heterocycle, whereas azophosphine 1 features an electronegative nitrogen bridging atom and the bonding nitrogen is acyclic. The Au–Au distance in 3 is 2.7782(5) Å, which is statistically similar to the shortest Au–Au distance known for gold dimers featuring 1,3-P,N ligands, namely the (2-pyridyl)dimethylphosphine (2.776(1) Å),^11a and is consistent with a significant aurophilic interaction.¹⁶ The hexafluoroantimonate anions in 3 only weakly interact with the Au centres, with the shortest Au–F separation being 3.222 Å, comparable to the shortest Au–O separation of [Au(midp)]₂²⁺ (midp = 1-methylimidazolediphenyl phosphine; entry 2 in Table 1) and its perchlorate anion (3.22(7) Å).^11b The Au–N bond length in 3 of 2.123(5) Å is particularly long, which is consistent with this nitrogen centre being a relatively poor donor due to being adjacent to an electronegative nitrogen atom. This finding prompted us to explore the hemilability of this Au–N bond.

Table 1 Selected bond distances from crystallographically characterised [{Au(μP,N-L)}₂] dimers featuring 1,3-P,N ligands shown in Scheme 1

1,3-P,N ligand	Au–P (Å)	Au–N (Å)	Au–Au (Å)	Ref.
a BF4^− counterions.b ClO4^− counterions.
A	2.215(6)	2.086(16)	2.776(1)	11a
B	2.235(3)^a	2.083(9)^a	2.8174(10)^a	11b
	2.238(3)^b	2.090(11)^b	2.8261(11)^b
C	2.245(4)	2.111(15)	2.8821(15)	11c
D	2.228(3)	2.074(4)	2.808(4)	11d
E	2.2586(15)	2.042(4)	2.8287(11)	11e
F	2.2526(7)	2.056(2)	2.8260(2)	11e
	2.2438(7)	2.040(2)	2.8278(3)
	2.2521(7)	2.042(2)
1	2.2513(15)	2.123(5)	2.7782(5)	This work

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The equilibrium between the dication in 3, [{Au(μP,N-1)}₂]²⁺, and its monomeric cation, [Au(κ¹P-1)]⁺, was probed by density functional theory (DFT); see Experimental for full details. Using the polarisable continuum model (PCM) to implicitly model solvent effects, the dimer is significantly favoured by 18.3 kcal mol⁻¹ and 21.1 kcal mol⁻¹ in dichloromethane and acetonitrile, respectively (Scheme 3A). When the system was modelled explicitly using acetonitrile to coordinate to the gold cation in the monomeric form, the dimer is still favoured but only by 4.7 kcal mol⁻¹ (Scheme 3B). These data suggest that in acetonitrile the monomeric form would be accessible at room temperature, and we theorised that the monomeric form might be similarly accessible in the presence of substrates that could coordinate to gold, and hence the hemilability of these ligands could enable catalysis.

Scheme 3 Computational equilibria for gold(I) azophosphine monomers and dimers with (A) implicit and (B) explicit solvent corrections. Ar = (p-OMe)C₆H₄.

To explore the nature of this equilibrium in acetonitrile experimentally, complex 3 was dissolved in acetonitrile and probed by NMR spectroscopy. The ¹H, ¹³C{¹H} and ³¹P NMR spectra for 3 in acetonitrile are all significantly different to those previously acquired in dichloromethane, consistent with a time-averaged signal for the monomer/dimer in the former. This difference is particularly apparent in the ³¹P NMR spectra, with a Δδ of 18.1 ppm at room temperature (δ(³¹P) = 108.1 in CD₂Cl₂, 126.2 in acetonitrile). Variable temperature ³¹P NMR measurements of 3 in CD₃CN are also consistent with this equilibrium, with a downfield shift at higher temperatures (up to 60 °C), and an upfield shift at lower temperatures (down to −30 °C) (see Fig. 3). Notably, all single crystals grown from the acetonitrile solution that were tested were the previously described [{Au(μP,N-1)}₂][SbF₆]₂ dimers and not the acetonitrile adduct; this is consistent with the computational data that even though there is an equilibrium in acetonitrile, the dimer is still favoured. Complex 3 is stable as a solid for at least six months without any signs of decomposition.

Fig. 3 Variable-temperature ³¹P NMR spectra of a CD₃CN solution of 3.

Catalysis

The efficacy of the gold complexes as catalysts was explored through their application to several different transformations (Scheme 4). The azophosphine gold dimeric complex 3 was employed directly, whilst the gold(I) chloride complexes including 2 were activated using AgSbF₆ to maintain counterion consistency across each system. Loadings were maintained at 1 mol% of Au(I). The reactions were not forced to completion to enable a comparison of catalytic activity.

Scheme 4 Assessing the catalytic function of azophosphine gold complexes in typical gold-mediated transformations. Metal complexes added as freshly prepared stock solutions in the reaction solvent. Product yields and ratios reflect an average of two runs and were determined using ¹H NMR spectroscopy relative to an internal standard. brsm = based on recovered starting material.

Direct alkyne hydrofunctionalisations were assessed through the cyclisation of propargylamide 4¹⁷ and propargyl ether 7.¹⁸ Heterocycles 5 and 8, respectively, were formed in comparable yield when compared to a Ph₃PAuCl pre-catalyst. None of the oxazole 6 from alkene-isomerisation was observed from reaction of 4.

Similar efficacy was seen across the tested catalysis systems in the rearrangement-cyclisation of propargylic carboxylate 9 affording the bicyclic enone 10 after hydrolysis.¹⁹ Despite the difference in ligand structure, cycloisomerisation of enyne 11 ²⁰ afforded similar yields and ratios of diene isomers 12 and 13 using either JohnPhosAuCl or 2, and slightly lower conversion was seen with the dimer 3.

While the relative efficacy varied across reaction type, both 2 and 3 prove to be effective pre-catalysts leading to product formation and selectivities that broadly match those obtained with commonly used triphenylphosphine or JohnPhos Au(I) complexes under the same conditions.

Conclusions

We have shown that azophosphines can act as bridging ligands between two gold(I) centres for the first time, extending the known coordination chemistry of azophosphines beyond mononuclear ruthenium(II) complexes. The dicationic dimeric species exists in equilibrium with the monocationic monomer, which has been probed by variable temperature NMR spectroscopy and computational methods. The hemilability of the azophosphines ensures the dimer does not represent a catalytic dead-end, and indeed that the dimer can be used as a stable form of the activated catalyst species. The gold(I) azophosphine complexes have been used in a range of cyclisation reactions that form C–C and C–O bonds. In most cases the dimer gives similar activity to the gold(I) azophosphine chloride activated by silver salts in situ, demonstrating that the dimer can act as a stable self-activating pre-catalyst in these reactions.

Experimental

General considerations

Except as otherwise noted, the syntheses were performed using standard Schlenk line technique under a flow of dry, oxygen-free nitrogen, or an MBraun ECO glovebox under an atmosphere of dry, oxygen-free nitrogen with water and oxygen levels maintained at <0.1 ppm. Room temperature (RT) refers to reactions where no thermostatic control was applied and the temperature was recorded as 16–25 °C. Unless otherwise stated, overnight reactions refer to a period of 16 hours. All glassware and Teflon-coated stirrer bars were dried in an 80 °C oven overnight prior to use. All molecular sieves are 3 Å and purchased from VWR chemicals and were activated by heating at 400 °C under vacuum prior to use. Unless otherwise stated, degassing refers to three freeze–pump–thaw cycles. Me₂SAuCl,²¹ azophosphine (p-OMe)C₆H₄N₂P(^tBu)₂ (1),^14b PPh₃AuCl²² and catalysis substrates 4,²³ 7,²⁴ 9 ²⁵ and 12 ^19b were prepared according to literature procedures. Details on solvents and instrument specifications can be found in the SI.

Synthetic procedures and characterisation

Synthesis of Au(κ¹P-(p-OMe)C₆H₄N₂P(^tBu)₂)Cl (2). A solution of azophosphine 1 (102 mg, 0.36 mmol, 1 equiv.) in 2 mL DCM (dichloromethane) was added dropwise to a solution of Me₂SAuCl (106 mg, 0.36 mmol, 1 equiv.) in 4 mL DCM. The resulting red solution was stirred for three hours before removal of the volatiles in vacuo yielded 2 as a red solid (158 mg, 86%). Single crystals suitable for SCXRD were grown from slow evaporation of a saturated DCM solution at room temperature. ¹H (400.1 MHz, CDCl₃, 298 K): δ 7.86 (d, ³J_H–H = 9 Hz, 2H; H_Ar (p-anisyl)), 7.01 (d, ³J_H–H = 9 Hz, 2H; H_Ar (p-anisyl)), 3.92 (s, 3H; p-OCH₃), 1.42 (d, ³J_H–P = 15 Hz, 18H; C(CH₃)₃). ¹³C{¹H} (100.6 MHz, CDCl₃): δ 164.5 (s; p-C_Ar (p-anisyl)), 149.1 (d, ³J_C–P = 43 Hz; i-C_Ar (p-anisyl)), 125.7 (s; CH_Ar (p-anisyl)), 114.6 (s; CH_Ar (p-anisyl)), 56.0 (s; p-OCH₃), 37.9 (d, ¹J_C–P = 27 Hz; C(CH₃)₃), 28.6 (d, ²J_C–P = 4 Hz; C(CH₃)₃). ³¹P{¹H} (161.7 MHz, CDCl₃): δ 124.6 (s). ³¹P (161.7 MHz, CDCl₃): δ 124.9 (br. s). UV-Vis (DCM, nm) λ_max: 241 (m), 344 (s), 497 (w). ASAP-HRMS m/z for C₁₅H₂₅ClN₂OPAu [M + H]⁺: calcd (found) 516.1140 (516.1085), 515.1111 (515.1113), 514.1168 (514.1165), 513.1137 (513.1133). IR (cm⁻¹) ν_max: 2964 (w), 2922 (w), 2861 (w), 1597 (m, N=N), 1577 (m). Elemental Analysis: calcd (found) C% 35.14 (35.05), H% 4.91 (4.86), N% 5.46 (5.58).

Synthesis of [{Au(μP,N-(p-OMe)C₆H₄N₂P(^tBu)₂)}₂][SbF₆]₂ (3). A solution of complex 2 (23 mg, 0.04 mmol, 1 equiv.) in 2 mL DCM was added dropwise to a stirring solution of AgSbF₆ (13 mg, 0.04 mmol, 1 equiv.) in 2 mL DCM resulting in the immediate formation of an orange solution and precipitate. This solution was filtered through Celite, and the solid washed with DCM. The solvent was removed from the resulting orange solution in vacuo to yield 3 as an orange solid (25 mg, 83%). Single crystals suitable for SCXRD were grown from slow evaporation of a saturated DCM solution at room temperature. NMR spectroscopy data below are provided in CD₂Cl₂ and CD₃CN. ¹H (400.1 MHz, CD₂Cl₂, 298 K): δ 8.29 (d, ³J_H–H = 9 Hz, 4H; H_Ar (p-anisyl)), 7.28 (d, ³J_H–H = 9 Hz, 4H; H_Ar (p-anisyl)), 4.08 (s, 6H; p-OCH₃), 1.60 (d, ³J_H–P = 18 Hz, 36H; C(CH₃)₃). ¹³C{¹H} (100.6 MHz, CD₂Cl₂): δ 170.0 (s; p-C_Ar (p-anisyl)), 149.3 (d, ³J_C–P = 26 Hz; i-C_Ar (p-anisyl)), 131.4 (s; CH_Ar (p-anisyl)), 117.8 (s; CH_Ar (p-anisyl)), 57.7 (s; p-OCH₃), 41.6 (d, ¹J_C–P = 24 Hz; C(CH₃)₃), 28.6 (d, ²J_C–P = 5 Hz; C(CH₃)₃). ³¹P{¹H} (161.7 MHz, CD₂Cl₂): δ 108.1 (s). ³¹P (161.7 MHz, CD₂Cl₂): δ 108.1 (m). ¹H (400.1 MHz, CD₃CN, 298 K): δ 7.91 (d, ³J_H–H = 9 Hz, 4H; H_Ar (p-anisyl)), 7.13 (d, ³J_H–H = 9 Hz, 4H; H_Ar (p-anisyl)), 3.93 (s, 6H; p-OCH₃), 1.43 (d, ³J_H–P = 16 Hz, 36H; C(CH₃)₃). ¹³C{¹H} (100.6 MHz, CD₃CN): δ 166.3 (s; p-C_Ar (p-anisyl)), 150.2 (d, ³J_C–P = 44 Hz; i-C_Ar (p-anisyl)), 126.9 (s; CH_Ar (p-anisyl)), 115.8 (s; CH_Ar (p-anisyl)), 56.9 (s; p-OCH₃), 38.6 (d, ¹J_C–P = 29 Hz; C(CH₃)₃), 28.4 (d, ²J_C–P = 4 Hz; C(CH₃)₃). ³¹P{¹H} (161.7 MHz, CD₃CN): δ 126.2 (s). UV-Vis (DCM, nm) λ_max: 237 (m), 354 (s), 422 (w). ESI-HRMS m/z for C₃₀H₅₀N₄O₂P₂Au₂ [M + Cl]⁺: calcd (found) 992.2436 (992.2459), 991.2414 (991.2423), 990.2460 (990.2480), 989.2429 (989.2442). ESI-HRMS m/z for SbF₆ [M]⁻: calcd (found) 236.8946 (236.8949), 234.8942 (234.8944). IR (cm⁻¹) ν_max: 2956 (w), 2926 (w), 2855 (w), 1601 (m, N=N), 1577 (m).

Computational details

DFT calculations were run with Gaussian 16 (C.01).²⁶ TPSS²⁷ optimizations including Grimme's D3 parameter set²⁸ were performed using the Def2TZVP basis set²⁹ with all stationary points being fully characterized via analytical frequency calculations as minima (all positive eigenvalues). Implicit solvation models were used to account for dichloromethane (ε = 8.93) and non-coordinated acetonitrile (ε = 35.688) using the polarizable continuum model (PCM).³⁰ Optimised coordinates and energies can be found in the SI.

Catalysis

For the cycloisomerisation reactions stock solutions of individual substrates 4 (0.2 M), 7 (0.2 M) and 9 (0.2 M), PPh₃AuCl (4 mM), 2 (4 mM), 3 (1 mM) and AgSbF₆ (4 mM) were prepared in CH₂Cl₂. For the hydrolytic cyclisation of 12 stock solutions of 12 (0.2 M), PPh₃AuCl (4 mM), 2 (4 mM), 3 (1 mM) and AgSbF₆ (4 mM) were prepared in CH₃CN.

Stock solutions of substrates (0.5 mL, 0.1 mmol) were added to a vial followed by stock solutions of the selected catalyst system [(a) PPh₃AuCl (0.25 mL, 1 µmol) then AgSbF₆ (0.25 mL, 1 µmol); (b) 2 (0.25 mL, 1 µmol) then AgSbF₆ (0.25 mL, 1 µmol); (c) 3 (0.5 mL, 0.5 µmol)] The vial was capped and the reaction stirred for 14 h at RT before being diluted with CH₂Cl₂ (2 mL) and passed through a pad of silica, eluting with CH₂Cl₂ (8 mL). The solvent was removed under reduced pressure. Product yield was determined by ¹H-NMR spectroscopy relative to an internal standard of 1,2,4,5-tetramethylbenzene and with comparison to the literature [Compounds: 5,²³ 8,³¹ 10,^19b 12,²⁰ 13 ²⁵]

Conflicts of interest

There are no conflicts to declare.

Data availability

Data for this article, including NMR spectra, IR spectra, UV-Vis spectra, mass spectra, and computational data are available at UBIRA at https://doi.org/10.25500/edata.bham.00001562.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6dt00553e.

CCDC 2465037 (3) and 2465038 (2) contain the supplementary crystallographic data for this paper.^15a,b

Acknowledgements

The authors thank the Royal Society (URF\R1\201636), the EPSRC (EP/W036908/1), and the University of Birmingham for funding. The computations described in this paper were performed using the University of Birmingham's BlueBEAR HPC service; see https://www.birmingham.ac.uk/bear for more details.

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Footnote

† These authors contributed equally.

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