Gold(I) complexes with a phosphinine ligand: synthesis and structural characterization

Abstract

The synthesis of two novel gold(I) complexes with a phosphinine ligand is reported. Reaction of a monodentate phosphinine ligand, 2,6-diphenyl-4-methylphosphorin (L_p; 1), with gold(I) molecular bricks leads to the preparation of two new mononuclear complexes with the general formulae [AuCl(L_p)] (2) and [Au(L_p)₂](OTf) (3). The new compounds were spectroscopically characterized by infrared and NMR (¹H, ¹³C and ³¹P). In addition, the molecular structure of [AuCl(L_p)] (2) was unequivocally ascertained by a single-crystal X-ray diffraction study. Furthermore the UV-Vis absorption and emission properties of these compounds were investigated. These complexes pave the way for the synthesis of a new class of phosphinine gold complexes displaying peculiar properties, which might hold promise as luminescent materials.

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Introduction

Gold(I) coordination complexes have emerged in the last few decades as an important class of compounds in many fields¹ and have found various applications in areas such as anion sensing,² luminescent materials,³ cytotoxicity,⁴ and catalysis,⁵ etc. On the other hand, phosphinines constitute an intriguing family of ligands due to their peculiar electronic properties, since they were found to behave as strong π-acceptor ligands. First discovered by Märkl et al.,⁶ the coordination chemistry of these ligands has witnessed a great breakthrough during the last two decades.⁷ However the chemistry of phosphinines has essentially been limited to low valence carbonyl transition metal complexes. The propensity of phosphinines for hydrolysis and low stability under harsh catalytic reaction conditions prevented the exploration of their coordination chemistry with most of the transition metals.⁸ More recently, some interesting phosphinine derivatives of terpyridine and bipyridine⁹ ligands have been reported, as well as the first enantiopure atropisomeric phosphinines.¹⁰ Moreover, very recently the first carbocyclometalation of a phosphinine ligand to the piano-stool “Cp*M” moieties (M = Rh, Ir) was described, providing the first C^P cyclometalated rhodium(III) and iridium(III) complexes.¹¹ Surprisingly only a few reports concerning the coordination chemistry of phosphinines towards gold(I) have been described.¹² This is rather surprising given the high affinity of phosphorus for soft gold(I) Lewis acid. We also note that no UV-Vis absorption nor photoluminescence data were provided for the few reported compounds.

Given the well established important properties of phosphane complexes of gold(I) we feel that phosphinine complexes of gold(I) could be very attractive building blocks to design a wide range of compounds with important properties. Hence we wish to report herein our recent results concerning the coordination chemistry with gold(I) of a monodentate phosphinine ligand, 2,6-diphenyl-4-methylphosphorin (L_p), described previously.⁶ Moreover we report the UV-Vis absorption and the preliminary photoluminescence results of the novel gold(I) phosphinine complexes.

Results and discussion

Synthesis and characterization

The ligand 2,6-diphenyl-4-methylphosphorin (L_p) was prepared according to the original procedure following the modular pyrylium-salt route.⁶ Moreover no silica gel chromatography was needed to purify the ligand thanks to crystallization from a dichloromethane–hexane mixture (90/10: v/v) at −16 °C. Having this ligand in hand, we decided to explore its coordination ability toward chlorogold(I) molecular bricks. Treatment of [AuCl(tht)] (tht = tetrahydrothiophene) with one equivalent of the ligand 2,6-diphenyl-4-methylphosphorin (L_p) in dichloromethane for two hours provided, after work up, complex 2 as a white microcrystalline solid in high yield. The novel compound formulated as [AuCl(L_p)] (2) was characterized spectroscopically (infra-red, ¹H, ¹³C and ³¹P NMR), by elemental analysis and by a single crystal X-ray diffraction study. When [AuCl(tht)] was treated with two equivalents of ligand L_p in the presence of silver(I) triflate, a new bis-phosphinine cationic gold(I) complex with the formula [Au(L_p)₂](OTf) (3) was obtained and was fully characterized spectroscopically (infra-red, ¹H, ¹³C and ³¹P NMR). The more informative data are the ³¹P{¹H} NMR chemical shifts of the phosphinine ligand L_p. We observed for both complex 2 and 3 an up-field shift with respect to the free ligand L_p (δ = 175.1 ppm). For instance, the ³¹P{¹H} NMR spectrum of compound [AuCl(L_p)] (2) recorded in CD₂Cl₂ exhibits a broad singlet at δ 149.3 ppm, which is up-field with respect to the free phosphinine ligand by 26 ppm. This is indicative of coordination of the ligand L_p to the gold(I) centre. Similarly a broad singlet is observed for compound [Au(L_p)₂](OTf) (3) at δ 160.5 ppm, which is up-field with respect to the free phosphinine ligand by 15 ppm. Furthermore the ¹H NMR spectra confirm the absence of signals due to the tetrahydrothiophene displaced by the phosphinine ligand (Scheme 1).

Scheme 1 Synthetic strategy for the preparation of [AuCl(L_p)] (2) and [Au(L_p)₂](OTf) (3).

In order to confirm the identity of these compounds without ambiguity, single crystals suitable for X-ray analysis were grown by slow diffusion of diethyl ether into a dichloromethane solution of complex [AuCl(L_p)] (2) (Fig. 1).

Fig. 1 The monodentate phosphinine ligand 2,6-diphenyl-4-methylphosphorin (L_p) and the novel gold(I) complexes described in this work [AuCl(L_p)] (2) and [Au(L_p)₂](OTf) (3).

X-ray structure analysis

Compound 2 crystalizes in the orthorhombic Pnma space group with Z = 8 molecules within the unit cell.§ The asymmetric unit consists of two independent half molecules (denoted I and II). Table S1† gives the experimental crystallographic data for [AuCl(L_p)] (2). A view of compound 2 is shown in Fig. 2. The X-ray molecular structure is in agreement with the spectroscopic data obtained for compound [AuCl(L_p)] (2). At first glance, it confirms the formation of the target gold(I) complex, where the metal centre is coordinated to one chloride and one phosphinine ligand (L_p). The X-ray structure displays the nearly linear geometry around the metal (Cl1–Au1–P1 = 178.48(3)° for molecule I and Cl2–Au2–P2 = 178.80(3)° for molecule II). Analysis of the bond distances and angles reveals some important features. The Au1–Cl1 (2.2666(7) Å) and Au2–Cl2 (2.2749(7) Å) bond distances lie in the normal range of triarylphosphane chlorogold(I) complexes, for example the Au–Cl bond distance is 2.279(3) Å for the benchmark [AuCl(PPh₃)]. Whereas the Au–Cl bond seems unaffected by the phosphinine ligand regarding triarylphosphane derivatives, the gold–phosphorus bond is significantly shortened in complex [AuCl(L_p)] (2) (Au1–P1 (2.2078(7) Å) and Au2–P2 (2.2047(7) Å)) in comparison to [AuCl(PPh₃)] where Au–P is 2.235(3) Å. This indicates perhaps some π-backdonation from the gold(I) centre to the phosphinine ligand. Even though π-backbonding is generally assumed to be negligible in gold(I) complexes, in recent papers this π-backdonation from gold(I) to alkenes and alkynes was considered to be quite intense.¹⁴ Therefore, we believe in our case that the short P–Au bond distance might be a consequence of modest π-backdonation, thanks to the powerful π-acidic nature of the phosphinine ligand (L_p).¹⁵ Another important feature is the aromaticity of the phosphorus ring. The CPC angles (C1–P1–C1 = 106.90(13)° and C11–P2–C11 = 106.87(12)°) are slightly larger than the values found in free phosphinines (generally between 101° and 103°). Also of interest are the C–C and P–C bond lengths within the ring in complex 2. The C–P bonds (C1–P1 = 1.723(2) Å and C11–P2 = 1.721(2) Å) remain unaffected compared to the free ligand, which is in stark contrast to the generally observed decrease in P–C bond lengths upon coordination.¹⁶ This is also an indication of some degree of π-backdonation. Moreover the C–C bond lengths within the phosphorus ring (between 1.393 Å and 1.396 Å in both molecules I and II) are in favour of an aromatic character, which also contrasts with the general alternating short and long C–C bonds as a consequence of a higher P–C double bond degree.

Fig. 2 Molecular structure of [AuCl(L_p)] (2) with an atom numbering system and selected bond distances (Å) and angles (°): Au1–P1 = 2.2078(7); Au2–P2 = 2.2047(7); Au1–Cl1 = 2.2666(7); Au2–Cl2 = 2.2749(7); P1–C1 = 1.723(2); P2–C11 = 1.721(2); C1–C2 = 1.395(2); C11–C12 = 1.396(2); C2–C3 = 1.394(2); C12–C13 = 1.393(2); P1–Au1–Cl1 = 178.48(3); P2–Au2–Cl2 = 178.80(3); C1–P1–Cl = 106.90(13); C11–P2–C11 = 106.87(12); C1–C2–C3 = 126.09(19); C11–C12–C13 = 125.70(18); C2–C3–C2 = 121.5(2); C12–C13–C12 = 122.1(2).

Analysis of the solid state packing of compound [AuCl(L_p)] (2) reveals that no particular features, such as aurophilic Au(I)⋯Au(I) or π–π interactions, could be found. However, we note the presence of a secondary weak Cl⋯Au interaction between the chloride anion of molecule I and the gold centre of molecule II (Cl1–Au2 = 3.6702(8) Å).¹⁷ Furthermore, careful analysis of the crystal packing reveals that a weak C–H⋯π interaction occurs between H9 of molecule I and the phenyl substituent (C15–C20) of molecule II (d = 2.770 Å). Likewise, a weak C–H⋯π interaction exists between H16 of molecule II and the phenyl ring of molecule I (C5–C10) (d = 2.670 Å).

UV-Vis absorption and emission properties

The UV-Vis spectra of the ligand 2,6-diphenyl-4-methylphosphorin (1; L_p) and compounds [AuCl(L_p)] (2) and [Au(L_p)₂](OTf) (3) were recorded in dichloromethane solutions (see ESI Fig. S1†). The spectrum of the ligand exhibits two strong absorptions at 274 nm (ε ≈ 39 300 M⁻¹ cm⁻¹) and 228 nm (ε ≈ 29 300 M⁻¹ cm⁻¹) with high molar absorptivity values. These high energy absorptions can be reasonably ascribed to π–π* transitions.¹⁸ The spectrum of compound [AuCl(L_p)] (2) shows two high energy absorption maxima at 281 nm (ε ≈ 32 100 M⁻¹ cm⁻¹) and 229 nm (ε ≈ 22 900 M⁻¹ cm⁻¹) with high molar absorption coefficient (ε) values that are characteristic of gold(I) perturbed π–π* ligand based transitions, in addition to a lower energy shoulder at λ ≈ 315 nm with a lower ε value (ε ≈ 13 300 M⁻¹ cm⁻¹). The spectrum profile of complex [Au(L_p)₂](OTf) (3) is quite different from those of the free ligand (L_p) and complex 2. It shows one high energy absorption maximum at λ = 229 nm (ε ≈ 54 500 M⁻¹ cm⁻¹) and two lower energy shoulders at λ ≈ 276 nm (ε ≈ 22 800 M⁻¹ cm⁻¹) and λ ≈ 319 nm (ε ≈ 12 700 M⁻¹ cm⁻¹) with high molar absorption coefficient values, indicating perturbed π–π* ligands based transitions.

It is well known that gold(I) complexes with phosphane ligands exhibit strong photoluminescence in the solid state and/or in fluid solutions.¹⁹ Many groups have reported compounds with useful luminescent properties since the discovery of the luminescence of [AuCl(PPh₃)₂] by Dori et al.²⁰ However to the best of our knowledge no examples of luminescent gold(I) assemblies with phosphinine ligands were described. Therefore we sought to explore the photoluminescence behaviour of our novel phosphinine gold(I) compounds [AuCl(L_p)] (2) and [Au(L_p)₂](OTf) (3), as well as the ligand 2,6-diphenyl-4-methylphosphorin (L_p) itself, especially because of the peculiar strong π-acidic character of phosphinines.^15,16 Preliminary analyses carried out on the new complexes 2 and 3 in fluid dichloromethane solutions did not reveal any luminescence at room temperature, but the ligand showed a weak luminescence at 405 nm upon photoexcitation at 320 nm. In the solid state, when polystyrene thin films were doped with ligand L_p, or complexes [AuCl(L_p)] (2) or [Au(L_p)₂](OTf) (3), luminescence was detected for the free ligand (L_p) at 409 nm upon photoexcitation at 320 nm. The neutral complex [AuCl(L_p)] (2) emits at 577 nm upon photoexcitation at 330 nm, with a large Stokes shift that might indicate it is phosphorescence. We also note two less intense maxima at 419 nm and 390 nm. In contrast, the cationic complex [Au(L_p)₂](OTf) (3) exhibits an emission in the blue region at 440 nm with a shoulder at around 500 nm (see ESI Fig. S2†). We assume that the detected emissions might involve metal centred and/or intraligand π–π* transitions, but a metal-to-ligand charge transfer (MLCT) from gold(I) to the phosphinine ligand cannot be excluded given the high π-acidic ability of the ligand.

Conclusions

In this paper we reported the synthesis of two novel gold(I) phosphinine assemblies. Reaction of [AuCl(tht)] with one or two equivalents of a phosphinine ligand, 2,6-diphenyl-4-methylphosphorin ligand (L_p), provided gold(I) complexes of the general formulae [AuCl(L_p)] (2) and [Au(L_p)₂](OTf) (3). These two novel complexes were characterized by spectroscopic methods (infrared, ¹H, ¹³C and ³¹P NMR). The structure of complex [AuCl(L_p)] (2) was unequivocally ascertained by a single crystal X-ray diffraction study. Moreover we have reported the UV-Vis absorption properties of these compounds, as well as preliminary photoluminescence results, which showed some emission properties in the solid state. Our future efforts are devoted to prepare some heteroleptic phosphinine gold(I) complexes with acetylides or N-heterocyclic carbene donor ligands.

Acknowledgements

Université Pierre et Marie Curie-Paris 6 and Centre National de la Recherche Scientifique are gratefully acknowledged.

Notes and references

1. Modern Supramolecular Gold Chemistry: Gold-Metal Interactions and Applications, ed. A. Laguna, Wiley-VCH, Weinheim, 2008; H. Schmidbauer and A. Schier, Gold Organometallicsin Comprehensive Organometallic Chemistry III, Elsevier, Amsterdam, The Netherlands, 2007, vol. 2, pp. 251–307; H. Schmidbauer and A. Schier, Chem. Soc. Rev., 2012, 41, 370–412.
2. X. He, N. Zhu and V. W. W. Yam, Dalton Trans., 2011, 40, 9703–9710; X. He and V. W. W. Yam, Coord. Chem. Rev., 2011, 255, 2111–2123; X. He and V. W. W. Yam, Inorg. Chem., 2010, 49, 2273–2279; S. H. Lim, M. M. Olmstead and A. L. Balch, Chem. Sci., 2013, 4, 311–318; V. W.-W. Yam and E. C.-C. Cheng, Chem. Soc. Rev., 2008, 37, 1806–1813.
3. I. O. Koshevoy, C.-L. Lin, C.-C. Hsieh, A. J. Karttunen, M. Haukka, T. A. Pakkanen and P.-T. Chou, Dalton Trans., 2012, 41, 937; I. O. Koshevoy, E. S. Smirnova, M. Haukka, A. Laguna, J. Camara Chueca, T. A. Pakkanen, S. P. Tunik, I. Ospino and O. Crespo, Dalton Trans., 2011, 40, 7412.
4. A. De Luca, C. G. Hartinger, P. J. Dyson, M. Lo Bello and A. Casini, J. Inorg. Biochem., 2013, 119, 38–42; E. Vergara, E. Cerrada, A. Casini, O. Zava, M. Laguna and P. J. Dyson, Organometallics, 2010, 29, 2596–2603; S. Tian, F. M. Siu, S. C. F. Kui, C. N. Lok and C. M. Che, Chem. Commun., 2011, 47, 9318–9320; C. F. Shaw III, Chem. Rev., 1999, 99, 2589–2600; Y. Y. Tong, J. J. R. Frausto da Silva, A. J. L. Pombeiro, G. Wagner and R. Herrmann, J. Organomet. Chem., 1998, 552, 17–21.
5. M. Joost, P. Gualco, S. Mallet-Ladeira, A. Amgoune and D. Bourissou, Angew. Chem., Int. Ed., 2013, 52, 7160–7163; P. Gualco, S. Ladeira, K. Miqueu, A. Amgoune and D. Bourissou, Angew. Chem., Int. Ed., 2011, 50, 8320–8324; M. Sircoglou, M. Mercy, N. Saffon, Y. Coppel, G. Bouhadir, L. Maron and D. Bourissou, Angew. Chem., Int. Ed., 2009, 48, 3454–3457; J. Dubarle-Offner, M. Barbazanges, M. Augé, C. Desmarets, J. Moussa, M. Rosa Axet, C. Ollivier, C. Aubert, L. Fensterbank, V. Gandon, M. Malacria, G. Gontard and H. Amouri, Organometallics, 2013, 32, 1665–1673; A. S. K. Hashmi, S. Schäfer, M. Wölfle, C. Diez, P. Fischer, A. Laguna, M. C. Blanco and M. C. Gimeno, Angew. Chem., Int. Ed., 2007, 46, 6184–6187; M. R. Axet, M. Barbazanges, M. Augé, C. Desmarets, J. Moussa, C. Ollivier, C. Aubert, L. Fensterbank, V. Gandon, M. Malacria, L.-M. Chamoreau and H. Amouri, Organometallics, 2010, 29, 6636–6638; A. Yanagisawa and T. Arai, Chem. Commun., 2008, 1165–1172; X.-Y. Liu, Z. Guo, S. S. Dong, X.-H. Li and C.-M. Che, Chem.–Eur. J., 2011, 17, 12932–12945; O. Kanno, W. Kuriyama, Z. J. Wang and F. D. Toste, Angew. Chem., Int. Ed., 2011, 42, 9919–9922.
6. G. Märkl, Angew. Chem., Int. Ed. Engl., 1966, 5, 846–847.
7. P. Le Floch, Top. Heterocycl. Chem., 2009, 20, 147–184; N. Mézailles and P. Le Floch, Curr. Org. Chem., 2006, 10, 3–25.
8. C. Müller, in Phosphorus Ligand Effects in Homogeneous Catalysis: Design and Synthesis, ed. P. C. J. Kamer and P. W. N. M. van Leeuwen, Wiley-VCH, 2012; C. Müller and D. Vogt, in Phosphorus Chemistry: Catalysis and Material Science Applications, ed. M. Peruzzini and L. Gonsalvi, Springer, 2011, vol. 36, ch. 6; A. Moores, N. Mézailles, L. Ricard and P. Le Floch, Organometallics, 2005, 24, 508–513.
9. A. Campos Carrasco, E. A. Pidko, A. M. Masdeu-Bultó, M. Lutz, A. L. Spek, D. Vogt and C. Müller, New J. Chem., 2010, 34, 1547–1550; C. Müller, E. A. Pidko, M. Lutz, A. L. Spek and D. Vogt, Chem.–Eur. J., 2008, 14, 8803–8807.
10. A. Campos-Carrasco, L. E. E. Broeckx, J. J. M. Weemers, E. A. Pidko, M. Lutz, A. M. Masdeu-Bultó, D. Vogt and C. Müller, Chem.–Eur. J., 2011, 17, 2510–2517; C. Müller, E. A. Pidko, M. Lutz, A. L. Spek, R. A. van Santen and D. Vogt, Dalton Trans., 2007, 46, 5372–5375.
11. L. E. E. Broeckx, M. Lutz, D. Vogt and C. Müller, Chem. Commun., 2011, 47, 2003–2005.
12. S. Komath Mallissery, M. Nieger and D. Gudat, Z. Anorg. Allg. Chem., 2010, 636, 1354–1360; S. Komath Mallissery and D. Gudat, Dalton Trans., 2010, 39, 4280–4284; S. B. Clendenning, P. B. Hitchcock, G. A. Lawless, J. F. Nixon and C. W. Tate, J. Organomet. Chem., 2010, 695, 707–720; A. Moores, N. Mézailles, N. Maigrot, L. Ricard, F. Mathey and P. Le Floch, Eur. J. Inorg. Chem., 2002, 2034–2039; N. Mézailles, L. Ricard, F. Mathey and P. Le Floch, Eur. J. Inorg. Chem., 1999, 2233–2241; N. Mézailles, N. Avarvari, N. Maigrot, L. Ricard, F. Mathey, P. Le Floch, L. Cataldo, T. Berclaz and M. Geoffroy, Angew. Chem., Int. Ed., 1999, 38, 3194–3197; Y. Mao, K. M. H. Lim, Y. Li, R. Ganguly and F. Mathey, Organometallics, 2013, 35, 3562–3565.
13. WinGX: L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837–838 ; SIR92: A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr., 1994, 27, 435 ; SHELX: G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112–122.
14. J. Schaefer, D. Himmel and I. Krossing, Eur. J. Inorg. Chem., 2013, 2712–2717; I. A. Das, C. Dash, M. Yousufuddin, M. A. Celik, G. Frenking and H. V. R. Dias, Angew. Chem., Int. Ed., 2012, 51, 3940–3943; I. Krossing, Angew. Chem., Int. Ed., 2011, 50, 11576–11578.
15. C. Müller and D. Vogt, Dalton Trans., 2007, 5505–5523.
16. P. Le Floch, Coord. Chem. Rev., 2006, 250, 627–681; F. Mathey, Angew. Chem., Int. Ed., 2003, 42, 1578–1604.
17. W. J. Hunks, M. C. Jennings and R. J. Puddephatt, Inorg. Chem., 2000, 34, 2699–2702; M. Hasan, I. V. Kozhevnikov, M. R. H. Siddiqui, A. Steiner and N. Winterton, Inorg. Chem., 1999, 38, 5637–5641; A. Bauer and H. Schmidbauer, J. Am. Chem. Soc., 1996, 118, 5324–5325.
18. UV-VIS Spectroscopy and Its Applications, ed. H.-H. Perkampus, Springer London, Limited, 2012.
19. R. Visbal, I. Ospino, J. M. López-de-Luzuriaga, A. Laguna and M. C. Gimeno, J. Am. Chem. Soc., 2013, 135, 4712–4715; C. Ma, C. T.-L. Chan, W.-M. Kwok and C.-M. Che, Chem. Sci., 2012, 3, 1883–1892; V. W.-W. Yam and K. M.-C. Wong, Chem. Commun., 2011, 47, 11579–11592; V. W.-W. Yam, E. C.-C. Cheng and Z.-Y. Zhou, Angew. Chem., Int. Ed., 2000, 39, 1683–1685; C. King, M. N. I. Khan, R. J. Staples and J. P. Fackler Jr, Inorg. Chem., 1992, 31, 3236–3238.
20. R. F. Ziolo, S. Lipton and Z. Dori, J. Chem. Soc., Chem. Commun., 1970, 1124–1125.

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Footnotes

† Electronic supplementary information (ESI) available: Synthetic procedures and spectroscopic data for compounds 1, 2 and 3. Crystallographic data and structure refinement for [Au(Cl)(L_p)] (2). CCDC 945516. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra47132b

‡ This manuscript is dedicated to Dr Michel Gruselle on the occasion of his 69th birthday.

§ Crystal data for 2. A colourless single crystal of compound 2 was selected, mounted onto a cryoloop, and transferred in a cold nitrogen gas stream. Intensity data were collected with a Bruker Kappa-APEXII with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Data collection was performed with an APEX2 suite (Bruker). Unit-cell parameters refinement, integration and data reduction were carried out with the Bruker-SAINT program. SADABS (Bruker) was used for multi-scan absorption corrections. In the WinGX¹³ suite of programs, the structure was solved with Sir92¹³ and refined by full-matrix least-squares methods using SHELXL-97.¹³ All non-hydrogen atoms were refined anisotropically. H atoms were placed at calculated positions and refined with a riding model. CCDC 945516 contains the supplementary crystallographic data for this paper, which is available in the ESI.†

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