Metal-only Lewis pairs featuring unsupported Pt→M (M = Zn or Cd) dative bonds

Abstract

Reactions of [Pt(PCy₃)₂] (Cy = cyclohexyl) with group 12 metal dihalides have afforded the novel metal only Lewis pair (MOLP) complexes, [(Cy₃P)₂Pt→MX₂] (M = Zn or Cd, X = Br or I), or the oxidative addition product, trans-[(Cy₃P)₂(I)PtHgI]. The zinc complex represents the first MOLP to contain an unsupported Pt→Zn linkage.

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The chemistry of heterobimetallic complexes featuring metal–metal bonds is of considerable importance, both for fundamental reasons, and because such systems have found a variety of applications in synthesis, catalysis, materials chemistry etc.¹ While the majority of such compounds are stabilised by ligands that bridge their metal–metal bonds, compounds with unsupported metal–metal bonds are generally considered as being more reactive species.^1,2 There are a variety of methodologies available for the construction of metal–metal bonded complexes, though the formation of heteronuclear M→M′ dative linkages from Lewis basic and Lewis acidic metal fragments is increasingly employed in this field.³ The popularity of this synthetic approach is derived from its simplicity, but also from the wide array of supported and unsupported M→M′ combinations that it offers the synthetic chemist. Compounds bearing such bonds have been termed metal-only Lewis pairs, or MOLPs.^3a

One of the more important metal donor Lewis bases that have been examined in recent years is the linear platinum(0) complex, [Pt(PCy₃)₂] (Cy = cyclohexyl). This has been employed to great effect, largely by Braunschweig and co-workers, in the formation of novel complexes with unsupported dative bonds between platinum and an array of s-,⁴ p-⁵ and d-block⁶ metal fragments.^5b,7 Structural and computational studies of several of these adducts have indicated that [Pt(PCy₃)₂] can act as a relatively strong Lewis base. Moreover, the oxidative addition of metal and non-metal halide bonds to its Pt⁰ centre has been demonstrated in some instances.^5b,7 It is surprising that there have been no reports of adducts between [Pt(PCy₃)₂] and group 12 metal halides, and to the best of our knowledge, unsupported Pt→Zn MOLPs of any kind are unknown. Here we report the preparation of the thermally stable complexes, [(Cy₃P)₂Pt→MX₂] (M = Zn or Cd; X = Br or I), and show that related mercury halide adducts are not easily accessible.

Treatment of toluene solutions of [Pt(PCy₃)₂] with either ZnBr₂ or CdI₂ led to high isolated yields of the yellow-orange crystalline products, 1 and 2, upon work-up (Scheme 1). Both compounds are thermally robust and show no tendency to convert to oxidative addition products, [(Cy₃P)₂(X)PtMX₂], in solution or the solid state at ambient temperature. In contrast, treatment of [Pt(PCy₃)₂] with HgI₂ afforded a high yield of the yellow addition product, trans-[(Cy₃P)₂(I)PtHgI] 3, with no evidence of adduct formation (though the reaction likely proceeds via [(Cy₃P)₂Pt→HgI₂]). Similar oxidative addition processes have been previously described for reactions of [Pt(PCy₃)₂] with, for example, GaX₃ (X = Br or I), BiCl₃ and group 11 metal halides.^5b,7 However, the outcome of these reactions can be dependent on the nature of the halide as much as the metal, as evidenced by the fact that GaCl₃ forms a stable adduct with the Pt⁰ Lewis base.^5b As a result, we investigated the reaction of HgCl₂ with [Pt(PCy₃)₂], and found that this led to deposition of mercury metal and the formation of the known complex, trans-[PtCl₂(PCy₃)₂],⁸ in addition to small amounts of trans-[(Cy₃P)₂(Cl)PtHgCl] (see ESI†). It is noteworthy that a similar result was obtained from the reaction of Hg₂Cl₂ with [Pt(PCy₃)₂].

Scheme 1 Reagents and conditions: i, ZnBr₂ or CdI₂; ii, HgI₂ (Cy = cyclohexyl).

The NMR spectroscopic data for 1 and 2 are similar and suggest both retain their solid state structures in solution.§ Their ³¹P{¹H} NMR spectra display singlet resonances at δ = 53.0 and 51.6 ppm respectively, each of which exhibits ¹⁹⁵Pt satellites (¹J_PtP = 2982 Hz 1; 2978 Hz 2). These values are comparable to those for the isostructural adduct, [(Cy₃P)₂Pt→BeCl₂] (δ = 53.6 ppm, ¹J_PtP = 3240 Hz),⁴ but lower in magnitude than the values for the precursor complex, [Pt(PCy₃)₂] (δ = 62.2 ppm, ¹J_PtP = 4155 Hz).^7a Furthermore, ^111/113Cd satellites (²J_CdP = 36 Hz) flank the signal for 2. Triplet resonances were observed in the ¹⁹⁵Pt{¹H} NMR spectra of the compounds (1: δ = −5425 ppm, 2: δ = −5260 ppm), which are significantly downfield of that for [Pt(PCy₃)₂] (δ = −6505 ppm).^7a The NMR spectroscopic data for the insertion product 3 (³¹P{¹H}: δ = 31.0 ppm, ¹J_PtP = 2448 Hz, ²J_HgP = 299 Hz; ¹⁹⁵Pt{¹H}: δ = −5052 ppm) compare well with data for similar compounds, e.g. trans-[(Cy₃P)₂(I)PtGaI₂] (³¹P{¹H}: δ = 23.1 ppm, ¹J_PtP = 2281 Hz).^5b

X-ray crystallographic studies determined 1 and 2 to be monomeric and isostructural in the solid state. Therefore only the molecular structure of 1 is depicted in Fig. 1 (see ESI† for an ORTEP diagram of 2).¶ Both compounds are also broadly isostructural with [(Cy₃P)₂Pt→BeCl₂] in that their Lewis acidic metal centres all possess trigonal planar geometries, while their Pt centres have T-shaped coordination environments with close to linear P–Pt–P fragments. In fact, this unit in 1 and 2 is more obtuse than in any other metal adduct of [Pt(PCy₃)₂]. The PtMX₂ and P₂PtM least squares planes of 1 and 2 bisect each other at angles of 71.8° 1 and 69.7° 2 (cf. 71.8° in [(Cy₃P)₂Pt→BeCl₂]⁴). With regard to the Pt–M distances in the compounds, that in 1 (2.4040(6) Å) is at the short end of the known range (2.343–3.003 Å),⁹ with only covalently bonded two-coordinate zinc compounds, e.g. [Cp*Pt(ZnCp*)₃],¹⁰ having shorter Pt–Zn bonds. Furthermore, the Pt–Cd bond in 2 (2.5867(6) Å) is shorter than all of the more than 50 other reported examples of such interactions (range: 2.606–3.129 Å).⁹ This, combined with the fact that 1 and 2 possess very rare examples of terminal MX₂ fragments, could indicate that [Pt(PCy₃)₂] acts as a strong nucleophile to those fragments.¹¹

Fig. 1 Molecular structure of 1 (25% thermal ellipsoids; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°) for 1: Pt(1)–P(1) 2.2760(6), Pt(1)–Zn(1) 2.4040(6), Br(1)–Zn(1) 2.3305(5), P(1)′–Pt(1)–P(1) 176.28(3), Br(1)–Zn(1)–Br(1)′ 118.75(3), Br(1)–Zn(1)–Pt(1) 120.626(15), Br(1)′–Zn(1)–Pt(1) 120.626(15); symmetry operation: ′−x + 1, y, −z + 1/2. Selected bond lengths (Å) and angles (°) for 2 (see ESI† for ORTEP diagram): Pt(1)–P(1) 2.2799(13), Pt(1)–P(2) 2.2920(13), Pt(1)–Cd(1) 2.5867(6), I(1)–Cd(1) 2.6974(8), I(2)–Cd(1) 2.6997(8), P(1)–Pt(1)–P(2) 176.92(3), Pt(1)–Cd(1)–I(1) 122.176(19), Pt(1)–Cd(1)–I(2) 117.531(19), I(1)–Cd(1)–I(2) 120.21(2).

The molecular structure of 3 (Fig. 2) confirms it to be an oxidative addition product. The compound is monomeric and its Pt center has a square planar coordination environment, with the two phosphine ligands trans- to each other. The Pt–Hg bond length is at the low end of the reported range for unsupported interactions of this nature (2.509–2.808 Å),⁹ while the terminal Hg–I and Pt–I distances are unexceptional. It is of note that several reports have detailed the oxidative addition of mercury(II) dihalides to platinum(II) centres,¹² though we are not aware of any prior structurally authenticated examples of such additions to platinum(0) complexes.

Fig. 2 Molecular structure of 3 (25% thermal ellipsoids; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°): Hg(1)–Pt(1) 2.5250(2), Hg(1)–I(2) 2.6764(16), Pt(1)–P(1) 2.3304(10), Pt(1)–P(2) 2.3403(10), Pt(1)–I(1) 2.6542(3), Pt(1)–Hg(1)–I(2) 173.05(3), P(1)–Pt(1)–P(2) 168.10(4), Hg(1)–Pt(1)–I(1) 178.850(11), P(1)–Pt(1)–Hg(1) 85.99(2), P(2)–Pt(1)–Hg(1) 87.17(3).

In conclusion, the first examples of adducts between the widely used metal donor Lewis base, [Pt(PCy₃)₂], and group 12 metal fragments have been prepared and structurally characterised. One synthesised complex, [(Cy₃P)₂Pt→ZnBr₂], represents the first MOLP containing an unsupported Pt→Zn linkage, while attempts to prepare a related Pt→Hg adduct, instead lead to an oxidative addition reaction. We are currently exploring the controlled reduction of 1–3 and related compounds with the aim of forming well defined, low valent bimetallic cluster compounds. Our results in this direction will be reported in due course.

CJ and AS gratefully acknowledge financial support from the Australian Research Council (fellowships for MM and LR). The EPSRC is also thanked for access to the UK National Mass Spectrometry Facility.

Notes and references

1. See for example: (a) C. M. Thomas, Comments Inorg. Chem., 2011, 32, 14; (b) M. Asay, C. Jones and M. Driess, Chem. Rev., 2011, 111, 354; (c) S. T. Liddle and D. P. Mills, Dalton Trans., 2009, 5592; (d) L. Gade, Angew. Chem. Int. Ed., 2000, 39, 2658; (e) N. Wheatley and P. Kalck, Chem. Rev., 1999, 99, 3379; (f) D. W. Stephan, Coord. Chem. Rev., 1989, 95, 41 , and references therein.
2. See for example: H. Lei, J.-D. Guo, J. C. Fettinger, S. Nagase and P. P. Power, J. Am. Chem. Soc., 2010, 132, 17399 , and references therein.
3. See for example: (a) J. Bauer, H. Braunschweig and R. D. Dewhurst, Chem. Rev., 2012, 112, 4329; (b) A. Amgoune and D. Bourissou, Chem. Commun., 2011, 47, 859 , and references therein.
4. H. Braunschweig, K. Gruss and K. Radacki, Angew. Chem. Int. Ed., 2009, 48, 4239.
5. See for example: (a) H. Braunschweig, A. Damme, R. D. Dewhurst, F. Hupp, J. O. C. Jiminez-Halla and K. Radacki, Chem. Commun., 2012, 48, 10410; (b) H. Braunschweig, K. Gruss and K. Radacki, Inorg. Chem., 2008, 47, 8595; (c) H. Braunschweig, K. Gruss and K. Radacki, Angew. Chem. Int. Ed., 2007, 46, 7782.
6. H. Braunschweig, K. Radacki and K. Schwab, Chem. Commun., 2010, 46, 913.
7. See for example: (a) J. Bauer, H. Braunschweig, A. Damme and K. Radacki, Angew. Chem. Int. Ed., 2012, 51, 10030; (b) J. Bauer, H. Braunschweig, K. Kraft and K. Radacki, Angew. Chem. Int. Ed., 2011, 50, 10457; (c) H. Braunschweig, P. Brenner, P. Cogswell, K. Kraft and K. Schwab, Chem. Commun., 2010, 46, 7894.
8. A. Del Pra and G. Zanotti, Inorg. Chim. Acta, 1980, 39, 137.
9. As determined from a survey of the Cambridge Crystallographic Database, October, 2012.
10. T. Bollermann, K. Freitag, C. Gemel, R. W. Seidel and R. A. Fischer, Organometallics, 2011, 30, 4123 , and references therein.
11. N.B. In order to draw structural comparisons with other adducts between sterically bulky and strongly nucleophilic Lewis bases and the ZnBr₂ and CdI₂ fragments, the N-heterocyclic carbene, IPr (:C{N(Dip)C(H)}₂, Dip = C₆H₃Prⁱ₂-2,6), was reacted with the metal halides. However, instead of monomeric adducts, the dimeric, halide bridged complexes, [{(IPr)MX(μ-X)}₂] (M = Zn or Cd, X = Br or I), were formed, thus discounting the proposed comparisons. See ESI† for synthetic, spectroscopic and structural details of these adducts.
12. See for example: (a) M. C. Janzen, M. C. Jennings and R. J. Puddephatt, Inorg. Chim. Acta, 2005, 358, 1614; (b) M. C. Janzen, M. C. Jennings and R. J. Puddephatt, Inorg. Chem., 2001, 40, 1728.

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Footnotes

† Electronic supplementary information (ESI) available: Further details and references for synthetic and crystallographic studies. CCDC 905516–905520. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc37442k

‡ Current address: Department of Chemistry, College of Science, Nanjing Forestry University, Jiangsu 210037, China.

§ [(Cy₃P)₂Pt→ZnBr₂] (1): ZnBr₂ (0.039 g, 0.173 mmol) was added to a solution of [Pt(PCy₃)₂] (0.131 g, 0.173 mmol) in toluene (80 cm³) at −70 °C. The mixture was warmed to room temperature overnight, then concentrated to ca. 25 cm³in vacuo. Yellow-orange crystals of 1 deposited from this solution over 1 week (0.14 g, 0.143 mmol, 83%). N.B. Crystals for the X-ray experiment were grown from benzene. M.p. 258–260 °C; ¹H NMR (300 MHz, 303 K, C₆D₆): δ = 1.29 (br., 18H, Cy-CH₂), 1.60–1.73 (m, 30H, Cy-CH₂), 2.06–2.12 (m, 12H, Cy-CH₂), 2.28 (m, 6H, Cy-CH); ¹³C{¹H} NMR (75.5 MHz, 303 K, C₆D₆): δ = 26.4 (s, C⁴, Cy), 27.6 (virtual triplet, J_PC = 11 Hz, C^2,6, Cy), 31.6 (s, C^3,5, Cy), 35.7 (virtual t, J_PC = 27 Hz, C¹, Cy); ³¹P{¹H} NMR (121.5 MHz, 303 K, C₆D₆): δ = 53.0 (s, ¹J_Pt-P = 2982 Hz); ¹⁹⁵Pt{¹H} NMR (85.6 MHz, 300 K, C₆D₆): δ = −5425 (s, ¹J_Pt-P = 2980 Hz); IR (Nujol) ν (cm⁻¹): 1601s, 1006s, 889s, 851s, 814s, 748s, 698s; MS (−ve CI/CH₄) m/z (%): 981.2 (M⁻, 6), 898.0 (M⁻-Br, 1); anal. calcd for C₃₆H₆₆Br₂P₂PtZn: C 44.07%, H 6.78%; found: C 44.10%, H 6.87%. [(Cy₃P)₂Pt→CdI₂] (2): A similar procedure to that used to prepare 1 was employed for the synthesis of 2 (yield: 74%). M.p. 229–231 °C; ¹H NMR (300 MHz, 303 K, C₆D₆): δ = 1.30 (br., 18H, Cy-CH₂), 1.61–1.74 (m, 30H, −CH₂), 2.06 (m, 12H, −CH₂), 2.22 (m, 6H, −CH); ¹³C{¹H} NMR (75.5 MHz, 303 K, C₆D₆): δ = 26.4 (s, C⁴, Cy), 27.7 (virtual t, J_PC = 11 Hz, C^2,6, Cy), 31.8 (s, C^3,5, Cy), 36.0 (virtual t, J_PC = 27 Hz, C¹, Cy); ³¹P{¹H} NMR (121.5 MHz, 303 K, C₆D₆): δ = 51.6 (s, ¹J_Pt-P = 2978 Hz, ²J_Cd-P = 36 Hz); ¹⁹⁵Pt{¹H} NMR (85.6 MHz, 300 K, C₆D₆): δ = −5260 (s, ¹J_Pt-P = 2974 Hz); IR (Nujol) ν (cm⁻¹): 1494w, 1027s, 914m, 887m, 851m, 799s, 732s, 695m; MS (−ve CI/CH₄) m/z (%): 1122.1 (M⁻, 1), 754.3 (M⁻-CdI₂, 1), 367.6 (CdI₂⁻, 2); anal. calcd for C₃₆H₆₆CdI₂P₂Pt: C 38.53%, H 5.93%; found: C 38.42%, H 5.90%. trans-[(Cy₃P)₂(I)PtHgI] (3): HgI₂ (0.101 g, 0.222 mmol) was added to a solution of [Pt(PCy₃)₂] (0.167 g, 0.221 mmol) in toluene (20 cm³) at −65 °C. The mixture was warmed to room temperature over 2 h, then concentrated to ca. 8 cm³in vacuo. Hexane (20 cm³) was added to this solution and the mixture allowed to stand in the absence of light for 1 day, yielding yellow crystals of 3 (0.24 g, 0.198 mmol, 90%). M.p. 270–272 °C; ¹H NMR (300 MHz, 303 K, C₆D₆): δ = 1.22 (m, 6H, Cy-CH₂), 1.37 (m, 12H, Cy-CH₂), 1.60–1.73 (m, 30H, Cy-CH₂), 2.14 (d, 12H, Cy-CH₂), 2.89 (br., 6H, Cy-CH); ¹³C{¹H} NMR (75.5 MHz, 303 K, C₆D₆): δ = 26.9 (s, C⁴, Cy), 27.7 (virtual t, J_PC = 11 Hz, C^2,6, Cy), 31.1 (virtual t, J_PC = 22 Hz, C^3,5, Cy), 39.0 (virtual t, J_PC = 29 Hz, C¹, Cy). ³¹P{¹H} NMR (121.5 MHz, 303 K, C₆D₆): δ = 31.0 (s, ¹J_Pt-P = 2448 Hz, ²J_Hg-P = 299 Hz); ¹⁹⁵Pt{¹H} NMR (85.6 MHz, 300 K, C₆D₆): δ = −5052 (s, ¹J_Pt-P = 2446 Hz); IR (Nujol) ν (cm⁻¹): 1445s, 1050m, 1002s, 916m, 891s, 848s, 816m, 733s; MS (EI/70 eV) m/z (%): 1082.3 (M⁺-I, 1), 755.4 (M⁺-HgI₂, 75); anal. calcd for C₃₆H₆₆HgI₂P₂Pt: C 35.72%, H 5.50%; found: C 35.63%, H 5.38%.

¶ Crystal data for 1·(benzene): C₄₂H₇₂Br₂P₂PtZn, M = 1059.22, monoclinic, space group C2/c, a = 16.7915(8) Å, b = 10.9089(17) Å, c = 23.5671(14) Å, β = 93.615(5)°, V = 4308.4(7) ų, Z = 4, D_c = 1.633 g cm⁻³, F(000) = 2128, μ(Mo-Kα) = 5.757 mm⁻¹, 123(2) K, 19036 collected reflections, 6274 unique reflections [R_(int) 0.0240], R (on F) 0.0251 (I > 2σI), wR (on F²) 0.0523 (all data); Crystal data for 2·(toluene): C₄₃H₇₄CdI₂P₂PtZn, M = 1214.25, monoclinic, space group C/c, a = 17.011(3) Å, b = 11.399(2) Å, c = 23.647(5) Å, β = 92.39(3)°, V = 4581.2(16) ų, Z = 4, D_c = 1.761 g cm⁻³, F(000) = 2376, μ(Mo-Kα) = 4.959 mm⁻¹, 123(2) K, 20129 collected reflections, 8275 unique reflections [R_(int) 0.0253], R (on F) 0.0192 (I > 2σI), wR (on F²) 0.0413 (all data); Crystal data for 3: C₃₆H₆₆HgI₂P₂PtZn, M = 1210.31, monoclinic, space group P2₁/c, a = 13.4975(5) Å, b = 13.4310(3) Å, c = 23.4845(8) Å, β = 107.180(3)°, V = 4067.4(2) ų, Z = 4, D_c = 1.976 g cm⁻³, F(000) = 2304, μ(Mo-Kα) = 8.825 mm⁻¹, 123(2) K, 26 184 collected reflections, 7991 unique reflections [R_(int) 0.0245], R (on F) 0.0233 (I > 2σI), wR (on F²) 0.0481 (all data).

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