Tuning the sulfur–heterometal interaction in organolead(IV) complexes of [Pt2(μ-S)2(PPh3)4]

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Abstract

Reactions of [Pt2(μ-S)2(PPh3)4] with Ph3PbCl, Ph2PbI2, Ph2PbBr2 and Me3PbOAc result in the formation of bright yellow to orange solutions containing the cations [Pt2(μ-S)2(PPh3)4PbR3]+ (R3 = Ph3, Ph2I, Ph2Br, Me3) isolated as PF6 or BPh4 salts. In the case of the Me3Pb and Et3Pb systems, a prolonged reaction time results in formation of the alkylated species [Pt2(μ-S)(μ-SR)(PPh3)4]+ (R = Me, Et). X-ray structure determinations on [Pt2(μ-S)2(PPh3)4PbMe3]PF6 and [Pt2(μ-S)2(PPh3)4PbPh2I]PF6 have been carried out, revealing different coordination modes. In the Me3Pb complex, the (four-coordinate) lead atom binds to a single sulfur atom, while in the Ph2PbI adduct coordination of both sulfurs results in a five-coordinate lead centre. These differences are related to the electron density on the lead centre, and indicate that the interaction of the heterometal centre with the {Pt2S2} metalloligand core can be tuned by variation of the heteroatom substituents. The species [Pt2(μ-S)2(PPh3)4PbR3]+ display differing fragmentation pathways in their ESI mass spectra, following initial loss of PPh3 in all cases; for R = Ph, loss of PbPh2 occurs, yielding [Pt2(μ-S)2(PPh3)3Ph]+, while for R = Me, reductive elimination of ethane gives [Pt2(μ-S)2(PPh3)3PbMe]+, which is followed by loss of CH4.

Graphical abstract

Reactions of [Pt2(μ-S)2(PPh3)4] with Me3PbOAc and Ph2PbI2 result in the formation of the cationic adducts [Pt2(μ-S)2(PPh3)4PbMe3]+ and [Pt2(μ-S)2(PPh3)4PbPh2I]+, which were found from X-ray structure determinations on their hexafluorophosphate salts to contain four- and five-coordinate lead atoms, respectively. Fragmentation pathways of these and other related adducts are reported.

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Introduction

In a series of recent papers we have been applying the technique of electrospray ionisation mass spectrometry (ESI MS) to probe the coordination chemistry of the metalloligand [Pt2(μ-S)2(PPh3)4] [1], [2], [3], [4]. This methodology allows the rapid screening of reaction systems on a small scale, with promising systems then being investigated on a more traditional macroscopic scale for full characterisation [5]. Because the ESI technique involves soft ionisation, solution ions are transferred in an essentially intact state into the gas phase, so such an MS-directed synthetic methodology gives a good picture of solution speciation.

Using this methodology we have previously investigated organo-tin(IV) adducts of [Pt2(μ-S)2(PPh3)4] [6] and of the related selenide complex [Pt2(μ-Se)2(PPh3)4] [7], leading to the isolation and characterisation of a range of new Pt2SnE2 trimetallic aggregates. In this paper we report the extension of this methodology to adducts formed between [Pt2(μ-S)2(PPh3)4] and organolead(IV) compounds. To date, only adducts of (inorganic) lead(II) have been described; reaction of [Pt2(μ-S)2(PPh3)4] and Pb(NO3)2 gave [Pt2(μ-S)2(PPh3)4Pb(NO3)2] which with NH4PF6 gave [Pt2(μ-S)2(PPh3)4Pb(NO3)]PF6, with both complexes characterised by X-ray crystallography [8]. The related complex [Pt2(μ-S)2(dppf)2] [dppf = 1,1′-bis(diphenylphosphino)ferrocene] forms a similar lead(II) adduct [Pt2(μ-S)2(dppf)2Pb(NO3)](NO3) [9] and lead(II) adducts of [Pt2(μ-S)2(PPh3)4] have been examined theoretically [10].

Section snippets

Results

The reaction of [Pt2(μ-S)2(PPh3)4] with Ph3PbCl in methanol rapidly led to the formation of a clear, bright yellow solution which was shown by ESI MS to contain solely the triphenyllead(IV) adduct [Pt2(μ-S)2(PPh3)4PbPh3]+ (m/z 1941). From this solution, solid products 1a and 1b were readily isolable in good yields by addition of NH4PF6 or NaBPh4, respectively. The complexes give satisfactory microanalytical data and are stable and soluble in polar solvents such as dichloromethane and

X-ray structure determinations on [Pt2(μ-S)2(PPh3)4PbMe3]PF6 (2) and [Pt2(μ-S)2(PPh3)4PbPh2I]PF6 (3)

The structure of the cation of the trimethyllead derivative 2 is illustrated in Fig. 1. It consists of the usual P4Pt2S2 core with the Me3Pb+ group attached to one of the two S atoms. There is no significant interaction with the other S atom, as evidenced by the longer Pb–S distance [3.835(1) Å c.f. 2.611(1) Å for the bonded one] and by the symmetrical geometry around the Pb atom (C–Pb–C angles essentially equal at 115.0 ± 1.6°). For comparison, Me3PbSMe has a Pb–S distance of 2.588 Å and C–Pb–C

Electrospray mass spectrometry study

A study of the cone voltage-induced fragmentation of the organolead(IV) adducts [Pt2(μ-S)2(PPh3)4PbMe3]+ and [Pt2(μ-S)2(PPh3)4PbPh3]+ has been carried out, in order to explore differences in fragmentation pathways imposed by the different organic groups. A number of studies have previously concerned the ESI MS analysis of organolead complexes. In an early study, the ESI MS behaviour of Me3PbCl and Et3PbCl were studied and yielded the parent [R3Pb]+ cations under low fragmentation conditions.[26]

Discussion

The metalloligand [Pt2(μ-S)2(PPh3)4] readily forms adducts with chemically soft lead(IV) species by displacement of halide ligands, analogous to the organo-tin(IV) systems [6]. The mode of binding of a PbR3+ fragment to the {Pt2S2} metalloligand core appears to depend on the Lewis acidity of the lead moiety. A study of the variation in 1J(PtP) coupling constants in a series of organo-tin(IV) adducts of [Pt2S2(PPh3)4] found that the stronger Lewis acids result in observation of higher 1J(PtP)

Experimental

General experimental procedures were as described in recent publications from these laboratories [1], [2], [3], [4]. ESI mass spectra were recorded on a VG Platform II instrument in positive-ion mode, using methanol as the mobile phase. Reaction solution aliquots were diluted with methanol prior to analysis, while isolated salts were dissolved in several drops of CH2Cl2 before diluting with MeOH to give a total solid concentration of ca. 0.1 mg mL−1. Confirmation of species was facilitated by

Synthesis of [Pt2(μ-S)2(PPh3)4PbPh3]PF6 (1a)

A suspension of [Pt2(μ-S)2(PPh3)4] (200 mg, 0.133 mmol) and Ph3PbCl (71 mg, 0.150 mmol) in methanol (30 mL) was stirred at room temperature, resulting in the rapid formation of a clear, bright yellow solution. After stirring for 24 h, the solution was filtered to remove a trace of insoluble matter, and to the filtrate was added NH4PF6 (100 mg, 0.613 mmol), resulting in the formation of a yellow microcrystalline solid. After stirring for 10 min water (10 mL) was added to complete precipitation. The

Synthesis of [Pt2(μ-S)2(PPh3)4PbPh3]BPh4 (1b)

Following the synthesis of 1a, complex 1b was prepared analogously, replacing NH4PF6 by NaBPh4 (150 mg, 0.439 mmol), resulting in the immediate formation of a light yellow precipitate; no water was added in this case. Following filtration and washing with cold methanol, complex 1b was obtained (201 mg, 67%). Anal. Calc. for C114H95BP4PbPt2S2: C, 60.6; H, 4.2. Found: C, 60.3; H, 4.5%.

Synthesis of [Pt2(μ-S)2(PPh3)4PbPh3]N(SO2C2F5)2 (1c)

Following the general procedure for 4a, Complex 1c was isolated in 52% yield by addition of LiN(SO2C2F5)2 to the filtered reaction solution, and was obtained as small orange-yellow crystals. The presence of the cation and anion were confirmed by positive- and negative-ion [N(SO2C2F5)2, m/z 380] ESI MS, respectively.

Synthesis of [Pt2(μ-S)2(PPh3)4PbPh3]PF6 (1a) from Pb2Ph6

A suspension of [Pt2(μ-S)2(PPh3)4] (200 mg, 0.133 mmol) and Pb2Ph6 (59 mg, 0.067 mmol) in methanol (30 mL) was stirred at room temperature, giving a clear, bright yellow solution after several hours. ESI MS showed predominantly the [Pt2(μ-S)2(PPh3)4PbPh3]+ cation. After stirring for 36 h, the solution was filtered, and NH4PF6 (100 mg, 0.613 excess) added to the stirred filtrate, giving a yellow precipitate. After addition of water (30 mL), the product was filtered off, washed with water (10 mL), and

Synthesis of [Pt2(μ-S)2(PPh3)4PbMe3]PF6 (2)

Following the general method for 1a (from Ph3PbCl), a suspension of [Pt2(μ-S)2(PPh3)4] (200 mg, 0.133 mmol) with Me3PbOAc (73 mg, 0.234 mmol) in methanol (30 mL) was stirred at room temperature, rapidly giving a bright yellow solution which was stirred for 1.5 h. Filtration, followed by addition of NH4PF6 (100 mg, 0.613 mmol) to the filtrate gave a bright yellow microcrystalline solid (199 mg, 79%). Anal. Calc. for C75H69F6P5PbPt2S2: C, 47.4; H, 3.7. Found: C, 47.6; H, 3.8%. ESI MS m/z 1755, [Pt2(μ-S)2

Synthesis of crude [Pt2(μ-S)(μ-SMe)(PPh3)4]PF6 using Me3PbOAc

A mixture of [Pt2(μ-S)2(PPh3)4] (100 mg, 0.067 mmol) with Me3PbOAc (36 mg, 0.116 mmol) in methanol (30 mL) was stirred, while being monitored periodically by positive-ion ESI MS. After 9 days, the dominant species observed was [Pt2(μ-S)(μ-SMe)(PPh3)4]+ at m/z 1518. The resulting light yellow solution was filtered to remove a trace of insoluble matter, and to the filtrate was added NH4PF6 (150 mg, 0.920 mmol), giving a light yellow precipitate. Water (10 mL) was added to assist precipitation, and the

Synthesis of [Pt2(μ-S)2(PPh3)4PbPh2I]PF6 (3)

A suspension of [Pt2(μ-S)2(PPh3)4] (200 mg, 0.133 mmol) and Ph2PbI2 (110 mg, 0.179 mmol) in methanol (30 mL) was stirred at room temperature, giving a clear orange solution after ca. 10 min. A positive-ion ESI mass spectrum showed [Pt2(μ-S)2(PPh3)4PbPh2I]+ as the dominant species. After filtration, solid NH4PF6 (200 mg, 1.227 mmol) was added to the filtrate, giving an orange solid, which was filtered off, washed with water, and dried to give 3 (198 mg, 70%). Anal. Calc. for C84H70F6IP5PbPt2S2: C, 47.2;

Synthesis of [Pt2(μ-S)2(PPh3)4PbPh2Br]PF6 (4)

[Pt2(μ-S)2(PPh3)4] (200 mg, 0.133 mmol) and Ph2PbBr2 (75 mg, 0.144 mmol) in methanol (30 mL) was stirred at room temperature for 40 min., giving a clear yellow-orange solution. After filtration, solid NH4PF6 (200 mg, 1.227 mmol) was added to the filtrate, giving a yellow-orange precipitate, which was filtered, washed with water, and dried to give 4 (195 mg, 70%). Anal. Calc. for C84H70BrF6P5PbPt2S2: C, 48.3; H, 3.4. Found: C, 48.1; H, 3.3%. ESI MS: [Pt2(μ-S)2(PPh3)4PbPh2Br]+ (m/z 1944, 70%) and [Pt2(μ-S)

Synthesis of [Pt2(μ-S)2(PPh3)4PbPh2(SCN)]SCN (5)

[Pt2(μ-S)2(PPh3)4] (281 mg, 0.187 mmol) and Ph2PbBr2 (113 mg, 0.218 mmol) in methanol (30 mL) was stirred at room temperature for 1 h. To the resulting clear yellow-orange solution was added KSCN (500 mg, large excess), resulting in the rapid deposition of a yellow precipitate. The mixture was stirred for 22 h, and the yellow product filtered, washed with water (2 × 10 mL) and dried under vacuum to give 5 (310 mg, 84%). Anal. Calc. for C86H70N2P4PbPt2S4: C, 52.1; H, 3.6; N, 1.4. Found: C, 51.1; H, 3.4; N,

X-ray crystallography

X-ray intensity data were collected on a Bruker CCD diffractometer using standard procedures and software. Empirical absorption corrections were applied (sadabs [33]). Structures were solved by direct methods and developed and refined on Fo2 using the shelx programmes [34] operating under wingx [35]. Hydrogen atoms were included in calculated positions.

Structure of [Pt2(μ-S)2(PPh3)4PbMe3]PF6 (2)

Yellow prismatic crystals were obtained from CH2Cl2/Et2O.

Crystal data: C75H69F6P5PbPt2S2, M = 1900.64, triclinic, space group P1¯, a = 13.6472(4) Å, b = 14.4319(4) Å, c = 19.3433(5) Å, α = 87.624(1)°, β = 80.817(1)°, γ = 67.297(1)°, U 3468.7(2) Å3, T 84(2) K, Z = 2, Dcalc = 1.820 g cm−3, μ (Mo Kα) = 6.679 mm−1, F(0 0 0) 1836; 33 905 reflections collected with 2° < θ < 26°, 14 132 unique (Rint 0.0188) used after correction for absorption (Tmax, min 0.281, 0.189). Crystal dimensions 0.32 × 0.28 × 0.24 mm3.

Refinement on Fo2 converged at R1

Structure of [Pt2(μ-S)2(PPh3)4PbPh2I]PF6 · 3CH2Cl2 (3 · 3CH2Cl2)

Orange prismatic crystals were obtained from CH2Cl2/Et2O.

Crystal data: C87H76Cl6F6IP5PbPt2S2, M = 2391.42, triclinic, space group P1¯, a = 12.7720(3) Å, b = 17.5564(4) Å, c = 19.5371(4) Å, α = 77.132(1)°, β = 84.108(1)°, γ = 86.387(1)°, U 4244.6(2) Å3, T 84(2) K, Z = 2, Dcalc = 1.871 g cm−3, μ (Mo Kα) = 6.02 mm−1, F(0 0 0) 2304; 41 036 reflections collected with 2° < θ < 26°, 17 189 unique (Rint 0.0305) used after correction for absorption (Tmax, min 0.350, 0.229). Crystal dimensions 0.32 × 0.26 × 0.24 mm3.

Refinement on Fo2 converged

Acknowledgements

We thank the University of Waikato and the National University of Singapore (NUS) for support of this work. We also thank Kelly Kilpin for recording the NMR spectra, and Dr. Tania Groutso (University of Auckland) for collection of the X-ray data sets of 2 and 3.

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