Direct synthesis of the Janus-head ligand (^MePy)₃Sn–Sn(^MePy)₃ using an unusual pyridyl-transfer reaction (^MePy = 6-methyl-2-pyridyl)

Abstract

The in situ reaction of the tripodal anion [EtAl(^MePy)₃]⁻ (^MePy = 6-Me-2-py) with SnCl₂ gives the Janus-head ligand (^MePy)₃Sn–Sn(^MePy)₃, containing a Sn–Sn bond, through a novel, one-pot reaction involving a combination of pyridyl transfer and metal-oxidation.

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Neutral tris(pyridyl) ligands of the general type [Y(Py)₃] (Py = 2-pyridyl, where Y can be a range of non-metallic bridgehead atoms or groups (e.g. CR, COR, CH, N, P, P = O, As; Fig. 1a)), have been studied extensively in the last 30 years.¹ These ligands and the related tris(pyrazolyl)borates and methanes^2,3 have found a broad range of applications in organometallic, coordination and bioinorganic chemistry. Their applications include catalysis, stoichiometric organic transformations and as models for the coordination in enzymatic metal sites.⁴ However, it is only relatively recently that attention has turned to Group 13 and 14 congeners containing heavier, more metallic main group bridgeheads.⁵ These metal-based ligands introduce the possibility of redox activity and variable oxidation states at the bridgehead, as well as previously unknown anionic tris(pyridyl) ligands,⁶ such as tris(2-pyridyl)aluminates of the type shown in Fig. 1b.

Fig. 1 (a) Connectivity of tris(2-pyridyl) ligand systems containing bridgehead atoms or groups (Y) and (b) the anionic tris(2-pyridyl)aluminate ligands.

In addition to the interesting coordination characteristics of this class of ligands revealed by a range of studies in the past 10 years,⁷ another potentially useful property of metal-based tris(pyridyl) arrangements is their ability to act as ‘soft’ pyridyl transfer reagents.^5a,6 This is the result of the relatively low polarity of the C–Al bond, which is roughly comparable to the C–Zn bond. Unlike the Zn counterparts, however, 2-pyridyl-aluminium compounds are thermodynamically stable at room temperature and are therefore storable reagents. This soft nucleophilic behaviour was first suggested by the reaction of the lithium complex [MeAl(Py)₃Li·THF] with CuCl, which gives the organo-Cu^I trimer [Cu(Py)]₃. In contrast, direct reaction of (2-Li-py) with CuCl results only in decomposition.⁶

Here we illustrate a further new facet of the reactivity of tris(2-pyridyl) reagents, with the discovery that the reaction of [EtAl(^MePy)₃]Li·THF (1a) (^MePy = 6-Me-2-pyridyl) with Sn^IICl₂ gives unique access to the Janus-head ligand system (^MePy)₃Sn–Sn(^MePy)₃ (2a) in a single step, a reaction that involves a combination of 2-pyridyl ligand transfer and oxidation of the Sn^II centre (Scheme 1). The significance of this new reaction is that it has provided the simplest access yet to a donor-functionalised Sn–Sn bonded framework of this type. For example, the most obvious alternative precursors to 2a are the hexahalides Sn₂X₆ (X = halogen); however, these are difficult to prepare and are either highly unstable (X = Cl)⁸ or mixed valence compounds (X = F).⁹

Scheme 1

The 1 : 1 reaction of [EtAl(^MePy)₃]Li·THF (1a) with SnCl₂ at room temperature in toluene–THF affords an orange reaction mixture containing several unidentified pyridyl compounds (see the discussion below and Fig. S6 of the ESI†) from which colourless crystals of [(MePy)₃Sn–Sn(MePy)₃] (2a) were isolated in a total yield up to 26%. A finely-divided black precipitate of the Sn metal was also visible upon prolonged storage of the reaction mixture. Compound 2a was fully characterized by analytical and spectroscopic methods and its structure was determined by single-crystal X-ray diffraction. Particularly diagnostic are its ¹¹⁹Sn NMR spectrum (singlet δ −218.9 ppm) and the presence of ^117,119Sn satellites for all of the C-atoms of the ^MePy ligands in the ¹³C{¹H} NMR spectrum (see Fig. S1–S5, ESI†). The single-crystal X-ray analysis of 2a (Fig. 2) confirms the formation of a Sn–Sn single bond in this reaction, the observed Sn–Sn bond length [2.7183(6) Å] being at the short end of the range of hexa-organo di-tin compounds [R₃Sn–SnR₃].¹⁰

Fig. 2 Structure of (^MePy)₃Sn–Sn(^MePy)₃ (2a). The molecule is centrosymmetric, with only one-half contained in the asymmetric unit. Selected bond lengths (Å) and angles (°): Sn(1)–Sn(1A) 2.7183(6), Sn–C range 2.143(4)–2.148(4), C–Sn–Sn range 103.1(1)–117.5(1), C–Sn–C range 106.7(2)–110.7(2).

Two obvious mechanisms might be responsible for the formation of the Sn–Sn bonded compound 2a, involving reductive elimination or disproportionation of Sn^II/IV intermediates. An in situ multinuclear NMR study of the reaction of an equimolar mixture of 1a with SnCl₂ in d₈-THF was used to gain insights into the mechanism by which 2a is formed. Within 15 min of mixing the reactants at ambient temperature an orange colouration developed and 2a was observed as the only Sn-containing species formed. The ²⁷Al NMR spectrum showed that a characteristic sharp resonance for AlCl₄⁻ was also present (δ 103 ppm),¹¹ together with other unidentified Al-containing species. No further changes in the ²⁷Al or ¹¹⁹Sn spectra were observed after 24 h. At this stage, the ¹H NMR spectrum confirmed that no precursor 1a remained. Importantly, no 6,6′-dimethyl-2,2′-dipyridyl is observed in the reaction at any point, despite the presence of several unidentified pyridyl signals (see Fig. S6 and S7, ESI†), ruling out a possible reductive elimination pathway to 2a which involves the Sn^IV intermediate Sn(^MePy)₄ (e.g., Scheme 2a). Confirming this, Sn(^MePy)₄ is not observed in the ¹¹⁹Sn NMR spectrum (lit. value δ −313 ppm in CDCl₃)¹² and we also find that heating an authentic sample of Sn(Py)₄ at 80 °C in toluene for 5 days results in only negligible decomposition (Fig. S8, ESI†).

Scheme 2

In a separate experiment, Sn^IICl₂ was treated with 2 equivalents of 2-Li-^MePy at −78 °C and the mixture was allowed to warm to room temperature. After 1 h, a mixture of at least five Sn species was formed, with the presence of 2a being apparent in both ¹H and ¹¹⁹Sn NMR spectra (Fig. S9, ESI†). This observation provided initial evidence that a disproportionation pathway involving a 2-pyridyl Sn^II species (of the type [Sn(^MePy)_n]^x−) is likely in the formation of 2a (e.g., Scheme 2b), which is also consistent with the formation of the Sn metal in the reaction.

The equimolar reaction of [EtAl(Py)₃Li·THF] (1b) (containing unsubstituted 2-pyridyl groups, Py) with SnCl₂ in d₈-THF was also investigated in order to determine the substrate-dependent behavior of this reaction. An in situ¹¹⁹Sn NMR spectroscopic investigation shows that three major Sn-containing products are generated after 24 h (Fig. S10, ESI†), an unidentified species at δ −130 ppm (s), [(Py)₃Sn–Sn(Py)₃] (2b) [assigned on the basis of its chemical shift, δ −218.6 ppm (s)] and another species (3) observed as two singlets (δ −264.5 and −348.3 ppm). The in situ NMR spectroscopic studies therefore indicate that a similar reactivity occurs for the unsubstituted 2-pyridyl groups, but the reaction becomes slower and less selective.

In both the in situ NMR spectroscopic investigations and on the preparative scale, 3 precipitates cleanly from the reaction mixture. The single-crystal X-ray structural analysis shows that 3 is the heterometallic Sn^II/Al^III complex [{Sn(Py)₃}Al{(μ-Cl)(Py)₂Sn(Cl)}] (Fig. 3), containing a six-coordinate Al^III centre that is complexed by a monoanionic [Sn^II(Py)₃]⁻ and a dianionic [(Py)₂Sn^IICl₂]²⁻ ligand in a sandwich-type arrangement. The presence of pyramidal and trigonal bipyramidal geometries at the two Sn centres (with stereochemically-active metal lone pairs) strongly supports the oxidation state formulation of the complex as Sn^II/Al^III/Sn^II. The structure of 3 is consistent with the previously mentioned ¹¹⁹Sn NMR spectrum, with the resonance at δ −266.3 being the Sn^II centre of the [Sn^II(Py)₃]⁻ ligand,¹³ while the resonance at δ −347.5 corresponds to the [(Py)₂Sn^IICl₂]²⁻ ligand.^14,15 Interestingly, there is also a very large change in chemical shift in the ²⁷Al NMR spectrum on reaction of the precursor [EtAl(Py)₃Li·THF] (1b) (δ 125) with the product 3 (δ 24), signifying the increase in the metal coordination number from four to six (Fig. S11–S13, ESI†).^11b,16

Fig. 3 Structure of the heterometallic complex 3. Only one of the two crystallographically-independent (chemically-identical) molecules found in the lattice is shown. H-atoms and THF lattice solvation have been omitted for clarity. Selected bond lengths (Å) and angles (°): Al–N range 2.049(9)–2.077(9), Al–Cl 2.300(4), Sn_{(Py)3(ligand)}–C range 2.19(1)–2.24(1), Sn_{(Py)2(Cl)2(ligand)}–C range 2.23(1)–2.25(1), C–Sn–C range 87.2(5)–91.1(5), N–Al–Cl range 172.3(3)–175.1(3).

Viewed as a model intermediate, 3 provides a snapshot of the likely stages of the reactions producing 2a and 2b. Although the precise details of the mechanism remain elusive at this stage, the key structural components of 3 provide further support for the conclusion that the formation of 2a and 2b involves (i) initial transfer of the 2-pyridyl groups from Al^III to Sn^II and (ii) the disproportionation of the resulting 2-pyridyl Sn^II complex into 2a and 2b (as concluded earlier in regard to Scheme 2b).

In summary, we have demonstrated in this paper a new feature of the reactivity of tris(2-pyridyl) aluminate anions, their ability to act as 2-pyridyl transfer reagents in combination with oxidation of metal centres. The formation of (^MePy)₃Sn–Sn(^MePy)₃ (2a) in a one-pot reaction avoids the use of highly unstable or poorly defined Sn^IV hexa-halides and may have extensive applications elsewhere in the p-block.¹⁷ Furthermore, the Janus-head ligand properties of 2a and related Group 14 arrangements should be of great interest in their own right. Future studies will be aimed at assessing the use of this new type of reaction for the synthesis of other main group Py′_nM-MPy′_n arrangements (Py′ = a substituted or unsubstituted pyridyl ligand) and the investigation of their coordination chemistry.

Experimental

All manipulations were carried out under dry, O₂-free argon on a vacuum line. Compounds 1a and 1b were synthesized as described previously.^7c,e

Synthesis of 2a

To a Schlenk tube were added 1a (200 mg, 0.49 mmol) and SnCl₂ (93 mg, 0.49 mmol) in a glove box. The Schlenk tube was transferred to a vacuum line, and toluene (15 mL) and THF (5 mL) were added. After the addition of THF a solution was obtained which quickly changed from pale yellow to deep orange. The mixture was stirred for 36 h at room temperature and then filtered through celite to remove a small amount of precipitate and an oily residue. The majority of the solvent was removed in vacuo until the precipitation of a solid was observed, which was heated gently back into solution. Storage at −15 °C (24 h) afforded a few colourless crystals of 2a along with an orange residue. The supernatant was transferred using a cannula to another Schlenk tube, concentrated and stored at −15 °C (48 h) affording colourless crystals of 2a. Total yield up to 26% (51 mg, 0.0645 mmol). Elem. anal. calc. for 2a: C: 54.7, H: 4.6, N: 10.6, found C: 54.4, H: 4.7, N: 10.2. ¹H NMR (298 K, 500 MHz, thf-d₈), δ = 7.68 (m, 6H, C(3)–H py), 7.36 (m, 6H, C(4)–H py), 6.94 (m, 6H, C(5)–H py), 2.39 (s, 18H, C(6)–Me). ¹³C{¹H} NMR (298 K, 125.7 MHz, thf-d₈), δ = 173.47 (C(2), ¹J(¹³C–¹¹⁹/¹¹⁷Sn) = 550/525 Hz, ²J(¹³C–¹¹⁹/¹¹⁷Sn) = 74/71 Hz), 158.77 (C(6) ³J(¹³C–¹¹⁹/¹¹⁷Sn) = 76/73 Hz, ⁴J(¹³C–¹¹⁹/¹¹⁷Sn) = 5 Hz), 134.16 (C(4) ³J(¹³C–¹¹⁹/¹¹⁷Sn) = 38/36 Hz, ⁴J(¹³C–¹¹⁹/¹¹⁷Sn) = 3 Hz), 131.82 (C(3) ²J(¹³C–¹¹⁹/¹¹⁷Sn) = 97/93 Hz, ³J(¹³C–¹¹⁹/¹¹⁷Sn) = 9 Hz), 122.03 (C(5) ⁴J(¹³C–¹¹⁹/¹¹⁷Sn) = 12 Hz), 24.43 (C(6)–Me, ⁴J(¹³C–¹¹⁹/¹¹⁷Sn) = 4 Hz. ¹¹⁹Sn{¹H} NMR (186.5 MHz, 298 K, rel. solution of SnMe₄), δ = −218.7 [s(ν_1/2 = 52 Hz)]. No ¹¹⁹Sn–¹¹⁷Sn coupling could be detected.

Synthesis of 3

To a Schlenk tube were added 1b (181 mg, 0.49 mmol) and SnCl₂ (93 mg, 0.49 mmol) in a glove box. The Schlenk tube was transferred to a vacuum line, and toluene (15 mL) and THF (5 mL) were added. After the addition of THF a yellow mixture was obtained which was stirred at room temperature for 40 h. A cloudy orange mixture was obtained. The supernatant was removed with a cannula and the resulting orange solid was dried in vacuo. Even though this orange solid was only partially soluble in THF, a few colourless crystals of 3 could be obtained from a concentrated sample in THF at room temperature. ¹H NMR (298 K, 500 MHz, thf-d₈), δ = 8.78 (d (6 Hz), 2H), 8.58 (d (7 Hz), 2H), 8.18 (d (7 Hz), 2H), 7.62–7.55 (m, 3H), 7.49 (t (7 Hz), 2H), 7.15 (m, 1H, overlapped with toluene resonances), 7.04 (t (7 Hz), 2H), 6.88 (d (6 Hz), 2H), 6.81 (t (7 Hz), 1H), 6.55 (t (7 Hz), 1H). ¹¹⁹Sn{¹H} NMR (186.5 MHz, 298 K, rel. solution of SnMe₄), δ = −264.5 (br, s), −348.3 (br, s). Changes up to 2 ppm were observed for the chemical shift. ²⁷Al NMR (298 K, d₈-thf-d₈, 130.3 MHz, rel. solution of AlCl₃·6H₂O–D₂O), δ = 23.8 (ν_1/2 = 210 Hz).

Crystal data: The data were collected on a Bruker SMART X2S diffractometer using a monochromatic Mo-Kα microfocus source with a BREEZE CCD detector, and equipped with an Oxford Cryosystem DeskTop cooler at 200 K. Data were collected using Bruker GIS, processed using Bruker SAINT and were refined using the SHELXTL-97 suite of programs. CCDC 1008866 and 1008867 contain the supplementary crystallographic data for 2a and 3, respectively.

R.G.-R. acknowledges the EU for a Marie Curie Intra European Fellowship and D.S.W. acknowledges the EU for an ERC Advanced Investigator Grant.

Notes and references

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17. It can be noted that a similar Pb-compound [(6-^tBuO-Py)₃PbPb(6-^tBuO-Py)₃] has been obtained using oxidative coupling of the plumbate ion [(6-^tBuO-Py)₃Pb]⁻ in the presence of Eu(II) (see ref. 5d). This reaction differs fundamentally from the current study in that no additional oxidising metal is required in the formation of 2a.

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Footnote

† Electronic supplementary information (ESI) available: Additional figures and tables and NMR spectra. CCDC 1008866 and 1008867. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02216e

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