A Zwitterionic Phosphonium Stannate(II) via Hydrogen Splitting by a Sn/P Frustrated Lewis-Pair and Reductive Elimination

Abstract

The reactivity of the frustrated Lewis pair (FLP) (F₅ C₂ )₃ SnCH₂ P(tBu)₂ (1) was investigated with respect to the activation of elemental hydrogen. The reaction of 1 at elevated hydrogen pressure afforded the intramolecular phosphonium stannate(II) (F₅ C₂ )₂ SnCH₂ PH(tBu)₂ (3). It was characterized by means of multinuclear NMR spectroscopy and single crystal X-ray diffraction. NMR experiments with the two isotopologues H₂ and D₂ showed it to be formed via an H₂ adduct (F₅ C₂ )₃ HSnCH₂ PH(tBu)₂ (2) and the subsequent formal reductive elimination of pentafluoroethane; this is supported by DFT calculations. Parahydrogen-induced polarization experiments revealed the formation of a second product of the reaction of 1 with H₂ , [HP(tBu)₂ Me][Sn(C₂ F₅ )₃ ] (4), in ¹ H NMR spectra, whereas 2 was not detected due to its transient nature.

Keywords: activation; fluoroalkyl groups; frustrated Lewis pair; hydrogen; tin.

Figure 1

¹H NMR spectra at 11.7 T of a) a sample of 1 (•) in CD₂Cl₂ and b) a sample of 1 in CD₂Cl₂ exposed to 10 bar of H₂ after different times of reaction.

Scheme 1

Reaction of FLP 1 with H₂ on an NMR scale. Conversions were determined using ¹H NMR integrals.

Scheme 2

Synthesis of 3 from FLP 1 and H₂.

Figure 2

Molecular structure of compound 3 in the solid state. Ellipsoids are set at 50 % probability; hydrogen atoms, apart from the P‐H moiety, are omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)‐C(5) 1.800(10), P(1)‐C(6) 1.830(8), P(1)‐C(10) 1.831(9), Sn(1)‐C(1) 2.238(13), Sn(1)‐C(3) 2.347(17), Sn(1)‐C(5) 2.275(5); P(1)‐C(5)‐Sn(1) 118.6(4), C(5)‐P(1)‐C(6) 118.5(12), C(5)‐P(1)‐P(10) 105.5(13), C(6)‐P(1)‐P(10) 118.1(4), C(1)‐Sn(1)‐C(3) 92.3(3), C(1)‐Sn(1)‐C(5) 93.9(7), C(3)‐Sn(1)‐C(5) 84.4(9).

Scheme 3

Theoretical equilibrium between the tautomers 3 and 5.

Figure 3

(a) ¹H NMR spectrum measured after addition of parahydrogen (5 bar) into a solution of 1 in CD₂Cl₂ at 60 °C and 9.4 T. Antiphase quartet signals of the P‐H proton are shown with blue arrows and underlined with a red dashed line. (b) A reference ¹H NMR thermal spectrum of the same sample measured after 24 hours. The signals other than those of the P−H group belong to HC₂F₅, 3, CHDCl₂ and H₂ (see Figure 1). The high‐intensity signals corresponding to thermally polarized protons are off‐scale and are cut in (b) for a better visibility of other signals.

Figure 4

Computed reaction profile for the H₂‐splitting reaction of 1. All data have been computed at the PBE0(D3BJ)/def2‐TZVPP level of theory. The substituents at tin and phosphorus have been omitted for the sake of clarity.