Hydrogen activation using a novel tribenzyltin Lewis acid

Abstract

Over the last decade there has been an explosion in the reactivity and applications of frustrated Lewis pair (FLP) chemistry. Despite this, the Lewis acids (LAs) in these transformations are often boranes, with heavier p-block elements receiving surprisingly little attention. The novel LA Bn₃SnOTf (1) has been synthesized from simple, inexpensive starting materials and has been spectroscopically and structurally characterized. Subtle modulation of the electronics at the tin centre has led to an increase in its Lewis acidity in comparison with previously reported R₃SnOTf LAs, and has facilitated low temperature hydrogen activation and imine hydrogenation. Deactivation pathways of the R₃Sn⁺ LA core have also been investigated.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Keywords: Lewis acids; frustrated Lewis pairs; hydrogen activation; hydrogenation; tin.

Figure 1.

Synthesis of 1.

Figure 2.

Molecular structure of 1 (molecule a). Ellipsoids shown at 50% probability, H atoms omitted for clarity (C atoms blue, O atoms red, F atoms green, S atoms yellow, Sn atoms pink). (a) View of one independent Bn₃SnOTf fragment in the asymmetric unit. (b) Extended view of the polymeric structure generated from the independent fragment along the b axis (all four independent molecules form similar polymeric chains).

Figure 3.

¹H NMR spectra of 1 and collidine in C₆D₆ before (a) and after (b) admission of H₂ (4 bar). Insets show Sn-H resonance and ¹J(^117/119Sn-¹H) satellites.

Figure 4.

(a,b) Proposed deactivation pathways for Bn₃SnX (X = TfO, H; 1, 2) during H₂ heterolysis reactions.