Exploiting the Electronic Tuneability of Carboranes as Supports for Frustrated Lewis Pairs

Abstract

The first example of a carborane with a catecholborolyl substituent, [1-Bcat-2-Ph-closo-1,2-C2B10H10] (1), has been prepared and characterized and shown to act as the Lewis acid component of an intermolecular frustrated Lewis pair in catalyzing a Michael addition. In combination with B(C6F5)3 the C-carboranylphosphine [1-PPh2-closo-1,2-C2B10H11] (IVa) is found to be comparable with PPh2(C6F5) in its ability to catalyze hydrosilylation, whilst the more strongly basic B-carboranylphosphine [9-PPh2-closo-1,7-C2B10H11] (V) is less effective and the very weakly basic species [μ-2,2′-PPh-{1-(1′-1′,2′-closo-C2B10H10)-1,2-closo-C2B10H10}] (IX) is completely ineffective. Base strengths are rank-ordered via measurement of the 1J 31P-77Se coupling constants of the phosphineselenides [1-SePPh2-closo-1,2-C2B10H11] (2), [9-SePPh2-closo-1,7-C2B10H11] (3), and [SePPh2(C6F5)] (4).

On the Basicity of Carboranylphosphines

“…2 Our interest in carboranylphosphines stems from our recent report of the use of such species as the Lewis base component of Frustrated Lewis Pairs (FLPs). 3 We were attracted to carboranylphosphines for this application for the reasons given above, in particular the possibility of almost limitless tuneability of the Lewis base strength that a carborane scaffold potentially affords. Control of the acid and/or base strength of Lewis acid/Lewis base components of FLPs is important in optimizing the use of FLPs in catalysis, 4 and therefore it is essential that, for carboranylphosphines, we understand the factors that influence their Lewis base strength.…”

“…Crystals suitable for diffraction were grown from slow evaporation of a concentrated solution in DCM. 1 H NMR (CDCl 3 ): δ 6.33 (d, 1 J PH = 468 Hz, 1H, PH), 4.75 (br s, 1H, C cage H), 1.43 [d, 3 J PH = 36 Hz, 9H, C(CH 3 ) 3 ]. 11 B{ 1 H} NMR (CDCl 3 ): δ 0.2 (1B), −2.4 (1B), −6.5 (1B), −7.4 (1B), −10.6 (1B), −11.8 (3B), −13.6 (1B), −14.4 (1B).…”

Crystal structure of 1-heptafluorotolyl-closo-1,2-dicarbadodecaborane

“…Van Nghia et al, 2018;Marsh et al, 2018). Very recently we have reported the first examples of substituted carboranes as components of intermolecular frustrated Lewis pairs (FLPs; Benton et al, 2018). In this field the ability to fine-tune the Lewis acidity or basicity of a functional group on a carborane support by the electron-withdrawing or electron-donating characteristics of a second substituent on the carborane is of potential importance in using these FLPs as catalysts.…”

Exploring the Reactivity of B‐Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

“…However, only a very small number of compounds containing a carborane moiety has been studied (or even synthesized) as part of a frustrated Lewis pair (Figure 2). [25–27] …”

“…[23,24] However, only a very small number of compounds containing a carborane moiety has been studied (or even synthesized) as part of a frustrated Lewis pair (Figure 2). [25][26][27] Figure 1. Dichotomy regarding the electronic effect of a carboranyl moiety on phosphine substituents.…”