Lewis acidity enhancement of triarylborane by appended phosphine oxide groups

Abstract

A series of mono-, di-, and tri-phosphine oxide substituted triarylboranes, Mes2BAr (1), MesBAr2 (2), and BAr3 (3) (Ar = 4-(Ph2PO)-2,6-Me2-C6H2) were prepared to investigate the effect of a phosphine oxide group (Ph2PO) on Lewis acidity enhancement of triarylboranes. The X-ray crystal structure of 3 revealed peripheral decoration of Ph2PO groups with a C3-axis perpendicular to the trigonal boron center. UV/Vis absorption and PL spectra indicated a significant contribution of π(Mes or phenylene) → pπ(B) charge transfer in the lower-energy electronic transition. The reduction potential measured by cyclic voltammetry showed apparent LUMO stabilization by introduction of phosphine oxide groups, the extent of which gradually increased with the increasing number of phosphine oxide groups. Lewis acidity enhancement was also supported by the gradual increase in fluoride ion affinity in the order 3 > 2 > 1. Theoretical calculations suggest that introduction of a Ph2PO group into a triarylborane significantly enhances the Lewis acidity of the boron center via an inductive electron-withdrawing effect and this effect is additive for multiple phosphine oxide groups.

Triarylborane‐Functionalized Au8Ag8(CCR)16 Nanocluster with Enhanced Lewis Acidity

“…Many efforts have been devoted to enhancing the Lewis acidity of triarylborane complexes by introducing strong electron‐withdrawing groups into the triarylboranes. [ 26–28 ] In our work, because the TAB‐CC‐ligands are bound to Au(I) via σ bonding interactions and bound to Ag(I) via π bonding interactions (Figure S3b, Supporting Information), the Au(I) and Ag(I) atoms of NCs probably have Columbic and inductive effects on the boron atoms of triarylborane moieties. [ 29 ] Thus, the Lewis acidity of the boron center in the NCs will be enhanced due to the electron‐withdrawing of Au(I) and Ag(I).…”

Synthesis of Highly Functionalizable Symmetrically and Unsymmetrically Substituted Triarylboranes from Bench‐Stable Boron Precursors

“…Overview of reported follow-up reactions, to which the BXyl 3 moiety (BAr 3 or BAr 2 Ar') is tolerant, and selected functional groups that have been attached. Examples of each path are given in the following references: a, [26][27][28][29][30] b, [27,[29][30][31][32][33] c, [34,35] d, [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] e, [52][53][54][55][56][57] f, [42,[58][59][60][61][62] g, [63][64][65][66][67] and h. [68][69][70][71]…”

“…For example, water solubility of triarylboranes can be facilitated by the introduction of cationic ammonium or phosphonium substituents (Figure 2e and f). [42,[52][53][54][55][56][57][58][59][60][61][62] Due to the limited synthetic access to unsymmetrically substituted triarylboranes, structure-property relationship studies have been mostly limited to symmetrically substituted triarylboranes (BAr 3 or BAr 2 Ar'). Consequently, even though a plethora of functional groups is available, a maximum of two can be realized at once.…”

“… Overview of reported follow‐up reactions, to which the BXyl 3 moiety (BAr 3 or BAr 2 Ar’) is tolerant, and selected functional groups that have been attached. Examples of each path are given in the following references: a,[ 26 , 27 , 28 , 29 , 30 ] b,[ 27 , 29 , 30 , 31 , 32 , 33 ] c,[ 34 , 35 ] d,[ 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ] e,[ 52 , 53 , 54 , 55 , 56 , 57 ] f,[ 42 , 58 , 59 , 60 , 61 , 62 ] g,[ 63 , 64 , 65 , 66 , 67 ] and h.[ 68 , …”

“…For example, water solubility of triarylboranes can be facilitated by the introduction of cationic ammonium or phosphonium substituents (Figure 2 e and f). [ 42 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 ]…”