A boron-transfer mechanism mediating the thermally induced revival of frustrated carbene–borane pairs from their shelf-stable adducts

Abstract

Chemists have designed strategies that trigger the conformational isomerization of molecules in response to external stimuli, which can be further applied to regulate the complexation between Lewis acids and bases. We have recently developed a system in which frustrated carbene–borane pairs are revived from shelf-stable but external-stimuli-responsive carbene–borane adducts comprised of N-phosphine-oxide-substituted imidazolylidenes (PoxIms) and triarylboranes. Herein, we report the detailed mechanism on this revival process. A thermally induced borane-transfer process from the carbene carbon atom to the N-phosphinoyl oxygen atom initiates the transformation of the carbene–borane adduct. Subsequent conformational isomerization via the rotation of the N-phosphinoyl group in PoxIm moieties eventually leads to the revival of frustrated carbene–borane pairs that can cleave H2. We believe that this work illustrates an essential role of dynamic conformational isomerization in the regulation of the reactivity of external-stimuli-responsive Lewis acid-base adducts that contain multifunctional substituents.

Complexation-Induced N‒P Axial Chirality in Sm(II) N-Phosphine-Oxide-Substituted Imidazolylidene and Imidazolinylidene Complexes

“…Given that the utility of the multifunctional NHCs reported so far has been limited to their use as multidentate ligands for the complexation of metals, the exploration of an unprecedented application of multifunctional NHCs would significantly advance the development of their chemistry. [7][8][9][10][11] Previously, we have proposed a concept of multifunctional multipurpose carbenes, [12][13][14][15][16][17][18][19][20][21][22] which are carbenes that include functional groups applicable to a variety of purposes beyond solely possessing additional coordination sites. In this context, the N-phosphine-oxidesubstituted imidazolylidene, PoxIm (Figure 1A), and the corresponding imidazolinylidene, SPoxIm, are pioneering examples.…”

“…12 kcal•mol -1 for the previously reported PoxIms). 12 Thus, complexation between PoxIm or SPoxIm and a main-group/transition-metal element (M) can proceed in an anti--C, 17,23 anti--O, 12,16,18,21 syn--C, 12,21,22 or syn--C,O 22,24 fashion when an equivalent amount of PoxIm or SPoxIm and M are reacted together. Additionally, PoxIm and SPoxIm can also serve as a platform for the formation of heterobimetallic M/M′ NHC complexes via a -O-M,-C-M′ form.…”

“…Additionally, PoxIm and SPoxIm can also serve as a platform for the formation of heterobimetallic M/M′ NHC complexes via a -O-M,-C-M′ form. 17 In this context, we have synthesized a variety of (anti--O-PoxIm)M and (anti--O-SPoxIm)M complexes using Lewis acidic compounds that tend to have high oxophilicity such as NaOTf, 16 LiOTf, 16 BAr3 (Ar = C6F5, 12 p-HC6F4 21 ), and Al(C6F5)3. 17,18 Some of these complexes offer rare examples of complexation-induced axial chirality around the N-P bonds (Figure 1B).…”

“…The presence of this structural motif was confirmed from the single-crystal lattice and optimized gasphase structures. [16][17][18]21 This axial chirality is caused by the restricted rotation around the N-P bond due to the steric repulsion between the C5-H atom(s) of the PoxIm or SPoxIm ligand and the Lewis acid. Thus, this effect is tunable depending on the oxophilicity (which would influence the M-O distance) and size of the Lewis acid.…”