Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

Abstract

SummaryThe concept of frustrated Lewis pairs (FLPs) has received considerable attention of late, and numerous reports have demonstrated the power of non- or weakly interacting Lewis acid–base pairs for the cooperative activation of small molecules. Although most studies have focused on the use of organic or main-group FLPs that utilize steric encumbrance to prevent adduct formation, a related strategy can be envisioned for both organic and inorganic complexes, in which "electronic frustration" engenders reactivity consistent with both nucleophilic (basic) and electrophilic (acidic) character. Here we propose that such a description is consistent with the behavior of many coordinatively unsaturated transition-metal species featuring metal–ligand multiple bonds, and we further demonstrate that the resultant reactivity may be a powerful tool for the functionalization of C–H and E–H bonds.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…[185,186] In these systems formation of aclassical Lewis acid base adduct is suppressed by the incorporation of sterically demanding ligands,b ut in the environment of the weakly interacting FLP,t he bond heterolysis of,f or example,H 2 into H + and H À is possible. [186][187][188][189][190][191][192][193][194][195][196][197] Therefore,i ti s not covered here. [186][187][188][189][190][191][192][193][194][195][196][197] Therefore,i ti s not covered here.…”

“…[185][186][187] This field has become extremely versatile and is intensively discussed in the literature. [186][187][188][189][190][191][192][193][194][195][196][197] Therefore,i ti s not covered here.…”

A Multifunctional Pincer Ligand Supports Unsaturated Cobalt: Five Functionalities in One Pincer

“…When deprotonated, the anionic pyrazolate becomes a peripherally directed nucleophile, coordinating to alkali metal cations, but also readily bridging to a second transition metal, leading to aggregation. In the event that the metal in a bis‐pyrazolate pyridyl complex is unsaturated, then it combines, in close proximity, both Lewis acid and Lewis base character, with the potential reactivity of a single‐molecule frustrated Lewis pair (FLP), that is, binding both nucleophile and electrophile; this concept has already been recognized for some metal–ligand interactions . The idea behind FLPs is that steric bulk prevents direct interaction between Lewis acid and Lewis base, but a substrate can bind between the FLP partners.…”

“…In the event that the metal in ab is-pyrazolate pyridyl complex is unsaturated, then it combines, in close proximity,b oth Lewis acid and Lewis base character,w ith the potentialr eactivity of as ingle-molecule frustrated Lewis pair (FLP), that is, binding both nucleo-phile and electrophile;t his concept hasa lready been recognized for somem etal-ligand interactions. [5] The idea behind FLPs is that steric bulk prevents direct interaction between Lewis acida nd Lewis base, but as ubstrate can bind between the FLP partners. In H 2 L-based complexes,L ewis acidic (metal) and Lewis basic (pyrazolyl Nb)s ites cannot undergo intramolecular quenching, but intermolecularb inding occurs rapidly, leadingt od imers or polymers.…”

A Combined “Electrochemical–Frustrated Lewis Pair” Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes

“…[3][4][5][6] For main-group FLPs, the Lewis acid component is typically based on electron-deficient boranes, typically B(C 6 F 5 ) 3 and its derivatives, [1,[7][8][9][10] although FLP H 2 activation has also been achieved by using analogous Al(C 6 F 5 ) 3 [11] and boranes that do not contain C 6 F 5 groups. [12][13][14][15][16][17] FLPs are not limited to the main group; [18][19][20] Wass and co-workers previously demonstrated the ability of zirconocene-phosphinoaryloxide complexes to mimic the reactivity of FLPs and offer additional, unprecedented reactivity towards small molecules. [19] The majority of literature reports focus on delivering the resulting hydride to reduce a wide range of functional groups including imines, enamines and nitriles; [21][22][23] aldehydes; [8] and activate small molecules, such as CO 2 .…”