Frustrated Lewis Pairs Bridged by Water and Immobilized in a Metal–Organic Framework as a Recyclable Transfer Hydrogenation Nanocatalyst

Abstract

Heterogenization of frustrated Lewis pairs (FLPs) through supporting on solid materials can realize FLP recycling. However, only a few successful examples have achieved the full recycling of both Lewis acid (LA) and Lewis base (LB). Herein, we proposed an ingenious strategy to construct heterogeneous FLP catalysts by using ordinary water molecules as the chemical bridge to connect the LA and the LB and achieved complete immobilization of the FLP in a metal–organic framework. The as-prepared FLP/MIL-101(Cr) was characterized by a series of characterizations, demonstrating that the LB of the FLP was immobilized by a coordination unsaturated CrIII site on MIL-101(Cr) directly, and then the LA was anchored via the dissociated water molecule in the formation of a [LB–H]+[LA–OH]− adduct. FLP/MIL-101(Cr) as a nanocatalyst exhibited excellent structural stability in different solvents and temperatures. More importantly, it showed high activity for the catalytic-transfer hydrogenation of various compounds including alkenes, aldehydes, ketones, and imines. During the transfer hydrogenation of alkenes, FLP/MIL-101(Cr) was recycled at least six times without a loss of active components. The excellent cyclic property of FLP/MIL-101(Cr) was revealed to depend on the content of water in the system. In addition, several heterogeneous FLPs with different LBs were developed in the same way, indicating that the strategy of using water molecules as the bridges to bond the LA and the LB and complete the full recycling of the FLP is universal. This work provides a valuable strategy for the universal preparation of highly active and stable heterogeneous FLP nanocatalysts for further utilization.

Design and Synthesis of Heterogeneous Frustrated Lewis Pairs for Hydrogenation: From Molecular Immobilization to Defects Engineering

“…FLP/MIL-101(Cr) [82] MIL-101(Cr CFLP@MOF [84] MIL-101(Cr) 2d). [79] The introduction of B(C 6 F 5 ) 3 molecules into the N-containing MOF-545 frameworks could form FLPs in situ and effectively hydrogenate CO 2 to methoxide at relatively low temperature and pressure (Figure 2e).…”

“…It also showed high activity for catalytic transfer hydrogenation of alkenes, aldehydes, ketones, imines and other compounds. [82] In addition, we immobilized B(C 6 F 5 ) 3 /1,4-dioxane (DO) into MIL-101 by the post-synthetic modification strategy to construct a heterogeneous B(C 6 F 5 ) 3 /DO-MIL-101 catalyst (Figure 3c). Under mild reaction conditions, the B(C 6 F 5 ) 3 /DO-MIL-101 showed excellent catalytic activity and high selectivity for C=C double bond in the hydrogenation of α, β-unsaturated ketones.…”

“…The LB was directly immobilized by a coordination unsaturated Cr III site on MIL‐101(Cr) and then the LA was anchored by dissociated water molecules to form a [LB−H] + [LA−OH] − adduct. It also showed high activity for catalytic transfer hydrogenation of alkenes, aldehydes, ketones, imines and other compounds [82] . In addition, we immobilized B(C 6 F 5 ) 3 /1,4‐dioxane (DO) into MIL‐101 by the post‐synthetic modification strategy to construct a heterogeneous B(C 6 F 5 ) 3 /DO‐MIL‐101 catalyst (Figure 3c).…”