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

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 Cr^III 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.