A New Type of Lewis Acid-Base Bifunctional M(salphen) (M=Zn, Cu and Ni) Catalysts for CO 2 Fixationby Yanwei Ren, Jungui Chen, Chaorong Qi, Huanfeng Jiang



Inorganic Chemistry / Organic Chemistry / Physical and Theoretical Chemistry / Catalysis


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A New Type of Lewis Acid–Base Bifunctional M(salphen) (M=Zn, Cu and Ni) Catalysts for CO2 Fixation

Yanwei Ren, Jungui Chen, Chaorong Qi, and Huanfeng Jiang*[a]

A new type of Lewis acid–base bifunctional M(salphen) complexes (M=ZnII, CuII, and NiII) pending two N-methylhomopiperazine groups as nucleophiles were prepared by a one-pot method. The Zn(salphen) complexes proved to be efficient and recyclable homogeneous catalysts towards the solvent-free synthesis of cyclic carbonates from epoxides and CO2 in the absence of a co-catalyst. The catalysts can be easily recovered and five times reused without significant loss of activity and selectivity.

The utilization of carbon dioxide (CO2) as a renewable C1 feedstock for the synthesis of value added chemicals currently receives considerable attention.[1] However, the exceptional kinetically and thermodynamically stability of CO2 is a major drawback regarding its economic use as reactant. Many procedures have been developed towards the easy and economical chemical fixation of CO2. Among them, the catalytic cycloaddition of CO2 with epoxides to form cyclic carbonates, which are used as electrolytes in lithium-ion batteries, raw materials for polycarbonate, and polar aprotic solvents, is one of the most promising environment-friendly reactions for the large-scale conversion of CO2. [2] To date, various homogeneous catalytic systems, including alkali metal halides,[3] metallosalen,[2c,4] metalloporphyrins,[5] and metal-free catalyst,[6] and various heterogeneous catalytic systems, including ion-exchange resins,[7] and quaternary ammonium or phosphonium supported on carbon nanotube or functional polymers,[8] and metallosalen based metal-organic frameworks,[9] have been developed to promote this transformation.

Prominent among these are a variety of metallosalen complexes because of their ease of synthesis, while varying the steric and electronic properties about the metal centers. Over the past decades, many successful examples of metallosalen catalysts that have been developed for the preparation of cyclic carbonates include both binary[4d–l] and bifunctional systems,[4a–c] with the latter category being less developed as a probable result of the more synthetically demanding characteristics of bifunctional catalysts preparation. Nonetheless, bifunctionality (as shown in Scheme 1a, a Lewis acid (metal center) and a halide anion X (nucleophile) are required) has proven to be highly useful in various cases to create more powerful catalyst mediators. For instance, Kleij and co-workers have developed a bifunctional Zn(salpyr) [salpyr=N,N’-bis(salicylidene)-3,4-pyridinediamine] catalyst that can be alkylated at the pyridyl-N atom, providing a complex with a built-in nucleophile (either I or Br).[4a] The catalysis data support the synergistic effect of the Lewis acidic site and the halide anion nucleophile resulting in markedly improved catalytic behavior compared with a system that lacks a Lewis acid activator. Alternatively, a bifunctional Al(salen) in conjunction with intramolecular quaternary ammonium salts as cocatalysts, has also been recently prepared and successfully applied in regioselective ring opening of three-membered heterocyclic compounds (epoxides or N-substituted aziridines) in coupling reactions with

CO2, affording the corresponding five-membered cyclic products with complete configuration retention at the methine carbon.[4b]

In addition to common halide anion nucleophiles, previously

Shi and co-workers reported that a binary system involving an organic base, triethylamine used as nucleophile and binaphthyladiamion M(salen)-type complexes can efficiently catalyze reactions of epoxides with CO2, and they proposed a Lewis acid and Lewis base cocatalyzed mechanism by isotope-labeling experiments.[4l] Combined these results and our interests in creating bifunctional (rather than binary) catalyst systems that could be prepared in a few steps from readily available materials, herein we present a new type of bifunctional single-component M-salphen [salphen=N,N’-bis(salicyladehyde-o-phenylenediamine)] catalysts system (Scheme 1b) that involves tertiary amine moiety as Lewis base and metal center as Lewis acid within one structure, and examine their catalytic activity for the cycloaddition of CO2 and various epoxides. This structural design increases the stability of catalysts compared with the ionic bifunctional systems, therefore enhances their recycling performance.

As shown in Scheme 2, via one-pot method, a series of

M(salphen) (Mn=ZnII, CuII, and NiII) catalysts with electron-donating group (1a–1c) and electron-withdrawing group (1d– 1e) were synthesized and characterized by IR spectroscopy, and mass spectrometry, as well as by elemental analysis (for

Scheme 1. Cooperative activation of epoxide with bifunctional catalysts. [a] Dr. Y. Ren, J. Chen, Dr. C. Qi, Prof. Dr. H. Jiang

School of Chemistry and Chemical Engineering

South China University of Technology

Guangzhou, 510640 (P.R. China)

E-mail : jianghf@scut.edu.cn

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201500113.

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CommunicationsDOI: 10.1002/cctc.201500113 further details see the Supporting Information). These selected divalent metal ions have been proven to be useful in cyclic carbonate synthesis.[2c] Single crystal X-ray diffraction analyses for 1e reveal that the central CuII ion coordinated in a nearly square-planar geometry with two nitrogen atoms and two oxygen atoms from the deprotonated salphen ligand and the seven-membered N-methylhomopiperazine adopts a shipshape conformation locating above and below the salphen plane (Figure 1), as we expected that the coordinatively unsaturated metal center can activate the epoxides ring, meanwhile the tertiary amine of N-methylhomopiperazine attacks the less-hindered carbon atom of the coordinated epoxide.