ONO pincer type ligand complexes of Al(III) as efficient catalyst for chemical fixation of CO₂ to epoxides at atmospheric pressure

Highlights

• Hydrazone-based ONO pincer mono-nuclear Al complexes were synthesized and characterized.

• These ONO-pincer Al-complexes are excellent catalyst for the fixation of CO₂.

• The coupling was catalyzed at atmospheric pressure with the highest conversion.

• For the mechanistic investigation intermediate species were detected.

Abstract

Carbon dioxide, the main cause of environmental pollution, its utilization to produce valuable products is of utmost interest. A series ONO pincer hydrazone based most active mono-nuclear Al(III) complexes were successfully synthesized and characterized with the help of NMR, IR, mass spectrometry and only complex 2a was confirmed by single-crystal analysis. The synthesized Al(III) complexes were then employed as capable catalysts for the solvent-free chemical fixation of CO₂ with epoxides at atmospheric pressure and could be reused five times without loss of any catalytic activity. In addition, the catalytic mechanism was investigated by analyzing intermediates via ¹H NMR, ¹³C NMR, and mass MALDI-TOF. The excellent catalytic performance could be due to simultaneous attack and the opening of the epoxide by metal centers to form an alkoxide ion which activates the CO₂ the same time.

Introduction

Worldwide industrialization and rapid urbanization are posing serious threats to our environment due to the large-scale anthropogenic production of CO_2, which is greatly affecting the concept of green chemistry. Global warming and greenhouse effect are mainly originating from the CO₂ emission¹. Nevertheless, the scientific community is taking a great and ever-increasing interest in the CO₂ transformation into valuable chemical products. Because of its abundance, ready availability, non-toxic and reusable nature, CO₂ is a valuable one-carbon (C1) building block in organic transformations [1], [2], [3]. As a well-known fact, CO₂ has a high thermodynamic stability and kinetic inertness due to the carbon is in its highest oxidation state hence making it a challenge to activate and valorise CO₂ to valuable products. Among the various approaches employed for CO₂ utilization, synthesis of cyclic carbonates and polycarbonates from the chemical fixation of CO₂ to epoxides, are extensively studied and even well commercialized [2], [4], [5]. Environment-friendly and 100% atom efficient, the redox neutral fixation of CO₂ to epoxides to form cyclic carbonates with no appreciable toxicity [6], [7], [8], [9] is specifically investigated in recent years because of its numerous uses as solvents, electrolytes, synthetic building blocks, fine chemicals, industrial lubricants and as monomers for polymers [10], [11], [12], [13], [14]. Over the past two decades, huge progress has been made in the chemical fixation of CO₂ to epoxides which can be clearly observed from the increase in the number of publications since 2000 [15]. Various metal complexes based on zinc [16], [17], [18], [19], chromium [20], [21], [22], iron [23], [24], [25], [26], cobalt [27], [28], [29], [30], [31], [32], [33], magnesium [17], [34], vanadium [35], [36], [37], tin [38] etc. have been reported as catalyst for the CO₂ cycloaddition to epoxides. Among the metal complexes, aluminium has been extensively studied for the CO₂/epoxides cycloaddition. This due to its environment-friendly nature, earth abundance, nontoxicity, and high Lewis acidity making aluminium the ideal choice for the ring-opening of epoxides [39]. Various research groups reported the CO₂/epoxides cycloaddition with salen and related ligand-based monometallic [27], [40], [41], [42], [43], [44] and bimetallic [39], [45], [46], [47], [48], [49], [50] aluminium complexes. Amongst the aluminium complexes, bimetallic aluminium(salen) complexes [51], [52] reported by North and other groups [50] are efficient catalysts for the CO₂ cycloaddition at ambient pressure. The most active catalyst amongst the mononuclear Al complexes is the hexachlorinated aluminium(III)–aminetriphenolate complex, reported by Kleij et al., in combination with tetrabutylammonium iodide as co-catalyst demonstrating the highest TOF of 24,000 h⁻¹ for the cycloaddition of CO₂ to 1,2-epoxy-hexane. This high TOF was obtained at 90 °C under a high pressure of 10 bar along with a high catalyst to co-catalyst ratio [39]. Similarly, the highest ratio of cat/co-cat (1:120) was reported by Qin et al. using the porphyrin ligand-based aluminium complex for synthesis of cyclic carbonates at a relatively high pressure of 10 bar [53]. Unfortunately, the synthesis of the porphyrin ligand requires a very long and tedious synthetic procedure. Till today, the reported literature suggests that most of the aluminium complexes studied for cycloaddition of CO₂ to epoxides are based on salen, salophen and phenolate scaffolds. Keeping these factors in minds, we hereby report a novel, highly active monometallic aluminium complexes supported by an ONO pincer type hydrazone moiety for the chemical fixation of CO₂ at atmospheric pressure. Furthermore, NMR and MALDI-TOF analyses were used to investigate the mechanistic insight of the CO₂/epoxides coupling reaction catalyzed by these Al complexes.