Crystal engineering of new lanthanide coordination polymers based on 3,3′,5,5′-azobenzenetetracarboxylic acid for use as catalysts

Abstract

To obtain high performance acid-base catalysts for use in the cycloaddition reactions of CO2 with ECH, four series of new LnCPs of H4abtc, including; series I: [LnIII(Habtc)(H2O)2]·2H2O (LnIII = SmIII (Ia), EuIII (Ib), GdIII (Ic), TbIII (Id), DyIII (Ie), HoIII (If), ErIII (Ig), TmIII (Ih), YbIII (Ii)), series II: [LnIII(Habtc)(H2O)4]·3H2O, (LnIII = EuIII (IIb), GdIII (IIc), TbIII (IId), DyIII (IIe), HoIII (IIf), TmIII (IIh)), series III: [EuIII2(Habtc)2(H2O)6]·2.75H2O, and series IV: [LnIII2(Habtc)2(DMSO)4]·(DMSO)·(H2O) (LnIII = SmIII (IVa), EuIII (IVb)), have been designed, synthesized and characterized. Their single crystal structures were elucidated and employed in the rationalization of the apparent catalytic activities. Evidently, the subtle change in the synthetic conditions imparted an immense affect on the yielded LnCPs. Based on the hydrothermal synthesis, the change in type of LnIII, reaction temperature and time as well as the cooling program led to diverse degrees of water included within the frameworks and consequently their framework architecture and porosity. With the addition of 3-nitrophthalic acid, series I were commonly yielded as pure phases disregarding the cooling program. The use of EuIII resulted in pure phases of Ib and III depending on the cooling program while IIb was obtained in the absence of 3-nitrophthalic acid. In contrast to series I, series II and III could be obtained only under specific conditions. Founded on these experimental results, we concluded that series I is thermodynamically and kinetically favorable than series II and III. The solvothermal synthesis using the mixed solvents made up of water and DMSO, on the other hand, led to the inclusion of DMSO in the framework, i.e. series IV. The presence of DMSO further diversified architecture and porosity of the yielded framework. These framework structures can however be transformed to each other under appropriate conditions, some of which are completely reversible. The reversible IVb ⇌ Ib and IVb ⇌ IIb and 13 irreversible IIb → Ib transformations have been demonstrated. Apparently, Ib is exceptionally stable that once it was formed, it was difficult or impracticable to convert to the other frameworks due to the most crowded environmental of LnIII node, the highest degree of linkage and the most condensed framework among all. Such disparity in reversibility and rapidity can be rationalized by coordination environment of the lanthanides, coordination moods of Habtc3- ligand, degree of framework stability, similarity in framework architecture and registration. With respect to the cycloaddition reactions of CO2 with ECH, the catalytic activities of the Ib, IIb and IVb CPs were evaluated and compared (Table 3.1). The excellent performance of all the CPs were revealed. The best performance was however achieved from IVb with the maximum turn over number and turn over frequency of 7,682 and 1,921 h−1 with activation and 7,142 and 1,768 h−1 without the activation. The fact that IVb showed excellent catalytic activities without the need to generate unoccupied coordination sites implies the direct substitution of the labile DMSO by the reagent of the reaction. The coordination of labile ligands such as DMSO to LnIII instead of water may therefore be an alternative approach to further simplify the catalysis procedure. This is extremely intriguing as the application of IVb without prior activation is a steppingstone from the practical viewpoint. In addition, all materials were also shown to be robust over ten cycles of catalysis and work-up process. However, transformation of IVb to IIb and Ib during the recycle apparently did not bother the catalytic performance.