Ligand Mediated Metal‐Metal and Metal‐Ligand Cooperativity in Cesium Coordination Polymers and its Beneficial Role in the ROP of Cyclic Esters

Designing a multimetallic catalyst based on the principle of metalloenzymes has emerged as a promising route to enhance catalytic performance owing to the combined action of different metal centers in a cooperative fashion. The cooperativity of metal centers with the surrounding ligand environment in metalloenzymes plays a crucial role in driving efficient catalysis. However, the replication of such cooperativity in a synthetic system is very challenging. In this work, we report the synthesis and characterization of diverse cesium coordination polymeric networks, a 2D sheet, [Cs{κ²-C₄H₃N-2(CH = N(2,6-iPr₂C₆H₃))}]_∞ (1), an infinite ladder chain, [Cs{κ³-C₄H₃N-2(CH = N(CHPh₂))}]_∞ (2), and a 3D cage [Cs₂{κ³-(C₄H₃N-2(CH = NCH₂CH₂N(CH₂CH₂)₂O))}]_∞ (3). This was accomplished by a variation of the substituent on the iminopyrrolyl ligand. Moreover, we studied a breakdown of the 2D sheet coordination polymer (1) into contact pair monomeric cesium complex [(18-Cr-6)Cs(κ²-C₄H₃N-2(CH = N(2,6-iPr₂C₆H₃)))] (4). In the coordination polymers, multiple Cs centers are held together within a proximal distance (3.5-6 Å) via an iminopyrrolyl ligand. The Cs coordination polymer 2 displays the highest catalytic efficiency in ring opening polymerization of rac-lactide (ROP), giving a turnover frequency (TOF) value of 49.5 h^-1 compared to 15.7 h^-1 and 24.0 h^-1 for 1 and 3. The higher reactivity of 2 in comparison to 1 and 3 is due to the metal-metal cooperativity in cesium coordination polymeric networks.