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{κ-CHN-2(CH = N(2,6-iPrCH))}] (1), an infinite ladder chain, [Cs{κ-CHN-2(CH = N(CHPh))}] (2), and a 3D cage [Cs{κ-(CHN-2(CH = NCHCHN(CHCH)O))}] (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(κ-CHN-2(CH = N(2,6-iPrCH)))] (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 compared to 15.7 h and 24.0 h 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.