Solvent effects on chelate cooperativity

Abstract

High-throughput UV/Vis absorption titrations have been used to characterise the properties of 120 different zinc porphyrin–pyridine complexes that contain between zero and four additional intramolecular H-bond interactions. Chemical double mutant cycles were used to measure the free energy contribution of each H-bond in each complex and hence determine the effective molarities (EM) for the intramolecular interactions. The experiments have been carried out in 1,1,2,2-tetrachloroethane (TCE) and previously in toluene. For an ester-phenol H-bond, the strength of the intermolecular H-bond does not change with solvent, and the values of EM for formation of intramolecular interactions are also solvent-independent. Intermolecular phosphonate diester–phenol H-bonds are one order of magnitude stronger than ester–phenol H-bonds in TCE and two orders of magnitude stronger in toluene. However, the differences in intermolecular interaction strength are not fully expressed in the intramolecular interactions. Thus the values of EM measured in TCE are up an order of magnitude larger than the values measured for the same complexes in toluene. This suggests that there may be some internal compensation in cooperative recognition interfaces, where stronger interactions lead to more organised structures reducing the value of EM. There is also evidence of structure specific effects due to differences in the details of the solvation shells of the complexes.