Influence of H-bond strength on chelate cooperativity

Intermolecular complexes formed between metalloporphyrins and pyridine ligands equipped with multiple H-bond donors and acceptors have been used to measure the free energy contributions due to intramolecular ether–phenol H-bonding in the 24 different supramolecular architectures using chemical double mutant cycles in toluene. The ether–phenol interactions are relatively weak, and there are significant populations of partially bound states where between zero and four intramolecular H-bonds are made in addition to the porphyrin–ligand coordination interaction. The complexes were analyzed as ensembles of partially bound states to determine the effective molarities for the intramolecular interactions by comparison with the corresponding intermolecular ether–phenol H-bonds. The properties of the ether–phenol interactions were compared with phosphonate diester–phenol interactions in a closely related ligand system, which has more powerful H-bond acceptor oxygens positioned at the same location on the ligand framework. This provides a comparison of the properties of weak and strong H-bonds embedded in the same 24 supramolecular architectures. When the product of the intermolecular association constant and the effective molarity KEM > 1, there is a linear increase in the free energy contribution due to H-bonding with log EM, because the intramolecular interactions contribute fully to the stability of the complex. When KEM < 1, the H-bonded state is not significantly populated, and there is no impact on the overall stability of the complex. Intermolecular phosphonate diester–phenol H-bonds are 2 orders of magnitude stronger than ether–phenol H-bonds in toluene, so for the phosphonate diester ligand system, 23 of the 24 supramolecular architectures make intramolecular H-bonds. However, only 8 of these architectures lead to detectable H-bonding in the ether ligand system. The other 15 complexes have a suitable geometry for formation of H-bonds, but the ether–phenol interaction is not strong enough to overcome the reorganization costs associated with making intramolecular contacts, i.e., KEM < 1 for the ether ligands, and KEM > 1 for the phosphonate diester ligands. The values of EM measured for two different types of H-bond acceptor are linearly correlated, which suggests that EM is a property of the supramolecular acrchitecture. However, the absolute value of EM for an intramolecular phosphonate diester H-bond is about 4 times lower than the corresponding value for an intramolecular ether–phenol interaction embedded in the same supramolecular framework, which suggests that there may be some interplay of K and EM.