Exploring the Strength of the H-Bond in Synthetic Models for Heme Proteins: The Importance of the N−H Acidity of the Distal Base

The distal hydrogen bond (H‐bond) in dioxygen‐binding proteins is crucial for the discrimination of O2 with respect to CO or NO. We report the preparation and characterization of a series of ZnII porphyrins, with one of three meso‐phenyl rings bearing both an alkyl‐tethered proximal imidazole ligand and a heterocyclic distal H‐bond donor connected by a rigid acetylene spacer. Previously, we had validated the corresponding CoII complexes as synthetic model systems for dioxygen‐binding heme proteins and demonstrated the structural requirements for proper distal H‐bonding to CoII‐bound dioxygen. Here, we systematically vary the H‐bond donor ability of the distal heterocycles, as predicted based on pKa values. The H‐bond in the dioxygen adducts of the CoII porphyrins was directly measured by Q‐band Davies‐ENDOR spectroscopy. It was shown that the strength of the hyperfine coupling between the dioxygen radical and the distal H‐atom increases with enhanced acidity of the H‐bond donor. The distal hydrogen bond in dioxygen‐binding proteins is crucial for the discrimination of O2 with respect to CO or NO. The H‐bond donor ability of distal heterocycles, as predicted based on pKa values, has been varied systematically. The H‐bond in the dioxygen adducts of the CoII porphyrins was directly measured by Q‐band Davies‐ENDOR spectroscopy, and it was shown that the strength of the hyperfine coupling between the dioxygen radical and the distal H‐atom increases with enhanced acidity of the H‐bond donor.