Influence of intramolecular secondary sphere hydrogen-bonding interactions on cytochrome c oxidase inspired low-spin heme–peroxo–copper complexes

Abstract

Dioxygen reduction by heme–copper oxidases is a critical biochemical process, wherein hydrogen bonding is hypothesized to participate in the critical step involving the active-site reductive cleavage of the O–O bond. Sixteen novel synthetic heme–(μ-O₂²⁻)–Cu(^XTMPA) complexes, whose design is inspired by the cytochrome c oxidase active site structure, were generated in an attempt to form the first intramolecular H-bonded complexes. Derivatives of the “parent” ligand (^XTMPA, TMPA = (tris((2-pyridyl)methyl)amine)) possessing one or two amine pendants preferentially form an H-bond with the copper-bound O-atom of the peroxide bridge. This is evidenced by a characteristic blue shift in the ligand-to-metal charge transfer (LMCT) bands observed in UV-vis spectroscopy (consistent with lowering of the peroxo π* relative to the iron orbitals) and a weakening of the O–O bond determined by resonance Raman spectroscopy (rR), with support from Density Functional Theory (DFT) calculations. Remarkably, with the TMPA-based infrastructure (versus similar heme–peroxo–copper complexes with different copper ligands), the typically undetected Cu–O stretch for these complexes was observed via rR, affording critical insights into the nature of the O–O peroxo core for the complexes studied. While amido functionalities have been shown to have greater H-bonding capabilities than their amino counterparts, in these heme–peroxo–copper complexes amido substituents distort the local geometry such that H-bonding with the peroxo core only imparts a weak electronic effect; optimal H-bonding interactions are observed by employing two amino groups on the copper ligand. The amino-substituted systems presented in this work reveal a key orientational anisotropy in H-bonding to the peroxo core for activating the O–O bond, offering critical insights into effective O–O cleavage chemistry. These findings indirectly support computational and protein structural studies suggesting the presence of an interstitial H-bonding water molecule in the CcO active site, which is critical for the desired reactivity. The results are evaluated with appropriate controls and discussed with respect to potential O₂-reduction capabilities.