A convergent fourfold hydrogen-bonding network controls a remote metal centre

Noncovalent interactions in the secondary coordination sphere play a critical role in regulating metal-centred reactivity, enabling metal centres with structurally similar primary coordination spheres to carry out vastly different chemistry. Despite the recognized importance of secondary-sphere hydrogen bonds (HBs), their quantitative energetic consequences for metal-centred redox chemistry remain poorly understood. To address this gap, we report a series of iron(ii) bis(tridentate) complexes featuring systematically varied intramolecular HBs in the secondary coordination sphere. In these complexes, para-functionalized pyridyl pendants installed around a common 2,6-bis(benzimidazolyl)pyridine core establish a converging network of intramolecular N-H⋯N contacts, the strength of which is finely regulated by the electron-donating ability of remotely located substituents. Despite sharing an essentially identical primary coordination sphere, these complexes exhibit dramatic variations in solution-state magnetic moments (µ _eff = 3.52-0.92µ _B), Fe^II/III redox potential (E _1/2 = +0.41 to -0.46 V vs. Fc/Fc⁺), and metal-to-ligand charge-transfer transitions (λ _max = 558-608 nm) as a function of HB strength. These findings highlight the functional role of what is often marginalized as the secondary environment, providing a quantitative framework for understanding how noncovalent interactions tune the chemistry of metalloenzymes and informing the design of bioinspired catalysts with tailored redox properties.