Tautomerizable Flavin Ligands for Constructing Primary and Secondary Coordination Spheres

Flavins─one of nature’s most ubiquitous organic cofactors─mediate proton and electron transfers in biological systems. Their heterocyclic (iso)alloxazine cores enable such reactivity through pronounced electro- and photochemical properties, as well as hydrogen bonding with surrounding residues. To harness these features in an organometallic context, we developed a redox-active, flavin-derived bidentate ligand ( all LH ) that engages both primary and secondary coordination spheres. Coordination with Fe(II) yields an octahedral complex, ( all LH) 2 FeX 2 (X = Cl, Br, OTf), stabilized by outer-sphere hydrogen bonds between the ligand and metal-bound (pseudo)halides. Upon deprotonation, all LH undergoes tautomerization to the isoalloxazine form ( iso L ), generating a hydrogen-bonded aqua complex, ( iso L) 2 Fe(OH 2 ) 2 . Furthermore, treatment of ( all LH) 2 FeCl 2 with cobaltocene triggers ligand tautomerization, affording [( all LH)( iso L)FeCl 2 ][CoCp 2 ] and highlighting the redox-responsive nature of the flavin scaffold. This work introduces a novel approach to repurpose flavin as a multifunctional ligand platform for constructing tunable coordination environments around transition metal centers, offering new opportunities in ligand design and bioinspired reactivity.