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 (^<b>all</b><b>LH</b>) that engages both primary and secondary coordination spheres. Coordination with Fe(II) yields an octahedral complex, <b>(</b>^<b>all</b><b>LH)</b>_<b>2</b><b>FeX</b>_<b>2</b> (X = Cl, Br, OTf), stabilized by outer-sphere hydrogen bonds between the ligand and metal-bound (pseudo)halides. Upon deprotonation, ^<b>all</b><b>LH</b> undergoes tautomerization to the isoalloxazine form (^<b>iso</b><b>L</b>), generating a hydrogen-bonded aqua complex, <b>(</b>^<b>iso</b><b>L)</b>_<b>2</b><b>Fe(OH</b>_<b>2</b><b>)</b>_<b>2</b>. Furthermore, treatment of <b>(</b>^<b>all</b><b>LH)</b>_<b>2</b><b>FeCl</b>_<b>2</b> with cobaltocene triggers ligand tautomerization, affording <b>[(</b>^<b>all</b><b>LH)(</b>^<b>iso</b><b>L)FeCl</b>_<b>2</b><b>][CoCp</b>_<b>2</b><b>]</b> 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.