Rigid scaffolds for the design of molecular catalysts and biomimetic active sites: A case study of anthracene-based ligands for modeling mono-iron hydrogenase (Hmd)

Abstract

Herein we examine the use of scaffold-based ligands for organometallic catalysis and bio-inorganic modeling studies. The use of scaffolds in catalyst development and complex design stems from researchers’ desire to install specific donor geometries (e.g., cis vs trans, fac vs mer) to generate desired structures and corresponding reactivities in mononuclear metal complexes. Starting from the use of polyaryl ligands for asymmetric catalysis (primarily Ru), we review successive implementations of scaffold-based ligands of ever-increasing complexity. Particular attention is paid to rigidly planar anthracene-based (and related) ligands that support both precious metal (Pd, Rh, Re) and base metal (Mn, Fe) centers in structural and reactivity studies. Previous work in scaffold design by others (Lu, Gelman) is considered in concert with our own contributions to this field. Ultimately, the complexity of such scaffolds has evolved to include non-symmetric anthracene scaffolds relevant to bio-inorganic synthetic modeling. As an illustrative example, work regarding the enzyme mono-iron hydrogenase is documented, wherein the ligand scaffold provides a biomimetic CNS chelate (containing an organometallic acyl-C donor) for structural and functional synthetic models. A quantitative analysis of structural relationships among torsion angles, donor atom distances, and bite angles of the ligand systems and resulting metal complexes is presented. This provides a foundation for a rational, target-driven syntheses of metal complexes derived from rigid, tricyclic scaffold ligands.