Unusual electronic structure of first row transition metal complexes featuring redox-active dipyrromethane ligands

Transition metal complexes (Mn --> Zn) of the dipyrromethane ligand, 1,9-dimesityl-5,5-dimethyldipyrromethane (dpm), have been prepared. Arylation of the dpm ligand alpha to the pyrrolic nitrogen donors limits the accessibility of the pyrrole pi-electrons for transition metal coordination, instead forcing eta(1),eta(1) coordination to the divalent metal series as revealed by X-ray diffraction studies. Structural and magnetic characterization (SQUID, EPR) of the bis-pyridine adducts of (dpm)Mn(II)(py)(2), (dpm)Fe(II)(py)(2), and (dpm)Co(II)(py)(2) reveal each divalent ion to be high-spin and pseudotetrahedral in the solid state, whereas the (dpm)Ni(II)(py)(2) is low-spin and adopts a square-planar geometry. Differential pulse voltammetry on the (dpm)M(II)(py)(2) series reveals a common two-electron oxidation pathway that is entirely ligand-based, invariant to the divalent metal-bound, its geometry or spin state within the dpm framework. This latter observation indicates that fully populated ligand-based orbitals from the dpm construct lie above partially filled metal 3d orbitals without intramolecular redox chemistry or spin-state tautomerism occurring. DFT analysis on this family of complexes corroborates this electronic structure assignment, revealing that the highest lying molecular orbitals are completely ligand-based. Chemical oxidation of the deprotonated dpm framework results in the four-electron oxidation of the dipyrrolide framework, although this oxidation product was not observed either in the electrochemical or chemical oxidation of the (dpm)M(II)(py)(2) complexes.