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 α to the pyrrolic nitrogen donors limits the accessibility of the pyrrole π-electrons for transition metal coordination, instead forcing η1,η1 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)MnII(py)2, (dpm)FeII(py)2, and (dpm)CoII(py)2 reveal each divalent ion to be high-spin and pseudotetrahedral in the solid state, whereas the (dpm)NiII(py)2 is low-spin and adopts a square-planar geometry. Differential pulse voltammetry on the (dpm)MII(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)MII(py)2 complexes.