Person:

King, Evan

Transition metal complexes (Mn f Zn) of the dipyrromethane ligand, 1,9-dimesityl-5,5- dimethyldipyrromethane (dpm), have been prepared. Arylation of the dpm ligand R 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.

In this Communication, we report an intramolecular C-H bond amination reaction of a dipyrromethene ferrous complex with organic azides. Monitoring of the spectral changes (variable temperature NMR and UV-vis) of the FeII complex reveals no buildup of an intermediate during conversion of the starting material into the nitrene-inserted product. The rate-determining step appears to be azide addition to the 14-electron FeII complex, hinting at the potential that these and related platforms may have to effect atom and group-transfer processes.