Probing the Protonation State and the Redox-Active Sites of Pendant Base Iron(II) and Zinc(II) Pyridinediimine Complexes

Utilizing
the pyridinediimine ligand [(2,6-iPrC6H3)­NCMe)­(N­(iPr)2­C2H4)­NCMe)­C5H3N] (didpa), the zinc­(II) and iron­(II) complexes Zn­(didpa)­Cl2 (1), Fe­(didpa)­Cl2 (2), [Zn­(Hdidpa)­Cl2]­[PF6] (3), [Fe­(Hdidpa)­Cl2]­[PF6] (4), Zn­(didpa)­Br2 (5), and [Zn­(Hdidpa)­Br2]­[PF6] (6), Fe­(didpa)­(CO)2 (7), and [Fe­(Hdidpa)­(CO)2]­[PF6] (8) were synthesized and characterized.
These complexes allowed for the study of the secondary coordination
sphere pendant base and the redox-activity of the didpa ligand scaffold.
The protonated didpa ligand is capable of forming metal halogen hydrogen
bonds (MHHBs) in complexes 3, 4, and 6. The solution behavior of the MHHBs was probed via pKa measurements and 1H NMR titrations of 3 and 6 with solvents of varying H-bond accepting
strength. The H-bond strength in 3 and 6 was calculated in silico to be 5.9 and 4.9 kcal/mol,
respectively. The relationship between the protonation state and the
ligand-based redox activity was probed utilizing 7 and 8, where the reduction potential of the didpa scaffold was
found to shift by 105 mV upon protonation of the reduced ligand in
Fe(didpa)(CO)2.