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-(PrC6H3)-Pr-i)N=CMe)(N(Pr-i)(2)C2H4)N=CMe)C5H3N] (didpa), the zinc(II) and iron(II) complexes Zn(didpa)Cl-2 (1), Fe(didpa)Cl-2 (2), [Zn(Hdidpa)Cl-2] [PF6] (3), [Fe(Hdidpa)Cl-2][PF6] (4), Zn(didpa)Br-2 (5), and [Zn(Hdidpa)Br-2][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 H-1 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 silica 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).