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

Abstract

Utilizing the pyridinediimine ligand [(2,6-(i)PrC6H3)N═CMe)(N((i)Pr)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 (1)H 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.

A guide to secondary coordination sphere editing

“…In acetonitrile, the pK a of the pendant group was estimated to be 18.4, as compared to Hu ¨nig's base (pK a = 18.8) (Scheme 6). 24 Diagnostic of a SCS interaction, the 1 H NMR of a diamagnetic Zn analogue showed a resonance at d H = 8.50 ppm, consistent with a hydrogen bonding interaction. Using a related dicarbonyl complex, [Fe( DiPrA PDI)(CO) 2 ], the authors estimated an approximate 105 mV change in reduction potential upon protonation.…”

“…In a series of contributions, Gilbertson and co-workers outlined the iron coordination chemistry of a pyridinediimine pincer ligand containing a pendant N,N-dialkylamino substituent. [24][25][26] Protonation of this pendant using [NH 4 ]PF 6 , provided an ammonium group, which allowed for SCS interactions with an iron-bound chloride or bromide. For the Zn analogue, this H-bonding interaction was estimated to be on the order of 5.9 kcal mol À1 (for H-Cl) and 4.9 kcal mol À1 (for H-Br).…”

Hemilabile Proton Relays and Redox Activity Lead to {FeNO}x and Significant Rate Enhancements in NO2– Reduction

“…41–47 Our group has been active in developing methodologies that control the movement of both protons and electrons for biologically relevant reactions by utilizing the redox-active pyridinediimine (PDI) scaffold merged with a proton-responsive secondary coordination sphere. 48–50 Those preliminary studies successfully showed: 1) the PDI scaffold can be tailored to facilitate NO 2 − reduction, and; 2) the NO 2 − reduction is dependent on the protonation state of the secondary coordination sphere (proton-responsivity). Furthermore, given the revelations of the role ligand hemilability plays in biomimicry, 51–53 we reasoned that the pendant base(s) could be utilized to introduce hemilability into the PDI, yielding ligand scaffolds that increase activity even further.…”

Uncoupled Redox-Inactive Lewis Acids in the Secondary Coordination Sphere Entice Ligand-Based Nitrite Reduction

“…We have been utilizing the PDI scaffold, which contains ligand-based redox sites to demonstrate a different paradigm, that of uncoupling the redox-activity from the effective “charge state” of the secondary coordination sphere. 70 This is achieved because of the redox-activity of PDI ligand scaffold, which allows the generation of highly reduced complexes, where the protonation state of the pendant base is very weakly coupled to the ligand-based reduction potential (Δ E 1/2 = 105 mV; see Figure 4, left). These integrated (yet uncoupled) complexes are capable of storing both protons and electrons for transfer to substrate.…”