Design, synthesis, and characterization of hybrid metal-ligand hydrogen-bonded (MLHB) supramolecular architectures.

Despite the prevalence of supramolecular architectures derived from metal-ligand or hydrogen-bonding interactions, few studies have focused on the simultaneous use of these two strategies to form discrete assemblies. Here we report the use of a supramolecular tecton containing both metal-binding and self-complementary hydrogen-bonding interactions that upon treatment with metal precursors assembles into discrete hybrid metal-ligand hydrogen-bonded assemblies with closed topology. (1)H NMR DOSY experiments established the stability of the structures in solution, and the measured hydrodynamic radii match those determined crystallographically, suggesting that the closed topology is maintained both in solution and in the solid state. Taken together, these results demonstrate the validity of using both hydrogen-bonding and metal-ligand interactions to form stable supramolecular architectures.

Selection for a Single Self-Assembled Macrocycle from a Hybrid Metal-Ligand Hydrogen-Bonded (MLHB) Ligand Subunit.

To expand the interface between self-assembled metal-ligand and hydrogen-bonded architectures, here we report the preparation, self-assembly, and metal-ligand binding of a pyridyl quinolone ligand (5-PYQ). The 5-PYQ ligand self-associates through quinolone hydrogen bonding, and it binds to metal centers through the pyridine ligand component. As a first step toward investigating more-complex hybrid metal-ligand hydrogen-bonded (MLHB) architectures, we report investigations of 5-PYQ with mono- and bis-platinated anthracene precursors. These results demonstrate that the 5-PYQ ligand maintains hydrogen bonding interactions while binding to square-planar platinum centers, but that generation of coordination compounds with closed topology erodes the hydrogen bonding fidelity to favor ambidentate coordination modes of the 5-PYQ ligand.

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-(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.

Chemically reversible reactions of hydrogen sulfide with metal phthalocyanines.

Hydrogen sulfide (H2S) is an important signaling molecule that exerts action on various bioinorganic targets. Despite this importance, few studies have investigated the differential reactivity of the physiologically relevant H2S and HS(-) protonation states with metal complexes. Here we report the distinct reactivity of H2S and HS(-) with zinc(II) and cobalt(II) phthalocyanine (Pc) complexes and highlight the chemical reversibility and cyclability of each metal. ZnPc reacts with HS(-), but not H2S, to generate [ZnPc-SH](-), which can be converted back to ZnPc by protonation. CoPc reacts with HS(-), but not H2S, to form [Co(I)Pc](-), which can be reoxidized to CoPc by air. Taken together, these results demonstrate the chemically reversible reaction of HS(-) with metal phthalocyanine complexes and highlight the importance of H2S protonation state in understanding the reactivity profile of H2S with biologically relevant metal scaffolds.