Zinc(II), Copper(II), and Nickel(II) Complexes of Bis(tripodal) Diamide Ligands – Reversible Switching of the Amide Coordination Mode upon Deprotonation

Abstract

A series of dinuclear Zn(II), Cu(II) and Ni(II) complexes of two new bis-tripodal ligands with aromatic (H 2 -4a) and aliphatic (H 2 -4b) diamide spacers has been synthesized and structurally characterized. Each of the two tripodal entities of the neutral dinucleating ligands coordinates to one metal ion via three amine functions and a carbonyl oxygen atom of the amide groups. Either trifluoroacetate counterions or solvent molecules complete the trigonal-bipyramidal (Zn, 5a), square-pyramidal (Cu, 6a and 6b), and octahedral (Ni, 7a and 7b) coordination environment at the metal centers. Complexes 5a, 6a, and 7a with the phenylene-bridged diamide ligand are readily deprotonated by potassium tert-butoxide effecting a rearrangement of the dinuclear complexes. In the resulting zinc and copper complexes 8 and 9, one of the metal centers is coordinated by three amine functions of a tripodal subunit and the two amidate nitrogen atoms of the deprotonated ligand 4a 2-while the coordination geometry of

Metal‐Templated, Tight Loop Conformation of a Cys‐X‐Cys Biomimetic Assembles a Dimanganese Complex

“…The synthetic paths to complex 5 emphasize metal exchange processes in intricate proton/metal competitions that are likely related to synthetic routes to metalloproteins and enzymes. Engagement of carboxamide oxygens as donor sites for zinc in peptide bioconjugates have been observed by the Alsfasser group, while Hahn et al., have reported a pH dependent, reversible switching of the N/O amide coordination modes in tripodal complexes of Zn II , Cu II , and Ni II . More closely related to our discovery are well‐designed studies via infrared multiple photon dissociation spectroscopy and DFT calculations from Dunbar et al., who explored the “basic tension” in small peptides between deprotonated amido nitrogens that bind to metals vs. the tautomeric, carboxy bound form …”

“…While the mimics of biological N 2 S 2 binding sites,i ncluding the ema ligand have been studied for decades,t his example of H 2 ema 2À as ab inucleating ligand is unknown or unreported until now.T he synthetic paths to complex 5 emphasize metal exchange processes in intricate proton/metal competitions that are likely related to synthetic routes to metalloproteins and enzymes.E ngagement of carboxamide oxygens as donor sites for zinc in peptide bioconjugates have been observed by the Alsfasser group, [16] while Hahn et al, have reported apHdependent, reversible switching of the N/ Oa mide coordination modes in tripodal complexes of Zn II , Cu II ,a nd Ni II . [17] More closely related to our discovery are well-designed studies via infrared multiple photon dissociation spectroscopy and DFT calculations from Dunbar et al, who explored the "basic tension" in small peptides between deprotonated amido nitrogens that bind to metals vs.t he tautomeric,carboxy bound form. [18] Theg reat stability of the dimanganese complex, yet with ligand exchange capability that implicates the likelihood of opening ac oordination site,o ffers an ew platform for examining bimetallic reactivity.The key factor that influences the folding of the free ligand is the preferred coordination environment of the targeting transition metal that promotes the iminol tautomer and lies at the center of the competition of metallation vs.protonation determining the folding behavior of Kruger, Peng, and Holmsema ligand.…”

Metal‐Templated, Tight Loop Conformation of a Cys‐X‐Cys Biomimetic Assembles a Dimanganese Complex

“…While the mimics of biological N 2 S 2 binding sites,i ncluding the ema ligand have been studied for decades,t his example of H 2 ema 2À as ab inucleating ligand is unknown or unreported until now.T he synthetic paths to complex 5 emphasize metal exchange processes in intricate proton/metal competitions that are likely related to synthetic routes to metalloproteins and enzymes.E ngagement of carboxamide oxygens as donor sites for zinc in peptide bioconjugates have been observed by the Alsfasser group, [16] while Hahn et al, have reported apHdependent, reversible switching of the N/ Oa mide coordination modes in tripodal complexes of Zn II , Cu II ,a nd Ni II . [17] More closely related to our discovery are well-designed studies via infrared multiple photon dissociation spectroscopy and DFT calculations from Dunbar et al, who explored the "basic tension" in small peptides between deprotonated amido nitrogens that bind to metals vs.t he tautomeric,carboxy bound form. [18] Theg reat stability of the dimanganese complex, yet with ligand exchange capability that implicates the likelihood of opening ac oordination site,o ffers an ew platform for examining bimetallic reactivity.The key factor that influences the folding of the free ligand is the preferred coordination environment of the targeting transition metal that promotes the iminol tautomer and lies at the center of the competition of metallation vs.protonation determining the folding behavior of Kruger, Peng, and Holmsema ligand.…”