Hydrogen-Bonding Cavities about Metal Ions:  Synthesis, Structure, and Physical Properties for a Series of Monomeric M−OH Complexes Derived from Water

Abstract

The tripodal ligand N[CH2CH2NHC(O)NHC(CH3)3]3 ([H61]) was used to synthesize a series of monomeric complexes with terminal hydroxo ligands. The complexes [Co(II/III)H31(OH)](2-/1-), [Fe(II/III)H31(OH)](2-/1-), and [Zn(II)H31(OH)](2-) have been isolated and characterized. The source of the hydroxo ligand in these complexes is water, which was confirmed with an isotopic labeling study for [Co(III)H31(OH)](1-). The synthesis of [M(II)H31(OH)](2-) complexes was accomplished by two routes. Method A used 3 equiv of base prior to metalation and water binding, affording yields of < or = 40% for [Co(II)H31(OH)](2-). When 4 equiv of base was used (method B), yields ranged from 50% to 70% for all of the M(II)H31(OH)](2-) complexes. This improvement is attributed to the presence of an intramolecular basic site within the cavity, which scavenges protons produced during formation of the M(II)-OH complexes. The molecular structures of [Zn(II)H31(OH)](2-), [Fe(II)H31(OH)](2-), [Co(II)H31(OH)](2-), and [Co(III)H31(OH)](1-) were examined by X-ray diffraction methods. The complexes have trigonal bipyramidal coordination geometry with the hydroxo oxygen trans to the apical nitrogen. The three M(II)-OH complexes crystallized with nearly identical lattice parameters, and each contains two independent anions in the asymmetric unit. The complexes have intramolecular H-bonds from the urea cavity of [H31](3-) to the coordinated hydroxo oxygen. All the complexes have long M-O(H) bond lengths (>2.00 A) compared to those of the few previously characterized synthetic examples. The longer bond distances in [M(II)H31(OH)](2-) reflect the intramolecular H-bonds in the complexes. The five-coordinate [Zn(II)H31(OH)](2-) has an average Zn-O(H) distance of 2.024(2) A, which is similar to that found for the zinc site in carbonic anhydrase II (2.05(2) A). The enzyme active site also has an extensive network of intramolecular H-bonds to the hydroxo oxygen. [Co(II)H31(OH)](2-) and [Fe(II)H31(OH)](2-) have one-electron redox processes at -0.74 and -1.40 V vs SCE. Both complexes can be chemically oxidized to yield their corresponding M(III)-OH complexes. [Co(III)H31(OH)](1-), with an S = 1 ground state, is a rare example of a paramagnetic Co(III) complex.

Isolation of Iron(II) Aqua and Hydroxyl Complexes Featuring a Tripodal H-bond Donor and Acceptor Ligand

“…The work by Borovik 12,18,19 is an elegant display of the importance of hydrogen bonding motifs in the preparation and characterization of iron complexes supported by the urea-based, tripodal ligand framework tris{(N′-tert-butylureayl)-N-ethylene]-amine. 23 Upon exposure to oxygen, formation of the iron-(II)/iron(III)−hydroxo species is observed, stabilized by the intramolecular hydrogen bonding framework. The work by Borovik has focused on intramolecular hydrogen bonds and intermolecular hydrogen bonds; 24 very bulky ligands 21,26b have also been used to stabilize monomeric Fe−OH complexes.…”

Bioinorganic Chemistry of Group 12 Complexes Supported by Tetradentate Tripodal Ligands Having Internal Hydrogen‐Bond Donors

“…[66,67] Masuda has also demonstrated that amide-appended tetradentate tripodal N 4 -donor ligands can be used to stabilize a variety of novel copper-and/or iron-hydroxo, -hydroperoxo, and -alkylperoxo species. [68][69][70][71][72][73] These latter complexes are particularly interesting in that each contains the {[6-(pivaloylamido)-2-pyridyl]methyl}amine structural component within the supporting chelate ligand. Further studies are clearly needed to determine how changes in the nature of the metal ion influence the amide-cleavage reactivity.…”

Connectivity patterns and rotamer states of nucleobases determine acid–base properties of metalated purine quartets

“…It took longer to recognize that additional factors and in particular the microenvironment around a potentially ionizable group and the medium (dielectric constant of solvent) markedly contribute to the overall effect. It was the undisputed achievement of model bioinorganic chemistry, among others, to provide a deeper understanding of this phenomenon [3][4][5][6][7][8][9]. When the field of metal-nucleic acid chemistry became popular during the second half of the last century, the topic of acidity/basicity alterations of nucleobases due to metal binding likewise started to receive attention, last but not least because of its potential impact on biorelevant features such as nucleobase tautomerism, base pairing and mispairing patterns, as well as acid-base catalysis involving nucleic acids .…”