http://www.chemie.uni-muenchen.de/ac/kluefers/homepage/L/bac/ca_2004.pdf

Synthetic analogues relevant to the structure and function of zinc enzymes.

Hydrogen bonds influence secondary coordination spheres around metal ions in many proteins. To duplicate these features of molecular architecture in synthetic systems, urea-based ligands have have been developed that create rigid organic frameworks when bonded to metal ions. These frameworks position hydro-gen bond donors proximal to metal ion(s) to form specific chem-ical microenvironments. Iron(II) and manganese(II) complexes with constrained cavities activate O2, yielding MIII (MIII = Fe and Mn) complexes with terminal oxo ligands. Installation of anionic sites within the cavity assists the formation of complexes with MII/III−OH and MIII−O units derived directly from water. Opening the cavity promotes M(μ-O)2M rhombs, as illustrated by isolation of a cobalt(III) analogue, the stability of which is promoted by the hydrogen bonds surrounding the bridging oxo ligands.

Bioinspired hydrogen bond motifs in ligand design: the role of noncovalent interactions in metal ion mediated activation of dioxygen.

Non-covalent interactions in the second coordination sphere of metalloenzymes play a significant role in determining activity and selectivity. The importance of these interactions is reflected in the increasing number of reports on model complexes with a functional second coordination sphere. These hybrid mimics were developed according to the strategies of modification with functional groups, combination with supramolecular hosts and coupling with polymers, to shed light on the role of non-covalent interactions in metalloenzyme catalysis. This review provides an overview of recent progress in this area. An introduction to native metalloenzymes with an emphasis on the second coordination sphere is also given.

Insights into metalloenzyme microenvironments: biomimetic metal complexes with a functional second coordination sphere.

Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear FeIII and MnIII complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N‘-tert-butylureaylato)-N-ethyl]aminato ([H3 1]3-), which creates a protective hydrogen bond cavity around the MIII−O(H) units (MIII = Fe and Mn). The MIII−O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the MIII−O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [FeIIIH3 1(O)]2- complex also can be made using sulfoxides. These findings support the proposal of a high valent MIV−oxo species as an interme...

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Utilization of hydrogen bonds to stabilize M-O(H) units: synthesis and properties of monomeric iron and manganese complexes with terminal oxo and hydroxo ligands.

The combination of transition metal ions and hydrogen-bonding interactions

The synthesis and properties of mononuclear zinc methoxide ([(ebnpa)Zn-OCH3]ClO4) (1) and hydroxide ([(ebnpa)Zn-OH]ClO4) (2) complexes of a new mixed nitrogen/sulfur ligand (ebnpa = N-2-(ethylthio)ethyl-N,N-bis(6-neopentylamino-2-pyridylmethyl)amine) are reported. The structures of 1 and 2 were determined by X-ray diffraction. Each possesses a single zinc-coordinated anion (methoxide or hydroxide) and exhibits an overall trigonal bipyramidal geometry. Structural and spectroscopic studies indicate the presence of two hydrogen-bonding interactions involving the oxygen atom of the zinc-bound anion in each complex. Treatment of [(ebnpa)Zn-OH]ClO4 with CH3OH results in the formation of an equilibrium mixture of 1 and 2. 1H NMR spectroscopic methods were used to examine the equilibrium as a function of temperature, yielding KMe (304 K) = 0.30(8), DeltaHMe = -0.9(1) kcal/mol, and DeltaSMe = -5(1) eu. The negative enthalpy indicates that spontaneous zinc alkoxide formation from a hydroxide precursor occurs in this system at low temperature. Using the experimentally determined DeltaHMe value, we found the homolytic Zn-O bond dissociation energy (BDE) in the Zn-OCH3 unit to be approximately -14 kcal/mol relative to the Zn-O BDE in the Zn-OH unit.

Mononuclear nitrogen/sulfur-ligated zinc methoxide and hydroxide complexes: investigating ligand effects on the hydrolytic stability of zinc alkoxide species.

Lisa M. Bezold

Papers

Mononuclear nitrogen/sulfur-ligated zinc methoxide and hydroxide complexes: investigating ligand effects on the hydrolytic stability of zinc alkoxide species.