Two novel
low-spin complexes,
[Et4N][FeL2]·1.5H2O
(1) and
[Et4N][CoL2]·2H2O
(2) (L is a deprotonated bis-amide ligand), have been
synthesized and characterized. Four relatively longer equatorial
M−Namide bonds and two significantly shorter axial
M−Npy bonds are the noteworthy features of their X-ray
structures. EPR and (34−300 K) magnetic studies of 1
confirm its low−spin character. Cyclic voltammetric studies
reveal a highly stabilized M(III) state. For 1 a linear
correlation between the Fe(III)−Fe(II) reduction potentials
and the reciprocal of solvent dielectric constants is
obtained.
A single hydrogen bond between an amide N-H and a thiolate sulfur in model complexes designed to mimic the binding site of zinc thiolate proteins, is shown to reduce the reactivity of the thiolate toward electrophiles by up to 2 orders of magnitude. In addition a single such bond is also sufficient to achieve nearly 100% regiospecificity of reaction between a strong, and hence inherently indiscriminate, alkylating agent like trimethyl oxonium tetrafluoroborate and a single sulfur in a dithiolate construct. The importance of these results in understanding how two systems such as the zinc fingers of the GATA family and the Escherichia coli DNA repair protein Ada which share the same pseudotetrahedral structure and tetrascysteinyl ligation around the zinc can fulfill such widely divergent (structural vs reactive) roles and how specificity of reaction in multithiolate-containing systems can be achieved is discussed.
Manganese-oxo complexes have been widely studied because of their importance in biological processes and their utility as synthetic reagents. 1 In biological systems, these species are proposed as intermediates in certain catalases 2 and peroxidases, 3 and in the oxidation of water to O 2 in the oxygen-evolving complex of photosynthesis. 4 Synthetic manganese-oxo complexes, such as those with imines 5 and porphyrin ligands, 6 have been postulated to be the reactive species in catalytic oxidations of various organic compounds. These oxo complexes are believed to have high-valent manganese centers in either 4+ or 5+ oxidation states. The isolation and structural analysis of highvalent, mononuclear manganese-oxo complexes is limited to only Mn(V)dO complexes of tetraanionic chelating ligands. 7 In contrast, low-valent manganese complexes (e.g., 3+ valence) with terminal oxo ligands are not known. Low-valent manganese complexes contain µ-oxo bridges, where Mn(III)-(O) n -Mn(III) dimers (n ) 1, 2) are the norm. 8 This report describes the preparation and properties of a monomeric Mn(III)-oxo complex and its Mn(III)-hydroxo analogue. The isolation of these complexes is accomplished by using the chelating ligand [H 3 1] 3-, which forms a protective hydrogen-bonding cavity around the Mn(III)-O(H) units.The importance of H-bonding in regulating metal ion reactivity is exemplified by metalloproteins. Several metalloproteins have active sites that contain either H-bond donors or acceptors that interact with external ligands that are covalently bonded to a metal center. 9,10 Efforts to mimic this multimode binding in synthetic complexes have involved both heme and non-heme systems. 11
It is shown in model complexes designed to mimic the binding site of zinc-thiolate proteins that a single hydrogen bond between an amide N-H and a Zn-coordinated thiolate reduces its reactivity toward electrophiles by up to 2 orders of magnitude. In addition, we show that a single N-H...S hydrogen bond is sufficient to achieve near 100% regiospecificity of reaction between a strong, and hence inherently indiscriminate, alkylating agent like trimethyloxonium tetraflouroborate and a single sulfur in a dithiolate construct. The importance of these results in understanding how systems such as the zinc fingers of the GATA family and the E. coli DNA repair protein Ada, which share the same pseudotetrahedral structure and tetracysteinyl ligation around the zinc, can fulfill such widely divergent (structural vs reactive) roles and how specificity of reaction in such multi-thiolate containing systems can be achieved is discussed.
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