Biomedical subjects
Publications and source records attributed to Ben L. Feringa.
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Explore the source record for details and available documents.
Explore the source record for details and available documents.
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In an effort to gain more insight into the factors controlling the formation of low-spin non-heme Fe(III)-peroxo intermediates in oxidation catalysis, such as activated bleomycin, we have synthesized a series of iron complexes based on the pentadentate ligand N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine). The following complexes have been prepared: [(N4Py)Fe(II)(CH(3)CN)](ClO(4))(2) (1), [(N4Py)Fe(II)Cl](ClO(4)) (2), [(N4Py)Fe(III)OMe](ClO(4))(2) (3), and [(N4Py)(2)Fe(2)O](ClO(4))(4) (4). Complexes 1 and 2 have low- and high-spin Fe(II) centers, respectively, whereas 3 is an Fe(III) complex that undergoes a temperature-dependent spin transition. The iron centers in the oxo-bridged dimer 4 are antiferromagnetically coupled (J = -104 cm(-)(1)). Comparison of the crystal structures of 1, 3, and 4 shows that the ligand is well suited to accommodate both Fe(II) and Fe(III) in either spin state. For the high-spin Fe(III) complexes 3 and 4 the iron atoms are positioned somewhat outside of the cavity formed by the ligand, while in the case of the low-spin Fe(II) complex 1 the iron atom is retained in the middle of the cavity with approximately equal bond lengths to all nitrogen atoms from the ligand. On the basis of UV/vis and EPR observations, it is shown that 1, 3, and 4 all react with H(2)O(2) to generate the purple low-spin [(N4Py)Fe(III)OOH](2+) intermediate (6). In the case of 1, titration experiments with H(2)O(2) monitored by UV/vis and (1)H NMR reveal the formation of [(N4Py)Fe(III)OH](2+) (5) and the oxo-bridged diiron(III) dimer (4) prior to the generation of the Fe(III)-OOH species (6). Raman spectra of 6 show distinctive Raman features, particularly a nu(O-O) at 790 cm(-)(1) that is the lowest observed for any iron-peroxo species. This observation may rationalize the reactivity of low-spin Fe(III)-OOH species such as "activated bleomycin".
The minimum energy conformations and racemization barriers for the chiral sterically overcrowded helical alkenes, trans- and cis-1,1',2,2',3,3',4,4'-octahydro-4,4'-biphenanthrylidenes (1 and 2), are reported. The trans-1 and cis-2 isomers can each adapt three different conformations, (P,P) and (M,M) (an enantiomeric pair) and an achiral (P,M) meso form, of which only the chiral isomers were obtained by synthesis. The conformations and heats of formation of (M,M)-(E)-1, (P,M)-(E)-1, (M,M)-(Z)-2, and (P,M)-(Z)-2 isomers were determined by MOPAC AM1 calculations. The racemization process for both the trans- and cis- isomers is postulated to occur via the (P,M) isomers by two successive inversions of the cyclohexenyl ring; (M,M) <--> (P,M) <--> (P,P). The (M,M) --> (P,M) and reverse (P,M) --> (M,M) isomerizations were simulated by reaction path calculations, providing the molecular structure and the activation energy of the transition state for each isomerization. For each racemization process, the activation enthalpy (DeltaH()) was calculated as 23.9 and 19.9 kcal mol(-)(1) for trans-olefin 1 and cis-olefin 2, respectively. These values reasonably agree with the experimental values obtained by temperature-dependent circular dichroism, optical rotation, and (1)H NMR magnetization transfer measurements: DeltaH() = 24.6 and 20.8 kcal mol(-)(1) for trans-olefin 1 and cis-olefin 2, respectively. While the racemization of cis-isomer 2 is controlled by the steric interaction of H5 with C4'a and C4'b, the surprisingly high barrier for trans-olefin 1 is due to the severe steric interaction between H5 and H3'alpha and/or H3'beta protons.
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