Circular dichroism (CD) spectra and density functional theory (DFT) calculations are reported for a series of conformationally bistable chiroporphyrins with 8-methylene bridles MBCP-8, which can display either an αααα or an αβαβ orientation of their meso substituents. From DFT geometry optimizations, the most stable form of ZnBCP-8 is found to be the αααα conformer. By passing to NiBCP-8, there is a strong stabilization of the αβαβ conformation with respect to the αααα conformation, consistent with the X-ray structures of αααα-ZnBCP-8 and αβαβ-NiBCP-8. A correlation between the sign of the CD signal in the Soret region and the conformation of the BCP-8 compounds is reported: the αααα conformers H2BCP-8 and ZnBCP-8 show a positive CD signal, whereas the αβαβ conformers NiBCP-8 and CuBCP-8 exhibit a negative signal. The possible contributions to the rotational strengths of αβαβ-NiBCP-8 and αααα-ZnBCP-8, calculated on the basis of their crystal structures, have been analyzed. The CD signals are found to result from a combination of both the inherent chirality of the porphyrin and of extrinsic contributions due to the chiral bridles. These results may have a broad significance for understanding the chiroptical properties of chiral porphyrins and hemoproteins and for monitoring stimuli-responsive, conformationally bistable chiroporphyrin compounds.
Magnetization measurements and variable temperature optical spectroscopy have been used to investigate, within the 4−300 K temperature range, the electronic structure of the reduced high-potential iron protein (HiPIP) from Chromatium vinosum and the model compounds (Cat)2[Fe4S4(SR)4], where RS- = 2,4,6-triisopropylphenylthiolate (1), 2,6-diphenylphenylthiolate (2), diphenylmethylthiolate (3), 2,4,6-triisopropylbenzylthiolate (4, 4‘), 2,4,6-triphenylbenzylthiolate (5, 5‘), 2,4,6-tri-tert-butylbenzylthiolate (6), and Cat+ = +NEt4 (1, 2, 3, 4‘, 5‘, 6), +PPh4 (4, 5). The newly synthesized 22-, 32-, 52-, and 62- complexes are, as 12- and 42-, excellent models of the reduced HiPIPs: they exhibit the [Fe4S4]3+/2+ redox couple, because of the presence of bulky ligands which stabilize the [Fe4S4]3+ oxidized core. Moreover, the presence of SCH2 groups in 42-, 52-, and 62-, as in the [Fe4S4] protein cores, makes them good biomimetic models of the HiPIPs. The X-ray structure of 2 is reported: it crystallizes in the orthorhombic space group Pcca with no imposed symmetry and a D2d-distorted geometry of the [Fe4S4]2+ core. Fit of the magnetization data of the reduced HiPIP and of the 1, 2, 3, 4, 5, and 6 compounds within the exchange and double exchange theoretical framework leads to exchange coupling parameters J = 261−397 cm-1. A firm determination of the double exchange parameters B or, equivalently, the transfer integrals β = 5B could not be achieved that way. The obtained |B| values remain however high, attesting thus to the strength of the spin-dependent electronic delocalization which is responsible for lowest lying electronic states being characterized by delocalized mixed-valence pairs of maximum spin 9/2. Electronic properties of these systems are then accounted for by the population of a diamagnetic ground level and excited paramagnetic triplet and quintet levels, which are respectively J and 3J above the ground level. Optical studies of 1, 2, 4‘, 5‘, and 6 but also of (NEt4)2[Fe4S4(SCH2C6H5)4] and the isomorph (NEt4)2[Fe4S4(S-t-Bu)4] and (NEt4)2[Fe4Se4(S-t-Bu)4] compounds reveal two absorption bands in the near infrared region, at 705−760 nm and 1270−1430 nm, which appear to be characteristic of valence-delocalized and ferromagnetically coupled [Fe2X2]+ (X = S, Se) units. The |B| and |β| values can be directly determined from the location at 10|B| of the low-energy band, and are respectively of 699−787 and 3497−3937 cm-1. Both absorption bands are also present in the 77 K spectrum of the reduced HiPIP, at 700 and 1040 nm (Cerdonio, M.; Wang, R.-H.; Rawlings, J.; Gray, H. B. J. Am. Chem. Soc. 1974, 96, 6534−6535). The blue shift of the low-energy band is attributed to the inequivalent environments of the Fe sites in the protein, rather than to an increase of |β| when going from the models to the HiPIP. The small differences observed in known geometries of [Fe4S4]2+ clusters, especially in the Fe−Fe distances, cannot probably lead to drastic changes in the direct Fe−Fe interactions (parameter β) responsible for the delocalization phenomenon. These differences are however magnetostructurally significant as shown by the 261−397 cm-1 range spanned by J. The cluster's geometry, hence the efficiency of the Fe−μ3-S−Fe superexchange pathways, is proposed to be controlled by the more or less tight fit of the cluster within the cavity provided by its environment.
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Last update Friday May 17 2013