Basket-handle structure

Structures of to date nonbiological Fe-S clusters (A) the thioprismane structure (B) basket-handle structure (C) monocapped prismatic structure (D) adamantane structure (E) circular FeigSso core unit. 

Another piece of evidence concerns the particularly small values of E ether-E TAPI in the case of the (0(CH2)4Ph(CH2)40)2-CT basket-handle structure. Furthermore, it is noticed that E ether-E TAP decreases as the coordinating power of the solvent increases. In this connection, the case of NMA is particularly striking whereas the (0(CH2)4Ph(CH2)40)2 structure shows, among the various basket-handle structures, the largest value of E ether-E TAP 1 for the Fe(I) + e- Fe("0 )2- reaction, the opposite is true for the Fe(II) + e- 5=AFe(I) reaction emphasizing the role of the freely rotating phenyls in sterically hindering the axial complexation of iron(II) by the solvent molecules. 

Figure 8. Schematic representation of the stabilization of the Fe(II)-1,2-Me2lm complex from the dipolar interactions with the NHCO groups in a (NHC0(CH2)ioCONH)2-CT (a) and -AC (b) basket-handle structure. The iron atom is represented at 0.4 A out of the porphyrin plane.

Figure 11 Structures of chiral basket-handle porphyrins [78, 223]...

Recently, several papers have appeared on various aspects of sterically crowded porphyrins. Nonplanarity in these systems has been induced either by substituting bulky groups on (3-pyrrole and/or meso-carbons or by attaching a short bridging chain across the porphyrin periphery in the so-called basket handle porphyrins [169-171]. Some typical examples of sterically crowded porphyrins are depicted in Fig. 17. An analysis of the crystal structure and the various spectral properties clearly reveal the effect of nonplanarity of the porphyrin rings. The details of these results are highlighted in this section. 

Fig. 17. Molecular structures of some highly substituted porphyrins and basket handle porphyrins...

Only one structure of basket handle porphyrin has been solved [182]. The macrocycle in Zn hexyl II is essentially planar with the largest displacement from the mean porphyrin plane at Cm and Cp of 0.08 and 0.06 A. The dihedral angle between the planes of two of the four nitrogen atoms of the macrocycle is also... 

Structure 5.17. Tetraphenylporphyrin Iron(III) chloride, with basket-handle of the picket-fence type with secondary amide groups in close vicinity to the porphyrin ring. Reprinted from Structure 16 J. Costamagna, G. Ferraudi, J. Canales and J. Vargas, Carbon dioxide activation by aza-macrocychc complexes, Coordination Chemistry Reviews 148 (1996) 221-248. Copyright 1996, with permission of Elsevier. 

Photochemical dissociation of CO from carboxyhemoglobin has been studied by time-resolved infrared (IR) measurements. The dissociation and recombination mechanism had been discussed in relation to protein structure [309,310]. The dissociated CO was supposed to be trapped in protein pocket below 200 K, while at a higher temperature the trapped CO was lib ated from the protein [311]. Ihe detailed time-resolved study indicated that the dissociation proceeded through the metastable heme protein with the lifetime of 2 1 ps, while the dissociated CO bounded with heme pocket for at least 500 ps [783]. Model compounds such as Fe and Ru porphyrins have also been studied. Rather high quantum yields of dissociation of CO (0.53 and 0.9) were reported on Fe"TPP(C0)C2H50H and Fe TPP (CO)Im in ethanol, respectively [312], Basket-handle porphyrins as model compounds for heme have been studied [314-316] (4) ... 

In the amide-linked series, steric hindrance to axial coordination of solvent molecules should also interfere in the modulation of the Fe(II) + e Fed) reactivity besides protection against solvation and "local solvation". If both protection against solvation and steric hindrance to axial ligation were the same with ether-linked and amide-linked basket-handles of corresponding structure, (E amide-E TPp)-(E ether-E TAp) should be a constant when changing superstructure and solvent. As seen in Table 3, this is grossly true, as expected from the fact that protection against solvation should vary less than in the case of the Fed) + e ... 

Although no En cluster molecules with n > 4 are known in the condensed phase, it has been shown [27] that neutral P6 can be generated in the gas phase by neutralization-reionization mass spectrometry with cp 2 6 as a precursor [cp = C5(CH3)5] [28]. Since the cp 2P6 precursor molecule already has a hetero-benzvalene P6 skeleton, it was inferred that the neutral P6 molecule should have the benzvalene structure as shown in Figure 2.6-5. This is in agreement with a multitude of quantum chemical calculations [8, 11, 27, 29-32]. Note that the handle of the basket of P6 includes two dicoordinate P atoms linked by a P=P double bond (cf. R-P=P-R 203 pm [33]). 

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