Structural Characterization of Five Sterically Protected Porphyrins

Abstract

The structures of the sterically protected “pocket” porphyrin H2(α-PocPivP) (1), and the “capped” porphyrins H2(C3-Cap) (2), Fe(C3-Cap)(CO)(1-MeIm) (3), H2(C4-Cap) (4), and Fe(C4-Cap)(Cl) (5) have been determined by single-crystal X-ray diffraction methods. Compounds 1-4 each pack with one independent porphyrin unit and solvate molecules in the unit cell. Compound 5 packs with two crystallographically independent porphyrins and solvate molecules in the unit cell. The structure of 1 is the first of a pocket porphyrin in which there are no ligands bound inside the protected region. This structure reveals that the main form of distortion that occurs when a ligand binds inside the protected region is the lateral movement of the benzene cap relative to a position above the centroid of the porphyrin. In 1 this lateral displacement is 1.86 Å whereas this displacement is 3.30 Å in Fe(β-PocPivP)(CO)(1,2-Me2Im). Comparison of the free-base structure 2 with the FeII carbonyl 3, where the CO ligand is bound under the cap and the 1-MeIm ligand is bound opposite the cap, reveals that there is little lateral distortion in the C3-Cap system, but there is significant vertical expansion of the cap upon coligation to the Fe center. The distance of the cap centroid from the mean porphyrin plane increases 2.37 Å to accommodate the CO ligand, from 3.49 Å in 2 to 5.86 Å in 3. The Fe−C−O angle in 3 is 178.0(13)°. In the structures of the C4-Cap system 4 and 5 there is sufficient space for the binding of small ligands, such as CO and O2, as well as larger ligands. Compound 4 crystallizes with a CHCl3 solvate molecule under the cap. The distances from cap centroid to porphyrin plane in the C4-Cap structures are 7.28, 7.12, and 7.66 Å for 4, 5A, and 5B, respectively. This is significantly greater than the distance of 5.6 Å in Fe(C2-Cap)(CO)(1-MeIm) and 5.86 Å in 3. The relation between these structural changes on ligation and the binding properties of these systems for CO and O2 is explored.

CO as a vibrational probe of heme protein active sites

“…Although the energy cost of such distortion is not as large as once thought [9,13], it still is likely to exceed the energy required for local conformation changes that move sterically intrusive protein residues out of the way. Even in highly constrained porphyrins with covalent superstructures, the crystal structures show small FeCO distortions, but large displacements of the superstructure, as well as distortion of the porphyrin ring [89][90][91].…”

Synthesis and solid-state molecular structures of nitrosoalkane complexes of iron porphyrins containing methanol, pyridine, and 1-methylimidazole ligands

“…Other iron porphyrin compounds containing axial imidazole-based ligands have been characterized by X-ray crystallography, and these include the bis-imidazole complexes (TPP)Fe(1-VinIm) 2 and (TPP)Fe(1-BzlIm) 2 (Fe-N(Im) ¼ 2.004(2), 2.017(4) A) [15], the dioxygen complex (TpivPP)Fe(O 2 )(1-MeIm) (Fe-N(Im) ¼ 2.068 (18) A) [16], the nitro compound (TpivPP)Fe(NO 2 )(HIm) (Fe-N(Im) ¼ 2.037 (10) A) [17], the carbonyl derivatives (por)Fe(CO)(1-MeIm) (Fe-N(Im) ¼ 2.027(5)-2.077 (3) A) [18][19][20][21][22][23][24], and the paramagnetic complex (TPP)Fe(NO)(1-MeIm) containing a bent NO ligand (Fe-N(Im) ¼ 2.173 (2) A) [25]. There are some interesting observations regarding the axial ligand orientations in the (por)Fe( i PrNO)(L) complexes described in this paper.…”

Myoglobin models and steric origins of the discrimination between O2 and CO

“…Structural characterization of this system makes it clear that solvents get trapped under the cap [46]. This seven-atom-linked capped porphyrin system is conceivably of use as a model for solvent displacement, such as a water in hemoglobin, but it is not of use as a probe of steric effects.…”

“…Additional energy is required to accomplish such movement, which is needed to accommodate the bent Fe-O-O linkage, and a higher value of P 1/2 (O 2 ) results. Comparison of the structure of the free base, H 2 (C 3 -Cap), with that of the carbonyl, Fe(C 3 -Cap) (CO)(1-MeIm), indicates that the vertical flexibility is available: the cap expands 2.3 Å, from F3.5 Å to F5.9 Å, upon CO ligation [46]. Similarly, C 2 -Cap expands F1.6 Å and OC 3 OPor expands F0.9 Å, to F5.6 Å, upon CO ligation (Table 2) [47][48][49].…”

“…From a comparison of the structure of H 2 (a-PocPivP) [46] with those of Ru(bPocPivP)(H 2 O) in (CO) out [50], Ru(a-PocPivP)(CO)(1-MeIm) [51], and Fe(b-PocPivP)(CO)(1,2-Me 2 Im) [52], it is apparent that horizontal displacement of the cap is the main type of distortion that occurs when CO or H 2 O binds under the cap ( Table 2). The cap moves laterally as much as 1.44 Å, from 1.86 Å in H 2 (a-PocPivP) to 3.30 Å in Fe(b-PocPivP)(CO)(1,2-Me 2 Im).…”