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 Fe(II) 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 C2, 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 C2 is explored.
ASJC Scopus subject areas
- Colloid and Surface Chemistry