STRUCTURAL CHARACTERIZATION OF 5 STERICALLY PROTECTED PORPHYRINS

The structures of the sterically protected ''pocket'' porphyrin H-2(al pha-PocPivP) (1), and the ''capped'' porphyrins H-2(C-3-Cap) (2), Fe(C -3-Cap)(CO)(1-MeIm) (3), H-2(C-4-Cap) (4), and Fe(C(4)Cap)(Cl) (5) hav e been determined by single-crystal X-ray diffraction methods. Compoun ds 1-4 each pack with one independent porphyrin unit and solvate molec ules in the unit cell. Compound 5 packs with two crystallographically independent porphyrins and solvate molecules in the unit cell. The str ucture of 1 is the first of a pocket porphyrin in which there are no l igands 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 d isplacement is 1.86 Angstrom whereas this displacement is 3.30 Angstro m in Fe(beta-PocPivP)(CO)(1,2-Me(2)Im). Comparison of the free-base st ructure 2 with the Fe-II carbonyl 3, where the CO ligand is bound unde r the cap and the 1-MeIm ligand is bound opposite the cap, reveals tha t there is little lateral distortion in the C-3-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 plan e increases 2.37 Angstrom to accommodate the CO ligand, from 3.49 Angs trom in 2 to 5.86 Angstrom in 3. The Fe-C-O angle in 3 is 178.0(13)deg rees. In the structures of the C-4-Cap system 4 and 5 there is suffici ent space for the binding of small ligands, such as CO and O-2, as wel l as larger ligands. Compound 4 crystallizes with a CHCl3 solvate mole cule under the cap. The distances from cap centroid to porphyrin plane in the C-4-Cap structures are 7.28, 7.12, and 7.66 Angstrom for 4, 5A , and 5B, respectively. This is significantly greater than the distanc e of 5.6 Angstrom in Fe(C-2-Cap)(CO)(1-MeIm) and 5.86 Angstrom in 3. T he relation between these structural changes on ligation and the bindi ng properties of these systems for CO and O-2 is explored.