Structural Characterization of Five Sterically Protected Porphyrins

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.