Solution and Solid State Structure of a Canted, Side-to-Face, Bis(porphyrin) Adduct

The coordination chemistry of meso-pyridyl/phenyl porphyrins (PyPs) toward metallo-porphyrins with substitutionally labile axial ligands has been exploited for the self-assembling of ordered arrays of pigments. PyPs are
particularly versatile bulding blocks:  the peripheral N atom can be either in the 4‘-position (4‘PyPs) or in the
3‘-position (3‘PyPs), and they can provide connections to as many as four metal centers by coordination of the
pyridyl groups. While 4‘PyPs lead to arrays of perpendicularly linked, side-to-face, porphyrins, 3‘PyPs yield the
corresponding canted analogues. Even though several perpendicular arrays have been described, the examples of
canted adducts are few and all very recent. We report here a detailed X-ray structure of the new adduct of two
axially ligated canted porphyrins [Ru(TPP)(CO)(3‘MPyP)] (1) (TPP = tetraphenylporphyrin, 3‘MPyP = 5-(3‘-pyridyl)-10,15,20-triphenylporphyrin) as well as its thorough spectroscopic characterization in solution, including
the dynamic 1H NMR investigation of the NH tautomeric exchange process. 1 crystallizes in the monoclinic
space group P21/a, Z = 4, with a = 13.508(1) Å, b = 29.972(1) Å, c = 19.084(1) Å, and β = 96.03(1)°. The
structural and spectroscopic data of 1 assume a strategic importance among axially ligated pyridylporphyrin systems,
since they can be fruitfully compared to those of perpendicular analogues. The importance of a comparative
investigation of the canted and perpendicular arrays of porphyrins is readily understood:  it might help understanding
how relevant physicochemical properties, such as photoinduced long-range electron or energy transfer processes,
depend on the mutual orientation of the pigments in the 3-dimensional architecture while the other parameters in
the supramolecular array (number of macrocycles, distance between them) are maintained substantially unchanged.