Interplay of Axial Ligation, Hydrogen Bonding, Self-Assembly, and Conformational Landscapes in High-Spin Ni(II) Porphyrins

The molecular structures of four bis-ligated high-spin Ni(II) complexes of the sterically crowded, nonplanar
2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetranitroporphyrin (NiOETNP) are reported. The ligands are imidazole
(Im), imidazole plus 2-methylimidazole (2-MeIm) in the crystal lattice, 1-methylimidazole (1-MeIm), and
2,1,3-benzoselenadiazole (BSeD). Extensive intermolecular hydrogen bonding is observed in the three
imidazole-ligated structures consisting of NH···O and CH···O bonding from the imidazoles to neighboring
nitro groups and of NH···N interactions to a nearby 2-MeIm. The different modes of hydrogen bonding,
typical of those frequently observed in proteins, mediate the self-assembly of discrete porphyrin dimers as
well as more extensive two- and three-dimensional arrays. Only the bis-BSeD complex remains monomeric.
The presence or absence of the different types of hydrogen bonds controls the orientations of the axial ligands
and also modulates the conformations of the porphyrin skeletons. This interplay of axial ligation, hydrogen
bonding, and self-assembly further illustrates the multiconformational landscapes that porphyrins can access
as a function of their microenvironment. Such nonplanar deformations have been shown to significantly affect
the optical, redox, magnetic, radical, and excited state properties of porphyrin derivatives. That hydrogen
bonding can influence ligand interactions with neighboring functional groups as well as macrocycle
conformations with their concomitant consequences on physical and chemical properties may thus be
particularly relevant to the bioenergetic roles of porphyrin in vivo. These results also raise the question whether
point mutations near porphyrins in vivo are structurally, and consequently functionally, innocent.