Die Sattelkonformation der hydroporphinoiden Nickel(II)‐Komplexe: Struktur, Ursprung und stereochemische Konsequenzen

摘要: The Saddle Conformation of Hydroporphinoid Nickel(II) Complexes: Structure, Origin, and Stereochemical Consequences Thirteen crystal structures document the general phenomenon coordination-hole contraction in hexahydro- tetrahydroporphinoid ligands complexes with small metal ions such as low-spin Ni(II). is characterized by a deformation ligand system towards saddle-shaped, ruffled conformation approximate S4 symmetry. central ion coplanar four coordinating N-centers whereas C(meso)-atoms are situated alternately above below this coordination plane. Increasing steepness saddle (parameter dm) associated decreasing metal-N distances. For pyrrocorphinates, dm increases order Cu(II) < (pyridine) Co(II) Ni(II), for Ni(II) it does so porphyrin chlorin bacteriochlorin isobacteriochlorin pyrrocorphin. In hydroporphinoid complexes, hydropyrrole rings assume half-chair conformations whereby individual half-chairs conformationally constrained way that inclination their peripheral single bond parallels (W-conformation ensemble 5-membered ring half-chairs). There only two available given complex; they interrelate inversion concomitant half-chairs. expected observed experimentally to exert control on stereochemistry reactivity periphery well axial electrophilicity ion. Tetracoordinate nickel(II) pyrrocorphinates found favor tctct configuration substituents at periphery, isobacteriochlorinates tct configuration, bacteriochlorinates ttt configuration. Relative rates regioselectivities autoxidation corresponding bacterio-and depend periphery. residual tetracoordinate octaethyl series appears increase ≲ All used X-ray structure studies were prepared part exploring porphyrinogen pyrrocorphin tautomerization, novel structural transformation porphyrinogens.