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

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

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