Structural consequences of spin conversion in a sterically encumbered Ni(II) porphyrin

The crystal structure of a pyridine-ligated, high-spin Ni(II) complex of 2, 3,7,8,12,13,17,18-octabromo-5,15-bis(isopropyl)-10,20-bis(isopropylidenyl) porphyrin, 2, is reported and compared to the unligated, levy-spin Ni(II) c omplex, 1, previously reported. The results demonstrate that conversion to high-spin Ni(II) in nonplanar, sterically encumbered porphyrins induces a s ignificant core expansion about the Ni while nonplanarity is still retained . The expansion of the core parameters (Ni-N, Ct-C alpha, Ct-Cmeso) and the Ni-N-axial distances are characteristic of the d(x2-y2) and d(z2) orbital occupancies in high-spin Ni(II) porphyrins and document the structural cons equences of the spin conversion in severely nonplanar NI(II) porphyrins. Th e stereochemical results are particularly relevant to ligation effects in n onplanar Ni biomolecules and synthetic porphyrins increasingly used as biom imetic models of conformational effects in chromophores and prosthetic grou ps in vivo, and to the remarkably wide range of lifetimes observed for exci ted (d,d) states in nonplanar, sterically constrained Ni(II) porphyrins in which the d(x2-y2) and d(z2) orbitals are also populated.