Structural Consequences of Spin Conversion in a Sterically Encumbered Ni(II) Porphyrin

Abstract

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, low-spin Ni(II) complex, 1, previously reported. The results demonstrate that conversion to high-spin Ni(II) in nonplanar, sterically encumbered porphyrins induces a significant core expansion about the Ni while nonplanarity is still retained. The expansion of the core parameters (Ni-N, Ct-CR, Ct-Cmeso) and the Ni-N axial distances are characteristic of the d x 2 -y 2 and d z 2 orbital occupancies in high-spin Ni(II) porphyrins and document the structural consequences of the spin conversion in severely nonplanar Ni(II) porphyrins. The stereochemical results are particularly relevant to ligation effects in nonplanar Ni biomolecules and synthetic porphyrins increasingly used as biomimetic models of conformational effects in chromophores and prosthetic groups in vivo, and to the remarkably wide range of lifetimes observed for excited (d,d) states in nonplanar, sterically constrained Ni(II) porphyrins in which the d x 2 -y 2 and d z 2 orbitals are also populated.

Synthesis, spectroscopic and electrochemical investigations of supramolecular nickel(II)tetraazaporphyrin complexes

“…S 1 state) did not show any detectable emission at longer wavelengths up to 800 nm for any of these complexes. The S 1 state in the nickel porphyrins generally undergoes radiationless decay to S 0 for which, energy transfer to lower lying LMCT [39], d-d [40] states or conformational relaxation [41,42] mechanisms have been suggested. The excitation profiles of the shorter wavelength emissions in the di-and pentanuclear complexes 2, 3, 4 and 5 show that emissions between 400-600 nm arise predominantly from S 2 level of the [Ni(OBTTAP)] core.…”

Bis{2-[N,N-bis(2-aminoethyl)amino]ethanaminium}nickel(II) hexacyanidoferrate(II)

“…The Ni II ion is coordinated by the two N,N,N-tridentate ligands in a distorted octahedral geometry (Table 1). The mean Ni-N bond length of of 2.129 (2)Å is in the normal range (Barkigia et al, 1999). The N-Ni-N chelate bite angles in (I) are comparable to those of other Ni II complexes with the tris(2-aminoethyl)amine ligand (Calatayud et al, 2003).…”

Spin-Forbidden Reactions in Transition Metal Chemistry

“…Obviously, this depends on the thermal rate of interconversion being low, and the design of systems having this property is dependent on having a full understanding of the nuclear factor of the corresponding spin-forbidden process. Spin crossover has also been observed for many bioinorganic systems [22].…”