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Conformational control of oxidation sites, spin states and orbital occupancy in nickel porphyrins

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Ni(II) porphyrin π cation radicals are known to undergo an internal electronic isomerization to L2Ni(III) cations upon complexation with ligands (L). Additional examples of the Ni(II) to Ni(III) conversion are presented for flexible, 'planar' NiOEP (2,3,7,8,12,13,17,18-octaethylporphyrin) and NiT(Pr)P (5,10,15,20-tetra-n-propylporphyrin) in which the Ni(III) orbital occupancy, dz2 or dx2-y2, is determined by the ligand field strength of the axial ligands (pyridine, imidazole, or cyanide). In contrast to these results, the nonplanar NiOETPP (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), which is easily oxidized because of its saddle-shape, yields a complex postulated to be a high spin Ni(II) π cation radical, based on crystallographic and optical data for (imidazole)2NiOETPP+ClO4-, in which the electron of high spin Ni(II) in the dx2-y2 orbital is antiferromagnetically coupled to the unpaired electron of the porphyrin radical leaving one electron in the Ni(II) dz2 orbital, i.e. a pseudo Ni(III). The sterically encumbered, nonplanar NiT(t-Bu)P (5,10,15,20-tetra-tertiary-butylporphyrin) yields Ni(III) complexes when ligated by pyridine, imidazole or cyanide, but in all cases only the Ni(III) dz2 orbital is occupied as evidenced by EPR spectroscopy. This anomalous chemistry is attributed to the fact that the macrocycle of NiT(t-Bu)P is so sterically constrained that it cannot readily expand to accommodate the longer equatorial Ni—N distances required by population of the dx2-y2 orbital in Ni(III) or high spin Ni(II). Further support for this postulate derives from NiD(t-Bu)P (5,10-di-tertiary-butylporphyrin) which is less sterically constrained and in which the Ni(III) dx2-y2 orbital is indeed occupied upon complexation with cyanide. These results thus illustrate the significant effects that the conformations, plasticity or rigidity of Ni porphyrin macrocycles can have on sites of oxidation (metal or porphyrin), spin states (low spin Ni(III) or high spin Ni(II)), and orbital occupancies (dz2 or dx2-y2 in Ni(III)).


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