CONFIGURATIONAL MULTIPLICITY OF PORPHYRIN PI-CATION RADICALS - NICKELPI-PI-DIMERS

The molecular structure of the Ni(II)OEP+ClO4- pi cation radical is re ported (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin). The vibrational spectrum of a single crystal, recorded with an FT-IR micrcospectromet er, shows the (2)A(lu), HOMO occupancy marker band predicted for an OE P cation radical. The radical crystallizes as a cofacial pi-pi dimer, (NiOEP+ClO4-)(2) . 24=H2Cl2, in an eclipsed configuration with the fol lowing parameters: mean interplanar separation 3.36 Angstrom, Ni-Ni = 3.41 Angstrom, Ct-Ct = 3.46 Angstrom, lateral shift = 0.83 Angstrom, t ranslational slip angle = 13.9 degrees, and zero rotation of equivalen t N-Ni-N axes. This dimeric configuration differs significantly from t hat of a related (NiOEP+ClO4-)(2) . 8CH(2)Cl(2) dimer and from those o f several other metallo (M) OEP radical dimers described previously. T he present results offer exceptions to generalizations recently propos ed regarding the geometry and extent of pi-pi interactions in fully an d partially oxidized MOEP+ dimeric pi cation radicals such as (MOEP+)( 2) and (MOEP)(2)(+). Indeed, the configuration and geometry of the (Ni OEP+ClO4-)(2) dimer reported here are more akin to those of a half-oxi dized dimer (NiOEP)(2)+SbCl6- than to those of a fully oxidized (NiOEP +ClO4-)2 dimer reported previously (Scheidt et al. Inorg. Chem. 1996, 35, 7500). The present results do demonstrate that identical porphyrin pi cation radicals can aggregate in and adopt more than one unique co facial geometry with different conformations, pi-pi overlaps, and inte rmolecular spacings. Such multiple configurational architectures may h elp explain why the oxidized bacteriochlophyll dimers (special pairs) of bacterial photosynthetic reaction centers exhibit varying unpaired spin density profiles.