Configurational Multiplicity of Porphyrin π Cation Radicals:  Nickel π−π Dimers

The molecular structure of the Ni(II)OEP+ClO4 - π cation radical is reported (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin). The vibrational spectrum of a single crystal, recorded with an FT-IR microspectrometer, shows the 2A1u HOMO occupancy marker band predicted for an OEP cation radical. The radical crystallizes as a cofacial π−π dimer, (NiOEP+ClO4 -)2·2CH2Cl2, in an eclipsed configuration with the following parameters: mean interplanar separation = 3.36 Å, Ni−Ni = 3.41 Å, Ct−Ct = 3.46 Å, lateral shift = 0.83 Å, translational slip angle = 13.9°, and zero rotation of equivalent N−Ni−N axes. This dimeric configuration differs significantly from that of a related (NiOEP+ClO4 -)2·8CH2Cl2 dimer and from those of several other metallo (M) OEP radical dimers described previously. The present results offer exceptions to generalizations recently proposed regarding the geometry and extent of π−π interactions in fully and partially oxidized MOEP+ dimeric π cation radicals such as (MOEP+)2 and (MOEP)2 +. Indeed, the configuration and geometry of the (NiOEP+ClO4 -)2 dimer reported here are more akin to those of a half-oxidized 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 π cation radicals can aggregate in and adopt more than one unique cofacial geometry with different conformations, π−π overlaps, and intermolecular spacings. Such multiple configurational architectures may help explain why the oxidized bacteriochlophyll dimers (special pairs) of bacterial photosynthetic reaction centers exhibit varying unpaired spin density profiles.