Consequences of nonplanarity in sterically crowded porphyrins. Antiferromagnetic coupling in Cu(II) {pi} cation radicals
- Brookhaven National Laboratory, Upton, NY (United States)
Crystallographic studies of sterically crowded 2,3,7,8,12,13,17,18 - octaethyl - 5,10,15,20 - tetraphenylporphyrins, H{sub 2}OETPP, H{sub 4}OETPP{lg_bullet}2Ac, NiOETPP, ZnOETPP and CuOETPP, indicate that all the OETPP derivatives adapt saddle conformations with the pyrrole {beta} carbons displaced by more than {+-}1{angstrom} from the average macrocycle planes. NMR, resonance Raman and EXAFS results further establish than the saddle distorations are maintained in solution. The ruffling affects the optical, redox, and excited state properties of the porphyrins but the {pi} cation radical of ZnOETPP retains an a{sub 2u} spin profile and the Cu and N hyperfine constants of the neutral CuOETPP differ little (10%) from those of CuTPP. The saddle conformation of CuOETPP affords a test of the premise advanced by Scheidt, Reed et al. (e.g. JACS 1989, 111, 6865) that magnetic coupling in {pi} cation radicals containing paramagnetic metals is ferromagnetic in planar a{sub 2u} radicals of CuOETPP is indeed diamagnetic in solution because of antiferromagnetic in S{sub 4} ruffled (saddle) macrocycles. In agreement with this proposal, the {pi} cation radical of CuOETPP is indeed diamagnetic in solution because of antiferromagnetic coupling of the Cu(II) with the sterically enforced saddle shape of the porphyrin.
- OSTI ID:
- 141115
- Report Number(s):
- CONF-930304--; CNN: DE-AC02-76CH00016
- Country of Publication:
- United States
- Language:
- English
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