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Copper β-Octakis(trifluoromethyl)corroles: New Paradigms for Ligand Substituent Effects in Transition Metal Complexes

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journal contribution
posted on 17.11.2008, 00:00 by Kolle Ekaney Thomas, Ingar H. Wasbotten, Abhik Ghosh
The reaction of copper β-octabromo-meso-triarylcorrole derivatives with methyl 2,2-difluoro-2-(fluorosulfonyl)acetate has provided four β-octakis(trifluoromethyl)corrole complexes, Cu[(CF3)8T(p-XP)C] (X = F, H, Me, OMe), in moderate yields. The new complexes present a conglomeration of remarkable substituent effects, both steric and electronic. DFT (OLYP/TZP) geometry optimization of Cu[(CF3)8TPC] (i.e., X = H) indicates a sterically hindered, strongly saddled geometry, with numerous short F···F nonbonded contacts of 2.5−2.9 Å and certain β carbons displaced by over 1.5 Å relative to the mean corrole plane. The CF3 groups generally appear as quartets in the 19F NMR spectra, with unexpectedly large 5JFF coupling constants of about 14 Hz, apparently a manifestation of the highly crowded structure. The eight CF3 groups together exert a powerful influence on the redox potentials of the copper corrole core. Thus, the E1/2ox of Cu[(CF3)8TPC] (1.4 V vs saturated calomel electrode) is a full half of a volt above that of Cu(TPC) (0.9 V) and a quarter of a volt above that of Cu(Br8TPC) (1.14 V). Intriguingly, the β CF3 groups also greatly intensify the influence of the meso aryl substituents on the redox potentials, relative to the other Cu[Y8T(p-XP)C] series, where Y = H, F, and Br. The Cu[(CF3)8T(p-XP)C] complexes also exhibit the most red-shifted optical spectra of any series of metallocorroles synthesized to date. Thus, between Cu(TPC) and Cu[(CF3)8T(p-MeO-P)C], the Soret maximum shifts by nearly 100 nm. The observed red-shifts are attributed in part to charge-transfer transitions of the Soret region and in part to the extreme nonplanar distortions.