Through-Space Shielding Effects of Metal-Complexed Phenyl Rings

Several naphthalene compounds containing a methyl group in a 1,8-relationship to a metal-complexed phenyl ring bearing various substituents have been synthesized. The through-space shielding effects of the phenyl ring, as a function of substituent and complexing metal species, were monitored by observing the H-1 NMR signal of the methyl group located in the shielding zone of the ring. In all cases, the methyl signal was slightly more downfield in the complexes than in the uncomplexed analogues. A comparison of available crystal structures, however, shows that the phenyl ring is slightly closer to the naphthyl methyl group in the complexes than in their metal-free counterparts. X-ray structures and DFT calculations also reveal a slight elongation in the average length of the carbon-carbon bonds of the phenyl ring upon complexation. The effect of substituents on the signal of the naphthyl methyl group is small but discernible in the uncomplexed derivatives, and consistent with our previous report. A similar trend is absent in the corresponding metal complexes, as exemplified by the chromium series, and the effect of the metal appears to be more dominant than that of the substituents. These observations were found to be in line with NMR shift calculations.