Structural evidence for an intramolecular CH-π interaction in ternary metal (Cu(II), Pd(II)) complexes involving o-/m-/p-methyl substituted phenylalanine and 1,10-phenanthroline

The existence of a CH-pi interaction has clearly been demonstrated by use of the ternary metal (Cu(II), Pd(II)) complexes involving o-/m-/p-methyl substituted L-phenylalanine (L-o-mPhe, L-m-mPhe, L-p-mPhe) and 1,10-phenanthroline (phen) on the basis of H-1 NMR, electronic absorption, and CD spectroscopies, and X-ray diffraction method. The crystal structure of [{Cu(p-mPhe)(phen)}(2){Cu(phen)(2)}(mu(3)-CO3)](ClO4)(2) has revealed a marked structural feature suggesting CH-pi and/or pi-pi interactions, although [Cu(L-o-mPhe)(phen)(H2O)]ClO4 does not show any interaction. CD spectra in the d-d region for [Cu(L-o-mPhe)(phen)]ClO4, [Cu(L-m-mPhe)(phen)]ClO4, and [Cu(L-p-mPhe)(phen)]ClO4 in aqueous solution have exhibited an obvious negative Cotton effect, and the magnitudes have significantly been reduced in dioxane/water in the order: [Cu(L-p-mPhe)(phen)]ClO4 less than or equal to [Cu(L-m-mPhe)(phen)]ClO4 < [Cu(L-o-mPhe)(phen)]ClO4. H-1 NMR spectra of [Pd(L-o-mPhe)(phen)]ClO4, [Pd(L-m-mPhe)(phen)]ClO4, and [Pd(L-p-mPhe)(phen)]ClO4 have exhibited larger ring current shifts for m- and p-CH3 protons of the latter two complexes both in D2O and in DMSO-d(6), whereas o-CH3 protons for the former complex do not. The magnitudes have greatly been reduced in this order: [Pd(L-p-mPhe)(phen)]ClO4 less than or equal to [Pd (L-m-mPhe)(phen)]ClO4 in D2O-dioxane-d(8), but not for [Pd(L-o-mPhe)(phen)]ClO4.