NH...pi interactions stabilize the most-hindered rotamer of the (S)-tyrosinato side group in bis[(S)-tyrosinato](diamine)cobalt(III) complexes: An NMR spectroscopic and DFT study

Conformational analysis based on an analysis of the vicinal alpha- and beta-proton coupling constants of (S)-tyrosinato ligands in diastereomers of a bis[(S)-tyrosinato](1,3-diaminopropane)cobalt(III) complex is used to calculate the mol fractions of the three most stable rotamers (t, g, h) of the (S)-tyrosinato ligand's side groups in D2O solution. The results of this conformational analysis indicate a population increase in the sterically least favorable rotamer h in diastereomers of Cl-molecular symmetry (complexes 5 and 6). The TOCSY spectrum of complex 6 in aqueous solution shows an exceptionally small chemical shift of one NH2 proton from the coordinated diamine, which is explained by an interligand NH center dot center dot center dot pi interaction. These findings demonstrate the persistence of this weak, noncovalent interaction in water solution. DFT calculations for complex 6, whose diamine ring is in a chair conformation, indicate that the complex with an h conformation in both its (S)-tyrosinato ligand side-residues, which yields the most frequent NH center dot center dot center dot pi interactions, represents an energy minimum. The fact that coordinated 1,3-diaminopropane in the examined complex 6 is in a chair conformation in aqueous solution is proved by the NMR analysis. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005