NH···π 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 α‐ and β‐proton coupling constants of ( S )‐tyrosinato ligandsin 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 D 2 O solution. The results of this conformational analysis indicate a population increase in the sterically least favorable rotamer h in diastereomers of C 1 ‐molecular symmetry (complexes 5 and 6 ). The TOCSY spectrum of complex 6 in aqueous solution shows an exceptionally small chemical shift of one NH 2 proton from the coordinated diamine, which is explained by an interligand NH ··· π 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 ··· π 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)