Characterization of an NH−π Interaction in Co(III) Ternary Complexes with Aromatic Amino Acids

Abstract

The NH-pi interaction has been detected in the crystal structures of Co(III) ternary complexes with N,N-bis(carboxymethyl)-(S)-phenylalanine (BCMPA) and aromatic amino acids including (S)-phenylalanine ((S)-Phe), (R)-phenylalanine ((R)-Phe), and (S)-tryptophan ((S)-Trp)). Additionally, this interaction has been studied in solution for Co(III) ternary complexes with BCMPA or NTA (NTA = nitrilotriacetic acid) and several amino acids (AA) by means of electronic absorption, circular dichroism (CD), and (1)H NMR spectroscopies. The CD intensities of the Co(III) complexes with aromatic amino acids measured in the d-d region ( approximately 20.5 x 10(3) cm(-)(1)) are significantly decreased in ethanol solutions relative to water. Analogous complexes with aliphatic amino acids do not exhibit this solvent effect. The (1)H NMR spectra of the Co(III) complexes with aromatic amino acids measured in DMSO-d(6) exhibit upfield shifts of the NH peaks compared with those with aliphatic amino acids, which suggest a shielding effect due to the aromaticity. The upshift values coincide with those experimentally evaluated from the crystal structures. The magnitude of the upfield shifts agrees well with Hammett's rule, indicating that the increase of pi-electron densities on the aromatic rings leads attractive NH-pi interaction that exerts a larger shielding effect for the NH protons. In ligand-substitution reactions of the carbonatocobalt(III) complexes with amino acids, the yields of those with aromatic amino acids are higher than the yields obtained for complexes with aliphatic amino acids. This observation is discussed in connection with the important contribution of the NH-pi interaction as one of the promotion factors in the reaction.

Electron‐Transfer Reactions through the Associated Interaction between Cytochrome c and Self‐Assembled Monolayers of Optically Active Cobalt(III) Complexes: Molecular Recognition Ability Induced by the Chirality of the Cobalt(III) Units

“…Spectroscopic measurements and X-ray crystal structures have indicated that the side chain is constrained to the equatorial plane for both complexes. [41] The spectroscopic and electrochemical data are summarized in Table 1.…”

“…[41] The CD spectral data for (S) )). [41] In the case of (R)-1, only the band near 610 nm, originating from a transition attributable to the chiral site, is inverted relative to that of (S)-1.…”

“…The reduction potentials of (S)-2 and (R)-2 are similar to that of trans-(N)-KA C H T U N G T R E N N U N G [Co III ((S)-bcmpa)((S)-phenylalanine)] (E pc = À330 mV). [41] As a redox wave is not observed near the redox potential of cyt c (+ 55 mV), [3] the Co III unit does not mediate an electrontransfer reaction between cyt c and the Au electrode in this redox range. [42] Cyclic voltammograms of cyt c in the pres- Table 1.…”

Influence of metal and ligand types on stacking interactions of phenyl rings with square-planar transition metal complexes

“…ligands coordinated to a metal, can interact with a π-system. In addition, interactions named metal-ligand-aromatic cation-π interactions (MLACπ) [14][15][16][17][18][19], can be considered as a type of XH/π hydrogen bond [20][21][22]. It was shown that some of these interactions are among the strongest XH/π hydrogen bonds [18].…”

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

“…Although NH···π interactions in proteins have been known since 1984, [15] in complexes of transition metals they have been discovered only recently [16] as, because of their low interaction energy, [17,18] they are difficult to detect in solution. One of the best ways to identify the presence of NH···π interactions in aqueous solution is the anomalous chemical shift of the NH protons involved in the interaction.…”