Coordination of Ru(NO)Cl3 to the tripeptides glyglygly and glyglyhis: N-terminal amine–amide and C-terminal imidazole–amide functionalities in bidentate chelation

[RuCl3(NO)(H2O)2] (1) reacts with triglycine (glyglygly) (***) and diglycylhistidine (glyglyhis) (***) (*** symbols represent formulas of the peptides for the monoanion deprotonated at the carboxylate; (glyglyglyH) or (glyglyhisH) represents ligands of one deprotonated amide, and (glyglygly2H) with two deprotonated amides) to form several complexes of the {RuII(NO+)}6 chromophore. Only coordination from the terminal amino group of (glyglygly) occurs below pH∼5. After heating 15 h at 55°C, pH 8.22, coordination through the terminal amine and adjacent ionized amido group as [Ru(NO)Cl3(glyglyglyH)]2− (2) is detected by 1H NMR with the appearance of one ‘AB’ quartet for the CH2 adjacent to the N-terminus. The NO stretching frequency suggests chelated N donors are cis to the NO+ ligand in Na2[Ru(NO)Cl3(glyglyglyH)] (νNO=1877 cm−1). Removal of 3Cl− by treatment with 3Ag(CF3CO2) at pH 7.6 allowed formation of the more chelated [Ru(NO)(H2O)2(glyglygly2H)] in approximately 56% yield, but the equilibrium could not be shifted fully to this two-amide bound chelate, apparently due to competitive hydrolysis or increased lability of the [Ru(NO)(H2O)3]3+ center. The species that have been detected for glyglyhis complexes (3) strongly depend upon the pH of the preparation. One main species and three lesser ones, in abundances of approximately 70, 20, and 10%, are produced in 3 h (55°C) at pH 5.2. The major isomer is coordinated only via the imidazole side chain. Approximately 20% adopts the C-terminal imidazole plus the adjacent amide chelation, distributed between two diastereomers with the terminal carboxylate placed above or below the RuN2Cl2 plane in nearly equal amounts. Another 10% adopts the N-terminal amine plus amide chelation of the kind discussed for glyglygly. The νNO value of 1871 cm−1 of the main imidazole-only coordinated product matches that observed for the 1:1 [RuCl3(NO)(imidazole)(H2O)] complex. The dissociation of the imidazole-bound species occurs when the pH is raised above 8. The distribution of species and the nearly equal amounts of the diastereomers have been supported by mmff94 and sybyl molecular mechanics structural energy minimization calculations. [NiII(glyglygly2H)]2− and [PdII(glyglygly2H)Cl]2− were examined by NMR methods for comparison. The presence or absence of the coordinated carboxylate was readily established by 13C NMR spectroscopy via a carboxylate chemical shift near 187 ppm for NiII and 173 ppm for {Ru(NO)}6 or PdII.