Self-aggregation of amino-acidate half-sandwich ruthenium(II) complexes in solution: From monomers to nanoaggregates

The aggregation tendency of [RuCl(AA)(Arene)] complexes (1, AA = amino acidate = Gly, Arene = p-cymene; 2, AA = Ala, Arene = p-cymene; 3, AA = N,N'-dimethyl-Gly, Arene = benzene; 3b, AA = N,N'-dimethyl-Gly, Arene = p-cymene; 3c, AA = N,N'-dimethyl-Gly, Arene = hexamethylbenzene; 4a, AA = t-Leu, Arene = benzene; 4b, AA = t-Leu, Arene = p-cymene; 4c, AA = t-Leu, Arene = hexamethylbenzene; 5, AA = alpha,alpha'-Me-2-Gly, Arene = p-cymene; 6, AA = alpha,alpha'-Ph-2-Gly, Arene = p-cymene; 7, AA = Pro, Arene = p-cymene) as a function of the concentration and solvent (CDCl3, CD2Cl2, acetone-d(6), and 2-propanol-d(8)) was investigated through diffusion NMR measurements. The equilibrium constant (K) and the standard variation of the free energy (Delta G degrees) for the aggregation process were determined by applying the Equal K self-aggregation model. The highest level of aggregation was observed for complexes 1, 2, and 4, bearing the NH2 moiety, which was involved in intermolecular H-bonding. Complex 2 formed aggregates with a hydrodynamic radius (r(H)) equal to 20.8 A in CDCl3 (Delta G degrees(296K) = -7.1 +/- 0.7 kcal mol(-1)) at a concentration of 124.9 mM, corresponding to an aggregation number (N) of 133. On the other hand, complex 3c did not show any tendency to aggregate (N = 1.1, 0.5 mM in CDCl3). The aggregation tendency decreased as the steric hindrance of arene (4a > 4b > 4c) and AA (1 approximate to 2 > 5 approximate to 4b > 6) and the polarity and proticity of the solvent increased. For complex 2, -Delta G degrees(kcal/mol) was 7.1 in CDCl3 (epsilon(r) = 4.81) > 5.6 in CD2Cl2 (epsilon(r) = 8.93) > 3.9 in acetone-d(6) (epsilon(r) = 20.56) > 3.0 in 2-propanol-d(8) (epsilon(r) = 19.92). While the two diastereoisomers of complexes 2 and 4b showed substantially the same tendency to self-aggregate, diastereoisomer (R-Ru, S-N, S-C)-7 showed a remarkably higher aggregation tendency than the other one [(S-Ru, S-N, S-C)-7] throughout the entire concentration range (1.4-178.0 mM) in CDCl3, indicating that a diastereoselective recognition process is occurring in solution [parallel to Delta(Delta G degrees(296K))parallel to = 1.8 +/- 0.5 kcal mol(-1)].