Hydrogen bonding association of a ruthenium(II) bipyridine barbituric acid guest to complementary 2,6-diaminopyridine amide hosts: guidelines for designing high binding hydrogen bonding cavities in both high-and low-polarity solvents

The binding between a ruthenium polypyridine guest RuG2, (where Ru = 4,4′-di-tert-butyl-bpy)2Ru (bpy = 2,2′-bipyridine) and G2 = 5-[4-(4′-methyl)-2,2′-bipyridyl]methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione, and a series of host acyl derivatives of 3,5-bis[(6-aminopyrid-2-yl) amino]carbonylpyridine (R/H = n-Pr/H, phenyl/H, CF3/H, t-Bu/H, —(CH2)3-CO2−H) and 3,5-bis[(6-amino-4-isopropoxypyrid-2-yl)amino]carbonylpyridine diacetyl derivative (R/X = CH3/i-OPr) was studied by fluorescence and NMR titrations. The RuG2 (which exists in the enolate form in the presence of the hosts) forms a number of H-bonds involving the amide groups of the hosts and the carbonyl groups of the G2 for all the hosts studied. Specific 1:1 association between RuG2 and all the complementary hosts was observed with binding constants, Ka (l mol−1), for R/H in CH2Cl2 of 3 × 105 (t-Bu/H), 5 × 106 (Ph/H), 3 × 107 (n-Pr/H), 9 × 107 (CF3/H) and >108 [—(CH2)3CO2H) and for R/X of 4 × 108 (Me/i-OPr). Similar, but weaker, binding was also observed in solvents of higher donor number such as d6-acetone, d3-acetonitrile and d6-DMSO with R/X = Me/i-OPr host showing the highest binding constant in CH2Cl2, d6-acetone and d6-DMSO. Differences in the binding constants of the ruthenium guest RuG2 to these hosts are analyzed in terms of the steric, electronic and solvational changes in the structure of the host amide substituents and the polarity of the solvents used.