The binding between a ruthenium polypyridine guest RuG2, (where Ru = 4,4'-di-tert-butyl-bpy) 2 Ru (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, CF 3 /H, t-Bu/H, -(CH 2 ) 3 -CO 2 À H) and 3,5-bis[(6-amino-4-isopropoxypyrid-2-yl)amino]carbonylpyridine diacetyl derivative (R/X = CH 3 /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, K a (l mol À1 ), for R/H in CH 2 Cl 2 of 3 Â 10 5 (t-Bu/H), 5 Â 10 6 (Ph/H), 3 Â 10 7 (n-Pr/H), 9 Â 10 7 (CF 3 /H) and b10 8 [-(CH 2 ) 3 CO 2 H) and for R/X of 4 Â 10 8 (Me/i-OPr). Similar, but weaker, binding was also observed in solvents of higher donor number such as d 6 -acetone, d 3 -acetonitrile and d 6 -DMSO with R/X = Me/i-OPr host showing the highest binding constant in CH 2 Cl 2 , d 6 -acetone and d 6 -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.
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