Hydrogen bonding association of a ruthenium(II) bipyridine barbituric acidguest 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'-d i-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, 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 = CH 3/i-OPr) was studied by fluorescence and NMR titrations. The RuG2 (which ex ists 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 th e G2 for all the hosts studied. Specific 1:1 association between RuG2 and a ll the complementary hosts was observed with binding constants, K-a (1 mol( -1)), for R/H in CH2Cl2 of 3 x 10(5) (t-Bu/H), 5 x 10(6) (Ph/H), 3 x 10(7) (n-Pr/H), 9 x 10(7) (CF3/H) and >10(8) [-(CH2)(3)CO2H) and for R/X of 4 x 1 0(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)-DM SO with R/X = Me/i-OPr host showing the highest binding constant in CH2Cl2, d(6)-acetone and d(6)-DMSO. Differences in the binding constants of the ru thenium guest RuG2 to these hosts are analyzed in terms of the steric, elec tronic and solvational changes in the structure of the host amide substitue nts and the polarity of the solvents used. Copyright (C) 1999 John Wiley & Sons, Ltd.