Host-Guest Complexation. 41. Preorganization of a Host Enhances Its Binding of Aryldiazonium Salts

Article abstract of DOI:10.1021/jo00376a004

Synthetic host 1, a spherand comprised of cyclic urea and aryl ether units, has been designed and examined.It displays markedly superior binding of aryldiazonium tetrafluoroborate salts over previously reported complexing agents.Qualitative experiments indicate a higher degree of stabilization of complexed diazonium salts toward thermal decomposition and azo dye formation than is given by 18-crown-6.Quantitative infrared studies yield an approximate binding free energy of -5.9 kcal mol^-1 for the complexation of p-(CH3)3CC6H4N2BF4 by 1 in ClCH2CH2Cl at 25 deg C, appr eciably higher than the -3.6 kcal mol^-1 determined for 18-crown-6 under identical conditions.Addition of an aqueous solution of Na2CO3 to a colorless solution of CH2Cl2 containing C6H5N(CH3)2 and 1 complexed to p-(CH3)3CC6H4N2BF4 instantaneously released the aryldiazonium ion which coupled to form the azxo dye.The alkali metal salts bind 1 much more strongly than does the aryldiazonium salt.Thus the Na⁺ ion acts as a trigger for dye formation, the system as a whole acting as an indicator for the presence of alkali metal ions.