Cryptate compounds

A method for coupling a first compound of formula (I):

• wherein:
• each R1 and R2 is independently selected from the group consisting of hydrogen, halogen, CH3, COOH, NO2, CH2OH, H2PO4, HSO3, CN, C(═O)NH2 and CHO;
• n=2,3 or 4;
• each X is independently NH or S, provided that at least one X is NH;
• each Z is an optional spacing group which, when present, is independently selected from the group consisting of optionally substituted C1-C10 alkyl, optionally substituted aryl and optionally substituted heterocycle;
• and
• each Y is independently selected from the group consisting of hydrogen, halogen, hydroxyl, nitro, optionally substituted amino, optionally substituted alkyl, optionally substituted aryl, optionally substituted aralkyl, optionally substituted heterocycle, isothiocyanate, cyano, —NH—COCH2Br, —COOR′ and —NH—CO—CH═CH—COOR′, wherein R′ is hydrogen, an optionally substituted alkyl, or an optionally substituted aryl, provided that at least one Y is or contains a primary amine;
• wherein the compound of formula (I) is not complexed with a metal ion;
• with a second compound that contains a carbonyl group that is an aldehyde or ketone, to form a coupling reaction product, the method comprising the steps of:
• (i) dissolving the compound of formula (I) in a solvent comprising water to form a solution;
• and
• (ii) adding to the solution the second compound that contains a carbonyl group;
• wherein the pH of the solution is from about 4 to about 5;
• whereby substantially all coupling will occur at the primary amine of the compound of formula (I);
• whereby
• the second compound reacts with and couples to the compound of formula (I) by way of a Schiff's base condensation between the primary amine of the compound of formula (I) and the carbonyl group of the second compound.