Synthesis based on the formation of noncovalent bonds provides a valuable alternative to the classical chemistry of covalent bonds. It has developed into an area of enormous interest for a wide variety of scientific and technological disciplines, ranging from materials science to molecular electronics. [1,2] The identification of small hydrogen-bonded dimers [3] or metal-coordinated assemblies [4] is relatively simple. However, for large multicomponent assemblies held together by weak forces the characterization is far from straightforward and is currently one of the major challenges in this field. [5] Most studies primarily rely on NMR data of compounds in solution, which solely provide information on the stoichiometry of the assembly, in combination with data from vapor-pressure osmometry (VPO) and/or gel-permeation chromatography (GPC). [1, 6] The latter techniques give values for the average molecular weight with an error of up to 20 %. Occasionally light-or neutron-scattering data [7] or single X-ray crystal structures are reported. [8] However, the characterization of weakly bound noncovalent assemblies by mass spectrometry, the only technique that provides quantitative data on the molecular composition, is still met with very limited success. [9] For hydrogen-bonded assemblies only two cases have been reported by Lehn et al. and Whitesides et al., [10] but the ion-labeling methods they use either require covalent attachment of benzo [18]crown-6 moieties to one of the components or work only for extremely stable assemblies.
Here we describe a novel Ag -labeling technique for the mass spectrometric characterization of multicomponent hydrogen-bonded assemblies. The method is based on the remarkably high affinity of Ag ions for a variety of aromatic p donors [11,12] and cyano groups, [13] and it provides for a nondestructive way to generate positively charged hydrogenbonded assemblies that can be easily detected by matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry. [14] The method is applicable to assemblies of both high (type A) and much lower thermodynamic stability (type B; Figure 1). Moreover, we show that the MALDI-TOF-MS data perfectly correlate with 1 H NMR