The guest-induced synthesis of a molecular box from methylaquacobaloxime and 1,4-phenylenebisboronic acid, with pyrazine (pz) as guest, is described. The resulting supramolecular species was characterized by X-ray structural analysis, H-1 and C-13 NMR spectroscopy, and low-resolution electrospray ionization (ES) mass spectrometry. The assembly was monitored by a time dependent H-1 NMR experiment, which showed that the guest thermodynamically drives the assembly of the host. The effectiveness of pyrazine in this role arises from its having both the correct geometry and a sufficiently low pK(a) value. Several other ditopic ligands were tested, but none led to formation of an analogous product. However, a second supramolecular species was formed in the case of ethylenediamine (en). X-ray structural analysis and H-1 and C-13 Spectroscopy revealed that this is closely related to the first, with a phenyl side removed and the pz bridge substituted by the en bridge.