Weak acids with super-electron-donor dimetal complexes: Synergy in bifunctional activity

Dimetal paddlewheel complexes with bicyclic guanidine ligands are extremely strong thermodynamic electron donors. As a probe of the chemical potential and sites for chemical reactivity of these complexes, the interaction of Mo₂(TEhpp)₄ with weak acids was investigated (TEhpp is the anion of the bicyclic guanidine 3,3,9,9-tetraethyl-1,5,7-triazabicyclo[4.4.0.]dec-4-ene). Mo₂(TEhpp)₄ is readily protonated by acetic acid and trifluoroacetic acid as expected, but surprisingly by a mechanism that is more complicated than a simple acid-base proton exchange. Electrochemistry measurements of the shifts in potentials with successive additions of acid reveals that the conjugate base of the acid plays a critical role throughout the reaction. Computations indicate that initially the acid hydrogen bonds to a TEhpp nitrogen atom bound to the metal, and then a facile rearrangement of the conjugate base toward the axial site of the Mo–Mo bond concomitantly results in protonation of the nitrogen atom. Interestingly, the dimetal complex is bifunctional in this process, acting as a nucleophile at the nitrogen atoms of the TEhpp ligands, and as an electrophile at the Mo–Mo axial bond sites. The protonation requires a novel synergism of these disparate bonding interactions, in which protonation is not favored without enhancement by the coordinated base, and coordination of the base is enhanced by the protonation.