Reversible switching of the sol-gel transition with ultrasound in rhodium(I) and iridium(I) coordination networks

Reversible coordination networks were prepared by combining diphenylphosphinite telechelic polytetrahydrofuran (2) with [RhCl(COD)]2 or [IrCl(COD)]2 in chloroform. Both systems resulted in stable gels at concentrations above 50 and 30 g/L for the rhodium(I) and iridium(I) networks, respectively. The rheological properties of the two coordination networks (100 g/L) were determined with oscillatory shear experiments, which showed that the elastic moduli are constant over a wide frequency range, indicating gel-like behavior; the iridium(I) gel has an elastic modulus distinctly higher (2.8x10(3) Pa) than that of the rhodium(I) gel (1.0x10(3) Pa). Ultrasonication of the rhodium(I) gel caused liquefaction after 3 min; regelation occurred 1 min after sonication was stopped. The iridium(I) gel was also liquefied after 3 min of sonication, but regelation took 1.5 h at room temperature and more than 10 days at -20 degrees C. 31P NMR measurements on model complexes showed that the large differences in gelation times are in agreement with the ligand exchange kinetics of the rhodium(I) and iridium(I) complexes. We propose that sonication of the gels results in ligand exchange, which changes the network topology without changing the coordination chemistry. Upon sonication, the fraction of metal centers in active cross-links decreases and thereby reduces the gel fraction to zero. The system is not at equilibrium, and upon standing the gel fraction increases at a rate that is determined by the exchange kinetics of the metal complex. The observed effects offer opportunities to use ultrasound in the activation of dormant transition metal catalysts.