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.

Dendrimer-based transient supramolecular networks.

Association of a 16-fold excess of a monodisperse telechelic oligo(THF) (Mw = 1270 g/mol) containing two end groups that selectively bind to the 32 binding sites of a fifth generation dendritic host (Mw = 18,511 g/mol and radius R(h) = 2.4 nm) results in the formation of reversible and dynamic supramolecular complexes. The structure of these complexes in solution depends strongly on the concentration. At low concentration, the two end groups of one guest are proposed to complex to the same host, and flowerlike structures are formed with a radius of R(h) = 3.7 nm. At higher concentrations, both end groups of one guest are complexed to different hosts, forming a bridge between them. This gives rise to the formation of larger associates, and eventually to a transient supramolecular network. Dynamic light scattering unequivocally showed that three distinct relaxation processes, associated with the proposed structures, are present in this system. In addition to the dynamics related to the flowerlike (fast) and the transient network structures (slow), an intermediate dynamic process is attributed to the cooperative motion of a few (approximately 6) connected flowerlike structures. Rheological data elucidate the nature of the intermittent network responsible for the slowest process. A monofunctional guest, not capable of forming a network structure, was used as a reference, and starlike supramolecular structures are formed at all concentrations, indeed.