Shape-persistent nanosize organometallic complexes:: Synthesis and application in a nanofiltration membrane reactor

Shape-persistent multi(NCN-palladium and/or -platinum) complexes having one- (5 and 6), two(1 and 2), and three-dimensional (3 and 4) geometries were prepared in moderate to good yields. Two different approaches were used to construct the multimetallic materials: (i) the construction of the multisite ligands followed by the permetalation step and (ii) selective and mild one-pot coupling of monometallic buiding blocks to a multifunctional shape-persistent organic core molecule. The first approach was used to prepare the palladated and/or platinated tris- (2) and bis(NCN-pincer) (5) complexes, while the second approach afforded the palladated and platinated octakis(3) and dodecakis(NCN-pincer) (4) complexes. Complexes 1-6 were subjected to nanofiltration (NF) experiments in order to investigate the influence of rigidity and geometry on the retention of these molecules by NF membranes. For this purpose, the corresponding (NCN-Pt-X)(n) complexes (1c-4c, 5, and 6) were used since exposing these complexes to sulfur dioxide in solution resulted in the formation of bright orange complexes, allowing the use of UV/vis spectroscopy to accurately determine the concentrations of 1-6 in both retentate and permeate. Using the MPF-60 (MWCO = 400) NF-membrane, retention rates of 82.4 (6), 93.9 (1c), 98.7 (2c), 99.5 (3c), 99.6 (5), and >99.9% (4c) were found, while 2c and 4c in combination with the MPF-50 (MWCO = 700) NF-membrane were retained in 97.6 and 99.9%, respectively. A clear relationship is observed between the dimensions calculated by molecular modeling and the retention rates of 1-6. The one-dimensional bis(pincer-platinum) complex 5, however, shows an unexpectedly high retention rate (99.6%) that can be due to precipitation of the complex in the membrane (clogging of the membrane) and/or to the formation of larger aggregates near the membrane. In addition, comparison of 2 and 4 with flexible nickelated G0- and G1-dendrimers with similar dimensions proved that a high degree of rigidity in the backbone of macromolecular complexes indeed leads to more efficient retentions of these multimetallic materials by NF-membranes.