Diagnostic organometallic and metallodendritic materials for SO2 gas detection: Reversible binding of sulfur dioxide to arylplatinum(II) complexes

A series of square-planar platinum(II) complexes of the N,C,N'-terdentate-c oordinating monoanionic "pincer" ligand, [PtX(4-E-2,6-{CH2NRR'}(2)-C6H2)] ( X = Cl, Br, l, tolyl; R, R' = Et, Me; E = H, OH, OSiMe2-Bu-t) has been prep ared. In the presence of sulfur dioxide, these complexes spontaneously adso rb this gas to form pentacoordinated adducts. Solid-state crystal-structure analyses of the SOL adducts 8c (X=I, R=R' = Me, E = OSiMe(2)tBu) and 11 (X = Cl, R = R' = Me, E = OH) show a square-pyramidal geometry around the met al center with SO2 in the apical position. Most interestingly, the adduct 1 1 forms similar Pt-Cl ... H-O hydrogen-bonded alpha-type networks as the co rresponding SO2-free complex 5. The conservation of the supramolecular info rmation (hydrogen-bonded self-assembly) throughout a reaction (SO2 adsorpti on) is unprecedented in crystal engineering. Adduct formation in the solid state or in solution is fast and reversible and is indicated by a character istic color change of the material from colorless to bright orange. Since f acile methods have been developed to remove SO2 from the adducts and to reg enerate the square-planar starting complexes, these complexes fulfill sever al essential prerequisites of sensor materials for repeated diagnostic SO2 detection. The platinum sensors have been found to be highly selective for sulfur dioxide and particularly sensitive Tor submilimolar to molar gas qua ntities. Their response capacity is tuneable by electronic and steric modif ications of the ligand array by introduction of, for example, different sub stituents on the nitrogen donors. The periphery of dendrimers is shown to b e an appropriate macromolecular support fur anchoring the detection-active sites, thus allowing full recovery of the sensor materials for repeated use . By using this concept, metallo-dendrimers 3 and 15 have been prepared. Ow ing to the dendritic connectivity, these sensors are suitable for repetitiv e qualitative and quantitative detection of small amounts of SO2.