A Prospective Study of the Use of the [Os(tpy)2]2+ (tpy = 2,2′;6′:2″-Terpyridine) Core as Signalling Scaffolding for the Development of Chemical Sensors

The heteroleptic osmium(n) complex [Os(L-1)(Me-phtpy)]-[PF6](2) (L-1 = 1-[4'-p-tolyl-(2,2':6',2"-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane, Me-phtpy = 4'-p-tolyl-2,2':6',2"-terpyridine) has been prepared, characterized and its potential use for chemical sensing purposes studied. The complex contains a polyazacycloalkane as "binding site" and a Os(tpy)(2)(2+) core that acts as a "signalling subunit". The emission intensity of the osmium complex was quenched in acetonitrile/water (1:1 v/v) by Cu2+ and Ni2+. Potentiometric titrations of [Os(L-1)(Me-phtpy)](2+) in the presence of Cu2+ were carried out in order to determine thermodynamic binding constants in acetonitrile/water (70:30 v/v, 0.1 mol(.)dm(-3) tetrabutylammonium perchlorate). With Cu2+ the receptor [Os(L-1)-(Me-phtpy)](2+) forms the complex {Cu[Os(L-1)(Me-phtpy)]}(4+) at neutral pH. Moreover it undergoes two stepwise protonation processes at acid pH related to the partial protonation of the cyclam core and forms two hydroxo complexes at basic pH. The Ni2+ complexes of the osmium receptor [Os(L-1)(Me-phtpy)](2+) are able to act as an anion fluorogenic chemosensor in acetonitrile/water mixtures. A displacement of the quenching effect to more acidic pH in the Ni2+-[Os(L-1)(Me-phtpy) 12, system was observed for anions in the order ATP > AMP > chloride. A prospective study of the use of the [Os(L-1)-(Me-phtpy)](2+) complex as a fluorogenic sensor for oxygen was also carried out by incorporation of the complex in sol-gel silica films made with TEOS and MTEOS. Significant quenching of the osmium fluorescence by oxygen was observed. The oxygen response of the films showed good stability and repeatability. The study suggested that the Os(tpy)(2)(2+) core might act as suitable signalling scaffolding in chemical sensing systems of cations, anions and gases.