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Two new ruthenium complexes Ru(bipy)(2)(PDA)(2+) (1) and Ru(phen)(2)(PDA)(2+) (2) (PDA = 1,10-phenanthroline-4,7-dicarboxaldehyde) have been synthesized to detect cyanide based on the well-known formation of cyanohydrins. Both 1PF(6)(2) and 2PF(6)(2) were fully characterized by various spectroscopic techniques and their solid state structures determined by single-crystal X-ray diffraction. Their anion binding properties in pure and aqueous acetonitrile were thoroughly examined using two different channels, i.e., UV-vis absorption and photoluminescence (PL). After addition of only 2 equiv of CN(-), the PL intensity of 1PF(6)(2) and 2PF(6)(2) was enhanced ∼55-fold within 15 s along with a diagnostic blue shift of the emission by more than 100 nm. PL titrations of 1PF(6)(2) and 2PF(6)(2) with CN(-) in CH(3)CN furnished the very high overall cyanohydrin formation constants log β(CN(-)) = 15.36 ± 0.44 (β(CN(-)) = 2.3 × 10(15) M(-2)) and log β(CN(-)) = 16.37 ± 0.53 (β(CN(-)) = 2.3 × 10(16) M(-2)), respectively. For both probes, the second constant, K(2), is about 57-84 times less than K(1), suggesting that the cyanohydrin reaction is stepwise. The stepwise mechanism is further supported by results of a (1)H NMR titration of 2PF(6)(2) with CN(-). The high selectivity of 2PF(6)(2) for CN(-) was established by PL in the presence of other competing anions. Furthermore, the color change from orange-red to yellow and the appearance of a orange luminescence, which can be observed by the naked eye, provides a simple real-time method for cyanide detection. Finally, theoretical calculations were carried out to elucidate the details of the electronic structure and transitions involved in the ruthenium probes and their cyanide adducts.
Khatua et al. (Wed,) studied this question.