Rapid and Highly Sensitive Dual-Channel Detection of Cyanide by Bis-heteroleptic Ruthenium(II) Complexes

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.