Balancing the acidity of the pendant urea arm of bis-heteroleptic ruthenium(II) complex containing pyridyl triazole for improved oxyanion recognition

Two new RuII-based bis-heteroleptic ditopic receptors 1[PF6](2) (C44H34F12N10OP2Ru) and 2[PF6](2) (C40H27F17N10OP2Ru), decorated with a 1-naphthyl and pentafluorophenyl urea pendant arm, respectively, along with the previously reported 3[PF6](2) (C40H31F13N10OP2Ru), containing a pendant 4-fluorophenyl urea unit and 4[PF6](2), devoid of a pendant urea arm, have been studied to establish the role of urea proton acidity on the sensing and extraction of oxyanions in the presence of triazole C-H as an additional hydrogen bonding motif. H-1-NMR, isothermal titration calorimetry (ITC) and photophysical experiments show selective binding of 1[PF6](2) and 2[PF6](2) toward oxyanions such as phosphates (e.g., H2PO4- and HP2O73-) and carboxylates (e.g., CH3CO2- and PhCO2-) like 3[PF6](2). This generalizes the role of triazole C-H and urea in the RuII-based bis-heteroleptic ditopic receptors towards recognition of such anions. Interestingly, complex 1[PF6](2) having intermediate acidic urea -NH protons showed the highest binding affinity with phosphates as compared to the other urea analogues 2[PF6](2) and 3[PF6](2) as well as the non-urea analogue, 4[PF6](2) (C32H24F12N8P2Ru). Moreover, 2[PF6](2) having the most acidic -NH protons showed higher binding affinity towards carboxylates as compared to that of 1[PF6](2/3)[PF6](2). Detailed photo-physical studies revealed that 1[PF6](2) is a farsuperior and more selective H2PO4- sensor compared to 2[PF6](2/3)[PF6](2/4)[PF6](2), as evidenced by the higher degree of amplification of RuII center-based MLCT emission, greater change in excited state lifetime, lower detection limit and higher degree of selectivity. Furthermore, 1[PF6](2) also acts as a moderate liquid-liquid extraction agent of H2PO4-, CH3CO2- and PhCO2- anions, which was comparable to 3[PF6](2) and much higher as compared to 2[PF6](2/4)[PF6](2).