Synthesis, Characterization, Solid-State Structures, and Spectroscopic Properties of Two Catechol-Based Luminescent Chemosensors for Biologically Relevant Oxometalates

The new heteroditopic ligand 2,3-dihydroxy-N-(1,10-phenanthroline-5-yl)benzamide (H2-L3) was synthesized and
coordinated to [Ru(bpy)2(phen)]2+- and [ReBr(CO)3(phen)]-type luminophores (bpy = 2,2‘-bipyridine and phen =
1,10-phenanthroline). The resulting chemosensors [Ru(bpy)2(H2-L3)]2+ and [ReBr(CO)3(H2-L3)] were fully characterized
and their solid-state structures and spectroscopic properties were investigated to assess how the photophysical
properties of the luminescent signaling units affect the performance of the sensors. [Ru(bpy)2(H2-L3)]2+ and [ReBr(CO)3(H2-L3)] both signal the presence and concentration of molybdate and vanadate in aqueous acetonitrile through
a decrease in emission intensity. [ReBr(CO)3(H2-L3)] also detects tungstate. Due to the higher emission intensity
of the Ru-based sensor, its detection limits for molybdate (43 μg L-1) and vanadate (24 μg L-1) are almost 1 order
of magnitude lower than the ones achieved with the Re-based sensor. The optimum working pH of the chemosensors
is determined by the pKa values of the 2-hydroxy-groups of the receptor units:  pH 4 for [ReBr(CO)3(H2-L3)] and pH
3 for [Ru(bpy)2(H2-L3)]2+. Both sensors are selective:  equimolar amounts of PO43-, SO42-, ReO4-, Mn(II), Fe(III),
Co(II), Ni(II), Cu(II), and Zn(II) do not interfere with the detection of molybdate or vanadate.