Substituent directed selectivity in anion recognition by a new class of simple osmium-pyrazole derived receptors†
Abstract
The present article deals with the structurally, spectroscopically and electrochemically characterised osmium-bipyridyl derived complexes [(bpy)2OsII(HL1)Cl]ClO4 [1]ClO4 and [(bpy)2OsII(HL2)Cl]ClO4 [2]ClO4 incorporating neutral and monodentate pyrazole derivatives (HL) with one free NH function (bpy = 2,2′-bipyridine, HL1 = pyrazole, HL2 = 3,5-dimethylpyrazole). The crystal structures of [1]ClO4 and [2]ClO4 reveal intramolecular hydrogen bonding interactions between the free NH proton of HL and the equatorially placed Cl− ligand (N–H⋯Cl) with donor–acceptor distances of 3.114(7) Å and 3.153(6) Å as well as intermolecular hydrogen bonding interactions between the NH proton and one of the oxygen atoms of ClO4− (N–H⋯O) with donor–acceptor distances of 2.870(10) Å and 3.024(8) Å, respectively. The effect of hydrogen bonding interactions has translated into the less acidic nature of the NH proton of the coordinated HL with estimated pKa > 12. 1+ and 2+ exhibit reversible Os(II)/(III) and irreversible Os(III)/(IV) processes in CH3CN within ±2.0 V versus SCE. The effect of 3,5-dimethyl substituted HL2 on 2+ has been reflected in the appreciable lowering (40 mV) of the OsII/III potential, along with the further decrease in the acidity of the NH proton (pKa > 13.0) with regard to HL1 coordinated 1+ (pKa: ∼12.3). The electronic spectral features of Os(II) (1+/2+) and electrochemically generated Os(III) (12+/22+) derived complexes have been analysed by TD-DFT calculations. The efficacy of the 1+ and 2+ encompassing free NH proton towards the anion recognition process has been evaluated by different experimental investigations using a wide variety of anions. It however establishes that receptor 1+ can recognise both F− and OAc− in acetonitrile solution, while 2+ is exclusively selective for the F− ion.