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)2Os(II)(HL1)Cl]ClO4 [1]ClO4 and [(bpy)2Os(II)(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-HCl) 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-HO) 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 Os(II/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) (1(2+)/2(2+)) 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.

Electrochemical Anion Sensing: Supramolecular Approaches

“…A variety of other redox-receptors based on metals have also been reported, whereby an advantageous double readout (electrochemical, luminescent) is often feasible. Most of these receptors are based on transition metal complexes of ruthenium or osmium with receptor modified nitrogenous ligands, such as bipy, terpy, imidazole, or pyrazole (for example, receptors 14 and 15 , Figure , in which anion sensing was reported via the M 2+ /M 3+ couple). − In a rare example of a receptor containing two redox transducers, Beer and co-workers reported tetraamide macrocycles 16a , b containing both a Ru­(bpy) 3 2+ as well as either a Fc or a Cp 2 Co + transducer . Cl – sensing in ACN was achieved both via the metallocene as well as bpy redox couples, with cathodic perturbation of 16a of −40 and −60 mV for the couples, respectively (anion in excess).…”

Indazole-Derived Mono-/Diruthenium and Heterotrinuclear Complexes: Switchable Binding Mode, Electronic Form, and Anion Sensing Events

“…Though ruthenium complexes of heterocyclic imidazole derivatives have widely been explored for selective anion sensing, , the effectivity of the analogous pyrazole moiety on the same ruthenium platform has surprisingly been overlooked. On the contrary, the recently reported corresponding osmium–pyrazole complex [Os II (bpy) 2 (Cl)­(H-Pz)]­ClO 4 (H-Pz = pyrazole) ( A in Scheme ) displayed varying selectivity as a function of substituents in the pyrazole frameworks: pyrazole versus 3,5-dimethylpyrazole …”

“…On the contrary, the recently reported corresponding osmium− pyrazole complex [Os II (bpy) pyrazole) (A in Scheme 1) displayed varying selectivity as a function of substituents in the pyrazole frameworks: pyrazole versus 3,5-dimethylpyrazole. 10 This indeed is the genesis of the present effort of examining the anion sensing ability of the ruthenium−acac and ruthenium−bpy complexes involving a less explored pyrazole derivative, that is, H-Iz (indazole) (Scheme 2). The industrially 11 and biologically 12 important indazole (H-Iz) heterocycle, that is, the benzene ring-fused pyrazole unit, exists in its stable 1H-indazole form along with the less stable tautomeric forms 2H-indazole and the elusive 3H-indazole (Scheme 2).…”