Multi-Ferrocene-Containing Silanols as Redox-Active Anion Receptors

The ability of diferrocenylsilanediol, Fc2Si­(OH)2 (5), and 1,1,3,3-tetraferrocenyldisiloxane-1,3-diol, Fc2(HO)­Si–O–Si­(OH)­Fc2 (6), to act as new electroactive anion receptors for either acetate or chloride anions has been investigated in solution, in the solid state, and in the gas phase. 1H NMR spectroscopic titrations with anions reveal that the binding interaction causes chemical-shift perturbations not only in the Si–OH hydrogen-bonding donor motif but also in the ferrocenyl protons of receptors 5 and 6. Square-wave voltammetric studies evidence that multiferrocenyl silanols 5 and 6 exhibit higher ability for electrochemical sensing of acetate than chloride, since the corresponding half-wave potentials (E1/2) for the successive ferrocene oxidations display a higher cathodic shift in the presence of such an anion. Furthermore, single-crystal X-ray diffraction analyses of the tetrabutylammonium salts of complexes [Fc2Si­(OH)2·CH3COO]− (8), [Fc2Si­(OH)2·Cl]− (9), [Fc2(HO)­Si–O–Si­(OH)­Fc2·CH3COO]− (10), [{Fc2(HO)­Si–O–Si­(OH)­Fc2}2·CH3COO]− (11), and [Fc2(HO)­Si–O–Si­(OH)­Fc2·Cl]− (12) confirm that redox-active silanol receptors 5 and 6 can bind the acetate and chloride anions in the solid state. Electronic structure calculations were carried out for 5 to explore the intrinsic ability of the silanediol group to bind these anions in a vacuum.