Cyanide Anion Binding by a Triarylborane at the Outer Rim of a Cyclometalated Ruthenium(II) Cationic Complex

As part of our ongoing interest in the design of boron-based cyanide anion receptors, we have synthesized a triaryl borane decorated by a cationic Ru(II) complex and have investigated its anion binding properties. This new borane, [(2,2′-bpy)Ru(κ-C,N-2-(dimesitylborylphenyl)pyridinato)]OTf ([2]OTf), binds both fluoride and cyanide anions in organic solvents to afford 2-F and 2-CN whose crystal structures have been determined. UV−vis titrations in 9/1 CHCl3/DMF (vol.) afforded K(F−) = 1.1(±0.1) × 104 M−1 and K(CN−) = 3.0(±1.0) × 106 M−1 indicating that [2]+ has a higher affinity for cyanide than for fluoride in this solvent mixture. These elevated binding constants show that the cationic Ru(II) complex increases the anion affinity of these complexes via Coulombic and inductive effects. The UV−vis spectral changes which accompany either fluoride or cyanide binding to the boron center are similar and include a 30 nm bathochromic shift of the metal-to-ligand charge transfer band. This shift is attributed to an increase in the donor ability of the boron-substituted phenylpyridine ligand upon anion binding to the boron center. Accordingly, cyclic voltammetry revealed that the RuII/III redox couple of [2]OTf (E1/2 = +0.051 V vs Fc/Fc+) undergoes a cathodic shift upon F− (ΔE1/2 = −0.242 V vs Fc/Fc+) or CN− (ΔE1/2 = −0.198 V vs Fc/Fc+) binding.