Reactions of a cyanosilane with an iron(II) centre. Synthesis and crystal structure of the isocyanotriphenylborate complex trans-[FeH(CNBPh3)(Ph2PCH2CH2PPh2)2] and anodic deprotonation of the hydrogen isocyanide (CNH) analogue

Abstract

Treatment of a thf solution of trans-[FeH(Cl)(dppe)₂] (dppe = Ph₂PCH₂CH₂PPh₂) with NCSiMe₃, in the presence of Na[BPh₄], formed trans-[FeH(CNBPh₃)(dppe)₂] 1 whose crystal structure was determined, whereas the use of Tl[BF₄] in an acidic medium, instead of Na[BPh₄], yielded trans-[FeH(CNH)(dppe)₂][BF₄] 2. Complex 1 is also formed by BPh₃ addition to trans-[FeH(CN)(dppe)₂] 3 which can be obtained by reaction of trans-[FeH(Cl)(dppe)₂] with [NBu₄]CN in the presence of Tl[BF₄]. Complexes 1 and 3 undergo single-electron reversible oxidations upon cyclic voltammetry in an aprotic medium, and the electrochemical P_L ligand parameter was estimated (–0.51 V) for the CNBPh₃^– ligand. Complex 2 undergoes anodically induced deprotonation to form the cyano-complex 3⁺ following a mechanism that has been established by digital simulation of cyclic voltammetry and allowed an estimate of the rate and acid/base equilibrium constants for the CNH/CN^– interconversion in the 2/3 and 2⁺/3⁺ pairs, showing that the oxidation of 2 leads to enhancements of its acidity constant and of the rate of proton loss by factors of ca. 1.5 × 10⁷ and 500, respectively.