Spectral properties and reactivity of the binuclear complex ion μ-(p-benzoquinonediimine)bis[pentacyanoferrate(II)]

Abstract

A sodium salt containing the binuclear complex anion μ-(p-benzoquinonediimine) bis(pentacyanoferrate(II)], [(NC)5FeHNC6H4NHFe(CN)5]6- [first described by Herington(3)) has been isolated as a solid from an oxygenated solution ofp-phenylenediamine and sodium amminepentacyanoferrate(II) in a 1∶2 molar ratio. The i.r. and u.v.-vis. spectra point to a quinonediimine structure for the bridging ligand. In aqueous solution this ion dissociates into aquopentacyanoferrate(II) andp-benzoquinonediiminepentacyanoferrate(II), with kobs=1.1×10−2s−1. The formation of the mononuclear pentacyanoferrate(II) complex of protonatedp-benzoquinonediimine is favoured at pH≤8. The pKa value for the coordinated diimine isca. two orders of magnitude higher than that for the free ligand, thus indicating thatp-benzoquinonediimine is stabilized by coordination to the [Fe(CN)5]3- moiety, though to a lesser extent than in rutheniumammine analogues.