Platinum(II) and palladium(II) metallomacrocycles derived from cationic 4,4′-bipyridinium, 3-aminopyrazinium and 2-aminopyrimidinium ligands

A series of cationic, ditopic N-donor ligands based on 4,4′-bipyridine (4,4′-bipy), 3-aminopyrazine (apyz) and 2-aminopyrimidine (apym), each incorporating two positively-charged N-heterocycles linked by a conformationally-flexible spacer unit, have been synthesised and treated with palladium(II) or platinum(II) precursors [M(2,2′-bipy)(NO₃)₂] (M = Pd(II) or Pt(II)) to form highly cationic metallocyclic species. Treatment of 1,6-bis(4,4′-bipyridinium)hexane nitrate with [M(2,2′-bipy)(NO₃)₂] in aqueous solution, followed by the addition of KPF₆, resulted in the formation of the [2+2] species [M₂(2,2′-bipy)₂{4,4′-bipy(CH₂)₆4,4′-bipy}₂](PF₆)₈. Treatment of [Pd(PhCN)₂Cl₂] with 1,3-bis(4,4′-bipyridinium)propane hexafluorophosphate in MeCN afforded [Pd₂Cl₄{4,4′-bipy(CH₂)₃4,4′-bipy}₂](PF₆)₄. When the cationic apyz or apym ligands were used in aqueous solution, the analogous metallomacrocycles did not form. Instead, deprotonation of the exocyclic amino group occurred upon coordination of the ligand to afford a tetranuclear [4+2] species in the case of platinum(II), with Pt(II)Pt(II) bonding supported by strong UV-vis absorption at λ = 428 nm which was assigned to a metal-metal-to-ligand charge transfer (MMLCT) band. Thus, treatment of 1,6-bis(3-aminopyrazinium)hexane nitrate with [Pt(2,2′-bipy)(NO₃)₂], followed by the addition of KPF₆, led to the formation of the red species [Pt₄(2,2′-bipy)₄{apyz(CH₂)₆apyz–2H}₂](PF₆)₈. No related products could be identified with palladium(II), consistent with the low propensity for this metal ion to form strong Pd(II)Pd(II) bonding interactions.