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 [M-2(2,2'-bipy)(2){4,4'-bipy(CH2)(6)4,4'-bipy}(2)](PF6)(8). Treatment of [Pd(PhCN)(2)Cl-2] with 1,3-bis(4,4'-bipyridinium)propane hexafluorophosphate in MeCN afforded [Pd2Cl4{4,4'-bipy(CH2)(3)4,4'-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)center dot center dot center dot Pt(II) bonding supported by strong UV-vis absorption at lambda = 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 [Pt-4(2,2'-bipy)(4){apyz(CH2)(6)apyz-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)center dot center dot center dot Pd(II) bonding interactions.