Discrete and polymeric heteronuclear constructs derived from triangular 2,2′-bipyrazine complexes of cis-a2PtII (with a = NH3 or a2 = en)

Abstract

cis-[Pta₂(H₂O)₂]²⁺ (with a = NH₃ or a₂ = en) and 2,2′-bipyrazine (2,2′-bpz) react in water preferentially to cyclic, triangular complexes of composition [{cis-Pta₂(2,2′-bpz-N4,N4′)}₃]⁶⁺ (a = NH₃, 1a; a₂ = en, 1b). In their C₃-symmetric conformation (all bridging pyrazine rings adopt cisoid orientations with respect to the central C2–C2′ bond), 1a and 1b provide three pairs of N1,N1′ donor sites each, which are capable of chelating additional (hetero)metal ions. The latter can in turn bridge to other N1,N1′ sites in an intermolecular fashion or simply complete their coordination spheres with other ligands (water, anions). Four previously not observed structural variants of heteronuclear (Pt^II, Cu^II; Pt^II, Ag^I; Pt^II, Cd^II) constructs have now been isolated by us and are reported here: a decanuclear Pt₆Cu₄ complex, [{(en)Pt(2,2′-bpz)}₃]₂Cu₄(H₂O)₆(NO₃)₂₀·11H₂O (2) with the appearance of a paddle-wheel, a dodecanuclear Pt₆Cu₆ capsule with a Cu²⁺ ion in its interior and additional Cu²⁺ ions in its periphery giving a total composition of [{(en)Pt(2,2′-bpz)}₃]₂Cu₁₁(NO₃)₃₄(H₂O)₁₈·3H₂O (3), as well as two coordination polymers, {[{(en)Pt(2,2′-bpz)}₃]Cd₂(H₂O)₇}(SO₄)₅·{[Cd(H₂O)₆](SO₄)}·15H₂O (4) and [{cis-(NH₃)₂Pt(2,2′-bpz)}₃]Ag(SiF₆)₃(BF₄)·7H₂O (5). The structures of these four compounds are dominated by host–guest interactions between the triangular metal vases of 1a and 1b and anions, respectively, as well as hydrogen bonding involving anions and water molecules and electrostatics. In aqueous solution considerable dissociation of the heteronuclear compounds 2–5 into the triangular Pt complexes 1 and the heterometal ions takes place. Preliminary AFM (atomic force microscopy) studies reveal that the +6 cations of 1a have a strong tendency to interact with double-stranded DNA with formation of condensed DNA states.