Multinuclear platinum(ii)–amine complexes containing bis(aminopropyl)dicarba-closo-dodecaborane(12) ligands

Abstract

Treatment of the bridging bidentate 1,Z-bis(aminopropyl)-1,Z-dicarba-closo-dodecaborane(12) (1,Z-bis(aminopropyl)-1,Z-carborane) ligands of the type 1,Z-{H₂N(CH₂)₃}₂-1,Z-C₂B₁₀H₁₀ (L¹, Z = 7, 5) or (L², Z = 12, 6) with two equivalents of trans-[PtClI₂(NH₃)]⁻, followed by halogen ligand metathesis with AgOTf and HCl_(aq) afforded the novel diplatinum(II)–amine species cis-[{PtCl₂(NH₃)}₂Lⁿ] (7 (n = 1) or 8 (n = 2), respectively). Similarly, the reaction of L¹ or L² with the labile trans-[PtCl(dmf)(NH₃)₂]⁺ afforded trans-[{PtCl(NH₃)₂}₂Lⁿ](OTf)₂ (9 (n = 1) or 10 (n = 2), respectively) in good yield and purity. However, isolation of the analogous 1,2-carborane complexes was not possible owing to decomposition reactions that led to extensive degradation of the carborane cage and reduction of the metal centre. The mixed dinuclear complex [cis-{PtCl₂(NH₃)}–L¹-trans-{PtCl(NH₃)₂}]OTf (19) was prepared by treatment of the Boc-protected amine ligand 1-{(Boc)₂N(CH₂)₃}-7-{H₂N(CH₂)₃}-1,7-C₂B₁₀H₁₀ (L³, 15) with trans-[PtCl(dmf)(NH₃)₂]⁺ to yield trans-[PtCl(NH₃)₂L³]OTf (16), followed by acid deprotection of the pendant amine group, complexation with trans-[PtClI₂(NH₃)]⁻, and halogen ligand metathesis using AgOTf and HCl_(aq). A novel trinuclear species containing 5 was prepared by the addition of two equivalents of 15 to the labile precursor cis-[Pt(dmf)₂(NH₃)₂]²⁺ followed by acid deprotection of the pendant amine groups. Further complexation with two equivalents of trans-[PtClI₂(NH₃)]⁻ followed by halogen ligand metathesis using AgOTf and HCl_(aq) afforded the triplatinum(II)–amine species [cis-{Pt(NH₃)₂(L¹)₂}-cis-{PtCl₂(NH₃)}₂](OTf)₂ (23). Complexes 7–10, 19 and 23 represent the first examples of multinuclear platinum(II)–amine derivatives containing carborane cages. Preliminary in vitro cytotoxicity studies for selected complexes are also reported.