Borane-Containing Ligands Based on Thioxanthene, Xanthene and Acridine: Syntheses and Platinum Coordination Chemistry

Abstract

<p> Ligand backbones based on xanthene, thioxanthene and acridine (XPB, TXPB and APB) have been targeted as scaffolds for the formation of rare compounds containing Lewis basic phosphine and Lewis acidic borane moieties. The resulting complexes with Pt(O) and Pt(II) reagents are expected to possess diverse reactivity compared to traditional platinum phosphine adducts owing to the strategically appended borane Lewis acid. </p> <p> Two ligands, 2,7-di-(tert-butyl)-4-diphenylboryl-5-diphenylphosphino-9,9dimethylthioxanthene (TXPB) and 2,7-di-(tert-butyl)-4-diphenylboryl-5diphenylphosphino-9,9-dimethylxanthene (XPB), have been synthesized and reacted with the platinum reagents [PtCl2(COD)], [PtMe2(COD)] and [Pt(nbe)3]. In the case of TXPB, two new Pt(II) compounds [PtCl(μ-Cl)(TXPB)] and [PtMePh(TXPB^(Ph,Me))] have been formed, isolated and characterized. In both compounds, X-Ray analysis shows that the S and the P atoms bind to the Pt center drawing the metal into the vicinity of the B group. In [PtCl( μ-Cl)(TXPB)], a chloride is seen to bridge between the Pt and B, whereas [PtMePh(TXPB^(Pb,Me))] shows that a methyl groups has been transferred to the boron with concomitant transfer of a phenyl group to Pt. Reaction with the platinum(O) precursor [Pt(nbe)3] led to formation of an unstable complex, tentatively assigned as [Pt(nbe)TXPB]. Analogous studies with the new xanthene-derived ligand, XPB, led to starkly different results. In the reaction with [PtCl2(COD)], an insoluble product, perhaps a dinuclear complex with bridging chlorides, was formed. No reaction occurred with [PtMe2(COD)], presumably because of non-participation of the 0 ligand. However, with [Pt(nbe)3], a stable complex, [Pt(nbe)2XPB], was observed. </p> <p> Synthesis of a third ligand scaffold APB has been initiated where the pyridine moiety is expected to allow strong chelation of Pt as observed with TXPB. An advanced intermediate AFBr has been synthesised where the backbone has been created through a ring closure reaction positioning two different halides as required for sequential addition of the phosphine and borane groups of the target compound. </p>