A Xanthene-based Ligand with Two Adjacent Malonate Binding Sites

The syntheses of two functionalized xanthenes are described which after deprotonation represent ligands for dinuclear metal complexes. For the previously prepared [^RXanthdim]H₂ - which after deprotonation leads to a ligand with two adjacent β -diiminato binding sites - a significantly improved synthetic procedure is described involving the Pd catalyzed coupling of two diethyl malonate moieties to the xanthene backbone. Deprotonation of the resulting compound [Xanthmal]H2 provides a ligand with two adjacent diethyl malonate functions. To demonstrate this, [Xanthmal]H₂ was reacted exemplarily with two equivalents of LDA to obtain the lithium salt {Li₂[Xanthmal]}2 (4) which can be treated with ZnBr₂ to yield the zinc complex [Xanthmal]₂Zn₂ (5). Alternatively, 5 can be obtained directly from [Xanthmal]H₂, when ZnEt₂ is chosen as the metal precursor. The crystal structures of 4 and 5 are discussed. In summary, the results show that [Xanthmal]²⁻ is a suitable ligand for the preparation of novel dinuclear metal complexes.