An Approach to Rigid, Optically Active Chelate Ligands with C2 Symmetry: Dialkylhexaphosphapentaprismanes

Diastereomeric Pd and Pt complexes with a highly rigid chelate cage ligand core have been designed on the basis of alkylated C-2-symmetric tetra-tert-butylhexaphosphapentaprismanes R(2)P(6)C(4)tBu(4). The ligands are accessible by substitution of the diiodo derivative I(2)P(6)C(4)tBu(4) with Grignard and organyllithium reagents. Depending on the optical properties of the organyl group, racemic or diastereomeric dialkylhexaphosphapentaprismanes R(2)P(6)C(4)tBu(4) can be treated with suitable Pd-II or Pt-II precursor complexes to form neutral square-planar cis-[(R(2)P(6)C(4)tBu(4))PdCl2] and cis-[(R(2)P(6)C(4)tBu(4))PtCl2] complexes, respectively. Monoalkylation products RIP(6)C(4)tBu(4) were also observed in the reaction mixtures, but in most cases were inactive as chelate ligands towards Pd-II and Pt-II. For R = (S)-2-methylbutyl, the diastereomers of the PdCl2 complex were efficiently separated by column chromatography and crystallization to give the two diastereomers in up to >99% de and they were fully characterized by NMR and CD spectroscopy as well as by X-ray diffraction analysis. The two diastereomers of cis-[{(S)-2-methylbutyl}(2)(P(6)C(4)tBu(4))PdCl2] represent the first example of a pair of isolated optically active, C-2-symmetric, square-planar Pd-II complexes with chelate P-C cage ligands. A square-planar Pd complex with two monoanionic RP(6)C(4)tBu(4) cage ligands can be obtained from the monoallylation product (allyl)IP(6)C(4)tBu(4). This reaction failed for the other monoalkylation products RIP(6)C(4)tBu(4) and Pt-II.