An Approach to Rigid, Optically Active Chelate Ligands with C 2 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 t Bu 4 . The ligands are accessible by substitution of the diiodo derivative I 2 P 6 C 4 t Bu 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 t Bu 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 t Bu 4 )PdCl 2 ] and cis ‐[(R 2 P 6 C 4 t Bu 4 )PtCl 2 ] complexes, respectively. Monoalkylation products RIP 6 C 4 t Bu 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 PdCl 2 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 t Bu 4 )PdCl 2 ] 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 t Bu 4 cage ligands can be obtained from the monoallylation product (allyl)IP 6 C 4 t Bu 4 . This reaction failed for the other monoalkylation products RIP 6 C 4 t Bu 4 and Pt II .