Synthesis and structure of two crowded trans-[PdCl2L2] complexes based on a chiral, calix[4]arene-fused phosphole

Abstract The chiral, calix[4]arene-fused phosphole rac - 1 reacts with [PdCl 2 (PhCN) 2 ] to afford a mixture of two diastereomeric complexes, trans -[PdCl 2 {( P , R )- 1 }{( M , S )- 1 }] ( 2a ) and trans -[PdCl 2 {( P , R )- 1 } 2 ]/ trans -[PdCl 2 {( M , S )- 1 } 2 ] ( 2b ), which can be separated by column chromatography. Ligand 1 contains an inherently chiral calix[4]arene skeleton (displaying either P or M helicity) as well as an inversion-stable phosphorus atom (with R or S configuration) having its lone pair located on the endo -face of the calixarene cavity. Both complexes, which equilibrate slowly in solution ( 2a : 2b  = 39:61 at equilibrium), have been isolated and characterised by single-crystal X-ray diffraction: Complex 2a : triclinic, space group P 1 ¯ , unit cell dimensions a  = 11.0070(3), b  = 21.9271(7), c  = 23.2270(7) A, α  = 116.054(2)°, β  = 95.823(2)°, γ  = 92.956(2)°, Z  = 2; Complex 2b : triclinic, space group P 1 ¯ , unit cell dimensions a  = 14.2643(6), b  = 17.3200(8), c  = 22.6335(10) A, α  = 102.496(2)°, β  = 103.774(2)°, γ  = 95.034(2)°, Z  = 2. The unit cell of 2a contains two inequivalent centrosymmetric molecules, which differ only slightly from each other. Both palladium atoms have close to a square planar environment and are sterically protected along the d z 2 axis by the two bulky calixarene units, which adopt markedly pinched conformations. Each chlorine atom sits close to an equatorial H atom (H eq ) of a methylenic ArC H 2 group (av. H eq …Cl separation 2.88 A). Consistent with an H eq …Cl bonding interaction, the corresponding 1 H NMR signal undergoes an unusual downfield shift upon complex formation. In the solid, 2b consists of a racemic mixture of trans -[PdCl 2 {( P , R )- 1 } 2 ] and trans -[PdCl 2 {( M , S )- 1 } 2 ]. Similar H eq …Cl interactions to those seen in 2a are found in 2b .