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 [PdCl2(PhCN)2] to afford a mixture of two diastereomeric complexes, trans-[PdCl2{(P,R)-1}{(M,S)-1}] (2a) and trans-[PdCl2{(P,R)-1}2]/trans-[PdCl2{(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 P1¯, unit cell dimensions a = 11.0070(3), b = 21.9271(7), c = 23.2270(7) Å, α = 116.054(2)°, β = 95.823(2)°, γ = 92.956(2)°, Z = 2; Complex 2b: triclinic, space group P1¯, unit cell dimensions a = 14.2643(6), b = 17.3200(8), c = 22.6335(10) Å, 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 dz2 axis by the two bulky calixarene units, which adopt markedly pinched conformations. Each chlorine atom sits close to an equatorial H atom (Heq) of a methylenic ArCH2 group (av. Heq…Cl separation 2.88 Å). Consistent with an Heq…Cl bonding interaction, the corresponding 1H NMR signal undergoes an unusual downfield shift upon complex formation. In the solid, 2b consists of a racemic mixture of trans-[PdCl2{(P,R)-1}2] and trans-[PdCl2{(M,S)-1}2]. Similar Heq…Cl interactions to those seen in 2a are found in 2b.