Chiral effects on a fluxional ligand: chiral diphosphine platinum(ii) complexes with thiacrowns

Abstract

We report the syntheses, crystal structures, and spectroscopic properties for two new Pt(II) heteroleptic complexes containing the chiral diphosphine ligands R-BINAP and R,R-Chiraphos along with crown trithioether [9]aneS₃ (1,4,7-trithiacyclononane). The complexes have the general formula [Pt([9]aneS₃)(P₂)](PF₆)₂ (P₂ = R-BINAP or R,R-Chiraphos) and form similar structures in which the Pt(II) center is surrounded by a cis arrangement of the two P donors from the diphosphine chelate and two sulfur atoms from the trithioether macrocycle. The third sulfur in each complex shows a long distance interaction with the metal, resulting in an elongated square pyramidal structure with platinum–axial sulfur distances ranging from 2.695(3) to 2.802(2) Å with the Chiraphos complex showing the shorter distance. The X-ray crystal structure of the Chiraphos complex exhibits an unusual example of co-crystallization of solid state conformational diastereoisomers within the same unit cell. The unit cell consists of two crystallographically independent complex cations which are related by a conformational change around one carbon–carbon bond in the [9]aneS₃ ligand. The ¹⁹⁵Pt NMR chemical shifts for both complexes fall in the range of −4167 to −4452 ppm, consistent with a cis-PtS₂P₂ coordination sphere. The effects of chirality of the diphosphine are most notably seen in the ¹³C NMR spectrum of the fluxional [9]aneS₃ ligand, resulting in two separate resonances as opposed to the single one which has always been observed in Pt complexes containing achiral diphosphines.