NMR and structural studies of gold(I) chloride adducts with bidentate 2-, 3- and 4-pyridyl phosphines

Abstract

The 1∶2 adducts of gold(I) chloride with the diphosphine ligands, 1,2-bis(di-n-pyridylphosphino)ethane (dnpype) for n = 2, 3 and 4 have been synthesized and solution properties characterized by NMR spectroscopy, including variable temperature one dimensional ³¹P{¹H} and 2D [³¹P, ³¹P] COSY experiments. The results show the 3-pyridyl (d3pype) and 4-pyridyl (d4pype) adducts to exist as bis-chelated monomeric [Au(d3pype)₂]⁺ and [Au(d4pype)₂]⁺ while the 2-pyridyl (d2pype) adduct forms an equilibrium mixture of monomeric [Au(d2pype)₂]⁺, dimeric [{Au(d2pype)₂}₂]²⁺ and possibly tetrameric [{Au(d2pype)₂}₄]⁴⁺ species in which the d2pype ligands coordinate in both bridging and chelated modes via the phosphorus atoms. The relative percentages of the species present are dependent on both temperature and solvent. A single crystal X-ray structure determination of the 4-pyridyl adduct obtained from ethanol/hexane shows the complex to crystallize as [Au(d4pype)₂H]Cl₂·6H₂O with monomeric cations and one of the 4-pyridyl rings likely to be protonated. Crystals of the 2-pyridyl complex obtained from methanol solution have been shown by crystal structure determination to be the dimer [(d2pype)Au(µ-d2pype)]₂Cl₂·14H₂O, in which each gold atom is coordinated by one chelated and two bridging d2pype ligands. The solubility properties and solution behaviour of these three systems are compared to the analogous 1,2-bis(diphenylphosphino)ethane (dppe) system and the potential significance of these results to the antitumour properties of chelated 1∶2 Au(I) diphosphine complexes are discussed.