Calixarene-monophosphines as supramolecular chelators

Abstract

The calix[4]arenes 5-diphenylphosphino-17-R¹-11,23-diR²-25,26,27,28-tetrapropoxycalix[4]arene (1, R¹ = R² = Br; 2, R¹ = Br, R² = H; 3, R¹ = R² = p-tolyl; 4, R¹ = p-tolyl, R² = H; 5, R¹ = R² = H; 20, R¹ = p-tolyl, R² = H), all bearing a diphenylphosphino group attached to the calixarene upper rim, have been synthesised starting from 5,11,17,23-tetrabromo-25,26,27,28-tetrapropoxycalix[4]arene. Reaction of 1–5 with [RuCl₂(p-cymene)]₂ leads quantitatively to monophosphine complexes, [RuCl₂(p-cymene)L], in which the endo-oriented ruthenium atom unit sits inside the cone delineated by the four phenoxy rings. The particular orientation of the P–Ru vector appears to result from π–π interactions between the p-cymene ligand and two aromatic cavity walls. Overall, in each case the calixarene end of the ligand behaves as a supramolecular receptor towards the Ru(p-cymene) fragment. Formation of complexes with exo-oriented P–M bonds were observed in the complexes cis-[PtCl₂·1₂] and in [RuCl₂(p-cymene)·20]. As shown by a variable temperature study carried out on [RuCl₂(p-cymene)·3], in which the ruthenium is embedded in an expanded cone, the p-cymene ring undergoes a fast oscillation about the Ru-arene bond which can be frozen out at low temperatures. Reaction of [RuCl₂(p-cymene)·3] with AgBF₄ in CH₃CN results in an intra-cavity reaction, with one chloride ligand being replaced by acetonitrile.