Calixarene-monophosphines as supramolecular chelators

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 pi-pi 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 x 1(2)] and in [RuCl2(p-cymene) x 20]. As shown by a variable temperature study carried out on [RuCl2(p-cymene) x 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) x 3] with AgBF4 in CH3CN results in an intra-cavity reaction, with one chloride ligand being replaced by acetonitrile.