Co-ordination chemistry of macrocyclic compounds with dangling phosphines. Unusual NMR shifts in metallo-calix[4]arenes †

Abstract

The chelating behaviour of three polyphosphines, cone-5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(diphenylphosphinomethoxy)calix[4]arene L1, cone-5,11,17,23-tetra-tert-butyl-25,26,27-tris(diphenylphosphinomethoxy)-28-methoxycalix[4]arene L2, and cone-5,11,17,23-tetra-tert-butyl-25,26-bis(diphenylphosphinomethoxy)-27,28-dihydroxycalix[4]arene L3, has been investigated. When [Mo(CO)3(C7H8)] and tetraphosphine L1 are heated together under reflux in tetrahydrofuran (THF) complex [Mo(CO)3L1] 1 is formed, for which the calixarene behaves as a fac-bonded tridentate ligand with one phosphine remaining free. Similar fac-chelating behaviour is found with [Mo(CO)3L2] 2, which is obtained from triphosphine L2. Formation of this latter complex is accompanied by the calixarene matrix adopting a partially flattened-cone conformation. In contrast, the conventional cone conformation is maintained in the trinuclear complex [(AuCl)3L2] 3, obtained quantitatively by treating L2 with [AuCl(THT)] (THT = tetrahydrothiophene). Reaction of L1 with [RuCl2(DMSO)4] (DMSO = Me2SO) in CH2Cl2 results in selective formation of the deep purple complex [RuCl2L2] 4 built around a fac-trigonal bipyramidal RuCl2P3 structure. Complex 4 reacts reversibly and stepwise with two equivalents of CH3CN. The calculated stability constants, as determined from a spectrophotometric titration, are log β1 = 9.1 and log β2 = 12.4. The proximally substituted calixarene L3 reacts with [PtCl2(COD)] (COD = cycloocta-1,5-diene) to afford the chelate complex cis-[PtCl2L3] 5. As revealed by an X-ray diffraction study, the P–Pt vectors point away from the calixarene axis in the solid state. The axial H atom of the C6H2CH2 group located between the two phosphine units of L3 undergoes a significant low-field shift upon complexation (δ 7.32 vs. 4.48 for free L3) presumably due to interaction with the lone pairs of the two neighbouring O-atoms. Complex 5 displays dynamic behaviour in solution, which can be rationalized as follows: (i) a fast flip-flop motion of the hydroxyl groups at low temperature, alternately forming hydrogen bonds with each of two neighbouring phenolic oxygens; (ii) a reversible inversion of the phenol ring through the lower-rim annulus, triggered by breakage of the hydrogen bonds at higher temperature. Reaction of [PtCl2(COD)] with one equivalent of L1, followed by in situ oxidation with NH2CONH2·H2O2, results in formation of a chelate complex, containing two proximal phosphines bonded to platinum as in 5 and two pending CH2P(O)Ph2 phosphine oxides. Stepwise reaction of [PtCl2(COD)] with one equivalent of L1 and two equivalents of [AuCl(THT)] gives a cis complex in which the platinum atom is again bonded to two proximal phosphines and the two AuCl units to the other two phosphine arms. As in 5, an anomalous low-field shift is observed for the axial C6H2CH belonging to the platinocycle of these complexes.