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.