Positioning of transition metal centres at the upper rim of cone-shaped calix[4]arenes. Filling the basket with an organometallic ruthenium unit

Abstract

A series of calix[4]arenes bearing diphenylphosphino groups tethered at the upper rim have been prepared by treatment of 5,11,17,23-tetrabromo-25,26,27,28-tetrapropoxycalix[4]arene with Bu^tLi (or BuⁿLi) followed by reaction with PPh₂Cl. The tetraphosphinated derivative 3 was found suitable for the formation of tetranuclear species, notably [3·(AuCl)₄], [3·{RuCl₂(p-cymene)}₄], and [3·{PdCl(o-C₆H₄CH₂NMe₂)}₄], all possessing an apparent C_4v-symmetry in solution. Reaction of [RuCl₂(p-cymene)]₂ with the diphosphines 5,17-di-X-11,23-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene (X = H, 1; X = Br, 4) afforded the C_2v-symmetrical dinuclear complexes [1·{RuCl₂(p-cymene)}₂] and [4·{RuCl₂(p-cymene)}₂], respectively. Reaction of the non-brominated diphosphine 1 with [PdCl(o-C₆H₄CH₂NMe₂)]₂ gave the complex [1·{PdCl(o-C₆H₄CH₂NMe₂)}₂]. Reaction at high dilution of [PtCl₂(1,5-cyclooctadiene)] with 4 or 1 resulted in quantitative formation of the corresponding cis-chelate complexes [4·PtCl₂] (12) and [1·PtCl₂] (13), respectively. The trans version of 13 could also be obtained, provided that [PtCl₂(PhCN)₂] was used as starting complex. In the solid state, the PtCl₂ unit of 12 is directed towards one bromine atom, resulting in a highly unsymmetrical calixarene structure where the metal plane is nearly parallel to the calix reference plane. The NMR spectra of 12 and 13 show an apparent C_2v-symmetrical structure, suggesting a fast fan-like motion in solution of the metal plane about the P⋯P axis. Similar dynamics are likely to occur in the related cationic complexes [1·Rh(norbornadiene)]BF₄ (15) and [1·Pd(Me-allyl)]BF₄ (16). As shown by variable temperature studies carried out on 12 and 16, these dynamics couple with a concomitant, restricted rotation of the two PPh₂ units about their coordination axis. The latter motion is probably a result of steric interactions within the phosphorus environment, two PPh rings being in competition for occupation of the cavity entrance. Reaction of the expanded cavity 5,11,17-tribromo-23-diphenylphosphino-25,26,27,28-tetrapropoxycalix[4]arene 5 with [RuCl₂(p-cymene)]₂ afforded the monophosphine complex [5·RuCl₂(p-cymene)]. In solution as well in the solid state, the p-cymene ligand fills the calixarene basket.