Generic calix[4]arenes became readily accessible in the late 70s. With their potential eight anchoring points, their utility for the production of sophisticated, highly functionalised macrocyclic molecules was rapidly recognised. While most studies in calixarene chemistry have focused on monocalixarene derivatives, there is now an increasing interest in developing multicalixarene compounds, especially those made of several linearly-arranged calix[4]arene units, the first examples of which were reported in 1989. This critical review will present the most important synthetic routes to such molecules together with an analysis of the properties that such cavity combinations may induce. In particular it will be shown that the nature of the links between the calixarene units plays a determinant role in the product properties and that singly-linked calixarenes can be exploited in varied applications, including those as efficient receptors of large molecules, as electrochemical and luminescent sensors in ion detection, or as new materials allowing capsule formation suitable for the storage of small guests (82 references).

Unschuldig? Mitnichten! „Nichtunschuldige“ sperrige Phosphane mit einer reduzierenden Cycloheptatrienyl‐Cyclopentadienyl‐Titan‐Einheit überführen bei Raumtemperatur glatt PdII‐ in Pd0‐Spezies (siehe Schema). Der entstehende Komplex katalysiert Suzuki‐Miyaura‐Kupplungen, die auch mit sterisch gehinderten Arylbromiden schnell ablaufen.

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.

The first diphosphines based on a double calixarene, namely 1,4 (or 1,3)-bis-(5-diphenylphosphino-25,26,27,28-tetrapropoxycalix[4]aren-17-yl)benzene (L(2), L(3)) were each prepared in four steps starting from 5,17-dibromo-25,26,27,28-tetrapropoxycalix[4]arene. Upon reaction of L(2) with [Au(tht)(thf)]BF(4), (tht = C(4)H(8)S) a rigid metallo-capsule was quantitatively formed, which adopts an oblique form owing to the distinct nature of the spacers linking the two calixarene half-spheres. In the solid state, the 1,4-substituted phenylene linker is turned towards the gold ion, suggesting the existence of weak bonding interactions between two aromatic CH protons of this ring and the metal centre (AuH =2.67 A). In contrast to this gold complex, the related silver complex shows dynamic behaviour in solution, the exchange between two enantiomeric oblique forms being facilitated by the greater stereochemical flexibility of Ag(I) vs. Au(I). A heteronuclear (109)Ag{(1)H} HMQC experiment established strong correlations between the CH protons of the phenylene linker and the (109)Ag ion. Dynamic behaviour similar to that observed for the silver complex was further observed in trans-[PtCl(2)L(2)], a chelate complex that could be obtained quantitatively from L(2) and [PtCl(2)(PhCN)(2)]. The intended formation of a chelate complex leading to a capsule with an endo-oriented metal centre was achieved by reacting L(3) with [Pd(allyl)(thf)(2)]BF(4). The complex thus formed constitutes the first organometallic transition metal complex embedded in a cavity with large portals. Binding of [RuCl(2)(p-cymene)] to L(2) and L(3) resulted in self-compacting bimetallic complexes in which each calixarene basket entraps a Ru(p-cymene) unit, thereby forming molecules occupying a minimal volume.

Unschuldig? Mitnichten! „Nichtunschuldige“ sperrige Phosphane mit einer reduzierenden Cycloheptatrienyl‐Cyclopentadienyl‐Titan‐Einheit überführen bei Raumtemperatur glatt PdII‐ in Pd0‐Spezies (siehe Schema). Der entstehende Komplex katalysiert Suzuki‐Miyaura‐Kupplungen, die auch mit sterisch gehinderten Arylbromiden schnell ablaufen.

Keywords: Calixarenes / Diphosphane / Nickel / Silver / Chelates / Strained ligands 5,26,27,arene (6) (6)]BF 4 quantitatively. These complexes were both characterised by a singlecrystal X-ray diffraction study. In the nickel complex the ligand shows a bite angle of 104.5°, which lies in the range

A unique, covalently constructed capsular catalyst obtained by reaction of [Ni(η 5 -C 5 H 5 )(1,5-cyclooctadiene)] BF 4 with the double-calixarene-derived diphosphine 1,3-bis (5-diphenylphosphino-25,26,27,28-tetrapropoxycalix[4]aren-17-yl)- [a]

The cover picture shows the structure of a calix[4]arene‐based ligand with an unusually large bite angle. Calix[4]arenes constitute versatile macrocyclic compounds, which can be obtained by condensation of phenols with formaldehyde. The so‐called CALDIPs, which are diphosphanes built upon such a skeleton, have recently found applications in a variety of C–C bond‐forming reactions, including polymerisation reactions. In the presence of group 8–10 transition metals, CALDIP ligands form straightforwardly dynamic chelate complexes, in which the ligand bite angle undergoes periodic variation. In the article by C. Jeunesse, D. Matt et al. on p. 4917 ff, a new chelate complex is described in which a CALDIP ligand adopts an unexpectedly large bite angle of 139°, which thus illustrates the great flexibility of the calix[4]arene core. The key ligand is depicted as superimposed over the sky of Strasbourg city (photo courtesy of Dr. R. Ruppert, UdS). Phenol–formaldehyde condensation products were already synthesised in Strasbourg by A. Baeyer in 1872.