The binding of copper(II) ions to membrane-bound synthetic receptors has been investigated. Complexation fitted a 4:1 receptor:copper(II) model, and the observed binding constants are significantly enhanced at the membrane relative to solution; these effects can be explained by the lower polarity of the membrane-water interface and the concentrating effect of the membrane, with no observed contribution from receptor preorganization. The stoichiometry of the complex formed is very sensitive to the concentration of the receptor in the membrane, and at low concentrations, binding is reduced relative to solution controls. This implies that by increasing or decreasing the number of receptors in their membranes, cells can finely tune biological responses such as chemotaxis that depend on the size of the receptor-ligand clusters formed.

The 1+1 reaction of the symmetrical dimers [ClP(m-N t Bu)] 2 1 and [H 2 NP(m-N t Bu)] 2 2 in thf-Et 3 N gives the tetrameric macrocycle [{P(m-N t Bu)} 2 NH] 4 3 (67%); consisting of four P 2 (m-N t Bu) 2 rings linked by endo N-H groups.

Inorganic macrocycles, possessing molecular frameworks composed entirely of elements other than carbon, are rare. Even rarer are species analogous to classical organic macrocycles (like crown ethers) that possess the potential ability to coordinate metal ions within their cavities. Recent studies reveal an emerging structural class of valence‐isoelectronic main‐group ligands of general formula [{E(µ‐E′R)}2Y]nx−, which possess fascinating coordination characteristics. Such species can coordinate metal ions within their cavities (using the donor centres Y and E′), their peripheral donor centres (E′) (in an exo fashion) and are also capable of coordinating anions (where Y = N−H). This short review highlights the recent synthetic and structural studies of these unusual ligand systems and pinpoints potential future areas of development. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The reaction of [ClP(mu-NtBu)]2 (1) with H2O (1 : 2 equivalents) in the presence of excess Et3N gives the new chain compound [(mu-O)[P(mu-NtBu)2P(H)=O]2] (3), consisting of two P2N2 rings linked by a mu-O atom and terminating in P(V)(H)=O groups. A similar chain species is obtained from the reaction of the lithiate of [(tBuNH)P(mu-NtBu)2P(H)=O] (5) with [ClP(mu-NtBu)2P(NHtBu)] (2), the product being [(mu-O)[P(mu-NtBu)2P(NHtBu)]2] (6). Compounds 3 and 6 are the first examples of O-bridged chain phosphazanes and potential precursors to new phosphorus-nitrogen macrocycles. The syntheses and X-ray structures of 3, 5 and 6 are reported.

Amination of [ClP(μ‐NtBu)]2 (1) using NH3 in THF gives the cyclophospha(III)zane dimer [H2NP(μ‐NtBu)]2 (2), in good yield. 31P NMR spectroscopic studies of the reaction of 1 with 2 in THF/Et3N show that almost quantitative formation of the cyclic tetramer [{P(μ‐NtBu)}2(μ‐NH)]4 (3) occurs. The remarkable selectivity of this reaction can (in part) be attributed to pre‐organisation of 1 and 2, which prefer cis arrangements in the solid state and solution. The macrocycle 3 can be isolated in yields of 58–67 % using various reaction scales. The isolation of the major by‐product of the reaction (ca. 0.5–1 % of samples of 3), the pentameric, host–guest complex [{P(μ‐NtBu)2}2(μ‐NH)]5(HCl)⋅2 THF] (4⋅2 THF), gives a strong indication of the mechanism involved. In situ 31P NMR spectroscopic studies support a stepwise condensation mechanism in which Cl− ions play an important role in templating and selection of 3 and 4. Amplification of the pentameric arrangement occurs in the presence of excess LiX (X=Cl, Br, I). In addition, the cyclisation reaction is solvent‐ and anion‐dependent. The X‐ray structures of 2 and 4⋅2 THF are reported.