Complexation of uncharged molecules and anions by crown-type host molecules

A new route for the synthesis of phosphate esters: 2,2'-[benzene-1,2-diylbis(oxy)]bis(5,5-dimethyl-1,3,2-dioxaphosphinane) 2,2'-dioxide

Podand-type ligands are an interesting class of acyclic ligands which can form host-guest complexes with many transition metals and can undergo conformational changes. Organic phosphates are components of many biological molecules. A new route for the synthesis of phosphate esters with a retained six-membered ring has been used to prepare 2,2'-[benzene-1,2-diylbis(oxy)]bis(5,5-dimethyl-1,3,2-dioxaphosphinane) 2,2'-dioxide, C6H4{O[cyclo-P(O)OCH2CMe2CH2O]}2 or C16H24O8P2, (1), 2-[(2'-hydroxybiphenyl-2-yl)oxy]-5,5-dimethyl-1,3,2-dioxaphosphinane 2-oxide, [cyclo-P(O)OCH2CMe2CH2O](2,2'-OC6H4-C6H4OH), (2), and oxybis(5,5-dimethyl-1,3,2-dioxaphosphinane) 2,2'-dioxide, O[cyclo-P(O)OCH2CMe2CH2O]2, (3). Compound (1) is novel, whereas the results for compounds (2) and (3) have been reported previously, but we record here our results for compound (3), which we find are more precise and accurate than those currently reported in the literature. In (1), two cyclo-P(O)OCH2CMe2CH2O groups are linked through a catechol group. The conformations about the two catechol O atoms are quite different, viz. one C-C-O-P torsion angle is -169.11 (11)° and indicates a trans arrangement, whereas the other C-C-O-P torsion angle is 92.48 (16)°, showing a gauche conformation. Both six-membered POCCCO rings have good chair-shape conformations. In both the trans and gauche conformations, the catechol O atoms are in the axial sites and the short P=O bonds are equatorially bound.

Control over molecular motion using the cis-trans photoisomerization of the azo group

Control over molecular motion represents an important objective in modern chemistry. Aromatic azobenzenes are excellent candidates as molecular switches since they can exist in two forms, namely the cis (Z) and trans (E) isomers, which can interconvert both photochemically and thermally. This transformation induces a molecular movement and a significant geometric change, therefore the azobenzene unit is an excellent candidate to build dynamic molecular devices. We describe selected examples of systems containing an azobenzene moiety and their motions and geometrical changes caused by external stimuli.

Coordination features of ditopic oxa-azamacrocycles toward Ni(ii) and Co(ii). Dioxygen uptake by their dinuclear Co(ii) complexes

The coordination properties of the ditopic oxa-aza macrocycles L1-L3 toward Ni(II) and Co(II) have been investigated by means of potentiometric and UV-vis spectrophotometric measurements. L1-L3 contain two triamine and/or tetraamine chains separated by two dioxa chains and form both mono- and dinuclear complexes in aqueous solution. In the [ML]2+ complexes, the metal ion is coordinated by one of the two polyamine moieties, while the other does not participate in the coordination. In the dinuclear complexes each metal ion is coordinated, almost independently, to a single polyamine moiety. Under aerobic conditions the binuclear Co(II) complexes of the ligands L1-L3 are able to bind molecular oxygen, with a bridging coordination of O2 between the two metals.

Near infrared spectral investigations of hydration of 18-crown-6 in HDO-D2O solutions at different temperatures

Near infrared spectra of solution of 6.4 M HDO in D(2)O have been obtained at 15, 25, 30, and 35 degrees C. It was observed that the bands of HDO in D(2)O occur at 1416, 1525, 1556, and 1666 nm, which are in good agreement with the similar data reported earlier by Worley and Klotz. The calculations of enthalpy change for hydrogen bond formation (DeltaH degrees ) yielded the value of -2.5+/-0.4 kcal mol(-1), which is in excellent agreement with the value reported by Walrafen. Similar spectra were recorded for 1 and 2 m 18-crown-6 (18C6) dissolved in the solution of HDO in D(2)O at different temperatures. The band positions remain unchanged, however, the variation of intensity as a function of concentration of 18C6 and temperature clearly indicate that 18C6 acts as a structure making solute. The structural temperature and DeltaH degrees values have been obtained for the 18C6 solutions. These results are explained on the basis of the stabilization of 18C6 in the D(3d) conformation through hydrogen bonding of HDO molecules [doubly hydrogen bonded, i.e. bridging, and singly hydrogen bonded] to the oxygen atoms of 18C6 molecules. Slightly different DeltaH degrees values obtained can be attributed to clathrate like structure at 1 m 18C6 concentration while at 2 m 18C6 concentration it is postulated that the hydrophobic interactions are contributing additionally.

Secondary hypervalent I(III)...O interactions: synthesis and structure of hypervalent complexes of diphenyl-lambda3-iodanes with 18-crown-6

Reported here for the first time are the synthesis and characterization of supramolecular complexes between diaryl-lambda(3)-iodanes and 18-crown-6 (18C6). Slow evaporation of solvents afforded 1:1 and 2:1 complexes between Ph(2)IBF(4) and 18C6 as stable crystals, depending on the conditions. X-ray crystal structures of these complexes indicated that each iodine atom contacts with the three adjacent oxygen atoms of 18C6 through two hypervalent secondary bonding and a weak interaction. (1)H NMR analyses and CSI-MS spectra showed that, in dichloromethane solution, Ph(2)IBF(4) exclusively forms the 1:1 complex with 18C6 (binding constant K(a), 1.02 x 10(3) M(-)(1)). The binding constants decrease with the increased solvent donor ability (Gutmann's DN). Changing the heteroatom ligand from BF(4) to the less nucleophilic PF(6) and AsF(6) increased the binding constant by about six times. Substitution of an electron-withdrawing group onto the para position of Ph(2)IBF(4) tends to increase in the complex stability. A linear Hammett relationship (rho = 0.59) between log K(a) and sigma(p)(+) values of substituents indicates that the diaryl-lambda(3)-iodanes with electropositive iodine(III) interact more efficiently with 18C6. Decreased binding magnitude was measured with 15C5, dibenzo-18C6, dibenzo-21C7, and dibenzo-30C10.

Synthesis and structure of chiral macrocycles containing 2,2'-bipyridine subunits

A series of macrocyclic receptors has been prepared containing bipyridine groups linked to two amino acids. Variations in both the amino acid and the linking spacer have been made. The structure of the resulting macrocycles has been investigated using 1H NMR spectroscopy and X-ray crystallography. The use of L-valine leads to an open conformation for the macrocycle in which the 2-propyl substituents are directed perpendicular to the plane of the ring leaving the bipyridine and amide groups accessible for binding to a metal or complementary substrate. Proline-based macrocycles take up a twisted arrangement with the linking chain stretched across the face of the bipyridine which takes up a trans conformation. The metal ion binding properties of these derivatives have been investigated and shown to occur only to the valine macrocyles which have the two pyridine rings preorganized for complexation. These macrocycles have also been shown to bind to phenolic hydroxyl groups by using hydrogen-bond donors and acceptors from the amide groups in the linking chain.