P-phenylene Spacer Research Articles

Palladium(II) Complexation by p-Cyclophane Receptors. A Solution and Solid State Study.

The coordination properties of the macrocyclic ligands 1,4,7,16,19,22-hexamethyl-1,4,7,16,19,22-hexaaza[9.9]-p-cyclophane (L1) and 1,4,7-trimethyl-19,22,28,31-tetraoxa-1,4,7,14,23-pentaaza[9.25]-p-cyclophane (L2) have been studied by means of potentiometric and (1)H and (13)C measurements in aqueous solution. L1 is composed of two equal triamine binding units connected by p-phenylene spacers. L2 presents a similar molecular architecture, a triamine moiety of L1 being replaced by a cyclic N(2)O(4) binding unit. L1 can form both mono- and dinuclear complexes in aqueous solution, while L2 gives only mononuclear species. The potentiometric data indicate that in the L1 dinuclear complex each metal is coordinated by a triamine moiety. In the L2 mononuclear complex the Pd(II) ion is coordinated by the N(3) unit. The N(2)O(4) moiety does not show any binding ability toward Pd(II), but exhibits a high tendency to protonate. These solution data are confirmed by the crystal structures of [Pd(2)Cl(2)L1](ClO(4))(2).H(2)O (a) and [PdClL2H(2)(H(2)O)](ClO(4))(3) (b). In complex a, each Pd(II) ion is four coordinated by the three amine groups of the triaza moiety and a chloride anion, in a square planar geometry. In the mononuclear complex b, the metal is coordinated by the N(3) moiety, with a coordination environment almost equal to that found in complex a. The N(2)O(4) moiety is diprotonated and encapsulates in its cavity a water molecule, held by a hydrogen-bond network. The solution structures of the L1 and L2 complexes have been studied by means of (1)H and (13)C NMR measurements. The analysis of the NMR data reveals that the dinuclear L1 complex and the L2 mononuclear one show structural features in solution almost equal to those found in the solid state. Solution and solid state data indicate that the Pd(II) complexation gives a marked stiffening of the macrocyclic structures.

Enantioselective Discrimination in the Self-Assembly of [2]Pseudorotaxanes

The combination of (i) an optically active, axially chiral π-electron-deficient tetracationic cyclophane derivative of cyclobis(paraquat-p-phenylene), in which both of the p-phenylene spacers have been replaced by axially-chiral 3,3′-disubstituted binaphthol spacers, and (ii) enantiomeric, π-electron-rich substrates, in which a hydroquinone ring is inserted into the polyether backbone terminated by carboxyl groups and substituted in a C2-symmetric manner by two methyl groups, thus creating two equivalent chiral centers in the substrate, produces in solution 1:1 complexes in which the π-electron-rich substrates are inserted into the π-electron-deficient cavities of the cyclophanes in a pseudorotaxane-like manner. The differences in the free energies of complexation for (RR) and (SS) enantiomers of the π-electron-rich substrates span the range from 0.1 to 0.7 kcal mol−1. Chiral recognition becomes more effective the closer the chiral centers are to the hydroquinone templating unit. CD spectroscopy reveals that the different modes of binding of the enantiomeric substrates by the axially chiral tetracationic cyclophane are not accompanied by drastically different core geometries for the [2]pseudorotaxanes. Thus, the chirality of the complex is governed primarily by the properties of the rigid receptor. The combination of the D2 symmetry of the receptor with the C2 symmetry of the substrates has been found to be particularly effective, considering that the chiral centers on the substrates are located on polyether chains which possess a high degree of conformational freedom.

Photoinduced Electron Transfer in Ruthenium(II)−Rhodium(III) Terpyridine Dyads

A series of Ru(II)−Rh(III) dyads of general formula (ttpy)Ru−tpy−(ph)n−tpy−Rh(ttpy)5+ with n = 0, 1, 2 [ttpy = 4‘-p-tolyl-2,2‘:6,2‘‘-terpyridine; tpy−(ph)n−tpy = bridging ligand where two 2,2‘:6‘,2‘‘-terpyridine units are connected at the 4‘-position through a variable number of p-phenylene spacers] have been studied, in acetonitrile at room temperature, by picosecond and nanosecond time-resolved emission spectroscopy. When n = 1, excitation of the Ru(II)-based molecular component is followed by efficient intramolecular quenching by electron transfer to the Rh(III) center. The rate constant, k ≥ 3 × 109 s-1, is high despite the relatively small driving force of the process (ca. 0.1 eV). When n = 2, with the same driving force as above, no intramolecular electron transfer quenching is observed (upper limit for the rate constant of the electron transfer process: k < 5 × 108 s-1). The decrease in electron transfer rate obtained in going from n = 1 to n = 2 is in line with the behavior of other systems containing poly-p-phenylene spacers. The dyad with n = 0 is definitely not homogeneous with the other two: the intercomponent electronic coupling is much stronger, the Ru(II)-based excited state is lower in energy, and the electron transfer has a smaller driving force (ΔG ≈ 0). The lifetime of the Ru(II)-based emission is 17 ns. The lack of an obvious model compound makes it difficult to draw firm conclusions for this system.

Mono- and bi-nuclear copper(II) complexes with polyazacyclophane receptors containing two different binding sites

The macrobicyclic compound 1,4,7,-trimethyl-19,22,28,31-tetraoxa-1,4,7,14,23-pentaaza[9.25]-p-cyclophane (L1) has been prepared. It comprises a N3 and a cyclic N2O4 subunit, connected by p-phenylene spacers. Protonation of L1 (and L2, in which the N2O4 unit is replaced by a N5 moiety) has been studied by means of potentiometric measurements. The compounds L1 and L2 bind up to five and six protons in aqueous solution above pH 2, respectively. Proton and 13C NMR spectra at different pH values allow the determination of the stepwise protonation sites. Considering the [H4L1]4+ species, the acidic protons are located on the benzylic nitrogens. Copper(II) co-ordination by L1 and L2 has been potentiometrically studied (298.1 K, 0.1 mol dm3 NMe4Cl aqueous solution): L1 forms only a mononuclear complex, while L2 gives both mono- and bi-nuclear species in aqueous solution. In the [CuL1]2+ complex the metal is co-ordinated by the three tertiary nitrogens of the N3 unit, while the N2O4 moiety shows a high tendency to protonation. These solution data are confirmed by the crystal structure of [Cu(HL1)Cl(H2O)][ClO4]2. In the [Cu(HL1)Cl(H2O)]2+ cation the metal is co-ordinated by the three amine groups of the triaza moiety and by a chloride anion, in a rather unusual square-planar geometry. A water molecule is encapsulated by the N2O4 moiety, held by a hydrogen-bond network. In the mononuclear [CuL2]2+ complex the CuII is preferentially lodged inside the pentaaza moiety. Such a complex can add a further copper(II) ion, which is co-ordinated by the N3 binding unit.

A Photochemically Driven Molecular Machine

The unthreading and rethreading of pseudorotaxane 1, which contains two p-phenylene spacers linking two bipyridinium units and a central 1, 5-naphthalenediyl unit, is reversible when an initially oxygen-free solution of 1 in water also containing 9-anthracenecarboxylic acid as a photosensitizes and triethanolamine as a “sacrificial” reluctant is irradiated and then aerated. The absorption and emission spectra of the solution provide conclusive evidence.