No abstract

Described are the preparation and functional characterization of nanocrystalline and/or amorphous thin films comprising of neutral "molecular squares" of the form [Re(CO) 3 (Cl)(µ-L)] 4 (L ) difunctional imine or azine ligand). The films are strongly adherent, stable in aqueous media, and characterized by comparatively few pinhole defects. Electrochemical transport experiments show that the materials are exceptionally porous with respect to sufficiently small solution-phase permeants but blocking toward larger permeants. Related thin-film experiments based on monometallic "corner" materials indicate efficient exclusion of all candidate permeant molecules evaluated. For the title materials, these experiments, together with additional electrochemical probe experiments, indicate that (1) membranelike permeation via pores or tunnels of about nanometer diameter is the primary mode of transport of molecular and ionic species through thin films and (2) the transportrelevant pore or tunnel diameter is defined by the cavity dimensions for the component molecular square. The crystal structure of a single isomer of [Re(CO) 3 (Cl)(µ-4,4′-bipyridine)] 4 is also reported. A packing view down the c axis of the tetragonal unit cell shows that the molecules, which are significantly puckered in the crystalline state, are arranged with cavities aligned to generate infinite zeolite-like channels.

Mesoporous thin films based on neutral, porphyrin-bridged, tetrarhenium "square" compounds (metallacycles) display permeability to molecules smaller than the dimensions of an individual square cavity, but blocking behavior toward larger species (see schematic representation). The characteristic permeant size cutoff can be systematically and predictably manipulated by decorating the walls of the porphyrin square with axially bound, mono-, bi-, and tetradentate ligands.

Reacting ReO 2 I(PPh 3 ) 2 with imidazole (ImH) and its methylated derivatives 1-MeIm, 1,2-Me 2 Im, 2-MeImH, and 4(5)-MeImH yielded salts of the trans-dioxo cation [ReO 2 L 4 ] + . Pure stable compounds were isolated for X -) Iand L ) ImH, 1,2-Me 2 Im, and 1-MeIm and X -) B(C 6 H 5 ) 4 -and L ) 1-MeIm, 2-MeImH and 5-MeImH. The ν as (OdRedO) IR bands were observed between 765 and 794 cm -1 , whereas the ν s Raman band appeared in the 900-925 cm -1 range. The compounds were also characterized by 1 H and 13 C NMR and UV-vis spectroscopies. Crystal structures were determined for three compounds:The complexes adopt the trans-dioxo geometry, and the octahedra are remarkably regular. Protonation of [ReO 2 L 4 ]I was studied in methanol and in water. Acid dissociation constants were determined in aqueous solution for the [ReO(OH)L 4 ] 2+ (K a1 ) and [ReO(OH 2 )L 4 ] 3+ (K a2 ) species of 1-MeIm (pK a1 ) 2.0, pK a2 ) ∼-4.0) and 1,2-Me 2 Im (pK a1 ) ∼3.8, pK a2 ) ∼-4.1). Iodide salts of [ReO(OH)L 4 ] 2+ were not stable enough to be isolated, but the B(C 6 H 5 ) 4 -salt of [ReO(OH)(1,2-Me 2 Im) 4 ] 2+ was obtained in pure form. The crystal structure of [ReO(OH)(1,2-Me 2 Im) 4 ](I 3 ) 0.5 -(ReO 4 ) 1.5 (monoclinic, I2/a, a ) 23.290(8) Å, b ) 10.863(7) Å, c ) 25.709(8) Å, β ) 94.36(3)°, Z ) 8, R ) 0.060) showed the presence of octahedral dications in which the oxo and hydroxo groups are trans to one another and the Re-N bonds are displaced to the Re-OH side by the steric effect of the multiple Re-oxo bond. The σ-donor ability of imidazole, greater than that of pyridine, leads to more stable [ReO 2 L 4 ] + compounds in which the Re)O double bond character is lower and the oxo groups are better nucleophiles.

Oxidative electropolymerization at glassy carbon and indium−tin−oxide electrodes from unbuffered aqueous solutions of Fe(5-NH2-phen)3 2+ and Ru(5-NH2-phen)3 2+ yields electrode-supported films that self-limit at thicknesses of ca. 50 and 2 nm, respectively. Electrodeposition is characterized by formation of nonelectroactive films, which are electronically insulating at least over the electrochemical potential range of 0 to +1 V. Electrochemical measurements of redox-active probe molecules demonstrate that these films exhibit highly selective molecular sieving. Neutral permeants and hydrophobic permeants are observed to penetrate the polymeric coating more rapidly than charged or hydrophilic forms. Atomic force microscopy, UV−vis spectroscopy, and X-ray photoelectron spectroscopy were used to correlate the polymeric molecular structure with the observed electrochemical reactivity. Molecular sieving behavior is discussed in terms of a membrane transport model implying that the films are comprised of metallopolymeric molecular cavities of uniform size and distribution.