A tris(2,2'-bipyridine)ruthenium(II) derivative tethered to a cis-PtCl2(amine)2 moiety: syntheses, spectroscopic properties, and visible-light-induced scission of DNA

A tris(2,2'-bipyridine)ruthenium(II) derivative having two N-(3-ammoniopropyl)carbamoyl pendant units has been prepared and reacted with cis-PtCl2(DMSO)2 (DMSO = dimethyl sulfoxide) to give a heteronuclear Ru(II)Pt(II) dimer having a cis-Pt(II)Cl2(aliphatic amine)2 unit, [Ru(bpy)2(mu-bridge)PtCl2](PF6)2 (bpy = 2,2'-bipyridine, bridge = 4,4'-bis(N-(3-aminopropyl)carbamoyl)-2,2'-bipyridine). The ESI-TOF mass spectrum of the Ru(II)Pt(II) dimer shows a set of signals corresponding to {[Ru(bpy)2(mu-bridge)PtCl2](PF6)}(+) (m/z 1181.1). The MLCT (metal-to-ligand charge transfer) luminescence intensity is enhanced upon the platination of two amine units, presumably due to the formation of a relatively rigid metallocycle. More interestingly, the luminescence intensity is further enhanced by the complexation of the Ru(II)Pt(II) dimer with either 5'-GMP (guanosine 5'-monophosphate disodium salt) or calf thymus DNA. Visible-light-induced scission of supercoiled pBR322 DNA is found to be efficiently enhanced in the presence of the title Ru(II)Pt(II) dimer.     

Parking and restarting a molecular shuttle in situ

Herein we report an easy-to-synthesize [2]rotaxane, which incorporates two ionic monopyridinium stations and one 2,2'-bipyridine station as the shaft of the dumbbell-shaped component and a bis-p-xylyl[26]crown-6 (BPX26C6) unit as the macrocyclic component. In this molecular shuttle, the BPX26C6 unit can be docked selectively on either the central 2,2'-bipyridine station or one of the two terminal pyridinium stations, and subsequently, returned to its shuttling molecular motion through the in situ addition of simple reagents (acid/base or metal ion/metal-ion-complexing ligand pairs).     

Stability of thin-film solid-state electroluminescent devices based on tris(2,2'-bipyridine)ruthenium(II) complexes

The factors affecting the operating life of the light-emitting electrochemical cells (LECs) based on films of tris(2,2'-bipyridine)ruthenium(II) both in sandwich (using an ITO anode and a Ga:Sn cathode) and planar (using interdigitated electrode arrays (IDAs)) configurations were investigated. Stability of these devices is greatly improved when they are produced and operated under drybox conditions. The proposed mechanism of the LEC degradation involves formation of a quencher in a small fraction of tris(2,2'-bipyridine)ruthenium(II) film adjacent to the cathode, where light generation occurs, as follows from the observed electroluminescence profile in the LECs constructed on IDAs, showing that the charge injection in such devices is highly asymmetric, favoring hole injection. Bis(2,2'-bipyridine)diaquoruthenium(II) is presumed to be the quencher responsible for the device degradation. A microscopic study of photo- and electroluminescence profiles of planar light-emitting electrochemical cells was shown as a useful approach for studies of charge carrier injection into organic films.     

Synthesis and characterization of tris(heteroleptic) Ru(II) complexes bearing styryl subunits

We have developed and optimized a well-controlled and refined methodology for the synthesis of substituted π-conjugated 4,4'-styryl-2,2'-bipyridine ligands and also adapted the tris(heteroleptic) synthetic approach developed by Mann and co-workers to produce two new representative Ru(II)-based complexes bearing the metal oxide surface-anchoring precursor 4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine. The two targeted Ru(II) complexes, (4,4'-dimethyl-2,2'-bipyridine)(4,4'-di-tert-butyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (1) and (4,4'-dimethyl-2,2'-bipyridine)(4,4'-dinonyl-2,2'-bipyridine)(4,4'-bis[E-(p-methylcarboxy-styryl)]-2,2'-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dnbpy)(p-COOMe-styryl-bpy)](PF(6))(2) (2) were obtained as analytically pure compounds in high overall yields (>50% after 5 steps) and were isolated without significant purification effort. In these tris(heteroleptic) molecules, NMR-based structural characterization became nontrivial as the coordinated ligand sets each sense profoundly distinct magnetic environments greatly complicating traditional 1D spectra. However, rational two-dimensional approaches based on both homo- and heteronuclear couplings were readily applied to these structures producing quite definitive analytical characterization and the associated methodology is described in detail. Preliminary photoluminescence and photochemical characterization of 1 and 2 strongly suggests that both molecules are energetically and kinetically suitable to serve as sensitizers in energy-relevant applications.     

Synthesis and structural,electrochemical, and optical properties of Ru(II) complexes with azobis(2,2'-bipyridine)s

Two new ditopic ligands, 5,5"-azobis(2,2'-bipyridine) (5,5"-azo) and 5,5"-azoxybis(2,2'-bipyridine) (5,5"-azoxy), were prepared by the reduction of nitro precursors. Mononuclear and dinuclear Ru(II) complexes having one of these bridging ligands and 2,2'-bipyridine terminal ligands were also prepared, and their properties were compared with previously reported Ru(II) complexes having 4,4"-azobis(2,2'-bipyridine) (4,4"-azo). The X-ray crystal structure showed that 5,5"-azo adopts the trans conformation and a planar rodlike shape. The X-ray crystal structure of [(bpy)(2)Ru(5,5"-azo)Ru(bpy)(2)](PF(6))(4) (Ru(5,5"-azo)Ru) showed that the bridging ligand is in the trans conformation and nearly planar also in the complex and the metal-to-metal distance is 10.0 A. The azo or azoxy ligand in these complexes exhibits reduction processes at less negative potentials than the terminal bpy's due to the low-lying pi level. The electronic absorption spectra for the complexes having 5,5"-azo or 5,5"-azoxy exhibit an extended low-energy metal-to-ligand charge-transfer absorption. The ligands, 5,5"-azo and 5,5"-azoxy, and the mononuclear complex, [(bpy)(2)Ru(5,5"-azo)](2+), isomerize reversibly upon light irradiation. The low-energy MLCT state sensitizes the isomerization of the azo moiety in this complex. While [(bpy)(2)Ru(4,4"-azo)Ru(bpy)(2)](PF(6))(4) exhibits light switch properties, namely, significant electrochromism and a large luminescence enhancement, upon reduction, Ru(5,5"-azo)Ru does not show these properties. The radical anion formation upon reduction of these complexes has been confirmed by ESR spectroscopy.     

Luminescently tagged 2,2'-bipyridine complex of FeII: synthesis and photophysical studies of 4-[N-(2-anthryl)carbamoyl]-4'-methyl-2,2'-bipyridine

The anthracene lumiphore was linked to the chelating ligand 2,2'-bipyridine, forming 4-[N-(2-anthryl)carbamoyl]-4'-methyl-2,2'-bipyridine (bpyAnth). Coupling through an amide linkage provides some electronic isolation of the anthracene lumiphore. Electrochemistry suggested little change of the anthracene oxidation whether free (1.35 V) linked to 2,2'-bipyridine as bpyAnth (1.30 V) or appended to Fe(II) (1.29 V). The bpyAnth ligand retained the structured luminescence characteristic of anthracene at 375, 400, 419, and 441 nm. This anthracene emission persists even when bpyAnth is complexed to an Fe(II) center. The complex [Fe(bpyAnth)3]2+ is emissive, in marked contrast to typical polyazine iron(II) complexes. This bpyAnth ligand serves as a luminescently tagged analogue of 2,2'-bipyridine, useful for coordination to a variety of metals.     

Multipolymer reaction system for selective aerobic alcohol oxidation: simultaneous use of multiple different polymer-supported ligands

A multipolymer reaction system has been developed in which a water-soluble polymer-supported 2,2'-bipyridine group and a similarly immobilized TEMPO derivative are used as ligands for copper to effect the mild and selective aerobic oxidation of primary alcohols in acetonitrile-water solvent. In this reaction system, poly(ethylene glycol) monomethyl ether (molecular weight = 5000 Da) was used as the support for both the 2,2'-bipyridine and TEMPO moieties because of its solubility properties. The use of these functionalized polymers simultaneously in catalytic quantities allows for primary alcohols to be oxidized selectively to the corresponding aldehydes in an environmentally friendly manner. This is the first reported example of using two different polymer-supported ligands together to form an organometallic species capable of catalyzing an organic reaction.     

A ruthenium(II) complex based turn-on electrochemiluminescence probe for the detection of nitric oxide

Electrochemiluminescence (ECL) detection technique using bipyridine-ruthenium(II) complexes as probes is a highly sensitive and widely used method for the detection of various biological and bioactive molecules. In this work, the spectral, electrochemical and ECL properties of a chemically modified bipyridine-ruthenium(II) complex, [Ru(bpy)(2)(dabpy)](2+) (bpy: 2,2'-bipyridine; dabpy: 4-(3,4-diaminophenoxy)-2,2'-bipyridine), were investigated and compared with those of its nitric oxide (NO)-reaction derivative [Ru(bpy)(2)(T-bpy)](2+) (T-bpy: 4-triazolephenoxy-2,2'-bipyridine) and [Ru(bpy)(3)](2+). It was found that the ECL intensity of [Ru(bpy)(2)(dabpy)](2+) could be selectively and sensitively enhanced by NO due to the formation of [Ru(bpy)(2)(T-bpy)](2+) in the presence of tri-n-propylamine. By using [Ru(bpy)(2)(dabpy)](2+) as a probe, a sensitive and selective ECL method with a wide linear range (0.55 to 220.0 μM) and a low detection limit (0.28 μM) was established for the detection of NO in aqueous solutions and living cells. The results demonstrated the utility and advantages of the new ECL probe for the detection of NO in complicated biological samples.     

Syntheses, characterization, and photo-hydrogen-evolving properties of tris(2,2'-bipyridine)ruthenium(II) derivatives tethered to a cis-Pt(II)Cl2 unit: insights into the structure-activity relationship

The photo-hydrogen-evolving activity (activity to enhance the photochemical EDTA-reduction of water into molecular hydrogen) was evaluated for three different Ru(II)Pt(II) dimers with a general formula of [(bpy)2Ru(micro-bridge)PtCl2]2+(bpy = 2,2'-bipyridine; bridge = 4,4'-bis(N-(3-aminopropyl)carbamoyl)-2,2'-bipyridine (L1), 2,3-bis(2-pyridyl)pyrazine (L2), and 4,4'-bis(N-(4-pyridyl)methylcarbamoyl)-2,2'-bipyridine (L3); EDTA = ethylenediaminetetraacetic acid disodium salt). A new Ru(II)Pt(II) complex, [(bpy)2Ru(micro-L3)PtCl2]2+, was synthesized and characterized. It was confirmed that all three compounds are ineffective towards photochemical H2 production. In each case, an acetate-buffer solution (pH = 5) containing the Ru(II)Pt(II) dimer and EDTA was photolysed using a 350-W Xe lamp under an Ar atmosphere, during which the amount of H2 evolved was analysed by gas chromatography. Additional photolysis experiments were carried out by adding [Ru(bpy)3]2+ and methylviologen (N,N'-dimethyl-4,4'-bipyridinium) to the photolysis solutions described above to test the H2-evolving activity of the Pt(II) unit involved in these Ru(II)Pt(II) dimers. As a result, the Pt(II) units involved in the L1 and L2 compounds were found to be active as an H2-evolving catalyst, while that of the L3 compound was found to show no activity at all. The extent of intramolecular electron-transfer quenching from the 3MLCT excited state of the [Ru(bpy)3]2+ derivative to the tethering Pt(II) catalyst centre was investigated by comparison of the luminescence spectra of these compounds, together with the related compounds. The results showed that the quenching of the 3MLCT luminescence is not at all enhanced in either the L1 or the L3 compounds. On the other hand, the L2 compound is strongly quenched as previously reported. In addition to the above studies, the H2-evolving activity of some Pt(II) monomers, cis-PtCl2(NH3)2, PtCl2(en)(en = ethylenediamine), cis-PtCl2(4-methylpyridine)2, PtCl2(2,2'-bipyrimidine), PtCl2(4,4'-dicarboxy-2,2'-bipyridine), and [PtCl(terpy)]+(terpy = 2,2':6',2'-terpyridine), were similarly investigated in the presence of EDTA, [Ru(bpy)3]2+ and methylviologen, since they were regarded as structural analogues of the Pt(II) units involved in the L1-L3 compounds. The compounds having a cis-Pt(II)Cl2 unit were generally found to show high H2-evolving activity. This was interpreted in terms of the ligation of negatively charged chloride anions leading to the destabilization of the Pt(II) dz2 orbital responsible for the hydrogenic activation. Importantly, cis-PtCl2(4-methylpyridine)2 exhibited relatively high activity as an H2-evolving catalyst, suggesting the importance of the flexible rotation of the pyridyl ligands for efficient hydrogenic activation at the axial site of the Pt(II) ion. The DFT calculations also showed the validity of the structure-activity relationship discussed above for the L3 compound.     

Electron-rich heteroaromatic conjugated bipyridine based ruthenium sensitizer for efficient dye-sensitized solar cells

A novel heteroleptic ruthenium complex carrying a heteroaromatic-4,4'-pi-conjugated 2,2'-bipyridine [Ru(II)LL'(NCS)(2)] (L = 4,4'-bis[(E)-2-(3,4-ethylenedioxythien-2-yl)vinyl]-2,2'-bipyridine, L' = 4,4'-(dicarboxylic acid)-2,2'-bipyridine) was synthesized and used in dye-sensitized solar cells, yielding photovoltaic efficiencies of 9.1% under standard global AM 1.5 sunlight.     

Room temperature phosphorescence from ruthenium(II) complexes bearing conjugated pyrenylethynylene subunits

We describe the synthesis, electrochemistry, and photophysical properties of several Ru(II) complexes bearing different numbers of pyrenylethynylene substituents in either the 5- or 5,5'-positions of 2,2'-bipyridine, along with the appropriate Ru(II) model complexes bearing either bromo- or ethynyltoluene functionalities. In addition, we prepared and studied the photophysical behavior of the diimine ligands 5-pyrenylethynylene-2,2'-bipyridine and 5,5'-dipyrenylethynylene-2,2'-bipyridine. Static and dynamic absorption and luminescence measurements reveal the nature of the lowest excited states in each molecule. All model Ru(II) complexes are photoluminescent at room temperature and exhibit excited-state behavior consistent with metal-to-ligand charge transfer (MLCT) characteristics. In the three Ru(II) molecules bearing multiple pyrenylethynylene substituents, there is clear evidence that the lowest excited state is triplet intraligand (3IL)-based, yielding long-lived room temperature phosphorescence in the red and near IR. This phosphorescence emanates from either 5-pyrenylethynylene-2,2'-bipyridine or 5,5'-dipyrenylethynylene-2,2'-bipyridine, depending upon the composition of the coordination compound. In the former case, the excited-state absorption difference spectra that were measured for the free ligand are easily superimposed with those obtained for the metal complexes coordinated to either one or two of these species. The latter instance is slightly complicated since coordination of the 5,5'-ligand to the Ru(II) center planarizes the diimine structure, leading to an extended conjugation on the long axis with a concomitant red shift of the singlet pi-pi absorption transitions and the observed room temperature phosphorescence. As a result, transient absorption measurements obtained using free 5,5'-dipyrenylethynylene-2,2'-bipyridine show a marked blue shift relative to its Ru(II) complex, and this extended pi-conjugation effect was confirmed by coordinating this ligand to Zn(II) at room temperature. In essence, all three pyrenylethynylene-containing Ru(II) complexes are unique in this genre of chromophores since the lowest excited state is 3IL-based at room temperature and at 77 K, and there is no compelling evidence of interacting or equilibrated excited states.     

A molecular light-driven water oxidation catalyst

Two mononuclear Ru(II) complexes, [Ru(ttbt)(pynap)(I)]I and [Ru(tpy)(Mepy)(2)(I)]I (tpy = 2,2';6,2"-terpyridine; ttbt = 4,4',4"-tri-tert-butyltpy; pynap = 2-(pyrid-2'-yl)-1,8-naphthyridine; and Mepy = 4-methylpyridine), are effective catalysts for the oxidation of water. This oxidation can be driven by a blue (λ(max) = 472 nm) LED light source using [Ru(bpy)(3)]Cl(2) (bpy = 2,2'-bipyridine) as the photosensitizer. Sodium persulfate acts as a sacrificial electron acceptor to oxidize the photosensitizer that in turn drives the catalysis. The presence of all four components, light, photosensitizer, sodium persulfate, and catalyst, are required for water oxidation. A dyad assembly has been prepared using a pyrazine-based linker to join a photosensitizer and catalyst moiety. Irradiation of this intramolecular system with blue light produces oxygen with a higher turnover number than the analogous intermolecular component system under the same conditions.     

Intramolecular metal...sulfur interactions of platinum(II) 1,4,7-trithiacyclononane complexes with bipyridyl ligands: the relationship between molecular and electronic structures

Five platinum(II) 1,4,7-trithiacyclononane (ttcn) complexes with bidentate-substituted 2,2'-bipyridine ligands have been prepared and structurally characterized: [Pt(bpy)(ttcn)](PF6)2 (bpy = 2,2'-bipyridine), triclinic, P1, a = 10.2529(3) A, b = 10.7791(3) A, c = 10.7867(3) A, alpha = 83.886(1) degrees, beta = 87.565(1) degrees, gamma = 84.901(1), V = 1179.99(6) A3, Z = 2; [Pt(4,4'-dmbpy)(ttcn)](PF6)2 x CH3CN x H2O (4,4'-dmbpy = 4,4'-dimethyl-2,2'-bipyridine), triclinic, P1, a = 10.1895(3) A, b = 11.8566(4) A, c = 13.1004(4) A, alpha = 77.345(1) degrees, beta = 79.967(1) degrees, gamma = 72.341(1) degrees, V = 1461.56(8) A3, Z = 2; [Pt(5,5'-dmbpy)(ttcn)](PF6)2 (5,5'-dmbpy = 5,5'-dimethyl-2,2'-bipyridine), triclinic, P1, a = 10.6397(4) A, b = 10.8449(4) A, c = 11.2621(4) A, alpha = 90.035(1) degrees, beta = 98.061(1) degrees, gamma = 91.283(1) degrees, V = 1286.32(8) A3, Z = 2; [Pt(dbbpy)(ttcn)](PF6)2 x CH3NO2 (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), triclinic, P1, a = 11.5422(7) A, b = 11.6100(7) A, c = 13.6052(9) A, alpha = 85.902(1) degrees, beta = 89.675(1) degrees, gamma = 74.942(1) degrees, V = 1755.90(19) A3, Z = 2; and [Pt(dtfmbpy)(ttcn)](PF6)2 x CH3CN (dtfmbpy = 5,5'-di-trifluoromethyl-2,2'-bipyridine): monoclinic, P2(1)/c, a = 13.1187(9) A, b = 20.9031(15) A, c = 11.3815(8) A, beta = 105.789(2) degrees, V = 3003.3(4) A3, Z = 4. For each salt, the platinum(II) center of the cation is bonded to two nitrogen atoms of the chelating diimine and two sulfur atoms of the thioether macrocycle. The third sulfur atom of ttcn forms a long apical interaction with the metal center (2.84-2.97 A), resulting in a flattened square pyramid structure. An examination of these and 17 other structures of platinum(II) ttcn complexes reveals a correlation between the apical Pt...S distance and the donor properties of the ancillary ligands, suggesting a means for using variations in ligand electronic properties to tune molecular structure. The room-temperature absorption spectra in acetonitrile solution show a broad and comparatively low-energy MLCT band maximizing near approximately 390 nm for the bpy and dialkyl-substituted bipyridyl derivatives. The maximum is dramatically red-shifted to 460 nm in the spectrum of the dtfmbpy complex as a result of the electron-withdrawing properties of the -CF(3) groups. The 3:1 EtOH/MeOH 77 K glassy solution emission spectra exhibit low-energy emission bands (lambdamax, 570-645 nm), tentatively assigned as originating from a lowest, predominantly spin-forbidden MLCT excited state that is stabilized by apical Pt...S interactions.     

Double-stranded helices and molecular zippers assembled from single-stranded coordination polymers directed by supramolecular interactions

Using three nonlinear dicarboxylates, isophthalate (ipa), 4,4'-oxybis(benzoate) (oba), and ethylenedi(4-oxybenzoate) (eoba), we have prepared five neutral infinite copper(II) dicarboxylate coordination polymers containing lateral aromatic chelate ligands, namely [Cu(ipa)(2,2'-bpy)]n.2nH2O (1), [Cu2(ipa)2(phen)2H2O]n (2), [Cu(oba)(phen)]n (3), [Cu(oba)(2,2'-bpy)]n (4), and [Cu(eoba)(phen)]n (5; 2,2'-bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline) by hydrothermal synthesis. X-ray single-crystal structural analyses of these complexes reveal that the nonlinear flexible or V-shaped dicarboxylates can induce the helicity or flexuousity of the polymeric chains and aromatic chelate ligands are important in providing potential supramolecular recognition sites for pi-pi aromatic stacking interactions. An appropriate combination of the bridging dicarboxylate and aromatic chelate can induce a pair of single-stranded helical or flexuous chains to generate a double-stranded helix or molecular zipper through supramolecular interactions, respectively.     

An unusual photosensitizer: dyad of eosin-tris(2,2'-bipyridine)Ru(II)

[structure: see text] A dyad of eosin and tris(2,2'-bipyridine)Ru(II) was prepared, and its photophysical properties were investigated. The photosensitization of eosin is greatly enhanced by introduction of tris(2,2'-bipyridine)Ru(II), which is verified via photooxygenation of anthracene derivatives. The electron-transfer mechanism of photosensitization is also discussed.     

Electrochemical and luminescent properties of new mononuclear ruthenium(II) and binuclear iridium(III)-ruthenium(II) terpyridine complexes.

Hexafluorophosphate salts of mononuclear complexes [Ru(II)Cl(L)(terpy)]+ (L = dmbpy (1); dpbpy (2), sambpy (3), and dpp (7), and binuclear complexes [Ru(II)2Cl2(dpp)(terpy)2]2+ (8) and [Ir(III)Ru(II)Cl2(dpp)(terpy)2]3+ (9) were prepared and characterized. Abbreviations of the ligands are bpy = 2,2'-bipyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine, dpbpy = 4,4'-diphenyl-2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, sambpy = 4,4'-bis((S)-(+)-alpha-1-phenylethylamido)-2,2'-bipyridine, and terpy = 2,2':6',2'-terpyridine. The absorption spectra of 8 and 9 are dominated by ligand-centered bands in the UV region and by metal-to-ligand charge-transfer bands in the visible region. The details of their spectroscopic and electrochemical properties were investigated. In both binuclear complexes, it has been found that the HOMO is based on the Ru metal, and LUMO is dpp-based. [Ir(III)Ru(II)Cl2(dpp)(terpy)2]3+, indicating intense emission at room temperature, and a lifetime of 154 ns. The long lifetime of this bimetallic chromophore makes it a useful component in the design of supramolecular complexes.     

First metal complexes of 6,6'-dihydroxy-2,2'-bipyridine: from molecular wires to applications in carbonylation catalysis

The first square planar rhodium(I) complexes containing the 6,6'-dihydroxy-2,2'-bipyridine ligand have been prepared. The complexes form molecular wires in the solid state and are active catalysts for the carbonylation of methyl acetate.     

Molecular structures and magnetic properties of the mixed-ligand complexes of bis(hexafluoroacetylacetonato)manganese(II), -copper(II), and -zinc(II) with 4,4'-bis(N-tert-butyl-N-oxylamino)-2,2'-bipyridine. Isosceles triangular hetero-three-spin systems consisting of aminoxyls and metal ions

4,4'-Bis(N-tert-butyloxylamino)-2,2'-bipyridine (4) and its 1:1 complexes with bis(hexafluoroacetylacetonato)manganese(II), -copper(II), and -zinc(II) were prepared. An X-ray structure analysis of free ligand 4 reveals that the molecule has a trans conformation with Ci symmetry and the aminoxyl radical center has a short contact of 2.36 A with one of the neighboring molecules. The three 1:1 complexes have mutually similar molecular structures in which the 2,2'-bipyridine moiety has a cis conformation and serves as a bidentate ligand and coordination geometry around the metal atom is a distorted octahedron. The EPR experiments for free ligand 4 and [Zn(hfac)2.4] in frozen solution suggested that the exchange couplings between the two aminoxyls (R) through the 2,2'-bipyridine rings are antiferromagnetic with JR-R/kB = -19.3 +/- 0.5 and -24.3 +/- 0.4 K, respectively. Isosceles triangular three-spin models were applied to the 1:1 magnetic metal complexes to give JR-M/kB = -19.1 +/- 0.2 K and JR-R/kB = -32.9 +/- 0.3 K for [Mn(hfac)2.4] and JR-M/kB = +73 +/- 18 K and JR-R/kB = -24.5 +/- 6.5 K for [Cu(hfac)2.4].     

Orientation tuning of a polypyridyl Ru(II) complex immobilized on a clay surface toward chiral discrimination

The present work reports an attempt to elucidate a stereoselective energy-transfer system by immobilizing a chiral metal complex on a clay surface. The metal complex used was [Ru(bpy)2L(i)]2+ with L1 = bpy (2,2'-bipyridine), L2 = 4,4'-diundecyl-2,2'-bipyridine, and L3 = 5,5'-diundecyl-2,2'-bipyridine. The adsorption structure of [Ru(bpy)2L(i)]2+ was studied by means of electric dichroism measurements on an aqueous dispersion of a colloidal clay. It was found that the molecular orientation of the adsorbed Ru(II) complex was affected remarkably by the positions of the alkyl chains on the bpy ligand; that is, the angle of the 3-fold or pseudo-3-fold symmetry axis of the Ru(II) complex with respect to the surface normal was obtained to be 24 degrees, 30 degrees, and 52 degrees for i = 1, 2, and 3, respectively. The efficiency of the energy-transfer was determined by photoluminescence quenching measurements between the adsorbed Ru(II) complex and [Ru(acac)3] (acac = acetylacetonate) in solution. As a result, stereoselectivity appeared most for the case of [Ru(bpy)2L3]2+ in which its two helically twisted bpy ligands were projected in an outward direction.     

A fast-moving [2]rotaxane whose stoppers are remote from the copper complex core

[reaction: see text] A new copper-complexed rotaxane is described. It consists of a two-coordination site ring threaded by a sterically non-hindering 2,2'-bipyridine derivative. An electrochemical signal (oxidation or reduction of the copper center, Cu(I) or Cu(II)) induces rearrangement of the system. By using long and flexible linkers between the stoppers and the central complex, ligand exchange is fast, which leads to short response times (on the millisecond time scale and even below).