Rhenium Center Research Articles

Solvothermal synthesis, characterization and properties of [ReCl5(py)]− complex with T (Néel) of 5.5 K

The synthesis and properties of a rhenium(IV) complex with pyridine as a ligand are presented. The complex, with the formula (2-methyl-pyH)[ReCl5(py)] 4,4′-bipyridyl 1 has been characterized by elemental analysis, IR spectroscopy, SQUID and X-ray measurements. The single crystal X-ray structure of 1 shows that the rhenium center is of distorted octahedral environment, consisting of five chloride atoms and one pyridine ligand. The solid-state magnetic measurements show antiferromagnetic interactions with T (Neel) of 5.5 K.

Bidentate Lewis Base Adducts of Methyltrioxidorhenium(VII): Ligand Influence on Catalytic Performance and Stability

AbstractMethyltrioxidorhenium (MTO) forms 1:1 adducts of the general formula CH3ReO3·L2 with bidentate Lewis bases (L2 = 5,5′‐dimethyl‐2,2′‐bipyridine, 5,5′‐diamino‐2,2′‐bipyridine, 4,4′‐dibromo‐2,2′‐bipyridine, 5,5′‐dibromo‐2,2′‐bipyridine, diethyl 2,2′‐bipyridine‐5,5′‐dicarboxylate, 1,10‐phenanthroline‐5,6‐dione, 3,6‐di(2‐pyridyl)pyridazine). Due to the steric demands of the ligands, the complexes display a distorted octahedral geometry as confirmed by solid state X‐ray crystallography. The rhenium center is disordered in all examined crystal structures. The complexes synthesized are thermally stable but sensitive to light and moisture. The 2,2′‐bipyridine derived complexes exhibit good catalytic activities for cyclooctene epoxidation in a biphasic H2O2/organic solvent catalytic system using hydrogen peroxide as oxidizing agent. The functional groups on the bipyridine rings play an important role with respect to the differences in formation, stability and activity of the complexes. Their influence depends largely on their electron donor capabilities. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Ortho-metalation, rotational isomerization, and hydride–hydride coupling at rhenium (V) polyhydride complexes stabilized by aromatic amine ligands

An ortho-metalated rhenium (V) polyhydride complex has been prepared through the reaction of ReH 7(PPh 3) 2 with 2-phenylpyridine. Additionally, a small series of neutral rhenium (V) pentahydride complexes, each of which is stabilized by an aromatic amine ligand, has been prepared. E and Z rotational isomers of the ReH 5(PPh 3) 2(aromatic amine) complexes have been observed at low temperatures by NMR spectroscopy. The E and Z rotational isomers arise from a combination of the lack of a mirror plane symmetry element orthogonal to the aromatic ring in the aromatic amine ligands and the restricted rotation about the Re–N bond in such complexes. Restricted rotation about the Re–N bond in the related complex, ReH 5(PPh 3) 2(Py) has previously been observed by Crabtree et al. The restricted rotation about the Re–N bond seems to result from π-donation of the lone electron pair on the rhenium (V) center to the π∗ system of the aromatic amine ligands. Different populations of the E and Z rotational isomers arise from interactions of substituents on the aromatic ring with the other ligands bound to rhenium. The values of Δ G ‡ for the restricted rotation about the Re–N bonds, for the complexes containing 4-phenylpyrimidine, 2-aminopyrimidine, or 2-aminopyridine, range from 9.9 to 11.3 kcal/mole. One of the new compounds reported herein, ReH 5(PPh 3) 2[1-(2-NH 2Pyr)] is the first rhenium (V) polyhydride complex to display hydride–hydride coupling in its 1H NMR spectrum.

Linker Conjugation Effects in Rhenium(I) Bifunctional Hole-Transport/Emitter Molecules

Spectroscopic, electrochemical and density functional theory (DFT) methods have been employed to investigate a group of [Re(CO)(3)(HT)(phen)](+) complexes (phen = 1,10-phenanthroline), and in particular the level of electronic communication between various hole-transporting (HT) ligands and the rhenium centre. Here, the HT ligand consists of a coordinating pyridine connected to dimethylaniline group through a single-, double- or triple-bond-connecting system. Electronic absorption, resonance Raman, and steady-state emission spectroscopy combined with lifetime studies and DFT calculations suggest that multiple dpi(Re)-->pi*(phen) metal-to-ligand charge transfers (MLCTs) exist for each complex, two of which significantly absorb at about 340 and 385 nm, and one that emits at approximately 540 nm. In the complexes containing more-conjugated HT ligands, non-emissive intraligand transitions (IL(HT)) exist with energies between the ground and MLCT excited states. The overlap of these IL(HT) transitions and the absorbing MLCT of lowest energy deactivates emission resulting from about 385 nm excitation, and lowers the quantum yield and excited-state lifetimes of these complexes. Cyclic voltammetry experiments indicate that throughout the series investigated, the highest occupied molecular orbital (HOMO) of each complex is centred on the HT ligand, while the occupied molecular orbitals localised on the rhenium are lower in energy.

Synthesis of Fluorinated ReCl(4,4′-R2-2,2′-bipyridine)(CO)3Complexes and Their Photophysical Characterization in CH3CN and Supercritical CO2

Two new CO(2)-soluble rhenium(I) bipyridine complexes bearing the fluorinated alkyl ligands 4,4'-(C(6)F(13)CH(2)CH(2)CH(2))(2)-2,2'-bipyridine (1a), and 4,4'-(C(8)F(17)CH(2)CH(2)CH(2))(2)-2,2'-bipyridine (1b) have been prepared and their photophysical properties investigated in CH(3)CN and supercritical CO(2). Electrochemical and spectroscopic characterization of these complexes in CH(3)CN suggests that the three methylene units effectively insulate the bipyridyl rings and the rhenium center from the electron-withdrawing effect of the fluorinated alkyl chains. Reductive quenching of the metal-to-ligand charge-transfer excited states with triethylamine reveals quenching rate constants in supercritical CO(2) that are only 6 times slower than those in CH(3)CN.

Synthesis, characterization, in vivo antitumor properties of the cluster rhenium compound with GABA ligands and its synergism with cisplatin

A new dirhenium(III) complex cis-[Re2(GABA)2Cl5(H2O)]Cl.2H2O with zwitterionic gamma-aminobutyrate ligands was prepared and characterized by spectral methods and crystallography. The structure of the compound is comprised of dinuclear complex cations (Re-Re 2.2437(3) A) involving cis-oriented double carboxylate bridges, four equatorial chloride ions and two weakly bonded aqua and chloride ligands in the axial positions at two rhenium centers (Re-O 2.363(3), Re-Cl 2.6735(12) A). Antitumor properties of the complex were studied in the model of tumor growth with the use of Wistar rats inoculated by tumor carcinoma Guerink cells. The introduction of the compound in dosage according to the scheme of antioxidant therapy, inhibited the tumor growth by ca. 60% and led to stabilization of red blood cells in the tumor-bearing organisms. The combined introduction of the compound and cisplatin had a significant impact on the tumor growth and the disappearance of the tumors in most of the animals.

Heterometallic complexes involving iron(ii) and rhenium(vii) centers connected by μ-oxido bridges

The reaction of FeI(2) with two equivalents of AgReO(4) in acetonitrile leads to yellow, crystalline Fe(ReO(4))(2)(CH(3)CN)(3) (1), and the treatment of 1 with four equivalents of CpFe(CO)(2)CN gives orange, crystalline Fe(ReO(4))(2)[CpFe(CO)(2)(CN)](4) (2). Compound 2 can also be prepared in one step by the reaction of FeI(2), AgReO(4) and CpFe(CO)(2)CN in dichloromethane. The structure of 1 consists of infinite chains in which alternating {Fe(CH(3)CN)(4)} and {Fe(ReO(4))(2)(CH(3)CN)(2)} units are linked by perrhenate anions to form a (-ReO(2)-O-Fe(ReO(4))(2)(CH(3)CN)(2)-O-ReO(2)-O-Fe(CH(3)CN)(4)-O-)(n) molecular wire. The structure of 2 shows a monomeric iron complex with a slightly distorted octahedral coordination environment consisting of four organometallic complexes coordinated in the equatorial plane via the cyanide groups and two monodentate perrhenates in the corresponding apical positions. Both compounds were further characterised in the solid state by IR spectroscopy, thermogravimetric analysis and magnetic susceptibility measurements. The magnetic data indicate that 1 behaves as a ferrimagnetic chain with 3D ordering below 8 K due to inter-chain interactions. Compound 2 has antiferromagnetic interactions within the Fe(ReO(4))(2)[CpFe(CO)(2)(CN)](4) clusters.

A New Mechanism for the Conversion of Transition Metal Azides to Imido Complexes

Density functional calculations reveal a new mechanism for the decomposition of rhenium alkyl azide. PhN3 binds to the rhenium center in an η1-diazoamino mode ([Re]—N(Ph)—N≡N) rather than the more common η1-terminal mode ([Re]—N═N═N—Ph). From this new intermediate direct loss of N2 is fast, while the previously postulated cyclic 4-centered intermediate/transition state are very unfavorable in this system.

Hetero-Bimetallic Complexes Involving Vanadium(V) and Rhenium(VII) Centers, Connected by Unsupported μ-Oxido Bridge: Synthesis, Characterization, and Redox Study

Heterobimetallic complexes of a vanadium(V) and rhenium(VII) combination connected by a mu-oxido bridge [LVO(mu-O)ReO 3].H 2O [H 2L = N, N'-ethylene bis(salicylideneimine) (H 2salen) and its methoxy derivative] ( 1, 2) are reported. The compounds have been prepared by a single-pot synthesis in which the precursor [V (IV)OL] complexes are allowed to be oxidized aerially in the presence of added perrhenate. The oxidized [V (V)OL] (+) species accommodate the ReO 4 (-) anion in their vacant coordination site, trans to the terminal oxido group, providing the complexes 1 and 2. The later generates a binuclear oxovanadium(V) compound [H 2en][(TBC)VO(mu-TBC) 2OV(TBC)].5H 2O ( 3) when treated with tetrabromocatechol. Single crystal X-ray diffraction analysis and (1)H NMR spectroscopy have been used to establish their identities. In compound 2, the Re(1)-O(11)-V(1) bridge angle is barely linear [170.2(3) degrees ] with a Re...V separation of 3.9647(9) A. The redox behavior of 1 and 2 are quite interesting, each undergoing two reductions both in the positive potential range at E 1/2 = 0.59 (process I) and E 1/2 = 0.16 V (process II) versus Ag/AgCl reference (corresponding potentials are 0.59 and 0.18 V for 2). Process I has a single-electron stoichiometry involving the [VO(salen)] part of the complexes as established by combined coulometry-Electron Paramagnetic Resonance (EPR) experiments which provide an eight-line isotropic EPR pattern at room temperature (<g> = 1.967; <A> = 87 x 10 (-4) cm (-1)), characteristic of an unpaired electron being coupled to a vanadium nuclear spin ( (51)V, I = 7/2). The almost linear V-O-Re bridge in 1 and 2 allows this unpaired electron to interact effectively with the neighboring Re nuclear spin, leading to familiar " two-line pattern" superhyperfine coupling ( A ( (185,187)Re) = 20.7 x 10 (-4) cm (-1)). Process II, on the other hand, is based on a Re(VII/VI) electron transfer as confirmed by differential pulse and normal pulse voltammetric experiments.

Rhenium chelate complexes with maltolate or kojate

Novel rhenium complexes containing the maltolate (mal) or kojate (koj) anions as chelating ligands have been synthesized: [ReOCl(mal) 2] ( 1), [ReOCl 2(mal)(PPh 3)] ( 2), [ReOBr 2(mal)(PPh 3)] ( 3), [ReOCl 2(koj)(PPh 3)] ( 4) and [ReOBr 2(koj)(PPh 3)] ( 5). The products have been characterized by FTIR, 1H, 13C, and 31P NMR spectroscopies and elemental analysis. The crystal and molecular structures of all complexes were determined. Complex 1 crystallizes monoclinic, space group C2/ c, Z = 8. It contains two O,O′-bidentate maltolate ligands and one chloro ligand at the (ReO) 3+ unit, so that a distorted octahedral geometry is adopted by the six-coordinated rhenium(V) center. The chloro ligand occupies a cis position to the oxo ligand. Complexes 2 and 3 are isostructural and crystallize orthorhombic, space group Pbca and Z = 8. The isostructural complexes 4 and 5 crystallize monoclinic, space group P2 1/ n and Z = 4. In complexes 2– 5, the (ReO) 3+ unit is coordinated by a monoanionic O, O-bidentate unit of the maltolate ( 2 and 3) or kojate ( 4 and 5) ligand, one triphenylphosphine and two halogeno ligands (Cl in 2 and 4; Br in 3 and 5), with the rhenium(V) center in a distorted octahedral environment. The halide ligands are in cis positions to each other.

A Modular Approach to Luminescent Dinuclear Ruthenium(II) and Rhenium(I) Complexes

AbstractA series of dinuclear (bipyridine)tricarbonylrhenium(I) and tris(bipyridine)ruthenium(II) complexes have been isolated and characterised, bridged by a flexible diamido ethylene glycol chain. A new stepwise synthetic pathway has been investigated to heterometallic complexes, with the rhenium(I) complexes exhibiting an unusual configuration and inequivalence of the metal centres potentially arising from a surprising hydrogen‐bonding interaction between an Re–CO group and an amide proton in low‐polarity solvents. This interaction appears to be broken by competing hydrogen‐bonding species such as dihydrogen phosphate. This effect was not observed in the corresponding ruthenium(II) complexes, which showed very little interaction with anions. The photophysical characterisation shows that the inclusion of two ester/amide groups to the rhenium centre effectively quenches the fluorescence at room temperature. The ruthenium(II) complexes have considerably stronger fluorescence than the rhenium species, and are less affected by theinclusion of ester and amide groups to the 2,2′‐bipyridine chelating group. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Synthesis and characterization of 2,5-bis(benzylthio)-1,3,4-thiadiazole complexes with fac- [formula omitted

Reactions of 2,5-bis(benzylthio)-1,3,4-thiadiazole ( 1) with a common organometallic rhenium starting material [NEt 4] 2[ fac-[Re(I)Br 3(CO) 3] yielded two distinct types of complexes. Both complexes coordinate only through the nitrogen of the thiadiazole ring. Reaction of 1 with the rhenium starting material alone yielded a bimetallic complex fac-(di-μ-bromo)(μ-2,5-bis(benzylthio)-1,3,4-thiadiazole-κN:κ′N)bis(tricarbonyl rhenium (I)) ( 2). The nitrogens of the thiadiazole ring of 1 each coordinate to a different rhenium center combined with two bridging bromide ligands in 2. A “2+1” complex was prepared in a two step process by reacting the rhenium starting material with picolinic acid followed by 1 to yield fac-Re(I)(2,5-bis(benzylthio)-1,3,4-thiadiazole)(CO) 3(picolinate) ( 3). Compounds 2 and 3 were characterized by NMR, IR, UV, elemental analysis, and single crystal X-ray diffraction.

Synthesis, Characterization, and Photophysical Properties of 2‐Hydroxybenzaldehyde [(1E)‐1‐pyridin‐2‐ylethylidene]hydrazone and Its Rhenium(I) Complexes

AbstractThe 2‐hydroxybenzaldehyde [(1E)‐1‐pyridin‐2‐ylethylidene]hydrazone (HL2) ligand was prepared and its rhenium(I) complexes [ReX(CO)3(HL2)] were obtained by reaction with fac‐[ReX(CO)3(CH3CN)2] (X = Cl, Br) in chloroform. The compounds were characterized by elemental analysis, mass spectrometry, and IR, UV/Vis, and 1H NMR spectroscopy. Furthermore, the structures were also established by X‐ray diffraction. The aromatic rings are almost coplanar in the ligand structure, and the configuration around the hydrazone group is strongly dominated by the presence of an intramolecular hydrogen bond between the OH group and the hydrazinic nitrogen atom N1. The rhenium center is coordinated through the pyridine and hydrazinic nitrogen atoms to give a five‐membered chelate ring. In addition, the free ligand shows interesting luminescence properties. The effect of the metal coordination on the photophysical behavior of the resulting complex was also studied.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

&lt;b&gt;Synthesis and structure of a rhenium(V) complex containing a tridentate imido-coordinated Schiff base&lt;/b&gt;

The six-coordinate complex [Re(mps)Cl2(PPh3)] (1) (H3mps = N-(2-amino-3-methylphenyl) salicylideneimine) was prepared by the reaction of [NH4][ReO4] with H3mps in the presence of triphenylphosphine and hydrochloric acid in glacial acetic acid. Crystallization from acetonitrile gave the product 1.CH3CN. The compound was characterized by spectroscopy and X-ray crystallography. The results show that the tridentate mps ligand coordinates via the doubly deprotonated 2-amino nitrogen (which is present in 1 as an imide), the neutral imino nitrogen and the phenolate oxygen. The imide and phenolate oxygen are trans to each other in a distorted octahedral geometry around the rhenium(V) centre, with the two chlorides in cis positions. KEY WORDS: Reduction of perrhenate, Rhenium(V), Imido donor atom, Crystal structure Bull. Chem. Soc. Ethiop. 2008, 22(1), 101-105.

Photochemical Synthesis and Reactivity Studies of Dirhenacarboranes

Ultraviolet irradiation of [PPh(4)][closo-1-CB(8)H(9)] with [Re(2)(CO)(10)] in THF (tetrahydrofuran) at ambient temperature affords the dirhenacarborane anion [6,10-{Re(CO)(4)}-10-(micro-H)-6,6,6-(CO)(3)-closo-6,1-ReCB(8)H(8)]-, isolated as its [PPh(4)]+ salt (1). Further irradiation of 1 yields a second isomeric anion [6,10-{Re(CO)(4)}-6-(micro-H)-10,10,10-(CO)(3)-closo-10,1-ReCB(8)H(8)]- that was characterized as a [N(PPh(3))(2)]+ salt (2). Reaction of 1 with NOBF(4) produces the neutral dirhenacarborane compound [8,10-{Re(CO)(4)}-8,10-(micro-H)2-6,6-(CO)(2)-6-NO-closo-6,1-ReCB(8)H(7)] (3). Compounds 1-3 all consist of a central {closo-ReCB(8)} cluster with a second rhenium center which is exo-polyhedral. Attempts to substitute the carbonyl ligands of 3 with other donor ligands such as phosphines, isocyanides, or alkynes resulted in loss of the exo-polyhedral rhenium moiety and formation of a monorhenium anion, [6,6-(CO)(2)-6-NO-closo-6,1-ReCB(8)H(9)]-, isolated as its [N(PPh(3))(2)]+ salt (4). The heterometallic dimetallacarborane species, [6,7,10-{Cu(PPh(3))}-7,10-(micro-H)2-6,6-(CO)(2)-6-NO-closo-6,1-ReCB(8)H(7)] (5) and [6,7-{Au(PPh(3))}-7-(micro-H)-6,6-(CO)(2)-6-NO-closo-6,1-ReCB(8)H(8)] (6) were formed from reactions of 4 with {Cu(PPh(3))}+ and {Au(PPh(3))}+, respectively. Similarly, reaction of 4 with {Ir(CO)(PPh(3))(2)}+ afforded two products, [6,10-{Ir(micro-PPh(2))(Ph)(CO)(PPh(3))}-10-(micro-H)-6-CO-6-NO-closo-6,1-ReCB(8)H(8)] (7) and [6,9,10-{Ir(micro-PPh(2))(H)(PPh(3))}-9-(micro-H)-6-CO-6-NO-10-Ph-closo-6,1-ReCB(8)H(8)] (8). The solid-state structures of compounds 1-8 were all unequivocally established by single-crystal X-ray diffraction experiments.

Synthesis of Hyperbranched Polymers with Rhenium(I) Bis(Stilbazoylimino)Acephthalene Complexes at the Branching Units

Two hyperbranched polymers that contains chlorotricarbonyl rhenium(I) bis(stilbazoylimino)acenaphthene complexes were synthesized. The AB2 typed monomers are based on 3- or 4-stilbazoylimino groups that coordinated to the rhenium center as the bidentate ligands. Self-polymerization could be initiated by the addition of silver triflate that activated the rhenium center by removing the chloride ligand. The sizes of the polymers were studied by various methods including gel permeation chromatography and light scattering, and atomic force microscopy. Addition of electrolyte to the polymer solutions resulted in decrease in molecular size, but after a critical concentration (ca. 10 mM), the sizes increased due to the formation of aggregates. These polymers were fabricated into multilayer films by the layer-by-layer electrostatic self-assembly with another polyanion. The multilayer films could function as the active layer in photovoltaic cells. The open-circuit voltage, short circuit current, and fill factor of the best device were measured to be 0.62 V, 3.7 μA/cm2, and 0.17, respectively. The power conversion efficiencies of the cells were in the order of 10−4 %. The contribution of metal complex in the photosensitization process was confirmed by the plot of incident photon-to-electron conversion efficiency as the function of incident light wavelength.

Monodentate imido coordination of 2-aminodiphenylamine to rhenium(V)

The complex trans-[Re(ada)Cl3(PPh3)2] (H2ada = 2-aminodiphenylamine) was prepared from the reaction of trans-[ReOCl3(PPh3)2] with H2ada in acetonitrile. The ligand ada is coordinated to the rhenium(V) centre solely through a dianionic imido nitrogen, with distorted octahedral coordination geometry around the metal ion. Surprisingly, the Re–Cl bond trans to the Re=N bond is shorter than the two equatorial Re–Cl bonds. The Re = N–C bond angle of the phenylimido moiety equals 178.7(4)°.

Synthesis and characterization of a neutral rhenium(V) complex containing a tridentate chelate with an imido donor atom

The six-coordinate complex cis-[Re(mps)Cl2(PPh3)] (1) (H3mps = N-(2-amino-3-methylphenyl)-salicylideneimine) was prepared the reaction of trans-[ReOCl3(PPh3)2] with a twofold molar excess of H3mps in benzene. The compound was characterized by spectroscopy and single-crystal X-ray crystallography. Mps coordinates as a tridentate chelate via the doubly deprotonated 2-amino nitrogen (which is present in 1 as an imide), the neutral imino nitrogen and the phenolate oxygen atoms. The imide and phenolate oxygen atoms coordinate trans to each other in a distorted octahedral geometry around the rhenium(V) centre, with the two chlorides in cis positions.

Mechanistic Insight into Hydrosilylation Reactions Catalyzed by High Valent Re⋮X (X = O, NAr, or N) Complexes: The Silane (SiH) Does Not Add across the Metal−Ligand Multiple Bond

Treatment of oxo and imido-rhenium(V) complexes Re(X)Cl3(PR3)2 (X = O, NAr, and R = Ph or Cy) (1-2) with Et3SiH affords Re(X)Cl2(H)(PR3)2 in high yields. Cycloaddition of silane across the ReX multiple bonds is not observed. Two rhenium(V) hydrides (X = O and R = Ph, 4a; X = NMes and R = Ph, 5a) have been structurally characterized by X-ray diffraction. The kinetics of the reaction of Re(O)Cl3(PPh3)2 (1a) with Et3SiH is characterized by phosphine inhibition and saturation in [Et3SiH]. Hence, formation of Re(O)Cl2(H)(PPh3)2 (4a) proceeds via a sigma-adduct followed by heterolytic cleavage of the Si-H bond and transfer of silylium (Et3Si+) to chloride. Oxo and imido complexes of rhenium(V) (1-2) as well as their nitrido analogues, Re(N)Cl2(PR3)2 (3), catalyze the hydrosilylation of PhCHO under ambient conditions, with the reactivity order imido > oxo > nitrido. The isolable oxorhenium(V) hydride 4a reacts with PhCHO to afford the alkoxide Re(O)Cl2(OCH2Ph)(PPh3)2 (6a) with kinetic dependencies that are consistent with aldehyde coordination followed by aldehyde insertion into the Re-H bond. The latter (6a) regenerates the rhenium hydride upon reaction with Et3SiH. These stoichiometric reactions furnish a possible catalytic cycle. However, quantitative kinetic analysis of the individual stoichiometric steps and their comparison to steady-state kinetics of the catalytic reaction reveal that the observed intermediates do not account for the predominant catalytic pathway. Furthermore, for Re(O)Cl2(H)(PCy3)2 and Re(NMes)Cl2(H)(PPh3)2 aldehyde insertion into the Re-H bond is not observed. Therefore, based on the kinetic dependencies under catalytic conditions, a consensus catalytic pathway is put forth in which silane is activated via sigma-adduct formation cis to the ReX bond followed by heterolytic cleavage at the electrophilic rhenium center. The findings presented here demonstrate the so-called Halpern axiom, the observation of "likely" intermediates in a catalytic cycle, generally, signals a nonproductive pathway.

Syntheses and Properties of Bimetallic Chromophore-Quencher Assemblies Containing Ruthenium(II) and Rhenium(I) Centers

We have prepared a family of six new bimetallic complexes containing electron-donating trans-{RuIICl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers linked to trans-[RuIICl(pdma)(2,2‘-bpy)(NO)] (bpy = bipyridyl) or fac-{ReI(biq)(CO)3}+-based (biq = 2,2‘-biquinolinyl) electron acceptors. The extended bridging units are based on 4,4‘-bpy or bpe [E-1,2-bis(4-pyridyl)ethylene] ligands, and these assemblies are designed to display long-lived photoinduced charge-separated states. A number of new monometallic precursor complexes have also been synthesized and fully characterized. The electronic absorption spectra of the bimetallic species are dominated by intense, visible d(RuII) → π*(4,4‘-bpy/bpe) metal-to-ligand charge-transfer (MLCT) bands and d(ReI) → π*(biq) absorptions in the near-UV region. Cyclic voltammetric studies show both metal-based oxidation and ligand-based reduction processes and provide evidence that these assemblies contain thermodynamic driving forces for charge separation. Single-crystal X-ray structures have been determined for the monometallic complex salts fac-[ReI(biq)(CO)3(MeCN)]CF3SO3·CHCl3, fac-[ReI(biq)(CO)3(4-BrCH2C5H4N)]PF6, fac-[ReI(biq)(CO)3(4-HCO2CH2C5H4N)]PF6·Me2CO, fac-[ReI(biq)(CO)3(ISNE)]CF3SO3 (ISNE = ethylisonicotinate), and fac-[ReI(biq)(CO)3(NC5H4CO2)]2·HPF6·3MeCN. Ultrafast time-resolved infrared and Raman spectroscopic measurements will be employed to investigate the photoexcitation properties of the bimetallic species and of monometallic reference complexes.