Multielectron Redox Chemistry of a Neutral,NIR‐Active,Indigo‐Pillared ReI‐Based Triangular Metalloprism

Self‐assembled, hexarhenium(I), triangular metalloprism compound [{(CO)₃Re(μ‐ 2 )Re(CO)₃}₃(μ₃‐ 1 )₂] ( 3 ) featuring three bis‐chelating pillarlike indigo dianions (μ‐ 2 ), each of which connects two fac‐Re(CO)₃ cores, which are interconnected by a tritopic N donor, that is, a 2,4,6‐tris(4‐pyridyl)‐1,3,5‐triazine (μ₃‐ 1 , tPyTz) ligand, has been synthesized in high yield and characterized. Metalloprism 3 exhibits a strong absorption in the near‐infrared (NIR) region. The reversible, multielectron redox properties of the electrogenerated 3 ⁿ species, where n=3+, 0, 3?, 4?, 5?, 8?, in the visible and especially in the NIR region were investigated in THF solution by cyclic voltammetry (CV), chronocoulometry, EPR spectroscopy, and thin‐layer UV/Vis/NIR spectroelectrochemistry (SEC). Stepwise, site‐specific electrochemical reductions lead to the formation of a series of highly stable ion (radical) species in which electrons associated with μ‐ 2 or μ₃‐ 1 components of the molecule can be clearly distinguished. An EPR investigation revealed interaction of unpaired electrons with the metal nuclei (^185,187Re, I=5/2) in the reduced intermediates. The framework has C₂ symmetry, and accidental degeneracies suffice. Detailed theoretical calculations by structure‐based DFT confirm that the triply degenerate HOMO has ≥70 % indigo character with a sizable dπ‐Re character, while the LUMO is dominated by the triply degenerate indigo ligands, and the LUMO+1 by doubly degenerate tPyTz ligands. A comparison of 3 and previously reported 2,2′‐bis‐benzimidazolate‐ (BiBzlm) or alkoxy‐pillared Re^I metalloprisms indicates a very low switching potential with a potential window of less than 1 V and reversibly accessible optical properties with higher stability of the intermediates. The properties exhibited by 3 appear to be due to the slight tuning of the bridging ligand from N,N^? to N,O^?.     

A new class of mixed-valence systems with orbitally degenerate organic redox centers. Examples based on hexa-rhenium molecular prisms

The ligand-centered mixed-valence (LCMV) properties of two supramolecular complexes are reported: triangular prisms of the form ([Re(CO)3]2X)3-mu,mu',mu' '-[tPyTz]2, where X is 2,2'-bisbenzimidazolate (1) or a pair of benzylthiols (2), and tPyTz is tri-(4-pyridyl)-1,3,5-triazine. Cyclic voltammetry demonstrates that the redox-accessible bridging ligands, tPyTz, are reduced in sequential, one-electron reactions. The singly reduced prisms, which represent an unusual type of mixed-valence compound in which the tPyTz ligands themselves are the redox centers, show intense, broad intervalance transfer (IT) bands in the NIR, consistent with highly coupled MV species. Electroabsorption (Stark spectroscopy) measurements reveal small dipole moment changes associated with intervalence excitation (|Deltamu12| = 0.30 +/- 0.02 eA for 1- and 0.48 +/- 0.02 eA for 2-), as well as noncollinear transition dipole moment (mu12) and dipole moment change vectors (zeta approximately 45 degrees ). DFT electronic structure calculations support this unusual result, along with a through-space electronic interaction mechanism. The neutral complexes (D3h symmetry) possess doubly degenerate, but spatially distinct, LUMO and LUMO+ orbitals. The orbital degeneracy of the tPyTz ligands is lifted in the MV forms, resulting in nonsymmetrical charge redistribution within the molecules upon on optical IT.     

Tricarbonylrhenium(I) complexes of highly symmetric hexaazatrinaphthylene ligands (HATN): structural, electrochemical and spectroscopic properties

The new mononuclear and dinuclear tricarbonylrhenium(I) complexes [(HATN)Re(CO)(3)Cl] (1-Cl) and [(μ-Me(6)-HATN)[Re(CO)(3)Cl](2)] (2-Cl(2)) of highly symmetric ligands HATN and Me(6)-HATN were synthesized and structurally characterized. X-Ray crystal structures reveal identical strained aromatic systems and out of the plane fac-Re(CO)(3)Cl units for both complexes. The packing geometry in the unit cell of 1 suggests intermolecular π-π association. Infrared spectroelectrochemistry (SEC) experiments confirmed ligand-based reductions. To get more insight into the reduction mechanism the triflate salts, [(HATN)Re(CO)(3)](OTf) (1-OTf) and [(μ-Me(6)HATN){Re(CO)(3)}(2)](OTf)(2) (2-OTf(2)), were synthesized. Their electrochemical and spectroelectrochemical behavior also exhibits reduction of the aromatic systems. The electronic absorption spectral features of the one electron reduced species were studied by UV-vis-NIR spectroscopy, which shows a broad shoulder at 1500 nm, confirming intra-ligand charge transfer (ILCT). Density functional theory (DFT) calculations on the complexes 1-Cl and 2-Cl(2) for structural optimization show good agreement with experimental bond lengths and bond angles. The spin density plot shows a metal based HOMO and HATN ligand centered LUMO.     

Towards carbohydrate derivatives of the ReI(CO)3 fragment

With the [Re(CO)(3)Br(3)](2-) ion as a precursor for the Re(I)(CO)(3) fragment, the diols (1R,2R)-cyclohexane-1,2-diol [(1R,2R)-Chxd], anhydroerythritol (AnEryt), and (1S,2S)-cyclopentane-1,2-diol [(1S,2S)-Cptd] form dinuclear monoanions in the salts (NBu(4))[(Re(2)(CO)(6){mu-(1R,2R)-ChxdH(-1)}(3)] (1), [K([18]crown-6)][Re(2)(CO)(6)(mu-OMe)(2)(mu-AnErytH(-1))] (2) and (NBu(4))[Re(2)(CO)(6){mu-(1S,2S)-CptdH(-1)}(3)] (3). The monoanionic diolato ligands in these triply bridged dirhenates(I) are monodentate. Bridging triolato ligation in the trirhenates(I) is supported by the anions of glycerol (Glyc) and methyl beta-D-ribopyranoside (Me-beta-D-Ribp), the latter binding in its (1)C(4) conformation, in (DBUH)(2)[Re(3)(CO)(9)(mu(3)-O)(mu(3)-GlycH(-3))]0.5 MeCN (4 a), (NEt(4))[Re(3)(CO)(9)(mu(3)-OMe)(mu(3)-GlycH(-3))] (4 b) and (DBUH)[Re(3)(CO)(9)(mu(3)-OMe)(mu(3)-(1)C(4)-Me-beta-D-Ribp2,3,4H(-3))] (5). The chiral sugar alcohols L-threitol (L-Thre) and D-arabitol (D-Arab) act as tetra- and pentadentate ligands, respectively, in (NEt(4))[Re(2)(CO)(6)(L-ThreH(-3))]MeCN (6) and (NEt(4))(2)(DBUH)(2)[Re(6)(CO)(18)(D-ArabH(-5))(2)] (7). Complexes 6 and 7 are free of supporting oxo or methoxo ligands and use solely the O-atom pattern of the polyol for the connection of the Re(I)(CO)(3) moieties.     

Electrochemical, computational, and photophysical characterization of new luminescent dirhenium-pyridazine complexes containing bridging OR or SR anions

A series of [Re(2)(μ-ER)(2)(CO)(6)(μ-pydz)] complexes have been synthesized (E = S, R = C(6)H(5), 2; E = O, R = C(6)F(5), 3; C(6)H(5), 4; CH(3), and 5; H, 6), starting either from [Re(CO)(5)O(3)SCF(3)] (for 2 and 4), [Re(2)(μ-OR)(3)(CO)(6)](-) (for 3 and 5), or [Re(4)(μ(3)-OH)(4)(CO)(12)] (for 6). Single-crystal diffractometric analysis showed that the two μ-phenolato derivatives (3 and 4) possess an idealized C(2) symmetry, while the μ-benzenethiolato derivative (2) is asymmetrical, because of the different conformation adopted by the phenyl groups. A combined density functional and time-dependent density functional study of the geometry and electronic structure of the complexes showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are the two lowest-lying π* orbitals of pyridazine, whereas the highest occupied molecular orbitals (HOMOs) are mainly constituted by the "t(2g)" set of the Re atoms, with a strong Re-(μ-E) π* character. The absorption spectra have been satisfactorily simulated, by computing the lowest singlet excitation energies. All the complexes exhibit one reversible monoelectronic reduction centered on the pyridazine ligand (ranging from -1.35 V to -1.53 V vs Fc(+)|Fc). The benzenethiolato derivative 2 exhibits one reversible two-electron oxidation (at 0.47 V), whereas the OR derivatives show two close monoelectronic oxidation peaks (ranging from 0.85 V to 1.35 V for the first peak). The thioderivative 2 exhibits a very small electrochemical energy gap (1.9 eV, vs 2.38-2.70 eV for the OR derivatives), and it does not show any photoluminescence. The complexes containing OR ligands show from moderate to poor photoluminescence, in the range of 608-708 nm, with quantum yields decreasing (ranging from 5.5% to 0.07%) and lifetimes decreasing (ranging from 550 ns to 9 ns) (3 > 4 > 6 ≈ 5) with increasing emission wavelength. The best emitting properties, which are closely comparable to those of the dichloro complex (1), are exhibited by the pentafluorophenolato derivative (3).     

Investigation of mixed oxidation state cyanide-bridged Re(V)oxo (acac(2)en/pn) and Re(I)(bipy)(CO)3 complexes

A series of cyanide-bridged complexes that combine a low-valent photoacceptor rhenium(I) metal center with an electroactive midvalent rhenium(V) complex were prepared. The synthesis involved the preparation of novel asymmetric rhenium(V) oxo compounds, cis-Re(V)O(CN)(acac(2)en) (1) and cis-Re(V)O(CN)(acac(2)pn) (2), formed by reacting trans-[Re(V)O(OH(2))(acac(2)en)]Cl or trans-Re(V)O(acac(2)pn)Cl with [NBu(4)][CN]. The μ-bridged cyanide mixed-oxidation Re(V)-Re(I) complexes were prepared by incubating the asymmetric complexes, 1 or 2, with fac-[Re(I)(bipy)(CO)(3)][OTf] to yield cis-[Re(V)O(acac(2)en)(μ-CN-1κC:2κN)-fac-Re(I)(bipy)(CO)(3)][PF(6)] (3) and [cis-Re(V)O(acac(2)pn)(μ-CN-1κC:2κN)-fac-Re(I)(bipy)(CO)(3)][PF(6)] (4), respectively.     

Photoreduction of carbon dioxide to carbon monoxide with hydrogen catalyzed by a rhenium(i) phenanthroline-polyoxometalate hybrid complex

A phenanthroline ligand decorated at the 5,6-position with a 15-crown-5 ether was used to prepare a metalorganic-polyoxometalate hybrid complex Re(I)(L)(CO)(3)CH(3)CN-MHPW(12)O(40) (L = 15-crown-5-phenanthroline, M = Na(+), H(3)O(+)). X-ray diffraction, (1)H and (13)C NMR, ESI-MS, IR, and elemental analysis were used to characterize this complex. In the presence of Pt/C, the polyoxometalate moiety in Re(I)(L)(CO)(3)CH(3)CN-MHPW(12)O(40) can oxidize H(2) to two protons and two electrons which in the presence of visible light can catalyze the photoreduction of CO(2) to CO with H(2) as the reducing agent instead of the universally used amines as sacrificial reducing agents. An EPR spectrum of a stable intermediate species under reaction conditions shows characteristics of a PW(V)W(VI)(11)O(40) and a Re(0) species with a tentative assignment of the intermediate as Re(0)(L)(CO)(3)(S)-MH(3)PW(V)W(VI)(11)O(40).     

Tuning the reactivity in classic low-spin d6 rhenium(I) tricarbonyl radiopharmaceutical synthon by selective bidentate ligand variation (L,L'-Bid; L,L'= N,N', N,O, and O,O' donor atom sets) in fac-[Re(CO)3(L,L'-Bid)(MeOH)]n complexes

A range of fac-[Re(CO)(3)(L,L'-Bid)(H(2)O)](n) (L,L'-Bid = neutral or monoanionic bidentate ligands with varied L,L' donor atoms, N,N', N,O, or O,O': 1,10-phenanthroline, 2,2'-bipydine, 2-picolinate, 2-quinolinate, 2,4-dipicolinate, 2,4-diquinolinate, tribromotropolonate, and hydroxyflavonate; n = 0, +1) has been synthesized and the aqua/methanol substitution has been investigated. The complexes were characterized by UV-vis, IR and NMR spectroscopy and X-ray crystallographic studies of the compounds fac-[Re(CO)(3)(Phen)(H(2)O)]NO(3)·0.5Phen, fac-[Re(CO)(3)(2,4-dQuinH)(H(2)O)]·H(2)O, fac-[Re(CO)(3)(2,4-dQuinH)Py]Py, and fac-[Re(CO)(3)(Flav)(CH(3)OH)]·CH(3)OH are reported. A four order-of-magnitude of activation for the methanol substitution is induced as manifested by the second order rate constants with (N,N'-Bid) < (N,O-Bid) < (O,O'-Bid). Forward and reverse rate and stability constants from slow and stopped-flow UV/vis measurements (k(1), M(-1) s(-1); k(-1), s(-1); K(1), M(-1)) for bromide anions as entering nucleophile are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) (50 ± 3) × 10(-3), (5.9 ± 0.3) × 10(-4), 84 ± 7; fac-[Re(CO)(3)(2,4-dPicoH)(MeOH)] (15.7 ± 0.2) × 10(-3), (6.3 ± 0.8) × 10(-4), 25 ± 3; fac-[Re(CO)(3)(TropBr(3))(MeOH)] (7.06 ± 0.04) × 10(-2), (4 ± 1) × 10(-3), 18 ± 4; fac-[Re(CO)(3)(Flav)(MeOH)] 7.2 ± 0.3, 3.17 ± 0.09, 2.5 ± 2. Activation parameters (ΔH(k1)(++), kJmol(-1); ΔS(k1)(), J K(-1) mol(-1)) from Eyring plots for entering nucleophiles as indicated are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) iodide 70 ± 1, -35 ± 3; fac-[Re(CO)(3)(2,4-dPico)(MeOH)] bromide 80.8 ± 6, -8 ± 2; fac-[Re(CO)(3)(Flav)(MeOH)] bromide 52 ± 5, -52 ± 15. A dissociative interchange mechanism is proposed.     

Quinonoid-bridged chair-shaped dirhenium(I) metallacycles: synthesis, characterization, and spectroelectrochemical studies

Self-assembled, chair-shaped dirhenium(I) macrocyclic compounds featuring the two different bis-chelating quinone dianions (1, L = dhnq(2-); 2, L = dhaq(2-); H(2)dhnq = 6,11-dihydroxy-5,12-naphthacenedione; H(2)dhaq = 1,4-dihydroxy-9,10-anthraquinone) that interface with two fac-Re(CO)(3) cores and a ditopic semirigid N-donor 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)naphthalene (L' = p-NBimM) ligand coordinated to the remaining orthogonal site were prepared in high yields. Their structures were confirmed by single-crystal X-ray diffraction analysis. Electrochemical assessments, using cyclic voltammetry (CV) and UV-vis-NIR spectroelectrochemistry (SEC), revealed the existence of two well-separated, single-electron quinone ligand-centered, reversibly accessible 0, -1, and -2 redox states. Among the two singly reduced radical complexes, the symmetrically bridged quinone complex 1(?-), showed a strong absorption in the NIR regions, which was not observed for the neutral and doubly reduced states, analogous to that of the free dhnq(3?-) quinone. In contrast, when 2 was reduced to 2(?-), a broad signal at 866 nm was observed, very similar to the reduced dhaq(3?-) quinone. This difference in spectral behavior in the singly reduced states is likely due to the annealed benzene ring in 1 and dhnq(2-) because of its symmetrical π-electron system, which is perturbed to a lesser degree compared to asymmetric 2 and dhaq(2-). Reduction to 1(?-) produces a small but not negligible g factor anisotropy (Δg = 0.024) in the electron spin resonance (ESR) signal, indicative of a very small metal-centered spin (5%), but 2(?-) shows a g value in the expected range for organic radicals (no detectable Δg). Thus, the combined investigations reveal that the singly reduced metallacycles are best described as being highly stable, noncommunicating, localized, quinonoid-centered radical complexes, [(CO)(3)Re(I)(μ-L(3?-))(μ-L')Re(I)(CO)(3)](?-).     

Electroswitchable photoluminescence activity: synthesis, spectroscopy, electrochemistry, photophysics, and X-ray crystal and electronic structures of [Re(bpy)(CO)3(C[triple bond]C[bond]C6H4[bond]C[triple bond]C)Fe(C5Me5)(dppe)][PF6](n) (n = 0, 1)

A novel heterobimetallic alkynyl-bridged complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)], 1, and its oxidized species, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 2, have been synthesized and their X-ray crystal structures determined. A related vinylidene complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond](H)C[double bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 3, has also been synthesized and characterized. The cyclic voltammogram of 1 shows a quasireversible reduction couple at -1.49 V (vs SCE), a fully reversible oxidation at -0.19 V, and a quasireversible oxidation at +0.88 V. In accord with the electrochemical results, density-functional theory calculations on the hydrogen-substituted model complex Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)H(5))(dHpe) (Cp = C(5)H(5), dHpe = H(2)P[bond](CH(2))(2)[bond]PH(2)) (1-H) show that the LUMO is mainly bipyridine ligand pi* in character while the HOMO is largely iron(II) d orbital in character. The electronic absorption spectrum of 1 shows low-energy absorption at 390 nm with a 420 nm shoulder in CH(2)Cl(2), while that of 2 exhibits less intense low-energy bands at 432 and 474 nm and additional low-energy bands in the NIR at ca. 830, 1389, and 1773 nm. Unlike the related luminescent rhenium(I)-alkynyl complex [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C[bond]H)], 4, complex 1 is found to be nonemissive, and such a phenomenon is attributed to an intramolecular quenching of the emissive d pi(Re) --> pi*(bpy) (3)MLCT state by the low-lying MLCT and LF excited states of the iron moiety. Interestingly, switching on of the luminescence property derived from the d pi(Re) --> pi*(bpy) (3)MLCT state can be demonstrated in the oxidized species 2 and the related vinylidene analogue 3 due to the absence of the quenching pathway.     

Systematic synthesis, isolation, and photophysical properties of linear-shaped Re(I) oligomers and polymers with 2-20 units

Systematic synthesis routes have been developed for the linear-shaped rhenium(I) oligomers and polymers bridged with bidentate phosphorus ligands, [Re(N--N)(CO)3-PP-{Re(N--N)(CO)2-PP-}(n)Re(N--N)(CO)3](PF6)(n+2) (N--N = diimine, PP = bidentate phosphine, n = 0-18). These were isolated by size exclusion chromatography (SEC) and identified by (1)H NMR, IR, electrospray ionization Fourier transform mass spectrometry, analytical SEC, and elemental analysis. Crystal structures of [Re(bpy)(CO)3-Ph2PC[triple bond]CPPh2-Re(bpy)(CO)3](PF6)2, [Re(bpy)(CO)3-Ph2PC[triple bond]CPPh2-Re(bpy)(CO)2-Ph2PC[triple bond]CPPh2-Re(bpy)(CO)3](PF6)3 and [Re(bpy)(CO)3-Ph2PC2H4PPh2-{Re(bpy)(CO)2Ph2PC2H4PPh2-}(n)Re(bpy)(CO)3](PF6)(n+2) (bpy = 2,2'-bipyridine, n = 1, 2) were obtained, showing that they have interligand pi-pi interaction between the bpy ligand and the phenyl groups on the phosphorus ligand. All of the oligomers and polymers synthesized were emissive at room temperature in solution. For the dimers, broad emission was observed with a maximum at 523-545 nm, from the (3)MLCT excited-state of the tricarbonyl complex unit, [Re(N--N)(CO)3-PP-]. Emission from the longer oligomers and polymers with > or = 3 Re(I) units was observed at wavelengths 50-60 nm longer than those of the corresponding dimers. This fact and the emission decay results clearly show that energy transfer from the edge unit to the interior unit occurs with a rate constant of (0.9 x 10(8))-(2.5 x 10(8)) s(-1). The efficient energy transfer and the smaller exclusive volume of the longer Re(I) polymers indicated intermolecular aggregation for these polymers in an MeCN solution.     

Proof of innocence for the quintessential noninnocent ligand TCNQ in its tetranuclear complex with four [fac-Re(CO)3(bpy)]+ groups: unusually different reactivity of the TCNX ligands (TCNX = TCNE, TCNQ, TCNB)

The reactions of [fac-Re(CO)(3)(bpy)(MeOH)](PF(6)), bpy = 2.2'-bipyridine, with the TCNX ligands (TCNE = tetracyanoethene, TCNQ = 7,7,8,8-tetracyano-p-quinodimethane, and TCNB = 1,2,4,5-tetracyanobenzene) in CH(2)Cl(2) gave very different results. No reaction was observed with TCNB whereas TCNE produced very labile intermediates which converted under mild conditions to structurally characterized [(mu-CN)[fac-Re(CO)(3)(bpy)](2)](PF(6)) with an eclipsed conformation relative to the almost linear Re-CN-Re axis (Re-N(NC) 2.134(8) A, Re-C(CN) 2.098(8) A). With TCNQ, a stable tetranuclear complex [(mu(4)-TCNQ)[Re(CO)(3)(bpy)](4)](BF(4))(4) was obtained. Its structural, electrochemical, and spectroscopic analysis indicates only negligible charge transfer from the rhenium(I) centers to the extremely strong pi acceptor TCNQ. Evidence includes a calculated charge of only -0.09 for coordinated TCNQ according to the empirical structure/charge correlation of Kistenmacher, a high-energy nitrile stretching band nu(CN) = 2235 cm(-1), and unprecedented large anodic shifts >0.7 V of the reduction potentials. DFT calculations were used to confirm and explain the absence of electron delocalization from the electron-rich metals to the TCNQ acceptor bridge. Correspondingly, the X-band and high-frequency (285 GHz) EPR data (g = 2.007) as well as the IR and UV-vis-NIR spectroelectrochemical results (marginal nu(CO) shifts, TCNQ(*-) chromophore bands) support the almost exclusive confinement of the added electron in [(mu(4)-TCNQ)[Re(CO)(3)(bpy)](4)](3+) to the TCNQ bridge.     

Reactions of phthalazine, quinazoline, 4,7-phenanthroline and 2,3'-bipyridine with ruthenium carbonyl

The reactions of [Ru(3)(CO)(12)] with four aromatic diazines have been studied in THF at reflux temperature. With phthalazine (L(1)), the compound [Ru(3)(μ-κ(2)N(2)N(3)-L(1))(μ-CO)(3)(CO)(7)] (1), which contains an intact phthalazine ligand in an axial position bridging an Ru-Ru edge through both N atoms, is initially formed but it reacts with more phthalazine to give [Ru(3)(κN(2)-L(1))(μ-κ(2)N(2)N(3)-L(1))(μ-CO)(3)(CO)(6)] (2), in which a π-π stacking interaction between the aromatic rings of both ligands determines their position in cluster axial sites on the same face of the Ru(3) triangle. With quinazoline (HL(2)), the cyclometalated hydrido decacarbonyl derivative [Ru(3)(μ-H)(μ-κ(2)N(3)C(4)-L(2))(CO)(10)] (3) is initially produced but it partially decarbonylates under the reaction conditions to give [Ru(6)(μ-H)(2)(μ-κ(2)N(3)C(4)-L(2))(μ(3)-κ(3)-N(1)N(3)C(4)-L(2))(CO)(19)] (4), which results from the displacement of a CO ligand of 3 by the uncoordinated N(1) atom of another molecule of 3. With 4,7-phenanthroline (H(2)L(3)), the stepwise formation of the cyclometalated derivatives [Ru(3)(μ-H)(μ-κ(2)N(4)C(3)-HL(3))(CO)(10)] (5) and two isomers of [Ru(6)(μ-H)(2)(μ(4)-κ(4)N(4)C(3)N(7)C(8)-L(3))(CO)(20)] (6a, 6b) takes place. In compounds 6a and 6b, two Ru(3)(μ-H)(CO)(10) trinuclear units are symmetrically (C(2) in 6a or C(S) in 6b) bridged by a doubly-cyclometalated 4,7-phenanthroline ligand. With 2,3'-bipyridine (HL(4)), two products have been isolated, [Ru(3)(μ-H)(μ-κ(2)N(3')C(4')-L(4))(CO)(10)] (7) and [Ru(3)(μ-H)(μ-κ(3)N(2)N(3')C(2')-L(4))(CO)(9)] (8). While compound 7 contains an N(3')C(4')-cyclometalated 2,3'-bipyridine, in compound 8 an N(3')C(2')-cyclometalation is accompanied by the coordination of the N(2) atom of the remaining pyridine fragment. The structures of compounds 2, 3, 4, 6a and 8 have been determined by X-ray diffraction crystallography.     

Proton-induced luminescence of mono- and dinuclear rhenium(I) tricarbonyl complexes containing 4-pyridinealdazine

New mono- and dinuclear rhenium(I) tricarbonyls, of formulas [Re(bpy)(CO)3(PCA)]+ (1), [(bpy)(CO)3Re(I)(PCA)Re(I)(CO)3(bpy)]2+ (2), and [(bpy)(CO)3Re(I)(PCA)Ru(II)(NH3)5]3+ (3) (bpy = 2,2'-bipyridine, PCA = 4-pyridinecarboxaldehydeazine), have been synthesized as PF6- salts and characterized by spectroscopic, electrochemical, and photophysical techniques. These species do not emit at room temperature in CH(3)CN; however, in aqueous solutions, a decrease in pH induces luminescence in all of them, due to protonation of one of the N atoms of the -C=N-N=C- chain of PCA, as indicated by the pKa values of the ground states, obtained by absorption measurements, which are ca. 3 orders of magnitude lower than the pKa value of the pyridine N of PCA in complex 1. On the other hand, the values of pKa* of the excited states, obtained by emission measurements, of complexes 1 and 2 are similar (pKa* = 2.7 +/- 0.1 at I = 0.1 M) and higher than those of the corresponding ground states. At low values of pH, chemical decomposition takes place rapidly in complex 3, but not in 1 and 2, supporting the possible use of these latter species as luminescent sensors of pH. The heterodinuclear complex, of formula [(bpy)(CO)3Re(I)(PCA)Ru(III)(NH3)5]4+, was obtained by bromine oxidation of the [Re(I), Ru(II)] precursor in CH3CN solution; from spectral and electrochemical measurements, the recombination charge-transfer reaction [Re(II), Ru(II) ] --> [Re(I), Ru(III)], which occurs after photoexcitation, is predicted to lie in the Marcus inverted region.     

Photoinduced intramolecular tryptophan oxidation and excited-state behavior of [Re(L-AA)(CO)3(α-diimine)](+) (L = pyridine or imidazole, AA = tryptophan, tyrosine, phenylalanine)

Re(I) carbonyl-diimine complexes [Re(L-AA)(CO)(3)(N,N)](+) (N,N = bpy, phen) containing an aromatic amino acid (AA), phenylalanine (Phe), tyrosine (Tyr), or tryptophan (Trp), linked to Re by a pyridine-amido or imidazole-amido ligand L have been synthesized and their excited-state properties investigated by nanosecond time-resolved IR (TRIR) and emission spectroscopy. Near-UV optical excitation populates a Re(I)(CO)(3)→N,N (3)MLCT excited state *[Re(II)(L-AA)(CO)(3)(N,N(?-))](+). Decay to the ground state (50-300 ns lifetime) is the only excited-state deactivation process observed in the case of Phe and Tyr complexes, whereas the Trp-containing species undergo a Trp(indole)→*Re(II) electron transfer (ET) producing a charge-separated (CS) state, [Re(I)(L-Trp(?+))(CO)(3)(N,N(?-))](+). The ET occurs with a 8-40 ns lifetime depending on L, N,N, and the solvent. The CS state is characterized by ν(CO) IR bands shifted to lower wavenumbers from their respective ground-state positions and two bands at 1278 and 1497 cm(-1) tentatively attributed to Trp(?+). The amido bridge is affected by both the MLCT excitation and the subsequent ET, manifested by the shifts and intensity changes of the amide-I IR band at about 1680 cm(-1). The CS state decays to the ground state by a N,N(?-)→Trp(?+) back-ET the rates of which are comparable to those of the forward ET, 30-60 ns. This study independently demonstrates that Trp can act as an electron-hopping intermediate in photodriven ET systems based on Re-labeled proteins and supramolecules. Photoinduced ET in Trp-containing Re complexes also can be used to generate Trp(?+) and investigate its spectral properties and reactivity.     

Intercalation of fac-[(4,4'-bpy)ReI(CO)3(dppz)]+, dppz = dipyridyl[3,2-a:2'3'-c]phenazine, in polynucleotides. On the UV-vis photophysics of the Re(I) intercalator and the redox reactions with pulse radiolysis-generated radicals

The intercalation of fac-[(4,4'-bpy)Re(I)(CO)3(dppz)]+ (dppz = dipyridyl[3,2-a:2'3'-c]phenazine) in polynucleotides, poly[dAdT]2 and poly[dGdC]2, where A = adenine, G = guanine, C = cytosine and T = thymine, is a major cause of changes in the absorption and emission spectra of the complex. A strong complex-poly[dAdT]2 interaction drives the intercalation process, which has a binding constant, Kb approximately 1.8 x 10(5) M(-1). Pulse radiolysis was used for a study of the redox reactions of e(-)(aq), C*H(2)OH and N3* radicals with the intercalated complex. These radicals exhibited more affinity for the intercalated complex than for the bases. Ligand-radical complexes, fac-[(4,4'-bpy*)Re(I)(CO)3(dppz)] and fac-[(4,4'-bpy)Re(I)(CO)3(dppz *)], were produced by e(-)(aq) and C*H(2)OH, respectively. A Re(II) species, fac-[(4,4'-bpy)Re(II)(CO)3(dppz)](2+), was produced by N3* radicals. The rate of annihilation of the ligand-radical species was second order on the concentration of ligand-radical while the disappearance of the Re(II) complex induced the oxidative cleavage of the polynucleotide strand.     

Electron-Transfer Reactions of Re(CO)(5): Atom-Transfer-Concerted Mechanism vs Bimetallic Intermediate Formation

Flash photochemically generated Re(CO)(5) reacts with halide complexes, Cu(Me(4)[14]-1,3,8,10-tetraeneN(4))X(+), Cu(Me(2)pyo[14]trieneN(4))X(+), and Ni(Me(2)pyo[14]trieneN(4))X(+) (X = Cl, Br, I) and ion pairs, [Co(bipy)(3)(3+), X(-)]. The rate constants for the electron transfers have values, k approximately 10(9) M(-1) s(-1), close to expectations for processes with diffusion-controlled rates. Reaction intermediates, probably bimetallic species, were detected in electron-transfer reactions of Re(CO)(5) with Cu(Me(6)[14]dieneN(4))X(+), (X = Cl, Br, I). In the absence of the halides X(-), the electron-transfer reactions between Re(CO)(5) and these complexes are slow, k < 10(6) M(-1) s(-1). The results are discussed in terms of inner-sphere pathways, namely an atom-transfer-concerted mechanism. The mediation of bimetallic intermediates in the electron transfer is also considered.     

A tetranuclear organorhenium(i) complex of the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane radical anion, TCNQF4 (-)

The radical complex {(mu(4)-TCNQF4)[Re(CO)(3)(bpy)](4)}(PF(6))(3), as prepared and isolated from the reaction between TCNQF4 and [Re(CO)(3)(bpy)(MeOH)](PF(6)), was studied electrochemically and by IR vibrational spectroscopy, UV-Vis-NIR absorption spectroscopy, and by EPR at 9.5, 190 and 285 GHz. The isotropic g factor of 2.0058, the detectable g anisotropy, and the (185,187)Re EPR hyperfine coupling of 0.95 mT for four equivalent metal nuclei support predominant, but not exclusive, spin localisation at the bridging ligand. Nitrile and metal carbonyl stretching frequencies as well as the typically structured near infrared absorption band lend further support to (TCNQF4 (-))(Re(I))(4) as the most appropriate oxidation state formulation. In comparison to the non-radical complex {(mu(4)-TCNQ)[Re(CO)(3)(bpy)](4)}(PF(6))(4) an X-ray structure analysis of {(mu(4)-TCNQF4)[Re(CO)(3)(bpy)](4)}(PF(6))(3) shows a marginally more twisted (ReNCCCNRe)(C(6)X(4))(ReNCCCNRe) configuration and a different up/down arrangement of the [Re(CO)(3)(bpy)](+) groups. This first isolation, electrochemical, structural and spectroscopic characterisation of a discrete tetranuclear radical complex of a TCNQ-type ligand suggests a link between the stability of such materials and the rather small structural changes on ligand-based electron transfer.     

Involvement of a binuclear species with the Re-C(O)O-Re moiety in CO2 reduction catalyzed by tricarbonyl rhenium(I) complexes with diimine ligands: strikingly slow formation of the Re-Re and Re-C(O)O-Re species from Re(dmb)(CO)3S (dmb = 4,4'-dimethyl-2,2'-bipyridine, S = solvent)

Excited-state properties of fac-[Re(dmb)(CO)(3)(CH(3)CN)]PF(6), [Re(dmb)(CO)(3)](2) (where dmb = 4,4'-dimethyl-2,2'-bipyridine), and other tricarbonyl rhenium(I) complexes were investigated by transient FTIR and UV-vis spectroscopy in CH(3)CN or THF. The one-electron reduced monomer, Re(dmb)(CO)(3)S (S = CH(3)CN or THF), can be prepared either by reductive quenching of the excited states of fac-[Re(dmb)(CO)(3)(CH(3)CN)]PF(6) or by homolysis of [Re(dmb)(CO)(3)](2). In the reduced monomer's ground state, the odd electron resides on the dmb ligand rather than on the metal center. Re(dmb)(CO)(3)S dimerizes slowly in THF, k(d) = 40 +/- 5 M(-1) s(-1). This rate constant is much smaller than those of other organometallic radicals which are typically 10(9) M(-1) s(-1). The slower rate suggests that the equilibrium between the ligand-centered and metal-centered radicals is very unfavorable (K approximately 10(-4)). The reaction of Re(dmb)(CO)(3)S with CO(2) is slow and competes with the dimerization. Photolysis of [Re(dmb)(CO)(3)](2) in the presence of CO(2) produces CO with a 25-50% yield based on [Re]. A CO(2) bridged dimer, (CO)(3)(dmb)Re-CO(O)-Re(dmb)(CO)(3) is identified as an intermediate. Both [Re(dmb)(CO)(3)](2)(OCO(2)) and Re(dmb)(CO)(3)(OC(O)OH) are detected as oxidation products; however, the previously reported formato-rhenium species is not detected.     

Phototriggering electron flow through Re(I)-modified Pseudomonas aeruginosa azurins

The [Re(I)(CO)(3)(4,7-dimethyl-1,10-phenanthroline)(histidine-124)(tryptophan-122)] complex, denoted [Re(I)(dmp)(W122)], of Pseudomonas aeruginosa azurin behaves as a single photoactive unit that triggers very fast electron transfer (ET) from a distant (2 nm) Cu(I) center in the protein. Analysis of time-resolved (ps-μs) IR spectroscopic and kinetics data collected on [Re(I)(dmp)(W122)AzM] (in which M=Zn(II), Cu(II), Cu(I); Az=azurin) and position-122 tyrosine (Y), phenylalanine (F), and lysine (K) mutants, together with excited-state DFT/time-dependent (TD)DFT calculations and X-ray structural characterization, reveal the character, energetics, and dynamics of the relevant electronic states of the [Re(I)(dmp)(W122)] unit and a cascade of photoinduced ET and relaxation steps in the corresponding Re-azurins. Optical population of [Re(I)(imidazole-H124)(CO)(3)]→dmp (1)CT states (CT=charge transfer) is followed by around 110 fs intersystem crossing and about 600 ps structural relaxation to a (3)CT state. The IR spectrum indicates a mixed Re(I)(CO)(3),A→dmp/π→π(*)(dmp) character for aromatic amino acids A122 (A=W, Y, F) and Re(I)(CO)(3)→dmp metal-ligand charge transfer (MLCT) for [Re(I)(dmp)(K122)AzCu(II)]. In a few ns, the (3)CT state of [Re(I)(dmp)(W122)AzM] establishes an equilibrium with the [Re(I)(dmp(.-))(W122(.+))AzM] charge-separated state, (3)CS, whereas the (3)CT state of the other Y, F, and K122 proteins decays to the ground state. In addition to this main pathway, (3)CS is populated by fs- and ps-W(indole)→Re(II) ET from (1)CT and the initially "hot" (3)CT states, respectively. The (3)CS state undergoes a tens-of-ns dmp(.-)→W122(.+) ET recombination leading to the ground state or, in the case of the Cu(I) azurin, a competitively fast (≈30 ns over 1.12?nm) Cu(I)→W(.+) ET, to give [Re(I)(dmp(.-))(W122)AzCu(II)]. The overall photoinduced Cu(I)→Re(dmp) ET through [Re(I)(dmp)(W122)AzCu(I)] occurs over a 2 nm distance in <50 ns after excitation, with the intervening fast (3)CT-(3)CS equilibrium being the principal accelerating factor. No reaction was observed for the three Y, F, and K122 analogues. Although the presence of [Re(dmp)(W122)AzCu(II)] oligomers in solution was documented by mass spectrometry and phosphorescence anisotropy, the kinetics data do not indicate any significant interference from the intermolecular ET steps. The ground-state dmp-indole π-π interaction together with well-matched W/W(.+) and excited-state [Re(II)(CO)(3)(dmp(.-))]/[Re(I)(CO)(3)(dmp(.-))] potentials that result in very rapid electron interchange and (3)CT-(3)CS energetic proximity, are the main factors responsible for the unique ET behavior of [Re(I)(dmp)(W122)]-containing azurins.