Excited states of nitro-polypyridine metal complexes and their ultrafast decay. Time-resolved IR Absorption, spectroelectrochemistry, and TD-DFT calculations of fac-[Re(Cl)(CO)3(5-nitro-1,10-phenanthroline)

The lowest absorption band of fac-[Re(Cl)(CO)3(5-NO2-phen)] encompasses two close-lying MLCT transitions. The lower one is directed to LUMO, which is heavily localized on the NO2 group. The UV-vis absorption spectrum is well accounted for by TD-DFT (G03/PBEPBE1/CPCM), provided that the solvent, MeCN, is included in the calculations. Near-UV excitation of fac-[Re(Cl)(CO)3(5-NO2-phen)] populates a triplet metal to ligand charge-transfer excited state, 3MLCT, that was characterized by picosecond time-resolved IR spectroscopy. Large positive shifts of the nu(CO) bands upon excitation (+70 cm(-1) for the A'1 band) signify a very large charge separation between the Re(Cl)(CO)3 unit and the 5-NO2-phen ligand. Details of the excited-state character are revealed by TD-DFT calculated changes of electron density distribution. Experimental excited-state nu(CO) wavenumbers agree well with those calculated by DFT. The 3MLCT state decays with a ca. 10 ps lifetime (in MeCN) into another transient species, that was identified by TRIR and TD-DFT calculations as an intraligand 3npi excited state, whereby the electron density is excited from the NO2 oxygen lone pairs to the pi system of 5-NO2-phen. This state is short-lived, decaying to the ground state with a approximately 30 ps lifetime. The presence of an npi state seems to be the main factor responsible for the lack of emission and the very short lifetimes of 3MLCT states seen in all d6-metal complexes of nitro-polypyridyl ligands. Localization of the excited electron density in the lowest 3MLCT states parallels localization of the extra electron in the reduced state that is characterized by a very small negative shift of the nu(CO) IR bands (-6 cm(-1) for A'1) but a large downward shift of the nu(s)(NO2) IR band. The Re-Cl bond is unusually stable toward reduction, whereas the Cl ligand is readily substituted upon oxidation.     

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.     

Mechanism of the photochemical ligand substitution reactions of fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.     

Photoexcitation in Cu(I) and Re(I) complexes containing substituted dipyrido[3,2-a:2',3'-c]phenazine: a spectroscopic and density functional theoretical study

Copper(I) and rhenium(I) complexes [Cu(PPh(3))(2)(dppz-11-COOEt)]BF(4), [Cu(PPh(3))(2)(dppz-11-Br)]BF(4), [Re(CO)(3)Cl(dppz-11-COOEt)] and [Re(CO)(3)Cl(dppz-11-Br)] (dppz-11-COOEt = dipyrido-[3,2a:2',3'c]phenazine-11-carboxylic ethyl ester, dppz-11-Br = 11-bromo-dipyrido[3,2a:2',3'c]-phenazine) have been studied using Raman, resonance Raman, and transient resonance Raman (TR(2)) spectroscopy, in conjunction with computational chemistry. DFT (B3LYP) frequency calculations with a 6-31G(d) basis set for the ligands and copper(I) centers and an effective core potential (LANL2DZ) for rhenium in the rhenium(I) complexes show close agreement with the experimental nonresonance Raman spectra. Modes that are phenazine-based, phenanthroline-based, and delocalized across the entire ligand structure were identified. The nature of the absorbing chromophores at 356 nm for ligands and complexes was established using resonance Raman spectroscopy in concert with vibrational assignments from calculations. This analysis reveals that the dominant chromophore for the complexes measured at 356 nm is ligand-centered (LC), except for [Re(CO)(3)Cl(dppz-11-Br)], which appears to have additional chromophores at this wavelength. Calculations on the reduced complexes, undertaken to model the metal-to-ligand charge transfer (MLCT) excited state, show that the reducing electron occupies a ligand MO that is delocalized across the ligand structure. Resonance Raman spectra (lambda(exc) = 514.5 nm) of the reduced rhenium complexes show a similar spectral pattern to that observed in [Re(CO)(3)Cl(dppz)](*-); the measured bands are therefore attributed to ligand radical anion modes. These bands lie at 1583-1593 cm(-1) for [Re(CO)(3)Cl(dppz-11-COOEt)] and 1611 cm(-1) for [Re(CO)(3)Cl(dppz-11-Br)]. The thermally equilibrated excited states are examined using nanosecond-TR(2) spectroscopy (lambda(exc) = 354.7 nm). The TR(2) spectra of the ligands provide spectral signatures for the (3)LC state. A band at 1382 cm(-1) is identified as a marker for the (3)LC states of both ligands. TR(2) spectra of the copper and rhenium complexes of dppz-11-Br show this (3)LC band, but it is not prominent in the spectra of [Cu(PPh(3))(2)(dppz-11-COOEt)](+) and [Re(CO)(3)Cl(dppz-11-COOEt)]. Calculations suggest that the lowest triplet states of both of the rhenium(I) complexes and [Cu(PPh(3))(2)(dppz-11-Br)](+) are metal-to-ligand charge transfer in nature, but the lowest triplet state of [Cu(PPh(3))(2)(dppz-11-COOEt)](+) appears to be LC in character.     

Metal-induced tautomerization of p- to o-quinone compounds: experimental evidence from CuI and ReI complexes of azophenine and DFT studies

Azophenine (7,8-diphenyl-2,5-bis(phenylamino)-p-quinonediimine, L(p)) reacts with [Cu(PPh3)4](BF4) or [Re(CO)(5)Cl] to yield the (Ph3P)(2)Cu(+) or [(OC)(3)ClRe] complex of the tautomeric form 7,8-diphenyl-4,5-bis(phenylamino)-o-quinonediimine, L(o), as evident from structure determinations and from very intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. Time-dependent DFT (TD-DFT) calculations on model complexes [(N intersection N)Re(CO)(3)Cl] confirm the spectroscopic results, showing considerably higher oscillator strengths of the MLCT transition for the o-quinonediimine complexes in comparison to compounds with N intersection N=1,4-dialkyl-1,4-diazabutadiene. The complexes are additionally stabilized through hydrogen bonding between two now ortho-positioned NHPh substituents and one fluoride of the BF(4) (-) anion (Cu complex) or the chloride ligand (Re complex). DFT Calculations on the model ligands p-quinonediimine or 2,5-diamino-p-quinonediimine and their ortho-quinonoid forms with and without Li(+) or Cu(+) are presented to discuss the relevance for metal-dependent quinoproteins.     

Synthesis and characterization of rhenium-copper sulfide cluster complexes [(Ph3P)2N][Re3(CuX)(mu3-S)4Cl6(PMe2Ph)3] (X = Cl, Br, I)

The reaction of a trinuclear rhenium sulfide cluster compound Re3S7Cl7 with dimethylphenylphosphine and CuX2 (X = Cl or Br) or CuX (X = Cl, Br, or I) formed tetranuclear cluster complexes [(Ph3P)2N][Re3(CuX)(mu3-S)4Cl6(PMe2Ph)3] (X = Cl, Br, or I). Their solutions have the characteristic intense blue color with visible spectral bands near 600 nm. Single-crystal X-ray structures show that three mu-S atoms in the intermediate trinuclear rhenium complex coordinate to a copper atom, forming elongated tetrahedral structures in which Re-Cu bonding interaction is negligible (Re-Cu distances are 3.50 to approximately 3.54 A as compared with Re-Re distances ranging from 2.69 to 2.81 A).     

Photochemical ligand substitution reactions of fac-[Re(bpy)(CO)3Cl] and derivatives

Excitation by high-energy light, such as that of 313 nm wavelength, induces a photochemical ligand substitution (PLS) reaction of fac-[Re(bpy)(CO)3Cl] (1a) to give the solvento complexes (OC-6-34)- and (OC-6-44)-[Re(bpy)(CO)2(MeCN)Cl] (2 and 3) in good yields. The disappearance quantum yield of 1a was 0.01+/-0.001 at 313 nm. The products were isolated, and X-ray crystallographic analysis was successfully performed for 2. Time-resolved IR measurements clearly indicated that the CO ligand dissociates with subpicosecond rates after excitation, leading to vibrationally hot photoproducts, which relax within 50-100 ps. Detailed studies of the reaction mechanism show that the PLS reaction of 1a does not proceed via the lowest vibrational level in the 3MLCT excited state. The PLS reaction gives 2 and (OC-6-24)-[Re(bpy)(CO)2(MeCN)Cl] (5) as primary products, and one of the products, 5, isomerizes to 3. This type of PLS reaction is more general, occurring in various fac-rhenium(I) diimine tricarbonyl complexes such as fac-[Re(X2bpy)(CO)3Cl] (X2bpy=4,4'-X2-bpy; X=MeO, NH2, CF3), fac-[Re(bpy)(CO)3(pyridine)]+, and fac-[Re(bpy)(CO)3(MeCN)]+. The stable photoproducts (OC-6-44)- and (OC-6-43)-[Re(bpy)(CO)2(MeCN)(pyridine)]+ and (OC-6-32)- and (OC-6-33)-[Re(bpy)(CO)2(MeCN)2]+ were isolated. The PLS reaction of rhenium tricarbonyl-diimine complexes is therefore applicable as a general synthetic method for novel dicarbonyls.     

Control of photochemical, photophysical, electrochemical, and photocatalytic properties of rhenium(I) complexes using intramolecular weak interactions between ligands

Intramolecular interactions between ligands have been successfully applied as a novel tool for controlling various properties of a series of cis,trans-[Re(dmb)(CO)(2)(PR(3))(PR'(3))](+)-type complexes (dmb = 4,4'-dimethyl-2,2'-bipyridine), in the ground state and in the excited state and in the one-electron reduced form. For rhenium complexes with two triarylphosphine ligands, P(p-XPh)(3), the dmb ligand was sandwiched by four aryl rings having CH(aryl)-pi(pyridine)-pi(aryl) interactions. On the other hand, complexes with one triarylphosphine ligand and one trialkylphosphite ligand, P(OR)(3), had pi-pi and CH-pi interactions between each pyridine ring in the dmb ligand and the aryl group in the P(p-XPh)(3). Various properties of these two series of rhenium complexes were compared with those of complexes having two trialkylphosphite ligands, which do not interact through space with the dmb ligand. Properties of the complexes associated mainly with the dmb ligand are strongly affected by the intramolecular interactions: (1) UV/vis absorptions to the pi-pi and (1)MLCT excited states were both red-shifted, but (2) emission from the (3)MLCT excited state was blue-shifted; (3) the lifetime of the (3)MLCT excited state was prolonged up to 3-fold; (4) the reduction potential in the ground state was positively shifted by 110 mV with pi-pi and CH-pi interactions and by 180-200 mV with the CH-pi-pi interactions. (5) In the excited states, the oxidation power of the complex was also enhanced by the intramolecular interactions. (6) In the corresponding one-electron-reduced species cis,trans-[Re(dmb(-.)(CO)(2)(PR(3))(PR'(3))], the intramolecular interactions are maintained and strongly affected their UV/vis spectra. (7) Photocatalysis for CO(2) reduction was significantly enhanced only by the CH-pi-pi interaction.     

Synthesis, structure, photophysical and electrochemiluminescence properties of Re(I) tricarbonyl complexes incorporating pyrazolyl-pyridyl-based ligands

Three rhenium carbonyl complexes 1-3 were synthesized by reaction of the appropriate bidentate pyrazolyl-pyridyl-based ligand L1, L2 (L1 = 2-[1-{4-(bromomethyl)benzyl}-1H-pyrazol-3-yl]pyridine; L2 = 1,4-bis(3-(2-pyridyl)pyrazol-1-ylmethyl)benzene) with [Re(CO)(5)Cl] in toluene. They were characterized by elemental analyses, ESI-MS, (1)H spectroscopy, and X-ray crystallography for 1 and 2. Compounds 1-3 exhibit bright yellow-green luminescence in the solid state and in solution at 298 K with the lifetimes in the microsecond range. It is noteworthy that the luminescent quantum efficiencies of compounds 1-3 are between 0.040 and 0.051, which are much higher than that of the [Re(bpy)(CO)(3)Cl] complex (= 0.019) (M. M. Richter et al., Anal. Chem., 1996, 68, 4370; J. Van Houten et al., J. Am. Chem. Soc., 1976, 98, 4853). Electrogenerated chemiluminescence (ECL) was observed in solutions of these complexes in the absence or presence of coreactant tri-n-propylamine (TPrA) or 2-(dibutylamino)ethanol (DBAE) by stepping the potential of a Pt disk working electrode. The ECL spectra are identical to the photoluminescence spectra, indicating that the chemical reactions following electrochemical oxidation or reduction form the same (3)MLCT excited states as that generated in the photoluminescence experiments. In most cases, the ECL quantum efficiencies of complexes 1-3 are comparable to that of the [Re(L)(CO)(3)Cl] (L = bpy or phen) system. Oxygen tends to substantially decrease ECL intensities of the three rhenium complexes-TPrA system, which could allow them to be used as oxygen sensors.     

Ligand-to-diimine/metal-to-diimine charge-transfer excited states of [Re(NCS)(CO)3(alpha-diimine)] (alpha-diimine = 2,2'-bipyridine, di-iPr-N,N-1,4-diazabutadiene). A spectroscopic and computational study

Two new complexes fac-[Re(NCS)(CO)3(N,N)] (N,N = 2,2'-bipyridine (bpy), di-iPr-N,N-1,4-diazabutadiene (iPr-DAB)) were synthesized and their molecular structures determined by X-ray diffraction. UV-vis absorption, resonance Raman, emission, and picosecond time-resolved IR spectra were measured experimentally and calculated with TD-DFT. A good agreement between experimental and calculated ground- and excited-state spectra is obtained, but only if the solvent (MeCN) is included into calculations and excited state structures are fully optimized at the TD-DFT level. The lowest excited states of the bpy and iPr-DAB complexes are assigned by TD-DFT as 3aA' by comparison of calculated and experimental IR spectra. Excited-state lifetimes of 23 ns and ca. 625 ps were determined for the bpy and DAB complex, respectively, in a fluid solution at room temperature. Biexponential emission decay (1.3, 2.7 micros) observed for [Re(NCS)(CO)3(bpy)] in a 77 K glass indicates the presence of two unequilibrated emissive states. Low-lying electronic transitions and excited states of both complexes have a mixed NCS --> N,N ligand-to-ligand and Re --> N,N metal-to-ligand charge-transfer character (LLCT/MLCT). It originates in mixing between Re d(pi) and NCS pi characters in high-lying occupied MOs. Experimentally, the LLCT/MLCT mixing in the lowest excited state is manifested by shifting the nu(CO) and nu(NC) IR bands to higher and lower wavenumbers, respectively, upon excitation. Resonant enhancement of both nu(CO) and nu(NC) Raman bands indicates that the same LLCT/MLCT character mixing occurs in the lowest allowed electronic transition.     

The effect of reduction on rhenium(I) complexes with binaphthyridine and biquinoline ligands: a spectroscopic and computational study

A number of rhenium complexes with binaphthyridine and biquinoline ligands have been synthesized and studied. These are [Re(L)(CO)3Cl] where L = 3,3'-dimethylene-2,2'-bi-1,8-naphthyridine (dbn), 2,2'-bi-1,8-naphthyridine (bn), 3,3'-dimethylene-2,2'-biquinoline (dbq), and 3,3'-dimethyl-2,2'-biquinoline (diq). This series represents ligands in which the electronic properties and steric preferences are tuned. These complexes are modeled using density functional theory (DFT). An analysis of the resonance Raman spectra for these complexes, in concert with the vibrational assignments, reveals that the accepting molecular orbital (MO) in the metal-to-ligand charge transfer (MLCT) transition is the LUMO and causes bonding changes at the inter-ring section of the ligand. The electronic absorption spectroelectrochemistry for the reduced complexes of [Re(dbn)(CO)3Cl], [Re(dbq)(CO)3Cl], and [Re(diq)(CO)3Cl] suggest that the singly occupied MO is delocalized over the entire ligand structure despite the nonplanar nature of the diq ligand in [Re(diq)(CO)3Cl]. The IR spectroelectrochemistry for [Re(dbn)(CO)3Cl], [Re(dbq)(CO)3Cl], and [Re(bn)(CO)3Cl] reveal that reduction lowers the CO ligand vibrational frequencies to a similar extent in all three complexes. The substitution of naphthyridine for quinoline has little effect on the nature of the singly occupied MO. These data are supported by DFT calculations on the reduced complexes, which reveal that the ligands are flattened out by reduction: This may explain the similarity in the properties of the reduced complexes.     

Proton-Coupled Electron Transfer between 4-Cyanophenol and Photoexcited Rhenium(I) Complexes with Different Protonatable Sites

Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2'-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls ([Re(bpz)(CO)(3)(py)](+)), and one tricarbonyl complex with a 2,2'-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand ([Re(bpy)(CO)(3)(pz)](+)) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the [Re(bpz)(CO)(3)(py)](+) complex while in the [Re(bpy)(CO)(3)(pz)](+) complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted proton-electron transfer is likely to play an important role in both cases, but a reaction sequence of proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol/[Re(bpz)(CO)(3)(py)](+) reaction couple. Interestingly, the rate constants for bimolecular excited-state quenching are on the same order of magnitude for both rhenium(I) complexes.     

Rhenium-to-benzoylpyridine and rhenium-to-bipyridine MLCT excited states of fac-[Re(Cl)(4-benzoylpyridine)(2)(CO)(3)] and fac-[Re(4-benzoylpyridine)(CO)(3)(bpy)](+): a time-resolved spectroscopic and spectroelectrochemical study

The lowest allowed electronic transition of fac-[Re(Cl)(CO)(3)(bopy)(2)] (bopy = 4-benzoylpyridine) has a Re --> bopy MLCT character, as revealed by UV-vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-lying, long-lived, excited state is Re --> bopy (3)MLCT. Electronic depopulation of the Re(CO)(3) unit and population of a bopy pi orbital upon excitation are evident by the upward shift of nu(CO) vibrations and a downward shift of the ketone nu(C=O) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of a single bopy ligand in the (3)MLCT excited state is indicated by time-resolved visible and resonance Raman (TR(3)) spectra that show features typical of bopy(*)(-). In contrast, the lowest allowed electronic transition and lowest-lying excited state of a new complex fac-[Re(bopy)(CO)(3)(bpy)](+) (bpy = 2,2'-bipyridine) have been identified as Re --> bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest MLCT excitation and the first reduction are different. (3)MLCT excited states of both fac-[Re(Cl)(CO)(3)(bopy)(2)] and fac-[Re(bopy)(CO)(3)(bpy)](+) are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic shifts of nu(C(triple bond)O) IR bands. The X-ray structure of fac-[Re(bopy)(CO)(3)(bpy)]PF(6).CH(3)CN has been determined.     

Complexes of functionalized dipyrido[3,2-a:2',3'-c]-phenazine: a synthetic, spectroscopic, structural, and density functional theory study

The ligands 11-bromodipyrido[3,2-a:2',3'-c]phenazine and ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate have been prepared and coordinated to ruthenium(II), rhenium(I), and copper(I) metal centers. The electronic effects of substitution of dipyrido[2,3-a:3',2'-c]phenazine (dppz) have been investigated by spectroscopy and electrochemistry, and some photophysical properties have been studied. The crystal structures of [Re(L)(CO)(3)Cl] (L = ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate or 11-bromodipyrido[3,2-a:2',3'-c]phenazine) are presented. Density functional theory calculations on the complexes show only small deviations in bond lengths and angles (most bonds within 0.02 Angstroms, most angles within 2 degrees) from the crystallographic data. Furthermore, the vibrational spectra of the strongest Raman and IR bands are predicted to within an average 6 cm(-1) for the complexes [Re(L)(CO)(3)Cl] and [Cu(L)(triphenylphosphine)(2)]BF(4) (in the 1000-1700 cm(-1) region). Spectroscopic and electrochemical evidence suggest that reduction of the complex causes structural changes across the entire dppz ligand. This is unusual as dppz-based ligands typically have electrochemical properties that suggest charge localization with reduction on the phenazine portion of the ligand. The excited-state lifetimes of the complexes have been measured, and they range from ca. 200 ns for the [Ru(L)(2,2'-bipyridine)(2)](PF(6))(2) complexes to over 2 mus for [Cu(11-bromodipyrido[3,2-a:2',3'-c]phenazine)(PPh(3))(2)](BF(4)) at room temperature. The emission spectra suggest that the unusually long-lived excited states of the copper complexes result from metal-to-ligand charge transfer (MLCT) transitions as they are completely quenched in methanol. Electroluminescent films may be fabricated from these compounds; they show MLCT state emission even at low doping levels [<0.1% by weight in poly(vinylcarbazole) polymer matrix].     

Rhenium(I) carbonyl complex of 4,7-diphenyl-1,10-phenanthroline-Spectroscopic properties, X-Ray structure, theoretical studies of ground and excited electronic states

The paper presents structural studies of the tricarbonyl complexes incorporating 4,7-diphenyl-1,10-phenanthroline (dpphen) - [Re(CO)₃(dpphen)Cl]₂·Me₂CO (1) and [Re(CO)₃(dpphen)Cl] (2). The absorption and emission spectra of [Re(CO)₃(dpphen)Cl] were discussed with the aid of DFT and time-dependent TD-DFT theories. The theoretical studies reveal mixing between the singlet (S₁) and triplet (T₁) excited states of MLCT character.     

Synthesis of Im3[Re3mu3-S)(mu-S3)Br9]Br (Im = 1-ethyl-3-methylimidazolium) by means of an ionic liquid

Reaction of [Re3(mu3-S)(mu-S2)3Cl6]Cl with an ionic liquid composed of 1-ethyl-3-methylimidazolium bromide (ImBr) and aluminum bromide (AlBr3) formed a new trinuclear rhenium sulfide cluster compound Im3[Re3(mu3-S)(mu-S)3Br9]Br with nine terminal bromo ligands.     

Excited-state dynamics of fac-[ReI(L)(CO)3(phen)]+ and fac-[ReI(L)(CO)3(5-NO2-phen)]+ (L = imidazole, 4-ethylpyridine; phen = 1,10-phenanthroline) complexes

The nature and dynamics of the lowest excited states of fac-[Re(I)(L)(CO)(3)(phen)](+) and fac-[Re(I)(L)(CO)(3)(5-NO(2)-phen)](+) [L = Cl(-), 4-ethyl-pyridine (4-Etpy), imidazole (imH); phen = 1,10-phenanthroline] have been investigated by picosecond visible and IR transient absorption spectroscopy in aqueous (L = imH), acetonitrile (L = 4-Etpy, imH), and MeOH (L = imH) solutions. The phen complexes have long-lived Re(I) --> phen (3)MLCT excited states, characterized by CO stretching frequencies that are upshifted relative to their ground-state values and by widely split IR bands due to the out-of-phase A'(2) and A"nu(CO) vibrations. The lowest excited states of the 5-NO(2)-phen complexes also have (3)MLCT character; the larger upward nu(CO) shifts accord with much more extensive charge transfer from the Re(I)(CO)(3) unit to 5-NO(2)-phen in these states. Transient visible absorption spectra indicate that the excited electron is delocalized over the 5-NO(2)-phen ligand, which acquires radical anionic character. Similarly, involvement of the -NO(2) group in the Franck-Condon MLCT transition is manifested by the presence of an enhanced nu(NO(2)) band in the preresonance Raman spectrum of [Re(I)(4-Etpy)(CO)(3)(5-NO(2)-phen)](+). The Re(I) --> 5-NO(2)-phen (3)MLCT excited states are very short-lived: 7.6, 170, and 43 ps for L = Cl(-), 4-Etpy, and imH, respectively, in CH(3)CN solutions. The (3)MLCT excited state of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) is even shorter-lived in MeOH (15 ps) and H(2)O (1.3 ps). In addition to (3)MLCT, excitation of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) populates a (3)LLCT (imH --> 5-NO(2)-phen) excited state. Most of the (3)LLCT population decays to the ground state (time constants of 19 (H(2)O), 50 (MeOH), and 72 ps (CH(3)CN)); in a small fraction, however, deprotonation of the imH.+ ligand occurs, producing a long-lived species, [Re(I)(im.)(CO)(3)(5-NO(2)-phen).-]+.     

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.     

Oxygen atom transfer reactions from dioxygen to phosphines via a bridging sulfur dioxide in a trinuclear cluster complex of rhenium, [(Ph(3)P)(2)N][Re(3)(mu(3)-S)(mu-S)(2)(mu-SO(2))Cl(6)(PMe(2)Ph)(3)

A trinuclear rhenium sulfide cluster complex, [(Ph(3)P)(2)N][Re(3)(mu(3)-S)(mu-S)(3)Cl(6)(PMe(2)Ph)(3)], synthesized from Re(3)S(7)Cl(7), dimethylphenylphosphine, and [(Ph(3)P)(2)N]Cl is readily converted to a bridging SO(2) complex, [(Ph(3)P)(2)N][Re(3)(mu(3)-S)(mu-S)(2)(mu-SO(2))Cl(6)(PMe(2)Ph)(3)], by reaction with O(2). The oxygen atoms on the SO(2) ligand react with phosphines or phosphites to form phosphine oxides or phosphates, and the original cluster complex is recovered. The reaction course has been monitored by (31)P NMR as well as by UV-vis spectroscopy. The catalytic oxygenation of PMePh(2) in the presence of the SO(2) complex shows that turnovers are 8 per hour at 23 degrees C in CDCl(3). The X-ray structures of the cluster complexes are described.