Structural investigations into the deactivation pathway of the CO2 reduction electrocatalyst Re(bpy)(CO)3Cl

We report a series of complexes synthesized from the chemical reduction of the fac-tricarbonyl complex Re(bpy)(CO)(3)Cl. Synthesis and characterization of [Re(bpy)(CO)(3)](2), [Re(bpy)(CO)(3)](2)(-), and Re(bpy)(CO)(3)(-) are presented. The Re(bpy)(CO)(3)(-) anion has long been postulated as the active species that reacts with carbon dioxide in the electrochemical reduction of CO(2).     

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.     

Ultrafast excited state dynamics controlling photochemical isomerization of N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium coordinated to a {Re I(CO)3(2,2'-bipyridine)} chromophore

Two multifunctional photoactive complexes [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) and [Re(MeDpe(+))(CO)(3)(bpy)](2+) (MeDpe(+)=N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium, bpy=2,2'-bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet-visible-infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time-resolved absorption spectroscopy in the visible and infrared regions; and time-resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about -1.1 V versus Fc/Fc(+) and is localized at MeDpe(+). Reduction alone does not induce a trans-->cis isomerization of MeDpe(+). [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) is photostable, while [Re(MeDpe(+))(CO)(3)(bpy)](2+) and free MeDpe(+) isomerize under near-UV irradiation. The lowest excited state of [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) has been identified as the Re(Cl)(CO)(3)-->MeDpe(+ 3)MLCT (MLCT=metal-to-ligand charge transfer), decaying directly to the ground state with lifetimes of approximately 42 (73 %) and approximately 430 ps (27 %). Optical excitation of [Re(MeDpe(+))(CO)(3)(bpy)](2+) leads to population of Re(CO)(3)-->MeDpe(+) and Re(CO)(3)-->bpy (3)MLCT states, from which a MeDpe(+) localized intraligand (3)pipi* excited state ((3)IL) is populated with lifetimes of approximately 0.6 and approximately 10 ps, respectively. The (3)IL state undergoes a approximately 21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited-state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe(+))(CO)(3)(bpy)](2+) reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe(+))(CO)(3)(bpy)](2+) emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.     

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.     

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.     

Dual modification of a triple-stranded β-helix nanotube with Ru and Re metal complexes to promote photocatalytic reduction of CO2

We have constructed a robust β-helical nanotube from the component proteins of bacteriophage T4 and modified this nanotube with Ru(II)(bpy)(3) and Re(I)(bpy)(CO)(3)Cl complexes. The photocatalytic system arranged on the tube catalyzes the reduction of CO(2) with higher reactivity than that of the mixture of the monomeric forms.     

Unusual, bifurcated photoreactivity of a rhenium(I) carbonyl complex of triethynylphosphine

Preparations of the first metal complexes of triethynylphosphine (TEP) are described. They are of the type fac-Re(bpy)(CO)(3)(TEP)(+) (1) and cis,trans-[Re(bpy)(CO)(2)(TEP)L](n)(+) (CH(3)CN, n = 1, complex 2; Cl, n = 0, complex 3), where bpy is 2,2'-bipyridine. Complex 1 displays unusual photochemical behavior compared to analogous fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes in that it emits from a state that has pi-pi* character but undergoes competitive photosubstitution of both TEP and CO. Density functional theory (DFT)/time-dependent DFT calculations predict that the lowest emitting state should, in fact, have pi-pi* character.     

Ultrafast excited-state dynamics preceding a ligand trans-cis isomerization of fac-[Re(Cl)(CO)3(t-4-styrylpyridine)2] and fac-[Re(t-4-styrylpyridine)(CO)3(2,2'-bipyridine)]+

UV-vis absorption and resonance Raman spectra of the complexes fac-[Re(Cl)(CO)3(stpy)2] and fac-[Re(stpy)(CO)3(bpy)]+ (stpy = t-4-styrylpyridine, bpy = 2,2'-bipyridine) show that their lowest absorption bands are dominated by stpy-localized intraligand (IL) pi pi* transitions. For the latter complex a Re --> bpy transition contributes to the low-energy part of the absorption band. Optical population of the 1IL excited state of fac-[Re(Cl)(CO)3(stpy)2] is followed by an intersystem crossing (< or =0.9 ps) to an 3IL state with the original planar trans geometry of the stpy ligand. This state undergoes a approximately 90 degrees rotation around the stpy C=C bond with a 11 ps time constant. An electronically excited species with an approximately perpendicular orientation of the phenyl and pyridine rings of the stpy ligand is formed. Conversion to the ground state and isomerization occurs in the nanosecond range. Intraligand excited states of fac-[Re(stpy)(CO)3(bpy)]+ show the same behavior. Moreover, it was found that the planar reactive 3IL excited state is rapidly and efficiently populated after optical excitation into the Re --> bpy 1MLCT excited state. A 1MLCT --> 3MLCT intersystem crossing takes place first with a time constant of 0.23 ps followed by an intramolecular energy transfer from the ReI(CO)3(bpy) chromophore to a stpy-localized 3IL state with a 3.5 ps time constant. The fast rate ensures complete conversion. Coordination of the stpy ligand to the ReI center thus switches the ligand trans-cis isomerization mechanism from singlet to triplet (intramolecular sensitization) and, in the case of fac-[Re(stpy)(CO)3(bpy)]+, opens an indirect pathway for population of the reactive 3IL excited state via MLCT states.     

Coordination Chemistry of Low-Valent Rhenium Polypyridyl Complexes: Synthesis, Reactivity, and Electrochemistry

The complex Re(III)(benzil)(PPh(3))Cl(3) (2) is used to synthesize a variety of Re(III) and Re(II) polypyridyl complexes of the type cis-[Re(III)(L(2))(2)Cl(2)](+), [Re(II)(L(2))(3)](2+), Re(III)(L(3))Cl(3), [Re(III)(L(3))(2)Cl](2+), and [Re(III)(L(4))Cl(2)](+), where L(2) = bpy (3and 6), tbpy (4 and 7), phen (5 and 8); L(3) = terpy (9and 10); L(4) = TMPA (11). The complex cis-[Re(III)(bpy)(2)Cl(2)](+) (3) is a useful synthon in the formation of complexes of the type [Re(bpy)(2)L(x)()](n)()(+) that are six- or seven-coordinate Re(III) complexes (13, 16, and 18) or octahedral Re(II) or Re(I) complexes (12 and 17). The [Re(III)(terpy)(2)Cl](2+) (10) complex can be reduced to form the Re(I) complex, [Re(I)(terpy)(2)](+) (21) and then electrochemically reoxidized to form new complexes of the type [Re(III)(terpy)(2)L](n)()(+). Similar behavior is observed for the [Re(II)(bpy)(3)](2+) (6) complex where [Re(III)(bpy)(3)((t)BuNC)](3+) (20) and [Re(I)(bpy)(3)](+) (19) may be formed. The electrochemistry of these complexes is discussed in relation to their reactivity and the observed pi-acidity of the polypyridyl ligands. In addition, X-ray crystal structures for cis-[Re(III)(bpy)(2)Cl(2)]PF(6) (3) and [Re(I)(bpy)(3)]PF(6) (19) are reported. cis-[Re(III)(bpy)(2)Cl(2)]PF(6) (3, ReC(20)H(16)N(4)Cl(2)F(6)P) crystallizes in the monoclinic space group C2/c with Z = 4 and lattice parameters a = 15.043(5) ?, b = 13.261(4) ?, c = 12.440(4) ?, and beta = 108.86(2) degrees at -100 degrees C. [Re(I)(bpy)(3)]PF(6) (19, ReC(30)H(24)N(6)F(6)P) crystallizes in the rhombohedral space group R&thremacr;c(h) (No. 167) with Z = 12 and lattice parameters a = 13.793(3) ? and c = 51.44(3) ? at -100 degrees C.     

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.     

Synthesis of heteroleptic bis(diimine)carbonylchlororuthenium(II) complexes from photodecarbonylated precursors

The reactions of bidentate diimine ligands (L2) with binuclear [Ru(L1)(CO)Cl2]2 complexes [L1 not equal to L2 = 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (4,4'-Me2bpy), 5,5'-dimethyl-2,2'-bipyridine (5,5'-Me2bpy), 1,10-phenanthroline (phen), 4,7-dimethyl-1,10-phenanthroline (4,7-Me2phen), 5,6-dimethyl-1,10-phenanthroline (5,6-Me2phen), di(2-pyridyl)ketone (dpk), di(2-pyridyl)amine (dpa)] result in cleavage of the dichloride bridge and the formation of cationic [Ru(L1)(L2)(CO)Cl]+ complexes. In addition to spectroscopic characterization, the structures of the [Ru(bpy)(phen)(CO)Cl]+, [Ru(4,4'-Me2bpy)(5,6-Me2phen)(CO)Cl]+ (as two polymorphs), [Ru(4,4'-Me2bpy)(4,7-Me2phen)(CO)Cl]+, [Ru(bpy)(dpa)(CO)Cl]+, [Ru(5,5'-Me2bpy)(dpa)(CO)Cl]+, [Ru(bpy)(dpk)(CO)Cl]+, and [Ru(4,4'-Me2bpy)(dpk)(CO)Cl]+ cations were confirmed by single crystal X-ray diffraction studies. In each case, the structurally characterized complex had the carbonyl ligand trans to a nitrogen from the incoming diimine ligand, these complexes corresponding to the main isomers isolated from the reaction mixtures. The synthesis of [Ru(4,4'-Me2bpy)(5,6-Me2bpy)(CO)(NO3)]+ from [Ru(4,4'-Me2bpy)(5,6-Me2bpy)(CO)Cl]+ and AgNO3 demonstrates that exchange of the chloro ligand can be achieved.     

Why is Re-Re bond formation/cleavage in [Re(bpy)(CO)3]2 different from that in [Re(CO)5]2? Experimental and theoretical studies on the dimers and fragments

The Re(NN)(CO)3(THF) (NN=bpy=2,2'-bipyridine or dmb=4,4'-dimethyl-2,2'-bipyridine) radical, produced by homolysis of [Re(NN)(CO)3]2 in THF solution by visible irradiation, dimerizes with rate constants kd=20 +/- 3 and 11 +/- 4 M(-1) s(-1) for NN=dmb and bpy, respectively. The dimerization processes are strikingly slow compared to those of typical metal radicals including Re(CO)5 (kd approximately 10(9) M(-1) s(-1)). In order to explain such slow reactions, we have performed B3LYP hybrid DFT and fully ab initio RHF and MP2 calculations on several conformations of [Re(bpy)(CO)3]2 (cis, trans, skewed cis, skewed trans) and [Re(CO)5]2 (staggered) and on their constituent monomer radicals and anions. The calculations show that the most stable geometry of [Re(bpy)(CO)3]2 is skewed cis, and the experimental infrared spectrum and photochemical properties of the [Re(bpy)(CO)3]2 dimer are best described by the calculated properties of the skewed cis conformer in which there is no low-lying unoccupied orbital that is predominantly sigma(MM) in character. The Re(bpy)(CO)3(THF) ligand radical is more stable than the 5-coordinate "17-electron" metal radical, Re(bpy)(CO)3, suggesting that the extremely slow dimerization rate most likely arises from the solvent blocking the binding site (i.e., the estimated fraction of the five-coordinate monomer is 1.6 x 10(-2)). Theoretical results are consistent with our experimental results that the dimerization process proceeds via the Re centered radical, which is involved in a pre-equilibrium favoring the ligand-centered radical. Furthermore, time-dependent DFT calculations on [Re(bpy)(CO)3]2 and [Re(bpy)(CO)3]- identify the origin of UV-vis absorption in THF.     

Ion-Pair Charge Transfer Photochemistry in Rhenium(I) Borate Salts

The photophysics and photochemistry of the salt [(bpy)Re(CO)(3)(py)(+)][BzBPh(3)(-)] (ReBo, where bpy = 2,2'-bipyridine, py = pyridine, Bz = C(6)H(5)CH(2) and Ph = C(6)H(5)) has been investigated in THF and CH(3)CN solutions. UV-visible absorption and steady-state emission spectroscopy indicates that in THF ReBo exists primairly as an ion-pair. A weak absorption band is observed for the salt in THF solution that is assigned to an optical ion-pair charge transfer transition. Stern-Volmer emission quenching studies indicate that BzBPh(3)(-) quenches the luminescent dpi (Re) --> pi (bpy) metal-to-ligand charge transfer excited state of the (bpy)Re(CO)(3)(py)(+) chromophore. The quenching is attributed to electron transfer from the benzylborate anion to the photoexcited Re(I) complex, (bpy(-)(*))Re(II)(CO)(3)(py)(+) + BzBPh(3)(-) --> (bpy(-)(*))Re(I)(CO)(3)(py) + BzBPh(3)(*). Laser flash photolysis studies reveal that electron transfer quenching leads to irreversible reduction of the Re(I) cation to (bpy(-)(*))Re(I)(CO)(3)(py). Photoinduced electron transfer is irreversible owing to rapid C-B bond fragmentation in the benzylboranyl radical, PhCH(2)BPh(3)(*) --> PhCH(2)(*) + BPh(3)(*). Quantitative laser flash photolysis experiments show that the quantum efficiency for production of the reduced complex (bpy(-)(*))Re(I)(CO)(3)(py) is unity, suggesting that C-B bond fragmentation in the benzylboranyl radical occurs more rapidly than return electron transfer within the geminate radical pair that is formed by photoinduced electron transfer.     

Enhancement of metal-metal coupling at a considerable distance by using 4-pyridinealdazine as a bridging ligand in polynuclear complexes of rhenium and ruthenium

Novel polynuclear complexes of rhenium and ruthenium containing PCA (PCA = 4-pyridinecarboxaldehyde azine or 4-pyridinealdazine or 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) as a bridging ligand have been synthesized as PF(6-) salts and characterized by spectroscopic, electrochemical, and photophysical techniques. The precursor mononuclear complex, of formula [Re(Me(2)bpy)(CO)(3)(PCA)](+) (Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine), does not emit at room temperature in CH(3)CN, and the transient spectrum found by flash photolysis at lambda(exc) = 355 nm can be assigned to a MLCT (metal-to-ligand charge transfer) excited state [(Me(2)bpy)(CO)(3)Re(II)(PCA(-))](+), with lambda(max) = 460 nm and tau < 10 ns. The spectral properties of the related complexes [[Re(Me(2)bpy)(CO)(3)}(2)(PCA)](2+), [Re(CO)(3)(PCA)(2)Cl], and [Re(CO)(3)Cl](3)(PCA)(4) confirm the existence of this low-energy MLCT state. The dinuclear complex, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(II)(NH(3))(5)](3+), presents an intense absorption in the visible spectrum that can be assigned to a MLCT d(pi)(Ru) --> pi(PCA); in CH(3)CN, the value of lambda (max) = 560 nm is intermediate between those determined for [Ru(NH(3))(5)(PCA)](2+) (lambda(max) = 536 nm) and [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](4+) (lambda(max) = 574 nm), indicating a significant decrease in the energy of the pi-orbital of PCA. The mixed-valent species, of formula [(Me(2)bpy)(CO)(3)Re(I)(PCA)Ru(III)(NH(3))(5)](4+), was obtained in CH(3)CN solution, by bromine oxidation or by controlled-potential electrolysis at 0.8 V in a OTTLE cell of the [Re(I),Ru(II)] precursor; the band at lambda(max) = 560 nm disappears completely, and a new band appears at lambda(max) = 483 nm, assignable to a MMCT band (metal-to-metal charge transfer) Re(I) --> Ru(III). By using the Marcus-Hush formalism, both the electronic coupling (H(AB)) and the reorganization energy (lambda) for the metal-to-metal intramolecular electron transfer have been calculated. Despite the considerable distance between both metal centers (approximately 15.0 Angstroms), there is a moderate coupling that, together with the comproportionation constant of the mixed-valent species [(NH(3))(5)Ru(PCA)Ru(NH(3))(5)](5+) (K(c) approximately 10(2), in CH(3)CN), puts into evidence an unusual enhancement of the metal-metal coupling in the bridged PCA complexes. This effect can be accounted for by the large extent of "metal-ligand interface", as shown by DFT calculations on free PCA. Moreover, lambda is lower than the driving force -DeltaG degrees for the recombination charge reaction [Re(II),Ru(II)] --> [Re(I),Ru(III)] that follows light excitation of the mixed-valent species. It is then predicted that this reverse reaction falls in the Marcus inverted region, making the heterodinuclear [Re(I),Ru(III)] complex a promising model for controlling the efficiency of charge-separation processes.     

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.     

Photochemistry of fac‐[Re(bpy)(CO)3Cl]

The photochemistry of fac‐[Re(bpy)(CO)₃Cl] ( 1 a ; bpy=2,2′‐bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer‐isomer 1 b . However, in the presence of a small amount of MeCN, the main product was the CO‐ligand‐substituted complex (OC‐6‐24)‐[Re(bpy)(CO)₂Cl(MeCN)] ( 2 c ; bpy=2,2′‐bipyridine). In MeCN, two isomers, 2 c and its (OC‐6‐34) form ( 2 a ), were produced. Only 2 c thermally isomerized to produce the (OC‐6‐44) form 2 b . A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV‐visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher‐energy light (e.g., 270 nm) and lower‐energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a . Irradiation of 1 a using the higher‐energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (³HES( 1 )). In THF, ³HES( 1 ) was competitively converted to both the triplet ligand field (³LF) and metal‐to‐ligand charge transfer (³M LCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)₂Cl(thf)]⁺ ( 1 c ) within 10 ps by means of a dissociative mechanism. Re‐coordination of CO to 1 c gives both 1 a and 1 b . In MeCN, irradiation of 1 a by using high‐energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c . A seven‐coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.     

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.     

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.     

Direct tyrosine oxidation using the MLCT excited states of rhenium polypyridyl complexes

Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional Re(I)(bpy)(CO)(3)CN framework via an amide linkage to the bipyridine (bpy) ligand. Comparative time-resolved emission quenching and transient absorption spectra of Re(bpy-Y)(CO)(3)CN and Re(bpy-F)(CO)(3)CN show that Y is oxidized only upon its deprotonation at pH 12. In an effort to redirect electron transport so that it is more compatible with intramolecular Y oxidation, polypyridyl Re(I) complexes have been prepared with the amide bond functionality located on a pendant phosphine ligand. A [Re(phen)(PP-Bn)(CO)(2)](PF(6)) (PP = bis(diphenylphosphino)ethylene) complex has been synthesized and crystallographically characterized. Electrochemistry and phosphorescence measurements of this complex indicate a modest excited-state potential for tyrosine oxidation, similar to that for the (bpy)Re(I)(CO)(3)CN framework. The excited-state oxidation potential can be increased by introducing a monodentate phosphine to the Re(I)(NN)(CO)(3)(+) framework (NN = polypyridyl). In this case, Y is oxidized at all pHs when appended to the triphenylphosphine (P) of [Re(phen)(P-Y)(CO(3))](PF(6)). Analysis of the pH dependence of the rate constant for tyrosyl radical generation is consistent with a proton-coupled electron transfer (PCET) quenching mechanism.     

Synthesis, characterization and photophysics of alkyne bridged bimetallic rhenium(I) and ruthenium(II) complexes

Six new homobimetallic and heterobimetallic complexes of rhenium(I) and ruthenium(II) bridged by ethynylene spacer [(CO)₃(bpy)Re(BL)Re(bpy)(CO)₃]²⁺ [Cl(bpy)₂Ru(BL)Ru(bpy)₂Cl]²⁺ and [(CO)₃(bpy)Re(BL)Ru(bpy)₂Cl]²⁺ (bpy = 2,2′-bipyridine, BL = 1,2-bis(4-pyridyl)acetylene (bpa) and 1,4-bis(4-pyridyl)butadiyne (bpb) are synthesized and characterized. The electrochemical and photophysical properties of all the complexes show a weak interaction between two metal centers in heterobimetallic complexes. The excited state lifetime of the complexes is increased upon introduction of ethynylene spacer and the transient spectra show that this is due to delocalization of electron in the bridging ligand. Also, intramolecular energy transfer from *Re(I) to Ru(II) in Re–Ru heterobimetallic complexes occurs with a rate constant 4 × 10⁷ s⁻¹.