Optosensing behavior of the first Ru(II)-compound of di-2-pyridylketone-p-nitrophenylhydrazone (dpknph), [Ru(bpy)2(dpknph)]Cl2

The reaction between Ru(bpy)(2)Cl(2) (bpy=2,2'-bipyridine) and di-2-pyridylketone-p-nitrophenylhydrazone (dpknph) in refluxing ethanol gave [Ru(bpy)(2)(dpknph)]Cl(2) in good yield. Optical measurements on [Ru(bpy)(2)(dpknph)]Cl(2) in non-aqueous media revealed the presence of two interlocked electronic states due to conformational changes associated with the hydrazone moiety of [Ru(bpy)(2)(dpknph)]Cl(2). The equilibrium distribution of the high-energy beta-conformation associated with the high-energy electronic state and the low-energy alpha-conformation associated with the low-energy electronic state is solvent and solute dependent controlled by the solvent-solute and solute-solute interactions. The interplay between the alpha- and beta-conformations of [Ru(bpy)(2)(dpknph)]Cl(2) allowed calculations of the extinction coefficients of electronic states by forcing the equilibrium to shift to one conformation using chemical stimuli. Extinction coefficients of 56000+/-2000 and 48500+/-2000 M(-1) cm(-1) were calculated in DMSO for the beta- and alpha-conformations of [Ru(bpy)(2)(dpknph)]Cl(2), respectively, using excess HgCl(2) in DMSO. Thermo-optical measurements on [Ru(bpy)(2)(dpknph)]Cl(2) in DMSO confirmed the interconversion between the alpha- and beta-conformations of [Ru(bpy)(2)(dpknph)]Cl(2) and gave changes in enthalpy (DeltaH(?)) of -35.5+/-4.0 and 13.0+/-0.5 kJ mol(-1), entropy (DeltaS(?)) of -126.9+/-20 and 45.2+/-4.5 kJ mol(-1), and free energy (DeltaG(?)) of 2.31+/-0.2 and -0.48+/-0.2 kJ mol(-1) in the absence and presence of NaBH(4) at 295 K. The high values for the extinction coefficients and low values and sensitivity of the activation parameters for the interconversion between the alpha- and beta-conformations of [Ru(bpy)(2)(dpknph)]Cl(2) in DMSO to solution composition allowed for the use of this system ([Ru(bpy)(2)(dpknph)]Cl(2) and surrounding solvent or solute molecules) as a spectrophotometric sensor for a variety of chemical stimuli that include metal ions. Group 12 metal ions in concentrations as low as 1.00x10(-8) M can be detected and determined using [Ru(bpy)(2)(dpknph)]Cl(2) in DMSO in the presence and absence of NaBH(4).     

Optosensing properties of fac-Re(CO)(3)(dpknph)Cl (dpknph=di-2-pyridyl ketone p-nitrophenyl hydrazone) toward chemotactic N-formylamino acids

Optical measurements on fac-Re(CO)(3)(dpknph)Cl in polar non-aqueous solvents in the presence and absence of NaBH(4)/KPF(6) revealed reversible interconversion between the high (beta-) and low (alpha-) energy electronic states of fac-Re(CO)(3)(dpknph)Cl. The reversibility of these interactions and the disturbance of the equilibrium distribution of the low (alpha-) and high (beta-) energy electronic state upon addition of NaBH(4)/KPF(6) mark improvement in the optosensing properties of fac-Re(CO)(3)(dpknph)Cl. The optical behavior of fac-Re(CO)(3)(dpknph)Cl in the presence and absence of the l-methionine and chemotactic N-formylamino acids: N-formyl-l-methionine (NFM), N-formyl-l-glycine (NFG) and N-formyl-l-phenylalanine (NFP) shows the alpha- and beta-electronic states of fac-Re(CO)(3)(dpknph)Cl to be insensitive to l-methinonine and highly sensitive to N-formylamino acids. N-formylamino acids in concentrations <1.0x10(-5) M can be determined using the optical sensor fac-Re(CO)(3)(dpknph)Cl in non-aqueous polar solvents. The optosensing power of fac-Re(CO)(3)(dpknph)Cl towards N-formylamino acids depends on the concentration and polarity of the side chain of the amino acids and increases in the following order: NFM>NFG>NFP.     

Re(CO)(3) complexes synthesized via an improved preparation of aqueous fac-[Re(CO)(3)(H(2)O)(3)](+) as an aid in assessing (99m)Tc imaging agents. Structural characterization and solution behavior of complexes with thioether-bearing amino acids as tridentate ligands

Parallel studies of the preparation of Re and (99m)Tc agents aid in interpreting the nature of tracer (99m)Tc radiopharmaceuticals. Aqueous solutions of the fac-[(99m)Tc(CO)(3)(H(2)O)(3)](+) cation are gaining wide use and are readily prepared, but such solutions of the fac-[Re(CO)(3)(H(2)O)(3)](+) cation (1) are not so easily accessible. Herein we describe a new, reliable, and straightforward preparation of aqueous solutions of 1, characterized by HPLC and ESI-MS. Treatment of solutions of 1 with thioether-bearing amino acids, AAH = S-methyl-l-cysteine (MECYSH), S-propyl-l-cysteine (PRCYSH), and methionine (METH), gave high yields of fac-Re(CO)(3)AA complexes. X-ray crystallographic and NMR analyses indicated that MECYS(-), PRCYS(-), and MET(-) were bound in fac-Re(CO)(3)AA complexes as tridentate monoanionic ligands through amino, thioether, and alpha-carboxyl groups. In CD(3)OD, (1)H NMR spectra have broad signals but have two sets of signals at -10 degrees C, consistent with two isomers with different configurations at the pyramidal sulfur; these interconvert slowly on the NMR time scale at low temperatures. Indeed, the crystal structure of the fac-Re(CO)(3)(PRCYS) reveals a mixture of the two possible diastereoisomers. S-(Carboxymethyl)-l-cysteine (CCMH(2)) and 1 gave two products, 5A (kinetically favored) and 5B (thermodynamically favored). X-ray crystallographic analyses of a crystal of 5B and of a 1:1 cocrystal of 5A and 5B showed that 5A and 5B are diastereoisomers with the CCMH(-) alpha-carboxyl group dangling. In addition to the amino and thioether groups, the S-(carboxymethyl) carboxyl group is coordinated, a feature that slows interconversion of diastereoisomers relative to the other fac-Re(CO)(3)AA complexes because interconversion can now occur only after the rupture of Re-ligand bonds. These N, O, and S tridentate adducts are quite stable, and the grouping has promise in (99m)Tc(CO)(3) tracer development.     

Synthesis and properties of fac-Re(dmbpy)(CO)3CHO (dmbpy = 4,4'-dimethyl-2,2'-bipyridine), a possible intermediate in reductions of CO2 catalyzed by fac-Re(dmbpy)(CO)3Cl

Synthesis of fac-Re(dmbpy)(CO)3CHO 2 and its reactions with CO2 in DMF and DMSO have been conducted; 2 transfers hydride to CO2 to give Re(dmbpy)(CO)4+ OCHO- 5 which is rapidly transformed to fac-Re(dmbpy)(CO)3(OCHO) 3 in DMF, thus supporting the viability of 2 in photocatalytic reactions of fac-Re(dmbpy)(CO)3Cl with CO2.     

Electro-optical properties of the first rhenium-hydrazone complex,fac-Re(CO)3(dpknph)*Cl

Reaction between Re(CO)₅Cl and dpknph in PhMe under reflux gave fac-Re(CO)₃(dpknph)Cl in good yield. Both dpknph and fac-Re(CO)₃(dpknph)Cl exhibit rich electro-optical properties that are sensitive to their surroundings and point to the potential use of these compounds in nonlinear optics and molecular sensing. Spectroscopic and electrochemical measurements on solutions of dpknph and fac-Re(CO)₃(dpknph)Cl show that the metal complex undergoes faster electron/charge-transfer than the free ligand. Solvent variations show that the rate increases in the following order: DMSO>DMF>MeCN.     

Influence of a gold(I)-Acetylide subunit on the photophysics of Re(phen)(CO)3Cl

The synthesis and photophysical properties of two new Re(I) complexes are reported: fac-Re(phenC triple-bond CH)(CO)(3)Cl (where phenC triple bond CH is 5-ethynyl-1,10-phenanthroline) and its Au(I)-acetylide analogue (fac-Re(phenC triple-bond CAuPPh(3))(CO)(3)Cl). Also reported are the photophysical measurements obtained for the benchmark fac-Re(phen)(CO)(3)Cl chromophore, as well as the phenC triple-bond CAuPPh(3) and phenC triple-bond CH ligands. The unstable nature of the precursor gold-containing ligand illustrates the advantage of using the "chemistry on the complex" approach, which facilitated preparation of the Re-Au binuclear complex. Where possible, all compounds were studied by static and transient absorption (TA), as well as steady-state and time-resolved photoluminescence (TRPL), at room temperature (RT) and 77 K, as well as nanosecond time-resolved infrared (TRIR) spectroscopy. The spectroscopic information provided by these techniques enabled a thorough evaluation of excited-state decay in most cases. In fac-Re(phenC triple bond CH)(CO)(3)Cl, the RT excited-state decay is most consistent with a metal-to-ligand charge transfer (MLCT) assignment, whereas at 77 K, the lowest excited state is dominated by the triplet intraligand ((3)IL) state, localized within the diimine ligand. The lowest excited state in fac-Re(phenC triple-bond CAuPPh(3))(CO)(3)Cl seems to result from an admixture of Re-based MLCT and (3)IL states resident on the phenC triple-bond CAuPPh(3) moiety. TA and TRIR methods indicate that these excited states are thermally equilibrated at room temperature. At 77 K, the MLCT energy of fac-Re(phenC triple-bond CAuPPh(3))(CO)(3)Cl is increased as a result of the glassy medium and the resulting excited state can be considered to be ligand-localized.     

Metal-assisted in situ formation of a tridentate acetylacetone ligand for complexation of fac-Re(CO)3+ for radiopharmaceutical applications

Reaction of [NEt4]2[ReBr3(CO)3] with 2,4-pentanedione (acac) yields a complex of the type fac-Re(acac)(OH2)(CO)3 (1) under aqueous conditions. 1 was further reacted with a monodentate ligand (pyridine) to yield a fac-Re(acac)(pyridine)(CO)3 complex (2). Complex 1 was found to react with primary amines to generate a Schiff base (imine) in aqueous solutions. When a mixed-nitrogen donor bidentate ligand, 2-(2-aminoethyl)pyridine, that has different coordination affinities for fac-Re(acac)(OH2)(CO)3 was utilized, a unique tridentate ligand was formed in situ utilizing a metal-assisted Schiff base formation to yield a complex fac-Re(CO)3(3[(2-phenylethyl)imino]-2-pentanone) (3). Tridentate ligand formation was found to occur only with the Re-coordinated acac ligand. Reactions of acac with fac-Re(CO)3Br(2-(2-aminoethyl)pyridine) (4) or a mixture of [NEt4]2[ReBr3(CO)3], acac, and 2-(2-aminoethyl)pyridine did not yield the formation of complex 3 in water.     

NMR studies of Ru(3)(CO)(10)(PMe(2)Ph)(2) and Ru(3)(CO)(10)(PPh(3))(2) and their H(2) addition products: detection of new isomers with complex dynamic behavior

The clusters Ru(3)(CO)(10)L(2), where L = PMe(2)Ph or PPh(3), are shown by NMR spectroscopy to exist in solution in at least three isomeric forms, one with both phosphines in the equatorial plane on the same ruthenium center and the others with phosphines in the equatorial plane on different ruthenium centers. Isomer interconversion for Ru(3)(CO)(10)(PMe(2)Ph)(2) is highly solvent dependent, with DeltaH decreasing and DeltaS becoming more negative as the polarity of the solvent increases. The stabilities of the isomers and their rates of interconversion depend on the phosphine ligand. A mechanism that accounts for isomer interchange involving Ru-Ru bond heterolysis is suggested. The products of the reaction of Ru(3)(CO)(10)L(2) with hydrogen have been monitored by NMR spectroscopy via normal and para hydrogen-enhanced methods. Two hydrogen addition products are observed with each containing one bridging and one terminal hydride ligand. EXSY spectroscopy reveals that both intra- and interisomer hydride exchange occurs on the NMR time scale. On the basis of the evidence available, mechanisms for hydride interchange involving Ru-Ru bond heterolysis and CO loss are proposed.     

Metal-to-ligand charge-transfer sensitisation of near-infrared emitting lanthanides in trimetallic arrays M2Ln (M=Ru, Re or Os; Ln=Nd, Er or Yb)

The new pro-ligand 4-methyl-4'-(carbonylamino(2-(tert-butoxycarbonylamino)ethyl))-2,2'-bipyridyl (L1) has been prepared and used to synthesise the complex fac-Re(I)Cl(CO)3(L1) 1 and the complex salts [M(II)(bipy)2(L1)](PF6)2 (M=RuII 8 or OsII 15). Deprotection with trifluoroacetic acid affords the amine-functionalised derivatives fac-Re(I)Cl(CO)3(L2) 2, [M(II)(bipy)2(L2)](PF6)2 (M=RuII 9 or OsII 16) which react with the dianhydride of diethylenetriamine pentaacetic acid (DTPA) to give the binuclear complex {fac-Re(I)Cl(CO)3}2(L3) 3 and the complex salts [{M(II)(bipy)2}2(L3)](PF6)4 (M = RuII 10 or OsII 17). The latter react with salts Ln(OTf)3 to afford a series of 12 heterotrimetallic compounds that contain a lanthanide (Ln) ion in the DTPA binding site; {fac-Re(I)Cl(CO)3}2(L3)LnIII (Ln=Nd 4, Er 5, Yb 6 or Y 7) and [{M(II)(bipy)2}2(L3)LnIII](PF6)(OTf)3 (M=RuII, Ln=Nd 11, Er 12, Yb 13 or Y 14; M=OsII, Ln=Nd 18, Er 19, Yb 20 or Y 21). All of these trimetallic species display absorption bands ascribed to metal-to-ligand charge-transfer (MLCT) excitations, and luminescence measurements show that these excited states can be used to sensitise near-infrared emission from LnIII (Ln=Nd, Er or Yb) ions. Single crystal X-ray structures of L1 and [RuII(bipy)2(L2H)](H2PO4)3.(CH3)2CO.0.8H2O were obtained, the latter revealing the presence of H2PO4- counter anions, the source of which is presumed to be hydrolysis of PF6- ions.     

Synthesis and ligand-based mixed valency of cis- and trans-Cr(III)(X(4)SQ)(X(4)Cat)(L)(n) (X = Cl and Br,n = 1 or 2) complexes: effects of solvent media on intramolecular charge distribution and ligand dissociation of Cr(III)(X(4)SQ)(3)

The treatment of Cr(III)(X(4)SQ)(3) (SQ = o-semiquinonate; X = Cl and Br) with acetonitrile affords trans-Cr(III)(X(4)SQ)(X(4)Cat)(CH(3)CN)(2) (X = Cl (1) and Br (2)). In the presence of 2,2'-bipyridine (bpy) or 3,4,7,8-tetramethyl-1,10-phenanthrene (tmphen), the reaction affords Cr(III)(X(4)SQ)(X(4)Cat)(bpy).nCH(3)CN (X = Cl, n = 1 (3); X = Br, n = 0.5 (4)) or Cr(III)(X(4)SQ)(X(4)Cat)(tmphen) (X = Cl (5) and Br (6)), respectively. All of the complexes show a ligand-based mixed-valence (LBMV) state with SQ and Cat ligands. The LBMV state was confirmed by the presence of the interligand intervalence charge-transfer band. Spectroscopic studies in several solvent media demonstrate that the ligand dissociation included in the conversion of Cr(III)(X(4)SQ)(3) to 1-6 occurs only in solvents with relatively high polarity. On the basis of these results, the effects of solvent media were examined and an equilibrium, Cr(III)(X(4)SQ)(3) <--> Cr(III)(X(4)BQ)(X(4)SQ)(X(4)Cat) (BQ = o-benzoquinone), is proposed by assuming an interligand electron transfer induced by solvent polarity.     

Reactions of the Dirhenium(II) Complexes Re(2)X(4)(&mgr;-dppm)(2) (X = Cl, Br; dppm = Ph(2)PCH(2)PPh(2)) with Isocyanides. 11.(1) A Triply Bonded Dirhenium Complex Containing a Labile Acetonitrile Ligand. Synthesis, Structural Characterization, and Reactivity of [(XylNC)(CH(3)CN)ClRe(&mgr;-dppm)(2)ReCl(2)(CO)]O(3)SCF(3)

The reaction of the open bioctahedral form of Re(2)Cl(4)(&mgr;-dppm)(2)(CO)(CNXyl) (1), where XylNC = 2,6-dimethylphenyl isocyanide, with TlO(3)SCF(3) in the presence of acetonitrile proceeds with retention of stereochemistry at the dirhenium unit to afford the complex [Re(2)Cl(3)(&mgr;-dppm)(2)(CO)(CNXyl)(NCCH(3))]O(3)SCF(3) (3). The single-crystal X-ray structure determination of 3 shows that a Re&tbd1;Re bond is retained (the Re-Re distance is 2.378(3) ?) and that it is the chloride ligand trans to the XylNC ligand of 1 which is labilized. Complex 1 reacts with TlO(3)SCF(3) in a noncoordinating solvent to produce the unsymmetrical complex [Re(2)Cl(3)(&mgr;-dppm)(2)(CO)(CNXyl)]O(3)SCF(3) (2), through loss of this same chloride ligand of 1 and CO transfer from the adjacent Re center. The acetonitrile ligand of 3 is very labile and is readily displaced by XylNC and t-BuNC, with retention of stereochemistry, to produce complexes of stoichiometry [Re(2)Cl(3)(&mgr;-dppm)(2)(CO)(CNXyl)(CNR)]O(3)SCF(3) (R = Xyl, 4a; R = t-Bu, 4b). In a noncoordinating solvent, the nitrile ligand of 3 is lost and 2 is formed following CO transfer; this conversion is reversed upon the reaction of 2 with acetonitrile. When 3 is treated with CO, the acetonitrile ligand is again displaced, but in this instance the reaction is accompanied by a structure change to produce an edge-sharing bioctahedral complex of the type [Re(2)(&mgr;-CO)(&mgr;-Cl)(&mgr;-dppm)(2)Cl(2)(CO)(CNXyl)]O(3)SCF(3) (5).     

Studies on metal carbonate equilibria-part 21 Study of the U(VI)H(2)OCO(2)(g) system by thermal lensing spectrophotometry

Equilibria in the U(VI)H(2)OCO(2)(g) system in 0.5M sodium perchlorate medium at 25 degrees have been studied. By using thermal tensing spectrophotometry (TLS) and a very low total concentration of U(V1) (4 x 10(-6)M) information could be obtained on equilibria involving UO(2)(CO(3))(2-)(2) without complications due to formation of the trimer (UO(2))(3)(CO(3))(6-)(6). The experimental data allowed a precise determination of the equilibrium constant log K(3) = 6.35 +/- 0.05 for the reaction UO(2)(CO(3))(2-)(2) + CO(2-)(3) right harpoon over left harpoonright harpoon over left harpoon UO(2)(CO(3))(4-)(3). The interpretation of TLS data is briefly discussed, as well as the potential use of this technique for studies of the speciation of trace elements in natural water systems.     

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.     

Photochemical reactions of (eta 5-cyclopentadienyl)bis(t-butylacrylate) rhodium with silanes: dynamics of isomer interconversion via Rh(eta 2-silane) species

The compound CpRh(C(2)H(3)CO(2)(t)Bu)(2) 1 has been synthesised as a mixture of two pairs of interconverting isomers which differ in the relative orientations of the alkene substituents. The four isomers have been fully characterised by NMR spectroscopy. When complex 1 is photolysed in the presence of a silane, HSiR(2)R'R(2)R'= Et(3), Me(3), HEt(2), (OMe)(3) and Me(2)Cl] the corresponding Si-H oxidative addition products CpRh(SiR(2)R')(H)(C(2)H(3)CO(2)(t)Bu) and CpRh(H)(2)(SiR(2)R')(2) are formed. The Rh(III) complexes CpRh(SiR(2)R')(H)(C(2)H(3)CO(2)(t)Bu) exist in two isomeric forms of comparable energy which interconvert in an intramolecular process that does not involve a reversible [1,3] hydride or [1,3] silyl migration. The hydride (1)H NMR resonances for these species consequently broaden before coalescing into a single peak. For R(2)R'= Et(3), the activation parameters for interchange from the major to minor isomer were Delta H++= 60.2 +/- 2 kJ mol(-1) and Delta S++= 8 +/- 9 J mol(-1) K(-1), while for R(2)R'= Me(3) and Et(2)H, Delta H++= 61.5 +/- 1 kJ mol(-1), Delta S++= 6 +/- 5 J mol(-1) K(-1), and Delta H++= 61.8 +/- 3 kJ mol(-1), Delta S++= 12 +/- 9 J mol(-1) K(-1) respectively for conversion from the major isomer to the minor. For these complexes an eta(2)-Rh-H-Si transition state or intermediate is consistent with the evidence. When R(2)R'=(OMe)(3) and Me(2)Cl the change in appearance of the hydride resonances is more complex, with the activation parameters for interchange from the major to minor isomer for the former species being Delta H++= 78.3 +/- 2 kJ mol(-1) and Delta S++= 30 +/- 7 J mol(-1) K(-1) while for Me(2)Cl the barrier proved too high to measure before decomposition occurred. The complex spectral changes could be simulated when a discrete eta(2)-Rh-H-Si intermediate was involved in the isomer interconversion process and hence silane rotation in all these systems is proposed to involve two isomers of CpRh(eta(2)-HSiR(2)R')(C(2)H(3)CO(2)(t)Bu).     

Spectroscopy of UO(2)Cl(4)(2)(-) in basic aluminum chloride-1-ethyl-3-methylimidazolium chloride

The optical absorption, emission, FT Raman, one-photon excitation, two-photon excitation, and luminescence lifetime measurements are reported for UO(2)Cl(4)(2)(-) in 40:60 AlCl(3)-EMIC (where EMIC identical with 1-ethyl-3-methylimidazolium chloride), a room-temperature ionic liquid. Comparison of the spectra with previous results from single crystals containing UO(2)Cl(4)(2)(-) allowed the characterization of four ground-state vibrational frequencies, two excited-state vibrational frequencies, and the location of eight electronic excited-state energy levels. The vibrational frequencies and electronic energy levels are found to be consistent with the UO(2)Cl(4)(2)(-) ion. Comparison of the one-photon and two-photon excitation spectra, and the relative intensities of the transitions in the emission spectrum indicate that the center of symmetry is perturbed by an interaction with the solvent.     

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.     

Contrasting photochemical and thermal reactivity of Ru(CO)2(PPh3)(dppe) towards hydrogen rationalised by parahydrogen NMR and DFT studies

The synthesis, characterisation and thermal and photochemical reactivity of Ru(CO)2(PPh3)(dppe) 1 towards hydrogen are described. Compound proved to exist in both fac (major) and mer forms in solution. Under thermal conditions, PPh3 is lost from 1 in the major reaction pathway and the known complex Ru(CO)2(dppe)(H)2 2 is formed. Photochemically, CO loss is the dominant process, leading to the alternative dihydride Ru(CO)(PPh3)(dppe)(H)2 3. The major isomer of 3, viz. 3a, contains hydride ligands that are trans to CO and trans to one of the phosphorus atoms of the dppe ligand but a second isomer, 3b, where both hydride ligands are trans to distinct phosphines, is also formed. On the NMR timescale, no interconversion of 3a and 3b was observed, although hydride site interchange is evident with activation parameters of DeltaH(double dagger) = 95 +/- 6 kJ mol(-1) and DeltaS(double dagger) = 26 +/- 17 J K(-1) mol(-1). Density functional theory confirms that the observed species are the most stable isomeric forms, and suggests that hydride exchange occurs via a transition state featuring an eta2-coordinated H2 unit.     

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.     

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.     

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.