A series of [3 + 2] cycloaddition products from the reaction of rhenium oxo complexes with diphenyl ketene

The reaction of rhenium (VII) trioxo complexes containing the ligand sets scorpionate, [HB(pz)3]ReO3 (6), [Ph-B(pz)3]ReO3 (7), and [[HC(pz)3]ReO3][ReO4] (8) and pyridine/pyridine-type ligands [(4,7-diphenyl-1,10-phen)(Br)ReO3] (12), [(4,4'-di-tert-butyl-2,2'-dipyridyl)(Cl)ReO3] (13), and [(py)2Re(Cl)O3] (4), with diphenyl ketene, has led to the isolation of six novel [3 + 2] cycloaddition products. These air-stable solids 9-11 and 15-17 are the result of [3 + 2] addition of the O=Re=O motif across the ketene C=C double bond. Five of the six [3 + 2] cycloaddition products have been structurally characterized by single-crystal X-ray diffraction and in all cases by 13C NMR and IR spectroscopies.     

2+2] versus [3+2] addition of metal oxides across C=C double bonds: toward an understanding of the surprising chemo- and periselectivity of transition-metal-oxide additions to ketene

The peri-, chemo-, stereo-, and regioselectivity of the addition of the transition-metal oxides OsO4 and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using density-functional methods. While metal-oxide additions to ethylene have recently been reported to follow a [3+2] mechanism only, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition: OsO4 again prefers a [3+2] pathway across the C=C moiety whereas, for the rhenium oxides LReO3, the [2+2] barriers are lowest. Furthermore, a divergent chemoselectivity arising from the ligand L was found: ReO4- and (H3PN)ReO3 add across the C=O bond while MeReO3 and CpReO3 favor the addition across the C=C moiety. The calculated energy profile for the MeReO3 additions differs from the CpReO3 energy profile by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting eta5, eta3, and eta1 bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor-acceptor interactions in the transition states. In contrast, metal-oxide additions to diphenylketene probably follow a different mechanism: We give theoretical evidence for a zwitterionic intermediate that is formed by nucleophilic attack at the carbonyl moiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.     

Ligand controlled dioxygen oxidation of rhenium nitrosyl complexes

The addition of an excess of phenyldiazomethane to chlorobenzene solutions of the cationic dinitrosyl bisphosphine rhenium(-I) complexes [Re(NO)2(PR3)2][BAr(F)4] (R = Cy 1a, R = (i)Pr 1b) gave the corresponding benzylidene complexes [Re{=CH(C6H5)}(NO)2(PR3)2][BAr(F)4] (2a and 2b) in good yields. The treatment of 2b with dioxygen resulted in the oxidation of one of the nitrosyl ligands into the corresponding eta2-nitrito (3b) and nitrato complexes (4b) both in the solid state and in solution. In the case of the tricyclohexylphosphine derivative 2a the analogous conversion was not observed. A mechanism for the reaction of 2b with O2 is proposed which is based on an initial SET to the O2 molecule and subsequent formation of a peroxynitrite complex followed by the formation of a dinuclear mU-N2O4 intermediate. This in turn would undergo fission of the peroxo bond to afford 3b. A related sequence of steps is anticipated for the transformation of 3b to 4b. Furthermore, a similar mechanism seems reasonable for the seemingly topochemical reaction of 2b to 3b and 4b in the solid state. The initial SET to dioxygen and subsequent formation of the peroxynitrite complex is supported by DFT calculations on the trimethylphosphine model complexes [Re=CH{C6H5})(NO)2(PMe3)2]n+ (n = 1 and 2).     

Rhenium complexes with triazine derivatives formed from semicarbazones and thiosemicarbazones

Benzil bis(semicarbazone), H2L(1), reacts with common rhenium(V) nitrido complexes such as [ReNCl2(PPh3)2] or [ReNCl2(PR2Ph)3] (R = Me, Et) under the release of one semicarbazone unit, cyclization, and formation of stable triazine-3-onato complexes of rhenium(V). The resulting 5,6-diphenyltriazine-3-one, HL (2), acts as monodentate or chelating, monoanionic ligand depending on the reaction conditions applied. Complexes of the compositions [ReNCl(L(2)-kappaN(2),kappaO)(PR2Ph)2] (R = Me, Et) or [ReN(L(2)-kappa N(2),O)(L(2)-kappaN(2))(PPh3)2] were isolated. The N(2) nitrogen atom is the preferred binding site of the monodentate form of the ligand. This contrasts the behavior of the analogous thione HL(3), which preferably coordinates to nitridorhenium(V) centers via the sulfur atom. HL(3) is readily formed by the abstraction of methanol from 5-methoxy-5,6-diphenyl-4,5-dihydro-2H-[1,2,4]triazine-3-thione, H2L(3)OCH 3. In the presence of [ReNCl2(PPh3)2] or [ReNCl2(PR2Ph)3] complexes (R = Me, Et), this reaction yields stable complexes of the composition [ReN(L(3)-kappaN(2),kappaS)(L(3)-kappaS)(PR2Ph)2] (R = Me, Et, Ph) in good yields. Reduction of the metal atom and formation of the seven-coordinate [Re(PPh3)(L(3)-kappaN(2),kappaS)3] was observed during reactions of H2L(3)OCH3 with [ReOCl3(PPh3)2] or [ReO2I(PPh3)2], while no rhenium complexes could be isolated during similar reactions with H2L(1), although cyclization of the bis(semicarbazone) and the formation of H 2L(2)OEt were observed.     

Syntheses and structures of rhenium(IV) and rhenium(V) complexes with ethanedithiolato ligands

A novel dimeric rhenium(IV) complex, [Re2(SCH2CH2S)4], and a monomeric methyloxorhenium(V) complex, [CH3ReO(SCH2CH2S)PPh3], were synthesized from methyloxorhenium(V) complexes and characterized crystallographically. The structure of [Re2(SCH2CH2S)4], the formation reaction of which showed surprising demethylation conceivably through the homolytic cleaveage of the rhenium-carbon bond, features distorted trigonal prismatic coordination of sulfurs around the metal center and a rhenium-rhenium triple bond. A revised structure, [Tc2(SCH2CH2S)4], is proposed for a related technetium complex, originally identified as [Tc2(SCH2CH2S)2(SCH=CHS)2] (Tisato et al. Inorg. Chem. 1993, 32, 2042). Additionally, a new compound, CH3Re(O)(SPh)2PPh3, was prepared.     

The surprising nitrogen-analogue chemistry of the methyltrioxorhenium-catalyzed olefin epoxidation

Density functional theory calculations at the B3LYP level have been carried out to investigate the mechanism of the reaction of ethylene with [Re(O)2(O-NH)Me], a formal hydroxylamine derivative of the industrial epoxidation catalyst methyltrioxorhenium(VII) (MTO). A variety of reaction pathways has been considered, including the concerted heteroatom-transfer mechanism postulated by Sharpless and the stepwise mechanism via a five-membered "organometallacycle" postulated by Mimoun. Ethylene has been found not to coordinate directly at the metal. The calculations reveal similar activation free enthalpies for the concerted nitrene-transfer event (aziridination) and for the formation of an organometallic rhena-2,3-oxazolidine via [2+2] addition of ethylene across the Re-N bond of the metallaoxaziridine moiety. The fragmentation of the organometallacycle is faster than its formation and gives ethylideneazane rather than aziridine. An additional pathway has a lower activation free enthalpy and leads to a rhena-3,2-oxazolidine. The formation of this organometallacycle proceeds via an intermediate ring-opening product, [Re(O)2(eta1-O-NH)Me], which undergoes [3+2] cycloaddition across the C=C bond of ethylene. Analysis of its electronic structure reveals that the eta1 species should be considered a metalla-analogue nitrosonium ylide rather than a metalla-analogue imine oxide. Fragmentation of the rhena-3,2-oxazolidine liberates acetaldehyde. The discovery of favorable pathways leading to organometallacycles upon reaction of C=C bonds with [Re(O)2(O-NH)Me] stands in sharp contrast to the strong preference of the concerted mechanism in the olefin epoxidation with rhenium peroxo complexes. The calculations show the multiple mechanisms to be distinguishable by four different products, calling for further experimental studies. The successful search for the five-membered organometallacycles parallels the computational prediction of four-membered organometallacycles derived from d0 metal oxo complexes (Deubel, D. V.; Frenking, G. Acc. Chem. Res. 2003, 36, 645) and the indirect observation of metalla-2-oxetanes in recent gas-phase experiments (Chen, X.; Zhang, X.; Chen, P. Angew. Chem., Int. Ed. 2003, 42, 3798).     

Reactivity of electrochemically generated rhenium (II) Tricarbonyl alpha-diimine complexes: a reinvestigation of the oxidation of luminescent Re(CO)3(alpha-Diimine)Cl and related compounds

The oxidative electrochemistry of luminescent rhenium (I) complexes of the type Re(CO) 3(LL)Cl, 1, and Re(CO) 3(LL)Br, 2, where LL is an alpha-diimine, was re-examined in acetonitrile. These compounds undergo metal-based one-electron oxidations, the products of which undergo rapid chemical reaction. Cyclic voltammetry results imply that the electrogenerated rhenium (II) species 1 ( + ) and 2 ( + ) disproportionate, yielding [Re(CO) 3(LL)(CH 3CN)] (+), 7, and additional products. Double potential step chronocoulometry experiments confirm that 1 ( + ) and 2 ( + ) react via second-order processes and, furthermore, indicate that the rate of disproportionation is influenced by the basicity and steric requirements of the alpha-diimine ligands. The simultaneous generation of rhenium (I) and (III) carbonyl products was detected upon the bulk oxidation of 1 using infrared spectroelectrochemistry. The rhenium (III) products are assigned as [Re(CO) 3(LL)Cl 2] (+), 5; an inner-sphere electron-transfer mechanism of the disproportionation is proposed on the basis of the apparent chloride transfer. Chemically irreversible two-electron reduction of 5 yields 1 and Cl (-). No direct spectroscopic evidence was obtained for the generation of rhenium (III) tricarbonyl bromide disproportionation products, [Re(CO) 3(LL)Br 2] (+), 6; this is attributed to their relatively rapid decomposition to 7 and dibromine. In addition, the 17-electron radical cations, 7 ( + ), were successfully characterized using infrared spectroelectrochemistry.     

Unprecedented ROMP activity of low-valent rhenium-nitrosyl complexes: Mechanistic evaluation of an electrophilic olefin metathesis system

The reaction of [Re(H)(NO)2(PR3)2] complexes (1 a: R = PCy3; 1 b: R = PiPr3) with [H(OEt2)2][BAr(F)4] ([BAr(F)4] = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) in benzene at room temperature gave the corresponding cations [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b). The addition of phenyldiazomethane to benzene solutions of 2 a and 2 b afforded the moderately stable cationic rhenium(I)-benzylidene-dinitrosyl-bis(trialkyl)phosphine complexes 3 a and 3 b as [BAr(F)4]- salts in good yields. The complexes 2 a and 2 b catalyze the ring-opening metathesis polymerization (ROMP) of highly strained nonfunctionalized cyclic olefins to give polymers with relatively high polydispersity indices, high molecular weights and over 80 % Z configuration of the double bonds in the chain backbone. However, these complexes do not show metathesis activity with acyclic olefins. The benzylidene derivatives 3 a and 3 b are almost inactive in ROMP catalysis with norbornene and in olefin metathesis. NMR experiments gave the first hints of the initial formation of carbene complexes from [Re(NO)2(PR3)2][BAr(F)4] (2 a and 2 b) and norbornene. In a detailed mechanistic study ESI-MS/MS measurements provided further evidence that the carbene formation is initiated by a unique reaction sequence where the cleavage of the strained olefinic bond starts with phosphine migration forming a cyclic ylide-carbene complex, capable of undergoing metathesis with alternating rhenacyclobutane formation and cycloreversion reactions ("ylide" route). However, even at an early stage the ROMP propagation route is expected to merge into an "iminate" route by attack by the ylide function on one of the N(NO) atoms followed by phosphine oxide elimination. The formation of phosphine oxide was confirmed by NMR spectroscopy. The proposed mechanism is supported further by detailed DFT calculations.     

Hetero-bimetallic complexes involving vanadium(V) and rhenium(VII) centers, connected by unsupported mu-oxido bridge: synthesis, characterization, and redox study

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

Mechanistic insight into hydrosilylation reactions catalyzed by high valent ReX (X = O, NAr, or N) complexes: the silane (Si-H) does not add across the metal-ligand multiple bond

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

Reactivity of rhenium(V) oxo Schiff base complexes with phosphine ligands: rearrangement and reduction reactions

The symmetric rhenium(V) oxo Schiff base complexes trans-[ReO(OH2)(acac2en)]Cl and trans-[ReOCl(acac2pn)], where acac2en and acac2pn are the tetradentate Schiff base ligands N,N'-ethylenebis(acetylacetone) diimine and N,N'-propylenebis(acetylacetone) diimine, respectively, were reacted with monodentate phosphine ligands to yield one of two unique cationic phosphine complexes depending on the ligand backbone length (en vs pn) and the identity of the phosphine ligand. Reduction of the Re(V) oxo core to Re(III) resulted on reaction of trans-[ReO(OH2)(acac2en)]Cl with triphenylphosphine or diethylphenylphosphine to yield a single reduced, disubstituted product of the general type trans-[Re(III)(PR3)2(acac2en)]+. Rather unexpectedly, a similar reaction with the stronger reducing agent triethylphosphine yielded the intramolecularly rearranged, asymmetric cis-[Re(V)O(PEt3)(acac2en)]+ complex. Reactions of trans-[Re(V)O(acac2pn)Cl] with the same phosphine ligands yielded only the rearranged asymmetric cis-[Re(V)O(PR3)(acac2pn)]+ complexes in quantitative yield. The compounds were characterized using standard spectroscopic methods, elemental analyses, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic data for the structures reported are as follows: trans-[Re(III)(PPh3)2(acac2en)]PF6 (H48C48N2O2P2Re.PF6), 1, triclinic (P), a = 18.8261(12) A, b = 16.2517(10) A, c = 15.4556(10) A, alpha = 95.522(1) degrees , beta = 97.130(1) degrees , gamma = 91.350(1) degrees , V = 4667.4(5) A(3), Z = 4; trans-[Re(III)(PEt2Ph)2(acac2en)]PF6 (H48C32N2O2P2Re.PF6), 2, orthorhombic (Pccn), a = 10.4753(6) A, b =18.4315(10) A, c = 18.9245(11) A, V = 3653.9(4) A3, Z = 4; cis-[Re(V)O(PEt3)(acac2en)]PF6 (H33C18N2O3PRe.1.25PF6, 3, monoclinic (C2/c), a = 39.8194(15) A, b = 13.6187(5) A, c = 20.1777(8) A, beta = 107.7730(10) degrees , V = 10419.9(7) A3, Z = 16; cis-[Re(V)O(PPh3)(acac2pn)]PF6 (H35C31N2O3PRe.PF6), 4, triclinic (P), a = 10.3094(10) A, b =12.1196(12) A, c = 14.8146(15) A, alpha = 105.939(2) degrees , beta = 105.383(2) degrees , gamma = 93.525(2) degrees , V = 1698.0(3) A3, Z = 2; cis-[Re(V)O(PEt2Ph)(acac2pn)]PF6 (H35C23N2O3PRe.PF6), 5, monoclinic (P2(1)/n), a = 18.1183(18) A, b = 11.580(1) A, c = 28.519(3) A, beta = 101.861(2) degrees , V = 5855.9(10) A(3), Z = 4.     

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(V) oxo complexes with acetylacetone derived Schiff bases: structure and catalytic epoxidation

Substitution reactions of rhenium(V) oxo precursors [ReOCl3(PPh3)2] or [NBu4][ReOCl4] with the bidentate acetylacetone-derived ketoamine ligands APOH = 4-anilino-3-penten-2-one, DPOH = 4-[2,6-dimethylanilino]-3-penten-2-one, and MTPOH = 4-[2-(methylthio)anilino]-3-penten-2-one gave the complexes [ReO(APO)Cl2(PPh3)] (1), [ReO(DPO)Cl2(PPh3)] (2), and [NBu4][ReOLCl3] (3, L = APO; 4, L = DPO; 5, L = MTPO), respectively. All complexes exhibit only one ketoamino chelate, independent of the amount of ligand added to the rhenium precursors. The complexes were characterized by 1H and 13C NMR spectroscopy. X-ray crystal structures of the complexes 1, 2, 4, and 5, including that of MTPOH, were determined, revealing the trans position of the two oxygen atoms and the trans-Cl,Cl conformation in 1 and 2, in contrast to most other rhenium complexes of this type where the cis-Cl,Cl conformation is observed. Coordination of the potentially tridentate ligand MTPOH in 5 is bidentate with a dangling thioether substituent. Compound 2 shows catalytic activity in the oxidation of cis-cyclooctene with tert-butylhydroperoxide.     

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.     

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.     

Hydride donor abilities and bond dissociation free energies of transition metal formyl complexes

The hydride complex [Pt(dmpe)2H]+ (dmpe = 1,2-bis(dimethylphosphino)ethane) reversibly transfers H- to the rhenium carbonyl complex [CpRe(PMe3)(NO)(CO)]+, giving the formyl CpRe(PMe3)(NO)(CHO). From the equilibrium constant for the hydride transfer (16.2), the DeltaGdegrees for the reaction was determined (-1.6 kcal/mol), as was the hydride-donating ability of the formyl (44.1 kcal/mol). The hydride-donating ability, DeltaGdegrees(H-), is defined as the energy required to release the hydride ion into solution by the formyl complex [i.e. M(CHO) right arrow M(CO)+ + H-]. Subsequently, the hydride-donating ability of a series of formyl complexes was determined, ranging from 44 to 55 kcal/mol. With use of this information, two rhenium carbonyl complexes, [CpRe(NO)(CO)2]+ and [Cp*Re(NO)(CO)2]+, were hydrogenated to formyls, employing [Pt(dmpp)2]2+ and Proton-Sponge. Finally, the E(1/2)(I/0) values for five rhenium carbonyl complexes were measured by cyclic voltammetry. Combined with the known DeltaGdegrees(H-) values for the complexes, the hydrogen atom donating abilities could be determined. These values were all found to be approximately 50 kcal/mol.     

Studies of C-S bond cleavage reactions of Re(V) dithiolates: synthesis, reactivity, and mechanism

A series of rhenium(V) complexes, [(X)(ReO)(dt)(PPh(3))] and [(o-SC(6)H(4)PPh(2))(ReO)(mtp)], were prepared to explore electronic effects on the C-S cleavage reaction that occurs upon reaction with PAr(3) at ambient temperature [where X = S(C(6)H(4)-p-Z) (Z = OMe, Me, H, F, Cl), OPh, Cl, and SC(2)H(5), and dt is the chelating dithiolate ligand derived from 2-(mercaptomethyl)thiophenol, 1,2-ethanedithiol, 1,3-propanedithiol, 1,3-butanedithiol, and 2,4-pentanedithiol]. The scope and selectivity of the C-S activation were examined. The C-S bond cleavage to form metallacyclic Re(V) complexes with a ReS core occurs only for the complexes with mtp and pdt frameworks and X = SAr and SC(2)H(5). The difference in reactivity is due to the different donating abilities of ancillary and dithiolate ligands, especially their pi-donating ability, which plays a critical role in C-S activation. The kinetics of the C-S activation process was determined; nucleophilic attack of PPh(3) on the oxo group of the Re(V)O core appears to be the rate-controlling step. The reaction is accelerated by electron-poor ArS ligands, but is unaffected by the substituents on phosphines. A detailed mechanistic study is presented. The results represent a rare example of migration of alkanethiolate leading to the formation of alkylthiolato complexes.     

Chemistry of re with N,N'-bis(2-pyridylmethyl)ethylenediamine (H2pmen): hydrolysis, dehydrogenation, and ternary complexes

A number of Re complexes with N,N'-bis(2-pyridylmethyl)ethylenediamine (H2pmen) have been made from [NH4][ReO4]. [ReOCl2(H2pmen)]Cl, [ReOCl(Hpmen)][ReO4], and [ReO2(H2pmen)][ReO4] are related by hydrolysis/HCl substitution. [ReOCl(Hpmen)][ReO4] was structurally characterized and found to contain a water-stable amido-Re bond. Dehydrogenation of the N-donor ligand from each amine to imine with concomitant two-electron reduction of the Re center occurs readily in these systems. With suitable 3-hydroxy-4-pyrones, ternary complexes such as [ReIIICl(ma)(C14H14N4)][ReO4].CH3OH, 5, were made from [NH4][ReO4], H2pmen.4HCl and pyrones in one-pot syntheses. 5, a seven-coordinate ReIII complex, was structurally characterized.     

Calix[4]arene rhenium(V) complexes as potential radiopharmaceuticals

The calix[4]arene platform was used for the syntheses of novel rhenium(V) complexes, that may have potential applications as radiopharmaceuticals. The reaction of ReO(PPh3)2Cl3 with tetradentate N2O2-calix[4]arene ligand 8 in ethanol gave the novel mixed-ligand rhenium complex 9 with the structure ReO(N2O2-calix)OEt. The configuration was elucidated by using a number of 1H NMR techniques. In 9, the ethoxy ligand could be easily and quantitatively exchanged for another monodentate ligand to give complex 12. Tetradentate N2S2-calix[4]arene ligand 15 formed the rhenium complex 16 either via reaction with ReO(PPh3)2Cl3 in an organic solvent or by reaction with rhenium gluconate in an aqueous solution. Complex 16 showed good stability in phosphate-buffered saline solution (37 degrees C, 5 d). The crystal structures of a mono- and a bimetallic complex were determined. The bimetallic N2O2-calixarene complex dimer 11 crystallized in the monoclinic space group C2/c, with a = 38.963(5) A, b = 23.140(6) A, c = 27.382(6) A, beta = 128.456(10) degrees, V = 19,333(7) A3, Z = 8, and final R = 0.0519. The monometallic N2S2 model complex 17 crystallized in the monoclinic space group Cc, with a = 15.715(2) A, b = 12.045(2) A, c = 20.022(3) A, beta = 94.863(12) degrees, V = 3776.3(10) A3, Z = 4, and final R = 0.0342.