Direct DNA photocleavage by a new intercalating dirhodium(II/II) complex: comparison to Rh2(mu-O2CCH3)4

Transition metal complexes possessing the intercalating dppz ligand (dppz = dipyrido[3,2-a:2',3'-c]phenazine) typically bind ds-DNA through intercalation (K(b) approximately 10(5)-10(6) M(-1)), and DNA photocleavage by these complexes with visible light proceeds through the generation of a reactive oxygen species. The DNA binding and photocleavage by [Rh(2)(mu-O(2)CCH(3))(2)(eta(1)-O(2)CCH(3))(CH(3)OH)(dppz)](+) (2) is reported and compared to that of Rh(2)(mu-O(2)CCH(3))(4) (1). Spectral changes and an increase in viscosity provide evidence for the intercalation of 2 to double stranded DNA with K(b) = 1.8 x 10(5) M(-1). DNA photocleavage by 2 is observed upon irradiation with lambda(irr) > 395 nm both in air and deoxygenated solution. DNA photocleavage is not observed for 1 or free dppz ligand under these irradiation conditions. The coupling of a single dppz ligand to a dirhodium(II/II) bimetallic core in 2 provides a means to access oxygen-independent DNA photocleavage with visible light.     

Insight into the photoinduced ligand exchange reaction pathway of cis-[Rh2(μ-O2CCH3)2(CH3CN)6]2+ with a DNA model chelate

We previously showed that [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) binds to dsDNA only upon irradiation with visible light and that photolysis results in a 34-fold enhancement of its cytotoxicity toward Hs-27 human skin fibroblasts, making it potentially useful for photodynamic therapy (PDT). With the goal of gaining further insight on the photoinduced binding of DNA to the complex, we investigated by NMR spectroscopy the mechanism by which 2,2'-bipyridine (bpy), a model for biologically relevant bidentate nitrogen donor ligands, binds to [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) upon irradiation in D(2)O. The photochemical results are compared to the reactivity in the dark in D(2)O and CD(3)CN. The photolysis of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) with equimolar bpy solutions in D(2)O with visible light affords [Rh(2)(O(2)CCH(3))(2)(eq/eq-bpy)(CH(3)CN)(2)(D(2)O(ax))(2)](2+) (eq/eq) with the reaction reaching completion in ~8 h. Only vestiges of eq/eq are observed at the same time in the dark, however, and the reaction is ~20 times slower. Conversely, the dark reaction of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+) with an equimolar amount of bpy in CD(3)CN affords [Rh(2)(O(2)CCH(3))(2)(η(1)-bpy(ax))(CH(3)CN)(5)](2+) (η(1)-bpy(ax)), which remains present even after 5 days of reaction. The photolysis results in D(2)O are consistent with the exchange of one equiv CH(3)CNeq for solvent, and the resulting species quickly reacting with bpy to generate eq/eq; the initial eq ligand dissociation is assisted by absorption of a photon, thus greatly enhancing the reaction rate. The photolytic reaction of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+):bpy in a 1:2 ratio in D(2)O affords the eq/eq and (eq/eq)(2) adducts. The observed differences in the reactivity in D(2)O vs CD(3)CN are explained by the relative ease of substitution of eq D(2)O vs CD(3)CN by the incoming bpy molecule. These results clearly highlight the importance of dissociation of an eq CH(3)CN molecule from the dirhodium core to attain high reactivity and underscore the importance of light for the reactivity of these compounds, which is essential for PDT agents.     

DNA binding and photocleavage in vitro by new dirhodium(II) dppz complexes: correlation to cytotoxicity and photocytotoxicity

Two new dirhodium(II) complexes possessing the intercalating dppz ligand (dppz = dipyrido[3,2-a:2',3'-c]phenazine), cis-[Rh(2)(mu-O(2)CCH(3))(2)(dppz)(eta(1)-O(2)CCH(3))(CH(3)OH)](+) (1) and cis-[Rh(2)(mu-O(2)CCH(3))(2)(dppz)(2)](2+) (2), were synthesized and characterized as potential agents for photochemotherapy. Various techniques show that 1 binds to DNA through intercalation, although some aggregation of the complex on the DNA surface is also present. In contrast, 2 does not intercalate between the DNA bases; however, strong hypochromic behavior is observed in the presence of DNA, which can be attributed to intermolecular pi-stacking of 2 enhanced by the polyanion. The apparent DNA binding constants determined using optical titrations are compared to those from dialysis experiments. Both complexes photocleave pUC18 plasmid in vitro under irradiation with visible light (lambda(irr) >or= 395 nm, 15 min), resulting in the nicked, circular form. Greater photocleavage is observed for 1 relative to 2, which may be due to the ability of 1 to intercalate between the DNA bases. The cytotoxicity toward human skin cells (Hs-27) measured as the concentration at which 50% cell death is recorded, LC(50), was found to be 135 +/- 8 microM for 2 in the dark (30 min), which is significantly lower than those of 1 (LC(50) = 27 +/- 2 microM) and Rh(2)(O(2)CCH(3))(4) (LC(50) = 15 +/- 2 microM). Irradiation of cell cultures containing 1 and Rh(2)(O(2)CCH(3))(4) with visible light (400-700 nm, 30 min) has little effect on their cytotoxicity, with LC(50) values of 21 +/- 3 and 13 +/- 2 microM, respectively. Interestingly, a 3.4-fold increase in the toxicity of 2 is observed when the cell cultures are irradiated (400-700 nm, 30 min), resulting in LC(50) = 39 +/- 1 microM. The greater toxicity of 1 compared to 2 in the dark may be related to the ability of the former compound to intercalate between the DNA bases. The lower cytotoxicity of 2, together with its significantly greater photocytotoxicity, makes this complex a potential agent for photodynamic therapy (PDT). These results suggest that intercalation or strong DNA binding may not be a desirable property of a potential PDT agent.     

Photocytotoxicity of a new Rh2(II,II) complex: increase in cytotoxicity upon irradiation similar to that of PDT agent hematoporphyrin

A new type of heteroleptic dirhodium complex cis-[Rh(2)(mu-O2CCH3)2-(bpy)(dppz)]2+ (3) was synthesized and its potential as a photodynamic therapy (PDT) agent was investigated. Although 27% hypochromicity of the absorption of 3 in the near-UV and visible regions is observed in the presence of duplex DNA, relative viscosity measurements reveal that the complex does not intercalate between the DNA bases. The DNA photocleavage with visible light by 3 proceeds via both oxygen dependent and independent mechanisms, and it is more efficient than that of related complexes. The increase in the cytotoxicity of 3 towards human skin cells is similar to that of hematoporphyrin, a key ingredient in a PDT drug currently in use. This feature makes this complex a useful candidate for further PDT studies.     

Isolation of the novel dirhodium(II/II) thiolate compound Rh(2)(eta(1)-C(6)H(5)S)(2)(mu-C(6)H(5)S)(2)(bpy)(2)

The reaction of the anticancer active compound [Rh(2)(mu-O(2)CCH(3))(2)(bpy)(2)(CH(3)CN)(2)][BF(4)](2) (1) (bpy = 2,2'-bipyridine) with NaC(6)H(5)S under anaerobic conditions yields Rh(2)(eta(1)-C(6)H(5)S)(2)(mu-C(6)H(5)S)(2)(bpy)(2).CH(3)OH (2), which was characterized by UV-visible, IR, and (1)H NMR spectroscopies as well as single-crystal X-ray crystallography. Compound 2 crystallizes as dark red platelets in the monoclinic space group C2/c with cell parameters a = 20.398(4) A, b = 11.861(2) A, c = 17.417(4) A, beta = 108.98 degrees, V = 3984.9(14) A(3), Z = 4. The main structural features are the presence of a [Rh(2)](4+) core with a Rh-Rh distance of 2.549(2) A bridged by two benzene thiolate ligands in a butterfly-type arrangement. The axial positions of the [Rh(2)](4+) core are occupied by two terminal benzene thiolates. Cyclic voltammetric studies of 2 reveal that the compound exhibits an irreversible oxidation at +0.046 V in CH(3)CN, which is in accord with the fact that the compound readily oxidizes in the presence of O(2). The fact that this unusual dirhodium(II/II) thiolate compound is formed under these conditions is an important first step in understanding the metabolism of dirhodium anticancer active compounds with thiol-containing peptides and proteins.     

Binding of DNA purine sites to dirhodium compounds probed by mass spectrometry

The adducts formed between the antitumor active compounds [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), Rh(2)(O(2)CCH(3))(4), and Rh(2)(O(2)CCF(3))(4) with DNA oligonucleotides have been assessed by matrix-assisted laser desorption ionization (MALDI) and nanoelectrospray (nanoESI) coupled to time-of-flight mass spectrometry (TOF MS). A series of MALDI studies performed on dipurine (AA, AG, GA, and GG)-containing single-stranded oligonucleotides of different lengths (tetra- to dodecamers) led to the establishment of the relative reactivity cis-[Pt(NH(3))(2)(OH(2))(2)](2+) (activated cisplatin) approximately Rh(2)(O(2)CCF(3))(4) > cis-[Pt(NH(3))(2)Cl(2)] (cisplatin) > [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) > Rh(2)(O(2)CCH(3))(4) approximately Pt(C(6)H(6)O(4))(NH(3))(2) (carboplatin). The relative reactivity of the complexes is associated with the lability of the leaving groups. The general trend is that an increase in the length of the oligonucleotide leads to enhanced reactivity for Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) and Rh(2)(O(2)CCH(3))(4) (except for the case of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+), which reacts faster with the GG octamers than with the dodecamers), whereas the reactivity of Rh(2)(O(2)CCF(3))(4) is independent of the oligonucleotide length. When monitored by ESI, the dodecamers containing GG react faster than the respectiveAA oligonucleotides in reactions with Rh(2)(O(2)CCF(3))(4) and Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), whereas AA oligonucleotides react faster with Rh(2)(O(2)CCH(3))(4). The mixed (AG, GA) purine sequences exhibit comparable rates of reactivity with the homopurine (AA, GG) dodecamers in reactions with Rh(2)(O(2)CCH(3))(4). The observation of initial dirhodium-DNA adducts with weak axial (ax) interactions, followed by rearrangement to more stable equatorial (eq) adducts, was achieved by electrospray ionization; the Rh-Rh bond as well as coordinated acetate or acetonitrile ligands remain intact in these dirhodium-DNA adducts. MALDI in-source decay (ISD), collision-induced dissociation (CID) MS-MS, and enzymatic digestion studies followed by MALDI and ESI MS reveal that, in the dirhodium compounds studied, the purine sites of the DNA oligonucleotides interact with the dirhodium core. Ultimately, both MALDI and ESI MS proved to be complementary, valuable tools for probing the identity and stability of dinuclear metal-DNA adducts.     

Excited-state properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py)

The photophysical properties of Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF = tetrahydrofuran, PPh(3) = triphenylphosphine, py = pyridine) were explored upon excitation with visible light. Time-resolved absorption shows that all the complexes possess a long-lived transient (3.5-5.0 micros) assigned as an electronic excited state of the molecules, and they exhibit an optical transition at approximately 760 nm whose position is independent of axial ligand. No emission from the Rh(2)(O(2)CCH(3))(4)(L)(2) (L = CH(3)OH, THF, PPh(3), py) systems was detected, but energy transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to the (3)pipi excited state of perylene is observed. Electron transfer from Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) to 4,4'-dimethyl viologen (MV(2+)) and chloro-p-benzoquinone (Cl-BQ) takes place with quenching rate constants (k(q)) of 8.0 x 10(6) and 1.2 x 10(6) M(-1) s(-1) in methanol, respectively. A k(q) value of 2 x 10(8) M(-1) s(-1) was measured for the quenching of the excited state of Rh(2)(O(2)CCH(3))(4)(PPh(3))(2) by O(2) in methanol. The observations are consistent with the production of an excited state with excited-state energy, E(00), between 1.34 and 1.77 eV.     

Chiral organometallic triangles with Rh-Rh bonds. 1. Compounds prepared from racemic cis-Rh2(C6H4PPh2)2(OAc)2

Reaction of racemic cis-Rh(2)(C(6)H(4)PPh(2))(2)(OAc)(2)(HOAc)(2) with excess Me(3)OBF(4) in CH(3)CN results in the formation of racemic cis-[Rh(2)(C(6)H(4)PPh(2))(2)(CH(3)CN)(6)](BF(4))(2).0.5H(2)O (1.0.5H(2)O), an ionic dirhodium complex which has two cisoid nonlabile orthometalated phosphine bridging anions and six labile CH(3)CN ligands in equatorial and axial positions. Reactions of 1 with tetraethylammonium salts of the linear dicarboxylates, oxalate, terephthalate, and 4,4'-biphenyl-dicarboxylate, in organic solvents, produced racemic crystals of the triangular compounds [Rh(2)(C(6)H(4)PPh(2))(2)](3)(C(2)O(4))(3)(py)(6).6MeOH.H(2)O (2.6MeOH.H(2)O), [Rh(2)(C(6)H(4)PPh(2))(2)](3)(O(2)CC(6)H(4)CO(2))(3)(DMF)(6).6.5DMF.0.5H(2)O (3.6.5DMF.0.5H(2)O), and [Rh(2)(C(6)H(4)PPh(2))(2)](3)(O(2)CC(6)H(4)C(6)H(4)CO(2))(3)(py)(6).4.5CH(3)OH.0.75H(2)O (4.4.5CH(3)OH.0.75H(2)O), respectively. All compounds are electrochemically active. The relative chiralities of the dirhodium units in each triangle have been established using a combination of data from X-ray crystallography and (31)P NMR spectroscopy.     

C-C coupling reactions in the coordination sphere of rhodium(I) and rhodium(III): New routes for the di- and trimerization of terminal alkynes

The alkynyl(vinylidene)rhodium(I) complexes trans-[Rh(C[triple bond, length as m-dash]CR)(=C=CHR)(PiPr3)2] 2, 5, 6 react with CO by migratory insertion to give stereoselectively the butenynyl compounds trans-[Rh{eta1-(Z)-C(=CHR)C[triple bond, length as m-dash]CR}(CO)(PiPr3)2](Z)-7-9, of which (Z)-7 (R=Ph) and (Z)-8 (R=tBu) rearrange upon heating or UV irradiation to the (E) isomers. Similarly, trans-[Rh{eta1-C(=CH2)C[triple bond, length as m-dash]CPh}(CO)(PiPr3)2] 12 and trans-[Rh{eta1-(Z)-C(=CHCO2Me)C[triple bond, length as m-dash]CR}(CO)(PiPr3)2](Z)-15, (Z)-16 have been prepared. At room temperature, the corresponding "non-substituted" derivative trans-[Rh{eta1-C(=CH2)C[triple bond, length as m-dash]CH}(CO)(PiPr3)2] 18 is in equilibrium with the butatrienyl isomer trans-[Rh(eta1-CH=]C=C=CH2)(CO)(PiPr3)2] 19 that rearranges photochemically to the alkynyl complex trans-[Rh(C[triple bond, length as m-dash]CCH=CH2)(CO)(PiPr3)2] 20. Reactions of (Z)-7, (E)-7, (Z)-8 and (E)-8 with carboxylic acids R'CO2H (R'=CH3, CF3) yield either the butenyne (Z)- and/or (E)-RC[triple bond, length as m-dash]CCH=CHR or a mixture of the butenyne and the isomeric butatriene, the ratio of which depends on both R and R'. Treatment of 2 (R=Ph) with HCl at -40 degrees C affords five-coordinate [RhCl(C[triple bond, length as m-dash]CPh){(Z)-CH=CHPh}(PiPr3)2] 23, which at room temperature reacts by C-C coupling to give trans-[RhCl{eta2-(Z)-PhC[triple bond, length as m-dash]CCH=CHPh}(PiPr3)2](Z)-21. The related compound trans-[RhCl(eta2-HC[triple bond, length as m-dash]CCH=CH2)(PiPr3)2] 27, prepared from trans-[Rh(C[triple bond, length as m-dash]CH)(=C=CH2)(PiPr3)2] 17 and HCl, rearranges to the vinylvinylidene isomer trans-[RhCl(=C=CHCH=CH2)(PiPr3)2] 28. While stepwise reaction of 2with CF3CO2H yields, via alkynyl(vinyl)rhodium(III) intermediates (Z)-29 and (E)-29, the alkyne complexes trans-[Rh(kappa1-O2CCF3)(eta2-PhC[triple bond, length as m-dash]CCH=CHPh)(PiPr3)2](Z)-30 and (E)-30, from 2 and CH3CO2H the acetato derivative [Rh(kappa2-O2CCH3)(PiPr3)2] 33 and (Z)-PhC[triple bond, length as m-dash]CCH=]CHPh are obtained. From 6 (R=CO2Me) and HCl or HC[triple bond, length as m-dash]CCO2Me the chelate complexes [RhX(C[triple bond, length as m-dash]CCO2Me){kappa2(C,O)-CH=CHC(OMe)=O}(PiPr3)2] 34 (X=Cl) and 35 (X=C[triple bond, length as m-dash]CCO2Me) have been prepared. In contrast to the reactions of [Rh(kappa2-O2CCH3)(C[triple bond, length as m-dash]CE)(CH=CHE)(PiPr3)2] 37(E=CO2Me) with chloride sources which give, via intramolecular C-C coupling, four-coordinate trans-[RhCl{eta2-(E)-EC[triple bond, length as m-dash]CCH=CHE}(PiPr3)2](E)-36, treatment of 37with HC[triple bond, length as m-dash]CE affords, via insertion of the alkyne into the rhodium-vinyl bond, six-coordinate [Rh(kappa2-O2CCH3)(C[triple bond, length as m-dash]CE){eta1-(E,E)-C(=CHE)CH=CHE}(PiPr3)2] 38. The latter reacts with MgCl2 to yield trans-[RhCl{eta2-(E,E)-EC[triple bond, length as m-dash]CC(=CHE)CH=CHE}(PiPr3)2] 39, which, in the presence of CO, generates the substituted hexadienyne (E,E)-EC[triple bond, length as m-dash]CC(=CHE)CH=CHE 40.     

A dinuclear manganese(II) complex with the [Mn(2)(mu-O(2)CCH(3))(3)](+) core: synthesis, structure, characterization, electroinduced transformation, and catalase-like activity

Reactions of Mn(II)(PF(6))(2) and Mn(II)(O(2)CCH(3))(2).4H(2)O with the tridentate facially capping ligand N,N-bis(2-pyridylmethyl)ethylamine (bpea) in ethanol solutions afforded the mononuclear [Mn(II)(bpea)](PF(6))(2) (1) and the new binuclear [Mn(2)(II,II)(mu-O(2)CCH(3))(3)(bpea)(2)](PF(6)) (2) manganese(II) compounds, respectively. Both 1 and 2 were characterized by X-ray crystallographic studies. Complex 1 crystallizes in the monoclinic system, space group P2(1)/n, with a = 11.9288(7) A, b = 22.5424(13) A, c =13.0773(7) A, alpha = 90 degrees, beta = 100.5780(10 degrees ), gamma = 90 degrees, and Z = 4. Crystals of complex 2 are orthorhombic, space group C222(1), with a = 12.5686(16) A, b = 14.4059(16) A, c = 22.515(3) A, alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees, and Z = 4. The three acetates bridge the two Mn(II) centers in a mu(1,3) syn-syn mode, with a Mn-Mn separation of 3.915 A. A detailed study of the electrochemical behavior of 1 and 2 in CH(3)CN medium has been made. Successive controlled potential oxidations at 0.6 and 0.9 V vs Ag/Ag(+) for a 10 mM solution of 2 allowed the selective and nearly quantitative formation of [Mn(III)(2)(mu-O)(mu-O(2)CCH(3))(2)(bpea)(2)](2+) (3) and [Mn(IV)(2)(mu-O)(2)(mu-O(2)CCH(3))(bpea)(2)](3+) (4), respectively. These results have shown that each substitution of an acetate group by an oxo group is induced by a two-electron oxidation of the corresponding dimanganese complexes. Similar transformations have been obtained if 2 is formed in situ either by direct mixing of Mn(2+) cations, bpea ligand, and CH(3)COO(-) anions with a 1:1:3 stoichiometry or by mixing of 1 and CH(3)COO(-) with a 1:1.5 stoichiometry. Associated electrochemical back-transformations were investigated. 2, 3, and the dimanganese [Mn(III)Mn(IV)(mu-O)(2)(mu-O(2)CCH(3))(bpea)(2)](2+) analogue (5) were also studied for their ability to disproportionate hydrogen peroxide. 2 is far more active compared to 3 and 5. The EPR monitoring of the catalase-like activity has shown that the same species are present in the reaction mixture albeit in slightly different proportions. 2 operates probably along a mechanism different from that of 3 and 5, and the formation of 3 competes with the disproportionation reaction catalyzed by 2. Indeed a solution of 2 exhibits the same activity as 3 for the disproportionation reaction of a second batch of H(2)O(2) indicating that 3 is formed in the course of the reaction.     

Formation of abundant [Pb(H2O)]2+ by ligand-exchange reaction between [Pb(N2)n]2+ (n = 1-3) and H2O

Doubly charged lead monohydrate, [Pb(H2O)]2+, was predicted to be unstable in the gas phase, but it has recently been observed to form in low yield via ligand change between [Pb(CH3CN)]2+ and H2O [Shi, T.; Orlova, G.; Guo, J.; Bohme, D. K.; Hopkinson, A. C.; Siu, K. W. M. J. Am. Chem. Soc. 2004, 126, 7975-7980]. Here we report that abundant [Pb(H2O)]2+ is formed in the gas phase by ligand-exchange reaction between [Pb(N2)n]2+ (n = 1-3) and water after collisional activation. Density functional theory has been used to examine the ligand-exchange reaction profile. A comparison of the potential-energy surfaces between [Pb(N2)]2+ and [Pb(CH3CN)]2+ reacting with H2O provides strong evidence that the ligand-exchange reaction of [Pb(N2)]2+ with H2O to form [Pb(H2O)]2+ is more efficient than that of [Pb(CH3CN)]2+ with H2O.     

Redox chemistry of the acetato-bridged clusters [M3(mu3-O)n(mu-O2CCH3)6(H2O)3]2+ (M = Mo, W, n = 1, 2): reversible redox between mono-micro3-oxo d8 M(III)2M(IV) and d9 M(III)3 forms

A cyclic voltammogram of aqueous 0.1 mol dm(-3) triflic acid solutions of the d6 bioxo-capped M-M bonded cluster [Mo3(mu3-O)2(O2CCH3)6(H2O)3]2+ at a glassy carbon electrode at 25 degrees C gives rise to an irreversible 3e- cathodic wave to a d9 Mo(III)3 species at -0.8 V vs. SCE which on the return scan gives rise to two anodic waves at +0.05 V vs. SCE (E(1/2), 1e- reversible to d8 Mo(III)2Mo(IV)) and +0.48 V vs. SCE (2e- irreversible back to d6 Mo(IV)3). The number of electrons passed at each redox wave has been confirmed by redox titration and controlled potential electrolysis which resulted in 90% recovery of [Mo3(mu3-O)2(O2CCH3)6(H2O)3]2+ following electrochemical re-oxidation at +0.8 V. A corresponding CV study of the d8 monoxo-capped W(III)2W(IV) cluster [W3(mu3-O)(O2CCH3)6(H2O)3]2+ gives rise to a reversible 1e- cathodic process at -0.92 V vs. SCE to give the d9 W(III)3 species [W3(mu3-O)(O2CCH3)6(H2O)3]+; the first authentic example of a W(III) complex with coordinated water ligands. However the cluster is too unstable (O2/water sensitive) to allow isolation. Comparisons with the cv study on [Mo3(mu3-O)2(O2CCH3)6(H2O)3]2+ suggest irreversible reduction of this complex to monoxo-capped [Mo(III)3(mu3-O)(O2CCH3)6(H2O)3]+ followed by reversible oxidation to its d8 counterpart [Mo3(mu3-O)(O2CCH3)6(H2O)3]2+ (Mo(III)2Mo(IV)) and finally irreversible oxidation back to the starting bioxo-capped cluster. Exposing the d9 Mo(III)3 cluster to air (O2) however gives a different final product with evidence of break up of the acetate bridged framework. Corresponding redox processes on d6 [W3(mu3-O)2(O2CCH3)6(H2O)3]2+ are too cathodic to allow similar generation of the monoxo-capped W(III)3 and W(III)2W(IV) clusters at the electrode surface.     

Large improvement in the catalytic activity due to small changes in the diimine ligands: new mechanistic insight into the dirhodium(II,II) complex-based photocatalytic H2 production

Two dirhodium(II) complexes, [Rh(II)(2)(μ-O(2)CCH(3))(2)(bpy)(2)](O(2)CCH(3))(2) (Rh(2)bpy(2); bpy = 2,2'-bipyridine) and [Rh(II)(2)(μ-O(2)CCH(3))(2)(phen)(2)](O(2)CCH(3))(2) (Rh(2)phen(2); phen = 1,10-phenanthroline) were synthesized, and their photocatalytic H(2) production activities were studied in multicomponent systems, containing [Ir(III)(ppy)(2)(dtbbpy)](+) (ppy = 2-phenylpyridine, dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) as the photosensitizer (PS) and triethylamine as the sacrificial reductant (SR). There is a more than 6-fold increase in the photocatalytic activity from Rh(2)bpy(2) to Rh(2)phen(2) just using phen in place of bpy. A turnover number as high as 2622 was obtained after 50 h of irradiation of a system containing 16.7 μM Rh(2)phen(2), 50 μM PS, and 0.6 M SR. The electrochemical, luminescence quenching, and transient absorption experiments demonstrate that Rh(I)Rh(I) is the true catalyst for the proton reduction. The real-time absorption spectra confirm that a new Rh-based species formed upon irradiation of the Rh(2)phen(2)-based multicomponent system, which exhibits an absorption centered at ～575 nm. This 575-nm intermediate may account for the much higher H(2) evolution efficiency of Rh(2)phen(2). Our work highlights the importance of N-based chelate ligands and opens a new avenue for pursuing more efficient Rh(II)(2)-based complexes in photocatalytic H(2) production application.     

Rhodium-103 NMR of [Rh(X)(PPh3)3] [X = Cl, N3, NCO, NCS, N(CN)2, NCBPh3, CNBPh3, CN] and derivatives containing CO, isocyanide, pyridine, H2 and O2. Ligand, solvent and temperature effects

Rhodium-103 chemical shifts are reported for 62 compounds, namely [Rh(X)(PPh3)3] [X = Cl, N3, NCO, NCS, N(CN)2, NCBPh3, CNBPh3, CN] and derivatives formed by replacement of a phosphine by CO, xylyl isocyanide (XNC) and pyridine and/or by oxidative addition of H2 or O2 to give trans-[Rh(X)(PPh3)2(CO)] (delta in the range -816 to -368 ppm) trans-[Rh(X)(PPh3)2(XNC)] (delta -817 to -250 ppm), cis-[Rh(X)(PPh3)2(py)] (the trans isomer is formed with X = CN, CNBPh3) (delta -233 to 170 ppm), [Rh(X)(H)2(PPh3)3] (delta -611 to 119), trans-[Rh(X)(H)2(PPh3)2(py)] (delta -30 to 566 ppm), [Rh(X)(O2)(PPh3)3] (delta 1393 to 3273 ppm) and cis-[Rh(X)(O2)(PPh3)2(py)] (delta 1949 to 3374 ppm). For the majority of these compounds data were obtained from solutions in chloroform and in toluene at temperatures of 247 and 300 K; for [Rh(X)(PPh3)3] (delta -562 to -4 ppm) data are reported at a number of temperatures in the range 195-300 K for solutions in chloroform, toluene and dichloromethane and at 300 K for solutions in DMSO. The expected trend to lower delta(103Rh) with decreasing temperature (vibrational shielding) is observed for the dichloromethane data, but data from solutions {of [Rh(X)(PPh3)3]} in chloroform and toluene show a number of features which diverge from this pattern, i.e. shifts to higher delta are found to accompany a decrease in temperature, most noticeably where X = CN and Cl [on changing the solvent from dichloromethane to chloroform changes in delta(103Rh) of up to 172 ppm are observed]. These results are interpreted in terms of a hydrogen-bonded interaction with the solvent that is enhanced by the presence of a polarizable ligand (CN, Cl). With a ligand (O2CCF3) that is only weakly polarizable the solvent dependence of delta(103Rh) is minimal.     

Chiral organometallic triangles with Rh-Rh bonds. 2. Compounds prepared from enantiopure cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 and their catalytic potentials

Enantiomers of the orthometalated dirhodium compound cis-Rh2(C6H4PPh2)2(OAc)2(HOAc)2 (R-1 and S-1) were prepared from carboxylate exchange reactions of the resolved diasteroisomers of cis-Rh2(C6H4PPh2)2(protos)2(H2O)2 (protos = N-4-methylphenylsulfonyl-l-proline anion) and acetic acid. These compounds react with excess Me3OBF4 in CH3CN, producing the enantiomers of [cis-Rh2(C6H4PPh2)2(CH3CN)6](BF4)2 (R-2 and S-2) which have six labile and replaceable CH3CN ligands in equatorial and axial positions. Reactions of R-2 and S-2 with tetraethylammonium salts of the linear dicarboxylic acids, terephthalic acid (HO2CC6H4CO2H), oxalic acid (HO2CCO2H), and 4,4'-diphenyl-dicarboxylic acid (HO2CC6H4C6H4CO2H) afford the enantiopure triangular supramolecules [cis-Rh2(C6H4PPh2)2(O2CC6H4CO2)(py)2]3, RRR-3 and SSS-3, Rh6(cis-C6H4PPh2)6(O2CCO2)3(py)5(CH2Cl2), RRR-4 and SSS-4, and Rh6(cis-C6H4PPh2)6(O2CC6H4C6H4CO2)3(py)4(CH2Cl2)2, RRR-5 and SSS-5, respectively. The absolute structures of each of the enantiomers of 1, 3, 4, and 5 were determined by X-ray diffraction analyses. The enantiomers of 3, 4, and 5 were found to be enantiomorphically isostructural, whereas R-1 and S-1 crystallized in different space groups. In 4 and 5, CH2Cl2 molecules coordinate to rhodium atoms in the axial positions. The 1H and 31P[1H] NMR spectra of all compounds are reported. The triangular compounds are redox-active, and their electrochemistry is also discussed. An assay of the catalytic activity/selectivity performance of the triangles for typical metal carbene transformation, using the model intermolecular cyclopropanation of styrene with ethyl diazoacetate in both homogeneous and heterogeneous phases, show that these chiral triangles are very active and have remarkable selectivity when compared with simple Rh2 paddle-wheel catalysts with chiral amidate ligands.     

Structural and functional characteristics of rhenium clusters derived from redox chemistry of the triangular [ReIII3(mu-Cl)3] core unit

The present study investigates structural and functional aspects of the redox chemistry of rhenium(III) chloride [Re3Cl9] (1) in aqueous and organic solvents, with emphasis on the dioxygen-activating capabilities of reduced rhenium clusters bearing the Re3(8+) core. Dissolution of 1 in HCl (6 M) generates [Re3(mu-Cl)3Cl9]3- (2a), which can be isolated as the tetraphenylphosphonium salt (2b). Anaerobic one-electron reduction of 1 by Hg in HCl (6-12 M) produces [(C6H5)4P]2[Re3(mu-Cl)3Cl7(H2O)2].H2O (3), the structure of which features a planar [Re3(mu-Cl)3Cl3] framework (Re3(8+) core), involving two water ligands that occupy out-of-plane positions in a trans arrangement. Compound 3 dissociates in the presence of CO, yielding [(C6H5)4P]2[ReIII2Cl8] (4) and an unidentified red carbonyl species. In situ oxidation (O2) of the reduced Re3(8+)-containing cluster in HCl (6 M) produces quantitatively 2a, whereas oxidation of 3 in organic media results in the formation of [(C6H5)4P]4[(Re3(mu-Cl)3Cl7(mu-OH))2].2CH2Cl2 (5). The structure of 5 reveals that two oxygen ligands (hydroxo units) bridge asymmetrically two Re3(9+) triangular clusters. The origin of these hydroxo units derives from the aquo ligands, rather than O2, as shown by 18O2 labeling studies. The hydroxo bridges of 5 can be replaced by chlorides upon treatment with Me3SiCl to afford the analogous [(C6H5)4P]4[(Re3(mu-Cl)3Cl7(mu-Cl))2].10CH2Cl2 (6). The reaction of 5 with Hg in HCl (6 M)/tetrahydrofuran regenerates compound 3. Complexes 1-3 exhibit nitrile hydratase type activity, inducing hydrolysis of CH3CN to acetamide. The reaction of 3 with CH3CN yields [(C6H5)4P]2[Re3(mu-Cl)3Cl6.5(CH3CN)1.5(CH3C(O)NH)0.5] (7), the structure of which is composed of [Re3(mu-Cl)3Cl7(CH3CN)2]2- (7a) and [Re3(mu-Cl)3Cl6(CH3CN)(CH3C(O)NH)]2- (7b) (Re3(8+) cores) as a disordered mixture (1:1). Oxidation of 7 with O2 in CH3CN affords [(C6H5)4P]2[Re3(mu-Cl)3Cl7(CH3C(O)NH)].CH3CN (8) and small amounts of [(C6H5)4P][ReO4] (9). Compound 8 is also independently isolated from the reaction of 2b with wet CH3CN, or by dissolving 5 in CH3CN. In MeOH, 5 dissociates to afford [(C6H5)4P]2[Re3(mu-Cl)3Cl8(MeOH)].MeOH (10).     

Dynamics of a cis-dihydrogen/hydride complex of iridium

Insertion of CS2 into one of the Ir-H bonds of [Ir(H)5(PCy3)2] takes place to afford the dihydrido dithioformate complex cis-[Ir(H)2(eta2-S2CH)(PCy3)2] accompanied by the elimination of H2. Protonation of the dithioformate complex using HBF4.Et2O gives cis-[Ir(H)(eta2-H2)(eta2-S2CH)(PCy3)2][BF4] wherein the H atom undergoes site exchange between the dihydrogen and the hydride ligands. The dynamics was found to be so extremely rapid with respect to the NMR time scale that the barrier to exchange could not be measured. Partial deuteration of the hydride ligands resulted in a J(H,D) of 6.5 and 7.7 Hz for the H2D and the HD2 isotopomers of cis-[Ir(H)(eta2-H2)(eta2-S2CH)(PCy3)2][BF4], respectively. The H-H distance (d(HH)) for this complex has been calculated to be 1.05 A, which can be categorized under the class of elongated dihydrogen complexes. The cis-[Ir(H)(eta2-H2)(eta2-S2CH)(PCy3)2][BF4] complex undergoes substitution of the bound H2 moiety with CH(3)CN and CO resulting in new hydride derivatives, cis-[Ir(H)(L)(eta2-S2CH)(PCy3)2][BF4] (L = CH3CN, CO). Reaction of cis-[Ir(H)2(eta2-S2CH)(PCy3)2] with electrophilic reagents such as MeOTf and Me3SiOTf afforded a new hydride aquo complex cis-[Ir(H)(H2O)(eta2-S2CH)(PCy3)2][OTf] via the elimination of CH4 and Me3SiH, respectively, followed by the binding of a water molecule (present in trace quantities in the solvent) to the iridium center. The X-ray crystal structures of cis-[Ir(H)2(eta2-S2CH)(PCy3)2] and cis-[Ir(H)(H2O)(eta2-S2CH)(PCy3)2][OTf] have been determined.     

Structural chemistry of "defect" cyanometalate boxes: (Cs subset [CpCo(CN)3]4[Cp*Ru]3) and (M subset [Cp*Rh(CN)3]4[Cp*Ru]3) (M = NH4, Cs)

A series of heptametallic cyanide cages are described; they represent soluble analogues of defect-containing cyanometalate solid-state polymers. Reaction of 0.75 equiv of [Cp*Ru(NCMe)3]PF6, Et(4)N[Cp*Rh(CN)3], and 0.25 equiv of CsOTf in MeCN solution produced (Cs subset [CpCo(CN)3]4[Cp*Ru]3)(Cs subset Rh4Ru3). 1H and 133Cs NMR measurements show that Cs subset Rh4Ru3 exists as a single Cs isomer. In contrast, (Cs subset [CpCo(CN)3]4[Cp*Ru]3) (Cs subset Co4Ru3), previously lacking crystallographic characterization, adopts both Cs isomers in solution. In situ ESI-MS studies on the synthesis of Cs subset Rh4Ru3 revealed two Cs-containing intermediates, Cs subset Rh2Ru2+ (1239 m/z) and Cs subset Rh3Ru3+ (1791 m/z), which underscore the participation of Cs+ in the mechanism of cage formation. 133Cs NMR shifts for the cages correlated with the number of CN groups bound to Cs+: Cs subset Co4Ru4+ (delta 1 vs delta 34 for CsOTf), Cs subset Rh4Ru3 where Cs+ is surrounded by ten CN ligands (delta 91), Cs subset Co4Ru3, which consists of isomers with 11 and 10 pi-bonded CNs (delta 42 and delta 89, respectively). Although (K subset [Cp*Rh(CN)3]4[Cp*Ru]3) could not be prepared, (NH4 subset [Cp*Rh(CN)3]4[Cp*Ru]3) (NH4 subset Rh4Ru3) forms readily by NH4+-template cage assembly. IR and NMR measurements indicate that NH4+ binding is weak and that the site symmetry is low. CsOTf quantitatively and rapidly converts NH4 subset Rh4Ru3 into Cs subset Rh4Ru3, demonstrating the kinetic advantages of the M7 cages as ion receptors. Crystallographic characterization of CsCo4Ru3 revealed that it crystallizes in the Cs-(exo)1(endo)2 isomer. In addition to the nine mu-CN ligands, two CN(t) ligands are pi-bonded to Cs+. M subset Rh4Ru3 (M = NH4, Cs) crystallizes as the second Cs isomer, that is, (exo)2(endo)1, wherein only one CN(t) ligand interacts with the included cation. The distorted framework of NH4 subset Rh4Ru3 reflects the smaller ionic radius of NH4+. The protons of NH4+ were located crystallographically, allowing precise determination of the novel NH4...CN interaction. A competition experiment between calix[4]arene-bis(benzocrown-6) and NH4 subset Rh4Ru3 reveals NH4 subset Rh4Ru3 has a higher affinity for cesium.