Importance of interfacial adsorption in the biphasic hydroformylation of higher olefins promoted by cyclodextrins: a molecular dynamics study at the decene/water interface

We report herein a molecular dynamics study of the main species involved in the hydroformylation of higher olefins promoted by cyclodextrins in 1-decene/water biphasic systems at a temperature of 350 K. The two liquids form a well-defined sharp interface of approximately 7 A width in the absence of solute; the decene molecules are generally oriented "parallel" to the interface where they display transient contacts with water. We first focused on rhodium complexes bearing water-soluble TPPTS(3-) ligands (where TPPTS(3-) represents tris(m-sulfonatophenyl)phosphine) involved in the early steps of the reaction. The most important finding concerned the surface activity of the "active" form of the catalyst [RhH(CO)(TPPTS)(2)](6-), the [RhH(CO)(2)(TPPTS)(2)](6-) complex, and the key reaction intermediate [RhH(CO)(TPPTS)(2)(decene)](6-) (with the olefin pi-coordinated to the metal center) which are adsorbed at the water side of the interface in spite of their -6 charge. The free TPPTS(3-) ligands themselves are also surface-active, whereas the -9 charged catalyst precursor [RhH(CO)(TPPTS)(3)](9-) prefers to be solubilized in water. The role of cyclodextrins was then investigated by performing simulations on 2,6-dimethyl-beta-cyclodextrin ("CD") and its inclusion complexes with the reactant (1-decene), a reaction product (undecanal), and the corresponding key reaction intermediate [RhH(CO)(TPPTS)(2)(decene)](6-) as guests; they were all shown to be surface-active and prefer the interface over the bulk aqueous phase. These results suggest that the biphasic hydroformylation of higher olefins takes place "right" at the interface and that the CDs promote the "meeting" of the olefin and the catalyst in this peculiar region of the solution by forming inclusion complexes "preorganized" for the reaction. Our results thus point to the importance of adsorption at the liquid/liquid interface in this important phase-transfer-catalyzed reaction.     

Theoretical elucidation of a classic reaction: protonation of the quadruple bond of the octachlorodimolybdate(II,II) [Mo2Cl8]4- anion

The protonation reaction of the unbridged quadruple metal-metal bond of [Mo(2)Cl(8)](4-) anion producing the triply bonded hydride [Mo(2)(μ-H)(μ-Cl)(2)Cl(6)](3-) is studied by accurate Density Functional Theory computations. The reactant, product, stable intermediates, and transition states are located on the potential energy surface. The water solvent is explicitly included in the calculations. Full reaction profiles are calculated and compared to experimental data. The mechanism of the reaction is fully elucidated. This involves two steps. The first is a proton transfer from an oxonium ion to the quadruple bond, being rate determining. The second, involves the internal rearrangement of chlorine atoms and is much faster. Activation energies with a mean value of 19 kcal/mol are calculated, in excellent agreement with experimental values.     

Experimental Raman spectra of dilute and laser-light-sensitive [Rh(4)(CO)(9)(mu-CO)(3)] and [(mu(4)-eta(2)-3-hexyne)Rh(4)(CO)(8)(mu-CO)(2)]. Comparison with theoretically predicted spectra

The reaction of [Rh(4)(CO)(9)(mu-CO)(3)] with 3-hexyne to form the butterfly cluster [(mu(4)-eta(2)-3-hexyne)Rh(4)(CO)(8)(mu-CO)(2)] was monitored viain-situ Raman spectroscopy using an NIR laser source, at room temperature and under atmospheric argon using n-hexane as solvent. The collected raw spectra were deconvoluted using band-target entropy minimization (BTEM). The pure component mid-Raman spectra of the [Rh(4)(CO)(9)(mu-CO)(3)] and the butterfly cluster [(mu(4)-eta(2)-3-hexyne)Rh(4)(CO)(8)(mu-CO)(2)], were reconstructed with a high signal-to-noise ratio. Full geometric optimization and Raman vibrational prediction were carried out using DFT. The experimental and predicted Raman spectra were in good agreement. In particular, the far-Raman vibrational modes in the region 100-280 cm(-1) provided characterization of the metal-metal bonds and direct confirmation of the structural integrity of the polynuclear frameworks in solution.     

Photoelectron imaging of hydrated carbon dioxide cluster anions

The effects of homogeneous and heterogeneous solvation on the electronic structure and photodetachment dynamics of hydrated carbon dioxide cluster anions are investigated using negative-ion photoelectron imaging spectroscopy. The experiments are conducted on mass-selected [(CO(2))(n)()(H(2)O)(m)()](-) cluster anions with n and m ranging up to 12 and 6, respectively, for selected clusters. Homogeneous solvation in (CO(2))(n)()(-) has minimal effect on the photoelectron angular distributions, despite dimer-to-monomer anion core switching. Heterogeneous hydration, on the other hand, is found to have the marked effect of decreasing the photodetachment anisotropy. For example, in the [CO(2)(H(2)O)(m)()](-) cluster anion series, the photoelectron anisotropy parameter falls to essentially zero with as few as 5-6 water molecules. The analysis of the data, supported by theoretical modeling, reveals that in the ground electronic state of the hydrated clusters the excess electron is localized on CO(2), corresponding to a (CO(2))(n)()(-).(H(2)O)(m)() configuration for all cluster anions studied. The diminishing anisotropy in the photoelectron images of hydrated cluster anions is proposed to be attributable to photoinduced charge transfer to solvent, creating transient (CO(2))(n)().(H(2)O)(m)()(-) states that subsequently decay via autodetachment.     

New insights into catalytic hydrogenation by phosphido-substituted triruthenium clusters: confirmation of intact cluster catalysis by parahydrogen NMR

The phosphido-substituted triruthenium cluster Ru(3)(CO)(9)(mu-H)(micro-PPh(2)) is shown to react with H(2) to form the trihydride cluster Ru(3)(CO)(9)(H)(mu-H)(2)(mu-PPh(2)), which undergoes a number of re-arrangement reactions on heating to yield other phosphido-substituted triruthenium clusters. In the presence of alkyne substrates, heating the system leads to catalytic hydrogenation via CO loss and the formation of a Ru(3)(eta(2)-PhC[double bond, length as m-dash]CHPh)(CO)(8)(micro-H)(PHPh(2)) resting state, in a reaction affected by the polarity of the solvent. No mononuclear fragments are observed in the catalytic transformation, confirming directly that the phosphido ligand is able to exert a stabilising influence on the cluster core.     

Adsorption at the liquid-liquid interface in the biphasic rhodium catalyzed hydroformylation of olefins promoted by cyclodextrins: a molecular dynamics study

Using molecular dynamics (MD) simulations, we investigate the interfacial distribution of partners involved in the phase transfer rhodium catalyzed hydroformylation of olefins promoted by beta-cyclodextrins (beta-CDs). The beta-CDs, the reactant (alkene), product (aldehyde), several rhodium complexes (the catalyst, its precursor, and its alkene adduct) are simulated at the water-"oil" interface, where oil is represented by chloroform or hexane. It is shown that unsubstituted beta-CD and its 6-methylated and 2,6-dimethylated analogues adsorb at the interface, whereas the liposoluble permethylated CD does not. The precursor of the catalyst [RhH(CO)(TPPTS)3]9- (with triphenylphosphine trisulfonated TPPTS3- ligands) sits in water, but the less charged [RhH(CO)(TPPTS)2]6- catalyst and the [RhH(CO)(TPPTS)2(alkene)]6- reaction intermediate are clearly surface active. The TPPTS3- anions also concentrate at the interface, where they adopt an amphiphilic conformation, forming an electrical double layer with their Na+ counterions. Thus, the most important key partners involved in the hydroformylation reaction concentrate at the interface, thereby facilitating the reaction, a process which may be further facilitated upon complexation by CDs. These results point to the importance of adsorption at the liquid-liquid interface in the two-phase hydroformylation reaction of olefins promoted by beta-CDs and provide microscopic pictures of this peculiar region of the solution.     

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer of a triruthenium cluster studied by in situ infrared spectroscopy

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru(3)(mu3-O)(mu-CH3COO)6(CO)(L1)(L2)] (L1 = [(NC5H4)CH2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coordination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures.     

Coadsorption of CO and O(2) on selected gold clusters: evidence for efficient room-temperature CO(2) generation

Spurred by the recent demonstrations of the size- and support-dependent reactivity of supported gold clusters, here we present results on the coadsorption of CO and O(2) on selected anionic gold clusters, Au(N)(-), in the gas phase. O(2) adsorbs in a binary (0,1) fashion as a one-electron acceptor on the Au(N)()(-) clusters, with even-N clusters showing varying reactivity toward O(2) adsorption, while odd-N clusters show no evidence of reactivity. CO shows a highly size-dependent reactivity for Au(N)(-) sizes from N = 4 to 19, but no adsorption on the gold dimer or trimer. When the gold clusters are exposed to both reactants, either simultaneously or sequentially, interesting effects have been observed. While the same rules pertaining to individual O(2) or CO adsorption continue to apply, the preadsorption of one reactant on a cluster may lead to the increased reactivity of the cluster to the other reactant. Thus, the adsorbates are not competing for bonding sites (competitive coadsorption), but, instead, aid in the adsorption of one another (cooperative coadsorption). New peaks also arise in the mass spectrum of Au(6)(-) under CO and O(2) coadsorption conditions, which can be attributed to the loss of a CO(2) molecule (or molecules). By studying the relative amount of reaction, and relating it to the reaction time, it is found that the gas-phase Au(6) anion is capable of oxidizing CO at a rate 100 times that reported for commercial or model gold catalysts.     

Chelate control of diiron(I) dithiolates relevant to the [Fe-Fe]- hydrogenase active site

The reaction of Fe2(S2C2H4)(CO)6 with cis-Ph2PCH=CHPPh2 (dppv) yields Fe2(S2C2H4)(CO)4(dppv), 1(CO)4, wherein the dppv ligand is chelated to a single iron center. NMR analysis indicates that in 1(CO)4, the dppv ligand spans axial and basal coordination sites. In addition to the axial-basal isomer, the 1,3-propanedithiolate and azadithiolate derivatives exist as dibasal isomers. Density functional theory (DFT) calculations indicate that the axial-basal isomer is destabilized by nonbonding interactions between the dppv and the central NH or CH2 of the larger dithiolates. The Fe(CO)3 subunit in 1(CO)4 undergoes substitution with PMe3 and cyanide to afford 1(CO)3(PMe3) and (Et4N)[1(CN)(CO)3], respectively. Kinetic studies show that 1(CO)4 reacts faster with donor ligands than does its parent Fe2(S2C2H4)(CO)6. The rate of reaction of 1(CO)4 with PMe3 was first order in each reactant, k = 3.1 x 10(-4) M(-1) s(-1). The activation parameters for this substitution reaction, DeltaH = 5.8(5) kcal/mol and DeltaS = -48(2) cal/deg.mol, indicate an associative pathway. DFT calculations suggest that, relative to Fe2(S2C2H4)(CO)6, the enhanced electrophilicity of 1(CO)4 arises from the stabilization of a "rotated" transition state, which is favored by the unsymmetrically disposed donor ligands. Oxidation of MeCN solutions of 1(CO)3(PMe3) with Cp2FePF6 yielded [Fe2(S2C2H4)(mu-CO)(CO)2(dppv)(PMe3)(NCMe)](PF6)2. Reaction of this compound with PMe3 yielded [Fe2(S2C2H4)(mu-CO)(CO)(dppv)(PMe3)2(NCMe)](PF6)2.     

Hydrogenation and carbon-sulfur bond hydrogenolysis of benzothiophene promoted by Re2(CO)10 and H4Re4(CO)12

In the search for metal complexes that promote the cleavage of C-S bonds in thiophenes, we observe that the reaction of Re(2)(CO)(10) and benzothiophene (BT) under a hydrogen atmosphere gives the trinuclear cluster Re(3)(mu-H)(2)(mu(3)-S-2-EtC(6)H(4))(mu-2,3-DHBT)(CO)(9) (1), which contains a hydrogenated BT ligand and a thiolate ligand resulting from the hydrogenation and cleavage of a C-S bond in BT. A detailed study of the reaction shows that Re(2)(CO)(10) initially reacts with H(2) to give H(3)Re(3)(CO)(12), which subsequently converts to H(4)Re(4)(CO)(12), which finally reacts with BT to give 1.     

Oxidative addition of iodo-acetonitrile and of elemental halogens to [Pt3(mu-CO)3(PCy3)3

The reaction of [Pt3(mu-CO)3(PCy3)3](1) with one mole-equivalent of iodo-acetonitrile was quantitative at -70 degrees C giving the oxidative addition product [Pt3(mu-CO)3(PCy3)3(I)(CH2CN)](2). Fragmentation of was observed in solution giving [Pt2I(CH2CN)(CO)2(PCy3)2](3) which is the major product at room temperature if the starting cluster/reactant ratio is equal to or less than 1 to 1.5. Dimer 3 decomposes slowly in solution giving [Pt2I2(CO)2(PCy3)2](4a) and succinonitrile. Monomer [PtI(CH2CN)(CO)(PCy3)] was the final product of the reaction when using excess of iodo-acetonitrile. The reactions of with one mole-equivalent of halogens X2 gave the new 44-electron clusters [Pt3X(micro-CO)2(micro-X)(PCy3)3](X = I2(7a) or Br2(7b)) by oxidative addition followed by substitution of CO by X-. Fragmentation of and took place in solution when using one and a half mole-equivalents of X2 giving dimers 4a and [Pt2Br2(CO)2(PCy3)2](4b) as well as [Pt2X2(mu-X)2(CO)2(PCy3)2]. Monomers cis-[PtX2(CO)(PCy3)] were the final products of the reaction of with excess of halogens. Insertion of SnCl2 was observed into the Pt-Pt bond but not into the Pt-X bond, when equimolar amounts of SnCl2 x 2H2O were added to a solution of 4a or its chloro-analogue giving [Pt2X2(micro-SnCl2)(CO)2(PCy3)2]. The Pt(I) dimers have unusually small J(Pt-Pt) values as observed by 195Pt NMR and calculated by DFT. These values showed periodic changes comparing 4a and its analogues with other halides and mixed halide dimers.     

Electroactive chain-like compounds constructed from trimetallic clusters and 4,4'-bipyridine spacers: one-pot synthesis, characterization and surface binding

This paper reports the synthesis and characterization of a novel series of chain-like compounds where oxo-centered triruthenium cluster moieties are bridged by 4,4'-bipyridine (4,4'-bpy) spacers. A reaction of solvent-coordinated triruthenium "monomer" precursor [Ru(3)O(CH(3)CO(2))(6)(CO)(CH(3)OH)(2)] with a 0.1 equimolar amount of 4,4'-bpy in CH(3)OH gave mixture of chain-like compounds containing "dimers" to "tetramers" which were cleanly separated by column chromatography and characterized by spectroscopic and electrochemical methods. Cyclic voltammetry revealed that all chain-like compounds exhibit reversible and stepwise redox processes in solution with very weak intramolecular coupling between the triruthenium components across the 4,4'-bpy bridge. Photo-induced dissociation of CO from the compounds and electrode surface binding were also investigated.     

Evidence for hydrogen bond network formation in microsolvated clusters of Pt(CN)4(2-): collision induced dissociation studies of Pt(CN)4(2-)·(H2O)(n) n = 1-4, and Pt(CN)4(2-)·(MeCN)(m) m = 1, 2 cluster ions

Low-energy collision induced dissociation has been used to investigate the structure and stability of microsolvated clusters of the prototypical, aprotic multiply charged anion, Pt(CN)(4)(2-), i.e. Pt(CN)(4)(2-)·(H(2)O)(n) n = 1-4, Pt(CN)(4)(2-)·(MeCN)(m) m =1, 2, and Pt(CN)(4)(2-)·(H(2)O)(3)·MeCN. For all of the systems studied, the lowest energy fragmentation pathway was found to correspond to decay of the cluster with loss of the entire solvent ensemble. No sequential solvent evaporation was observed. These observations suggest that the Pt(CN)(4)(2-) solvent clusters studied here form hydrogen-bonded "surface solvated" structures. Electronic structure calculations are presented to support the experimental results. In addition, the detailed fragmentation patterns observed are interpreted with reference to the differential solvation of the ionic fragmentation and electron detachment potential energy surfaces of the core Pt(CN)(4)(2-) dianion. The results described represent some of the first experiments to probe the microsolvation of this important class of multiply charged anions.     

Remarkable dynamical opening and closing of platinum and palladium pentaruthenium carbido carbonyl cluster complexes

The reaction of Ru(5)(CO)(15)(mu(5)-C), 1, with Pt(PBu(t)(3))(2) at room temperature yielded the mixed-metal cluster complex PtRu(5)(CO)(15)(PBu(t)(3))(C), 2, in 52% yield. Compound 2 consists of a mixture of two interconverting isomers in solution. One isomer, 2A, can be isolated by crystallization from benzene/octane solvent. The second isomer, 2B, can be isolated by crystallization from diethyl ether. Both were characterized crystallographically. Isomer 2A consists of a square pyramidal cluster of five ruthenium atoms with a phosphine-substituted platinum atom spanning the square base. Isomer 2B consists of a square pyramidal cluster of five ruthenium atoms with a phosphine-substituted platinum atom on an edge on the square base. The two isomers interconvert rapidly on the NMR time scale at 40 degrees C, deltaG(313)++ = 11.4(8) kcal mol(-1), deltaH++ = 8.8(5) kcal mol(-1), deltaS++ = -8.4(9) cal mol(-1) K(-1). The reaction of Pd(PBu(t)(3))(2) with compound 1 yielded two new cluster complexes: PdRu(5)(CO)(15)(PBu(t)(3))(mu(6)-C), 3, in 50% yield and Pd(2)Ru(5)(CO)(15)(PBu(t)(3))(2)(mu(6)-C), 4, in 6% yield. The yield of 4 was increased to 47% when an excess of Pd(PBu(t)(3))(2) was used. In the solid state compound 3 is structurally analogous to 2A, but in solution it also exists as a mixture of interconverting isomers; deltaG(298)++ = 10.6(6) kcal mol(-1), deltaH++ = 9.7(3) kcal mol(-1), and deltaS++ = -3(1) cal mol(-1) K(-1) for 3. Compound 4 contains an octahedral cluster consisting of one palladium atom and five ruthenium atoms with an interstitial carbido ligand in the center of the octahedron, but it also has one additional Pd(PBu(t)(3)) grouping that is capping a triangular face of the ruthenium cluster. The Pd(PBu(t)(3)) groups in 4 also undergo dynamical interchange that is rapid on the NMR time scale at 25 degrees C; deltaG(298)++ = 11(1) kcal mol(-1), deltaH++ = 10.2(4) kcal mol(-1), and deltaS++ = -3(2) cal mol(-1) K(-1) for 4.     

Dinuclear ruthenium and iron complexes containing palladium and platinum with tri-tert-butylphosphine ligands: synthesis, structures, and bonding

The reaction of Pd(PBu(t)(3))(2) with Ru(CO)(5) yielded the dipalladium-diruthenium cluster complex Ru(2)(CO)(9)[Pd(PBu(t)(3))](2), 10. The reaction of Pt(PBu(t)(3))(2) with Ru(CO)(5) at room temperature afforded the diplatinum-diruthenium cluster complex Ru(2)(CO)(9)[Pt(PBu(t)(3))](2), 12, and the monoplatinum-diruthenium cluster PtRu(2)(CO)(9)(PBu(t)(3)), 11. All three complexes contain a diruthenium group with bridging Pd(PBu(t)(3)) or Pt(PBu(t)(3)) groups. Compound 11 can be converted to 12 by reaction with an additional quantity of Pt(PBu(t)(3))(2). The reaction of 12 with hydrogen at 68 degrees C yielded the dihydrido complex Pt(2)Ru(2)(CO)(8)(PBu(t)(3))(2)(micro-H)(2), 13. This complex contains a Ru(2)Pt(2) cluster with hydride ligands bridging two of the Ru-Pt bonds. The reaction of Fe(2)(CO)(9) with Pt(PBu(t)(3))(2) yielded the platinum-diiron cluster complex PtFe(2)(CO)(9)(PBu(t)(3)), 14, which is analogous to 11. All new complexes were characterized crystallographically. Molecular orbital calculations of 10 reveal an unusual delocalized metal-metal bonding system involving the Pd(PBu(t)(3)) groups and the Ru(2)(CO)(9) group.     

Photodetachment and photofragmentation pathways in the [(CO2)2(H2O)m]- cluster anions

The mass-selected [(CO(2))(2)(H(2)O)(m)](-) cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO(2) (-)(H(2)O)(m)CO(2) and (O(2)CCO(2))(-)(H(2)O)(m) parent cluster structures, particularly for m=2 and 3, only the CO(2) (-) based clusters are both required and sufficient to explain all fragmentation pathways for m>/=3. Three types of anionic photofragments are observed: CO(2) (-)(H(2)O)(k), O(-)(H(2)O)(k), and CO(3) (-)(H(2)O)(k), k6) is attributed to hindrance from the H(2)O molecules.     

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.     

DFT and MP2 Calculations on Tautomers and Water-Assisted Proton Transfer on 1,2,5-Oxadiazol-4,3-diamine

In this work a detailed quantum-chemical comparison of the relative stability of six tautomers of 1,2,5-oxadiazol-4,3-diamine studied in the gas phase and solution. Theoretical calculations are carried out by the density functional theory (DFT/B3LYP) and MP2 methods using the standard 311++G(d,p) basis set. The results indicate that A is the most stable form in the gas phase and also is the predominant tautomer in solution at the DFT and MP2 methods. The transition states of proton transfer reaction are calculated. The variation of dipole moments and charges on atoms are studied in various solvent. Specific solvent effects with addition of one water molecule near the electrophilic centers of tautomer investigated. Also the transition state of proton transfer assisted by a water molecule was investigated.     

Luminescent hydrido-carbonyl clusters of rhenium containing bridging 1,2-diazine ligands

The reaction of the electronically unsaturated (56 valence electrons, ve) tetrahedral cluster [Re4(mu3-H)4(CO)12] (1) with pyridazine (pydz) gives as the main initial product the tetranuclear cluster [Re4(mu-H)4(mu-pydz)(pydz)2(CO)12] (2a), with 64 ve and four hydrogen-bridged metal-metal interactions, with a spiked-triangle geometry. One of the three pydz ligands bridges, in a cis configuration, the cluster edge opposite to the vertex bearing the spike, as indicated by the X-ray single-crystal analysis. This species slowly decomposes, affording the dinuclear unsaturated (32 ve) complex [Re2(mu-H)2(mu-pydz)(CO)6] (3a) and two isomers of the tetranuclear cluster [Re4(mu-H)4(mu-pydz)2(CO)12] (64 ve), sharing an unusual square cluster geometry and differing in the trans (major, 85%, 4a) or cis (4a') configuration of the bridging pydz ligands. The structures of 3a and 4a have been ascertained by X-ray analysis, while the characterization of 4a' was hampered by its instability (slowly transforming into 3a in THF solution). Both the dimer and the square cluster 4a are also formed directly (and quickly) from 1, being present in solution since the beginning of the reaction. Cluster 4a is the main final reaction product. The reaction with phthalazine follows a similar course, with some differences in the relative amount of the final products 3b and 4b. Most of the novel complexes are able to emit light in solution at room temperature, and photophysical measurements were performed in CH2Cl2 solution on the main stable reaction products (i.e., the dinuclear species 3a and 3b and the trans square clusters 4a and 4b). The emission was in the range of 580-645 nm, from MLCT excited states, with lifetimes on the order of a hundred nanoseconds (50-473 ns). The quantum yields were 1 order of magnitude higher for the squares (1.7 and 1.3% for 4a and 4b, respectively, in CH2Cl2) than for the dinuclear complexes ( approximately 0.1%). In the case of 4a, a blue shift and an increase of the emission intensity were observed upon decreasing the solvent polarity.     

1,2,3,4-Tetraphenyl-1,2,3,4-tetraphospholane,a highly versatile cyclocarbaphosphine ligand: reactions with activated triosmium clusters and characterization of the products

Reaction of 1,2,3,4-tetraphenyl-1,2,3,4-tetraphospholane (I) with [Os(3)(CO)(11)(NCMe)] at ambient temperature affords substituted clusters: the monosubstituted trinuclear cluster [Os(3)(CO)(11)[(PPh)(4)CH(2)]] (1) and the isomeric linked bis-trinuclear clusters [[Os(3)(CO)(11)](2)[mu-1,4-eta(2)-(PPh)(4)CH(2)]] (2) and [[Os(3)(CO)(11)](2)[mu-1,3-eta(2)-(PPh)(4)CH(2)]] (3). Clusters 2 and 3 can also be prepared by further reaction of 1 with [Os(3)(CO)(11)(NCMe)]. The reaction at 100 degrees C gives, apart from cluster 2, the disubstituted 1,4-bridged trinuclear cluster [Os(3)(CO)(10)[mu-1,4-eta(2)-(PPh)(4)CH(2)]] (4). The conversion of 1 into 4 can be achieved through the pyrolysis of a solution of 1. When 1 reacts with an equimolar amount of [Os(3)(CO)(10)(mu-H)(2)] at 100 degrees C in toluene, the 1,2,4-linked bis-trinuclear cluster [Os(3)(CO)(11)[mu(3)-1,2,4-eta(3)-(PPh)(4)CH(2)]Os(3)(CO)(8)(mu-H)(2)] (5) is obtained. When I reacts with a 2-fold molar amount of [Os(3)(CO)(10)(mu-H)(2)], the 1,2,3,4-linked bis-trinuclear hydride cluster [[Os(3)(CO)(8)(mu-H)(2)](2)[mu(4)-1,2,3,4-eta(4)-(PPh)(4)CH(2)]] (6) is obtained. Cluster 1 exists as two conformational isomers (1y and 1r) in the crystalline state, due to different conformational arrangements of pseudoaxial carbonyls in the cluster. Cluster 3 shows two interconvertible conformers (3y and 3r) due to the inversion of the configuration of the uncoordinated outer phosphorus atom, and a pair of enantiomers exists in 3r. All of the new compounds obtained have been characterized by spectroscopic and analytical techniques, and their structures have been established by X-ray crystallography.