Bridging CO Research Articles

The Umbrella-Shaped Trimethylenemethane Ligand in Iron Carbonyl Chemistry: Comparison with Butadiene and Cyclobutadiene Analogues

Reaction of Fe2(CO)9 with 3-chloro-2-(chloromethyl)propene is known to give the stable iron tricarbonyl complex [η4-(CH2)3C]Fe(CO)3, containing the umbrella-shaped trimethylenemethane ligand. The mononuclear complexes (CH2)3CFe(CO)n (n = 1–4) and the binuclear complexes [(CH2)3C]2Fe2(CO)n (n = 3–5) of this trimethylenemethane ligand have been studied here. The lowest energy [(CH2)3C]2Fe2(CO)5 structure has one bridging CO group and an Fe–Fe single bond but is disfavored with respect to dissociation into the mononuclear fragments [(CH2)3C]2Fe2(CO)3 + [(CH2)3C]2Fe2(CO)2. The lowest energy [(CH2)3C]2Fe2(CO)4 structure has two semibridging CO groups and an Fe═Fe double bond but is disfavored with respect to disproportionation into [(CH2)3C]2Fe2(CO)5 + [(CH2)3C]2Fe2(CO)3. In contrast to [(CH2)3C]2Fe2(CO)5 and [(CH2)3C]2Fe2(CO)4, the tricarbonyl [(CH2)3C]2Fe2(CO)3 appears to be a reasonable synthetic objective. The lowest energy structure has one terminal CO group, one semibridging CO group, one symmetrical bri...

Binuclear fluoroborylene (BF) cobalt carbonyls: Comparison with homoleptic cobalt carbonyls

The recent synthesis by Vidović and Aldridge of the complex (η5-C5H5)2Ru2(CO)4(μ-BF) containing the fluoroborylene ligand (BF), isoelectronic with CO, suggests the possibility of introducing the BF ligand into binuclear cobalt carbonyl derivatives of the type Co2(BF)2(CO)n (n=7, 6, 5, 4, 3). This has now been explored using density functional theory. Structures in which both BF groups bridge a cobalt–cobalt bond are energetically preferred for the Co2(BF)2(CO)n (n=6, 5, 4, 3) derivatives over other types of structures. Thus the Co2(μ-BF)2(CO)6 structure with two bridging BF groups lies more than 18kcal/mol below the next lowest Co2(BF)2(CO)6 structure and appears to be a reasonable synthetic objective. This differs from the well-known isoelectronic Co2(CO)8 for which the structures with two bridging CO groups and with all terminal CO groups are so close in energy that they are found experimentally to exist in equilibrium. Examples of Co2(B2F2)(CO)n (n=7, 4, 3) derivatives are found in which the two BF groups have coupled to form a bridging difluorodiborene (B2F2) ligand with a B–B distance of ∼1.9Å. However, the Co2(B2F2)(CO)7 structures do not appear to be viable since loss of a CO group to give Co2(BF)2(CO)6 is predicted to be an exothermic process.

Fluoroborylene ligands in binuclear ruthenium carbonyls: Comparison with their iron analogues

The qualitative aspects of the chemistry of the fluoroborylene iron and ruthenium carbonyls M(BF)(CO)m (m=4, 3) and M2(BF)2(CO)n (n=7, 6) (M=Fe, Ru) are predicted to be very similar. For Ru(BF)(CO)4 the trigonal bipyramidal structures with the BF group in an equatorial position and in an axial position are both found with the equatorially substituted isomer lying ∼2kcal/mol below the axially substituted isomer. For the coordinately unsaturated Ru(BF)(CO)3 both singlet and triplet structures are found derived from the equatorially substituted Ru(BF)(CO)4 structure by removal of an equatorial CO group. The structures for the binuclear derivatives Ru2(BF)2(CO)n (n=7, 6) provide examples of lower energy structures with bridging BF groups relative to similar structures with bridging CO groups. The lowest energy Ru2(BF)2(CO)7 structure has two bridging BF groups and one CO group and is qualitatively similar to the well-known Fe2(CO)9 structure with three bridging CO groups. All of the structures of the unsaturated Ru2(BF)2(CO)6 within 30kcal/mol of the global minimum have two bridging groups. The structure with one bridging CO group and one bridging BF group lies ∼16kcal/mol above the global minimum with two bridging BF groups again showing the preference for bridging BF groups over bridging CO groups. A triplet Ru2(BF)2(CO)6 structure with two bridging BF groups was also found but at the high energy of ∼26kcal/mol above that of the corresponding singlet global minimum. This singlet–triplet splitting is much larger than the singlet–triplet splitting of the corresponding Fe2(BF)2(CO)6.

Cu(I)/O2 Chemistry Using a β-Diketiminate Supporting Ligand Derived from N,N-Dimethylhydrazine: A [Cu3O2]3+ Complex with Novel Reactivity

A Cu(I) complex, LCu(CH(3)CN), was prepared and characterized, where L(-) is a sterically unencumbered β-diketiminate ligand, the deprotonated version of 4-(2,2-dimethylhydrazino)dimethylhydrazone-3-penten-2-one (LH). Analysis of FTIR spectra of the products of the reaction of LCu(CH(3)CN) with CO indicate that L(-) is strongly electron donating, and support an equilibrium in solution between monomeric and dimeric forms with terminal and bridging CO ligands, respectively. Low temperature oxygenation of LCu(CH(3)CN) generated a bis(μ-oxo)tricopper complex with a S = 1 [Cu(3)O(2)](3+) core that was identified on the basis of UV-vis (λ(max) (ε, M(-1) cm(-1) per Cu) = 328 (10700), 420 (1500), 590 (835) nm) and X-band electron paramagnetic resonance (EPR) spectroscopy (Δm(s) = 2 transition at 1500 G), electrospray ionization (ESI) mass spectrometry, and spectrophotometric titration (0.35(2) equiv of O(2) per copper atom), magnetic susceptibility (μ(eff) = 2.8(1) BM), and H(2)O(2) detection experiments (no H(2)O(2) evolved upon acidification). Unlike other reported variants supported by neutral N-donor ligands, L(3)Cu(3)O(2) is not reduced by ferrocene, does not abstract H-atoms from phenols or 1,2-dihydroanthracene, oxidizes PPh(3) to Ph(3)P═O, and generates carbonate species upon exposure to CO(2). This unique reactivity for a [Cu(3)O(2)](3+) complex may be traced to the anionic charge and strong electron donating characteristics of L(-).

Bonding in homoleptic iron carbonyl cluster cations: a combined infrared photodissociation spectroscopic and theoretical study

Infrared spectra of mass-selected homoleptic iron carbonyl cluster cations including mononuclear Fe(CO)5+ and Fe(CO)6+, dinuclear Fe2(CO)8+ and Fe2(CO)9+, and trinuclear Fe3(CO)12+ are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The structures are established by comparison of the experimental spectra with simulated spectra derived from density functional calculations. Only one IR band is observed for the Fe(CO)5+ cation, which is predicted to have a C4v structure. The Fe(CO)6+ cation is determined to be a weakly bound complex involving a Fe(CO)5+ core ion. In contrast to neutral clusters which have symmetric structures with two and three bridging carbonyl ligands, the dinuclear Fe2(CO)8+ and Fe2(CO)9+ cations are characterized to have unbridged asymmetric (OC)5Fe–Fe(CO)n+ (n = 3 and 4) structures. The trinuclear Fe3(CO)12+ cluster cation is determined to have an open chain like (OC)5Fe–Fe(CO)2–Fe(CO)5 structure instead of the triangular structure with two bridging CO groups for the Fe3(CO)12 neutral. The di- and trinuclear cluster cations all involve a square pyramid like Fe(CO)5 building block that satisfies the 18-electron configuration of this iron center. The Fe(CO)5 building block is isolobal to the CH3 fragment in hydrocarbon chemistry, the Fe2(CO)9+ and Fe3(CO)12+ cluster cations may be considered through isolobality to be metal carbonyl analogues of the ethyl and isopropyl cations.

A novel decanuclear Co(ii) cluster with adamantane-like metallic skeleton supported by 8-hydroxyquinoline and in situ formed CO32− anions

A novel decanuclear Co(II) cluster, [Co(10)(Q)(12)(μ(6)-CO(3))(4)]·2.5DMF (1) (Q = 8-hydroxyquinoline), has been synthesized under solvothermal conditions and characterized by powder XRD, TGA and IR spectroscopy. Single-crystal X-ray diffraction analysis shows that 1 represents a new type of decanuclear cobalt cluster with an approximate supertruncated tetrahedral shape coordinated with coexisted Q ligands and in situ formed μ(6)-bridging CO(3)(2-) anions. Meanwhile, 1 is the largest aggregate in metal-Q coordination chemistry. Another distinguishing feature of 1 is the adamantane-like metallic skeleton with point symbol {6(3)}(2){6}(3), which is observed in the polynuclear metal complexes for the first time. Furthermore, it is considered that the in situ formed CO(3)(2-) anions have a crucial effect on the formation of the resultant unexpected polynuclear structures. The magnetic studies show that antiferromagnetic (AF) interactions exist within 1.

Isolation, observation, and computational modeling of proposed intermediates in catalyticprotonreductions with the hydrogenase mimic Fe2(CO)6S2C6H4

Proposed electrocatalytic proton reduction intermediates of hydrogenase mimics were synthesized, observed, and studied computationally. A new mechanism for H(2) generation appears to involve Fe(2)(CO)(6)(1,2-S(2)C(6)H(4)) (3), the dianions {[1,2-S(2)C(6)H(4)][Fe(CO)(3)(μ-CO)Fe(CO)(2)](2-) (3(2-)), the bridging hydride {[1,2-S(2)C(6)H(4)][Fe(CO)(3)(μ-CO)(μ-H)Fe(CO)(2)]}(-), 3H(-)(bridging), and the terminal hydride 3H(-)(term-stag), {[1,2-S(2)C(6)H(4)][HFe(CO)(3)Fe(CO)(3)]}(-), as intermediates. The dimeric sodium derivative of 3(2-), {[Na(2)(THF)(OEt(2))(3)][3(2-)]}(2) (4) was isolated from reaction of Fe(2)(CO)(6)(1,2-S(2)C(6)H(4)) (3) with excess sodium and was characterized by X-ray crystallography. It possesses a bridging CO and an unsymmetrically bridging dithiolate ligand. Complex 4 reacts with 4 equiv. of triflic or benzoic acid (2 equiv. per Fe center) to generate H(2) and 3 in 75% and 60% yields, respectively. Reaction of 4 with 2 equiv. of benzoic acid generated two hydrides in a 1.7 : 1 ratio (by (1)H NMR spectroscopy). Chemical shift calculations on geometry optimized structures of possible hydride isomers strongly suggest that the main product, 3H(-)(bridging), possesses a bridging hydride ligand, while the minor product is a terminal hydride, 3H(-)(term-stag). Computational studies support a catalytic proton reduction mechanism involving a two-electron reduction of 3 that severs an Fe-S bond to generate a dangling thiolate and an electron rich Fe center. The latter iron center is the initial site of protonation, and this event is followed by protonation at the dangling thiolate to give the thiol thiolate [Fe(2)H(CO)(6)(1,2-SHSC(6)H(4))]. This species then undergoes an intramolecular acid-base reaction to form a dihydrogen complex that loses H(2) and regenerates 3.

Structural changes upon replacing carbonyl groups with thiocarbonyl groups in first row transition metal derivatives: new insights

The chemistry of metal thiocarbonyls is much more limited than that of metal carbonyls because of the instability of CS as a synthetic reagent. In view of the many gaps remaining in experimentally realized metal thiocarbonyl chemistry, theoretical studies using density functional methods have been used to explore the possible future scope of metal thiocarbonyl chemistry. This paper reviews such theoretical studies on binuclear metal carbonyl derivatives of the types M(2)(CS)(2)(CO)(n) and Cp(2)M(2)(CS)(2)(CO)(n) (Cp = η(5)-C(5)H(5); M = V through Ni) as well as the trinuclear and tetranuclear iron carbonyls Fe(3)(CS)(3)(CO)(n) (n = 9, 8, 7, 6) and Fe(4)(CS)(4)(CO)(n) (n = 12, 11, 10, 9). The substitution of one or two CO groups with CS groups to give less symmetrical structures leads to many more isomers. Structures in which a four-electron donor thiocarbonyl group uses its sulfur atom to bridge a metal-metal bond as a η(2)-μ-CS ligand are more favorable in binuclear metal thiocarbonyl chemistry than corresponding structures in metal carbonyl chemistry owing to the basicity of the sulfur atom. Six-electron donor thiocarbonyl groups bridging clusters of three or four iron atoms are also found in low-energy structures including a particularly favorable Fe(4)(CS)(4)(CO)(10) structure suggested as a possible target for future synthetic chemistry. In thiocarbonyl substitution products of simple binuclear metal carbonyls such as Fe(2)(CO)(9) [= Fe(2)(CO)(6)(μ-CO)(3)], Co(2)(CO)(8) [= Co(2)(CO)(6)(μ-CO)(2)], and Cp(2)Fe(2)(CO)(4) [= Cp(2)Fe(2)(CO)(2)(μ-CO)(2)], structures with bridging CS groups are invariably lower energy structures than isomeric structures with bridging CO groups.

Effect of the Calcination Temperature on the Catalyst Performance of ZrO<sub>2</sub>-Supported Cobalt for Fischer-Tropsch Synthesis

The effect of the calcination temperature on the catalyst performance of ZrO2-supported cobalt for Fischer-Tropsch synthesis is investigated. Results show that the size of the cobalt species particles of the ZrO2-supported cobalt catalysts increases and their reducibility is enhanced with increasing calcination temperature. In addition, the extent of CO linear absorption and bridge absorption peak increases and then decreases with increasing calcination temperature. The results from the Fishcer-Tropsch synthesis show that the CO conversion rate increases and then decreases as the calcination temperature is increased. Catalyst selectivity for C1 monotonically decreases, whereas that for C5+ increases. The changes in the CO conversion rate demonstrate a regularity consistent with the trend of the CO absorption peak extent. Meanwhile, the growth and enhanced reducibility of the cobalt species particles contribute to the generation of heavy hydrocarbons and explain the differences in product selectivity. Therefore, the appropriate calcination temperature facilitates an increase in the CO conversion rate of the ZrO2-supported cobalt catalysts and results in a better Fischer-Tropsch synthesis product selectivity.

Chemisorption-Induced 2D–3D–2D Structural Transitions in Gold Heptamer: (CO)nAu7– (n = 1–4)

CO chemisorption onto the Au7– cluster is investigated using photoelectron spectroscopy (PES) and ab initio calculations. It is found that CO binding can induce previously unreported 2D–3D–2D structural changes. The gold motif in the most stable structure of COAu7– is an intermediate between the two known stable 2D isomers of Au7–. Two minor isomers are observed in the PES of (CO)2Au7–; one is due to an unprecedented 3D Au7– species with Cs symmetry. This 3D Cs Au7 motif becomes a major isomer in (CO)3Au7–. The most stable isomers of COAu7– and (CO)2Au7– are planar with identical Au7 motifs; the stable planar isomers of (CO)3Au7– include not only the global-minimum structure of Au7– but also a planar hexagonal Au7 motif. The PES spectrum of (CO)4Au7– is markedly different, and its most stable structure consists of the global-minimum structure of Au7–, three terminal CO, and one bridging CO.

Adsorption and Oxidation of Carbon Monoxide on Pt/C, Pt3Co/C, and PtRu/C Catalysts Studied by In-Situ Attenuated Total Reflection Fourier-Transform Infrared

The adsorption and oxidation of CO on commercial nanoparticle catalysts supported on carbon black (Pt/C, Pt3Co/C, PtRu/C) were examined at 23, 40, and 60 degrees C in 0.1 M HClO4 by use of in situ ATR-FTIR (attenuated total reflection Fourier-transform infrared) spectroscopy. Absorption bands for the adsorbed CO assigned to linear (atop) CO (CO(L)) and bridge CO (CO(B)) were observed around 2040 cm(-1) and 1850 cm(-1), respectively, at high CO coverage theta(CO) close to 0.8 on all three types of catalysts. The adsorption rates of both CO(L) and CO(B) at the initial stage were found to decrease in the order Pt/C > Pt3Co/C > PtRu/C, indicating that the interaction of CO with PtRu is weakest. The adsorption of CO on these catalysts resulted in the growth of a sharp O-H stretching band around 3630 to 3640 cm(-1), which was assigned to non-hydrogen-bonded water molecules (isolated H2O) co-adsorbed with CO. For the electrooxidation reaction of CO, PtRu/C exhibited the highest activity at all temperatures. It was confirmed that the dominant factor for determining CO oxidation activity was the onset potential for the oxidation of isolated H2O, E(onset)(H2O), to provide an oxygen species that is consumed in either a Langmuir-Hinshelwood mechanism (Pt/C, Pt3Co/C) or the bi-functional mechanism (PtRu/C). In addition, PtRu/C exhibited the weakest Pt-CO interaction. The values of E(onset)(H2O) at PtRu/C were lowest among the three catalysts from 23 to 60 degrees C. With increasing temperature, the E(onset)(H2O) at Pt/C and Pt3Co/C shifted to less positive potential, resulting in increased CO oxidation activity, while the shift in E(onset)(H2O) at PtRu/C was relatively small.

Unsaturation in binuclear cyclopentadienyliron carbonyl thiocarbonyls: Four-electron donor bridging thiocarbonyl groups versus metal–metal multiple bonds

Theoretical studies on the binuclear cyclopentadienyliron carbonyl thiocarbonyl derivatives Cp2Fe2(CO)2(μ-CS)(μ-CO) and Cp2Fe2(CO)2(μ-CS)2 indicate that the trans and cis isomers are nearly degenerate in energy, consistent with experiment. Structures with bridging CS groups are of lower energy than corresponding structures with bridging CO groups. The corresponding unbridged Cp2Fe2(CS)(CO)3 and Cp2Fe2(CS)2(CO)2 isomers are predicted to lie 11 and 16kcal/mol, respectively, above their global minima, indicating increasing activation energies for the cis/trans interconversion as bridging CO groups are replaced by bridging CS groups. The unsaturated species Cp2Fe2(μ-CS)(μ-CO)2 and Cp2Fe2(μ-CS)2(μ-CO) are predicted to have triply bridged triplet spin state structures with FeFe double bonds of lengths 2.26Å, analogous to the experimentally known triplet (Me5C5)2Fe2(μ-CO)3. However, low-lying singlet Cp2Fe2(CS)(CO)2 and Cp2Fe2(CS)2(CO) structures with four-electron donor bridging η2-μ-CS groups and formal Fe–Fe single bonds are also found. The lowest lying Cp2Fe2(CS)(CO) and Cp2Fe2(CS)2 structures have two bridging groups and very short FeFe distances of ∼2.14Å, suggesting formal triple bonds. Several higher energy four-electron donor η2-μ-CS bridged structures are also found for Cp2Fe2(CS)(CO) and Cp2Fe2(CS)2. In addition, singlet and triplet structures are found for Cp2Fe2(CS)2 in which the two CS ligands have coupled to form a bridging SCCS group with a carbon–carbon bond. Only a η2-μ-CS bridged singlet structure is predicted for Cp2Fe2(CS), rather than the normal bridged structure with a FeFe quadruple bond such as that predicted for the carbonyl analog Cp2Fe2(CO).

Metal–Metal Bonding in Bis(alkylthio)hexacarbonyldicobalt Complexes: Open Structures vs. Butterfly and Tetrahedrane Structures

AbstractDensity functional calculations on the Co2(CO)6(SR)2 compounds (R = CH3, CF3) predict both open and butterfly structure types of similar energies. The open Co2(CO)6(μ‐SR)2 structures have non‐planar central Co2S2 units with two bridging RS groups and ca. 3.4 Å Co···Co distances indicating a lack of direct metal–metal bonding. The butterfly Co2(CO)5(μ‐SR)(μ‐CO)(SR) structures have direct Co–Co bonds of lengths ca. 2.5 Å forming the “body” of the butterfly, one terminal RS group, one bridging RS group, and one bridging CO group. The lowest energy Co2(CO)6(SCH3)2 structure is an open isomer. However, this open isomer lies only 0.4 kcal/mol below the corresponding butterfly isomer. For the corresponding fluorinated derivative Co2(CO)6(SCF3)2 a butterfly structure with a direct Co–Co bond and a bridging CO group lies at a slightly lower energy than the lowest energy open structures. The relative energy difference between open and butterfly Co2(CO)6(SCF3)2 structures is more than five times higher than for the Co2(CO)6(SCH3)2 structures. The electronegativity of the RS group in the Co2(CO)6(SR)2 structures has little effect on the geometric parameters but exerts a significant influence on the atomic charge distribution. The butterfly structures with a direct metal–metal bond are predicted to be nearly isoenergetic to open structures without a Co–Co bond. Unsuccessful attempts to optimize a previously proposed Co2(CO)6(μ‐η2:η2‐S2R2) structure with a central Co2S2 tetrahedrane unit with one Co–Co bond, one S–S bond, and four Co–S bonds are consistent with the previous reformulation of the originally claimed Co2S2 tetrahedranes Co2(CO)6[μ‐η2:η2‐S2(C6X5)2] (X = F, Cl) as the trinuclear derivatives Co3(μ3‐S)(C6F5)(CO)8.

Influence of electronic effects from bridging groups on synthetic reaction and thermally activated polymerization of bisphenol‐based benzoxazines

AbstractSix bis‐benzoxazines based on bisphenols with different bridging groups, C(CH3)2, CH2, O, CO, SO2, and single bond, were synthesized in toluene. The influence of electronic effects from bridging groups on ring‐forming reaction and thermal ring‐opening polymerization were relatively discussed in detail. Their structures were characterized by high‐performance liquid chromatography, Fourier transform infrared, 1H NMR, differential scanning calorimetry, and elementary analysis. The quantum chemistry parameters of the bisphenols and bis‐benzoxazines were calculated by molecular simulation. The results indicated that the electron‐withdrawing groups inhibited the synthetic reaction by decreasing the charge density of α‐Cs of bisphenols and increasing energy barriers of the synthetic reactions. However, the electron‐withdrawing groups promoted the thermally activated polymerization, which resulted from their activation energy and curing temperature decrease by increasing the bond length and lowering the bond energy of CO on oxazine rings. Besides, because of stronger electron‐withdrawing sulfone group, there were more arylamine methylene Mannich bridge structure in the polybenzoxazine. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Unraveling the Electronic Properties of the Photoinduced States of the H‐Cluster in the [FeFe] Hydrogenase from D. desulfuricans

Abstract[FeFe] hydrogenases belong to a class of enzymes that catalyze the reversible heterolytic splitting of molecular hydrogen. The structure of their active site is rather unusual consisting of a CN– and CO‐coordinated [2Fe] subcluster connected to a ferredoxin‐like [4Fe4S] subcluster through one of the Cys‐S ligands. The iron, distal to the “cubane” has an open coordination site that is believed to be the substrate binding site. In the active forms of the H‐cluster, this site can be inhibited by carbon monoxide, which results in an EPR‐active state, called Hox‐CO. At temperatures below 100 K, illumination in the visible range causes conversion of this state into two other EPR‐active states. One of these is the Hox state, i.e. the catalytically active form, characterized by a vacant external coordination site. The other state (HLI) is probably characterized by dissociation of the bridging CO ligand. The photodissociation of the Hox‐CO state provides a convenient way to study changes in the electronic structure caused by dissociation of various CO ligands. In this article, we show results of an advanced EPR study of the light‐induced species of the active site, in particular, HLI. On the basis of the resolved hyperfine signals of the 1H, 14N, 13C, and 57Fe nuclei, we obtained a detailed picture of the electronic structure of the H‐cluster. It turned out that dissociation of the bridging CO ligand causes a major rearrangement of the structure of the H‐cluster and large changes in the distribution of the unpaired spin. Moreover, it appears that the extent of spin delocalization in the 2Fe subcluster in the case of HLI is intermediate between that of Hox and Hox‐CO.

Refined solution structure and backbone dynamics of the archaeal MC1 protein

The 3D structure of methanogen chromosomal protein 1 (MC1), determined with heteronuclear NMR methods, agrees with its function in terms of the shape and nature of the binding surface, whereas the 3D structure determined with homonuclear NMR does not. The structure features five loops, which show a large distribution in the ensemble of 3D structures. Evidence for the fact that this distribution signifies internal mobility on the nanosecond time scale was provided by using (15)N-relaxation and molecular dynamics simulations. Structural variations of the arm (11 residues) induced large shape anisotropy variations on the nanosecond time scale that ruled out the use of the model-free formalism to analyze the relaxation data. The backbone dynamics analysis of MC1 was achieved by comparison with 20 ns molecular dynamics trajectories. Two β-bulges showed that hydrogen bond formation correlated with ϕ and ψ dihedral angle transitions. These jumps were observed on the nanosecond time scale, in agreement with a large decrease in (15)N-NOE for Gly17 and Ile89. One water molecule bridging NH(Glu87) and CO(Val57) through hydrogen bonding contributed to these dynamics. Nanosecond slow motions observed in loops LP3 (35-42) and LP5 (67-77) reflected the lack of stable hydrogen bonds, whereas the other loops, LP1 (10-14), LP2 (22-24), and LP4 (50-53), were stabilized by several hydrogen bonds. Dynamics are often directly related to function. Our data strongly suggest that residues belonging to the flexible regions of MC1 could be involved in the interaction with DNA.

Boronyl Ligand as a Member of the Isoelectronic Series BO− → CO → NO+: Viable Cobalt Carbonyl Boronyl Derivatives?

Recently the first boronyl (oxoboryl) complex [(c-C(6)H(11))(3)P](2)Pt(BO)Br was synthesized. The boronyl ligand in this complex is a member of the isoelectronic series BO(-) → CO → NO(+). The cobalt carbonyl boronyls Co(BO)(CO)(4) and Co(2)(BO)(2)(CO)(7), with cobalt in the formal d(8) +1 oxidation state, are thus isoelectronic with the familiar homoleptic iron carbonyls Fe(CO)(5) and Fe(2)(CO)(9). Density functional theory predicts Co(BO)(CO)(4) to have a trigonal bipyramidal structure with the BO group in an axial position. The tricarbonyl Co(BO)(CO)(3) is predicted to have a distorted square planar structure, similar to those of other 16-electron complexes of d(8) transition metals. Higher energy Co(BO)(CO)(n) (n = 3, 2) structures may be derived by removal of one (for n = 3) or two (for n = 2) CO groups from a trigonal bipyramidal Co(BO)(CO)(4) structure. Structures with a CO group bridging 17-electron Co(CO)(4) and Co(BO)(2)(CO)(3) units and no Co-Co bond are found for Co(2)(BO)(2)(CO)(8). However, Co(2)(BO)(2)(CO)(8) is not viable because of the predicted exothermic loss of CO to give Co(2)(BO)(2)(CO)(7). The lowest lying Co(2)(BO)(2)(CO)(7) structure is a triply bridged (2BO + CO) structure closely related to the experimental Fe(2)(CO)(9) structure. However, other relatively low energy Co(2)(BO)(2)(CO)(7) structures are found, either with a single CO bridge, similar to the experimental Os(2)(CO)(8)(μ-CO) structure; or with 17-electron Co(CO)(4) and Co(BO)(2)(CO)(3) units joined by a single Co-Co bond with or without semibridging carbonyl groups. Both triplet and singlet Co(2)(BO)(2)(CO)(6) structures are found. The lowest lying triplet Co(2)(BO)(2)(CO)(6) structures have a Co(CO)(3)(BO)(2) unit coordinated to a Co(CO)(3) unit through the oxygen atoms of the boronyl groups with a non-bonding ∼4.3 Å Co···Co distance. The lowest lying singlet Co(2)(BO)(2)(CO)(6) structures have either two three-electron donor bridging η(2)-μ-BO groups and no Co···Co bond or one such three-electron donor BO group and a formal Co-Co single bond.

A Synthesis of Carbasugar−Sugar Pseudodisaccharides via a Cycloaddition−Cycloreversion Reaction of 2H-Pyran-2-ones

Cycloaddition of 3-carbomethoxy-2H-pyran-2-one to a vinylated sugar followed by the loss of bridging CO(2) from the cycloadduct affords a cyclohexadiene which can be manipulated to a carbasugar-sugar pseudodisaccharide.

The effects of the dimethylether bridging moiety in the H-cluster of the Clostridium pasteurianum hydrogenase on the mechanism of H 2 production: A quantum mechanics/molecular mechanics study

[Fe–Fe]-hydrogenases are naturally occurring metalloenzymes that catalyze the reversible production of H 2 from two protons and two electrons. [Fe–Fe]-hydrogenases found in two species – Clostridium pasteurianum (CpI) and Desulfovibrio desulfuricans (DdH) – were shown with x-ray crystallography to have active sites that are very similar, although several atoms that bridge the dithiolate ligand were unresolved. In earlier work, we employed density functional theory (DFT) within a QM/MM method to investigate two previously proposed mechanisms of hydrogen production by DdH and CpI hydrogenases. In one mechanism (I), a CO ligand bridging two Fe atoms in the active site rotates to a terminal position while in the other (II) the CO bridge remains intact throughout the catalytic cycle. We previously assumed that the active sites for the two hydrogenases were identical; each had a dimethylamine bridging moiety, whose basicity is important for Mechanism II. Our overall conclusion, taking into consideration an energy comparison for the two mechanisms and activation energies for the CO-unbridging step in Mechanism I, was that Mechanism II was favored for both hydrogenases. In this paper, we extend our previous work to show that Mechanism II is favored over Mechanism I even if the bridging moiety in CpI hydrogenase is dimethylether, a significantly weaker base than dimethylamine, providing further support for Mechanism II even though experimental verification of the bridging moiety for the CpI H-cluster is lacking.

Interactions between interfacial water and CO adsorbed on Pt and Pt–Ru alloy surfaces under electrochemical conditions: Density-functional theory study

The structural and electronic properties of interfacial water and adsorbed CO on platinum and platinum/ruthenium alloy have been studied via density-functional theory calculations to gain insight into the water–adsorbate interaction under electrochemical conditions. The computational simulations reveal a new interpretation for the interaction of adsorbed CO and water at the electrochemical interfaces. The new interaction model rationalizes the observed quantitative relationship between infrared intensities for adsorbed bridging CO and water molecules that impart a high-frequency O–H stretch, ca. 3630–3660 cm −1 on pure Pt and 3600–3620 cm −1 on PtRu alloy. The theoretical modeling indicates that the observed feature common to both pure Pt and PtRu alloy surfaces is due to interfacial water molecules firmly hydrogen-bonded to bridging CO.