Terminal CO Groups Research Articles

Trinuclear and Tetranuclear Ruthenium Carbonyl Nitrosyls: Oxidation of a Carbonyl Ligand by an Adjacent Nitrosyl Ligand.

Trinuclear and tetranuclear ruthenium carbonyls of the types Ru3(CO)n(NO)2, Ru3(N)(CO)n(NO), Ru3(N)2(CO)n, Ru3(N)(CO)n(NCO), Ru3(CO)n(NCO)(NO), Ru4(N)(CO)n(NO), Ru4(N)(CO)n(NCO), and Ru4(N)2(CO)n related to species observed experimentally in the chemistry of Ru3(CO)10(µ-NO)2 have been investigated using density functional theory. In all cases, the experimentally observed structures have been found to be low-energy structures. The low-energy trinuclear structures typically have a central strongly bent Ru-Ru-Ru chain with terminal CO groups and bridging nitrosyl, isocyanate, and/or nitride ligands across the end of the chain. The low-energy tetranuclear structures typically have a central Ru4N unit with terminal CO groups and a non-bonded pair of ruthenium atoms bridged by a nitrosyl or isocyanate group.

Metalloenzyme mimic: diironhexacarbonyl cluster coupled to redox-active 4-mercapto-1,8-naphthalic anhydride ligands

The non-innocent redox-active ligand, 4-mercapto-1,8-naphthalic anhydride (HS-NAH), has been used in the design and synthesis of a diironhexacarbonyl complex, [μ-(S-NAH)2Fe2(CO)6]. [μ-(S-NAH)2Fe2(CO)6] has been characterized by spectroscopic methods and cyclic voltammetry. Infrared spectrum of [μ-(S-NAH)2Fe2(CO)6] displays peaks corresponding to terminal metal CO groups (2081, 2044, 2006 cm−1) and peaks assigned to C=O of the naphthalic anhydrides (1780, 1740 cm−1). Electrochemical measurements of [μ-(S-NAH)2Fe2(CO)6] feature redox events that are metal-based (irreversible, Epc: − 1.13 V, − 1.60 V vs Fc/Fc+) and naphthalic anhydride centered (partially chemically reversible, Epc: − 1.99 V; Epa: − 1.90 V vs Fc/Fc+). Cyclic voltammetric analysis of [μ-(S-NAH)2Fe2(CO)6] in the presence of acetic acid show that the complex mimics the active site of [Fe–Fe]-hydrogenase by catalyzing the electrochemical reduction of protons to hydrogen with an overpotential of − 0.67 V. The bi-functional model, [μ-(S-NAH)2Fe2(CO)6], exhibits electronic coupling with synergistic metal–ligand interactions leading to transformation of protons to molecular hydrogen.

Bridging cyclobutadiene ligands with agostic hydrogen atoms in binuclear chromium carbonyl derivatives

The binuclear cyclobutadiene chromium carbonyls (C4H4)2Cr2(CO)n (n = 7, 6, 5, 4, 3), potentially accessible from the experimentally known (η4-C4H4)Cr(CO)4, have been examined by density functional theory. The low-energy structures for the heptacarbonyl (C4H4)2Cr2(CO)7 have terminal (η4-C4H4)2Cr2(CO)7 rings with a central Cr–Cr single bond bridged by one or three CO groups and the remaining CO groups as terminal ligands evenly distributed among the chromium atoms. The lowest energy structure for the hexacarbonyl (C4H4)2Cr2(CO)6 has a (η4-C4H4)2Cr(CO)2 complex functioning as a bidentate chelating ligand to a Cr(CO)4 fragment through the formation of two three-center C–H–Cr bonds with agostic hydrogen atoms exhibiting short Cr–H distances of ∼1.85 Å. The pentacarbonyl (η4-C4H4)2Cr2(CO)5 resembles the analogous experimentally known (η5-C5H5)2V2(CO)5 by having terminal η4-C4H4 rings, two bridging CO groups, and a central CrCr triple bond. The three lowest energy structures for the tetracarbonyl (η4-C4H4)2Cr2(CO)4 are singlet structures with two four-electron donor bridging η2-μ-CO groups, a terminal CO group on each chromium atom, and a central CrCr double bond. The lowest energy structure for the more highly unsaturated tricarbonyl (η4-C4H4)2Cr2(CO)3 by a margin of more than 12 kcal/mol is a triplet spin state structure with a terminal η4-C4H4 ring, a bridging η4,η1-C4H4 ring through an agostic C–H–Cr interaction, and a central CrCr triple bond. Thermochemistry suggests that the lowest energy structures for all of the (C4H4)2Cr2(CO)n (n = 7, 6, 5, 4) systems, especially the pentacarbonyl, are viable towards carbonyl dissociation.

The role of surface N[sbnd]H groups on the selective hydrogenation of cinnamaldehyde over Co/BN catalysts

The surface chemistry of catalyst plays a vital role in the catalytic process. And the effects of surface groups of porous boron nitride (p-BN) on the hydrogenation reaction have not been studied in detailed. In this work, two main surface species of Co/p-BN catalyst were tuned thermally and their roles on the hydrogenation reaction were discriminated by α, β-unsaturated aldehyde hydrogenation reaction. The surface BO content decreased at higher reduction temperature and the cobalt phase changed from CoO to the mixture of CoO and Co0. The Co/p-BN-500 exhibited the highest selectivity to CO hydrogenation. And its turnover frequency (TOF) of 13.2 h−1 is close to the optimal value of reported cobalt catalysts. When the reaction proceeds, hydrogenation rate of CO increases but that of CC decreases gradually, thus a surface groups variation was expected at the initial period. Further in-situ FTIR spectra showed the band intensity of edge NH increases as the adsorption proceeds at 50°С but it decreases as the temperature rise to 200°С, in combined with the shift of CO adsorption peak, a intermolecular hydrogen bond between edge NH and the terminal CO group was suggested, which account for the better selectivity to CO than CC.

Synthesis and characterization of triruthenium carbonyl incorporating 4-pyridones as potential antitumor agents

Four new organometallic triruthenium carbonyl incorporating 4-pyridones: [Ru3(CO)8(2L-2H)] (1, L = N-benzyl-3-hydroxy-2-methyl-4-pyridone; 2, L = 3-hydroxy-2-methyl-N-phenyl-4-pyridone; 3, L = 3-hydroxy-N,2-dimethyl-4-pyridone; 4, L = 2-methyl-4-pyridone) were synthesized and characterized. Proton NMR spectrum of 1 shows doublets for the two-nonequivalent benzylic hydrogens due to restricted rotation around C-N bond upon bonding to Ru(2)---Ru(3) as opposed to a singlet in the unbound N-benzyl-3-hydroxy-2-methyl-4-pyridone. Compounds 1 and 3 were characterized by X-ray diffraction analyses. Compounds 1 and 3 consists of two ruthenium-ruthenium bonds Ru(1)-Ru(2), Ru(1)-Ru(3) and a non-bonded separation of two ruthenium atoms defined by Ru(2) … Ru(3), which is bridged by two 3 electron donar oxygen atoms through two-pyridinonato ligands above and below triruthenium plane, and 8 terminal CO groups. Atoms Ru(2) and Ru(3) ligated by two CO ligands each and Ru(1) with four. Antitumor activity of the compounds 1–4 has been evaluated against five types of human cancer cell lines. Compounds 1 and 2 show excellent antitumor activity due to lipophilic nature of N-substituted arene pyridinonato ligands than compounds 3 and 4. Structure-activity studies show that there is a correlation between the N-substituted arene pyridinonato ligands and an increase in activity following the order 1 > 2>3 > 4. The anticancer activity of compounds 1 and 2 is between 1–2 orders of magnitude more active than cisplatin. This is possibly due to substitution of CO ligands at Ru(1) by transferrin delivery vehicle transporting to cancer cells and lipophilic N-substituted arenes diffuse through lipophic membrane leading to apoptosis and other carbonyl groups under physiological conditions likely function as carbonyl releasing molecules (CORMs).

Binuclear vanadium dimethylphosphino carbonyls: vanadium-vanadium multiple bonds and four-electron donor carbonyl groups as structural features in unsaturated systems

Abstract The structures and energetics of the binuclear dimethylphosphido vanadium carbonyls (Me2P)2V2(CO)n (n = 10, 9, 8, 7, 6) have been examined using density functional theory. The experimentally known (Me2P)2V2(CO)8 with two bridging Me2P groups and all terminal CO groups is found to be the lowest energy structure by a substantial margin. The predicted V V distance in this structure of 2.696 A is close to the experimental distance of 2.733 A as determined by X-ray crystallography. The Wiberg bond index (WBI) of 0.94 supports the formulation of this V V interaction as a formal double bond, consistent with the 18-electron rule. Decarbonylation of (Me2P)2V2(CO)8 is predicted to give a singly CO-bridged (Me2P)2V2(CO)6(µ-CO) structure with a shorter V V distance of 2.374 A corresponding to a WBI of 1.30 and suggesting a formal triple bond. Further decarbonylation to (Me2P)2V2(CO)6 is predicted to lead to structures with four-electron donor bridging η2-µ-CO groups rather than structures with formal V V quadruple bonds. For the carbonyl-richer system (Me2P)2V2(CO)9 the lowest energy structure retains two bridging Me2P ligands and all terminal CO groups. Its V V distance of 3.138 A coupled with a WBI value of 0.23 suggests the formal single bond required by the 18-electron rule. The lowest energy (Me2P)2V2(CO)10 structure has a similar V V single bond. However, only one of its Me2P ligands bridges the central V V bond; the other Me2P group is a terminal ligand bonded to a single vanadium atom. Structures with intact Me2P-PMe2 ligands having direct P P bonds are very high energy structures relative to their isomers with two separate Me2P units lacking a direct P P bond.

Binuclear pentalene titanium carbonyls involved in the deoxygenation of carbon dioxide

The complete series of binuclear pentalene titanium carbonyls Pn2Ti2(CO)n and Pn†2Ti2(CO)n (Pn = pentalene; Pn† = 1,4-(iPr3Si)2C8H4; n = 0, 1, 2, 3, 4), including Pn†2Ti2(CO)2 obtained experimentally from Pn†2Ti2 and carbon dioxide, have been investigated by density functional theory. For the carbonyl-free Pn2Ti2 system the singlet structure is found to lie 22.5 and 18.5 kcal/mol in energy below the isomeric triplet and quintet structures, respectively. However, for the experimentally known Pn†2Ti2 system (Pn† = 1,3-(iPr3Si)2C8H4) the singlet, triplet, and quintet spin state structures have essentially equal energies within 0.3 kcal/mol suggesting the possibility of complicated magnetic behavior. The Ti–Ti distance of 2.399 Å found in crystalline Pn†2Ti2 by X-ray crystallography is closest to the 2.415 Å Ti-Ti distance predicted for the triplet spin state structure. Carbonylation of Pn2Ti2 results in successive lengthening of the Ti-Ti distance as CO groups are added. Three CO groups are the maximum number that can be introduced into a viable Pn2Ti2(CO)n derivative as indicated by the predicted exothermic CO dissociation energy of the tetracarbonyl Pn2Ti2(CO)4. The lowest energy structure for the monocarbonyl Pn2†Ti2(CO) has a four-electron donor bridging η2-μ-CO group in accord with the experimental structure. The lowest energy structure for the dicarbonyl Pn†2Ti2(CO)2 has exclusively terminal CO groups in a cis configuration, again in accord with the experimental Pn†2Ti2(CO)2 structure. Two terminal CO groups are accompanied by a third highly unsymmetrical semibridging CO group in the lowest energy structures for the tricarbonyls Pn2Ti2(CO)3 and Pn†2Ti2(CO)3. However, the lowest energy structures for the tetracarbonyls Pn2Ti2(CO)4 and Pn†2Ti2(CO)4 have exclusively terminal CO groups.

Binuclear chromium carbonyl complexes of the highly basic small bite bidentate diphosphine bis(dimethylphosphino)methane

The structure and thermochemistry of the triads of binuclear chromium carbonyl complexes [(Me2P)2CH2]Cr2(CO)n (n=10, 9, 8), [(Me2P)2CH2]2Cr2(CO)n (n=8, 7, 6) and [(Me2P)2CH2]3Cr2(CO)n (n=6, 5, 4) have been investigated using density functional theory. The lowest-energy structures of the carbonyl-richest triad members [(Me2P)2CH2]Cr2(CO)10, [(Me2P)2CH2]2Cr2(CO)8, and [(Me2P)2CH2]3Cr2(CO)6 have long Cr⋯Cr distances indicating the absence of direct chromium bonds. The lowest energy structures of the intermediate triad members [(Me2P)2CH2]2Cr2(CO)7 and [(Me2P)2CH2]3Cr2(CO)5 systems have Cr–Cr single bonds of lengths of 3.13 and 3.39Å, respectively, and intact diphosphine ligands. However, in the lowest energy [(Me2P)2CH2]Cr2(CO)9 structure, one of the terminal CO groups is converted into a four-electron donor bridging η2-μ-CO group, retaining a long non-bonding Cr⋯Cr distance of 4.80Å. The lowest energy structure of the carbonyl-poorest [(Me2P)2CH2]Cr2(CO)8, [(Me2P)2CH2]2Cr2(CO)6, and [(Me2P)2CH2]3Cr2(CO)4 members of each triad show different features. In the lowest energy [(Me2P)2CH2]Cr2(CO)8 structure the ligand splits into separate Me2P and Me2PCH2 units, both of which bridge a Cr–Cr bond of length 3.06Å. The lowest energy [(Me2P)2CH2]2Cr2(CO)6 structure has a four-electron donor bridging η2-μ-CO group and a Cr–Cr single bond of length 2.91Å. The lowest energy [(Me2P)2CH2]3Cr2(CO)4 structure is a triplet structure with a Cr–Cr single bond of length 3.03Å.

Metal-metal multiple bonds with “half-bond” components in paramagnetic organometallics of f-block metals: Cyclopentadienyluranium carbonyls as molecular relatives of diuranium

The experimentally observed reductive CO coupling in organouranium systems observed by Cloke and co-workers makes of interest the exploration of the structures and thermochemistry of simple Cp2U2(CO)n (n = 5, 4, 3, 2) derivatives. In this connection, the low-energy Cp2U2(CO)n (n = 2, 3, 4) structures contain two to four bridging η2-μ-CO groups supplemented, in some cases, by one or two terminal CO groups. No examples of CO coupling to form C2O2 ligands are found in any of these Cp2U2(CO)n structures in contrast to the previously studied Cp2Th2(CO)4 system in which CO coupling to form C2O2 ligands is observed. The retention of uranium valence electrons in low-energy Cp2U2(CO)n (n = 3, 4) structures, even after partial donation of uranium electrons to reduce Cp to Cp– and CO to CO2-, leads to a rich and complicated variety of UU bonding modes as well as spin states from singlet to quintet. Of particular interest are uranium-uranium multiple bonds containing multiple single electron half-bond components in triplet and quintet spin state Cp2U2(CO)n structures. The formal uranium-uranium multiple bonds in such Cp2U2(CO)n structures exhibit some of the features of the quintuple bond of diuranium, U2, in which a typical σ + 2π triple bond similar to the CC triple bond of acetylene is supplemented by no less than four separate one-electron half-bonds. Thus the UU bonds in the lowest energy Cp2U2(CO)n (n = 2, 3) structures can be interpreted as formal triple bonds of various types with unusually short bond distances of ∼2.5 Å and Wiberg bond indices of ∼3.

Iron-Carbonyl Aqueous Vesicles (MCsomes) by Hydration of [Fe(CO){CO(CH2)5CH3}(Cp)(PPh3)] (FpC6): Highly Integrated Colloids with Aggregation-Induced Self-Enhanced IR Absorption (AI-SEIRA).

Self-assembly of hydrophobic molecules into aqueous colloids contradicts common chemical intuition, but has been achieved through hydration of [Fe(CO){CO(CH2)5CH3}(Cp)(PPh3)] (FpC6). FpC6 has no surface activity, no NMR signals in D2O and no critical aggregation concentration (CAC) in H2O. The molecule, however, contains both acyl and terminal CO groups that are prone to being hydrated. By adding water to a solution in THF, self-assembly of FpC6 can be initiated through water-carbonyl interactions (WCIs) with the highly polarized acyl CO groups. This aggregation subsequently enhances the hydration of the acyl CO groups and also induces the WCI of otherwise unhydrated terminal CO groups. The resultant metal-carbonyl aggregates have been proved to be bilayer vesicles with iron complexes exposed towards water and alkyl chains forming inner walls (MCsomes). These MCsomes show high structure integration upon dilution due to the hydrophobic nature of the building blocks. The highly polarized CO groups on the surface of the MCsomes result in a negative zeta potential (-65 mV) and create a local electric field, which significantly enhances the IR absorption of CO groups by more than 100-fold. This is the first discovery of aggregation-induced self-enhanced IR absorption (AI-SRIRA) without the assistant of external dielectric substrates. Highly integrated MCsomes are, therefore, promising as a novel group of materials, for example, for IR-based sensing and imaging.

Influence of the pretreatment conditions on the development and performance of active sites of Pt/TiO2 catalysts used for the selective citral hydrogenation

The influence of pretreatment conditions of Pt/TiO2 catalysts was analyzed by different techniques and changes in the textural and chemical properties correlated with the catalytic performance in citral hydrogenation. After H2 pretreatment, the formation of two-dimensional Pt nanoparticles and oxygen vacancies favors the diffusion of Pt species into the TiO2 structure, while three-dimensional Pt crystallites are formed after He pretreatment. The coupled effect of Pt 2-D nanoparticles and oxygen vacancies strongly enhances the catalytic activity and the selective hydrogenation of citral to unsaturated alcohols by favoring the citral adsorption on the reduced surface through the terminal CO groups in an environment rich in hydrogen provided by the highly dispersed Pt nanoparticles. These coupled metal-vacancy active sites are formed also after a combined He/H2 pretreatment; nevertheless, the formation of three-dimensional Pt particles during the previous He treatment strongly reduce the extent of TiO2–Pt interface and consequently the surface site concentration and the catalytic activity.

Homoleptic tetranuclear rhodium carbonyls: comparison with their iridium analogues.

Density functional theory confirms the experimentally known triply bridged Rh4(CO)9(μ-CO)3 structure to be the lowest-energy structure. The lowest-energy structures of the unsaturated systems Rh4(CO)n (n = 11, 10, 9, 8) are also triply bridged structures with central Rh4 tetrahedra that can be derived from this Rh4(CO)9(μ-CO)3 structure by removal of terminal CO groups in various ways. The M-M distances in these central M4 tetrahedra change very little as CO groups are lost, suggesting reluctance to form metal-metal multiple bonds in these unsaturated systems. The natural bond orbital (NBO) Wiberg bond indices provide depth to this analysis. All of these unsaturated systems are predicted to be highly fluxional, as two to three isomeric structures lie within ∼4 kcal/mol of the global minima. The Rh4(CO)8(μ-CO)2(μ4-CO) structure analogous to the lowest-energy Co4(CO)11 structure with all four atoms of a central Co4 butterfly bridged by a μ-CO group is predicted to lie ∼6 kcal/mol in energy above the lowest-energy Rh4(CO)11 structure. Comparisons of the relative energies of analogous Rh4(CO)n and Ir4(CO)n structures indicate that more highly bridged M4(CO)n structures are energetically much more favorable for rhodium than for iridium. Dissociation energies (for loss of CO) and disproportionation energies are reported.

New titanium carbonyls: Ti2(CO)10, Ti2(CO)11, and Ti2(CO)12.

Structural and energetic features of the binuclear titanium carbonyls Ti2(CO)n (n = 12, 11, 10) have been examined using density functional theory. The lowest-energy Ti2(CO)12 structure is a singlet structure consisting of two Ti(CO)6 units linked by Ti═Ti double bonds of lengths 3.0-3.2 Å. A similar slightly higher energy triplet Ti2(CO)12 structure is found with longer Ti-Ti bonds (3.37-3.62 Å), considered to be formal single bonds. The energy required for the dissociation of Ti2(CO)12 into two Ti(CO)6 fragments is 18.5 ± 2 kcal/mol higher than the energy required for the dissociation of V2(CO)12 into two V(CO)6 fragments. For the unsaturated Ti2(CO)11 system, the lowest-energy structures contain a four-electron-donor bridging η(2)-μ-CO group and 10 terminal CO groups with formal Ti═Ti double bonds in the singlet structures and formal Ti-Ti single bonds in the triplet structures. Similarly, the most favored geometries for the more highly unsaturated Ti2(CO)10 contain two four-electron-donor bridging η(2)-μ-CO groups with formal Ti═Ti double bonds for the singlet structures and formal Ti-Ti single bonds for the triplet structures. Higher-energy triplet Ti2(CO)10 structures are found with two or three two-electron-donor semibridging CO groups and formal Ti≡Ti triple bonds of length 2.7-2.8 Å.

The diversity of iron−sulfur bonding in binuclear iron carbonyl sulfides

The potential accessibility of Fe2S(CO)n derivatives with 1:2 sulfur to iron ratios by the decarboxylation of iron carbonyl thionyls has led to their investigation using density functional theory. The lowest energy Fe2S(CO)n (n = 8, 7, 6) structures are predicted to be singlet structures with all terminal CO groups, a bridging sulfur atom, and a formal Fe–Fe single bond of length ∼2.5 Å. The Fe−S distances in these structures shorten from ∼2.3 to ∼2.1 Å as CO groups are lost, suggesting an increase in the formal Fe−S bond orders. The thermochemistry of CO dissociation suggests that both Fe2S(CO)8 and Fe2S(CO)7 are viable synthetic objectives. A similar density functional theory study of Fe2S2(CO)n derivatives (n = 7, 6, 5) finds the experimentally known Fe2S2(CO)7 structure with a bridging S2CO group and the Fe2S2(CO)6 structure with a bridging disulfide ligand to be the lowest energy structures by substantial margins of ∼17 and ∼21 kcal/mol, respectively. The low-energy structures for the unsaturated Fe2S2(CO)5 are derived from the low-energy Fe2S2(CO)6 structures by loss of a CO group in various ways with relatively little change in the underlying Fe2S2 framework.

Effects of hetero atoms in the preferred structures of binuclear cyclopentadienyl manganese carbonyl analogues

The complete series of (C4H4E)2Mn2(CO)n derivatives (E=N, P, As, Sb, Bi; n=5, 4, 3) has been examined by density functional theory in order to ascertain the effect of the basic properties of the heteroatom E on the lowest energy structures. For the pentacarbonyls (C4H4E)2Mn2(CO)5 the lowest energy structures for all but the phospholyl derivative are the same as that for the cyclopentadienyl derivative (C5H5)2Mn2(CO)5, namely structures with terminal rings, a single bridging CO group, and an MnMn single bond of lengths ranging from 2.84 to 2.88Å. The phospholyl derivative (C4H4P)2Mn2(CO)5 has a different type of structure with a bridging η5,η1-μ-C4H4P ring, all terminal CO groups, and a long Mn–Mn distance of 4.35Å indicating the lack of a bond. The lowest energy structures for the tetracarbonyls (C4H4E)2Mn2(CO)4 of all five group 15 elements (E=N, P, As, Sb, Bi) have two such bridging η5,η1-C4H4E rings, all terminal CO groups, and no MnMn bond. However, the lowest energy structures for the tricarbonyls (C4H4E)2Mn2(CO)3 are the same as that for the cyclopentadienyl derivative (C5H5)2Mn2(CO)3, namely structures with terminal rings, three bridging CO groups, and a formal MnMn triple bond of length ~2.17Å.

Metal chains versus metal triangles in trinuclear trimethylenemethane iron carbonyls related to Fe3(CO)12

The trimethylenemethane ligand is found in the experimentally known and structurally characterized mononuclear iron carbonyl derivative (CH2)3CFe(CO)3, which may be regarded as a substitution product of Fe(CO)5. The structures of trinuclear trimethylenemethane iron carbonyl derivatives similarly related to the well-known doubly bridged Fe3(CO)12 structure have now been investigated by density functional theory. The lowest energy [(CH2)3C]Fe3(CO)10 structures likewise have two CO bridges and can be derived from the known Fe3(CO)12 structure by replacement of two terminal CO groups with either an unsymmetrical edge-bridging or face-bridging trimethylene methane ligand while preserving the central Fe3 triangle. However, replacing four CO groups in Fe3(CO)12 with two trimethylenemethane ligands to give [(CH2)3C]2Fe3(CO)8 is predicted unexpectedly to give a very different type of structure. In this structure one edge of the Fe3 triangle in Fe3(CO)12 is broken to form a bent Fe–Fe–Fe chain with the trimethylenemethane ligands bonded to the end iron atoms of this triiron chain. This [(CH2)3C]2Fe3(CO)8 structure appears to be very favorable, since it lies ∼22kcal/mol below alternative structures. We anticipate that both [(CH2)3C]Fe3(CO)10 and [(CH2)3C]2Fe3(CO)8 should be sufficiently stable to be synthesized from the known (CH2)3CFe(CO)3 by thermolysis or photolysis with suitable iron carbonyls.

Surface Chemistry on Small Ruthenium Nanoparticles: Evidence for Site Selective Reactions and Influence of Ligands

The reactivity of two classes of ruthenium nanoparticles (Ru NPs) of small size, either sterically stabilized by a polymer (polyvinylpyrrolidone, PVP) or electronically stabilized by a ligand (bisdiphenylphosphinobutane, dppb) was tested towards standard reactions, namely CO oxidation, CO2 reduction and styrene hydrogenation. The aim of the work was to identify the sites of reactivity on the nanoparticles and to study how the presence of ancillary ligands can influence the course of these catalytic reactions by using NMR and IR spectroscopies. It was found that CO oxidation proceeds at room temperature (RT) on Ru NPs but that the system deactivates rapidly in the absence of ligands because of the formation of RuO2. In the presence of ligands, the reaction involves exclusively the bridging CO groups and no bulk oxidation is observed at RT under catalytic conditions. The reverse reaction, CO2 reduction, is achieved at 120 °C in the presence of H2 and leads to CO, which coordinates exclusively in a bridging mode, hence evidencing the competition between hydrides and CO for coordination on Ru NPs. The effect of ligands localized on the surface is also evidenced in catalytic reactions. Thus, styrene is slowly hydrogenated at RT by the two systems Ru/PVP and Ru/dppb, first into ethylbenzene and then into ethylcyclohexane. Selectively poisoning the nanoparticles with bridging CO groups leads to catalysts that are only able to reduce the vinyl group of styrene whereas a full poisoning with both terminal and bridging CO groups leads to inactive catalysts. These results are interpreted in terms of location of the ligands on the particles surface, and evidence site selectivity for both CO oxidation and arene hydrogenation.

A binuclear trimethylenemethane cobalt carbonyl providing the first example of a low-energy perpendicular structure with acyclic hydrocarbon ligands

Density functional theory predicts an unusual perpendicular structure for [μ-(CH2)3C]2Co2(CO)2with bridging η4-TMM ligands, terminal CO groups, and a relatively short Co–Co distance of ∼2.29 Å.

Binuclear butadiene chromium carbonyls: Comparison with their trimethylenemethane isomers

Butadiene is known to form a chromium carbonyl complex (η4-C4H6)Cr(CO)4, which has been structurally characterized by X-ray crystallography. The structures and energetics of the butadiene chromium carbonyl complexes (C4H6)Cr(CO)m (m=5, 4, 3, 2) and (C4H6)2Cr2(CO)n (n=7, 6, 5) have now been investigated by density functional theory. The lowest energy (C4H6)2Cr2(CO)7 structure is a singlet singly bridged structure with a Cr–Cr single bond distance of ∼3.1Å. This structure is predicted to be disfavored with respect to dissociation into mononuclear fragments. The lowest energy singlet (C4H6)2Cr2(CO)6 structure is an unbridged structure with a CrCr double bond distance of ∼2.7Å. However, a doubly bridged isomeric singlet (C4H6)2Cr2(CO)4(μ-CO)2 structure with two bridging CO groups and a similar CrCr double bond lies only slightly higher in energy. Related triplet (C4H6)2Cr2(CO)6 structures with longer Cr–Cr distances of ∼3.0Å corresponding to formal single bonds are comparable in energy to the singlet structures. The hexacarbonyl (C4H6)2Cr2(CO)6 is predicted to be energetically disfavored with respect to disproportionation into (C4H6)2Cr2(CO)7+(C4H6)2Cr2(CO)5. The lowest energy (C4H6)2Cr2(CO)5 structure has all terminal CO groups and a short CrCr triple bond distance of ∼2.4Å. A higher energy (C4H6)2Cr2(CO)5 structure by ∼4kcal/mol has a four-electron donor η2-μ-CO group and a CrCr double bond distance of ∼2.7Å. These (C4H6)2Cr2(CO)n (n=7, 6, 5) structures are similar to the isomeric [(CH2)3C]2Cr2(CO)n structures that are previously reported except for some differences in the arrangements of the bridging carbonyl groups.

Extreme Metal Carbonyl Back Bonding in Cyclopentadienylthorium Carbonyls Generates Bridging C2O2 Ligands by Carbonyl Coupling

Laboratory studies of the interaction of carbon monoxide with organoactinides result in the formation of isolable complexes such as Cp3UCO derivatives (Cp = cyclopentadienyl) as well as coupling reactions to give derivatives of the oligomeric anions C(n)O(n)(2-) (n = 2, 3, 4). To gain some insight into actinide carbonyl chemistry, binuclear cyclopentadienylthorium carbonyls Cp2Th2(CO)n (n = 2 to 5) as model compounds have been investigated using density functional theory. The most favorable such structures in terms of energy and thermochemistry are the tricarbonyl Cp2Th2(η(2)-μ-CO)3 having three four-electron donor bridging carbonyl groups and the tetracarbonyl Cp2Th2(η(4)-μ-C2O2)(η(2)-μ-CO)2 having not only two four-electron donor bridging carbonyl groups but also a bridging ethynediolate ligand formed by coupling two CO groups through C-C bond formation. The bridging infrared ν(CO) frequencies ranging from 1140 to 1560 cm(-1) in these Cp2Th2(CO)n (n = 3, 4) derivatives indicate extremely strong Th→CO back bonding in these structures, corresponding to formally dianionic CO(2-) and C2O2(2-) ligands and the favorable +4 thorium oxidation state. A characteristic of the Cp2Th2(η(2)-μ-CO)3 and Cp2Th2(η(4)-μ-C2O2)(η(2)-μ-CO)2 structures is their ability to add terminal CO groups, preferably to the thorium atom bonded to the fewest oxygen atoms. These terminal CO groups exhibit ν(CO) frequencies in a similar range as terminal CO groups in d-block metal carbonyls. However, these terminal CO groups are relatively weakly bonded to the thorium atoms as indicated by predicted CO dissociation energies of 14 kcal/mol for Cp2Th2(CO)5. Two low energy structures for the dicarbonyl Cp2Th2(CO)2 are found with two separate four-electron donor bridging CO groups and relatively short Th-Th distances of 3.3 to 3.4 Å suggesting formal single bonds and +3 thorium formal oxidation states. However, a QTAIM analysis of this formal Th-Th bond does not reveal a bond critical point thus suggesting a multicenter bonding model involving the bridging CO groups.