Terminal CO 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.

Highly Activated Terminal Carbon Monoxide Ligand in an Iron-Sulfur Cluster Model of FeMco with Intermediate Local Spin State at Fe.

Nitrogenases, the enzymes that convert N2 to NH3, also catalyze the reductive coupling of CO to yield hydrocarbons. CO-coordinated species of nitrogenase clusters have been isolated and used to infer mechanistic information. However, synthetic FeS clusters displaying CO ligands remain rare, which limits benchmarking. Starting from a synthetic cluster that models a cubane portion of the FeMo cofactor (FeMoco), including a bridging carbyne ligand, we report a heterometallic tungsten-iron-sulfur cluster with a single terminal CO coordination in two oxidation states with a high level of CO activation (νCO = 1851 and 1751 cm-1). The local Fe coordination environment (2S, 1C, 1CO) is identical to that in the protein making this system a suitable benchmark. Computational studies find an unusual intermediate spin electronic configuration at the Fe sites promoted by the presence the carbyne ligand. This electronic feature is partly responsible for the high degree of CO activation in the reduced cluster.

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst.

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

A rotational isomerization of diiron complexes of ferrole type involved in the catalytic hydroboration of aldehydes under mild conditions

In this work, two new diiron(I) complexes of ferrole or ferracyclopentadiene type, [Fe2(CO)6{RC2(CH2)4C2R}] (R = H, 1; CH2CH3, 2) were synthesized by reacting the [Fe2(CO)9] with diynes (1,7-octadiyne and 3,9-dodecanediyne). The ferrole-type complexes were fully characterized, wherein the absolute structure of 2 was determined by single-crystal X-ray diffraction analysis. Interestingly, a rotational isomerization of the diiron(I) complexes (1 and 2) between the sawhorse conformer and the non-sawhorse conformer is verified, originated from the switch of CO coordination. Additionally, the sawhorse conformer with the terminal CO is dominant in solution due to the tricarbonyl iron units undergo a drastic geometrical turnstile rotation, whereas the non-sawhorse conformer with a semi-bridging CO is rigid in a solid state. Particularly, the isomerism of the diiron(I) complexes of ferrole type leads to a unique catalytic activity for hydroboration of aldehydes with pinacolborane (HBpin). Notably, twelve aldehydes (3a-3l) were exampled to be converted quantitatively into the corresponding organoboronates (4a-4l) mediated by complex 2 under mild conditions. Mechanistically, the 1H NMR and FTIR spectroscopic results proved that the ferrole-type complex 2 bound with the aldehyde to activate the carbonyl of the substrate rather than to cleave the H-B bond of pinacolborane. Collectively, a plausible mechanism for the catalytic hydroboration is proposed, with the rotational isomerization of the diiron(I) complexes involved in.

One-pot esters synthesis from secondary alcohols and CO catalyzed byPd-phosphine systems

The aim of the work is to develop the esters syntheses by combining in one reactor the acid–catalytic intramolecular alcohols dehydration and the resulting alkenes alkoxycarbonylation. Esters were synthesized from cycloalkanols or secondary alkanols C5–C9 and CO under mild conditions. For the first time, a wide range of catalytic systems including palladium precursor, organophosphine, strong protonic acid, and additive for binding water have been tested in the model one-pot process based on cyclohexanol and CO. For the first time, the dehydration processes of secondary linear alcohols C6, C7 and C9 were combined in one reactor with isomerizing alkoxycarbonylation catalyzed by Pd-systems with the XANTPHOS promoter. These processes led to the isomeric esters formation, among which 60–73% were linear carboxylic acid esters as products of terminal CO addition to alkenes.

Construction of Two-Faced (Hetero)hydrocarbyl Diiron Complexes Mediated by the Interplay of Ligands

The functionalized allylidene complex [Fe2Cp2(CO)(μ-CO){μ-η1:η3-Cγ(Fc)CβHCα(CN)NMe2}], 1 [Cp = η5-C5H5; Fc = CpFe(η5-C5H4)], reacted with isocyanides (CNR), in isopropanol solution at ca. 100 °C, to give the CO-substitution products [Fe2Cp2(CO)(μ-CNR){μ-η1:η3-Cγ(Fc)CβHCα(CN)NMe2}] [R = CH2P(O)(OEt)2, 2a; R = 2-naphthyl, 2b; R = CH2C(O)(OEt), 2c], which were isolated in 61–84% yields. The bridging coordination of CNR in 2a–c is forced by a stabilizing electrostatic interaction between the nitrogen lone pair belonging to the {NMe2} group and the terminal CO ligand. Isocyanide methylation with methyl triflate proceeded with the inversion of stereochemistry at the Cα carbon and led to [Fe2Cp2(CO){μ-CN(Me)R}{μ-η1:η3-Cγ(Fc)CβHCα(CN)NMe2}]CF3SO3 (R = CH2P(O)(OEt)2), ([3a]CF3SO3; R = 2-naphthyl, [3b]CF3SO3), containing bridging allylidene and aminocarbyne ligands (68–74% yields). All products were fully characterized by IR and multinuclear NMR spectroscopy, and the structures of 2a and [3a]CF3SO3 were elucidated by X-ray diffraction studies. Density functional theory (DFT) calculations were extensively carried out to shed light on structural, mechanistic, and thermodynamic features.

Fourier-transform microwave spectroscopy of the s-trans-3-propenalyl (CH2CHĊO) and 3-propenolyl (ĊH2CHCO) radicals.

Pure rotational transitions of two conformers of the CH2CHCO radical have been observed by Fourier-transform microwave spectroscopy, where one conformer is called the s-trans-3-propenalyl radical and the other the 3-propenolyl radical. The observed two conformers have different electronic states. The former, the s-trans-3-propenalyl radical, has the 2A' electronic state and can be written as CH2CHĊO, where the unpaired electron resides mainly on the terminal CO carbon. On the other hand, the latter, 3-propenolyl radical has the 2A'' electronic state and can be written as ĊH2CHCO. We were able to observe pure rotational transitions of the two conformers. Since both of the species have an unpaired electron, there exist spin-rotation interactions due to the unpaired electron and the magnetic hyperfine interactions due to the three coupling protons. The observed very complicated spectra, caused by these interactions, were assigned, leading to detailed molecular constants including the fine and hyperfine coupling constants for both of the species. The determined molecular constants support the electronic structures of the two conformers. There exists a controversy as to which of the two conformers is the lowest energy one. Our present observation led to the conclusion that s-trans-3-propenalyl is the lowest energy conformer.

Effect of boron nitride overlayers on Co@BNNSs/BN-Catalyzed aqueous phase selective hydrogenation of cinnamaldehyde

The confinement effect known as efficient strategy to enhance the heterocatalytic activity and stability, but a clear view regarding the role of encapsulated overlayers is far from convincing at present, especially, the penetration of substrates with different size. Herein, the experimental evidence about the impacts of BN overlayers on hydrogenation is obtained over Co@BN/BN model catalysts with tuned thinness, in which BN overlayers encapsuled Co particles that dispersed on the defective BN supports, fabricating by the nitridizing of ball-milled BN microplates under NH3 atmosphere. The thinness and crystallinity of BN shells was simply tuned by controlling the pyrolyzed temperature (600–900 °C). The cinnamaldehyde (CAL) selective hydrogenation is taken as probe reaction due to (1) both larger CAL and small H2 molecule involved simultaneously, (2) the middle CC and terminal CO bonds all involved. Combined with structural analysis, the results demonstrate that small H2 molecule can penetrate into the metal-cover interface through the defect sites, then inner Co core dissociate it to atomic H, and the hydrogenation mainly resulting from the spillover of H atoms which occurred on the BN surface. As a result, the thickest BN shells (∼3.4 nm) with ordered-layer-lattice significantly hindered the adsorption and activation of CAL, also the longest H spillover distance led to the lowest activity of Co@BN/BN-900. Instead, Co@BN/BN-600 with merely 2 ∼ 3 overlayers presented the most efficient and highly chemoselective hydrogenation activity. The ultrathin but turbostratic BN overlayers provide more migrating sites also shortened the H spillover distance effectively, and the more favorable CO hydrogenation was also achieved driven by the steric hindrance effect of thinner BN shells. These observations provide new insights towards understanding of confined effect on catalysis process through the accurate regulation of BN shells properties.

Front Cover: Heterobridged Hexa‐ and Octanuclear Ruthenium‐Thiolate/Halide Wheels: Synthesis, X‐ray Crystal Structure, and Spectroscopic Characterization (Eur. J. Inorg. Chem. 33/2022)

The Front Cover shows the structure of an octanuclear heterobridged molecular wheel featuring six-coordinate ruthenium ions, terminal CO ligands, and thiolate/iodide mixed bridging ligands. This wheel represents one of the crystallographically characterized {Run(μ-SR)n(μ-X)n}(CO)2n (X=Cl, Br, n=6; X=I, n=8) wheels composed of edge-shared RuC2S2X2 (X=halide) octahedra. The present work demonstrates the construction of hexa- and octanuclear wheels which feature fused Mn(μ-SR)n/Mn(μ-X)n (n=6, 8) metallamacrocycles as well as six-coordinate metal molecular wheels with their metal ions linked by mixed S/X-donor ligands. More information can be found in the Research Article by J.-S. Huang, C.-M. Che and co-workers.

Spectroscopic Identification of the Heterocumulenic Isocyanatoborane Radical HBNCO.

The highly elusive isocyanatoborane radical HBNCO has been generated by the reaction of laser-ablated boron atoms with HNCO and also by the light-induced chemical transformation of the hydrogen-bonded molecule-radical complex BNH···CO in solid neon matrix. IR spectroscopic and theoretical studies indicate that the HBNCO radical possesses a quasilinear B═N═C═O heterocumulenic structure with the unpaired electron mainly located at the boron atom. This is in sharp contrast to the bonding properties of the isoelectronic analogues HCCCO and NCCO, in which the unpaired electron is located at the terminal CO moiety.

Biosynthesis of the [FeFe] hydrogenase H-cluster via a synthetic [Fe(II)(CN)(CO)2(cysteinate)]- complex.

The H-cluster of [Fe-Fe] hydrogenase consists of a [4Fe]H subcluster linked by the sulfur of a cysteine residue to an organometallic [2Fe]H subcluster that utilizes terminal CO and CN ligands to each Fe along with a bridging CO and a bridging SCH2NHCH2S azadithiolate (adt) to catalyze proton reduction or hydrogen oxidation. Three Fe-S "maturase" proteins, HydE, HydF, and HydG, are responsible for the biosynthesis of the [2Fe]H subcluster and its incorporation into the hydrogenase enzyme to form this catalytically active H-cluster. We have proposed that HydG is a bifunctional enzyme that uses S-adenosylmethione (SAM) bound to a [4Fe-4S] cluster to lyse tyrosine via a transient 5'-deoxyadenosyl radical to produce CO and CN ligands to a unique cysteine-chelated Fe(II) that is linked to a second [4Fe-4S] cluster via the cysteine sulfur. In this "synthon model", after two cycles of tyrosine lysis, the product of HydG is completed: a [Fe(CN)(CO)2(cysteinate)]- organometallic unit that is vectored directly into the synthesis of the [2Fe]H sub-cluster. However our HydG-centric synthon model is not universally accepted, so further validation is important. In this Frontiers article, we discuss recent results using a synthetic "Syn-B" complex that donates [Fe(CN)(CO)2(cysteinate)]- units that match our proposed HydG product. Can Syn-B activate hydrogenase in the absence of HydG and its tyrosine substrate? If so, since Syn-B can be synthesized with specific magnetic nuclear isotopes and with chemical substitutions, its use could allow its enzymatic conversions on the route to the H-cluster to be monitored and modeled in fresh detail.

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.

Spectroscopic Characterization of μ-η1:η1-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium Clusters.

Although oxygen is a common ligand in supported metal catalysts, its coordination has been challenging to elucidate. We now characterize a diiridium complex that has been previously shown by X-ray diffraction crystallography to incorporate a μ-η1:η1-peroxo ligand. We observe markedly enhanced intensity at 788 cm-1 in the Raman spectrum of this complex, which is a consequence of bonding of the peroxo ligand but does not shift upon 18O labeling. Electronic structure calculations at the density functional theory level suggest that this increase in Raman intensity results from bands associated with rocking of CH2 substituents directly attached to P(Ph)2 groups coupling with the O-O band. These results provide part of the foundation for understanding oxygen ligands on a silica-supported tetrairidium carbonyl cluster stabilized with bulky electron-donating phosphine ligands [p-tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl)]. Reaction of the cluster with O2 also led to the growing in of a Raman band at 788 cm-1, similar to that in the diiridium complex and also assigned to the bonding of a bridging peroxo ligand. Infrared spectra recorded as the supported cluster reacted in sequential exposures to (i) H2, (ii) O2, (iii) H2, and (iv) CO indicate that two bridging peroxo ligands were bonded irreversibly per tetrairidium cluster, replacing bridging carbonyl ligands without altering either the cluster frame or the phosphine ligands. X-ray absorption near edge and infrared spectra include isosbestic points signifying a stoichiometrically simple reaction of the cluster with O2, and mass spectra of the effluent gas show that CO2 formed by oxidation of one terminal CO ligand per cluster as H2 (and not H2O) formed, evidence that hydride ligands had been present on the cluster following treatment (i). The understanding of how O2 reacts with the metal polyhedron provides a foundation for understanding of how oxidation catalysis may proceed on the surfaces of noble metals.

Heterobimetallic Chromium Manganese Carbonyl Nitrosyls: Comparison with Isoelectronic Homometallic Binuclear Chromium Carbonyl Nitrosyls and Manganese Carbonyls

The heterometallic chromium-manganese carbonyl nitrosyls CrMn(NO)(CO)n (n = 9, 8) have been investigated by density functional theory. The lowest energy CrMn(NO)(CO)9 structures have unbridged staggered conformations with a ~2.99 Å Cr–Mn single bond similar to the experimental and lowest energy structures of the isoelectronic Mn2(CO)10 and Cr2(NO)2(CO)8. A significantly higher energy CrMn(NO)(CO)9 isomer has a nearly symmetrical bridging nitrosyl group and a very weakly semibridging carbonyl group. The two lowest energy structures of the unsaturated CrMn(NO)(CO)8 have a five-electron donor bridging η2-µ-NO nitrosyl group or a four-electron donor bridging η2-µ-CO group, as well as a Cr–Mn single bond of length ~2.94 Å. The next higher energy CrMn(NO)(CO)8 structure has exclusively terminal CO and NO ligands and a shorter Cr–Mn single bond of ~2.85 Å, suggesting an 18-electron configuration for the manganese atom and a 16-electron configuration for the chromium atom indicated by a vacant coordination site nearly perpendicular to the Cr–Mn bond.

Three-Coordinate Iron(0) Complexes with N-Heterocyclic Carbene and Vinyltrimethylsilane Ligation: Synthesis, Characterization, and Ligand Substitution Reactions.

Low-coordinate iron(0) species are implicated as intermediates in a range of iron-catalyzed organic transformations. Isolable iron(0) complexes with coordination numbers of less than four, however, are rarely known. In continuing with our interests in three-coordinate iron(0) complexes with N-heterocyclic carbene (NHC) and alkene ligation, we report herein the synthesis and ligand substitution reactivity of three-coordinate iron(0) complexes featuring monodentate alkene ligands, [(NHC)Fe(η2-vtms)2] (vtms = vinyltrimethylsilane, NHC = 1,3-bis(2',6'-diisopropylphenyl)-imidazol-2-ylidene (IPr), 1; 1,3-bis(2',6'-diisopropylphenyl)-4,5-tetramethylene-imidazol-2-ylidene (cyIPr), 2; 1,3-bis(2',6'-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene (7-IPr), 3). Complexes 1-3 were synthesized from the one-pot reactions of ferrous dihalides with the N-(2,6-diisopropylphenyl)-substituted NHC ligands, vtms, and KC8. Reactivity study of 1 revealed its facile ligand substitution reactions with terminal aryl alkynes, ketones, isocyanides, and CO, by which iron(0) complexes [(IPr)Fe(η2-HCCAr)] (Ar = Ph, 5; p-CH3C6H4, 6; 3,5-(CF3)2C6H3, 7), [(IPr)Fe(η2-OCPh2)2] (8), [(IPr)Fe(CNR)4] (R = 2,6-Me2C6H3, 9; But, 10), and (IPr)Fe(CO)4 (11) were prepared in good yields. These iron(0) complexes have been characterized by 1H NMR, solution magnetic susceptibility measurement, single-crystal X-ray diffraction study, 57Fe Mössbauer spectroscopy, and elemental analysis. Characterization data and computational studies suggest S = 1 ground-spin states for three-coordinate iron(0) complexes 1-3 and 5-8 and S = 0 ground states for 9-11. Theoretical studies on the three-coordinate complexes 1, 6, and 8 indicated pronounced metal-to-ligand backdonation from occupied Fe 3d orbitals to the π* orbitals of the C═C, C≡C, and C═O moieties of the π ligands. In addition, 1 proved an effective precatalyst for the cyclotrimerization reaction of alkynes.

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.

Photoelectron velocity map imaging spectroscopic and theoretical study of heteronuclear vanadium-nickel carbonyl anions VNi(CO) n - (n = 2-6).

Mass-selected heteronuclear vanadium-nickel carbonyl anions VNi(CO) n - (n = 2-6) were investigated by photoelectron velocity-map imaging spectroscopy and quantum chemical calculations to obtain their chemical bonding and intrinsic electronic structure in the gas phase. The calculated energies (adiabatic detachment energies)/vertical detachment energies (VDEs) match well with experimental values: 1.30/1.49, 1.66/1.95, 2.22/2.48, 2.70/2.89, and 2.95/3.15 eV. The VDE value of VNi(CO) n - increases with an increase of cluster size, implying that the negative electron is stabilized upon the bonding of CO molecules. VNi(CO)2 - consists of one bridging carbonyl and one terminal carbonyl, whose feature is different from MNi(CO)2 - (M = Sc, Y, La, and Ce) with the involvement of one side-on-bonded carbonyl and one terminal CO carbonyl. The building block composed of three bridging carbonyls is favored for VNi(CO)3 -, the structure of which persists up to n = 6. The additional CO ligands are preferentially coordinated in the terminal mode to the Ni atom at n = 4 and then to the V atom at n = 5 and 6. The results obtained in this work would provide a molecular-level understanding about chemisorbed CO molecules on alloy surfaces/interfaces, which is important to understand CO molecule activation processes.

Applications of Radical Carbonylation and Amine Addition Chemistry: 1,4-Hydrogen Transfer of 1-Hydroxylallyl Radicals.

1,4-Hydrogen transfer from the 1-hydroxyallyl radical to give the enoxyl (α-keto) radical is an exothermic process with a high activation energy based on DFT calculations. The lack of experimental examples of such 1,4-H shift reactions lies in the difficulty of generating the 1-hydroxyallyl radical. We have shown that radical carbonylation of alkenyl radicals with CO followed by nucleophilic trapping of the carbonyl portion of the resulting radical by amines gives rise to 1-amino-substituted 1-hydroxyallyl radicals in situ. At the outset of this chemistry, we examined intramolecular trapping reactions via radical carbonylation of alkynylamines mediated by tributyltin hydride. Consequently, α-methylene lactams were obtained, for which the initially formed 1-amino-substituted 1-hydroxyallyl radical underwent a 1,4-H shift followed by subsequent β-scission, which led to the expulsion of a tributyltin radical. A competing pathway of the 1,4-H shift of 1-amino-substituted 1-hydroxyallyl radicals involving hydrogen abstraction was observed, which led to the formation of α-stannylmethylene lactams as a major byproduct. However, in contrast, when intermolecular trapping of α-ketenyl radicals by amines was carried out, the 1,4-H shift from the 1-amino-substituted 1-hydroxyallyl radical became the major pathway, which gave good yields of α,β-unsaturated amides. Thus, we were able to develop three-component reactions comprising terminal alkynes, CO, and amines that led to α,β-unsaturated amides via the 1,4-H shift reaction. DFT calculations support the observation that the 1,4-H shift is more facile when 1-hydroxyallyl radicals have both 1-amino and 3-tin substituents. The choice of substituents on the amine nitrogen is also important, since N-C bond cleavage via an SH2-type reaction can become a competing pathway. Such an unusual SH2-type reaction at the amine nitrogen is favored when the leaving alkyl radicals are stable, such as PhC(•)H(CH3) and t-Bu•. Interestingly, even nucleophilic attack of tertiary amines onto α-ketenyl radicals causes cleavage of the C-N bond. For this reaction, DFT calculations predict an indirect homolytic substitution mechanism involving expulsion of alkyl radicals through the zwitterionic radical intermediate arising from nucleophilic amine addition onto the α-ketenyl radical. In contrast, the carbonylation of aryl radicals, generated from aryl iodides, in the presence of amines gave aromatic carboxylic amides in good yields. It is proposed that radical anions originating from acyl radicals and amines undergo electron transfer to aryl iodides to give aminocarbonylation products.

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).