Carbonyl Analogues Research Articles

1,5-Bis[2-(Dioxyphosphoryl)-4-ethylphenoxy]-3-oxapentane and Its Analogs: Acidity and Complexation in Aqueous Media Containing Copper(II) Cation

Dissociation and complexation with Cu(II) cation is studied for phosphoryl-containing podands (tetrabasic 1,5-bis[2-(dioxyphosphoryl)-4-ethylphenoxy]acidic-type-3-oxapentane (H4LP) and dibasic 1,5-bis[2-(oxyethoxyphosphoryl)-4-ethylphenoxy]-pentane (H2LP)) and their carbonyl analog (polyether dibasic acid 1,5-bis[2-(oxycarbonylphenoxy)]oxapentane (H2LC) in water in the presence of 5% dimethylformamide potentiometrically, spectrophotometrically, and conductometrically. Phosphoryl podands exhibit good complexing properties and can selectively bind metal cations. The dissociation constants are determined and the distribution diagram of ionized species of the compounds under study depending on pH is plotted. The molar ratio between the metal and podand in all complexes is found to be 1 : 1. The stability constants of complexes with Cu2+ cation are calculated. The most stable copper(II) complexes are formed with the tetrabasic podand.

Kallikrein 5 inhibitors identified through structure based drug design in search for a treatment for Netherton Syndrome

Netherton syndrome (NS) is a rare and debilitating severe autosomal recessive genetic skin disease with high mortality rates particularly in neonates. NS is caused by loss-of-function SPINK5 mutations leading to unregulated kallikrein 5 (KLK5) and kallikrein 7 (KLK7) activity. Furthermore, KLK5 inhibition has been proposed as a potential therapeutic treatment for NS. Identification of potent and selective KLK5 inhibitors would enable further exploration of the disease biology and could ultimately lead to a treatment for NS. This publication describes how fragmentation of known trypsin-like serine protease (TLSP) inhibitors resulted in the identification of a series of phenolic amidine-based KLK5 inhibitors 1. X-ray crystallography was used to find alternatives to the phenol interaction leading to identification of carbonyl analogues such as lactam 13 and benzimidazole 15. These reversible inhibitors, with selectivity over KLK1 (10–100 fold), provided novel starting points for the guided growth towards suitable tool molecules for the exploration of KLK5 biology.

Simple Schiff‐Base Cu(II) Complexes as Efficient Catalysts for Benzyl Alcohol Oxidation

AbstractHerein, we present synthesis and characterization of new ligand with N2O‐donor moiety based on benzothiazole unit and its two complexes with Cu(II) cations. X‐ray diffraction measurements revealed solid state structures of presented compounds as mono‐ and dinuclear entities. Both complexes appeared to be highly effective catalysts in oxidation process of benzyl alcohol and its derivatives in aqueous media. Copper(II)‐(2,2,6,6‐tetramethylpiperidine‐1‐oxyl) system provides selective conversion in the presence of 0.25 mol% and 1 mol% for complex 1 and 2, respectively. Obtained results revealed that both copper complexes are highly selective towards benzyl alcohols oxidation to its carbonyl analogues. It was tested in the presence of several electron donating and accepting substituents. Consequently, presented systems can provide effective catalytic oxidation of aforementioned groups in the presence of aliphatic alcohols.

Disilaruthena- and Ferracyclic Complexes Containing Isocyanide Ligands as Effective Catalysts for Hydrogenation of Unfunctionalized Sterically Hindered Alkenes.

Disilaferra- and disilaruthenacyclic complexes containing mesityl isocyanide as a ligand, 3' and 4', were synthesized and characterized by spectroscopy and crystallography. Both 3' and 4' showed excellent catalytic activity for the hydrogenation of alkenes. Compared with iron and ruthenium carbonyl analogues, 1' and 2', the isocyanide complexes 3' and 4' were more robust under the hydrogenation conditions, and were still active even at higher temperatures (∼80 °C) under high hydrogen pressure (∼20 atm). The iron complex 3' exhibited the highest catalytic activity toward hydrogenation of mono-, di-, tri-, and tetrasubstituted alkenes among currently reported iron catalysts. Ruthenium complex 4' catalyzed hydrogenation under very mild conditions, such as room temperature and 1 atm of H2. The remarkably high catalytic activity of 4' for hydrogenation of unfunctionalized tetrasubstituted alkenes was especially notable, because it was comparable to the activity of iridium complexes reported by Crabtree and Pfaltz, which are catalysts with the highest activity in the literature. DFT calculations suggested two plausible catalytic cycles, both of which involved activation of H2 assisted by the metal-silicon bond through σ-bond metathesis of late transition metals (oxidative hydrogen migration). The linear structure of M-C≡N-C (ipso carbon of the mesityl group) played an essential role in the efficient hydrogenation of sterically hindered tetrasubstituted alkenes.

Novel Carbonyl Analogs of Tamoxifen: Design, Synthesis, and Biological Evaluation.

Aim of this work was to provide tamoxifen analogs with enhanced estrogen receptor (ER) binding affinity. Hence, several derivatives were prepared using an efficient triarylethylenes synthetic protocol. The novel compounds bioactivity was evaluated through the determination of their receptor binding affinity and their agonist/antagonist activity against breast cancer tissue using a MCF-7 cell-based assay. Phenyl esters 6a,b and 8a,b exhibited binding affinity to both ERα and ERβ higher than 4-hydroxytamoxifen while compounds 13 and 14 have shown cellular antiestrogenic activity similar to 4-hydroxytamoxifen and the known ER inhibitor ICI182,780. Theoretical calculations and molecular modeling were applied to investigate, support and explain the biological profile of the new compounds. The relevant data indicated an agreement between calculations and demonstrated biological activity allowing to extract useful structure-activity relationships. Results herein underline that modifications of tamoxifen structure still provide molecules with substantial activity, as portrayed in the inhibition of MCF-7 cells proliferation.

Mimicking the pineapple scent: Synthesis and properties of (semi)conjugated triene carbonyl derivatives

ABSTRACTThe unexpected formation of previously nondescribed (semi)conjugated ethyl trienoate revealed its powerful aroma of fresh pineapple. Thus, we have designed, prepared and evaluated a set of its carbonyl analogues as mixtures of (E/Z)-isomers. Although their synthesis from natural ocimene led to target compounds in low yields, optimized preparation from geranyl acetate furnished an aldehyde as high-yielding common intermediate on multigram scale. A series of its Wittig olefinations provided corresponding (E,E)-configured carbonyl dienes. Final acid-catalyzed elimination of allylic acetates provided the desired “pineapple” target in moderate yield. Sensory analysis revealed that only the parent compound possesses the typical pineapple aroma. Although analogous tBu- and/or Bn-esters feature an additional green note, the most similar Me-ester differs by its fresh woody aroma analogously to methyl ketone.

Synthesis and Light-absorption Characteristics of Thiophene Derivatives Bearing Ferrocenylthiocarbonyl Groups

Thiophene-based dyes bearing two ferrocenylcarbonyl groups were obtained from a short synthesis involving direct arylations of the thiophene rings. A subsequent thionation of the carbonyl groups resulted in the formation of new thiophene-based dyes bearing two ferrocenylthiocarbonyl groups. These dyes absorb in a wide range of the visible spectrum due to their π–π* and d–π* transitions, and show a remarkable red-shift of the absorption band relative to their carbonyl analogues without increasing the molecular size.

Kinetic Analysis and Probing with Substrate Analogues of the Reaction Pathway of the Nitrile Reductase QueF from Escherichia coli

The enzyme QueF catalyzes a four-electron reduction of a nitrile group into an amine, the only reaction of this kind known in biology. In nature, QueF converts 7-cyano-7-deazaguanine (preQ0) into 7-aminomethyl-7-deazaguanine (preQ1) for the biosynthesis of the tRNA-inserted nucleoside queuosine. The proposed QueF mechanism involves a covalent thioimide adduct between preQ0 and a cysteine nucleophile in the enzyme, and this adduct is subsequently converted into preQ1 in two NADPH-dependent reduction steps. Here, we show that the Escherichia coli QueF binds preQ0 in a strongly exothermic process (ΔH = −80.3 kJ/mol; −TΔS = 37.9 kJ/mol, Kd = 39 nm) whereby the thioimide adduct is formed with half-of-the-sites reactivity in the homodimeric enzyme. Both steps of preQ0 reduction involve transfer of the 4-pro-R-hydrogen from NADPH. They proceed about 4–7-fold more slowly than trapping of the enzyme-bound preQ0 as covalent thioimide (1.63 s−1) and are thus mainly rate-limiting for the enzyme's kcat (=0.12 s−1). Kinetic studies combined with simulation reveal a large primary deuterium kinetic isotope effect of 3.3 on the covalent thioimide reduction and a smaller kinetic isotope effect of 1.8 on the imine reduction to preQ1. 7-Formyl-7-deazaguanine, a carbonyl analogue of the imine intermediate, was synthesized chemically and is shown to be recognized by QueF as weak ligand for binding (ΔH = −2.3 kJ/mol; −TΔS = −19.5 kJ/mol) but not as substrate for reduction or oxidation. A model of QueF substrate recognition and a catalytic pathway for the enzyme are proposed based on these data.

A novel double carbonyl analog of curcumin induces the apoptosis of human lung cancer H460 cells via the activation of the endoplasmic reticulum stress signaling pathway.

Curcumin can inhibit the growth of a variety of cancer cells; however, its poor bioavailability and pharmacokinetic profiles, which are attributed to its instability under physiological conditions, have limited its application in anticancer therapy. In the present study, we screened a double carbonyl analog of curcumin (A17) and analyzed its effects and mechanism of inducing apoptosis in human lung cancer H460 cells. The results showed that A17 not only induced CHOP expression in human lung cancer H460 cells, but also induced the apoptosis of H460 cells in a dose-responsive manner, and this effect was related to corresponding activation of some important components in the endoplasmic reticulum (ER) stress-mediated apoptosis pathway. When CHOP was knocked down by specific siRNA, A17-induced cell apoptosis was attenuated, thereby further demonstrating that the apoptotic pathway is ER stress‑dependent. Our studies demonstrated that A17 has better stability and antitumor activity than curcumin in H460 cells via an ER stress-mediated mechanism. These results imply that A17 could be further explored as a potential anticancer agent for the treatment of human non-small cell lung cancer (NSCLC).

Signaling of chloramine: a fluorescent probe for trichloroisocyanuric acid based on deoximation of a coumarin oxime

A new chloramine signaling probe, based on a coumarin oxime, was developed. The coumarin oxime 1 exhibited efficient off–on type fluorescent signaling behavior toward trichloroisocyanuric acid (TCCA) in an aqueous acetonitrile solution. The signaling is due to the TCCA-assisted transformation of the oxime function to its carbonyl analogue. The presence of common metal ions and anions did not interfere with the TCCA signaling of this probe. Probe 1 was found to be useful for the sensitive determination of the concentration of the practical oxidant TCCA in an aqueous environment, with a detection limit of 7.58×10−7M.

Protonation Studies of a Tungsten Dinitrogen Complex Supported by a Diphosphine Ligand Containing a Pendant Amine

Treatment of trans-[W(N2)2(dppe)(PEtNMePEt)] (dppe = Ph2PCH2CH2PPh2; PEtNMePEt = Et2PCH2N(Me)CH2PEt2) with 3 equiv of tetrafluoroboric acid (HBF4·Et2O) at −78 °C generated the seven-coordinate tungsten hydride trans-[W(N2)2(H)(dppe)(PEtNMePEt)][BF4]. At higher temperatures, protonation of a pendant amine is also observed, affording trans-[W(N2)2(H)(dppe)(PEtNMe(H)PEt)][BF4]2, with formation of the hydrazido complex [W(NNH2)(dppe)(PEtNMe(H)PEt)][BF4]2 as a minor product. A similar product mixture was obtained using triflic acid (HOTf). The protonated products are thermally sensitive and do not persist at ambient temperature. Upon acid addition to the carbonyl analogue cis-[W(CO)2(dppe)(PEtNMePEt)], the seven-coordinate carbonyl hydride complex trans-[W(CO)2(H)(dppe)(PEtNMe(H)PEt)][OTf]2 was generated. A mixed diphosphine complex without the pendant amine in the ligand backbone, trans-[W(N2)2(dppe)(depp)] (depp = Et2P(CH2)3PEt2), was synthesized and treated with HOTf, selectively generating a hydrazido complex, [W(NNH2)(OTf)(dppe)(depp)][OTf]. Computational analysis probed the proton affinity of three sites of protonation in these complexes: the metal, pendant amine, and N2 ligand. Room-temperature reactions with 100 equiv of HOTf produced NH4+ from reduction of the N2 ligand (electrons come from W). The addition of 100 equiv of HOTf to trans-[W(N2)2(dppe)(PEtNMePEt)] afforded 0.81 equiv of NH4+, while 0.40 equiv of NH4+ was formed upon treatment of trans-[W(N2)2(dppe)(depp)] with HOTf, showing that the complexes containing proton relays produce more products of reduction of N2.

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.

Microwave assisted synthesis of bis and tris(ω-bromoacetophenones): versatile precursors for novel bis(imidazo[1,2-a]pyridines), bis(imidazo[1,2-a]pyrimidines) and their tris-analogs

Backgroundα-Bromination of the side chain of aromatic ketones using NBS in the presence of p-toluenesulfonic acid (p-TsOH) in acetonitrile is very common. However, regioselective bromination of bis and tris(ω-bromoacetophenones) with NBS in the presence of p-TsOH in acetonitrile under microwave irradiation is quite novel. The bis- and tris(ω-bromoacetophenones) are used in synthesis of bis and tris(heterocycles). bis(heterocycles) have received a great deal of attention, because many biologically active natural and synthetic products have molecular symmetry. The use of the pressurized microwave irradiation is very advantageous to many syntheses and provide a large rate enhancement.ResultsBis and tris(ω-bromoacetophenones) were obtained as single monobrominated derivatives in a shorter time than the conventional conditions. The results clearly demonstrate the better reactivity and selectivity of NBS/p-TsOH/CH3CN as a brominating mixture under microwave conditions. The reaction of bis and tris(ω-bromoacetophenone) with 2-aminopyridine and 2-aminopyrimidine proceeded smoothly in a mixture of anhydrous ethanol and DMF under reflux or using 300 W/105°C/ 20 min microwave irradiation conditions to afford the corresponding bis(imidazo[1,2-a]pyridine), bis(imidazo[1,2-a]pyrimidine) and tris(imidazo[1,2-a]pyridine) derivatives in moderate to excellent yields. The carbonyl analogue of the targeted bis(imidazopyridines) could be synthesized by the reaction of N,N-dimethyl-N'-(pyridin-2-yl)formimidamide with bis(ω-bromoacetophenone) in refluxing ethanol. The structures of the newly synthesized compounds were confirmed by their spectral data as well as their elemental analyses.ConclusionIn conclusion, selective α-bromination of bis- and tris(acetophenones) has been accomplished efficiently utilizing NBS/p-TsOH/CH3CN under microwave irradiation. In addition, a facile synthesis of novel series of bis- and tris(imidazopyridine) and bis(imidazopyrimidine) derivatives.

Tandem semi-hydrogenation/isomerization of propargyl alcohols to saturated carbonyl analogues by dodecanethiolate-capped palladium nanoparticle catalysts.

The efficient one-pot conversion of propargyl alcohols to their saturated carbonyl analogues is carried out for the first time using metal nanoparticle catalysts, dodecanethiolate-capped Pd nanoparticles. Kinetic studies reveal that the reaction progresses through a semi-hydrogenation intermediate (allyl alcohols) followed by isomerization to carbonyls.

Controlling Surface Ligand Density and Core Size of Alkanethiolate-Capped Pd Nanoparticles and Their Effects on Catalysis

This article presents systematic investigations on the relationship between the catalytic property and the surface ligand density/core size of thiolate ligand-capped Pd nanoparticles (PdNPs). The systematic variations in the two-phase synthesis of PdNPs generated from sodium S-dodecylthiosulfate were performed. The resulting PdNPs were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and (1)H NMR and UV-vis spectroscopy. The decrease in the molar equivalent of sodium S-dodecylthiosulfate (Bunte salts) resulted in the formation of nanoparticles with lower surface ligand density and larger particle core size. A decrease in the molar equivalent of tetra-n-octylammonium bromide or an increase in reaction temperature generated nanoparticles with higher surface ligand density and smaller particle core size. As the molar equivalent of NaBH(4) decreased, the particle core size increased. The catalysis studies on various PdNPs with different surface ligand density and average core size showed a strong correlation between the PdNP composition and the turnover frequency (TOF) of the isomerization of allyl alcohol. Optimized "good" PdNPs with lower surface ligand coverage and larger core size catalyzed the isomerization of various allyl alcohols to carbonyl analogues with high activity and selectivity.

Pd Nanoparticle-Catalyzed Isomerization vs Hydrogenation of Allyl Alcohol: Solvent-Dependent Regioselectivity

Our previous work has shown that alkanethiolate-capped Pd nanoparticles generated from sodium S-dodecylthiosulfate are excellent catalysts for selective isomerization of various allyl alcohols to the carbonyl analogues. The present work focuses on understanding the mechanism and the regioselectivity of Pd nanoparticles in different environments. First, the presence of H2 gas has turned out to be essential for the efficient catalytic isomerization reaction. This suggests that the mechanism likely involves the Pd-alkyl intermediate rather than the η3 π-allyl Pd hydride intermediate. Second, the Pd nanoparticles are found to convert allyl alcohol selectively to either propanal or 1-propanol depending on the type of solvent used for the catalytic reactions. The reaction pathway is most likely determined by steric hindrance, which is the result of the interaction between substrate and alkylthiolate ligands on Pd nanoparticles. Presumably, the conformation of alkylthiolate ligands changes upon the type of solvents, resulting in varying degree of available space close to the nanoparticle surface. In general, nonpolar or weakly polar solvents such as benzene and chloroform, respectively, promote the isomerization of allyl alcohol to propanal via the formation of the branched Pd-alkyl intermediate. On the other hand, polar protic solvents such as methanol and water foster the hydrogenation of allyl alcohol to 1-propanol involving the steric induced formation of a linear Pd-alkyl intermediate. Third, the use of sodium S-hexylthiosulfate instead of sodium S-dodecylthiosulfate for the synthesis of Pd nanoparticles results in nanoparticle catalysts with a lower regioselectivity toward isomerization over hydrogenation. This is due to the higher surface ligand density of hexanethiolate-capped Pd nanoparticles, which negatively impacts the formation of branched Pd-alkyl intermediate. The results clearly indicate that controlling the structure and surface density of alkanethiolate ligands around Pd nanoparticles can provide an opportunity to tune the activity and selectivity of nanoparticle catalysts. Lastly, the high stability of soluble nanocatalysts is demonstrated by recycling dodecanethiolate-capped Pd nanoparticles over 10 times for the isomerization reaction of allyl alcohol.

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.

Thiocarbonyl quinacridone-based “turn on” fluorescent chemodosimeters for highly sensitive and selective detection of Hg(II)

Two new thiocarbonyl quinacridone compounds ( TQA-H and TQA-TPA) were designed and synthesized, and their fluorescent behaviors toward various metal ions were investigated. The measurements of sensing behavior to various metal ions reveal that both of them are excellent “turn on” fluorescent chemosensors for mercury ion. Addition of Hg 2+ to the chloroform solution of TQA-TPA gave the lowest detection limit at 4.7 nM. In addition, colorimetric changes of sensors interacted with Hg 2+ are also successfully demonstrated. The recognition is attributed to Hg 2+-induced conversion of thiocarbonyl quinacridones into their carbonyl analogues.

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).

Unsaturation in binuclear cyclopentadienylchromium carbonyl thiocarbonyls: Viability of (η 5-C 5H 5) 2Cr 2(CS) 2(CO) 3 in contrast to (η 5-C 5H 5) 2Cr 2(CO) 5

The cyclopentadienylchromium carbonyl thiocarbonyls Cp 2Cr 2(CS) 2(CO) n ( n = 4, 3, 2, 1) have been studied by density functional theory using the B3LYP and BP86 functionals. The lowest energy Cp 2Cr 2(CS) 2(CO) 4 structure can be derived from the experimentally characterized unbridged Cp 2Cr 2(CO) 6 structure by replacing the two terminal carbonyl groups furthest from the Cr–Cr bond with two terminal CS groups. The two lowest energy Cp 2Cr 2(CS) 2(CO) 3 structures have a single four-electron donor η 2-μ-CS group and a formal Cr–Cr single bond of length ∼3.1 Å. In contrast to the carbonyl analogue Cp 2Cr 2(CO) 5 these Cp 2Cr 2(CS) 2(CO) 3 structures are viable with respect to disproportionation into Cp 2Cr 2(CS) 2(CO) 4 and Cp 2Cr 2(CS) 2(CO) 2 and thus are promising synthetic targets. The lowest energy Cp 2Cr 2(CS) 2(CO) 2 structures have all two-electron donor CO and CS groups and short Cr Cr distances around ∼2.3 Å suggesting the formal triple bonds required to give the chromium atoms the favored 18-electron configurations. These Cp 2Cr 2(CS) 2(CO) 2 structures are closely related to the known structure for Cp 2Cr 2(CO) 4. In addition, several doubly bridged structures with four-electron donor η 2-μ-CS bridges are found for Cp 2Cr 2(CS) 2(CO) 2 at higher energies. The global minimum Cp 2Cr 2(CS) 2(CO) structure is a triply bridged triplet with a Cr Cr triple bond (2.299 Å by BP86). A higher energy singlet Cp 2Cr 2(CS) 2(CO) structure has a shorter Cr–Cr distance of 2.197 Å (BP86) suggesting the formal quadruple bond required to give each chromium atom the favored 18-electron configuration.