Reductive Decarbonylation Research Articles

Softer Is Better for Titanium: Molecular Titanium Arsenido Anions Featuring Ti≡As Bonding and a Terminal Parent Arsinidene.

We introduce the arsenido ligand onto the TiIV ion, yielding a remarkably covalent Ti≡As bond and the parent arsinidene Ti═AsH moiety. An anionic arsenido ligand is assembled via reductive decarbonylation involving the discrete TiII salt [K(cryptand)][(PN)2TiCl] (1) (cryptand = 222-Kryptofix) and Na(OCAs)(dioxane)1.5 in thf/toluene to produce the mixed alkali ate-complex [(PN)2Ti(As)]2(μ2-KNa(thf)2) (2) and the discrete salt [K(cryptand)][(PN)2Ti≡As] (3) featuring a terminal Ti≡As ligand. Protonation of 2 or 3 with various weak acids cleanly forms the parent arsinidene [(PN)2Ti═AsH] (4), which upon deprotonation with KCH2Ph in thf generates the more symmetric anionic arsenido [(PN)2Ti(As){μ2-K(thf)2}]2 (5). Experimental and computational studies suggest the pKa of 4 to be ∼23, and the bond orders in 2, 3, and 5 are all in the range of a Ti≡As triple bond, with decreasing bond order in 4.

Activation of heteroallenes by metal-substituted electron-rich tetrylenes

The metal-substituted germylene LGa(μ-Cl)GeDMP (L = HC[C(Me)NDipp]2, Dipp = 2,6-iPr2C6H3, DMP = 2,6-Mes2C6H3, Mes = 2,4,6-Me3C6H2) 1 reacts with CO2 with reductive decarbonylation and formation of the oxygen-bridged complex L(Cl)GaOGeDMP 2, which was also formed in the reaction of 1 with N2O. The CO2 reaction most likely proceeds via intermediate formation of a gallacarboxylate germylene complex as is observed in reactions of complex 1 with both ethyl isocyanate and N,N’-dicyclohexylcarbodiimide, yielding the corresponding heteroallenes insertion complexes L(Cl)Ga-[C(NEt)O]-GeDMP 3 and L(Cl)Ga-[C(NCy)2]-GeDMP 4. In addition, the gallanediyle stannylene adduct LGa-Sn[N(SiMe3)(Dipp)] 5 reacts with CO2 to form the gallacarboxylate tin complex L(Cl)Ga-(CO2)-Sn[N(Dipp)(SiMe3)] 6. Complexes 2–4 and 6 are characterized spectroscopically (1H, 13C, 119Sn (6), IR) and by single crystal X-ray diffraction (sc-XRD).

Construction of a 21-Membered-Ring Orifice on [60]Fullerene.

Open-[60]fullerenes possessing a huge orifice with a ring-atom count exceeding 19 have been confined to only a few examples. Herein, we report a 20-membered-ring orifice which enables for a guest molecule such as H2 , N2 , and CH3 OH to be encapsulated inside the [60]fullerene cavity. In addition, a 21-membered-ring orifice was prepared via a reductive decarbonylation, in which one of the carbon atoms was moved out of the [60]fullerene skeleton as an N,N-dimethylamide group. At a low temperature of -30 °C, an Ar atom was encapsulated with an occupation level up to 52 %. At around room temperature, the amide group on the orifice rotates along with the C(amide)-C(fullerene) bond axis, realizing a self-inclusion of the methyl substituent on the amide group as confirmed NMR spectroscopically and computationally.

Phosphorus‐Atom Transfer from Phosphaethynolate to an Alkylidyne

AbstractA low‐spin and mononuclear vanadium complex, (Menacnac)V(CO)(η2‐P≡CtBu) (2) (Menacnac−=[ArNC(CH3)]2CH, Ar=2,6‐iPr2C6H3), was prepared upon treatment of the vanadium neopentylidyne complex (Menacnac)V≡CtBu(OTf) (1) with Na(OCP)(diox)2.5 (diox=1,4‐dioxane), while the isoelectronic ate‐complex [Na(15‐crown‐5)]{([ArNC(CH2)]CH[C(CH3)NAr])V(CO)(η2‐P≡CtBu)} (4), was obtained via the reaction of Na(OCP)(diox)2.5 and ([ArNC(CH2)]CH[C(CH3)NAr])V≡CtBu(OEt2) (3) in the presence of crown‐ether. Computational studies suggest that the P‐atom transfer proceeds by [2+2]‐cycloaddition of the P≡C bond across the V≡CtBu moiety, followed by a reductive decarbonylation to form the V−C≡O linkage. The nature of the electronic ground state in diamagnetic complexes, 2 and 4, was further investigated both theoretically and experimentally, using a combination of density functional theory (DFT) calculations, UV/Vis and NMR spectroscopies, cyclic voltammetry, X‐ray absorption spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X‐ray single‐crystal structural characterization. In combination, these data are consistent with a low‐valent vanadium ion in complexes 2 and 4. This study represents the first example of a metathesis reaction between the P‐atom of [PCO]− and an alkylidyne ligand.

Phosphorus-Atom Transfer from Phosphaethynolate to an Alkylidyne.

A low-spin and mononuclear vanadium complex, (Me nacnac)V(CO)(η2 -P≡Ct Bu) (2) (Me nacnac- =[ArNC(CH3 )]2 CH, Ar=2,6-i Pr2 C6 H3 ), was prepared upon treatment of the vanadium neopentylidyne complex (Me nacnac)V≡Ct Bu(OTf) (1) with Na(OCP)(diox)2.5 (diox=1,4-dioxane), while the isoelectronic ate-complex [Na(15-crown-5)]{([ArNC(CH2 )]CH[C(CH3 )NAr])V(CO)(η2 -P≡Ct Bu)} (4), was obtained via the reaction of Na(OCP)(diox)2.5 and ([ArNC(CH2 )]CH[C(CH3 )NAr])V≡Ct Bu(OEt2 ) (3) in the presence of crown-ether. Computational studies suggest that the P-atom transfer proceeds by [2+2]-cycloaddition of the P≡C bond across the V≡Ct Bu moiety, followed by a reductive decarbonylation to form the V-C≡O linkage. The nature of the electronic ground state in diamagnetic complexes, 2 and 4, was further investigated both theoretically and experimentally, using a combination of density functional theory (DFT) calculations, UV/Vis and NMR spectroscopies, cyclic voltammetry, X-ray absorption spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X-ray single-crystal structural characterization. In combination, these data are consistent with a low-valent vanadium ion in complexes 2 and 4. This study represents the first example of a metathesis reaction between the P-atom of [PCO]- and an alkylidyne ligand.

Chemical Ecology, Biochemistry, and Molecular Biology of Insect Hydrocarbons.

Insect cuticular hydrocarbons (CHCs) consist of complex mixtures of straight-chain alkanes and alkenes, and methyl-branched hydrocarbons. In addition to restricting water loss through the cuticle and preventing desiccation, they have secondarily evolved to serve a variety of functions in chemical communication and play critical roles as signals mediating the life histories of insects. In this review, we describe the physical properties of CHCs that allow for both waterproofing and signaling functions, summarize their roles as inter- and intraspecific chemical signals, and discuss the influences of diet and environment on CHC profiles. We also present advances in our understanding of hydrocarbon biosynthesis. Hydrocarbons are biosynthesized in oenocytes and transported to the cuticle by lipophorin proteins. Recent work on the synthesis of fatty acids and their ultimate reductive decarbonylation to hydrocarbons has taken advantage of powerful new tools of molecular biology, including genomics and RNA interference knockdown of specific genes, to provide new insights into the biosynthesis of hydrocarbons.

A Planar Ti2 P2 Core Assembled by Reductive Decarbonylation of - O-C≡P and P-P Radical Coupling.

The complex [(nacnac)Ti(OAr)]2 (μ2 :η2 ,η2 -P2 ) (1) is formed via reductive decarbonylation of the phosphaethynolate ion - [OCP], which serves as a P atom source. Complex 1 is the first structurally characterized Group 4 transition metal P2 complex and its structure reveals the rhombic Ti2 P2 core is essentially planar with short bond lengths suggesting some degree of multiple bonding character between the Ti-P and P-P sites. Computational studies of 1 provide an understanding of the Ti2 P2 core as well as the origin of the highly downfield 31 P NMR spectroscopic signal.

Phthalic anhydride production from hemicellulose solutions: Technoeconomic analysis and life cycle assessment

The process synthesis, technoeconomic analysis, and life cycle assessment (LCA) of a novel route for phthalic anhydride (PAN) production from hemicellulose solutions are presented. The production contains six steps including dehydration of xylose to furfural, reductive decarbonylation of furfural to furan, oxidation of furfural to maleic anhydride (MA), Diels‐Alder cycloaddition of furan, and MA to exo−4,10‐dioxa‐tricyclo[5.2.1.0]dec‐8‐ene‐3,5‐dione followed by dehydration to PAN in the presence of mixture of methanesulfonic acid and acetic anhydride (AAN) which is converted to acetyl methanesulfonate and acetic acid (AAD), and dehydration of AAD to AAN. The minimum selling price of PAN is determined to be $810/metric ton about half of oil‐based PAN. The coproduction of high‐value products is essential to improve the economics. Biomass feedstock contributes to the majority of cost. LCA results shows that biomass‐based PAN has advantages over oil‐based PAN to reduce climate change and fossil depletion however requires more water usage. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3708–3718, 2015

Radiosynthesis of 4-[18F]fluoro-l-tryptophan by isotopic exchange on carbonyl-activated precursors

Several 18F-labeled aromatic amino acids have been developed primarily for tumor imaging with positron-emission-tomography (PET). Also, 18F-labeled tryptophan derivatives were synthesized by electrophilic 18F-fluorination or by introducing a [18F]fluoroalkyl group. Here, a 3-step method for a nucleophilic radiosynthesis of 4-[18F]fluoro-l-tryptophan was developed. A carbonyl activated precursor containing a chiral amino acid building block was radiofluorinated by isotopic exchange, followed by removal of the activating formyl group by reductive decarbonylation and subsequent cleavage of the building block under acidic conditions. The title compound was obtained within 100min with a radiochemical yield of about 13%, a molar activity of >70MBq/mmol and an enantiomeric excess of >99%.

Exploring Small Bite-Angle Ligands for the Rhodium-Catalyzed Intermolecular Hydroacylation of β-S-Substituted Aldehydes with 1-Octene and 1-Octyne

A comparative study of seven crystallographically characterized rhodium precatalysts, which contain a variety of chelating diphosphine ligands, for the hydroacylation of 1-octyne or 1-octene with 2-(methylthio)benzaldehyde has been undertaken. These studies show that the best performing catalyst for 1-octyne, [Rh(L)(η6-C6H5F)][BArF4], L = iPr2PNMePiPr2, delivers alkyne selective hydroacylation with high efficiencies at low loadings (1 mol %, 2.0 M aldehyde, 25 °C, ToN = 100, 97% conversion in 5 min), and also shows high selectivity for the linear product. Experiments suggest that the alkyne selectivity arises from the alkyne being more competitive for metal binding compared to the alkene. Labeling experiments using the [Rh(tBu2PCH2PtBu2)(η6-C6H5F)][BArF4] system, that gives the final product in a linear:branched ratio of 6:1, indicate that the pathway that produces the branched product operates via an irreversible hydride insertion. Intermediate acyl hydride complexes, [Rh(L)(H)(COC6H4SMe)(acetone)][BArF4], have been characterized by low temperature NMR spectroscopy, as have their subsequent reductive decarbonylation products, one of which has also been crystallographically characterized: [Rh(iPr2PNMePiPr2)(SMePh)(CO)][BArF4].

Aromatic‐Amide‐Derived Olefins as a Springboard: Isomerization‐Initiated Palladium‐Catalyzed Hydrogenation of Olefins and Reductive Decarbonylation of Acyl Chlorides with Hydrosilane

A highly efficient catalytic protocol for the isomerization of substituted amide-derived olefins is presented that successfully uses a hydride palladium catalyst system generated from [PdCl(2)(PPh(3))(2)] and HSi(OEt)(3). The Z to E isomerization was carried out smoothly and resulted in geometrically pure substituted olefins. Apart from the cis-trans isomerization of double bonds, the selective reduction of terminal olefins and activated alkenes was performed with excellent functional group tolerance in the presence of an amide-derived olefin ligand, and the products were obtained in high isolated yields (up to >99 %). Furthermore, the palladium/hydrosilane system was able to promote the reductive decarbonylation of benzoyl chloride when a (Z)-olefin with an aromatic amide moiety was used as a ligand.

Comparison of the Thermal and Reductive Decarbonylation of a Rhodium Trifluoroacetyl Diphosphine Complex

Acyl complexes of rhodium(III) with chelating diphosphine ligands are well known for their stability toward decarbonylation. However, we have found that the corresponding perfluoro complexes do decarbonylate under mild conditions. In this report we address specifically Rh(dppp)(COCF3)I2 (dppp = 1,3-bis(diphenylphosphino)propane). Another difference between this compound and rhodium acyl complexes containing monodentate phosphines is that it does not undergo reductive elimination of alkyl halide after decarbonylation. Instead, slow CO dissociation from Rh(dppp)(CO)(CF3)I2 occurs to yield Rh(dppp)(CF3)I2 as the final product. Reduction of the trifluoroacetyl complex is a net two-electron process that also involves decarbonylation, but through a quite different mechanism. The products consist of Rh(dppp)(CO)I, CF3− anion, and iodide. X-ray crystal structures of both Rh(dppp)(COCF3)I2 and Rh(dppp)(CF3)I2 have been obtained and show the expected square-pyramidal geometry with the acyl/alkyl group in the apical...

Controlling Selectivity in Intermolecular Alkene or Aldehyde Hydroacylation Reactions Catalyzed by {Rh(L2)}+Fragments

Rhodium(III) dihydrido complexes [Rh(L(2))(H)(2)(acetone)][BAr(4)(F)] (Ark = C(6)H(3)(CF(3))(2)) containing the potentially hemilabile ligands L(2) = 9,9-dimethy1-4,5-bis(diphenylphosphino)xanthene (Xantphos) and [Ph(2)P(CH(2))(2)](2)O (POP') have been prepared from their corresponding norbornadiene rhodium(I) precursors. In solution these complexes are fluxional by proposed acetone dissociation, which can be trapped out by addition of MeCN to form [Rh(L(2))(H)(2)(NCMe)][BAr(4)(F)], which have been crystallographically characterized. Addition of alkene (methyl acrylate) to these complexes results in reduction to a rhodium(I) species and when followed by addition of the aldehyde HCOCH(2)CH(2)SMe affords the new acyl hydrido complexes [Rh(L(2))(COCH(2)CH(2)SMe)H][BAr(4)(F)] in good yield. The solid-state and solution structures show a tight binding of the POP' and Xantphos ligands, having a trans-arrangement of the phosphines with the central ether linkage bound. This is similar to the previously reported complex [Rh(DPEphos)(COCH(2)CH(2)SMOHNBAr(4)(F)] (DPEphos = [Ph(2)P(C(6)H(4))](2)O). Unlike the DPEphos complex, the Xantphos and POP' ligated complexes are not effective catalysts for the hydroacylation reaction between methyl acrylate and HCOCH(2)CH(2)SMe. This is traced to their inability to dissociate the central ether link in a hemilabile manner to reveal a vacant site necessary for alkene coordination. Consistent with this lack of availability of the vacant site, these complexes also are stable toward reductive decarbonylation. Complexes [Rh(Ph(2)P(CH(2))PPh(2))(acetone)(2)][BAr(4)(F)] (n = 2-5) have also been studied as catalysts for the hydroacylation reaction between methyl acrylate and HCOCH(2)CH(2)SMe at 22 degrees C. As found previously, for n=2 this affords the product of alkene hydroacylation, but as the chain length is progressively increased to n= 5, the reaction also progressively changes to favor the product of aldehyde hydroacylation. This is suggested to occur by a decrease in the accessibility of the metal site on increasing the bite angle of the chelate ligand, so that alkene coordination to a putative Rh(III)-acyl hydrido intermediate is progressively disfavored and aldehyde coordination (followed by hydride transfer) is progressively favored. These, and previous, results show that the overall conversion in the hydroacylation reaction can be controlled by the hemilabile nature of the chelating phosphine in the catalyst (e.g.. DPEphos versus Xantphos), and the course of the reaction can also be tuned by changing the bite angle of the phosphine, cf. Ph(2)P(CH(2))(2)PPh(2) and Ph(2)P(CH(2))(5)PPh(2)).

Formation and Reactivity of [(tacn)-N-CO-ReIIIBr(CO)2]+in Water: a Theoretical and Experimental Study

The chemistry of [(tacn)-N-CO-Re(III)(CO)(2)Br]X (X = Cl or Br), obtained in good yield from the reaction of fac-[(tacn)Re(I)(CO)(3)]Br (1, tacn = 1,4,7-triazacyclononane) with X(2) in water, is described. The [(tacn)-N-CO-Re(III)(CO)(2)Br]X complex (2 with X = Br(-); 2a with X = BrCl(2)(-)), which we have previously communicated, is characterized by an unusual three-membered ring acyl amide bond. Complex 2 is stable as a solid but is reactive in aqueous solution. Under basic conditions (1 M NaOH), reductive decarbonylation was observed, and the bis-carbonyl complex [(tacn)Re(I)(CO)(2)Br] (3) was obtained in quantitative yield. The Br(-) ligand in 3 could be replaced by CN(-), giving the neutral complex [(tacn)Re(I)(CO)(2)(CN)] (4). In acidic media (1 M HBr), complex 2 partially converted to the monocarbonyl mu-oxo bridged dinuclear complex {[(tacn)Re(III)(CO)Br](2)O}(2+) (5 as [PF(6)](-) salt). Under mild oxidative conditions the trioxo [(tacn)Re(VII)O(3)](ReO(4)(-)) (6) was formed almost quantitatively, and small amounts of the uncommon Re(VI) complex [(tacn)Re(VI)O(2)Br](ReO(4)(-)) (7) were identified. Mechanistic investigations at the density functional level of theory (DFT) showed that the elementary steps in the formation of 2 from 1 and 3 from 2 involved reactions of the complexes with hydroxide. The calculated pathway is strongly exothermic (ca. -137 kcal/mol), confirming the energetically and kinetically highly favored formation of 3. The X-ray structures of 2a and 3-5 are reported and discussed.

Intermolecular Alkene and Alkyne Hydroacylation with β‐S‐Substituted Aldehydes: Mechanistic Insight into the Role of a Hemilabile P–O–P Ligand

A straightforward to assemble catalytic system for the intermolecular hydroacylation reaction of beta-S-substituted aldehydes with activated and unactivated alkenes and alkynes is reported. These catalysts promote the hydroacylation reaction between beta-S-substituted aldehydes and challenging substrates, such as internal alkynes and 1-octene. The catalysts are based upon [Rh(cod)(DPEphos)][ClO(4)] (DPEphos=bis(2-diphenylphosphinophenyl)ether, cod=cyclooctadiene) and were designed to make use of the hemilabile capabilities of the DPEphos ligand to stabilise key acyl-hydrido intermediates against reductive decarbonylation, which results in catalyst death. Studies on the stoichiometric addition of aldehyde (either ortho-HCOCH(2)CH(2)SMe or ortho-HCOC(6)H(4)SMe) and methylacrylate to precursor acetone complexes [Rh(acetone)(2)(DPEphos)][X] [X=closo-CB(11)H(6)Cl(6) or [BAr(F) (4)] (Ar(F)=3,5-(CF(3))(2)C(6)H(3))] reveal the role of the hemilabile DPEphos ligand. The crystal structure of [Rh(acetone)(2)(DPEphos)][X] shows a cis-coordinated diphosphine ligand with the oxygen atom of the DPEphos distal from the rhodium. Addition of aldehyde forms the acyl hydride complexes [Rh(DPEphos)(COCH(2)CH(2)SMe)H][X] or [Rh(DPEphos)(COC(6)H(4)SMe)H][X], which have a trans-spanning DPEphos ligand and a coordinated ether group. Compared to analogous complexes prepared with dppe (dppe=1,2-bis(diphenylphosphino)ethane), these DPEphos complexes show significantly increased resistance towards reductive decarbonylation. The crystal structure of the reductive decarbonylation product [Rh(CO)(DPEphos)(EtSMe)][closo-CB(11)H(6)I(6)] is reported. Addition of alkene (methylacrylate) to the acyl-hydrido complexes forms the final complexes [Rh(DPEphos)(eta(1)-MeSC(2)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X] and [Rh(DPEphos)(eta(1)-MeSC(6)H(4)-eta(1)-COC(2)H(4)CO(2)Me)][X], which have been identified spectroscopically and by ESIMS/MS. Intermediate species in this transformation have been observed and tentatively characterised as the alkyl-acyl complexes [Rh(CH(2)CH(2)CO(2)Me)(COC(2)H(4)SMe)(DPEphos)][X] and [Rh(CH(2)CH(2)CO(2)Me)(COC(6)H(4)SMe)(DPEphos)][X]. In these complexes, the DPEphos ligand is now cis chelating. A model for the (unobserved) transient alkene complex that would result from addition of alkene to the acyl-hydrido complexes comes from formation of the MeCN adducts [Rh(DPEphos)(MeSC(2)H(4)CO)H(MeCN)][X] and [Rh(DPEphos)(MeSC(6)H(4)CO)H(MeCN)][X]. Changing the ligand from DPEphos to one with a CH(2) linkage, [Ph(2)P(C(6)H(4))](2)CH(2), gave only decomposition on addition of aldehyde to the acetone precursor, which demonstrated the importance of the hemiabile ether group in DPEphos. With [Ph(2)P(C(6)H(4))](2)S, the sulfur atom has the opposite effect and binds too strongly to the metal centre to allow access to productive acetone intermediates.

A New Radical Route to C4-Unsubstituted β-Lactams

C4-Unsubstituted β-lactams 3 are conveniently prepared from easily available 4-formyl-β-lactams 1, in a sequential three step synthesis, using as the key step a radical reductive decarbonylation of 4-carboxy derivatives through their phenyl selenoesters 2. © 1997 Elsevier Science Ltd. All rights reserved.

Synthesis of deuterated cyclopropene fatty esters structurally related to palmitic and myristic acids

AbstractTo develop a synthesis of tritiated cyclopropene fatty acids (CPFA), compounds that should prove useful for affinity labeling of desaturases in insect pheromone biosynthetic studies, a series of novel, selectively deuterated CPFA analogues was prepared and characterized. In methyl [16‐2H]12,13‐methylene‐12‐hexadecenoate, the incorporation of deuterium was achieved by treatment of the corresponding ω‐chloro derivative with sodium borodeuteride in dimethylsulfoxide at 70°C for 24 h (67% yield) following conventional procedures. Alkylation of the tetrahydropyranyl derivative of 13‐tridecynol in the presence of lithium diisopropylamide in tetrahydrofuran at −20°C with 1‐chloro‐3‐iodopropane in hexamethylphosphoramide, followed by Jones oxidation of the crude product, yielded 16‐chloro‐12‐hexadecynoic acid (54%), which was esterified to the corresponding methyl ester by treatment with potassium carbonate and methyl iodide in dimethylformamide. Treatment of this acetylenic ester with ethyldiazoacetate in the presence of activated copper‐bronze as catalyst followed by hydrolysis in KOH solution at room temperature yielded 16‐chloro‐12,13‐(carboxymethylene)‐12‐hexadecenoic acid. This diacid was treated with excess oxalyl chloride to give the corresponding diacyl chloride, which was decarbonylated in a diethyl ether solution with zinc chloride, and the cyclopropenium ions thus formed were added at −40°C to a methanolic sodium hydroxide solution of sodium borohydride to give methyl 16‐chloro‐12,13‐methylene‐12‐hexadecenoate. Analogous procedures were followed to prepare methyl [17‐2H]10,11‐methylene‐10‐hexadecenoate, methyl [17‐2H]11,12‐methylene‐11‐hexadecenoate and methyl [17‐2H]12,13‐methylene‐12‐hexadecenoate from the corresponding diacids using sodium borodeuteride in the reduction of the cyclopropenium ions. Alternatively, methyl [2,2,3,3‐2H4]hexadecynoate, prepared by reaction of methyl 2,11‐hexadecadiynoate with magnesium in deuterated methanol at room temperature, was submitted to the above cyclopropenylation and reductive decarbonylation sequence to give methyl [2,2,3,3,17‐2H5]‐11,12‐methylene‐11‐hexadecenoate. In summary, complementary methods for the selective incorporation of one to five deuterium atoms into cyclopropene fatty acids, at different sites, in moderate to high yields have been developed. The methods should easily be applicable to the preparation of the corresponding tritiated analogues.

Studies in negative ion mass spectrometry: XIV—Electron attachment reactions of a series of η6‐arenetricarbonylchromium(O) complexes

AbstractElectron attachment reactions of a series of (η6‐arene)tricarbonylchromium(O) complexes have been examined in the gas phase. The electron capture process has been shown to be influenced by the structure of the η6‐arene ligand and its substituents. Whereas (η6‐benzene)‐ and (η6‐mesitylene)tricarbonylchromium(O) undergo dissocative electron capture, or reductive decarbonylation, yielding [MCO]−˙ ions of highest abundance in their negative ion mass spectra, [η6‐(2,2‐dimethylindan‐1,3‐dione)]tricarbonylchromium(O) forms a molecular negative ion which undergoes sequential CO eliminations and finally a demetallation to give the arene radical ion. A localization of charge on the coordinated arene ligand is proposed for the formation of [M]−˙ in this case. (η6‐Methylbenzoate)tricarbonylchromium(O) also forms a molecular negative ion by secondary electron attachment which decomposes by simultaneous and consecutive eliminations of up to four CO molecules. The elucidation of a mechanism and sequence for these CO eliminations has been achieved by synthesizing and examining the negative ion mass spectrum of [η6‐(C6H5·13CO2Me)]Cr(CO)3. The first CO loss in the principal fragmentation pathway occurs solely from the –Cr(CO)3 group of [M]−˙. The effect of para substituent groups on the stabilities of molecular negative ions and their fragmentations has been ascertained using a series of para‐substituted (η6‐methylbenzoate)tricarbonylchromium(O) compounds containing the groups NH2, OH, OCH3, CL and COOMe. The stabilities of the [M]−˙ ions have been rationalized in terms of the Hammett and Taft parameters σP, σI, σRP, σPO and σRO. The overall electronic substituent effect transmitted to the carbonyl groups of the –Cr(CO)3 unit involves both resonance and inductive components. In this series of compounds the stability of [M]−˙ decreases as the electron withdrawing capacities of the para substituents increase.

Studies in negative ion mass spectrometry. VIII—Electron capture by some monomeric and dimeric η5—cylcopentadienyl transition metal carbonyls

AbstractThe negative ion mass spectra of a series of monomeric and dimeric η5‐cyclopentadienyl transition metal carbonyls have been examined. The base peak in the case of the monomeric compounds (η5‐C5H5)V(CO)4, (η5‐C5H5)Mn(CO)3 and (η5‐CH3C5H4)Mn(CO)3 arises from a reductive decarbonylation of the parent molecule—the resulting radical anion [M–CO]− is formally isoelectronic with the molecular cations [M]− observed in the positive ion mass spectra of these compounds and subsequently undergoes successive decarbonylations to the ‘aromatic’ cyclopentadienyl anions. For the compound (η5‐C5H5)Co(CO)2, however, a molecular anion was observed as the base peak which has been formulated as [(η3‐C5H5)Co(CO)2]− in the light of considerations based on the rare gas rule. As expected, the dimeric molecules [(η5‐C5H5)M(CO)3]2 (where M = Cr or Mo) and [(η5‐C5H5)Fe(CO)2]2 (and its methyl analogue) undergo reductive cleavage of their metal‐metal bonds to give the anions [(η5‐C5H5)M(CO)3]− and [(η5‐C5H5)Fe(CO)2]− as the base peaks in their negative ion mass spectra. The dimeric nickel compound [(η5‐C5H5)Ni(CO)]2, however, reductively decarbonylates to the [M‐CO]− radical anion as its predominant fragmentation in the gas phase. Very low abundances of [(η5‐C5H5)Fe(CO)2] and [(η5‐CH3C5H4)Fe(CO)2] were also observed.