Metal Carbonyl Dimers Research Articles

Frustrated Al/M Heterobimetallic Complexes (M = Cr, Mo, W) That Exhibit Both Lewis and Radical Pair Behavior.

Exploration of new heterobinuclear Al/M combinations is relevant to contemporary strategies for cooperative bond activation. Here, we report the synthesis and characterization of six new Al/M heterobimetallic complexes (M = Cr, Mo, W) that exhibit end-on "isocarbonyl"-type Al─O═C═M bridges with metalloketene character rather than featuring Al─M─C≡O motifs with metal-metal bonding. The new compounds were characterized experimentally by nuclear magnetic resonance and infrared spectroscopies and theoretically using density functional theory, natural bond orbital, and quantum theory of atoms in molecules calculations. Factors influencing Al─O═C═M vs Al─M─C≡O isomerism were probed both experimentally and computationally. Crossover experiments between different group VI Al/M derivatives and regioselective epoxide ring opening indicate that the Al/M complexes act as masked frustrated Lewis pairs in solution under certain conditions. However, crossover experiments between group VI Al/M complexes and a previously studied Al-Fe complex, as well as computational modeling, imply that the same complexes can also reasonably act as masked frustrated radical pairs (FRPs). FRP reactivity with the group VI Al/M complexes was achieved under photochemical conditions, producing unsaturated metal-carbonyl dimers [(CpCr)2(CO)3]2- and [Mn2(CO)8]2-, which would otherwise be unstable under standard conditions but that are isolable here due to Al(III) coordination. The metal-metal bonding in these unsaturated metal-carbonyl dimers was also analyzed theoretically.

The effect of titanium alkoxides in the synthesis of heterobimetallic complexes by titanocene(III) alkoxide-induced metal–metal bond cleavage of metal carbonyl dimers

A series of titanocene(III) alkoxides L 2Ti(III)OR where L = Cp, R = Et( 1b), t Bu( 1a), 2,6-Me 2C 6H 3( 1c), 2,6- t Bu 2-4-Me–C 6H 2( 1d), or L = Cp*, R = Me( 2e), t Bu( 2a), Ph( 2f) was synthesized and subjected to reaction with [CpM(CO) 3] 2 [M = Mo, W], [CpRu(CO) 2] 2, and Co 2(CO) 8. The Ti(III) precursors 1a, 1c, 2a, 2e, and 2f reacted with [CpM(CO) 3] 2 [M = Mo, W] to form heterobimetallic complexes L 2Ti(OR)(μ-OC)(CO) 2MCp [M = Mo, W], of which Ti and M are linked by an isocarbonyl bridge. Reactions of these Ti(III) complexes with Co 2(CO) 8 resulted in formation of Ti–Co 1 heterobimetallic complexes, Cp 2 ∗ Ti ( O t Bu ) ( μ - OC ) Co ( CO ) 3 from 2a, 2e, or 2f, or Ti–Co 3 tetrametallic complexes, Cp 2Ti(O t Bu)(μ-OC)Co 3(CO) 9 from 1a, 1b, or 1c. The products were characterized by NMR, IR, and X-ray crystallography. Reaction mechanisms were proposed from these results, in particular, from steric/electronic effects of titanium alkoxides.

Thermally or Photochemically Induced Reductive Cleavage of Metal−Metal Bonds of Metal Carbonyl Dimers by a Titanocene(III) tert-Butoxide: Novel Reversible Access to Heterobimetallic Complexes

Novel synthetic methods for Ti−Mo, Ti−W, and Ti−Ru heterobimetallic complexes are established by the reaction of a certain titanium(III) alkoxide and the corresponding metal carbonyl dimer, [CpM(CO)n]2 [M = Mo, W (n = 3); M = Ru (n = 2)]. Thus, a novel monomeric titanium complex, Cp2Ti(OtBu) (1), which is synthesized from Cp2TiCl and KOtBu and characterized by spectroscopy and crystallography, smoothly reacts with [CpM(CO)3]2 (M = Mo (2a), W (2b)) to give heterobimetallic complexes, Cp2Ti(OtBu)(μ-OC)M(CO)2Cp (M = Mo (3a) and W (3b)), of which two metallic moieties are linked by the isocarbonyl bridge. Similar reaction of 1 with [CpRu(CO)2]2 (4) does not occur thermally, but is accomplished under photoirradiation to afford Cp2(OtBu)Ti−Ru(CO)2Cp (5), of which two metallic moieties are linked by a direct metal−metal bond. Use of 1 is particularly important for these reactions; other titanium alkoxides such as [Cp2Ti(OMe)]2 and Cp2Ti[O(2,6-tBu2-4-Me)C6H2] do not react with the metal carbonyl dimers. An interesting feature of these new heterobimetallic complexes, 3a, 3b, and 5, is the existence of the thermal fragmentation process to regenerate the starting materials: 3a or 3b is actually in equilibrium with a mixture of 1 and 2a or 1 and 2b, respectively [ΔG0298K= −4.1 ± 0.2 kcal mol-1 (3a), −4.3 ± 0.2 kcal mol-1 (3b)]. The Ti−Ru compound 5 thermally undergoes fragmentation to regenerate 1 and 4. The formation of these heterobimetallic complexes is formally considered as the metal−metal bond cleavage of metal carbonyl dimers by a Ti(III) reducing reagent; possible mechanisms are discussed.

IR spectroelectrochemical studies of the metal carbonyl dimers Mn2(CO)10 and [CpM(CO)3]2 (M = W, Mo). Cross-coupling and ligand substitution reactions of electrochemically generated organometallic radicals

Abstract Reflectance IR spectroelectrochemistry (IR SEC) has been used to study the electrochemical reactions of dinuclear metal carbonyl compounds. Reduction of the dinuclear metal carbonyl complexes by two electrons yields two equivalents of the corresponding monomeric metal carbonyl anions. Upon re-oxidation, the dimeric starting materials can be re-formed in almost quantitative yields. Reactions of the dinuclear metal carbonyls include metal-metal bond homolysis reactions such as electrochemical cross-coupling reactions between Mn2(CO)10 and [CpM(CO)3]2, where M = W, Mo. The ligand substitution reaction of Mn2(CO)10 and PPh3, which occurs only during the metal-metal bond homolysis reaction, is also described. The design of the anaerobic specular reflectance IR SEC cell used in these studies is also reported.

ELECTRON TRANSFER REACTIONS OF METAL CARBONYL ANIONS

Abstract Metal carbonyl anions exhibit one- and two-electron reactions. The two-electron processes involving transfer of groups (hydrogen, alkyl, and halogen) between metal centers are related to the nucleophilicity. The one-electron processes are primarily outer-sphere electron transfer for the metal carbonyl anions. These reactions are observed in the presence of oxidants such as coordination complexes, pyridinium salts, metal carbonyl dimers and metal carbonyl clusters. However, in contrast to organic reactions, the metal carbonyl anions may undergo inner-sphere electron transfer. Reactions of metal carbonyl anions of low nucleophilicity with metal carbonyl cations or halides are best interpreted as inner-sphere, one-electron transfer.

Kinetics and mechanism of the outer-sphere oxidation of metal carbonyl anions with coordination complexes containing chloride

Reactions of metal carbonyl anions, CpFe(CO){sub 2}{sup -}, Re(CO){sub 5}{sup -}, Mn(CO){sub 5}{sup -}, CpMo(CO){sub 3}{sup -}, CpCr(CO){sub 3}{sup -}, and Co(CO){sub 4}{sup -}, with CrCl{sub 3}{center_dot}3S (S = THF, CH{sub 3}CN) and reactions of Mn(CO){sub 5}{sup -} and Re(CO){sub 5}{sup -} with [Co(o-phen){sub 2}Cl{sub 2}]CIO{sub 4} are reported. Net oxidation/reduction chemistry is observed with formation of metal carbonyl dimers and CrCl{sub 2}{center_dot}4S or Co(o-phen){sub 2}Cl{sub 2}. Metal carbonyl halides are also observed and shown to arise from a secondary reaction of the metal carbonyl dimer with the oxidant. The products and rates are most consistent with outer-sphere electron-transfer reactions. Reactions of CpFe(CO){sub 2}{sup -} with CpFe(CO){sub 2}X (X = Cl, Br, I) are also reported. The rate dependence on X is very small and in the order expected for nucleophilic substitution. 21 refs. 1 fig., 4 tabs.

A new mechanism for the substitution reactions of metal carbonyl dimers

A new mechanism in which the heterolytic fission of the metal—metal bond is considered to be the key initial step in the substitution reactions of the metal carbonyl dimers MM′(CO) 10(M = M′ = Mn or Re; M = Mn, M′ = Re) and Co( 2(CO) 8 is described.

Electron transfer between mononuclear metal carbonyl anions (M(CO)5-, M = Mn, Re; CpFe(CO)2-; CpM(CO)3-, M = Cr, Mo) and trinuclear clusters (M3(CO)12, M = Fe, Ru, Os) and between trinuclear dianions (M3(CO)112-, M = Fe, Ru, Os) and metal carbonyl dimers (Mn2(CO)10 and Cp2M2(CO)6, M = Cr, Mo, W)

Electron transfer between mononuclear metal carbonyl anions (M(CO)5-, M = Mn, Re; CpFe(CO)2-; CpM(CO)3-, M = Cr, Mo) and trinuclear clusters (M3(CO)12, M = Fe, Ru, Os) and between trinuclear dianions (M3(CO)112-, M = Fe, Ru, Os) and metal carbonyl dimers (Mn2(CO)10 and Cp2M2(CO)6, M = Cr, Mo, W)

Reaction of metal carbonyl anions with metal carbonyl dimers: thermodynamic and kinetic factors that control the reactions

Reaction of metal carbonyl anions with metal carbonyl dimers: thermodynamic and kinetic factors that control the reactions

Laser flash photolysis of bis[tricarbonyl(cyclopentadienyl)tungsten] under visible irradiation

Continuous photolysis studies of single-bonded metal carbonyl dimers show evidence for a homolytic cleavage of the metalmetal bond and a carbonyl-loss process. Most time-resolved experiments have been performed with UV excitation and the nature of the excited state(s) involved has not been completely elucidated. Using visible laser irradiation of bis[tricarbonyl(cyclopentadienyl)tungsten] in pure toluene, the spectra of two intermediates can be established at different times after the pulse: a seventeen-electron radical in the microsecond region and a species due to the loss of a carbonyl ligand on the millisecond scale. The processes follow a quadratic dependence on the light intensity; this indicates a simultaneous two-photon absorption populating a higher excited state which subsequently contributes to the observed photochemistry. Kinetic analysis of the fast process gives a nearly diffusion-controlled second-order decay in agreement with a bimolecular recombination in neat toluene. The disappearance of the longer-lived intermediate is found to be first-order, independent of the concentration of added CO. The latter observation suggests that the rate-determining step in the recapture of CO is the opening of a CO-bridged isomer. The molybdenum analog displays a similar behavior.

Reactions of anionic transition metal carbonyl hydrides with electrophilic metal carbonyls: Nucleophilic addition (hydride transfer) vs. electron transfer mechanisms

The relative reactivity of anionic transition metal carbonyl hydrides towards hydride transfer to monomeric metal carbonyls is consistent with an ionic route, generating, in the best-matched cases of electrophilic carbonyls (highest f CO or average v(CO)) and most nucleophilic hydrides (such as HW(CO) 4P(OMe) 3 −), metal formyls as sole product. With the metal carbonyl dimers Co 2(CO) 8, [(η 5-C 5H 5)M(CO) 3] 2 (M = Mo, W), and Mn 2(CO) 10 reductive cleavage to the respective anions occurs at rates which correlate with the nucleophilicity of the metal hydride, without observation of intermediate formyls. No reaction occurs with Re 2(CO) 10 or [(η 5-C 5H 5)Fe(CO) 2] 2.

Kinetics and mechanism of electron transfer from pentacarbonylrhenate to metal carbonyl dimers by infrared stopped-flow spectroscopy

The results of the reactions of Re(CO){sub 5}{sup {minus}} with the dimers Mn{sub 2}(CO){sub 10}, CO{sub 2}(CO){sub 8} and cp{sub 2}Mo{sub 2}(CO){sub 6} are reported herein. Infrared spectroscopy was used for product examination. The reaction rates observed had a first order dependence on the oxidant and reductant, and the dependence on the dimer was of the order Cp{sub 2}Mo{sub 2}(CO){sub 6}, CO{sub 2}(CO){sub 8}, Mn{sub 2}(CO){sub 10}. Both inner-sphere and outer-sphere mechanisms are ruled out by the data. 10 refs., 2 figs., 3 tabs.

Reaction of the hydrido-bridged, group 6 metal carbonyl dimers with organic alkoxides. An apparent beta-hydride elimination reaction pertinent to catalysis

IR spectroscopic and GC-MS studies have been made of the reaction between the Group 6 metal carbonyl dimers [μ-HM 2(CO) 10] − (where M = Cr and W) and organic alkoxides, R 1R 2CHO −, both as the PPN + salts. Where R 1 = R 2 = Ph or Me, the reactions result in the oxidation of the alkoxide to benzophenone and acetone, respectively, as determined by GC-MS. IR spectroscopic studies indicate that the reaction involves cleavage of the hydrido-bridged dimer reactants to give the hydride monomers, [HM(CO) 5] −, and an intermediate alkoxo complex, [R 1R 2CHOM(CO) 5] −. The overall rate of oxidation of a given alkoxide is slower when M = W than when M = Cr. It is proposed that the alkoxo intermediate is responsible for the formation of the ketone products.

Bimetallic anionic formyl complexes: synthesis and properties

Three bimetallic anionic complexes, (2) lithium + dimanganese nonacarbonyl formyl/sup -/, (3) lithium + rhenium manganese nonacarbonyl formyl/sup -/, and (4) lithium + cisdirhenium nonacarbonyl formyl, are prepared by the reaction of lithium triethylboron hydride with the corresponding neutral metal carbonyl dimers, dimanganese decacarbonyl and manganese rhenium decarbonyl. 2 has a half-life of ca 8 min at room temperature, 4 is stable for days and is easily isolated as a tetrahydrofuran solvate. When 2 - 4 are treated with electrophiles such as benzaldehyde, iron pentacarbonyl, n-octyl iodide, hydride transfer occurs to produce benzyl alcohol, lithium + iron tetracarbonyl formyl/sup -/ complex, and octane, respectively. 3 is the weakest hydride donor. Reaction of 4 with methyl iodide produces ca 52% methane. The only identifiable product from the pyrolysis of 4 is dirhenium decacarbonyl; photolysis of 4 produces lithium + dirhenium nonacarbonyl hydride. 1 figure.

Synthesis of metal carbonyl monoanions by trialkylborohydride cleavage of metal carbonyl dimers: a convenient one-flask preparation of metal alkyls, metal acyls, and mixed-metal compounds

Commercially available trialkylborohydrides Li(C2H5),BH, Li(sec-C4H9),BH, and K(sec-C4H9),BH effect the nearquantitative, homogeneous, room-temperature synthesis of transition-metal monoanions [Co(CO),]-, [(C,H,)Mo(CO),]-, [Mn(CO),]-, and [ (C,HS)Fe(CO),]- from the corresponding metal carbonyl dimers. Reactions are nearly instantaneous except in the case of [(C5Hs)Fe(CO)2]2, which is reduced over a 3-h period by K(sec-C4H9),BH in THF or within 2 h by Li(C2HB these aspects are discussed in detail.

Insertion Reactions of Germanium Dibromide with Metal Carbonyl Dimers

Abstract Germanium dibromide reacts with [π-C5H5Fe(CO)2]2 [π-C5H5Mo(CO)3]2 and Fe2 (CO)9 to give insertion products, [π-C5H5Fe(CO)2]2GeBr2, (π-C5H5) Mo (CO)3GeBr3 and [(CO)4Fe]2 GeBr2, respectively. Dimanganese decacarbonyl reacts with germanium dibromide to give MnBr2 and GeO2 in refluxing p-dioxane. Although a rapid reaction between dicobalt octacarbonyl and germanium dibromide was observed, no organometallic product could be isolated from the reaction mixture.

A convenient preparation of metal carbonyl monoanions by trialkylborohydride cleavage of metal carbonyl dimers; observation and reactions of a bimetallic manganese-formyl intermediate

Commercially available Li(C 2H 5) 3BH rapidly and quantitatively cleaves [Mn(CO) 5] 2, [Co(CO) 4] 2, and [(C 5H 5)Mo(CO) 3] 2 in THF at room temperature to the corresponding transition metal monoanions. [(C 5H 5)Fe(CO) 2] 2 can be cleaved in hexamethylphosphorous triamide (HMPA) or by use of K((unsec)-C 4H 9) 3BH. The mechanism postulated for Li[Mn(CO) 5] formation (Scheme I) is supported by the direct observation of a manganese-formyl intermediate ((un4)) and rate data. ((un4)) acts as a hydride donor toward CH 3SO 3F, Fe(CO) 5, and HMn(CO) 5.

The chemistry of metal carbonyl anions : III. Sodium-potassium alloy: An efficient reagent for the production of metal carbonyl anions

Reduction of several metal carbonyl dimers including Mn 2](CO) 10, [C 5H 5Fe(C0) 2] 2, Co 2(CO) 8, and [C 5H 5M(CO) 3] 2 (M = Cr, Mo and W) by sodium—potassium alloy (NaK) in tetrahydrofuran at room temperature provides a rapid and clean method for the production of the corresponding metal carbonyl anions in high yield. Isolation and characterization of [n-Bu 4N] [Fe(CO) 2C 5H 5] from the iron dimer reduction is described. Reductions of other carbonyls including M(CO) 6 (M = Cr, Mo and W) and Re 2(CO) 10 proceed more slowly than previously established methods and provide principally M 2(CO) 10 2− and Re(CO) 5 5 −. Methods for the preparation of Re(CO) 5 − are critically considered. The reaction of NaK with [C 5H 5NiCO] 2 is discussed in relation to previously reported results. Infrared solution spectra of a number of carbonyl anions in THF, obtained in a special infrared solution cell, are reported.

Reactivity of metalmetal bonds : XII. The preparation of some phenylchloro group IVB derivatives of pentacarbonylmanganese and (π-cyclopentadienyl)dicarbonyliron and their reactivities towards pentafluorophenyllithium

The reaction of the appropriate phenylgermanium halide with the transition metal anions Mn(CO) — 5 or Fe(CCO) 2Cp — (M′) in THF gave the compounds Ph 3— n Cl n GeM′ ( n = 1 or 2), while the analogous silicon compounds were prepared by the reaction of the appropriate phenylchloro- or phenylfluorophenyl-silane with the transition metal carbonyl dimers. The reactivities of the compounds Ph 3— n Cl n MM′ [ n = 1 to 3; M = Si, Ge or Sn and M′ = Mn(CO) 5 or Fe(CO) 2Cp] towards pentafluorophenyllithium have been found to depend upon M, M′ and teh number of phenyl groups bonded to the Group IV metal. With M′  Mn(CO) 5 the reativity decreased in the order Sn ≈ Ge > Si. For infrared, NMR and 57Fe Mössbauer spectra of these and related compounds are discussed.