Phosphine-substituted Complexes Research Articles

Synthesis, characterization and electrocatalytic H2 production of phosphine-substituted CpFe complexes

Reaction of Ph2P(Se)CSNHPh with Cp2Fe2(CO)4 in refluxing toluene affords complexes CpFe(CO)(k2P,S-Ph2PC(S)NPh) (1) and CpFe(CO)2(kP-P(Se)Ph2) (2). Further, reactions of 2 with 1 equiv of PPh2R in the presence of Me3NO generate complexes CpFe(CO)(kSe-SeP(Se)Ph2)(L) (3, L = PPh3; 4, L = PPh2Py). Unlike 2, reactions of CpFe(CO)2(kP-P(S)Ph2) with PPh2R under the same conditions give the corresponding phosphine-substituted complexes CpFe(CO)(kP-P(S)Ph2)(L) (5, L = PPh3; 6, L = PPh2Py). All the new compounds have been characterized by elemental analyses, IR, 1H NMR, 13C NMR and 31P NMR spectroscopy and structurally determined by X-ray crystallography. Electrochemical studies indicate that using HOAc or TFA as a proton source complexes 1–6 can catalyze H2 evolution.

Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe3(CO)7(μ-edt)2] and Phosphine Derivatives [Fe3(CO)7-x (PPh3) x (μ-edt)2] (x = 1, 2) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] as Proton Reduction Catalysts.

The mixed-valence triiron complexes [Fe3(CO)7–x(PPh3)x(μ-edt)2] (x = 0–2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(μ-edt)2]+ and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)–Fe(II)–Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(μ-edt)2] is oxidized at −0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between −1.47 V for [Fe3(CO)7(μ-edt)2] and around −1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. In comparison to the diiron complex [Fe2(CO)6(μ-edt)], [Fe3(CO)7(μ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron–iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.

Density Functional Study on the Mechanism of Nickel-Mediated Diazo Carbonylation

The mechanism of diazo activation as well as the carbonylation of the resulting carbene complexes has been investigated by means of DFT calculations at the PBE0/TZVP level of theory. The free energy profile of all elementary steps of the reaction, i.e., diazo coordination, dinitrogen extrusion, carbene–CO coupling, CO coordination, and ketene elimination, have been elucidated for diazomethane and ethyl diazoacetate as substrates and for Ni(CO)3, Ni(CO)2(PH3), and Ni(dtbpe)(CO) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) as precursors. The reaction rate is determined by the formation of the coordinatively unsaturated precursor followed by the dinitrogen extrusion, regardless of the initial diazo compound and the catalyst precursor. For the homoleptic precursor Ni(CO)3 the free energy of activation for diazomethane and ethyl diazoacetate (EDA) is 16.9 and 22.4 kcal/mol, respectively. The replacement of one carbonyl ligand with PH3 results in a decrease in the activation barrier, modifying the energies to 15.7 and 20.5 kcal/mol, respectively. The activation free energy of the diazo extrusion step promoted by Ni(dtbpe)(CO) is 24.3 kcal/mol for diazomethane and 28.1 kcal/mol for EDA. The formation of carbene complexes is slightly endergonic starting from the homoleptic precursor and exergonic for the phosphine-substituted complex. The carbene–CO coupling, resulting in coordinatively unsaturated ketene complexes, is a fast and highly exergonic process with reaction free energies between −32.3 and −42.1 kcal/mol, depending on the substituents. Under a carbon monoxide atmosphere the ketene complexes may uptake one CO, forming coordinatively saturated ketene complexes via low barriers in exergonic reactions. The final step, i.e., the dissociation of ketene or ethoxycarbonylketene from the saturated ketene complex, regenerates the corresponding nickel–carbonyl precursor. The azine formation side reaction for the simplest case between Ni(CO)3 and diazomethane has been also examined and found to be highly exergonic; however, the large free energy barrier prevents the formation of benzophenone-azine when CO is also present.

Reactions of cyclometalated carbonyliron complex derived from thienyl Schiff base

Cyclometalated hexacarbonyldiiron complex 2, which derived from N-(2-thienylmethylidene)aniline, undergoes (1) thermolysis to recover the original Schiff base 1, (2) reduction to form a hydrogenation product of the original thienyl Schiff base 3, (3) substitution to form a phosphine-substituted complex 4, and (4) chemical as well as electrochemical oxidation to produce a γ-lactam 5.

Hydrido-sulfido-bridged triangular Os3 cluster compounds with different phosphine ligand substitution patterns

In the course of our studies of trinuclear osmium cluster complexes with bridging sulfido and hydrido ligands, the new compounds Os3(μ-H)(μ-SR)(CO)9(PHCy2) (Cy = cyclo­hexyl) with R = phenyl, (I) (nona­carbonyl-1κ3C,2κ3C,3κ3C-di­cyclo­hexyl­phosphine-3κP-μ-hydrido-1:2κ2H-μ-phenyl­thio-1:2κ2S-triangulo-triosmium), [Os3H(C6H5S)(C12H23P)(CO)9], and R = naphthyl, (II) [nona­carbonyl-1κ3C,2κ2C,3κ4C-di­cyclo­hexyl­phosphine-2κP-μ-hydrido-1:2κ2H-μ-(2-naphthyl­thio)-1:2κ2S-triangulo-triosmium], [Os3H(C10H7S)(C12H23P)(CO)9], were prepared. We report on these two phosphine-substituted complexes, which exhibit perceptible changes of the Os—Os bond parameters due to the ligand-substitution pattern.

Regiospecificity in reactions of alkynes and phosphines with the phosphido-bridged iron–cobalt complex [(OC) 4Fe(μ-PPh 2)Co(CO) 3

The iron–cobalt phosphido-bridged complex [(OC) 4Fe(μ-PPh 2)Co(CO) 3] ( 1) can be prepared conveniently and in good yield from the reaction of [Co 2(μ-PPh 2) 2(CO) 6] with [Fe(CO) 5]. A study of the reactivity of 1 towards symmetrical and unsymmetrical alkynes, R 1CCR 2 (R 1=R 2=CO 2Me, Ph; R 1=H, R 2=Ph), has been undertaken. In all cases, five-membered ferracycle-containing products of the type [(OC) 3Fe{μ-PPh 2C(O)CR 1CR 2}Co(CO) 3] (R 1=R 2=CO 2Me ( 2a), Ph ( 2c); R 1=H, R 2=Ph ( 2b)), are initially obtained in which a molecule of CO and a molecule of R 1CCR 2 have been inserted regiospecifically into a CoP bond in 1. Decarbonylation of 2a occurs during its preparation or in low yield on its thermolysis to give the four-membered ferracyclic species [(OC) 3Fe{μ-PPh 2C(CO 2Me)C(CO 2Me)}Co(CO) 3] ( 3a). Similar thermolysis of 2b results not only in the related decarbonylation product [(OC) 3Fe(μ-PPh 2CHCPh)Co(CO) 3] ( 3b), but additionally in three other products all in low yield, namely the regioisomer of 3b, [(OC) 3Fe(μ-PPh 2CPhCH)Co(CO) 3] ( 4b), the aldehyde-substituted complex [(OC) 3Fe{μ-PPh 2C(CHO)CPh}Co(CO) 3] ( 5b) and the five-membered ferracycle-containing species [(OC) 3Fe{μ-PPh 2CHCPhC(O)}Co(CO) 3] ( 6b). Treatment of 2a and 2b with PPhMe 2 and P(OMe) 3 results in substitution of an iron-bound carbonyl group to give, respectively, [(PhMe 2P)(OC) 2Fe{μ-PPh 2C(O)C(CO 2Me)C(CO 2Me)}(μ-CO)Co(CO) 2] ( 7a) and [{(MeO) 3P}(OC) 2Fe{μ-PPh 2C(O)CHCPh}(μ-CO)Co(CO) 2] ( 7b) in high yield. In contrast, substitution of a cobalt-bound carbonyl is achieved on reaction of 3a with PPhMe 2 or PPh 2H to give [(OC) 3Fe{μ-PPh 2C(CO 2Me)C(CO 2Me)}Co(CO) 2(L)] (L=PPhMe 2 ( 8a), PPh 2H ( 9a)). Thermolysis of the secondary phosphine-substituted complex 9a results in phosphorushydrogen bond cleavage to give [(OC) 3Fe{μ-PPh 2C(CO 2Me)CH(CO 2Me)}(μ-PPh 2)Co(CO) 2] ( 10a). Single-crystal X-ray diffraction studies have been performed on complexes 7b, 8a and 10a.

Linking of benzyne and carbon monoxide at a triruthenium centre by migratory insertion

The μ 3-benzyne complex [Ru 3(CO) 7( μ-PPh 2) 2( μ 3-C 6H 4)] ( 1a) reacts at room temperature (r.t.) with carbon monoxide (1 atm), trimethylphosphite, or t-butylisocyanide to give complexes [Ru 3(CO) 6L 2( μ-PPh 2) 2( μ 3-C 6H 4CO)] ( 2a, L=CO; 2b, L=P(OMe) 3; 2c, L= t BuNC) containing an ortho-metallated μ 3-benzoyl ligand. Studies with 13CO confirm that the reaction involves a migratory insertion of CO into a benzyne-CO σ-bond. The μ 3-benzoyl products are formed exclusively when a large excess of the π-acceptor ligand is used, but at lower concentrations substitution of CO by the incoming ligand also occurs to give the complexes [Ru 3(CO) 6L( μ-PPh 2) 2( μ 3-C 6H 4)] [ 1b, L=P(OMe) 3; 1c, L= t BuNC]. Although 13C-labelling studies show that the reaction of 1a with CO is irreversible at r.t., it is readily reversed by warming complex 2a in hexane. No reaction occurs at r.t. between complex 1a and triphenylphosphine or tri-isopropylphosphine, but on heating in toluene the phosphine-substituted complexes [Ru 3(CO) 6(PR 3)( μ-PPh 2) 2( μ 3-C 6H 4)] ( 1d, R=Ph; 1e, R= i Pr) are formed, which upon carbonylation give complexes [Ru 3(CO) 7(PR 3)( μ-PPh 2) 2( μ 3-C 6H 4CO)] ( 2d, R=Ph; 2e, R= i Pr). The structure of the complexes 2 has been established by an X-ray diffraction study on the tri-isopropylphosphine-substituted cluster 2e. Benzaldehyde is obtained from complex 2a by bubbling hydrogen through a heated solution of the complex. A mechanism for the carbonylation of μ 3-benzyne is proposed and implications for metal surface chemistry discussed.

The photochemical generation of heterobimetallic complexes containing carbon disulfide

Photochemistry has been used to generate a number of heterobimetallic complexes containing π + σ CS2 bridging groups. Broad-band UV photolysis of tetrahydrofuran (thf) solutions of [M(CO)6] (M = Cr, Mo or W) with [Fe(CO)2(PR3)2(η2-CS2)] (R = Et or Ph) leads to the formation of the bimetallic complexes [(PR3)2(CO)2Fe(µ-η2∶η1-CS2)M(CO)5]. Photolysis of acetonitrile solutions of [Os3(CO)12] with [Fe(CO)2(PR3)2(η2-CS2)] leads to [(PR3)2(CO)2Fe(µ-η2∶η1-CS2)Os3(CO)11]. Photolysis of a thf solution of (PEt3)2(CO)2Fe(µ-η2∶η1-CS2)M(CO)5] with PEt3 results in the formation of the phosphine-substituted complex [(PPh3)2(CO)2Fe(µ-η2∶η1-CS2)M(CO)4(PEt3)]. Single-crystal X-ray diffraction has been used to determine the molecular structures of [(PPh3)2(CO)2Fe(µ-η2∶η1-CS2)W(CO)5], [(PEt3)2(CO)2Fe(µ-η2∶η1-CS2)Os3(CO)11] and [(PPh3)2(CO)2Fe(µ-η2∶η1-CS2)Cr(CO)4(PEt3)].

Cylindrical internal reflectance (CIR) evidence for the facile fragmentation of PhCCO 3(CO) 7( cis-Ph 2PCHCHPPh 2) during the hydroformylation of 1-pentene

The hydroformylation of 1-pentene has been examined with the catalyst precursor PhCCo 3(CO) 7(LL) (where LL= cis-Ph 2PCHCHPPh 2) under mild batch conditions at 130°C and 600 psi H 2/CO (1:1). Extensive cluster fragmentation is observed at the end of the hydroformylation reaction based on the isolation of the initially charged cluster in < 10% yield. Cylindrical internal reflectance (CIR) spectroscopy has been used to study the stability of the PhCCo 3(CO) 7(LL) cluster and the nature of cluster fragmentation species present in the working catalyst solution. Cluster fragmentation to [Co(CO) 4] − occurs early in the reaction prior to alkene hydroformylation, followed by the formation of the phosphine-substituted complexes [Co(CO) 3(LL)] + and [Co(CO) 2(LL)] −. The CIR data allow for the timing of the fragmentation of the cluster to be assigned with confidence and support the batch reaction data regarding the extensive fragmentation cluster observed. The effect of the ancillary phosphine ligand on the stability of the initially charged cluster is discussed and the CIR data is compared with the results obtained from the parent cluster PhCCo 3(CO) 9 under similar conditions.

Synthesis of bis[µ-bis(diphenylphosphino)methane]-tri-µ-carbonyl-tricarbonyl-triangulo-trirhodium(1+) perchlorate, [Rh3(CO)3(µ-CO)3(µ-dppm)2]ClO4. An unusual 46-electron cluster possessing the A-frame structure with a bridging Rh(CO)3fragment

The A-frame complexes [Rh2(µ-OR)(CO)2(µ-dppm)2]ClO4(R = H, Me, or Et; dppm = Ph2PCH2PPh2) react under carbon monoxide to give the 46-electron, trinuclear cluster [Rh3(CO)3(µ-CO)3(µ-dppm)2]ClO4 whose structure resembles that of an A-frame complex with a bridging Rh(CO)3group. The unique terminal carbonyl on the bridging rhodium atom undergoes rapid exchange with 13CO and is readily substituted by tertiary phosphines PR3(R3= MePh2, Et3, or Ph3; PR3= dppm). Unlike the parent cluster, the phosphine-substituted complexes [Rh3(CO)2(µ-CO)3(Pr3)(µ-dppm)2]ClO4 undergo rapid exchange of the 1,3 and 2,3 bridging carbonyl groups only. In addition, [Rh3(CO)2(µ-CO)3(dppm)(µ-dppm)2]ClO4 is fluxional, showing relatively rapid flipping between one donor atom and the other in the diphosphine. All complexes have been characterized on the basis of i.r., 1H, 13C, and 31P n.m.r. spectroscopy. The structure of [Rh3(CO)3(µ-CO)3(µ-dppm)2]ClO4 is discussed and a rationale proposed for the stability of the 46-electron cluster by comparison to the expected 48-electron species [Rh3(CO)7(µ-dppm)2]+.

A new synthetic method for the preparation of [(η 5-C 5H 5)Fe(CO)(L)(un)]BF 4 (L = CO, PPh 3; un = unsaturated hydrocarbon) complexes and reduction of the η 2-acetylene complexes

The reaction of (η 5-C 5H 5)Fe(CO) 2I with AgBF 4 in CH 2Cl 2 generates the reactive intermediate [(η 5-C 5H 5)Fe(CO) 2] + in solution which, when mixed with 2 or 3 equivalents of an olefinic or acetylene ligand produces η 2-olefin and η 2-acetylene complexes in ca. 70% yield. The analogous reaction of (η 5-C 5H 5)Fe(CO)(PPh 3)I with AgBF 4 in CH 2Cl 2 produces [(η 5-C 5H 5)Fe(CO)(PPh 3)] + in solution which also reacts readily with olefins and acetylenes to yield new π-complexes. The dicarbonyl complexes are generally more stable than the phosphine-substituted complexes. The η 2-acetylene complexes in both systems can be reduced with hydride reagents to yield the corresponding neutral σ-vinyl compounds. Although unstable in the dicarbonyl system, these vinyl compounds in the phosphine system are quite stable and show no tendency to isomerize or thermally decompose to a metal hydride as previously established for the analogous alkyl derivatives. Attempts to prepare cyano-substituted olefin complexes lead instead to new complexes in which the iron bonds to these ligands through the nitrogen lone-pair.