Michael G. Richmond

Models of the iron-only hydrogenase: a comparison of chelate and bridge isomers of Fe2(CO)4{Ph2PN(R)PPh2}(μ-pdt) as proton-reduction catalysts

Reactions of Fe2(CO)6(μ-pdt) (pdt = SCH2CH2CH2S) with aminodiphosphines Ph2PN(R)PPh2 (R = allyl, (i)Pr, (i)Bu, p-tolyl, H) have been carried out under different conditions. At room temperature in MeCN with added Me3NO·2H2O, dibasal chelate complexes Fe2(CO)4{κ(2)-Ph2PN(R)PPh2}(μ-pdt) are formed, while in refluxing toluene bridge isomers Fe2(CO)4{μ-Ph2PN(R)PPh2}(μ-pdt) are the major products. Separate studies have shown that chelate complexes convert to the bridge isomers at higher temperatures. Two pairs of bridge and chelate isomers (R = allyl, (i)Pr) have been crystallographically characterised together with Fe2(CO)4{μ-Ph2PN(H)PPh2}(μ-pdt). Chelate complexes adopt the dibasal diphosphine arrangement in the solid state and exhibit very small P-Fe-P bite-angles, while the bridge complexes adopt the expected cisoid dibasal geometry. Density functional calculations have been carried out on the chelate and bridge isomers of the model compound Fe2(CO)4{Ph2PN(Me)PPh2}(μ-pdt) and reveal that the bridge isomer is thermodynamically favourable relative to the chelate isomers that are isoenergetic. The HOMO in each of the three isomers exhibits significant metal-metal bonding character, supporting a site-specific protonation of the iron-iron bond upon treatment with acid. Addition of HBF4·Et2O to the Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-pdt) results in the clean formation of the corresponding dibasal hydride complex [Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-H)(μ-pdt)][BF4], with spectroscopic measurements revealing the intermediate formation of a basal-apical isomer. A crystallographic study reveals that there are only very small metric changes upon protonation. In contrast, the bridge isomers react more slowly to form unstable species that cannot be isolated. Electrochemical and electrocatalysis studies have been carried out on the isomers of Fe2(CO)4{Ph2PN(allyl)PPh2}(μ-pdt). Electron accession is predicted to occur at an orbital that is anti-bonding with respect to the two metal centres based on the DFT calculations. The LUMO in the isomeric model compounds is similar in nature and is best described as an antibonding Fe-Fe interaction that contains differing amounts of aryl π* contributions from the ancillary PNP ligand. The proton reduction catalysis observed under electrochemical conditions at ca. -1.55 V is discussed as a function of the initial isomer and a mechanism that involves an initial protonation step involving the iron-iron bond. The measured CV currents were higher at this potential for the chelating complex, indicating faster turnover. Digital simulations showed that the faster rate of catalysis of the chelating complex can be traced to its greater propensity for protonation. This supports the theory that asymmetric distribution of electron density along the iron-iron bond leads to faster catalysis for models of the Fe-Fe hydrogenase active site.

1,1′-Bis(diphenylphosphino)ferrocene ligand substitution in the benzylidyne-capped cluster PhCCo3(CO)9. Synthesis, X-ray structure, and redox reactivity of PhCCo3(CO)7(dppf)

Abstract The reaction between 1,1′-bis(diphenylphosphino)ferrocene (dppf) and the tricobalt cluster PhCCo 3 (CO) 9 ( 1 ) yields the disubstituted cluster PhCCo 3 (CO) 7 (dppf) ( 2 ). The dppf ligand in 2 bridges adjacent cobalt centers via axial coordination. Ligand substitution leading to 2 may be achieved by thermolysis, oxidative decarbonylation using trimethylamine oxide, and by electrontransfer chain (ETC) catalysis using sodium benzophenone ketyl. The isolated yield of 2 ranged from 50 to 70% in all cases. Solution characterization of 2 by FT-IR and 31 P NMR spectroscopy is presented along with the single-crystal X-ray diffraction results. The dppf-bridged cluster PhCCo 3 (CO) 7 (dppf)·(toluene) crystallized in the monoclinic space group P 2 1 / c with a = 13.560(3), b = 17.339(3), c = 21.482(3) A, β = 106.81(1)°, V = 4835(1) A 3 and Z = 4. Block-cascade least-squares refinement yielded R = 0.0630 for 5055 ( I > 3σ( I )) reflections. The redox reactivity of 2 was examined by cyclic voltammetry, which revealed the presence of two irreversible oxidations that are attributed to the oxidation of the dppf ligand and the cluster core along with the observation of an irreversible reduction that exhibits cyclic voltammetric curve crossing. All of the redox processes are discussed with respect to existing tricobalt cluster redox chemistry.

Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions

Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L(1)) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L(2)), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [Fe(II)(CH3CN)(L)](2+) (L = L(1) (1); L(2) (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe(IV)(O)(L)](2+) (L = L(1) (3); L(2) (4)), which were characterized by UV-vis spectroscopy, high resolution mass spectrometry, and Mössbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives at room temperature of 40 h (L = L(1)) and 2.5 h (L = L(2)). The redox potentials of 1 and 2, as well as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex ([Fe(IV)(O)(N4Py)](2+)), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement of a pyridyl moiety; i.e., [Fe(IV)(O)(L(2))](2+) exhibits the highest rates. The second-order HAT rate constants can be directly related to the substrate C-H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction proceeds via a high spin transition state.

Bidentate ligand substitution in PhCCo3(CO)9. Synthesis, molecular structure, and redox reactivity of PhCCo3(CO)7(cis-Ph2PCHCHPPh2

Abstract The synthesis of PhCCo 3 (CO) 7 ( cis -Ph 2 PCHCHPPh 2 ) ( 2 ) from PhCCo 3 (CO) 9 ( 1 ) and the bidentate phosphine cis (9), b = 18.385 (2), c = 15.943 (1) A 3 , β = 98.025 (6)°, V = 3688.0 (5) A 3 , and Z = 4. Full-matrix least-squares refine an irreversible, multi-electron oxidation. The electrochemistry of 2 is compared to the known cluster PhCCo 3 (CO) 7 (dppe).

Annual survey of organometallic metal cluster chemistry for the year 2000

Abstract The synthetic, mechanistic, and structural chemistry of organometallic metal cluster compounds is reviewed for the year 1999.

1-pentene hydroformylation using the mixed-metal cluster Fe2Co2(Co)11(μ4-PPh)2: cylindrical internal reflectance evidence for cluster catalysis

Abstract The mixed-metal cluster Fe 2 Co 2 (CO) 11 (μ 4 -PPh) 2 (1) has been found to catalyze the hydroformylation of 1 -pentene to hexanal and 2-methyl-pentanal in moderate to high yield under mild batch conditions. Cluster catalysis is suggested based on FT-IR and HPLC analyses of the final reaction solutions, and in situ Cylindrical Internal Reflectance measurements of the working catalyst solution. A closo→nido polyhedral transformation in cluster 1 is proposed as the entry point into the catalytic cycle.

Regioselective phosphine attack on the coordinated alkyne in Co2(μ-alkyne) complexes Reactivity studies and X-ray diffraction structures of Co2(CO)4(bma)(μ-HCCtBu) and the zwitterionic hydrocarbyl complexes Co2(CO)4[μ-η2:η2:η1:η1-RCC(R′)PPh2CC(PPh2)C(O)OC(O)]

Abstract The alkyne-bridged compounds Co 2 (CO) 6 (μ-R′CCR) where R′  H, R,  Ph, t Bu; R′  Me, R  Ph) have been examined for their reactivity with the redox-active diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma). Thermal and Me 3 NO-induced activation of each Co 2 (CO) 6 (μ-R′CCR) compound initiallyfurnishes the binuclear compounds Co 2 (CO) 4 (bma)(μ-R′CCR) ( 1c , R′  H, R  Ph; 2c , R′  H, R  t Bu; 3c , R′  Me, R  Ph), all of which are shown to possess a chelating bma ligand. These compounds exhibit a reversible chelate-to-bridge bma ligand isomerization that proceeds by a pathway involving dissociative CO loss. The stability of the chelating isomers is dependent on the steric bulk of the alkyne substituents, with the bridging isomer ( 1b, 2b, 3b ) being favored at ambient temperature only in the case of Co 2 (CO) 4 (bma)(μ-HCC t Bu) ( 2b ). Regioselective phosphine attack at the least-substituted alkyne carbon is observed at temperatures above 60 °C to afford the corresponding zwitterionic hydrocarbyl compounds Co 2 (CO) 4 [μ-η 2 :η 2 :η 1 :η 1 -RCC(R′)PPh 2 CC(PPh 2 )C(O)OC (O)] ( 1z, 2z, 3z ). Depending on the ease of separation, the new compounds have been characterized by IR and NMR spectroscopy, and the molecular structure of the bridged compound 2b and all three zwitterionic hydrocarbyl compounds have been determined by X-ray crystallography. 1z crystallizes in the triclinic space group P 1 : a = 9.8093(7) A , b = 12.215(1) A , c = 15.744(2) A , α = 98.251(8)°, β = 95.060(8)°, γ = 90.640(7)°, V = 1859.1(3) A 3 , Z = 2, d calc = 1.426 g cm −3 ; R = 0.0365, R w = 0.0469 for 2749 observed reflections. 2b crystallizes in the triclinic space group P 1 : a = 11.1272(7) A , b = 12.142(1) A , c = 13.565(1) A , α = 97.123(7)°, β = 94.951(6)°, γ = 100.197(6)°, V = 1778.8(2) A 3 , Z = 2, d calc = 1.453 g cm −3 ; R = 0.0469, R w = 0.0484 for 3357 observed reflections. 2z crystallizes in the monoclinic space group P2 1 /n: a = 9.465(7) A , b = 20.071(8) A , c = 18.389(6) A , β = 98.88(5)°, V = 3452(3) A 3 , Z = 4, d calc = 1.498 g cm −3 ; R = 0.0544, R w = 0.0589 for 3114 observed reflections. 3z , as the CH 2 Cl 2 solvate, crystallizes in the triclinic space group P 1 : a = 9.732(3) A , b = 12.286(2) A , c = 16.715(3) A , α = 98.39(1)°, β = 102.91(2)°, γ = 91.93(2)°, V = 1922.6(8) A 3 , Z = 2, d calc = 1.550 g cm −3; R = 0.0730, R w = 0.0825 for 2896 observed reflections. The X-ray data on 1z, 2z, and 3z confirm the site of phosphine attack on the coordinated alkyne ligand and the presence of the eight-electron hydrocarbyl ligand that accompanies this reaction. The redox properties of the zwitterionic compounds Co 2 (CO) 4 [μ-η 2 :η 2 :η 1 :η 1 -RCC(R′)PPh 2 CC(PPh 2 )C(O)OC (O)] have been examined by cyclic coltammetry, and the data discussed relative to the coordination mode adopted by the bma ligand and the redox properties exhibited by the related alkyne-bridged compound Co 2 (CO) 4 (bma)(μ-PhCCPh) (chelating and bridging isomers). Extended Huckel calculations on the model compound Co 2 (CO) 4 [μ-η 2 :η 2 :η 1 :η 1 -HCC(H)PH 2 CC(PH 2 )C(O)OC (O)] have been carried out and are used in a discussion concerning the site of electron reduction and oxidation in 1z, 2z , and 3z .

Cp∗Ru(NO) (catecholate) compounds. Synthesis, redox behavior, extended Hückel molecular orbital calculations, and X-ray diffraction structure of the pentamethylcyclopentadienylruthenium complex Cp∗Ru(NO) (2,3-naphthalenediolate)·CH2Cl2

Abstract Eight new catecholate- and naphthalenediolate-substituted pentamethylcyclopentadienylruthenium, compounds have been prepared from the reaction between Cp ∗ Ru(NO)Cl 2 and the appropriate aromatic diol in methanolic KOH. Each new compound has been isolated and fully characterized in solution by IR and NMR ( 1 H and 13 C) spectroscopy, in addition to X-ray diffraction analysis which has established the solid-state structure of the naphthalenediolate compound Cp ∗ Ru(NO)(2,3-O 2 C 10 H 6 ). Cp ∗ Ru(NO)(2,3-O 2 C 10 H 6 ) crystallizes, as the CH 2 Cl 2 solvate, in the monoclinic space group P 2 1 / n with a = 11.0176(8), b = 16.041(2), c = 13.0504(9) A , β = 112.024(6)° , V = 2138.1(3) A 3 and Z =4. Full-matrix least-squares refinement yielded R =0.0376 for 1893 ( l .3 ϱ ( l )) reflections. Cyclic voltammetric studies have been carried out, and two redox responses were observed, assignable to the 0/1− and 0/1+ redox couples. Whereas the reduction wave in each of the compounds is essentially independent of the nature of the ancillary diolate ligand, the stability of the oxidation couple is modulated by the electronic properties of the diolate ligand. The molecular orbital properties of several model CpRu(NO)(diolate) compounds were explored by extended Huckel calculations. The calculated HOMO and LUMO energies of each compound displayed good agreement with the cyclic voltammetry data, allowing for generalizations to be made concerning the site of electron addition to and electron removal from the new Cp ∗ Ru(NO)(diolate) compounds.

In situ cluster stability studies explored by cylindrical internal reflectance (CIR) spectroscopy: 1-pentene hydroformylation using phosphine-substituted Co4(CO)8P2(μ4-PPh)2 clusters

Abstract The stability of diphosphine-substituted cobalt clusters derived from Co4(CO)10(μ4-PPh)2 has been examined under hydroformylation conditions using Cylindrical Internal Reflectance (CIR) spectroscopy. The monodentate phosphine clusters Co4(CO)8(PPh3)2(μ4-PPh)2 (1) and Co4(CO)8[P(OMe)3]2(μ4-PPh)2 (2) undergo rapid P-ligand loss to give the corresponding monosubstituted cluster Co4(CO)9P(μ4-PPh)2 [where P=PPh3 or P(OMe)3] and Co4(CO)10(μ4-PPh)2 at 130 °C. No hydroformylation activity was observed at 130 °C. Increasing the reaction temperature to 150 °C affords the parent cluster as the only observable organometallic species. 1-Pentene hydroformylation is observed at 150 °C, and the working catalyst solution shows no evidence for the fragmentation of Co4(CO)10(μ4-PPh)2 to HCo(CO)4. In contrast, the bidentate-substituted dimethylphosphino(ethane) (dmpe) cluster Co4(CO)8(dmpe)(μ4-PPh)2 (3) appears to function as a hydroformylation catalyst without phosphine loss or declusterification at 130 °C. The greater stabilization of the dmpe-substituted cluster relative to the monodentate phosphine systems and the absence of cluster fragmentation, which has recently been reported in the hydroformylation of 1-pentene using Co4(CO)10(μ4-PPh)2, are discussed.

Reaction of bis(dimethylphosphino)ethane with the tetracobalt cluster Co4(CO)10(μ4-PPh)2; synthesis, structure, and solution dynamics of Co4(CO)8(μ4-PPh)2(dmpe)

Abstract The reaction of the tetracobalt cluster Co4(CO)10(μ4-PPh)2 (1) with the bidentate ligand 1,2-bis(dimethylphosphino)ethane (dmpe) gives the disubstituted cluster Co4(CO)8(μ4-PPh)2(dmpe) (3) in high yield. The dmpe ligand is bound to a single cobalt atom in a chelating fashion as determined by FTIR and NMR (31P and 13C) spectroscopy and singlecrystal X-ray crystallography. Co4(CO)8(μ4-PPh)2(dmpe) · 1 2 toluene crystallizes in the triclinic space group P 1 with a 11.673(1), b 15.986(5), c 20.276(7) A, α 94.40(3), β 106.28(2), γ 94.89(2)°, V 3599(2) A3 and Z  4. Blockcascade least squares refinement yielded R  0.0521 for 6830 reflections. The temperaturedependent 13C NMR spectra of 3 reveal two distinct fluxional processes which serve to equilibrate the carbonyl ligands about the cluster polyhedron. The stability of 3 under different conditions has been examined by Cylindrical Internal Reflectance (CIR) spectroscopy. In benzene solution 3 is stable under 250 psi of H2 at 150°C; partial decomposition to Co(CO)−4 is observed using CO and H2/CO under analogous conditions.