Shishir Ghosh

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.

The rational synthesis of tetranuclear heterometallic butterfly clusters: reactions of [M2(CO)6(μ-pyS)2] (M = Re, Mn) with group VIII metal carbonyls

Heating [Os3(CO)10(NCMe)2] with [M2(CO)6(μ-pyS)2] (M = Re, Mn) (1–2) in benzene affords tetranuclear mixed-metal butterfly clusters [MOs3(CO)13(μ3-pyS)] (3–4). Similar reactions with Ru3(CO)12 give [MRu3(CO)13(μ3-pyS)] (5–6); however, when the latter is carried out in toluene the tetraruthenium sulfido cluster [Ru4(CO)12(μ-py)2(μ4-S)] (7) is the major product. Treatment of Fe3(CO)12 with 1 in refluxing toluene affords the mixed-metal sulfido cluster, [Fe2Re2(CO)13(μ-py)(μ-pyS)(μ4-S)] (8), while a similar reaction with 2 furnishes the tetrairon cluster [Fe4(CO)12(μ-py)2(μ4-S)] (9). Addition of PPh3 to 3 in the presence of Me3NO affords both the mono- and bis(phosphine)-substituted products [ReOs3(CO)12(PPh3)(μ3-pyS)] (10) and [ReOs3(CO)11(PPh3)2(μ3-pyS)] (11), respectively. All new complexes have been characterized by a combination of spectroscopic data and single-crystal X-ray diffraction studies.

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(κ(2)-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.

Combining anti-cancer drugs with artificial sweeteners: synthesis and anti-cancer activity of saccharinate (sac) and thiosaccharinate (tsac) complexes cis-[Pt(sac)2(NH3)2] and cis-[Pt(tsac)2(NH3)2].

The new platinum(II) complexes cis-[Pt(sac)2(NH3)2] (sac=saccharinate) and cis-[Pt(tsac)2(NH3)2] (tsac=thiosaccharinate) have been prepared, the X-ray crystal structure of cis-[Pt(sac)2(NH3)2] x H2O reveals that both saccharinate anions are N-bound in a cis-arrangement being inequivalent in both the solid-state and in solution at room temperature. Preliminary anti-cancer activity has been assessed against A549 human alveolar type-II like cell lines with the thiosaccharinate complex showing good activity.

Backbone Modified Small Bite-Angle Diphosphines: Synthesis, Structure, Fluxionality and Regioselective Thermally-Induced Transformations of Ru3(CO)10{µ-Ph2PCH(Me)PPh2}

The synthesis of Ru3(CO)10{µ-Ph2PCH(Me)PPh2} (1) has been achieved from the radical-catalysed reaction of Ru3(CO)12 with 1,1′-bis(diphenylphosphino)ethane and the fluxionality, protonation and regioselective thermally-induced on-metal transformations of the small bite-angle diphosphine have been studied. Cluster 1 is fluxional in solution and variable temperature 13C{1H} NMR spectroscopy shows that the six carbonyls on the phosphine-bound metal centers interconvert rapidly on the NMR timescale. Protonation of 1 is facile at room temperature and affords the cationic-hydride [Ru3(CO)10{µ-Ph2PCH(Me)PPh2}(μ-H)][BF4] (1H+) which is fluxional, the hydride migrating between bridged and non-bridged ruthenium–ruthenium vectors, location across an unbridged metal–metal bond being thermodynamically favoured. Thermolysis of 1 in heptane affords moderate amounts of the expected benzene-CO elimination product, Ru3(CO)8(µ-CO){µ3-PhPCH(Me)PPh(C6H4)} (2), along with smaller amounts of Ru3(CO)10{μ-PhP(CHMe)(C6H4)PPh} (3) containing a novel doubly-bridged diphosphine ligand. Hydrogenation of 1 in refluxing cyclohexane affords the hydride cluster Ru3(CO)9{μ3-PhPCH(Me)PPh2}(μ-H) (4), the same species also being obtained when 2 was treated with hydrogen under similar conditions. All thermally-induced transformations are regioselective, with only a single isomer being generated. In light of the observed regioselectivity a mechanism is proposed for the formation of 2 from 1 which results from an intermediate in which the methyl-group is held over the triruthenium framework.Graphical AbstractCluster Ru3(CO)10{µ-Ph2PCH(Me)PPh2} has been synthesized from the radical-catalysed reaction between Ru3(CO)12 and 1,1′-bis(diphenylphosphino)ethane and the fluxionality, protonation and regioselective thermally-induced on-metal transformations of the small bite-angle diphosphine have been investigated.

The First Carbonyl-Substituted Derivative of [Mn2(CO)6(µ-pyS)2]

Reaction of [Mn2(CO)6(μ-pyS)2] (1) with (Ph3P)2Ni(CO)2 at room temperature affords [Mn2(CO)5(PPh3)(μ-pyS)2] (3) which is the first carbonyl-substituted derivative of 1. A mononuclear complex fac-[Mn(CO)3(PPh3)(κ2-pyS)] (4) is also isolated as a minor product in this reaction. The formation of 3 allows us to propose a different mechanism operating in this reaction. A similar reaction between [Re2(CO)6(μ-pyS)2] (2) and (Ph3P)2Ni(CO)2 gives only mononuclear fac-[Re(CO)3(PPh3)(κ2-pyS)] (5). Both 4 and 5 undergo CO substitution to produce [M(CO)2(PPh3)2(κ2-pyS)] (6, M = Mn; 7, M = Re) when treated with (Ph3P)2Ni(CO)2 in boiling THF. Complex 3 also reacts with further (Ph3P)2Ni(CO)2 at room temperature to give 4 and 6. The molecular structures of 3 and 4 have been established by single crystal X-ray diffraction analyses.

Triosmium Clusters Containing 2-Mercaptobenzothiazolate Ligands

Reaction of the labile cluster [Os3(CO)10(NCMe)2] with 2,2′-benzothiazyl disulfide leads to the isolation of four products, namely the known hydride complex [Os3(CO)10(µ-H)(µ- S2NC7H4)], [Os3(CO)10(µ-S2NC7H4)2] (1) in which both thiolate ligands act as three-electron donor ligands and span a single osmium-osmium vector and two isomers of [Os3(CO)9(µ-S2NC7H4)(µ3-η2-S2NC7H4)] (2, 3) in which one thiolate caps a face of the triosmium cluster via secondary nitrogen coordination. In a separate experiment thermolysis of 1 in n-heptane affords only 2. Cluster 1 contains two 2-mercaptobenzothiazolate ligands in a µ-η1 mode of bonding, while 2 and 3 are isomers differing in the relative disposition of the two 2-mercaptobenzothiazolate ligands.

Iron carbonyl complexes bearing phenazine and acridine ligands: X-ray structures of Fe(CO)3(η4-C12H8N2), Fe(CO)2{P(OMe)3}(η4-C12H8N2), Fe(CO)2(PPh3) (η4-C13H9N), and Fe(CO)2(κ1-dppm) (η4-C12H8N2)

Reactions of Fe3(CO)12 with the heterocycles phenazine and acridine in refluxing benzene afforded the mononuclear complexes Fe(CO)3(η4-C12H8N2) (1a) and Fe(CO)3(η4-C13H9N) (1b), respectively. Treatment of 1a with P(OMe)3 and PPh3 in the presence of Me3NO at room temperature yielded the carbonyl substitution products Fe(CO)2{P(OMe)3}(η4-C12H8N2) (2a) and Fe(CO)2(PPh3) (η4-C12H8N2) (3a), respectively. Similar reactions of 1b yielded Fe(CO)2{P(OMe)3}(η4-C13H9N) (2b) and Fe(CO)2(PPh3) (η4-C13H9N) (3b). Treatment of 1a with the diphosphines dppm and dppf under similar conditions afforded the mononuclear compounds Fe(CO)2(κ1-dppm) (η4-C12H8N2) (4a) and Fe(CO)2(κ1-dppf) (η4-C12H8N2) (4b). Compounds 1a, 2a, 3b, and 4a have been structurally characterized by X-ray crystallography. The ancillary phenazine and acridine ligands in these products adopt an η4-coordination mode by using only the peripheral carbon atoms in one of the carbocyclic rings. Given the rarity of this coordination mode in metal carbonyl complexes, we have performed electronic structure calculations on 1a, and these data are discussed relative to the solid-state structure

Experimental and computational preference for phosphine regioselectivity and stereoselective tripodal rotation in HOs3(CO)8(PPh3)2(μ-1,2-N,C-η1,κ1-C7H4NS)

The site preference for ligand substitution in the benzothiazolate-bridged cluster HOs3(CO)10(μ-1,2-N,C-η1,κ1-C7H4NS) (1) has been investigated using PPh3. 1 reacts with PPh3 in the presence of Me3NO to afford the mono- and bisphosphine substituted clusters HOs3(CO)9(PPh3)(μ-1,2-N,C-η1,κ1-C7H4NS) (2) and HOs3(CO)8(PPh3)2(μ-1,2-N,C-η1,κ1-C7H4NS) (3), respectively. 2 exists as a pair of non-interconverting isomers where the PPh3 ligand is situated at one of the equatorial sites syn to the edge-bridging hydride that shares a common Os–Os bond with the metalated heterocycle. The solid-state structure of the major isomer establishes the PPh3 regiochemistry at the N-substituted osmium center. DFT calculations confirm the thermodynamic preference for this particular isomer relative to the minor isomer whose phosphine ligand is located at the adjacent C-metalated osmium center. 2 also reacts with PPh3 to give 3. The locus of the second substitution occurs at one of the two equatorial sites at the Os(CO)4 moiety in 2 and gives rise to a pair of fluxional stereoisomers where the new phosphine ligand is scrambled between the two equatorial sites at the Os(CO)3P moiety. The molecular structure of the major isomer has been determined by X-ray diffraction analysis and found to represent the lowest energy structure of the different stereoisomers computed for HOs3(CO)8(PPh3)2(μ-1,2-N,C-η1,κ1-C7H4NS). The fluxional behavior displayed by 3 has been examined by VT NMR spectroscopy, and DFT calculations provide evidence for stereoselective tripodal rotation at the Os(CO)3P moiety that serves to equilibrate the second phosphine between the two available equatorial sites.

CCDC 1499029: Experimental Crystal Structure Determination

Related Article: Shahin A. Begum, Md. Arshad H. Chowdhury, Shishir Ghosh, Derek A. Tocher, Michael G. Richmond, Edward Rosenberg, Shariff E. Kabir|2017|J.Organomet.Chem.|849-850|337|doi:10.1016/j.jorganchem.2016.10.024