Reactions of diphosphine-stabilized Os3 clusters with triphenylantimony: syntheses and structures of new antimony-containing Os3 clusters via Sb-Ph bond cleavage.

The reactivity of the trimetallic clusters [Os3(CO)10(µ-dppm)] [dppm = bis(diphenylphosphino)methane] and [HOs3(CO)8{µ3-Ph2PCH2PPh(C6H4-µ2,σ1)}] with triphenylantimony (SbPh3) has been examined. [Os3(CO)10(µ-dppm)] reacts with SbPh3 in refluxing toluene to yield three new triosmium clusters [Os3(CO)9(SbPh3)(µ-dppm)] (1), [HOs3(CO)7(SbPh3){µ3-Ph2PCH2PPh(C6H4-µ2,σ1)}] (2), and [HOs3(CO)7(SbPh3)(µ-C6H4)(µ-SbPh2)(µ-dppm)] (3). [HOs3(CO)8{µ3-Ph2PCH2PPh(C6H4-µ2,σ1)}] reacts with SbPh3 (excess) at room temperature to afford [Os3(CO)8(SbPh3)(η1-Ph)(µ-SbPh2)(µ-dppm)] (4) as the sole product. A series of control experiments have also been conducted to establish the relationship between the different products. The molecular structure of each product has been determined by single-crystal X-ray diffraction analysis, and the bonding in these new clusters has been investigated by electronic structure calculations.

Palladium(II) ortho-cyano-aminothiophenolate (ocap) complexes.

A series of Pd(II) complexes containing ortho-cyano-aminothiophenolate (ocap) ligands have been prepared and their molecular structures elucidated. Hg(II) ocap complexes, [Hg{SC6H3XN(CN)}]n (X = H, Me) (1), react with Na2S to afford HgS and Na2[ocap] which reacts in situ with K2[PdCl4] to afford palladium ocap complexes [Pd{SC6H3XN(CN)}]n (2). A second route to these coordination polymers has also been developed from reactions of 2-aminobenzothiazole (abt) complexes, trans-PdCl2(abt)2 (3), with NaOH. We have not been able to crystallographically characterise coordination polymers 2, but addition of PPh3, a range of phosphines and cyclic diamines affords mono and binuclear complexes in which the ocap ligand adopts different coordination geometries. With PPh3, binuclear [Pd(µ-κ2,κ1-ocap)(PPh3)]2 (4) results, in which the ocap bridges the Pd2 centre acting as an S,N-chelate to one metal centre and binding the second via coordination of the cyanide nitrogen. In contrast, with diphosphines, Ph2P(CH2)nPPh2 (n = 1-4), mononuclear species predominate as shown in the molecular structures of Pd(κ2-ocap){κ2-Ph2P(CH2)nPPh2} (5-7; n = 1-3). With 2,2'-bipy and 1,10-phen we propose that related monomeric chelates Pd(κ2-ocap)(κ2-bipy) (9) and Pd(κ2-ocap)(κ2-phen) (10) result but we have been unable to substantiate this crystallographically. Addition of HgCl2(phen) to 9a (generated in situ) affords heterobimetallic Pd(κ2-phen)(µ-κ2,κ1-ocap)HgCl2(κ2-phen) (11), in which Hg(II) is coordinated through the ring sulfur.

Biomimics of [FeFe]-hydrogenases incorporating redox-active ligands: synthesis, redox properties and spectroelectrochemistry of diiron-dithiolate complexes with ferrocenyl-diphosphines as Fe4S4 surrogates.

[FeFe]-Ase biomimics containing a redox-active ferrocenyl diphosphine have been prepared and their ability to reduce protons and oxidise H2 studied, including 1,1'-bis(diphenylphosphino)ferrocene (dppf) complexes Fe2(CO)4(µ-dppf)(µ-S(CH2)nS) (n = 2, edt; n = 3, pdt) and Fe2(CO)4(µ-dppf)(µ-SAr)2 (Ar = Ph, p-tolyl, p-C6H4NH2), together with the more electron-rich 1,1'-bis(dicyclohexylphosphino)ferrocene (dcpf) complex Fe2(CO)4(µ-dcpf)(µ-pdt). Crystallographic characterisation of four of these show similar overall structures, the diphosphine spanning an elongated Fe-Fe bond (ca. 2.6 Å), lying trans to one sulfur and cis to the second. In solution the diphosphine is flexible, as shown by VT NMR studies, suggesting that Fe2⋯Fe distances of ca. 4.5-4.7 Å in the solid state vary in solution. Cyclic voltammetry, IR spectroelectrochemistry and DFT calculations have been used to develop a detailed picture of electronic and structural changes occurring upon oxidation. In MeCN, Fe2(CO)4(µ-dppf)(µ-pdt) shows two chemically reversible one-electron oxidations occurring sequentially at Fe2 and Fc sites respectively. For other dppf complexes, reversibility of the first oxidation is poor, consistent with an irreversible structural change upon removal of an electron from the Fe2 centre. In CH2Cl2, Fe2(CO)4(µ-dcpf)(µ-pdt) shows a quasi-reversible first oxidation together with subsequent oxidations suggesting that the generated cation has some stability but slowly rearranges. Both pdt complexes readily protonate upon addition of HBF4·Et2O to afford bridging-hydride cations, [Fe2(CO)4(µ-H)(µ-dcpf)(µ-pdt)]+, species which catalytically reduce protons to generate H2. In the presence of pyridine, [Fe2(CO)4(µ-dppf)(µ-pdt)]2+ catalytically oxidises H2 but none of the other complexes do this, probably resulting from the irreversible nature of their first oxidation. Mechanistic details of both proton reduction and H2 oxidation have been studied by DFT allowing speculative reaction schemes to be developed.

Synthesis, structure and reactivity with phosphines of Hg(II) ortho-cyano-aminothiophenolate complexes formed via C-S bond cleavage and dehydrogenation of 2-aminobenzothiazoles.

Addition of 2-aminobenzothiazole (abt) and substituted derivatives to Hg(OAc)2 leads to the high yield formation of ortho-cyano-aminothiophenolate (ocap) complexes [Hg{SC6H3XN(CN)}]n (X = H, Me, Cl, Br, NO2) resulting from dehydrogenation and C-S bond cleavage. The reaction appears to be unique to Hg(OAc)2 and with HgCl2 the product [HgCl2(abt)]n contains an intact abt ligand, but reacts with acetate to afford the ocap complex [Hg{SC6H4N(CN)}]n. Binding of abt to Hg(II) has previously been probed in molecular structures of [Hg(sac)2(abt)L] (L = MeOH, DMSO) and these have been reexamined to understand the perturbation of abt upon coordination. When the reaction of abt and Hg(OAc)2 was carried out at low temperatures the intermediate [Hg(κ2-OAc)(EtOH)(µ-HNCNSC6H4)]2 was isolated resulting from a single ligand deprotonation thus allowing a mechanism for ring-opening to be proposed. Reactions of [Hg{SC6H3XN(CN)}]n with mono- and bidentate phosphines have been studied, affording a series of complexes in which the ocap ligands adopt four different binding modes in the solid state, as shown by a number of crystallographic studies. In all, the ligand chelates to a single mercury centre but spans to the second via either: (i) a simple S,N-chelate, (ii) coordination through nitrogen of the CN group, (iii) the sulfur acting as a thiolate-bridge, (iv) both thiolate bridging and cyanide coordination. With PPh3 two different binding modes are seen in complexes [Hg{SC6H3XN(CN)}(PPh3)]2 being dependant upon the nature of the arene-substituent, while addition of excess PPh3 affords mononuclear [Hg{SC6H3XN(CN)}(PPh3)2]. With dppm, binuclear [Hg{SC6H3XN(CN)}(κ1-dppm)]2 result in which the diphosphine binds in a monodentate fashion. With the more flexible diphosphines, dppe and dppb, coordination polymers [Hg{SC6H3XN(CN)}(κ1,κ1-diphosphine)]n result in which ocap binds in a simple chelate fashion. Somewhat unexpectedly, with dppp, binuclear complexes [Hg2{SC6H3XN(CN)}2(µ,κ1,κ1-dppp)] result in which two diphosphines bridge the Hg2 centre, while with dppf mononuclear chelates are proposed to result. Thus, the simple and high-yielding ring-opening of 2-aminobenzothiazole and substituted derivatives by mercuric acetate provides ready access to a range of novel ortho-cyano-aminothiophenolate complexes, being shown to be a highly versatile ligand that can adopt a number of different coordination modes.

Reactions of triosmium and triruthenium clusters with 2-ethynylpyridine: new modes for alkyne C-C bond coupling and C-H bond activation.

The reaction of the trimetallic clusters [H2Os3(CO)10] and [Ru3(CO)10L2] (L = CO, MeCN) with 2-ethynylpyridine has been investigated. Treatment of [H2Os3(CO)10] with excess 2-ethynylpyridine affords [HOs3(CO)10(µ-C5H4NCH=CH)] (1), [HOs3(CO)9(µ3-C5H4NC[double bond, length as m-dash]CH2)] (2), [HOs3(CO)9(µ3-C5H4NC[double bond, length as m-dash]CCO2)] (3), and [HOs3(CO)10(µ-CH[double bond, length as m-dash]CHC5H4N)] (4) formed through either the direct addition of the Os-H bond across the C[triple bond, length as m-dash]C bond or acetylenic C-H bond activation of the 2-ethynylpyridine substrate. In contrast, the dominant pathway for the reaction between [Ru3(CO)12] and 2-ethynylpyridine is C-C bond coupling of the alkyne moiety to furnish the triruthenium clusters [Ru3(CO)7(µ-CO){µ3-C5H4NC[double bond, length as m-dash]CHC(C5H4N)[double bond, length as m-dash]CH}] (5) and [Ru3(CO)7(µ-CO){µ3-C5H4NCCHC(C5H4N)CHCHC(C5H4N)}] (6). Cluster 5 contains a metalated 2-pyridyl-substituted diene while 6 exhibits a metalated 2-pyridyl-substituted triene moiety. The functionalized pyridyl ligands in 5 and 6 derive via the formal C-C bond coupling of two and three 2-ethynylpyridine molecules, respectively, and 5 and 6 provide evidence for facile alkyne insertion at ruthenium clusters. The solid-state structures of 1-3, 5, and 6 have been determined by single-crystal X-ray diffraction analyses, and the bonding in the product clusters has been investigated by DFT. In the case of 1, the computational results reveal a rare thermodynamic preference for a terminal hydride ligand as opposed to a hydride-bridged Os-Os bond (3c,2e Os-Os-H bond).

A new synthetic route for the preparation of [Os3(CO)10(µ-OH)(µ-H)] and its reaction with bis(diphenylphosphino)methane (dppm): syntheses and X-ray structures of two isomers of [Os3(CO)8(µ-OH)(µ-H)(µ-dppm)] and [Os3(CO)7(µ3-CO)(µ3-O)(µ-dppm)].

The triosmium cluster [Os3(CO)10(µ-OH)(µ-H)] containing bridging hydride and hydroxyl groups at a common Os-Os edge was obtained in good yield (ca. 75%) from the hydrolysis of the labile triosmium cluster [Os3(CO)10(NCMe)2] in THF at 67 °C. [Os3(CO)10(µ-OH)(µ-H)] reacts with dppm at 68 °C to afford the isomeric clusters 1 and 2 with the general formula [Os3(CO)8(µ-OH)(µ-H)(µ-dppm)] that differ by the disposition of bridging dppm ligand. Cluster 1 is produced exclusively from the reaction of [Os3(CO)10(µ-OH)(µ-H)] with dppm in CH2Cl2 at room temperature in the presence of added Me3NO. Heating cluster 1 at 81 °C furnishes 2 in a process that likely proceeds by the release of one arm of the dppm ligand, followed by ligand reorganization about the cluster polyhedron and ring closure of the pendent dppm ligand. The oxo-capped [Os3(CO)7(µ3-CO)(µ3-O)(µ-dppm)] (3) has been isolated starting from the thermolysis of either 1 or 2 at 139 °C. Reactions of [Os3(CO)10(µ-dppm)] with ROH (R = Me, Et) in the presence of Me3NO at 80 °C furnish [Os3(CO)8(µ-OH)(µ,η1,κ1-OCOR)(µ-dppm)] (4, R = Me; 5, R = Et). Clusters 1-5 have been characterized by a combination of analytical and spectroscopic studies, and the molecular structure of each product has been established by X-ray crystallography. The bonding in these products has been examined by electronic structure calculations, and cluster 1 is confirmed as the kinetic product of substitution, while cluster 2 represents the thermodynamically favored isomer.

Models of the iron-only hydrogenase enzyme: structure, electrochemistry and catalytic activity of Fe2(CO)3(µ-dithiolate)(µ,κ1,κ2-triphos).

A series of diiron bis(2-diphenylphosphinoethyl)phenylphosphine (triphos) complexes Fe2(CO)3(µ-dithiolate)(µ,κ1,κ2-triphos) (1-4) [dithiolate = 1 pdt; 2 edt; 3 adt (R = Bz), 4 (SMe)2] have been prepared and investigated as biomimics of the diiron site of [FeFe]-hydrogenases. The triphos ligand bridges the diiron vector whilst also chelating to one iron and 1-3 exist as a mixture of basal-basal-apical (bba) and basal-basal-basal (bbb) isomers which differ in the mode of chelation. In solution the bba and bbb forms do not interconvert on the NMR time scale, but the bba isomers are fluxional, and at low temperature four forms of 1bba are seen as the conformations for the pdt ring and triphos methylene groups are frozen. Crystallographic studies have established bba (pdt) and bbb (adt) ground state conformations and in both there is a significant deviation away from the expected eclipsed conformation (Lap-Fe-Fe-Lap torsion angle 0°) by 49.4 and 24.9° respectively, suggesting that introduction of triphos leads to significant strain and DFT calculations have been used to understand the relative energies of isomers. The electron rich nature of the diiron centre in 1-4 would suggest rapid protonation, but while bridging hydride complexes such as [Fe2(CO)3(µ-pdt)(µ,κ1,κ2-triphos)(µ-H)][BF4] (1H+) can be formed the process is slow. This behavior is likely a result of the high energy barrier in forming the initial (not observed) terminal hydride which requires a significant conformational change in triphos coordination. CV studies show that all starting compounds oxidize at low potentials and the addition of [Cp2Fe][PF6] to 1 affords [Fe2(CO)3(µ-pdt)(µ,κ1,κ2-triphos)][PF6] (1+) which has been characterised by IR spectroscopy. DFT studies suggest a ground state for 1+ with a partially rotated Fe(CO)2P moiety that yields a weak semi-bridging carbonyl with the adjacent Fe(CO)P2 group. No reduction peaks are seen for 1-4 within the solvent window but 1H+ undergoes reduction at -1.7 V. All complexes act as proton-reduction catalysts in the presence of HBF4·Et2O. For 1, three separate processes are observed and their dependence on acid concentration has been probed, and a mechanistic scheme is proposed based on formation via a CECE process of 1(µ-H)H which can either slowly release H2 or undergo further reduction. Relative contributions of the three processes to the total current were found to be highly dependent upon the background electrolyte, being attributed to their relative abilities to facilitate proton transfer processes. While 2 and 4 show similar proton reduction behaviour, the adt complex 3 is quite different being attributed to facile protonation of nitrogen which is followed by addition of a second proton at the diiron centre.

Hydrogenase biomimics containing redox-active ligands: Fe2(CO)4(µ-edt)(κ2-bpcd) with electron-acceptor 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) as a potential [Fe4-S4]H surrogate.

[FeFe]-hydrogenases contain strongly electronically coupled diiron [2Fe]H and tetrairon [Fe4-S4]H clusters, and thus much recent effort has focused on the chemistry of diiron-dithiolate biomimics with appended redox-active ligands. Here we report on the synthesis and electrocatalytic activity of Fe2(CO)4(µ-edt)(κ2-bpcd) (2) in which the electron-acceptor 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) acts as a surrogate of the [Fe4-S4]H sub-cluster. The complex is prepared in low yield but has been fully characterised, including a crystallographic study which shows that the diphosphine adopts a basal-apical coordination geometry in the solid state. Cyclic voltammetry shows that 2 undergoes four reduction events with DFT studies confirming that the first reduction is localised on the low-lying π* system of the diphosphine ligand. The addition of the second electron furnishes a triplet dianion that exhibits spin density distributed over the diphosphine and diiron subunits. Protonation at the Fe-Fe bond of the triplet dianion furnishes the corresponding bridging hydride as the thermodynamically favoured species that contains a reduced bpcd ligand. Complex 2 functions as a catalyst for proton-reduction at its second reduction potential, in contrast to the related 2,3-bis(diphenylphosphino)maleic anhydride (bma) complex, Fe2(CO)4(µ-pdt)(κ2-bma) (1), which shows similar electrochemical behaviour but is not catalytically active. The difference in chemical behaviour is attributed to greater stability of the 4-cyclopenten-1,3-dione platform in 2 as compared to the maleic anhydride ring of the bma ligand in 1 following the uptake of the second electron. Thus protonation of the Fe-Fe bond in the 22- affords a species which is stable enough to undergo a further reduction-protonation event, unlike the bma ligand whose maleic anhydride ring undergoes deleterious C-O bond scission upon protonation or reaction with adventitious moisture. DFT studies, however, suggest that electron-transfer from the diphosphine to the diiron centre is not significant, probably due to their poor redox levelling. Thus, while the diphosphine is readily reduced, the added electron is apparently not utilised in proton-reduction and hence cannot truly be considered as an [Fe4-S4]H surrogate.

New molecular architectures containing low-valent cluster centres with di- and trimetalated 2-vinylpyrazine ligands: synthesis and molecular structures of Ru5(CO)15(µ5-C4H2N2CH[double bond, length as m-dash]CH)(µ-H)2 and Ru8(CO)24(µ7-C4H2N2CH[double bond, length as m-dash]C)(µ-H)3.

Reaction of 2-vinylpyrazine with Ru3(CO)12 results in multiple C-H bond activations to afford penta- and octa-ruthenium clusters, Ru5(CO)15(µ5-C4H2N2CH[double bond, length as m-dash]CH)(µ-H)2 (2) and Ru8(CO)24(µ7-C4H2N2CH[double bond, length as m-dash]C)(µ-H)3 (3), in which a Ru3 sub-unit is linked to Ru2 and Ru5 centres via di- and tri-metalated 2-vinylpyrazine ligands, exhibiting novel coordination modes including the loss of ring aromaticity in 2. The bonding of 2 and the mechanism for the fluxional behaviour of the hydrides have been examined by electronic structure calculations.

Mixed-valence dimolybdenum complexes containing hard oxo and soft carbonyl ligands: synthesis, structure, and electrochemistry of Mo2(O)(CO)2(µ-κ2-S(CH2)nS)2(κ2-diphosphine).

Mixed-valence dimolybdenum complexes Mo2(O)(CO)2{µ-κ2-S(CH2)nS}2(κ2-Ph2P(CH2)mPPh2) (n = 2, 3; m = 1, 2) (1-4) have been synthesized from one-pot reactions of fac-Mo(CO)3(NCMe)3 and dithiols, HS(CH2)nSH, in the presence of diphosphines. The dimolybdenum framework is supported by two thiolate bridges, with one molybdenum carrying a terminal oxo ligand and the second two carbonyls. The dppm (m = 1) products exist as a pair of diastereomers differing in the relative orientation of the two carbonyls (cis and trans) at the Mo(CO)2(dppm) center, while dppe (m = 2) complexes are found solely as the trans isomers. Small amounts of Mo(CO){κ3-S(CH2CH2S)2}(κ2-dppe) (5) also result from the reaction using HS(CH2)2SH and dppe. The bonding in isomers of 1-4 has been computationally explored by DFT calculations, trans diastereomers being computed to be more stable than the corresponding pair of cis diastereomers for all. The calculations confirm the existence of Mo[triple bond, length as m-dash]O and Mo-Mo bond orders and suggest that the new dimeric compounds are best viewed as Mo(v)-Mo(i) mixed-valence systems. The electrochemical properties of 1 have been investigated by CV and show a reversible one-electron reduction associated with the Mo(v) centre, while two closely spaced irreversible oxidation waves are tentatively assigned to oxidation of the Mo(i) centre of the two isomers as supported by DFT calculations.

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.

Alkyne activation and polyhedral reorganization in benzothiazolate-capped osmium clusters on reaction with diethyl acetylenedicarboxylate (DEAD) and ethyl propiolate.

The reactivity of the face-capped benzothiazolate clusters HOs3(CO)9[µ3-C7H3(R)NS] (1a, R = H; 1b, R = 2-CH3) with alkynes has been investigated. 1a reacts with DEAD at 67 °C to furnish the isomeric alkenyl clusters Os3(CO)9(µ-C7H4NS)(µ3-EtO2CCCHCO2Et) (2a and 3a). X-ray crystallographic analyses of 2a and 3a have confirmed the stereoisomeric relationship of these products and the regiospecific polyhedral expansion that follows the formal transfer of the hydride to the coordinated alkyne ligand in HOs3(CO)9(µ-C7H4NS)(η2-DEAD). The significant structural differences between the two isomers, as revealed by the solid-state structures, derives from the regiospecific cleavage of one of the three Os-Os bonds in the intermediate alkenyl cluster Os3(CO)9(µ-C7H4NS)(η1-EtO2CCCHCO2Et), which follows hydride transfer to the coordinated alkyne ligand in the pi compound HOs3(CO)9(µ-C7H4NS)(η2-DEAD). Control experiments confirm the reversibility of the reaction leading to the formation of 2a and 3a. Whereas heating either isomer in refluxing THF or benzene affords a binary mixture containing 2a and 3a, thermolysis in refluxing toluene leads to the activation of the alkenyl ligand and formation of the new cluster Os3(CO)9(µ-C7H4NS)(µ3-EtO2CCCH2) (4). 4 was independently synthesized from 1a and ethyl propiolate at room temperature. The computed mechanisms that account for the formation of 2a and 3a are presented, along with the mechanism for the reaction of 1a with ethyl propiolate to give 4.

Formation of ortho-cyano-aminothiophenolate ligands with versatile binding modes via facile carbon-sulfur bond cleavage of 2-aminobenzothiazoles at mercury(II) centres.

Addition of 2-aminobenzothiazole and substituted derivatives to mercuric acetate in warm ethanol leads to the high yield formation of [Hg{SC6H3XN(C[triple bond, length as m-dash]N)}]n resulting from loss of hydrogen and sulfur-carbon bond cleavage. Addition of phosphines affords a series of complexes in which the new ortho-cyano-aminothiophenolate ligands adopt three different binding modes.

Copper catalysed amidation of aryl halides through chelation assistance.

A copper mediated C-N bond formation for the amidation of aryl halides using 8-aminoquinoline has been developed. This strategy provides efficient access to amides bearing two contiguous heterocyclic moieties and does not require the presence of additional ligands.

Electrocatalytic proton reduction catalysed by the low-valent tetrairon-oxo cluster [Fe4(CO)10(κ(2)-dppn)(µ4-O)](2-) [dppn = 1,1'-bis(diphenylphosphino)naphthalene].

The 62-electron oxo-capped tetrairon butterfly cluster, Fe4(CO)10(κ(2)-dppn)(µ4-O) (1) {dppn = 1,8-bis(diphenylphosphino)naphthalene}, undergoes reversible one-electron oxidation and reduction events to generate the 61- and 63-electron radicals [Fe4(CO)10(κ(2)-dppn)(µ4-O)](+) (1+) and [Fe4(CO)10(κ(2)-dppn)(µ4-O)](-) (1-) respectively. Addition of a second electron affords the 64-electron cluster [Fe4(CO)10(κ(2)-dppn)(µ4-O)](2-) (1(2-)) which has more limited stability but is stable within the time frame of the electrochemical experiment. While 1 and 1(-1) are inactive as proton reduction catalysts, dianionic 1(2-) is active for the formation of hydrogen from both CHCl2CO2H and CF3CO2H. This occurs via two separate mechanistic cycles branching at the mono-protonated species [Fe4(CO)10(κ(2)-dppn)(µ4-O)H](-) (1H-) resulting from the rapid protonation of 1(2-). This intermediate then undergoes competing protonation and reduction events leading to EECC and ECEC catalytic cycles respectively with 1- being pivotal to both. In order to understand the nature of [Fe4(CO)10(κ(2)-dppn)(µ4-O)](2-) (1(2-)) and its protonated products density functional theory (DFT) calculations have been employed. Theoretical calculations reveal that the cluster core remains intact in 1(2-), but the two consecutive one-electron reductions lead to an expansion of one of the trigonal-pyramids of this trigonal-bipyramidal cluster. The two-electron reduced cluster 1(2-) protonates at dppn-bound iron, accompanied by a wingtip-hinge iron-iron bond scission, and then reacts with a second proton to evolve hydrogen.

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.

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.

Hydrogenase biomimetics: Fe2(CO)4(µ-dppf)(µ-pdt) (dppf = 1,1'-bis(diphenylphosphino)ferrocene) both a proton-reduction and hydrogen oxidation catalyst.

Fe2(CO)4(µ-dppf)(µ-pdt) catalyses the conversion of protons and electrons into hydrogen and also the reverse reaction thus mimicing both types of binuclear hydrogenase enzymes.

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.

Bio-inspired hydrogenase models: mixed-valence triion complexes as proton reduction catalysts.

Mixed-valence triiron complexes Fe(3)(CO)(7-x)(PPh(3))(x)(µ-edt)(2) (x = 0-2) have been prepared and are shown to act as proton reduction catalysts. Catalysis takes place via an ECEC mechanism with a reduced overpotential of ca. 0.45 V for Fe(3)(CO)(7)(µ-edt)(2) as compared to the corresponding diiron complex.