Syntheses of Group 5 Amide Amidinates and Their Reactions with Water: Different Reactivities of Nb(V) and Ta(V) Complexes.

Chemistries of Nb(V) and Ta(V) compounds are essentially identical as a result of lanthanide contraction. Hydrolysis of M(NMe2)5 (M = Nb, Ta), for example, yields [M(µ3-O)(NMe2)3]4 (M = Nb, 1; Ta, 2) reported earlier. The similar reactivities of Nb(V) and Ta(V) compounds make it challenging, for example, to separate the two metals from their minerals. We have found that the reactions of H2O with amide amidinates M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) show that the niobium and tantalum analogues take different principal paths. For the Nb(V) complex 3, the amidinate and one amide ligand are liberated upon treatment with water, yielding [Nb(µ3-O)(NMe2)3]4 (1). For the Ta(V) complex 4, the amide ligands are released in the reaction with H2O, leaving the amidinate ligand intact. [Ta(µ3-O)(NMe2)3]4 (2), the analogue of 1, was not observed as a product in the reaction of 4 with H2O. To our knowledge, this is the first example of the formation of two different complexes that maintain the (V) oxidation state in both metals. The new complexes M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) have been prepared by the aminolysis of M(NMe2)5 (M = Nb, Ta) with iPrN(H)C(Me)=NiPr (5). The hydrolysis of 3 and 4 has been investigated by DFT electronic structure calculations. The first step in each hydrolysis reaction involves the formation of a hydrogen-bonded complex that facilitates a proton transfer to the amidinate ligand in 3 and protonation of an axial dimethylamide ligand in 4. Both proton transfers furnish an intermediate metal-hydroxide species. The atomic charges in 3 and 4 have been computed by Natural Population Analysis (NPA), and these data are discussed relative to which of the ancillary ligands is protonated initially in the hydrolysis sequence. Ligand exchanges in 3 and 4 as well as the exchange in iPrN(H)C(Me)=NiPr (5) were probed by EXSY NMR spectroscopy, giving rate constants of the exchanges: 0.430(13) s-1 (3), 0.033(6) s-1 (4), and 2.23(7) s-1 (5), showing that the rate of the Nb complex Nb(NMe2)4[MeC(NiPr)2] (3) is 13 times faster than that of its Ta analogue 4.

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.

Synthesis, Structure, and Conformational Dynamics of Rhodium and Iridium Complexes of Dimethylbis(2-pyridyl)borate.

Rhodium(I) and Iridium(I) borate complexes of the structure [Me2B(2-py)2]ML2 (L2 = (tBuNC)2, (CO)2, (C2H4)2, cod, dppe) were prepared and structurally characterized (cod = 1,5-cyclooctadiene; dppe = 1,2-diphenylphosphinoethane). Each contains a boat-configured chelate ring that participates in a boat-to-boat ring flip. Computational evidence shows that the ring flip proceeds through a transition state that is near planarity about the chelate ring. We observe an empirical, quantitative correlation between the barrier of this ring flip and the π acceptor ability of the ancillary ligand groups on the metal. The ring flip barrier correlates weakly to the Tolman and Lever ligand parameterization schemes, apparently because these combine both σ and π effects while we propose that the ring flip barrier is dominated by π bonding. This observation is consistent with metal-ligand π interactions becoming temporarily available only in the near-planar transition state of the chelate ring flip and not the boat-configured ground state. Thus, this is a first-of-class observation of metal-ligand π bonding governing conformational dynamics.

Ruthenium complexes of 1,4-diazabutadiene ligands with a cis-RuCl2 moiety for catalytic acceptorless dehydrogenation of alcohols: DFT evidence of chemically non-innocent ligand participation.

The acceptorless dehydrogenative coupling (ADC) of primary alcohols to esters by diazabutadiene-coordinated ruthenium compounds is reported. Treatment of cis-Ru(dmso)4Cl2 in acetone at 56 °C with different 1,4-diazabutadienes [p-XC6H4N[double bond, length as m-dash]C(H)(H)C[double bond, length as m-dash]NC6H4X-p; X = H, CH3, OCH3, and Cl; abbreviated as DAB-X], gives trans-Ru[κ2-N,N-DAB-X]2Cl2 as the kinetic product of substitution. Heating these products in o-xylene at 144 °C gives the thermodynamically favored cis-Ru[κ2-N,N-DAB-X]2Cl2 isomers. Electronic structure calculations confirm the greater stability of the cis diastereomer. The molecular structures for each pair of geometric isomers have been determined by X-ray diffraction analyses. Cyclic voltammetry experiments on the complexes show an oxidative response and a reductive response within 0.50 to 0.93 V and -0.76 to -1.24 V vs. SCE respectively. The cis-Ru[κ2-N,N-DAB-X]2Cl2 complexes function as catalyst precursors for the acceptorless dehydrogenative coupling of primary alcohols to H2 and homo- and cross-coupled esters. When 1,4-butanediol and 1,5-pentanediol are employed as substrates, lactones and hydroxyaldehydes are produced as the major dehydrogenation products, while secondary alcohols afforded ketones in excellent yields. The mechanism for the dehydrogenation of benzyl alcohol to benzyl benzoate and H2 using cis-Ru[κ2-N,N-DAB-H]2Cl2 (cis-1) as a catalyst precursor was investigated by DFT calculations. The data support a catalytic cycle that involves the four-coordinate species Ru[κ2-N,N-DAB-H][κ1-N-DAB-H](κ1-OCH2Ph) whose protonated κ1-diazabutadiene moiety functions as a chemically non-innocent ligand that facilitates a ß-hydrogen elimination from the κ1-O-benzoxide ligand to give the corresponding hydride HRu[κ2-N,N-DAB-H][κ1-N-DAB-H](κ2-O,C-benzaldehyde). H2 production follows a Noyori-type elimination to give (H2)Ru[κ2-N,N-DAB-H][κ1-N-DAB-H](κ1-O-benzaldehyde) as an intermediate in the catalytic cycle.

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.

An investigation of steric influence on the reactivity of FeV(O)(OH) tautomers in stereospecific C-H hydroxylation.

Two new tetradentate N4 ligands (LN4), LN4 = Me2,Me2PyzTACN (1-(2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)-4,7-dimethyl-1,4,7-triazacyclononane) and Me2,MeImTACN (1-((1-methyl-1H-imidazol-1-yl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane) have been synthesized and their corresponding Fe(II) complexes [FeII(Me2,Me2PyzTACN)(CF3SO3)2], 1Pz, and [FeII(Me2,MeImTACN)(CF3SO3)2], 1Im, have been prepared and characterized. Complexes 1Pz and 1Im catalyse the hydroxylation of C-H bonds of alkanes with excellent efficiencies, using hydrogen peroxide as oxidant. The high H/D kinetic isotope effect values for C-H hydroxylation, large normalized tertiary/secondary C-H (C3/C2) bond selectivities in adamantane oxidation, and high degrees of stereoretention in the oxidation of cis-1,2-dimethylcyclohexane are indicative of metal-based oxidation processes. The complexes also catalyse the oxidation of cyclooctene to form its corresponding epoxide and syn-diol. For 1Pz the epoxide is the main product, while for the analogous complex 1Im the syn-diol predominates. The active oxidant is proposed to be an [(LN4)FeV(O)(OH)]2+ species (2Pz, LN4 = Me2,Me2PyzTACN and 2Im, LN4 = Me2,MeImTACN) which may exist in two tautomeric forms related by a proton shift between the oxo and hydroxo ligands. Isotope labelling experiments show that the oxygen atom in the hydroxylated products originates from both water and hydrogen peroxide, and labelling experiments involving oxygen atom transfer to sterically bulky substrates provide indirect information on the steric influence exerted by the two ligands in the relative reactivities of the two hypervalent iron tautomers. Based on these labelling studies, the steric influence exerted by each of the ligands towards the relative reactivity of the oxo ligands of the corresponding pair of Fe(V)(O)(OH) tautomers can be derived. Furthermore, this steric influence can be gauged relative to related complexes/ligands.

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.

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.

Electrocatalytic proton-reduction behaviour of telluride-capped triiron clusters: tuning of overpotentials and stabilization of redox states relative to lighter chalcogenide analogues.

Reaction of [Fe3(CO)9(µ3-Te)2] (1) with the corresponding phosphine has been used to prepare the phosphine-substituted tellurium-capped triiron clusters [Fe3(CO)9(µ3-Te)2(PPh3)] (2), [Fe3(CO)8(µ3-Te)2(PPh3)] (3) and [Fe3(CO)7(µ3-Te)2(µ-R2PXPR2)] (X = CH2, R = Ph (4), Cy (5); X = NPri, R = Ph (6)). The directly related cluster [Fe3(CO)7(µ3-CO)(µ3-Te)(µ-dppm)] (7) was isolated from the reaction of [Fe3(CO)10(µ-Ph2PCH2PPh2)] with elemental tellurium. The electrochemistry of these new clusters has been probed by cyclic voltammetry, and selected complexes have been tested as proton reduction catalysts. Each 50-electron dicapped cluster exhibits two reductive processes; the first has good chemical reversibility in all cases but the reversibility of the second is dependent upon the nature of the supporting ligands. For the parent cluster 1 and the diphosphine derivatives 4-5 this second reduction is reversible, but for the PPh3 complex 3 it is irreversible, possibly as a result of CO or phosphine loss. The nature of the reduced products of 1 has been probed by DFT calculations. Upon addition of one electron, an elongation of one of the Fe-Te bonding interactions is found, while the addition of the second electron affords an open-shell triplet which is more stable by 8.8 kcal mol-1 than the closed-shell singlet dianion and has two elongated Fe-Te bonds. The phosphine-substituted clusters also exhibit oxidation chemistry but with poor reversibility in all cases. Since the reduction potentials for the tellurium-capped clusters occur at more positive potentials than for the sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues, the tellurium-capped clusters 1 and 3-5 have been examined as proton reduction catalysts. In the presence of p-toluenesulfonic acid (TsOH) or trifluoroacetic acid (TFA), these clusters reduce protons to H2 at both their first and second reduction potentials. Electron uptake at the second reduction potential is far greater than the first, suggesting that the open-shell triplet dianions are efficient catalysts. As expected, the catalytic overpotential increases upon successive phosphine substitution but so does the current response. A mechanistic scheme that takes the roles of the supporting ligands on the preferred route(s) to H2 production and release into account is presented.

Asymmetric hydrogenation of an α-unsaturated carboxylic acid catalyzed by intact chiral transition metal carbonyl clusters - diastereomeric control of enantioselectivity.

Twenty clusters of the general formula [(µ-H)2Ru3(µ3-S)(CO)7(µ-P-P*)] (P-P* = chiral diphosphine of the ferrocene-based Walphos or Josiphos families) have been synthesised and characterised. The clusters have been tested as catalysts for asymmetric hydrogenation of tiglic acid [trans-2-methyl-2-butenoic acid]. The observed enantioselectivities and conversion rates strongly support catalysis by intact Ru3 clusters. A catalytic mechanism involving an active Ru3 catalyst generated by CO loss from [(µ-H)2Ru3(µ3-S)(CO)7(µ-P-P*)] has been investigated by DFT calculations.

Electron Transfer Mediated by Iron Carbonyl Clusters Enhance Light-Driven Hydrogen Evolution in Water by Quantum Dots.

Photocatalytic water splitting has become a promising strategy for converting solar energy into clean and carbon-neutral solar fuels in a low-cost and environmentally benign way. Hydrogen gas is such a potential solar fuel/energy carrier. In a classical artificial photosynthetic system, a photosensitizer is generally associated with a co-catalyst to convert photogenerated charge into (a) chemical bond(s). In the present study, assemblies consisting of CdSe quantum dots that are coupled with one of two molecular complexes/catalysts, that is, [Fe2 S2 (CO)6 ] or [Fe3 Te2 (CO)9 ], using an interface-directed approach, have been tested as catalytic systems for hydrogen production in aqueous solution/organic solution. In the presence of ascorbic acid as a sacrificial electron donor and proton source, these assemblies exhibit enhanced activities for the rate of hydrogen production under visible light irradiation for 8 h in aqueous solution at pH 4.0 with up to 110 µmol of H2 per mg of assembly, almost 8.5 times that of pure CdSe quantum dots under the same conditions. Transient absorption and time-resolved photoluminescence spectroscopies have been used to investigate the charge carrier transfer dynamics in the quantum dot/iron carbonyl cluster assemblies. The spectroscopic results indicate that effective electron transfer from the molecular iron complex to the valence band of the excited CdSe quantum dots significantly inhibits the recombination of photogenerated charge carriers, boosting the photocatalytic activity for hydrogen generation; that is, the iron clusters function as effective intermediaries for electron transfer from the sacrificial electron donor to the valence band of the quantum dots.

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.

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.

A new diphosphine-carbonyl complex of ruthenium: an efficient precursor for C-C and C-N bond coupling catalysis.

Reaction of 1,2-bis(diphenylphosphino)benzene (dppbz) with [{Ru(CO)2Cl2}n] affords [Ru(dppbz)(CO)2Cl2], where the two carbonyls are mutually cis and the two chlorides are trans. The molecular structure of [Ru(dppbz)(CO)2Cl2], has been determined by X-ray crystallography, and the stability of the different available stereoisomers has been computationally evaluated. [Ru(dppbz)(CO)2Cl2] has been found to serve as an excellent pre-catalyst for catalytic Suzuki-type C-C coupling and Buchwald-type C-N coupling reactions.

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.

Palladium(0)-mediated C-H bond activation of N-(naphthyl)salicylaldimine and related ligands: utilization of the resulting organopalladium complexes in catalytic C-C and C-N coupling reactions.

N-(Naphthyl)-4-R-salicylaldimines (R = OCH(3), H and Cl; H(2L)(1)-H(2)L(3)) and 2-hydroxy-N-(naphthyl)naphthaldimine (H(2)L(4)) readily undergo, upon reaction with Na(2)[PdCl(4)] in the presence of triphenylphosphine, cyclopalladation via C-H bond activation at the peri-position to afford complexes of type [Pd(L)(PPh(3))] (L = L(1)-L(4)). The C-H bond activation has been found to be mediated by palladium(0) formed in situ. A similar reaction of H(2)L(1) with Na(2)[PdCl(4)] in the presence of 1,2-bis(diphenylphosphino)ethane (dppe), in a 2 : 2 : 1 mole ratio, yields a dinuclear complex of type [{Pd(L(1))}(2)(dppe)]. Reaction of H(2)L(1) with Na(2)[PdCl(4)] in the presence of 4-picoline (pic) yields [Pd(L(1))(pic)]. The molecular structures of the six complexes have been determined by X-ray crystallography. The aldiminate ligand in each compound is coordinated to the metal center as a di-anionic tridentate ONC-donor, with the fourth coordination site occupied by a phosphine or picoline ligand. The new complexes show intense absorptions in the visible and ultraviolet regions, and the nature of the optical transitions has been analyzed by TDDFT calculations. The palladium complexes display notable efficiency in catalyzing C-C and C-N bond coupling reactions. The thermodynamics for the formation of the cyclometalated catalyst precursor [Pd(L(2))(PPh(3))] has been evaluated by DFT calculations.