Greg A. N. Felton

Novel organometallic complexes Mn(benzene-1,2-diamine)(CO)3Br, Mn-1, Mn(3-methylbenzene-1,2-diamine)(CO)3Br, Mn-2, and Re(benzene-1,2-diamine)(CO)3Cl, Re-1, have been synthesized and characterized by IR, UV/Vis, 1H-NMR, EA and HRMS. The structures of Mn-2 and Re-1 were confirmed by X-ray crystallography. The three novel compounds were studied for their electrocatalytic reduction of carbon dioxide to carbon monoxide using cyclic voltammetry in acetonitrile solutions. Controlled potential electrolysis was used to obtain information on faradaic yield, with product formation being confirmed by GC. Using earth-abundant manganese, compounds Mn-1 and Mn-2 display turnover frequencies of 108 s−1 and 82 s−1, respectively, amid selective production of carbon monoxide (faradaic yields ~85%), with minimal co-production of dihydrogen (<2%), and low overpotential of 0.18 V. The rhenium congener, Re-1, displays no activity as an electrocatalyst for carbon dioxide reduction under identical conditions.

The X-ray structure of the novel compound [Fe2(μ-SC6H4-p-tBu)(μ-SC6H4-p-NO2)(CO)6] is shown, with arrows indicating the direction of electron density donation/withdrawal. Highlighting the presence of both features in a single compound. [Display omitted] The novel compounds [Fe2(μ-SC6H4-p-NO2)2(CO)6] (1), [Fe2(μ-SC6H4-p-tBu)(μ-SC6H4-p-NO2)(CO)6] (2), [Fe2(μ-SC6H4-p-tBu)2(CO)6] (3), [Fe2(μ-SC6H4-p-tBu)(μ-SC6H4-p-CF3)(CO)6] (4) and [Fe2(μ-SC6H4-p-OCH3)(μ-SC6H4-p-CF3)(CO)6] (5) have been characterized by 1H NMR, FTIR, elemental analysis, and cyclic voltammetry. X-ray crystallography of compounds 1 and 2 reveal an anti (ax,eq) S-aryl ligand orientation in the solid state. Carbonyl IR spectra and reduction potentials correlate strongly with Hammett constants of the electron donating/withdrawing groups. Evaluation for electrocatalytic formation of dihydrogen from acetic acid indicates a low overpotential of activity of 0.26 V for compound 5.

Eight novel manganese carbonyl complexes of the type [Mn(bpy-Bu-t)(CO)(3)PR3](+) (bpy-Bu-t = 4,4 '-di-tert-butyl-2,2 '-bipyridine; R = Cy, Bu-n, Me, p-tol, Ph, p-F-Ph, OEt, and OMe), have been synthesized and characterized by H-1 NMR, FTIR, UV/Vis, HRMS and CV. X-ray crystallographic structures of [Mn(bpy-Bu-t)(CO)(3)(PCy3)](+) and [Mn(bpy-Bu-t)(CO)(3)(PPh3)](+) were obtained. The short Mn-P bond length allows for close proximity of the bipyridine ligand and the phosphine R groups, resulting in strong anisotropic shielding of certain bipyridine protons by aryl R groups (reordering the bipyridine H-1 NMR pattern in the most extreme case). Electrochemical analysis of the compound series reveals that while each is a competent precatalyst for electrochemical carbon dioxide reduction (to carbon monoxide), the lability of the PR3 ligand results in similar catalytic performance amongst the series.

X-ray crystallographic structure of [Fe2(µ-3,6-dichloro-1,2-benzenedithiolato)(CO)5Sb(O)Ph3] above the linear voltammetric response obtained for [Fe2(µ-3,6-dichloro-1,2-benzenedithiolato)(CO)5L] when L = CO (blue), L = Sb(O)Ph3 (purple), L = AsPh3 (green) and L = PPh3 (red). [Display omitted] •Synthesis and characterization of novel pnictogen containing organometallic compounds.•Electrochemical comparison of pnictogen iron-sulfur dithiolates.•Hydrogen-generation from weak acid sources.•Bio-inspired electrocatalysts.•Hypervalent antimony. The effect of replacing one CO ligand of the competent proton reduction electrocatalyst [(μ-Cl2-bdt)Fe2(CO)6] (“Cl2-bdt” = 3,6-dichloro-1,2-benzenedithiolato), A, with a pnictogen-based ligand has been explored. Four novel compounds have been synthesized and characterized, including X-ray crystallographic structures: [Fe2(µ-Cl2-bdt)(CO)5PPh3] (1), [Fe2(µ-Cl2-bdt)(CO)5AsPh3] (2), [Fe2(µ-Cl2-bdt)(CO)5Sb(O)PPh3] (3), and [Fe2(µ-Cl2-bdt)(CO)4(SbPPh3)2] (4). Notably, 3 represents an uncommon hypervalent antimony 12-Sb-5 compound. The three mono-pnictogen compounds (1–3) were produced in yields of 44–58% and have been investigated electrochemically. Evaluation for electrocatalytic formation of dihydrogen in the presence of acetic acid for compounds 1–3 shows negligible advantage over compound A.

A series of rhenium diimine carbonyl complexes was prepared and characterized in order to examine the influence of axial ligands on electronic structure. Systematic substitution of the axial carbonyl and acetonitrile ligands of [Re(deeb)(CO)(3)(NCCH3)](+) (deeb = 4,4'-diethylester-2,2'-bipyridine) with trimethylphosphine and chloride, respectively, gives rise to red-shifted absorbance features. These bathochromic shifts result from destabilization of the occupied dorbitals involved in metal-to-ligand charge-transfer transitions. Time-Dependent Density Functional Theory identified the orbitals involved in each transition and provided support for the changes in orbital energies induced by ligand substitution.

The synthesis and characterization of a series of compounds of the type Re(diimine)(CO)(2)(L)Cl (where L = P(OEt)(3), PMe3) is reported. The non-photochemical ligand substitution (non-PLS) utilizes the trans-labilizing effect of phosphorus ligands to facilitate carbonyl replacement. The non-PIS pathway leads to good yields (>60%) of single isomer products, comparing favorably to the lower yield, mixed isomer products of analogous PLS pathways. The substitution of a carbonyl group by a phosphine/phosphite ligands shifts absorption further into the visible region without diminishing molar absorptivity. The synthesis presented provides a general facile ligand substitution pathway for related light-sensitive neutral rhenium/manganese carbonyl halide compounds. (C) 2015 Elsevier B.V. All rights reserved.

Three new complexes of the type Mn(NN)(CO)(3)Br have been synthesized, where NN = 5-substituted-1,10-phenanthroline. The novel compounds have been characterized by H-1 NMR, FTIR, CV, HRMS, and UV/Vis, as well as X-ray crystallography in one case. The known NN = phenanthroline compound, and four analogous Re(I) compounds are also presented; their photophysical and electrochemical properties are discussed. A correlation is apparent between the MLCT absorbance bands and the electrochemical reduction of both the Re and Mn series of compounds. (C) 2014 Elsevier B.V. All rights reserved.

Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4'-bipyridyl-3,3'-disulfide (1) and the di-N-methylated derivative (2(2+)) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S S bond to form the dithiolate. In contrast, compound 2(2+) is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials.

Three variations of the common [FeFe]-Hydrogenase active site mimic μ–(SCH2CH2S)–Fe2(CO)6, 1, have been studied: μ-(SCH(CH3)CH2S)–Fe2(CO)6, 2; μ-(SCH(CH3)CH(CH3)S)–Fe2(CO)6, 3; and μ-(SCH2CH(CH2OH)S)–Fe2(CO)6, 4. The electrochemistry of these four species is compared, indicating that 3 displays greater durability upon electrochemical cycling. This enhanced stability is ascribed to steric blockage of decomposition routes, rather than any change to the Fe2S2 butterfly structure. Electrocatalysis using a weak acid (4-tert-butylphenol, pKa = 27.5 in acetonitrile) is observed for each catalyst with similar overpotential. The enhanced stability of the reduction products of 3 leads to a modest increase of the apparent catalytic rate. Compound 4 was tested for an enhancement of the rate of the acid deprotonation step; compound 4 failed to behave in such a manner, providing no improvement in electrochemical catalysis compared to the parent compound. Structures of the four compounds studied above their cyclic voltammetric response. [Display omitted] ► Electrochemical comparison of iron–sulphur dithiolates. ► Hydrogen-generation from weak acid sources. ► Bio-inspired electrocatalysts.