Co-reporter:Tiankun Zhao, Pokhraj Ghosh, Zachary Martinez, Xufeng Liu, Xianggao Meng, and Marcetta Y. Darensbourg
Organometallics May 8, 2017 Volume 36(Issue 9) pp:1822-1822
Publication Date(Web):April 25, 2017
DOI:10.1021/acs.organomet.7b00176
Four novel bidentate phosphine-Pd complexes coordinated by NiN2S2 metallodithiolate ligands were synthesized and characterized, including XRD analysis; these redox-active complexes are stable in air, and they survive column chromatography on silica gel. The Ni–Pd bimetallic complexes are demonstrated to be precatalyst for Suzuki–Miyaura cross-coupling reactions. The best of these has an overall yield of over 99% under only 1% catalyst loading. The role of NiN2S2 as a stabilizing bidentate ligand that is also oxidizable may account for its efficacy in the support of the catalysis.
Co-reporter:Pokhraj Ghosh;Manuel Quiroz;Ning Wang;Nattamai Bhuvanesh
Dalton Transactions 2017 vol. 46(Issue 17) pp:5617-5624
Publication Date(Web):2017/05/02
DOI:10.1039/C6DT04666E
The control of aggregation at sulfur by metallodithiolates (MN2S2) has made them prime candidates as building blocks for the synthesis of biomimetics of various bimetallic enzyme active sites, with reactivity consequences implicating redox control by both metal centers. Recent studies of MN2S2 (M = Ni2+, Fe(NO)2+) bound to [(η5-C5H5)Fe(CO)]+ as electrocatalysts for proton reduction, the hydrogen evolution reaction, demonstrated reduction-induced hemi-lability of the bridging cis-dithiolates as a key step in the electrochemical proton reduction process (Ding, et al., J. Am. Chem. Soc., 2016, 138, 12920–12927). The MN2S2·Fe(η5-C5R5)(CO) platform offers numerous possibilities for tuning the electronic character of the M(μ-S2)Fe core. As well as modifying M within the metallodithiolate ligand, replacing H by CH3 at the η5-C5R5 moiety increases the electron density at the Fe center, which might facilitate the reductive Fe–S bond cleavage. Although release of a free thiolate in these hemi-labile ligands creates a needed internal pendant base, this benefit might be countered by the increase in over-potential for addition of the first electron. Herein we report the preparation and characterization of four bimetallic aggregates with the (η5-C5R5)Fe(CO) (R = H, CH3; Fe′ or Fe*′, respectively) or the dicarbonyl (η5-C5R5)Fe(CO)2 scaffold (R = H, CH3; Fe′′ or Fe*′′, respectively) bound to redox active MN2S2 ligands (M = Ni2+, Co(NO)2+; N2S2 = bismercaptoethane diazacycloheptane) Co-Fe*′, Ni-Fe*′, Co-Fe′ and Co-Fe*′′ complexes. The bidentate complexes were found to be electrocatalysts for proton reduction, although at high over-potential, especially for the derivatives of the electron-rich (η5-C5(CH3)5)Fe(CO)+. The turnover (TON) and turnover frequencies (TOF) were determined and found to be comparable to the previously reported MN2S2·Fe(η5-C5H5)(CO)+ analogues.
Co-reporter:Randara Pulukkody;Rachel B. Chupik;Steven K. Montalvo;Sarosh Khan;Nattamai Bhuvanesh;Soon-Mi Lim
Chemical Communications 2017 vol. 53(Issue 6) pp:1180-1183
Publication Date(Web):2017/01/17
DOI:10.1039/C6CC08659D
Both monomeric and dimeric tetraacetylglucose-containing {Fe(NO)2}9 dinitrosyl iron complexes (DNICs) were prepared and examined for NO release in the presence of both chemical NO-trapping agents and endothelial cells.
Co-reporter:Pokhraj Ghosh;Shengda Ding;Rachel B. Chupik;Manuel Quiroz;Chung-Hung Hsieh;Nattami Bhuvanesh;Michael B. Hall
Chemical Science (2010-Present) 2017 vol. 8(Issue 12) pp:8291-8300
Publication Date(Web):2017/11/20
DOI:10.1039/C7SC03378H
Experimental and computational studies address key questions in a structure–function analysis of bioinspired electrocatalysts for the HER. Combinations of NiN2S2 or [(NO)Fe]N2S2 as donors to (η5-C5H5)Fe(CO)+ or [Fe(NO)2]+/0 generate a series of four bimetallics, gradually “softened” by increasing nitrosylation, from 0 to 3, by the non-innocent NO ligands. The nitrosylated NiFe complexes are isolated and structurally characterized in two redox levels, demonstrating required features of electrocatalysis. Computational modeling of experimental structures and likely transient intermediates that connect the electrochemical events find roles for electron delocalization by NO, as well as Fe–S bond dissociation that produce a terminal thiolate as pendant base well positioned to facilitate proton uptake and transfer. Dihydrogen formation is via proton/hydride coupling by internal S–H+⋯−H–Fe units of the “harder” bimetallic arrangements with more localized electron density, while softer units convert H−⋯H−via reductive elimination from two Fe–H deriving from the highly delocalized, doubly reduced [Fe2(NO)3]− derivative. Computational studies also account for the inactivity of a Ni2Fe complex resulting from entanglement of added H+ in a pinched –Sδ−⋯H+⋯δ−S− arrangement.
Co-reporter:Allen M. Lunsford;Kristina F. Goldstein;Matthew A. Cohan;Jason A. Denny;Nattamai Bhuvanesh;Shengda Ding;Michael B. Hall
Dalton Transactions 2017 vol. 46(Issue 16) pp:5175-5182
Publication Date(Web):2017/04/19
DOI:10.1039/C7DT00600D
The bipyridine ligand is renowned as a photo- and redox-active ligand in catalysis; the latter has been particularly explored in the complex Re(bipy)(CO)3Cl for CO2 reduction. We ask whether a bidentate, redox-active MN2S2 metallodithiolate ligand in heterobimetallic complexes of Mn and Re might similarly serve as a receptor and conduit of electrons. In order to assess the electrochemical features of such designed bimetallics, a series of complexes featuring redox active MN2S2 metallodithiolates, with M = Ni2+, {Fe(NO)}2+, and {Co(NO)}2+, bound to M′(CO)3X, where M′ = Mn and Re, were synthesized and characterized using IR and EPR spectroscopies, X-ray diffraction, cyclic voltammetry, and density functional theory (DFT) computations. Butterfly type structures resulted from binding of the convergent lone pairs of the cis-sulfur atoms to the M′(CO)3X unit. Bond distances and angles are similar across the M′ metal series regardless of the ligand attached. Electrochemical characterizations of [MN2S2·Re(CO)3Cl] showed the redox potential of the Re is significantly altered by the identity of the metal in the N2S2 pocket. DFT calculations proved useful to identify the roles played by the MN2S2 ligands, upon reduction of the bimetallics, in altering the lability of the Re–Cl bond and the ensuing effect on the reduction of ReI to Re0.
Co-reporter:Shengda Ding, Pokhraj Ghosh, Allen M. Lunsford, Ning Wang, Nattamai Bhuvanesh, Michael B. Hall, and Marcetta Y. Darensbourg
Journal of the American Chemical Society 2016 Volume 138(Issue 39) pp:12920-12927
Publication Date(Web):August 19, 2016
DOI:10.1021/jacs.6b06461
Synthetic analogues and computationally assisted structure–function analyses have been used to explore the features that control proton–electron and proton–hydride coupling in electrocatalysts inspired by the [NiFe]-hydrogenase active site. Of the bimetallic complexes derived from aggregation of the dithiolato complexes MN2S2 (N2S2 = bismercaptoethane diazacycloheptane; M = Ni or Fe(NO)) with (η5-C5H5)Fe(CO)+ (the Fe′ component) or (η5-C5H5)Fe(CO)2+, Fe″, which yielded Ni–Fe′+, Fe–Fe′+, Ni–Fe″+, and Fe–Fe″+, respectively, both Ni–Fe′+ and Fe–Fe′+ were determined to be active electrocatalysts for H2 production in the presence of trifluoroacetic acid. Correlations of electrochemical potentials and H2 generation are consistent with calculated parameters in a predicted mechanism that delineates the order of addition of electrons and protons, the role of the redox-active, noninnocent NO ligand in electron uptake, the necessity for Fe′–S bond breaking (or the hemilability of the metallodithiolate ligand), and hydride-proton coupling routes. Although the redox active {Fe(NO)}7 moiety can accept and store an electron and subsequently a proton (forming the relatively unstable Fe-bound HNO), it cannot form a hydride as the NO shields the Fe from protonation. Successful coupling occurs from a hydride on Fe′ with a proton on thiolate S and requires a propitious orientation of the H–S bond that places H+ and H– within coupling distance. This orientation and coupling barrier are redox-level dependent. While the Ni–Fe′ derivative has vacant sites on both metals for hydride formation, the uptake of the required electron is more energy intensive than that in Fe–Fe′ featuring the noninnocent NO ligand. The Fe′–S bond cleavage facilitated by the hemilability of thiolate to produce a terminal thiolate as a proton shuttle is a key feature in both mechanisms. The analogous Fe″–S bond cleavage on Ni–Fe″ leads to degradation.
Co-reporter:Jason A. Denny, Marcetta Y. Darensbourg
Coordination Chemistry Reviews 2016 Volume 324() pp:82-89
Publication Date(Web):1 October 2016
DOI:10.1016/j.ccr.2016.06.013
•Metallodithiolate ligands are stronger donors than phosphines.•Highly asymmetric metallodithiolates are characterized by wedge and hinge angles.•Customizable donor and steric properties.•Butterfly type structures expected useful in bimetallic catalysis.The well-known steric and electronic factors developed for phosphines by Tolman have influenced the design of organometallic complexes for catalytic processes for nearly a half century. Metallodithiolates as in square planar MN2S2, MP2S2, and MS′2S2 complexes utilize cis-dithiolates as mono- or bidentate donors to exogeneous metals, such as low valent metal carbonyls and nitrosyls wherein their established donor abilities are similar to phosphines. The highly asymmetric heterobimetallic complexes have folded “butterfly” structures deriving from their hinged M(μ-SR)2M′ cores. Analysis of the electronic factors of representatives from a broad series of MN2S2 complexes is referenced to electrochemical redox events as well as the vibrational spectroscopy of diatomic ligand reporters. Steric properties are assessed from XRD studies that yield common metric parameters such as bite angle; they define as well the hinge angle of the bridging thiolate, and the wedge that encompasses the largely planar ligands in bidentate bonding. Adaptation of various approaches to defining ligand steric factors such as the solid angle and percent-buried-volume permits comparison with ligands such as phosphines or simple imines. According to the percent buried volume, spacial requirements of MN2S2 ligands are within a range of 33–36% and in-between the 1,1-bis(diphenylphosphino)methane, dppm, (38%) and bipyridine (30%). The asymmetry and tunability of such MN2S2 metalloligands for both electronic and steric control are prospects for applications in catalytic processes.
Co-reporter:Allen M. Lunsford, Christopher C. Beto, Shengda Ding, Özlen F. Erdem, Ning Wang, Nattamai Bhuvanesh, Michael B. Hall and Marcetta Y. Darensbourg
Chemical Science 2016 vol. 7(Issue 6) pp:3710-3719
Publication Date(Web):29 Feb 2016
DOI:10.1039/C6SC00213G
Developing from certain catalytic processes required for ancient life forms, the H2 processing enzymes [NiFe]- and [FeFe]-hydrogenase (H2ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H2ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent FeIFeI unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66–70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]–CN–Fe ⇌ [FeFe]–NC–Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.
Co-reporter:Allen M. Lunsford; Jan H. Blank; Salvador Moncho; Steven C. Haas; Sohail Muhammad; Edward N. Brothers; Marcetta Y. Darensbourg;Ashfaq A. Bengali
Inorganic Chemistry 2016 Volume 55(Issue 2) pp:964-973
Publication Date(Web):December 30, 2015
DOI:10.1021/acs.inorgchem.5b02601
Studies focused on the dehydrogenation of amine-borane by diiron complexes that serve as well-characterized rudimentary models of the diiron subsite in [FeFe]-hydrogenase are reported. Complexes of formulation (μ-SCH2XCH2S)[Fe(CO)3]2, with X = CH2, CMe2, CEt2, NMe, NtBu, and NPh, 1-CO through 6-CO, respectively, were determined to be photocatalysts for release of H2 gas from a solution of H3B ← NHMe2 (B:As), dissolved in THF. The thermal displacement of the tertiary amine-borane, H3B ← NEt3 (B:At) from photochemically generated (μ-SCH2XCH2S)[Fe(CO)3][Fe(CO)2(μ-H)(BH2–NEt3)], 1-B:At through 6-B:At, by P(OEt)3 was monitored by time-resolved FTIR spectroscopy. Rates and activation barriers for this substitution reaction were consistent with a dissociative mechanism for the alkylated bridgehead species 2-CO through 6-CO, and associative or interchange for 1-CO. DFT calculations supported an intermediate [I] for the dissociative process featuring a coordinatively unsaturated diiron complex stabilized by an agostic interaction between the metal center and the C–H bond of an alkyl group on the central bridgehead atom of the SRS linker. The rate of H2 production from the initially formed 1-B:As through 6-B:As complexes was inversely correlated with the lifetime of the analogous 1-B:At through 6-B:At adducts. Possible mechanisms are presented which feature involvement of the pendent nitrogen base as well as a separate mechanism for the all carbon bridgeheads.
Co-reporter:Dr. Jan H. Blank;Dr. Salvador Moncho;Allen M. Lunsford; Edward N. Brothers; Marcetta Y. Darensbourg; Ashfaq A. Bengali
Chemistry - A European Journal 2016 Volume 22( Issue 36) pp:12752-12760
Publication Date(Web):
DOI:10.1002/chem.201601677
Abstract
The mechanism and energetics of CO, 1-hexene, and 1-hexyne substitution from the complexes (SBenz)2[Fe2(CO)6] (SBenz=SCH2Ph) (1-CO), (SBenz)2[Fe2(CO)5(η2-1-hexene)] (1-(η2-1-hexene)), and (SBenz)2[Fe2(CO)5(η2-1-hexyne)] (1-(η2-1-hexyne)) were studied by using time-resolved infrared spectroscopy. Exchange of both CO and 1-hexyne by P(OEt)3 and pyridine, respectively, proceeds by a bimolecular mechanism. As similar activation enthalpies are obtained for both reactions, the rate-determining step in both cases is assumed to be the rotation of the Fe(CO)2L (L=CO or 1-hexyne) unit to accommodate the incoming ligand. The kinetic profile for the displacement of 1-hexene is quite different than that for the alkyne and, in this case, both reaction channels, that is, dissociative (SN1) and associative (SN2), were found to be competitive. Because DFT calculations predict similar binding enthalpies of alkene and alkyne to the iron center, the results indicate that the bimolecular pathway in the case of the alkyne is lower in free energy than that of the alkene. In complexes of this type, subtle changes in the departing ligand characteristics and the nature of the mercapto bridge can influence the exchange mechanism, such that more than one reaction pathway is available for ligand substitution. The difference between this and the analogous study of (μ-pdt)[Fe(CO)3]2 (pdt=S(CH2)3S) underscores the unique characteristics of a three-atom S−S linker in the active site of diiron hydrogenases.
Co-reporter:Jason A. Denny and Marcetta Y. Darensbourg
Chemical Reviews 2015 Volume 115(Issue 11) pp:5248
Publication Date(Web):May 7, 2015
DOI:10.1021/cr500659u
Co-reporter:Randara Pulukkody and Marcetta Y. Darensbourg
Accounts of Chemical Research 2015 Volume 48(Issue 7) pp:2049
Publication Date(Web):June 19, 2015
DOI:10.1021/acs.accounts.5b00215
Resulting from biochemical iron–NO interactions, dinitrosyl iron complexes (DNICs) are small organometallic-like molecules, considered to serve as vehicles for NO transport and storage in vivo. Formed by the interaction of NO with cellular iron sulfur clusters or with the cellular labile iron pool, DNICs have been documented to be the largest NO-derived adduct in cells, even surpassing the well-known nitrosothiols (RSNOs). Continuing efforts in biological chemistry are aimed at understanding the movement of DNICs in and out of cells, and their important role in NO-induced iron efflux leading to apoptosis in cells.Intrigued by the integrity of the unique dinitrosyl iron unit (DNIU) and the possibility of roles for it in human physiology or medicinal applications, the understanding of fundamental properties such as ligand effects on its ability to switch between two redox levels has been pursued through biomimetic complexes. Using metallodithiolates and N-heterocyclic carbenes (NHCs) as ligands to Fe(NO)2, the synthesis of a library of novel DNICs, in both the oxidized, {Fe(NO)2}9, and reduced, {Fe(NO)2}10, forms (Enemark–Feltham notation), offers opportunity to examine structural, reactivity, and spectroscopic features.The raison d’etre for the MN2S2·Fe(NO)2 synthesis development is for the potential to exploit the ease of accessing two redox levels on two different metal sites, a property presumably required for achieving two electron redox processes in base metals. Hence such molecules may be viewed as synthetic analogues of [NiFe]- or [FeFe]-hydrogenase active sites in nature, both of which use bridging thiolates for connection of the two centers. A particular success was the development of an Fe(NO)N2S2·Fe(NO)2+/0 redox pair for proton reduction electrocatalysis.Monomeric, reduced NHC-DNICs of the L2Fe(NO)2 type are synthesized via the Fe(CO)2(NO)2 precursor, and oxidized thiolate-containing forms are derived from the dimeric (μ-RS)2[Fe(NO)2]2. Monomeric NHC-DNICs are four coordinate, pseudotetrahedral compounds with planar Fe(NO)2 units in which the slightly bent Fe–NO groups are directed symmetrically inward at both redox levels. They serve as stable analogues of biological histidine binding sites. In agreement with IR data, Mössbauer spectroscopic parameters, and DFT computations, the prototypic NHC-DNICs indicate extensive delocalization of the electron density of iron via π-backbonding. Such π-delocalization presents an unusual reaction path for the one electron process of RS–/RSSR interconversion. Comparisons with imidazole-DNICs find NHCs to be the “better” ligands to Fe(NO)2 and prompted investigations in (a) possible relationships between such imidazole- and NHC-containing DNICs, (b) systems that might mimic the reactivity of DNICs with the endogenous gaseotransmitter CO, and (c) mechanistic details of such processes.In a broader context, these studies aim to further describe the behavior of the {Fe(NO)2} unit as a single molecular entity when subjected to various ligand environments and reaction conditions.
Co-reporter:J. A. Denny, W. S. Foley, A. D. Todd and M. Y. Darensbourg
Chemical Science 2015 vol. 6(Issue 12) pp:7079-7088
Publication Date(Web):07 Sep 2015
DOI:10.1039/C5SC02269J
The effect of S-acetylation in MN2S2 complexes on metal exchange reactivity was examined in a series of MN2S2O2 complexes. While clean exchange processes do not occur for the MN2S2 derivatives where formation of S-bridged aggregates predominate, acetylation permits the metal exchange with hierarchy that follows the Irving–Williams series of stability for first row transition metals: Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. The rate determining step consistent with kinetic parameters depends on both M and M′, supporting a mechanism of exchange that involves ligand unwrapping/rewrapping process as earlier defined by Margerum et al. for M(EDTA) systems. The enhanced metal exchange deriving from S-acetylation is of significance to probes and detection of cysteine-S metallo-proteins and metallo-enzyme active sites, and highlights a new role for S-acetylation.
Co-reporter:Danielle J. Crouthers;Shengda Ding;Jason A. Denny;Dr. Ryan D. Bethel;Dr. Chung-Hung Hsieh; Michael B. Hall; Marcetta Y. Darensbourg
Angewandte Chemie International Edition 2015 Volume 54( Issue 38) pp:11102-11106
Publication Date(Web):
DOI:10.1002/anie.201504574
Abstract
The Ph3PAu+ cation, renowned as an isolobal analogue of H+, was found to serve as a proton surrogate and form a stable Au2Fe2 complex, [(μ-SAuPPh3)2{Fe(CO)3}2], analogous to the highly reactive dihydrosulfide [(μ-SH)2{Fe(CO)3}2]. Solid-state X-ray diffraction analysis found the two SAuPPh3 and SH bridges in anti configurations. VT NMR studies, supported by DFT computations, confirmed substantial barriers of approximately 25 kcal mol−1 to intramolecular interconversion between the three stereoisomers of [(μ-SH)2{Fe(CO)3}2]. In contrast, the largely dative SAu bond in μ-SAuPPh3 facilitates inversion at S and accounts for the facile equilibration of the SAuPPh3 units, with an energy barrier half that of the SH analogue. The reactivity of the gold-protected sulfur atoms of [(μ-SAuPPh3)2{Fe(CO)3}2] was accessed by release of the gold ligand with a strong acid to generate the [(μ-SH)2{Fe(CO)3}2] precursor of the [FeFe]H2ase-active-site biomimetic [(μ2-SCH2(NR)CH2S){Fe(CO)3}2].
Co-reporter:Danielle J. Crouthers;Shengda Ding;Jason A. Denny;Dr. Ryan D. Bethel;Dr. Chung-Hung Hsieh; Michael B. Hall; Marcetta Y. Darensbourg
Angewandte Chemie 2015 Volume 127( Issue 38) pp:11254-11258
Publication Date(Web):
DOI:10.1002/ange.201504574
Abstract
The Ph3PAu+ cation, renowned as an isolobal analogue of H+, was found to serve as a proton surrogate and form a stable Au2Fe2 complex, [(μ-SAuPPh3)2{Fe(CO)3}2], analogous to the highly reactive dihydrosulfide [(μ-SH)2{Fe(CO)3}2]. Solid-state X-ray diffraction analysis found the two SAuPPh3 and SH bridges in anti configurations. VT NMR studies, supported by DFT computations, confirmed substantial barriers of approximately 25 kcal mol−1 to intramolecular interconversion between the three stereoisomers of [(μ-SH)2{Fe(CO)3}2]. In contrast, the largely dative SAu bond in μ-SAuPPh3 facilitates inversion at S and accounts for the facile equilibration of the SAuPPh3 units, with an energy barrier half that of the SH analogue. The reactivity of the gold-protected sulfur atoms of [(μ-SAuPPh3)2{Fe(CO)3}2] was accessed by release of the gold ligand with a strong acid to generate the [(μ-SH)2{Fe(CO)3}2] precursor of the [FeFe]H2ase-active-site biomimetic [(μ2-SCH2(NR)CH2S){Fe(CO)3}2].
Co-reporter:Subal Dey ; Atanu Rana ; Danielle Crouthers ; Biswajit Mondal ; Pradip Kumar Das ; Marcetta Y. Darensbourg ;Abhishek Dey
Journal of the American Chemical Society 2014 Volume 136(Issue 25) pp:8847-8850
Publication Date(Web):May 20, 2014
DOI:10.1021/ja5021684
The instability of [Fe-Fe]-hydrogenase and its synthetic models under aerobic conditions is an inherent challenge in their development as practical H2 producing electrodes. The electrochemical oxygen reduction reaction of a series of synthetic model complexes of the [Fe-Fe] hydrogenase is investigated, and a dominant role of the bridgehead nitrogen in reducing the amount of partially reduced oxygen species (PROS), which is detrimental to the stability of these complexes, is discovered.
Co-reporter:Randara Pulukkody, Samuel J. Kyran, Michael J. Drummond, Chung-Hung Hsieh, Donald J. Darensbourg and Marcetta Y. Darensbourg
Chemical Science 2014 vol. 5(Issue 10) pp:3795-3802
Publication Date(Web):16 Jun 2014
DOI:10.1039/C4SC01523A
The displacement of RS˙ from [(NHC)(SPh)Fe(NO)2] (NHC = N-heterocyclic carbene) by carbon monoxide follows associative kinetics, rate = k [CO]1 [(NHC)(SPh)Fe(NO)2]1, resulting in reduction of the oxidized form of the dinitrosyliron unit, {Fe(NO)2}9 (Enemark–Feltham notation) to {Fe(NO)2}10. Thermodynamically driven by the release of PhS–SPh concomitant with formation of [(NHC)(CO)Fe(NO)2], computational studies suggested the reactant dinitrosyliron unit serves as a nucleophile in the initial slanted interaction of the π* orbital of CO, shifting into normal linear Fe–CO with weakening of the Fe–SPh bond. The current study seeks to experimentally test this proposal. A series of analogous {Fe(NO)2}9 [(NHC)(p-S–C6H4X)Fe(NO)2] complexes, with systematic variation of the para-substituents X from electron donor to electron withdrawing groups was used to monitor variation in electron density at the Fe(NO)2 unit via Hammett analyses. Despite the presence of non-innocent NO ligands, data from ν(NO) IR spectroscopy and cyclic voltammetry showed consistent tracking of the electron density at the {Fe(NO)2} unit in response to the aryl substituent. The electronic modifications resulted in systematic changes in reaction rates when each derivative was exposed to CO. A plot of the rate constants and the Hammett parameter σp is linear with a negative slope and a ρ value of −0.831; such correlation is indicative of rate retardation by electron-withdrawing substituents, and provides experimental support for the unique role of the delocalized frontier molecular orbitals of the Fe(NO)2 unit.
Co-reporter:Tiffany A. Pinder, Steven K. Montalvo, Chung-Hung Hsieh, Allen M. Lunsford, Ryan D. Bethel, Brad S. Pierce, and Marcetta Y. Darensbourg
Inorganic Chemistry 2014 Volume 53(Issue 17) pp:9095-9105
Publication Date(Web):August 21, 2014
DOI:10.1021/ic501117f
Metallodithiolate ligands are used to design heterobimetallic complexes by adduct formation through S-based reactivity. Such adducts of dinitrosyl iron were synthesized with two metalloligands, namely, Ni(bme-daco) and V≡O(bme-daco) (bme-daco = bismercaptoethane diazacyclooctane), and, for comparison, an N-heterocyclic carbene, namely, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (Imes), by cleavage of the (μ-I)2[Fe(NO)2]2 dimer of electronic configuration {Fe(NO)2}9 (Enemark–Feltham notation). With Fe(NO)2I as Lewis acid acceptor, 1:1 adducts resulted for both the IMes·Fe(NO)2I, complex 2, and V≡O(bme-daco)·Fe(NO)2I, complex 4. The NiN2S2 demonstrated binding capability at both thiolates, with two Fe(NO)2I addenda positioned transoid across the NiN2S2 square plane, Ni(bme-daco)·2(Fe(NO)2I), complex 3. Enhanced binding ability was realized for the dianionic vanadyl dithiolate complex, [Et4N]2[V≡O(ema)], (ema = N,N′-ethylenebis(2-mercaptoacetamide)), which, unlike the neutral (V≡O)N2S2, demonstrated reactivity with the labile tungsten carbonyl complex, cis-W(CO)4(pip)2, (pip = piperidine), yielding [Et4N]2[V≡O(ema)W(CO)4], complex 1, whose ν(CO) IR values indicated the dianionic vanadyl metalloligand to be of similar donor ability to the neutral NiN2S2 ligands. The solid-state molecular structures of 1–4 were determined by X-ray diffraction analyses. Electron paramagnetic resonance (EPR) measurements characterize the {Fe(NO)2}9 complexes in solution, illustrating superhyperfine coupling via the 127I to the unpaired electron on iron for complex 2. The EPR characterizations of 3 [Ni(bme-daco)·2(Fe(NO)2I)] and 4 [V≡O(bme-daco)·Fe(NO)2I] indicate these complexes are EPR silent, likely due to strong coupling between paramagnetic centers. Within samples of complex 4, individual paramagnetic centers with localized superhyperfine coupling from the 51V and 127I are observed in a 3:1 ratio, respectively. However, spin quantitation reveals that these species represent a minor fraction (<10%) of the total complex and thus likely represent disassociated paramagnetic sites. Computational studies corroborated the EPR assignments as well as the experimentally observed stability/instability of the heterobimetallic DNIC complexes.
Co-reporter:Tiffany A. Pinder, Steven K. Montalvo, Allen M. Lunsford, Chung-Hung Hsieh, Joseph H. Reibenspies and Marcetta Y. Darensbourg
Dalton Transactions 2014 vol. 43(Issue 1) pp:138-144
Publication Date(Web):04 Oct 2013
DOI:10.1039/C3DT52295D
Development of square planar cis-dithiolate nickel complexes as metallo S-donor ligands focuses on the synthesis and structures of gold(I) heterometallic clusters derived from assemblage with three NiN2S2 complexes: Ni(bme-daco), Ni(bme-dach) and Ni(ema)2− (bme-daco = (bismercaptoethanediazacyclooctane); bme-dach = bismercaptoethanediazacycloheptane; and ema = N,N′-ethylenebis-2-mercaptoacetamide). With Ph3PAuCl as the gold source, examples of simple S-aurolation retaining the PPh3 on Au+ were obtained for [{Ni(bme-daco)}AuPPh3]+Cl− and [{Ni(ema)}2Au4(PPh3)4], where the latter complex demonstrated unsupported aurophilic interactions between [{Ni(ema)}Au2(PPh3)2] units in its X-ray crystal structure (Au–Au = 3.054 and 3.127 Å). Three compounds containing fully-supported digold units with Au–Au distances in the aurophilic range of 3.11 to 3.13 Å were found as stair-step structures in which planar NiN2S2 step treads are connected by planar S2Au2S2 risers at ca. 90°: [{Ni(bme-daco)}2Au2]2+(Cl−)2; [{Ni(bme-dach)}2Au2]2+(Cl−)2; and (Et4N+)2[{Ni(ema)}2Au2]2−. Electrochemical data from cyclic voltammograms demonstrated a positive shift in NiII/I couples for the [{NiN2S2}xAuy] complexes as compared to the NiN2S2 precursors and a ca. 700 mV decrease in communication between multiple NiN2S2 units as compared to [{NiN2S2}2Ni]2+ analogues in slant chair conformation. The analogy between NiN2S2 metallodithiolate ligands and diphosphine ligands holds here as in other examples of inorganic and organometallic complexes.
Co-reporter:Danielle J. Crouthers, Jason A. Denny, Ryan D. Bethel, David G. Munoz, and Marcetta Y. Darensbourg
Organometallics 2014 Volume 33(Issue 18) pp:4747-4755
Publication Date(Web):May 12, 2014
DOI:10.1021/om500023j
Dynamic NMR (13C and 1H) studies of (μ-SCH2XCH2S)[Fe(CO)3]2 complexes (X = CR2, NR) were utilized to examine the fluxional processes that are important in the [FeFe]-hydrogenase active site models, where an open site for proton/hydrogen binding, achieved by configurational mobility of the Fe(CO)3 unit, is required for electrocatalysis of proton reduction. In order to interrogate the effects of fluxional mobility on electrochemical response to added acid, energy barriers for the CO site exchange in Fe(CO)3 rotors were determined for nitrogen- and carbon-based bridgehead complexes. The effect of the methyl substituent in both the NH/NCH3 and CH2/C(CH3)2 cases is to lower the Fe(CO)3 rotational activation barrier relative to the NH or CH2 analogues. Although the C(CH3)2 case has the lowest Fe(CO)3 rotational barrier, its performance as a proton reduction electrocatalyst is 2-fold less than that for the X = NR species, indicating the proton-directing effect of the pendent base on catalytic efficiency.
Co-reporter:Chung-Hung Hsieh, Randara Pulukkody and Marcetta Y. Darensbourg
Chemical Communications 2013 vol. 49(Issue 81) pp:9326-9328
Publication Date(Web):16 Aug 2013
DOI:10.1039/C3CC45091K
An N-alkyl imidazole bearing a neutral {Fe(NO)2}10 dinitrosyliron complex (DNIC) when treated with sodium t-butoxide undergoes base-promoted conversion to the N-heterocyclic carbene (NHC)–DNIC, while maintaining the Fe(NO)2 unit intact. Subsequent alkylation led to the isolation of the NHC–DNIC product; further nitrosylation led to trinitrosyl (NHC)Fe(NO)3+. Both were isolated and structurally characterized.
Co-reporter:Jennifer L. Hess ; Chung-Hung Hsieh ; Scott M. Brothers ; Michael B. Hall
Journal of the American Chemical Society 2011 Volume 133(Issue 50) pp:20426-20434
Publication Date(Web):November 10, 2011
DOI:10.1021/ja208384d
Imidazolate-containing {Fe(NO)2}9 molecular squares have been synthesized by oxidative CO displacement from the reduced Fe(CO)2(NO)2 precursor. The structures of complex 1 [(imidazole)Fe(NO)2]4, (Ford, Li, et al.; Chem. Commun.2005, 477–479), 2 [(2-isopropylimidazole)Fe(NO)2]4, and 3 [(benzimidazole)Fe(NO)2]4, as determined by X-ray diffraction analysis, find precise square planes of irons with imidazolates bridging the edges and nitrosyl ligands capping the irons at the corners. The orientation of the imidazolate ligands in each of the complexes results in variations of the overall structures, and molecular recognition features in the available cavities of 1 and 3. Computational studies show multiple low energy structural isomers and confirm that the isomers found in the crystallographic structures arise from intermolecular interactions. EPR and IR spectroscopic studies and electrochemical results suggest that the tetramers remain intact in solution in the presence of weakly coordinating (THF) and noncoordinating (CH2Cl2) solvents. Mössbauer spectroscopic data for a set of reference dinitrosyl iron complexes, reduced {Fe(NO)2}10 compounds A ((NHC-iPr)2Fe(NO)2), and C ((NHC-iPr)(CO)Fe(NO)2), and oxidized {Fe(NO)2}9 compounds B ([(NHC-iPr)2Fe(NO)2][BF4]), and D ((NHC-iPr)(SPh)Fe(NO)2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene) demonstrate distinct differences of the isomer shifts and quadrupole splittings between the oxidized and reduced forms. The reduced compounds have smaller positive isomer shifts as compared to the oxidized compounds ascribed to the greater π-backbonding to the NO ligands. Mössbauer data for the tetrameric complexes 1–3 demonstrate larger isomer shifts, most comparable to compound D; all four complexes contain cationic {Fe(NO)2}9 units bound to one anionic ligand and one neutral ligand. At room temperature, the paramagnetic, S = 1/2 per iron, centers are not coupled.
Co-reporter:Jennifer L. Hess, Chung-Hung Hsieh, Joseph H. Reibenspies, and Marcetta Y. Darensbourg
Inorganic Chemistry 2011 Volume 50(Issue 17) pp:8541-8552
Publication Date(Web):August 8, 2011
DOI:10.1021/ic201138f
N-heterocyclic carbenes (NHCs) are shown to be reasonable mimics of imidazole ligands in dinitrosyl iron complexes determined through the synthesis and characterization of a series of {Fe(NO)2}10 and {Fe(NO)2}9 (Enemark–Feltham notation) complexes. Monocarbene complexes (NHC-iPr)(CO)Fe(NO)2 (1) and (NHC-Me)(CO)Fe(NO)2 (2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene and NHC-Me = 1,3-dimethylimidazol-2-ylidene) are formed from CO/L exchange with Fe(CO)2(NO)2. An additional equivalent of NHC results in the bis-carbene complexes (NHC-iPr)2Fe(NO)2 (3) and (NHC-Me)2Fe(NO)2 (4), which can be oxidized to form the {Fe(NO)2}9 bis-carbene complexes 3+ and 4+. Treatment of complexes 1 and 2 with [NO]BF4 results in the formation of uncommon trinitrosyl iron complexes, (NHC-iPr)Fe(NO)3+ (5+) and (NHC-Me)Fe(NO)3+ (6+), respectively. Cleavage of the Roussin’s Red “ester” (μ-SPh)2[Fe(NO)2]2 with either NHC or imidazole results in the formation of (NHC-iPr)(PhS)Fe(NO)2 (7) and (Imid-iPr)(PhS)Fe(NO)2 (10) (Imid-iPr = 2-isopropylimidazole). The solid-state molecular structures of complexes 1, 2, 3, 4, 5+, and 7 show that they all have pseudotetrahedral geometry. Infrared spectroscopic data suggest that NHCs are slightly better electron donors than imidazoles; electrochemical data are also consistent with what is expected for typical donor/acceptor abilities of the spectator ligands bound to the Fe(NO)2 unit. Although the monoimidazole complex (Imid-iPr)(CO)Fe(NO)2 (8) was observed via IR spectroscopy, attempts to isolate this complex resulted in the formation of a tetrameric {Fe(NO)2}9 species, [(Imid-iPr)Fe(NO)2]4 (9), a molecular square analogous to the unsubstituted imidazole reported by Li and Wang et al. Preliminary NO-transfer studies demonstrate that the {Fe(NO)2}9 bis-carbene complexes can serve as a source of NO to a target complex, whereas the {Fe(NO)2}10 bis-carbenes are unreactive in the presence of a NO-trapping agent.
Co-reporter:Chung-Hung Hsieh, Rachel B. Chupik, Scott M. Brothers, Michael B. Hall and Marcetta Y. Darensbourg
Dalton Transactions 2011 vol. 40(Issue 22) pp:6047-6053
Publication Date(Web):06 May 2011
DOI:10.1039/C1DT10438A
The reaction of Fe(CO)2(NO)2 and Ni(N2S2) (N2S2 = N,N′-Bis(2-mercaptoethyl)-1,4-diazacycloheptane) by a single CO replacement yields [Ni(N2S2)]Fe(NO)2(CO), while an excess of Fe(CO)2(NO)2 leads to triply bridging thiolate sulphurs in a cluster of core composition Ni2S4Fe3, lacking one Fe(NO)2 unit to complete the adamantane-like structure. This structural type was earlier identified in a CuICl aggregate of MII(N2S2) (MII = Ni, Cu), in which complete MII2S4CuI4 core structures were obtained as the major, and, in the case of CuII(N2S2), the incomplete CuII2S4CuI3 as a minor, product. The full Ni2S4Fe4 cluster has not yet been realized for Fe = Fe(NO)2. Computational analysis of the NiFe-heterobimetallic complex addresses structural issues including a ∠Ni–S–Fe of 90° in the bimetallic complex.
Co-reporter:Ryan D. Bethel;Michael L. Singleton ; Marcetta Y. Darensbourg
Angewandte Chemie 2010 Volume 122( Issue 46) pp:8747-8749
Publication Date(Web):
DOI:10.1002/ange.201003747
Co-reporter:Ryan D. Bethel;Michael L. Singleton ; Marcetta Y. Darensbourg
Angewandte Chemie International Edition 2010 Volume 49( Issue 46) pp:8567-8569
Publication Date(Web):
DOI:10.1002/anie.201003747
Co-reporter:Randara Pulukkody, Rachel B. Chupik, Steven K. Montalvo, Sarosh Khan, Nattamai Bhuvanesh, Soon-Mi Lim and Marcetta Y. Darensbourg
Chemical Communications 2017 - vol. 53(Issue 6) pp:NaN1183-1183
Publication Date(Web):2016/12/12
DOI:10.1039/C6CC08659D
Both monomeric and dimeric tetraacetylglucose-containing {Fe(NO)2}9 dinitrosyl iron complexes (DNICs) were prepared and examined for NO release in the presence of both chemical NO-trapping agents and endothelial cells.
Co-reporter:Allen M. Lunsford, Kristina F. Goldstein, Matthew A. Cohan, Jason A. Denny, Nattamai Bhuvanesh, Shengda Ding, Michael B. Hall and Marcetta Y. Darensbourg
Dalton Transactions 2017 - vol. 46(Issue 16) pp:NaN5182-5182
Publication Date(Web):2017/03/13
DOI:10.1039/C7DT00600D
The bipyridine ligand is renowned as a photo- and redox-active ligand in catalysis; the latter has been particularly explored in the complex Re(bipy)(CO)3Cl for CO2 reduction. We ask whether a bidentate, redox-active MN2S2 metallodithiolate ligand in heterobimetallic complexes of Mn and Re might similarly serve as a receptor and conduit of electrons. In order to assess the electrochemical features of such designed bimetallics, a series of complexes featuring redox active MN2S2 metallodithiolates, with M = Ni2+, {Fe(NO)}2+, and {Co(NO)}2+, bound to M′(CO)3X, where M′ = Mn and Re, were synthesized and characterized using IR and EPR spectroscopies, X-ray diffraction, cyclic voltammetry, and density functional theory (DFT) computations. Butterfly type structures resulted from binding of the convergent lone pairs of the cis-sulfur atoms to the M′(CO)3X unit. Bond distances and angles are similar across the M′ metal series regardless of the ligand attached. Electrochemical characterizations of [MN2S2·Re(CO)3Cl] showed the redox potential of the Re is significantly altered by the identity of the metal in the N2S2 pocket. DFT calculations proved useful to identify the roles played by the MN2S2 ligands, upon reduction of the bimetallics, in altering the lability of the Re–Cl bond and the ensuing effect on the reduction of ReI to Re0.
Co-reporter:Chung-Hung Hsieh, Randara Pulukkody and Marcetta Y. Darensbourg
Chemical Communications 2013 - vol. 49(Issue 81) pp:NaN9328-9328
Publication Date(Web):2013/08/16
DOI:10.1039/C3CC45091K
An N-alkyl imidazole bearing a neutral {Fe(NO)2}10 dinitrosyliron complex (DNIC) when treated with sodium t-butoxide undergoes base-promoted conversion to the N-heterocyclic carbene (NHC)–DNIC, while maintaining the Fe(NO)2 unit intact. Subsequent alkylation led to the isolation of the NHC–DNIC product; further nitrosylation led to trinitrosyl (NHC)Fe(NO)3+. Both were isolated and structurally characterized.
Co-reporter:Allen M. Lunsford, Christopher C. Beto, Shengda Ding, Özlen F. Erdem, Ning Wang, Nattamai Bhuvanesh, Michael B. Hall and Marcetta Y. Darensbourg
Chemical Science (2010-Present) 2016 - vol. 7(Issue 6) pp:NaN3719-3719
Publication Date(Web):2016/02/29
DOI:10.1039/C6SC00213G
Developing from certain catalytic processes required for ancient life forms, the H2 processing enzymes [NiFe]- and [FeFe]-hydrogenase (H2ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H2ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent FeIFeI unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66–70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]–CN–Fe ⇌ [FeFe]–NC–Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.
Co-reporter:J. A. Denny, W. S. Foley, A. D. Todd and M. Y. Darensbourg
Chemical Science (2010-Present) 2015 - vol. 6(Issue 12) pp:NaN7088-7088
Publication Date(Web):2015/09/07
DOI:10.1039/C5SC02269J
The effect of S-acetylation in MN2S2 complexes on metal exchange reactivity was examined in a series of MN2S2O2 complexes. While clean exchange processes do not occur for the MN2S2 derivatives where formation of S-bridged aggregates predominate, acetylation permits the metal exchange with hierarchy that follows the Irving–Williams series of stability for first row transition metals: Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. The rate determining step consistent with kinetic parameters depends on both M and M′, supporting a mechanism of exchange that involves ligand unwrapping/rewrapping process as earlier defined by Margerum et al. for M(EDTA) systems. The enhanced metal exchange deriving from S-acetylation is of significance to probes and detection of cysteine-S metallo-proteins and metallo-enzyme active sites, and highlights a new role for S-acetylation.
Co-reporter:Randara Pulukkody, Samuel J. Kyran, Michael J. Drummond, Chung-Hung Hsieh, Donald J. Darensbourg and Marcetta Y. Darensbourg
Chemical Science (2010-Present) 2014 - vol. 5(Issue 10) pp:NaN3802-3802
Publication Date(Web):2014/06/16
DOI:10.1039/C4SC01523A
The displacement of RS˙ from [(NHC)(SPh)Fe(NO)2] (NHC = N-heterocyclic carbene) by carbon monoxide follows associative kinetics, rate = k [CO]1 [(NHC)(SPh)Fe(NO)2]1, resulting in reduction of the oxidized form of the dinitrosyliron unit, {Fe(NO)2}9 (Enemark–Feltham notation) to {Fe(NO)2}10. Thermodynamically driven by the release of PhS–SPh concomitant with formation of [(NHC)(CO)Fe(NO)2], computational studies suggested the reactant dinitrosyliron unit serves as a nucleophile in the initial slanted interaction of the π* orbital of CO, shifting into normal linear Fe–CO with weakening of the Fe–SPh bond. The current study seeks to experimentally test this proposal. A series of analogous {Fe(NO)2}9 [(NHC)(p-S–C6H4X)Fe(NO)2] complexes, with systematic variation of the para-substituents X from electron donor to electron withdrawing groups was used to monitor variation in electron density at the Fe(NO)2 unit via Hammett analyses. Despite the presence of non-innocent NO ligands, data from ν(NO) IR spectroscopy and cyclic voltammetry showed consistent tracking of the electron density at the {Fe(NO)2} unit in response to the aryl substituent. The electronic modifications resulted in systematic changes in reaction rates when each derivative was exposed to CO. A plot of the rate constants and the Hammett parameter σp is linear with a negative slope and a ρ value of −0.831; such correlation is indicative of rate retardation by electron-withdrawing substituents, and provides experimental support for the unique role of the delocalized frontier molecular orbitals of the Fe(NO)2 unit.
Co-reporter:Chung-Hung Hsieh, Rachel B. Chupik, Scott M. Brothers, Michael B. Hall and Marcetta Y. Darensbourg
Dalton Transactions 2011 - vol. 40(Issue 22) pp:NaN6053-6053
Publication Date(Web):2011/05/06
DOI:10.1039/C1DT10438A
The reaction of Fe(CO)2(NO)2 and Ni(N2S2) (N2S2 = N,N′-Bis(2-mercaptoethyl)-1,4-diazacycloheptane) by a single CO replacement yields [Ni(N2S2)]Fe(NO)2(CO), while an excess of Fe(CO)2(NO)2 leads to triply bridging thiolate sulphurs in a cluster of core composition Ni2S4Fe3, lacking one Fe(NO)2 unit to complete the adamantane-like structure. This structural type was earlier identified in a CuICl aggregate of MII(N2S2) (MII = Ni, Cu), in which complete MII2S4CuI4 core structures were obtained as the major, and, in the case of CuII(N2S2), the incomplete CuII2S4CuI3 as a minor, product. The full Ni2S4Fe4 cluster has not yet been realized for Fe = Fe(NO)2. Computational analysis of the NiFe-heterobimetallic complex addresses structural issues including a ∠Ni–S–Fe of 90° in the bimetallic complex.
Co-reporter:Tiffany A. Pinder, Steven K. Montalvo, Allen M. Lunsford, Chung-Hung Hsieh, Joseph H. Reibenspies and Marcetta Y. Darensbourg
Dalton Transactions 2014 - vol. 43(Issue 1) pp:NaN144-144
Publication Date(Web):2013/10/04
DOI:10.1039/C3DT52295D
Development of square planar cis-dithiolate nickel complexes as metallo S-donor ligands focuses on the synthesis and structures of gold(I) heterometallic clusters derived from assemblage with three NiN2S2 complexes: Ni(bme-daco), Ni(bme-dach) and Ni(ema)2− (bme-daco = (bismercaptoethanediazacyclooctane); bme-dach = bismercaptoethanediazacycloheptane; and ema = N,N′-ethylenebis-2-mercaptoacetamide). With Ph3PAuCl as the gold source, examples of simple S-aurolation retaining the PPh3 on Au+ were obtained for [{Ni(bme-daco)}AuPPh3]+Cl− and [{Ni(ema)}2Au4(PPh3)4], where the latter complex demonstrated unsupported aurophilic interactions between [{Ni(ema)}Au2(PPh3)2] units in its X-ray crystal structure (Au–Au = 3.054 and 3.127 Å). Three compounds containing fully-supported digold units with Au–Au distances in the aurophilic range of 3.11 to 3.13 Å were found as stair-step structures in which planar NiN2S2 step treads are connected by planar S2Au2S2 risers at ca. 90°: [{Ni(bme-daco)}2Au2]2+(Cl−)2; [{Ni(bme-dach)}2Au2]2+(Cl−)2; and (Et4N+)2[{Ni(ema)}2Au2]2−. Electrochemical data from cyclic voltammograms demonstrated a positive shift in NiII/I couples for the [{NiN2S2}xAuy] complexes as compared to the NiN2S2 precursors and a ca. 700 mV decrease in communication between multiple NiN2S2 units as compared to [{NiN2S2}2Ni]2+ analogues in slant chair conformation. The analogy between NiN2S2 metallodithiolate ligands and diphosphine ligands holds here as in other examples of inorganic and organometallic complexes.
Co-reporter:Pokhraj Ghosh, Manuel Quiroz, Ning Wang, Nattamai Bhuvanesh and Marcetta Y. Darensbourg
Dalton Transactions 2017 - vol. 46(Issue 17) pp:NaN5624-5624
Publication Date(Web):2017/02/08
DOI:10.1039/C6DT04666E
The control of aggregation at sulfur by metallodithiolates (MN2S2) has made them prime candidates as building blocks for the synthesis of biomimetics of various bimetallic enzyme active sites, with reactivity consequences implicating redox control by both metal centers. Recent studies of MN2S2 (M = Ni2+, Fe(NO)2+) bound to [(η5-C5H5)Fe(CO)]+ as electrocatalysts for proton reduction, the hydrogen evolution reaction, demonstrated reduction-induced hemi-lability of the bridging cis-dithiolates as a key step in the electrochemical proton reduction process (Ding, et al., J. Am. Chem. Soc., 2016, 138, 12920–12927). The MN2S2·Fe(η5-C5R5)(CO) platform offers numerous possibilities for tuning the electronic character of the M(μ-S2)Fe core. As well as modifying M within the metallodithiolate ligand, replacing H by CH3 at the η5-C5R5 moiety increases the electron density at the Fe center, which might facilitate the reductive Fe–S bond cleavage. Although release of a free thiolate in these hemi-labile ligands creates a needed internal pendant base, this benefit might be countered by the increase in over-potential for addition of the first electron. Herein we report the preparation and characterization of four bimetallic aggregates with the (η5-C5R5)Fe(CO) (R = H, CH3; Fe′ or Fe*′, respectively) or the dicarbonyl (η5-C5R5)Fe(CO)2 scaffold (R = H, CH3; Fe′′ or Fe*′′, respectively) bound to redox active MN2S2 ligands (M = Ni2+, Co(NO)2+; N2S2 = bismercaptoethane diazacycloheptane) Co-Fe*′, Ni-Fe*′, Co-Fe′ and Co-Fe*′′ complexes. The bidentate complexes were found to be electrocatalysts for proton reduction, although at high over-potential, especially for the derivatives of the electron-rich (η5-C5(CH3)5)Fe(CO)+. The turnover (TON) and turnover frequencies (TOF) were determined and found to be comparable to the previously reported MN2S2·Fe(η5-C5H5)(CO)+ analogues.
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