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Marcetta Y Darensbourg

Publications and source records attributed to Marcetta Y Darensbourg.

At least 19 recordsLinked to original sources
PubMed2007

Control of S-based aggregation: designed synthesis of NiM2 and Ni2M trinuclear complexes.

Modification of the nickel dithiolate metalloligand, Ni(bme-daco) [where bme-daco = bis(mercaptoethyl)diazacyclooctane] or Ni-1, by oxygenation of one thiolate into a sulfinate, Ni(mese-daco) [where mese-daco = (mercaptoethyl)(sulfinatoethyl)diazacyclooctane] or Ni-2, restricts the ligating ability to monodentate and is expected to reduce the donor ability of the remaining thiolate S. Nevertheless, the Ni-2 complex forms a stable thiolate S-bound adduct of W0(CO)5, (Ni-2)W(CO)5, a complex whose upsilon(CO) IR spectrum reports insignificant differences in the donor abilities of Ni-1 and Ni-2 in (eta1-NiN2S2)W(CO)5 complexes. In the presence of the strong sulfophile CuI, a CuNi2 trimetallic, (Ni-2)2CuBr, was isolated. Another trimetallic, (mu-eta2-Ni-1)[W(CO)5]2, demonstrated the Ni(bme-daco), Ni-1, unit to bridge low-valent metals in a transoid configuration, yielding W-W distances of over 5 A.

Journal Article↗
PubMed2006

NiN(2)S(2) complexes as metallodithiolate ligands to Rh(I), Rh(II) and Rh(III).

Three molecular structures are reported which utilize the NiN(2)S(2) ligands -, (bis(mercaptoethyl)diazacyclooctane)nickel and -', bis(mercaptoethyl)diazacycloheptane)nickel, as metallodithiolate ligands to rhodium in oxidation states i, ii and iii. For the Rh(I) complex, the NiN(2)S(2) unit behaves as a bidentate ligand to a square planar Rh(I)(CO)(PPh(3))(+) moiety with a hinge or dihedral angle (defined as the intersection of NiN(2)S(2) and S(2)Rh(C)(P) planes) of 115 degrees . Supported by -' ligands, the Rh(II) oxidation state occurs in a dirhodium C(4) paddlewheel complex wherein four NiN(2)S(2) units serve as bidentate bridging ligands to two singly-bonded Rh(II) ions at 2.893(8) A apart. A compilation of the remarkable range of M-M distances in paddlewheel complexes which use NiN(2)S(2) complexes as paddles is presented. The Rh(III) state is found as a tetrametallic [Rh(-')(3)](3+) cluster, roughly shaped like a boat propeller and structurally similar to tris(bipyridine)metal complexes.

Ligands↗
PubMed2006

A nickel tripeptide as a metallodithiolate ligand anchor for resin-bound organometallics.

The molecular structure of the acetyl CoA synthase enzyme has clarified the role of individual nickel atoms in the dinickel active site which mediates C-C and C-S coupling reactions. The NiN2S2 portion of the biocatalyst (N2S2 = a cysteine-glycine-cysteine or CGC4- tripeptide ligand) serves as an S-donor ligand comparable to classical bidentate ligands operative in organometallic chemistry, ligating the second nickel which is redox and catalytically active. Inspired by this biological catalyst, the synthesis of NiN2S2 metalloligands, including the solid-phase synthesis of resin-bound Ni(CGC)2-, and sulfur-based derivatization with W(CO)5 and Rh(CO)2+ have been carried out. Through comparison to analogous well-characterized, solution-phase complexes, Attenuated Total Reflectance FTIR spectroscopy establishes the presence of unique heterobimetallic complexes, of the form [Ni(CGC)]M(CO)x, both in solution and immobilized on resin beads. This work provides the initial step toward exploitation of such an evolutionarily optimized nickel peptide as a solid support anchor for hybrid bioinorganic-organometallic catalysts.

Acetate-CoA Ligase↗
PubMed2006

Iron nitrosyl complexes as models for biological nitric oxide transfer reagents.

Owing to the indiscriminate reactivity of the free NO radical, intricate control mechanisms are required for storage, transport and transfer of NO to its various biological targets. Among the proposed storage components are protein-bound thionitrosyls (Rprotein-SNO) and protein-bound dinitrosyl iron complexes. Current knowledge suggests the latter are derived from iron-sulfur cluster degradation in the presence of excess NO. Mobilization of protein-bound NO could involve NO or Fe(NO)2 unit transfer to small serum molecules such as glutathione, free cysteine, or iron-porphyrins. The study reported is of a reaction model which addresses the key steps in NO transfer from a prototypal iron dinitrosyl complex. While the N,N'-bis(2-mercaptoethyl)-N,N'-diazacyclooctane (bme-daco) ligand typically binds in square-planar N2S2 coordination, it also serves as a bidentate dithiolate donor for tetrahedral structures in the preparation of the (H+bme-daco)Fe(NO)2 derivative (Chiang et al., J. Am. Chem. Soc. 126:10867-10874, 2004). The removal of one NO produces the mononitrosyl complex, (bme-daco)Fe(NO), and simplifies studies of NO release mechanisms. We have used heme-type model complexes, Fe or Co porphyrins as NO acceptors, yielding (porphyrin)M(NO), where M is Fe or Co, and monitored reactions by nu(NO) Fourier transform IR spectroscopy. Reaction products were verified by electrospray ionization mass spectrometry. Rudimentary mechanistic studies suggest a role for HNO in the NO release from the dinitrosyl; the mononitrosyl benefits as well from acid catalysis. Other NO uptake complexes such as [(N2S2)Fe]2 [N2S2 is bme-daco or N,N'-bis(2-mercapto-2-methylpropyl)-daco] are shown to form Fe(NO) mononitrosyls with stability and spectroscopic signatures similar to those of the porphyrins.

Indicators and Reagents↗
PubMed2006

De novo design of synthetic di-iron(I) complexes as structural models of the reduced form of iron-iron hydrogenase.

Simple synthetic di-iron dithiolate complexes provide good models of the composition of the active site of the iron-iron hydrogenase enzymes. However, the formally Fe(I)Fe(I) complexes synthesized to date fail to reproduce the precise orientation of the diatomic ligands about the iron centers that is observed in the molecular structure of the reduced form of the enzyme active site. This structural difference is often used to explain the fact that the synthetic di-iron complexes are generally poor catalysts when compared to the enzyme. Herein, density functional theory computations are used for the rational design of synthetic complexes as structural models of the reduced form of the enzyme active site. These computations suggest several possible synthetic targets. The synthesis of complexes containing five-atom S-to-S linkers of the form S(CH2)2X(CH2)2S (X = CH2, NH, or O) or pendant functionalities attached to the three-carbon framework is one method. Another approach is the synthesis of asymmetrically substituted complexes, in which one iron center has strongly electron donating ligands and the adjacent iron center has strongly electron accepting ligands. The combination of a sterically demanding S-to-S linker and asymmetric substitution of the CO ligands is predicted to be a particularly effective synthetic target.

Hydrogenase↗
PubMed2006

A kinetic study of the ring-opening process in tungsten carbonyl complexes containing hemilabile metallodithiolate ligands.

The synthesis of the metallodithiolate derivative of tungsten pentacarbonyl from the reaction of photogenerated W(CO)(5)THF and Ni-1 ((1,5-bis(2-mercapto-2-methylpropane)-1,5-diazacyclooctanato)nickel(II)) is described, along with its crystal structure. In N,N-dimethylformamide solution, the pentacarbonyl exists in equilibrium with its tetracarbonyl analogue and carbon monoxide. The pentacarbonyl complex stereoselectively loses cis carbonyl ligands, as is apparent from (13)CO-labeling studies, where the thus-formed tetracarbonyl tungsten complex resulting from chelate ring-closure is preferentially (13)CO-labeled among the two mutually trans CO groups. The kinetics of the addition of CO to the tetracarbonyl to afford the metal pentacarbonyl were monitored by means of in situ infrared spectroscopy in the nu(CO) region at CO pressures between 28 and 97 bar and temperatures over the range 45-60 degrees C. Under these conditions, there was no evidence for W-S bond cleavage in the pentacarbonyl complex with concomitant formation of W(CO)(6). These studies reveal that the tetracarbonyl complex and CO are only slightly unstable with respect to the formation of the pentacarbonyl complex, with an equilibrium constant at 50 degrees C of about 2.8 M(-1) or DeltaG degrees = -1.4 kJ/mol. The activation parameters determined for the ring-opening process (DeltaH = 89.1 kJ/mol and DeltaS = -37.2 J/mol.K) suggest a solvent-assisted concerted ring-opening mechanism.

Journal Article↗
PubMed2006

Correlation between computed gas-phase and experimentally determined solution-phase infrared spectra: models of the iron-iron hydrogenase enzyme active site.

Gas-phase density functional theory calculations (B3LYP, double zeta plus polarization basis sets) are used to predict the solution-phase infrared spectra for a series of CO- and CN-containing iron complexes. It is shown that simple linear scaling of the computed C--O and C--N stretching frequencies yields accurate predictions of the the experimentally determined nu(CO) and nu(CN) values for a variety of complexes of different charges and in solvents of varying polarity. As examples of the technique, the resulting correlation is used to assign structures to spectroscopically observed but structurally ambiguous species in two different systems. For the (mu-SCH2CH2CH2S)[Fe(CO)3]2 complex in tetrahydrofuran solution, our calculations show that the initial electrochemical reduction process leads to a simple one-electron reduced product with a structure very similar to the (mu-SCH2CH2CH2S)[Fe(CO)3]2 parent complex. For the iron-iron hydrogenase enzyme active site, our computations show that the absence or presence of a water molecule near the distal iron center (the iron center further from the [4Fe4S] cluster and protein backbone) has very little effect on the predicted infrared spectra.

Binding Sites↗
PubMed2005

Characterization of steric and electronic properties of NiN2S2 complexes as S-donor metallodithiolate ligands.

The physical properties and structures of a series of six complexes of the type (NiN(2)S(2))W(CO)(4) have been used to establish electronic and steric parameters for square planar NiN(2)S(2) complexes as bidentate, S-donor ligands. According to the nu(CO) stretching frequencies and associated computed Cotton-Kraihanzel force constants of the tungsten carbonyl adducts, there is little difference in donor abilities of the five neutral NiN(2)S(2) metallodithiolate ligands in the series. The dianionic Ni(ema)(2)(-) (ema = N,N'-ethylenebis(2-mercaptoacetamide)) complex transfers more electron density onto the W(CO)(4) moiety. A ranking of donor abilities and a comparison with classical bidentate ligands is as follows: Ni(ema)(=) > {[NiN(2)S(2)](0)} > bipy approximately phen > Ph(2)PCH(2)CH(2)PPh(2) > Ph(2)PCH(2)PPh(2). Electrochemical data from cyclic voltammetry find that the reduction event in the (NiN(2)S(2))W(CO)(4) derivatives is shifted to more positive potentials by ca. 0.5 V compared to the ca. -2 V Ni(II/I) redox event in the free NiN(2)S(2) ligand, consistent with the electron drain from the nickel-dithiolate ligands by the W(CO)(4) acceptor. Differences in Ni(II/I) DeltaE(1/2) values appear to have a ligand dependence which is related to a structural feature of the hinge angle imposed by the (mu-SR)(2) bridges. Thus the angle formed by the intersection of NiN(2)S(2)/WS(2)C(2) planes has been established by X-ray diffraction analyses as a unique orientational feature of the nickel-dithiolate ligands in contrast to classical diphosphine or diimine ligands and ranges in value from 136 to 107 degrees . Variable-temperature (13)C NMR studies show that the spatial orientations of the ligands remained fixed with respect to the W(CO)(4) moiety to temperatures of 100 degrees C.

Binding Sites↗
PubMed2005

Synthesis and molecular structures of mononitrosyl (N2S2)M(NO) complexes (M = Fe, Co).

A series of tetragonally distorted square pyramids of formula N2S2M(NO) (M = Fe, Co) is prepared and characterized by nu(NO) IR and EPR spectroscopies, magnetism and electrochemical properties, as well as solid-state crystal structure determinations. While the nu(NO) IR frequencies and the angleM-N-O angles indicate differences in the electronic environment of NO consistent with the Enemark-Feltham notation of [Fe(NO)]7 and [Co(NO)]8, the reduction potentials, assigned to [Fe(NO)]7 + e- <==> [Fe(NO)]8 and [Co(NO)]8 + e- <==> [Co(NO)]9 respectively, are very similar, and in cases identical, for most members of the series. Coupled with the potential for the M(NO) units to breathe out of and into the N2S2 core plane are unique S-M-N-O torsional arrangements and concomitant pi-bonding interactions which may account for the unusual coherence of reduction potentials within the series.

Journal Article↗
PubMed2005

Dual electron uptake by simultaneous iron and ligand reduction in an N-heterocyclic carbene substituted [FeFe] hydrogenase model compound.

An N-heterocyclic carbene containing [FeFe]H(2)ase model complex, whose X-ray structure displays an apical carbene, shows an unexpected two-electron reduction to be involved in its electrocatalytic dihydrogen production. Density functional calculations show, in addition to a one-electron Fe-Fe reduction, that the aryl-substituted N-heterocyclic carbene can accept a second electron more readily than the Fe-Fe manifold. The juxtaposition of these two one-electron reductions resembles the [FeFe]H(2)ase active site with an FeFe di-iron unit joined to the electroactive 4Fe4S cluster.

Binding Sites↗
PubMed2005

N2S2Ni metallodithiolate complexes as ligands: structural and aqueous solution quantitative studies of the ability of metal ions to form M-S-Ni bridges to mercapto groups coordinated to nickel(II). implications for acetyl coenzyme A synthase.

The nickel(II) complex of an N2S2 ligand, derived from a diazacycle, N,N'-bis(mercaptoethyl)-1,5-diazacycloheptane, (bme-dach)Ni, Ni-1', serves as a metallodithiolate ligand to NiII, CuI, ZnII, Ag, and PbII. The binding ability of the NiN2S2 ligand to the metal ions was established through spectrochemical titrations in aqueous media and compared to classical S-donor ligands. For M = Ni, Zn, Pb, binding constants, log K = ca. 2. were computed for 1:1 Ni-1'/M(solvate) adducts; for Ag+ and Cu+, the 3:2 (Ni-1')3M2 adducts were the first formed products even in water with log beta3,2 values of 26 and >30, respectively. In all cases, the binding ability of Ni-S-R is intermediate between that of a free thiolate and a free thioether. The great specificity for copper over nickel and zinc by N2S2Ni, which serves as a reasonable structural model for the distal nickel of the acetyl CoA synthase active site, relates to biochemical studies of heterogeneity (metal content and type) in various preparations of acetyl CoA synthase enzyme.

Acetate-CoA Ligase↗
PubMed2004

The hydrophilic phosphatriazaadamantane ligand in the development of H2 production electrocatalysts: iron hydrogenase model complexes.

As functional biomimics of the hydrogen-producing capability of the dinuclear active site in [Fe]H(2)ase, the Fe(I)Fe(I) organometallic complexes, (mu-pdt)[Fe(CO)(2)PTA](2), 1-PTA(2), (pdt = SCH(2)CH(2)CH(2)S; PTA = 1,3,5-triaza-7-phosphaadamantane), and (mu-pdt)[Fe(CO)(3)][Fe(CO)(2)PTA], 1-PTA, were synthesized and fully characterized. For comparison to the hydrophobic (mu-pdt)[Fe(CO)(2)(PMe(3))](2) and [(mu-H)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+) analogues, electrochemical responses of 1-PTA(2) and 1-(PTA.H(+))(2) were recorded in acetonitrile and in acetonitrile/water mixtures in the absence and presence of acetic acid. The production of H(2) and the dependence of current on acid concentration indicated that the complexes were solution electrocatalysts that decreased over-voltage for H(+) reduction from HOAc in CH(3)CN by up to 600 mV. The most effective electrocatalyst is the asymmetric 1-PTA species, which promotes H(2) formation from HOAc (pK(a) in CH(3)CN = 22.6) at -1.4 V in CH(3)CN/H(2)O mixtures at the Fe(0)Fe(I) redox level. Functionalization of the PTA ligand via N-protonation or N-methylation, generating (mu-pdt)[Fe(CO)(2)(PTA-H(+))](2), 1-(PTA.H(+))(2), and (mu-pdt)[Fe(CO)(2)(PTA-CH(3)(+))](2), 1-(PTA-Me(+))(2), provided no obvious advantages for the electrocatalysis because in both cases the parent complex is reclaimed during one cycle under the electrochemical conditions and H(2) production catalysis develops from the neutral species. The order of proton/electron addition to the catalyst, i.e., the electrochemical mechanism, is dependent on the extent of P-donor ligand substitution and on the acid strength. Cyclic voltammetric curve-crossing phenomena was observed and analyzed in terms of the possible presence of an eta(2)-H(2)-Fe(II)Fe(I) species, derived from reduction of the Fe(I)Fe(I) parent complex to Fe(0)Fe(I) followed by uptake of two protons in an ECCE mechanism.

Adamantane↗
PubMed2004

Accommodation of the irregular coordination geometry of lead(II) by a square planar N2S2 ligand and its preference for zinc(II).

The N2S2 ligand, bis-mercaptoethanediazacyclooctane, coordinates to Pb(II) largely through sulfur donors, enlisting a second unit to fulfill an irregular, hemispherical N2S3 coordination environment in which a void suggests the location of a stereochemically active lone pair on Pb(II). That the highly exposed lead is vulnerable to metal ion displacement is demonstrated on reaction with zinc which results in a regular square pyramidal coordination about zinc within a [N2S2Zn]2 dimer. Analysis of the two dimeric structures finds different connectivities of the monomeric subunits account for the stability of the zinc structure over that of the lead.

Journal Article↗
PubMed2004

Bismercaptoethanediazacyclooctane as a N2S2 chelating agent and Cys-X-Cys mimic for Fe(NO) and Fe(NO)2.

The N-protonated bismercaptoethanediazacyclooctane serves as a bidentate dithiolate ligand to oxidized Fe(NO)(2) of Enemark-Feltam notation, E-F [Fe(NO)(2)],(9) mimicking Cys-X-Cys binding of Fe(NO)(2) to proteins or thio-biomolecules. The neutral compound is characterized by the well-known g = 2.03 EPR signal which is a hallmark of dinitrosyl iron complexes, DNIC's. The Fe(NO)(2) unit can be removed from the chelate by excess PhS(-), producing (PhS)(2)Fe(NO)(2)(-). Transfer of NO from Fe(H(+)bme-daco)(NO)(2) (nu(NO) = 1740, 1696 cm(-)(1)) to Fe(II) of [(bme-daco)Fe](2) yields the five-coordinate, square-pyramidal N(2)S(2)Fe(NO) (nu(NO) = 1649 cm(-)(1)), where NO is in the apical position. Its isotropic EPR signal at g = 2.05 is consistent with E-F [Fe(NO)](7) formulation. In excess NO, Roussin's red ester-type molecules are formed as dinuclear or tetranuclear species, [(micro-SRS)[Fe(2)(NO)(4)]](n)() (n =1, 2). These well-characterized molecules furnish reference points for positions and patterns in nu(NO) vibrational spectroscopy expected to be useful for in vivo studies of NO degradation of iron-sulfur clusters in ferredoxins.

Chelating Agents↗