Chung-Hung Hsieh

Structural and Spectroscopic Features of Mixed Valent FeIIFeI Complexes and Factors Related to the Rotated Configuration of Diiron Hydrogenase

The compounds of this study have yielded to complementary structural, spectroscopic (Mössbauer, EPR/ENDOR, IR), and computational probes that illustrate the fine control of electronic and steric features that are involved in the two structural forms of (μ-SRS)[Fe(CO)2PMe3]2(0,+) complexes. The installation of bridgehead bulk in the -SCH2CR2CH2S- dithiolate (R = Me, Et) model complexes produces 6-membered FeS2C3 cyclohexane-type rings that produce substantial distortions in Fe(I)Fe(I) precursors. Both the innocent (Fc(+)) and the noninnocent or incipient (NO(+)/CO exchange) oxidations result in complexes with inequivalent iron centers in contrast to the Fe(I)Fe(I) derivatives. In the Fe(II)Fe(I) complexes of S = 1/2, there is complete inversion of one square pyramid relative to the other with strong super hyperfine coupling to one PMe3 and weak SHFC to the other. Remarkably, diamagnetic complexes deriving from isoelectronic replacement of CO by NO(+), {(μ-SRS)[Fe(CO)2PMe3] [Fe(CO)(NO)PMe3](+)}, are also rotated and exist in only one isomeric form with the -SCH2CR2CH2S- dithiolates, in contrast to R = H ( Olsen , M. T. ; Bruschi , M. ; De Gioia , L. ; Rauchfuss , T. B. ; Wilson , S. R. J. Am. Chem. Soc. 2008 , 130 , 12021 -12030 ). The results and redox levels determined from the extensive spectroscopic analyses have been corroborated by gas-phase DFT calculations, with the primary spin density either localized on the rotated iron in the case of the S = 1/2 compound, or delocalized over the {Fe(NO)} unit in the S = 0 complex. In the latter case, the nitrosyl has effectively shifted electron density from the Fe(I)Fe(I) bond, repositioning it onto the spin coupled Fe-N-O unit such that steric repulsion is sufficient to induce the rotated structure in the Fe(II)-{Fe(I)((•)NO)}(8) derivatives.

Hydride binding to the active site of [FeFe]-hydrogenase.

[FeFe]-hydrogenase from green algae (HydA1) is the most efficient hydrogen (H2) producing enzyme in nature and of prime interest for (bio)technology. Its active site is a unique six-iron center (H-cluster) composed of a cubane cluster, [4Fe4S]H, cysteine-linked to a diiron unit, [2Fe]H, which carries unusual carbon monoxide (CO) and cyanide ligands and a bridging azadithiolate group. We have probed the molecular and electronic configurations of the H-cluster in functional oxidized, reduced, and super-reduced or CO-inhibited HydA1 protein, in particular searching for intermediates with iron-hydride bonds. Site-selective X-ray absorption and emission spectroscopy were used to distinguish between low- and high-spin iron sites in the two subcomplexes of the H-cluster. The experimental methods and spectral simulations were calibrated using synthetic model complexes with ligand variations and bound hydride species. Distinct X-ray spectroscopic signatures of electronic excitation or decay transitions in [4Fe4S]H and [2Fe]H were obtained, which were quantitatively reproduced by density functional theory calculations, thereby leading to specific H-cluster model structures. We show that iron-hydride bonds are absent in the reduced state, whereas only in the super-reduced state, ligand rotation facilitates hydride binding presumably to the Fe-Fe bridging position at [2Fe]H. These results are in agreement with a catalytic cycle involving three main intermediates and at least two protonation and electron transfer steps prior to the H2 formation chemistry in [FeFe]-hydrogenases.

Self-Assembly of Dinitrosyl Iron Units into Imidazolate-Edge-Bridged Molecular Squares: Characterization Including Mössbauer Spectroscopy

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 (CH(2)Cl(2)) solvents. Mössbauer spectroscopic data for a set of reference dinitrosyl iron complexes, reduced {Fe(NO)(2)}(10) compounds A ((NHC-iPr)(2)Fe(NO)(2)), and C ((NHC-iPr)(CO)Fe(NO)(2)), and oxidized {Fe(NO)(2)}(9) compounds B ([(NHC-iPr)(2)Fe(NO)(2)][BF(4)]), 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.

Ambidentate thiocyanate and cyanate ligands in dinitrosyl iron complexes.

To explore the effect of delocalization in the Fe(NO)(2) unit on possible linkage isomerism of ambidentate ECN(-) ligands, E = S and O, anionic DNICs, dinitrosyl iron complexes, (SCN)(2)Fe(NO)(2)(-) (1) and (OCN)(2)Fe(NO)(2)(-) (2) were synthesized by the reaction of in situ-generated [Fe(CO)(2)(NO)(2)](+) and PPN(+)ECN(-). Other {Fe(NO)(2)}(9) (Enemark-Feltham notation) complexes, (N(3))(2)Fe(NO)(2)(-) and (PhS)(2)Fe(NO)(2)(-), were prepared for comparison. The X-ray diffraction analysis of 1 and 2 yielded the typical tetrahedral structures of DNICs with two slightly bent Fe-N-O oriented toward each other, and linear FeNCE units. The ν(NO) IR values shift to lower values for 1 > 2 > (N(3))(2)Fe(NO)(2)(-) > (PhS)(2)Fe(NO)(2)(-), reflecting the increasing donor ability of the ancillary ligands and consistent with the redox potentials of the complexes, and the small trends in Mössbauer isomer shifts. Computational studies corroborate that the {Fe(NO)(2)}(9) motif prefers N-bound rather than E-bound isomers. The calculated energy differences between the linkage isomers of 1 (Fe-NCS preferred over Fe-SCN by about 6 kcal/mol) are smaller than those of 2 (Fe-NCO preferred over Fe-OCN by about 16 kcal/mol), a difference that is justified by the frontier molecular orbitals of the ligands themselves.

Carbon monoxide induced reductive elimination of disulfide in an N-heterocyclic carbene (NHC)/thiolate dinitrosyl iron complex (DNIC).

Dinitrosyliron complexes (DNICs) are organometallic-like compounds of biological significance in that they appear in vivo as products of NO degradation of iron-sulfur clusters; synthetic analogues have potential as NO storage and releasing agents. Their reactivity is expected to depend on ancillary ligands and the redox level of the distinctive Fe(NO)2 unit: paramagnetic {Fe(NO)2}(9), diamagnetic dimerized forms of {Fe(NO)2}(9) and diamagnetic {Fe(NO)2}(10) DNICs (Enemark-Feltham notation). The typical biological ligands cysteine and glutathione themselves are subject to thiolate-disulfide redox processes, which when coupled to DNICs may lead to intricate redox processes involving iron, NO, and RS(-)/RS•. Making use of an N-heterocyclic carbene-stabilized DNIC, (NHC)(RS)Fe(NO)2, we have explored the DNIC-promoted RS(-)/RS• oxidation in the presence of added CO wherein oxidized {Fe(NO)2}(9) is reduced to {Fe(NO)2}(10) through carbon monoxide (CO)/RS• ligand substitution. Kinetic studies indicate a bimolecular process, rate = k [Fe(NO)2](1)[CO](1), and activation parameters derived from kobs dependence on temperature similarly indicate an associative mechanism. This mechanism is further defined by density functional theory computations. Computational results indicate a unique role for the delocalized frontier molecular orbitals of the Fe(NO)2 unit, permitting ligand exchange of RS• and CO through an initial side-on approach of CO to the electron-rich N-Fe-N site, ultimately resulting in a 5-coordinate, 19-electron intermediate with elongated Fe-SR bond and with the NO ligands accommodating the excess charge.

A dinitrosyl iron complex as a platform for metal-bound imidazole to N-heterocyclic carbene conversion

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.

Regioselectivity in ligand substitution reactions on diiron complexes governed by nucleophilic and electrophilic ligand properties.

The discovery of a diiron organometallic site in nature within the diiron hydrogenase, [FeFe]-H2ase, active site has prompted revisits of the classic organometallic chemistry involving the Fe-Fe bond and bridging ligands, particularly of the (μ-SCH2XCH2S)[Fe(CO)3]2 and (μ-SCH2XCH2S)[Fe(CO)2L]2 (X = CH2, NH; L = PMe3, CN(-), and NHCs (NHC = N-heterocyclic carbene)), derived from CO/L exchange reactions. Through the synergy of synthetic chemistry and density functional theory computations, the regioselectivity of nucleophilic (PMe3 or CN(-)) and electrophilic (nitrosonium, NO(+)) ligand substitution on the diiron dithiolate framework of the (μ-pdt)[Fe(CO)2NHC][Fe(CO)3] complex (pdt = propanedithiolate) reveals the electron density shifts in the diiron core of such complexes that mimic the [FeFe]-H2ase active site. While CO substitution by PMe3, followed by reaction with NO(+), produces (μ-pdt)(μ-CO)[Fe(NHC)(NO)][Fe(CO)2PMe3](+), the alternate order of reagent addition produces the structural isomer (μ-pdt)[Fe(NHC)(NO)PMe3][Fe(CO)3](+), illustrating how the nucleophile and electrophile choose the electron-poor metal and the electron-rich metal, respectively. Theoretical explorations of simpler analogues, (μ-pdt)[Fe(CO)2CN][Fe(CO)3](-), (μ-pdt)[Fe(CO)3]2, and (μ-pdt)[Fe(CO)2NO][Fe(CO)3](+), provide an explanation for the role that the electron-rich iron moiety plays in inducing the rotation of the electron-poor iron moiety to produce a bridging CO ligand, a key factor in stabilizing the electron-rich iron moiety and for support of the rotated structure as found in the enzyme active site.

A reduced 2Fe2S cluster probe of sulfur-hydrogen versus sulfur-gold interactions.

The Ph3 PAu(+) cation, renowned as an isolobal analogue of H(+) , was found to serve as a proton surrogate and form a stable Au2 Fe2 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 SAu 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]H2 ase-active-site biomimetic [(μ2 -SCH2 (NR)CH2 S){Fe(CO)3 }2 ].

Hammett correlations as test of mechanism of CO-induced disulfide elimination from dinitrosyl iron complexes

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.

Structural and Electronic Responses to the Three Redox Levels of Fe(NO)N2S2-Fe(NO)2

The nitrosylated diiron complexes, Fe2 (NO)3 , of this study are interpreted as a mono-nitrosyl Fe(NO) unit, MNIU, within an N2 S2 ligand field that serves as a metallodithiolate ligand to a dinitrosyl iron unit, DNIU. The cationic Fe(NO)N2 S2 ⋅Fe(NO)2 + complex, 1+ , of Enemark-Feltham electronic notation {Fe(NO)}7 -{Fe(NO)2 }9 , is readily obtained via myriad synthetic routes, and shown to be spin coupled and diamagnetic. Its singly and doubly reduced forms, {Fe(NO)}7 -{Fe(NO)2 }10 , 10 , and {Fe(NO)}8 -{Fe(NO)2 }10 , 1- , were isolated and characterized. While structural parameters of the DNIU are largely unaffected by redox levels, the MNIU readily responds; the neutral, S= 1 / 2 , complex, 10 , finds the extra electron density added into the DNIU affects the adjacent MNIU as seen by the decrease its Fe-N-O angle (from 171° to 149°). In contrast, addition of the second electron, now into the MNIU, returns the Fe-N-O angle to 171° in 1- . Compensating shifts in FeMNIU distances from the N2 S2 plane (from 0.518 to 0.551 to 0.851 Å) contribute to the stability of the bimetallic complex. These features are addressed by computational studies which indicate that the MNIU in 1- is a triplet-state {Fe(NO)}8 with strong spin polarization in the more linear FeNO unit. Magnetic susceptibility and parallel mode EPR results are consistent with the triplet state assignment.