Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

You’ve supercharged your research process with ACS and Mendeley!

STEP 1:
Click to create an ACS ID

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

MENDELEY PAIRING EXPIRED
Your Mendeley pairing has expired. Please reconnect
ACS Publications. Most Trusted. Most Cited. Most Read
My Activity
CONTENT TYPES

Figure 1Loading Img

Electronic Control of the Protonation Rates of Fe–Fe Bonds

View Author Information
Energy Materials Laboratory, School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
Cite this: J. Am. Chem. Soc. 2014, 136, 37, 13038–13044
Publication Date (Web):August 12, 2014
https://doi.org/10.1021/ja506693m

Copyright © 2014 American Chemical Society. This publication is licensed under CC-BY.

  • Open Access

Article Views

2679

Altmetric

-

Citations

LEARN ABOUT THESE METRICS
PDF (1 MB)
Supporting Info (2)»

Abstract

Protonation at metal–metal bonds is of fundamental interest in the context of the function of the active sites of hydrogenases and nitrogenases. In diiron dithiolate complexes bearing carbonyl and electron-donating ligands, the metal–metal bond is the highest occupied molecular orbital (HOMO) with a “bent” geometry. Here we show that the experimentally measured rates of protonation (kH) of this bond and the energy of the HOMO as measured by the oxidation potential of the complexes (E1/2ox) correlate in a linear free energy relationship: ln kH = ((F(c – βE1/2ox))/(RT)), where c is a constant and β is the dimensionless Brønsted coefficient. The value of β of 0.68 is indicative of a strong dependence upon energy of the HOMO: measured rates of protonation vary over 6 orders of magnitude for a change in E1/2ox of ca. 0.55 V (ca. 11 orders of magnitude/V). This relationship allows prediction of protonation rates of systems that are either too fast to measure experimentally or that possess additional protonation sites. It is further suggested that the nature of the bridgehead in the dithiolate ligand can exert a stereoelectronic influence: bulky substituents destabilize the HOMO, thereby increasing the rate of protonation.

Introduction

ARTICLE SECTIONS
Jump To

The protonation of diiron dithiolate complexes such as those based on the [Fe2(SCH2XCH2S)(CO)(6–n)Ln] assembly (X = alkyl, N-alkyl, NH, O, S, Se; n = 0 to 4) to give bridging or terminal hydride products has received considerable attention over the past 10 years. (1, 2) This has been primarily driven by the need to understand the structure and function of the subsite of [FeFe]-hydrogenase and provide knowledge for the design of artificial (electro)catalytic systems for hydrogen production/uptake. (3, 4) The generation of bridging hydrides at metal–metal bonded systems is of relevance to other metallosulfur enzyme active sites, for example, the FeMoco center of nitrogenase, (5) the [NiFe]-hydrogenase, (1, 2) CO dehydrogenase (6) and also in establishing general mechanistic principles of protonation at metal centers.
Protonation of Fe(I)Fe(I) diiron dithiolate units can occur at the metal–metal bond to give bridging hydrides (7) or at a single iron site to give a terminal hydride. (8-11) In earlier work it was thought that a terminal hydride intermediate was on the pathway to the formation of the thermodynamically more stable bridging hydride. (9) However, it was later shown that at low temperature bridging hydrides are formed more rapidly in certain electron-rich systems than are the terminal species. (10) Furthermore, terminal hydrides have not been detected as intermediates in less basic systems, which give bridging hydrides under ambient conditions. While the natural system possesses CN coligands, herein we have examined diiron units with PMe3 substituents. (12) This has the advantage in that alternative protonation on CN is avoided, but the electron-donating properties are retained; notably, in the natural system the cyanide ligands are hydrogen bonded and do not present protonation sites.
In earlier studies we have shown that the rate of protonation of dithiolate systems is dependent on the nature of the X in the dithiolate ligand. (13-15) For example, the complex Fe2(odt)(CO)4(PMe3)2 (odt = 2-oxapropane-1,3-dithiolate) is protonated at the metal–metal bond roughly 1 order of magnitude more slowly than Fe2(pdt)(CO)4(PMe3)2 (pdt = propane-1,3-dithiolate): in neither case was a terminal hydride detected. In this study we sought to unravel how the nature of the bridging dithiolate and the coligand(s) control the overall rate of protonation at {2Fe2S} and {2Fe3S} cores (containing two iron atoms and two or three sulfur atoms, respectively). We show that bulky bridgehead units, which are known to stabilize mixed valence Fe(I)Fe(II) cations, (16) can also influence the of rate of formation of Fe(II)(μ-H)Fe(II) cations. Noting the earlier pioneering work of Norton and co-workers, which showed that protonation at metal centers can be slow, (17) and studies by Henderson and co-workers on the protonation of Mo and W hydrides (18, 19) and on iron–sulfur clusters, (20) which leads to hydrogen evolution or substrate reduction, studies of the factors that control protonation rates at metal centers are relatively few.

Results and Discussion

ARTICLE SECTIONS
Jump To

Synthesis

The compounds used in this study were synthesized by literature methods or by modification of these as described in the Supporting Information (SI). Among the new compounds reported are hexacarbonyl precursors with one or two isopropyl substituents on the dithiolate bridgehead, together with their PMe3 derivatives, and related monomethyl species.

Kinetic Measurements

Stopped-flow (SF) methods have previously allowed examination and protonation rates for 1(pdt), 1(edt) and 1(odt) (Scheme 1, edt = ethane-1,2-dithiolate), revealing significant variation in the primary protonation rate. (13-15) However, the reasons underpinning this variation was not apparent; notably the closely similar (CO) infrared data suggests minimal electronic influence by the bridghead substituent X. (16, 21, 22) To probe the relationship between structure and reactivity further we have studied a series of established and new diiron dithiolates of general formula Fe2(xdt)(CO)(6–n)(PMe3)n (n = 1 or 2) and have determined protonation rates under directly comparable conditions.

Scheme 1

Scheme 1. Protonation of Fe2(xdt)(CO)4(PMe3)2a

Scheme aThere is turnstile interchange of the CO and PMe3 ligands at the metal centers. (23) General conditions: complex concentration 0.12–0.50 mM, acid (HBF4·Et2O) concentrations cover the range 5–250 mM, 21 °C, reaction under N2 or Ar.

The reaction of 1(iPr-pdt) (iPr-pdt =2-isopropylpropane-1,3-dithiolate) with HBF4·Et2O in MeCN proceeds quantitatively to give the bridging hydride product [1H(iPr-pdt)]+. This species was isolated and fully characterized, and its structure was confirmed by X-ray crystallography (see SI for details). Figure 1 (left) shows a typical time-course for the protonation of 1(iPr-pdt) under pseudo-first-order conditions of acid, as measured at 348 nm in an SF UV–visible experiment. Over a range of concentrations of HBF4·Et2O the decays each fit to single exponential curves from which the pseudo-first-order rate constants (kobs) were estimated. Figure 1 (right) shows the plot of kobs versus [HBF4·Et2O] from which the second order rate constant (kH) for protonation was estimated to be 1190(40) M–1 s–1. Rate constants (kH) for the compounds listed in Table 1 were similarly determined either by SF UV–visible or SF FT-IR techniques.
Our previous stopped-flow studies have established that rapid primary protonation in these systems is followed by isomerization on a slower time scale, on the order of tens of seconds to minutes depending on the bridge. (13, 14) In the current paper we focus on the primary protonation step: this is distinguished from the later isomerizations as only the first phase of reaction leads to a change in the UV spectrum and discernible change in the IR. At room temperature the dynamics of isomer interconversion of the unprotonated complexes is fast, whereas product interconversion is slow. (23)

Figure 1

Figure 1. Left: Decay of UV signal at 348 nm over time on protonation of 1(iPr-pdt) (circles); pseudo-first-order fit (line); [1(iPr-pdt)]0 0.13 mM, [HBF4·Et2O]0 125 mM. Right: Rates of protonation of 1(iPr-pdt) in MeCN as measured by UV over a range of acid concentrations. The linear fit for kobs versus [HBF4·Et2O] plot is that for a fixed zero intercept.

Table 1. Second Order Rate Constants for Protonationa and Oxidation Potentials, E1/2ox, for Parent Complexesb
Table a

Protonation carried out using HBF4·Et2O in MeCN at 21 °C; protonation rates determined by stopped-flow UV unless otherwise noted.

Table b

Measured at vitreous carbon electrode in 0.1 M [Bu4N][BF4]-MeCN, under argon.

Table c

Protonation rates determined by stopped-flow IR.

Inspection of Table 1 reveals the gross trends in protonation rates. First, for those complexes for which kH can be directly determined, the rate constants span 6 orders of magnitude. Second, the more electron-donating PMe3 groups installed at the dithiolate core, the faster is the protonation rate: protonation of the tetrakis(trimethylphosphine) complex 7 is immeasurably fast, while that for the hexacarbonyl 2 is not observed because HBF4·Et2O is an insufficiently strong acid to protonate 2. (24) We do not detect terminal hydride intermediates in any of the systems amenable to SF FT-IR study at 294 K.
For the five bis(trimethylphosphine) complexes 1 in which the bridgehead 2-substitution is varied on the propanedithiolate framework the reaction rate ranges increase by a factor of 5 on going from the unsubstituted pdt complex to the bis(isopropyl) substituted species. While this variation accords with the enhanced inductive resulting from the bridgehead dialkyl substituents on the diiron unit, the enhancement of rate for monoalkyl substitution is minimal (Table 1). Notably, FT-IR frequencies for the 1(pdt, Me-pdt, iPr-pdt) are essentially indistinguishable (Table 2), which is consistent with the closely similar rates. (15, 16, 21, 22)
Table 2. Comparison IR Stretching Frequencies (MeCN) for 1(xdt)
EntryBridge/cm–1
1edt1982, 1944, 1908, 1898 sh (14)
2odt1984, 1947, 1913, 1898 sh (15)
3pdt1980, 1943, 1898 (13)
4Me-pdt1980, 1943, 1899
5iPr-pdt1980, 1943, 1899
6Me2-pdt1980, 1939, 1900
7iPr2-pdt1978, 1971, 1939, 1899

The Nature of the Site of Protonation: The Energy of the HOMO

The site of protonation in all the complexes studied by SF FT-IR is the metal–metal bond. Photoelectron spectroscopy and DFT calculations have shown that in a typical Fe2S2(CO)6 unit the orbital character of the HOMO (highest occupied molecular orbital) corresponds closely to the classical “bent” Fe–Fe bond. (25-27) Thus, the protonation of the metal–metal bond and the oxidation of diiron complexes engages the HOMO directly. If a reversible one-electron oxidation process in solution is considered, then the value of formal potential E0’ (close to E1/2) (28) can be viewed as a relative measure of the energy of the HOMO for a series of complexes where solvation energy differences between oxidized and reduced forms are very similar or vary systematically. (29)
Cyclic voltammetry (CV) of the complexes was carried out under an argon atmosphere in 0.1 M [Bu4N][BF4]-MeCN solutions, and revealed a clear variation in E1/2 values (Table 1). The voltammograms also demonstrate that the presence of sterically demanding groups results in a much more stable product: oxidation of 1(iPr2-pdt) and 1(Me2-pdt) is fully reversibly at low scan rates (50 mV s–1), whereas in all other cases only partially reversible waves were observed (see SI). E1/2ox values for the monophosphine complexes are significantly more positive than those for the related disubstituted analogues. The extent to which the HOMO is raised in energy by increasing the degree of substitution of PMe3 ligands at the diiron core is illustrated in Figure 2. The linear relationship conforms to an additivity of substituent influence previously recognized in progressive substitution of CO in mononuclear complexes by donor ligands. (30) It is perhaps surprising that the single (asymmetric) substitution fits with this correlation; this presumably reflects extensive delocalization of electron density in the {2Fe2S} core.

Figure 2

Figure 2. Correlation of oxidation potential with degree of phosphine substitution in the series Fe2(pdt)(CO)(6–n)(PMe3)n, n = 0, 1, 2, 4. Oxidation potentials were recorded at a vitreous carbon electrode in 0.1 M [Bu4N][BF4]-MeCN and are reported relative to a Fc+/Fc internal standard.

There is a linear correlation (r2 = 0.985) between the oxidation potential of the complexes and the activation energy of the protonation reaction at 294 K, as is evident from the plot of (RT/F) ln kH versus E1/2ox (Figure 3). Taking the value of the gas constant R = 8.314 J mol–1 K–1 and that of the Faraday constant F = 9.649 × 104 J mol–1 V–1, there is parity in the units of the x- and y-axes, i.e., volts. The dimensionless slope is −0.68, which is strongly indicative that the ground-state energy of the HOMO has a determining influence on the activation energy. Explicitly, for the series of dialkyl dithiolate PMe3 complexes, an increase in the energy of the HOMO by 100 kJ mol–1 lowers the activation energy for protonation by 68 kJ mol–1 at 294 K. (31)

Figure 3

Figure 3. Correlation of rate of protonation with oxidation potential for {2Fe2S} and {2Fe3S} systems. Where not specified, substrates are of general structure 1. The line shows the best-fit for the filled circles: substrates of general structure Fe2(xdt)(CO)(6–n)(PMe3)n, xdt = alkyl dithiolate, n = 1, 2. Oxidation potentials were recorded at a vitreous carbon electrode in 0.1 M [Bu4N][BF4]-MeCN and are reported relative to a Fc+/Fc internal standard. Protonation rates were measured by stopped-flow IR [3(edt), 3(pdt), 4, 5] or UV (all others).

Whereas the dominant correlation of ln kH against E1/2ox is readily understood in terms of the donicity of the substituent groups at the diiron core, as discussed above, the infrared data for the bridgehead alkyl substituted compounds would suggest that there is at first sight a minimal electronic effect of these groups on the core. In accord with this, for the CH2, CHMe and CH(iPr) bridgeheads [1(pdt), 1(Me-pdt) and 1(iPr-pdt), respectively], the measured E1/2ox values and infrared frequencies are closely similar, as are the magnitudes of kH. However, the dialkyl bridgehead complexes 1(Me2-pdt) and 1(iPr2-pdt) show enhanced rates and more negative E1/2ox values than might be expected from the FT-IR data (Table 2). Changes in (CO) have previously been correlated with E1/2ox in mononuclear monocarbonyl species, where Δ(CO)/ΔE1/2ox is of the order of 100 cm–1/V. (32) Using data reported by Ott and co-workers for the protonated and unprotonated forms of Fe2(Bn-adt)(CO)4(PMe)2 (Bn-adt = N-benzyl-2-azapropanedithiolate, Bn = benzyl), (33) Δave(CO)/ΔE1/2ox may be estimated at ca. 60 cm–1/V for diiron dithiolate systems bearing four CO groups. On this basis we might expect Δave(CO) of ca. 5 cm–1 between 1(pdt) and 1(iPr2-pdt) (ΔE1/2ox = 75 mV), which rather emphasizes the poor sensitivity of changes in IR frequencies and judging the effect of bridgehead structural change vis à vis ΔE1/2ox. (34)
It is therefore rather more pertinent to consider the substantial change in E1/2ox between the monoalkylated and dialkylated bridgehead against the minimal change observed between the unsubstituted and monoalkylated forms and the possibility of a stereoelectronic effect. The solid-state structures of the complexes 1(iPr-pdt) and 1(iPr2-pdt) are shown in Figure 4, from which it is clear that the dialkyl bridgehead complex has a semibridging CO group, whereas the monoalkyl bridgehead complex has an essentially all-terminal arrangement of CO ligands. The former parallels the solid state for a diethyl bridgehead complex by Darensbourg and co-workers, which also displays a semibridging CO group. (22, 35) Infrared studies show that in MeCN solution there is not a detectable population of the semibridged carbonyl form. In addition 31P NMR spectra at room temperature for unsubstituted, mono- and dialkyl-substituted show single 31P resonances. This apparently contradictory spectroscopic, kinetic and E1/2 data can be reconciled as follows.

Figure 4

Figure 4. Solid state structures of 1(iPr-pdt) and 1(iPr2-pdt) showing spheres of arbitrary radius (top) and 50% probability ellipsoids (bottom).

Density functional theory (DFT) calculations were undertaken for 1(Me-pdt) and 1(Me2-pdt). For 1(Me-pdt) minimization from either an all-terminal or semibridging initial geometry results in identical minima in which all of the carbonyl groups are terminal. In contrast, minimizations of 1(Me2-pdt) leads to the formation of a clearly semibridging CO group (closest nonbonding Fe···C distance 2.78 Å). Both the semibridging and all-terminal CO coordination modes have a predominantly metal–metal bond character in the HOMOs (Figure 5). The relative gas-phase energy of the HOMO in the dimethyl bridgehead (semibridged) complex is higher than that of the monomethyl bridgehead (all-terminal) analogue by approximately 8 kJ mol–1. This would be consistent with bulky bridgehead groups destabilizing the ground state HOMO, lowering E1/2 by ca. 83 mV and diminishing activation energies. Notably the experimental difference in oxidation potentials of the monomethyl and dimethyl complexes is ca. 70 mV, in surprisingly good accord with the ground state energy difference of ca. 80 mV for the parent complexes as estimated by DFT.
The potential impact of this isomerism on the solution electrochemistry and kinetics can be accommodated by a fast equilibrium between semibridged and terminal isomers as set out in Scheme 2. Although, as clear from the spectroscopic data, the semibridged forms of the dialkyl complexes must be present in low concentrations, a fast pre-equilibrium would shift the redox potential of the dialkyl bridgehead substituted species negative toward that of the more easily oxidized semibridged species, conserving the general correlation between ln kH, E1/2ox and the energy of the HOMO. Cyclic voltammetry of 1(Me2-pdt) in MeCN down to −52 °C does not freeze out an equilibrium. There is a negative shift in E1/2ox of ca. 30 mV on cooling from 24 to −52 °C, suggesting that the equilibrium must remain fast and shifts toward the semibridging form at lower temperature.

Figure 5

Figure 5. DFT gas-phase HOMO for 1(Me-pdt) (left) and 1(Me2-pdt) (right), showing the two (arbitrary) phases of the orbital in red and green.

Scheme 2

Scheme 2. Equilibria between All-Terminal and Semibridging Isomers (E1/2ox,1 > E1/2ox,2)
Given in the plot of (RT/F) ln kH against E1/2ox (Figure 3, open circles) are data for related systems, 46, the two {2Fe3S} complexes and a complex with a benzenedithiolate bridging unit. While broadly following the trend of the correlation, the complex with the phenylene bridge (5) and the thioether complex (4) are outliers. In the former case, delocalization of the charge by the noninnocent benzene dithiolate may stabilize the oxidized form accounting for the lower oxidation potential than would be expected from the observed rate. In the case of the thioether complex it might be argued that the SMe group is less polarizable than a PMe3 substituent, and so is less effective in stabilizing the oxidized form.

Relationship between HOMO Energy and the Basicity of the Metal–Metal Bond

Work by Bordwell and co-workers established linear free energy correlations between pKa values and the oxidation potential of a series of carbon bases and established for a series of fluorenide anions an absolute (dimensionless) Brønsted coefficient close to unity. (36, 37) Moreover, linear free energy correlations have been established between the oxidation potentials and energy of activation of these anions in nucleophilic single electron transfer substitution reactions: (38) in this case the Brønsted coefficient was reported to be also close to one. For the series of complexes discussed herein we have similarly established a linear free energy relationship between protonation rate and oxidation potential of the metal–metal bond based HOMO. The magnitude of the Brønsted coefficient for (RT/F) ln kH versus E1/2ox is 0.68, and it thus is reasonable to conclude that the activation energy for protonation will correlate to the basicity of the metal–metal bond. (29) If we assume that the measured E1/2 values for the diiron series are linearly related to the pKa of the metal–metal bond with a Brønsted coefficient close to unity, ΔpKaE1/2 = 16.9 units/V, then the pKa values of the complexes for which we have directly measured protonation rates span about 9 pKa units.
The linear free energy relationship derived from the (RT/F) ln kH against E1/2ox can be expressed in the general form of eq 1, where β is the Brønsted coefficient and c is a constant that depends on the chosen reference system.(1)
With the estimated value of β = 0.68 and potentials based relative to the Fc+/Fc system, c = 0.012, the expression can be rearranged using known values for the physical constants at 294 K to that shown in eq 2.(2)
This relationship allows the prediction of the rate of protonation of systems that are not directly amenable to measurement. For example, protonation of 7(pdt) is too fast to measure by SF UV–vis methods (kH > 8 × 105 M–1 s–1), the predicted rate from eq 2 is 1.3 × 1013 M–1 s–1; bimolecular diffusion-controlled rate constants are typically <109 M–1 s–1, and it is therefore likely that protonation of this complex is not rate limiting. The complex Fe2(Bn-adt)(CO)4(PMe3)2 first protonates at the NBn group, (33) the metal–metal bond is subsequently protonated at a rate retarded by deactivation by the electron-withdrawing bridgehead cationic group NBnH+. Using the reported value for E1/2ox for this complex (−0.26 V vs Fc+/Fc), eq 2 predicts a rate of direct protonation of the metal–metal bond as ca. 1700 M–1 s–1, comparable to the other bis(trimethylphosphine) complexes (Table 1). The dicyanide complex [Fe2(pdt)(CO)4(CN)2]2– similarly protonates kinetically at basic CN site rather than the metal–metal bond, subsequently decomposing. (39) With the caveat that we are comparing CN and PMe3 substituents, which have different donicities and size, protonation at the metal–metal bond in the dicyanide species may be estimated to have a rate of ca. 8.6 × 105 M–1 s–1 based on the reported E1/2ox (−0.49 V vs Fc+/Fc). (40) It is interesting to note that Reihner and co-workers have predicted theoretically that protonation of the dicyanide subsite in the enzyme to give a bridging hydride has a high activation energy barrier (39 kcal mol–1) at the Fe(I)Fe(I) level in the enzyme. (41) Although we are very wary of the oversimplification, at an operating potential of −420 mV versus SHE and at pH 7 eq 2 predicts a turnover frequency of ca. 103 s–1 for the formation of a bridging hydride at the enzyme site, this is an order of magnitude slower than is observed for the enzyme (42) and consistent with the faster kinetics being associated with protonation to give a terminal hydride.

Conclusions

ARTICLE SECTIONS
Jump To

The synthesis and characterization of a range of new diiron dithiolate complexes and their bridging hydride derivatives has allowed systematic study of how structure influences protonation at the metal–metal bond. The complexes display second order kinetics on reaction with HBF4·Et2O in MeCN to form the μ-hydrido species. It is found that ln kH shows a strong linear correlation with the energy of the HOMO as measured by E1/2ox. The linear free energy relationship allows prediction of rates of protonation where the oxidation potential of the complex is known. Bulky bridgehead substituents are suggested to exert a stereoelectronic influence on the protonation rate. This is explained by the switching of a terminal carbonyl to a semibridging mode, which destabilizes the HOMO, enhancing the protonation rate at the metal–metal bond.

Supporting Information

ARTICLE SECTIONS
Jump To

Experimental details for synthesis, stopped-flow and electrochemistry, characterization data, X-ray crystallographic data and DFT details. This material is available free of charge via the Internet at http://pubs.acs.org.

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

ARTICLE SECTIONS
Jump To

  • Corresponding Author
    • Christopher J. Pickett - Energy Materials Laboratory, School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
  • Authors
    • Aušra Jablonskytė - Energy Materials Laboratory, School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
    • Lee R. Webster - Energy Materials Laboratory, School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
    • Trevor R. Simmons - Energy Materials Laboratory, School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
    • Joseph A. Wright - Energy Materials Laboratory, School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United Kingdom
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

ARTICLE SECTIONS
Jump To

We thank the Biotechnology and Biological Sciences Research Council (Grant BB/E023290/1) and Engineering and Physical Sciences Research Council (Grants EP/F047878/1 and EP/H019480/1) for funding. A.J. thanks the University of East Anglia for a studentship. We are grateful to the National Crystallographic Service, University of Southampton and the National Mass Spectrometry Service, Swansea, for data collection. DFT calculations were performed using the High Performance Computing Cluster supported by the Research and Specialist Computing Support Service at the University of East Anglia. We thank Drs. Vasily Oganesyan and Richard Stephenson for providing access to Gaussian 09, Dr. S. K. Ibrahim for help with the electrochemical measurements, and James Box and Max Mason-Gransby for the preparation of compounds 1(odt) and 5, respectively.

References

ARTICLE SECTIONS
Jump To

This article references 42 other publications.

  1. 1
    Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245 2274
  2. 2
    (a) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Chem. Rev. 2014, 114, 4081 4148
    (b) Simmons, T. R.; Berggren, G.; Bacchi, M.; Fontecase, M.; Artero, V. Coord. Chem. Rev. 2014, 270-271, 127 150
  3. 3
    Camara, J. M.; Rauchfuss, T. B. Nat. Chem. 2012, 4, 26 30
  4. 4
    Ghosh, S.; Hogarth, G.; Hollingsworth, N.; Holt, K. B.; Kabir, S. E.; Sanchez, B. E. Chem. Commun. 2014, 50, 945 947
  5. 5
    Hoffman, B. M.; Lukoyanov, D.; Dean, D. R.; Seefeldt, L. C. Acc. Chem. Res. 2013, 46, 587 595
  6. 6
    Lindahl, P. A. Biochemistry 2002, 41, 2097 2105
  7. 7

    For an early biomimetic example, see:

    Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710 9711
  8. 8
    Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 2007, 46, 3426 3428
  9. 9
    Barton, B. E.; Rauchfuss, T. B. Inorg. Chem. 2008, 47, 2261 2263
  10. 10
    Zaffaroni, R.; Rauchfuss, T. B.; Gray, D. L.; De Gioia, L.; Zampella, G. J. Am. Chem. Soc. 2012, 134, 19260 19269
  11. 11
    Wang, W.; Rauchfuss, T. B.; Zhu, L.; Zampella, G. J. Am. Chem. Soc. 2014, 136, 5773 5782
  12. 12
    van der Vlugt, J. I.; Rauchfuss, T. B.; Wilson, S. R. Chem.—Eur. J. 2005, 12, 90 98
  13. 13
    Wright, J. A.; Pickett, C. J. Chem. Commun. 2009, 45, 5719 5721
  14. 14
    Jablonskytė, A.; Wright, J. A.; Pickett, C. J. Dalton Trans. 2010, 39, 3026 3034
  15. 15
    Jablonskytė, A.; Wright, J. A.; Pickett, C. J. Eur. J. Inorg. Chem. 2011, 1033 1037
  16. 16
    Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492 9495
  17. 17
    Kramarz, K. W.; Norton, J. R. Prog. Inorg. Chem. 1994, 42, 1 65
  18. 18
    Henderson, R. A.; Oglieve, K. E. J. Chem. Soc., Dalton Trans. 1993, 22, 3431 3439
  19. 19
    Henderson, R. A.; Ibrahim, S. K.; Oglieve, K. E.; Pickett, C. J. J. Chem. Soc., Chem. Commun. 1995, 31, 1571 1572
  20. 20
    Grönberg, K. L. C.; Henderson, R. A.; Oglieve, K. E. J. Chem. Soc., Dalton Trans. 1998, 27, 3093 3104
  21. 21
    Singleton, M. L.; Jenkins, R. M.; Klemashevich, C. L.; Darensbourg, M. Y. C. R. Chim. 2008, 11, 861 874
  22. 22
    Hsieh, C.-H.; Erdem, Ö. F.; Harman, S. D.; Singleton, M. L.; Reijerse, E.; Lubitz, W.; Popescu, C. V.; Reibenspies, J. H.; Brothers, S. M.; Hall, M. B.; Darensbourg, M. Y. J. Am. Chem. Soc. 2012, 134, 13089 13102
  23. 23
    Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.; Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917 3928
  24. 24

    Protonation of 2(pdt) has been reported under superacidic conditions:

    Matthews, S. L.; Heinekey, D. M. Inorg. Chem. 2010, 49, 9746 9748
  25. 25
    Teo, B. K.; Hall, M. B.; Fenske, R. F.; Dahl, L. F. Inorg. Chem. 1975, 14, 3103 3117
  26. 26
    Andersen, E. L.; Fehlner, T. P.; Foti, A. E.; Salahub, D. R. J. Am. Chem. Soc. 1980, 102, 7422 7429
  27. 27
    Walther, B.; Hartung, H.; Reinhold, J.; Jones, P. G.; Mealli, C.; Bottcher, H. C.; Baumeister, U.; Krug, A.; Mockelt, A. Organometallics 1992, 11, 1542 1549
  28. 28
    Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd Ed.; John Wiley & Sons: Chichester, U.K., 2001.
  29. 29
    Sarapu, A. C.; Fenske, R. F. Inorg. Chem. 1975, 14, 247 253
  30. 30
    Pickett, C. J.; Pletcher, D. J. Organomet. Chem. 1975, 102, 327 333
  31. 31

    For reversible systems E1/2 is independent of temperature if, as is usually the case, the ratio of the diffusion coefficient for the oxidized and reduced partners is closely similar at all temperatures. Whether the slope, viz. ΔΔGE1/2, varies with T will essentially depend on whether changes in the entropy of activation ΔΔS varies significantly across the series. The observed correlation indicates the entropic contribution to ΔG associated with the ordering of the transition state is similar for all of the complexes or varies monotonically with ΔH. In either case ΔΔS is probably small, and consequently the effect of T on the slope is likely to be small with ΔΔG and dominated by the enthalpic ΔΔH terms.

  32. 32
    Chatt, J.; Leigh, G. J.; Neukomm, H.; Pickett, C. J.; Stanley, D. R. J. Chem. Soc., Dalton Trans. 1980, 121 127
  33. 33
    Eilers, G.; Schwartz, L.; Stein, M.; Zampella, G.; de Gioia, L.; Ott, S.; Lomoth, R. Chem.—Eur. J. 2007, 13, 7075 7084
  34. 34

    A further factor that we have considered that might trend with protonation rates is variation in metal–metal bond distances in the reactants and products. There is not a systematic dependence on reactant or product bond lengths, or changes of these (Table S1, SI).

  35. 35

    “Rotated” states at the Fe(I)Fe(I) level have been generated by the introduction of steric bulk at the bridgehead (ref 22) and by combing steric bulk with electronic asymmetry:

    Sabrina; Munery; Capon, J.-F.; De Gioia, L.; Elleouet, C.; Greco, C.; Pétillon, F. Y.; Schollhammer, P.; Zampella, G. C. Chem.—Eur. J. 2013, 19, 15458 15461
    Wang, W.; Rauchfuss, T. B.; Moore, C. E.; Rheingold, A. L.; De Gioia, L.; Zampella, G. Chem.—Eur. J. 2013, 19, 15476 15479
    De Gioia, L.; Elleouet, C.; Munery, S.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J.; Zampella, G. Eur. J. Inorg. Chem. 2013, 3456 3461

    The latter exhibit fully rotated states and bridging carbonyl groups, while the former are best viewed as partially “twisted”. In the natural system, the geometry at the metal center is fully rotated with a bridging CO present, although the influence of the enzyme scaffold is also likely responsible for maintaining the rotated state.

  36. 36
    Bordwell, F. G.; Bausch, M. J. J. Am. Chem. Soc. 1986, 108, 1979 1985
  37. 37
    Bordwell, F. G.; Bausch, M. J. J. Am. Chem. Soc. 1986, 108, 1985 1988
  38. 38
    Angelici, R. J. Acc. Chem. Res. 1995, 28, 51 60
  39. 39
    Wright, J. A.; Webster, L.; Jablonskytė, A.; Woi, P. M.; Ibrahim, S. K.; Pickett, C. J. Faraday Discuss. 2011, 148, 359 371
  40. 40
    Le Cloirec, A.; Best, S. P.; Borg, S.; Davies, S. C.; Evans, D. J.; Hughes, D. L.; Pickett, C. J. Chem. Commun. 1999, 35, 2285 2286
  41. 41
    Finkelmann, A. R.; Stiebritz, M. T.; Reiher, M. Chem. Sci. 2014, 5, 215 221
  42. 42
    Frey, M. ChemBioChem 2002, 3, 153 160

Cited By

ARTICLE SECTIONS
Jump To

This article is cited by 30 publications.

  1. Rakesh C. Puthenkalathil, Bernd Ensing. Linear Scaling Relationships to Predict pKa’s and Reduction Potentials for Bioinspired Hydrogenase Catalysis. Inorganic Chemistry 2022, 61 (1) , 113-120. https://doi.org/10.1021/acs.inorgchem.1c02429
  2. Husain N. Kagalwala, Noémie Lalaoui, Qian-Li Li, Liang Liu, Toby Woods, Thomas B. Rauchfuss. Redox and “Antioxidant” Properties of Fe2(μ-SH)2(CO)4(PPh3)2. Inorganic Chemistry 2019, 58 (4) , 2761-2769. https://doi.org/10.1021/acs.inorgchem.8b03344
  3. Xin Yu, Maofu Pang, Shengnan Zhang, Xinlong Hu, Chen-Ho Tung, Wenguang Wang. Terminal Thiolate-Dominated H/D Exchanges and H2 Release: Diiron Thiol–Hydride. Journal of the American Chemical Society 2018, 140 (36) , 11454-11463. https://doi.org/10.1021/jacs.8b06996
  4. Shihuai Wang, Alexander Aster, Mohammad Mirmohades, Reiner Lomoth, and Leif Hammarström . Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst. Inorganic Chemistry 2018, 57 (2) , 768-776. https://doi.org/10.1021/acs.inorgchem.7b02687
  5. Noémie Lalaoui, Toby Woods, Thomas B. Rauchfuss, and Giuseppe Zampella . Characterization of a Borane σ Complex of a Diiron Dithiolate: Model for an Elusive Dihydrogen Adduct. Organometallics 2017, 36 (11) , 2054-2057. https://doi.org/10.1021/acs.organomet.7b00236
  6. Noémie Elgrishi, Daniel A. Kurtz, and Jillian L. Dempsey . Reaction Parameters Influencing Cobalt Hydride Formation Kinetics: Implications for Benchmarking H2-Evolution Catalysts. Journal of the American Chemical Society 2017, 139 (1) , 239-244. https://doi.org/10.1021/jacs.6b10148
  7. David Schilter, James M. Camara, Mioy T. Huynh, Sharon Hammes-Schiffer, and Thomas B. Rauchfuss . Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides. Chemical Reviews 2016, 116 (15) , 8693-8749. https://doi.org/10.1021/acs.chemrev.6b00180
  8. Yulong Li and Thomas B. Rauchfuss . Synthesis of Diiron(I) Dithiolato Carbonyl Complexes. Chemical Reviews 2016, 116 (12) , 7043-7077. https://doi.org/10.1021/acs.chemrev.5b00669
  9. Xiaoyuan Zhou, Bryan E. Barton, Geoffrey M. Chambers, and Thomas B. Rauchfuss , Federica Arrigoni and Giuseppe Zampella . Preparation and Protonation of Fe2(pdt)(CNR)6, Electron-Rich Analogues of Fe2(pdt)(CO)6. Inorganic Chemistry 2016, 55 (7) , 3401-3412. https://doi.org/10.1021/acs.inorgchem.5b02789
  10. Yu-Chiao Liu, Kai-Ti Chu, Yi-Lan Huang, Cheng-Huey Hsu, Gene-Hsiang Lee, Mei-Chun Tseng, and Ming-Hsi Chiang . Protonation/Reduction of Carbonyl-Rich Diiron Complexes and the Direct Observation of Triprotonated Species: Insights into the Electrocatalytic Mechanism of Hydrogen Formation. ACS Catalysis 2016, 6 (4) , 2559-2576. https://doi.org/10.1021/acscatal.5b02646
  11. Neil T. Hunt, Joseph A. Wright, and Christopher Pickett . Detection of Transient Intermediates Generated from Subsite Analogues of [FeFe] Hydrogenases. Inorganic Chemistry 2016, 55 (2) , 399-410. https://doi.org/10.1021/acs.inorgchem.5b02477
  12. Charlotte L. Montgomery, Jaruwan Amtawong, Aldo M. Jordan, Daniel A. Kurtz, Jillian L. Dempsey. Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions. Chemical Society Reviews 2023, 52 (20) , 7137-7169. https://doi.org/10.1039/D3CS00355H
  13. Atheer M. Madlool, Grace E. Wingrove, Ben J. Paran Rutterford, Ahmad Malik, River Kit, Joseph A. Wright. Improved Access to ‘Butterfly’ Di‐Iron Dithiolates Fe 2 (μ‐SR) 2 (CO) 6 and their Mono‐ and Bis(phosphine) Adducts. ChemistrySelect 2023, 8 (33) https://doi.org/10.1002/slct.202302935
  14. Lintang Hizbullah, Ahibur Rahaman, Seyedeh Safavi, Matti Haukka, Derek A. Tocher, George C. Lisensky, Ebbe Nordlander. Synthesis of phosphine derivatives of [Fe2(CO)6(μ-sdt)] (sdt = SCH2SCH2S) and investigation of their proton reduction capabilities. Journal of Inorganic Biochemistry 2023, 246 , 112272. https://doi.org/10.1016/j.jinorgbio.2023.112272
  15. Graeme Hogarth. An unexpected leading role for [Fe2(CO)6(μ-pdt)] in our understanding of [FeFe]-H2ases and the search for clean hydrogen production. Coordination Chemistry Reviews 2023, 490 , 215174. https://doi.org/10.1016/j.ccr.2023.215174
  16. Lucile Chatelain, Jean-Baptiste Breton, Federica Arrigoni, Philippe Schollhammer, Giuseppe Zampella. Geometrical influence on the non-biomimetic heterolytic splitting of H 2 by bio-inspired [FeFe]-hydrogenase complexes: a rare example of inverted frustrated Lewis pair based reactivity. Chemical Science 2022, 13 (17) , 4863-4873. https://doi.org/10.1039/D1SC06975F
  17. Xiao-Li Gu, Bo Jin, Xiao Tan, Pei-Hua Zhao. Influence of pendant amine of phosphine ligands on the structural, protophilic, and electrocatalytic properties of diiron model complexes related to [FeFe]-hydrogenases. Inorganic Chemistry Communications 2021, 133 , 108934. https://doi.org/10.1016/j.inoche.2021.108934
  18. Pei-Hua Zhao, Meng-Yuan Hu, Jian-Rong Li, Yan-Zhong Wang, Bao-Ping Lu, Hong-Fei Han, Xu-Feng Liu. Impacts of coordination modes (chelate versus bridge) of PNP-diphosphine ligands on the redox and electrocatalytic properties of diiron oxadithiolate complexes for proton reduction. Electrochimica Acta 2020, 353 , 136615. https://doi.org/10.1016/j.electacta.2020.136615
  19. Alba Collado, Alejandro Torres, Mar Gómez‐Gallego, Luis Casarrubios, Miguel A. Sierra. A Model for the Prediction of the Redox Potentials in [FeFe]‐Clusters from the Electronic Properties of Isocyanide Ligands. ChemistrySelect 2020, 5 (24) , 7177-7182. https://doi.org/10.1002/slct.202001820
  20. Shishir Ghosh, Ahibur Rahaman, Georgia Orton, Gregory Gregori, Martin Bernat, Ummey Kulsume, Nathan Hollingsworth, Katherine B. Holt, Shariff E. Kabir, Graeme Hogarth. Synthesis, Molecular Structures and Electrochemical Investigations of [FeFe]‐Hydrogenase Biomimics [Fe 2 (CO) 6‐ n (EPh 3 ) n (µ‐edt)] (E = P, As, Sb; n = 1, 2). European Journal of Inorganic Chemistry 2019, 2019 (42) , 4506-4515. https://doi.org/10.1002/ejic.201900891
  21. Takehiko Shimamura, Yuki Maeno, Kazuyuki Kubo, Shoko Kume, Claudio Greco, Tsutomu Mizuta. Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge. Dalton Transactions 2019, 48 (44) , 16595-16603. https://doi.org/10.1039/C9DT03427G
  22. David G. Unwin, Shishir Ghosh, Faith Ridley, Michael G. Richmond, Katherine B. Holt, Graeme Hogarth. Models of the iron-only hydrogenase enzyme: structure, electrochemistry and catalytic activity of Fe 2 (CO) 3 (μ-dithiolate)(μ,κ 1 ,κ 2 -triphos). Dalton Transactions 2019, 48 (18) , 6174-6190. https://doi.org/10.1039/C9DT00700H
  23. Young-Do Kwon, Shinwoo Kang, Hyunjun Park, Il-koo Cheong, Keun-A Chang, Sang-Yoon Lee, Jae Ho Jung, Byung Chul Lee, Seok Tae Lim, Hee-Kwon Kim. Novel potential pyrazolopyrimidine based translocator protein ligands for the evaluation of neuroinflammation with PET. European Journal of Medicinal Chemistry 2018, 159 , 292-306. https://doi.org/10.1016/j.ejmech.2018.09.069
  24. Moritz Senger, Konstantin Laun, Florian Wittkamp, Jifu Duan, Michael Haumann, Thomas Happe, Martin Winkler, Ulf‐Peter Apfel, Sven T. Stripp. Protonengekoppelte Reduktion des katalytischen [4Fe‐4S]‐Zentrums in [FeFe]‐Hydrogenasen. Angewandte Chemie 2017, 129 (52) , 16728-16732. https://doi.org/10.1002/ange.201709910
  25. Moritz Senger, Konstantin Laun, Florian Wittkamp, Jifu Duan, Michael Haumann, Thomas Happe, Martin Winkler, Ulf‐Peter Apfel, Sven T. Stripp. Proton‐Coupled Reduction of the Catalytic [4Fe‐4S] Cluster in [FeFe]‐Hydrogenases. Angewandte Chemie International Edition 2017, 56 (52) , 16503-16506. https://doi.org/10.1002/anie.201709910
  26. Felix Koch, Andreas Berkefeld, Hartmut Schubert, Claudius Grauer. Redox and Acid–Base Properties of Binuclear 4‐Terphenyldithiophenolate Complexes of Nickel. Chemistry – A European Journal 2016, 22 (41) , 14640-14647. https://doi.org/10.1002/chem.201603060
  27. Shishir Ghosh, Ben E. Sanchez, Idris Richards, Mohammed N. Haque, Katherine B. Holt, Michael G. Richmond, Graeme Hogarth. Biomimetics of the [FeFe]-hydrogenase enzyme: Identification of kinetically favoured apical-basal [Fe2(CO)4(μ-H){κ2-Ph2PC(Me2)PPh2}(μ-pdt)]+ as a proton-reduction catalyst. Journal of Organometallic Chemistry 2016, 812 , 247-258. https://doi.org/10.1016/j.jorganchem.2015.09.036
  28. Kai‐Ti Chu, Yu‐Chiao Liu, Yi‐Lan Huang, Gene‐Hsiang Lee, Mei‐Chun Tseng, Ming‐Hsi Chiang. Redox Communication within Multinuclear Iron–Sulfur Complexes Related to Electronic Interplay in the Active Site of [FeFe]Hydrogenase. Chemistry – A European Journal 2015, 21 (18) , 6852-6861. https://doi.org/10.1002/chem.201406101
  29. Ahmed Alwaaly, William Clegg, Richard A. Henderson, Michael R. Probert, Paul G. Waddell. Mechanisms and rates of proton transfer to coordinated carboxydithioates: studies on [Ni(S 2 CR){PhP(CH 2 CH 2 PPh 2 ) 2 }] + (R = Me, Et, Bu n or Ph). Dalton Transactions 2015, 44 (7) , 3307-3317. https://doi.org/10.1039/C4DT03543G
  30. Ralf Trautwein, Laith R. Almazahreh, Helmar Görls, Wolfgang Weigand. Steric effect of the dithiolato linker on the reduction mechanism of [Fe 2 (CO) 6 {μ-(XCH 2 ) 2 CRR′}] hydrogenase models (X = S, Se). Dalton Transactions 2015, 44 (43) , 18780-18794. https://doi.org/10.1039/C5DT01387A
  • Abstract

    Scheme 1

    Scheme 1. Protonation of Fe2(xdt)(CO)4(PMe3)2a

    Scheme aThere is turnstile interchange of the CO and PMe3 ligands at the metal centers. (23) General conditions: complex concentration 0.12–0.50 mM, acid (HBF4·Et2O) concentrations cover the range 5–250 mM, 21 °C, reaction under N2 or Ar.

    Figure 1

    Figure 1. Left: Decay of UV signal at 348 nm over time on protonation of 1(iPr-pdt) (circles); pseudo-first-order fit (line); [1(iPr-pdt)]0 0.13 mM, [HBF4·Et2O]0 125 mM. Right: Rates of protonation of 1(iPr-pdt) in MeCN as measured by UV over a range of acid concentrations. The linear fit for kobs versus [HBF4·Et2O] plot is that for a fixed zero intercept.

    Figure 2

    Figure 2. Correlation of oxidation potential with degree of phosphine substitution in the series Fe2(pdt)(CO)(6–n)(PMe3)n, n = 0, 1, 2, 4. Oxidation potentials were recorded at a vitreous carbon electrode in 0.1 M [Bu4N][BF4]-MeCN and are reported relative to a Fc+/Fc internal standard.

    Figure 3

    Figure 3. Correlation of rate of protonation with oxidation potential for {2Fe2S} and {2Fe3S} systems. Where not specified, substrates are of general structure 1. The line shows the best-fit for the filled circles: substrates of general structure Fe2(xdt)(CO)(6–n)(PMe3)n, xdt = alkyl dithiolate, n = 1, 2. Oxidation potentials were recorded at a vitreous carbon electrode in 0.1 M [Bu4N][BF4]-MeCN and are reported relative to a Fc+/Fc internal standard. Protonation rates were measured by stopped-flow IR [3(edt), 3(pdt), 4, 5] or UV (all others).

    Figure 4

    Figure 4. Solid state structures of 1(iPr-pdt) and 1(iPr2-pdt) showing spheres of arbitrary radius (top) and 50% probability ellipsoids (bottom).

    Figure 5

    Figure 5. DFT gas-phase HOMO for 1(Me-pdt) (left) and 1(Me2-pdt) (right), showing the two (arbitrary) phases of the orbital in red and green.

    Scheme 2

    Scheme 2. Equilibria between All-Terminal and Semibridging Isomers (E1/2ox,1 > E1/2ox,2)
  • References

    ARTICLE SECTIONS
    Jump To

    This article references 42 other publications.

    1. 1
      Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245 2274
    2. 2
      (a) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Chem. Rev. 2014, 114, 4081 4148
      (b) Simmons, T. R.; Berggren, G.; Bacchi, M.; Fontecase, M.; Artero, V. Coord. Chem. Rev. 2014, 270-271, 127 150
    3. 3
      Camara, J. M.; Rauchfuss, T. B. Nat. Chem. 2012, 4, 26 30
    4. 4
      Ghosh, S.; Hogarth, G.; Hollingsworth, N.; Holt, K. B.; Kabir, S. E.; Sanchez, B. E. Chem. Commun. 2014, 50, 945 947
    5. 5
      Hoffman, B. M.; Lukoyanov, D.; Dean, D. R.; Seefeldt, L. C. Acc. Chem. Res. 2013, 46, 587 595
    6. 6
      Lindahl, P. A. Biochemistry 2002, 41, 2097 2105
    7. 7

      For an early biomimetic example, see:

      Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710 9711
    8. 8
      Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 2007, 46, 3426 3428
    9. 9
      Barton, B. E.; Rauchfuss, T. B. Inorg. Chem. 2008, 47, 2261 2263
    10. 10
      Zaffaroni, R.; Rauchfuss, T. B.; Gray, D. L.; De Gioia, L.; Zampella, G. J. Am. Chem. Soc. 2012, 134, 19260 19269
    11. 11
      Wang, W.; Rauchfuss, T. B.; Zhu, L.; Zampella, G. J. Am. Chem. Soc. 2014, 136, 5773 5782
    12. 12
      van der Vlugt, J. I.; Rauchfuss, T. B.; Wilson, S. R. Chem.—Eur. J. 2005, 12, 90 98
    13. 13
      Wright, J. A.; Pickett, C. J. Chem. Commun. 2009, 45, 5719 5721
    14. 14
      Jablonskytė, A.; Wright, J. A.; Pickett, C. J. Dalton Trans. 2010, 39, 3026 3034
    15. 15
      Jablonskytė, A.; Wright, J. A.; Pickett, C. J. Eur. J. Inorg. Chem. 2011, 1033 1037
    16. 16
      Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492 9495
    17. 17
      Kramarz, K. W.; Norton, J. R. Prog. Inorg. Chem. 1994, 42, 1 65
    18. 18
      Henderson, R. A.; Oglieve, K. E. J. Chem. Soc., Dalton Trans. 1993, 22, 3431 3439
    19. 19
      Henderson, R. A.; Ibrahim, S. K.; Oglieve, K. E.; Pickett, C. J. J. Chem. Soc., Chem. Commun. 1995, 31, 1571 1572
    20. 20
      Grönberg, K. L. C.; Henderson, R. A.; Oglieve, K. E. J. Chem. Soc., Dalton Trans. 1998, 27, 3093 3104
    21. 21
      Singleton, M. L.; Jenkins, R. M.; Klemashevich, C. L.; Darensbourg, M. Y. C. R. Chim. 2008, 11, 861 874
    22. 22
      Hsieh, C.-H.; Erdem, Ö. F.; Harman, S. D.; Singleton, M. L.; Reijerse, E.; Lubitz, W.; Popescu, C. V.; Reibenspies, J. H.; Brothers, S. M.; Hall, M. B.; Darensbourg, M. Y. J. Am. Chem. Soc. 2012, 134, 13089 13102
    23. 23
      Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.; Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917 3928
    24. 24

      Protonation of 2(pdt) has been reported under superacidic conditions:

      Matthews, S. L.; Heinekey, D. M. Inorg. Chem. 2010, 49, 9746 9748
    25. 25
      Teo, B. K.; Hall, M. B.; Fenske, R. F.; Dahl, L. F. Inorg. Chem. 1975, 14, 3103 3117
    26. 26
      Andersen, E. L.; Fehlner, T. P.; Foti, A. E.; Salahub, D. R. J. Am. Chem. Soc. 1980, 102, 7422 7429
    27. 27
      Walther, B.; Hartung, H.; Reinhold, J.; Jones, P. G.; Mealli, C.; Bottcher, H. C.; Baumeister, U.; Krug, A.; Mockelt, A. Organometallics 1992, 11, 1542 1549
    28. 28
      Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd Ed.; John Wiley & Sons: Chichester, U.K., 2001.
    29. 29
      Sarapu, A. C.; Fenske, R. F. Inorg. Chem. 1975, 14, 247 253
    30. 30
      Pickett, C. J.; Pletcher, D. J. Organomet. Chem. 1975, 102, 327 333
    31. 31

      For reversible systems E1/2 is independent of temperature if, as is usually the case, the ratio of the diffusion coefficient for the oxidized and reduced partners is closely similar at all temperatures. Whether the slope, viz. ΔΔGE1/2, varies with T will essentially depend on whether changes in the entropy of activation ΔΔS varies significantly across the series. The observed correlation indicates the entropic contribution to ΔG associated with the ordering of the transition state is similar for all of the complexes or varies monotonically with ΔH. In either case ΔΔS is probably small, and consequently the effect of T on the slope is likely to be small with ΔΔG and dominated by the enthalpic ΔΔH terms.

    32. 32
      Chatt, J.; Leigh, G. J.; Neukomm, H.; Pickett, C. J.; Stanley, D. R. J. Chem. Soc., Dalton Trans. 1980, 121 127
    33. 33
      Eilers, G.; Schwartz, L.; Stein, M.; Zampella, G.; de Gioia, L.; Ott, S.; Lomoth, R. Chem.—Eur. J. 2007, 13, 7075 7084
    34. 34

      A further factor that we have considered that might trend with protonation rates is variation in metal–metal bond distances in the reactants and products. There is not a systematic dependence on reactant or product bond lengths, or changes of these (Table S1, SI).

    35. 35

      “Rotated” states at the Fe(I)Fe(I) level have been generated by the introduction of steric bulk at the bridgehead (ref 22) and by combing steric bulk with electronic asymmetry:

      Sabrina; Munery; Capon, J.-F.; De Gioia, L.; Elleouet, C.; Greco, C.; Pétillon, F. Y.; Schollhammer, P.; Zampella, G. C. Chem.—Eur. J. 2013, 19, 15458 15461
      Wang, W.; Rauchfuss, T. B.; Moore, C. E.; Rheingold, A. L.; De Gioia, L.; Zampella, G. Chem.—Eur. J. 2013, 19, 15476 15479
      De Gioia, L.; Elleouet, C.; Munery, S.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J.; Zampella, G. Eur. J. Inorg. Chem. 2013, 3456 3461

      The latter exhibit fully rotated states and bridging carbonyl groups, while the former are best viewed as partially “twisted”. In the natural system, the geometry at the metal center is fully rotated with a bridging CO present, although the influence of the enzyme scaffold is also likely responsible for maintaining the rotated state.

    36. 36
      Bordwell, F. G.; Bausch, M. J. J. Am. Chem. Soc. 1986, 108, 1979 1985
    37. 37
      Bordwell, F. G.; Bausch, M. J. J. Am. Chem. Soc. 1986, 108, 1985 1988
    38. 38
      Angelici, R. J. Acc. Chem. Res. 1995, 28, 51 60
    39. 39
      Wright, J. A.; Webster, L.; Jablonskytė, A.; Woi, P. M.; Ibrahim, S. K.; Pickett, C. J. Faraday Discuss. 2011, 148, 359 371
    40. 40
      Le Cloirec, A.; Best, S. P.; Borg, S.; Davies, S. C.; Evans, D. J.; Hughes, D. L.; Pickett, C. J. Chem. Commun. 1999, 35, 2285 2286
    41. 41
      Finkelmann, A. R.; Stiebritz, M. T.; Reiher, M. Chem. Sci. 2014, 5, 215 221
    42. 42
      Frey, M. ChemBioChem 2002, 3, 153 160
  • Supporting Information

    Supporting Information

    ARTICLE SECTIONS
    Jump To

    Experimental details for synthesis, stopped-flow and electrochemistry, characterization data, X-ray crystallographic data and DFT details. This material is available free of charge via the Internet at http://pubs.acs.org.


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.