D Exchanges and H2 Release: Diiron

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Terminal Thiolate-Dominated H/D Exchanges and H2 Release: Diiron Thiol–Hydride Xin Yu, Maofu Pang, Shengnan Zhang, Xinlong Hu, Chen-Ho Tung, and Wenguang Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06996 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Journal of the American Chemical Society

Terminal Thiolate-Dominated H/D Exchanges and H2 Release: Diiron Thiol–Hydride Xin Yu, Maofu Pang, Shengnan Zhang, Xinlong Hu, Chen-Ho Tung and Wenguang Wang* Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, No.27 South Shanda Road, Jinan, 250100, P. R. China ABSTRACT. To determine the reaction pathways at a metal-ligand site in enzymes, we incorporated a terminal thiolate site into a diiron bridging hydride. Trithiolato diiron hydride, (µ-H)Fe2(pdt)(dppbz)(CO)2(SR) (1(µ-H)) [pdt2− = 1,3-(CH2)3S22−, dppbz = 1,2-C6H4(PPh2)2, RS− = 1,2-Cy2PC6H4S−)] was synthesized directly by photo-assisted oxidative addition of 1,2Cy2PC6H4SH to Fe2(pdt)(dppbz)(CO)4. The terminal thiolate in 1(µ-H) undergoes protonation, affording a thiol-hydride complex [1(µ-H)H]+. Placing an acidic SH site adjacent to the Fe-H-Fe site allows intramolecular thiol-hydride coupling, and releases H2 from [1(µ-H)H]+. A diiron η2H2 intermediate in the formation of H2 is proposed, and is evidenced by the H/D exchange reactions of [1(µ-H)H]+ with D2, D2O and CD3OD. Isotopic exchange in [1(µ-D)H]+ is driven by an equilibrium isotope effect with 2.1 kJ/mol difference in free energy that favors [1(µ-H)D]+. [1(µ-H)H]+ catalyzes H/D scrambling between H2 and D2O or CD3OD to produce HD. The reactions based on such a “proton-hydride” model provide insights into the reversible heterolytic cleavage of H2 by H2ases. INTRODUCTION Sulfur is essential to life since it is contained in the amino acids methionine and cysteine, which are key components of most proteins.1 It is not only a minor constituent of organisms but

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it plays significant roles in microbial transformations.2 In particular, thiolate sulfur is widely thought to serve as a basic site for proton transfer in biological transformations.3-5 A premier example is exhibited by the active site [NiFe]-H2ase (Figure 1a), which employs one of its cysteinated-S at the Ni center to mediate protons for the interconversion of H2, protons and electrons at a high rate.6-10 Many of the states in these pervasive enzymes are related by simple protonation/deprotonation of one or both terminal thiolate ligands.11,12 It is also widely perceived that the hydride states arise by proton transfer from S to the Ni-Fe center.13 Finally, the evolution of H2, which is characteristically heterolytic,14 is thought to proceed by coupling of the NiFehydride and one of the protonated cysteine residues.15 More generally, it is postulated that in nitrogenase the proton delivery to the sulfur site is coupled with electron transfer to the FeMoco active site. After accumulating four electrons and four protons in the active site, the E4/H4 state featuring SH ligands and hydrides is formed (Figure 1b).16

Figure 1. Schematic representations of (a) the active site of [NiFe]-H2ase (Ni-R state), (b) the E4/H4 sate of nitrogenase, (c) proposed “proton-hydride” exchange in the “thiol-hydride” systems. Investigating the reactivity toward protons of metal hydrides bearing thiolate ligands is of obvious significance and interest with respect to the reaction pathways at a metal-ligand site in


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such enzymes.17,18 Most importantly, protonation of metal hydrides with proton-responsive coordination spheres (HM-X-) gives the “proton-hydride” state (XH+---−HM), which is envisioned as the key intermediate species in many catalytic reactions such as H2 production and oxidation,19 and hydrogenation and transfer hydrogenation reactions.20-23 Significant progress has been made in research into synthetic models of hydrogenases aimed at exploration of the highefficiency bioinspired H2 evolution catalyst.24-29 In the synthetic [NiFe] and [FeFe] analogues, sulfur was predicted to respond to the uptake of H2 which proceeds SH+---−HFe decoupling.30-33 Intramolecular H/D exchange of a thiol proton with hydride has been demonstrated for Ir,34,35 Ru36 and Rh,37 and an η2-HD intermediate state is assumed for the exchange reaction (Figure 1c). Although iron dihydrogen complexes Fe(η2-H2) are well known,38-40 “SH+---−HFe” models are rare. The only example of an iron(II) thiol-hydride complex was one generated from the protonation of (PhS)Fe(H)(CO)2(P(OPh)3)2, but it is unstable and fails to perform the important H/D exchange function.17

Figure 2. (a) The proposed “proton-hydride” intermediate for the active site of [FeFe]-H2ase; (b) a representative synthetic “proton-hydride” model; (c) reported isomerization of diiron terminal hydride to the bridging hydride.


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The active site of [FeFe]-H2ases on the other hand, also operates the “proton-carried metal hydride” protocol for catalytic proton reduction and H2 splitting but it employs its amine in a second coordination sphere to facilitate proton transfer.41-43 As a consequence, instead of the “thiol-bridging hydride”, the transient state was thought to be an “ammonium-terminal hydride” (Figure 2a).44-46 Most modelling efforts, especially those containing the phosphine ligand Fe2(pdt)(CO)6(PR3)6-x47-51 focus on substituted derivatives of Fe2(pdt)(CO)6 lacking the amine cofactor. Protonation of these reduced Fe(I)Fe(I) compounds eventually produces the bridging diiron hydride [Fe(II)HFe(II)]+,52,53 although a number of terminal diiron hydrides [Fe(II)Fe(II)H]+ were characterized as kinetic intermediates by NMR spectra at low temperature.54,55 One attractive “ammonium-hydride” model is [(t-H)Fe2(adtNH2)(CO)2(dppv)2]2+ ((adtNH)2- = HN(CH2S)22-), in which the NH---HFe distance of 1.88(7) Å is indicative of dihydrogen bonding.56 Despite the fact that the half-life of the terminal hydrides can be prolonged by such an NH---HFe interaction or by the increased steric hindrance of phosphines at the diiron center, again they slowly isomerize to the bridging hydrides.57 Puzzling is that Fe(II)H-Fe(II) sites are typically inert with respect to protonolysis and liberation of H2,46,57d and the hydride ligand even serves as a spectator in H2 evolution reactions.58-60

Scheme 1. Hypothetical “thiol-hydride” model arising from protonation of the terminal thiolate site in a diiron bridging hydride platform


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It is noteworthy that cooperative metal-sulfur reactivity enables the heterolysis of H2,61 and activation of the B-H or Si-H bonds for organic transformations.62 The geometry of Fe-H-Fe resembles that found in the active site of [NiFe]-H2ase, which contains terminal thiolate sulfurs. Hence, we hypothesized that the bridging hydride in a FeFe(pdt) platform can be activated by the introduction of a terminal thiolate (Scheme 1). In this paper, we demonstrate many of the reactions based on a well-characterized iron(II) thiol-hydride complex in a synthetic FeFe system, [(µ-H)Fe2(pdt)(dppbz)(CO)2(1,2-Cy2PC6H4SH)]+ ([1(µ-H)H]+). The distinctive feature of the new complexes disclosed is the co-existence of terminal thiolate ligand with a bimetallic hydride center. The terminal thiolate is susceptible to protonation that can place an acidic SH site adjacent to the Fe-H-Fe site, allowing the release of H2 and catalysis of H2/D2O scrambling. RESULTS AND DISCUSSION Synthesis of 1(µ-H). Photo-assisted oxidative addition of X-H bonds (X = H, Si and N) to iron carbonyls is a powerful approach to synthesize iron hydride complexes.63-66 Oxidative addition of the S-H bond to Mo(II)67 and Rh(I) precursors68 has been reported for the synthesis of thiolato hydride complexes. Accordingly, we synthesized compound 1(µ-H) by photolysis (λ = 365 nm) of Fe2(pdt)(dppbz)(CO)4 with 1,2-PCy2C6H4SH in toluene (Scheme 2). As monitored by IR spectra, the formation of 1(µ-H) was indicated by the appearance of a new broad νCO band at 1930 cm−1 and with an increase in its intensity upon photolysis (Figure S1). Compared to [(µH)Fe2(pdt)(CO)2(dppv)2]+ (νCO = 1968, 1951 cm−1),56 the νCO for 1(µ-H) is about 20-35 cm-1 lower in energy. The 31P NMR spectrum of 1(µ-H) displayed two set of signals at δ 92.44 (s) and δ 85.33 (m) in a ratio of 1:2, which are assigned respectively to the PCy2 and dppbz groups (Figure S3). The two overlapping doublets at δ 85.33 with JP-P =19.9 Hz suggest the two P atoms


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at dppbz are chemically non-equivalent. In agreement with the phosphorus assignments, the 1H NMR spectrum exhibits a well-resolved triplet of doublets at δ -12.87 (JP-H = 23.2 and 9.6 Hz) for the hydride signal (Figure S2). Following the same procedure as 1(µ-H), the corresponding deuteride 1(µ-D) (95% enriched) was synthesized using 1,2-PCy2C6H4SD (Figure S5).

Scheme 2. Preparation, Sulfur-Protonation and Methylation of 1(µ-H) PCy2

Ph2P

Cy2 P

S S

OC Fe

Fe

PPh2 H (D)

CO

SH(D) hv, - 2CO

OC OC

S

1(µ -H) or 1(µ -D)

Ph2 P

S S

OC Fe

Fe

P Ph2 CO

Fe2(pdt)(dppbz)(CO)4 +[ Me

+H

BF

4

Cy2 P

S S

OC Ph2P

3 O]

+

Fe PPh2 H (D)

Fe CO S

H [1( µ-H)H]+ or [1(µ -D)H]+

Fe

Ph2P

Cy2 P

S S

OC

PPh2

Fe H

CO S

Me [1( µ-H)(Me)] +

The molecular structure of 1(µ-H) was revealed by single crystal X-ray diffraction (Figure 3). Crystallographic analysis agrees with the NMR spectroscopic assignments of the diiron triphosphine complex as a hydride. The dppbz occupies the two basal sites of a Fe center, while the P and S atoms in the P-S ligand respectively are located at the apical and a basal site of the other Fe center. The bridging hydride ligand was located and refined, and is more strongly coordinated at the Fe(dppbz) site (∆(Fe-H) = 0.377 Å) and trans to the terminal CO. At the Fe(PS) site, the Fe2---Sterminal distance is 2.300(1) Å, approximately 0.02 Å longer than the average Fe2-Sbridging distance.


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Figure 3. Structure of 1(µ-H) with thermal ellipsoids drawn at 50% probability and H atoms (except for the hydride ligand) omitted. The phenyl and cyclohexyl groups at phosphine site are drawn in lines. Selected distances (Å): Fe1−Fe2, 2.6107(7); Fe1−H, 1.4872; Fe1−S1, 2.272(1); Fe1−S2, 2.2719(9); Fe2−H, 1.8646; Fe2−S1, 2.261(1); Fe2−S2, 2.284(1); Fe2−S3, 2.300(1). Angles (°): Fe1−H−Fe2, 101.726; P3−Fe2−S3, 85.69(4). S-Protonation of 1(µ-H). The neutral hydride complex 1(µ-H) undergoes S-protonation affording the thiol-hydride complex [1(µ-H)H]+. Treatment of a CH2Cl2 solution of 1(µ-H) with HBF4·Et2O led to the color change from black to brown. This reaction is also reflected in a 28 cm-1 higher energy shift for νCO to 1958 cm-1 (Figure S7), which is agreement with the ∆νCO that has been observed for S-protonation.17,33 For comparison, protonation of the amido site in the diiron bridging hydride HFe2(pdt)(CO)2(Ph2PC6H4NH2)(Ph2PC6H4NH) causes a shift in νCO by about 20 cm-1.66 The νS-H vibration which is known to be weak, was observed at 2471 cm-1.69 The S-protonation also leads to the upfield shifts of the phosphorus resonances. The

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P signals

appearing as doublets at δ 84.34 (JP-P = 19.4 Hz) and 83.71 (JP-P = 19.3 Hz) are assigned to dppbz, and a singlet at δ 89.30 is assigned to the P-S ligand (Figure S9). The 1H NMR spectrum of [1(µ-H)H]BF4 features the hydride signal at δ -14.35 (vs δ -12.87 for 1(µ-H)) and the proton signal for SH at δ 3.68 (Figure S8).


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Figure 4. Structure of endo-[1(µ-H)H]BF4. Selected distances (Å): Fe1−Fe2, 2.616(1); Fe1−H, 1.67(4); Fe1−S1, 2.262(2); Fe1−S2, 2.261(2); Fe2−H, 1.82(4); Fe2−S1, 2.244(2); Fe2−S2, 2.280(2); Fe2−S3, 2.270(2); S3−H3A, 1.34(5); H−H3A, 2.809. Angles (°): Fe1−H1−Fe2, 96.85(4); P3−Fe2−S3, 84.81(6). The structure of major isomer, endo-[1(µ-H)H]BF4, was confirmed by X-ray crystallography (Figure 4). The structures of [1(µ-H)] and [1(µ-H)H]+ are very similar although some bond lengths are subtly changed. The semi-bridging hydride is closer to Fe(dppbz) site with ∆(Fe-H) = 0.15(4) Å and a Fe-H-Fe angle of 96.85(4)°. The protonation causes the Fe−S distance at the Fe(P-S) site to decrease from 2.300(1) to 2.270(2) Å. A similar contraction in the Fe-S distance was observed in the conversion of CpFe(CO)2SPh to [CpFe(CO)2(HSPh)]+.70 Such a change in bond length was attributed to the relief of the four-electron destabilization in the Fedπ-Spπ antibonding. The H atom bound to the sulfur is 2.069 Å from a F atom of the BF4- anion and with ∠S-HF = 167.9°, suggestive of the existence of a hydrogen bond.33,70,71


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+

Ph2P

Cy2 P

S S

OC Fe

+

Fe CO S

PPh2 H

Ph2P

Cy2 P

S S

OC Fe

Fe

PPh2 H

H

CO S H

(1)

exo

endo

Figure 5. 1H NMR spectra of [1(µ-H)H]BArF4 in CD2Cl2 at (a) 25 °C, and (b) -60 °C; 31P NMR spectra of [1(µ-H)H]BArF4 at (c) 25 °C, and (d) -60 °C. On the basis of the X-ray crystallography, S-protonation is diastereoselective (eq 1). However, the NMR data obtained at room temperature do not distinguish the presence of a single diastereomer in a rapidly equilibrating mixture. Even in the 1H NMR spectrum of [1(µH)H]BArF4, the peaks of the hydride and thiol are both broadened at room temperature (Figure 5). However, upon cooling the solution to -60 oC, the broad hydride signal of [1(µ-H)H]BArF4 splits into two sets of a triplet of doublets at δ -14.30 (dt, JP-H = 21.8 and 9.1 Hz) and -15.40 (dt, JP-H = 22.1 and 14.1 Hz) with a ratio of 86:14. Meanwhile, a split in the SH peak was also observed. Consistent with the 1H NMR spectrum analysis, the 31P NMR spectrum collected at -60 oC definitely suggests the existence of two isomers. Judging from the hydride resonance, the major diastereomer with the

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P NMR signals at δ 89.1 (s), 84.8 (d, JP-P= 19.0 Hz) and 83.3 (d, JP-P=


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19.0 Hz) are assigned to endo-[1(µ-H)H]BArF4, and the

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P signals at δ 90.8 (s), 85.4 (d, JP-P=

19.0 Hz) and 84.0 (d, JP-P= 19.0 Hz) correspond to the minor isomer exo-[1(µ-H)H]BArF4. Raising the temperature to 0 oC results in equilibration of the diastereomers as indicated by coalescence of the hydride and thiol resonances. The structure of the major diastereomer, endo[1(µ-H)H]BArF4, was also characterized by X-ray crystallography (Figure S10).

K eq S S

OC Ph2 P

Fe

Cy2 P

+ [HP(p-tol)3 ]+

CO

- [HP(p-tol)3] +

Fe

PPh2 H

S

Ph2P

Cy2 P

S S

OC Fe

Fe

PPh2 H

(2)

CO S H

[1(µ -H)H] +

[1(µ -H)]

Acid-Base Properties of [1(µ-H)H]+ in CD2Cl2. [1(µ-H)H]+ undergoes deprotonation by conventional organic bases such as trimethylamine, pyridine, and tricyclohexylphosphine, producing 1(µ-H) quantitatively. Equilibria were established between 1(µ-H) and [HP(ptol)3]BF4 (pKaCH2Cl2 = 2.7, eq 2), and the pKa value of [1(µ-H)H]BF4 was determined in CD2Cl2 by 1H NMR and

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P NMR spectroscopic analysis.72 The chemical shift of the hydride signal is

sensitive to the amount of [HP(p-tol)3]BF4 that was added (Figure S16). For instance, the addition of one equiv of [HP(p-tol)3]BF4 shifts the hydride signal by about 1 ppm. Separate NMR signals are not observed for 1(µ-H) and [1(µ-H)H]+ at room temperature because apparently, rapid proton exchange occurs between the thiol complex and its conjugate base. The ratios of conjugate acid-base pairs were both determined from the weighted averages of the chemical shifts.73 Using these ratios, an equilibrium constant (Keq) of 0.34 was determined, and a pKa of 3.17 was calculated for [1(µ-H)H]+. Overall, these data confirm that [1(µ-H)H]+ is very acidic, and that is consistent with the strong acidic character of the benzenethiol ligand reported for [Fe(C5H5)(CO)2(PhSH)]BF4.74


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Figure 6. Structure of [1(µ-H)(Me)]BF4. Selected distances (Å): Fe1−Fe2, 2.6219(6), Fe1−H, 1.69(3); Fe1−S1, 2.2600(8); Fe1−S2, 2.2638(9); Fe2−H, 1.63(3); Fe2−S1, 2.2471(9); Fe2−S2, 2.3004(8); Fe2−S3, 2.2681(9); S3−C54, 1.812(3); Angle (°): Fe1−H−Fe2, 104.802; P3−Fe2−S3, 84.58(3). S-Methylation of 1(µ-H). The nucleophilicity of the terminal thiolate in 1(µ-H) was further addressed by the S-methylation with the oxonium salt [Me3O]BF4, yielding [1(µ-H)(Me)]BF4 (Scheme 2). Addition of one equivalent of [Me3O]BF4 to a CH2Cl2 solution of 1(µ-H) resulted in a rapid color change from black to brown. This new product displayed a νCO band at 1955 cm-1 (Figure S18) that is very close to 1958 cm-1 observed for [1(µ-H)H]+. The formation of a methylated cationic complex was deduced from the ESI-MS spectral analysis, which showed an ion peak at m/z 1041.1642 corresponding to [1(µ-H)(Me)]+. At room temperature, its 1H NMR spectrum exhibits the hydride signal at δ -14.15 (td, JP-H = 20.4 and 9.6 Hz). X-ray crystallographic analysis confirmed the methylation at the terminal thiolate site (Figure 6). The overall geometry of endo-[1(µ-H)(Me)]+ is similar to that of [1(µ-H)H]+ with respect to Smethylation vs S-protonation. In particular, the Fe2-S bond distance 2.2681(9) Å in [1(µH)(Me)]+ is comparable to the 2.270(2) Å that was observed for [1(µ-H)H]+. In addition, the SCH3 distance of 1.812(3) Å is comparable to those reported for dithiolato diiron hydride


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complexes

with

bound

thioethers,

i.e.

1.815(3)

Page 12 of 31

Å

for

[(µ-

H)Fe2(MeSCH2C(Me)(CH2S)2)(CO)4(PMe3)]+ and 1.813(7) Å for [(µ-H)Fe2(Me2pdt)(1,2Cy2PC6H4SMe)(PPh3)(CO)3]+.75,76

H2 Evolution. As with the synthetic diiron bridging hydride containing phosphine ligands, complex [1(µ-H)(Me)]BF4 is stable in solution; no decomposition was found after several hours in ambient light. Despite its thermal stability in the dark, solutions of [1(µ-H)H]+ slowly release H2 once exposed to room light. After several hours, the solution becomes colorless, which is indicative of the decomposition of the diiron complex by loss of CO.77

Scheme 3. Conversions of [1(µ-H)H]+ to [1]+, [1(CO)]+ and 1(µ-H)

By irradiation of [1(µ-H)H]+ in CH2Cl2 with a blue-LED (λ = 410 nm) at room temperature, the H2 evolution was completed in 10 min. The yield of H2 was quantified by GC analysis, which was determined from three experiments as 90 ± 5%. The IR spectrum of the reaction solution suggests a new organoiron species is produced with two νCO bands at 2025 cm-1, 1972 cm-1 (Figure S22). In the 31P NMR spectrum, three new phosphorus signals were observed at 90.8 (s),


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84.7 (d, JP-P = 28.3 Hz) and 79.9 (d, JP-P = 28.3 Hz). According to ESI-MS analysis (m/z = 1025.1328), the reaction product is [Fe2(pdt)(CO)2(dpbz)(Cy2PC6H4S)]+ ([1]+). As a diferrous 32-electron species, [1]+ is a 32-electron diiron species and unstable for isolation. It degrades to insoluble species with loss of CO. Interestingly, the anticipated [1]+ converts to 1(µ-H) by the addition of Cp2Co and 1.1 equiv of H(OEt2)2BArF4 to the reaction solution (Scheme 3).

Figure 7. Structure of [1(CO)]+. Selected distances (Å): Fe1−Fe2, 3.141; Fe1−S1, 2.346(1); Fe1−S2, 2.362(1); Fe1−S3, 2.332(1); Fe2−S1, 2.347(1); Fe2−S2, 2.312(1); Fe1−S3, 2.284(1); Fe2−S3, 2.284(1); Angles (°): Fe1−S1−Fe2, 84.03(5); Fe1−S2−Fe2,84.44(4); Fe1−S3−Fe2, 85.77(4); P3−Fe2−S3, 85.38(4). The generation of [1]+ was trapped by CO to provide a stable adduct [1(CO)]+. After photolysis, the solution was bubbled by CO gas, and then [1(CO)]+ was isolated and characterized crystallographically (Figure 7). [1(CO)]+ is a cationic diiron trithiolate complex with a framework of [Fe2(pdt)(Cy2PC6H4S)(dppbz)(CO)3]+. Both of the Fe centers present welldefined octahedral geometry. As with the dithiolate of the pdt2- ligand, the S- of the phosphinethiolate also bridges the two Fe centers. The dppbz switches to the apical-basal binding mode, compared to that of the dibasal chelating mode in [1(µ-H)H]+ and 1(µ-H).


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When

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CO was used as trapping reagent, a single isotopomer [1(13CO)]+ was obtained, as

indicated by characteristic changes in the IR spectrum (Figure S27). Specifically, the νCO band at 1970 cm-1 in [1(CO)]+ was split in [1(13CO)]+ to give two absorptions at 1970 cm-1 and 1921 cm1

(calculated: 1926 cm-1). This isotopic labeling does not affect the νCO band at 2023 cm-1 for

[1(CO)]+. The single label was also confirmed by the ESI-MS spectrum analysis, which features m/z = 1054.1337 for [1(13CO)]+, vs m/z = 1053.1303 for [1(CO)]+ (Figure S31) . [1(CO)]+ tends to resemble the CO-inhibited model of [FeFe]-H2ase78 and also the new inactivated form of [NiFe]-hydrogenase.79 Scheme 4. Proposed Pathway for Dihydrogen Release from [1(µ-H)H]+ Fe Fe

+

Fe H

SR

Fe H

Fe

−H 2 hv

+

+

Fe S R 1+

H SR

H [1(µ -H)H]+

[1(η-H2)]+

D2

Fe H2

+

Fe D

SR D

[1(µ -D)D]+

Photochemical evolution of H2 by metal hydrides is interesting and known.80,81 Previously described intermolecular H2 evolution by [(µ-H)Fe2(pdt)(dppv)(CO)4]+ requires strong acids such as triflic acid (HOTf).77 Surprisingly, the terminal thiol proton in [1(µ-H)H]+ is sufficiently acidic to react with the bridging hydride, releasing H2 photochemically. We propose that an intramolecular thiol-hydride coupling82 leads to access to a diiron η2-H2 intermediate, which losses H2 upon photolysis (Scheme 4). Evidence for such a diiron η2-H2 intermediate is observation of H/D exchange between [1(µ-H)H]+ and D2.


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Figure 8. FTIR spectra of Nujol mulls for the comparison of [1(µ-H)H]+ (black) with the reaction sample of [1(µ-H)H]+ with D2 after 6 h (blue). Pressurized CH2Cl2 solutions of [1(µ-H)H]+ in J. Young NMR tube by D2 (15 psi), results in increases in the intensity of the SD (δ 3.65) and µ-D (δ -14.35) peaks in the 2H NMR spectrum (Figure S32). Interestingly, the 1H NMR spectrum of the solution (Figure S33) displays a characteristic HD signal at δ 4.55 (t, JD-H = 42.5 Hz), as well as an H2 peak at δ 4.59. After the reaction, the sample was evaporated under vacuum and further analyzed by the solid state IR spectrum. In addition to the νS-H vibration at 2471 cm-1, the IR spectrum also exhibits the S-D stretching frequency at 1781 cm-1, agreeing with calculated value of 1774 cm-1 (Figure 8). In contrast, no H/D exchange was found for [1(µ-H)(Me)]+ with D2 under the identical reaction conditions carried for [1(µ-H)H]+. The results are convincing that the intramolecular thiolhydride coupling is responsible for H-H bond formation and D-D bond heterolytic cleavage. To gain insight into the H/D exchange process, many related reactions were subsequently considered (see below).


15


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+

Ph2P

Cy2 P

S S

OC Fe

Fe CO

PPh2 H [1(µ -H)H]

S H

+

excess CD3OD k1 k -1 excess CH3OH

Ph2P

+

Cy2 P

S S

OC Fe

Fe

(3)

CO PPh2 D [1(µ -D)D]

S D +

Figure 9. (left) Integral of the µ-H signal vs time for the reactions of (◊) [1(µ-H)H]BArF4 with excess CD3OD, and (♦) [1(µ-D)D]BArF4 with excess CH3OH; (right) plot of the integration vs time for ◊: y = 1.79 × 10-4x + 0.06; for ♦: y = 3.17 × 10-4x - 0.07. H/D exchange. [1(µ-H)H]+ undergoes H/D exchange not only with D2 but also with CD3OD or D2O to produce a mixture of [1(µ-D)D]+, [1(µ-H)D]+ and [1(µ-D)H]+, a reaction which surprisingly, takes place in the dark. In a reaction of [1(µ-H)H]+ with excess CD3OD (20 equiv) in 0.6 mL CD2Cl2, 1H NMR spectroscopic analysis showed that the S-H signal disappeared in 10 min. Upon addition of CD3OD to [1(µ-H)H]+ in CH2Cl2, appearance of the S-D signal was observed at δ 3.65 in 2H NMR spectrum. The results indicate that H/D exchange between S-H and CD3OD is fast, due no doubt to the strong acidity of the thiol.35 Compared to S-H, the H/D exchange for the hydride is relative slow (Figure S37). A plot of the integration vs time for the hydride signal suggests the SD/Fe-H-Fe exchange follows unimolecular kinetics with a rate


16

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constant of 1.79 × 10-4 s-1 at 25 oC (Figure 9). After evaporation of the reaction solution under vacuum, the residue was re-dissolved in CH2Cl2 and examined by 2H NMR spectroscopy, which unambiguously showed signals for S-D and Fe-D-Fe (Figure S36). Such H/D exchange was also examined for the reversible reaction of [1(µ-D)D]+ with excess CH3OH. Kinetics studies provided a rate constant of 3.17 × 10-4, which is slightly faster than the exchange between [1(µH)H]+ and CD3OD. It is more likely that the intramolecular Fe-H-Fe/SD exchange is slower than the opposing reaction of Fe-D-Fe/SH.

Ph2P

Cy2 P

S S

OC Fe

K(298) = 2.36

Fe CO

PPh2

D H

S

Ph2P

Cy2 P

S S

OC Fe PPh2

Fe H D

+

[1(µ -D)H]

CO

(4)

S

[1(µ -H)D]+

Figure 10. The integral of the µ-H signal vs time in 1H NMR spectra for the reaction of 1(µ-D) and one equiv of H(OEt2)2BArF4 in CD2Cl2 at 298 K. To investigate the isotopic exchange, we conducted the protonation of 1(µ-D) by one equivalent of H(OEt2)2BArF4 in CD2Cl2 in an NMR tube. Analysis of the

31

P NMR spectrum


17


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suggests that the terminal thiolate is quantitatively protonated within 5 min. After that, the conversion of [1(µ-D)H]+ to [1(µ-H)D]+ was further monitored by the 1H NMR spectrum. The integral of the hydride signal vs time is shown in Figure 10. Indeed, Fe-D-Fe does not completely convert to the Fe-H-Fe species. After 26 h, an equilibrium between [1(µ-H)D]+ and [1(µ-D)H]+ is reached and Keq of 2.36 was measured. Accordingly, a difference in free energy of -2.1 kJ/mol was calculated for the conversion of [1(µ-D)H]+ to [1(µ-H)D]+. A similar intramolecular equilibrium isotope effect (EIE) has been reported for the cases of the “ammonium-hydride”

[(P2PhN2BnD)MnH(CO)(bppm)]+

and

the

“amido-hydride”

Me4C5(H))2ZrH(N(D)tBu,83,84 in which the deuterium atom prefers the N-D position to the M-D position. In the present case, the anomalous kinetics observed for isotopic exchange is driven by an EIE that favors [1(µ-H)D]+.

Figure 11. 1H NMR spectra for the reactions of (a) H2/D2O, (b) H2/CD3OD catalyzed by [1(µH)H]+ in CD2Cl2 and (c) the control experiment for b in the absence of catalyst. Catalysis of H2/D2O and H2/CD3OD Scrambling. Further evidence for the postulated diiron η2-H2 intermediate is the observation of the catalysis of H/D scrambling between H2 and D2O, as well as for H2/CD3OD and D2/CH3OH by [1(µ-H)H]+. The H/D scrambling experiments were


18

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conducted in a J. Young tube in CD2Cl2 at room temperature.85-88 Typically, D2O (5 µL) was carefully injected under an N2 atmosphere into an NMR tube containing 0.6 mL CD2Cl2 solution of [1(µ-H)H]+ (5.3 µmol). The solution was frozen in liquid nitrogen and evaporated and then purged with H2 at 15 psi. The sample was allowed to warm to room temperature and placed in a dark cabinet. After 6 h, the reaction mixture was subjected to NMR spectroscopic analysis. The HD appeared as a characteristic triplet peak at 4.55 ppm in the 1H NMR spectrum (Figure 11). The doublet at δ 4.67 with JH-D = 42.5 Hz in the 2H NMR spectrum agrees with the assignment for HD. In addition to HD, the reaction also produces D2, identified by the 2H signal at δ 4.62. Similar scrambling was also found for H2/CD3OD, which generates a mixture of HD, D2 and CD3OH (Figure S44). This reaction is further confirmed by the catalysis of D2/CH3OH scrambling by [1(µ-H)H]+, which produces HD, H2 and CH3OD (Figure S46).

Scheme 5. Proposed Catalytic Scrambling Process for H2/D2O Fe

+

Fe

D 2O

Fe

H

SR H [1(µ -H)H] +

Fe H

Fe H

+

H

SR D [1(µ -H)D] +

+

Fe H

SR

[1(η-H2)]+

Fe

H2

HD

Fe D

+

SR

[1(η-HD)]+

D2 H2 Fe D

+

Fe D

SR

[1( η-D2)] +

Fe

Fe D

+

SR D [1(µ -D)D] +

D2 O H2 O

Fe

Fe

+

D

SR H [1(µ -D)H] +

CONCLUSIONS


19


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Based on the results described above, we propose a terminal thiolate-dominated H/D exchange process between H2 and D2O for [1(µ-H)H]+ (Scheme 5). The strong acidic character of the terminal thiol proton facilitates the fast H+/D+ exchange from D2O or CD3OD. The acidic SD(H) site adjacent to the Fe-H-Fe site allows intramolecular SD/Fe-H-Fe exchange and a dynamic process via a η2-HD complex as the intermediate state is postulated. Replacing η2-HD by H2 leads to release of HD and recovery of [1(µ-H)H]+. Intramolecular isotopic exchange studies indicate the conversion of [1(µ-H)D]+ to [1(µ-D)H]+ is feasible. The thiol in [1(µ-D)H]+ exchanges with D2O to provide [1(µ-D)D]+, which reacts with H2 resulting in formation of D2 and [1(µ-H)H]+. The main challenge for reactions or catalysis based on bridging diiron hydrides is to create an open site for the substrate binding.59c Recalling the H/D exchanges based on [(µH)Fe2(pdt)(PMe3)2(CO)4]+ reported by Darensbourg et al.85, photolysis induces the CO ligand to dissociate from the diiron center creating an open site for H2 or D2 binding. The H/D exchanges were suggested through deprotonation of (η2-H2)FeHFe intermediate by the internal hydride base, or by external water.85b In comparison, cooperative “diiron-terminal thiolate” reactivity enables [1(µ-H)H]+ to catalyze such H/D exchanges in the absence of light. Reversible H2 activation by [NiFe]-hydrogenases via η2-H2 intermediates is widely predicted by DFT studies. The key feature of such activation is heterolytic cleavage of H2 through a conjunction of vacant metal sites and basic terminal thiolate sites with the H2 molecule. Such a thiol-hydride species is difficult to establish experimentally.33 Aside from the FeFe platform, H/D exchange reactions probed at the metal-thiolate centers of [1(µ-H)H]+ provide the insights that have been envisioned for the η2-H2 complex.

Experimental Section


20

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General Information. Unless otherwise noted, all the experimental methods followed those recently

described.58

Fe2(pdt)(dppbz)(CO)4,77

1,2-Cy2PC6H4SH89

and

[(µ-

H)Fe2(edt)(PMe3)2(CO)4]+ 85 were prepared according to published procedures. 1(µ-H). In a 50-mL Pyrex Schlenk tube, Fe2(pdt)(dppbz)(CO)4 (200 mg, 0.26 mmol) and 1,2Cy2PC6H4SH (80 mg, 0.26 mmol) were dissolved in toluene (30 mL). The solution was irradiated with a 365 nm LED array (15 W) until the conversion, as monitored by IR spectroscopy was complete (∼ 8 h). Then the solvent was removed under vacuum and the crude product was extracted with toluene (5 mL). Recrystallization from toluene and hexane give the product as dark crystals. Yield: 190 mg, 71%. 1H NMR (CD2Cl2, 500 MHz): δ 7.66−6.78 (m, 28H, 28 × ArH), 2.74−0.86 (m, 28H, SCH2CH2CH2S, P(C6H11)2), -12.88 (td, JP-H = 23.2, 9.6 Hz, 1H, Fe-H-Fe). 31P NMR (CD2Cl2, 202 MHz): δ 92.42 (s), 85.42−85.11 (m). FT-IR (CH2Cl2): νCO 1930 cm−1. ESI-MS: calcd for 1(µ-H), 1026.1432; found, 1026.1408. Anal. Calcd for C53H57Fe2O2P3S3 (found): C, 62.00 (62.35); H, 5.60 (5.76). [1(µ-H)H]BF4. At room temperature, HBF4·Et2O (16 µL, 1.1 equiv) was added to a solution of 1(µ-H) (100 mg, 0.10 mmol) in CH2Cl2 (20 mL). After 5 min, IR spectroscopy indicated the reaction was complete. After removal of solvent, the product was washed with Et2O, and then recrystallized from CH2Cl2/hexane at or below -30 °C. Yield: 103 mg, 92%. 1H NMR (CD2Cl2, 500 MHz): δ 7.84−7.28 (m, 28 × ArH), 3.68 (broad, 1H, SH), 2.94−0.86 (m, 28H, SCH2CH2CH2S, P(C6H11)2), -14.32 (broad, 1H, Fe-H-Fe). 31P NMR (CD2Cl2, 202 MHz): δ 89.30 (s), 84.34 (d, JP-P = 19.4 Hz), 83.71 (d, JP-P = 19.3 Hz). FT-IR (CH2Cl2): νCO 1958 cm−1. Anal. Calcd for C53H58BF4Fe2O2P3S3 (found): C, 57.11 (56.89); H, 5.25 (5.34).


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[1(µ-H)H]BArF4. The synthesis of [1(µ-H)H]BArF4 followed the procedures used for [1(µH)H]BF4, except that one equiv of H(OEt2)2BArF4 was used as the proton source. After removal of solvent, the product was recrystallized from CH2Cl2/hexane under -30 °C. Yield: 170 mg, 90%. 1H NMR (CD2Cl2, 500 MHz): δ 7.88−7.27 (m, 40 × ArH), 3.69 (broad, 1H, SH), 2.95−0.89 (m, 28H, SCH2CH2CH2S, P(C6H11)2), -14.35 (broad, 1H, Fe-H-Fe).

31

P NMR (CD2Cl2, 202

MHz): δ 89.41 (s), 84.25 (d, JP-P = 18.8 Hz), 83.71 (d, JP-P = 18.8 Hz). FT-IR (CH2Cl2): νCO 1958 cm−1. [1(µ-H)(Me)]BF4. 1(µ-H) (41 mg, 0.04 mmol) in CH2Cl2 (4 mL) was treated with one equivalent of [Me3O]BF4 (6 mg, 0.04 mmol). IR spectroscopy indicated that the reaction was complete in 15 min. After removal of the solvent, the product was recrystallized from CH2Cl2/Et2O. Yield: 43 mg, 91%. 1H NMR (CD2Cl2, 500 MHz): δ 7.78−7.20 (m, 28 × ArH), 2.93−0.84 (m, 31H, SCH3, SCH2CH2CH2S, P(C6H11)2), -14.15 (td, JP-H = 20.4, 9.6 Hz, 1H, FeH-Fe). 31P NMR (CD2Cl2, 202 MHz): δ 86.21 (s), 82.40 (d, JP-P = 20.4 Hz), 75.86 (d, JP-P = 20.4 Hz). FT-IR (CH2Cl2): νCO 1955 cm−1. ESI-MS: calcd for [1(µ-H)(Me)]+, 1041.1667; found, 1041.1642. Anal. Calcd for C54H60BF4Fe2O2P3S3 (found): C, 57.47 (57.27); H, 5.36 (5.45). [1(CO)]BF4. A solution of [1(µ-H)H]BF4 (80 mg, 0.072 mmol) in CH2Cl2 (15 mL) was irradiated by a 410 nm LED array (48 W) under an N2 atmosphere. After 10 min, the photolysis was terminated, and CO gas was bubbled into the reaction solution for 10 min. The solvent was removed under vacuum and the product was extracted into CH2Cl2 (3 mL), and then hexane was layered on top. Crystallization at -30 oC provided [1(CO)]BF4 as brown minor crystals (yield: 78 mg, 92%). 1H NMR (CD2Cl2, 500 MHz): δ 7.75−5.61 (m, 28 × ArH), 2.95−0.89 (m, 28H, SCH2CH2CH2S, P(C6H11)2). 31P NMR (202 MHz, CD2Cl2) δ 90.95 (s), 78.86 (d, JP-P = 28.7 Hz),


22

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73.73 (d, JP-P = 28.8 Hz). FT-IR (CH2Cl2):νCO 2023, 1970 cm−1. ESI-MS: calcd for [1(CO)]+, 1053.1303; found, 1053.1277. Anal. Calcd for C54H56BF4Fe2O2P3S3 (found): C, 56.86 (56.66); H, 4.95 (5.05). H/D Exchange between [1(µ-H)H]+ and D2. A solution of [1(µ-H)H]+ (0.015 mmol) in CD2Cl2 (0.6 mL) was added to a J. Young NMR tube. The solution was gently degassed outside a glovebox. The tube was then immersed in a liquid nitrogen bath and pressurized with D2 (15 psi). After the tube was resealed, it was shaken to mix the D2 gas into the solution. The tube was then put in the dark at room temperature for 6 h, and then the 1H NMR spectrum was recorded. For the study of [1(µ-D)D]+, sample was prepared in CH2Cl2 and the 2H NMR spectra was recorded after 6 h. H/D Exchange between [1(µ-H)H]+ and D2O (or CD3OD). A 20-fold excess of D2O (5.2 µL, 0.26 mmol) or CD3OD (12 µL, 0.26 mmol) was added to a CD2Cl2 solution (0.6 mL) of [1(µH)H]+ (0.013 mmol) and [(µ-H)Fe2(edt)(PMe3)2(CO)4]+ (0.013 mmol) in a J. Young NMR tube. The tube was sealed immediately and frozen in a liquid nitrogen bath. The solution was mixed well and placed immediately in an NMR probe that was pre-adjusted to 298 K. The reaction was monitored by 1H NMR spectroscopy, which showed the hydride signal to be gradually diminishing. H/D Exchange between [1(µ-D)D]+ and CH3OH. The procedures are the same as were used for the H/D exchange between [1(µ-H)H]+ and CD3OD. The reaction was monitored by 1H NMR spectra in which the growth of the hydride signal was observed.


23


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Isotopic Exchange of [1(µ-D)H]+. One equiv of H(OEt2)2BArF4 (14.8 mg, 0.015 mmol) was carefully added to a CD2Cl2 (0.6 mL) solution of 1(µ-D) (15 mg, 0.015 mmol) and [(µH)Fe2(edt)(PMe3)2(CO)4]+ (0.015 mmol) in a J. Young NMR tube. The tube was immediately taken out of the glovebox and frozen in a liquid nitrogen bath. After it was allowed to warm up to room temperature, it was shaken to mix up all the components and placed in an NMR probe that was pre-adjusted to 298 K. The reaction was monitored by 1H NMR spectra, in which the hydride signal increased gradually. General Procedure for Catalysis H/D Scrambling. In a typical experiment, CD3OD (0.22 mmol) was injected under N2 atmosphere into a CD2Cl2 (0.6 mL) solution of [1(µ-H)H]+ (5.3 µmol) in a J. Young NMR tube. The tube was then immersed in a liquid nitrogen bath and degassed gently. Subsequently, the reaction mixture was pressurized with H2 (15 psi). The tube was resealed and was well shaken to mix the components. The tube was put in a dark cabinet at room temperature for 6 h, and then 1H NMR spectra were recorded. For the identification of deuterium signals, the experiments were conducted in CH2Cl2 and 2H NMR spectra were recorded after 6 h. ASSOCIATED CONTENT Supporting Information. NMR and IR spectra, and crystallographic data ((CCDC 18525161852520) in CIF format. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge “Thousand Plan” Youth Program and the National Natural Science Foundation of China (21402107 and 91427303) for the funding supported. We also thank Prof. Di Sun for assistance with the X-ray crystallography. REFERENCES (1) Greenwood, N. N.; Earnshaw, A.; Chemistry of the Elements, 2nd Ed. Oxford; Boston: Butterworth-Heinemann, 1997; pp 645−746. (2) Lamers, L. P. M.; van Diggelen, J. M. H.; Op den Camp, H. J. M.; Visser, E. J. W.; Lucassen, E. C. H. E. T.; Vile, M. A.; Jetten, M. S. M.; Smolders, A. J. P.; Roelofs, J. G. M. Front. Microbiol. 2012, 3, 1−12. (3) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245−2274. (4) Shafaat, H. S.; Rüdiger, O.; Ogata, H.; Lubitz, W. Biochim. Biophys. Acta, Bioenerg. 2013, 1827, 986−1002. (5) Kovacs, J. A.; Brines, L. M. Acc. Chem. Res. 2007, 40, 501−509. (6) Ash, P. A.; Hidalgo, R.; Vincent, K. A. ACS Catal. 2017, 7, 2471−2485. (7) Ogata, H.; Nishikawa, K.; Lubitz, W. Nature 2015, 520, 571−574. (8) Evans, R. M.; Brooke, E. J.; Wehlin, S. A. M.; Nomerotskaia, E.; Sargent, F.; Carr, S. B.; Phillips, S. E. V.; Armstrong, F. A. Nat. Chem. Biol. 2015, 12, 46−50. (9) Frey, M. ChemBioChem 2002, 3, 153−160. (10) Ohki, Y.; Yasumura, K.; Kuge, K.; Tanino, S.; Ando, M.; Li, Z.; Tatsumi, K. Proc. Natl. Acad. Sci. 2008, 105, 7652−7657.


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