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H2 Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex Demyan E. Prokopchuk, Geoffrey M. Chambers, Eric D. Walter, Michael T. Mock, and R. Morris Bullock J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Journal of the American Chemical Society

H2 Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex Demyan E. Prokopchuk,‡,† Geoffrey M. Chambers,‡ Eric D. Walter,§ Michael T. Mock,‡ and R. Morris Bullock*,‡ Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, Richland, WA 99352, United States § Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, United States ‡

ABSTRACT: While diamagnetic transition metal complexes that bind and split H 2 have been extensively studied, paramagnetic complexes that exhibit this behavior remain rare. The square planar S = ½ FeI(P4N2)+ cation (FeI+) reversibly binds H2/D2 in solution, exhibiting an inverse equilibrium isotope effect of KH2/KD2 = 0.58(4) at -5.0 °C. In the presence of excess H2, the dihydrogen complex FeI(H2)+ cleaves H2 at 25 °C in a net hydrogen atom transfer reaction, producing the dihydrogen-hydride trans-FeII(H)(H2)+. The proposed mechanism of H2 splitting involves both intra- and intermolecular steps, resulting in a mixed first- and second-order rate law with respect to initial [FeI+]. The key intermediate is a paramagnetic dihydride complex, trans-FeIII(H)2+, whose weak FeIII-H bond dissociation free energy (calculated BDFE = 44 kcal/mol) leads to bimetallic H-H homolysis, generating trans-FeII(H)(H2)+. Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations support the proposed mechanism.

The coordination and reactivity of H2 ligands in diamagnetic transition metal complexes has been intensely studied for decades.1 Recent efforts in sustainable energy have focused on using dihydrogen (H2) or protons/electrons (H+/e-) as energy carriers that are interconverted using molecular electrocatalysts.2 The thermodynamic bias for electrocatalytic H2 production or the reverse reaction, H2 oxidation, can be strongly influenced by modification of the ligand. Reactivity of these diamagnetic dihydrogen complexes is easily probed by NMR spectroscopy.3 Reactions of paramagnetic metal complexes with H2 have important implications for biological processes involving hydrogenase4 and nitrogenase5 enzymes, yet detailed reports of H2 binding/splitting with paramagnetic complexes are rare.1a,6 H2 binding to paramagnetic metal complexes has been postulated in a few instances,7 and three paramagnetic H2 complexes have been thoroughly characterized using advanced EPR spectroscopic techniques (Figure 1A-B).8 The S = ½ FeI(η2-H2)(SiPiPr3) and Co0(η2-H2)(BPiPr3) complexes (A) reversibly bind H2. The recently reported complex [FeI(H2)(depe)2]+ (B) reacts irreversibly with H2 to generate trans-[FeII(H)(H2)(depe)2]+. In terms of H2 cleavage by open-shell complexes, early kinetic studies of [CoII(CN)5]3- 9 and Co2(CO)810 provide evidence for a bimetallic transition state, [M---H---H---M]‡, where H2 homolysis furnishes diamagnetic metal hydride products.11 For example, the porphyrin complex RhII(TMP) (TMP = tetramesitylporphyrinato) cleaves H2 to generate diamagnetic RhIIIH(TMP), as demonstrated by clean second-

Figure 1. Selected paramagnetic M-H2 complexes (A, B) and terminal M-H complexes (C, D).

order kinetics with respect to [Rh].11d,11e Reduction of the three-coordinate complex LtBuFeIICl (L = bulky βdiketiminate ligand) by 1 equiv. KC8 followed by exposure to H2 yields the bridging dihydride complex (LtBuFeIIH)2,7e which is in equilibrium with the terminal hydride complex LtBuFeIIH.12 However, a discrete dihydrogen complex was not observed in any of these cases. Open-shell terminal metal hydride complexes are often unstable at room temperature, with M-H cleavage occurring by release of a proton or hydrogen atom.6a,13 In some instances, electrochemically induced hydrogen atom loss has been observed, producing H2 and diamagnetic products (Figure 1C-D).14 The anion RuIIH(OEP)- (C)14a,14b is electrochemically oxidized to generate an open-shell metal hydride complex that undergoes second-order M-H homolysis to release H2. Recently, the thermally sensitive S = ½ FeIIIH(N2)(PPSi-thiolate) complex (D) was

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characterized, with kinetic studies of H2 loss revealing a second-order dependence on [Fe].14c Notwithstanding the examples of open-shell dihydrogen or hydride complexes mentioned above, the binding of H2 and its reactivity in paramagnetic complexes remains poorly understood. We previously reported the relationship between N2 binding affinity, metal oxidation state, and catalytic N2 silylation activity for a series of [Fen(P4N2)]n+ complexes (n = 0, 1, 2), where P4N2 is a tetradentate ligand.15 We now report studies of H2 binding and splitting at d7 [FeI(P4N2)]+ (FeI+) using kinetic, thermodynamic, spectroscopic, and computational studies. Our data support a mechanism involving intramolecular H2 cleavage at FeI and net hydrogen atom transfer via bimetallic H-H coupling at FeIII to generate trans-[FeII(H)(H2)(P4N2)]+ (transFeII(H)(H2)+). Exposing deep purple [FeI(P4N2)][B(C6F5)4] (FeI+)15 to H2 or D2 (1 atm) in fluorobenzene results in an equilibrium with the S = ½ adduct FeI(H2)+ (FeI(D2)+). Monitoring the temperature-dependent equilibria by UV-vis spectroscopy from 268-288 K (Figures 2A and S17) leads to a van’t Hoff analysis of FeI+/FeI(H2)+ (ΔG268 = -0.20(7) kcal/mol, ΔH = 2.48(7) kcal/mol, ΔS = -8.5(2) cal/mol･K) and FeI+/FeI(D2)+ (ΔG268 = -0.49(2) kcal/mol, ΔH = -5.49(2) kcal/mol, ΔS = 18.6(5) cal/mol･K; Figure 2B). Binding of D2 is thermodynamically more favorable than H2 (ΔΔG268 = 0.29(7) kcal/mol, ΔΔH = -3.01(7) kcal/mol, ΔΔS = -10.1(5) cal/mol･K), resulting in an inverse equilibrium isotope effect (EIE); KH2/KD2 = 0.58(4) at 268K. While inverse EIEs are well-established for diamagnetic H2 complexes,1b,16 this is the first reported EIE for H2 binding to a paramagnetic dihydrogen complex. Cooling a purple solution of FeI+ under H2 or D2 below 268K results in a color change to pale yellow, which we attribute to the exclusive formation of FeI(H2)+ (FeI(D2)+), based on the thermodynamic data. Consistent

Scheme 1. Reactivity of Fe complexes with H2.

with the assignment of H2 or D2 binding at Fe, experimental EPR spectra were simulated by changing only the coupling constant found for the FeI(H2)+ spectrum (AH = 35 MHz) and dividing by the gyromagnetic ratios to obtain γH/γD ≅ 6.51,17 yielding AD = 5.4 MHz for FeI(D2)+ (Figure 2C).18 Warming a fluorobenzene solution of FeI+/FeI(H2)+ above 288K under H2 slowly and irreversibly produces the d6 dihydrogen-hydride complex, trans-FeII(H)(H2)+ (Scheme 1). This reaction constitutes an unusual net hydrogen atom transfer reaction from H2 to FeI(H2)+. The 1H NMR spectrum of trans-FeII(H)(H2)+ at 25 °C shows characteristic resonances for the η2-H2 ligand at -10.49 ppm (T1 = 26 ms, 298K) and hydride at -15.53 ppm (T1 = 513 ms, 298K), akin to the trans-M(H)(H2)(diphosphine)2 (M = Fe, Ru, Os) family of complexes.19 The dihydrogen and hydride sites of transFeII(H)(H2)+ are fluxional in solution, undergoing intramolecular chemical exchange that can be observed by isotopic labeling experiments (Figure S11). The coupling constant JHD = 31.5 Hz for coordinated HD was used to calculate an H-H distance of 0.89 Å in transFeII(H)(H2)+.1a,1d,20 The H2 ligand in trans-FeII(H)(H2)+ is stable under argon, but N2 (1 atm) readily displaces H2, yielding trans-FeII(H)(N2)+ (see SI).

Figure 2. A: Binding equilibrium of H2/D2 to FeI+ with equilibrium constants KH2 and KD2. B: van’t Hoff plot for H2 (red circles) and D2 (blue squares). C: Experimental (black) and simulated (red) X-band EPR spectra (2-MeTHF glass) of FeI(H2)+ (80K) and FeI(D2)+ (90K).


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Journal of the American Chemical Society Kinetic data for the reaction of FeI+ and H2 to generate trans-FeII(H)(H2)+ were obtained by monitoring the decay of FeI+ (λmax = 559 nm) by UV-vis spectroscopy under 1 atm H2 over several days. If H2 cleavage occurs through bimetallic H2 homolysis, a second-order dependence on [FeI+] would be expected; however, the data adheres to neither first- nor second-order kinetics (Figure S19). A loglog plot of the kinetic runs reveals a change in slope during the course of the reaction, which is characteristic of a mixed-order reaction where the second-order kinetic term is dominant at higher [FeI+] and the first-order term is prevalent at lower [FeI+] (Figure 3, top).11b,21 This kinetic behavior is analyzed by taking into account the total concentration of FeI+ and FeI(H2)+ ([FeI]tot) according to eq. 1.

Figure 3. Top: Log-Log plot of the kinetics of FeI+ under H2, showing the change in slope (reaction order) as [FeI+]tot decreases at longer reaction times (total reaction time of 114 h). Bottom: Kinetics to 75% completion under H2, fit to 𝐴 −𝐴 a mixed order integrated rate law where α = 𝑙𝑛 ( 𝑡 ∞ ) + 𝑘1 𝑡 (see eq. 1 and SI ). −

𝑑[𝐹𝑒 𝐼 ]𝑡𝑜𝑡 𝑑𝑡

= 𝑘1 [𝐹𝑒 𝐼 ]𝑡𝑜𝑡 + 𝑘2 [𝐹𝑒 𝐼 ]2𝑡𝑜𝑡

𝐴0 −𝐴∞

(1)

An excellent fit is achieved by plotting eα vs. Absorbance (Figure 3, bottom). The observed rate constant k1 is obtained by plotting the kinetic data as first-order dependent and measuring the slope at low [FeI+]tot (>75% completion), which gives k1 = 5.0(7) × 10-6 s-1. Analysis of this data using the integrated rate law gives the observed second-order rate constant k2 = 6(1) × 10-2 M-1 s-1 (see SI for derivation). Experiments with D2 follow clean first-order kinetics (k1D = 1.8 × 10-6 s-1), indicating that the first-order process is significantly slower than the second-order process under these conditions. Thus, a KIE of k1H/k1D = 2.8 is observed for the first-order process with respect to [FeI+]tot.

After formation of FeI(H2)+, we rationalized that the firstorder kinetic term could involve either pendant amineassisted H2 heterolysis2b or oxidative addition to generate a d5 FeIII(H)2+ intermediate (Figure 4A). DFT calculations indicate that oxidative addition is energetically preferred, proceeding through a proposed dihydride intermediate cisFeIII(H)2+ with ΔG‡calc = 22.7 kcal/mol. The computed barrier is consistent with our kinetic data, where ΔG‡exp(k1) = 24.7(1) kcal/mol, calculated using conventional transition state theory. Calculations also indicate that heterolytic H2 cleavage, forming an Fe-H bond and protonating the pendant amine to form cis-FeI(H)(NH)+ is possible (ΔG‡calc = 18.9 kcal/mol), but an energetically viable pathway to afford trans-FeIII(H)2+ was not found in our computations (Figure S22). Instead, intramolecular rearrangement of the cis-dihydride intermediate yields the intermediate transFeIII(H)2+. The wide bite angle in FeI+ (Ph2P-Fe-PPh2 = 108.71(3)°)15 may also facilitate intramolecular hydride rearrangement without dissociation of a Fe-P bond. The calculated k1H/k1D = 2.4 for oxidative addition (Table S6) is in good agreement with the aforementioned experimental KIE.

Figure 4. A: Computed mechanistic pathways to generate trans-FeIII(H)2+ with computed free energies relative to FeI(H2)+. B: Synthetic procedure yielding FeIII(H)2+. C: Molecular structure of FeII(H)2 with 50% probability ellipsoids; most H atoms are not shown. D: Experimental (black) and simulated (red) X-band EPR spectra (PhF, 105K) of trans-FeIII(H)2+.

We sought experimental support for the proposed d5 intermediate trans-FeIII(H)2+. The diamagnetic dihydride complex, trans-FeII(H)2, was easily prepared in 97% yield by exposing the d8 complex Fe0(N2)15 to 1 atm H2 in THF (Scheme 1 and Figure 4C). Cyclic voltammograms of transFeII(H)2 in fluorobenzene indicate that trans-FeIII(H)2+ is stable on the CV timescale, with E1/2(III/II) = -0.34 V vs. Cp2Fe0/+ at scan rates >30 V/s (Figure S15). Monitoring the chemical oxidation of trans-FeII(H)2 by EPR spectroscopy supports the formation of a thermally sensitive low spin d5 complex (g = 2.001, 2.079, 2.198)22 that we assign to trans-



FeIII(H)2+ (Figures 4B and 4D). Therefore, the combined computational, kinetic, and spectroscopic data suggest that the first-order step in the H2 cleavage step involves formation of trans-FeIII(H)2+ by oxidative addition at FeI. We then turned our attention to understanding the second-order rate term that forms trans-FeII(H)(H2)+. When chemical oxidation of trans-FeII(H)2 is performed at 298K in a sealed tube under 1 atm H2, trans-FeII(H)(H2)+ is generated in 69% yield (Scheme 1, right). This observation provides compelling evidence that oxidation to generate trans-FeIII(H)2+ triggers bimetallic H-H coupling in the presence of excess H2 to generate trans-FeII(H)(H2)+. The calculated weak Fe-H bond dissociation free energy (BDFE) of trans-FeIII(H)2+ (44 kcal/mol) indicates spontaneous H2 release is thermodynamically favored (see SI). The FeIII/II redox couple is only reversible at fast scan rates (>30 V/s), however, which is inconsistent with rate constant k2. We suggest that a rapid electrochemically induced sidereaction produces an unknown intermediate, precluding us from obtaining meaningful kinetic information via electrochemical analysis (Figures S15, S16).14b,23 The proposed mechanism of H2 cleavage is shown in Scheme 2. Initially, dihydrogen coordination through the FeI+/FeI(H2)+ equilibrium generates the oxidative addition product, trans-FeIII(H)2+, with rate constant k1. Next, the mechanism could bifurcate into kinetically indistinguishable H atom transfer routes for the formation of trans-FeII(H)(H2)+ via rate constant k2, one of which has been discussed above and is shown in Scheme 2 (right). The second mechanism considered involves comproportionation through bimetallic hydrogen atom transfer from trans-FeIII(H)2+ to FeI(H2)+ (Scheme 2, left). To test this hypothesis, we treated trans-FeII(H)2 with Cp2Fe+ for 3 min at 25 °C under H2 to generate transFeIII(H)2+ in situ, followed by addition of FeI+ (forming FeI(H2)+ in situ) dissolved in fluorobenzene (Scheme 1, middle). Formation of trans-FeII(H)(H2)+ occurs in 65% yield, indicating that both bimetallic mechanistic pathways Scheme 2. Proposed mechanism of H2 splitting by FeI+. KH2 FeI+

H H

+

+

H

k1

+

H N

P P

=

FeIII

FeI

Ph

H

Ph

N

FeIII H

2+

HFe H H FeH

(H2)FeIIH+ +

FeIIH+

+ H2

H

FeII H H

+

k3 k-3

H H

FeII H

+

Ph

The Supporting Information is available free of charge on the ACS Publications website. Experimental details, syntheses, spectra (NMR, IR), DFT structures and Cartesian coordinates (PDF) Crystallographic data (CIF)

AUTHOR INFORMATION †Current

address: Department of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102, United States  Current address: Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717, United States

Corresponding Author

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

k2 (H2)Fe H FeH

Supporting Information

Notes

FeIII(H)2+

2+

ASSOCIATED CONTENT

* [email protected]

PPh2 PPh2

BDFEFeH = 44 kcal/mol FeI(H2)+

(rate constant k2) appear to be possible. After bimetallic hydrogen atom transfer, one equivalent of transFeII(H)(H2)+ is produced, and H2 coordinates to the unobserved FeIIH+, generating the final product, transFeII(H)(H2)+. To further probe the intramolecular dihydrogen-hydride exchange in trans-FeII(H)(H2)+ mentioned earlier, high temperature 1H-1H-EXSY NMR experiments on transFeII(H)(H2)+ at 100 °C in C6D5Cl confirm the presence of chemical exchange cross-peaks with a rate constant k3(k-3) of ca. 7 s-1 (ΔG‡373 = 20 kcal/mol; Table S1). DFT calculations indicate that this process involves a transient sevencoordinate FeIV(H)3+ cation24 with a calculated free energy barrier of 20.5 kcal/mol for oxidative addition, in excellent agreement with experiment. (Scheme 2 and Figure S23). Complementary kinetic, spectroscopic, electrochemical, and computational evidence provide strong support for the mechanism of H2 cleavage at a paramagnetic FeI complex. Detailed kinetic analysis reveals a mixed first- and secondorder rate law that involves reversible dihydrogen coordination at FeI, FeI/FeIII oxidative addition of H2, and net hydrogen atom transfer involving an observable FeIII transdihydride intermediate with a weak Fe-H BDFE. We hope that these results provide a foundation for discovery of new reactivity of paramagnetic H2 complexes.

+

H N

via

P P

FeIV

N

H

PPh2 H PPh2

Ph DG‡exp = 20.0 kcal/mol DG‡calc = 20.5 kcal/mol

This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. EPR experiments were performed using Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. DOE. Computational resources were provided by the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory. We thank Dr. Thilina Gunasekara and Dr. Andrew Preston for helpful discussions on the analysis of the kinetics. This work is dedicated to the memory of Prof. Jack Halpern


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Journal of the American Chemical Society (1925-2018), whose ground-breaking studies of H2 and metal hydrides provided remarkable insights.

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