H2 Binding, Splitting, and Net Atom Transfer at a Paramagnetic Iron Complex

Demyan E. Prokopchuk,‡,† Geoffrey M. Chambers,‡ Eric D. Walter,§ Michael T. Mock,‡à 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: The reactivity of H2 with abundant transition metals is crucial for developing catalysts for energy storage in chemical bonds. While diamagnetic transition metal complexes that bind and split H2 have been extensively studied, paramagnetic complexes I + I+ that exhibit this behavior remain rare. We describe the reactivity of a square planar S = ½ Fe (P4N2) cation (Fe ) that 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, I + the dihydrogen complex Fe (H2) cleaves H2 at 25 °C in a net hydrogen atom transfer reaction to give the dihydrogen-hydride cation II + trans-Fe (H)(H2) . The proposed mechanism of H2 splitting involves both intra- and intermolecular steps, resulting in a mixed first- I+ III + and second-order rate law with respect to initial [Fe ]. The key intermediate is a paramagnetic dihydride complex, trans-Fe (H)2 , whose weak FeIII-H bond dissociation free energy (calculated BDFE = 44 kcal/mol) leads to bimetallic H-H , generating II + trans-Fe (H)(H2) . Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations all support the proposed reaction mechanism.

The coordination and reactivity of H2 at diamagnetic transition metals 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 H production or the 2 Figure 1. Selected paramagnetic M-H complexes (A, B) and reverse reaction, H oxidation, can be controlled by 2 2 terminal M-H complexes (C, D). modification of the . Reactivity of these diamagnetic 3 tBu II dihydrogen complexes is easily probed by NMR spectroscopy. L Fe Cl (L = bulky β-diketiminate ligand) with KC8 followed Reactions of paramagnetic metal complexes with H2 have by exposure to H2 (1 atm) yields the bridging dihydride important implications for biological processes involving complex (LtBuFeIIH) ,7e which is equilibrium with the terminal 4 5 2 hydrogenase and nitrogenase enzymes, yet detailed reports of hydride complex LtBuFeIIH.13 However, a discrete dihydrogen 1a,6 H2 binding/splitting with paramagnetic complexes are rare. complex was not observed in any of these cases. H binding to paramagnetic metal complexes has been 2 Open-shell terminal metal hydride complexes are often postulated only in a few instances,7 and three paramagnetic H 2 unstable at room temperature, with M-H cleavage occurring by complexes have been isolated and thoroughly characterized 6a,14 I 2 i 0 2 proton or hydrogen atom release. In some instances, (Figure 1A-B). The S = ½ Fe (η -H2)(SiP Pr3) and Co (η - i 8 electrochemically induced H-atom loss has been observed, H2)(BP Pr3) complexes (A) reversibly bind H2. The recently 15 9 I + producing H2 and diamagnetic products (Figure 1C-D). The reported complex [Fe (H )(depe) ] (B) reacts irreversibly with II - 15a,15b 2 2 anion Ru H(OEP) (C) (OEP = octaethylporphyrinato) is H to generate trans-[FeII(H)(H )(depe) ]+, but the reaction 2 2 2 electrochemically oxidized to generate an open-shell metal mechanism has not been reported. hydride complex that undergoes second-order M-H homolysis In terms of H2 cleavage by open-shell complexes, early to release H2. Recently, the thermally sensitive S = ½ II 3- 10 11 III kinetic studies of [Co (CN)5] and Co2(CO)8 provide Fe H(N )(PPSi-thiolate) complex (D) was spectroscopically ‡ 2 evidence for a bimetallic transition state, [M---H---H---M] , and structurally characterized, with kinetic studies of H2 loss 15c where H2 homolysis furnishes diamagnetic metal hydride revealing a second-order dependence on [Fe]. products.12 For example, the porphyrin complex RhII(TMP) Notwithstanding the above examples of open-shell (TMP = tetramesitylporphyrinato) cleaves H to generate 2 dihydrogen or hydride complexes, the binding of H and its diamagnetic RhIIIH(TMP), as demonstrated by clean second- 2 reactivity in paramagnetic complexes remains poorly order kinetics with respect to [Rh];12d rate enhancements are understood. We previously reported the relationship between observed when using covalently tethered RhII porphyrin 12e N2 binding affinity, metal oxidation state, and catalytic N2 bimetalloradicals. Reacting the three-coordinate complex n n+ silylation activity with a series of [Fe (P4N2)] complexes (n =

a 0, 1, 2), where P4N2 is a tetradentate ligand containing an 8- Scheme 1. Reactivity of Fe complexes with H2. 16 membered ring and flanking PPh2 arms. We now present the I + mechanism of H2 binding and splitting at S = ½ [Fe (P4N2)] (FeI+) using kinetic, thermodynamic, spectroscopic, and computational studies. The data support a mechanism involving I intramolecular H2 cleavage at Fe and net hydrogen atom transfer via bimetallic H-H coupling at FeIII to generate trans- II + II + [Fe (H)(H2)(P4N2)] (trans-Fe (H)(H2) ). I I+ 16 Exposing deep purple [Fe (P4N2)][B(C6F5)4] (Fe ) to an atmosphere of H2 or D2 in fluorobenzene results in a temperature-dependent equilibrium with the S = ½ adduct I + I + Fe (H2) (Fe (D2) ). Monitoring the equilibria by UV-vis spectroscopy from 268-288 K (Figures 2 and S17) leads to a I+ I + van’t Hoff analysis of Fe /Fe (H2) (ΔG268 = -0.20(7) kcal/mol, ΔH = -2.48(7) kcal/mol, ΔS = -8.5(2) cal/mol･K) and FeI+/FeI(D )+ (ΔG = -0.49(2) kcal/mol, ΔH = -5.49(2) 2 268 kcal/mol, ΔS = -18.6(5) cal/mol･K). Binding of D2 is thermodynamically more favorable than H (ΔΔG = -0.29(7) 2 268 aThe B(C F ) - counteranion is not shown. kcal/mol, ΔΔH = -3.01(7) kcal/mol, ΔΔS = -10.1(5) cal/mol･ 6 5 4 K,), resulting in an inverse equilibrium isotope effect (EIE); ppm (T1 = 26 ms, 298 K) and hydride at -15.53 ppm (T1 = 513 KH2/KD2 = 0.58(4) at 268 K. While inverse EIEs are well- 1b,17 ms, 298 K), akin to the trans-M(H)(H2)(diphosphine)2 (M = Fe, established for diamagnetic H2 complexes, this is the first Ru, Os) family of complexes.20 The dihydrogen and hydride reported EIE for a paramagnetic dihydrogen complex. Cooling II + I+ sites of trans-Fe (H)(H2) are fluxional in solution, undergoing a purple solution of Fe under H2 or D2 below 268 K results in intramolecular chemical exchange that can be observed by a color change to pale yellow, which we attribute to the I + I + isotopic labeling experiments (Figure S11). The coupling exclusive formation of Fe (H2) (Fe (D2) ) based on the constant JHD = 31.5 MHz for coordinated HD was used to thermodynamic data. Consistent with the assignment of either II + 1a,1d,21 calculate an H-H distance of 0.89 Å in trans-Fe (H)(H2) . H2 or D2 binding at Fe, experimental EPR spectra are simulated II + I + The H2 ligand in trans-Fe (H)(H2) is stable under argon, but by changing only the coupling constant found for the Fe (H2) II + N2 (1 atm) readily displaces H2, yielding trans-Fe (H)(N2) spectrum (A1H = 35 MHz) and dividing by the gyromagnetic 18 (see SI). ratios to obtain γH/γD ≅ 6.51, yielding A2D = 5.4 MHz for I+ Kinetic data for the reaction of Fe and H2 to generate trans- II + I+ Fe (H)(H2) were obtained by monitoring the decay of Fe (λmax = 559 nm) by UV-vis spectroscopy under 1 atm H2 over several days. If H2 cleavage occurs through bimetallic H2 homolysis, a clean second-order dependence on [FeI+] would be observed; however, the dataset adheres to neither first- nor second-order kinetics (Figure S19). A log-log 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 low [FeI+] (Figure 3, left).12b,22 This kinetic behavior is modelled by taking into account the I+ I + I total concentration of Fe and Fe (H2) ([Fe ]tot) according to eq. 1. − = � �� + � �� (1)

I+ Figure 2. Top: Binding equilibrium of H2/D2 to Fe , and van’t Hoff plot for H2 (red circles) and D2 (blue squares). Bottom: Experimental (black) and simulated (red) X-band EPR spectra (2- I + I + MeTHF glass) of Fe (H2) and Fe (D2) at 80 K and 90 K, - respectively. The B(C6F5)4 counterion is not shown.

I + 19 Fe (D2) (Figure 2). Warming a solution of FeI+/FeI(H )+ in fluorobenzene above 2 288 K under H2 slowly and irreversibly produces the II + dihydrogen-hydride complex, trans-Fe (H)(H2) , in 98% I+ Figure 3. Left: Log-Log plot of the kinetics of Fe under H2, isolated yield (Scheme 1). This unusual reaction constitutes a I+ I + showing the change in slope (reaction order) as [Fe ]tot decreases net hydrogen atom transfer reaction from H2 to Fe (H2) , and at longer reaction times (total reaction time of 114 h). Right: the mechanism will be discussed in detail below. At room Kinetic plot to 75% completion under H , fit to a mixed order 1 II + 2 temperature, the H NMR spectrum of trans-Fe (H)(H2) 2 integrated rate law where α = �� + 2�� (see eq. 1 and shows characteristic resonances for the η -H2 ligand at -10.49 SI for details).

We sought experimental support for the proposed α III + An excellent fit is achieved when plotting variable e vs. intermediate trans-Fe (H)2 . The diamagnetic dihydride II Absorbance (Figure 3, right). The observed rate constant k1 is complex, trans-Fe (H)2, was easily prepared in 97% yield by 0 16 obtained by plotting the kinetic data as first-order dependent reacting Fe (N2) in THF under H2 (Scheme 2 and Figure 3C). I+ II and measuring the slope at low [Fe ]tot (>75% completion), Cyclic voltammograms of trans-Fe (H)2 in fluorobenzene -6 -1 III + which gives k1 = 2.5(4) × 10 s . Analysis of this data using the indicate that trans-Fe (H)2 is stable on the CV timescale, with 0/+ integrated rate law gives the observed second-order rate E1/2(III/II) = -0.34 V vs. Cp2Fe at scan rates greater than 30 -2 -1 -1 constant k2 = 2.8(7) × 10 M s (see SI for derivation). V/s (Figure S15). Monitoring the chemical oxidation of trans- II Experiments with D2 follow clean first-order kinetics (k1D = 9.2 Fe (H)2 by EPR spectroscopy indicates the formation of a × 10-7 s-1), indicating that the first-order process is significantly thermally unstable S = ½ complex with rhombic23 features in slower than the second-order process under these conditions. the EPR spectrum (g = 2.001, 2.079, 2.198) that we assign to III + Thus, a KIE of k1H/k1D = 3.0 is observed for the first-order trans-Fe (H)2 (Figures 3B and 3D). Therefore, the combined I+ process with respect to [Fe ]tot. computational, kinetic, and spectroscopic data suggest that the I + first-order step in the H2 cleavage step involves formation of After formation of Fe (H2) , we rationalized that the first- III + I order kinetic term could involve either pendant amine assisted trans-Fe (H)2 by oxidative addition at Fe . 2b III H2 or oxidative addition to generate a Fe (H)2 We then turned our attention to understanding the second- II + intermediate (Figure 3A). DFT calculations indicate that order rate term to form trans-Fe (H)(H2) . When chemical II oxidative addition is energetically preferred, proceeding oxidation of trans-Fe (H)2 is performed at 298 K in a sealed III + II + through a proposed dihydride intermediate cis-Fe (H)2 with tube under 1 atm H2, trans-Fe (H)(H2) is generated in 69% ‡ ΔG calc = 22.7 kcal/mol. The computed barrier is consistent with yield (Scheme 1, right). This observation provides compelling ‡ III + our kinetic data, where ΔG exp(k1) = 24.7(1) kcal/mol, calculated evidence that oxidation to generate trans-Fe (H)2 triggers using conventional transition state theory. Calculations also bimetallic H-H coupling in the presence of excess H2 to yield II + indicate that heterolytic H2 cleavage, forming an Fe-H bond and trans-Fe (H)(H2) . Furthermore, the calculated weak Fe-H I + III + protonating the pendant amine to form cis-Fe (H)(NH) is bond dissociation free energy (BDFE) of trans-Fe (H)2 (44 ‡ possible (ΔG calc = 18.9 kcal/mol), but an energetically viable kcal/mol) indicates spontaneous H2 release is III + III/II pathway to afford trans-Fe (H)2 was not found in our thermodynamically favored (see SI). However, the Fe redox computations (see SI). Instead, intramolecular rearrangement of couple is only reversible at fast scan rates (>30 V/s), which is III + the cis-dihydride intermediate yields trans-Fe (H)2 , with inconsistent with rate constant k2. We believe that a rapid I+ ΔGrxn = 4.9 kcal/mol. The wide bite angle in Fe (Ph2P-Fe-PPh2 electrochemically induced side-reaction produces an unknown = 108.71(3)°)16 may also facilitate intramolecular hydride intermediate, precluding us from obtaining meaningful kinetic rearrangement without dissociation of a Fe-P bond. information via electrochemical analysis (Figures S15, 15b,24 Furthermore, the calculated k1H/k1D = 2.4 for oxidative addition S16). (Table S6) is in good agreement with the aforementioned The proposed mechanism of H2 cleavage is shown in Scheme experimental KIE. I+ I + 2. Initially, dihydrogen coordination through the Fe /Fe (H2) equilibrium generates the oxidative addition product, trans- III + Fe (H)2 , with rate constant k1. Next, the mechanism could bifurcate into kinetically indistinguishable H atom transfer II + routes for the formation of trans-Fe (H)(H2) via rate constant I+ a Scheme 2. Proposed mechanism of H2 splitting by Fe .

Figure 3. A: Computed mechanistic pathways to generate trans- III + III + Fe (H)2 . B: Synthetic procedure yielding Fe (H)2 (the - B(C6F5)4 counteranion is not shown). C: Molecular structure of II Fe (H)2 with 50% probability ellipsoids; most H atoms are not shown. D: Experimental (black) and simulated (red) X-band EPR III + spectra (PhF, 105 K) of trans-Fe (H)2 . a - The B(C6F5)4 counteranion is not shown. k2, one of which has been discussed above and is shown below Fundamentals of H2 Binding and Reactivity on Transition Metals in Scheme 2 (right). The second mechanism involves Underlying Hydrogenase Function and H2 Production and Storage Chem. comproportionation via bimetallic hydrogen atom transfer from Rev. 2007, 107, 4152-4205; (c) Morris, R. H. Dihydrogen, dihydride and in III + I + between: NMR and structural properties of iron group complexes Coord. trans-Fe (H)2 to Fe (H2) (Scheme 2, left). To test this Chem. Rev. 2008, 252, 2381-2394; (d) Crabtree, R. H. Dihydrogen II + hypothesis, we reacted trans-Fe (H)2 with Cp2Fe for 3 min at Complexation Chem. Rev. 2016, 116, 8750-8769. III + 25 °C under H2 to generate trans-Fe (H)2 in situ, followed by (2) (a) Bullock, R. M.; Appel, A. M.; Helm, M. L. Production of hydrogen I+ I + by electrocatalysis: making the H-H bond by combining protons and addition of Fe (forming Fe (H2) in situ) dissolved in fluorobenzene (Scheme 1, middle). Formation of trans- hydrides Chem. Commun. 2014, 50, 3125-3143; (b) Bullock, R. M.; Helm, II + M. L. Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth- Fe (H)(H2) occurs in 65% yield, indicating that both Abundant Metals: Shoving Protons Around with Proton Relays Acc. Chem. bimetallic mechanistic pathways with rate constant k2 appear to Res. 2015, 48, 2017-2026; (c) Lee, K. J.; Elgrishi, N.; Kandemir, B.; be possible. After bimetallic hydrogen atom transfer, one Dempsey, J. L. Electrochemical and spectroscopic methods for evaluating II + equivalent of trans-Fe (H)(H2) is produced, and an equivalent molecular electrocatalysts Nat. Rev. Chem. 2017, 1, 0039; (d) Robinson, S. II + J. C.; Heinekey, D. M. Hydride & dihydrogen complexes of earth abundant of H2 coordinates to an unobserved Fe H cation, generating II + metals: structure, reactivity, and applications to catalysis Chem. Commun. the final product trans-Fe (H)(H2) . 2017, 53, 669-676; (e) Bellini, M.; Bevilacqua, M.; Marchionni, A.; Miller, To further probe the intramolecular dihydrogen-hydride H. A.; Filippi, J.; Grützmacher, H.; Vizza, F. Energy Production and II + Storage Promoted by Organometallic Complexes Eur. J. Inorg. Chem. exchange in trans-Fe (H)(H2) mentioned earlier, high temperature 1H-1H-EXSY NMR experiments on trans- 2018, 2018, 4393-4412. II + (3) (a) Yang, J. Y.; Bullock, R. M.; Shaw, W. J.; Twamley, B.; Fraze, K.; Fe (H)(H2) at 100 °C in C6D5Cl confirm the presence of DuBois, M. R.; DuBois, D. L. Mechanistic Insights into Catalytic H2 chemical exchange cross-peaks with a rate constant k3(k-3) of ca. Oxidation by Ni Complexes Containing a Diphosphine Ligand with a -1 ‡ 7 s (ΔG 373 = 20 kcal/mol; Table S1). DFT calculations Positioned Amine Base J. Am. Chem. Soc. 2009, 131, 5935-5945; (b) Liu, indicate that this process involves a transient seven-coordinate T.; Chen, S.; O'Hagan, M. J.; Rakowski DuBois, M.; Bullock, R. M.; FeIV(H) + cation25 with a calculated oxidative addition free DuBois, D. L. Synthesis, Characterization, and Reactivity of Fe Complexes 3 Containing Cyclic Diazadiphosphine Ligands: The Role of the Pendant energy barrier of 20.5 kcal/mol, which is in excellent agreement Base in Heterolytic Cleavage of H2 J. Am. Chem. Soc. 2012, 134, 6257- with experiment. (Scheme 2 and Figure S23). 6272; (c) Liu, T.; DuBois, D. L.; Bullock, R. M. An iron complex with Complementary kinetic, spectroscopic, electrochemical, and pendent amines as a molecular electrocatalyst for oxidation of hydrogen computational evidence provide strong support for the Nat. Chem. 2013, 5, 228--233; (d) Hulley, E. B.; Welch, K. D.; Appel, A. I M.; DuBois, D. L.; Bullock, R. M. Rapid, Reversible Heterolytic Cleavage mechanism of H2 cleavage at a paramagnetic Fe complex. of Bound H2 J. Am. Chem. Soc. 2013, 135, 11736-11739; (e) Hulley, E. B.; Detailed kinetic analysis reveals a mixed first- and second-order Helm, M. L.; Bullock, R. M. Heterolytic cleavage of H2 by bifunctional rate law that involves (1) reversible dihydrogen coordination at manganese(I) complexes: impact of ligand dynamics, electrophilicity, and FeI, (2) FeI/FeIII oxidative addition, and (3) net hydrogen atom base positioning Chem. Sci. 2014, 5, 4729-4741; (f) Zhang, S.; Appel, A. transfer involving an observable FeIII trans-dihydride M.; Bullock, R. M. Reversible Heterolytic Cleavage of the H–H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between intermediate with a weak Fe-H BDFE. These results on Proton and Hydride J. Am. Chem. Soc. 2017, 139, 7376-7387. reactivity of paramagnetic H2 complexes provide a foundation (4) (a) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Hydrogenases Chem. of new reactivity that will be expanded in future studies. Rev. 2014, 114, 4081-4148; (b) Schilter, D.; Camara, J. M.; Huynh, M. T.; Hammes-Schiffer, S.; Rauchfuss, T. B. Hydrogenase Enzymes and Their AUTHOR INFORMATION Synthetic Models: The Role of Metal Hydrides Chem. Rev. 2016, 116, † 8693-8749. Current address: Department of Chemistry, Rutgers University, (5) (a) Hoffman, B. M.; Lukoyanov, D.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, 73 Warren Street, Newark, NJ 07102, United States L. C. Mechanism of Nitrogen Fixation by Nitrogenase: The Next Stage à Current address: Department of Chemistry and Biochemistry, Chem. Rev. 2014, 114, 4041-4062; (b) Lukoyanov, D.; Yang, Z.-Y.; Montana State University, Bozeman, MT 59717, United States Khadka, N.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. Identification of a Key Catalytic Intermediate Demonstrates That Nitrogenase Is Activated Corresponding Author by the Reversible Exchange of N2 for H2 J. Am. Chem. Soc. 2015, 137, * [email protected] 3610-3615; (c) Lukoyanov, D.; Khadka, N.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. Reductive Elimination of H2 Activates Notes Nitrogenase to Reduce the N≡N Triple Bond: Characterization of the E4(4H) Janus Intermediate in Wild-Type Enzyme J. Am. Chem. Soc. 2016, The authors declare no competing financial interest. 138, 10674–10683. (6) (a) Hoff, C. D. Thermodynamic and kinetic studies of stable low valent ACKNOWLEDGMENTS transition metal radical complexes Coord. Chem. Rev. 2000, 206, 451-467; (b) Capps, K. B.; Bauer, A.; Kiss, G.; Hoff, C. D. The rate and mechanism This research was supported as part of the Center for Molecular of oxidative addition of H to the Cr(CO) C Me radical—generation of a Electrocatalysis, an Energy Frontier Research Center funded by the 2 3 5 5 model for reaction of H2 with the Co(CO)4 radical J. Organomet. Chem. U.S. Department of Energy (DOE), Office of Science, Office of 1999, 586, 23-30. Basic Energy Sciences. EPR experiments were performed using (7) (a) Baya, M.; Houghton, J.; Daran, J.-C.; Poli, R.; Male, L.; Albinati, Environmental Molecular Sciences Laboratory, a national A.; Gutman, M. Synthesis, Structure, and Electrochemical Properties of scientific user facility sponsored by the DOE’s Office of Biological Sterically Protected Molybdenum Trihydride Redox Pairs: A Paramagnetic and Environmental Research and located at Pacific Northwest “Stretched” Dihydrogen Complex? Chem. Eur. J. 2007, 13, 5347-5359; (b) National Laboratory (PNNL). PNNL is operated by Battelle for the Hetterscheid, D. 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