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

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H2 Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex 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,‡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 homolysis, 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 ligands 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 ligand. 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 I+ I + Fe /Fe (D2) (ΔG268 = -0.49(2) kcal/mol, ΔH = -5.49(2) 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. % &'( # )*) 0 0 $ − = � �� + � �� (1) $ %+ # +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). I+ I + Warming a solution of Fe /Fe (H2) in fluorobenzene above 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) 8)98: 2 integrated rate law where α = �� 67 < + 2�#�? (see eq.
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