Mechanistic Insight into Switching between H2-or O2-Activation by

Oct 9, 2017 - O2-tolerant [NiFe]hydrogenases ([NiFe]H2ases) are bifunctional enzymes that catalyze the reduction of O2 to H2O in addition to their mai...
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Mechanistic Insight into Switching between H2- or O2‑Activation by Simple Ligand Effects of [NiFe]hydrogenase Models Takahiro Matsumoto,†,‡,§ Takahiro Kishima,†,‡ Takeshi Yatabe,†,‡,§ Ki-Seok Yoon,†,‡,§ and Seiji Ogo*,†,‡,§ †

Center for Small Molecule Energy, ‡Department of Chemistry and Biochemistry, Graduate School of Engineering, and §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: We present a mechanistic investigation for the activation of H2 and O2, induced by a simple ligand effect within [NiFe] models for O2-tolerant [NiFe]hydrogenase. Kinetic study reveals Michaelis−Menten type saturation behaviors for both H2 and O2 activation, which is the same behavior as that found in O2-tolerant [NiFe]hydrogenase. Such saturation behavior is caused by H2 complexation followed by heterolytic cleavage of H2 by an outer-sphere base, resulting in the formation of a hydride species showing hydridic character.

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INTRODUCTION O2-tolerant [NiFe]hydrogenases ([NiFe]H2ases) are bifunctional enzymes that catalyze the reduction of O2 to H2O in addition to their main role of the oxidation of H2 to 2H+ and 2e−.1−4 Due to the relevance of these reactions to a potential hydrogen economy, chemists have sought to elucidate this bifunctional mechanism with structurally simpler compounds.5−10 We have previously reported two series of [NiRu]-based H2ase models that demonstrate H2 or O2 activation depending on the nature of the Ru-coordinated ligand.5 The [NiRu] complexes show different activation mechanisms for H2 and O2. Whereas the binding of O2 shows the expected Michaelis− Menten type kinetics, the heterolytic activation of H2 is irreversible and therefore does not properly mirror the kinetics of H2 binding to O2-tolerant [NiFe]H2ases (see Table S1).4 As a result of our efforts to improve these [NiRu] models, we can now report two series of [NiFe]-based H2ase models by employing either trialkyl phosphite derived or Cp (cyclopentadienyl)-derived ligands. These complexes show Michaelis−Menten type kinetics for both H2 and O2 activation.5 Here, we report the mechanistic elucidation of heterolytic H−H bond cleavage with a Lewis base to form a hydride species and two-electron reduction of O2 to form a peroxo species. On the basis of kinetic investigation, we demonstrate that a Michaelis−Menten type binding of H2 originates from the interaction of an outer-sphere Lewis base and the Lewis acid metal center, resulting in the heterolytic cleavage of H2 to produce a hydridic ligand (see Table S1). We prepared two series of NiIIFeII complexes: trialkyl phosphite complexes [NiII(X)FeII(RCN)(L)](BPh4)2 [X = © XXXX American Chemical Society

N,N′-diethyl-3,7-diazanonane-1,9-dithiolato, R = Me, L = {P(OMe)3}3: 1a, {P(OEt)3}3: 1b, {P(OnBu)3}3: 1c] and the methyl-substituted cyclopentadienyl complexes [NiII(X)FeII(RCN)(L)](BPh4) (R = Et, L = η5-C5Me4H: 1d, η5C5Me5: 1e). The trialkyl phosphite complexes can activate H2 and the methyl-substituted cyclopentadienyl complexes activate O2 (Figure 1). NiIIFeII complexes 1a, 1c, and 1d as starting materials were synthesized according to the similar methods for 1b and 1e.9,10

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RESULTS AND DISCUSSION Synthesis and Characterization of NiFe Complexes. The series of trialkyl phosphite complexes 1a−1c and methylsubstituted cyclopentadienyl complexes 1d and 1e were prepared according to the literature and its modified methods.9,10 These complexes were characterized by X-ray analysis (Figures 2 and 3) and electrospray ionization mass spectrometry (ESI-MS, see Figures S1−S3). Crystals of [1a](BPh4)2 and [1d](BPh4) suitable for X-ray analysis were obtained from diffusion of diethyl ether into the acetonitrile and propionitrile solutions, respectively. The ORTEP drawings of 1a and 1d show the butterfly structures composed of the NiFe dinuclear center joined by thiolate-linkages (Figures 2 and 3). The interatomic distance between the Ni and Fe atoms of 1a is 3.2789(7) Å, and the Ni−S−Fe angles are 93.84(4) and 93.70(4)°, which are similar to those of 1b.9 The Ni···Fe distance of 1d {3.2199(6) Å} is slightly shorter than that of 1a due to the smaller Ni−S−Fe Received: June 20, 2017

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DOI: 10.1021/acs.organomet.7b00471 Organometallics XXXX, XXX, XXX−XXX

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Reactivity of NiFe Complexes toward H2 and O2. Trialkyl phosphite complexes 1a−1c activate H2 in the presence of MeONa as a base to form corresponding hydride species 2a−2c via heterolytic cleavage of H2 (see the Figures S1, S2, and S4).9 It was confirmed that no hydride species were formed by three control experiments: (1) H2 + no base, (2) no H2 + base, and (3) no H2 + no base. Methyl-substituted cyclopentadienyl complexes 1d and 1e activate O2 to form the corresponding peroxo species 3d and 3e via two-electron reduction of O2 (see Figure S3).10 The FeIV state of the peroxo complex was characterized by 57Fe Mössbauer spectroscopy {isomer shift (δ) = 0.42 mm s−1 and a quadrupole doublet (ΔEQ) = 0.33 mm s−1} and a superconducting quantum interference device (SQUID) measurement {magnetic moment (S) = 0 in the ground state}.10 Complexes 1a−1c were unreactive toward O2, while 1d and 1e were unreactive toward H2 and base. The ORTEP drawing of 2a revealed that the heterolytic cleavage of H2 by 1a to form 2a resulted in a narrowing of the Ni−S−Fe angles, 75.36(4) and 75.44(4)° (see Figure S4) with the Ni···Fe distance shortened {2.7822(8) Å}, which is also seen for the conversion of 1b to 2b.9 Electrochemical Properties of NiFe Complexes. We investigated the electrochemical properties of the NiFe complexes 1a−1e by cyclic voltammetry (see Figures S5 and S6). The quasi-reversible cyclic voltammograms of FeIII/FeII in 1a−1e were observed. The large gap of redox potentials between trialkyl phosphite complexes 1a−1c and methylsubstituted cyclopentadienyl complexes 1d and 1e is derived from electron-donating ability of the supporting ligands, though this large gap should be also caused by the difference in charges between dicationic and monocationic complexes. The negatively charged cyclopentadienyl ligand should be more strongly electron-donating than the neutral trialkyl phosphite ligand. Owing to such difference of electron-donating ability, the potentials of cyclopentadienyl complexes shift to more negative side compared to the phosphite complexes. The anodic potentials (Epa) of FeIII/FeII in 1a−1c indicate that the order of electron-donating ability is P(OEt)3 (0.71 V vs Fc+/ Fc) > P(OnBu)3 (0.80 V vs Fc+/Fc) > P(OMe)3 (0.84 V vs Fc+/Fc) (see Figure S5). The redox potentials (E1/2) of FeIII/ FeII in 1d and 1e reveal that electron-donating ability of η5C5Me5 (1e, −0.57 V vs Fc+/Fc) is higher than that of η5C5Me4H (1d, −0.53 V vs Fc+/Fc) due to the difference of one methyl group (see Figure S6).11 Kinetic Analysis of H2 and O2 Activation. For the kinetic investigation of H2 activation (Figures 4−7 and S7−S17), we followed the reactions of 1a−1c with H2 in the presence of MeONa to form the hydride complexes 2a−2c by monitoring the increase of absorption bands around 400 nm (Figures 4a, S7, S11, and S14). These reactions obey pseudo-first-order kinetics (v = kobs[NiFe complex]). The plot of the observed rate constants (kobs) against the concentration of H2 shows Michaelis−Menten type saturation curves (Figures 4b, S8, S12, and S15), while the plot of kobs against the concentration of MeONa is proportional (Figures 4d, S10, and S17). These results suggest that the heterolytic activation of H2 involves a preliminary binding of H2 to the NiFe compound, with subsequent formation of the hydride complex via bimolecular deprotonation by MeONa (Figure 5), which enables us to formulate the rate (eqs 1 and 2). The double-reciprocal plots (Figures 4c, S9, S13, and S16) yielded the equilibrium constants K and the rate constants k2 as shown in Table 1.

Figure 1. (a) Selective activation of H2 and O2 modulated by the electron-donating ability of the ligand(s). (b) Ligands (L). Ni-R: EPRsilent reduced state. Ni-SIa: EPR-silent active state. †OBS: Oxygenbound species5a = Eact−O2.4 n = 2: 1a−1c. n = 1: 1d and 1e.

Figure 2. ORTEP drawing of [1a](BPh4)2 with ellipsoids at the 50% probability level. The counteranions (BPh4), solvent (MeCN), and hydrogen atoms are omitted for clarity. Selected interatomic distances (l/Å) and angles (ϕ/deg): Ni1···Fe1 = 3.2789(7), Ni1−S1 = 2.1645(10), Ni1−S2 = 2.1607(10), Ni1−N1 = 2.026(3), Ni1−N2 = 2.019(3), Fe1−S1 = 2.3226(10), Fe1−S2 = 2.3310(10), Fe1−N3 = 1.960(3), Fe1−P1 = 2.1974(10), Fe1−P2 = 2.1907(11), Fe1−P3 = 2.1772(11), Ni1−S1−Fe1 = 93.84(4), Ni1−S2−Fe1 = 93.70(4).

Figure 3. ORTEP drawing of [1d](BPh4) with ellipsoids at the 50% probability level. The counteranion (BPh4) and hydrogen atoms are omitted for clarity. Selected interatomic distances (l/Å) and angles (ϕ/deg): Ni1···Fe1 = 3.2199(6), Ni1−S1 = 2.1670(4), Ni1−S2 = 2.1722(4), Ni1−N1 = 2.0279(13), Ni1−N2 = 2.0327(14), Fe1−S1 = 2.2835(4), Fe1−S2 = 2.2907(4), Fe1−N3 = 1.8943(14), Ni1−S1−Fe1 = 92.654(16), Ni1−S2−Fe1 = 92.322(16).

angles {92.654(16) and 92.322(16)°} rather than those of 1a. The structural parameters of 1d are similar to those of 1e.10 B

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Figure 5. Proposed mechanism for H2-activation by the NiIIFeII complexes 1a−1c. Z = Me: a, Et: b, nBu: c.

Figure 6. Eyring plots based on k2 for H2 activation by (a) 1a, (b) 1b, and (c) 1c, with error bars.

Figure 4. (a) UV−vis spectral change for the reaction of 1b (0.15 mM) with H2 (100%, 4.4 mM, 0.1 MPa) in acetonitrile/methanol (1/ 1) in the presence of MeONa (1200 mM) at 15 °C. Inset: Time profile of the absorbance at 400 nm (curve, red open circle) and the pseudo-first-order plot (linear plot, blue open circle) with least-squares fits (solid curve and line). (b) Plot of kobs against concentration of H2 {1.2 mM (28%, 0.028 MPa), 2.2 mM (50%, 0.05 MPa), 3.3 mM (75%, 0.075 MPa), and 4.4 mM (100%, 0.1 MPa)} in acetonitrile/methanol (1/1) at 15 °C. (c) Double-reciprocal plot of kobs against concentration of H2 {1.2 mM (0.028 MPa), 2.2 mM (0.05 MPa), 3.3 mM (0.075 MPa), and 4.4 mM (0.1 MPa)} for the reaction of 1b with H2 in acetonitrile/methanol (1/1) at 15 °C in the presence of MeONa (1200 mM). (d) Plot of kobs against concentration of MeONa for the reaction of 1b with H2 (100%, 4.4 mM, 0.1 MPa) in acetonitrile/methanol (1/1) in the presence of MeONa at 15 °C.

kobs = 1 kobs

=

Kk 2[H 2][MeONa] 1 + K[H 2] 1 1 + k 2[MeONa] Kk 2[H 2][MeONa]

Figure 7. Plot of ΔG⧧ at 15 °C against Epa (FeIII/FeII) in 1a−1c, with error bars. Fc+ = ferrocenium ion. Fc = ferrocene.

Figure 5, and [H2] and [MeONa] are concentrations of H2 and MeONa, respectively. Such Michaelis−Menten type kinetics is usually applied for the reaction of metal complexes with substrates.12 This H2-activation mechanism performed by the [NiFe] model is the same as that of the natural [NiFe]H2ase1,4 but different from that of the [NiRu] model.5 There are two notable differences for H2 activation between the [NiFe] and the [NiRu] systems. The first is the Lewis base, i.e., outer- or inner-sphere bases. The [NiFe] model cleaves H2 by the use of an outer-sphere Lewis base, MeONa, which is similar to the

(1)

(2)

where kobs is observed rate constant, K is equilibrium constant (= k1/k−1), k1, k−1, and k2 are the rate constants shown in C

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Organometallics Table 1. Kinetic Parameters for H2 Activation by 1a−1c and O2 Activation by 1d and 1e15 NiFe complex a

1a 1ba 1ca 1db 1eb a

ΔH⧧ (kJ mol−1) 32.7 75.4 61.2 59.5 55.2

± ± ± ± ±

0.86 1.1 1.7 1.0 1.4

ΔS⧧ (J mol−1 K−1) −175 −40.2 −80.8 21.8 −25.2

± ± ± ± ±

K (k1/k−1)

3.0 3.9 5.8 2.0 2.6

496 349 33 969 422

± ± ± ± ±

0.85 2.7 2.4 9.0 3.5

k2 (4.8 (9.4 (3.4 (7.9 (2.7

± ± ± ± ±

0.62) 0.24) 0.24) 0.25) 0.15)

× × × × ×

10−3 10−4 10−3 10−2 10−3

M−1 s−1 M−1 s−1 M−1 s−1 s−1 s−1

Values of K and k2 of 1a−1c at 15 °C. bValues of K and k2 of 1d and 1e at −65 °C.

recently proposed, outer-sphere Lewis base, arginine (Arg509) residue in O2-tolerant [NiFe]H2ase.13 In contrast, the [NiRu] model heterolytically activates H2 by using an inner-sphere Lewis base, which is an aqua ligand coordinated to the Ru center.5 The second point is the property of the hydride ligand coordinated to the Fe and Ru centers, i.e., a hydridic character in [NiFe]H2ase and the [NiFe] model, but a protic character in the [NiRu] model. On the basis of our results, the outer-sphere Lewis base and hydridic character should cause the formation of a Michaelis−Menten type H2-complex before heterolytic cleavage of H2 (Figure 5). The rate of the reaction with H2 is faster in NiFe complexes with the more strongly electron-withdrawing ligands (k2 = 4.8 × 10−3 M−1 s−1 for 1a > 3.4 × 10−3 M−1 s−1 for 1c > 9.4 × 10−4 M−1 s−1 for 1b at 15 °C), which should be caused by electrondeficient FeII center favorably binding hydride ion. The equilibrium constants are correlated to the bulkiness of the trialkyl phosphite ligand (K = 496 for 1a > 349 for 1b > 33 for 1c at 15 °C), which suggests that the more bulky ligand inhibits approach of H2 to the FeII center. Such tendencies of k2 and K should also support the proposed Michaelis−Menten type mechanism shown in Figure 5. From Eyring plots based on k2 (Figure 6) using transition-state theory according to eqs 3 and 4, we determined the Eyring activation parameters for H2 activation with 1a−1c to investigate the key step of heterolytic cleavage of H2 (Table 1). ⎛ −ΔG⧧ ⎞ ⎛ κk T ⎞ ⎟ k 2 = ⎜ B ⎟ exp⎜ ⎝ h ⎠ ⎝ RT ⎠ ⎛ ΔS ⧧ ⎞ ⎛ −ΔΗ⧧ ⎞ ⎛ κk T ⎞ ⎟ ⎟ exp⎜ = ⎜ B ⎟ exp⎜ ⎝ h ⎠ ⎝ R ⎠ ⎝ RT ⎠ ⎛ κk ⎞ ΔH ⧧ ⎛k ⎞ ΔS ⧧ ln⎜ 2 ⎟ = ln⎜ B ⎟ − + ⎝T ⎠ ⎝ h ⎠ RT R

Figure 8. (a) UV−vis spectral change for the reaction of 1d (0.060 mM) with O2 (100%, 8.8 mM, 0.1 MPa) in propionitrile at −65 °C. Inset: Time profile of the absorbance at 414 nm (curve, red open circle) and the pseudo-first-order plot (linear plot, blue open circle) with least-squares fits (solid curve and line). (b) Plot of kobs against concentration of O2 {0.88 mM (0.01 MPa), 2.64 mM (0.03 MPa), 4.4 mM (0.05 MPa), and 8.8 mM (0.1 MPa)} in propionitrile at −65 °C. (c) Double-reciprocal plot of kobs against concentration of O2 {0.88 mM (0.01 MPa), 2.64 mM (0.03 MPa), 4.4 mM (0.05 MPa), and 8.8 mM (0.1 MPa)} in propionitrile at −65 °C.

(3)

(4)

⧧

where k2 is the rate constant, ΔG is the activation free energy, ΔH⧧ is the activation enthalpy, ΔS⧧ is the activation entropy, kB is the Boltzmann’s constant, T is the temperature, h is Planck’s constant, R is the gas constant, and κ is the transmission coefficient (taken to be unity). As the electron-donating ability of ligand decreases, activation enthalpy ΔH⧧ is more favorable, indicating that an electron-deficient FeII center facilitates heterolytic cleavage of H2 to strongly bind the negatively charged hydride ligand. Calculation of activation free energy ΔG⧧ from the equation ΔG⧧ = ΔH⧧ − TΔS⧧ allows us to plot ΔG⧧ against Epa, which provides a linear free energy relationship (Figure 7). This trend is opposite to the case of our previous [NiRu] system where the inner-sphere aqua ligand can acts as a Lewis base. For the kinetics of O2 activation (Figures 8−11 and S18− S23), we investigated the oxygenation of complexes 1d and 1e in propionitrile at −80 to −45 °C, which obeys pseudo-first-

order kinetics (v = kobs[NiFe complex]) (Figures 8a, S18, and S21). The plots of pseudo-first-order rate constants (kobs) against the concentrations of O2 afford a Michaelis−Menten type saturation curve (Figures 8b, S19, and S22), which indicates the formation of a Michaelis−Menten type O2complex before the formation of peroxo complexes (Figure 9). This reaction should obey the general rate given in eqs 5 and 6. This reaction mechanism is the same as that for the previous [NiRu] system.5b The double-reciprocal plots (Figures 8c, S20, and S23) yielded the equilibrium constants K and the rate constants k2 for the oxygenation process. We estimated the D

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where kobs is observed rate constant, K is equilibrium constant (= k1/k−1), k1, k−1, and k2 are the rate constants shown in Figure 9, and [O2] is the concentration of O2. A correlation between ΔG⧧ for O2 reduction and the redox potential (E1/2) of the Fe center indicates that the rate of the reaction with O2 was accelerated as the number of the electrondonating methyl groups decreased (Figure 11). This graph can visualize the tendency of ΔG⧧ to E1/2 in the O2 activation by 1d and 1e even though it has only two data points. The result suggests that the oxygenation rate is affected by steric bulkiness of the ligand, which is supported by K and k2 values (K = 969 for 1d > 422 for 1e, k2 = 7.9 × 10−2 s−1 for 1d > 2.7 × 10−3 s−1 for 1e at −65 °C). Both constants of 1d are higher than those of 1e, which should be correlated to more favorable ΔS⧧ of 1d than that of 1e (ΔS⧧ = 21.8 J mol−1 K−1 for 1d > − 25.2 J mol−1 K−1 for 1e).14 The difference of only one methyl group causes the large difference in the activation entropies of 1d between 1e (ΔΔS⧧ = 47.0 J mol−1 K−1). The electron-donating ability of such ligands allows the NiIIFeII complex to determine H2 or O2 activation (Figure 12).

Figure 9. Proposed mechanism for O2-activation by the NiIIFeII complexes 1d and 1e. Z = H: d, Me: e. †OBS: Oxygen-bound species5a = Eact−O2.4

Figure 10. Eyring plots based on k2 for O2 activation by (a) 1d and (b) 1e, with error bars.

Figure 11. Plot of ΔG⧧ at −65 °C against E1/2 (FeIII/FeII) in 1d and 1e, with error bars. Fc+ = ferrocenium ion. Fc = ferrocene. Figure 12. Selective activation of H2 and O2 controlled by redox potential (FeIII/FeII). †OBS: Oxygen-bound species5a = Eact−O2.4

kinetic parameters for O2 activation with 1d and 1e by Eyring plots on the basis of k2 (Figure 10) using transition-state theory according to eqs 3 and 4 for insight into the formation process of peroxo species (Table 1). kobs = 1 kobs

=

Kk 2[O2 ] 1 + K[O2 ] 1 1 + k2 Kk 2[O2 ]

For H2 activation, the relatively electron-deficient FeII center is favorable to accept a hydride ligand being formed by heterolytic cleavage of H2. For O2 activation, the relatively electron-rich FeII center is favorable to generate the peroxo complex via twoelectron reduction of O2.

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(5)

CONCLUSIONS X-ray crystallographic analysis of O2-tolerant [NiFe]H2ase has revealed that it can switch the electron flow to the [Fe−S] clusters (under an H2 atmosphere) or to the oxidized NiFe

(6) E

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added P(OnBu)3 (2.00 mL, 7.39 mmol). Addition of NaBH4 (320 mg, 8.46 mmol) into the solution caused a color change from dark red to dark brown. The reaction mixture was stirred at room temperature for 1 h, to which was added NaBPh4 (966 mg, 2.82 mmol). After removing the solvent by evaporation, the precipitate was dissolved into dichloromethane (40 mL), and the insoluble residue was filtered. The solvent of the filtrate was removed under reduced pressure to yield dark brown materials, to which was added HBF4·Et2O (600 μL, 4.41 mmol) in dichloromethane/acetonitrile (1/1, 20 mL). To the solution was added NaBPh4 (724 mg, 2.12 mmol) in methanol (10 mL), and the reaction mixture was stirred for 2 h. The solvent was removed under reduced pressure. Dichloromethane (20 mL) was added into the residue and insoluble material was removed by filtration. After removing the solvent by evacuation, the resulting red oil was washed with pentane and diethyl ether. The red powder was collected by filtration and dried in vacuo (yield: 61% based on [NiII(X)FeII(Cl)2]). 1 H NMR (300 MHz, in acetonitrile-d3, referenced to TMS, 25 °C): δ 0.91−0.96 {t, 27H, O−(CH2)3−CH3}, 1.55 (t, 6H, N−CH2−CH3), 1.34−3.15 (m, 54H, −CH2−), 3.97−4.13 {m, 18H, O−CH2− (CH2)2−CH3}, 6.81−6.86, 6.96−7.01, 7.26−7.32 {B(C6H5)4}; ESIMS (in acetonitrile): m/z 556.4 ([1c − MeCN]2+, I = 100% in the range of m/z 100−2000). FT-IR (cm−1, KBr disk): 2872−3055 ( a l i p h a t i c C − H) ; A na l . C a l c d f o r [ 1 c ] ( B P h 4 ) 2 · H 2 O: C97H150B2FeN3NiO10P3S2: C, 64.32; H, 8.35; N, 2.32%. Found: C, 64.22; H, 8.62; N, 2.32%. [NiII(X)FeII(EtCN)(η5-C5Me4H)](BPh4) {[1d](BPh4)}. [FeII(MeCN)(CO)2(η5-C5Me4H)](BF4) (347 mg, 961 μmol) was dissolved in acetonitrile (100 mL). The resulting solution was slowly evaporated with irradiation by USHIO Optical ModuleX (Deep UV 500, BA-M500) for 4 h to afford purple powder. To the purple powder was added a propionitrile solution (30 mL) of [NiII(X)] (295 mg, 961 μmol). The resulting mixture was stirred for 18 h, to which was added NaBPh4 (477 mg, 1.40 mmol). The solution was concentrated under reduced pressure to generate insoluble materials, which were removed by filtration. Diffusion of methanol into the filtrate yielded purple powder of [1d](BPh4), which were collected by filtration, washed with methanol and diethyl ether, and dried in vacuo {yield: 76% based on [FeII(MeCN)(CO)2(η5-C5Me4H)](BF4)}. 1H NMR (300 MHz, in acetonitrile-d3, reference to TMS, 25 °C): δ 1.37−1.42 (t, 6H, N− CH2−CH3), 1.53, 1.58 {s, 12H, C5(CH3)4H}, 1.60−2.61, 2.84−2.95 (m, 18H, −CH2−), 3.19 {s, 1H, C5(CH3)4H}, 6.80−6.85, 6.95−7.00, 7.23−7.27 {B(C6H5)4}; ESI-MS (in propionitrile): m/z 483.1 ([1d − EtCN]+, I = 100% in the range of m/z 100−2000); FT-IR (cm−1, KBr disk): 2222 (CN), 2857−3035 (aliphatic C−H); Anal. Calcd for [1d](BPh4)·CH3OH: C48H66BFeN3NiOS2: C, 64.74; H, 7.47; N, 4.72%. Found: C, 64.72; H, 7.21; N, 4.91%. Typical Procedure for H2 Activation by NiIIFeII Complexes (1a−1c) to Form NiIIFeII Hydride Complexes (2a−2c). H2 was bubbled through acetonitrile/methanol (1:1) solutions of [1a](BPh4)2, [1b](BPh4)2, and [1c](BPh4)2 (0.15 mM, 2.0 mL), to which was added MeONa in methanol (300 mM, 50 μL), resulted in formation of the NiIIFeII hydride complexes [NiII(X)(μ-H)FeII{P(OMe)3}3](BPh4) {[2a](BPh4)}, [NiII(X)(μ-H)FeII{P(OEt)3}3](BPh4) {[2b](BPh4)},9 and [NiII(X)(μ-H)FeII{P(OnBu)3}3](BPh4) {[2c](BPh4)}, respectively. The ESI-MS results in acetonitrile/ methanol showed m/z 735.1 ([2a]+; I = 100% in the range of m/z 100−2000) for 2a, m/z 861.2 ([2b]+; I = 100% in the range of m/z 100−2000) for 2b,9 and m/z 1113.6 ([2c]+; I = 100% in the range of m/z 100−2000) for 2c. Typical Procedure for D2 Activation by NiIIFeII Complexes (1a−1c) to Form NiIIFeII Deuteride Complexes (D-Labeled 2a− 2c). D2 was bubbled through acetonitrile/methanol (1:1) solutions of [1a](BPh4)2, [1b](BPh4)2, and [1c](BPh4)2 (0.15 mM, 2.0 mL), to which was added MeONa in methanol (300 mM, 50 μL) resulted in formation of the NiIIFeII deuteride complexes [NiII(X)(μ-D)FeII{P(OMe)3}3](BPh4) {[D-labeled 2a](BPh4)}, [NiII(X)(μ-D)FeII{P(OEt)3}3](BPh4) {[D-labeled 2b](BPh4)},9 and [NiII(X)(μ-D)FeII{P(OnBu)3}3](BPh4) {[D-labeled 2c](BPh4)}, respectively. The ESI-MS results in acetonitrile/methanol showed m/z 736.1 ([D-labeled 2a]+; I = 100% in the range of m/z 100−2000) for D-labeled 2a, m/z 862.2

cluster (in the presence of O2) by the electronic effects of the proximal [Fe−S] cluster.3b,16 Such switching mechanism should enable the O2-tolerant [NiFe]H2ase to activate H2 (as a hydrogenase) or O2 (as an oxidase) depending on the situation. Our mimic is a simple model for such switching mechanism of O2-tolerant [NiFe]H2ase. Our results show that simple ligand effects can switch the mimic between H2 and O2 activation (Figure 12).

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EXPERIMENTAL SECTION

Materials and Methods. All experiments were carried out under an N2 or Ar atmosphere by using standard Schlenk techniques and a glovebox. Methanol was distilled over Mg/I; pentane and diethyl ether were distilled over Na/benzophenone. Acetonitrile, acetonitrile-d3, propionitrile, and dichloromethane were distilled over CaH2 under an N2 atmosphere prior to use. H2 (99.9999%), H2/N2 mixtures (75/ 25%, 50/50%, and 28/72%), O2 (99.9999%), O2/N2 mixtures (50/ 50%, 30/70%, and 10/90%), and D2 gas (99.5%) were purchased from Sumitomo Seika Chemicals Co. 18O2 was purchased from Shoko Co., Ltd.; P(OMe)3 and NaBPh4 were purchased from Tokyo Kasei Industries Co. HBF4·Et2O, P(OnBu)3, and nBu4NPF6 were purchased from Sigma-Aldrich, and 28% MeONa methanol solution was purchased from Wako Pure Chemical Industries, Ltd.; these were used without further purification. [NiII(X)] (X = N,N′-diethyl-3,7diazanonane-1,9-dithiolato), 17 [Fe II (MeCN)(CO) 2 (η 5 -C 5 Me 5 )](BF 4 ), 18 [Fe II (MeCN)(CO) 2 (η 5 -C 5 Me 4 H)](BF 4 ), 18 [Ni II (X)FeII(Cl)2],19 [NiII(X)FeII(MeCN){P(OEt)3}3](BPh4)2,9 and [NiII(X)FeII(EtCN)(η5-C5Me5)](BPh4)10 were prepared by the methods described in the literature. Electrospray ionization mass spectrometry (ESI-MS) data were obtained by a JEOL JMS-T100LC AccuTOF. IR spectra of solid compounds in KBr disks were recorded on a Thermo Nicolet NEXUS 8700 FT-IR instrument from 650 to 4000 cm−1 using 2 cm−1 standard resolution. UV−vis spectra were recorded on an Otsuka Electronics MCPD-2000 photodiode array spectrometer with an Otsuka Electronics optical fiber attachment, a JASCO V-670 UV−visible− NIR Spectrophotometer and Agilent Cary 8454 UV−visible Spectroscopy System (light pass length: 1.0 cm). 1H NMR spectra were recorded on a JEOL JNM-AL300 spectrometer, in which tetramethylsilane (TMS) is used as an internal standard. Elemental analysis data were obtained by a PerkinElmer 2400II series CHNS/O analyzer. [Ni II (X)Fe II (MeCN){P(OMe) 3 } 3 ](BPh 4 ) 2 {[1a](BPh 4 ) 2 }. To [NiII(X)FeII(Cl)2] (831 mg, 1.92 mmol) in methanol (25 mL) was added P(OMe)3 (1.00 mL, 8.43 mmol). Addition of NaBH4 (320 mg, 8.46 mmol) into the resulting solution caused a color change from dark yellow to dark brown. The resulting mixture was stirred at room temperature for 1 h, to which was added NaBPh4 (960 mg, 2.81 mmol) to form a dark brown precipitate, which was collected by filtration and dried in vacuo. The precipitate was dissolved into dichloromethane (40 mL), and the insoluble residue was removed by filtration. The solvent of the filtrate was removed under reduced pressure to yield dark brown materials, to which was added HBF4· Et2O (600 μL, 4.41 mmol) in acetonitrile (20 mL) at 0 °C, and the resulting solution was stirred for 30 min at room temperature. To the solution was added NaBPh4 (734 mg, 2.15 mmol) in methanol (10 mL), and the mixture was stirred for 15 min. Methanol (120 mL) was added into the solution to afford an orange precipitate, which was collected by filtration, washed with methanol and diethyl ether, and dried in vacuo (yield: 64% based on [NiII(X)FeII(Cl)2]). 1H NMR (300 MHz, in acetonitrile-d3, referenced to TMS, 25 °C): δ 1.56 (t, 6H, N−CH2−CH3), 1.82−1.86, 2.18−2.55, 2.82−2.94, 3.13−3.23 (m, 18H, −CH2−), 3.76 (m, 27H, O−CH3), 6.82−6.87, 6.97−7.02, 7.25− 7.29 {B(C6H5)4}. ESI-MS (in acetonitrile): m/z 367.1 {[1a − MeCN]2+, relative intensity (I) = 100% in the range of m/z 100− 2000}. FT-IR (cm−1, KBr disk): 2844−3054 (aliphatic C−H); Anal. Calcd for [1a](BPh4)2: C70H94B2FeN3NiO9P3S2: C, 59.43; H, 6.70; N, 2.97%. Found: C, 59.60; H, 6.78; N, 3.05%. [Ni II (X)Fe II (MeCN){P(O n Bu) 3 } 3 ](BPh 4 ) 2 {[1c](BPh 4 ) 2 }. To [NiII(X)FeII(Cl)2] (817 mg, 1.88 mmol) in methanol (30 mL) was F

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Organometallics

rate of 200 mV s−1. Potentials are referenced to Fc+/Fc (ferrocenium/ ferrocene) couple. X-ray Crystallographic Analysis. X-ray quality crystals of [1a](BPh4)2 were prepared by diffusion of diethyl ether into its acetonitrile solution. X-ray-quality crystals of [1d](BPh4) were prepared by diffusion of diethyl ether into its propionitrile solution. [2a](BPh4) was crystallized by the same method as [2b](BPh4).9 Measurements were made on a Rigaku/MSC Saturn CCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71070 Å). Data were collected and processed using the CrystalClear program. All calculations were performed using the teXsan crystallographic software package of Molecular Structure Corp. or the CrystalStructure crystallographic software package except for refinement, which was performed using SHELXL-97. Crystallographic data for [1a](BPh4)2, [1d](BPh4), and [2a](BPh4) have been deposited with the Cambridge Crystallographic Data Centre under reference numbers CCDC 1507696 (1a), 1507697 (1d), and 1507698 (2a), respectively.

([D-labeled 2b]+; I = 100% in the range of m/z 100−2000) for Dlabeled 2b,9 and m/z 1114.5 ([D-labeled 2c]+; I = 100% in the range of m/z 100−2000) for D-labeled 2c. Typical Procedure for O2 Activation by NiIIFeII Complexes (1d and 1e) to Form NiIIFeIV Peroxo Complexes (3d and 3e). O2 was bubbled through propionitrile solutions of the NiIIFeII complexes [NiII(X)FeII(EtCN)(η5-C5Me4H)](BPh4) {[1d](BPh4)} and [NiII(X)FeII(EtCN)(η5-C5Me5)](BPh4) {[1e](BPh4)} (0.15 mM, 2.0 mL) at −78 °C to afford the NiIIFeIV peroxo complexes [NiII(X)FeIV(η2O2)(η5-C5Me4H)](BPh4) {[3d](BPh4)} and [NiII(X)FeIV(η2-O2)(η5C5Me5)](BPh4) {[3e](BPh4)},10 respectively. The ESI-MS results in propionitrile showed m/z 515.1 ([3d]+; I = 100% in the range of m/z 100−2000) for 3d, and m/z 529.1 ([3e]+; I = 100% in the range of m/ z 100−2000) for 3e.10 Typical Procedure for 18O2 Activation with NiIIFeII Complexes (1d and 1e) to Form 18O-labeled NiIIFeIV Peroxo Complexes (18O-labeled 3d and 3e). 18O2 was bubbled through propionitrile solutions of the NiIIFeII complexes [1d](BPh4) and [1e](BPh4) (0.15 mM, 2.0 mL) at −78 °C to afford the NiIIFeIV peroxo complexes [NiII(X)FeIV(η2-18O2)(η5-C5Me4H)](BPh4) {[18Olabeled 3d](BPh4)} and [NiII(X)FeIV(η2-18O2)(η5-C5Me5)](BPh4) {[18O-labeled 3e](BPh4)},10 respectively. The ESI-MS results in propionitrile showed m/z 519.1 ([18O-labeled 3d]+; I = 100% in the range of m/z 100−2000) for 18O-labeled 3d, and m/z 533.1 ([18Olabeled 3e]+; I = 100% in the range of m/z 100−2000) for 18O-labeled 3e.10 Kinetic Measurements. Kinetic measurements of H2 activation with NiIIFeII complexes 1a−1c (0.15 mM) in acetonitrile/methanol (1/1, 2.50 mL) at 5−25 °C were carried out by monitoring the spectral change of the absorption band at 400 nm. 100% H2 or H2/N2 gas mixture (H2/N2 ratio 75/25%, 50/50%, or 28/72%) was bubbled through an acetonitrile/methanol (1/1) solution of MeONa at 25 °C for 15 min to give an acetonitrile/methanol solution with a constant concentration of H2. The resulting solution was allowed to stand at 5, 10, 15, 20, or 25 °C for 20 min. The NiIIFeII complexes, [1a](BPh4)2, [1b](BPh4)2, and [1c](BPh4)2, in acetonitrile (12.5 mM, 30 μL) were added to H2- or H2-/N2-bubbled acetonitrile/methanol solutions (2.47 mL) at 5, 10, 15, 20, or 25 °C. Pseudo-first-order rate constants (kobs) were determined by least-squares curve fitting. The final concentrations of NiIIFeII complexes were MeONa are 0.15 and 150−1200 mM, respectively. The concentration of H 2 in H2-saturated acetonitrile/methanol (1:1) (4.4 mM) was calculated on the basis of the literature,20 and the concentration under the kinetic conditions was estimated based on a ratio of H2 in H2/N2 gas mixture. The reaction rate of O2 activation with NiIIFeII complexes 1d and 1e (0.060 mM) in propionitrile at −80 to −45 °C was followed by the spectral change around 400 nm; 100% O2 or O2/N2 gas mixture (O2/ N2 ratio 50/50%, 30/70%, or 10/90%) was bubbled through propionitrile (4.90 mL) at 25 °C for 30 min to give a propionitrile solution with a constant concentration of O2. The resulting solution was allowed to stand at −80, −75, −70, −65, −60, −55, −50, or −45 °C for 40 min. The NiIIFeII complexes, [1d](BPh4) and [1e](BPh4), in propionitrile (3.0 mM, 100 μL) were added to O2- or O2-/N2bubbled propionitrile solutions in a Schlenk flask at −80, −75, −70, −65, −60, −55, −50, or −45 °C. Pseudo-first-order rate constants (kobs) were determined by least-squares curve fitting. The final concentration of NiIIFeII complexes was 0.06 mM. The concentration of O2 in O2-saturated propionitrile solution (8.8 mM) was estimated based on the literature,21 and the concentration under the kinetic conditions was calculated based on a ratio of O2 in O2/N2 gas mixture. Uncertainties in the activation parameters for H2 and O2 activation were determined by the use of the equations described in the literature.15 Electrochemical Analysis. Electrochemical measurements were performed in an acetonitrile solution of NiIIFeII complexes 1a−1e (1.0 mM) with nBu4NPF6 (100 mM) as a supporting electrolyte on a BAS660A electrochemical analyzer using a carbon working electrode at room temperature. Counteranion of BPh4− is exchanged with BF4− by using PPh4BF4 because oxidation peak of BPh4− is observed in cyclic voltammogram. The cyclic voltammograms were collected using a scan

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00471. UV−vis and ESI mass spectra, cyclic voltammograms, and kinetic analyses (PDF) Accession Codes

CCDC 1507696−1507698 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Seiji Ogo: 0000-0003-2078-6349 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid: JP26000008 (Specially Promoted Research), JP16K05727, and JP15K05566 from Japan Society for the Promotion of Science (JSPS), and the World Premier International Research Centre Initiative (WPI), Japan.

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REFERENCES

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