English

• Nowadays, hydrogen (H₂) is widely considered a keystone for replacing depleting fossil fuels^[1-2], due to its highly efficient energy carrier and environmental friendliness^[3-4]. One promising method for H₂ production involves natural [FeFe]-hydrogenases that can efficiently catalyze the reduction of protons (H⁺) into H₂^[5-6]. The highly catalytic activities of these metalloenzymes are attributed to their unique active sites, which consist of a butterfly-like [2Fe-2S] subcluster and a cubic-shaped [4Fe-4S] subcluster^[5-8]. It is interesting to note that this key [2Fe-2S] subcluster core is coordinated by a bridging azadithiolate (adt^NH=(SCH₂)₂NH) and several terminal diatomic ligands (CO and CN^-)^[5-9]. Based on the structural elucidation for the active sites of [FeFe]-hydrogenases, a great variety of biomimetic diiron dithiolate complexes have been widely reported to develop the novel and promising diiron-based molecular electrocatalysts for H₂ evolution^[10-20]. Among these bioinspired complexes, the bridgehead amines (NH or NR) of azadithiolate-bridged diiron hexacarbonyl complexes are beneficial for fast bonding H⁺ to efficiently catalyze H₂ production^[21], strongly revealing the key role of the basic NH moiety of the adt cofactor in the highly-active catalysis of natural [FeFe]-hydrogenases^[5-6]. With these findings in mind, a progressive improvement has motivated us to introduce the amine-bearing phosphine ligands into diiron hexacarbonyl complexes to obtain a new family of aminophosphine-coordinated diiron model complexes for electrochemical H₂ evolution.

  Under the above consideration and as a continuation of our project on diiron model complexes inspired by [FeFe]-hydrogenases^[22-28], we herein present the substitutions of diiron hexacarbonyl complexes [Fe₂(μ-X₂pdt)(CO)₆] (X₂pdt=(SCH₂)₂CX₂, X=Me, H)^[29] with aminodiphosphine ligands (Ph₂PCH₂)₂NY (Y=(CH₂)₂OH, (CH₂)₃OH)^[25] to form two new diiron monophosphine complexes [Fe₂(L1)(μ-Me₂pdt)(CO)₅] (1) and [Fe₂(L2)(μ-H₂pdt)(CO)₅] (2) (L1=3-[(diphenylphosphaneyl)methyl]oxazolidine, L2=3-[(diphenylphosphaneyl)methyl]-1, 3-oxazinane) having a pendant amine. Further structural features and electrochemical properties of the as- obtained complexes 1 and 2 were studied and compared by employing spectroscopic techniques (IR and NMR), single-crystal X-ray diffraction analysis, and cyclic voltammetry (CV).

  1.   Experimental

  1.1   Materials and methods

  The reagents were of chemical analytical purity. The characterization methods, containing elemental analysis and various spectroscopies, and their detailed parameters were provided in the Supporting information.

  1.2   Synthesis of complex 1

  A mixture of diiron hexacarbonyl complex [Fe₂(μ-Me₂pdt)(CO)₆] (Fe-Me₂pdt, 0.4 mmol, 164.6 mg), (Ph₂PCH₂)₂NCH₂CH₂OH (0.4 mmol, 260 mg), and Me₃NO·2H₂O (0.4 mmol, 44.4 mg) was dissolved in 20 mL of MeCN. The mixture solution was stirred at room temperature for 5 h until the TLC monitor showed starting precursor Fe-Me₂pdt was completely consumed. After the solvent was removed in vacuo, the residue was subjected to preparative TLC separation eluting with petroleum ether/CH₂Cl₂ (1∶1, V/V). The red band (R_f=0.6) was collected to obtain complex 1 as a red solid. Yield: 84.7 mg (32.2%). Anal. Calcd. for C₂₆H₂₈Fe₂NO₆PS₂(%): C, 47.51; H, 4.29; N, 2.13. Found(%): C, 47.38; H, 4.40; N, 2.31. FTIR (KBr disk, cm^-1): ν_C≡O 2 039 (vs), 1 984 (s), 1 971 (vs), 1 925 (s). ¹H NMR (600 MHz, CDCl₃, TMS): δ 7.71 (t, J=7.8 Hz, 4H, o-H of C₆H₅), 7.44 (d, J=7.8 Hz, 6H, m/p-H of C₆H₅), 3.95 (s, 2H, NCH₂O), 3.76 (d, J_PH=1.2 Hz, 2H, PCH₂N), 3.63 (t, J=7.2 Hz, 2H, OCH₂), 2.67 (t, J=7.2 Hz, 2H, NCH₂), 1.86 (d, J=13.8 Hz, 2H, SCH₂), 1.54 (d, J=13.8 Hz, 2H, SCH₂), 0.94, 0.77 (2s, 6H+6H, CH₃). ³¹P{¹H} NMR (243 MHz, CDCl₃, 85% H₃PO₄): δ 62.2 (s).

  1.3   Synthesis of complex 2

  To a toluene (15 mL) solution of [Fe₂(μ-H₂pdt) (CO)₆] (Fe-H₂pdt, 0.4 mmol, 154.8 mg) and (Ph₂PCH₂)₂ NCH₂CH₂CH₂OH (0.4 mmol, 200 mg) was added the acetonitrile (5 mL) solution of Me₃NO·2H₂O (0.4 mmol, 44.4 mg). The mixture solution was refluxed at 110 ℃ for 6 h until the TLC monitor showed starting precursor Fe-H₂pdt was completely consumed. After the solvent was removed in vacuo, the residue was subjected to preparative TLC separation eluting with petroleum ether/EtOAc (3∶1, V/V). The main red band (R_f=0.6) was collected to obtain complex 2 as a red solid. Yield: 110.9 mg (43.1%). Anal. Calcd. for C₂₅H₂₆Fe₂NO₆PS₂(%): C, 46.68; H, 4.07; N, 2.18. Found(%): C, 46.83; H, 4.28; N, 2.41. FTIR (KBr disk, cm^-1): ν_C≡O 2 040 (vs), 1 978 (vs), 1 954 (w), 1 925 (w). ¹H NMR (600 MHz, CDCl₃, TMS): δ 7.75 (t, J=7.8 Hz, 4H, o-H of C₆H₅), 7.44 (s, 6H, m/p-H of C₆H₅), 4.01 (s, 2H, NCH₂O), 3.93 (s, 2H, PCH₂N), 3.77 (t, J=4.8 Hz, 2H, OCH₂), 2.71 (t, J=4.8 Hz, 2H, NCH₂), 1.78 (m, 2H, NCH₂CH₂CH₂O), 1.59 (s, 2H, SCH₂), 1.49 (s, 2H, CH₂), 1.29 (s, 2H, SCH₂). ³¹P{¹H} NMR (243 MHz, CDCl₃, 85% H₃PO₄): δ 60.1 (s).

  1.4   Crystal structure determination

  Single crystals of complexes 1 and 2 suitable for X-ray diffraction analysis were grown by slow evaporation of the CH₂Cl₂/n-hexane solution at -5 ℃. The crystals were mounted on a Bruker-CCD diffractometer. Data were collected at 150(2) K using a graphite monochromator with Mo Kα radiation (λ=0.071 073 nm) in the ω-φ scanning mode. The structure was solved by direct methods using the SHELXS-97 program^[30] and refined by full-matrix least-squares techniques (SHELXL-97) on F ^{2 [31]}. Hydrogen atoms were located using the geometric method. The crystallographic parameters, data collection, and structure refinement details of 1 and 2 are summarized in Table S1 (Supporting information).

  1.5   Electrochemical measurement

  All electrochemical experiments of 1 and 2 were done using a typical three-electrode system. Experimental details and parameters were available in the Supporting information.

  2.   Results and discussion

  2.1   Synthesis and spectroscopic characterization

  To obtain a new family of aminophosphine-coordinated diiron model complexes of [FeFe]-hydrogenases, the Me₃NO-induced substitution reactions of diiron hexacarbonyl precursor [Fe₂(μ-X₂pdt)(CO)₆] (Fe-X₂pdt) with aminodiphosphine (Ph₂PCH₂)₂N(Y) (PCN^YCP) resulted in the unexpected formation of two new diiron monophosphine complexes having a pendant amine, namely 1 obtained in room-temperature MeCN solution and 2 prepared in refluxing MeCN-toluene solution, as shown in Scheme 1.

  Scheme 1

  

  Scheme 1.  Synthesis of complexes 1 and 2

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  Further, a tentative process for the formation of complexes 1 and 2 is described in Scheme 2. First, the oxidative decarbonylation of Fe-X₂pdt with one equivalent of Me₃NO in MeCN produced [Fe₂(μ-X₂pdt)(CO)₅(L)] (A, L=NMe₃ or NCMe)^{[20, 32-34]}. Afterwards, the in-situ substitution of A with one equivalent of (Ph₂PCH₂)₂N(CH₂)_nOH formed the diphosphine-monosubstituted species [Fe₂(μ-X₂pdt)(CO)₅((Ph₂PCH₂)₂ N(CH₂)_nOH)] (B)^{[20, 33, 34]}. Lastly, the intramolecular nucleophilic attack of the oxygen atom into the methylene-C atom attached to the free P atom happened in species B, leading to the unexpected formation of complexes 1 and 2.

  Scheme 2

  

  Scheme 2.  Proposed formation process of complexes 1 and 2

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  The as-obtained new complexes 1 and 2 were fully characterized by elemental analysis, FTIR, and NMR (¹H, ³¹P) spectroscopies (Fig.S1-S6). First of all, the FTIR spectra displayed four apparent absorption peaks in the region of 2 040-1 925 cm^-1 for terminal-coordinated carbonyls^{[15-16, 20, 22, 24, 26]}. The first ν(C≡O) peaks of both 1 and 2 were found to be 2 039 and 2 040 cm^-1, respectively, indicating the very similar electron density of the diiron center in them^{[24, 26]}. At the same time, the ¹H NMR spectra showed two groups of aryl proton signals in the low-field NMR region from δ 7.8 to 7.4 for the phenyls attached to phosphorus atom and four sets of alkyl proton signals in the high-field NMR region from δ 4.0 to 2.6 for the methylenes linked to phosphorus, oxygen and nitrogen atoms in their monophosphine ligands. In addition, the CDCl₃- solution ³¹P{¹H} NMR spectra showed a sole singlet at δ 62.2 and 60.1, suggesting the coordination of only one phosphorus atom to one of the diiron cores.

  2.2   Crystal structure

  More intuitively, the molecular structures of 1 and 2 were confirmed by single-crystal X-ray diffraction analysis, as depicted in Fig.1 with their selected bond lengths and angles. Complexes 1 and 2 feature a butterfly-shaped [Fe₂S₂] cluster core with five terminal carbonyls, one propanedithiolate bridge, and one monophosphine ligand. And the phosphorus atom of phosphine ligand is bonded apically to a single Fe core, being well accordant with those observed in many previously-reported diiron monophosphine complexes^{[20, 24, 26, 35-37]}. Further, the Fe1—Fe2 distance of 1 [0.249 46(5) nm] is shorter than that of 2 [0.252 09(7) nm], but which are shorter than those found in natural [FeFe]-hydrogenases (0.255-0.260 nm)^[7-8]. Meanwhile, the Fe2—P1 distance of 1 [0.222 36(7) nm] is a little shorter than that of 2 [0.223 10(9) nm]. Moreover, the P_apical—Fe—Fe angles (P1—Fe2—Fe1) are 3.25° smaller for 1 and larger 8.60° for 2 in contrast to the respective C_apical—Fe—Fe angles (C1—Fe1—Fe2). These findings are probably due to the electron effects of the bridgehead CH₃ moiety versus the H atom of the dithiolato bridge and their steric interaction with the apical monophosphine ligands.

  Figure 1

  

  Figure 1.  Molecular structures of complexes 1 (a) and 2 (b) drawn at a 30% probability level ellipsoids

  All hydrogen atoms and solvents are omitted for clarity; Selected bond distances (nm) and angles (°) for 1 and 2: Fe1—Fe2 0.249 46(5) and 0.252 09(7), Fe2—P1 0.222 36(7) and 0.223 10(9), P1—Fe2—Fe1 156.19(2) and 155.25(3), C1—Fe1—Fe2 159.44(9) and 146.65(11).

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  2.3   Electrochemical properties

  To explore the electrochemical reduction of protons into H₂ evolution catalyzed by this class of aminophosphine-coordinated diiron model complexes, the electrochemical properties of complexes 1 and 2 were studied and compared in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN solution without or with HOAc using the CV technique.

  The electrochemical redox properties of 1 or 2 were examined in free acid through the CV method as depicted in Fig.2a. Both 1 and 2 showed an irreversible reduction peak at E_pc1=-1.85 V, indicating their very similar reduction abilities due to the analogous electron density around diiron cores as indicated in the above IR result. This reduction peak may be assigned to one-electron reduction of Fe^ⅠFe^Ⅰ into Fe^ⅠFe^{0 [36-39]}. Meanwhile, the Cottrell plots of reduction peak current (i_pc) against the square root of scan rate (v^1/2) for 1 and 2 displayed the linear relationship between them (Fig.S7 and S8). This finding indicates that the homogeneous electrochemical processes are diffusion-controlled^[26-28].

  Figure 2

  

  Figure 2.  Electrochemical properties of complexes 1 and 2: (a) CV curves in free acid and (b, c) CV curves with 0-10 mmol·L^-1 HOAc; (d) plots of k_obs vs c_HOAc (2-10 mmol·L^-1)

  Conditions: 1.0 mmol·L^-1 complexes 1 and 2 were respectively contained in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1, and all potentials were versus the ferrocene/ferrocenium (Fc/Fc⁺) couple; Inset: plot of catalytic currents (i_cat) vs c_HOAc (2-10 mmol·L^-1).

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  Further electrocatalytic H₂ evolution properties of 1 and 2 were investigated with 0-10 mmol·L^-1 HOAc (pK_a=22.3 in MeCN)^[40] by using CV as displayed in Fig.2b and 2c. Both 1 and 2 exhibited the initial reduction peak currents at E_pc1=-1.85 V, which didn't almost rise with increasing the added acid concentrations, but a new reduction peak at a more negative potential (E_pc2) appeared when 2 mmol·L^-1 acid was added. The new reduction peak currents (i_cat) at E_pc2=-2.37 or -2.34 V showed a linear response to the increasing acid concentrations (c_HOAc) as seen in the insets of Fig.2b and 2c, suggesting complexes 1 and 2 are electrocatalytically active for proton reduction into H₂ evolution with HOAc as a proton source^[36-39].

  Remarkably, the electrocatalytic H₂ evolution activities of 1 and 2 were estimated through turnover frequency (TOF), which can be measured by the kinetics rate constant (k_obs) of hydrogen evolution reaction (HER) catalyzed by them. The k_obs (s^-1) was calculated using the following equation: k_obs=1.94v(i_cat/i_p)^{2 [41-43]}, wherein v is scan rate (V·s^-1), i_cat is catalytic peak current (A), and i_p is reduction peak current (A). Based on this equation and the CV data of Fig.2b and 2c, the plots of the calculated k_obs against the added acid concentrations (c_HOAc, 2-10 mmol·L^-1) were established for 1 and 2 as illustrated in Fig.2d. Both 1 and 2 exhibited the linear acid-dependent response of the k_obs to the added c_HOAc, implying they have good electrochemical durability with HOAc as proton source. In the acid- dependent region of Fig.2d, the maximum i_cat/i_p values at 10 mmol·L^-1 of c_HOAc were estimated to be 6.97 for 1 and 7.11 for 2 (Table 1), leading to the slightly smaller maximum TOF value of 1 (9.4 s^-1) in contrast to 2 (9.8 s^-1). This outcome demonstrates that complexes 1 and 2 have similar electrocatalytic H₂ evolution activities with HOAc as a proton source.

  Table 1

  

  Table 1.  Relevant electrochemical data for complexes 1 and 2 without or with HOAc

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  Complex	Epc1 / V	Epc2a / V	(icat/ip)maxb	kobsb / s-1
  1	-1.85	-2.37	6.97	9.4
  2	-1.85	-2.34	7.11	9.8
  a Epc2 was denoted as the new reduction potential when cHOAc was 2 mmol·L-1; b The maximum icat/ip and kobs, namely maximum turnover frequency (TOF) value, were estimated when cHOAc was 10 mmol·L-1.

  3.   Conclusions

  In summary, we prepared two new diiron monophosphine complexes 1 and 2 with a pendant amine, following the structural similarity to the active site of [FeFe]-hydrogenases. Further structural features and electrochemical properties of 1 and 2 were studied and compared through spectroscopic, crystallographic, and CV methods. On one hand, 1 and 2 show a similar electron density around the iron core as indicated by IR and the apical coordination of monophosphine to one iron core as observed by X-ray crystallography. On the other hand, 1 and 2 exhibit the analogous electrochemical reduction property (i.e., E_pc1=-1.85 V) and the nearly equivalent electrocatalytic proton reduction activity (i.e., TOF=ca. 10 s^-1) with HOAc as a proton source.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Synthesis of complexes 1 and 2

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  Scheme 2  Proposed formation process of complexes 1 and 2

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  Figure 1  Molecular structures of complexes 1 (a) and 2 (b) drawn at a 30% probability level ellipsoids

  All hydrogen atoms and solvents are omitted for clarity; Selected bond distances (nm) and angles (°) for 1 and 2: Fe1—Fe2 0.249 46(5) and 0.252 09(7), Fe2—P1 0.222 36(7) and 0.223 10(9), P1—Fe2—Fe1 156.19(2) and 155.25(3), C1—Fe1—Fe2 159.44(9) and 146.65(11).

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  Figure 2  Electrochemical properties of complexes 1 and 2: (a) CV curves in free acid and (b, c) CV curves with 0-10 mmol·L^-1 HOAc; (d) plots of k_obs vs c_HOAc (2-10 mmol·L^-1)

  Conditions: 1.0 mmol·L^-1 complexes 1 and 2 were respectively contained in 0.1 mol·L^{-1 n}Bu₄NPF₆/MeCN at a scan rate of 0.1 V·s^-1, and all potentials were versus the ferrocene/ferrocenium (Fc/Fc⁺) couple; Inset: plot of catalytic currents (i_cat) vs c_HOAc (2-10 mmol·L^-1).

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  Table 1.  Relevant electrochemical data for complexes 1 and 2 without or with HOAc

  Complex	Epc1 / V	Epc2a / V	(icat/ip)maxb	kobsb / s-1
  1	-1.85	-2.37	6.97	9.4
  2	-1.85	-2.34	7.11	9.8
  a Epc2 was denoted as the new reduction potential when cHOAc was 2 mmol·L-1; b The maximum icat/ip and kobs, namely maximum turnover frequency (TOF) value, were estimated when cHOAc was 10 mmol·L-1.

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