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Jun 4, 2018 - As the active site mimics of [NiFe]-H2ases, the first heterodinuclear Ni/M (M = Mo, W)-based complexes that contain both azadiphosphine ...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Heterodinuclear Ni/M (M = Mo, W) Complexes Relevant to the Active Site of [NiFe]-Hydrogenases: Synthesis, Characterization, and Electrocatalytic H2 Evolution Li-Cheng Song,*,†,‡ Long-Duo Zhang,† Wei-Wei Zhang,† and Bei-Bei Liu† †

Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China S Supporting Information *

ABSTRACT: As the active site mimics of [NiFe]-H2ases, the first heterodinuclear Ni/M (M = Mo, W)-based complexes that contain both azadiphosphine and cyclopentadienyl ligands have been prepared by simple and convenient methods in quite high yields. Thus, treatment of the azadiphosphinechelated Ni dithiolates R1N(PPh2)2Ni(SEt)2 (R1 = p-MeC6H4CH2, EtO2CCH2) with an equimolar parent or acetyl-substituted cyclopentadienyl Mo halides (η5-R2C5H4)(CO)3MoY (R2 = H, MeCO; Y = Cl, Br) gave dinuclear Ni/Mo halide complexes [R1N(PPh2)2Ni(μ-SEt)2Mo(η5-R2C5H4)(CO)2]Y (1a, R1 = p-MeC6H4CH2, R2 = H, Y = Br; 1b, R1 = EtO2CCH2, R2 = H, Y = Cl; 1c, R1 = EtO2CCH2, R2 = MeCO, Y = Br), while further treatment of 1a−c with a slight excess of NaBPh4 afforded dinuclear Ni/Mo tetraphenylboron complexes [R1N(PPh2)2Ni(μ-SEt)2Mo(η5-R2C5H4)(CO)2]BPh4 (2a, R1 = p-MeC6H4CH2, R2 = H; 2b, R1 = EtO2CCH2, R2 = H; 2c, R1 = EtO2CCH2, R2 = MeCO). Similarly, while dinuclear Ni/W halide complexes [R1N(PPh2)2Ni(μ-SEt)2W(η5-R2C5H4)(CO)2]Y (3a, R1 = p-MeC6H4CH2, R2 = H, Y = Cl; 3b, R1 = EtO2CCH2, R2 = H, Y = Cl; 3c, R1 = EtO2CCH2, R2 = MeCO, Y = Br) could be prepared by treatment of mononuclear Ni dithiolates R1N(PPh2)2Ni(SEt)2 with mononuclear W halides (η5-R2C5H4)(CO)3WY, further treatment of 3a−c with NaBPh4 resulted in formation of dinuclear Ni/W tetraphenylboron complexes [R1N(PPh2)2Ni(μ-SEt)2W(η5-R2C5H4)(CO)2]BPh4 (4a, R1 = p-MeC6H4CH2, R2 = H; 4b, R1 = EtO2CCH2, R2 = H; 4c, R1 = EtO2CCH2, R2 = MeCO). The X-ray crystallographic study of dinuclear Ni/Mo and Ni/W tetraphenylboron complexes 2a−c and 4a,b confirmed that they have a butterfly [NiIIMII(μ-S)2] (M = Mo, W) core in which the azadiphosphine R1N(PPh2)2-chelated Ni fragment is combined with an (η5-R2C5H4)(CO)2M moiety via two thiolate ligands. In addition, the electrochemical and electrocatalytic properties of representative dinuclear complexes 2a,b and 4b,c have been investigated, and particularly, they have been found to be catalysts for proton reduction to H2 from TFA under CV conditions.

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INTRODUCTION Currently, using molecular H2 as an alternative energy source to replace fossil fuel has been considered to be one of the best ways to solve the global energy crisis and environmental pollution problems.1−3 This consideration has stimulated intensive studies on the synthetic modeling of the active sites of [FeFe]and [NiFe]-hydrogenases (H2ases), since the two classes of H2ases are the highly efficient H2-producing catalysts and contain the earth-abundant and cheap metals Fe and Ni present in the active sites.4−9 X-ray crystallographic studies revealed that the active site of [NiFe]-H2ases features a unique heterodinuclear Ni/Fe complex in which a nickel tetrathiolate is connected via its two thiolates to an Fe(CO)(CN)2 moiety; in addition, while the oxidized state of [NiFe]-H2ases contains a bridging oxygenous ligand X, the reduced state of [NiFe]-H2ases has a bridging hydride ligand or a Ni−Fe metal−metal bond (Figure 1a).10−13 Inspired by the well-elucidated active site structure of [NiFe]-H2ases, researchers in bioinorganic and organometallic communities have designed and prepared a large number of heterodinuclear transition-metal complexes as useful models for the active site of [NiFe]-H2ases, such as the heterodinuclear © XXXX American Chemical Society

Figure 1. (a) Active site structure of [NiFe]-H2ases determined by X-ray crystallography. (b) Targeted Ni/M (M = Mo, W) complexes as models for [NiFe]-H2ases.

Ni/Fe,14−30 Ni/Ru,31−36 Ni/Mn,37−39 and Ni/M (M = Mo, W)40 complexes. To gain an insight into the catalytic mechanism for H2 production catalyzed by [NiFe]-H2ases and to search the highly efficient and non-noble-metal-containing H2-producing biomimetic catalysts, we recently launched a study attempted to synthesize the hitherto unknown dinuclear NiII/MII (M = Mo, W)-based Received: April 13, 2018

A

DOI: 10.1021/acs.organomet.8b00228 Organometallics XXXX, XXX, XXX−XXX

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Organometallics complexes in which an azadiphosphine R1N(PPh2)2-chelated NiII center is linked via two bridging thiolate ligands to an η5-R2C5H4(CO)2MII moiety (Figure 1b). It should be noted that complexes of this type have the following features: (i) They contain a butterfly [NiIIMII(μ-S)2] (M = Mo, W) core that is structurally similar to the butterfly [NiIIFeII(μ-S)2] core present in the active site of [NiFe]-H2ases. (ii) They have an azadiphosphine (PNP) ligand in which the pendant amine N atoms could be able to facilitate the electrocatalytic H2 production.41 (iii) They bear different R1 and R2 substituents with different electronic and steric effects, which might be able to modify their physical and chemical properties. Fortunately, we have successfully prepared such a new type of model complexes and particularly some of them have been found to be catalysts for proton reduction to H2 under CV conditions. Now, we wish to report the interesting results obtained by this study.

Scheme 2. Synthesis of Dinuclear Ni/Mo Tetraphenylboron Complexes 2a−c

Complexes 2a−c are also air-stable brown solids and were characterized by elemental analysis and various spectroscopic techniques (see Figures S5−S9). For example, the IR spectra of 2a−c displayed two absorption bands in the region 1956−1889 cm−1 for their terminal carbonyls and one absorption band in the range 818−815 cm−1 for the stretching vibration of the P−N−P skeleton in their azadiphosphine ligands.39,42 The 1H NMR spectra of 2a,b showed one singlet at 5.30 and 5.31 ppm for their parent η5-cyclopentadienyl groups, while 2c exhibited one singlet at 2.08 ppm for its acetyl group bound to the cyclopentadienyl ring. While the 31P{1H} NMR spectra of 2a−c exhibited one singlet in the region 60−65 ppm for their azadiphosphine P atoms, the 13C{1H} NMR spectra displayed one singlet in the range 242−246 ppm for their terminal carbonyl C atoms. In addition, the 11B{1H} NMR spectra of 2a−c displayed one singlet at −6.5 ppm for B atoms in their Ph4B− monoanions. Of particular note is that the molecular structures of dinuclear Ni/Mo tetraphenylboron complexes 2a−c were unambiguously confirmed by X-ray crystallography (Figures 2−4), although

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RESULTS AND DISCUSSION Synthesis and Characterization of Dinuclear Ni/Mo Complexes [R1N(PPh2)2Ni(μ-SEt)2Mo(η5-R2C5H4)(CO)2]Y (1a, R1 = p-MeC6H4CH2, R2 = H, Y = Br; 1b, R1 = EtO2CCH2, R2 = H, Y = Cl; 1c, R1 = EtO2CCH2, R2 = MeCO, Y = Br) and [R1N(PPh2)2Ni(μ-SEt)2Mo(η5-R2C5H4)(CO)2]BPh4 (2a, R1 = p-MeC6H4CH2, R2 = H; 2b, R1 = EtO2CCH2, R2 = H; 2c, R1 = EtO2CCH2, R2 = MeCO). When azadiphosphine R1N(PPh2)2chelated mononuclear Ni dithiolates R1N(PPh2)2Ni(SEt)2 were treated with 1 equiv of mononuclear Mo carbonyl halides (η5-R2C5H4)(CO)3MoY in CH2Cl2 at room temperature, dinuclear Ni/Mo halide complexes 1a−c were produced via nucleophilic attack of the two S atoms in R1N(PPh2)2Ni(SEt)2 at Mo atom of (η5-R2C5H4)(CO)3MoY followed by loss of one molecule of CO ligand in 70−83% yields (Scheme 1). Scheme 1. Synthesis of Dinuclear Ni/Mo Halide Complexes 1a−c

Complexes 1a−c are air-stable brown solids and were characterized by elemental analysis and various spectroscopic methods (see Figures S1−S4). For instance, the IR spectra of 1a−c showed two absorption bands in the range 1964−1869 cm−1 for their terminal carbonyls and one absorption band in the region 820−813 cm−1 for the stretching vibration of the P−N−P skeleton in their azadiphosphine ligands.39,42 The 1H NMR spectra of 1a,b displayed one singlet at 5.41 and 5.46 ppm for their parent η5-cyclopentadienyl groups, whereas 1c displayed one singlet at 2.20 ppm for its acyl group attached to the cyclopentadienyl ring. In addition, while the 31P{1H} NMR spectra of 1a−c exhibited one singlet in the region 60−65 ppm for their azadiphosphine P atoms, the 13C{1H} NMR spectra exhibited one singlet in the region 242−245 ppm for their terminal carbonyl C atoms. Having prepared dinuclear Ni/Mo halide complexes 1a−c, we further found that they could react with slightly excess of NaBPh4 in CH2Cl2 at room temperature to give dinuclear Ni/Mo tetraphenylboron complexes 2a−c via the anionic Cl−/Ph4B− and Br−/Ph4B− exchange reactions in 88−90% yields (Scheme 2).

Figure 2. Molecular structure of the monocation of 2a with ellipsoids drawn at a 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Mo(1)− S(1) 2.5371(4), Ni(1)−S(1) 2.2070(4), Ni(1)−P(1) 2.1682(4); Mo(1)−S(1)−Ni(1) 79.543(13), P(1)−Ni(1)−P(2) 73.676(15), S(1)−Mo(1)−S(2) 68.421(13), S(1)−Ni(1)−S(2) 80.573(16).

those of their precursor complexes, 1a−c, could not be determined by X-ray diffraction techniques due to the difficulty to obtain their single crystals suitable for X-ray diffraction analysis. The X-ray crystallographic study indicated that complexes 2a−c indeed comprise one monocation [R1N(PPh2)2Ni(μ-SEt)2Mo(η5-R2C5H4)(CO)2]+ and one monoanion Ph4B−. In their monocations the Ni and Mo centers are bridged by two EtS ligands, which results in a butterfly [NiIIMoII(μ-S)2] core. While the Ni center is tetracoordinate and adopts a distorted square-planar geometry, the Mo center is heptacoordinate due to the B

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the azadiphosphine R1N(PPh2)2-chelated mononuclear Ni dithiolates R1N(PPh2)2Ni(SEt)2 with mononuclear W carbonyl halides (η5-R2C5H4)(CO)3WY under the same conditions as for preparation of their Ni/Mo analogues 1a−c via nucleophilic attack of the two S atoms in R1N(PPh2)2Ni(SEt)2 at W atom of (η5-R2C5H4)(CO)3WY followed by loss of one molecule of CO ligand in 48−66% yields (Scheme 3). Scheme 3. Synthesis of Dinuclear Ni/W Halide Complexes 3a−c

Figure 3. Molecular structure of the monocation of 2b with ellipsoids drawn at a 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Mo(1)− S(1) 2.5150(6), Ni(1)−S(1) 2.1893(6), Ni(1)−P(1) 2.1815(6); Mo(1)−S(1)−Ni(1) 80.261(17), P(1)−Ni(1)−P(2) 73.70(2), S(1)−Mo(1)−S(2) 68.876(16), S(1)−Ni(1)−S(2) 81.02(2).

Complexes 3a−c, like their Ni/Mo analogues 1a−c, are airstable brown solids and were characterized by elemental analysis and various spectroscopic methods (see Figures S10−S13). For example, the IR spectra of 3a−c showed two or three absorption bands in the region 1990−1840 cm−1 for their terminal carbonyls and one absorption band in the range 816− 815 cm−1 for the stretching vibration of the P−N−P skeleton in their azadiphosphine ligands.39,42 The 1H NMR spectra of 3a,b displayed one singlet at 5.66 and 5.77 ppm for their η5-cyclopentadienyl groups, while 3c exhibited one singlet at 2.27 ppm for its acetyl group attached to the cyclopentadienyl ring. In addition, the 31P{1H} NMR spectra of 3a−c exhibited one singlet in the range 73−81 ppm for their azadiphosphine P atoms, while the 13C{1H} NMR spectra displayed one singlet in the range 207−225 ppm for their terminal carbonyl C atoms. After dinuclear Ni/W halide complexes 3a−c were prepared, we further prepared their tetraphenylboron analogues 4a−c (to enhance their stability and to facilitate growing the single crystals, see below) by the anionic exchange reactions of 3a−c with NaBPh4 under the same conditions as for preparation of their Ni/Mo analogues 2a−c in 66−91% yields (Scheme 4). Scheme 4. Synthesis of Dinuclear Ni/W Tetraphenylboron Complexes 4a−c

Figure 4. Molecular structure of the monocation of 2c with ellipsoids drawn at a 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Mo(1)− S(1) 2.5126(7), Ni(1)−S(1) 2.1980(8), Ni(1)−P(1) 2.1541(7); Mo(1)−S(1)−Ni(1) 80.46(2), P(1)−Ni(1)−P(2) 73.73(3), S(1)− Mo(1)−S(2) 69.03(2), S(1)−Ni(1)−S(2) 81.09(3).

coordination number of the parent and substituted cyclopentadienyl ligands being equal to 3. Similar to the previously reported NiII/Mo0 complexes,40,43 the NiII/MoII internuclear separation in 2a (3.0455 Å), 2b (3.0423 Å), and 2c (3.0519 Å) is much longer than the sum of the covalent radii (Ni, 1.24 Å; Mo, 1.54 Å),44 which implies that the Ni/Mo metal−metal bonding in 2a−c could be negligible. Synthesis and Characterization of Dinuclear Ni/W Complexes [R1N(PPh2)2Ni(μ-SEt)2W(η5-R2C5H4)(CO)2]Y (3a, R1 = p-MeC6H4CH2, R2 = H, Y = Cl; 3b, R1 = EtO2CCH2, R2 = H, Y = Cl; 3c, R1 = EtO2CCH2, R2 = MeCO, Y = Br) and [R1N(PPh2)2Ni(μ-SEt)2W(η5-R2C5H4)(CO)2]BPh4 (4a, R1 = p-MeC6H4CH2, R2 = H; 4b, R1 = EtO2CCH2, R2 = H; 4c, R1 = EtO2CCH2, R2 = MeCO). Dinuclear Ni/W halide complexes 3a−c could be similarly prepared by treatment of

Complexes 4a−c, like their Ni/Mo analogues 2a−c, are airstable brown solids and were characterized by elemental analysis and various spectroscopic techniques (see Figures S14−S18). For instance, the IR spectra of 4a−c displayed two or three absorption bands in the region 1988−1863 cm−1 for their terminal CO ligands and one absorption band in the region 817−814 cm−1 for the stretching vibration of the P−N−P skeleton in their azadiphosphine ligands.39,42 The 1H NMR spectra of 4a,b showed one singlet at 5.26 and 5.20 ppm for their η5-cyclopentadienyl groups, whereas 4c displayed one singlet at 2.02 ppm for its acetyl group bonded to the cyclopentadienyl ring. While the 31P{1H} NMR spectra of 4a−c C

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[NiIIWII(μ-S)2] core. However, in contrast to the internuclear separation between NiII and MoII in 2a,b being much longer than the sum of the covalent radii (Ni, 1.24 Å; Mo, 1.54 Å), the NiII/WII internuclear separation in 4a (2.6201 Å) and 4b (2.5904 Å) is much less than the sum of the covalent radii (Ni, 1.24 Å; W, 1.62 Å),44 which implies that there exists a Ni/W metal−metal bond in each of complexes 4a,b. It should be noted that the Ni/W metal−metal bonds in 4a,b are slightly longer than that (2.5358 Å) of the previously reported for NiI/ WV complex.45 Electrochemical Properties of Dinuclear NiII/MII (M = Mo, W) Complexes 2a,b and 4b,c. Although the electrochemical properties of some dinuclear NiII/M0 (M = Mo, W),40,43,46 NiI/MoV or Ni0/MoVI,47 and NiI/WV45 complexes were previously studied by means of CV techniques, the electrochemical and electrocatalytic properties of dinuclear NiII/MII (M = Mo, W)-based [NiFe]-H2ase models have not been systematically investigated, so far. To know the electrochemical properties of our NiII/MII complexes, we chose 2a,b and 4b,c as representatives (note that 2a,b and 4b,c are suitable representatives of 2a−c and 4a−c since they contain all the different metals Mo/W and different substituents R1/R2 included in 2a−c and 4a−c) to determine their cyclic voltammograms by using n-Bu4NPF6 as the supporting electrolyte in MeCN at a scan rate of 100 mV s−1. As shown in Figure 7 and Table 1, complexes

exhibited one singlet in the region 72−81 ppm for their azadiphosphine P atoms, the 13C{1H} NMR spectra displayed one singlet in the region 221−225 ppm for their terminal carbonyl C atoms. The 11B{1H} NMR spectra of 4a−c, like those of 2a−c, exhibited one singlet at −6.5 ppm. Just like dinuclear Ni/Mo tetraphenylboron complexes 2a−c, Ni/W tetraphenylboron analogues 4a,b were crystallographically characterized, although the molecular structures of precursors 3a−c were unable to be confirmed by X-ray cryatallography owing to the difficulty to grow their qualified single crystals. As can be seen in Figures 5 and 6, dinuclear Ni/W complexes

Figure 5. Molecular structure of the monocation of 4a with ellipsoids drawn at a 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): W(1)−S(1) 2.4440(5), Ni(1)−S(1) 2.2064(5), Ni(1)−P(1) 2.1628(5); W(1)− S(1)−Ni(1) 68.366(15), P(1)−Ni(1)−P(2) 74.112(19), S(1)− W(1)−S(2) 75.964(16), S(1)−Ni(1)−S(2) 82.444(19).

Figure 7. Cyclic voltammograms of 2a,b and 4b,c (1.0 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1.

Table 1. Electrochemical Data of 2a,b and 4b,ca compound

Epc1 (V)

Epc2 (V)

Epa1 (V)

Epa2 (V)

2a 2b 4b 4c

−1.36 −1.31 −1.28 −1.22

−2.04 −2.02 −2.10 −1.95

0.20 0.12 0.09 0.16

0.44 0.46 0.45 0.44

a All potentials are versus Fc/Fc+ in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1.

Figure 6. Molecular structure of the monocation of 4b with ellipsoids drawn at a 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): W(1)−S(1) 2.4927(12), Ni(1)−S(1) 2.3286(14), Ni(1)−P(1) 2.1448(12); W(1)−S(1)−Ni(1) 64.89(3), P(1)−Ni(1)−P(2) 73.94(5), S(1)− W(1)−S(2) 76.92(4), S(1)−Ni(1)−S(2) 84.83(5).

2a,b and 4b,c each display two irreversible reduction waves from −1.22 to −2.04 V and two irreversible oxidation waves from 0.16 to 0.46 V, respectively. All these redox waves could be assigned as one-electron processes since their final Q values determined by controlled-potential electrolysis are close to half that of the known two-electron reduction process of dimer [CpFe(CO)2]2 (see Figures S19−S22).48,49 In addition, since the first reduction wave of stronger electron-donating group p-MeC6H4CH2-substituted complex 2a (−1.36 V) is remarkably shifted negatively up to 50 mV relative to that of weaker

4a,b are both composed of one monocation [R1N(PPh2)2Ni (μ-SEt)2W(η5-R2C5H4)(CO)2]+ and one Ph4B− monoanion. In their monocations, the tetracoordinated Ni and heptacoordinated W are bridged by two EtS ligands to form a butterfly D

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Organometallics electron-donating group EtO2CCH2-substituted 2b (−1.31 V) and the second reduction wave of 2a (−2.04 V) is negatively shifted only 20 mV relative to that of 2b (−2.02 V), the first reduction wave could be assigned to the nickel-centered reduction from the NiII/MoII species to NiI/MoII species, and the second reduction wave could be assigned to the molybdenumcentered reduction from the NiI/MoII species to NiI/MoI species.50,51 However, in contrast to NiII/MoII complexes 2a,b, the first and second reduction waves of NiII/WII complex 4c are positively shifted to 60 and 150 mV relative to those of 4b, respectively; this is consistent with 4c having an electron-withdrawing acetyl group on its cyclopentadienyl ring. Therefore, the first reduction waves of 4b displayed at −1.28 V and those of 4c at −1.22 V might be attributed to the nickel-centered reduction from the NiII/WII species to NiI/WII species, while the second reduction waves of 4b displayed at −2.10 V and those of 4c at −1.95 V could be attributed to the tungsten-centered reduction from the NiI/WII species to NiI/WI species. It is worth noting that all these reduction processes are diffusion-controlled, since the reduction peak currents versus the square root of the scan rates (25−2000 mV s−1) are linearly correlated (see Figure S23).52 Electrocatalytic H2 Production Catalyzed by Dinuclear NiII/MII (M = Mo, W) Complexes 2a,b and 4b,c. To examine whether our NiII/MII complexes 2a,b and 4b,c could be able to catalyze proton reduction to H2, we further determined their cyclic voltammograms in the presence and absence (for comparison) of trifluoroacetic acid (TFA) (pKaMeCN = 12.7).53 Note that 2a,b and 4b,c are also the suitable representatives of 2a−c and 4a−c for the electrocatalytic study. This is because they not only contain all the different metals Mo/W and different substituents R1/R2 included in 2a−c and 4a−c but also are more stable than 2c and 4a under the electrocatalytic conditions. As shown in Figures 8, 9, S24, and S25, when

Figure 9. Cyclic voltammograms of 2b (1.0 mM) with TFA (0, 4, 8, 12, 16, and 20 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1 (reverse scans were partly truncated for clarity).

Figure 10. Influence of acid concentration upon current ratio ratio (icat/ip) for 2a,b and 4b,c (1.0 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1.

the influence of various TFA concentrations upon the icat/ip values and the [icat/ip]max values of 2a,b and 4b,c in the acidindependent range. As the acid concentration increases, the catalytic current for each complex initially increases and then becomes independent of the acid concentration. The observed values of [icat/ip]max for NiII/MoII complexes 2a,b are 37.7 and 33.4, which correspond to the acid-independent turnover frequencies (TOFs) of 275.7 and 216.4 s−1, respectively (see Table 2 and the Supporting Information). For NiII/WII complexes 4b,c, the observed [icat/ip]max values are 17.9 and 20.1, which correspond to the much lower acid-independent TOFs of 62.2 and 78.4 s−1, respectively. It follows that the TOFs values of our NiII/WII complexes 4b,c are much higher than that (12 s−1) of the previously reported NiI/WV complex.45 The overpotential (defined as the difference between the potential at the half-maximum of the catalytic current and the standard potential E0HA of the acid) is another important parameter for evaluating the catalytic efficiency for a catalytic reaction; note that a lower overpotential represents a better catalytic reaction.58−60 We determined the overpotentials of 2a,b and 4b,c by using Evans’ methods.61 At the concentration of 50 mM TFA, the overpotentials for 2a,b and 4b,c were calculated to be 783−810 mV (see Table 2 and Figure S26). As shown in Table 2, p-MeC6H4CH2-substituted 2a exhibits an overpotential

Figure 8. Cyclic voltammograms of 2a (1.0 mM) with TFA (0, 4, 8, 12, 16, and 20 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V s−1 (reverse scans were partly truncated for clarity).

TFA was sequentially added to MeCN solutions of 2a,b and 4b,c, considerable increase for the cathodic currents occurred. Such observations feature the electrocatalytic proton reduction processes catalyzed by 2a,b and 4b,c.54,55 To compare the electrocatalytic activity of 2a,b and 4b,c, we determined the ratio of the catalytic current (icat) to reductive peak current (ip) in the absence of TFA as well as the corresponding turnover frequencies (TOFs); note that a higher icat/ip or TOF value means faster catalysis.56−58 Figure 10 indicates E

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Organometallics Table 2. Electrocatalytic Data of 2a,b and 4b,c in MeCNa compound

TOF (s−1)

overpotential (mV)

2a 2b 4b 4c

275.7 216.4 62.2 78.4

783 810 790 810

R2 = MeCO, Y = Br)64,65 were prepared according to the published procedures. 1H, 13C{1H}, 31P{1H}, and 11B{1H} NMR spectra were obtained on a Bruker Avance 400 NMR spectrometers. IR spectra were recorded on a Bio-Rad FTS 135 infrared spectrophotometer. Elemental analyses were performed on an Elementar Vario EL analyzer. Melting points were determined on a SGW X-4 microscopic melting point apparatus with a microscope and were uncorrected. Preparation of [p-MeC6H4CH2N(PPh2)2Ni(μ-SEt)2Mo(η5-C5H5)(CO)2]Br (1a). A 50 mL three-necked flask fitted with a magnetic stirbar, two serum caps, and a nitrogen inlet tube was charged with p-MeC6H4CH2N(PPh2)2Ni(SEt)2 (0.335 g, 0.50 mmol), (η5-C5H5)(CO)3MoBr (0.163 g, 0.50 mmol), and CH2Cl2 (20 mL). The mixture was stirred at room temperature for 12 h until TLC showed that the starting materials were completely consumed. Solvent was removed at reduced pressure and the residue was subjected to TLC separation using CH2Cl2/MeOH (v/v = 20:1) as eluent. From the brown band, 1a (0.403 g, 83%) was obtained as a brown solid. Mp 112−113 °C. Anal. Calcd for C43H44BrMoNNiO2P2S2: C 53.38; H, 4.58; N, 1.45. Found: C, 53.64; H, 4.83; N, 1.75. IR (KBr disk): νCO 1956 (vs), 1869 (vs); νP−N−P 817 (s) cm−1. 1H NMR (400 MHz, CDCl3): 0.78 (br.s, 6H, 2CH2CH3), 1.87 (br.s, 4H, 2CH2CH3), 2.11 (s, 3H, CH3C6H4), 3.83−3.88 (m, 2H, CH2N), 5.41 (s, 5H, C5H5), 6.12, 6.58 (dd, J = 7.6 Hz, 4H, C6H4), 7.57−7.79 (m, 20H, 4C6H5) ppm. 13 C{1H} NMR (100 MHz, CDCl3): 20.8, 28.6, 29.5 (3s, CH2CH3, CH3C6H4), 51.7 (s, CH2N), 94.7 (s, C5H5), 126.0−138.3 (m, C6H4, C6H5), 244.8 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 60.4 (s) ppm. Preparation of [EtO2CCH2N(PPh2)2Ni(μ-SEt)2Mo(η5-C5H5)(CO)2]Cl (1b). The same procedure was followed as for preparation of 1a, except that EtO2CCH2N(PPh2)2Ni(SEt)2 (0.326 g, 0.50 mmol) and (η5-C5H5)(CO)3MoCl (0.140 g, 0.50 mmol) were utilized instead of p-MeC6H4CH2N(PPh2)2Ni(SEt)2 and (η5-C5H5)(CO)3MoBr, respectively. From the brown band, 1b (0.365 g, 81%) was obtained as a brown solid. Mp 83−84 °C. Anal. Calcd for C39H42ClMoNNiO4P2S2: C 51.76; H, 4.68; N, 1.55. Found: C, 52.00; H, 4.67; N, 1.80. IR (KBr disk): νCO 1954 (vs), 1869 (vs); νC=O 1732 (s); νP−N−P 820 (s) cm−1. 1H NMR (400 MHz, CDCl3): 0.84−0.89 (m, 9H, 2SCH2CH3, OCH2CH3), 1.93 (br.s, 4H, 2CH2CH3), 3.32−3.38 (m, 2H, OCH2CH3), 3.58−3.61 (m, 2H, CH2N), 5.46 (s, 5H, C5H5), 7.64− 7.92 (m, 20H, 4C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 13.6, 18.0 (2s, SCH2CH3, OCH2CH3), 28.6 (s, SCH2CH3), 47.9 (s, CH2N), 61.9 (s, OCH2CH3), 94.8 (s, C5H5), 126.0−133.9 (m, C6H5), 166.6 (s, CO), 244.0 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 64.2 (s) ppm. Preparation of [EtO 2 CCH 2 N(PPh 2 ) 2 Ni(μ-SEt) 2 Mo(η 5 MeCOC5H4)(CO)2]Br (1c). The same procedure was followed as for preparation of 1b, except that (η5-MeCOC5H4)(CO)3MoBr (0.184 g, 0.50 mmol) was employed instead of (η5-C5H5)(CO)3MoCl. From the brown band, 1c (0.347 g, 70%) was obtained as a brown solid. Mp 85−86 °C. Anal. Calcd for C41H44BrMoNNiO5P2S2: C 49.67; H, 4.47; N, 1.41. Found: C, 49.89; H, 4.55; N, 1.71. IR (KBr disk): νCO 1964 (vs), 1885 (s); νC=O 1735 (s), 1681 (s); νP−N−P 813 (s) cm−1. 1H NMR (400 MHz, CDCl3): 0.86−0.93 (m, 9H, 2SCH2CH3, OCH2CH3), 1.91−2.03 (m, 4H, 2SCH2CH3), 2.20 (s, 3H, CH3CO), 3.30−3.38 (m, 2H, OCH2CH3), 3.58−3.65 (m, 2H, CH2N), 5.79, 5.96 (2s, 4H, C5H4), 7.28−8.03 (m, 20H, 4C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 13.7, 18.2 (2s, SCH2CH3, OCH2CH3), 27.4, 28.8 (2s, SCH2CH3, CH3CO), 47.9 (s, CH2N), 62.0 (s, OCH2CH3), 92.5, 102.4 (2s, C5H4), 129.9−134.2 (m, C6H5), 166.6 (s, CH3CH2OCO), 194.2 (s, CH3CO), 242.3 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 63.0 (s) ppm. Preparation of [p-MeC6H4CH2N(PPh2)2Ni(μ-SEt)2Mo(η5-C5H5)(CO)2]BPh4 (2a). A 50 mL three-necked flask fitted with a magnetic stir-bar, two serum caps, and a nitrogen inlet tube was charged with 1a (0.145 g, 0.15 mmol), NaBPh4 (0.072 g, 0.21 mmol), and CH2Cl2 (15 mL). The mixture was stirred at room temperature for 1 h until TLC showed that starting material 1a was completely consumed. Solvent was removed at reduced pressure and the residue was subjected to TLC separation using CH2Cl2/MeOH (v/v = 20:1) as eluent. From the brown band, 2a (0.163 g, 90%) was obtained as a brown solid.

a

The TOF and overpotential values of 2a,b and 4b,c were determined by the same methods.

27 mV lower than that of EtO2CCH2-substituted 2b, while η5-C5H5-coordinated 4b exhibits an overpotential 20 mV lower than that of η5-MeCOC5H4-coordinated 4c. In addition, it should be noted that the overpotential values of our NiII/WII complexes are higher than that (700 mV) of the previously reported NiI/Wv complex.45 To further confirm the H2-producing ability catalyzed by 2a,b and 4b,c, we carried out the bulk electrolysis for each of 2a,b and 4b,c (0.5 mM) in a MeCN solution with excess TFA (25 mM). During 1 h of the bulk electrolysis, a total of 60.6− 65.6 F mol−1 passed. Gas chromatographic (GC) analysis demonstrated that the Faradaic efficiency for all 2a,b and 4b,c is above 90%.

■

SUMMARY AND CONCLUSIONS To gain an insight into the catalytic mechanism for H2 production catalyzed by [NiFe]-H2ases and to search for the highly efficient non-noble metal-containing H2-producing catalysts, we have designed and synthesized the first Ni/M (M = Mo, W)based [NiFe]-H2ase model complexes (1a−c to 4a−c) with a general formula [R1N(PPh2)2Ni(μ-SEt)2M(η5-R2C5H4)(CO)2] Y (M = Mo, W; R1 = p-MeC6H4CH2, EtO2CCH2; R2 = H, MeCO; Y = Cl, Br, Ph4B). While Ni/M (M = Mo, W) complexes 1a−c and 3a−c are prepared by coordination reactions of the two SEt ligands in R1N(PPh2)2-chelated Ni dithiolates R1N(PPh2)2Ni(μ-SEt)2 with Mo or W atom in the (η5-R2C5H4)(CO)3M moiety followed by loss of one CO ligand, Ni/M complexes 2a−c and 4a−c are prepared by anionic Cl−/ Ph4B− or Br−/Ph4B− exchange reactions between 1a−c and NaBPh4 or between 3a−c and NaBPh4, respectively. X-ray crystallographic study reveals that (i) Ni/M (M = Mo, W) complexes 2a−c and 4a,b contain a butterfly [NiIIMII(μ-S)2] core that is similar to [NiIIFeII(μ-S)2] core present in [NiFe]H2ases and (ii) while Ni/Mo complexes 2a−c do not have metal−metal bonds between their two metal centers due to the internuclear distances being much longer than the sum of the covalent radii (Ni, 1.24 Å; Mo, 1.54 Å), Ni/W complexes 4a,b have the metal−metal bonds between their two metal centers owing to the internuclear distances being much shorter than the sum of the covalent radii (Ni, 1.24 Å; W, 1.62 Å). Particularly interesting is that complexes 2a,b and 4b,c have been demonstrated to be catalysts for proton reduction to H2 from TFA under CV conditions. It follows that complexes 2a,b and 4b,c could be regarded not only as the structural models but also as the functional models for [NiFe]-H2ases.

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EXPERIMENTAL SECTION

General Comments. All reactions were carried out by using standard Schlenk and vacuum-line techniques under an atmosphere of highly purified nitrogen. Solvents CH2Cl2 and MeOH were dried and distilled under N2 prior to use. While NaBPh4 was available commercially, R1N(PPh2)2Ni(SEt)2 (R1 = p-MeC6H4CH2, EtO2CCH2),39 (η5-R2C5H4)(CO)3MoY (R2 = H, Y = Cl; R2 = H, Y = Br; R2 = MeCO, Y = Br),62,63 and (η5-R2C5H4)(CO)3WY (R2 = H, Y = Cl; F

DOI: 10.1021/acs.organomet.8b00228 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Found: C, 47.12; H, 4.44; N, 1.43. IR (KBr disk): νCO 1988 (s), 1941 (s); νC=O 1734 (s); νP−N−P 815 (m) cm−1. 1H NMR (400 MHz, CDCl3): 0.88−0.96 (m, 9H, 2SCH2CH3, OCH2CH3), 2.18−2.20 (m, 4H, 2SCH2CH3), 3.25−3.31 (m, 2H, OCH2CH3), 3.62− 3.70 (m, 2H, CH2N), 5.77 (s, 5H, C5H5), 7.58−7.90 (m, 20H, 4C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 13.6, 17.9 (2s, SCH2CH3, OCH2CH3), 31.0 (s, SCH2CH3), 48.1 (s, CH2N), 61.6 (s, OCH2CH3), 92.1 (s, C5H5), 128.9−133.5 (m, C6H5), 167.3 (s, CO), 207.1 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 80.2 (s) ppm. Preparation of [EtO 2 CCH 2 N(PPh 2 ) 2 Ni(μ-SEt) 2 W(η 5 MeCOC5H4)(CO)2]Br (3c). The same procedure was followed as for preparation of 3b, except that (η5-MeCOC5H4)(CO)3WBr (0.177 g, 0.39 mmol) was employed in place of (η5-C5H5)(CO)3WCl. From the brown band, 3c (0.214 g, 66%) was obtained as a brown solid. Mp 91−92 °C. Anal. Calcd for C41H44BrNNiO5P2S2W: C 45.63; H, 4.11; N, 1.30. Found: C, 45.29; H, 4.40; N, 1.35. IR (KBr disk): νCO 1990 (s), 1951 (vs), 1871 (vs); νC=O 1732 (s), 1685 (s); νP−N−P 816 (s) cm−1. 1H NMR (400 MHz, CDCl3): 0.87−0.98 (m, 9H, 2SCH2CH3, OCH2CH3), 1.82−2.19 (m, 4H, 2SCH2CH3), 2.27 (s, 3H, CH3CO), 3.29−3.40 (m, 2H, OCH2CH3), 3.61−3.79 (m, 2H, CH2N), 6.03, 6.16 (2s, 4H, C5H4), 7.30−8.02 (m, 20H, 4C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 13.7, 18.2 (2s, SCH2CH3, OCH2CH3), 27.7, 30.5 (2s, SCH2CH3, CH3CO), 48.0 (s, CH2N), 61.8 (s, OCH2CH3), 90.0, 99.8 (2s, C5H4), 127.3−133.5 (m, C6H5), 167.0 (s, CH3CH2OCO), 193.5 (s, CH3CO), 225.0 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 73.6 (s) ppm. Preparation of [p-MeC6H4CH2N(PPh2)2Ni(μ-SEt)2W(η5-C5H5)(CO)2]BPh4 (4a). A 50 mL three-necked flask equipped with a magnetic stir-bar, two serum caps, and a nitrogen inlet tube was charged with 3a (0.152 g, 0.15 mmol), NaBPh4 (0.072 g, 0.21 mmol), and CH2Cl2 (15 mL). The mixture was stirred at room temperature for 1 h until TLC showed that starting material 3a was completely consumed. Solvent was removed at reduced pressure and then the residue was subjected to TLC separation using CH2Cl2/MeOH (v/v = 20:1) as eluent. From the brown band, 4a (0.161 g, 83%) was obtained as a brown solid. Mp 100−101 °C. Anal. Calcd for C67H64BNNiO2P2S2W: C 62.16; H, 4.98; N, 1.08. Found: C, 62.09; H, 5.10; N, 1.04. IR (KBr disk): νCO 1984 (s), 1943 (vs), 1863 (s); νP−N−P 814 (m) cm−1. 1H NMR (400 MHz, CDCl3): 0.78−0.82 (m, 6H, 2CH2CH3), 1.67−1.98 (m, 4H, 2CH2CH3), 2.14 (s, 3H, CH3C6H4), 3.80−3.91 (m, 2H, CH2N), 5.26 (s, 5H, C5H5), 6.12, 6.61 (dd, J = 8.0 Hz, 4H, C6H4), 6.84−7.76 (m, 40H, 8C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 17.9, 21.1, 29.8 (3s, CH2CH3, CH3C6H4), 52.1 (s, CH2N), 92.2 (s, C5H5), 121.7−165.1 (m, C6H4, C6H5), 224.7 (s, CO) ppm. 31 1 P{ H} NMR (162 MHz, CDCl3, 85% H3PO4): 73.5 (s) ppm. 11 1 B{ H} NMR (128 MHz, CDCl3, BF3·Et2O): −6.5 (s) ppm. Preparation of [EtO2CCH 2N(PPh 2) 2Ni(μ-SEt)2W(η5-C5H5)(CO)2]BPh4 (4b). The same procedure was followed as for preparation of 4a, but 3b (0.149 g, 0.15 mmol) was employed instead of 3a. From the brown band, 4b (0.174 g, 91%) was obtained as a brown solid. Mp 76−77 °C. Anal. Calcd for C63H62BNNiO4P2S2W: C 59.27; H, 4.90; N, 1.10. Found: C, 59.52; H, 4.73; N, 0.95. IR (KBr disk): νCO 1988 (s), 1943 (vs); νC=O 1737 (s); νP−N−P 815 (m) cm−1. 1H NMR (400 MHz, CDCl3): 0.85−0.89 (m, 9H, 2SCH2CH3, OCH2CH3), 1.71− 2.11 (m, 4H, 2SCH2CH3), 3.19−3.25 (m, 2H, OCH2CH3), 3.60−3.65 (m, 2H, CH2N), 5.20 (s, 5H, C5H5), 6.77−7.88 (m, 40H, 8C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 13.7, 18.0 (2s, SCH2CH3, OCH2CH3), 29.8 (s, SCH2CH3), 48.3 (s, CH2N), 61.7 (OCH2CH3), 91.8 (s, C5H5), 121.7−165.1 (m, C6H5), 167.5 (s, CO), 221.2 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 80.3 (s) ppm. 11B{1H} NMR (128 MHz, CDCl3, BF3·Et2O): −6.5 (s) ppm. Preparation of [EtO 2 CCH 2 N(PPh 2 ) 2 Ni(μ-SEt) 2 W(η 5 MeCOC5H4)(CO)2]BPh4 (4c). The same procedure was followed as for preparation of 4a, but 3c (0.162 g, 0.15 mmol) was utilized in place of 3a. From the brown band, 4c (0.131 g, 66%) was obtained as a brown solid. Mp 91−92 °C. Anal. Calcd for C65H64BNNiO5P2S2W: C 59.21; H, 4.89; N, 1.06. Found: C, 59.38; H, 5.03; N, 0.94. IR (KBr disk): νCO 1953 (s), 1872 (s); νC=O 1737 (s), 1684 (s); νP−N−P 817 (m) cm −1 . 1 H NMR (400 MHz, CDCl 3 ): 0.80−0.88

Mp 100−101 °C. Anal. Calcd for C67H64BMoNNiO2P2S2: C 66.68; H, 5.35; N, 1.16. Found: C, 66.55; H, 5.40; N, 1.35. IR (KBr disk): νCO 1956 (vs), 1874 (vs); νP−N−P 815 (s) cm−1. 1H NMR (400 MHz, CDCl3): 0.76 (s, 6H, 2CH2CH3), 1.83 (br.s, 4H, 2CH2CH3), 2.15 (s, 3H, CH3C6H4), 3.75−3.85 (m, 2H, CH2N), 5.30 (s, 5H, C5H5), 6.13, 6.61 (dd, J = 7.4 Hz, 4H, C6H4), 6.86−7.74 (m, 40H, 8C6H5) ppm. 13 C{1H} NMR (100 MHz, CDCl3): 18.0, 21.1, 28.8 (3s, CH2CH3, CH3C6H4), 51.9 (s, CH2N), 94.9 (s, C5H5), 121.6−165.1 (m, C6H4, C6H5), 245.1 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 60.4 (s) ppm. 11B{1H} NMR (128 MHz, CDCl3, BF3·Et2O): −6.5 (s) ppm. Preparation of [EtO2CCH2N(PPh2)2Ni(μ-SEt)2Mo(η5-C5H5)(CO)2]BPh4 (2b). The same procedure was followed as for preparation of 2a, but 1b (0.136 g, 0.15 mmol) was used in place of 1a. From the brown band, 2b (0.161 g, 90%) was obtained as a brown solid. Mp 91−92 °C. Anal. Calcd for C63H62BMoNNiO4P2S2: C 63.65; H, 5.26; N, 1.18. Found: C, 63.59; H, 5.30; N, 1.34. IR (KBr disk): νCO 1958 (vs), 1877 (vs), νCO 1738 (s); νP−N−P 818 (m) cm−1. 1H NMR (400 MHz, CDCl3): 0.83 (s, 9H, 2SCH2CH3, OCH2CH3), 1.85 (br.s, 4H, 2SCH2CH3), 3.28 (t, J = 12.0 Hz, 2H, OCH2CH3), 3.47−3.74 (m, 2H, CH2N), 5.31 (s, 5H, C5H5), 6.85−7.88 (m, 40H, 8C6H5) ppm. 13 C{1H} NMR (100 MHz, CDCl3): 13.7, 18.2 (2s, SCHCH3, OCHCH 3 ), 28.6 (2s, SCH 2 CH 3 ), 47.9 (s, CH 2 N), 61.9 (s, OCH2CH3), 94.8 (s, C5H5), 121.6−165.0 (m, C6H5), 166.7 (s, CO), 244.1 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 64.3 (s) ppm. 11B{1H} NMR (128 MHz, CDCl3, BF3· Et2O): −6.5 (s) ppm. Preparation of [EtO 2 CCH 2 N(PPh 2 ) 2 Ni(μ-SEt) 2 Mo(η 5 MeCOC5H4)(CO)2] BPh4 (2c). The same procedure was followed as for preparation of 2a, but 1c (0.149 g, 0.15 mmol) was employed instead of 1a. From the brown band, 2c (0.162 g, 88%) was obtained as a brown solid. Mp 81−82 °C. Anal. Calcd for C65H64BMoNNiO5P2S2: C 63.43; H, 5.24; N, 1.14. Found: C, 63.19; H, 5.32; N, 0.98. IR (KBr disk): νCO 1963 (vs), 1889 (vs); νC=O 1733 (s), 1683 (s); νP−N−P 818 (s) cm−1. 1H NMR (400 MHz, CDCl3): 0.81−0.87 (m, 9H, 2SCH2CH3, OCH2CH3), 1.84−1.86 (m, 4H, 2SCH2CH3), 2.08 (s, 3H, CH3CO), 3.24−3.32 (m, 2H, OCH2CH3), 3.56−3.61 (m, 2H, CH2N), 5.40, 5.68 (2s, 4H, C5H4), 6.83−7.88 (m, 40H, 8C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 13.7, 18.1 (2s, SCH2CH3, OCH2CH3), 27.2, 28.6 (2s, CH3CO, SCH2CH3), 46.0 (s, CH2N), 62.0 (s, OCH2CH3), 92.3, 102.0 (2s, C5H4), 121.7− 165.1 (m, C6H5), 166.5 (s, CH3CH2OCO), 194.0 (s, CH3CO), 242.3 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 62.9 (s) ppm. 11B{1H} NMR (128 MHz, CDCl3, BF3·Et2O): −6.5 (s) ppm. Preparation of [p-MeC6H4CH2N(PPh2)2Ni(μ-SEt)2W(η5-C5H5)(CO)2]Cl (3a). A 50 mL three-necked flask equipped with a magnetic stir-bar, two serum caps, and a nitrogen inlet tube was charged with p-MeC6H4CH2N(PPh2)2Ni(SEt)2 (0.200 g, 0.30 mmol), (η5-C5H5)(CO)3WCl (0.144 g, 0.39 mmol) and CH2Cl2 (15 mL). The mixture was stirred at room temperature for 12 h to give a brown mixture. Solvent was removed at reduced pressure and then the residue subjected to TLC separation using CH2Cl2/MeOH (v/v = 15:1) as eluent. From the brown band, 3a (0.197 g, 65%) was obtained as a brown solid. Mp 91−92 °C. Anal. Calcd for C43H44ClNNiO2P2S2W: C 51.09; H, 4.39; N, 1.39. Found: C, 50.90; H, 4.21; N, 1.46. IR (KBr disk): νCO 1987 (s), 1940 (vs); νP−N−P 816 (s) cm−1. 1H NMR (400 MHz, CDCl3): 0.83 (br.s, 6H, 2CH2CH3), 1.78−2.15 (m, 7H, 2CH2CH3, CH3C6H4), 3.81−3.86 (m, 2H, CH2N), 5.66 (s, 5H, C5H5), 6.11, 6.59 (dd, J = 6.4 Hz, 4H, C6H4), 7.46−7.83 (m, 20H, 4C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 17.9, 21.0, 29.6 (3s, CH2CH3, CH3C6H4), 52.0 (s, CH2N), 92.4 (s, C5H5), 129.0−138.1 (m, C6H4, C6H5), 224.6 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 73.2 (s) ppm. Preparation of [EtO2CCH2N(PPh2)2Ni(μ-SEt)2W(η5-C5H5)(CO)2] Cl (3b). The same procedure was followed as for preparation of 3a, except that EtO2CCH2N(PPh2)2Ni(SEt)2 (0.196 g, 0.30 mmol) was utilized in place of p-MeC6H4CH2N(PPh2)2Ni(SEt)2. From the brown band, 3b (0.143 g, 48%) was obtained as a brown solid. Mp 84−85 °C. Anal. Calcd for C39H42ClNNiO4P2S2W: C 47.18; H, 4.26; N, 1.41. G

DOI: 10.1021/acs.organomet.8b00228 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Table 3. Crystal Data and Structure Refinements Details for 2a−c

mol formula mol wt cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g·cm−3) abs coeff (mm−1) F(000) index ranges no. of reflns no. of indep reflns 2θmax (deg) R Rw goodness of fit largest diff peak, hole (e Å−3)

2a

2b

2c

C134H128B2Mo2N2Ni2 O4P4S4·C3H6O 2471.49 triclinic P1̅ 13.6092(14) 14.3622(15) 15.4504(15) 93.465(3) 92.202(3) 99.384(3) 2970.5(5) 1 1.382 0.700 1284 −17 ≤ h ≤ 17 −18 ≤ k ≤ 18 −19 ≤ l ≤ 20 38544 13449 55.22 0.0246 0.0640 1.034 0.515/−0.402

C63H62BMoNNiO4 P2S2·C3H6O 1246.73 triclinic P1̅ 13.636(3) 14.247(3) 15.465(3) 92.053(5) 96.252(4) 99.454(4) 2941.6(10) 2 1.408 0.710 1296 −17 ≤ h ≤ 17 −18 ≤ k ≤ 18 −20 ≤ l ≤ 20 37856 13318 55.24 0.0286 0.0808 1.073 1.131/−0.640

C65H64BMoNNiO5 P2S2 1230.69 triclinic P1̅ 13.6702(4) 14.3036(4) 17.2283(4) 107.027(2) 109.417(3) 99.367(2) 2908.74(15) 2 1.405 0.717 1276 −16 ≤ h ≤ 17 −17 ≤ k ≤ 15 −21 ≤ l ≤ 21 28664 12017 53.00 0.0403 0.0878 1.025 0.462/−0.459

with Mo Kα radiation (λ = 0.71073 Å) in the ω scanning mode at 113 K. A single crystal of 2b was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Rigaku XtaLab P200 accessory,

(m, 9H, 2SCH2CH3, OCH2CH3), 1.72−1.94 (m, 4H, 2SCH2CH3), 2.02 (s, 3H, CH3CO), 3.21−3.31 (m, 2H, OCH2CH3), 3.56−3.66 (m, 2H, CH2N), 5.23, 5.53 (2s, 4H, C5H4), 6.82−7.85 (m, 40H, 8C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 13.7, 18.1 (2s, SCH2CH3, OCH2CH3), 27.1, 29.8 (2s, SCH2CH3, CH3CO), 48.1 (s, CH2N), 61.9 (s, OCH2CH3), 89.3, 99.8 (2s, C5H4), 121.7− 165.1 (m, C6H5), 167.0 (s, CH3CH2OCO), 193.1 (s, CH3CO), 225.0 (s, CO) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 72.6 (s) ppm. 11B{1H} NMR (128 MHz, CDCl3, BF3·Et2O): −6.5 (s) ppm. Electrochemical and Electrocatalytic Experiments. Cyclic voltammograms were recorded on a BAS Epsilon electrochemical analyzer using a three-electrode cell with a 3 mm diameter glassy carbon working electrode, a platinum counter electrode and an Ag/Ag+ (0.01 M AgNO3/0.1 M n-Bu4NPF6 in MeCN) reference electrode. The working electrode was polished with 0.05 μm alumina paste and sonicated in water for about 10 min prior to use. Solutions were degassed by bubbling with dry Ar for at least 10 min, and during measurements, a blanket of Ar was maintained over the solutions. All experiments were performed at room temperature in a MeCN (Amethyst Chemicals, HPLC-grade) solution of 1.0 mM analyte and 0.1 M supporting electrolyte n-Bu4NPF6. The electrolyte n-Bu4NPF6 was dried in an oven at 110 °C for at least 24 h. Bulk electrolysis and controlled-potential electrolysis experiments were run on a vitreous carbon rod (A = 2.9 cm2) in a two-compartment, gas-tight, H-type electrolysis cell containing ca. 25 mL of MeCN. Ferrocene (Fc) served as the internal reference, and all potentials are reported relative to the Fc/Fc+ couple at 0.00 V. Gas chromatography was performed with a Shimadzu gas chromatograph GC-2014 under isothermal conditions with nitrogen as a carrier gas and a thermal conductivity detector. X-ray Crystal Structure Determinations of 2a−c and 4a,b. While single crystals of 2c and 4a were grown by slow evaporation of their CH2Cl2/n-hexane solutions at −5 °C, those of 2a, 2b and 4b were grown by slow evaporation of their acetone/n-hexane solutions at −5 °C. A single crystal of 2a was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with Rigaku Pilatus 200 K accessory, and data were collected using a confocal monochromator

Table 4. Crystal Data and Structure Refinements Details for 4a,b

mol formula mol wt cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g·cm−3) abs coeff (mm−1) F(000) index ranges no. of reflns no. of indep reflns 2θmax (deg) R Rw goodness of fit largest diff peak, hole (e Å−3) H

4a

4b

C67H64BNNiO2 P2S2W 1294.62 triclinic P1̅ 9.9682(10) 16.6703(16) 18.6493(19) 111.267(2) 95.783(2) 92.5240(10) 2862.4(5) 2 1.502 2.512 1316 −11 ≤ h ≤ 11 −19 ≤ k ≤ 19 −22 ≤ l ≤ 18 25146 9890 50.02 0.0171 0.0449 1.043 0.615/−0.879

C63H62BNNiO4 P2S2W 1276.56 triclinic P1̅ 9.826(2) 17.199(3) 18.955(4) 75.13(3) 82.89(3) 81.87(3) 3052.2(12) 2 1.389 2.357 1296 −12 ≤ h ≤ 12 −22 ≤ k ≤ 22 −24 ≤ l ≤ 24 36788 14481 55.82 0.0493 0.1120 1.055 1.628/−2.228

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and data were collected using a confocal monochromator with Mo Kα radiation (λ = 0.71073 Å) in the ω scanning mode at 113 K. A single crystal of 4a was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Rigaku Pilatus 200 K accessory, and data were collected using a confocal monochromator with Mo Kα radiation (λ = 0.71073 Å) in the ω scanning mode at 113 K. A single crystal of 4b was mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Bruker P4 accessory, and data were collected using a confocal monochromator with Mo Kα radiation (λ = 0.71073 Å) in the ω scanning mode at 113 K. A single crystal of 2c was mounted on a SuperNova diffractometer equipped with a AtlasS2 accessory, while the data of 2c were collected using a mirror monochromator with Mo Kα radiation (λ = 0.71073 Å) in the ω scanning mode at 154 K. Data collection, reduction, and absorption correction were performed by the CRYSTALCLEAR program.66 The structures were solved by direct methods using the SHELXS program67 and refined by full-matrix leastsquares techniques (SHELXL)68 on F2. Hydrogen atoms were located by using the geometric method. Details of crystal data, data collections, and structure refinements are summarized in Tables 3 and 4.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00228. Corresponding IR and NMR spectra for 1a, 2c, 3a, and 4b, some electrochemical and electrocatalytic data and plots (PDF) Accession Codes

CCDC 1820572−1820576 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

Li-Cheng Song: 0000-0003-0964-8869 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Ministry of Science and Technology of China (973 program 2014CB845604) and the National Natural Science Foundation of China (21472095 and 21772106) for financial support.

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REFERENCES

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