Synthesis of Cobaltocenes Bearing 4-(2,6-Dimethylpyridin-1-ium-4-yl)phenyl Moiety and Their Stoichiometric and Catalytic Reactivity toward Ammonia Formation
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Synthesis of Cobaltocenes Bearing 4-(2,6-Dimethylpyridin-1-ium-4-yl)phenyl Moiety and Their Stoichiometric and Catalytic Reactivity toward Ammonia Formation
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Organometallics

Cite this: Organometallics 2026, XXXX, XXX, XXX-XXX
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https://doi.org/10.1021/acs.organomet.5c00492
Published March 22, 2026

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Abstract

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Novel cobaltocenium compounds bearing a 4-(2,6-dimethylpyridin-4-yl)phenyl group are designed and synthesized for the use of proton-coupled electron transfer (PCET) reagents. Reduction and protonation of these compounds afford the corresponding cobaltocenes bearing a 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety with bond dissociation free energies (BDFEs) of 34.4–43.0 kcal/mol. A superstoichiometric amount of ammonia is formed from the catalytic reduction of dinitrogen with these cobaltocenes in the presence of a molybdenum-nitride complex under ambient reaction conditions.

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Introduction

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Ammonia formation from dinitrogen is one of the most essential industrial processes because ammonia becomes more and more important not only as a feedstock but also as a new energy carrier for a carbon-neutral society. (1−3) The Haber–Bosch process as the current main method for industrial ammonia production from dinitrogen and dihydrogen requires harsh reaction conditions such as high temperature and high pressure, and releases a substantial amount of carbon dioxide by the synthesis of dihydrogen from a considerable amount of fossil fuels. (4,5) Therefore, a new method for ammonia synthesis without the use of fossil fuels under milder reaction conditions is desired for the future of humanity.
For the last 20 years, transition-metal-catalyzed ammonia formation under mild reaction conditions has been intensively studied. (6−9) Especially, our research group found that molybdenum complexes bearing benzimidazole-based PCP-type pincer ligands worked as excellent catalysts for ammonia formation from dinitrogen under ambient reaction conditions. (10,11) In our reaction system as shown in Figure 1a, molybdenum-nitride complex [Mo(N)I(PCP)] as a key reactive intermediate is generated, and then three electrons and protons transfer to the nitride complex proceeds to afford ammine complex [Mo(NH3)I(PCP)] via the stepwise formation of imide complex [Mo(NH)I(PCP)] and amide complex [Mo(NH2)I(PCP)] (Figure 1b). Our experimental and theoretical studies indicate that the imide complex formation is the rate-determining step of the catalytic ammonia formation. (12,13)

Figure 1

Figure 1. (a) Catalytic cycle for ammonia production from dinitrogen and water using [MoI3(PCP)] as a catalyst. (b) Stepwise conversion of [Mo(N)I(PCP)] into [Mo(NH3)I(PCP)] via reduction and protonation. (c) Catalytic ammonia formation reaction using the combination of SmI2 and water and that of cobaltocene and pyridinium salt.

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After detailed investigations, we found that the combination of samarium diiodide (SmI2) and water (14,15) exhibited substantially higher catalytic activity than that of cobaltocene derivatives and pyridinium salts under ambient reaction conditions (Figure 1c). (10,11) In fact, up to 60,000 equiv of ammonia was produced based on the molybdenum atom of the catalyst when SmI2 and water were used as reductant and proton source, respectively. (14) In our reaction system, proton-coupled electron transfer (PCET) process occurs between the molybdenum-nitride complex and a samarium aqua complex (SmI2(H2O)n) (16) formed in situ from SmI2 and water. We consider that the PCET plays an important role in achieving excellent catalytic activity.
The hybridization of reductant and Brønsted acid has attracted attention as a typical strategy to develop PCET reagents. Recently, Peters and co-workers found that a cobaltocene derivative bearing N,N-dimethyl-4-anilinium moiety (17) worked as an efficient PCET reagent toward the reduction of dinitrogen into ammonia under electrochemical conditions (Figure 2a). (18) After the pioneering work by Peters and co-workers, we designed and prepared neutral and anionic iron sandwich complexes bearing π-phenol ligand as PCET reagents (Figure 2b). (19,20) Separately, we also designed and prepared samarium complexes bearing cyclopentadienyl-amine ligands as PCET reagents (Figure 2c). Interestingly, the samarium complexes worked as efficient cocatalysts in the molybdenum-catalyzed ammonia formation under ambient reaction conditions. (21)

Figure 2

Figure 2. (a) Previous work by Peters and co-workers: cobaltocene derivative bearing N,N-dimethylanilinium. (17,18) (b) Our previous work: iron complexes bearing π-phenol ligand. (19,20) (c) Our previous work: samarium complexes bearing cyclopentadienyl-amine chelate ligands. (21) (d) This work: cobaltocenes bearing 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety [1-H]+.

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Based on these research backgrounds, we have newly designed cobaltocenes bearing a 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety, which hybridizes cobaltocene and 2,6-dimethylpyridinium linked via a phenylene group because cobaltocene and 2,6-dimethylpyridinium worked as suitable reductant and proton source, respectively. Herein, we report the preparation of three cobaltocenes bearing 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety such as [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2NH))]OTf ([1a-H]OTf; Tf = SO2CF3), [(η5-C5Me5)Co(η5-C5H4(C6H4–C5H2Me2NH))]OTf ([1b-H]OTf), and [(η5-C5H5)Co(η5-C5H4(C6H4–C5H2Me2NH))]OTf ([1c-H]OTf), and their stoichiometric and catalytic activity with the molybdenum-nitride complex under ambient reaction conditions (Figure 2d).

Results and Discussion

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DFT Calculations

We carried out density functional theory (DFT) calculations at the B3LYP-D3 level of theory. The BDFEN–H (i.e., BDFEcalcd) values of the newly designed cobaltocenes [1-H]+ in the doublet spin state are as follows (Table 1): 32.9 kcal/mol ([1a-H]+), 37.9 kcal/mol ([1b-H]+), and 47.6 kcal/mol ([1c-H]+). For comparison, the BDFEN–H of the molybdenum-imide complex [Mo(NH)I(PCP)] was previously reported to be 33.8 kcal/mol (14) because the molybdenum-imide complex [Mo(NH)I(PCP)] has the weakest N–H bond among the reactive intermediates [Mo(NHx)I(PCP)] (x = 1–3). The result of the DFT calculations indicates that [1a-H]+ has the potential to work as a PCET reagent toward the molybdenum-nitride complex [Mo(N)I(PCP)] under ambient reaction conditions.
Table 1. BDFEsN–H of [1-H]+ in THF
cobaltocenespKaaE(CoII/III) (V)bBDFEexp (kcal/mol)cBDFEcalcd (kcal/mol)d
[1a-H]+9.0–1.6434.432.9
[1b-H]+8.8–1.5237.037.9
[1c-H]+8.9–1.2643.047.6
a

For [1-H](OTf)2 measured in THF using UV–vis titration.

b

For [1]OTf in the presence of 10 equiv of [ColH]OTf as a proton source in THF (vs Fc/Fc+, Figure S20).

c

Estimated from eq 1 with each pKa and E(CoII/III) of [1-H]2+.

d

Determined by DFT calculations.

Preparation of Cobaltocenes Bearing 4-(2,6-Dimethylpyridin-1-ium-4-yl)phenyl Moiety

First, we prepared a cobaltocenium compound bearing 4-(2,6-dimethylpyridin-4-yl)phenyl group [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2N))]OTf ([1a]OTf). The reaction of nonamethylcobaltocenium hexafluorophosphate [(η5-C5Me5)Co(η5-C5Me4H)]PF6 with 1.1 equiv of (4-(2,6-dimethylpyridin-4-yl)phenyl)lithium, generated in situ from 4-(4′-bromophenyl)-2,6-dimethylpyridine and nBuLi, (22) was carried out in tetrahydrofuran (THF) at −78 °C for 3 h, and then warmed to room temperature overnight. After the addition of 1 equiv of ferrocenium triflate (FcOTf), the reaction mixture was stirred at room temperature for 1 h to afford [1a]OTf in a 10% yield (Scheme 1). This compound was characterized by 1H and 13C NMR spectroscopy together with elemental analysis. The detailed structure of [1a]OTf was confirmed by X-ray analysis. An ORTEP drawing of [1a]OTf is shown in Figure 3a.

Scheme 1

Scheme 1. Synthesis of [1a]OTf, [1a], and [1a-H](OTf)2
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Figure 3

Figure 3. ORTEP drawings of the cationic part of (a) [1a]OTf and (b) [1a-H](OTf)2. Hydrogen atoms except for a NH hydrogen atom in [1a-H](OTf)2 were omitted for clarity.

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Protonation of [1a]OTf with 5 equiv of HOTf in dimethoxyethane (DME) at room temperature for 2 h gave the corresponding cobaltocenium compound bearing the 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl group [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2NH))](OTf)2 [1a-H](OTf)2 in 63% yield (Scheme 1). A peak assigned as a proton on the N atom of pyridinium was observed at 12.8 ppm in 1H NMR. IR spectrum exhibited an N–H stretching at 3292 cm–1. The detailed structure of [1a-H](OTf)2 was confirmed by an X-ray analysis. An ORTEP drawing of [1a-H](OTf)2 is shown in Figure 3b.
On the other hand, reduction of [1a]OTf with 1 equiv of KC8 in THF at −10 °C for 3 h and then warmed to room temperature for 15 h gave the corresponding cobaltocene bearing 4-(2,6-dimethylpyridin-4-yl)phenyl group [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2N))] ([1a]) in 64% yield (Scheme 1). Unfortunately, no single crystal of [1a] suitable for X-ray analysis was obtained; however, 1H NMR, magnetic susceptibility measurement (vide infra), and elemental analysis support the formation of the corresponding cobaltocene [1a].
Next, we tried to isolate cobaltocene bearing 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2NH))]OTf ([1a-H]OTf) from the reaction of [1a] with 1 equiv of [PicH]OTf (Pic = 2-methylpyridine) as the proton source in THF at −78 °C (Scheme 2). After the addition of [PicH]OTf, a green solution derived from [1a] was immediately changed to a brown solution. We consider that the color change of the solution suggests the formation of [1a-H]OTf from [1a]. However, unfortunately, the complex is unstable at room temperature and decomposes gradually into [1a]OTf together with the formation of dihydrogen.

Scheme 2

Scheme 2. Formation of [1a-H]OTf
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To confirm the formation of [1a-H]OTf generated from the reaction of [1a] with 1 equiv of [PicH]OTf, we compared the electron paramagnetic resonance (EPR) of [1a] with that of [1a-H]OTf. Complex [1a] in frozen 2-methyltetrahydrofuran (2-MeTHF) at −196 °C exhibited no signal (Figure 4, black line) probably because of fast relaxation of the ground state, which was previously observed in other cobaltocenes. (23,24) We confirmed paramagnetic species (S = 1/2) by the magnetic susceptibility measurement of [1a]. On the other hand, after reacting [1a] with 1 equiv of [PicH]OTf as a proton source at −78 °C, we observed a new EPR signal of paramagnetic species (S = 1/2) assigned as [1a-H]OTf in frozen 2-MeTHF at −196 °C (Figure 4, red line).

Figure 4

Figure 4. X-band EPR spectra of [1a] (black) and [1a-H]OTf (observed: red, simulated: blue) in 2-MeTHF at −196 °C. giso = 1.99911, line width = 4.65 mT.

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The formation of [1a-H]OTf generated from the reaction of [1a] with 1 equiv of [PicH]OTf was also confirmed by a UV–vis absorption spectroscopic measurement. After the addition of [PicH]OTf to a solution of [1a] in THF at −78 °C, the spectral change with the isosbestic point was observed (Figure 5). The spectral change was almost completed when 1 equiv of [PicH]OTf was added. This experimental result strongly indicates that [1a] was almost fully converted into [1a-H]OTf by the addition of 1 equiv of [PicH]OTf to [1a].

Figure 5

Figure 5. Absorption spectra of [1a] (0.05 mM) in THF at −78 °C by the addition of [PicH]OTf as the proton source.

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According to similar procedures for the preparation of [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2NH))]OTf [1a-H]OTf, we prepared [(η5-C5Me5)Co(η5-C5H4(C6H4–C5H2Me2NH))]OTf ([1b-H]OTf) and [(η5-C5H5)Co(η5-C5H4(C6H4–C5H2Me2NH))]OTf ([1c-H]OTf) from reactions of pentamethylcobaltocenium hexafluorophosphate [(η5-C5Me5)Co(η5-C5H5)]PF6 and cobaltocenium hexafluorophosphate [(η5-C5H5)Co(η5-C5H5)]PF6, in place of nonamethylcobaltocenium hexafluorophosphate [(η5-C5Me5)Co(η5-C5Me4H)]PF6. Detailed experimental results for the preparation of the series of [1b] and [1c] are shown in the Supporting Information. Detailed molecular structures of the corresponding [1]OTf, [1], and [1b-H](OTf)2 are confirmed by X-ray analysis (Figures S3–S7). Generation of [1b-H]OTf and [1c-H]OTf by the addition of 1 equiv of [PicH]OTf is also confirmed by EPR and absorption spectroscopic studies (Figures S8–S11).

Property of Cobaltocenes Bearing 4-(2,6-Dimethylpyridin-1-ium-4-yl)phenyl Moiety

We measured the pKa value of [1a-H](OTf)2 using absorption spectroscopic titration (Figure S14). By the addition of [PicH]OTf as a proton source to a THF solution of [1a]OTf, the spectrum was gradually changed by the formation of [1a-H](OTf)2. According to the equilibrium constant between [1a]OTf/[PicH]OTf and [1a-H](OTf)2/Pic and the pKa of [PicH]+ (8.6 in THF), (25) we estimated that the pKa value of [1a-H](OTf)2 in THF is 9.0 (Table 1). According to the same procedure, we estimated that the pKa values of [1b-H](OTf)2 and [1c-H](OTf)2 in THF are 8.8 and 8.9, respectively, indicating that the substituents on the cyclopentadienyl do not substantially affect the pKa values (Figures S15 and S16, Table 1). These pKa values of [1-H](OTf)2 are lower than that of lutidinium triflate (9.5 in THF), (25) due to the electron-withdrawing ability of the presence of cobaltoceniumphenyl moiety in [1-H](OTf)2. We also confirmed the absorption spectral change of [1a-H](OTf)2 and [1b-H](OTf)2 to the corresponding [1]OTf by the addition of triethylamine as a base, showing the reversibility of conversion between each [1-H](OTf)2 and [1]OTf (Figures S17 and S18).
We avoided the use of isolated [1a-H](OTf)2 because of the poor solubility of isolated [1a-H](OTf)2 in THF. We measured cyclic voltammetry of [1a-H](OTf)2, which was generated in situ from the reaction of [1a]OTf with 10 equiv of [ColH]OTf (Col = 2,4,6-trimethylpyridine). As a result, a quasi-reversible redox couple assignable as the redox of the Co(II/III) center was observed at E1/2 = −1.64 V vs Fc/Fc+ (Figure S20). According to the same procedure, similar quasi-reversible redox couples for [1b]OTf and [1c]OTf were observed at E1/2 = −1.52 ([1b]OTf) and −1.26 ([1c]OTf) V vs Fc/Fc+, respectively (Table 1 and Figure S20). The E1/2 of [1-H](OTf)2 was slightly shifted to a more positive potential (by 30–40 mV) compared with that of [1]OTf (E1/2 = −1.68 ([1a]OTf), −1.56 ([1b]OTf), and −1.29 ([1c]OTf) V vs Fc/Fc+, Figure S19).
BDFEsN–H of [1-H]OTf experimentally (i.e., BDFEsexp) were determined by the combination of the pKa and the redox potential E1/2 values of each [1-H](OTf)2 according to eq 1 (26) and solvent-dependent constant CG (= 59.9 kcal/mol in THF). (27)
BDFEexp=1.37pKa+23.06E°+CG
(1)
As a result, BDFEsexp of [1-H]OTf are estimated as 34.4 ([1a-H]OTf), 37.0 ([1b-H]OTf), and 43.0 kcal/mol ([1c-H]OTf), respectively (Table 1). Reflecting the trend of E(CoII/III), [1a-H]OTf has the lowest BDFEexp among [1-H]OTf due to the electron-donating nature of the methyl groups on the cobaltocenes. The BDFEsexp are similar to the BDFEscalcd shown in the first section (vide supra). Unfortunately, the BDFEsexp of [1-H]OTf is slightly higher than that of the molybdenum-imide complex [Mo(NH)I(PCP)] (33.8 kcal/mol). (14)

Reactivity of Cobaltocenes Bearing 4-(2,6-Dimethylpyridin-1-ium-4-yl)phenyl Moiety with Molybdenum-Nitride Complex

With the cobaltocene complexes as new PCET reagents in hand, we examined the stoichiometric reaction of the molybdenum-nitride complex [Mo(N)I(PCP)] with [1a-H]OTf. The reaction of [Mo(N)I(PCP)] (0.0075 mmol) with an excess amount of [1a-H]OTf, generated in situ from [1a] (0.050 mmol) and [PicH]OTf (0.050 mmol) in THF at −78 °C to suppress dihydrogen evolution, under 1 atm of Ar, afforded ammonia in 71% yield based on the molybdenum atom together with dihydrogen in 16% yield (Table 2, Entry 1). When [1b-H]OTf and [1c-H]OTf were used in place of [1a-H]OTf, ammonia was obtained in 70% and 18% yields, respectively, together with dihydrogen in 37% and 12% yields, respectively (Table 2, Entries 2 and 3). These experimental results suggest that net H atom (i.e., one electron and one proton) transfer from [1a-H]OTf and [1b-H]OTf to the molybdenum complex proceeded successfully; however, net H atom transfer from [1c-H]OTf did not proceed smoothly. Thus, only [1a-H]OTf and [1b-H]OTf worked as PCET reagents toward the molybdenum-nitride complex.
Table 2. Stoichiometric Reaction of [1-H]OTf with the Molybdenum-Nitride Complex [Mo(N)I(PCP)]
  NH3H2
entrycobaltocenes(%/Mo)(%/[1-H]+)
11a7116
21b7037
31c1812
We also carried out catalytic ammonia formation with [1a-H]OTf and [1b-H]OTf in the presence of [Mo(N)I(PCP)]. The reaction of an atmospheric pressure of dinitrogen with [1a-H]OTf, generated in situ from [1a] (0.075 mmol) and [PicH]OTf (0.075 mmol), in the presence of a catalytic amount of [Mo(N)I(PCP)] (0.0025 mmol) in THF at −78 °C to room temperature gave only 1.9 equiv of ammonia based on the molybdenum atom of the catalyst (19% yield based on [1a-H]OTf) together with only a small amount of dihydrogen (0.3 equiv) (Table 3, Entry 1). When [ColH]OTf (Col = 2,4,6-trimethylpyridine) was used as a proton source for the preparation of [1a-H]OTf under the same reaction conditions, (28) 4.9 equiv of ammonia was obtained based on the molybdenum atom of the catalyst (49% yield based on [1a-H]OTf) (Table 3, Entry 2). Next, we investigated the catalytic reaction with [1b-H]OTf, generated in situ from [1b] and [ColH]OTf under the same reaction conditions to afford 6.1 equiv of ammonia based on the molybdenum atom of the catalyst in 61% yield (Table 3, Entry 3). For comparison, the reaction with the combination of [Cp*2Co] (Cp* = η5-C5Me5) and [ColH]OTf as reductant and proton source, respectively, under the same reaction conditions gave 2.7 equiv of ammonia and 8.2 equiv of dihydrogen based on the molybdenum atom of the catalyst in 27% and 55% yields, respectively (Table 3, Entry 4). These experimental results suggest that the hybridization of cobaltocene and 2,6-dimethylpyridinium linked via a phenylene group may contribute to the promotion of net H atom transfer to the molybdenum-nitride complex, leading to the formation of ammonia.
Table 3. Catalytic Ammonia Formation from Dinitrogen with [1-H]OTf in the Presence of [Mo(N)I(PCP)]
entrycobaltocenesproton sourceNH3 (equiv)aNH3 (%)bH2 (equiv)aH2 (%)b
11a[PicH]OTf1.9 ± 0.019 ± 00.3 ± 0.02 ± 0
21a[ColH]OTf4.9 ± 0.249 ± 20.3 ± 0.02 ± 0
31b[ColH]OTf6.1 ± 0.361 ± 31.4 ± 0.29 ± 1
4[Cp*2Co][ColH]OTf2.7 ± 0.127 ± 18.2 ± 0.055 ± 0
a

Equiv based on [Mo(N)I(PCP)].

b

Yield based on the Co atom of cobaltocenes.

Conclusions

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A series of cobaltocenes bearing a 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety with bond dissociation free energies (BDFEs) of 34.4–43.0 kcal/mol in THF were newly designed and prepared. Although the BDFEsexp of these cobaltocenes bearing 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety are slightly higher than that of the molybdenum-imide complex, these cobaltocenes bearing 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety worked as PCET reagents toward stoichiometric and catalytic ammonia formation with the molybdenum-nitride complex. We have not yet cleared the exact reason why [1a-H]OTf and [1b-H]OTf worked as PCET reagents for ammonia formation. We believe that the present experimental results may provide some hints to design more efficient PCET reagents for ammonia formation with molybdenum catalysts in the future.

Experimental Section

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General Methods

1H NMR (400 MHz) and 13C{1H} NMR (100 MHz) spectra were recorded on a JEOL ECZS-400 spectrometer in suitable solvents, and the spectra were referenced to the residual solvent. Magnetic susceptibility was measured in C6D6 using the Evans’ method. (29−31) IR spectra were recorded on a JASCO FT/IR 4100 Fourier transform infrared spectrometer. UV–vis absorption spectra were recorded on a Shimadzu UV-1280 or a Shimadzu UV-1850 instrument (equipped with a Unisoku CoolSpek USP-203-B instrument for low-temperature experiments). Evolved dihydrogen (H2) was quantified by gas chromatography (GC) using a Shimadzu GC-8A with a TCD detector and SHINCARBON ST (6 m × 3 mm). Elemental analyses were carried out using an Exeter analytical CE-440 Elemental Analyzer. X-band EPR spectra were recorded at 9.45 GHz on a Bruker Magnettech ESR5000 spectrometer, and the spectra were simulated by EasySpin toolbox (32) on MATLAB with Simultispin GUI. (33) Cyclic voltammetry (CV) was carried out using a GAMRY Interface 1010 jp with a one-compartment, three-electrode cell. All manipulations were carried out under an atmosphere of nitrogen or argon using standard Schlenk techniques or glovebox techniques unless otherwise stated. Solvents were dried by general methods and degassed before use. ([(η5-C5Me5)Co(η5-C5H5)]PF6), (34) (Na(η5-C5Me4H)), (35) [(η5-C5Me5)Co(acac)] (acac = acetylacetonate), (36) 4-(4′-bromophenyl)-2,6-dimethylpyridine, (22) (FcOTf), (37) KC8, (38) [Mo(N)I(PCP)], (14) [PicH]OTf, (10) and [ColH]OTf (10) were prepared according to the literature methods. All of the other reagents were commercially available.

Synthesis of [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2N))]OTf ([1a]OTf)

To a solution of [(η5-C5Me5)Co(acac)] (1.56 g, 5.3 mmol) in THF (20 mL) was added a suspension of Na(η5-C5Me4H) (721.8 mg, 5.0 mmol) in THF (20 mL), and the mixture was refluxed for 13 h. After the resultant dark brown solution was evaporated, the residue was extracted with pentane (10 mL × 3). The dark green solution was evaporated, and water (50 mL) was added under air. This suspension was stirred at 40 °C for 4 h, and then activated charcoal (379.0 mg) was added. The resultant suspension was filtered, and the filtrate was concentrated to ca. 5 mL. The solution was cooled to 0 °C, then an aqueous solution of KPF6 (0.5 M, 10 mL) was added. The resultant yellow precipitate was collected by filtration and washed with cold water and Et2O. Recrystallization was carried out by vapor diffusion of Et2O to the CH3CN solution at room temperature to afford [(η5-C5Me5)Co(η5-C5Me4H)]PF6 as an orange-yellow crystalline solid (1.31 g, 2.8 mmol, 54% yield). This compound was used for the next reaction without further purification. 1H NMR (CD3CN) δ 4.66 (s, 1H), 1.78 (s, 6H), 1.77 (s, 15H), 1.68 (s, 6H).
To a solution of 4-(4′-bromophenyl)-2,6-dimethylpyridine (412.6 mg, 1.6 mmol) in THF (24 mL) was added dropwise nBuLi (1.56 M in hexane, 1.1 mL, 1.7 mmol) at −78 °C, and the mixture was stirred at −78 °C for 3 h. (22) The resultant green-yellow solution was added to a suspension of [(η5-C5Me5)Co(η5-C5Me4H)]PF6 (687.3 mg, 1.5 mmol) in THF (16 mL) at −78 °C, followed by stirring at −78 °C for 3 h. The resultant dark green solution was allowed to warm to room temperature and stirred overnight. After the reaction, the solution was evaporated in vacuo. Then the residue was extracted with pentane, until the filtrate became colorless. The filtrate was evaporated, and the residue was redissolved in THF (14 mL). FcOTf (460.4 mg, 1.4 mmol) was added, and the mixture was stirred at room temperature for 1 h. The resultant yellow-green suspension was evaporated and washed with hexane to afford an orange-yellow residue. The residue was purified by column chromatography on basic alumina (eluent: Et2O/CH3CN, 1:2, v/v) under air, and the yellow fraction was collected. The resultant yellow oil was redissolved in CH3CN, and then Et2O was layered. After recrystallization at −30 °C, [1a]OTf was obtained as yellow crystals, which were collected and washed with cold CH3CN/Et2O (v/v = 1/10) and cold Et2O (99.7 mg, 0.15 mmol, 10% yield). 1H NMR (CD3CN) δ 7.86 (d, J = 8.0 Hz, 2H), 7.51 (d, J = 8.4 Hz, 2H), 7.35 (s, 2H), 2.53 (s, 6H), 1.84 (s, 6H), 1.80 (s, 6H), 1.69 (s, 15H). 13C NMR (CD3CN) δ 159.54, 148.30, 139.71, 131.79, 130.65, 128.25, 118.68, 100.05, 96.50, 95.94, 94.19, 24.59, 9.87, 8.65, 8.54. Anal. Calcd for C33H39CoF3NO3S: C, 61.39; H, 6.09; N, 2.17. Found: C, 61.78; H, 5.66; N, 1.81.

Synthesis of [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2NH))](OTf)2 ([1a-H](OTf)2)

To a solution of [1a]OTf (69.1 mg, 0.11 mmol) in dimethoxyethane (15 mL) was added a solution of HOTf (0.33 M, 1.5 mL, 0.50 mmol) in DME at room temperature, and the mixture was stirred at room temperature for 3 h. After the resultant yellow suspension was evaporated, the residue was redissolved in CH3CN. The solution was filtered through Celite. The filtrate was concentrated to ca. 1 mL, and Et2O was layered. After recrystallization at −30 °C, [1a-H](OTf)2 was obtained as yellow crystals, which were collected and washed with Et2O (53.7 mg, 0.067 mmol, 63% yield). 1H NMR (CD3CN) δ 12.79 (br s, 1H), 8.00 (d, J = 8.0 Hz, 2H), 7.92 (s, 2H), 7.60 (d, J = 9.2 Hz, 2H), 2.72 (s, 6H), 1.84 (s, 6H), 1.81 (s, 6H), 1.69 (s, 15H). Anal. Calcd for C34H40CoF6NO6S2: C, 51.32; H, 5.07; N, 1.76. Found: C, 51.37; H, 5.11; N, 1.96. IR (KBr): 3292 cm–1 (νNH).

Synthesis of [(η5-C5Me5)Co(η5-C5Me4(C6H4–C5H2Me2N))] ([1a])

To a solution of [1a]OTf (83.6 mg, 0.13 mmol) in THF (2.5 mL) was added a suspension of KC8 (19.5 mg, 0.14 mmol) in THF (3.5 mL) at −10 °C, and the mixture was stirred at room temperature for 15 h. After the resultant dark green suspension was evaporated, the residue was redissolved in pentane (6 mL), and the solution was filtered through Celite. The dark green filtrate was concentrated to ca. 1 mL. After recrystallization at −78 °C, [1a] was obtained as a green solid, which was collected and washed with cold pentane (41.2 mg, 0.083 mmol, 64% yield). Magnetic susceptibility (Evans’ method): μeff = 1.69 μB in C6D6. 1H NMR (C6D6) δ 53.79 (br s, 6H), 48.42 (br s, 15H), 17.47 (s, 2H), 2.02 (s, 2H), 1.39 (s, 6H), −14.79 (s, 2H), −19.02 (s, 6H). Anal. Calcd for C32H39CoN: C, 77.40; H, 7.92; N, 2.82. Found: C, 77.70; H, 7.69; N, 2.42.

EPR Observation of [1a-H]OTf

Typical procedures are described as follows. To a solution of [1a] (1.1 mM, 900 μL, 1.0 μmol) in 2-methyltetrahydrofuran (2-MeTHF) was added a solution of [PicH]OTf (10 mM, 100 μL, 1.0 μmol) in 2-MeTHF at −78 °C. The color changed immediately. The resultant mixture was transferred to a chilled EPR tube, and the EPR measurement was carried out at −196 °C.

UV–Vis Observation of [1a-H]OTf

The typical procedure is described as follows. A solution of [1a] (0.05 mM) in THF (3 mL) was placed in an optical cell (l = 1 cm) equipped with a stir bar, sealed with a septum, and then cooled to −78 °C. A solution of [PicH]OTf (5 mM) in THF was added to the cell. After the mixture was stirred at −78 °C for 1 min, the absorption spectrum was recorded.

Stoichiometric Reaction of Molybdenum-Nitride Complex with [1a-H]OTf

The typical experimental procedure is described as follows. To a solution of [1a] (0.050 mmol) in THF (7 mL) was added a solution of [ColH]OTf (0.050 mmol) in THF (1 mL) at −78 °C, and the mixture was stirred for 3.5 min under Ar (1 atm). A solution of [Mo(N)I(PCP)] (0.0075 mmol) in THF (2 mL) was added to the mixture at −78 °C. The mixture was stirred at room temperature for 3 h. After the reaction, the amount of generated H2 was quantified by gas chromatography. The mixture was evaporated in vacuo, and the distillate was trapped in a dilute H2SO4 solution (0.1 M, 10 mL). The amount of ammonia present in the H2SO4 solution was determined by the indophenol method. (39)

Reactions of N2 (1 atm) with Molybdenum-Nitride Complex as Catalyst and [1a-H]OTf

Typical experimental procedures are described as follows. To a solution of [1a] (0.075 mmol) in THF (8 mL) was added a solution of [ColH]OTf (0.075 mmol) in THF (1 mL) at −78 °C, and the mixture was stirred for 3.5 min under N2 (1 atm). A solution of [Mo(N)I(PCP)] (0.0025 mmol) in THF (1 mL) was added to the mixture. The reaction mixture was stirred at room temperature for 3 h. After the reaction, the amount of generated H2 was quantified by gas chromatography. An aqueous solution of KOH (30 wt %, 5 mL) was added to the reaction mixture and then distilled into the dilute H2SO4 solution (0.1 M, 10 mL). The amount of ammonia present in the H2SO4 solution was determined by the indophenol method. (39)

X-ray Crystallography

Crystallographic data of [1a]OTf, [1a-H](OTf)2, [1b]OTf, [1b-H](OTf)2, [1b], [1c]OTf, and [1c] are summarized in Tables S1–S3, and their ORTEP drawings are shown in Figures S1–S7. Diffraction data were collected for the 2θ range of 4 to 60° at −180 °C on a Rigaku XtaLAB Synergy-S diffractometer equipped with a HyPix-6000HE Hybrid Photon Counting (HPC) detector and VariMax optics using multilayer mirror monochromated Mo Kα radiation (λ = 0.71073 Å). Intensity data were corrected for Lorentz and polarization effects and for empirical absorption (CrysAlisPro), (40) whereas structure solutions and refinements were carried out by using the Olex2 crystallographic software package. (41) The positions of non-hydrogen atoms were determined by direct methods (SHELXT Version 2018/2) (42) and subsequent Fourier syntheses (SHELXL Version 2018/3) (43) and were refined on F02 using all unique reflections by full-matrix least-squares with anisotropic thermal parameters. The disordered solvent molecules in the voids for [1a-H](OTf)2 and [1b]OTf were not modeled explicitly. The electron density associated with some solvent molecules was removed by the Solvent mask routine (44) of Olex2.

Computational Details

DFT calculations were performed with the Gaussian 16 program. (45) Geometry optimizations were carried out with the B3LYP-D3 functional, which is the B3LYP hybrid functional (46−49) combined with an empirical dispersion correction developed by Grimme. (50) In the present calculations, the SDD (Stuttgart/Dresden) basis sets (51,52) were employed for the cobalt atom and the 6–31G(d) basis sets (53−56) for the other atoms. All optimized structures were confirmed to have the appropriate number of imaginary frequencies by vibrational analysis. To determine the free energy profiles, single-point energy calculations were performed at the optimized geometries using the SDD and 6–311+G(d,p) basis sets. (57−59) In the single-point calculations, solvation effects of THF (ε = 7.4257) were taken into account by using the polarizable continuum model (PCM). (60) The bond dissociation free energies (BDFEs) of the N–H bond of [1-H]+ at 298 K in THF were calculated based on reaction [1-H]+ → [1]+ + H, and therefore [1-H]+ loses a proton and an electron at the same time. Detailed data on SCF energies, thermal energy corrections at 298 K, and SCF energies in THF are summarized in Table S4. Cartesian coordinates of all optimized structures are given in a separate .xyz file.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.5c00492.

  • Detailed experiment procedures and computational details, Synthesis and reactivity of cobaltocenes, X-ray crystallography, EPR study, UV–vis absorption spectroscopy, cyclic voltammetry, NMR and IR spectra, and DFT calculations (PDF)

  • Cartesian coordinates of all optimized geometries (XYZ)

Accession Codes

Deposition Numbers 2512896–2512902 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.

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Author Information

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  • Corresponding Authors
  • Authors
    • Hiroki Otsuka - Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
    • Kazuya Arashiba - Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
    • Taiji Nakamura - Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishibiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan;  Orcidhttps://orcid.org/0000-0002-7301-9752
    • Hiromasa Tanaka - School of Liberal Arts and Sciences, Daido University, Takiharu-cho, Minami-ku, Nagoya 457-8530, Japan;  Orcidhttps://orcid.org/0000-0001-8516-8280
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We acknowledge Grants-in-Aid for Scientific Research (20H05671, 22K19041, 24H00049, 24H01834, 24K21245, and 24K21778) from JSPS and MEXT. This paper is based on results obtained from a project, JPNP21020, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

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  • Abstract

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    Figure 1

    Figure 1. (a) Catalytic cycle for ammonia production from dinitrogen and water using [MoI3(PCP)] as a catalyst. (b) Stepwise conversion of [Mo(N)I(PCP)] into [Mo(NH3)I(PCP)] via reduction and protonation. (c) Catalytic ammonia formation reaction using the combination of SmI2 and water and that of cobaltocene and pyridinium salt.

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    Figure 2

    Figure 2. (a) Previous work by Peters and co-workers: cobaltocene derivative bearing N,N-dimethylanilinium. (17,18) (b) Our previous work: iron complexes bearing π-phenol ligand. (19,20) (c) Our previous work: samarium complexes bearing cyclopentadienyl-amine chelate ligands. (21) (d) This work: cobaltocenes bearing 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety [1-H]+.

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    Scheme 1

    Scheme 1. Synthesis of [1a]OTf, [1a], and [1a-H](OTf)2
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    Figure 3

    Figure 3. ORTEP drawings of the cationic part of (a) [1a]OTf and (b) [1a-H](OTf)2. Hydrogen atoms except for a NH hydrogen atom in [1a-H](OTf)2 were omitted for clarity.

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    Scheme 2

    Scheme 2. Formation of [1a-H]OTf
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    Figure 4

    Figure 4. X-band EPR spectra of [1a] (black) and [1a-H]OTf (observed: red, simulated: blue) in 2-MeTHF at −196 °C. giso = 1.99911, line width = 4.65 mT.

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    Figure 5

    Figure 5. Absorption spectra of [1a] (0.05 mM) in THF at −78 °C by the addition of [PicH]OTf as the proton source.

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  • Supporting Information

    Supporting Information

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    • Detailed experiment procedures and computational details, Synthesis and reactivity of cobaltocenes, X-ray crystallography, EPR study, UV–vis absorption spectroscopy, cyclic voltammetry, NMR and IR spectra, and DFT calculations (PDF)

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