Silylation of Dinitrogen Catalyzed by Hydridodinitrogentris(Triphenylphosphine)Cobalt(I)

1. Introduction

Effective catalytic activation of dinitrogen under ambient conditions remains one of the biggest challenges in synthetic chemistry. Although such a transformation is feasible, as evidenced by the biosynthesis of ammonia enabled by nitrogenase enzymes [1,2,3,4], the activation of dinitrogen on transition metal complexes is not trivial and the best man-made homogeneous catalysts for ammonia synthesis can perform only several dozens of catalytic turnovers [5,6,7,8,9,10,11,12,13].

In this context, most of the effort has been focused on molybdenum and iron systems [14,15,16,17,18]. Two major strategies for catalytic N₂ activation by homogeneous systems were developed: reductive protonation to yield ammonia and reductive silylation to yield silylamines (Figure 1).

Catalytic formation of ammonia on well-defined systems was pioneered by the group of Schrock who developed a molybdenum-based system that catalyzed the formation of almost eight equivalents of NH₃ per metal atom [19]. The group of Nishibayashi developed binuclear catalysts which were able to catalyze the formation of up to 63 equivalents of NH₃ per Mo atom [20,21]. Iron also proved to be catalytically active as shown by the group of Peters, whose catalysts could produce up to 64 equivalents of NH₃ at very low temperatures [22,23].

However, these catalysts suffer from poisoning with ammonia and dihydrogen formed as side products. This is usually not the case in catalytic silylation reactions and therefore this reaction generally yields more turnovers of N₂ fixation (Figure 2). Catalytic reductive silylation of N₂ was pioneered by Shiina who used CrCl₃ as the pre-catalyst. This led to formation of 5.4 equivalents of N(SiMe₃)₃ when N₂ was reacted with lithium and Me₃SiCl [24]. Other salts of various metals yielded much less of the desired product. For instance, the use of CoCl₂ which is relevant to the research presented here led to the formation of 1.2 equivalents of N(SiMe₃)₃ per metal centre. This chemistry was further developed for well-defined Mo and W systems by the groups of Hidai [25] and Nishibayashi [26]. Another well-defined molybdenum catalyst was recently disclosed by Mézailles and co-workers [27]. The group of Nishibayashi also reported that simple organometallic iron compounds can be used as pre-catalysts in this reaction and up to 34 equivalents of N(SiMe₃)₃ could be formed when a ferrocene derivative was used [28]. Interestingly, none of the studied iron compounds contain coordinated dinitrogen, and an approximately 1 h incubation period was observed, indicating that the catalytically active species is formed by reaction of the iron pre-catalyst with silylchlorides under reductive conditions. A two-coordinate iron(0) complex supported with a cyclic (alkyl)(amino)carbene ligand which revealed comparable activity was recently reported by Ung and Peters [29].

Despite the fact that cobalt-containing catalysts [30] are active in the Haber–Bosch reaction, only a handful of homogeneous systems based on this metal have been reported in the context of N₂ activation. Yamamoto and co-workers showed that treatment of {[Co(PPh₃)₃N₂]⁻}_xM^x+(THF)_2+x (M = Li, Na, Mg) complexes with acids leads to the formation of ammonia and hydrazine in sub-stoichiometric quantities [31]. Peters and co-workers reported methylation and trimethylsilylation of dinitrogen bound to [(PhB(CH₂PⁱPr₂)₃)CoN₂]₂Mg(THF)₄ (Figure 3) [32]. Last year, Gagliardi, Lu and co-workers reported a dinuclear cobalt complex that catalyzes the formation of N(SiMe₃)₃ from dinitrogen with an impressive turnover number of 195 [33], while the group of Nishibayashi showed that in the presence of 2-2’-bipyridine, Co₂(CO)₈ can catalyze the formation of up to 49 equivalents of N(SiMe₃)₃ (Figure 2) [34]. Homogeneous cobalt complexes with tris(phosphine)borate ligands were also shown by Peters and co-workers to catalyze the formation of over two equivalents of ammonia [35].

2. Results and Discussion

Since Yamamoto’s [Co(PPh₃)₃N₂]⁻ complex promoted the reduction of the coordinated N₂ in THF to ammonia and hydrazine, it seemed plausible that it could also promote the silylation of dinitrogen. Indeed, recently the group of Nishibayashi mentioned that CoH(PPh₃)₃N₂ catalyzes the reductive silylation of N₂ [34]. Given the fact that [Co(PPh₃)₃N₂]⁻ can be formed, e.g., by the reduction of CoH(PPh₃)₃N₂ with alkali metals, we decided to investigate the activity of the cobalt triphenylphosphine system in silylation of N₂ in more detail.

One could expect that [CoN₂(PPh₃)₃]⁻ should directly react with Me₃SiCl, resulting in the formation of Si–N bonds, similarly to the dinuclear system reported by the groups of Gagliardi and Lu [33]. However, as shown by Nishibayashi, Yoshizawa and co-workers, many simple cobalt complexes reveal long incubation periods before the onset of the catalytic reaction [34]. We were thus interested in the mechanism behind the catalytic dinitrogen silylation by the cobalt triphenylphosphine system. Is [Co(PPh₃)₃N₂]⁻ the actual catalyst like the bimetallic Lu–Gagliardi complex or merely a pre-catalyst which undergoes a transformation into the catalytically active species through reaction with chlorosilane? To answer this question we started our investigations by studying the performance of the in-situ formed [Co(PPh₃)₃N₂]⁻ anion in the catalytic silylation of dinitrogen.

Treating a 0.0025 M solution of CoH(PPh₃)₃N₂ in THF with 180 equivalents of Na and 190 equivalents of Me₃SiCl led to the formation of 6.7 ± 1.0 equivalents (per Co atom) of N(SiMe₃)₃ as evidenced by GC. This corresponded to approximately 12% yield based on sodium (

Table 1. Screening of catalytic conditions of reductive silylation of N₂ ^a.

#	Catalyst	Solvent	Vol./mL	Reduct.	Yield/% b	TON c
1 d	CoH(PPh3)3N2	THF	10	Na	11.7 ± 1.6	6.7 ± 1.0
2 e	Co(acac)3	THF	10	Na	0.9 ± 0.6	0.5 ± 0.4
3 e	CoBr2	THF	10	Na	0	0
4	Co	THF	10	Na	0	0
5 f	Co(Cp)2	THF	10	Na	15.9 ± 3.4	8.3 ± 2.3
6 e	-	THF	10	Na	0	0
7 e	CoCl2(PPh3)2	THF	10	Na	1.2 ± 0.6	0.7 ± 0.3
8 e	CoCl(PPh3)3	THF	10	Na	6.1 ± 1.2	3.7 ± 0.7
9	CoH(PPh3)3N2	Et2O	10	Na	9.1	5.8
10	CoH(PPh3)3N2	C6H6	10	Na	0	0
11	CoH(PPh3)3N2	THF	5	Na	10.8	6.8
12	CoH(PPh3)3N2	THF	20	Na	11.6	7.3
13	CoH(PPh3)3N2	THF	10	Li	8.7	5.5
14	CoH(PPh3)3N2	THF	10	K	0	0
15	CoH(PPh3)3N2	THF	10	KC8	5.8	3.5

^a Conditions: 0.025 mmol catalyst, 4.5 mmol reductant; 4.72 mmol Me₃SiCl, time = 48 h. Me₃Si–SiMe₃, (Me₃Si)₂NH and Me₃Si–C₄H₈O–SiMe₃ were observed as the main side products; ^b Yield was determined with GC, using decane (10 μL) as an internal standard, and is based on the maximum theoretical yield of (Me₃Si)₃N (1.5 mmol); ^c TON is the amount of (Me₃Si)₃N formed per cobalt atom; ^d average of four runs; ^e average of two runs; ^f average of three runs.

). The identity of N(SiMe₃)₃ was confirmed with mass spectrometry. Analysis of the ammonia content using indophenol method after saponification of the reaction mixture with 1 N H₂SO₄ was in accord with the GC analysis.

For comparison, we also investigated the behavior of cobalt compounds that do not have a coordinated dinitrogen molecule. The use of Co(acac)₃ led to the substoichiometric formation of N(SiMe₃)₃, while the use of CoBr₂ or Co powder did not result in the formation of N(SiMe₃)₃ (Table 1, entries 2–4). The use of cobaltocene led to the formation of N(SiMe₃)₃ in a slightly higher (but comparable) yield to the one obtained with CoH(PPh₃)₃N₂ (entry 5). N(SiMe₃)₃ was not formed when no cobalt was present (entry 6). CoCl₂(PPh₃)₂ which features the triphenylphosphine ligand had significantly lower activity (entry 7), while CoCl(PPh₃)₃ (which upon two-electron reduction under dinitrogen atmosphere could form [Co(PPh₃)₃N₂]⁻) was less active than CoH(PPh₃)₃N₂ (entry 8). The use of diethyl ether as a solvent led to a slightly lower yield of N(SiMe₃)₃, and in benzene no formation of this compound was detected (entries 9,10). No significant changes in yield were measured when the amount of solvent varied (entries 11,12). The use of lithium as the reductant led to a comparable yield, while for potassium no desired product was formed (entries 13,14). Potassium gave good results with other systems [26] and the failure to obtain the desired product with this reductant is likely caused by a very fast reaction of potassium (disappearance of all solids within less than an hour) with all Me₃SiCl before the catalytically active species could be formed (see below). The use of potassium graphite instead of metallic potassium led to the formation of 3.5 equivalents of N(SiMe₃)₃. The exact reason why Me₃SiCl is consumed in unproductive reaction with metallic K at higher rates than with metallic Li or Na or KC₈ is currently not clear but remains outside of the scope of this work. For an example of a somewhat related low performance of metallic K compared to Na and KC₈ in reductive coupling of alkyl chlorides see e.g., [36].

Overall, CoH(PPh₃)₃N₂ showed a rather moderate activity, and although it performed better than its non-dinitrogen-containing analogs CoCl(PPh₃)₃ and CoCl₂(PPh₃)₂, it was still less effective than cobaltocene. Next, we undertook mechanistic studies of its catalytic activity, and attempted to detect any possible cobalt intermediates in the trimethylsilylation reaction. The time profile of the formation of N(SiMe₃)₃ reveals an induction period of ca. 4 h followed by a period of moderate activity (ca. 16 h) during which all the sodium is consumed (Figure 4). A somewhat shorter (1 h) induction period was reported by the group of Nishibayashi for the organometallic cobalt systems. This induction period was proposed to account for the formation of catalytically active species that features trimethylsilyl groups directly bound to the cobalt center [34].

We attempted to shed light on the possible initial transformations of CoH(PPh₃)₃N₂ using IR spectroscopy. Yamamoto et al. reported that the reduction of CoH(PPh₃)₃N₂ with sodium results in the formation of [Co(PPh₃)₃N₂]Na(THF)₃ [31]. Indeed, the addition of five equivalents of Na to a THF solution of CoH(PPh₃)₃N₂ resulted in the disappearance of its characteristic N≡N stretch vibration at 2091 cm⁻¹ and the appearance of a new peak at 1912 cm⁻¹ attributable to [Co(PPh₃)₃N₂]Na(THF)₃. When three equivalents of Me₃SiCl were added to a solution of CoH(PPh₃)₃N₂ in THF, the disappearance of the IR signal corresponding to the cobalt-dinitrogen complex was observed within 20 min and no peaks attributable to a reduced N₂ moiety could be observed. When the addition of Me₃SiCl was performed in the presence of metallic sodium, the disappearance of the signal of CoH(PPh₃)₃N₂ was retarded, suggesting a possible concurrent reaction of Me₃SiCl with sodium. Still, however, it seemed plausible that the attack of Me₃SiCl on [Co(PPh₃)₃N₂]⁻ would lead to the formation of the active catalyst. Therefore, we investigated whether the reaction of pre-formed [Co(PPh₃)₃N₂]Na(THF)₃ with six equivalents of trimethylchlorosilane would lead to the formation of N(SiMe₃)₃. GC-MS analysis of this reaction mixture did not reveal the formation of either N(SiMe₃)₃ or any organic compound containing an N–SiMe₃ moiety. The subsequent addition of 180 equivalents of sodium and Me₃SiCl to the reaction mixture resulted in the formation of N(SiMe₃)₃, albeit an induction period still was observed.

Since in the first several hours of reaction no formation of N(SiMe₃)₃ is observed, and the stoichiometric reaction of Me₃SiCl with [Co(PPh₃)₃N₂]⁻ does not lead to the formation of N–Si bonds, it seemed rather unlikely that the catalytic reaction proceeds on CoH(PPh₃)₃N₂ (or [Co(PPh₃)₃N₂]Na(THF)₃). Indeed, IR spectroscopy analysis of an aliquot of a catalytically active mixture taken after 8 h of reaction revealed no peaks that could be assigned to any Co–N₂ species. This suggests that after the induction period, virtually all CoH(PPh₃)₃N₂ is decomposed and the bulk of the cobalt is not coordinated with a terminally bound N₂.

The majority of the coordination compounds of cobalt are paramagnetic, therefore we used EPR spectroscopy to probe the catalytic reaction mixture. Investigation of the reaction mixture after 3.5 h, i.e., before the onset of the formation of N(SiMe₃)₃, revealed the presence of a strong signal (Figure 5, red line) with clearly visible hyperfine couplings with the cobalt center and a very strong and broad signal spanning through the whole measurement window. Its spectral features (g_x = 2.13; g_y = 2.12; g_z = 2.02; A_x = 215; A_y = 195; A_z = 180 MHz) are indicative of a low-spin cobalt(II) species [35,37,38]. This species corresponded to approximately 8% of the total cobalt concentration. The amount of paramagnetic material was calculated from double integrals of the EPR signal of the catalytic reaction mixture and of a solution of Cobalt(II) meso-tetraphenylporphine used as the external standard (see Figure S1 in the supplementary materials). An aliquot from the reaction mixture taken after 48 h revealed the presence of the same paramagnetic species (with less resolved hyperfine structure, due to the lack of formation of good glass upon freezing) corresponding to approximately 5% of the total cobalt concentration (Figure 5, blue line). These data suggest that during the reaction of the CoH(PPh₃)₃N₂ with Me₃SiCl in THF, a small amount of paramagnetic cobalt(II) species is formed which is not a catalytic intermediate in the N₂ fixation reaction. This species can be formed, e.g., by a transfer of a chloro radical from Me₃SiCl to the cobalt(I) center, or by the attack of the trimethylsilyl radical formed after a one-electron reduction of Me₃SiCl. Formation of other cobalt species which could not be easily identified either using IR or EPR spectroscopy cannot be excluded; however, the reduction of the pre-catalyst to metallic cobalt has not been observed during the reaction.

Monitoring of the catalytic reaction with ³¹P NMR did not reveal any substantial signal of triphenylphosphine during the incubation period, and only after two days the signal at −6.5 ppm corresponding to free triphenylphosphine was observed, constituting less than 10% of the initial amount of CoH(PPh₃)₃N₂ (triphenylphosphine oxide added after the reaction stopped was used as an internal standard). This points to the possibility that triphenylphosphine remains bound to cobalt during catalysis; however, a fast exchange of triphenylphosphines on the cobalt center can also account for the lack of NMR signal.

To investigate whether cobalt nanoparticles are the active species, we analyzed the supernatant of the reaction mixture for the presence of nanoparticles using dynamic light scattering (DLS) after the formation of N(SiMe₃)₃ reached plateau. Measurements of the clear, brown solution obtained after sedimentation of the insoluble material formed during the catalytic reaction revealed a bimodal distribution of particles with an average size of 20 and 100 nm (see Figures S2 and S3 in the supplementary materials). This solution was, however, no longer active in the silylation of N₂ when additional sodium and Me₃SiCl were added. This suggests that even if cobalt nanoparticles are formed, they are an unlikely catalyst for the title reaction.

From the above results it is clear that neither CoH(PPh₃)₃N₂ nor its reduction product [Co(PPh₃)₃N₂]Na(THF)₃ activate N₂ towards the formation of N–Si bonds. Dinitrogen complexes of cobalt(II) are known [39]; however, it is unlikely that the observed paramagnetic complex is the active species given that it is already present in the reaction mixture before the onset of N(TMS)₃ formation is reached. It is possible that this species is being transformed into the actual catalyst during the incubation period. Since good molecular catalysts for the silylation of dinitrogen can reach over 200 turnovers [25,33], it is plausible that the actual very active catalyst is present only in minute amounts, which prevents its spectroscopic characterization. Based on the work of Nishibayashi, Yoshizawa and co-workers, an in-situ formed metallosilane species [28,34] can be responsible for the catalytic activity of CoH(PPh₃)₃N₂.

3. Materials and Methods

All manipulations were performed using Schlenk techniques under nitrogen or in a nitrogen-filled glovebox. CoH(PPh₃)₃N₂ was synthesized from Co(acac)₃ (99.99% trace metal basis, Sigma-Aldrich, (St. Louis, MO, USA) (acac = acetylacetonato) as reported previously [40]. CoCl(PPh₃)₃ [41] and CoCl₂(PPh₃)₂ [42] were synthesized according to published procedures. THF and Et₂O were distilled from sodium wire/benzoquinone, benzene and pentane was distilled from sodium wire. Me₃SiCl (99%, purified by redestillation) was degassed using freeze-pump-thaw method (three cycles) and stored under nitrogen. Solvents used for catalytic runs were degassed using freeze-pump-thaw method (three cycles) immediately before use. GC measurements were performed on a Shimadzu GC-17A Gas Chromatograph (Shimadzu Corporation, Kyoto, Japan) with a Supelco SPB-1 fused silica capillary column. X-band EPR measurements were performed on a Bruker EMX spectrometer equipped with a He temperature control cryostat system (Oxford Instruments, Tubney Woods, UK). DLS measurements were performed on an ALV/LSE 5003 electronics and multiple Tau digital correlator (ALV-Laser Vertriebsgesellschaft m-b.H, Langen, Germany).

General procedure for catalytic silylation of dinitrogen: 320 mg of sodium (30%–35% in mineral oil, 4.5 mmol) was placed in a 100 mL Schlenk flask equipped with a glass stirring bar. Sodium was washed three times with 5 mL of pentane and a 22 mg (0.025 mmol) amount of CoH(PPh₃)₃N₂ was added followed by 10 mL of THF. The solution was vigorously stirred and 0.6 mL (4.7 mmol) of Me₃SiCl was added with syringe. The initially orange-brown mixture turned gray/black after several hours of stirring. After 48 h, 10 μL of decane as an internal standard was added with a micropipette, and the mixture was centrifuged giving a purple precipitate and green-brown supernatant which was used for analysis of the amount of fixed nitrogen.

4. Conclusions

The cobalt-dinitrogen complex CoH(PPh₃)₃N₂ is a pre-catalyst in the reductive silylation of the dinitrogen reaction. This complex itself does not mediate the formation of N–Si bonds; however, under the applied catalytic conditions it transforms to the active species after approximately 4 h. Spectroscopic analysis of the reaction mixture did not reveal the presence of a resting state with a terminally bound N₂, and the nanoparticles formed during the course of the reaction are not catalytically active. It is therefore likely that in-situ formed homogeneous cobaltosilane species proposed by Nishibayashi, Yoshiawa and co-workers are responsible for the catalytic activity. Further studies should aim at an unambiguous determination of the structure of the catalytically active species generated from this mononuclear cobalt precursor.