ARTICLE pubs.acs.org/JACS
Molybdenum-Catalyzed Transformation of Molecular Dinitrogen into Silylamine: Experimental and DFT Study on the Remarkable Role of Ferrocenyldiphosphine Ligands Hiromasa Tanaka,† Akira Sasada,† Tomohisa Kouno,† Masahiro Yuki,‡ Yoshihiro Miyake,‡ Haruyuki Nakanishi,§ Yoshiaki Nishibayashi,*,‡ and Kazunari Yoshizawa*,† †
Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka, Fukuoka 819-0395, Japan Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan § Fuel Cell System Development Center, Toyota Motor Corporation, Mishuku, Susono, Shizuoka 410-1193, Japan ‡
bS Supporting Information ABSTRACT: A molybdenum-dinitrogen complex bearing two ancillary ferrocenyldiphosphine ligands, trans-[Mo(N2)2(depf)2] (depf = 1,10 -bis(diethylphosphino)ferrocene), catalyzes the conversion of molecular dinitrogen (N2) into silylamine (N(SiMe3)3), which can be readily converted into NH3 by acid treatment. The conversion has been achieved in the presence of Me3SiCl and Na at room temperature with a turnover number (TON) of 226 for the N(SiMe3)3 generation for 200 h. This TON is significantly improved relative to those ever reported by Hidai's group for mononuclear molybdenum complexes having monophosphine coligands [ J. Am. Chem. Soc. 1989, 111, 1939]. Density functional theory (DFT) calculations have been performed to figure out the mechanism of the catalytic N2 conversion. On the basis of some pieces of experimental information, SiMe3 radical is assumed to serve as an active species in the catalytic cycle. Calculated results also support that SiMe3 radical is capable of working as an active species. The formation of five-coordinate intermediates, in which one of the N2 ligands or one of the Mo-P bonds is dissociated, is essential in an early stage of the N2 conversion. The SiMe3 addition to a “hydrazido(2-)” intermediate having the NN(SiMe3)2 group will give a “hydrazido(1-)” intermediate having the (Me3Si)NN(SiMe3)2 group rather than a pair of a nitrido (tN) intermediate and N(SiMe3)3. The N(SiMe3)3 generation would not occur at the Mo center but proceed after the (Me3Si)NN(SiMe3)2 group is released from the Mo center. The flexibility of the Mo-P bond between Mo and depf would play a vital role in the high catalysis of the Mo-Fe complex.
’ INTRODUCTION Nitrogen fixation under ambient reaction conditions is one of the most important and challenging topics in chemistry.1 Molecular dinitrogen (N2) is chemically inert due to its extremely strong, nonpolar triple bond (225 kcal/mol) as well as the large HOMO-LUMO gap. Industrial nitrogen fixation typified by the Haber-Bosch process requires drastic reaction conditions of high pressures and high temperatures. Since the discovery of the first transition metal-N2 complex [Ru(N2)(NH3)5]2þ by Allen and Senoff in 1965,2 a great deal of effort has been devoted to the development of artificial nitrogen fixation systems that are capable of working under mild reaction conditions. Nowadays, a great number of well-defined N2 complexes are known for almost all d-block transition metals as well as some f-block metals, and a lot of studies have been reported so far on stoichiometric transformation of their coordinated N2 into ammonia (NH3) and hydrazine (NH2NH2).3-14 In 1975, for example, Chatt and co-workers found the formation of NH3 by protonolysis of tungsten-N2 and molybdenum-N2 complexes [M(N2)2(PR3)4] (M = Mo, W).15 In sharp contrast to such stoichiometric reactivity of transitionmetal-N2 complexes, there are only a few examples of the catalytic r 2011 American Chemical Society
transformation of N2 into NH3 and/or its NH3 equivalent using transition metal complexes under mild reaction conditions.16 In 1972, Shiina discovered the metal-chloride-catalyzed reductive transformation of N2 into tris(trimethylsilyl)amine (N(SiMe3)3), which can be readily converted into NH3 by acid treatment, in the presence of Me3SiCl and lithium.17 The turnover number (TON) of N(SiMe3)3 generation was up to 5.4 (CrCl3). Hidai, Mizobe, and co-workers reported in 1989 a more effective method for N(SiMe3)3 generation using Me3SiCl and sodium together with molybdenum-N2 complexes such as [Mo(N2)2(PMe2Ph)4] as a catalyst.18 In their reaction system, the TON reached 24 based on the Mo atom. The mechanism of this catalytic reaction still remains to be elucidated although they proposed a mechanism involving a silyldiazenido intermediate formed by the attack of SiMe3 radical on the N2 ligand. Recently, Yandulov and Schrock reported the direct conversion of N2 into NH3 using a molybdenum-N2 complex bearing an ancillary triamidoamine ligand.19 They adopted a pair of proton and electron Received: October 19, 2010 Published: February 22, 2011 3498
dx.doi.org/10.1021/ja109181n | J. Am. Chem. Soc. 2011, 133, 3498–3506
Journal of the American Chemical Society
ARTICLE
donors, lutidinium and decamethylchromocene, for the nitrogen fixation and proposed a catalytic mechanism containing successive hydrogenation of N2 through alternating steps of protonation and reduction. The hydrogenation of a Mo-NNH2 intermediate gives a nitrido (tN) intermediate and the first molecule of NH3, and then the nitrido ligand on Mo is converted into the second molecule of NH3. At present, the validity of this mechanism is strongly supported by the isolation and observation of a large part of reactive intermediates as well as intensive theoretical studies on the catalytic cycle.20-24 Unfortunately, the TON for the NH3 generation was not very high (up to 8 based on the Mo atom).25 Quite recently, some of us have reported the synthesis and the stoichiometric reactivity of molybdenum-N2 and tungsten-N2 complexes bearing ferrocenyldiphosphines as auxiliary ligands, trans-[M(N2)2(depf)2] (M = Mo (1a), W (2a); depf = 1,10 bis(diethylphosphino)ferrocene), where the electron transfer process from the ferrocene moiety to the tungsten or molybdenum center may be expected to assist the reduction of the coordinated N2 into NH3.26 During the continuous study on the development of novel nitrogen fixation system under mild reaction conditions,27 a more efficient catalytic nitrogen fixation system was found by using 1a as a catalyst. Dinitrogen under an atmospheric pressure was catalytically converted into N(SiMe3)3 in the presence of Me3SiCl and sodium in quite a high TON.
experimental information that can be directly associated with the determination of the catalytic mechanism. Computational quantum chemistry will provide a valuable insight into our understanding. Actually, some of us have computationally revealed the mechanisms on the activation and triple-bond cleavage of N2 by a cubane-type RuIr3S4 cluster28 as well as a hydride-bridged diniobium complex.29 In this Article, we describe typical results of the catalytic conversion of N2 into N(SiMe3)3 using 1a as a catalyst and propose a possible reaction pathway by quantum chemical calculations.
’ RESULTS AND DISCUSSION Catalytic Conversion of N2 into N(SiMe3)3 by 1a. Typical experimental results are summarized in Table 1 for the conversion of N2 into N(SiMe3)3 by using 1a as a catalyst. Details are described in the Supporting Information. Treatment of Na (60 mmol) and Me3SiCl (60 mmol) in the presence of a catalytic amount of 1a (0.015 mmol) in THF (tetrahydrofuran; 40 mL) under an atmospheric pressure of N2 at room temperature for 20 h gave 1.35 mmol of N(SiMe3)3 together with some side products such as Me3SiSiMe3, Me3Si(CH2)4OSiMe3, and nBuOSiMe3 (eq 1; Table 1, run 1). The generated N(SiMe3)3 was isolated from the reaction mixture and confirmed by 1H and 13C NMR and MS. catalyst ð0:015 mmolÞ
N2 þ reductant þ Me3 SiCl sf NðSiMe3 Þ3 ð1Þ
ð1 atmÞ
An intriguing subject is to elucidate how the coordinated N2 is converted in this multimetallic system and how the depf ligands enhanced the catalysis for the conversion of N2 into N(SiMe3)3. For the present reaction system, unfortunately, we have not obtained
ð60 mmolÞ
ð60 mmolÞ
THF; rt
The amounts of N(SiMe3)3 and other side-products were determined by GLC analysis. The TON for the formation of N(SiMe3)3 was 90 based on the Mo atom of 1a. A longer reaction time (100 h) increased the amount of N(SiMe3)3 up to 1.77 mmol, corresponding to a TON of 118 (Table 1, run 2). In the reaction with Li as a reductant in place of Na (Table 1, run 3) for 100 h, a smaller amount of N(SiMe3)3 (0.56 mmol, TON = 37) was formed together with a larger amount of Me3SiSiMe3 (16.53 mmol). The reaction in a more dilute solution (60 mL of THF) yielded a larger amount of N(SiMe3)3 (2.25 mmol, TON = 150; Table 1, run 4) than the result of run 2 of Table 1. The TON reached 184 after 150 h, and Me3SiCl was confirmed to be depleted in the reaction solution (Table 1, run 5). Here, we considered that the presence of more Na and Me3SiCl may produce an extra amount of N(SiMe3)3 from the reaction mixture
Table 1. Reactions Using Reductant (60 mmol) and Me3SiCl (60 mmol) Were Carried Out in the Presence of a Catalyst (0.015 mmol) under Atmospheric Pressure of N2 at Room Temperature in THF
a
time (h)
amount of N(SiMe3)3 (mmol)a
40
20
1.35
90
40 40
100 100
1.77 0.55
118 37
run
catalyst
reductant
THF (mL)
1
1a
Na
2 3
1a 1a
Na Li
TONb
4
1a
Na
60
100
2.25
150
5c
1a
Na
60
200
3.39
226
6
3
Na
40
20
0.48
32
7
1b
Na
40
20
0.81
54
8
4
Na
40
20
0.01
