Synthesis and Protonation of Molybdenum– and Tungsten–Dinitrogen

Feb 23, 2012 - Jesús A. Luque-Urrutia and Albert Poater ... Matthew F. Cain , William P. Forrest , Jr. , Dmitry V. Peryshkov , Richard R. Schrock , an...
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Synthesis and Protonation of Molybdenum− and Tungsten− Dinitrogen Complexes Bearing PNP-Type Pincer Ligands Kazuya Arashiba,† Kouitsu Sasaki,† Shogo Kuriyama,† Yoshihiro Miyake,† Haruyuki Nakanishi,‡ and Yoshiaki Nishibayashi*,† †

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

‡

S Supporting Information *

ABSTRACT: Novel molybdenum− and tungsten−dinitrogen complexes bearing PNP-type pincer ligands are prepared and characterized by X-ray analysis. Reactions of these molybdenum− and tungsten−dinitrogen complexes with an excess amount of sulfuric acid in THF at room temperature afford ammonia and hydrazine in good yields.

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INTRODUCTION Since the discovery of the first transition metal−dinitrogen complex,1 the preparation of various transition metal− dinitrogen complexes and their stoichiometric transformation of the coordinated dinitrogen have so far been well investigated toward the goal of achieving a nitrogen fixation system under mild reaction conditions.2,3 In sharp contrast to many studies on the stoichiometric reactivity of transition metal−dinitrogen complexes,2,3 there are only a few examples of the catalytic transformation of molecular dinitrogen using these complexes as catalysts under mild reaction conditions.4 In 2003, Schrock and his co-worker reported the molybdenum-catalyzed direct conversion of molecular dinitrogen into ammonia by using a molybdenum−dinitrogen complex bearing triamidomonoamine as a tetradentate ligand, where less than 8 equiv of ammonia was produced based on the catalyst.5 After Schrock’s report, there is no example of the catalytic conversion of molecular dinitrogen into ammonia under mild reaction conditions. Recently, we have found another successful example of the molybdenum-catalyzed direct conversion of molecular dinitrogen into ammonia by using a dinitrogen-bridged dimolybdenum complex bearing a tridentate PNP-type pincer ligand (1a: R = tBu) as a catalyst.6 In our reaction system, molecular dinitrogen under an atmospheric pressure was catalytically converted into ammonia in the presence of both electron source and proton source, where 23 equiv of ammonia was produced based on the catalyst (12 equiv of ammonia was produced based on the molybdenum atom of the catalyst). It is noteworthy that only 1a works as an effective catalyst, although some related molybdenum dinitrogen complexes were investigated toward the catalytic conversion of molecular dinitrogen into ammonia. In this paper, we describe detailed methods for the preparation of some derivatives of molybdenum− and tungsten−dinitrogen complexes bearing © 2012 American Chemical Society

PNP-type pincer ligands including 1a and their reactivity toward protonolysis to produce ammonia and hydrazine from the coordinated molecular dinitrogen.

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RESULTS AND DISCUSSION Treatment of [MoCl3(thf)3] (thf = tetrahydrofuran) with 1 equiv of tBu-PNP-type pincer ligand (2a: R = tBu) in THF (tetrahydrofuran) at 50 °C for 18 h gave [MoCl3(2a)] (3a: R = t Bu) in 94% yield (Scheme 1). The complex 3a is paramagnetic. Scheme 1. Preparation of Molybdenum Complexes Bearing PNP-Type Pincer Ligand

As shown in the previous report,6 the molecular structure of 3a was confirmed by X-ray crystallography. Similarly, reactions of [MoCl3(thf)3] with 1 equiv of R-PNP-type pincer ligands (2b: R = iPr; 2c: R = Ph) under the same reaction conditions afforded [MoCl3(2b)] (3b: R = iPr) and [MoCl3(2c)] (3c: R = Ph) in 92% and 71% yields, respectively. The molecular structure of 3b was confirmed by X-ray crystallography. An ORTEP drawing of 3b is shown in Figure 1. Selected bond lengths and angles for 3b are listed in Table 1. The crystal Received: January 6, 2012 Published: February 23, 2012 2035

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Scheme 2. Preparation of a Dinitrogen-Bridged Dimolybdenum Complex Bearing a tBu-PNP Pincer Ligand

Figure 1. Molecular structure of [MoCl3(2b)]·CH2Cl2 (3b·CH2Cl2). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

structure of 3b displays a distorted octahedral geometry around the molybdenum center. The metrical parameters of 3b are comparable to those of 3a. Reduction of 3a with an excess amount of Na−Hg (6 equiv) in THF at room temperature for 12 h under an atmospheric pressure of dinitrogen gave the diamagnetic dinitrogen-bridged dimolybdenum complex [Mo(N2)2(2a)]2(μ-N2) (1a: R = tBu) in 63% yield (Scheme 2). The molecular structure of 1a was confirmed by NMR, IR, and Raman spectra together with an Xray crystallographic study, as shown in the previous paper.6 In sharp contrast to the formation of 1a from 3a, unfortunately, no formation of the corresponding dinitrogen-bridged dimolybdenum complexes such as [Mo(N2)2(2b)]2(μ-N2) (1b: R = iPr) and [Mo(N2)2(2c)]2(μ-N2) (1c: R = Ph) was observed by the same method. In both cases, only unidentified products were formed. When reduction of 3a with an excess amount of Na−Hg (6 equiv) in THF at room temperature for 12 h under an atmospheric pressure of dinitrogen was carried out in the presence of 10 equiv of dimethylphenylphosphine (PMe2Ph), the corresponding mononuclear molybdenum dinitrogen complex bearing both 2a and the phosphine as auxiliary ligands, trans-[Mo(N2)2(2a)(PMe2Ph)] (4a), was obtained in 48% yield (Scheme 2). The 31P{1H} NMR spectrum of 4a shows a set of a doublet due to the PNP ligand 2a at 92.7 ppm and a triplet at 18.5 ppm due to the PMe2Ph ligand with a coupling constant of 5 Hz, respectively. The IR spectrum exhibits a strong absorption for N−N stretching at 1915 cm−1, which is comparable with that in the related mononuclear molybdenum dinitrogen complex trans-[Mo(N2)2(NC5H5)(PMePh2)3] (νNN = 1914 cm−1).7 In the presence of trimethylphosphine (PMe3) in place of PMe2Ph, a similar

reduction of 3a proceeded smoothly to afford trans-[Mo(N2)2(2a)(PMe3)] (4a′) in 46% yield (Scheme 2). The 1H and 31 P{1H} NMR spectra of 4a′ are consistent with the formulation. The IR spectrum of 4a′ displays one ν(NN) band at 1915 cm−1, consistent with the trans conformation. The molecular structure of 4a′ was confirmed by X-ray crystallography. An ORTEP drawing of 4a′ is shown in Figure 2. Selected bond lengths and angles for 4a′ are listed in Table 2. The PMe3 ligand occupies the position trans to the pyridine group of a PNP ligand. The Mo−N (2.01 Å, mean) and N−N (1.13 Å, mean) bond distances of trans-molybdenum bis(dinitrogen) in 4a′ are not unusual. Treatment of 1a in THF at room temperature for 12 h under an atmospheric pressure of carbon monoxide gave the corresponding dinitrogen-bridged dimolybdenum complex

Table 1. Selected Bond Lengths (Å) and Angles (deg) for 3b·CH2Cl2 Mo(1)−Cl(1) Mo(1)−Cl(3) Mo(1)−P(1) P(1)−Mo(1)−P(2) P(1)−Mo(1)−Cl(2) P(2)−Mo(1)−Cl(2) P(1)−Mo(1)−Cl(1) P(2)−Mo(1)−Cl(1) N(1)−Mo(1)−Cl(1) Cl(1)−Mo(1)−Cl(2) Cl(1)−Mo(1)−Cl(3)

2.4251(9) 2.4164(9) 2.531(1) 157.40(3) 103.77(3) 98.76(3) 83.62(3) 94.39(3) 87.49(7) 90.87(3) 176.26(4)

Mo(1)−Cl(2) Mo(1)−N(1) Mo(1)−P(2) P(1)−Mo(1)−N(1) P(2)−Mo(1)−N(1) N(1)−Mo(1)−Cl(2) P(1)−Mo(1)−Cl(3) P(2)−Mo(1)−Cl(3) N(1)−Mo(1)−Cl(3) Cl(2)−Mo(1)−Cl(3)

2036

2.409(1) 2.229(3) 2.530(1) 78.50(9) 78.92(9) 177.04(7) 94.04(3) 86.65(3) 89.19(7) 92.52(3)

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Scheme 3. Preparation of a Tungsten Complex Bearing a PNP-Type Pincer Ligand

Figure 2. Molecular structure of trans-[Mo(N2)2(2a)(PMe3)] (4a′). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

bearing carbon monoxide as an auxiliary ligand, [Mo(CO)2(2a)]2(μ-N2) (5a), in 64% yield (Scheme 2). Elemental analysis of 5a indicates that four dinitrogen ligands in 1a are replaced by four carbonyl ligands. The Raman spectrum of a THF solution of 5a shows one strong Raman band corresponding to the bridging-dinitrogen ligand at 1893 cm−1, which is similar to that of the parent dinitrogen-bridged dimolybdenum complex 1a (νNN = 1890 cm−1). As shown in the previous report,6 the molecular structure of 5a was confirmed by X-ray crystallography. Complex 5a is isomorphous with 1a and keeps the dinuclear structure during the course of reaction of 1a with CO. On the other hand, heating of 1a with 4 equiv of PMe2Ph and PMe3 in THF at 50 °C for 14 h gave 4a and 4a′ in moderate yield, respectively (Scheme 2). The result of the ligand exchange of 1a with carbon monoxide is in sharp contrast to that of the ligand exchange of 1a with the phosphines. The regioselectivity of the substitution reaction of 1a is likely due to the steric repulsion with tBu groups of a PNP ligand neighboring the terminal-dinitrogen ligands in 1a. Thus, a small molecule such as CO or N2 can coordinate at the apical position in 1a, whereas phosphines, which are more bulky than CO and N2, are not substituted with terminal-dinitrogen ligands but the bridging-dinitrogen ligands even under heating reaction conditions. Treatment of [WCl4(dme)] (dme = MeOCH2CH2OMe) with 1 equiv of 2a and 1 equiv of KC8 in THF at room temperature for 24 h gave [WCl3(2a)] (6a: R = tBu) in 59% yield (Scheme 3). The complex 6a is paramagnetic. An ORTEP drawing of 6a is shown in Figure 3. The unit cell includes two crystallographically independent molecules, but their structure is essentially the same. Selected bond lengths and angles for

Figure 3. Molecular structure of [WCl3(2a)]·C4H8O (6a·C4H8O). One of the two crystallographically independent molecules is shown. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

one of the independent molecules are listed in Table 3. The total coordination geometry of 6a is comparable to that of 3a, suggesting the tungsten center was reduced from W(IV) to W(III). Unfortunately, no formation of the tungsten congener of 1a by the reduction of 6a with an excess of Na−Hg or KC8 was observed, where only unidentified products were formed. In the presence of PMe2Ph, a similar reaction of 6a with Na− Hg afforded the corresponding tungsten dinitrogen complex trans-[W(N2)2(2a)(PMe2Ph)] (7a) in 28% isolated yield (Scheme 4). In the IR spectrum, complex 7a exhibits one strong ν(NN) band at 1895 cm−1 indicative of the trans conformation. The detailed structure of 7a has been determined by X-ray crystallography (Figure 4). Selected bond lengths and angles for 7a are listed in Table 4. The coordination geometry of 7a is similar to that of 4a′ except for PMe2Ph. With the novel molybdenum− and tungsten−dinitrogen complexes bearing PNP ligands in hand, we have investigated the reactivity of these complexes toward protonolysis. Treatment of the molybdenum− and tungsten−dinitrogen com-

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 4a′ Mo(1)−N(1) Mo(1)−N(4) N(4)−N(5) Mo(1)−P(2) Mo(1)−N(2)−N(3) P(1)−Mo(1)−P(2) P(1)−Mo(1)−P(3) P(2)−Mo(1)−P(3) P(1)−Mo(1)−N(2) P(2)−Mo(1)−N(2) P(3)−Mo(1)−N(2) N(1)−Mo(1)−N(2) N(2)−Mo(1)−N(4)

2.258(3) 2.013(4) 1.126(7) 2.487(1) 178.8(3) 154.52(4) 103.61(4) 101.85(4) 87.58(11) 92.16(11) 88.53(12) 89.78(15) 176.30(17)

Mo(1)−N(2) N(2)−N(3) Mo(1)−P(1) Mo(1)−P(3) Mo(1)−N(4)−N(5) P(1)−Mo(1)−N(1) P(2)−Mo(1)−N(1) N(1)−Mo(1)−P(3) P(1)−Mo(1)−N(4) P(2)−Mo(1)−N(4) P(3)−Mo(1)−N(4) N(1)−Mo(1)−N(4)

2037

2.005(4) 1.134(7) 2.500(1) 2.425(1) 177.2(4) 77.30(9) 77.22(9) 178.05(11) 91.85(11) 89.92(12) 88.05(12) 93.66(16)

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Table 3. Selected Bond Lengths (Å) and Angles (deg) for 6a·C4H8O W(1)−Cl(1) W(1)−Cl(3) W(1)−P(1) P(1)−W(1)−P(2) P(1)−W(1)−Cl(2) P(2)−W(1)−Cl(2) P(1)−W(1)−Cl(1) P(2)−W(1)−Cl(1) N(1)−W(1)−Cl(1) Cl(1)−W(1)−Cl(2) Cl(1)−W(1)−Cl(3)

2.393(3) 2.409(3) 2.560(2) 156.85(8) 99.59(7) 103.55(6) 89.45(7) 90.13(8) 88.81(15) 91.52(8) 177.18(10)

W(1)−Cl(2) W(1)−N(1) W(1)−P(2) P(1)−W(1)−N(1) P(2)−W(1)−N(1) N(1)−W(1)−Cl(2) P(1)−W(1)−Cl(3) P(2)−W(1)−Cl(3) N(1)−W(1)−Cl(3) Cl(2)−W(1)−Cl(3)

2.416(3) 2.176(6) 2.577(2) 78.16(14) 78.69(14) 177.73(13) 91.25(6) 88.16(7) 88.66(14) 91.05(8)

small amount of hydrazine (Table 5, runs 1−3). It is noteworthy that the yield of ammonia by the protonation of 4a was 1.38 equiv based on the molybdenum atom (Scheme 5; Table 5, run 2). Unfortunately, the protonation of the dinitrogen-bridged dimolybdenum carbonyl complex 5a afforded ammonia in only a lower yield (Table 5, run 4). In contrast to the results of the protonation of these molybdenum dinitrogen complexes, where ammonia is mainly produced, when the reaction of the tungsten−dinitrogen complex 7a with an excess amount of sulfuric acid in THF at room temperature for 24 h was performed, 0.62 equiv of hydrazine was obtained based on the tungsten atom together with 0.17 equiv of ammonia based on the tungsten atom (Scheme 5; Table 5, run 5). These results indicate that the nature of the metal atom in the dinitrogen complexes bearing a PNP ligand affects the course of the protonation reaction. It is well known that the yields of ammonia and/or hydrazine formed by protonolysis of [M(N2)2L4] (M = Mo, W) are affected by not only the metal center but also the sort of solvents, ligands, and acids.2,8,9 In fact, Chatt and co-workers reported that the reaction of a tungsten−dinitrogen complex bearing monodentate phosphines as auxiliary ligands, cis[W(N2)2(PMe2Ph)4], with an excess amount of sulfuric acid in MeOH at room temperature produced ammonia in high yield (1.98 equiv/W), while the molybdenum analogue afforded only a small amount of ammonia (0.64 equiv/Mo) due to the poorer electron-donating ability of the molybdenum atom than that of the tungsten atom.8 Many efforts toward the formation of ammonia from the molybdenum−dinitrogen complex suggest a reaction pathway involving disproportionation of the hydrazido complex.8−10 We suggest that the high yield of ammonia for 4a, which is comparable to those obtained from conventional tungsten−dinitrogen complexes such as cis-

Scheme 4. Preparation of a Tungsten−Dinitrogen Complex Bearing a Phosphine Ligand

Figure 4. Molecular structure of trans-[W(N2)2(2a)(PMe2Ph)] (7a). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

plexes 1a, 4a, 4a′, 5a, and 7a with an excess amount of sulfuric acid in THF at room temperature for 24 h led to the formation of ammonia and hydrazine. Typical results are shown in Table 5. Reactions of molybdenum−dinitrogen complexes 1a, 4a, and 4a′ exclusively gave ammonia in good yields together with a Table 4. Selected Bond Lengths (Å) and Angles (deg) for 7a W(1)−N(1) W(1)−N(4) N(4)−N(5) W(1)−P(2) W(1)−N(2)−N(3) P(1)−W(1)−P(2) P(1)−W(1)−P(3) P(2)−W(1)−P(3) P(1)−W(1)−N(2) P(2)−W(1)−N(2) P(3)−W(1)−N(2) N(1)−W(1)−N(2) N(2)−W(1)−N(4)

2.232(4) 1.993(5) 1.121(7) 2.483(2) 178.7(4) 153.25(5) 104.83(5) 101.80(6) 94.02(17) 87.67(15) 91.50(13) 90.33(16) 178.9(3)

W(1)−N(2) N(2)−N(3) W(1)−P(1) W(1)−P(3) W(1)−N(4)−N(5) P(1)−W(1)−N(1) P(2)−W(1)−N(1) N(1)−W(1)−P(3) P(1)−W(1)−N(4) P(2)−W(1)−N(4) P(3)−W(1)−N(4) N(1)−W(1)−N(4)

2038

1.991(4) 1.131(7) 2.492(2) 2.415(1) 179.7(5) 77.03(13) 76.27(13) 177.29(11) 86.82(16) 91.90(15) 87.59(13) 90.56(16)

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Table 5. Protonolysis of Molybdenum− and Tungsten−Dinitrogen Complexes

a

run

M(N2) complex

NH3 (mmol)

NH3 (equiv/M)a

N2H4 (mmol)

N2H4 (equiv/M)a

1 2 3 4 5

1a 4a 4a′ 5a 7a

0.049 0.055 0.034 0.008 0.007

0.61 1.38 0.85 0.10 0.17

0.005 0.005 0.003 0 0.025

0.06 0.13 0.08 0 0.62

Equiv based on M atom (M = Mo, W).

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Scheme 5. Protonolysis of Molybdenum− and Tungsten− Dinitrogen Complexes

EXPERIMENTAL SECTION

General Methods. 1H NMR (270 MHz) and 31P{1H} NMR (109 MHz) spectra were recorded on a JEOL Excalibur 270 spectrometer in a suitable solvent, and spectra were referenced to the residual solvent (1H) or external standard (31P{1H}: 85% H3PO4). IR spectra were recorded on a JASCO FT/IR 4100 Fourier transform infrared spectrometer. Raman spectra were recorded on a JASCO NRS-2000. Absorption spectra were recorded on a Shimadzu MultiSpec-1500. Elemental analyses were performed at the Microanalytical Center of The University of Tokyo. All manipulations were carried out under an atmosphere of nitrogen by using standard Schlenk techniques or glovebox techniques unless otherwise stated. Solvents were dried by the usual methods, then distilled and degassed before use. [Mo(N2)2(2a)]2(μ-N2) (1a),6 2,6bis(di-tert-butylphosphinomethyl)pyridine (2a), 13 2,6-bis(diisopropylphosphinomethyl)pyridine (2b),14 2,6-bis(diphenylphosphinomethyl)pyridine (2c),15 [MoCl3(2a)] (3a),6 [Mo(N 2 ) 2 (2a)(PMe 2 Ph)] (4a), 6 [Mo(CO) 2 (2a)] 2 (μ-N 2 ) (5a), 6 [MoCl3(thf)3],16 and [WCl4(dme)]17 were prepared according to the literature methods. Preparation of [MoCl3(2b)] (3b). A mixture of 2b (503.6 mg, 1.48 mmol) and [MoCl3(thf)3] (516.3 mg, 1.23 mmol) in THF (20 mL) was stirred at 50 °C for 24 h. The resultant yellow suspension was concentrated under reduced pressure, and the residue was recrystallized from CH2Cl2 (12 mL)−hexane (40 mL) to give yellow needles of 3b·CH2Cl2, which were collected by filtration, washed with Et2O (5 mL × 3), and dried in vacuo to afford 3b as a yellow crystalline solid (617.3 mg, 1.14 mmol, 92% yield). Anal. Calcd for C19H35Cl3MoNP2: C, 42.12; H, 6.51; N, 2.59. Found: C, 41.93; H, 6.33; N, 2.47. Preparation of [MoCl3(2c)] (3c). A mixture of 2c (114.6 mg, 0.24 mmol) and [MoCl3(thf)3] (100.1 mg, 0.24 mmol) in THF (10 mL) was stirred at 50 °C for 13 h. The resultant yellow suspension was filtered, washed with Et2O (3 mL × 2), and dried in vacuo to afford 3c as a yellow solid (115.2 mg, 0.17 mmol, 71% yield). Anal. Calcd for C31H27Cl3MoNP2: C, 54.93; H, 4.01; N, 2.07. Found: C, 54.61; H, 4.09; N, 1.99. Preparation of trans-[Mo(N2)2(2a)(PMe2R)] (4a, R = Ph; 4a′, R = Me). From 3a: To Na−Hg (0.5 wt %, 4.2309 g, 0.91 mmol) was added THF (10 mL). After the sequential addition of PMe3 (1 M solution in toluene, 1.5 mL, 1.5 mmol) and 3a (89.7 mg. 0.15 mmol), the mixture was stirred at room temperature for 12 h under a N2 atmosphere. The resultant dark purple solution was decanted, and then the supernatant solution was concentrated in vacuo. To the residue was added Et2O (6 mL), the solution was filtered through Celite, and the filter cake was washed with Et2O (2 mL × 5). After the combined filtrate was concentrated in vacuo to about 1 mL, the obtained solution was stood at −40 °C to afford 4a′ as purple crystals, which were collected by filtration and dried in vacuo (43.3 mg, 0.07 mmol, 46% yield). 31P{1H} NMR (C6D6): δ 94.6 (d, 2JPP = 5 Hz, PNP), 6.6 (t, 2JPP = 5 Hz, PMe3). 1H NMR (C6D6): δ 6.74−6.68 (m, PNP, 3H), 3.25 (br s, CH2PtBu2, 4H), 1.70 (d, 2JPH = 5.1 Hz, PMe3, 9H), 1.26 (pseudo t, CH2PtBu2, 36H). IR (KBr, cm−1): 1915 (s, νNN). Calcd for C26H52MoN5P3: C, 50.08; H, 8.41; N, 11.23. Found: C, 50.10; H, 8.33; N, 11.21. The synthetic method for 4a from 3a was previously reported.6 From 1a: To a solution of 1a (73.1 mg. 0.065

[W(N2)2(PMe2Ph)4], is due to the higher reducing ability of the molybdenum center in 4a bearing both the PNP ligand 2a and PMe2Ph.11 A related example of the protonation of 4a is the reaction of a molybdenum−dinitrogen complex bearing mixed tridentate and monodentate ligands, trans-[Mo(N2)2(triphos)(PPh3)] (triphos = PhP(CH2CH2PPh2)2), with an excess amount of HBr in THF at room temperature, giving 0.87 equiv of ammonia.9a On the other hand, the protonation reaction of the tungsten−dinitrogen complex 7a preferentially produced hydrazine. Previously, Hidai and co-workers reported that the reaction of cis-[M(N2)2(PMe2Ph)4] (M = Mo, W) with an excess amount of HCl in DME at room temperature gave hydrazine as a main product in moderate yield.12 The reaction pathway involving the hydrido−hydrazido complex formed by protonation of the corresponding hydrazido complex at the metal center is proposed for the formation of hydrazine. Thus, it might be possible that the more electron-rich nature of the tungsten center in 7a than that of the molybdenum analogue 4a causes the proton attack on the tungsten center of the hydrazido intermediate (WN−NH2) to give hydrazine. In conclusion, we have synthesized a series of molybdenum− and tungsten−dinitrogen complexes bearing a PNP-type pincer ligand. Protonation of the molybdenum−dinitrogen complexes with an excess of sulfuric acid in THF afforded ammonia in good yields. In contrast, hydrazine was obtained upon the reaction of the more electron-rich tungsten complex with sulfuric acid under the same reaction conditions. We believe that the presence of the pincer ligand in the molybdenum− and tungsten−dinitrogen complexes realizes this unique reactivity toward the protonolysis. We will report the detailed catalytic activity of molybdenum−dinitrogen complexes bearing a pincer ligand in due course. Further studies on the preparation and reactivity of related early transition metal (Re, V, Nb, Ta, Cr, Mo, and W)−dinitrogen complexes bearing pincer ligands including other PCP-type pincer ligands are currently under way. 2039

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Table 6. X-ray Crystallographic Data for [MoCl3(2b)]·CH2Cl2 (3b·CH2Cl2), trans-[Mo(N2)2(2a)(PMe3)] (4a′), [WCl3(2a)]·C4H8O (6a·C4H8O), and trans-[W(N2)2(2a)(PMe2Ph)] (7a) chemical formula fw dimens of cryst cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρcalcd, g cm−3 F(000) μ, cm−1 transmn factors range no. reflns measd no. unique reflns no. params refined R1 (I > 2 σ(I))a wR2 (all data)b GOF (all data)c Flack param max. diff peak/hole, e Å−3

3b·CH2Cl2

4a′

6a·C4H8O

7a

C20H37Cl5MoNP2 626.67 0.50 × 0.35 × 0.20 monoclinic C2/c 19.9052(5) 10.7519(3) 27.3135(8) 90 107.3880(9) 90 5578.5(3) 8 1.492 2568 10.715 0.665−0.807 26 830 6224 (Rint = 0.021) 299 0.0508 0.1259 1.022

C26H52MoN5P3 623.59 0.25 × 0.10 × 0.10 monoclinic P21/c 8.8589(8) 20.731(2) 17.840(2) 90 106.435(3) 90 3142.5(5) 4 1.318 1320 5.924 0.480−0.942 19 707 5487 (Rint = 0.090) 368 0.0330 0.1090 1.002

C31H54N5P3W 773.57 0.45 × 0.25 × 0.03 triclinic P1̅ 10.6999(6) 13.0508(6) 13.9197(7) 73.199(1) 79.697(2) 69.329(2) 1734.6(2) 2 1.481 788 34.996 0.444−0.900 16 833 7818 (Rint = 0.046) 415 0.0406 0.0925 1.017

2.28/−1.92

0.47/−0.30

C27H51Cl3NOP2W 757.86 0.40 × 0.30 × 0.15 monoclinic C2 29.2304(8) 14.9824(4) 19.7839(5) 90 126.2874(7) 90 6983.9(3) 8 1.441 3064 36.517 0.461−0.578 55 741 15 123 (Rint = 0.041) 676 0.0316 0.0785 1.009 0.007(5) 3.98/−0.95

R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = − (4a′); p = 0, q = 3.3 (6a·C4H8O); p = 0, q = 1.45 (7a)]. GOF = a

b

[∑w(Fo2

Fc2)2/∑w(Fo2)2]1/2, c

w = 4Fo2/[pFo2 + qσ(Fo2)] [p = 0, q = [∑w(Fo2 − Fc2)2/(No − Nparams)]1/2.

mmol) in THF (5 mL) was added PMe2Ph (40 μL, 0.281 mmol), and then the mixture was stirred at 50 °C for 14 h. The resultant dark purple solution was concentrated, and the residue was extracted with Et2O (10 mL). After the extract was concentrated to about 3 mL, the obtained solution was stood at −40 °C to afford 4a as purple crystals (45.5 mg, 0.066 mmol, 51% yield). The spectroscopic data are in agreement with those previously reported. The synthetic method for 4a′ from 1a is similar to that of 4a. 4a′: purple crystals, 49% yield. Preparation of [WCl3(2a)] (6a). A mixture of 2a (198.9 mg, 0.50 mmol), [WCl 4 (dme)] (208.0 mg, 0.50 mmol; dme = MeOCH2CH2OMe), and KC8 (67.8 mg, 0.50 mmol) in THF (20 mL) was stirred at room temperature for 24 h. The resultant dark purple solution was filtered. Slow addition of hexane (40 mL) to the filtrate afforded 6a·C4H8O as dark brown crystals, which were collected by filtration, washed with Et2O (5 mL), and dried in vacuo to afford 6a as a brown crystalline solid (202.4 mg, 0.30 mmol, 59% yield). Anal. Calcd for C23H43Cl3WNP2: C, 40.28; H, 6.32; N, 2.04. Found: C, 40.31; H, 6.25; N, 1.82. Preparation of trans-[W(N2)2(2a)(PMe2Ph)] (7a). To Na−Hg (0.43 wt %, 7.96 g, 1.51 mmol) were added THF (15 mL), PMe2Ph (75 μL, 0.53 mmol), and 6a (178 mg. 0.26 mmol), and then the mixture was stirred at room temperature for 13 h under N2. The resultant dark reddish-purple solution was decanted, and then the supernatant solution was concentrated in vacuo. To the residue was added Et2O (10 mL), the solution was filtered through Celite, and the filter cake was washed with Et2O (5 mL × 3). After the combined filtrate was concentrated in vacuo to about 1 mL, the obtained solution was stood at −40 °C to afford 7a as purple crystals, which were collected by filtration and dried in vacuo (55.6 mg, 0.07 mmol, 28% yield). 31P{1H} NMR (C6D6): δ 73.1 (d with W satellites, 2JPP = 5.5 Hz, 2JPW = 328.2 Hz, PNP), −17.4 (t with W satellites, 2JPP = 5.5 Hz, 2 JPW = 405.2 Hz, PMe2Ph). 1H NMR (C6D6): δ 7.65 (m, PMe2Ph, 2H), 7.27 (m, PMe2Ph, 2H), 7.02 (m, PMe2Ph, 1H), 6.81−6.70 (m, PNP, 3H), 3.35 (br s, CH2PtBu2, 4H), 2.13 (d, 2JPH = 5.9 Hz, PMe2Ph, 6H), 1.26 (pseudo t, CH2PtBu2, 36 H). IR (KBr, cm−1): 1889 (s, νNN).

2.15/−1.98

12.0 (3b·CH2Cl2); p = 0.0006, q = 0.16

Anal. Calcd for C31H54N5P3W: C, 48.13; H, 7.04; N, 9.05. Found: C, 48.11; H, 7.17; N, 8.85. Protonolysis of Molybdenum− and Tungsten−Dinitrogen Complexes. To a solution of molybdenum− or tungsten−dinitrogen complex (0.040 mmol) in THF (5 mL) was added concentrated sulfuric acid (50 μL). The mixture was stirred at room temperature for 24 h. The reaction mixture was evaporated under reduced pressure. Potassium hydroxide aqueous solution (30 wt %; 5 mL) was added to the residue, and the mixture was distilled into dilute H2SO4 solution (0.5 M, 10 mL) under reduced pressure. The amount of NH3 or N2H4 was determined by the indophenol18 or p-(dimethylamino)benzaldehyde method,19 respectively. Detailed results are shown in Table 5. X-ray Crystallography. Crystallographic data of 3b·CH2Cl2, 4a′, 6a·C4H8O, and 7a are summarized in Table 6. Selected bond lengths and angles are summarized in Tables 1−4, and their ORTEP drawings are shown in Figures 1−4. Diffraction data were collected at −100 °C on a Rigaku RAXIS RAPID imaging plate area detector with graphitemonochromated Mo Kα radiation (λ = 0.71075 Å). Reflections were collected for the 2θ range of 5° to 55°. Intensity data were collected for Lorentz−polarization effects and for empirical absorption (REQAB). The structure solution and refinements were carried out by using the CrystalStructure crystallographic software package.20 The positions of the non-hydrogen atoms were determined by heavy atom Patterson methods (PATTY21 for 3b·CH2Cl2, SHELX-9722 for 6a·C4H8O) or direct methods (SHELX-9722 for 4′, SIR-2004 for 7a) and subsequent Fourier syntheses (DIRDIF-9923) and were refined on Fo2 using all unique reflections by full-matrix least-squares with anisotropic thermal parameters except for a few solvated molecules (O1−O4 and C47−C61 for 6a·C4H8O), which were refined isotropically. All other hydrogen atoms were placed at calculated positions with fixed isotropic parameters. 2040

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(10) Molybdenum− and tungsten−hydrazido complexes (MN− NH2) are well known as one of the intermediates in the transformation of dinitrogen into ammonia. See ref 2. (11) (a) We have confirmed in the previous report that the reaction of the molybdenum hydrazido complex [MoF(2a)(NNH2)(pyridine)] BF4, which is prepared by the reaction of 1a with HBF4 followed by addition of pyridine, with an excess amount of 2,6-lutidinium trifluoromethanesulfonate produced ammonia (see ref 6). On the basis of the experimental results, it is generally believed that tungsten complexes have a stronger reducing ability via π back-donation than molybdenum complexes do. (b) Hidai, M.; Mizobe, Y.; Sato, M.; Kodama, T.; Uchida, Y. J. Am. Chem. Soc. 1978, 100, 5740. (c) Tuczek, F.; Horn, K. H.; Lehnert, N. Coord. Chem. Rev. 2003, 245, 107. (d) See ref 2. (12) Takahashi, T.; Mizobe, Y.; Sato, M.; Uchida, Y.; Hidai, M. J. Am. Chem. Soc. 1980, 102, 7461. (13) Kawatsura, M.; Hartwig, J. F. Organometallics 2001, 20, 1960. (14) Leung, W.-P.; Ip, Q. W.-Y.; Wong, S.-Y.; Mak, T. C. W. Organometallics 2003, 22, 4604. (15) Cochran, B. M.; Michael, F. E. J. Am. Chem. Soc. 2008, 130, 2786. (16) Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001, 2699. (17) Persson, C.; Andersson, C. Inorg. Chim. Acta 1993, 203, 235. (18) Weatherburn, M. W. Anal. Chem. 1967, 39, 971. (19) Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006. (20) CrystalStructure 3.80: Single Crystal Structure Analysis Software; Rigaku Corp: Tokyo, Japan, and MSC: The Woodlands, TX, 2000− 2007. (21) Beurskens, P. T., Admiraal, G., Beurskens, G., Bosman, W. P., Garcia-Granda, S., Gould, R. O., Smits, J. M. M., Smykalla, C. PATTY: The DIRDIF Program System; Technical Report of the Crystallography Laboratory, University of Nijmegen: Nijmegen, The Netherlands, 1992. (22) Sheldrick, G. M. SHELX-97: Program for the Refinement of Crystal Structure; University of Göttingen: Göttingen, Germany, 1997. (23) Beurskens, P. T., Beurskens, G., de Gelder, R., García-Granda, S., Gould, R. O., Israël, R., Smits, J. M. M. The DIRDIF-99 Program System; Crystallography Laboratory, University of Nijmegen: Nijmegen, The Netherlands, 1999.

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data for 3b·CH2Cl2, 4a′, 6a·C4H8O, and 7a are available in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Funding Program for Next Generation World-Leading Researchers (GR025). Financial support from Toyota Motor Corporation is also gratefully acknowledged. We thank the Research Hub for Advanced Nano Characterization at The University of Tokyo.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on Feb 23, 2012, with errors in ref 5. The corrected version was reposted on Feb 29, 2012.

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