Synthesis and Catalytic Activity of Molybdenum–Dinitrogen

Nov 29, 2012 - Markus Hölscher , Walter Leitner ... Práxedes Sánchez , Martín Hernández-Juárez , Eleuterio Álvarez , Margarita Paneque , Nuria ...
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Synthesis and Catalytic Activity of Molybdenum−Dinitrogen Complexes Bearing Unsymmetric PNP-Type Pincer Ligands Eriko Kinoshita,† Kazuya Arashiba,† Shogo Kuriyama,† Yoshihiro Miyake,† Ryuji Shimazaki,‡ Haruyuki Nakanishi,§ and Yoshiaki Nishibayashi*,† †

Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan Future Project Division, Toyota Motor Corporation, Mishuku, Susono, Shizuoka 410-1193, Japan § Fuel Cell System Development Center, Toyota Motor Corporation, Mishuku, Susono, Shizuoka 410-1193, Japan ‡

S Supporting Information *

ABSTRACT: Novel dinitrogen-bridged dimolybdenum complexes bearing unsymmetric PNP-type pincer ligands are prepared and characterized by X-ray analysis. A molybdenum−dinitrogen complex bearing 2-(di-1-adamantylphosphino)methyl-6(di-tert-butylphosphino)methylpyridine has been found to work as an effective catalyst toward the formation of ammonia from molecular dinitrogen under ambient conditions.

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INTRODUCTION Toward the goal of achievement of a nitrogen fixation system under mild reaction conditions,1 we have recently found another successful example2 of the molybdenum-catalyzed direct conversion of molecular dinitrogen into ammonia by using a dinitrogen-bridged dimolybdenum complex bearing tridentate PNP-type pincer ligands (1) as a catalyst (Scheme 1).3 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 the dinitrogen-

bridged dimolybdenum complex 1 works as an effective catalyst, although some related molybdenum−dinitrogen complexes bearing monodentate and bidentate phosphines were investigated toward the catalytic conversion of molecular dinitrogen into ammonia. As an extension of our study, we have already prepared some molybdenum complexes bearing other types of PNP pincer ligands, where both phosphorus atoms have the same substituents such as isopropyl and phenyl groups; however, the corresponding dinitrogen-bridged dimolybdenum complexes cannot be prepared according to the previous method.4 These results prompted us to investigate the preparation of dinitrogen-bridged dimolybdenum complexes bearing unsymmetric PNP-type pincer ligands, where one phosphorus atom in the pincer ligand has two tert-butyl groups as substituents and the other has two other substituents such as 1-adamantyl, phenyl, isopropyl, and cyclohexyl groups (Chart 1). In this paper, we describe detailed methods for the preparation of

Scheme 1. Molybdenum-Catalyzed Reduction of Molecular Dinitrogen into Ammonia under Ambient Conditions

Chart 1

Received: November 2, 2012 Published: November 29, 2012 © 2012 American Chemical Society

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unsymmetric PNP-type pincer ligands and the corresponding dinitrogen-bridged dimolybdenum complexes. Interestingly, a dinitrogen-bridged dimolybdenum complex bearing 2-(di-1adamantylphosphino)methyl-6-(di-tert-butylphosphino)methylpyridine has been found to work as an effective catalyst toward the formation of ammonia from molecular dinitrogen under ambient conditions.

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RESULTS AND DISCUSSION Treatment of 2,6-lutidine with 1 equiv of nBuLi in diethyl ether at 0 °C for 1 h and then the addition of tBu2PCl at −78 °C to room temperature for 2 h gave 2a (R = tBu) in 54% yield (Scheme 2). After lithiation of 2a with 3 equiv of nBuLi in

Figure 1. Molecular structure of [MoCl3(3b)] (4b). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

Scheme 2. Preparation of Unsymmetric PNP-Type Pincer Ligands

diethyl ether at 0 °C to reflux temperature for 15 h, the addition of Ad2PCl (Ad = 1-adamantyl) at −78 °C to room temperature for 22 h gave 3a (R = tBu, R′ = Ad) in 72% yield as an unsymmetric PNP-type pincer ligand. According to the same method, other unsymmetric pincer ligands (3c and 3d) were prepared in high yields and characterized spectroscopically. Although the reaction of 2a with Ph2PCl gave only a complex mixture, the reaction of 2-(diphenylphosphinomethyl)-6methylpyridine (2b) as starting material with tBu2PCl afforded 3b in 89% yield. Treatment of [MoCl3(thf)3] (thf = tetrahydrofuran) with 1 equiv of 3a in THF (tetrahydrofuran) at 50 °C for 20 h gave the paramagnetic molybdenum pincer complex [MoCl3(3a)] (4a: R = tBu, R′ = Ad) in 97% yield (Scheme 3). Similarly,

Figure 2. Molecular structure of [MoCl3(3d)] (4d). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

(N2)2(3b)]2(μ-N2) (5b: R = tBu, R′ = Ph) in 49% and 54% yields, respectively (Scheme 4). The IR spectrum of 5a show a strong absorption at 1939 cm−1 assignable to ν(NN), while the ν(NN) value of 5b (1957 cm−1) is higher than that of 1 (1936 cm−1), suggesting the less electron-donating ability of the pincer ligand 3b. The detailed structure of 5b has been confirmed by X-ray crystallography. An ORTEP drawing of 5b is shown in Figure 3. Selected bond lengths and angles for 5b are listed in Table 3. As shown in Figure 3, complex 5b has a dinuclear structure, where two trans-[Mo(N2)2(3b)] units are bridged by one dinitrogen ligand in an end-on fashion. The total coordination geometry of 5b is comparable to that of 1 except for the PNP pincer ligands. The bridging N−N bond distance (1.131(4) Å) is close to that of the dinitrogen-bridged Mo(0) complex [Mo(CO)(Et2PCH2CH2PEt2)2]2(μ-N2) (N− N 1.127(5) Å).5 On the other hand, reduction of 4c and 4d with an excess amount of Na−Hg under the same reaction conditions gave also the dinitrogen-bridged dimolybdenum complexes [Mo(N2)2(3c)]2(μ-N2) (5c: R = tBu, R′ = iPr) and [Mo(N2)2(3d)]2(μ-N2) (5d: R = tBu, R′ = Cy) in 43% and 61% yields, respectively (Scheme 4). In contrast to 5a and 5b, the IR spectrum of 5c exhibits four strong absorptions at 1968, 1949, 1897, and 1875 cm−1 attributable to ν(NN) bands. The 31 1 P{ H} NMR spectrum of 5c shows a set of doublets at 89.5 ppm due to the tBu2P group and a doublet at 75.5 ppm due to the iPr2P group with a coupling constant of 143 Hz, and a set of doublets at 88.4 ppm due to the tBu2P group and a doublet at 75.7 ppm due to the iPr2P group with a coupling constant of 130 Hz, respectively. The IR and 31P{1H} NMR spectra of 5d are similar to those of 5c. These results of spectroscopical data for 5c and 5d indicate that the molecular structures of 5c and

Scheme 3. Preparation of Molybdenum Trichloride Complexes Bearing Unsymmetric PNP-Type Pincer Ligands

reactions of [MoCl3(thf)3] with 1 equiv of other unsymmetric PNP-type pincer ligands (3b, 3c, and 3d) under the same reaction conditions afforded [MoCl3(3)] (4b: R = tBu, R′ = Ph; 4c: R = tBu, R′ = iPr; 4d: R = tBu, R′ = Cy (Cy = cyclohexyl)) in 88%, 76%, and 92% yields, respectively. The molecular structures of 4b and 4d were confirmed by X-ray crystallography. ORTEP drawings of 4b and 4d are shown in Figures 1 and 2. Selected bond lengths and angles for 4b and 4d are listed in Tables 1 and 2. The crystal structures of 4b and 4d display a distorted octahedral geometry around the molybdenum center. Reduction of 4a and 4b 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 corresponding dinitrogen-bridged dimolybdenum complexes [Mo(N2)2(3a)]2(μ-N2) (5a: R = tBu, R′ = Ad) and [Mo8438

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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 4b 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)

2.4157(7) 2.4052(7) 2.5900(6) 155.12(2) 102.84(2) 102.02(2) 96.62(2) 84.24(2) 92.10(5) 90.46(2) 170.62(2)

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)

2.3970(6) 2.216(2) 2.5288(6) 77.73(5) 77.39(5) 177.30(5) 91.71(2) 86.39(2) 85.46(5) 91.88(2)

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)

2.3997(6) 2.214(1) 2.5376(6) 77.52(4) 78.49(4) 177.22(4) 89.58(2) 90.44(2) 84.50(4) 92.81(2)

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 4d 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)

2.4259(5) 2.4220(5) 2.6139(6) 155.90(2) 103.21(2) 100.86(2) 94.66(2) 83.92(2) 92.16(4) 90.46(2) 173.93(2)

Scheme 4. Preparation of Dinitrogen-Bridged Dimolybdenum Complexes Bearing Unsymmetric PNPType Pincer Ligands

5d are quite different from those of 5a and 5b. The detailed structure of 5c has been unambiguously determined by X-ray crystallography. An ORTEP drawing of 5c is shown in Figure 4. Selected bond lengths and angles for 5c are listed in Table 4. The distinct structural feature of 5c is the existence of trans[Mo(N2)2(3c)] and cis-[Mo(N2)2(3c)], which are bridged by one dinitrogen ligand in an end-on fashion. The difference in the coordination geometry around each molybdenum center is in accord with the spectral data, suggesting that dimolybdenum complex 5c also maintains this configuration in solution. The bridging N−N (1.148(4) Å) and terminal N−N (1.12 Å, mean) bond distances are comparable to those of 5b, and the Mo−N−N−Mo array in 5c is almost linear. The iPr2P groups of PNP ligands in 5c are oriented to avoid steric interactions, where the dihedral angle between the planes defined by P(1)− N(1)−P(2)−N(6) and P(3)−N(7)−P(4)−N(9) is 70.73(3)°. Next, the catalytic reduction of dinitrogen into ammonia by using 5 was investigated according to a modified procedure.3 To a suspension of 5a and 2,6-lutidinium trifluoromethanesulfonate (96 equiv to 5a; [LutH]OTf; Lut = 2,6-lutidine; OTf =

Figure 3. Molecular structure of [Mo(N2)2(3b)]2(μ-N2) (5b·3C6H6). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

OSO2CF3) as a proton source in toluene was added a solution of cobaltocene (72 equiv to 5a; CoCp2; Cp = η5-C5H5) as a reductant in toluene via a syringe pump at room temperature over a period of 1 h, followed by stirring at room temperature for another 19 h under an atmospheric pressure of dinitrogen. After the reaction, ammonia (14 equiv to 5a; 7 equiv to Mo atom in 5a) was catalytically formed together with the formation of dihydrogen (8 equiv to 5a) (Table 5, run 1). Ammonia and dihydrogen were formed in 58% and 22% yields, respectively, based on the cobaltocene. The amount of ammonia by using 5a as a catalyst is larger than that by using 1 as a catalyst (12 equiv of ammonia to 1; Table 5, run 7).3 It is noteworthy that 5a worked as a more effective catalyst than 1 under the present reaction conditions. 8439

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Table 3. Selected Bond Lengths (Å) and Angles (deg) for 5b·3C6H6 Mo(1)−N(1) Mo(1)−N(4) Mo(1)−P(1) N(2)−N(3) N(6)−N(6*) Mo(1)−N(2)−N(3) Mo(1)−N(6)−N(6*) P(1)−Mo(1)−N(1) P(2)−Mo(1)−N(6) N(1)−Mo(1)−N(6) P(1)−Mo(1)−N(4) P(2)−Mo(1)−N(4) N(1)−Mo(1)−N(4) N(4)−Mo(1)−N(6)

2.195(3) 2.007(3) 2.466(1) 1.111(5) 1.131(4) 175.5(3) 175.4(3) 78.49(8) 106.40(9) 175.1(1) 95.5(1) 85.26(9) 92.8(1) 88.3(1)

Mo(1)−N(2) Mo(1)−N(6) Mo(1)−P(2) N(4)−N(5)

2.036(3) 2.045(3) 2.3884(9) 1.124(5)

Mo(1)−N(4)−N(5) P(1)−Mo(1)−P(2) P(1)−Mo(1)−N(6) P(2)−Mo(1)−N(1) P(1)−Mo(1)−N(2) P(2)−Mo(1)−N(2) N(1)−Mo(1)−N(2) N(2)−Mo(1)−N(6) N(2)−Mo(1)−N(4)

175.9(4) 156.97(4) 96.64(9) 78.48(8) 92.1(1) 87.54(9) 88.2(1) 91.3(1) 172.4(1)

However, the use of larger amounts of both cobaltocene (216 equiv) and [LutH]OTf (288 equiv) only slightly increased the amount of ammonia (16 equiv of ammonia to 5a; Table 5, run 2), in contrast to the catalytic reaction by using 1 as a catalyst under the same reaction conditions (23 equiv of ammonia to 1; Table 5, run 8). When we reduced the time of addition of a toluene solution of cobaltocene from 1 h to 30 min in the reaction using 5a as a catalyst, substantially a larger amount of ammonia was produced based on the catalyst (19 equiv of ammonia to 5a; Table 5, run 3).6 These results indicate that 5a has been found to be a less effective catalyst than 1 toward the formation of ammonia from molecular dinitrogen under ambient reaction conditions. In contrast to 5a, other dinitrogen-bridged dimolybdenum complexes such as 5b, 5c, and 5d did not work as catalysts. In these cases, only a stoichiometric amount of ammonia was produced based on the catalyst (Table 5, runs 4−6). We have not yet obtained the exact reason that these dinitrogen-bridged

Figure 4. Molecular structure of [Mo(N2)2(3c)]2(μ-N2) (5c). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvated molecules are omitted for clarity.

Table 4. Selected Bond Lengths (Å) and Angles (deg) for 5c Mo(1)−N(1) Mo(1)−N(4) Mo(2)−N(7) Mo(2)−N(9) Mo(1)−P(1) Mo(2)−P(3) N(2)−N(3) N(6)−N(7) N(11)−N(12) Mo(1)−N(2)−N(3) Mo(1)−N(6)−N(7) Mo(2)−N(9)−N(10) P(1)−Mo(1)−P(2) P(1)−Mo(1)−N(1) P(2)−Mo(1)−N(6) P(1)−Mo(1)−N(2) P(2)−Mo(1)−N(2) N(1)−Mo(1)−N(2) N(2)−Mo(1)−N(6) P(3)−Mo(2)−N(8) P(4)−Mo(2)−N(11) P(3)−Mo(2)−N(7) P(4)−Mo(2)−N(7) N(7)−Mo(2)−N(8) N(8)−Mo(2)−N(9)

2.219(3) 2.029(3) 2.094(3) 1.977(3) 2.479(1) 2.472(1) 1.126(5) 1.148(4) 1.133(5) 175.3(3) 177.5(3) 178.9(3) 156.69(4) 78.06(9) 102.48(9) 94.27(9) 83.19(9) 93.0(1) 90.9(1) 78.59(7) 101.37(9) 94.60(8) 85.22(8) 95.7(1) 87.2(1) 8440

Mo(1)−N(2) Mo(1)−N(6) Mo(2)−N(8) Mo(2)−N(11) Mo(1)−P(2) Mo(2)−P(4) N(4)−N(5) N(9)−N(10)

2.020(3) 1.991(3) 2.213(3) 1.999(4) 2.424(1) 2.424(1) 1.118(5) 1.122(5)

Mo(1)−N(4)−N(5) Mo(2)−N(7)−N(6) Mo(2)−N(11)−N(12) P(3)−Mo(2)−P(4) P(1)−Mo(1)−N(6) P(2)−Mo(1)−N(1) P(1)−Mo(1)−N(4) P(2)−Mo(1)−N(4) N(1)−Mo(1)−N(4) N(4)−Mo(1)−N(6) P(3)−Mo(2)−N(11) P(4)−Mo(2)−N(8) P(3)−Mo(2)−N(9) P(4)−Mo(2)−N(9) N(7)−Mo(2)−N(11) N(9)−Mo(2)−N(11)

175.6(3) 173.4(3) 177.9(3) 157.26(4) 100.72(9) 78.94(9) 89.52(8) 92.06(8) 84.6(1) 91.5(1) 101.36(9) 78.80(8) 89.10(9) 92.24(9) 88.4(1) 88.7(1)

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dinitrogen complexes did not work as catalysts. These results indicate that the modification of the PNP-type pincer ligand dramatically affects the catalytic activity. We believe that the result described in this paper provides useful information to design a more effective catalyst toward the catalytic formation of ammonia under mild reaction conditions. Further work is currently in progress to develop a more effective reaction system including the elucidation of the reaction pathway.9

Table 5. Molybdenum-Catalyzed Reduction of Molecular Dinitrogen into Ammonia under Ambient Conditionsa

run

catalyst

Cp2Co (equiv)

[LutH]OTf (equiv)

NH3 (equiv)b

H2 (equiv)b

1 2 3c 4 5 6 7 8

5a 5a 5a 5b 5c 5d 1 1

72 216 216 72 72 72 72 216

96 288 288 96 96 96 96 288

14 16 19 2 3 1 12 23

8 47 48 22 21 22 10 46

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EXPERIMENTAL SECTION

General Methods. 1H NMR (270 MHz) and 31P{1H} NMR (109 MHz) spectra were recorded on a JEOL Excalibur 270 spectrometer in suitable solvent, and spectra were referenced to residual solvent (1H) or external standard (31P{1H}: 85% H3PO4). IR spectra were recorded on a JASCO FT/IR 4100 Fourier transform infrared spectrometer. Absorption spectra were recorded on a Shimadzu MultiSpec-1500. Elemental analyses were performed at 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. Dinitrogen-bridged dimolybdenum complex 1,3 2-(diphenylphosphinomethyl)-6-methylpyridine (2b),10 Ad2PCl,11 and [MoCl3(thf)3]12 were prepared according to the literature methods. Preparation of 2-(Di-tert-butylphosphino)methyl-6-methylpyridine (2a). To a solution of 2,6-lutidine (1.38 g, 12.8 mmol) in Et2O (30 mL) at 0 °C was added nBuLi (1.65 M in hexane, 7.8 mL, 12.9 mmol). The reaction mixture was stirred at 0 °C for 1 h and then cooled to −78 °C. tBu2PCl (2.4 mL, 12.6 mmol) was added and stirred at −78 °C for 1 h, and the reaction mixture was warmed to room temperature and stirred at room temperature for 1 h. The reaction was quenched with degassed water and extracted with Et2O. The extracts were dried over anhydrous MgSO4, and the solvent was removed under reduced pressure. Recrystallization from hexane at −40 °C afforded 2a as a white solid, which was collected by filtration and dried in vacuo (1.72 g, 6.8 mmol, 54% yield). This sample was used for subsequent reaction without further purification. 1H NMR (C6D6): δ 7.26 (d, J = 7.6 Hz, Ar-H, 1H), 7.09 (t, J = 7.6 Hz, Ar-H, 1H), 6.57 (d, J = 7.6 Hz, Ar-H, 1H), 3.09 (d, J = 3.0 Hz, CH2PtBu2, 2H), 2.42 (s, ArCH3, 3H), 1.11 (d, J = 10.5 Hz, CH2PtBu2, 18H). 31P{1H} NMR (C6D6): δ 34.6 (s, PtBu2). Preparation of 2-(Di-1-adamantylphosphino)methyl-6-(ditert-butylphosphino)methylpyridine (3a). To a solution of 2a (357 mg, 1.42 mmol) in Et2O (30 mL) at 0 °C was added nBuLi (1.62 M in hexane, 2.7 mL, 4.37 mmol). The reaction mixture was heated to reflux temperature for 15 h and then cooled to −78 °C. Ad2PCl (457.1 mg, 1.36 mmol) was added, and the reaction mixture was warmed to room temperature and then stirred for 22 h. The reaction was quenched with degassed water and extracted with Et2O. The extracts were dried over anhydrous MgSO4, and the solvent was removed under reduced pressure. Recrystallization from Et2O at −40 °C afforded 3a as a white solid, which was collected by filtration and dried in vacuo (542 mg, 0.98 mmol, 72% yield). 1H NMR (C6D6): δ 7.40 (d, J = 6.5 Hz, Ar-H, 1H), 7.23 (t, J = 6.5 Hz, Ar-H, 2H), 3.12 (br s, CH2P, 4H), 1.97−1.58 (m, CH2PAd2, 30H), 1.15 (d, J = 10.5 Hz, CH2PtBu2, 18H). 31P{1H} NMR (C6D6): δ 35.0 (s, PtBu2), 30.4 (s, PAd2). Anal. Calcd for C35H55NP2: C, 76.19; H, 10.05; N, 2.54. Found: C, 76.16; H, 9.69; N, 2.62. Preparation of 2-(Di-tert-butylphosphino)methyl-6(diphenylphosphino)methylpyridine (3b). To a solution of 2b (580 mg, 1.99 mmol) in Et2O (20 mL) at 0 °C was added nBuLi (1.59 M in hexane, 3.8 mL, 6.04 mmol). The reaction mixture was heated to reflux temperature for 15 h and then cooled to −78 °C. tBu2PCl (380 μL, 2.00 mmol) was added, and the reaction mixture was warmed to room temperature and then stirred for 6 h. The reaction was quenched with degassed water and extracted with Et2O. The extracts were dried over anhydrous MgSO4, and the solvent was removed under reduced pressure. Recrystallization from pentane at −40 °C afforded 3b as a

a

To a suspension of catalyst (5 or 1; 0.010 mmol) and 2,6-lutidinium trifluoromethanesulfonate (equiv to catalyst; [LutH]OTf; Lut = 2,6lutidine; OTf = OSO2CF3) as a proton source in toluene was added a solution of cobaltocene (equiv to catalyst; CoCp2; Cp = η5-C5H5) as a reductant in toluene via a syringe pump at room temperature over a period of 1 h, followed by stirring at room temperature for another 19 h under an atmospheric pressure of dinitrogen. bMolar equiv based on the catalyst. cAddition of a solution of cobaltocene over a period of 30 min.

dimolybdenum complexes did not work as catalysts toward the catalytic formation of ammonia under the same reaction conditions. We consider that complex 5b has only less activated dinitrogen ligands to exhibit a catalytic activity, based on the IR absorbance of 5b in comparison with those of 1 and 5a. Goldman and co-workers previously reported that iridium complexes bearing various alkyl-substituted PCP-type pincer ligands [Ir(CO)(RPCP)] (RPCP = κ3-C6H3-2,6-(CH2PR2)2; R = tBu, iPr, and Ad) show almost the same electronic properties by the measurement of the CO frequency.7 These results indicate that the electron density of the molybdenum centers in 5c and 5d is comparable to that of 5a. Although complexes 5c and 5d have a similar electronic effect to 5a, both complexes 5c and 5d have different molecular structures (vide supra). At present, we consider that the coordination environment on the molybdenum atom may affect the thermodynamic stability of catalytically active intermediates in the protonation and reduction process, leading to the formation of ammonia. Although we have not yet obtained detailed information on the reaction pathway, dinitrogen-bridged dimolybdenum complexes bearing PNP-type pincer ligands (5b, 5c, and 5d), where less sterically demanding substituents such as Ph, iPr, and Cy groups8 are present at the phosphorus atoms, did not work as effective catalysts toward the catalytic formation of ammonia from molecular dinitrogen. These results indicate that the steric hindrance of PtBu2 and PAd2 groups in the PNPpincer ligand is necessary to stabilize some reactive intermediates during the catalytic reaction. In summary, we have synthesized a series of unsymmetric PNP-type pincer ligands and the corresponding dinitrogenbridged dimolybdenum complexes bearing these unsymmetric pincer ligands. A molybdenum−dinitrogen complex bearing 2(di-1-adamantylphosphino)methyl-6-(di-tert-butylphosphino)methylpyridine has been found to work as an effective catalyst toward the formation of ammonia from molecular dinitrogen under ambient conditions. In contrast, other molybdenum− 8441

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Organometallics

Article

white solid, which was collected by filtration and dried in vacuo (771 mg, 1.77 mmol, 89% yield). 1H NMR (C6D6): δ 7.50−7.46 (m, CH2PPh2, 4H), 7.08−6.95 (m, CH2PPh2 and Ar-H, 8H), 6.64 (d, J = 7.3 Hz, Ar-H, 1H), 3.60 (s, CH2PPh2, 2H), 3.02 (d, J = 2.7 Hz, CH2PtBu2, 2H), 1.10 (d, J = 10.5 Hz, CH2PtBu2, 18H). 31P{1H} NMR (C6D6): δ 35.0 (s, PtBu2), −11.5 (s, PPh2). Anal. Calcd for C27H35NP2: C, 74.46; H, 8.10; N, 3.22. Found: C, 74.11; H, 7.98; N, 3.06. Preparation of 2-(Di-tert-butylphosphino)methyl-6(diisopropylphosphino)methylpyridine (3c). To a solution of 2a (369 mg, 1.47 mmol) in Et2O (15 mL) at 0 °C was added nBuLi (1.65 M in hexane, 2.7 mL, 4.46 mmol). The reaction mixture was heated to reflux temperature for 15 h and then cooled to −78 °C. i Pr2PCl (235 μL, 1.48 mmol) was added, and the reaction mixture was warmed to room temperature and then stirred for 6 h. The reaction was quenched with degassed water and extracted with Et2O. The extracts were dried over anhydrous MgSO4, and the solvent was removed under reduced pressure to afford 3c as a yellow oil (452 mg, 1.23 mmol, 84% yield). This sample was used for subsequent reaction without further purification. 1H NMR (C6D6): δ 7.22−7.00 (m, Ar-H, 3H), 3.07 (d, J = 3.0 Hz, CH2P, 2H), 2.98 (d, J = 1.6 Hz, CH2P, 2H), 1.77−1.66 (m, CHMe2, 2H), 1.12 (d, J = 10.8 Hz, CH2PtBu2, 18H), 1.08−1.00 (m, CHMe2, 12H). 31P{1H} NMR (C6D6): δ 34.9 (s, PtBu2), 10.8 (s, PiPr2). Preparation of 2-(Di-tert-butylphosphino)methyl-6(dicyclohexylphosphino)methylpyridine (3d). To a solution of 2a (526 mg, 2.09 mmol) in Et2O (35 mL) at 0 °C was added nBuLi (1.65 M in hexane, 3.8 mL, 6.27 mmol). The reaction mixture was heated to reflux temperature for 15 h and then cooled to −78 °C. Cy2PCl (460 μL, 2.08 mmol) was added, and the reaction mixture was warmed to room temperature and stirred at room temperature for 22 h. The reaction was quenched with degassed water and extracted with Et2O. The extracts were dried over anhydrous MgSO4, and the solvent was removed under reduced pressure. Recrystallization from hexane at −40 °C afforded 3d as a white solid, which was collected by filtration and dried in vacuo (545 mg, 1.22 mmol, 59% yield). 1H NMR (C6D6): δ 7.25−7.05 (m, Ar-H, 3H), 3.09 (d, J = 2.7 Hz, CH2P, 2H), 3.04 (d, J = 1.6 Hz, CH2P, 2H), 1.85−1.20 (m, CH2PCy2, 22H), 1.13 (d, J = 10.5 Hz, CH2PtBu2, 18H). 31P{1H} NMR (C6D6): δ 34.4 (s, PtBu2), 2.7 (s, PCy2). Anal. Calcd for C27H47NP2: C, 72.45; H, 10.58; N, 3.13. Found: C, 72.19; H, 10.30; N, 3.14. Preparation of [MoCl3(3a)] (4a). A mixture of 3a (298 mg, 0.539 mmol) and [MoCl3(thf)3] (207 mg, 0.495 mmol) in THF (20 mL) was stirred at 50 °C for 20 h. The resultant red-orange solution was concentrated under reduced pressure, and the residue was recrystallized from CH2Cl2 (3 mL)−hexane (18 mL) to give orange-brown needles of 4a, which were collected by filtration and dried in vacuo (363 mg, 0.482 mmol, 97% yield). Anal. Calcd for C35H55Cl3MoNP2: C, 55.75; H, 7.35; N, 1.86. Found: C, 55.75; H, 7.34; N, 1.71. Preparation of [MoCl3(3b)] (4b). A mixture of 3b (276 mg, 0.633 mmol) and [MoCl3(thf)3] (249 mg, 0.595 mmol) in THF (11 mL) was stirred at 50 °C for 20 h. The resultant yellow-orange suspension was concentrated under reduced pressure, and the residue was recrystallized from CH2Cl2 (6 mL)−Et2O (17 mL) to give orange needles of 4b, which were collected by filtration and dried in vacuo (334 mg, 0.523 mmol, 88% yield). Anal. Calcd for C27H35Cl3MoNP2: C, 50.84; H, 5.53; N, 2.20. Found: C, 50.32; H, 5.59; N, 2.04. Preparation of [MoCl3(3c)] (4c). A mixture of 3c (198 mg, 0.539 mmol) and [MoCl3(thf)3] (203 mg, 0.485 mmol) in THF (10 mL) was stirred at 50 °C for 20 h. The resultant orange-brown solution was concentrated under reduced pressure and washed with Et2O (2 mL × 2), and the residue was recrystallized from CH2Cl2 (3 mL)−hexane (18 mL) to give an orange-brown solid of 4c, which was collected by filtration and dried in vacuo (210 mg, 0.369 mmol, 76% yield). Anal. Calcd for C21H39Cl3MoNP2: C, 44.26; H, 6.90; N, 2.46. Found: C, 44.02; H, 7.11; N, 2.30. Preparation of [MoCl3(3d)] (4d). A mixture of 3d (407 mg, 0.909 mmol) and [MoCl3(thf)3] (397 mg, 0.947 mmol) in THF (30 mL) was stirred at 50 °C for 20 h. The resultant dark orange-brown solution was concentrated under reduced pressure, and the residue was

recrystallized from CH2Cl2 (3 mL)−hexane (20 mL) to give orangebrown needles of 4d, which were collected by filtration and dried in vacuo (545 mg, 0.839 mmol, 92% yield). Anal. Calcd for C27H47Cl3MoNP2: C, 49.90; H, 7.29; N, 2.16. Found: C, 49.55; H, 7.39; N, 2.00. Preparation of [Mo(N2)2(3a)]2(μ-N2) (5a). To Na−Hg (0.5 wt %, 10.8 g, 2.36 mmol) were added THF (27 mL) and 4a (300 mg. 0.398 mmol), and then the mixture was stirred at room temperature for 12 h under N2. The resultant solution was decanted, and then the supernatant solution was concentrated under reduced pressure. To the residue was added benzene (10 mL), the solution was filtered through Celite, and the filter cake was washed with benzene (3 mL × 6). Slow addition of hexane (15 mL) to the concentrated filtrate (ca. 5 mL) gave 5a as a dark green solid, which was collected by filtration and dried in vacuo (140 mg, 0.097 mmol, 49% yield). Unfortunately, NMR spectra of 5a have not been obtained because 5a is not soluble enough in THF-d8 and C6D6 for measurement of 1H and 31P{1H} NMR. ESITOF-MS of 5a shows the existence of dinitrogen-bridged dimolybdenum species, where three or four dinitrogen ligands in 5a are lost. ESI-TOF-MS (THF): 1354 (M − 3N2), 1326 (M − 4N2). IR (KBr, cm−1): 1939 (s, νNN). Anal. Calcd for C70H110Mo2N12P4: C, 58.57; H, 7.72; N, 11.71. Found: C, 59.02; H, 8.04; N, 7.51. Preparation of [Mo(N2)2(3b)]2(μ-N2) (5b). To Na−Hg (0.5 wt %, 10.3 g, 2.26 mmol) were added THF (25 mL) and 4b (239 mg. 0.375 mmol), and then the mixture was stirred at room temperature for 12 h under N2. The resultant dark green suspension was decanted, and then the supernatant solution was concentrated under reduced pressure. To the residue was added benzene (7 mL), the solution was filtered through Celite, and the filter cake was washed with benzene (3 mL × 5). Slow addition of hexane (18 mL) to the concentrated filtrate (ca. 5 mL) gave dark brown crystals of 5b·3C6H6, which was collected by filtration and dried in vacuo to afford 5b as a dark brown solid (122 mg, 0.102 mmol, 54% yield). Unfortunately, NMR spectra of 5b have not been obtained because 5b is not soluble enough in THF-d8 and C6D6 for measurement of 1H and 31P{1H} NMR. ESI-TOF-MS of 5b shows the existence of dinitrogen-bridged dimolybdenum species, where four dinitrogen ligands in 5b are lost. ESI-TOF-MS (THF): 1094 (M − 4N2). IR (KBr, cm−1): 1957 (s, νNN). Anal. Calcd for C54H70Mo2N12P4: C, 53.91; H, 5.86; N, 13.97. Found: C, 54.37; H, 6.04; N, 8.70. Preparation of [Mo(N2)2(3c)]2(μ-N2) (5c). To Na−Hg (0.50 wt %, 8.23 g, 1.79 mmol) were added THF (20 mL) and 4c (170 mg. 0.298 mmol), and then the mixture was stirred at room temperature for 24 h under N2. The resultant dark green suspension was decanted, and then the supernatant solution was concentrated in vacuo. To the residue was added benzene (8 mL), the solution was filtered through Celite, and the filter cake was washed with benzene (3 mL × 6). Pentane (18 mL) was added to the concentrated filtrate (ca. 2 mL) and the solution was kept at −40 °C to give 5c as a dark green solid, which was collected by filtration and dried in vacuo (68 mg, 0.064 mmol, 43% yield). 1H NMR (C6D6): δ 6.67−6.48 (m, Ar-H, 6H), 3.37−2.76 (m, CH2P, 8H), 2.46−1.97 (m, CHMe2, 4H), 1.79−0.79 (m, CHMe2, 24H) 1.57 (d, J = 11.6 Hz, CH2PtBu2, 9H), 1.53 (d, J = 11.6 Hz, CH2PtBu2, 9H), 1.30 (d, J = 11.6 Hz, CH2PtBu2, 9H), 1.07 (d, J = 11.6 Hz, CH2PtBu2, 9H). 31P{1H} NMR (C6D6): δ 89.5 (d, J = 143 Hz, PtBu2), 88.4 (d, J = 130 Hz, PtBu2), 75.7 (d, J = 130 Hz, PiPr2), 75.5 (d, J = 143 Hz, PiPr2). IR (KBr, cm−1): 1968 (s, νNN), 1949 (s, νNN), 1897 (s, νNN), 1875 (s, νNN). Anal. Calcd for C42H78Mo2N12P4: C, 47.28; H, 7.37; N, 15.75. Found: C, 47.16; H, 7.48; N, 9.08. Preparation of [Mo(N2)2(3d)]2(μ-N2) (5d). To Na−Hg (0.50 wt %, 10.9 g, 2.38 mmol) were added THF (26 mL) and 4d (258 mg. 0.397 mmol), and then the mixture was stirred at room temperature for 24 h under N2. The resultant dark green suspension was decanted, and then the supernatant solution was concentrated under reduced pressure. To the residue was added benzene (3 mL), the solution was filtered through Celite, and the filter cake was washed with benzene (2 mL × 8). Pentane (15 mL) was added to the concentrated filtrate (ca. 3 mL) and the solution was kept at −40 °C to give 5d as dark green crystals, which were collected by filtration and dried in vacuo to afford 5d as a dark green solid (149 mg, 0.121 mmol, 61% yield). 1H NMR 8442

dx.doi.org/10.1021/om301046t | Organometallics 2012, 31, 8437−8443

Organometallics

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(C6D6): δ 6.69−6.52 (m, Ar-H, 6H), 3.66−2.71 (m, CH2P, 8H), 2.22−1.20 (m, CH2PCy2, 44H), 1.68 (d, J = 12.7 Hz, CH2PtBu2, 9H), 1.59 (d, J = 11.9 Hz, CH2PtBu2, 9H), 1.27 (d, J = 11.1 Hz, CH2PtBu2, 9H), 1.00 (d, J = 11.3 Hz, CH2PtBu2, 9H). 31P{1H} NMR (C6D6): δ 89.6 (d, J = 143 Hz, PtBu2), 87.5 (d, J = 130 Hz, PtBu2), 67.7 (d, J = 130 Hz, PCy2), 66.6 (d, J = 143 Hz, PCy2). IR (KBr, cm−1): 1966 (s, νNN), 1950 (s, νNN), 1896 (s, νNN), 1877 (s, νNN). Anal. Calcd for C54H94Mo2N12P4: C, 52.85; H, 7.72; N, 13.70. Found: C, 52.44; H, 7.71; N, 5.04. Catalytic Reduction of Dinitrogen to Ammonia. A typical experimental procedure3 for the catalytic reduction of dinitrogen into ammonia using the dinitrogen complex 5a is described below. In a 50 mL Schlenk flask were placed 5a (14.4 mg, 0.010 mmol) and 2,6lutidinium trifluoromethanesulfonate, [LutH]OTf (741 mg, 2.88 mmol). Toluene (1 mL) was added under N2 (1 atm), and then a solution of CoCp2 (408 mg, 2.16 mmol) in toluene (4 mL) was slowly added to the stirred suspension in the Schlenk flask with a syringe pump at a rate of 4 mL per hour. After the addition of CoCp2, the mixture was further stirred at room temperature for 19 h. The amount of dihydrogen of the catalytic reaction was determined by gas chromatography (GC). The reaction mixture was evaporated under reduced pressure, and the distillate was trapped in dilute H2SO4 solution (0.5 M, 10 mL). Potassium hydroxide aqueous solution (30 wt %; 5 mL) was added to the residue, and the mixture was distilled into another dilute H2SO4 solution (0.5 M, 10 mL). NH3 present in each of the H2SO4 solutions was determined by the indophenol method.13 The amount of ammonia was 0.017 mmol of NH3 collected before base distillation of the reaction mixture and 0.143 mmol of NH3 collected after base distillation to fully liberate NH3, respectively. The total amount of ammonia was 0.160 mmol (16.0 molar equiv based on 5a). X-ray Crystallography. Crystallographic data of 4b, 4d, 5b·3C6H6, and 5c are summarized in Table S1. Selected bond lengths and angles are summarized in Tables 1−4, and their ORTEP drawings are shown in Figures 1−4. Diffraction data for 4b were collected for the 2θ range of 6° to 55° at −180 °C on a Rigaku Saturn 70 CCD diffractometer with graphite-monochromated Mo Kα (λ = 0.71075 Å) radiation with VariMax optics. Diffraction data for 4d, 5b·3C6H6, and 5c were collected for the 2θ range of 5° to 55° at −90 °C (for 4d and 5b·3C6H6) or −75 °C (for 5c) on a Rigaku RAXIS RAPID imaging plate area detector with graphite-monochromated Mo Kα radiation (λ = 0.71075 Å), with VariMax optics. 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.14 The positions of the non-hydrogen atoms were determined by direct methods (SIR 9715 for 4b, SIR 200216 for 4d, SHELX-9717 for 5b·3C6H6 and 5c) and subsequent Fourier syntheses (DIRDIF-9918) and were refined on Fo2 using all unique reflections by full-matrix least-squares with anisotropic thermal parameters except for a few solvated molecules (C28−C39 for 5b·3C6H6), which were refined isotropically. All the other hydrogen atoms were placed at the calculated positions with fixed isotropic parameters.

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ACKNOWLEDGMENTS This work was supported by Funding Program for Next Generation World-Leading Researchers (GR025) and a Grantin-Aid for Scientific Research on Innovative Areas “Advanced Molecular Transformations by Organocatalysts” from MEXT, Japan. 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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REFERENCES

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S Supporting Information *

Crystallographic data for 4b, 4d, 5b·3C6H6, and 5c are available in CIF format, including a table of crystallographic data. 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. 8443

dx.doi.org/10.1021/om301046t | Organometallics 2012, 31, 8437−8443