Synthesis of Group IV (Zr, Hf)−Group VIII (Fe, Ru) Heterobimetallic

ARTICLE pubs.acs.org/Organometallics

Synthesis of Group IV (Zr, Hf) Group VIII (Fe, Ru) Heterobimetallic Complexes Bearing Metallocenyl Diphosphine Moieties and Their Application to Catalytic Dehydrogenation of Amine Boranes Takamasa Miyazaki, Yoshiaki Tanabe, Masahiro Yuki, Yoshihiro Miyake, and Yoshiaki Nishibayashi* Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan

bS Supporting Information ABSTRACT: A series of heterobimetallic complexes, including [M1Me(μη5:η1-C5H4PEt2)2M2Cp*] (5a, M1 = Zr, M2 = Ru; 5b, M1 = Hf, M2 = Ru; 5c, M1 = Zr, M2 = Fe) have been prepared and characterized by X-ray analyses. Dehydrogenation of Me2NH 3 BH3 takes place in the presence of a catalytic amount of these heterobimetallic complexes. Studies on the reaction pathway of the catalytic dehydrogenation of Me2NH 3 BH3 lead us to propose a catalytic cycle where two metal centers cooperatively participate in the catalysis. Here, heterobimetallic hydride species serve as key reaction intermediates.

’ INTRODUCTION Multimetallic complexes are expected to supply different reactivities in comparison with mononuclear complexes, because electronic interactions between metals and cooperative activation of the substrates can be anticipated.1 The preparation and stoichiometric reactivity of multimetallic complexes have been widely investigated, but relatively little is known about their catalytic reactivity.1 Meanwhile, we have disclosed propargylic substitution reactions of propargylic alcohols with various nucleophiles catalyzed by thiolato-bridged dinuclear ruthenium complexes,2 where the electronic interaction between the two ruthenium atoms plays an important role in facilitating the catalysis.3 As part of our effort to develop intriguing reactions using multimetallic complexes, we recently reported the transformation of molecular dinitrogen into silylamine4 catalyzed by a molybdenum dinitrogen complex bearing two ferrocenyldiphosphines.5 In this reaction, the presence of the ferrocenyldiphosphine moiety was essential to achieve high catalytic activity. The remarkable catalytic activity induced by the complex prompted us to develop similar complexes bearing metallocenyldiphosphines.6 We have now focused on the use of the heterobimetallic complexes bearing group IV metallocenyldiphosphine1c moieties, which can provide heterobimetallic coordination sites for substrates by ligand exchange, and cooperative activation of substrates can be expected. In this paper, we describe the synthesis of a series of heterobimetallic complexes consisting of group IV metallocenes and pentamethylcyclopentadienyl iron or ruthenium moieties and their catalytic activity toward the dehydrogenation of amine boranes,7 where both of the metal centers of heterobimetallic complexes work cooperatively to promote the reaction. r 2011 American Chemical Society

’ RESULTS AND DISCUSSION Preparation and Characterization of Zr Ru, Hf Ru, and Zr Fe Heterobimetallic Complexes. For constructing a new

heterobimetallic system, we focused upon the use of group IV metallocenyl diphosphines [(η5-C5H4PR2)2MCl2] (M = Ti, Zr, Hf) as appropriate building blocks1c previously reported to incorporate group VI (Cr, Mo, W),8 group VII (Re),9 group VIII (Ru),10 group IX (Rh, Ir),11 group X (Pd, Pt),12and group XI (Cu, Au) metals.13 Indeed, treatment of [ZrCl4(thf)2] with 2 equiv of Na[C5H4PEt2] (1) in THF at room temperature for 12 h afforded the desired zirconocenyl diphosphine [(η5C5H4PEt2)2ZrCl2] (2a) in 71% isolated yield as a new compound (Scheme 1). Formation of 2a was confirmed by its 1H and 31 1 P{ H} NMR spectra. We next investigated the preparation of heterobimetallic complexes that include a ruthenium atom ligated by the zirconocenyl diphosphine 2a. Accordingly, treatment of 2a with 0.25 equiv of the tetraruthenium complex [{Cp*Ru(μ3-Cl)}4] (Cp* = η5-C5Me5) in toluene at room temperature for 12 h afforded the Zr Ru heterobimetallic trichloride complex [ZrCl2(μ-η5:η1C5H4PEt2)2RuClCp*] (3a) in 93% isolated yield (Scheme 1). The 1H NMR spectrum of 3a exhibits four C5H4 (δ 7.83, 6.82, 6.62, 5.75, 2H each) and a set of two ethyl resonances (δ 2.37 (2H), 1.89 (4H), 1.70 (2H) for CH2; 1.10 (6H), 0.61 (6H) for CH3), while the 31P{1H} NMR spectrum shows only one singlet resonance at δ 34.6. These spectroscopic features suggest that 3a has a Cs symmetry where two cyclopentadienylphosphines Received: February 9, 2011 Published: March 30, 2011 2394

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Organometallics Scheme 1

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Scheme 2

Scheme 3

Figure 1. ORTEP drawing of 3a. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Ru(1) 3 3 3 Zr(1), 5.0017(4); Ru(1) Cl(3), 2.4741(8); Ru(1) P(1), 2.3152(11); Ru(1) P(2), 2.3091(10); Zr(1) Cl(1), 2.4440(10); Zr(1) Cl(2), 2.4472(9); Cl(3) Ru(1) P(1), 87.36(3); Cl(3) Ru(1) P(2), 92.44(3); P(1) Ru(1) P(2), 97.09(3); Cl(1) Zr(1) Cl(2), 96.46(3).

bridging the ZrCl2 and Cp*RuCl moieties are located tilted toward each other. Details of the molecular structure of 3a were unambiguously determined by single-crystal X-ray crystallographic analysis. As depicted in Figure 1, the zirconium and ruthenium atoms are bridged by a set of cyclopentadienylphosphines with an interatomic distance of 5.0017(4) Å between the two metals, which is even greater than the values of the Zr 3 3 3 Mo (4.8524(3) Å) and Zr 3 3 3 Pd (4.723(1) Å) separations found in the Zr Mo and Zr Pd heterobimetallic complexes [ZrCl2(μ-η5:η1-C5H4PPh2)2Mo(CO)4]8a and [ZrCl2(μ-η5:η1C5H4PPh2)2PdCl2],12a ruling out any direct bonding interaction between the two metals. The configuration around the ruthenium atom adopts a three-legged piano-stool coordination geometry, typical of coordinatively saturated cyclopentadienyl Ru(II) bis(phosphine) complexes.14 The X-ray crystallographic study also revealed that the two cyclopentadienyl rings attached to the Zr(IV) dichloride moiety are located almost eclipsed with each other in the solid state. This geometrical feature is in contrast to the aforementioned Zr Mo heterobimetallic complex8a or well-known Cp2ZrCl2 (Cp = η5-C5H5),15 which take staggered arrangements for the two cyclopentadienyl ligands. In contrast to the conversion of 2a, the analogous zirconocenyl diphosphines [(η5-C5H4PR2)2ZrCl2] (R = iPr, tBu, Ph, Cy),11c

having more bulky substituents on the phosphorus atoms than 2a, did not react with [{Cp*Ru(μ3-Cl)}4] at all, and the corresponding Zr Ru heterobimetallic complexes were not obtained. These results suggest that the bulkiness of the substituents on the phosphorus atoms directly affects the complexation of the zirconocenyl diphosphines to afford heterobimetallic complexes. Next, we surveyed the reactivity of 3a. When 3a was treated with 2 equiv of LiBHEt3 at room temperature for 4 h, the Zr Ru heterobimetallic monochloride complex [ZrCl(μ-η5:η1C5H4PEt2)2RuCp*] (4a) was obtained as the sole product in 73% isolated yield (Scheme 2). In this reaction, evolution of H2 gas (74% yield) was confirmed by gas chromatography, suggesting that the reaction should proceed via the successive ligand exchanges of two chlorides with two hydrides to afford the Zr Ru heterobimetallic monochloride bis(hydride) species [ZrCl(H)(μη5:η1-C5H4PEt2)2RuHCp*] (A), followed by the reductive elimination of dihydrogen from A to give 4a (Scheme 3). The 1H NMR spectrum of 4a exhibits four C5H4 (δ 6.89, 6.78, 5.04, 3.25, 2H each) and a set of two ethyl resonances (δ 3.11, 2.41, 1.76, 0.07, 2H each; 1.45, 0.68, 6H each), while the 31P{1H} NMR spectrum shows only one singlet resonance at δ 34.4. 2395

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Organometallics

Figure 2. ORTEP drawing of 4a. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Ru(1) Zr(1), 2.8620(10); Ru(1) P(1), 2.260(2); Ru(1) P(2), 2.245(2); Zr(1) Cl(1), 2.466(2); Zr(1) Ru(1) P(1), 76.84(6); Zr(1) Ru(1) P(2), 77.99(6); P(1) Ru(1) P(2), 100.62(7); Ru(1) Zr(1) Cl(1), 106.60(5).

These spectroscopic features are comparable to those of 3a and suggest that 4a also has a Cs symmetry. The detailed molecular structure of 4a was also confirmed by X-ray crystallography. As shown in Figure 2, the short interatomic distance between zirconium and ruthenium centers is 2.8620(10) Å, which is comparable to the calculated sum of the covalent radii of zirconium and ruthenium atoms (2.79 3.21 Å)16 or the reported values for the zirconium ruthenium metal metal bond lengths (2.74 3.08 Å),17 suggesting that there is a metal metal single bond between them. The existence of such a metal-to-metal interaction is in good accordance with the diamagnetic nature of 4a, with an electron count of 32 within the RuZr core, which is also the case for the Zr Rh heterobimetallic complex [ZrMe(μ-η5:η1C5H4PPh2)2Rh(CO)(L)] (L = CO, PPh3).11c The X-ray crystallographic data also revealed that the ruthenium center still adopts a three-legged piano-stool coordination geometry,14 while the two cyclopentadienyl rings attached to the zirconium center are geometrically staggered with respect to each other in the solid state.8a,15 The remaining chloride ligand bound to the zirconium atom in 4a can be easily substituted by nucleophiles such as MeLi, NaBH4, and LiNMe2. When the reaction of 4a and 1 equiv of MeLi was carried out in toluene at room temperature for 18 h, the corresponding Zr Ru heterobimetallic methyl complex [ZrMe(μ-η5:η1-C5H4PEt2)2RuCp*] (5a) was isolated in 83% yield (Scheme 2). Moreover, treatment of 4a with an excess amount of NaBH4 and LiNMe2 resulted in the formation of [Zr (η2-BH4)(μ-η5:η1-C5H4PEt2)2RuCp*] (6a) and [Zr(NMe2) (μ-η5:η1-C5H4PEt2)2RuCp*] (7a) in 52% and 56% yields, respectively (Scheme 2). As expected, 5a presented 1H and 31P{1H} NMR spectra similar to those of 4a except for a 1H NMR singlet resonance at δ 1.25, which is assigned to the methyl group ligated to the zirconium atom, demonstrating that the molecular geometry of 5a is essentially the same as that of 4a. In fact, the X-ray crystallographic study of 5a revealed that the interatomic distance between the zirconium and ruthenium centers is 2.8606(6) Å, suggesting the interatomic bond

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Figure 3. ORTEP drawing of 5a. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Ru(1) Zr(1), 2.8606(6); Ru(1) P(1), 2.2584(15); Ru(1) P(2), 2.2562(13); Zr(1) C(29), 2.307(5); Zr(1) Ru(1) P(1), 77.09(3); Zr(1) Ru(1) P(2), 75.78(3); P(1) Ru(1) P(2), 101.43(5); Ru(1) Zr(1) C(29), 108.71(13).

is retained in this reaction.16,17 Other metric parameters of 5a are also similar to those of 4a (Figure 3). 6a and 7a also have spectroscopic and structural features similar to those of 5a.18 We also investigated the preparation of Hf Ru heterobimetallic complexes analogous to 3a 5a. When [HfCl4(thf)2] was treated with 2 equiv of 1 in toluene including a trace amount of THF at room temperature for 3 h, formation of the corresponding hafnocenyl diphosphine [(η5-C5H4PEt2)2HfCl2] (2b) in situ was confirmed by crude NMR analyses, though 2b was not obtained in pure form. However, further addition of 0.25 equiv of [{Cp*Ru(μ3-Cl)}4] to the solution of 2b afforded the expected Hf Ru heterobimetallic trichloride complex [HfCl2(μ-η5:η1-C5H4PEt2)2RuClCp*] (3b) in 62% isolated yield based on [HfCl4(thf)2] after 6 h of stirring at room temperature (Scheme 1). Analogous to the preparation of 4a from 3a, treatment of 3b with 2 equiv of LiBHEt3 gave rise to the formation of the metal metal-bonded Hf Ru heterobimetallic monochloride complex [HfCl(μ-η5:η1-C5H4PEt2)2RuCp*] (4b) as a major product together with H2 gas evolution (45% GC yield per 3b), but 1H and 31P{1H} NMR observation of the crude mixture also revealed that several minor products were additionally formed and isolation of these complexes was not successful. One of the minor products can be identified as the Hf Ru heterobimetallic trihydride complex [HfH2(μ-η5:η1-C5H4PEt2)2RuHCp*] (B), which exhibits one broad signal at δ 2.49 (2H) due to the ZrH2 moiety and a triplet signal at δ 14.2 (2JPH = 33.8 Hz, 1H) due to the RuH moiety, as shown in Scheme 3. On the other hand, when 3b was reacted with 3 equiv of KC8 in toluene at room temperature for 24 h, the reaction proceeded cleanly to give 4b as the sole product, which was isolated in 91% yield. The corresponding Hf Ru heterobimetallic methyl complex [HfMe(μ-η5:η1-C5H4PEt2)2RuCp*] (5b) was also obtained by reacting 4b with 2 equiv of MeLi in 26% isolated yield (Scheme 2). 2396

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Organometallics Chart 1

Hf Ru heterobimetallic complexes 3b 5b were also characterized by 1H and 31P{1H} NMR spectroscopy as well as by X-ray crystallographic analyses (Figures S1 S3, Supporting Information). X-ray crystallography revealed that the metric parameters among the Hf Ru cores of 3b 5b are almost the same as those of the Zr Ru cores of 3a 5a, except for the shortening of the corresponding metal metal bond distance (4b, 2.8396(2) Å; 5b, 2.8441(2) Å) in the Hf Ru heterobimetallic complexes as compared to those (4a, 2.8620(10) Å; 5a, 2.8606(6) Å) in the Zr Ru heterobimetallic complexes, due to the influence of the lanthanide contraction on the postlanthanides.16,19 Thus, we have succeeded in the preparation of a series of Zr Ru (3a 5a) and Hf Ru (3b 5b) heterobimetallic complexes, respectively. Unfortunately, however, the corresponding Ti Ru heterobimetallic complexes could not be prepared in pure forms by similar procedures. Next, we investigated the preparation of Zr Fe heterobimetallic complexes analogous to 3a 5a and 3b 5b. Treatment of [Cp*FeCl(TMEDA)] with 1 equiv of 2a resulted in the formation of [ZrCl2(μ-η5:η1-C5H4PEt2)2FeClCp*] (3c) in 30% yield (Scheme 1). Treatment of 3c with 2 equiv of LiBHEt3 afforded a complex mixture containing 4c with H2 gas evolution. However, analogously to the preparation of 4b from 3b, treatment of 3c with 3 equiv of KC8 resulted in the formation of [ZrCl(μ-η5:η1C5H4PEt2)2FeCp*] (4c) in 33% yield. Similarly, [ZrMe(μη5:η1-C5H4PEt2)2FeCp*] (5c) was obtained in 47% isolated yield by reacting 4c with 1.8 equiv of MeLi (Scheme 2). 1H NMR spectra of 3c and 4c are significantly broadened, and we could not get detailed structural information of these complexes. In contrast, the 1H NMR spectrum of 5c is similar to that of 5a, while the 31 1 P{ H} NMR spectrum shows only one singlet resonance at δ 34.6, suggesting that 5c also has Cs symmetry. All structures of 3c 5c were also characterized by X-ray crystallographic analyses (Figures S4 S6, Supporting Information). It was revealed that the metal metal bond distances of 4c 5c (4c, 2.8404(8) Å; 5c, 2.8385(10) Å) are slightly shorter than those of 4a 5a, corresponding to the values found for Zr Fe single bonds.16,17 Catalytic Dehydrogenation of Amine Boranes Using Zr Ru, Hf Ru, and Zr Fe Heterobimetallic Complexes. As shown in Scheme 2, the facile evolution of dihydrogen was observed in the formation of 4a, which prompted us to investigate the catalytic reactivity of the heterobimetallic Zr Ru complexes and their analogous Hf Ru and Zr Fe complexes

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Table 1. Catalytic Dehydrogenation of Me2NH 3 BH3 (i) by Heterobimetallic Complexes a

run

catalyst (amt (mol %))

time (h)

GC yield of H2 (%)

1 2

3a (10) 4a (10)

24 24

8 11

3

5a (10)

2

98b

4

5a (5)

3

95b

3

87b

c

5

5a (5)

6

5a (2)

6

93b

7

5b (10)

24

25

8

5c (5)

3

75

9 10

[Cp*RuH(depe)] (5) 6a (5)

3 3

18 2

11

7a (5)

3

97 b

12

8a (5)

24

94b

a

Reaction was carried out with Me2NH 3 BH3 (i; 0.20 mmol) in toluene (2 mL) at 50 °C. b Complete conversion of Me2NH 3 BH3 (i) was observed by 11B NMR. c Hg (0.1 mL) was added to the reaction mixture.

Figure 4. H2 gas evolved for the dehydrogenation of Me2NH 3 BH3 (i; 0.20 mmol) using 5a (10 mol % or 5 mol %) or 8a (5 mol %) as a catalyst.

toward dehydrogenation of polar substrates such as amine boranes because they are attractive candidates for hydrogen storage materials.20 We assumed that the formation of heterobimetallic dihydride and subsequent reductive elimination of dihydrogen should occur rapidly to form molecular dihydrogen when the N H bond and B H bond of amine boranes are activated by heterometallic moieties, as shown in Chart 1. At first, we investigated their catalytic activity for the dehydrogenation of dimethylamine borane, Me2NH 3 BH3 (i), which has been known to undergo dehydrogenation in the presence of transition-metal catalysts to give dihydrogen gas and cyclic (dimethylamino)borane dimer, (Me2NBH2)2 (ii), as final products.20 The conversion has been monitored by evaluating the amount of H2 gas evolved by GC, while the formation of 2397

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Organometallics borane derivatives was monitored by nondecoupled 11B NMR spectra. Typical results are shown in Table 1. The Zr Ru heterobimetallic trichloride and monochloride complexes 3a and 4a were shown to be almost inactive in catalytic quantities for the dehydrogenation of i (Table 1, runs 1 and 2). However, when using the Zr Ru heterobimetallic methyl complex 5a as a catalyst (10 mol %), dehydrogenation proceeded smoothly with the evolution of 98% GC yield of dihydrogen gas in 2 h, corresponding to a TON value of 9.8 (Table 1, run 3). As drawn in Figure 4, the time-interval sampling of the catalytic reaction mixture monitored by GC revealed that the evolution of dihydrogen gas reached 91% yield within 1 h, corresponding to a TOF value of 9.1 h 1. Lower catalyst loading (5 mol % of 5a) did not affect the catalytic activity, with a TON value of 19, but a longer reaction time (3 h) was required to complete the catalysis (Table 1, run 4). In this case, the evolution of dihydrogen gas reached 95% GC yield within 2 h, corresponding to a TOF value of 9.5 h 1. Addition of Hg did not inhibit catalysis, suggesting this catalytic process is mediated by a homogeneous system (Table 1, run 5).21 Further decreasing the catalyst loading to 2 mol % required a longer reaction time to complete the catalysis in comparison to the loadings of 10 and 5 mol % of 5a, but the maximum TON value up to 47 was achieved in 6 h (Table 1, run 6). Interestingly, the Hf Ru and Zr Fe heterobimetallic methyl complexes 5b and 5c were also shown to be catalytically active, although less reactive than 5a (Table 1, runs 7 and 8). These results are in sharp contrast to the previously reported catalytic dehydrogenation of i using mononuclear group IV metallocenes,22,23 where hafnocene and zirconocene derivatives such as [Cp2MCl2] (M = Zr, Hf) were completely inactive toward the dehydrogenation of i,7b and to some zirconocene derivatives such as [(η5-C5Me4H)2ZrH2], which are almost inactive toward the catalytic dehydrogenation of i (TOF = 0.02 0.34 h 1 in C6D6 at 65 °C).23 Moreover, the monomeric ruthenium(II) diphosphine complex [Cp*RuH(depe)] (depe = Et2PCH2CH2PEt2),24 which can be regarded as an alternative to 5a without zirconium moieties, was much less catalytically active than 5a toward the dehydrogenation of i (Table 1, run 9). Thus, the considerable catalytic activity found in the Zr Ru, Hf Ru, and Zr Fe heterobimetallic complexes 5a c as well as the almost identical geometrical features in the Zr Ru, Hf Ru, and Zr Fe cores of 5a c (Figure 3 and Figures S3 and S6 (Supporting Information)) strongly suggest that both of the metal centers participate in the catalysis, and the electronic nature of the metal centers in heterobimetallic complexes directly affects the catalytic activity. In fact, changing the group IV metal center from zirconium to hafnium resulted in a significant decrease of the catalytic activity, which is probably due to the stability of the oxidized Hf(IV) hydride species such as Hf Ru heterobimetallic trihydride complex B shown in Scheme 3, preventing the facile reductive elimination of H2. In contrast, treatment of i with a catalytic amount (5 mol %) of 6a in toluene at 50 °C for 3 h resulted in the evolution of a trace amount of H2 gas (2% GC yield), so that the reaction did not proceed catalytically at all (Table 1, run 10). This is not surprising, as some transition-metal borohydride complexes have been isolated, have shown no catalytic activity toward the dehydrogenation of amine boranes, and have been proposed to be deactivated states of the catalyst.25 Rather surprisingly, treatment of i with a catalytic amount of 7a (5 mol %) in toluene at 50 °C for 3 h resulted in complete conversion of i with 97% GC yield of H2 gas evolution and showed almost the same catalytic

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Scheme 4

Scheme 5

activity as 5a (Table 1, run 11). Furthermore, treatment of i with a catalytic amount of the tetranuclear Zr2Ru2 heterobimetallic bis(hydride)-bridged complex [{Zr(μ-η5:η1-C5H4PEt2)(μ3η5:η1:η1-C5H3PEt2)(μ-H)RuCp*}2] (8a) (5 mol %) (vide infra) in toluene at 50 °C for 24 h resulted in complete conversion of i to ii with evolution of H2 gas in 94% GC yield (Table 1, run 12). The amount of H2 gas evolved was almost the same with that using 5a as a catalyst, but a longer reaction time was required (Figure 4), where the conversion rate was calculated to be 5.6 times slower compared to that of 5a (5 mol %). We also investigated the catalytic activity of 5a toward the dehydrogenation of other amine boranes. Treatment of the tertiary amine borane Me3N 3 BH3 or the more sterically encumbered secondary amine borane iPr2NH 3 BH326 with a catalytic amount of 5a (5 mol %) in toluene at 50 °C for 24 h resulted in no reaction. In contrast, treatment of ammonia borane, NH3 3 BH3, and the primary amine borane MeNH2 3 BH326 with a catalytic amount of 5a (10 mol %) brought about dehydrogenation with the evolution of 56% and 92% H2 gas, respectively, with the formation of B N polymers27 such as [MeNHBH2]322 (Scheme 4). However, higher catalyst loadings and reaction times were necessary compared to those for i. This result is also distinct from that for the titanocene(II)catalyzed dehydrogenation, where dehydrogenation of ammonia borane and primary amine boranes did not occur.7b Stoichiometric Reactions of 5a and 7a with i. To elucidate the reaction pathway for these catalytic dehydrogenation reactions, 2398

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Figure 5. ORTEP drawing of 8a. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity, except for the hydride atoms coordinated to Ru. Selected interatomic distances (Å) and angles (deg): Ru(1) 3 3 3 Zr(1), 3.3619(12); Ru(1) P(1), 2.298(2); Ru(1) P(2), 2.320(2); Ru(1) H(1), 1.49; Zr(1) Zr(1)*, 3.2438(13); Zr(1) C(3), 2.376(9); Zr(1) C(3)*, 2.367(8); Zr(1) H(1), 2.29; P(1) Ru(1) P(2), 95.50(9); Zr(1)* Zr(1) C(3), 46.7(2); Zr(1)* Zr(1) C(3)*, 47.0(2); Zr(1) C(3) Zr(1)*, 86.3(2).

we investigated the stoichiometric reactions of 5a and 7a with i in order to isolate reaction intermediates. First, we investigated the reaction of 5a with 1 equiv of i in C6D6 at 50 °C for 12 h, which afforded the tetranuclear Zr2Ru2 heterobimetallic bis(hydride)bridged complex [{Zr(μ-η5:η1-C5H4PEt2)(μ3-η5:η1:η1-C5H3PEt2) (μ-H)RuCp*}2] (8a) in 91% NMR yield together with the formation of Me2NdBHMe (iii)28 (Scheme 5). The GC analysis of the reaction revealed that dihydrogen gas (1.10 equiv to 5a) was evolved in this reaction. Formation of the hydride species 8a was assigned by the 1H NMR resonance at δ 17.4, which appears as a triplet (2JPH = 20.3 Hz) displaying cis coupling to the two nonequivalent phosphine moieties coordinated to the ruthenium centers. The existence of hydrides was also reinforced by a weak absorbance at 1933 cm 1 in the IR spectrum of 8a. The 1H NMR of 8a also exhibits a set of seven broad resonances at δ 6.01, 5.88, 5.26, 5.20, 4.88, 4.51, and 4.43 with an equal intensity ratio, due to two cyclopentadienyl moieties where one of the eight bis(cyclopentadienyl) C H bonds was cleaved, and four sets of resonances due to four symmetrically independent ethyl groups bound to two phosphorus atoms. The 13C{1H} NMR of 8a showed a low-field-shifted resonance at δ 190.1 in its NMR spectrum, which is comparable to the reported value (δ 190.43) for the cyclopentadienyl quaternary carbon atom that bridges two zirconium centers in [{CpZr(μ-η5:η1-C5H4)(PMe2Ph)}2].29 Complex 8a was also isolable as brown block crystals suitable for X-ray crystallography. As shown in Figure 5, two Zr Ru heterobimetallic units are bridged by two of the cyclopentadienylphosphine derivatives {C5H3PEt2}2 , which actually bridge three metals (RuZr2) in a μ3-η5:η1 fashion. The position of the hydride atoms, determined at the maximum peak in the final difference Fourier map, satisfies a three-legged piano-stool coordination geometry at the ruthenium centers, typical of coordinatively saturated cyclopentadienyl Ru(II) bis(phosphine) complexes14 with an Ru H distance of 1.49 Å. At the same time, the interatomic distance between the hydride hydrogen and zirconium atoms (2.29 Å) is within the range of the values found in hydride-bridged dinuclear zirconium complexes (1.82 2.35 Å),30

suggesting that the hydride bridges the zirconium and ruthenium atoms, while the interatomic distance between the zirconium and ruthenium atoms is 3.3619(12) Å, slightly too long to imply a metal metal single bond.16 Interestingly, the only previously reported Zr Ru heterobimetallic complexes where zirconium and ruthenium atoms are bridged by hydrides are the related cyclopentadienylphosphine-bridged complexes [ZrX(μ-η5:η1C5H4PPh2)2(μ-Cl)(μ-H)RuH(PPh3)] and [ZrCl(μ-η5:η1C5H4PPh2)2(μ-H)2RuX(PPh3)] (X = H, Cl), where a slightly shorter Zr 3 3 3 Ru separation was observed (3.130(3) Å) for [ZrCl(μ-η5:η1-C5H4PPh2)2(μ-Cl)(μ-H)RuH(PPh3)].10 On the other hand, the interatomic distance between two zirconium atoms (3.2438(13) Å) is within the range of Zr Zr single bonds (3.08 3.50 Å),16 significantly shorter than the values (4.00 4.07 Å) found in the bis(zirconium(IV)) complex [{CpZr(η3-(Me3Si)3CdCHR)(μ-η5:η1-C5H3)}2] (R = SiMe3, Ph)31 but comparable to the values found in the diamagnetic bis(zirconium(III)) complexes [{(C5H4tBu)(μ-C5H3tBu)Zr(μH)Na}2 3 OEt2]2 (3.2930(16) Å) and [{(C5H4tBu)(μC5H3tBu)Zr(μ-H)Na}2]4 (3.3076(7) Å).32 The existence of two bridging hydrides between ruthenium and zirconium centers and a metal metal single bond between the two zirconium centers is compatible with the diamagnetic nature of 8a, with an electron count of 66 within the tetranuclear Ru2Zr2 core (or 30 within the Zr2 core without counting two Ru H bonds), corresponding to the formal oxidation states of Ru(II) and Zr(III). A proposed reaction pathway is also shown in Scheme 5. At first, ligand exchange of a hydride of i with the methyl group at Zr in 5a occurs to afford the hydride-bridged complex C, with the formal oxidation state of Zr(II)Ru(II), and Me2NH 3 BH2Me. Next, oxidative addition of the N H bond to low-valent Zr(II) proceeds to form the amidoborane dihydride complex D, followed by reductive elimination of dihydrogen to afford the amidoborane complex E. Subsequent activation of the B H bond of the amidoborane moiety results in the regeneration of the hydride-bridged complex C with liberation of iii. Because there is no further amine borane to react with C under the 2399

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Organometallics Scheme 6

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Scheme 8

Scheme 7

stoichiometric reaction conditions, intermolecular C H bond activation on low-valent Zr(II) centers occurs to afford the thermodynamically stable tetranuclear Zr2Ru2 heterobimetallic bis(hydride)-bridged complex 8a with the formal oxidation state of Zr(III)2Ru(II)2 with reductive elimination of dihydrogen (0.5 equiv with respect to 5a).32 Indeed, more than 1 equiv of H2 gas was observed for the stoichiometric reaction of 5a with i in Scheme 5. Similarly, the stoichiometric reaction of 7a with 1 equiv of i in C6D6 at 50 °C for 12 h resulted in almost quantitative conversion of 7a into 8a, together with the quantitative formation of iv as the sole boron-containing product and evolution of more than 1 equiv of H2 (Scheme 6). This result strongly indicates that the dimethylamide complex 7a is converted into 8a via C, which is the plausible intermediate in the dehydrogenation of i catalyzed by 5a: i.e., the common reactive species C is formed from the reaction of both 5a and 7a with i. As described in the previous section, 8a has a catalytic activity toward the dehydrogenation of i similar to that of 5a and 7a, although a longer reaction time was necessary to complete the reaction. This result indicates that C can be slowly generated in situ from 8a during the catalytic reaction. In fact, after the dehydrogenation of i by using 8a as a catalyst, 8a was almost recovered together with a small amount of 6a. On the other hand, only the formation of 6a was observed after the dehydrogenation of i by using 5a or 7a as a catalyst.33 These results indicate that reactive species C was finally converted into 6a during the catalytic dehydrogenation. Stoichiometric Reaction of 7a with BH3. We next tried to isolate the dimethylamidoborane complex which we propose as another key reaction intermediate, because such complexes were

postulated as possible reaction intermediates in the transitionmetal-catalyzed dehydrogenation of amine boranes.7b,c Group IV amidoborane complexes were previously prepared by reactions of the corresponding amide complexes with boranes.34 Therefore, we examined the stoichiometric reaction of 7a with BH3 in order to prepare amidoborane complexes. However, treatment of 7a with 1 equiv of BH3 3 THF resulted in the formation of 6a and about half of 7a remained unreacted. Complete conversion of 7a was observed when 7a was treated with 2 equiv of BH3 3 THF in C6D6 at room temperature for 1 h to form 6a and ii in 87% and 95% NMR yields, respectively (Scheme 7). First, the reaction of 7a with 1 equiv of BH3 affords the Zr Ru heterobimetallic dimethylamidoborane complex G. Then, G transforms into C with the liberation of Me2NdBH2 (v), which readily dimerizes to afford ii. Finally, C quickly reacts with another BH3 to afford stable 6a. The almost quantitative formation of ii in this reaction suggests that an amidoborane complex such as G was indeed formed in situ and G is so reactive that the amidoborane moiety smoothly converted into v to form C. Plausible Reaction Pathway of the Catalytic Reaction. A plausible reaction pathway of the catalytic dehydrogenation of i is shown in Scheme 8. At first, 5a or 7a reacts with i to produce C. Next, oxidative addition of the N H bond of another i to lowvalent Zr(II) proceeds to form the amidoborane dihydride complex F, followed by reductive elimination of dihydrogen to afford the amidoborane complex G. Finally, the B H bond of the amidoborane moiety in G is activated on the Ru center to regenerate C. In this step, activation of the B H bond on Zr is also possible, because a mononuclear zirconocene amidoborane complex, where the B H bond of the amidoborane moiety is activated by the Zr center, has been isolated by Roesler et al.35 However, this isolated complex is so stable that subsequent liberation of the amidoborane moiety as an aminoborane does not occur, which prompts us to propose the reaction pathway where the B H bond of the amidoborane moiety of G is 2400

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Organometallics activated by the Ru center. This idea is also supported by the reaction of 7a with 2 equiv of BH3 shown in Scheme 7, where facile liberation of the amidoborane moiety of G with the formation of v was observed. Furthermore, in the dehydrogenation of amine boranes catalyzed by late transition metals, including Ru, B H bond activation by the metal center is proposed as a key step of the reaction36 and related σ-amine borane complexes have been reported to be isolated,37 which also supports our proposed reaction pathway.

’ CONCLUSION We have prepared a novel series of Zr Ru, Hf Ru, and Zr Fe heterobimetallic complexes bridged by a set of cyclopentadienylphosphine moieties and revealed that these heterobimetallic complexes were applicable to the catalytic dehydrogenation of secondary and primary amine boranes, including NH3 3 BH3. Although the catalytic activity of our system is not high enough in comparison to that in recent works dealing with the catalytic dehydrogenation of amine boranes,7 the observed moderate catalytic activities enabled us to investigate their detailed reaction mechanism, especially for dehydrogenation of i. The result of the stoichiometric reaction of 5a and 7a with i and the reaction of 7a with BH3 prompted us to propose the catalytic cycle summarized in Scheme 8, where both zirconium and ruthenium centers of 5a and 7a participate in the catalysis. We consider that the cooperative activation of N H and B H bonds of amine boranes leads to the formation of heterobimetallic hydride complex C, which is the plausible key reaction intermediate. Notably, the ruthenium hydride moiety in C would stabilize the reactive low-valent zirconium(II) center to facilitate the catalysis. Although we have not succeeded in the isolation of C, an analogue of C has been successfully isolated and structurally characterized as 8a. These results demonstrate that the design and location of an electron-deficient early transition metal and an electron-rich late transition metal in close proximity lead to the unique activation and effective transformation of substrate molecules through cooperative effects between these two metals. Furthermore, it should be noted that successful examples of the application of cooperative effects induced by heterobimetallic moieties to catalytic reactions such as our dehydrogenation system are still quite limited in number, although the preparation of various heterobimetallic complexes and investigations of their reactivity have been widely reported.1 We believe the result described here opens up a new aspect of the design and preparation of homogeneous bimetallic catalysts. Future studies will address the influence of the pair of metals in the heterobimetallic complexes on the catalytic activities. In addition, application of this catalytic system to other polar substrates and intriguing catalytic reactions are also ongoing in our laboratory. ’ EXPERIMENTAL SECTION General Considerations. 1H NMR (270 MHz), 31P NMR (109 MHz), 11B NMR (86.6 MHz), and 13C NMR (67.8 MHz) spectra were recorded on a JEOL Excalibur 270 spectrometer in suitable solvents. 31P or 11B NMR chemical shifts were quoted relative to an external standard of 85% H3PO4 or of BF3 3 Et2O. NMR yields were determined using ferrocene as an internal standard. Elemental analyses were performed at the Microanalytical Laboratory of The University of Tokyo or on an Exeter Analytical CE-440 elemental analyzer. IR spectra were recorded on a JASCO FT/IR 4100 Fourier transform infrared spectrophotometer. All reactions were carried out under a dry nitrogen atmosphere

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or in an argon-filled glovebox. The volume of evolved dihydrogen was quantified by gas chromatography using a Shimadzu GC-8A instrument with a TCD detector and a SHINCARBON ST column (6 m  3 mm). Solvents were dried by general methods and degassed before use. [{Cp*Ru(μ3-Cl)}4],38 [Cp*RuCl(depe)],24 [Cp*FeCl(TMEDA)],39 CpLi,40 MeNH2 3 BH3, and iPr2NH 3 BH326 were prepared according to the literature procedures. [Cp*RuH(depe)] was prepared by a method similar to that for the preparation of [Cp*RuH(dppe)].24 NH3 3 BH3, Me2NH 3 BH3 (i), and Me3N 3 BH3 were purchased commercially and sublimed before use. Other reagents such as [ZrCl4(thf)2], [HfCl4(thf)2], Et2PCl, NaN(SiMe3)2, LiBHEt3, MeLi, NaBH4, and LiNMe2 were purchased commercially and used as received. Preparation of [(η5-C5H4PEt2)2ZrCl2] (2a). To a THF (85 mL) solution of [ZrCl4(thf)2] (2.61 g, 6.92 mmol) was added dropwise a THF solution (85 mL) of 1 (2.50 g, 14.2 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. After the solvent was removed in vacuo, toluene (40 mL) was added to the residue, and then the slurry was filtered through a pad of Celite. The solvent was then removed from the filtrate, and the resulting yellow solid was washed with hexane (20 mL  2) to afford 2a as a pale yellow solid (2.30 g, 4.91 mmol, 71% isolated yield). 1H NMR (C6D6): δ 6.45 (pseudo t, J = 2.7 Hz, 4H, C5H4), 6.25 (pseudo t, J = 2.7 Hz, 4H, C5H4), 1.59 1.41 (m, 8H, PCH2), 0.88 (dt, 3JHP = 14.9 Hz, 3JHH = 7.4 Hz, 12H, 25.0 (s). Anal. Calcd for Me). 31P{1H} NMR (C6D6): δ C18H28Cl2P2Zr: C, 46.15; H, 6.02. Found: C, 46.22; H, 6.03. Preparation of [ZrCl2(μ-η5:η1-C5H4PEt2)2RuClCp*] (3a). To a slurry of [{Cp*Ru(μ3-Cl)}4] (870 mg, 0.800 mmol) in THF (50 mL) was added 2a (1.51 g, 3.22 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. Solvent was removed in vacuo, and the resulting orange product was washed with pentane (10 mL  2) to afford 3a as an orange solid (2.20 g, 2.97 mmol, 93% isolated yield). Red thick needles of 3a 3 0.5C6H14 suitable for X-ray crystallography were obtained by layering hexane onto a toluene solution of 3a, which was then cooled to 35 °C. 1H NMR (C6D6): δ 7.83 (br, 2H, C5H4), 6.82 (br, 2H, C5H4), 6.63 6.60 (m, 2H, C5H4), 5.75 (br, 2H, C5H4), 2.45 2.29 (m, 2H, PCH2), 1.98 1.80 (m, 4H, PCH2), 1.74 1.66 (m, 2H, PCH2), 1.43 (s, 15H, Cp*), 1.10 (dt, 3JHP = 15.1 Hz, 3JHH = 7.4 Hz, 6H, CH2Me), 0.61 (dt, 3JHP = 14.3 Hz, 3JHH = 7.4 Hz, 6H, CH2Me). 31 1 P{ H} NMR (C6D6): δ 34.6 (s). Anal. Calcd for C28H43Cl3P2RuZr: C, 45.43; H, 5.86. Found: C, 45.28; H, 5.72. Preparation of [ZrCl(μ-η5:η1-C5H4PEt2)2RuCp*] (4a). To a solution of 3a (1.87 g, 2.53 mmol) in THF (50 mL) was added a THF solution of LiBHEt3 (1.01 M, 5.00 mL, 5.05 mmol) at room temperature, and the mixture was stirred at room temperature for 4 h. After the solvent was removed, toluene (30 mL) was added to the residue, and the slurry was filtered through a pad of Celite. After solvent was removed from the filtrate, the resulting green residue was washed with hexane (15 mL  4) to afford 4a (1.23 g, 1.84 mmol, 73%) as a green solid. Brownish green plates of 4a suitable for X-ray crystallography were obtained by cooling a toluene solution of 4a to 35 °C. 1H NMR (C6D6): δ 6.89 (br, 2H, C5H4), 6.78 (br, 2H, C5H4), 5.04 (br, 2H, C5H4), 3.26 3.24 (m, 2H, C5H4), 3.19 3.03 (m, 2H, PCH2), 2.47 2.34 (m, 2H, PCH2), 1.89 (s, 15H, Cp*), 1.82 1.69 (m, 2H, PCH2), 1.45 (dt, 3JHP = 13.2 Hz, 3JHH = 7.0 Hz, 6H, CH2Me), 0.68 (dt, 3 JHP = 16.5 Hz, 3JHH = 7.9 Hz, 6H, CH2Me), 0.12 0.02 (m, 2H, PCH2). 31 1 P{ H} NMR (C6D6): δ 34.4 (s). Anal. Calcd for C28H43ClP2RuZr: C, 50.24; H, 6.48. Found: C, 49.97; H, 6.32. Preparation of [ZrMe(μ-η5:η1-C5H4PEt2)2RuCp*] (5a). To a solution of 4a (107 mg, 0.160 mmol) in toluene (2 mL) was added an ethereal solution of MeLi (1.6 M, 0.100 mL, 0.160 mmol) at room temperature, and the mixture was stirred at room temperature for 18 h. The resulting reaction mixture was filtered through a pad of Celite, and then solvent was removed from the filtrate in vacuo to afford 5a as a green solid (86.1 mg, 0.133 mmol, 83% isolated yield). Brownish green 2401

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Organometallics plates of 5a suitable for X-ray crystallography were obtained by cooling a THF solution of 5a to 35 °C. 1H NMR (C6D6): δ 6.86 6.81 (m, 2H, C5H4), 6.54 (br, 2H, C5H4), 4.80 (br, 2H, C5H4), 3.51 3.50 (m, 2H, C5H4), 3.20 3.04 (m, 2H, PCH2), 2.43 2.30 (m, 2H, PCH2), 1.96 1.86 (m, 2H, PCH2), 1.80 (s, 15H, Cp*), 1.52 (dt, 3JHP = 13.2 Hz, 3JHH = 7.5 Hz, 6H, CH2Me), 0.71 (dt, 3JHP = 16.2 Hz, 3JHH = 7.6 Hz, 6H, CH2Me), 0.21 0.08 (m, 2H, PCH2), 1.25 (s, 3H, ZrMe). 31 1 P{ H} NMR (C6D6): δ 35.7 (s). Anal. Calcd for C29H46P2RuZr: C, 53.68; H, 7.15. Found: C, 53.45; H, 7.02. Preparation of [HfCl2(μ-η5:η1-C5H4PEt2)2RuClCp*] (3b). To a slurry of 1 (440 mg, 2.50 mmol) in toluene (35 mL) and a trace amount of THF (0.1 mL) was added [HfCl4(thf)2] (553 mg, 1.20 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. Then, [{Cp*Ru(μ3-Cl)}4] (324 mg, 0.298 mmol) was added to the reaction mixture and stirring was continued for a further 6 h. The resulting reaction mixture was filtered through a pad of Celite, and then solvent was removed from the filtrate in vacuo. The resulting orange residue was washed with hexane (10 mL  3) to afford 3b as an orange solid (610 mg, 0.737 mmol, 62% isolated yield per [HfCl4(thf)2]). Orange blocks of 3b 3 1.5C6H6 suitable for X-ray crystallography were obtained by layering hexane onto a benzene solution of 3b. 1H NMR (C6D6): δ 7.78 (br, 2H, C5H4), 6.73 (br, 2H, C5H4), 6.53 6.50 (m, 2H, C5H4), 5.65 (br, 2H, C5H4), 2.46 2.30 (m, 2H, PCH2), 1.98 1.81 (m, 4H, PCH2), 1.80 1.71 (m, 2H, PCH2), 1.62 (s, 15H, Cp*), 1.08 (dt, 3 JHP = 15.4 Hz, 3JHH = 7.6 Hz, 6H, CH2Me), 0.57 (dt, 3JHP = 14.0 Hz, 3 JHH = 7.4 Hz, 6H, CH2Me). 31P{1H} NMR (C6D6): δ 35.0 (s). Anal. Calcd for C28H43Cl3HfP2Ru: C, 40.64; H, 5.24. Found: C, 40.44; H, 5.20. Preparation of [HfCl(μ-η5:η1-C5H4PEt2)2RuCp*] (4b). To a slurry of KC8 (81.0 mg, 0.599 mmol) in toluene (10 mL) was added 3b (165 mg, 0.199 mmol) at room temperature, and the mixture was stirred at room temperature for 24 h. The resulting mixture was filtered through a pad of Celite. After solvent was removed from the filtrate, 4b was obtained as a greenish brown solid (138 mg, 0.182 mmol, 91% isolated yield). Red blocks of 4b suitable for X-ray crystallography were obtained by layering hexane onto a toluene solution of 4b, which was then cooled to 35 °C. 1H NMR (C6D6): δ 6.65 (br, 2H, C5H4), 6.47 (br, 2H, C5H4), 5.22 (br, 2H, C5H4), 3.40 (br, 2H, C5H4), 3.04 2.88 (m, 2H, PCH2), 2.38 2.25 (m, 2H, PCH2), 1.97 (s, 15H, Cp*), 1.88 1.71 (m, 2H, PCH2), 1.37 (dt, 3JHP = 13.2 Hz, 6H, 3JHH = 6.9 Hz, CH2Me), 0.73 (dt, 3JHP = 16.2 Hz, 3JHH = 7.8 Hz, 6H, CH2Me), 0.29 0.15 (m, 2H, PCH2). 31P{1H} NMR (C6D6): δ 30.3 (s). Anal. Calcd for C28H43ClHfP2Ru: C, 44.45; H, 5.73. Found: C, 44.83; H, 5.71. Preparation of [HfMe(μ-η5:η1-C5H4PEt2)2RuCp*] (5b). To a solution of 4b (72.6 mg, 0.0960 mmol) in toluene (4 mL) was added an ethereal solution of MeLi (1.06 M, 0.180 mL, 0.191 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. Then, the mixture was filtered through a pad of Celite, and solvent was removed from the filtrate in vacuo. The resulting reddish brown residue was recrystallized from toluene Et2O to afford 5b as brown platelets (18 mg, 0.025 mmol, 26% isolated yield). 1H NMR (C6D6): δ 6.51 (br, 2H, C5H4), 6.44 (br, 2H, C5H4), 4.90 (br, 2H, C5H4), 3.65 (br, 2H, C5H4), 3.09 2.93 (m, 2H, PCH2), 2.36 2.23 (m, 2H, PCH2), 2.06 1.93 (m, 2H, PCH2), 1.88 (s, 15H, Cp*), 1.46 (dt, 3JHP = 13.0 Hz, 3JHH = 6.9 Hz, 6H, CH2Me), 0.74 (dt, 3JHP = 16.2 Hz, 3JHH = 7.8 Hz, 6H, CH2Me), 0.31 0.18 (m, 2H, PCH2), 1.43 (s, 3H, HfMe). 31 1 P{ H} NMR (C6D6): δ 35.7 (s). Anal. Calcd for C29H46HfP2Ru: C, 47.31; H, 6.30. Found: C, 46.84; H, 5.82. Preparation of [ZrCl2(μ-η5:η1-C5H4PEt2)2FeClCp*] (3c). To a solution of [Cp*FeCl(TMEDA)] (34.3 mg, 0.100 mmol) in THF (2 mL) was added 2a (46.8 mg, 0.100 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. Solvent was removed in vacuo, and the resulting purple product was recrystallized from toluene hexane at 35 °C to afford 3c 3 1.5C7H8 as purple

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platelets (25.4 mg, 0.030 mmol, 30% isolated yield). (C6D6): δ 53.7 (s).

31

P{1H} NMR

Preparation of [ZrCl(μ-η5:η1-C5H4PEt2)2FeCp*] (4c). To a slurry of KC8 (40.8 mg, 0.302 mmol) in toluene (3 mL) was added 3c (69.6 mg, 0.100 mmol) at room temperature, and the mixture was stirred at room temperature for 24 h. The resulting mixture was filtered through a pad of Celite. After solvent was removed from the filtrate, 4c was obtained as a brown solid. Brown platelets of 4c suitable for X-ray crystallography were obtained by layering hexane onto a toluene solution of 4c, which was then cooled to 35 °C (20.6 mg, 0.033 mmol, 33% isolated yield). 31P{1H} NMR (C6D6): δ 42.6 (s). Anal. Calcd for C28H43FeClP2Zr: C, 53.88; H, 6.94. Found: C, 53.06; H, 6.63. Preparation of [ZrMe(μ-η5:η1-C5H4PEt2)2FeCp*] (5c). To a solution of 4c (305.1 mg, 0.489 mmol) in toluene (15 mL) was added an ethereal solution of MeLi (1.06 M, 0.820 mL, 0.869 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. Then, the mixture was filtered through a pad of Celite. After solvent was removed from the filtrate, 5c was obtained as a brown solid (140 mg, 0.232 mmol, 47% isolated yield). Brown platelets of 5c suitable for X-ray crystallography were obtained by layering hexane onto a toluene solution of 5c, which was then cooled to 35 °C. 1H NMR (C6D6): δ 7.19 (br, 2H, C5H4), 6.61 (br, 2H, C5H4), 4.75 (br, 2H, C5H4), 3.47 (br, 2H, PCH2), 3.11 (br, 2H, C5H4), 2.53 2.39 (m, 2H, PCH2), 1.86 1.72 (m, 2H, PCH2), 1.64 (s, 15H, Cp*), 1.64 (dt, 3JHP = 19.2 Hz, 3 JHH = 8.4 Hz, 6H, CH2Me), 0.64 (dt, 3JHP = 15.4 Hz, 3JHH = 7.8 Hz, 6H, CH2Me), 0.07 to 0.26 (m, 2H, PCH2), 1.34 (s, 3H, ZrMe). 31 1 P{ H} NMR (C6D6): δ 44.4 (s). Anal. Calcd for C29H46FeP2Zr: C, 57.70; H, 7.68. Found: C, 57.17; H, 7.39. Preparation of [Zr(η2-BH4)(μ-η5:η1-C5H4PEt2)2RuCp*] (6a). To a solution of 4a (100 mg, 0.149 mmol) in THF (6 mL) was added an excess amount of NaBH4 (57.0 mg, 1.51 mmol), and the mixture was stirred at 50 °C for 6 h. After removal of the solvent in vacuo, the residue was extracted with toluene (10 mL) and filtered through a pad of Celite. After solvent was removed from the filtrate, the resulting brown solid was recrystallized from THF hexane at 35 °C to afford 6a (50.3 mg, 0.0775 mmol, 52% isolated yield) as dark greenish brown platelets. 1H NMR (C6D6): δ 6.40 (br, 2H, C5H4), 6.07 (br, 2H, C5H4), 4.84 (br, 2H, C5H4), 3.21 3.19 (m, 2H, C5H4), 2.81 2.66 (m, 2H, PCH2), 2.26 2.13 (m, 2H, PCH2), 1.90 (s, 15H, Cp*), 1.85 1.72 (m, 2H, PCH2), 1.34 (dt, 3JHP = 13.0 Hz, 3JHH = 7.4 Hz, 6H, CH2Me), 0.74 (dt, 3JHP = 16.5 Hz, 3JHH = 7.6 Hz, 6H, CH2Me), 0.14 0.01 (m, 2H, PCH2), 1.37 (br, 2H, BH4), 1.72 (br, 2H, BH4). 31P{1H} NMR (C6D6): δ 33.9 (s). 11B{1H} NMR (C6D6): δ 4.8 (br). Anal. Calcd for C28H47BP2RuZr: C, 51.84; H, 7.30. Found: C, 51.62; H, 7.23.

Preparation of [Zr(NMe2)(μ-η5:η1-C5H4PEt2)2RuCp*] (7a).

To a solution of 4a (100 mg, 0.149 mmol) in THF (6 mL) was added LiNMe2 (17.0 mg, 0.333 mmol), and the mixture was stirred at room temperature for 3 h. After removal of the solvent in vacuo, the residue was extracted with toluene (10 mL) and filtered through a pad of Celite. After solvent was removed from the filtrate, 7a (56.3 mg, 0.0830 mmol, 56% isolated yield) was obtained as an orange solid. Brown platelets of 7a suitable for X-ray crystallography were obtained by layering Et2O onto a toluene solution of 7a, which was then cooled to 35 °C. 1H NMR (C6D6): δ 6.30 (br, 2H, C5H4), 5.71 (br, 2H, C5H4), 5.21 (br, 2H, C5H4), 3.54 (br, 2H, C5H4), 2.75 (s, 6H, NMe2), 2.81 2.66 (m, 2H, PCH2), 2.28 2.10 (m, 2H, PCH2), 1.97 (s, 15H, Cp*), 1.94 1.84 (m, 2H, PCH2), 1.38 (dt, 3JHP = 10.8 Hz, 3JHH = 7.4 Hz, 6H, CH2Me), 0.87 (dt, 3JHP = 16.2 Hz, 3JHH = 7.4 Hz, 6H, CH2Me), 0.49 0.36 (m, 2H, PCH2). 31P{1H} NMR (C6D6): δ 34.4 (s). Anal. Calcd for C30H49NP2RuZr: C, 53.15; H, 7.28; N, 2.07. Found: C, 53.34; H,6.98; N, 2.21.

Preparation of [{Zr(μ-η5:η1-C5H4PEt2)(μ3-η5:η1:η1C5H3PEt2)(μ-H)RuCp*}2] (8a). To a solution of 5a (64.9 mg, 0.100 mmol) in benzene (3 mL) was added i (5.9 mg, 0.10 mmol) at 2402

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Organometallics room temperature, and the mixture was heated at 50 °C for 18 h. Solvent was removed in vacuo, and the resulting purple residue was recrystallized from benzene hexane to afford 8a (14.4 mg, 0.0114 mmol, 23% isolated yield) as brownish purple blocks. 1H NMR (C6D6): δ 6.01 (br, 2H, ZrCp), 5.88 (br, 2H, ZrCp), 5.26 (br, 2H, ZrCp), 5.20 (br, 2H, ZrCp), 4.88 (br, 2H, ZrCp), 4.51 (br, 2H, ZrCp), 4.43 (br, 2H, ZrCp), 2.15 2.07 (m, 4H, PCH2), 1.91 (s, 30H, Cp*), 1.73 1.60 (m, 4H, PCH2), 1.46 1.37 (m, 4H, PCH2), 1.22 (dt, 3JHP = 13.0 Hz, 3JHH = 6.4 Hz, 12H, CH2Me), 1.08 0.86 (m, 16H, PCH2 and CH2Me), 17.4 (t, 2 JPH = 20.3 Hz, 2H, RuH). 31P{1H} NMR (C6D6): δ 19.8 (d, 2JPP = 29.5 Hz), 18.6 (d, 2JPP = 29.5 Hz). 13C{1H} NMR (C6D6): δ 190.1 (s, μ-C of C5H3), 113.8 (d, 2JCP = 11.1 Hz, C5H3), 110.1 (d, 2JCP = 8.9 Hz, C5H4), 106.8 (d, 2JCP = 8.9 Hz, C5H3), 101.0 (d, 3JCP = 6.2 Hz, C5H3), 99.7 (d, 3 JCP = 4.4 Hz, C5H4), 97.7 (d, 1JCP = 13.4 Hz, ipso-C of C5H3), 94.8 (d, 1 JCP = 47.4 Hz, ipso-C of C5H4), 90.7 (s, C5Me5), 23.1 (dd, 1JCP = 15.0 Hz, 3JCP = 1.7 Hz, PCH2), 21.5 (dd, 1JCP = 19.2, 3JCP = 1.7 Hz, PCH2), 20.4 (dd, 1JCP = 15.4 Hz, 3JCP = 2.5 Hz, PCH2), 18.9 (dd, 1JCP = 19.8 Hz, 3 JCP = 3.6 Hz, PCH2), 13.6 (s, C5Me5), 8.6 (d, 2JCP = 8.3 Hz, CH2Me), 8.2 (d, 2JCP = 8.9 Hz, CH2Me), 8.1 (d, 2JCP = 3.8 Hz, CH2Me), 6.9 (d, 2 JCP = 5.6 Hz, CH2Me). IR (KBr, cm 1): 1933 (w, νRuH). Anal. Calcd for C56H86P4Ru2Zr2: C, 53.05; H, 6.84. Found: C, 52.59; H, 6.57. Catalytic Dehydrogenation of Amine Boranes. A typical procedure for the conversion of i using 5a as a catalyst is given as follows. To a toluene solution (2 mL) of i (11.8 mg, 0.200 mmol) in a sealed Schlenk flask (25 mL) was added 5a (6.5 mg, 0.010 mmol), and the mixture was stirred at 50 °C for 3 h. The amount of the evolved H2 gas (0.190 mmol, 95% yield) was measured by GC sampling of the gas in the sealed flask (50 mL) using a 100 μL microsyringe. Complete conversion of i and formation of ii as a major product as well as formation of iii and iv as minor products were confirmed by 11B NMR spectroscopy of an aliquot of the final crude reaction mixture. Detailed results are summarized in Table 1. Reaction of 5a with 1 equiv of i. To a C6D6 solution (0.6 mL) of 5a (19.5 mg, 0.0300 mmol) was added 1 equiv of i (0.30 M in C6D6, 0.100 mL, 0.0300 mmol). After the mixture was heated to 50 °C for 12 h in a sealed NMR tube, 8a was obtained in 91% yield (or 0.46 equiv per 5a), confirmed by 1H NMR spectroscopy, together with the formation of iii (0.84 equiv per 5a), ii (a trace amount), and iv (a trace amount) confirmed by 11B NMR, and the evolution of H2 gas (0.033 mmol, 1.10 equiv per 5a), detected by GC. Reaction of 7a with 1 equiv of i. To a C6D6 solution (0.6 mL) of 7a (21.3 mg, 0.0314 mmol) was added 1 equiv of i (0.31 M in C6D6, 0.100 mL, 0.0310 mmol). After the mixture was heated to 50 °C for 12 h in a sealed NMR tube, complete conversion of 7a was observed, and 8a was obtained quantitatively, confirmed by 1H NMR spectroscopy, together with the formation of H2 gas (0.035 mmol, 1.11 equiv per 7a), confirmed by GC. 11B NMR of the crude product also confirmed the formation of iv as the sole boron-containing product. Reaction of 7a with 2 equiv of BH3. To a C6D6 solution (1 mL) of 7a (33.2 mg, 0.0490 mmol) was added 2 equiv of BH3 (0.98 M in THF, 0.100 mL, 0.0980 mmol). After the mixture was stirred at room temperature for 1 h, an aliquot of the reaction mixture was analyzed by NMR spectroscopy. 6a and ii were obtained in 87% and 95% yields, respectively, confirmed by 1H NMR spectroscopy.

’ ASSOCIATED CONTENT Supporting Information. Text, figures, tables, and CIF files giving experimental details and X-ray crystallographic data for 3a 3 0.5C6H14, 3b 3 1.5C6H6, 3c 3 1.5C7H8, 4a c, 5a c, and 6a 8a. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by a a Grant-in-Aid for the Scientific Research for Young Scientist (S) (No. 19675002) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and Funding Program for Next Generation WorldLeading Researchers (GR025). Y.N. thanks Ube Industries Ltd. T.M. acknowledges the Global COE program for Chemistry Innovation. ’ REFERENCES (1) (a) Wheatley, N.; Kalck, P. Chem. Rev. 1999, 99, 3379. (b) Gade, L. A. Angew. Chem., Int. Ed. 2000, 39, 2658. (c) Erker, B.; Kehr, G.; Fr€ohlich, R. Coord. Chem. Rev. 2006, 250, 36. (2) For recent examples, see: (a) Fukamizu, K.; Miyake, Y.; Nishibayashi, Y. J. Am. Chem. Soc. 2008, 130, 10498. (b) Ikeda, M.; Miyake, Y.; Nishibayashi, Y. Angew. Chem., Int. Ed. 2010, 49, 7289. For a recent review, see: (c) Miyake, Y.; Uemura, S.; Nishibayashi, Y. ChemCatChem 2009, 1, 342. (3) Ammal, S. C.; Yoshikai, N.; Inada, Y.; Nishibayashi, Y.; Nakamura, E. J. Am. Chem. Soc. 2005, 127, 9428. (4) Tanaka, H.; Sasada, A.; Kouno, T.; Yuki, M.; Miyake, Y.; Nakanishi, H.; Nishibayashi, Y.; Yoshizawa, K. J. Am. Chem. Soc. 2011, 133, 3498. (5) Yuki, M.; Miyake, Y.; Nishibayashi, Y.; Wakiji, I.; Hidai, M. Organometallics 2008, 27, 3947. (6) (a) Yuki, M.; Midorikawa, T.; Miyake, Y.; Nishibayashi, Y. Organometallics 2009, 28, 4741. (b) Yuki, M.; Miyake, Y.; Nishibayashi, Y. Organometallics 2009, 28, 5821. (7) For recent examples of catalytic dehydrogenation of amine boranes, see: (a) Hill, M. S.; Kociok-K€ohn, G.; Robinson, T. P. Chem. Commun. 2010, 46, 7587. (b) Sloan, M. E.; Staubitz, A.; Clark, T. J.; Russell, C. A.; Lloyd-Jones, G. C.; Manners, I. J. Am. Chem. Soc. 2010, 132, 3831. (c) Kawano, Y.; Uruichi, M.; Shimoi, M.; Taki, S.; Kawaguchi, T.; Kakizawa, T.; Ogino, H. J. Am. Chem. Soc. 2009, 131, 14946. (d) Jiang, Y.; Blacque, O.; Fox, T.; Frech, C. M.; Berke, H. Organometallics 2009, 28, 5493. (e) K€ass, M.; Friedrich, A.; Drees, M.; Schneider, S. Angew. Chem., Int. Ed. 2009, 48, 905. (f) Friedrich, A.; Drees, M.; Schneider, S. Chem. Eur. J. 2009, 15, 10339. (g) Conley, B. L.; Williams, T. J. Chem. Commun. 2010, 46, 4815. (h) Blaquiere, N.; DialloGarcia, S.; Gorelsky, S. I.; Black, D. A.; Fagnou, K. J. Am. Chem. Soc. 2008, 130, 14034. (i) Staubitz, A.; Sloan, M. E.; Robertson, A. P. M.; Friedrich, A.; Schneider, S.; Gates, P. J.; Schmedt auf der G€unne, J.; Manners, I. J. Am. Chem. Soc. 2010, 132, 13332. (j) Dietrich, B. L.; Goldberg, K. I.; Heinekey, D. M.; Autrey, T.; Lindehan, J. C. Inorg. Chem. 2008, 47, 8583. (k) Ciganda, R.; Garralda, M. A.; Ibarlucea, L.; Pinilla, E.; Torres, M. R. Dalton Trans. 2010, 39, 7226. (l) Kim, S.-K.; Han, W.-S.; Kim, T.-J.; Kim, T.-Y.; Nam, S. W.; Mitoraj, M.; Piekos, y.; Michalak, A.; Hwang, S.-J.; Kang, S. O. J. Am. Chem. Soc. 2010, 132, 9954. (m) Mal, S. S.; Stephens, F. H.; Baker, R. T. Chem. Commun. 2011, 47, 2922. (8) (a) Tikkanen, W.; Fujita, Y.; Petersen, J. L. Organometallics 1986, 5, 888. (b) Morcos, D.; Tikkanen, W. J. Organomet. Chem. 1989, 371, 15. (c) Schenk, W. A.; Labude, C. Chem. Ber. 1989, 122, 1489. (d) Szymoniak, J.; Kubicki, M. M.; Besanc-on, J.; Moïse, C. Inorg. Chim. Acta 1991, 180, 153. (e) Schenk, W. A.; Neuland-Labude, C. Z. Naturforsch., B: Chem. Sci. 1991, 46b, 573. (f) Ara, I.; Delgado, E.; Fornies, J.; Hernandez, E.; Lalinde, E.; Mansilla, N.; Moreno, M. T. J. Chem. Soc., Dalton Trans. 1996, 3201. (g) Schenk, W. A.; Gutmann, T. J. Organomet. Chem. 1997, 544, 69. (9) Gutmann, T.; Dombrowski, E.; Burzlaff, N.; Schenk, W. A. J. Organomet. Chem. 1998, 552, 91. 2403

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