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Synthesis and Structural Characteristics of Discrete Organoboron and Organoaluminum Hydrides Incorporating Bulky Eind Groups Takahiro Murosaki,† Shohei Kaneda,† Ryota Maruhashi,† Kazuya Sadamori,† Yoshiaki Shoji,‡ Kohei Tamao,‡ Daisuke Hashizume,∥ Naoki Hayakawa,† and Tsukasa Matsuo*,†,‡ †

Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan ‡ Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute (ASI), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ∥ Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: The bulky Eind-based aryllithium, (Eind)Li (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl), reacted with BF3·OEt2 in Et2O to form the corresponding aryldifluoroborane (Eind)BF2 (1), along with a trace amount of the sterically congested diarylborane, (Eind)2BH (2). The reaction of 1 with LiAlH4 in THF led to the isolation of the corresponding lithium trihydroborate [Li(thf)]2[(Eind)BH3]2 (3), which can be transformed into the diborane(6), (Eind)HB(μ-H)2BH(Eind) (4), with treatment with Me3SiCl. T h e E in d - b a s e d l i t h i u m t r i h y d r o a lu m in a t e [ L i(OEt2)]2[(Eind)AlH3]2 (5) has been synthesized by the reaction of (Eind)Li with LiAlH4 in Et2O. The subsequent addition of Me3SiCl to a solution of 5 in toluene produced the dialumane(6), (Eind)HAl(μ-H)2AlH(Eind) (6), the heavier congener of 4. The dialumane(6) 6 reacted with lithium metal in a mixed solvent of Et2O and toluene to give the diarylalumane, (Eind)2AlH (7), via a disproportionation reaction along with the cleavage and recombination of the Al−C bond. The discrete monomeric structures of 1, 2, and 7 and dimeric structures of 3, 4, 5, and 6 have been determined by X-ray crystallography.

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INTRODUCTION In the chemistry of the group 13 elements, boron and aluminum hydrides have been among the most widely employed and effective reagents for chemical syntheses.1 Particularly, NaBH4 and LiAlH4 are an important class of inorganic reducing agents,2 and their hydride reactivities are essentially based on the electronegativities of the electropositive boron and the more electropositive aluminum atoms. The structural features of the hydrogen-substituted group 13 compounds have also long been studied both experimentally and theoretically for a better understanding of the nature of the chemical bonding in electron-deficient compounds.3,4 Organoboron and organoaluminum hydrides have also proven to be useful and versatile tools in a wide range of chemistries, mainly due to their excellent solubility in common organic solvents with some enhanced reducing ability, as represented by the commercially available lithium triethylborohydride (LiBHEt3)5 and diisobutylaluminum hydride (iBu2AlH).6 However, these hydride species, especially with relatively small alkyl or aryl groups, are not very thermally stable and sometimes cause side reactions such as a disproportionation reaction.7,8 The introduction of a sterically © XXXX American Chemical Society

large substituent may more effectively stabilize highly reactive hydrides on the boron and aluminum atoms, thus providing many opportunities to clarify their structures and bonding nature as well as to undergo various chemical transformations.9,10 Recently, we focused on the development of low-coordinate compounds of the main group elements and transition metals using the bulky aryl protecting groups based on a fused-ring octa-R-substituted s-hydrindacene skeleton, called “Rind” groups (Chart 1).11,12 The Rind groups have some unique features, e.g., an excellent chemical stability due to the full substitution at all benzylic positions, tunability of physical properties by the outer R1 groups, and controllability of the steric size by the inner R2 groups. As part of these studies, we have obtained two types of structurally well-defined diborane(4) compounds; one is the doubly hydrogen-bridged butterfly-shaped diborane(4), (μ-H)2(Eind)BB(Eind) (A), with bulky Eind groups (R1 = R2 = Et),13a,b and the other is the hydrogen-terminal staggered diborane(4), (MPind)HBBHReceived: August 5, 2016

A

DOI: 10.1021/acs.organomet.6b00633 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Chart 1. Rind Groups

Scheme 1

(MPind) (B), with bulkier MPind groups (R1 = Me, R2 = nPr) (Figure 1).13c Thus, the steric effects of the bulky groups are

Compound 1 was characterized by spectroscopic and crystallographic methods (Tables 1 and 2). The 11B NMR Table 1. NMR Data for 1−4 (ppm, Benzene-d6, Room Temperature) cmpd 1 2 3 4

Figure 1. Doubly hydrogen-bridged diborane(4) (A), hydrogenterminal diborane(4) (B), diborane(6) (C), and diborane(6) dianion (D).

11

B(δ)

27.7 65.4 −32.3 19.6

1

7

H(δ) (B−H)

Li(δ)

19

F(δ)

−56.4 8.36 4.23 3.13, 5.36

2.03

Table 2. Structural Parameters for 1−4 essential for the structure determination of the central B2H2 unit by changing the B−H bonding modes. We have also found that the two-electron reduction of the diborane(6), (Rind)HB(μ-H)2BH(Rind) (C), produces the diborane(6) dianion, [(Rind)H2BBH2(Rind)]2− (D),13b,c being an isolable B−B σbonded species,14 in which the undesirable clustering and disproportionation reactions can be suppressed by the introduction of the bulky groups on the boron atoms. In this article, we report the synthesis and characterization of some discrete organoboron and organoaluminum hydrides bearing fused-ring bulky Eind groups. The introduction of an Eind group on the boron and aluminum atoms, the isolation of the Eind-based lithium trihydroborate and trihydroaluminate derivatives, and the structural elucidation of the Eind-based diborane(6) and dialumane(6) compounds will be reported together with some related compounds.

cmpd 1 2 3 4 (A) 4 (B) 4 (C) 4 (D) 4 (E)

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RESULTS AND DISCUSSION Reaction of (Eind)Li with BF3·OEt2. As we previously reported,13a the bulky Eind-based difluoroborane (Eind)BF2 (1) can be synthesized using a salt metathesis reaction by the addition of BF3·OEt2 to a solution of the bulky aryllithium (Eind)Li prepared by the reaction of (Eind)Br with 2 equiv of t BuLi in Et2O (Scheme 1). After removal of any insoluble materials by filtration, the filtrate was evaporated to dryness, and the resulting residue was subjected to Kugelrohr distillation to give the crude 1. Recrystallization from hexane furnished airstable colorless crystals of 1 with a yield of up to 60%. We have found that the use of Et2O as the reaction solvent is crucial for the introduction of the Eind group on the boron atom; a similar reaction of (Eind)Li, which was prepared by the reaction of (Eind)Br with 2 equiv of nBuLi in tetrahydrofuran (THF),12c with BF3·OEt2 yielded only a small amount of 1.

B−C/Å

B−F/Å or B···Li/Å

1.576(3) 1.5702(11) 1.5700(11) 1.613(4)

1.3146(15)

1.586(4) 1.579(4) 1.596(4) 1.583(4) 1.585(4) 1.581(4) 1.581(4) 1.577(4) 1.579(4) 1.580(4)

2.376(6) 2.240(6)

B···B/Å

C−B···B−C /deg

3.739(4) 1.806(4)

−170.1(3)

1.801(4)

168.8(3)

1.819(4)

−172.8(3)

1.795(4)

169.0(2)

1.789(4)

−171.4(2)

spectrum of 1 in C6D6 showed a broad signal at around 27.7 ppm (W1/2 = 780 Hz). In the 19F NMR spectrum, one signal was observed at −56.4 ppm. As shown in Figure 2, the molecule has C2-symmetry with the 2-fold axis passing through the B1, C1, and C7 atoms. The boron center has a trigonal planar geometry with bond lengths of B1−C1 = 1.576(3) and B1−F1 = 1.3146(15) Å. This is a rare example of the X-ray structural analysis of an aryldifluoroborane.15 As for the synthesis of 1, we also isolated a trace amount of the sterically congested diarylborane (Eind)2BH (2) from the residual materials after the Kugelrohr distillation (Scheme 1). The diarylborane 2 is a rare example of a monomeric tricoordinate dialkyl- or diaryl-boron hydride16 and is extraordinarily air-stable; it can survive for more than several years in the solid state, thus indicating the efficient protection of the reactive B−H bond by the two bulky Eind groups. The B


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Organometallics

reported for (Trip)2BH [128.0(4)°],16b ascribed to the steric congestion around the boron atom. The B−C bond distances in 2 [B1−C1 = 1.5702(11) and B1−C29 = 1.5700(11) Å] are similar to those in (Trip)2BH [1.570(6) and 1.564(6) Å]. The space-filling model of 2 revealed that the central B−H bond is encapsulated by the two Eind groups (Figure 3b). We have investigated the possibility of performing the second introduction of the Eind group to the Eind-bonded boron center.17 However, the reaction between 1 and (Eind)Li met with failure; no proof was obtained to date for the formation of (Eind)2BH 2 and (Eind)2BF. Although the formation mechanism of 2 is not clear at this moment, a unique over-reduction may occur to give 2 in the above reaction conditions.18 Isolation of [Li(thf)]2[(Eind)BH3]2. We previously reported that the reaction of 1 with LiAlH4 followed by trimethylchlorosilane (Me3SiCl) produced the Eind-based diborane(6), (Eind)HB(μ-H)2BH(Eind) (4), in 78% yield.13b We have also examined the possibility of isolating the Eind-based lithium trihydroborate as an intermediate (Scheme 2). After the

Figure 2. Molecular structure of (Eind)BF2 (1). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms are omitted for clarity. Selected distances (Å) and angles (deg): B1−C1 = 1.576(3), B1−F1 = 1.3146(15), C1−B1−F1 = 122.61(9), F1−B1−F1′ = 114.78(18). 1 H NMR spectrum of 2 in C6D6 showed a relatively broad signal due to the terminal B−H group at 8.36 ppm, which is similar to that observed for (Trip)2BH (Trip = 2,4,6-iPr3C6H2) (8.5 ppm).16b In the 11B NMR spectrum of 2, a rather broad signal mainly appeared at 65.4 ppm (W1/2 = 2600 Hz), comparable to that reported for (Trip)2BH (73.5 ppm).16b The molecular structure of 2 was determined by X-ray crystallography. Figure 3a shows the molecular structure of monomeric 2. The hydrogen atom on the boron atom was definitely located on the difference Fourier maps and isotropically refined. The boron atom adopts a trigonal planar geometry with a sum of the bond angles of ca. 360.0°. The C1− B1−C29 bond angle in 2 [131.09(7)°] is larger than that

Scheme 2

reaction of 1 with LiAlH4 in THF, the reaction mixture was evaporated to dryness. To the residue was added benzene, and the resulting suspension was centrifuged to remove the insoluble materials. The supernatant was evaporated to dryness, and the residue was washed with hexane to give the crude product, [Li(thf)]2[(Eind)BH3]2 (3), as a grayish white solid. This solid was extracted with a mixture of benzene and THF, and the supernatant was evaporated to dryness to afford pure 3 in 33% yield. In the 1H NMR spectrum of 3 in C6D6, the B−H signal was found at 4.23 ppm. The 7Li NMR spectrum showed one resonance at 2.03 ppm. The characteristic 11B NMR signal was observed at −32.3 ppm as a quartet with a 1J(1H−11B) coupling constant of 74.7 Hz, which are comparable to those reported for Li[PhBH3] [δ = −26.4 ppm (q, 1J(1H−11B) = 76.0 Hz)]19 and the organyl-substituted lithium trihydroborate derivatives.9a,e−g,20 A solution containing 3 in benzene was allowed to stand at room temperature, from which colorless single crystals suitable for an X-ray structure analysis were formed. As depicted in Figure 4, the Eind-based lithium trihydroborate 3 assumes a dimeric structure and forms two contact ion pairs in the crystals, together with multiple B−H···Li+ interactions. The dimer molecule has a crystallographically imposed C2hsymmetry with a 2-fold axis running across the center of the two lithium atoms (Li1 and Li1′), perpendicular to the plane containing Li1, B1, C1, C7, and C17 atoms. Each lithium atom is stabilized by the coordination of one THF molecule and five hydrogen atoms of the B−H bonds. The distances between the lithium and boron atoms are 2.376(6) Å for B1···Li1 and 2.240(6) Å for B1···Li1′, which are assignable to the coordination modes of (Eind)BH3-η2-Li and (Eind)BH3-η3-Li, respectively.20 The multiple B−H···Li+ interactions cause a

Figure 3. (a) Molecular structure of (Eind)2BH (2) (top view). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the B−H group, are omitted for clarity. Selected distances (Å) and angles (deg): B1−C1 = 1.5702(11), B1− C29 = 1.5700(11), B1−H1 = 1.103(13), C1−B1−C29 = 131.09(7), C1−B1−H1 = 115.0(7), C29−B1−H1 = 113.9(7). (b) Space-filling model of (Eind)2BH (2) (front view): blue, boron; gray, carbon; white, hydrogen, except for B−H group (orange). C


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Organometallics

Figure 5. Molecular structure of (Eind)HB(μ-H)2BH(Eind) (4) (form I, molecule A). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the B−H groups, are omitted for clarity. Selected distances (Å) and angle (deg): B1A−C1A = 1.586(4), B2A−C29A = 1.579(4), B1A−H1A = 1.05(2), B1A−H2A = 1.25(2), B1A−H3A = 1.22(3), B2A−H2A = 1.19(2), B2A−H3A = 1.25(3), B2A−H4A = 1.12(3), B1A···B2A = 1.806(4), C1A−B1A··· B2A−C29A = −170.1(3).

Figure 4. Molecular structure of [Li(thf)]2[(Eind)BH3]2 (3). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the B−H groups, disordered oxygen and carbon atoms of the THF molecule, and benzene molecule are omitted for clarity. Selected distances (Å): B1−C1 = 1.613(4), B1−H1A = 1.13(5), B1−H1B = 1.19(3), Li1−O1 = 1.875(6), B1···Li1 = 2.376(6), B1···Li1′ = 2.240(6), Li1···Li1′ = 2.710(10), B1···B1′ = 3.739(4).

As described above (Figure 1), the Rind-based diborane(6) C (Eind: 4) can be converted into the corresponding diborane(6) dianion D by two-electron reduction with lithium naphthalenide (LiNaph).13b,c Moreover, the hydride abstraction of D can provide the Rind-based diborane(4), A and B, without B−B bond scission. These results prompted us to further investigate the relevance of the hydrogen-substituted group 13 compounds, including the heavier congeners of 2, 3, and 4. Synthesis of [Li(OEt2)]2[(Eind)AlH3]2. Power et al. reported the synthesis of structurally well-defined bulky arylsubstituted lithium trihydroaluminate derivatives by the reaction of the aryllithium with LiAlH4 or AlH3·NMe3.10c,e For the introduction of the Eind group to the aluminum atom, we have examined the reaction of (Eind)Li with LiAlH4 in Et2O (Scheme 3). After the reaction mixture was evaporated to

relatively short Li1···Li1′ distance of 2.710(10) Å, with a long B1···B1′ separation of 3.739(4) Å. The B−C bond length in 3 [1.613(4) Å] is longer than those in 1 [1.576(3) Å] and 2 [1.5702(11) and 1.5700(11) Å] and similar to those reported for the organyl-substituted lithium trihydroborate derivatives.9e,g,20 We have also examined the reaction of (Eind)Li with LiBH4, but no evidence was obtained for the formation of 3. This is in contrast to the fact that the Eind-based lithium trihydroaluminate (5) can be synthesized by the reaction of (Eind)Li with LiAlH4 (vide inf ra). Structure of (Eind)HB(μ-H)2BH(Eind). The structure of the diborane(6) 4 was deduced on the basis of the NMR spectroscopic data.13a,b In the 1H NMR spectrum of 4 in C6D6, two broad B−H signals were found at 3.13 and 5.36 ppm, thus being consistent with a dimeric structure in solution at room temperature on the NMR time scale. The 11B NMR signal due to 4 appeared at 19.6 ppm (W1/2 = 850 Hz), which is shifted to a lower field than that of 3 (−32.3 ppm). We have also examined the structure of 4 in the solid state. On the basis of the X-ray diffraction measurements, the diborane(6) 4 was found to form two polymorphs (forms I and II) of colorless crystals depending on the crystallization conditions. While form I was crystallized from a saturated hexane solution of 4, generated by gentle heating,13a colorless prisms of form II were obtained from a toluene solution of 4 at −30 °C. The X-ray analysis of form I at 100 K shows the presence of three independent molecules (A, B, and C) in the unit cell. For form II at 273 K, an asymmetrical unit consists of two crystallographically independent molecules (D and E). Since the five molecules are structurally similar, some structural features are mentioned only for molecule A (Figure 5). The two terminal and two bridging hydrogen atoms on the boron atoms were located on the difference Fourier maps and isotropically refined. The two boron atoms, the two hydrogen atoms of the terminal B−H bonds, and the two ipso-carbon atoms of the Eind groups adopt an essentially coplanar arrangement; the C−B···B−C torsion angle is −170.1(3)°. The B···B separation [1.806(4) Å] is slightly longer than that reported in the parent molecule of B2H6 at 94 K [1.743(1) Å].21 The B−C bond lengths [B1A−C1A = 1.586(4) and B2A−C29A = 1.579(4) Å] are similar to those of 1 [1.576(3) Å] and 2 [1.5702(11) and 1.5700(11) Å].

Scheme 3

dryness, benzene was added to the residue and the resulting suspension was centrifuged to remove any insoluble materials. This procedure was repeated three times to fully extract the generated benzene-soluble materials. The collected supernatant was evaporated to dryness, and the resulting residue was washed with hexane to give the crude product, [Li(OEt2)]2[(Eind)AlH3]2 (5), as a white solid. Purification by recrystallization using a mixture of hexane and Et2O produced air- and moisture-sensitive colorless crystals of 5 in 62% yield. In the 1H NMR spectrum of 5 in C6D6, a broad Al−H signal D


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Organometallics was found at 3.75 ppm (Table 3). The 7Li NMR spectrum of 5 exhibited one signal at 2.55 ppm. The 27Al NMR spectrum of 5 displays a very broad signal at 48.3 ppm (W1/2 = 7100 Hz).

one Et2O molecule and three hydrogen atoms of the Al−H bonds. The Al1···Li1 and Al2···Li2 distances [3.102(3) and 3.098(3) Å] are longer than the Al1···Li2 and Al2···Li1 distances [2.628(3) and 2.652(3) Å], according to the (Eind)AlH3-η1-Li and (Eind)AlH3-η2-Li coordination modes. The lowering of the coordination number of the lithium ions in 5 compared with that in 3 is mainly due to the larger covalent atomic radius of aluminum [1.21(4) Å] relative to that of boron [0.84(3) Å].22 Thus, the Li···Li and Al···Al separations in 5 [3.915(4) and 4.2196(8) Å] are much longer than those corresponding to 3 [Li1···Li1′ = 2.710(10) and B1···B1′ = 3.739(4) Å]. The Al−C bond lengths in 5 [Al1−C1 = 2.0110(16) and Al2−C29 = 2.0146(16) Å] are similar to those reported for the aryl-substituted lithium trihydroaluminate derivatives [2.007(1)−2.030(5) Å].10c,f Synthesis of (Eind)HAl(μ-H)2AlH(Eind). With 5 in hand, we next investigated the synthesis of the Eind-substituted dialumane(6) (Eind)HAl(μ-H)2AlH(Eind) (6), the heavier congener of 4, using a similar hydride abstraction reaction with Me3SiCl (Scheme 4).10d,e,23 After removal of any insoluble

Table 3. NMR Data for 5−7 (ppm, Benzene-d6, Room Temperature) 27

cmpd

Al(δ)

48.3 71.2 −a

5 6 7 a

1

H(δ) (Al−H) 3.75 4.84, 5.10 6.63

7

Li(δ) 2.25

Not observed.

The structure of 5 was determined by a single-crystal X-ray diffraction analysis (Figure 6), which is closely related to that of

Scheme 4

Figure 6. Molecular structure of [Li(OEt2)]2[(Eind)AlH3]2 (5). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the Al−H groups, are omitted for clarity. Selected distances (Å): Al1−C1 = 2.0110(16), Al2−C29 = 2.0146(16), Al1−H1 = 1.597(19), Al1−H2 = 1.59(2), Al1−H3 = 1.571(17), Al2−H4 = 1.58(2), Al2−H5 = 1.57(2), Al2−H6 = 1.545(19), Li1−O1 = 1.933(3), Li2−O2 = 1.904(3), Al1···Li1 = 3.102(3), Al1···Li2 = 2.628(3), Al2···Li1 = 2.652(3), Al2···Li2 = 3.098(3), Li1···Li2 = 3.915(4), Al1···Al2 = 4.2196(8).

materials by filtration, the filtrate was evaporated to dryness and the resulting residue was washed with hexane. The remaining white solid was recrystallized from toluene to give 6 as colorless crystals in 94% yield. In the 1H NMR spectrum of 6 in C6D6, two broad signals were observed at 4.84 and 5.10 ppm, assignable to the Al−H groups. Thus, 6 can form a dimeric structure in solution at room temperature on the NMR time scale, as observed in the diborane(6) 4 (vide supra). The 27Al NMR spectrum of 6 showed a very broad signal at 71.2 ppm (W1/2 = 5300 Hz), which is somewhat downfield shifted relative to that of 5 [48.3 ppm (W1/2 = 7100 Hz)]. The Eind-based dialumane(6) 6 was found to be thermally stable. For example, no decomposition was observed in the 1H NMR spectrum of 6 in C6D6 upon heating at 65 °C overnight, which is in sharp contrast to the fact that the diborane(6) 4, the lighter congener of 6, is thermally labile and is smoothly converted to the fused monoborane compound via an intramolecular C−H activation with the evolution of H2.13a This difference of the thermal stability between 4 and 6 may be ascribed to the delicate balances between the dynamic equilibrium between the dimers and the monomers in solution and the intramolecular distances between the C−H bonds of the inner ethyl side chains of the Eind groups and the polar B− H and Al−H bonds as well as the inherent electron-deficient nature of the boron and aluminum atoms. As shown in Figure 7, the molecular structure of the doubly hydrogen-bridged dimer of monoalumane, dialumane(6) 6, was confirmed by X-ray crystallography. The molecule has a C2symmetry in the crystal with the 2-fold axis passing through the two bridging hydrogen atoms (H2 and H3). The two terminal and two bridging hydrogen atoms on the aluminum atoms are located on the difference Fourier maps and isotropically refined.

3 (Figure 4). The structural parameters for 5 are summarized in Table 4. The discrete molecule shows a dimeric nature in the Table 4. Structural Parameters for 5−7 cmpd

Al−C/Å

Al···Li/Å

Al···Al/Å

5

2.0110(16) 2.0146(16)

3.102(3) 2.628(3) 2.652(3) 3.098(3)

4.2196(8)

6

1.970(2)

7

1.991(5) 1.992(5)

2.6253(13)

C−Al···Al−C/deg

1.96(11)

solid state, in which the two anionic [(Eind)AlH3]− units are bridged by two lithium ions with an attractive Al−H···Li+ interaction. Similar dimer formation was also reported for the bulky aryl-substituted lithium trihydroaluminate derivatives [Li(THF)2]2[(Mes*)AlH3]2 (Mes* = 2,4,6-tBu3C6H2),10c [Li2(OEt2)3][(Triph)AlH3]2 (Triph = 2,4,6-Ph3C6H2),10c and [Li] 2 [(Ar iPr 8 )AlH 3 ] 2 (Ar iPr8 = 2,6-(2,4,6- i Pr 3 C 6 H 2 ) 2 3,5-iPr2C6H).10f In the crystals of 5, the hydrogen atoms on the aluminum atoms were located on the difference Fourier maps and isotropically refined. The lithium atoms are bound to E


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Organometallics

without donor coordination.24 In the 1H NMR spectrum of 7 in C6D6, a broad Al−H signal appeared at 6.63 ppm. No detectable 27Al NMR signal was observed despite a prolonged measurement. Although the X-ray crystallographic analysis of 7 was not sufficient, with a high R value (R = 0.1789) due to the poor crystallinity, preliminary X-ray studies showed the monomeric structure of 7 in the solid state, as depicted in Figure 8a. The aluminum atom assumes a trigonal planar

Figure 7. Molecular structure of (Eind)HAl(μ-H)2AlH(Eind) (6). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the Al−H groups, are omitted for clarity. Selected distances (Å) and angle (deg): Al1−C1 = 1.970(2), Al1−H1 = 1.46(2), Al1−H2 = 1.693(17), Al1−H3 = 1.731(18), Al1···Al1′ = 2.6253(13), C1−Al1···Al1′−C1′ = 1.96(11).

The C−Al···Al′−C′ torsion angle was found to be 1.96(11)°, indicative of the highly coplanar arrangement of the six skeletal atoms (Al1, Al1′, C1, C1′, H1, and H1′). The Al···Al separation in 6 [2.6253(13) Å] is much longer than the B···B separation in 4 [1.789(4)−1.819(4) Å] and comparable to those previously reported for the bulky aryl-substituted dialumane(6) [2.630(2)−2.652(2) Å].10b,d,e,23 The Al−C bond length in 6 [1.970(2) Å] is shorter than those in 5 [2.0110(16) and 2.0146(16) Å], but in the range of those reported for the arylsubstituted dialumane(6) derivatives [1.956(3)−1.989(2) Å].10b,d,e,23 Reaction of (Eind)HAl(μ-H)2AlH(Eind) with Li. The twoelectron reduction of the diborane(6) C (Eind: 4) produced the corresponding diborane(6) dianion D, via the B−B σ-bond formation (Figure 1).13b,c These studies prompted us to investigate the course of the reduction of the dialumane(6) 6. Thus, we examined the reaction of 6 with lithium naphthalenide (LiNaph) in various solvents, but always resulting in a complex mixture; no aluminum product could be isolated from the reaction mixture. We also examined the reaction of 6 with lithium metal in a mixed solvent of Et2O and toluene, from which the sterically congested diarylalumane (Eind)2AlH (7), with the two bulky Eind groups, was obtained as a white powder (Scheme 5). The detailed mechanism for the

Figure 8. (a) Molecular structure of (Eind)2AlH (7) (top view). The thermal ellipsoids are shown at the 50% probability level. The hydrogen atoms, except for the Al−H group, and disordered carbon atoms of the ethyl group are omitted for clarity. Selected distances (Å) and angles (deg): Al1−C1 = 1.991(5), Al1−C29 = 1.992(5), C1− Al1−C29 = 128.4(2). (b) Space-filling model of (Eind)2AlH (7) (front view): light blue, aluminum; gray, carbon; white, hydrogen, except for Al−H group (orange).

geometry. The C−Al−C bond angle in 7 [128.4(2)°] is slightly smaller than the C−B−C bond angle in 2 [131.09(7)°] and is also smaller than those in (Mes*)2AlH [131.7(3)°]24a and (2,6Mes2C6H3)2AlH [137.62(5)°].24b The Al−C bond distances in 7 [Al1−C1 = 1.991(5) and Al1−C29 = 1.992(5) Å] are similar to those in 6 [1.970(2) Å], (Mes*)2AlH [1.976(6) and 2.007(6) Å],24a and (2,6-Mes2C6H3)2AlH [1.9936(13) and 1.9864(13) Å].24b Figure 8b shows a space-filling model of 7, indicating that the steric congestion around the Al−H bond in 7 is smaller than that around the B−H bond in 2 (Figure 3b), because of the larger atomic radius of aluminum relative to that of boron.22

Scheme 5

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CONCLUSIONS In this study, we have demonstrated the synthesis and structural characterization of discrete organoboron and organoaluminum hydrides bearing bulky Eind groups. The Eind-based lithium trihydroborate and trihydroaluminate have been successfully isolated as dimeric forms, which can be transformed into the corresponding diborane(6) and dialumane(6) species, respectively. We have also obtained the sterically congested diarylborane and diarylalumane monomers having two bulky Eind groups. A series of structurally well-defined hydrogen-

formation of 6 is unclear at the moment, but the facile disproportionation should occur in this reaction along with cleavage and recombination of the Al−C bond. The diarylalumane 7 was found to be highly air- and moisture-sensitive, even in the solid state, in sharp contrast to the air-stable diarylborane 2. We have also examined the reaction of 6 with (Eind)Li in Et2O, but no evidence was obtained for the formation of 7. The diarylalumane 7 is a rare example of a neutral monomeric tricoordinate dialkyl- or diaryl-aluminum hydride F


Article

Organometallics

2597 cm−1. Anal. Calcd for C56H91B: C, 86.77; H, 11.83. Found: C, 86.16; H, 11.91. Synthesis of [Li(thf)]2[(Eind)BH3]2 (3). To a solution of 1 (2.01 g, 4.67 mmol) in THF (50 mL) was dropwise added LiAlH4 (1.0 M in Et2O, 14.0 mL, 14.0 mmol) at −40 °C. The mixture was allowed to warm to room temperature, stirred for 2 h, and evaporated to dryness. To the residue was added benzene (50 mL × 2), and the resulting suspension was centrifuged (3000 rpm, 10 min), after which the supernatant was collected and evaporated to dryness. The residue was washed with hexane, and to the resulting grayish-white solid were added benzene and THF. The resulting suspension was allowed to stand at room temperature; then the supernatant was evaporated to dryness to give 3 as a white solid (729 mg, 0.77 mmol, 33%): mp 152−155 °C (dec) (in a vacuum-sealed tube); 1H NMR (C6D6) δ 1.08 (t, J = 7.6 Hz, 12 H, CH3), 1.13 (t, J = 7.3 Hz, 12 H, CH3), 1.27−1.32 (m, 8 H, THF), 1.78−1.96 (m, 8 H, CH2), 2.05 (s, 4 H, CH2), 2.30− 2.47 (m, 8 H, CH2), 3.54−3.59 (m, 8 H, THF), 4.23 (br s, 3 H, BH), 6.78 (s, 1 H, ArH); 7Li NMR (C6D6) δ 2.03; 11B NMR (C6D6) δ −32.3 (q, 1J(1H−11B) = 74.7 Hz); 13C NMR (C6D6) δ 9.8, 10.4, 25.5, 32.2, 33.9, 44.1, 47.2, 52.9, 68.8, 115.7, 146.9, 151.3 (one peak of the aromatic carbon, ipso position to the boron, was not observed); IR (THF), ν(B−H) = 2237 cm−1. Synthesis of [Li(OEt2)]2[(Eind)AlH3]2 (5). To a solution of (Eind)Br (2.31 g, 5.00 mmol) in Et2O (140 mL) was dropwise added t BuLi (1.61 M in pentane, 6.80 mL, 10.9 mmol) at −78 °C. The mixture was stirred and allowed to warm to 0 °C. To the resulting solution was dropwise added LiAlH4 (1.0 M in Et2O, 5.50 mL, 5.50 mmol), and the mixture was stirred for 5 h at room temperature. Then the mixture was concentrated in vacuo. To the residue was added benzene (50 mL × 3), and the resulting suspension was centrifuged (3000 rpm, 10 min), after which the supernatant was collected and evaporated to dryness. The residue was washed with hexane, and the resulting white solid was recrystallized from hexane and Et2O to give 5 as colorless crystals (1.53 g, 1.55 mmol, 62%): mp 125−130 °C (dec) (in a vacuum-sealed tube); 1H NMR (C6D6) δ 0.96 (t, J = 7.3 Hz, 12 H, CH3), 1.03 (t, J = 7.3 Hz, 12 H, CH3), 1.03 (t, J = 7.3 Hz, 6 H, Et2O), 1.70−1.80 (m, 8 H, CH2), 1.92 (s, 4 H, CH2), 2.26 (q, J = 7.3 Hz, 8 H, CH2), 3.16 (q, J = 7.3 Hz, 4 H, Et2O), 3.75 (br s, 3 H, AlH), 6.86 (s, 1 H, ArH); 7Li NMR (C6D6) δ 2.55; 13C NMR (C6D6) δ 9.6, 10.0, 14.6, 33.6, 33.8, 42.9, 47.9, 52.9, 66.7, 119.9, 147.0, 158.4 (one peak of the aromatic carbon, ipso position to the aluminum, was not observed); 27Al NMR (C6D6) δ 48.3 (br s, W1/2 = 7100 Hz); IR (benzene), ν(Al−H) = 1693 cm−1. Anal. Calcd for C64H116Al2Li2O2: C, 78.00; H, 11.86. Found: C, 77.91; H, 11.86. Synthesis of (Eind)HAl(μ-H)2AlH(Eind) (6).23 To a solution of 5 (1.01 g, 1.02 mmol) in toluene (80 mL) was added Me3SiCl (270 μL, 232 mg, 2.13 mmol) at room temperature, and the solution was stirred for 30 min and evaporated to dryness. To the residue was added toluene (100 mL), and the resulting suspension was filtered through a glass filter. The filtrate was evaporated to dryness, and the residue was washed with hexane (15 mL). The remaining white solid was recrystallized from toluene to give 6 as colorless crystals (792 mg, 0.96 mmol, 94%): mp 212−214 °C (dec) (in a vacuum-sealed tube); 1H NMR (C6D6) δ 0.92 (t, J = 7.4 Hz, 24 H, CH3), 0.99 (t, J = 7.3 Hz, 24 H, CH3), 1.59−1.79 (m, 16 H, CH2), 1.83 (s, 8 H, CH2), 1.92−2.06 (m, 8 H, CH2), 2.09−2.23 (m, 8 H, CH2), 4.84 (br s, 2 H, AlH), 5.10 (br s, 2 H, AlH), 6.92 (s, 2 H, ArH); 13C NMR(C6D6) δ 9.5, 9.8, 33.5, 34.8, 41.7, 48.4, 52.4, 121.6, 147.5, 158.5 (one peak of the aromatic carbon, ipso position to the aluminum, was not observed.); 27Al NMR (C6D6) δ 71.2 (W1/2 = 5300 Hz). Anal. Calcd for C56H94Al2: C, 81.89; H, 11.54. Found: C, 80.47; H, 11.22. Reaction of (Eind)HAl(μ-H)2AlH(Eind) with Li. To a suspension of 6 (292 mg, 0.356 mmol) in toluene (10 mL) and Et2O (5 mL) was added lithium metal (25.1 mg, 3.62 mmol) at room temperature, and the solution was stirred for 4 d and evaporated to dryness. To the residue was added hexamethyldisiloxane (HMDSO), and the resulting suspension was filtered through a glass filter to remove (Eind)H, which is soluble in HMDSO. To the remaining grayish-white solid was added hexane, and the resulting suspension was filtered through a glass filter to remove any insoluble materials. The filtrate was concentrated

substituted organoboron and organoaluminum compounds are now available. Further studies on the reactivity of the boron and aluminum hydrides described here and other group 13 compounds incorporating bulky Rind groups are now in progress.

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EXPERIMENTAL SECTION

General Considerations. All manipulations of the air- and/or moisture-sensitive compounds were performed either using standard Schlenk-line techniques or in a glovebox under an inert atmosphere of argon. Anhydrous hexane, benzene, toluene, diethyl ether (Et2O), and tetrahydrofuran were dried by passage through columns of activated alumina and a supported copper catalyst supplied by Hansen & Co., Ltd. Deuterated benzene (benzene-d6, C6D6) was dried and degassed over a potassium mirror in vacuo prior to use. 4-Bromo-1,1,3,3,5,5,7,7octaethyl-s-hydrindacene, (Eind)Br, was synthesized according to literature methods.11 All other chemicals and gases were used as received. Nuclear magnetic resonance measurements were carried out using a JEOL ECS-400 spectrometer (399.8 MHz for 1H, 155.4 MHz for 7Li, 128.3 MHz for 11B, 100.5 MHz for 13C, 376.2 MHz for 19F, and 104.2 MHz for 27Al). Chemical shifts (δ) are given by definition as dimensionless numbers and determined with respect to the residual solvent for 1H (residual C6D5H in C6D6: 1H(δ) = 7.15 and residual C4D7HO in C4D8O: 1H(δ) = 1.73) and for 13C (C6D6: 13C(δ) = 128.0 and C4D8O: 13C(δ) = 25.3). The absolute values of the coupling constants are given in hertz (Hz) regardless of their signs. Multiplicities are abbreviated as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br). The infrared spectroscopy was obtained on a JASCO FT/IR-4100 spectrometer. Elemental analyses (C and H) were performed at the Materials Characterization Support Unit of the RIKEN Center for Emergent Matter Science (CEMS). Melting points (mp) were determined by a Stanford Research Systems OptiMelt instrument. We were unable to obtain a satisfactory elemental analysis for several compounds. For example, diffraction-quality single crystals afforded a rather lower-than-expected carbon analysis, even when combustion aids were used. On the basis of the NMR spectra (Figures S1−S13) and the X-ray crystal structure, we believe that these compounds are authentic and analytically pure but that its lability on heating or incomplete combustion is responsible for the disappointing elemental analysis. Reaction of (Eind)Li with BF3·OEt2.13a To a suspension of (Eind)Br (9.99 g, 21.6 mmol) in Et2O (200 mL) was dropwise added t BuLi (1.61 M in pentane, 30.0 mL, 48.3 mmol) at −78 °C. The mixture was stirred and allowed to warm to 0 °C. To the resulting solution was dropwise added BF3·OEt2 (14.0 mL, 111 mmol), and the mixture was stirred for 2 h at room temperature. Then the mixture was concentrated in vacuo. To the residue was added toluene (150 mL), and the resulting suspension was filtered through Celite (Wako Pure Chemicals Industries, Ltd.). The filtrate was concentrated in vacuo and subjected to Kugelrohr distillation under reduced pressure (bp 120− 180 °C, 0.1 mmHg). The resulting white solid was recrystallized from hexane to give (Eind)BF2 (1) as colorless crystals (5.59 g, 13.0 mmol, 60%). The residual material after the Kugelrohr distillation was washed with hexane to afford a trace amount of (Eind)2BH (2) as a white powder. 1: Analytical data were identical to the reported values.13a 1H NMR (C6D6) δ 0.81 (t, J = 7.6 Hz, 12 H, CH3), 0.83 (t, J = 7.6 Hz, 12 H, CH3), 1.50−1.81 (m, 16 H, CH2), 1.73 (s, 4 H, CH2), 6.88 (s, 1 H, ArH); 11B NMR (C6D6) δ 27.7 (W1/2 = 780 Hz); 13C NMR (C6D6) δ 9.4, 9.6, 33.3, 34.2, 41.6, 48.4, 51.0, 121.9, 148.5, 151.7 (one peak of the aromatic carbon, ipso position to the boron, was not observed.); 19 F NMR (C6D6) δ −56.4. 2: mp 177−178 °C (in a vacuum-sealed tube); 1H NMR (C6D6) δ 0.79 (t, J = 7.3 Hz, 24 H, CH3), 0.91 (t, J = 7.3 Hz, 24 H, CH3), 1.50− 1.82 (m, 24 H, CH2), 1.77 (s, 8 H, CH2), 1.92−2.03 (m, 8 H, CH2), 6.82 (s, 2 H, ArH), 8.36 (br s, 1 H, BH); 13C NMR (C6D6) δ 9.3, 10.2, 33.2, 36.1, 43.3, 48.2, 53.4, 122.6, 143.7, 148.7, 154.1; 11B NMR (C6D6) δ 65.4 (br.s, W1/2 = 2600 Hz); IR (KBr, pellet), ν(B−H) = G


Article

Organometallics

g cm−3, μ(Mo Kα) = 0.056 mm−1, 63 039 reflections collected, 23 311 unique reflections, and 1086 refined parameters. The final R(F) value was 0.0788 [I > 2σ(I)]. The final wR(F2) value was 0.2158 (all data). The goodness-of-fit on F2 was 1.025. Crystal data for 5: C64H116Al2Li2O2, M = 985.46, crystal size 0.39 × 0.33 × 0.33 mm, monoclinic, space group P21/c (#14), a = 20.016(2) Å, b = 16.3120(17) Å, c = 20.747(2) Å, β = 105.0125(15)°, V = 6542.9(12) Å3, Z = 4, Dx = 1.000 g cm−3, μ(Mo Kα) = 0.082 mm−1, 105 209 reflections collected, 14 873 unique reflections, and 675 refined parameters. The final R(F) value was 0.0612 [I > 2σ(I)]. The final wR(F2) value was 0.1410 (all data). The goodness-of-fit on F2 was 1.147. Crystal data for 6: C56H94Al2, M = 821.32, crystal size 0.18 × 0.12 × 0.06 mm, monoclinic, space group C2/c (#15), a = 17.3426(7) Å, b = 22.7052(14) Å, c = 13.2870(7) Å, β = 105.341(5)°, V = 5045.6(5) Å3, Z = 4, Dx = 1.081 g cm−3, μ(Mo Kα) = 0.092 mm−1, 41 122 reflections collected, 5807 unique reflections, and 278 refined parameters. The final R(F) value was 0.0655 [I > 2σ(I)]. The final wR(F2) value was 0.1695 (all data). The goodness-of-fit on F2 was 1.008. Crystal data for 7: C56H91Al, M = 791.26, crystal size 0.18 × 0.13 × 0.08 mm, monoclinic, space group C2/c (#15), a = 32.777(12) Å, b = 18.557(6) Å, c = 16.936(6) Å, β = 104.135(5)°, V = 9989(6) Å3, Z = 8, Dx = 1.052 g cm−3, μ(Mo Kα) = 0.074 mm−1, 80 057 reflections collected, 11 447 unique reflections, and 542 refined parameters. The final R(F) value was 0.1789 [I > 2σ(I)]. The final wR(F2) value was 0.2429 (all data). The goodness-of-fit on F2 was 1.333.

and allowed to stand at −30 °C to give 7 as a white powder (145 mg, 0.183 mmol, 52%): 1H NMR (C6D6) δ 0.81 (t, J = 7.3 Hz, 24 H, CH3), 0.89 (t, J = 7.3 Hz, 24 H, CH3), 1.58−1.73 (m, 16 H, CH2), 1.76 (s, 8 H, CH2), 1.77−1.85 (m, 8 H, CH2), 1.99−2.10 (m, 8 H, CH2), 6.63 (br s, 1 H, AlH), 6.78 (s, 2 H, ArH); 13C NMR(C6D6) δ 9.4, 10.0, 33.6, 37.1, 42.6, 48.4, 52.7, 120.7, 147.5, 157.8 (one peak of the aromatic carbon, ipso position to the aluminum, was not observed); IR (benzene), ν(Al−H) = 1847 cm−1. X-ray Crystallography. Single crystals suitable for X-ray diffraction were obtained from hexane for 1, 2, 4 (form I), and 7, from benzene for 3, from toluene for 4 (form II) and 6, and from a mixture of hexane and Et2O for 5 as colorless blocks or prisms. The single crystals were immersed in oil (Immersion Oil, type B: code 1248, Cargille Laboratories, Inc.) and mounted on a Rigaku XtaLAB P200 with a PILATUS200 K detector for 1 and 6, Rigaku AFC-8 diffractometer with a Saturn70 CCD detector for 2 and 3, and a Rigaku AFC-10 diffractometer with a Saturn724+ CCD detector for 4 (forms I and II), 5, and 7. The diffraction data were collected using Mo Kα radiation (λ = 0.710 73 Å), which was monochromated and focused by a curved graphite monochromator. The specimens were cooled at 90 K for 2 and 3, at 100 K for 1, 4 (form I), 5, 6, and 7, and at 273 K for 4 (form II) in a cold nitrogen stream during the measurements. The integration and scaling of the diffraction data were carried out using the programs of CrystalClear.25 Lorentz−polarization and absorption corrections were also performed. The structures were solved by a direct method with the programs of SIR9226 for 4 (form II), SHELXS-9727 for 2 and 5, SIR200428 for 3 and 7, and SIR201129 for 1, 4 (form I), and 6 and refined on F2 by a full-matrix least-squares method using the programs of SHELXL-9727 and SHELEXL-2014.30 Anisotropic atomic displacement parameters were applied to all the non-hydrogen atoms. The hydrogen atoms, except for the B−H and Al−H groups, were placed at the calculated positions and refined by applying riding models. The B−H and Al−H hydrogen atoms were located on difference Fourier maps and isotropically refined. Crystal data for 1: C28H45BF2, M = 430.47, crystal size 0.20 × 0.10 × 0.05 mm, monoclinic, space group C2/c (#15), a = 20.1669(6) Å, b = 8.6860(2) Å, c = 15.4565(5) Å, β = 111.566(3)°, V = 2517.97(14) Å3, Z = 4, Dx = 1.135 g cm−3, μ(Mo Kα) = 0.073 mm−1, 25 806 reflections collected, 2892 unique reflections, and 146 refined parameters. The final R(F) value was 0.0580 [I > 2σ(I)]. The final wR(F2) value was 0.1551 (all data). The goodness-of-fit on F2 was 1.028. Crystal data for 2: C56H91B, M = 775.10, crystal size 0.35 × 0.35 × 0.20 mm, monoclinic, space group P21/n (#14), a = 11.9190(11) Å, b = 19.2340(17) Å, c = 21.530(2) Å, β = 100.0436(13)°, V = 4860.2(8) Å3, Z = 4, Dx = 1.059 g cm−3, μ(Mo Kα) = 0.058 mm−1, 122 733 reflections collected, 20 965 unique reflections, and 552 refined parameters. The final R(F) value was 0.0573 [I > 2σ(I)]. The final wR(F2) value was 0.1489 (all data). The goodness-of-fit on F2 was 1.087. Crystal data for 3: C64H112B2Li2O2·C6H6, M = 1027.14, crystal size 0.37 × 0.19 × 0.12 mm, monoclinic, space group C2/m (#12), a = 17.4943(5) Å, b = 17.8447(7) Å, c = 10.9847(5) Å, β = 106.5391(19)°, V = 3287.3(2) Å3, Z = 2, Dx = 1.038 g cm−3, μ(Mo Kα) = 0.058 mm−1, 37 065 reflections collected, 5255 unique reflections, and 222 refined parameters. The final R(F) value was 0.0854 [I > 2σ(I)]. The final wR(F2) value was 0.2339 (all data). The goodness-of-fit on F2 was 1.089. Crystal data for 4 (form I): C56H94B2, M = 788.93, crystal size 0.18 × 0.16 × 0.09 mm, triclinic, space group P1̅ (#2), a = 12.396(2) Å, b = 23.243(4) Å, c = 26.501(5) Å, α = 97.983(4)°, β = 90.176(3)°, γ = 94.192(4)°, V = 7540(2) Å3, Z = 6, Dx = 1.042 g cm−3, μ(Mo Kα) = 0.057 mm−1, 112 149 reflections collected, 26 553 unique reflections, and 1663 refined parameters. The final R(F) value was 0.0777 [I > 2σ(I)]. The final wR(F2) value was 0.2019 (all data). The goodness-offit on F2 was 1.085. Crystal data for 4 (form II): C56H94B2, M = 788.98, crystal size 0.15 × 0.15 × 0.05 mm, triclinic, space group P1̅ (#2), a = 12.4926(8) Å, b = 18.6202(12) Å, c = 23.2945(14) Å, α = 104.261(2)°, β = 94.6326(19)°, γ = 101.4262(19)°, V = 5099.5(5) Å3, Z = 4, Dx = 1.028

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00633. NMR spectra of the compounds 2, 3, 5, and 7 (PDF) Crystallographic details for 1, 2, 3, 4 (form I), 4 (form II) 5, 6, and 7 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Crystallographic data for 1 (CCDC 1490935), 2 (CCDC 1490956), 3 (CCDC 1490957), 4 (form I) (CCDC 1490961), 4 (form II) (CCDC 1490962), 5 (CCDC 1490963), 6 (CCDC 1490969), and 7 (CCDC 1490964) can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.ac.uk/data-request.

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ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules (No. 2408)” (JSPS KAKENHI Grant Nos. JP20109003 for T.M. and JP15H00964 for D.H.) and Scientific Research (B) (JP15H03788). This study was also partially supported by a MEXT-Supported Program for the Strategic Research Foundation at Private Universities 2014−2018 subsidy from MEXT and Kindai University. We thank Dr. K. Nagata, Dr. T. Agou, Prof. T. Sasamori, and Prof. N. Tokitoh for their valuable discussions. We are grateful to the Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science (CEMS), for the elemental analyses. H


Article

Organometallics

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