Article pubs.acs.org/Organometallics
Reactivity of Scandium Terminal Imido Complex toward Boranes: C(sp3)−H Bond Borylation and B−O Bond Cleavage Jiaxiang Chu, Chen Wang, Li Xiang, Xuebing Leng, and Yaofeng Chen* State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, PR China S Supporting Information *
ABSTRACT: Scandium terminal imido complex [(NNNN)ScNDIPP] (2; NNNN = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMeCH2CH2NMe2)]−, DIPP = 2,6-iPr2C6H3) reacts with 9borabicyclononane (9-BBN) to give scandium borohydride [(NNNN(B)H)Sc(N(H)DIPP)] (3; NNNN(B)H = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMeCH2CH2N(Me)CH2(BBN)]2−), and C(sp3)−H bond borylation of the NNNN ligand occurs during this reaction. In contrast, the reaction between complex 2 and catecholborane (CatBH) gives scandium catecholate [(NNNN)Sc(Cat)] (4), and B−O bond cleavage happens during this reaction. Both 3 and 4 have been well-characterized including the single-crystal X-ray diffraction analysis. Reaction of 2 with bis(catecholato)diboron (CatB−BCat) also gives a B−O bond cleavage product.
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INTRODUCTION Terminal imido complexes of early transition metals have been of research interest in the past three decades.1 Rich reactivity and applications in both stoichiometric and catalytic reactions have been revealed during the research on such complexes. However, terminal imido complexes of rare-earth metals remain less studied compared with other transition metals.2 Due to the highly polarized nature of the LnN double bond, once the terminal species is formed, it can easily assemble into more stable bimetallic or multimetallic species3 or undergo C−H bond activation to give the amides.4 Recently, we isolated and structurally characterized the first rare-earth metal terminal imido complex [(NNN)ScNDIPP(DMAP)] (1; NNN = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMe2)]−, DIPP = 2,6-iPr2C6H3, DMAP = 4-(dimethylamino)pyridine)5a and then a DMAP-free scandium terminal imido complex [(NNNN)ScNDIPP] (2; NNNN = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMeCH2CH2NMe2)]−) (Scheme 1).5b Later, Cui’s and Piers’ groups reported several other scandium terminal imido complexes,6a−c and Anwander’s group extended
the scope of rare-earth metal terminal imido species to yttrium and lutetium.6d Very recently, Schelter and co-workers reported the first imido complexes of tetravalent rare-earth metal, alkali metal capped cerium(IV) imido complexes and a cerium(IV) terminal imido complex.7 Scandium terminal imido complexes 1 and 2 showed intriguing reactivity toward elemental selenium,5b unsaturated organic substrates,8a metal halides,8b silane,8c and a variety of C−H bonds.8d−f The observed reactivity is in line with the highly polarized nature of the ScN bond. Herein, we report reactions of complex 2 with two different boranes, 9borabicyclononane (9-BBN) and catecholborane (CatBH); the reactions give scandium borohydride [(NNNN(B)H)Sc(N(H)(DIPP))] (3) via C(sp3)−H bond borylation and scandium catecholate (NNNN)Sc(Cat)] (4) via B−O bond cleavage, respectively. To study the mechanism of formation of 4 from 2 and CatBH, reaction between 2 and bis(catecholato)diboron (CatB−BCat) was also performed.
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RESULTS AND DISSCUSION Reaction of 2 with 9-BBN. Reaction of complex 2 with 1 equiv of 9-BBN in toluene at 50 °C gave a new complex, 3, in nearly quantitative yield. Complex 3 was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography, confirming that 3 is a scandium borohydride as shown in Scheme 2. Therefore, C(sp3)−H bond borylation of the tetradentate ligand occurs, which is different from the reaction
Scheme 1. Scandium Terminal Imido Complexes 1 and 2
Special Issue: Organometallic Actinide and Lanthanide Chemistry Received: June 15, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.7b00452 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
dimer (1.27(2) Å)11 but is shorter than that in molecule 3a (1.34(3) Å). In the 1H NMR spectrum of 3 (Figure S1), the two broad signals at 0.71 and −1.04 ppm with the integration of 1H can be assigned as the Sc−H−B and NCH2B signals, respectively; another NCH2B signal can be found at 1.09 ppm as a multiplet. The slightly deviated chemical shift of one of the NCH2B signals from another may be attributed to the constrained geometry formed by the NNNN(B)H ligand; for comparison, Mindiola and co-workers reported a scandium triethylborohydride complex with CHB signal shown at 0.51 ppm.9c The 11B{1H} NMR chemical shift of 3 (−9.4 ppm) clearly indicates a four-coordinate borane. The different Sc−H and B−H distances in molecules 3a and 3b raised the question of whether two extreme resonance structures of a metal borohydride can be observed at low temperature: one for a metal hydride with weak interaction with a neutral borane and another for a metal cation with weak interaction with a R3BH− borohydride. Therefore, variable-temperature NMR experiment of 3 in toluene-d8 was performed (Figure S6). Lowering the temperature to −90 °C only leads to a resonance very similar to that at room temperature, and the resonance decoalescence does not occur, excluding the possibility of observing these resonance structures at such a low temperature. C−H bond borylation provides an economic pathway to organoboranes as versatile regents in organic synthesis and has long been the regime of middle to late transition metals.12 C− H bond borylation of alkanes turned out to be more challenging compared with that of arenes due to the strong C(sp3)−H bond.13 Besides, little is known about the C−H bond borylation mediated by rare-earth metals.14a A rare example was reported by Evans and co-workers from the reaction of (C5Me5)2Y(η3-C3H5) with 9-BBN, and boranesubstituted allyl complex [(C5Me5)2Y(η3-C3H4(BC8H14))] was obtained.9d Arnold and co-workers reported the C−H borylation of arenes mediated by an actinide complex [U(ODtbp)3] (Dtbp = 2,6-tBu2C6H3).14b To study the mechanism for the formation of complex 3, the reaction between complex 2 and 1 equiv of 9-BBN was monitored by 1H NMR, but no intermediate was observed (Figure S10). In another attempt to gain insight into the mechanism of this reaction, isotopic labeling experiment was carried out using 2 and 9-D-BBN, which cleanly produced isotopomer [(NNNN(B)D)Sc(N(H)DIPP)] (3-D). The 1H NMR spectrum of 3-D (Figure S3) resembles that of 3 with the same integration at 6.31 ppm for the hydrogen of the amido ligand [N(H)DIPP]− but with no resonance at 0.71 ppm, which was previously observed for Sc−H−B in 3. In addition, the 2H NMR spectroscopy for 3-D showed a signal at 0.69 ppm (Figure S4). Therefore, the hydrogen of [N(H)DIPP]− in 3 is from the −NMe2 group of the tetradentate ligand in 2. On the basis of these results, we proposed the mechanism for the formation of complex 3 as follows (Scheme 3): First, C−H bond activation occurs at one of the methyl group (−CH2NMe2) of the
Scheme 2. Reactions of Complex 2 with 9-BBN and CatBH
between DMAP-containing scandium terminal imido complex 1 and 9-BBN, where B−H bond addition to ScN double bond was observed.8f The NNNN tetradentate monoanionic ligand in 2 becomes a NNNN(B)H pentadentate dianionic ligand in 3. Although rare-earth metal borohydrides are wellknown,9 complex 3 is a rare example having the borohydride as a part of the dianionic ligand. There are two crystallographically independent molecules in the unit cell of 3 which are enantiomers, and the position of the bridged hydride is determined from electron-density differential Fourier map (Figure 1). In both molecules, the scandium ions adopt
Figure 1. Molecular structures of 3 with ellipsoids set at the 30% probability level. Isopropyl groups at the DIPP substituents and hydrogen atoms (except the bridged hydride and anilido hydrogen) have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Sc1−H1, 2.01(3); Sc2−H2, 2.11(2); B1−H1, 1.34(3); B2−H2, 1.27(2); B1−C35, 1.644(4); B2−C35A, 1.646(3); N4−C35−B1, 113.61(19); N5−C35A−B2, 113.64(17); N4−Sc1−N5, 148.18(8); N9−Sc2−N10, 148.70(8).
pseudo-octahedron coordination geometry. Three nitrogen atoms (N1, N2, and N3 or N6, N7, and N8) and a hydrogen atom (H1 or H2) form the equatorial plane. In molecule 3a, the nitrogen atom of the anilido ligand (N5) lies above this equatorial plane (from N1, N2, and N3 to H1, counterclockwise), and the nitrogen atom of the pentadentate ligand (N4) lies under this plane. In molecule 3b, the opposite situations were observed for the two nitrogen atom positions (N9 and N10) relative to the equatorial plane. The bond lengths and bond angles are very similar in both independent molecules except for those of Sc−H and B−H bonds, which are different. The Sc−H distance in molecule 3a (2.01(3) Å) is shorter than that in 3b (2.11(2) Å), but both lie in the range of reported Sc−H bond lengths (1.87−2.29 Å).10 The B−H distance in molecule 3b (1.27(2) Å) is same as that in 9-BBN
Scheme 3. Proposed Mechanism for the Formation of Complex 3
B
DOI: 10.1021/acs.organomet.7b00452 Organometallics XXXX, XXX, XXX−XXX
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observed only after 1 h. After 13 h, the 1H NMR spectrum showed resonances assignable to 2, 4a, and 4 in a 1:10:5 molar ratio. Signals for complex 2 and 4a disappeared after the reaction mixture was heated to 75 °C for 18.5 h. Attempts to isolate complex 4a failed due to its instability. On the basis of these observations, a mechanism due to successive B−O cleavage was proposed (Scheme 4). The first B−O cleavage by
tetradentate ligand generates a scandium alkyl anilido intermediate; then, this intermediate undergoes a nucleophilic attack on the boron atom of 9-BBN to give borohydride 3. However, our group previously found that the DMAP in scandium terminal imido complex [(NNN)ScNDIPP(DMAP)] (1) can be removed by 9-BBN to generate a DMAP → BBN adduct and a coordinatively unsaturated scandium terminal imido intermediate, which is extremely reactive.8f To check the possibility of coordination between 9BBN and the terminal amino group (−NMe2) of the tetradentate ligand in 2, the VT 1H NMR spectra of a toluene-d8 solution of complex 2 and 9-BBN from −60 to 25 °C were recorded (Figure S7), and no coordination between 9BBN and terminal amino group was observed. Although this cannot firmly exclude the participation of the −NMe2 → BBN coordination in the formation of complex 3b, it does indicate that the direct C−H bond activation by ScN bond is a more plausible mechanism. Reaction of 2 with CatBH. To further study the reaction between scandium terminal imido complex and borane, CatBH, which bears oxygen substitutes, was chosen as the reactant. From the reaction of 2 with CatBH, a scandium catecholate complex [(NNNN)Sc(Cat)] (4) was isolated in 83% yield (Scheme 2). Complex 4 was characterized by 1H and 13C{1H} NMR spectroscopy, elemental analysis, and single crystal X-ray diffraction. B−O bond cleavage of catecholborane has been reported by Eisen and co-workers by using early transition metal complexes (C5Me5)2M(CH3)2 (M = Th,15a U,15a and Zr).15b Our group reported B−O bond cleavage of pinacolborane and catecholborane by a bis-scandium-bridged phosphinidene complex.15c There are two crystallographically independent molecules in the unit cell of 4, and one is shown in Figure 2.
Scheme 4. Proposed Mechanism for the Formation of Complex 4
the ScN bond in complex 2 can generate intermediate 4a, while the second B−O cleavage, that is, σ-bond metathesis between Sc−N bond and B−O bond, results in scandium catecholate complex 4. It is noteworthy that the bond dissociation energy of B(H)−O bond (890 kJ/mol) is much higher than that of B(O)−H bond (416 kJ/mol).16 The preferred cleavage of B−O bond instead of B−H bond during this reaction is due to the strong oxophilicity of the scandium ion. To verify of the mechanism proposed in Scheme 4, we attempted to isolate proposed iminoborane [HBNDIPP]. To start with, the 1H NMR spectrum at the end of the in situ reaction was compared against that of isolated complex 4 (Figure S12). The 1H NMR spectrum from the in situ reaction showed broad signals at 3.40 and 1.20 ppm which were not present in that of the isolated 4. These resonances are tentatively assigned to the isopropyl protons of the iminoborane. Moreover, the 11B NMR of this reaction solution showed a broad signal at 28 ppm. However, our efforts to isolate this compound were not successful, possibly due to the extreme reactivity of iminoboranes.17a In fact, monomeric iminoboranes are known to be stabilized only by using sterically demanding ligands17b−d or in the environment of transition metals17e or carbenes.17f,g Very recently, Braunschweig and coworkers reported a MB/CN metathesis pathway to generate iminoboranes.17d Our results shows that reactions between early transition metal−main group element multiple bonding species and catecholborane could be an “umpolunged” metathesis pathway to generate this kind of species. Reaction of 2 with CatB−BCat. Another way to test the mechanism proposed in Scheme 4 is the reaction between 2 and a catecholborane derivative, with the commercially available CatB−BCat being an ideal substrate. Thus, the reaction between complex 2 and CatB−BCat at a 1:1 ratio was monitored in a NMR tube. Two products were generated in 1:1 ratio based on 1H NMR spectra after the reaction solution was allowed to stand at room temperature for 12 h; one product can be assigned as 4 with another unknown complex 5 (Scheme 5 and Figure S13). We initially assigned 5 as the reaction intermediate generated from the first B−O bond cleavage of CatB−BCat by 2, which is akin to the generation of intermediate 4a as shown in Scheme 4. However, heating the reaction solution to 75 °C or lowering the amount of CatB− BCat to 0.5 equiv could not alter the distribution of the
Figure 2. Molecular structure of 4 with ellipsoids set at the 30% probability level. Isopropyl groups at the DIPP substituents and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Sc1−O1, 2.0251(13); Sc1−O2, 2.0141(13); C23−O1, 1.345(2); C24−O2, 1.352(2); C23−C24, 1.416(3); O1− Sc−O2, 79.91(6).
In 4, the scandium ion adopts a pseudo-octahedron coordination geometry and all the four nitrogen atoms in the tetradentate ligand (NNNN) are coordinated to scandium ion. Three nitrogen atoms (N1, N2, and N3) and one oxygen atom (O1) form the equatorial plane, while the remaining nitrogen atom (N4) of the tetradentate ligand and another oxygen atom (O2) occupy the apical positions. To probe the mechanism for the formation of complex 4, the reaction between complex 2 and 1 equiv of CatBH was monitored by 1H NMR spectroscopy (Figure S11). Upon standing at room temperature for 15 min, the 1H NMR spectrum of the reaction mixture showed resonances assignable to intermediate 4a, while the signals for complex 4 were C
DOI: 10.1021/acs.organomet.7b00452 Organometallics XXXX, XXX, XXX−XXX
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mixture was allowed to stand at 50 °C for 1 day. It was cooled to room temperature, filtered, and evaporated to provide a yellow powder. Recrystallization in hexane afforded complex 3 as a yellow powder (116 mg, 96% yield). 1H NMR (400 MHz, C6D6, 25 °C): δ = 7.20− 7.06 (m, 4H; ArH), 7.02 (t, 3JH−H = 8.4 Hz, 1H; ArH), 6.92 (t, 3JH−H = 8.0 Hz, 1H; ArH), 6.32 (br, 1H; HNAr), 4.80 (s, 1H; MeC(N)CH), 3.71 (m, 1H; NCH2), 3.61 (sept, 3JH−H = 7.6 Hz, 3H; ArCHMe2), 2.95 (sept, 3JH−H = 7.6 Hz, 1H; ArCHMe2), 2.85 (m, 2H; NCH2), 2.65 (m, 1H; NCH2), 2.45−2.15 (m, 9H; NCH2 or CH and CH2 of 9-BBN), 2.31 (s, 3H; NMe), 2.10−1.95 (m, 3H; NCH2 or CH2 of 9-BBN), 1.99 (s, 3H; NMe), 1.92−1.82 (m, 2H; CH2 of 9-BBN), 1.70 (s, 3H; CMe), 1.67 (s, 3H; CMe), 1.57−1.52 (m, 2H; CH2 of 9-BBN), 1.48 (d, 3JH−H = 6.8 Hz, 3H; ArCHMe2), 1.45−1.40 (br, 3H; ArCHMe2), 1.37 (d, 3 JH−H = 6.8 Hz, 6H; ArCHMe2), 1.24 (d, 3JH−H = 6.8 Hz, 6H; ArCHMe2), 1.09 (m, 1H; NCH2B), 1.03 (d, 3JH−H = 6.8 Hz, 3H; ArCHMe2), 0.96 (d, 3JH−H = 6.8 Hz, 3H; ArCHMe2), 0.71 (br, 1H; ScH-B), −1.04 (br, 1H; NCH2B), 2.11 (s, Me of toluene). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ = 170.0, 166.1 (imine C), 150.4 (oPhC of DIPP), 144.7 (i-PhC of DIPP), 144.2 (o-PhC of DIPP), 142.9 (o-PhC of DIPP), 126.9 (m-PhC of DIPP), 125.9 (i-PhC of DIPP), 125.2 (p-PhC of DIPP), 123.9 (m-PhC of DIPP), 117.5 (p-PhC of DIPP), 97.5 (MeC(N)CH), 55.5, 55.3, 54.9 (NCH2), 51.2 (NCH2B), 49.1 (NCH2), 47.0, 46.4 (NMe), 36.6, 36.5, 32.8 (CH2 of BBN), 31.7 (NCH2), 28.4, 28.0 (ArCHMe2), 27.7 (ArCHMe2), 27.1 (CMe), 26.8, 26.0, 25.2, 25.1 (ArCHMe2), 23.8 (CMe), 22.9 (CH of BBN), 22.6 (CH of BBN). 137.9, 129.3, 128.6, 125.7, 21.4 (toluene). 11B{1H} NMR (192 MHz, C6D6, 25 °C): δ = −9.4. Elemental analysis calcd (%) for C44H73BN5Sc·0.5toluene: C 73.72, H 10.03, N 9.05; found: C 73.40, H 10.23, N 8.61. Synthesis of Complex 3-D. Following the procedure described for complex 3, reaction of complex 2 with 9-D-BBN afforded complex 3-D as a yellow powder. The 1H NMR spectrum of 3-D (400 MHz, C6D6, 25 °C) is nearly the same as that of 3 except no resonance at 0.71 ppm. 2H NMR (400 MHz, C6H6, 25 °C): δ = 0.69 (Sc−D−B). Synthesis of Complex 4. A solution of catecholborane in toluene (20 mg, 0.167 mmol, 1 mL of toluene) was added to a solution of complex 2 in toluene (100 mg, 0.165 mmol, 5 mL of toluene). The reaction solution was allowed to stand at room temperature for 1 day and then at 75 °C for 5 days. It was cooled to room temperature and evaporated to provide a yellow solid. After washing with hexane (2 × 2 mL), complex 3 was obtained as a yellow powder (75 mg, 83% yield). 1 H NMR (400 MHz, C6D6, 25 °C): δ = 7.15−6.96 (m, 3H; ArH of DIPP), 6.72−6.68 (m, 2H; ArH of catecholate), 6.66−6.62 (m, 2H; ArH of catecholate), 4.82 (s, 1H; MeC(N)CH), 3.57−3.45 (m, 2H; NCH2 and ArCHMe2), 3.26 (sept, 3JH−H = 6.8 Hz, 1H; ArCHMe2), 2.80 (dd, 2JH−H = 15.2 Hz, 3JH−H = 6.0 Hz, 1H; NCH2), 2.70 (dt, 2JH−H = 14.0 Hz, 3JH−H = 4.4 Hz, 1H; NCH2), 2.47 (s, 3H; NMe2), 2.32 (m, 1H; NCH2), 2.29 (s, 3H; NMe2), 1.96 (m, 1H; NCH2), 1.79 (s, 3H; NMe), 1.78 (d, 3JH−H = 6.8 Hz, 3H; ArCHMe2), 1.75 (d, 3JH−H = 6.8 Hz, 3H; ArCHMe2), 1.69 (s, 3H; CMe), 1.66 (dd, 2JH−H = 12.4 Hz, 3 JH−H = 4.8 Hz, 1H; NCH2), 1.55 (s, 3H; CMe), 1.41 (m, 1H; NCH2), 1.38 (m, 1H; NCH2), 1.25 (d, 3JH−H = 6.8 Hz, 3H; ArCHMe2), 1.19 (d, 3JH−H = 6.8 Hz, 3H; ArCHMe2), 2.11 (s, Me of toluene). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ = 167.3, 166.4 (imine C), 160.7 (CO of catecholate), 144.3 (i-PhC of DIPP), 143.6, 142.5 (o-PhC of DIPP), 125.7, 124.4, 123.4 (m- and p-PhC of DIPP), 116.6, 113.1 (CH of catecholate), 98.0 (MeC(N)CH), 57.9, 55.7, 50.6 (NCH2), 48.7 (NMe2), 47.8 (NCH2), 47.2 (NMe2), 44.8 (NMe), 29.3, 28.7 (ArCHMe2), 25.9, 25.8 (CMe), 25.6, 24.6, 24.4, 24.0 (ArCHMe2), 22.7 (CMe), 137.9, 129.3, 128.6, 125.7, 21.4 (toluene). Elemental analysis calcd (%) for C30H45N4O2Sc·0.5toluene: C 68.81, H 8.45, N 9.58; found: C 68.81, H 8.41, N 9.30. Reaction between 2 and Bis(catecholato)diboron. A solution of bis(catecholato)diboron in C6D6 (2.4 mg, 0.01 mmol, 0.2 mL of C6D6) was added to a solution of complex 2 in C6D6 (6.1 mg, 0.01 mmol, 0.3 mL of C6D6). The reaction solution was allowed to stand at room temperature and was monitored by 1H NMR spectroscopy, which showed the generation of 4 and unidentified product 5 (see Figure S13).
Scheme 5. Reaction between Complex 2 and CatB−BCat to Generate Complex 4 and Unidentified Product 5
products. Thus, we tentatively assign 5 as the reaction product between 2 and monomeric CatB−BNDipp which was generated from the reaction of 2 and CatB−BCat. Although attempts to isolate 5 failed, the generation of 4 during this reaction supports the proposed mechanism shown in Scheme 4 that B−H bond cleavage may not occur during the reaction. We note that the different fates of the two proposed iminoborane species, [HBNDIPP] versus [CatB−BNDIPP], were probably due to the different substituents on borane atoms and thus different reactivities: While iminoborane bearing hydrogen substitute on borane can easily form clusters or decompose, that with bulkier −BCat group may be more stable and could be trapped by other species, such as scandium imido complex 2 in our work.
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SUMMARY AND CONCLUSIONS We have studied the reactivity of scandium terminal imido complex 2 toward boranes, and two kinds of reactivity were demonstrated. The reaction with 9-BBN generates a scandium borohydride, in which a C−H bond in the −NMe2 group of the tetradentate ligand (NNNN) is borylated. Reaction with CatBH or CatB−BCat results in B−O cleavage to afford a scandium catecholate. Although the proposed iminoboranes, [HBNDIPP] or [CatB−BNDipp], could not be isolated due to their extreme reactivity, our work raised the possibility to generate this kind of species in a new way, that is, metathesis between early transition metal−main group element multiple bonding species and catecholborane (or its derivatives). This work is an example that studying the reaction between rareearth complexes and boranes enriched our understanding of the chemistry in both areas.
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EXPERIMENTAL SECTION
General Methods. All operations related to air- or moisturesensitive rare-earth-metal complexes were carried out under an atmosphere of argon using Schlenk techniques or in a nitrogen-filled glovebox. Complex 25b and 9-D-BBN18 were prepared according to literature procedures. 9-BBN, CatBH, and CatB−BCat were purchased from Sigma-Aldrich and used as received. Toluene, hexane, benzened6, and toluene-d8 were dried over Na/K alloy, transferred under vacuum, and stored in the glovebox. 1H, 2H, 11B, and 13C NMR spectra were recorded on a Varian Mercury 300 MHz, Varian 400 MHz, or Agilent 600 MHz spectrometer. Variable-temperature NMR spectra were recorded on a Bruker 500 MHz or Agilent 600 MHz spectrometer. All chemical shifts were reported as δ units with reference to the residual solvent resonance of the deuterated solvents for proton and carbon chemical shifts and reference to boron trifluoride ether complex for borane chemical shifts. Quartz tubes were used when collecting 11B NMR spectra. Elemental analyses (C, H, and N) were performed by the Analytical Laboratory of the Shanghai Institute of Organic Chemistry. Synthesis of Complex 3. A solution of 9-BBN in toluene (20 mg, 0.164 mmol, 1 mL of toluene) was added to a solution of complex 2 in toluene (100 mg, 0.165 mmol, 5 mL of toluene), and the reaction D
DOI: 10.1021/acs.organomet.7b00452 Organometallics XXXX, XXX, XXX−XXX
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Organometallics X-ray Crystallography. Single crystals of complexes 3 and 4 suitable for single-crystal X-ray diffraction were grown from toluene solutions. The single crystals of 3 and 4 were mounted under a nitrogen atmosphere on a glass fiber, and data collection was performed on a Bruker APEX2 diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The SMART program package was used to determine the unit cell parameters. The absorption correction was applied using SADABS. The structures were solved by direct methods and refined on F2 by full-matrix least-squares techniques with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were placed at calculated positions and were included in the structure calculations, except for H1, H2, H5, and H10 in complex 3, which were located from the Fourier map. Calculations were carried out using SHELXL-97, SHELXL-2014, and Olex2.19 Crystallographic data and refinement for 3 and 4 are given in Table S1.
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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.7b00452. NMR spectra and crystallographic details (PDF) Accession Codes
CCDC 1556193−1556194 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21325210 and 21421091) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000).
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