Synthesis of Base-Stabilized Hydrido (hydroborylene) tungsten

Article Cite This: Organometallics XXXX, XXX, XXX-XXX

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Synthesis of Base-Stabilized Hydrido(hydroborylene)tungsten Complexes and Their Reactions with Terminal Alkynes To Give η3‑Boraallyl Complexes Zeping Hui, Takahito Watanabe, and Hiromi Tobita* Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan S Supporting Information *

ABSTRACT: Reaction of the methyl(pyridine)tungsten complex Cp*(CO)2W(py)Me with borane−NHC (NHC = Nheterocyclic carbene; MeIMe, MeIiPr) (MeIMe = 1,3,4,5tetramethylimidazol-2-ylidene, MeIiPr = 1,3-diisopropyl-4,5dimethylimidazol-2-ylidene) adducts gave the NHC-stabilized hydrido(hydroborylene)tungsten complexes Cp*(CO)2W(H)(BH·NHC) (1a: NHC = MeIMe, 1b: NHC = MeIiPr) as dark brown crystals in high yields accompanied by liberation of pyridine and methane. X-ray crystal structure analysis revealed that the W−B bond of 1b is bridged by a hydrido ligand. Reactions of 1a,b with phenylacetylene at room temperature afforded NHC-stabilized η3-boraallyl complexes 2a,b, respectively, through a new type of hydroboration of the alkyne triple bond: i.e., addition of a hydrogen and a borylene. Reaction of 1a with 3butyn-2-one also led to hydroboration of the triple bond, not the CO double bond, to give η3-boraallyl complex 3. In the reaction of 1a with (trimethylsilyl)acetylene, 1-oxo-4-borabutenyl complex 4 was formed as a primary product through carboboration of the alkyne triple bond. The subsequent slow reaction of 4 at ambient temperature resulted in the formation of η3-boraallyl complex 5 and carbyne complex 6 in the ratio of 5:4. In all of these reactions, regioselective boration of the terminal carbon of alkynes was observed.

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INTRODUCTION Syntheses of terminal borylene complexes have attracted considerable attention because of their intriguing structure and bonding mode.1 Syntheses of the first structurally wellcharacterized terminal borylene complexes were reported independently in 1998 by Cowley et al.2 and Braunschweig et al.3 Since these reports, a number of terminal borylene complexes have been synthesized and various synthetic approaches to them have been developed. A representative example of the synthetic approaches is 1,2-migration of a boron-bound substituent to a metal center in boryl complexes that leads to borylene complexes. Roper and co-workers reported the synthesis of the first intramolecularly basestabilized terminal borylene complex Os(BNHC9H9 N)Cl2(CO)(PPh3)2 by the reaction of the dichloroboryl complex Os(BCl2)Cl(CO)(PPh3)2 with 8-aminoquinoline, which proceeds through 1,2-chloride migration.4 Braunschweig et al. also reported that the cationic base-stabilized borylene complex [trans-(Cy3P)2Pt(Br){B(Fc)(NC5H4-4-Me)}][BArF4] (Fc = ferrocenyl, ArF = C6H3-3,5-(CF3)2) was formed by addition of 4-methylpyridine to a cationic ferrocenyl(bromoboryl)platinum complex via 1,2-bromide migration from the boron center to the platinum center.5 Direct conversion of dihydroboranes to borylene complexes via double B−H bond activation was reported. Sabo-Etienne et al. reported the formation of the borylene complex RuHCl(BMes)(PCy3)2 by the reaction of RuHCl(H2)(PCy3)2 with H2BMes accompanied by evolution of H2.6 Aldridge et al. reported that oxidative © XXXX American Chemical Society

addition of a B−H bond of dihydroaminoboranes H2BNR2 (R = iPr, Cy) to an iridium center and subsequent 1,2-hydride migration triggered by chloride abstraction from the iridium center resulted in the formation of cationic borylene complexes [fac-Ir(PMe3)3(H)2(BNR2)][BArF4].7 Aldridge et al. also reported another approach to the synthesis of cationic borylene complexes [Cp*Fe(CO)2(BMes)]+ and [CpFe(dmpe)2(BNMe2)]+ through halide abstraction from the haloboryl complexes.8 In this regard, the authors have recently revealed the unique reactivity of the hydrido(hydrosilylene)tungsten complex Cp*(CO)2(H)WSi(H)C(SiMe3)3 (A), which was synthesized by the reaction of a methyltungsten complex with a trihydrosilane through double Si−H bond activation and reductive elimination of methane (Scheme 1).9 Because a base-stabilized borylene ligand is isoelectronic with a base-free Scheme 1. Synthesis of a Hydrido(hydrosilylene)tungsten Complex9e

Received: September 24, 2017

A

DOI: 10.1021/acs.organomet.7b00723 Organometallics XXXX, XXX, XXX−XXX

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Organometallics silylene ligand (Chart 1), we thought that this synthetic method of A in Scheme 1 would be applicable to the synthesis of a baseChart 1. Base-Stabilized Borylene and Base-Free Silylene

stabilized hydrido(hydroborylene) complex using a trihydroborane−base adduct instead of the trihydrosilane. Although borane−base adducts are known to react with transition-metal complexes to form base-stabilized boryl complexes,10 formation of base-stabilized borylene complexes from borane−base adducts has never been reported. Here we report the synthesis of base-stabilized hydrido(hydroborylene) complexes and their reactions with some terminal alkynes.

Figure 1. ORTEP drawing of 1b with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity except for H(1) and H(2) on the B atom. Selected bond lengths (Å) and angles (deg): W−C(1) 1.928(4), W−C(2) 1.948(4), W−B 2.174(4), W− H(1) 1.79(4), B−C(13) 1.590(5), B−H(1) 1.21(4), B−H(2) 1.09(4), C(1)−O(1) 1.177(4), C(2)−O(2) 1.167(5), C(1)−W−C(2) 82.69(14), C(1)−W−B 78.35(14), C(2)−W−B 111.56(15), B−W− H(1) 33.9(12), W−B−H(1) 55.3(17), W−B−H(2) 116(2), W−B− C(13) 130.4(3), C(13)−B−H(1) 117.1(19), C(13)−B−H(2) 112(2).

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RESULTS AND DISCUSSION Synthesis of NHC-Stabilized Hydrido(hydroborylene) Complexes. Treatment of the methyl(pyridine)tungsten complex Cp*W(CO)2(py)Me11 with borane−NHC adducts gave NHC-stabilized hydrido(hydroborylene)tungsten complexes Cp*(CO)2W(H)(BH·NHC) (1a, NHC = MeIMe; 1b, NHC = MeIiPr) as dark brown crystals in high yields accompanied by liberation of pyridine and methane (Scheme 2). Complex 1a was thermally unstable and slowly decomposed

borylene complex Cp(CO)2W(H){BN(SiMe3)2} (2.025(9) Å).14c The W−H−B three-membered-ring framework of 1b is constructed by two bonding interactions, i.e. a W−B σ bond and a W−H−B three-center−two-electron bond, and the structure of 1 can be drawn as the resonance of three canonical forms: hydrido(borylene) complex form, α-agostic boryl complex form, and zwitterionic boryl complex form (Figure 2).

Scheme 2. Synthesis of Hydrido(hydroborylene) Complexes

at 60 °C in C6D6 (t1/2 = 1 week) to give a complicated mixture including [Cp*W(CO)2]212 (29% NMR yield). On the other hand, no decomposition of 1b in C6D6 was observed at 60 °C within 1 week. An X-ray crystal structure analysis of 1b shows that the tungsten center adopts a distorted four-legged piano-stool geometry consisting of a Cp* ligand, a hydrido ligand, a borylene ligand, and two carbonyl ligands (Figure 1). The W− H(1) bond length (1.79(4) Å) is longer than the normal values of the W−H bond length for related tungsten hydrido complexes (1.61−1.71 Å).13 The B···H(1) distance (1.21(4) Å), on the other hand, is very short and is slightly longer than that of the terminal B−H(2) bond (1.09(4) Å). These data clearly suggest the existence of a strong bonding interaction between the hydrido ligand and the borylene ligand. A similar interligand interaction has been reported for hydrido(aminoborylene) complexes.14 Interestingly, in spite of the increase in the coordination number around the boron atom by this interaction, the W, B, H(2), and C(13) atoms are nearly coplanar, as reflected in the sum of three bond angles around the B atom (358(3)°). The W−B bond distance (2.174(4) Å) is clearly shorter than that of the W−B single bond in the phosphine-stabilized boryl complex Cp*(CO)3W(BH2·PMe3) (2.476(7) Å)10a but longer than that of the W−B bond in the

Figure 2. Three canonical forms for 1.

The 11B NMR spectra of 1a,b show broad signals at 74.0 and 74.4 ppm, respectively (see Figures S4 and S7 in the Supporting Information). These are assigned to the borylene ligand, and the chemical shifts are comparable to those of aminoborylene complexes (CO) 5WBN(SiMe 3 ) 2 (86.6 ppm)3 and Cp(CO)2W(H){BN(SiMe3)2} (81.6 ppm).14c In the 13C{1H} NMR spectra of 1a,b, two CO ligands on the W atom are observed inequivalently (1a, 230.3 and 248.0 ppm; 1b, 231.3 and 248.2 ppm) (see Figures S3 and S6 in the Supporting Information). The 1H NMR spectra of 1a,b show the signal for methyl groups at 4- and 5-positions of NHC equivalently, whereas 1b shows two doublet signals for two diastereotopic Me groups in the iPr groups of MeIiPr (see Figures S2 and S5 in the Supporting Information). These observations indicate that the W centers of 1a,b are chiral but that the B−C(NHC) bond rotates freely in solution. In the 1H NMR spectrum of 1a, there are two very broad signals around −14 and 10 ppm (see Figure S2 in the Supporting Information). The 11B-decoupled 1H NMR spectrum shows two sharp signals at −13.75 and 9.82 ppm B


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of 1a with 2 equiv of 3-butyn-2-one for 1 h gave the η3-boraallyl complex Cp*(CO)2W(η3-Me(O)CHCCHBH·MeIMe) (3) as the sole product (Scheme 4). This result shows that the CC

that are coupled to each other with JHH = 2.0 Hz. The upfield signal is assigned to the hydrido ligand, because it is accompanied by 183W satellites (1JWH = 87.1 Hz). The broadening of this signal indicates the existence of the bridging W−H−B interaction in solution. The downfield signal is assigned to the terminal BH proton, and the chemical shift value is much larger (downfield shifted) than those of the borane−NHC adduct BH3·IMe (1.95 ppm) (IMe = 1,3dimethylimidazol-2-ylidene),15 the dihydroboryl complex Cp*(CO)3W(BH2·PMe3) (1.92 ppm),10a and the phosphinestabilized hydroborylene complex [(HB)(μ-Ph 2 PN(H)PPh2)2Fe(H)(CO)2](BF4) (2.83 ppm).16 This downfield shift is attributable to a deshielding effect caused by the WB double bond. A similar downfield shift has been observed for the terminal SiH proton in A.9a The IR spectrum of 1a (see Figure S36 in the Supporting Information) shows two νCO bands (1761 and 1842 cm−1) in the terminal carbonyl region, which are significantly lower in wavenumber than those of A (1853 and 1928 cm−1).9a This lower-wavenumber shift implies that the σ donation from the NHC-stabilized borylene ligand is strong while the π backdonation from the W to the borylene ligand is comparatively weak. Unfortunately, no bands corresponding to the terminal and bridging BH stretching vibrations were observed, probably because of weakness or broadening of the bands and/or overlap of the other bands. Reactions of NHC-Stabilized Hydrido(hydroborylene) Complexes with Terminal Alkynes. The hydrido(hydrosilylene) complex A is known to react with polar unsaturated molecules such as acetone9a,e and nitriles9b under mild conditions to give hydrosilylation products. In sharp contrast, complex 1a, which is isoelectronic with complex A, did not react with acetone in C6D6 at room temperature. However, we found that complexes 1 reacted with nonpolar unsaturated molecules, i.e. alkynes.17 Thus, the reaction of 1a with phenylacetylene in toluene at room temperature afforded the NHC-stabilized η3-boraallyl complex Cp*(CO)2W(η3PhHCCHBH·MeIMe) (2a) through a new type of hydroboration: i.e., addition of a hydrogen and an NHC-coordinated borylene moiety (Scheme 3). Complex 1b, the MeIiPr analogue

Scheme 4. Reaction of 1a with 3-Butyn-2-one

triple bond is more reactive than the polar CO double bond toward the W−B−H unit of 1a. Complex 3 was isolated by recrystallization from a toluene/hexane solution in 77% yield as yellow crystals. The structures of 2a and 3 were unambiguously determined by X-ray crystal structure analysis. The ORTEP drawings of 2a (Figure 3a) and 3 (Figure 3b) clearly show the formation of NHC-stabilized η3-boraallyl ligands with exo conformation. Both the phenyl group in 2a and the acetyl group in 3 are syn to the central allyllic hydrogen while the NHCs are anti to it. The same geometries are also observed in the MeIiPr analogue 2b (see Figure S1 in the Supporting Information). Importantly, in all cases, the formation of a single isomer adopting syn-R and anti-NHC conformation in the η3-boraallyl ligand was observed. The B−C(1) bond lengths (2a, 1.524(10) Å; 3, 1.513(6) Å) are clearly shorter than the B−C single bond lengths of the NHC−vinylborane adduct IMe·BH 2 {C(SiMe 3 )=CHPh} (1.629(5) Å)18 and slightly longer than the B−C unsaturated bond lengths of an NHC−borabenzene adduct 4-iPr-C6H4B· IMes (IMes = 1,3-dimesitylimidazol-2-ylidene) (1.482(5), 1.486(5) Å).19 These indicate the existence of partial doublebond character in the B−C bonds of 2a and 3. The C(1)−C(2) bond lengths (2a, 1.413(11) Å; 3, 1.431(5) Å) are comparable to the C−C bond lengths of the η3-allyl ligand of the tungsten allyl complex Cp*(CO) 2 W[η 3 -PhHCCHCMeOSiH 2 {C(SiMe3)3}] (1.418(6) and 1.434(6) Å).20 These data support the η3 coordination of the boraallyl ligands in 2a and 3. These η3-boraallyl complexes 2a,b and 3 were also characterized by 1H, 11B, and 13C{1H} NMR spectroscopy (see Figures S8−S18 in the Supporting Information). In the 1 H{11B} NMR spectrum of 2a, a resonance assigned to BH is observed at 2.57 ppm as a doublet (3J = 6.4 Hz). A doublet of doublets (3J = 12.4 Hz, 3J = 6.4 Hz) signal at 2.79 ppm and a doublet (3J = 12.4 Hz) signal at 3.84 ppm are assigned to the central allyl hydrogen and the terminal CH, respectively. These coupling constants are consistent with the η3-boraallyl ligand with syn B−C and anti C−C bonds. In the 11B NMR spectrum of 2a, a broad signal appears at −9.5 ppm. This signal, assignable to the boron in the η3-boraallyl ligand, is shifted upfield from that of 1a (74.0 ppm). In the 13C{1H} NMR spectrum of 2a, a broad signal of the central allyl carbon is observed at 56.3 ppm. This broadening is caused by the coupling with the quadrupolar boron nucleus. The carbene carbon is also observed as a broad signal at 166.5 ppm. Similar spectroscopic features were also observed for complexes 2b and 3.

Scheme 3. Reaction of 1 with Phenylacetylene

of 1a, also reacted with phenylacetylene to give the hydroboration product 2b, but the reaction time (12 h) was longer than that for 1a (1 h). The elongation of the reaction time indicates that the reactivity of 1b toward phenylacetylene was decreased by the sterically more hindered NHC. Complexes 2a,b were isolated by crystallization from toluene/hexane solutions in 77% and 84% yield, respectively, as yellow crystals. The hydroboration of a CC triple bond also occurred in the reaction of acetylenes bearing an acyl group. Thus, reaction C


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Organometallics

Figure 4. Monitoring of the reaction of 1a with phenylacetylene by NMR spectroscopy. 1H{11B} NMR spectra at 300 K: (a) initial, (b) after 10 min, and (c) after 50 min.

with (trimethysilyl)acetylene, by comparison of their spectroscopic data. Isolation of an Intermediate Corresponding to B. The reaction of 1a with (trimethylsilyl)acetylene, a bulky terminal alkyne, was next examined. This reaction in toluene at room temperature was complete within 1 h. Subsequent cooling of the reaction mixture in a freezer at −30 °C led to isolation of 4 as yellow crystals in 51% yield (Scheme 5). Scheme 5. Reaction of 1a with (Trimethylsilyl)acetylene

Figure 3. ORTEP drawings of 2a (a) and 3 (b) with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity except for H(1), H(2), and H(3) on the η3-boraallyl ligands. Selected bond lengths (Å) and angles (deg): 2a, W−C(1) 2.231(7), W−C(2) 2.331(7), W−C(3) 1.942(8), W−C(4) 1.911(9), W−B 2.512(7), C(1)−C(2) 1.413(11), C(3)−O(1) 1.172(10), C(4)−O(2) 1.197(10), B−H(1) 1.09(9), B−C(1) 1.524(10), B−C(5) 1.579(11), C(3)−W−C(4) 80.9(3), B−C(1)−C(2) 128.5(7), C(1)−C(2)−C(12) 121.1(7), C(1)−B−C(5) 126.9(6); 3, W−C(1) 2.232(3), W−C(2) 2.318(3), W−C(5) 1.949(4), W−C(6) 1.925(4), W−B 2.515(4), C(1)−C(2) 1.431(5), C(5)−O(2) 1.166(5), C(6)− O(3) 1.169(4), B−H(1) 1.15(5), B−C(1) 1.513(6), B−C(7) 1.574(5), C(5)−W−C(6) 79.83(15), B−C(1)−C(2) 125.9(3), C(1)−C(2)−C(3) 121.3(4), C(1)−B−C(7) 128.1(3).

The structure of 4 was determined by X-ray crystal structure analysis. The ORTEP drawing of 4 is shown in Figure 5. Intermediate 4 can be described as a 1-oxo-4-borabutenyl complex formed through carboboration of (trimethylsilyl)acetylene: i.e., addition of a CO and an NHC-coordinated boryl group. The 1-oxo-4-borabutenyl ligand is coordinated to the tungsten center with three different bonding interactions that are σ coordination of C(3), π coordination of the C(1)− C(2) double bond, and a B−H agostic interaction. The C(1)− C(2) bond length (1.443(5) Å) is elongated from the normal CC double-bond length (1.34 Å) through π coordination to the tungsten center. The 1H{11B} NMR spectrum of 4 (see Figure S19 in the Supporting Information) shows that the signal of the agostic BH proton appears as a slightly broadened doublet at −10.07 ppm with tungsten satellite signals (1JWH = 79.0 Hz). This splitting to a doublet (2JHH = 15.6 Hz) is attributable to coupling with the other BH proton (terminal), whose signal is observed at 2.68 ppm as a doublet of doublets (2JHH = 15.6 Hz, 3 JHH = 6.6 Hz). The signal of the BCH proton appears at 2.16 ppm as a doublet of doublets coupled with the terminal BH proton (3JHH = 6.6 Hz) and the agostic BH proton (3JHH = 1.4 Hz). In the 11B NMR spectrum of 4 (see Figure S21 in the Supporting Information), the resonance appears at −21.8 ppm and this chemical shift is close to that of H3B·IMe (−37.2

The intermediate B was observed during the reaction of 1a with phenylacetylene, which was monitored by 1H{11B} NMR spectroscopy (Figure 4). After 10 min (Figure 4b), the signals of 1a decreased and two sets of new signals appeared. One of them is the set of signals of 2a, while the other set can be assigned to the signals of the intermediate B, which shows a characteristic signal assigned to a W−H−B three-center bond at −9.52 ppm as a doublet with a 183W satellite (2JHH = 15.2 Hz and 1JWH = 79.4 Hz). After 50 min (Figure 4c), the intermediate B mostly changed to η3-boraallyl complex 2a. Although intermediate B was not able to be isolated, its structure can be estimated to be analogous to that of the aforementioned intermediate 4 that forms in the reaction of 1a D


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Organometallics

Figure 5. ORTEP drawing of 4 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity except for H(1), H(2), and H(3) on the 1-oxo-4-borabutenyl ligand. Selected bond lengths (Å) and angles (deg): W−C(1) 2.255(4), W−C(2) 2.276(3), W−C(3) 2.137(3), W−C(4) 1.922(4), W−B 2.470(4), W− H(2) 1.68(4), C(1)−C(2) 1.443(5), C(2)−C(3) 1.448(5), C(3)− O(1) 1.232(4), C(4)−O(2) 1.167(5), B−C(1) 1.542(6), B−C(5) 1.597(5), B−H(1) 1.09(4), B−H(2) 1.41(4), C(3)−W−C(4) 66.78(14), C(2)−C(1)−B 126.3(3), C(1)−C(2)−C(3) 113.3(3), W−C(3)−O(1) 149.0(3), C(2)−C(3)−O(1) 134.2(3), C(1)−B− C(5) 123.2(3).

Figure 6. ORTEP drawing of 5 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity except for H(1), H(2), and H(3) on the η3-boraallyl ligand. Selected bond lengths (Å) and angles (deg): W−C(1) 2.238(2), W−C(2) 2.346(2), W−C(3) 1.943(2), W−C(4) 1.923(2), W−B 2.488(2), Si−C(2) 1.860(2), C(1)−C(2) 1.438(2), C(3)−O(1) 1.163(3), C(4)−O(2) 1.176(3), B−H(1) 1.12(3), B−C(1) 1.519(3), B−C(5) 1.586(3), C(3)−W−C(4) 81.05(10), B−C(1)−C(2) 124.9(2), C(1)−C(2)−Si 122.87(16), C(1)−B−C(5) 126.0(2).

ppm), supporting the fact that the boron atom in 4 is fourcoordinate. In the 13C{1H} NMR spectrum (see Figure S20 in the Supporting Information), the BCH carbon appears at 49.6 ppm as a broad signal caused by coupling with the directly bonded boron atom. The IR spectrum of 4 shows a strong band at 1870 cm−1 that can be assigned to the stretching of the terminal CO ligand. Another band at 1776 cm−1 is assignable to the CO group in the 1-oxo-4-borabutenyl ligand. Intermediate 4 slowly changed in C6D6 at ambient temperature to form η3-boraallyl complex 5 and carbyne complex 6 in a ratio of 5:4 after 24 h (Scheme 6). This ratio did not change even at elevated temperatures.

low field (336.4 ppm). A broad signal assigned to the carbon atom bound to both the boron and silicon atoms appears at 55.2 ppm, whose broadening is attributable to the quadrupole broadening and the coupling with the boron atom. In the 11B NMR spectrum, a triplet signal is observed at −27.7 ppm due to the coupling with two hydrogen atoms on the boron atom. These data are all consistent with the structure of 6 depicted in Scheme 6. Possible Mechanism for the Reactions of 1 with Terminal Alkynes. A possible reaction mechanism for the reaction of 1a with alkynes is shown in Scheme 7. The triple bond of terminal alkynes initially coordinates to the tungsten center accompanied by cleavage of the W−H bond to form C. Subsequent migratory insertion of the CC triple bond into

Scheme 6. Conversion of 4 in Benzene-d6 at Room Temperature

Scheme 7. Possible Mechanism for the Reactions of 1a with Alkynes

15

Complex 5 was isolated from the reaction mixture by rinsing it with cold n-hexane and was fully characterized by spectroscopy as well as by X-ray crystal structure analysis (Figure 6). On the other hand, complex 6 could not be isolated by recrystallization due to its high solubility. Therefore, complex 6 was characterized on the basis of the NMR spectra of the reaction mixture (see Figures S30−S34 in the Supporting Information). In the 13C{1H} NMR spectrum, a signal characteristic of the carbyne ligand is observed at extremely E


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Organometallics the W−B bond generates the γ-B−H agostic intermediate D. This intermediate D is considered to be relatively unstable because of the steric repulsion between a substituent of the alkyne and the ligands on the tungsten center and changes rapidly to the kinetically controlled product B or 4 through insertion of a CO ligand into the W−C bond. This reaction is reversible, and from D a σ-CAM21 reaction forming E followed by coordination of a CC double bond to the tungsten center gives the thermodynamically controlled product 2a or 5. In the case of (trimethylsilyl)acetylene, a 1,2-silyl migration of a SiMe3 group also occurs possibly through the zwitterionic intermediate F generated from D to give carbyne complex 6. To confirm the proposed mechanism for the hydroboration, the reaction of 1a with deuterated phenylacetylene, PhCCD (90% deuterium content determined by 1H NMR), was examined. Treatment of 1a with deuterated phenylacetylene resulted in the formation of a deuterated analogue of 2a, Cp*(CO)2W{η3-PhHCCDBH(MeIMe)} (2a-d) (Scheme 8).

for its MeIiPr analogue 1b. The intermediate B was observed during the reaction of 1a with phenylacetylene. In the case of the reaction of 1a with (trimethylsilyl)acetylene, intermediate 4, which corresponds to B, was isolated and fully characterized. Slow rearrangement of 4 then proceeded at room temperature to give η3-boraallyl complex 5 and carbyne complex 6. The formation of complexes 4−6 can be explained by a mechanism involving the generation of hypothetical γ-B−H agostic intermediate D. The research on the reactivity of the hydrido(hydroborylene) complexes toward terminal alkynes in this work would provide beneficial information for developing a new catalytic transformation reaction of alkynes involving a new type of hydroboration using borylene complexes as catalyst.

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EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under an argon atmosphere in dried Schlenk glasswares or vacuum lines, and airand moisture-sensitive chemicals and reagents were handled in a glovebox under an argon atmosphere. Materials. n-Hexane and diethyl ether were dried using a Glass Contour alumina column (Nikko Hansen & Co., Ltd.). Toluene was dried using an MBRAUN Solvent Purification System, further dried over CaH2, and vacuum-transferred. Benzene-d6 was dried over CaH2 and vacuum-transferred. All solvents were stored under argon over 4 Å molecular sieves in a glovebox. Cp*W(CO)2(py)Me,11 BH3·MeIMe,23 and BH3·MeIiPr24 were prepared according to the literature procedures. Spectroscopic Measurements. The NMR spectra were obtained by used of a Bruker AVANCE III 400 Fourier transform spectrometer (1H, 400.1 MHz; 11B, 128.4 MHz; 13C, 100.6 MHz; 29Si, 79.5 MHz). Chemical shifts are reported in parts per million. Coupling constants (J) are given in Hz. 29Si{1H} NMR measurements were performed using the DEPT pulse sequence. The 1H and 1H{11B} NMR chemical shifts were referenced to the residual proton of benzene-d6 (7.15 ppm). The 11B NMR chemical shifts were referenced to BF3·OEt2 (0 ppm) as an external standard. The 13C{1H} NMR chemical shifts were referenced to benzene-d6 (128 ppm). The signals of carbon atoms connected to boron atoms were assigned using 1H−13C HSQC and HMBC experiments because those can rarely be observed in the usual 13 C{1H} NMR spectra due to quadrupole broadening and coupling with the boron atom. The 29Si{1H} NMR chemical shifts were referenced to SiMe4 (0 ppm) as an external standard. All NMR data were collected at room temperature unless otherwise indicated. IR spectra were recorded on a HORIBA FT-720 spectrometer at room temperature. Elemental analysis was performed using a J-Science Lab JM11 microanalyzer at the Research and Analytical Center for Giant Molecules, Tohoku University. Synthesis of Cp*(CO)2W(H)(BH·MeIMe) (1a). To a toluene solution (20 mL) of Cp*W(CO)2(py)Me (1.00 g, 2.13 mmol) in a Schlenk tube was added BH3·MeIMe (295 mg, 2.14 mmol) at room temperature with constant stirring. After the reaction mixture was stirred for 1 h, the resulting solution was filtered through a PTFE membrane syringe filter. The filtrate was evaporated under reduced pressure. The residue was dissolved in a mixture of n-hexane (10 mL) and toluene (5 mL). The solution was stored in a freezer at −30 °C for 24 h to give complex 1a as dark brown crystals. Yield: 932 mg (1.82 mmol, 85%). 1H{11B} NMR (C6D6): δ −13.75 (d, 2JHH = 2.0 Hz, 1JWH = 87.1 Hz, 1H, WH), 1.10 (s, 6H, NCMe), 2.32 (s, 15H, Cp*), 3.01 (s, 6H, NMe), 9.82 (d, 2JHH = 2.0 Hz, 1H, BH). 13C{1H} NMR (C6D6): δ 7.6 (NCMe), 12.3 (C5Me5), 31.8 (NMe), 103.3 (C5Me5), 123.6 (NCMe), 170.4 (br, NCN), 230.3 (CO), 248.0 (CO). 11B NMR (C6D6): δ 74.0 (br s). IR (KBr pellet, cm−1): 1761 (s, νCO), 1842 (s, νCO). Anal. Calcd for C19H29BN2O2W: C, 44.56; H, 5.71; N, 5.47. Found: C, 44.61; H, 5.86; N, 5.49. Synthesis of Cp*(CO)2W(H)(BH3·MeIiPr) (1b). By a procedure similar to the synthesis of 1a, Cp*W(CO)2(py)Me (235 mg, 0.501 mmol) was allowed to react with BH3·MeIiPr (97 mg. 0.500 mmol) in toluene (5 mL) for 6 h to give complex 1b as dark reddish brown

Scheme 8. Reaction of 1a with Phenylacetylene-d

The 1H{11B} NMR spectrum of the reaction mixture after 1 h shows that the deuterium is mainly located on the central allyl carbon of 2a-d (see Figure S35 in the Supporting Information). As a result of this experiment using PhCCD, the signal of BCH in 2a-d significantly decreased in comparison with that in 2a and the PhCH and BH protons appeared as slightly broadened singlets due to the three-bond coupling with deuterium. These results support that this hydroboration reaction involves the cleavage of the terminal or bridging BH bond of 1a and rule out the formation of a vinylidene intermediate via 1,2-H migration from C (see Scheme S1 in the Supporting Information). This alkyne−vinylidene rearrangement on a metal center is often proposed for hydroboration of terminal alkynes.22

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CONCLUSIONS The NHC-stabilized hydrido(hydroborylene)tungsten complexes 1a,b were synthesized by the reaction of Cp*W(CO)2(py)Me with borane−NHC adducts and structurally fully characterized. A strong interligand interaction between the borylene and the hydrido ligands was revealed by X-ray crystal structure analysis and NMR study. In the IR spectra, the νCO bands of these borylene complexes were greatly shifted to a lower wavenumber region in comparison with the corresponding hydrido(hydrosilylene)tungsten complex A. This implies that the electron-donating ability of the NHC-stabilized borylene ligand is much stronger than that of the isoelectronic silylene ligand. The reactions of 1 with terminal alkynes at room temperature led to a new type of selective hydroboration of a CC triple bond: i.e., addition of a hydrogen and an NHCcoordinated borylene moiety, to give η3-boraallyl complexes with the syn B−C and anti C−C geometry as a sole product. The reaction rate for 1a with MeIMe as NHC is faster than that F


Article

Organometallics crystals (256 mg, 0.451 mmol, 90%). 1H{11B} NMR (C6D6): δ −13.54 (s, 1JWH = 84.5 Hz, 1H, WH), 1.08 (d, 3JHH = 7.1 Hz, 6H, NCHMe2), 1.34 (d, 3JHH = 7.1 Hz, 6H, NCHMe2), 1.47 (s, 6H, NCMe), 2.28 (s, 15H, Cp*), 4.95 (sept, 3JHH = 7.1 Hz, 2H, NCHMe2), 9.75 (s, 1H, BH). 13C{1H} NMR (C6D6): δ 9.9 (NCMe), 12.2 (C5Me5), 20.9, 21.3 (NCHMe2), 50.1 (NCHMe2), 102.8 (C5Me5), 124.3 (NCMe3), 170.5 (br, NCN), 231.3 (CO), 248.2 (CO). 11B NMR (C6D6): δ 74.4 (br s). IR (KBr pellet, cm−1): 1772 (s, νCO), 1857 (s, νCO). Anal. Calcd for C23H37BN2O2W: C, 48.62; H, 6.56; N, 4.93. Found: C, 48.90; H, 6.54; N, 5.05. Synthesis of Cp*(CO)2W(η3-PhHCCHBH·MeIMe) (2a). To a toluene solution (10 mL) of 1a (100 mg, 0.195 mmol) in a Schlenk tube was added phenylacetylene (40 mg, 0.39 mmol), and the reaction mixture was stirred at room temperature for 1 h. The resulting solution was filtered through a glass filter, and volatiles were removed from the filtrate under reduced pressure. The residual solid was washed with nhexane (3 mL) and dissolved in a mixture of n-hexane (3 mL) and toluene (1 mL). The solution was stored at −30 °C in a freezer for 3 days to give complex 2a as yellow crystals. Yield: 93 mg (0.15 mmol, 77%). 1H{11B} NMR (C6D6): δ 1.12 (s, 6H, NCMe), 1.88 (s, 15H, Cp*), 2.57 (d, 3J = 6.4 Hz, 1H, BH), 2.79 (dd, 3J = 12.4 Hz, 3J = 6.4 Hz, 1H, BCH), 3.11 (s, 6H, NMe), 3.84 (d, 3J = 12.4 Hz, 1H, PhCH), 7.08 (t, 3J = 7.8 Hz, 1H, Ph), 7.28 (t, 3J = 7.8 Hz, 2H, Ph), 7.95 (d, 3J = 7.8 Hz, 2H, Ph). 13C{1H} NMR (C6D6): δ 7.9 (NCMe), 10.9 (C5Me5), 33.5 (NMe), 56.3 (br, BCH), 57.6 (PhCH), 101.0 (C5Me5), 124.3 (NCMe), 124.7, 126.9, 146.6 (Ph), 166.5 (br, NCN), 231.2 (CO), 236.7 (CO). One of the four signals assigned to the Ph group was not observed due to its overlap with the signals of C6D6. 11B NMR (C6D6): δ −9.5 (br s). IR (KBr pellet, cm−1): 1766 (s, νCO), 1865 (s, νCO). Anal. Calcd for C27H35BN2O2W: C, 52.80; H, 5.74; N, 4.56. Found: C, 53.12; H, 5.83; N, 4.58. Synthesis of Cp*(CO)2W(η3-PhHCCHBH·MeIiPr) (2b). By a procedure similar to the synthesis of 2a, 1b (240 mg, 0.422 mmol) was allowed to react with phenylacetylene (90 mg. 0.881 mmol) in toluene (10 mL) for 12 h to give complex 2b as yellow crystals (238 mg, 0.355 mmol, 84%). 1H{11B} NMR (C6D6): δ 1.12 (d, 3JHH = 7.1 Hz, 6H, NCHMe2), 1.21 (d, 3JHH = 7.1 Hz, 6H, NCHMe2), 1.53 (s, 6H, NCMe), 1.83 (s, 15H, Cp*), 2.65 (br s, 1H, BH), 2.77 (dd, 3J = 12.4 Hz, 3J = 6.8 Hz, 1H, BCH), 3.95 (d, 3J = 12.4 Hz, 1H, PhCH), 5.19 (sept, 3JHH = 7.1 Hz, 2H, NCHMe2) 7.05 (t, 3J = 7.8 Hz, 1H, Ph), 7.24 (t, 3J = 7.8 Hz, 2H, Ph), 7.88 (d, 3J = 7.8 Hz, 2H, Ph). 13C{1H} NMR (C6D6): δ 9.9 (NCMe), 10.6 (C5Me5), 21.07, 21.09 (NCHMe2) 50.0 (NCHMe2), 56.7 (br, BCH), 57.0 (PhCH), 100.3 (C5Me5), 124.3 (NCMe), 124.7, 126.2, 146.3 (Ph), 165.9 (br, NCN), 230.4 (CO), 236.5 (CO). One of the four signals assigned to the Ph group was not observed due to its overlap with the signals of C6D6. 11B NMR (C6D6): δ −8.9 (br s). IR (KBr pellet, cm−1): 1786 (s, νCO), 1869 (s, νCO). Anal. Calcd for C31H43BN2O2W: C, 55.54; H, 6.47; N, 4.18. Found: C, 55.40; H, 6.39; N, 4.28. Synthesis of Cp*(CO)2W(η3-Me(O)CHCCHBH·MeIMe) (3). To a toluene solution (10 mL) of 1a (100 mg, 0.195 mmol) was added 3butyn-2-one (27 mg, 0.40 mmol) in a Schlenk tube with stirring at room temperature. After 1 h, the resulting solution was filtered through a glass filter and then the filtrate was evaporated to dryness. The residue was washed with n-hexane (2 mL) and dissolved in a mixture of n-hexane (3 mL) and toluene (1 mL). Storing the solution at room temperature for 5 days gave complex 3 as orange crystals. Yield: 86 mg (0.15 mmol, 77%). 1H{11B} NMR (C6D6): δ 1.13 (s, 6H, NCMe), 2.02 (s, 15H, Cp*), 2.51 (s, 3H, C(O)Me), 2.62 (d, 3J = 6.5 Hz, 1H, BH), 2.91 (d, 3J = 11.6 Hz, 1H, Me(O)CCH), 3.03 (s, 6H, NMe), 3.19 (dd, 3J = 6.5 Hz, 3J = 11.6 Hz, 1H, BCH). 13C{1H} NMR (C6D6): δ 7.9 (NCMe), 10.6 (C5Me5), 29.5 (C(O)Me), 33.5 (NMe), 56.8 (Me(O)CCH), 56.9 (br, BCH), 101.5 (C5Me5), 124.5 (NCMe), 164.8 (NCN), 201.3 (C(O)Me), 231.4 (CO), 236.3 (CO). 11B NMR (C6D6): δ −7.8 (br s, BH). IR (KBr pellet, cm−1): 1649 (s, νCO), 1786 (s, νCO), 1890 (s, νCO). Anal. Calcd for C23H33BN2O3W: C, 47.61; H, 5.73; N, 4.83. Found: C, 47.79; H, 5.87; N, 4.87. Synthesis of Cp*(CO)W(η5-C(O)C(SiMe3)CHBH2·MeIMe) (4). (Trimethylsilyl)acetylene (20 mg, 0.20 mmol) was added to a toluene solution (10 mL) of 1a (100 mg, 0.195 mmol) in a Schlenk tube, and

the mixture was stirred at room temperature for 1 h. The resulting solution was filtered through a glass filter, and then the filtrate was evaporated under reduced pressure. The residue was dissolved in a mixture of toluene (1 mL), diethyl ether (0.5 mL), and n-hexane (5 mL). The solution was stored in a freezer at −30 °C for 12 h to give complex 4 as yellow crystals. Yield: 63 mg (0.10 mmol, 51%). 1H{11B} NMR (C6D6): δ −10.07 (d, 2JHH = 15.6 Hz, 1JWH = 79.0 Hz, 1H, WHB), 0.55 (s, 9H, SiMe3), 1.10 (s, 6H, NCMe), 2.06 (s, 15H, Cp*), 2.16 (dd, 3JHH = 6.6 Hz, 2JHH = 1.4 Hz, 1H BCH), 2.68 (dd, 3JHH = 6.6 Hz, 2JHH = 15.6 Hz, 1H, BH), 3.23 (s, 6H, NMe). 13C{1H} NMR (C6D6): δ 0.6 (SiMe3), 7.8 (NCMe), 12.0 (C5Me5), 32.2 (CSiMe3), 32.7 (NMe), 49.6 (br, BCH), 100.3 (C5Me5), 122.9 (NCMe), 160.1 (br, NCN), 228.6 (CO), 236.3 (CO). 11B NMR (br, C6D6): δ −21.8 (s, BH). IR (KBr pellet, cm−1): 1776 (m, νCO), 1870 (s, νCO). Anal. Calcd for C24H39BN2O2SiW: C, 47.23; H, 6.44; N, 4.59. Found: C, 46.84; H, 6.50; N, 4.64. Thermal Rearrangement of 4. A C6D6 solution of 4 (10 mg, 0.020 mmol) was kept at room temperature in the NMR tube. After 24 h, the formation of Cp*(CO)2W(η3-Me3SiCHCHBH·MeIMe) (5) and Cp*(CO)2WCCH(SiMe3)(BH2·MeIMe) (6) in a ratio of 5:4 was observed by NMR spectroscopy. Complex 5 was isolated from the reaction mixture as described in the next section, but isolation of complex 6 was unsuccessful. Complex 6 was therefore characterized by the NMR spectra of the reaction mixture of 5 and 6. 6: 1H{11B} NMR (C6D6): δ 0.63 (s, 9H, SiMe3), 1.42 (s, 6H, NCMe), 1.92, 1.93 (AB of ABX, 2H, BH and BCH), 2.03 (s, 15H, Cp*), 2.30 (X of ABX, 1H, BH), 3.28 (s, 6H, NCH3); 13C{1H} NMR (C6D6) δ 0.1 (SiMe3), 8.0 (NCMe), 11.6 (C5Me5), 32.4 (NMe), 55.2 (CHSiB), 102.6 (C5(CH3)5), 123.2 (NCCH3), 168.7 (NCN), 229.1 (CO), 229.2 (CO), 336.4 (WC); 11B NMR (C6D6) δ −27.7 (t, 1JBH = 91 Hz, BH2); 29Si NMR (C6D6) δ −0.31. Synthesis of Cp*(CO)2W(η3-Me3SiHCCHBH·MeIMe) (5). To a toluene solution (10 mL) of 1a (100 mg, 0.195 mmol) in a Schlenk tube was added (trimethylsilyl)acetylene (40 mg, 0.41 mmol) with stirring at room temperature. After the mixture was stirred at room temperature for 48 h, the solvent was removed under reduced pressure. The residue was extracted with diethyl ether (20 mL) and filtered through a PTFE membrane syringe filter. The filtrate was evaporated to dryness. The residue was washed with cold n-hexane (3 mL × 3) and then dried under reduced pressure to give complex 5 as yellow crystals. Yield: 31 mg (0.051 mmol, 26%). 1H{11B} NMR (C6D6): δ 0.49 (s, 9H, SiMe3), 0.99 (d, 3J = 15.3 Hz, 1H, Me3SiCH), 1.17 (s, 6H, NCMe), 2.09 (s, 15H, Cp*), 2.22 (dd, 3J = 5.4 Hz, 3J = 15.3 Hz, 1H, BCH), 2.53 (d, 3J = 5.4 Hz, 1H, BH), 3.17 (s, 6H, NCH3). 13C{1H} NMR (C6D6): δ 1.8 (SiMe3), 8.0 (NCMe), 11.6 (C5Me5), 33.9 (NMe), 43.2 (Me3SiCH), 65.7 (br, BCH), 100.5 (C5(CH3)5), 124.2 (NCMe), 167.0 (NCN), 229.8 (CO), 233.8 (CO). 11 B NMR (C6D6): δ −7.6 (br s, BH). 29Si NMR (C6D6): δ 1.08. IR (KBr pellet, cm−1): 1772 (s, νCO), 1868 (s, νCO). Anal. Calcd for C24H39BN2O2SiW: C, 47.23; H, 6.44; N, 4.59. Found: C, 47.34; H, 6.55; N, 4.64. Reaction of Cp*(CO)2W(H)(BH·MeIMe) (1a) with PhCCD. PhCCD (90% deuterium content determined by 1H NMR, 4.0 mg, 0.039 mmol) was added to a C6D6 solution of 1a (10 mg, 0.020 mmol) in an NMR tube at room temperature using a microsyringe. Ferrocene (2.0 mg) was added as an internal standard. The color of the reaction mixture changed from dark red to bright red within 1 h by the quantitative formation of Cp*(CO)2W{η3-PhHCCDBH(MeIMe)} (2a-d). Complex 2a-d was characterized by 1H{11B} NMR spectroscopy. 2a-d: 1H{11B} NMR (C6D6): δ 1.12 (s, 6H, NCMe), 1.87 (s, 15H, Cp*), 2.57 (br, 1H, BH), 3.11 (s, 6H, NMe), 3.84 (br, 1H, PhCH), 7.09 (t, 3J = 7.2 Hz, 1H, p-Ph), 7.28 (t, 3J = 7.6 Hz, 2H, mPh), 7.95 (d, 3J = 8 Hz, 2H, o-Ph). X-ray Crystal Structure Determination. X-ray-quality single crystals of 1b, 2a, and 3−5 were obtained from a toluene/hexane solution at −35 °C. Intensity data for the analysis were collected on a Rigaku RAXIS-RAPID imaging plate diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) under a cold nitrogen stream (T = 140 K). Numerical absorption corrections were applied to the data. The structures of 1b, 2a, and 3−5 were solved by G


Article

Organometallics

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Patterson methods using the SHELXS-97 program and refined by fullmatrix least-squares techniques on all F2 data with SHELXL-97.25 Anisotropic refinement was applied to all non-hydrogen atoms. All calculations were carried out using Yadokari-XG 2009.26 Crystallographic data are available as a CIF file, and selected crystallographic data are also given in Table S1 (for 1b, 2a, and 2b) and Table S2 (for 3−5) in the Supporting Information. The crystal structure of 2b is depicted in Figure S1 in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

A (Figures S2−S36). The Supporting Information is available free of charge on the ACS Publications Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00723. Crystal structure of complex 2b, selected crystallographic data, and NMR and IR spectra of complexes 1a−6 (PDF) Accession Codes

CCDC 1576227−1576232 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for H.T.: [email protected]. ORCID

Takahito Watanabe: 0000-0002-6495-9961 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Numbers 15H03782 and 17K05798 from the Japan Society for the Promotion of Science (JSPS). We are grateful to the Research and Analytical Center for Giant Molecules, Tohoku University, for elemental analysis.

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REFERENCES

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Organometallics (26) Wakita, K. Yadokari-XG, Software for Crystal Structure Analyses, 2001. Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E. Nippon Kessho Gakkaishi 2009, 51, 218−224.

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Synthesis of Base-Stabilized Hydrido (hydroborylene) tungsten

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