Synthesis of Rhodaboratranes Bearing Phosphine-Tethered Boranes

A series of rhodaboratranes [{o(Ph2P)C6H4}3BRhHn(CO)]m (1, n = 1, m = 0; 4, n = 0, m = +1; 5, n = 0, m = −1) with different electron charges ranging...
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Synthesis of Rhodaboratranes Bearing Phosphine-Tethered Boranes: Evaluation of the Metal−Boron Interaction Hajime Kameo, Yasuhiro Hashimoto, and Hiroshi Nakazawa* Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *

ABSTRACT: A series of rhodaboratranes [{o(Ph 2 P)C6H4}3BRhHn(CO)]m (1, n = 1, m = 0; 4, n = 0, m = +1; 5, n = 0, m = −1) with different electron charges ranging from −1 to +1 have been synthesized. X-ray diffraction, IR, NMR, and DFT calculation studies have demonstrated that the σacceptor borane ligand produces a unique electron distribution in these systems and significantly weakens the Rh−L bond (L = CO, PR3) trans to the boron. The reversible CO/PR3 (R = Me or Ph) substitution reactions of 1 and 5 are attributed to these properties.

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INTRODUCTION Ligands of transition-metal complexes are generally classified as L-type, X-type, or Z-type (Chart 1).1 L-type ligands serve as

and theoretical analyses, indicating that the coordination of the σ-acceptor ligand is not necessarily accompanied by formal twoelectron oxidation.5c They also quantified the electron-withdrawing effect of the σ-accepting borane moiety through IR spectroscopic analyses of the structurally similar rhodium complexes (DPB)Rh(CO)Cl and (i-Pr2PhP)2Rh(CO)Cl.5d Further studies of new frameworks are ongoing,6 and insight into metal−boron interactions has been accumulated. Nevertheless, examples of metal-based reactivity in such compounds are still scarce. To the best of our knowledge, there are only a few examples of redox reactions that preserve the metal−boron bond. One such example was reported by Hill and co-workers, in which they demonstrated deprotonation of the [{B(mimMe)3}Pt(H)(PPh3)]+ (mimMe = 2-mercapto-1-methylimidazolyl) and oxidative additions to the resulting complex [{B(mimMe)3}Pt(PPh3)].3c,j Another interesting report has been presented by Peters and Moret,7 in which they prepared a (TPB)Fe(Br) complex (TPB = triphosphine-borane {(iPr)2P(C6H4)}3B), which was converted into dinitrogen (TPB)Fe(N2) and Na[(TPB)Fe(N2)] complexes by reduction with sodium naphthalenide under dinitrogen.7a The resulting (TPB)Fe(N2) reacted with N3Ar (Ar = p-methoxyphenyl) to give the imido complex (TPB)FeNAr. The series of isolated iron complexes spanned four different electronic states ranging from (Fe−B)6 to (Fe−B)9. The successful syntheses of the (TPB)Fe complexes with the various electron configurations are likely to have resulted from the ability of the Fe−B interaction to respond to changes in the electronic properties of the metal center. These results demonstrate one of the useful properties of Z-type ligands, which is the ability to tune the electron density of the metal center.

Chart 1. Three Coordination Styles of Ligands

two-electron donors for transition metals and are represented by amines, phosphines, CO, and alkenes. X-type ligands typically are composed of halides, alkyls, and hydrides and are considered to donate one electron to the central metal. In comparison, Z-type ligands accept electrons from transition metals through a σ-bonding interaction. Group 13 BR3 Lewis acids are known to act as such σ electron acceptor ligands. These types of ligands potentially have two useful functions: (1) they can tune the electron density of the metal center, and (2) they exhibit a strong trans-labilizing effect. However, Z-type ligands are much less well understood than L- and X-type ligands, and their functions are in the early stages of investigation. A structurally authenticated metallaboratrane was reported by Hill et al. in 1999 for a cage complex of ruthenium.2 After this historical and pioneering work, tri- and tetradentate 2mercapto-1-R-imidazolyl-tethered (R = Me, t-Bu) borane ligands have become irreplaceable scaffolds in inducing transition-metal−boron interactions.3,4 Another important scaffold featuring ambiphilic ligands was developed by Bourissou et al. Their phosphine-based system has widened the field of the chemistry of Z-type ligands.5 For example, the (DPB)AuCl complex (DPB = diphosphineborane {o-R2P(C6H4)}2BPh, R = i-Pr, Ph) provided informative evidence for the d10 configuration of the Au atom through Mössbauer © 2012 American Chemical Society

Received: January 19, 2012 Published: March 22, 2012 3155

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Parallel studies of complexes with σ-acceptor borane ligands and their borane-free complexes would provide fresh insight into the chemistry of Z-type ligands. Hence, we focused on the carbonyl rhodium hydride [{o-(Ph2P)C6H4}3B]RhH(CO) (1), bearing a new triphosphine-borane (TPB) ligand, which has a framework similar to that of the well-known tris(triphenylphosphine) carbonyl rhodium hydride RhH(CO)(PPh3)3 (2), except for the borane moiety (Chart 2). The Chart 2. TPB Complex 1 and Its Related Borane-Free Complex 2

Figure 1. ORTEP drawings of 3 (left) and 1 (right) with 40% probability ellipsoids. Hydrogen atoms and phenyl groups (except for the ipso carbon) in 1 are omitted for clarity. Selected bond distances (Å) and angles (deg) are as follows. 3: B1−C1 = 1.574(4), B1−C19 = 1.580(4), B1−C37 = 1.569(4); C1−B1−C19 = 120.2(2), C1−B1− C37 = 122.0(2), C19−B1−C37 = 114.9(2). 1: Rh1−P1 = 2.3116(13), Rh1−P2 = 2.2733(13), Rh1−P3 = 2.3355(13), Rh1−C1 = 1.960(6), Rh1−B1 = 2.370(5), Rh1−H1 = 1.5367(4), C1−O1 = 1.113(7), B1− C2 = 1.632(7), B1−C20 = 1.637(7), B1−C38 = 1.604(7); P1−Rh1− P2 = 134.19(5), P1−Rh1−P3 = 105.30(5), P2−Rh1−P3 = 107.40(5), P1−Rh1−C1 = 108.47(16), P2−Rh1−C1 = 100.10(18), P3−Rh1−C1 = 94.40(18), P1−Rh1−B1 = 78.90(13), P2−Rh1−B1 = 77.65(14), P3−Rh1−B1 = 77.82(13), C1−Rh1−B1 = 170.6(2), Rh1−C1−O1 = 173.6(5), C2−B1−C20 = 114.4(4), C2−B1−C38 = 109.6(4), C20− B1−C38 = 108.5(4).

comparison should clearly reveal the functions of a Z-type ligand. We focused on the TPB ligand [{o-(Ph2P)C6H4}3B] (3), similar to Bourissou’s TPB ligand,8 but with the isopropyl groups replaced with phenyl groups. The preparation and characterization of complex 1 allowed us to directly compare the properties of the Rh→B interaction in 1 with that in 2.

is comparable in length to the sum of the covalent bonds (2.22 Å).11 These results indicate the presence of a strong Rh→B interaction in 1. Furthermore, the Rh−C1 distance (1.960 Å) is considerably longer than typical Rh−CO bond distances,12 which is attributed to the strong trans influence of boron. To estimate the σ-electron acceptability of the Z-type borane ligand through the Rh→B interaction, we attempted making drastic changes to the electron density of the metal center of 1 through abstraction of H on rhodium as H− or H+. The H− abstraction can be accomplished by treating 1 with a strong Brønsted acid. Treatment of 1 with (HOEt2)(BF4) at ambient temperature immediately afforded the cationic rhodium complex [{o-(Ph2P)C6H4}3BRh(CO)](BF4) (4) and dihydrogen (Scheme 2). The formation of dihydrogen was confirmed by monitoring the reaction of 1 with (HOEt2)(BF4) using 1H NMR spectroscopy (δ 4.59 in CD2Cl2). The 31P{1H} NMR spectrum of 4 showed two mutually coupled resonances at δ 33.1 and 45.0 (JP−P = 33.0 Hz) in a 2:1 ratio, which is consistent with the Cs-symmetric structure observed in the solid state. The H+ abstraction of 1 was achieved by deprotonation using a mixture of KH and 18-crown-6, and the anionic complex [{o-(Ph2P)C6H4}3BRh(CO)][K(18-crown-6)] (5) containing an electron-rich Rh center was obtained in a moderate yield. The 31P{1H} NMR spectrum of 5 recorded at 23 °C showed a sharp doublet at 60.7 ppm (JRh−P = 183.6 Hz).

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RESULTS AND DISCUSSION Compound 3 was prepared in an excellent yield by treating olithiated-phenyldiphenylphosphine with borane trichloride at 100 °C (Scheme 1). The 31P{1H} NMR resonance of 3 was observed at −9.1 ppm as a sharp singlet, which is consistent with an approximate C3v-symmetric structure observed in the solid state. Allowing 3 to react with 2 at 50 °C gave the desired rhodium complex 1 in a moderate yield via phosphine ligand exchange. In the 31P{1H} NMR spectrum of 1 at −100 °C, two broad resonances were observed at δ 55 and 65 with an intensity ratio of 2:1, which coalesced near −60 °C (see the Experimental Section).9 These results corresponded to a Cssymmetric geometry observed in the solid state of 1 (see below) and also suggested the fluxionality of 1. The carbonyl stretching frequency of 1 (1994 cm−1) was higher than that of 2 (1918 cm−1). The shift strongly implies that the borane ligand withdraws a significant amount of electron density through its interaction with rhodium. The molecular structures of 3 and 1 were confirmed by X-ray analysis (Figure 1). Compound 3 has a planar boron configuration (∑(C−B−C) = 357.2°).10 In contrast, the geometry of the boron in 1 deviates noticeably from planarity (∑(C−B−C) = 332.5°), and the Rh−B distance (2.370 Å) in 1

Scheme 1. Synthetic Pathway for the Carbonyl Rhodium Hydride [{o-(Ph2P)C6H4}3B]RhH(CO) (1)

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Scheme 2. Synthetic Pathways for Cationic and Anionic Complexes [{o-(Ph2P)C6H4}3BRh(CO)](BF4) (4) and [{o(Ph2P)C6H4}3BRh(CO)][K(18-crown-6)] (5)

In the 11B{1H} NMR spectra, the resonance shifted to a higher magnetic field in the order 4 < 1 ≈ 5 (4, δ 0.0; 1, δ −7.5; 5, δ −7.6), which implies the strengthening of the σ-electron donation from the Rh center. The crystallographically determined structure of compound 4 is shown in Figure 2. The nonequivalence of the phosphine

assumption of a structure featuring trigonal-bipyramidal geometry is strongly supported by density functional theory (DFT) calculations.13 In the 31P{1H} NMR spectrum of 5, only one sharp doublet was observed, as mentioned above. Furthermore, the full width at half-maximum of the signal remained sharp even at −80 °C (w1/2 = 1.8 Hz), suggesting that 5 has an approximate C3v-symmetric structure. Hence, the NMR data for 5 are entirely consistent with the structure optimized by DFT calculations. Hope et al. prepared the cationic complex [Rh(CO)(PPh3)3](BF4) (6).14 The structure was similar to that of 4, except for the borane anchor (Chart 3). The related anionic Chart 3. Structures of [Rh(CO)(PPh3)3][BF4] (6) and [Rh(CO)(PPh3)3][K(18-crown-6)] (7)

Figure 2. ORTEP drawing of 4 with 40% probability ellipsoids. The anion BF4, hydrogen atoms, and phenyl groups (except for ipso carbon) are omitted for clarity. Selected bond distances (Å) and angles (deg): Rh1−P1 = 2.3104(16), Rh1−P2 = 2.3276(15), Rh1−P3 = 2.3773(15), Rh1−C1 = 1.949(6), Rh1−B1 = 2.286(6), C1−O1 = 1.113(7), B1−C2 = 1.614(8), B1−C20 = 1.619(8), B1−C38 = 1.608(8); P1−Rh1−P2 = 93.61(5), P1−Rh1−P3 = 163.45(5), P2− Rh1−P3 = 94.08(5), P1−Rh1−C1 = 86.99(18), P2−Rh1−C1 = 177.15(17), P3−Rh1−C1 = 84.64(18), P1−Rh1−B1 = 86.50(16), P2−Rh1−B1 = 82.09(17), P3−Rh1−B1 = 80.07(16), C1−Rh1−B1 = 95.2(2), Rh1−C1−O1 = 176.3(5), C2−B1−C20 = 110.7(4), C2− B1−C38 = 118.0(5), C20−B1−C38 = 113.0(5).

complex Na[Rh(CO)(PPh3)3] was characterized by IR spectroscopy studies,15 but its structure remained unconfirmed. In this study, the novel [Rh(CO)(PPh3)3][K(18-crown-6)] (7) (Chart 3) was synthesized by the deprotonation of 2 with a mixture of KH and 18-crown-6, and the structure of the rhodium d10 complex featuring tetrahedral geometry was determined (detailed data from the structural analysis are shown in the Supporting Information). The carbonyl stretching frequencies of 6 and 7 were found to be at 1988 and 1800 cm−1, respectively. These values are considerably lower than those of their ionic complexes bearing borane anchors (4, ν(CO) 2086 cm−1; 5, ν(CO) 1887 cm−1). These shifts are attributable to the presence of the strong Rh→ B interaction. To investigate the rhodium−boron interaction in the series of 1−5, we performed DFT calculations on the actual mononuclear complexes 1−5 (Figure S9, Supporting Information). The B3PW9116/SDD17(Rh),6-31G(d) (C, H, O, P, B) level of theory was found to well reproduce the key features of the complexes 1−4, and there were virtually no deviations in the Rh−B distance and the boron pyramidalization relative to those measured by X-ray crystallography (see the Supporting Information). Natural population analysis data18 together with ν(CO) data for 1−5 are summarized in Table 1. A comparison of 1 (rhodaboratrane), 2 (borane-free Rh complex), and 3 (metal-free borane compound) revealed interesting features of the Rh→B interaction. The natural charge of boron (qB) is lower for 1 than for 3 (0.43 for 1 and 0.88 for 3), indicating that the Rh→B electron donation occurs

groups noted in the 31P{1H} NMR spectrum is consistent with the observed square-pyramidal geometry. The sum of the C− B−C angles is larger in 4 (341.7°) than in 1 (332.5°), indicating that the amount of electron transfer from rhodium to boron decreases, owing to the lower Lewis basicity of the Rh center. This result corresponds to the observation made above in the 11B{1H} NMR spectra. Curiously, the Rh−B bond is shorter in the cationic complex 4 (2.286 Å) than in the neutral complex 1 (2.370 Å), although other results indicated that the borane ligand in 1 withdraws more electrons than that in 4 does. This appears to be due to the absence of a ligand in a position trans to the borane moiety. The unexpected shortening of the Rh−B distance from 1 to 4 can be also explained by the fact that the Rh−B interaction in 1 has more of a 5p orbital character than that in 4 (see the results of NLMO analyses in the Experimental Section). The radius of the 5p orbital is generally larger than that of the 4d orbital; thus, the Rh−B bond increases in length with increasing 5p orbital character of the hybridization. Although the structure of 5 was not determined by X-ray diffraction analysis because of poor crystal quality, the 3157

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Table 1. NBO Atomic Charges (qRh and qB) and ν(CO)

qRh qB ΔqB ν(CO) (cm−1)

1

2

−1.54 0.43 −0.45 1994

−1.56

1918

in 1 to a considerable extent. As mentioned above, the Rh−B distance (2.370 Å) in 1, corresponding to the sum of the covalent bonds (2.22 Å), also suggests the occurrence of a Rh→B interaction. Interestingly, in contrast, the qRh value of 1 (−1.54) is almost equal to that of 2 (−1.56), where the Rh→B interaction is absent. This means that the Rh in 1 increases the electron density of B, while the electron density of Rh does not decrease. The apparent discrepancy may be explained by considering π back-donation from Rh to CO. The value of ν(CO) of 1 (1994 cm−1) is much greater than that of 2 (1918 cm−1), showing that π back-donation from Rh to CO is weaker for 1 than for 2. Therefore, the Rh in 1 gives its electron density to B at the expense of π back-donation to CO. Next, we consider the NBO data of 4, 1, and 5 with negative, neutral, and positive charges on the rhodaboratranes, respectively. Comparison of 1 and 5 reveals that values of qRh are very close (−1.54 and −1.56, respectively) irrespective of the total charge variation (0 to −1). In addition, the values of qB are not significantly different (0.43 for 1 and 0.38 for 5). This means that the Rh→B electron donation is the same for 1 and 5, which is, as mentioned above, supported by the 11B{1H} NMR chemical shift (−7.5 ppm for 1 and −7.6 ppm for 5). In contrast, the value of ν(CO) for 5 (1887 cm−1) is more than 100 cm−1 lower than that for 1 (1994 cm−1). These data show that anionization has only a slight contribution to the enhancement of electron density of either Rh or B, but it significantly strengthens the back-donation from Rh to CO. Therefore, it is considered that the borane ligand in 1 has accumulated almost the maximum possible electron density, and the negative charge given in 5 is mainly distributed to the CO ligand through π back-donation from the Rh center. With 4 bearing a positive charge, the values of qRh (−0.90) and qB (0.65) indicate lower electron densities of Rh and B than for 1 (−1.54 and 0.43) (and also 5). The value of ν(CO) in 4 (2086 cm−1) is about 90 cm−1 larger than that of 1 (1994 cm−1), indicating the decrease in π back-donation to CO. Therefore, it is considered that the positive charge in 4 weakens the Rh→B donation and the π back-donation of Rh→CO and thus decreases the electron density of the Rh. The unique electron flow induced by the Rh→B interaction is expected to influence the reactivity of rhodaboratranes. Hence, we examined the CO/PMe3 substitution reaction. Complex 1 reacted with a slight excess of PMe3 even at ambient temperature to afford the phosphine complex [{o-(Ph2P)C6H4}3B]RhH(PMe3) (8) through a CO/PMe3 exchange reaction (eq 1). Compound 8 was fully characterized by 1H and 31P{1H} NMR spectroscopy as well as elemental analysis.9,19 The substitution reaction was perfectly reversible;

3

4

5

0.88 0

−0.90 0.65 −0.23 2086

−1.56 0.38 −0.50 1887

the exposure of 8 to CO immediately resulted in the almost quantitative regeneration of 1. Neutral monocarbonyl complexes of rhodium(I) rarely show the CO replacement reaction, because of the strong back-donation from the metal center. For example, Baird et al. reported that the rhodium(I) monocarbonyl complex RhMe(CO)(η3-MeC(CH2PPh2)3) on reaction with PMe3 gave RhMe(CO)(PMe3)(η2-MeC(CH2PPh2)3) via a process involving dissociation of one arm of the triphos ligand, but the subsequent dissociation of CO has not been observed.20 Therefore, the properties of the σ-acceptor borane ligand certainly contribute to the CO elimination. Surprisingly, the anionic complex 5 also exhibited a substitution reaction of a carbonyl ligand with a phosphorus ligand under relatively mild conditions (80 °C) to produce the carbonyl-free anionic complex 9 (eq 2). This substitution

reaction is also reversible; 9 reacted with CO to regenerate 5. In comparison, we examined the reaction of the boron-free complex 7 with an excess amount of PPh3 at 100 °C and found the recovery of 7. The strong trans-labilizing effect21 and σelectron acceptor properties of borane are probably responsible for the weakening of the Rh−CO bonds in 1 and 5. Rh(PPh3)4− containing an anionic charge has not been reported. In fact, Rh(−I) complexes with no strong π-acceptor ligand, such as a carbonyl ligand, are rare.22 The presence of a σ-acceptor borane ligand is therefore responsible for the stability of 9. Single crystals of 8 suitable for X-ray diffraction analysis were obtained from slow diffusion of hexane into a dichloromethane solution of 8. The molecular framework of 8 is basically similar to that of 1 (Figure 3). The Rh−B distance (2.354 Å) pointed to the presence of a Rh→B interaction. The boron pyramidalization in 8 (∑(C−B−C) = 330.8°) is slightly more than that of 1, which is probably due to the enhancement 3158

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EXPERIMENTAL SECTION

General Procedures. The compounds described below were handled under a dinitrogen atmosphere, and air and water were removed completely using Schlenk techniques. o-(PPh2)C6H4Br23 and RhH(CO)(PPh3)3 (2)24 were prepared according to literature procedures. Benzene-d6, toluene-d8, tetrahydrofuran-d8, diethyl ether, and benzene were dried over sodium benzophenone ketyl and distilled under the dinitrogen atmosphere. Tetrahydrofuran, hexane, and toluene were purified using a two-column solid-state purification system. Chloroform-d, dichloromethane-d2, and dichloromethane were dried over P2O5 and stored over 4 Å molecular sieves. The other reagents used in this study were purchased from commercial sources and used without further purification. The 1H, 11B, 13C, and 31P NMR spectra were recorded on a JNM-AL-400 spectrometer. 1H and 13C NMR data were referenced to the residual peaks of the solvent, and 11 B and 31P NMR chemical shifts were referenced to external BF3·Et2O and 85% H3PO4 samples, respectively. IR spectra were recorded on a Perkin-Elmer FTIR-Spectrum instrument. Elemental analyses were recorded on a Perkin-Elmer 2400 II elemental analyzer. Preparation of [o-PPh2(C6H4)]3B (3). A 200 mL Schlenk tube was filled with 4.17 g of o-PPh2(C6H4)Br and 60 mL of dimethyl ether, and the mixture was cooled to −80 °C. To the cold solution was slowly added 7.7 mL of nBuLi (1.6 M in hexane, 12.3 mmol), and then the mixture was gradually warmed to room temperature. Additionally, the reaction was stirred at room temperature for 1 h, before the volatile materials were removed under vacuum. The white residue was washed with 2 mL of Et2O to afford 3.88 g of {o-PPh2(C6H4)}Li·Et2O (11.3 mmol) as a white solid in 93% yield. After {o-PPh2(C6H4)}Li·Et2O (3.88 g, 11.3 mmol) was dissolved in toluene (50 mL), the solution was cooled again to −80 °C. To this cooled toluene solution was added a 1 M heptane solution of BCl3 (3.75 mL, 3.75 mmol), and the reaction mixture was stirred at 110 °C for 12 h. The mixture was cooled to room temperature, before the volatile materials were removed under vacuum. The residue was washed with hexane (15 mL × 1, 5 mL × 2) to afford 2.42 g of 3 (3.00 mmol) as a white solid in 82% yield. The white powder was used in the next step without further purification. A sample suitable for elemental analysis was prepared by crystallization by the slow diffusion of hexane into a dichloromethane solution followed by washing with Et2O. 1H NMR (400 MHz, CDCl3): δ 6.94−7.00 (m, 15H), 7.08−7.14 (m, 12H), 7.16−7.22 (m, 12H), 7.33−7.39 (m, 3H). 31P{1H} NMR (163 MHz, CDCl3): δ −9.1 (s). 13C{1H} NMR (100 MHz, CDCl3): δ 128.2, 129.4, 133.9, 134.0, 134.3, 134.7, 135.3, 138.9, 141.3, 157.4. 11B{1H} NMR (128 MHz, CDCl3): δ −7.6. Anal. Found for C54H42P3B: C, 82.00; H, 5.71. Calcd: C, 81.62; H, 5.33. Preparation of [{o-PPh2(C6H4)}3B]RhH(CO) (1). A 100 mL Schlenk tube was filled with 1.24 g of 3 (1.60 mmol), 1.15 g of 2 (1.30 mmol), and 20 mL of benzene, and the reaction mixture was stirred at 50 °C. After 10 h, the mixture was cooled to room temperature, and the volatile materials were removed under vacuum. The orange residue was extracted with 50 mL of Et2O, before the volatile materials were removed under vacuum. The residue was washed with hexane (20 mL) and cold Et2O to afford 0.760 g of 1 (0.80 mmol) as a pale brown solid in 65% yield. Variable-temperature 31P{1H} NMR spectra of 1 are shown in Figure 4. 1H NMR (400 MHz, CDCl3): δ −7.84 (dq, 1JH−Rh = 36.6 Hz, 2JH−P = 10.1 Hz, 1H, RhH), 6.66−6.76 (m, 12H, Ar), 6.76−6.84 (m, 12H, Ar), 6.93−7.06 (m, 12H, Ar), 7.27−7.34 (m, 3H, Ar), 7.89−7.94 (m, 3H, Ar). 31P{1H} NMR (163 MHz, CDCl3): δ 54.8 (d, JP−Rh = 128.5 Hz). 13C{1H} NMR (100 MHz, CDCl3): δ 123.2, 124.4, 124.9, 126.5, 127.3, 127.7, 128.2, 128.5, 128.8, 129.7, 131.5, 132.0, 132.9, 133.4, 133.6, 136.4, 137.2, 140.0, 141.5, 169.8, 192.4. 11B{1H} NMR (128 MHz, THF-d8): δ −7.5. Anal. Found for C55H43P3OBRh: C, 71.28; H, 5.04. Calcd: C, 71.29; H, 4.68. IR (KBr): 1994 cm−1 (νCO). Preparation of [{o-PPh2(C6H4)}3B]Rh(CO)[BF4] (4). A 50 mL Schlenk tube was filled with 117.2 mg of 1 (0.126 mmol) and 10 mL of dichloromethane, and then 18 μL of tetrafluoroboric acid ether complex (0.13 mmol) was added slowly to the solution. The reaction mixture was stirred at room temperature for 1 h, before the volatile

Figure 3. ORTEP drawings of 8 (left) and 9 (right) with 40% probability ellipsoids. Hydrogen atoms and phenyl groups (except for ipso carbon) are omitted for clarity. Selected bond distances (Å) and angles (deg) are as follows. 8: Rh1−P1 = 2.3463(10), Rh1−P2 = 2.3033(11), Rh1−P3 = 2.3002(11) Rh1−P4 = 2.4077(11), Rh1−B1 = 2.354(4), Rh1−H1 = 1.59(6), B1−C1 = 1.628(6), B1−C19 = 1.616(6), B1−C37 = 1.636(6), Rh1−H1 = 1.59(6); P1−Rh1−P2 = 102.79(4), P1−Rh1−P3 = 102.25(4), P1−Rh1−P4 = 105.42(4), P2− Rh1−P3 = 142.16(4), P2−Rh1−P4 = 99.02(4), P3−Rh1−P4 = 101.13(4), P1−Rh1−B1 = 78.31(11), P2−Rh1−B1 = 81.05(12), P3− Rh1−B1 = 76.78(11), P4−Rh1−B1 = 176.11(11), C1−B1−C19 = 112.8(3), C1−B1−C37 = 107.3(3), C19−B1−C37 = 110.7(3). 9: Rh1−P1 = 2.332(4), Rh1−P2 = 2.270(4), Rh1−P3 = 2.284(3), Rh1− P4 = 2.286(4), Rh1−B1 = 2.501(15), B1−C1 = 1.52(3), B1−C19 = 1.61(2), B1−C37 = 1.573(19); P1−Rh1−P2 = 109.17(14), P1−Rh1− P3 = 108.36(13), P1−Rh1−P4 = 104.88(10), P2−Rh1−P3 = 121.61(10), P2−Rh1−P4 = 105.77(14), P3−Rh1−P4 = 105.77(13), P1−Rh1−B1 = 72.5(4), P2−Rh1−B1 = 76.9(3), P3−Rh1−B1 = 73.8(3), P4−Rh1−B1 = 176.9(3), C1−B1−C19 = 108.9(13), C1− B1−C37 = 112.0(13), C19−B1−C37 = 112.2(11).

of the Lewis basicity of the rhodium center derived from a better σ-donating ability of the aliphatic phosphine. In a fashion similar to that for 1, the σ-acceptor borane elongates the Rh−P bond (2.4077 Å) trans to the boron, which is likely to be at the origin of the phosphine elimination. The molecular structure of 9 was also determined by X-ray crystallography using a single crystal obtained from a 8:1 mixed solvent of 1,4-dioxane and tetrahydrofuran (Figure 3). The Xray study exhibited a distorted-trigonal-bipyramidal geometry with P−Rh−P angles of 109.17(14), 108.36(13), and 121.61(10)°, and the phosphine and borane ligands occupied the apical positions. In the 31P{1H} NMR spectrum of 9, mutually coupled signals were observed at δ 34.6 and 45.5 ppm (JP−P = 25.6 Hz) with an intensity ratio of 1:3, which is consistent with a C3v-symmetric structure observed in the solid state.

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Article

SUMMARY

We synthesized a series of rhodaboratrane compounds with different electron charges ranging from −1 to +1. These compounds provide conclusive evidence for the adaptability of σ-acceptor borane ligands to tune the electron density of the metal center. Studies of the electron distribution using NBO analysis show a unique electron flow induced by the σ-acceptor borane ligand. Furthermore, we showed that the σ-acceptor borane ligand potentially exhibits an extremely strong translabilizing effect. The properties of these σ-acceptor borane ligands make them promising candidates for the development of molecular catalysts. 3159

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Organometallics

Figure 4. Variable-temperature THF-d8) of 1.

Article

31

toluene, and then a 1 M toluene solution of PMe3 (300 μL, 0.30 mmol) was added slowly to the solution. The reaction mixture was stirred at room temperature overnight, before the volatile materials were removed under vacuum. The residue was washed with hexane (2 mL × 2) to afford 120.5 mg of 8 (0.124 mmol) as a white solid in 81% yield. 1H NMR (400 MHz, CDCl3): δ −10.56 (ddq, 1JH,Rh = 31.7 Hz, 2 JH,P = 14.6 Hz, 2JH,P = 6.1 Hz, 1H, RhH), 0.89 (d, 1JH,P = 4.9 Hz, 3H, PMe3), 6.52−6.62 (m, 3H, Ar), 6.63−6.74 (m, 3H, Ar), 6.92−7.06 (m, 12H, Ar), 7.07−7.19 (m, 15H, Ar), 7.27−7.41 (m, 9H, Ar). 31P{1H} NMR (163 MHz, CDCl3): δ −34.1 (d, JP−Rh = 91.8 Hz, PMe3), 52.1 (d, JP−Rh = 135.8 Hz, Ph2P). 13C{1H} NMR (100 MHz, CDCl3): δ 21.8, 123.0, 123.7, 125.3, 127.1, 127.4, 127.9, 128.4, 128.6, 129.0, 129.8, 131.5, 132.1, 133.0, 133.5, 133.6, 133.8, 137.4, 138.9, 144.8, 168.8. 11B{1H} NMR (128 MHz, CDCl3): δ −7.6. Anal. Found for C57H52P4BRh: C, 70.14; H, 5.67. Calcd: C, 70.24; H, 5.38. Preparation of [{o-PPh2(C6H4)}3B]Rh(PPh3)[K(18-crown-6)] (9). Method A. A 50 mL Schlenk tube was filled with 39.7 mg of 5 (0.0323 mmol), 28.3 mg of PPh3 (0.108 mmol), and 3 mL of THF, and the reaction mixture was stirred at 80 °C. After 5 h, the mixture was cooled to room temperature and filtered with a cannula. Slow diffusion of hexane into the mixture provided 12.1 g of 9 (0.00827 mmol) as red-brown crystals in 26% yield. Method B. A 50 mL Schlenk tube was filled with 104.3 mg of 1 (0.113 mmol), 57.5 mg of 18-crown-6 (0.218 mmol), 24.9 mg of KH (0.621 mmol), 70.3 mg of PPh3 (0.268 mmol), and 4 mL of THF, and the reaction mixture was stirred at 80 °C. After 3 h, the mixture was cooled to room temperature and filtered with a cannula. Slow diffusion of hexane into the filtrate provided 67.8 g of 9 (0.0463 mmol) as redbrown crystals in 41% yield. 31P{1H} NMR (163 MHz, THF-d8): δ 34.6 (dq, JP−Rh = 100.4 Hz, JP−P = 25.6 Hz, PPh3), 45.5 (dd, JP−Rh = 191.0 Hz, JP−P = 25.6 Hz, Ph2P). 13C{1H} NMR (100 MHz, THF-d8): δ 70.3, 115.7, 121.1, 124.2, 124.6, 125.1, 126.1, 128.0, 128.5, 131.4, 133.1, 131.5, 134.6, 145.0, 145.7. 11B{1H} NMR (128 MHz, THF-d8): δ −7.5. Anal. Found for C84H81P4O6BKRh: C, 69.01; H, 5.37. Calcd: C, 68.95; H, 5.58. Structure Determination by X-ray Diffraction. Preparations of single crystals are described in the Supporting Information. The prepared single crystals were mounted on a CryoLoop with Palaton oil, and the X-ray diffraction experiments were carried out using a Rigaku/MSC Mercury CCD diffractometer with a graphite-monochromated Mo Kα radiation source (λ = 0.710 69 Å). Cell refinement and data reduction were carried out using the CrystalClear program.25 The intensity data were corrected for Lorentz−polarization effects and empirical absorption. The structures were determined by direct methods (SIR 97). All non-hydrogen atoms were found by a difference Fourier synthesis and were refined anisotropically unless otherwise stated. Refinement using the SHELXL-97 package26 was carried out by least-squares methods based on F2 with all measured reflection data. The crystal data and results of the analyses are given in Tables S1−S14 (Supporting Information). CCDC 859005, 859006, 859007, 859008, 859009, 859010, and 859011 contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Density Functional Theory (DFT) Calculations. Density functional theory (DFT) calculations were carried out at the B3PW91 level16 in conjunction with the Stuttgart/Dresden ECP17 and associated with triple-ξ basis sets for Rh. For H, B, C, O, and P, 631-G(d) was employed. All calculations were performed without symmetry constraints utilizing the Gaussian09 program.27 Natural localized molecular orbital (NLMO) analysis data for 1, 4, and 5 are given in Table 2. NBO analysis data for 6 and 7 are given in the Supporting Information (Table S15). Cartesian coordinates are given in Tables S17−S23 (Supporting Information).

P{1H} NMR spectra (163 MHz,

materials were removed under vacuum. The white residue was washed with Et2O (1 mL × 2) to afford 115.4 mg of 4 (0.114 mmol) as a white solid in 90% yield. A 13C NMR spectrum could not be obtained because of its poor solubility. 1H NMR (400 MHz, CDCl3): δ 6.62− 6.71 (m, 3H), 6.76−6.88 (m, 12H), 6.96−7.08 (m, 9H), 7.16−7.23 (m, 3H), 7.28−7.44 (m, 6H), 7.48−7.64 (m, 6H), 7.92−8.00 (m, 3H). 31 1 P{ H} NMR (163 MHz, CDCl3): δ 33.1 (d, JP−Rh = 110.1 Hz, JP−P = 33.0 Hz), 45.0 (dt, JP−Rh = 124.8 Hz, JP−P = 33.0 Hz). 11B{1H} NMR (128 MHz, THF-d8): δ −0.8 (BF4), 0.0 (BAr4). Anal. Found for C55H43P3OBF4Rh: C, 65.19; H, 4.28. Calcd: C, 65.25; H, 4.18. IR (KBr): 2086 cm−1 (νCO). Preparation of [{o-PPh2(C6H4)}3B]Rh(CO)[K(18-crown-6)] (5). A 50 mL Schlenk tube was filled with 41.2 mg of 1 (0.0445 mmol), 20.1 mg of 18-crown-6 (0.0760 mmol), 18.2 mg of KH (0.454 mmol), and 4 mL of THF and the reaction mixture was stirred at 60 °C. After 3 h, the mixture was cooled to room temperature and filtered with a cannula. Slow diffusion of hexane into the filtrate provided 23.3 mg of 5 (0.0190 mmol) as red-brown crystals in 43% yield. 1H NMR (400 MHz, THF-d8): δ 6.47−6.58 (m, 3H), 6.65−6.71 (m, 6H), 6.77−6.85 (m, 18H), 6.98−7.05 (m, 3H), 7.20−7.22 (m, 12H). 31P{1H} NMR (163 MHz, THF-d8): δ 60.7 (d, JP−Rh = 183.6 Hz). 13C{1H} NMR (100 MHz, THF-d8): δ 70.3, 121.1, 124.7, 125.1, 125.8, 128.0, 128.5, 131.2, 131.8, 133.1, 145.0, 172.2. 11B{1H} NMR (128 MHz, THF-d8): δ −7.6. Anal. Found for C67H66P3O7BKRh: C, 65.56; H, 5.42. Calcd: C, 65.48; H, 5.41. IR (KBr): 1887 cm−1 (νCO). Preparation of [Rh(CO)(PPh3)3][BF4] (6). Compound 6 was synthesized in a manner slightly different from that reported by Hope et al.14 A 50 mL Schlenk tube was filled with 57.2 mg of 2 (0.0623 mmol) and 7 mL of dichloromethane, and then 9.0 μL of tetrafluoroboric acid ether complex (0.066 mmol) was added slowly to the solution. The reaction mixture was stirred at room temperature for 2 h, before the volatile materials were removed under vacuum. The residue was washed with Et2O (1 mL × 3) to afford 56.9 mg of 6 (0.0566 mmol) as a white solid in 91% yield. The NMR data were identical with those reported by Hope et al.14 Preparation of [Rh(CO)(PPh3)3][K(18-crown-6)] (7). A 50 mL Schlenk tube was filled with 53.1 mg of 2 (0.0578 mmol), 31.8 g of 18crown-6 (0.120 mmol), 23.7 mg of KH (0.591 mmol), and 2 mL of THF, and the reaction mixture was stirred at 60 °C. After 3 h, the mixture was cooled to room temperature, and filtered with a cannula. Slow diffusion of hexane into the filtrate provided 35.0 g of 7 as redbrown crystals in 50% yield. The IR spectral data were identical with those reported by Zotti et al.15 1H NMR (400 MHz, THF-d8): δ 3.54 (s, 24H, C12H24O6), 6.71−6.75 (m, 15H, Ar), 6.83 (m, 6H, Ar), 7.21− 7.25 (m, 12H, Ar), 7.27−7.33 (m, 9H, Ar), 7.38−7.42 (m, 3H, Ar). 13 C{1H} NMR (100 MHz, THF-d8): δ 70.3, 124.6, 125.7, 128.2, 133.7, 147.0. 31P{1H} NMR (163 MHz, THF-d8): δ 47.7 (d, JP−Rh = 171.2 Hz). Anal. Found for C67H69P3O7KRh: C, 65.90; H, 5.70. Calcd: C, 65.57; H, 5.33. IR (KBr): 1800 cm−1 (νCO). Preparation of [{o-PPh2(C6H4)}3B]RhH(PMe3) (8). A 50 mL Schlenk tube was filled with 141.5 mg of 1 (0.153 mmol) and 2 mL of 3160

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(h) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2006, 25, 289. (i) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2007, 26, 3891. (j) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2008, 27, 312. (k) Crossley, I. R.; Foreman, M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J.; Willis, A. C. Organometallics 2008, 27, 381. (l) Crossley, I. R.; Hill, A. F. Dalton Trans. 2008, 201. (m) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2010, 29, 326. (4) (a) Mihalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.; Rheingold, A. L.; Rabinovitch, D. Dalton Trans. 2004, 1626. (b) Landry, V. K.; Melnick, J. G.; Buccella, D.; Pang, K.; Ulichny, J. C.; Parkin, G. Inorg. Chem. 2006, 45, 2588. (c) Figueroa, J. S.; Melnick, J. G.; Parkin, G. Inorg. Chem. 2006, 45, 7056. (d) Blagg, R. J.; Charmant, J. P. H.; Connelly, N. G.; Haddow, M. F.; Orpen, A. G. Chem. Commun. 2006, 2350. (e) Senda, S.; Ohki, Y.; Hirayama, T.; Toda, D.; Chen, J.-L.; Matsumoto, T.; Kawaguchi, H.; Tatsumi, K. Inorg. Chem. 2006, 45, 9914. (f) Pang, K.; Quan, S. M.; Parkin, G. Chem. Commun. 2006, 5015. (g) Pang, K.; Tanski, J. M.; Parkin, G. Chem. Commun. 2008, 1008. (5) (a) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem., Int. Ed. 2006, 45, 1611. (b) Bontemps, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. J. Am. Chem. Soc. 2006, 128, 12056. (c) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583. (d) Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.; Howard, J. A. K.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Chem. Eur. J. 2008, 14, 731. (e) Bontemps, S.; Bouhadir, G.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2008, 47, 1481. (f) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729. (g) Sircoglou, M.; Bontemps, S.; Mercy, M.; Miqueu, K.; Ladeira, S.; Saffon, N.; Maron, L.; Bouhadir, G.; Bourissou, D. Inorg. Chem. 2010, 49, 3983. (6) (a) Tsoureas, N.; Haddow, M. F.; Hamilton, A.; Owen, G. R. Chem. Commun. 2009, 2538. (b) Tsoureas, N.; Bevis, T.; Butts, C. P.; Hamilton, A.; Owen, G. R. Organometallics 2009, 28, 5222. (7) (a) Moret, M.-E.; Peters, J. C. Angew. Chem., Int. Ed. 2011, 50, 2063. (b) Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc. 2011, 133, 18118. (8) Bontemps, S.; Bouadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2007, 46, 5149. (9) The low-temperature fluxionality of compounds 1 and 8 is somewhat unexpected for octahedral complexes, and a possible explanation would be transient migration of the hydride to the boron atom to form a hydrogenoborate ligand (R3BH−). Related hydride migrations are known; see for example: Tsoureas, N.; Kuo, Y.-Y.; Haddow, M. F.; Owen, G. R. Chem. Commun. 2011, 484. (10) Unlike Bourissou’s isopropyl analogue, in which the boron is pyramidalized owing to the presence of an intramolecular P→B interaction, compound 3 has planar boron (∑(C−B−C) = 357.2°). This difference probably originates from the lower electron-donating ability of the aromatic phosphine. (11) Covalent radius: Rh, 134 pm; B, 88 pm: Atkins, P. W.; Overton, T.; Rourke, J.; Weller, M.; Armstrong, F. Shriver and Atkins Inorganic Chemistry, 4th ed.; Oxford University Press: New York, 2006; p 24. (12) We checked 1943 rhodium complexes with a carbonyl ligand in a terminal configuration via the Cambridge Structural database (version 5.32) and found that only six complexes have a longer Rh−CO bond (over 1.960 Å) than complex 1. Recently, Owen reported an interesting explanation on the very large trans influence observed in the octahedral system.1f (13) Density functional theory (DFT) calculations were carried out at the B3PW9116/(Rh, SDD;17 H, C, B, O, P, 6-31G*) level of theory. To confirm that the aforementioned basis sets were able to reproduce the geometry of the anionic complex 5, the geometry of anionic complex 9, which is similar to that of 5 (whose structure was accurately characterized by X-ray diffraction studies), was optimized. This optimized geometry, shown in Figure S15 (Supporting

Table 2. Hybridization of NLMO Involving the Rh and B Atoms and NLMO/NPA Bond Orders28 1 % d(Rh) % p(B) % s(Rh) % p(Rh) % d(Rh) % s(B) % p(B)

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NLMO d(Rh)→p(B) 76.21 86.19 19.14 11.84 Donor NBO d(Rh) 0.23 1.83 8.14 0.43 91.62 97.74 Acceptor NBO p(B) 18.80 11.08 81.19 88.86 Rh−B Bond Order 0.571 0.443

5 56.69 30.36 2.34 21.31 76.35 20.05 79.95 0.582

ASSOCIATED CONTENT

S Supporting Information *

Tables, figures, and CIF files giving bond lengths and angles, structure refinement details, and ORTEP drawings of 3, 1, 4, 6, 7, 8, and 9 and details of the DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was supported by a Challenging Exploratory Research grant (No. 23655056) and a Grant-inAid for Young Scientists(B) (No. 23750064) from Japan Society of the Promotion of Science. H.K. acknowledges financial support from the Sasakawa Scientific Research Grant from the Japan Science Society. Finally, we thank Prof. A. F. Hill, Prof. G. Parkin, and Prof. D. Bourissou for attracting our attention to this field.

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REFERENCES

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