Trioxorhena(VII)carborane Anion and Its Methyl-Substituted Analogue

Article pubs.acs.org/Organometallics

Trioxorhena(VII)carborane Anion and Its Methyl-Substituted Analogue: Synthesis, Structure, DFT, and Catalytic Studies Kothanda Rama Pichaandi, Phillip E. Fanwick, and Mahdi M. Abu-Omar* Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Synthesis and characterization of trioxorhena(VII)carborane [Bu4N][(η1-C2B9H11)ReO3] (1a) and its methyl-substituted analogue [Bu4N][(7,8-Me2-η1-C2B9H11)ReO3] (1b) are reported. The single-crystal X-ray structures of 1a and 1b display η1 coordination between Re and the carborane cage. Density functional theory computations at B3LYP/LANL2DZ (Gaussian 09 suite) level show that the strong π donor character of the oxo ligands results in a weak interaction between the d orbitals on Re and the π orbitals of the boron cage, the consequence of which is favoring η1 Re−B coordination. Complexes 1a and 1b exhibit reversible oneelectron reduction in cyclic voltammetry experiments at E1/2 potentials of −1.83 and −1.88 V versus Cp2Fe/Cp2Fe+, respectively. Complex 1a catalyzes the hydrosilylation of aldehydes and ketones in excellent yields and with high tolerance to a variety of functional groups. Mechanistic investigation of the hydrosilylation reaction revealed potential involvement of multirhenium−boron cluster.

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biomass-derived compounds.11 We report herein the synthesis of trioxorhena(VII)carborane [Bu4N][(η1-C2B9H11)ReO3] (1a) and its methyl-substituted analogue [Bu4N][(7,8-Me2-η1C2B9H11)ReO3] (1b). Unlike Cp*ReO3, the carborane complexes 1 are formally 14-electron complexes featuring η1 ligation to the carborane cage. Density functional theory (DFT) studies were undertaken to shed light on the reason behind the preference for η1 ligation versus η5. The latter is observed for the rhenium(I) carbonyl precursor complexes. Trioxorhenium carborane 1a was investigated as a catalyst for the hydrosilylation of aldehydes and ketones, and it gave good to excellent yields of the corresponding silyl ethers. It should be noted that the only known trioxo complex of carborane is that of molybdenum [Me4N]2[(η1-C2B9H11)MoO3], which was reported 15 years ago.12 It also adopts η1 ligation to the carborane cage.

INTRODUCTION The catalytic diversity of methyltrioxorhenium (CH3ReO3, MTO) has fueled significant research development in organometallic oxorhenium chemistry.1 In addition to being an epoxidation catalyst with hydrogen peroxide, MTO was shown to catalyze aldehyde olefination and a myriad of oxygen atom transfer (OAT) reactions.2 Upon immobilization, heterogeneous MTO also catalyzes olefin metathesis.2a,3 More recently oxorhenium complexes have received attention as hydrosilylation catalysts.4 Even though transition metal oxo complexes are often employed in oxidation reactions, the utility of oxorhenium compounds in reduction catalysis has stimulated new mechanistic paradigms for high oxidation state organometallic complexes.5 In comparison, the 18-electron species Cp*ReO3 has been more limited in its catalytic applications. One noteworthy reaction that has garnered renewed interest in the context of biomass conversion is deoxydehydration of vicinal diols.6 Gable et al. studied the mechanism of alkene extrusion from rhenium diolate complexes.7 While the carborane complex [Me4N][(η5C2B9H11)Re(CO)3] has been known for nearly half a century,8 the trioxorhenium(VII) carborane (an analogue of Cp*ReO3) has not been prepared. Limited exchange of the CO ligand with other π acceptor ligands such as nitrosyl and alkylidene has been noted. Fischer and co-workers have reported exchange of CO on rhenium with halides such as Cl− and I− using Me3NO as a CO scavenger.9 Our research group has had a long interest in the reaction chemistry of oxorhenium complexes in the context of OAT,10 catalytic hydrosilylation,4d and selective deoxygenation of © 2012 American Chemical Society

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RESULTS AND DISCUSSION Synthesis. The [Bu4N][nido-7,8-C2B9H12] and its rhenium tricarbonyl complex [Bu4N][(η5-C2B9H11)Re(CO)3] (2a) as well as [Bu4N][7,8-Me2-nido-7,8-C2B9H10] were prepared according to published procedures.8,13 The rhenium tricarbonyl complex [Bu4N][(7,8-Me2-η5-C2B9H11)Re(CO)3] (2b) was prepared similarly to the procedure described for 2a. Oxidation of 2a by PhIO in the presence of Ph2S2 or PhS(O2)SPh, with catalytic amounts of CH3COOH at 6−8 and 15−20 °C, respectively, yielded 1a in 83% yield (Scheme 1). In the Received: December 8, 2011 Published: February 13, 2012 1888

dx.doi.org/10.1021/om201222r | Organometallics 2012, 31, 1888−1896

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absence of Ph2S2 or PhS(O2)SPh, the oxidation was not successful and gave a poor yield. Similar observations were reported by Kim et al. during the direct oxidation of [Me4N]2[(η5-C2B9H11)Mo(CO)3] to [Me4N]2[(η1-C2B9H11)MoO3] by PhIO.12 Ph2S2 as well as PhS(O2)SPh acts as CO scavengers. The need for a CO scavenger for the conversion of 2a to 1a is not surprising since the oxidation of 2a to [Me4N][(η5-C2B9H11)Re(CO)2Cl2]− by [N(C6H4Br-4)3][SbCl6] took place only in the presence of Me3NO, a recognized CO scavenger.9 Also, PhS(O2)SPh is used as a CO transfer agent in the synthesis of thiol esters via a radical mechanism.14 The possibility of making an oxidative-addition product of 2a with Ph2S2 analogous to [Me4N]2[(η5-C2B9H11)Mo(CO)2(SPh)2]12 is unlikely because no reaction was observed between 2a and Ph2S2 in the absence of an oxidizing agent. Oxidation of 2a to 1a can also be carried out with H2O2/ CH3COOH in the presence of Ph2S2 in benzene under reflux.15 PhS(O2)SPh and [Bu4N][ReO4] were obtained as byproducts in this reaction. However, the reaction with H2O2/CH3COOH gave a lower yield of 1a (42%) due to decomposition of 1a at higher temperature. Compound 2b was converted to its

Scheme 1. Synthesis of Trioxorhenium Carborane Compounds 1

Table 1. Crystal Data and Structure Refinement for 2b, 1a, and 1b 2b formula fw space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z dcalc, g cm−3 cryst dimens, mm temperature, K radiation (wavelength, Å) monochromator linear abs coef, mm−1 absorp corr applied transmn factors: min., max. diffractometer h, k, l range 2θ range, deg mosaicity, deg programs used F000 weighting data collected unique data Rint data used in refinement cutoff used in R-factor calculations data with I > 2.0σ(I) no. of variables largest shift/esd in final cycle R(Fo) Rw(Fo2) goodness of fit

1a

1b

C18H47B9NO3Re C31H35B9O3PRe 770.1 609.08 P 1 21/c 1 (No. 14) P1̅ (No. 2) 15.6225(4) 11.3208(6) 7.4327(2) 12.0687(8) 28.7002(10) 12.5154(5) 90 66.350(4) 95.433(3) 78.506(4) 90 62.778(4) 3317.62(17) 1392.58(13) 4 2 1.542 1.452 0.20 × 0.10 × 0.04 0.20 × 0.14 × 0.10 150 150 Cu Kα (1.54184) Cu Kα (1.54184) confocal optics confocal optics 7.858 8.661 empirical empirical 0.59, 0.73 0.30, 0.42 Rigaku RAPID-II Rigaku RAPID-II 0 to 19, 0 to 9, −31 to 33 0 to 13, −12 to 14, −14 to 15 3.09−145.18 7.71−144.38 0.62 1.3 SHELXTL SHELXTL 1520 612 1/[σ2(Fo2) + (0.0612P)2 + 9.4122P] where P = (Fo2 + 2Fc2)/3 34 553 21 937 6166 4882 0.055 0.075 6166 4882 Fo2 > 2.0σ(Fo2) Fo2 > 2.0σ(Fo2) 5963 4772 444 337 0 0 0.039 0.05 0.109 0.138 1.101 1.114 1889

C20H51B9NO3Re 637.13 P 1 21/n 1 (No. 14) 14.3733(10) 11.3361(7) 19.5318(14) 90 95.052(5) 90 3170.1(4) 4 1.335 0.20 × 0.10 × 0.03 293 Cu Kα (1.54184) confocal optics 7.633 empirical 0.61, 0.80 Rigaku RAPID-II 0 to 16, 0 to 13, −21 to 20 4.54−126.76 0.72 SHELXTL 1288 23 105 3998 0.075 3998 Fo2 > 2.0σ(Fo2) 2874 347 0 0.063 0.194 1.058


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and steric effect of methyl substitution on the cage carbon atoms. Single-crystal X-ray structures of 1a and 1b are given in Figure 2. Their comparison of bond lengths and bond angles, in addition to the DFT-predicted structure for 1a, is given in Table 2. Complexes 1a and 1b have η1 coordination and distorted tetrahedral geometry around the rhenium center. The Re−B bond lengths in 1a and 1b are comparable within the error limits. All three Re−O bond lengths in 1a and 1b are also comparable among themselves as well as with the Re−O bond lengths in CpReO317 and Cp*ReO3.18 DFT Studies. Earlier DFT studies by Fisher et al. on 2a were limited to explaining its nonemissive behavior.9 To shed light on the η5 versus η1 bonding preference observed in 2 and 1, respectively, we carried out DFT calculations at the B3LYP/ LANL2DZ level (Gaussian 09 suite).19 Geometry optimizations for 1a, 1b, and 2b were obtained from their respective crystal structures reported in this work, and for 2a, the crystal structure reported by Zalkin et al.16 Table 2 displays DFT-calculated bond lengths and bond angles for 1a alongside experimental values obtained from X-ray (for 1b, 2a, and 2b, see Supporting Information). The bond lengths and angles of the optimized structures are in reasonable agreement with experimental values. As expected, computed complexes 2a and 2b feature η5 bonding, whereas 1a and 1b have η1 bonding in their global minimum structure, consistent with experimental structures. When 1a and 1b were forced to have η5 coordination at the onset of the geometry optimization, the calculation converged to give η1 coordination, demonstrating that DFT was successfully predicting the preferred geometry irrespective of the starting structure. The compositions of the molecular orbitals (MO) responsible for bonding between Re and the boron cage were examined. Representative MOs for 2a and 1a are compared in Figure 3. In complex 2a, as expected for metal carbonyl compounds, there is strong π back-donation from the d orbitals of Re to the π* orbitals of CO. This creates an electron deficiency on the Re center and favors the same d orbitals to interact robustly with the in-plane π orbitals of the boron cage. This synergy results in a covalent interaction of all the π orbitals on the boron cage and the rhenium atom, hence, η 5 coordination of the carborane. Ten more such molecular orbitals (Supporting Information) are involved in this bonding

trioxorhenium carborane 1b using H2O2/CH3COOH in refluxing benzene in the presence of Ph2S2 or PhS(O2)SPh with 33% yield. Oxidation of 2b by PhIO was not successful, resulting in partial oxidation and decomposition to boric acid. Structural Characterization. Table 1 represents the crystal data and structure refinement for 2b, 1a, and 1b. Xray quality crystals of 2b as the tetraphenylphosphonium salt were obtained as needles belonging to the P21/c space group. The structure of 2b (Figure 1) resembles that of 2a reported by

Figure 1. ORTEP drawing of compound 2b. Ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å), angles (deg), and torsions (deg): Re1−B4 = 2.338(5), Re1−B3 = 2.300(6), Re1−C1 = 2.347(5), Re1−C5 = 1.919(5), B3−C2 = 1.733(7), C1−C2 = 1.664(7), C5−Re1−B4 = 126.1(3), Re1−B4−B5 = 66.8(2), C5−Re1−C7 = 89.4(2), C6−Re1− C7 = 89.3(2), Re1−C1−B5−B4 = −57.1(3), C5−Re1−B4−B5 = −87.6(3).

Zalkin et al.16 It has η5 coordination between Re and the boron cage, and all three CO groups are near perpendicular to each other. One notable difference between 2b and 2a is the shortening of the Re−B bond by 0.027 Å and elongation of the Re−C as well as the C−C bonds of the cage by 0.057 and 0.048 Å, respectively. This difference is attributed to the electronic

Figure 2. ORTEP drawing of compounds 1a and 1b. Ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity. 1890


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Table 2. Comparison of X-ray (1a, 1b) and DFT (1a) Bond Lengths (Å) and Bond Angles (deg) bond length Re1−B4 Re1−O1 Re1−O2 Re1−O3 B4−B5 B4−B3 B5−C1 C1−C2 Re1−B3 Re1−C1

experimental (X-ray)

DFT

(1b)

(1a)

(1a)

2.16(1) 1.715(8) 1.72(1) 1.69(1) 1.77(2) 1.79(2) 1.67(2) 1.52(1) 2.59(1) 3.156

2.19(1) 1.727(3) 1.695(3) 1.730(5) 1.797(8) 1.81(1) 1.626(9) 1.545(7) 2.574(8) 3.104

2.162 1.743 1.737 1.743 1.838 1.838 1.619 1.566 2.738 3.361

bond angle Re1−B4−B5 Re1−B4−B3 O2−Re1−B4 O3−Re1−B4 O2−Re1−O1 O1−Re1−B4 B4−B5−C1

experimental (X-ray)

DFT

(1b)

(1a)

(1a)

80.6(6) 81.6(6) 105.35(5) 103.3(8) 105.4(4) 126.7(4) 104.3(8)

80.9(4) 79.5(4) 126.1(3) 101.7(3) 109.1(2) 104.1(3) 104.9(5)

86.00 85.96 120.05 104.42 109.73 104.39 104.04

contributions from nonbonding π orbitals on the oxygen atoms as well as dxy on Re. The frontier unoccupied molecular orbitals (LUMO, LUMO+1, and LUMO+2) are predominantly π* and d orbitals of the ReO bond and Re metal, respectively, as well as a minor contribution from the π* orbitals of the boron cage. The frontier orbitals responsible for the observed UV−vis spectrum of 1a (Figure 5) and a comparison of the excitation energy between experimental and DFT of 1a and 1b are given in Table 3 and are discussed next. UV−Vis, IR, and Electrochemistry. Compounds 1a and 1b have the longest wavelength absorptions at 344 and 385 nm with extinction coefficients (ε) of 2306 and 2418 L mol−1 cm−1, respectively in CH2Cl2 (Figure 5). This transition can be assigned to the LMCT transition from HOMO to LUMO+1 and LUMO+2 (Figure 4 for 1a, Supporting Information for 1b) from TD-DFT calculations. The excitation energy calculated from the UV−vis spectra is in reasonable agreement with the value obtained from TD-DFT calculations (Table 3). The red shift of 41 nm for 1b versus 1a is due to the electron-donating methyl substituents on the carborane cage and is comparable to the red shift of 22 nm obtained from TD-DFT calculations. Tables 4 and 5 represent the comparison of IR frequencies of 2 and 1 with their structural analogues reported in the literature.17,20 Complexes 2a and 2b have lower stretching νCO compared with their Cp and Cp* analogues. This implies that the carborane cage upon η5 coordination is a better electron donor compared to Cp and Cp*. The νRe−O stretching frequencies of 1 are in between MTO, PhReO3, and η5coordinated Cp/Cp*ReO3, indicating that the Re−O bond strength of 1 is lower than MTO and PhReO3 but higher than Cp/Cp*ReO3. This implies that the carborane cage upon η1 coordination is a weaker donor compared to Cp and Cp*, but a stronger electron donor than Ph or methyl. These results support the DFT calculations that show the π acceptor and π donor characteristics of CO and oxo ligands are majorly responsible for η5 versus η1 bonding modes in 2 and 1, respectively. Compounds 1a and 1b show a one-electron (Re7+/Re6+) reversible reduction wave in the CV with E1/2 values of −1.83 and −1.88 V versus Cp2Fe/Cp2Fe+ in CH2Cl2, respectively (Figure 6). The shift in the reduction potential (50 mV) for 1b versus 1a is attributed to the electron-donating methyl groups on the carborane cage. The observed reduction potentials for 1a and 1b are more negative than the reported value for MTO (−1.24 V) in CH2Cl2.21 Catalytic Studies. To explore the catalytic potential of 1, hydrosilylation of aldehydes and ketones with Et3SiH was carried out using 1a as a catalyst (Scheme 2). The reaction was

Figure 3. Representative MOs of 2a and 1a responsible for their respective η5 and η1 bonding.

mode in addition to the HOMO and HOMO−1 illustrated in Figure 3. This bonding synergy does not exist in compound 1a, primarily because of the π donor character of the oxo ligands. Here the bonding between Re and the boron cage revolves around HOMO−2 and HOMO−12 (Figure 3). These involve a unidirectional weak interaction between dz2 and a small portion of the π orbitals of the boron cage and p orbitals of the boron atom attached to Re, thereby explaining the observed η1 coordination. A similar observation was found in comparing the methyl-substituted carborane analogues 1b and 2b (Supporting Information). Figure 4 represents the frontier MOs of 1a. The HOMO orbital is dominated by π orbitals of the boron cage with minor 1891


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Figure 4. Frontier MO orbitals of 1a based on TD-DFT calculations.

Table 4. Comparison of IR Frequencies (cm−1) of 2 with Their Structural Analogues20a,b CpRe(CO)3

Cp*Re(CO)3

2a

2b

2035 1946

2005 1906

1999 1895

1991 1875

νsym(CO) νas(CO)

Table 5. Comparison of IR Frequencies (cm−1) of 1 with Their Structural Analogues17 CpReO3

Cp*ReO3

νsym(Re−O)

926

909

Figure 5. UV−vis spectra of 1a and 1b in CH2Cl2.

νas(Re−O)

886

878

carried out in CD2Cl2 under reflux. The yields of silyl ether products were good to excellent (Table 6). Catalyst 1a is tolerant of various functional groups such as OMe, NMe2, Cl, and NO2 (entries 3, 2, 4, and 6 in Table 6). Chemoselective reduction of bifunctional molecules (entries 9, 12, and 14 in Table 6) ascertains the versatility of 1a. The selectivity of trans over cis (25:1) for the corresponding ether formation with tertbutylcyclohexanone showcases the catalyst’s stereoselectivity,

similar to the results obtained for the dioxorhenium(V) catalyst (PPh3)2Re(O)2I.5a Longer time and higher mol % of catalyst were required for 4-nitrobenzaldehyde and pyridine-2-carboxaldehyde (entries 6 and 7; Table 6) due to their coordinating ability with the catalyst. Moderate yield with 2-cyanobenzaldehyde (entry 5) can be understood by competitive hydrosilylation of the cyano group with the formation of corresponding imines.22 This agrees with the lower yields

1a

1b

954, 930 916, 903

954, 929 916, 901

PhReO3

MTO

986

1005

956

953

Table 3. Comparison of Excitation Energies from UV−Vis Experiment and TD-DFT experimental

DFT

compound

λmax

excitation energy (eV)

λmax

excitation energy (eV)

orbital transitions

oscillator strength

% contribution

1a

344

3.61

1b

385

3.22

400 353 422 366

3.10 3.52 2.94 3.39

HOMO−LUMO+1 HOMO−LUMO+2 HOMO−LUMO+1 HOMO−LUMO+2

0.053 0.013 0.047 0.010

92 98 93 98

1892


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Table 6. Hydrosilylation of Ketones and Aldehydes Catalyzed by Trioxorhenium 1aa,b

Figure 6. Cyclic voltametry of 1a and 1b in CH2Cl2 at 100 mV scan rate using Bu4NPF6 (0.1 M) as a supporting electrolyte.

Scheme 2. Hydrosilylation of Carbonyl Compounds Using 1a As Catalyst

obtained when acetonitrile was used as solvent for the hydrosilylation of benzaldehyde with triethylsilane. When a lower temperature (25 °C) was tried for hydrosilylation of benzaldehyde with Et3SiH, there was no impact on the yield; however, longer reaction times (48 h) were required. When THF-d8 was used as a solvent for the hydrosilylation of benzaldehyde, the result was the same as that of CD2Cl2. However when CDCl3 was used as a solvent, a competitive Si− H/C−Cl exchange reaction was observed with the formation of CDHCl2 and Et3SiCl, in addition to the hydrosilylation reaction. The hydrosilylation of 2-butanone with Et3SiH (1:1) under neat reaction conditions gave 55% of the corresponding ether. The lower yield was due to catalyst precipitation as the solvent polarity changes during the course of the reaction. The precipitated catalyst can be reused with fresh reactants without altering the yield or reaction time. Moderate to good yields were obtained (Table 7) with various organosilanes. In the case of PhSiH3, the lower yield of PhSiH2OCH2Ph was observed due to consecutive hydrosilylation of the formed product with the formation of PhSiH(OCH2Ph)2 and PhSi(OCH2Ph)3 as observed by GC-MS. However, reduction with sterically encumbered iPr3SiH was not successful.23 Observation of an induction period (ca. 1 h) in the hydrosilylation of benzaldehye with Et3SiH indicated the formation of an active intermediate(s) over prolonged reaction times. Reaction of 1a with a 20-fold excess of Et3SiH in the absence of benzaldehyde, under reflux, resulted in a dark brown mixture that is sensitive toward moisture. Formation of molecular H2 was observed by 1H NMR. Removal of the solvent and excess silane under vacuum from the brown reaction mixture afforded a solid residue that, after washing with hexane, acted as an effective catalyst for the hydrosilylation of benzaldehyde with Et3SiH without exhibiting an induction period. Within 1 h, 43% of the corresponding silyl ether was formed, in comparison with only 11% under similar conditions starting with precatalyst 1a. The absence of an induction period indicated that this mixture contained the active catalyst. Another plausible interpretation of the long induction period is formation of a heterogeneous Re metal (or nanoparticles)

a

Reaction conditions: 1 mol % of 1a, substrate = 0.23 mmol, 1.8−2 equiv of Et3SiH, CD2Cl2, 40 °C. bNMR yields. c10 mol % of 1a. d6 mol % of 1a.

that is responsible for catalysis. A common test for heterogeneous catalyst activity is poisoning by metallic Hg.24 Catalytic hydrosilylation of benzaldehyde with 1a was carried out in the presence of a 275-fold excess of Hg. The yields of silyl ether and catalyst activity were unaffected by the presence of Hg. This result rules out heterogeneous Re metal being the catalyst. The catalytic mixture gave a complex EPR spectrum with a g value of 2.3 (Supporting Information), indicating the presence of paramagnetic species. The complexity and the hyperfine splitting pattern indicated the presence of multiple rhenium clusters.25 The broad 11B{1H} NMR spectrum that extends from δ +60 to −60 in contrast to the sharp chemical shifts observed for 1a is very typical for paramagnetic transition metal dicarbollides and exhibits the characteristic boron cage having a large paramagnetic contact and pseudocontact shift (Supporting Information).26 The presence of chemical shifts corresponding to OSiEt3 groups in the 1H and 13C{1H} NMR indicated the possibility of active species having Et3SiO− ligands 1893


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Re(CO)3] (2a), as well as [Bu4N][7,8-Me2-nido-7,8-C2B9H10] were prepared according to published procedures.8,13,27 Rhenium tricarbonyl complex [Bu4N][(7,8-Me2-η5-C2B9H11)Re(CO)3] (2b) was prepared similarly to 2a. 1H NMR spectra were recorded on Varian Inova 300 and 600 MHz and Bruker DRX 500 and 800 MHz instruments. 13C{1H} NMR were recorded in Varian Inova 75 MHz and Bruker DRX 125 MHz instruments. 11B{1H} NMR spectra were recorded in Bruker DRX 165 MHz and Varian Inova 99 MHz instruments. EPR spectra were recorded on a Bruker ESP 300E EPR spectrometer equipped with an hp 5350B microwave frequency counter, an Oxford ITC4 temperature controller, and a VC40 gas flow controller (for liquid He) and a Eurotherm temperature control unit (liquid N2). IR spectra and UV−vis were recorded using ThermoNicolet Nexus FT-IR and Shimadzu 2501-A instruments, respectively. A PARSTAT 2273 potentiostat-galvanostat was used in cyclic voltammetric experiments in dichloromethane with Bu4NPF6 (0.1 M) as a supporting electrolyte with a scan rate of 100 mV/s. A silver wire immersed in 0.010 M AgNO3/0.10 M Bu4NPF6/acetonitrile served as a reference electrode. The working electrode was a glassy carbon electrode with an area of 0.074 cm2. All potentials are reported vs Cp2Fe/Cp2Fe+, and the E1/2 was calculated by the average of cathodic and anodic peak potentials. All data fittings were carried out using KaleidaGraph. Mass spectrometry was performed by the Purdue University Campus Wide Mass Spectrometry Center using a HewlettPackard Engine mass spectrometer (GC/MS). All electrospray ionization analyses were carried out on a FinniganMAT LCQ Classic (ThermoElectron Corp, San Jose, CA, USA) mass spectrometer system. The electrospray needle voltage was set at 4.0 kV, the heated capillary voltage was set to 10 V, and the capillary temperature was 207 °C. Typical background source pressure was 1.2 × 10−5 Torr as read by an ion gauge. The sample flow rate was approximately 8 μL per minute. The drying gas was nitrogen. The LCQ is typically scanned to 2000 amu for these experiments. The electrospray ionization highresolution mass measurements were obtained in the peak matching mode using a FinniganMAT XL95 (FinniganMAT Corp., Bremen, Germany) mass spectrometer. The instrument was calibrated to a resolution of 10 000 with a 10% valley between peaks using the appropriate polypropylene glycol standards. Characterization of [Bu4N][(7,8-Me2-η5-C2B9H11)Re(CO)3] (2b). Yield: 0.82 g (70%). 1H NMR (500 MHz, CD2Cl2): δ 3.11 (t, 8H); 2.27 (s, 6H); 1.61 (m, 8H); 1.42 (m, 8H); 1.00 (t, 12H). 13 C{1H} NMR (125 MHz, CD2Cl2): δ 200.4 (CO), 62.5 (C of dicarbollide), 59.5 (Me2C of dicarbollide), 59.8, 24.7, 20.5, 14.2. 11 1 B{ H} NMR (160 MHz, CD2Cl2): δ −6.3, −9.8, −15.4. IR ν (KBr): 2962, 2933, 2876, 2554, 2530, 1991, 1875. HRMS (m/z): calcd for C5H15B9ReO3 [M−] 431.1388, found 431.1392. X-ray quality crystals for 2b were obtained by dissolving its Li salt in acetone, precipitated with water as its tetraphenylphosphonium (PPh4) salt by using PPh4Br, and then vapor diffusion of pentane to the THF solution of the PPh4 salt of 2b, in sequence. Synthesis of [Bu4N][(η1-C2B9H11)ReO3] (1a). Method 1. To a suspension of iodosobenzene (0.5 g, 2.27 mmol) in 55 mL of THF were added 2a (0.20 g, 0.31 mmol) and Ph2S2 (0.07 g, 0.31 mmol) with stirring, and the mixture was cooled to −5 to 0 °C in an ice salt bath. Acetic acid (67 μL) was added to the above suspension, and the temperature was slowly raised to 6− 8 °C. The suspension became clear and turned a yellowishorange color. After stirring for 2 h at room temperature, the solvent was removed under vacuum. The solid obtained was washed with hexane (50 mL) twice to remove the iodobenzene and Ph2S2. It was further dissolved in CH2Cl2 and filtered through Celite to remove any boric acid formed during the reaction and recrystallized by the solvent diffusion method by using ether as diffusing solvent to give 1a as bright yellow crystals. Yield: 0.16 g (83%). 1H NMR (500 MHz, CD2Cl2): δ 3.11 (t, 8H); 2.83 (b, 2H); 1.61 (m, 8H); 1.42 (h, 8H); 1.00 (t, 12H). 13C{1H} NMR (125 MHz, CD2Cl2): δ 60.4 (CH of dicarbollide), 59.8, 24.7, 20.5, 14.2. 11B{1H} NMR (160 MHz, CD2Cl2): δ −2.4 (3B), −2.8 (1B), −8.8 (2B), −12.5 (2B), −25.6 (1B). IR ν (KBr): 2962, 2933, 2876, 2554, 2530, 1469,

Table 7. Hydrosilylation of Benzaldehyde Using Different Organosilanes Catalyzed by Trioxorhenium 1aa,b

a

Reaction conditions: 1 mol % of 1a, substrate = 0.23 mmol, 1.8−2 equiv of silane, CD2Cl2, 40 °C. bNMR yields.

attached to Re. However no signals were observed in the 29Si NMR. The IR spectrum showed characteristic bands for B−H (2536 cm−1) and Re−O (906 cm−1) stretching modes. Efforts to obtain single crystals from the reaction of 1a and Et3SiH for further characterization were not successful. In the absence of Et3SiH, 1a was inert toward benzaldehyde and showed no significant reaction. No kinetic isotope effect was observed with C6H5CDO and Et3SiD. In summary, we believe the active catalytic mixture is composed of multinuclear paramagnetic rhenium decorated with the carborane cage, oxo ligands, and possibly Et3SiO−. In order to compare the catalytic activity of compound 1b with 1a, a model reaction of hydrosilylation of benzaldehyde with Et3SiH was carried out under the same conditions used for 1a. The results were the same as those of 1a, except with a relatively longer reaction time of 24 h.

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CONCLUSION We have reported herein the efficient synthesis of [Bu4N][(η1C2B9H11)ReO3] (1a) and its methyl-substituted analogue [Bu4N][(1,2-Me2-η1-C2B9H11)ReO3] (1b) from their respective carbonyl precursors 2a and 2b in the presence of Ph2S2 or PhS(O2)SPh. Through DFT calculations the coordination mode of η5 versus η1 in compounds 2 and 1 was rationalized by the strong π acceptor and π donor character of CO and oxo ligands, respectively. The catalytic utility of 1a in hydrosilylation of aldehydes and ketones was demonstrated. Complex 1a exhibits high chemoselectivity and tolerance of various functional groups. The intermediates formed from the reaction of Et3SiH with 1a act as the active homogeneous catalyst(s). Extension of the catalytic activity of this family of trioxorhenium carborane compounds to other organic reactions is underway in our laboratory.

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EXPERIMENTAL SECTION

Reactions were performed in a nitrogen-filled glovebox or using standard Schlenk techniques using argon. Solvents were degassed and purified with a solvent purification system developed by Anhydrous Engineering Inc. prior to use. CD2Cl2 was dried with CaH2, distilled under argon, and stored over molecular sieves. o-Carborane and bromopentacarbonylrhenium(I) were purchased from Strem and used as received. Organosilanes, aldehydes, and ketones were purchased from Gelest or Aldrich and used as received. The [Bu4N][nido-7,8C2B9H12] and its rhenium tricarbonyl complex, [Bu4N][(η5-C2B9H11)1894


Organometallics

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954, 930, 916, 903. HRMS (m/z): calcd for C2H11B9ReO3 [M−] 367.1075, found 367.1080. When PhS(O2)SPh was used instead of Ph2S2, the reaction mixture became clear at 15−20 °C and the remaining procedure and yield were the same. Method 2. To a solution of 2a (0.20 g, 0.31 mmol) and Ph2S2 (0.07 g, 0.31 mmol) in 7.0 mL of benzene were added 7.0 mL of H2O2 (30% solution) and 70 μL of acetic acid, and the solution was heated to reflux for 3 h. The yellowish-orange organic phase was extracted with methylene chloride (20 mL twice), dried with Na2SO4, and rotary evaporated to remove the solvent. The residue was column chromotographed starting with hexane and ethylacetate in a gradient fashion up to 50%. Yield: 0.08 g (42%). PhS(O2)SPh and [Bu4N][ReO4] were obtained as byproducts in this reaction. Synthesis of [Bu4N][(7,8-Me2-η1-C2B9H11)ReO3] (1b). To a solution of 2b (0.17 g, 0.25 mmol) and Ph2S2 (0.04 g, 0.25 mmol) in 11.0 mL of benzene were added 5.0 mL of H2O2 (30% solution) and 40 μL of acetic acid, and the solution was heated to reflux. After 5 and 7.5 h two lots of H2O2 (4.5 mL) and acetic acid (30 μL) were added, and the reaction mixture was refluxed further until the disappearance of peaks in the 11B{1H} NMR corresponding to 2b . The yellowishorange organic phase was extracted with methylene chloride (20 mL twice), dried with Na2SO4, and rotary evaporated to remove the solvent. The solid obtained was washed with hexane twice (50 mL each) to remove the iodobenzene and PhS(O2)SPh. It was dissolved in THF and recrystallized by the solvent diffusion method by using pentane as diffusing solvent to give 1b as bright yellow crystals. Yield: 0.03 g (33%). 1H NMR (500 MHz, CD2Cl2): δ 3.11 (t, 8H); 1.61 (m, 8H); 1.51 (s, 6H); 1.42 (m, 8H); 1.00 (t, 12H). 13C{1H} NMR (125 MHz, CD2Cl2): δ 72.9 (C(CH3)2 of dicarbollide), 59.5, 24.4, 22.6 (C(CH3)2 of dicarbollide) 20.3, 13.9. 11B{1H} NMR (160 MHz, CD2Cl2): δ −4.1 (1B), −7.2 (3B), −8.8 (2B), −12.8 (2B), −24.9 (1B). IR ν (Nujol): 2962, 2933, 2876, 2554, 2530, 1469, 954, 929, 916, 901. HRMS (m/z): calcd for C4H15B9ReO3 [M−] 397.1416, found 397.1412. Computational Methods. The geometry optimization of compounds 1 and 2 was done using the x, y, z coordinates from their corresponding crystal structures reported in this paper as well as from the Cambridge database.28 We used the DFT method based on the B3LYP density functional model and LANL2DZ basis sets within the Gaussian 09 suite of programs.19

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of hydrosilylation, spectral details, tables for X-ray crystallography of 2b, 1a, and 1b, DFT details (pdf), and X-ray crystallographic data (CIF) of 2b, 1a, and 1b. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by DOE-BES, grant no. DE-FG-02-06ER15794. We thank Dr. Alice Rene and Prof. Dennis H. Evans of Purdue University for their help with the electrochemistry experiments. We thank Prof. Hilkka Kenttämaa and Dr. Nelson Venueza of Purdue University for their help with the mass spectrometry measurements. We thank Mr. Zhi Cao of Purdue University for useful discussions on DFT. This research was supported through computational resources provided by Information Technology at Purdue−Rosen Center for Advanced Computing, West Lafayette, Indiana. 1895


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Trioxorhena(VII)carborane Anion and Its Methyl-Substituted Analogue

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