NCN-Pincer Cobalt Complexes Containing Bis(oxazolinyl)phenyl

Satomi Hosokawa, Jun-ichi Ito*, and Hisao Nishiyama*. Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, ...
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NCN-Pincer Cobalt Complexes Containing Bis(oxazolinyl)phenyl Ligands Satomi Hosokawa, Jun-ichi Ito,* and Hisao Nishiyama* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: We describe the preparation and characterization of new NCN-pincer Co(III) complexes containing bis(oxazolinyl)phenyl (phebox) ligands as auxiliary ligands. The reaction of Co2(CO)8 with the 2-bromo-substituted ligand precursor (phebox-R)Br (1a, R = Me2; 1b, R = iPr) resulted in the formation of the tricarbonyl Co(I) complex (pheboxR)Co(CO)3 (2a, R = Me2; 2b, R = iPr), in which NC-bidentate coordination of the phebox ligand was observed. Complexes 2 underwent oxidative addition of I2 to give the Co(III) aqua complex (phebox-R)CoI2(H2O) (4a, R = Me2; 4b, R = iPr) by a change in the coordination geometry to the NCN-tridentate mode. Ligand exchange reactions of H2O or I ligand with CNtBu or AgOAc smoothly proceeded to give the isocyanide complex (phebox-dm)CoI2(CNtBu) (5) or the acetate complex (pheboxdm)Co(κ1-OAc)(κ2-OAc) (6), respectively.

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the first pincer NCN-Co complex, [C6H3(CH2NMe2)2]CoCl(py), by transmetalation of a lithium salt with CoCl2(py)2.5 Recently, Heinekey described pincer Co complexes containing the POCOP ligand (POCOP = C6H3[O(PtBu)2]2), which produced unique hydride and dihydrogen complexes by reaction with H2.6 Therefore, further studies on ECE-Co pincer ligands have the potential to develop new types of complexes. In this context, we examined the introduction of the Co atom into the phebox ligand framework to fabricate new NCN-pincer complexes. Here, we report the preparation and characterization of several types of phebox-Co complexes containing Co(I) and Co(III) centers. In this work, the phebox ligand was found to serve as both a bidentate or tridentate ligand in accordance with the oxidation state of the Co center.

INTRODUCTION Recently, we developed chiral bis(oxazolinyl)phenyl (phebox) ligands to construct several types of NCN-pincer transitionmetal catalysts for asymmetric catalysis and stoichiometric transformations.1 The phebox-Rh complex was found to serve as a highly selective and active catalyst for several types of asymmetric reduction and reductive coupling reactions, such as reductive aldol reactions of aldehydes with α,β-unsaturated carbonyl compounds.2 In addition, other functionalization reactions and C−C bond formation reactions, including βboration and alkynylation, were achieved with high efficiency and enantioselectivity.2d,e The phebox Rh and Ir acetate complexes were also utilized in stoichiometric C−H bond cleavage reactions of benzene derivatives as well as n-alkanes, giving the corresponding aryl and alkyl complexes.3 These results encouraged us to prepare a Co analogue containing the phebox ligand. Pincer complexes based on ECE meridional ligands (E = donor groups) have been extensively studied in the area of organometallic chemistry because of their unique reactivities in stoichiometric and catalytic transformations of organic and inorganic molecules.4 A number of pincer complexes containing group 9 metals, namely Rh and Ir, have been extensively studied in terms of fundamental reactivity as well as application to catalysis. In contrast, cobalt pincer complexes containing the ECE ligand have been limited. Previously, van Koten reported © 2013 American Chemical Society

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RESULTS AND DISCUSSION Previously, cyclometalated Co complexes have been synthesized by transmetalation of Co salts with lithio ligand precursors,5−7 C−H bond activation by alkyl Co complexes,8 and oxidative addition of C−X bonds with low-valence complexes.9 We recently described that oxidative addition of the bromo-substituted ligand precursors 1 with an iron Received: May 21, 2013 Published: July 1, 2013 3980

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carbonyl complex, such as Fe2(CO)9, was a useful method to construct pincer phebox-Fe complexes.10 To synthesize Co complexes with the phebox ligand, we examined the reaction of 1 with Co2(CO)8 as a feasible preparation method for NCNpincer complexes. The reaction of (phebox-dm)Br (1a) with Co2(CO)8 (1.5 equiv of Co) in toluene proceeded smoothly at room temperature to afford the Co carbonyl complex (η2-pheboxdm)Co(CO)3 (2a) (eq 1). Purification by column chromatog-

Figure 1. ORTEP diagram of 2a at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Co1−C1 = 1.9867(17), Co1−C17 = 1.7902(19), Co1−C18 = 1.7641(19), Co1−C19 = 1.787(2), Co1−N1 = 2.0189(15), C17−O3 = 1.136(2), C18−O4 = 1.142(2), C19−O5 = 1.135(2), Co2−C20 = 1.9878(18), Co2−C36 = 1.787(2), Co2−C37 = 1.762(2), Co2−C38 = 1.788(2), Co2−N3 = 2.0071(15), C36−O8 = 1.133(3), C37−O9 = 1.140(3), C38−O10 = 1.138(2). Selected angles (deg): C1−Co1− C17 = 177.06(8), C18−Co1−N1 = 121.60(8), C19−Co1−N1 = 114.67(8), C18−Co1−C19 = 122.07(9), C36−Co2−C20 = 175.99(9), C37−Co2−N3 = 120.62(8), C38−Co2−N3 = 109.69(8), C37−Co2−C38 = 127.24(9).

raphy on silica gel with ethyl acetate afforded 2a in 67% yield (based on 1a) as a yellowish solid. Reaction of the chiral ligand 1b with Co2(CO)8 also produced the corresponding chiral phebox-Co complex 2b (eq 1). Complex 2b was isolated in 39% yield (based on 1b) by column chromatography on silica gel performed at 0 °C. Complexes 2a,b were identified on the basis of their 1H and 13 C NMR spectra, IR spectra, and elemental analyses. The 1H NMR spectrum of 2a measured in C6D6 showed two singlet signals for the methyl groups at δ 0.88 and 1.36 in the intensity ratio of 6H:6H and two singlet signals for the oxazoline methylene groups at δ 3.58 and 3.89. The signals for the aromatic protons were observed as three nonequivalent peaks at δ 6.94, 7.57, and 8.28. In the 13C NMR spectrum of 2a, the signals for the methyl groups on the oxazoline rings were observed at δ 28.0 and 29.0 ppm. The IR spectrum showed the absorption of the CO ligands at 2055, 1986, and 1962 cm−1. These spectral data suggest that 2a has an unsymmetrical structure, in which the phebox ligand is coordinated as an NCbidentate supporting ligand. In the 1H NMR spectrum of 2b, signals for the isopropyl groups appeared as four doublet peaks at δ 0.44, 0.45, 0.88, and 1.20 ppm, indicating an unsymmetrical structure as described in 2a. The molecular structure of 2a was confirmed by X-ray analysis (Figure 1). There are two independent molecules, which contain different orientations of noncoordinated oxazoline rings. Both molecules are described as trigonal bipyramidal (TBP) with the phenyl fragment (C1 or C20) and one of the CO ligands (C17 or C36) in axial positions. The C1−Co1− C17 and C36−Co2−C20 angles are 177.06(8) and 175.99(9)°, respectively. The phebox ligand serves as the NC-bidentate supporting ligand. Consequently, one of the oxazoline rings is not coordinated to the Co center. The Co1−C1 and Co2−C20 bond lengths of 1.9867(17) and 1.9878(18) Å, respectively, are close to those of other Co bidentate metallacycle complexes (1.91−2.09 Å).7b−e,8,9a−c Similarly, the Co−N bond lengths (2.0189(15), 2.0071(15) Å) are in the range of those of NCchelated Co complexes (1.97−2.09 Å).7c−e,9b,c Such NCbidentate coordination of the phebox ligand was also observed in the phebox-Fe complex (phebox)Fe(PPhMe2)2(CO)Br.11 The line shapes of the 1H NMR spectrum of 2a depended significantly on temperature due to fluxional behavior (Figure

2). The 1H NMR spectrum measured at 20 °C in toluene-d8 exhibited two singlet signals for the oxazoline methyl groups at

Figure 2. VT 1H NMR spectra of 2a (500 MHz, toluene-d8).

δ 0.93 and 1.32. These signals gradually broadened upon an increase in temperature and then coalesced at 80 °C. At 90 °C, a broad signal appeared at δ 1.16. Simultaneously, two singlet signals for the oxazoline CH2 and two doublet signals for the aromatic protons also broadened and coalesced. This fluxional behavior can be explained by a process that involves attachment and detachment of the oxazoline fragments, as shown in Scheme 1. This exchange process might involve symmetric pseudo-octahedral geometry A as a transition state or the square-planar geometry B as an intermediate of the dissociation mechanism. In the case of a square-planar Rh(I) complex having a NC-bidentate coordination of C6H3(CH2NMe2)2, the exchange process between two amino groups was proposed to be a dissociation mechanism through a T-shaped intermediate.12 Next, we examined oxidation of the phebox-Co(I) complex 2 to prepare NCN-pincer Co complexes by elimination of the CO ligands. For example, the Co(I) complex CpCo(CO)2 afforded the Co(III) complex CpCoI2 by the reaction with I2.13 In addition, the Mo(II) and W(II) carbonyl complexes Cp*M(CO)3Me (M = Mo, W) were oxidized by treatment with PCl5 to give the Mo(V) and W(V) complexes Cp*MCl4.14 3981

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The chiral phebox-Co complex 4b was synthesized in 72% yield as a dark brown solid using the same procedure. In contrast, reactions of other oxidants, such as Cl2, Br2, NBS, and PhI(OAc)2, with 2a resulted in the formation of unidentified products and elimination of the phebox ligand from the Co center. The change in the geometry of a NCN ligand from a NC-bidentate coordination to a NCN-tridentate coordination was reported in the square-planar Rh(I) complex [η2C6H3(NMe2CH2)2]Rh(cod), which was converted to the square-pyramidal NCN-pincer Rh(III) complex [η 3 (NMe2CH2)2C6H3]Rh(Me)I by oxidative addition of MeI.15 Complex 4a was identified on the basis of the 1H and 13C NMR and IR spectra and elemental analysis. The spectral features of 4a indicate that it has a C2v-symmetrical geometry. In the 1H NMR spectrum of 4a in CDCl3, two singlet signals for the methyl groups and methylene groups were observed at δ 1.61 and 4.46, respectively. Two triplet and doublet signals for the aromatic protons appeared at δ 7.38 and 7.74. The molecular structure of 4a was unambiguously confirmed by X-ray analysis (Figure 3). The ORTEP diagram shows that

Scheme 1. Proposed Mechanism for the Exchange Process of the Oxazoline Fragments

Reaction of 2a with I2 in THF at room temperature proceeded smoothly to give a new Co complex. Although characterization of the crude products by 1H NMR spectroscopy failed, due to the formation of paramagnetic species, the IR spectrum exhibited one peak from absorption of the CO ligand at 2087 cm−1, suggesting the formation of the carbonyl complex 3a and/or 3a′. However, purification by column chromatography on silica gel led to ligand exchange of the CO ligand to the H2O ligand. The NMR spectra and elemental analysis revealed the phebox-Co(III) aqua complex (phebox)CoI2(H2O) (4a). The formation of 4a is considered to be the result of oxidative addition of I2 to 2a followed by ligand exchange with H2O on silica gel. Consequently, successive reaction of H2O with the crude product obtained by the reaction of 2a with I2 gave 4a in 65% isolated yield (Scheme 2). Scheme 2. Reaction of 2 with I2

Figure 3. ORTEP diagram of 4a at the 50% probability level. Hydrogen atoms and THF have been omitted for clarity. Selected bond lengths (Å): Co1−C1 = 1.845(10), Co1−N1 = 1.979(4), Co1− O2 = 2.041(8), Co1−I1 = 2.5958(5). Selected angle (deg): N1− Co1−N1 = 162.3(3).

the phebox ligand is coordinated to the Co atom as the NCNtridentate ligand. The geometry of the Co center is described to be pseudo-octahedral. The Co1−C1 bond length of 1.845(10) Å is shorter than those of 2a (1.9867(17) and 1.9878(18) Å). This bond length is also shorter than those of the related Rh and Ir phebox complexes (phebox)MCl2(H2O) (M = Rh, 1.921 Å; M = Ir, 1.930 Å).16 The Co−Cipso bond length is comparable to those of the phebox-Ni complexes (phebox)NiX (1.849− 1.859 Å).17 The N−Co−N bond angle of 162.3(3)° is close to those of (phebox)NiX complexes (161.0−162.2 Å).17 Two I ligands are coordinated to positions vertical to the phebox plane, and H2O is coordinated to the position trans to the phenyl fragment. This structural feature is similar to those of the phebox-Rh and -Ir complexes (phebox)MCl2(H2O).1 The molecular structure of 4b was also confirmed by X-ray analysis (Figure 4). The ORTEP diagram shows pseudo-octahedral geometry with a Co1−C1 bond length of 1.849(10) Å. To obtain information about the carbonyl complex 3a formed by the reaction of 2a with I2, we examined the reaction of 4a with CO gas. The reaction quickly proceeded in CH2Cl2 at room temperature to give the corresponding carbonyl 3982

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Scheme 3. Ligand Exchange Reaction of 4a

Figure 4. ORTEP diagram of 4b at the 50% probability level. Hydrogen atoms and CHCl3 have been omitted for clarity. Selected bond lengths (Å): Co1−C1 = 1.849(10), Co1−N1 = 1.952(9), Co1− N2 = 1.957(9), Co1−O3 = 2.033(7), Co1−I1 = 2.5892(15), Co1−I2 = 2.6040(14). Selected bond angle (deg): N1−Co1−N2 = 162.4(3).

signals for the two acetate groups were also observed to be equivalent at δ 1.73. In contrast to the case for 4a, there is no coordination of the H2O ligand in 6 on the basis of the NMR spectra and elemental analysis. The molecular structure of 6 was confirmed by X-ray analysis (Figure 5). The phebox ligand is bound to the Co atom as

complex (phebox)CoI2(CO) (3a) quantitatively. Complex 3a was identified on the basis of 1H and 13C NMR and IR spectra as well as elemental analysis. In the case of 3a, the NMR spectra are consistent with C2v symmetry. The 1H NMR spectrum exhibited two singlet signals at δ 1.54 (12H) and 4.43 (4H), stemming from four methyl groups and methylene protons on the oxazoline rings. In the 13C NMR spectrum, a signal for the CO ligand was observed at δ 208.9. The IR spectrum of 3a showed a peak for absorption of the CO ligand at 2089 cm−1, which was identical with that of the crude product formed by the reaction of 2a with I2, as described in Scheme 2. Thus, the reaction of 2a with I2 likely yielded 3a. However the formation of the isomer 3a′ cannot be ruled out because of a lack of detailed spectroscopic data for the crude product. The CO wavenumber of 3a was significantly higher than that of CpCoI2(CO) (νCO 2045 cm−1).13b Judging from the wavenumber of 3a, the coordination of the CO ligand is considered to be weak due to weak back-donation by the Co(III) center. In order to obtain further insight into the fundamental reactivity of 4a, we examined ligand exchange reactions with neutral and anionic ligands (Scheme 3). Complex 4a reacted with tert-butyl isocyanide at room temperature to give the isocyanide complex (phebox-dm)CoI2(CNtBu) (5) in 89% yield via exchange with the H2O ligand. The 1H NMR spectrum of 5 showed the signal for the phebox methyl group at δ 1.53 and the signal for the tert-butyl group at δ 1.98. The IR spectrum of 5 showed the CN absorption at 2177 cm−1. These spectral features indicate that the isocyanide ligand is coordinated at an equatorial position, which is trans to the phenyl fragment of the phebox ligand. The phebox-Rh(III) chloride complex (phebox)RhCl2(H2O) underwent ligand exchange of the H2O ligand with isocyanide to give the isocyanide complex (phebox)RhCl2(CNR).18 Thus, there are similarities in the reactivity toward the ligand exchange reaction of the H2O ligand in the phebox-Co and -Rh complexes. Reaction of 4a with excess AgOAc at room temperature proceeded smoothly to give the corresponding acetate complex (phebox)Co(OAc)2 (6) in 96% yield. In the 1H NMR spectrum, a single signal for four methyl groups on the oxazoline rings was observed at δ 1.25. Similarly, the methyl

Figure 5. ORTEP diagram of 6 at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Co1− C1 = 1.849(2), Co1−N1 = 1.9630(17), Co1−N2 = 1.9622(17), Co1− O3 = 1.8749(14), Co1−O5 = 1.9523(15), Co1−O6 = 2.1125(16). Selected angles (deg): N1−Co1−N2 = 162.75(8), O5−Co1−O6 = 64.81(6).

NCN-tridentate coordination. One of the acetate ligands is ligated as a κ1 coordination, while another is ligated as a κ2 coordination. The structure of 6 is different from those of the phebox-Rh and -Ir complexes, which have one H2O ligand at the equatorial position and two κ1-acetate ligands at the apical positions.2b,3a,19 The Co1−O3 bond length of 1.8749(14) Å is shorter than the Co1−O5 and Co1−O6 bond lengths of 1.9523(15) and 2.1125(16) Å. The Co−O bond length of the κ1-acetate ligand in 6 is also shorter than those of the pheboxRh and -Ir complexes (Rh, 2.0473(14)−2.0265(13) Å;2b Ir, 2.037−2.058 Å3a). The Co1−C1 bond length of 1.849(2) Å 3983

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to give a dark brown solid of 4a (117.3 mg, 0.195 mmol, 65%). A similar procedure using 2b (132.7 mg, 0.30 mmol) gave a red solid of 4b (135.8 mg, 0.22 mmol, 72%). 4a: 1H NMR (300 MHz, CDCl3, room temperature) δ 1.61 (s, 12H, CH3), 2.29 (br, 2H, OH2), 4.46 (s, 4H, CH2), 7.38 (t, J = 7.6 Hz, 1H, C6H3), 7.74 (d, J = 7.6 Hz, C6H3); IR (KBr, cm−1): 2977, 1622, 1588, 1491, 1403, 1210, 982. Anal. Calcd for C16H21CoI2N2O3: C, 31.92; H, 3.52; N, 4.65. Found: C, 32.01; H, 3.62; N, 4.43. We were unable to obtain a clear 13C NMR spectrum due to low solubility in organic solvents. 4b: 1H NMR (300 MHz, CDCl3) δ 0.88 (d, J = 6.9 Hz, 6H, CHMe2), 1.04 (d, J = 7.2 Hz, 6H, CHMe2), 2.39−2.49 (m, 2H, CHMe2), 3.33 (br, 2H, OH2), 4.03−4.10 (m, 2H), 4.66−4.72 (m, 4H), 7.39 (t, J = 7.6 Hz, 1H, C6H3), 7.72 (d, J = 7.6 Hz, 2H, C6H3); 13C NMR (75 MHz, CDCl3, room temperature) δ 16.3, 20.7, 29.3, 67.2, 73.3, 123.9, 127.8, 135.7, 171.2, 185.8; IR (KBr, cm−1): 2958, 1623, 1569, 1476, 1386, 1248, 1195, 1148, 943. Anal. Calcd for C18H25CoI2N2O3·0.33C4H8O2: C, 35.21; H, 4.23; N, 4.25. Found: C, 35.36; H, 4.12; N, 4.30. Reaction of 4a with CO. A suspension of 4a (30.1 mg, 0.050 mmol) in CH2Cl2 (5 mL) was bubbled with CO gas for 30 min. The suspension immediately changed to a red solution. Removal of the solvent under reduced pressure afforded a red solid of 3a (30.6 mg, 0.050 mmol, 99%). 1H NMR (300 MHz, CDCl3, room temperature) δ 1.54 (s, 12H, CH3), 4.43 (s, 4H, CH2), 7.49 (t, J = 7.8 Hz, 1H, C6H3), 7.93 (d, J = 7.8 Hz, 2H, C6H3); 13C NMR (125 MHz, CDCl3, room temperature) δ 29.0, 66.1, 82.7, 124.5, 128.8, 133.4, 173.2, 203.4, 208.9 (CoCO); IR (KBr, cm−1) 2974, 2925, 2089 (νCO), 1619, 1593, 1491, 1403, 1387, 1208, 1148, 982. Anal. Calcd for C17H19CoI2N2O3: C, 33.36; H, 3.13; N, 4.58. Found: C, 33.11; H, 3.14; N, 4.49. (phebox-dm)CoI2(CNtBu) (5). To a solution of 4a (301 mg, 0.50 mmol) in toluene was added a toluene solution of tert-butyl isocyanide (550 μL, 0.55 mmol 1.0 M). The mixture was stirred at 50 °C for 12 h. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel with hexane/ethyl acetate (3/1) as eluent to give 5 (298 mg, 0.447 mmol, 89%): 1H NMR (300 MHz, CDCl3, room temperature) δ 1.53 (s, 12H, CH3), 1.98 (s, 9H, CMe3), 4.39 (s, 4H, CH2), 7.42 (t, J = 7.3 Hz, 1H, C6H3), 7.88 (d, J = 7.3 Hz, 2H C6H3); 13C NMR (75 MHz, CDCl3, room temperature) δ 29.1, 30.8, 65.8, 82.7, 123.3, 127.8, 134.2, 156.3, 172.3, 210.7; IR (KBr, cm−1): 2980, 2925, 2177 (νCN), 1615, 1487, 1402, 1382, 1205, 980. Anal. Calcd for C21H28CoI2N3O2: C, 37.80; H, 4.23; N, 6.30. Found: C, 37.70; H, 4.27; N, 6.21. (phebox-dm)Co(OAc)2 (6). Silver acetate (18.7 mg, 0.112 mmol, 2.2 equiv) was added to a THF solution of 4a (30.1 mg, 0.050 mmol). The suspension was stirred at room temperature for 5 h. The resulting yellow suspension was centrifuged to give a yellow solution. After removal of the solvent, the residue was crystallized from slow evaporation of a dichloromethane and hexane solution to give red crystals of 6 (21.4 mg, 0.0478 mmol, 96%): 1H NMR (500 MHz, C6D6, room temperature) δ 1.25 (s, 12H, CH3), 1.73 (s, 6H, COCH3), 3.77 (s, 4H, CH2), 7.06 (t, J = 7.5 Hz, 1H, C6H3), 7.47 (d, J = 7.5 Hz, 2H, C6H3); 13C NMR (125 MHz, C6D6, room temperature) δ 23.7 (COCH3), 26.6, 64.3, 82.9, 124.2, 125.8, 137.5, 171.1, 182.4 (COMe), 193.4; IR (KBr, cm−1): 2974, 2927, 1637, 1545, 1458, 1402, 1316, 1210, 1142, 945. Anal. Calcd for C20H25CoN2O6: C, 53.58; H, 5.62; N, 6.25. Found: C, 53.50; H, 5.64; N, 6.24. X-ray Diffraction. The diffraction data for 2a, 4a,b, and 6 were collected on a Bruker SMART APEX CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). An empirical absorption correction was applied by using SADABS. The structure was solved by direct methods and refined by full-matrix least squares on F2 using SHELXTL. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were located on calculated positions and refined as rigid groups. Crystallographic data for 2a, 4a,b, and 6 are summarized in the Supporting Information.

and the Co1−N1 and N2 bond lengths of 1.9630(17) and 1.9622(17) Å are similar to those of 4a. It was found that the coordination modes of the two OAc ligands were not equivalent in the solid-state structure, while the OAc ligand in the 1H NMR spectrum at room temperature was equivalent. In order to determine the fluxional process of the OAc ligands, we performed variable-temperature 1H NMR studies of 6 in toluene-d8. However, only one slightly broad peak for the OAc ligands was observed, even at −90 °C. This result indicates that the two OAc ligands underwent a rapid exchange process between κ1 and κ2 coordination modes in solution. Such an exchange process was proposed in Cp*Rh(κ1OAc)(κ2-OAc)20 and M(PMe3)3(κ1-O2CtBu)(κ2-O2CtBu)H2 (M = Mo, W).21 In summary, we have described new cyclometalated cobalt complexes containing bis(oxazolinyl)phenyl ligands. The NCchelated Co(I) complexes have been successfully synthesized by C−Br bond cleavage of the phebox-Br ligand precursors with Co2(CO)8. The successive oxidation by I2 resulted in the formation of the NCN-Co(III) complex. The phebox-Co complex underwent ligand exchange reactions with CNtBu and AgOAc to give the corresponding isocyanide and acetate complexes, respectively. Further reactivity and catalytic reactions mediated by the phebox-Co complex are now being conducted in our laboratory.

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EXPERIMENTAL SECTION

General Information. All air- and moisture-sensitive compounds were manipulated under standard Schlenk and vacuum-line techniques under an argon atmosphere. 1H NMR and 13C NMR spectra were obtained at 25 °C on a Varian Mercury 300 spectrometer and a Varian Inova 500 spectrometer. Infrared spectra were recorded on JASCO FT/IR-4200 and 230 spectrometers. Compounds 1a,b were prepared by the reported method.16a (phebox-dm)Co(CO)3 (2a). A solution of 1a (351 mg, 1.0 mmol) and Co2(CO)8 (256 mg, 0.75 mmol) in THF was stirred at room temperature for 1 h in darkness. After removal of the solvent, the residue was purified by column chromatography on silica gel with ethyl acetate as eluent to give a yellow solid of 2a (277 mg, 0.67 mmol, 67%). A similar procedure using 1b (491 mg, 1.0 mmol) gave a yellow solid of 2b (173 mg, 0.39 mmol, 39%). 2a: 1H NMR (300 MHz, C6D6, room temperature) δ 0.88 (s, 6H, CH3), 1.36 (s, 6H, CH3), 3.58 (s, 2H, CH2), 3.89 (s, 2H, CH2), 6.94 (t, J = 7.5 Hz, 1H, C6H3), 7.57 (d, J = 7.5 Hz, 1H, C6H3), 8.28 (d, J = 7.5 Hz, 1H, C6H3); 13C NMR (125 MHz, C6D6, room temperature) δ 27.6, 28.5, 66.1, 68.2, 78.9, 81.2, 124.1, 128.8, 133.3, 133.4, 138.6, 162.9, 165.1, 170.1, 197.7, 212.4; IR (KBr, cm−1) 2972, 2055, 1986, 1962, 1628, 1559, 1487, 1378, 1197, 1145, 976. Anal. Calcd for C19H19CoN2O5: C, 55.08; H, 4.62; N, 6.76. Found: C, 54.78; H, 4.73; N, 6.76. 2b: 1H NMR (300 MHz, C6D6, room temperature) δ 0.44 (d, J = 6.6 Hz, 3H, CHMe2), 0.45 (d, J = 6.9 Hz, 3H, CHMe2), 0.88 (d, J = 6.9 Hz, 3H, CHMe2), 1.20 (d, J = 6.6 Hz, 3H, CHMe2), 1.78 (m, 1H, CHMe2), 1.92 (m, 1H, CHMe2), 3.53 (m, 1H), 3.69 (t, J = 9.0 Hz), 3.84−3.93 (m, 2H), 4.02 (dd, J = 8.0, 16.1 Hz), 4.14 (t, J = 8.3 Hz), 6.93 (dd, J = 6.6, 6.9 Hz, 1H, C6H3), 7.58 (d, J = 6.6 Hz, 1H, C6H3), 8.27 (d, J = 6.9 Hz, 1H, C6H3); 13C NMR (75 MHz, C6D6, room temperature) δ 14.5, 18.8, 19.5, 19.9, 31.3, 34.3, 69.9, 70.5, 70.8, 74.3, 124.2, 129.0, 132.4, 133.5, 138.6, 164.1, 165.7, 171.7, 197.4; IR (KBr, cm−1) 2963, 2057, 1995, 1971, 1628, 1559, 1428, 1376, 1155, 975. Anal. Calcd for C21H23CoN2O5: C, 57.02; H, 5.24; N, 6.33. Found: C, 56.69; H, 5.24; N, 6.04. (phebox-dm)CoI2(H2O) (4a). A solution of 2a (124.3 mg, 0.30 mmol) and I2 (228 mg, 0.90 mmol) in toluene (6 mL) was stirred at 50 °C for 1 day in darkness. After removal of the solvent under reduced pressure, the residue was dissolved in wet methanol and acetone and stirred at room temperature for 6 h. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel with hexane/ethyl acetate (3/1) as eluent 3984

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Organometallics

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Article

(c) Chen, Y.; Sun, H.; Flörke, U.; Li, X. Organometallics 2008, 27, 270−275. (d) Li, J.; Wang, C.; Li, X.; Sun, H. Dalton Trans. 2012, 41, 8715−8722. (10) Hosokawa, S.; Ito, J.; Nishiyama, H. Organometallics 2010, 29, 5773−5775. (11) Hosokawa, S.; Ito, J.; Nishiyama, H. Organometallics 2012, 31, 8283−8290. (12) van der Zeijden, A. A. H.; van Koten, G.; Nordemann, R. A.; Kojić-Prodić, B.; Spek, A. L. Organometallics 1988, 7, 1957−1966. (13) (a) Heck, R. F. Inorg. Chem. 1965, 4, 855−857. (b) King, R. B. Inorg. Chem. 1966, 5, 82−87. (14) Murray, R. C.; Blum, L.; Llu, A. H.; Schrock, R. R. Organometallics 1985, 4, 953−954. (15) van der Zeijden, A. A. H.; van Koten, G.; Ernsting, J. M.; Elsevier, C. J.; Krijnen, B.; Stam, C. H. J. Chem. Soc., Dalton Trans. 1989, 317−324. (16) (a) Motoyama, Y.; Okano, M.; Narusawa, H.; Makihara, N.; Aoki, K.; Nishiyama, H. Organometallics 2001, 20, 1580−1591. (b) Ito, J.; Shiomi, T.; Nishiyama, H. Adv. Synth. Catal. 2006, 348, 1235−1240. (17) (a) Fossey, J. S.; Richards, C. J. J. Organomet. Chem. 2004, 689, 3056−3059. (b) Stol, M.; Snelders, D. J. M.; Godbole, M. D.; Havenith, R. W. A.; Haddleton, D.; Clarkson, G.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van Koten, G. Organometallics 2007, 26, 3985− 3994. (18) Motoyama, Y.; Shimozono, K.; Aoki, K.; Nishiyama, H. Organometallics 2002, 21, 1684−1696. (19) Ito, J.; Shiomi, T.; Nishiyama, H. Adv. Synth. Catal. 2006, 348, 1235−1240. (20) Boyer, P. M.; Roy, C. P.; Bielski, J. M.; Merola, J. S. Inorg. Chim. Acta 1996, 245, 7−15. (21) Zhu, G.; Parkin, G. Inorg. Chem. 2005, 44, 9637−9639.

ASSOCIATED CONTENT

S Supporting Information *

CIF files and tables giving experimental and crystallographic data for 2a, 4a,b, and 6. 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] (J.I.); hnishi@apchem. nagoya-u.ac.jp (H.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Nos. 22245014, 24750084).

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REFERENCES

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