(Tris(pyrazolyl) - American Chemical Society

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Structures, Physicochemical Properties, and Reactivities of Cobalt(II) Complexes Supported by a Homoscorpionate (Tris(pyrazolyl)borate) Ligand TpPh,Me Atsushi Kunishita, Thomas L. Gianetti, and John Arnold* Department of Chemistry, University of California, Berkeley, California 94720-1460, United States S Supporting Information *

ABSTRACT: The coordination chemistry of a mononuclear cobalt complex supported by the monoanionic hydrotris(pyrazolyl)borate tridentate ligand [TpPh,Me] containing substituted phenyl groups at the 3-position of the pyrazole rings and methyl groups at the 5-position of the pyrazole rings is examined. The Co(II) halide complex [TpPh,Me]CoCl (1) serves as a precursor to the organometallic species [TpPh,Me]CoMe (2), [TpPh,Me]CoCH2SiMe3 (3), [TpPh,Me]CoCH2CH(Me) 2 (4), [TpPh,Me]Co(CH2)2Me (5), [TpPh,Me]CoEt (6), [TpPh,Me]Co(CH2)5Me (7), and [TpPh,Me]Co(CH2)3Me (8). All of the cobalt(II) organometallic complexes have been characterized by X-ray crystallography, NMR spectroscopy, FT-IR, and elemental analysis. The X-ray crystallographic analyses reveal that complexes 2−6 and 8 have slightly distorted tetrahedral structures. Solution-state magnetic susceptibility measurements of all complexes in C6D6 indicate that they all have 3/2 ground states. The reaction of 1 with tBuLi and iPrMgCl in hexane under N2 results in the production of the i-butyl and n-propyl complexes 4 and 5, respectively, in ∼56% yield. These isomerizations and exchange reactions of the alkyl group proceed via formation of a Co−H species, as determined by trapping experiments.

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complexes toward small molecules,7 we were interested in preparing first-row metal species with potentially reactive M−C bonds. Here, we report the synthesis and characterization of a series of organometallic cobalt(II) species that confirm the usefulness of the [TpPh,Me] ligand set ([TpPh,Me] = tris(3-phenyl5-methyl-pyrazolyl)borate) toward this end. Work toward supporting transient Co−H species that undergo isomerizations and olefin-exchange reactions are also described.

INTRODUCTION The tris(pyrazolyl)borate (Tp) moiety is one of the most popular tridentate ligands in organometallic, bioinorganic, and coordination chemistry.1 A large number of transition-metal complexes of this ligand have been extensively studied in order to gain insight into their structures, physicochemical properties, and reactivities.1a−d Tp ligands typically coordinate to a metal center in a κ3 fashion. These resemble the shape of a hunting scorpion and are, therefore, nicknamed “scorpionates”. Recently, several types of Tp derivatives have also been developed in order to tune the structure and reactivity of the supported complexes, where steric constraints and/or electronic effects induced by the substituents are important.1a−d,2 Sterically hindered tris(pyrazolyl)borate ligands [TpR,R′ = (3-R-5-R′-pyrazolyl)borate] can be used to stabilize coordinatively unsaturated and low-coordinate molecules.1a−c,h,j,o−t,3 For instance, a variety of metal-dioxygen species have been prepared by the reaction of transition-metal complexes of tetrahedral TpM−X type with oxygenating reagents.1e,h,3,4 Recently, Akita et al., Parkin et al., Theopold et al., and others have disclosed several thermally stable, tetrahedral, coordinatively unsaturated complexes supported by tris(pyrazolyl)borate ligands.1h−t,3,5 Moreover, Jones and co-workers have reported the C−H bond activation in a series of alkyl nitriles and chloroalkanes by using the [TpRh] fragment.6 The ability of Tp ligands to adapt to the steric and electronic effects of the metal center renders them useful in supporting a range of metal-based reactivities. In connection with our recent studies on the reactivity of a range of transition-metal © 2011 American Chemical Society

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EXPERIMENTAL SECTION

All reactions were performed using standard Schlenk techniques or in an MBraun drybox (425 K. Hexane and diethyl ether were purified by passage through a column of activated alumina and degassed with nitrogen prior to use.8 Deuterated benzene was vacuum-transferred from sodium/benzophenone. NMR spectra were recorded at ambient temperature on Bruker AVQ-400, AVB-400, AV-500, or AV-600 spectrometers. Proton chemical shifts are given relative to residual solvent peaks. Infrared samples were prepared as Nujol mulls. Magnetic susceptibility measurements were performed in C6D6 using the Evans NMR method.9 Melting points were determined using sealed capillaries prepared under a nitrogen atmosphere. [TpPh,Me], [TpPh,Me]CoCl (1), and d7-isopropylmagnesium bromide were prepared using literature procedures,10 and unless otherwise noted, all reagents were acquired from commercial sources. Elemental analyses were determined at the College of Chemistry, University of California, Berkeley. The X-ray Received: October 12, 2011 Published: December 13, 2011 372

dx.doi.org/10.1021/om200973x | Organometallics 2012, 31, 372−380

Organometallics

Article

[TpPh,Me]CoEt (6). EtMgCl (2.0 M in hexane, 0.30 mL, 0.60 mmol) was added to a suspension of 1 (0.40 mmol) in 30 mL of ether at −78 °C. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from diethyl ether at −40 °C gave 6 as dark green crystals (123 mg, 54%). 1H NMR (C6D6, 400 MHz, 298 K): δ(ppm) −3.95 (br), 7.55 (6H, s, m-Ph), 9.46 (3H, s, p-Ph), 16.6 (9H, s, 9H, −CH3), 53.7 (3H, s, −CH−). IR (cm−1): 2526 (BH, m), 1541 (w), 1505 (w), 1474 (w), 1433 (m), 1414 (m), 1381 (w), 1367 (w), 1345 (w), 1306 (w), 1283 (w), 1174 (m), 1093 (w), 1057 (w), 1030 (m), 1001 (w), 978 (w) 913 (w), 832 (w), 805 (w), 774 (m), 759 (m), 691 (m), 657(w). Anal. Calcd for [Tp]CoCH2CH3 (C34H35.5BCoN6O0.25): C, 67.19; H, 6.07; N, 14.25. Found: C, 66.92; H, 5.89; N, 14.51. mp: 204−205 °C. μeff = 4.2(2) μB. [TpPh,Me]Co(CH2)5Me (7). Me(CH2)5MgCl (1.0 M in ether, 0.30 mL, 0.30 mmol) was added to a suspension of 1 (0.20 mmol) in 50 mL of hexane at −78 °C. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from hexane at −40 °C gave 7 as green crystals (95 mg, 75%). 1H NMR (C6D6, 400 MHz, 298 K): δ(ppm) −5.31 (br), 4.94 (3H, s), 9.56 (3H, s, p-Ph), 10.3 (2H, s), 16.4 (9H, s, −CH3), 16.7 (2H, s), 53.9 (3H, s, −CH−). IR (cm−1): 2540 (BH, m), 1543 (w), 1507 (w), 1477 (w), 1435 (m), 1414 (m), 1367 (w), 1345 (w), 1180 (m), 1093 (m), 1061 (s), 1031 (w), 978 (m), 832 (w), 777 (m), 763 (s), 695 (s), 658 (w). Anal. Calcd [Tp]Co(CH2)5CH3 (C36H41BCoN6): C, 68.91; H, 6.59; N, 13.39. Found: C, 68.60; H, 6.83; N, 13.09. mp: 137−138 °C (dec). μeff = 4.0(2) μB. [TpPh,Me]Co(CH2)3Me (8). nBuLi (2.5 M in pentane, 0.2 mL, 0.35 mmol) was added to a suspension of 1 complex (0.30 mmol) in 50 mL of hexane at −78 °C. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from hexane at −40 °C gave 8 as green crystals (100 mg, 56%). 1 H NMR (C6D6, 400 MHz, 298 K): δ(ppm) −4.16 (br), −2.33 (br), 7.58 (6H, s, m-Ph), 9.59 (3H, s, p-Ph), 16.4 (9H, s, −CH3), 18.8 (3H, s, −CH2−CH3), 54.0 (3H, s, −CH−). IR (cm−1): 2539 (BH, m), 1542 (w), 1506 (w), 1474 (w), 1434 (m), 1413 (m), 1367 (w), 1346 (w), 1181 (s), 1093 (w), 1060 (s), 1030 (w), 978 (w), 834 (m), 777 (w), 761 (s), 695 (s), 657 (w). Anal. Calcd [Tp]Co(CH 2 ) 3 CH 3 (C37H44BCoN6): C, 69.16; H, 6.90; N, 13.08. Found: C, 68.89; H, 6.85; N, 12.97. mp: 130−131 °C (dec). μeff = 4.2(2) μB. Trapping Experiment. (CD3)2CDMgCl (1 equiv) was added to a solution of 1 (1 equiv) and 9,10-dihydroanthracene (9,10-DHA) (20 equiv) in C6H6 at room temperature using a J. Young NMR tube containing a C6D6 capillary tube. The solution was analyzed using 2H NMR spectroscopy (see below). Olefin-Exchange Experiment. Method A. Me2CHMgCl (2.0 M in THF, 0.15 mL, 0.30 mmol) was added to a suspension of 1 (0.2 mmol) in diethyl ether (20 mL) at −78 °C under an ethylene atmosphere. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration and removal of solvent gave a mixture of [TpPh,Me]Co(CH2)2CH3 5 and [TpPh,Me]CoEt 6 (1.7:1). Method B. Me2CHMgCl (2.0 M in THF, 0.2 mL, 0.40 mmol) was added to a suspension of 1 (0.30 mmol) in 50 mL of 1-hexene at −78 °C. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred for 4 h. Filtration, removal of solvent, and recrystallization from hexane at −40 °C gave 7 as dark green crystals (37.0 mg, 20%). Crystallographic Analyses. X-ray structural determinations were performed on a Bruker SMART 1000 or SMART APEX diffractometer. Both are 3-circle diffractometers that couple a CCD detector11 with a sealed-tube source of monochromated Mo Kα radiation (λ = 0.71073 Å). A crystal of appropriate size was coated in Paratone-N oil and mounted on a Kaptan loop. The loop was transferred to the diffractometer, centered in the beam, and cooled by a nitrogen flow low-temperature apparatus that had been previously calibrated by a thermocouple placed at the same position as the crystal. Preliminary orientation matrices and cell constants were determined by collection of 60 10 s frames, followed by spot

structural determinations were performed at CHEXRAY, University of California, Berkeley. DFT calculations were performed at the MGCF (Molecular Graphics and Computation Facility), University of California, Berkeley. [TpPh,Me]CoMe (2). MeLi (1.6 M in Et2O, 0.19 mL, 0.3 mmol) was added to a suspension of 1 (0.2 mmol) in 50 mL of diethyl ether at −78 °C. The solution immediately turned dark blue. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from diethyl ether at −40 °C gave 2 as dark blue crystals (89 mg, 80%). 1 H NMR (C6D6, 400 MHz, 298 K): δ(ppm) 0.08 (br), 8.29 (6H, s, m-Ph), 9.67 (3H, s, p-Ph), 16.4 (9H, s, −CH3), 55.3 (3H, s, −CH−). IR (cm−1): 2544 (BH, m), 1541 (w), 1505 (w), 1477 (w), 1435 (m), 1415 (m), 1369 (m), 1347 (m), 1308 (w), 1181 (w), 1121 (w), 1095 (m), 1062 (m), 1029 (w), 980 (w), 918 (w) 834 (w), 789 (m), 777 (m), 759 (m), 693 (m), 681 (m). Anal. Calcd for [Tp]CoMe·0.5 CH3CH2OCH2CH3 (C33H36BCoN6O0.5): C, 66.68; H, 6.10; N, 14.14. Found: C, 66.26; H, 6.07; N, 13.99. mp: 258−259 °C. μeff = 4.2(2) μB. [TpPh,Me]CoCH2SiMe3 (3). Me3SiCH2MgCl (1.0 M in pentane, 0.30 mL, 0.30 mmol) was added to a suspension of 1 (0.20 mmol) in 50 mL of hexane at −78 °C. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from hexane at −40 °C gave 3 as green crystals (54 mg, 43%). 1 H NMR (C6D6, 400 MHz, 298 K): δ(ppm) −5.31 (br) 5.34 (6H, s, m-Ph), 8.96 (3H, s, p-Ph), 14.5 (9H, s, −CH3), 22.3 (9H, brs, −Si− (CH3)3), 55.9 (3H, s, −CH−). IR (cm−1): 2550 (BH, m), 1544 (w), 1506 (w), 1475 (w), 1449 (w), 1436 (w), 1413 (w), 1369 (w), 1344 (w), 1233 (w), 1181 (m), 1168 (m), 1093 (w), 1060 (m), 1028 (w), 980 (w), 922 (w), 888 (w), 853 (w), 832 (w), 817 (w), 802 (w), 778 (m), 761 (m), 732 (m), 717 (w), 706 (w), 690 (m) 657 (w). Anal. Calcd [Tp]CoCH2Si(CH3)3 (C34H39BCoN6Si): C, 64.87; H, 6.24; N, 13.35. Found: C, 64.68; H, 6.27; N, 13.09. mp: 222−223 °C. μeff = 4.2(2) μB. [TpPh,Me]CoCH2CHMe2 (4). Method A. tBuLi (1.7 M in pentane, 0.2 mL, 0.35 mmol) was added to a suspension of 1 (0.30 mmol) in 50 mL of hexane at −78 °C. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from hexane at −40 °C gave 4 as dark green crystals (100 mg, 56%). 1H NMR (C6D6, 400 MHz, 298 K): δ(ppm) −4.24 (br), 6.84 (6H, s, m-Ph), 9.13 (3H, s, p-Ph), 16.2 (9H, s, −CH3), 40.6 (6H, brs, −CH(CH3)2), 53.0 (3H, s, −CH−). IR (cm−1): 2546 (BH, m), 1545 (w), 1506 (w), 1474 (w), 1434 (w), 1412 (w), 1369 (w), 1345 (w), 1308 (w), 1282 (w), 1191 (m), 1092 (m), 1065 (w), 1032 (m), 975 (w), 910 (w), 834 (w), 789 (m), 758 (s), 707 (w), 693 (s), 659 (w). Anal. Calcd [Tp]CoCH2CHMe2 (C34H37BCoN6): C, 68.12; H, 6.22; N, 14.02. Found: C, 67.93; H, 6.29; N, 13.72. mp: 250−251 °C. μeff = 4.1(2) μB. Method B. Me2CHCH2MgCl (2.0 M in Et2O, 0.3 mmol, 0.15 mL) was added to a suspension of 1 (0.2 mmol) in 50 mL of hexane at −78 °C. The solution immediately turned green. The reaction mixture was warmed to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from hexane at −40 °C gave 6 as dark green crystals (56 mg, 47%). [TpPh,Me]Co(CH2)2Me (5). Me2CHMgCl (2.0 M in THF, 0.3 mmol, 0.15 mL) was added to a suspension of 1 (0.2 mmol) in 50 mL of hexane at −78 °C. The solution immediately turned green. The reaction mixture was allowed to warm to room temperature and was stirred overnight. Filtration, removal of solvent, and recrystallization from hexane at −40 °C gave 5 as green crystals (59 mg, 50%). 1H NMR (C6D6, 400 MHz, 298 K): δ(ppm) −3.95 (br), 7.65 (6H, s, mPh), 9.65 (3H, s, p-Ph), 16.7 (9H, s, −CH3), 23.0 (3H, brs, −CH2− CH3), 54.1 (3H, s, −CH−). IR (cm−1): 2544 (BH, m), 1545 (w), 1506 (w), 1475 (w), 1434 (m), 1413 (m), 1345 (w), 1310 (w), 1181 (m), 1093 (w), 1061 (m), 1030 (w), 978 (w), 913(m), 833 (w), 789 (w), 777 (w), 761 (s), 695 (s), 657 (w). Anal. Calcd [Tp]Co(CH2)2CH3·0.5CH3(CH2)4CH3 (C36H42BCoN6): C, 68.80; H, 6.74; N, 13.37. Found: C, 68.38; H, 6.58; N, 13.56. mp: 247−248 °C. μeff = 4.3(2) μB. 373


Organometallics

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Scheme 1

integration and least-squares refinement. The reported cell dimensions were calculated from all reflections with I > 10 σ. The data were corrected for Lorentz and polarization effects; no correction for crystal decay was applied. An empirical absorption correction based on comparison of redundant and equivalent reflections was applied using SADABS.12 All software used for diffraction data processing and crystal-structure solution and refinement are contained in the APEX2 program suite (Bruker AXS, Madison, WI).13 Thermal parameters for all non-hydrogen atoms were refined anisotropically. For all structures, R1 = ∑(|Fo| − |Fc|)/∑(|Fo|); wR2 = [∑{w(Fo2 − Fc2)2}/∑{w(Fo2)2}]1/2. ORTEP diagrams were created using the ORTEP-3 software package and POV-ray.14 Computational Methods. All structures and energies were calculated using the Gaussian 09 suite of programs.15 Self-consistent field computations were performed with tight convergence criteria on ultrafine grids, while geometry optimizations were converged to tight geometric convergence criteria for all compounds. Spin expectation values 2 indicated that spin contamination was not significant in any result. Frequencies were calculated analytically at 298.15 K and 1 atm. Structures were considered true minima if they did not exhibit imaginary vibration modes and were considered as transition states when only one imaginary vibration mode was found. Intrinsic reaction coordinates (IRC) calculations were performed to ensure that the transition-state geometries connected the reactants and the products. Optimized geometries were compared using the sum of their electronic and zero-point energies. To reduce the computational time, the system was structurally simplified from Ph,MeTp to H,HTp by replacing the methyl and phenyl groups by hydrogen atoms. In one case, the steric effect of the phenyl group was considered and, therefore, was not substituted by a hydrogen atom (the ligand used is described as Ph,HTp). The B3LYP,16 TPSS-TPSS,17 and WB97XD18 functionals were used throughout this computational study. For geometry optimizations and frequency calculations, all the atoms (H, C, N, B, and Co) were treated with Pople’s 6-31G+2(d,p) double-ζ splitvalence basis.19 5d and 7f diffusional functions was used for the H, C, N, and Co atoms.

Figure 1. ORTEP drawings of (A) 2 and (B) 3 showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

(1.995(5) Å, C.N. = 5).7a The 1H NMR spectrum of 2 exhibits four well-resolved, isotropically shifted, and moderately broadened resonances attributed to the [TpPh,Me] ligand (see Figure S1, Supporting Information). Resonances for the BH proton and o-Ph protons were not identified. The resonances at δ = 8.29 ppm (6H) and 9.67 ppm (3H) are assigned to m-phenyl and o-phenyl protons, respectively. The resonances at δ = 16.4 ppm (9H) and 55.3 ppm (3H) are assigned to the methyl groups at the 5-position of the pyrazole ring and the protons at the 4-position of the pyrazole ring, respectively. Resonances for the hydrogen atom bonded directly to the α-carbon atom were not detected. The solution magnetic susceptibility (μeff) of complex 2 is 4.2(2) μB, indicating the presence of three unpaired electrons (S = 3/2). Addition of trimethylsilylmethylmagnesium chloride to a suspension of 1 in hexane at room temperature resulted in a color change of the solution from blue to green (Scheme 2). Following evaporation of the solvent under vacuum, extraction with hexane, and cooling at −40 °C, [TpPh,Me]CoCH2SiMe3 3 was isolated as dark green crystals in 43% yield. Results of an X-ray diffraction study are shown in the ORTEP diagram, Figure 1B; selected bond distances and angles are shown in Table 1. The cobalt atom lies in a slightly distorted tetrahedral geometry (τ = 0.77) with the Co(1)−C(1) bond distance of 2.017(2) Å very close to the values seen in three previously reported examples [N2P2]CoCH2SiMe3 (2.033(2) Å),7a (P-P)CoCl(CH2SiMe3) (2.013(2) Å), and (P-P)Co(CH2Si-Me3)2 (2.039(7) and 2.043(7) Å).24 The 1H NMR spectrum of 3 shows [TpPh,Me] ligand resonances that are similar to those in 2. The resonance at δ = 22.3 ppm (9H) is assigned to SiMe3 methyl groups. The solution magnetic susceptibility (μeff) of complex 3 is 4.2 μB, again consistent with an S = 3/2 electronic configuration. Mindiola and co-workers recently reported that reaction of a cobalt(II) chloride complex supported by a pincer-type PNP ligand with tBuLi under N2 and Ar atmospheres led to

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RESULTS AND DISCUSSION Alkyl Derivatives of [TpPh,Me]CoCl (1). The starting material for our studies, [TpPh,Me]CoCl (1), was reported earlier by Akita and co-workers.10a Reaction of 1 with MeLi (1.6 M in Et2O) in Et2O resulted in the formation of [TpPh,Me]CoMe (2) as greenish-blue crystals in 80% yield following crystallization from Et2O at −40 °C (Scheme 1). Crystals suitable for X-ray diffraction were grown from Et2O at −40 °C. An ORTEP diagram is shown in Figure 1A, and selected bond lengths and angles are summarized in Table 1. The cobalt center in 2 exhibits a slightly distorted tetrahedral geometry (τ = 0.78).20 The Co(1)−C(1) bond distance of 1.923(5) Å is slightly shorter than the values observed in closely related Co(II) compounds [Ph2PC6H4N(H)]CoMe(PMe3)2 (2.019(3) Å, C.N. = 5),21 [PhTttBu]CoMe (2.052(3) Å, C.N.= 4),22 PhB(tBuIm)3CoMe (2.042(2) Å, C.N. = 4),23 [Tpt‑Bu,Me]CoMe (2.115(14) Å, C.N. = 4),1j and [N2P2]CoMe 374


Organometallics

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Table 1. Selected Bond Lengths (Å) and Angles (deg) of Complexes 2 and 3 complex 2 Co(1)−N(1) Co(1)−N(3) N(1)−Co(1)−N(2) N(1)−Co(1)−C(1) N(2)−Co(1)−C(1)

2.081(4) 2.074(4) 120.12(19) 122.2(2) 93.65(15)

Co(1)−N(2) Co(1)−C(1) N(1)−Co(1)−N(3) N(2)−Co(1)−N(3) N(3)−Co(1)−C(1)

2.064(4) 1.923(5) 89.07(15) 94.57(17) 128.2(2)


2.064(2) 2.017(2) 90.63(8) 97.61(8) 119.71(9)

complex 3 Co(1)−N(1) Co(1)−N(3) N(1)−Co(1)−N(2) N(1)−Co(1)−C(1) N(2)−Co(1)−C(1)

2.069(2) 2.074(2) 88.63(8) 131.27(9) 120.20(9)

Scheme 2

Scheme 3

Figure 2. ORTEP drawings of 4 (A) and 5 (B) showing 50% probability thermal ellipsoids. The hydrogen atoms are omitted for clarity. Only the major partial position of the disordered isobutyl group is shown for 4.

tBuLi in hexane under nitrogen yields instead the isobutyl isomer complex 4 (Scheme 3). The structure of 4 has been confirmed by X-ray crystallography (Figure 2A), and this reaction is similar to that of the

reduction. Two products were isolated from these reactions: the dianionic cobalt(I) dinitrogen compound [{(PNP)Co}2(μ2-N2)]]2− and a dianionic cobalt(I) compound [{(μ2-PNP)Co}2].25 In the present system, treatment of complex 1 with 375


Organometallics

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Scheme 4

Table 2. Selected Bond Lengths (Å) and Angles (deg) of Complexes 4 and 5 complex 4 Co(1)−N(1) Co(1)−N(3) N(1)−Co(1)−N(2) N(1)−Co(1)−C(1) N(2)−Co(1)−C(1)

2.073(3) 2.087(3) 90.11(12) 125.73(15) 125.19(15)

Co(1)−N(1) Co(1)−N(3) N(1)−Co(1)−N(2) N(1)−Co(1)−C(1) N(2)−Co(1)−C(1)

2.084(2) 2.052(2) 94.36(8) 122.51(10) 126.07(10)


2.078(3) 2.008(4) 94.70(12) 90.97(12) 120.72(15)


2.084(2) 2.009(3) 92.65(8) 91.49(8) 120.96(9)

complex 5

Scheme 5

Scheme 6

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(CD3)2CDMgBr instead of (CH3)2CHMgCl. The 2H NMR spectrum of [TpPh,Me]Co(CD2)2CD3 exhibits a peak at 23.0 ppm (see Figure S5, Supporting Information). The hydrogen atoms bonded directly to α- and β-carbon atoms were not detected in either 1H or 2H NMR spectra. To gain more insight into the mechanism and the nature of the various potential intermediates, we turned to density functional theory (DFT). We investigated the β-hydride elimination mechanism from Co−CHMe2 complex (I−1) using the simplified κ3-R,HTpCo complexes (quartet electronic state, S = 3/2, R = H, Ph) to the Co−(CH2)2Me complex (S = 3/2) with the B3LYP, TPSSTPSS, and WB97XD functionals (see the Supporting Information, Figure S9). In each case, the transition state corresponding to the β-hydride abstraction step was found to be very high (>60 kcal/mol), which is in poor agreement with the expected activation energy for a reaction at room temperature. We modeled possible rearrangements prior to the first transition state, including a spin flip to form a doublet state, along with variable Tp ligand denticities (e.g., the κ2-H,HTpCo moiety; see Figure S10, Supporting Information). However, these did not decrease the energy of the transition state. Regardless, R,HTpCo−(CH2)2Me (R = H, Ph) was found to be more stable than R,HTpCo−CHMe2 (R = H, Ph), which is fully consistent with our experimental results. This DFT analysis suggests that the mechanism for this olefin rearrangement may involve a radical process (Scheme 5). To probe the viability of a radical mechanism, a hydrogenatom trapping experiment was performed. The reaction of 1 equiv of 1 and 1 equiv of (CD3)2CDMgCl was carried out in the presence of 20 equiv of 9,10-dihydroanthracene (9,10DHA) in C6H6 at room temperature. Notably, HD and d6propylene were detected in the reaction mixture by 2HNMR spectroscopy (Figure S11, Supporting Information).28 This presumably suggests that the d7-isopropyl radical, generated by the Co−C bond homolysis of the TpCo−CD(CD3)2 moiety, undergoes rearrangement to give deuterium radical (D•) and d6-propylene. The deuterium radical (D•), generated by the C−D bond breaking of the d7-isopropyl radical, was trapped by the reaction with 9,10-dihydroanthracene (9,10-DHA) (Scheme 6). Isomerization and Olefin-Exchange Reactions. Further evidence for the intermediacy of a [TpPh,Me]Co−H species was obtained via olefin-exchange experiments. The reaction of 1 and isopropylmagnesium chloride in ether under an ethylene atmosphere produced complex 5 and a new compound. This new compound was found to be identical to a sample of [TpPh,Me]CoEt (6), prepared independently by the reaction of 1 and ethylmagnesium chloride. The X-ray structure of 6 is

Figure 3. ORTEP drawings of 6 showing 50% probability thermal ellipsoids. The hydrogen atoms are omitted for clarity.

isobutyliron complex of β-diketiminate ligands reported by Holland et al.26 More recently, they have described T-shaped coordinatively unsaturated transition-metal complexes supported by β-diketiminate ligands.27 Complex 4 can also be prepared directly by reaction of 1 with isobutylmagnesium chloride. The structure of 4 showed a slightly distorted tetrahedral geometry (τ = 0.77), and the Co(1)−C(1) bond distance of 2.008(4) Å is also very close to the value seen in 3 (selected bond distances and angles are shown in Table 1). The solution magnetic susceptibility was determined to be 4.1 μB, which is in agreement with a tetrahedral Co(II) center. Attempts to isolate [TpPh,Me]Co(I) derivatives using 1 and reducing agents, such as KC8 and Mg, resulted in dark red-brown solutions that did not yield tractable materials. In common with the t-Bu example, reaction of 1 with isopropylmagnesium chloride gave the isomerized complex 5 in 50% yield as dark green crystals following crystallization from hexane at −40 °C (Scheme 4). The results of an X-ray diffraction study of 5 are shown in Figure 2B, with selected bond lengths and distances provided in Table 2. The geometry around the Co center is pseudotetrahedral; the solution magnetic susceptibility of 5 (4.2 μB) is in accord with an S = 3/2 ground state. Peaks from the [TpPh,Me] ligand in the 1H NMR spectrum of 5 are similar to that of 2. The resonance at δ = 23.0 ppm (3H) is assigned to the hydrogen atoms of methyl group in the isomerized i-propyl ligand. This peak is absent in the 1H NMR spectrum of the perdeutero analogue [TpPh,Me]Co(CD2)2CD3 prepared using

Table 3. Selected Bond Lengths (Å) and Angles (deg) of Complex 6 complex 6 Co(1)−N(1) Co(1)−N(3) N(1)−Co(1)−N(2) N(1)−Co(1)−C(1) N(2)−Co(1)−C(1) Co(2)−N(7) Co(2)−N(9) N(7)−Co(2)−N(8) N(7)−Co(2)−C(33) N(8)−Co(2)−C(33)

2.057(2) 2.057(2) 88.94(9) 127.76(11) 96.2(2) 2.091(2) 2.061(2) 93.87(9) 124.88(12) 123.80(12)

Co(1)−N(2) Co(1)−C(1) N(1)−Co(1)−N(3) N(2)−Co(1)−N(3) N(3)−Co(1)−C(1) Co(2)−N(8) Co(2)−C(33) N(7)−Co(2)−N(9) N(8)−Co(2)−N(9) N(9)−Co(2)−C(33) 377

2.076(2) 1.992(3) 119.63(12) 96.05(9) 91.49(9) 2.076(2) 1.986(3) 91.57(9) 92.96(9) 120.98(12)


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Scheme 7

identified using NMR spectroscopy as being identical to the same complex prepared independently from 1 and hexylmagnesium chloride. These results also imply that the [TpPh,Me]Co−H generated from [TpPh,Me]Co−alkyl species can be involved in the isomerization and olefin-exchange reactions (Scheme 7).26,29 We recently reported the formation of the Co(I) hydride [N2P2]Co−H resulting from β-hydride elimination from a fivecoordinate cobalt(II) [N2P2] alkyl complex prepared using n BuLi.7a In the present work, however, it was not surprising to discover that the corresponding reaction of 1 with nBuLi led not to a new hydride, but to the n-butyl complex 8. An ORTEP diagram of the molecule is shown in Figure 4 with selected bond lengths and angles summarized in Table 4. The solidstate structure of 8 reveals a four-coordinate tetrahedral geometry with the [TpPh,Me] ligand bound N3 and alkyl moiety. In common with the other alkyl species described above, the solution susceptibility of 8 is 4.2 (S = 3/2). Attempts to prepare [TpPh,Me]Co−H species by treatment of 1 and hydride reagents, such as KBH(Et)3, LiAlH4, or Red-Al, were unsuccessful.

Figure 4. ORTEP drawings of 8 showing 50% probability thermal ellipsoids. The hydrogen atoms are omitted for clarity.

shown in Figure 3 together with the selected bond lengths and angles listed in Table 3. In addition, the reaction of 1 with isopropylmagnesium chloride in a solution of neat 1-hexene led to the isolation of [TpPh,Me]Co(CH2)5Me (7). This compound was also

Table 4. Selected Bond Lengths (Å) and Angles (deg) of Complexes 8 complex 8 Co(1)−N(1) Co(1)−N(3) N(1)−Co(1)−N(2) N(1)−Co(1)−C(1) N(2)−Co(1)−C(1)

2.089(2) 2.055(2) 93.06(9) 125.08(12) 126.78(11)

Co(1)−N(2) Co(1)−C(1) N(1)−Co(1)−N(3) N(2)−Co(1)−N(3) N(3)−Co(1)−C(1) 378

2.080(2) 2.000(3) 94.23(9) 90.15(9) 104.3(2)


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(2) (a) Imai, S.; Fujisawa, K.; Kobayashi, T.; Shirasawa, N.; Fujii, H.; Yoshimura, T.; Kitajima, N.; Moro-oka, Y. Inorg. Chem. 1998, 37, 3066. (b) Nabika, M.; Kiuchi, S.; Miyatake, T.; Fujisawa, K. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 5720. (c) Trofimenko, S. Polyhedron 2004, 23, 197. (d) Trofimenko, S. J. Chem. Educ. 2005, 82, 1715. (e) Trofimenko, S.; Calabrese, J. C.; Kochi, J. K.; Wolowiec, S.; Hulsbergen, F. B.; Reedijk, J. Inorg. Chem. 1992, 31, 3943. (3) (a) Hess, A.; Horz, M. R.; Liable-Sands, L. M.; Lindner, D. C.; Rheingold, A. L.; Theopold, K. H. Angew. Chem., Int. Ed. 1999, 38, 166. (b) Qin, K.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2002, 124, 14008. (c) Qin, K.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. Angew. Chem., Int. Ed. 2002, 41, 2333. (d) Reinaud, O. M.; Theopold, K. H. J. Am. Chem. Soc. 1994, 116, 6979. (e) Reinaud, O. M.; Yap, G. P. A; Rheingold, A. L.; Theopold, K. H. Angew. Chem., Int. Ed. 1995, 34, 2051. (f) Thyagarajan, S.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. Chem. Commun. 2001, 2198. (g) Thyagarajan, S.; Shay, D. T.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2003, 125, 4440. (4) (a) Chen, P.; Root, D. E.; Campochiaro, C.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2002, 125, 466. (b) Fujisawa, K.; Tanaka, M.; Moro-oka, Y.; Kitajima, N. J. Am. Chem. Soc. 1994, 116, 12079. (c) Kitajima, N.; Katayama, T.; Fujisawa, K.; Iwata, Y.; Morooka, Y. J. Am. Chem. Soc. 1993, 115, 7872. (d) Hikichi, S.; Komatsuzaki, H.; Akita, M.; Moro-oka, Y. J. Am. Chem. Soc. 1998, 120, 4699. (e) Hikichi, S.; Okuda, H.; Ohzu, Y.; Akita, M. Angew. Chem., Int. Ed. 2009, 48, 188. (f) Hikichi, S.; Yoshizawa, M.; Sasakura, Y.; Akita, M.; Moro-oka, Y. J. Am. Chem. Soc. 1998, 120, 10567. (g) Kitajima, N.; Komatsuzaki, H.; Hikichi, S.; Osawa, M.; Moro-oka, Y. J. Am. Chem. Soc. 1994, 116, 11596. (5) (a) Gorrell, I. B.; Parkin, G. Inorg. Chem. 1990, 29, 2452. (b) Kisko, J. L.; Hascall, T.; Parkin, G. J. Am. Chem. Soc. 1998, 120, 10561. (6) (a) Evans, M. E.; Li, T.; Vetter, A. J.; Rieth, R. D.; Jones, W. D. J. Org. Chem. 2009, 74, 6907. (b) Tanabe, T.; Evans, M. E.; Brennessel, W. W.; Jones, W. D. Organometallics 2011, 30, 834. (7) (a) Chomitz, W. A.; Arnold, J. Inorg. Chem. 2009, 48, 3274. (b) Chomitz, W. A.; Arnold, J. Chem. Commun. 2008, 3648. (c) Chomitz, W. A.; Arnold, J. Chem.Eur. J. 2009, 15, 2020. (d) Chomitz, W. A.; Sutton, A. D.; Krinsky, J. L.; Arnold, J. Organometallics 2009, 28, 3338. (e) Minasian, S. G.; Arnold, J. Dalton Trans. 2009, 106. (f) Rozenel, S. S.; Chomitz, W. A.; Arnold, J. Organometallics 2009, 28, 6243. (g) Rozenel, S. S.; Kerr, J. B.; Arnold, J. Dalton Trans. 2011, 40, 10397. (8) Alaimo, P. J.; Peters, D. W.; Arnold, J.; Bergman, R. G. J. Chem. Educ. 2001, 78, 64. (9) Piguet, C. J. Chem. Educ. 1997, 74, 815. (10) (a) Uehara, K.; Hikichi, S.; Akita, M. J. Chem. Soc., Dalton Trans. 2002, 3529. (b) McNally, J. P.; Cooper, N. J. Organometallics 1988, 7, 1704. (11) SMART: Area-Detector Software Package; Brucker Analytic X-ray Systems, I.: Madison, WI, 2001−2003. (12) SADABS: Bruker-Nonius Area Detector Scaling and Absorption V2.05; Bruker Analytical X-ray Systems, I.: Madison, WI, 2003. (13) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. (14) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;

CONCLUSIONS Reactions of cobalt(II) chloride complex 1 supported by hydrotris(pyrazolyl)borate ligand [TpPh,Me] and a series of organometallic molecules have been examined in detail. The [TpPh,Me] ligand has allowed the isolation of four-coordinate organometallic cobalt complexes. The X-ray crystallographic analyses reveal that complexes 2−6 and 8 exhibit a four-coordinate pseudotetrahedral geometry of approximate C3v symmetry. Solutionstate magnetic susceptibility measurements in C6D6 indicate that they all exhibit 3/2 spin ground states. The isomerization and olefin-exchange reactions of [TpPh,Me]Co−alkyl complexes implicate the intermediacy of a [TpPh,Me]Co−H species, which may be formed via a radical process.

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ASSOCIATED CONTENT

S Supporting Information *

Details of X-ray crystallographic studies, NMR spectra (Figures S1−S8, S11, and S12), and DFT results (Figures S9 and S10). 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]. ACKNOWLEDGMENTS We are grateful to the National Science Foundation (Grant No. 0848931) for financial support and Dr. Antonio DiPasquale for crystallographic assistance. A.K. thanks JSPS for a postdoctoral fellowship for research abroad.

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