Article pubs.acs.org/Organometallics
Synthesis and Reactivity of Manganese(II) Complexes Containing N‑Heterocyclic Carbene Ligands Malik H. Al-Afyouni,†,‡,§ V. Mahesh Krishnan,†,§ Hadi D. Arman,† and Zachary J. Tonzetich*,† †
Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
‡
S Supporting Information *
ABSTRACT: A series of manganese(II) complexes containing arylsubstituted N-heterocyclic carbene (NHC) ligands have been synthesized and characterized. Chloride complexes of Mn(II) containing the NHC ligands 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3dimesitylimidazol-2-ylidene (IMes) were prepared in straightforward fashion by direct carbene addition to MnCl2(THF)1.6. These complexes exist as chloride-bridged dimers in solution of formula [Mn2Cl2(μCl)2(NHC)2]. The monomeric complex [MnCl2(IMes)2] has also been prepared and structurally characterized, although NMR studies are consistent with facile dissociation of one of the IMes ligands in solution. [Mn2Cl2(μ-Cl)2(IPr)2] serves as a precursor to dimeric alkyl and aryl compounds of Mn(II) including [Mn2R2(μ-Cl)2(NHC)2] (R = Bn, o-tolyl, and Ph) and the bridging methyl complex [Mn2Me2(μ-Me)2(IPr)2]. Stoichiometric reactions of these hydrocarbyl species with bromocyclohexane demonstrate that they are not chemically competent in C−C coupling reactions involving alkyl electrophiles.
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INTRODUCTION The fundamental organometallic chemistry of the 3d transition metals continues to enjoy an upsurge in activity as new catalytic methodologies utilizing earth-abundant metals proliferate. Yet among the base metals examined for catalytic organic transformations, especially those involving C−C bond formation, manganese finds much less use than other mid 3d metals.1−6 Despite this scarcity, manganese remains popular for use in oxidation catalysis and select C−X bond forming reactions.7−10 Many of the difficulties surrounding the use of manganese in classical organometallic reactions stem from the lack of well-characterized examples of organomanganese complexes in oxidation states other than +1. Although manganese is most frequently encountered in the +2 oxidation state, the majority of its organometallic chemistry concerns the univalent state.11,12 Such Mn(I) compounds exhibit almost exclusively low-spin d6 configurations and typically feature very strong-field ligands such as carbonyl. Complexes of divalent manganese containing metal−carbon bonds are known, albeit much less common.13−35 The bonding in these high-spin Mn(II) complexes is believed to be much more ionic than that in Mn(I) compounds, invoking parallels with Grignard reagents.34,36 Even in those Mn(II) complexes containing strong-field ligands such as cyclopentadienyl, the ionic nature of metal−carbon bonding persists due to the large spin-pairing energy of manganese. Thus, high-spin configurations are observed for most Mn(II) organometallics.37,38 These species exhibit no spin-allowed ligand field transitions and broadened or nearly featureless NMR spectra, resulting in few reported studies featuring their detailed solution characterization. © XXXX American Chemical Society
However, these complexes have been shown to take part in reactions unique from those observed for other 3d organometallics.39,40 As part of our interest in exploring the organometallic chemistry of first-row transition metals containing arylsubstituted N-heterocyclic carbene ligands (NHCs), we have turned our focus to manganese NHC complexes.41 We envisioned that such complexes might display parallels to the chemistry of Fe and Co, both of which demonstrate activity in C−C cross-coupling reactions.42−53 Scattered examples of Mn(II) NHC compounds are present in the literature dating back to Cowley and Jones’ isolation of a manganocene NHC complex in 2001.54 Subsequent work by Roesky reported a variety of NHC adducts to Mn(II) salts and Mn(II) βdiketiminate species.55−57 More recently, both Mulvey and Goicoechea have reported examples of three-coordinate Mn(II)-alkyl species by treatment of manganese hydrocarbyl aggregates, [MnR2]n (R = CH2SiMe3, CH(SiMe3)2, or mesityl), with IPr.58,59 A variant of the NHC ligand, the cyclic alkyl amino carbene, has also been used to generate low-coordinate Mn species.60 In addition to these examples that feature monodentate carbenes, chelating variants of NHC ligands have been employed to stabilize manganese complexes in the +2 through +5 oxidation states.61−65 Despite this precedent, no comprehensive study has examined the preparation and reactivity of simple manganese carbene complexes of the type likely to be encountered in binary mixtures of Mn(II) salts and Received: August 7, 2015
A
DOI: 10.1021/acs.organomet.5b00684 Organometallics XXXX, XXX, XXX−XXX
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Organometallics the commercially available IPr and IMes NHC coligands. In this paper we describe the synthesis of well-defined Mn(II) halide and alkyl complexes containing aryl-substituted NHC ligands. The structure and reactivity of these compounds are discussed in the context of potential catalytic C−C bond forming reactions with Mn(II).
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RESULTS AND DISCUSSION Reaction of MnCl2(THF)1.6 with 1 equiv of IPr in THF afforded the chloride-bridged dimer [Mn2Cl2(μ-Cl)2(IPr)2] (1), in analogous fashion to that observed for Fe and Co (eq 1).47,50 The equivalent reaction with IMes yielded the
Addition of 2 equiv of IMes to MnCl2(THF)1.6 produced the monomeric bis-NHC complex [MnCl2(IMes)2] (3, eq 3). The
corresponding [Mn2Cl2(μ-Cl)2(IMes)2] (2). Both complexes are soluble in arene solvents, albeit sparingly in the case of 2. 1 H NMR spectra of 1 and 2 in benzene-d6 are nearly featureless, displaying only a few very broadened peaks between 0 and 10 ppm (see Supporting Information). Magnetic susceptibility measurements of 1 in benzene-d6 gave an effective magnetic moment of 6.7(2) μB, which is consistent with weakly antiferromagnetically coupled high-spin Mn(II) centers. Such a description is further corroborated by the solid-state structure of 1 (Figure 1), which displays a Mn−Mn separation of 3.355(1) Å.66 No evidence for the formation of the manganate salt (IPrH)[MnCl3(IPr)] was observed during the synthesis of 1 and 2, although this species has been reported to be formed in reactions of IPr with Mn(II) halides.67 Halide-bridged dimers analogous to 1 and 2 containing Fe and Co are known to cleave readily in the presence of Lewis bases.47,48,51 In a similar fashion, treatment of 1 with 4cyanopyridine resulted in cleavage of the dimer and formation of the monomeric pyridine adduct [MnCl2(4-CNpy)(IPr)] (1py, eq 2). The solid-state structure of 1py was determined crystallographically and can be found in the Supporting Information. Magnetic susceptibility measurements of 1py in benzene were consistent with a monomeric high-spin Mn(II) complex, giving an effective magnetic moment of 5.7(2) μB.
solid-state structure of 3 depicted in Figure 1 compares well to the previously reported [MnCl2([C(Me)N(iPr)]2C)2] complex in both Mn−CNHC (ΔavM−C = 0.021 Å) and Mn−Cl (ΔavM− Cl = 0.011 Å) bond distance.56 However, the tetrahedral geometry about the Mn center in 3 is substantially more distorted, with a CNHC−Mn−CNHC angle of 127.41(7)°, very similar to that observed in the Fe and Co analogues.47,50 In contrast to the analogous Fe and Co complexes, however, observation of 3 by 1H NMR spectroscopy demonstrated that carbene dissociation occurs readily in solution. Spectra of 3 in benzene-d6 are analogous to those of 2, but show additional peaks consistent with free IMes. Titration of 2 with IMes resulted in a spectrum identical to that of 3 with no change to the broadened, paramagnetically shifted peaks of the Mn-bound carbene ligand (see Supporting Information). We therefore propose that the equilibrium depicted in eq 4 is established upon dissolution of 3 and that it lies predominantly toward 2 and free IMes. We note that the ability of NHCs to dissociate from mid 3d metals was also recently observed in studies with iron.68,69
Figure 1. Thermal ellipsoid renderings (50%) of the solid-state structures of 1 (left) and 3 (right). Hydrogen atoms and cocrystallized solvent molecules are omitted for clarity. Bond distances and angles can be found in the Supporting Information. B
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Figure 2. Thermal ellipsoid renderings (50%) of the solid-state structures of 4 (left) and 6 (right). Hydrogen atoms and crystallographically independent molecules (4) are omitted for clarity. Bond distances and angles can be found in the Supporting Information.
elongated Mn−Mn contact of 3.421(1) Å (see Supporting Information). Similar reactions to those displayed in eq 5 were also examined for MeMgCl. Surprisingly, however, no halidebridged species was formed from this reaction. Instead, the methyl-bridged complex [MnMe2(μ-Me)2(IPr)2] (8) was isolated as a pale orange solid (eq 6). Solution magnetic
Given the observation that carbene dissociation occurs readily for 3, we chose to focus on compound 1 as a precursor to alkylated species. This species is more soluble than 2, and we reasoned that any hydrocarbyl complex containing the bulkier IPr ligand might prove more stable than the corresponding analogue with IMes. Accordingly, treatment of 1 with 2 equiv of RMgCl (R = Ph, o-tolyl, or Bn) led to formation of chloridebridged monoalkyl species of formula [Mn2R2(μ-Cl)2(IPr)2] (4−6, eq 5). The 1H NMR spectra of 4−6 are essentially
susceptibility measurements on 8 yielded a value of 2.4(2) μB. This value is indicative of substantial antiferromagnetic coupling between the Mn centers and is in line with values reported previously for other methyl-bridged complexes of Mn(II).71,72 The solid-state structure of 8 is depicted in Figure 3. In like fashion to compounds 4 and 6, 8 crystallizes on an inversion center. The complex displays a Mn−Mn distance of 2.680(2) Å, which is shorter than that observed in select systems featuring metal−metal bonding between two low-spin Mn(II) centers.66,73,74 It is therefore likely that a Mn−Mn interaction in 8 arising from the spatial overlap of d orbitals results in stronger antiferromagnetic coupling as compared with the chloride-bridged species, consistent with solution phase magnetic susceptibility measurements. The Mn(1)−C(1) distance of 2.219(4) Å is comparable to that observed in compounds 1, 4, and 6, and the Mn(1)−C(30) distance of 2.136(4) Å is consistent with the terminal Mn−Cbenzyl distance of 2.142(3) Å in 4. The planar diamond-shaped Mn2C2 core features nearly symmetric Mn−C bonds at 2.254(5) and 2.262(5) Å with internal angles of 72.79(14)° and 107.21(14)° for Mn(1)−C(29)−Mn(1A) and C(29)−Mn(1)−C(29A), respectively. Similar metric parameters were reported for the diamond Mn2C2 motifs of other methyl-bridged Mn(II) complexes.30,71,72 To the best of our knowledge, 8 represents the only example of a bridging-methyl complex of any first-row transition metal containing NHC ligands.75 In order to establish the versatility of 1 as a general starting material for alkylated Mn NHC complexes, we also examined its ability to serve as precursor to the previously reported three-
featureless, similar to what is observed for 1 and 2 (see Supporting Information). In no instance were dialkyl complexes isolated from these reactions even when 4 equiv of the corresponding Grignard reagent was employed. The isolation of monoalkyl-monochloride species for Mn(II) contrasts findings with Fe and Co IPr species, where only monometallic dialkyl complexes (Fe) or decomposition products (Co) were obtained upon treatment with these Grignard reagents.45,47,50,70 Moreover, no high-spin phenyl complex of either Fe or Co has been isolated to date with the IPr ligand. Compounds 4−6 all proved very sensitive to ambient conditions, decomposing in a matter of seconds when exposed to air. The compounds are also thermally sensitive, especially 4 and 6, which were found to decompose in the solid state to an intractable black solid when stored at ambient temperature for several days. The solid-state structures of complexes 4 and 6 are depicted in Figure 2. The Mn−Mn distances for both compounds are similar (cf. 3.393(1) Å for 6) and close to that observed for complex 1 (3.355(1) Å). In line with the Mn−Mn separation for 4 and 6 is the solution magnetic moment value of 7.0(2) μB measured for 6, which is consistent with weakly antiferromagnetically coupled Mn(II) centers. Synthesis of the IMes analogue of 6, [Mn2Ph2(u-Cl) 2(IMes)2] (7), was also accomplished by the treatment of 2 with PhMgCl. The solidstate structure of 7 is very similar to that of 6 with a slightly C
DOI: 10.1021/acs.organomet.5b00684 Organometallics XXXX, XXX, XXX−XXX
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bromocyclohexane and 4-bromotoluene with phenyl Grignard (Scheme 2). Complex 1 demonstrated no activity for the Scheme 2. Attempted Catalytic Reactions of Complex 1
coupling of PhMgCl with 4-bromotoluene. To our surprise, however, the reaction of PhMgCl with bromocyclohexane in the presence of 5 mol % of 1 was found to lead to small amounts (∼25%) of cross-coupled product. This result suggests that in the presence of excess Grignard, species other than 6 may form that are active for C−C cross-coupling with alkyl electrophiles. The identity of such species, although unknown at this time, may likely be per-alkylated Mn(II) complexes given the precedent for such compounds2,25,78,79 and the propensity for NHC dissociation observed in the present study.
Figure 3. Thermal ellipsoid renderings (50%) of the solid-state structures of 8. Hydrogen atoms are omitted for clarity. Bond distances and angles can be found in the Supporting Information.
coordinate Mn(II) species [Mn(CH2SiMe3)2(IPr)].58 Treatment of 1 with 4 equiv of Me3SiCH2MgCl produced, after workup, a mixture of two crystalline species. Mechanical separation of the mixture afforded a set of white crystals and a set of pale yellow crystals (Scheme 1). Examination of the
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CONCLUSIONS We have demonstrated that simple aryl-substituted NHC complexes of Mn(II) can be prepared in straightforward fashion by direct carbene addition to MnCl2(THF)1.6. The solid-state structures and aggregation behavior of manganese chloride complexes containing IPr and IMes ligands are akin to those of the corresponding Fe and Co compounds. Despite this similarity, carbene dissociation from [MnCl2(IMes)2] appears to occur much more readily in solution. Treatment of [Mn2Cl2(μ-Cl)2(IPr)2] with a series of Grignard reagents leads to new hydrocarbyl complexes, nearly all of which are unprecedented in Fe and Co NHC chemistry. The diversity of hydrocarbyl complexes isolable with the IPr-Mn(II) system is greater than that of either Fe or Co and suggests that such species are more stable. In agreement with this apparent stability is the lack of reactivity with the prototypical electrophile, bromocyclohexane. Therefore, the prospect of catalytic cross-coupling by well-defined Mn(II) NHC complexes appears uncertain. Nonetheless, these new complexes may in fact offer clues as to the structures of intermediate species in Fe- and Co-catalyzed reactions.
Scheme 1. Alkylation of Complex 1 with TMSCH2MgCl
yellow crystals by X-ray crystallography demonstrated that they were in fact the desired [Mn(CH2SiMe3)2(IPr)] complex. However, X-ray analysis of the white crystals revealed that they corresponded to the magnesium complex [Mg(CH2SiMe3)2(IPr)], also reported previously by Mulvey.58 These results are consistent with less robust IPr coordination to Mn(II) than in analogous Fe and Co complexes, further consistent with our observation of carbene dissociation from 3.45,50 As a final set of experiments, we investigated the activity of several of the Mn NHC complexes in C−C bond forming reactions. Several paramagnetic iron(II) alkyl complexes have been reported to react rapidly and cleanly with alkyl bromides to generate the products of C−C coupling.53,76,77 In contrast, treatment of complexes 6 and 8 with bromocyclohexane in THF at room temperature was found to result in no consumption of the electrophile, as judged by GC-MS. Therefore, the Mn(II) NHC alkyl complexes examined here do not appear to be chemically competent in C−C bond forming reactions. To determine if this lack of reactivity shuts down any possible catalytic activity, we also investigated the efficacy of 1 as a pre-catalyst for the cross-coupling of
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EXPERIMENTAL SECTION
General Comments. All manipulations were performed under an atmosphere of purified nitrogen gas in a Vacuum Atmospheres glovebox. Tetrahydrofuran, diethyl ether, pentane, and toluene were purified by sparging with argon gas and passage through two columns packed with 4 Å molecular sieves. Benzene-d6 was dried over sodium ketyl and vacuum-distilled prior to use. 1H NMR spectra were recorded in benzene-d6 on a Varian spectrometer operating at 500 MHz (1H) and referenced to the residual proton resonance of the solvent (δ 7.16 ppm). FT-IR spectra were recorded with a ThermoNicolet iS 10 spectrophotometer running the OMNIC software; solid samples were pressed into KBr disks. Solution magnetic susceptibility measurements were determined by the Evans method without a solvent correction using reported diamagnetic D
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Organometallics corrections.80 Combustion analyses were obtained from the CENTC Elemental Analysis Facility at the University of Rochester. Materials. IMes, IPr, and MnCl2(THF)1.6 were prepared according to published procedures or slight modification thereof.81−84 4Cyanopyridine was purchased from Acros Organics and used as received. All Grignard reagents were obtained from commercial suppliers and titrated prior to use with 1-octanol employing 1,10phenanthroline as an indicator. X-ray Data Collection and Structure Solution Refinement. Crystals suitable for X-ray diffraction were mounted in Paratone oil onto a glass fiber and frozen under a nitrogen cold stream maintained by an X-Stream low-temperature apparatus. The data were collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.710 73 Å). Data collection and unit cell refinement were performed using Crystal Clear software. Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with Crystal Clear and ABSCOR, respectively.85,86 All structures were solved by direct methods and refined on F2 using full-matrix, least-squares techniques with SHELXL97.87,88 All non-hydrogen atoms were refined with anisotropic displacement parameters. All carbon-bound hydrogen atom positions were determined by geometry and refined by a riding model. Crystallographic data and refinement parameters for each structure can be found in the Supporting Information. [Mn2Cl2(μ-Cl)2(IPr)2], 1. A solution of IPr (500 mg, 1.28 mmol) in THF (10 mL) was combined with MnCl2(THF)1.6 (331 mg, 1.37 mmol) and stirred at room temperature for 12 h. The beige suspension was then filtered through Celite, and all volatiles were removed under reduced pressure. The resulting white residue was extracted into 10 mL of toluene and filtered through Celite. The volatile components were again removed under reduced pressure to ensure removal of THF. The white solid was then redissolved in 6 mL of toluene and set aside at −30 °C for 20 h, during which time colorless crystals formed. The crystals were isolated by decanting the mother liquor and washed with pentane to give 498 mg (75%). Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution of the complex. μeff (Evans, C6D6, 25 °C): 6.7(2) μB. Anal. Calcd for C54H72N4Cl4Mn2: C, 63.04; H, 7.05; N, 5.45. Found: C, 62.81; H, 7.10; N, 5.81. [MnCl2(4-CNpy) (IPr)], 1py. A solution of 4-CNpy (15 mg, 0.14 mmol) in THF (5 mL) was added to a solution of 1 (75 mg, 0.073 mmol) in THF (5 mL) at room temperature. This colorless solution was stirred at room temperature for 4 h. All volatiles were removed under reduced pressure. The residue was extracted into 7 mL of toluene and filtered through Celite. The resulting solution was dried under reduced pressure, and the remaining colorless solid was rinsed with pentane (5 mL). The crude product was redissolved in 3 mL of toluene and set aside at −30 °C for 20 h, during which time colorless crystals formed. The crystals were decanted and washed with pentane to give 56 mg (62%). Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution of the complex. μeff (Evans, C6D6, 25 °C): 5.7(2) μB. IR (νCN, KBr): 2242 cm−1. Anal. Calcd for C33H40N4Cl2Mn: C, 64.08; H, 6.52; N, 9.06. Found: C, 64.74; H, 6.40; N, 9.03. [Mn2Cl2(μ-Cl)2(IMes)2], 2. A solution of IMes (139 mg, 0.455 mmol) in THF (7 mL) was combined with MnCl2(THF)1.6 (118 mg, 0.489 mmol) to give a beige mixture. The mixture was stirred at room temperature for 12 h. The volatile components were removed under reduced pressure to give an off-white solid, which was washed with 5 mL of toluene. The yield of the crude product is essentially quantitative. The crude material was extracted into 10 mL of hot 2MeTHF and filtered through Celite. The resulting solution was concentrated (∼4 mL) and set aside at −30 °C for 20 h, during which time colorless crystals formed. The crystals were isolated by decanting the mother liquor and washed with pentane to give 50 mg (13%). The poor solubility of this complex in most organic solvents resulted in low yields of recrystallized material. Repeated combustion analyses of the compound consistently gave low values for carbon and nitrogen. Anal. Calcd for C42H48Cl4Mn2N4: C, 58.62; H, 5.62; N, 6.51. Found: C, 56.63; H, 5.58; N, 5.73.
[MnCl2(IMes)2], 3. A solution of IMes (36 mg, 0.118 mmol) in THF (7 mL) was added to a vial containing 2 (50 mg, 0.058 mmol) to form a colorless solution. The solution was stirred at room temperature for 4 h. The volatile components were removed under reduced pressure, and the residue was rinsed with pentane (5 mL) to give an off-white solid. The solid was extracted into 5 mL of 2MeTHF, filtered, and then concentrated to ∼2 mL. The solution was set aside at −30 °C for 20 h, during which time colorless crystals formed. The crystals were isolated by decanting the mother liquor and washed with pentane to give 43 mg (50%). Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a saturated benzene solution of the complex. Due to the observed dissociation of IMes from the complex in solution, no magnetic susceptibility was recorded. As with 2, repeated elemental analyses gave low values for carbon and nitrogen. Anal. Calcd for C42H48Cl2MnN4: C, 68.66; H, 6.59; N, 7.63. Found: C, 63.49; H, 6.46; N, 6.60. [Mn2(CH2C6H5)2(μ-Cl)2(IPr)2], 4. A solution of C6H5CH2MgCl (90 μL, 1.0 M, 0.090 mmol) in THF was added dropwise to a solution of 1 (50 mg, 0.049 mmol) in THF (7 mL). The resulting solution immediately became yellow. The reaction mixture was stirred at room temperature for 1 h. All volatile materials were removed under reduced pressure, and the resulting residue was extracted into 5 mL of toluene. The solution was filtered through Celite, and the toluene was removed under reduced pressure. The residue was dissolved in 2 mL of Et2O and set aside at −30 °C for 20 h, during which time yellow crystals formed. The yellow crystals were isolated by decanting the mother liquor and washed with pentane to give 25 mg (45%). Crystals suitable for X-ray diffraction were grown by slow cooling of a saturated toluene solution at −30 °C. μeff (Evans, C6D6, 25 °C): 6.6(2) μB. Combustion analyses of the compound were attempted, but satisfactory values could not be obtained due its sensitivity. Anal. Calcd for C68H86Cl2Mn2N4: C, 71.63; H, 7.60; N, 4.91. Found: C, 66.55; H, 7.42; N, 3.80. [Mn2(2-MeC6H4)2(μ-Cl)2(IPr)2], 5. A solution of 2-MeC6H4MgCl (90 μL, 1.0 M, 0.090 mmol) was added dropwise to a solution of 1 (50 mg, 0.049 mmol) in THF (7 mL) to give a light brown solution. The solution was stirred for 4 h at room temperature. All volatiles were removed under reduced pressure. The brown residue was extracted with toluene (∼7 mL) and filtered through Celite. The filtrate was dried under reduced pressure. The residue was redissolved in 2 mL of toluene, filtered, and set aside at −30 °C for 20 h, during which time brown crystals formed. The crystals were isolated by decanting the mother liquor and washed with pentane to give 36 mg (65%). μeff (Evans, C6D6, 25 °C): 7.6(2) μB. As with 4, the sensitivity of the compound hampered combustion analysis. Anal. Calcd for C68H86N4Cl2Mn2: C, 71.63; H, 7.60; N, 4.91. Found: C, 70.40; H, 7.62; N, 4.83. [Mn2(C6H5)2(μ-Cl)2(IPr)2], 6. A solution of C6H5MgCl (70 μL, 2.78 M, 0.19 mmol) in THF was added dropwise to a solution of 1 (100 mg, 0.097 mmol) in THF (7 mL) to give a light yellow solution. The solution was stirred at room temperature for 4 h. The volatile components were removed under reduced pressure, and the resulting residue was extracted into 7 mL of toluene. After filtration, the toluene solution was evaporated under reduced pressure. The residue was dissolved in 2 mL of Et2O, filtered, and set aside at −30 °C for 20 h, during which time pale yellow-brown crystals formed. The crystals were isolated by decanting the mother liquor and washed with pentane to give 71 mg (66%). μeff (Evans, C6D6, 25 °C): 7.0(2) μB. As with 4, the sensitivity of the compound hampered combustion analysis. Anal. Calcd for C66H82Cl2Mn2N4: C, 71.28; H, 7.43; N, 5.04. Found: C, 66.22; H, 9.02; N, 4.53. [Mn2(C6H5)2(μ-Cl)2(IMes)2], 7. A solution of C6H5MgCl (0.136 mL, 2.0 M, 0.27 mmol) in THF was added dropwise to a suspension of 2 (100 mg, 0.136 mmol) in Et2O cooled to −30 °C. The solution was then stirred at room temperature for 1 h. The volatile materials were removed under reduced pressure. The residue was extracted into 8 mL of toluene and filtered through Celite. Concentration of the toluene solution and storage at −30 °C produced crystals of 7 suitable for X-ray diffraction. E
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Organometallics [Mn2(CH3)2(μ-CH3)2(IPr)2], 8. A solution of MeMgCl (110 mL, 3.0 M, 0.33 mmol) in THF was added dropwise to a solution of 1 (83 mg, 0.081 mmol) in THF (7 mL) to give an orange solution. The solution was allowed to stir at room temperature for 1 h. The volatile components were removed under reduced pressure. The residue was extracted into 7 mL of toluene and filtered through Celite. The filtrate was concentrated to ∼2 mL and stored at −30 °C for 20 h, during which time pale orange crystals formed. The crystalline solid was isolated by decanting the mother liquor and washed with pentane to give 58 mg (76%). Crystals suitable for X-ray diffraction were grown by slow cooling of a saturated THF/pentane solution to −30 °C. μeff (Evans, C6D6, 25 °C): 2.4(2) μB. Anal. Calcd for C58H84N4Mn2: C, 73.55; H, 8.94; N, 5.92. Found: C, 72.52; H, 8.76; N, 5.50. Preparation of [Mn(CH2SiMe3)2(IPr)] from 1. A solution of Me3SiCH2MgCl (0.11 mL, 0.92 M, 0.10 mmol) in THF was added dropwise to a solution of 1 (54 mg, 0.053 mmol) in THF (7 mL). Upon addition of Grignard, the reaction mixture became yellow. The solution was allowed to stir at room temperature for 1 h. All volatile components were removed under reduced pressure, and the resulting yellow residue was extracted with toluene and filtered through Celite. The toluene was removed under reduced pressure. The solid was then dissolved in ∼1 mL of Et2O and stored at −30 °C for 48 h. A mixture of colorless and yellow crystals suitable for X-ray diffraction formed and was separated mechanically. Crystallographic analysis identified the colorless crystals as [Mg(CH2SiMe3)2(IPr)] and the yellow crystals as [Mn(CH2SiMe3)2(IPr)].58 Stoichiometric and Catalytic C−C Coupling Reactions. For stoichiometric reactions, a solution of bromocyclohexane (16 μL, 0.34 M, 0.54 μmol) in THF was added to a solution of either 6 (5.9 mg, 5.3 μmol) or 8 (5.0 mg, 5.3 μmol) in THF (2.5 mL). The solution was stirred at room temperature for 1 h. An aliquot (0.10 mL) of the reaction mixture was injected into a vial containing oxalic acid (0.30 mL) and Et2O (5 mL). The mixture was shaken, and the organic phase separated from the aqueous components. The organic phase was diluted further by Et2O before being subjected to GC-MS analysis. For catalytic trials, a solution of bromocyclohexane or 4bromotoluene (0.10 mmol) in THF was combined with a solution of 1 (5.4 mg, 5.3 μmol) in THF (2.5 mL). A solution of C6H5MgCl (0.11 μmol) in THF was then added. The reaction mixture was allowed to stir at room temperature for 1 h. An aliquot (0.10 mL) of the reaction mixture was injected into a vial containing oxalic acid (0.30 mL) and Et2O (5 mL). The mixture was shaken, and the organic phase separated from the aqueous components. An aliquot of the organic phase (0.1 mL) was then diluted with Et2O and subjected to GC-MS analysis.
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ACKNOWLEDGMENTS
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REFERENCES
The authors acknowledge UTSA and the Welch Foundation (AX-1772 to Z.J.T.) for financial support of this work. The CENTC Elemental Analysis Facility is supported by NSF (CHE-0650456).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00684. Additional spectra of compounds 1−4, thermal ellipsoid drawings of 1-py and 7, and tables of crystallographic data and refinement parameters for all structures (PDF) Crystallographic data (CIF)
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions §
M. H. Al-Afyouni and V. M. Krishnan contributed equally.
Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.organomet.5b00684 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.5b00684 Organometallics XXXX, XXX, XXX−XXX
