Article pubs.acs.org/Organometallics
New Alkylidyne Complexes Featuring a Flexible Trianionic ONO3− Pincer-Type Ligand: Inorganic Enamine Effect versus Sterics in Electrophilic Additions Sudarsan VenkatRamani, Nicholas B. Huff, Muhammad Tariq Jan, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige* Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *
ABSTRACT: Metal−carbon multiple bonds exhibit enhanced nucleophilicity at the α carbon within ONO trianionic pincer alkylidyne complexes. Defined as the inorganic enamine ef fect, this phenomenon is a result of the overlap of the N atom lone pair within the ONO3− ligand and a π bond from the metal− carbon multiple bond. Treating the proligand [OCH2NCH2O]H3 (2) with (tBuO)3WCR (where R = Et, tBu) results in the formation of the dianionic pincer complexes [OCH2NHCH2O]WCR(OtBu) (where R = Et (3-Et), tBu (3-tBuanti)). Deprotonation of 3-tBuanti by treatment with Ph3PCH2 forms the anionic alkylidyne complex {[OCH2NCH2O]W CtBu(OtBu)}{CH3PPh3} (4-tBu). DFT calculations modeling 4-tBu reveal overlap of the N atom lone pair with a WC π bond. However, 4-tBu reacts with electrophiles preferentially at the pincer N atom as opposed to the WCα group. Multinuclear NMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography are employed to characterize complexes 3-Et, 3-tBuanti, and 4-tBu.
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INTRODUCTION Tailored ancillary ligands are critical to the further development of homogeneous transition-metal catalysis.1 Ligand modifications enable exquisite control over the electronic and geometric properties of metal complexes. Among the various classes of ligands available, pincer and pincer-type ligands2 garner particular attention. Emerging as a class of their own, trianionic pincer ligands,3 thus far, have been well-suited for stabilizing early transition metals in their high oxidation states. Trianionic [NCN], 4−7 [OCO], 8−20 [NNN], 21−25 [CCC], 26 and [ONO]27−36 ligands are some of the common donor motifs and in combination with the appropriate metal ion will catalyze nitrene and carbene transfer,21−23,27,37 aerobic oxidation,14 ethylene and alkyne polymerization,5,6,15,16,36 and alkene isomerization.5 Important structure−bonding relationships29,33 within trianionic pincer complexes are emerging and serve to elevate the level of electronic and geometric control over metal ions. For example, tungsten−alkylidene and −alkylidyne complexes featuring the [ONO]3− pincer-type ligand [CF3-ONO]H3 exhibit enhanced nucleophilicity at the metal−carbon multiple bond.28−30 The enhanced nucleophilicity is a consequence of the “inorganic enamine effect” (Figure 1).28−30 Analogously to enamines, an inorganic enamine involves constraining a nitrogen atom lone pair to be collinear with a metal−carbon π bond. The interaction accentuates the nucleophilicity by © 2015 American Chemical Society
delocalizing electron density from the nitrogen atom lone pair onto the α carbon, and being π* in character, the HOMO orbital is destabilized. The trianionic pincer ligand [CF3-ONO]3−, first used to illustrate the inorganic enamine effect, is relatively rigid and contains a biaryl amido ligand.28−30 One question to answer is as follows: what influence do the N atom substituents (aryl vs alkyl) and the flexibility of the ligand have on the nucleophilicity of metal−carbon multiple bonds? Described herein is the synthesis of the trianionic pincer ligand precursor [OCH2NCH2O]H3 (2), its metalation with (tBuO)3WCEt and (tBuO)3WCtBu to afford complexes 3-Et and 3-tBuanti, access to the trianionic version {[OCH2NCH2O]WCtBu(OtBu)}{CH3PPh3} (4-tBu), and in situ solution-phase experiments to assess the inorganic enamine effect on 4-tBu.
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RESULTS AND DISCUSSION In four steps, starting from commercially available 2-(tertbutyl)phenol, or alternatively in three steps from 3-(tert-butyl)2-hydroxybenzaldehyde, the proligand 238,39 was synthesized (Scheme 1). o-Formylation of 2-(tert-butyl)phenol using the procedure 40 described by Skattebol et al. affords the salicylaldehyde 3-(tert-butyl)-2-hydroxybenzaldehyde in gram Received: February 24, 2015 Published: May 28, 2015 2841
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Organometallics
Scheme 2. Synthesis of [OCH2NHCH2O]WCEt(OtBu) (3Et) and [OCH2NHCH2O]WCtBu(OtBu) (3-tBuanti)
ONO]H3, as result of proton migration to the alkylidyne.29 Cooling concentrated diethyl ether solutions of 3-Et and 3-tBuanti to −35 °C provides single crystals amenable to X-ray interrogation. Figures 2 and 3 depict the solid-state molecular structures of 3-Et and 3-tBuanti. During the crystallization step, 3-Et binds a diethyl ether molecule to complete the distorted-octahedral coordination environment around the W(VI) ion (see Figure 2, right). The W1−C31 bond length of 1.760(2) Å and the ∠W1−C31−C32 bond angle of 178.11(18)° are consistent with those of known high-oxidation-state tungsten− and molybdenum−alkylidynes featuring a pincer ligand (either dianionic or trianionic).15,29,30,32,44 The sum of angles around the central nitrogen donor of 339.33(13)° implies a pyramidal geometry; indeed, the difference Fourier Map reveals electron density for a proton on the nitrogen that was refined freely. The long W1− N1 bond distance of 2.2743(18) Å is consistent with a protonated “amino” rather than “amido” nitrogen, which serves as an L-type donor. A 1H NMR spectrum (C6D6) of recrystallized 3-Et exhibits signals consistent with a Cs-symmetric complex. The higher symmetry is in contrast with the C1-symmetric structure observed in the solid state. Presumably, a fluxional process flips the C14 methylene from the up position to the down position and vice versa for C7. The dynamic process occurs more quickly than the NMR time scale, therefore imparting a mirror plane that bisects the complex. As a consequence, the two −tBu groups from the pincer ligand are symmetric and resonate as a singlet at 1.63 ppm. The −OtBu protons resonate slightly downfield of the pincer ligand tBu protons at 1.78 ppm. The alkylidyne ethyl group CH2 protons appear as a quartet centered at 3.85 ppm, and the CH3 protons resonate upfield at 0.69 ppm. The methylene protons on the pincer ligand are diastereotopic and exhibit different scalar couplings with the amino proton. The net result is the appearance of two doublets of doublets: one centered at 3.33 ppm (with 2JHH = 13.1 Hz and 3JHH = 2.4 Hz) and another centered at 4.51 ppm (with 2 JHH = 13.1 Hz and 3JHH = 12.2 Hz). The protons that resonate at 4.51 ppm must be anti relative to the NH due to its large coupling constant (12.2 Hz). In contrast, the coupling constant for the syn protons is only 2.4 Hz.45,46 A resonance at 283.5
Figure 1. Enhancement of metal−carbon multiple bond nucleophilicity due to overlap of the amido lone pair with the tungsten alkylidyne π bonds within [CF3-ONO]WCEt(OtBu)− (orbital pictures presented at isovalue 0.056187).
quantities and sufficient purity. Methylation of the phenolic oxygen followed by reductive amination using the Williamson method41 provides the protected secondary amine derivative 1 in reasonable yield and purity. Demethylation using BBr3 in CH2Cl2 at 0 °C, followed by successive acid and base washes and a pentane recrystallization of the crude mixture, provides access to the proligand 2 in 16−25% overall yield. The low yield is a consequence of the BBr3 deprotection step. Fresh BBr3 must be used to achieve the modest yields. The 1H NMR spectrum of 2 in CDCl3 is characteristic of a C2-symmetric molecule. The methylene spacer in 2 resonates as a singlet, integrating to four protons at 3.94 ppm. The other resonances pertaining to the tert-butyl group and the aromatic protons are unexceptional. Syntheses and Solid-State Structures of [OCH2NHCH2O]WCR(OtBu) (3-R; R = Et, tBu). Combining solutions of [OCH2NCH2O]H3 (2) and (tBuO)3WCEt42 or (tBuO)3WCtBu43 in benzene generates the dianionic pincertype complexes [OCH2NHCH2O]WCEt(OtBu) (3-Et) and [O CH2 NH CH2O]WC tBu(OtBu) (3-t Buanti ), respectively (Scheme 2). Complexes 3-Et and 3-tBuanti retain the proton on the N atom, and this is the first clear indication that ligand 2 is significantly different from the rigid [CF3-ONO]H3 ligand.29 The alkylidenes [CF3-ONO]WCHEt(OtBu) and [CF3ONO]WCHtBu(OtBu) form upon metalation with [CF3-
Scheme 1. Synthesis of [OCH2NCH2O]Me2 (1) and [OCH2NCH2O]H3 (2)a
a
Legend: (i) Ti(OiPr)4, NH4Cl, NEt3, NaBH4, ethanol (200 proof); (ii) BBr3, CH2Cl2, 0 °C. 2842
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Figure 2. (left) Solid-state structure of 3-Et with hydrogen atoms (except H1) and disordered C33 removed for clarity. (right) Truncated thermal ellipsoid plot (40% probability) of the W(VI) core. Selected bond distances (Å): W1−C31−1.760(2), W1−O1−1.9918(13), W1−O2−1.9929(14), W1−O3−2.4973(14), W1−O4−1.9004(14), W1−N1−2.2743(18), N1−H1−0.83(2). Selected bond angles (deg): ∠W1−C31−C32−178.11(18), ∠C31−W1−O3−172.32(7), ∠N1−W1−O4−158.59(6), ∠O1−W1−O2−150.42(6), ∠C14−N1−C7−110.03(16), ∠C14−N1−W1−112.10(12), ∠C7−N1−W1−117.20(12).
Figure 3. (left) Solid-state structure of 3-tBuanti with hydrogen atoms (except H1) and lattice solvent diethyl ether removed for clarity. (right) Truncated thermal ellipsoid plot (40% probability) of the W(VI) core. Selected bond distances (Å): W1−C23−1.761(2), W1−O1−1.9992(14), W1−O2−1.9956(13), W1−O3−1.8956(14), W1−N1−2.2709(18), N1−H1−0.82(2). Selected bond angles (deg): ∠W1−C23−C24−177.82(16), ∠N1−W1−O3−155.08(7), ∠O1−W1−O2−149.96(6), ∠C18−N1−C7−109.03(17), ∠C18−N1−W1−113.70(13), ∠C7−N1−W1−116.91(13).
stopped early and tBuOH is removed in vacuo, a 1H NMR spectrum of the resulting solid only displays signals for 3-tBuanti with little or no resonances for 3-tBusyn. The cross peaks in the gHMBC spectrum of the mixture indicate that both compounds are Cs symmetric with an intact NH bond, a tBu alkylidyne, and a tBuO ligand. Given the C2 symmetry of the ligand, the only possible isomers are those resulting from the relative orientation of the NH hydrogen and the alkylidyne/tBuO. An NOE between the protons in the tBu alkylidyne of 3-tBuanti (0.95 ppm) and the methylene protons anti to the NH (5.09 ppm; Figure S43 in the Supporting Information), confirms the anti disposition of the NH protons relative to the alkylidyne. Heating a solution of 3-tBuanti for 9 days at 70 °C establishes equilibrium between the isomers in an anti:syn ratio of 36:64, respectively. Depicted in Figure 3 is the solid-state molecular structure of 3-tBuanti. Complex 3-tBuanti does not bind a diethyl ether molecule during crystallization. A simple explanation is that the ∠N1−W1−O3 angle in 3-tBuanti is 3.51(9)° smaller than that in 3-Et, thus precluding the diethyl ether from binding. At 0.09,
ppm in the 13C{1H} NMR spectrum of 3-Et (C6D6) for W Cα confirms the presence of an alkylidyne. Adding proligand 2 to (tBuO)3WCtBu in C6D6 initially provides 3-tBuanti. Monitoring the reaction overtime by 1H NMR spectroscopy reveals signals attributable to 3-tBuanti and the syn isomer 3-tBusyn (Scheme 3). When the reaction is Scheme 3. Interconversion of 3-tBuanti and 3-tBusyn
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Article
Organometallics the Addison parameter47 τ is consistent with a square-pyramidal geometry. Around the nitrogen atom, the sum of angles measures 339.64(14)°, indicating pyramidalization, and again, the proton was refined freely (N1−H1 = 0.82(2) Å). Consistent with 1D-NOE data (see Figure S43 in the Supporting Information), the proton on the nitrogen adopts an anti configuration with respect to the alkylidyne fragment. From previous work,29 using the more rigid [CF3-ONO]H3 ligand, we observed facile proton migration from the central nitrogen to the alkylidyne carbon upon metalation, leading to alkylidenes.29,30,32 3-Et and 3-tBuanti/3-tBusyn are interesting because the −NH proton does not migrate to the alkylidyne. A straightforward explanation is that the N group in [OCH2NCH2O]H3 is an aliphatic secondary amine and is therefore more basic and the flexibility within the ligand framework places the NH anti to the alkylidyne, thereby precluding activation. Other alkylidynes that retain an −NH bond include Mo(NHArCl)(CtBu)[biphen]48 and W(NHAr)(CtBu)[OCMe(CF3)2]2(DME);49 these alkylidynes do not interconvert to their corresponding alkylidene tautomers Mo(NAr Cl )(CH t Bu)[biphen] 48 and W(NAr)(CH t Bu)[(OCMe(CF3)2]2.49 Synthesis and Molecular Structure of {[P(CH3)(C6H5)3]}{[OCH2NCH2O]WCtBu(OtBu)} (4-tBu). Treating 3-tBuanti with freshly prepared Ph3PCH250 in pentane at ambient temperature followed by overnight stirring precipitates 4-tBu as a fine orange powder (Scheme 4). The syntheses of trianionic pincer complexes [tBuOCO]WCtBu(OtBu)15 and [CF3-ONO]WCtBu(OtBu)30 employ a similar deprotonation strategy.
Figure 4. Molecular structure of 4-tBu with hydrogen atoms and lattice solvent benzene removed for clarity. Selected bond distances (Å): W1−C23−1.758(4), W1−O1−2.011(3), W1−O2−2.018(3), W1−O3−1.934(2), W1−N1−2.026(3), N1−C7−1.469(5), N1− C18−1.466(5) P1−C50−1.785(5). Selected bond angles (deg): ∠W1−C23−C24−173.0(3), ∠N1−W1−O3−151.29(13), ∠O1− W1−O2−146.47(11), ∠C18−N1−C7−113.9(3), ∠C18−N1−W1− 118.2(3), ∠C7−N1−W1−116.8(3).
W(VI) ion in a distorted-square-pyramidal geometry (τ = 0.08)47 and a triphenylmethylphosphonium countercation. A striking feature found in the solid-state structure of 4-tBu is the sum of angles around the central nitrogen donor. In spite of being sp2 hybridized, the N atom substituents deviate from planarity with angles adding up to only 348.4(3)°; this is 10.4(4)° short of the corresponding angles (358.8(3)°) found in the neutral alkylidyne complex [CF3-ONO]WCtBu(OEt2).30 Nonetheless, other data support the nitrogen as being sp2 hybridized. The difference Fourier map did not reveal electron density attributable to a proton on the nitrogen; attempts to place a proton riding on the nitrogen atom led to chemically meaningless N−H bond lengths. In addition, the W1−N1 bond length of 2.026(3) Å in 4-tBu is in agreement with the corresponding bond distance of 2.008(2) Å found in [CF3-ONO]WCtBu(OEt2)30 and the 2.161(3) Å W1−N1 bond distance in {[P(CH3)(C6H5)3]}{[pyr-ONO]WCtBu(OtBu)}.32 Conversely, in 3-Et and 3-tBuanti, which feature an sp3-hybridized N atom, the W1−N1 bond distances are longer at 2.2743(18) and 2.2709(18) Å, respectively. Ground-State DFT Study of {[P(CH 3 )(C 6 H 5 ) 3 ]}{[OCH2NCH2O]WCtBu(OtBu)} (4-tBu). Using the hybrid functional B3LYP51,52/LANL2DZ53 and M0654/LANL2DZ53 basis sets from the Gaussian 09 program suite,55 geometry optimization and single point analysis of 4-tBu were performed using spin-restricted density functional theory (DFT). Atomic coordinates from the crystal structure serve as the initial input for the geometry-optimized structure of 4-tBu. 4-tBu′ is calibrated by comparison to the experimental bond lengths and angles determined for 4-tBu. Table 1 gives pertinent bond lengths and angles for 4-tBu and 4-tBu′. Molecular orbitals generated from the program Gabedit are reported at their stated isovalues. As is evident from Figure 5 (only the B3LYP result depicted), the calculation overestimates the bond lengths by ∼0.02 Å. In addition, the sum of the angles (358.15°) around the nitrogen atom within 4- t Bu′ is larger than the
Scheme 4. Synthesis of {[P(CH3)(C6H5)3]}{[OCH2NCH2O]WCtBu(OtBu)} (4-tBu)
The 1H NMR spectrum (C6D6) of 4-tBu exhibits resonances consistent with the successful deprotonation event. The resonances of the tBu directly bound to the WC unit, the methylene, and the aryl protons from the phosphonium countercation are broadened. Perhaps as a consequence of this broadening and fast relaxation, the resonance corresponding to WCα was not located either in a 13C{1H} or in a 2D 1 H−13C gHMBC NMR experiment. Evidence from the 31 1 P{ H} NMR spectrum for the deprotonation event comes in the form of a singlet at 21.3 ppm corresponding to the phosphonium countercation. This value in the 31P{1H} NMR spectrum of 4-tBu is consistent with other anionic alkylidynes generated employing a similar deprotonation strategy.29,30,32 Conclusive evidence for the identity of 4-tBu comes from an X-ray crystallography experiment performed on single crystals that grow via vapor diffusion of pentane into a benzene solution of 4-tBu. Depicted in Figure 4 is the thermal ellipsoid plot of the solid-state molecular structure of 4-tBu. Complex 4-tBu is C1 symmetric in the solid state, featuring a core comprising the 2844
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version involves removal of the bound −OtBu ligand. For example, treating {[P(CH3)(C6H5)3]}{[CF3-ONO]WCtBu(OtBu)} with MeOTf in Et2O precipitates CH3PPh3OTf and releases MeOtBu to produce the neutral alkylidyne [CF3ONO]WCtBu(OEt2).30 Treating 4-tBu with methylating reagents MeOTf and Me3OBF4 in C6D6 (Scheme 5) results in a color change from orange to yellow and precipitation of CH3PPh3OTf. The 1H NMR spectrum after addition reveals the formation of several unidentifiable compounds. However, within the mixture are resonances attributable to Nmethylation. In addition, deprotonation occurs with an instant color change to deep brown upon dissolving 4-tBu in CDCl3 or CHCl3 to give 3-tBu (Scheme 5). Unfortunately, we were unable to conclusively identify the conjugate base product from the deprotonation of either CDCl3 or CHCl3. Preferential attack at the N atom over the −OtBu group is understandable. The inorganic enamine effect increases the nucleophilicity of the alkylidyne α carbon, but sterics play an important role as well. For example, treating the ethyl alkylidyne {[CF3-ONO]WCEt(OtBu)}− 29 with MeOTf results in attack at the α carbon of the alkylidyne, whereas in the larger tBu derivative {[CF3-ONO]WCtBu(OtBu)}− 30 attack occurs at the −OtBu group. For 4-tBu, though the inorganic enamine effect may even be enhanced relative to that for {[CF3-ONO]WCtBu(OtBu)}− (see strong overlap in HOMO(-3) of Figure 6), the most open site is the nitrogen atom. The lack of steric encumbrance around the N atom renders it more reactive in comparison to WCα. Attack at nitrogen in the presence of alkoxide and alkylidyne ligands is normal. For example, electrophilic additions occur preferentially at the aryl−amido ligand for the imido alkylidyne anion {(NAr)MoCtBu[OCMe(CF3)2]2}−.57 In addition, trianionic pincer-type complexes of the general formula [NNN]TaMe224 exhibit divergent reactivity as a function of π bonding between the equatorial nitrogen atom and the Ta ion. [NNN]TaMe2 complexes with significant π bonding from the central N atom do not react with protons or Lewis acids, whereas complexes with minimal π bonding do.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Single-Crystal X-ray Structure of 4-tBu and DFT Geometry Optimized Structure of 4-tBu′ bond length
4-tBu
W1−O1 W1−O2 W1−O3 W1−N1 W1−C23 C23−C24 N1−C7 N1−C18
2.011(3) 2.018(3) 1.934(2) 2.026(3) 1.758(4) 1.515(6) 1.469(5) 1.466(5)
4-tBu′ (B3LYP/ LANL2DZ)
bond angle
4-tBu
2.038 2.045 1.950 2.014 1.779 1.505 1.478 1.467 4-tBu′ (B3LYP/ LANL2DZ)
∠W1−C23−C24 ∠N1−W1−O3 ∠O1−W1−O2 ∠C18−N1−C7 ∠C18−N1−W1 ∠C7−N1−W1 ∠W1−O3−C28
173.0(3) 151.29(13) 146.47(11) 113.9(3) 118.2(3) 116.8(3) 139.3(3)
177.23 148.42 150.19 117.61 119.92 120.52 146.48
4-tBu′ (M06/ LANL2DZ) 2.018 2.026 1.947 2.020 1.767 1.495 1.470 1.462 4-tBu′ (M06/ LANL2DZ) 170.51 147.82 153.26 117.49 119.51 122.44 140.63
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Figure 5. Geometry-optimized structure of 4-tBu′ using B3LYP/ LANL2DZ level theory.
CONCLUSION The new flexible pincer [OCH2NCH2O]H3 (2) is a viable ligand for stabilizing tungsten alkylidyne complexes in both the dianionic ([O CH2 NH CH2 O]WCEt(O t Bu) (3-Et) and [OCH2NHCH2O]WCtBu(OtBu) (3-tBuanti/3-tBusyn)) and trianionic forms ({[OCH2NCH2O]WCtBu(OtBu)}{CH3PPh3} (4-tBu)). The methylene spacers in [OCH2NCH2O]3− allow flexibility in the backbone to maximize overlap between the N atom lone pair and an alkylidyne π bond. The intent is to accentuate the inorganic enamine effect and therefore the nucleophilicity of the alkylidyne α carbon. At least from computation results, the flexible ligand does increase the orbital overlap between the N atom lone pair and the filled metal− carbon π orbital in {[O CH2 N CH2 O]WC t Bu(O t Bu)}{CH3PPh3} (4-tBu) versus {[CF3-ONO]WCtBu(OtBu)}{CH3PPh3} (see Figure 6). However, attack at the exposed N atom in the backbone on 4-tBu occurs for both H+ and Me+, thus precluding the isolation of a neutral alkylidyne complex.
experimentally determined value of 348.4(3)°. In spite of these differences, the calculation correctly reproduces the experimentally observed twist in the ONO ligand backbone. Evidence for the twist comes from the dihedral angle between the N atom lone pair and the W1−C23 bond. A vector perpendicular to the C7−W1−C18 plane within 4-tBu represents the idealized position of the N atom lone pair. In the crystal structure, the vector subtends 44.02(4)° with respect to the W1−C23 bond, and the computed structure matches with an angle of 44.32°. As a consequence, the N atom lone pair strongly overlaps with the π bond in the WCtBu fragment, giving rise to an “inorganic enamine” (Figure 6; HOMO(-3)). The corresponding antibonding combination is the HOMO and is analogous to the HOMO orbital observed for a typical organic enamine.56 The overlap between the amido lone pair and the alkylidyne π bonds raises the energy of the HOMO and delocalizes electron density onto the alkylidyne α carbon, thus increasing the nucleophilicity of the metal−carbon multiple bond (similar to the increased nucleophilicity of an enamine’s β carbon). Ligand-Centered Reactivity. An established protocol15,29 to convert the anionic alkylidyne into the corresponding neutral
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EXPERIMENTAL SECTION
General Considerations. Unless specified otherwise, all manipulations were performed under an inert atmosphere using standard Schlenk or glovebox techniques. Pentane, hexanes, toluene, diethyl ether, tetrahydrofuran, and acetonitrile were dried using a Glass2845
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Figure 6. Truncated molecular orbital diagrams (B3LYP/LANL2DZ) exhibiting inorganic enamine bonding combinations for 4-tBu′ (left) and {[CF3-ONO]WCEt(OtBu)}− anion (right). The HOMO(-3) of 4-tBu′ exhibits more overlap between the amido lone pair and a tungsten alkylidyne π bond. The comparative bonding combination within [CF3-ONO]WCEt(OtBu)− is represented by HOMO(-2) (isovalue 0.056187). vacuum-transferred and stored over 4 Å molecular sieves. CDCl3 (Cambridge Isotopes) was dried over CaH2, distilled, and stored over 4 Å molecular sieves. THF-d8 (Cambridge Isotopes) was used as received. (tBuO)3WCtBu,43 (tBuO)3WCEt,42 3-(tert-butyl)-2hydroxybenzaldehyde,40 3-(tert-butyl)-2-methoxybenzaldehyde,58 and Ph3PCH250 were prepared according to published procedures. All other reagents were purchased from commercial vendors and used without further purification. 1H and 13C{1H} 2D NMR spectra were obtained on an Inova 500 MHz spectrometer, and 31P{1H} spectra were acquired on either a Varian Mercury broad band 300 MHz or Varian Mercury 300 MHz spectrometer. The chemical shifts are reported in δ (ppm) and were referenced to the lock signal on the TMS scale for 1H and 13C NMR spectra. For 1H and 13C{1H} NMR spectra, the residual solvent peak was used as an internal reference. Elemental analyses were performed at Complete Analysis Laboratory Inc., Parsippany, NJ. DFT Calculations. Geometry optimization and single point analysis of 4-tBu′ were performed using spin-restricted density functional theory calculations, with the hybrid functional B3LYP51,52/LANL2DZ53 and M0654/LANL2DZ53 basis sets as implemented in the Gaussian 09 program suite55. The atomic coordinates from the crystal structures were used as an initial input for the geometry-optimized structures. Molecular orbital pictures were generated from Gabedit at their reported isovalues.
Scheme 5. Nitrogen-Centered Reactivity: MeOTf Addition and Protonation of the Anionic Alkylidyne 4-tBu in C6D6
Contour drying column. Benzene-d6 and toluene-d8 (Cambridge Isotopes) were dried over sodium benzophenone ketyl, distilled, or 2846
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Article
Organometallics
JHH = 12.3 Hz, −NCH2), 3.30 (dd, 2H, 2JHH = 12.1 Hz, 3JHH = 2.2 Hz, −NCH2), 1.81 (s, 9H, −OC(CH3)3), 1.63 (s, 18H, Ar C(CH3)3), 0.98 (s, 9H, −WCC(CH3)3) ppm. 13C{1H} NMR (C6D6, 125 MHz): δ 288.9 (s, WCtBu), 161.9 (s, Ar C), 139.3 (s, Ar C), 127.5 (s, Ar C), 127.2 (s, Ar C), 122.5 (s, Ar C), 118.6 (s, Ar C), 80.5 (s, −OC(CH3)3), 66.2 (s, WCC(CH3)3), 59.4 (s, −NCH2), 50.8 (s, Ar C(CH3)3), 35.8 (s, WCC(CH3)3), 34.2 (s, −OC(CH3)3), 30.6 (s, Ar C(CH3)3) ppm. Anal. Calcd for C31H47NO3W: C, 55.94; H, 7.12; N, 2.10. Found; C, 55.86; H, 7.10; N, 2.09. Data for 3-Et (yield 0.11 g, 57%) are as follows. 1H NMR (C6D6, 300 MHz): δ 7.40 (dd, 2H, 3JHH = 7.6 Hz, 4JHH = 1.8 Hz, Ar H), 6.82 (t, 2H, 3JHH = 7.6 Hz, Ar H), 6.75 (dd, 2H, 3JHH = 7.3 Hz, 4JHH = 1.5 Hz, Ar H), 4.51 (dd, 2H, 2JHH = 13.1 Hz, 3JHH = 12.2 Hz, −NCH2), 3.85 (q, 2H, WCC(CH2CH3)3), 3.33 (dd, 2H, 2JHH = 13.1 Hz, 3JHH = 2.4 Hz, −NCH2), 1.78 (s, 9H, −OC(CH3)3), 1.63 (s, 18H, Ar C(CH3)3), 0.69 (t, 3H, −WC(CH2CH3)) ppm. 13C{1H} NMR (C6D6, 75 MHz): δ 283.5 (s, WCEt), 162.1 (s, Ar C), 139.4 (s, Ar C), 127.4 (s, Ar C), 127.3 (s, Ar C), 122.9 (s, Ar C), 118.8 (s, Ar C), 81.4 (s, −OC(CH3)3), 59.4 (s, −NCH2), 55.7 (s, Ar C(CH3)3), 35.8 (s, −OC(CH3)3), 33.9 (s, WC(CH2CH3)), 30.7 (s, Ar C(CH3)3) 18.0 (s, WC(CH2CH3)) ppm. Anal. Calcd for C29H43NO3W: C, 54.64; H, 6.80; N, 2.20. Found; C, 54.09; H, 6.37; N, 2.12. Synthesis of {[P(CH3)(C6H5)3]}{[OCH2NCH2O]WCtBu(OtBu)} (4-tBu). A pentane solution (5 mL) of Ph3PCH2 (0.044 g, 0.16 mmol, 1.01 equiv) was added dropwise to a stirred pentane solution of 3-tBuanti (0.105 g, 0.157 mmol, 1.0 equiv), resulting in the precipitation of a pale orange powder. The mixture was stirred for 4 h ,and the solid was separated by filtration and washed with fresh pentane. The solid was dried under vacuum for 1 h to afford 4-tBu as a fine pale orange powder (yield 0.13 g, 89%). 1H NMR (C6D6, 500 MHz): δ 7.55 (br, 4H, Ar H), 7.40 (dd, 2H, 3JHH = 7.1 Hz, 4JHH = 2.5 Hz, Ar H), 6.96−7.08 (br, 8H, Ar H), 6.80 (dd, 2H, 3JHH = 7.4 Hz, 4 JHH = 2.5 Hz, Ar H), 6.77 (t, 2H, 3JHH = 7.4 Hz, Ar H), 3.47 (br, 4H, −NCH2), 1.82 (s, 9H, −OC(CH3)3), 1.70 (s, 18H, Ar C(CH3)3), 0.72 (s, 9H, WCC(CH3)3) ppm. 31P{1H} NMR (C6D6, 121 MHz): δ 21.3 ppm. 13C{1H} NMR (C6D6, 125 MHz): δ not observed (W CtBu), 165.2 (s, Ar C), 138.2 (s, Ar C), 134.5 (s, Ar C), 133.1 (d, Ar C, 3 JCP = 9.10 Hz), 129.5 (d, Ar C, 2JCP = 11.2 Hz), 128.7 (s, Ar C), 127.7 (s, Ar C), 126.7 (s, Ar C), 125.2 (s, Ar C), 119.1 (s, Ar C), 80.5 (s, OC(CH3)3), 56.7 (NCH2), 50.8 (s, WCC(CH3)3), 35.8 (s, Ar C(CH3)3), 33.6 (s, OC(CH3)3), 33.1 (s, WCC(CH3)3), 31.2 (s, Ar C(CH3)3) ppm. Anal. Calcd for C50H64NO3PW: C, 63.76; H, 6.85; N, 1.49. Found; C, 63.42; H, 6.60; N, 1.63. 3
Synthesis of Bis(3-(tert-butyl)-2-methoxybenzyl)amine, [OCH2NHCH2O]Me2 (1). In a round-bottom flask were placed 3-(tertbutyl)-2-methoxybenzaldehyde (8.22 g, 43.0 mmol), Ti(OiPr)4 (25.6 mL, 85.0 mmol, 2.0 equiv), ammonium chloride (4.57 g, 85.0 mmol, 2.0 equiv), and trimethylamine (12.0 mL, 85.0 mmol, 2.0 equiv) in that order, and the mixture was slurried using absolute ethanol. The reaction mixture was stirred overnight under a positive pressure of argon; the solution color changed from yellow-orange to deep orange. NaBH4 (2.42 g, 64 mmol, 1.5 equiv) was then added as a solid to the reaction mixture, and this mixture was further stirred for another 7 h. The reaction mixture was quenched by pouring into ammonium hydroxide (2 M, 200 mL); the copious white precipitate that formed was filtered off and extracted with ethyl acetate (×3). The aqueous layer was extracted with ethyl acetate, and the organic fractions were combined, dried over MgSO4, and concentrated in vacuo to afford an orange oil (4.5 g, crude). Hexane was added until all the oil dissolved; 2 M HClaq was added carefully, and the mixture was stirred for 15 min. While still warm, the layers were separated. A white solid precipitated from the organic layer upon cooling. The solid was filtered off and extensively dried to give 1·HCl (yield 2.60 g, 30%). 1H NMR (CDCl3, 300 MHz): δ 7.50 (dd, 2H, 3JHH = 7.4 Hz, 4JHH = 1.7 Hz, Ar H), 7.36 (dd, 2H, 3JHH = 7.9 Hz, 4JHH = 1.7 Hz, Ar H), 7.07 (t, 2H, 3JHH = 7.6 Hz, Ar H), 4.18 (s, 4H, −NCH2), 3.55 (s, 6H, −OCH3), 1.36 (s, 18H, −C(CH3)3) ppm. 1·HCl was slurried in diethyl ether and treated with saturated aqueous NaOH (1 M), upon which the organic layer turned turbid; at this point the layers were separated. The organic layer was collected, dried over MgSO4 and concentrated in vacuo to yield neutral 1. 1H NMR (CDCl3, 300 MHz): δ 7.29 (dd, 2H, 3JHH = 7.4 Hz, 4JHH = 1.8 Hz, Ar H), 7.24 (dd, 2H, 3JHH = 7.9 Hz, 4JHH = 1.8 Hz, Ar H), 7.02 (t, 2H, 3JHH = 7.6 Hz, Ar H), 3.89 (s, 4H, −NCH2), 3.77 (s, 6H, −OCH3), 1.40 (s, 18H, −C(CH3)3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ 158.3 (s, Ar), 142.7 (s, Ar), 133.9 (s, Ar), 128.6 (s, Ar), 126.0 (s, Ar), 123.5 (s, Ar), 61.9 (s, OCH3), 49.0 (s, NCH2), 35.1 (s, C(CH3)3), 31.1 (s, C(CH3)3) ppm. Synthesis of 6,6′-(Azanediylbis(methylene))bis(2-(tertbutyl)phenol), [OCH2NCH2O]H3 (2). [OCH2NHCH2O]Me2 (1; 4.5 g, 0.012 mol) was dissolved in dichloromethane and cooled in an ice− water bath. Freshly purchased BBr3 (10 mL, 8 equiv) was then syringed carefully into the reaction vessel, leading to an immediate color change from orange to yellow. The reaction mixture was stirred under a positive pressure of argon at the same temperature and slowly warmed to ambient temperature. The reaction mixture was stirred for an additional 6 h and then cooled back to 0 °C using an ice−water bath and carefully quenched with an ice-cold solution of methanol. HClaq (1 M) was then added to the reaction mixture and stirred for 30 min. The layers were separated, and the organic fraction was treated with aqueous NaHCO3. Ether extraction of the organic layer followed by drying over MgSO4 and concentration in vacuo afforded a greasy solid. Recrystallization from cold pentane provided pure 2 (yield 0.67 g, 16%). 1H NMR (CDCl3, 300 MHz): δ 7.25 (dd, 2H, 3JHH = 7.6 Hz, 4 JHH = 1.5 Hz, Ar H), 6.97 (dd, 2H, 3JHH = 7.6 Hz, 4JHH = 1.5 Hz, Ar H), 6.80 (t, 2H, 3JHH = 7.6 Hz, Ar H), 3.93 (s, 4H, −NCH2), 1.44 (s, 18H, −C(CH3)3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ 155.1 (s, Ar), 136.7 (s, Ar), 127.7 (s, Ar), 126.5 (s, Ar), 123.5 (s, Ar), 119.4 (s, Ar), 50.9 (s, NCH2), 34.6 (s, C(CH3)3), 29.7 (s, C(CH3)3) ppm. Synthesis of [OCH2NHCH2O]WCCR(OtBu) (R = Et (3-Et), tBu (3-tBuanti)). A representative procedure is given for the synthesis of 3-tBuanti; the same procedure was utilized for the synthesis of 3-Et. A benzene solution (2 mL) containing 2 (0.096 g, 0.28 mmol, 1 equiv) was added dropwise to a benzene (1 mL) solution of (tBuO)3WCtBu (0.133 g, 0.280 mmol, 1 equiv). The reaction mixture was stirred for 0.5 h. All volatiles were evaporated under vacuum for 1 h. The golden brown powder was triturated with pentane (×3). After removal of all volatiles, the golden brown powder was dissolved in a minimal amount of Et2O and filtered. Cooling the solution to −35 °C precipitated crystals of 3-tBuanti (yield 0.13 g, 68%). 1H NMR (C6D6, 500 MHz): δ 7.39 (dd, 2H, 3JHH = 7.7 Hz, 4 JHH = 1.4 Hz, Ar H), 6.80 (t, 2H, 3JHH = 7.7 Hz, Ar H), 6.75 (dd, 2H, 3 JHH = 7.7 Hz, 4JHH = 1.4 Hz, Ar H), 4.68 (dd, 2H, 2JHH = 12.1 Hz,
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, tables, and CIF files giving full experimental procedures, NMR spectra, X-ray crystallographic details, and details of the calculated structures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00155.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for A.S.V.:
[email protected]fl.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.S.V. thanks the University of Florida and the National Science Foundation for financial support of this project (CHE1265993). K.A.A. thanks the University of Florida and the National Science Foundation (CHE-0821346) for funding the purchase of X-ray equipment. Computational resources and support were provided by the University of Florida HighPerformance Computing Center. 2847
DOI: 10.1021/acs.organomet.5b00155 Organometallics 2015, 34, 2841−2848
Article
Organometallics
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DOI: 10.1021/acs.organomet.5b00155 Organometallics 2015, 34, 2841−2848
