ARTICLE pubs.acs.org/Organometallics
Allyl Ligand Reactivity in Tantalum(V) Compounds: Experimental and Computational Evidence for Allyl Transfer to the Formamidinate Ligand in fac-Ta(NMe2)3(η1-allyl)[iPrNC(H)NiPr] via a Metallo-Claisen Rearrangement Shih-Huang Huang,† Xiaoping Wang,‡ Vladimir Nesterov,† David A. Hrovat,†,§ Michael B. Hall,*,^ and Michael G. Richmond*,† †
Department of Chemistry, University of North Texas, Denton, Texas 76203, United States Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Center for Advanced Scientific Computing and Modeling, University of North Texas, Denton, Texas 76203, United States ^ Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States ‡
bS Supporting Information ABSTRACT: Treatment of TaCl(NMe2)4 (1) with allylMgCl furnishes the allyl-substituted compound Ta(NMe2)4(η1-allyl) (2) in moderate yield. The X-ray structure of 2 reveals a trigonal-bipyramidal geometry at the tantalum center with an equatorially situated η1-allyl moiety. VT 1H NMR measurements confirm that the molecule is fluxional in solution over the temperature range 298193 K, and DFT calculations indicate that the time-averaged environment exhibited by the allyl moiety in fluid solution derives from a rapid η1-to-η3 equilibration, with Ta(NMe2)4(η3-allyl) serving as the transition state for this process. 1 reacts rapidly with the formamidine iPrNC(H)NHiPr to yield fac-TaCl(NMe2)3[iPrNC(H)NiPr] (5) and Me2NH, and the tantalum product has been characterized by NMR spectroscopy and X-ray diffraction analysis. The five-coordinate compound Ta(NMe2)3[iPrNCH(allyl)NiPr] (7), whose origin is traced to the putative octahedral species fac-Ta(NMe2)3(η1-allyl)[iPrNC(H)NiPr] (6), has been obtained from the reaction of 2 with iPrNC(H)NHiPr; 7 may also be prepared from the reaction of 5 with allylMgCl. The rearrangement of the allyl moiety in facTa(NMe2)3(η1-allyl)[iPrNC(H)NiPr] to the formamidinate carbon atom in 7 has been investigated by DFT calculations. Here the DFT calculations have provided crucial insight into the reaction mechanism and the composition of those transient species that do not lend themselves to direct spectroscopic observation. The computed barrier for this metallo-Claisen rearrangement is sensitive to the nature of the density functional employed, and the barrier computed using the meta-GGA TPSS functional provides the best agreement with the experimental conditions. The related alkenyl derivatives Ta(NMe2)4(η1-3-butenyl) (3) and Ta(NMe2)3(η1-3butenyl)[iPrNC(H)NiPr] (8) have been synthesized, and their reactivity is contrasted with the corresponding allyl-substituted analogues.
’ INTRODUCTION The first alkyl-amido tantalum(V) compounds having the composition Ta(NMe2)4R (where R = Et, iPr, tBu, CH2SiMe3) were reported by Chisholm et al. in 1982.1 These monoalkyl derivatives were prepared from TaCl(NMe2)4 and the appropriate organolithium reagent. The molecular structure of the tBu derivative revealed a square-pyramidal tantalum compound, and the alkyl group was situated at the apical site. More recently, Xue and co-workers have prepared the related silyl-substituted Ta(NMe2)4R derivatives (where R = SiMe3, SitBuPh2), and the molecular structure of Ta(NMe2)4(SitBuPh2) was found to exhibit a trigonal-bipyramidal geometry with the silyl substituent residing at an equatorial site.2 To our knowledge, Ta(NMe2)4(tBu) and Ta(NMe2)4(SitBuPh2), whose structures are shown r 2011 American Chemical Society
below, represent the only structurally characterized derivatives for this genre of compounds.
Industrial demands for new, single-source tantalum compounds remain high. In particular, new tantalum compounds Received: July 25, 2011 Published: October 17, 2011 5832
dx.doi.org/10.1021/om200683q | Organometallics 2011, 30, 5832–5843
Organometallics that are capable of serving as precursors for the construction of TaN barrier layers and Ta2O5 gate insulators in ULSI devices are actively sought after by the microelectronics industry.3 In the case of TaN diffusion barriers, the major impetus is traced to the fabrication of electronic devices beyond the 45 nm mode, and this is best achieved through an ALD protocol employing a volatile tantalum-based precursor.4 The low molecular weight and expected volatility of these Ta(NMe2)4R derivatives possessing small-chain R groups make them ideal candidates for ALD screening studies. While the R group in Ta(NMe2)4R has the potential to serve as a source of carbon impurity in the product thin film, the use of a group that can undergo a facile homolytic cleavage of the TaC bond during the exposure period of the ALD cycle may circumvent this undesirable manifold. ALD purge removal of the generated organic radical would, in theory, help to eliminate/minimize carbon incorporation in the nascent TaN film. The allyl derivative Ta(NMe2)4(C3H5) has the requisite features needed in an ALD precursor (i.e., low molecular weight and weak TaC bond),5 coupled with the added attraction as a direct precursor for the generation of the reactive d1 species Ta(NMe2)4, after the formal TaC bond homolysis.6,7 Herein we report our results on the synthesis and physical characterization of Ta(NMe2)4(η1-allyl) (2). The introduction of a formamidinate ligand into the coordination sphere of 2 is accompanied by a rapid metallo-Claisen reaction, leading to the formation of the allyl-substituted derivative Ta(NMe2)3[iPrNCH(allyl)NiPr] (7). The energetics and mechanistic details associated with (1) the addition of the formamidine i PrNC(H)NHiPr to compound 2 and (2) the rearrangement of the allyl ligand in transient species fac-Ta(NMe2)3(η1-allyl)[iPrNC(H)NiPr] to afford 7 have been elucidated by DFT calculations.
’ EXPERIMENTAL SECTION General Methods. The TaCl(NMe2)4 (1) starting material employed in our studies was prepared from TaCl5 and LiNMe2 (1:4 stoichiometry) or from the reaction of Ta(NMe2)5 with Me3SiCl, according to the published procedures.8 The formamidine iPrNC(H)NHiPr was synthesized from the silyl-substituted formamidinate derivative iPrNC(H)N(SiEt3)iPr according to the procedure of Ojima.9 TaCl5 was purchased from Pressure Chemical and used as received. The chemicals Me3SiCl, diisopropylcarbodiimide, Et3SiH, allylMgCl (2.0 M in THF), 3-butenylMgBr (0.5 M in THF), and LiNMe2 (95%) were purchased from Aldrich Chemical Co.; the latter three reagents were stored in the glovebox after they were received. All syntheses and the handling of the new tantalum compounds described here were conducted in the glovebox due to the highly sensitive nature of these compounds to moisture and oxygen. The deuterated solvents benzened6 (99.6% D) and toluene-d8 (99.6% D) were purchased from Cambridge Isotope Laboratories. The solvents pentane, toluene, and benzene were distilled from Na-benzophenone ketyl under argon; when not in use, these solvents were stored in Schlenk storage vessels equipped with high-vacuum Teflon stopcocks.10 The NMR solvents benzene-d6 and toluene-d8 were purified by bulb-to-bulb distillation from Nabenzophenone ketyl. The 1H and 13C NMR data were recorded at 400 or 500 MHz on Varian VXR-400 and VXR-500 spectrometers, respectively. The DEPT and gradient-HMQC experiments that were employed in the assignment of the 13C resonances were recorded on the latter spectrometer. The 1H and 13C spectral data have been referenced against the residual protiated and carbon resonance(s) of the NMR solvent. The reported
ARTICLE
mass spectral data were collected at the UBC mass spectrometry facility, which specializes in the handling of highly air- and moisture-sensitive samples. The EI mass spectra were recorded on a Kratos MS50 doublefocusing mass spectrometer configured with a MASPEC data system. The samples were handled under dry nitrogen and introduced via a direct insertion probe, employing a source temperature of 120 C and an ionization energy of 70 eV. The combustion analysis was performed by Galbraith Laboratories, Knoxville, TN.
Synthesis of Ta(NMe2)4(η1-allyl) (2) from 1 and allylMgCl.
To 0.80 g (2.04 mmol) of 1 in 50 mL of pentane at 30 C was added dropwise 1.02 mL (2.04 mmol) of 2.0 M allylMgCl in THF. The solution was allowed to warm slowly to room temperature, and stirring continued for an additional 30 min before the solution was filtered. The insoluble material remaining on the frit was extracted with 3 30 mL portions of pentane. The pentane fractions were combined and then concentrated to ca. 5 mL; the concentrated pentane solution was placed in the glovebox freezer at 30 C for one week, yielding 0.48 g (59% yield) of 2 as a yellow solid. 1H NMR (C7D8, 298 K): δ 3.18 (s, 24H, NMe2), 3.41 (d, 4H, CH2, 3J = 11.3 Hz), 6.14 (quintet, 1H, CH, 3J = 11.3 Hz). 13C NMR (C6D6): δ 47.26 (Me), 88.37 (CH allyl), 138.37 (CH2 allyl). EI-MS: m/e 398 [M]+, 357 [M C3H5]+, 310 [M 2Me2N]+.
Synthesis of Ta(NMe2)4(η1-3-butenyl) (3) and TaBr(NMe2)3(η1-3-butenyl) (4) from 1 and 3-butenylMgBr. A 0.50 g (1.27
mmol) amount of 1 in 50 mL of pentane at 30 C was treated with 2.55 mL (1.27 mmol) of 0.5 M 3-butenylMgBr in THF. After the addition of the Grignard reagent, the solution was allowed to warm to room temperature, with stirring continued for an additional 0.5 h. The solution was filtered, and the residue was then extracted with 3 30 mL portions of pentane. The pentane extracts were combined, and all volatiles removed under vacuum. 1H NMR analysis of the crude product confirmed the existence of an 80/20 mixture of compounds 3 and 4. Yield (combined): 0.25 g. 1H NMR (C6D6) for 3: δ 1.16 (m, 2H, α-CH2), 2.53 (m, 2H, β-CH2), 3.16 (s, 24H, NMe2), 5.05 (m, 1H, CH, vinyl, 2J = 2.0, 3J = 10.3, 4J = 1.2 and 1.0 Hz), 5.22 (m, 1H, CH, vinyl, 2J = 2.0, 3J = 16.3, 4J = 1.4, 1.3 Hz), 6.16 (m, 1H, CH, vinyl, 3J = 16.3, 10.3, 7.20, 6.0 Hz). 13C NMR (C6D6) for 3: δ 32.61 (Cβ, butenyl), 45.35 (amido), 62.27 (Cα, butenyl), 112.03 (CH2, vinyl), 145.30 (CH, vinyl). EI-MS for 3: m/e 412 [M]+, 368 [M NMe2]+, 357 [M C4H7]+. 1H NMR (C6D6) for 4: δ 1.08 (m, 2H, α-CH2), 2.53 (m, 2H, β-CH2), 3.16 (s, 18H, NMe2), 5.00 (m, 1H, CH, vinyl, 2J = 2.0, 3J = 10.1, 4J = 1.2 and 1.0 Hz), 5.12 (m, 1H, CH, vinyl, 2J = 2.0, 3J = 16.7, 4J = 2.0, 1.3 Hz), 6.02 (m, 1H, CH, vinyl 3J = 16.7, 10.1, 7.20, 6.2 Hz). 13C NMR (C6D6) for 4: δ 33.58 (Cβ, butenyl), 45.35 (amido), 71.46 (Cα, butenyl), 112.41 (CH2, vinyl), 144.89 (CH, vinyl). EI-MS for 4: m/e 392/394 [M C4H7]+, 368 [M Br]+.11 i
Synthesis of fac-TaCl(NMe2)3[iPrNC(H)NiPr] (5) from 1 and PrNC(H)NHiPr. To 0.61 g (1.6 mmol) of 1 in 100 mL of pentane at
30 C was added 0.20 g (1.6 mmol) of iPrNC(H)NHiPr in one portion. The reaction solution was allowed to warm slowly to room temperature, and stirring was continued overnight. The homogeneous solution was concentrated to ca. 4 mL, and the contents were placed in the glovebox freezer at 30 C. Yellow crystals of 5 were obtained after several days, and these were isolated by filtration to furnish 0.55 g (75% yield) of the desired product. 1H NMR (C6D6, 298 K): δ 1.15 (d, 6H, iPr Me, 3J = 5.0 Hz), 1.19 (d, 6H, iPr Me, 3J = 5.0 Hz), 3.42 (s, 6H, amido), 3.68 (s, 12H, amido), 3.76 (sept, 2H, iPr methine, 3J = 5.0 Hz), 8.32 (s, 1H, formamidinate). 13C NMR (C6D6, 298 K): δ 24.15 (iPr Me), 25.73 (iPr Me), 47.98 (NMe2), 48.74 (NMe2), 51.11 (iPr methine), 160.82 (CH formamidinate). EI-MS: m/e 475 [M]+, 431 [M Me2N]+, 386 [M 2Me2N]+. Anal. Calcd (found) for C13H33ClN5Ta: C, 32.81 (32.57); H, 6.99 (7.14). Synthesis of Ta(NMe2)3[iPrNCH(allyl)NiPr] (7). A. Reaction of 2 with iPrNC(H)NHiPr. To 0.40 g (1.0 mmol) of 2 in 50 mL of pentane 5833
dx.doi.org/10.1021/om200683q |Organometallics 2011, 30, 5832–5843
Organometallics was added 0.13 g (1.0 mmol) of iPrNC(H)NHiPr in one portion. The solution was stirred overnight, after which time the liberated HNMe2 was removed under vacuum while the solution was concentrated to ca. 2 mL. The concentrated pentane solution was recrystallized at 30 C to furnish 0.34 g (70% yield) of compound 7 as a highly air- and moisture-sensitive pale yellow solid. 1H NMR (C6D6, 298 K): δ 1.15 (d, 6H, iPr Me, 3J = 6.4 Hz), 1.23 (d, 6H, iPr Me, 3J = 6.4 Hz), 2.53 (m, 2H, CH2), 3.24 (s, 18H, NMe2), 3.91 (septet, 2H, iPr methine, 3J = 6.4), 5.15 (dd, 1H, vinyl, 3J = 10.1, 1.7 Hz), 5.22 (dd, 1H, vinyl, 3J = 17.4, 1.7 Hz), 5.68 (t, 1H, diamido methine, 3J = 3.6 Hz), 6.26 (m, 1H, vinyl, 3J = 17.4, 10.1, 6.7 Hz). 13C NMR (C6D6): δ 25.45 (Me, iPr), 25.92 (Me, iPr), 40.78 (CH2, allyl), 46.54 (amido), 51.60 (methine, iPr), 73.60 (CH, diamido), 116.15 (CH2, alkene), 137.57 (CH, alkene). EI-MS: m/e 481 [M]+, 440 [M C3H5]+, 393 [M 2Me2N]+. B. Reaction of 5 with allylMgCl. To 0.50 g (1.05 mmol) of 5 in ca. 50 mL of pentane at 30 C was slowly added 0.53 mL (1.06 mmol) of 2.0 M allylMgCl in THF. The solution was allowed to warm to room temperature, and stirring was continued for an additional 30 min. The solution was next filtered, and the residue was extracted with 3 30 mL portions of pentane. The combined pentane solutions were concentrated to ca. 3 mL, and the solution was allowed to sit in the freezer at 30 C before the recrystallized product was isolated by filtration. Yield: 64% yield (0.32 g) of 7. The spectroscopic properties of the isolated product were identical with the material obtained from the formamidinolysis reaction described above.
ARTICLE
Table 1. X-ray Crystallographic Data and Processing Parameters for Ta(NMe2)4(η1-allyl) (2) and fac-TaCl(NMe2)3[iPrNC(H)NiPr] (5)
75 mL of pentane at 30 C was added, slowly by syringe, 2.10 mL (1.05 mmol) of 0.5 M 3-butenylMgBr in THF. The solution was allowed to warm slowly to room temperature, after which the solution was filtered and the residue extracted with 3 30 mL portions of pentane. The organic solutions were combined and then concentrated to ca. 3 mL before the solution was placed in the freezer at 30 C for recrystallization. 8 was isolated as a light yellow solid in 67% yield (0.35 g) after ca. one week at 30 C. 1H NMR (C6D6, 298 K): δ 1.05 (d, 12H, iPr Me, 3 J = 6.4 Hz), 1.06 (m, 2H, α-CH2), 2.80 (m, 2H, β-CH2), 3.42 (s, 18H, NMe2), 3.45 (septet, 2H, iPr methine, 3J = 6.4), 5.08 (m, 1H, CH, vinyl, 2 J = 2.5, 3J = 10.1, 4J = 1.2 and 1.2 Hz), 5.27 (m, 1H, CH, vinyl, 2J = 2.5, 3 J = 16.9, 4J = 1.5, 1.5 Hz), 6.26 (m, 1H, CH, vinyl, 3J = 16.9, 10.1, 7.0, 6.7 Hz), 8.31 (s, 1H, formamidinate). 13C NMR (C6D6): δ 25.05 (iPr Me), 35.72 (Cβ, butenyl), 47.58 (NMe2), 50.85 (iPr methine), 63.09 (Cα, butenyl), 111.05 (CH2, vinyl), 148.27 (CH, vinyl), 160.13 (CH formamidinate). EI-MS: m/e 493 [M 2H]+, 440 [M C4H7]+. X-ray Diffraction Data for Compounds 2 and 5. Single crystals of 2 and 5 suitable for X-ray diffraction analyses were grown from pentane at 30 C in the glovebox freezer; in each case the selected crystal was picked up with a cryoloop from a dry ice cooled pentane solution containing the crystals using a Hampton cryoclamp. The X-ray data were collected on a Bruker X8 APEX CCD diffractometer at 100(2) K. The frames were integrated with the available APEX212 software package, using a narrow-frame algorithm; and the structures were solved and refined using the SHELXTL program package.13 The molecular structures were checked using PLATON,14 and the non-hydrogen atoms were refined anisotropically, except where otherwise noted. The crystal of 2 was found to be split. The unit cell for 2 was indexed and integrated successfully as a four-component twin (twin ratio 0.569:0.287:0.106:0.037), and these data were refined accordingly to convergence using the unmerged intensity data in HKLF5 format. All hydrogen atoms were assigned calculated positions and allowed to ride on the attached carbon atom. Table 1 summarizes the pertinent X-ray data and processing parameters for the two structures. Computational Methodology and Modeling Details. The bulk of our calculations were performed with the hybrid DFT functional B3LYP, as implemented by the Gaussian 09 program package.15
fac-TaCl(NMe2)3-
(η1-allyl)
[iPrNC(H)NiPr]
CCDC entry no.
776971
776969
cryst syst
triclinic
triclinic
space group
P1
P1
a, Å b, Å
7.951(1) 9.631(1)
7.6476(6) 15.091(1)
c, Å
10.339(2)
18.283(2)
α, deg
90.741(1)
113.307(1)
β, deg
95.894(1)
91.927(1)
γ, deg
97.351(1)
96.393(1)
V, Å3
780.8(2)
1918.8(3)
mol formula
C11H29N4Ta
C13H33ClN5Ta
fw formula units per cell (Z)
398.33 2
475.84 4
Dcalcd, Mg/m3
1.694
1.647
λ (Mo Kα), Å
0.71073
0.71073
μ, mm1
7.025
5.867
semiempirical from
numerical
absorp corr
Synthesis of Ta(NMe2)3(η1-3-butenyl)[iPrNC(H)NiPr] (8) from 5 and 3-butenylMgBr. To 0.50 g (1.05 mmol) of 5 in ca.
Ta(NMe2)4-
equivalents
a
F(000)
392
944
cryst size, mm3 abs corr factor
0.51 0.43 0.14 0.3742/0.0281
0.39 0.37 0.32 0.2530/0.2075
total reflns
3015
23 947
indep reflns
3009
8497
data/res/params
3009/2/164
8497/0/381
R1a (I g 2σ(I)]
0.0385
0.0160
wR2b
0.1023
0.0351
GOF on F2
1.069
1.013
ΔF(max), ΔF(min), e/Å3
2.787/3.498
0.824/0.872
R1 = ∑||Fo| |Fc||/∑|Fo|. b R2 = {∑[w(F2o F2c)2/∑[w(F2o)2]}1/2.
This functional utilizes the Becke three-parameter exchange functional (B3),16 combined with the correlation functional of Lee, Yang, and Parr (LYP).17 The generalized gradient approximation (meta-GGA) TPSS was employed in the study of the η1-to-η3 allyl equilibrium in Ta(NMe2)4(η1-allyl) and the transfer of the allyl to the formamidinate ligand in Ta(NMe2)3(η1-allyl)[iPrNC(H)NiPr] since the B3LYP functional did not afford realistic barrier heights for these particular reactions.18,19 The Ta and Cl atoms were described by StuttgartDresden effective core potentials (ecp) and the SDD basis set, while the 6-31G(d0 ) basis set, as implemented in the Gaussian09 program suite, was employed for the remaining atoms. All reported geometries were fully optimized and evaluated for the correct number of imaginary frequencies through calculation of the vibrational frequencies, using the analytical Hessian. Zero imaginary frequencies (positive eigenvalues) correspond to an intermediate or minimum (local or global), whereas an imaginary frequency (negative eigenvalue) designates a transition state. All transition states on the potential energy surface were evaluated by IRC calculations. The computed frequencies were used to make zero-point and thermal corrections to the electronic energies; the reported potential energies and enthalpies are quoted in kcal/mol relative to the specified standard.20 The geometry-optimized structures have been drawn with the JIMP2 molecular visualization and manipulation program.21 5834
dx.doi.org/10.1021/om200683q |Organometallics 2011, 30, 5832–5843
Organometallics
ARTICLE
Scheme 1
’ RESULTS AND DISCUSSION I. Preparation and Characterization of the Alkenyl-Substituted Compounds 2 and 3. The reactions of 1 with the
Grignard reagents allylMgCl and 3-butenylMgBr were performed in pentane at 243 K, and the stirred solution was allowed to warm to room temperature with stirring. Scheme 1 shows the principal products that were isolated from these reactions. The depicted tantalum products, which are highly soluble in hydrocarbon solvents, were isolated as yellow solids that are extremely oxygen and moisture sensitive. The workup protocol is straightforward and simply involves filtration to remove the pentaneinsoluble material that accompanied the reaction, followed by recrystallization of the crude product(s) from a concentrated pentane solution at 243 K. In the case of 2, the isolated yields are lower than the crude reaction yields determined by 1H NMR analysis, and this is attributed to the high solubility of 2 in hydrocarbon solvents. The coordination geometry at the tantalum center and the bonding mode displayed by the ancillary allyl ligand in 2 were established by X-ray crystallography. The solid-state structure of 2 (Figure 1) reveals the compound adopts a trigonal-bipyramidal geometry and possesses an η1-allyl moiety that is situated at an equatorial site. The four amido groups are trigonal planar (∑ of the angles subtended at each nitrogen ≈359), and this allows these groups to function as 3e donor ligands through both σ and π coordination manifolds.22 The N2 and N3 amido groups occupy the axial sites in the trigonal bipyramid, and the N2Ta(1)N3 linkage reveals a bond angle of 169.2(2) that is canted toward the allyl ligand. Presumably, this deviation from linearity is influenced by the two equatorial amido groups, which serve to repel the two axial substituents. The Ta(1)C(9) distance of 2.225(6) Å in 2 agrees closely with those TaC bond distances reported for the amido-substituted compounds Ta(NMe2)4(tBu) (2.24(2) Å),1 TaBr(NMe2)3(p-tolyl) (2.18(2) Å),1 Ta(NMe2)3(CH2Ph)[(iPrN)2CN(SiMe3)iPr] (2.275(7) Å),23 and TaCl(NMe2)3(Me) (2.19(1) Å).24 The allyl C(9)C(10) and C(10)C(11) bond distances of 1.470(9) and 1.33(1) Å are unremarkable in nature and require no comment. The room-temperature 1H NMR spectrum of 2, recorded in toluene-d8, reveals a singlet at δ 3.18 (24H) for the four amido groups, but more interesting are the doublet at δ 3.41 (4H) and
Figure 1. Thermal ellipsoid plot of the molecular structure of Ta(NMe2)4(η1-allyl) (2) at the 50% probability level. Selected bond distances (Å) and angles (deg): Ta(1)N(1) = 1.982(5), Ta(1) N(2) = 2.027(5), Ta(1)N(3) = 2.050(5), Ta(1)N(4) = 1.976(5), Ta(1) C(9) = 2.225(6), C(9)C(10) = 1.470(9), C(10)C(11) = 1.33(1), N(4)Ta(1)N(1) = 115.6(2), N(4)Ta(1)C(9) = 117.9(2), N(1)Ta(1)C(9) = 126.5(2), N(2)Ta(1)N(3) = 169.2(2), N(2)Ta(1)C(9) = 86.5(2), N(3)Ta(1)C(9) = 82.7(2).
binomial pentet at δ 6.14 (1H). The latter two resonances are readily assigned to the allyl moiety, which is fluxional in solution on the NMR time scale.25 The 13C NMR spectrum exhibits three resonances at δ 47.26, 88.37, and 138.37, and these are assignable to the amido and the allyl CH and CH2 groups, respectively. The NMR assignments were further corroborated by 1H COSY, DEPT, and HMQC experiments. The 1H and 13C chemical shifts in 2 are consistent with the NMR data reported for compounds that contain a conformationally static η3-C3H5 moiety, as well as those allyl-substituted derivatives whose allyl moiety exhibits a time-average environment due to a rapid equilibration involving the η1 and η3 isomers.26 The VT 1H NMR behavior of 2 in toluene-d8 over the temperature range 298193 K was next investigated, in order 5835
dx.doi.org/10.1021/om200683q |Organometallics 2011, 30, 5832–5843
Organometallics to probe the dynamics associated with the fluxional allyl moiety.27 Lowering the temperature to 253 K promoted the collapse of the 4H doublet resonance at δ 3.41, with coalescence achieved at 223 K. The remaining amido and allyl pentet resonances did not exhibit any significant temperature-dependent changes over the course of this experiment. Attempts to record spectra below 193 K were met with excessive line broadening due to decreased solubility and viscosity problems. The VT NMR data are, nonetheless, informative in terms of establishing the fact that the η1η3 allyl fluxionality remains rapid at 193 K. The ΔGq for the η1η3 allyl equilibration in 2 is estimated as 9.0 kcal/mol.28 A detailed discussion of the process responsible for the generation of the time-average environment displayed by the allyl moiety in 2, based on the data at hand, is problematic absent the slow-exchange spectrum. Complicating matters are the fluxional processes involving Berry pseudorotation and allyl ligand rotation about the TaC bond of the η1 isomer; however, these two processes cannot, in and by themselves, give rise to a symmetrically bound η3 isomer or the observed time-average environment displayed by the allyl moiety in 2.29,30 The dynamics associated with the η1η3 allyl isomerization in 2 were addressed through the use of electronic structure calculations. Of the three minima that were found for 2, the global minimum is represented by species A1, which also happens to coincide with the solid-state structure. The structures of A1 and A10 are based on a trigonal bipyramid, while A100 consists of a square pyramid with an apical allyl group.31 The energies, reported as ΔE(ΔH) relative to A1, are listed below. The energy difference between the isomeric compounds is small and in keeping with the highly fluxional nature of five-coordinate d0 compounds.
DFT calculations employing A10 and A100 failed to provide structures that contained a symmetrical η3-allyl ligand. Calculations using A10 collapsed to A1, and the C4v propeller orientation of the amido groups in A100 precluded the formation of both a stable minimum and a bona fide transition state with a symmetrical η3-allyl ligand. Equilibration of the allyl moiety in A1 occurs via a sliding motion and gives rise to transition state TSA1A2, which contains a mirror plane that bisects the migrating allyl ligand and the amido group that is distal to the CH vector of the central allyl carbon. Coupled with the allyl transit across the equatorial face of 2 is the gearlike motion of the methyl groups associated with the equatorial amido ligands. That the species TSA1A2 is an actual transition state and not a local minimum on the PES was verified by normal-mode analysis. Figure 2 shows this process and the optimized structures. B3LYP gives a ΔGq value of 15.9 kcal/mol for isomerization of the allyl ligand; the computed magnitude of ΔGq, relative to the VT NMR data, was deemed unrealistically high, and this prompted us to examine the TPSS functional to better model this particular process.19 The ΔGq barrier computed by TPSS is 8.6 kcal/mol, and this represents a magnitude more in line with the VT NMR data.
ARTICLE
Figure 2. Optimized B3LYP (black) and TPSS (red) potential energy surfaces for the dynamic equilibrium between Ta(NMe2)4(η1-allyl) (A1, A2) and Ta(NMe2)4(η3-allyl) (TSA1A2). Energy values are ΔE(ΔH) in kcal/mol.
As shown in Figure 2, TSA1A2 from TPSS is computed to have a 4.3 kcal/mol lower enthalpy compared with the transition-state structure from B3LYP. The reaction of 1 with the nonconjugated Grignard reagent 3-butenylMgBr was next examined in an effort to prepare Ta(NMe2)4(η1-3-butenyl) (3), as this particular compound would serve as a useful NMR benchmark against the fluxional allyl moiety in 2. Treatment of 1 with 3-butenylMgBr gave a mixture of 3 and 4 (Scheme 1), as assessed by NMR spectroscopy and mass spectrometry. 3 is produced as the major product in yields that varied between 60% and 80%. While the source of bromide ligand in 4 derives from the commercially available Grignard reagent,1,32 the nature of the precursor that gives 4 is not obvious and is complicated by the known redistribution reactions exhibited by 1 and related derivatives.8a,b Ligand substitution in 1 likely occurs in a bimodal fashion through a formal replacement of the Cl and NMe2 ligands by the 3-butenyl group, giving 3 and TaCl(NMe2)3(3-butenyl). Once formed, TaCl(NMe2)3(3-butenyl) undergoes Cl/Br metathesis to furnish 4. A related scenario involving the reaction of 1 with the reducing agents ZnMe2 and MeMgCl has recently been reported by us.24 The high solubility of 3 and 4 in hydrocarbon solvents prevented their effective separation by fractional recrystallization, prompting us to examine the reaction of 4, in this mixture, with added LiNMe2 as a route to 3. Mixtures of 3 and 4 react with excess LiNMe2 in pentane to give 3, along with Ta(NMe2)5 (33 kcal/mol. (53) For other reports on structurally characterized five-coordinate tantalum compounds that display a distorted trigonal-bipyramidal geometry, see:(a) Ref 36. (b) Tin, M. K. T.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 1998, 37, 6728. (c) Lavoie, N.; Gorelsky, S. I.; Liu, Z.; Burchell, T. J.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 2010, 49, 5231.
’ NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper that was published on October 17, 2011, the chemical formula given in the title was incorrect. The version of the title that appears as of November 7, 2011 is correct.
5843
dx.doi.org/10.1021/om200683q |Organometallics 2011, 30, 5832–5843
