Advances in Structural Studies on Alkylaluminum Species in the Solid

Article pubs.acs.org/JPCC

Advances in Structural Studies on Alkylaluminum Species in the Solid State via Challenging 27Al−13C NMR Spectroscopy and X‑ray Diffraction Frédérique Pourpoint,* Yohann Morin, Régis M. Gauvin,* Julien Trébosc, Frédéric Capet, Olivier Lafon, and Jean-Paul Amoureux* Université Lille Nord de France, CNRS, UMR 8181 UCCS; ENSCL, 59652 Villeneuve d’Ascq France S Supporting Information *

ABSTRACT: Advanced multinuclear solid state NMR experiments were developed to probe the structure of two organometallic aluminum derivatives, Li[Al(CH3)3CH2Si(CH3)3] (1) and Li[Al(CH3)4] (2), which are relevant to olefin polymerization processes. For the first time, NMR observation of 27Al−13C covalent bonds in solids is performed with the natural abundance material 1. Unprecedented triple-resonance (1H−13C−27Al) and quadruple-resonance (1H−7Li−13C−27Al) heteronuclear correlation two-dimensional NMR experiments are also introduced to probe 27Al−13C and 13C−7Li proximities for 2. High-resolution solid-state NMR spectra thus obtained provide information on the local structure of these representative organometallic derivatives that proved to be most complementary and in full agreement with the structures obtained by X-ray diffraction.

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INTRODUCTION Organometallic derivatives of aluminum are a major class of compounds within the worldwide chemical industry. Since the development of their direct synthesis in the 1950s,1 they represent the organometallic species with the largest commercial use, ranging up to multithousand tons per year.2 Their application field encompasses stoichiometric and catalytic processes, allowing access for instance to olefins or alcohols. Additionally, they can also be involved as cocatalysts in olefin and diene polymerization processes.3 In this context, both neutral trisalkyl (AlR3) and anionic tetraalkyl (MAlR4) derivatives can be used, depending on the specificity of the considered catalytic system.4,5 Moreover, tetraalkylaluminates have been extensively used as organometallic building blocks, allowing access to a wide library of heterobimetallic organometallic species.6,7 Obviously, the deepest understanding of the structure of these species is a prerequisite to optimize their use and to rationalize further developments.8 However, the structure of several organoaluminum solids, such as Li[Al(CH3)3CH2Si(CH3)3] (1) and Li[Al(CH3)4] (2), have not been reported hitherto.8−11 Furthermore, the characterization of supported organoaluminum species has remained challenging.12−15 In this context, X-ray diffraction (XRD) and solid-state NMR constitute two complementary methods to probe respectively the long- and short-range orders in solid organoaluminum materials.16−19 Single-crystal XRD provides detailed information on the internal lattice including cell dimensions, bondlengths, and bond angles, but it is not an element specific method. Moreover, the preparation of single crystals may sometimes prove to be very troublesome. Of course, X-ray diffraction on single crystals can be applied to highly air © 2013 American Chemical Society

sensitive species, while requiring extreme care in sample preparation and crystal handling. This generates complications for their characterization by XRD techniques. Furthermore, XRD is not applicable for noncrystalline solids, such as organoaluminum species supported on oxide that are widely used in industrial olefin polymerization processes.12−15 On the contrary, NMR is a powerful element specific local probe and hence it is suitable for both ordered and disordered solids. In particular, it is a method of choice to characterize the structure of supported organometallic species.20 In this case only simple sample preparation is required (even if NMR is more demanding than XRD in terms of material quantity), and strictly airtight equipment is widely available for NMR analysis of highly air-sensitive materials. Moreover, NMR allows a priori an easy detection of the Al−C bond, which is the reactive moiety of the organometallic aluminum derivatives, since both 27 Al and 13C isotopes benefit from favorable NMR properties, including wide chemical shift ranges, large gyromagnetic ratios of about one forth that of protons and 100% natural abundance for 27Al. However, the observation of connectivities and proximities between these two nuclei has so far been limited by the specifications of current NMR probes. Actually, the conventional HXY triple resonance probes usually require at least ca. 30% difference in the resonance frequencies of X and Y channels,21−23 whereas the 27Al and 13C Larmor frequencies are only 3.6% apart. Therefore, conventional triple resonance probes cannot be tuned simultaneously to the 27Al and 13C Received: June 4, 2013 Revised: July 19, 2013 Published: July 22, 2013 18091

dx.doi.org/10.1021/jp4055044 | J. Phys. Chem. C 2013, 117, 18091−18099

The Journal of Physical Chemistry C

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Spectra were recorded with a three-channels HXY probe with a 4 mm rotor spinning at νR = 10 kHz. The probe was used in double resonance mode for triple-resonance (1H−13C−27Al) experiments and in triple resonance mode for quadrupleresonance (1H−7Li−13C−27Al) experiments. The Larmor frequencies are of 400, 155.4, 104.2, and 100.6 MHz, for 1H, 7 Li, 27Al, and 13C, respectively. Samples 1 and 2 are air sensitive, and hence rotors were packed under inert atmosphere in a glovebox. Diplexers. It has been demonstrated that diplexers, also called overcoupled resonators, are an efficient solution to tune and match a probe simultaneously to two close Larmor frequencies,22,23,31 such as 27Al and 13C.24−26 Here, commercially available diplexer was employed.32 This device allows the conversion of a triple-resonance probe into a quadrupleresonance one. The only limitation of these diplexers is the inability to apply simultaneously two radio frequency (rf) fields at the two Larmor frequencies. The rf pulses on both 27Al and 13 C channels were generated by two rf-synthesizers, one for each isotope, connected to a single amplifier. This spectrometer setup allows an accurate control of the phases of 27Al and 13C pulses, which is essential for 27Al−13C HETCOR 2D experiments. In a previous 27Al−13C dipolar dephasing sequence,26 the phase control of the two saturation pulses was not required, which allowed the use of only one rfsynthesizer. 27 Al− 13 C HETCOR Experiments. The first 27 Al− 13 C HETCOR 2D experiments are introduced. These experiments were achieved using heteronuclear multiple quantum correlation (HMQC) sequences relying on coherence transfers via Jor dipolar couplings, denoted hereafter J-HMQC and DHMQC, respectively. J-HMQC methods aim at probing the 27 Al−13C J-coupling in solids in order to observe chemical connectivities between these nuclei. To the best of our knowledge, these J-couplings have never been measured or used in solids. They have only been determined in solutions.16,17 27Al−13C D-HMQC methods complement JHMQC since they provide information on spatial proximities between these isotopes. In the D-HMQC sequences, 27Al−13C dipolar interactions were reintroduced using the simultaneous frequency and amplitude modulation (SFAM1) scheme,33 since (i) this m = 1 heteronuclear dipolar recoupling benefits from a √2 faster dipolar dephasing than the m = 2 recoupling sequences, such as SR421,34 thus lowering the losses, and (ii) 13 C−13C dipolar couplings are negligible with respect to the spinning speed, especially in NA13C samples.34,35 The SFAM1 sequence was employed with peak values for frequency and max amplitude sweeps equal to (Δvmax ref,13C, Δvnut,13C) = (60, 40−50) 33 kHz. These peak values correspond to the regime 2, where the effective field is always much larger than the apparent frequency. This regime was chosen since it benefits from a high robustness to 13C electronic shielding.34,35 As SFAM1 is not γencoded, the reintroduced dipolar dephasing depends on the initial rotor-phase, and the beginning of the two SFAM1 recoupling parts must be perfectly rotor-synchronized in order to avoid imperfect echo formations.36 13 C-{27Al} J- or D-HMQC started by a 1H → 13C crosspolarization (CP) transfer to increase the 13C signal and to decrease the recycle delay to τRD = 30 s, with the use of presaturation. For 27Al-{13C} J- or D-HMQC, no initial CP transfer was used since 27Al longitudinal relaxation times are

Larmor frequencies. These probe specifications are not only a limitation for the 27Al−13C pairs, but they preclude the observation of proximities and connectivities between any pairs of isotopes displaying close Larmor frequencies. Recently this instrumental limitation has been circumvented using diplexers,24−26 and 13C−27Al distances have been measured by solid-state NMR spectroscopy. However, to the best of our knowledge, no 27Al−13C heteronuclear correlation (HETCOR) two-dimensional (2D) NMR experiment has been reported hitherto for both liquids and solids and 27Al−13C J-couplings have only been measured for organoaluminum in solution.16,17 In this article, we introduce 27Al−13C HETCOR 2D experiments relying on coherence transfers via J or dipolar couplings to observe 27Al−13C connectivities and proximities, respectively. These methods are first proved in natural isotopic abundance for 1, and they provide structural insights for this compound, without the use of XRD. Then, NMR and XRD are combined to elucidate the solid-state structure of 2; the crystal arrangement of this compound being determined for the first time using single crystal diffraction. These 2D experiments provide valuable insights into its crystal structure. In particular, we report the first NMR observation of 27Al−13C covalent bonds via J-coupling, and we show that 27Al−13C HETCOR spectra allow distinguishing crystallographically inequivalent carbon sites. In addition, quadruple-resonance 1 H−7Li−13C−27Al HETCOR experiments are introduced with triple resonance probe to observe 13C−7Li spatial proximities in 2.

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EXPERIMENTAL METHODS Synthesis. 1 was prepared with 13C natural abundance (NA13C) following the literature procedure.27 2 was synthesized under inert argon atmosphere using a glovebox, following a two-step reaction: first, methyllithium, [13C]-CH3Li was prepared by reacting 99% 13 C enriched iodomethane (purchased from CortecNet Company) with lithium in dry diethylether. After the reaction mixture was stirred overnight at room temperature, it was added to a toluene solution of excess trimethylaluminum with NA13C. The resulting solution was stirred overnight, and volatiles were evacuated. The resulting waxy solid was stirred over pentane, resulting in a white precipitate of lithium−tetramethylaluminum, which was collected by filtration and washed with pentane. In this material, site exchange may occur with high probability during the reaction,28 resulting in Li[Al(13CH3)n(12CH3)4‑n] species, with n = 0−4. This compound is called [25%-13C]-2 in the following. X-ray Diffraction. Single-crystal X-rays measurements were performed at 100 K under the N2 stream of a Cryostream 700 device (Oxford Cryosystem) because of the high air sensitivity of the samples. Data were collected using an Apex II CCD 4K Bruker diffractometer with λMoKα = 0.71073 Å. Several samples were checked and revealed to be systematically twinned (180° around a⃗*; domain-fraction parameter, 0.519 (2)). The twin law was determined with the program Cell-now. Then intensities were extracted from Saint frames.29 Data were corrected for absorption using the program Twinabs.29 The structure was solved by direct methods using the Shelxs program and then refined with Shelxl.30 All non-H thermal parameters were refined anisotropically. Hydrogen atoms were located from Fourier difference maps and refined isotropically. NMR Experiments. Experiments were performed on a 9.4 T Bruker spectrometer equipped with AVANCE-II console. 18092



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Figure 1. Pulse-sequences of (a) 13C-{27Al} and (b) 27Al-{13C} J-HMQC, as well as of (c) 13C-{27Al} and (d) 27Al-{13C} D-HMQC and (e) 13C{7Li} D-HMQC experiment with 27Al decoupling during the 13C acquisition. The 13C-{7Li} D-HMQC experiment without decoupling is identical to that of panel e, but no pulse is applied to the 27Al channel. The phase cycling is the one reported in the literature.34 For sequences a to d, all pulses on 27Al and 13C channels were rotor-synchronized. This also holds true for the 13C and 7Li channels of 13C-{7Li} D-HMQC. SPINAL-64 1H decoupling,40 was used during recoupling, evolution, and acquisition times (τ, t1, and t2). Quadrature detection in the indirect dimension of 2D NMR experiment was achieved using the States-TPPI method.41.

short and 1H → 27Al CP transfers are inefficient.37 The τRD delay for 27Al-{13C} J- or D-HMQC experiments was 2 s. 13 C-{7Li} D-HMQC Experiment with 27Al Decoupling. 13 C−7Li proximities in [25%-13C]-2 were probed using 13C7 { Li} D-HMQC sequences (Figure 1e) recorded with and without 27Al decoupling during the acquisition. The experiments were acquired with τRD = 15 s and presaturation pulses. 27 Al decoupling during 13C acquisition has not been reported hitherto. The 13C-{7Li} D-HMQC experiment with 27Al decoupling is a quadruple-resonance (1H−7Li−13C−27Al) NMR experiment, which was carried out using a triple resonance probe equipped with a diplexer. The 27Al decoupling consists of rotor asynchronized multiple pulses (RA-MP) during the acquisition with pulse amplitude and length of 37 kHz and 10 μs, respectively. The period between the centers of two consecutive RA-MP pulses was 110 μs, which slightly exceeds one rotor period (TR = 100 μs) as usual with RA-MP decoupling. This decoupling suppresses the J-coupling with the central and satellite transitions of 27Al nuclei and is more efficient than continuous wave (CW) decoupling since the latter reintroduces the heteronuclear dipolar couplings.38,39 Moreover, the RA-MP decoupling is compatible with the use of diplexers since the irradiation of the quadrupolar isotope is discontinuous and 13C signals can be acquired during the

windows. The 13C signal between two consecutive RA-MP pulses is calculated by averaging signals acquired every 50 ns, after the ring-down of the probe. Additional parameters for NMR experiments are given in the Supporting Information.

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RESULTS AND DISCUSSION Al−13C HMQC of 1 in Natural Abundance. Hitherto, 27 Al−13C double-resonance NMR experiments have been limited to dipolar dephasing methods, such as S-RESPDOR,26 REAPDOR,24,25 and TRAPDOR.42−45 These double-resonance experiments allow the measurement of 27Al−13C distances for isolated spin-pairs and identifying carbon sites that are close to aluminum atoms. However, they do not indicate which aluminum sites are near to carbon atoms, because there is no 27 Al spectral dimension. The HMQC sequences have been chosen for 27Al−13C HETCOR since they do not require simultaneous irradiation of 27 Al and 13C isotopes and hence are compatible with the use of diplexers. Furthermore, the efficiency and robustness of the Jand D-HMQC experiments have been demonstrated to correlate the signals of spin-1/2 and quadrupolar nuclei exhibiting remote Larmor frequencies (e.g., 31P−27Al or 31 P−11B).35,46−49 The 27Al−13C J- and D-HMQC sequences 18093

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Figure 2. Sample 1. (a) Structure of the anionic Al(CH3)3CH2Si(CH3)3 moiety displaying carbon atom labels. (b−e) 27Al−13C HMQC 2D spectra of 1 in natural abundance recorded at 9.4 T with νR = 10 kHz. 27Al-{13C} J- (b) and D- (c) HMQC spectra with τ = 3.5 and 2 ms, respectively. 13C{27Al} J- (d) and D- (e) HMQC spectra with τ = 3.5 and 1 ms. The total experiment times are 15, 27, 45, and 36 h for spectra b, c, d, and e, respectively. The 27Al DP-MAS and 1H→13C CP-MAS are shown.

and (ii) two broad resonances at 0 and −5 ppm attributed respectively to the C3 and C1 atoms bonded to the Al atom. Figure 2b,c shows the 27Al-{13C} J- and D-HMQC 2D spectra of 1 in natural abundance corresponding respectively to the sequences illustrated in Figure 1b,d. These spectra were acquired in 15 and 27 h, respectively, showing the feasibility of 27 Al-{13C} HMQC experiments in natural abundance. The spectrum of 27Al-{13C} J-HMQC in Figure 2b represents the first solid-state NMR observation of 27Al−13C J-couplings and hence of 27Al−13C covalent bonds. This 2D-experiment displays two cross-peaks Al−C1 and Al−C3, which correspond to one-bond 27Al−13C proximities, and hence validates the structure given in Figure 2a. The lower Al−C3 cross-peak intensity originates from the lower multiplicity of the C3 site and the broader width of its signal. The correlation Al−C2 is absent owing to the very low three-bond 3JAl−C2 scalar coupling. Figure 2c displays the 27Al-{13C} D-HMQC 2D spectrum of 1. Besides Al−C1 and Al−C3 cross-peaks, Al−C2 correlation is also visible in this spectrum since coherence transfer via dipolar

are shown in Figure 1a−d. The HMQC experiments with 13C (see Figure 1a,c) or 27Al (see Figure 1b,d) detection are denoted 13C-{27Al} and 27Al-{13C}, respectively, in the following. The structure of 1 is given in Figure 2a where the three types of carbon are annotated. No crystallographic structure of this compound has been published. Through the example of 1, we aim at showing that 27Al−13C HMQC experiments (i) provide valuable structural insights in the absence of XRD crystal structure and (ii) can be used in natural isotopic abundance. 27 Al direct polarization MAS (DP-MAS) and 1H→13C CP-MAS spectra of 1 are presented and assigned in Figure 2b−e and Supporting Information. The 27Al DP-MAS 1D spectrum exhibits a single CT (Central Transition) with a typical secondorder quadrupolar line shape, which can be fitted with the following parameters: δCS,27Al = 151 ppm, CQ = 2.8 MHz, and ηQ = 0.5.26 The 1H→13C CP-MAS spectrum can be modeled by three resonances: (i) a sharp resonance at 4.25 ppm, corresponding to the three C2 atoms bonded to the Si atom, 18094



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coupling is effective between noncovalently bonded 27Al and 13 C atoms. The 13C-{27Al} J- and D-HMQC 2D spectra are shown in Figure 2d,e corresponding respectively to the sequences illustrated in Figure 1a,c. However, these spectra exhibit lower resolution along the 13C projection since the J-couplings with 27Al CT transition are not refocused. Furthermore, the 27 Al-{13C} HMQC experiments exhibit higher sensitivity than their 13C-{27Al} counterparts (see Supporting Information, Table S1), owing to (i) shorter recycle delay and (ii) weaker 1 H−27Al dipolar couplings than the 1H−13C ones. However, 13 C-{27Al} HMQC experiments can be advantageous for compounds exhibiting narrow 13C resonances spread out over broad spectral width. The sensitivity decrease is especially important for the C3 site, since (i) it experiences two strong 1 H−13C dipolar couplings and is hence related to fast signal decay, especially during SFAM1 recoupling, and (ii) it is submitted to non-negligible quadrupolar-dipolar crossterms.39,50 These losses prevent the observation of Al−C3 correlation. These NMR experiments performed on 1 with NA13C allow validation of the J- and D-HMQC methods for close Larmor frequencies nuclei with the use of a diplexer and two rfsynthesizers. As this kind of heteronuclear couplings has never been explored in the literature, we set forth to implement such a methodology on a further example that proved to be deceivingly simple. Structural Studies of [25%-13C]-2. Previous examples of alkaline tetramethylaluminate derivatives have shown that the M[Al(CH3)4] species (M = Na,11 K,9 Rb,9,10 Cs9) adopt a slightly distorted tetrahedral structure in the solid-state, with unremarkable features. It was expected that the lithium derivative [25%-13C]-2 should afford spectral resolution resulting from such a structure (i.e., low CQ value for the aluminum center and a single type of 13C signal resulting from the presence of very similar AlCH3 moieties). Figure 3 and Figure S2 in Supporting Information, display the 1H → 13C CP-MAS and 27Al DP-MAS spectra of the [25%-13C]-2. To the best of our knowledge, no solid-state NMR study of this compound has been reported so far. As shown in Figure S2b, the 27Al spectrum can be simulated with the following parameters: δCS,27Al = 152 ppm, which agrees with shifts reported for methyl−aluminum compounds in solution,51 CQ = 2.6 MHz, and ηQ = 0.65. The significant CQ value reveals the slightly distorted tetrahedral geometry of the Al site in [25%-13C]-2. The 1H → 13C CP-MAS spectrum, shown in Figure 3 and Figure S2a, exhibits a broad signal spanning from −2 to −10 ppm, a chemical shift range typical of direct aluminum−carbon bonds. This range of 13C chemical shifts is consistent with those measured for 1 in solution.17,18 The complexity of the 13C signal results from (i) the existence of several crystallographically inequivalent 13C sites, (ii) the one-bond 27Al−13C scalar-coupling,16,17 and (iii) the second-order cross-terms between quadrupolar and dipolar interactions.39,50 The 1JAl−C coupling constants have not been determined for solid-state [25%-13C]-2, but they were found equal to 71 Hz for 1 dissolved in 1,2-dimethoxyethane at room temperature.17 In [25%-13C]-2, 27Al−13C distances are of about 200 pm, which corresponds to a dipolar coupling constant of bAl−C/(2π) = −985 Hz, and the second-order shift of the outermost lines of a 13 C sextuplet is thus ca. Δ = −3CQbAl−C/(20πν027Al) = 7 Hz,50

Figure 3. Sample [25%-13C]-2. 27Al-{13C} J- (a) and D- (b) HMQC 2D spectra recorded at 9.4 T and νR = 10 kHz, with τ = 3.5 and 1 ms and total experiment time of 11 and 7 h, respectively. The 27Al DPMAS, 1H→13C CP-MAS and 13C 2D projections are shown in panels a and b.

since ν027Al = 104.2 MHz at 9.4 T. Hence, second-order shifts are much smaller than the 1JAl−C coupling and they do not produce any crossing of the sextuplet lines. Under these conditions, the width of each 13C sextuplet should be about 5 × 71 = 355 Hz. Furthermore, J-HMQC experiments rely mainly on the coherence transfers via the 1JAl−C couplings, since they dominate the second-order dipolar-quadrupolar cross-terms. Hence, these experiments allow the observation of 27Al−13C covalent bonds, which are a distinctive feature of organoaluminum species. Figure 3a displays the 27Al-{13C} J-HMQC 2D spectrum of [25%-13C]-2, which exhibits intense cross-peaks. The discontinuities of 27Al CT are not resolved, because of insufficient 1H decoupling. Conversely, the projection along the indirect 13C dimension exhibits higher spectral resolution than the 1H→13C CP-MAS 1D spectrum since (i) the 27Al-{13C} HMQC experiment uses 27Al pulses selective for the CT, and hence eliminates the J-coupling with the satellite transitions, and (ii) the CT-selective π-pulse on the 27Al channel suppresses the Jcoupling with the CT. The 13C projection shows two peaks and a shoulder, at ca. −7.3, −6.0, and −5.3 ppm. The peak at −6.0 18095



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ppm is twice more intense than the other 13C resonances (see Figure S3). In the absence of transverse irreversible magnetization decay, T2′, during a spin echo the optimal τ/2 delay in J-HMQC experiment for such isotope distribution is about 0.42/1JAl−C = 6 ms. Experimentally, the optimal delay is τ/2 ≈ 3.5 ms since the coherence transfer via 1JAl−C coupling is damped by T2′ relaxation with a decay rate of 5 ms. Figure 3b shows the 27Al-{13C} D-HMQC 2D spectrum of [25%-13C]-2. Its 13C projection exhibits three resolved signals since the 27Al−13C J-couplings are fully suppressed by the sequence. As shown in Figure S4, the 13C signals detected in 27 Al-{13C} J- and D-HMQC experiments exhibit identical isotropic chemical shifts. Hence, D-HMQC allows observations of the proximities between 27Al nuclei and the 13C sites in their vicinity. Furthermore, compared to J-HMQC, the higher rf field strength for 1H decoupling during t1 and t2 periods for DHMQC improves the resolution of 13C and 27Al projections. 13 C-{27Al} J- and D-HMQC 2D spectra of [25%-13C]-2 are shown in Figure S5. These spectra exhibit lower spectral resolution than the 27Al-{13C} variants, since the J-couplings between 13C nuclei and the 27Al CT are not refocused during the acquisition. The elimination of these couplings would require the application of RA-MP 27Al decoupling during the acquisition.38 Furthermore, the sensitivity of 13C-{27Al} variants is lower than the 27Al-{13C} ones, as explained above for 1. With the presence of three types of clearly distinct methyl groups found with the NMR data, the structure of [25%-13C]-2 is surprisingly different from the known structure of the higher alkaline derivatives (Na,11 K,9 Rb,9,10 Cs9). This called for deeper investigation of the solid-state structure of lithium derivative [25%-13C]-2, which was thus studied by X-ray diffraction on a single crystal grown from diethylether. The crystal data and details of the data collection are summarized in Table 1. 2 adopts an original monoclinic P21/n structure composed of layers (see Figure 4a) with an interlayer distance of 4.08 Å. The asymmetric unit contains a single aluminum site surrounded by four different crystallographic carbon sites as well as a lithium crystallographic site (see Figure 4b). Tables 2 and 3 feature selected distances and angles, respectively. The Al(CH3)4 moiety exhibits marked distortion from tetrahedral symmetry, as already observed for other alkaline tetramethylaluminate solids,9−11 which is consistent with the 27Al NMR data (Figure S2b). The Al−C and C−Li distances are not markedly different (Table 2). The C−Li distances are particularly short for ionic bonds, which could suggest that the C−Li bond has substantial covalent character. The covalency of the C−Li bond has been suggested in the literature for organolithium compounds but to the best of our knowledge, never for lithium organoaluminum.52−54 Interestingly, the tetraethylaluminate derivative LiAl(CH2CH3)4 has been characterized by XRD: its solid state structure is markedly different from that of [25%-13C]-2, as it consists of linear chains with alternating lithium and aluminum atoms connected by bridging ethyl groups, and it features a single type of Al−C fragment.55,56 Along the same line, the heavier analogue, LiIn(CH3)4 derivative has been structurally characterized by XRD on single crystals: in contrast to [25%-13C]-2, it features interconnected In(CH3)4 and Li(CH3)4 tetrahedra.57 It should be noticed that the low density of Dx = 0.890 Mg m−3 of [25%-13C]-2 is in agreement with the one of lithium aluminum tetraethyl LiAl(CH2CH3)4.55,56

Table 1. Crystal Data and Details of the Data Collection Crystal Data C4H12AlLi Mr = 94.06 monoclinic, P21/n a = 6.2978 (2) Å b = 11.6066 (3) Å c = 9.6537 (3) Å β = 95.748 (1)° V = 702.10 (4) Å3 Bruker Apex2 diffractometer 2640 independent reflections Rint = 0.0385 h = −9→9 k = 0→17 l = 0→14 Refinement on F2 R[F2 > 2σ(F2)] = 0.036 wR(F2) = 0.116 S = 1.12 2640 reflections 104 parameters 0 restraints

Z=4 F(000) = 208 Dx = 0.890 Mg m−3 Mo Kα radiation, λ = 0.71073 Å θ = 4−33° μ = 0.16 mm−1 T = 100 K 0.25 × 0.21 × 0.12 mm Data Collection ω−scans and ϕ−scans 2361 reflections with I > 2σ(I) Abs.corr.:multiscan Tmin = 0.64, Tmax = 0.75 θmax = 34.4°, θmin = 2.8° Refinement w = 1/[σ2(Fo2) + (0.0801P)2 + 0.0219P] where P = (Fo2 + 2Fc2)/3 (Δ/σ)max = 0.003 Δρmax = 0.37 e Å−3 Δρmin = −0.28 e Å−3

Figure 4. ORTEP representations of the solid state structure of [25%-13C]-2: (a) view of the global arrangement in a layer; (b) asymmetric unit and selected neighboring atoms.

The presence of only one aluminum site is fully consistent with the observation of a single 27Al NMR resonance (Figure S2b). Regarding the number of carbon sites, the XRD structural analysis shows four different sites. However, two of them, C1 and C4, have very similar environments as they are almost located symmetrically on both sides of the plane defined by the 18096



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Table 2. Selected Al−C and C−Li distances in [25%-13C]-2 C1 C2 C3 C4

dC−Al (Å)

dC−Li (Å)

2.024 2.002 2.002 2.028

2.293 2.318 2.352 2.265

(1) (1) (1) (1)

(2) (2) (2) (2)

Table 3. Selected C−Al−C, C−Li−C and Al−C−Li Angles in [25%-13C]-2 angles (deg) C1−Al−C2 C2−Al−C3 C3−Al−C4 C4−Al−C1 C3−Li−C4 C2−Li−C1 Al−C1−Li Al−C2−Li Al−C3−Li Al−C4−Li

108.88 112.46 108.67 111.57 112.12 108.45 76.48 171.80 175.65 77.08

(4) (4) (5) (4) (9) (8) (6) (8) (7) (6)

Al and Li centers. This translates into very close chemical shifts. On the other hand, C2 and C3 are not equivalent, as their respective neighboring [Li−C4−Al−C1] moieties from the next unit are arranged differently (Figure 4b). More precisely, one can also decompose the layer into intertwined ribbons composed of enchainment of [Li−C4−Al−C1] units with either C2 (vertically arranged in Figure 4a) or C3 (horizontally arranged in Figure 4a). In the first case, the [Li−C4−Al−C1] units are parallel, whereas they point toward alternate directions in the second case. In both cases, it is expected that C2 and C3 feature significantly different chemical shifts that can be distinguished using NMR spectroscopy. From the relative intensities of the 13C signals, one can assign the peak at −6.1 to C4 and C1, while the two others account for C2 and C3. More precise assignment cannot be provided at this stage, and would require Density Functional Theory (DFT) calculations of the C2 and C3 chemical shifts. Thus, the information extracted from the 27Al−13C correlation studies was correct in pointing the occurrence of three types of methyl species: such a level of sensitivity clearly illustrates the power of our approach to the understanding of alkylaluminum species in the solid-state. A further example of the usefulness of joint correlation observation was provided in additional 13C−7Li HMQC experiments. 13 C−7Li proximities were probed using 13C-{7Li} D-HMQC 2D experiments with and without RA-MP 27Al decoupling during the acquisition. The corresponding spectra are shown in Figure 5. The 7Li projections of these spectra exhibit a single symmetrical signal at 0 ppm, which is typical of 7Li site in diamagnetic material. The three 13C chemical shifts measured in 27Al-{13C} D-HMQC spectra are hardly resolved in the 13C projection of the spectra without 27Al decoupling (see Figure 5a) since the 13C signals are broaden by the 27Al−13C Jcouplings. As shown in Figure 5b, the spectral resolution in the 13 C dimension can be improved by applying RA-MP 27Al decoupling during the acquisition. The 13C projection under 27 Al decoupling can be modeled by three signals with isotropic chemical shifts identical to those measured in 27Al-{13C} DHMQC experiments. Because of the weak rf field of 27Al pulses, the Bloch-Siegert58 shift is limited (below 2 Hz) and was not

Figure 5. 13C-{7Li} D-HMQC 2D spectra of [25%-13C]-2 (a) without and (b) with 27Al RA-MP decoupling during acquisition at 9.4 T, with νR = 10 kHz and τ = 0.5 ms. The 13C and 7Li projections of the 2D spectra are shown. The total experiment times are 12 h for spectrum a and 20 h for spectrum b.

detected experimentally. As the relative integrals of the three C signals are similar to those determined for 27Al-{13C} DHMQC experiments, we can conclude that all 13C sites are at comparable distances from 7Li nuclei. This NMR information is consistent with the structure proposed by XRD (Table 2). As shown in Figure S6, the resolution of 13C projection in 13C{7Li} D-HMQC experiment with 27Al RA-MP decoupling is lower than that of 27Al-{13C} D-HMQC since the rf field strength of 27Al RA-MP decoupling is only 37 kHz, which is not sufficient to fully suppress 27Al−13C J-coupling with the satellite transitions. In the field of NMR methodology, Figure 5b is the first example of 13C spectrum acquired with 27Al decoupling. Furthermore, the 13C-{7Li} D-HMQC experiment with 27Al decoupling demonstrates that diplexers allow carrying out quadruple resonance NMR experiments using a triple resonance probe. 13

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CONCLUSIONS This article provides new insights into the structure of two lithium organoaluminum compounds, which are used as cocatalysts in the polymerization of olefins. In particular, we have reported the crystal structure of Li[Al(CH3)4], which significantly differs from that of other alkaline tetramethylaluminates. The numbers of crystallographically inequivalent sites for Li, C, and Al elements have been confirmed by solid-state NMR spectroscopy. The 27Al−13C covalent bonds in solids have also been detected for the first time by NMR spectroscopy through the introduction of 27Al−13C J-HMQC 2D experi18097



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ments. Novel 27Al−13C D-HMQC 2D experiments have also been introduced to identify 27Al−13C spatial proximities and to improve the spectral resolution of 13C spectrum. These 27 Al−13C HMQC experiments are also employed for the structural characterization of Li[Al(CH3)3CH2Si(CH3)3], the crystal structure of which has not been reported so far. Furthermore, we have shown that 27Al−13C J- and D-HMQC experiments with 27Al detection benefit from higher sensitivity than their variants with 13 C detection. Through the introduction of 13C−7Li D-HMQC experiments with 27Al decoupling, we also demonstrate (i) that the resolution of 13C spectra of organoaluminum can be improved by 27Al decoupling and (ii) that four-resonance NMR experiments can be acquired using three-channel probes and diplexers. These novel 27Al−13C solid-state NMR methods are applicable for noncrystalline compounds and are thus expected to have important implications for a wide range of materials, including supported organoaluminum species, metal−organic frameworks, nanocomposites, and soils.

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ASSOCIATED CONTENT

S Supporting Information *

1 with NA13c: 27Al and 13C 1D spectra. [25%-13C]-2: 27Al and 13 C 1D spectra. 13C-{27Al} J- and D-HMQC 2D spectra; comparison of the 13C spectra and their deconvolution; comparison of the sensitivity of the HMQC. CCDC reference number 939819. 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]; jean-paul. [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Region Nord/Pas de Calais, Europe (FEDER), CNRS, French Minister of Science, USTL, ENSCL, CortecNet, Bruker BIOSPIN and Contract No. ANR-2010-jcjc-0811-01.

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