Article pubs.acs.org/Organometallics
Deprotonation of Al2Me6 by Sterically Bulky NHCs: Scope, Rationale through DFT Studies, and Application in the Methylenation of Carbonyl Substrates Gilles Schnee,† David Specklin,† Jean-Pierre Djukic,⧧ and Samuel Dagorne*,† †
SRCO Group and ⧧Laboratoire de Chimie et Systémique Organo-métalliques, Institut de Chimie de Strasbourg, CNRS-Université de Strasbourg, UMR 7177, 1 Rue Blaise Pascal, F-67000 Strasbourg, France S Supporting Information *
ABSTRACT: The sterically bulky NHCs 1,3-di-tert-butylimidazol-2-ylidene (ItBu), 1,3-di-tert-butylimidazolin-2-ylidene (StBu), and 1,3-di-tert-butyl-3,4,5,6-tetrahydripyrimidin-2-ylidene (C6-tBu) were found to readily react with excess Al2Me6 at room temperature to form salts 2, 3, and 4, respectively, consisting of the polynuclear Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− anion associated with either the cation ItBu-H+, StBu-H+, or C6-tBu-H+. Such a reaction involving the deprotonation of an Al2Me6 moiety by a NHC does not proceed with less sterically hindered NHCs such as 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), for which only the classical Lewis pair adducts (NHC)AlMe3 were isolated. In line with experimental and density functional theory (DFT) calculations data, such reactivity thus appears to be driven by steric frustration, resulting in the destabilization of the corresponding (NHC)AlMe3 adducts, then more prone to dissociate and hence allowing Al2Me6 activation/deprotonation. The DFT-estimated profile of the reaction of model adduct (ItBu)AlMe3 (I) with Al2Me6 agrees with a ready adduct dissociation at low energy cost, with a subsequent deprotonation of Al2Me6 by the NHC fragment to afford model salt II (isostructural to salt 2). Anion Me3Al(μ3CH2)(AlMe2)2(μ2-CH3)− behaves as an efficient CH22− group transfer agent with the methylenation of aldehydes and ketones to afford the corresponding methylene organics in good conversions. Bonding analysis of the latter anion agrees with an enhanced nucleophilicity of the Al-CH2 moiety (compared to the Al-Me groups), in line with the observed reactivity. DFT calculations also allowed a detailed bonding description of the pentacoordinate methylene carbon in the anion Me3Al(μ3-CH2)(AlMe2)2(μ2CH3)−.
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INTRODUCTION Due to their exceptional σ-donating properties and steric tunability, N-heterocyclic carbenes (NHCs) are a privileged class of ligands for the stabilization of a wide range of metal/ heteroatom centers.1 Consequently, NHC-bearing metal complexes have found numerous applications in various areas ranging from fundamental reactivity to their widespread use in homogeneous catalysis.1 The use of NHCs across transition metal and main group chemistry for the stabilization of various unusual structural motifs and study of the derived reactivity is also currently attracting attention.1,2 Although the steric properties of NHCs (through the nature of the N-substituents) are typically exploited to protect the coordinated metal center (kinetic stabilization), the coordination of sterically hindered NHC ligands to metal centers/heteroatoms may lead to the © XXXX American Chemical Society
formation of unstable and reactive NHC-M adducts due to severe steric congestion or even prevent adduct formation in the case of frustrated Lewis pairs (FLPs). Such compounds are of particular interest for small-molecule activation.3 In this regard and most notably, Stephan and Tamm showed that no adduct forms upon combination of the bulky NHC ItBu with B(C6F5)3 and that the ItBu/B(C6F5)3 FLP readily cleaves H2 at room temperature to afford the corresponding imidazolium salt [ItBu-H][HB(C6F5)3].4 Also, several sterically bulky imidazol2-ylidene discrete metal adducts [based on Al(III), Ga(III), Mn(II), and Fe(II)] were recently reported to be quite reactive and to isomerize to the corresponding abnormal/mesoionic Received: February 26, 2016
A
DOI: 10.1021/acs.organomet.6b00159 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Such a hydrodynamic volume value is similar to that estimated for salt 2 from its solid-state structure (VX‑ray = 433 Å3, Figure S11),9 which is in line with salt 2 consisting of discrete ItBuH+/Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− ion pairs (and not as fully dissociated cations/anions) in CD2Cl2 at room temperature. For comparison, the DOSY NMR data for carbene ItBu (nearly identical in size to the ItBu-H+ cation) agree with a much smaller molecule volume (hydrodynamic radius = 3.2(7) Å and VDOSY = 146(3) Å3, Figure S10). To better understand the formation of salt 2 from adduct 1 and Al2Me6, the stability of adduct 1 toward dissociation in the presence/absence of Al2Me6 was studied. Species 1 on its own shows no observable sign of dissociation (CD2Cl2 or C6D6, room temperature) according to 1H NMR data. In contrast, the 1 H NMR spectrum (CD2Cl2, room temperature) of a 1/4 mixture of (ItBu)AlMe3 and Al2Me6 contains two broad AlMe3 singlet signals (Figure S12), indicative of a dynamic process.10 For comparison, the 1H NMR resonances of the less sterically bulky adduct (IDipp)AlMe 3 11 (IDipp = (1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene) are not affected at all by the presence of 4 equiv of Al2Me6 (CD2Cl2, room temperature; C6D6, from 25 to 75 °C; Figures S13 and S14). The observed dynamic behavior for the (ItBu)AlMe3/Al2Me6 mixture is thus clearly related to the severe steric congestion in adduct 1 when not observed with the less sterically hindered adduct (IDipp)AlMe3. It seems reasonable to assume a decoordination/coordination of adduct (ItBu)AlMe3 in the presence of AlMe3 under the studied conditions, although a complete VT NMR analysis (along with the determination of the associated parameters) could not be performed.10 Note that the ready dissociation of adduct (ItBu)AlMe3 in the presence of Al2Me6 is suggested by DFT studies (vide inf ra). A 1H NMR monitoring of the reaction of adduct 1 in the presence of 4 equiv of Al2Me6 (CD2Cl2, room temperature) led to no observable intermediate prior to the generation of the final product 2. In the present studies, we also found that salt 2 may be produced in a straightforward manner by direct reaction of carbene ItBu in neat Al2Me6, the latter thus acting both as solvent and as reagent (quantitative reaction after 2 h at room temperature). Such an improved procedure (i.e., in neat AlMe3) was also tested with other carbene sources to probe/extend the scope of such a deprotonation reactivity. In this regard, unlike what is observed with carbene ItBu, the reaction of the less sterically demanding NHCs IDipp and IMes (1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene) in neat Al2Me6 (room temperature, 20 h) led only to the isolation of the corresponding Al-NHC adducts (IDipp)AlMe3 and (IMes)AlMe3, as deduced from NMR data and comparison with literature data.6a,11 Also, species (IDipp)AlMe3 and (IMes)AlMe3 are stable in the presence of an excess Al2Me6 (4 equiv, CD2Cl2 or C6D6) either at room temperature or upon heating (40 and 75 °C). These experiments further highlight and confirm the key role of steric bulk for Al2Me6 deprotonation by NHCs to occur. In contrast, the more sterically demanding carbenes such as StBu and C6-tBu are also readily protonated in neat Al2Me6 to afford the corresponding salt species [StBuH][Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)] (3) and [C6-tBu-H][Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)] (4), which were isolated in nearly quantitative yields as analytically pure colorless oils (Scheme 2). The 1H and 13C NMR data for 3 and 4 (CD2Cl2, room temperature) are identical to those for salt 2, except the resonances for cations StBu-H+ and C6-tBu-H+, respectively. In
NHC (aNHC, C4/C5-bonded NHC) metal/heteroatom species for steric relief.5 In the case of sterically bulky Al(III) derivatives, the formation of the aNHC metal derivatives was shown to be thermodynamically favored for steric reasons despite the electronic preference of Al(III) for NHC coordination at the C2 carbene carbon.5a In the area of Al(III)-NHC chemistry, 6 we earlier communicated that the bulky adduct (ItBu)AlMe3 (1, Scheme 1) is unstable in the presence of excess Al2Me6 to yield the Scheme 1
quantitative formation of the imidazolium salt [ItBu-H]][Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)] (2, Scheme 1), a reaction that may be formally described as a deprotonation of Al2Me6 (i.e., dimeric AlMe3) by ItBu, although the mechanism of such a process remained unclear at the time.7 Besides its peculiarity, the interest in this deprotonation reaction also arises from the formation of an unusual polynuclear AlMe3-stabilized Al-CH2− anion, the Brønstedt conjugate base of dimer Al2Me6 stabilized by an AlMe3 group. In addition to its structural interest (including its bonding features), such a readily accessible anion may act as a methylenating agent of carbonyl substrates by analogy with the reactivity of the Tebbe reagent and related metal methylene/methylidene species.8 In the present paper, we provide a full account of our studies in the area including (i) the scope of such a deprotonation reactivity, (ii) DFT studies to establish the mechanism converting 1 (in the presence of excess Al2Me6) to salt 2, (iii) the electronic and bonding characteristics of adduct 1 and salt 2, and (iv) the use of such Al-CH2−-based anions for the methylenation of carbonyl substrates.
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RESULTS AND DISCUSSION Access to the Aluminate Salts 2−4 by Deprotonation of Al2Me6: Scope and Stability. Preliminary studies showed that the Al-NHC adduct (ItBu)AlMe3 (1, Scheme 1) slowly reacts with excess Al2Me6 (2.5 equiv of Al2Me6, CH2Cl2, room temperature, 20 h) to quantitatively afford the corresponding salt [ItBu-H][Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)] (2), which was isolated in a pure form as a colorless oil. The formation of 2 formally arises from the deprotonation of dimer Al2Me6 by ItBu (in adduct 1) to yield trinuclear aluminate anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− along with the corresponding imidazolium cation ItBu-H+, as rationalized by DFT calculations (vide inf ra). According to X-ray crystallographic analysis, salt 2 crystallizes as discrete ItBu-H+ and Me3Al(μ3CH2)(AlMe2)2(μ2-CH3)− ions with no apparent anion−cation interactions. In solution, a DOSY (diffusion ordered spectroscopy) measurement for 2 (CD2Cl2, room temperature, Figure S9), allowing an estimation of its hydrodynamic radius and volume, suggests associated ion pairs in solution under the studied conditions. Thus, a hydrodynamic radius and volume of 4.6(5) Å and 420(5) Å3, respectively, were measured for salt 2. B
DOI: 10.1021/acs.organomet.6b00159 Organometallics XXXX, XXX, XXX−XXX
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presence of excess InMe3 (20 equiv of InMe3, CH2Cl2, room temperature, overnight).13 DFT Studies of Model Species (ItBu)AlMe3 (I). The peculiar reactivity of adduct (ItBu)AlMe3 (1) [relative to adducts such as (IDipp)AlMe3 and (IMes)AlMe3] in the presence of Al2Me6 prompted us to investigate its bonding and thermodynamic properties through DFT calculations to gain insight into the observed reactivity. The singlet ground-state geometry of model species I was faithfully reproduced at the meta-GGA and GGA levels of theory, i.e., respectively with ZORA-TPSS-D3(BJ)/all-electron TZP and ZORA-PBED3(BJ)/all-electron TZP methods, with a rather better fit with experimental values (most importantly with the Al− Ccarbene distance) being obtained with the meta-GGA method (Table S1). Natural bond order (NBO)14 analysis of the Al− Ccarbene bond indicated a Wiberg bond index of 0.51, indicative of a dative bond, and a strongly polarized bond with a charge at Ccarbene and Al(III) of +0.07 and +1.37, respectively. The natural bonding orbital for the Al−Ccarbene bond, i.e., ψC1−Al = 0.915(sp1.4)C1 + 0.403(sp4.8)Al (population 1.93 electrons), indicates a major contribution of the sp1.4 hybrid at Ccarbene, as illustrated by the isosurface plot in Figure 1. These data agree
Scheme 2
particular, the 13C NMR spectra for 3 and 4 (CD2Cl2, room temperature) both contain two upfield signals at δ −5.8 and −4.7 ppm assigned to the Al-Me and Al-CH2 groups, respectively. On the basis of NMR data, ionic liquids 3 and 4 are most probably isostructural to salt 2 in solution and in the solid state. Although stable for days in CD2Cl2, salts 2−4 decompose within a few hours at room temperature in C6D6 to unknown species, with a reaction apparently involving both the anion and cation according to 1H NMR data. Such a difference in stability possibly reflects a somewhat better cation−anion separation in CD2Cl2 vs C6D6, hence an improved stability of these salts in CD2Cl2. Interestingly, the observed deprotonation reaction of Al2Me6 by sterically bulky carbenes may be reversible, as demonstrated by the reactivity of salt 3. Thus, the reaction of [StBuH][Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)] (3) with an excess of THF (10 equiv, C6D6, 15 min, room temperature) led to the immediate and quantitative formation of free carbene StBu along with 3 equiv of adduct THF-AlMe3 (Scheme 3; Figure Scheme 3
Figure 1. Natural bonding orbital ψAl−C(NHC) and associated parameters for the Al−Ccarbene bond in I.
with a covalent Al−Ccarbene bond with a strong ionic character, a bond only slightly fragilized by the much longer Al−Ccarbene bond (2.161 Å) compared to that in less sterically cluttered NHC-AlMe3 Lewis adducts (2.11 Å on average based on literature).6a The nature of the Al−Ccarbene bond in model I was also investigated by the quantum theory of atoms in molecule (QTAIM), that is, the analysis of the electron density topology. The bond critical point (3,−1) located in the Al−Ccarbene segment bears a residual electron density ρ of 0.052 au that is slightly lower than that expected for a typical shared interaction (covalent bond). The sign of the Laplacian of the density −1/4∇ρ = −0.04 suggests some degree of ionicity in the bonding interaction. In other terms the ρ and −1/4∇ρ values point at an intermediate situation between a closed-shell interaction and a shared interaction in that the density at the bond critical point remains relatively high with a negative value of the −1/4∇ρ Laplacian. This further supports the stance on the role of electrostatic interactions in the overall cohesion of I. A NOCV-EDA (natural orbitals for chemical valence−energy decomposition analysis) analysis was performed for model species I to get a more precise picture of the type of interactions (and their extent) between the Me3Al and ItBu fragments.15,17 As depicted in the Supporting Information (Figure S20), such an analysis reveals, as expected, the major contribution due to the Ccarbene→Al σ-bond formation, representing about 65% of the orbital interaction energy. A Rauk−Ziegler energy decomposition analysis16 was carried out
S14). In the presence of excess THF, the deprotonation of the imidazolinium StBu-H+ in salt 3 presumably results from the cleavage of the aggregate anion [Me3Al(μ3-CH2)(AlMe2)2(μ2CH3)] by THF coordination to Al(III) center(s), which in turn enhances the basicity of the Al-CH2− moiety so that it may then deprotonate the StBu-H+ cation to yield carbene StBu and adduct THF-AlMe3. Despite the much stronger σ-donating character of StBu vs THF, the formation of free StBu + THFAlMe3 is preferred over that of adduct (StBu)AlMe3 + free THF, reflecting severe steric congestion in adduct (StBu)AlMe3.12 A similar preference for THF coordination was earlier observed with carbene ItBu.5a Higher Al alkyls such as Al2Et6 (dimeric AlEt3) and Al(iBu)3 as well as lower group 13 metal analogues GaMe3 and InMe3 were also studied. The reaction of StBu or ItBu in neat Al2Et6 or Al(iBu) 3 (room temperature, 2 to 18 h) produced a complicated mixture of compounds that could not be identified, thus in sharp contrast with the clean formation of salts 2 and 3 observed with AlMe3. On the other hand, the reaction of ItBu in neat GaMe3 (room temperature, overnight) led only to the isolation of the corresponding adduct (ItBu)GaMe3. Similarly, adduct (ItBu)InMe3 was isolated upon reaction of ItBu in the C
DOI: 10.1021/acs.organomet.6b00159 Organometallics XXXX, XXX, XXX−XXX
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Organometallics to extract the orbital interaction energy term, ΔEorbital, from the total interaction energy, ΔEint (−53.1 kcal/mol), defined as
0.06, qAl = 1.39). The only major and significant difference between I and I′ is the Gibbs enthalpy of formation (computed at the gas-phase PBE-D3(BJ) level), which is nearly two times lower for I with respect to I′ (ΔGf(298.15 K)= −17 and −28 kcal/mol for I and I′, respectively). The higher thermodynamic stability of I′ vs I purportedly arises from the lower steric hindrance in I′ compared to I. DFT-Estimated Reaction Profile for the Formation of Salt II/II′ from the Corresponding Model Adduct I/I′ and Al2Me6. The formation of model salt II/II′ was considered to proceed through the direct reaction of dimeric AlMe3, i.e., Al2Me6, with the corresponding Al-NHC adduct I/I′ [I′, model of adduct (IMe)AlMe3]. The two energy profiles are depicted in Figure 3, while the corresponding model structures are
ΔE int = ΔE Pauli + ΔEelectrostat + ΔEorbital + ΔEdispersion
the terms standing respectively for Pauli repulsion (+99.5 kcal/ mol), electrostatic (−91.2 kcal/mol), orbital (−50.4 kcal/mol), and dispersion (−11.6 kcal/mol) interaction energy components. It must be pointed out here that the EDA suggests that the orbital interaction alone is not exclusively responsible for the stabilization of the bond. Very much in line with Frenking’s previous statements,17 the Al−Ccarbene bond in I can be described as an electrostatically supported donor−acceptor bond. Indeed, among all attractive energy components obtained by EDA, the electrostatic is the strongest: ΔEelectrostat > ΔEorbital > ΔEdispersion. In the case of model I′ wherein the tBu fragments have been replaced by Me, EDA leads to a similar picture of the attractive terms ΔEorbital (−44.4 kcal/mol), ΔEdisp (−6.8 kcal/ mol), and ΔEelectrostat (−90.1 kcal/mol), thus dominated by electrostatics. The value of ΔEint (−55.3 kcal/mol) in I′ differs only by 2 kcal/mol with that for I. Yang’s noncovalent interaction visualization method18 based on the discrimination of the reduced density gradient offers a good intuitive way to visualize the importance of noncovalent interactions (NCIs) because they can be readily sorted into two classes, attractive NCIs and either repulsive or van der Waals NCIs. The reduced gradient method applied to the electron density associated with I produced the isosurface plot depicted in Figure 2. Regarding the Al−Ccarbene bond, the large red
Figure 3. Gibbs energy profiles (in kcal/mol, T = 298.15 K) for the formation of salt II (a) and fictitious salt II′ (b).
shown in Figure 4 and Figures S21−23. The reaction was DFTinvestigated by standard methods (PBE-D3(BJ)) for model I and the less sterically bulky I′. The systems were considered in the gas phase in order to evaluate the neat thermodynamics that rule the process. Although the deprotonation process I′ in the presence of Al2Me6 is fictitious, it served as a reference case to highlight the peculiar reactivity of I. The reaction of I with Al2Me6 first proceeds with the formation of reactant complex RC-I at little energetic cost (+1.7 kcal/mol), in which adduct I is already dissociated into AlMe3 and ItBu fragments that are stabilized by the incoming Al2Me6 through noncovalent interactions. Such a ready formation of RC-I, and thus dissociation of adduct I in the presence of Al2Me6, is consistent with the experimental data. From RC-I, a C−H bond cleavage of the Al(μ-CH3)Al methyl group (that interacts with the AlMe3 and ItBu fragments from the dissociation of 1) occurs via a low barrier transition state TS-I (7 kcal/mol) to afford salt II, consisting of Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− and ItBu-H+ anions, with an overall exoergonic process (ΔG° = −6.7 kcal/ mol at T = 298.15 K). In TS-I, the activated H atom, located midway between the two carbon centers (at ca. 1.45 Å), is clearly the one abstracted by the basic carbenic carbon atom via a typical proton exchange reaction. The overall reaction profile features an activation free Gibbs energy of roughly 9 kcal/mol, which is certainly consistent with a reaction occurring at room temperature, especially in neat AlMe3. Alternatively, a
Figure 2. ADFview2013 plots of noncovalent interaction (NCI) regions18 materialized by reduced density gradient isosurfaces (cutoff value s = 0.02 au, ρ = 0.05 au) colored according to the sign of the signed density λ2ρ (red and blue colors are associated with negatively and positively signed terms) for gas-phase relaxed singlet ground state models of I (a) and I′ (b). The “through” within the red-colored isosurface is associated with the covalent component of the C(NHC)−Al bond.
isosurface of attractive NCIs located in between the two atoms contains a “covalent through”, which provides an intuitive illustration of how noncovalent interactions support the dative bond in I. For comparison, the model compound of the much less sterically hindered (IMe)AlMe3 (I′, IMe = 1,3-dimethylimidazol-2-ylidene) was also analyzed within the NBO Lewisstructural framework. It features an Al−Ccarbene bond (2.105 Å) in line with other reported NHC-AlMe3 Lewis adducts and thus 0.06 Å shorter than that for adduct I. However, the bonding features of the Al−Ccarbene in I′ are quite similar to those of I, as reflected by an identical or nearly identical Wiberg bond index (0.51) and natural population analysis (NPA) charges (qC = D
DOI: 10.1021/acs.organomet.6b00159 Organometallics XXXX, XXX, XXX−XXX
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Figure 4. Optimized geometries for the main structures involved in the reaction profile (depicted in Figure 3a) resulting in the formation of salt II from adduct I and Al2Me6. Distances are in Å.
species have been structurally characterized and typically incorporate at least an M-(μ-CH2)-Al motif, where M is, for the most part, a high oxidation state early transition/rare-earth metal center.21 In particular, akin to Schrock-type carbene complexes, this class of heterometallic species is well known to readily methylenate carbonyls, including esters in some instances. To our knowledge, the presently studied AlMe3stabilized Al-CH2− anion (in salts 2−4) is the first wellcharacterized homometallic Al(III) methylene anion, and its reactivity toward carbonyl species was thus studied. Salts 2 and 3 were found to readily react with aldehyde and ketone substrates to produce the corresponding methylene products with good to quantitative conversions, thus showing the Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− anion to be an effective CH22− transfer agent under the studied conditions. In all cases, salts 2 and 3 behaved identically and 1H NMR data agree with the spectator role of the ItBu-H+/StBu-H+ cation, as expected for these methylation reactions. For some substrates, the addition of an external Lewis base such as tetrahydrofuran (THF) was required to achieve effective methylenation. The results are compiled in Table 1. Thus, the reaction of benzaldehyde/benzophenone with 1 equiv of 2 or 3 (CD2Cl2, room temperature, 15 min) led to the immediate and quantitative formation of styrene and 1,1′-diphenylethylene, respectively, as deduced from 1H NMR data. Such a functionalization may also be performed with enolizable and/or sterically bulky ketones such as acetophenone and 1-phenyl-2methyl-1-propanone, although a lower conversion to the desired products was observed in these cases (66% and 70% conversion). Interestingly, despite the presence of eight Al-Me groups in anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)−, which are well-known moieties for the methylation of carbonyls, all methylenation reactions proceeded with little to no formation of methylation products. Unlike Tebbe’s reagent, anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− was found to be much less reactive toward ester methylenation, as reflected by the low conversion of benzyl benzoate to the corresponding methyl-
mechanism involving the complete dissociation of adduct 1 and a subsequent interaction of the incoming Al2Me6 with the formed ItBu and AlMe3 fragments seems less favored energetically given the Gibbs enthalpy for the dissociation of adduct 1 (ΔG(298.15 K) = 17 kcal/mol, computed at the gasphase PBE-D3(BJ) level). Although the latter would also be compatible with experimental data (which does not rule out the dissociation of adduct 1 on its own), an Al2Me6-promoted dissociation of adduct (ItBu)AlMe3 is energetically preferred on the basis of DFT calculations. In contrast, for the reaction involving the less sterically hindered model I′ no reactant complex akin to RC-I could be isolated on the potential map. Instead, calculations systematically led to the formation of Ad-I′ (Figure S21), which bears an intact Al−Ccarbene bond (2.101 Å). Even though transition state TS-I′ could be isolated on the energy surface map (Figure S22), its direct affiliation with Ad-I′ was not established. Anyhow, the high barrier of activation necessary to perform the transformation of Ad-I′ into anion II′ (ΔG0≠ = +22.8 kcal/ mol) combined with the endergonic character of the reaction (ΔG0 = +9 kcal/mol) strongly disfavors the formation of salt II′ (Figure S22) from adduct I′ and Al2Me6. Thus, in agreement with experimental observations, the present calculations rationalize that the deprotonation reactivity of model adduct I toward Al2Me6 can be attributed to the steric cluttering around the carbene carbon, which destabilizes the formed (NHC)AlMe3 Lewis pair adduct. Steric hindrance kinetically and thermodynamically favors a deprotonation reaction with the carbene moiety acting as a Brønstedt base. Reactivity of the Aluminum Methylene Complexes 2 and 3 with Carbonyl Substrates. Apart from their structural interest, metal methylene/methylidene (CH22−) complexes may be useful entities for the functionalization of unsaturated organics and have most notably been exploited in olefin metathesis and methylenation of carbonyl substrates. Since Tebbe’s initial studies on Ti(IV)/Al(III) methylidene systems,8,19,20 various Al-containing heterometallic methylidene E
DOI: 10.1021/acs.organomet.6b00159 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Methylenation of Various Carbonyl Substrates with the Al Methylene Species 2 and 3
substrate R R R R R
= = = = =
Ph, R′ = Hb R′ = Pha,c Me, R′ = Phb i Pr, R′ = Phb,c Ph, R′ = OPhb
time (h)
T (°C)
conversion (%)
0.25 0.25 1 1 12
RT RT −30 to RT −30 to RT RT
>95 >95 50 70 18
a Salt 2 was used for the methylenation reaction. bSalt 3 was used for the methylenation reaction. cReaction carried out in the absence of THF.
ation product (1-(benzyloxy)vinyl)benzene (CD2Cl2, room temperature, 12 h, 18% conversion). Bonding Characteristics of Anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)−. As experimentally shown above, anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− may preferentially methylenate (rather than methylate) carbonyls, indicating that the reaction of the Al-CH2-Al methylene moiety with such unsaturated organics is kinetically favored over that of the AlMe and Al-Me-Al groups. DFT analysis of the natural charges indicates that the Al-CH2 carbon atom (C9, Figure 5) bears an
Figure 6. ETS-NOCV22 deformation densities Δρ and their stabilization energy ΔEorb (in kcal/mol) for the interaction of prepared fragments of anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)−. Red- and blue-colored isosurfaces materialize regions where charge density depletion and buildup occur, respectively: electron density transfer operates from red- to blue-colored areas upon bonding. Deformation density isosurface contour was set to 0.005 e/bohr3.
Using the A3B− formula and decomposing it into an interaction between A3+ and B2− so-called prepared moieties, where A3+ stands for the Al2Me5+ and AlMe3 dyad, and B2− for CH22−, it can readily be concluded that the shortest Al2−C9 bond results from the buildup of a σ bond, whereas the longer Al1−C9 and Al3−C9 bonds arise from interactions of the two Al(III) centers Al1 and Al3 with electrons of the shared p orbital at C9: each such individual Al−p interaction represents about a fourth of the orbital interaction energy responsible for the Al2−C9 bond. NBO analysis confirms this bonding feature for Al1−C9 and Al2−C9 bonds (Figure S23). The electron population for Al3−C9 lies under the standard occupation population threshold of 1.5 electron to be accounted for in the inferred Lewis structure: ψAl1−C9 (1.82 e) = 0.36(sp3.5)Al + 0.93(sp25)C9, ψAl2−C9 = 0.40(sp2.7)Al2 + 0.92(sp1.2)C9. Further, NCI analysis of Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− was also performed (Figure S24), indicating that noncovalent interactions also play a role in the stability of such a polynuclear anionic aggregate.
Figure 5. Gas-phase singlet ground state geometry of anion Me3Al(μ3CH2)(AlMe2)2(μ2-CH3)− (left) and its associated electrostatic potential map drawn on a density isosurface (0.04 e/Å3) computed at the ZORA-PBE-D3(BJ)/all-electron TZP level. Distances are in Å.
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SUMMARY AND CONCLUSION Sterically bulky NHCs such as ItBu, StBu, and C6-tBu may behave as Brønstedt bases in the presence of excess Al2Me6 (dimer of AlMe3) to afford an unusual polynuclear AlMe3stabilized Al-CH2− anion through a formal deprotonation of an Al2Me6 unit. In contrast, no such reactivity is observed with the less sterically bulky IDipp and IMes carbenes, for which only classical Lewis base/acid chemistry was observed, with the isolation of robust (NHC)AlMe3 adducts found to be stable in AlMe3. According to experimental and computational data, the observed deprotonation or C−H activation reactivity at AlMe3 thus arises from severe steric congestion in the corresponding (NHC)AlMe3 Lewis adducts. Such destabilization favors dissociation with the decoordinated fragments that may subsequently deprotonate Al2Me6: in that sense, such reactivity is akin to small-molecule activation by FLP-type systems. This is best substantiated by the DFT-estimated reaction profile for the ItBu/AlMe3 system, with the reaction between (ItBu)AlMe3 and Al2Me6 proceeding via an initial (and low energy cost) dissociation of the sterically bulky (ItBu)AlMe3 adduct. The
NPA charge (−1.58) that is 0.4 unit more negative than those of the Al-Me carbons (average value −1.17), consistent with an enhanced nucleophilicity of the Al-CH2 group toward carbonyl functions. Although several discrete metal-supported species containing a pentacoordinate methylene carbon are known, this bonding scheme has rarely been thoroughly assessed through computations. The bonding of the methylene moiety in anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− was studied using a fragment analysis point of view. In this regard, such an anion may be seen as an sp2-hybridized CH22− dianion stabilized by the Lewis acidic moieties Al2Me5+ and AlMe3, formally resulting in a pentacoordinate methylene carbon (Figure 6, top). Interatomic Al−C distances range from 2.007 to 2.125 Å (in agreement with experimental data), with the Al2−C9 bond being shortest. The Wiberg indices, which follow the order Al2−C9 (0.51) > Al1−C9 (0.44) > Al3−C9 (0.37), indicate unequal organometallic bonding to C9. Again, the NOCV-EDA method4 provides a clear picture of the bonding situation in anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− (Figure 6, bottom). F
DOI: 10.1021/acs.organomet.6b00159 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
species 2 in a pure form (as a colorless liquid) on the basis of 1H and 13 C NMR data, X-ray crystallographic data, and elemental analysis. Due to the dynamic behavior of the aluminate anion Me3Al(μ3CH2)(AlMe2)2(μ2-CH3)− in solution at room temperature, 13C{1H} and DEPT NMR were recorded at room temperature and at −30 °C; these data are consistent with the aluminate anion in complex 2 essentially retaining its solid-state structure in solution. In neat AlMe3, in a glovebox, AlMe3 (1 mL) was added at room temperature via a pipet to the free carbene 1,3-di-tert-butylimidazol-2-ylidene (50.0 mg, 0.28 mmol). The resulting colorless solution was vigorously stirred at room temperature. After 24 h the excess AlMe3 was evaporated under vacuum and the residue was washed with pentane (2 × 5 mL) to yield a colorless oil. The latter was dried in vacuo to afford the ionic liquid compound 2 as a colorless oil (105 mg, 96% yield). Anal. Calcd for C20H47Al3N2 (396.56): C 60.57; H 11.95; N 7.06. Found: C, 60.82; H 11.63. 1H NMR (400 MHz, CD2Cl2): δ −0.84 (s, 2H, AlCH2), −0.72 (br s, 24H, Al-CH3), 1.70 (s, 18H, CH3-tBu), 7.49 (d, 4JHH = 1.8 Hz, 1H, NCHCHN), 8.20 (d, 4JHH = 1.8 Hz, 1H, NCHN). 1H NMR (400 MHz, CD2Cl2, 243 K): δ −1.04 (br s, 2H, AlCH2), −0.88 (br s, 24H, AlMe), 1.74 (s, 18H, tBu), 7.42 (s, 1H, CH), 8.15 (s, 1H, CH). 13 C{1H} NMR (75 MHz, CD2Cl2): δ −5.9 (AlCH2), −4.7 (AlCH3), 30.1 (CH 3 - t Bu), 59.3 (C(CH 3 ) 3 ), 61.5 (C(CH 3 ) 3 ), 121.1 (NCHCHN), 129.5 (NCHN). 13C{1H} NMR (100 MHz, CD2Cl2, 243 K): δ −6.4 (AlCH2), −5.3 (AlMe), −4.5 (AlMe), 30.2 (tBu), 61.4 (tBu), 121.2 (CH), 129.4 (CH). [StBu-H][Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)] (3). In a glovebox, AlMe3 (2 mL) was added at room temperature via a pipet to the free carbene 1,3-di-tert-butylimidazolin-2-ylidene (367 mg, 2.01 mmol). The resulting colorless, cloudy solution was vigorously stirred at room temperature. Over the course of 2 h, a massive precipitation of a colorless solid occurred. The resulting suspension was then filtered through a glass frit, and the residue was washed three times with pentane (3 × 10 mL) to yield a colorless oil. The latter was dried in vacuo to afford the ionic liquid compound 3 as an analytically pure solid (606 mg, 79% yield). Anal. Calcd for C20H47Al3N2 (398.56): C 60.27; H 12.39; N 7.03. Found: C 60.01; H 12.23; N 6.89. 1H NMR (300 MHz, CD2Cl2): δ −0.83 (br s, 26H, AlCH2 and AlMe3), 1.45 (s, 18H, CH3-tBu), 4.01 (s, 4H, NCH2CH2N), 7.53 (s, 1H, NCHN). 13 C{1H} NMR (100 MHz, CD2Cl2): δ −5.8 (AlCH2), −4.6 (AlMe3), 28.3 (CH3-tBu), 45.8 (NCH2CH2N), 57.9 (C(CH3)3), 151.0 (NCHN). [C6-tBu-H][Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)] (4). The salt species 4 was prepared following an identical procedure to that used for the synthesis of 3 but using 1,3-di-tert-butyltetrahydropyrimidin-2-ylidene (C6-tBu, 72.7 mg, 0.37 mmol) as a carbene source. The ionic liquid 4 was isolated as an analytically pure pale yellow oil (152 mg, 99% yield). Anal. Calcd for C21H51Al3N2 (412.59): C 61.13; H 12.46; N 6.79. Found: C 61.31; H 12.54. 1H NMR (300 MHz, CD2Cl2): δ −0.83 (br s, 17H, AlMe3), 1.46 (s, 18H, CH3-tBu), 2.10 (q, 3JHH = 5.7 Hz, 2H, NCH2CH2CH2N), 3.44 (t, 3JHH = 5.7 Hz, 4H, NCH2CH2CH2N), 7.83 (s, 1H, NCHN). 13C{1H} NMR (100 MHz, CD2Cl2): δ −5.8 (AlCH2), −4.7 (AlMe3), 20.1 (NCH2CH2CH2N), 27.9 (CH3-tBu), 40.3 (NCH2CH2CH2N), 62.1 (C(CH3)3), 146.2 (NCHN). Typical Procedure for the Methylenation of Carbonyl Substrates with Salts 2 and 3. All runs were carried out on an NMR scale. To a solution of the appropriate carbonyl substrate (0.1 mmol) in CD2Cl2 (0.2 mL) was added a CD2Cl2 solution (0.5 mL) of the salt species 2 and 3 (0.1 mmol). For the methylenation of acetophenone, 3-methyl-2-phenylbutanone, and benzyl benzoate, THF (2 equiv) was also added along with the carbonyl substrate. The mixture was stirred for 1 h at room temperature to yield the corresponding methylenation products as deduced by 1H NMR and comparison with literature data.31 Thus, the reaction of benzaldehyde, acetophenone, benzophenone, 3-methyl-2-phenyl-1-propanone, and benzyl benzoate with salt 2 or 3 led to the respective formation of styrene (>95% conversion), 2-phenylpropene (66% conversion), 1,1′diphenylethylene (>95% conversion), 3-methyl-2-phenyl-1-butene (70% conversion), and (1-(benzyloxy)vinyl)benzene (18% conversion). In the case of 3-methyl-2-phenylbutanone, the identity of the
subsequent deprotonation of Al2Me6 (occurring at a bridging Me group) by ItBu may then readily occur to yield salt 2 as the kinetic and thermodynamic product. On the other hand, such a deprotonation reaction was computed to be unfavorable both kinetically and thermodynamically with the much less sterically bulky and thus more robust model adduct (IMe)AlMe3, in line with the importance of adduct dissociation for the reaction to proceed. Anion Me3Al(μ3-CH2)(AlMe2)2(μ2-CH3)− was shown to efficiently methylenate various carbonyl substrates such as aldehydes and ketones, a reactivity clearly preferred over methylation. This agrees well with the DFT bonding analysis concluding on a more nucleophilic Al-CH2-Al (vs Al-Me moieties) group.
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EXPERIMENTAL SECTION
General Procedures. All experiments were carried out under N2 using standard Schlenk techniques or in an MBraun Unilab glovebox. Toluene, pentane, and dichloromethane were collected after going through drying columns and stored over activated molecular sieves (4 Å) for 24 h in a glovebox prior to use. Tetrahydrofuran was distilled over Na/benzophenone and stored over activated molecular sieves (4 Å) for 24 h in a glovebox prior to use. CD2Cl2, C6D6, d8-toluene, and d8-THF were distilled from CaH2, degassed under a N2 flow, and stored over activated molecular sieves (4 Å) in a glovebox prior to use. All deuterated solvents were obtained from Eurisotop (CEA, Saclay, France). All chemicals were purchased from Aldrich or Strem Chemicals. Elemental analysis for all compounds was performed at the Service de Microanalyse of the Université de Strasbourg (Strasbourg, France). 1,3-Di-tert-butylimidazol-2-ylidene (ItBu) was purchased from TCI Europe and was used as received. 1,3-Bis(2,6diisopropylphenyl)imidazol-2-ylidene (IDipp), 1,3-di-tert-butylimidazolin-2-ylidene (StBu), and 1,3-di-tert-butyl-3,4,5,6-tetrahydripyrimidin-2-ylidene (C6-tBu) were synthesized according to literature procedures.23−25 The Al-NHC adduct (ItBu)AlMe3 (1) was also synthesized according to a literature procedure. For all compounds, the combination of 13C and DEPT NMR data allowed the unambiguous assignment of all resonances. Caution: AlMe3 is a hazardous and highly pyrophoric colorless liquid that should be handled with great care under a controlled atmosphere devoid of H2O and O2. All manipulations involving AlMe3 (whether in solution or neat) were carried out in a N2-filled (
