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A Frustrated Lewis Pair Based on a Cationic Aluminum Complex and Triphenylphosphine Tom E. Stennett, Jürgen Pahl, Harmen S. Zijlstra, Falk W. Seidel, and Sjoerd Harder* Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: The highly Lewis acidic, cationic aluminum species [DIPP-nacnacAlMe]+[B(C6F5)4]− (1, DIPP-nacnac = [HC{C(Me)N(2,6-iPr2C6H3)}2]−) has been shown to undergo reactions with a wide variety of small molecules, in both the presence and absence of an external weak phosphine base, PPh3. Cycloaddition reactions of unsaturated C−C bonds across the aluminum diketiminate framework are reported, and the first structural confirmation of this type of cycloaddition product is presented. Addition of PPh3 to 1 produces the cationic aluminum phosphine complex [DIPP-nacnacAl(Me)PPh3]+[B(C6F5)4]−, which undergoes fluxional dissociation/ coordination of the phosphine in solution. This weak Al−P interaction can be utilized in frustrated Lewis pair type reactions to activate alkenes, alkynes, CO2, propylene oxide, and the C−Cl bonds of CH2Cl2. The CO2 adduct [DIPP-nacnacAl(Me)OC(PPh3)O]+[B(C6F5)4]− undergoes further stoichiometric reduction with Et3SiH to produce an aluminum formate species.

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alkenes,17,18 hydrogen,19 isocyanates,20 CO220−22 and other carbonyl groups,23 and carbodiimides,16 as well as to complex alkali-metal hydrides24 and to stoichiometrically25 and catalytically13 dehydrocouple amine−boranes. HX ion pairs (X = F, Cl) have also been shown to add across intramolecular Al−P pairs without causing protonolysis of the adjacent Al−C bonds.26 A recent report also details the FLP capture of elemental chalcogens, with oxidation of the phosphine moiety to phosphorus(V).14 A number of these Al-based FLPs are not “frustrated” in the original sense, as they contain Al−P bonds, existing rather as dimers. However, on account of a mismatch between the hard Lewis acid (Al) and soft Lewis base (P), the Al−P bond is not particularly strong, with the result that such dimers show FLP-type reactivity.20,21 The possibility of increasing the reactivity of FLPs by using a cationic Lewis acid component has been exploited by the groups of Stephan,27 Wass,28−30 and Erker,31−33 using transition-metal (Zr and Ti) complexes. In particular, cationic zirconocene complexes bearing pendant phosphines were shown to activate alkyl chlorides and fluorides−unprecedented reactivity for B/P, B/N, and Al/P systems. With the exception of a very recent report from Erker et al. on FLP-type reactivity based upon cationic zirconocene amide and phosphide complexes,34 these systems require the basic component to be integrated into a bifunctional ligand. Main-group FLPs containing cationic Lewis acids have also recently been developed for silicon,35,36 carbon,37 boron,38 and phosphorus39 and are already proving useful in a variety of bond activations.

INTRODUCTION Since the advent of frustrated Lewis pairs (FLPs, Figure 1), the powerful combination of unquenched Lewis acid/Lewis base

Figure 1. Selected previously reported FLPs.

pairs has allowed a wealth of reactivity to be accessed with main-group compounds, in domains previously dominated by transition metals.1−5 Various catalytic conversions typical in dblock chemistry, including hydrogenation,2,6−8 alkyne hydroamination,9 and the hydrosilylation of CO2,10 have been extended to FLPs. As for stoichiometric reactions, an everincreasing range of substrates is also being reported.11−14 Although the majority of systems consist of P/B or N/B pairs, replacement of the Lewis acidic component with aluminum species, notably by the groups of Stephan, Uhl, and Fontaine, has provided a variety of FLPs with unique properties. Such compounds have been shown to activate alkynes, 15,16 © XXXX American Chemical Society

Received: November 6, 2015

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broad signals, irrespective of stoichiometry. Taking into account the previously reported reversible cycloaddition of ethylene between Al and the backbone methine carbon of the βdiketiminate ligand,40 we postulated that a dynamic process was occurring. Although with ethylene a stable cycloaddition product was characterized in this previous case, theoretical studies on this reaction indicate that alkyl substituents on the alkene favor the formation of a π complex and disfavor cycloaddition.52 The 1H NMR spectrum at 258 K, with a slight excess of 1 relative to 1-hexene, displays slightly broadened multiplet signals at −0.25, 0.50, and 2.42 ppm, each integrating as one H atom, which are assigned to the inserted alkene. These values are in the range observed in Jordan’s ethylene cycloaddition product, 3, where the corresponding signals appear at 0.42 (2H) and 2.46 ppm (2H). In contrast, they differ significantly from the alkene protons in previously reported aluminum alkene π complexes, which appear within a range of 5.4−6.8 ppm.17 The product was thus formulated as [κ3-N,N,C{HC(C(Me)N(DIPP))2(nBuCHCH2)}AlMe]+[B(C6F5)4]− (4; Scheme 1). The observation of two signals at high field, and a

Despite a number of cationic aluminum compounds being known, and having noteworthy applications in catalysis, all aluminum-based FLPs have so far been based upon neutral precursors. In the late 1990s, Jordan and co-workers reported a series of highly Lewis acidic, cationic aluminum compounds, with the objective of developing new catalysts for olefin polymerization,40−43 the mechanism of which is still a matter of debate.44,45 Of particular interest to us was the complex [DIPPnacnacAlMe]+[B(C6F5)4]− (1, DIPP-nacnac = [HC{C(Me)N(2,6-iPr2C6H3)}2]−), which was isolated base-free. Although one fluorine atom of the diffuse [B(C6F5)4]− anion coordinates to aluminum in the solid state, in solution compound 1 can be considered to be coordinatively unsaturated. Reaction of this product with ethylene and 2-butyne resulted in interesting cycloaddition reactions between the electrophilic aluminum atom and the nucleophilic backbone carbon of the βdiketiminate.40 Otherwise, the reactivity of these compounds was limited to coordination of bases to the vacant metal site. We decided to revisit this chemistry in light of recent advances in FLP chemistry, to further investigate the reactivity of this highly Lewis acidic cation. Our results show that the presence of a simple, weakly Lewis basic phosphine such as PPh3 diverts the reaction pathway of compound 1 with a variety of small molecules.

Scheme 1. Cycloaddition Reactions of 1 with Alkenes and Alkynes

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RESULTS AND DISCUSSION Lewis Acidity of 1. While the cationic nature of compound 1 and the inherently electron-deficient nature of aluminum suggested high Lewis acidity, we sought to quantify this for the purposes of comparison with other Lewis acids. The Gutmann−Beckett method determines relative Lewis acidity by measuring the perturbation of the 31P chemical shift of triethylphosphine oxide upon coordination to an acceptor molecule.46,47 On this scale, n-hexane is defined as having an acceptor number (AN) of 0, whereas the powerful Lewis acid SbF5 has an AN value of 100. Addition of compound 1 to a solution of Et3PO in C6D6/C6D5Br (1/1) produced a 31P resonance at 81.5 ppm, resulting in an AN of 89.7 according to the method of Beckett.48 Under identical conditions, the ubiquitous FLP Lewis acid B(C6F5)3 gave values of δ31P 75.8 ppm and AN = 77.1, respectively, which correlates well to literature values.49 This indicates that 1 possesses a Lewis acidity markedly higher than that of B(C6F5)3, and it is even marginally higher than that of AlCl3 (AN = 87).50 Due to concerns about the reliability of the Gutmann−Beckett method when comparing neutral and cationic species,51 the result was confirmed with a competitive binding experiment, in which Et3PO was added to a solution of a mixture of 1 and B(C6F5)3. The initial 31P NMR spectrum after 5 min showed signals for 1· OPEt3 and (C6F5)3B·OPEt3 in a ratio of 1:1.25. However, heating to 60 °C for 1 h resulted in complete conversion to 1· OPEt3, indicating the higher Lewis acidity of 1. The compound crystallized from a benzene/bromobenzene solution, and analysis by X-ray crystallography confirmed the formation of the product as [DIPP-nacnacAl(Me)OPEt3]+[B(C6F5)4]− (2). The structure (see the Supporting Information) displayed the expected tetrahedral geometry at Al. Encouraged by this result, we set about examining the reactivity of 1. Phosphine-Free Reactions of 1. In order to compare the reactivity of 1 in the presence and absence of a phosphine base, the base-free complex was treated with a variety of small molecules. Addition of 1-hexene to a C6D5Br solution of 1 resulted in a room-temperature 1H NMR spectrum with very

single signal at 2.42 ppm, suggests a regiochemistry in 4 in which the terminal alkene carbon forms a bond with the Al center (see Scheme 1). Furthermore, the two methyl groups in the nacnac backbone of 4 resolve into two separate peaks at 1.65 and 1.68 ppm, consistent with the generation of a new chiral center. The reversibility of this cycloaddition reaction was proved by the observation that the removal of volatiles under vacuum resulted in the complete regeneration of compound 1. Treatment of 1 with phenylacetylene resulted in an immediate reaction, as evidenced by a new set of signals in the 1H NMR spectrum. A downfield shift of the ligand backbone CH proton to 5.76 ppm and its coupling to a new resonance at 7.32 ppm (J = 1.9 Hz) suggested the formation of a cycloaddition product (6; Scheme 1). Heating of the compound as a clathrate with toluene/pentane at 45 °C yielded crystals suitable for X-ray analysis. The structure confirmed the cycloaddition, with the terminal carbon of phenylacetylene bound to aluminum (Figure 2). Although related structures have been reported for Ru and Pt complexes,53,54 compound 6 is the first structurally characB

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CO2 to form cyclic carbonates59 with aluminum complexes, reacts unselectively with 1 alone. Analysis by 1H NMR spectroscopy revealed roughly 20 different species, possibly as a result of cationic polymerization reactions. Reaction of 1 with Triphenylphosphine. Combination of 1 and PPh3 produces a new, somewhat broadened signal at −9.1 ppm in the 31P{1H} NMR spectrum at room temperature (cf. −6.0 ppm for PPh3). The 1H NMR spectrum revealed new signals for the DIPP-nacnac ligand relative to 1. The iPr groups of the DIPP moieties also appear broadened. Due to the steric bulk of the DIPP-nacnac ligand and the relatively large size of triphenylphosphine, along with the preference of aluminum for hard donor ligands, we suspected that a dynamic process may be occurring, involving coordination and dissociation of the phosphine. The results of low-temperature 1H NMR experiments are displayed in Figure 3. A decoalescence was observed at around 258 K, below which temperature four different methyl environments and two methine environments within the DIPP iPr groups became apparent. This indicated conversion of C2v-symmetric [DIPP-nacnacAlMe]+ to Cssymmetric [DIPP-nacnacAlMe(PPh3)]+, rendering all methyl groups of each DIPP moiety inequivalent. A decoalescence also occurs in the 31P NMR spectrum at low temperature (Figure 4)the main peak sharpens and shifts slightly to −10.9 ppm, while a small peak at −6.6 ppm begins to appear, which we attribute to free PPh3. These results indicate the fluxional coordination/dissociation of triphenylphosphine on the NMR time scale, with an estimated ΔG⧧(258 K) value of 11 kcal/ mol. Combination of 1 with substoichiometric quantities of PPh3 also results in a single set of signals in the roomtemperature 1H NMR spectra, which resolve into the signals for 1 and 7 upon cooling to 238 K. The concentration of PPh3 was too low to be observed by NMR in this instance. The coalescence temperature (258 K) is not affected by the 1/PPh3 ratio. These observations confirm that the exchange takes place via a dissociative mechanism. The 1/PPh3 combination can thus be considered an intermediate between “classical” FLPs, with no observable acid/base interaction, and recently developed “masked”, dimeric pairs.

Figure 2. Crystal structure of compound 6. Hydrogen atoms (except for H30), DIPP iPr groups, and the [B(C6F5)4]− anion are omitted for clarity. Selected bond distances (Å): Al1−N1 1.959(1), Al1−N2 1.983(1), Al1−C30 1.979(2), Al1−C38 1.931(2), N1−C13 1.289(2), N2−C15 1.285(2), C13−C14 1.518(2), C14−C15 1.523(2), C30− C31 1.338(2).

terized example of a cycloaddition product with an aluminum β-diketiminate species. The regioselective formation of the observed isomer can be attributed to the charge densities at the acetylene carbon atoms55the more negatively charged terminal carbon preferentially attacks the aluminum center. Examples of carbon dioxide activation by β-diketiminate metal complexes are relatively scarce. Nonetheless, reactions in which CO2 undergoes cycloaddition between a Lewis acidic metal and the nucleophilic backbone methine have been reported for Mg,56 Ca,56 and Sc.57 Reaction of 1 with carbon dioxide (1 bar) results in a dynamic process, as judged by broadened peaks in the 1H NMR spectrum that unfortunately do not allow conclusions to be drawn about the possible reaction products. Freeze−thaw degassing of the solution results in the complete regeneration of 1. Attempts to crystallize the product(s) of this reaction under CO2 pressure resulted in precipitation of a liquid clathrate. This reaction remains the subject of current investigations. Propylene oxide, an important industrial intermediate that can be catalytically polymerized58 to polyethers or coupled with

Figure 3. Variable-temperature 1H NMR spectra of 1/PPh3 in C6D5Br/C6D5CD3. C

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180° in the fully delocalized diketiminate, is only 148.8(1)°. This is also reflected in the bond lengths of the diketiminate framework; the N2−C5 (1.360(2) Å) and C4−C3 (1.412(2) Å) bond lengths are somewhat elongated in comparison to the fully delocalized ligand (for example in DIPP-nacnacAlMe2, where N−C = 1.345 Å and C−C = 1.399 Å),63 while the N1− C3 (1.332(2) Å) and C5−C4 (1.378(2) Å) bonds are contracted. This indicates that the ligand possesses some amido-imine character in 7. Reactivity of 7 with Small Molecules. With the nature of the interaction between 1 and PPh3 clarified, we began to explore its utility in FLP reactions (Scheme 2). Triphenylphosphine, despite being one of the cheapest and most readily available phosphines, is far from a ubiquitous base in FLP chemistry due to its modest steric bulk and relatively poor donor properties, in comparison to PMes3 or PtBu3, for example. It forms adducts with the boranes B(C6F5)3 and B(pC6F4H)3, both of which are inert toward H2 activation.64,65 The classical Lewis adduct Ph3P·B(C6F5)3 has, nonetheless, been reported to add across the triple bond of phenylacetylene.66 This reaction is believed to occur due to the presence of a small equilibrium concentration, not observable by NMR spectroscopy, of the free phosphine and borane. In contrast, combination of B(C6F5)3 with bulkier phosphines of similar basicity, such as P(o-C6H4Me)3 and PMes3, provides increased reactivity. Due to the fluxional coordination of PPh3 to the aluminum center of 1, we anticipated that the acid−base pair would display appreciable FLP-type reactivity. The species were combined in situ before addition of the substrate. The 1/PPh3 solution was typically freshly prepared, although no visible degradation was observed after 1 week at room temperature under N2. Treatment of 1/PPh3 with phenylacetylene at room temperature initially resulted predominantly in the formation of cycloaddition product 6. However, a small amount (ca. 10%) of a second product was also observed in the 1H NMR spectrum. Whereas the nacnac backbone CH group in 6 appeared as a doublet at 5.76 ppm, the new species gave rise to a singlet at 4.96 ppm, typical for delocalized β-diketiminate species. Significantly, the 31P{1H} NMR spectrum showed a signal for free triphenylphosphine, alongside a sharp singlet at 23.7 ppm. Heating the sample to 60 °C for 20 h resulted in consumption of PPh3 and complete conversion to the new product, whose identity was confirmed by X-ray crystallography as trans-[DIPP-nacnacAl(Me)C(H)C(Ph)PPh 3 ] + [B(C6F5)4]− (8; Figure 6), the result of Al/P addition across the triple bond. Again, a single regioisomer is formed. Regarding the mechanism of the conversion of the kinetic product 6 to the thermodynamic product 8, we suggest that this occurs via initial dissociation of phenylacetylene from the cycloaddition product. Nucleophilic attack of the neutral phosphine at the Ph-substituted β-carbon of 6 seems unlikely for steric reasons, and the reversibility of the cycloaddition reaction between 1 and PhCCH was supported by a 2Hlabeling experiment: heating 6 with labeled PhCCD for 16 h at 60 °C resulted in significant deuterium incorporation in the cycloaddition product. Whereas P/B FLPs generally add across alkene double bonds,67 the picture is somewhat different with aluminum. Stephan and co-workers reported reversible addition reactions of R3P/AlX3 (R = mesityl, o-tolyl; X = Cl, Br, I, C6F5) pairs to ethylene.17 However, reactions with propene and isobutene resulted in deprotonation to form allyl species, and in some

Figure 4. Variable-temperature 31P{1H} NMR spectrum of 1/PPh3 in C6D5Br/C6D5CD3.

Heating the clathrate formed from 1 and PPh3 in mesitylene and pentane (ca. 1/4) at 45 °C resulted in the formation of colorless crystals. X-ray crystallography confirmed the formation of phosphine complex 7, whose structure is displayed in Figure 5. To the best of our knowledge, 7 is the first example of

Figure 5. Crystal structure of compound 7. Hydrogen atoms and the [B(C6F5)4]− anion are omitted for clarity. Selected bond distances (Å): Al1−P1 2.5191(6), Al1−N1 1.915(1), Al1−N2 1.871(1), Al1− C1 1.940(2), N1−C3 1.332(2), C3−C4 1.412(2), C4−C5 1.378(2), N2−C5 1.360(2).

a cationic aluminum triphenylphosphine complex. The Al−P bond length of 2.5191(6) Å is slightly shorter than that in Me3Al·PPh3 (2.535(1) Å)60 but markedly longer than those in which the Al fragment is similarly Lewis acidic to 1 (Cl3Al·PPh3 , 2.430(1) Å;61 ((CF3)3CO)3Al·PPh3, 2.447 Å (average)62)a fact that we attribute to the large steric hindrance afforded by the DIPP-nacnac ligand. Importantly, the bond is longer than that in Uhl and Lammertsma’s dimeric, “masked” FLPs, e.g. [Me2AlCH2PtBu2]2 (Al−P = 2.5001(5) Å20), which remain reactive toward CO2 and tBuNCO. The higher basicity of the PtBu2 fragment may contribute to the shorter bond length in this case. The DIPP-nacnac ligand in 7 is also visibly distorted from planarity in the crystal structure by the presence of PPh3. The C4−C5−N2−C7 torsion angle, which would ideally be D

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Organometallics Scheme 2. Summary of the FLP-Type Reactivity of 1/PPh3a

a

In all cases the counteranion is [B(C6F5)4]−.

Figure 7. Crystal structure of compound 9. Hydrogen atoms, DIPP iPr groups, and the [B(C6F5)4]− anion are omitted for clarity. Selected bond distances (Å): Al1−C31 1.998(4), Al1−N1 1.921(3), Al1−N2 1.919(3), Al1−C1 1.960(5), C31−C32 1.520(7), P1−C32 1.866(5).

Figure 6. Crystal structure of compound 8. Hydrogen atoms (except for H31), DIPP iPr groups, and the [B(C6F5)4]− anion are omitted for clarity. Selected bond distances (Å): Al1−C31 2.011(1), C31−C32 1.343(2), P1−C32 1.820(2), Al1−N1 1.896(1), Al1−N2 1.910(1), Al1−C1 1.953(1).

Carbon dioxide also rapidly undergoes an FLP addition between Al and P upon exposure of a degassed solution of 1/ PPh3 to 1 bar of CO2. The product, 10, precipitates as a liquid clathrate upon addition of n-hexane to a solution in chlorobenzene but could be crystallized by heating at 45 °C in the presence of toluene and pentane (ca. 1/4). The structure is displayed in Figure 8. The Al1−O1 (1.825(1) Å), C31−O1 (1.296(2) Å), and C31−O2 (1.200(2) Å) bond lengths closely match those in the previously reported compound tBu3PC(O)OAl(C6F5)3.68 The high rate of reaction to form 10 was confirmed by in situ IR spectroscopy; upon exposure of the 1/ PPh3 solution to CO2 introduced via a balloon, the band corresponding to the CO stretch of 10 at 1711 cm−1 was observed immediately. The characteristic signal for free CO2 at 2339 cm−1 was only detected once conversion to 10 was complete, indicating that the reaction between 10 and CO2 is instantaneous. In light of previous work in which FLPs are shown to be capable of the catalytic hydrosilylation of CO2,10 we treated compound 10 with Et3SiH. A slow but selective reaction

cases C−C coupling was observed.18 Addition of 1-hexene to a solution of 1/PPh3 in PhCl/C6D6 (1/1) resulted in a clean reaction within 10 min to produce a species with a peak at 29.7 ppm in the 31P{1H} NMR spectrum. The product could be precipitated as a colorless solid by addition of n-hexane. The subsequent 1H NMR spectrum in C6D5Br revealed multiplet signals at −0.26 to −0.41 ppm (2H) and 2.81 ppm (1H), corresponding to the phosphonium alane product 9, with the alkene having inserted into the Al−P bond. The product can be dried under vacuum at 45 °C without any evidence for this reaction being reversible. X-ray-quality crystals were grown by slow diffusion of n-hexane into a PhCl solution of the product. The structure is displayed in Figure 7. The reduction of the former double bond of the alkene to a single bond is confirmed by the bond length (C31−C32) of 1.520(7) Å. E

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reported by Ménard et al., [(C6F5)3Al−O−C(H)−O−Al(C6F5)3]−[HPtBu3]+, prepared by the insertion of CO2 into a bridged dialuminum hydride, in which the dialuminum formate fragment bears a formal negative charge.69 Despite the difference in charge, the bond lengths in the formyl group are rather similar. However, in contrast to the previously reported compound, 11 has a “trans” geometry at both C−O bonds, with Al−O−C−O torsion angles approaching 180°. The 31 1 P{ H} NMR spectrum after the reaction between 10 and Et3SiH displays two major signals−that for triphenylphosphine and a second peak at −3.4 ppm. We attribute this second peak to the recently reported salt [Et3SiPPh3]+[B(C6F5)4]− (12) by comparison with the literature 31P NMR shift of −2.9 ppm.70 We suggest that the hydride of Et3SiH attacks the electrophilic central carbon atom of 10 (Scheme 3), eliminating triphenylphosphine and a triethylsilylium cation, which then combine to form the silylphosphonium salt 12 as reported previously.70 The carboxyl aluminum compound generated could then coordinate to another cationic aluminum moiety (1) to produce 11, although the source of excess 1 is unclear at this point. In this regard, we suspected that the addition of CO2 to 1/PPh3 may be reversible; however, heating compound 10 to 90 °C under dynamic vacuum (0.1 mbar) for 1 h produced no detectable quantity of 1, with 10 remaining largely intact alongside traces (total