Influence of the Lewis Acidity of Gallium Atoms on the Reactivity of a

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Influence of the Lewis Acidity of Gallium Atoms on the Reactivity of a Frustrated Lewis Pair: Experimental and Theoretical Studies Josephine Possart and Werner Uhl* Institut für Anorganische und Analytische Chemie der Universität Münster, Corrensstraße 30, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: The reactivity of the Ga/P-based frustrated Lewis pair (FLP) Mes2P− C[C(H)−Ph]−GatBu2 (3) is influenced by the relatively weak Lewis acidity of its Ga atom and differs significantly from that of the analogous Al compound 1. The adduct of 3 with CO2 was only detectable at low temperature by NMR spectroscopy. Benzaldehyde was coordinated only via a Ga−O bond; the P atom was not involved. In contrast, a relatively persistent adduct was formed with soft CS2 to yield a fivemembered GaCPCS heterocycle. Dehydrocoupling with H3B←NHMe2 afforded the dimeric amidoborane (H2B−NMe2)2, while an adduct with a GaCPBN heterocycle was isolated with the sterically less shielded ammonia−borane H3B←NH3. The latter product was unstable in solution and decomposed by H2 elimination and formation of oligomeric BN compounds. Small quantities of 3 catalyzed hydrogen transfer from H3B←NH3 to an imine. The Lewis acidities of the Al/P- and Ga/P-based FLPs were examined by experiments (Gutmann−Beckett method) and by calculation of the fluoride ion affinity (including the B and In analogues). The Al compound is the strongest Lewis acid; the Ga FLP is significantly weaker but is a stronger F− acceptor in comparison to the unknown analogues of B and In. These results reflect the different reactivities of these FLPs and may help to develop FLPs with finely adjusted properties.

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available alkynylphosphines with dialkylaluminum hydrides.5 This procedure allowed the modification of FLPs. A P−H functionalized compound (2) has been obtained which showed the coordination of substrates via the typical FLP functionality followed by the transfer of the phosphorus-bound hydrogen atom to the activated substrates and the generation of unusual structural motifs such as a 1-phosphatetrazene.12 Transmetalation starting with 1 afforded a unique phosphinylvinyl Grignard reagent as an important starting material for the synthesis of Ga/P-based (3; Chart 1) or In/P2-based FLPs13,14 which allow systematic investigations into the influence of the hardness or softness of the metal atoms on reaction courses. Gallium atoms in organometallic compounds are softer and weaker Lewis acids in comparison to aluminum atoms, and adducts of e.g. ethers with gallium-based Lewis acids are less stable than those with aluminum. The reason is the comparatively high electronegativity of gallium, which results in a lower polarity of Ga−C bonds and a lower partial positive charge at gallium in comparison to aluminum. These differences should influence the FLP properties and allow a careful adjustment of the acceptor capability and reactivity of FLPs, which is important for their specific application in coordination, activation, or catalytic transformations. Preliminary investigations into the chemical behavior of the Ga/Pbased FLP 3 revealed similarities (formation of azide adducts) but also significant differences (reactivity toward alkynes) in comparison to the aluminum analogue 1.13 On the basis of

INTRODUCTION Molecular activation and the optimization of catalytic transformations have been the focus of numerous research activities for many decades. Transition-metal-based systems were traditionally applied, but recently frustrated Lewis pairs (FLPs) exclusively based on main-group elements have attracted enormous interest in this field. FLPs have coordinatively unsaturated Lewis acidic and basic atoms in single molecules or bimolecular systems.1 Their specific functionality results in a unique reactivity and allows the bipolar coordination or activation of various substrates in stoichiometric or catalytic processes. Several research groups have shown2−5 that Al/P-based compounds form an interesting class of highly efficient FLPs.6 They coordinate a wide variety of substrates such as CO2, HX, benzaldehyde, alkynes, alkenes, azides, nitrenes, diazomethane, BX3,3−5,7 chalcogen (S to Te),8 and transition metal atoms.9 They have been applied in phase transfer catalysis with the solubilization of alkali-metal hydrides in organic solvents10 or in the catatylic dehydrogenation of dimethylamine−borane.11 A facile method for the selective generation of monomolecular, geminal FLPs on a multigram scale (e.g., 1; Chart 1) comprises hydroalumination of easily Chart 1. Schematic Drawings of FLPs 1−3

Received: February 8, 2018

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.8b00075 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics experiments and quantum chemical calculations, the influence of the acidity of aluminum and gallium atoms on the reactivity of FLPs is presented in this article.

constant of 35.3 Hz (3: δ 7.40, 3JPH = 16.8 Hz), which indicates the formation of an adduct featuring a four-coordinate phosphorus atom6−8,10 (4, Scheme 1; CO2 adduct of 1,5a δ

RESULTS AND DISCUSSION Reactivity of 3 toward CO2, Benzaldehyde, and CS2. The coordination of CO2 by FLPs was first observed in 200915 using inter- and intramolecular P/B-based systems and resulted in the formation of stoichiometric 1:1 complexes.1,3−5,15 In a continuation of this work the application of FLPs in the catalytic reduction of CO2 was of particular interest because it would allow the transformation of this cheap greenhouse gas to valuable C1 building blocks for applications in synthetic chemistry.3b,16 Catalytic processes require substrates that are not too tightly bound and the reversibility of adduct formation, which both may be favored by weakly Lewis acidic centers. The gallium atoms of the Ga/P FLP 3 seem to be useful for such reactions. They are relatively soft in comparison to aluminum atoms and have a low polarizing capability. The analogous Al/P FLP 1 showed the effective complexation of CO2 at room temperature and normal pressure to yield a heterocyclic compound with the oxygen atom coordinated to aluminum, the carbon atom bound to phosphorus, and an exocyclic CO bond. The coordinated CO2 was released only under drastic conditions upon heating the neat material under vacuum to 135 °C.5a As briefly noted in a communication,13 the Ga/P FLP 3 does not react with CO2 at room temperature by formation of an isolable adduct. 31P{1H} NMR spectra in d8-toluene (1 atm of CO2; Figure 1) revealed an unusually broad resonance at

Scheme 1. Reactions of 3 with CO2, CS2, and Benzaldehyde

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7.78, 3JPH = 40.1 Hz). The reaction is reversible upon warming to room temperature. Storing a reaction mixture under a CO2 atmosphere at −75 °C did not result in the formation of single crystals. Quantum chemical calculations revealed a mechanism for the reaction of intermolecular P/B-based FLPs with CO2, in which the phosphorus atom binds in the first step to the carbon atom of CO2.17 A different reaction pathway was found for the reaction of the Al/P-based FLP 1 with CO2. It starts with the formation of a van der Waals complex between the phosphorus and aluminum atoms and a CO bond.5a In contrast to the results for the B/P FLPs the formation of a transition state with an Al−O bond was calculated for the next step, which is followed by an attack of the phosphorus atom at the polarized and activated carbonyl carbon atom. The lower polarization of the CO bond by the weaker Lewis acid gallium in 3 results in a less activated carbon atom and a weaker P−C(O) interaction and prevents the formation of an isolable adduct. Only kinetic control allowed coordination of a C−O group to phosphorus and gallium atoms, as detected by NMR spectroscopy. A similar behavior of 3 was observed toward benzaldehyde. It reacted with the Al/P FLP 1 by formation of a five-membered heterocycle in which phosphorus was bound to the carbonyl carbon atom and oxygen to the aluminum atom. A relatively long P−C distance (193.8(2) pm) and a dynamic behavior in solution confirmed a comparably weak P−C interaction.7e Treatment of 3 with benzaldehyde in cyclopentane yielded immediately a red solution indicating the formation of an adduct. After it was stirred overnight, the solution turned yellow and pure compound 5 was obtained as an amorphous solid from the reaction mixture at −40 °C (Scheme 1). The 31 1 P{ H} and 1H NMR spectra confirmed the coordination of the aldehyde to the FLP 3 only via a Ga−O bond without significant P−C(O) interactions. The phosphorus atom of 5 resonated at δ −4.3 and had a relatively small coupling constant to the vinylic hydrogen atom (3JPH = 21.0 Hz). These values are close to the data observed for 3 and are consistent with a threecoordinate phosphorus atom. The benzaldehyde adduct of the Al/P FLP 1, which contains a five-membered AlCPCO heterocycle and a four-coordinate phosphorus atom, had different values of δ(31P) 31.7 and 3JPH = 37.5 Hz.7e The NMR parameters of 5 are similar to those of an adduct which has been obtained previously by treatment of Al/P FLP 1 with benzophenone (δ −7.9, 3JPH = 20.6 Hz). It had only the carbonyl oxygen atom coordinated to the aluminum atom, while the phosphorus atom was not affected.18 Adduct formation via an Al−O bond was in this case calculated to be energetically favored over the coordination of the CO bond to phosphorus and aluminum.18 The resonance of the

Figure 1. 31P{1H} NMR spectra of FLP 3 treated with 1 bar of CO2 in d8-toluene at various temperatures (300−220 K).

ambient temperature. Its chemical shift (δ −13.3) corresponds to that of the starting FLP 3 (δ −14.6 in C6D6), which together with its shape may indicate the formation of a weak adduct and a fast exchange between bound and free CO2. At 280 K this resonance disappeared, and at about 260 K a new broad resonance was found at δ 0. At 220 K a sharp singlet was observed at δ −0.1, which is close to the CO2 adduct of 1 (δ 5.6; C6D6, room temperature).5a The 1H NMR spectrum at this temperature showed the resonance of the vinylic hydrogen atom as a doublet at δ 7.72 with a relatively large 3JPH coupling B


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Organometallics

is bound to phosphorus atoms and a Lewis acid.19,22 The difference between both C−S bonds is only 5.5 pm and indicates delocalization of π-electron density. The Ga1−S1 bond is 4 pm longer than in the phenyl isothiocyanate adduct of 3.23 The heterocycle adopts an envelope conformation in which P1 is 57.1 pm above the plane C11−Ga1−S1−C7. Reactivity of 3 toward H3B←NHMe2 and H3B←NH3. The ammonia−borane adduct, H3B←NH3, is a promising hydrogen storage material, as it contains 19.6 wt % of hydrogen.24 Transition-metal complexes have been used as efficient catalysts for dehydrogenation, but reversibility is still a limiting factor for further application. Current activities have been directed toward the formation of oligomeric or polymeric amido- and imidoboranes by catalytic dehydrocoupling reactions.24 Only recently, also main-group compounds have been applied as catalysts for the dehydrogenation of amine− boranes.11,25 The Al/P-based FLP 1 proved to be an excellent reagent and reacted with H3B←NHR2 (R = H, Me) already at −60 °C (Scheme 2). Hydrogen release gave the corresponding

hydrogen atom of the aldehyde in 5 (δ 8.96) is shifted to high field in comparison to the free aldehyde (δ 9.68). The chemical shifts in the NMR spectra of 5 vary depending on concentration and indicate dynamic behavior in solution, which may be caused by hindered rotation about the Ga−O bond, ligand exchange or ring closure by a weak P−C interaction. The formation of a relatively weak adduct with FLP 3 in contrast to FLP 1 concurred with the results of the reaction with CO2. According to Pearson’s HSAB concept, 3 was expected to favor coordination of soft CS2 molecules. A similar reaction has previously been reported only with a B/P FLP.19 Ga/P FLP 3 reacted with CS2 in n-hexane to yield a brown solution. Red crystals of 6 precipitated from the concentrated reaction mixture at room temperature. The 31P{1H} NMR spectrum showed a sharp singlet at δ 33.6. In the 1H NMR spectrum the resonance of the vinylic hydrogen atom was observed as a doublet at δ 7.62 with a relatively large 3JPH coupling constant of 33.4 Hz, indicating a four-coordinate phosphorus atom. The 13 C{1H} NMR spectrum showed the resonance of the endocyclic, sulfur bound carbon atom at δ 238.4 with 1JPC = 49.4 Hz, confirming the dipolar fixation of a CS bond. Compound 6 slowly decomposed in solution, and complete spectroscopic characterization proved to be difficult. It is interesting to note that the relatively soft GaR2 group forms an isolable product with the soft Lewis base sulfur, while CO2 gave only an unstable adduct.20 The Al/P-based FLP 1 and CS2 gave an unclear reaction course, and no product could be isolated so far in a pure form. The influence of the hardness of metal atoms on the reactivity of such FLPs may become important in further secondary reactions. The molecular structure of 6 is depicted in Figure 2 and confirms the coordination of a CS bond to phosphorus (P− C) and gallium (Ga−S) with formation of a five-membered heterocycle. The endocyclic C7−S1 bond (169.8(1) pm) is slightly longer than the exocyclic C7−S2 bond (164.3(1) pm). Both bond lengths are in the typical range of CS double bonds21 and compare well to similar compounds in which CS2

Scheme 2. Reactivity of the Al/P FLP 1 toward Amine− Borane Adducts (R = H, Me)

amidoboranes, which were captured by the FLP and afforded five-membered AlCPBN heterocycles via the formation of Al− N and P−B bonds.11 Only the H2B−NH2 adduct was stable at room temperature in solution, while the H2B−NMe2 derivative dissociated slowly, probably caused by steric repulsion between the bulky tert-butyl and NMe2 groups. The corresponding cyclodiborazane (H2B−NMe2)2 with a B2N2 heterocycle was selectively formed, and FLP 1 was completely recovered.11 This behavior allowed catalytic reactions starting with H3B← NHMe2 and low loadings of 1 (0.4%) and resulted in quantitative transformations.11 The Ga/P FLP 3 behaved differently toward the adduct H3B←NHMe2. Dehydrocoupling led to the quantitative formation of (H2B−NMe2)2 after 1 day in benzene at room temperature, as was evident from NMR spectra of the reaction mixture (Scheme 3). The reaction is faster than that of the Al/P FLP 1. The amidoborane adduct of FLP 3 was not observed as an intermediate, which may be caused by the weak Lewis acidity of gallium and the facile dissociation favored by relatively weak Ga−N interactions.

Figure 2. Molecular structure and atomic numbering scheme of compound 6. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H12, arbitrary radius) have been omitted. Selected bond lengths (pm) and angles (deg): C11−C12 134.5(2), C11−P1 178.8(1), C11−Ga1 205.0(1), P1−C7 184.9(1), C7−S1 169.8(1), C7−S2 164.3(1), Ga1−S1 246.0(1); P1−C11−Ga1 114.7(1), C11−Ga1−S1 89.5(1), Ga1−S1−C7 104.0(1), S1−C7− P1 115.7(1), S2−C7−P1 117.5(1), S1−C7−S2 126.8(1), C7−P1− C11 105.0(1), C11−Ga1−S1−C7 32.9.

Scheme 3. Dehydrocoupling of H3B←NMe2H

C


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Organometallics

We treated 7 with an equimolar quantity of FLP 3 because we hoped to release a second equivalent of dihydrogen and to form an imido borane. A small quantity of dihydrogen was formed, but the reaction was unselective and afforded a mixture of unknown products. Storing a solution of 7 in C6D6 in an NMR tube at 75 °C overnight led to decomposition of 7, full recovery of FLP 3, and formation of dihydrogen. Although the resulting byproducts of both reactions could not be identified, these results demonstrate the reactivity and feasible cleavage of adduct 7, which contrasts with the behavior of the corresponding, thermally stable adduct of the Al FLP 1. Several years ago, the transfer of dihydrogen from H3B← NH3 to benzylidene-tert-butylamine was reported. The reaction proceeded at 60 °C without a catalyst with an NMR yield of 46% after 4 days.26 We supposed that FLP 3 could act as a catalyst and treated equimolar quantities of H3B←NH3 and benzylidene-tert-butylamine in a preliminary experiment in toluene with 4 mol % of the FLP at room temperature. Immediate evolution of gas occurred (Scheme 5). After 1 h, the

Treatment of FLP 3 with the sterically less shielded ammonia−borane H3B←NH3 in toluene at room temperature resulted in the evolution of dihydrogen and the formation of the amidoborane adduct 7, which features a five-membered PCGaNB heterocycle (Scheme 4). The reaction was complete after one night. 7 was selectively formed but was isolated in only 43% yield after recrystallization from n-pentane. Scheme 4. Reaction of 3 with H3B←NH3

The 11B{1H} and 31P{1H} NMR spectra of 7 showed broad singlets at δ −12.0 and 19.6, respectively. The resonance of the vinylic hydrogen atom was detected in the 1H NMR spectrum as a doublet at δ 7.97 with a 3JPH coupling constant of 35.7 Hz, which confirms the coordination number of 4 at phosphorus. BH and NH hydrogen atoms were found at δ 3.19 and 1.23. The latter showed a 3JPH coupling constant of 15.0 Hz. The constitution of 7 was confirmed by a crystal structure determination (Figure 3). The molecule contains the five-

Scheme 5. Transfer Hydrogenation with Benzylidene-tertbutylamine and Ammonia−Borane Catalyzed by 3

resonances of the starting materials were still observed in the NMR spectra: e.g., the signal of the hydrogen atom of the N CH group was detected at δ 8.09 in the 1H NMR spectrum. However, additional resonances indicated the beginning hydrogenation of the imine by reduction of the CN bond. The hydrogen atoms of the CH2 group of the resulting amine resonated as a doublet at δ 3.56 (3JHH = 7.6 Hz). The 31P{1H} NMR spectrum showed the resonance of 7 at δ 19.6. A 25% conversion to the amine was observed after 19 h at room temperature and full conversion after 1 month. The 11B NMR spectrum did not show the characteristic signal of H3B←NH3, but broad resonances at δ 22−31 ppm indicated the formation of oligomeric or polymeric polyborazylenes, including borazine.26 Higher temperatures of 60 °C did not increase the conversion rate. The formation of polyborazylenes is consistent with the release of more than 1 equiv of dihydrogen from H3B←NH3. The finally recorded 31P{1H} NMR spectrum showed exclusively the resonance of recovered FLP 3 at δ −14.6. This experiment under nonoptimized conditions showed the promising capability of 3 to act as a catalyst in hydrogen transfer reactions. Control experiments did not show a reaction between H3B←NH3 and benzylidene-tert-butylamine at room temperature after 2 weeks. FLP 3 and pure 7 were unreactive toward the imine, and no reaction occurred between FLP 3 and the imine under a hydrogen atmosphere (4 bar). Oligomeric BN compounds hydrogenate imines faster in comparison to H3B←NH3.26 Such compounds may be formed by release of H2BNH2 from 7 and secondary oligomerization reactions. We verified this hypothesis by treatment of H3B← NH3 (δ(11B) −22.5 (q)) with catalytic quantities of 4 mol % of FLP 3. After 1 day in toluene at room temperature, the formation of the heterocyclic compound H2B(μ-NH2)2BH−

Figure 3. Molecular structure of 7. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms with the exception of BH2 and NH2 (arbitrary radius) have been omitted for clarity. Selected bond lengths (pm) and angles (deg): C11−C12 135.0(2), C11−P1 180.7(2), C11− Ga1 204.8(2), P1−B1 200.4(2), Ga1−N1 206.0(2), B1−N1 155.8(2), P1−C31 183.9(2), P1−C41 184.0(2); P1−C11−Ga1 112.4(1), P1− B1−N1 107.2(2), Ga1−N1−B1 116.3(1).

membered heterocycle P1−C11−Ga1−N1−B1 with five different elements in a slightly distorted envelope conformation. The distance of the nitrogen atom to the plane defined by the remaining four atoms (B1−P1−C11−N1) is 49 pm. The Ga1− N1 and P1−B1 bonds are slightly longer by 4 and 2 pm in comparison to the respective bonds (Al−N and P−B) in the corresponding Al/P amidoborane adduct,11 while the B1−N1 bond of 7 is shorter by 3 pm. Interestingly, the endocyclic M− N−B angles differ considerably (116.3(1)° in 7 versus 107.6(1)°) and may reflect an approach of the nitrogen atom in 7 to sp2 hybridization. These observations may support a weaker complexation of H2BNH2 in the gallium compound, which is consistent with the capability of 7 to undergo secondary reactions in contrast to the unreactive aluminumbased adduct. Other bond lengths and angles are similar in both amidoborane adducts. D


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Organometallics NH2−BH3 was confirmed by its characteristic broad 11B NMR resonances at δ −24.2, −11.8, and −5.1.26 After 4 days additional broad signals were observed at δ 26.9 and 31.2 in the characteristic range of polyborazylenes.26 After 18 days only a small quantity of H3B←NH3 was left. Characteristic signals of (H2B−NH2)3 or (HB−NH)3 (δ(11B) −10.8 and 30.1)26 were not detected. The reaction may start by the formation of the amidoborane adduct 7 and hydrogen evolution. The corresponding adduct of the Al/P FLP 1 is thermally stable and does not allow the catalytic degradation of H3B←NH3. In contrast, 7 containing the softer gallium atom reacts further and affords oligomeric BN compounds. Catalytic quantities of the Ga FLP 3 are sufficient for a quantitative transformation. The intermediate oligomeric amidoboranes may allow the slow quantitative reduction of the imine, as experimentally observed. The different Lewis acidities of aluminum and gallium atoms influence the reaction courses and the stability of the intermediately formed complexes. However, for a sound understanding quantitative data would be helpful, and we investigated the Lewis acidity of these FLPs by experimental and theoretical methods. Experimental Determination of the Lewis Acidity of 1 and 3. The most commonly applied concept for the experimental determination of Lewis acidities is the Gutmann−Beckett method. In 1975, Gutmann used Et3PO as a standard Lewis base to determine the Lewis acidity of solvents.27 This method was extended to boron-based polymerization catalysts by Beckett in 1996.28 Alteration of the charge on the phosphorus atoms of Et3PO upon adduct formation and coordination of the oxygen atom to a Lewis acid can easily be monitored by 31P NMR spectroscopy. The difference between the chemical shifts of free Et3PO (δ 45.7 in C6D6 at room temperature) and the Lewis acid−base adducts is a measure of the relative Lewis acidity. The “Childs method” (adduct formation with crotonaldehyde)29 did not work, because complexes that were too weak were formed with gallium, while unclear side reactions occurred with the Al/P FLP 1. FLP 3 was treated with an equimolar quantity of Et3PO in toluene. Coordination of the oxygen atom to gallium afforded adduct 8 (Scheme 6), which was isolated in 54% yield after

Figure 4. 31P{1H} NMR spectra of FLP 3 treated with 1 equiv of Et3PO in CD2Cl2 at different temperatures (300−240 K).

C6D6 revealed a singlet assigned to the hydrogen atoms of the Bu groups at δ 1.35. The downfield shift in comparison to 3 (δ 1.05) is typical for an increased coordination number at gallium.13 The hydrogen atoms of the ethyl groups showed a doublet of quartets at δ 1.13 with 2JPH = 13.0 Hz and a doublet of triplets at 0.61 ppm with a 3JPH coupling constant of 17.0 Hz. The relatively small 3JPH coupling constant to the vinylic hydrogen atom (22.8 Hz) is in accordance with a threecoordinate phosphorus atom. Three equivalents of the Lewis base shifted the equilibrium completely to adduct 8 at room temperature and resulted in a sharp 31P NMR signal of the phosphorus atom of coordinated Et3PO at δ 61.0. Determination of the molecular structure of 8 (Figure 5) confirmed coordination of an Et3PO molecule by FLP 3 via Ga−O bond formation. The atoms Ga1 and P2 have distortedtetrahedral coordination spheres, while P1 of the PMes2 group is three-coordinate with a pyramidal surrounding. The bond lengths and angles of the FLP backbone are almost unchanged in comparison to uncoordinated 3. The increase of the coordination number results in a lengthening of the Ga−C bonds by about 4 pm (204.3 pm versus 200.2 pm for 3). The Ga1−O1−P2 group approaches linearity with an angle of 164.0(1)°. The P2−O1 distance is almost unchanged in comparison to that of uncoordinated Et3PO, which is consistent with a relatively weak Ga1−O1 bond.30 Accordingly, the Ga1−O1 distance (203.7(1) pm) is significantly longer than that usually observed for Ga−O−P moieties (about 189 pm).31 For comparison, the Al/P FLP 1 was treated with 1 equiv of Et3PO in C6D6 to form adduct 8a in an NMR experiment. In contrast to the gallium compound 8 the 31P{1H} NMR spectrum showed two sharp signals already at ambient temperature, indicating the formation of a persistent complex. The resonance of the PMes2 phosphorus atom was detected at t

Scheme 6. Reaction of the Ga FLP 3 with Et3PO

concentration and cooling of the reaction mixture to −15 °C. The 31P{1H} NMR spectrum in C6D6 showed two broad signals at δ −2.7 and 58.4, which may indicate a dynamic equilibrium in solution at room temperature. The signal at δ 58.4 was assigned to the phosphorus atom of coordinated Et3PO and the resonance at δ −2.7 to the PMes2 group (δ −14.6 for 3). The dynamic behavior of 8 in solution was confirmed by VT NMR experiments in CD2Cl2 at temperatures between 300 and 240 K (Figure 4). Broad resonances of two different phosphorus atoms were observed at δ 57 and −6 at 300 K. At 270 K the line width of the signals decreased, and the spectrum at 240 K showed two sharp singlets at δ −2.3 for the PMes2 group and δ 63.3 for the phosphorus atom of coordinated Et3PO in a 1:1 ratio. The 1H NMR spectrum in E


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Organometallics

I) gave the strongest interactions with hard fluoride and chloride atoms. The gallium-based compounds preferred coordination of softer methanide and hydride ions and showed in these cases a similar or even stronger Lewis acidity in comparison to their aluminum analogues. Quantum chemical calculations gave insight into the acceptor capabilities of B-, Al (1)-, Ga (3)- and In-based FLPs which have simple alkyl groups attached to their Lewis acidic atoms. Structure optimization was carried out with GAUSSIAN0937 using the B3LYP38,39 functional and the Def2TZVP40 basis set. London dispersion forces were considered by using GD3BJ41−43 as a correction scheme for dispersive interactions.44 On the basis of these structures, reaction enthalpies for the capture of an F− ion by 1, 3, and the up to now unknown analogous B and In FLPs in the gas phase were calculated; the results were referenced against the COF2/COF3− system (Table 1). Table 1. Calculated Reaction Enthalpies ΔH for Binding a Fluoride Ion to the Lewis Acidic Atom of FLPs Mes2P−C( CH−Ph)−MtBu2 in the Gas Phase (FIA; B3LYP/Def2TZVP (GD3BJ))37−43,45 and Results of the Gutmann−Beckett Method for FLPs 1 and 3

Figure 5. Molecular structure of compound 8. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms with the exception of H12 have been omitted. Selected bond lengths (pm) and angles (deg): C11−C12 135.3(2), C11−P1 184.2(1), C11−Ga1 204.8(1), P1−C31 185.8(1), P1−C41 184.9(1), Ga1−C5 204.2(2), Ga1−C6 203.7(2), Ga1−O1 203.7(1), O1−P2 150.5(1); P1−C11−Ga1 116.3(1), Ga1− O1−P2 164.0(1).

δ −0.5 (δ −14.2 in 1), while the phosphorus atom of Et3PO resonated at δ 66.9. The 1H NMR chemical shifts are similar to those of 8. In accordance with the increased coordination number at the aluminum atoms the 1H NMR resonance of the tert-butyl groups is shifted to a lower field (δ 1.29 versus 0.96). The difference between the 31P NMR chemical shifts of free Et3PO (δ 45.7) and the corresponding Lewis acid base adducts of FLPs 1 (δ 66.9) and 3 (δ 58.4) was used to determine the relative Lewis acidity. In the case of 3 the data of the equimolar reaction at room temperature were considered. The Al system showed a larger difference between the chemical shifts of free and bound Et3PO, indicating a higher Lewis acidity (Δδ 21.1 ppm) in comparison to 3 (Δδ 12.7 ppm). The resulting relatively weak acceptor strength of 3 is consistent with the observed exchange process in solution. Tris(pentafluorophenyl)borane, B(C6F5)3, is often applied in FLP chemistry as a strong Lewis acid and has a large Δδ value of 30.2 ppm (Δδ(BPh3) = 19.6 ppm).32 Quantum Chemical Calculations of the Fluoride Ion Affinity. Fluoride ion affinity (FIA) is a quantum chemical method for the calculation of Lewis acidities, which was established in 1984 by Bartlett.33 The FIA concept correlates Lewis acidity with the enthalpy of fluoride binding. As it is problematic to calculate an isolated fluoride ion in the gas phase, Christe suggested the experimental FIA of F2CO (208.8 kJ mol−1) as a reference point.34 In 2008, Krossing compared the FIA values of strong Lewis acids MX3 containing electronwithdrawing substituents (M = B, Al, Ga; X = halide, ORF, RF).35 AlBr3 and AlI3 were found to be the strongest acceptors, followed by the corresponding gallium and boron compounds.7a,14e,35 A disadvantage of the FIA method is the hardness of the fluoride ion,20 which may result in an irrelevant assessment of acceptor strengths for soft Lewis acids that preferably coordinate soft bases. The affinity of Lewis acids toward ions of varying hardness (F−, Cl−, H−, CH3−) was therefore investigated.36 For simple halogen compounds the results showed that the harder Lewis acids AlX3 (X = F, Cl, Br,

M

FIA (kJ/mol)

Δδ(31P) (ppm)

B Al (1) Ga (3) In

280.17 376.77 302.42 285.10

21.1 12.7

The Lewis acidity of these FLPs follows the expected trend of group 13 elements as derived from their chemical behavior. The boron compound is the weakest and the aluminum based FLP the strongest Lewis acid (280 vs 377 kJ/mol). The latter value may be compared to that of BF3 (342 kJ/mol) or BCl3 (405 kJ/mol).36 The FIA of the Ga FLP 3 (302 kJ/mol) is slightly larger than those of the boron and indium analogues. The gallium and indium compounds may be more efficient Lewis acids toward softer Lewis bases such as a hydride anion. Nevertheless, these results clearly confirm our observations with respect to the relative stability of complexes with FLPs 1 and 3 and help in a sound understanding of the different reactivities of these compounds toward carbonyl compounds or amine−boranes H3B←NHR2 (hard O and N donors).

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CONCLUSION The chemical properties of FLPs depend on the Lewis acidity of the acceptor and Lewis basicity of the donor atoms. Systematic variation of the donor or acceptor strengths should influence the reactivity of FLPs and help to optimize their behavior in chemical transformations. In this article, we compared Al/P- and Ga/P-based FLPs (1 and 3) which are available in facile routes by hydroalumination of alkynylphosphines or salt elimination starting with a phosphinylvinyl Grignard reagent. The differing acceptor strengths of 1 and 3 were experimentally determined by the alteration of the 31P NMR chemical shift of Et3PO after coordination to the respective FLP (Gutmann−Beckett method). The strongest shift difference was observed for the Al FLP 1. Quantum chemical calculations of the fluoride ion affinities of 1 and 3 and the corresponding boron and indium compounds gave a similar result, with the aluminum FLP 1 as the strongest acceptor. The gallium FLP 3 is a weaker Lewis acid, while the smallest FIAs F


Article

Organometallics

1602 m, 1585 w, 1554 w, 1535 s phenyl, ν(CC), ν(CO); 1490 m, 1465 vs, 1446 vs, 1403 m, 1382 m, 1353 m, 1342 m, 1290 m, 1241 s δ(CH3); 1189 m, 1155 w, 1116 vs, 1070 m, 1027 m, 1012 w, 985 vw, 923 m, 883 w, 848 m, 809 m, 786 s, 769 m, 746 vs, 703 s ν(CC); 692 m, 665 m, 645 s phenyl; 587 s, 572 w, 557 m, 535 m, 495 m, 460 m, 447 s ν(PC), ν(GaC), δ(CC). MS (EI, 20 EV, 373 K): m/z (%) 554 (3) [3+]. Anal. Calcd for C41H52GaPO (660.3): C, 74.4; H, 7.9. Found: C, 74.2; H, 8.0. Reaction of 3 with CS2: Synthesis of 6. A solution of 3 (0.21 g, 0.38 mmol) in 5 mL of n-hexane was treated with CS2 (23 μL, 29 mg, 0.38 mmol). The brown mixture was stirred at room temperature. Red single crystals were obtained from the solution at room temperature (91 mg, 38%). Compound 6 decomposed slowly in the solid state and in solution. Therefore, we did not observe satisfactory analytical data, and the 13C NMR spectra showed impurities of unknown substances. Mp (argon, sealed capillary): 105 °C dec. 1H NMR (C6D6, 300 K): δ 1.10 (s, 18H, GatBu2), 1.95 (s, 6H, p-CH3), 2.30 (s, 12H, o-CH3), 6.00 [d, 4JPH = 3.7 Hz, 4H, m-H(Mes)], 7.05 [t, 3JHH = 7.5 Hz, 1H, pH(Ph)], 7.14 [pseudo-t, 3JHH = 7.5 Hz, 2H, m-H(Ph)], 7.33 [d, 3JHH = 7.5 Hz, 2H, o-H(Ph)], 7.62 (d, 3JPH = 33.4 Hz, 1H, PCCH). 13C NMR (C6D6, 300 K): δ 20.8 (s, p-CH3), 24.9 (d, 3JPC = 3.7 Hz, oCH3), 25.7 (d, 3JPC = 4.1 Hz, Ga(CMe3)2), 32.5 (s, Ga(CMe3)2), 121.3 [d, 1JPC = 72.3 Hz, ipso-C(Mes)], 128.1 [s, o-C(Ph)], 129.4 [s, mC(Ph)], 129.7 [s, p-C(Ph)], 132.6 [d, 3JPC = 11.0 Hz, m-C(Mes)], 139.5 (d, 1JPC = 8.0 Hz, PCCH), 140.8 [d, 3JPC = 29.2 Hz, ipsoC(Ph)], 142.9 [d, 4JPC = 2.9 Hz, p-C(Mes)], 145.0 [d, 2JPC = 8.5 Hz, oC(Mes)], 156.4 (d, 2JPC = 2.2 Hz, PCCH), 238.4 (d, 1JPC = 49.4 Hz, SCS). 31P{1H} NMR (C6D6, 300 K): δ 33.6. MS (EI, 20 EV, 353 K): m/z (%) 554 (3) [3+]. Reaction of Dimethylamine−Borane with FLP 3: NMR Experiment. H3B←NHMe2 (2 mg, 0.04 mmol) and 3 (20 mg, 0.04 mmol) were dissolved in 0.6 mL of C6D6 in a Young NMR tube. Full conversion to the dimeric amidoborane (H2B−NMe2)2 was observed after 22 h at room temperature. The NMR spectra showed the resonances of unchanged FLP 3 and the amidoborane. 1H NMR (C6D6, 300 K): δ 1.05 (s, 18H, GatBu2, 3), 2.09 (s, 6H, p-CH3, 3), 2.22 (s, 6H, NMe2, amidoborane), 2.51 (s, 12H, o-CH3, 3), 3.02 (q, 1JBH = 113.3 Hz, 2H, BH2, amidoborane), 6.77 (d, 4JPH = 2.8 Hz, 4H, m-HMes, 3), 6.81 (pseudo-d, 3JHH = 7.5 Hz, 1H, p-HPh, 3), 6.92 (pseudo-t, 3JHH = 7.5 Hz, 2H, o-HPh, 3), 7.03 (pseudo-t, 3JHH = 7.5 Hz, 2H, m-HPh, 3), 7.40 (d, 3JPH = 16.8 Hz, 1H, PCCH, 3). 31P{1H} NMR (C6D6, 300 K): δ −14.2 (3). 11B{1H} NMR (C6D6, 300 K): δ 5.45 (s, BH2, amidoborane). Reaction of 3 with the Ammonia−Borane Adduct: Synthesis of 7. A solution of 3 (0.20 g, 0.36 mmol) in 5 mL of toluene was treated with the ammonia−borane adduct (13 mg, 0.42 mmol) at room temperature. The mixture was stirred overnight at room temperature. The solvent was removed in vacuo, and the residue was recrystallized from n-pentane at 2 °C (90 mg, 43%). Small quantities of uncoordinated FLP 3 were observed in the NMR spectra and may indicate the beginning of secondary reactions. Mp (argon, sealed capillary): 141 °C. 1H NMR (C6D6, 300 K): δ 1.16 (s, 18H, GatBu2), 1.23 (d, 3JPH = 15.0 Hz, 2H, NH2), 1.99 (s, 6H, p-CH3), 2.33 (s, 12H, o-CH3), 3.19 (s br, 2H, BH2), 6.64 [d, 4JPH = 2.8 Hz, 4H, m-H(Mes)], 7.03 [d, 3JHH = 7.6 Hz, 1H, o-H(Ph)], 7.12 [pseudo-t overlap, 2H, mH(Ph)], 7.41 [d, 3JHH = 7.9 Hz, 2H, o-H(Ph)], 7.97 (d, 3JPH = 35.7 Hz, 1H, PCCH). 13C NMR (C6D6, 300 K): δ 20.8 (d, 5JPC = 1.1 Hz, pCH3), 22.5 [d, 3JPC = 5.2 Hz, Ga(CMe3)2], 25.3 (d, 3JPC = 4.2 Hz, oCH3), 32.7 [s, Ga(CMe3)2], 128.5 [d, 4JCP = 1.5 Hz, o-C(Ph)], 128.8 [d, 1JPC = 46.6 Hz, ipso-C(Mes)], 128.95 [s, m-C(Ph)], 129.01 [s, pC(Ph)], 131.4 [d, 3JPC = 8.7 Hz, m-C(Mes)], 140.1 [d, 4JPC = 2.5 Hz, p-C(Mes)], 142.3 [d, 3JPC = 24.3 Hz, ipso-C(Ph)], 142.5 [d, 2JPC = 8.4 Hz, o-C(Mes)], 145.4 (d, 1JPC = 21.7 Hz, PCCH), 157.4 (d, 2JPC = 7.9 Hz, PCCH). 31P{1H} NMR (C6D6, 300 K): δ 19.6. 11B{1H} NMR (C6D6, 300 K): δ −12.0. IR (KBr pellets, cm−1): 3356 m, 3306 m ν(NH2); 3080 vw, 3059 vw, 3022 m, 2957 s, 2928 s, 2911 s, 2864 s, 2826 vs, 2760 w, 2731 vw, 2698 w ν(C−H); 2426 s, 2351 m ν(BH2); 2170 vw, 1969 vw, 1948 vw, 1803 vw, 1738 w, 1722 w, 1653 w, 1603 s, 1547 s phenyl, ν(CC, δ(NH2); 1487 m, 1464 s, 1447 s, 1400 m, 1379 m, 1356 m, 1333 vw, 1286 w, 1246 w δ(CH3), ν(BN); 1190 m

were calculated for the B- or In-based FLPs. In order to increase the acceptor strength of boron-based FLPs, they usually contain electron-withdrawing substituents such as pentafluorophenyl groups attached to boron. In accordance with these results adducts of Al FLPs with various substrates such as CO2 and amidoboranes are comparably inert and do not show secondary reactions in many cases. They may be considered as thermodynamic sinks. In contrast, adducts of the Ga FLP 3 are less stable. The CO2 complex was detectable by NMR spectroscopy only at low temperature, and benzaldehyde coordinated to 3 only via a Ga−O bond. The polarizing capability of gallium atoms is obviously not strong enough to activate the CO bonds, to increase the positive charge at the carbonyl carbon atoms, and to facilitate P−C interactions. For the same reason, the H2B−NH2 adduct of 3 shows a unique reactivity. In contrast to the comparable adduct of 1 it dissociates in solution, and the secondary products react further by H2 elimination. Small quantities of 3 are sufficient to catalyze hydrogen transfer reactions or dehydrocoupling with formation of oligomeric or polymeric B−N compounds. These results will stimulate systematic investigations into the generation of FLPs with finely adjusted properties, which by systematic exchange of the Lewis acidic B, Al, Ga, or In atoms may help in finding optimized systems for a specific application in various transformations.

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EXPERIMENTAL SECTION

General Considerations. All procedures were carried out under an atmosphere of purified argon in dried solvents (n-hexane and cyclopentane with LiAlH4; toluene with Na/benzophenone). NMR spectra were recorded in C6D6 at ambient probe temperature using the Bruker instruments Avance I (1H, 400.13 MHz; 13C, 100.62 MHz; 31P, 161.98 MHz; 11B, 128 MHz) and Avance III (1H, 400.03 MHz; 13C, 100.59 MHz; 31P, 161.92 MHz) and referenced internally to residual solvent resonances (chemical shift data in δ). 13C NMR spectra were all proton decoupled. IR spectra were recorded as paraffin mulls between CsBr plates or as KBr pellets on a Shimadzu Prestige 21 spectrometer. The Al/P-based FLP Mes2P−C(CH−Ph)−Al(CMe3)2 (1) and the Ga/P-based FLP Mes2P−C(CH−Ph)− Ga(CMe3)2 (3) were obtained according to literature procedures.5a,13 The assignment of NMR spectra is based on HMBC, H,H-ROESY, HSQC, and DEPT135 data. Reaction of 3 with Benzaldehyde: Synthesis of 5. A solution of 3 (0.13 g, 0.23 mmol) in 2 mL of cyclopentane was treated with benzaldehyde (20 μL, 24 mg, 0.23 mmol). The mixture was stirred overnight to afford a red solution. Cooling of the solution to −40 °C yielded the pure compound 5 as a yellow solid (45 mg, 30%). Attempts at recrystallization from several polar or nonpolar solvents failed. Mp (argon, sealed capillary): 97 °C dec. 1H NMR (C6D6, 300 K): δ 1.15 (s, 18H, GatBu2), 2.05 (s, 6H, p-CH3), 2.42 (s, 12H, oCH3), 6.71 [d, 4JPH = 2.7 Hz, 4H, m-H(Mes)], 6.96 [t, 3JHH = 7.4 Hz, 1H, p-H(Ph)], 6.98 [pseudo-t overlap, 3JHH = 7.1 Hz, 2H, m-H(O CPh)], 7.01 [d overlap, 3JHH = 7.4 Hz, 2H, o-H(Ph)], 7.05 [m overlap, 1H, p-H(OCPh)], 7.07 [pseudo-t, 3JHH = 7.4 Hz, 2H, m-H(Ph)], 7.48 [d, 3JHH = 7.1 Hz, 2H, o-H(OCPh)], 7.55 (d, 3JPH = 21.0 Hz, 1H, PCCH), 8.96 (s, 1H, OCH). 13C NMR (C6D6, 300 K): δ 20.9 (s, p-CH3), 24.4 (s, o-CH3), 28.1 (d, 3JPC = 1.8 Hz, Ga(CMe3)2), 31.0 (d, 4JPC = 1.4 Hz, Ga(CMe3)2), 125.1 [d, 4JPC = 1.3 Hz, o-C(Ph)], 128.0 [s, p-C(Ph)], 128.6 [s, m-C(OCPh)], 129.3 [s, o-C(O CPh)], 130.1 [s, m-C(Ph)], 130.2 [d, 1JPC = 7.1 Hz, ipso-C(Mes)], 130.5 [d, 3JPC = 4.8 Hz, m-C(Mes)], 132.5 [s, p-C(OCPh)], 138.1 [s, ipso-C(OCPh)], 139.1 [d, 4JPC = 0.8 Hz, p-C(Mes)], 143.3 [d, 2 JPC = 12.9 Hz, o-C(Mes)], 145.0 [d, 3JPC = 16.6 Hz, ipso-C(Ph)], 145.4 (s, PCCH), 155.6 (d, 1JPC = 46.7 Hz, PCCH), 168.9 (s, CO). 31P{1H} NMR (C6D6, 300 K): δ −4.3. IR (KBr pellets, cm−1): 3052 m, 3025 m, 2952 s, 2929 s, 2916 s, 2865 s, 2813 vs, 2752 m, 2692 m ν(C−H); 1945 w, 1893 vw, 1743 vw, 1720 vw, 1702 w, 1654 vw, G


Organometallics

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δ(BH2); 1130 m, 1080 s, 1028 m, 959 s, 924 m, 883 w, 858 s, 810 m, 793 s, 746 vs ν(CC); 696 s, 611 s phenyl; 584 w, 559 m, 530 w, 519 vw, 478 m, 455 s, 411 m ν(PC), ν(PN), ν(PB), ν(GaC). MS (EI, 20 EV, 393 K): m/z (%) 554 (3) [3+] Anal. Calcd for C34H50BGaNP (584.3): C, 69.9; H, 8.6, N, 2.4. Found: C, 69.3; H, 8.6, N, 2.3. Reaction of Ammonia−Borane and Benzylidene-tert-butylamine in the Presence of 4 mol % of FLP 3. Ammonia−borane H3B←NH3 (24 mg, 0.78 mmol) and benzylidene-tert-butylamine (0.14 mL, 0.13 mg, 0.78 mmol) were dissolved in 0.6 mL of d8-toluene and treated with 4 mol % of FLP 3 (8 mg, 0.014 mmol). The reaction was monitored for 21 days at room temperature. See the Results and Discussion for details. Reaction of Ammonia−Borane with 4 mol % of FLP 3. A suspension of ammonia−borane H3B←NH3 (40 mg, 1.3 mmol) in 8 mL of toluene was treated with 4 mol % of FLP 3 (36 mg, 0.06 mmol) in 2 mL of toluene and monitored for 18 days at room temperature. See the Results and Discussion for details. Synthesis of Mes2P−C(CH−Ph)−Ga(CMe3)2(OPEt3) (8). Compound 3 (0.22 g, 0.40 mmol) and triethylphosphine oxide (54 mg, 0.40 mmol) were dissolved in 3 mL of toluene and stirred overnight at room temperature. Single crystals of 8 were obtained from a concentrated solution at −15 °C (0.15 g, 54%). Mp (argon, sealed capillary): 171 °C. 1H NMR (C6D6, 300 K): δ 0.61 (dt, 3JPH = 17.0 Hz, 3JHH = 7.7 Hz, 9H, CH2CH3), 1.13 (dq, 2JPH = 13.0 Hz, 3JHH = 7.7 Hz, 6H, CH2CH3), 1.35 (s, 18H, GatBu2), 2.11 (s, 6H, p-CH3), 2.57 (s, 12H, o-CH3), 6.73 [d, 4JPH = 2.0 Hz, 4H, m-H(Mes)], 7.00 [t, 3JHH = 7.4 Hz, 1H, p-H(Ph)], 7.14 [pseudo-t, 3JHH = 7.4 Hz, 2H, m-H(Ph)], 7.34 [d, 3JHH = 7.4 Hz, 2H, o-H(Ph)], 7.57 (d, 3JPH = 22.8 Hz, 1H, PCCH). 13C{1H} NMR (C6D6, 300 K): δ 6.2 (d, 2JPC = 4.0 Hz, CH2CH3), 19.2 (d, 1JPC = 1.13 Hz, CH2CH3), 20.9 (s, p-CH3), 24.2 (d, 3 JPC = 13.6 Hz, o-CH3), 25.2 [s br, Ga(CMe3)2], 33.0 [s, Ga(CMe3)2], 126.7 [s, p-C(Ph)], 127.7 [s, o-C(Ph)], 128.6 [s, m-C(Ph)], 130.0 [d, 3 JPC = 2.9 Hz, m-C(Mes)], 135.4 [d, 1JPC = 29.1 Hz, ipso-C(Mes)], 137.0 [s, p-C(Mes)], 143.7 [d, 2JPC = 15.0 Hz, o-C(Mes)], 144.4 (s, PCCH), 144.7 [d, 3JPC = 12.3 Hz, ipso-C(Ph)], 154.5 (d, 1JPC = 78.0 Hz, PCCH). 31P{1H} NMR (C6D6, 300 K): δ −2.7 (s br, PCGa), 58.4 (s br, OPEt3). IR (CsBr plates, paraffin, cm−1): 1578 s, 1558 s phenyl, ν(CC); 1456 vs, 1402 w, 1375 s (paraffin); 1339 sh, 1306 s, 1281 s, 1242 s δ(CH3); 1209 m, 1168 m, 1153 m, 1118 s, 1074 s, 1045 m, 1028 m, 1015 m, 978 m, 966 m, 934 m, 920 m, 851 m, 810 m, 770 m ν(PO), ν(CC); 721 s (paraffin); 698 w, 644 w phenyl; 602 w, 559 m, 521 w, 484 m, 449 m, 397 w, 361 w, 302 m δ(CC), ν(GaC), ν(PC), ν(GaO). MS (EI, 20 EV, 393 K): m/z (%) 554 (3) [2+]. Anal. Calcd for C40H61GaOP2 (689.6): C, 69.7; H, 8.9. Found: C, 69.6; H, 8.7. Synthesis of Mes2P−C(CH−Ph)−Al(CMe3)2(OPEt3) (8a): NMR Experiment. Compound 1 (20 mg, 0.04 mmol) and triethylphosphine oxide (5 mg, 0.04 mmol) were dissolved in 0.6 mL of C6D6 in a Young NMR tube. 1H NMR (C6D6, 300 K): δ 0.52 (dt, 4JPH = 18.0 Hz, 3JHH = 7.7 Hz, 9H, CH2CH3), 0.86 (dq br, 3JHH = 7.7 Hz, 6H, CH2CH3), 1.29 (s, 18H, AltBu2), 2.11 (s, 6H, p-CH3), 2.56 (s, 12H, o-CH3), 6.73 [d, 4JPH = 2.5 Hz, 4H, m-H(Mes)], 7.02 [t, 3 JHH = 7.5 Hz, 1H, p-H(Ph)], 7.19 [pseudo-t, 3JHH = 7.5 Hz, 2H, mH(Ph)], 7.51 [d, 3JHH = 7.5 Hz, 2H, o-H(Ph)], 7.61 (d, 3JPH = 27.8 Hz, 1H, PCCH). 31P{1H} NMR (C6D6, 300 K): δ −0.5 (s, PCGa), 66.9 (s, OPEt3). X-ray Crystallography. Crystals suitable for X-ray crystallography were obtained from n-hexane (6), n-pentane (7), or toluene (8). Intensity data were collected on Bruker Quazar and D8-Venture diffractometers with monochromated Mo Kα radiation. The collection method involved ω scans. Data reduction was carried out using the program SAINT+.46 The crystal structures were solved by Direct Methods using SHELXTL.47 Non-hydrogen atoms were first refined isotropically followed by anisotropic refinement by full matrix leastsquares calculation based on F2 using SHELXTL.47 Hydrogen atoms were positioned geometrically and allowed to ride on their respective parent atoms. Further details of the crystal structure determinations are available from the Cambridge Crystallographic Data Center on quoting the depository numbers CCDC 1817325 (6), 1817324 (7), and 1817326 (8).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00075. Results of quantum chemical calculations and NMR spectra of compounds 5−8 (PDF) Cartesian coordinates of the calculated structures (XYZ) Accession Codes

CCDC 1817324−1817326 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for W.U.: [email protected]. ORCID

Werner Uhl: 0000-0002-7178-5517 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft (IRTG 2027) for generous financial support. REFERENCES

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Organometallics

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