Differences in the Reactivity of Geminal Si-O-P and Al-O-P Frustrated Lewis Pairs

Abstract

The new oxygen-bridged geminal Si/P Frustrated Lewis Pair (FLP) tBu₂ P-O-Si(C₂ F₅ )₃ (2) is able to reversibly bind carbon dioxide at ambient temperature. We compared its reactivity towards benzil, but-3-en-2-one, nitriles and phenylacetylene to that of the Al/P FLP tBu₂ P-O-AlBis₂ (Bis=-CH(SiMe₃ )₂ ) (1). When reacted with benzil, both, 1 and 2, form the 1,2-addition product, but in the Si/P FLP 2, the second carbonyl function additionally binds to the silicon atom. With but-3-en-2-one 2 forms the 1,2-addition product, while 1 binds in 1,4-position. The reaction with acetonitrile yielded an unexpected etheneimine adduct for both systems, while only 1 reacted with tert-butylnitrile. With benzonitrile and acrylonitrile, 2 showed reversible addition to the C≡N bond and 1 forms a stable adduct with benzonitrile. Solely 1 shows reactivity towards phenylacetylene affording a mixture of the CH deprotonation adduct tBu₂ P(H)-O-AlBis₂ (CCPh) and the FLP -C≡C 1,2-addition adduct under ring formation. All compounds were characterized by multinuclear NMR spectroscopy, XRD and elemental analysis.

Keywords: activation; aluminum; frustrated Lewis Pairs; silicon; small molecules.

Scheme 1

Synthesis of FLP 2 from lithiated di‐tert‐butylphosphane oxide and chlorotris(pentafluoroethyl)silane.

Figure 1

Molecular structure of 2 in the crystalline state. Ellipsoids are set at 50 % probability. Hydrogen atoms and the minor occupied parts of the disordered C₂F₅ groups were omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−O(1) 1.708(1), P(1)−C(7) 1.871(2), P(1)−C(11) 1.867(2), Si(1)−O(1) 1.583(1), Si(1)−C(1) 1.943(2), Si(1)−C(3) 1.930(2), Si(1)−C(5) 1.946(2); O(1)−P(1)−C(7) 98.8(1), O(1)−P(1)−C(11) 99.0(1), C(11)−P(1)−C(7) 111.0(1), O(1)−Si(1)−C(1) 113.0(1), O(1)−Si(1)−C(3) 110.9(1), O(1)−Si(1)−C(5) 108.0(1), Si(1)−O(1)−P(1) 142.0(1).

Scheme 2

Reversible, temperature‐dependent binding of CO₂ by 2.

Figure 2

Stacked ¹H NMR spectra for the reaction of FLP 2 with CO₂ in a closed NMR tube under an atmosphere of CO₂ (1 bar). a) free tBu₂P−O−Si(C₂F₅) ₃ (*) in CD₂Cl₂, b) after addition of CO₂, the adduct 3 (+) is formed at 293 K and c) sample cooled to 273 K. An excess of CO₂ is present in b) and c).

Figure 3

Molecular structure of 3 in the solid state. Ellipsoids are set at 50 % probability; hydrogen atoms were omitted for clarity. Only one of two independent molecules of the asymmetric unit is shown. Selected bond lengths [Å] and angles [°]: P(1)−O(1) 1.576(1), P(1)−C(15) 1.860(2), Si(1)−O(1) 1.730(1), Si(1)−O(2) 1.798(1), Si(1)−C(1) 1.982(2), Si(1)−C(5) 2.013(2), O(2)−C(15) 1.317(2), O(3)−C(15) 1.199(2); O(1)−P(1)−C(15) 96.1(1), O(1)−Si(1)−O(2) 87.8(1), O(1)−Si(1)−C(1) 132.3(1), O(1)−Si(1)−C(3) 119.3(1), O(1)−Si(1)−C(5) 85.9(1), P(1)−C(15)−O(3) 125.5(1), O(2)−C(15)−O(3) 127.0(1), P(1)−O(1)−Si(1) 123.6(1).

Scheme 3

Reaction of 1 and 2 with benzil and proposed weak coordination by the second carbonyl function.

Figure 4

Molecular structure of 4 (left) and 5 (right) in the solid state. Hydrogen atoms were omitted for clarity; ellipsoids are set at 50 %. Selected bond lengths [Å] and angles [°]: 4: P(1)−O(3) 1.549(1), P(1)−C(2) 1.950(1), Si(1)−O(1) 2.014(1), Si(1)−O(2) 1.751(1), Si(1)−O(3) 1.833(1), O(1)−C(1) 1.247(2), O(2)−C(2) 1.403(1), C(1)−C(2) 1.547(2), O(3)−P(1)−C(2) 94.6(1), O(2)−Si(1)−O(1) 80.1(1), O(2)−Si(1)−O(3) 85.5(1), O(3)−Si(1)−O(1) 87.8(1), C(1)−O(1)−Si(1) 113.4(1), C(2)−O(2)−Si(1) 112.7(1), P(1)−O(3)−Si(1) 119.5(1), O(1)−C(1)−C(2) 110.5(1), O(2)−C(2)−P(1) 98.4(1), O(2)−C(2)−P(1) 98.4, C(1)−C(2)−P(1) 104.4(1). 5: P(1)−O(1) 1.548(1), P(1)−C(23) 1.933(1), Al(1)−O(1) 1.853(1), Al(1)−O(2) 1.808(1), O(2)−C(23) 1.387(1), O(3)−C(24) 1.224(1), C(23)−C(24) 1.535(1); O(1)−P(1)−C(23) 98.9(1), O(2)−Al(1)−O(1) 89.4(1), P(1)−O(1)−Al(1) 118.1(1), C(23)−O(2)−Al(1) 123.1(1), O(2)−C(23)−P(1) 102.8(1), O(2)−C(23)−C(24) 116.1(1), C(24)−C(23)−P(1) 106.8(1), O(3)−C(24)−C(23) 116.5(1).

Scheme 4

Different coordination behaviors of FLP 1 and 2 towards but‐3‐en‐2‐one to form the 1,2‐ (6) and 1,4‐ (7) addition products.

Figure 5

Molecular structure of 6 in the solid state. Ellipsoids are set at 50 % probability, hydrogen atoms and the minor occupied disordered positions in the tBu and C₂F₅ groups were omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−O(1) 1.561(1), P(1)−C(1) 1.907(2), Si(1)−O(1) 1.807(1), Si(1)−O(2) 1.676(1), O(2)−C(1) 1.421(2), C(1)−C(2) 1.504(2), C(1)−C(4) 1.534(3), C(2)−C(3) 1.315(3); O(1)−P(1)−C(1) 97.4(1), O(2)−Si(1)−O(1) 87.7(1), P(1)−O(1)−Si(1) 121.6(1), C(1)−O(2)−Si(1) 127.9(1), O(2)−C(1)−P(1) 100.8(1), O(2)−C(1)−C(2) 111.3(2), C(3)−C(2)−C(1) 124.7(2).

Figure 6

Molecular structure of 7 in the solid state. Ellipsoids are set at 50 % probability; hydrogen atoms and solvent benzene molecules were omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−O(1) 1.534(1), P(1)−C(23) 1.812(1), Al(1)−O(1) 1.847(1), Al(1)−O(2) 1.755(1), O(2)−C(25) 1.336(2), C(25)−C(26) 1.502(2), C(23)−C(24) 1.508(2), C(24)−C(25) 1.338(2); O(1)−(P(1)−C(23) 109.3(1), O(2)−Al(1)−O(1) 101.4(1), P(1)−O(1)−Al(1) 132.3(1), C(25)−O(2)−Al(1) 145.2(1), C(24)−C(23)−P(1) 117.6(1), C(25)−C(24)−C(23) 125.3(1), O(2)−C(25)−C(24) 125.2(1), O(2)−C(25)−C(26) 112.8(1).

Scheme 5

Reactions of 1 and 2 with aceto‐, pivalo‐ benzo‐ and acrylonitrile.

Figure 7

Molecular structure of 8 (left) and 9 (right) in the solid state. Hydrogen atoms of the tBu groups were omitted for clarity. Ellipsoids are set at 50 % except for hydrogen atoms (20 %). Selected bond lengths [Å] and angles [°]: 8: P(1)−O(1) 1.556(1), P(1)−C(1) 1.800(1), Si(1)−O(1) 1.799(1), Si(1)−N(1) 1.755(1), N(1)−C(1) 1.396(1), C(1)−C(2) 1.340(1); C(1)−P(1)−O(1) 97.9(1), N(1)−Si(1)−O(1) 86.7(1), Si(1)−O(1)−P(1) 122.6(1), C(1)−N(1)−Si(1) 124.3(1), N(1)−C(1)−P(1) 106.6(1), C(2)−C(1)−P(1) 126.3(1), C(2)−C(1)−N(1) 127.0(1); 9: [°]: P(1)−O(1) 1.540(1), P(1)−C(1) 1.824(1), Al(1)−O(1) 1.854(1), Al(1)−N(1) 1.898(1), N(1)−C(1) 1.372(1), C(1)−C(2) 1.357(1); C(1)−P(1)−O(1) 103.1(1), N(1)−Al(1)−O(1) 88.3(1), Al(1)−O(1)−P(1) 119.1(1), C(1)−N(1)−Al(1) 121.1(1), N(1)−C(1)−P(1) 108.3(1), C(2)−C(1)−P(1) 122.5(1), C(2)−C(1)−N(1) 129.3(1).

Figure 8

Molecular structure of 10 in the solid state. Ellipsoids are set at 50 % probability; hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−O(1) 1.623(1), Al(1)−O(1) 1.757(1), Al(1)−N(1) 2.005(1), N(1)−C(1) 1.145(1), C(1)−C(2) 1.475(1); O(1)−Al(1)−N(1) 98.3(1), P(1)−O(1)−Al(1) 133.6(1), C(1)−N(1)−Al(1) 177.4(1), N(1)−C(1)−C(2) 179.1(1).

Scheme 6

Reaction of 1 with phenyl acetylene to form 14 a and 14 b (ratio about 85 : 15, determined by integration of the ¹H NMR signals).

Figure 9

Molecular structure of 14 a in the solid state. Ellipsoids are set at 50 % probability; hydrogen atoms except H(1) and the minor occupied disordered OPHtBu₂ group were omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−O(1) 1.518(2), Al(1)−O(1) 1.864(2), Al(1)−C(23) 1.963(1), C(23)−C(24) 1.223(1), C(24)−C(25) 1.428(1); C(23)−Al(1)−O(1) 104.4(1), Al(1)−O(1)−P(1) 168.8(2), C(24)−C(23)−Al(1) 172.2(1), C(25)−C(24)−C(23) 177.7(1).

Figure 10

Molecular structure of 14 b in the solid state. Ellipsoids are set at 50 % probability; hydrogen atoms were omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−O(1) 1.542(1), P(1)−C(23) 1.819(2), Al(1)−O(1) 1.858(1), Al(1)−C(24) 2.015(2), C(23)−C(24) 1.350(2), C(23)−C(25) 1.502(2); O(1)−P(1)−C(23) 104.0(1), O(1)−Al(1)−C(24) 87.7(1), P(1)−O(1)−Al(1) 119.5(1), C(24)−C(23)−P(1) 111.0(1), C(24)−C(23)−C(25) 123.3(2), C(25)−C(23)−P(1) 125.6(1), C(23)−C(24)−Al(1) 117.7(1).