Reactivity of the Bicyclic Amido-Substituted Silicon(I) Ring Compound Si4 {N(SiMe3 )Mes}4 with FLP-Type Character

Abstract

The bicyclic amido-substituted silicon(I) ring compound Si₄ {N(SiMe₃ )Mes}₄ 2 (Mes=Mesityl=2,4,6-Me₃ C₆ H₂ ) features enhanced zwitterionic character and different reactivity from the analogous compound Si₄ {N(SiMe₃ )Dipp}₄ 1 (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ) due to the smaller mesityl substituents. In a reaction with the N-heterocyclic carbene NHC ${}^{\mathrm{Me}{}_4}$ (1,3,4,5-tetramethyl-imidazol-2-ylidene), we observe adduct formation to give Si₄ {N(SiMe₃ )Mes}₄ ⋅ NHC ${}^{\mathrm{Me}{}_4}$ (3). This adduct reacts further with the Lewis acid BH₃ to yield the Lewis acid-base complex Si₄ {N(SiMe₃ )Mes}₄ ⋅ NHC ${}^{\mathrm{Me}{}_4}$ ⋅ BH₃ (4). Coordination of AlBr₃ to 2 leads to the adduct 5. Calculated proton affinities and fluoride ion affinities reveal highly Lewis basic and very weak Lewis acidic character of the low-valent silicon atoms in 1 and 2. This is confirmed by protonation of 1 and 2 with Brookharts acid yielding 6 and 7. Reaction with diphenylacetylene only occurs at 111 °C with 2 in toluene and is accompanied by fragmentation of 2 to afford the silacyclopropene 8 and the trisilanorbornadiene species 9.

Keywords: N-heterocyclic carbenes; amines; frustrated Lewis pairs; inorganic ring systems; silicon.

Figure 1

Silicon‐based FLP system A and four‐membered silicon ring compounds 1, B–D. A similar compound to B with a different substituent is missing in Figure 1. A new Figure was generated and was sent together with this document. This also affects reference 3.

Scheme 1

Synthesis of 2 (R=Mesityl=2,4,6‐Me₃C₆H₂).

Figure 2

Molecular structure of 2 (hydrogen atoms are omitted for clarity and thermal ellipsoids are set at a 50 % probability level). Selected bond lengths/Å and angles/°: Si1−Si2 2.2375(6), Si2−Si3 2.2514(6), Si3−Si4 2.3617(6), Si1−Si4 2.3560(6), Si1−Si3 2.523(5), Si2⋅⋅⋅Si4 3.5633, Si1−N1 1.7267(13), Si2−N2 1.7105(13), Si3−N3 1.7221(12), Si4−N4 1.7912(13).

Figure 3

Kohn Sham molecular orbitals of 2 (TPSS−D3(BJ)/def2‐TZVP; isovalue set at ±0.04 a.u.; representation of 2 similar to that in Figure 2). Figure 3 has very small pictures of the molecular orbitals and small labels. Please replace it by the new Figure 3 that was sent together with this document

Scheme 2

Synthesis of 3 (R=Mesityl).

Figure 4

Molecular structure of 3 (hydrogen atoms are omitted for clarity, and thermal ellipsoids are set at the 50 % probability level). Selected bond lengths/Å and angles/°: Si1−Si2 2.3637(5), Si2−Si3 2.3679(5), Si3−Si4 2.3299(5), Si1−Si4 2.3195(5), Si1−Si3 2.3567(5), Si2⋅⋅⋅Si4 3.717(1), Si1−N1 1.7591(13), Si2−N2 1.7914(12), Si3−N3 1.7622(12), Si4−N4 1.8002(12), Si2−C25 1.943(2).

Scheme 3

Synthesis of 4 (R=Mesityl).

Figure 5

Molecular structure of 4 (hydrogen atoms are omitted for clarity, except those at B1, and thermal ellipsoids are set at the 50 % probability level). Selected bond lengths/Å and angles/°: Si1−Si2 2.3324(5), Si2−Si3 2.3728(5), Si3−Si4 2.3216(5), Si1−Si4 2.3395(5), Si1−Si3 2.4234(5), Si2⋅⋅⋅Si4 3.638(6), Si1−N1 1.7576(12), Si2−N2 1.7870(12), Si3−N3 1.7447(12), Si4−N4 1.7741(12), Si4−B1 2.0327(16), Si2−C25 1.9322(14).

Scheme 4

Synthesis of 5 (R=Mesityl).

Figure 6

Molecular structure of 5 (hydrogen atoms are omitted for clarity, and thermal ellipsoids are set at the 50 % probability level). Selected bond lengths/Å and angles/°: Si4−Al1 2.4899(10), Si1−Si2 2.2466(9), Si2−Si3 2.2678(9), Si3−Si4 2.3540(9), Si1−Si4 2.3690(9), Si1⋅⋅⋅Si3 2.691(9), Si1−N1 1.710(2), Si2−N2 1.704(2), Si3−N3 1.710(2), Si4−N4 1.755(2); Si2−Si1−Si4 94.42(3), Si1−Si2−Si3 73.18(3), Si2−Si3−Si4 94.28(3), Si3−Si4−Si1 69.47(3).

Scheme 5

Synthesis of 6 (R=Dipp=2,6‐iPr₂C₆H₃) and 7 (R=Mesityl).

Figure 7

Molecular structure of the cation in 7 (hydrogen atoms, except H4 and the counter‐anion, are omitted for clarity, and thermal ellipsoids are set at the 50 % probability level). Selected bond lengths/Å and angles/°: Si1−Si2 2.2413(10), Si2−Si3 2.2263(9), Si1−Si4 2.2997(9), Si3−Si4 2.3161(10), Si1⋅⋅⋅Si3 2.695(9), Si1−N1 1.686(2), Si2−N2 1.681(2), Si3−N3 1.689(2), Si4−N4 1.714(2); Si3−Si2−Si1 74.20(3), Si2−Si1−Si4 91.72(3), Si2−Si3−Si4 91.67(3), Si1−Si4−Si3 71.44(3).

Figure 8

Molecular structure of 8 and 9 (hydrogen atoms are omitted for clarity, and thermal ellipsoids are set at the 50 % probability level). Selected bond lengths/Å and angles/° for 5: C1−C1′ 1.354(5), C1−C2 1.462(4), Si1−N1 1.7135(2), Si1−C1 1.8045(3), Si2−N1 1.762(2), C1−C1′−Si1 67.97(8), C1−Si1−C1‘ 44.064(17); for 6: Si1−Si2 2.3339(12), Si1−C1 1.887(3), Si1−C8 1.889(3), C1−C8′ 1.359(4) Si2−C15 1.816(3), Si1−N1 1.724(3), Si3−N1 1.758(3).

Scheme 6

A) Reaction of 2 with diphenylacetylene at 111 °C. B) Proposed formal fragmentation of 2 into a silylene and a silacyclopropenylidene.

Scheme 7

Proposed formation of 9 (R=Mesityl).