Reactions of an Anionic Gallylene with Azobenzene or Azide Compounds Through C(sp2)-H and C(sp3)-H Activation

Abstract

The activation of inert C-H bonds remains a challenge in current chemistry. Here, we report the excellent reactivity of the anionic gallylene species [LGa:][Na(THF)₃] (L = [(2,6-iPr₂C₆H₃)NC(CH₃)]₂^2-, 1) that allows the selective activation one ortho sp² C-H bond of several azobenzene and azide derivatives at ambient temperature, with the transfer of the hydrogen atom to one of the nitrogen atoms. The process leads to the formation of the aryl amido products [LGa-κ²N,C-PhNN(H)(p-R-C₆H₃)][Na(solvent)₃] (2, R = H solvent = DME (1,2-Dimethoxyethane); 3, R = -OMe, solvent = DME; 4, R = -NMe₂ solvent = THF), [LGa-κ²N,C-(m-CH₃-C₆H₄)NN(H)(m-CH₃-C₆H₃)][Na(15-C-5)₂] (5) with new Ga-C and Ga-N bonds. Moreover, 1 is also effective for the C-H activation of two azides RN₃ (R = 2,4,6-Me₃C₆H₂ or 2,6-iPr₂C₆H₃), resulting in the formation of gallium amides [LGa(NH-2-(CH₂)-4,6-Me₂C₆H₂)][Na(15-C-5)₂] (6) and [LGa(NH-2,6-iPr₂C₆H₃)₂][Na(THF)₅] (7) through intra- or intermolecular sp³ C-H amination. Significantly, these reactions occur for the highly challenging activation of inert C(sp²)-H and C(sp³)-H bonds, thus demonstrating the excellent reactivity of the Ga(I) species 1. The products 2-7 were characterized by X-ray crystallography, ¹H and ¹³C NMR, UV-vis spectroscopy, and density functional theory (DFT) calculations.

Keywords: C–H activation; azobenzene; gallylene; organic azides; α-diimine ligands.

Scheme 1

Examples of C−H bond activation at low-valent Al(II/I) centers. The blue bonds represent newly formed chemical bonds. (a) C–H activation of benzene by aluminyl anion [K{Al(NON)}]₂. (b,c) C–H activation of unactivated arenes (benzene, toluene, xylenes) and Cp*H by [(^DippNacNac)]Al(I). (d) C–H activation of pyridine by [LAl(THF)]₂.

Scheme 2

Examples of C–H bond activation at low-valent Ga(I) centers. The blue bonds represent newly formed chemical bonds. (a) Sequential oxidation/C–H activation on ^DippNacNacGa(I). (b) C–H activation of mrthyl in toluene by gallylene species [LGa:][Na(THF)₃] (1).

Scheme 3

Reaction of 1 with azobenzene derivatives to yield complexes 2–5. The blue bonds represent newly formed chemical bonds.

Figure 1

Molecular structure of 2. Thermal ellipsoid plot (30%). H atoms except N4–H, Na(DME)₃ counterion, and iPr groups on the ligands are omitted for clarity. The green bonds represent newly formed chemical bonds.

Figure 2

Molecular structure of 4. Thermal ellipsoid plot (30%). H atoms except N4–H and iPr groups on the ligands are omitted for clarity. The green bonds represent newly formed chemical bonds.

Figure 3

Molecular structure of 5. Thermal ellipsoid plot (30%). H atoms except N4–H and iPr groups on the ligands are omitted for clarity. The green bonds represent newly formed chemical bonds.

Scheme 4

Possible mechanism for the reaction of gallylenes with azobenzene. The blue bonds represent newly formed chemical bonds.

Scheme 5

Reaction of 1 with 2,4,6-trimethylphenylN₃ and 2,6-iPr₂C₆H₃N₃ to yield 6 and 7. The blue bonds represent newly formed chemical bonds.

Figure 4

Molecular structure of 6. Hydrogen atoms and iPr groups have been omitted. The green bonds represent newly formed chemical bonds. Selected bond lengths [Å] and angles [°]: Ga–N1 1.913(4), Ga–N2 1.912(4), N–C1 1.419(5), N2–C2 1.428(6), C1–C2 1.356(6), Ga–N3 1.929(4), Ga–C37 1.995(5), N3–C29 1.372(6), C29–C34 1.407(7), C34–C37 1.506(7), N3–H 0.880, N1-Ga-N3 116.06(17), N1-Ga-C37 128.6(2), N2-Ga-N1 86.86(16), N2-Ga-N3 118.46(17), N2-Ga-C37 122.73(19), N3-Ga-C37 87.6(2).

Figure 5

Molecular structure of 7. Thermal ellipsoid plot (30%). H atoms except N3–H and N4–H, Na(THF)₅ counterion, and the isopropyl groups are omitted for clarity. The green bonds represent newly formed chemical bonds. Selected bond lengths [Å] and angles [°]: Ga–N1 1.910(4), Ga–N2 1.915(4), N1–C1 1.417(4), N2–C2 1.424(5), C1–C2 1.353(5), Ga–N3 1.913(3), Ga–N4 1.908(4), N3–C29 1.397(5), N4–C41 1.401(4), N3–H3 0.8700, N4–H4 0.8700, N1-Ga-N2 88.05(19), N1-Ga-N3 118.07(14), N3-Ga-N2 107.3(2), N4-Ga-N1 116.2(2), N4-Ga-N2 123.07(15), N4-Ga-N3 104.3(2).