A Mesoionic Diselenolene Anion and the Corresponding Radical Dianion

Abstract

Dichalcogenolenes are archetypal redox non-innocent ligands with numerous applications. Herein, a diselenolene ligand with fundamentally different electronic properties is described. A mesoionic diselenolene was prepared by selenation of a C2-protected imidazolium salt. This ligand is diamagnetic, which is in contrast to the paramagnetic nature of standard dichalcogenolene monoanions. The new ligand is also redox-active, as demonstrated by isolation of a stable diselenolene radical dianion. The unique electronic properties of the new ligand give rise to unusual coordination chemistry. Thus, preparation of a hexacoordinate aluminum tris(diselenolene) complex and a Lewis acidic aluminate complex with two ligand-centered unpaired electrons was achieved.

Keywords: X-ray diffraction; aluminum complex; biradical; diselenolene; electronic structure; radical.

Figure 1

The three redox states of standard dichalcogenolene ligands and the mesoionic diselenolene ligands described herein (Dpt=4,6‐diphenyltriazinyl; Dipp=2,6‐diisopropylphenyl).

Scheme 1

Synthesis of the potassium salt 2 containing a diselenolene monoanion, and its molecular structure in the crystal (thermal ellipsoids at 50 % probability, hydrogen atoms are omitted for clarity, THF molecules are simplified as wireframes).

Figure 2

a) Surface diagram of the LUMO of 2 (isovalue=0.04; calculated at the B3LYP‐D3(BJ)/def2‐SVP level). b) Cyclic voltammogram of 2 at 0.1 V s⁻¹ scan rate in THF / 0.1 M [nBu₄N][PF₆].

Scheme 2

Synthesis of the potassium salt 3 containing a diselenolene radical dianion, and its molecular structure in the crystal (only two of the eight independent diselenolene dianions are shown, thermal ellipsoids at 50 % probability, hydrogen atoms are omitted for clarity).

Figure 3

a) Experimental (THF, 290 K) and simulated EPR spectra of the diselenolene radical dianion 3. b) Calculated spin density of the radical dianion (isovalue=0.003; calculated at the UB3LYP‐D3(BJ)/def2‐SVP level).

Scheme 3

Synthesis of the neutral aluminum bis(diselenolene) and tris(diselenolene) complexes 4 and 5.

Figure 4

Molecular structures of the complexes 4 and 5 in the crystal (thermal ellipsoids at 50 % probability, hydrogen atoms are omitted for clarity).

Scheme 4

Synthesis of the cationic bis(diselenolene) complex 6 and the anionic, paramagnetic bis(diselenolene) complex 7.

Figure 5

a) Molecular structure of complex 6 in the crystal (thermal ellipsoids at 50 % probability, hydrogen atoms and the [B(C₆F₅)₄]⁻ anion are omitted for clarity.

Figure 6

a) Molecular structure of complex 7 (thermal ellipsoids at 50 % probability, hydrogen atoms and K⁺[18‐C‐6] unit are omitted for clarity), and b) Calculated spin density of the open‐shell singlet ground state of 7 (α‐spin: green; β‐spin: orange; isovalue=0.003; calculated at the UB3LYP‐D3(BJ)/def2‐SVP level).