Chiral Bis(tetrathiafulvalene)-1,2-cyclohexane-diamides

Abstract

Chiral bis(TTF) diamides have been obtained in good yields (54-74%) from 1,2-cyclohexane-diamine and the corresponding TTF acyl chlorides. The (R,R)-1 and (S,S)-1 enantiomers have been characterized by circular dichroism and the racemic form by single-crystal X-ray diffraction. The neutral racemic bis(TTF)-diamide shows the formation of a pincer-like framework in the solid state, thanks to the intramolecular S···S interactions. The chemical oxidation in a solution using FeCl₃ provides stable oxidized species, while the electrocrystallization experiments provided radical cation salts. In particular, single-crystal resistivity measurements on the racemic donor with AsF₆^- as a counterion demonstrate semiconductor behavior in this material. The DFT and TD-DFT calculations support the structural and chiroptical features of these new chiral TTF donors.

Keywords: DFT calculations; chirality; chiroptical properties; crystal structure determination; magnetic properties; tetrathiafulvalene.

Figure 1

Examples of chiral TTF-amides; 1 is described in the present work.

Scheme 1

Synthesis of compounds 1.

Figure 2

CD (a) and UV-Vis (b) spectra of compound 1 pair of enantiomers. Green line: (S,S) enantiomer, red line: (R,R) enantiomer.

Figure 3

UV-Vis absorption spectra of (rac)-1 during chemical oxidation by successive addition of FeCl₃ in DCM at room temperature.

Figure 4

Cyclic voltammogram of (rac)-1 in 0.1 M Bu₄NPF₆/CH₂Cl₂, Pt electrode, scan rate 100 mV s⁻¹. The E_1/2 values are shown for both reversible peaks.

Figure 5

Asymmetric unit structure of (rac)-1 (a) and top view of molecule A (b).

Figure 6

Packing of (rac)-1 (a) in the ac plane and the intra- and intermolecular S···S short contacts (b).

Figure 7

H bonding interaction between A- and B-type molecules.

Figure 8

View of crystals (a) and the partial structure of (rac)-1 in the radical cation salt with AsF₆⁻ (b). The anions were omitted due to insufficient crystallographic data.

Figure 9

Temperature dependence of the electrical resistivity (blue line) for a single crystal of the radical cation salt of (rac)-1 with AsF₆⁻. The red line is the fit to the data with a law of the type ρ = ρ₀exp(E_a/T).

Figure 10

Equilibrium geometries of the neutral compound (R,R)-1, optimized at the following DFT levels: (a) B3LYP/Def2-SVPD, (b) B3LYP-D3/Def2-SVPD.

Figure 11

Experimental and theoretical (TD-B3LYP/Def2-SVPD) CD and UV-Vis spectra for neutral compound (R,R)-1.

Figure 12

Most relevant frontier MOs involved in the monoelectronic excitations computed for compound (R,R)-1 at the B3LYP/Def2-SVPD level of theory. Brief description of the MOs: HOMO-3 to HOMO: π orbital on the two TTF units; LUMO and LUMO+1: π* orbitals on the two different arms of compound (R,R)-1; LUMO+2 and LUMO+3: σ* orbital on the TTF units.

Figure 13

NTOs computed for the neutral (R,R)-1 enantiomer, describing the following monoelectronic excitations: (a) S0 → S2, (b) S0 → S4, (c) S0 → S13, (d) S0 → S14, (e) S0 → S16 and (f) S0 → S25.