Radical Anions, Radical‐Anion Salts, and Anionic Complexes of 2,1,3‐Benzochalcogenadiazoles

Abstract

With cyclic voltammetry (CV) and DFT calculations, it was found that electron acceptor ability of the 2,1,3-benzochalcogenadiazoles 1-3 (chalcogen: S, Se and Te, respectively) increases with the atomic number of chalcogen. This trend is nontrivial since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1]*- and [2]*-, RA [3]*- was not detected by EPR under the CV conditions. Chemical reduction of 1-3 was performed and new thermally-stable RA salts [K(THF)]+[2]*- (8) and [K(18-crown-6)]+[2]*- (9) were isolated in addition to known salt [K(THF)]+[1]*- (7). Upon contact with air, RAs [1]*- and [2]*- underwent fast decomposition in solution with the formation of anions [ECN]- isolated in the form of salts [K(18-crown-6)]+[ECN]- (10, E = S; 11, E = Se). In the case of 3, RA [3]*- was detected by EPR to be the first representative of tellurium-nitrogen π-heterocyclic RAs but not isolated. Instead, salt [K(18-crown-6)]+2[3-Te2]2- (12) featuring a new anionic complex with the coordinate bond Te-Te was obtained. Upon contact with air, salt 12 transformed into salt [K(18-crown-6)]+2[3-Te4-3]2- (13) revealing an anionic complex with two coordinate bonds Te-Te. The structures of 8-13 were confirmed by X-ray diffraction and the nature of the coordinate bonds Te-Te in [3-Te2]2- and [3-Te4-3]2- was studied with DFT calculations and QTAIM analysis.

A Sterically Hindered Derivative of 2,1,3‐Benzotelluradiazole: A Way to the First Structurally Characterised Monomeric Tellurium–Nitrogen Radical Anion

“…The EPR spectrum recorded for 3 .− is less resolved and has very similar parameters ( g= 1.9952, a ( 14 N)=0.585 mT) [4] . DFT calculations predict for 3 .− the localisation of approximately 75 % spin density on the NTeN fragment and EPR parameters ( g= 1.9947, a ( 14 N)=0.512 mT) in good agreement with the experiment [4] …”

“…Thus, Figure 5 shows the room‐temperature EPR spectrum of 1 .− , which gives a five‐line pattern at g= 1.9948, originating from the HFI of an unpaired electron with two equivalent nitrogen nuclei with an HFI constant of a ( 14 N)=0.50 mT. The EPR spectrum recorded for 3 .− is less resolved and has very similar parameters ( g= 1.9952, a ( 14 N)=0.585 mT) [4] . DFT calculations predict for 3 .− the localisation of approximately 75 % spin density on the NTeN fragment and EPR parameters ( g= 1.9947, a ( 14 N)=0.512 mT) in good agreement with the experiment [4] …”

“…Thus, the EPR spectra unambiguously prove the radical‐anion nature of salt 2 and demonstrate the substantial spin population of the nitrogen and tellurium atoms in 1 .− Therefore, 1 .− is the first example of a structurally characterised monomeric radical anion containing a telluradiazole‐like TeN 2 fragment. As mentioned above, the radical anion 3 .− was recently detected by EPR spectroscopy, [4] but its isolation failed [6] . Several other reports claimed isolation of paramagnetic derivatives of trivalent tellurium [38] .…”

“…[4] To no surprise, therefore, the S-and Se-based radical anions of type B have been known sincet he 1960s, [5] whereas their first Te congener was characterised (by EPR spectroscopy in solution) only in 2019. [4] Ar ecent attempt [6] to isolate the RA of 2,1,3-benzotelluradiazole led to the crystallisation of ad iamagnetica dduct in which formally two RAs B are bondedt ot he neutralm olecule A by as trongT e ••••Nb ondingi nteraction (chalcogen bonding, ChB). [7] The formation of oligo-or polymeric structures due to ChB is another feature of chalcogenadiazoles.…”

“…However, the chemical properties of such compounds strongly depend on the nature of the chalcogen; for example, Te‐based RAs are extremely sensitive to oxygen and moisture, and decompose with breakdown of the heterocyclic rings [4] . To no surprise, therefore, the S‐ and Se‐based radical anions of type B have been known since the 1960s, [5] whereas their first Te congener was characterised (by EPR spectroscopy in solution) only in 2019 [4] . A recent attempt [6] to isolate the RA of 2,1,3‐benzotelluradiazole led to the crystallisation of a diamagnetic adduct in which formally two RAs B are bonded to the neutral molecule A by a strong Te ⋅⋅⋅⋅ N bonding interaction (chalcogen bonding, ChB) [7] .…”