Use of the TCNQF₄²⁻ Dianion in the Spontaneous Redox Formation of [Fe^III(L⁻)₂][TCNQF₄^⋅−]

Abstract

The reaction of [FeII(L.)2](BF4)2 with Li2TCNQF4 results in the formation of [FeIII(L−)2][TCNQF4.−] (1) where L. is the radical ligand, 4,4‐dimethyl‐2,2‐di(2‐pyridyl)oxazolidine‐N‐oxide and TCNQF4 is 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane. This has been characterised by X‐ray diffraction, Raman and Fourier transform infrared (FTIR) spectroscopy, variable‐temperature magnetic susceptibility, Mössbauer spectroscopy and electrochemistry. X‐ray diffraction studies, magnetic susceptibility measurements and Raman and FTIR spectroscopy suggest the presence of low‐spin FeIII ions, the anionic form (L−) of the ligand and the anionic radical form of TCNQF4; viz. TCNQF4.−. Li2TCNQF4 reduces the [FeII(L.)2]2+ dication, which undergoes a reductively induced oxidation to form the [FeIII(L−)2]+ monocation resulting in the formation of [FeIII(L−)2][TCNQF4.−] (1), the electrochemistry of which revealed four well‐separated, diffusion‐controlled, one‐electron, reversible processes. Mössbauer spectroscopy and electrochemical measurements suggest the presence of a minor second species, likely to be [FeII(L.)2][TCNQF42−].

[FeII(L•)2][TCNQF4•−]2: A Redox-Active Double Radical Salt

“…Assigning the oxidation and spin states of the [Fe II (L • ) 2 ] 2+ dication and confirmation of both the [TCNQF 4 ] •radical anion and the neutral radical form of the ligand L • , is well established and relies on a rigorous examination of the bond lengths and angles contained in both the cation [17][18][19][20][21][22][23] and anion [24][25][26] (Tables 1-3). The nitroxide N-O (N3-O1) bond length is 1.30 Å in 1 which strongly suggests the ligand (L • ) is in the neutral radical form as found previously when relating nitroxide bond lengths to the redox form of the NO group.…”

“…The nitroxide N-O (N3-O1) bond length is 1.30 Å in 1 which strongly suggests the ligand (L • ) is in the neutral radical form as found previously when relating nitroxide bond lengths to the redox form of the NO group. [17][18][19][20][21][22][23] Equally the [17] Full bond lengths with errors can be found in the CIF file. The sign of the torsion angle is positive if the bond A-B is rotated in a clockwise direction through less than 180° in order that it may eclipse the bond C-D: a negative torsion angle requires rotation in the opposite sense.…”

“…This is in comparison to the trigonal pyramidal geometry found when the ligand is in the hydroxylamino anionic form, L -. [23] The…”

“…Cyclic voltammetric data have been reported for [Fe III (L -) 2 ][TCNQF 4 •-] [23] and [Fe II (L • ) 2 ](BF 4 ) [17] and on this basis, processes I and III are assigned to reduction of the TCNQF 4 •anion to electrochemically reversible couple at 23 °C. [31] A steady-state voltammogram of 1 obtained at a Pt microdisc electrode over the potential range of -0.3 to +1.5 V vs Ag/Ag + is shown in Fig.…”

“…The TCNQF 4 2dianion was subsequently oxidised to its anionic radical form, TCNQF 4 •-, which then crystallised as [Fe III (L -) 2 ][TCNQF 4 •-]. [23] To confirm our initial hypothesis that the TCNQF 4 2- [17] , pre-dissolved in a mixture of acetonitrile and dichloromethane, crystalline product of [Fe II (L • ) 2 ][TCNQF 4 •-] 2 •2CH 3 CN (1) was indeed obtained in 69% yield (Fig. 1).…”