Molecular and Electronic Structure of the Square Planar Bis(o-amidobenzenethiolato)iron(III) Anion and Its Bis(o-quinoxalinedithiolato)iron(III) Analogue

Crystalline purple [PPh4][(FeL2)-L-III] (1), where L2- represents the closed-shell dianion of 4,6-di-tert-butyl-2[(pentafluorophenyl)amino]benzenethiol, has been synthesized from the reaction of H2L and FeBr2 (2:1) in acetonitrile with excess NEt3, careful, brief exposure of the solution to air, and addition of [PPh4]Br. The monoanion has been shown by X-ray crystallography to be square planar. The oxidation of 1 with 1 equiv of iodine produces the neutral species [Fel(L-center dot)(2)(2)(0) where (L-center dot)(1-) represents the one-electron oxidized pi radical anion of L2-. The reaction of H2L and PtCl2 (2:1) and NEt3 in CH3CN in the presence of air produced green, crystalline [Pt-II(L-center dot)2] (3). From temperature-dependent (2-300 K) magnetic susceptibility measurements, it was established that 1 possesses a central intermediate spin ferric ion (S-Fe = 3/2), whereas neutral 2 has a doublet ground state (S-t = 1/2) comprising an intermediate spin ferric ion coupled antiferromagnetically to two ligand pi radicals (L-center dot)(1-) (S-rad = 1/2). Complex 3 is diamagnetic. Almeida et al.'s complexes in ref 1, [N(n-Bu)(4)][Fe-III(qdt)(2)] (A), and [PPh4](2)[Fe-2(III)(qdt)(4)] (B), have been revisited. It is shown here that the square planar anion in mononuclear [Fe-III(qdt)(2)](-) also possesses an S-Fe = 3/2 ground state. The zero-field Mossbauer spectra of 1, 2, A, and B have been recorded and the molecular and electronic structures of all mononuclear iron species have been calculated by density functional theoretical methods. It is shown that the S = 3/2 ground state in 1 and A is lower in energy by 8.5 and 16.6 kcal mol(-1), respectively, than the S = 1/2 state.