Electronic Structure of Mononuclear Bis(1,2-diaryl-1,2-ethylenedithiolato)iron Complexes Containing a Fifth Cyanide or Phosphite Ligand:  A Combined Experimental and Computational Study

A series of mononuclear square-based pyramidal complexes of iron containing two 1,2-diaryl-ethylene-1,2-dithiolate ligands in various oxidation levels has been synthesized. The reaction of the dinuclear species [Fe^III₂(¹L^•)₂(¹L)₂]⁰, where (¹L)^2- is the closed shell di-(4-tert-butylphenyl)-1,2-ethylenedithiolate dianion and (¹L^•)^1- is its one-electron-oxidized π-radical monoanion, with [N(n-Bu)₄]CN in toluene yields dark green crystals of mononuclear [N(n-Bu)₄][Fe^II(¹L^•)₂(CN)] (1). The oxidation of 1 with ferrocenium hexafluorophosphate yields blue [Fe^III(¹L^•)₂(CN)] (1^ox), and analogously, a reduction with [Cp₂Co] yields [Cp₂Co][N(n-Bu)₄][Fe^II(¹L^•)(¹L)(CN)] (1^red); oxidation of the neutral dimer with iodine gives [Fe^III(¹L^•)₂I] (2). The dimer reacts with the phosphite P(OCH₃)₃ to yield [Fe^II(¹L^•)₂{P(OCH₃)₃}] (3), and [Fe^III₂(³L^•)₂(³L)₂] reacts with P(OC₆H₅)₃ to give [Fe^II(³L^•)₂{P(OC₆H₅)₃}] (4), where (³L)^2- represents 1,2-diphenyl-1,2-ethylenedithiolate(2−). Both 3 and 4 were electrochemically one-electron oxidized to the monocations 3^ox and 4^ox and reduced to the monoanions 3^red and 4^red. The structures of 1 and 4 have been determined by X-ray crystallography. All compounds have been studied by magnetic susceptibility measurements, X-band EPR, UV−vis, IR, and Mössbauer spectroscopies. The following five-coordinate chromophores have been identified:  (a) [Fe^III(L^•)₂X]ⁿ, X = CN^-, I^- (n = 0) (1^ox, 2); X = P(OR)₃ (n = 1+) )3^ox, 4^ox) with S_t = ¹/₂, S_Fe = ³/₂; (b) [Fe^II(L^•)₂X]ⁿ, X = CN^-, (n = 1−) (1); X = P(OR)₃ (n = 0) (3, 4) with S_t = S_Fe = 0; (c) [Fe^II(L^•)(L)X]ⁿ ↔ [Fe^II(L)(L^•)X]ⁿ, X = CN^- (n = 2−) (1^red); X = P(OR)₃ (n = 1−) (3^red, 4^red) with S_t = ¹/₂, S_Fe = 0 (or 1). Complex 1^ox displays spin crossover behavior:  S_t = ¹/₂ ↔ S_t = ³/₂ with intrinsic spin-state change S_Fe = ³/₂ ↔ S_Fe = ⁵/₂. The electronic structures of 1 and 1^ox have been established by density functional theoretical calculations:  [Fe^II(¹L^•)₂(CN)]^1- (S_Fe = 0, S_t = 0) and [Fe^III(¹L^•)₂(CN)]⁰ (S_Fe = ³/₂, S_t = ¹/₂).