Preparation and Protonation of Fe₂(pdt)(CNR)₆, Electron-Rich Analogues of Fe₂(pdt)(CO)₆

The complexes Fe₂(pdt)(CNR)₆ (pdt^2- = CH₂(CH₂S^-)₂) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C₆H₄-4-OMe (1), C₆H₄-4-Cl (2), Me (3). These complexes represent electron-rich analogues of the parent Fe₂(pdt)(CO)₆. Unlike most substituted derivatives of Fe₂(pdt)(CO)₆, these isocyanide complexes are sterically unencumbered and have the same idealized symmetry as the parent hexacarbonyl derivatives. Like the hexacarbonyls, the stereodynamics of 1-3 involve both turnstile rotation of the Fe(CNR)₃ as well as the inversion of the chair conformation of the pdt ligand. Structural studies indicate that the basal isocyanide has nonlinear CNC bonds and short Fe-C distances, indicating that they engage in stronger Fe-C π-backbonding than the apical ligands. Cyclic voltammetry reveals that these new complexes are far more reducing than the hexacarbonyls, although the redox behavior is complex. Estimated reduction potentials are E_1/2 ≈ -0.6 ([2]^+/0), -0.7 ([1]^+/0), and -1.25 ([3]^+/0). According to DFT calculations, the rotated isomer of 3 is only 2.2 kcal/mol higher in energy than the crystallographically observed unrotated structure. The effects of rotated versus unrotated structure and of solvent coordination (THF, MeCN) on redox potentials were assessed computationally. These factors shift the redox couple by as much as 0.25 V, usually less. Compounds 1 and 2 protonate with strong acids to give the expected μ-hydrides [H1]⁺ and [H2]⁺. In contrast, 3 protonates with [HNEt₃]BAr^F₄ (pK_a^MeCN = 18.7) to give the aminocarbyne [Fe₂(pdt)(CNMe)₅(μ-CN(H)Me)]⁺ ([3H]⁺). According to NMR measurements and DFT calculations, this species adopts an unsymmetrical, rotated structure. DFT calculations further indicate that the previously described carbyne complex [Fe₂(SMe)₂(CO)₃(PMe₃)₂(CCF₃)]⁺ also adopts a rotated structure with a bridging carbyne ligand. Complex [3H]⁺ reversibly adds MeNC to give [Fe₂(pdt)(CNR)₆(μ-CN(H)Me)]⁺ ([3H(CNMe)]⁺). Near room temperature, [3H]⁺ isomerizes to the hydride [(μ-H)Fe₂(pdt)(CNMe)₆]⁺ ([H3]⁺) via a first-order pathway.