Influences on the rotated structure of diiron dithiolate complexes: electronic asymmetry vs. secondary coordination sphere interaction

Abstract

In the pursuit of a “rotated” structure, and exploration of the influence of the aza nitrogen lone pair, the Fe^IFe^I model complexes wherein two Fe(CO)_3−xP_x moieties are significantly twisted from the ideal configuration (torsion angle >30°) are reported. [Fe₂(μ-S(CH₂)₂NⁱPr(X)(CH₂)₂S)(CO)₄(κ²-dppe)]₂²⁺ (X = H, 4; Me, 5) prepared from protonation and methylation, respectively, of [Fe₂(μ-S(CH₂)₂NⁱPr(CH₂)₂S)(CO)₄(κ²-dppe)]₂, 1, possess Φ angles of 34.1 and 35.4° (av.), respectively. Such dramatic twist is attributed to asymmetric substitution within the Fe₂ unit in which a dppe ligand is coordinated to one Fe site in the κ²-mode. In the presence of the N⋯C(CO_ap) interaction, the torsion angle is decreased to 10.8°, suggesting availability of lone pairs of the aza nitrogen sites within 1 is in control of the twist. Backbones of the bridging diphosphine ligands also affect distortion. For a shorter ligand, the more compact structure of [Fe₂(μ-S(CH₂)₂NⁱPr(CH₂)₂S)(μ-dppm)(CO)₄]₂, 7, is formed. Dppm in a bridging manner allows achievement of the nearly eclipsed configuration. In contrast, dppe in [Fe₂(μ-S(CH₂)₂NⁱPr(CH₂)₂S)(μ-dppe)(CO)₄]₂, 6, could twist the Fe(CO)_3−xL_x fragment to adopt the least strained structure. In addition, the N⋯C(CO_ap) interaction would direct the twist towards a specific direction for the closer contact. In return, the shorter N⋯C(CO_ap) distance of 3.721(7) Å and larger Φ angle of 26.5° are obtained in 6. For comparison, 3.987(7) Å and 3.9° of the corresponding parameters are observed in 7. Conversion of (μ-dppe)[Fe₂(μ-S(CH₂)₂NⁱPr(CH₂)₂S)(CO)₅]₂, 2, to complex 1via an associative mechanism is studied.