Chemistry of the Si−Si and Fe−Fe Bonds in the Cyclic Structure (Me₂SiSiMe₂)[η⁵-C₅H₄Fe(CO)₂]₂. Selective Cleavage of the Fe−Fe Bond by I₂ and Unexpected Properties of the Iodide

The Si−Si bond in the title cyclic structure (1) exhibited unexpected stability toward I₂. Thus, the reaction of 1 with 1 equiv of I₂ in chloroform resulted in selective cleavage of the Fe−Fe bond to afford diiodide (Me₂SiSiMe₂)[η⁵-C₅H₄Fe(CO)₂I]₂ (2) with retention of the Si−Si bond. When excess (2−4 equiv) I₂ was used to react with 1 in either benzene or chloroform, iodonium-bridged diiron complex {(Me₂SiSiMe₂)[η⁵-C₅H₄Fe(CO)₂]₂I⁺}(I₅^-) (4) was obtained, in which the Si−Si bond was still retained. It is noteworthy that 4 contains a counteranion I₅^- rather than the expected I₃^-, which is the first example for an iodonium-bridged diiron complex to combine a polyiodide anion larger than I₃^-. UV irradiation of 2 did not affect the stability of the silicon−silicon bond and, in the presence of PR₃, resulted in CO substitution to give (Me₂SiSiMe₂)[η⁵-C₅H₄Fe(CO)(PR₃)I]₂ (5, R = Ph; 6, R = OPh). The molecular structure of 2 was determined by the X-ray diffraction method. It is noteworthy that the structure of 2 does not take the expected anti conformation but adopts a gauche one. The length of the Si−Si bond of 2 [2.353(3) Å] is about the same as that of 1 [2.346(4) Å], which can be direct evidence to demonstrate that the Si−Si bond in the cyclic structure of 1 is not subject to significant ring strain. The molecular structure of 4 was also determined by the X-ray diffraction method. It is noted that the structure of 4 contains an abnormally large Fe−I⁺−Fe bond angle of 121.25(7)°. Of particular interest is the observation that the I₅^- anions of 4 are self-assembled into novel layered, two-dimensional networks with the (Me₂SiSiMe₂)[η⁵-C₅H₄Fe(CO)₂]₂I⁺ cations as the template.