Chemistry of the Si-Si and Fe-Fe Bonds in the Cyclic Structure (Me2SiSiMe2)[(eta)^5-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)[(eta)^5-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)[(eta)^5-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)[(eta)^5-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) (angstrom)] is about the same as that of 1 [2.346(4) (angstrom)], 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)[(eta)^5-C5H4Fe(CO)2]2I+ cations as the template.