Synthesis and structures of one- and two-electron oxidized forms of bis(acetylene)tetrairon clusters Cp'(4)Fe(4)(HCCH)(2) (Cp' = Cp,eta5-C(5)H(4)Me)

Air-oxidation of Cp'(4)Fe(4)(HCCH)(2) (Cp' = Cp (1a), C(5)H(4)Me (1b)) in an NH(4)PF(6)/CH(3)CN solution afforded the one-electron oxidized clusters Cp'(4)Fe(4)(HCCH)(2)](PF(6)). Oxidation of 1a with excess AgBF(4) in THF afforded 1a](BF(4)), while that of 1b with excess AgBF(4) gave 1b](BF(4))(2). The X-ray crystal structure analysis of 1a](BF(4)) revealed that the monocationic cluster retains the butterfly-type Fe(4)(mu4-eta(2):eta(2):eta(1):eta(1)-HCCH)(2) framework similar to that of the neutral cluster. The average Fe-Fe bond length is shorter by 0.029 A than that in the neutral cluster. Electrochemical oxidation of 1a and 1b in 0.1 M NH(4)PF(6)/CH(3)CN solution at +0.30 and +0.25 V versus Ag/10 mM AgNO(3), respectively, afforded the two-electron oxidized clusters 1a](PF(6))(2) and 1b](PF(6))(2). The X-ray crystal structure analysis for 1b](BF(4))(2) shows that the butterfly-type cluster core is retained but shrinks more of those of neutral and monocationic clusters. The four Fe-Fe bonds in 1b](BF(4))(2) are unequivalent: one Fe-Fe bond (2.397(1) A) is apparently shorter than the others (2.439(2)-2.461(2) A).