Naphthalene and anthracene complexes sandwiched by two {(Cp*)Fe(I)} fragments: strong electronic coupling between the Fe(I) centers

The reactions of the half-sandwich iron(II) complex [FeCl(Cp*)(tmeda)] (1; Cp*=η(5)-C(5)Me(5), TMEDA=N,N,N',N'-tetramethylethylenediamine) with potassium naphthalenide or potassium anthracenide gave the diamagnetic complexes [(Cp*)Fe(μ-polyarene)Fe(Cp*)] (polyarene=naphthalene (2), anthracene (3a)), which have two {(Cp*)Fe} units bound to opposite faces of the polyarene. One of two {(Cp*)Fe} units in 3a is located over the central ring of anthracene while the other is positioned over an outer ring. The {(Cp*)Fe} unit bound to the central ring of 3a migrates to the outer ring upon heating in the solid state to give the isomer 3b. The electrochemical potential separations between successive one-electron redox events for complexes 2 and 3b are large. The mixed valence complexes [2](+) and [3b](+) were synthesized by chemical oxidation. The mixed-valence complex [3b](+) is charge delocalized on the Mössbauer timescale at 78 K, and its absorption spectrum shows an intervalence charge-transfer band. Complex [2](+) exhibits two absorption bands in the near-IR region and a slightly broadened doublet in the Mössbauer spectrum. DFT calculations were carried out to examine the electronic structures of these dinuclear iron(I) complexes to elucidate the factors responsible for their diamagnetism and to determine the degree of charge delocalization in the mixed-valence complexes.