Di-organoiron mixed valent complexes featuring "(eta2-dppe)(eta5-C5Me5)Fe" endgroups: smooth class-III to class-II transition induced by successive insertion of 1,4-phenylene units in a butadiyne-diyl bridge.

The synthesis and study of a new redox family of symmetric dinuclear iron(II/III) complexes featuring "(eta(2)-dppe)(eta(5)-C(5)Me(5))Fe(CC)" endgroups connected by a bis(diethynyl)-4,4'-biphenyl spacer are reported. The solid-state structures were determined (X-rays) for the homovalent Fe(II)/Fe(II) and Fe(III)/Fe(III) parents. In contrast, the mixed valent (MV) complex 5[PF(6)] has a low thermodynamic stability (Kc around 10) and cannot be isolated in a pure form, but was studied in solution. According to the Robin and Day classification, it constitutes a remarkable example of well-behaved weakly coupled class-II organometallic MV compound. The photodriven metal-metal electron-transfer process takes place over ca. 16 A and corresponds to an electronic coupling of ca. 150 cm(-1) with a reorganization energy of ca. 6250 cm(-1) in dichloromethane. A similar investigation was also conducted in the near-IR range for the known and much more stable MV analogue 3[PF(6)] featuring the 1,4-phenyl unit instead of the 4,4'-biphenyl one (K(c) = 2.6 10(4)). The latter also exhibits a localized valency, but presents a very intense intervalence charge-transfer band (IVCT) with a cutoff on the low-energy side. A much stronger electronic coupling is derived (ca. 1700 cm(-1)) from the band shape for this MV complex in the frame of the two-level model. Although slowed, the electron exchange is not disrupted by insertion of an additional para-phenylene moiety into a 1,4-diethynylaryl bridge. Thus, starting from a compound with a butadiyne-diyl spacer, stepwise para-phenylene insertions in the bridge produce a smooth Class-III to Class-II transition for the corresponding MV complexes.