Structures and photoactivation of the charge-transfer complexes of Bis(arene)iron(II) dications with ferrocene and arene donors

Article abstract of DOI:10.1021/ja00002a018

Ferrocene forms a series of unusual charge-transfer crystals with the isoelectronic bis(arene)iron(II) dication, in which X-ray crystallography establishes the alternate (heterosoric) stacking of sandwich structures comprising the donor-acceptor pairs. In acetonitrile, the 1:1 complex [Cp₂Fe, Ar₂Fe²⁺] shows a broad absorption band centered at λ_max ～ 630 nm. Bis(arene)iron(II) acceptors also form highly colored crystals with various arene donors, in which the charge-transfer absorption (hv_CT) derives from the same heterosoric stacking of donor-acceptor pairs-despite the structurally divergent nature of the arene (planar) and ferrocene (sandwich) donors. These absorptions undergo a predictable red-shift with increasing acceptor strength in the order (BZ)₂Fe²⁺ > (MES)₂Fe²⁺ > (DUR)₂Fe²⁺ > (HMB)₂Fe²⁺, as judged by the reduction potentials E^o_red of the benzene, mesitylene, durene, and hexamethylbenzene derivatives, respectively. Common to both classes of 1:1 bis(arene)iron(II) complexes [Ar₂Fe²⁺, D], where D is either a ferrocene or arene donor, is the efficient photoinduced substitution of the arene ligands that occurs in solution when the charge-transfer bands are selectively irradiated with monochromatic light. Time-resolved picosecond spectroscopy identifies such a charge-transfer deligation to occur via the direct photoexcitation of the complex to the ion radical pair, i.e., [Ar₂Fe²⁺, D] ^hvCT→ [Ar₂Fe⁺, D^?+], followed by the spontaneous loss of arene ligands by the acceptor moiety. Indeed, the substitution lability of the 19-electron intermediate Ar₂Fe⁺ can be independently demonstrated by the application of transient electrochemical methods to the reduction of various bis(arene)iron(II) dications.