Organometallic Fe–Fe Interactions: Beyond Common Metal–Metal Bonds and Inverse Mixed‐Valent Charge Transfer

Abstract

The compounds [Fe(CO) (dRpf)] , n=0, 1, 2 and dRpf=1,1'-bis(dicyclohexylphosphino)ferrocene ([1] ) or 1,1'-bis(diisopropylphosphino)ferrocene ([2] ), were obtained as two-step reversible redox systems by photolytic and redox reactions. The iron-iron distance decreases from about 4 Å to about 3 Å on oxidation, which takes place primarily at the tricarbonyliron moiety. Whereas ferrocene oxidation is calculated to occur only in excited states, the near infrared absorptions of the mixed-valent monocations are due to an unprecedented "inverse" inter-valence charge transfer from the electron-rich iron(II) in the ferrocene backbone to the electron-deficient tricarbonyliron(I). Protonation of complex 1 results in the formation of the structurally characterized hydride [1H]BF , which reacts with acetone to form the dication, 1 , and isopropanol. While the hydride [2H]BF was found to be unstable, protonation of 2 in acetone resulted in the clean formation of 2 formally a hydrogen transfer.

Beyond Common Metal–Metal Bonds, κ3‐Bis(donor)ferrocenyl→Transition‐Metal Interactions

“…The molecular structures of the dicationic species [ 25 ] 2+ and [ 24 ] 2+ show the Fe−Fe distance 3.026 and 2.945 Å, and α 23.44° and 26.69°, respectively . The IR SEC for the formation of the dication showed a high energy weak band and lower energy strong band, consistent with the quasi‐octahedral configuration at the Fe(CO) 3 site, and meridonal carbonyls for all three complexes [ 24 ] 2+ , [ 25 ] 2+ and [ 26 ] 2+ .…”

“…The isotropic EPR spectrum of the monocations [ 25 ] + and [ 26 ] + showed a triplet at g =2.019 (2xA P =13 G), which signified a lower spin density on the Fe(CO) 3 moiety, especially when compared to similar tricarbonlyiron(I) species, for example, [Fe(CO) 3 (PCy 3 ) 2 ] + g =2.055 (2xA P =17.5 G) . The reduced electron spin density at the Fe(CO) 3 moiety was further supported by DFT calculations, which showed the SOMO resided over both Fe centers in a 2‐center‐3‐electron interaction …”

“…Nearly 30 years later, [ 24 ] and the similar complexes [Fe(CO) 3 (dippf)] ([ 25 ]), and [Fe(CO) 3 (dchpf)] ([ 26 ]) (dchpf=1,1′‐bis(dicyclohexylphosphino)ferrocene) were reexamined . Using photochemical techniques improved the synthesis and yield of these tricarbonyliron complexes.…”

“…Spectroelectrochemical (SEC) studies of the monocation [ 25 ] + and [ 26 ] + showed two low energy bands in the UV/Vis/NIR SEC at λ max =535 and 790 nm . These bands were assigned based on TD‐DFT calculations, with the higher energy absorption corresponding to a SOMO (antibonding orbital between the two Fe sites) to LUMO+1 (d orbital of Fe(CO) 3 ) transition and the second band belonged to a transition from HOMO−3 to SOMO.…”

“…This absorption was also formally an “inverse” inter‐valence charge transfer from the electron‐rich Fe II in the ferrocene to the electron‐deficient tricarbonyliron(I) (Fe II →Fe I ) (Scheme ). This behavior could be considered noninnocent …”