Five-Membered Heterocycles as Linking Units in Strongly Coupled Homobimetallic Group 8 Metal Half-Sandwich Complexes

The synthesis and characterization
of a series of alkynyl half-sandwich
complexes of type 2,5-((η5-C5H5)­(dppe)­MCC)2-cC4H2E (E = O (11), S (12); M = Fe (a), Ru (b); dppe = 1,2-bis­(diphenylphosphino)­ethane)
are reported. The molecular structures of 11a and 12a,b in the solid state have been determined
by single-crystal X-ray diffraction. The influence of different metals
and the variation of the heterocyclic bridge on the electronic interactions
between the terminal redox-active units in 11a,b and 12a,b was studied using electrochemical
(cyclic and square wave voltammetry) and spectroelectrochemical (in situ UV–vis/NIR, ESR, and IR spectroscopy) methods
and DFT calculations. Electrochemical studies demonstrated that mixed-valent
species 11a,b+ and 12a,b+ exhibit high thermodynamic
stabilities with respect to disproportionation (KC values from 6.87 × 104 to 9.33 ×
105). In situ spectroelectrochemical ESR
and IR measurements display delocalization of the single electron
between the metal centers M/M+, revealing that within this
setup five-membered heterocycles are well suited to promote intramolecular
metal–metal interactions. Furthermore, the UV–vis/NIR
spectra of mixed-valent 11a,b+ and 12a,b+ show intense, narrow, and nonsolvatochromic π­(dπ)
→ π*­(dπ) absorptions in the NIR region with a high-energy
shoulder. Both experimental and computational results suggest that
at least two thermally accessible rotation conformers of the organometallic
termini of 11a,b and 12a,b contribute to the electronic spectra of these compounds.
One of the conformers can clearly be characterized as a delocalized
class III system, while the other shows a more localized behavior.