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-^<i>c</i>C₄H₂E (E = O (<b>11</b>), S (<b>12</b>); M = Fe (<b>a</b>), Ru (<b>b</b>); dppe = 1,2-bis­(diphenylphosphino)­ethane) are reported. The molecular structures of <b>11a</b> and <b>12a</b>,<b>b</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 <b>11a</b>,<b>b</b> and <b>12a</b>,<b>b</b> was studied using electrochemical (cyclic and square wave voltammetry) and spectroelectrochemical (<i>in situ</i> UV–vis/NIR, ESR, and IR spectroscopy) methods and DFT calculations. Electrochemical studies demonstrated that mixed-valent species <b>11a</b>,<b>b</b>^<b>+</b> and <b>12a</b>,<b>b</b>^<b>+</b> exhibit high thermodynamic stabilities with respect to disproportionation (<i>K</i>_C values from 6.87 × 10⁴ to 9.33 × 10⁵). <i>In situ</i> 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 <b>11a</b>,<b>b</b>^<b>+</b> and <b>12a</b>,<b>b</b>^<b>+</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 <b>11a</b>,<b>b</b> and <b>12a</b>,<b>b</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