Synthesis of a Bis(indenyl) Co(I) Anion: A Reactive Source of a 14 Electron Indenyl Co(I) Equivalent

Alkali
metal reduction of (η5-C9H5-1,3-(SiMe3)2)2Co (1) in tetrahydrofuran (THF) permits isolation of the unusual and reactive
20 electron Co­(I) anion [Na­(THF)6]­[(η5-C9H5-1,3-(SiMe3)2)2Co] (2). Crystallographic characterization of
both 1 and 2 provide support for the one
electron reduction from Co­(II) to Co­(I). Reactivity studies of 2 are further consistent with a Co­(I) equivalent, based on
both one electron chemical oxidation to reform 1 and
reaction with a variety of σ and π donors. Upon addition
of pyridines or vinyltrimethylsilane to 2, known dimer
[(C9H5-1,3-(SiMe3)2)2Co2] (3) is formed, likely through
16 electron (η5-C9H5-1,3-(SiMe3)2)­Co­(L) intermediates. Ethylene addition to 2 establishes an equilibrium between (η5-C9H5-1,3-(SiMe3)2)­Co­(η2-H2CCH2)2 (8) and 2, suggestive of reversible ligand ejection from 2. Crossover experiments between
a related metal indenide salt and 2 confirm ligand extrusion
from the anion, even in the absence of strong supporting donors. Reaction
of 2 with PMe3 results in formation of 3, (η5-C9H5-1,3-(SiMe3)2)­Co­(PMe3)2 (13), and a paramagnetic species, with the product ratios being highly
dependent on the conditions of synthesis. Collectively, 2 demonstrates an alternative entry point into the chemistry of 14
electron Co­(I) equivalents when compared to typical ligand loss from
neutral 18 electron cyclopentadienyl cobalt bis­(ligand) complexes,
perhaps permitting generation of low electron count species more effective
for small molecule activation.