Photoelectron spectroscopy and DFT calculations of easily ionized quadruply bonded Mo₂⁴⁺ compounds and their bicyclic guanidinate precursors

A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo₂⁴⁺ units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo ₂(TEhpp)₄ complex which has a broad first ionization band centered around 4.27 ± 0.03 eV and an ionization onset at the very low energy of 3.93 ± 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo-Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (η⁵-C ₉Me₇)₂Co and (η⁵-C ₅Me₅)₂Cr.