Intriguing methylation effects in cobaltocene revealed by high-resolution MATI spectroscopy and <i>ab initio</i> calculations of (η5-C5H4Me)2Co

Cobaltocene, such as ferrocene, is a key member of the organometallic family. Many of its applications in molecular electronics are based on the low ionization energy (IE), which can be tuned by ring substituents. Mass-analyzed threshold ionization (MATI) spectroscopy provides unprecedented accuracy in determining IE. In this work, the MATI spectrum of 1,1'-dimethylcobaltocene, (Cp')2Co, has been obtained for the first time. The spectrum shows signals of the (Cp')2Co individual conformers with eclipsed methyl groups and Me fragments rotated by 144° relative to each other. Their adiabatic IEs are 5.1000(6) and 5.0966(6) eV, respectively. The rich MATI vibronic structure was fully interpreted on the basis of DFT simulations. Experimental MATI data combined with DFT and CCSD(T) calculations reveal non-trivial methylation effects in cobaltocenes. The IE reduction caused by the introduction of Me groups in cobaltocene is greater than in bis(η6-benzene)chromium. In contrast, the mutual influence of Me substituents in (Cp')2Co is weaker than in (η6-PhMe)2Cr. The molecular structure of neutral (Cp')2Co0 is perturbed by vibronic interactions associated with two closely lying electronic states. The largest structural distortions correspond to out-of-plane deformations of the carbocycles. Methylation leads to a dramatic increase in the energy barriers corresponding to the pseudorotation of the neutrals along the vibronic-active normal coordinate. Surprisingly, the symmetry of the (Cp')2Co0 doublet ground electronic state changes during the Cp' ligand rotation around the Co-ring centroid axis. This may open up new possibilities for controlling the spin distribution in substituted cobaltocenes.