Electronic Factors for Protonation of an Organometallic Molecule. Photoelectron Spectroscopy and Electron Paramagnetic Resonance Study of [(η6-C6H6)Mo(TRIPOD)]0/+

Abstract

We have previously shown that the arene complex (η6-C6H6)Mo(TRIPOD), where TRIPOD = 1,1,1-tris((diphenylphosphino)methyl)ethane, is protonated by exo addition of H+ to the arene ring to give the transient cyclohexadienyl complex [(η5-C6H7)Mo(TRIPOD)]+, which eventually yields the thermodynamic molybdenum hydride [(η6-C6H6)Mo(TRIPOD)(H)]+. The present study is a combined experimental and theoretical investigation that reveals the fundamental basis for this mechanism. Photoelectron spectroscopy (PES) is used to probe the electronic structure of (η6-C6H6)Mo(TRIPOD) and the production of the [(η6-C6H6)Mo(TRIPOD)]+ cation in the gas phase. The initial ionizations of (η6-C6H6)Mo(TRIPOD) are from energetically closely spaced orbitals of predominantly metal d character (2A1 and 2E cation states using C3v symmetry) that are shifted over 2 eV to lower energy with respect to the comparable ionizations of (η6-C6H6)Mo(CO)3. The oxidized species [(η6-C6H6)Mo(TRIPOD)]+ is also prepared in solution by electrochemical means and through the use of chemical oxidants. The electron paramagnetic resonance (EPR) spectrum of this cation shows arene-proton hyperfine coupling that indicates substantial arene character in the highest occupied orbital. The photoelectron and EPR spectra both provide evidence for Jahn−Teller distortion of the 2E positive ion states. Electronic structure calculations show that this distortion involves opening of one L−Mo−L angle, which effectively creates an open coordination site on the metal for the hydride to occupy in the final thermodynamic product. These experimental and computational results show that, in terms of their energy, the e symmetry and a1 symmetry metal-based orbitals are similarly favored for oxidative protonation directly at the metal. The e symmetry orbital has a portion of its density on the arene ring, making access to this orbital by proton approach to the exo position of the arene ring possible. For (η6-C6H6)Mo(TRIPOD), exo attack at the arene is favored because the TRIPOD ligand shields the e symmetry orbital from direct attack at the metal by the solvated proton. Thus, exo attack is not initiated by proton interaction with an arene-based orbital but is initiated by proton interaction with the arene portion of the same e symmetry orbital that directs attack at the metal. Calculations predict low barriers for both direct attack at the metal and exo attack at the arene, with attack at the arene favored for longer metal−proton distances.