Free H2 Rotation vs Jahn鈥揟eller Constraints in the Nonclassical Trigonal (TPB)Co鈥揌2 Complex

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• 作者：William A. Gunderson ; Daniel L. M. Suess ; Henry Fong ; Xiaoping Wang ; Christina M. Hoffmann ; George E. Cutsail III ; Jonas C. Peters ; Brian M. Hoffman
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：October 22, 2014
• 年：2014
• 卷：136
• 期：42
• 页码：14998-15009
• 全文大小：640K
• ISSN：1520-5126

文摘

Proton exchange within the M鈥揌₂ moiety of (TPB)Co(H₂) (Co鈥揌₂; TPB = B(o-C₆H₄PⁱPr₂)₃) by 2-fold rotation about the M鈥揌₂ axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP^iPr₃)Fe(H₂) (Fe鈥揌₂) (SiP^iPr₃ = [Si(o-C₆H₄PⁱPr₂)₃]). The g-values for Co鈥揌₂ and Fe鈥揌₂ show that both have the Jahn鈥揟eller (JT)-active ²E ground state (idealized C₃ symmetry) with doubly degenerate frontier orbitals, (e)³ = [|m_L 卤 2>]³ = [x² 鈥?y², xy]³, but with stronger linear vibronic coupling for Co鈥揌₂. The observation of ¹H ENDOR signals from the Co鈥揌D complex, ²H signals from the Co鈥揇₂/HD complexes, but no ¹H signals from the Co鈥揌₂ complex establishes that H₂ undergoes proton exchange at 2 K through rotation around the Co鈥揌₂ axis, which introduces a quantum-statistical (Pauli-principle) requirement that the overall nuclear wave function be antisymmetric to exchange of identical protons (I = 1/2; Fermions), symmetric for identical deuterons (I = 1; Bosons). Analysis of the 1-D rotor problem indicates that Co鈥揌₂ exhibits rotor-like behavior in solution because the underlying C₃ molecular symmetry combined with H₂ exchange creates a dominant 6-fold barrier to H₂ rotation. Fe鈥揌₂ instead shows H₂ localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)鈫?H₂(蟽*) 蟺-backbonding that becomes dependent on the H₂ orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L₂鈥揗鈥揈 axis (E = Si for Fe鈥揌₂; E = B for Co鈥揌₂). Notably, the isotropic ¹H/²H hyperfine coupling to the diatomic of Co鈥揌₂ is nearly 4-fold smaller than for Fe鈥揌₂.