Free H_2 Rotation vs Jahn−Teller Constraints in the Nonclassical Trigonal (TPB)Co−H_2 Complex

Abstract

Proton exchange within the M–H_2 moiety of (TPB)Co(H_2) (Co–H_2; TPB = B(o-C_6H_4PiPr_2)_3) by 2-fold rotation about the M–H_2 axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiP^(iPr)_3)Fe(H_2) (Fe–H_2) (SiP^(iPr)_3 = [Si(o-C_6H_4PiPr_2)_3]). The g-values for Co–H_2 and Fe–H_2 show that both have the Jahn–Teller (JT)-active ^2E ground state (idealized C_3 symmetry) with doubly degenerate frontier orbitals, (e)^3 = [|m_L ± 2>]^3 = [x^2 – y^2, xy]^3, but with stronger linear vibronic coupling for Co–H_2. The observation of ^1H ENDOR signals from the Co–HD complex, ^2H signals from the Co–D_2/HD complexes, but no ^1H signals from the Co–H_2 complex establishes that H_2 undergoes proton exchange at 2 K through rotation around the Co–H_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–H_2 exhibits rotor-like behavior in solution because the underlying C_3 molecular symmetry combined with H_2 exchange creates a dominant 6-fold barrier to H_2 rotation. Fe–H_2 instead shows H_2 localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)→ H_2(σ^*) π-backbonding that becomes dependent on the H_2 orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L_2–M–E axis (E = Si for Fe–H_2; E = B for Co–H_2). Notably, the isotropic ^1H/^2H hyperfine coupling to the diatomic of Co–H_2 is nearly 4-fold smaller than for Fe–H_2.

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