Proton exchange within the M-H2 moiety of (TPB)Co(H2) (Co-H2; TPB = B(o-C6H4PiPr2)3) by 2-fold rotation about the M-H2 axis is probed through EPR/ENDOR studies and a neutron diffraction crystal structure. This complex is compared with previously studied (SiPiPr3)Fe(H2) (Fe-H2) (SiPiPr3 = [Si(o-C6H4PiPr2)3]). The g-values for Co-H2 and Fe-H2 show that both have the Jahn-Teller (JT)-active 2E ground state (idealized C3 symmetry) with doubly degenerate frontier orbitals, (e)3 = [|mL ± 2>]3 = [x2 - y2, xy]3, but with stronger linear vibronic coupling for Co-H2. The observation of 1H ENDOR signals from the Co-HD complex, 2H signals from the Co-D2/HD complexes, but no 1H signals from the Co-H2 complex establishes that H2 undergoes proton exchange at 2 K through rotation around the Co-H2 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-H2 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-H2 instead shows H2 localization at 2 K because a dominant 2-fold barrier is introduced by strong Fe(3d)→ H2(Σ∗) π-backbonding that becomes dependent on the H2 orientation through quadratic JT distortion. ENDOR sensitively probes bonding along the L2-M-E axis (E = Si for Fe-H2; E = B for Co-H2). Notably, the isotropic 1H/2H hyperfine coupling to the diatomic of Co-H2 is nearly 4-fold smaller than for Fe-H2.
ASJC Scopus subject areas
- Colloid and Surface Chemistry