Free H2 Rotation vs Jahn–Teller Constraints in the Nonclassical Trigonal (TPB)Co–H2 Complex

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.