Free H<sub>2</sub> Rotation vs Jahn–Teller
Constraints in the Nonclassical Trigonal (TPB)Co–H<sub>2</sub> Complex
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Abstract
Proton
exchange within the M–H<sub>2</sub> moiety of (TPB)Co(H<sub>2</sub>) (Co–H<sub>2</sub>; TPB = B(<i>o</i>-C<sub>6</sub>H<sub>4</sub>P<sup><i>i</i></sup>Pr<sub>2</sub>)<sub>3</sub>) by 2-fold rotation about the M–H<sub>2</sub> axis
is probed through EPR/ENDOR studies and a neutron diffraction crystal
structure. This complex is compared with previously studied (SiP<sup><i>i</i>Pr</sup><sub>3</sub>)Fe(H<sub>2</sub>) (Fe–H<sub>2</sub>) (SiP<sup><i>i</i>Pr</sup><sub>3</sub> = [Si(<i>o</i>-C<sub>6</sub>H<sub>4</sub>P<sup><i>i</i></sup>Pr<sub>2</sub>)<sub>3</sub>]). The <i>g</i>-values for
Co–H<sub>2</sub> and Fe–H<sub>2</sub> show that both
have the Jahn–Teller (JT)-active <sup>2</sup><i>E</i> ground state (idealized <i>C</i><sub>3</sub> symmetry)
with doubly degenerate frontier orbitals, (e)<sup>3</sup> = [|<i>m</i><sub><i>L</i></sub> ± 2<i>></i>]<sup>3</sup> = [<i>x</i><sup>2</sup> – <i>y</i><sup>2</sup>, <i>xy</i>]<sup>3</sup>, but with
stronger linear vibronic coupling for Co–H<sub>2</sub>. The
observation of <sup>1</sup>H ENDOR signals from the Co–HD complex, <sup>2</sup>H signals from the Co–D<sub>2</sub>/HD complexes, but <i>no</i> <sup>1</sup>H signals from the Co–H<sub>2</sub> complex establishes that H<sub>2</sub> undergoes proton exchange
at 2 K through rotation around the Co–H<sub>2</sub> axis, which
introduces a quantum-statistical (Pauli-principle) requirement that
the overall nuclear wave function be antisymmetric to exchange of
identical protons (<i>I</i> = 1/2; Fermions), symmetric
for identical deuterons (<i>I</i> = 1; Bosons). Analysis
of the 1-D rotor problem indicates that Co–H<sub>2</sub> exhibits
rotor-like behavior in solution because the underlying <i>C</i><sub>3</sub> molecular symmetry combined with H<sub>2</sub> exchange
creates a dominant 6-fold barrier to H<sub>2</sub> rotation. Fe–H<sub>2</sub> instead shows H<sub>2</sub> localization at 2 K because a
dominant 2-fold barrier is introduced by strong Fe(3d)→ H<sub>2</sub>(σ*) π-backbonding that becomes dependent on the
H<sub>2</sub> orientation through quadratic JT distortion. ENDOR sensitively
probes bonding along the L<sub>2</sub>–M–E axis (E =
Si for Fe–H<sub>2</sub>; E = B for Co–H<sub>2</sub>).
Notably, the isotropic <sup>1</sup>H/<sup>2</sup>H hyperfine coupling
to the diatomic of Co–H<sub>2</sub> is nearly 4-fold smaller
than for Fe–H<sub>2</sub>
