NMR and INS Line Shapes of Transition Metal Hydrides in the Presence of Coherent and Incoherent Dihydrogen Exchange

Abstract

In this paper a unified description of the effects of the coherent and incoherent dihydrogen exchange on the NMR and INS spectra of transition metal hydrides based on the quantum-mechanical density matrix formalism of Alexander-Binsch is proposed. The dynamic parameters of the line shape analyses are the exchange couplings or rotational tunnel splittings J of the coherent exchange and the rate constants k of the incoherent exchange. As experimental examples, we report the temperature dependent values J and k for 1 ≡ Cp*RuH 3 (PCy 3 ) (Cp* ≡ C 5 (CH 3 ) 5 and Cy ≡ cyclohexyl) including the kinetic HH/HD/DD isotope effects on the incoherent exchange, determined by NMR, and for the tungsten dihydrogen complex 2 ≡ W(PCy 3 ) 2 (CO) 3 -(η-H 2 ), determined by INS. The temperature dependence of J and k is interpreted qualitatively in terms of a simple reaction scheme involving at each temperature a ground state and a dominant ro-vibrationally excited state. Using formal kinetics it is shown that a coherent exchange in the excited state contributes to J only if this exchange presents the rate limiting reaction step, i.e., if vibrational deactivation is fast. This is the case for levels located substantially below the top of the barrier. A very fast coherent exchange of levels located close to the top of the barrier contributes only to k. This result reproduces in a simple way the quantummechanical results of Szymanski, S.

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

“…One of the mechanisms proposed by Limbach and co-workers involves a pre-equilibrium with a hydride-dihydrogen complex in which quantum exchange occurs between the two nuclei of the dihydrogen ligand. [42,69,70] Strong evidence to support this theory comes from the [Cp 2 TaH 2 L] + series of complexes, examples of which can be isolated with dihydride or dihydrogen structures according to the choice of L. The complex with L = CO was identified as a dihydrogen complex both by X-ray diffraction and NMR spectroscopic data while the species with L = 3 whereas a value greater than 3200 Hz was estimated for L = CO.…”

Dihydrogen Transfer and Symmetry: The Role of Symmetry in the Chemistry of Dihydrogen Transfer in the Light of NMR Spectroscopy

“…In this regime, a transition from coherent to incoherent exchange will take place (see below), as was shown by density matrix theory [19].…”

“…The results of this elaborate theory can be reproduced by a combination of density matrix theory and formal kinetics as was shown in Ref. [19]. The transition from coherent to incoherent tunneling is most easily understood by considering the ground state of the system and the first excited state.…”

“…3 Sketch of the transition from coherent to incoherent tunneling: (a) Quantitative four-level model [86]; (b) corresponding chemisty of coherent and incoherent dihydrogen exchange [19]; (c) dependence of coherent and incoherent rates [19].…”

“…21.17). It displays the superimposed experimental and calculated 1 H liquid state NMR hydride signals of (C5Me5)RuH3(PCy3) (Cy = cyclohexyl) 1 dissolved in tetrahydrofuran-d8 [19]. At low temperatures site 2 exhibits a triplet splitting characterized by a temperature dependent exchange coupling constant J 12 = J 23 .…”