Observation of New Intermediates in the Reaction of Dihydrogen with Iridium, Rhodium, and Mixed Metal A-Frame Complexes with Parahydrogen-Induced Polarization

Abstract

The reaction of hydrogen with iridium and rhodium µ-S A-frame complexes has been investigated using parahydrogen-induced polarization (PHIP) NMR spectroscopy. The reaction of Ir 2 (µ-S)(CO) 2 (dppm) 2 where dppm ) bis(diphenylphosphino)methane with hydrogen proceeds by two addition pathways corresponding to oxidative addition inside (endo) and outside (exo) the pocket of the A-frame complex. The exo addition pathway yields a kinetically significant dihydride that has not been previously reported. This species is a key intermediate in the formation of the thermodynamic product of the reaction system. Also present are polarized resonances for the addition of hydrogen to the mixed metal A-frame, IrRh(µ-S)(CO) 2 (dppm) 2 , formed as a contaminant from rhodium present in the iridium trichloride starting material. To confirm this observation, IrRh(µ-S)(CO) 2 (dppm) 2 was prepared independently and its reaction with parahydrogen was investigated, leading to results supporting endo addition H 2 as the major pathway and exo addition as a minor reaction channel. The complex Rh 2 (µ-S)(CO) 2 (dppm) 2 , which had not previously been found to react with dihydrogen, generates polarized hydride signals under para-enriched hydrogen, showing that an equilibrium is established between the complex and its H 2 adduct. In all of these cases, PHIP is used to detect species not readily seen by normal NMR spectroscopy and to investigate the mechanism of these reactions.

Parahydrogen Allows Ultrasensitive Indirect NMR Detection of Catalytic Hydrogen Complexes

“…The study of the characteristic line shape caused by the interaction of dissolved p-H 2 with the catalyst [1,4bis(diphenylphosphino)butane](1,5-cyclooctadiene)rhodium(I) tetrafluoroborate was performed in acetoned 6 . Parahydrogen enriched hydrogen gas is prepared at 77 K with about 50% of p-H 2 spin-isomer.…”

Mechanisms of Dipolar Ortho/Para-H2O Conversion in Ice

“…(11) The coherent evolution of the density operator under the influence of the Hamiltonian is governed by the following equation of motion (12) In this equation we have followed the NMR convention and set the constant ћ = 1, which is equivalent to measuring energies in angular frequency units. Since the Hamilton operator does not explicitly depend on time, the equation of motion of the density matrix can be solved with the help of the following unitary propagator (13) The time dependent density matrix is (14) Using r ˆ(t) the expectation value of any physical observable <Â> is given as: (15) In the case of conversion of para-ice to ortho-ice we are interested in the relative amount of molecule pairs which are still both in the para-state or which both are in the ortho-state after time t. These numbers are proportional to the expectation values of the matrices ( 16) and (17) Inserting the time dependent density matrix [14] gives (18) and (19) For the further discussion it is very useful to employ a base set where the Hamiltonian Eq. 10 is diagonal.…”

Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion

“…The mechanisms of the ortho/para conversion of hydrogen have been the topic of various experimental and theoretical studies. This interest in the conversion was triggered by three main reasons, namely (i) the application of combined studies of the ortho/para spin conversion of dihydrogen and of the hydrogen/deuterium scrambling reactions as tools for the elucidation of the mechanisms of catalytic hydrogenation reactions; 3,4 (ii) the application of para-hydrogen as an agent for the sensitivity enhancement of liquid state NMR, [5][6][7] in particular for studies of catalysis; and (iii) the quantum mechanical nature of the ortho-and para-states, which is interesting by itself; (iv) the usage of ortho/para-water ratios as remote sensors for the thermal history of the ice core of comets; 8-11 (v) to understand the mechanisms of the conversion of para-formaldehyde to ortho-formaldehyde by surface adsorption. 12 The studies of dihydrogen metal complexes started when Wilkinson found the first catalysts which are able to take up and easily activate molecular dihydrogen during the hydrogenation of double bonds.…”