Rapid, Reversible Heterolytic Cleavage of Bound H₂

Abstract

Heterolytic cleavage of dihydrogen into a proton and a hydride ion is a fundamentally important step in many reactions, including the oxidation of hydrogen by hydrogenase enzymes and ionic hydrogenation of organic compounds. We report the facile, reversible heterolytic cleavage of H2 in a manganese complex bearing a pendant amine, leading to the formation of a manganese hydride and a protonated amine that undergo H(+)/H(-) exchange at an estimated rate of >10(7) s(-1) at 25 °C.

Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H2

“…47,48 Although κ 3 -coordination is uncommon with the P 2 N 2 ligand family, the positioned amine could potentially bind to Fe; crystal structures of CrCl 3 (κ 3 -P Ph 2 N Bn 2 ), 62 [Mn(CO)(κ 3 -P Ph 2 N Bn 2 )(bppm)] + (bppm = (PAr F 2 ) 2 CH 2 ), 63,64 and [Mn(CO)(κ 3 -P Ph 2 N Bn 2 )-(dppm)] +63 show that P 2 N 2 ligands can adopt a κ 3 -bonding mode when the metal center is sufficiently electrophilic. Moreover, Rauchfuss and co-workers also reported a diiron complex with the amine of its azadithiolate ligand exhibiting κ 3 -bonding to one Fe site.…”

“…We believe that the HD experiment is critically important in exploring bifunctional H−H cleavage because it qualitatively establishes that rapid exchange is occurring and because quantifying the EIE provides a fundamental benchmark for future theoretical studies. Information about the EIE for [4-FeD(NH)] + ⇄ [4-FeH(ND)] + can be obtained from NMR spectroscopic studies; it requires knowledge of the "frozen out" chemical shifts of the This Fe system has a sufficiently low exchange barrier that this information cannot be directly obtained experimentally, as we were unable to observe decoalescence for 63 Chirik and co-workers reported an intramolecular EIE of K eq = 0.41 for the equilibrium between (C 5 Me 4 H) 2 Zr-(H)(ND t Bu) and (C 5 Me 4 H) 2 Zr(D)(NH t Bu) at 56°C; as in the reactions reported here, the MH/ND isotopomer is favored over MD/NH. 71 In CD 2 Cl 2 , the averaged resonance for [4-FeH(NH)] + appears at −5.36 ppm (slightly shifted from its value of −7.06 ppm in PhCl-d 5 ) and remains almost unchanged upon cooling to −80°C.…”

Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays

“…Abstraction of bromide from Mn(Br)(P Ph 2 N Bn 2 )(bppm)(CO) (Scheme 10) gives a cationic Mn I complex that has a κ 3 coordination mode of the P Ph 2 N Bn 2 ligand, as shown by crystallography. 50 Accounts of Chemical Research Article Use of HD rather than H 2 provided further evidence for the extremely fast, reversible heterolytic cleavage of H 2 . The 1 H NMR spectrum at −20°C had a resonance at δ −2.78, while the 2 H NMR spectrum exhibited a resonance at δ 7.8.…”

The Mononuclear Molybdenum Enzymes

“…The signal-giving species seen with H 2 -reduced enzyme is attributed to an η 2 complex of H 2 bound side-on at the copper (for which chemical precedents exist 243 ), analogous to the CO complex inferred from the ENDOR work described above. The bound H 2 is expected to be polarized by interaction with the copper, 244 and the reaction has been proposed to proceed by deprotonation to a copper hydride, which subsequently deprotonates to populate the delocalized redox-active orbital of the binuclear cluster. 242 …”