Reversible Heterolytic Cleavage of the H–H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride

Controlling
the heterolytic cleavage of the H–H bond of
dihydrogen is critically important in catalytic hydrogenations and
in the catalytic oxidation of H2. We show how the rate
of reversible heterolytic cleavage of H2 can be controlled,
spanning 4 orders of magnitude at 25 °C, from 2.1 × 103 s–1 to ≥107 s–1. Bifunctional Mo complexes, [CpMo­(CO)­(κ3-P2N2)]+ (P2N2 =
1,5-diaza-3,7-diphosphacyclooctane diphosphine ligand with alkyl/aryl
groups on N and P), have been developed for heterolytic cleavage of
H2 into a proton and a hydride, akin to frustrated Lewis
pairs. The H–H bond cleavage is enabled by the basic amine
in the second coordination sphere. The products of heterolytic cleavage
of H2, Mo hydride complexes bearing protonated amines,
[CpMo­(H)­(CO)­(P2N2H)]+, were characterized by spectroscopic studies and by
X-ray crystallography. Variable-temperature 1H, 15N, and 2-D 1H–1H ROESY NMR spectra indicated
rapid exchange of the proton and hydride. The exchange rates are in
the order [CpMo­(H)­(CO)­(PPh2NPh2H)]+ > [CpMo­(H)­(CO)­(PtBu2NPh2H)]+ >
[CpMo­(H)­(CO)­(PPh2NBn2H)]+ > [CpMo­(H)­(CO)­(PtBu2NBn2H)]+ >
[CpMo­(H)­(CO)­(PtBu2NtBu2H)]+. The pKa values
determined in acetonitrile range from 9.3 to 17.7 and show a linear
correlation with the logarithm of the exchange rates. This correlation
likely results from the exchange process involving key intermediates
that differ by an intramolecular proton transfer. Specifically, the
proton-hydride exchange appears to occur by formation of a molybdenum
dihydride or dihydrogen complex, resulting from proton transfer from
the pendant amine to the metal hydride. The exchange dynamics are
controlled by the relative acidity of the [CpMo­(H)­(CO)­(P2N2H)]+ and
[CpMo­(H2)­(CO)­(P2N2)]+ isomers, providing a design principle for controlling
heterolytic cleavage of H2.