Proton-responsive Ruthenium(II) Catalysts for the Solvolysis of Ammonia-Borane

Two
water-soluble ruthenium­(II) complexes (1 and 2) containing the proton-responsive 6,6′-dihydroxy-2,2′-bipyridine
(dhbp) ligand were studied as precatalysts for the solvolysis of ammonia-borane
(AB) in water and/or alcohols. Both precursors generate in situ the
same catalytic system. In spite of the excellent initial activities
observed, it suffered a sudden deactivation at extended conversions.
Detailed experimental studies combined with theoretical (density functional
theory) calculations permitted us to describe the reaction mechanism
as two interrelated pH-dependent catalytic cycles operating within
a single run. According to this hypothesis, the deactivation observed
occurred as a consequence of the increase in NH3 concentration
and pH along the reaction progress. It was attributed to deprotonation
of the dhbp ligand and amination or hydroxylation of the resulting
compound. Accordingly, reactions performed in buffered solutions at
neutral pH retarded this process, and up to 2.5 equiv of H2 per mole of AB were liberated in less than 1 min (TOF50% up to 33 113 h–1 (TOF = turnover frequency)).
When the reaction was run in alcohols (MeOH, EtOH, or iPrOH) it proceeded
at faster rates than in water, the one discussed here being the first
homogeneous catalytic system active for this process. Actually, in
MeOH, the initial TOFs surpasses that of any metallic or organometallic
catalyst reported for AB alcoholysis.