Establishing the Family of Diruthenium Water Oxidation Catalysts Based on the Bis(bipyridyl)pyrazolate Ligand System

A bis­(bipyridyl)­pyrazolate (Mebbp–) has recently been introduced as a
rugged dinucleating, bis­(tridentate) ligand for the formation of efficient
diruthenium water oxidation catalysts (J. Am. Chem. Soc. 2014, 136, 24–27). Now, detailed
protocols for the synthesis of a whole family of such dinuclear ruthenium
complexes [{Ru­(pyR2)2}2­(μ-R1bbp)­(X,Y)]2+ based on the bis­(bipyridyl)­pyrazolate
scaffold are reported. The isolation of a synthetic key intermediate
allowed the straightforward introduction of different pyridines as
axial ligands. Thereby, a set of complexes with different substituents
at the pyrazolate backbone (R1 = Br, H, Me), different
pyridines as axial ligand (R2 = H, NMe2, SO3), and different (non)­bridging units in the in,in-position (X,Y = Cl, H2O, OAc) has been prepared and thoroughly
characterized. Complexes of the type [{Ru­(pyR2)2}2­(μ-R1bbp)­(μ-OAc)]2+, with an exogenous acetato bridge, have been used as catalyst precursors
in catalytic water oxidation experiments with a sacrificial oxidant.
The effect of substitution on the pyrazole core of the R1bbp– ligand as well as on the pyridine ligands
on both electrochemistry and catalytic activity has been systematically
investigated. The catalyst stability, reflected by the turnover number,
is crucially determined by the substituent at the pyrazolate ligand
(R1 = Me > H > Br). In contrast, the axial pyridine
ligands modulate the rate of the catalytic process, expressed by the
initial turnover frequency (R2 = H > NMe2H+).