Mechanistically Driven Control over Cubane Oxo Cluster Catalysts

Predictive and mechanistically driven
access to polynuclear oxo
clusters and related materials remains a grand challenge of inorganic
chemistry. We here introduce a novel strategy for synthetic control
over highly sought-after transition metal {M4O4} cubanes. They attract interest as molecular water oxidation catalysts
that combine features of both heterogeneous oxide catalysts and nature’s
cuboidal {CaMn4O5} center of photosystem II.
For the first time, we demonstrate the outstanding structure-directing
effect of straightforward inorganic counteranions in solution on the
self-assembly of oxo clusters. We introduce a selective counteranion
toolbox for the controlled assembly of di­(2-pyridyl) ketone (dpk)
with M­(OAc)2 (M = Co, Ni) precursors into different cubane
types. Perchlorate anions provide selective access to type 2 cubanes with the characteristic {H2O-M2(OR)2-OH2} edge-site, such as [Co4(dpy-C­{OH}­O)4(OAc)2(H2O)2]­(ClO4)2. Type 1 cubanes with separated polar faces
[Co4(dpy-C­{OH}­O)4(L2)4]n+ (L2 = OAc, Cl, or OAc and H2O) can be
tuned with a wide range of other counteranions. The combination of
these counteranion sets with Ni­(OAc)2 as precursor selectively
produces type 2 Co/Ni-mixed or {Ni4O4} cubanes. Systematic mechanistic experiments in combination with
computational studies provide strong evidence for type 2 cubane formation through reaction of the key dimeric building block
[M2(dpy-C­{OH}­O)2(H2O)4]2+ with monomers, such as [Co­(dpy-C­{OH}­O)­(OAc)­(H2O)3]. Furthermore, both experiments and DFT calculations
support an energetically favorable type 1 cubane formation
pathway via direct head-to-head combination of two [Co2(dpy-C­{OH}­O)2(OAc)2(H2O)2] dimers. Finally, the visible-light-driven water oxidation activity
of type 1 and 2 cubanes with tuned ligand
environments was assessed. We pave the way to efficient design concepts
in coordination chemistry through ionic control over cluster assembly
pathways. Our comprehensive strategy demonstrates how retrosynthetic
analyses can be implemented with readily available assembly directing
counteranions to provide rapid access to tuned molecular materials.