Supramolecular Selection in Molecular Alloys

Complexes of the type [M­(phen)3]­(PF6)2 (M = Ni­(II), Fe­(II), Ru­(II) and phen = 1,10-phenanthroline)
were found to co-crystallize to form molecular alloys (solid solutions
of molecules) with general formula [MAxMB1–x(phen)3]­(PF6)2·0.5H2O in which
the relative concentrations of the metal complexes in the crystals
closely match those in the crystallizing solution. Consequently, the
composition of the co-crystals can be accurately predicted and controlled
by modulating the relative concentrations of the metal complexes in
the crystallizing solution. Although they are chemically and structurally
similar, complexes of the type [M­(bipy)3]­(PF6)2 (M = Ni­(II), Fe­(II), Ru­(II) and bipy = 2,2′-bipyridine)
display markedly different behavior upon co-crystallization. In this
case, the resulting co-crystals of general formula [MAxMB1–x(bipy)3]­(PF6)2 have relative
concentrations of the constituent complexes that are markedly different
from the relative concentrations of the complexes initially present
in the crystallizing solution. For example, when the nickel and iron
complexes are co-crystallized from a solution containing a 50:50 ratio
of each, the result is the formation of some crystals with a higher
proportion of iron and others with a higher proportion of nickel.
The relative concentrations of the metal complexes in the crystals
can vary from those in the crystallizing solutions by as much as 15%.
This result was observed for a range of combinations of metal complexes
(Ni/Fe, Ni/Ru, and Fe/Ru) and a range of starting concentrations in
the crystallizing solutions (90:10 through to 10:90 in 10% increments).
To explain this remarkable result, we introduce the concept of “supramolecular
selection”, which is a process driven by molecular recognition
that leads to the partially selective aggregation of like molecules
during crystallization.