Crystal Structure and Solution Species of Ce(III) and Ce(IV) Formates: From Mononuclear to Hexanuclear Complexes

Cerium­(III)
and cerium­(IV) both form formate complexes. However,
their species in aqueous solution and the solid-state structures are
surprisingly different. The species in aqueous solutions were investigated
with Ce K-edge EXAFS spectroscopy. Ce­(III) formate shows only mononuclear
complexes, which is in agreement with the predicted mononuclear species
of Ce­(HCOO)2+ and Ce­(HCOO)2+. In
contrast, Ce­(IV) formate forms in aqueous solution a stable hexanuclear
complex of [Ce6(μ3-O)4(μ3-OH)4(HCOO)x(NO3)y]12–x−y. The structural differences reflect
the different influence of hydrolysis, which is weak for Ce­(III) and
strong for Ce­(IV). Hydrolysis of Ce­(IV) ions causes initial polymerization
while complexation through HCOO– results in 12 chelate
rings stabilizing the hexanuclear Ce­(IV) complex. Crystals were grown
from the above-mentioned solutions. Two crystal structures of Ce­(IV)
formate were determined. Both form a hexanuclear complex with a [Ce6(μ3-O)4(μ3-OH)4]12+ core in aqueous HNO3/HCOOH solution.
The pH titration with NaOH resulted in a structure with the composition
[Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)2(H2O)3]·(H2O)9.5, while
the pH adjustment with NH3 resulted in [Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)4]·(NO3)3(NH4)5(H2O)5. Furthermore,
the crystal structure of Ce­(III) formate, Ce­(HCOO)3, was
determined. The coordination polyhedron is a tricapped trigonal prism
which is formed exclusively by nine HCOO– ligands.
The hexanuclear Ce­(IV) formate species from aqueous solution is widely
preserved in the crystal structure, whereas the mononuclear solution
species of Ce­(III) formate undergoes a polymerization during the crystallization
process.