Stereoisomerism in Lanthanide Complexes: Enumeration, Chirality, Identification, Random Coordination Ratios

The concept of random
coordination ratios, RCRs, is advanced for lanthanide complexes. RCRs
describe the relative probabilities of occurrence of subsets of stereoisomers
of same-symmetry point groups in the limiting situation when energetic
effects are equivalent. We then introduce a method to uniquely identify
the stereoisomer of the coordination polyhedron of a given crystallographic
structure and introduce a notation that fully characterizes its stereochemistry
in an unambiguous manner, from which absolute configuration naturally
follows. De facto, the coordination chirality in lanthanide complexes
is a frequently overlooked property, even though these compounds often
exhibit, when luminescent, high dissymmetry factors. With our methodology,
we even managed to recognize a known dilanthanide complex as a meso
compound, with both metal ions functioning as stereogenic centers.
To achieve these results, we enumerate all possible stereoisomers
of lanthanide complexes with coordination numbers from 4 to 9 for
all combinations of monodentate, symmetric and asymmetric bidentate
ligands, and for several shapes of coordination polyhedra. We confirmed
the number of stereoisomers for each case by means of Pólya’s
theorem. We further classified all stereoisomers according to their
symmetry point groups and generated their Cartesian coordinates. This
collection of all coordination polyhedra stereoisomer geometries,
which is made available in the Supporting Information, can also be used to easily build starting-point geometries for
theoretical calculations of metal complexes.