Incorrect atom connectivity in X-ray structure solutions associated with a “partial polar ambiguity”: a non-macrocyclic structure for the macrocyclic lead complex, [η4-Me8taa]Pb

Abstract

The divalent lead complex [η⁴-Me₈taa]Pb has been synthesized by metathesis of PbI₂ with Li₂[Me₈taa] in THF (Me₈taaH₂=octamethyldibenzotetraaza[14]annulene). [η⁴-Me₈taa]Pb exists as (at least) three structural modifications, namely the triclinic chloroform solvate [η⁴-Me₈taa]Pb·CHCl₃, and the primitive and C-centered orthorhombic polymorphs of the dichloromethane solvate [η⁴-Me₈taa]Pb·CH₂Cl₂. The structures of each of these modifications have been determined by single crystal X-ray diffraction. Of most significance, the C-centered orthorhombic polymorph [η⁴-Me₈taa]Pb·CH₂Cl₂ is subject to the existence of a false minimum in the structure solution refinement procedure, the result of which is the generation of a non-macrocyclic structure for a compound that is actually macrocyclic. Significantly, even though it does not exhibit the true connectivity of the molecule, the errant structure is characterized by a low R value and well-behaved displacement parameters. The manifestation of the false minimum is a consequence of the crystal belonging to a polar space group, which results in a “partial polar ambiguity.” As such, the result serves to emphasize that care must be taken to ensure that, for polar space groups, all atoms of the derived structure belong to a single true polar configuration, rather than a hybrid of the two possible true polar configurations.