The Grubbs catalyst – dichlorido(1-(2,6-diethylphenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene)-({5-nitro-2-[(propan-2-yl)oxy]phenyl}methylidene)ruthenium(II) – some observations on the crystallography and stereochemistry of a racemic mimic pair

Abstract The title species has been studied in two crystalline forms resulting from reports describing (1) a centrosymmetric form [ P 2 1 / n , Z ′ = 1.0] and (2) a Sohncke form [ P 2 1 , Z ′ = 2.0; Flack parameter = −0.04(2)]. They are related by the fact that (a) the two have, approximately, the same cell constants, while (b) Z ′ is doubled in the latter in the course of losing the n-glide plane. The existence of species displaying such a remarkable relationship was noted approximately 75 years ago, but, ignored probably because the discoverers (see below) never completed the necessary experimental work to fully document the phenomenon, inasmuch as (1) crystallography was in its infancy then and (2) synthesizing the necessary, pure, chiral species and growing the necessary high-quality crystals was a major challenge which has, not even yet, been accomplished. Moreover, O. Hassel, the major player in this field, was distracted by his work on the chair-boat equilibrium in cyclohexane and related rings systems for which he eventually won the Nobel Prize (1969). Herein, we have attempted to contribute a chapter to this curious crystallographic phenomenon, which has been made even more fascinating by the observation that a ruthenium Grubbs catalyst belongs in the racemic mimic class.