Resolution of Seven‐Coordinate Complexes

Abstract

The crystal structures of [Pr(dbm)3H2O] (1), [Sm(dbm)3H2O] (2), and [Er(dbm)3H2O] (3) have been determined (dbm=dibenzoylmethane). They display seven-coordinate propeller-shaped molecules, which are chiral and crystallize as conglomerates in space group R3. Analysis of the crystal structures reveals supramolecular interactions, including formation of a quadruple helix, which explain how stereochemical information can be transferred between stacks of molecules. A method to quantify the ee in bulk samples of stereochemically labile compounds by using solid-state CD spectroscopy is described. Using this method, it has been shown that compounds 1-3 undergo total spontaneous resolution directly after synthesis, forming a microcrystalline reaction product that is essentially enantiopure. The resolution of bulk quantities of seven-coordinate complexes (without chiral or polydentate ligands) is thus reported for the first time. Because the crystallization starts without seeding, the overall preparation may be regarded as absolute asymmetric synthesis.

Absolute Asymmetric Synthesis of Stereochemically Labile Aldehyde Helicates and Subsequent Chirality Transfer Reactions

“…Crystallographic data for complexes 2-6. 153 (2) 193 (2) 143 (2) 123 (2) 123 (2) [8,9,[26][27][28] that yield enantiopure crystal batches without stirring. In these cases preferential crystallization is achieved by single-colony growth, a mechanism that also depends on secondary nucleation.…”

“…Solutions of enantiopure samples of 1-3 are CD silent owing to rapid racemization in solution, but solid-state CD spectroscopy is a viable option. [27][28][29] Indeed, by grinding a single crystal of L-3 with KBr and pressing a thin disk (the disk preparation is similar to IR spectroscopy, but much less sample is needed and thorough grinding is essential) it is possible to record the CD spectrum of the L enantiomer. The absolute configuration of the crystal was known since diffraction data (including Friedel pairs) could be collected by using the very same crystal before grinding.…”

“…Solid-state CD spectroscopy is a powerful method by which to determine the ee (and indirectly, the absolute configuration) of batches of stereochemically labile molecules. [27] Information concerning the mechanism of the solid-state reaction of the ATPH-tolualdehyde complex (3) with organometallic reagents has been obtained since the absolute configuration of both the substrate complex and the product are known. Although still at a moderate level of enantioselectivity, it is apparent that stereochemical information can be transferred in "crystal-to-crystal" organometallic reactions (using stereochemically labile complexes).…”

“…Further verification of the enantiopurity of batches of 1-3 has now been obtained since alkylation of a single crystal of 1-3 gave similar ee values to the alkylation of a batch sample of 1-3. Actually, solid-state reactions could provide an alternative method (to solid-state CD spectroscopy) [27] by which to determine the ee of stereochemically labile samples. [34] It is evident from entries 3 and 10 in Table 3 that the use of suspensions of ATPH complexes in hexane at low temperature is indeed detrimental to the enantioselectivity.…”

Total Spontaneous Resolution of Chiral Covalent Networks from Stereochemically Labile Metal Complexes

“…[1,2] Owing to the stereochemical lability of these complexes in solution and hence their tendency to enantiomerise rapidly in solution, conventional solution-based methods for resolution of such enantiomers are ineffective. If, however, the complex crystallises as a conglomerate, it may undergo crystallisation-induced asymmetric transformation, also known as total spontaneous resolution.…”