Design and Enantioresolution of Homochiral Fe(II)–Pd(II) Coordination Cages from Stereolabile Metalloligands: Stereochemical Stability and Enantioselective Separation

Abstract

The stereochemistry of chiral-at-metal complexes is much more abundant, albeit complicated, than chiral-at-carbon compounds, but how to make use of stereolabile metal-centers remains a formidable challenge due to the highly versatile coordination geometry of metal ions and racemization/epimerization problem. We demonstrate herein a stepwise assembly of configurationally stable [Pd 6 (FeL 3 ) 8 ] 28+ (Δ/Λ-MOCs-42) homochiral octahedral cages from unstable D 3 -symmetry trischelate-Fe type metalloligands via strong face-directed stereochemical coupling and facile chiral-induced resolution processes based on stereodifferentiating host−guest dynamics. Kinetic studies reveal that the dissociation rate of MOC-42 cages is 100-fold slower than that of Femetalloligands and the racemization is effectively inhibited, making the cages retain their chirality over extended periods of time (>5 months) at room temperature. Recyclable enantioseparation of atropisomeric compounds has been successfully achieved, giving up to 88% ee.

A Tris(3‐pyridyl)stannane as a Building Block for Heterobimetallic Coordination Polymers and Supramolecular Cages

“…The 19 Fa nd 1 HNMR spectra are also concentrationdependent, with the low-temperature species being favoured in more concentrated samples and the high-temperature species being favoured in dilute solutions ( Figure S7 in the Supporting Information). The concentration-and temperature-dependenceo ft he 1 Ha nd 19 FNMR spectra are consistentw ith a dynamic process involving dissociation of the cage [Cu(1)(NC-Me)] 4 [PF 6 ] 4 into smaller species at higher temperatures and lower concentrations.V ariable-temperature NMR experiments also confirm that the observed equilibrium is completely reversible. On the basis of the integrationo ft he 19 FNMR signals at 228 K, we assign the resonance at d = À71.0 to the cage-encapsulated PF 6 À environment of the [{Cu(1)(NCMe)} 4 (&PF 6 )] 3 + cation and the resonance at d = À72.4 ppm to the free PF 6 À counter-anions (found in the expected 1:3r atio, respectively).…”

“…Ar adically different outcomeiso bserved in the coordination of 1 to CuPF 6 ,c ontaining aw eakly-coordinating anion. The 1:1 reaction of 1 with [Cu(NCMe) 4 ]PF 6 gives the complex [Cu(1)(NCMe)] 4 [PF 6 ] 4 in 91 %i solated yield ( Figure 4) (see Exper-imentalS ection). The X-ray crystal structure showsacage arrangementi nw hich four of the ligands 1 bridge the four Cu centres,u sing all three pyridyl arms, into ad iscrete [{Cu(1)(NC-Me)} 4 (&PF 6 )] 3 + cation.…”

“…arises from an encapsulated PF 6 À anion within ac age closely related to the [Cu(1)(NCMe)] 4 4 + cation. In contrast, the 1:1r eaction of 1 with AgPF 6 in MeCN does not result in the formation of ad iscrete molecular structure.…”

“…In contrast, the 1:1r eaction of 1 with AgPF 6 in MeCN does not result in the formation of ad iscrete molecular structure. While the ligands 1 adopt as imilar coordination mode to that in [Cu(1)(NCMe)] 4 [PF 6 ] 4 ,i nvolving all three of the pyridyl-N atoms, the product {[Ag(1)(NCMe)]PF 6 ·OEt 2 } n is a2 D-coordination polymer (Figure 7) (see Experimental Section).T he network is composed of fused Ag 3 Sn 3 rings, with the PF 6 À anions interspersed within the 2D network, andf orming weak F···Ag interactions with some of the Ag + cations (less than the sum of the van der Waals radii). The presence of these F···Ag interactions is not surprising bearing in mind the larger ionic radius and more electropositive character of Ag + ,a nd provides ap otential reason why ap olymeric structure is formed in this case rather than ad iscrete cage (cf.…”

“…[2] Severin and co-workersh ave overcome this tendency by incorporatinga dditional steric groups into bis(3-pyridyl) Fe clathrochelate ligandst op roduce Pd 6 L 12 12 + cages, [2] and have found that tuningt he sterics alters the size of the cage. [3] This metalloligand strategy for the construction of heterometallic supramolecular architectures has been successfully enacted for av ariety of mixed transition metals ystems, for example, Fe/Pd, [2,[4][5][6] Fe/Ru, [7] Fe/Co, [8,9] Co/ Mn [5] and Ru/Pd. [10,11] However,t he use of main group metalloligands to createmixed main group/transitionmetal heterometallic systems is rare.…”