Modular, homochiral, porous coordination polymers: Rational design, enantioselective guest exchange sorption and ab initio calculations of host-guest interactions

Two new, homochiral, porous metal-organic coordination polymers [Zn ₂(ndc){(R)-man)(dmf)]·3DMF and [Zn₂(bpdc){(R)-man} (dmf)]·2DMF (ndc = 2,6-naphthalenedicarboxylate ; bpdc=4,4′- biphenyldicarboxylate ; man = mandelate; dmf=N,N'-dimethylformamide) have been synthesized by heating Zn^IInitrate, H₂ndc or H ₂bpdc and chiral (R)-mandelic acid (H₂man) in DMF. The colorless crystals were obtained and their structures were established by single-crystal X-ray diffraction. These isoreticular structures share the same topological features as the previously reported zinc(II) terephthalate lactate [Zn₂(bdc){(S)-lac}(dmf)]·DMF framework, but have larger pores and opposite absolute configuration of the chiral centers. The enhanced pores size results in differing stereoselective sorption properties: the new metal-organic frameworks effectively and stereoselectively (ee up to 62%) accommodate bulkier guest molecules (alkyl aryl sulfoxides) than the parent [Zn₂(bdc)((S)-lac}(dmf)]·DMF, while the latter demonstrates decent enantioselectivity toward precursor of chiral anticancer drug sulforaphane, CH₃SO(CH₂)₄OH. The new homochiral porous metal-organic coordination polymers are capable of catalyzing a highly selective oxidation of bulkier sulfides (2-NaphSMe (2-C₁₀H ₇SMe) and PhSCH₂Ph) that could not be achieved by the smaller-pore [Zn₂(bdc)((S)-lac}(dmf)]·DMF. The sorption of different guest molecules (both R and S isomers) into the chiral pores of [Zn₂(bdc){(S)-lac)(dmf)]·DMF was modeled by using ab initio calculations that provided a qualitative explanation for the observed sorption enantioselectivity. The high stereo-preference is accounted for by the presence of coordinated inner-pore DMF molecule that forms a weak C-H⋯O bond between the DMF methyl group and the (S)PhSOCH₃ sulfinyl group.