Abiotic Metallofoldamers as Electrochemically Responsive Molecules

Abstract

Described are the design, synthesis, and study of nonbiological molecules based on salophen and salen ligands that fold into single-stranded helices in the presence of either Ni(II) or Cu(II). X-ray diffraction studies show that the materials fold into helical structures in the solid state, and a series of NMR studies provide strong evidence that the folded structures are conserved in solution. Metal coordination is required for folding, as NMR and X-ray show that the free ligands do not adopt helical structures. Two of the racemic metallofoldamers spontaneously resolve during crystallization from CHCl3/acetonitrile, and CD spectroscopy and optical rotation show that the resolved, crystalline materials racemize quickly when dissolved at 5 degrees C. This shows that the secondary structures can reorganize easily and can, therefore, provide the basis for responsive materials. By comparison, an analogue from enantiomerically pure (R,R)-(-)-trans-cyclohexanediamine showed a strong CD signal and a large specific rotation. Electrochemical experiments show that a structural reorganization occurs upon metal-centered reduction of a Cu(II)-containing foldamer. When the reduction is carried out in the presence of coordinating ligands, it is proposed that apical binding of those ligands gives square pyramidal complexes. Semiempirical (AM1) calculations support that the helical structure would be disrupted by the reduction to Cu(I) with concomitant reorganization to a square pyramidal complex.

Conformational Control in Metallofoldamers: Design, Synthesis and Structural Properties

“…[6] "Helicates", molecules that template an abiotic helix, have been broadly investigated and reviewed. [1,7] This field includes short single-stranded oligopyridines, oligo(m-phenyleneethynylene) and zinc bilinones, all fold into helices upon coordination to metal ions.…”

Dynamic Helicity Control of Oligo(salamo)-Based Metal Helicates

“…In particular, the 1,2-disubstituted ethylene bridge in the chiral salen unit facilitated the synthesis of dynamic helical structures in which the preferred helicity is rationally predicted from the structure of the organic framework. In addition to the oligo(salamo) helical structures reported in this article, there have already been a number of reports on various kinds of helical structures based on the monomeric [31][32][33][34][35][36][37][38][39][40][41], dimeric [42,43] and polymeric [44][45][46] salen-type ligands, as well as the tris(saloph) triple-helical cages [93,94] and single-helix with different types of donor sets [90,[95][96][97]. It has already been demonstrated that the oligo(salamo) helical complexes show unique physical properties and reactivities [98][99][100][101], and the dynamic helicity control would be useful for switching of the chiroptical properties of the helical structures.…”

“…As one of the possible candidates of dynamic helical complexes, we designed metal complexes having a series of acyclic oligo(salen)-type ligands (H 2 salen = N,N'-disalicylideneethylenediamine) ( Figure 5a) [25][26][27][28][29][30], while there are several types of related helical structures based on monomeric [31][32][33][34][35][36][37][38][39][40][41], dimeric [42,43] and polymeric [44][45][46] salen-type ligands. We employed salamo derivatives, the oxime analog of salen, as the constituent of the oligomers (H 2 salamo = 1,2-bis(salicylideneaminooxy)ethane) [47,48].…”

Stimuli-responsive Synthetic Metallopeptides

“…The coordination number, change, and geometry of redoxactive metals enable the metal center to serve as a hub for electron-mediated, dynamic structural changes of metallopeptides, in light of previous examples of metal-mediated structure controls using catenane, 50 rotaxane, 51 helicate, 52,53 and rotor. 54 Arora, Canary, et al synthesized a new 17-residue peptide, Ac-SIRKLEYEIEELRLRIG-NH 2 , 25 with a view to realizing ¡-helix-to-¢-sheet transition by redox-active Cu ions.…”