Conformational Flexibility of Hybrid [3]- and [4]-Rotaxanes

Abstract

The synthesis, structures, and properties of [4]- and [3]-rotaxane complexes are reported where [2]-rotaxanes, formed from heterometallic {Cr 7 Ni} rings, are bound to a fluoride-centered {CrNi 2 } triangle. The compounds have been characterized by single-crystal X-ray diffraction and have the formulas [CrNi 2 (F)(O 2 C t Bu) 6 ]{( B H)[Cr 7 NiF 8 (O 2 C t Bu) 16 ]} 3 ( 3 ) and [CrNi 2 (F)(O 2 C t Bu) 6 (THF)]{( B H)[Cr 7 NiF 8 (O 2 C t Bu) 16 ]} 2 ( 4 ), where B = py-CH 2 CH 2 NHCH 2 C 6 H 4 SCH 3 . The [4]-rotaxane 3 is an isosceles triangle of three [2]-rotaxanes bound to the central triangle while the [3]-rotaxane 4 contains only two [2]-rotaxanes bound to the central triangle. Studies of the behavior of 3 and 4 in solution by small-angle X-ray scattering and atomistic molecular dynamic simulations show that the structure of 3 is similar to that found in the crystal but that 4 has a different conformation to the crystal. Continuous wave and pulsed electron paramagnetic resonance spectroscopy was used to study the structures present and demonstrate that in frozen solutions (at 5 K) 4 forms more extended molecules than 3 and with a wider range of conformations.

Gold(i) bridged dimeric and trimeric heterometallic {Cr7Ni}-based qubit systems and their characterization

“…The results also indicate that DEER must be used with some caution in supramolecular systems. Previously 15 we have published examples where SAXS demonstrates that a complicated assembly exists in solution, and the DEER is then consistent with the SAXS. Here we demonstrate by SAXS that the oligomers do not exist in mobile solution but that we can freeze them out and see the dipolar interaction by DEER even in these equilibrium mixtures.…”

“…Even if 4 were intact we would expect the shorter distance peak to be more intense based on precedent. 15 Here where we have significant dissociation we are unable to measure any ring⋯ring distances. A further complication may be the possibility of different conformers of 4 (see Scheme 2).…”

“…This feature is known and is due to S = 1/2 ground state of the heterometallic ring which centres on a g ≈ 1.78. 15 A well resolved g z feature of the {Cr 7 Ni} ring is seen in the powder spectra of both compounds, but is absent in solution spectra (Fig. 5a).…”

“…The solution spectra are less well-resolved and lines are very slightly shifted down field, however the spectra are all typical of {Cr 7 Ni} rings when isolated 3 or when in weakly coupled oligomers. 15 Therefore the c.w. EPR spectra do not show whether the structures are maintained in solution.…”

Pb2+-Containing Metal-Organic Rotaxane Frameworks (MORFs)

“…Specifically, metal-organic rotaxane framework (MORF) structures involve mechanically interlocking molecules (MIMs; i.e., rotaxanes) [ 8 , 9 , 10 , 11 , 12 ] and attract interest of the chemical community. To date, crown ether [ 13 , 14 , 15 , 16 , 17 , 18 ], “blue box” (CBPQT 4+ ) [ 19 , 20 ], cucurbiurils [ 21 , 22 , 23 ], octametallic metallacrown [ 24 , 25 , 26 , 27 ], and polyamide [ 28 , 29 ] have been utilized to produce a series of MORF structures. The Texas-sized molecular box ([cyclo[2](2,6-di(1 H -imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene), 1 4+ ) [ 30 , 31 ] was developed to construct MORFs cooperating with metal cations (e.g., Zn 2+ , Ag + , or lanthanide) and aromatic dicarboxylate anions ( Figure 1 ).…”