A bistable [2]pseudorotaxane 1⊂CBPQT·4PF 6 and a bistable [2]rotaxane 2·4PF 6 have been synthesised to measure the height of an electrostatic barrier produced by double molecular oxidation (0 to +2). Both systems have monopyrrolotetrathiafulvalene (MPTTF) and oxyphenylene (OP) as stations for cyclobis(paraquat- p -phenylene) (CBPQT 4+ ). They have a large stopper at one end while the second stopper in 2 4+ is composed of a thioethyl (SEt) group and a thiodiethyleneglycol (TDEG) substituent, whereas in 1⊂CBPQT 4+ , the SEt group has been replaced with a less bulky thiomethyl (SMe) group. This seemingly small difference in the substituents on the MPTTF unit leads to profound changes when comparing the physical properties of the two systems allowing for the first measurement of the deslipping of the CBPQT 4+ ring over an MPTTF 2+ unit in the [2]pseudorotaxane. Cyclic voltammetry and 1 H NMR spectroscopy were used to investigate the switching mechanism for 1⊂CBPQT·MPTTF 4+ and 2·MPTTF 4+ , and it was found that CBPQT 4+ moves first to the OP station producing 1⊂CBPQT·OP 6+ and 2·OP 6+ , respectively, upon oxidation of the MPTTF unit. The kinetics of the complexation/decomplexation process occurring in 1⊂CBPQT·MPTTF 4+ and in 1⊂CBPQT·OP 6+ were studied, allowing the free energy of the transition state when CBPQT 4+ moves across a neutral MPTTF unit (17.0 kcal mol −1 ) or a di-oxidised MPTTF 2+ unit (24.0 kcal mol −1 ) to be determined. These results demonstrate that oxidation of the MPTTF unit to MPTTF 2+ increases the energy barrier that the CBPQT 4+ ring must overcome for decomplexation to occur by 7.0 kcal mol −1 .
more »
« less
Salts accelerate the switching kinetics of a cyclobis(paraquat- p -phenylene) [2]rotaxane
The rate at which the macrocyclic cyclobis(paraquat- p -phenylene) ring of a bistable [2]rotaxane moves from a tetrathiafulvalene station to an oxyphenylene station upon oxidation of the tetrathiafulvalene station is found to be increased in the presence of added salts. Compared to the salt-free case, 0.1 M solutions of a series of tetraalkylammonium hexafluorophosphate salts (R 4 N·PF 6 , R = H, Me, Et or n -Bu) and of tetrabutylammonium perchlorate ( n -Bu 4 N·ClO 4 ) all afford an increased switching rate, which is largest in the case of n -Bu 4 N·ClO 4 with smaller anions. Variation in the size of the ammonium cation has no significant effect. These results indicate that the addition of excess ions can be used as an accelerator to speed up shuttling processes in rotaxanes and catenanes based on the mobile cyclobis(paraquat- p -phenylene) ring, and that the choice of anion offers a convenient means of controlling the extent of this effect.
more »
« less
- Award ID(s):
- 1709909
- NSF-PAR ID:
- 10095269
- Date Published:
- Journal Name:
- Organic & Biomolecular Chemistry
- Volume:
- 17
- Issue:
- 9
- ISSN:
- 1477-0520
- Page Range / eLocation ID:
- 2432 to 2441
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract The self‐assembly in aqueous solution of the well‐known cyclophane, cyclobis(paraquat‐p‐phenylene) (BB4+), and two cucurbit[7]uril (CB7) hosts around a simple hydroquinol‐based, diamine guest (GH22+) was investigated by1H NMR and electronic absorption spectroscopies, electrospray mass spectrometry and DFT computations. The formation of a quaternary supramolecular assembly [GH22+⋅BB4+⋅ (CB7)2] was shown to be a very efficient process, which takes place not only because of the attractive forces between each of the hosts and the guest, but also because of the lateral interactions between the hosts in the final assembly. This complementary set of attractive interactions results in clear cooperative binding effects that help overcome the entropic barriers for multiple component assembly.
-
more » « less
Abstract Complexation between a viologen radical cation (
V.+ ) and cyclobis(paraquat‐p ‐phenylene) diradical dication (CBPQT2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair ofV.+ using radical‐pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+ ) in a size‐matched bisradical dicationic host — namely, cyclobis(paraquat‐2,6‐naphthalene)2(.+), i.e.,CBPQN2(.+) . Central to this dual recognition process was the choice of 2,6‐bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations inCBPQN2(.+) suitable for binding twoMV.+ with relatively short (3.05–3.25 Å) radical‐pairing distances. The size‐matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the twoMV.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex — namely, [(MV)2 ⊂CBPQN ]4( .+) – in MeCN was confirmed by VT1H NMR, as well as by EPR spectroscopy. The solid‐state superstructure of [(MV)2 ⊂CBPQN ]4( .+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three differentV.+ , suggesting that localization of the radical‐pairing interactions has a strong influence on the packing of the twoMV.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host‐guest radical‐pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox‐switchable recognition motif for the construction of MIMs and AMMs. -
more » « less
Abstract Complexation between a viologen radical cation (
V.+ ) and cyclobis(paraquat‐p ‐phenylene) diradical dication (CBPQT2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair ofV.+ using radical‐pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+ ) in a size‐matched bisradical dicationic host — namely, cyclobis(paraquat‐2,6‐naphthalene)2(.+), i.e.,CBPQN2(.+) . Central to this dual recognition process was the choice of 2,6‐bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations inCBPQN2(.+) suitable for binding twoMV.+ with relatively short (3.05–3.25 Å) radical‐pairing distances. The size‐matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the twoMV.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex — namely, [(MV)2 ⊂CBPQN ]4( .+) – in MeCN was confirmed by VT1H NMR, as well as by EPR spectroscopy. The solid‐state superstructure of [(MV)2 ⊂CBPQN ]4( .+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three differentV.+ , suggesting that localization of the radical‐pairing interactions has a strong influence on the packing of the twoMV.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host‐guest radical‐pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox‐switchable recognition motif for the construction of MIMs and AMMs. -
Functionalization of planar aromatic rings is very straightforward, up scalable, and economical in comparison with many azole, caged, linear or cyclic structures. In our present work, a facile synthesis of N , N ′-(4,6-dinitro-1,3-phenylene)dinitramide (3) is obtained by a single-step nitration of 4,6-dinitrobenzene-1,3-diamine (2). Compound 3 exhibits a surprisingly high density of 1.90 g cm −3 at 100 K (1.87 g cm −3 at 298 K). Its reactions with bases result in the formation of a series of energetic salts (4–7) which exhibit relatively high densities (1.74 to 1.83 g cm −3 ), and acceptable thermal sensitivities (177 to 253 °C). Energetic salt formation increases intermolecular hydrogen bonding while the planarity of the aromatic ring maximizes weak non-covalent interactions (π-stacking, cation/π, anion-π, X-H/π, etc. ,). The synergetic effect of these stabilizing interactions plays a crucial role in increasing thermal stability and decreasing sensitivity toward the external stimuli. Overall, these easily accessible new energetic compounds exhibit high densities and good denotation properties with potential applications as new high-energy materials.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9ob00085b
