An official website of the United States government
Abstract Mechanically interlocked molecules (MIMs) represent an exciting yet underexplored area of research in the context of carbon nanoscience. Recently, work from our group and others has shown that small carbon nanotube fragments—[n]cycloparaphenylenes ([n]CPPs) and related nanohoop macrocycles—may be integrated into mechanically interlocked architectures by leveraging supramolecular interactions, covalent tethers, or metal‐ion templates. Still, available synthetic methods are typically difficult and low yielding, and general methods that allow for the creation of a wide variety of these structures are limited. Here we report an efficient route to interlocked nanohoop structures via the active template Cu‐catalyzed azide‐alkyne cycloaddition (AT−CuAAC) reaction. With the appropriate choice of substituents, a macrocyclic precursor to 2,2′‐bipyridyl embedded [9]CPP (bipy[9]CPP) participates in the AT−CuAAC reaction to provide [2]rotaxanes in near‐quantitative yield, which can then be converted into the fully π‐conjugated catenane structures. Through this approach, two nanohoop[2]catenanes are synthesized which consist of a bipy[9]CPP catenated with either Tz[10]CPP or Tz[12]CPP (whereTzdenotes a 1,2,3‐triazole moiety replacing one phenylene ring in the [n]CPP backbone).
more »
« less
Spin‐Crossover Properties of an Iron(II) Coordination Nanohoop
Abstract Addition of the bipyridyl‐embedded cycloparaphenylene nanohoop bipy[9]CPP to [Fe{H2B(pyz)2}] (pyz=pyrazolyl) produces the distorted octahedral complex [Fe(bipy[9]CPP){H2B(pyz)2}2] (1). The molecular structure of1shows that the nanohoop ligand contains a non‐planar bipy unit. Magnetic susceptibility measurements indicate spin‐crossover (SCO) behaviour with aT1/2of 130 K, lower than that of 160 K observed with the related compound [Fe(bipy){H2B(pyz)2}2] (2), which contains a conventional bipy ligand. A computational study of1and2reveals that the curvature of the nanohoop leads to the different SCO properties, suggesting that the SCO behaviour of iron(II) can be tuned by varying the size and diameter of the nanohoop.
more »
« less
- Award ID(s):
- 1800586
- PAR ID:
- 10236752
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Angewandte Chemie International Edition
- Volume:
- 60
- Issue:
- 7
- ISSN:
- 1433-7851
- Page Range / eLocation ID:
- p. 3515-3518
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Exploration of the reduction chemistry of the 2,2’‐bipyridine (bipy) lanthanide metallocene complexes Cp*2LnCl(bipy) and Cp*2Ln(bipy) (Cp* = C5Me5) resulted in the isolation of a series of complexes with unusual composition and structure including complexes with a single Cp* ligand, multiple azide ligands, and bipy ligands with close parallel orientations. These results not only reveal new structural types, but they also show the diverse chemistry displayed by this redox‐active platform. Treatment of Cp*2NdCl(bipy) with excess KC8resulted in the formation of the mono‐Cp* Nd(III) complex, [K(crypt)]2[Cp*Nd(bipy)2],1, as well as [K(crypt)][Cp*2NdCl2],2, and the previously reported [K(crypt)][Cp*2Nd(bipy)]. A mono‐Cp* Lu(III) complex, Cp*Lu(bipy)2,3, was also found in an attempt to make Cp*2Lu(bipy) from LuCl3, 2 equiv. of KCp*, bipy, and K/KI. Surprisingly, the (bipy)1−ligands in neighboring molecules in the structure of3are oriented in a parallel fashion with intermolecular C⋅⋅⋅C distances of 3.289(4) Å, which are shorter than the sum of van der Waals radii of two carbon atoms, 3.4 Å. Another product with one Cp* ligand per lanthanide was isolated from the reaction of [K(crypt)][Cp*2Eu(bipy)] with azobenzene, which afforded the dimeric Eu(II) complex, [K(crypt)]2[Cp*Eu(THF)(PhNNPh)]2,4. Attempts to make4from the reaction between Cp*2Eu(THF)2and a reduced azobenzene anion generated instead the mixed‐valent Eu(III)/Eu(II) complex, [K(crypt)][Cp*Eu(THF)(PhNNPh)]2,5, which allows direct comparison with the bimetallic Eu(II) complex4. Mono‐Cp* complexes of Yb(III) are obtained from reactions of the Yb(II) complex, [K(crypt)][Cp*2Yb(bipy)], with trimethylsilylazide, which afforded the tetra‐azido [K(crypt)]2[Cp*Yb(N3)4],6, or the di‐azido complex [K(crypt)]2[Cp*Yb(N3)2(bipy)],7 a, depending on the reaction stoichiometry. A mono‐Cp* Yb(III) complex is also isolated from reaction of [K(crypt)][Cp*2Yb(bipy)] with elemental sulfur which forms the mixed polysulfido Yb(III) complex [K(crypt)]2[Cp*Yb(S4)(S5)],8 a. In contrast to these reactions that form mono‐Cp* products, reduction of Cp*2Yb(bipy) with 1 equiv. of KC8in the presence of 18‐crown‐6 resulted in the complete loss of Cp* ligands and the formation of [K(18‐c‐6)(THF)][Yb(bipy)4],9. The (bipy)1−ligands of9are arranged in a parallel orientation, as observed in the structure of3, except in this case this interaction is intramolecular and involves pairs of ligands bound to the same Yb atom. Attempts to reduce further the Sm(II) (bipy)1−complex, Cp*2Sm(bipy) with 2 equiv. of KC8in the presence of excess 18‐crown‐6 led to the isolation of a Sm(III) salt of (bipy)2−with an inverse sandwich Cp* counter‐cation and a co‐crystallized K(18‐c‐6)Cp* unit, [K2(18‐c‐6)2Cp*]2[Cp*2Sm(bipy)]2 ⋅ [K(18‐c‐6)Cp*],10.more » « less
-
Abstract Co‐crystallization of the spin‐crossover (SCO) cationic complex, [Fe(1‐bpp)2]2+(1‐bpp=2,6‐bis(pyrazol‐1‐yl)pyridine) with fractionally charged organic anion TCNQδ−(0<δ<1) afforded hybrid materials [Fe(1‐bpp)2](TCNQ)3.5 ⋅ 3.5MeCN (1) and [Fe(1‐bpp)2](TCNQ)4 ⋅ 4DCE (2), where TCNQ=7,7,8,8‐tetracyanoquinodimethane, MeCN=acetonitrile, and DCE=1,2‐dichloroethane. Both materials exhibit semiconducting behavior, with the room‐temperature conductivity values of 1.1×10−4 S/cm and 1.7×10−3 S/cm, respectively. The magnetic behavior of both complexes exhibits strong dependence on the content of the interstitial solvent. Complex1undergoes a gradual temperature‐driven SCO, with the midpoint temperature ofT1/2=234 K. The partial solvent loss by1leads to the increase in theT1/2value while complete desolvation renders the material high‐spin (HS) in the entire studied temperature range. In the case of2, the solvated complex shows a gradual SCO withT1/2=166 K only when covered with a mother liquid, while the facile loss of interstitial solvent, even at room temperature, leads to the HS‐only behavior.more » « less
-
Abstract Using optical characterization, it is evident that the spin state of the spin crossover molecular complex [Fe{H2B(pz)2}2(bipy)] (pz = tris(pyrazol-1-1y)-borohydride, bipy = 2,2ʹ-bipyridine) depends on the electric polarization of the adjacent polymer ferroelectric polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) thin film. The role of the PVDF-HFP thin film is significant but complex. The UV–Vis spectroscopy measurements reveals that room temperature switching of the electronic structure of [Fe{H2B(pz)2}2(bipy)] molecules in bilayers of PVDF-HFP/[Fe{H2B(pz)2}2(bipy)] occurs as a function of ferroelectric polarization. The retention of voltage-controlled nonvolatile changes to the electronic structure in bilayers of PVDF-HFP/[Fe{H2B(pz)2}2(bipy)] strongly depends on the thickness of the PVDF-HFP layer. The PVDF-HFP/[Fe{H2B(pz)2}2(bipy)] interface may affect PVDF-HFP ferroelectric polarization retention in the thin film limit.more » « less
-
Abstract In this work, we provide clear evidence of magnetic anisotropy in the local orbital moment of a molecular thin film based on the SCO complex [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol−1−yl, bipy = 2,2′−bipyridine). Field dependent x-ray magnetic circular dichroism measurements indicate that the magnetic easy axis for the orbital moment is along the surface normal direction. Along with the presence of a critical field, our observation points to the existence of an anisotropic energy barrier in the high-spin state. The estimated nonzero coupling constant of ∼2.47 × 10−5eV molecule−1indicates that the observed magnetocrystalline anisotropy is mostly due to spin–orbit coupling. The spin- and orbital-component anisotropies are determined to be 30.9 and 5.04 meV molecule−1, respectively. Furthermore, the estimatedgfactor in the range of 2.2–2.45 is consistent with the expected values. This work has paved the way for an understanding of the spin-state-switching mechanism in the presence of magnetic perturbations.more » « less
