news & views on whether an axial (109°) or an equatorial (114°) fluorine substituent was present. In addition, the overall crystal packing tended towards a general electrostatic ordering with fluorous faces in direct contact with hydrocarbon faces. This feature is consistent with facial polarization of the individual molecules, a finding further corroborated by quantum-mechanical computations. Calculations suggest a molecular dipole of 6.2 D — a figure that is significantly higher than the calculated dipole moment of the related 1,2,4,5-tetrafluorocyclohexane moiety (5.24 D)4 or any other aliphatic compound reported so far. Variable-temperature 19F NMR spectroscopy was used to determine the thermodynamic properties associated with the degenerate ring flip of 1. Although the enthalpic contribution ΔH‡ to the barrier (ΔG‡) was found to be similar to the value reported for cyclohexane (13.3 kcal mol−1 versus 10.8 kcal mol−1, respectively), the entropic contribution ΔS‡ was found to be negative (−3.8 cal mol−1 K−1). This differs significantly from the positive value

reported in the literature for cyclohexane (+2.8 cal mol−1 K−1), and implicates a rigidified transition state for the fluorinated compound. Additional computational studies were undertaken to delineate the role of dispersion interactions in bonding. Interestingly, the most important stabilizing contributions stem from donation of fluorine atom lone-pairs into antibonding σ*C–H and σ*C–C orbitals and are in the range of 7–8 and 10–11 kcal mol−1, respectively. Additional hyperconjugative interactions of the type σC–H → σ*C–F only account for roughly 6 kcal mol−1 each. It is interesting to note that in the 1,2-difluoroethane scaffold, these donor–acceptor interactions contribute significantly to the characteristic gauche conformation5. This seminal synthesis of a facially polarized ring will have important consequences for molecular design and molecular recognition in the broadest sense. Compound 1 may be conceived as an intersection of benzene (partially positively charged face) and

hexafluorobenzene (partially negatively charged face), where the properties of hydrocarbons and their perfluorinated analogues have been unified. Moreover, this fluorous Janus-head molecule exhibits the largest reported molecular dipole moment of any alkane reported so far. Overall, this study of the highest energy isomer of C6F6H6 is a masterclass in organofluorine chemistry. ❐ Nico Santschi and Ryan Gilmour are in the Organic Chemistry Institute, Westfälische-Wilhelms Universität Münster, Corrensstr. 40, 48149 Münster, Germany. e-mail: [email protected]; [email protected] References

1. Kirsch, P. Modern Fluoroorganic Chemistry: Synthesis, Reactivity, Applications 2nd edn (Wiley-VCH, 2013). 2. Hunter, L., Kirsch, P., Slawin, A. M. Z. & O’Hagan, D. Angew. Chem. Int. Ed. 48, 5457–5460 (2009). 3. Keddie, N. S., Slawin, A. M. Z., Lebl, T., Philip, D. & O’Hagan, D. Nature Chem. 7, 483–488 (2015). 4. Durie, A. J., Slawin, A. M. Z., Lebl, T., Kirsch, P. & O’Hagan, D. Chem Commun. 48, 9643–9645 (2012). 5. O’Hagan, D. Chem. Soc. Rev. 37, 308–319 (2008).

Published online 13 April 2015

MAIN GROUP CHEMISTRY

Small silicon oxides isolated

Bulk SiO2 is widespread in nature, and silicon oxide clusters are important to a variety of applications, yet molecular silicon oxides have remained elusive. Two molecular compounds featuring silicon oxide moieties, Si2O3 and Si2O4, have now been isolated by oxidation of a carbene-stabilized disilicon precursor.

Yitzhak Apeloig

A

lthough it is carbon’s closest neighbour in group 14 of the periodic table, silicon offers dramatically different chemistry. This is particularly striking when it comes to multiply bonded compounds: alkenes and acetylenes are common in nature and play an important role in organic chemistry, yet their doubly and triply bonded silicon counterparts were essentially unknown — and thus believed to be unstable1 — until the early 1980s. The first disilene, Mes2Si=SiMes2 (Mes, mesityl)2, and silene, (Me3Si)2Si=C(OSiMe3)Ad (Ad, 1–adamantyl)3, were synthesized, isolated and characterized by X-ray crystallography only in 1981. These advances were rendered possible by the use of bulky substituents, which prevent the dimerization or oligomerization of these highly reactive molecules. Other stable silenes (R2Si=CR2, with R being alkyl, aryl or heteroatom substituents), disilenes (R2Si=SiR2) and 468

disilyne (RSi ≡ SiR) soon followed, along with compounds featuring Si atoms doubly bonded to a variety of heteroatoms such as N, P and S, and compounds exhibiting double bonds to the heavier group 14 elements (Ge, Sn and Pb)1. Yet the apparently trivial molecular oxide SiO2 — analogue of the ubiquitous CO2 — other stable compounds featuring a non-coordinated Si=O double bond (silanones), and other silicon oxides remained unknown. This is in stark contrast again with carbon chemistry, which exhibits a myriad of stable ketones, esters, carboxylic acids, amides and other carbonyl-containing compounds. Writing in Nature Chemistry, Gregory Robinson and co-workers now report the isolation and characterization by X-ray crystallography of carbene-stabilized Si2O3 and Si2O4, which represent the first monomeric silicon oxides exhibiting a formal Si=O double bond isolated at ambient conditions4.

The first attempts to synthesize compounds with Si=O bonds go back as far as 150 years ago, with studies by Friedel and Crafts5, then 35 years later at the beginning of the twentieth century by Kipping 6,7. These groups reported the synthesis of compounds they named ‘silicones’ in analogy to ketones, owing to the presence of a RRʹSiO moiety, determined by elemental analysis. The very different reactivity and physical properties (such as their extremely high boiling points) of the ‘silicones’ compared with analogous ketones, along with molecular weight determinations by the ebullioscopic method, led Kipping to conclude that the synthesized materials were not monomers, but (RRʹSiO)3 trimers6,7. Kipping, considered the ‘father’ of organosilicon chemistry, did not envisage the wide potential of these ‘silicones’ and other polysiloxanes8 that are now important building blocks for organic–inorganic

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news & views a

Lewis base R R

A

DMAP

L

dipp N

N Lewis acid

Cr

Si O

Si O

Si O

N

N

Si

B(C6F5)3

O

L = N-heterocyclic carbene (NHC) or 4-(dimethylamino)pyridine (DMAP)

R

b 3 N2O R

N

N

Toluene, r.t.

R

R

N

1

R

2

R

R

R

R = 2,6-diisopropylphenyl

Si

R

N

R

R

Si O

N N

N

O

O

2 O2 Toluene, r.t.

NHC-dipp dipp = 2,6-diisopropylphenyl

O

Si

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dipp

N Si

N

R

N Si

Si

N

N

O

O

NHC-dipp

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O 3

Figure 1 | The search for silicon oxides. a, Structures of several known compounds with Si=O moieties that are stabilized by coordination of a Lewis base to the Si atom, as well as coordination of either a Lewis acid or a metal to the O atom. b, Synthesis and structure of the carbene-stabilized silicon oxides Si2O3 (2) and Si2O4 (3). Si, purple; O, red; N, blue; C, grey. Ball and stick structures by Miriam Karni.

hybrid polymers, with applications ranging from the construction industry to microelectronics to medical devices. A major challenge that prevents the isolation of monomeric silicon oxide compounds is their facile dimerization and oligomerization resulting from the high polarity of the Si=O bond (Pauling scale electronegativities: 1.9 for Si and 3.4 for O), the relatively weak π-bond and the high exothermicity of their dimerization and addition reactions. These reactions are suppressed in solid noble gas matrices at low temperatures, so that a variety of silanones have been detected in those environments9, but isolable R2Si=O compounds remain unknown at ambient conditions. Great progress however has recently been achieved in the synthesis of compounds with formal Si=O bonds that are stabilized by coordination of Lewis bases to the Si atom, and in the addition of Lewis acids or a metal to the oxygen atom10,11 (Fig. 1a). Of special interest is monomeric SiO2, which so far has been detected only in solid noble gas matrices. In contrast, naturally occurring silica (SiO2)n is highly stable and widespread, and silicates form 90% of the Earth’s crust. It is worth noting, however,

that silica does not contain Si=O bonds: it is polymeric and consists instead of a covalent network in which each silicon atom is singly bonded to four neighbouring oxygen atoms. Other small monomeric silicon oxides have also proved elusive. Now Robinson and co-workers have succeeded in stabilizing two silicon oxide moieties Si2O3 (2) and Si2O4 (3). This was achieved by reacting the soluble carbene-stabilized disilicon compound L:Si=Si:L (1; ref. 12) (where L: is a N-heterocyclic carbene) with N2O and O2, respectively (Fig. 1b): oxidation of 1 with N2O yields only compound 2, whereas oxidation with O2 yields only compound 3. Thus, the syntheses of 2 and 3 were made possible through an important moiety in main group and organometallic chemistry, stable N-heterocyclic carbenes13, which have proved over the years to be valuable coordinating ligands able to stabilize a wide variety of highly reactive species10,11,12,14 — including precursor 112. The wonderful ability of N-heterocyclic carbenes to stabilize highly reactive species such as 2 and 3 arises from their high Lewis basicity, which provides electron density that stabilizes the electron-deficient

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Si atoms. The syntheses of 2 and 3 require, in addition to outstanding experimental skills, careful control of the stoichiometry: excess oxidant in both cases leads to decomposition of the small oxides to polymeric SiO2. The structures of the novel silicon oxides 2 and 3 were examined by spectroscopic methods and single-crystal X-ray diffraction, as well as by density functional theory quantum mechanical calculations to gain insight into the nature of the bonding in these silicon–oxygen compounds. In 2, the Si2O3 core features a Si2O threemembered ring retaining the Si–Si bond and two terminal oxygen atoms (in Si=O units) with the Si atoms having a formal oxidation state of +3. In 3, the Si–Si bond is further oxidized, forming a four-membered (SiO)2 ring, also featuring two formal terminal Si=O bonds. In both 2 and 3 each silicon atom is 4-coordinate with one coordination bond to an N-heterocyclic carbene molecule. The spectroscopic evidence, along with bond distances and indices inferred from the theoretical analysis, indicate that the Si=O terminal bonds have a partial double bond character with a dominant Si+-O– zwitterionic character. Hopefully, in the future a 469

news & views genuine 3-coordinate silanone, (R3E)2Si=O, where E is C or Si, can be synthesized and characterized at ambient conditions. The isolation of 2 and 3 is not a mere chemical curiosity. Silicon oxide clusters play an important role in many technological fields including various areas of the semiconductor industry, in silicon-etching processes and in the formation of silicon-based nanowires. The new possibilities created by the isolation of 2 and 3 at ambient conditions to study the structural properties and chemistry of silicon oxides may help to model microscopic aspects of bulk silicon oxide as well as the oxidation and doping of

silicon surfaces. In addition, this work also points to a route towards the preparation of similar novel silicon chalcogenides — for example how would 1 react with sulfur, selenium or phosphorous? — and heavier main-group oxides, which have so far eluded isolation. ❐ Yitzhak Apeloig is in the Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Haifa 32000, Israel. e-mail: [email protected] References 1. Lee, V. Y. & Sekiguchi, A. Organometallic Compounds of Low-Coordinate Si, Ge, Sn, and Pb (Wiley, 2010). 2. West, R., Fink, M. J. & Michl, J. Science 214, 1343–1344 (1981).

3. Brook, A. G., Abdesaken, F., Gutekunst, B., Gutekunst, G. & Kallury, R. K. J. Chem. Soc. Chem. Commun. 191–192 (1981). 4. Wang, Y. et al. Nature Chem. 7, 509–513 (2015). 5. Friedel, C. & Crafts, J. Ann. Chim. Phys. 9, 5 (1866). 6. Kipping, F. S. & Lloyd, L. L. J. Chem. Soc. Trans. 79, 449–459 (1901). 7. Robison, R. K. & Kipping, F. S. J. Chem. Soc. Trans 93, 439–456 (1908). 8. Kipping, F. S. Proc. R. Soc. 159, 139–148 (1937). 9. Khabashesku, V. N., Kerzina, Z. A., Kudin, K. N. & Nefedov, O. M. J. Organomet. Chem. 566, 45–59 (1998). 10. Xiong, Y., Yao, S. L. & Driess, M. Angew. Chem. Int. Ed. 52, 4302–4311 (2013). 11. Filippou, A. C., Baars, B., Chernov, O., Lebedev, Y. N. & Schnakenburg, G. Angew. Chem. Int. Ed. 53, 565–570 (2014). 12. Wang, Y. et al. Science 321, 1069–1071 (2008). 13. Arduengo, A. J. III, Harlow, R. L. & Kline, M. J. Am. Chem. Soc. 113, 361–363 (1991). 14. Wang, Y. & Robinson, G. H. Inorg. Chem. 53, 11815–11832 (2014).

METAL–ORGANIC FRAMEWORKS

Shuttling in the solid state

Incorporating mechanically interlocked molecular shuttles within a metal–organic framework that has enough free space in the crystal lattice to permit volume-conserving translational motion sets the stage for defect-free molecular-electronic device fabrication and more.

Mark A. Olson

A

rtificial molecular machines — such as switches and actuators — in which the mechanical bond is the distinguishing characteristic are arguably examples of some of the more exotic architectures that can be found in chemistry 1–3. Mechanically interlocked molecules (MIMs) started out as mere academic curiosities, but are now used in a range of applications, including molecular electronic devices, nanoelectromechanical systems, plasmonic devices and mechanized nanoparticles capable of delivering both chemical and biological payloads. The ability to control the solution-phase molecular dynamics of MIMs such as rotaxanes — compounds in which one or more macrocycles are threaded onto a linear component and trapped there by bulky end groups — has been carefully honed over the years to permit relatively long-range translational motion of the interlocked components relative to one another. The first reports4,5 of rotaxanes in the literature date back to the work of Schill, Zollenkopf and Harrison in the late 1960s. Fast-forward almost half a century and there are now numerous studies published each year that report some aspect of research related to so-called mechanical bonds. Nevertheless, rotaxanes — and all MIMs for that matter — suffer from 470

a lack of material processability from a device-fabrication standpoint. The effort to overcome this obstacle has resulted in a number of different approaches to incorporating switchable MIMs into a variety of different settings — for example, within two-terminal crossbar junctions, in polymeric scaffolds, onto the surfaces of metal nanoparticles and into self-assembled monolayer half-devices. The scientific consensus is that the molecular switching property of MIMs is universal6 across all environments, but their repeated dynamics operate in an incoherent manner. The challenge still remains, therefore, to further focus the coherence of the molecular motion in these systems by taking solvent out of the picture and organizing individual molecules in a rigid, highly dense, well-defined manner to enable their operation in two or three dimensions in the solid state. Writing in Nature Chemistry, Robert Schurko, Stephen Loeb and co-workers7 have now succeeded in producing a crystalline material (Fig. 1) that exhibits the ‘to-andfro’ linear translational motion (often referred to as shuttling) of a [24]crown-8 macrocycle in a metal–organic rotaxane framework in the solid state. By combining 8 the very-much-dynamic chemistry of the mechanical bond with the robust structures of metal–organic frameworks (MOFs),

Schurko, Loeb and co-workers have produced7 an elegant example of a material that exhibits ‘robust dynamics’9. A degenerate two-station rotaxane — incorporating two benzimidazole recognition units in a linear component that is encircled by a single [24]crown-8 macrocycle — was used as a crossbar unit to bridge two triphenyl-dicarboxylic acid struts (Fig. 1d) in crystalline Zn(ii)-based MOFs. These compounds, which contain on the order of 1021 shuttling units per cm3 of material, were produced in greater than 70% yield. The key design strategy was to incorporate the rotaxane as a bridging unit between two struts rather than using the struts themselves as the foundation for the molecular interlocking. This approach effectively disconnects the rigid framework of the MOF from the components that are intended to be dynamic. Moreover, the molecular architecture includes what has been identified10 as ‘free-space’ in the crystal lattice, which is necessary to permit the volume-conserving molecular motion in which the [24]crown-8 macrocycles shuttle back and forth between benzimidazole recognition units (Fig. 1e). Confirming the long-range shuttling motion and kinetics of the intramolecular movement of the [24]crown-8 macrocycles within the MOF crystal lattice required a touch of synthetic ingenuity. Schurko,

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