Chemical Science Blog

The mighty gear is essential in machines. Even when scaling down the size of the machine, like from cars to small wristwatches, gears are necessary to transmit motion and mechanical power across the system. Machines can be decreased in size all the way to the nanoscale with molecular machines, where individual molecules can produce mechanical motion in response to external stimuli. Just as in macroscopic machines (e.g. cars), the addition of gears to nanomachines is needed for creating more complex assemblies with controlled motion, extending the applications of these molecules beyond the fundamental.

Figure 1. A schematic representation of the design for trains of molecular gears.

A team of researchers from France and Japan have now reported a series of molecular gears, with the aim of achieving correlated motion within trains of gears across a surface (Figure 1). To achieve this correlated motion, the researchers designed desymmetrised organometallic molecular gears based around star-shaped ruthenium piano-stool complexes. These molecular gears incorporated a facially capping hydrotris(indazolyl)borate ligand at one end, which both anchors the complex to the surface and lifts the central metal centre away to enable a rotational axis around the ruthenium. At the other end, the molecular gears have a cyclopentadienyl core to act as the cogwheel, functionalised with bulky groups that mimic the teeth that allow correlated motion between the gears (Figure 2). The researchers set out to make these molecular gears with lower symmetry to allow for detailed on-surface mechanical studies, by changing one of the five teeth (i.e. the functionalised groups around the cyclopentadienyl core) to include a steric or chemical tag– this is shown in Figures 1 and 2 by the green section.

Figure 2. Chemical structure of the molecular gears, with the anchoring ligand in black beneath the ruthenium centre and the rotating cogwheel cyclopentadienyl ligand in blue. The rectangles represent the teeth of the cogwheel as the bulky groups added to the central cyclopentadienyl core, where one of the five teeth (coloured green rather than blue) is sterically or chemically changed to lower the symmetry.

The researchers developed a modular synthetic approach to achieve desymmetrisation of the star-shaped ruthenium molecular gears, based on post-functionalisation of the central cyclopentadienyl core with Ni(II) porphyrins to act as the teeth of the cogwheels. They used an unsymmetrical 1,2,3,4,5-penta(p-halogenophenyl)cyclopentadienyl as the core; the p-halogenophenyl groups are all pre-activated to allow for further functionalisation, but one of the five is a p-iodophenyl group that chemoselectively reacts over the other four p-bromophenyl groups. Scheme 1 shows a sequential synthetic route towards one of the desymmetrised molecular gears: the p-iodophenyl group is first functionalised with a unique porphyrin (shown in green), before subsequent functionalisation of the four other p-bromophenyl groups with the same porphyrins (shown in blue), all using palladium-catalysed cross-coupling reactions.

Scheme 1. An example synthetic route towards desymmetrised molecular This example shows a sterically tagged cogwheel, where the unique porphyrinic tooth (in green) contains a longer linker than the four other teeth (in blue).

The researchers varied their approach to changing the unique porphyrinic tooth for the molecular gear, using either steric tagging (with one longer linker between the porphyrin and p-halogenophenyl group) or chemical tagging, using either one distinct electron-deficient porphyrin (achieved by using p-cyanophenyl substituents on the tetrapyrrole core) or one distinct metal porphyrin (Zn(II) instead of Ni(II)). The synthesised desymmetrised molecular gears were characterised using spectroscopic and electrochemical techniques, and the researchers are currently undertaking further mechanical studies to understand the correlated motion of these gears on surfaces.

To find out more, please read:

Desymmetrised pentaporphyrinic gears mounted on metallo-organic anchors

Seifallah Abid, Yohan Gisbert, Mitsuru Kojima, Nathalie Saffon-Merceron, Jérôme Cuny, Claire Kammerer* and  Gwénaël Rapenne*

Chem. Sci., 2021, Advance Article

About the blogger:

Dr. Samantha Apps recently finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

New month, new HOT articles!

We are pleased to share a selection of our referee-recommended HOT articles for February 2021. We hope you enjoy reading these articles, congratulations to all the authors whose articles are featured! As always, Chemical Science is free to read & download.

You can explore our full 2021 Chemical Science HOT Article Collection here!

Browse a selection of our February HOT articles below:

Towards the rational design of ylide-substituted phosphines for gold(i)-catalysis: from inactive to ppm-level catalysis
Jens Handelmann, Chatla Naga Babu, Henning Steinert, Christopher Schwarz, Thorsten Scherpf, Alexander Kroll and Viktoria H. Gessner;
Chem. Sci., 2021, Advance Article

Ruthenium-catalyzed formal sp³ C–H activation of allylsilanes/esters with olefins: efficient access to functionalized 1,3-dienes
Dattatraya H. Dethe, Nagabhushana C. Beeralingappa, Saikat Das and Appasaheb K. Nirpal
Chem. Sci., 2021, Advance Article

Symmetry-related residues as promising hotspots for the evolution of de novo oligomeric enzymes
Jaeseung Yu, Jinsol Yang, Chaok Seok and Woon Ju Song
Chem. Sci., 2021, Advance Article

Desymmetrised pentaporphyrinic gears mounted on metallo-organic anchors
Seifallah Abid, Yohan Gisbert, Mitsuru Kojima, Nathalie Saffon-Merceron, Jérôme Cuny, Claire Kammerer and Gwénaël Rapenne
Chem. Sci., 2021, Advance Article

The atomic-level regulation of single-atom site catalysts for the electrochemical CO₂ reduction reaction
Qingyun Qu, Shufang Ji, Yuanjun Chen, Dingsheng Wang and Yadong Li
Chem. Sci., 2021, Advance Article

Chemical tuning of spin clock transitions in molecular monomers based on nuclear spin-free Ni(ii)
Marcos Rubín-Osanz, François Lambert, Feng Shao, Eric Rivière, Régis Guillot, Nicolas Suaud, Nathalie Guihéry, David Zueco, Anne-Laure Barra, Talal Mallah and Fernando Luis
Chem. Sci., 2021, Advance Article

Submit to Chemical Science today! Check out our author guidelines for information on our article types or find out more about the advantages of publishing in a Royal Society of Chemistry journal.

Keep up to date with our latest articles, reviews, collections & more by following us on Twitter. You can also keep informed by signing up to our E-Alerts.

Methylene (-CH₂) is one of the simplest and most important building blocks for chemical synthesis. Methylenation reactions add methylene groups to molecules and often proceed using transition metal methylene complexes. Titanium methylene complexes are excellent for methylenations and have been used in a variety of reactions such as olefin metathesis, polymerisations or olefination of carbonyls. Early examples of such titanium methylenation reagents include Tebbe’s reagent that can generate a terminally bound mononuclear titanium methylidene, Cp₂Ti=CH₂ (Figure 1a), or a methylenation reagent prepared from CH₂Br₂, Zn and TiCl₄ (with catalytic lead), referred to as ‘CH₂X₂-Zn(Pb)-TiCl₄’.

Figure 1. (a) The titanium methylidene methylenating reagent from the Tebbe or Petasis reagents. (b) The first key step for the ‘CH₂X₂-Zn(Pb)-TiCl₄’ methylenating reagent: generation of the zinc methylene. (c) The second key step for ‘CH₂X₂-Zn(Pb)-TiCl₄’ methylenating reagent: reduction of Ti(IV) to Ti(III).

Researchers from Japan have been interested in the ‘CH₂X₂-Zn(Pb)-TiCl₄’ methylenation reagent and in particular, deducing the molecular structure of the reactive species. Earlier studies have revealed two key steps in the preparation of this methylenating reagent: the first is that a zinc methylene species, ‘CH₂(ZnX₂)’, is formed by the reaction of CH₂X₂ with Zn and catalytic lead (Figure 1b), and the second is that the Ti(IV) chloride reagent is reduced to Ti(III) chloride by Zn(0) simultaneously (Figure 1c). The researchers hypothesised that a reactive titanium methylidene species (similar to that generated from Tebbe’s reagent in Figure 1a) should form via a transmetallation event between the zinc methylene species and the Ti(III) chloride, and thus be the reactive methylenating species of the ‘CH₂X₂-Zn(Pb)-TiCl₄’ methylenation reagent.

Scheme 1. The synthesis of the titanium methylene complex 3 generated via transmetallation.

To confirm their hypothesis, the researchers studied the reactivity of multiple combinations of a zinc methylene species (1) and titanium(III or IV) chloride reagents, with and without additional ligands (such as phosphines, amines or ethers). The researchers found that most combinations of reagents resulted in methylene loss via the generation of methane or ethylene, but the combination of TMEDA adducts of the zinc methylene (1a) and Ti(III) chloride (2) gave clean conversion to a new titanium methylene species 3 (Scheme 1). Although the researchers originally hypothesised the formation of a mononuclear titanium methylidene via methylene transmetallation from zinc to titanium, the new species 3 was revealed to be a dinuclear, bridging methylene complex. The dinuclear species was characterised using NMR spectroscopy and single-crystal X-ray diffraction techniques, and the connectivity of the bridging methylene was conclusively established by the X-ray crystal structure.

After elucidating the structure of the dinuclear titanium methylene complex, the researchers tested 3 as a methylenating reagent and observed successful methylene transfer reactions from 3 to esters, terminal olefins and 1,3-dienes. A further computational mechanistic study for the reactivity of 3 and a 1,3-diene was performed, where the DFT calculations indicated a mononuclear titanium methylidene as the reactive species, generated from the dinuclear titanium methylene complex. These calculations corroborate the researchers’ initial hypothesis and correlate with Tebbe’s reagent, where the reactive methylenating agent is also a mononuclear titanium methylidene that is generated from a dinuclear bridging methylene complex.

To find out more, please read:

Structural elucidation of a methylenation reagent of esters: synthesis and reactivity of a dinuclear titanium(III) methylene complex

Takashi Kurogi,* Kaito Kuroki, Shunsuke Moritani and Kazuhiko Takai*

Chem. Sci., 2021, Advance Article

About the blogger:

Dr. Samantha Apps recently finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.