The authors thank the Fonds der Chemischen Industrie for a Liebig fellowship and the Hertha and Helmut Schmauser foundation for financial support. Support by K. Meyer is gratefully acknowledged.

CAUTION: Carry out all syntheses in a well-ventilated fume hood. Wear appropriate personal protective equipment (PPE) including a lab coat and safety goggles.
NOTE: The starting materials were synthesized according to the literature: 1-(2,6-diisopropylphenyl)-2,2,4,4-tetramethyl-3,4-dihydro-2H-pyrrol-1-ium tetrafluoroborate (1prot) (For the synthesis of CAACs, see:18,30,31,64,65) and 1,3-dimethyl-4,5-dihydro-1H-imidazol-3-ium iodide (2prot)65. We suggest drying these salts at 120 &#176;C in vacuo overnight in order to ensure the absence of water or halogenated solvents. Silver triflate and potassium hexamethyldisilazide (KHMDS) were obtained by commercial vendor and used as is without further purification. All manipulations were performed using Schlenk techniques or in a dinitrogen filled glovebox (O2 &#60; 0.1 ppm; H2O &#60; 0.1 ppm). Solvents were dried by a two-column, solid-state purification system and stored over activated molecular sieves. Tetrahydrofuran, diethylether, hexanes, pentane, benzene and toluene were deoxygenated by three freeze-pump-thaw cycles. Deuterated benzene was dried over molecular sieves, deoxygenated by three freeze-pump-thaw cycles and stored over a mirror of potassium, deuterated acetonitrile was distilled from calcium hydride and stored over molecular sieves. Glassware was oven-dried at 150 &#176;C for at least 12 h prior to use and brought hot directly into the glovebox (cycling the antechamber at least three times over the course of at least 15 min). Glass micro fiber filters were stored at 150 &#176;C; cannulas were either oven-dried or thoroughly purged with air prior to use in order to ensure the absence of residual organic solvent (water, respectively).
1. Synthesis of cyclic (alkyl)(amino) carbene (Compound 1)
<ol>
	<li>Transfer a hot, oven-dried 100 mL Schlenk flask equipped with a stir bar and a rubber septum into a dinitrogen filled glovebox.</li>
	<li>Weigh out the iminium salt 1-(2,6-diisopropylphenyl)-2,2,4,4-tetramethyl-3,4-dihydro-2H-pyrrol-1-ium tetrafluoroborate (1prot) (2.00 g, 5.36 mmol, 1.0 eq.) and potassium hexamethyldisilazide (KHMDS) (1.05 g, 5.25 mmol, 0.98 eq.) and combine in the 100 mL Schlenk flask. Cap the flask with a rubber septum.</li>
	<li>Transfer the flask to the Schlenk line. Evacuate and refill all connecting hoses with dinitrogen three times in order to remove any traces of water and air.</li>
	<li>Connect a second oven-dried 100 mL Schlenk flask capped with a rubber septum to the Schlenk line. Evacuate/refill the connecting hose three times.</li>
	<li>Open the solid containing flask to dinitrogen and cool the flask using an isopropanol slush bath (-88 &#176;C) or a dry ice/acetone (-78 &#176;C) cooling bath.</li>
	<li>Add 20 mL of diethylether (dry, degassed) over the course of 3 min along the cold flask wall using a syringe. Stir the suspension for 10 min before allowing the reaction mixture to warm to room temperature.</li>
	<li>Once the mixture reaches room temperature, discontinue stirring and allow the potassium tetrafluoroborate salt to settle.</li>
	<li>Prepare a steel cannula equipped with a glass micro fiber filter, which is fitted to one end of the cannula by polytetrafluoroethylene (PTFE) tape. Wind the PTFE tape around the end of the cannula to obtain an overall diameter of about 0.6 cm (0.25 inch; Figure 3a, b). Then fit the glass micro fiber filter by winding further PTFE tape around (Figure 3c).</li>
	<li>Perforate a septum with a small needle (with a smaller diameter than the cannula) and subsequently push the filter cannula through the tiny hole. Swiftly exchange this septum under a gentle flow of dinitrogen with the septum on the Schlenk flask containing the crude carbene. Purge the cannula for at least 1 min with dinitrogen.</li>
	<li>Perforate the second septum capping the second empty Schlenk flask as well with a small needle and introduce the other end of the steel cannula.</li>
	<li>Additionally, insert a thin needle through the septum of the empty flask and close the Schlenk valve connecting this flask to the Schlenk line. Note that overpressure will be released through the extra needle (Figure 4).</li>
	<li>Lower the filter cannula into the overlying solution to start the filtration of the solution containing the free carbene into the second Schlenk flask using slight dinitrogen overpressure provided by the line. Eventually, also lower the filter cannula into the suspension with the settled salt at the bottom of the flask.</li>
	<li>After quantitative transfer of the carbene, reopen the valve of the second Schlenk flask to the Schlenk line for dinitrogen supply. Remove the small needle as well as the steel cannula and seal the perforated septum of the Schlenk flask with adhesive tape. 
	Alternatively, replace the perforated septum by a well-greased glass stopper.</li>
	<li>Remove the solvent in vacuo to obtain the free carbene 1 quantitatively as a colorless to slightly yellow and greasy solid (1.53 g). Quantitative removal of hexamethyldisilazane 
	[HN(SiMe3)2] requires typically a vacuum around 1 * 10-3 mbar or gentle heating. Transfer&#160;1 to a glovebox for storage.</li>
</ol>
2. Synthesis of the N-heterocyclic carbene (Compound 2)
<ol>
	<li>Transfer a hot, oven-dried 100 mL Schlenk flask, a rubber septum and a stir bar into a dinitrogen filled glovebox.</li>
	<li>Weigh out the imidazolium salt 1,3-dimethyl-4,5-dihydro-1H-imidazol-3-ium iodide 2prot 
	(2.00 g, 8.93 mmol, 1.0 eq.) and KHMDS (1.75 g, 8.75 mmol, 0.98 eq.). Combine both in the Schlenk flask, add the stir bar and seal the flask with the rubber septum.</li>
	<li>Transfer the Schlenk flask to the Schlenk line and evacuate/refill the connecting hose three times. Additionally, connect a second oven-dried 100 mL Schleck flask equipped with a septum to the Schlenk line. Evacuate/refill with dinitrogen three times.</li>
	<li>Add 10 mL of diethylether (dry, degassed) via a syringe to the 2prot / KHMDS mixture and stir for 20 min at room temperature.</li>
	<li>To separate the precipitated salt, use a steel cannula equipped with a glass micro fiber filter to one end and transfer the solution into the second Schlenk flask as described previously (steps 1.8 &#8211; 1.13).</li>
	<li>Remove the solvent in vacuo to afford the free carbene 2 as a slightly yellow oil in a yield of 390 mg (45%). Transfer 2 to a glovebox for storage and the next step.</li>
</ol>
3. Synthesis of the CAAC&#8211;NHC salt (Compound 3prot)
<ol>
	<li>Transfer a hot, oven-dried 100 mL Schlenk flask equipped with a stir bar and a rubber septum into a dinitrogen filled glovebox.</li>
	<li>Weigh out the cyclic iminium salt 1prot (1.50 g, 4.02 mmol, 1.0 eq.) and the free carbene 2 
	(409 mg, 4.22 mmol, 1.05 eq.). Combine both in the Schlenk flask and seal the flask with a rubber septum.</li>
	<li>Transfer the Schlenk flask to a Schlenk line. Evacuate/refill the connecting hoses with dinitrogen three times.</li>
	<li>Add 20 mL of tetrahydrofuran (dry, degassed) via a syringe according to description in steps 1.5 - 1.6. Swiftly replace the perforated septum by a well-greased glass stopper. Stir the reaction mixture for at least 12 h at room temperature.</li>
	<li>Allow the precipitate to settle. Exchange the glass stopper by a rubber septum with a steel&#160;cannula equipped with a glass micro fiber filter to one end to transfer the yellow&#160;supernatant solution into the second Schlenk flask as described previously (1.8 &#8211; 1.12)</li>
	<li>Exchange the glass stopper by a rubber septum and wash the residue with tetrahydrofuran: Add dry tetrahydrofuran (20 mL) via a syringe and stir until you obtain a fine suspension. Remove the supernatant using a filter cannula as described previously (1.8 &#8211; 1.12). If the residue is still&#160;yellow/orange repeat the washing step with additional 20 mL tetrahydrofuran. Exchange&#160;&#160;the perforated septum along with the filter cannula by a well-greased glass stopper.</li>
	<li>Dry the residue in vacuo to afford the protonated heterodimer quantitatively as an off-white powder. Transfer 3prot to a glovebox for storage and the next step.</li>
</ol>
4. Synthesis of the mixed Wanzlick CAAC&#8211;NHC dimer (Compound 3)
<ol>
	<li>Transfer a hot, oven-dried 100 mL Schlenk flask equipped with a stir bar and a rubber septum into a dinitrogen glovebox.</li>
	<li>Weigh out 3prot (1.5 g, 3.19 mmol, 1.0 eq.) and KHMDS (624 mg, 3.13 mmol, 0.98 eq.). Combine both in the Schlenk flask and cap the flask with the rubber septum.</li>
	<li>Connect this Schlenk flask and a second oven-dried empty 100 mL Schlenk flask equipped with a rubber septum to the Schlenk line. Evacuate/refill the connecting hoses with dinitrogen three times.</li>
	<li>Add 10 mL of toluene (dry, degassed) via a syringe to the mixture of 3prot and KHMDS. Stir for 12 h at room temperature, then stop stirring and allow the precipitate to settle.</li>
	<li>Transfer the supernatant solution, containing the dimer 3, into the second Schlenk flask using a filter cannula as described previously (steps 1.8 &#8211; 1.13).</li>
	<li>Remove the solvent in vacuo.</li>
	<li>Wash the residue with hexanes to remove residual HN(SiMe3)2: Add 5 mL hexanes (dry, degassed) and stir until you obtain a fine suspension. Remove the supernatant using a filter cannula as described previously (steps 1.8 &#8211; 1.13). Exchange the perforated septum along with the filter cannula by a well-greased glass stopper.</li>
	<li>Dry the residue in vacuo to obtain CAAC&#8211;NHC heterodimer 3 as a off white powder in a yield of 970 mg (80%). Transfer 3 to a glovebox for storage.</li>
</ol>
5. Synthesis of the organic radical CAAC&#8211;NHC-2 (compound 4)
<ol>
	<li>Transfer a hot, 20 mL Schlenk flask equipped with a stir bar and a rubber septum into a dinitrogen glovebox.</li>
	<li>Weigh out the silver trifluoromethanesulfonate [Ag(OTf); 134 mg, 0.52 mmol, 1.0 eq.] and compound 3 (200 mg, 0.52 mmol, 1.0 eq.). Combine both in the 20 mL Schlenk flask and cap with a rubber septum.</li>
	<li>Connect this Schlenk flask and a second oven-dried empty 20 mL Schlenk flask equipped with a stir bar and a septum to the Schlenk line. Evacuate/refill the connecting hoses with dinitrogen three times.</li>
	<li>Add 5 mL of tetrahydrofuran (dry, degassed) via syringe to receive a deep maroon mixture.</li>
	<li>Filter the solution into the second Schlenk flask using a filter cannula as described previously (steps 1.8 &#8211; 1.13).</li>
	<li>Remove the solvent in vacuo to obtain the stable radical quantitatively as a red powder. Transfer 4 to a glovebox for storage.</li>
</ol>

The authors have nothing to disclose.

Herein, we present a general and adaptable protocol for the synthesis of stable carbenes (NHC, CAAC) and their electron rich dimer. All steps can readily be upscaled to at least a 25&#160;g scale. Crucial for a successful synthesis are the strict exclusions of moisture (air, respectively) for the synthesis of the carbenes, and of oxygen (air, respectively) for the electron rich olefin. The herein applied filtration cannula technique in combination with a Schlenk line is a very convenient method to separate solutions from...

More than half a century ago, Wanzlick reported arguably the first attempts to synthesize N-heterocyclic carbenes1,2,3. However, instead of isolating the free carbenes, he succeeded only in characterizing their dimers. This observation prompted him to suggest an equilibrium between the olefin dimer and the respective free carbenes, which is now commonly referred to as &#8220;Wanzlick&#8217;s equilibrium&#8221; (Figure 1, I.)4,5,6. Later on, it was argued that the dimerization of free carbenes and of course equally the reverse reaction (i.e., the dissociation of the related olefin dimers), is catalyzed by protons7,8,9,10,11,12. It took another 30 years until the first &#8220;bottleable&#8221; carbene, which did not dimerize at room temperature, was reported by Bertrand13,14. Especially N-heterocyclic carbenes (NHCs; imidazolin-2-ylidenes) became the subject of intensive research after Arduengo had reported a stable crystalline NHC, 1,3-diadamantyl-imidazolin-2-ylidene15. The surprising stability of this carbene was first rationalized by a combination of steric effects due to the bulky adamantyl substituents as well as electronic effects associated with the aromatic N-heterocycle. However, it was shown later in an elegant study by Murphy that even &#8220;monomeric&#8221; 1,3-dimethyl-imidazolin-2-ylidene16 (i.e., the free carbene derived from N,N-dimethylimidazolium salts) with very small methyl substituents is more stable than its dimer17. Lavallo and Bertrand showed on the contrary, that also the removal of one stabilizing nitrogen atom, as reported by the isolation of a cyclic (alkyl)(amino) carbene (CAAC), can be balanced by introduction of a bulky 2,6-diisopropylphenyl (Dipp) substituent18.
NHCs and CAACs proved extraordinarily fruitful for the coordination chemistry of the d- and p-block elements, transition metal catalysis, or organocatalysis (For thematic issues and books on NHCs, see19,20,21,22,23, for reviews on CAACs, see24,25,26,27,28, for the synthesis of CAACs, see18,29,30,31). The impressive success story of cyclic carbene ligands is mainly due to two reasons32. First, both electronic and steric properties can be readily tuned to fit the requirements of a specific application. Second, the isolation of stable free carbenes offers a convenient method to synthesize metal complexes by straightforward combination with a metal precursor. Accordingly, it is important to understand the factors which control whether a free carbene is stable at or below room temperature or whether it dimerizes to form an olefin. Note that the derived electron rich olefins usually33 do not form complexes upon treatment with a metal precursor, which is at least in part due to their highly reducing character.
Not only are free carbenes key players in synthetic chemistry nowadays. In fact, their electron rich olefin dimers34,35,36 (e.g., tetraazafulvalenes in case of NHCs37 or tetrathiafulvalenes TTF38,39,40 in case of 1,3-dithiol-2-ylidenes; Figure 1, II.), have not only found broad application as reductants41,42,43, but even more so in organic electronics.
TTF is in fact called the &#8220;brick-and-mortar&#8221; of organic electronics44. This is largely due to the particular electronic properties of electron rich olefins &#8211; notably, many of those show three stable redox states upon oxidation, including the open-shell organic radical (For reviews on carbene derived organic radicals, see:45,46,47, for recent contributions in the area of carbene stabilized organic radicals, see:48,49,50,51,52,53,54). Accordingly, TTF allows for the fabrication of conductive/semiconductive material as required for magnetic materials, organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs) and molecular switches or sensors55,56,57,58,59.
Herein, we present convenient protocols for the isolation of two stable carbenes with enormous impact in coordination chemistry and homogeneous catalysis (Figure 2), viz. the cyclic (alkyl)(amino) carbene 1 18, and the dimethylimidazolin-2-ylidene NHC 2 15. We will discuss why both carbenes are stable at room temperature and do not dimerize. We will then elaborate on proton catalysis related to Wanzlick&#8217;s equilibrium and the formation of the mixed CAAC&#8211;NHC heterodimer 360,61,62. The exciting electronic properties of such triaza-alkenes is connected with the impressive stability of the related organic radical 4 63.
Methodological focus lies on the Schlenk technique using filter cannulas equipped with a glass micro fiber filter for the separation of a supernatant from a precipitate under inert conditions. A dinitrogen filled glovebox is used for weighing in starting material and the storage of air sensitive compounds.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glass micro fiber filter, 691, 24 mm. Particle retention 1.6 mm</td>
			<td>VWR</td>
			<td>516-0859</td>
			<td></td>
		</tr>
		<tr>
			<td>magnetic stir bar</td>
			<td>FengTecEx</td>
			<td>various</td>
			<td></td>
		</tr>
		<tr>
			<td>PTFE tape</td>
			<td>Sigma-Aldrich</td>
			<td>Z148814-1PAK</td>
			<td>PTFE tape used in this manuscript was obtained from a local supplier. Tape from Sigma Aldrich should show comparable performance.</td>
		</tr>
		<tr>
			<td>rubber septum</td>
			<td>FengTecEx</td>
			<td>RS112440</td>
			<td>Joint size: 24/29</td>
		</tr>
		<tr>
			<td>rubber septum</td>
			<td>FengTecEx</td>
			<td>RS111420</td>
			<td>Joint size: 14/23</td>
		</tr>
		<tr>
			<td>rubber septum</td>
			<td>FengTecEx</td>
			<td>RS111922</td>
			<td>Joint size: 19/26</td>
		</tr>
		<tr>
			<td>schlenk flasks</td>
			<td>FengTecEx</td>
			<td>various</td>
			<td>100 mL</td>
		</tr>
		<tr>
			<td>steel cannula</td>
			<td>FengtecEx</td>
			<td>C702024</td>
			<td>Attachment of a steel joint by a machine shop not required, but facilitates preparation of filter cannula</td>
		</tr>
		<tr>
			<td>syringe cannula</td>
			<td>FengtecEx</td>
			<td>S380221</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Reactants</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>1-(2,6-diisopropylphenyl)-2,2,4,4-tetramethyl-3,4-dihydro-2H-pyrrol-1-ium tetrafluoroborate</td>
			<td></td>
			<td></td>
			<td>Synthesized according to: Jazzar, R., Dewhurst, R. D., Bourg, J. B., Donnadieu, B., Canac, Y., Bertrand, G. Intramolecular &#8220;Hydroiminiumation&#8221; of alkenes: Application to the synthesis of conjugate acids of cyclic alkyl amino carbenes (CAACs). Angewandte Chemie International Edition 46 (16), 2899-2902, (2007).</td>
		</tr>
		<tr>
			<td>1,3-dimethyl-4,5-dihydro-1H-imidazol-3-ium iodide</td>
			<td></td>
			<td></td>
			<td>Synthesized according to: Benac, B. L., Burgess, E. M., Arduengo, A. J. 1,3-Dimethylimidazole-2-Thione. Organic Synthesis 64, 92, (1986).</td>
		</tr>
		<tr>
			<td>potassium hexamethyldisilazide</td>
			<td>Sigma-Aldrich</td>
			<td>324671-100G</td>
			<td>CAS 40949-94-8</td>
		</tr>
		<tr>
			<td>silver trifluoromethanesulfonate</td>
			<td>Sigma-Aldrich</td>
			<td>85325-25G</td>
			<td>CAS 2923-28-6</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Solvents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>acetonitrile-D3</td>
			<td>Deutero</td>
			<td>00202-10m</td>
			<td>distilled from CaH2, stored over activated molecular sieves</td>
		</tr>
		<tr>
			<td>benzene-D6</td>
			<td>Deutero</td>
			<td>00303-100ml</td>
			<td>dried over activated molecular sieves, stored over potassium</td>
		</tr>
		<tr>
			<td>diethylether</td>
			<td>-</td>
			<td>-</td>
			<td>dried by two-column, solid-state purification system and degassed by three freeze-pump-thaw cycles, stored over activated molecular sieves</td>
		</tr>
		<tr>
			<td>hexanes</td>
			<td>-</td>
			<td>-</td>
			<td>dried by two-column, solid-state purification system and degassed by three freeze-pump-thaw cycles, stored over activated molecular sieves</td>
		</tr>
		<tr>
			<td>tetrahydrofuran</td>
			<td>-</td>
			<td>-</td>
			<td>dried by two-column, solid-state purification system and degassed by three freeze-pump-thaw cycles, stored over activated molecular sieves</td>
		</tr>
		<tr>
			<td>toluene</td>
			<td>-</td>
			<td>-</td>
			<td>dried by two-column, solid-state purification system and degassed by three freeze-pump-thaw cycles, stored over activated molecular sieves</td>
		</tr>
	</tbody>
</table>

isolating free carbenes, their mixed dimers and organic radicals

Protocols for the isolation of the commonly employed cyclic (alkyl)(amino) carbene (CAAC) and N-heterocyclic carbene (NHC) are reported. Furthermore, the synthesis of their mixed CAAC&#8211;NHC &#8220;Wanzlick&#8221; dimer and the synthesis of the related stable organic &#8220;olefin&#8221; radical are presented. The main goal of this manuscript is to give a detailed and general protocol for the synthetic chemist of any skill level on how to prepare free heterocyclic carbenes by deprotonation using filter cannulas. Due to the air-sensitivity of the synthesized compounds, all experiments are performed under inert atmosphere using either Schlenk technique or a dinitrogen filled glovebox. Controlling Wanzlick&#8217;s equilibrium (i.e., the dimerization of free carbenes), is a crucial requirement for the application of free carbenes in coordination chemistry or organic synthesis. Thus, we elaborate on the specific electronic and steric requirements favoring the formation of dimers, heterodimers, or monomers. We will show how proton catalysis allows for the formation of dimers, and how the electronic structure of carbenes and their dimers affects the reactivity with either moisture or air. The structural identity of the reported compounds is discussed based on their NMR spectra.

Free carbenes react typically readily with water66. Hence, carefully dried glassware and solvents are required67. In the procedure described above, we used cannulas fitted with a glass micro fiber filter in order to separate air sensitive solutions from a precipitate under inert conditions. We used this technique for both the extraction of solids (i.e., the desired product is dissolved) as well as the washing of solid compounds (i.e., the de...

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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

We present protocols for the isolation of stable heterocyclic carbenes. The synthesis of a cyclic (alkyl)(amino) carbene (CAAC) and an N-heterocyclic carbene (NHC) is demonstrated using filter cannulas and Schlenk technique. We furthermore present the synthesis of the related oxygen-sensitive, electron-rich mixed &#8220;Wanzlick dimer&#8221; and the reduced stable organic radical.

Protocols for the isolation of the commonly employed cyclic (alkyl)(amino) carbene (CAAC) and N-heterocyclic carbene (NHC) are reported. Furthermore, the ...

isolating-free-carbenes-their-mixed-dimers-and-organic-radicals

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10.6K Views.  Friedrich-Alexander-Universität Erlangen-Nürnberg. We present protocols for the isolation of stable heterocyclic carbenes. The synthesis of a cyclic (alkyl)(amino) carbene (CAAC) and an N-heterocyclic carbene (NHC) is demonstrated using filter cannulas and Schlenk technique. We furthermore present the synthesis of the related oxygen-sensitive, electron-rich mixed “Wanzlick dimer” and the reduced stable organic radical.

Video: Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Free Radicals in Chemical Biology: from Chemical Behavior to Biomarker Development

The involvement of free radicals in life sciences has constantly increased with time and has been connected to several physiological and pathological processes. This subject embraces diverse scientific areas, spanning from physical, biological and bioorganic chemistry to biology and medicine, with applications to the amelioration of quality of life, health and aging. Multidisciplinary skills are required for the full investigation of the many facets of radical processes in the biological environment and chemical knowledge plays a crucial role in unveiling basic processes and mechanisms. We developed a chemical biology approach able to connect free radical chemical reactivity with biological processes, providing information on the mechanistic pathways and products. The core of this approach is the design of biomimetic models to study biomolecule behavior (lipids, nucleic acids and proteins) in aqueous systems, obtaining insights of the reaction pathways as well as building up molecular libraries of the free radical reaction products. This context can be successfully used for biomarker discovery and examples are provided with two classes of compounds: mono-trans isomers of cholesteryl esters, which are synthesized and used as references for detection in human plasma, and purine 5',8-cyclo-2'-deoxyribonucleosides, prepared and used as reference in the protocol for detection of such lesions in DNA samples, after ionizing radiations or obtained from different health conditions.

Radical-based biomimetic chemistry has been applied to building-up libraries necessary for biomarker development.

The involvement of free radicals in life sciences has constantly increased with time and has been connected to several physiological and pathological ...

Analysis of Volatile and Oxidation Sensitive Compounds Using a Cold Inlet System and Electron Impact Mass Spectrometry

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The analysis of volatile and oxidation sensitive compounds by mass spectrometry is not easily achieved, as all state-of-the-art mass spectrometric methods require at least one sample preparation step, e.g., dissolution and dilution of the analyte (electrospray ionization), co-crystallization of the analyte with a matrix compound (matrix-assisted laser desorption/ionization), or transfer of the prepared samples into the ionization source of the mass spectrometer, to be conducted under atmospheric conditions. Here, the use of a sample inlet system is described which enables the analysis of volatile metal organyls, silanes, and phosphanes using a sector field mass spectrometer equipped with an electron impact ionization source. All sample preparation steps and the sample introduction into the ion source of the mass spectrometer take place either under air-free conditions or under vacuum, enabling the analysis of compounds highly susceptible to oxidation. The presented technique is especially of interest for inorganic chemists, working with metal organyls, silanes, or phosphanes, which have to be handled using inert conditions, such as the Schlenk technique. The principle of operation is presented in this video.

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The presented technique is especially of interest for inorganic chemists, working with metal organyls, silanes, or phosphanes which have to be handled using inert conditions, such as the Schlenk technique.

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The ...

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus

Here we present a protocol for the synthesis of two distinct carbonyl-decorated carbenes. Both carbenes can be prepared using nearly identical procedures in multi-gram scale quantities. The goal of this manuscript is to clearly detail how to handle and prepare these unique carbenes such that a synthetic chemist of any skill level can work with them. The two carbenes described are a diamidocarbene (DAC, carbene 1) and a monoamidoaminocarbene (MAAC 2). These carbenes are highly electron-deficient and as such display reactivity profiles that are atypical of more traditional N-heterocyclic carbenes. Additionally, these two carbenes only differ in their electrophilic character and not their steric parameters, making them ideal for studying how carbene electronics influence reactivity. To demonstrate this phenomenon, we are also describing the activation of white phosphorus (P4) using these carbenes. Depending on the carbene used, two very different phosphorus-containing compounds can be isolated. When the DAC 1 is used, a tris(phosphaalkenyl)phosphane can be isolated as the exclusive product. Remarkably however, when MAAC 2 is added to P4 under identical reaction conditions, an unexpected carbene-supported P8&#160;allotrope of phosphorus is isolated exclusively. Mechanistic studies demonstrate that this carbene-supported P8allotrope forms via a [2+2] cycloaddition dimerization of a transient diphosphene which has been trapped by treatment with 2,3-dimethyl-1,3-butadiene.

Here, we present a protocol for the synthesis of two carbonyl-decorated carbenes. The protocol makes these interesting compounds readily available to chemists of all skill levels. In addition to the synthesis of these two carbenes, their use in the activation of white phosphorus is also described.

Here we present a protocol for the synthesis of two distinct carbonyl-decorated carbenes. Both carbenes can be prepared using nearly identical procedures ...

Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single molecule level. An electrostatic self-assembly and covalent fixation (ESA-CF) process is used for the synthesis of the cyclic poly(tetrahydrofuran) (poly(THF)). The diffusive motion of individual cyclic polymer chains in a melt state is visualized using single molecule fluorescence imaging by incorporating a fluorophore unit in the cyclic chains. The diffusive motion of the chains is quantitatively characterized by means of a combination of mean-squared displacement (MSD) analysis and a cumulative distribution function (CDF) analysis. The cyclic polymer exhibits multiple-mode diffusion which is distinct from its linear counterpart. The results demonstrate that the diffusional heterogeneity of polymers that is often hidden behind ensemble averaging can be revealed by the efficient synthesis of the cyclic polymers using the ESA-CF process and the quantitative analysis of the diffusive motion at the single molecule level using the MSD and CDF analyses.

A protocol for the synthesis and characterization of diffusive motion of cyclic polymers at the single molecule level is presented.

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single ...

Grafting Multiwalled Carbon Nanotubes with Polystyrene to Enable Self-Assembly and Anisotropic Patchiness

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a free-radical polymerization strategy to enable the modulation of the nanotube surface properties and produce supramolecular self-assembly of the nanostructures. First, a selective hydroxylation of the pristine nanotubes through a biphasic catalytically mediated oxidation reaction creates superficially distributed reactive sites at the sidewalls. The latter reactive sites are subsequently modified with methacrylic moieties using a silylated methacrylic precursor to create polymerizable sites. Those polymerizable groups can address further polymerization of styrene to produce a hybrid nanomaterial containing PS chains grafted to the nanotube sidewalls. The polymer-graft content, amount of silylated methacrylic moieties introduced and hydroxylation modification of the nanotubes are identified and quantified by Thermogravimetric Analysis (TGA). The presence of reactive functional groups hydroxyl and silylated methacrylate are confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Polystyrene-grafted carbon nanotube solutions in tetrahydrofuran (THF) provide wall-to-wall collinearly self-assembled nanotubes when cast samples are analyzed by transmission electron microscopy (TEM). Those self-assemblies are not obtained when suitable blanks are similarly cast from analogous solutions containing non-grafted counterparts. Therefore, this method enables the modification of the nanotube anisotropic patchiness at the sidewalls which results into spontaneous auto-organization at the nanoscale.

A procedure for the synthesis of polystyrene-grafted multiwalled carbon nanotubes using successive chemical modification steps to selectively introduce the polymer chains to the sidewalls and their self-assembly via anisotropic patchiness is presented.

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a ...

Time-resolved Photophysical Characterization of Triplet-harvesting Organic Compounds at an Oxygen-free Environment Using an iCCD Camera

Here, we present a sensible method of the acquisition and analysis of time-resolved photoluminescence using an ultrafast iCCD camera. This system enables the acquisition of photoluminescence spectra covering the time regime from nanoseconds up to 0.1 s. This enables us to follow the changes in the intensity (decay) and emission of the spectra over time. Using this method, it is possible to study diverse photophysical phenomena, such as the emission of phosphorescence, and the contributions of prompt and delayed fluorescence in molecules showing thermally activated delayed fluorescence (TADF). Remarkably, all spectra and decays are obtained in a single experiment. This can be done for solids (thin film, powder, crystal) and liquid samples, where the only limitations are the spectral sensitivity of the camera and the excitation wavelength (532 nm, 355 nm, 337 nm, and 266 nm). This technique is, thus, very important when investigating the excited state dynamics in organic emitters for their application in organic light-emitting diodes and other areas where triplet harvesting is of paramount importance. Since triplet states are strongly quenched by oxygen, emitters with efficient TADF luminescence, or those showing room temperature phosphorescence (RTP), must be correctly prepared in order to remove any dissolved oxygen from solutions and films. Otherwise, no long-lived emission will be observed. The method of degassing solid samples as presented in this work is basic and simple, but the degassing of liquid samples creates additional difficulties and is particularly interesting. A method of minimizing solvent loss and changing the sample concentration, while still enabling to remove oxygen in a very efficient and a repeatable manner, is presented in this work.

Here, we present a method of the spectroscopic characterization of organic molecules by means of time-resolved photoluminescence spectroscopy on the nanosecond-to-millisecond timescale in oxygen-free conditions. Methods to efficiently remove oxygen from the samples and, thus, limit luminescence quenching are also described.

Here, we present a sensible method of the acquisition and analysis of time-resolved photoluminescence using an ultrafast iCCD camera. This system enables ...

Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

Here, we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, allowing for the synthesis of isotactic poly(&#945;-hydroxy acids) with expected molecular weights (&#62;140 kDa) and narrow molecular weight distributions (Mw/Mn &#60; 1.1). This ring-opening polymerization is mediated by Ni and Zn complexes in the presence of an alcohol initiator and a photoredox Ir catalyst, irradiated by a blue LED (400 - 500 nm). The polymerization is performed at a low temperature (-15 &#176;C) to avoid undesired side reactions. The complete monomer consumption can be achieved within 4 - 8 hours, providing a polymer close to the expected molecular weight with narrow molecular weight distribution. The resulted number-average molecular weight shows a linear correlation with the degree of polymerization up to 1000. The homodecoupling 1H NMR study confirms that the obtained polymer is isotactic without epimerization. This polymerization reported herein offers a strategy for achieving rapid, controlled O-carboxyanhydrides polymerization to prepare stereoregular poly(&#945;-hydroxy acids) and its copolymers bearing various functional side-chain groups.

Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

A protocol for the controlled photoredox ring-opening polymerization of O-carboxyanhydrides mediated by Ni/Zn complexes is presented.

Here, we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, ...

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu2(O2C)4&#160;paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science &#38; Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of CuII ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites.

Robust functional catechol coatings were produced in one step by direct reaction of the material known as HKUST with synthetic catechols under anaerobic conditions. The formation of homogeneous coatings surrounding the entire crystal is ascribed to the biomimetic catalytic activity of Cu(II) dimers on the external surface of the crystals.

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. ...

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes and determine the corresponding photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC-photogenerating system. To photogenerate IMes, a system comprising 2-isopropylthioxanthone (ITX) as the sensitizer and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the protected form of NHC is employed. IMesH+BPh4- can be obtained in a single step by anion exchange between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate. A real-time steady-state photolysis setup is described, which hints that the photochemical reaction proceeds in two consecutive steps: 1) ITX triplet is photo-reduced by the borate anion and 2) subsequent proton transfer takes place from the imidazolium cation to produce the expected NHC IMes. Two separate characterization protocols are implemented. Firstly, CS2 is added to the reaction media to evidence the photogeneration of NHC through formation of the IMes-CS2 adduct. Secondly, the amount of NHC released in situ is quantified using acid-base titration. The use of this NHC photo-generating system for the ROMP of norbornene is also discussed. In solution, a photopolymerization experiment is conducted by mixing ITX, IMesH+BPh4-, [RuCl2(p-cymene)]2 and norbornene in CH2Cl2, then irradiating the solution in a UV reactor. In a dispersed medium, a monomer miniemulsion is first formed then irradiated inside an annular reactor to produce a stable poly(norbornene) latex.

We describe a protocol to photogenerate N-heterocyclic carbenes (NHCs) by UV irradiation of a 2-isopropylthioxanthone/imidazolium tetraphenylborate salt system. Methods to characterize the photoreleased NHC and elucidate the photochemical mechanism are proposed. The protocols for ring-opening metathesis photopolymerization in solution and miniemulsion illustrate the potential of this 2-component NHC photogenerating system.

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes ...