Analytical and preparative separation and isolation of functionalized fullerenes by conventional HPLC stationary phases: method development and column screening

Abstract

Isolation and purification of functionalized fullerenes from often complex reaction mixtures is challenging. Here, a simple and efficient HPLC method is presented.

Playing with Protic Additives to Improve the Outcome of Rhodium‐Catalyzed Hydroarylation of Fullerene C60 with Arylboronic Acids

“…To reliably quantify conversion while performing extended parameter screening, and allow for screening of a range of boronic acids without necessarily isolating all products, we decided to standardize post‐reaction treatment to improve quantification accuracy and to employ different methods for quantifying C 60 conversion (by UV‐vis spectroscopy) and for product quantity (by 1 H‐NMR spectroscopy employing internal standard integration), see supporting information. UV‐vis quantification of C 60 can be achieved with high accuracy after mixture separation using HPLC [31] while components with unknown molar extinction coefficients cannot be quantified, not even against a known “standard” as structurally highly similar compounds may have drastically different molar extinction coefficients [32] . In contrast, in 1 H‐NMR spectroscopy, there is a good agreement between the number of hydrogens in a certain environment and the signal area regardless of which compound or part of a compound constitutes the environment, which allows for accurate quantification using added internal standard [33] …”

“…Experiments at anhydrous conditions using o‐DCB in the absence of protic co‐solvent gave a 1 H‐NMR‐determined product yield of less than 20 % based on total C 60 loaded (20–40 % based on consumed C 60 determined using UV‐vis after HPLC separation, [31] Table 1). The product yield observed from reactions carried out in the presence of dry alcohol was 26–46 % (33–62 % based on consumed C 60 ), with the highest amount obtained for methanol which is the alcohol most similar to water.…”

“…[32] In contrast, in 1 H-NMR spectroscopy, there is a good agreement between the number of hydrogens in a certain environment and the signal area regardless of which compound or part of a compound constitutes the environment, which allows for accurate quantification using added internal standard. [33] Experiments at anhydrous conditions using o-DCB in the absence of protic co-solvent gave a 1 H-NMRdetermined product yield of less than 20 % based on total C 60 loaded (20-40 % based on consumed C 60 determined using UV-vis after HPLC separation, [31] Table 1). The product yield observed from reactions carried out in the presence of dry alcohol was 26-46 % (33-62 % based on consumed C 60 ), with the highest amount obtained for methanol which is the alcohol most similar to water.…”

“…Arylboronic acids with methyl in position 2 and 6 or with electron‐withdrawing substituents in the 4‐position resulted in lower conversion of C 60 . Separation of hydroarylated products from unreacted C 60 as well as isolation of pure hydroarylated product can be achieved using preparative HPLC methods analogous to the analytical one employed in the present study [31] …”

“…Standard and reaction mixture sample solutions were subjected to chromatographic separation using a Synergi 4 μm Hydro‐RP 80 Å HPLC column (250×4.6 mm, Phenomenex, USA) together with a Gemini C 18 pre‐guard cartridge (2.0×4.0 mm, Phenomenex, USA) using an isocratic flow rate of 1 mL/min eluting with 40 % (v/v) acetonitrile in toluene with detection at 285 nm [31] . All injections were made within 30 minutes of sample preparation.…”