Plasma-Assisted Immobilization of aPhosphonium Salt

and Its Use as aCatalyst in the Valorization of CO2

catalysts allows facile separation from the product without te-

the carbon-basedscaffold, which allows facile structuralmodi-

fication,catalyst tuning,and catalystimmobilization.[5]

properties, a-C:H thin films are highlyattractive materials for

the immobilization of catalysts. An additional advantage in the

use of plasma-generated a-C:H films is the directattachment

ment.Compared to other coating procedures, it reduces prep-

arative steps and allows, in principle, the direct incorporation

So far,there are only alimited number of reports regarding

the immobilization of catalysts by plasma techniques.For ex-

ample,Kruth et al. encapsulated Ru dyes[11] and Ir dyes[12] with

lyst assemblies. There are also some examples concerning

plasma immobilization techniques in biology,for instance, the

entrapmentofenzymes.Inthis respect, Belhacene et al.[13] and

chemicalvapor deposition. Furthermore, Heyse et al.[15] de-

The first plasma-assisted immobilization of an organocatalyst,

namely abifunctionalphosphonium salt in an amorphous hy-

drogenated carbon coating, is reported. This method makes

catalystand even the homogeneous counterpart. After optimi-

bonates in yields of up to 99%. Furthermore, the possibility to

acetylene or pyrrole into an atmospheric plasma to immobilize

enzymeswhile preserving their bioactivity.

yieldingcyclic carbonates is an interesting and frequently stud-

ighly active systemsb

on OH-functionalized organocatalysts were reported for the

catalysts is attributed to epoxide activation andstabilization of

tion of abifunctional phosphonium bromidebearing aphenol

moiety utilizing functionalized polystyreneand silica supports

ure 1c). Furthermore, the efficiency and recyclability of the im-

mobilized catalysts were studied in the synthesis of cyclic car-

bonates.

Results and Discussion

the 2-position proved to be asuperior structuralmotif in the

We envisioned that an allyl substituent might allow subse-

quent immobilization in an a-C:H thin film generated by

plasma techniques. Thus, bifunctional phosphonium salts 5a

nyl)phenol(3)with allyl bromide (4a)and allyl iodide (4b), as

shown in Scheme 1a.The incorporationof5a and 5b into the

immobilized catalyst 6(Scheme 1b). Hence, we additionally

prepared salts 5c and 5d bearingasaturated side chain for

Subsequently,wetested catalysts 5(1 mol%) in the model

reactionof1,2-butylene oxide (1a)with CO2to generate cyclic

carbonate 2a (Table 1). At 908Cand aCO

Initially,the supports were tested in the model reaction and

provednot to facilitate the reactionof1a with CO2(Table 2,

entries 1–3). Subsequently,these supports were treated with

the presence of the plasma-treated supports, the formation of

(Table 2, entries 7–9). The catalystretained its catalytic activity,

oxide (1a)to1,2-butylene carbonate (2a)in93and 99%yield

catalystamoderate yield of 72%was obtained (Table 2,

the impregnatedsupports (Table 2, entries 7–9 versus 10–12).

The nominallayer thickness of the a-C:H coating is related

ments of the nominal layer thickness of a-C:H films obtained

after 6.5, 25, and 39 min of plasma treatment gave layer thick-

nesses of 53.3, 136.8, and 190 nm, respectively.Westudied the

impact of different plasma-treating times (6.5, 25, and 39 min)

on the catalytic activity of 5b on TiO2,FeO, and SiO2and the

and 5b@SiO2was initially investigated (Figure 2). In the model

reactionall three catalysts gave good yields of up to 88%after

yield achievedinthe second run was only 31%with 5b@SiO2.

We assumed that the low yields can be explainedbyleaching

of catalyst 5b into the liquid phase. This is easily possible be-

We studied immobilized catalyst 5b on differentsupports

(TiO2,FeO, and SiO2)after 6.5 min plasma-treating time under

Hence, the same set of experiments was repeated with cata-

lysts 5bb3

25 min plasma-treating time (Figure 3b). In the first run, all

extendedto39min, and the prepared catalysts were tested

provedimmobilization (Figure 3a and b). However,ifthe

plasma-treating time is too long, for example, 39 min for

mogeneous catalyst 5b and 5b-impregnated SiO2(5b@SiO2).

showedabroad signal at d=16.9 ppm, which is in asimilar

range to that in the 31PNMR spectrum of homogeneous cata-

lyst 5b (d=20.2 ppm), indicating the presence of aphosphoni-

um motif. The solid-state 13CNMR spectra of impregnated

In contrast, the impregnated and plasma-treated materials

showedsignals at 2.04 keV (P Ka), which indicatethe presence

low-intensity signal at 4.07 keV (I La1andL

face-associated iodine, which resultsfrom the high energy re-

quired for excitation of the iodine Ltransitions. This can be

overcomebychanging the sample pretreatment; for example,

Moreover,westudied 5bb3

of the silica support and 5bb3

and IIa. The carbon EDX mapping of theseparticles shows

clearly an increaseincarbon surface coating owing to plasma

dicatesthat the catalyst is evenly distributed over the support,

the reactiontime to 3hgave 2a in 99%yield of isolated prod-

On the basis of theseresultswedetermined reactioncondi-

vent-free). As shown in Scheme 2, terminal aliphatic epoxides

popularterm for the fatty acid methyl esters formed by trans-