2
58
C. Gnad et al. / Journal of Catalysis 375 (2019) 257–266
Scheme 1. The Mizoroki-Heck reaction of an aryl halide or aryl triflate with an alkene. X = Cl, Br, I, OTf, OTs.
from the field of Surface Organometallic Chemistry (SOMC). By the
application of high vacuum techniques in combination with high
temperatures, the distribution and number of hydroxyl groups on
oxide surfaces can be controlled. This dehydroxylation can result
in a homogeneous distribution of isolated surface hydroxyl groups.
These functional groups can be converted with transition metal
complexes, which possess at least one ligand being able to react
with a hydroxyl group. For example, alkyl, amide, alkoxy or halo-
genide metal complexes are suitable precursors for the reaction
with the hydroxyl groups of the oxide surface. Applying said tech-
nique known as Grafting, solid catalysts with molecular defined
and isolated metal centers can be synthesized [23–25].
siloxane groups without significant loss of specific surface area.
This process can be monitored by IR spectroscopy (see Fig. 5,
experimental). The surface of the resulting material consists of
2
approximately 0.8 isolated silanol groups per nm [23–25,30–32].
Silicon dioxide, which was dehydroxylated at 700 °C is denoted
7
00
as SiO
2
.
To obtain heterogeneous single-site palladium catalysts, the
dimethylpalladium(II) complexes dimethyl(N,N,N’,N’-tetramethyle
thanediamine)palladium(II) and dimethyl[1,2-bis(diphenyl-phos
phino)ethane]palladium(II) were chosen as metal-organic precur-
sors. The synthesis of both precursor compounds was described
by the group of de Graaf [33]. The prepared metal precursors were
7
00
Richmond et al. successfully grafted Pd(II) complexes with the
structural motif PdMeCl or dimeric palladium compounds on par-
tially dehydroxylated silicon dioxide and applied them as catalysts
for hydroamination reactions [26]. Immobilization of palladium
complexes can result in potent catalysts for the Heck coupling reac-
tions [27,28]. In these works, grafting of different Pd(II) complexes
on silicon dioxide or on a MOF support provides novel solid cata-
lysts with high to maximal metal dispersions and palladium cen-
ters with defined coordination spheres. To achieve exclusively
uniform molecular metal centers and to avoid self-reduction of
palladium in particular with very sensitive Pd methyl complexes
as applied in the present work, the grafting procedure had to be
further improved regarding the pre-treatment of the support, the
reaction temperature as well as precursor concentrations. In con-
trast to other approaches in which grafted noble metal catalysts
function as precursors in order to generate small particles through
defined pre-treatments [29], the focus of this contribution lays
upon the isolated structurally defined supported metal complexes
themselves. The variation of the ligands allows the investigation of
the influence of different stabilities of the surface complexes on
their reactivity towards the Heck coupling reaction. The synthe-
sized catalysts were applied for the coupling of aryl bromides as
well as chlorides. To get a fair relation to supported palladium
and palladium oxide particles, all catalysts were directly compared
to well-characterized supported palladium oxide materials known
and reported for its extraordinarily high activity in the conversion
of aryl bromides.
grafted on SiO
2
as shown in Scheme 2.
A noble metal complex comprising donating ligands like methyl
groups is likely to be reduced to the oxidation state of zero. The
grafting reaction is always in direct competition with the reductive
elimination of ethane from the precursor complex [34]. The danger
of this undesired reaction to happen is ubiquitous and can already
be observed in the pure solid complex. Quantitative decomposition
2
of PdMe (L) with L = tmeda and dppe occur at T = 398 K and 439 K
respectively [33,35], however partial decomposition can be
deduced from formation of palladium black during storage in case
of the tmeda-complex even at 253 K [36]. Reductive elimination
during the grafting reaction can be suppressed by working at low
temperatures with very diluted solutions of the precursor. Also, fil-
tration of precursor solutions removes any metal black which
could have formed on storage or dissolution of the complexes. Such
low temperatures on the other hand, decrease reaction rates, so
that long reaction times are necessary to achieve maximum con-
version which in turn enables more undesired side reactions.
Grafting alkyl palladium(II) complexes is thus always a balance
between undesired and wanted reactivity in order to obtain iso-
lated surface species. The corresponding optimization led to the
following reaction parameters (given more detailed in the experi-
mental part): 20 h reaction time at À30 to À40 °C, argon atmo-
sphere (Schlenk techniques). The amount of applied precursor
complexes was chosen in order to achieve metal loadings between
0.1 and 1.0 wt% palladium. The prepared white to slightly yellow
powders were stored under inert conditions at À30 °C.
2
. Results and discussion
2.2. Characterization of the silica-bound species
2.1. Preparation of the silica supported palladium catalysts
2.2.1. Elemental analysis
To verify the metal loadings of the prepared catalysts, the palla-
dium content was examined by digestion and subsequent photo-
metric quantification. The same procedure was conducted to
quantify the phosphorus content. Carbon, hydrogen and nitrogen
contents were determined by combustion analysis (and, if not
mentioned otherwise, with the exclusion of air contact to prevent
contamination of the samples with carbon-containing substances
or water). The results are summarized in Table 1.
In order to gain catalysts with isolated surface complexes the
method of grafting was applied. It relies on the conversion of sur-
face functional groups with defined organometallic complexes
under the formation of a covalent bond (at least in the first immo-
bilization step) [23–25]. To achieve metal site isolation, it is neces-
sary to pre-treat the support. In the case of oxides such as SiO
2
,
Al or TiO , the number and properties of certain surface hydro-
2
O
3
2
xyl groups can be controlled by dehydroxylation [30]. Silicon diox-
ide is a frequently applied support for the grafting step due to its
well-defined surface properties. The surface hydroxyl groups can
be classified into neighboring (vicinal and geminal hydroxyl
groups) and isolated hydroxyl groups. By the thermal treatment
Since the precursors tend to decompose by a reductive elimina-
tion, it is apparent that the lower palladium and consequently car-
bon, hydrogen and nitrogen values are caused by the loss of
precursor complex. To avoid the contamination of the catalysts
with palladium metal, the precursor complexes were filtered
before the grafting step (and the excessive ligand is removed by
careful washing of the catalysts). Consequently, the determined
Ò
of AEROSIL 200 at 700 °C in high vacuum, the neighboring hydro-
xyl groups are forced to react with each other and condensate to