Generation of Phosphonium Sites on Sulfated Zirconium Oxide: Relationship to Br?nsted Acid Strength of Surface -OH Sites

Article abstract of DOI:10.1021/jacs.8b13204

The reaction of (^tBu)₂ArP (1a-h), where the para position of the Ar group contains electron-donating or electron-withdrawing groups, with sulfated zirconium oxide partially dehydroxylated at 300 °C (SZO₃₀₀) forms [(^tBu)₂ArPH][SZO₃₀₀] (2a-h). The equilibrium binding constants of 1a-h to SZO₃₀₀ are related to the pK_a of [(^tBu)₂ArPH]; R₃P that form less acidic phosphoniums (high pK_a values) bind stronger to SZO₃₀₀ than R₃P that form more acidic phosphoniums (low pK_a values). These studies show that Br?nsted acid sites on the surface of SZO₃₀₀ are not superacidic.

Full text of DOI:10.1021/jacs.8b13204

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Communication
Generation of Phosphonium Sites on Sulfated Zirconium Oxide:
Relationship to Brønsted Acid Strength of Surface –OH Sites
Jessica Rodriguez, Damien B. Culver, and Matthew P. Conley
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13204 • Publication Date (Web): 09 Jan 2019
Downloaded from http://pubs.acs.org on January 10, 2019
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Generation of Phosphonium Sites on Sulfated Zirconium
Oxide: Relationship to Brønsted Acid Strength of Surface –OH
Sites
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Jessica Rodriguez, Damien B. Culver, and Matthew P. Conley*
Department of Chemistry, University of California, Riverside, California 92521, United States
Supporting Information Placeholder
ABSTRACT: The reaction of (tBu)2ArP (1a – 1h),
where the para position of the Ar group contains
electron donating or electron withdrawing groups,
with
sulfated
zirconium
oxide
partially
forms
dehydroxylated at 300 oC (SZO300
)
[(tBu)2ArPH][SZO300] (2a – 2h). The equilibrium
binding constants of 1a – 1h to SZO300 is related to the
pKa of [(tBu)2ArPH]; R3P that form less acidic
phosphoniums (high pKa values) bind stronger to
SZO300 than R3P that form more acidic phosphoniums
(low pKa values). These studies show that Brønsted
acid sites on the surface of SZO300 are not superacidic.
Figure 1. Chemisorption of organometallic complexes onto oxides
to form A or B.
Brønsted acidity of the –OH sites on oxides is often
presumed to affect the formation of A or B. The example
reactions of Cp2ZrMe2 with oxides are illustrative of this
argument. Neutral oxides containing weak Brønsted –OH
sites (e.g. SiO2) tend to form A, and oxides containing
stronger Brønsted –OH sites (e.g. SO) tend to form B.
The control of active site structures in heterogeneous
catalysts is important to realize the long standing goal of
more selective and reactive solid catalysts.1 A common
method to control the structures of these sites is to support
an organometallic complex onto a partially dehydroxylated
high surface area oxide to form M–OX sites (A, Figure 1, OX
= surface oxygen) or electrostatic ion-pairs [M][OX] (B,
Figure 1).1c,1d Determining the factors that promote
formation of A, B, or mixtures of these two extremes is
fundamentally and practically significant. For example, the
reaction of partially dehydroxylated SiO2 with Cp2ZrMe2
forms Cp2Zr(Me)(OSi≡), a type A surface species, and is
inactive in the polymerization of ethylene.2 However,
supporting Cp2ZrMe2 on partially dehydroxylated sulfated
oxides (SO) forms [Cp2ZrMe][SO], a type B surface
species, that is very active in ethylene polymerization.3 The
reaction of organometallic complexes with SO usually forms
type B surface species that are also active in hydrogenation
of arenes4 and activation of C–H bonds.5 Sulfated zirconium
oxide (SZO) also supports the formation of strong Lewis
acid [iPr3Si][SZO] sites that initiate C–F bond activation
reactions.6
The Brønsted acidity of the –OH sites on SO are the
subject of a long-standing debate. Initial reports showed that
sulfated zirconium oxide (SZO) catalyzed the isomerization
of n-butane to isobutane at lower temperatures than neat
H2SO4.7 This result suggests that the Brønsted sites on SZO
are stronger than neat H2SO4, which would place SZO
across the threshold of superacidity (Hammett acidity
parameter; H0  –12.0).8 This observation was supported by
adsorption of aromatic colorimetric superacid indicators
onto SZO showing that H0  –16.04,7b suggesting that the –
OH sites on SZO are about four orders of magnitude more
acidic than H2SO4 (H0 = –12.0). However, butane
isomerization catalyzed by SZO occurs only in the presence
of olefin impurities in the reaction feed.9 In addition,
isothermal calorimetry showed that SZO binds pyridine
weaker than zeolites.10 Solid-state NMR studies of SZO
after adsorption of probe molecules showed that, in addition
to Brønsted sites, the SZO surface contains Lewis sites,11
and pyrosulfates that are implicated in oxidative reaction
pathways.5a Indeed, pyrosulfate sites were suggested to be
active in reactions of SZO with C-H bonds,12 and are also
implicated in the reaction of organometallic Ir-complexes
with SZO.5a
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This discussion shows some of the complexities in
[tBu3PH][SZO300] contains a P-H at 2441 cm-1 (Figure
2b), and the 31P{1H} MAS NMR spectrum contains a
signal at 52 ppm (Figure 2c). 31P NMR signals for other
phosphorus species are not observed, consistent with
the sole formation of a phosphonium under these
conditions. These results indicate that bulky strong
donor R3P are selective probes for Brønsted sites on
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studying the Brønsted acidity of SZO. The situation is
complicated by the nature of H0, which is a property of
solution acids. SZO is a solid. Among the available methods
to assess the Brønsted acidity of solids are temperature
programmed desorption of NH3,13 measurement NH
stretches of ammonium salts by FTIR,14 and changes in
chemical shift of adsorbed probe molecules by NMR
spectroscopy.15 This paper describes the reaction of SZO
t
SZO300. The clean reactivity of SZO300 with Bu3P to
9
form [tBu3PH][SZO300] contrasts with NH3, a well-
known probe for Brønsted sites, because NH3 can also
react with Lewis and strained sites on oxide surfaces.17
with R3P to form [R3PH][SZO] (eq 1). The 31P{1H} NMR
chemical shifts of R3P and [R3PH] are clearly
distinguishable and the acidity of [R3PH] spans ~15
pKa units in MeCN.16 These properties allow rapid
assignment of [R3PH][SZO] and provide an
understanding of how pKa of [R3PH] affects the
formation of [R3PH][SZO]. These studies show that the
Brønsted sites on SZO are certainly not superacidic.
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SZO was prepared by suspending precipitated
zirconium oxide in dilute aqueous H2SO4, followed by
o
calcination at 600 C. This temperature was chosen
because calcination at 600 oC produces SZO containing
the strongest Brønsted acid sites based on colorimetric
titrations. After cooling to room temperature under air,
SZO was partially dehydroxylated under high
3c
vacuum (10-6 mbar) at 300 oC to form SZO300
.
The reaction of SZO300 with excess gas phase Me3P
forms [Me3PH][SZO300] as the major product. The
31P{1H} Magic Angle Spinning (MAS) spectrum of
[Me3PH][SZO300] contains a major signal at -4 ppm,
which is characteristic of [Me3PH] and a minor peak at
-33 ppm, assigned to small amounts of Me3P bound to
Lewis sites (Figure S1). This result is important
because several previous studies showed that SZO
reacts with gas phase Me3P to form mixtures of
O=PMe3, [Me3PH], and [Me4P].11 Indeed, the reaction
of PMe3 with SZO dehydroxulated at 500 oC results in
formation of significant amounts of [Me4P] byproducts
(Figure S1).The formation of [Me3PH] as the major
surface species indicates that SZO300 does not contain
significant quantities of Lewis sites or pyrosulfates that
are implicated in the formation of O=PMe3 or [Me4P]
byproducts.
Figure 2. Reaction of tBu3P with SZO300 to form
[tBu3PH][SZO300] (a); FTIR of [tBu3PH][SZO300], the nPH is
labelled for clarity (b); 31P{1H} MAS NMR spectrum of this
material (c).
(tBu)2ArP (1a – 1h, eq 2), where the para position of the
Ar group contains substituents that donate or withdraw
electron density, were chosen to systematically evaluate how
the electronics at phosphorous affects formation of
Reacting bulkier tBu3P with SZO300 (1 equiv/OH) in
Et2O slurry results in the formation of
[tBu3PH][SZO300], Figure 2a. The FTIR of
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phosphonium sites on SZO300. The reaction of (tBu)2ArP (1a
1h) with SZO300 in MeCN slurry forms
[ퟐ]
퐾a =
θ =
(3)
(4)
1
2
3
4
5
6
7
8
–
[
]
[ퟏ] HOx
[(tBu)2ArPH][SZO300] (2a – 2h), eq 2. Table 1 contains key
spectroscopic data for 2a – 2h. The 31P NMR chemical shifts
of [1H][BF4] are between 5 – 10 ppm downfield from 1a –
1h in CD3CN solution, indicating that 31P NMR chemical
shift can discriminate between free phosphine and
[1H][BF4]. The 31P{1H} MAS spectra of 2a – 2h contain
signals for the phosphonium at, or near, the values observed
퐾a[ퟏ][HOx]0
1 + Ka[ퟏ]
in [1H][BF4]. Similar to the grafting of tBu3P onto SZO300
,
9
signals for oxidized products or (tBu)2ArP interacting with
Lewis sites were not observed in 31P MAS spectra of 2a –
2h. In addition to the 31P MAS NMR spectra, the FTIR
spectra of 2a – 2h contain P–H stretches, decreasing from
2448 cm-1 for 2a to 2432 cm-1 for 2h.
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Table 1. 31P NMR data for 1a – 1h, [1H][BF4], and 2a – 2h.a
(tBu)2ArP
31P
31P (ppm)
[1H][BF4]c
31P (ppm)
2a – 2hd

P–H (cm-1)
(ppm)b
2a – 2h
1a
1b
1c
1d
1e
1f
36.4
41.8
37.6
39.9
38.9
37.1
38.4
39.0
46.8
46.7
47.6
44.7
48.0
46.8
47.2
47.4
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46
2448
2445
2438
2441
2439
2438
2433
2432
Figure 3. Langmuir isotherm of 1a binding to SZO300. The study
was performed in triplicate using a phosphine stock solution of 0.3
mM, the error bars are standard deviations from these three binding
studies.
Table 2. Binding constants for formation of 2 and pKa of
phosphoniums.
(tBu)2ArP
Ka
pKa
(x104)a
[1H][BF4]b
1g
1h
1a
1b
1c
1d
1e
1f
7.4(4)
5.5(3)
5.5(4)
4.7(1)
4.3(3)
3.0(2)
2.6(1)
1.9(4)
16.4(1)
15.8(1)
15.7(2)
14.9(1)
14.7(1)
14.0(1)
12.9(1)
12.6(2)
a – referenced to 85 % H3PO4; b – CD3CN solution; c – generated in
CD3CN solution (refer to the Supporting Information for details); d
– 10 kHz MAS spinning speed.
Equilibrium binding studies were performed in
anhydrous MeCN slurries at 25 oC under rigorously
anaerobic conditions, experimental details are provided in
the Supporting Information. A representative plot of the
binding of 1a to SZO300 to form 2a is shown in Figure 3. The
data in Figure 3 relates to the equilibrium adsorption
constant Ka, shown in eq 3, where [HOx] are Brønsted sites
on SZO300 reported in mmol/g. Ka is extracted from fits of
the data in Figure 3 to a single-site Langmuir isotherm,
shown in eq 4, where [HOx]0 is the initial OH loading and 
is the surface coverage of the phosphine on SZO300. Ka for
the reaction of SZO300 with 1a to form 2a is 7.4(4) x 104 M-1.
The extracted binding constants from fits to eq 4 for other
phosphines in this study, as well as pKa values of [1H][BF4]
measured in CD3CN solution,18 are given in Table 2. The
prevailing trend of these data is that Ka decreases as pKa of
[1H] decreases. A Hammett plot from the data in Table 2 is
linear with  = –0.14 (R2 = 0.96, Figure S2), consistent with
buildup of positive charge on phosphorus in the formation of
2a – 2h.
1g
1h
a
– average of three binding studies in MeCN slurries, errors in
parentheses are standard deviations from these data; b – determined
in CD3CN solution using methods described in ref 17, refer to the
Supporting Information for details.
The data in Table 2 are inconsistent with Brønsted
superacid behavior. Superacids are known to protonate
MeCN to form [(MeCN)xH][X] solvates.8 The pKa of
solvated protons in MeCN is 0,19 which should result in
much stronger binding for bases whose conjugate acids have
pKa values as those in Table 2.
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implicated in reactions with C–H bonds,12 and can also play
Page 4 of 7
To test the strength of the acid sites on SZO300 we
contacted this material with Ph3P. The pKa of
[Ph3PH][BF4] is 7.6 in MeCN, therefore Ph3P should
bind to SZO300 weaker than (tBu)2ArP described in
Table 2. The reaction of SZO300 with Ph3P forms
[Ph3PH][SZO300] (4) in MeCN. Similar to 2, the 31P{1H}
MAS NMR chemical shift of 4 appears at a chemical
shift close to those observed for solutions of
[Ph3PH][BF4]. 31P{1H} NMR studies of SZO300
suspended in MeCN solutions containing PPh3 show
that Ka ~ 3 M-1, indicating that PPh3 binds much weaker
than 1a – h. This is consistent with the hypothesis that
pKa of [R3PH] influences binding of phosphines to
a role in organometallic grafting reactions.5a Since the
quantity of pyrosulfate is related to thermal activation
temperatures appropriate thermal treatment of these
materials prior to grafting and/or catalysis requires special
attention.
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ASSOCIATED CONTENT
Supporting Information.
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The Supporting Information is available free of charge
on the ACS Publications website:
Experimental details, solid-state NMR spectra, pKa
determinations, Langmuir titration data (PDF)
Brønsted sites on SZO300
.
AUTHOR INFORMATION
The reaction of SZO300 with (2-F-C6H4)Ph2P in
MeCN slurry, followed by successive washings with
MeCN, does not contain a signals in the 31P{1H} MAS
NMR spectrum. Removal of MeCN of SZO300
contacted with a solution of (2-F-C6H4)Ph2P results in
Corresponding Author
*matthew.conley@ucr.edu
Funding Sources
a
31P{1H} MAS NMR spectrum containing a single
No competing financial interests have been declared.
signal at -8 ppm, the chemical shift of (2-F-C6H4)Ph2P
in CD3CN solution is -8.3 ppm. These data indicate that
(2-F-C6H4)Ph2P does not react with Brønsted sites on
SZO300. [(2-F-C6H4)Ph2PH][BF4] has a pKa of 6.11 in
MeCN.20 This result establishes that the –OH sites on
SZO300 do not protonate bases whose conjugate acid
has a pKa of 6.11 or below. This assertion is supported
by the reaction of SZO300 with an acetonitrile solution
ORCID
Matthew P. Conley: 0000-0001-8593-5814
ACKNOWLEDGMENT
M. P. C. is a member of the UCR Center for
Catalysis. This work was supported by the
National Science Foundation (CHE-1800561).
Solid-state NMR measurements were recorded on
an instrument supported by the National Science
Foundation (CHE-1626673).
of p-nitroaniline (pKa(anilinium) = 6.22 in CH3CN),21
a
common Hammett indicator. Contacting SZO300 with
bright yellow MeCN solutions of p-nitroaniline results
in a bright yellow solution and a white solid after
successive washing with MeCN (Figure S4). This result
indicates that p-nitroaniline is not protonated by the
acid sites on SZO300. Consistent with this observation,
the FTIR spectrum of SZO300 contacted with p-
nitroaniline lacks the NH stretch of p-nitroanilinium
(Figure S5).
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] allowed an
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