Reactions of phosphinites with oxide surfaces: A new method for anchoring organic and organometallic complexes

Article abstract of DOI:10.1039/c0dt01369b

When a pincer-ligated iridium complex with a phosphinite substituent in the para-position of the aromatic backbone is immobilized on γ-alumina, it becomes a highly effective supported catalyst for the transfer-dehydrogenation of alkanes. The nature of the interaction between the organometallic complex and the support was investigated using solid-state ³¹P MAS NMR spectroscopy, solution-state ¹H and ³¹P{¹H} NMR spectroscopy, IR and GC/MS analysis of extracted reaction products. The phosphinite substituent is cleaved from the pincer ligand by its reaction with hydroxyl groups on the γ-alumina surface, resulting in covalent anchoring of the complex via the aryl ring. A similar reaction occurs on silica, allowing for ready grafting onto this support as well. A strategy for anchoring homogeneous catalysts on hydroxyl-terminated oxide supports though the selective cleavage of [POR]-containing ligand substituents is suggested.

Full text of DOI:10.1039/c0dt01369b

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PAPER
Reactions of phosphinites with oxide surfaces: a new method for anchoring
organic and organometallic complexes†
Brian C. Vicente,a Zheng Huang,b Maurice Brookhart,*b Alan S. Goldman*c and Susannah L. Scott*a
Received 11th October 2010, Accepted 3rd February 2011
DOI: 10.1039/c0dt01369b
When a pincer-ligated iridium complex with a phosphinite substituent in the para-position of the
aromatic backbone is immobilized on g-alumina, it becomes a highly effective supported catalyst for
the transfer-dehydrogenation of alkanes. The nature of the interaction between the organometallic
complex and the support was investigated using solid-state 31P MAS NMR spectroscopy, solution-state
1
1H and 31P{ H} NMR spectroscopy, IR and GC/MS analysis of extracted reaction products. The
phosphinite substituent is cleaved from the pincer ligand by its reaction with hydroxyl groups on the
g-alumina surface, resulting in covalent anchoring of the complex via the aryl ring. A similar reaction
occurs on silica, allowing for ready grafting onto this support as well. A strategy for anchoring
homogeneous catalysts on hydroxyl-terminated oxide supports though the selective cleavage of
[POR]-containing ligand substituents is suggested.
alkane dehydrogenation.4 An alternative immobilization strategy
was successfully implemented, in which several pincer complexes
were designed with Lewis basic substituents (i.e., -N(CH3)2, -
Introduction
Pincer complexes of iridium catalyze the dehydrogenation of
alkanes in solution under mild conditions (50–200 ◦C), and show
high selectivity for the formation of a-olefins from n-alkanes.1
This reactivity has been used to achieve alkane metathesis via a
tandem catalytic sequence involving dehydrogenation of alkanes,
metathesis of the olefinic products, and rehydrogenation. In the
tandem reactions, short-chain linear alkanes yield longer-chain
alkanes,2 while cycloalkanes yield ring-expanded cycloalkanes.3
When supported on g-alumina, these catalysts show good activity
and much improved recyclability, e.g., in transfer-dehydrogenation
reactions, relative to their homogeneous analogues.4 The inter-
actions between the complexes and the g-alumina surface, and
the effectiveness of the latter in promoting catalyst stability, were
therefore investigated.
OCH3, -O-K+, -OP(t-Bu)2) in the para-position of the aromatic
backbone of the pincer ligand in order to promote their im-
mobilization onto Lewis acidic g-alumina.4 The nature of the
interaction was presumed to be binding of the para-substituent
to a coordinatively-unsaturated surface Al site, Scheme 1. The
organometallic complexes vary in their synthetic accessibility,
however, the one bearing a phosphinite substituent in the para-
position is particularly easy to prepare, starting from 1,3,5-tris(di-
tert-butylphosphinooxy)benzene. It was found to bind strongly to
g-alumina, and is highly effective as a transfer-dehydrogenation
catalyst.4
Our initial attempts to immobilize iridium pincer complexes
via incorporation of a remote trimethoxysilyl substituent resulted
in heterogeneous catalysts with low activity.4 Methanol generated
as a byproduct of the tethering reaction with surface hydroxyls
results in the formation of a carbonyl complex that is inactive for
aDepartment of Chemical Engineering, University of California, Santa
Barbara, 93106-5080. E-mail: sscott@engineering.ucsb.edu
bDepartment of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, NC, 27599. E-mail: brookhar@email.unc.edu
cDepartment of Chemistry and Chemical Biology, Rutgers University,
Piscataway, NJ, 08854. E-mail: alan.goldman@rutgers.edu
† Electronic supplementary information (ESI) available: Solution-state 31
P
Scheme 1 Proposed immobilization of para-substituted pincer complexes
at Lewis acid sites on g-alumina.
1
{ H} and 1H NMR spectra of products extracted by THF from the reaction
of (t-Bu)2(PhO)P with g-alumina, GC/MS of products extracted by water
from the reaction of (t-Bu)2(PhO)P with g-alumina, and solid-state 31P
MAS NMR of 1,3,5-((t-Bu)2PO)3-C6H3 partially oxidized by air exposure
supported on g-alumina. See DOI: 10.1039/c0dt01369b
Lewis acidity in g-alumina arises due to the presence of four-
and five-coordinate Al sites on the partially dehydrated surface.
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The four-coordinate Lewis acid sites are proposed to be highly
distorted from tetrahedral geometry, while the five-coordinate
Lewis acid sites are square-pyramidal.5 Although frequently
invoked in computational studies, very energetic three-coordinate
Al sites are probably not present in appreciable amounts.6–8 Such
sites may be present at surfaces created in silico by terminating
the bulk structure of g-alumina, but they are proposed to
reconstruct to increase the coordination of highly-unsaturated
cations.9 Tricoordinate Al sites also undergo strongly exothermic
reactions with atmospheric water, and are not restored unless the
g-alumina is dehydrated at temperatures higher than 550 ◦C.5
Recently, we reported that the solid-state 31P magic-angle
spinning (MAS) NMR spectrum of {p-[(t-Bu)2PO]-C6H2-2,6-
[OP(t-Bu)2]2}Ir(C2H4) supported on g-alumina contains a signal
far upfield from the position expected for a phosphinite interacting
with a Lewis acid site, suggesting a more complex chemical
interaction with the surface than that shown in Scheme 1.4
Similar, unexplained upfield signals were observed for the free
pincer ligand, 1,3,5-tris(di-tert-butylphosphinooxy)benzene, and
the model compound phenyl di-tert-butylphosphinite. In this
contribution, the nature of the surface reaction is elucidated
through studies of the catalyst, the pincer ligand, and model
compounds adsorbed onto g-alumina, and the generality of the
immobilization strategy is explored with another commonly-used
catalyst support: silica.
Fig. 1 Comparison of 31P NMR spectra of {p-[(t-Bu)2PO]-C6H2-
2,6-[OP(t-Bu)2]2}Ir(C2H4), 1: (a) solid-state MAS spectrum, for 1 sup-
ported on g-alumina, and (b) solution-state spectrum, for 1 dissolved in
C6D6; and 31P NMR spectra of 1,3,5-((t-Bu)2PO)3-C6H3: (c) solid-state
MAS spectrum, for the triphosphinite supported on g-alumina, and
(d) solution-state spectrum, for the triphosphinite dissolved in CDCl3.
Solid-state spectra were recorded with 14 kHz MAS; asterisks denote
spinning sidebands.4,10
Results and discussion
Adsorption of phosphinites onto c-alumina
When a red solution of the pincer complex {p-[(t-Bu)2PO]-C6H2-
2,6-[OP(t-Bu)2]2}Ir(C2H4), 1, in hexanes is stirred over g-alumina,
the support turns red while the solution becomes colorless,
indicating that 1 is adsorbed onto the support, as previously
reported.4 The solid-state 31P MAS NMR spectrum of 1 supported
in this way on g-alumina is shown in Fig. 1a. The most intense
signal, at 176 ppm, is assigned to the two phosphinite groups
coordinated to iridium, and is shifted slightly upfield compared
to the chemical shift of the phosphinite donors in the molecular
complex (181.1 ppm in C6D6). However, the solid-state 31P signal
of the uncoordinated para-phosphinite substituent at 155 ppm
(150.8 ppm in C6D6) is much weaker than expected. It appears
that much of its 31P signal has undergone a dramatic upfield shift,
resulting in a broad feature at 75 ppm. By integration of the
peaks at 176 and 75 ppm, the ratio of phosphorus coordinated
to iridium to the new phosphorus environment represented by
the isotropic signal at 75 ppm is 2.1, which corresponds closely
to the expected ratio of Ir-coordinated and non-Ir-coordinated
phosphinite groups.
Adsorption of the free ligand, 1,3,5-tris(di-tert-butylphosphino-
oxy)benzene, onto g-alumina gave the 31P MAS NMR spectrum
shown in Fig. 1c. The signal at 155 ppm is assigned to non-
interacting or weakly-interacting phosphinite groups by analogy
to their solution-state chemical shift (151.2 ppm in CDCl3).4 The
spectrum of the supported triphosphinite ligand also contains a
signal at 75 ppm, with the same upfield shift as that observed for
adsorbed 1. The intensity ratio of the two major signals, including
the isotropic peaks at 155 and 75 ppm and their incompletely-
resolved spinning side-bands, is ca. 1.5, suggesting that either one
or two of the three phosphinite groups for each molecule have
reacted. We infer that the free ligand and the iridium complex 1
undergo closely-related reactions with g-alumina surfaces.
Adsorption of phosphine oxides onto c-alumina
The chemical shift of the unexpected signal at 75 ppm in the solid-
state 31P MAS NMR spectra of both the free triphosphinite ligand
and its iridium complex supported on g-alumina (Fig. 1) resembles
that of an aryl(di-tert-butyl)phosphine oxide.11 We considered
the possibility that arylphosphinites undergo g-alumina-induced
rearrangement to the corresponding arylphosphine oxides, which
can then interact with g-alumina via the P O moiety through
either adsorption onto Lewis acidic Al sites or hydrogen bonding
to surface hydroxyl groups.
In order to test this hypothesis, authentic samples of (t-
Bu)2(PhO)P and (t-Bu)2(Ph)P O were adsorbed from toluene
onto g-alumina. The solid-state 31P MAS NMR spectrum of
adsorbed (t-Bu)2(PhO)P, Fig. 2a, exhibits a broad signal with
a large upfield shift, to 71 ppm, relative to its solution value
(153 ppm in CDCl3),4 similar to that observed for some of the
phosphinite groups of the triphosphinite ligand supported on g-
alumina (Fig. 1c). The 31P signals of adsorbed (t-Bu)2(Ph)P
O
appear at similar positions: 69 (major) and 64 ppm (minor),
Fig. 2b. Both are shifted downfield relative to the solution-
state chemical shift of (t-Bu)2(Ph)P O (52 ppm),12 due to its
interactions with the g-alumina surface.
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The IR spectra of both (t-Bu)2(H)P O and (t-Bu)2(PhO)P on g-
alumina show a broad band at ca. 2350 cm-1, Fig. 3a,b. This feature
occurs in the P–H stretching region,14,15 and further confirms
that (t-Bu)2(PhO)P is transformed into (t-Bu)2(H)P O when it
is adsorbed onto g-alumina. The bands appear to be comprised
of at least two components, at 2360 and 2340 cm-1.15 These
are tentatively assigned to (t-Bu)2(H)P O interacting with two
different types of Lewis acid sites, e.g., four- and five-coordinate
Al. For each type of adsorption site, the heterogeneity of acidities
is likely to be responsible for the width of the peaks observed in
Fig. 3a,b.
Fig. 2 Solid-state 31P MAS NMR spectra of three molecular model
compounds, following their adsorption on g-alumina: (a) phenyl
di-tert-butylphosphinite;4,10 (b) di(tert-butyl)phenylphosphine oxide; and
(c) di-tert-butylphosphine oxide. All spectra were recorded with 14 kHz
MAS; asterisks denote spinning sidebands.
Although the locations of the isotropic 31P signals for (t-
Bu)2(Ph)P O and (t-Bu)2(PhO)P adsorbed on g-alumina are
similar, Fig. 2 shows that their linewidths are quite different.
In addition, the spectrum of (t-Bu)2(Ph)P O on g-alumina
lacks the prominent spinning-sidebands observed in the spectrum
of (t-Bu)2(PhO)P on g-alumina, indicating that adsorbed (t-
Bu)2(Ph)P O is considerably more mobile (consistent with its
principal interaction with Brønsted acid sites via H-bonding rather
than with Lewis acid sites). Thus, supporting (t-Bu)2(PhO)P on
g-alumina yields a species with a solid-state 31P MAS NMR
spectrum similar, but not identical, to that of adsorbed (t-
Bu)2(Ph)P O.
Fig. 3 Transmission FT-IR spectra of (a) (t-Bu)2(H)P O supported
on g-alumina; (b) (t-Bu)2(PhO)P supported on g-alumina; and (c)
1,3,5-((t-Bu)2PO)3-C6H3 supported on silica.
A plausible route for the transformation of (t-Bu)2(PhO)P to (t-
Bu)2(H)P O is shown in Scheme 2. Since phenoxide is an effective
leaving group, the attack of a surface hydroxyl group on the aryl
phosphinite could result in phenoxide displacement, creating the
coordinated phosphinous acid shown in the first step. The surface
of g-alumina bears several types of hydroxyls (acidic, basic and
neutral) differing in the coordination number of the Al and the
disposition of the hydroxyl group (terminal, doubly- or triply-
bridging).5,16 The most nucleophilic hydroxyls are the terminal
[OnAlOH] sites, as represented in Scheme 2.
Identification of reaction products
When a portion of the g-alumina modified with (t-Bu)2(PhO)P was
washed with dry THF, no 31P MAS NMR signals were detected
in the washed solid, confirming that all phosphorus-containing
compounds had been removed from the support. The solution-
Subsequent phenoxide displacement of di-(tert-butyl)phos-
phinous acid from the coordination sphere of aluminium will
yield a phenoxide group covalently attached to the surface.
Phosphinous acids tautomerize readily to phosphine oxides,17,18
and the favorable isomerization provides an additional driving
force for phenoxide attachment. In related chemistry, hydrolysis
of (t-Bu)2(PhO)P in solution was reported to give (t-Bu)2(H)P O;
the presumed phosphinous acid intermediate was not-
observed.19
1
state 31P{ H} NMR spectrum of the soluble material, recovered
from the washing solvent and redissolved in THF-d8, contains
1
a major signal at 61.9 ppm, Fig. S1 (see ESI†). The 31P and H
NMR chemical shifts, as well as the JPH coupling constant (419
Hz) of the extracted species, match precisely those reported for (t-
Bu)2(H)P O.13 For direct comparison, an authentic sample of (t-
Bu)2(H)P O was supported on g-alumina. Both the appearance
and the chemical shift of the signal in the 31P MAS NMR spectrum
at 71 ppm, Fig. 2c, closely resemble those of the spectrum of (t-
Bu)2(PhO)P on g-alumina.
Di-(tert-butyl)phosphine oxide, but not phenoxide, was ex-
tracted from the alumina support by washing with dry THF, as
1
shown by solution-state 1H and 31P{ H} NMR spectroscopy of the
filtrate, Fig. S2 (see ESI†). This experiment demonstrates that the
phenoxide is covalently attached to g-alumina, since physisorbed
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acidic water: in this experiment, both phenol and (t-Bu)2(H)P
O
were detected in the aqueous phase by GC/MS, Fig. S3 (see ESI†).
The solid-state 31P MAS NMR spectrum of 1/g-alumina
(Fig. 1a) suggests that the para-phosphinite substituent of the
coordinated pincer ligand reacts with terminal hydroxyl groups
of g-alumina in a manner analogous to that observed for (t-
Bu)2(PhO)P and 1,3,5-(t-Bu2PO)3-C6H3, resulting in detachment
of the uncoordinated phosphinite group. The resulting supported
Ir complex closely resembles the presumed product of direct
grafting of [{p-O-C6H2-2,6-[OP(t-Bu)2]2}Ir(C2H4)]-, 2, onto Lewis
acidic sites on the g-alumina surface. The grafting reactions
are compared in Scheme 3. The solid-state 31P MAS NMR
chemical shift of the phosphinite arms that remain coordinated
to iridium in supported 1 (176 ppm, Fig. 1a) is very similar to
that of an authentic sample of [K+][2] supported on g-alumina
(177 ppm).4 In addition, the supported monocarbonyl complexes
generated by the reactions of 1 and [K+][2] with CO(g) followed
by their adsorption onto g-alumina have the same u(IrC O)
frequency, at 1945 cm-1.4 Since the synthesis of 1 is considerably
more convenient than that of [K+][2],4 in situ generation of the
phenoxide-substituted pincer complex provides a much simpler
route to this highly-active supported dehydrogenation catalyst.
Scheme
2
Proposed mechanism for the reaction of phenyl
Decomposition of c-alumina-supported iridium pincer complexes
di(tert-butyl)phosphinite with a basic surface hydroxyl group on g-alumina
to give a phosphinous acid which rearranges to the corresponding
phosphine oxide, and its eventual displacement by THF.
The observed cleavage of uncoordinated phosphinite substituents
on g-alumina suggests a general decomposition pathway for this
family of supported pincer iridium complexes. The coordinated
phosphinite arms may also react with the surface hydroxyl groups,
albeit more slowly due to the need for initial de-coordination
from Ir. This hypothesis was explored using complex 2, since
phenol would be expected to desorb even in the absence of a proton
source. As expected, phenoxide was displaced by protonolysis
when (t-Bu)2(PhO)P-treated g-alumina was washed with slightly
Scheme
3
Proposed structures of the immobilized ethylene complexes {p-[(t-Bu)2PO]-C6H2-2,6-[OP(t-Bu)2]2}Ir(C2H4), 1, and
[K+][{p-O-C6H2-2,6-[OP(t-Bu)2]2]}Ir(C2H4)-], [K+][2], on g-alumina, as well as the corresponding adsorbed carbonyl complexes.
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it does not generate phosphine oxide upon initial adsorption.
The 31P MAS NMR spectrum of fresh [K+][{p-O-C6H2-2,6-[OP(t-
Bu)2]2}Ir(C2H4)-], [K+][2], supported on g-alumina showed only
the expected isotropic signal at 177 ppm,4 confirming that both
phosphinite arms of the pincer ligand are intact and coordinated
to Ir. After being used as a transfer hydrogenation catalyst,4,20 the
31P MAS NMR spectrum of [K+][2]/g-alumina shows a second
isotropic signal at ca. 66 ppm.4 Its chemical shift is similar to the
signal observed at 71 ppm in the spectrum of 1/g-alumina, and
assigned to adsorbed (t-Bu)2(H)P O (see above).
Another possible decomposition pathway for the supported pin-
cer complexes is the direct oxidation of an Ir-coordinated phosphi-
nite to give the corresponding phosphinate, (t-Bu)2(ArO)P O. A
sample of 1,3,5-((t-Bu)2PO)3-C6H3 that had been exposed to
air was supported on g-alumina. In addition to the expected
signals at 155 ppm and 71 ppm (in common with an intact
sample), a new peak at 59 ppm was present for the air-
exposed sample, Fig. S4 (see ESI†). It is assigned to (di-tert-
butyl)phenylphosphinate, (t-Bu)2(PhO)P O (see ESI†). The sim-
ilarity in chemical shifts for the products of phosphinite oxidation
and hydrolysis/isomerization makes precise assessment of the
decomposition pathway difficult for the supported complex, and
it is possible that both occur.
Fig. 4 Solid-state 31P MAS NMR spectra of (a) 1,3,5-((t-Bu)2PO)3-C6H3
supported on Aerosil-380 silica (previously dehydrated at 500 ◦C); and (b)
1 supported on the same silica. Spectra were recorded with 14 kHz MAS;
asterisks denote spinning sidebands.
Adsorption onto silica
Other oxide supports whose surfaces are terminated by nucle-
ophilic hydroxyl groups are expected to induce similar phosphinite
transformations, leading to anchored organic and organometallic
compounds. We therefore investigated the reaction of phosphinites
with silica. In terms of anchoring pincer catalysts, we envisage the
formation of a covalent linkage between the phenoxide and the
support, as illustrated in Scheme 4.
latter was observed to be hydrogen bonded to residual hydroxyl
groups.15
When a red solution of 1 in hexanes was stirred with an excess
of silica (i.e., containing more surface hydroxyl groups than 1),
the support acquired the color of the Ir pincer complex while
the solvent became completely decolorized. The 31P MAS NMR
spectrum of 1 supported on silica, Fig. 4b, shows a major signal
at 173 ppm with a shoulder at 183 ppm, assigned to the Ir-
coordinated phosphinites of the pincer complex. The minor signal
at 72 ppm corresponds to adsorbed (t-Bu)2(H)P O. These results
confirm that a grafting reaction similar to that observed on g-
alumina also occurs on silica.
The 31P MAS NMR spectrum of 1/silica also contains a
broad peak at 158 ppm, assigned to uncoordinated phosphinite
groups on the basis of its chemical shift. It appears that, in
addition to covalent anchoring via phosphinite cleavage, 1 can
also adsorb on the silica surface via hydrogen bonding, so that the
uncoordinated phosphinite at the para-position remains intact. We
were unable to remove this species by washing the silica-supported
pincer complex with hexanes. The ratio of the signals at 72 and
158 ppm, representing reacted and unreacted para-phosphinite
groups, respectively, is ca. 1.1. Thus the two adsorbed species are
present in roughly equal amounts. The ratio of the area of the
signal at 173 ppm to the sum of the areas of the signals at 72 and
158 ppm is 2.1, close to the original ratio of 2.0 for Ir-coordinated
and non-Ir-coordinated phosphinite groups of intact 1.
Scheme 4 Proposed grafting of {p-[(t-Bu)2PO]-C6H2-2,6-[OP(t-Bu)2]2}-
Ir(C2H4), 1, onto silica.
The 31P MAS NMR spectrum of 1,3,5-((t-Bu)2PO)3-C6H3
supported on silica is shown in Fig. 4a. The signal at 160 ppm
is assigned to intact phosphinite groups, while the signal at
71 ppm is attributed to the phosphine oxide formed by surface-
mediated hydrolysis/isomerization. The appearance of a P–H
stretch at 2326 cm-1 in the IR spectrum of 1,3,5-((t-Bu)2PO)3-
C6H3 supported on silica, Fig. 3c, confirms the generation of (t-
Bu)2(H)P O, which is presumably attached to unreacted surface
hydroxyl groups via hydrogen bonding (since silica lacks Lewis
acidity). Other phosphine oxides have been shown to interact with
silica in this way.21 A similar reaction of P(OMe)3 with silica was
reported to generate SiOMe and (MeO)2(H)P O, where the
Finally, we tested 1/silica in the transfer dehydrogenation of
cyclooctane and tert-butylethylene (TBE) to cyclooctene and 2,2-
dimethylbutane at 200 ◦C. In order to rule out leaching of 1 into
solution during reaction, the catalyst was washed repeatedly with
hexanes prior to use. After 1 h, a turnover number of 612 was
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achieved. No TBE isomerization products (2,3-dimethyl-2-butene
or 2,3-dimethyl-1-butene) were detected by GC/MS, consistent
with the low Brønsted acidity of silica relative to g-alumina.
dissolved in dry hexanes, stirred over g-alumina or silica under N2,
filtered, and washed with excess hexanes. Residual hexanes were
allowed to evaporate without application of vacuum, under air-
and moisture-free conditions.
Conclusions
NMR characterization
The uncoordinated phosphinite substituents of pincer ligands
react with surface hydroxyl groups on g-alumina as well as silica,
resulting in covalent anchoring of the ligands to the oxide surface
by elimination of (t-Bu)2(H)P O. The latter interacts with oxide
surfaces at Lewis acid sites and/or hydroxyl sites, but it can be
displaced by other Lewis bases. The surface-mediated phosphinite
rearrangement allows for the in situ generation of a supported
analogue of [p-O-C6H2-2,6-[OP(t-Bu)2]2]Ir(C2H4)-, using the more
synthetically accessible complex {p-[(t-Bu)2PO]-C6H2-2,6-[OP(t-
Bu)2]2}Ir(C2H4). A similar transformation of the Ir-coordinated
phosphinite arms represents a possible decomposition pathway
for these supported pincer complexes.
Solid samples (approx. 120 mg) were packed under N2 into 4 mm
zirconia rotors, then closed with tight-fitting rotor caps equipped
with O-ring seals (Wilmad). Solid-state MAS NMR spectra were
recorded at room temperature on a Bruker Avance DSX300
spectrometer equipped with a 4-mm broadband MAS probehead
operating at 121.49 MHz for 31P, and using 14 kHz MAS. 31P
MAS NMR experiments were performed with a 90◦ pulse length
of 3 ms, an acquisition time of 0.25 ms, and a recycle delay of
3 s. Between 5000 and 12 800 scans were acquired to achieve an
adequate signal-to-noise ratio. Solution-state NMR spectra were
recorded on a Bruker Avance DMX500 spectrometer operating
1
at 202.45 MHz and 500.13 MHz for 31P and H, respectively. 31
P
We envisage that reactions of phosphinite-, phosphonite- or
phosphite-containing ligands (P(OR1)nR2 where n = 1, 2, 3)
NMR chemical shifts are referenced to aqueous 85% H3PO4.
3-n
with surface hydroxyls to be a new, general method for the
non-hydrolytic immobilization of homogeneous catalysts onto
silica, alumina, or other oxide supports bearing surface hydroxyl
groups. The formation of a phosphine oxide provides a driving
force for the attachment reaction. The use of [POR]-containing
substituents remote from the active center of the catalyst may
produce immobilized metal catalysts with activities similar to
their homogeneous analogues, but with longer lifetimes due to
the suppression of bimolecular decomposition pathways. Catalyst
loadings can be controlled by adjusting the hydroxyl density of
the support, which also minimizes decomposition induced by
reactions with residual hydroxyl groups.
Infrared characterization
Self-supporting pellets of silica or g-alumina, pretreated thermally
to removed adsorbed water and with the appropriate phosphine
oxide or phosphinite adsorbed, were pressed under N2. Spectra
were recorded in transmission mode on a Bruker ALPHA-T
spectrophotometer under N2. Background and sample spectra
were recorded by co-addition of 64 scans at a resolution of 4 cm-1.
Identification of reaction products by GC/MS
Analyses were performed on a Shimadzu GCMS-QP2010 fitted
with an Agilent DB-1 column (100% dimethylpolysiloxane, 30 m
¥ 0.25 mm i.d., 0.25 mm film thickness). A typical temperature
Experimental methods
◦
Materials
program involved 5 min isothermal operation at 40 C followed
◦
by a 25 C min-1 ramp to 220 ◦C. The inlet and ion source
g-Alumina◦(Strem, 185 m2 g-1) was calcined overnight in flowing
O2 at 550 C. Silica Aerosil-380 (Evonik Industries, 363 m2 g-1)
was dried under dynamic vacuum (10-4 Torr) at 500 ◦C overnight.
After thermal pretreatment, the supports were handled under
strictly air-free conditions to prevent re-adsorption of atmospheric
moisture and CO2. Solvents were rigorously dried and standard
glovebox and Schlenk techniques were used to keep samples
air-free. Chloroform-d (99.8% D) and THF-d8 (99.5% D) were
purchased from Cambridge Isotopes Laboratories, Inc. (Andover,
MA).
temperatures were 250 ◦C and 260 ◦C, respectively.
Transfer dehydrogenation
A glass reactor was loaded with 50 mg of 1/silica (1 wt% Ir),
0.40 mL cyclooctane, and 0.30 mL tert-butylethylene (TBE). The
reactor was tightly sealed under N2 with a Teflon plug equipped
with a Viton O-ring. The suspension was stirred in an oil bath at
200 ◦C for 1 h, then cooled to room temperature. The liquid in the
reactor was analyzed by GC/MS, and the turnover number was
calculated as the molar ratio of 2,2-dimethylbutane formed to Ir
present.
The syntheses of phosphinite compounds and the iridium-
pincer complexes were reported previously.4 Bis(tert-butyl)phenyl-
phosphine oxide was prepared from di-tert-butylchlorophosphine
(Strem, 96%), as describedbyGray et al.11 A solution-state31P{ H}
1
NMR spectrum in CDCl3 confirmed its identity (d = 51.7 ppm, s).
Di-tert-butylphosphine oxide was purchased from Strem, and used
as received. To prepare the supported phosphorus compounds,
the phosphinite or phosphine oxide (approx. 20 mg) dissolved in
toluene (3 mL) was stirred under N2 over the appropriate oxide
(g-alumina or silica, 400 mg) for 2 h, filtered, washed with excess
toluene, and dried in vacuum. To prepare g-alumina- or silica-
supported iridium-pincer complexes, the appropriate complex was
Acknowledgements
This work was supported by the NSF under the auspices of
the Center for Enabling New Technologies through Catalysis
(CENTC). Portions of this work made use of facilities of
the Materials Research Laboratory, supported by the MRSEC
Program of the National Science Foundation under award No.
DMR05-20415.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4268–4274 | 4273
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higher MAS speed to reduce overlap between spinning sidebands and
isotropic signals. The new spectra are shown here to allow the reader
to make comparisons with the spectra of authentic phosphine oxides.
11 M. Gray, B. J. Chapell, J. Felding, N. J. Taylor and V. Snieckus, Synlett,
1998, 422.
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4274 | Dalton Trans., 2011, 40, 4268–4274
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The Royal Society of Chemistry 2011
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