An organometallic sol-gel route to layered zinc phenylphosphonate and encapsulation studies with η6-(C6H6)Cr(CO)3

Article abstract of DOI:10.1016/S0022-328X(99)00209-0

Phenylphosphonic acid reacts with diethylzinc in toluene/THF to give a white gel. The gel was dried in air at room temperature for 8 h to give the layered compound [ZnC₆H₅PO₃]·H₂O (1). This process represents a convenient non-aqueous organometallic sol-gel route to a layered metal phosphonate material. Zinc phenylphosphonate synthesized using the sol-gel method is identical to that prepared using the conventional aqueous precipitation technique as determined by IR spectroscopy, thermal gravimetric analysis and powder X-ray diffraction. η⁶-(C₆H₆)Cr(CO)₃ can be incorporated into the gel during the condensation reaction.

Full text of DOI:10.1016/S0022-328X(99)00209-0

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 585 (1999) 162–166
An organometallic sol–gel route to layered zinc phenylphosphonate
and encapsulation studies with h6-(C6H6)Cr(CO)3
Robert W. Deemie, Muralidhar Rao, D. Andrew Knight *
Department of Chemistry, The George Washington Uni6ersity, 725 21st Street, N.W. Washington, DC 20052, USA
Received 9 October 1998
Abstract
Phenylphosphonic acid reacts with diethylzinc in toluene/THF to give a white gel. The gel was dried in air at room temperature
for 8 h to give the layered compound [ZnC6H5PO3]·H2O (1). This process represents a convenient non-aqueous organometallic
sol–gel route to a layered metal phosphonate material. Zinc phenylphosphonate synthesized using the sol–gel method is identical
to that prepared using the conventional aqueous precipitation technique as determined by IR spectroscopy, thermal gravimetric
analysis and powder X-ray diffraction. h6-(C6H6)Cr(CO)3 can be incorporated into the gel during the condensation reaction.
© 1999 Elsevier Science S.A. All rights reserved.
Keywords: Sol–gel; Layered phosphonate; Chromium; Carbonyl
1. Introduction
materials, and exciting new applications for layered
metal phosphonates including catalysts [11], catalyst
supports [12], chemical sensors [13], sorbents [14] and
ferromagnets [15], continue to appear in the literature.
The development of a low-temperature, non-aqueous
sol–gel process for the fabrication of these materials
would expand their future utility. Our involvement in
metal phosphonate chemistry stems from an interest in
The advantages of the sol–gel process as a technique
are well documented and include the processing meth-
ods such as monolith formation, spin and spray coat-
ing, and fiber formation [1]. The process has recently
been used to encapsulate homogeneous catalysts [2],
form thin films of molecular sieves [3] and high-temper-
ature superconductors [4], and to prepare ceramics [5]
and ferromagnetic alloys [6] to name but a few. The
sol–gel method for the synthesis of hybrid inorganic–
organic materials, particularly using non-aqueous and
non-hydrolytic methods, is currently receiving a great
deal of interest, with the commonly explored routes
involving the condensation of metal chlorides with oxy-
gen donors such as ethers, alcohols or alkoxides [7].
The field of layered metal phosphonate chemistry has
also been the subject of a great deal of interest in recent
years. The research groups of Clearfield [8], Mallouk [9]
and others [10] have made important progress towards
understanding the nature of this important class of
the unique microenvironment that
a
lamellar
mesophase can afford on a supported or ‘heteroge-
nized’ organometallic catalyst, and we recently reported
the synthesis of a zinc organometallic phosphonate
containing a covalently bonded, bisphosphonic acid
derived xylenyl chromium tricarbonyl via direct reac-
tion of hydrated zinc nitrate and the arene chromium
tricarbonyl in aqueous methanol (Eq. (1)) [16]. In un-
published work we have also shown that this catalyst is
active for the selective oligomerization of phenyl-
acetylene [17].
In contrast, molecular metal organophosphonates
have been prepared via the condensation of a metal
alkyl with an organophosphonic acid where a sterically
demanding organic substituent is present. For example,
* Corresponding author.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00209-0

R.W. Deemie et al. / Journal of Organometallic Chemistry 585 (1999) 162–166
163
Barron described the synthesis of gallophosphonate
2.2. Preparation of [ZnC6H5PO3]·H2O (1)
[tBu2Ga{m-O2P(Ph)OGatBu2}]2 via the reaction of
phenylphosphonic acid with 3 mol equivalents of t-
Bu3Ga [18] and Mason reported the synthesis of a series
of molecular gallophosphonates via the condensation of
t-Bu3Ga and phosphonic or phosphoric acids [19].
Roesky and Kuchen independently reported the first
molecular borophosphonate cages via the reaction of
t-butylphosphonic acid with BEt3 [20] and t-
Bu2P(O)(OSiMe3)2 with PhBCl2 [21]. More recently,
molecular titanophosphonates, have been synthesized
via the reaction of Cp*TiMe3 with organophosphonic
acids [22]. During our search for non-aqueous, low-
temperature routes to organometallic phosphonates we
discovered that the reaction of diethylzinc and
phenylphosphonic acid in THF/toluene led to the for-
mation of a zinc phenylphosphonate sol–gel instead of
the expected solid zinc phenylphosphonate. In addition,
the catalytically active organometallic molecule h6-
(C6H6)Cr(CO)3 can be incorporated into [ZnC6H5-
PO3]·H2O during the gelation process.
A Schlenk flask was charged with phenylphosphonic
acid (1.739 g, 11.00 mmol), THF (50 ml), a stirrer bar,
and cooled to −78°C (acetone/N2). Then, diethylzinc
(10.0 ml, 11.0 mmol, 1.1 M in toluene) was added
dropwise with stirring. The cold bath was removed and
the reaction mixture was allowed to warm to room
temperature. On warming, the solution became viscous,
stirring was stopped and eventually a homogeneous
white gel formed. The gel was collected in air on a
medium porosity frit, washed exhaustively with THF
and dried overnight under water aspirator vacuum to
give 1 as a white powder (2.447 g, 93%).
Encapsulation of h6-(C6H6)Cr(CO)3 into 1.
A
Schlenk flask was charged with phenylphosphonic acid
(1.739 g, 11.00 mmol), h6-(C6H6)Cr(CO)3 (0.173 g,
0.807 mmol) THF (50 ml), a stirrer bar, and cooled to
−78°C (acetone/N2). Then, diethylzinc (10.0 ml, 11.0
mmol, 1.1 M in toluene) was added dropwise with
stirring. The cold bath was removed and the reaction
mixture was allowed to warm to room temperature. On
warming, the solution became viscous, stirring was
stopped and eventually a homogeneous yellow gel
formed. Analogous work-up to 1 gave 2 as a yellow
powder (2.739 g,) which gradually turns pale green on
prolonged exposure to air.
(1)
2. Experimental
3. Results and discussion
2.1. General data
3.1. Organometallic sol–gel route to
[ZnC6H5PO3]·H2O
All reactions were conducted under dry N2 using
standard Schlenk-line and catheter-tubing techniques
unless otherwise stated. IR spectra were recorded on a
Mattson Galaxy 6020 (FT) spectrometer using KBr
pellets. X-ray powder patterns were recorded on a
Scintag XDS-2000 automated powder diffractometer,
fitted with a graphite monochrometer, using Cu–Ka
radiation. The diffractometer was operated in constant
scan mode of 2.00° min−1 over the range 5°B2qB60°
with zero point determined from a silicon internal
standard. Step scanning was performed with a step size
of 0.03° 2q and a count time of 27.5 min. Powder
samples were prepared by manual grinding using a
mortar and pestle and were then bound to the sample
holder with petroleum jelly. TGA was recorded on a
Perkin–Elmer 7 Series Thermal Analysis System. Mi-
croanalysis was performed by Atlantic Microlab.
Atomic absorption was performed by Galbraith Labo-
ratories.Tetrahydrofuran was distilled from sodium/
benzophenone. Diethylzinc (Aldrich), 1.0 M in toluene,
and phenylphosphonic acid were used as received. h6-
(C6H6)Cr(CO)3 was prepared according to the literature
procedure [23].
The preparation of layered zinc phenylphosphonate
[ZnC6H5PO3]·H2O (1) was first reported by Cunning-
ham et al. [24] and involved the reaction of zinc chlo-
ride or zinc sulfate with phenylphosphonic acid in
water at room temperature. Using this method, the
metal phosphonate immediately precipitated out of so-
lution, although crystalline samples of 1 could be pre-
pared hydrothermally using more dilute aqueous
solutions at higher temperatures. Clearfield and co-
workers reported the single-crystal X-ray analysis of 1
[25] and made extensive studies on the intercalation of
small molecules, such as primary amines, within the
interlayer spacing [8b,d].
When a toluene solution of diethylzinc is added
dropwise via syringe to a solution of phenylphosphonic
acid in THF cooled to −78°C, and slowly warmed to
room temperature, a noticeable increase in viscosity of
the resulting clear solution is observed. The reaction
presumably results in the elimination of ethane, and
bubbles of an unidentified gas were observed immedi-
ately prior to gel formation. After a period of 5 min the
solution eventually reached gelation point and the

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R.W. Deemie et al. / Journal of Organometallic Chemistry 585 (1999) 162–166
white gel was poured onto a frit in air and washed
thoroughly with THF. The gel maintained its integrity
for several hours on standing in air but after drying
under aspirator vacuum overnight, a white powder
formed which was subsequently identified as the known
hydrated zinc phenylphosphonate [ZnC6H5PO3]·H2O
(1) by several techniques (vide supra) (Eq. (2)).
In order to show that the sol–gel method ultimately
gives an identical compound to that prepared using the
aqueous precipitation route,
a sample of [ZnC6-
H5PO3]·H2O was synthesized using the literature proce-
dure [8d] and a direct comparison of the infrared
spectra was made. Fig. 1 shows the IR spectra of
compound 1 prepared by both the aqueous precipita-
tion method (a), and via the organometallic sol–gel
method (b). The spectra contain two bands centered at
3466 and 3430 cm−1 corresponding to the zinc-coordi-
nated water O–H stretching modes and a band at 1626
cm−1 due to the bending mode. The three intense
(2)
Fig. 1. Infrared spectra of 1 (KBr disk) prepared via (a) aqueous precipitation method (b) non-aqueous sol–gel method.

R.W. Deemie et al. / Journal of Organometallic Chemistry 585 (1999) 162–166
165
bands at 1144, 1084 and 986 cm−1 correspond to –PO3
stretching modes and the four weaker bands at 748,
723, 694 and 579 cm−1 belong to the monosubstituted
phenyl ring. The formation of the monohydrate form of
zinc phenylphosphonate using the sol–gel method is
not unexpected, as the sample was dried in air.
Clearfield has reported that anhydrous zinc
phenylphosphonate rapidly rehydrates on exposure to
air [8d].
spectroscopy for several independently prepared sam-
ples. Evaluation of chromium leaching was attempted
using Soxhlet extraction (THF) but extensive decompo-
sition was observed.
The presence of zinc phenylphosphonate coordinated
chromium tricarbonyl subunits would give rise to ex-
pansion of the interlayer distance due to the additional
steric bulk. The X-ray powder pattern of compound 2
is identical to that present in 1 with an intense peak at
A direct comparison of the powder X-ray diffraction
patterns of [ZnC6H5PO3]·H2O prepared by the aqueous
and sol–gel methods further support the suggestion
that the two samples are identical. Both patterns con-
,
14.34 A indicating that no layer expansion has occurred
and that the arene chromium tricarbonyl has not been
incorporated within the zinc phenylphosphonate layers.
Thus the exact location of [h6-(C6H6)Cr(CO)3] within
the zinc phenylphosphonate matrix is unknown and
may simply involve the random attachment of surface
bound molecules within the micro-structure of the
metal phosphonate. The adsorption of arene chromium
tricarbonyl compounds onto the surfaces of silica and
alumina is well known and it has been suggested that
metal carbonyl bonding to the surface of these supports
is via carbonyl oxygen coordination to Lewis acid sites
(e.g. Al3+) or hydrogen bonding of surface hydroxy
groups to the benzene rings [28]. Further studies on
compound 2 are underway and the catalytic activity of
2 will be reported in due course.
,
tain an intense reflection at 14.34 A corresponding to
the interlayer spacing, which is in agreement with the
value reported previously [8d]. An overlay of the two
patterns shows a direct peak match, although the peaks
present in the sol–gel prepared sample are slightly
broader than those in the aqueous sample. A thermal
gravimetric analysis was also performed on the air-
dried gel sample and indicated weight losses of 7.0% at
95°C and 24.7% and 525°C which correspond respec-
tively to dehydration and loss of the organic portion of
the phosphonate. These values match those reported in
the literature for 1 [8b]. We are therefore reasonably
confident that the sol–gel route gives an identical mate-
rial to that prepared via the aqueous precipitation
method.
In conclusion, we have studied a non-aqueous
organometallic sol–gel route to layered hydrated zinc
phenylphosphonate and an example of an encapsulated
arene chromium tricarbonyl molecule, which are likely
to broaden the application of metal phosphonates as
materials. We are currently investigating the synthesis
of other metal phosphonates using this method, includ-
ing water-sensitive and thermally sensitive organometal-
lic phosphonates which have catalytic activity, and
these will be reported in future publications.
3.2. Encapsulation of p6-(C6H6)Cr(CO)3 into 1
When the condensation reaction described above was
performed in the presence of the small organometallic
molecule [h6-(C6H6)Cr(CO)3] a new yellow gel formed,
indicating that the carbonyl had been incorporated into
the zinc phenylphosphonate. After exhaustive washing
of the sample with THF to remove excess [h6-
(C6H6)Cr(CO)3], the sample was dried in air to give
compound 2. The infrared spectrum of 2 contains two
medium intensity absorptions at 1962 and 1885 cm−1
in the carbonyl region of the spectrum corresponding to
the A1 and E bands and confirms the presence of intact
[h6-(C6H6)Cr(CO)3] molecules with negligible perturba-
tion of the chromium coordinated carbonyl ligands [26].
We previously reported the synthesis and characteriza-
tion of the organometallic phosphonate [h6-
(C6H5PO3Et2)Cr(CO)3] which has intense CO
absorptions at 1981 and 1910 cm−1 [27] The lack of
absorptions with these values may indicate the absence
of chromium coordinated phenyphosphonate subunits
which could possibly arise from arene exchange be-
tween [h6-(C6H6)Cr(CO)3] and [ZnC6H5PO3]·H2O, al-
though we suggest this with some caution due to the
electronic differences between the phosphonic acid sub-
stituents (CH3CH2– vs. zinc ion). A chromium loading
of 0.19–0.21% was determined using atomic absorption
Acknowledgements
The donors of the Petroleum Research Fund admin-
istered by the American Chemical Society are thanked
for support of this work. We would also like to thank
Professor F.R. Siegel of the Geology Department
(GWU) for use of the powder diffractometer purchased
under NSF Grant EAR-9218692 and Michael Miggins
for preliminary observations.
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