Syntheses, Spectroscopy, Structures, and Reactivity of Neutral Cubic Group 13 Molecular Phosphonates

Article abstract of DOI:10.1021/ic970346j

The syntheses of cubic group 13 molecular phosphonate cages of the formulas [RPO₃MR′]₄ (R = Me, Et, t-Bu, Ph; MR′ = BEt, BBu-s, GaMe, InMe; all combinations, 1-16), [t-BuPO₃AlBu-i]₄ (17), and [MePO₃AlBu-t]4 (18) have been achieved by the facile reactions of alkyl- or arylphosphonic acids with the corresponding alkyl compounds under appropriately chosen reaction conditions. Compounds 1-18 have been characterized by analytical and spectroscopic techniques. In all cases, the central M₄O₁₂P₄ inorganic cubic framework is shielded by alkyl/ aryl groups. The phosphonates bearing tert-butyl groups on the phosphorus are highly soluble in common organic solvents such as n-hexane, Et₂O, and THF. The molecular structure of representative examples [t-BuPO₃BEt]₄ (3) and [t-BuPO₃BBu-s]₄ (7) as determined by single-crystal X-ray diffraction studies reveal the presence of a central cubic M₄O₁₂P₄ framework that is analogous to those found in alumino- and gallophosphate materials. Most of these cubic phosphonates are air and moisture stable. In the presence of excess phenol, it is possible to react one of the M-C bonds at the corners of the cube 3 and prepare its monophenoxy derivative [t-BuPO₃BEt]₃[t-BuPO₃B(OPh)] (19) in very low yields. However, in spite the presence of reactive M-C bonds, these cubic phosphonates are unreactive toward alcohols, amines, carboxylic acids, H₂O, and air under ambient conditions.

Full text of DOI:10.1021/ic970346j

4202
Inorg. Chem. 1997, 36, 4202-4207
Syntheses, Spectroscopy, Structures, and Reactivity of Neutral Cubic Group 13 Molecular
Phosphonates†
Mrinalini G. Walawalkar, Ramaswamy Murugavel, Herbert W. Roesky,* and
Hans-Georg Schmidt
Institut fu¨r Anorganische Chemie, Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany
ReceiVed March 27, 1997X
The syntheses of cubic group 13 molecular phosphonate cages of the formulas [RPO3MR′]4 (R ) Me, Et, t-Bu,
Ph; MR′ ) BEt, BBu-s, GaMe, InMe; all combinations, 1-16), [t-BuPO3AlBu-i]4 (17), and [MePO3AlBu-t]4
(18) have been achieved by the facile reactions of alkyl- or arylphosphonic acids with the corresponding alkyl
compounds under appropriately chosen reaction conditions. Compounds 1-18 have been characterized by analytical
and spectroscopic techniques. In all cases, the central M4O12P4 inorganic cubic framework is shielded by alkyl/
aryl groups. The phosphonates bearing tert-butyl groups on the phosphorus are highly soluble in common organic
solvents such as n-hexane, Et2O, and THF. The molecular structure of representative examples [t-BuPO3BEt]4
(3) and [t-BuPO3BBu-s]4 (7) as determined by single-crystal X-ray diffraction studies reveal the presence of a
central cubic M4O12P4 framework that is analogous to those found in alumino- and gallophosphate materials.
Most of these cubic phosphonates are air and moisture stable. In the presence of excess phenol, it is possible to
react one of the M-C bonds at the corners of the cube 3 and prepare its monophenoxy derivative [t-BuPO3-
BEt]3[t-BuPO3B(OPh)] (19) in very low yields. However, in spite the presence of reactive M-C bonds, these
cubic phosphonates are unreactive toward alcohols, amines, carboxylic acids, H2O, and air under ambient conditions.
Introduction
advanced materials.5 This is partly due to the large success in
using sol-gel methods for the preparation of a variety of glasses
and related ceramic materials.6 Since the molecular level control
over the final formation of three-dimensional extended structures
would provide rational routes to a new class of materials with
tailored properties, we have been concentrating on the use of
discrete silanetriols and a variety of organometallic derivatives
as starting materials for the synthesis of metallasiloxane cages
which show excellent solubility properties.7 In particular, our
recent success with the synthesis of a multitude of framework
group 13 silicates8 prompted us to extend this approach to the
isoelectronic phosphate and phosphonate systems.9,10 Continu-
ing our studies in this area, we wish to report the synthesis and
structural characterization of a series of phosphonate cage
molecules of B, Al, Ga, and In. This series of compounds,
Studies on metal organophosphates and phosphonates have
received considerable attention due to the interest in their
structural chemistry, intercalation behavior, ion-exchange prop-
erties, and catalytic applications. Early studies on phosphates
and phosphonates have focused on the incorporation of transition
metals such as V, Zr, and Zn into their frameworks.1,2 However,
the discovery3 of a new group of aluminophosphate-based
molecular sieves with exceptional properties has recently shifted
the focus toward the synthesis of new group 13 element based
phosphates and phosphonates.4 Most of these materials are
synthesized either under hydrothermal conditions in the presence
of structure-directing organic templates or by high-temperature
solid-state synthesis routes.
On the other hand, in the last few years there has been an
increasing realization of the importance of devising soft chemical
routes or the so-called molecular routes for the synthesis of
(4) Group 13 phosphonates/phosphates: (a) Estermann, M.; McCusker,
L. B.; Baerlocher, C.; Merrouche, A.; Kessler, H. Nature 1991, 352,
320. (b) Chippindale, A. M.; Walton, R. I. J. Chem. Soc., Chem.
Commun. 1994, 2453. (c) Jones, R. H.; Thomas, J. M.; Huo, Q.; Xu,
R.; Hursthouse, M. B.; Chen, J. J. Chem. Soc., Chem. Commun. 1991,
1520. (d) Chippindale, A. M.; Brech, S. J.; Cowley, A. R. Chem.
Mater. 1996, 8, 2259. (e) Oliver, S.; Kuperman, A.; Lough, A.; Ozin,
G. A. Chem. Mater. 1996, 8, 2391. (f) Jones, R. H.; Chippindale, A.
M.; Natarajan, S.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1994,
565. (g) Yin, X.; Nazar, L. F. J. Chem. Soc., Chem. Commun. 1994,
2349. (h) Oliver, S.; Kuperman, A.; Lough, A.; Ozin, G. A. Inorg.
Chem. 1996, 35, 6373.
(5) For some recent examples on precursor routes to materials, see: (a)
Su, K.; Tilley, D. T. Chem. Mater. 1997, 9, 588 and references cited
therein. (b) Cowley, A. H.; Jones, R. A. Angew. Chem. 1989, 101,
1235; Angew. Chem., Int. Ed. Engl. 1989, 28, 1208. (c) Better
Ceramics Through Chemistry V; Materials Research Society Symposia
Poceedings, Vol. 271; Hampden-Smith, M. J., Klemperer, W. G.,
Brinker, C. J., Eds.; Materials Research Society: Pittsburgh, PA, 1992.
(d) Lugmair, C. G.; Tilley, T. D.; Rheingold, A. L. Chem. Mater.
1997, 9, 339.
† Dedicated to Professor Anton Meller on the occasion of his 65th
birthday.
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
(1) Reviews: (a) Zubieta, J. Comments Inorg. Chem. 1994, 16, 153. (b)
Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420.
(c) Clearfield, A. Comments Inorg. Chem. 1990, 10, 89. (d) Alberti,
G.; Constantino, U. In Inclusion Compounds 5; Atwood, J. L., Davis,
J. E. D., MacNicol, D. D., Eds.; Oxford University Press: Oxford,
U.K., 1991.
(2) For some recent work on phosphonates and phosphates, see : (a) Byrd,
H.; Clearfield, A.; Poojary, D.; Reis, K. P.; Thompson, M. E. Chem.
Mater. 1996, 8, 2239. (b) Song, P.; Xu, J.; Zhao, Y.; Yue, Y.; Xu, Y.;
Xu, R.; Hu, N.; Wie, G.; Jia, H. J. Chem. Soc., Chem. Commun. 1994,
1171. (c) Gendraud, P.; de Roy, M. E.; Besse, J. P. Inorg. Chem.
1996, 35, 6108. (d) Bonavia, G.; Haushalter, R. C.; O’Conner, C. J.;
Zubieta, J. Inorg. Chem. 1996, 35, 5603. (e) Soghomonian, V.;
Haushalter, R. C.; Zubieta, J. Chem. Mater. 1995, 7, 1648. (f) Roca,
M.; Marcos, M. D.; Amoros, P.; Beltran-Porter, A.; Edwards, A. J.;
Beltran-Porter, D. Inorg. Chem. 1996, 35, 5613. (g) Bellito, C.;
Federici, F.; Ibrahim, S. A. J. Chem. Soc., Chem. Commun. 1996,
759.
(6) (a) Mehrotra, R. C. J. Non-Cryst. Solids 1988, 100, 1. (b) Brinker, C.
J.; Scherer, G. W. Sol-Gel Science; Academic Press: Boston, MA,
1990.
(7) (a) Murugavel, R.; Chandrasekhar, V.; Roesky, H. W. Acc. Chem.
Res. 1996, 29, 183. (b) Murugavel, R.; Voigt, A.; Walawalkar, M.
G.; Roesky, H. W. Chem. ReV. 1996, 96, 2205.
(3) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen,
E. M. J. Am. Chem. Soc. 1982, 104, 1146.
S0020-1669(97)00346-7 CCC: $14.00 © 1997 American Chemical Society

Group 13 Molecular Phosphonates
Inorganic Chemistry, Vol. 36, No. 19, 1997 4203
Scheme 1
Scheme 2
longer reaction periods (48 h).15 The change of ethyl groups
on boron to sec-butyl groups further slows the reaction rate,
and hence the reactions involving B(Bu-s)3 were carried out in
a toluene/1,4-dioxane mixture typically for more than 55 h under
reflux conditions. Under these forcing conditions, however, a
slight decomposition of the reaction products was observed.
During the reactivity studies of an anionic lithium gallium
phosphonate cage molecule Li4[(MeGa)6(µ3-O)2(t-BuPO3)6]‚
4THF (A) (derived from LiGaMe4 and t-BuP(O)(OH)2),16 we
also discovered that it is also possible to quantitatively prepare
the cubic gallophosphonate 11 by a delithiation reaction of A
in the presence of 12-crown-4 and D2O as shown in Scheme 2.
The details of this investigation are described elsewhere.16
It should be emphasized that, although in principle other
synthetic routes can be used for the preparation of cubic
phosphonates,17 in our hands, the alkane elimination reaction
between the group 13 trialkyls and an acidic hydrogen contain-
ing compound as shown in Scheme 1 proved to be the most
facile route for synthesizing the title compounds in good yields.
Moreover, the workup procedure in this case is very simple
and most of the products were normally isolated in an
analytically pure form after washing the crude reaction product
with either cold n-hexane or THF depending upon the solubility
of the final product. In the case of borophosphonate 3, it is
possible to obtain the pure product as single-crystals in almost
quantitative yield after crystallizing the crude product from
n-hexane. With the exception of indium compounds 13-16,
all other phosphonates described here can be handled in air
without any detectable decomposition for sufficiently long
periods. When the indium phosphonates were left open in air
for extended periods, a considerable amount of decomposition
was observed, as evidenced by 31P NMR spectroscopy.
existing in the form of three-dimensional cubic cage structures,
possess a M4O12P4 core that is analogous to the D4R (double-
four-tetrahedral-atom-ring) secondary building units (SBUs) of
AlPO4 materials. A preliminary communication of this work
has already appeared.10 While this paper was in preparation, a
few interesting structurally analogous soluble group 13 phos-
phonates were also reported by Mason (Al and Ga),11,12 Barron
(Ga),13 and Kuchen (B)14 and their co-workers.
Results and Discussion
Synthesis. The reaction between phosphonic acids RP(O)-
(OH)2 (R ) Me, Et, t-Bu, Ph) and an equimolar quantity of
trialkyl derivatives of group 13 elements yield cubic phospho-
nates of the formula [RPO3MR′]4 (1-18) in good yields
(Scheme 1). In the case of Al, Ga, and In phosphonates, the
reactions are very facile in THF even at room temperature (or
in few cases at 65 °C), leading to good yields of the products.
When the synthesis of the borophosphonates were carried out
under similar conditions (e.g. in THF at 65 °C for 24 h), a
substantial amount of unreacted phosphonic acid was also
recovered from the reaction mixture. Hence, the syntheses of
borophosphonates starting from BEt3 were carried out using
high-boiling solvents (a toluene/1,4-dioxane mixture, 1:1) and
Spectra. The new phosphonates 1-18 have been fully
characterized by means of analytical and spectroscopic (EI MS,
IR, and NMR) techniques. Single-crystal X-ray diffraction
studies were carried out to establish the solid-state structures
of representative examples [t-BuPO3BEt]4 (3) and [t-BuPO3-
BBu-s]4 (7). Selected spectroscopic data obtained on these
compounds are gathered in Table 1. The compounds that
contain bulky alkyl groups either on phosphorus (e.g., t-Bu) or
on group 13 metals (e.g., BBu-s, AlBu-i, or AlBu-t) yield
phosphonates that are completely soluble in all common organic
solvents. On the other hand, the phosphonates that contain only
phenyl or methyl groups as substituents show poor solubility,
(8) (a) Ritter, U.; Winkhofer, N.; Murugavel, R.; Voigt, A.; Stalke, D.;
Roesky, H. W. J. Am. Chem. Soc. 1996, 118, 8580. (b) Montero, M.;
Voigt, A.; Teichert, M.; Uso´n, I.; Roesky, H. W. Angew. Chem. 1995,
107, 2761; Angew. Chem., Int. Ed. Engl. 1995, 34, 2504. (c)
Chandrasekhar, V.; Murugavel, R.; Voigt, A.; Roesky, H. W.; Schmidt,
H.-G.; Noltemeyer, M. Organometallics 1996, 15, 918. (d) Voigt, A.;
Murugavel, R.; Parisini, E.; Roesky, H. W. Angew. Chem. 1996, 108,
823; Angew. Chem., Int. Ed. Engl. 1996, 35, 748. (e) Montero, M.;
Uso´n, I.; Roesky, H. W. Angew. Chem. 1994, 106, 2198; Angew.
Chem., Int. Ed. Engl. 1994, 33, 2103.
(9) Yang, Y.; Schmidt, H.-G.; Noltemeyer, M.; Pinkas, J.; Roesky, H.
W. J. Chem. Soc., Dalton Trans. 1996, 3609.
(10) Walawalkar, M. G.; Murugavel, R.; Roesky, H. W.; Schmidt, H.-G.
Organometallics 1997, 16, 516.
(11) Mason, M. R.; Matthews, R. M.; Mashuta, M. S.; Richardson, J. F.
Inorg. Chem. 1996, 35, 5756.
(12) Mason, M. R.; Mashuta, M. S.; Richardson, J. F. Angew. Chem. 1997,
109, 249; Angew. Chem., Int. Ed. Engl. 1997, 36, 239.
(13) Keys, A.; Bott, S.; Barron, A. R. J. Chem. Soc., Chem. Commun. 1996,
2339.
(15) Although this observation contrasts with the instantaneous reaction
between tert-butylphosphonic acid and aluminum/gallium alkyls at
ambient temperature,10 it is consistent with the higher reactivity of
the Al-C bond compared to the B-C bond.
(16) Walawalkar, M. G.; Murugavel, R.; Voigt, A.; Roesky, H. W.; Schmidt,
H.-G. J. Am. Chem. Soc. 1997, 119, 4656.
(17) Other synthetic routes known for group 13 phosphonates: Mason et
al. have used the reaction between the silyl phosphate ester P(O)-
(OSiMe3)3 and t-BuAlCl2 to prepare the cubic aluminophosphate
[(OSiMe3)PO3AlBu-t]4; likewise, Kuchen et al. employed the same
strategy to prepare [t-BuPO3BPh]4.
(14) Dimert, K.; Englert, U.; Kuchen, W.; Sandt, F. Angew. Chem. 1997,
109, 251; Angew. Chem., Int. Ed. Engl. 1997, 36, 241.

4204 Inorganic Chemistry, Vol. 36, No. 19, 1997
Walawalkar et al.
Table 1. Selected Physical, Analytical, and Spectroscopic Data for Cubic Metallophosphonates
C, H anal., %
yield,
%
IR (ν(M-O-P)),a
31P NMR,
ppm
compound
mp, °C
>310
found
calcd
cm-1
EI MS,b m/z
ref
[MePO3BEt]4 (1)
[EtPO3BEt]4 (2)
82
85
>95
90
66
72
80
77
14
85
80
88
75
88
60
62
55
51
75
65
27.4, 6.3 26.9, 6.0
210 dec 32.6, 6.8 32.5, 6.8
1070, 1087
1069, 1108
1080, 1097
1072, 1087
1055, 1093
1056, 1092
1033, 1097
1071, 1086
507 (M+ - Et, 40%)
563 (M+ - Et, 100%)
675 (M+ - Et, 100%)
755 (M+ - Et, 60%)
591 (M+ - Bu, 80%)
647 (M+ - Bu, 100%)
759 (M+ - Bu, 40%)
839 (M+ - Bu, 100%)
897 (M+H+, 100%)h
701 (M+ - Me, 70%)
757 (M+ - Me, 60%)
869 (M+ - Me, 100%)
949 (M+ - Me, 100%)
1075 (M+ - Bu, 100%)
880 (M+ - Me, 100%)
937 (M+ - Me, 100%)
1049 (M+ - Me, 70%)
1129 (M+ - Me, 60%)
824 (M+ - Bu, 100%)
655 (M+ - Bu, 100%)
d
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14
33.3e
7.3f
19.3g
31.9e
34.6e
6.7f
17.7e
9.3f
d
[t-BuPO3BEt]4 (3)
[PhPO3BEt]4 (4)
[MePO3BBu-s]4 (5)
[EtPO3BBu-s]4 (6)
[t-BuPO3BBu-s]4 (7)
[PhPO3BBu-s]4 (8)
[t-BuPO3BPh]4
311
>280
>250
>250
283
41.6, 8.2 41.0, 8.0
48.8, 5.2 49.0, 5.2
36.8, 7.4 37.1, 7.5
40.3, 8.0 41.0, 8.0
47.5, 8.9 47.1, 8.9
280 dec 54.0, 6.3 53.6, 6.3
300 dec 53.4, 6.2 53.6, 6.3
[MePO3GaMe]4 (9)
[EtPO3GaMe]4 (10)
[t-BuPO3GaMe]4 (11)
[PhPO3GaMe]4 (12)
[PhPO3GaBu-t]4
[MePO3InMe]4 (13)
[EtPO3InMe]4 (14)
[t-BuPO3InMe]4 (15)
[PhPO3InMe]4 (16)
[t-BuPO3AlBu-i]4 (17)
[MePO3AlBu-t]4 (18)
>280
>280
>280
14.6, 3.8 13.4, 3.4
19.1, 4.3 18.7, 4.2
28.0, 5.7 27.2, 5.5
1055, 1090
1064, 1085
1030, 1073
1040, 1070
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12
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17.4g
21.0g
d
250 dec 35.6, 3.5 34.9, 3.3
42.4, 5.1 42.5, 5.0
3.2g
d
>280
11.0, 3.2 10.7, 2.7
15.6, 3.9 15.1, 3.4
23.0, 4.7 22.6, 4.5
29.6, 3.5 29.4, 2.8
1040, 1095
1034, 1123
1045, 1074
1025, 1097
1070, 1090
>280
>250
>300
d
31.1g
d
16.9f
12.0f
>280
a Recorded as Nujol mulls between KBr/CsI plates. b Recorded at 70 eV. c Reference 85% H3PO4. d Poor solubility precluded 31P NMR analysis.
e In C6D6/THF. f In C6D6. g In CDCl3. h DCI-MS spectrum.
the integrated 1H NMR intensities reveal that there is only one
alkyl group remaining on the metal (B, Al, Ga, or In). The
protons of these alkyl groups attached to the metals show, as
expected, a high-field shift. The protons of the phosphorus
substituents show coupling to the phosphorus centers to which
1
they are bonded. Particularly interesting are the observed H
NMR spectra for the borophosphonates derived from B(Bu-s)3
(Figure 1). Due to the presence of a vicinal chiral center, the
geminal methylene protons in 7 are nonequivalent and appear
Figure 1. 1H NMR (400 MHz, CD2Cl2) spectrum of 7.
and hence it has not been possible to obtain 31P NMR chemical
shifts in these cases (Table 1).
as two separate complex multiplets (0.98 and 1.57 ppm,
respectively). The resonance of the methylene proton appears
as broad high-field multiplet (0.33 ppm) as a result of coupling
with boron and other protons of the sec-butyl group. The
various coupling constants for this resonance are unresolved
even at 400 MHz. The methyl groups of the sec-butyl group
appear as a doublet and a triplet (0.82 and 0.88 ppm,
respectively). In the case of borophosphonates, the 11B NMR
spectra show a single broad peak for all the four boron nuclei
in the molecule.
As it is evident from Table 1, all the cubic phosphonates are
thermally stable and do not melt below 200 °C. In most cases,
the compounds were found to either melt or decompose only
above 280 °C. Further, it is of interest to note that under
electron impact mass spectral conditions (70 eV), the cubic core
in these molecules (1-18) remains intact. In all cases, the peak
due to the M+ - R′ fragment is observed as the most prominent
peak (often base peak) apart from the presence of molecular
ion (M+) peaks in a few cases with a lower intensity. The
subsequent fragmentations are due to the loss of alkyl groups
from the group 13 metals. The IR spectra are devoid of any
absorptions in the region 3000-3500 cm-1, indicating complete
reaction of all P-OH groups with MR3′. Further, all the
metallaphosphonates show two very strong absorptions between
1030 and 1110 cm-1 that are assignable to ν(P-O-M)
vibrations (Table 1). These vibrations are also observed as
prominent peaks in other transition metal containing phospho-
nates synthesized by us.
Although it has not been possible to obtain the 31P NMR
shifts for all the compounds because of poor solubility, the
observed 31P NMR chemical shifts of these compounds show a
periodic trend. In some cases, the spectra were recorded in a
mixture of solvents (THF 90%/C6D6 10%) for solubility reasons.
In the case of indium phosphonates, it was difficult to obtain
good NMR spectra because of the presence of many minor
soluble impurities, and hence the compounds were repeatedly
washed and crystallized with cold pentane to obtain the NMR
samples. It was also not possible to obtain analytically pure
samples for the indium phosphonates (Table 1).
1
The H NMR spectral data are consistent with the structure
of the products. In all cases where spectra could be obtained,

Group 13 Molecular Phosphonates
Inorganic Chemistry, Vol. 36, No. 19, 1997 4205
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 7
P(1)-O(3)
P(1)-O(2)
P(2)-O(6)
P(3)-O(7)
P(3)-O(8)
P(4)-O(11)
B(1)-O(1)
B(1)-O(7)
B(2)-O(10)
B(2)-O(5)
B(3)-O(11)
B(3)-O(3)
B(4)-O(6)
B(4)-O(12)
1.492(6)
1.497(6)
1.494(5)
1.495(5)
1.499(5)
1.503(5)
1.467(9)
1.483(10)
1.453(10)
1.484(10)
1.452(10)
1.493(10)
1.467(10)
1.473(9)
P(1)-O(1)
P(2)-O(4)
P(2)-O(5)
P(3)-O(9)
P(4)-O(10)
P(4)-O(12)
B(1)-O(4)
B(1)-C(21)
B(2)-O(2)
B(2)-C(41)
B(3)-O(8)
B(3)-C(61)
B(4)-O(9)
B(4)-C(81)
1.494(5)
1.490(6)
1.499(5)
1.500(5)
1.498(5)
1.506(5)
1.482(11)
1.591(11)
1.483(10)
1.577(12)
1.487(9)
1.574(11)
1.468(9)
1.582(11)
O(3)-P(1)-O(1)
O(1)-P(1)-O(2)
O(1)-P(1)-C(11)
O(1)-B(1)-O(4)
O(4)-B(1)-O(7)
O(4)-B(1)-C(21)
B(1)-O(1)-P(1)
B(3)-O(3)-P(1)
B(2)-O(5)-P(2)
B(1)-O(7)-P(3)
B(4)-O(9)-P(3)
B(3)-O(11)-P(4)
110.4(4)
113.1(4)
107.4(3)
109.2(6)
107.4(6)
110.7(7)
147.4(5)
146.2(5)
146.6(5)
146.9(5)
148.2(5)
148.2(5)
O(3)-P(1)-O(2)
O(3)-P(1)-C(11)
O(2)-P(1)-C(11)
O(1)-B(1)-O(7)
O(1)-B(1)-C(21)
O(7)-B(1)-C(21)
B(2)-O(2)-P(1)
B(1)-O(4)-P(2)
B(4)-O(6)-P(2)
B(3)-O(8)-P(3)
B(2)-O(10)-P(4)
B(4)-O(12)-P(4)
111.6(4)
107.0(4)
107.0(3)
107.4(6)
111.5(7)
110.6(7)
147.5(5)
147.1(5)
148.5(5)
147.4(5)
148.9(5)
146.6(5)
Figure 2. Molecular structure of the cubic borophosphonate 7.
defined as the central core which comprises four boron and four
phosphorus atoms occupying the alternate vertices. Each of
the B‚‚‚P edges of the polyhedron is bridged by an oxygen atom
in a µ2-fashion, which results in the formation of six nonplanar
B2O4P2 eight-membered rings. Each of these eight-membered
rings adopt a pseudo-C4 crown conformation (Figure 3). While
the two B and two P atoms are coplanar, the four oxygen atoms
of the ring form another almost parallel plane just above the
former plane. The periphery of the central B4O12P4 polyhedron
is surrounded by hydrophobic alkyl groups (tert-butyl and ethyl
or sec-butyl), explaining the high solubility of 3 and 7 in
common organic solvents.
Figure 3. Shape of the core B4O12P4 polyhedron in 7 also revealing
the C4-crown conformation of the faces of the cube.
The bond parameters of 7 compare well with those observed
for 3. Selected bond lengths and angles are listed in Table 2.
There are no formal P-O and PdO bonds in the molecule. In
case of 7, the observed average P-O bond length (1.50 Å) is
much shorter than a formal P-O bond (1.59-1.60 Å) and
considerably longer than a formal PdO bond (1.45-1.46 Å).18
Moreover, the observed values are slightly shorter than the P-O
distances found in many phosphates and phosphonates (1.506-
1.543 Å).4 The average B-O distance in the molecule is 1.47
Å. The observed B-O distances in 7 are much longer than
those found in the eight-membered ring borosiloxane [t-Bu2-
SiO2BPh]2 (average 1.35 Å)19 and other borosiloxanes20 but
comparable to the values found in [t-BuPO3BPh]4 (average 1.46
Å).14 The average B-O-P angle (147.5°) is comparable to
the M-O-Si angles found in many of the cubic heterosiloxanes
described in the literature. The B and P atoms adopt nearly
ideal tetrahedral geometries with the average angles around them
being 109.4 and 109.4°, respectively.
The comparison of 31P chemical shift data is hence made from
the spectra obtained for the tert-butylphosphonic acid derived
boron, aluminum, gallium, and indium phosphonates (com-
pounds 3, 17, 11, and 15, respectively) whose spectral values
are listed in Table 1. As can be seen from Table 1, the chemical
shifts of all metal-containing phosphonates are high-field-shifted
compared to that of tert-butylphosphonic acid (δ 42.4 ppm).
The highest shift was observed in the case of borophosphonate
3 (δ 7.3 ppm, ∆δ ) 35.1 ppm), while the least high-field shift
was observed for the indium compound 15 (δ 31.1 ppm, ∆δ )
11.3 ppm). In the case of [t-BuPO3BEt]4 and [t-BuPO3BBu-
s]4, the observed chemical shifts compare well with that reported
for [t-BuPO3BPh]4 (δ 9.3 ppm).14
Crystal Structure Descriptions for 3 and 7. In order to
further ascertain the solid-state structures of these compounds,
single-crystal X-ray diffraction studies were carried out on
borophosphonates 3 and 7. In a previous publication, the
molecular structure of [t-BuPO3AlBu-i]4 (17) was described.9
The cubic borophosphonate 3 crystallizes in the centrosymmetric
tetragonal space group I41/a with one-fourth of the molecule in
the asymmetric part of the unit cell, while borophosphonate 7
crystallizes in the centrosymmetric triclinic space group P1h with
one full molecule in the asymmetric unit. The final refined
molecular structure of 7 is shown in Figure 2, and the central
B-O-P framework is depicted in Figure 3. The details of
molecular structure determination of 3 have been described in
detail elsewhere in a preliminary communication.10 In the
structure of both 3 and 7, a cube-shaped polyhedron can be
A structural comparison of the cubic borophosphonates 3 and
7 with other group 13 cubic phosphonates recently described is
illustrated in Table 3. Unfortunately, the data remain incomplete
without a crystal structure determination for a cubic indium-
(18) Corbridge, D. E. C. The Structural Chemistry of Phosphorus;
Elsevier: Amsterdam, 1974.
(19) Mazzah, A.; Haoudi-Mazzah, A.; Noltemeyer, M.; Roesky, H. W. Z.
Anorg. Allg. Chem. 1991, 604, 93.
(20) (a) Foucher, D. A.; Lough, A. J.; Manners, I. Inorg. Chem. 1992, 31,
3034. (b) Feher, F. J.; Budzichowski, T. A.; Ziller, J. W. Inorg. Chem.
1992, 31, 5100.

4206 Inorganic Chemistry, Vol. 36, No. 19, 1997
Walawalkar et al.
Table 3. Comparision of Molecular Structural Parameters of Cubic Group 13 Phosphonates
structural feature
space group
polyhedral shape
[t-BuPO3BEt]4
[t-BuPO3BBu-s]4
[t-BuPO3BPh]4
[t-BuPO3AlBu-i]4
[PhPO3GaBu-t]4
I41/a
cubic
C4-crown
1.50
1.47
2.85
3.92
4.12
4.93
P1h
cubic
C4-crown
1.50
1.47
2.85
3.93
4.13
4.94
P21/c
cubic
C4-crown
1.50
1.46
2.84
3.93
4.15
4.92
I41/a
P1h
cubic
C4-crown
1.52
1.76
3.17
4.42
4.56
5.50
cubic
C4-crown
1.51
M2O4P2 ring conformn
P-O length (av), Å
M-O length (av), Å
P‚‚‚M length (av), Å
P‚‚‚P length (av), Å
M‚‚‚M length (av), Å
body diagonal (av), Å
P-O-M angle (range), deg
P-O-M angle (av), deg
ref
1.85
3.22
4.42
4.69
5.57
135.6-175.2
150
146.1-149.3
147.8
this work
146.2-148.9
147.5
this work
140.7-154.0
146.8
14
147.5-154.9
150.5
9
12
Scheme 3
aluminum-, gallium-, and indium-containing phosphonates
through a facile alkane elimination reaction. The structural
similarities of the central M/O/P framework in these molecules
to the basic structural units found in cloverite, ULM-5,21 and
gallophosphate-A22 offer the possibility of using this class of
compounds for the preparation of boron-, aluminum-, gallium-,
and indium-containing phosphonate molecular sieves under mild
conditions using sol-gel techniques. We also plan to investi-
gate the use of these compounds as secondary building units in
synthesizing supramolecular group 13 phosphate structures
through cage fusion reactions utilizing the reactive M-C bonds
located on the four vertices of the cube under forcing conditions.
Moreover, owing to the well-known use of oligophosphate-based
boron compounds in boron neutron capture therapy (BNCT),23
it should also be possible to extend the chemistry described in
this paper to develop new water-soluble borophosphonate
superstructures and study their use in cancer therapy.
containing phosphonate. So far, our attempts to crystallize an
indium compound have been unsuccessful. As can be seen from
Table 3, the size of the central cubic core increases on going
from borophosphonates to gallophosphonates. For example,
while the average length of the B‚‚‚P edge of the cube is 2.85
Å in 3 and 7, the Ga‚‚‚P distance in [PhPO3GaBu-t]4 is 3.22 Å.
Similarly, the average lengths of the face diagonals in the M‚‚‚M
and P‚‚‚P directions increase on going from B to Ga (Table 3).
The length of the body diagonal in the cage of Ga compounds
is ca. 0.63 Å longer than the corresponding values observed in
the case of borophosphonates. Another interesting structural
feature that stems from the data in Table 2 is the very small
change in the M‚‚‚P, M‚‚‚M, P‚‚‚P, and body diagonal distances
between the aluminum and gallium derivatives [t-BuPO3AlBu-
i]4 and [PhPO3GaBu-t]4. In short, the changes observed in the
size of the cubic core on going from B to Al are more
pronounced than the corresponding changes observed on going
from Al to Ga.
Reactivity Studies. Owing to its good isolated yield and
ease of purification, compound 3 was chosen for the reactivity
studies. The B-C bonds of compound 3 do not react either
with alcohols or with primary or secondary amines in THF under
reflux conditions. The reaction of 3 with a large excess of acetic
acid under reflux conditions does not yield the expected acetate
derivative. The 31P NMR of the reaction mixture in this case
reveals extensive decomposition of the starting material. When
compound 3 was reacted with phenol under reflux in high
boiling solvents, the resulting material was analyzed to be
[t-BuPO3BEt]3[t-BuPO3B(OPh)] (19) (Scheme 3). However,
the conversion is less than 5%, as evidenced by 31P NMR
spectroscopy, even after reaction periods of 72 h. The EI mass
spectrum of 19 shows a peak due to M+ - Et at m/e 739 with
the expected isotope pattern. The results of this experiment
clearly demonstrate that it is possible to functionalize the cubic
framework with appropriately chosen reagents (which would
however require forcing conditions). In the future, it should
be possible to adopt this strategy and link these cubic phos-
phonates to prepare supramolecular phosphonate frameworks.
Experimental Section
General Information. All experimental manipulations were carried
out under a dry, prepurified nitrogen atmosphere, using Schlenk
techniques and rigorously excluding moisture and air.24 The samples
for spectral measurements were prepared in a drybox. Solvents were
purified by conventional procedures and were freshly distilled prior to
1
use.25 All H and 31P NMR spectra were recorded on a Bruker AM
200 or a Bruker AS 400 instrument. The chemical shifts are reported
1
in ppm with reference to SiMe4 (external) for H and to 85% H3PO4
(external) for 31P nuclei. The upfield shifts from the reference are
negative. IR spectra were recorded on a Bio-Rad Digilab FTS7
spectrometer (only strong absorptions are reported; vide infra). Mass
spectra were obtained on a Finnigan MAT system 8230 or a Varian
MAT CH5 mass spectrometer. Melting points were obtained on an
HWS-SG 3000 apparatus and are reported uncorrected. CHN analyses
were performed at the analytical laboratory of the Institute of Inorganic
Chemistry, University of Go¨ttingen.
Starting Materials. Methylphosphonic acid, ethylphosphonic acid,
tert-butylphosphonic acid, and phenylphosphonic acid (Aldrich) were
dried under high vacuum prior to use. Triethylboron (1 M solution in
THF, Janssen), tri-sec-butylboron (Aldrich), diisobutylaluminum hy-
dride (1 M solution in hexanes, Aldrich), trimethylgallium (Strem),
and trimethylindium (Strem) were used as received.
Synthesis of Borophosphonates [RPO3BEt]4 (1-4) and [RPO3BBu-
s]4 (5-8). All borophosphonates were prepared by similar procedures.
To a solution of the respective phosphonic acid (1 mmol) in toluene
(20 mL) and 1,4-dioxane (5 mL) was added slowly BR′3 (1 mL of 1 M
solution in THF, 1 mmol) via a syringe at room temperature, and the
(21) Loiseau, T.; Ferey, G. J. Solid State Chem. 1994, 111, 403.
(22) Merrouche, A.; Patarin, J.; Soulard, M.; Kessler, H.; Angelrot, D. Synth.
Microporous Mater. 1992, 1, 384.
(23) Hawthorne, M. F. Angew. Chem. 1993, 105, 997; Angew. Chem., Int.
Ed. Engl. 1993, 32, 1033.
(24) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-SensitiVe
Compounds, 2nd ed.; Wiley-Interscience: New York, 1986.
(25) Perrin, D. D.; Armargeo, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: London, 1988.
Conclusion
In this paper we have demonstrated the use of commonly
available starting materials as precursors for framework boron-,

Group 13 Molecular Phosphonates
Inorganic Chemistry, Vol. 36, No. 19, 1997 4207
mixture was stirred for 2 h. No evolution of ethane gas was noted
during this period. The reaction mixture was subsequently heated under
reflux for 48 h (for 1-4) or 55 h (for 5-8), during which the evolution
of ethane gas ceased. After the reaction mixture was cooled, the solvent
was removed under reduced pressure at room temperature, the residue
was redissolved in a THF (6 mL)/n-hexane (2 mL) mixture, and the
solution was cooled to yield analytically pure products. In the case of
the synthesis of 1, 2, and 4, subsequent to the heating under reflux, the
reaction mixtures were stirred for an additional 2 days.
Synthesis of Gallium Phosphonates [RPO3GaMe]4 (9-12). All
of these phosphonates were synthesized by similar procedures. To a
solution of RP(O)(OH)2 (1 mmol) in THF was added slowly a solution
of GaMe3 (1 mmol) in THF (5 mL) via a syringe at 0 °C. The reaction
mixture was allowed to warm to room temperature, during which
evolution of methane gas ceased. In order to ensure the completion of
the reaction, the reaction mixtures were additionally heated under reflux
for 30 min, after which the volatiles were removed in Vacuo. In the
case of 11, the crude product was purified by crystallization from a
THF/n-hexane mixture. The other derivatives (9, 10, and 12) were
purified by repeatedly washing with cold THF.
Synthesis of Indium Phosphonates [RPO3InMe]4 (13-16). All
of these phosphonates were synthesized by similar procedures. To a
solution of InMe3 (1 mmol) in THF (50 mL) was added slowly a
solution of RP(O)(OH)2 (1 mmol) in THF (5 mL) via a syringe at 0
°C. The reaction mixture was allowed to warm to room temperature,
during which evolution of methane gas ceased. In order to ensure the
completion of the reaction, the reaction mixtures were additionally
refluxed for 1 h, after which the volatiles were removed in Vacuo. In
the case of 15, the crude product was purified by crystallization from
a THF/n-hexane mixture. The other derivatives (13, 14, and 16) were
purified by repeatedly washing with cold THF.
Table 4. Crystal Data and Structure Refinement Details for 7
empirical formula
fw
C32H72B4O12P4
816.02
T, K
250(2)
0.710 73
triclinic
P1h
10.678(2)
wavelength, Å
crystal system
space group
a, Å
b, Å
c, Å
10.775(2)
21.171(4)
R, deg
â, deg
γ, deg
V, Å3
77.19(3)
80.14(3)
85.95(3)
2338.9(8)
Z
D
2
calcd, g‚cm-3
1.159
0.211
880
µ, mm-1
F(000)
crystal size, mm
θ range, deg
index range
1.0 × 0.4 × 0.2
3.5 to 22.0
-11 e h e 11, -11 e k e 11,
-18 e l e 22
4934
no. of total reflns
no. of independent reflns
refinement method
data/restraints/params
R1, R2 (I > 2σ(I))
R1, R2 (all data)
4345 (Rint ) 0.081)
full-matrix least-squares on F2
4246/0/489
0.0809, 0.2323
0.1032, 0.2724
1.060
S
largest diff peak and hole, e‚Å-3
0.976, -0.335
tropically, and the hydrogen atoms were placed in calculated positions
and refined isotropically. Other details pertaining to data collection
and structure solution and refinement are listed in Table 4.
Synthesis of Aluminum Phosphonates 17 and 18. Compounds
17 and 18 were prepared from the respective phosphonic acids and
26
Al(Bu-i)2H (for 17) or Al(Bu-t)3 (for 18) by a procedure similar to
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft, Witco GmbH, and Hoechst AG.
Thanks are also expressed to Dr. U. Englert and the CCDC for
the fractional coordinates of crystal structures of [t-BuPO3BPh]4
and [PhPO3GaBu-t]4, respectively.
that described for gallium phosphonates 9-12. The products were
purified by cooling an n-hexane solution to -20 °C.
The yields, melting points, analytical data, and selected spectroscopic
data for the new phosphonates 1-18 are presented in Table 1.
Additional IR spectral data for 1-17 are provided in the Supporting
Information.
Supporting Information Available: A listing of additional IR
spectral data for 1-18 and tables of structure refinement details,
positional and thermal parameters, bond distances and angles, and
anisotropic thermal parameters for 7 (8 pages). Ordering information
is given on any current masthead page.
X-ray Structure Determination. Crystals of 7 suitable for X-ray
diffraction studies were grown from a dilute 3:1 toluene/n-heptane
solution at room temperature over 72 h by slow evaporation. Intensity
data were collected on a Siemens-Stoe AED2 four-circle diffractometer
using an ω-2θ scan mode. The cell parameters were determined from
randomly chosen and well-centered high-angle reflections. The
structure solution (direct methods) and refinement (by full-matrix least-
squares calculations on F2) were carried out using SHELXS-9027 and
SHELXL-9328 programs. All non-hydrogen atoms were refined aniso-
IC970346J
(27) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(28) Sheldrick, G. M. SHELXL-93: Program for crystal structure refine-
ment. University of Go¨ttingen, 1993.
(26) Uhl, W. Z. Anorg. Allg. Chem. 1989, 570, 37.