A nonanuclear organostiboxane cage

Article abstract of DOI:10.1021/om900113q

The first example of a nonanuclear organostiboxane cage,[(Ph2SbMPhSbMfi-O) 11(fi3-OMfi-OH)2(fi-cycPO2)2(cyc- PO2)2(H2O)2] · 2CH3CN· H2O, containing Sb(V) has been assembled by a mild hydrolysis and Sb-C bond cleavage reaction of [(Ph3Sb)2(fi-O)(fi-cycPO2)

Full text of DOI:10.1021/om900113q

Organometallics 2009, 28, 2637–2639
2637
A Nonanuclear Organostiboxane Cage
Vadapalli Chandrasekhar* and Ramalingam Thirumoorthi
Department of Chemistry, Indian Institute of Technology-Kanpur, Kanpur-208 016, India
ReceiVed February 12, 2009
reactions involving organoantimony halides. Herein, we report
the synthesis and structural characterization of [(Ph3Sb)2(µ-O)(µ-
cycPO2)2] (1) and [(Ph2Sb)2(PhSb)7(µ-O)11(µ3-O)3(µ-OH)2(µ-
cycPO2)2(cycPO2)2(H2O)2] · 2CH3CN · H2O (2) (cycPO2 ) 1,1,-
2,3,3-pentamethyltrimethylene phosphinate). The latter is an
unprecedented nonanuclear organostiboxane cage containing a
Sb9O16 core. Unlike the mixed-valent Sb(III)/Sb(V) cages
reported earlier, 2 is an all Sb(V) cage. Additionally, two
putative stibinic acid motifs Ph2Sb(O)(OH) are trapped in the
cage structure of 2 and serve as the two extremes of this ellipsoid
molecule. Another interesting structural feature of 2 is that it
possesses a partially hydrolyzed antimony center containing two
molecules of water. Remarkably, 2 is formed by a concomitant
Sb-C bond cleavage and hydrolysis reaction that occurs at very
mild reaction conditions.
The reaction of Ph3SbCl2 · H2O with cycP(O)(OH) in the
presence of 2 equiv of triethylamine afforded 1 (Chart 1, Figure
1), whose molecular structure was determined by single-crystal
X-ray analysis8 (Vide infra). The 31P NMR spectrum of 1 in
C6D6 revealed the presence of two closely spaced resonances
at 48.02 and 48.56 ppm, respectively, which may be due to the
slight nonequivalence of the phosphorus centers in 1, which
may be a result of variation of coordination modes (monodentate
and bidentate).3a The 1H NMR spectrum of 1 also is consistent
with the presence of two nonequivalent phosphinate ligands.
The 31P chemical shifts of 1 are slightly upfield shifted in
comparison to cycP(O)(OH) (59.83 ppm, CDCl3). Such an
upfield shift of 31P resonance was noted before in [(nBu2Sn)2(µ-
OH)(µ-cycPO2)3]n and [(nBu3Sn)(µ-cycPO2)]4.7c
The crystal structure of 1 reveals that its asymmetric unit
contains one full molecule. The molecular structure of 1 (Figure
1) shows that it is made up of two triphenylantimony units that
are bridged to each other by a µ-O and two (cycPO2)- ligands.
The latter bind the two antimony centers in a 2.11 mode,9
affording two six-membered Sb2O3P rings, which are nearly
perpendicular to each other (dihedral angle 86.7°) (Supporting
Information). The six-membered rings themselves are nearly
planar, although one oxygen atom (O2) deviates from the mean
plane by 0.4 Å. The Sb-O-Sb angle found in 1 is quite wide
(146.40(16)°) in comparison with (Ph3SbO)2 (av 102.5(2)°).3d
Two types of Sb-O distances are found in 1. The bond distance
involving the µ-O is shorter (av 1.9537(25) Å) in comparison
with that involving the phosphinate ligand (av 2.1926(31) Å).
Only three other organoantimony compounds are known in the
Summary: The first example of a nonanuclear organostiboxane
cage, [(Ph2Sb)2(PhSb)7(µ-O)11(µ3-O)3(µ-OH)2(µ-cycPO2)2(cyc-
PO2)2(H2O)2] · 2CH3CN · H2O, containing Sb(V) has been as-
sembled by a mild hydrolysis and Sb-C bond cleaVage reaction
of [(Ph3Sb)2(µ-O)(µ-cycPO2)2] (cycPO2 ) 1,1,2,3,3-pentam-
ethyltrimethylene phosphinate).
Introduction
In contrast to the well-studied family of organostannoxanes,1
the corresponding organostiboxanes have received much less
attention, although initial efforts by the group of Sowerby were
successful in the isolation of a diphenylantimony oxide whose
cation consisted of a planar Sb6O6 ring apart from two Sb2O2
rings.2 The latter motif was also found in di- and tetranuclear
oxodiarylantimony phosphinates or arsinates.3 Breunig and co-
workers have also synthesized many Sb2O2-cored antimony
complexes.4 Recently in a significant breakthrough Beckmann
and co-workers have isolated the first example of a well-defined
stibonic acid, [2,6-Mes2C6H3Sb(O)(OH)2]2, which also consists
of a four-membered distiboxane ring.5a This group also suc-
ceeded in preparing mixed-valent antimony(III/V) oxo clusters
[(2,6-Mes2C6H3Sb)4(SbCl)4(SbOH)2O14] by a controlled hy-
drolysis of 2,6-Mes2C6H3SbCl2.5b In a new dimension to
stiboxane chemistry, Winpenny and co-workers have recently
shown that [(ArSb)4O2(PhPO3)4(PhPO3H)] and [(ArSb)2(t-
BuPO3H)6O] (Ar ) p-chlorophenyl) function as ligands to afford
a variety of polymetallic cages.6a,b Such approaches have also
been extended to other systems.6c We have been involved for
some time in delineating the chemistry arising out of the
reactions of phosphonic and phosphinic acids with organotin
oxides, hydroxides, and oxide-hydroxides.7 In view of the rich
structural diversity among the products obtained in such
reactions we initiated a program on the study of analogous
* To whom correspondence should be addressed. E-mail: vc@iitk.ac.in.
Phone: (+91) 512-259-7259. Fax: (+91) 512-259-0007/7436.
(1) (a) Chandrasekhar, V.; Sasikumar, P.; Singh, P.; Thirumoorthi, R.;
Senapati, T. J. Chem. Sci. 2008, 120, 105–113. (b) Chandrasekhar, V.;
Gopal, K.; Thilagar, P. Acc. Chem. Res. 2007, 40, 420–434. (c) Chan-
drasekhar, V.; Nagendran, S.; Baskar, V. Coord. Chem. ReV. 2002, 235,
1–52. (d) Holmes, R. R. Acc. Chem. Res. 1989, 22, 190–197.
(2) Southerington, I. G.; Begley, M. J.; Sowerby, D. B. J. Chem. Soc.,
Chem. Commun. 1991, 1555–1556.
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1997, 2785–2792. (b) Said, M. A.; Swamy, K. C. K.; Poojary, D. M.;
Clearfield, A.; Veith, M.; Huch, V. Inorg. Chem. 1996, 35, 3235–3241. (c)
Liu, B.-Y.; Ku, Y.-T.; Wang, M.; Wang, B.-Y.; Zheng, P.-J. J. Chem. Soc.,
Chem. Commun. 1989, 651–652. (d) Bordner, J.; Doak, G. O.; Everett, T. S.
J. Am. Chem. Soc. 1986, 108, 4206–4213. (e) Sowerby, D. B.; Begley,
M. J.; Millington, P. L. J. Chem. Soc., Chem. Commun. 1984, 896–897.
(4) (a) Breunig, H. J.; Kru¨ger, T.; Lork, E. J. Organomet. Chem. 2002,
1-2, 209–213. (b) Breunig, H. J.; Kru¨ger, T.; Lork, E. Angew. Chem., Int.
Ed. Engl. 1997, 36, 615–617. (c) Breunig, H. J.; Probst, J.; Ebert, K. H.;
Lork, E.; Cea-Olivares, R.; Aivarado-Rodr´ıguez, J.-G. Chem. Ber. 1997,
130, 959–961.
(6) (a) Ali, S.; Baskar, V.; Muryn, C. A.; Winpenny, R. E. P. Chem.
Commun. 2008, 6375–6377. (b) Baskar, V.; Shanmugam, M.; Helliwell,
M.; Teat, S. J.; Winpenny, R. E. P. J. Am. Chem. Soc. 2007, 129, 3042–
3043. (c) Clark, C. J.; Nicholson, B. K.; Wright, C. E. Chem. Commun.,DOI:
10.1039/b816113e.
(7) (a) Chandrasekhar, V.; Sasikumar, P.; Thilagar, P. Organometallics
2007, 26, 4386–4388. (b) Chandrasekhar, V.; Gopal, K. Appl. Organomet.
Chem. 2005, 429–436. (c) Chandrasekhar, V.; Baskar, V.; Steiner, A.;
Zacchini, S. Organometallics 2004, 23, 1390–1395. (d) Chandrasekhar, V.;
Baskar, V.; Steiner, A.; Zacchini, S. Organometallics 2002, 21, 4528–4532.
(5) (a) Beckmann, J.; Finke, P.; Hesse, M.; Wettig, B. Angew. Chem.,
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10.1021/om900113q CCC: $40.75
2009 American Chemical Society
Publication on Web 04/02/2009

2638 Organometallics, Vol. 28, No. 8, 2009
Notes
Chart 1. Schematic Picture of 1 and 2
literature possessing similar structural features to those shown
by 1.3a,10 Interestingly the reaction of cycPO2H with organotin
substrates results in the formation of polymeric or macrocyclic
compounds.7c
In view of the recent interest in the hydrolytic chemistry of
organoantimony and organotin compounds and recognizing that
compounds such as 1 are hydrolytically sensitive,3a we effected
a mild hydrolysis of 1 in a mixture of CH3CN and water (99:1,
v/v) at 45 °C. In contrast to previous literature observations,
where Sb(V)-C bonds were found to be stable,5b we find a
facile Sb-C cleavage reaction accompanied by hydrolysis
to afford [(Ph2Sb)2(PhSb)7(µ-O)11(µ3-O)3(µ-OH)2(µ-cycPO2)2-
(cycPO2)2(H2O)2] · 2CH3CN · H2O (2) (Chart 1), which could be
characterized in the solid state by single-crystal X-ray analysis.8
Its poor solubility, however, precluded its characterization in
solution.
2 is an unprecedented nonanuclear organostiboxane cage
containing a Sb9O16 core (Figure 2). Seven of the nine antimony
atoms have only one phenyl substituent, while two others
possess two phenyl groups each. Although four phosphinate
ligands are involved in binding to six of the antimony atoms,
the bulk of the core structure is entirely built from bridging
oxide (µ3 and µ2) and hydroxide ligands. Interestingly, 2 also
j
(8) Crystal data for 1: triclinic; P1; a ) 10.8548(17) Å, b ) 12.1442(19)
Å, c ) 19.956(3) Å; R ) 95.080(3)°, ꢀ ) 99.305(3)°, γ ) 103.353(2)°; V
) 2504.1(7) Å3, T ) 273(2) K, Z ) 2; Fcalcd ) 1.422 g cm-3; crystal
dimensions 0.2 × 0.2 × 0.1 mm; 2θmax ) 25.00; 13 051 reflections
measured; 8680 unique reflections; 564 parameters; R1 ) 0.0428; wR2 )
0.1071. Crystal data for 2: monoclinic; P2/n; a ) 15.420(3) Å, b )
15.122(3) Å, c ) 25.599(5) Å; ꢀ ) 92.52(3)°; V ) 5963(2) Å3, T ) 100(2)
K, Z ) 2; Fcalcd ) 1.692 g cm-3; crystal dimensions 0.3 × 0.2 × 0.2 mm;
2θmax ) 25.00; 30 217 reflections measured; 10 482 unique reflections; 630
parameters; R1 ) 0.0395; wR2 ) 0.0981. CCDC 716145 and 716146
contain the crystallographic data for this publication.
Figure 1. ORTEP diagram of 1. Thermal ellipsoids are set at 30%,
and hydrogen atoms have been omitted for clarity. Selected bond
distances (Å) and bond angles (deg): Sb(1)-O(5) 1.959(3),
Sb(2)-O(5) 1.949(3), Sb(1)-O(1) 2.160(3), Sb(1)-O(4) 2.204(3),
Sb(2)-O(2) 2.214(3), Sb(2)-O(3) 2.194(3), Sb(2)-O(5)-Sb(1)
146.40(16).
(9) Coxall, R. A.; Harris, S. G.; Henderson, D. K.; Parsons, S.; Tasker,
P. A.; Winpenny, R. E. P. J. Chem. Soc., Dalton Trans. 2000, 2349–2356.

Notes
Organometallics, Vol. 28, No. 8, 2009 2639
procedures. 2,4,4-Trimethyl-2-pentene and triphenylantimony were
purchased from Sigma Aldrich Ltd. Aluminum chloride was
purchased from s. d. Fine Chemicals, Mumbai. Triphenylantimony
dichloride and 1,1,2,3,3-pentamethyltrimethylenephosphinic acid
were synthesized according to well-known literature procedures.12
Instrumentation. Melting points were measured using a JSGW
melting point apparatus in glass capillaries and are uncorrected.
Elemental analyses were carried out using a Thermoquest CE
Instruments model EA/110 CHNS-O elemental analyzer. IR spectra
were recorded from 4000 to 400 cm-1 on a Bruker FT-IR Vector
22 model in the solid state using KBr pellets and in the solution
state using dichloromethane. 1H and 13C{1H} NMR were recorded
on a JEOL-JNM LAMBDA model 400 spectrometer using C6D6
and C7D8 operating at 400 and 100 MHz, respectively. 31P{1H}
NMR spectra were recorded on a JEOL-JNM DELTA model 500
spectrometer using C6D6 operating at 162 MHz. ESI-MS analyses
were performed on a Waters Micromass Quattro Micro triple
quadrupole mass spectrometer. Electrospray ionization (positive ion,
full scan mode) was used keeping acetonitrile as solvent and
nitrogen gas for desolvation. Capillary voltage was maintained at
2 kV, and cone voltage was kept at 31 kV. The temperature
maintained for the ion source was 100 °C and for desolvation 350
°C.
Figure 2. ORTEP diagram of the Sb9O16 core of 2. Thermal
ellipsoids are set at 30%; phenyl groups, phosphinate ligands, and
water molecules are omitted for clarity. Selected bond distances
(Å) and bond angles (deg): Sb(1)-O(7) 1.925(3), Sb(1)-O(4)
1.985(3), Sb(1)-O(3) 2.058(3), Sb(2)-O(9) 1.962(3). Sb(2)-O(4)
2.051 (3), Sb(2)-O(3) 2.092(3), Sb(3)-O(6) 1.953(3), Sb(3)-O(8)
1.955(3), Sb(3)-O(7) 1.955(3), Sb(3)-O(12) 2.107(3), Sb(3)-O(5)
2.116(3), Sb(4)-O(9) 1.940(3), Sb(4)-O(13) 1.959(3), Sb(4)-O(8)
1.998(3), Sb(4)-O(5) 2.157(3), Sb(5)-O(13) 1.917(3), Sb(5)-O(12)
2.016(4), Sb(1)-O(4)-Sb(2) 105.97(14), Sb(1)-O(3)-Sb(2)
101.87(14).
contains an antimony atom (Sb5) that is partially hydrolyzed
and contains two water molecules of coordination. It is of
interest to note that recently we have shown that hydrated
organotin cations are the precursors to further hydrolyzed
products.11 In light of this, it is clear that the coordination
environment around Sb5 represents a snapshot of an intermediate
before its final hydrolysis to form hydroxides or their condensed
products.
Synthesis. Synthesis of [(Ph3Sb)2(µ-O)(µ-cycPO2)2] (1): Hydrated
triphenylantimony dichloride (0.19 g, 0.45 mmol) was added to a
clear solution of 1,1,2,3,3-pentamethyltrimethylenephosphinic acid
(0.08 g, 0.45 mmol) and triethylamine (0.2 mL) in 20 mL of
benzene. The solution was stirred for 24 h. Filtration followed by
evaporation produced a white solid. Block-shaped crystals of 1 were
formed, which were recrystallized from n-hexane. Yield: 0.2 g, 83%.
Mp: 192-193 °C. Anal. Calcd for C52H62P2O5Sb2 (1): C, 58.31;
H, 5.84. Found: C, 58.12; H, 5.76. IR (KBr, cm-1): 1110 (νasymPO2)
The molecular structure of 2 contains two symmetrically related
trimer units that are present an either end of the structure. Thus,
Sb1, Sb2, and Sb4 are part of a six-membered Sb3O3 ring, where
Sb2 and Sb1 are further bridged by a µ-OH (O3) ligand to generate
a bicyclic ring system. The central portion of the cage contains
two symmetrically related antimony atoms (Sb3 and Sb3*), which
are connected to each other by two µ-O ligands (O12 and O6).
The partially hydrolyzed antimony (Sb5) containing two water
molecules is associated in bridging the two trimeric (through O13
and O13*) as well as the central dimeric unit (through O12). The
latter is connected to the trimeric poles through the bridging action
of two µ-O (O7 and O8) and one µ3-O (O5) ligand. The formation
of this Sb9O16 core leads to the presence of many Sb-O ring
systems including Sb2O2, Sb3O3, and Sb4O4 rings (Supporting
Information). Two of the four phosphinate ligands act as a 2.11
coordination mode9 and bridge the Sb2O(OH) motifs present on
either end of the cage. The other two phosphinate ligands are
monodentate and bind to the third antimony of the trimeric units.
The three antimony atoms present in the center of the cage do not
have phosphinate ligands.
1
and 1031, 1008 (νsym PO2). H NMR (400 MHz, C6D6, ppm): δ
8.27 (Ar, m), 8.09 (Ar, m), 7.96 (Ar, m), 7.84 (Ar, d, J ) 6.83
Hz), 7.30-7.00 (Ar, m), 1.41-1.34 (-CH3, dd, J ) 8.79 and 9.51
Hz), 1.30-1.22 (-CH, m), 1.18 (-CH3, d, J ) 4.8757 Hz), 1.13
(-CH3, d, J ) 5.16 Hz), 0.80 (-CH3, d, J ) 18.78 Hz), 0.61
(-CH3, d, J ) 19.02). 31P NMR (202.5 MHz, C6D6, ppm): δ 48.02
(s), 48.56 (s). ESI-MS: m/z (%) 177.1065 [cyc-PO2H+H]+ (18),
369.0209 [(C6H5)3SbOH]+ (100), 411.0345 [(C6H5)3Sb(OH2)(CH3-
CN)] (47), 429.0620 [(C6H5)3Sb(OH2)2(CH3CN)] (16), 527.1167
[(C6H5)3Sb(cyc-PO2)] (8), 739.0409 [{(C6H5)3Sb}2(O)(OH)]+ (8).
Synthesis of [(Ph2Sb)2(PhSb)7(µ-O)11(µ3-O)3(µ-OH)2(µ-cycPO2)2-
(cycPO2)2(H2O)2] · 2CH3CN · H2O (2): Compound 1 (0.1 g, 0.09
mmol) was dissolved at 50 °C in 50 mL of a solution consisting of
acetonitrile and water (99:1). After filtration, the filtrate was kept
for slow evaporation at 45 °C. After a few weeks, parallelopiped-
shaped crystals of 2 were isolated. Yield: 0.02 g, 32%. Mp > 300
°C dec. Anal. Calcd for C102H133N2O27P4Sb9 (2): C, 40.31; H, 4.41;
N, 0.92. Found: C, 40.20; H, 4.32; N, 0.74. IR (KBr, cm-1): 3431
(ν O-H), 1073, 1061 (νasym PO2), and 1023, 993 (νsym PO2).
Some of the bond parameters of 2 are summarized in the caption
for Figure 2. The others are given in the Supporting Information.
The cage structure of 2 represents the largest structure containing
Sb(V) centers. Another novel feature of the structure of 2 is that
the two extremes of the cage are in fact capped by two putative
stibinic acid [Ph2Sb(O)(OH)] units. Thus, 2 is a unique cage
containing motifs related to stibonic and stibinic acids as well as
a partially hydrolyzed organoantimony center.
Acknowledgment. V.C. is thankful to DST for a J.C. Bose
fellowship. R.T. thanks CSIR for Senior Research Fellowships.
Supporting Information Available: CIF files for 1 and 2 and
Figures S1-S14. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM900113Q
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hankar, R.; Singh, S.; Steiner, A.; Zacchini, S. Organometallics 2002, 21,
4575–4577.
Experimental Section
Reagents and General Procedures. Solvents and other general
reagents used in this work were purified according to standard
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