Syntheses and structures of Ar3Sb5 and Ar 4Sb4 compounds (Ar = C6H3-2,6- (CH2NMe2)2)

Article abstract of DOI:10.1021/om800227m

The Sb₅Ar₃ (Ar = N,C,N chelating ligand, C ₆H₃-2,6-(CH₂NMe₂)₂) trigonal bipyramidal cluster was obtained as one of two products (along with a tetrameric Sb₄Ar₄ compound) by reduction of the parent ArSbCl₂ compound.

Full text of DOI:10.1021/om800227m

Organometallics 2008, 27, 2169–2171
2169
Syntheses and Structures of Ar3Sb5 and Ar4Sb4 Compounds
(Ar ) C6H3-2,6-(CH2NMe2)2)
Libor Dosta´l,* Roman Jambor, Alesˇ Ru˚zˇicˇka, and Jaroslav Holecˇek
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, UniVersity of
ˇ
Pardubice, na´m. Cs. Legi´ı 565, Pardubice 53210, Czech Republic
ReceiVed March 11, 2008
Scheme 1. Preparation of Sb5Ar3 (1) and Sb4Ar4 (2)
Summary: The Sb5Ar3 (Ar ) N,C,N chelating ligand, C6H3-2,6-
(CH2NMe2)2) trigonal bipyramidal cluster was obtained as one
of two products (along with a tetrameric Sb4Ar4 compound) by
reduction of the parent ArSbCl2 compound.
Introduction
Considerable progress has recently been achieved in the field
of low-valent organoantimony compounds in formal oxidation
states lower than +III, and obtained results have been reviewed.1
Although successful isolation of “naked” monomeric stib-
inidenes is still lacking, dimeric distibenes, trans-Ar′SbdSbAr′
(Ar′ ) terphenyl-type ligands), were obtained by Okazaki,
Tokitoh, and Power using effective steric protection.2 There are
many examples of monocyclic stibines RnSbn (n ) 3-6), where
the ring size strongly depends on the group R, and many of
these compounds display ring-ring equilibrium in solution.3
Polycyclic stibines RnSbm (n < m) constitute a very interesting
class of low-valent organoantimony compounds. The only
examples of these species, R4Sb8, R6Sb8, and R5Sb7 (R )
(Me3Si)2CH), were prepared as side products by the reduction
of corresponding chloride RSbCl2 with Mg or by treating of
RSbH2 with SbCl3 in the presence of pyridine. The molecular
structures of the first two compounds were determined by X-ray
diffraction.4
Dehydrocoupling reactions have recently been recognized as
a convenient path to element-element bond formation.5 Using
this synthetic strategy, we report here on the synthesis of the
Sb5 cluster Sb5Ar3 (1) (Ar ) C6H3-2,6-(CH2NMe2)2). Compound
1 features a trigonal bipyramidal framework and, to our
knowledge, represents the first organoantimony cluster of the
general formula R3Sb5. This coordination arrangement of five
antimony atoms is unknown even among inorganic antimony
3+
polycations and polyanions,6 and the isolation of the stable Sb5
3+
cation (as an analogue of the known Bi5 cation) remained
unsuccessful, although its presence was proposed.7
8
The reaction of ArSbCl2 with 2 equiv of K[B(i-Bu)3H] in
THF (Scheme 1) gives, depending on the reaction time, a
mixture of two products, 1 and tetrameric Sb4Ar4 (2) (10 min),
or compound 2 as a sole product (24 h).9 Compound 1 can be
isolated from the reaction mixture as red X-ray quality crystals
by crystallization from hexane solution at room temperature in
a low yield of 20% (1 displayed only limited solubility in
hydrocarbons after crystalization, so compound 2 could be easily
washed out if necessary). It is likely that the reduction of
ArSbCl2 proceeds via intermediate formation of the correspond-
ing organoantimony hydride ArSbH2, which, in turn, reductively
eliminates hydrogen gas and ArH and gives compound 1.
(6) (a) Lindsjo¨, M.; Fischer, A.; Kloo, L. Angew. Chem., Int. Ed. 2004,
43, 2540. (b) Kuznetsov, A. N.; Popovnik, B. A.; Stahl, K.; Lindsjo¨, M.;
Kloo, L. Eur. J. Inorg. Chem. 2005, 4907. (c) Adolphson, D. G.; Corbett,
J. D.; Merryman, D. J. J. Am. Chem. Soc. 1976, 98, 7234. (d) Bolle, U.;
Tremel, W. J. Chem. Soc. Chem. Commun. 1992, 91.
(7) Ulvenlund, S.; Stahl, K.; Kloo, L. Inorg. Chem. 1996, 35, 223.
(8) Atwood, D. A.; Cowley, A. H.; Ruiz, J. Inorg. Chim. Acta 1992,
198-200, 271.
(9) Preparation of Sb5Ar3 (1): A solution of K[B(i-Bu)3H] (2.35 mL,
2.35 mmol, 1 M solution in THF) was added to a precooled (0 °C) solution
of ArSbCl28 (0.450 g, 1.17 mmol) in 20 mL of THF, immediately warmed
to rt, and stirred for an additional 10 min. The color of the solution turned
to red-orange, and evolution of hydrogen was obvious. Then the solvent
was evaporated in Vacuo and the remaining mixture was extracted with 30
mL of hexane. The suspension was filtered, and the volume of the filtrate
was reduced to ca. 10 mL. After 5 h, compound 1 was isolated as red crystals
after filtration and washing with hexane (0.056 g, 20%), mp 167 °C (dec).
1H NMR (C7D8; 500.13 MHz): δ (ppm) 2.14 (s, 12H, N(CH3)2), 3.96 (s,
4H, CH2N), 6.95 (d, 2H, Ar-H3,5), 7.02 (t, 1H, Ar-H4). 13C NMR (C7D8;
125.77 MHz): δ (ppm) 44.0 (N(CH3)2), 66.7 (CH2N), 126.6 (Ar-C4), 127.7
(Ar-C3,5), 146.8 (Ar-C1), 148.5 (Ar-C2,6). UV-vis (hexane, nm): 338
(shoulder) and 435 (shoulder). Preparation of Sb4Ar4 (2): Procedure similar
as for 1: THF solution of K[B(i-Bu)3H] (3.24 mL, 3.24 mmol, 1 M solution);
ArSbCl28 (0.620 g, 1.61 mmol) in 40 mL of THF, 24 h. Compound 1 was
isolated as yellow-orange crystals from a saturated hexane solution at 0 °C
(0.228 g, 45%), mp 156 °C (dec). Anal. Calcd for C48H76N8Sb4: C, 46.0;
H 6.1. Found: C, 46.3; H 6.4. 1H NMR (C7D8; 500.13 MHz): δ (ppm) 2.00
(s, 12H, N(CH3)2), 3.86 (s, 4H, CH2N), 7.11 (t, 1H, Ar-H4), 7.28 (d, 2H,
Ar-H3,5). 13C NMR (C7D8; 125.77 MHz): δ (ppm) 45.2 (N(CH3)2), 69.9
(CH2N), 126.2 (Ar-C4), 128.3 (Ar-C3,5), 140.6 (Ar-C1), 147.7 (Ar-C2,6).
UV-vis (hexane, nm): 368 (shoulder).
* Corresponding author. E-mail: libor.dostal@upce.cz. Fax: +420
466037068. Tel.: +420 466037163.
(1) (a) Bala´zs, L.; Breunig, H. J. Coord. Chem. ReV. 2004, 284, 603.
(b) Tokitoh, N. Pure Appl. Chem. 1999, 71, 495. (c) Breunig, H. J.; Ro¨sler,
R. Chem. Soc. ReV. 2000, 29, 403. (d) Sasamori, T.; Tokitoh, N. Dalton
Trans. 2008, 1395.
(2) (a) Tokitoh, N.; Arai, Y.; Sasamori, T.; Okazaki, R.; Nagase, S.;
Uekusa, H.; Ohashi, Y. J. Am. Chem. Soc. 1998, 120, 433. (b) Twamley,
B.; Solfield, C. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1999,
121, 3357.
(3) For examples see: (a) Bala´zs, G.; Bala´zs, L.; Breunig, H. J.; Lork,
E. Organometallics 2003, 22, 2919. (b) Garc´ıa, F.; Hopkins, A. D.;
Kowenicki, R. A.; McPartlin, M.; Tesa, Y. Dalton Trans. 2004, 2051. (c)
Breunig, H. J.; Bala´zs, L. Organometallics 2004, 23, 304.
(4) (a) Bala´zs, G.; Breunig, H. J.; Lork, E.; Mason, S. Organometallics
2003, 22, 576. (b) Breunig, H. J.; Ro¨sler, R.; Lork, E. Angew. Chem., Int.
Ed. Engl. 1997, 36, 2237.
(5) (a) Waterman, R.; Tilley, T. D. Angew. Chem., Int. Ed. 2006, 45,
2926. (b) Rivard, E.; Steiner, J.; Fettinger, J. C.; Guiliani, J. R.; Augustine,
M. P.; Power, P. P. Chem. Commun. 2007, 4919. (c) Jambor, R.; Kasˇna´,
B.; Kirschner, K. N.; Schu¨rmann, M.; Jurkschat, K. Angew. Chem., Int.
Ed. 2008, 47, 650.
10.1021/om800227m CCC: $40.75
2008 American Chemical Society
Publication on Web 04/18/2008

2170 Organometallics, Vol. 27, No. 10, 2008
Communications
Similar ligand and hydrogen elimination from an organotin(II)
hydride was detected previously.5b
1
Indeed, H and 13C NMR spectra of the mother liquor after
isolation of 1 indicate the presence of a significant amount of
free ArH ligand together with the second product, 2. Compound
2 was preferably isolated after prolonged reaction time as the
only product in the form of orange crystals in 45% yield.9 These
findings point to a limited stability of 1 in the reaction mixture,
and only an immediate isolation leads to compound 1 in
reasonable yield. Both compounds 1 and 2 were characterized
by a combination of 1H and 13C NMR, elemental analysis (for
2), UV-vis spectroscopy, and X-ray crystallography.10
The molecular structure of 1 was unambiguously determined
by X-ray diffraction (Figure 1) and shows a trigonal-bipyramidal
framework of five antimony atoms. Each of the antimony atoms
Sb(1) to Sb(3) in the equatorial plane carry one N,C,N chelating
ligand, and the other two antimony atoms, Sb(4) and Sb(5),
forming axial positions, remain “naked”, or ligand free. The
distances within the equatorial plane (Sb(1) to Sb(3)) lie between
3.7658(7) and 3.8558(7) Å. The Sb-Sb distances describing
connection between the vertices (Sb(4), Sb(5) atoms) and the
antimony atoms (Sb(1-3)) in the equatorial plane fall within
the narrow interval 2.8459(6)-2.8699(7) Å. The distance
between both axial atoms Sb(4) · · · Sb(5) is 3.6664(8) Å, still
shorter than ∑vdW(Sb-Sb) ) 4.52 Å. The internal ring angles
in the equatorial plane are in the range 118.9(1)-121.9(2)°, close
to the ideal value of 120°. The virtual angle Sb(4)-centroid of
TBP-Sb(5) is 179.0(2)°, and although it deviates from linearity,
the overall geometry of the Sb5 core is very close to D3h
symmetry.
Figure 1. ORTEP plot of a molecule of 1 showing 30% probability
displacement ellipsoids and the atom-numbering scheme and view
of the central Sb5 framework. Hydrogen atoms were omitted for
clarity. Selected bond lenghts (Å) and angles (deg): Sb(1)-Sb(2)
3.7658(7), Sb(1)-Sb(3) 3.7811(7), Sb(2)-Sb(3) 3.558(7), Sb(4)-
Sb(1) 2.8584(7), Sb(4)-Sb(2) 2.8459(6), Sb(4)-Sb(3) 2.8699(7),
Sb(5)-Sb(1) 2.8512(6), Sb(5)-Sb(2) 2.8667(7), Sb(5)-Sb(3)
2.8655(6), Sb(4) · · · Sb(5) 3.6664(8), Sb(1)-C(1) 2.202(7), Sb(2)-
C(13) 2.195(6), Sb(3)-C(25) 2.181(6), Sb(1)-N(1) 3.143(6),
Sb(1)-N(2) 2.826(7), Sb(2)-N(3) 2.859(7), Sb(2)-N(4) 3.146(6),
Sb(3)-N(5) 2.967(6), Sb(3)-N(6) 2.834(6); the internal angle in
the equatorial Sb3 plane, Sb(1)-centroid-Sb(2) 118.9(1), Sb(1)-
centroid-Sb(3) 119.2(1), Sb(2)-centroid-Sb(3) 121.9(2), angle
between axial positions Sb(4)-centroid-Sb(5) 179.0(2), average
angle of N-Sb-N angles 115.4(2).
The Sb-C(ligand) distances are 2.181(6)-2.202(7) Å, re-
sembling expected values for covalent bonds. Regarding the
six Sb-N intramolecular coordinations, they are in the region
2.826(7)-3.143(6) Å, approaching the limit of the weakest
intramolecular interactions, which can be taken into account
(∑vdW(Sb-N) ) 3.74 Å). Thus, one can only tentatively assume
that stabilization of the Sb5 framework can be rationalized only
by intramolecular dative Sb-N bonds. Most probably, the
stabilization is based on synergic steric and electronic influences
of the pincer ligands.
Wade’s rules have proven to be a convenient tool for
3+
estimation of the geometries of main group clusters.11 The Sb5
cation obeys these rules and should possess a trigonal-bipyra-
midal geometry. If we take N,C,N chelating ligands as monoan-
ionic substituents [C6H3-2,6-(CH2NMe2)2]-, the central trigonal-
bipyramidal Sb5 core of 1 obeys Wade’s rules.
In general, compound 1 represents only the second example
(and the first neutral and organometallic) of a trigonal-
bipyramidal cluster within the group 15 elements and, to our
knowledge, a very rare example of a neutral trigonal-bipyramidal
organometallic cluster in the field of all main group elements12
(a few inorganic charged Zintl-type compounds with this shape
have recently been isolated7,13).
The molecular structure of 2 is depicted in Figure 2.
Compound 2 is formed as a cyclic tetramer and closely
resembles the structure of the tetrameric N,C-chelated analogue
Sb4Ar4* (Ar* ) C6H4-2-CH2NMe2).14 The presence of more
(10) Suitable single crystals of 1 and 2 were mounted on a glass fiber
with oil and measured on a four-circle KappaCCD diffractometer with CCD
area detector by monochromatized Mo KR radiation (λ ) 0.71073 Å). The
numerical15 absorption correction from crystal shape was applied for crystals
of 2 and multi-scan16 for 1. The structures were solved by the direct method
(SIR9217) and refined by a full matrix least-squares procedure based on F2
(SHELXL9718). Hydrogen atoms were fixed into idealized positions (riding
model) and assigned temperature factors Hiso(H) ) 1.2Ueq(pivot atom); for
the methyl moiety a multiple of 1.5 was chosen. The final difference maps
displayed no peaks of chemical significance, as the highest peaks and holes
are in close vicinity (∼1 Å) of heavy atoms. 1: C36H57N6Sb5, M ) 1182.63,
monoclinic, P21/c, red block, a ) 10.3120(8) Å, b ) 34.361(4) Å, c )
13.8470(9) Å, ꢀ ) 117.132(6)°, V ) 4366.5 Å3, Z ) 4, T ) 150(1) K,
22 529 total reflections, 9180 independent (Rint ) 0.069, R1 (obs data) )
0.055, wR2 (all data) 0.108. CCDC 679245. 2: C48H76N8Sb4.2C6H6, M )
1406.54, orthorhombic, Pnma, yellow needle, a ) 22.9280(5) Å, b )
22.6020(17) Å, c ) 12.2360(8) Å, V ) 6340.9 Å3, Z ) 4, T ) 150(1) K,
50 512 total reflections, 7355 independent (Rint ) 0.085, R1 (obs data) )
0.043, wR2 (all data) 0.110. CCDC 679246. There is disordered solvent
(benzene) in this structure. Attempts were made to model this disorder or
split it into two positions, but were unsuccessful. PLATON/SQUEZZE19
was used to correct the data for the presence of disordered solvent. A
potential solvent volume of 702 Å3 was found. 337 electrons per unit cell
worth of scattering were located in the void. The stoichiometry of the solvent
was calculated to be eight molecules of benzene per unit cell, which results
in 336 electrons per unit cell. The crystals of 2 with no solvent were also
obtained, and their structure was determined in hexagonal point group P3221,
orange block, a ) 13.2623(18) Å, b ) 13.2623(9) Å, c ) 52.515(6) Å, V
) 7999.2 Å3, Z ) 6, but the low quality of the data collected resulted in
a large disorder of two of the ligands present.
(11) (a) Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 1. (b)
Kuznetsov, A. N.; Kloo, L.; Lindsjo¨, M.; Rosdahl, J.; Stoll, H. Chem.-
Eur. J. 2001, 7, 2821.
(12) A similar propellane-like organotin Sn5 cluster was prepared and
characterized by X-ray diffraction: Sita, L. R.; Bickerstaff, R. D. J. Am.
Chem. Soc. 1989, 111, 6454.
(13) (a) For example see: Cordier, G.; Ludwig, M.; Stahl, D.; Schmidt,
P. C.; Kniep, R. Angew. Chem., Int. Ed. 1995, 34, 1761. (b) Goicoechea,
J. M.; Sevov, S. C. J. Am. Chem. Soc. 2004, 126, 6860.
(14) Opris, L. M.; Silvestru, A.; Silvestru, C.; Breunig, H. J.; Lork, E.
Dalton Trans. 2004, 3575.
(15) Coppens, P. In Crystallographic Computing; Ahmed F. R., Hall,
S. R., Huber, C. P., Eds.; Munksgaard: Copenhagen, 1970; pp 255-270.
(16) Blessing, R. H. J. Appl. Crystallogr. 1997, 30, 421.
(17) Altomare, A.; Cascarone, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 1045.

Communications
Organometallics, Vol. 27, No. 10, 2008 2171
situation is more complex in 2. Both nitrogen atoms of the
ligands bonded to the antimony atoms Sb(2) and Sb(3) can be
regarded as coordinated, although very weakly, the range of
Sb-N distances being 3.016(4)-3.342(5) Å. On the other hand,
in the case of ligands on the Sb(1) and Sb(1a) atoms only one
of the donor atoms (N(1) and N(1a)) remains in contact with
the antimony atom (Sb-N ) 3.304(5) Å), and the second donor
group (N(2) and N(2a)) is unambiguously pendant (Sb · · · N
4.950(5) Å).
Both compounds 1 and 2 retain their structures in solution.
1
The H and 13C NMR spectra9 contain one set of signals for
both NCH2 and N(CH3)2 at various concentrations, ruling out
the presence of any ring-ring equilibria in solution, similarly
to a tetrameric Sb4Ar4* N,C-chelated compound.14 The 1H NMR
signals corresponding to NCH2 groups are in the case of both
compounds observed as singlets at 300 K, indicating that both
ligand’s donor groups undergo a fluxional association/dissocia-
tion process, as is common for pincer ligands and complexes
and indicating only weak Sb-N intramolecular interactions in
solution of 1 and 2 (similar results were observed in the solid
state).
In conclusion, a rare example of a trigonal bipyramidal main
group cluster (1) was obtained by dehydrocoupling reaction,
and its molecular structure was determined by X-ray diffraction.
An investigation concerning the theoretical background of the
bonding structure in 1 and the reactivity of 1 and 2 is currently
underway.
Figure 2. ORTEP plot of a molecule of 2 showing 30% probability
displacement ellipsoids and the atom numbering scheme and view
of the central Sb4 framework. Hydrogen atoms were omitted for
clarity. Selected bond lenghts (Å) and angles (deg): Sb(1)-Sb(2)
2.8650(5), Sb(1)-Sb(3) 2.8808(5), Sb(1)-C(1) 2.181(4), Sb(2)-
C(13) 2.195(6), Sb(3)-C(20) 2.168(7), Sb(1)-N(1) 3.304(5),
Sb(1) · · · N(2) 4.950(5), Sb(2)-N(3) 3.342(5), Sb(3)-N(4) 3.016(4);
Sb(1)-Sb(2)-Sb(1a) 72.99(1), Sb(2)-Sb(1)-Sb(3) 86.13(1),
Sb(1)-Sb(3)-Sb(1a) 72.53(1), N(3)-Sb(2)-N(3a) 93.38(11),
N(4)-Sb(3)-N(4a) 109.09(11).
ˇ
Acknowledgment. The authors thank the GACR (project
203/07/P094) and the Ministry of Education of the Czech
Republic (projects MSM0021627501 and LC523) for finan-
cial support.
sterically demanding N,C,N ligands leads only to marginal
elongation of the Sb-Sb bond within the central ring, average
2.873(2) Å compared to 2.854(3) Å. Whereas in Sb4Ar4* each
of the four nitrogen donor atoms is coordinated to one antimony
atom (range of Sb-N contacts 2.915(3)-3.040(3) Å), this
1
Supporting Information Available: H, 13C NMR spectra of
1, all crystal data and structure refinement, atomic coordinates,
anisotropic displacement parameters, and geometric data for
compounds 1and 2 (cif files) are available free of charge via the
Internet at http://pubs.acs.org.
(18) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; University of Go¨ttingen: Germany, 1997.
(19) Spek, A. L. Acta Crystalllogr., Sect. A 1990, 46, C34.
OM800227M