[LAl(μ-S3)2AIL]: A homobimetallic derivative of the sulfur crown S8

Article abstract of DOI:10.1002/anie.200461209

Pieces of eight: The reaction of [LAI^I] with elemental sulfur yields the crown shaped complex [LAI(μS₃)₂AIL] (L= HC(CMe-NAr)₂, Ar = 2,6-iPr₂C₆H₃) containing an AI₂S₆ eight-membered ring, in which two (μ-S₃) chains are bridging two aluminum atoms (see structure). The relative stabilities of the possible [L₂AI₂S_n] (n = 2-8) species are estimated by theoretical calculation.

Full text of DOI:10.1002/anie.200461209

Communications
Metal Polysulfides
[LAl(m-S3)2AlL]: A Homobimetallic Derivative of
the Sulfur Crown S8**
Ying Peng, Hongjun Fan, Vojtech Jancik,
Herbert W. Roesky,* and Regine Herbst-Irmer
Scheme 1. Synthesis of 2.
Dedicated to Professor Tobin J. Marks
on the occasion of his 60th birthday
and characterized by its characteristic melting point and EI
mass spectrum. Pale yellow crystals of 2 were obtained from
the concentrated filtrate at 48C, as well as at room temper-
ature. It is noted that even when the above reaction was
employed in a molar ratio of 2:3,[7] the isolated product is also
2 however in lower yield (about 10%). Compound 2 was
characterized by 1H and 13C NMR spectroscopy, EI mass
spectrometry, and elemental analysis. The most intense peak
in the EI mass spectrum of 2 appears at m/z 508 [M+ꢀLAlS4].
The signals at 540 (38%) and 572 (15%) are assigned to the
[M+ꢀLAlS3] and [M+ꢀLAlS2] fragments, respectively. Com-
pound 2 is sparingly soluble in [D6]benzene, and the solubility
does not improve even when heated. 2 does not dissolve in
hexane and pentane. When the reaction mixture or the
isolated compound is exposed to traces of moisture, the free
ligand LH can be detected by 1H NMR spectroscopy.
Metal polysulfides, synthesized by a variety of methods using
various reagents as sulfur source, such as S8, M2Sn (M = alkali
metal), P4S10, H2S, and organic polysulfanes, have attracted
much attention not only regarding their structure and
reactivity but also owing to their potential uses.[1] Metal
polysulfide complexes[1b] may be viewed as derivatives of the
2ꢀ
Sx (x ꢁ 2) ion. Transition-metal polysulfides have attracted
interest as catalysts and intermediates in enzymatic processes
and in catalytic reactions of industrial importance, such as the
hydrodesulfurization (HDS) of fossil fuels.[2] Furthermore,
the metal polysulfides can be used as precursor for metal–
sulfur clusters. In contrast, such complexes containing heavier
main-group elements, such as the Group 13, 14, and 15 metals
have been much less explored.[1a,d] Among the numerous
investigations of the metal polysulfides, complexes with the
(m-S3) chain are rarely reported. The most common examples
are those of transition-metal complexes [{(h5-RC5H4)2Ti(m-
S3)}2] (R = H, Me)[3] and [{(h5-MeC5H4)Ru(PPh3)(m-S3)}2][4]
which were obtained by treatment of [(h5-RC5H4)2TiS5]
(R = H, Me) with PPh3, and [{(h5-MeC5H4)Ru(PPh3)2S}2]
[SbF6]2 with (NBu4)2S6, respectively. [LAlI] (1) (L =
HC(CMeNAr)2, Ar= 2,6-iPr2C6H3)[5] with its nonbonding
lone pair of electrons at aluminum has a singlet carbene-
like character and may show unprecedented chemical reac-
tion behavior.[6] Herein, we describe the synthesis and
structural characterization of [LAl(m-S3)2AlL] (2) containing
two (m-S3) chains.
Single crystals of 2[9] suitable for X-ray structural analysis
were obtained in toluene at 48C. Compound 2 crystallizes in
the monoclinic space group P21/n with two co-crystallized
molecules of toluene per molecule of 2 (Figure 1). Two (m-S3)
chains connect two aluminum atoms to form an aluminum
polysulfide with an Al2S6 eight-membered ring. The two
L ligands are almost coplanar. The symmetry of the structure
is Ci. In the S8 structure,[10] the two S3 units are eclipsed,
whereas in 2 they are staggered, thus we cannot simply argue
that the two aluminum atoms are replacing the corresponding
ꢀ
sulfur atoms in S8. The S S bond length (av 2.08 Š) in 2 is
Compound 2 was synthesized by the reaction of 1 with
sulfur in a molar ratio of 2:6[7] (Scheme 1). Cold toluene was
added to the mixture of 1 and sulfur at ꢀ788C. After several
minutes a suspension was obtained which was kept at ꢀ788C
for 2 h. Subsequently the suspension was slowly warmed to
room temperature under formation of more precipitate. The
compound [LAl(m-S)2AlL][8] was isolated from the precipitate
[*]Dipl.-Chem. Y. Peng, V. Jancik, Prof. Dr. H. W. Roesky,
Dr. R. Herbst-Irmer
Institut für Anorganische Chemie der Universität Göttingen
Tammannstrasse 4
37077 Göttingen (Germany)
Fax: (+49)551-39-3373
E-mail: hroesky@gwdg.de
Dr. H. Fan
Figure 1. Thermal ellipsoids plot of 2 (thermal ellipsoids set at 50%
probability). Hydrogen atoms and solvent molecules are omitted for
clarity. Selected bond lengths [Š]and angles [ 8]: Al(1)-N(1) 1.882(2),
Al(1)-N(2) 1.904(2), Al(1)-S(1) 2.223(1), Al(1)-S(3A) 2.248(1), S(1)-
S(2) 2.095(1), S(2)-S(3) 2.073(1); N(1)-Al(1)-N(2) 97.7(1), S(2)-S(1)-
Al(1) 98.0(1), S(1)-Al(1)-S(3A) 116.9(1), S(3)-S(2)-S(1) 104.7(1).
Labor für Physikalische und Theoretische Chemie
Universität Siegen, 57068 Siegen (Germany)
[**]L =HC(CMeNAr)2, Ar=2,6-iPr2C6H3. This work was supported by
the Deutsche Forschungsgemeinschaft, the Göttinger Akademie der
Wissenschaften and the Fonds der Chemischen Industrie.
6190
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200461209
Angew. Chem. Int. Ed. 2004, 43, 6190 –6192

Angewandte
Chemie
Table 1: The reaction energy of [L2Al2Sn] (n=2–8).[a]
n
2
3
4
5
6
7
8
reaction energy
[kJmolꢀ1
ꢀ648 (S1S1)
ꢀ641 (S1S2)
ꢀ644 (S1S3)
ꢀ571 (S2S2)
ꢀ602 (S1S4)
ꢀ581 (S2S3)
ꢀ621 (S1S5)
ꢀ608 (S3S3)
ꢀ594 (S1S6)
ꢀ570 (S3S4)
ꢀ609 (S1S7)
ꢀ524 (S4S4)
]
[a]The reaction energy is obtained from E([L2Al2Sn])ꢀ2E([LAl])ꢀn/8E(S8). The notes next to the energies show the conformations. For example, S3 S4
means one bridge has three S atoms, while the other one has four S atoms
slightly longer than that in S8 (av 2.05 Š), [{(h5-MeC5H4)2-
[10]
suspension was stirred at this temperature for 2 h and slowly
warmed to room temperature. The mixture was stirred for additional
15 h. After removal of the precipitate by filtration, the concentrated
solution was kept at room temperature for 3 days to afford pale
Ti(m-S3)}2][3] (av 2.06 Š), and [{(h5-MeC5H4)Ru(PPh3)(m-
S3)}2][4] (av 2.05 Š). The Al S bond lengths (av 2.24 Š) are
ꢀ
comparable with those in the dimer [LAl(m-S)2AlL] (av
yellow crystals of 2. (0.150 g, 22%). M.p. 1858C (decomp); EI-MS: m/
2.24 Š)[8] and in [LAl(SH)2] (av 2.22 Š).[11] The Al N bond
ꢀ
z (%) 508 (100) [M+ꢀLAlS4], 540 (38) [M+ꢀLAlS3], 572 (15)
1
[M+ꢀLAlS2]. H NMR (500.13 MHz, C6D6): d = 7.20–6.90 (m, 12H,
length (av 1.89 Š) falls within the range of those in aluminum
derivatives bearing the L ligand.[12] The S-S-S angle (104.78) is
Ar-H), 4.72 (s, 2H, g-H), 3.30 (sept, 8H, J = 6.8 Hz, CHMe2), 1.50 (s,
close to those found in [{(h5-MeC5H4)2Ti(m-S3)}2][3] (109.18)
12H, Me), 1.37 (d, 24H, J = 6.8 Hz, CHMe2), 1.00 ppm (d, 24H, J =
6.8 Hz, CHMe2); 13C NMR (125.77 MHz, C6D6): d = 171.78 (CN),
and [{(h5-MeC5H4)Ru(PPh3)(m-S3)}2][4] (105.28).
145.74, 143.69, 128.20, 123.83 (Ar), 97.74 (g-C), 28.92 (CHMe2),
The S8 ring can have different cleavage patterns and thus
25.45(CHMe2), 23.93 (CHMe2), 23.55 ppm (Me); 27Al NMR: reso-
nance is silent. Elemental analysis (%) calcd for C58H82Al2N4S6
(1081.66): C 64.40, H 7.64, N 5.18; found C 64.65, H 7.88, N 4.76%.
forms various types of structures.[1a] For [L2Al2Sn] species the
product with n = 2 is known,[8] and herein compound 2 has n =
6. Theoretical calculations were carried out to estimate the
relative stability of the compounds with n = 2–8. The method
used is RI-BP86/TZVP within the TURBOMOLE[13] pro-
gram. The optimized structure of [L2Al2S6] is in good agree-
ment with the X-ray values for 2 (mean deviation < 0.04 Š),
which shows the reliability of the selected theoretical method.
In the calculation the CHMe2 groups in the ligand L were
replaced by H atoms. The relative stability of [L2Al2Sn] was
estimated by the reaction energy of 2[LAl] + n/8S8!
[L2Al2Sn]. The calculated reaction energies are listed in
Table 1. All the reactions are exothermic. Compound
[L2Al2S2][8] is the most stable. However, there are quite a
few conformations with only a slightly smaller reaction
energy, and thus thermodynamically they are all possible.
Most [L2Al2Sn] species prefer only one bridging S atom, while
all the other S atoms are arranged in the second bridge
(Table 1). We could isolate 2 but not the (theoretically
favored) S1S5 isomer, this might be due to the very low
solubility of the latter species.
Therefore, the theoretical work shows that all [L2Al2Sn]
(n = 2–8) species have rather stable conformations (and that
there can be more than one).[14] These compounds may
coexist in the product, and under different reaction conditions
another species may be preferentially formed. To obtain a
single crystal out of such a mixture is difficult except for n = 2
where one can use excess of 1 to react with sulfur.
In summary, we report herein the first compound of
Group 13 with two (m-S3) chains connecting two aluminum
atoms under formation of an eight-membered Al2S6 ring.
Studies of the other possible conformations of [L2Al2Sn]
estimated by calculation are underway.
Received: July 6, 2004
Keywords: aluminum · bridging ligands · density functional
.
calculations · structure elucidation · sulfur
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Experimental Section
All manipulations were performed under a dry and oxygen-free
nitrogen atmosphere using Schlenk-line and glovebox techniques.
2: Toluene (30 mL) was added to a mixture of 1 (0.580 g,
1.3 mmol) and S8 (0.125 g, 3.9 mmol) at ꢀ788C. The resulting
Angew. Chem. Int. Ed. 2004, 43, 6190 –6192
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6191

Communications
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[7] The ratio corresponds to the stoichiometry of S.
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[9] Crystal data for 2·2toluene: C72H98Al2N4S6, Mr = 1265.86, mono-
clinic, space group P21/n, a = 14.277(1), b = 16.387(1), c =
15.786(1) Š, b = 109.66(1)8, V= 3478(1) Š3, Z = 2, 1calcd
=
1.209 Mgmꢀ3
,
F(000) = 1360, l = 1.54178 Š, T= 100(2) K,
m(CuKa) = 2.386 mmꢀ1. Data for the structure were collected on
a Bruker three-circle diffractometer equipped with a SMART
6000 CCD detector. Intensity measurements were performed on
a rapidly cooled crystal (dimensions 0.30 ” 0.20 ” 0.10 mm3) in
the range of 7.24 ꢂ 2q ꢂ 117.848. Of the 15352 measured
reflections, 4907 were independent (R(int) = 0.0363). The struc-
ture was solved by direct methods (SHELXS-97)[15] and refined
with all data by full-matrix least-squares methods on F2.[16] The
ꢀ
hydrogen atoms of C H bonds were placed in idealized
positions. The final refinements converged at R1 = 0.0328 for
I > 2s(I), wR2 = 0.0868 for all data. The final difference Fourier
synthesis gave a min/max residual electron density ꢀ0.263/
ꢀ3 [14]
+ 0.619 eŠ
.
CCDC-241840 (2) contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-336-033; or
deposit@ccdc.cam.ac.uk).
[10] A. C. Gallacher, A. A. Pinkerton, Acta. Cryst. 1993, C49, 125 –
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[14] The residual electron density 0.619 eŠꢀ3 can be explained by the
presence of a higher homologue in the crystal (ca. 3%). It can be
refined as either [L2Al2S7] or [L2Al2S8]. Owing to the inversion
center it is not possible to distinguish between these two
homologues. Although this disordered model shows good
geometry and leads to a lower R value, the ordered model for
2 was used for the discussion and theoretical calculations.
[15] SHELXS-97, Program for Structure Solution: G. M. Sheldrick,
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6192
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 6190 –6192