Synthesis and characterization of β-diketiminato complexes of antimony (III) halides

Article abstract of DOI:10.1016/j.jorganchem.2006.06.036

Treatment of SbX₃ (X = Br, Cl) with DippnacnacLi (Dippnacnac = [{N(C₆H₃ⁱPr₂2,6)C(Me)}₂CH]^-) or Mesnacnac (Mesnacnac = [{N(Mes)C(Me)}₂CH]^-, Mes = 2,4,6, trim

Full text of DOI:10.1016/j.jorganchem.2006.06.036

Journal of Organometallic Chemistry 691 (2006) 4250–4256
www.elsevier.com/locate/jorganchem
Synthesis and characterization of b-diketiminato complexes
of antimony (III) halides
*
Leslie A. Lesikar, Anne F. Richards
Department of Chemistry, Texas Christian University, Fort Worth, TX 76129, United States
Received 16 June 2006; received in revised form 28 June 2006; accepted 28 June 2006
Available online 6 July 2006
Abstract
i
Treatment of SbX3 (X = Br, Cl) with DippnacnacLi (Dippnacnac = [{N(C6H3 Pr22,6)C(Me)}2CH]ꢀ) or Mesnacnac (Mesnac-
nac = [{N(Mes)C(Me)}2CH]ꢀ, Mes = 2,4,6, trimethyl benzene) affords different products that are dependent on the stoichiometry of
the reaction and the halide precursor. When DippnacnacLi is reacted with SbBr3, C–H activation of the ligand backbone is observed
and an asymmetric, bridged bromide dimer is isolated. In comparison, the reaction of SbCl3 with MesnacnacLi affords monomeric
MesnacnacSbCl2. The solid-state structures were determined using X-ray crystallography.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: b-Diketiminato (nacnac); Antimony (III) complex; C–H activation; Crystal structure
1. Introduction
acterized.1 From the work of Burford and Lappert it would
appear that phosphorus preferentially coordinates at the
c-C rather than the more commonly observed nitrogen
chelation [5,6].1 To date, the preferred coordination of
the heavier group 15 elements is unknown. The syntheses
of compounds featuring group 15 elements were desirable
for a variety of reasons. Firstly, examples of low valent
antimony complexes are less common than their phospho-
rus counterparts [7], therefore structural comparisons to
related main group structures were of interest and secondly
for the potential applications of group 15 heterocyclic com-
pounds [8]. Moreover, we wished to explore the further
reactivity and chemistry of these compounds as precursors
for organometallic syntheses.
In recent years use of b-diketiminato ligands (nacnac),
(Dippnacnac = [{N(C6H3 Pr22,6)C(Me)}2CH]ꢀ) or (Mes-
i
nacnac = [{N(Mes)C(Me)}2CH]ꢀ, Mes = 2,4,6, trimethyl
benzene), have become increasingly common [1]. Com-
plexes of transition metals, main group elements and lan-
thanides are all well documented [2]. These ligands are
particularly useful as they can be prepared in high yields,
crystallize easily and offer various coordination modes,
thus have the ability to stabilize low oxidation state com-
pounds [3]. Furthermore, steric and electronic properties
of the ligands can be varied through manipulation of the
R groups [4]. Complexes of the group 15 elements have
been reported by Burford [5] and Lappert [6] (Fig. 1).1
To date, group 15 compounds of the type nanacEX2
(E = P, As, Sb, Bi, X = halide) remain structurally unchar-
2. Experimental
Solvents used were dried over sodium or potassium and
degassed before use. All manipulations were performed
under anaerobic conditions using standard Schlenk tech-
niques. DippnacnacH, MesnacnacH and their correspond-
ing lithium salts were prepared according to published
procedures [9]. All other reagents were purchased from
Aldrich and used as received.
*
Corresponding author. Tel.: +1 18172576220; fax: +1 18172575851.
E-mail address: a.richards@tcu.edu (A.F. Richards).
During the review process, a manuscript reporting the synthesis and
1
characterization of the first Phosphorus b-diketiminato was reported; D.
Vidovic, Z. Lu, G. Reeske, J.A. Moore, A.H. Cowley, Chem. Commun.
(2006) advance article, June 22, 2006.
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.06.036

L.A. Lesikar, A.F. Richards / Journal of Organometallic Chemistry 691 (2006) 4250–4256
4251
Fig. 1. b-Diketiminato compounds of the group 15 elements.
Crystal data were collected with a Bruker SMART dif-
fractometer in using graphite monochromated molybde-
num radiation (k = 0.7107 A). The data were corrected
low yield. M.p. 122–125 ꢁC, due to low yield no spectro-
scopic data were obtained.
˚
for absorption. Structures were solved by direct methods
[10] and refined [10] via full-matrix least squares.
2.3. Preparation of MesnacnacSbCl2 (3)
A 100 mL Schlenk flask was charged with 0.3361 g of
SbCl3 (1.47 mmol) and 20 mL of THF. MesnacnacLi
(0.5 g, 1.46 mmol) was dissolved in 30 mL of THF and
added dropwise to a stirred THF solution of SbCl3 at
ꢀ78 ꢁC. The clear yellow solution was stirred overnight.
Following removal of the THF under vacuum, 20 mL of
toluene was added and stirred. The clear dark yellow solu-
tion was filtered, concentrated under reduced pressure, and
placed in a ꢀ30 ꢁC freezer to obtain crystals (34.25%). M.p.
2.1. Preparation of 1
A THF solution of DippnacnacLi (0.5 g, 1.2 mmol) was
added rapidly by cannula to a stirred THF solution of
SbBr3 (0.29 g, 1.2 mmol) at ꢀ78 ꢁC. The resultant yellow
colored reaction mix was immediately removed from the
dry-ice bath and allowed to reach ambient temperature.
Stirring was maintained for a further 12 h, after which time
the THF was removed in vacuo and the yellow solid
extracted into toluene. Concentration of the toluene solu-
tion and storage at ꢀ5 ꢁC for 1 day afforded 1 as crystalline
yellow plates in 48% yield. M.p. 103–108 ꢁC melts, 1H
NMR (d8 PhMe, 25 ꢁC): d (ppm) 1.08, 1.11, 1.13, 1.14 (4
doublets, 1JH–H = 6.6 Hz, iPr–Me), 1.38 (s, 3H, CH3, back-
bone), 1.99 (s, CH2), 2.36, 3.02 (iPr C–H Dipp, multiplet,
1JH–H = 6.9 Hz), 4.59 (br, s), 4.86 (s, N–H), 6.75 (ortho,
1
149–151 ꢁC. H NMR (d8 PhMe, 25 ꢁC): d 1.45 (s, 6H,
Me), 1.85 (s, 18H, Mearyl), 3.87 (d, JH–H 5.2 Hz, CH),
6.45 (s, 4H, Haryl); 13C NMR (C6H5CH3, 25 ꢁC): d 17.1
(Co–Me), 22.3 (a-Me), 25.9 (Cp–Me), 39.2 (Cb), 39.5 (Ca),
126.3–127.9 (br, Co, Cm, Cp), 140.5 (C–N).
2.4. Preparation of [MesnacnacH2]þ2 [SbCl4ꢀ][Iꢀ] (4)
To a flask charged with 0.0737 g (0.375 mmol) of ‘GaI’
in 20 mL of toluene a solution of 0.1897 g (0.359 mmol)
of 3 in 20 mL of Toluene were added dropwise at room
temperature. The resulting dark green solution was stirred
overnight, filtered and the reaction mixture concentrated
under reduced pressure. Storage of the solution at room
temperature for 5 days afforded green crystals of 4.
1
H, aromatic, JH–H = 6.2 Hz), 6.92 (triplet, meta H Dipp,
1JH–H = 6.2 Hz); 13C NMR (C6H5CH3, 25 ꢁC): d (ppm)
24.7, 23.3 (C–iPr), 27.9 (Me-nacnac backbone) 121.7,
122.2, 123.9, 124.5, 124.7, 127.2 (aromatic 13C), 141.4
(C-backbone N–CCN), 160.1 (Sb–C).
1
2.2. Preparation of 2
Yield = 42.3%, m.p. 124–126 ꢁC. H NMR (C6D6, 25 ꢁC):
d 1.13 (s, 6H, Me, backbone), 2.06 (s, 18H, Mearyl), 3.72
A ꢀ78 ꢁC THF solution of DippnacnacH (0.5 g,
1.22 mmol) had 1 equiv. of n-BuLi (0.76 mL of a 1.6 M
solution) added dropwise. The solution was slowly warmed
to room temperature and stirred for 4 h, after which time it
was rapidly added to a stirred THF solution of SbCl3
(0.33 g, 1.44 mmol) at ꢀ78 ꢁC. An immediate color change
was observed and the resultant amber colored reaction
mixture was removed from the dry-ice bath and allowed
to warm to ambient temperature. Stirring was maintained
for a further 12 h, after which time the reaction mixture
was filtered from the LiCl precipitate. Repeated filtration
and concentration over a period of weeks afforded 2 in
(s, 2H, NH), 4.48 (br, s), 6.13 (s, 2H, Haryl), 6.50 (s, 2H,
H
Mes), 24.2 (Me, backbone), 93.8 (C–H, backbone), 127.4
(Co), 127.8 (Cp), 129.7 (Cm), 139.0 (CN); IR (m cmꢀ1, Nujol
mull), 3172.29 (m-N–H), 2853 (w), 1608, 1550, 1205 (m),
874.2 (m).
aryl); 13C NMR (C6D6, 25 ꢁC): d 17.8, 22.0 (2 · Me,
2.5. Preparation of
2[MesnacnacH2]þ ꢁ [AsCl4ꢀ][AsCl3][Clꢀ] (5)
To a stirred toluene suspension of ‘GaI’ (0.28 g, 14.3
mmol) was added a toluene solution of MesnacnacAsCl2

4252
L.A. Lesikar, A.F. Richards / Journal of Organometallic Chemistry 691 (2006) 4250–4256
(0.4 g, 7.0 mmol) at room temperature. No immediate
color change was observed. After stirring for 16 h, a metal-
lic precipitate was observed and a brown solution. The
solution was decanted and placed at ꢀ30 ꢁC for 2 days to
3. Results and discussion
The crystalline compounds 1–3, Scheme 1, were pre-
pared from the reaction of nacnacLi (Dipp, compounds 1
and 2 or Mes, compound 3) with SbBr3 or SbCl3 in a
1:1, 1:1.2 and 1:1 ratio respectively.
The structures (1–3, Scheme 1) were unequivocally
established using X-ray crystallography. Table 1 displays
the crystallographic data.
The crystallographic study revealed that the backbone
of the Dippnacnac ligand in compounds 1 and 2 had
undergone intramolecular C–H activation. This is not a
1
yield compound 5. Yield = 35.89%, m.p. 145–146 ꢁC. H
NMR (C6D6, 25 ꢁC): d 1.18 (s, 6H, Me, backbone), 2.10
(s, 18H, Mearyl), 3.75 (s, 2H, NH), 4.52 (br, s), 6.21 (s,
2H, Haryl), 6.55 (s, 2H, Haryl); 13C NMR (C6D6, 25 ꢁC): d
18.2, 22.5 (2 · Me, Mes), 24.6 (Me, backbone), 94.4(C–H,
backbone), 127.9 (Co), 128.2 (Cp), 130.1 (Cm), 139.0
(CN); IR (m cmꢀ1, Nujol mull), 3152.29 (m-N–H), 2853
(w), 1608 (m), 1560 (m) 1215 (m).
Scheme 1. Synthesis of compounds 1–3.
Table 1
Crystallographic data for compounds 1–5
Compound name
1
2
3
4
5
Chemical formula
Formula weight
Crystal system
Space group
T (K)
C
129H177Br4N8Sb4
C33H48Cl4N2OSb2
874.03
Triclinic
C30H37Cl2N2Sb
618.28
Triclinic
C46H62Cl4IN4Sb
1057.41
Tetragonal
P42/nmc
91(2)
11.5310(5)
11.5310(5)
22.9540(14)
90
90
90
3052.1(3)
2
C23H31As2Cl8N4
1105.44
Orthorhombic
Pca2(1)
2646.43
Triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
91(2)
91(2)
91(2)
91(2)
˚
a (A)
14.8326(9)
15.9125(10)
17.6414(11)
96.8930(10)
114.6680(10)
112.8090(10)
3284.6(4)
1
12.123(7)
12.137(7)
13.495(7)
91.783(9)
90.617(9)
97.652(9)
1966.8(19)
2
8.6608(14)
12.453(2)
16.148(3)
103.161(2)
103.096(3)
100.270(3)
1602.5(5)
2
17.8963(15)
10.6789(9)
28.319(2)
90
90
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
5412.1(8)
8
Z
Reflections collected
Independent reflections
Data/restraints/parameter ratio
28094
11804
11804/6/630
0.0800
1.338
10146
6886
6886/0/378
0.1123
1.476
13615
5759
5759/0/324
0.1012
1.281
24739
1527
1527/0/83
0.0464
1.151
44415
9644
9644/1/534
0.0951
1.355
Unique data (Rint
Dcalc (Mg mꢀ3
)
)
Absorption coefficient (mmꢀ1
F(000)
)
2.077
1351
1.671
876
1.046
632
1.160
1068
1.664
2272
Crystal size (mm)
h Range for collection (ꢁ)
R indices (all data)
0.29 · 0.26 · 0.23
1.56–25.25
R1 = 0.0882,
wR2 = 0.1110
R1 = 0.0504,
wR2 = 0.0994
1.201 and ꢀ0.718
0.13 · 0.11 · 0.09
1.51–25.25
R1 = 0.1094,
wR2 = 0.2674
R1 = 0.0812,
wR2 = 0.2403
1.765 and ꢀ1.417
0.14 · 0.14 · 0.13
1.35–28.24
R1 = 0.03653,
wR2 = 0.0999
R1 = 0.0346,
wR2 = 0.0981
1.051 and ꢀ1.334
0.07 · 0.035 · 0.012
1.77–24.40
R1 = 0.0983,
wR2 = 0.2852
R1 = 0.0910,
wR2 = 0.2759
1.387 and ꢀ4.324
0.15 · 0.13 · 0.12
1.44–25.25
R1 = 0.1325,
wR2 = 0.2244
R1 = 0.0732,
wR2 = 0.1860
2.063 and ꢀ1.583
Final R indices [I > 2r(I)]
Largest difference in
peak and hole (e A
ꢀ3
˚
)

L.A. Lesikar, A.F. Richards / Journal of Organometallic Chemistry 691 (2006) 4250–4256
4253
Fig. 2. ORTEP of 1 at 50% probability, hydrogen atoms are removed for clarity.
new phenomenon for these ligands, however, is usually
associated with more reactive species, for example the early
transition elements and the b-diketiminato compounds of
the alkaline earth metals [11]. Intramolecular C–H activa-
tion is thought to be attributed to the presence of the reac-
tive Sb–X (X = halide) [12] and the conditions which the
reactions were performed. In order to observe C–H activa-
tion the addition of DippnacnacLi to the antimony halide
must proceed rapidly and the reaction mixture brought to
room temperature quickly. Compound 1 was isolated as
a yellow crystalline solid and is a bridged Sb(III) com-
pound. The structure and selected bond lengths are
depicted in Fig. 2 (see Table 2).
Sb–Br bonds [13,14] and this asymmetry appears to be a
common characteristic in the 32 reported Sb–Br bridged
structures [14]. Each Sb center can be viewed as a distorted
trigonal bipyramidal structure with a vacant coordination
site presumably occupied by the Sb lone pair. A related
compound 2 can be isolated if a slight excess of SbCl3 is
present in the reaction. Compound 2 was isolated serendip-
itously in low yield from the reaction when DippnacnacLi
was prepared in situ from DippnacnacH, and the reaction
mixture is kept in THF (see Fig. 3).
Compound 2 is somewhat unusual in that C–H bond
activation and a subsequent deprotonation has occurred.
LiCl is displaced and Sb(1) sits in the C–N pocket. The
geometry around this antinomy (Sb1) is T-shaped and
Cl(1)–Sb(1)–N(1) lie almost at an 180ꢁ angle. The excess
SbCl3 in the reaction mixture is coordinated as an SbCl2
fragment to the adjacent carbon atom in the nacnac back-
bone. The +1 charge on Sb(1) is balanced by a chloride ion
that sits in close proximity to a Dipp (2,6-diisopropyl ben-
The bond lengths in 1 compare well to those of the opti-
mized C–H activated Dippnacnac2ꢀ ion as predicted by
ab initio calculations [11a]. Each antimony atom is
attached to a nitrogen atom of the ligand, a terminal and
bridged bromide (Scheme 1, Fig. 2). The Sb–Br bond
˚
˚
lengths vary from 2.446 A, Sb(1)–Br(1), to 2.98 A, Sb(2)–
Br(2), and highlight the lack of symmetry in the bridged
Sb–Br conformation. These values are not unusual for
˚
zene) group at a distance of 2.967 A from the SbCl2 frag-
ment. While the exact mechanism for formation is
unclear, it appears that from isolation of compound 1, this
is the primary product formed. It is envisaged that the reac-
tion proceeds in a similar manner as observed in the forma-
tion of (C, Fig. 1) [5], nucleophilic displacement of the
chloride on the excess SbCl3 present in the reaction and
inter- or intramolecular tautomerism that gives access to
the carbon for coordination.
In an attempt to isolate nacnacSbCl2 the reaction of
MesnacnacLi (R = Mes = 2,4,6 trimethylbenzene) with
SbCl3 was carried out. Compound 3 was isolated in a mod-
erate yield. Examination of the solid-state structure
revealed it as monomeric MesnacnacSbCl2, as shown in
Fig. 4.
Table 2
˚
Selected bond lengths (A) and bond angles (ꢁ) of 1
Sb(1)–Br(3)
Sb(1)–Br(4)
Sb(2)–C(46)
Sb(2)–N(3)
Sb(2)–Br(2)
Sb(2)–Br(4)
2.7911(16)
2.9119(16)
2.141(6)
2.188(4)
2.5315(17)
2.7234(16)
C(17)–Sb(1)–N(1)
C(17)–Sb(1)–Br(1)
N(1)–Sb(1)–Br(1)
Br(1)–Sb(1)–Br(3)
81.96(19)
90.05(15)
92.54(12)
89.52(5)

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L.A. Lesikar, A.F. Richards / Journal of Organometallic Chemistry 691 (2006) 4250–4256
˚
Fig. 3. Solid-state structure of 2, thermal ellipsoids at 30% probability, selected bond lengths (A) and bond angles (ꢁ) are depicted – hydrogen atoms are
removed for clarity. A molecule of THF is omitted from the diagram.
Fig. 4. X-ray structure of MesnacnacSbCl2. Thermal ellipsoids are shown at 50% probability.
The crystallographic analysis revealed ‘normal’ Sb–Cl
˚
(2.5662(7) A) and Sb–N (Sb(1)–N(1) 2.088(2)) bond
lengths and bond angles [7]. The coordination geometry
of the Sb is as predicted by VSEPR for an AX5E structure.
Within the ‘see-saw’ structure the Cl(1)–Sb(1)–Cl(2)
arrangement is almost linear with an angle of 178.72ꢁ.
An angle of 88.38ꢁ for N(1)–Sb(1)–N(2) is also close to
the 90ꢁ right angle expected. These small deviations from
the predicted values are associated with the sterics of the
bulky ligand and the possibility of lone pair delocalization,
which is also responsible for the asymmetry of the mole-
cule. The fact that the bond lengths from the chelating
nitrogen atoms to their respective carbons and antimony
within the Mesnacnac are of similar lengths reinforces the
probability of delocalization within the ligand.
Attempts to reduce 1 and 3 using potassium, sodium,
KC8 or Mg with the aim of forming low valent Sb species
were generally unsuccessful. Treatment of MesnacnacSbCl2
with 2 equiv. of the softer reducing agent, gallium (I) iodide
afforded compound 4. From the X-ray analysis of 4 it can
be seen that the Mesnacnac ligand opens up forming a
Fig. 5. Structure and selected bond lengths and angles for
2½MesnacnacH2ꢂþ ꢁ SbCl4ꢀIꢀ, compound 4, the second MesnacnacHþ2 and
hydrogen atoms are omitted for clarity.
highly symmetrical structure that is protonated at both
nitrogen atoms. The cationic structure is balanced by the
presence of an SbClꢀ4 anion. Within the asymmetric unit
there are two Mesnacnac cations with an iodide ion from
the ‘GaI’ as the anion for the second cation (see Fig. 5
and Table 3).
Numerous attempts to crystallize DippnacnacAsCl2 and
the Bi analogue using a variety of solvents and R substitu-
ents on the ligand all failed. MesnacnacAsCl2 was con-
firmed spectroscopically and in order to try to form a

L.A. Lesikar, A.F. Richards / Journal of Organometallic Chemistry 691 (2006) 4250–4256
4255
Table 3
Acknowledgements
˚
Selected bond lengths (A) and bond angles (ꢁ) of 4
Sb(1)–Cl(1)
N(1)–C(2)
N(1)–C(4)
C(1)–C(2)
2.165(2)
1.314(10)
1.437(9)
1.504(9)
A.F.R. thanks Professor Cameron Jones, Cardiff Uni-
versity, for helpful discussions.
We are grateful for the financial Support provided by
TCU, TCU-RCAF and the Robert A Welch Foundation.
Cl(1)#1–Sb(1)–Cl(1)
C(2)–N(1)–C(4)
N(1)–C(2)–C(3)
107.53(8)
122.9(6)
119.1(7)
Appendix A. Supplementary information
Crystallographic data for the structural analysis has
been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 611223 for compound 1, CCDC No.
611229 for compound 2, CCDC No. 611225 for compound
3, CCDC No. 611226 for compound 4 and CCDC No.
611224 for compound 5. Copies of this information may
be obtained free of charge from: The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ UK (fax: (int code)
+44 1223 336 033 or email: deposit@ccdc.cam.ac.uk or
www:http://www.ccdc.cam.ac.uk). Supplementary data
associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.jorganchem.2006.06.036.
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˚
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derivative for structural characterization the reaction with
‘GaI’ was performed. The reaction outcome shows similar-
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atomꢀis displaced from the ligand and forms the anion
AsCl4 , which is the counter ion for the MesnacnacHþ2 cat-
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4. Conclusions
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Compounds 1–5 are novel b-diketiminato antimony
complexes and show that through manipulation of the
halide precursor, reaction stoichiometry and the R substi-
tuent on the nacnac different reaction outcomes can be
achieved. Reactions using DippnacnacLi with SbBr3 and
SbCl3 proceed rapidly with C–H activation observed.
Using SbCl3 and MesnacnacLi, monomeric Mesnacnac
antimony (III) chloride is isolated. Further work will focus
on exploring the chemistry of 1–3 as synthons for organo-
metallic syntheses and the preparation of the heavier cong-
eners of the group 15 elements.
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