Reactions of 2-arsa- and 2-stiba-1,3-dionato lithium complexes with group 4-7 metal halides

Article abstract of DOI:10.1016/j.jorganchem.2004.02.020

The reactions of a range of 2-arsa- and 2-stiba-1,3-dionato lithium complexes with group 4-7 metals have been investigated. These have given rise to several complexes in which an arsadionate acts as a chelating ligand; [V{η²-O,O-OC(Bu^t) AsC(Bu^t)O}₃], [M{η²-O,O-OC (Bu^t)AsC(Bu^t)O}₂(DME)], M=Cr or Mn; or as an η¹-As-diacylarsenide, [MnBr(CO)₄ {As[C(O)Bu^t]₂Li(DME)}]₂. In addition, reactions of lithium arsadionates with TaCl₅ have led to metal mediated arsadionate decomposition reactions and arsadionate oxidative coupling reactions to give the known arsaalkyne tetramer, As₄C₄Bu^t₄, and the new tetraacyldiarsane, [{As[C(O)Mes]₂}₂] Mes=mesityl, respectively. The treatment of several lithium arsadionates with [MoBr₂(CO)₂(PPh₃)₂] has also initiated arsadionate decomposition reactions and the formation of the metal carboxylate complexes, [MoBr(CO)₂{η² -O₂C(R)}(PPh₃)₂] R=Bu^t, Ph, Mes. The X-ray crystal structures of six of the prepared complexes are discussed.

Full text of DOI:10.1016/j.jorganchem.2004.02.020

Journal of Organometallic Chemistry 689 (2004) 1648–1656
www.elsevier.com/locate/jorganchem
Reactions of 2-arsa- and 2-stiba-1,3-dionato lithium complexes
with group 4–7 metal halides
*
Cameron Jones , Thomas C. Williams
Department of Chemistry, Cardiff University, P.O. Box 912, Park Place, Cardiff CF10 3TB, UK
Received 1 February 2004; accepted 17 February 2004
Abstract
The reactions of a range of 2-arsa- and 2-stiba-1,3-dionato lithium complexes with group 4–7 metals have been investigated.
These have given rise to several complexes in which an arsadionate acts as a chelating ligand; [V{g2-O,O-OC(But)AsC(But)O}3],
[M{g2-O,O-OC(But)AsC(But)O}2(DME)], M ¼ Cr or Mn; or as an g1-As-diacylarsenide, [MnBr(CO)4{As[C(O)But]2Li(DME)}]2.
In addition, reactions of lithium arsadionates with TaCl5 have led to metal mediated arsadionate decomposition reactions and
arsadionate oxidative coupling reactions to give the known arsaalkyne tetramer, As4C4But4, and the new tetraacyldiarsane,
[{As[C(O)Mes]2}2] Mes ¼ mesityl, respectively. The treatment of several lithium arsadionates with [MoBr2(CO)2(PPh3)2] has also
initiated arsadionate decomposition reactions and the formation of the metal carboxylate complexes, [MoBr(CO)2{g2-
O2C(R)}(PPh3)2] R ¼ But, Ph, Mes. The X-ray crystal structures of six of the prepared complexes are discussed.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Arsenic; Antimony; Low coordination; Pnictadionates; Crystal structure
1. Introduction
[E{C(O)R}2]ꢀ, 1 E ¼ As or Sb, R ¼ alkyl or aryl [3].
These are valence isoelectronic analogues of b-diket-
onates, the complexes of which are ubiquitous in
inorganic chemistry [4] and have found numerous
applications in areas ranging from catalysis to
chemical vapour deposition (CVD) processes [5].
Considering the importance of b-diketonates as li-
gands, we have begun to explore the coordination
chemistry of 1 with s-, p- and d-block metals. This is
proving a fertile and versatile area of study. For
example, whereas b-diketonates generally bind to
main group and transition metals in variations of the
chelating g2-O,O-mode, the presence of 2-coordinate
pnicogen centres in 1 has led to them exhibiting a
variety of ligating modes which include g2-O,O-; g1-
E; g1-E:g1-O:g2-CO; g1-E:g2-O,O- and l-g1:g1-E (as
in, for example, 2–5) [6]. In addition, the weakness of
the E–C bonds in transition metal complexes of 1
can lead to their cleavage and the formation of novel
and previously inaccessible complex types, for exam-
ple the only known functionalised distibene which is
stabilised in the complex, cis-[Pt(PEt3)2{g2-Sb,Sb-
(But)(O)CSb@SbC(O)(But)}] [7].
The low coordination chemistry of phosphorus has
become well established over the last 20 years. Studies in
this area have revealed a close analogy between phos-
phorus and the valence isoelectronic CR fragment. In
this respect phosphorus has been described as a ‘‘car-
bon-copy’’ [1]. In contrast, relatively little attention has
been paid to the chemistry of low coordination arsenic
and antimony compounds, probably due to the low
thermal stability of these species, their toxicity and the
absence of As or Sb isotopes with nuclear spin I ¼ 1=2
[2]. Despite these problems, we have developed routes to
several low coordination arsenic and antimony systems
in recent years and have shown that their chemistry can
be similar, but can also be significantly different to that
of their phosphorus analogues.
One class of compound that we have been par-
ticularly interested in are the 2-pnicta-1,3-dionates,
* Corresponding author. Tel.: +44(0)29-20-874-060; fax: +44(0)-29-
20-874-030.
E-mail address: jonesca6@cardiff.ac.uk (C. Jones).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.02.020

C. Jones, T.C. Williams / Journal of Organometallic Chemistry 689 (2004) 1648–1656
1649
But
O
Despite the coordinative versatility of 1 towards
As
But
But
As
Li
But
transition metals, to date we have only reported on
complexes of 1 with group 8–12 metals [6d], whilst
complexes with group 4–7 metals remain unknown. In
this paper we describe the preparation and characteri-
sation of a series of such complexes, in addition to a
number of unexpected results.
But
As
VCl3(THF)3
Me
O
O
O
O
O
O
O
V
But
O
But
O
2
Me
6
As
But
8
TaCl5
But
But
But
O
O
O
O
But
As
As
Cl
Pt
TaCl5-n
But
TaL5-n
But
But
Me
O
As
+
C
O
O
Et3P
PEt3
R
O
n
As
Fe
But
n
Cl
11
Ru
E
O
O
R
But
10
Ph3P
O
O
Me
PPh3
O
O
As
But
4
But
As
2
3
E = As or Sb
As
But
R = Ph or Mes
9
Me
Me
As
As
But
O
Li
O
Li
But
Cl
O
O
Cl
Me
Me
But
But Zn
O
O
Scheme 1.
O
O
As
As
5
equivalent of the highly oxygen sensitive, 6. It is worthy
of mention that all attempts to prepare the antimony
analogue of 8 by reaction of 7 with [VCl3(THF)3] led to
decomposition processes involving the deposition of
antimony metal.
But
But
All attempts to obtain the X-ray crystal structure of 8
met with failure. It is likely, however, that it is an octa-
hedral tris-chelate complex similar to related b-diketo-
nate complexes, e.g., [V(acac)3], acac ¼ acetylacetonoate
[8]. Evidence for this proposal comes from its infrared
spectrum which exhibits C–O stretching absorptions at
1502 and 1490 cmꢀ1, in the region consistent with delo-
calised, chelating arsadionate ligands [6e]. In contrast, it is
known that when 1, E ¼ As, acts as a g1-As-ligand it
contains largely localised C–O double bonds which dis-
play stretching absorptions at significantly higher fre-
quencies, ca. 1610–1680 cmꢀ1 [6d,9]. This ligating mode is
common in late transition metal complexes, e.g., 3, but is
unlikely for the oxophilic early transition metals. Com-
pound 8 is paramagnetic with a leff of 3.0 lB (Evans
method [10]), as would be expected for a V3þ complex. As
a result, no meaningful NMR data could be obtained on
the compound.
2. Results and discussion
2.1. Group 4
Numerous examples of group 4 b-diketonate com-
plexes have been reported in the literature [4]. We have
made many attempts to prepare pnictadionato ana-
logues of these via the reactions of [{[Li{g2-O,O-
OC(But)EC(But)O}(DME)0:5]2}1], E ¼ As 6, Sb 7, with
MCl4 and Cp2MCl2, M ¼ Ti or Zr, using various stoi-
chiometries of reagents. In all cases, however, rapid
decomposition of the reaction mixtures at room tem-
perature led to the deposition of either elemental arsenic
or antimony and intractable mixtures of products. As a
result, no further efforts were made in this area.
2.2. Group 5
An attempt to form a tantalum–arsadionate complex
led to an unexpected result. The reaction of three
equivalents of 6 with TaCl5 in DME yielded the known
tetraarsacubane, 9 [11], in a 14% yield (Scheme 1). The
same product was obtained in lower yields when the
reaction was carried out in 1:1, 2:1 or 4:1 stoichiome-
tries. All attempts to elucidate the mechanism of this
remarkable reaction failed but presumably it involves
the initial formation of a chelated tantalum–arsadionate
complex, 10. This could then decompose via As–C
and C–O bond cleavage reactions to give a transient
arsaalkyne, AsBCBut, and tantalum carboxylate
The reaction of three equivalents of
6 with
[VCl3(THF)3] afforded a moderate yield of the first
group 5 arsadionato complex, 8, as large black crystals
(Scheme 1). Interestingly, an attempt to prepare the re-
lated oxo-vanadium complex, [V( ¼ O){g2-O,O-
OC(But)AsC(But)O}2], via the 2:1 reaction of 6 with
VOCl2 in DME led to 8 in a low yield. When this re-
action was repeated in a 4:1 stoichiometry, moderate
yields (ca. 40%) of 8 resulted. The mechanism of this
reaction is unclear but presumably involves the deoxy-
genation of the vanadium starting material by one

1650
C. Jones, T.C. Williams / Journal of Organometallic Chemistry 689 (2004) 1648–1656
complexes, e.g., 11, or perhaps oxo-tantalum by-
products. The generated arsaalkyne could then tetram-
erise to give 9. Although this mechanism is speculative,
especially as no tantalum carboxylate by-products were
isolated from the reaction mixture, the products of re-
actions of 6 with group 6 halide complexes give it cre-
dence (vide infra). It is also interesting that the only
previously reported preparation of the tetraarsacubane,
9, involved the arsaalkyne, AsBCBut, as a transient
intermediate in its formation [11]. In the present study 9
was characterised by its X-ray crystal structure which is
isomorphous to that described in the prior report and
therefore will not be commented on here. In addition, its
13C NMR spectrum displays a characteristic high field
shift (d )46.5 ppm) for its framework carbon centres.
Arsaalkynes are very rare and indeed there is only
one example that is stable at room temperature,
AsBCMes*, Mes* ¼ C6H2But3-2,4,6 [12]. Unfortunately,
the kinetic protection afforded by the bulky supermesityl
substituent in this compound has limited the study of its
further chemistry [13]. With this in mind, the formation
of 9 raised the interesting possibility of preparing a va-
riety of less hindered arsaalkynes (or arsaalkyne oligo-
mers) using a similar synthetic procedure. To this end
TaCl5 or NbCl5 were reacted with [Li{OC(R)As-
C(R)O}(DME)], R ¼ Ph 12 or mesityl (Mes) 13, but in
all cases the tetraacyl diarsanes, [{As[C(O)R]2}2] R ¼ Ph
14, Mes 15, were formed in moderate yields, presumably
via oxidative coupling of the arsadionate ligands by the
M(V) precursors. It is not known why such couplings
occur in these reactions when tetraarsacubane formation
is seen in the reaction involving 6, though perhaps the
As–C bonds in the aryl substituted arsadionates are
stronger and therefore less susceptible to cleavage.
Compound 14 has previously been reported by us [6d]
but 15 is new. Its spectroscopic data are consistent with
O1
C3
C1
As1
As1’
O2
C2
C12
ꢀ
Fig. 1. Molecular structure of 15. Selected bond lengths (A) and angles
(°): As(1)–C(2) 2.022(4), As(1)–C(1) 2.051(4), As(1)–As(10) 2.4300(8),
O(1)–C(1) 1.203(4), O(2)–C(2) 1.211(5), C(2)–As(1)–C(1) 89.62(15),
C(2)–As(1)–As(10) 98.13(11), C(1)–As(1)–As(10) 94.86(10), O(1)–C(1)–
As(1) 120.3(3), O(2)–C(2)–As(1) 117.5(3). Symmetry operationÕ
y; x; ꢀz þ 1=2.
its proposed formulation which was confirmed by a
single crystal X-ray structure determination (Fig. 1,
Table 1). This shows the molecule to be dinuclear with
two pyramidal arsenic centres generated by a crystallo-
graphic centre of inversion. The structure is very similar
to that of 14 and the only other crystallographically
authenticated tetraacyldiarsane, [{As[C(O)But]2}2] [14].
Both its As–C and As–As bond lengths are typical for
single bonds and all the C–O distances are in the lo-
calised double bond range [15].
There have been no reported syntheses of stibaalky-
nes (SbBCR) or stibaalkyne oligomers. Given the
preparation of 9, it was thought that the reaction of
the lithium stibadionate, 7, with TaCl5 might lead to
Table 1
Summary of crystallographic data for complexes 15, 16, 17 ꢁ Et2O, 18 ꢁ Et2O, 21 and 22
15 16 18 ꢁ Et2O
17 ꢁ Et2O
Chemical Formula C40H44As2O4 C24H46As2CrO6 C47H49BrMoO5P2 C49H45BrMoO5P2
21
22
C24H46As2MnO6
635.39
150(2)
C36H56As2Br2Li2Mn2O16
Formula weight
T (K)
738.59
150(2)
632.45
150(2)
Monoclinic
P2ð1Þ=n
10.986(2)
15.334(5)
18.633(6)
90
931.65
150(2)
Monoclinic
P2ð1Þ=n
15.366(3)
10.592(2)
26.250(5)
90
951.64
150(2)
Monoclinic
P2ð1Þ=n
13.164(3)
17.657(4)
19.377(4)
90
1178.22
150(2)
Monoclinic
P2ð1Þ=n
10.548(2)
17.295(4)
13.578(3)
90
Crystal system
Space group
Hexagonal
ꢁ
Monoclinic
C2=c
24.615(4)
17.055(4)
21.094(6)
90
P3c1
ꢀ
a (A)
17.984(3)
17.984(3)
22.606(5)
90
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
90
120
92.93(2)
90
94.61(3)
90
103.71(3)
90
128.20(2)
90
92.90(3)
90
3
ꢀ
V (A )
6331.8(18)
6
1.616
3134.8(15)
4
2.491
4258.5(15)
4
1.367
4375.6(15)
4
1.332
6959(3)
8
2.295
2473.8(9)
2
3.513
Z
l(Mo Ka) (mmꢀ1
)
Unique reflections 3734
R1ðI > 2rðIÞÞ
5651
0.0572
0.1729
7761
0.0583
0.1334
7620
0.1072
0.3110
6252
0.0470
0.1350
5673
0.0372
0.0909
0.0572
0.1080
wR02(all data)

C. Jones, T.C. Williams / Journal of Organometallic Chemistry 689 (2004) 1648–1656
1651
examples of these compounds. Unfortunately, however,
the reaction only resulted in the deposition of elemental
antimony and an oily intractable mixture of products.
Analysis of this mixture by 13C NMR did not reveal any
high field (<)20 ppm) resonances that would be ex-
pected if a tetrastibacubane (cf. 9) was present.
The X-ray crystal structure of 16 was obtained
(Fig. 2, Table 1) and shows the complex to be mono-
meric with a distorted octahedral geometry, as is the
case for the isomorphous Fe(II) complex, 2 [6d]. An-
other similarity with 2 is the fact that the arsadionate
ligands are essentially planar and appear to be largely
ꢀ
delocalised as all As–C bond lengths are similar (1.91 A
avg.) and lie between values normally seen for As–C
2.3. Group 6
ꢀ
ꢀ
single (1.96 A) [16] and double bonds, e.g., 1.821(3) A in
Chromium tris(b-diketonate) complexes are ubiqui-
tous in the literature [4]. In order to prepare an arsadi-
onate analogue of this complex type, 6 was reacted in a
3:1 stoichiometry with CrCl3 in DME. Surprisingly, this
resulted in the moderate yield formation of the deep red
Cr(II) complex, 16 (Scheme 2). The chromium reduction
in this reaction is accompanied by the oxidative coupling
of the arsadionate to form the known diarsane,
[{As[C(O)But]2}2], as a yellow crystalline solid. Solu-
tions of 16 were shown to be paramagnetic with a leff of
5.0 lB (Evans method [10]), corresponding to the metal
centre having a high spin d4 electronic configuration.
The paramagnetic nature of the complex restricted the
acquisition of meaningful NMR data but its infra-red
spectrum displays a strong, broad absorption centred at
1501 cmꢀ1. This is consistent with the arsadionate li-
gands acting in a chelating mode, cf. the situation in 8.
[CpFe(CO)2As@CBut(OSiMe3)] [17]. The Cr–O bonds
ꢀ
associated with the arsadionate ligand (2.01 A avg.) are
significantly shorter than those arising from the DME
ligand but similar to those in Cr(II)-b-diketonate com-
ꢀ
plexes, e.g., 1.98 A avg. in [Cr(acac)2] [18].
The reaction of the stibadionate, 7, with CrCl3 ꢁ DME
did not lead to the antimony analogue of 16 but instead
to an intractable mixture of products. Decomposition
products also resulted from the reactions of 6 with
[MoBr3(THF)3], [MoI2(NCMe)3], WCl6 and [CpMoCl4]
under a range of stoichiometries. In contrast, The 1:1 or
1:2 reactions of [MoBr2(CO)2(PPh3)2] with a series of
lithium arsadionates, 6, 12 and 13, led to very unusual
outcomes, namely the moderate yield formations of the
molybdenum carboxylate complexes, 17–19 (Scheme 2).
In the 1:2 reactions the products, 17–19, are apparently
too hindered to allow reaction with the second equiva-
lent of 6 which was recovered from the reaction mixture.
The intermediates in these reactions are presumably the
g2-O,O-arsadionate complexes, 20, which are unstable
R
As
R
R
O
O
O
O
Ph3P
OC
Mo
PPh3
CO
"-
As CR"
Ph3P
OC
Mo
PPh3
CO
Br
Br
R = But 17
Ph 18
20
As2
Mes 19
C2
MoBr2(CO)2(PPh3)2
C12
As1
But
O
O2
O4
As
But
But
Me
C11
As
R
R
CrCl3
O
O
C1
O
O
M
O
O3
Li
(DME)n
or MnCl2
O
O
Cr1
O1
Me
6 R = But, n = 0.5
12 R = Ph, n = 1
13 R = Mes, n = 1
As
But
O5
O6
M = Cr 16
Mn 21
Mn(CO)5Br
ꢀ
Fig. 2. Molecular structure of 16. Selected bond lengths (A) and angles
(°): As(1)–C(1) 1.903(5), As(1)–C(2) 1.916(5), As(2)–C(11) 1.907(6),
As(2)–C(12) 1.921(7), Cr(1)–O(3) 1.995(4), Cr(1)–O(4) 2.009(5), Cr(1)–
O(2) 2.017(4), Cr(1)–O(1) 2.024(4), Cr(1)–O(5) 2.150(5), Cr(1)–O(6)
2.175(4), O(1)–C(1) 1.266(6), O(2)–C(2) 1.263(6), O(3)–C(11) 1.256(7),
O(4)–C(12) 1.249(8), C(1)–As(1)–C(2) 101.8(2), C(11)–As(2)–C(12)
101.0(3), O(1)–Cr(1)–O(2) 90.8(2), O(3)–Cr(1)–O(4) 92.4(2), O(5)–
Cr(1)–O(6) 76.8(2), As(1)–C(1)–O(1) 127.5(4), O(2)–C(2)–As(1)
126.6(4), O(3)–C(11)–As(2) 128.3(4), O(4)–C(12)–As(2) 127.9(5).
But
CO
O
CO
Mn
Br
Li
O
O
2
As
OC
OC
Li
O
O
But
22
Scheme 2.

1652
C. Jones, T.C. Williams / Journal of Organometallic Chemistry 689 (2004) 1648–1656
with respect to As–C and C–O bond cleavage and sub-
sequent rearrangement to the observed carboxylate
complexes. The by-products in these processes could be
the arsaalkynes, AsBCR, R ¼ But, Ph or Mes, or their
oligomers, though these could not be isolated from the
reaction mixtures or detected by 13C NMR, TLC or GC/
MS analyses. The parallels with the reaction between 6
and TaCl5 are clear, and the formations of 17–19 add
weight to the proposed formation of tantalum carbox-
ylate by-products in that case.
C2
C1
O2
O1
All the spectroscopic data for 17–19 support their
proposed structures and those for 18 are in close agree-
ment with a previous report on this complex [19]. The
crystal structures of 17 and 18 were obtained and are
shown in Figs. 3 and 4 respectively (see Table 1). Both
complexes are isostructural to each other and the closely
related complex, [MoBr(CO)2{g2-O2C(CF3)}(PPh3)2]
[20]. As in this complex, the molybdenum centres in 17
and 18 can be described as 7-coordinate, having 4:3 ge-
ometries with the bromine and carboxylate oxygens oc-
cupying the triangular face, whilst the phosphine and
carbonyl ligands form the quadrilateral face. The bond
lengths and angles in both complexes are unexceptional
and compare well to those in [MoBr(CO)2{g2-
O2C(CF3)}(PPh3)2].
Br1
Mo1
P1
C9
O4
P2
C8
O3
ꢀ
Fig. 4. Molecular structure of 18. Selected bond lengths (A) and angles
(°): Mo(1)–O(2) 2.232(6), Mo(1)–O(1) 2.233(6), Mo(1)–P(2) 2.519(2),
Mo(1)–P(1) 2.482(2), Mo(1)–Br(1) 2.500(2), C(1)–O(1) 1.262(11),
O(2)–C(1) 1.281(10), O(1)–C(1)–O(2) 119.1(7), O(2)–Mo(1)–O(1)
58.8(2), P(1)–Mo(1)–P(2) 131.23(7), C(8)–Mo(1)–C(9) 104.6(3).
2.4. Group 7
In the final stage of this study the reactivity of
lithium arsa- and stibadionates towards manganese
halide complexes was investigated. The reaction of
two equivalents of 6 with MnCl2 in DME afforded a
moderate yield of the manganese-bis(arsadionate)
complex, 21, as an orange crystalline solid (Scheme 2).
A magnetic susceptibility measurement (Evans method
[10]) on this compound revealed a leff of 5.3 lB which
equates to a high spin d5 complex, as would be ex-
pected. As in 8 and 16 its infrared spectrum exhibits
C–O stretching absorptions in the normal range for
chelating, delocalised arsadionate ligands. This was
confirmed by a crystal structure determination (Fig. 5,
Table 1) which shows the complex to be isostructural
but not isomorphous with its chromium analogue, 16,
and thus to contain a distorted octahedrally coordi-
nated metal centre. The average As–C and C–O bond
C2
C1
O1
O2
Br1
Mo1
P1
C6
O3
P2
C7
O4
ꢀ
lengths (1.91 and 1.25 A respectively) are close to
those in 16 and are suggestive of delocalised arsadio-
nate ligands. Finally, the average Mn–O (arsadionate)
ꢀ
distance (2.09 A) compares well with the Mn–O sep-
arations in related manganese b-diketonate complexes,
t
ꢀ
e.g., 2.03 A avg. in [Mn{[OC(Bu )]2CH}3] [21].
In previous studies [6d] we have shown that 2-pnicta-
1,3-dionato ligands display a preference for g1-E-coor-
dination to the softer late transition metals, as in 3,
whereas in this study g2-O,O-chelation to the harder
early transition metals has proved to be the norm. As
manganese is a mid transition metal, we predicted that it
ꢀ
Fig. 3. Molecular structure of 17. Selected bond lengths (A) and angles
(°): Mo(1)–O(2) 2.231(4), Mo(1)–O(1) 2.246(4), Mo(1)–P(2)
2.4591(16), Mo(1)–P(1) 2.5297(15), Mo(1)–Br(1) 2.6069(9), C(1)–O(1)
1.262(6), O(2)–C(1) 1.279(6), O(1)–C(1)–O(2) 117.9(5), O(2)–Mo(1)–
O(1) 58.17(13), P(1)–Mo(1)–P(2) 130.37(5), C(7)–Mo(1)–C(6) 105.4(2).

C. Jones, T.C. Williams / Journal of Organometallic Chemistry 689 (2004) 1648–1656
1653
O8
C18
O6
C16
O5
C15
Br1
As2
C12
O4
Mn1
As1
C17
O7
C1
O2
O3
Li1
C2
O4
C2
Li1’
O4’
As1
O2
O6
O3’
O1
C1
C1
Mn1
O1
O3
O5
ꢀ
Fig. 6. Molecular structure of 22. Selected bond lengths (A) and angles
(°): As(1)–C(2) 2.017(3), As(1)–C(1) 2.026(3), As(1)–Mn(1) 2.5596(7),
Mn(1)–Br(1) 2.5163(7), O(3)–Li(1) 2.197(6), O(3)–Li(10) 2.142(6),
O(1)–C(1) 1.225(4), O(1)–Li(1) 1.963(6), O(2)–C(2) 1.215(4), O(2)–
Li(1) 1.988(6), O(4)–Li(1) 1.976(6), C(1)–As(1)–C(2) 92.77(12), C(2)–
As(1)–Mn(1) 102.00(9), C(1)–As(1)–Mn(1) 105.92(8). Symmetry op-
erationÕ ꢀx; ꢀy; ꢀz.
ꢀ
Fig. 5. Molecular structure of 21. Selected bond lengths (A) and angles
(°): As(1)–C(1) 1.917(5), As(1)–C(2) 1.906(4), As(2)–C(11) 1.914(4),
As(2)–C(12) 1.925(5), Mn(1)–O(3) 2.100(3), Mn(1)–O(4) 2.088(3),
Mn(1)–O(2) 2.091(3), Mn(1)–O(1) 2.109(3), Mn(1)–O(5) 2.232(4),
Mn(1)–O(6) 2.279(4), O(1)–C(1) 1.261(5), O(2)–C(2) 1.255(5), O(3)–
C(12) 1.248(6), O(4)–C(11) 1.238(5), C(1)–As(1)–C(2) 101.33(19),
C(11)–As(2)–C(12) 100.0(2), O(1)–Mn(1)–O(2) 85.32(12), O(3)–
Mn(1)–O(4) 85.38(12), O(5)–Mn(1)–O(6) 71.99(16), As(1)–C(1)–O(1)
126.9(3), O(2)–C(2)–As(1) 126.6(3), O(3)–C(12)–As(2) 127.3(3), O(4)–
C(11)–As(2) 127.8(3).
CO stretching absorption at 1644 cmꢀ1 which is in the
region normally observed for localised diacylarsenido li-
gands. Moreover, there are four strong overlapping bands
between 1926 and 2057 cmꢀ1, as might be expected for a
cis-substituted octahedral metal carbonyl complex.
The structure of 22 was confirmed by an X-ray crystal
structure analysis (Fig. 6, Table 1) which shows the
molecule to be dimeric through bridging DME mole-
cules, giving rise to a planar Li2O2 ring. The coordina-
tion environment about the manganese centre is
distorted octahedral with the Br and As centres in cis-
positions. Interestingly, the As–Mn separations
may show intermediate properties. It seemed possible
that although g2-O,O-chelation was observed for the
Mn(II) complex, 21, g1-As-ligation might predominate
in Mn(I) systems. To test this hypothesis 6 was reacted
with [Mn(CO)5Br] in a 1:1 stoichiometry. This reaction
led to an unexpected outcome, namely the moderate
yield formation of the dimeric complex, 22, after re-
crystallisation from hexane (Scheme 2). In this reaction
CO displacement from the manganese precursor has
occurred instead of the intended salt elimination reac-
tion which would give [Mn(CO)5{g1-As[C(O)But]2}],
23. As compound 22 could be considered as an inter-
mediate in the expected salt elimination reaction, it was
slowly heated as a DME solution to 80 °C under an
atmosphere of CO. This did not, however, lead to 23 but
instead to decomposition and the formation of an in-
tractable mixture of products. Likewise, the reaction of
7 with [Mn(CO)5Br] led to the deposition of elemental
antimony and no other reaction products could be
identified.
Complex 22 can be regarded as an anionic diacylar-
senido-manganese(I) complex which is coordinated to its
lithium counter-ion. It is of note that we have previ-
ously observed a range of related complexes which are
putative salt elimination reaction intermediates, e.g.,
[(COD)Rh{As[C(O)Ar]2[LiCl(OEt2)]}] Ar ¼ C6H2Pri3-
2,4,6; [Zn{As[C(O)But]2[LiCl(DME)]}2] and [LiCl(Et2O)
(DME){g2-O,O-Cl3InAs[C(O)R]2}] [6]. The spectro-
scopic data for 22 support its proposed formulation.
Specifically, its infra-red spectrum exhibits a strong acyl
ꢀ
[2.5596(7) A] are easily the longest yet reported for 3-
coordinate arsenic centres (the mean As–Mn distance of
ꢀ
structurally characterised compounds is 2.413 A) [15].
This anomaly likely arises due to the steric bulk of the
ꢀ
diacylarsenido ligands. The Mn–Br bonds [2.5163(7) A]
are in the normal region for such interactions. In con-
trast to near planar, delocalised arsadionate ligands in
complexes such as 16 and 21, the diacylarsenide ligands
in 22 are not planar and contain pyramidal arsenic
centres (R angles ¼ 300.7°), each presumably having a
stereochemically active lone pair. In addition, both the
ꢀ
ꢀ
As–C (2.03 A avg.) and C–O distances (1.22 A avg.) are
indicative of localised single and double bonds respec-
tively. The oxygen centres of these ligand chelate the 5-
coordinate lithium counter-ions of the complex.
3. Conclusion
In summary, we have described the reactions of a
range of 2-arsa- and 2-stiba-1,3-dionato lithium com-
plexes with group 4–7 metal halide complexes. Although
little success was had with the formation of stibadionato

1654
C. Jones, T.C. Williams / Journal of Organometallic Chemistry 689 (2004) 1648–1656
complexes, as series of g2-O,O-arsadionato metal com-
plexes have been prepared and structurally character-
ised. In addition, in one complex, 22, the pnictadionate
acts as an g1-As:g2-O,O-bridging diacylarsenido ligand.
A remarkable tantalum mediated decomposition of an
arsadionate ligand has also been reported to give rise to
the known arsaalkyne tetramer, As4C4But4. The mech-
anism of this reaction has been suggested to involve an
intermediate tantalum–arsadionate complex which de-
composes to give As4C4But4 and an unidentified tanta-
lum carboxylate by-product. The formation of
molybdenum carboxylate complexes in related reactions
adds some weight to this proposal. The results of the
work described herein have been compared with those of
our earlier studies on pnictadionato late transition and
main group metal complexes and highlight the coordi-
native versatility of this ligand class.
trated to ca. 10 ml. This was placed at )30 °C overnight
yielding 8 as black crystals (yield: 0.16 g, 41%), m.p.
>200 °C; leff ¼ 3:0 lB; IR (Nujol, m/cmꢀ1) AsCO 1502
s, 1490 s; MS APCI: m/z 787 (MHþ, 100%); calc. for
C30H54O6As3V: C 45.76, H 6.93. Found: C 45.70, H
6.86%.
4.2. As4C4But4 9
A solution of 6 (0.15 g, 0.51 mmol) in DME (40 ml)
was added dropwise to a solution of TaCl5 (0.06 g, 0.17
mmol) in DME (30 ml) at )78 °C. The resulting yellow
solution was allowed to warm to room temperature and
stirred for 24 h. Volatiles were removed in vacuo leaving
a dark yellow residue which was extracted with hexane
(3 ꢂ 30 ml), filtered and the filtrate concentrated to ca.
15 ml. This was placed at )30 °C overnight yielding 9 as
yellow crystals (yield: 0.01 g, 14%). Spectroscopic data
and melting point as reported [11].
4. Experimental
4.3. [{As[C(O)Mes]2}2] 15
All manipulations were carried out using standard
Schlenk and glove box techniques under an atmosphere
of high purity argon. DME and hexane were distilled
over potassium whilst diethyl ether was distilled over
Na/K alloy. 1H and 13C NMR spectra were recorded on
a Bruker DXP400 spectrometer operating at 400.13 and
100.6 MHz respectively and were referenced to the re-
sidual 1H or 13C resonances of the solvent used. 31P{1H}
NMR spectra were recorded on a Jeol Eclipse 300
spectrometer and were referenced to external 85%
H3PO4, 0.0 ppm. Mass spectra were recorded using a
VG Fisons Platform II instrument operating under
APCI conditions. IR spectra were recorded using a
Nicolet 510 FT-IR spectrometer as Nujol mulls between
NaCl plates. Magnetic susceptibilities were determined
using the Evans method, corrected for super-conducting
NMR instruments [10]. Melting points were determined
in sealed glass capillaries under argon, and are uncor-
rected. Microanalyses were carried out by the Warwick
Analytical Service. The 2-arsa- and 2-stiba-1,3-diona-
tolithium starting materials, 6, 7, 12 and 13, were pre-
pared by variations of literature methods [3,6].
[MoBr2(CO)2(PPh3)2] was prepared by a literature
procedure [22]. All other reagents were obtained com-
mercially and used as received.
A solution of 13 (0.854 g, 2.03 mmol) in DME (50 ml)
was added dropwise to a solution of TaCl5 (0.240 g, 0.68
mmol) in DME (40 ml) at )78 °C. The resulting dark
red solution was allowed to warm to room temperature
and stirred for 24 h. Volatiles were removed in vacuo
leaving a dark brown residue which was washed with
hexane (10 ml), extracted with DME (20 ml), filtered
and the filtrate concentrated to ca. 10 ml. This was
placed at )30 °C overnight yielding 15 as colourless
needles (yield: 0.28 g, 38%), m.p. 153–154 °C (dec.);
1
NMR: H (400 MHz, C6D6), d 2.11 (s, 12H, p-ArCH3),
2.52 (s, 24H, o-ArCH3), 6.76 (s, 8H, ArH); 13C (100.6
MHz, C6D6), d 19.1, 19.6 (ArCH3), 128.1 (m-Aryl C),
132.8 (o-Aryl C), 138.2 (p-Aryl C), 140.2 (ipso-Aryl C),
219.1 (As–C); IR (Nujol, m/cmꢀ1) AsCO 1667 s, 1641 s;
MS APCI: m/z: 369 (Mþ/2, 100%).
4.4. [Cr{g2-O,O-OC(But)AsC(But)O}2(DME)] 16
A solution of 6 (0.36 g, 1.21 mmol) in DME (40 ml)
was added dropwise to a suspension of CrCl3 (0.064 g,
0.40 mmol) in DME (30 ml) at )78 °C. The resulting
purple suspension was allowed to warm to room tem-
perature and stirred for 24 h. Volatiles were removed in
vacuo leaving a dark red residue which was extracted
with hexane (3 ꢂ 30 ml), filtered and the filtrate con-
centrated to ca. 20 ml. This was placed at )30 °C
overnight yielding 16 as red crystals (yield: 0.114 g,
41%), m.p. 112–113 °C (decomp); leff ¼ 5.0 lB; IR
(Nujol, m/cmꢀ1) AsCO 1501 br; MS APCI: m/z (%): 58
(HButþ, 100), 543 (Mþ)DME, 15), 245 (AsO2C2Bu2t þ ,
13); calc. for C24H46As2CrO6: C 45.57, H 7.20. Found:
C 45.81, H 6.97%.
4.1. [V{g2-O,O-OC(But)AsC(But)O}3] 8
A solution of 6 (0.443 g, 1.49 mmol) in DME (40 ml)
was added dropwise to a solution of [VCl3(THF)3] (0.186
g, 0.50 mmol) in DME (30 ml) at )78 °C. The resulting
dark red solution was allowed to warm to room temper-
ature and stirred for 24 h. Volatiles were removed in vacuo
leaving a dark brown residue which was extracted with
hexane (3 ꢂ 30 ml), filtered and the filtrate concen-

C. Jones, T.C. Williams / Journal of Organometallic Chemistry 689 (2004) 1648–1656
1655
4.5. [MoBr(CO)2{g2-O2C(But)}(PPh3)2] 17
g, 0.90 mmol) in DME (40 ml) at )78 °C. The resulting
red solution was allowed to warm to room temperature
and stirred for 24 h. Volatiles were removed in vacuo
leaving a dark orange/red residue which was extracted
with hexane (3 ꢂ 30 m1), filtered and the filtrate con-
centrated to ca. 25 ml. This was placed at )30 °C
overnight yielding 22 as orange/red crystals (yield: 0.15
A solution of 6 (0.203 g, 0.69 mmol) in DME (50 ml)
was added dropwise to a suspension of [MoBr2(CO)2-
(PPh3)2] (0.286 g, 0.34 mmol) in DME (50 ml) at )78 °C.
The resulting blue suspension was allowed to warm to
room temperature and stirred for 24 h. Volatiles were
removed in vacuo leaving a dark brown residue which
was extracted with diethyl ether (3 ꢂ 30 ml), filtered and
the filtrate concentrated to ca. 10 ml. This was placed at
)30 °C overnight yielding 17 as red crystals (yield: 0.13
1
g, 29%), m.p. 59–60 °C (dec.); NMR: H (400 MHz,
C6D6), d 1.30 (36H, s, But), 3.10 (s, 12H, OMe), 3.31 (s,
8H, OCH2); 13C (100.6 MHz, C6D6), d 25.2 (C(CH3)3),
50.5 (C Me3), 57.9 (OMe), 68.9 (OCH2), 215.9 (MnCO),
253.1 (AsC); IR (Nujol, m/cmꢀ1) AsCO 1645 s, 1598 s;
MnCO 2057, 1982, 1961, 1926; MS APCI m/z (%): 245
(As{C(O)But}2þ, 100), 679 (MnBr(CO)4{As[C(O)But]2-
Li(DME)2}þ, 23); calc. for C36H56As2Br2Li2Mn2O16: C
36.67, H 4.75. Found: C 35.46, H 4.63%.
1
g, 41%), m.p. 127–129 °C (dec.); NMR: H (400 MHz,
C6D6), d 0.92 (9H, s, But), 6.91 (m, 12H, m-ArH), 7.13
(m, 12H, o-ArH), 7.72 (m, 6H, p-ArH); 13C (100.6 MHz,
C6D6), d 27.3 [C(CH3)], 66.3 [C(CH3)], 131.0 (m-aryl C),
133.5 (o-aryl C), 134.0 (p-aryl C), 135.1 (ipso-aryl C),
197.5 (CO2); 31P{1H} (121.7 MHz, C6D6) d 64.3 (s); IR
(Nujol, m/cmꢀ1) CO 1941(m), 1854 (s), CO2 1580 (m);
MS: APCI m/z (%): 263 (HPPhþ3 , 100).
4.9. Crystallographic studies
Crystals of 15, 16, 17, 18, 21 and 22 suitable for X-ray
structural determination were mounted in silicone oil.
Crystallographic measurements were made using either
Nonius CAD-4 or Kappa-CCD diffractometers. The
structures were solved by direct methods and refined on
F2 by full matrix least squares (SHELX 97) [23] using all
unique data. The data for 18 were poor, leading to a
high r-factor, but the molecular connectivity of this
complex is unambiguous. All non-hydrogen atoms are
anisotropic with H-atoms included in calculated posi-
tions (riding model). Crystal data, details of data col-
lections and refinement are given in Table 1.
4.6. [MoBr(CO)2{g2-O2C(Ph)}(PPh3)2] 18 and
[MoBr(CO)2{g2-O2C(Mes)}(PPh3)2] 19
Complexes 18 (yield: 25%) and 19 (yield: 20%) were
prepared by a similar procedure used for the prepara-
tion of 17. The spectroscopic data for 18 are identical to
those reported [19]. The data for 19 are as follows; m.p.
1
132–133 °C (dec.); NMR: H (400 MHz, C6D6), d 2.01
(s, 3H, p-ArCH3), 2.53 (s, 6H, o-ArCH3), 6.75 (s, 2H,
Mes-ArH), 7.2–8.0 (m, 30H, ArH); 13C (100.6 MHz,
C6D6), d 19.6, 21.4 (ArCH3), 127.0, 127.1, 127.4, 128.8,
129.3, 133.4, 138.7, 139.1 (ArC), 185.0 (CO2); 31P{1H }
(121.7 MHz, C6D6) d 64.6 (s); IR (Nujol, m/cmꢀ1) CO
1954 (m), 1866 (s), CO2 1603 (m); MS: APCI m/z (%):
263 (HPPhþ3 , 100).
5. Supplementary material
Crystallographic data (excluding structure factors)
for the structures of 15, 16, 17, 18, 21 and 22 have been
deposited with the Cambridge Crystallographic Data
Centre 15: CCDC no. 230102; 16: CCDC no. 230103; 17
CCDC no. 230104; 18: CCDC no. 230105; 21: CCDC
no. 230106; 22: CCDC no. 230107. Copies of this in-
formation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336033; email: deposit@
ccdc.cam.ac.uk or www.ccdc.cam.ac.uk).
4.7. [Mn{g2-O,O-OC(But)AsC(But)O}2(DME)] 21
A solution of 6 (0.327 g, 1.10 mmol) in DME (40 ml)
was added dropwise to a solution of MnCl2 (0.069 g, 0.55
mmol) in DME (30 ml) at )78 °C. The resulting yellow/
orange solution was allowed to warm to room tempera-
ture and stirred for 24 h. Volatiles were removed in vacuo
leaving an orange/brown residue which was extracted
with hexane (3 ꢂ 30 ml), filtered and the filtrate concen-
trated to ca. 10 ml. This was placed at )30 °C overnight
yielding 21 as orange crystals (yield: 0.123 g, 35%), mp
Acknowledgements
110–111 °C (dec.); leff ¼ 5:3 lB; IR (Nujol, m/cmꢀ1
)
AsCO 1562 s, 1592 s; MS APCI: m/z (%): 635.9 (Mþ, 100);
calc. for C24H46As2MnO6: C 45.35, H 7.24%. Found: C
45.73, H 7.42%.
We thank the EPSRC for funding (studentship for
TCW).
4.8. [MnBr(CO)4{As[C(O)But]2Li(DME)}]2 22
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