Synthesis and structural characterisation of a soluble, metastable indium(I) halide complex, [InBr(tmeda)]

Article abstract of DOI:10.1039/b814658f

The first example of an indium(I) halide complex, [InBr(tmeda)], has been prepared by the reversible dissolution of InBr in a tmeda-toluene mixture. The structural characterisation of the metastable compound shows it to be monomeric with weak In...In interactions in the solid state. In solution, it decomposes to either InBr or [In₂Br₄(tmeda)₂]. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b814658f

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Synthesis and structural characterisation of a soluble, metastable
indium(I) halide complex, [InBr(tmeda)]w
Shaun P. Green,ab Cameron Jones*a and Andreas Stascha
Received (in Cambridge, UK) 22nd August 2008, Accepted 7th October 2008
First published as an Advance Article on the web 4th November 2008
DOI: 10.1039/b814658f
The first example of an indium(I) halide complex, [InBr(tmeda)],
has been prepared by the reversible dissolution of InBr in a
tmeda–toluene mixture. The structural characterisation of the
metastable compound shows it to be monomeric with weak
Inꢀ ꢀ ꢀIn interactions in the solid state. In solution, it decomposes
to either InBr or [In2Br4(tmeda)2].
We have begun to examine the mechanism by which solid
indium(I) halides interact with donor solvents and other Lewis
bases. This study was inspired by a number of early reports
which indicated that InX can form soluble adducts with
N-donor ligands without undergoing disproportionation.1 Of
most relevance here, is the work of Tuck et al. who found InX
(X = Br or I) to dissolve in toluene–tmeda (tmeda =
N,N,N0,N0-tetramethylethylenediamine) mixtures to yield so-
lutions having concentrations of up to 15.7 ꢁ 10ꢂ3 M.14 From
these, solids analysing as InX(tmeda)0.5 were precipitated by
the addition of hexane. In the case of indium(I) iodide, we have
recently shown that the precipitated solid is, in fact, the novel
indium sub-halide cluster complex, [In6I8(tmeda)4] 1 (average
In oxidation state: +1.33).15 We have also demonstrated that
the reaction of InBr with toluene–quinuclidine mixtures gives
the related anionic indium cluster complex, 2.15,16 As an
extension of that work, we now report the synthesis and
structural characterisation of the first molecular indium(I)
halide complex, [InBr(tmeda)], and briefly discuss the bearing
this result will have on synthetic chemists.
The commercial availability of the indium(I) halides, InX
(X = Cl, Br or I), has allowed the low oxidation state
chemistry of the metal to flourish over the past two decades.1
For example, these salts have been used as precursors in the
preparation of molecular indium(I) organometallics,2 mixed
valence indium species,3 heterometallic clusters4 and inorganic
materials containing In+ cations.1 In addition, indium(I)
halides are finding increasing use in stoichiometric and cata-
lytic organic transformations.5 Despite their importance, there
are problems associated with these reagents. The most limiting
of these is their vanishingly low solubility in non-coordinating
solvents. As a result, many of their reactions require coordi-
nating solvents to be effective to any degree. However, given
that indium(I) halides can rapidly disproportionate to indium(II)
and indium(III) halides in coordinating solvents,1 the outcomes
of these reactions are not always straightforward or predict-
able. Remarkably, almost nothing is known about the me-
chanism by which indium(I) halides dissolve in coordinating
solvents, and whether or not this involves soluble indium(I)
species at all.6 Indeed, there are no known examples of
structurally characterised molecular indium(I) halide com-
plexes.7,8 This is perhaps surprising when it is considered that
a range of soluble, metastable aluminium(I) and gallium(I)
halide complexes, [MmXm(L)n] (M = Al or Ga; L = ether,
amine or phosphine), have been prepared and their remark-
able further chemistry well developed.9 The structural
characterisation of several examples of these complexes
([Al4I4(PEt3)4],10 [Al4Br4(NEt3)4]11 and [Ga8I8(PEt3)6]12) show
all to possess aluminium or gallium centres covalently bonded
to one halide and two other metals. As a result, although
formally in the +1 oxidation state, their metal centres should
be considered as trivalent.13 To some extent, this must explain
the metastability of the complexes, [MmXm(L)n].
Using a modification of the method of Tuck et al.,14 InBr
powder was suspended in a 10% v/v tmeda–toluene mixture at
ꢂ85 1C (Scheme 1). Upon warming, dissolution of the InBr
commenced at ꢂ60 1C and was complete by ꢂ30 1C, yielding
an yellow-orange solution. Concentration of the solution at
this temperature led to the deposition of a blue-violet micro-
crystalline solid which, upon isolation, decomposed at 20 1C
over several hours to a grey solid. Similarly, warming the
yellow-orange solution above ꢂ20 1C led to indium metal
deposition and the eventual isolation of the known indium(II)
complex, [In2Br4(tmeda)2] (see ESIw for details of its X-ray
crystal structure).14,17 In order to obtain X-ray quality crystals
of the blue-violet solid, the yellow-orange solution was filtered
and the filtrate carefully layered with hexane at ꢂ80 1C in a
long, thin Schlenk flask (ca. 40 cm by 1 cm). After approxi-
mately two weeks at this temperature, yellow crystals had
grown at the solution/hexane interface and red-purple crystals
had grown ca. 2 cm above it. The crystals were isolated and,
surprisingly, upon warming above ꢂ30 1C, the yellow crystals
rapidly and irreversibly changed colour to blue-violet with
retention of their crystallinity. This material was structurally
a School of Chemistry, Monash University, Melbourne, PO Box 23,
Victoria, 3800, Australia
b School of Chemistry, Main Building, Cardiff University, Cardiff
UK CF10 3AT
w Electronic supplementary information (ESI) available: Full synthetic
and spectroscopic details; ORTEP diagrams for [In2Br4(tmeda)2]
and [(quin)2H][In5Cl8(quin)5]; full details and references for the
theoretical studies. CCDC 699631–699633. For ESI and crystallo-
graphic data in CIF or other electronic format, see DOI:
10.1039/b814658f
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Fig. 1 Molecular structure of 3 (25% thermal ellipsoids; hydrogen
atoms omitted). Selected bond lengths (A) and angles (1):
In(1)ꢀ ꢀ ꢀIn(1)0 3.678(2), In(1)–N(2) 2.500(5), In(1)–N(1) 2.531(4),
In(1)–Br(1) 2.7579(8), N(2)–In(1)–N(1) 72.98(14), N(2)–In(1)–Br(1)
92.21(12), N(1)–In(1)–Br(1) 88.44(11). Symmetry operation: ꢂx,
1 ꢂ y, ꢂz.
Scheme 1 Reagents and conditions: i, tmeda–toluene; ii, ꢂtmeda;
iii, >ꢂ20 1C, ꢂIn(s).
characterised as [InBr(tmeda)] 3 (vide infra), while the red-
purple crystals were found to be a known structural modifica-
tion of InBr.18 Compound 3 could also be obtained in a 28%
yield by placing the aforementioned yellow-orange solution at
ꢂ80 1C for 7 days.
well outside the sum of two In covalent radii (2.84 A19) but just
inside double the sum of the van der Waals radius for the
metal (3.86 A20). Accordingly, they should be considered as no
more than weak, non-directional interactions.21 This situation
differs to that in all structurally characterised aluminium(I)
and gallium(I) halide complexes which possess metal–metal
covalent bonds. It also highlights an analogy between 3 and
monomeric group 13 metal(I) diyls, :MR (R = bulky alkyl,
aryl etc.), which have a substantial coordination chemistry
derived from their Lewis basic metal lone pairs.2 The geometry
of the indium centre in 3 is distorted pyramidal with acute
angles about the metal (S angles = 253.61) that are suggestive
of a high degree of s-character to its lone pair. Although the In
centre is only 3-coordinate, both its N–In distances and the
In–Br distance are at the upper end of the known ranges.22
This is not surprising, considering that the metal is in the
monovalent state and would be expected to have a larger
radius than in higher oxidation state systems. It is also of no
surprise that the In–Br distance is shorter than the closest
contact in the crystal structure of InBr (3.01 A - closest of 7
contacts in the range 3.00–3.90 A18), but significantly longer
than that for monomeric InBr in the gas phase (2.543 A1).
We were intrigued by the colour change (from yellow to
blue-violet) that crystals of 3 underwent at ꢂ30 1C. A number
of attempts were made to obtain the crystal structure of the
yellow form of the compound, but all were thwarted by the
temperature sensitivity of this modification. It seems likely,
however, that the colour change is associated with an irrever-
sible, non-destructive phase change of the crystals.
These experiments confirm Tuck’s assertion that InBr dis-
solves in tmeda–toluene mixtures without disproportionation
below ꢂ20 1C, and that disproportionation to [In2Br4(tmeda)2]
occurs above ꢂ20 1C. Moreover, the presence of single crystals
of InBr above those of 3 in the crystallisation Schlenk flask,
indicates that solutions of 3 are in equilibrium with tmeda and
solid InBr. In this equilibrium, the deposition of InBr is
favoured as the tmeda concentration diminishes by diffusion
of hexane into the tmeda–toluene solution of 3. We believe
that such a crystallisation of an indium(I) halide from an
organic solvent is unprecedented. Based on our results, it
seems likely that the solid material Tuck obtained from his
experiments, viz. InBr(tmeda)0.5, was a mixture of two or more
of the following: 3, InBr, [In2Br4(tmeda)2] or In(s)
.
Considering the different outcomes from the reactions of InI
and InBr with tmeda in toluene, we examined the related
reaction with InCl. This led to the dissolution of the salt and
the formation of a red-orange solution which was found to be
much more sensitive to disproportionation processes than
solutions of the other two materials. Efforts to obtain crystalline
solids from this solution were not successful. It is noteworthy,
however, that the reaction of quinuclidine with InCl in toluene
led to a good yield of a mixed oxidation state cluster complex
closely related to 2, viz. [(quin)2H][In5Cl8(quin)5], but with one
of its InCl2 fragments coordinated by two quinuclidine ligands
(see ESIw for details of its X-ray crystal structure). These
differences between 2 and [(quin)2H][In5Cl8(quin)5] likely arise
from the lesser steric strain placed upon the anion by the smaller
size of the halide ligands in the latter, relative to those in former.
Compound 3 is not soluble in normal deuterated solvents
without the addition of tmeda. As a result, meaningful NMR
spectroscopic data for the compound could not be obtained
and, therefore, its characterisation relied on a crystallographic
analysis.z The molecular structure of the compound is dis-
played in Fig. 1, which shows it to consist of monomeric units
having long range Inꢀ ꢀ ꢀIn interactions (3.678(2) A). These are
In order to probe the electronic structure of 3, ab initio
calculations were carried out on it at the MP2 level of theory.
The geometry of the optimised gas phase structure, which was
found to be a minimum by analysis of vibrational modes, is in
excellent agreement with that from the X-ray crystal structure.
The In–Br (2.796 A) and In–N bond lengths (2.6055 A, mean)
are overestimated by ca. 1% and 4%, respectively, whilst the
Br–In–N angles (86.71 and 90.01) are underestimated by less
than 3%. An NBO analysis indicated that the indium lone pair
is predominantly associated with the HOMO (Fig. 2) and
has high s-character (93.2% s-, 6.76% p-character), while
the In–Br bond exhibits appreciable ionic character
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2 C. Gemel, T. Steinke, M. Cokoja, A. Kempter and R. A. Fischer,
Eur. J. Inorg. Chem., 2004, 4161, and references therein.
3 See for example: (a) B. E. Eichler, N. J. Hardman and P. P. Power,
Angew. Chem., Int. Ed., 2000, 39, 383; (b) N. Wiberg, T. Blank,
H. Noth and W. Ponikwar, Angew. Chem., Int. Ed., 1999, 38, 839;
¨
(c) W. Uhl, S. Melle, G. Geiseler and K. Harms, Organometallics,
2001, 20, 3355, and references therein.
4 (a) F. P. Gabbaı, S.-C. Chung, A. Schier, S. Kruger, N. Rosch and
¨
¨
H. Schmidbaur, Inorg. Chem., 1997, 36, 5699; (b) F. P. Gabbaı,
¨
¨
A. Schier, J. Riede and H. Schmidbaur, Inorg. Chem., 1995, 34,
3855.
5 For recent uses of InI halides in organic synthesis, see:
(a) U. Schneider, I.-H. Chen and S. Kobayashi, Org. Lett., 2008,
10, 5; (b) U. Schneider and S. Kobayashi, Angew. Chem., Int. Ed.,
2007, 46, 5909; (c) B. C. Ranu, K. Chattopadhyay and S. Banerjee,
J. Org. Chem., 2006, 71, 423; (d) S. A. Babu, M. Yasuda,
Y. Okabe, I. Shibata and A. Baba, Org. Lett., 2006, 8, 3029;
(e) T. Hirashita, S. Kambe, H. Tsuji and S. Araki, Chem. Com-
mun., 2006, 2595; (f) A. L. Braga, P. H. Schneider, M. W. Paixao,
A. M. Deobald, C. Peppe and D. P. Bottega, J. Org. Chem., 2006,
71, 4305.
Fig. 2 Representations of (a) the HOMO and (b) the LUMO of
[InBr(tmeda)].
(natural charges: In +0.74, Br ꢂ0.81; Wiberg bond index:
0.33). The LUMO (Fig. 2) corresponds principally to a vacant
p-orbital at the In centre.
In conclusion, InBr has been shown to dissolve in tmeda–
toluene mixtures to give solutions that are stable with respect
to disproportionation up to ꢂ20 1C. Crystallisation of the first
indium(I) halide complex, [InBr(tmeda)], from these solutions
has been achieved, and the compound shown to be monomeric
by X-ray crystallography. This result differs significantly
from the dissolution of InI in tmeda–toluene mixtures, which
yields the cluster complex [In6I8(tmeda)4]. The propensity of
indium(I) halides to disproportionate in most coordinating
solvents is a shortcoming of these important reagents that has,
no doubt, led to many reactions involving them being
unsuccessful in the past. We suggest that synthetic chemists
requiring well defined, soluble indium(I) halide reagents
consider using solutions of InBr in 10% tmeda–toluene at
temperatures below ꢂ20 1C.23 In our laboratory we are
currently examining the synthetic utility of such solutions, in
addition to the use of 3 as an In-donor ligand towards
transition metal fragments (cf. group 13 metal(I) alkyls).24
We will report on these endeavours in due course.
6 N.B. The mechanisms of the disproportionation of InII halides to
mixed oxidation state (InI/InIII) species have been studied. See:
(a) C. G. Andrews and C. L. B. Macdonald, Angew. Chem., Int.
Ed., 2005, 44, 7453; (b) D. G. Tuck, Chem. Soc. Rev., 1993, 22, 269;
(c) H. Schmidbaur, Angew. Chem., Int. Ed. Engl., 1985, 24, 893,
and references therein.
7 N.B. The structures of two complexes containing crown ether
coordinated In+ ions have been reported. See ref. 6a and B. F.
T. Cooper and C. L. B. Macdonald, J. Organomet. Chem., 2008,
693, 1707.
8 N.B. The structure of a complex, [Cl3InInCl(dibenzo-18-crown-6)],
with an In–In bond has been reported (see ref. 6a). This can be
viewed as a mixed oxidation state (InI/InIII) donor–acceptor
complex.
9 See for example: (a) A. Schnepf and H. Schnockel, Angew. Chem.,
¨
Molecular Clusters of the Main Group Elements, ed. M. Driess
¨
Int. Ed., 2002, 41, 3532; (b) G. Linti and H. Schnockel, in
and H. Noth, Wiley-VCH, Weinheim, 2004, pp. 126–168;
¨
¨
(c) A. Schnepf and H. Schnockel, Adv. Organomet. Chem., 2001,
47, 235.
10 A. Ecker and H. Schnockel, Z. Anorg. Allg. Chem., 1998, 624, 813.
¨
ckel, Angew. Chem., Int. Ed.
¨
11 M. Mocker, C. Robl and H. Schno
Engl., 1994, 33, 1754.
12 C. U. Doriat, M. Friesen, E. Baum, A. Ecker and H. Schnockel,
¨
Angew. Chem., Int. Ed. Engl., 1997, 36, 1969.
13 G. Parkin, J. Chem. Educ., 2006, 83, 791.
14 C. Peppe, D. G. Tuck and L. Victoriano, J. Chem. Soc., Dalton
Trans., 1982, 2165.
We gratefully acknowledge financial support from the
EPSRC (partial studentship for SPG), the Australian Re-
search Council (fellowships for CJ and AS), and the EPSRC
Mass Spectrometry Service, Swansea.
15 S. P. Green, C. Jones and A. Stasch, Angew. Chem., Int. Ed., 2007,
46, 8618.
16 N.B. Compound 2 can also be formed by the thermal decomposi-
tion of [InH2Br(quin)]. M. L. Cole, C. Jones and M. Kloth, Inorg.
Chem., 2005, 44, 4909.
17 N.B. The crystal structure of [In2Br3I(tmeda)2] has previously been
reported. M. A. Khan, C. Peppe and D. G. Tuck, Can. J. Chem.,
1984, 62, 601.
18 T. Staffel and G. Meyer, Z. Anorg. Allg. Chem., 1987, 552, 113.
19 B. Cordero, V. Gomez, A. E. Platero-Prats, M. Reves,
J. Echeverria, E. Cremades, F. Barragan and S. Alvarez, Dalton
Trans., 2008, 2832.
References
z Crystal data for 3: C6H16BrInN2, M = 310.94, monoclinic, space
group P21/c, a = 7.3609(15) A, b = 14.693(3) A, c = 10.264(2) A,
b = 104.82(3)1, V = 1073.2(4) A3, Z = 4, Dc = 1.925 g cmꢂ3
,
F(000) = 600, m(Mo-Ka) = 5.870 mmꢂ1, 123(2) K, 1869 unique
reflections [R(int) 0.0293], R (on F) 0.0360, wR (on F2) 0.0902
(I > 2sI); [In2Br4(tmeda)4]: C12H32Br4In2N4, M = 781.70, orthor-
hombic, space group Pbca, a = 17.327(4) A, b = 12.110(2) A, c =
22.420(5) A, V = 4704.4(16) A3, Z = 8, Dc=2.207 g cmꢂ3, F(000) =
2960, m(Mo-Ka) = 8.757 mmꢂ1, 123(2) K, 4317 unique reflections
[R(int) 0.1382], R (on F) 0.0552, wR (on F2) 0.1493 (I > 2sI).
[(quin)2H][In5Cl8(quin)5]: C49H92Cl8In5N7, M = 1637.00, triclinic,
20 J. Emsley, The Elements, Clarendon, Oxford, 2nd edn, 1995.
21 P. Pyykko, Chem. Rev., 1997, 97, 597.
¨
22 As determined from a survey of the Cambridge Crystallographic
Database, August, 2008.
ꢀ
space group P1, a = 12.079(2) A, b = 21.690(4) A, c = 12.363(3)
A, a = 90.38(3)1, b = 97.84(3)1, g = 103.65(3)1, V = 3115.5(11) A3, Z
23 N.B. A soluble indium(I) pseudo-halide, [In(CF3SO3)], has been
reported. C. L. B. Macdonald, A. M. Corrente, C. G. Andrews,
A. Taylor and B. D. Ellis, Chem. Commun., 2004, 250.
24 N.B. GaI has recently been shown to act as a terminal ligand
towards a transition metal fragment. N. D. Coombs, W. Clegg,
A. L. Thompson, D. J. Willock and S. Aldridge, J. Am. Chem.
Soc., 2008, 130, 5449.
= 2, Dc = 1.745 g cmꢂ3, F(000) = 1632, m(Mo-Ka) = 2.204 mmꢂ1
,
150(2) K, 10 832 unique reflections [R(int) 0.0477], R (on F) 0.1097, wR
(on F2) 0.3065 (I > 2sI). For crystallographic data in CIF or other
electronic format, see DOI: 10.1039/b814658f
1 J. A. J. Pardoe and A. J. Downs, Chem. Rev., 2007, 107, 2, and
references therein.
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Chem. Commun., 2008, 6285–6287 | 6287