Controlled decomposition of an indium trihydride adduct: Synthesis and characterization of the first mixed-oxidation-state indium sub-halide complex anion, [In5Br8(quinuclidine)4]-

Article abstract of DOI:10.1021/ic050415r

The first example of a compound containing a mixed-oxidation-state indium sub-halide complex anion, [In₅Br₈(quinuclidine) ₄]^-, has been accessed by the controlled decomposition of an indium trihydride adduct, [InH₃(quinuclidine)], in the presence of LiBr. An intermediate in this reaction, [InH₂Br(quinuclidine) ₂], has been isolated and suggests that its mechanism involves hydride-bromide exchange, reductive dehydrogenation, and disproportionation processes.

Full text of DOI:10.1021/ic050415r

Inorg. Chem. 2005, 44, 4909−4911
Controlled Decomposition of an Indium Trihydride Adduct: Synthesis
and Characterization of the First Mixed-Oxidation-State Indium
-
Sub-halide Complex Anion, [In5Br8(quinuclidine)4]
Marcus L. Cole, Cameron Jones,* and Marc Kloth
Center for Fundamental and Applied Main Group Chemistry, School of Chemistry, Main Building,
Cardiff UniVersity, Cardiff, U.K. CF10 3AT
Received March 21, 2005
The first example of a compound containing a mixed-oxidation-
state indium sub-halide complex anion, [In5Br8(quinuclidine)4]-, has
been accessed by the controlled decomposition of an indium
trihydride adduct, [InH3(quinuclidine)], in the presence of LiBr. An
intermediate in this reaction, [InH2Br(quinuclidine)2], has been
isolated and suggests that its mechanism involves hydride−bromide
exchange, reductive dehydrogenation, and disproportionation
processes.
L ) Lewis base, X ) halide, n ) 0; L ) X ) halide, n )
2, are known,7 as are a range of binary sub-halides, e.g.,
In5Br7 and In4Br7.8 The latter of these are, however, generally
“saltlike” and do not consist of discrete molecular units. It
should also be mentioned that a number of alkyl-9 or alkyl/
halide10-substituted mixed-oxidation-state indium clusters and
related compounds have been recently reported.
Part of the reason behind the paucity of indium sub-halide
cluster complexes must come from the very low solubility
of indium(I) halides in most organic solvents. In addition, it
is reasonable to assume that indium(I) halides should be more
stable toward disproportionation reactions than their lighter
group 13 counterparts. It seemed to us that an alternative
route to polynuclear indium sub-halide complexes could
involve the controlled decomposition of thermally labile
indium(III) species. In recent years, we have developed
synthetic and stabilization strategies that have allowed us to
access the first indium trihydride and related indium hydride
complexes.11 Although these can be exceptionally thermally
stable, e.g., [InH3{C[N(Mes)C(H)]2}], where Mes ) mesityl
(dec 115 °C), they normally decompose to indium metal,
dihydrogen, and free ligand at close to room temperature.
Herein, we report the remarkable controlled decomposition
reaction of one of these, [InH3(quin)], where quin )
quinuclidine, in the presence of LiBr, which has yielded the
anionic indium sub-halide complex, [H(quin)2]+[In5Br8-
(quin)4]-.
The chemistry of polyhedral and nonpolyhedral complexes
of aluminum and gallium sub-halides has rapidly expanded
in recent years.1 The advances made in this field have mainly
come from the group of Schno¨ckel, who have developed a
specialized reactor to generate metastable metal(I) halide
complexes of the type [{MX(L)}n], where M ) Al or Ga, X
) Cl, Br, or I, and L ) ether, amine, or phosphine. The
controlled decomposition of these halides has led to an array
of clusters and related complexes in which the average
oxidation state of the metal is either greater than or less than
+1, e.g., [Al22Cl20(THF)12],2 [Al5Br6(THF)6]+[Al5Br8(THF)4]-,3
[Ga24Br22(THF)10],4 and [Ga5Cl7(OEt2)5].5 It is of note that
other lower nuclearity gallium sub-halide complexes have
come from the partial disproportionation of “GaI” in the
presence of Lewis bases.6 Perhaps surprisingly, indium sub-
halide cluster complex chemistry is nonexistent, though a
variety of complexes of the type [(L)X2InInX2(L)]n-, where
It is known that pure samples of [InH3(quin)], prepared
from LiInH4 and quin‚HCl, decompose in solution or in the
solid state above -5 °C to give indium metal, H2, and
quinuclidine.11,12 However, we observed that when an
* To whom correspondence should be addressed. E-mail: jonesca6@
cardiff.ac.uk.
(1) (a) Schnepf, A.; Schno¨ckel, H.-G. Angew. Chem., Int. Ed. 2002, 41,
3532. (b) Linti, G.; Schno¨ckel, H.-G. In Molecular Clusters of the
Main Group Elements; Driess, M., No¨th, H., Eds.; Wiley-VCH:
Weinheim, Germany, 2004; pp 126-168 and references therein.
(2) Klemp, C.; Bruns, M.; Gauss, J.; Ha¨ussermann, U.; Sto¨sser, G.; Wu¨llen,
L.; Jansen, M.; Schno¨ckel, H.-G. J. Am. Chem. Soc. 2001, 123, 9099.
(3) Klemp, C.; Sto¨sser, G.; Krossing, I.; Schno¨ckel, H.-G. Angew. Chem.,
Int. Ed. 2000, 39, 3691.
(4) Duan, T.; Baum, E.; Burgert, R.; Schno¨ckel, H.-G. Angew. Chem.,
Int. Ed. 2004, 43, 3190.
(5) Loos, D.; Schno¨ckel, H.-G.; Fenske, D. Angew. Chem., Int. Ed. 1993,
32, 1059.
(7) Chemistry of Aluminium, Gallium, Indium and Thallium; Downs, A.
J., Ed.; Blackie Academic: Glasgow, 1993; and references therein.
(8) Scholten, M.; Ko¨lle, P.; Dronskowski, R. J. Solid State Chem. 2003,
174, 349 and references therein.
(9) Eichler, B. E.; Hardman, N. J.; Power, P. P. Angew. Chem., Int. Ed.
2000, 39, 383.
(10) Uhl, W.; Melle, S.; Geiseler, G.; Harms, K. Organometallics 2001,
20, 3355.
(11) Jones, C. Chem. Commun. 2001, 2293 and references therein.
(12) Cole, M. L. Ph.D. Thesis, Cardiff University, Cardiff, U.K., 2001.
(6) Baker, R. J.; Jones, C. Dalton Trans. 2005, 1341 and references therein.
10.1021/ic050415r CCC: $30.25
Published on Web 06/15/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 14, 2005 4909

COMMUNICATION
Scheme 1
the likely presence of highly reactive transient intermediates,
e.g., radicals, in the reaction solution. There is a recent
precedent for the generation of such radicals from a complex
closely related to [InHBr2(quin)1or2], viz., [InHCl2(THF)n],
which has been shown to reduce alkyl halides at room
temperature via a process involving •InCl2 radicals as
intermediates.14 In addition, [GaHCl2(L′)2], where L′ ) 3,5-
dimethylpyridine, has been reported to undergo a reductive
dehydrogenation reaction to give [(L′)Cl2GaIIGaIICl2(L′)] in
boiling toluene, presumably via homolytic Ga-H bond
cleavage.15
A more rapid route to 2 was devised whereby an in situ
generated ethereal solution of LiInH4/LiBr was treated with
2 equiv of quin‚HCl.16 Placement of the reaction mixture
overnight at -30 °C gave a 24% isolated yield of the
complex. When the reaction was repeated but left at -30
°C for 3 days, an orange solution resulted, which upon
workup gave 1 in a 17% yield. This alternate route
circumvents the initial hydride-bromide exchange reaction,
thus leading to the accelerated reaction times. The low but
reproducible yield of 1 from both synthetic pathways
suggests that the deeply colored reaction solutions contain
other indium sub-halide species. None of these have yet been
isolated in a pure form despite repeated attempts.
Complex 1 is thermally unstable in solution above -15
°C, or in the solid state above 5 °C, and decomposes via
disproportionation to give indium metal and [InBr3(quin)2],
among other products. Although 2 decomposes in solution
at temperatures greater than -5 °C, it does not decompose
rapidly in the solid state until 72 °C, though it slowly (over
3 days) deposits indium metal upon standing at 25 °C. Both
compounds are diamagnetic, though little information could
be obtained from their 1H NMR spectra. These display broad
signals resulting from the quinuclidine ligands, which did
not resolve in the temperature range from -20 to -50 °C.
The 1H NMR spectrum of 2 does, however, exhibit a
characteristically broad hydride signal centered at δ 3.56
ppm, which is in the normal region for indium hydride
unpurified diethyl ether reaction solution containing [InH3-
(quin)] and LiBr (an ether-soluble byproduct in the synthesis
of LiInH4 from LiH and InBr311) was stored at -30 °C, it
took on a deep orange color over 5 days and some indium
metal was deposited. Concentration of this solution afforded
the complex [H(quin)2]+[In5Br8(quin)4]- (1, <5%; Scheme
1), which is closely related to both the salt, [Al5Br6(THF)6]+-
[Al5Br8(THF)4]-,3 and the neutral complex, [Ga5Cl7(OEt2)5].5
Concentration of another such reaction solution after standing
for only 2 days at -30 °C gave the new colorless crystalline
complex [InH2Br(quin)2] (2), in low yield, again with some
indium metal deposition. These observations prompted an
investigation of the mechanisms of the unusual reactions that
yielded 1 and 2.
It seemed that 2 could possibly be an intermediate in the
formation of 1 and may have been formed via a bromide-
hydride exchange reaction between [InH3(quin)] and LiBr.
The second-coordinated equivalent of quinuclidine in 2 likely
arises from the full decomposition of some [InH3(quin)] in
the solution, as evidenced by the deposition of indium metal.
To test these hypotheses, solutions of pure [InH3(quin)] or
2 were treated with anhydrous LiBr (20 equiv) and placed
at -30 °C for 5 days. In both cases, orange solutions resulted
and 1 could be isolated in low yield. It seems, therefore,
that the mechanism of formation of 1 does involve initial
hydride-bromide exchange reactions to give [InH2Br-
(quin)1or2] and perhaps [InHBr2(quin)1or2], though we have
no evidence for the presence of any dibromide complex as
yet. These exchange reactions are presumably driven by the
insolubility of the LiH product relative to the LiBr reactant.
In this respect, we have recently shown that anion-hydride
exchange and subsequent alkali-metal hydride deposition
processes involving [InH3(quin)] can be facile.13 Both [InH2-
Br(quin)1or2] and [InHBr2(quin)1or2] could then undergo
homolytic In-H cleavage reactions (H2 elimination) and a
subsequent series of partial disproportionation/compropor-
tionation and bromide transfer reactions to give the anion,
[In5Br8(quin)4]- (cf. the formation of [Al5Br8(THF)4]- 3). The
proton of the [H(quin)2]+ cation probably results from a
solvent abstraction process, which is reasonable considering
(14) Inoue, K.; Sawada, A.; Shibata, I.; Baba, A. J. Am. Chem. Soc. 2002,
124, 906.
(15) Nogai, S. D.; Schmidbaur, H. Organometallics 2004, 23, 5877.
(16) Experimental data for 1: To an in situ generated solution of LiInH4
(1.41 mmol) containing ca. 4.2 mmol of LiBr in diethyl ether (60
cm3) at -78 °C was added solid quin‚HCl (0.42 g, 2.81 mmol) over
5 min. The resultant colorless suspension was warmed to -30 °C,
stirred for 2 h, filtered, and kept at this temperature for 72 h to yield
an orange solution. Volatiles were removed in vacuo and the residue
extracted into toluene (15 cm3). Placement at -50 °C overnight yielded
1 as yellow-orange prisms (0.09 g, 17% based on indium); mp 5-6
°C (dec); 1H NMR (400 MHz, C6D5CD3, 243 K) δ 1.89 (br m, 36 H,
CH2), 2.19 (br m, 6 H, CH), 3.26 (br m, 36 H, CH2N), NH resonance
not observed; IR ν/cm-1 (Nujol) 2580 v br (N-H), 1318 m, 1047 s,
981 s, 826 m, 791 m, MS/APCI m/z (%): 112 [quinH+, 100].
Experimental data for 2: reaction conditions as for 1 except the ether
reaction solution was concentrated to 5 cm3 immediately after filtration
and placed at -30 °C for 8 h to yield 2 as colorless prisms (0.14 g,
24%); mp 72-74 °C (dec); 1H NMR (300 MHz, C6D5CD3, 243 K) δ
1.29 (br m, 12 H, CH2), 1.49 (br m, 2 H, CH), 2.79 (br s, 12 H, CH2N),
3.56 (br s, 2 H, InH); 13C NMR (100.6 MHz, C6D5CD3, 243 K) δ
26.6 (CH), 30.4 (CH2), 47.8 (NCH2); IR ν/cm-1 (Nujol) 1707 (br s,
In-H); MS/APCI m/z (%) 112 [quinH+, 100]. Reproducible mi-
croanalyses could not be obtained for 1 or 2 because of their high air
sensitivity and thermal instability at 25 °C. The low solubility of 1 at
243 K in C6D5CD3 precluded the acquisition of interpretable 13C NMR
data for this compound.
(13) Baker, R. J.; Jones, C.; Kloth, M.; Platts, J. A. Angew. Chem., Int.
Ed. 2003, 43, 2660.
4910 Inorganic Chemistry, Vol. 44, No. 14, 2005

COMMUNICATION
Figure 2. Molecular structure of 2. Selected bond lengths (Å) and angles
(deg): In1-N1 2.429(3), In1-Br1 2.5930(8), In1-H1 1.60(5), N1-In1-
N1′ 178.03(14), H1-In1-Br1 113.7(17), H1-In1-H1′ 132.6(5), Br1-
In1-N1 89.02(7), H1-In1-N1 90.0(17). Symmetry operation ′: -x, y,
1
-z + /2.
which has no significant contact with the anion, has been
previously structurally characterized.20 Complex 2 (Figure
2) is isomorphous with both its aluminum and gallium
analogues, [MH2Cl(quin)2], where M ) Al21 or Ga,22 and
represents the first structurally characterized tertiary amine
adduct of an indium hydride fragment. Its crystallographically
independent hydride ligand was located from difference maps
and refined isotropically without restraints. This shows the
geometry of the indium center to be distorted trigonal
bipyramidal with the quinuclidine ligands in axial positions.
The In-H distance [1.60(5) Å] compares well with those in
the few other crystallographically authenticated indium
hydride complexes.11 All other geometric parameters within
this complex are unexceptional.
Figure 1. Structure of the anionic component of 1. Selected bond lengths
(Å) and angles (deg): In1-In2 2.7460(14), In1-In3 2.7471(14), In1-In4
2.7484(14), In1-In5 2.7451(13), In2-N1 2.260(13), In3-N2 2.297(11),
In4-N3 2.298(11), In5-N4 2.268(14), In2-Br1 2.5652(19), In2-Br2
2.5519(19), In3-Br3 2.588(2), In3-Br4 2.565(2), In4-Br5 2.566(2), In4-
Br6 2.586(2), In5-Br7 2.5534(19), In5-Br8 2.5663(19), In2-In1-In3
110.19(4), In2-In1-In4 112.42(5), In2-In1-In5 110.02(4), In5-In1-
In3 112.47(5), In5-In1-In4 110.07(4).
species.11 In addition, a strong, broad In-H stretching
absorption is present in its infrared spectrum at 1707 cm-1.
This is at a significantly higher wavenumber than is normal
for indium trihydride complexes, e.g., [InH3(quin)] at 1642
cm-1,12 because of a negative inductive effect from the
bromide ligand, which reduces the polarity of the In-H
bonds relative to those in InH3 complexes.
Complex 1 was crystallographically characterized,17 and
the structure of its anionic component is depicted in Figure
1. This shows it to have a central indium atom tetrahedrally
coordinated by four InBr2(quin) fragments. A very similar
arrangement has been seen for the M5 fragments in the anion,
[Al5Br8(THF)4]-, and the neutral complex, [Ga5Cl7(OEt2)5],
which, as in 1, have formal average metal oxidation states
of 1.4. The average In-In bond length in 1 (2.747 Å) is
shorter than the mean for all crystallographically character-
ized examples (3.02 Å)18 but similar to those in related
homoleptic complexes, e.g., 2.696(2) Å in [In{In(Trip)2}3],
where Trip ) C6H2Pri3-2,4,6.19 Its In-N and In-Br bond
lengths are in the normal range, and its cationic component,
In conclusion, we have devised an unusual synthetic route
to the first example of a mixed-oxidation-state indium sub-
halide complex anion. This has involved the controlled
decomposition of an indium trihydride adduct in the presence
of LiBr. Although this reaction is relatively low-yielding and
its mechanism is not yet fully understood, it is reproducible.
This lends hope to the possibility of utilizing similarly
controlled indium hydride decomposition reactions in the
preparation of a range of higher nuclearity indium sub-halide
complexes and clusters that will be comparable to their now
well-studied aluminum and gallium counterparts. We will
report on our efforts in this direction in a later publication.
Acknowledgment. We gratefully acknowledge financial
support from EPSRC in the form of a studentship for M.L.C.
and a partial studentship for M.K.
(17) Crystal data for 1: C42H79Br8In5N6, M ) 1881.49, hexagonal, space
group P3, a ) b ) 21.359(3) Å, c ) 15.206(3) Å, V ) 6007.7(17)
Å3, Z ) 3, Dc ) 1.560 g cm-3, F(000) ) 2694, µ(Mo KR) ) 5.433
mm-1, T ) 150(2) K, 14 109 unique reflections [R(int) ) 0.1067], R
(on F) ) 0.0711, Rw (on F2) ) 0.1724 (I > 2σI). Crystal data for 2:
C14H28BrInN2, M ) 419.11, orthorhombic, space group Pbcn, a )
11.215(2) Å, b ) 12.033(2) Å, c ) 12.634(3) Å, V ) 1705.0(6) Å3,
Supporting Information Available: Crystallographic CIF files
for 1 and 2. This material is available free of charge via the Internet
at http://pubs.acs.org.
Z ) 4, Dc ) 1.633 g cm-3, F(000) ) 840, µ(Mo KR) ) 3.718 mm-1
,
IC050415R
T ) 150(2) K, 1950 unique reflections [R(int) ) 0.0513], R (on F) )
0.0321, Rw (on F2) ) 0.0623 (I > 2σI).
(18) As determined from a survey of the Cambridge Crystallographic
Database, Mar 2005.
(20) Yutronic, N.; Merchan, J.; Gonzales, G.; Garland, M. T. J. Chem.
Soc., Perkin Trans. 2 2002, 1956.
(21) Jones, C.; Junk, P. C.; Cole, M. L. Main Group Met. Chem. 2001, 24,
249.
(19) Brothers, P. J.; Hu¨bler, K.; Hu¨bler, U.; Noll, B. C.; Olmstead, M. M.;
Power, P. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 2355.
(22) Luo, B.; Young, V. G.; Gladfelter, W. L. Chem. Commun. 1999, 123.
Inorganic Chemistry, Vol. 44, No. 14, 2005 4911