Low valent and hydride complexes of NHC coordinated gallium and indium

Article abstract of DOI:10.1039/c1dt11202c

The reactions of the N-heterocyclic carbene 1,3-dimesitylimidazol-2-ylidene (IMes) with Ga[GaCl₄], "GaI", InCl₂ and GaBr₃ have been examined. All reactions using a low valent gallium or indium starting material led to species of the form [{MX₂(IMes)} ₂], where M = Ga, X = Cl (1), I (2); M = In, X = Cl (3), with disproportionation and loss of gallium metal in the case of 2. Reaction of IMes with gallium tribromide yields the air and moisture stable complex [GaBr ₃(IMes)] (4), which has been used as a precursor to the mixed bromohydrides [GaBrH₂(IMes)] (5) and [GaBr₂H(IMes)] (6) by (i) ligand redistribution with [GaH₃(IMes)], (ii) hydride-bromide exchange with triethylsilane, and (iii) alkylation with ⁿbutyllithium followed by β-hydride elimination (6 only). Attempts to prepare 1, or monovalent analogues such as [{GaCl(IMes)}_n], by thermally induced reductive elimination of dihydrogen from the chlorohydride congeners of 5 and 6 resulted in isolation of the known compounds [IMesCl][Cl] (IMesCl = 1,3-dimesityl-2-chloroimidazolium), and/or 1,3-dimesityl-2-dihydroimidazole, and gallium metal. Preliminary photochemical NMR spectroscopy and catalytic studies of 5 and 6 aimed at reductive dehydrogenation under milder conditions are reported. Compounds 1 and 4 have been characterised by single crystal X-ray structure determination.

Full text of DOI:10.1039/c1dt11202c

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PAPER
Low valent and hydride complexes of NHC coordinated gallium and indium†
Graham E. Ball, Marcus L. Cole* and Alasdair I. McKay
Received 24th June 2011, Accepted 16th September 2011
DOI: 10.1039/c1dt11202c
The reactions of the N-heterocyclic carbene 1,3-dimesitylimidazol-2-ylidene (IMes) with Ga[GaCl4],
“GaI”, InCl2 and GaBr3 have been examined. All reactions using a low valent gallium or indium
starting material led to species of the form [{MX2(IMes)}2], where M = Ga, X = Cl (1), I (2); M = In,
X = Cl (3), with disproportionation and loss of gallium metal in the case of 2. Reaction of IMes with
gallium tribromide yields the air and moisture stable complex [GaBr3(IMes)] (4), which has been used
as a precursor to the mixed bromohydrides [GaBrH2(IMes)] (5) and [GaBr2H(IMes)] (6) by (i) ligand
redistribution with [GaH3(IMes)], (ii) hydride–bromide exchange with triethylsilane, and (iii) alkylation
with nbutyllithium followed by b-hydride elimination (6 only). Attempts to prepare 1, or monovalent
analogues such as [{GaCl(IMes)}n], by thermally induced reductive elimination of dihydrogen from the
chlorohydride congeners of 5 and 6 resulted in isolation of the known compounds [IMesCl][Cl]
(IMesCl = 1,3-dimesityl-2-chloroimidazolium), and/or 1,3-dimesityl-2-dihydroimidazole, and gallium
metal. Preliminary photochemical NMR spectroscopy and catalytic studies of 5 and 6 aimed at
reductive dehydrogenation under milder conditions are reported. Compounds 1 and 4 have been
characterised by single crystal X-ray structure determination.
Introduction
Arduengo’s report of the first main group element NHC complex;
[AlH3(IMes)] (IMes = 1,3-dimesitylimidazol-2-ylidene),1 began
a two decade period of sustained interest in the stabilisation
of group 13 hydride and low valent complexes by NHCs.2 In
this vein, notable achievements include Robinson and Schleyer’s
Scheme 1 Preparation of compounds 1–3. Reagents and conditions (i)
isolation of a neutral [Ga6Mes4(IPrMe)2] octahedron (Mes = 2,4,6-
M = Ga, X = Cl, 1.0 eq. Ga[GaCl4], -78 ◦C to RT, toluene; (ii) M = Ga,
trimethylphenyl, IPrMe = 1,3-diisopropyl-4,5-dimethylimidazol-
X = I, 1.0 eq. “GaI”, -Ga(s), -78 ◦C to RT, toluene; (iii) M = In, X = Cl,
2-ylidene),2d Jones and Stasch’s isolation of the first ambi-
1.0 eq. InCl2, RT, 16 h, toluene.
ent temperature stable dialane [{AlH2(IDipp)}2] (IDipp = 1,3-
di(diisopropylphenyl)imidazol-2-ylidene),2e and Jones’ isolation
pathways (Scheme 4). We also report on preliminary studies of the
of the first indium trihydride complexes.2a,3 As part of a broad
research program focusing on methods for the stabilisation of
heavy group 13 hydrides, we have reported extensively on the use
reductive elimination of H2 from 5, 6 and their chloride analogues.
Results and discussion
of sterically hindered ligands to stabilise aluminium and gallium
hydrides.4 This includes the NHC complexes [MX2H(IMes)],
NHC supported tetrahalo dimetallanes
M = Al5,6 and Ga,6,7 X = Cl and Br, which have exceptionally
Jones and co-workers have reported the disproportionation
reaction of monovalent indium bromide with IMes to afford
[{InBr2(IMes)}2] and elemental indium,2b and the reaction of
IDipp with in situ generated “GaI”8 to yield the mono-NHC,
iodide adduct of tetraiodo digallane; [HIDipp][I3GaGaI2(IDipp)],
which exhibits an unusual eclipsed structure.9
As illustrated in Scheme 1, reaction of the IMes with Ga[GaCl4]
or “GaI” at -78 ◦C followed by gradual warming to room
temperature, or InCl2 at ambient temperature, affords colourless
complexes 1–3 in moderate to good yield. For the preparations of
1 and 2, addition of IMes initially affords bright orange reaction
mixtures that fade to pale yellow, with deposition of gallium for 2
high decomposition temperatures for molecular alumino- and
gallohydrides, and are entirely air stable. Herein we report
the preparation of IMes adducts of tetrachloro and tetraiodo
digallane; [{GaX2(IMes)} ] (X = Cl (1) and I (2)), and tetrachloro
diindane; [{InCl2(IMes)}22] (3) (Scheme 1), the air-stable gallium
tribromide complex [GaBr3(IMes)] (4) and its use as a precursor
to [GaBrH2(IMes)] (5) and [GaBr2H(IMes)]6 (6) by three synthetic
School of Chemistry, University of New South Wales, Sydney, NSW, 2052,
Australia. E-mail: m.cole@unsw.edu.au; Fax: +61 (0)2 9385 6141
† CCDC reference numbers 830949 and 830950. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt11202c
946 | Dalton Trans., 2012, 41, 946–952
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(solid with m.p. 30 ◦C), upon slow warming to room temperature.
All three compounds are air and moisture sensitive, particularly
1 and 2 for which satisfactory elemental analyses could not be
acquired. Compound 1 has been characterised by single crystal
X-ray structure determination (Fig. 1, Table 1).
Table 1 Summary of crystal measurement and refinement data for
compounds 1 and 4
[{GaCl2(IMes)}2] (1)
[GaBr3(IMes)] (4)
Mol. formula moiety
Mol. weight
T/K
C42H48Cl4Ga2N4
890.08
173(2)
P21
10.345(2)
14.917(3)
14.442(3)
90
104.27(3)
90
2160.0(8)
2
C21H24Br3GaN2
613.87
150(2)
Pca21
16.4429(5)
16.6293(5)
17.4316
90
90
90
4766.4(3)
8
1.711
Space group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
Volume/A
Z
Dc/g cm-3
m/mm-1
1.369
1.528
18728
9453
6.193
Reflections collected
Unique reflections
Parameters varied
Flack parameter
R(int)
75609
13580
499
0.283(16)
0.1644
0.0595
0.1201
501
0.00(3)
0.1327
0.0906
0.2289
R1
wR2 (all data)
Table 2 Spectroscopic (1H NMR and IR) and physical data for the
complexes reported herein and related literature compounds
Compound
ao-CH3 p-CH3 4,5-C2H2 M-H IR M-H dec. (◦C)
[{GaCl2(IMes)}2] (1) 2.17
[{GaI2(IMes)}2] (2) 2.19
[{InCl2(IMes)}2] (3) 2.11
2.13 5.76
2.13 5.70
2.17 5.76
2.14 5.74
2.05 6.04
2.05 5.95
2.05 5.94
2.04 5.84
2.05 5.88
2.06 5.79
2.05 5.77
2.38 6.35
2.17 6.09
—
—
—
—
—
—
—
—
230
216
234
n/a
214
263
270
274
286
327
176
115
119
[{InBr2(IMes)}2]2b
[GaH3(IMes)]3
2.14
2.02
2.03
3.96 1780
4.72 1870
4.51 1874
n/a 1917
5.30 1925
—
—
[GaClH2(IMes)]7
[GaBrH2(IMes)] (5) 2.04
[GaCl2H(IMes)]7
2.02
[GaBr2H(IMes)] (6)6 2.03
Fig. 1 (a) Molecular structure of 1, and (b) view down Ga(1)–Ga(2)
axis showing staggered arrangement of ligands (POV-Ray illustrations,
50% thermal elipsoids). All hydrogen atoms omitted for clarity.
[GaBr3(IMes)] (4)
[GaCl3(IMes)]2c
[InH3(IMes)]3
2.02
2.00
2.29
2.13
—
—
5.20 1650
6.84 1737
◦
˚
[InClH2(IMes)]3
Selected bond lengths (A) and angles ( ): Ga(1)–Ga(2) 2.4243(17),
Ga(1)–C(1) 2.047(11), Ga(2)–C(22) 2.062(13), Ga(1)–Cl(1) 2.241(4),
Ga(1)–Cl(2) 2.212(4), Ga(2)–Cl(3) 2.237(3), Ga(2)–Cl(4) 2.248(3),
C(1)–Ga(1)–Ga(2) 120.3(3), C(1)–Ga(1)–Cl(1) 104.7(3), C(1)–Ga(1)–Cl(2)
a All 1H NMR spectroscopic data in C6D6.
101.7(3),
Cl(1)–Ga(1)–Ga(2)
102.07(11),
Cl(1)–Ga(1)–Cl(2)
In previous studies we have shown that the imidazol-2-ylidene
4,5-C2H2 1H NMR resonance provides a useful handle for the
Lewis acidity of the coordinated group 13 metal.4a,5–7 Based on this
measure, comparison of the 4,5-C2H2 resonances of compounds
1–3 with that of [GaCl3(IMes)] (Table 2),2c,10 and the likely trend
in [InHnClm(IMes)] resonances (Table 2),3 indicates that tetrahalo
digallane and diindane possess similar Lewis acidities to those
of their trivalent analogues. It is noteworthy that the 1H and
13C NMR spectra of 2 do not contain more than one set of
IMes resonances nor an imidazolium-2-CH resonance, as would
be consistent with the ionic formulation of the species reported
by Jones and co-workers.9 All three compounds exhibit carbenic
carbon resonances in the expected region (1; 139.5, 2; 139.8, 3;
140.2 ppm, [GaCl2H(IMes)]; 140.3 ppm, also C6D6).7
103.57(17), Cl(2)–Ga(1)–Ga(2) 122.17(11), N(1)–C(1)–N(2) 104.3(9),
C(22)–Ga(2)–Ga(1) 121.3(3), C(22)–Ga(2)–Cl(3) 97.5(4), C(22)–Ga(2)–
Cl(4) 105.8(4), Cl(3)–Ga(2)–Ga(1) 124.45(11), Cl(3)–Ga(2)–Cl(4)
102.33(12), Cl(4)–Ga(2)–Ga(1) 103.01(10), N(3)–C(22)–N(4) 104.4(12).
1
The H NMR spectra of compounds 1 and 2 exhibit sharp
signals, are near identical, and possess a unique feature relative
to IMes adducts of trivalent hydrido and halo gallium species
such as [GaH3(IMes)],3 [GaCl3(IMes)]2c and [GaCl2H(IMes)],7
wherein the mesityl methyl resonances are markedly shifted
downfield (Table 2). This is particularly pronounced for the H
ortho-methyl resonances of 1 and 2. For example, the H ortho-
methyl resonance of 2 is 0.19 ppm downfield of the analogous
resonance of [GaCl3(IMes)] (both C6D6), and the 1H para-methyl
resonance is 0.08 ppm downfield of that for the same complex.10
An analogous shift is not observed for compound 3, which
exhibits similar chemical shifts to those of [InClH2(IMes)] (2.13
and 2.17 ppm3, respectively, 3; 2.11 and 2.13 ppm) and those of
[{InBr2(IMes)}2] (single broad resonance at 2.14 ppm) (Table 2).2b
1
1
Cooling of a saturated toluene solution of 1 to -25 ◦C afforded
colourless prisms suitable for single crystal X-ray structure deter-
mination. Compound 1 crystallises in the monoclinic space group
P21 with one staggered molecule in the asymmetric unit (Fig. 1).
The bonds of 1 are expectedly longer than those of its trivalent
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˚
analogue [GaCl3(IMes)] (1; Ga–C 2.047(11) and 2.062(13) A, Ga–
halides has been observed before when heating [TlX3(IMes)] (X =
Cl or Br) compounds to reflux in 1,3,5-mesitylene.17
˚
˚
Cl 2.212(4)–2.248(3) A, [GaCl3(IMes)]; 1.954(4) A and 2.1674(8)–
2.1910(8) A, respectively). Surprisingly, its bonding exhibits
2c
˚
NHC supported gallium tribromide and bromohydrides
marked differences relative to the eight known structurally char-
acterised tetrachloro digallanes [{GaCl2(L)}2] (L = Lewis base),11
of which seven exhibit a staggered conformation. For instance, the
Cl–Ga–Cl angles of 1 (103.57(17) and 102.33(12)◦) are below the
range described by the aforementioned complexes (104.8–110.2
11
˚
˚
A), the Ga–Cl bonds are extended (2.212(4)–2.248(3) A versus
˚
mean average of 2.20 A), and the Cl–Ga–Ga bonding evidences
two distinct chloride environments at each gallium (Cl(1)–Ga(1)–
Ga(2) 102.33(12)◦ and Cl(4)–Ga(2)–Ga(1) 103.01(10)◦, Cl(2)–
Ga(1)–Ga(2) 122.17(11)◦ and Cl(3)–Ga(2)–Ga(1) 124.45(11)◦,
range of observed Cl–Ga–Ga angles for literature [{GaCl2(L)}2]
species 111.6–118.0◦).11 Comparison of 1 with the IDippGaI2
fragment of the eclipsed anion of [HIDipp][Ga2I5(IDipp)]9 also
indicates greater distortion despite the decreased size of the NHC
herein. This can be seen in the Ga–Ga–I, I–Ga–I and C–Ga–
I angles about the IDippGaI2 of the anion, which range from
103.24(4)–109.81(19)◦, while the analogous Ga–Ga–Cl, Cl–Ga–
Cl and C–Ga–Cl angles of 1 lie between 101.7(3) and 122.17(11)◦.
The thermally induced reductive dehydrogenation of group 13
halometallanes (MXHn, M = Ga or In, X = Cl or Br)12,13 is an under
utilised pathway to low valent group 13 species. This is most likely
as the reactions are complicated by the competing requirements of
adduct stabilisation (good ligand donation) and dihydrogen elimi-
nation (weak ligand donation). A further consideration is the ther-
mal stability of the low valent generated, which may decompose at
thetemperatures employed. Withcomplex1 fully characterised, we
attempted to prepare 1 or its monovalent congener via the NMR
scale dehydrogenation of [GaClH2(IMes)] and [GaCl2H(IMes)],
which we have reported previously.7 The successes of Jones and
Stasch,2e and Robinson and Schleyer2d in isolating low valent
aluminium and gallium species, respectively, provides evidence
that NHCs can stabilise low valent group 13 compounds.
Scheme 3 Original preparation of compound 6 via hydride–bromide
exchange at an NHC.6
In previous studies we have reported the preparation of
[GaBr2H(IMes)] (6) from the reaction of quinuclidine coordinated
gallane and 4,5-dibrominated IMes (IMesBr) (Scheme 3).6 This
reaction is selective for the dibromogallane (GaBr2H) and requires
heating of the intermediate complex [GaH3(IMesBr)] to 50 ◦C
in toluene. This path differs from the conventional syntheses of
halogallanes,4b introduced independently by Greenwood18 and
Schmidbaur,19 wherein redistribution of halide and hydride ligands
about gallium or hydride-bromide exchange with a silane is used
respectively. Seeking a larger scale preparation of 6 for reactivity
studies, we attempted the preparation of [GaBrH2(IMes)] (5)
and 6 utilising both Greenwood and Schmidbaur’s routes from
a common [GaBr3(IMes)] (4) precursor, as-well-as attempting
b-hydride elimination from a [GaBr2(nBu)(IMes)] intermediate
prepared from 4 (Scheme 4).
The stoichiometric reaction of gallium tribromide with IMes in
diethyl ether affords [GaBr3(IMes)] (4) (Scheme 4) as a white pre-
cipitate that may be recrystallised from toluene to afford colourless
square plates suitable for single crystal X-ray structure determina-
tion (Fig. 2). Like [GaCl3(IMes)],2c compound 4 is air and moisture
stable. Spectroscopic data for 4 are also in accordance with those
expected based on the aforementioned trend in 4,5-C2H2 1H NMR
resonances of the coordinated IMes (Table 2).5–7
Unlike their alane counterparts, which are stable for extended
periods in toluene at reflux,5 the chlorohydrides [GaClH2(IMes)]
and [GaCl2H(IMes)], which represent ◦chloride analogues of 5
1
and 6 vide infra, decompose above 100 C (d8-toluene, H NMR
spectroscopy) to afford gallium metal and, for [GaClH2(IMes)];
1,3-dimesityl-2-dihydroimidazole,14 or, for [GaCl2H(IMes)], a
mixture of 1,3-dimesityl-2-dihydroimidazole and 2-chloro-1,3-
dimesitylimidazolium chloride (Scheme 2).15 The identity of these
Scheme 4 Preparation of compounds 4–6. Reagents and conditions (i)
0.5 (6) or 2.0 (5) eq. [GaH3(IMes)], 50 ◦C, 36h, toluene; (ii) 1.0 (6) or 2.0
(5) eq. HSiEt3, -BrSiEt3, -20 ◦C to RT, toluene; (iii) 1.0 eq. nBuLi (6 only),
-LiBr, -nbutene, RT to 70 ◦C, 6 h, toluene.
1
products was confirmed by preparative scale reactions and H
NMR spectroscopic and melting point analysis on the species
formed.15 In view of the typical high coloration of low valent gal-
lium cluster compounds,2d,16 it is noteworthy that heated solutions
remained pale yellow or colourless while being heated. Moreover,
at no time were resonances attributable to 1 observed when heating
either chlorohydride at lower temperatures (d8-toluene,1H NMR
spectroscopy). The formation of 2-halo-1,3-dimesitylimidazolium
Compound 4 crystallises in the orthorhombic space group
Pca21 with two unique molecules in the asymmetric unit. The
two distinct molecules exhibit comparable bonding parameters
(Fig. 2). Accordingly, only one molecule is discussed here. It
is noteworthy that 4 is isomorphous with [AlCl2H(IMes)]5 and
[GaCl2H(IMes)],7 and crystallises in the same space group as
[GaCl3(IMes)].2c Despite the considerable bulk of its NHC ligand,
the molecular structure of 4 (Fig. 2) exhibits remarkably regular
bond angles about gallium, and gallium to bromine contacts
that are comparable to the mean of those for [GaBr3(L)] (L =
Lewis base) adducts (2.3103(12)–2.3226(12) A versus 2.32 A).11 For
˚
˚
Scheme
2
Thermal decomposition of [GaClH2(IMes)] and
[GaCl2H(IMes)] in toluene.
instance, the Br–Ga–Br angles range from 107.22(5)–108.96(5)◦
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isobutene at room temperature t-butyl studies)24,25 and study n-
butene elimination at varying temperatures, or simple use of
lower conversion temperatures, resulted in Ga–H free intractable
mixtures. A 1H NMR spectrum of the impure material displays 4,5-
C2H2 resonances at 5.79 and 5.88 in a 1 : 3 ratio. These resonances
can be attributed to compounds 4 and 6, respectively. The
deposition of gallium metal during this reaction, and subsequent
isolation of 4 and 6, indicates a mixture of compounds 4–6
and [GaH3(IMes)] is generated, and that this mixture exists as a
series of equilibria at elevated temperatures. These are most likely
perturbed by solution phase decomposition of 5 and [GaH3(IMes)]
at the temperatures used.
Fig.
2
Molecular structure of
4
(POV-Ray illustration, 50%
thermal elipsoids). All hydrogen atoms omitted for clarity. Se-
lected bond lengths (A) and angles (◦): Ga(1)–C(1) 2.006(8),
˚
Ga(1)–Br(1) 2.3103(12), Ga(1)–Br(2) 2.3226(12), Ga(1)–Br(3) 2.3162(14),
C(1)–Ga(1)–Br(1) 112.6(2), C(1)–Ga(1)–Br(2) 112.2(2), C(1)–Ga(1)–Br(3)
107.3(2), Br(1)–Ga(1)–Br(2) 107.22(5), Br(1)–Ga(1)–Br(3) 108.96(5),
Br(2)–Ga(1)–Br(3) 108.37(5), N(1)–C(2)–N(2) 104.6(7).
Preliminary dehydrogenation studies of 5 and 6
The poor success of thermal methods in generating low valent
compounds from chloride analogues of 5 and 6 led us to consider
alternative methods for reductive dehydrogenation. To this end,
and the C–Ga–Br angles range from 107.3(2)–112.6(2)◦. These
are the reverse of most structural authenticated [GaBr3(L)] species
(mean literature values; Br–Ga–Br 111.4◦, L–Ga–Br 107.4◦),11 and
indicate considerable pyramidalisation of the gallium that is only
1
d8-toluene solutions of 5 and 6 were irradiated in situ during H
NMR spectroscopy experiments using broadband light output
from a 100 W mercury arc lamp at a range of temperatures with
spectra collected after irradiation periods of up to twenty two
minutes. Initial experiments using 6 at 188 K led to a loss of signal
intensity without new signals. This is indicative of the formation of
an insoluble or paramagnetic product, the latter being inconsistent
21
22
rivalled by the related P(SiMe3)3,20 PH(tBu)2 and N(SnMe3)2
adducts (mean Br–Ga–Br 108.5◦ and mean L–Ga–Br 110.5◦).
Reaction of
4 with two or one half equivalent(s) of
[GaH3(IMes)]3 in toluene, with heating to 50 ◦C for one and a half
days, yields 5 and 6, respectively, in good to moderate yield without
further purification (Scheme 4). Likewise, reaction of 4 with one or
two equivalents of triethylsilane at low temperature yields 6 and 5,
respectively, in good to excellent yield after extraction into toluene
and washing with diethyl ether (6), or simply washing with diethyl
ether (5) (Scheme 4). The infrared Ga–H stretches of 5 and 6 (1874
and 1925 cm-1, Table 2) are comparable to those of their chloride
relatives (1870 and 1917 cm-1, Nujol),7 but contrast the stretch
listed in our original report of 6 (1880 cm-1), which may have been
due to contamination of samples with 5.6 These demonstrate in-
ductive strengthening of the Ga–H bond relative to [GaH3(IMes)]
1
with diamagnetic compounds 1 and 2, cf sharp H NMR reso-
nances. To enable observation of low solubility products, repeat
1H NMR experiments on 5 and 6 were conducted at 273 K, wherein
irradiation resulted in loss of 4,5-C2H2 singlets (5; 5.88 ppm, 6; 5.76
ppm in d8-toluene) and broad hydride singlet resonances (5; 4.38
ppm, 6; 5.18 ppm in d8-toluene) and formation of new resonances
in the 4,5-C2H2 region of the spectrum (5.77 ppm and 5.69 ppm
respectively) as-well-as a new singlet resonance at 4.52 ppm. The
chemical shift of the latter resonance corresponds exactly to that of
dissolved dihydrogen in d8-toluene, as confirmed by a separate con-
trol experiment. Equilibration of both systems after irradiation (15
min) results in a stable set of 4,5-C2H2 1H NMR resonances (5; 5.74
ppm, 6; 5.65 ppm, shifts of 14.41 Hz and 21.03 Hz respectively) that
are accompanied by singlets attributable to the methyl and meta-
aryl protons of IMes (5; 1.86 (12H), 2.05 (6H), 6.65 (4H) ppm, 6;
1.85 (12H), 2.07 (6H), 6.66 (4H) ppm). It is noteworthy that overall
no loss of signal intensity was observed throughout the experiment
or after equilibration, as confirmed by post irradiation monitor-
ing. Furthermore, no gallium metal was generated, and resonances
attributable to ‘free’ IMes, [GaH3(IMes)], 4, 1,3-dimesityl-2-
dihydroimidazole, or a 2-haloimidazolium (cf thermal studies of
[GaCl2H(IMes)] and [GaClH2(IMes)]) were not observed. From
these data it is clear that 5 and 6 generate different species upon
irradiation, and that these do not exhibit the downfield shift
of methyl resonances noted for 1 and 2 in d6-benzene. Line
broadening, compound insolubility and solvent melting point
precluded the use of d6-benzene for analogous experiments.
Baker and co-workers have reported the catalytic dehydro-
genation of aminoborane [BH3(NH3)] (2.8 equivalents of H2 per
molecule) using the catalyst [Ni(N3NHC)2], where N3NHC = 1,3,4-
triphenyl-1,2,4-triazol-5-ylidene.26 Himmel and co-workers have
reported a thermally induced catalytic dehydrogenation of the
guanidinate supported gallium hydride [{GaCl(hpp)H}2] (hppH =
1,4,6-triazabicyclo[3.3.0]oct-4-ene) using [TiCl2Cp2]/nBuLi and
1
(Ga–H, 1780 cm-1).3 The H NMR spectra of 5 and 6 (C6D6)
exhibit the expected resonances (Table 2) and are consistent with
the trend in Lewis acidity GaBr3 > GaBr2H > GaBrH2 > GaH3 as
denoted by the 4,5-C2H2 resonances, which shift downfield with
1
increasing acidity (5.79 > 5.88 > 5.94 > 6.04 ppm).3,4a The H
NMR spectra of 5 and 6 exhibit broad Ga–H resonances due to the
quadrupole moment of gallium. The chemical shift of these (Table
2) is consistent with increased deshielding of the hydride(s) upon
going from 5 to 6 (4.51 and 5.30 ppm, [GaH3(IMes)] 3.96 ppm)3
and is consistent with the chemical shift of related hydrides like
[GaClH2(Quin)] (Quin = quinuclidine, 4.72 ppm).23 The solid state
decomposition temperatures (Table 2) of 5 and 6 are remarkable
(270 and 286 ◦C, respectively) and are accompanied by appreciable
tolerance to air and moisture, making 5 and 6 two of the most
stable molecular gallium hydrides prepared.
The room temperature elimination of isobutene from sterically
hindered t-butylgallium compounds has been reported by Power24
and Gillan.25 Impure 6 was isolated after heating a solution of
in situ generated [GaBr2(nBu)(IMes)] to 70 ◦C in toluene (n-
butene loss). This occurs with deposition of a grey solid that melts
just above room temperature (gallium m.p. 29.8 ◦C). Attempts
to isolate the intermediate alkylgallium compounds (cf the use
of n-butyl group relative to the aforementioned elimination of
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purification system and freeze-thaw degassed prior to use. ‘GaI’,8
[GaH3(IMes)],3 IMes,31 [GaCl2H(IMes)]7, [GaClH2(IMes)]7 and
[Ni(IMes)2]28 were prepared according to literature procedures,
all other reagents were purchased from Sigma-Aldrich and used
as received. Infrared spectra were recorded as Nujol mulls using
sodium chloride plates on a Nicolet Avatar 320 FTIR spectropho-
[{RhCl(COD)}2] precatalysts.27 In an effort to further explore
the catalytic dehydrogenation of our NHC supported bromogal-
lanes, the dehydrogenation of 5 and 6 on an NMR scale using
[Ni(IMes)2]28 (5 mol%) was attempted, wherein each species was
◦
heated to 50 C (cf thermal catalytic studies of Himmel)27 with
1
acquisition of H NMR spectra over a period of twelve hours
1
1
tometer. H and 13C{ H} NMR spectroscopic characterisations
were recorded on Bruker spectrometers (see below for MHz) at
298 K unless otherwise stated, with chemical shifts referenced
to the residual 1H and 13C resonances of d6-benzene (d 7.16
and 128.06 ppm, respectively). See separate section below for
1H NMR spectroscopic studies of reductive dehydrogenation.
Melting points were determined in sealed glass capillaries under
argon and are uncorrected. Microanalyses were conducted at
the University of Otago, P.O. Box 56, Dunedin, New Zealand.
Single crystal X-ray diffraction data collections were undertaken
at UNSW using a Bruker Apex II CCD diffractometer.†
(d8-toluene). During this time signals attributable to the parent
species reduced in intensity with the growth of new singlets at 1.85,
2.07 and 6.66 ppm. The new resonances are in accordance with
those generatedduring the photochemical studywith the exception
of signals attributable to the 4,5-C2H2 moiety, which are absent.
This could indicate nickel catalysed hydrogen–deuterium exchange
with the solvent. 4,5-Deuterium labelling of ‘free’ 1,3-disubstituted
imidazol-2-ylidenes is well established using weakly acidic solvents
such as d6-DMSO, d3-acetonitrile and d6-acetone.15,29 However, to
our knowledge, deuteration using a nickel catalyst with d8-toluene,
as per that suggested here, is unprecedented. It is noteworthy that
[Ni(IMes)2] can be characterised in d8-toluene without loss of 4,5-
C2H2 resonance intensity, and the limited literature concerning this
compound makes no mention of deuterium-hydrogen exchange.
Moreover, the literature pertaining to bis(imidazol-2-ylidene) or
bis(triazol-5-ylidene) complexes of nickel contains a single report
of H/D exchange, this being at an activated benzylic carbon using
D2O as a deuterium source.30
[{GaCl2(IMes)}2] (1)
A solution of Ga[GaCl4] (209 mg, 0.74 mmol) in toluene (15 mL)
was treated dropwise with a solution of IMes (434 mg, 1.43 mmol)
◦
in toluene (40 mL) at -78 C with stirring. The resultant orange
solution was allowed to warm to ambient temperature over several
hours, during which the colour faded to pale yellow. Filtration
and concentration in vacuo (~ 30 mL), followed by slow cooling to
-25 ◦C afford◦ed colourless prisms (110 mg, 26%) after one week,
m.p. 182–184 C, dec. 230–234 ◦C. 1H NMR (400.13 MHz, C6D6);
d 2.13 (s, 12 H, p-CH3), 2.17 (s, 24 H, o-CH3), 5.76 (s, 4 H, NCH),
Conclusions
In conclusion, we have prepared a catalogue of new di- and
trivalent gallium N-heterocyclic carbene species (1, 2, 4–6) and
a related bis(NHC) complex of tetrachloro diindane (3). The
divalent gallium complexes 1 and 2 exhibit diagnostic downfield
1
6.75 (s, 8 H, m-C6H2). 13C{ H} NMR (100.62 MHz, C6D6); d
18.6 (o-CH3), 21.2 (p-CH3), 123.6 (NCH), 129.3 (m-C6H2), 129.9
(p-C6H2), 134.5 (o-C6H2), 135.5 (ipso-C6H2), 139.5 (NCN). IR
(Nujol) n/cm-1; 1608 (w), 1539 (w), 1261 (w), 1230 (w), 1097 (br,
m), 1031 (br, m), 854 (w), 800 (br, m), 727 (w), 665 (w).
1
shifted H NMR methyl signals in d6-benzene that differentiate
them from trivalent species such as 4, and exhibit 1H NMR
4,5-C2H2 signals that are indicative of similar Lewis acidity to
their GaX3 analogues. The first air stable NHC adduct of gallium
tribromide has been prepared (4) and used as a precursor to mono-
and dibromogallium hydride species 5 and 6. Several methods have
been employed in this regard, with the greatest success (as judged
by yield) achieved using triethylsilane (Scheme 4). Attempts to
generate low valent species such as 1 by thermal dehydrogenation
of [GaClH2(IMes)] and [GaCl2H(IMes)] led to decomposition and
isolation of 1,3-dimesityl-2-dihydroimidazole, and 1,3-dimesityl-
2-dihydroimidazole and 1,3-dimesityl-2-chloroimidazolium chlo-
ride, respectively. Preliminary NMR spectroscopic studies of the
photochemical and catalytic dehydrogenation of 5 and 6 under
milder conditions than those applied to [GaClH2(IMes)] and
[GaCl2H(IMes)] led to stable gallium compounds with the gen-
eration of dihydrogen in solution (photochemical studies) or the
loss of 4,5-C2H2 resonances (nickel catalysed studies). The latter
is consistent with nickel catalysed H/D exchange at the gallium
coordinated NHC. These preliminary studies show promise. Fur-
ther catalytic and photochemical studies of group 13 halohydrides
and their synthetic application are ongoing in our laboratory.
[{GaI2(IMes)}2] (2)
A solution of IMes (257 mg, 0.84 mmol) in toluene (40 mL) was
added dropwise to a stirred slurry of ‘GaI’ (0.84 mmol) in toluene
(20 mL) at -78 ◦C. The resultant orange mixture was allowed to
warm to ambient temperature over several hours, during which
the colour of the solution faded to pale yellow with a grey deposit.
Filtration, followed by solvent removal in vacuo, afforded a brown
powder. Washing with diethyl ether (30 mL) gave 2 as an off white
◦
powder (180 mg, 34%), m.p. 216–220 ◦C, dec. 300–320 C. H
1
NMR (300.30 MHz, C6D6); d 2.13 (s, 12 H, p-CH3), 2.19 (s, 24
1
H, o-CH3), 5.70 (s, 4 H, NCH), 6.75 (s, 8 H, m-C6H2). 13C{ H}
NMR (75.51 MHz, C6D6); d 18.1 (o-CH3), 21.2 (p-CH3), 124.4
(NCH), 129.3 (m-C6H2), 130.5 (p-C6H2), 135.0 (o-C6H2), 136.1
(ipso-C6H2), 139.8 (NCN). IR (Nujol) n/cm-1: 1608 (w), 1261 (w),
1223 (w), 1112 (w), 1032 (m, br), 929 (w), 857 (w), 800 (m, br), 761
(w), 727 (w, br).
Elemental analyses for compounds 1 and 2 were routinely high
in carbon and hydrogen due to decomposition in transit. Example
analysis for 1: Anal. Calc. for C42H48N4Ga2Cl4: C, 56.67; H, 5.44;
N, 6.29. Found: C, 57.85 (+1.18%); H, 6.06 (+0.60%); N, 5.90%.
Experimental
All manipulations were performed using conventional Schlenk
or glovebox techniques under an atmosphere of ultra high
purity argon in flame-dried glassware. Diethyl ether and toluene
were collected from an Innovative Technology MD-7 solvent
[{InCl2(IMes)}2] (3)
InCl2 (302 mg, 1.63 mmol) was added to a solution of IMes
(520 mg, 1.71 mmol) in toluene (30 mL) with stirring at room
950 | Dalton Trans., 2012, 41, 946–952
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temperature. The reaction mixture was stirred for 16 h, whereupon
a white precipitate was isolated by filtration and dried in vacuo to
afford 3 as an analytically pure white powder (620 mg, 64%),
m.p. 234–238 ◦C (dec.). 1H NMR (500.13 MHz, C6D6); d 2.11 (s,
24 H, o-CH3), 2.17 (s, 12 H, p-CH3), 5.76 (s, 4 H, NCH), 6.80
of a small quantity of Ga metal. The mixture was filtered whilst
hot and volatiles removed in vacuo to afford analytically pure 6 as
an off-white powder (480 mg, 71%). (ii) Triethylsilane (0.16 mL,
1.01 mmol) was added with stirring to a solution of 4 (626 mg, 1.02
mmol) in toluene (10 mL) at -20 ◦C. The reaction mixture was
allowed to warm to ambient temperature overnight and stirred
for a further 48 h. Drying in vacuo afforded impure 6. Washing
with diethyl ether (15 mL) followed by extraction into toluene (30
mL) afforded analytically pure 6 as a white powder after removal
1
(s, 8 H, m-C6H2). 13C{ H} NMR (100.61 MHz, C6D6, 273 K);
18.3 (o-CH3), 21.4 (p-CH3), 123.5 (NCH), 129.9 (m-C6H2), 130.2
(p-C6H2), 133.7 (o-C6H2), 135.1 (ipso-C6H2), 140.2 (NCN). IR
(Nujol) n/cm-1: 2277 (w, br), 1608 (w), 1541 (w), 1231 (m), 1109
(w), 1035 (w, br), 931 (w), 850 (m), 768 (m), 728 (w). Anal. Calc.
for C22H48Cl4In2N4: C, 51.46; H, 4.94; N, 5.72. Found: C, 51.51;
H, 5.11; N, 5.82%.
n
of volatiles in vacuo (400 mg, 75%). (iii) Butyllithium (0.10 mL,
1.6 M, 0.16 mmol) was added dropwise with vigorous stirring
to a solution of 4 (95 mg, 0.15 mmol) in toluene (40 mL) at
room temperature. The resultant mixture was heated to 70 ◦C
for 6 h with deposition of a grey solid. This occurred with light
effervescence. Volatiles were removed in vacuo to yield an off-
white powder containing 6 contaminated with 4 as evidenced by
1H NMR (28 mg, ◦29%, calculated as 6). Updated data for 6: m.p.
[GaBr3(IMes)] (4)
A solution of IMes (840 mg, 2.76 mmol) in diethyl ether (30
mL) was added dropwise with stirring to a solution of gallium
tribromide (844 mg, 2.73 mmol) in diethyl ether (30 mL) at -40 ◦C.
The reaction was allowed to warm to ambient temperature with
stirring overnight. An off white precipitate (880 mg, 53%) was
isolated by filtration. Recrystallisation (saturated toluene solution)
188 ◦C, dec. 286 C. H NMR (300.13 MHz, C6D6); d 2.03 (s,
1
12 H, o-CH3), 2.05 (s, 6 H, p-CH3), 5.30 (br s, 1 H, Ga-H), 5.88
1
(s, 2 H, NCH), 6.70 (s, 4 H, m-C6H2). 13C{ H} NMR (100.62
MHz, C6D6); 18.0 (o-CH3), 21.1 (p-CH3), 125.7 (NCH), 127.9 (m-
C6H2), 129.7 (p-C6H2), 135.2 (o-C6H2), 135.4 (ipso-C6H2), 140.5
(NCN). IR (Nujol) n/cm-1; 1925 (br, w, Ga-H). Anal. Calc. for
C21H25Br2GaN2: C, 47.15; H, 4.71; N, 5.24. Found: C, 47.59; H,
4.70; N, 5.36%.
at -25 ◦C afforded colourless square plates suitable for X-ray
◦
diffraction studies, m.p. 278 ◦C, dec. 327 C. H NMR (300.13
1
MHz, C6D6); d 2.02 (s, 12 H, o-CH3), 2.06 (s, 6 H, p-CH3), 5.79 (s,
1
2 H, NCH), 6.71 (s, 4 H, m-C6H2). 13C{ H} NMR (75.47 MHz,
C6D6); 18.2 (o-CH3), 21.1 (p-CH3), 124.4 (NCH), 128.9 (m-C6H2),
129.9 (p-C6H2), 133.1 (o-C6H2), 135.5 (ipso-C6H2), 140.9 (NCN).
IR (Nujol) n/cm-1; 3137 (w), 2727 (w), 2670 (br, w), 2411 (br, w),
1605 (m), 1230 (m), 1168 (br, w), 1128 (w), 1031 (br, m), 932 (m),
856 (sh, s), 722 (w), 703 (w). Anal. Calc. for C21H24Br3GaN2: C,
41.09; H, 3.94; N, 4.56. Found: C, 41.28; H, 3.93; N, 4.41%.
NMR irradiation experiments
General procedure for preparation of NMR samples
All photolysis experiments were conducted using screw cap 5 mm
NMR tubes and septa. NMR tubes were oven dried (120 ◦C)
followed by flame drying under vacuum to remove residual water
prior to use. d8-Toluene stock solutions of 5 and 6 were prepared
in a glovebox (1.0 mg mL-1). A solution (0.6 mL) containing the
hydride to be irradiated or catalytically dehydrogenated was added
to the NMR tube via syringe using an inert UHP argon manifold
followed by freeze-pump-thaw cycling.
[GaBrH2(IMes)] (5)
(i) A solution of 4 (258 mg, 0.42 mmol) in toluene (15 mL) was
added dropwise with stirring to a solution of [GaH3(IMes)] (320
mg, 0.85 mmol) in toluene (35 mL). The stirred reaction mixture
◦
was heated to 50 C for 36 h, filtered, and the volatiles removed
in vacuo to afford 5 as an off-white powder (260 mg, 45%); (ii)
Triethylsilane (0.40 mL, 2.53 mmol) was added with stirring to a
solution of 4 (775 mg, 1.26 mmol) in toluene (30 mL) at -20 ◦C.
The stirred reaction mixture was allowed to warm to ambient
temperature overnight. Drying in vacuo and washing with diethyl
ether (3 ¥ 5 mL) afforded analytically pure 5 as a white powder (165
mg, 86%), m.p. 194 ◦C, dec. 270 ◦C. 1H NMR (500.13 MHz, C6D6);
d 2.04 (s, 12 H, o-CH3), 2.05 (s, 6 H, p-CH3), 4.51 (br s, 2 H, Ga-H),
Procedure for photochemical studies32
1
Photolysis of the solution was monitored using H NMR spec-
troscopy on a Bruker DMX 600 instrument fitted with a 1H/31P/X
TBI probe. Irradiation was achieved using an Oriel 100 W
mercury-arc lamp, with light transmitted using a 2 m long, 1.5
mm diameter, silica core single fibre UV transmitting fibre optic
1
1
5.94 (s, 2 H, NCH), 6.71 (s, 4 H, m-C6H2). 13C{ H} NMR (75.47
cable. The H NMR spectrum of the sample was collected prior
to irradiation. During irradiation and equilibration spectra were
collected at 65 s intervals (32 scans). Samples were irradiated for
22 min (6 at 188 K), 17 min (5 at 273 K), 18 min (6 at 273 K)
after which the lamp was turned off and spectra were collected for
another 102, 33 or 29 min, respectively.
MHz, C6D6); 17.9 (o-CH3), 21.1 (p-CH3), 122.8 (NCH), 127.9 (m-
C6H2), 129.6 (p-C6H2), 134.4 (o-C6H2), 135.0 (ipso-C6H2), 140.0
(NCN). IR (Nujol) n/cm-1; 1874 (br, m, Ga-H). Anal. Calc. for
C21H26BrGaN2: C, 55.30; H, 5.75; N, 6.14. Found: C, 55.88; H,
5.92; N, 6.45%.
[GaBr2H(IMes)] (6)6
Photochemical control reaction – dissolved H2 in d8-toluene
(i) A solution of 4 (516 mg, 0.84 mmol) in toluene (12 mL) was
added dropwise, with stirring, to a room temperature solution
of [GaH3(IMes)] (160 mg, 0.42 mmol) in toluene (12 mL). The
reaction mixture was then heated to 50 ◦C for 36 h with deposition
d8-Toluene (0.6 cm3) was added to a 5 mm NMR tube. The solution
was degassed by freeze-pump-thaw cycling and H2 gas (500 cm3)
was bubbled through the solution over a period of five minutes.
1H NMR (600.13 Hz, 273 K); d 4.52 (s, 2 H, H2).
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Procedure for catalytic studies
2 (a) D. E. Hibbs, M. B. Hursthouse, C. Jones and N. A. Smithies, Chem.
Commun., 1998, 869; (b) R. J. Baker, R. D. Farley, C. Jones, M. Kloth
and D. M. Murphy, Chem. Commun., 2002, 1196; (c) N. Marion, E.
C. Escudero-Ada´n, J. Benet-Buchholz, E. D. Stevens, L. Fensterbank,
M. Malacria and S. P. Nolan, Organometallics, 2007, 26, 3256; (d) B.
Quillian, P. Wei, C. S. Wannere, P. v. R. Schleyer and G. H. Robinson,
J. Am. Chem. Soc., 2009, 131, 3168; (e) S. J. Bonyhady, D. Collis, G.
Frenking, N. Holzmann, C. Jones and A. Stasch, Nat. Chem., 2010, 2,
865.
Samples for catalytic dehydrogenation were prepared as follows; a
stock solution of [Ni(IMes)2] (50 mL, 0.5 mg mL-1, d8-toluene) was
added to an NMR tube containing a stock solution of 5 or 6 (500
mL, 1.0 mg mL-1, d8-toluene). 1H NMR spectra were recorded on
a Bruker Avance III 400 spectrometer (400.13 MHz).
3 C. D. Abernethy, M. L. Cole and C. Jones, Organometallics, 2000, 19,
4852.
X-ray structure determination
4 (a) S. G. Alexander, M. L. Cole, M. Hilder, J. C. Morris and J. B.
Patrick, Dalton Trans., 2008, 6361; (b) S. G. Alexander and M. L. Cole,
Eur. J. Inorg. Chem., 2008, 4493; (c) S. G. Alexander, M. L. Cole, C. M.
Forsyth, S. K. Furfari and K. Konstas, Dalton Trans., 2009, 2326.
5 S. G. Alexander, M. L. Cole, S. K. Furfari and M. Kloth, Dalton Trans.,
2009, 2909.
6 S. G. Alexander, M. L. Cole and C. M. Forsyth, Chem.–Eur. J., 2009,
15, 9201.
7 M. L. Cole, S. K. Furfari and M. Kloth, J. Organomet. Chem., 2009,
694, 2934.
8 (a) M. L. H. Green, P. Mountford, G. J. Smout and S. R. Speel,
Polyhedron, 1990, 9, 2763; (b) M. Wilkinson and I. J. Worral, J.
Organomet. Chem., 1975, 93, 39.
Crystalline samples of 1 and 4 were mounted on glass fibres in
silicone oil at -100(2) ◦C and -123(2) ◦C, respectively. A summary
of crystallographic data can be found in Table 1. Data were
collected using graphite monochromated Mo-Ka X-ray radiation
˚
(l = 0.71073 A) on a Bruker Apex II CCD diffractometer, and
data were corrected for absorption by empirical methods using
SADABS. Structural solution and refinement was carried out
using the SHELX33 suite of programs and the interface X-Seed.34
Hydrogen atoms were refined in calculated positions (riding
model).
9 R. J. Baker, H. Bettentrup and C. Jones, Eur. J. Inorg. Chem., 2003,
2446.
10 Unpublished data, M. L. Cole, M. Kloth, University of New South
Variata
Wales.
11 As determined by a survey of the Cambridge Structural Database v.
5.32 with updates for February 2011 and May 2011.
12 S. D. Nogai and H. Schmidbaur, Organometallics, 2004, 23, 5877.
13 M. L. Cole, C. Jones and M. Kloth, Inorg. Chem., 2005, 44, 4909.
14 A. J. Arduengo, J. R. Goerlich and W. J. Marshall, J. Am. Chem. Soc.,
1995, 117, 11027.
Data for compound 1 were poor. Compound 1 undergoes a
destructive phase change at -123(2) ◦C leading to data collection
at the higher temperature of -100(2) ◦C. Unlike the IMes ligand
coordinated to Ga(1), the IMes ligand coordinated to Ga(2) (N(3),
N(4), C(22)–C(42)) exhibits a significant number of prolate and
oblate atoms across the two aryl rings and imidazole heterocycle.
Attempts to model two separate occupancies for (i) each ring sep-
arately, (ii) rings in combination, or (iii) together as two separate
IMes occupancies, resulted in unacceptable thermal parameters
(non positive definite). Only one occupancy has been refined for
the second IMes with the exception of meta-carbon C(27) and
ortho-methyl C(31), which were successfully modelled over two
sites with occupancies of 83 : 17% (a:b) (highest occupancy shown
in Fig. 1). The remaining suspected ‘disordered’ atoms (C(22)–
C(26), C(28)–C(30), C(32), C(33), N(1), N(2)) have been refined
using ISOR 0.05 restraints (isotropic displacement parameters).
15 M. L. Cole, C. Jones and P. C. Junk, New J. Chem., 2002, 262, 1296.
16 (a) C. U. Doriat, M. Friesen, E. Baum, A. Ecker and H. Schno¨ckel,
H., Angew. Chem., Int. Ed. Engl., 1997, 36, 1969; (b) A. Schnepf, E.
Weckert, G. Linti and H. Schno¨ckel, H., Angew. Chem., Int. Ed., 1999,
38, 3381; (c) H. Schno¨ckel and A. Schnepf, Adv. Organomet. Chem.,
2001, 47, 235.
17 M. L. Cole, A. J. Davies and C. Jones, J. Chem. Soc., Dalton Trans.,
2001, 2451.
18 N. N. Greenwood and A. Storr, J. Chem. Soc., 1965, 3426.
19 H. Schmidbaur, W. Findeiss and E. Gast, Angew. Chem., 1965, 77, 170.
20 J. F. Janik, R. A. Baldwin, R. L. Wells, W. T. Pennington, G. L. Schimek,
A. L. Rheingold and L. M. Liable-Sands, Organometallics, 1996, 15,
5385.
21 F. Dornhaus, S. Scholz, I. Sanger, M. Bolte, M. Wagner and H.-W.
Lerner, Z. Anorg. Allg. Chem., 2009, 635, 2263.
22 Q. M. Cheng, O. Stark, K. Merz, M. Winter and R. A. Fischer, J.
Chem. Soc., Dalton Trans., 2002, 2933.
23 B. Luo, V. G. Young and W. L. Gladfelter, Chem. Commun., 1999, 123.
24 R. J. Wehmschulte, J. J. Ellison, K. Ruhlandt-Senge and P. P. Power,
Inorg. Chem., 1994, 33, 6300.
25 L. Grocholl, S. A. Cullison, J. Wang, D. C. Swenson and E. G. Gillan,
Inorg. Chem., 2002, 41, 2920.
26 R. J. Keaton, J. M. Blacquiere and R. T. Baker, J. Am. Chem. Soc.,
2007, 129, 1844.
27 H.-J. Himmel, P. Roquette, H. Wadepohl, S. Leingang, O. Ciobanu, F.
Allouti and M. Enders, Eur. J. Inorg. Chem., 2008, 5482.
28 A. J. Arduengo, S. F. Gamper, J. C. Calabrese and F. Davidson, J. Am.
Chem. Soc., 1994, 116, 4391.
-3
˚
The largest residual hole and peak of electron density (-2.18 e A
-3
˚
˚
˚
and 1.33 e A ) are located 0.17 A and 0.70 A from meta-carbon
C(38). Attempts to model disorder of this atom failed repeatedly.
Compound 4 crystallises with two unique molecules in the
asymmetric unit. No additional space group symmetry elements
were identified. The bonding parameters for both molecules are
comparable. Parameters for the lowest numbered molecule have
been used in the discussion throughout this article and this
molecule has been used in Fig. 2.
29 M. K. Denk and J. M. Rodezno, J. Organomet. Chem., 2000, 608,
122.
Acknowledgements
30 M. V. Baker, B. W. Skelton, A. H. White and C. C. Williams,
Organometallics, 2002, 21, 2674.
31 A. J. Arduengo, H. V. R. Dias, R. L. Harlow and M. Kline, J. Am.
Chem. Soc., 1992, 114, 5530.
32 For further information on this technique see: J. A. Calladine, O. Torres,
M. Anstey, G. E. Ball, R. G. Bergman, J. Curley, S. B. Duckett, M. W.
George, A. I. Gilson, D. J. Lawes, R. N. Perutz, X.-Z. Sun and K. P. C.
Vollhardt, Chem. Sci., 2010, 1, 622 and references therein.
33 G. M. Sheldrick, SHELXL-97 and XS-97, University of Go¨ttingen,
Germany, 1997.
The authors would like to thank the Australian Research Council
(DP110104759 and DP0881692) for financial support of this
research, and the Australian Commonwealth for the provision
of an Australian Postgraduate Award (AIM).
References
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34 L. J. Barbour, J. Supramol. Chem., 2001, 1, 189.
952 | Dalton Trans., 2012, 41, 946–952
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The Royal Society of Chemistry 2012
©