Hydride-bromide exchange at an NHC - A new route to brominated alanes and gallanes

Article abstract of DOI:10.1039/b821367d

The reaction of [MH₃(Quinuclidine)] (M = Al or Ga) with an air stable dibrominated N-heterocyclic carbene (NHC) affords the hydride-bromide exchange product [MBr₂H(NHC)].

Full text of DOI:10.1039/b821367d

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Number 16 | 28 April 2009 | Pages 2885–3112
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HOT ARTICLE
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Hambley et al.
Cole et al.
HOT ARTICLE
Investigations using fluorescent
ligands to monitor platinum(iv)
Hydride–bromide exchange at an
NHC—a new route to brominated
alanes and gallanes
Pombeiro et al.
Single-pot template transformations of reduction and platinum(ii) reactions
cyanopyridines on a PdII centre
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Hydride–bromide exchange at an NHC—a new route to brominated
alanes and gallanes†
Sean G. Alexander,b Marcus L. Cole,*a Samantha K. Furfaria and Marc Klothb
Received 28th November 2008, Accepted 11th February 2009
First published as an Advance Article on the web 18th February 2009
DOI: 10.1039/b821367d
The reaction of [MH3(Quinuclidine)] (M = Al or Ga)
with an air stable dibrominated N-heterocyclic carbene
(NHC) affords the hydride–bromide exchange product
[MBr2H(NHC)].
Himmel.15 Typical access routes to these species are the stoichio-
metric redistribution of gallane Lewis base adducts with gallium
trichloride species,16 and the treatment of gallium trichloride with
trialkylsilanes followed by Lewis base coordination.17 To date,
these routes have not been successfully applied to aluminium
compounds.9
The hydrides of the group 13 elements have a multitude of
applications that span synthetic chemistry.1 In 1991 Arduengo
and co-workers reported the first main group metal complex co-
ordinated by an N-heterocyclic carbene (NHC), the alane (AlH3)
[AlH3(IMes)] (1) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-
2-ylidene) (Fig. 1).2 This compound is remarkable for its thermal
stability, which exceeds that of all◦ other complexes of alane,
Herein we report the attempted preparations of [AlH3(IMesBr)]
(3) and [GaH3(IMesBr)] (4) by ligand substitution of quinuclidine
(Quin) with the air-stable 4,5-dibrominated derivative of IMes;
IMesBr18 (Scheme 1). Contrary to expectation, both reactions
afford the dibromometallane IMes complexes [MBr2H(IMes)]
(M = Al; 5, M = Ga; 6) either directly (5) or after heating of
4 to afford 6. The putative hydride–bromide exchange process that
yields 5 and 6 has been studied using trideuteride precursors, e.g.
[GaD3(Quin)], to establish the source of the 4,5-dihydrogens in the
products, and the conversion of 4 to 6 has also been examined by
◦
e.g. [AlH3(Quinuclidine)] (dec. 160 C;3 1 m.p. 246–7 C,2 dec.
◦
257 C).4 In 2000, Jones et al. reported the gallane (GaH3) and
indane (InH3) analogues of 1, completing the most thermally
stable series of group 13 trihydride homologues reported to date
1
◦
(M = Ga; dec. 214 ◦C, M = In; dec. 115 C) (Fig. 1).5
variable temperature H NMR. The molecular structure of 5 is
also reported.
The unsurpassed stability of these metallanes (MH3) has been
attributed to the high nucleophilicity of the NHC6 and the
steric protection afforded to the MH3 moiety by the N-mesityl
substituents.2,5,7 This ‘shielding’ is thought to stifle M–H–M
bridge formation, which expedites M–H bond decomposition by
reductive dehydrogenation.8 We have an interest in halogenated
variants of 1,9 e.g. [AlCl2H(IMes)] (2) (Fig. 1),10 as these possess
considerable air stability and exhibit reactivities distinct to those
of trihydride counterparts. For example, compound 2 reduces a-
halogenated ketone functionalities to alcohols without dehalo-
genation and may be used to regioselectively ring-open oxiranes.4
By contrast, compounds such as 1 and [AlH3(Quin)]11 exhibit little
if any selectivity in the same processes.
Scheme 1 Reagents and conditions (i) [MH3(Quin)], -Quin, M = Al,
toluene–THF; M = Ga, Et2O; (ii) toluene, 50 ◦C, 5 h.
The treatment of [AlH3(Quin)] and [GaH3(Quin)] with an
equimolar quantity of IMesBr18 in toluene–THF or diethyl ether
respectively affords the colourless compounds 5 and 4,‡ which
absorb strongly in the metal–hydride stretching region of their
respective IR spectra (Table 1).
The Ga–H stretching absorption of 4 is some 30 wavenumbers
higher than that of [GaH3(IMes)] (Table 1).5 This is consistent
with a reduction in NHC nucleophilicity for IMesBr relative
to IMes due to s-electron withdrawal at the 4,5-heterocycle
Fig. 1 Solid-state decomposition temperatures for IMes complexes of
group 13 trihydrides and dichloroalane.
Table 1 1H NMR (C6D6) and IR (Nujol mull) data for the compounds
Routes to haloalanes like 2 are scarce relative to chlorogallanes.9
The latter have been studied in some detail by Nogai and
Schmidbaur,12 Raston,13 Gladfelter14 and more recently Down and
discussed herein.
1H NMR
1H NMR
IR M–H
Compound
4,5-C2H2 d (ppm) MHn d (ppm) stretch n/cm-1
aSchool of Chemistry, University of New South Wales, Sydney, NSW 2052,
Australia. E-mail: m.cole@unsw.edu.au; Fax: +61 2 93856141; Tel: +61 2
93854678
[AlH3(IMes)], 1
6.014
6.16
3.804
3.96
—
3.89
—
17432
1780
1851
1807
1888
1880
[GaH3(IMes)]5
[AlCl2H(IMes)], 210 5.86
[GaH3(IMesBr)], 4 n/a
[AlBr2H(IMes)], 5 5.86
[GaBr2H(IMes)], 6 5.82
bSchool of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005,
Australia
† CCDC reference number 711718 for 5. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b821367d
4.50
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positions, thereby suggesting the composition [GaH3(IMesBr)] for
4. A related trend in metal–hydride stretching absorptions is not
observed for 5 relative to 1 (Table 1). Compound 5 exhibits a strong
IR absorption 140 wavenumbers higher than the Al–H stretching
absorption of 1. Indeed, the absorption of 5 is of comparable
energy to that of the dichloroalane complex 2 (Table 1),10 in which
the Al–H bond is inductively strengthened by chloride ligands.
Thus, 5 is most likely a bromoalane (AlBrnH3-n, n = 1 or 2).
Further evidence for the composition of 5 may be found in its 1H
NMR spectrum (C6D6), which clearly exhibits a singlet resonance
at 5.86 ppm (Table 1) integrating in a 1:2 ratio with the m-aryl
In contrast to the spontaneous hydride–bromide exchange that
generates 5, the characterisation of 4 by infrared and H NMR
1
spectroscopy (C6D6) (see Table 1) indicates preparation of the an-
ticipated 4,5-bromoimidazol-2-ylidene adduct [GaH3(IMesBr)].
Solvent dependent, analytically pure, samples of 4 were acquired
upon recrystallisation of reaction mixtures from diethyl ether.
Further to the strengthening of the Ga–H bond evidenced by
IR spectroscopy,‡ the electronic impact of 4,5-dibromination of
1
the NHC of 4 can be gauged by the upfield shift of its H GaH3
and 13C 4,5-C2 resonances relative to those of [GaH3(IMes)] (see
Table 1 for 1H NMR GaH3 resonance, 4,5-C2 13C NMR resonance
4; 110.0 ppm, [GaH3(IMes)]; 122.4 ppm).5 Compound 4 is less
thermally stable than its IMes relative (152 ◦C dec. and 214 ◦C
dec. respectively).5
1
hydrogen resonance of 5.‡ A comparison of the H NMR data
for compounds 1, 2 and 5 indicates that this resonance can be
assigned to a heterocycle 4,5-C2H2 unit, which is inconsistent with
4,5-bromination of the NHC in 5. One can therefore conclude
that the reaction of IMesBr with [AlH3(Quin)] (Scheme 1) results
in (i) bromination of the alane to afford a bromoalane, and
(ii) 4,5-dihydrogen substitution of IMesBr to generate an IMes
ligand. It is noteworthy that fast deuterium labelling of the 4,5-
positions of ‘free’ imidazol-2-ylidenes in moderately acidic deutero
solvents is an established pathway.19 However, in the present case
it is unlikely that bromane (Br+)/H+ exchange occurs to give 5
as the NHC most likely coordinates (cf. 4) prior to exchange,
and the present chemistry is not conducive to the formation of
H+. The molecular structure of 5§ (Fig. 2, POV-RAY illustration,
50% thermal ellipsoids) confirms that the product of this reaction
(Scheme 1) is [AlBr2H(IMes)].
Inspired by the spontaneous formation of 5, the hydride–
◦
bromide exchange of 4 was attempted by heating (50 C) of 4 in
toluene over a 5 h period.‡ This resulted in quantitative conversion
to a bromogallane, 6, as evidenced by a single high wavenumber
Ga–H stretching absorption in the IR spectra of vacuum dried
reaction mixtures (Table 1; ca. 80 cm-1 greater than that of 4), and
the observation of a single set of IMes (cf. not IMesBr) resonances
1
in the H NMR spectrum of 6, that are positioned analogously
to those of the IMes of compound 5 (Table 1), in addition to a
GaH resonance (4.50 ppm) shifted downfield relative to that of
4 (3.89 ppm). Microanalytical data for 6 are consistent with the
composition [GaBr2H(IMes)].‡
To gain insight into the hydride–bromide exchange process that
affords 5 and 6, deuteride analogues of 4–6 were prepared using
[MD3(Quin)] precursors, affording 5D (M = Al), 4D and 6D
(M = Ga) (Scheme 2), the latter after heating of 4D in H8-toluene
as per 4 above.
Fig. 2 Molecular structure of5.§ All hydrogen atoms excepting H(1) omit-
◦
˚
ted for clarity. Selected bond lengths (A) and angles ( ): Al(1)–H(1) 2.07(3),
Al(1)–Br(1) 2.3164(13), Al(1)–Br(2) 2.2973(13), Al(1)–C(1) 2.028(4),
N(1)–C(1)–N(2) 104.2(3), Br(1)–Al(1)–Br(2) 110.01(5), C(1)–Al(1)–Br(1)
107.93(12), C(1)–Al(1)–Br(2) 107.15(12).
Scheme 2 Preparation of deuterated analogues of compounds 4–6.
The ‘Al(IMes)’ subunit of 5 is statistically identical to that
of its trihydride congener 1.2 The Al–C bonding contact and
The IR and 1H NMR spectra of 4D are identical to those of 4
with the exception of a broad IR absorption at 1286 cm-1, that may
be attributable to the Ga–D stretches of 4D (calculated Ga–D n
1292 cm-1). Spectroscopic characterisation of 5D and 6D indicate
deuteride–bromide exchange by the absence of the expected
IMes 1H NMR 4,5-C2H2 and MHn resonances. Furthermore,
the IR spectra of both compounds exhibit broad metal–deuteride
absorptions at 1341 cm-1 (5D) and 1340 cm-1 (6D) (calculated
◦
˚
endocyclic NCN angle of 5 (2.028(4) A and 104.2(3) ) cannot
◦
2
˚
be distinguished from those of 1 (2.034(3) A and 104.2(2) ).
It is therefore probable that the increased Lewis acidity of the
dibromoalane unit of 5, which one would expect to shorten the
4
˚
Al–C contact relative to that of 1 (cf. 2; mean Al–C 2.01 A),
is counterbalanced by the spatial bulk of the bromide ligands
vis-a`-vis hydrides. The hydride ligand of 5 was located and
1
refined isotropically.§ The contact distance of 2.07(3) A, although
M–D n 1359 and 1348 cm-1 respectively) and their 13C{ H} NMR
˚
unreliably determined by XRD methods, is within the range
spectra (C6D6) exhibit 13C-D 1JCD couplings of magnitude 25.6 Hz
and 28.2 Hz for the heterocyclic 4,5-C2 resonances.
To further probe the hydride–bromide exchange process, 4 was
studied by variable temperature (VT) 1H NMR spectroscopy
(C6D6). The onset of exchange was observed at 45 ◦C, and
conversion to 6 was complete after 20 minutes at this temperature;
unfortunately no long-lived intermediates were observed during
of Al–H bonding contacts established by X-ray methods20 but
2
˚
extended relative to the bonds of 1 (mean 1.54 A). The Al–Br
˚
˚
distances of 5 (2.3164(13) A Br(1) and 2.2973(13) A Br(2)) are
similar to those of four-coordinate dibromoaluminiumaryls such
i
as [AlBr2(2,4,6- Pr3C6H2)(OEt2)] (2.311(6) A),21 reported by Power
˚
and co-workers.
2910 | Dalton Trans., 2009, 2909–2911
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this study. This data, in combination with the deuteride studies
above, conclusively identifies the metal trihydride as the source
of the 4,5-hydrogens of 5 and 6 rather than the solvent medium
employed.
§ Crystal data for 5: C21H25AlBr2N2, M = 492.23, orthorhombic, Pbca (No.
3
˚
61), a = 14.1065(2), b = 16.5232(3), c = 19.4059(3) ;, V = 4523.21(13) A ,
Z = 8, Dc = 1.446 g cm-3, F000 = 1984, m = 3.630 mm-1, 2qmax = 59.14◦,
46827 reflections collected, 6173 unique (Rint = 0.1178). Final GooF =
1.021, R1 = 0.0537, wR2 = 0.1403, R indices based on 3642 reflections
with I > 2s(I) (refinement on F2), 245 parameters, 0 restraints. Data were
collected at 123(2) K on an Enraf-Nonius Kappa CCD diffractometer.
In conclusion, we have developed a new high yielding synthetic
route to dibrominated group 13 metallanes coordinated by imi-
dazole NHCs. Preliminary studies in our laboratory suggest this
is a general reaction pathway for 4,5-dibrominated imidazol-2-
ylidenes.4 These studies and those concerning the synthetic utility
of haloalanes and -gallanes are ongoing in our laboratory.
The authors would like to thank Dr Craig M. Forsyth (Monash
University) for the collection of single crystal X-ray diffraction
data for 5, the Australian Research Council (DP0558562) for finan-
cial support of this research and the Australian Commonwealth
Government for an Australian Postgraduate Award (SGA).
1 (a) Chemistry of Aluminium, Gallium, Indium and Thallium, ed. A. J.
Downs, 1993, Blackie, Glasgow, UK; (b) C. Jones, Chem. Commun.,
2001, 2293–2298; (c) J. A. Jegier and W. L. Gladfelter, Coord. Chem.
Rev., 2000, 206–207, 631–650; (d) M. G. Gardiner and C. L. Raston,
Coord. Chem. Rev., 1997, 166, 1–34.
2 A. J. Arduengo, H. V. R. Dias, J. C. Calabrese and F. Davidson, J. Am.
Chem. Soc., 1992, 114, 9724–9725.
3 N. N. Greenwood and B. S. Thomas, J. Chem. Soc. A, 1971, 814–817.
4 S. G. Alexander, S. K. Furfari and M. L. Cole, Universities of New
South Wales and Adelaide, unpublished results.
5 C. D. Abernethy, M. L. Cole and C. Jones, Organometallics, 2000, 19,
4852–4857.
6 (a) D. Bourissou, O. Guerret, F. P. Gabba¨ı and G. Bertrand, Chem. Rev,
2000, 100, 39–91; (b) F. E. Hahn and M. C. Jahnke, Angew. Chem. Int.
Ed., 2008, 47, 3122–3172.
Notes and references
7 A related complex to 1, [AlH3(IDip)] (IDip
=
1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene), has been prepared by Jones and
found to have a lesser thermal stability; dec. 229 ◦C: R. J. Baker, A.
J. Davies, C. Jones and M. Kloth, J. Organomet. Chem., 2002, 656,
203–210.
‡ Compound 4: A solution of IMesBr (0.33 g, 0.71 mmol) in diethyl ether
(7 mL) was added to a solution of [GaH3(Quin)] (0.13 g, 0.71 mmol) in
diethyl ether (8 mL) at room temperature and stirred for 2 days. Filtration
followed by concentration to the point of crystallisation resulted in the
isolation of 4 as solvent dependent colourless plates after placement at
-5 ◦C overnight (0.32 g, 84%), dec. 152 ◦C. Elemental analysis calculated
(%) for C21H25N2GaBr2 (vacuum dried sample): C 47.15, H 4.71, N 5.24;
Found: C 47.08, H 4.80, N 5.27; 1H NMR (300 MHz, C6D6, 300 K):
d 2.02 (s, 12H, o-CH3), 2.05 (s, 6H, p-CH3), 3.89 (br s, 3H, GaH3),
8 A. J. Downs and C. R. Pulham, Chem. Soc. Rev., 1994, 23, 175–84.
9 S. G. Alexander and M. L. Cole, Eur. J. Inorg. Chem., 2008, 4493–4506.
10 S. G. Alexander, M. L. Cole, M. Hilder, J. C. Morris and J. B. Patrick,
Dalton Trans., 2008, 6361–6363.
11 C. L. Raston, A. F. H. Siu, C. J. Tranter and D. J. Young, Tetrahedron
Lett., 1994, 35, 5915–5918.
6.73 (s, 4H, m-ArCH); 13C{ H} NMR (75.5 MHz, C6D6, 300 K): d 18.3
1
12 see for example: S. D. Nogai and H. Schmidbaur, Organometallics,
2004, 23, 5877–5880.
(o-CH3), 21.5 (p-CH3), 110.0 (C2Br2), 130.0 (m-ArCH), 134.1 (p-ArC),
135.8 (o-ArC), 140.7 (ipso-ArC), 183.4 (NCN); IR (Nujol) v/cm-1: 1807 (s
br, Ga-H). Compound 5: A THF solution (20 mL) of IMesBr (1.25 g,
2.7 mmol) was added dropwise to a stirred solution of [AlH3(Quin)]
(0.38 g, 2.7 mmol) in toluene (100 mL) at room temperature. The pale
orange reaction mixture was stirred for 12 h and volatiles were removed in
vacuo. Washing with hexane (10 mL) and extraction into toluene (50 mL)
afforded 5 as colourless prisms after placement at -30 ◦C for several days
(0.89 g, 67%), m.p. 272 ◦C (dec.). Elemental analysis calculated (%) for
C21H25AlBr2N2: C 51.24, H 5.12, N 5.69; Found: C 51.92, H 5.09, N 5.67;
1H NMR (300 MHz, C6D6, 300 K): d 2.03 (s, 12H, o-CH3), 2.05 (s, 6H,
p-CH3), 5.86 (s, 2H, 4,5-C2H2), 6.71 (s, 4H, m-ArCH); IR (Nujol), cm-1:
1888 (s br, Al-H). Compound 6: A toluene (30 mL) solution of 4 (0.11 g,
0.204 mmol) was heated to 50 ◦C for 5 h to afford a deep yellow solution
that was filtered and dried under reduced pressure to afford compound
6 as an off-white powder (0.10 g, 90%), dec. 278 ◦C. Elemental analysis
calculated (%) for C21H25N2GaBr2: C 47.15, H 4.71, N 5.24; Found: C
47.27, H 4.57, N 5.61; 1H NMR (300.13 MHz, C6D6); d 2.02 (s, 12H,
o-CH3), 2.06 (s, 6H, p-CH3), 4.50 (br s, 1H, GaBr2H), 5.82 (s, 2H, 4,5-
C2H2), 6.73 (s, 4H, m-ArCH); IR (Nujol) v/cm-1: 1880 (s br, Ga-H).
13 F. M. Elms, G. A. Koutsantonis and C. L. Raston, Chem. Commun.,
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19 M. K. Denk and J. M. Rodezno, J. Organomet. Chem., 2000, 608,
122–125.
20 As determined by a survey of the Cambridge Structural Database
(CSD), version 5.29, 2008.
21 M. A. Petrie, P. P. Power, H. V. R. Dias, K. Ruhlandt-Senge, K. M.
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 2909–2911 | 2911
©