Imidazolium formation from the reaction of N-heterocyclic carbene stabilised group 13 trihydride complexes with organic acids

Article abstract of DOI:10.1016/j.ica.2004.07.036

Stoichiometric reaction of NHC stabilised aluminium trihydride species with 3.0 equivalents of phenol generates imidazolium tetraphenoxyaluminate species. Reaction of [InH₃(IMes)] (IMes = CN(2,4,6-Me₃C ₆H₂)C

Full text of DOI:10.1016/j.ica.2004.07.036

Inorganica Chimica Acta 358 (2005) 102–108
www.elsevier.com/locate/ica
Imidazolium formation from the reaction of N-heterocyclic
carbene stabilised group 13 trihydride complexes with organic acids
a,b,
Marcus L. Cole
Peter C. Junk , Neil A. Smithies
a
*, David E. Hibbs a,1, Cameron Jones ,
c
a
a
School of Chemistry, Cardiff University, P.O. Box 912, Park Place, Cardiff, CF10 3TB, UK
Department of Chemistry, University of Adelaide, Adelaide, South Australia 5005, Australia
b
c
School of Chemistry, Monash University, Victoria 3800, Australia
Received 10 June 2004; accepted 24 July 2004
Available online 26 August 2004
Abstract
The reactions of the N-heterocyclic carbene (NHC) stabilised group 13 trihydride complexes [AlH3(IMeMe)] (1)
(IMeMe = 1,3,4,5-tetramethylimidazol-2-ylidene), [AlH3(IiPrMe)] (2) (IiPrMe = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene)
with three molar equivalents of phenol, and [InH3(IMes)] (3) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) with one
molar equivalent of 1,1,1,5,5,5-hexafluoropentan-2,4-dione (F6acacH) are presented. These render the imidazolium tetraphenoxya-
luminate species; [IMeMe Æ H][Al(OPh)4] (4) and [IiPrMe Æ H][Al(OPh)4] (5), and 1,3-bis(2,4,6-trimethylphenyl)imidazolium
1,1,1,5,5,5-hexafluoropentan-2,4-dionate; [IMes Æ H][CH{C(O)CF3}2] (6), the latter leading to metallohydride decomposition. The
molecular structures of 4 and 6 are described.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Carbene complexes; Group 13 hydrides; Hydrogen bonding; Imidazolium salts; Organic acids
1. Introduction
[TlCl3(IMes)] [10], which exhibits a divalent carbon
coordinated to a highly oxidative thallium(III) metal
centre [11].
Recently we have systematically studied the prepara-
tion, stability and synthetic utility of N-heterocyclic car-
bene (NHC) adducted group 13 trihydrides [1–7] and
trihalides [8–10]. Our success in this area is exemplified
by the thermally stable indium trihydride complex
[InH3(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imi-
dazole-2-ylidene), 3 [3], (decomposition in the solid-state
115 ꢁC, cf. [InH3(IiPrMe)] decomposition; ꢀ5 ꢁC [1],
where IiPrMe = 1,3-diisopropyl-4,5-dimethylimidazol-
2-ylidene) and the thallium trihalide adduct
Two logical steps in exploring the chemistry of the tri-
hydride derivatives have been their application toward
the preparation of group 13–16 [M(ER)3L] species
(M = Al, Ga, In; E = group 16 element; L = Lewis base)
[2,12], which have potential as set-stoichiometry precur-
sors to III/VI semiconductor materials [13], and the
study of their hydrometallation chemistry when treated
with unsaturated organic substrates [6,14]. Herein we
present two examples of these approaches that, contrary
to the anticipated outcomes, yield imidazolium species.
These reactions suggest group 13 coordinated N-hetero-
cyclic carbenes (NHCs) remain available for secondary
acid–base reactions and hence, unlike their amine and
phosphine congeners [15,16], may be flawed in terms
of their synthetic utility.
*
Corresponding authors. Tel.: +61 8 8303 5497; fax: +61 8 8303
4358.
E-mail address: marcus.cole@adelaide.edu.au (M.L. Cole).
1
Present address: Faculty of Pharmacy, University of Sydney,
Sydney, New South Wales 2006, Australia.
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.07.036

M.L. Cole et al. / Inorganica Chimica Acta 358 (2005) 102–108
103
Table 1
Summary of crystal data and refinement parameters for compounds 4
and 6
2. Results and discussion
2.1. Reaction of [AlH3(NHC)] species with phenol
[IMeMe Æ H]
[Al(OPh)4] (4)
[IMes Æ H]
[CH{C(O)CF3}2] (6)
The established preparation of compounds of type
[M(ER3)L] via the reaction of a parent trihydride with
1.5 equivalents of diaryl- or alkyldichalcogenide [2,12],
e.g., PhSeSePh, is undesirable for systems where
E = oxygen due to the requirement for hazardous organ-
ic peroxides. The alternative reaction of NHC stabilised
aluminium trihydride species with three equivalents of
phenol potentially offers a preferable path to these
compounds.
Molecular formula
Molecular weight
Temperature (K)
Crystal system
C
31H33N2O4Al
C26H26F6N2O2
512.49
150(2)
monoclinic
P21/n
524.57
150(2)
triclinic
ꢀ
Space group
˚
P1
a (A)
˚
10.397(8)
10.574(9)
14.432(11)
108.43(6)
107.47(6)
94.13(5)
1411.2(19)
2
14.801(3)
11.687(2)
16.161(3)
90
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
115.78(3)
90
Contrary to the expected [Al(OPh)3(NHC)] products,
low temperature (ꢀ50 ꢁC) treatment of tetrahydrofuran
solutions of [AlH3(IMeMe)] (1) (IMeMe = 1,3,4,5-tetra-
methylimidazol-2-ylidene) or [AlH3(IiPrMe)] (2) with
three equivalents of phenol led to the isolation of colour-
less solids characterising as the imidazolium tetraphen-
oxyaluminate salts; [IMeMe Æ H][Al(OPh)4] (4) and
[IiPrMe Æ H][Al(OPh)4] (5) (Scheme 1). The 1H NMR
spectra of 4 and 5 (CDCl3) display the expected methyl
and, for the latter, isopropyl resonances in addition to
broad singlet resonances integrating for one proton at
8.79 and 8.88 ppm, respectively. These are in the normal
region for 1,3-disubstituted imidazolium 2-position pro-
tons, those of the related species [IMeMe Æ H][InBr4] [9]
and [IiPrMe Æ H][InCl4(IiPrMe)] [8] occurring at 8.30
and 9.54 ppm, respectively. The 13C NMR spectra of
both compounds are as expected with the protonated
carbene centres resonating at 134.1 ppm (4) and 132.3
ppm (5). This is considerably upfield from the 2-position
carbon resonances of the parent carbenes (212.7 and
205.9, respectively, in C6D6 [17], resonances for InX4
species listed above: 133.4 and 131.8 ppm, respectively)
[8,9]. Lastly, the base peaks in the positive ion APCI
mass spectra of both compounds correspond to the imid-
azolium cations (4: 125 m/z, 5: 181 m/z).
3
˚
Volume (A )
2517.3(8)
4
1.352
Z
Dc (g cmꢀ3
)
1.235
0.934
l (mmꢀ1
)
0.116
4527
Reflections collected
Unique reflections
Parameters varied
R (all data)
4265
4265
4527
457
347
0.1640
0.0606
0.1799
0.1479
0.2152
0.0533
0.1210
0.0905
R(I > 2r(I))
wR0 (all data)
wR0(I > 2r(I))
Unlike compound 5, which proved insoluble in ethe-
real solvents, recrystallisation of compound 4 from fresh
THF gave yellow prismatic crystals suitable for X-ray
structure determination (see Table 1). The molecular
ꢀ
structure of 4, which crystallises in the space group P1
with one ion pair in the asymmetric unit, is depicted in
Fig. 1 (see caption for selected bond lengths and angles).
Compound 4 contains a rare example of the tetraaryl-
oxyaluminate anion, 2 and indeed the first that is devoid
of supplementary donors or alkyl groups appended to
the aryl ring. Moreover, the configuration of the aryl
rings about the ‘‘AlO4’’ tetrahedral centre of 4 is akin
to that of the only structurally characterised
[Al(OAr)4]ꢀ compound (Ar = phenyl ring without sup-
plementary
donors);
[Li(THF)4][Al{O(2,4-
R2
R2
ButC6H3)}4][18]. The 1,3,4,5-tetramethylimidazolium
cation of 4 has intra-ring parameters consistent with
those of the related imidazolium salts; [IMeMe Æ -
N
N
R1
R1
H][InBr4] [9] and [IiPrMe Æ H][Br], the N(1)–C(1)–
3
1-3
(ii)
(i)
M
N(2) angle being 107.8(4)ꢁ ([IMeMe Æ H][InBr4];
108.1(8)ꢁ, [IiPrMe Æ H][Br]; 109.6(8)). This angle is ꢀopenꢁ
relative to that of the parent carbene (IMeMe; 101.5(1)ꢁ)
[17] indicating a significant enhancement in electronic
H
H
H
R2
R2
R2
R2
CF
3
OPh
Al
O
O
N
N
N
N
R1
R1 PhO
R1
R1
OPh
OPh
2
H
CF
3
H
The Cambridge Crystallographic Structural Database (CSD
version 2.5 with updates for Jan and Apr 2004) lists 16 examples of
an aluminium centre bound by four aryloxides, of which most
incorporate either calix-[4]-arene, binaphthol or Schiff base derived
phenolates.
4 and 5
6
Scheme 1. Reagents and conditions: (i) M = Al, 3.0 eq. ethereal
phenol, THF, ꢀ50 ꢁC, ꢀ3/2 H2(g), ꢀ0.25 [AlH3(NHC)], 4; R1 = Me,
R2 = Me, 5; R1 = iPr, R2 = Me; (ii) M = In, R1 = 2,4,6-Me3C6H2,
R2 = H, 1.0 eq. F6acacH, THF, ꢀ50 ꢁC, ꢀ3/2 H2(g), ꢀIn(s).
3
The single X-ray structure determination of [IiPrMe Æ H][Br] is
included as supplementary material for this article.

104
M.L. Cole et al. / Inorganica Chimica Acta 358 (2005) 102–108
confirmed by FTIR spectra of the dried reaction mix-
tures, which both exhibit Al–H stretching modes attrib-
utable to the parent trihydride (FTIR Al–H stretch 1; br
s 1731 cmꢀ1 [24], 2; br s 1730 cmꢀ1 [1]). As pronounced
steric crowding about the aluminium centre of the in-
tended [Al(OPh)3(NHC)] product is unlikely, the gener-
ation of 4 and 5 suggests both imidazolium cations
possess pKas above that of phenol (9.98) [25]. Further-
more, it is noteworthy that there are no Lewis base ad-
5
ducts of triphenoxyalane in the structural database,
the closest example being the tris(trimethylsilyl)silanide
salt; 2,2,6,6-tetrapiperidinium triphenoxy-tris(trimethyl-
silyl)silanidoaluminate [26]. Indeed, protection of the
mononuclear neutral tris(aryloxide)aluminium unit with
respect to addition of a further aryloxide, and hence alu-
minate formation, requires steric hindrance at the 2,6-
phenyl positions of the aryloxide [27–32].
Fig. 1. Molecular structure of compound 4 (POV-RAY 40% thermal
ellipsoids). All hydrogen atoms except H(1) omitted for clarity.
˚
Selected bond lengths (A) and angles (ꢁ): C(1)ꢁ ꢁ ꢁO(1) 3.126(6),
H(1)ꢁ ꢁ ꢁO(1) 2.22, N(1)–C(1) 1.315(5), N(2)–C(1) 1.343(5), N(1)–C(2)
1.368(5), N(2)–C(3) 1.364(5), C(2)–C(3) 1.337(6), Al(1)–O(1) 1.730(3),
Al(1)–O(2) 1.742(3), Al(1)–O(3) 1.709(3), Al(1)–O(4) 1.736(3), C(1)–
H(1)ꢁ ꢁ ꢁO(1) 158.1, N(1)–C(1)–N(2) 107.8(4), C(1)–N(1)–C(2) 109.7(4),
C(1)–N(2)–C(3) 108.0(4), N(1)–C(2)–C(3) 106.5(3), N(2)–C(3)–C(2)
108.1(4), O(1)–Al(1)–O(2) 113.2(2), O(1)–Al(1)–O(3) 109.8(2), O(1)–
Al(1)–O(4) 98.5(2), O(2)–Al(1)–O(3) 108.7(1), O(2)–Al(1)–O(4)
113.5(1), O(3)–Al(1)–Al(4) 111.9(2).
2.2. Reaction of [InH3(IMes)] with 1,1,1,5,5,5-
hexafluoroacetylacetone
Since the seminal work of Brown [33], growing inter-
est in group 13 hydrides has seen their synthetic utility
extended to organic chemistry [14]. Both boro- and alu-
minohydrides have an extensive history in this field,
however, despite numerous attempts, notably by our-
selves [6] and the group of Raston et al. [14], the acqui-
sition of similar applications for gallium and indium
trihydride analogues has been hampered by a marked
increase in thermal and aerobic instability for these met-
allohydrides [11]. Accordingly, given our success in the
stabilisation of indane (InH3) species [1,3,4] we sought
to elaborate upon the reductive character of these by
isolating hydrometallation intermediates that would
perhaps shed light on the diastereoselectivity observed
for the reduction of di- and hydroxyketones by Lewis
base stabilised indane reagents [6]. A report by Raston
et al. highlighted 1,1,1,5,5,5-hexafluoropentan-2,4-dione
(F6acacH) as a candidate substrate for this study as
intermediates had been isolated by stoichiometric
hydrometallation of F6acacH using alane and gallane
reagents [34]. This proceeded before the formation of
tris(dionato) compounds, which result from the use of
more bulky and/or electron-rich 2,4-pentandiones, e.g.,
delocalisation about the heterocycle. Hydrogen bonding
involving the 2-position C–H of imidazolium salts is
now well documented [19,20], however, the composition
of 4 is novel in that it represents the first structurally
characterised imidazolium group 13 ꢀateꢁ species to dis-
play a distinct cation–anion hydrogen bond [8,9,21,22].
This results from the inclusion of oxygen atom hydrogen
bond acceptors on the anion, one of which; O(1), accept-
˚
ing donation from the C(1)–H(1) bond (dꢁ ꢁ ꢁA 2.22 A,
4
˚
Dꢁ ꢁ ꢁA 3.126(6) A, D–Hꢁ ꢁ ꢁA 158.1ꢁ). This renders a
hydrogen to oxygen interaction that is extended relative
to the only structurally characterised imidazolium phe-
[IMes Æ H][O(2,6-tBu2-4-MeC6H2)]
(IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylid-
nolate;
(7)
˚
˚
ene) (dꢁ ꢁ ꢁA 1.87(3) A, Dꢁ ꢁ ꢁA 2.801(4) A, D–Hꢁ ꢁ ꢁA
4
175(2)ꢁ) [23], the hydrogen bonding interaction within
7 benefiting from an anionic hydrogen bond acceptor,
and distortion of the AlO4 tetrahedra such that the
O(1)–Al(1)–O(4) angle (the O–Al–O ꢀbiteꢁ that is orien-
tated toward the imidazolium) is greater than 10ꢁ smal-
ler (98.5(2)ꢁ) than the other O–Al–O angles (108.7(1)–
113.5(1)ꢁ). Interestingly, the bonding of 4 is metrically
similar to that described by the IMes adduct of diphe-
nylamine, wherein dꢁ ꢁ ꢁA and Dꢁ ꢁ ꢁA1 distances of
2,2,6,6-tetramethylheptan-3,5-dione,
in place
of
F6acacH indicating unconventional group 13 hydride
basicity [34]. Accordingly, F6acacH was treated with
our most stable indane; [InH3(IMes)], 3 [3].
Contrary to analogous reactions with a-diketones or
a-hydroxyketones [6], addition of one molar equivalent
˚
2.30(1) and 3.196(2) A are observed [23].
On stoichiometric grounds, the generation of 4 and 5
leaves 0.25 equivalents of 1 and 2 in solution. This was
5
The Cambridge Crystallographic Structural Database (CSD
version 2.5 with updates for Jan and Apr 2004) lists 82 examples of
an aluminium bound by at-least three aryloxides (inclusive of those in
reference 17). Of these, seven constitute free or Lewis base adducted
Al(OAr)3 compounds, where Ar = a substituted phenyl.
4
Notation employed for hydrogen bond: D = hydrogen bond
donor, A = hydrogen bond acceptor, ꢀdꢁ used to define hydrogen of
hydrogen bond donor in ‘‘dꢁ ꢁ ꢁA’’.

M.L. Cole et al. / Inorganica Chimica Acta 358 (2005) 102–108
105
of F6acacH (a b-diketone) to low temperature tetra-
hydrofuran solutions of 3 (Scheme 1) repeatedly led to
immediate decomposition of the indane, as evidenced
by the deposition of an indium mirror on reaction ves-
sels. Filtration, followed by removal of all volatiles in
vacuo gave a light coloured free flowing powder that
characterised as [IMes Æ H][CH{C(O)CF3}2] (6). This
compound was also prepared from the direct reaction
of IMes with F6acacH.
1
As per 4 and 5, the H and 13C NMR spectra of 6
indicate protonation of the carbenic 2-position (10.39
and 132.8 ppm CD2Cl2, respectively), wherein the 13C
resonance is considerably shifted from that of the parent
carbene (219.7 ppm, d8-THF) [35]. The FTIR of 6 dis-
plays absorbances between 1700 and 1500 cmꢀ1 that
are consistent with the inclusion of a symmetrical dion-
ate anion and absence of indium hydride bonds (3; In–H
stretch 1650 cmꢀ1 [3]). Recrystallisation from diethyl
ether gave colourless blocks that characterised as 6.
An X-ray structure determination of these was under-
taken, for which the molecular structure is depicted in
Fig. 2 (see caption for selected bond lengths and angles,
Table 1 for a summary of crystal data and refinement
parameters). In comparison to 4, and akin to several lit-
erature IMes based imidazolium species [19,20,23], com-
pound 6, which crystallises in the space group P21/n with
one ion pair in the asymmetric unit, exhibits increased
C–Hꢁ ꢁ ꢁO hydrogen bonding interactions to the counte-
rion. These interactions are bifurcated, as is typical of
the hydrogen bond acceptor [36–38], and incorporate
both the expected 2-position C–H and one of the
unsaturated imidazolium backbone C–H bonds (C(2)–
H(2)) thereby generating an unusual ꢀzig-zagꢁ polymer.
Further to an imidazolium N(1)–C(1)–N(2) angle of
108.2(3)ꢁ that is ꢀopenꢁ relative to the 101.4(2)ꢁ of the
parent carbene [35] (4; 107.8(4)ꢁ, 7; 107.3(2)ꢁ [23], [IiPr-
Fig. 2. Molecular structure of compound 6 (POV-RAY 40% thermal
ellipsoids). All hydrogen atoms except those of the imidazolium cations
˚
omitted for clarity. Selected bond lengths (A) and angles (ꢁ): C(1)–H(1)
0.96(3), C(1)ꢁ ꢁ ꢁO(1) 2.995(5), C(1)ꢁ ꢁ ꢁO(2) 3.182(5), H(1)ꢁ ꢁ ꢁO(1) 2.38(3),
H(1)ꢁ ꢁ ꢁO(2) 2.29(3), C(2)–H(2) 0.95(4), C(2)ꢁ ꢁ ꢁO(1)#1 3.586(5),
C(2)ꢁ ꢁ ꢁO(2)#1 3.099(5), H(2)ꢁ ꢁ ꢁO(1)#1 2.83(4), H(2)ꢁ ꢁ ꢁO(2)#1 2.23(4),
N(1)–C(1) 1.327(4), N(2)–C(1) 1.331(4), N(1)–C(2) 1.380(4), N(2)–C(3)
1.389(4), C(2)–C(3) 1.340(5), O(1)–C(22) 1.243(4), O(2)–C(24) 1.248(4),
C(22)–C(23) 1.396(5), C(23)–C(24) 1.373(5), C(1)–H(1)ꢁ ꢁ ꢁO(1) 121(2),
C(1)–H(1)ꢁ ꢁ ꢁO(2) 154(3), C(2)–H(2)ꢁ ꢁ ꢁO(1)#1 138(3), C(2)–
H(2)ꢁ ꢁ ꢁO(2)#1 151(3), N(1)–C(1)–N(2) 108.2(3), C(1)–N(1)–C(2)
108.8(3), C(1)–N(2)–C(3) 108.8(3), N(1)–C(2)–C(3) 107.6(4), N(2)–
C(3)–C(2) 106.6(4), N(1)–C(1)–H(1) 130(2), N(2)–C(1)–H(1) 122(2),
N(1)–C(2)–H(2) 120(2), C(3)–C(2)–H(2) 132(2), O(1)ꢁ ꢁ ꢁH(1)ꢁ ꢁ ꢁO(2)
78(1), O(1)ꢁ ꢁ ꢁH(2)#2ꢁ ꢁ ꢁO(2) 70(1), O(1)–C(22)–C(23) 128.9(4), C(22)–
C(23)–C(24) 124.1(4), O(2)–C(24)–C(23) 130.3(4), plane to plane angle
of C(4) aryl ring:imidazol-2-ylidyl ring 88.5(1), C(13) aryl ring:imidazol-
2-ylidyl ring 72.6(2), C(4) aryl ring:C(13) aryl ring 34.6(1). Symmetry
operations used to define # atoms: #1 1/2 ꢀ x, 1/2 + y, 3/2 ꢀ z; #2 1/
2 ꢀ x, y ꢀ 1/2, 3/2 ꢀ z.
3
Me Æ H][Br]; 109.6(6)ꢁ), the interconnection of the IM-
es Æ H units of 6 with proximal F6acac counterions
provides the following contact parameters: H(1);
˚
dꢁ ꢁ ꢁO(1) 2.38(3) A, D–Hꢁ ꢁ ꢁO(1) 121(2)ꢁ, dꢁ ꢁ ꢁO(2)
NHCs
recently
studied
by
Cowley
˚
(e.g.,
˚
2.29(3) A, D–Hꢁ ꢁ ꢁO(2) 154(3)ꢁ, H(2); dꢁ ꢁ ꢁO(1)#
[IMeMe Æ H][Fluorenide]; dꢁ ꢁ ꢁA 2.26(2) A, where
˚
˚
2.83(4) A, D–Hꢁ ꢁ ꢁO(1)# 138(2)ꢁ, dꢁ ꢁ ꢁO(2) 2.23(4)# A,
A = C5 centroid) [40].
4
D–Hꢁ ꢁ ꢁO(2)# 151(3)ꢁ. With the exception of the
The structure of compound 6 makes a novel contri-
bution to the growing catalogue of IMes derived imida-
zolium salts that exhibit significant hydrogen bond
donation from the 2-, 4- and 5-imidazolium C–H bonds
[19,20,23,40,41]. To this end, IMes appears unique in its
ability to participate simultaneously in numerous con-
tacts; however, this may simply reflect the prevalence
H(2)–O(1)# contact, these are within the combined
van der Waals radii contact of hydrogen and oxygen
˚
(2.40 A) despite bifurcation [39]. Furthermore, the sol-
id-state ꢀbindingꢁ of the F6acac anion of 6 appears to
be augmented by C–Hꢁ ꢁ ꢁaryl contact between the
C(3)–H(3) bond and C(6)# (see Fig. 2 for atom loca-
tions; dꢁ ꢁ ꢁA 2.72(4)ꢁ, D–Hꢁ ꢁ ꢁA 175(3)ꢁ), this type of
contact has precedent most notably in the C–Hꢁ ꢁ ꢁp con-
tacts displayed by 7 [23], wherein one mesityl ring partic-
ipates as a hydrogen bond acceptor to an imidazolium
ꢀalkenylꢁ C–H (dꢁ ꢁ ꢁA 2.30ꢁ, D–Hꢁ ꢁ ꢁA 149ꢁ, where
A = mesityl centroid), and the species derived from the
treatment of unsaturated cyclic hydrocarbons with
6
of IMes relative to other NHCs.
6
Of the 426 structures deposited in ꢀThe Cambridge Crystallo-
graphic Structural Databaseꢁ (CSD version 2.5 with updates for Jan
and Apr 2004) that contain an imidazol-2-ylidene or 4,5-dihydroimid-
azol-2-ylidene coordinated to a metal via the 2-position carbon (348
and 78 examples, respectively), 47 employ the NHC IMes.

106
M.L. Cole et al. / Inorganica Chimica Acta 358 (2005) 102–108
The immediate metal deposition observed upon
3.2. Synthetic procedure for compounds 4 and 5
F6acacH reaction with 3 (as is consistent with the
destabilisation of an indium trihydride fragment) indi-
cates ‘‘acid–base’’ reaction of the highly nucleophilic
NHC ligand prior to b-diketone hydrometallation by
the indane. As noted, this is contrary to previous sur-
veys using alane and gallane [34] and controverts stud-
ies of the reducing agent nature of 3 undertaken with
a-diketones or a-hydroxyketones [6]. These provide
reasonable yields of the hydrometallation product al-
beit with reduced yields compared to tricyclohexyl-
phosphine stabilised indane [2]. This lessened activity
has been attributed to the increased steric bulky of
IMes relative to P(c-C6H11)3, which may impede hy-
dride delivery.
The composition of species 4–6 highlights the availa-
bility of NHCs toward undesirable acid–base secondary
reactions with organic substrates despite coordination to
an aluminium or indium metal centre. This further
emphasises the potential of NHCs to promote unusual
solid-state interactions.
Phenol (1.81 g, 19.23 mmol) in Et2O (20 cm3) was
added dropwise to either [AlH3(IMeMe)] (1.00 g, 6.49
mmol) or [AlH3(IiPrMe)] (1.35 g, 6.42 mmol) (for 4
and 5 respectively) in THF (50 cm3) at low temperature
(ꢀ50 ꢁC). Effervescence was observed, and the solution
was warmed to ambient temperature and stirred for a
further 2 hours. Removal of volatiles in vacuo rendered
compounds 4 and 5 as a light yellow solid and pale
brown oil, respectively. These worked up as follows.
3.3. [IMeMe Æ H][Al(OPh)4] (4)
Extraction of the light yellow solid into fresh THF
(20 cm3) followed by placement at ꢀ35 ꢁC overnight
yielded yellow prismatic crystals of 4 (1.18 g, 47 %),
m.p. 123 ꢁC. 1H NMR (CDCl3): d 2.02 (s, 6H,
C2(CH3)2), 3.49 (s, 6H, N(CH3)), 6.60–7.00 (m, 20H,
Ar–H), 8.79 (br s, 1H, NC(H)N). 13C NMR (CDCl3):
d 9.6 (s, C2(CH3)2), 32.3 (s, N(CH3)), 116.4, 118.9 (s,
Ar–C), 126.3 (s, C2(CH3)2), 129.9 (s, Ar–C), 134.1 (s,
NC(H)N), 152.4 (s, CO). FTIR (nujol) m/cmꢀ1: 1592
(m), 1285 (m), 1163 (m), 877 (m), 695 (m). MS APCI:
m/z (%) 125 [(IMeMe Æ H)+, 100].
3. Experimental
3.1. General
3.4. [IiPrMe Æ H][Al(OPh)4] (5)
The group 13 trihydride compounds 1 [1] and 3 [3]
were prepared using literature procedures. Compound
2 was prepared using a modification of the literature
method for compound 1 [1,24]. Phenol was purchased
from Aldrich and used as received. 1,1,1,5,5,5-Hexa-
fluoropentan-2,4-dione and CDCl3 were purchased
Repeated washing of the pale brown oil with hexane
(3 · 5 cm3) and Et2O (2 · 5 cm3) provided 5 as a pale
brown solid that proved insoluble in ethereal solvents
1
(0.87 g, 31%), m.p. 67 ꢁC. H NMR (CDCl3): d 1.28
(s, 6H, C2(CH3)2), 1.94 (br s, 12H, CH(CH3)2), 4.11
3
(septet, 2H, CH(CH3)2, JHH 7.0 Hz), 6.60–7.70 (m,
from Aldrich, dried over molecular sieves (3 A) and
20H, Ar–H), 8.88 (br s, 1H, NC(H)N). 13C NMR
(CDCl3): d 8.4 (s, C2(CH3)2), 22.0 (s, CH(CH3)2), 51.0
(s, CH(CH3)2), 117.1, 119.8 (s, Ar–C), 124.9 (s,
C2(CH3)2), 126.4 (s, Ar–C), 132.3 (s, NC(H)N), 150.4
(s, CO). FTIR (nujol) m/cmꢀ1: 1550 (m). MS APCI: m/
z (%) 181 [(IiPrMe Æ H)+, 100].
˚
freeze-thaw degassed prior to use. Tetrahydrofuran
(THF), diethyl ether and hexane were dried over so-
dium, freshly distilled from sodium benzophenone ke-
tyl and freeze-thaw degassed prior to use. Deutero
dichloromethane was stored over CaH2 under argon
and freeze-thaw degassed prior to use. All manipula-
tions were performed using conventional Schlenk or
glovebox techniques under an atmosphere of high pur-
ity argon in flame-dried glassware. Infrared spectra
were recorded as Nujol mulls using sodium chloride
plates on a Nicolet Nexus FTIR spectrophotometer.
1H NMR spectra were recorded at 400 MHz and
13C NMR spectra were recorded at 100 MHz using
a Bruker DPX 400 spectrometer at ambient tempera-
ture, and chemical shifts were referenced to the 13C
or residual 1H resonances of the deutero chloroform
or dichloromethane solvent employed. Mass Spectra
were recorded using a VG Fisons Platform II instru-
ment under APCI conditions. Melting points were
determined in sealed glass capillaries under argon
and are uncorrected.
3.5. [IMes Æ H][CH{C(O)CF3}2] (6)
Method (i): F6acacH (155 ll, 1.10 mmol) was added
dropwise to a solution of 3 (0.23 g, 0.54 mmol) in
THF (20 cm3) at ꢀ50 ꢁC. The clear colourless solution
immediately deposited a metallic mirror onto the surface
of the reaction vessel and was warmed to room temper-
ature and stirred for 2 h. Removal of volatiles under re-
duced pressure rendered the title compound as a free
flowing lightly coloured powder. Extraction into fresh
Et2O (ca. 10 cm3) followed by concentration to the point
of crystallisation and placement at ꢀ35 ꢁC yielded 6 as
small colourless blocks (0.21 g, 76%), m.p. 158 ꢁC.
Method (ii): F6acacH (237 ll, 1.67 mmol) was
added dropwise to a solution of IMes (0.51 g, 1.68

M.L. Cole et al. / Inorganica Chimica Acta 358 (2005) 102–108
107
mmol) in THF (35 cm3) at ꢀ30 ꢁC. The resulting clear
Appendix A. Supplementary material
yellow solution was warmed to room temperature and
stirred for 2 h. Concentration in vacuo followed by
placement at ꢀ35 ꢁC overnight gave 6 as colourless
blocks. These were isolated by filtration and washed
with cold (0 ꢁC) hexane (2 · 3 cm3) (0.71 g, 83%),
m.p. 162 ꢁC. 1H NMR (CD2Cl2): d 2.14 (s, 12H, o-
CH3), 2.35 (s, 6H, p-CH3), 5.09 (s, 1H,
CH{C(O)CF3}2), 7.00 (s, 4H, m-H), 7.42 (s, 2H,
C2H2), 10.39 (s, 1H, N2CH). 13C NMR (CD2Cl2): d
17.9 (s, o-CH3), 21.6 (s, p-CH3), 84.1 (s,
CH{C(O)CF3}2), 124.3 (s, C2H4), 130.4 (s, m-CH),
131.6 (s, p-CCH3), 132.8 (s, NC(H)N), 135.1 (s, o-
CCH3), 142.1 (s, ipso-C), carbonyl carbon signals not
observed. FTIR (nujol) m/cmꢀ1: 1669 (sh s), 1544 (sh
s), 1527 (sh s), 1237 (m), 1177 (m), 1126 (br m), 851
(m). MS APCI: m/z (%) 305 [{M ꢀ F6acac}+, 100].
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.ica.2004.07.036.
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