Synthesis and characterisation of anionic and neutral gallium(i) N-heterocyclic carbene analogues

Article abstract of DOI:10.1039/c2dt30299c

A high yield synthesis of a new, extremely bulky anionic gallium(i) N-heterocyclic carbene analogue, [(DAB*)Ga:]^- (DAB* = {N(Ar*)C(H)}₂, Ar* = C₆H₂{C(H) Ph₂}₂Me-2,6,4) has been developed and four monomeric sodium complexes of the heterocycle have been crystallographically characterised. The gallium(i) heterocycle has been utilised in the preparations of the heteroleptic zinc and cadmium gallyl complexes, [(DAB*)GaMX(tmeda)] (M = Zn or Cd, X = Br or I), which were crystallographically characterised. In addition, [(DAB*)Ga:]^- was oxidatively coupled to give the diamagnetic digallane(4), [(DAB*)GaGa(DAB*)]. The moderate yield synthesis of the six-membered gallium(i) heterocycle, [(^ButMesNacnac) Ga:] (^ButMesNacnac = [(MesNCBu^t)₂CH] ^-, Mes = mesityl), is described, and the compound found to be a monomer in the solid state by an X-ray crystallographic analysis. A low yield by-product from this synthesis, [Ga₅I₄( ^ButMesNacnac)₃], was also isolated and shown by X-ray crystallography to be a rare example of a compound bearing a group 13 metal-metal bonded chain stabilised by β-diketiminate ligands. A preliminary analysis of the bonding in the compound was carried out using DFT calculations.

Full text of DOI:10.1039/c2dt30299c

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PAPER
Synthesis and characterisation of anionic and neutral gallium(I)
N-heterocyclic carbene analogues†‡
Deepak Dange, Sam L. Choong, Christian Schenk, Andreas Stasch and Cameron Jones*
Received 10th February 2012, Accepted 29th March 2012
DOI: 10.1039/c2dt30299c
A high yield synthesis of a new, extremely bulky anionic gallium(I) N-heterocyclic carbene analogue,
[(DAB*)Ga:]− (DAB* = {N(Ar*)C(H)}2, Ar* = C6H2{C(H)Ph2}2Me-2,6,4) has been developed and
four monomeric sodium complexes of the heterocycle have been crystallographically characterised.
The gallium(I) heterocycle has been utilised in the preparations of the heteroleptic zinc and cadmium
gallyl complexes, [(DAB*)GaMX(tmeda)] (M = Zn or Cd, X = Br or I), which were crystallographically
characterised. In addition, [(DAB*)Ga:]− was oxidatively coupled to give the diamagnetic digallane(4),
[(DAB*)GaGa(DAB*)]. The moderate yield synthesis of the six-membered gallium(I) heterocycle,
[(ButMesNacnac)Ga:] (ButMesNacnac = [(MesNCBut)2CH]−, Mes = mesityl), is described, and the
compound found to be a monomer in the solid state by an X-ray crystallographic analysis. A low yield
by-product from this synthesis, [Ga5I4(ButMesNacnac)3], was also isolated and shown by X-ray
crystallography to be a rare example of a compound bearing a group 13 metal–metal bonded chain
stabilised by β-diketiminate ligands. A preliminary analysis of the bonding in the compound was carried
out using DFT calculations.
all blocks of the periodic table. In addition, their reactive gallium
Introduction
centres are capable of undergoing cycloaddition reactions,
and activating a variety of E–H bonds.3a Moreover, the neutral
six-membered N-heterocycle, 4a, is emerging as a specialist
reducing agent in main group and transition metal organometal-
lic synthesis.9
Since the isolation of the first stable N-heterocyclic carbene
(NHC) in 1991,1 these versatile compounds have proved of enor-
mous importance to many areas of coordination chemistry, syn-
thesis, catalysis etc.2 It is, therefore, not surprising that
analogues of NHCs incorporating various p-block elements in
place of the carbene centre have been intensively investigated
over the past two decades. While most work has centred on
heavier group 14 element(II) NHC counterparts, considerable
attention has also been paid to N-heterocycles containing a
group 13 element in the +1 oxidation state.3,4 Of these,
gallium(I) NHC analogues have been most studied and examples
of four-,5 five-,6,7 and six membered systems8 have been reported
(Fig. 1). Like NHCs, gallium(I) N-heterocycles have proven
widely applicable as ligands, as evidenced by the fact that they
have been employed in the synthesis of complexes containing
unsupported bonds between gallium and ca. 50 elements from
Despite the importance of gallium(I) N-heterocycles, and the
large number of publications devoted to their chemistry, there are
only a handful of such compounds (1–4) that are available to the
synthetic chemist. In order to remedy this situation, and to allow
for more in-depth comparisons of the their further reactivity with
that of NHCs, we set out to prepare new examples of anionic
five-membered, and neutral six-membered gallium(I) hetero-
cycles. With regard to the former, we were particularly interested
in accessing extremely bulky heterocycles and investigating their
use as Ga-donor ligands. This interest stems from recent reports
which have shown that related, very sterically hindered NHC
ligands can influence the stability of low-valent transition metal
complexes and their catalytic activity.10 With regard to the latter,
we wished to prepare less bulky examples of 4, as we believed
these may prove to be more active as reducing agents in organo-
metallic synthesis. Our initial findings in this study are detailed
herein.
School of Chemistry, Monash University, PO Box 23, VIC, 3800,
Australia. E-mail: cameron.jones@monash.edu
†Dedicated to Professor David Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
‡Electronic supplementary information (ESI) available: Crystal data,
details of data collections and refinements for 6a–d, 7, 8, 10, 11, DAB*,
Ar*NH2, a cyclic isomer of DAB*H2 and [NaI(18-crown-6)]; ORTEP
diagrams for 7, DAB*, Ar*NH2, a cyclic isomer of DAB*H2 and [NaI
(18-crown-6)]; and further details of the theoretical calculations. CCDC
866214–866221, 866224 and 866225. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt30299c
Results and discussion
(i) Bulky anionic gallium(I) N-heterocycles
One of the most widely utilised NHCs in homogeneous catalysis and
coordination chemistry is the bulky system IPr, :C{N(Dip)C(H)}2
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Fig. 1 Reported examples of gallium(I) N-heterocycles (Dip = 2,6-diisopropylphenyl).
Scheme 1 (i) “GaI”, toluene, −Ga; (ii) Na, THF, −NaI; (iii) tmeda–diethyl ether; (iv) toluene, toluene–hexane or diethyl ether; (v) 6a, THF (Ar* =
C6H2{C(H)Ph2}2Me-2,6,4).
(Dip = C6H3Pri2-2,6).11 The utility of this compound as a ligand
is derived from its ease of preparation and its ability to stabilise
a variety of low-valent transition metal and main group element
complexes.2,12 Similarly, the isostructural gallium(I) anion, 2b, is
the most studied of the group 13 metal(I) NHC analogues, and
its high nucleophilicity has lent it to the stabilisation of many
novel complex types.13 Over the past two years several much
bulkier versions of IPr have been developed, which either
exhibit substitution at the para-positions of its Dip groups, as in
:C{N(Ar)C(H)}2 (Ar = C6H2Pri2R-2,6,4; R = CPh3, 1-adamantyl
etc.),10c or a combination of para-substitution and phenyl substi-
tution at the methine centres of the Dip group, as in IPr*, :
C{N(Ar*)C(H)}2 (Ar* = C6H2{C(H)Ph2}2Me-2,6,4).10a Not
only is the latter NHC one of the bulkiest yet reported, but its
steric profile shows flexibility with respect to the size of coordi-
nated metal fragments in its complexes. It is noteworthy that the
“flexible sterics” of very bulky NHCs have been harnessed by
several groups (e.g. those of Glorius14 and Bertrand15), and
shown to be of significant benefit to the reactivity of complexes
derived from such NHCs. Given the current interest in sterically
very imposing NHCs, we set out to prepare and explore the
reactivity of the anionic gallium(I) analogues of IPr* and related
heterocycles.
It seemed reasonable that a variation of the synthetic pro-
cedure we used to access 2 in prior studies would be successful
here.6c,d Accordingly, DAB* ({N(Ar*)C(H)}2) was prepared by
a condensation reaction between Ar*NH2 and glyoxal, as
reported by Berthon-Gelloz, Markó et al.10a This was treated
with two equivalents of the gallium(I) iodide source, “GaI”,16
which led to a single electron reduction of DAB* and concomi-
tant disproportionation (evidenced by Ga metal deposition) to
give the paramagnetic gallium(III) complex, [(DAB*˙)GaI2] 5, in
high yield (89%) as a red brown crystalline solid (Scheme 1).
Meaningful NMR spectroscopic data for 5 could not be obtained
due to its paramagnetic nature and it was not possible to crystal-
lographically characterise the microcrystalline material. It is
likely, however, that it is monomeric in the solid state, as
are several closely related compounds, e.g. [(DAB˙)GaI2]
(DAB = {N(Dip)C(H)}2).6c,17 It is noteworthy that during this
phase of the study, the X-ray crystal structures of the known
compounds, Ar*NH2 and DAB*, were obtained, full details of
which can be found in the ESI.‡
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Previously, we showed that potassium salts of anion 2b can be
prepared in high yields via the potassium metal reduction of
[(DAB˙)GaI2] in THF, followed by treatment of the product
mixture with various Lewis bases. For example, when tmeda is
used as the Lewis base, the dimeric complex, [{(DAB)GaK-
(tmeda)}2], is obtained.6c In contrast, the treatment of 5 with an
excess of potassium in THF, followed by treatment with tmeda,
led to an unidentified mixture of products. The reduction of a
THF solution of 5 was subsequently carried out with the less
reducing metal, sodium. After treatment of the crude product
with an excess of a tmeda–diethyl ether mixture, and subsequent
crystallisation from toluene, a high isolated yield (82%) of the
monomeric sodium–gallyl complex, 6a, resulted (Scheme 1).
It seems likely that the coordinated toluene molecule of 6a is
quite labile, as in one attempt to obtain crystals from a benzene
solution of the compound by layering it with hexane, the
toluene-free complex, 6b, resulted. Presumably, one of the
phenyl groups of the complex successfully competes with
toluene (and the benzene solvent) for coordination to the sodium
centre in the absence of excess toluene, and when the concen-
tration of benzene in the solution is sufficiently diluted by
hexane in the layering experiment. Moreover, when 6a is recrys-
tallised from the strong donor solvent, THF, both the
coordinated toluene and tmeda molecules are displaced and the
THF coordinated product, 6c, results. Finally, if diethyl ether is
used instead of toluene for the crystallisation of the initially
reduced product, a low isolated yield of the etherate complex,
6d, was obtained.
An alternative route to 6 was explored whereby the new
aminoimine, Ar*N(H)C(H)2C(H)vNAr* (prepared by quench-
ing [(DAB*)Mg] with NMe3·HCl), was treated with a mixture of
GaCl3 and the strong base, NPri2Et, with the expectation of
obtaining the diamagnetic compound, [(DAB*)GaCl], via a
dehydrochlorination reaction. If accessible, it was proposed that
[(DAB*)GaCl] could be subsequently reduced with sodium to
give 6. Although a similar methodology has been successfully
employed for the formation of the related lithium boryl, [(DAB)-
BLi(THF)2], via a lithium reduction of [(DAB)BBr],18 in the
current study we saw no evidence for the initial formation of the
precursor complex, [(DAB*)GaCl], and instead obtained
complex product mixtures. This outcome possibly results from
the likely acidity of the C(H)Ph2 methine protons of the aminoi-
mine, which could lead to the formation of side products in the
dehydrochlorination reaction. A hint at the “non-innocence” of
these methine protons comes from the fact that one preparation
of Ar*N(H)C(H)2C(H)vNAr* afforded a small amount of a
cyclised isomeric form of the compound, which resulted from a
net addition of one methine C–H bond across the imine frag-
ment. The spectroscopic and crystallographic data for this
by-product can be found in the ESI.‡ It is of note that we have
also explored the possibility of preparing gallium(I) hetero-
cycles related to the bulky NHC family, :C{N(Ar)C(H)}2
(Ar = C6H2Pri2R-2,6,4; R = CPh3, 1-adamantyl etc.), recently
reported by Holland et al.10c In attempts to access precursors to
these targets (related to 5), the diazabutadienes, {N(Ar)C(H)}2
(Ar = C6H2Pri2R-2,6,4; R = CPh3, C(H)Ph2 or 1-adamantyl)
were treated with two equivalents of “GaI” in THF. However, all
of these reactions instead led to unidentified product mixtures
and were not pursued further.
The NMR spectroscopic data for 6a, 6c and 6d appear consist-
ent with them retaining their solid state structures in solution,
though given that the chemical shifts associated with the toluene
and diethyl ether ligands of 6a and 6d are identical to those of
the uncoordinated molecules, it is likely that these ligands were
displaced from the sodium centres of 6a and 6d by the solvent
(neat d6-benzene) used in the NMR experiments (N.B. the yield
of 6b was too low to allow acquisition of solution state NMR
spectroscopic data). All of the sodium gallyl complexes, 6a–d,
were crystallographically characterised and all exhibit similar
monomeric structures in the solid state (Fig. 2, see also Table 1).
The gallium centre of each compound has a distorted trigonal
planar coordination environment, and the bond lengths and
angles within the gallium heterocycles are suggestive of minimal
intra-cyclic electronic delocalisation, as is the case for alkali
metal salts of 2.6 There are significant differences between the
Ga–Na distances in the compounds, which vary over the range
2.910(3) Å to 3.1208(16) Å. These distances reflect the degree
of electronic saturation of the sodium centres, in that compounds
6a and 6b, which display weak η2-phenyl–toluene interactions,
have the shortest Ga–Na interactions, whereas the four- and five-
coordinate ether ligated compounds, 6d and 6c, have signifi-
cantly longer Ga–Na distances. With that said, a similar range
of Ga–Na distances has been reported for related mono-
meric sodium gallyl complexes, e.g. [(MeDAB)GaNa(THF)3]
(
MeDAB {N(Dip)C(Me)}2) 2.972(4) Å;6e [(Ar-BIAN)-
=
GaNa(L)n] ((Ar-BIAN)Ga = 3), (L)n = (DME)2 2.938(12) Å,
(OEt2)3 3.0490(7) Å, (THF)3(OEt2) 3.1110(11) Å.7 It is note-
worthy that DFT studies (B3LYP/DZP) of a model of [(MeDAB)-
GaNa(THF)3] have been carried out in a previous study,6e
and these indicated that the compound has a very polar covalent
Ga–Na interaction with a Wiberg bond index of 0.41. It is
reasonable to assume that similar, largely electrostatic metal–
metal bonding is present in 6a–d.
Preliminary studies of the reactivity of the [(DAB*)Ga:]−
anion in 6a were carried out for sake of comparison with the
well established coordination and other chemistry of the [(DAB)
Ga:]− anion, which is normally utilised as its potassium salt,
[{(DAB)GaK(tmeda)}2]. In a recent study this compound was
reacted with [MX2(tmeda)] (M = Zn or Cd, X = Br or I) to give
the isostructural monomeric complexes, [(DAB)GaMX-
(tmeda)].13c Attempts to reduce these compounds to group
12 metal(I) dimers, [(DAB)GaMMGa(DAB)], were unsuccess-
ful. It was thought that the bulkier gallyl fragment in 6a might
allow the kinetic stabilisation of similar M–M bonded com-
pounds. Accordingly, [MX2(tmeda)] were treated with one
equivalent of 6a in THF, though in both cases no reaction
occurred. This is likely a result of the extreme steric bulk of the
gallyl ligand, in combination with the more covalent Ga–Na
bond of 6a, relative to the Ga–K interactions in [{(DAB)GaK-
(tmeda)}2]. In order to increase the reactivity/nucleophilicity of
the gallyl fragment of 6a, the reactions were repeated, but in the
presence of 18-crown-6. It was thought this would lead to the
cleavage of the Ga–Na bond of 6a, similar to the formation of
the ion separate salt, [(DAB)Ga:]2[K2(18-crown-6)3],6c from the
reaction of [{(DAB)GaK(tmeda)}2] with the crown ether. This
strategy was successful and the group 12 metal gallyl complexes,
7 and 8, were isolated from the reactions, albeit in low yields
(Scheme 2).19 Subsequent attempts to reduce both compounds to
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Fig. 2 Molecular structures of (a) 6a, (b) 6b, (c) 6c and (d) 6d (25% thermal ellipsoids are shown; hydrogen atoms omitted).
group 12 metal(I) dimers by treating them with the mild
and soluble magnesium(I) reducing agent, [{(MesNacnac)Mg}2]
(
successful, and instead led to the deposition of elemental zinc
and cadmium.
figure caption. Both compounds are monomeric with trigonal
planar Ga centres and distorted tetrahedral geometries at the
group 12 metal atoms. Moreover, their Ga–M distances are only
marginally longer than those in the less bulky counterparts,
MesNacnac = [(MesNCMe)2CH]−, Mes = mesityl),20 were not
[(DAB)GaZnBr(tmeda)] 2.3829(8)
Å
and [(DAB)GaCdI-
The solution state spectroscopic data for 7 and 8 are similar
and in the solid state the compounds are isomorphous and iso-
structural. Only the molecular structure of 8 is depicted in Fig. 3
(see ESI‡ for the molecular structure of 7), while selected geo-
metrical parameters for both compounds are included in the
(tmeda)] 2.509(1) Å.13c There are no other structurally character-
ised complexes bearing unsupported Ga–Cd bonds for compari-
son, but the Ga–Zn bond length in 7 lies well within the known
range (2.323–2.463 Å, 8 examples).21 The M–X distances in 7
and 8 are unexceptional.
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We have previously shown that the [(DAB)Ga:]− anion and its
slightly bulkier counterpart, [(MeDAB)Ga:]−, can be oxidatively
coupled by treatment with either the ferrocenium ion or Tl2SO4
to give the digallane(4) compounds, [(DAB)GaGa(DAB)] and
[(MeDAB)GaGa(MeDAB)], respectively.6d,13s,22 The synthetic
utility of such compounds is highlighted by the fact that [(DAB)-
GaGa(DAB)] has been utilised as a source of the gallyl fragment
in oxidative addition reactions with low-valent transition metal
complexes on several occasions.13 Given the extreme steric bulk
of [(DAB*)Ga:]−, it seemed plausible that its oxidation may not
lead to a coupling reaction, but could instead yield a stable,
monomeric gallium(II) radical, [(DAB*)Ga:]˙ (cf. the recently
reported stable germanium(I) radical, [(ButNacnac)Ge:]˙ ButNac-
nac = [(DipNCBut)2CH]−).23 To this end, 6a was treated with
half an equivalent of Tl2SO4 which saw the deposition of thal-
lium metal and the formation of the deep purple, diamagnetic
digallane(4), [(DAB*)GaGa(DAB*)] 9 in high yield. While a
crystal structure of publishable quality could not be obtained for
9, a low quality structure (not included here) did establish that it
is dimeric in the solid state with a Ga–Ga distance of 2.396(2)
Å. It is of note that this is only marginally longer that the same
bond in [(DAB)GaGa(DAB)] (2.3482(2) Å).22
(ii) Neutral gallium(I) N-heterocycles
To date, only two examples of structurally authenticated six-
membered gallium(I) heterocycles, 4, have appeared in the litera-
ture.8 Although no further chemistry has yet come forward for
Table 1 Selected interatomic distances (Å) and angles (°) for 6a–da
6a
6b
6c
6d
Ga–Na
Ga–N
2.9404(13) 2.910(3)
3.1208(16) 3.0384(16)
1.9682(19) 1.951(2)
1.960(2)
1.943(2)
1.967(5)
1.911(5)
1.445(5)
1.386(6)
1.332(4)
2.436(9)
2.459(8)
—
—
1.389(30
—
1.347(5)
—
—
2.377(2)
2.349(3)
—
—
81.81(11)
1.958(3)
1.397(4)
1.380(4)
1.333(5)
2.488(4)
2.491(4)
2.356(3)
—
—
—
81.98(12)
145.50(9)
132.19(9)
N–C (Ga-cycle) 1.397(3)
1.399(3)
C–C (Ga-cycle) 1.328(4)
Na–N
Na–O
Na–C
2.434(3)
2.482(3)
—
Fig. 3 Molecular structure of 8 (25% thermal ellipsoids are shown;
hydrogen atoms omitted). Selected bond lengths (Å) and angles (°) 8:
I(1)–Cd(1) 2.7212(17), Cd(1)–N(3) 2.370(5), Cd(1)–N(4) 2.380(5),
Cd(1)–Ga(1) 2.5300(16), Ga(1)–N(1) 1.883(4), Ga(1)–N(2) 1.886(4);
N(1)–Ga(1)–N(2) 86.33(18), N(1)–Ga(1)–Cd(1) 133.32(13), N(2)–
Ga(1)–Cd(1) 139.89(13), N(3)–Cd(1)–N(4) 78.00(18), Ga(1)–Cd(1)–
I(1) 129.79(4). Selected bond lengths (Å) and angles (°) 7: Br(1)–Zn(1)
2.3582(8), Ga(1)–N(1) 1.881(3), Ga(1)–N(2) 1.891(3), Ga(1)–Zn(1)
2.3957(7), Zn(1)–N(4) 2.135(5), Zn(1)–N(3) 2.142(4); N(1)–Ga(1)–
N(2) 86.03(14), N(1)–Ga(1)–Zn(1) 133.90(11), N(2)–Ga(1)–Zn(1)
139.96(10), N(4)–Zn(1)–N(3) 84.98(16), Br(1)–Zn(1)–Ga(1) 123.20(3).
—
—
3.079(4)
2.823(3)
82.70(8)
142.91(6)
134.39(7)
3.078(11)
3.049(9)
82.07(19)
150.14(16) 139.09(5)
121.11(15)
N–Ga–N
Na–Ga–N
—
a Where more than one value is given in a table entry, the order of the
entries follows the atom numbering scheme.
Scheme 2 (i) [ZnBr2(tmeda)] or [CdI2(tmeda)], −NaX; (ii) Tl2SO4, −Tl(s), −Na2SO4.
9308 | Dalton Trans., 2012, 41, 9304–9315
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Scheme 3 (i) “GaI”, toluene, −NaI.
4b, the less bulky system, 4a, has been widely studied as
a ligand and as a specialist reducing agent in inorganic syn-
theses.3a,4c,9 The emerging utility of the compound as a reducing
agent in the formation of low valent p-block complexes is
especially impressive. Just one example here is its reaction with
SnCl2 which yielded, amongst other products, the largest
known mixed valence Sn cluster, [Sn17{Ga(Cl)(DipNacnac)}4]
Ignoring dimeric compounds containing M–M bonds
(M = group 13 metal), and the numerous group 13 metalloid
clusters that have been pioneered by the group of Schnöckel,25
homocatenation for the group 13 metals is rare.26 Indeed, com-
pounds containing three or more covalently bonded group 13
atoms and mono-anionic, bidentate N,N′-donor ligands (e.g.
β-diketiminates) number only a few.27 Of these, the most
celebrated is the mixed valence compound, [(XylNacnac)(I)In{In-
(
DipNacnac = [(DipNCMe)2CH]−).9c It is reasonable that if less
bulky examples of such heterocycles were available, they would
be more reactive, and therefore potentially more synthetically
useful. In this respect, in a series of recent studies we have
shown that the β-diketiminate, ButMesNacnac ([(MesNC-
But)2CH]−), is sufficiently bulky to kinetically stabilise very low
oxidation state p-block complexes, but these are more reactive
than related compounds derived from the larger DipNacnac and
ButNacnac ligands.24 Accordingly, we explored the possibility of
preparing a gallium(I) heterocycle incorporating the ButMesNac-
nac ligand.
The synthetic procedure we employed was similar to the prep-
arations of 4a and 4b, in that [Li(ButMesNacnac)] was treated
with “GaI” in toluene, which gave a moderate yield (54%) of the
monomeric compound, 10, as a yellow crystalline solid after
work-up (Scheme 3). The compound is very thermally stable
and decomposes only at temperatures in excess of 235 °C. Its
solution state spectra are compatible with its proposed structure,
though a spectroscopic analysis of the total reaction mixture
prior to work-up suggested the presence of at least one other
compound. This prompted a re-examination of the reaction,
which included further work-up of the mother liquor obtained
after the isolation of 10. This afforded a very low yield (ca. 3%)
of the unusual yellow, mixed valence penta-gallium compound,
11, which has an average Ga oxidation state of 1.4, and which
was likely formed via a partial disproportionation process. Given
the low yield of the compound, only limited spectroscopic data
for the compound could be obtained.
(
XylNacnac)}4In(I)(XylNacnac)] (XylNacnac = [{(3,5-xylyl)NCMe}2-
CH]−) which is thought to exhibit σ-delocalisation along its In6
chain.27c Compound 11 is somewhat related to that linear oligo-
mer, though, in contrast, it does possess two M–M bonds that
are bridged by iodide ligands.
The molecular structures of 10 and 11 are displayed in Fig. 4
and 5, respectively. That for 10 reveals it to be monomeric with
a two-coordinate Ga centre (N–Ga–N 89.42(10)°). The hetero-
cycle of the compound is essentially planar, with a delocalised
β-diketiminate backbone, and Ga–N distances similar to those in
4a (2.054 Å mean) and 4b (2.063 Å mean). It is of note that
although the N-substituents of 10 are smaller than the Dip
groups of 4a, its C–N–C angles (125.2° mean) are markedly
more obtuse than those of 4a (119.0° mean, cf. 126.4° mean for
4b). This presumably results from its backbone tert-butyl groups
buttressing its mesityl substituents, thereby adding greater steric
protection to the GaI centre. The molecular structure of 11 shows
the compound to have the overall composition of [Ga5I4(ButMes-
Nacnac)3]. It possesses a branched Ga5 chain with all Ga–Ga
distances in a narrow range, three terminal ButMesNacnac
ligands, two terminal iodide ligands, and two unsymmetrically
bridging iodides. The Ga–I distances of the bridging iodides
suggest that I(2) is principally associated with Ga(2), but acts as
a donor towards Ga(5), while I(3) is associated with Ga(3) and
donates a lone pair to Ga(2). Another interesting feature of the
complex is that while all the Ga centres are four-coordinate, the
N2Ga2 fragment incorporating Ga(5) is close to planar (Σ angles
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Fig. 4 Molecular structure of 10 (25% thermal ellipsoids are shown; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°): Ga(1)–
N(2) 2.034(3), Ga(1)–N(1) 2.060(3), N(1)–C(1) 1.340(4), N(2)–C(3) 1.336(4), C(1)–C(2) 1.379(6), C(2)–C(3) 1.403(5); N(2)–Ga(1)–N(1) 89.42(10).
Fig. 5 Molecular structure of 11 (25% thermal ellipsoids are shown; hydrogen atoms and methyl groups omitted). Selected bond lengths (Å) and
angles (°): I(1)–Ga(1) 2.6393(8), I(2)–Ga(2) 2.7643(8), I(2)–Ga(5) 2.8448(9), I(3)–Ga(3) 2.7559(10), I(3)–Ga(2) 2.8513(9), I(4)–Ga(4) 2.6183(10),
Ga(1)–N(1) 1.948(5), Ga(1)–N(2) 1.997(6), Ga(1)–Ga(2) 2.4493(11), Ga(2)–Ga(3) 2.4517(10), Ga(3)–Ga(4) 2.4330(10), Ga(3)–Ga(5) 2.4722(10),
Ga(4)–N(3) 1.929(6), Ga(4)–N(4) 1.997(5), Ga(5)–N(6) 1.956(6), Ga(5)–N(5) 1.968(5); Ga(1)–Ga(2)–Ga(3) 162.02(4), Ga(4)–Ga(3)–Ga(2)
140.72(4), Ga(2)–Ga(3)–Ga(5) 92.88(3), Ga(4)–Ga(3)–Ga(5) 125.27(4), Ga(2)–I(2)–Ga(5) 79.00(2), Ga(3)–I(3)–Ga(2) 51.82(2), Ga(2)–Ga(1)–I(1)
110.53(3), Ga(3)–Ga(4)–I(4) 104.65(3).
about Ga(5): 357.6°). As a result, the compound could be
viewed as containing a Ga(5) → Ga(3) dative bond, i.e. it is
an adduct between [(ButMesNacnac)Ga:] and the covalently
bonded Ga4 chain [(ButMesNacnac)(I)Ga{Ga(I)}2Ga(I)(ButMes-
Nacnac)], with I(2) donating electron density into the empty
p-orbital at Ga(5).
DFT calculations (RI-DFT/BP86/def2-SVP) were carried out
on 11 (full molecule) in the gas phase, which led to the geome-
try of the molecule optimising to be very close to that of
the compound in the solid state, but with slightly overestimated
Ga–Ga and Ga–I bond distances. An orbital analysis revealed
that the Ga–Ga σ-bonds of the compound are most obviously
associated with its HOMO and HOMO − 4, whilst the LUMO +
3 is clearly the highest in Ga–Ga σ* character (Fig. 6). An NBO
analysis of the metal–metal bonding in the compound showed
that the Ga(5) s-orbital component of the Ga(5)–Ga(3) bond is
very high (78.7%), while the Ga(3) p-orbital contribution to the
same bond is also high (75.6%). This situation could indicate
significant dative character to this bond, as was suggested from
its crystal structure.
9310 | Dalton Trans., 2012, 41, 9304–9315
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Fig. 6 (a) HOMO − 4, (b) HOMO and (c) LUMO + 3 of 11 (Ga = red; I = pink).
yield by-product from this synthesis, [Ga5I4(ButMesNacnac)3],
was isolated and its crystal structure showed it to be a rare
example of a compound bearing a group 13 metal–metal bonded
chain stabilised by β-diketiminate ligands. A preliminary analy-
sis of the bonding in the compound was carried out using DFT
calculations. We continue to explore the further chemistry of
[(DAB*)Ga:]− and [(ButMesNacnac)Ga:].
Conclusion
In summary, a high yield synthesis of a new, extremely bulky
anionic gallium(I) N-heterocyclic carbene analogue, [(DAB*)-
Ga:]−, has been developed and four monomeric sodium com-
plexes of the heterocycle have been crystallographically charac-
terised. Each of these displays
a different coordination
environment at the sodium centre. The reactivity of the gallium(I)
heterocycle appears to be lower than that of closely related
but less bulky systems, though heteroleptic zinc and cadmium
gallyl complexes, [(DAB*)GaMX(tmeda)] (M = Zn or Cd,
X = Br or I), have been prepared and structurally characterised.
It has also been found that the steric bulk of the anion is not
sufficient to hinder the formation of the diamagnetic digallane
(4), [(DAB*)GaGa(DAB*)] upon oxidation of the anion. In the
second arm of this study, a new neutral, six-membered gallium(I)
heterocycle, [(ButMesNacnac)Ga:], has been prepared in moder-
ate yield, and the compound found to be a monomer in the solid
state by an X-ray crystallographic analysis. Furthermore, a low
Experimental section
General considerations
All manipulations were carried out using standard Schlenk and
glove box techniques under an atmosphere of high purity dinitro-
gen. THF, toluene and hexane were distilled over molten potass-
ium, diethyl ether was distilled over a Na–K alloy, while tmeda
was distilled over molten sodium. Melting points were deter-
mined in sealed glass capillaries under dinitrogen and are
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uncorrected. Mass spectra were recorded at the EPSRC National
Mass Spectrometric Service at Swansea University. The micro-
analysis was carried out at the Science Centre, London Metro-
politan University. Reproducible microanalyses could not be
obtained for the majority of compounds reported here due to
difficulties in completely removing the solvent of crystallisation
by vacuum drying prior to analysis, and/or the extreme air and
moisture sensitivity of the compounds. That said, the purity of
all diamagnetic compounds was judged by NMR spectroscopy
to be >95%. IR spectra were recorded using a Perkin-Elmer RX1
45); anal. calcd for C81H81N4GaNa: C 80.84%, H 6.79%,
N 4.66%; found C 80.17%, H 6.70%, N 4.33%.
N.B. During an attempt to obtain good quality crystals of 6a
by slow diffusion of a benzene solution of the compound into
hexane, a few crystals of the toluene-free compound, [{(DAB*)
Ga}Na(tmeda)] 6b, were instead obtained. This compound was
crystallographically characterised though no spectroscopic data
were obtained for it.
1
Preparation of [{(DAB*)Ga}Na(THF)4] 6c
FT-IR spectrometer as Nujol mulls between NaCl plates. H and
13C{1H} NMR spectra were recorded on either Bruker DPX300
or AvanceIII 400 spectrometers and were referenced to the reson-
ances of the solvent used. DAB*,10a H2NC6H2Pri2{C(H)Ph2}-
2,6,4,10c [ZnBr2(tmeda)],28 [CdI2(tmeda)],28 ButMesNacnacH29
and “GaI”16 were prepared by variations of literature procedures,
while all other reagents were used as received.
Compound 6a (0.30 g, 0.25 mmol) was dissolved in THF
(20 cm3) and the resultant solution concentrated to ca. 10 cm3.
Cooling to −30 °C overnight afforded yellow crystals of 6c
1
(0.26 g, 81% yield). Mp 230–232 °C (dec.); H NMR (C6D6,
400 MHz, 296 K): δ = 1.40 (br, 16H, CH2), 1.89 (s, 6H,
ArCH3), 3.57 (br, 16H, CH2O), 5.91 (s, 2H, NCH), 6.15 (s, 4H,
CHPh2), 6.93–7.28 (m, 44H, ArH); 13C{1H} NMR (C6D6,
100 MHz, 296 K): δ = 21.2 (ArCH3), 25.7 (CH2), 52.0 (CHPh2),
67.9 (CH2O), 120.4 (NCH), 126.5, 126.6, 128.1, 128.5, 128.9,
129.7, 130.3, 134.6, 142.0, 144.51, 144.5, 145.6 (ArC);
IR (Nujol) ν/cm−1: 1598(w), 1537(w), 1377(s), 1261(m),
1094(bm), 800(m), 721(w); MS EI m/z (%): 900.5 (DAB*+, 15),
Preparation of [(DAB*˙)GaI2] 5
To a suspension of Ga metal (0.44 g, 6.2 mmol) in toluene
(15 cm3) at 25 °C was added solid I2 (0.79 g, 3.1 mmol).
The mixture was ultra-sonicated until the solution had
become colourless and a pale green precipitate was produced
(ca. 2 hours). This suspension was added to a solution of DAB*
(2.80 g, 3.1 mmol) in toluene (15 cm3) at −78 °C and the
mixture slowly warmed to ambient temperature. It was then
stirred for 2 days to yield a deep red-brown solution, whereupon
volatiles were removed in vacuo. The residue was extracted into
THF (100 cm3), the extract concentrated to ca. 10 cm3 and
cooled to −30 °C overnight to give dark brown crystals of
5 (3.40 g, 89% yield). Mp 248–252 °C; MS/EI m/z (%):
+
+
439.2 (Ar*NH2 , 50), 167.0 (CHPh2 , 67).
Preparation of [{(DAB*)Ga}Na(OEt2)(tmeda)] 6d
A solution of [(DAB*·)GaI2] (2.0 g, 1.6 mmol) in THF (30 cm3)
was stirred over a sodium mirror (0.35 g, 20 mmol) at 25 °C for
2 days. The resultant yellow-red solution was filtered and vola-
tiles were removed in vacuo. The residue was suspended in a
mixture of diethyl ether (20 cm3) and tmeda (2 cm3), and the
suspension stirred at room temperature for 1 hour, whereupon
volatiles were removed in vacuo. The residue was then extracted
into diethyl ether (50 cm3) and filtered. The filtrate was concen-
trated to ca. 10 cm3 and cooled to −30 °C overnight to give
yellow crystals of 6d (0.25 g, 13% yield). Mp 238–242 °C
(dec.); 1H NMR (400 MHz, C6D6, 296 K): δ = 1.10 (t, 6H, 3JHH
= 6.8 Hz, OCH2CH3), 1.56 (s, 12H, NCH3), 1.66 (4H, NCH2),
+
+
900.5 (DAB*+, 40), 439.3 (Ar*NH2 , 100), 167.0 (CHPh2 , 18);
IR (Nujol) ν/cm−1: 1598(s), 1377(s),1259(s), 1094(s), 1020(s),
871(m), 700(s). Meaningful NMR spectroscopic data could not
be obtained for 5 due to its paramagnetic nature.
Preparation of [{(DAB*)Ga}Na(η2-C7H8)(tmeda)] 6a
3
2.01 (s, 6H, ArCH3), 3.26 (q, 4H, JHH = 6.8 Hz, OCH2), 6.45
A solution of [(DAB*˙)GaI2] (2.00 g, 1.6 mmol) in THF
(30 cm3) was stirred over a sodium mirror (0.35 g, 20 mmol) at
25 °C for 2 days. The resultant yellow-orange solution was
filtered and volatiles removed in vacuo. The residue was sus-
pended in a mixture of diethyl ether (20 cm3) and tmeda
(2 cm3), and the suspension stirred at room temperature for
1 hour, whereupon volatiles were removed in vacuo. The residue
was extracted into toluene (50 cm3) and filtered. The filtrate was
concentrated to ca. 10 cm3 and cooled to −30 °C overnight to
give 6a as yellow crystals. (1.60 g, 82% yield). Mp 222–226 °C
(s, 2H, NCH), 6.86 (s, 4H, CHPh2), 6.87–7.18 (m, 26H, ArH),
7.41–7.48 (m, 18H, ArH); 13C{1H} NMR (C6D6, 100 MHz,
296 K): δ = 15.5 (OCH2CH3), 21.4 (ArCH3), 45.7 (NCH3), 51.6
(CHPh2), 57.4 (NCH2), 65.8 (OCH2), 122.5 (NCH), 124.76,
125.8, 128.3, 128.5, 129.3, 130.2, 130.7, 139.2, 142.9, 146.8,
148.6, 150.3 (ArC); IR (Nujol) ν/cm−1: 1598(m), 1493(m), 1377
(s), 1260(s), 1094(bs), 1020(bs), 799(s); MS EI m/z (%): 970.2
((DAB*)Ga+, 10), 900.2 (DAB*+, 45).
1
(dec.); H NMR (C6D6, 400 MHz, 296 K): δ = 1.47 (s, 12H,
Preparation of Ar*N(H)C(H)2C(H)vNAr*
NCH3), 1.55 (s, 4H, NCH2), 2.01 (s, 6H, ArCH3), 2.10 (s, 3H,
PhCH3), 6.45 (s, 2H, NCH), 6.85 (s, 4H, CHPh2), 6.88–7.48 (m,
49H, ArH); 13C{1H} NMR (C6D6, 100 MHz, 296 K): δ = 21.5
(overlapping PhCH3/ArCH3), 45.6 (NCH3), 51.6 (CHPh2), 57.3
(NCH2), 122.6 (NCH), 124.8, 125.8, 127.6, 127.8, 128.3, 129.3,
130.8, 130.9, 140.6, 146.8, 148.6, 150.4 (ArC); IR ν/cm−1
(Nujol): 1598(s), 1562(m), 1260(s), 1094(s), 1020(s), 871(m),
799(s); MS EI m/z (%): 970.2 ((DAB*)Ga+, 12), 900.2 (DAB*+,
A suspension of DAB* (1.0 g, 1.11 mmol) in THF (30 cm3) was
added to a suspension of Mg powder (0.40 g, 16.7 mmol) in
diethyl ether (5 cm3). Three drops of 1,2-dibromoethane were
added to the suspension which was then heated at reflux for
4 hours yielding a red solution. Excess magnesium was filtered
off this solution and the filtrate cooled to −78 °C. Solid
NMe3·HCl (0.21 g, 2.22 mmol) was added to the solution,
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which was then warmed to ambient temperature and stirred until
the mixture became colourless. Volatiles were removed in vacuo
and residue extracted into toluene (30 cm3) and filtered.
Removal of volatiles from the filtrate in vacuo yielded an off-
white solid which was washed with diethyl ether to give Ar*N
(H)C(H)2C(H)vNAr* as a white powder (0.50 g, 50% yield).
1.95 (s, 6H, ArCH3), 6.04 (s, 2H, NCH), 6.58 (s, 4H, CHPh2),
6.88–7.54 (m, 44H, Ar); 13C{1H} NMR (C6D6, 100 MHz,
296 K): δ = 21.3 (ArCH3), 47.5 (NCH3), 51.5 (CHPh2),
70.2 (br, NCH2), 123.7 (NCH), 126.1, 126.2, 127.8, 128.2,
128.4, 128.7, 129.2, 129.7, 129.9, 130.5, 133.4, 146.0 (ArC);
IR (Nujol) ν/cm−1: 1558(w), 1377(s), 1266(w), 1103(bm), 949
(m), 798(w), 700(m); MS EI m/z (%): 900.5 (DAB*+, 22), 439.2
1
Mp 236–238 °C; H NMR (CDCl3, 400 MHz, 296 K): δ = 2.00
3
+
+
(s, 3H, ArCH3), 2.03 (s, 3H, ArCH3), 2.97 (br. d, JHH = ca.
(Ar*NH2 , 40), 167.0 (CHPh2 , 100).
3
6 Hz, 2H, NCH2), 3.83 (br. t, JHH = ca. 6 Hz, 1H, NH), 5.25
(s, 2H, CHPh2), 5.86 (s, 2H, CHPh2), 6.05 (br, 1H, NCH), 6.49
(s, 2H, m-ArH), 6.52 (s, 2H, m-ArH), 6.86–7.16 (m, 40H, ArH);
13C{1H} NMR (CDCl3, 100 MHz, 296 K): δ = 21.4 (ArCH3),
21.5 (ArCH3), 51.4 (CHPh2), 51.9 (CHPh2), 55.2 (NCH2),
126.6 (NCH), 128.3, 128.4, 128.9, 129.7, 129.8, 130.0, 132.1,
132.3, 133.4, 138.3, 143.0, 143.9, 144.0, 144.4, 147.5, 166.3
(ArC); IR (Nujol) ν/cm−1: 3126(bw, NH), 1598(m), 1156(m),
1075(s), 1028(s), 854(w), 725(s); MS EI m/z (%): 902.2
(M+, 52), 423.1 (Ar*+, 55).
N.B. A small amount of a cyclised isomeric form of Ar*N(H)
C(H)2C(H)vNAr* was isolated from this preparation. Spectro-
scopic and crystallographic data for this isomer can be found in
the ESI.‡ The reaction of Ar*N(H)C(H)2C(H)vNAr* with
GaCl3 in the presence of NPri2Et led to an intractable mixture of
products.
Preparation of [{(DAB*)Ga}CdI(tmeda)] 8
A solution of 6a (0.20 g, 0.17 mmol) and 18-crown-6 (0.05 g,
0.17 mmol) in THF (15 cm3) was added to a solution of [CdI2-
(tmeda)] (0.09 g, 0.18 mmol) in THF (10 cm3) at −80 °C,
whereupon it was warmed to room temperature over 12 hours.
Volatiles were then removed in vacuo and the residue extracted
into toluene and filtered. The filtrate was concentrated to ca.
10 cm3 and stored at −30 °C overnight to give yellow crystals of
8 (0.06 g, 26%). Mp 198–202 °C (dec.); 1H NMR (C6D6,
400 MHz, 296 K): δ = 1.64 (br, 12H, NCH3), 1.91 (s, 6H,
ArCH3), 2.10 (s, 4H, NCH2), 6.00 (s, 2H, NCH), 6.52 (s, 4H,
CHPh2), 7.02–7.50 (m, 44H, Ar); 13C{1H} NMR (C6D6,
100 MHz, 296 K): δ = 21.3 (ArCH3), 47.0 (NCH3), 51.7
(CHPh2), 69.4 (NCH2), 125.6 (NCH), 126.1, 126.5, 127.8,
128.4, 128.6, 129.1, 129.6, 129.9, 130.2, 130.4, 133.6, 145.6
(ArC); IR (Nujol) ν/cm−1: 1598(w), 1377(s), 1355(m), 1261(w),
1099(bw), 977(m), 852(w); MS EI: m/z (%): 900.4 (DAB*+,
Preparation of [N{C6H2Pri2(CHPh2)-2,6,4}C(H)]2
+
+
The aniline H2NC6H2Pri2(CHPh2)-2,6,4 (2.0 g, 5.8 mmol) was
dissolved in CH3CN (50 cm3) and glyoxal (40% in H2O,
0.42 cm3, 2.9 mmol) added to the solution. The reaction mixture
was stirred overnight at 65 °C during which time the diimine
product precipitated out as a bright yellow powder (1.7 g, 83%
100), 439.2 (Ar*NH2 , 48), 167.0 (CHPh2 , 95).
Preparation of [{(DAB*)Ga}2] 9
Tl2SO4 (0.064 g, 12.5 mmol) was added to a solution of 6a
(0.30 g, 0.25 mmol) in THF (15 cm3) at 25 °C and the mixture
stirred for 4 days. This resulted in the deposition of thallium
metal and the formation of an intense purple coloured solution.
The solution was filtered, the filtrate concentrated to ca. 10 cm3,
and then cooled to −30 °C overnight to yield purple crystals of 9
1
yield). Mp 202–204 °C; H NMR (C6D6, 300 MHz, 296 K):
3
δ = 1.08 (d, 24H, JHH = 6.8 Hz, CH(CH3)2), 3.07 (sept., 4H,
3JHH = 6.8 Hz, CH(CH3)2), 5.54 (s, 2H, CHPh2), 7.04–7.23
(m, 24H, ArH), 8.11 (s, 2H, NCH); 13C{1H} NMR (C6D6,
100 MHz, 296 K): δ = 23.3 CH(CH3)2, 28.6 (CH(CH3)2), 57.4
(CHPh2), 124.8 (NCH), 126.5, 128.5, 129.8, 137.0, 140.9,
144.8, 147.3, 163.6 (ArC); IR (Nujol) ν/cm−1: 1630(s) (CvN),
1377(s), 1031(s), 817(m), 700(s), 628(s); MS EI m/z (%): 709.3
(MH+, 5), 344.2 (H3NC6H2Pri2(CHPh2)-2,6,4+, 100); acc. mass
calc. for C52H56N2: 708.443, found: 708.4513.
1
(0.20 g, 82% yield). Mp 238–242 °C (dec.); H NMR (C6D6,
400 MHz, 296 K): δ = 1.91 (s, 12H, ArCH3), 5.02 (s, 8H,
CHPh2), 6.04 (s, 4H, NCH), 6.88–7.14 (m, 88H, ArH); 13C{1H}
NMR (C6D6, 100 MHz, 296 K): δ = 21.1 (ArCH3), 52.2
(CHPh2), 115.4 (NCH), 120.4, 126.5, 128.62, 128.9, 129.7,
129.9, 130.3, 133.2, 134.6, 139.3, 140.4, 142.0, 144.3, 144.4,
144.6, 145.6 (ArC); IR (Nujol) ν/cm−1: 1654(w), 1598(s), 1541
(m), 1377(s), 1260(s), 1094(s), 1029(s), 861(m), 799(s);
N.B. The reaction of [N{C6H2Pri2(CHPh2)-2,6,4}C(H)]2 with
“GaI” led to the formation of an intractable mixture of products.
+
MS EI m/z (%): 900.5 (DAB*+, 10), 439.2 (Ar*NH2 , 18),
+
Preparation of [{(DAB*)Ga}ZnBr(tmeda)] 7
167.0 (CHPh2 , 67).
A solution of 6a (0.30 g, 0.25 mmol) and 18-crown-6 (0.07 g,
0.26 mmol) in THF (15 cm3) was added to a solution of
[ZnBr2(tmeda)] (0.09 g, 0.26 mmol) in THF (10 cm3) at
−80 °C, whereupon it was warmed to room temperature over
12 hours. Volatiles were then removed in vacuo and the residue
extracted into toluene and filtered. The filtrate was concentrated
to ca. 2 cm3 and then layered with n-hexane to give yellow crys-
tals of 7 (0.06 g, 29%). N.B. Crystals of 7 suitable for an X-ray
diffraction experiment were grown from a benzene solution of
the compound. Mp 260–262 °C (dec.); 1H NMR (400 MHz,
C6D6, 296 K): δ = 1.79 (s, 4H, NCH2), 1.88 (s, 12H, NCH3),
Preparation of [(ButMesNacnac)Ga:] 10 and
[Ga5I4(ButMesNacnac)3] 11
To a solution of ButMesNacnacH (0.60 g, 1.43 mmol) in toluene
(20 cm3) at −78 °C was added a 1.6 M solution of n-butyl-
lithium in hexane (0.96 cm3, 0.15 mmol). The reaction mixture
was allowed to warm to room temperature and was stirred for
1 hour. The resultant solution was cooled to −78 °C and a sus-
pension of “GaI” (2.14 mmol) in toluene (20 cm3) added to it.
The mixture was allowed to warm to ambient temperature,
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resulting in the formation of a dark brown suspension. This was
filtered to give a golden-orange solution which was concentrated
to ca. 8 cm3 in vacuo. X-ray quality crystals of pale yellow 10
were formed upon letting the solution stand at room temperature
overnight. These crystals were isolated and hexane (4 cm3)
added to supernatant, which was then placed at −30 °C to give
yellow crystals of 11 overnight. Data for 10 (0.32 g, 54% yield):
Mp 234–238 °C (dec.); 1H NMR (300 MHz, C6D6, 298 K):
δ = 1.15 (s, 18H, C(CH3)3), 2.14 (s, 6H, p-CH3), 2.20 (s, 12H,
o-CH3), 5.79 (s, 1H, NCCHCN), 6.80 (s, 4H, ArH); 13C{1H}
NMR (400 MHz, C6D6, 300 K): δ = 20.5 (o-CH3), 20.8
(p-CH3), 32.0 (C(CH3)3), 42.2 (C(CH3)3), 97.7 (NCCHCN),
129.0, 132.4, 133.7, 145.2 (ArC), 171.3 (NCCH); IR v/cm−1
(Nujol): 1532(w), 1377(s), 1260(m), 1093(m), 1019(m),
722(m); MS (EI 70 eV), m/z (%): 486.2 (M+, 100); Data for 11
(0.02 g, 3% yield): Mp (dec.) 160–162 °C; 1H NMR (300 MHz,
C6D6, 298 K): δ = 1.02, 1.15, 1.21 (3 × v.br s, 54H, C(CH3)3),
1.59–2.42 (6 × v.br overlapping s, 54H, ArCH3), 5.87 (br., 3H,
NCCHCN), 6.80–7.12 (br., 12H, ArH); IR v/cm−1 (Nujol): 1377
(s), 1260(s), 864(w), 722(w), 660(w); MS EI m/z (%): 613.2
((ButMesNacnac)GaI+, 100), 487.3 ((ButMesNacnac)GaH+, 5),
418.3 (ButMesNacnacH+, 10).
References
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2 Selected recent reviews:
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2005, 1815.
3 Selected recent reviews: (a) M. Asay, C. Jones and M. Driess, Chem.
Rev., 2011, 111, 354; (b) S. Yao, Y. Xiong and M. Driess, Organometal-
lics, 2011, 30, 1748; (c) Y. Mizuhata, T. Sasamori and N. Tokitoh, Chem.
Rev., 2009, 109, 3479; (d) S. Nagendran and H. W. Roesky, Organome-
tallics, 2008, 27, 457; (e) N. J. Hill and R. West, J. Organomet. Chem.,
2004, 689, 4165.
4 Selected reviews: (a) C. Jones and A. Stasch, in The Group 13 Metals
Aluminium, Gallium, Indium and Thallium. Chemical Patterns and
Peculiarities, ed. A. J. Downs and S. Aldridge, Wiley-Blackwell, Chiche-
ster, 2011, ch. 5, pp. 285–341; (b) C. Jones, Coord. Chem. Rev., 2010,
254, 1273; (c) S. Gonzalez-Gallardo, G. Prabusankar, T. Cadenbach,
C. Gemel, M. von Hopffgarten, G. Frenking and R. A. Fischer, Struct.
Bonding, 2010, 136, 147; (d) M. Yamashita and K. Nozaki, Bull. Chem.
Soc. Jpn., 2008, 81, 1377; (e) R. J. Baker and C. Jones, Coord. Chem.
Rev., 2005, 249, 1857; (f) N. J. Hardman and P. P. Power, ACS Symp.
Ser., 2002, 822, 2; (g) Y.-C. Tsai, Coord. Chem. Rev., 2012, 256, 722.
5 (a) C. Jones, P. C. Junk, J. A. Platts and A. Stasch, J. Am. Chem. Soc.,
2006, 128, 2206; (b) J. Overgaard, C. Jones, D. Dange and J. A. Platts,
Inorg. Chem., 2011, 50, 8418.
6 (a) E. S. Schmidt, A. Jockisch and H. Schmidbaur, J. Am. Chem. Soc.,
1999, 121, 9758; (b) E. S. Schmidt, A. Schier and H. Schmidbaur,
J. Chem. Soc., Dalton Trans., 2001, 505; (c) R. J. Baker, R. D. Farley,
C. Jones, M. Kloth and D. M. Murphy, J. Chem. Soc., Dalton Trans.,
2002, 3844; (d) R. J. Baker, C. Jones, D. P. Mills, G. A. Pierce and
M. Waugh, Inorg. Chim. Acta, 2008, 361, 427; (e) Y. Liu, S. Li,
X.-J. Yang, Q.-S. Li, Y. Xie, H. F. Schaefer and B. Wu, J. Organomet.
Chem., 2011, 696, 1450.
7 (a) I. L. Fedushkin, A. N. Lukoyanov, G. K. Fukin, S. Y. Ketkov,
M. Hummert and H. Schumann, Chem.–Eur. J., 2008, 14, 8465;
(b) I. L. Fedushkin, A. N. Lukoyanov, A. N. Tishkina, G. K. Fukin,
K. A. Lyssenko and M. Hummert, Chem.–Eur. J., 2010, 16, 7563.
8 (a) N. J. Hardman, B. E. Eichler and P. P. Power, Chem. Commun., 2000,
1991; (b) S. L. Choong, W. D. Woodul, A. Stasch, C. Schenk and
C. Jones, Aust. J. Chem., 2011, 64, 1173.
9 See for example: (a) G. Prabusankar, C. Gemel, P. Parameswaran,
C. Flener, G. Frenking and R. A. Fischer, Angew. Chem., Int. Ed., 2009,
48, 5526; (b) A. Seifert, D. Scheid, G. Linti and T. Zessin, Chem.–Eur.
J., 2009, 15, 12114; (c) G. Prabusankar, A. Kempter, C. Gemel,
M.-K. Schröter and R. A. Fischer, Angew. Chem., Int. Ed., 2008, 47,
7234; (d) A. Kempter, C. Gemel, N. J. Hardman and R. A. Fischer,
Inorg. Chem., 2006, 45, 3133.
X-Ray crystallography
Crystals of 6a–d, 7, 8, 10, 11, DAB*, Ar*NH2, a cyclic isomer
of DAB*H2 and [NaI(18-crown-6)] suitable for X-ray structural
determination were mounted in silicone oil. Crystallographic
measurements were made using an Oxford Gemini Ultra diffracto-
meter using a graphite monochromator with Mo Kα radiation
(λ = 0.71073 Å). The structures were solved by direct methods
and refined on F2 by full matrix least squares (SHELX97)30
using all unique data. Hydrogen atoms have been included in
calculated positions (riding model) for all structures, except the
amino protons of Ar*NH2 and the cyclic isomer of DAB*H2,
the positional and thermal parameters of which were refined.
Crystal data, details of data collections and refinement are given
in Table S1 in the ESI.‡
Theoretical studies
10 See for example: (a) G. Berthon-Gelloz, M. A. Siegler, A. L. Spek,
B. Tinant, J. N. H. Reek and I. Markó, Dalton Trans., 2010, 39, 1444;
(b) A. Gomez-Suarez, R. S. Ramon, O. Songis, A. M. Z. Slawin,
C. S. J. Cazin and S. P. Nolan, Organometallics, 2011, 30, 5463;
(c) B. R. Dible, R. E. Cowley and P. L. Holland, Organometallics, 2011,
30, 5123.
Quantum-chemical calculations were carried out with the Turbo-
mole (version 5.9) program package,31 employing the Becke–
Perdew 86-functional32 at the RI-DFT level33 with the def2-SVP
basis set.34 The molecular orbitals of the global minimum
energy structure (showing zero imaginary frequencies) were
visualized with the aid of Molden.35 NBO orbital analyses were
performed using the program package Gaussian0936 and NBO
3.037 at the very same combination of functional and basis sets
as above.
11 L. Jafarpour, E. D. Stevens and S. P. Nolan, J. Organomet. Chem., 2000,
606, 49.
12 Selected recent reviews: (a) Y. Wang and G. H. Robinson, Dalton Trans.,
2012, 41, 337; (b) Y. Wang and G. H. Robinson, Inorg. Chem., 2011, 50,
12326; (c) Y. Wang and G. H. Robinson, Chem. Commun., 2009, 5201.
13 (a) A. V. Protchenko, L. M. A. Saleh, D. Vidovic, D. Dange, C. Jones,
P. Mountford and S. Aldridge, Chem. Commun., 2010, 46, 8546;
(b) C. Jones, D. P. Mills, R. P. Rose, A. Stasch and W. D. Woodul,
J. Organomet. Chem., 2010, 695, 2410; (c) O. Bonello, C. Jones,
A. Stasch and W. D. Woodul, Organometallics, 2010, 29, 4914;
(d) C. Jones, A. Stasch and W. D. Woodul, Chem. Commun., 2009, 113;
(e) S. T. Liddle, J. McMaster, D. P. Mills, A. J. Blake, C. Jones and
W. D. Woodul, Angew. Chem., Int. Ed., 2009, 48, 1077; (f) S. T. Liddle,
D. P. Mills, B. M. Gardner, J. McMaster, C. Jones and W. D. Woodul,
Inorg. Chem., 2009, 48, 3520; (g) C. Jones, D. P. Mills, R. P. Rose and
A. Stasch, Dalton Trans., 2008, 4395; (h) C. Jones, R. P. Rose and
Acknowledgements
CJ thanks the Australian Research Council and the donors of
The American Chemical Society Petroleum Research Fund. CS
thanks the Alexander von Humboldt Foundation for a Feodor-
Lynen Fellowship. The EPSRC Mass Spectrometry Service at
Swansea University is also thanked.
9314 | Dalton Trans., 2012, 41, 9304–9315
This journal is © The Royal Society of Chemistry 2012

View Online
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9304–9315 | 9315