Bulky aryl functionalized carbazolyl ligands: Amido alternatives to the 2,6-diarylphenyl ligand class?

Article abstract of DOI:10.1039/b716674e

Sterically encumbered amido ligands based on a 1,8-diarylcarbazol-9-yl backbone have been investigated as electronically distinct alternatives to the widely-used terphenyl ligand class in the stabilization of low-coordinate metal complexes, and structural

Full text of DOI:10.1039/b716674e

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Bulky aryl functionalized carbazolyl ligands: amido alternatives to the
2,6-diarylphenyl ligand class?†‡§¶
Natalie D. Coombs,a,b Andreas Stasch,b,c Andrew Cowley,a Amber L. Thompsona and Simon Aldridge*a
Received 30th October 2007, Accepted 26th November 2007
First published as an Advance Article on the web 11th December 2007
DOI: 10.1039/b716674e
Sterically encumbered amido ligands based on a 1,8-diarylcarbazol-9-yl backbone have been
investigated as electronically distinct alternatives to the widely-used terphenyl ligand class in the
stabilization of low-coordinate metal complexes, and structurally characterized for the first time. While
1,8-diphenylcarbazol-9-yl derivatives are readily available, facile rotation about the Cipso–Cipso bonds
leads to structurally characterized main group derivatives {e.g. [(1,8-Ph2-3,6-Me2C12H4N)K]2} in which
the coordination geometry at the metal centre is augmented by secondary metal–arene interactions. By
contrast, the extra bulk inherent in the corresponding 1,8-dimesityl ligand results in essentially
perpendicular alignments of the arene and carbazole planes, and a substituent-enforced sterically
protected pocket. Comparative structural studies of complexes containing the GaCl2 fragment imply
that the 1,8-dimesitylcarbazol-9-yl framework offers greater steric protection at the metal centre than
does the corresponding 2,6-dimesitylphenyl ligand.
1. Introduction
Carbon donor ligands offering the possibility for extremely
high steric loading at a metal centre have been exploited to
great effect in recent years in the stabilization of electronically
unsaturated complexes.1,2 Such species have been of interest not
only from a fundamental structure/bonding perspective (e.g.
unusual multiple bonding or very low coordination numbers),3,4
but also from the viewpoint of applied reactivity (e.g. in the
activation of dihydrogen).5 Thus, in recent years 2,6-diarylphenyl
ligands (I, Scheme 1), which were pioneered for main group
metal systems,3 have been further exploited in the synthesis of
landmark multiply-bonded transition metal compounds.4 In view
Scheme 1 The sterically encumbered 2,6-diarylphenyl (I) and 1,8-diaryl-
carbazol-9-yl (II) ligand families examined in the current study.
of the key steric shielding in such complexes provided by the
flanking aryl substituents, and with a view to extending the
range of bulky ligands available to the synthetic chemist, we
have targeted 1,8-diarylcarbazol-9-yl ligands (II) as alternative
systems of potential interest. Such an approach would allow
for the exploitation, for example, of the differing electronic
properties of amido (vs. aryl) ligands. To date, a number of metal
complexes containing the parent, unsubstituted carbazol-9-yl
ligand have been reported, including homoleptic derivatives of
both main group and transition metals.6 Moreover, systems
featuring additional N-donor substituents tethered to the 1- and
8-positions have also been examined in the context of transition
metal catalyst design.7
However, given the versatility and widespread application of
2,6-diarylphenyl ligand systems (I), we were surprised to find
that the structural characteristics of the potentially analogous
1,8-diarylcarbazol-9-yl derivatives have yet to be investigated.
With this in mind, and with a view to comparing the size and
angular extent of the sterically protected cavities generated by
these two ligand families, we have synthesized and structurally
characterized a number of species containing ligands of type
II. Given that complexes of empirical composition 2,6-(2,4,6-
R3C6H2)C6H3GaCl2 have been structurally characterized (for
aInorganic Chemistry, University of Oxford, South Parks Road, Oxford,
UK OX1 3QR. E-mail: simon.aldridge@chem.ox.ac.uk; Fax: +44 (0)1865
272690; Tel: +44 (0)1865 285201
bSchool of Chemistry, Cardiff University, Main Building, Park Place, Cardiff,
UK CF10 3TB
cSchool of Chemistry, Monash University, PO Box 23, Victoria, 3800,
Australia
i
R = Me, Pr),8 and are known to exhibit different degrees of
† Dedicated to Professor Ken Wade on the occasion of his 75th birthday.
‡ The HTML version of this article has been enhanced with colour images.
aggregation dependent on the steric bulk of R, we targeted
analogous type II dichlorogallium complexes as comparative
structural probes. It should be noted that target monomeric three-
coordinate amido(dihalo)gallanes are as yet unknown, despite a
range of interesting further chemistry for which they might serve
as useful starting materials.
§ CCDC reference numbers 665746–665748. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b716674e
¶ Electronic supplementary information (ESI) available: Details of all
three crystal structures and details of the synthesis of (1,8-diphenyl-3,6-
dimethylcarbazol-9-yl)potassium dimer. See DOI: 10.1039/b716674e
332 | Dalton Trans., 2008, 332–337
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˚
2. Experimental
Ka radiation source (k = 0.71073 A). Data collection and cell
refinement were carried out using DENZO, and structure solu-
tion and refinement (by full-matrix least-squares) using SIR-92,
SHELXS-97 and SHELXL-97, respectively; empirical absorption
corrections were carried out using SORTAV.10 For 1b, the hexanes
solvent is disordered on an inversion centre and was modelled
as a quarter molecule of n-hexane and a quarter molecule of
cyclohexane in the asymmetric unit. The solvent carbon atoms
were refined isotropically.
General
Manipulations involving air-sensitive compounds were carried out
under a nitrogen or argon atmosphere using standard Schlenk
line or dry box techniques. Hexanes and toluene were pre-dried
over sodium wire and purged with nitrogen prior to distillation
from potassium (hexanes) or sodium (toluene). Benzene-d6 and
chloroform-d (both Goss) were degassed and dried over potassium
(benzene-d6) or molecular sieves (chloroform-d) prior to use.
Ethanol, Pd(PPh3)4, GaCl3, MesB(OH)2 (Mes = mesityl, 2,4,6-
Syntheses
n
Me3C6H2-) and BuLi (1.6 M solution in hexanes) were used as
Synthesis of (1,8-diphenyl-3,6-dimethylcarbazol-9-yl)gallium
dichloride (3a). To a suspension of (1,8-diphenyl-3,6-dimethyl-
carbazol-9-yl)lithium (2a, 0.825 g, 2.33 mmol) in hexanes (25 cm3)
at −15 ◦C was added dropwise a solution of GaCl3 (0.411 g,
2.33 mmol) also in hexanes (15 cm3). The reaction mixture was
allowed to warm slowly to 20 ◦C and was stirred for a further
12 h, with the formation of a pale yellow solution and orange
precipitate. Isolation of the precipitate by filtration and extraction
into toluene (10 cm3) followed by layering with hexanes (30 cm3)
received; 1,8-dibromo-3,6-dimethylcarbazole and (1,8-diphenyl-
3,6-dimethylcarbazol-9-yl)lithium (2a) were prepared by minor
modification of literature methods.9 NMR spectra were measured
on a Bruker AM-400 or Jeol Eclipse 300 Plus FT-NMR spectrom-
eter; residual protons of solvent were used for reference for 1H and
13C NMR spectra. Mass spectra were measured by the EPSRC
National Mass Spectrometry Service Centre, University of Wales
Swansea and by the departmental service. Perfluorotributylamine
was used as the standard for high-resolution EI mass spectra.
Abbreviations: s = singlet, m = multiplet.
◦
led to the isolation of 3a as orange crystals at −30 C. Isolated
yield: 0.146 g, 13%. 1H NMR (300 MHz, C6D6): dH 2.34 (s,
6H, CH3 of carbazol-9-yl), 6.89 (overlapping s, 4H, carbazol-
9-yl and phenyl CH), 7.01 (m, 4H, phenyl CH), 7.26 (m, 4H,
phenyl CH), 7.76 (s, 2H, carbazol-9-yl CH). 13C NMR (76 MHz,
C6D6): dC 20.0 (CH3), 119.2 (aromatic CH), 124.8 (aromatic CH),
125.8 (quaternary aromatic carbon), 126.1 (aromatic CH), 127.8
Crystallographic method
Data for compounds 1b, 3b and (1,8-dimesityl-3,6-dimethyl-
carbazol-9-yl)potassium dimer (see Table 1) were collected on an
Enraf Nonius Kappa CCD diffractometer equipped with a Mo-
Table 1 Crystallographic data for 1b, 3b and (1,8-dimesityl-3,6-dimethylcarbazol-9-yl)potassium dimer
1
8
1
8
1b· (C6H12)· (C6H14)
3b
[(1,8-Ph2-3,6-Me2C12H4N)K]2
Empirical formula
Formula weight
Crystal system
C33.5H36.25
452.89
Triclinic
P-1
N
C32H32Cl2GaN
571.24
Monoclinic
P21/c
C52H40K2N2
771.06
Monoclinic
P21/n
Space group
Unit cell dimensions:
˚
a/A
7.939(2)
13.589(3)
25.682(5)
78.42(3)
81.45(3)
86.56(3)
2683.0(9)
4, 1.121
0.064
17.611(1)
7.962(1)
20.376(1)
90
104.70(1)
90
2763.4(1)
4, 1.373
1.211
11.050(5)
11.472(5)
16.097(5)
90
105.146(5)
90
1969.7(14)
2, 1.300
0.280
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
Volume/A
Z, Density (calc.)/Mg m−3
Absorption coefficient/mm−1
F(000)
977
1184
808
Crystal size/mm3
0.30 × 0.15 × 0.10
0.24 × 0.24 × 0.20
0.12 × 0.10 × 0.10
Theta range for data colln./◦
Index ranges:
2.97 to 25.00
5.0 to 27.5
3.13 to 27.00
h
−9 to 9
−22 to 22
0 to 10
0 to 26
29657
6730 (0.048)
99.1
0.78, 0.75
3990/0/325
1.084
R1 = 0.0459
wR2 = 0.0535
R1 = 0.0779
wR2 = 0.0773
0.77, −1.26
−14 to 14
−14 to 14
−20 to 20
8311
4287 (0.0408)
99.7
0.868, 0.778
4287/0/255
1.034
R1 = 0.0498
wR2 = 0.1065
R1 = 0.0801
wR2 = 0.1178
0.220, −0.303
k
−15 to 16
−30 to 30
14902
l
Reflections collected
Independent reflections, Rint
Completeness to theta max. (%)
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
9075 (0.0416)
96.0
0.998, 0.665
9075/4/636
1.018
R1 = 0.0874
wR2 = 0.2172
R1 = 0.1178
wR2 = 0.2372
1.451, −0.309
R indices (all data)
−3
˚
Largest diff. pk and hole/e A
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(quaternary aromatic carbon), 128.1 (quaternary aromatic car-
bon), 128.6 (aromatic CH), 135.0 (aromatic CH), 141.1 (quater-
nary aromatic carbon), 144.1 (quaternary aromatic carbon). EI-
MS (m/z): 485.0 {5%, [M]+}, 347.3 {100%, [M–GaCl2]+}: exact
mass: calc. for [M]+ 485.0223, meas. 485.0222. Microanalysis: calc.
C 64.11, H 4.14, N 2.88; meas. C 64.58, H 4.44, N 2.55.
(2H, s, CH of carbazole). 13C NMR (75.5 MHz, CDCl3): dC 19.3
(para-CH3 of Mes), 20.0 (ortho-CH3 of Mes), 20.4 (carbazol-9-
yl CH3), 119.7 (quaternary aromatic carbon), 123.6 (quaternary
aromatic carbon) 126.1 (quaternary aromatic carbon), 129.4
(aromatic CH), 131.8 (aromatic CH), 132.8 (quaternary aromatic
carbon), 136.3 (quaternary aromatic carbon), 140.0 (aromatic
CH), 140.9 (quaternary aromatic carbon), 142.8 (quaternary
aromatic carbon). EI-MS (m/z): 571.1 {10%, [M]+}, 431.2 {100%,
[M–GaCl2]+}; exact mass: calc. for [M]+ 569.1168, meas. 569.1149.
Microanalysis: calc. C 67.28, H 5.65, N 2.45; meas. C 67.66, H 5.88,
N 2.33.
Synthesis of 1,8-dimesityl-3,6-dimethylcarbazole (1b). To a
solution of 1,8-dibromo-3,6-dimethylcarbazole (3.184 g,
9.32 mmol) and Pd(PPh3)4 (1.068 g, 0.96 mmol) in toluene
(420 cm3) was added a solution of mesitylboronic acid (21.024 g,
128.35 mmol) in ethanol (100 cm3), followed by an aqueous
solution of 1M Na2CO3 (92.4 cm3, 92.4 mmol). After degassing
(argon) the reaction mixture was stirred at 80 ◦C for 18 h, filtered
while hot, and the filtrate washed with 1M NaOH (2 × 180 cm3)
and water (2 × 180 cm3). After drying over MgSO4 and filtration,
the volatiles were removed in vacuo yielding a dark brown solid
which was recrystallised from hot ethanol yielding 1 as a beige
solid. Crystals suitable for X-ray diffraction were obtained by
cooling an ethanol/hexanes solution to −30 ◦C. Isolated yield:
1.655 g, 43%. 1H NMR (300 MHz, CDCl3): dH 1.87 (s, 12H, ortho-
CH3 of Mes), 2.25 (s, 6H, para-CH3 of Mes), 2.51 (s, 6H, CH3 of
carbazol-9-yl), 6.85 (s, 2H, aromatic CH of carbazol-9-yl), 6.88
(s, 4H, aromatic CH of Mes), 7.05 (s, 1H, NH of carbazol-9-yl),
7.78 (s, 2H, aromatic CH of carbazol-9-yl). 13C NMR (76 MHz,
CDCl3): dC 19.2 (ortho-CH3 of Mes), 20.0 (para-CH3 of Mes),
20.4 (carbazol-9-yl CH3), 117.7 (quaternary aromatic carbon),
122.2 (quaternary aromatic carbon) 122.6 (quaternary aromatic
carbon), 127.0 (quaternary aromatic carbon), 127.4 (aromatic
CH), 127.7 (quaternary aromatic carbon), 133.6 (quaternary
aromatic carbon), 135.3 (quaternary aromatic carbon), 135.5
(quaternary aromatic carbon), 136.0 (aromatic CH). EI-MS
(m/z): 431.3 {100%, [M]+}; exact mass: calc for [M]+ 431.2608,
meas. 431.2610. Reproducible microanalysis was impossible to
obtain for 1b, presumably due to the presence of hexanes in the
lattice of crystalline samples.
3. Results and discussion
2,6-Diarylphenyl ligand systems (I) have been widely exploited
in the isolation of complexes containing highly unsaturated
metal centres protected, in part, by the remarkable degree of
steric shielding afforded by the flanking aryl rings.3,4 Given that
monoanionic carbazol-9-yl ligands have been reported to act as
very strong donors in transition metal chemistry,6,7 and that the
ability to functionalize such ligands at the 1 and 8 positions
via Suzuki/Miyaura coupling has previously been reported,9 we
have targeted the 1,8-diarylcarbazol-9-yl framework (II) as the
basis for a range of possible alternative ligands. In particular,
we sought answers to the following questions: (i) how do the
ligand families I and II compare in terms of the spatial and
angular extent of the sterically shielded cavity produced? and
(ii) how are the properties of II influenced by the bulk of the
flanking aryl rings? Additionally, we were interested to examine
(as a function of aryl substituent bulk) to what extent the
coordination geometry at the metal is likely to be augmented
via either intra- or inter-molecular interactions. With this in
mind, and in the knowledge that the coordination geometry
at the metal centre (and the degree of aggregation) found for
complexes of the type 2,6-(2,4,6-R3C6H2)C6H3GaCl2 is highly
sensitive to the bulk of the peripheral R substituents (Scheme 2),8
we targeted dichlorogallium complexes of the type (1,8-Ar2–3,6-
Me2C12H4)GaCl2 for comparative structural studies.
Synthesis of (1,8-dimesityl-3,6-dimethylcarbazol-9-yl)gallium
dichloride (3b). To a solution of 1,8-dimesityl-3,6-dimethyl-
carbazole (1.211 g, 2.81 mmol) in hexane (100 cm3) was added
nBuLi (2.08 cm3 of a 1.6 M solution in hexanes, 3.33 mmol) and the
reaction mixture stirred at 20 ◦C for 3 h, after which the solution
was concentrated in vacuo ca. 15 cm3. The resulting precipitate
was isolated by filtration and dried in vacuo yielding an off-white
powder (0.905 g, 74%) of (1,8-dimesityl-3,6-dimethylcarbazol-9-
yl)lithium (2b) which was used in the subsequent step without
further purification. To a suspension of 2b (0.700 g, 1.60 mmol)
in hexanes (20 cm3) was added a solution of GaCl3 (0.282 g,
1.60 mmol) also in hexanes (30 cm3) at −30 ◦C. The reaction was
warmed slowly to 20 ◦C and stirred for 12 h. The resulting grey pre-
cipitate was isolated by filtration, extracted into toluene (15 cm3),
and filtered. Hexanes (50 cm3) were added to the concentrated
toluene solution yielding (1,8-dimesityl-3,6-dimethylcarbazol-9-
yl)gallium dichloride (3b) as a pale purple crystalline solid.
Crystals suitable for X-ray diffraction were obtained by layering
a concentrated toluene solution with hexanes (ca. 1 : 5) and
Scheme 2 Variation in the degree of aggregation for 2,6-diarylphenylgal-
lium dichloride complexes as a function of the steric bulk of the flanking
aryl rings.
Ligand syntheses can readily be carried out using 1,8-dibromo-
3,6-dimethylcarbazole and the corresponding arylboronic acids
(R = Ph, Mes) according to the methodology developed by
Spitzmesser and Gibson (Scheme 3).9 The propensity of carbazole
itself to undergo bromination reactions at the 3 and 6 positions,
◦
1
storage at −30 C for 1 week. Isolated yield: 0.488 g, 54%. H
NMR (300 MHz, CDCl3): dH 1.75 (12H, s, ortho-CH3 of Mes),
1.78 (6H, s, para-CH3 of Mes), 2.13 (6H, s, CH3 of carbazole),
6.49 (2H, s, CH of carbazole), 6.61 (4H, s, CH of Mes), 7.59
334 | Dalton Trans., 2008, 332–337
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12
˚
(ca. 4.0 A). These secondary interactions also fall within the
range of previously reported potassium–arene contacts,14 and are
presumably responsible for the observed disrotatory rotation of
the phenyl substituents about the Cipso–Cipso bonds. This orientation
of the phenyl rings allows for further intermolecular interactions
involving the potassium centre. Thus, a centrosymmetric dimeric
structure is adopted in the solid state through pairwise interaction
of each potassium with C(5) and C(6) of the second carbazolyl
Scheme 3 Syntheses of carbazolylgallium dichloride complexes 3a and
3b. Key reagents and conditions: (i) ArB(OH)2 (10–15 equiv.), Pd(PPh3)4,
Na2CO3, toluene : ethanol (4 : 1), 80 ◦C, 18 h, 43% for 1b; (ii) nBuLi
(1.2 equiv.), hexanes, 20 ◦C, 3 h, then GaCl3 (1.0 equiv.), hexanes, −30 to
20 ◦C, 12 h, 54% for 3b.
˚
ligand [at distances of 3.059(2) and 3.080(2) A]. These intermolec-
ular contacts lead to significant bending of the K–N vector out of
the parent carbazolyl ligand plane; the angle defined by K(1), N(1)
and the centroid of the central five-membered carbazolyl ring is
150.37(5)◦.
necessitates the use of the protected 3,6-dimethyl derivative, which
then undergoes bromination at the desired 1 and 8 positions.
In common with previous observations of Suzuki/Miyaura
chemistry,11 the yields of the final coupling reactions used to
generate 1a and 1b are strongly dependent on the steric bulk of the
boronic acid. Thus, while 1a has been reported to be synthesized
in yields of up to 85% using a Pd(PPh3)4/Na2(CO)3/10 : 3
ethanol : water protocol, 1b is synthesized from mesitylboronic
acid in 43% yield, and only a trace of the corresponding 2,4,6-
triisopropylphenyl (Trip) derivative is obtained from TripB(OH)2
using the same catalytic cocktail. Nevertheless 1a and 1b are
available using this methodology in multi-gramme quantities and
can be used to synthesize the desired dichlorogallium complexes
3a and 3b in moderate yields by a simple one-pot lithiation,
electrophilic GaCl3 quenching procedure. While the lithiated
intermediates 2a and 2b were typically generated in situ without
isolation we were successful in structurally characterizing the
potassium analogue of 2a (see Fig. 1 and ESI¶).
Dichlorogallium complexes 3a and 3b have been characterized
by standard spectroscopic and analytical techniques and 3b
by single crystal X-ray diffraction. While 1H and 13C NMR
data are consistent with the presence of the respective 1,8-
diaryl-3,6-dimethylcarbazol-9-yl units, and mass spectrometric
data (including accurate mass determination) are consistent with
the formulation of each species as the desired GaCl2 complex,
determination of the state of aggregation hinges on single crystal
X-ray diffraction studies. Several sets of crystals of 3a were
obtained (from various solvent systems), but these were sufficient
only to obtain very poor quality structure solutions. While such
data are consistent with a monomeric structure for 3a, it is
apparent from the structure of [(1,8-Ph2-3,6-Me2C12H4N)K]2 that
the conformational flexibility inherent in diphenyl substituted
ligands of this type is unlikely to be compatible with the isolation of
low coordinate metal systems, allowing as it does close secondary
inter- or intramolecular metal–arene interactions. Attention was
therefore switched to the corresponding dimesityl derivatives in
the hope that the increased bulk of the flanking arene rings would
restrict such flexibility. In practice the 1,8-dimesitylcarbazol-9-yl
ligand does indeed prove to offer a more rigid sterically protected
‘pocket’. Thus, the solid state structure of the parent ligand itself
(Fig. 2) features mesityl substituents which, despite the minimal
steric demands of the carbazolyl substituent (i.e. hydrogen) are
Fig. 1 Molecular structure of (1,8-diphenyl-3,6-dimethylcarbazol-9-yl)-
potassium dimer. Hydrogen atoms omitted for clarity; ORTEP el-
˚
lipsoids set at the 50% probability level. Key bond lengths (A)
and angles (◦): K(1)–N(1) 2.745(2), K(1)–C(15) 3.181(2), K(1)–C(20)
3.227(2), K(1)–C(22) 3.202(2), K(1)–C(5ꢀ) 3.059(2), K(1)–C(6ꢀ) 3.080(2)
K(1)–N(1)–centroid of five membered ring 150.4(1). Symmetry operations
used to generate equivalent atoms: −x + 1, −; y, −z + 1.
Fig. 2 Molecular structures of (left) one of the two independent
molecules in the unit cell of 1b and (right) of 3b. Solvent molecules
and hydrogen atoms [except that attached to N(2)] omitted for clarity;
ORTEP ellipsoids set at the 50% probability level. Key bond lengths
The solid state structure of 1,8-diphenyl-3,6-dimethylcarbazol-
9-yl potassium provides evidence that rotation of the phenyl rings
within the 1,8-diphenyl-substituted ligand about the Cipso–Cipso
bonds is sufficiently facile such as to accommodate secondary
◦
˚
(A) and angles ( ): (for 1b) N(2)–C(33) 1.392(4), N(2)–C(44) 1.389(4),
angles between least squares mesityl and carbazolyl planes 74.2, 82.3;
(for 3b) Ga(1)–Cl(1) 2.127(1), Ga(1)–Cl(2) 2.112(1), Ga(1)–N(1) 1.852(2),
Ga(1)–C(13) 3.179(2), Ga(1)–C(24) 2.851(2), Ga–C(24) N(1)–Ga(1)–Cl(1)
120.36(8), N(1)–Ga(1)–Cl(2) 124.75(8), Cl(1)–Ga(1)–Cl(2) 113.28(4), an-
gles between least squares mesityl and carbazolyl planes 87.7, 89.0.
metal–arene interactions. Each (1,8-Ph2-3,6-Me2C12H4N)K unit
12,13
˚
features not only a relatively short K–N distance [2.745(2) A],
but also intramolecular contacts with the ipso and ortho carbons
˚
of both pendant phenyl rings [3.181(2)-3.227(2) A] which are
also well within the sum of the respective van der Waals radii
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inclined at angles with respect to the carbazolyl backbone which
are significantly closer to perpendicular (74.1–82.3◦ for the two
independent molecules in the asymmetric unit). The correspond-
ing angles measured for GaCl2 complex 3b are even closer to 90◦
(89.0 and 87.7◦); in addition relatively long Ga · · · Cipso contacts
ing 1,8-dibromo derivative by Suzuki/Miyaura chemistry and
have been investigated as potential alternatives to the widely-
used terphenyl ligand class in the stabilization of low-coordinate
metal complexes. Studies of the parent 1,8-diphenylcarbazol-9-yl
derivatives are consistent with facile rotation about the Cipso–Cipso
bonds leading to structurally characterized main group derivatives
in which the coordination geometry at the metal centre is aug-
mented by close metal–arene interactions. By contrast, the extra
bulk inherent in the corresponding 1,8-dimesityl ligand results in
essentially perpendicular alignments of the arene and carbazole
planes, and a substituent-enforced sterically protected pocket.
Moreover, comparative structural studies imply that the 1,8-
dimesitylcarbazol-9-yl framework offers significantly greater steric
protection at the metal centre than does the corresponding 2,6-
dimesitylphenyl ligand. As such, further studies of the reactivity
of 3b with respect to reduction, halide abstraction and halide
metathesis chemistries are currently ongoing.
˚
[2.851(2) and 3.179(2) A] are observed. While the shorter of these
distances is actually shorter than that measured by Schmidbaur
and co-workers for 1,2,4,5-tetramethylbenzene coordinated to
˚
gallium [closest Ga–C contact 2.965(4) A, albeit for Ga(I) rather
than Ga(III)],15,16 much shorter M · · · Cipso contacts have been
observed by Power and co-workers for complexes containing
˚
2,6-diarylphenyl ligands [e.g. 2.294(1) A for (2,6-Dipp2C6H3Cr)2;
Dipp = 2,6-iPr2C6H3].4c Thus, despite the noticeable asymmetry
in the positioning of the GaCl2 unit between the flanking arene
rings in 3b, the relatively long Ga–Cipso distances and the trigonal
ꢀ
planar coordination geometry [ angles at gallium = 358.95(8)◦]
are consistent with weak secondary contacts for this ligand.
To our knowledge 3b represents the first monomeric three-
coordinate amidogallium dihalide to be reported in the liter-
Acknowledgements
˚
ature, although the Ga–N distance [1.852(2) A] is similar to
that reported for other trigonal planar gallium monoamides
We acknowledge the EPSRC for funding and for access to the
National Mass Spectrometry Service Centre, Swansea University.
17
˚
featuring sterically bulky substituents [1.829(9)–1.937(3) A].
˚
Ga–Cl distances [2.127(1), 2.112(1) A] are similar to those
˚
measured for 2,6-(Trip)2C6H3GaCl2 [2.113(4), 2.124(3) A] which
References
also features a trigonal planar gallium centre, and significantly
shorter than the terminal Ga–Cl bonds found in dimeric
[2,6-(Mes)2C6H3GaCl2]2 which features tetra-coordinate gallium
1 See for example: (a) P. P. Power, J. Organomet. Chem., 2004, 689, 3904.
2 C. Stanciu, A. F. Richards, J. C. Fettinger, M. Brynda and P. P. Power,
J. Organomet. Chem., 2006, 691, 2540.
8
˚
[2.172(5), 2.290(4) A]. Further analysis of the structures of 3b
3 For recent reports in main group chemistry, see for example: (a) P. P.
Power, Organometallics, 2007, 26, 4362; (b) Y. Wang and G. H.
Robinson, Organometallics, 2007, 36, 2; (c) Z. Zhu, M. Brynda, R. J.
Wright, R. C. Fischer, W. A. Merrill, E. Rivard, R. Wolf, J. C. Fettinger,
M. M. Olmstead and P. P. Power, J. Am. Chem. Soc., 2007, 129, 10847;
(d) B. Qunllian, Y. Wang, P. Wei, A. Handy and G. H. Robinson,
J. Organomet. Chem., 2006, 691, 3765; (e) X.-J. Yang, Y. Wang, B.
Quillian, P. Wei, Z. Chen, P. v. R. Schleyer and G. H. Robinson,
Organometallics, 2006, 25, 925; (f) R. J. Wright, M. Brynda, J. C.
Fettinger, A. R. Betzer and P. P. Power, J. Am. Chem. Soc., 2006,
128, 12498; (g) R. J. Wright, M. Brynda and P. P. Power, Angew. Chem.,
Int. Ed., 2006, 45, 5953; (h) Y. Wang, B. Quillian, X.-Y. Yang, P. Wei,
Z. Chen, C. S. Wannere, P. v. R. Schleyer and G. H. Robinson, J. Am.
Chem. Soc., 2005, 127, 7672; (i) J. D. Young, M. A. Khan and R. J.
Wehmschulte, Organometallics, 2004, 23, 1965.
4 For recent reports in transition metal chemistry, see for example: (a) R.
Wolf, M. Brynda, C. Ni, G. J. Long and P. P. Power, J. Am. Chem. Soc.,
2007, 129, 6076; (b) D. L. Kays and A. Cowley, Chem. Commun., 2007,
1053; (c) T. Nguyen, A. D. Sutton, M. Brynda, J. C. Fettinger, G. J.
Long and P. P. Power, Science, 2005, 310, 844.
5 See, for example:G. H. Spikes, J. C. Fettinger and P. P. Power, J. Am.
Chem. Soc., 2005, 127, 12232.
6 For examples of homoleptic main group and transition metal complexes
containing the carbazol-9-yl ligand see: (a) M. A. Beswick, C. N.
Harmer, P. R. Raithby, A. Steiner, K. L. Verhorevoort and D. S. Wright,
J. Chem. Soc., Dalton Trans., 1997, 2029; (b) D. Barr, A. J. Edwards, P. R.
Raithby, M.-A. Rennie, K. Verhorevoort and D. S. Wright, J. Chem.
Soc., Chem. Commun., 1994, 1627.
7 For examples of carbazol-9-yl ligands bearing additional donors
tethered at the 1 and 8 positions see: (a) M. Moswer, B. Wucher, D. Kunz
and F. Rominger, Organometallics, 2007, 26, 1204; (b) J. A. Gaunt, V. C.
Gibson, A. Haynes, S. K. Spitzmesser, A. J. P. White and D. J. Williams,
Organometallics, 2004, 23, 1015; (c) V. C. Gibson, S. K. Spitzmesser,
A. J. P. White and D. J. Williams, Dalton Trans., 2003, 2718; (d) G. J. P.
Britovsek, V. C. Gibson, O. D. Hoarau, S. K. Spitzmesser, A. J. P. White
and D. J. Williams, Inorg. Chem., 2003, 42, 3454.
and of [2,6-(Ar)2C6H3GaCl2]n (Scheme 2; Ar = Trip, n = 1; Ar =
Mes, n = 2) allows useful comparison of the steric properties of the
1,8-diarylcarbazol-9-yl and 2,6-diarylphenyl ligand classes. Thus,
from a superficial perspective, the fact that 3b is mononuclear,
featuring a three-coordinate trigonal planar gallium centre while
2,6-(Mes)2C6H3GaCl2 dimerizes via bridging chloride ligands
(to give four-coordinate metal centres) implies that for a given
flanking aryl ring (i.e. Mes in this case) the carbazolyl framework
offers greater bulk in the vicinity of the metal centre.8 Indeed,
comparison of the spatial and angular extent of the cavity between
the flanking aryl rings confirms this inference. The separation
˚
between the two mesityl ring centroids in 3b is 6.53 A, while
˚
˚
the mean distance in [2,6-(Mes)2C6H3GaCl2]2 is 7.44 A (7.35 A
for the corresponding mononuclear Trip complex); similarly the
angle between the two mesityl least squares planes in 3b is 35.1◦,
compared to 58.7◦ (mean) for [2,6-(Mes)2C6H3GaCl2]2 and 64.6◦
for the Trip analogue.8 All-in-all such, geometric data imply
a significantly wider cone angle for the 1,8-dimesitylcarbazol-
9-yl ligand system than its 2,6-dimesitylphenyl counterpart, an
inference which is confirmed by calculation of the Tollman cone
angle (H) using an established method for ligands of this type
(i.e. with H > 180◦).18 Thus, a cone angle of 269◦ is calculated
for the 1,8-dimesitylcarbazol-9-yl ligand based on the distances
measured from the crystal structure of 3b, with the analogous
values of 185◦ and 201◦ being calculated for the 2,6-Mes2 and
2,6-Trip2 derivatized phenyl ligands, respectively.8,18
4. Conclusions
8 (a) B. Twamley and P. P. Power, Chem. Commun., 1999, 1805; (b) J. Su,
X.-W. Li and G. H. Robinson, Chem. Commun., 1998, 2015; (c) R. C.
Crittendon, X.-W. Li, J. Su and G. H. Robinson, Organometallics, 1997,
16, 2443.
Sterically encumbered amido ligands based on a 1,8-diaryl-
carbazol-9-yl backbone can be synthesized from the correspond-
336 | Dalton Trans., 2008, 332–337
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˚
9 S. K. Spitzmesser and V. C. Gibson, J. Organomet. Chem., 2003, 673,
95.
13 Related K–N distances in the range: 2.740(7)-3.019(4) A are listed in
the Cambridge Structural Database (27/09/2007) with a bond length
of 2.774(2) A having been determined for the [K(18-crown-6)] salt of
the parent carbazol-9-yl ligand: H. Esbak and U. Behrens, Z. Anorg.
+
˚
10 (a) Denzo: Z. Otwinowski and W. Minor, in Methods in Enzymology,
ed. C.W. Carter and R. M. Sweet, Academic Press, New York. 1996,
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Sheldrick, University of Go¨ttingen, 1997; (e) R. H. Blessing, Acta
Crystallogr., Sect. A, 1995, A51, 33.
11 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
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1579.
15 H. Schmidbaur, R. Nowak, B. Huber and G. Mu¨ller, Polyhedron, 1990,
9, 283.
12
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16 Covalent radii for Ga(I) and Ga(III): 1.13 and 0.62 A, respectively .
17 C. J. Carmalt, Coord. Chem. Rev., 2001, 223, 217.
18 C. A. Tollman, Chem. Rev., 1977, 77, 313.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 332–337 | 337
©