Dimethylxanthene- and dibenzofuran-diamido complexes of titanium

Article abstract of DOI:10.1016/j.poly.2005.09.011

The new ligands 4,5-dialkyl-and-diaryl-amido-9,9-dimethylxanthene and 4,6-dialkyl-and-diaryl-amido dibenzofuran serve as rigid, pre-organised and tunable 'spectators' for group 4 metals. The structure of a titanium complex with 4,5-di-cyclohexylamido-9,9-dimethylxanthene is reported.

Full text of DOI:10.1016/j.poly.2005.09.011

Polyhedron 25 (2006) 859–863
www.elsevier.com/locate/poly
Dimethylxanthene- and dibenzofuran-diamido complexes of titanium
*
Robin M. Porter, Andreas A. Danopoulos
School of Chemistry, University of Southampton, Highfield Campus, Southampton, Hants SO17 1BJ, UK
Received 4 July 2005; accepted 21 September 2005
Available online 27 October 2005
Honouring the outstanding chemist and special friend Professor Michael B. Hursthouse on the occasion of his retirement.
Abstract
The new ligands 4,5-dialkyl-and-diaryl-amido-9,9-dimethylxanthene and 4,6-dialkyl-and-diaryl-amido dibenzofuran serve as rigid,
pre-organised and tunable ÔspectatorsÕ for group 4 metals. The structure of a titanium complex with 4,5-di-cyclohexylamido-9,9-
dimethylxanthene is reported.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Amido titanium; Crystal structure; Xanthene dibenzofuran; Catalytic amination
1. Introduction
the n-/i-aldehyde ratio is influenced by the bite angle of
the chelating diphosphine [1d].
The development of new rigid, pre-organised ligands is
an active area of research. Of particular interest is their
use in homogeneous catalysis, where a geometric perturba-
tion introduced by such ligands could improve the activity
or selectivity of the catalyst. Demonstrations of this
concept in the catalytic aziridination, cyclopropanation,
C–H/C–C activation and hydroformylation reactions ap-
peared recently [1]. The use of rigid preorganised ligands
with early transition metals is less common. In addition
to certain ansa-cyclopentadienyls [2], recent work has fo-
cussed on amido ligands, which could offer wide scope
for electronic and steric tuning and ligand design [3]. The
generation of low coordination number, electronically/
sterically unsaturated metal species, the suppression of b-
hydrogen elimination and the modulation of the rate of
insertion reactions are attractive goals, where ligand preor-
ganisation may prove useful. Rigid xanthene and dibenzo-
furan bridged chelating diphosphines have successfully
been used in Rh catalysed hydroformylation. In this case,
As part of our ongoing research aiming at the devel-
opment of novel ligands for the early transition metals,
we report here a facile method for the synthesis of
chelating dialkyl- and diaryl-amido ligands with xanthene
and dibenzofuran backbones and initial results on the
synthesis of their titanium complexes. The tunable rigid-
ity, bite-angle, N-attached bulk and donor characteristics
make these ligands useful building blocks for the design
of the metal coordination environment. They are comple-
mentary to the family of the tridentate diamido ligands
developed by Schrock et al. [4]. Hagadorn et al. [5] has
recently prepared bimetallic xanthene bis-amidinate com-
plexes of Zr and Ti. A review on the catalytic activity of
Ti complexes in C–C and C–N bond formation reactions
has recently appeared [6].
2. Experimental
2.1. General methods
Elemental analyses were carried out by the University
College, London, microanalytical laboratory. All manipu-
lations were performed under nitrogen in a Braun
*
Corresponding author. Tel.: +02380594116; fax: +02380596805.
E-mail address: ad1@soton.ac.uk (A.A. Danopoulos).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.09.011

860
R.M. Porter, A.A. Danopoulos / Polyhedron 25 (2006) 859–863
glove-box or using standard Schlenk techniques, unless sta-
ted otherwise. Solvents were dried using standard methods
and distilled under nitrogen prior use.
The starting materials 4,5-dibromo-2,7-di-tert-butyl-9,9-
dimethylxanthene [7], 4,5-dibromo-dibenzofuran [8], Ti
(NMe2)2Cl2 [9] and Ti(benzyl)4 [10] were prepared by liter-
ature methods.
NMR data were recorded on Bruker AMX-300 and
DPX-400 spectrometers, operating at 300 and 400 MHz
(1H), respectively. The spectra were referenced internally
using the signal from the residual protio-solvent (1H) or
the signals of the solvent (13C). Mass spectra (ES+)
were run from acetonitrile solutions on a VG Biotec
platform.
moved in vacuo and the residue was recrystallised from
boiling ethanol to yield the product as a cream coloured so-
lid. Yield: 2.53 g, 69%. Mass spectrometry (EI): m/z, 590
(M + 1)+. dH (CDCl3): 1.39 (18H, s, But), 1.89 (6H, s, xan-
thene CH3), 2.38 (12H, s, mesityl CH3), 2.44 (6H, s, mesityl
CH3), 5.73 (2H, s, NH), 6.37 (2H, s, aromatics), 7.03 (2H,
s, aromatics), 7.09 (4H, s, mesityl aromatics). 13C{1H}
(CDCl3): 18.98, 21.55 (mesityl CH3), 32.15 (xanthene
CH3), 32.20 (But), 35.18, 35.67 (tertiary carbons), 108.34,
111.49 (aromatic CHs), 129.86 (mesityl aromatic CHs),
130.35, 134.60, 135.43, 135.77, 136.45, 137.55, 146.11 (qua-
ternary aromatics).
2.4. 4,6-(N,N0-bis-mesitylamino)-dibenzofuran (LbmesH2)
2.2. 4,5-(N,N0-bis-cyclohexylamino)-2,7-di-tert-butyl-9,9-
dimethyl-xanthene (LacyH2)
An ampoule was charged with 4,6-dibromo-dibenzo-
furan (1.00 g, 3.1 mmol), sodium tert-butoxide (0.88 g,
9.2 mmol), Pd(dba)2 (0.18 g, 0.3 mmol), 1,3-bis-(2,6-diiso-
propylphenyl)imidazolinium chloride (0.26 g, 0.6 mmol)
and 1,4-dioxane (20 cm3). To this was added 2,4,6-trime-
thylaniline (0.99 g, 7.3 mmol) and the ampoule was
closed under reduced pressure and heated at 105 ꢁC for
18 h. The reaction was allowed to cool and water
(100 cm3) was added before extracting the reaction into
diethyl ether (3 · 100 cm3). The organic extracts were
combined, filtered and dried with MgSO4. Volatiles were
removed in vacuo and the residue was recrystallised from
boiling ethanol to yield the product as a white solid.
Yield: 0.82 g, 61%. Mass spectrometry (EI): m/z, 435
(M + 1)+ dH (CDCl3): 2.28 (12H, s, mesityl methyls),
2.36 (6H, s, mesityl methyls), 5.69 (2H, s, NH), 6.28
(2H, d, aromatics), 7.01 (4H, s, mesityl aromatics), 7.09
(2H, t, aromatics), 7.33 (2H, d, aromatics). 13C{1H}
(CDCl3): 18.94, 21.64 (mesityl methyls), 109.86, 110.46,
124.27 (aromatic CHs), 125.85 (quaternary aromatic),
129.98 (mesityl aromatic CHs), 133.23, 135.55, 136.43,
136.79, 145.01 (quaternary aromatics).
An ampoule was charged with 4,5-dibromo-2,7-di-tert-
butyl-9,9-dimethyl-xanthene (2.80 g, 5.83 mmol), sodium
tert-butoxide (1.70 g, 17.5 mmol), Pd(dba)2 (0.10 g,
0.175 mmol), 1,3-bis-(2,6-diisopropylphenyl)imidazolinium
chloride (0.15 g, 0.35 mmol) and 1,4-dioxane (40 cm3). To
the mixture was added cyclohexylamine (1.27 g,
12.83 mmol) and the ampoule was closed under reduced
pressure and heated at 100 ꢁC for 60 h. The reaction was
allowed to cool and water (40 cm3) was added. The mixture
was extracted with diethyl ether (2 · 100 cm3 then 50 cm3).
The combined organic extracts were filtered and dried with
MgSO4. Evaporation of the volatiles under reduced pres-
sure yielded a pale yellow solid that was recrystallised from
boiling petroleum (60/80) to give LacyH2 as a pale yellow
solid. Yield: 2.29 g, 76%. Mass spectrometry (EI): m/z,
517 (M + 1)+ dH (CDCl3): 1.20–2.15 (20H, m, cyclohexyl
CH2), 1.35 (18H, s, But), 1.62 (6H, s, CH3), 3.40–3.50
(2H, m, cyclohexyl CH a to amino N), 4.03 (2H, d, NH),
6.62 (2H, s, aromatics), 6.77 (2H, s, aromatics). 13C{1H}
(CDCl3): 25.30, 26.75 (cyclohexyl CH2s), 32.25 (methyls),
32.32 (But), 34.07 (cyclohexyl CH2), 35.30, 35.58 (tertiary
carbons), 52.04 (cyclohexyl CH a to amino N), 107.61,
110.58 (aromatic CHs), 130.12, 135.59, 137.36, 146.13
(quaternary aromatics).
2.5. Bis-(N,N0-mesitylamido-dibenzofuran) dilithium salt
(LbmesLi2)
To a stirred, cooled (ꢀ78 ꢁC) solution of (LbmesH2)
(0.22 g, 0.5 mmol) in petroleum (20 cm3), was added slowly
via cannula BunLi (0.4 cm3 of 2.45 M solution in hexanes,
1.0 mmol). The mixture was stirred at ꢀ78 ꢁC for 1 h, al-
lowed to reach room temperature and stirred for 15 h. This
gave a colourless precipitate, which was isolated by filtra-
tion and dried in vacuo. Yield: 0.190 g, 85%. dH (C6D6):
1.65 (12H, s, mesityl methyls), 2.09 (6H, s, mesityl meth-
yls), 6.32 (2H, d, aromatics), 6.63 (4H, s, mesityl aromat-
ics), 6.90–7.05 (4H, m, aromatics). 13C{1H} (C6D6):
19.70, 21.33 (mesityl methyls), 107.27, 112.15 (aromatic
CHs), 126.94 (quaternary aromatics), 127.84 (aromatic
CHs), 131.27 (mesityl aromatic CHs), 131.94, 132.12,
142.25, 145.49, 146.40 (quaternary aromatics). The dili-
thium salts of (LacyH2), and (LamesH2) were prepared in
an analogous way.
2.3. 4,5-(N,N0-bis-mesitylamino)-2,7-di-tert-butyl-9,9-
dimethyl-xanthene (LamesH2)
An ampoule was charged with 4,5-dibromo-2,7-di-tert-
butyl-9,9-dimethyl-xanthene (3.00 g, 6.25 mmol), sodium
tert-butoxide (1.80 g, 18.75 mmol), Pd(dba)2 (0.18 g,
0.3 mmol), 1,3-bis-(2,6-diisopropylphenyl)imidazolinium
chloride (0.26 g, 0.6 mmol) and 1,4-dioxane (60 cm3). To
this was added 2,4,6-trimethylaniline (1.86 g, 13.74 mmol),
the ampoule was closed under reduced pressure and heated
at 105 ꢁC for 4 days. The reaction was allowed to cool and
water (40 cm3) was added. The mixture was extracted with
diethyl ether (3 · 100 cm3). The combined organic extracts
were filtered and dried with MgSO4. Volatiles were re-

R.M. Porter, A.A. Danopoulos / Polyhedron 25 (2006) 859–863
861
2.6. (Lbmes)Ti(NMe2)2
dH (C6D6): 1.10–2.65 (20H, m, cyclohexyl CH2s), 1.35
(18H, s, But), 1.57 (6H, s, methyls), 2.92 (4H, s, benzyl
CH2s), 5.78 (2H, br m, cyclohexyl CH a to amido N),
6.70–6.85 (4H, m, aromatics), 6.95–7.05 (6H, m, aromat-
ics), 7.10–7.20 (4H, m, aromatics). 13C{1H}: 27.57, 28.24,
29.86 (cyclohexyl CH2s), 32.57 (But), 33.05 (methyls),
35.02, 35.74 (cyclohexyl CH2s), 38.56, 38.86 (tertiary car-
bons), 60.57 (cyclohexyl CH a to amido N), 84.53 (benzyl
CH2s), 107.85, 111.75, 123.50 (aromatic CHs), 127.94 (qua-
ternary aromatic), 128.02, 129.16 (aromatic CHs), 142.32,
143.74, 148.52, 149.01 (quaternary aromatics).
To a stirred, cooled (ꢀ78 ꢁC) solution of (LbmesH2)
(0.24 g, 0.41 mmol) in diethyl ether (10 cm3), was added
slowly via cannula BunLi (0.34 cm3 of 2.45 M solution in
hexanes, 0.81 mmol). The mixture was stirred at ꢀ78 ꢁC
for 1 h, allowed to reach room temperature and stirred
for a further hour to give an orange solution with a colour-
less precipitate. The mixture was cooled to ꢀ25 ꢁC and a
cooled (ꢀ25 ꢁC) solution of Ti(NMe2)2Cl2 in diethyl ether
(7 cm3) was added slowly via cannula. The reaction was
slowly allowed to reach room temperature and stirred for
ca. 15 h to give a red solution and grey precipitate. The vol-
atiles were removed in vacuo and the residue extracted into
petroleum (20 cm3) and filtered through Celite. Removal of
the volatiles in vacuo gave (Lbmes)Ti(NMe2)2 as an orange/
red oil. Yield: 0.157 g, 53%. dH (C6D6): 1.23 (18H, s, But),
1.74 (6H, s, xanthene methyls), 2.17 (6H, s, mesityl meth-
yls), 2.22 (12H, s, mesityl methyls), 2.83 (12H, s, dimethyla-
mides), 6.26 (2H, s, aromatics), 6.80–7.00 (6H, m,
aromatics). 13C{1H} (C6D6): 19.92, 21.64 (mesityl methyls),
29.20 (xanthene methyls), 32.64 (But), 35.76, 37.38 (tertiary
carbons), 45.99 (dimethylamides), 108.18, 109.01 (aromatic
CHs), 129.79 (mesityl aromatic CHs), 132.04, 133.66,
134.23, 142.57, 147.73, 148.01, 149.39 (quaternary
aromatics).
2.9. Structure determination of (Lacy)Ti(NMe2)2
The data set was collected on a Bruker Nonius Kappa
CCD area detector diffractometer with rotating anode
FR591 and an Oxford Cryosystems low-temperature
device operating in omega scanning mode with phi and
omega scans to fill the Ewald sphere. The programs used
for control and integration were COLLECT, SCALEPACK and
DENZO [11,12]. An orange air sensitive crystal was mounted
on a glass fibre with silicon grease, from Fomblin vacuum
oil. Chemical formula C39H62N4OTi. C4H10O; crystal
ꢀ
system triclinic; space group P1; cell dimensions a =
˚
˚
˚
10.2581(4) A, b = 12.2629(6) A, c = 18.6460(11) A, a =
94.693(2)ꢁ, b = 95.234(3)ꢁ, c = 107.671(2)ꢁ; Z = 2,
l/mmꢀ1 0.230; F(000) 708.0; number of data collected
23,300; number of unique data 7739; Rint 0.0981; final
R(|F|) for I > 2r(I) 0.0955; final R(F2) for all data 0.1547.
All solutions and refinements were performed using the
WINGX package [13] and all software packages within. All
non-hydrogen atoms were refined using anisotropic ther-
mal parameters and hydrogens were added using a riding
model. The quality of the data was affected by the small
size of the crystal and the rapid deterioration of the crystal
quality during crystal selection possibly due to solvent loss.
The structure contains a highly disordered molecule of
diethylether and attempts to satisfactorily model this
failed. As a result it was removed using the squeeze algo-
rithm (P.v.d. Sluis, A.L. Spek, Acta Crystallogr., Sect. A
46 (1990) 194).
2.7. (Lacy)Ti(NMe2)2
This was prepared as the complex described above from
LacyH2 (0.18 g, 0.35 mmol) and Ti(NMe2)2Cl2 (0.072 g,
0.35 mmol). It was isolated as orange solid from petroleum.
X-ray quality crystals were obtained from concentrated
diethyl ether solutions by cooling at 0 ꢁC for one week.
Yield: 0.14 g, 65%.
d
H (C6D6) 1.10–2.10 (20H, m, cyclohexyl), 1.44 (18H, s,
But), 1.64 (6H, s, methyls), 3.10 (12H, s, dimethylamides),
4.04 (2H, br m, cyclohexyl CH a to amido N), 6.77 (2H, s,
aromatic), 7.12 (2H, s, aromatic).
2.8. (Lacy)Ti(Bz)2
In the absence of light an NMR tube equipped with
YoungÕs tap was charged with a C6D6 solution of (LacyH2)
(0.020 g, 0.039 mmol) and Ti(benzyl)4 (0.016 g, 0.039 mmol)
3. Results and discussion
The ligands LamesH2, LacyH2 and LbmesH2 can be easily
synthesised in multigram quantities from commercially
available or easily accessible dibromo-xanthene or -di-
benzofurans by Buchwald–Hartwig amination (Scheme 1).
The best conversions and selectivities are obtained with
primary alkyl and aryl amines with a catalytic system com-
prising ½1; 3-bisð2; 6-Pr2i C6H3Þimidazoliniumꢁchloride=Pd2
ðdbaÞ3=ButONa in dioxane [14]. Low catalyst loadings
result in monoamination and dehalogenation giving rise
to monoamines. The only alternative methods that have
been used for the introduction of the amino (NH2) group
to xanthene ring were based on Curtius rearrangement with
diphenylphosphorylazide and benzyl alcohol followed by
1
and the base of the NMR tube was covered in foil. A H
NMR spectrum was run after the mixing and showed only
the presence of the two starting materials. The sample was
left at RT for ca. 15 h before the 1H NMR spectrum was re-
corded again. The spectrum now contained a small set of
peaks assignable to the (Lacy)Ti(benzyl)2 complex and free
toluene as well as the more pronounced peaks due to the
two starting materials. The NMR tube was then heated at
80 ꢁC for 3 h and appeared to show complete conversion of
the starting materials to the product. The volatiles were re-
moved in vacuo and the dark red residue redissolved in
C6D6 to obtain the spectra of the pure product.

862
R.M. Porter, A.A. Danopoulos / Polyhedron 25 (2006) 859–863
But
But
But
But
O
O
NH
HN
Br
Br
R1
LamesH2
R1
LacyH2
O
O
R1
R1
NH
HN
Br
Br
LbmesH2
R1=mesityl (Lmes), cyclohexyl (Lcy
)
Scheme 1. Reagents and conditions: 2.2 equiv. R1NH2, 6 equiv. But
ONa, 0.2 equiv. ½1; 3-bisð2; 6-Pr2i C6H3Þimidazoliniumꢁ chloride, 0.2 equiv.
Pd(dba)2, dioxane 100 ꢁC, 12 h.
Fig. 1. ORTEP representation of the structure of LacyTi(NMe2)2. One
disordered ether molecule present in the unit cell as well as hydrogen atoms
˚
are omitted for clarity. Selected bond lengths (A) and angles (ꢁ): N(1)–
Ti(1) = 2.057(4); N(2)–Ti(1) = 2.040(4); N(3)–Ti(1) = 1.887(4); N(4)–Ti(1)
= 1.893(4): O(1)–Ti(1) = 2.132(3) and N(3)–Ti(1)–N(4) = 105.15(18); N(3)
–Ti(1)–N(2) = 103.92(17); N(4)–Ti(1)–N(2) = 101.64(17); N(3)–Ti(1)–N
(1) = 105.55(17); N(4)–Ti(1)–N(1) = 97.36(17); N(2)–Ti(1)–N(1) = 139.08
(15); N(3)–Ti(1)–O(1) = 99.97(16); N(4)–Ti(1)–O(1) = 154.80(16); N(2)–
Ti(1)–O(1) = 73.73(13); N(1)–Ti(1)–O(1) = 73.72(13).
hydrogenolysis of the resulting carbamate [15]. Introduc-
tion of one dimethylamino (NMe2) group has been accom-
plished by Ullman amination of the iodides under forcing
conditions [16] and is of limited scope.
The new diamines were converted in almost quantitative
yields to extremely air/moisture sensitive dilithium salts by
reaction with BunLi in petroleum.
Ômer fashionÕ as anticipated. The two Ti–dimethylamido
˚
The introduction of the La and Lb to group 4 transition
metals has been successfully carried out by salt elimination
from dilithium amides and metal halides or by alkane elimi-
nation from Ti tetralkyls and LacyH2 as shown in Scheme 2.
bond lengths [Ti(1)–N(3) 1.888(4) A; Ti(1)–N(2) 1.892(4)
˚
A] are equal within the observed e.s.d.Õs and shorter than
˚
the cyclohexylarylamido bonds [Ti(1)–N(1) 2.056(4) A;
˚
Ti(1)–N(2) 2.039(4) A]. The latter are in the range reported
1
for less rigid ÔpincerÕ or NON2ꢀ diamido ligands [18]. All
the amido groups are virtually planar. Of particular inter-
est is the deviation of the ligand backbone from planarity
due to structural constraint imposed by the linking groups
(O and CMe2). The angle defined by the two aromatic rings
is ca. 150. In solution the structure is possibly non-rigid
(idealised C2v) with equivalent dimethylamido groups and
methyl substituents on the 9-position.
Even though the conversions are quantitative (by H
NMR spectroscopy) the yields of the isolated products
are lower due to their high solubility in non-polar solvents.
The new complexes were characterised by 1H and 13C{1H}
NMR spectroscopy. Elemental analysis data were not
reproducible, possibly due to the formation of solvates
(see below). The spectroscopic data support the presence
of a Cs symmetric structure in solution with the plane of
symmetry being perpendicular to the aromatic plane of
the ligand and passing through the metal centre and the
oxygen donor. The structure of LacyTi(NMe2)2 in the solid
state was determined by single crystal X-ray diffraction. A
diagram of the molecule is shown in Fig. 1.
In solution, complexes LbmesTi(NMe2)2 and LacyTi(Bz)2
exhibit spectra with features similar to LacyTi(NMe2)2 and,
therefore, are expected to adopt analogous structures. We
have been unable to grow crystals of LbmesTi(NMe2)2 and
LacyTi(Bz)2 suitable for X-ray diffraction. The electron defi-
cient LacyTi(Bz)2 is stable at room temperature in solution.
This behaviour contrasts with the more flexible and less
bulky (NON)Ti(alkyl)2 which show limited stability in
solution and the solid state [18b].
The coordination geometry around the metal can be
best described as distorted square pyramidal (s = 0.0186),
[17] with one dimethylamide nitrogen occupying the axial
position. The diamidoxanthene ligand coordinates in the
In summary, we have developed a general method
based on Pd catalysed aromatic amination for the prep-
aration of preorganised xanthene and dibenzofuran
diamines. The diamido ligands that originate from them
provide a useful tunable environment for the stabilisation
of early transition metals. Extension of the scope of the
catalytic amination to the synthesis of preorganised car-
bazole and dibenzothiophenes as well as studies of the
chemistry of the new early transition metal amides as a
function of ligand structural parameters is currently in
progress.
(i)
Li2L + Ti(NMe2)2Cl2
H2L + Ti(CH2Ph)4
LTi(NMe2)2
mes
cy
L= La L=Lb
)
(ii)
LTi(CH2Ph)2
L=Lacy
Scheme 2. Reagents and conditions: (i) ether, ꢀ78 ꢁC to room temper-
ature; (ii) Me3SiCl, toluene; (iii) toluene, 80 ꢁC, 3 h.

R.M. Porter, A.A. Danopoulos / Polyhedron 25 (2006) 859–863
863
(c) R. Baumann, R.R. Schrock, J. Organomet. Chem. 557 (1998)
69;
4. Supplementary material
(d) R. Baumann, R. Stumpf, W.M. Davis, L.C. Liang, R.R.
Schrock, J. Am. Chem. Soc. 121 (1999) 7822;
(e) R.R. Schrock, R. Baumann, S.M. Reid, T.J. Goodman, R.
Stumpf, W.M. Davis, Organometallics 18 (1999) 3649;
(f) J.T. Goodman, R.R. Schrock, Organometallics 20 (2001)
5205.
The crystallographic data for this paper have been pre-
deposited at the Cambridge Crystallographic Database
(CCDC 276930). These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336033).
[5] (a) M.J. McNevin, J.R. Hagadorn, Inorg. Chem. 43 (2004) 8547;
(b) J.R. Hagadorn, M.J. McNevin, G. Wiedenfeld, R. Shoemaker,
Organometallics 22 (2003) 4818, and references cited therein.
[6] A.L. Odom, Dalton Trans. (2005) 225.
Acknowledgement
[7] J.S. Novick, P. Ballester, F. Ebmeyer, J. Rebek, J. Am. Chem. Soc.
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[8] E.B. Schwartz, C.B. Knobler, D.J. Cram, J. Am. Chem. Soc. 114
(1992) 10775.
[9] E. Benzing, W. Kornicker, Chem. Ber. 94 (1961) 2263.
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Orpen, J. Am. Chem. Soc. 117 (1995) 3008.
We thank Professor M.B. Hursthouse for helpful discus-
sions and the EPSRC for support and provision of X-ray
facilities.
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