The synthesis, structure and reactivity of 4-nonafluorobiphenyl complexes

Article abstract of DOI:10.1039/b817252h

The Grignard reagent Ar_F′MgBr (Ar_F′ = 4-(C₆F₅)C₆F₄) reacts with Me ₃SiCl, Me₂SiCl₂ and Me₃SnCl to give the 4-nonafluorobiphenyl group 14 complexes Ar_F′Me ₃Si, (Ar_F′)₂Me₂Si and Ar _F′Me₃Sn respectively. Ar_F′Me ₃Sn undergoes only methyl group exchange when treated with BBr ₃, yielding Ar_F′Me₂SnBr. The solid state structures of Ar_F′Me₃Sn and Ar_F′ Me₂SnBr have been determined and exhibit the expected distorted tetrahedral geometries at tin. The reaction between three equivalents of Ar _F′MgBr and BF₃ was not selective, while one equivalent of Ar_F′MgBr and (Ar_F)₂BF (Ar_F = C₆F₅) reacted cleanly to give (Ar _F)₂Ar_F′B. Treatment of BCl₃ with three equivalents of Ar_F′Li, prepared at low temperature from the reaction between Ar_F′Br and n-BuLi, yielded (Ar _F′)₃B. The molecular structures of the acetonitrile adducts of (Ar_F)₂Ar_F′B and (Ar _F′)₃B closely resemble that of (Ar_F) ₃B·NCMe. During the course of the boron investigations, reaction with adventitious water led to the structural characterization of (Ar_F′)₂BOH·OH₂ as a hydrogen-bonded dimer. The Grignard reagent reacts selectively with ZnCl ₂ in diethyl ether giving first [(Ar_F′)Zn(μ-Cl) (OEt₂)]₂ then (Ar_F′) ₂Zn(OEt₂)₂, both of which have been characterised by X-ray diffraction. The corresponding reaction with HgCl ₂ requires the use of tetrahydrofuran as the solvent and yields (Ar_F′)₂Hg(THF)₂. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b817252h

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www.rsc.org/dalton | Dalton Transactions
The synthesis, structure and reactivity of 4-nonafluorobiphenyl complexes†
Eddy Martin,a David L. Hughes,a Michael B. Hursthouse,b Louise Maleb and Simon J. Lancaster*a
Received 2nd October 2008, Accepted 7th November 2008
First published as an Advance Article on the web 19th January 2009
DOI: 10.1039/b817252h
The Grignard reagent ArF¢MgBr (ArF¢ = 4-(C6F5)C6F4) reacts with Me3SiCl, Me2SiCl2 and Me3SnCl to
give the 4-nonafluorobiphenyl group 14 complexes ArF¢Me3Si, (ArF¢)2Me2Si and ArF¢Me3Sn
respectively. ArF¢Me3Sn undergoes only methyl group exchange when treated with BBr3, yielding
ArF¢Me2SnBr. The solid state structures of ArF¢Me3Sn and ArF¢Me2SnBr have been determined and
exhibit the expected distorted tetrahedral geometries at tin. The reaction between three equivalents of
ArF¢MgBr and BF3 was not selective, while one equivalent of ArF¢MgBr and (ArF)2BF (ArF = C6F5)
reacted cleanly to give (ArF)2ArF¢B. Treatment of BCl3 with three equivalents of ArF¢Li, prepared at low
temperature from the reaction between ArF¢Br and n-BuLi, yielded (ArF¢)3B. The molecular structures
of the acetonitrile adducts of (ArF)2ArF¢B and (ArF¢)3B closely resemble that of (ArF)3B·NCMe. During
the course of the boron investigations, reaction with adventitious water led to the structural
characterization of (ArF¢)2BOH·OH2 as a hydrogen-bonded dimer. The Grignard reagent reacts
selectively with ZnCl2 in diethyl ether giving first [(ArF¢)Zn(m-Cl)(OEt2)]2 then (ArF¢)2Zn(OEt2)2, both
of which have been characterised by X-ray diffraction. The corresponding reaction with HgCl2 requires
the use of tetrahydrofuran as the solvent and yields (ArF¢)2Hg(THF)2.
In contrast to boron, reports of further-substituted derivatives
of bis(pentafluorophenyl)zinc are scarce. Bochmann et al. have
The pentafluorophenyl group has been widely employed to
reported the elegant, but somewhat unconventional, synthesis
Introduction
of (2-(C6F5)C6F4)2Zn from EtZnCl and 2-(C6F5)C6F4Li.13 The
only other example of the preparation of a relevant derivative
of (C6F5)2Zn of which we are aware is that of (4-(CF3)C6F4)2Zn.14
A number of synthetic routes to the parent compound, (C6F5)2Zn,
have been reported, including the thermal decomposition of
the carboxylate15 and the salt metathesis reactions between
magnesium, lithium and silver reagents with zinc halides.3,5,16 The
ligand exchange reaction between R2Zn and (C6F5)3B17 is the most
convenient, since the preparation and purification of (C6F5)3B is
facile.18
This report describes an exploration of the main group
chemistry of the 4-nonafluorobiphenyl group utilizing lithium,
magnesium and tin reagents for the preparation of novel zinc
complexes and includes the attempted isolation of a base-free
borane as a potential intermediate.
prepare metal complexes because it combines the highly electron-
withdrawing character of a halide with the steric requirements
of an arene ligand.1,2 Main group complexes of the pentafluo-
rophenyl ligand were subject to a first flourish of interest in the
1960s with the publication of the homoleptic zinc and boron
compounds.3,4 Following a period of dormancy, they are currently
enjoying considerable attention across a remarkably broad range
of applications.
The majority of reports on bis(pentafluorophenyl)zinc have
been concerned with its utility as a C6F5-transfer reagent.5
Recently it has found direct application as an initiator component
for the carbocationic polymerisation of isobutene.6 We have
focused upon the supramolecular assembly of Lewis adducts of
the form (C6F5)2ZnL2 (where L is an amine, pyridine or benzoni-
trile donor) through N–H ◊ ◊ ◊ F–C, face-to-face C6F5 ◊ ◊ ◊ C6H5 and
offset C6F5 ◊ ◊ ◊ C6F5 interactions.7–9
The importance of (ArF)3B (ArF = C6F5) as a Lewis acidic
activator, or as the basis for poorly coordinating anions, in 1-
alkene polymerisation catalysis has led to a number of studies
directed towards elaborating the perfluoroaryl group;10 the most
significant example being (2-(C6F5)C6F4)3B.11 (ArF¢)3B (ArF¢ = 4-
(C6F5)C6F4) appears in the patent literature, but few synthetic and
characterisation details are presented.12
Results and discussion
The Grignard reagent, ArF¢MgBr (1), was prepared at room
temperature in diethyl ether solution and has not been isolated.
The reaction is accompanied by a darkening of the solution,
similar to that seen for ArFMgBr, but the reagent is indefinitely
stable if stored in solution under nitrogen at 5 ◦C. Reactions
involving 1 can be conveniently followed by 19F NMR. The
chemical shift of the fluorine in the 2-position (F2) (1: d = -114.1
ppm) is sensitive to the (metal) substituent in the 1-position and
is characteristic for these compounds.19–21 Not surprisingly, this
sensitivity drops off dramatically with the distance from the metal,
such that the remaining 19F chemical shifts are similar for all the
complexes reported herein.
aWolfson Materials and Catalysis Centre, School of Chemical Sciences and
Pharmacy, University of East Anglia, Norwich, UK NR4 7TJ. E-mail:
S.Lancaster@uea.ac.uk; Fax: +44 1603 592003; Tel: +44 1603 592009
bSchool of Chemistry, University of Southampton, Highfield, Southampton,
UK SO17 1BJ
† Electronic supplementary information (ESI) available: Torsion angles
describing the twist between the two phenyl rings of each ArF¢ ligand.
CCDC reference numbers 704294–704300. For ESI and crystallographic
data in CIF format see DOI: 10.1039/b817252h
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The success of the Grignard reagent preparation was confirmed
by quenching with either one equivalent of trimethylsilylchloride
or 0.5 equivalents of dimethylsilyldichloride giving 49 and 84%
isolated yields of ArF¢Me3Si (2) and (ArF¢)2Me2Si (3), respectively
(Scheme 1). The 19F NMR spectra of 2 and 3 are very similar and
in both cases the F2 resonances are found at considerably lower
frequency than those of the Grignard reagent, 1.21 Compounds 2
and 3 gave elemental analyses in very good agreement with the
expected values.
Fig. 1 Molecular structure of 4. Displacement ellipsoids are drawn
˚
at the 50% probability level. Selected bond lengths (A) and angles
(◦), Esd’s are in parentheses: Sn(1)–C(21) 2.131(4); Sn(1)–C(22)
2.133(4); Sn(1)–C(23) 2.137(4); Sn(1)–C(11) 2.196(4); C(22)–Sn(1)–C(21)
113.76(18);
C(21)–Sn(1)–C(23)
112.76(19);
C(22)–Sn(1)–C(23)
110.36(18); C(21)–Sn(1)–C(11) 106.95(17); C(22)–Sn(1)–C(11) 109.25(16);
C(23)–Sn(1)–C(11) 103.11(18).
Scheme 1
While perfluoroaryl silane reagents undergo defluorosilylation
reactions with silver fluoride,22 there is little precedent for their
application in the preparation of boron compounds and no
reaction occurs between 2 or 3 and BF3·OEt2, BCl3 or BBr3.23 In
contrast, stannane reagents have been widely used for the selective
introduction of perfluoroaryl substituents to boron.10b,f
Fig. 2 Molecular structure of 4a. Displacement ellipsoids are drawn at
Treatment of 1 with one equivalent of trimethyltin chloride in
diethyl ether gave the tin reagent Me3ArF¢Sn (4) in 71% yield
(Scheme 1). The F2 chemical shift of 4 is found at d = -121.4 ppm
and is therefore intermediate between those of 1 and the silicon
homologue 2. The structure of 4 was confirmed by single-crystal
X-ray diffraction. Compound 4 and BCl3 do not react under
a range of experimental conditions, including refluxing toluene.
When 4 was heated to 60 ◦C in neat BBr3 a new crystalline
the 50% probability level. The acetonitrile molecule has been omitted
◦
˚
for clarity. Selected bond lengths (A) and angles ( ), Esd’s are in paren-
theses: Sn(1)–C(11) 2.166(3); Sn(1)–C(21) 2.128(3); Sn(1)–C(22) 2.119(3);
Sn(1)–Br(2) 2.5587(3); C(22)–Sn(1)–C(21) 123.04(12); C(22)–Sn(1)–C(11)
113.71(11); C(21)–Sn(1)–C(11) 117.33(11); C(11)–Sn(1)–Br(2) 93.87(7);
C(21)–Sn(1)–Br(2) 99.52(8); C(22)–Sn(1)–Br(2) 100.64(8).
substantial distortion from ideal tetrahedral geometry in 4a. At
1
compound, 4a, was isolated from the reaction mixture. The H
˚
2.196(4) and 2.166(3) A in 4 and 4a respectively the Sn(1)–C(11)
NMR spectrum was sufficient to determine that a reaction had
taken place but the signal for F2 at d = -121.9 ppm was rather
similar to that of 4 and not in the range expected for a boron
compound. Unequivocal identification was achieved through X-
ray diffraction, whereby 4a was determined to be ArF¢Me2SnBr.
Rather than ArF¢/Br exchange, the reaction between 4 and BBr3
proceeds through Me/Br exchange (Scheme 2). This reaction
resembles that observed by Piers et al. when attempting the
synthesis of related binapthyl derivatives.24
˚
bonds are slightly longer than the 2.126(8) A found for the
homoleptic tetrakis(pentafluorophenyl)tin.25
The preferred procedure for the preparation of (C6F5)3B sees
BF3 treated with three equivalents of ArFMgBr in diethyl ether–
toluene solution and warmed to 100 ◦C to ensure complete
conversion.18 The analogous reaction between BF3 and three
equivalents of 1 proceeds to completion with respect to the
Grignard reagent. However, the plethora of signals in the 11B
and 19F NMR spectra suggest that the crude product consists
of a mixture of [XMg(OEt2)n][B(ArF¢)4-yXy] and B(ArF¢)3-zXz
compounds (where X = F or Br). Whereas extraction of the
ArF system with hexane gives the borane in typically 60% yield,18
the analogous procedure did not yield pure (ArF¢)3B; however, it
remains probable that it is present in the product mixture.
A small quantity of crystalline material was isolated during
an attempted preparation of (ArF¢)3B. Attempted NMR spec-
troscopy was frustrated by the very small quantity of material
present and provided signals for more than one compound.
Despite this and difficulties caused by the low melting point
Scheme 2
The molecular structures of 4 (Fig. 1) and 4a (Fig. 2) are rather
similar, with essentially tetrahedral geometries at tin. Replacement
of a methyl group with a bromine substituent results in more
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of the crystals, an X-ray diffraction data set was ultimately
collected. The structure was determined to be that of the
deuterated benzene solvate (ArF¢)2B(OH)(OH2).(C6D6)3 (5), which
was presumably formed through reaction between adventitious
water and (ArF¢)3B or (ArF¢)2BF.26 This compound is noteworthy
since bis(pentafluorophenyl)borinic acid has received considerable
attention as a promoter in synthetic chemistry because of the
unusual combination of Lewis acidic, Lewis basic and Brønsted
acidic sites.27–29
The molecular structure of 5 is presented in Fig. 3. Bond length
B(1)–O(2) is significantly shorter than that for B(1)–O(1), which is
consistent with B(1)–O(1) having dative character. In addition, the
hydrogen atoms were located in the difference map and have been
freely refined, further supporting the assertion that 5 is a Lewis
adduct between bis(4-nonafluorophenyl)borinic acid and water.
Molecules of 5 are strongly hydrogen bonded in pairs (Fig. 4),
resembling the pattern in [(C6F5)2(HO)BO(Me)H]2.28 As wellas the
hydrogen bond, there are a number of O–H ◊ ◊ ◊ F–C close contacts
(Table 1).
Fig. 4 Illustrating the hydrogen-bond driven dimerisation of 5. Nonaflu-
orobiphenyl ligands are represented by the first carbon atom for clarity.
The two halves of the molecule are related by the symmetry operator 2 -
x, 1 - y, 1 - z.
nonafluorobiphenyl lithium can be handled without incident if the
correct solvent and temperature conditions are employed.11 Fol-
lowing the procedure of Marks and co-workers for the preparation
of tris(2-nonafluorobiphenyl)borane, the lithiation of ArF¢Br at
-78 ◦C was performed in a 1 : 1 solvent mixture of diethyl ether and
light petroleum (Scheme 3).11 Three equivalents of the lithium salt
were treated with boron trichloride to give a mixture of (ArF¢)3B (7)
(Scheme 3) and a second unidentified by-product. Solvent removal
from the light petroleum fraction yielded only a crude brown oil.
The isolation of a crystalline solid was again facilitated by the
addition of acetonitrile, giving rise to the adduct, 7·NCMe, the
structure of which was elucidated by X-ray crystallography. The
11B NMR signals for the neutral four-coordinate adducts 6·NCMe
and 7·NCMe are similar at d -10.8 and -9.1 ppm respectively. Both
6·NCMe and 7·NCMe gave elemental analyses in good agreement
with the calculated values.
The mixed-aryl boron compound 6·NCMe crystallises from
acetonitrile with one molecule of solvent coordinated to the boron
and a second of crystallisation (Fig. 5). Compound 7·NCMe
(Fig. 6) was recrystallised from dichloromethane and there are
two disordered solvent molecules in the lattice. The molecular
structures of both of the acetonitrile adducts reported here and
also the known (ArF)3B·NCMe32 exhibit distorted tetrahedral
environments about boron. The angles at boron between the
nitrogen and the perfluoroaryl groups are generally more acute
than those between perfluoroaryl groups, reflecting the steric
demands of the substituents. The B–N distances for 6·NCMe,
Fig. 3 Molecular structure of (ArF¢)2B(OH)(OH2) (5). Displacement
ellipsoids are drawn at the 50% probability level. Solvent molecules
˚
have been omitted for clarity. Selected bond lengths (A) and angles
(◦), Esd’s are in parentheses: B(1)–O(1), 1.542(5); B(1)–O(2), 1.462(5);
B(1)–C(11), 1.637(6); B(1)–C(21), 1.630(5); O(2)–B(1)–O(1), 102.9(3);
O(2)–B(1)–C(21), 114.7(3); O(1)–B(1)–C(21), 107.9(3); O(1)–B(1)–C(11),
110.8(3); O(2)–B(1)–C(11), 109.7(3); C(21)–B(1)–C(11), 110.6(3).
In contrast to the rather poor selectivity observed for the
reaction between three equivalents of 1 and boron trifluoride,
treating bis(pentafluorophenyl)boron fluoride30 with one equiv-
alent of 1 gave (ArF¢)(ArF)2B (6) in near quantitative yield
(Scheme 3). The ether adduct of 6 proved not to share the
high crystallinity of (ArF)3B·OEt2 and isolation of a crystalline
solid was only achieved after addition of acetonitrile, giving
(ArF¢)(ArF)2B·NCMe (6·NCMe), the identity of which was con-
firmed through diffraction methods.
The initial reluctance to employ the 4-nonafluorobiphenyl
lithium reagent stems from the thermal sensitivity of related
lithium salts and the associated explosion hazard.11,31 However, 2-
Table 1 Hydrogen bonds and close O–H ◊ ◊ ◊ F contacts in compound 5
D–H ◊ ◊ ◊ Aa
Symmetry operation for interactions
Distance (H ◊ ◊ ◊ A)/A
Distance (D ◊ ◊ ◊ A)/A
Angle (DHA)/◦
˚
˚
O(1)–H(1b) ◊ ◊ ◊ O(2)
O(1)–H(1a) ◊ ◊ ◊ F(12)
O(1)–H(1a) ◊ ◊ ◊ F(26)
O(2)–H(2) ◊ ◊ ◊ F(22)
O(2)–H(2) ◊ ◊ ◊ F(16)
2 - x, 1 - y, 1 - z
x, y, z
1.64(2)
2.36(4)
2.10(3)
2.27(5)
2.40(4)
2.464(4)
2.710(4)
2.724(4)
2.772(3)
3.075(3)
173(6)
108(4)
134(4)
122(5)
146(5)
x, y, z
x, y, z
1 - x, 1 - y, 1 - z
a A is the hydrogen bond acceptor and D the hydrogen bond donor.
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Scheme 3 The synthesis of boron complexes of the ArF¢ ligand.
˚
7·NCMe and (ArF)3B·NCMe at 1.583(3), 1.600(5) and 1.610(5) A
are essentially identical.
Fig. 6 Molecular structure of 7·NCMe. Displacement ellipsoids are
drawn at the 50% probability level. The hydrogen atoms and the
CH2Cl2 molecules of crystallisation have◦ been omitted for clarity.
˚
Selected bond lengths (A) and angles ( ), Esd’s are in parenthe-
Fig.
5
Molecular structure of 6·NCMe. Displacement ellipsoids
ses: B(1)–N(41), 1.600(5); B(1)–C(11), 1.621(5); B(1)–C(21), 1.627(6);
B(1)–C(31), 1.632(5); N(41)–B(1)–C(11), 103.6(3); N(41)–B(1)–C(21),
104.8(3); N(41)–B(1)–C(31), 105.8(3); C(11)–B(1)–C(21), 115.1(3);
C(11)–B(1)–C(31), 114.0(3); C(21)–B(1)–C(31), 112.2(3).
are drawn at the 50% probability level. Hydrogen atoms and the
acetonitrile solvate molecule have been omitted for clarity. Se-
lected bond lengths (A) and angles (◦), Esd’s are in parenthe-
˚
ses: B(1)–N(4), 1.583(3); B(1)–C(11), 1.624(3); B(1)–C(21), 1.640(3);
B(1)–C(31), 1.624(3); N(4)–B(1)–C(11), 103.38(18); N(4)–B(1)–C(21),
104.63(17); N(4)–B(1)–C(31), 109.50(19); C(11)–B(1)–C(21), 115.70(19);
C(11)–B(1)–C(31), 115.91(18); C(31)–B(1)–C(21), 106.99(18).
was examined. Treatment of HgCl2 with two equivalents of 1
in tetrahydrofuran gave complex 8 as a beige solid in modest
yield (Scheme 4). The formulation of 8 as the expected product,
(ArF¢)2Hg(THF)2, was based upon the NMR and elemental
analysis results. The 1H NMR spectrum simply indicated the
presence of coordinated tetrahydrofuran. No reaction takes place
if, instead of tetrahydrofuran, a mercury dichloride suspension in
diethyl ether is treated with the Grignard reagent, presumably due
to the mercury reagent’s poor solubility.
Refining the synthesis of 7, to yield the pure, base-free pre-
cursor required for the ligand exchange reaction with R2Zn,
was considered unlikely to provide the most expedient route to
(ArF¢)2Zn. In order to gauge the effectiveness of a more direct
salt metathesis route to the (ArF¢)2M complexes of group 12, the
reaction between mercury dichloride and the Grignard reagent
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tively. However, compounds 9 and 10 were both isolated in only
moderate yields of 51 and 54%, respectively, because of their poor
solubility and difficulties associated with their separation from the
magnesium halide by-products. The spectroscopic data are similar
and the two compounds were ultimately distinguished through
elemental analysis and X-ray crystallography. The deviation
between the calculated and observed elemental analysis results
for compound 9 is believed to reflect the facile loss of coordinated
diethyl ether.
Compound 9 crystallises as a chloride bridged dimer, in which
zinc has an essentially tetrahedral coordination environment
(Fig. 7). The dative Zn–Cl bond in 9 is distinguished by a slightly
greater interatomic separation. Both the Zn–C and Zn–Cl bond
lengths in compound 9 are similar to those observed for the
closely related and recently reported dimer [(C6Me6)(ArF)Zn(m-
Cl)]2.33 Predictably, the Cl(1)–Zn(1)–Cl(1¢) and C(11)–Zn(1)–Cl(1)
angles are greater in 9 than in Bochmann’s arene adduct, since the
Scheme 4
The good solubility of ZnCl2 meant that not only does it react
readily with 1 in diethyl ether, but the product stoichiometry
can easily be controlled by the reactant ratio. Thus, when ZnCl2
was treated with one equivalent of 1 in diethyl ether, a mono-
substituted compound was obtained [(ArF¢)Zn(m-Cl)(OEt2)]2 (9)
(Scheme 5). If two equivalents of Grignard reagent are employed
the final product is (ArF¢)2Zn(OEt2)2 10. Following these reactions
spectroscopically indicated that they proceed essentially quantita-
3
h -hexamethylbenzene ligand is sterically more demanding than
diethyl ether.
Scheme 5
Fig. 7 Molecular structure of 9. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. The two
monomers are related by an inversion centre lying at the centre of the Zn2Cl2 core. The two halves of the molecule are related by the symmetry operator
◦
˚
1 - x, 1 - y, 1 - z. Selected bond lengths (A) and angles ( ), Esd’s are in parentheses: Zn(1)–C(11), 1.984(4); Zn(1)–O(1), 2.050(3); Zn(1)–Cl(1), 2.3470(12);
Zn(1)–Cl(1¢), 2.3746(12); C(11)–Zn(1)–O(1), 116.97(16); C(11)–Zn(1)–Cl(1), 121.92(13); O(1)–Zn(1)–Cl(1), 99.73(10); C(11)–Zn(1)–Cl(1¢), 119.03(13);
O(1)–Zn(1)–Cl(1¢), 101.46(10); Cl(1)–Zn(1)–Cl(1¢), 93.13(4).
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The zinc-bonded ligands in 10 adopt a somewhat distorted
tetrahedral arrangement (Fig. 8). The origin of this distortion
is the steric mismatch between the perfluoroaryl and ether ligands.
So, while the C–Zn–O angle is close to the tetrahedral ideal, that of
C–Zn–C is more obtuse and that of O–Zn–O is much more acute.
As one would expect, the Zn–O bond length is similar to that
rane (ArF¢)3B was isolated and crystallographically characterised
as the acetonitrile adduct (7·NCMe) after synthetic recourse to the
lithium reagent. In contrast to the lack of selectivity of the reaction
between the Grignard reagent and boron trifluoride, treatment of
zinc dichloride with one or two equivalents of 1 gave [(ArF¢)Zn(m-
Cl)(OEt2)]2 (9) or (ArF¢)2Zn(OEt2)2 (10) respectively.
Our observations during the preparation and isolation of
the boron and zinc complexes suggest that, while the 4-
nonafluorobiphenyl ligand may well be regarded as both sterically
and electronically very similar to the pentafluorophenyl ligand
there are subtle differences in both reactivity and solubility.
From a molecular structure standpoint, at the metal(loid) centre,
complexes of the 4-nonafluorobiphenyl ligand resemble their
pentafluorophenyl analogues.
34
found for (ArF)2Zn(THF)2 but is slightly longer than that found
for 9, which enjoys significantly less steric hindrance. The Zn–C
bond lengths are unremarkable and within the range observed in
studies of bis(pentafluorophenyl) zinc adducts.3,5,7–9,34
Experimental
Syntheses were performed under anhydrous oxygen-free nitrogen
using standard Schlenk techniques. Solvents were distilled over
sodium–benzophenone (diethyl ether, tetrahydrofuran), sodium
(toluene), sodium–potassium alloy (light petroleum, bp 40–
60 ◦C), or CaH2 (dichloromethane, 1,2-difluorobenzene). NMR
˚
solvents (CDCl3, C6D6, C7D8) were dried over activated 4 A
molecular sieves and degassed by several freeze–thaw cycles. NMR
spectra were recorded using a Bruker DPX300 spectrometer at
293 K. Chemical shifts are reported in ppm and referenced to
residual solvent resonances (1H, 13C); 19F is relative to CFCl3; 11
B
is relative to BF3·OEt2. The atom numbering scheme is given in
Chart 1. All coupling constants are given in Hz. The 19F NMR
spectra of reaction mixtures, including the ethereal solution of
1, were obtained by adding 0.1 cm3 of the reaction medium to
0.5 cm3 benzene-d6. Elemental analyses were performed at the
Department of Health and Human Sciences, London Metropoli-
tan University. 4-Bromo-2,3,5,6,2¢,3¢,4¢,5¢,6¢-nonafluoro-biphenyl
(ArF¢Br) was prepared according to the literature procedure.36 The
remaining reagents were purchased from Aldrich or Alfa Aesar
and used without further purification.
Fig. 8 Molecular structure of 10. Displacement ellipsoids are drawn at
the 50% probability level. The hydrogen atoms have been omitted for
clarity. The two pairs of ligands are related by a two-fold symmetry
axis passing through the zinc atom. The two halves of the molecule
are related by the symmetry operator 1 - x, y, 1/2 - z. Selected bond
lengths (A) and angles ( ), Esd’s are in parentheses: Zn(1)–C(11), 2.013(6);
Zn(1)–O(2), 2.084(3); C(11)–Zn(1)–C(11¢), 124.2(3); C(11)–Zn(1)–O(2),
108.6(2); O(2)–Zn(1)–O(2¢), 85.98(19); C(11)–Zn(1)–O(2¢), 111.43(19).
◦
˚
In each of the seven crystal structures reported here the two
phenyl groups of each 4-nonafluorobiphenyl ligand are twisted, in
order to minimise steric repulsion between the fluorine atoms.35
Isolation of compound 9 facilitated the preparation of the mixed
perfluoroaryl compound (ArF¢)(ArF)Zn(THF)2 (11) through treat-
ment of 9 with one equivalent of ArFMgBr (Scheme 5). The 1 : 1
ratio of ArF to ArF¢ groups was confirmed by 19F NMR and the
identity of the sample was affirmed by the elemental analysis.
Conclusion
A broad range of 4-nonafluorobiphenyl complexes is accessible
through reaction of the nucleophilic lithium and, preferably,
magnesium reagents with main group electrophiles. The Grignard
reagent ArF¢MgBr (1) reacts selectively with Me3SiCl, Me2SiCl2,
Me3SnCl and (ArF)2BF. However, the reaction of BF3 with three
equivalents of 1 gives an intractable mixture of products. In
one such reaction, adventitious hydrolysis lead to crystals of
(ArF¢)2B(OH)(OH2) (5) being obtained. In light of the elegant
studies on bis(pentafluorophenyl)borinic acids and their synthetic
importance, the borinic acid precursor to compound 5 and the
structurally characterised aqua adduct merit further study. The bo-
Chart 1 The fluorine atom numbering scheme in ArF¢ groups.
Compound 5 was obtained serendipitously in small quantity as
described in the results and discussion above.
Preparation of ArF¢MgBr (1) solution
A solution of 4-(C6F5)C6F4Br (4 g, 10 mmol) in 30 cm3 diethyl
ether was added dropwise to a suspension of iodine-activated Mg
turnings (0.5 g, 21 mmol) in 10 cm3 of diethyl ether. The resulting
brown solution was stirred for a further 2 h at room temperature
1598 | Dalton Trans., 2009, 1593–1601
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to ensure complete reaction and the excess magnesium separated
by filtration. (For a 0.1 cm3 aliquot of diethyl ether solution) dF
(282 MHz, C6D6) -114.1 (2F, m, F2), -139.7 (4F, m, F3 and F2¢),
-153.0 (1F, t, 3JF,F 22, F4¢), -162.4 (2F, m, F3¢).
CH3CN). dF (282 MHz, C6D6) -122.0 (2F, m, F2), -137.0 (2F, m,
F3 or F2¢), -139.0 (2F, m, F3 or F2¢), -150.2 (1F, t, 3JF,F 21, F4¢),
-160.9 (2F, m, F3¢). Elemental analysis found: C, 32.75; H, 1.52;
N, 2.38. Calculated for C16H9F9NBrSn: C, 32.86; H, 1.55; N, 2.40.
(ArF)2(ArF¢)B(NCMe) (6·NCMe)
ArF¢Me3Si (2)
C6F5Br (0.95 cm3, 7.6 mmol) was added dropwise to a suspension
of Mg turnings (0.20 g, 8.4 mmol) in diethyl ether (10 cm3)
and the mixture was stirred for 1 h. The resulting Grignard
reagent solution was filtered and added to a solution of BF3·Et2O
(0.47 cm3, 3.8 mmol) in diethyl ether (10 cm3) at 0 ◦C. The solution
was warmed slowly to room temperature and stirred for 2 h, before
all the volatile materials were removed under vacuum. A solution
of 1 (20 cm3, 0.19 M in Et2O, 3.8 mmol) was added directly to the
solid residue and the resulting mixture stirred overnight at room
temperature. After removing the solvent under vacuum the residue
was extracted with warm (40 ◦C) light petroleum. Distilling off
the light petroleum gave a crude pale brown solid. Compound
6-NCMe·MeCN was obtained as colourless crystals following
crystallisation from a mixture of acetonitrile and light petroleum
at -27 ◦C (1.31 g, 48%). dH (300 MHz, C6D6) 0.61 (3H, s, free
MeCN), 0.34 (3H, s, bound MeCN). dB (96.3 MHz, C6D6) -10.8
(br). dF (282 MHz, C6D6) -133.5 (2F, d, 3JF,F 23, F2), -134.6 (4F, d,
3JF,F 23, o-F C6F5), -138.9 (2F, m, F3 or F2¢), -139.8 (2F, m, F3 or
F2¢), -150.8 (1F, t, 3JF,F 22, F4¢), -155.3 (2F, t, 3JF,F 21, p-F C6F5),
-161.2 (2F, m, F3¢), -163.1 (4F, m, m-F C6F5). Elemental analysis
found: C, 45.26; H, 0.74; N, 3.86. Calculated for C28H6BF19N2: C,
45.32; H, 0.81; N, 3.77.
A solution of 12.7 mmol of 1 in diethyl ether (20 cm3) at room
temperature was treated with chlorotrimethylsilane (1.79 cm3,
14 mmol) and stirred overnight. The soluble part was separated by
filtration and the volatile materials removed under vacuum. After
extraction with light petroleum, and distillation of the solvent
under vacuum, the residue was crystallised from light petroleum to
give ArF¢SiMe3 as colourless needles (2.43 g, 49%). dH (300 MHz,
CDCl3) 0.46 (9H, s, Si(CH3)3). dF (282 MHz, CDCl3) -127.4 (2F,
m, F2), -137.9 (2F, m, F3 or F2¢), -139.2 (2F, m, F3 or F2¢), -151.1
(1F, t, 3JF,F 21, F4¢), -161.2 (2F, m, F3¢). Elemental analysis found:
C, 46.51; H, 2.20. Calculated for C15H9F9Si: C, 46.40; H, 2.34.
(ArF¢)2Me2Si (3)
Dichlorodimethylsilane (1.9 mmol, 0.24 cm3) was added to a
solution of 1 (3.8 mmol) in diethyl ether (15 cm3). The mixture
was stirred for 36 h before filtration and removal of the volatile
materials under vacuum afforded 3 as a brown solid (1.1 g, 84%),
which did not require further purification. dH (300 MHz, CDCl3)
0.93 (6H, s, Si(CH3)2). dF (282 MHz, CDCl3): d -127.2 (4F, m,
F2), -137.7 (4F, m, F3 or F2¢), -138.1 (4F, m, F3 or F2¢), -150.7
(2F, t, 3JF,F 21, F4¢), -161.1 (4F, m, F3¢). Elemental analysis found:
C, 45.44; H, 0.94. Calculated for C26H6F18Si: C, 45.37; H, 0.88.
(ArF¢)3B(NCMe) (7·NCMe)
ArF¢Me3Sn (4)
4-(C6F5)C6F4Br (1.16 g, 2.9 mmol) was dissolved in a mixed
solvent of diethyl ether (16 cm3) and light petroleum (16 cm3)
and the solution was cooled to -78 ◦C. A solution of n-butyl
lithium (1.4 cm3, 2.1 M in hexane) was added dropwise and the
mixture was stirred for 2 h. A solution of BCl3 (1 cm3, 1.0 M in
heptane, 1 mmol) was then introduced and the reaction mixture
was allowed to warm slowly to room temperature. The LiCl was
separated by filtration and acetonitrile (1 cm3) was added to the
filtrate. The solvents were distilled off under vacuum to give a
crude brown oil. Crystallisation by layering light petroleum over a
dichloromethane solution at -27 ◦C gave colourless crystals of 7-
NCMe·2(CH2Cl2) (0.53 g, 55%). dH (300 MHz, C6D6) 0.23 (s, 3H,
MeCN). dF (282 MHz, C6D6) -132.7 (6F, m, F2), -138.4 (6F, m, F3
or F2¢), -138.8 (6F, m, F3 or F2¢), -150.2 (3F, t, 3JF,F 22, F4¢), -160.7
(6F, m, F3¢). dB (96.3 MHz, C6D6) -9.1 (br). Elemental analysis
found: C, 45.74; H, 0.15; N, 1.45. Calculated for C38H3BF27N: C,
45.77; H, 0.30; N, 1.40.
A solution of chlorotrimethyltin (0.64 g, 3.2 mmol) in diethyl ether
(10 cm3) was treated with 3.8 mmol of 1 dissolved in diethyl ether
(15 cm3) and the resulting reaction mixture was stirred overnight.
The solvents were distilled off under reduced pressure and the
product extracted with warm (40 ◦C) light petroleum (3 ¥ 15 cm3).
The light petroleum solution was concentrated until the point of
crystallisation and cooled to -27 ◦C yielding crystals of compound
4 (1.2 g, 71%) suitable for X-ray crystallography. dH (300 MHz,
C6D6) 0.26 (s, 9H, Sn(CH3)). dF (282 MHz, C6D6) -121.4 (m,
2F, F2), -138.3 (m, 2F, F3 or F2¢), -139.2 (m, 2F, F3 or F2¢),
3
-150.0 (t, 1F, JF,F = 22 Hz, F4¢), -161.2 (m, 2F, F3¢). In order
to provide a comparison and to illustrate the independence of 19
F
NMR spectra upon the solvent, the data for chloroform-d1 are also
presented here.16 dH (300 MHz, CDCl3) 0.56 (s, 9H, Sn(CH3)). dF
(282 MHz, CDCl3) -121.3 (m, 2F, F2), -137.6 (m, 2F, F3 or F2¢),
-138.6 (m, 2F, F3 or F2¢), -151.0 (t, 1F, 3JF,F 21, F4¢), -161.0 (2F,
m, F3¢). Elemental analysis found: C, 37.68; H, 1.81. Calculated
for C15H9F9Sn: C, 37.62; H, 1.89.
(ArF¢)2Hg(THF)2 (8)
To a suspension of HgCl2 (0.52 g, 1.9 mmol) in tetrahydrofuran
(15 cm3) 3.8 mmol of 1 dissolved in THF (15 cm3) was added
and the resulting reaction mixture stirred overnight. The solvent
was then removed under vacuum and the residue extracted with
toluene (3 ¥ 15 cm3). The toluene was concentrated under reduced
pressure to 10 cm3, 0.5 cm3 of tetrahydrofuran was added and
the solution cooled to -27 ◦C precipitating a pale brown solid.
Separation by filtration and drying under vacuum gave 8 as a beige
ArF¢Me2SnBr (4a)
A sample of compound 4 (0◦.24 g, 0.5 mmol) was dissolved in
BBr3 (2 cm3) and heated at 60 C overnight. The volatile materials
were removed under vacuum. The residue was extracted with light
petroleum and crystallised from acetonitrile to afford crystals of
4a·MeCN as colourless plates suitable for X-ray diffraction (0.13 g,
42%). dH (300 MHz, C6D6) 0.66 (6H, s, Sn(CH3)), 0.57 (3H, s,
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solid (0.57 g, 32%). dH (300 MHz, C6D6) 3.57 (8H, m, 2,5-THF),
1.40 (8H, m, 3,4-THF). dF (282 MHz, C6D6) -119.6 (4F, m, F2),
-136.9 (4F, m, F3 or F2¢), -138.0 (4F, m, F3 or F2¢), -150.5 (2F,
t, 3JF,F 21, F4¢), -160.7 (4F, m, F3¢). Elemental analysis found: C,
39.14; H, 1.55. Calculated for C32H16F18HgO2: C, 39.42; H, 1.65.
reagent. The solvent was then removed under vacuum and the
resulting crude solid washed with light petroleum and extracted
with a 1 : 4 mixture of dichloromethane–light petroleum (3 ¥
20 cm3). The solvent was then distilled off under reduced pressure
affording a white solid. The product was purified by crystallisation
from a diethyl ether–light petroleum solvent mixture (0.93 g, 54%).
Slow diffusion of light petroleum into a diethyl ether solution
held at -27 ◦C afforded crystals suitable for a single crystal X-
ray diffraction study. dH (300 MHz, C6D6) 3.62 (4H, q, J 7.1,
OCH2CH3), 1.22 (6H, t, J 7.1, OCH2CH3). dF (282 MHz, C6D6)
-118.4 (4F, m, F2), -138.0 (4F, m, F3 or F2¢), -138.7 (4F, m,
[(ArF¢)Zn(l-Cl)(OEt2)]2 (9)
This compound was prepared using essentially the same procedure
as outlined below for 10, using only one equivalent of the Grignard
reagent 1 (1.5 mmol in 10 cm3 of diethyl ether) to one equivalent of
ZnCl2 (1.5 mmol, 0.20 g). [(ArF¢)Zn(m-Cl)(OEt2)]2 was crystallised
by slow diffusion of light petroleum into diethyl ether at -27 ◦C,
leading to colourless crystals suitable for a single crystal X-ray
diffraction study (0.25 g, 51%). dH (300 MHz, C6D6) 3.45 (4H, q,
J 7.1, OCH2CH3), 0.85 (6H, t, J 7.1, OCH2CH3). dF (282 MHz,
C6D6) -117.8 (4F, m, F2), -138.9 (4F, m, F3 or F2¢), -139.4 (4F,
3
3
F3 or F2¢), -151.8 (2F, t, JF,F 21, F4¢), -161.6 (4F, t, JF,F 21,
F3¢). Elemental analysis found: C, 45.24; H, 2.32. Calculated for
C32H20F18O2Zn: C, 45.55; H, 2.39.
(ArF)(ArF¢)Zn(THF)2 (11)
3
m, F3 or F2¢), -151.7 (2F, t, 3JF,F 22, F4¢), -161.7 (4F, t, JF,F 22,
To a solution of 9 (0.1 g, 0.2 mmol) in tetrahydrofuran (10 cm3)
C6F5MgBr (0.64 cm3 of a 0.625 M solution, 0.4 mmol) was
added. The reaction mixture was then stirred for 2 h before all
the volatile materials were removed under vacuum. The residue
was washed with light petroleum (20 cm3) and extracted with
toluene (3 ¥ 20 cm3). After the toluene was distilled off under
reduced pressure, the residue was purified by precipitation from
a tetrahydrofuran solution following addition of light petroleum
to give compound 11 as a colourless solid (0.12 g, 0.18 mmol,
45%). dH (300 MHz, CDCl3) 3.94 (8H, br, 2,5-THF), 1.92 (8H, m,
F3¢). Elemental analysis found: C, 37.80; H, 1.78. Calculated for
C32H20Cl2F18O2Zn2: C, 39.21; H, 2.06.
(ArF¢)2Zn(OEt2)2 (10)
A solution of ZnCl2 (2.5 mmol, 0.34 g) in room temperature
diethyl ether (10 cm3) was treated with a solution of 1, (20 cm3,
0.25 M, 5 mmol). The reaction was appreciably exothermic and
some white solid precipitated within a few minutes. After 2 h the 19F
NMR spectrum confirmed complete consumption of the Grignard
Table 2 Crystal data and refinement results for all samples
6-
Crystal
4
4a·MeCN
5·3(C6D6)
NCMe·MeCN
7-NCMe·2(CH2Cl2)
9
10
Empirical formula
C15H9F9Sn
478.9
C14H6F9BrSn· C24H3BF18O2· C26H3BF19N·
C38H3BF27N·
2(CH2Cl2)
1167.1
C32H20Cl2F18- C32H20F18O2Zn
O2Zn2
C2H3N
584.8
3(C6D6)
928.5
C2H3N
742.2
Formula wt/g mol-1
Crystal system, space
group
980.1
Triclinic, P1
843.8
Monoclinic,
C2/c
¯
¯
¯
¯
Triclinic, P1
Monoclinic,
P21/n
Triclinic, P1
Monoclinic,
P21/c
Triclinic, P1
˚
a/A
6.7641(4)
11.2250(8)
11.2842(8)
66.841(7)
83.121(6)
81.090(6)
776.59(9)
2
2.048
1.739
Colourless
blocks
0.46 ¥ 0.16 ¥
0.13
3.5 to 27.5
10427/3543/
3192
0.022
0.84/1.24
3543/0/229
9.8151(2)
10.5666(3)
18.0686(4)
90
96.468(2)
90
7.0808(2)
14.5619(7)
18.4705(8)
92.210(2)
95.502(3)
99.797(2)
1865.15(13)
2
10.1502(4)
9.9420(3)
27.4704(11)
90
99.069(4)
90
9.2820(4)
13.8838(5)
17.4899(7)
104.808(3)
89.136(3)
105.715(3)
2094.05(14)
2
7.9926(12)
10.3081(15)
11.1367(13)
78.391(11)
88.745(11)
76.934(13)
875.2(2)
1
19.078(4)
8.1176(8)
20.494(3)
90
91.363(14)
90
3172.9(9)
4
1.767
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
1862.01(8)
4
2737.47(17)
4
Z
Dc/Mg m-3
2.086
3.605
1.653
0.160
1.801
0.197
1.851
0.439
Colourless plates
1.860
Abs. coeff. m/mm-1
Colour, habit
1.651
Colourless
blocks
0.38 ¥ 0.20 ¥
0.11
0.912
Colourless
blocks
0.29 ¥ 0.15 ¥
0.07
Colourless
plates
0.18 ¥ 0.04 ¥
0.005
Colourless
plates
0.29 ¥ 0.19 ¥
0.11
Colourless
needles
0.59 ¥ 0.09 ¥
0.06
Crystal dimensions/mm3
0.52 ¥ 0.16 ¥ 0.09
q range/◦
Reflections collected/
unique/observed
Rint
Absorption Tmin/Tmax
No. of
3.2 to 30.0
28886/5427/
3624
2.9 to 22.5
24007/4873/
3638
3.5 to 25.0
28704/4798/
3031
3.1 to 25.0
23775/7309/
3380
0.078
0.89/1.09
7309/18/694
3.9 to 27.5
4.0 to 20.0
11781/3976/ 8721/1460/
2532
0.052
997
0.104
0.057
0.062
0.087
0.86/1.17
5427/0/254
0.82/1.00
4873/3/580
0.84/1.16
4798/0/476
0.88/1.08
3976/0/272
0.91/1.13
1460/0/240
data/restraints/parameters
Final R indices [F2 >
2s(F2)]: R1, wR2
R indices (all data):
R1, wR2
0.038, 0.097
0.043, 0.099
0.028, 0.049
0.061, 0.055
0.052, 0.094
0.084, 0.104
0.038, 0.058
0.088, 0.066
0.047, 0.074
0.143, 0.091
0.45 and -0.41
0.054, 0.133
0.093, 0.145
0.044, 0.051
0.089, 0.058
Largest diff. peaks and
4.59 and -0.90 0.70 and -0.86 0.24 and -0.26 0.22 and -0.20
1.24 and -0.81 0.33 and -0.33
-3
˚
hole/e A
1600 | Dalton Trans., 2009, 1593–1601
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3,4-THF). dF (282 MHz, CDCl3) -117.7 (4F, br, F2 and o-F C6F5),
-137.8 (2F, br, F3 or F2¢), -140.0 (2F, br, F3 or F2¢), -152.5 (1F, t,
3JF,F 20, p-F C6F5), -157.0 (1F, br, F4¢), -161.7 (2F, m, m-F C6F5),
-162.1 (br, 2F, F¢3). Elemental analysis found: C, 44.99; H, 2.37.
Calculated for C26H16F14O2Zn: C, 45.14; H, 2.33.
of patent examples of the application of salts of the tetrakis(4-
nonafluorobiphenyl)borate anion in alkene polymerisation, see for
example: ExxonMobil, L. S. Baugh, A. O. Patil, C. P. Mehnert,
K. R. Squire, K. A. Cook, C. Gong, E. Berluche, K. S. Colle, A. J.
Oshinski, S. Zushma and B. J. Poole, Int. Pat. Appl., WO2008008112,
2008.
13 A. Guerrero, E. Martin, D. L. Hughes, N. Kaltsoyannis and M.
Bochmann, Organometallics, 2006, 25, 3311–3313.
14 V. I. Krasnov, A. S. Vinogradov and V. E. Platonov, Mendeleev
Commun., 2006, 16, 168–170.
15 P. Sartori and M. Weidenbruch, Chem. Ber., 1967, 100, 3016.
16 W. T. Miller and K. K. Sun, J. Am. Chem. Soc., 1970, 92, 6985–6987.
17 D. A. Walker, T. J. Woodman, D. L. Hughes and M. Bochmann,
Organometallics, 2001, 20, 3772–3776.
Crystal structure analyses†
A suitable crystal was selected and data for 5 were measured
at 120 K at the National Crystallography Service on a Bruker
Nonius APEX II CCD Area Detector equipped with a Bruker
˚
Nonius FR591 rotating anode (lMo-Ka = 0.71073 A) driven by
18 S. J. Lancaster, http://www.syntheticpages.org/pages/215; J. L. W.
Pohlmann and F. E. Brinckman, Z. Naturforsch., 1965, 20b, 5–11.
19 For 4-bromo-2,3,5,6,2¢,3¢,4¢,5¢,6¢-nonafluoro-biphenyl: dF (282 MHz,
293 K, C6D6) -131.7 (m, 2F), -137.3 (m, 2F), -138.3 (m, 2F), -149.4
(t, 1F, 3JF,F = 22 Hz), -160.3 (m, 2F).
COLLECT37 and processed by DENZO38 software. For 4, 4a,
6, 7, 9 and 10, data were collected at 140 K at UEA on an
Oxford Diffraction Xcalibur-3 CCD diffractometer equipped with
Mo-Ka radiation and graphite monochromator, the data were
processed using the CrysAlis-CCD and -RED39 programs. The
structures were determined in SHELXS-97 and refined using
SHELXL-97.40 Crystal data and refinement results for all samples
are collated in Table 2.
20 We note that the 19F NMR spectra show little sensitivity to solvent, as is
illustrated by comparison of the benzene-d6 and chloroform-d1 spectra
of compound 4, both of which are given in the experimental section.
21 The respective 19F NMR chemical shifts for the F2 position are:
-118 ppm in C6D6 for Zn (9–11), -120 ppm for Hg (8), -121 ppm
for Sn (4, 4a), -127 ppm for Si (2 and 3) and -133 ppm for B (6·NCMe
and 7·NCMe).
The large difference peak in compound 4 was thought to be of
a minor, disordered tin atom (the rest of this molecule was not
identified); refinement including this partially occupied atom gave
better R-indices and coordinate Esd’s but the results presented
here are for the complete major component only.
22 W. Tyrra and M. S. Wickleder, Z. Anorg. Allg. Chem., 2002, 628, 1841–
1847.
23 M. V. Metz, D. J. Schwartz, C. L. Stern, T. J. Marks and P. N. Nickias,
Organometallics, 2002, 21, 4159–4168.
24 D. J. Morrison, S. D. Riegel, W. E. Piers, M. Parvez and R. McDonald,
Chem. Commun., 2006, 2875–2877.
25 A. Karipides, C. Forman, R. H. P. Thomas and A. T. Reed, Inorg.
Chem., 1974, 13, 811–815.
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therefore no supporting spectroscopic data are provided.
27 K. Ishihara and H. Yamamoto, Eur. J. Org. Chem., 1999, 527–538.
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