Syntheses and molecular structures of some saturated N-heterocyclic plumbylenes

Article abstract of DOI:10.1039/b808717b

Cyclic diamino plumbylenes derived from saturated heterocycles are obtained from deprotonation of diamines and subsequent reaction with PbCl₂, or by reaction of a suitable diamine with Pb[N(SiMe₃) ₂]₂. Single crystal X-ray studies have been used to probe the solid state structures of a range of these complexes and have shown the fine balance between monomer and dimer formation which is related to the bulk of the organic group attached to the nitrogen atoms. Dimerisation is also shown to effect structural changes within the core of the heterocyclic plumbylene.

Full text of DOI:10.1039/b808717b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Syntheses and molecular structures of some saturated N-heterocyclic
plumbylenes†
Jonathan P. H. Charmant,a Mairi F. Haddow,a F. Ekkehardt Hahn,*b Dennis Heitmann,b Roland Fro¨hlich,c
Stephen M. Mansell,a Christopher A. Russell*a and Duncan F. Wassa
Received 22nd May 2008, Accepted 22nd July 2008
First published as an Advance Article on the web 23rd September 2008
DOI: 10.1039/b808717b
Cyclic diamino plumbylenes derived from saturated heterocycles are obtained from deprotonation of
diamines and subsequent reaction with PbCl2, or by reaction of a suitable diamine with Pb[N(SiMe3)2]2.
Single crystal X-ray studies have been used to probe the solid state structures of a range of these
complexes and have shown the fine balance between monomer and dimer formation which is related to
the bulk of the organic group attached to the nitrogen atoms. Dimerisation is also shown to effect
structural changes within the core of the heterocyclic plumbylene.
nulated N-heterocyclic plumbylenes;10,11 we also note the report
on the Cambridge Structural Database of the crystal structure
Introduction
of [Me3SiN(CH2)2N(SiMe3)Pb]2.12 In this paper we report the
synthesis and X-ray crystal structures of three examples of N-
heterocyclic plumbylenes, two with a saturated C2 backbone (i.e.
analogous to a 4,5-dihydroimidazol-2-ylidenes) and the corre-
sponding compound with a saturated C3 backbone.
N-heterocyclic carbenes belong to the most intensively re-
searched class of compounds in modern synthetic chemistry.1
The most commonly studied subset of this family are the so-
called “Arduengo type” carbenes based on the imidazol-2-ylidene
2
=
framework, where the mesomeric interaction of the C C double
bond on the backbone with the two N atoms allows for efficient
stabilisation of the formally divalent carbon atom. The versions
of these ligands with saturated backbones, 4,5-dihydroimidazol-
2-ylidenes,3 are less stable as free carbenes and are more strongly
basic than their unsaturated analogues. An intermediate position
regarding stability and reactivity is observed for the benzannulated
N-heterocyclic carbenes.4
In searching for higher homologues of N-heterocyclic carbenes,
most attention has focussed on the lighter elements Si5 and
Ge,6,8b and several structurally characterised examples of these
have been reported containing both saturated and unsaturated
backbones. Furthermore, several of the compounds proved to
be preparable on a sufficient scale to enable their reactivity, in
particular regarding their use as ligands towards transition metals,
to be investigated.6c,7
We were intrigued by the fact that there are comparatively
few examples of analogues of N-heterocyclic carbenes utilising
the heavier group 14 elements, tin and lead, despite the well-
known inert pair effect which suggests that the divalent group 14
species should become more stable upon descending the group.
Only recently have tin analogues of Arduengo-type carbenes8
and their benzannulated derivatives been described.9 Structural
evidence for related lead analogues is limited to a few benzan-
Results and discussion
The plumbylenes, DippN(CH2)nN(Dipp)Pb (Dipp
=
2,6-
diisopropylphenyl, n = 2, 1; n = 3, 3) were synthesised by the
reaction of the dilithium salt of the appropriate diamine with
PbCl2 in diethyl ether at -78 ◦C (Scheme 1). This produced
deep red-coloured solutions from which the solvent was removed
in vacuo after 2 h. The solids were extracted into n-hexane and
crystallisation at -20 ◦C yielded dark red crystals of the pure
plumbylenes 1 and 3 respectively. Plumbylene 2 was obtained in
a transamination reaction between 1,2-(dimesitylamino)ethane13
and bis[bis(trimethylsilyl)amido]lead(II)14 in toluene at ambient
temperature. It precipitates from the reaction solution and can be
recrystallised from toluene to give red crystals.
aSchool of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK
BS8 1TS. E-mail: Chris.Russell@bris.ac.uk; Fax: +44 (0)117 929 0509
bInstitut fu¨r Anorganische und Analytische Chemie, Universita¨t Mu¨nster,
Corrensstraße 36, 48149, Mu¨nster, Germany. E-mail: fehahn@
uni-muenster.de; Fax: +49 (0)251 833 310
cOrganisch-Chemisches Institut, Universita¨t Mu¨nster, Corrensstraße 40,
48149, Mu¨nster, Germany
† CCDC reference numbers CCDC reference numbers 689239 (1), 689240
(2), and 689241 (3). For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b808717b
Scheme 1 Synthesis of plumbylenes 1–3. Dipp = 2,6-diisopropylphenyl,
Mes = 2,4,6-trimethylphenyl.
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1H NMR spectra (C6D6, 25 ◦C) of both 1 and 3 show two
doublets for the isopropyl CH3 groups and only one signal for
the isopropyl CH groups as is commonly encountered for the
diisopropylphenyl ligand owing to restricted rotation around the
N–Ar bond. Furthermore, the 1H NMR spectra show two broad
satellite resonances associated with the signals from the CH2
3
groups adjacent to nitrogen atoms (for 1, d = 5.47 ppm, JPb,H
ª
14 Hz; for 3, d = 4.91 ppm, 3JPb,H ª 18 Hz) from coupling with the
207Pb nucleus (I = 12 , 22.6% abundant). Formation of plumbylene
2 was detected by the downfield shift for the resonance of the N–
CH2 protons from d = 3.2 ppm for the diamine11 to d = 5.2 ppm
for 2.
Fig. 1 Molecular structure of 1 (thermal ellipsoids are set to 50%
probability, hydrogen atoms have been removed for clarity and only one
orientation of the disordered five-membered ring is shown). Selected bond
Single crystal X-ray diffraction experiments were carried out on
1–3. Thermal ellipsoid plots of the molecular structures are shown
in Fig. 1–3 and selected structural parameters are presented in the
legend of each figure; crystallographic data are shown in Table 1.
Each structure contains a lead atom as part of a non-planar
five- or six-membered ring. Complexes 1 and 3 are monomeric
in the solid state (although there are long-range interactions with
aryl groups of adjacent molecules, vide infra) whereas complex 2
adopts a dimeric motif.
The structure of 1 revealed that the molecule was disordered
over two sites that were related by a twofold rotation axis oriented
through the N1–N1* vector. Consequently, atoms Pb, C21 and
C22 are disordered over two positions (refinement with SOF =
0.5). The lead atoms of the monomeric units of 1 (Fig. 1) maintain
weak intermolecular interactions with aryl rings of neighbouring
molecules. Owing to the two orientations of the five-membered
◦
˚
lengths (A) and angles ( ) for the depicted orientation of the five-membered
ring: Pb–N1 2.087(5), Pb–N1* 2.096(4); N1–Pb–N1* 80.1(2). Bond
parameters for the second orientation of the five-membered ring are
identical to those of the first one for symmetry reasons.
ring this leads to the appearance of an extended structure in the
crystal lattice.
The N1–Pb–N1* angle in 1 (80.1(2)◦) is larger than the
equivalent angles in benzannulated N-heterocyclic plumbylenes
(range 75.4(2)◦ to 75.8(3)◦),11 with the smaller angles presumably
a consequence of the annulation of the five-membered ring. By
analogy to the situation found for equivalent parameters in N-
˚
heterocyclic carbenes, longer Pb–N separations (2.150(7) A to
2.180(4) A)11 were observed for the benzannulated N-heterocyclic
˚
plumbylenes compared to 1 which exhibits some of the shortest
Table 1 Selected crystallographic and data collection parameters for compounds 1–3
Compound
1
2
3
Data collection diffractometer
Data collection method
Absorption correction
Bruker Nonius KappaCCD
Bruker SMART CCD area-detector
Bruker SMART CCD area-detector
w and j scansa
w and j scansb
Multi-scan (based on symmetry
related measurements)d
Yellow, prism
0.15 ¥ 0.08 ¥ 0.04
C20H26N2Pb
501.62
w scansb
Multi-scan (based on symmetry
Multi-scan (based on symmetry
related measurements)c
Red, plate
0.35 ¥ 0.15 ¥ 0.06
C26H38N2Pb
585.77
related measurements)d
Red, needle
0.25 ¥ 0.10 ¥ 0.05
C27H40N2Pb
599.81
Colour, habit
Size/mm
Empirical formula
Mw
T/K
223(2)
153(2)
Triclinic
P-1
1.876
8.7974(5)
10.0046(6)
11.9931(7)
66.2800(10)
74.3180(10)
68.1540(10)
887.91(9)
173(2)
Crystal system
Space group
Monoclinic
C2/c
1.490
20.2881(5)
6.5360(1)
20.2041(6)
90
102.915(1)
90
Monoclinic
P21/n
1.477
12.986(2)
10.107(2)
11.9931(7)
90
95.445(2)
90
r
a/A
b/A
calc/g cm-3
˚
˚
˚
c/A
a/◦
b/◦
g /◦
V/A-3
2611.36(11)
4
6.474
2697.6(6)
4
6.268
Z
2
m/mm-1
9.502
10817/3178/0.017
Reflections:
total/independent/Rint
q range/◦
Final R1 and wR2
Largest peak, hole
9680/3178/0.038
28092/6184/0.0825
0.41–27.88
0.0541, 0.1145
1.518, -1.438
1.87–31.59
0.0201, 0.049
1.528, -1.336
2.00–27.48
0.0424, 0.0836
1.995, -0.729
a R. W. W. Hooft, COLLECT, Nonius BV, Delft, The Netherlands, 1998. b SMART diffractometer control software, Bruker Analytical X-ray Instruments
Inc., Madison, WI, 1998. c DENZO: Z. Otwinowski and W. Minor, “Processing of X-ray Diffraction Data Collected in Oscillation Mode”, Methods in
Enzymology, Macromolecular Crystallography, Part A, ed. C. W. Carter, Jr. and R. M. Sweet, Academic Press, New York, 1997, vol. 276, pp. 307–326.
d G. M. Sheldrick, SADABS: A program for absorption correction with the Siemens SMART system, University of Go¨ttingen, Germany, 1996.
6056 | Dalton Trans., 2008, 6055–6059
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we take as an indication for the strengths of the interaction. The
N1–Pb–N2 angle within one five-membered ring in 2 (77.28(8)◦)
is slightly narrower than the value in 1 but wider than those in
the benzannulated N-heterocyclic plumbylenes.11 Nitrogen atom
N2, which is not involved in the intermolecular interaction, is
surrounded in a trigonal-planar fashion (sum of angles 358.7◦)
while nitrogen atom N1 is strongly pyramidalized (sum of angles
336.1◦). The dimerisation via strong intermolecular Pb–N interac-
tions obviously has a strong influence on the geometric parameters
of N-heterocyclic plumbylenes, a view that is reinforced by the
fact that the solid state structure of [Me3SiN(CH2)2N(SiMe3)Pb]2
shows a very similar pattern of bond dimensions (Pb–N distances
Fig. 2 Molecular structure of a dimer of 2. Hydrogen atoms are omitted
for clarity and thermal ellipsoids are set to 50% probability. Selected bond
˚
of 2.171, 2.349 and 2.437 A and an intramolecular N–Pb–N angle
◦
˚
of 79.1◦ were observed in this species).12
lengths (A) and angles ( ): Pb–N1 2.350(2), Pb–N2 2.203(2), Pb–N1*
2.471(2); N1–Pb–N2 77.28(8), N2–Pb–N1* 95.43(8).
Yet another mode of aggregation is found for plumbylene 3
(Scheme 1) which contains a six-membered N-heterocyclic ring.
Plumbylene 3 also bears the sterically demanding N,N¢-Dipp
substituents which leads to a similar mode of intermolecular
interactions as observed for 1. Two monomers of 3, which are
related by a crystallographic inversion centre (Fig. 3), form long-
range interactions between the Pb atoms and the aryl rings of
an adjacent molecule (Fig. 3). For 3 the distance between Pb
˚
and the centroid of the closest aryl ring measures 3.83 A; the
˚
corresponding distance in 1 is 3.40 A for each of the disordered
lead sites. However, these distances are longer than those seen
6
in the benzannulated plumbylene exhibiting a similar Pb ◊ ◊ ◊ h -
aryl interaction between the lead atom of one plumbylene and an
aromatic six-membered ring of a second plumbylene at a distance
11
˚
of 3.17 A. Furthermore, the well-known interaction between a
6
PbII centre and a benzene molecule in [(h -C6H6)Pb(AlCl4)2]·C6H6
17
˚
was observed to be only 3.11 A. Hence, although the alignment
Fig. 3 Molecular structure showing two loose-contact monomers of 3
(hydrogen atoms have been omitted for clarity, thermal ellipsoids are set to
50% probability). The distance between the lead atom and the centroid of
of the monomeric units of 3 is suggestive of dimer formation,
the long bond lengths suggest that this is, at best, a very weak
interaction.
˚
the aryl ring is 3.827 A, and intermolecular Pb–C distances between 4.167
◦
In contrast to the strong Pb ◊ ◊ ◊ N intermolecular contacts
and asymmetric intramolecular NPbN units observed in 2, the
˚
˚
and 3.951 A. Selected bond lengths (A) and angles ( ): Pb–N1 2.125(4),
Pb–N2 2.136(5); N1–Pb–N2 89.9(2).
6
weak intermolecular Pb ◊ ◊ ◊ h -aryl interaction in 3 manifests itself
II
˚
˚
Pb –N bond distances observed so far (2.087(5) A to 2.096(4) A).
The nitrogen atoms in 1 adopt distorted trigonal-planar geometry
with the sum of bond angles at each nitrogen centre being 356.5◦
while the five-membered ring shows, as expected, a non-planar
geometry.
in short and almost equally long intramolecular Pb–N bond
distances within the six-membered ring, which are similar to the
equivalent parameters observed for 1 and some benzannulated N-
heterocyclic plumbylenes.11 The nitrogen atoms are surrounded in
a distorted trigonal-planar fashion (sum of bond angles at N1 and
N2 359.7◦ and 359.9◦) and the six-membered ring is not planar.
Within the context of the wider field of heavier group 14
element (E) analogues of N-heterocyclic carbenes, the syntheses
and structures of these N-heterocyclic plumbylenes possessing
saturated organic backbones extends the boundaries on this class
of compound. The volume of structural data on compounds of
this type is relatively limited, with only a few reported examples
based on five-membered rings for E = Si5,7 and a handful of
examples with the same ring size for E = Ge;6,8b no examples
with saturated organic backbones have been reported for E = Sn
and the only other example for E = Pb has been discussed above.12
It is noteworthy that there are no previous reports of structures
of heavier homologues of N-heterocyclic carbenes involving a six-
membered heterocyclic ring with a saturated organic backbone,
although we note examples of analogues for all the heavier
group 14 elements save lead involving six-membered rings with
unsaturated backbones.18
Plumbylene 2 (Scheme 1) which bears the less sterically demand-
ing N-mesityl substituents crystallises as a dimer exhibiting strong
intermolecular Pb–N interactions. This mode of dimerisation
has been described previously for benzannulated N-heterocyclic
stannylenes with sterically non-demanding N,N¢-substituents9a
and was present in the solid state structures of the lead-containing
heterocycles, [PhB(ButN)2Pb]2,15 [(C3H6)Si(ButN)2Pb]2,16 and in
[Me3SiN(CH2)2N(SiMe3)Pb]2.12 The dimer resides on a crystallo-
graphic inversion centre (Fig. 2). This type of dimerisation has
a profound effect on the molecular parameters of 2. The Pb–N
bond distances in 2 are significantly longer than found in 1. In
addition, the Pb centre is not bound symmetrically to the nitrogen
atoms within the five-membered ring. An elongated Pb–N1 bond
˚
distance (2.350(2) A) is observed for the nitrogen atom involved
in intermolecular coordination while the Pb–N2 bond distance is
˚
significantly shorter (2.203(2) A). The length of the intermolecular
˚
Pb–N1* separation (2.471(2) A) is longer than these values which
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◦
It would be inappropriate to attempt to identify any trends given
such limited crystallographic data, however, a few observations
are pertinent. The heterocycles appear to adopt non-planar
geometries in the solid state and the structures do not show
considerable changes upon coordination to a transition metal.
Furthermore, the angle subtended at E by the nitrogen centres
becomes increasing acute as the group is descended; whereas it
is tempting to ascribe this to being a consequence of the inert
pair effect, it may simply reflect the geometric effect that is a
consequence of increasing E–N bond lengths as the group is
descended.
respectively) in diethyl ether (10 mL) at -78 C. Upon warming,
the solution turned dark red. The reaction mixture was stirred at
ambient temperature for another 2 h. Subsequently all solvents
were removed in vacuo. The dark solid was extracted with n-
hexane (20 mL) and the light red solution filtered through a sinter
(porosity 3 with Celite) and washed through with a further portion
of hexane (10 mL). The volume of solvent was then reduced to
about 10 mL. Storage of the solution for 16 h at -20 ◦C gave red
crystals of the respective plumbylene (for 1, 0.276 g, 0.47 mmol,
26%; for 3, 0.59 g, 0.99 mmol, 53%).
Synthesis of 2. Plumbylene 2 was prepared from 0.264 g
(0.5 mmol) of bis[bis(trimethylsilyl)]lead and 0.147 g (0.5 mmol) of
1,2-bis(2,4,6-trimethylphenylamino)ethane in toluene at ambient
temperature using a method reported earlier for benzannulated
N-heterocyclic plumbylenes.10 Yield: 0.213 g (0.42 mmol, 85%).
Conclusions
In summary, we have demonstrated facile syntheses of three
new N-heterocyclic plumbylenes. Single crystal X-ray diffraction
studies showed the plumbylenes to possess extremely short
and symmetrical Pb–N bonds when only weak intermolecular
Analytical data for 1. 1H NMR (300 MHz, C6D6): d 7.34 (d,
7.5 Hz, 2H, m-H Dipp), 7.12 (t, 1H, p-H Dipp), 5.47 (s, with
broad 207Pb satellites ª 14 Hz, 4H, NCH2), 3.76 (sept, 7.0 Hz,
4H, CH Dipp), 1.34 (d, 6.8 Hz, 12H, CH3 Dipp), 1.27 (d, 6.8 Hz,
12H, CH3 Dipp). 13C NMR (75.4 MHz, C6D6): d 151.0 (ipso-C
Dipp), 147.5 (ortho-C Dipp), 125.1 (para-C Dipp), 123.3 (meta-C
Dipp), 69.2 (NCH2), 27.5 (CH3 Dipp), 26.5 (CH3 Dipp), 25.1 (CH
Dipp). 207Pb NMR (63.0 MHz, C6D6): 3504 (s, br). Anal. Calc. for
1 (C26H38N2Pb): C, 53.29; H, 6.54; N, 4.78. Found: C, 54.02; H,
6.73; N, 4.96.
6
Pb ◊ ◊ ◊ h -aryl interactions exist. However, for plumbylene 2, with
the sterically less demanding N,N¢-mesityl substituents, formation
of strong intermolecular Pb–N interactions was observed, leading
to longer and unsymmetrical intramolecular Pb–N separations
and the pyramidalisation of the nitrogen atoms involved in the
intermolecular interaction. Work is ongoing towards probing the
coordination ability of these heavier analogues of N-heterocyclic
carbenes.
Analytical data for 2. 1H NMR (400 MHz, C6D6): d 7.18 (s,
4H, ArH Mes), 5.19 (s, 4H, NCH2), 2.57 (s, 12H, ortho-CH3 Mes),
2.48 (s, 6H, para-CH3 Mes). 13C NMR (100 MHz, C6D6): d 153.2
(ipso-C Mes), 135.2 (ortho-C Mes), 131.9 (para-C Mes), 129.9
(meta-C Mes), 65.1 (NCH2), 20.1 (para-CH3 Mes), 19.5 (ortho-
CH3 Mes). MS (EI): m/z (rel. int.): 502 (53, [M]+).
Experimental
General
All manipulations were performed under an inert atmosphere of
argon or dinitrogen using standard Schlenk-line and glove-box
techniques. Water- and oxygen-free solvents were employed. All
chemicals used were purchased from commercial sources and used
as supplied with the exception of the diamine ligand precursors
which were synthesised according to established literature pro-
cedures. Melting points were determined in sealed tubes under
argon. Elemental compositions (C, H and N) were determined
by sealing samples in air-tight aluminium boats in a glove-box
and recorded on a Carlo Erba EA1108 CHN elemental analyser.
Solution NMR spectroscopy samples were prepared using dry and
degassed deuterated solvent in air-tight NMR tubes sealed with a
Young’s tap. 207Pb spectra were run on an ECP Eclipse 300 MHz
spectrometer and externally referenced to aqueous Pb(NO3)2, the
reported shift19 of which is -2965.7 ppm relative to tetramethyllead
at 295 K. 1H NMR and 13C spectroscopy samples were run
on either an ECP Eclipse 300 MHz spectrometers, a Lambda
300 MHz spectrometer, or a Bruker Avance 400 MHz spectrometer
and were referenced to the internal solvent peaks.
Analytical data for 3. 1H NMR (300 MHz, C6D6): d 7.30 (d,
7.7 Hz, 2H, meta-H Dipp), 7.13 (t, 7.7 Hz, 1H, para-H Dipp),
4.91 (t, 5.1 Hz with broad 207Pb satellites ª 18 Hz, 4H, NCH2),
3.71 (sept, 7.0 Hz, 4H, CH Dipp), 2.50 (quintet, 2H, CH2-CH2-
CH2), 1.35 (d, 7.0 Hz, 12H, CH3 Dipp), 1.26 (d, 7.0 Hz, 12H,
CH3 Dipp). 13C NMR (75.4 MHz, C6D6): d 149.2 (ipso-C Dipp),
147.6 (ortho-C Dipp), 125.5 (para-C Dipp), 123.5 (meta-C Dipp),
58.6 (NCH2), 43.7 (CH2-CH2-CH2), 27.4 (CH3 Dipp), 27.3 (CH3
Dipp), 25.0 (CH Dipp). Anal. Calc. for 3 (C27H40N2Pb): C, 54.06;
H, 6.72; N, 4.68. Found: C, 54.35; H, 7.11; N, 4.63.
X-Ray crystallography
Single crystals of 1–3 were coated in paraffin oil and mounted on a
glass fibre. Data collection and refinement parameters are given in
Table 1. X-Ray measurements were made using Mo-Ka radiation
˚
(l = 0.71073 A). The structures were solved by direct methods and
Synthesis of 1 and 3. To a solution at 0 ◦C of the appro-
priate diamine (for 1, 0.684 g, 1.8 mmol of 1,2-bis(diisopropyl-
phenylamino)ethane; for 3, 0.743 g, 0.188 mmol of 1,3-bis-
(diisopropylphenylamino)propane) in diethyl ether (30 mL) was
added BunLi (1.6 M in hexane, 2.25 mL, 3.6 mmol and 2.35 mL,
3.8 mmol, respectively) and the clear, pale yellow solution was
allowed to warm to room temperature. The reaction mixture was
then stirred for 2 h and was subsequently added dropwise to a
suspension of PbCl2 (0.50 g, 1.8 mmol and 0.524 g, 1.88 mmol
refined by least squares on weighted F2 values for all reflections.20
All non-hydrogen atoms were assigned anisotropic displacement
parameters and refined without positional constraints. Hydrogen
atoms were constrained to ideal geometries and refined with
fixed isotropic displacement parameters. Refinement proceeded
smoothly to give the residuals shown in Table 1. Complex neutral-
atom scattering factors were used.21
As described in the main text, the structure of 1 was disordered
with atoms Pb, C21 and C22 disordered over two positions (refined
6058 | Dalton Trans., 2008, 6055–6059
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with SOF = 0.5). In addition, all crystals of 3 suitable for X-ray
diffraction were found to be twinned and so the data which were
collected were detwinned using the programme GEMINI.22
8 (a) T. Gans-Eichler, D. Gudat and M. Nieger, Angew. Chem., Int. Ed.,
2002, 41, 1888–1891; (b) T. Gans-Eichler, D. Gudat, K. Na¨ttinen and
M. Nieger, Chem.–Eur. J., 2006, 12, 1162–1173.
9 (a) F. E. Hahn, L. Wittenbecher, D. Le Van and A. V. Zabula, Inorg.
Chem., 2007, 46, 7662–7667; (b) A. V. Zabula, T. Pape, A. Hepp, F. M.
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10 B. Gehrhus, P. B. Hitchcock and M. F. Lappert, J. Chem. Soc., Dalton
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