Reactions of LiCHR2 and related lithium alkyls with a-H free nitriles and the crystal structures of eleven representative lithium 1,3-diazaallyls, 1-azaallyls and β-diketiminates

Article abstract of DOI:10.1039/b002376k

Reactions between bis(trimethylsilyl)methyllithium LiCHR2 and a nitrile R'CN gave lithium 1-azaallyls, β-diketiminates or 1,3-diazaallyls. The products of related processes involving the lithium alkyl LiCH2R, LiCH(R)Ph or [Li(tmen)]2[l,2-{C(H)R}2C6H4] hav

Full text of DOI:10.1039/b002376k

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Reactions of LiCHR2 and related lithium alkyls with ꢀ-H free
nitriles and the crystal structures of eleven representative lithium
1,3-diazaallyls, 1-azaallyls and ꢁ-diketiminates
Peter B. Hitchcock, Michael F. Lappert,* Marcus Layh, Dian-Sheng Liu, Rafael Sablong and
Tian Shun
The Chemistry Laboratory, University of Sussex, Brighton, UK BN1 9QJ
Received 27th March 2000, Accepted 19th May 2000
Published on the Web 30th June 2000
Reactions between bis(trimethylsilyl)methyllithium LiCHR2 and a nitrile RЈCN gave lithium 1-azaallyls, β-diketim-
inates or 1,3-diazaallyls. The products of related processes involving the lithium alkyl LiCH2R, LiCH(R)Ph or
[Li(tmen)]2[1,2-{C(H)R}2C6H4] have also been obtained (R = SiMe3 and RЈ = But, Ph or C6H4Me-4). Analytical and
spectroscopic data served to identify the following crystalline compounds: [Li{N(R)C(But)C(H)R}(D)]n [D absent
and n = 2; D = tmen or dme and n = 1], [Li{N(R)C(Ph)C(H)R}(D)]n [D = tmen or pmdien and n = 1, or D = thf
and n = 2], [Li{N(R)C(Ph)C(H)C(RЉ)NR}(D)]n [D absent, n = 2 and RЉ = Ph (4a), C6H4Me-4 (4b) or But; or
D = tmen, (thf)2 (9b), NEt3 or thf and NCPh and n = 1 and RЉ = Ph], [Li{N(R)C(Ph)C(H)C(Ph)NR}(D)Li(CHR2)]
[D = OEt or thf], [Li{N(R)C(Ar)NC(Ph)᎐C(H)R}(D)] [D = tmen, n = 1 and Ar = Ph or C H Me-4; or D = thf, n = 2
᎐
2
n
6
4
and Ar = Ph], [Li{N(Ph)C(R)NC(Ph)᎐C(H)R}(tmen)], [Li{N(R)C(But)CH ] , [Li{N(R)C(Ph)᎐C(H)Ph}(tmen)], and
᎐
᎐
2 2
[Li(tmen)]2[1,2-{N(R)C(But)CH}2C6H4]. Each of the three classes of ligands has a diversity of ligating possibilities,
functioning towards lithium variously in a terminal or bridging and mono- or bi-dentate fashion, the role of a neutral
donor D often being crucial. For an isomeric pair, the lithium β-diketiminate was thermodynamically preferred over
the 1,3-diazaallyl. From [Li{N(R)C(But)C(H)R}]2 and successively CH2Br2 and LiBun, the further lithium 1-azaallyl
compound [Li{N(R)C(But)C(H)C(H)(R)Bun}]2 was obtained. From 4a and CH2Br2 or (BrCH2)2 a bis(β-diketiminyl)-
methane CH2[C{C(Ph)NR}C(Ph)N(H)R]2 or 9b or β-diketimine HN(R)C(Ph)C(H)C(Ph)NR were obtained, while
4b with successively KOBut and H2O gave the β-diketimine HN(R)C(C6H4Me-4)C(H)C(C6H4Me-4)NR. Mechanistic
pathways are proposed, which involve Me3Si migrations: from C to N or N to N; or, for the latter a 1,2-dyotropic
Me3Si/H exchange. The crystal structures of eleven of these compounds have been determined. Each of the new
lithium compounds may in principle behave as an N-, C-, (N,NЈ)-, or (N,C)-centred nucleophile. Reactions demon-
strating the first two alternatives for the β-diketiminate [Li{N(R)C(Ar)C(H)C(Ar)NR}]2 are (i) those with water
(Ar = C6H4Me-4) or (BrCH2)2 (Ar = Ph) which gave the diketimine N(R)C(Ar)C(H)C(Ar)N(H)R, whereas (ii)
that with CH2Br2 (Ar = Ph) yielded CH2[C{C(Ph)NR}C(Ph)N(H)R]2.
aluminoxane ((MeAlO)n, MAO), were effective catalysts for
Introduction
olefin polymerisation.9 Related chemistry has been based on
In a series of communications we have shown that the inter-
action of a trimethylsilylmethyllithium reagent Li[CH3ϪnRn]
the reactions of [Li{C(R)(R1)(C5H4N-2)}]2 or [Li{C(R)(R1)-
(C9H6N-2)}]2 with R2CN;10 some of the derived zirconium()
(n = 1, 2, or 3 and R = SiMe3) and an α-hydrogen-free nitrile
RЈCN can yield a 1-azaallyl-, β-diketiminato- or 1,3-diazaallyl-
lithium compound depending on n, the nature of RЈ, the stoi-
chiometry and the absence or presence of a neutral co-ligand.1–4
Furthermore, we have demonstrated that each of such ligands
(illustrated below for the case of n = 2 in the delocalised
mode as A, B and C, respectively) has a diversity of ligating
or hafnium() complexes [M{N(R)C(R2)C(R1)(C5H4N-2 or
C9H6N-2)}4ϪmClm] were effective catalysts with MAO for the
polymerisation of ethylene [R1 = H or R (= SiMe3), R2 = But
or Ph and m = 1 or 2].11 Further similar reactions were
those between Li2[η5-C5H4Si(Me)2C(H)R] and 2 ButCN in the
absence or presence of tmen.12
The aim of this paper is to provide (i) full details of the
material briefly described in the above mentioned communi-
cations,1–4 (ii) extensions to related trimethylsilyl(methyl)-
lithium–RЈCN systems, and (iii) details of the crystal structures
of eleven representative lithium 1-azaallyls, β-diketiminates and
1,3-diazaallyls.
Compounds related to the lithium 1-azaallyls and β-diketim-
inates here reported, but prepared by different routes include
possibilities, functioning in a terminal or bridging fashion. The
1-azaallyl may bind to the metal in a chelating (η3-1-azaallyl) or
open chain (η1-1-azaallyl or enamido) mode.
We have used these lithium compounds as ligand transfer
agents, generating 1-azaallyls, β-diketiminates or 1,3-diazaallyls
of various metals [e.g. there are papers on derivatives of K,5,6
CuI,7 SnII,6 HgII,6 and AuI 7b]; this aspect,8 as well as the reaction
pathways to Li(A), Li(B) and Li(C),9 have been reviewed. Some
of the zirconium() complexes, when treated with methyl-
the following: [Li{η3-CH ᎐C(But)NPh}(OEt )] ,13 [Li{2,6-
᎐
2
2
2
[C(H)R]2C5H3N}Li],14 [Li{2-(CR2)C5H4N}(OEt2)n]2 (n = 0 or
1),15 [Li{2-(CR2)(C5H4N)}(tmen)],15 [Li{2-(CR2)(C5H4N)-
{NC5H4[C(H)R2]-2}],15 [Li{2-[C(H)R]C5H4N}],15 [Li{NC5-
H )᎐C(Ph)R-2}(tmen)],16 [Li{2-(CR2)(C5H3NMe-6)}(tmen)],17
᎐
4
[Li{(C(H)R)(C5H3NCHR2-6)}(tmen)],17 [Li{N(H)᎐C(But)C-
᎐
(H)Prn}(hmpa)],18 [Li{(2-NC5H4)2CH2}(thf)2],19 [Li{(2-NC5-
H4)2CH2}]2,19
and
[Li{N(H)C(H)C(H)C(H)NH}{µ-OP-
(NMe2)3}]2.20 The 1,3-diazaallyls presented in this paper belong
DOI: 10.1039/b002376k
J. Chem. Soc., Dalton Trans., 2000, 2301–2312
This journal is © The Royal Society of Chemistry 2000
2301

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Scheme 1
to the amidinates, a review of which, dealing with compounds
Although we have not been able to separate this mixture into
such as the benzamidinate [Li{N(R)C(Ph)NR}]2 is available.21
its individual components, 5a was independently and conveni-
ently produced (vi in Scheme 1) in satisfactory yield by
dissolving LiCHR2 and 4a in the presence of a slightly
larger than stoichiometric amount of Et2O; its thf analogue 5b
was prepared from 5a by replacing the co-ordinated Et2O with
thf.
Results and discussion
The various reactions of LiCHR2 (R = SiMe3) with ButCN and
PhCN are summarised in Scheme 1. Treatment of LiCHR2 with
an equivalent portion of ButCN in Et2O or a hydrocarbon led
(i in Scheme 1) in high yield to the 1:1 adduct, the lithium
1-azaallyl 1. Use of two equivalents of ButCN proceeded no
further; affording the same product 1 after removal of solv-
ents and heating the residue at 75 ЊC/10Ϫ2 mmHg for 14 h. It
is possible that a presumably unstable Lewis base adduct
1ؒn(NCBut) (n = 1 or 2), similar to 2a and 2b, had formed but
dissociated at work-up. The tmen (2a) and dme (2b) adducts
were, in contrast, much more stable and obtained (ii in
Scheme 1) in high yield from 1 after addition of one equiv-
alent of tmen or dme to 1 and subsequent recrystallisation
from pentane. Treating 1 with the less bulky nitrile PhCN
led (iii in Scheme 1) in a clean reaction to the lithium
β-diketiminate 3.
Although our attempts to isolate the solvent-free lithium
1-azaallyl Li{N(R)C(Ph)C(H)R}, by variation of reaction
conditions or stoichiometry of the reactants, have not been
successful, it was discovered that the reaction of LiCHR2 and
PhCN in the presence of a stoichiometric amount of a neutral
(chelating) electron donor such as tmen or pmdien led (vii
in Scheme 1) in high yield to the donor-co-ordinated lithium
1-azaallyl complexes 6a and 6b. Use of the weaker electron-
donor thf gave the corresponding thf complex 6c in a much
lower yield (32%), with the lithium β-diketiminate 4a as a
co-product. When the same reaction was carried out in the
presence of a large excess of thf (thf as solvent) and at higher
temperature 6c was isolated in high yield (78%). Interestingly
the reaction of 6a with a second equivalent of PhCN gave not,
as anticipated, the β-diketiminate 9a (a donor adduct of 4a) but
instead (viii in Scheme 1) the lithium 1,3-diazaallyl 7a, which
is a regioisomer of 9a. Treatment of 6a with 4-MeC6H4CN
or PhNC gave the 1,3-diazaallyls 7c or 8, while reaction of
LiCHR2 with 2 equivalents of PhCN in thf gave 7b (viii, ix and
xi, respectively in Scheme 1).
Investigation of the thermal stability of the 1,3-diazaallyl by
heating a solution of compound 7b in toluene-d8 in a sealed
NMR tube showed that it was converted (x in Scheme 1) within
5 min at 100 ЊC into the isomeric diketiminate 9b, thus indi-
cating that 9b is the thermodynamically more stable isomer. A
wider range of β-diketiminates 9 was easily prepared (xii in
Scheme 1) on a preparative scale from 4a by adding the
appropriate neutral donor to the solvent-free compound 4,
followed by recrystallisation from a hydrocarbon solvent.
The reaction of LiCHR2 with PhCN is much more complex
than that with ButCN. It was initially shown that treatment of
these reagents at room temperature in Et2O, independent of the
ratio of LiCHR2 to PhCN, gave (iv in Scheme 1) the lithium
β-diketiminate 4. Using a 1:1 ratio of reagents, there was no
evidence for the formation of a lithium 1-azaallyl (an analogue
of 1) as an intermediate. Further investigation of this reaction
showed that when it was performed in the presence of an excess
of LiCHR2 (1.5 fold to PhCN) at Ϫ78 ЊC in Et2O, and the
mixture worked up immediately after warming up to room
temperature, the complex 5a, a co-crystal of LiCHR2 and the
lithium β-diketiminate 4a were obtained (v in Scheme 1) in
moderate yield (45%), while the 1H NMR spectrum of the
mother liquor showed signals corresponding to a mixture of
LiCHR2 (as its Et2O adduct) and Li{N(R)C(Ph)C(H)R}.
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The colourless (1, 2) or pale yellow (6) lithium 1-azaallyls
were highly air-sensitive, extremely hydrocarbon-soluble com-
pounds. The lithium 1,3-diazaallyls 7 and 8 and the lithium
β-diketiminates 3, 4, 5 and 9 were, in contrast, yellow (3, 4, 7, 8,
9a, 9c, 9d), yellow-green (9b) or orange-red (5), but also soluble
in hydrocarbons (except the solvent-free compounds 4 which
were only moderately soluble) and slightly less air-sensitive than
the 1-azaallyls.
Previously we suggested pathways to the lithium 1-azaallyl
Li(A), β-diketiminate Li(B) and 1,3-diazaallyl Li(C) from
LiCHR2 and RЈCN.8 Thus, the formation of Li(A) was attrib-
uted to initial nucleophilic attack of the ϪCHR2 carbanion at the
nitrile carbon atom followed by a 1,3-Me3Si shift from carbon
to nitrogen. Li(A) was proposed to be an intermediate along the
reaction path to Li(B) or Li(C), depending on whether Li(A)
behaved as a C- or N-centred nucleophile, respectively; a final
1,3-Me3Si migration from C to N for Li(B) or N to N for Li(C)
completed the reaction sequence.
The closely related lithium starting materials LiCH2R,
[Li(tmen)]2[{1,2-CH(R)}2C6H4]
and
Li{CH(R)Ph}(tmen)
behaved very similarly to LiCHR2 with regard to their reactions
with nitriles, as illustrated in eqns. (1)–(3) for the synthesis of
the lithium-1-azaallyls 10–12. The outcome of reaction (4) lead-
The major features of these proposals seem to us still to be
persuasive. However, some new experimental observations have
come to light which need to be accommodated. These relate to
(i) the role of a neutral coligand D, (ii) different ligand-to-metal
bonding modes of Li(A) and Li(A)D (see 2 or 6 in Scheme 1)
and (iii) the isomerism of Li(B) and Li(C). As for (i), we draw
attention to the contrast between steps iv {Ar = Ph yielding
[Li(B)]2 4} and vii [giving Li(A)D 6] of Scheme 1. Regarding
(iii), it is now evident (step x of Scheme 1) that the β-diketim-
inates (e.g. 9a) are the thermodynamically preferred isomers of
the 1,3-diazaallyls (e.g. 7a). We now propose the modified reac-
tion pathways summarised in Scheme 3. This shows the initial
(1)
(2)
(3)
(4)
ing to the 1-azaallyllithium complex 13 was unexpected. The
purpose of adding CH2Br2 to compound 1 was to generate a
methylene-bridged bis(1-azaallyl) precursor. Since a crystalline
complex was not obtained, n-butyllithium was added, yielding
13 in modest yield.
The β-diketiminatolithium compounds 4a (Ar = Ph) and 4b
(Ar = C6H4Me-4) generally behaved as N-centred (ii or iii in
Scheme 2, or reactions with metal halides1,8,9,13) nucleophiles, but
exceptionally (i in Scheme 2) functioned as C-centred reagents.
Scheme 3 Proposed reaction pathways from LiCHR2 ϩ RЈCN to
Li(A) (III; 1, 2 or 6), Li(B) (3, 4, 5 or 9) and Li(C) (7 or 8).
formation of the donor–acceptor complex I, leading to the
1,2-insertion product the imidolithium compound II, which
rearranges to the 1-azaallyllithium compound III.
The formation of [Li{N(R)C(But)C(H)C(H)RBun}]2 13,
from [Li{N(R)C(But)C(H)R}]2 1, according to eqn. (4),
requires further comment. It is likely that compound 1 behaves
as a C-centred nucleophile generating D, which with LiBun may
first yield the dyotropic isomer E of 13. The use of the various
lithium 1-azaallyls, β-diketiminates and 1,3-diazaallyls as
ligand transfer reagents with metal halides will be reported in
later papers (see also refs. 5–7). The role of the lithium 1-azallyl 1
in phosphorus chemistry, yielding diazaphosphetidines such as
F or cyclic phosphonium salts such as G, has been described.22
The 1-azallyl ligands are ambidentate nucleophiles; hence
lithium derivatives are likely to exist in solution as a tautomeric
mixture of a monomeric lithium iminoalkyl IIIa, a monomeric
Scheme 2 Some reactions of the β-diketiminatolithium compounds 4a
(Ar = Ph) and 4b (Ar = C6H4Me-4) (R = SiMe3).
J. Chem. Soc., Dalton Trans., 2000, 2301–2312
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Table 1 The effect of temperature and time on the products from the
reaction between LiCHR2 and 2 PhCN in thf
Table 2 Characteristic NMR spectroscopic data for lithium 1-aza-
allyls, 1,3-diazaallyls and β-diketiminates
Ratio of compound
9b to 7b
Compound
δ(7Li) δ(CH,a 1H)
δ(C(H,R),a 13C)
δ(CN, 13C)
T/ЊC
Ϫ78
0
65
t/min
1
2a
2b
6a
6b
6c
10
11
12
7a
7b
7c
8
—
0.34
Ϫ0.62
0.88
0.79
0.76
—
—
—
1.89
1.62
1.90
1.54
—
4.47
4.02
4.06
3.67
3.53
4.37
3.80/4.48
5.53
5.36
4.45
4.67
4.43
4.79
5.85
5.49
5.69
5.51
5.09
5.57
5.34
5.37
5.60
95.6
83.4
83.6
83.1
84.1
93.9
—
185.5
185.6
184.8
175.9
176.3
174.7
—
—
120
60
5
78:22
53:47
5:95
—
—
—
108.2
113.6
107.9
96.7
—
105.2
106.9
105.4
105.4
105.4
105.3
105.2
105.6
164.2/175.0
164.0/176.2
164.3/175.3
154.1/165.7
—
175.5
176.5
177.2
176.8
174.9
175.7
3
4a
4b
5a
5b
9a
9b
9c
9d
2.80
2.29
2.34
—
2.65
—
175.2
175.8
—
lithium enamide IIIb and, in most cases in the absence of a
neutral donor, a dimeric species IIIc. These three species are
expected to be in equilibrium with one another. Dimeric species
IIIc have been established for crystalline complexes 1 and 6c
by single crystal X-ray diffraction studies. The lithium 1-azaallyl
[Li{N(R)C(Ph)᎐CR }(thf)], related to 6c,4 was shown to be
a Olefinic proton or carbon.
NMR Spectroscopy
The NMR spectroscopic data on compounds 1–12 are sum-
marised in Table 2. Use of Li NMR spectroscopy provided
a powerful tool for distinguishing between lithium 1-azaallyls
(δ Ϫ0.6 to 0.9), lithium 1,3-diazaallyls (δ 1.6–1.9) and lithium
β-diketiminates (δ 2.3–2.8) as the principal products of the reac-
tion of LiCHR2 with nitriles. Further help in the identification
᎐
2
monomeric in the crystal and a low temperature 1H NMR spec-
troscopic study provided no evidence for more than one species
in solution. We suggest that IIIa is predominant in pentane
or Et2O solution at room temperature, since under these
reaction conditions an excess of nitrile led to the formation of
a β-diketiminate (3, 4, 5, or 9). Use of a chelating donor such
as tmen or pmdien, or refluxing in thf as solvent, is likely to
increase the abundance of IIIb; the presence of a powerful
neutral donor increases the ionicity of the lithium 1-azaallyl
and therefore favours structure IIIb, in which the more electro-
negative nitrogen atom is the anionic centre. As a consequence,
a lithium 1,3-diazaallyl (7) is favoured in strongly polar solvents
(thf > Et2O) over an isomeric β-diketiminate (4 or 9). As men-
tioned above (item iii) the lithium 1,3-diazaallyl 7a was convert-
ible into the thermodynamically more stable β-diketiminate 9a.
This process was very fast in toluene (conversion within 5 min
at 100 ЊC) but much slower in thf (not fully completed after 1
week at 65 ЊC) and involves, we believe, an equilibrium between
7, IIIb, IIIa and 4. A cross-over experiment, in which a mixture
of 7a and 7c was heated in toluene, was inconclusive; slow
decomposition of the sample occurred and a mixture of
unidentified products was obtained and not, as expected, a
mixture of lithium β-diketiminates.
7
1
of the products came from H NMR spectra, which showed
characteristic, with the exception of compounds 11 and 12
(these have a very different backbone compared to the other
compounds), resonances for the protons of the CH groups in
the region of δ 3.5–4.5 (1-azaallyl), 4.4–4.7 (1,3-diazaallyl) and
5.1–5.9 (β-diketiminate). Additional support for the identifi-
cation of a compound was provided by the 13C NMR spectra;
the lithium 1-azaallyls showed shift values at δ 85–96 (CH) and
175–186 (CN), as compared to values close to δ 105 (CH) and
δ 175 (CN) in lithium 1,3-diazaallyls or β-diketiminates. A dis-
tinction between the latter two isomeric compounds was easily
accomplished on the basis of the lower molecular symmetry of
the former, which gave rise to two distinct CN signals (and also
two for the Me3Si groups), as compared with one signal for the
β-diketiminates.
In order to establish the structures of the above twentythree
crystalline lithium complexes more securely, single crystal X-ray
diffraction studies were undertaken on eleven representative
samples: 1, 3, 4a, 5b, 6c, 7b, 8, 9a, 9b, 11 and 13. The molecular
structures of 1,2 4a1 and 5b3 were described in preliminary
communications. We now present such data on the remaining
eight complexes, discuss trends and comment on bonding
modes. Such problems have previously been discussed in the
context of complexes of 1-azaallyls7,23 or 1,3-diazaallyls6 with
other metals (AuI,7b SnII,6,23 CuI,7 K6 and HgII 6).
In a series of additional experiments LiCHR2 was treated
with PhCN (2 equivalents) in thf, varying both reaction time
and temperature (Table 1). After a selected reaction time the
solvent was removed and the residue analysed by NMR
spectroscopy and shown to be a mixture of the lithium 1,3-
diazaallyl 7b and β-diketiminate 9b. These results appear to
contradict the earlier discussed notion of 9b being thermo-
dynamically more stable than 7b, because the proportion of 7b
to 9b increased with increasing temperature. A possible explan-
ation is that there is a higher proportion of the dimeric species
IIIc at lower temperatures, and that due to steric shielding of
the nitrogen atoms IIIc behaves more as a C- than a N-centred
nucleophile and therefore facilitates the formation of the
lithium β-diketiminate 9b. At higher temperature, on the other
hand, the abundance of the monomer IIIb is increased, which
behaving as a N-centred nucleophile affords 7b. Refluxing for
a longer period, or heating to a higher temperature, shifts the
equilibrium back to 9b.
Crystal structures of representative lithium complexes
1,3-Diazaallyls [Li{N(R)C(Ph)NC(Ph)᎐CHR}(thf)] 7b and
᎐
2
[Li{N(Ph)C(R)NC(Ph)᎐CHR}(tmen)] 8. The molecular struc-
᎐
tures of compounds 7b and 8 are illustrated in Figs. 1 and 2,
respectively. Selected bond distances and angles are listed in
Table 3. N,NЈ-Bis(trimethylsilyl)benzamidines HN(R)C(Ar)NR
and their metal complexes containing the conjugate base [N(R)-
C(Ar)NR]Ϫ as ligand have been reviewed (R = SiMe3, Ar = aryl
group).21 The 1,3-diazaallyllithium complexes 7b and 8 belong
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Table 3 Some important geometric data (bond lengths in Å, angles
in Њ) for compounds 7b and 8
7b
8
Li–N(1)
Li–N(2)
Li–N(2)Ј
Li–D
1.99(1)
—
1.99(1)
1.89(1)
2.025(9)
2.003(9)
—
2.06(1)
2.10(1)
1.355(6)
1.379(6)
C(1)–N(1)
C(1)–N(2)
1.302(7)
1.364(7)
N(1)–C(1)–N(2)
119.0(5)
3
113.2(4)
4
Co-ordination
number at Li
Aggregation
Dimer
Monomer
Fig. 3 Molecular structure of compound 6c.
Complex 7b is a centrosymmetric dimer. Each anionic ligand
is bound to the two lithium atoms in a bridging manner.
Although this is also the case for H–K (in L it is simply a
chelate), 7b is unique in having no close Li ؒ ؒ ؒ N contact to a
third nitrogen atom: Li–N(1) 1.989(11), Li–N(2) 1.985(11) and
Li ؒ ؒ ؒ NЈ 3.26(1) Å. In contrast, the third shortest Li ؒ ؒ ؒ N
contact in H–K is 2.145(8), 2.260(9), 2.137(7) and 2.231(9) Å,
respectively. The mean of the two shorter Li–N contacts in 7b is
1.99(1) Å compared with 2.05(1), 2.06(1), 2.05(1) and 2.06(1) Å
in H–K, respectively. The core in 7b is an eight-membered
(LiNCN)2 ring in chair conformation. Each of the lithium,
nitrogen, oxygen and endocyclic carbon atoms is in a distorted
trigonal planar environment, the distortion being minimal at
carbon. The widest angle at each lithium atom is that subtended
by the two neighbouring nitrogen atoms, while the C–N–Li and
C–O–C angles are the narrowest at the nitrogen or oxygen
atoms, respectively.
Fig. 1 Molecular structure of compound 7b.
In the mononuclear crystalline complex 8 the geometric
parameters of the chelating LiNCN core are very similar to
those found in M27 or N.26 Thus, the mean Li–N [2.014(9) Å] and
C–N [1.367(6) Å] bond lengths are close to the 2.02(1) (M) or
2.13(11) Å (N) and 1.33(1) Å (M and N); cf.28 also the mean
Li–N distance of 2.04 Å in the LiNC(sp2)N unit of [Li{NC5-
H3(NR)Me-2,6}(tmen)]. The angles subtended at Li, N (mean)
and C are 67.5(3), 89.5(5) and 132.2(9)Њ for 8, 67.6(2), 88.7(2)
and 115.1(2)Њ for M,27 and 64.4(2), 89.4(2) and 116.7(3)Њ for N.26
A comparison between the NCN fragments of the LiNCN
unit of compounds 7b and 8 shows there to be (i) a significant
2
contraction from the Csp value of the angle subtended at the
carbon atom only for the mononuclear complex 8 [113.2(4)Њ],
attributable to the chelate effect, and (ii) a more pronounced
similarity in the two C–N distances for 8 (they are identical for
M
27 and N26) than 7b: ∆r = 0.029(6) Å for 8 and 0.062(7) Å for
7b. In respect of (ii), 7b also differs from the other binuclear
complexes H–L. We conclude that 7b is unique in containing
the shortest [1.302(7) Å] and to a less marked extent the longest
[1.364(7) Å] C–N bonds among this family of nine lithium 1,3
diazaallyls and hence the least degree of NCN π-electron
delocalisation. These data may be compared with the single and
double NC bonds lengths in the bis(hydrogen-bonded) dimer
[HN(Ph)C(Ph)NPh]2 of 1.369(8) and 1.310(8) Å, respectively.29
Fig. 2 Molecular structure of compound 8.
to the latter class. They differ in that 7b is dinuclear, while 8 is
mononuclear. Although in both the 1,3-diazaallyl ligand is
bidentate and the LiNCN core is planar (8) or almost planar
(7b), in 8 it is chelating whereas in 7b it is bridging. The molec-
ular structure of 7b may be compared with those of five other
dinuclear amidinates [Li{N(R)C(C6H4Me-4)NR}(thf)2] H,24
[Li{N(R)C(Ph)NR}(NCC6H4Me-4)]2 I,25 [Li{N(Ph)C(Ph)N-
(Ph)}{OP(NMe2)3}]2 J,26 [Li{N(C6H4Me-4)C(H)NC6H4Me-4}-
(OEt)2]2 K27 and [Li{N(Ph)C(Me)NPh}{µ-OP(NMe2)3}]2 L26
which like 7b have an additional neutral ligand at each
lithium atom; that of 8 is related to those of the mononuclear
1-Azaallyls [Li{N(R)C(But)CHR}]2 1, [Li{N(R)C(Ph)CHR}-
(thf)]2
6c,
[Li{N(R)C(Ph)C(H)Ph}(tmen)]
11
and
[Li{N(R)C(But)CH(R)Bun}]2 13. The molecular structure of
complex 12 is shown schematically while those of 6c, 11 and 13
are illustrated in Figs. 3–5, respectively. Selected bond distances
and angles are listed in Table 4.
lithium amidinates [Li{N(Ph)C(Ph)NPh}(tmen)] M27 and
[Li{N(Ph)C(Ph)NPh}(pmdien)] N.26 In each of H–N each
The crystal structures of three lithium 1-azaallyls [Li{N-
(Ph)C(But)CH2}(OEt2)]2 O,13 [Li{N(Ph)(C(᎐CH[CH ] CH )}-
᎐
2 3
2
i
NCNLi core is planar.
(NHPr2 )]2 P30 and [Li{N(R)C(But)CH2}]3 Q31 have previously
J. Chem. Soc., Dalton Trans., 2000, 2301–2312
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Table 4 Some important geometric data (bond lengths in Å) for com-
pounds 1, 6c, 11 and 13
1
6c
11
13
Li–N
Li–NЈ
Li–C(1)
Li–C(2)
N–C(1)
C(1)–C(2)
2.02(1) av.
1.99(1) av.
2.43(2)
2.32(1)
1.397(7)
1.368(7)
2.051(3)
2.063(3)
2.622(3)
3.011(3)
1.380(2)
1.367(2)
1.983(4)
—
2.416(5)
2.510(5)
1.361(3)
1.381(3)
2.035(9)
1.983(9)
2.27(1)
2.36(1)
1.392(6)
1.361(7)
Co-ordination
number at Li
Aggregation
2–3
3
4–6
3–4
Dimer
Dimer
Monomer
Dimer
Fig. 4 Molecular structure of compound 11.
Fig. 5 Molecular structure of compound 13.
2.08(2) Å (P)30]. Whereas 1 and 13 are free of neutral coligands,
each of the remainder has a single such molecule at each
lithium atom. Complexes 1 and 13 have Li–Cα and Li–Cβ dis-
tances which are within bonding range [2.43(1) and 2.32(1) (1),
2.36(1) and 2.27(1) Å (13)]; consequently 1 and 13 are formu-
lated as η3-1-azaallyllithium compounds. By contrast, com-
plexes 6c, O13 and P,30 which contain a neutral coligand at each
lithium atom, have appreciably longer Li–Cα and Li–Cβ con-
tacts [3.01(1) and 2.62(1) (6c) and 2.99(2) and 2.57(1) Å (O)];
hence these are assigned as lithium enamides, i.e. η1-1-
azaallyllithium complexes.
The molecular structures of the two mononuclear lithium
compounds differ in that 11 is an enamide, whereas S is an η3-1-
azallyl, as evident from the Li–Cα and Li–Cβ distances: 2.81(4)
and 2.77(4) (11) and 2.23(2) and 2.32(2) Å (S)4 Å. In both
complexes the Li–N distances are rather short: 1.97(4) (11) and
1.93(2) (S) Å. For 11 this is particularly noteworthy since its
lithium atom is more co-ordinatively saturated than that in S,
having not only a bidentate coligand (tmen; cf. thf in S) but also
short Li-C contacts to the ipso-[2.77(4) Å] and ortho-[2.56(4)
Å] carbon atoms of the phenyl substituent of Cα.
been published and brief reference has been made to
two others: the tetranuclear complex R12 obtained from [Li{η-
C5H4Si(Me)2C(H)R}(tmen)]2 and 2 ButCN, and S.10 Closely
related compounds are the 2-pyridylmethyls,14,17,32 pioneered by
Raston and exemplified by T and U.32a They differ from the
1-azaallyls in that, unlike in the latter, the pyridyl nitrogen atom
cannot bear a negative charge unless the ring loses its aroma-
ticity, which almost invariably is not the case. Thus, the rela-
tionship of the 1-azaallyls to the 2-pyridylmethyls is analogous
to that of allyls to benzyls.
The C–C and N–C bond distances of the 1-azaallyl ligands in
these complexes (see Table 4) do not appear to be diagnostic as
to the nature of their bonding mode.
ꢁ-Diketiminates [Li{N(R)C(Ph)C(H)C(But)NR}]2 3,
[Li{N(R)C(Ph)C(H)C(Ph)NR}]2 4a,
The molecular structures of the dinuclear compounds 1, 6c
and 13 resemble one another and also those of O13 and P,30
in that each is a centrosymmetric dimer containing as core a
nearly planar LiNLiN ring, with the angles subtended at the
nitrogen atoms narrower [73.0(4) (1), 74.1(1) (6c) and 72.7(4)Њ
(13)] than those at the Li atoms [107.5(5) (1), 106.0(1) (6c) and
107.3(4)Њ (13)]. The Li–N distances are closely similar {2.00(1)
and 2.04(1) Å (1), 2.051(3) and 2.063(3) Å (6c), 1.983(9) and
2.035(9) Å (13), 2.00(1) and 2.08(1) (O)13 and 2.01(2) and
[Li{N(R)C(Ph)C(H)C(Ph)NR}(LiCHR2)(thf)] 5b,
[Li{N(R)C(Ph)C(H)C(Ph)NR}(tmen)] 9a and
[Li{N(R)C(Ph)C(H)C(Ph)NR}(thf)2] 9b. The molecular struc-
tures of complexes 4a1 and 5b3 are shown schematically while
those of 3, 9a, and 9b are illustrated in Figs. 6–8. Important
bond distances and angles are summarised in Table 5.
The crystal structure of a lithium β-diketiminate [Li{N-
(Ph)(CH)3NPh}(µ-OP(NMe2)3}]2 V has been published.20 The
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Table 5 Some important geometric data (bond lengths in Å, angles in Њ) on compounds 3, 4a, 5b, 9a and 9c (R = SiMe3)
3
4a
5b
9a
9b (av.)
Li–N(1)
Li–N(2)
Li–N(1)Ј
N(1)–C(1)
N(2)–C(3)
C(1)–C(2)
C(2)–C(3)
1.97(2)
2.04(2)
2.17(2)
1.27(1)
1.33(1)
1.46(1)
1.41(5)
1.965(9)
1.952(10)
2.095(9)
1.337(6)
1.299(6)
1.394(7)
1.439(6)
2.01(2)
—
2.01(2)
1.32(1)
1.32(1)
1.41(1)
1.41(1)
2.016(7)
2.024(6)
—
1.326(4)
1.320(4)
1.408(4)
1.402(4)
2.02(2)
2.02(2)
—
1.32(1)
1.32(1)
1.41(1)
1.41(1)
N(1)–Li–N(2)
100.0(7)
3
103.0(4)
3
94.4
97.3(3)
4
102.3(7)
4
Co-ordination
number at Li
Aggregation
3
Dimer
Dimer
Monomer
Monomer
Monomer
Fig. 6 Molecular structure of compound 3.
Fig. 7 Molecular structure of compound 9a.
bis(2-pyridyl)methyllithium compounds [Li{(2-NC5H4)2CH}-
(thf)2] W, [Li{(2-NC5H4)2CH}{(2-NC5H4)2CH2}] X and [Li{(2-
NC5H4)2CH}2] Y19 are closely related to lithium 1-azaallyls,
while the complexes obtained from PhCN or ButCN and an
appropriate 2-pyridylmethyllithium precursor10 are similarly
related to lithium β-diketiminates.
Fig. 8 Molecular structure of compound 9b.
The dinuclear complexes 3 and 4a1 differ only in the substitu-
ents (Ph, But in 3, but Ph, Ph in 4a) at the C(N) atoms. Each
molecule is a centrosymmetric dimer built around a planar
LiNLiN core, the angle at the lithium atom being wider
[103.8(7) (3) and 105.0(4)Њ (4a)] than that at nitrogen [76.2(6) (3)
and 75.0(4)Њ (4a)]. One of the nitrogen atoms is four-co-
ordinate and bridging, the other is three-co-ordinate. The
2
shorter Li–Nsp bond [1.973(15) Å (3) and 1.952(10) Å (4a)].
Within the LiNCCCN ring, the NCCC fragment is planar and
there is some degree of π-electron delocalisation, although the
pairs of N–C [1.271(11) and 1.333(11) Å (3) and 1.299(6) and
1.337(6) Å (4a)] and C–C bonds [1.460(11) and 1.415(11) Å (3)
and 1.439(6) and 1.394(7) Å (4a)] are far from being identical.
The lithium β-diketiminate V20 differs from 3 and 4a in that,
although dinuclear, it has bridging hmpa ligands and thus it is
3
intramolecular Li–Nsp bond is slightly shorter [2.004(2) (3) and
3
1.965(9) Å (4a)] than the intermolecular Li–Nsp [2.17(2) (3) and
2.095(9) Å (4a)]. The β-diketiminato ligand has a marginally
J. Chem. Soc., Dalton Trans., 2000, 2301–2312
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best considered with the mononuclear complexes. Complex 5b3
is distinct from 3, 4a and V in that it contains a single β-
diketiminato ligand. Its skeleton is nearly planar and π-electron-
delocalised. The two three-co-ordinate lithium atoms are on
opposite sides of this plane; the Li–N distances are very similar.
The mononuclear complexes 9a and 9b, like V,20 have a
π-electron-delocalised and almost planar β-diketiminate
skeleton, as evident by the essentially equivalent pairs of N–C
[1.32(1) Å] and C–C [1.40(1) Å (9a), 1.39(1) and 1.42(1) Å (9b)
and 1.39(1) Å (V)] bonds. The four-co-ordinate lithium atom in
(s, CN). Compound 2b: mass spectrum m/z (%) 243 (20, [HL]ϩ),
1
228 (43, [HL Ϫ Me]ϩ) and 186 (79, [HL Ϫ But]ϩ); H NMR
(C6D6) δ 0.32 (s, SiMe3), 0.47 (s, SiMe3), 1.34 (s, But), 2.60
7
(m, OCH2), 2.96 (s, OMe) and 4.06 (s, CH); Li NMR(C6D6)
δ Ϫ0.62; 13C NMR (C6D6) δ 1.1 (s, SiMe3), 5.7 (s, SiMe3), 31.0
(s, CMe3), 40.6 (s, CMe3), 59.4 (s, OMe), 69.6 (s, OCH2), 83.6 (s,
CH) and 184.8 (s, CN).
[Li{N(R)C(But)C(H)C(Ph)NR}]2 3. Benzonitrile (0.4 g, 5.09
mmol) was slowly added to
a
solution of [Li{N-
2
(R)C(But)CH(R)}]2 1 (1.33 g, 2.05 mmol) in Et2O (30 cm3) at
ca. 25 ЊC. The resulting yellow solution was stirred for ca. 12 h.
Volatiles were removed during 1 h at 90 ЊC/10Ϫ2 Torr. Crystal-
lisation from hexane afforded yellow crystals of complex 3 (1.40
g, 81%) (Found: C, 63.4, H, 9.50; N, 7.68. C19H33LiSi2 requires
C, 64.7; H, 9.43; N, 7.94%). 1H NMR (360 MHz, C6D6) δ 0.03
and 0.04 (SiMe3, s, 9 H), 1.25 (But, d, 9 H), 5.85 (CH, s, 1 H),
7.07–7.13 (phenyl, m, 3 H) and 7.37 (phenyl, d, 2 H).
each molecule is about 0.7 Å out of this plane and the Li–Nsp
distances are similar [2.02(1) Å in 9a and 9b and 2.04(2) Å in V].
Experimental
General procedures
All manipulations were carried out under argon, using standard
Schlenk techniques. Solvents were distilled from drying agents
and degassed. The NMR spectra were recorded in C6D6,
C6D5CD3 or CDCl3 at 298 K using the following Bruker
[Li{N(R)C(Ph)C(H)C(Ph)NR}]2 4a. PhCN (2.30 cm3, 22.53
mmol) was added slowly by syringe to a cooled (0 ЊC) and
stirred solution of LiCHR2 (1.85 g, 11.15 mmol) in diethyl ether
(50 cm3). There was an immediate change from colourless to
yellow. The resultant mixture was slowly warmed to room tem-
perature and stirred for 2 h; the diethyl ether was removed in
vacuo, and the resultant yellow-white solid heated to boiling,
then cooled and filtered. The yellow precipitate was dried in
vacuo and identified as compound 4a (3.16 g, 76%), mp 167.5–
171 ЊC (Found: C, 66.1; H, 8.32; N, 7.85. C21H29LiN2Si2
7
instruments DPX 300 (1H, 300.1; 13C, 75.5; Li, 116.6 MHz)
and AMX 500 (1H, 500.1; 13C, 125.7; 7Li, 194.3 MHz) and ref-
erenced internally to residual solvent resonances (data in δ) in
1
7
the case of H and 13C spectra. The Li were referenced exter-
nally to LiCl. Unless otherwise stated all NMR spectra other
1
than H were proton-decoupled. Electron impact mass spectra
were taken from solid samples using a Kratos MS 80 RF
instrument. Melting points were recorded in sealed capillaries
and are uncorrected. Elemental analyses (empirical formulae
shown) were determined by Medac Ltd., Brunel University. In
the experimental section, the abbreviation L for mass spectra
denotes the ligand A, B or C.
1
requires C, 67.7; H, 7.85; N, 7.52%); H NMR (C6D6–C4D8O)
δ Ϫ0.07, (s, SiMe3), 5.49 (s, CH), 7.07–7.13 (m, Ph, 6 H) and
7.50–7.53 (m, Ph, 4 H); 13C NMR (C6D6) δ 3.0 (s, SiMe3), 105.2
(s, CH), 127.4, 127.8, 127.9 (s, o, m, p-C of Ph), 149.8 (s, ipso-
C) and 175.5 (CN).
Preparations
[Li{N(R)C(But)C(H)R}]2 1. Method (i). ButCN (0.40 cm3,
3.61 mmol) was added dropwise to a cooled (Ϫ20 ЊC) and
stirred solution of LiCHR2 (0.60 g, 3.61 mmol) in diethyl ether
(ca. 15 cm3). The resultant solution was warmed to room
temperature and stirred overnight. The solvent was removed in
vacuo. The residue was dried at room temperature (10Ϫ2 Torr)
for 1 h and extracted with pentane (ca. 20 cm3). The resultant
extract was filtered. Concentrating the filtrate by slow evapor-
ation of the solvent produced white crystals of compound 1
(0.98 g, 99%).
[Li{N(R)C(C6H4Me-4)C(H)C(C6H4Me-4)NR}]2 4b. Com-
pound 4b (1.09 g, 52%) (Found: C, 68.1; H, 8.66; N, 6.99.
C23H35LiN2Si2 requires C, 69.0; H, 8.24; N, 7.00%) was pre-
1
pared similarly, mp 188–191 ЊC. H NMR (C6D5CD3) δ 0.0 (s,
SiMe3), 2.07 (s, Me), 5.69 (s, CH), 6.93, 7.41 (d, Ph, 4 H). 13C
NMR (C6D5CD3) δ 3.0 (s, SiMe3), 21.2 (s, Me) 106.9 (s, CH),
128.2, 128.6 (s, m, p-C of Ph), 137.7, 140.0 (s, ipso-C) and
176.5 (CN).
Method (ii). ButCN (0.98 cm3, 8.87 mmol) was added drop-
wise to a cooled (Ϫ20 ЊC) and stirred solution of LiCHR2 (0.74
g, 4.46 mmol) in diethyl ether (ca. 25 cm3). The mixture was
warmed to room temperature and stirred for 14 h. The solvent
was removed in vacuo; the residual solid was dried at 75 ЊC/10Ϫ2
Torr for 3 h and dissolved in hexane (ca. 15 cm3), filtering to
remove an insoluble impurity. The filtrate was concentrated and
set aside for a few days at room temperature to give white
crystals of compound 1 (0.98 g, 89%), mp (decomp.) 145–
148 ЊC (Found: C, 58.0; H, 11.61; N, 5.74. C12H28LiNSi2
Reaction of LiCHR2 with PhCN. A solution of PhCN (0.30 g,
2.88 mmol) in Et2O (10 cm3) was added slowly to a solution of
LiCHR2 (0.72 g, 4.33 mmol) in Et2O (30 cm3) at Ϫ80 ЊC. The
resulting yellow solution was allowed to warm to room tem-
perature and stirred for 90 min. The solvent was removed in
vacuo, the residue “stripped” with pentane (this procedure
refers to adding the solvent and then removing it in vacuo) and
then dissolved in pentane (45 cm3). Filtration from a yellow
precipitate (0.2 g) and cooling the filtrate gave yellow crystals of
[Li{N(R)C(Ph)C(H)C(Ph)NR}]2 4a (0.24 g, 44.7% based on
PhCN). Concentration of the mother liquor yielded orange red
crystals of compound 5a (0.20 g, 23%), while further concen-
1
requires C, 55.6; H, 11.91; N, 5.90%); H NMR (C6D5CD3) δ
0.16, 0.24 (s, SiMe3), 1.11 (s, CMe3) and 4.47 (s, CH); 13C NMR
(C6D5CD3) δ 0.8, 4.8 (s, SiMe3), 31.3 (s, CMe3), 41.6 (s, CMe3),
95.6 (s, CH) and 185.5 (s, CN).
1
tration gave colourless crystals which, according to H NMR
spectroscopic data, correspond to a mixture of LiCHR2 and
Li{N(R)C(Ph)C(H)R}.
[Li{N(R)C(But)C(H)R}(tmen)] 2a and [Li{N(R)C(But)C-
(H)R}(dme)] 2b. Compounds 2a and 2b were obtained from
LiCHR2 and ButCN as described for 1, except that reactions
were carried out in C5H12 with 1 equivalent of tmen or dme.
Compound 1 (0.53 g, 2.12 mmol) and tmen (0.32 cm3, 2.12
mmol) gave 2a (0.71 g, 91.6%): 1H NMR (C6D6) δ 0.33 (s,
SiMe3), 0.47 (s, SiMe3), 1.34 (s, But), 1.58 (m, NCH2), 1.85 (s,
[Li{N(R)C(Ph)C(H)C(Ph)NR}(LiCHR2)(OEt2)] 5a. Pentane
(10 cm3) and Et2O (0.1 cm3) were added to a mixture of solid
[Li{N(R)C(Ph)C(H)C(Ph)NR}]2 4a (0.50 g, 0.67 mmol) and
LiCHR2 (0.22 g, 1.34 mmol). The resulting orange-red solution
was concentrated. Cooling to Ϫ25 ЊC gave red crystals of com-
pound 5a (0.47 g, 57%). From the mother liquor another 0.19 g
(21.9%) was obtained, mp (decomp.) 129 ЊC (Found: C, 62.9;
H, 9.48; N, 4.72. C29H65Li2N3OSi4 requires C, 62.7; H, 9.54; N,
7
NMe) and 4.02 (s, CH); Li NMR(C6D6) δ 0.34; 13C NMR
(C6D6) δ 1.8 (s, SiMe3), 6.1 (s, SiMe3), 31.2 (s, NMe), 40.6 (s,
CMe3), 46.1 (s, CMe3), 56.4 (s, NCH2), 83.4 (s, CH) and 185.6
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7
4.57%); 1H NMR (C6D6) δ Ϫ1.94 (s, CHSi2), 0.05 (s, NSiMe3),
0.36 (s, SiMe3), 0.92 [t, CH3 (Et2O), 3J(1H–1H) 7.1], 3.20 [q, CH2
(Et2O), 3J(1H–1H) 7.1 Hz], 5.51 (s, CH), 7.03–7.14 (Ph, 6 H) and
7.44 (d, Ph, 4 H); 7Li NMR (C6D6) δ 2.3; 13C NMR (C6D6) δ 1.8
(s, CHSi2), 3.0 (s, NSiMe3), 5.7 (s, SiMe3), 14.3 [s, CH3 (Et2O)],
65.9 [s, CH2 (Et2O)], 105.4 (s, CH), 127.4, 128.0, 128.8 (s, Ph),
148.0 (s, ipso-C) and 177.2 (s, CN).
(Ph, 3 H) and 7.40 [d, J(1H–1H) 7.8 Hz, o-H of Ph, 2 H]. Li
NMR(C6D6) δ 0.76. 13C NMR (C6D6) δ 2.1 [s, SiMe3], 3.9
(s, NSiMe3), 25.4 (s, CH2), 68.8 (s, OCH2), 94.0 (s, CH), 127.0
(s, p-Ph), 127.6 and 128.9 (s, o- and m-Ph), 148.4 (s, ipso-C) and
174.7 ppm (s, CN).
[Li{N(R)C(Ph)NC(Ph)᎐C(H)R}(tmen)] 7a. A mixture of
᎐
PhCN (0.39 cm3, 3.85 mmol) and tmen (0.58 cm3, 3.85 mmol)
in Et2O (10 cm3) was added dropwise to a solution of LiCHR2
(0.64 g, 3.85 mmol) in Et2O at Ϫ60 ЊC. The clear solution was
warmed to room temperature and stirred for 15 h. The solvent
was removed in vacuo; Et2O (20 cm3) and PhCN (0.39 cm3, 3.85
mmol) were added and the solution stirred for 15 h. Removing
the solvent and recrystallisation of the residue yielded pale
yellow crystals of compound 7a (1.42 g, 86%), mp (decomp.)
110 ЊC (Found: C, 65.3; H, 9.08; N, 11.38. C27H45LiN4Si2
requires C, 66.3, H, 9.28, N, 11.46%); 1H NMR (C6D6) δ Ϫ0.10
(s, SiMe3), 0.13 (s, SiMe3), 1.63 (s, NCH2), 1.85 (s, NCH3), 4.45
(s, CH), 7.05–7.19 (Ph, 6 H), 7.43 [dd, o-H of Ph, J(1H–1H) 8.1,
[Li{N(R)C(Ph)C(H)C(Ph)NR}(LiCHR2)(thf)] 5b. Com-
pound 5a (0.3 g, 0.49 mmol) was dissolved in a mixture of
pentane (8 cm3) and thf (0.1 cm3). Volatiles were completely
removed in vacuo; the residue was treated with pentane (20
cm3). Filtration and concentration of the red filtrate gave upon
1
cooling red crystals of compound 5b (0.1 g, 33%); H NMR
(C6D6) δ Ϫ1.90 (s, CHSi2), 0.05 (s, NSiMe3), 0.396 (s, SiMe3),
1.19 (m, thf), 3.40 (m, thf), 5.09 (s, CH), 7.09–7.12 (Ph, 6 H)
and 7.48 (d, Ph, 4 H); 7Li NMR (C6D6) δ 2.3; 13C NMR (C6D6)
δ 1.4 (s, CHSi2), 2.8 (s, NSiMe3), 5.9 (s, SiMe3), 25.1 [s, CH2
(thf)], 68.6 [s, OCH2 (thf)], 105.4 (s, CH), 127.5–128.3 (s, Ph),
148.3 (s, ipso-C) and 176.8 (s, CN).
7
2 H], 7.51 [dd, o-H of Ph, J(1H–1H) 8.0 Hz]; Li NMR(C6D6)
δ 1.89; 13C NMR (C6D6) δ 1.2 (s, SiMe3), 3.6 (s, SiMe3), 45.3
(s, NCH3), 56.2 (s, NCH2), 108.2 (s, CH), 126.1–128.4 (s, Ph),
[Li{N(R)C(Ph)᎐C(H)R}(tmen)] 6a, [Li{N(R)C(Ph)᎐C(H)R}
᎐
᎐
(pmdien)] 6b and [Li{N(R)C(Ph)᎐C(H)R}(thf)] 6c. The
᎐
2
144.0, 147.2 (s, ipso-C), 164.2 (s, C᎐CH) and 175.0 (s, CN ).
᎐
2
procedure for the preparation of each of compounds 6a–6c
was very similar and is therefore reported in detail only for 6b.
A solution of PhCN (0.31 cm3, 3.07 mmol) and pmdien (0.64
cm3, 3.07 cm3) in Et2O (10 cm3) was added dropwise to a solu-
tion of LiCHR2 (0.51 g, 3.07 mmol) in Et2O (20 cm3). The
mixture was allowed to warm to room temperature and
stirred for 15 h. All volatiles were removed in vacuo and the
residue was recrystallised from Et2O to give colourless crystals
of 6b (1.23 g, 90%), mp (decomp.) 132 ЊC (Found: C, 61.6; H,
10.74; N, 12.47. C23H47LiN4Si2 requires C, 62.4; H, 10.74; N,
12.65%); mass spectrum m/z (%) 538 (75, [LiL2]ϩ), 523 (20,
[LiL2 Ϫ Me]ϩ), 465 (10 [LiL2 Ϫ SiMe3]ϩ), 276 (100, [Li2L]ϩ),
[Li{N(R)C(Ph)NC(Ph)᎐C(H)R}(thf)] 7b. Benzonitrile(2cm3,
᎐
2
19.60 mmol) was added to a boiling solution of LiCHR2 (1.37
g, 8.24 mmol) in thf (30 cm3) under reflux. The orange solution
was stirred for 5 min and the volatiles were removed in vacuo.
The residual yellow solid was dissolved in hexane (20 cm3);
cooling at –30 ЊC afforded pale yellow crystals of compound 7b
(1.14 g, 31%), mp 145 ЊC (Found: C, 66.8; H, 8.22; N, 6.29.
C25H37LiN2OSi2 requires C, 67.5; H, 8.29; N, 6.30%); mass
spectrum m/z (%) 366 (17, [½M Ϫ Li Ϫ thf]ϩ), 351 (4,
[½M Ϫ Li Ϫ thf Ϫ Me]ϩ), 293 (19, [½M Ϫ Li Ϫ thf Ϫ SiMe3]ϩ)
and 263 (82 [½M Ϫ Li Ϫ thf Ϫ NSiMe3 Ϫ Me]ϩ); 1H NMR
(C6D5CD3) δ Ϫ0.16 (s, SiMe3), 0.01 (s, NSiMe3), 1.14 (m, CH2,
4 H), 3.60 (m, OCH2, 4 H), 4.67 (s, CH), 6.92–7.17 (Ph, 8 H)
1
269 (56, [LiL]ϩ), 262 (65, [L]ϩ); H NMR (C6D6) δ 0.08 (s,
SiMe3), 0.16 (s, SiMe3), 1.74 (s, broad, NMe), 1.92 (s, NCH2),
2.14 (s, NMe2), 3.53 (s, CH), 7.15 (d, p-H of Ph), 7.24 [dt, m-H
of Ph, J(1H–1H) 6.8] and 7.57 [o-H of Ph, J(1H–1H) 6.8 Hz]; 7Li
NMR(C6D6) δ 0.79; 13C NMR (C6D6) δ 3.1 (s, SiMe3), 4.7 (s,
SiMe3), 45.6 (s, NMe), 45.9 (s, NMe2), 53.9 (s, NCH2), 57.1
(NCH2), 84.1 (s, CH), 126.0 (s, p-C), 127.3, 129.7 (s, o/m-C),
150.7 (s, ipso-C) and 176.3 (s, CN).
7
and 7.26 (d, Ph, J(1H–1H) 6.9 Hz, 2 H); Li NMR (C6D5CD3)
δ 1.62; 13C NMR (C6D5CD3) δ 1.1 (s, SiMe3), 3.2 (s, NSiMe3),
25.8 (s, CH2), 68.5 (s, OCH2), 113.6 (s, CH), 119.5, 127.3, 127.7,
127.9, 128.2 and 129.0 (s, aromatic carbons), 143.3 (s, ipso-C),
145.3 (s, ipso-C), and 164.0, 176.2 (s, CN).
Similarly, compound 6a (1.27 g, 85.6%) was obtained from
LiCHR2 (0.64 g, 3.85 mmol), PhCN (0.39 cm3, 3.85 mmol) and
tmen (0.58 cm3, 3.85 mmol). It was recrystallised from pentane,
mp (decomp.) 90 ЊC (Found: C, 60.9; H, 10.17; N, 11.48.
C20H40LiN3Si2 requires C, 62.3; H, 10.45; N, 10.89%); mass
spectrum m/z (%) 263 (40, [HL]ϩ), 248 (10, [HL Ϫ Me]ϩ), 186
(15, [HL Ϫ Ph]ϩ) and 176 (50, [Me SiC᎐NPh]ϩ); 1H NMR
(C6D6) δ 0.11 (s, SiMe3), 0.22 (s, SiMe3), 1.55 (s, NCH2), 1.79 (s,
NCH3), 3.67 (s, CH), 7.09–7.20 (Ph, 3 H) and 7.49 [dd, o-H of
Ph, J(1H–1H) 6.6 Hz]; 7Li NMR(C6D6) δ 0.88; 13C NMR (C6D6)
δ 2.6 (s, SiMe3), 3.5 (s, SiMe3), 45.5 (s, NCH3), 56.2 (s, NCH2),
83.1 (s, CH), 126.2–128.7 (s, o/m/p-C), 150.1 (s, ipso-C) and
175.9 (s, CN).
Likewise compound 6c (0.35 g, 31.5%) was obtained from
LiCHR2 (0.54 g, 3.25 mmol), PhCN (0.33 cm3, 3.25 mmol) and
thf (0.52 cm3, 3.25 mmol) and was recrystallised from pentane;
the second crop of crystals (0.44 g) consisted of a mixture of
Li{N(R)C(Ph)C(H)R} and 9b. Compound 6c was also pre-
pared by addition of benzonitrile (2.9 cm3, 28.42 mmol) to a
boiling solution of LiCHR2 (4.95 g, 29.79 mmol) in thf (70 cm3)
under reflux. The deep green mixture was stirred for 2 min and
thf was removed in vacuo. The pale green residue was extracted
into hexane (30 cm3). Cooling at Ϫ30 ЊC afforded pale yellow
crystals of compound 6c (7.95 g, 78%) (Found: C, 62.2; H, 9.35;
N, 3.96. C18H32LiNOSi2 requires C, 63.3; H, 9.44; N, 4.10%); 1H
NMR (C6D6) δ 0.04 (s, SiMe3, 9 H), 0.10 (s, NSiMe3, 9 H), 1.36
(m, CH2, 4 H), 3.69 (m, OCH2, 4 H), 4.37 (s, CH), 7.10–7.17
[Li{N(R)C(C H Me-4)NC(Ph)᎐C(H)R}(tmen)] 7c. Toluoni-
᎐
6
4
trile (0.31 cm3, 2.6 mmol) was added at Ϫ30 ЊC to a solution of
compound 6a (1 g, 2.6 mmol) in Et2O (30 cm3). The mixture
was allowed to warm to room temperature and stirred for 12 h.
The solvent was removed in vacuo, the residue extracted into
pentane and the extract filtered. Concentration of the filtrate
and cooling gave yellow crystals of compound 7c (0.79 g, 60%);
1H NMR (C6D6) δ Ϫ0.10, 0.15 (s, SiMe3), 1.65 (s, NCH2), 1.86
(s, NCH3), 2.14 (s, Me), 4.43 (s, CH), 6.98 [d, Ph, J(1H–1H)
7.80, 2 H], 7.14 (m, Ph, 3 H), 7.36 [d, Ph, J(1H–1H) 7.80] and
᎐
3
7.52 [d, Ph 3J(1H–1H) 6.68 Hz, 2 H]; 7Li NMR(C6D6) δ 1.90; 13
C
NMR (C6D6) δ 1.2, 3.7 (s, SiMe3), 21.2 (s, Me), 45.4 (s, NCH3),
56.3 (s, NCH2), 107.9 (s, CH), 119.5 (s, ipso-C), 126.6, 127.3,
128.1, 129.1, 129.7 (s, o-, m-, p-C), 135.8, 141.2, 147.4 (s, ipso-
C), 164.3 (s, C᎐CH) and 175.3 (s, CN ).
᎐
2
[Li{N(Ph)C(R)NC(Ph)᎐C(H)R}(tmen)] 8. A solution of
᎐
phenyl isocyanide (0.20 g, 2.0 mmol) in pentane (10 cm3) was
slowly added to a solution of compound 6a (0.77 g, 2.0 mmol)
in pentane (30 cm3) at Ϫ78 ЊC. After stirring for 3 h at this
temperature the reaction mixture was allowed to warm slowly
to room temperature and stirred for 8 h. All volatiles were
removed in vacuo and the residue was recrystallised from pen-
tane or methylcyclohexane to give compound 8 (0.51 g, 52%),
mp (decomp.) 98 ЊC; elemental analysis was unsatisfactory due
to incorporation of solvent; mass spectrum m/z (%) 366 (42,
J. Chem. Soc., Dalton Trans., 2000, 2301–2312
2309

View Article Online
[HL]ϩ), 351 (8, [HL Ϫ Me]ϩ), 293 (70, [HL Ϫ SiMe3]ϩ), 191 (15,
[Li{N(R)C(Ph)᎐C(H)Ph}(tmen)] 11. PhCN (2.0 cm3, 20
᎐
[(Ph)NC(SiMe )NH]ϩ) and 176 (86, [Me SiC᎐NPh]ϩ); 1H NMR
mmol) was added by syringe to Li{CH(R)Ph}(tmen) (5.7 g,
᎐
3
3
(C6D6) δ 0.24 (s, SiMe3), 0.45 (s, SiMe3), 1.49 (s, NCH2), 1.68 (s,
NCH3), 4.79 (s, CH), 6.95 [t, p-H of Ph, J(1H–1H) 7.2, 1 H],
7.04 [d, o-H of Ph, J(1H–1H) 7.2], 7.17–7.29 (Ph, 5 H) and 7.72
[dd, o-H of Ph, J(1H–1H) 6.8 Hz, 2 H]; 7Li NMR(C6D6) δ 1.54;
13C NMR (C6D6) δ 1.9 (s, SiMe3), 4.2 (s, SiMe3), 45.0 (s, NCH3),
56.2 (s, NCH2), 96.7 (s, CH), 120.7, 123.2, 127.1, 127.3, 128.9,
19.93 mmol) in pentane (ca. 30 cm3) at ambient temperature.
The mixture was stirred for 6 h. Filtration gave the yellow solid
11 (7.0 g, 90%) (Found: C, 70.8; H, 9.17; N, 10.26. C23H36LiN3Si
requires C, 70.9; H, 9.31; N, 10.79%); 1H NMR (C6D6) δ 0.31 (s,
SiMe3), 1.41 (s, NCH2), 1.68 (s, NMe), 5.53 (s, CH), 6.79 [t, Ph,
1 H], 7.11 (m, Ph, 4 H), 7.23 (d, Ph, 2 H) and 8.12 (m, Ph, 2 H).
129.4 (s, o/m-C of Ph), 119.4, 146.9 (s, ipso-C), 154.1 (s, C᎐CH)
and 165.7 (s, CN2).
᎐
[Li(tmen)]2[{1,2-N(R)C(But)C(H)}2C6H4] 12. ButCN (1.39
cm3, 12.6 mmol) was added dropwise to a stirred solution of
[Li(tmen)]2[{1,2-CH(R)}2C6H4] (3.1 g, 6.28 mmol) in Et2O (ca.
40 cm3) at 25 ЊC. The resulting orange solution was stirred for
12 h. Volatiles were removed at 90 ЊC/10Ϫ2 Torr, and the residue
was extracted into hexane (ca. 50 cm3). The filtered extract
upon concentrating yielded yellow crystals of complex 12
(3.1 g, 75%); 1H NMR (C6D6) δ 0.54 (s, SiMe3), 1.53 (s, CMe3),
1.69 (s, broad, tmen), 5.36 (s, CH), 6.92 (m, Ph, 2 H) and 9.04
(m, Ph, 2 H).
[Li{N(R)C(Ph)C(H)C(Ph)N(R)}(tmen)] 9a. Addition of
tmen (2 cm3, 1.3 mmol) to a stirred suspension of compound 4a
(0.35 g, 0.47 mmol) in hexane (30 cm3) at ca. 25 ЊC immediately
gave a clear solution. After stirring for 6 h this was concentrated
to ca. 3 cm3. Cooling to Ϫ30 ЊC afforded the yellow crystalline
compound 9a (0.4 g, 87%) (Found: C, 65.8; H, 9.09; N, 10.7.
1
C21H29LiN2Si2 requires C, 66.3; H, 9.28; N, 11.5%); H NMR
(C6D6) δ Ϫ 0.12 (s, SiMe3), 1.90 (s, NCH2, 4 H), 2.01 (s, NMe, 12
H), 5.57 (s, CH), 7.10–7.33 (m, Ph, 6 H) and 7.62–7.67 (m, Ph, 4
H); 29Si NMR (C6D6) δ Ϫ6.7, 13C NMR (C6D6) δ 3.9 (s, SiMe3),
46.9 (s, NCH2), 57.8 (s, NMe), 105.4 (s, CH), 127.4–127.8 (s,
o-, m-, p-C of Ph), 150.0 (s, ipso-C) and 174.9 (s, CN).
[Li{N(R)C(But)C(H)CH(R)Bun}]2 13. Dibromomethane
(0.45 cm3, 6.88 mmol) was added dropwise to a stirred solution
of [Li{N(R)C(But)C(H)(R)}]2 1 (3.60 g, 6.88 mmol) in hexane
(ca. 30 cm3) at 25 ЊC. The resulting suspension was stirred for 12
h and filtered. n-Butyllithium (1.6 mmol dmϪ3 in hexane, 7 cm3)
was added to the filtrate at 25 ЊC; the solution was stirred for
4 h. Concentration yielded white crystals of compound 13
(0.60 g, 27%) (Found: C, 63.0; H, 11.78; N, 4.49. C17H38LiNSi2
[Li{N(R)C(Ph)C(H)C(Ph)NR}(thf)2] 9b. From compound 4a
(1.12 g, 3.01 mmol) in thf (20 cm3) at room temperature, using
procedures similar to those for 9a, then recrystallisation from
hexane, yellow-green crystals of compound 9b (0.46 g, 30%)
were obtained. When separated from the mother liquor these
crystals lost thf rapidly. 1H NMR (C6D5CD3): δ Ϫ0.02 (s,
NSiMe3), 1.37 (m, CH2, 8 H), 3.52 (m, OCH2, 8 H), 5.34 (s,
CH), 7.01–7.07 (Ph, 6 H) and 7.39 (Ph, 4 H). 7Li NMR
(C6D5CD3) δ 2.65. 13C NMR (C6D5CD3) δ 3.2 (s, NSiMe3), 25.7
(s, CH2), 68.4 (s, OCH2), 105.3 (s, CH), 127.5 (s, p-C of Ph),
128.3 and 129.2 (s, o- and m-Ph), 149.8 (s, ipso-C) and 175.7
(CN).
1
requires C, 64.1; H, 11.71; N, 4.40%); H NMR (C6D6) δ 0.15
(s, SiMe3), 0.33 (s, SiMe3), 1.27 (s, CMe3), 4.97 (d, CH), 0.92,
1.04 and 2.08 (Bun).
CH2[C{C(Ph)NR}C(Ph)N(H)R]2 14. 1,2-Dibromoethane
(0.34 g, 1.95 mmol) was added to
a
solution of
[Li{N(R)C(Ph)C(H)C(Ph)NR}]2 4a (1.32 g, 1.77 mmol) in
hexane (ca. 20 cm3). The mixture was refluxed for 10 h. The
white precipitate of lithium bromide was filtered off. Concen-
trating the filtrate by slow evaporation of the solvent in vacuo
gave yellow crystals of compound 14 (0.57 g, 43%), which were
washed with hexane (ca. 5 cm3) and dried in vacuo, mp 193–
198 ЊC (Found: C, 67.9; H, 8.44; N, 7.41. C43H60N4Si4 requires
[Li{N(R)C(Ph)C(H)C(Ph)N(R)}(thf)(NCPh)] 9c. Likewise,
from compound 4a (1.0 g, 1.37 mmol) and PhCN (0.28 g, 2.7
mmol) in thf (50 cm3) at 25 ЊC, there was obtained, after
1
recrystallisation from toluene, compound 9c (1.3 g, 90%). H
1
C, 69.3; H, 8.12; N, 7.52%); H NMR (C6D5CD3) δ 0.08 (s,
NMR (C6D5CD3) δ 0.01 (s, SiMe3), 1.34 (m, CH2), 3.51 (m,
OCH2), 5.37 (s, CH), 6.60–7.15 (m, Ph, 9 H) and 7.43–7.50
(m, Ph, 6 H). 29Si NMR (C6D5CD3) δ Ϫ5.7 and 0.5. 13C NMR
SiMe3), 2.65 (s, CH2), 6.67 (d, Ph, 4 H), 7.01 (m, Ph, 6 H) and
11.80 (s, NH); 13C NMR (C6D5CD3) δ 2.0 (s, SiMe3), 33.1 (s,
CH2), 110.5 (s, CH), 127.6, 128.4, 129.3 (s, o-, m-, p-C of Ph),
142.9 (s, ipso-C) and 170.1 (s, CN).
(C6D5CD3) δ 3.1 (s, SiMe3), 25.5 (s, CH2), 68.2 (s, OMe), 105.2
᎐
(s, CH), 127.1–132.6 (s, o-, m-, p-C of Ph and C᎐N), 149.8 (s,
᎐
ipso-C), 170.5, 175.2 (s, CN).
HN(R)C(Ph)C(H)C(Ph)NR 15a. BrCH2CH2Br (0.01 cm3,
0.82 mmol) was added by syringe to a solution of K{N(R)-
C(Ph)C(H)C(Ph)NR} [prepared from 4a and KOBut (0.41 g,
1.01 mmol) in hexane (ca. 20 cm3)]. The mixture was main-
tained at 50 ЊC for 5 h. After filtration, the filtrate was con-
centrated in vacuo to yield yellow needles of compound 15a
(0.36 g, 97%), mp 85–88 ЊC (Found: C, 68.9; H, 8.25; N, 7.82.
Li{N(R)C(Ph)C(H)C(Ph)N(R)}(NEt3) 9d.
A mixture of
[Li{N(R)C(Ph)C(H)C(Ph)N(R)}]2 4a (0.18 g, 0.24 mmol) and
LiCHR2 (0.08 g, 0.48 mmol) was dissolved in pentane (8 cm3)
and NEt3 (0.05 cm3). After cooling to Ϫ25 ЊC, yellow crystals
1
of 9d (0.1 g, 43.7%) were obtained. H NMR (C6D6) δ 0.08 (s,
SiMe3), 0.90 [t, CH3, J(1H–1H) 7.2], 2.52 [q, NCH2, J(1H–1H)
7.2 Hz], 5.60 (s, CH) and 7.10–7.13 (Ph, 6 H), 7.54 (d, o-H of
Ph). 13C NMR (C6D6): δ 3.3 (s, SiMe3), 10.4 (s, Me), 45.7 (s,
NCH2), 105.6 (s, CH), 127.6–128.7 (s, Ph), 149.1 (s, ipso-C) and
175.8 (s, CN).
1
C21H20N2Si2 requires C, 86.8; H, 8.25; N, 7.64%); H NMR
(C6D6) δ 0.04 (s, SiMe3), 5.39 (s, CH), 6.99 (m, Ph, 6 H), 7.21
(m, Ph, 4 H) and 12.38 (s, NH); 13C NMR (C6D6) δ 1.9 (s,
SiMe3), 102.8 (s, CH), 127.4, 128.9, 132.8 (s, o-, m-, p-C of Ph),
143.7 (s, ipso-C) and 170.7 (s, CN).
[Li{N(R)C(But)CH2}]2 10. ButCN (2.21 cm3, 20.0 mmol) was
added dropwise to a stirred diethyl ether solution of LiCH2R
(1.0 mol dmϪ3, 20 cm3) at ca. 25 ЊC. The resulting solution was
stirred for 12 h. Volatiles were removed during 1 h at 70 ЊC/10Ϫ2
Torr. The residue was washed with pentane to give the white
solid 10 (3.45 g, 97%) (Found: C, 58.6; H, 11.37; N, 7.33.
C9H20Li requires C, 61.0; H, 11.37; N, 7.90%); 1H NMR (C6D6)
δ 0.25 (s, SiMe3), 1.15 (s, CMe3), 3.80 (s, CH2, 1 H), 4.48 (s, CH2,
1 H) and 9.04 (m, Ph, 2 H).
HN(R)C(C6H4Me-4)C(H)C(C6H4Me-4)NR 15b. A suspen-
sion of [Li{N(R)C(C6H4Me-4)C(H)C(C6H4Me-4)NR}]2 4b
(0.33 g, 0.44 mmol) in hexane (ca. 10 cm3) was stirred under
aerobic conditions until nearly all the solid had dissolved,
changing from red to yellow. Slow evaporation of the solvent
afforded yellow crystals of compound 15b (0.31 g, 96%), mp
120–124 ЊC (Found: C, 70.1; H, 9.17; N, 7.22. C23H34N2Si2
requires C, 70.0; H, 8.78; N, 7.10%); 1H NMR (CDCl3) δ 0.06 (s,
SiMe3), 2.39 (s, Me), 5.28 (s, CH) 7.16, 7.28 (d, Ph, 2 H) and
2310
J. Chem. Soc., Dalton Trans., 2000, 2301–2312

View Article Online
12.08 (s, NH); 13C NMR (CDCl3) δ 1.8 (s, SiMe3), 21.3 (s, Me),
102.1 (s, CH), 127.2, 128.6 (s, m-, p-C of Ph), 138.1, 140.5 (s,
ipso-C) and 170.5 (s, CN).
Crystal data and refinement details
Data were collected on an Enraf-Nonius CAD4 diffractometer
using monochromatic Mo-Kα or Cu-Kα radiation [λ 0.71073 Å
or 1.5418 Å (13)]. Crystals were either sealed in a Lindemann
capillary under argon or else directly mounted on the diffract-
ometer under a stream of cold nitrogen gas. Cell dimensions
were calculated from the setting angles for 25 reflections with
7 < θ < 10Њ. Intensities were measured by an ω–2θ scan. Correc-
tions were made for Lorentz and polarisation effects and in the
case of compound 13 for absorption correction (DIFABS).33
The programs used for structure solutions and refinement were
SHELXS 8634 and SHELXL 9335 (or MOLEN36). There was
no crystal decay as measured by two standard reflections.
Further details are found in Table 6.
CCDC reference number 186/2001.
See http://www.rsc.org/suppdata/dt/b0/b002376k/ for crystal-
lographic files in .cif format.
Acknowledgements
We thank the Sir Run Run Shaw Foundation (studentship for
D.-S. L.), SPECS-BIOSPECS (fellowship for D.-S. L.),
E.P.S.R.C. (fellowship for M. L.) and the European Com-
mission (Marie Curie fellowship for R. S.) for support.
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