NMR spectroscopic study of the adduct formation and reactivity of homoleptic rare earth amides with alkali metal benzyl compounds, and the crystal structures of [Li(TMEDA)2][Nd{N(SiMe3)2}3(CH2Ph)] and [{Li(TMP)}2{Li(Ph)}]2

Article abstract of DOI:10.1016/j.jorganchem.2017.10.022

An NMR spectroscopic study has been conducted into the reactivity of alkali metal benzyls [M(CH₂Ph)], (M = Li, Na, K) with lanthanide tris(amide) complexes [Ln(N″)₃] (Ln = Y, Ce, Nd; N″ = N(SiMe₃)₂) and [Ce(TMP)₃] (TMP = 2,2,6,6-tetramethylpiperidide). It was found that for [Ln(N″)₃], benzyl adducts [M][Ln(N″)₃(CH₂Ph)] were initially formed, and the molecular structure for M = Li(TMEDA)₂ and Ln = Nd was determined revealing a distorted tetrahedral [Nd(N″)₃(CH₂Ph)] anion. In all cases, these adduct complexes were unstable, intramolecularly deprotonating a methyl arm of a N″ ligand via benzyl basicity and eliminating toluene to prepare cyclometallated complexes of the form [M][Ln(N″)₂{κ²-CH₂Si(Me)₂N(SiMe₃)}]. In parallel studies, reactions of [Li(Ph)] with [Ln(N″)₃] (Ln = Ce, Nd) afforded [Li(N″)], whilst for (Ln = Y) adduct formation was observed. [Ce(TMP)₃] did not generate any characterisable bimetallic adducts. The reaction of [Li(Ph)] with [Li(TMP)] afforded the hexanuclear [{Li(TMP)}₂{Li(μ-Ph)}]₂, which features lithium in three different coordination environments.

Full text of DOI:10.1016/j.jorganchem.2017.10.022

Journal of Organometallic Chemistry xxx (2017) 1e9
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
NMR spectroscopic study of the adduct formation and reactivity of
homoleptic rare earth amides with alkali metal benzyl compounds,
and the crystal structures of [Li(TMEDA)2][Nd{N(SiMe3)2}3(CH2Ph)]
and [{Li(TMP)}2{Li(Ph)}]2
,
, *
Simon G. Rachor a, Peter A. Cleaves a, Stuart D. Robertson b **, Stephen M. Mansell a
a Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK
b WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
An NMR spectroscopic study has been conducted into the reactivity of alkali metal benzyls [M(CH2Ph)],
(M ¼ Li, Na, K) with lanthanide tris(amide) complexes [Ln(N00)3] (Ln ¼ Y, Ce, Nd; N00 ¼ N(SiMe3)2) and
[Ce(TMP)3] (TMP ¼ 2,2,6,6-tetramethylpiperidide). It was found that for [Ln(N00)3], benzyl adducts [M]
[Ln(N00)3(CH2Ph)] were initially formed, and the molecular structure for M ¼ Li(TMEDA)2 and Ln ¼ Nd
was determined revealing a distorted tetrahedral [Nd(N00)3(CH2Ph)] anion. In all cases, these adduct
complexes were unstable, intramolecularly deprotonating a methyl arm of a N00 ligand via benzyl basicity
Received 14 August 2017
Received in revised form
16 October 2017
Accepted 17 October 2017
Available online xxx
and eliminating toluene to prepare cyclometallated complexes of the form [M][Ln(N00)2{ 2-CH2Si(-
k
Keywords:
Me)2N(SiMe3)}]. In parallel studies, reactions of [Li(Ph)] with [Ln(N00)3] (Ln ¼ Ce, Nd) afforded [Li(N00)],
whilst for (Ln ¼ Y) adduct formation was observed. [Ce(TMP)3] did not generate any characterisable
f-block organometallics
Metal amide
Bimetallic
bimetallic adducts. The reaction of [Li(Ph)] with [Li(TMP)] afforded the hexanuclear [{Li(TMP)}2{Li(
Ph)}]2, which features lithium in three different coordination environments.
m-
Benzyl
Alkali-metal
Cyclometallate
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
{N(SiMe3)2}] and [M(TMP)]; M ¼ Li, Na, K; TMP ¼ 2,2,6,6-
tetramethylpiperidide
e are ubiquitous throughout synthetic
Research into mixed-metal organometallics has demonstrated
that certain bi- and multi-metallic compounds have patterns of
reactivity that can be both wider in scope and more selective than
the individual components on their own [1]. This has led to the
systematic development of mixed-metal compounds with synergic
reactivity [2] that have many potential applications including se-
lective metalation reactions [3] for more efficient organic synthesis
[4], as well as for use in catalysis (e.g. mixed f-block/Mg polymer-
isation catalysts) [5,6]. One particularly well known example of
enhanced reactivity of a bimetallic system is the widely used
Lochmann-Schlosser base, which is a mixture of n-butyllithium and
potassium tert-butoxide [3].
chemistry [7], and their synergic behaviour with, for example, Mg,
Mn and Zn organometallics leads to unusual reactivity that can be
understood through detailed knowledge of the structure of these
reagents (e.g. I, Fig. 1) [1,3]. Rare earth amides and organometallics,
on the other hand, are not widely used throughout chemistry with
their use mainly restricted to synthetic f-block chemistry [8].
Simple homoleptic starting materials, such as [Ln(N00)3] [9] and [Ln
{CH(SiMe3)2}3] [10e12], and the THF solvated [Ln(CH2Ph)3(THF)n]
(n ¼ 2 [13,14] or 3 [13e16]) and [Ln(CH2SiMe3)3(THF)n] (n ¼ 2
[17,18] or 3 [17,19]), are well known, although they are very air- and
moisture-sensitive and can be non-trivial to synthesise. The rare
earth elements, with their large ionic radii, high coordination
numbers and interactions dominated by ionic bonding, also have a
well-developed ‘ate’ chemistry producing charge separated
lanthanide complexes when additional anionic ligands bind to the
Ln metal centres [20]. Examples of charge-separated organome-
tallic lanthanide ‘ate’ compounds include [Li(L)n][LnR4] {R ¼ tBu
[21e23], CH2SiMe3 [22,24,25]; (L)n ¼ (THF)4, (TMEDA)2, (DME)3;
DME ¼ MeOC2H4OMe}, the heteroleptic [Li(DME)3][Nd(Cp)3(Ph)]
Alkali metal secondary amides e in particular [M{N(iPr)2}], [M
* Corresponding author.
** Corresponding author.
E-mail addresses: stuart.d.robertson@strath.ac.uk (S.D. Robertson), s.mansell@
hw.ac.uk (S.M. Mansell).
https://doi.org/10.1016/j.jorganchem.2017.10.022
0022-328X/© 2017 Elsevier B.V. All rights reserved.
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S.G. Rachor et al. / Journal of Organometallic Chemistry xxx (2017) 1e9
stable for several days, long enough to crystallise a heteroleptic Nd
‘ate’ species (type III, Fig. 1).
2. Results and discussion
2.1. Homometallic species
Initially, we investigated the reaction of a lithium organome-
tallic with a lithium amide in order to study adduct formation using
the same very electropositive metal (Pauling electronegativity, cP
,
of Li ¼ 0.98) [38]. This would allow us to investigate ligand
compatibility and bonding modes that could then be applicable to
heterobimetallic systems including rare earth elements.
Fig. 1. Bimetallic mixed-metal organometallic compounds.
The 1:1 reaction of [Li(Ph)] with [Li(TMP)] in benzene at 80 ꢀC
for 2 h afforded a brown suspension, wherein only some of the
lithium phenyl was observed to dissolve. Upon cooling, pale yellow
crystals suitable for X-ray diffraction studies were grown, which
revealed this complex to be the hexanuclear species [{Li(TM-
[26] and [YbII(CH2Ph)(THF)5][YbIII(CH2Ph)4(THF)2] which formed in
preference to neutral [YbIII(CH2Ph)3(THF)3] [13]. Contacted ion-
pairs are also known (Fig. 2, top), for example, with Me: [Ln{(
Me)2Li(TMEDA)}3] [27,28], N(iPr)2: [Li( -NiPr2)2Y(NiPr2)2] [29], N00:
[Li( -CH2Ph)5],
-N00)2LnIIN00] [30] and CH2Ph: [K(THF)K(THF)2Sc(
m-
m
P)}2{Li(m-Ph)}]2, (1). Repeating the reaction with a 1:2 ratio of
m
m
[Li(Ph)]:[Li(TMP)] (Scheme 1) resulted in a clear solution from
which the same product crystallised upon cooling in improved
yield (37%). Both triclinic (P1) and monoclinic (P21/n) polymorphs
have been characterised, with virtually identical molecular
structures.
which contains a complicated extended structure in the solid state
[15].
We were interested in lanthanide motifs with two different
bridging atoms similar to I, because only a handful of structurally
characterised examples are known (Fig. 2, bottom) that show
lanthanide complexes with mixed alkyl/amide bridging in-
teractions (type II, Fig. 1) [31e33]. These include cyclometallated
complexes that result from N00 ligand deprotonation [34e36], such
Compound 1 is a hexanuclear centrosymmetric complex (see
Fig. 3), containing six lithium atoms in three different coordination
geometries. The first environment is the h1 1-Li(1), which is bound
:h
to the amide nitrogen atoms of two TMP molecules. Although this is
formally only two-coordinate at lithium, the flanking methyl
groups of the TMP ligand must provide some degree of steric
protection, as is true of the tetrameric lithium reagent [{Li(TMP)}4],
as [K][Y(N00)2{ 2-CH2Si(Me)2N(SiMe3)}] [30] that becomes a solvent
k
separated ion pair when a co-ligand such as 18-crown-6 is used
[37]. These cyclometallated structures demonstrate one potential
drawback from using the ubiquitous N00 anion as a supporting
ligand because group 1 organometallics are strong bases; [Na(N00)],
[K{Si(SiMe3)3}] and nBuLi/TMEDA react with [Ln(N00)3] (Ln ¼ Y, Yb)
to deprotonate the N00 ligand forming a dianionic ( 2-N,C) ligand
k
[30,34] instead of generating a stable bimetallic adduct. In contrast,
the reaction of [Yb(N00)3] with tBuLi caused reduction to Yb(II) and
formation of [LiYb(N00)3] [30].
Synergic reactivity based upon bimetallic complexes of group 1
and the rare earths has not been systematically investigated, so this
paper details our first investigations in this field targeting bime-
tallic complex formation using weaker bases than butyl lithium
(type II). We show that adducts are initially formed, which can be
TMP
Scheme 1. Preparation of [{Li(TMP)}2{Li(m-Ph)}]2 (1).
Fig. 2. Organometallic lanthanide contacted ion pairs.
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S.G. Rachor et al. / Journal of Organometallic Chemistry xxx (2017) 1e9
3
Fig. 3. Solid state structure of [{Li(TMP)}2{Li(m
-Ph)}]2, (1). Thermal ellipsoids set at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (ꢀ)
for the triclinic polymorph: Li(1)-N(1) 1.976(2), Li(1)-N(2) 1.979(2), Li(2)-N(1) 2.008(2), Li(2)-C(19) 2.217(2), Li(2)-C(19A) 2.346(2), Li(3)-N(2) 1.948(3), Li(3)-Phplane 1.944, Li(3)-CPh
range 2.361(3)e2.430(3); Li(1)-N(2)-Li(3) 90.07(11), N(2)-Li(1)-N(1) 165.18(14), Li(1)-N(1)-Li(2) 99.65(10), N(1)-Li(2)-C(19) 124.50(11), N(1)-Li(2)-C(19A) 125.65(10), C(19)-Li(2)-
C(19A) 109.85(9). Values for the monoclinic polymorph, all of which fall within the standard deviations of those listed above, can be found in the ESI.
which features these near linear N-Li-N motifs [39,40]. Additional
electrostatic interactions with the TMP methyl groups will be
tempered by their distance from the lithium centre [range of Li(1)-
CTMP-Me distances: 2.941e3.443 Å; cf Li(TMP) tetramer range Li-
CTMP-Me 2.824e3.357 Å] [40]. The second environment is the
they presented the first complex containing both lithium-p-system
contacts and lithium-carbanion interactions, so we find it pertinent
to highlight the three differing lithium environments present in
complex 1.
Unfortunately, isolated crystals of 1 do not redissolve in non-
polar solvents, however, samples of 1 prepared in situ in C6D6
were successfully analysed by 1H and 7Li NMR spectroscopy. Three
environments could be observed in the 7Li NMR spectrum (see
Figure S4, ESI): the sharp signal at 2.53 ppm we attribute to Li(1), as
this is a very similar chemical shift to that of [{Li(TMP)}4], wherein
the lithium resides in a similar environment; a sharp signal at
1.27 ppm and a broad resonance at 0.23 ppm. The 1H NMR spec-
trum features well-resolved signals in the aromatic region, and two
broad resonances at lower chemical shift attributed to the H atoms
of the TMP ligands.
h1 h1 1-Li(2), which is bound to one of the TMP amide nitrogen
: :h
atoms and bridges between two ipso-carbon atoms of two phenyl
rings. These Li-C interactions form a flat Li-C-Li-C core, at the
midpoint of which is situated the inversion centre. This Li2C2
dimeric core is a signature feature of phenyl lithium structural
chemistry, featuring as the repeating unit in unsolvated [Li(Ph)]
[41] and as the core when solvated by (ꢁ)-sparteine [42], N,N0,N00-
trimethyl-1,3,5-triazacyclohexane [43], tetrahydropyran [44] and
(R,R)-N,N,N0,N0-tetramethyl-1,2-diaminocyclohexane [45]. The sum
of the bond angles at Li(2) is 360ꢀ, indicating a trigonal planar co-
ordination geometry for these lithium ions. The third coordination
geometry is the h1
:h
6-Li(3) centre which bridges a TMP amide and
2.2. Heterometallic species
the -system of the phenyl ring [46e49]. The phenyl ring is tilted
p
with respect to the two lithium atoms coordinated to the ipso-
carbon [angle Li(2)-CPh plane ¼ 11.35ꢀ, Li(2A)-CPh plane ¼ 31.80ꢀ].
Encouraged by the preparation of the homometallic hex-
anuclear lithium species 1, we then introduced rare earth amides in
an attempt to prepare heterobimetallic complexes using very
electropositive metals. In this initial study, we chose to investigate
the reactivity of [M(CH2Ph)] and [Li(Ph)] with [Ln(N00)3], (M ¼ Li, Na,
K; Ln ¼ Y, Ce, Nd; cP for Y ¼ 1.22, Ce ¼ 1.12, Nd ¼ 1.14) [38] utilising
the ubiquitous bis(trimethylsilyl)amide ligand, as well as
[Ce(TMP)3]. We focussed on these rare earth elements due to the
reasonably interpretable NMR spectra for Ce3þ (4f1) and Nd3þ (4f3),
and in order to cover a variety of ionic radii, from Y as the smallest
to Ce as the largest [53].
This may be driven by the symmetrical h
6-coordination of Li(3) to
the phenyl
p-system by enforcing all the Li(3)-CPh distances to be
similar [2.361(3)e 2.430(3) Å]. The TMP-Li1-TMP-Li3 unit is remi-
niscent of the Li(TMP) unit seen in Williard's TMEDA hemi-solvated
‘open dimer’ [50], or the tail of the heteroalkali-metallic complex
[(PMDETA)K(
m-TMP)Li(
m
-TMP)Li(TMP)] (PMDETA ¼ N,N,N0,N00,N00-
pentamethyldiethylenetriamine, Me2NC2H4N(Me)C2H4NMe2) [51].
Consequently, complex 1 can be envisaged as either a “Li(TMP)
solvated” derivative of cyclic phenyl lithium or alternatively as
having been formed by the insertion of phenyl lithium into the
Li(TMP) tetramer. Aryl lithium insertion into the [Li(TMP)] tetramer
has been witnessed previously by Klett and co-workers who
inserted monomeric cyclopentadienyl lithium units into the same
lithium amide [52]. Having multiple coordination geometries pre-
sent for lithium within one complex has been highlighted recently
by Strohmann et al. as an interesting structural feature [46], where
2.2.1. [Ln{amide}3] þ [Li(Ph)]
Firstly, we investigated the reaction of [Ln(N00)3] (Ln ¼ Ce, Nd)
with [Li(Ph)] (Scheme 2). In both cases, the insoluble [Li(Ph)]
instantaneously dissolves, the reaction mixtures turn black, and
subsequently a substantial amount of solid material is precipitated.
1H NMR spectroscopy of the reaction mixtures in C6D6 only show
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S.G. Rachor et al. / Journal of Organometallic Chemistry xxx (2017) 1e9
proceeds similarly by the reaction of [Li{CH(SiMe3)2}] with
[Sn(N00)2], which eliminates [Li(N00)] [57].
2.2.2. [Ln{amide}3] þ [M(CH2Ph)]
TMP
The benzyl anion is less basic than the phenyl anion (pKa of
benzene ¼ 43, toluene ¼ 40) [58], and we selected the readily
synthesised [Na(CH2Ph)] and [K(CH2Ph)] for reaction studies. The
lithium analogue was an oil that required the addition of one
equivalent of donor ligand in order to isolate [Li(TMEDA)(CH2Ph)]
as a solid. We started our investigations of [Ln(N00)3] þ [M(CH2Ph)]
with the diamagnetic yttrium complex. For all metal benzyl species,
upon mixing with [Y(N00)3] in C6D6 the insoluble alkali metal re-
agents dissolve, and 1H NMR spectroscopy suggests the formation
of adduct species, of the form [M][Y(N00)3(CH2Ph)], wherein there is
an interaction between the benzyl anion and the yttrium ion
(Scheme 3, Fig. 4).
Scheme 2. Reactivity studies of lanthanide amides with phenyl lithium.
resonances attributable to residual [Ln(N00)3] and [Li(N00)], (see ESI,
Figures S5 and S6), which suggests than [Li(Ph)] is acting as a
nucleophile, displacing [N00]-, and has not formed a hetero-
bimetallic species.
The parallel reactivity of [Li(Ph)] with diamagnetic [Y(N00)3] in a
1:1 ratio in C6D6 instead afforded a colourless solution, and [Li(N00)]
was not observed by 1H NMR spectroscopy. Indeed, upon increasing
the ratio of [Li(Ph)] to [Y(N00)3], the solution became pale blue at 2:1,
and only in excess of this ratio did the solution darken in colour
with noticeable amounts of insoluble material. As this ratio was
increased to two equivalents of [Li(Ph)], the relative intensity of the
resonance for the yttrium starting material (0.30 ppm) slowly
decreased. The 1H NMR spectrum of this new species (see Figure S7,
ESI) is in agreement with this ratio as the sum of the three Ph
resonances (at 8.39, 6.93 and 6.59 ppm) integrate to 10, and the N00
ligands integrate to 54, split into two singlet resonances integrating
to 18 and 36 hydrogen atoms, suggesting an empirical formula of
[Li2Ph2Y(N00)3]. Using 1H NMR DOSY, all five resonances gave the
same diffusion coefficient supporting its formulation as an adduct
that diffuses slower than [Y(N00)3]. Unfortunately, we were unable
to isolate crystals of this 2:1 reaction product to conclusively
identify its structure, and therefore are unable to comment on how
the Li cations are associated with the Y ‘ate’ complex, despite the
apparent stability of this compound in solution (>2 weeks).
In light of the deleterious formation of [Li(N00)] with the larger
rare earth metals (6 coordinate M3þ ionic radii: Ce ¼ 101, Nd ¼ 98,
Y ¼ 90 pm) [54], we postulated that a more sterically encumbered
amide could stabilise any possible adduct species that might be
formed. As [Ce(TMP)3] was readily prepared [55,56], we utilised
this complex for reactivity studies. However, as was the case for the
reaction of [Ce(N00)3] with [Li(Ph)], the reaction of [Ce(TMP)3] with
[Li(Ph)] afforded [Li(TMP)] as the only soluble product. In this case,
the increased steric encumbrance slowed the rate of reaction so
that the reaction was only complete after 24 h (cf [Ce(N00)3] where
the reaction was instantaneous). At intermediate times, resonances
due to intermediates were observed, but these could not be char-
acterised further. At the end of the reaction, whilst we observed
resonances corresponding to [Li(TMP)], there is also a resonance for
residual [Ce(TMP)3] that suggests that the reaction does not pro-
ceed in exact 1:1 stoichiometry. Ligand exchange of this type is not
unknown. For instance, the preparation of [Sn{CH(SiMe3)2}2]
For the adduct generated with [K(CH2Ph)], the 1H NMR reso-
nance attributed to the N00 ligands has shifted from 0.30 to
0.64 ppm, and there are well resolved resonances in the aromatic
region that correspond to a benzyl fragment (Figure S13, ESI).
Perhaps most indicative of the formation of a Y-C bond is the
presence of a doublet for the CH2 resonance at 2.22 ppm
(2JYH ¼ 2.6 Hz), which is attributed to the interaction of the 100%
I ¼ ½ 89Y isotope with the CH2 of the benzyl moiety. This two-bond
coupling is readily observed, as in [Y(BIPM)(CH2Ph)(THF)]
(BIPM ¼ {C(PPh2NSiMe3)2}2-
,
2JYH ¼ 1.6 Hz) [59] or [K][Y(N00)2{k2
-
CH2Si(Me)2N(SiMe3)}] (2JYH ¼ 2.6 Hz) [30]. Similarly, for reactions
with [Na(CH2Ph)], adduct formation was instantaneous and the CH2
resonance was observed as a doublet (2JYH ¼ 3.2 Hz). For Li(T-
MEDA), 10 min after mixing two benzyl adducts were observed
(Fig. 4), with resonances for species a (displaying no resolvable 2JYH
coupling) gone within 1 h at room temperature, whereas reso-
nances for species b (2JYH ¼ 3.4 Hz) reduce in intensity over 2
weeks.
For all of the reactions, the adducts formed are not stable and
convert to the cyclometallated product (Scheme 3), assigned by
comparison of its 1H NMR spectrum with that of the previously
reported solvent free species, [K][Y(N00)2{ 2-CH2Si(Me)2N(SiMe3)}]
k
[30]. The reaction is almost complete within 1 week for K (Fig. 5) or
2 weeks for Li(TMEDA), but three species were observed by 1H NMR
spectroscopy for Na even after 2 weeks. Close inspection of 1H NMR
spectra for M ¼ Na and K over intermediate times also revealed
resonances for a second benzyl adduct, labelled b (Fig. 5 for M ¼ K).
For K, a CH2 resonance was observed at 1.98 ppm (2JHY ¼ 3.7 Hz)
together with Ph resonances and a singlet at 0.50 ppm for N00. For
Na, the CH2 resonance for the second species was observed at
2.37 ppm (2JHY ¼ 4.0 Hz). The 1J(13C-1H) coupling constant for the
benzyl CH2 group is a sensitive probe of the hybridisation of the C
atom, with pyramidalised sp3 carbanions displaying a coupling
constant of approximately 125 Hz that increases to 167 Hz upon
becoming planar and sp2 hybridised [60]. Observation of the
1J(13C-1H) coupling constants in the benzyl adducts should
Scheme 3. Proposed reactivity for [Ln(N00)3] with [M(CH2Ph)].
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S.G. Rachor et al. / Journal of Organometallic Chemistry xxx (2017) 1e9
5
1
Fig. 4. Comparison of the benzylic region of the H NMR spectra for the reactions of [Y(N00)3] with [M(CH2Ph)] [M ¼ Li(TMEDA), Na, K] 10 min after mixing. For Li(TMEDA), two
species were observed labelled a and b. * ¼ toluene CH3.
1
Fig. 5. Comparison of the H NMR spectra for the reaction of [Y(N00)3] with [K(CH2Ph)] after various time, to show ultimate conversion to [K][Y(N00)2{
k
2-CH2Si(Me)2N(SiMe3)}].
therefore give more information about the hybridisation in these
compounds. For M ¼ K, 13C satellites for the CH2 resonances in both
benzyl species were observed and gave values of 118 Hz (a) and
121 Hz (b) indicating sp3 hybridisation. It is likely that neither of
these species is therefore a contact ion pair with a planar benzyl
anion bridging the two centres. For Li, species a was not sufficiently
stable in solution to record 1H NMR spectra with a sufficient signal
to noise ratio in order to observe the 13C satellites, but species b
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S.G. Rachor et al. / Journal of Organometallic Chemistry xxx (2017) 1e9
Scheme 4. Preparation of [Li(TMEDA)2][Nd(N00)3(CH2Ph)], (2).
displayed a 1J coupling consonant of 122 Hz, ruling out a planar
benzyl anion that would be conducive for a Y-C-Li interaction, and
instead suggesting a solvent separated ion pair with a pyr-
amidalised terminal CH2.
The NMR spectroscopic data suggests that over time the benzyl
moiety deprotonates the silyl amide ligand with the formation of a
new Y-C bond. The conversion of [M][Y(N00)3(CH2Ph)] to [M]
[Y(N00)2{
k
2-CH2Si(Me)2N(SiMe3)}] for M ¼ Na is much slower than for
K, whereas for [Li(TMEDA)(CH2Ph)], loss of the initial adduct species
is fast, but the conversion to metallacycle happens over two weeks.
We have been unable to definitively characterise the two different
benzyl adducts, and can only tentatively propose that they might
represent a contact ion pair (most likely to be species a for M ¼ Li)
and a solvent separated ion pair, as crystallographically identified for
Nd (see below). The relative stability of these two species is likely to
be greatly influenced by the nature of the alkali metal and the
presence of TMEDA as a solvent-separating co-ligand [61].
We then turned our attention to the analogous reactivity with
the f-block metals Ce and Nd. Again, the insoluble alkali metal
benzyl reagents dissolved instantaneously upon mixing with
[Ln(N00)3] (Ln ¼ Ce and Nd) and new resonances are observed.
Fortunately, single crystals suitable for X-ray diffraction studies
were grown from the reaction of [Li(TMEDA)(CH2Ph)] and
[Nd(N00)3] after storage of a toluene solution overnight at ꢁ25 ꢀC as
pale blue needles (Scheme 4), despite this adduct being the least
stable (see below). The anion of 2 is shown in Fig. 6 (see Figure S2,
ESI for both cation and anion).
Fig. 6. Solid-state structure of the anionic moiety of [Li(TMEDA)2][Nd(N00)3(CH2Ph)],
(2). Thermal ellipsoids set at 50% probability. Hydrogen atoms and lattice solvent
(toluene) are omitted for clarity. Selected bond lengths (Å) and angles (ꢀ): Nd(1)-N(1)
2.3545(18), Nd(1)-N(2) 2.3746(18), Nd(1)-N(3) 2.3756(17), Nd(1)-C(1) 2.601(2); Nd(1)-
C(1)-C(2) 131.5(2).
The anion features Nd in a distorted tetrahedral geometry
comprising one h1-benzyl and three N00 ligands. The neodymium
ion sits out of the plane of the three amides by 0.62 Å, and the sum
of the three N-Nd(1)-N bond angles is 340.25ꢀ. This indicates an
increase in the displacement of the Nd compared to the homoleptic
precursor, [Nd(N00)3], where the sum of bond angles is 353.4ꢀ, and
the Nd ion was found to be disordered 0.34 Å above and below the
plane of the three N donors [62]. Commensurate with the increased
coordination number, the Nd-N bond distances have increased (av.
of 2.37 Å in 2 vs 2.29(2) Å in [Nd(N00)3]) [62]. The Nd-C bond dis-
tance is slightly longer than in the homoleptic benzyl species
[Nd(CH2Ph)3(THF)3] [2.567(2) Å vs 2.601(2) Å in 2] [13], which
could reflect the increased steric bulk of the N00 groups. The carbon
atom of the CH2 unit of the benzyl moiety is also distorted away
from the tetrahedral ideal [Nd(1)-C(1)-C(2) 131.5(2)ꢀ]. The analo-
gous yttrium adduct, which has a smaller ionic radius [Nd3þ 112.3
vs Y3þ 104.0 pm] [54], underwent more facile deprotonation of the
silyl amide moiety, and this could be due to increased steric pres-
sure or closer proximity in space between the benzyl and N00 li-
gands. The lithium cation adopts a distorted tetrahedral geometry
as a consequence of the small bite angle of the tethered Namine
donors of the TMEDA ligands (internal N-Li-N bond angles av.
88.7ꢀ), but is otherwise unremarkable.
with Ce, Nd and the three alkali metals very difficult. For Ce, three
characteristic paramagnetically shifted Ph resonances were
observed directly after mixing the reagents (Figure S25, ESI),
however, the CH2 resonance could not be definitively identified. As
seen for the reactions involving [Y(N00)3], these adduct complexes
are not stable and undergo further reactivity with the Li(TMEDA)
complex being the least stable. Inspection of the 1H NMR spectra
after several hours revealed (in all cases) resonances for both
toluene and [M(N00)], as well as for a new product that is assigned to
be the analogous [M][Ce(N00 2-CH2Si(Me)2N(SiMe3)}] cyclo-
{k
2
metallated species, data for which could be generated by waiting
for the reactions to go to completion at room temperature or
through heating (Figure S26, ESI).
For [Nd(N00)3], its initial reactivity with [M(CH2Ph)] (M ¼ Na, K)
was found to follow the same pathway as for cerium and new
characteristic paramagnetically shifted resonances were observed
for the benzyl group, including a resonance at very low frequency
(at ca. ꢁ45 ppm) assigned to the benzyl CH2. However, several
broad resonances were observed for the N00 ligands which could not
be definitively assigned to the benzyl adduct because they could
not be differentiated from [Nd(N00)3] or other by-products. None of
the benzyl adducts were stable and they converted to the
Due to paramagnetic effects, the resonances observed by 1H
NMR spectroscopy were broad and cover a large chemical shift
range, making complete characterisation of the benzyl adducts
Please cite this article in press as: S.G. Rachor, et al., Journal of Organometallic Chemistry (2017), https://doi.org/10.1016/
j.jorganchem.2017.10.022

S.G. Rachor et al. / Journal of Organometallic Chemistry xxx (2017) 1e9
7
cyclometallated species [M][Nd(N00)2{
k
2-CH2Si(Me)2N(SiMe3)}]
collection and absorption corrections were performed and struc-
tures were solved using direct methods (SHELXT) [74] and refined
by full-matrix least-squares (SHELXL) [74] interfaced with the
programme OLEX2 [75] (Table S1). Diffraction experiments were
carried out at Heriot-Watt University (Bruker X8 APEXII at 100 K),
the University of Edinburgh (Oxford Diffraction SuperNova, Dual at
120 K) and the University of Strathclyde (Oxford Diffraction Gemini
at 153 K). CCDC deposition numbers: 1568980e1568983. Elemental
analyses were performed by Mr Stephen Boyer at London Metro-
politan University. Although elemental analysis was repeatedly
attempted, unsatisfactory results were obtained. This is an
acknowledged problem with some organometallic compounds,
often those containing silicon [76e78], which can lead to unsatis-
factory elemental analyses. In addition, the thermal instability of
these compounds added to these difficulties as well.
(Figure S28, ESI) with elimination of toluene over the course of
several days. [M(N00)] can also be observed by 1H NMR spectros-
copy, suggesting that again the mechanisms here are not
straightforward.
In light of the preparation of 2, we can then comment on the
stability of benzyl species in the presence of silyl amide ligands. As
has been observed in the NMR studies detailed above, it is evident
that benzyl adducts are initially formed, however, deprotonation of
a silyl amide ligand is relatively facile and proceeds at room tem-
perature. This follows literature precedent for the reaction of
[Y(N00)3] with nBuLi [30]. There is also a general lack of literature
examples for benzyl ligands in the presence of silyl amide ligands
indicating their incompatibility, with only examples of Sc, Ti, Al or
Ta benzyls in the presence of N00 [36,63e65], as well as Zr or Th
benzyls with tris(silylamido)amines [66,67], known.
Reactions of [M(CH2Ph)] [M
¼
Li(TMEDA), Na, K] with
4.1. Preparation of [{Li(TMP)2}{Li(Ph)}]2, (1)
[Ce(TMP)3] in C6D6 were attempted. However, large amounts of
solid material precipitated which we have been unable to charac-
terise. For M ¼ K and Na, the major soluble product appears to be
free tetramethylpiperidine, presumably arising from a reaction
with the solvent. A stable bimetallic product was therefore not
obtained.
[Li(Ph)] (21.0 mg, 0.25 mmol) and [Li(TMP)] (73.6 mg,
0.50 mmol) were added to a 5 mL ampoule and dissolved in ben-
zene (3 mL). The mixture was heated to 80 ꢀC for 2 h. Upon slow
cooling to room temperature, pale yellow crystals of 1 suitable for
X-ray diffraction studies were grown (35 mg, 0.05 mmol, 37%). In-
situ 1H NMR (400 MHz, 25 ꢀC, C6D6):
d 8.28 (4H, d, o-Ph,
2
2JHH ¼ 6.4 Hz), 7.08 (4H, t, m-Ph, JHH ¼ 7.2 Hz), 6.91 (2H, t, p-Ph,
3. Conclusion
2JHH ¼ 7.6 Hz), 1.70 (8H, br, TMP), 1.18 (64H, br, TMP); In-situ 7Li
The synthesis of bimetallic species based on very electropositive
metals made through the combination of metal amides and metal
phenyl or benzyl compounds was studied, and different results
were obtained for lithium as opposed to group 1/rare earth mix-
tures. Lithium, with its greater tendency for multi-centre bonding,
generated a hexametallic species with three different Li coordina-
tion environments. [Ln(N00)3]/alkali metal organometallics, with
very ionic bonding, demonstrated the initial formation of ‘ate’
complexes by 1H NMR spectroscopy, and [Li(TMEDA)2]
[Nd(N00)3(CH2Ph)] was structurally characterised. However, these
adducts were not stable with respect to ligand deprotonation and
NMR (155 MHz):
d 2.53 (s, N-Li-N), 1.27 (s), 0.23 ppm (br).
Elemental analysis of different crystalline samples was attempted
three times, but was consistently low on C.
4.2. NMR studies
Lanthanide tris(amides) [Y(N00)3], [Ce(N00)3], [Nd(N00)3] and
[Ce(TMP)3], were reacted on an NMR scale in a 1:1 ratio with the
alkali metal reagents [Li(Ph)], [Li(TMEDA)(CH2Ph)], [Na(CH2Ph)]
and [K(CH2Ph)], in approximately 0.5 mL of C6D6. Initial spectra
were taken, and then the samples were left to stand at room
temperature. Further spectra were taken after appropriate time
intervals (stated in the stack plots for these spectra in the ESI).
formation of the cyclometallated species [M][Ln(N00)2{ 2-CH2Si(-
k
Me)2N(SiMe3)}]. Despite the greater resistance of the TMP anion to
strong bases, no bimetallic species could be identified with
[Ce(TMP)3]. Future work will focus on the combination of lantha-
nide amides with the softer organometallics of groups 2 and 12.
4.2.1. [Ln(N00)3] þ [Li(Ph)] (Ln ¼ Ce, Nd)
1H NMR (400 MHz):
d
0.14 ppm (18H, s, Me, [Li(N00)]) and
d
ꢁ3.39 ppm (54H, s, Me, [Ce(N00)3]) or ꢁ6.24 ppm (54H, s, Me,
4. Experimental
[Nd(N00)3]).
All reactions were performed under an oxygen-free (H2O,
O2 < 0.5 ppm) nitrogen atmosphere using standard Schlenk line
techniques or by using an MBRAUN UNIlab Plus glovebox. Anhy-
drous toluene was obtained from an MBRAUN SPS-800 and 40e60
petroleum ether was distilled from sodium wire; benzene and
benzene-d6 were dried over molten potassium and distilled. All
anhydrous solvents were degassed before use and stored over
activated molecular sieves. TMP(H) was dried over activated 4 Å
molecular sieves, TMEDA was dried over CaH2 and distilled, and
LiN00 was sublimed prior to use. The following compounds were
prepared according to literature methods: [Li(Ph)] [68], [Li(TMP)]
[39], [Li(TMEDA)(CH2Ph)] [69], [Na(CH2Ph)] [60], [K(CH2Ph)] [70],
[Y(N00)3] [71], [Ce(N00)3] [9,72], [Nd(N00)3] [73], and [Ce(TMP)3]
[55,56]. NMR spectra were recorded on Bruker AVI400 or AVIII400
4.2.2. [Y(N00)3] þ [Li(Ph)]
[Li]2[Y(N00)3(Ph)2] 1H NMR (400 MHz):
d 8.39 (4H, dt, Ph,
2JHH ¼ 7.6, 1.6 Hz), 6.93 (4H, t, Ph, JHH ¼ 7.2 Hz), 6.59 (4H, t, Ph,
2
2JHH ¼ 7.2, 2 Hz), 0.51 (36H, s, 2 ꢂ N00), ꢁ0.20 ppm (18H, s, 1 ꢂ N00).
Residual [Y(N00)3] is observed in the 1:1 reaction:
d
0.30 ppm. 13C
NMR (100 MHz):
d
187.84 (d, Y-CPh
,
1JYC ¼ 49 Hz), 139.89 (s, Ph),
127.21 (s, Ph), 125.07 (s, Ph), 5.18 (s, 2 ꢂ N00), 5.10 ppm (s, 1 ꢂ N00),
4.01 ppm (s, [Y(N00)3]). 29Si NMR (80 MHz):
d
ꢁ8.64 (1 ꢂ N00), ꢁ11.52
(2 ꢂ N00), ꢁ11.37 ppm (s, [Y(N00)3], 3 ꢂ N00). 7Li NMR (155 MHz):
d
1.08 and ꢁ6.39 ppm. For 2D DOSY and associated diffusion rates,
see ESI, Figure S11 and Table S2.
4.2.3. [Y(N00)3] þ [K(CH2Ph)]
spectrometers and the chemical shifts
d
are noted in parts per
[K][Y(N00)3(CH2Ph)] 1H NMR (400 MHz):
d
6.85 (2H, d, Ph,
2
million (ppm) calibrated to the residual proton resonances of the
deuterated solvent. X-ray diffraction experiments were performed
on single crystals of the samples covered in inert oil and placed
under the cold stream of the diffractometer, and exposures were
2JHH ¼ 7.2 Hz), 6.56 (2H, t, Ph, JHH ¼ 6.8 Hz), 6.05 (1H, t, Ph,
2JHH ¼ 6.8 Hz), 2.22 (2H, d, Y-CH2, 2JYH ¼ 2.6 Hz), 0.64 ppm (54H, s,
N00); 13C NMR (100 MHz):
d 137.92 (s, Ph), 129.34 (s, Ph), 125.68 (s,
Ph), 123.38 (s, Ph), 24.06 (d, Y-CH2, 1JYC ¼ 23.6 Hz), 5.97 (s, N00). 29Si
collected using Mo K
a
radiation (
l
¼ 0.71073 Å). Indexing, data
NMR (80 MHz):
d
ꢁ10.84 ppm.
Please cite this article in press as: S.G. Rachor, et al., Journal of Organometallic Chemistry (2017), https://doi.org/10.1016/
j.jorganchem.2017.10.022

8
S.G. Rachor et al. / Journal of Organometallic Chemistry xxx (2017) 1e9
4.2.4. [Y(N00)3] þ [Na(CH2Ph)]
Acknowledgements
[Na][Y(N00)3(CH2Ph)] 1H NMR (400 MHz):
d 6.96 (2H, d, Ph,
2JHH ¼ 7.6 Hz), 6.73 (2H, t, Ph, JHH ¼ 8 Hz), 6.07 (1H, t, Ph,
2JHH ¼ 6.8 Hz), 1.91 (2H, d, Y-CH2, 2JYH ¼ 3.2 Hz), 0.30 ppm (54H, s,
N00). Additional resonances at 0.44 and 0.01 ppm are unidentified.
Selected 13C NMR resonances (100 MHz): 52.98 (d, Y-CH2,
1JYC ¼ 29.3 Hz).
The authors wish to acknowledge the support of the Leverhulme
Trust (RPG-2016-338 for supporting PAC), the Royal Society
(Research grant RG130436 to SMM), the Royal Society of Edinburgh
(BP Trust Fellowship to SDR) and the University of Strathclyde
(Chancellor's Fellowship to SDR). SGR is an Erasmus exchange
2
€
student from the Julius-Maximilians-Universitat Würzburg, Ger-
4.2.5. [Y(N00)3] þ [Li(TMEDA)(CH2Ph)]
many. Dr Gary Nichol (University of Edinburgh) is thanked for his
assistance with collecting data on 1 (triclinic polymorph).
[Li(TMEDA)][Y(N00)3(CH2Ph)] as a mixture of two benzyl spe-
cies. 1H NMR (400 MHz):
b) 6.72 (2H, t, Ph, JHH z 7 Hz, a), 6.46 (t, b), 6.16 (1H, t, Ph,
2JHH z 7 Hz, a), 2.37 (s, a) 2.16 (2H, d, Y-CH2, 2JYH ¼ 3.4 Hz, b), 1.46
(bs, TMEDA), 0.61 (bs), 0.54 (s, N00), 0.32 (bs, N00).
d
7.04 (2H, d, Ph, 2JHH ¼ 7.7 Hz, a), 6.99 (m,
2
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.jorganchem.2017.10.022.
4.2.6. [Ce(N00)3] þ [K(CH2Ph)]
[K][Ce(N00)3(CH2Ph)] 1H NMR (400 MHz):
d 11.98 (br, 2H, Ph),
References
6.83 (br, 2H, Ph), 4.45 (br, 1H, Ph), 0.17 (s, [K(N00)] impurity), ꢁ1
to ꢁ3 (overlapping br resonances, ca. 36H), e5.13 (br, ca. 18H). CH2
not identified and may be obscured by the broad resonances.
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[K][Ce(N00)2{ 2-CH2Si(Me)2N(SiMe3)}] 1H NMR (400 MHz):
k
10.03 (s, 6H, SiMe2), 0.17 (s, [K(N00)] impurity), e3.71 (36H, s, 2 ꢂ
N00), e3.77 (s, 9H, SiMe3), e14.39 (s, 2H, CH2). 29Si NMR (80 MHz):
d
1.19, ꢁ20.60, ꢁ25.04 ppm.
4.2.7. [Ce(N00)3] þ [Li(TMEDA)(CH2Ph)]
[Li(TMEDA)][Ce(N00)3(CH2Ph)] 1H NMR (400 MHz):
d 14.93 (2H,
br, Ph), 6.52 (2H, br, Ph), 3.92 (1H, br, Ph), 1.58e1.82 (br, 12 þ 4H,
TMEDA), 0.35 (s, [Li(N00)] impurity), e3.25 (br s, ca. 36H, N00),
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4.2.8. [Nd(N00)3] þ [K(CH2Ph)]
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d 8.63 (2H, br, Ph),
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[Na][Nd(N00)3(CH2Ph)] 1H NMR (400 MHz):
d 9.74 (2H, br, Ph),
6.42 (2H, br, Ph), 0.60 (54H, s, N00), ꢁ14.03 (1H, br, Ph), ꢁ44.8 ppm
(2 h, br, Nd-CH2);
[Na][Nd(N00)2{ 2-CH2Si(Me)2N(SiMe3)}]
k d 13.37 (6H, s, SiMe2),
ꢁ0.83 (9H, s, SiMe3), ꢁ4.75 (36H, s, 2xN00), ꢁ12.2 ppm (2H, s, Nd-
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d
ꢁ6.26 ppm (54H, s, 3 ꢂ N00).
4.2.10. [Nd(N)3] þ [Li(TMEDA)(CH2Ph)]
[Li(TMEDA)][Nd(N00)3(CH2Ph)] 1H NMR (400 MHz):
d 8.69 (2H,
br, Ph), 1.47e1.73 (br, TMEDA), e3.34 (54H, br), ꢁ34.1 ppm (2H, br,
Nd-CH2). Other resonances from the Ph group could not be defin-
itively
assigned.
Some
[Li(TMEDA)2][Nd(N00)2{ 2-CH2Si(-
k
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d
14.26 (6H, s, SiMe2), 1.46e1.74
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N00), ꢁ12.14 ppm (2H, br, Nd-CH2).
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tiple samples to be mixtures of Li(TMEDA)2][Nd(N00)3(CH2Ph)],
[Li(TMEDA)][Nd(N00)2{k2-CH2Si(Me)2N(SiMe3)}] and [Nd(N00)3] in
varying proportions.
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Please cite this article in press as: S.G. Rachor, et al., Journal of Organometallic Chemistry (2017), https://doi.org/10.1016/
j.jorganchem.2017.10.022