Structural Diversity in Alkali Metal and Alkali Metal Magnesiate Chemistry of the Bulky 2,6-Diisopropyl-N-(trimethylsilyl)anilino Ligand

Article abstract of DOI:10.1002/chem.201602683

Bulky amido ligands are precious in s-block chemistry, since they can implant complementary strong basic and weak nucleophilic properties within compounds. Recent work has shown the pivotal importance of the base structure with enhancement of basicity and extraordinary regioselectivities possible for cyclic alkali metal magnesiates containing mixed n-butyl/amido ligand sets. This work advances alkali metal and alkali metal magnesiate chemistry of the bulky arylsilyl amido ligand [N(SiMe₃)(Dipp)]^?(Dipp=2,6-iPr₂-C₆H₃). Infinite chain structures of the parent sodium and potassium amides are disclosed, adding to the few known crystallographically characterised unsolvated s-block metal amides. Solvation by N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) or N,N,N′,N′-tetramethylethylenediamine (TMEDA) gives molecular variants of the lithium and sodium amides; whereas for potassium, PMDETA gives a molecular structure, TMEDA affords a novel, hemi-solvated infinite chain. Crystal structures of the first magnesiate examples of this amide in [MMg{N(SiMe₃)(Dipp)}₂(μ-nBu)]_∞(M=Na or K) are also revealed, though these breakdown to their homometallic components in donor solvents as revealed through NMR and DOSY studies.

Full text of DOI:10.1002/chem.201602683

DOI: 10.1002/chem.201602683
Full Paper
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Alkali Metals |Hot Paper|
Structural Diversity in Alkali Metal and Alkali Metal Magnesiate
Chemistry of the Bulky 2,6-Diisopropyl-N-(trimethylsilyl)anilino
Ligand
[
a]
M. ꢀngeles Fuentes, Andoni Zabala, Alan R. Kennedy, and Robert E. Mulvey*
Abstract: Bulky amido ligands are precious in s-block
chemistry, since they can implant complementary strong
basic and weak nucleophilic properties within compounds.
Recent work has shown the pivotal importance of the base
structure with enhancement of basicity and extraordinary re-
gioselectivities possible for cyclic alkali metal magnesiates
containing mixed n-butyl/amido ligand sets. This work ad-
crystallographically characterised unsolvated s-block metal
amides. Solvation by N,N,N’,N’’,N’’-pentamethyldiethylenetria-
mine (PMDETA) or N,N,N’,N’-tetramethylethylenediamine
(TMEDA) gives molecular variants of the lithium and sodium
amides; whereas for potassium, PMDETA gives a molecular
structure, TMEDA affords a novel, hemi-solvated infinite
chain. Crystal structures of the first magnesiate examples of
vances alkali metal and alkali metal magnesiate chemistry of
this amide in [MMg{N(SiMe )(Dipp)} (m-nBu)] (M=Na or K)
3 2 1
ꢀ
the bulky arylsilyl amido ligand [N(SiMe )(Dipp)] (Dipp=2,6-
are also revealed, though these breakdown to their homo-
metallic components in donor solvents as revealed through
NMR and DOSY studies.
3
iPr -C H ). Infinite chain structures of the parent sodium and
2
6
3
potassium amides are disclosed, adding to the few known
Introduction
agent mixtures is increased by the fact their chemistry can be
considered cooperative in origin, implemented through dis-
tinct components working together through either synchron-
ised or stepwise mechanisms. A leading example of the former
Alkali metal amides of bulky secondary amines, [(MNR ) ], are
2
n
popular tools in many synthetic campaigns due to a comple-
mentary combination of strong deprotonating power and
weak nucleophilicity. Most common are lithium 1,1,1,3,3,3-hex-
amethyldisilazide (LiHMDS), lithium diisopropylamide (LiDA)
and lithium 2,2,6,6-tetramethylpiperidide (LiTMP), referred to
[8]
is hexanuclear sodium magnesiate [Na Mg (TMP) (nBu) ], a bi-
4
2
6
2
metallic ring complex with two pendant butyl base units, in
functioning as a template to effect ortho–meta’ or meta–meta’
dideprotonation of a suite of aromatic substrates, superseding
[
1]
[9]
collectively as the “utility amides”. Though all these amides
are useful in their monometallic form, the performance of the
TMP anion in CꢀH deprotonation applications can be signifi-
cantly enhanced by incorporating it within multicomponent
directed ortho-metallation (DoM), in reactions irreproducible
by the separated monometallic components.
Casting the net wider for other bulky amides that might
possess interesting cooperativity in magnesiate modifications
[2]
systems such as the turbo-Hauser base [TMPMgCl·LiCl] or
our attention was drawn to the arylsilyl amido ligand [N(Si-
[
3]
ꢀ
magnesiate [(TMEDA)Na(m-TMP)(m-nBu)Mg(TMP)]
(TMEDA=
Me )(Dipp)]
(where Dipp=2,6-diisopropylphenyl=2,6-iPr -
2
3
[10]
N,N,N’,N’-tetramethylethylenediamine) or alternatively by ad-
C H ). Its metallo chemistry is well developed, with Power re-
6 3
ministering LiTMP in combination with an organometallic [e.g.,
cently reporting the extraordinary finding that CꢀH···HꢀC dis-
persion forces between two such eclipsed amido ligands
helped stabilise linear, two-coordinate Fe, Co and Ni
[
4]
[5]
[6]
Al(iBu) , (TMP)Al(iBu)2, Zn(TMP)2 ] or salt (MgCl , ZnCl or
3
2
2
[
7]
CuCN) trapping agent, which can drive equilibria to products
by stabilising sensitive intermediates through lower polarity
MꢀC bonds (M=Al, Mg, Zn). Curiosity in these new TMP re-
[11]
bis(amides), while Tilley demonstrated the catalytic capability
of the Ni bis(amide) in carbon–carbon cross-coupling reactions.
Though [(Li{N(SiMe )(Dipp)}) ] commonly acts as a transfer
3
2
[
a] Dr. M. ꢀ. Fuentes, A. Zabala, Dr. A. R. Kennedy, Prof. R. E. Mulvey
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde Glasgow G1 1XL (UK)
Fax: (+44)141-548-4787
agent to generate such transition-metal amides, surprisingly
the alkali metal chemistry of this amide has many gaps. While
[12]
Roesky
has determined the dimeric structure of [(Li{N(Si-
E-mail: r.e.mulvey@strath.ac.uk
Me )(Dipp)}) ] and solvated structures such as Anwander’s
3
2
[13]
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
chem.201602683.
[Li{N(SiMe )(Dipp)}(THF) ] are known, the sodium and potas-
3 3
sium congeners have not been crystallographically character-
ised. However, potassium has been observed in interesting
ꢁ
2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
[14]
transition-metal bimetallic systems reported by Tilley
that
This is an open access article under the terms of the Creative Commons At-
tribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
demonstrate the large, soft alkali metal’s propensity for engag-
[15]
[16]
ing with arene-p systems (Figure 1). While Ruhlandt-Senge
Chem. Eur. J. 2016, 22, 1 – 12
1
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has described two-coordinate [Mg{N(SiMe )(Dipp)} ] and the
mined its monomeric structure. In our study we observed no
3
2
THF solvate [(THF) Mg{N(SiMe )(Dipp)}(nBu)], and [Mg{N(Si-
benefit in making the PMDETA solvates 4 and 5 through the
2
17]
3
[
Me )(Dipp)} (OEt )] was described by our group, to the best
salt metathesis of the lithium amide [Li{N(SiMe )(Dipp)}] with
3
2
2
3
of our knowledge no alkali metal magnesiate has hitherto
sodium tert-butoxide and potassium tert-butoxide, respectively,
as crystalline yields of all the products were about 50% from
both deprotometallation and metathesis methods (Scheme 1).
It is worth noting that irrespective of the method employed,
been reported. Therefore this study sets out to fill in important
ꢀ
gaps in the alkali metal chemistry of [N(SiMe )(Dipp)] and to
3
synthesise and characterise the first magnesiate examples as
a prerequisite to determining whether this amide also possess-
es synergistic template base potential.
1
H NMR monitoring of the filtrates from reaction solutions
showed essentially quantitative conversions of the amine to
amide in all cases. Both the starting metal reagent and, where
relevant, the donor solvent employed in these reactions were
added in slight excess compared to the parent amine.
Figure 1. Low-coordinate Ni and Fe complexes showing interaction of potas-
[
14]
sium with p-system as reported by Tilley.
Result and Discussion
Synthetic studies
A total of ten new crystalline compounds were prepared in
this study. These comprise the solvent-free parent sodium and
Scheme 1. Alternative methods to prepare complexes 4 and 5.
potassium amides [Na{N(SiMe )(Dipp)}]
(1) and [K{N(Si-
3
1
Me )(Dipp)}] (2); the PMDETA (N,N,N’,N’’,N’’-pentamethyldi-
3
1
ethylenetriamine) solvates [Li{N(SiMe )(Dipp)}(PMDETA)] (3),
Deprotometallation was used also in the syntheses of mag-
nesiates 9 and 10 (Scheme 2). n-Butylsodium and trimethylsilyl-
methylpotassium were utilised as above to generate the re-
spective heavier alkali metal amide from the amine, which in
turn was reacted in a co-complexation approach with the n-
butylmagnesium amide to afford the monoalkyl–bisamidomag-
nesiates 9 and 10. Obtained as crystalline solids in yields of
76% and 63% respectively, these can be described as lower
order ates, in the sense that their alkali metal to magnesium
ratio is 1:1; whereas higher order ates would have 2:1 ratios.
Note that these structures were formed independent of the
starting stoichiometry as reaction mixtures that have 3:1
amide/nBu ratios, as in [Na Mg (TMP) (nBu) ], still afforded 9
3
[
(
Na{N(SiMe )(Dipp)}(PMDETA)] (4) and [{K{N(SiMe )(Dipp)}-
3 3
PMDETA)} ] (5); the TMEDA (N,N,N’,N’-tetramethylethylenedi-
2
amine) solvates [Li{N(SiMe )(Dipp)}(TMEDA)] (6), [{Na{N(SiMe )-
3
3
(
Dipp)}(TMEDA)} ] (7) and [{K{N(SiMe )(Dipp)}} (TMEDA)] (8);
2 3 2 1
and the heavier alkali metal magnesiates [NaMg{N(SiMe3)-
Dipp)} (m-nBu)] (9) and [KMg{N(SiMe )(Dipp)} (m-nBu)] (10).
(
2
1
3
2
1
Polyamines PMDETA and TMEDA are popular chelating and sol-
[
18]
ubility-enhancing ligands in alkali metal chemistry; they gen-
erally help to deaggregate the parent unsolvated compounds
as reflected here, though interestingly deaggregation has not
[
19]
occurred in the hemi-TMEDA solvate 8.
Monometallic complexes 1–8 were all synthesised by depro-
4
2
6
2
tometallation of the starting amine N(H)(SiMe )(Dipp) by
and 10 as the main products. To the best of our knowledge
these are the first alkali metal magnesiates of 2,6-diisopropyl-
N-(trimethylsilyl)aniline to be synthesised, isolated from solu-
tion and crystallographically characterised (vide infra). Howev-
er, sodium and potassium Group 13 ate complexes are known
3
a metal alkyl reagent. In the lithium and sodium cases this was
the relevant metal n-butyl reagent, whereas the lower stability
of potassium alkyls necessitated switching to the more stable
silylalkyl reagent KCH SiMe , which unlike the n-butyl reagent
2
3
[20]
does not possess any b-hydrogen atoms, and so avoids possi-
for aluminium.
ble decomposition by such an elimination reaction. Ruhlandt-
[
16]
Senge
previously made [K{N(SiMe )(Dipp)}] (probably as
3
Crystallographic characterisation
a THF solvate) by deprotonation of the amine with potassium
hydride in THF and used it in situ to generate Group 2 bis(a-
mides) by salt metathesis with the relevant Group 2 metal
Tables S7 and S8 in the Supporting Information list the crystal
data for all ten new compounds, while Table S9 compares
selected metric data across the series.
[
13]
iodide. Anwander used a similar salt metathesis approach to
make a series of rare earth metal amide complexes, during
In the absence of interrupting donor molecules, the parent
sodium amide 1 and potassium amide 2 adopt infinite chain
structures, though each has a distinctive propagation (Fig-
which he isolated [K{N(SiMe )(Dipp)}] in powder form and crys-
3
tallised the tris(THF) solvate of [Li{N(SiMe )(Dipp)}] and deter-
3
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Scheme 2. Method to prepare alkali metal magnesiates 9 and 10.
ures 2 and 3, respectively). The former has a linear arrange-
ment with the amido substituents syn with respect to the
[
ML] chain and hence eclipsing each other along the chain,
n
whereas the latter has an anti conformation with the Dipp sub-
stituent on one molecular unit facing the SiMe substituent of
3
the adjacent unit. Structure 1 is a channel solvate with both
the solvent channels and the one-dimensional polymers propa-
gating parallel to the a direction. For 2 the one-dimensional
polymeric chains propagate parallel to c and neighbouring
chains link through Me···K agostic interactions. Such interchain
agostic interactions are not observed in 1. The strongest
metal–ligand bond in each case is to the amido N atom (Na1ꢀ
N1 2.2585(16) ꢂ; K1ꢀN1 2.6755(12) ꢂ), but as these can only
provide the alkali metal with a coordination number of one,
the Dipp substituents also engage with the metal. The propa-
gating interactions between monomeric units in 1 are Na con-
3 1
Figure 2. a) Infinite linear chain structure of [Na{N(SiMe )(Dipp)}] (1), show-
ing atomic connectivity between the metal and the ligand. b) Packing dia-
gram of complex 1 (viewed along a-axis). Thermal ellipsoids are displayed at
35% probability and hydrogen atoms have been omitted for clarity. The
dashed lines represent short Na···C contacts. Symmetry operation to gener-
ate equivalent atoms denoted ’: x, ꢀy+1/2, z; ’’: xꢀ1, y, z; ’’’: xꢀ1, ꢀy+1/2,
z. Selected bond lengths [ꢂ] and angles [8]: Na1ꢀN1 2.2585(16); Na1ꢀC4’’
2.696(2); Na1ꢀC3’’’ 2.7514(16); Na1ꢀC1 3.1279(18); Si1ꢀN1 1.6816(16); N1ꢀ
C1 1.381(2); C1ꢀC2 1.4260(17); N1-Na1-C4’’ 130.07(6); N1-Na1-C3’’’ 137.70(5);
C1-N1-Si1 123.82(11); C1-N1-Na1 116.41(11); Si1-N1-Na1 119.77(8); N1-C1-C2
tacts to the para and meta/meta’ Dipp C atoms (2.696(2) and
121.69(8).
3
2
.7514(16) ꢂ) making the Dipp unit h -bound, though there is
an additional intramolecular Na1···C1 contact (3.1279(18) ꢂ) to
the ipso-C atom within the monomeric unit. Possessing the
that 3 and 4 are isomorphic and isostructural. The more
crowded distorted tetrahedral environment about the smaller
Li compared to that of Na in 4 is reflected in a less obtuse C1-
N1-Si1 bond angle (128.77(11)8). Andrews has reported a mono-
meric lithium amide PMDETA complex in [Li(NPhMe)(PMDE-
6
larger, softer K cation, 2 propagates through h -CꢀK engage-
ment (centroid length, 2.81 ꢂ; range of KꢀC lengths,
3.1014(14)–3.1647(16) ꢂ) and there is an additional close intra-
monomeric contact (KꢀC1, 3.0735(14) ꢂ). The N1-K1-centroid’
angle is 154.538 reflecting the bulky heteroleptic nature of the
substituents attached to the amido nitrogen atom. Crystallo-
graphically characterised polymeric sodium and potassium
[22]
TA)].
A single tridentate PMDETA ligand is not sufficient to coordi-
natively saturate the K centre of the amide 5, which therefore
exists as a centrosymmetric dimer (Figure 6). Dimerisation is
manifested by a long K1ꢀC13’ intermolecular interaction
[
21]
amides are comparatively rare.
PMDETA deaggregates the polymer of 1 all the way down to
a monomer within 4 (Figure 4). Accompanied by the amido N,
the tridentate solvent surrounds the Na centre within a distort-
ed (Nꢃ4) tetrahedron in which the shortest bond is to the
amido N atom (NaꢀN1, 2.3206(12) ꢂ) with the PMDETA bonds
to Na spanning the range 2.4188(14)–2.5201(14) ꢂ. There is an
additional Na···ipso-C contact (NaꢀC1, 3.0937(14) ꢂ), presuma-
bly an unavoidable artefact of the short NaꢀN1 bond, though
this is longer than that in polymeric 1, intimating that PMDETA
[23]
(3.2272(17) ꢂ) involving a methyl of the SiMe group, which
3
closes an eight-membered (KNSiC) ring similar to that found
2
by Coles and Cloke in [K(N{SiMe (C H -2-OMe)} )] with an anal-
2
6
4
2
[24]
ogous KꢀC intermolecular interaction of 3.222(2) ꢂ. The pri-
mary K coordination sphere of 5 is composed of four N atoms,
with the shortest bond being to the amido N1 (2.7174(13) ꢂ)
with those to PMDETA (N2, N3, N4) having a mean length of
2.8913 ꢂ. This intramolecular tetranitrogen coordination brings
K into close proximity to both the ipso-C1 of the Dipp substitu-
ent and one terminal Me of the PMDETA ligand (lengths of
2.9806(14) ꢂ and 3.2166(2) ꢂ, respectively). Small PMDETA bite
3
is more effective than the h -bound Dipp at binding to Na,
a point reinforced by an increased C1-N1-Si1 bond angle
(140.098 cf., 123.85(19)8 in 2). As reported by Westerhausen,
less bulky sodium amides can still dimerise in the presence of
angles dictate that this KN coordination sphere is far removed
4
[
21i]
PMDETA in [{Na(NPh )(PMDETA)} ].
from a perfect tetrahedron (range of bond angles, 60.58(4)8 to
127.56(4)8; mean 103.578).
Only with the lithium amide is a monomeric structure re-
tained upon substituting PMDETA by the smaller TMEDA
ligand. The resulting structure, 6, is shown in Figure 7. There is
2
2
Unsurprisingly, the PMDETA lithium congener 3 is also mon-
omeric (Figure 5) with a short Li1ꢀ(amido)N1 bond (2.020(3) ꢂ)
and three longer bonds to PMDETA N atoms (Li1ꢀN2 2.314(3);
Li1ꢀN3 2.148(3); Li1ꢀN4 2.216(3); mean value, 2.226 ꢂ). Note
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Figure 5. Molecular structure of [Li{N(SiMe
3
)(Dipp)}(PMDETA)] (3). Thermal el-
lipsoids are displayed at 35% probability and hydrogen atoms have been
omitted for clarity. Selected bond lengths [ꢂ] and angles [8]: Li1ꢀN1
2
1
.020(3); Li1ꢀN2 2.314(3); Li1ꢀN3 2.148(3); Li1ꢀN4 2.216(3); Si1ꢀN1
.6813(14); N1ꢀC1 1.3898(19); C1ꢀC2 1.429(2); N1-Li1-N2 122.13(13); N1-Li1-
N3 125.62(14); N1-Li1-N4 122.30(14); N3-Li1-N4 84.00(11); N3-Li1-N2
83.16(10); N4-Li1-N2 108.19(13); C1-N1-Si1 128.77(11); C1-N1-Li1 115.86(13);
Si1-N1-Li1 114.96(10); N1-C1-C2 122.22(14).
3 1
Figure 3. a) Infinite zig-zag chain structure of [K{N(SiMe )(Dipp)}] (2), show-
ing atomic connectivity between the metal and the ligand. b) Packing dia-
gram of complex 2 (viewed along c-axis). Thermal ellipsoids are displayed at
35% probability and hydrogen atoms have been omitted for clarity. The
dashed lines represent short K···C contacts. Symmetry operation to generate
equivalent atoms denoted ’: x, ꢀy+3/2, zꢀ1/2. Selected bond lengths [ꢂ]
and angles [8]: K1ꢀN1 2.6755(12); K1ꢀC1 3.0735(14); K1ꢀCentroid’ 2.81; Si1ꢀ
N1 1.6615(14); N1ꢀC1 1.355(2); C1ꢀC2 1.445(2); C1-N1-Si1 137.47(10); C1-N1-
K1 93.59(8); Si1-N1-K1 128.62(7); N1-K1-Centroid’ 154.53.
3 2
Figure 6. Molecular structure of [{K{N(SiMe )(Dipp)}(PMDETA)} ] (5). Thermal
ellipsoids are displayed at 35% probability and hydrogen atoms have been
omitted for clarity. The dashed lines illustrate the K···C contacts. Symmetry
operation to generate equivalent atoms denoted ’: ꢀx+1, ꢀy, ꢀz. Selected
bond lengths [ꢂ] and angles [8]: K1ꢀN1 2.7174(13); K1ꢀN2 2.9203(15); K1ꢀ
N3 2.9306(15); K1ꢀN4 2.8230(15); K1ꢀC1 2.9806(14); K1ꢀC13’ 3.2272(17);
K1ꢀC20 3.2166(2); Si1ꢀN1 1.6569(13); N1ꢀC1 1.3716(18); C1ꢀC2 1.432(2);
N1-K1-N4 99.71(4); N1-K1-N2 127.56(4); N4-K1-N2 115.64(5); N1-K1-N3
1
57.12(4); N4-K1-N3 60.85(4); N2-K1-N3 60.58(4); N1-K1-C1 27.36(4); N1-K1-
C13’ 90.96(4); C1-K1-C20 98.760(3); C1-K1-C13’ 109.373(3); C1-N1-Si1
33.05(10); C1-N1-K1 87.06(8); Si1-N1-K1 139.87(7); N1-C1-C2 122.09(13).
1
3
Figure 4. Molecular structure of [Na{N(SiMe )(Dipp)}(PMDETA)] (4). Thermal
ellipsoids are displayed at 35% probability and hydrogen atoms have been
omitted for clarity. The dashed lines illustrate the Na···C contact. Selected
bond lengths [ꢂ] and angles [8]: Na1ꢀN1 2.3206(12); Na1ꢀN2 2.4862(13);
Na1ꢀN3 2.4188(14); Na1ꢀN4 2.5201(14); Na1ꢀC1 3.0937(14); Si1ꢀN1
the case of the K PMDETA complex 5 dimerisation takes place
through Na···CꢀSi interactions to close a (NaNSiC) ring
2
(
Figure 8). Na approaches the anionic amido N atom more
1
.6565(12); N1ꢀC1 1.3729(17); C1ꢀC2 1.4321(19); N1-Na1-N2 125.66(5); N1-
closely (2.2847(14) ꢂ) than the neutral TMEDA N atoms (mean
length, 2.4668 ꢂ), giving rise to a trigonal pyramidal primary
Na1-N3 125.63(5); N3-Na1-N2 75.39(4); N1-Na1-N4 123.87(5); N3-Na1-N4
4.71(5); N2-Na1-N4 109.45(5); C1-N1-Si1 140.09(10); C1-N1-Na1 111.17(8);
7
Si1-N1-Na1 107.98(6); N1-C1-C2 121.51(12).
NaN coordination (sum of bond angles, 339.088), but as in 5
3
this crowded coordination brings the metal into close proximi-
ty with intramolecular C1 (2.9379(16) ꢂ) and intermolecular
C14’ (2.869(2) ꢂ), making it five-coordinate overall.
The crystallographic determination of potassium amide 8
(Figure 9) established it as a hemi-TMEDA solvate though the
reaction was carried out using a 1:1 K/TMEDA stoichiometric
ratio. Note that this structure contains two crystallographically
independent molecules within the asymmetric unit cell, with
disorder present which somewhat compromises the LiꢀN
(
TMEDA) metric data, but the primary coordination about Li is
unequivocal comprising three nitrogen atoms—one amido
and two TMEDA types.
In contrast the larger sodium centre in 7 cannot be coordi-
natively satisfied by the amide/TMEDA combination so akin to
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slightly different structural parame-
ters, but for brevity only one is dis-
cussed here. Because of this donor
deficiency, 8 exhibits an infinite
zig-zag chain structure, propagat-
ing parallel to the a direction, with
two distinctly different K environ-
ments. K2 occupies a highly dis-
torted tetrahedral N4 site [bond
angles ranging from 66.72(9)8 to
Figure 7. Molecular structure
of [Li{N(SiMe )(Dipp)}(TMEDA)]
6). Thermal ellipsoids are dis-
1
48.95(9)8, with the extremities
3
representing the TMEDA bite angle
(
played at 35% probability, hy- and amido-K-amido angle, respec-
drogen atoms, and the disor-
dered component of TMEDA
N atoms and two anionic N atoms,
ligand and two methyl
tively] comprising the two TMEDA
belonging to two amide ligands
Figure 9. a) Asymmetric unit of the structure of [{K{N(Si-
Me )(Dipp)}} (TMEDA)] (8). b) Section of extended framework structure
showing atom connectivity between the metal and the [N(SiMe )(Dipp)]
ligand. Thermal ellipsoids are displayed at 35% probability and hydrogen
atoms have been omitted for clarity. Dashed lines denote the K···aryl con-
tacts. Symmetry operation to generate equivalent atoms denoted ’: x+1/2,
groups of one iPr unit are
3
2
1
omitted for clarity. Selected
bond lengths [ꢂ] and angles
(range of bond lengths, 2.780(3) to
.830(3) ꢂ). In contrast, K1 lies be-
3
2
[
1
8]: Li1ꢀN1 1.905(4); N1ꢀC1
tween the aryl rings of two trans-
oid disposed Dipp ligands, in
a near linear sandwich arrange-
ment (centroid-K-centroid angle,
.388(2); Si1ꢀN1 1.6756(17);
Si1-N1-Li1 121.50(15); C1-N1-
Li1 108.20(17); C1-N1-Si1
ꢀ
y+2, z. Selected bond lengths [ꢂ] and angles [8]: K1ꢀCentroid1 2.7943(8);
K1ꢀCentroid2 2.7909(8); N1ꢀC1 1.354(4); N1ꢀSi1 1.663(3); Si2ꢀN2 1.651(3);
K2ꢀN3 2.780(3); K2ꢀN1’ 2.830(3); K2ꢀN2 2.831(3); K2ꢀN4 2.932(3); N2ꢀC16
1.350(4); Centroid1-K1ꢀCentroid2 165.8; C1-N1-Si1 135.1(2); C1’-N1’-K2
130.30(13).
1
65.88; mean KꢀC(centroid) length,
112.00(19); Si1’-N1’-K2 110.56(12); N3-K2-N1’ 93.22(9); N3-K2-N2 103.45(9);
N1’-K2-N2 148.95(9); N3-K2-N4 66.72(9); N1’-K2-N4 97.86(9); N2-K2-N4
12.70(9); C16-N2-Si2 143.5(2); C16-N2-K2 108.7(2); Si2-N2-K2 107.61(13).
1
were expected to adopt different structural architectures from
this TMP derivative. X-ray crystallographic determinations duly
confirmed this expectation. The salient difference is that both
9
and 10 have infinite helical chain structures (Figures 10–13)
and not the ring architecture that appears to be the key fea-
ture behind the special templating metallation ability of
[8]
[
Na Mg (TMP) (nBu) ].
4 2 6 2
Sodium magnesiate 9 displays two distinct types of (half oc-
cupancy) Na atom separated by a magnesiate anion, neither of
which engage with the amido N atom. Na1 lies upon a twofold
rotation axis and bonds to two symmetrically equivalent butyl
C atoms (C16/C16’, 2.779(2) ꢂ) and two symmetrically equiva-
lent aryl groups (C4/C4’ and C5/C5’) at the para/meta positions
3 2
Figure 8. Molecular structure of [{Na{N(SiMe )(Dipp)}(TMEDA)} ] (7). Thermal
ellipsoids are displayed at 30% probability and hydrogen atoms have been
omitted for clarity. Dashed lines illustrate the Na···C contacts. Symmetry op-
eration to generate equivalent atoms denoted ’: ꢀx+1, ꢀy+2, ꢀz+1. Se-
lected bond lengths [ꢂ] and angles [8]: Na1ꢀN1 2.2847(14); Na1ꢀN2
(
2.793(2) ꢂ and 2.913(2) ꢂ, respectively), making it six-coordi-
2
1
.4726(17); Na1ꢀN3 2.461(2); Na1ꢀC14’ 2.869(2); Na1ꢀC1 2.9379(16); Si1ꢀN1
.6631(13); N1ꢀC1 1.3785(19); C1ꢀC2 1.424(2); N1-Na1-N3 133.21(7); N1-
nate overall. The next shortest distance between Na1 and the
aryl group is to C3 (at 3.2535(20) ꢂ), which seems too long, so
Na1-N2 130.49(6); N3-Na1-N2 75.38(6); N1-Na1-C14’ 103.78(6); N3-Na1-C14’
106.10(7); N2-Na1-C14’ 103.19(7); C1-N1-Si1 132.08(11); C1-N1-Na1 103.88(9);
2
the hapticity of the aryl substituent is best regarded as h .
Si1-N1-Na1 124.02(7); N1-C1-C2 121.54(14).
Lying on a crystallographic inversion centre sandwiched be-
tween two aryls from neighbouring magnesiate ions, Na2
6
2
.7926 ꢂ) architecturally akin to classic structures such as
binds in a h arrangement to both of them (range of Na2ꢀC
bis(benzene)chromium, though the bonding in potassium
bond lengths, 2.684(2)–3.1565(18) ꢂ; Naꢀcentroid distance
2.5311(1) ꢂ; centroid-Na-centroid angle, 180.08). Propagation of
this contacted ion pair structure into an infinite helical chain
(Figure 12) is through these distinct types of NaꢀC interaction.
[
25,15c]
+
sandwiches is electrostatic in origin.
Formally this [(K)
ꢀ
(R K·TMEDA) ] formulation of 8 in which K1 does not engage
2
with an anionic centre (discounting any delocalisation within
the Dipp aryl ring) can be described as a potassium potassiate.
Though rare, potassium potassiate structures have been re-
Mg1 occupies a N C distorted trigonal planar coordination
2
(sum of bond angles; 359.698) comprising two amide ligands
(mean MgꢀN distance, 2.0328 ꢂ) and one butyl ligand (Mg1ꢀ
C16, 2.1533(19) ꢂ). The helical chain structure of 10 (Figure 13)
is less intricate than that of 9 as it contains only one alkali
metal (K) site. Sandwiched between two aryl rings (K1ꢀcent-
[
26]
ported previously.
Magnesiates 9 and 10, which both have a 2:1 amido/butyl
stoichiometric ratio different to the 3:1 ratio in the aforemen-
tioned pre-inverse-crown template base [Na Mg (TMP) (nBu) ],
4
2
6
2
Chem. Eur. J. 2016, 22, 1 – 12
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Figure 10. a) Asymmetric unit of the structure of [NaMg{N(SiMe
nBu)] (9). Note that Na1 and Na2 are at half occupancy. b) Section of ex-
tended framework structure showing atomic connectivity between the
metals, n-butyl and connecting N atom of the [N(SiMe )(Dipp)] ligands. Ther-
mal ellipsoids are displayed at 35% probability and hydrogen atoms and iPr
groups have been omitted for clarity. The dashed lines illustrate the Na···aryl
contacts. Symmetry operation to generate equivalent atoms denoted ’:
3 2
)(Dipp)} (m-
3 2 1
Figure 11. a) Structure of [KMg{N(SiMe )(Dipp)} (m-nBu)] (10), showing the
contents of the asymmetric unit. b) Section of extended framework structure
showing atomic connectivity between the metals, n-butyl and connecting N
1
3
atom of the [N(SiMe
3
)(Dipp)] ligands. Thermal ellipsoids are displayed at
5% probability, hydrogen atoms, iPr groups, one disordered component of
3
3
a -SiMe group and one disordered methylcyclohexane molecule of crystalli-
sation have been omitted for clarity. The dashed lines illustrate the K···aryl
contacts. Symmetry operation to generate equivalent atoms denoted ’: x,
ꢀ
x+1, y, ꢀz+1/2. Selected bond lengths [ꢂ] and angles [8]: Na1ꢀC16
2
.779(2); Na1ꢀC4 2.793(2); Na1ꢀC5 2.913(2); Na1ꢀC3 3.2535(20); Na2ꢀCent-
ꢀ
y+1, zꢀ1/2; ’’: x, ꢀy+1, z+1/2. Selected bond lengths [ꢂ] and angles [8]:
K1ꢀC31 3.046(3); K1ꢀCentroid1 2.9162(1); K1ꢀCentroid2 2.9239(1); Mg1ꢀN1
.0327(18); Mg1ꢀN2’ 2.0341(17); Mg1ꢀC31 2.150(2); Si1ꢀN1 1.7092(17); N1ꢀ
C1 1.415(2); N2ꢀC16 1.411(2); N2ꢀMg1’’ 2.0340(17); Centroid1-K1-Centroid2
47.4; C31-K1-Centroid1 91.943(3); C31-K1-Centroid2 117.537(2); Mg1-C31-
K1 94.26(9); N1-Mg1-N2’ 133.18(7); N1-Mg1-C31 110.36(9); N2’-Mg1-C31
16.34(9); C1-N1-Si1 121.87(13); C1-N1-Mg1 111.36(12); Si1-N1-Mg1 126.69(9);
roid 2.5311(1); Mg1ꢀN1 2.0263(16); Mg1ꢀN2 2.0393(15); Mg1ꢀC16
2.1533(19); Si1ꢀN1 1.7030(16); Si2ꢀN2 1.7042(16); N1ꢀC1 1.414(2); N2ꢀC20
1.405(2); centroid-Na2-centroid 180.0; Mg1-C16-Na1 126.94(8); C16’-Na1-C16
132.53(9); N1-Mg1-N2 129.36(7); N1-Mg1-C16 112.50(7); N2-Mg1-C16
2
1
117.83(7); C1-N1-Si1 120.85(12); C1-N1-Mg1 105.48(11); Si1-N1-Mg1
1
1
33.55(9); C20-N2-Si2 122.86(11); C20-N2-Mg1 109.11(11); Si2-N2-Mg1
27.99(8).
1
C16-N2-Mg1’’ 122.16(13).
roid1 2.9162(1) ꢂ; K1ꢀcentroid2 2.9239(1) ꢂ; centroid1-K1-cent-
roid2, 147.48), K1 also interacts with the a-C of the Bu ligand
Solution NMR spectroscopic characterisation
(
K1ꢀC31 3.046(3) ꢂ), while there is a contact with one of the
CH (Si) groups though the disorder in this group negates an
All the new compounds 1–10 were also characterised in solu-
3
1
13
1
accurate bond length. Consistent with the interaction between
the alkali metal and the magnesiate ion being predominately
electrostatic in nature the dimensions of the Mg1 centre in 10
tion by H and C{ H} NMR spectroscopy recorded at ambient
1
1
temperature. Assignments were aided by H, H-COSY and
1
13
H, C-HSQC experiments. In general, the data obtained (see
(
mean MgꢀN, 2.0334 ꢂ; MgꢀC31, 2.150(2) ꢂ; sum of bond
the full experimental details in the Supporting Information)
supported the formulae established by X-ray crystallography of
the solid samples.
angles subtending Mg1, 359.828) match those in the sodium
magnesiate 9.
Figure 12. Part of the polymeric helical chain of [NaMg{N(SiMe
Chem. Eur. J. 2016, 22, 1 – 12 www.chemeurj.org
3
)(Dipp)}
2
(m-nBu)]
1
(9) that propagates parallel to the crystallographic c-axis.
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3 2 1
Figure 13. Part of the polymeric helical chain of [KMg{N(SiMe )(Dipp)} (m-nBu)] (10) propagating parallel to the c-axis.
Since the polymeric solids 1 and 2 were poorly soluble in
arene solvents, their solutions were made up in a mixture of
C D and [D ]THF solvents. This meant we were viewing THF
tures distinct from that of the cyclic hexanuclear sodium mag-
nesiate [Na Mg (TMP) (nBu) ], which shows excellent solubility
4
2
6
2
1
13
in hydrocarbon media. The H and C NMR spectra of 9 show
two distinct types of amido group. For example, the former
shows two p-CH-Ar triplet resonances at 6.85 and 6.64, two
CH(CH ) septets at 4.27 and two Si(CH ) singlets at 0.30 ppm
6
6
8
solvates instead of the pure, unsolvated parent amides. All the
expected resonances were observed and irrespective of wheth-
er the alkali metal was sodium or potassium the chemical
shifts essentially matched (e.g., the quaternary C atom bound
3
2
3 3
and 0.25 ppm. This could be due to two distinct amido groups
within the same molecule or alternatively two distinct amido-
13
to N was located at 157.5 ppm and 156.8 ppm in the C spec-
tra of 1 and 2, respectively), giving credence to the view that
the bonding between the amide and alkali metal is predomi-
nately electrostatic. The presence of PMDETA in 3–5 enhanced
the solubility of the amides such that they could dissolve well
containing molecules. The chemical shift of the CH -nBu mul-
2
tiplet appearing at ꢀ0.26 ppm indicates that this ligand is
more likely to be attached to Mg as it would appear more up-
field if attached to the more electropositive metal Na {for ex-
ample, the same resonance appears in C D solutions of
7
in C D without the addition of polar THF. The Li NMR spec-
6
6
6
6
1
[27]
trum of 3 revealed a single species at 258C. Its H NMR spec-
trum showed one methine CH resonance and two distinct Me
resonances for the Dipp substituent, consistent with one type
of amido group. The aforementioned quaternary C atom
[{Mg(TMP)(nBu)} ] at 0.05 ppm and [Mg(TMP)(nBu)·IPr] (IPr=
2
1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene)
at
[28]
ꢀ0.42 ppm}.
Due to the ambiguity of these spectra we
[29]
turned to diffusion-ordered spectroscopy (DOSY) to attempt
to find a resolution. For an accurate molecular weight (MW)
determination we utilised the external calibration curve (ECC)
approach with normalised diffusion coefficients that has re-
13
bound to N appears at 157.5 ppm in the C spectrum, essen-
tially identical to those in the THF solvates of 1 and 2, and to
those in the PMDETA solvates of the sodium and potassium
amide 4 and 5 (at 157.8 ppm and 157.0 ppm, respectively).
There are differences but surprisingly only modest ones for the
PMDETA resonances across the series 3–5. For example, the
[30]
cently been developed by the Stalke group. This novel ap-
proach takes into account the shape of the molecule allowing
accurate MW predictions with a maximum error of less than
9%. The beauty of this approach is that only one internal refer-
ence (that also can be the solvent) is necessary, whereas previ-
ous approaches required multiple references. In this case tetra-
methylsilane was employed. DOSY determined MWs of species
in solution were estimated using the diffusion coefficient
CH resonances appear at 46.0/44.9 ppm, 45.3/43.9 ppm, and
3
4
4.9/40.9 ppm for terminal/central positions respectively in the
1
3
C NMR spectra. Both the Li and Na TMEDA solvates 6 and 7,
being molecular solids, dissolve in C D , in contrast to the poly-
6
6
meric hemi-TMEDA solvate 8, which requires some [D ]THF to
8
exhibit solubility in the arene. Consistent with the monomeric
values for 9 in [D ]THF (Figure S1, Tables S2–S3 in the Support-
8
7
structure in the crystal, the Li NMR spectrum of 6 shows a soli-
tary resonance. TMEDA is bound to the Li as evidenced by the
ing Information). Two distinct diffusion coefficients were ob-
tained consistent with two distinct species. The best fit for
these were found to be the homometallic THF solvates
[([D ]THF) Mg{N(SiMe )(Dipp)}(m-nBu)] 9a (surprisingly the error
chemical shift and order of the TMEDA resonances in the
1
H NMR spectrum (Me, 1.61 ppm; CH , 1.44 ppm), which bear
2
8
x
3
a close resemblance to those in the Na TMEDA solvate
when x=1, ꢀ6%, was found to be much less than that for x=
2, ꢀ21%, a result at odds with the aforementioned crystal
structure, which contained two THF ligands) and
[([D ]THF) Na{N(SiMe )(Dipp)}] 9b with the error from their cal-
(1.57 ppm and 1.49 ppm, respectively). As anticipated, the
mixed solvent solution of 8 contains free TMEDA, with the
order of its resonances reversed in comparison (CH , 2.12 ppm;
2
8
2
3
Me, 1.97 ppm), as [D ]THF will now be solvating the larger
culated MWs being ꢀ6% and ꢀ7%, respectively. Significantly
the heterometallic ate 9 would have an error of 36% if unsol-
vated, or 43% if solvated by one THF ligand. Clearly the mag-
nesiate that crystallises from hydrocarbon/arene solution
breaks down to homometallic species in the presence of the
8
alkali metal.
Magnesiates 9 and 10 conform to the pattern seen with the
other polymeric solids 1, 2 and 8 in requiring the addition of
[
D ]THF for their dissolution in C D . Hence, the solutions will
8
6
6
[31]
contain THF solvates. The reduced solubility of the donor-free
compounds in hydrocarbon solvents such as methylcyclohex-
ane immediately signalled that we were dealing with struc-
strong Lewis base THF (Scheme 3). Organomagnesium com-
pounds are synonymous with redistribution reactions, most
notably the Schlenk equilibrium. Unsurprisingly, the potassium
Chem. Eur. J. 2016, 22, 1 – 12
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magnesiate 10 was found to undergo the same disproportio-
formed at room temperature dropping from 80% to 10% and
11%, respectively. Rerunning the metallations under reflux con-
ditions greatly improves the yields of the iodo product 13a,
though they still fall short of that obtained by [Mg{N(Si-
nation reaction in [D ]THF (Scheme 3), with the corresponding
8
DOSY data consistent with the two homometallic species
[
([D ]THF)Mg{N(SiMe )(Dipp)}(m-nBu)]
10a
and
8
3
[
([D ]THF)K{N(SiMe )(Dipp)}] 10b with small errors of ꢀ4% and
Me )(Dipp)}(nBu)]. Only when the amount of ate base is dou-
8
3
3
+
1%, respectively, versus the calculated MWs. Supporting evi-
bled do the yields obtained approach 100%. Yields of the iodo
product 13b from reaction of 9 or 10 with the benzamide
never reach quantitative even at reflux temperature with the
best just over 50%.
dence that 9a and 10a are the same solution species comes
from the close similarity of their chemical shifts [for example:
1
H, CH -nBu at ꢀ0.26 ppm and ꢀ0.22 ppm, respectively;
2
13
Si(CH ) at 0.30 ppm in both; C, CH -nBu at 9.3 ppm
Attempts were made to gain insight into the intermediate
metallated complexes prior to the iodination step. A modicum
of success was made in the reaction between “sodium mag-
nesiate 9” and the oxazoline 12a. The reaction solution depos-
ited a crystalline solid in the magnesium complex [Mg{N(Si-
3
3
2
and 9.0 ppm, respectively; (o-C -Ar) appearing at 145.1 ppm
q
in
both].
Ruhlandt-Senge
also
reported
[
(THF) Mg{N(SiMe )(Dipp)}(nBu)] and NMR data are consistent
2
3
with that found in 9a and 10a (e.g., CH -nBu at 9.4 ppm in
2
neat C D ).
Me )(Dipp)}R] (11), in which R is ortho-deprotonated 4,4-di-
6
6
3
methyl-2-phenyl-oxazoline. Unfortunately X-ray crystallographic
studies of 11 revealed a highly disordered structure that neg-
ates its inclusion here, though NMR studies confirmed its for-
mula. Though monometallic 11 could result from a dispropor-
tionation of bimetallic ate 9, it is possible that 9 never formed
in the hydrocarbon medium, but was in fact a mixture of
8
Scheme 3. Breakdown of the complexes 9 and 10 in [D ]THF.
1
and [Mg{N(SiMe )(Dipp)}(nBu)], with 11 forming as a result of
3
the latter deprotonating the oxazoline substrate. However,
Reactivity studies
given that the yield of iodo product 12a using [Mg{N(Si-
Our failure to synthesise
a
template ring complex as
Me )(Dipp)}(nBu)] on its own was 80% at 258C, but is only
3
[
[
Na Mg (TMP) (nBu) ] incorporating the arylsilyl amido ligand
10% using 9 as the base under the same conditions, this sug-
gests that the presence of the sodium amide component sig-
nificantly inhibits reactivity of the alkylmagnesium amide.
Since it was clear that these magnesiates do not possess
robust bimetallic structures like that of the cyclic hexanuclear
sodium magnesiate [Na Mg (TMP) (nBu) ], it was deemed not
4
2
6
2
N(SiMe )(Dipp)] in place of TMP reduced our expectation of re-
3
alising enhanced or special reactivities, but for completeness
we carried out some representative metallation–iodination re-
actions. Table S10 in the Supporting Information compiles
those carried out between 4,4-dimethyl-2-phenyl-oxazoline
4
2
6
2
(
12a) and each of the following complexes: the solvent-free
worthwhile to explore their reactivity any further.
parent sodium and potassium amides 1 and 2; the heavier
alkali metal magnesiates 9 and 10; and for comparison, the al-
kylmagnesium amide [Mg{N(SiMe )(Dipp)}(nBu)] (Mg). Table S11
3
Conclusion
lists the reactions between N,N-diisopropylbenzamide (12b)
and the same set of potential bases. In both cases the at-
tempted metallations were performed in methylcyclohexane
and the subsequent iodine quenches were done in THF. Full
details are provided in the Experimental Section.
This study has uncovered a remarkable s-block complex struc-
ꢀ
tural diversity based on the amide [N(SiMe )(Dipp)] function-
3
ing as a ligand on its own or in combination with an n-butyl
ligand. The parent sodium and potassium amides adopt infin-
ite chain structures, with linear or zig-zag arrangements, re-
spectively, adding to the relatively few known crystallographi-
cally characterised unsolvated s-block metal amides. Tridentate
PMDETA deaggregates the sodium polymer to a monomer and
also generates a monomeric lithium amide, but it is not suffi-
cient to coordinatively saturate the K centre of the amide,
which exists as a centrosymmetric dimer, with dimerisation ex-
pressed by a long KꢀCH SiMe intermolecular interaction that
Owing mainly to their poor hydrocarbon solubility, unsolvat-
ed 1 and 2 proved unreactive with both of the aromatic sub-
strates even under reflux conditions. Though magnesium
bases are generally significantly less reactive than alkali metal
bases, heteroleptic [Mg{N(SiMe )(Dipp)}(nBu)] proved effective
3
at deprotonating the oxazoline substrate (the iodo product
13a was quantitative when the metallation was performed
under reflux conditions), since it contains a butyl anion as well
as an amide and is more soluble in hydrocarbon media. The
magnesium base also metallated the slightly more challenging
benzamide substrate, but much less effectively (yield of iodo
product 13b, 42%). In both cases the metallation–iodination
operation took place regioselectively at the ortho position in
3
2
closes an eight-atom (KNSiC) ring. Reducing the chelating ca-
2
pability of the donor solvent via TMEDA has little effect on the
lithium amide, which remains monomeric, but the sodium
amide dimerises by means of Na···CꢀSi interactions mimicking
the case of the PMDETA potassium amide. Refusing half
a molar equivalent of TMEDA despite a 1:1 K/TMEDA stoichio-
metric ratio in the solution, the potassium amide crystallises as
a hemi-TMEDA solvate. Its infinite chain structure has distinct K
[9]
keeping with the directed ortho-metallation (DoM) principle.
Interestingly, when the alkylmagnesium amide is incorporated
within the sodium and potassium ates 9 and 10, the reactivity
towards 12a diminishes especially when the metallation is per-
coordination environments, one occupying a N site, the other
4
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sandwiched between aryl rings of two Dipp ligands, classifies it
as a novel potassium potassiate.
phenyl-oxazoline (12a; 341.9 mL,
mmol) was added by syringe. Next
the mixture was heated to reflux for
.5 h to give a yellow solution, which
was allowed to cool down to ambi-
ent temperature. After a period of
one week, colourless crystals of 11
grew from the reaction mixture. At-
tempts to analyse the crystalline ma-
terial by X-ray diffraction studies
2
Both crystalline sodium and potassium magnesiates display
a 2:1 amido/butyl stoichiometric ratio in infinite helical chain
structures conflicting with the 3:1 ratio of the template base
1
[
Na Mg (TMP) (nBu) ], which inspired this study. Lack of hydro-
4 2 6 2
carbon solubility denied any opportunity for these ates to dis-
play special reactivities like that of the template base. THF was
needed for solubility but DOSY studies indicate that the donor
promotes fragmentation of the magnesiates into homometallic
moieties. The ring architecture is the key feature behind the
special templating metallation ability of [Na Mg (TMP) (nBu) ],
were unsuccessful due to the highly disordered nature within the
structure of 11. These were filtered, washed with n-hexane (3ꢃ
4
0
mL) and dried under vacuum (unrefined yield: 290 mg,
.60 mmol, 30%). The absolute yield was higher since the filtrate
4
2
6
2
but such architectures proved inaccessible with the [N(Si-
contained a mixture of the product 11 and starting material. The
reaction was also studied using two and three molar equivalents
of complex 9 and one equivalent of the substrate (2 mmol) giving
the same compound 11. The NMR spectra of isolated crystalline 11
Me )(Dipp)] ligand. Despite this disappointment the study is
3
important, as it has clearly established that the presence of
aryl groups can be an inhibiting factor to molecular ring for-
mation as their p-faces can engage intermolecularly with alkali
metals to help construct polymeric chains. Future work will
focus on circumventing this solubility/structural problem by
using bulky lipophilic non-aryl ligands.
are in agreement with a 1:1 ratio of [N(SiMe
)(Dipp)]:[2-C
H -1-(oxa-
6 4
3
1
zoline(Me) )] in 11. H NMR (400.13 MHz, C D /[D ]THF, 258C): d=
2
6
6
8
8.24 (ddd, J(H,H)=6.8, 1.2, 0.8 Hz, 1H; m-CH-Ar), 7.99 (dt, J(H,H)=
7.6, 0.9 Hz, 1H; o-CH-Ar), 7.42 (td, J(H,H)=7.2, 1.2 Hz, 1H; p-CH-Ar),
7.20 (td, J(H,H)=7.5, 1.4 Hz, 1H; m-CH-Ar), 7.17 (d, 2H; C D over-
6
6
lapping, m-CH-Ar, Dipp), 6.97 (t, J(H,H)=7.5 Hz, 1H; CH-Ar, Dipp),
.30 (brsept, J(H,H)=6.7 Hz, 2H; -CH(CH ) ), 3.69 (s, 2H; -CH -), 1.43
4
3
2
2
Experimental Section
(d, J(H,H)=6.9 Hz, 6H; -CH(CH ) ), 1.28 (brd, J(H,H)=6.5 Hz, 6H;
3 2
General procedures: All reactions and manipulations were per-
formed under a protective atmosphere of dry pure argon gas
using standard Schlenk tube or glovebox techniques. Solvents
were dried by heating to reflux over sodium benzophenone ketyl
and distilled under nitrogen prior to use. Methylcyclohexane was
distilled over sodium metal and stored with molecular sieves (4 ꢂ).
Deuterated NMR solvents were degasified and stored over molecu-
lar sieves (4 ꢂ) prior to use. 2,6-diisopropylaniline, N,N,N’,N’’,N’’-
-CH(CH
3
)
2
), 0.94 (brs, 6H; (-CH
/[D
-Ar, Dipp), 145.0 (o-C
-Ar), 131.0 (p-CH-Ar), 125.7 (o-CH-Ar), 125.1 (m-CH-
Ar), 123.3 (m-CH-Ar, Dipp), 119.5 (p-CH-Ar, Dipp), 81.3 (-CH -), 64.5
(C ), 28.1 (br, (-CH ), 27.3 (-CH(CH ), 25.3 (-CH(CH ), 25.1
(-CH(CH ), 3.6 ppm (-Si(CH ). NMR spectra also revealed small
3
)
2
), 0.33 ppm (s, 9H; -Si(CH
]THF, 258C): d=178.9 (C -Mg),
-Ar, Dipp), 140.1 (m-
3 3
) );
13
1
C{ H} NMR (100.6 MHz, C
176.0 (C -Ar), 152.5 (N-C
CH-Ar), 135.0 (C
D
6
6
8
q
q
q
q
q
2
)
3
)
2
)
3 2
q
2
3
)
3
)
3 3
2
amounts of methylcyclohexane (from crystallisation) as well as
minute traces of 4,4-dimethyl-2-phenyl-oxazoline and [N(H)(Si-
pentamethyldiethylenetriamine
(PMDETA)
and
N,N,N’,N’-
tetramethylethylenediamine (TMEDA) were purchased from Al-
drich, dried by heating to reflux over calcium hydride and stored
with molecular sieves (4 ꢂ) under nitrogen prior to use. nBuLi
Me
calcd (%) for C26
70.38, H 8.49, N, 6.55.
3
)(Dipp)] as a result of unavoidable hydrolysis. Elemental analysis
H38MgN
OSi: C 69.86, H 8.57, N 6.27; found: C
2
(
1.6m in n-hexane) and nBu Mg (1m in n-heptane) solutions were
2
purchased from Aldrich and titrated prior to use. Trimethylsilyl
chloride, tetramethylsilane, sodium tert-butoxide, potassium tert-
butoxide, 4,4-dimethyl-2-phenyl-oxazoline, N,N-diisopropylbenza-
mide and 1,10-Phenanthroline were purchased from Aldrich and
Application of metallated compounds in organic synthesis
by electrophilic addition reaction
[32]
[33]
General procedure: The aryl substrates 4,4-dimethyl-2-phenyl-oxa-
zoline (12a) and N,N-diisopropylbenzamide (12b) were treated
with the appropriate metal complex in methylcyclohexane. All re-
actions were stirred at/for different temperatures/times and sub-
strate:base stoichiometries of 1:1 and 1:2 were probed. Following
metallation, the corresponding 2-monoiodo derivatives 13a and
used as received. nBuNa,
K(CH SiMe )
and [N(H)(Si-
2
3
[34]
Me )(Dipp)] (Dipp=2,6-iPr -C H ) were prepared according to lit-
3
2
6
3
erature procedures. NMR spectra were recorded on a Bruker DPX
1
4
00 NMR spectrometer, operating at 400.13 MHz for H, 155.5 MHz
7
13
1
13
1
for Li and 100.6 MHz for C. H and C{ H} spectra were refer-
enced to the appropriate solvent signal, Li NMR spectra were ref-
7
1
3b were obtained by in situ reaction with an iodine solution in
erenced against LiCl in D O at 0.00 ppm. Elemental analyses of the
2
tetrahydrofuran (1m) at ꢀ788C. The reaction mixture was allowed
compounds 1–10 were carried out using a PerkinElmer 2400 ele-
mental analyser. Full characterisation details are given in the Sup-
porting Information.
to warm up to ambient temperature over a period of 16 h. A satu-
rated aqueous NH Cl solution was added, followed by saturated
4
aqueous Na S O solution. Extraction of the organic crude with
2
2
3
ethyl acetate (3ꢃ10 mL), then it was washed with brine (10 mL)
Reactivity studies
and dried over anhydrous MgSO . The solvent was removed under
4
reduced pressure and the crude reaction product was dissolved in
Isolation and characterisation of metallated intermediate 11:
Complex 9 was chosen as an example. Freshly prepared n-butylso-
dium (168.2 mg, 2.1 mmol) was suspended in methylcyclohexane
CDCl . 1,10-Phenanthroline was added as internal standard, the
3
1
yields being calculated by H NMR spectroscopy (see Tables S10
and S11 in the Supporting Information for details).
(
15 mL) and then 2,6-diisopropyl-N-(trimethylsilyl)aniline (998.0 mg,
4
1
mmol) was added. The resulting beige suspension was stirred for
The aromatic substrate 12a or 12b (2 mmol) was added to a reac-
tion mixture of the corresponding monometallic complexes 1, 2,
h and then commercial nBu Mg (2.1 mL, 1m solution in n-hep-
2
tane, 2.1 mmol) was introduced by syringe. The reaction mixture
was stirred for an additional 1 h. At this juncture, 4,4-dimethyl-2-
[Mg{N(SiMe )(Dipp)}(nBu)] (2 mmol) in methylcyclohexane (15 mL)
or the mixed-metal complexes 9/10 (2 mmol/4 mmol) in methylcy-
3
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
9
ꢁ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
clohexane (15 mL/30 mL, respectively). All reactions were stirred at/
for 258C/24 h and 1018C/1.5 h respectively. The iodination reaction
was carried out as outlined above.
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3
2
,6-diisopropyl-N-(trimethylsilyl)aniline (499.0 mg, 2 mmol) was
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Chem. Eur. J. 2016, 22, 1 – 12
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ꢁ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 12
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FULL PAPER
&
Alkali Metals
M. ꢀ. Fuentes, A. Zabala, A. R. Kennedy,
R. E. Mulvey*
&& – &&
Structural Diversity in Alkali Metal and
Alkali Metal Magnesiate Chemistry of
the Bulky 2,6-Diisopropyl-N-
(trimethylsilyl)anilino Ligand
Broken promise: Rare examples of
polymeric unsolvated alkali metal mag-
formation, a requirement for template
bases, as aryl p-faces can engage inter-
molecularly with alkali metals to con-
struct chains (see figure), which break in
donor solvents to homometallic species
ruling out any synergistic reactivity.
nesiates containing the bulky amide
ꢀ
[
N(SiMe )(Dipp)] (Dipp=2,6-iPr -C H )
3
2
6
3
are uncovered. Their structures show
aryl groups can inhibit molecular ring
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
12
ꢁ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!