Probing the metallating ability of a polybasic sodium alkylmagnesiate supported by a bulky bis(amido) ligand: Deprotomagnesiation reactions of nitrogen-based aromatic substrates

Article abstract of DOI:10.1039/c3dt52639a

Exploring the reactivity of sodium butylmagnesiate reagent [{Na(THF) ₆}⁺{(Ph₂Si(NAr*)₂)Mg(Bu)(THF)} ^-] (1) supported by the bulky chelating silyl(bisamido) ligand {Ph₂Si(NAr*)₂}^2- (Ar* = 2,6-iPr₂-C₆H₃) towards N-methylbenzimidazole (bIm^Me), pyrrole and 2,6-diisopropylaniline (NH₂Ar*) , this study provides new insights into the ability of this bimetallic base to facilitate direct Mg-H exchange reactions as well as to exhibit polybasicity. Thus 1 effectively promotes the deprotomagnesiation of bIm^Me under mild reaction conditions to give the α-metallated intermediate [{Na(THF)₅}₂⁺{(Ph₂Si(NAr*) ₂)Mg(bIm^Me*)}₂^-] (2) (bIm ^Me* = 2-N-methylbenzimidazolyl). Analysis of crystallographic and NMR data of 2 combined with DFT calculations show that the metallated C in the bIm^Me* ligands possesses a significant carbenic character. Contrasting with previous studies of benzothiazole (btz), 1 does not react with an excess of bIm^Me even under forcing refluxing conditions. Contrastingly, the amination reactions of equimolar amounts of 1 with pyrrole and 2,6-diisopropylaniline allowed the isolation of [{(Ph ₂Si(NAr*)(NHAr*))Mg(NC₄H₄) ₂(THF)Na(THF)₂}] (3) and [{Na(THF)₆} ⁺{(Ph₂Si(NAr*)(NHAr*))Mg(NHAr*) ₂(THF)}^-] (4) respectively as crystalline solids. Highlighting the ability of 1 to act as a polybasic reagent, 3 and 4 are formed as the result of the deprotonation of two molecules of the relevant amine via its butyl group and one amido arm of the silyl(bisamido) ligand. Similarly, the reactions of 1 with 3 molar equivalents of the relevant amine yielded homoleptic tris(amido) compounds [(THF)₂NaMg(NC₄H₄) ₃] (5) and [{Na(THF)₆}⁺{Mg(NHAr*) ₃}^-] (7), with the concomitant formation of bis(amine) Ph₂Si(NHAr)₂, as a result of the complete amination of 1 using its three basic sites. The structures in the solid state of 3 and 4 were elucidated by X-ray crystallography. Despite their similar constitution, these heteroleptic tris(amido)magnesiates adopt contrasting structures, with the former displaying a contacted ion-pair structure, where Na and Mg are connected by two bridging pyrrolyl anions, whereas the latter gives rise to a solvent-separated ion pair motif. To the best of our knowledge 3 represents the first crystallographically characterized magnesium compound containing an anionic non-substituted form of pyrrole. Noticeably, Mg interacts exclusively with the N atoms of the pyrrolyl ligands, forming strong σ-bonds, whereas Na prefers to engage with the π-systems of both NC₄-rings.

Full text of DOI:10.1039/c3dt52639a

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Probing the metallating ability of a polybasic
sodium alkylmagnesiate supported by a bulky
bis(amido) ligand: deprotomagnesiation reactions
of nitrogen-based aromatic substrates†
Cite this: Dalton Trans., 2014, 43,
4361
David R. Armstrong,a William Clegg,b Alberto Hernán-Gómez,a Alan R. Kennedy,a
Zoe Livingstone,a Stuart D. Robertson,a Luca Russob and Eva Hevia*a
Exploring the reactivity of sodium butylmagnesiate reagent [{Na(THF)6}+{(Ph2Si(NAr*)2)Mg(Bu)(THF)}−] (1)
supported by the bulky chelating silyl(bisamido) ligand {Ph2Si(NAr*)2}2− (Ar* = 2,6-iPr2-C6H3) towards
N-methylbenzimidazole (bImMe), pyrrole and 2,6-diisopropylaniline (NH2Ar*), this study provides new
insights into the ability of this bimetallic base to facilitate direct Mg–H exchange reactions as well as to
exhibit polybasicity. Thus 1 effectively promotes the deprotomagnesiation of bImMe under mild reaction
conditions to give the α-metallated intermediate [{Na(THF)5}2+{(Ph2Si(NAr*)2)Mg(bImMe*)}2
] (2)
−
(bImMe* = 2-N-methylbenzimidazolyl). Analysis of crystallographic and NMR data of 2 combined with
DFT calculations show that the metallated C in the bImMe* ligands possesses a significant carbenic
character. Contrasting with previous studies of benzothiazole (btz), 1 does not react with an excess of
bImMe even under forcing refluxing conditions. Contrastingly, the amination reactions of equimolar
amounts of 1 with pyrrole and 2,6-diisopropylaniline allowed the isolation of [{(Ph2Si(NAr*)(NHAr*))Mg-
(NC4H4)2(THF)Na(THF)2}] (3) and [{Na(THF)6}+{(Ph2Si(NAr*)(NHAr*))Mg(NHAr*)2(THF)}−] (4) respectively
as crystalline solids. Highlighting the ability of 1 to act as a polybasic reagent, 3 and 4 are formed as the
result of the deprotonation of two molecules of the relevant amine via its butyl group and one amido
arm of the silyl(bisamido) ligand. Similarly, the reactions of 1 with 3 molar equivalents of the relevant
amine yielded homoleptic tris(amido) compounds [(THF)2NaMg(NC4H4)3] (5) and [{Na(THF)6}+{Mg-
(NHAr*)3}−] (7), with the concomitant formation of bis(amine) Ph2Si(NHAr)2, as a result of the complete
amination of 1 using its three basic sites. The structures in the solid state of 3 and 4 were elucidated by
X-ray crystallography. Despite their similar constitution, these heteroleptic tris(amido)magnesiates adopt
contrasting structures, with the former displaying a contacted ion-pair structure, where Na and Mg are
connected by two bridging pyrrolyl anions, whereas the latter gives rise to a solvent-separated ion pair
motif. To the best of our knowledge 3 represents the first crystallographically characterized magnesium
compound containing an anionic non-substituted form of pyrrole. Noticeably, Mg interacts exclusively
with the N atoms of the pyrrolyl ligands, forming strong σ-bonds, whereas Na prefers to engage with the
π-systems of both NC4-rings.
Received 23rd September 2013,
Accepted 23rd October 2013
DOI: 10.1039/c3dt52639a
www.rsc.org/dalton
Introduction
The synthesis of magnesium compounds supported by steri-
cally demanding ligands, such as β-diketiminates, guanidi-
aWestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde,
Glasgow G1 1XL, UK. E-mail: eva.hevia@strath.ac.uk
bSchool of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
†Electronic supplementary information (ESI) available: CIF files giving crystallo-
graphic results, experimental and computational details and proposed
nates, N-heterocyclic carbenes or bis(amides), is attracting
increasing widespread interest.1 Providing a protective steric
shelter, the use of these ligands has been instrumental in
recent landmarks in Mg chemistry, which include the isolation
of the first stable Mg–Mg bonded compounds,2 as well as the
trapping of novel high nuclearity magnesium-hydride clusters,
having important implications as models for hydrogen
mechanism for the formation of [Na2Mg2(L1)2(THF)5] (Scheme 1)]. CCDC
960101–960103. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt52639a
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4361–4369 | 4361

View Article Online
Paper
Dalton Transactions
storage.3 Within the context of catalysis, magnesium com- at its C2 position in quantitative yields.10 Our initial studies
plexes supported by β-diketiminate ligands have proved to be show that rather than act as a mere spectator, the bis(amido)
extremely active single-site initiators for polymerisation of rac- ligand {Ph2Si(NAr*)2}2− plays a prominent role in facilitating
lactide4 as well as efficient precatalysts for intramolecular this cascade of fast intramolecular reactions, acting as a base
hydroamination reactions of a wide range of aminoalkenes.5 through both its amido arms with concomitant formation of
Contrastingly, the effects bulky ligands may have on the reac- bis(amine) Ph2Si(NHAr*)2 (Scheme 1).9 Furthermore, it
tivity/structures of alkali-metal magnesiate compounds remain appears that the deprotonation of the first molecule of btz, by
largely unstudied. This is particularly surprising as members the Bu base in 1, is the rate determining step in the process.
of this mixed-metal family are versatile and efficient polar Further evidence supporting this proposal has been found by
organometallic reagents, finding extensive applications in investigating the reactivity of btz with β-diketiminate stabilised
many important synthetic transformations, including nucleo- magnesium reagents, which show that a closely related
philic alkylation, magnesium–halogen exchange or deprotona- cascade activation process is also in operation when a kineti-
tion (C–H bond breaking) reactions, to name a few.6 By cally resilient butyl derivative is employed, although in this
operating synergistically, alkali-metal magnesiates can exhibit case the β-diketiminate ligand acts as a steric stabiliser rather
superior chemo- and regioselectivities and/or functional group than a base.11
tolerances to conventional monometallic reagents. Recent
Stimulated by these intriguing initial findings and aiming
breakthroughs in this area include Knochel’s turbo Grignard to gain a better understanding of the metallating ability of
reagents RMgCl·LiCl, whose enhanced nucleophilicity allows sodium magnesiate 1 and its involvement in the above men-
direct magnesiation (via Mg–halogen exchange reactions) of tioned btz activation process, herein we extend our reactivity
aromatic molecules with sensitive organic functional groups, studies towards the related 1,3-benzoazole N-methylbenzimid-
without the need for cryogenic conditions,7 as well as the use azole (bImMe) as well as exploring its amination reactions with
of heterobimetallic amides in alkali-metal-mediated magnesia- the amines pyrrole and 2,6-diisopropylphenylaniline, NH2Ar*.
tion (AMMMg) processes to facilitate in certain cases the regio-
selective deprotonation of organic substrates in remote
positions, not available using single-metal bases.8 We recently
Results and discussion
reported the synthesis of the solvent-separated sodium mag-
nesiate [{Na(THF)6}+{(Ph2Si(NAr*)2)Mg(Bu)(THF)}−] (1) result-
Direct magnesiation of N-methylbenzimidazole
ing from the reaction of homoleptic NaMgBu3 with the silyl-
bis(amine) precursor Ph2Si(NHAr*)2, which contains bulky 2,6-
diisopropylphenyl substituents (Ar*).9 On treatment with benzo-
thiazole (btz), 1 is transformed to the complex molecular
assembly [Na2Mg2(L1)2(THF)5], where L1 is a novel trianionic
ligand resulting from the deprotonation and coupling of three
btz molecules, one of which ring opens (Scheme 1; the pro-
posed mechanism for this cascade process involving magne-
siation, C–C coupling, ring opening, nucleophilic addition and
intramolecular deprotonation is depicted in Scheme S1 in ESI
file†). The unique outcome of this activation process contrasts
sharply with the straightforward reactivity reported for more
conventional organomagnesium reagents such as Hauser
bases or Grignard reagents, which can readily deprotonate btz
We started our studies by studying the reaction of sodium
magnesiate
with N-methylbenzimidazole bImMe whose
1
hydrogen atom at the C2 position between the N atoms is sub-
stantially less acidic than that in benzothiazole (calculated pKa
values, 32.5 for bImMe vs. 27.5 for btz).12 The room-tempera-
ture 1 : 1 reaction in THF solution afforded a pale green solu-
tion which on cooling deposited a crop of yellow crystals of
+
[{Na(THF)5}2 {(Ph2Si(NAr*)2)Mg(bImMe*)}2−] (2) (bImMe* = 2-N-
methylbenzimidazolyl) in a 71% isolated yield (Scheme 2).
X-ray crystallographic studies of 2 confirmed that C2-mag-
nesiation of bImMe has taken place, giving rise to a solvent sep-
arated ion pair complex containing two sodium cations, each
solvated by five THF molecules, and a novel dinuclear dianion
comprising two {(Ph2Si(NAr*)2)Mg} fragments connected by
two bridging N-methylbenzimidazolyl bImMe* ligands. These
ligands coordinate in an asymmetric fashion via their metal-
lated carbon and one of their N atoms (C1 and N1 in Fig. 1).
This distinct bonding mode closes
a
six-membered
{MgCNMgCN} ring, which is fused through each of its Mg
Scheme 1 Magnesium-mediated cascade activation of btz.
4362 | Dalton Trans., 2014, 43, 4361–4369
Scheme 2 Direct magnesiation of bImMe using sodium magnesiate 1.
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
for Mg–N-heterocyclic carbene complexes [ranging from
2.200(2) to 2.288(5) Å].14 Closely related, seminal studies by
Boche et al. on the structure of lithiated 1-methyl-4-tert-butyl-
imidazole have also established the significant carbenic charac-
ter of the imidazolyl ligands, by eloquently comparing the
structural differences of this compound with those found for
imidazole and for the related NHC 1,3-bis(1-adamantyl-imid-
azol-2-ylidene).15,16 Interestingly, the coordination flexibility of
imidazolyl ligands has recently been highlighted by Hayton
et al. for the synthesis of bimetallic compounds containing
uranyl-nacnac fragments, where the binding modes of these
ligands to U (as carbanions or N donors in their carbene
resonance form) can be finely controlled by modifying the
electronic properties of the accompanying metal.17 Contrasting
with this tunability, computational studies on the tauto-
merization of free imidazole to its NHC form show that this
Fig. 1 Structure of the anion of 2 with displacement ellipsoids drawn at
the 50% probability level and hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (°): Mg–N1 2.091(2), Mg–C1A
2.233(3), Mg–N3 2.028(2), Mg–N4 2.160(2), C1–N1 1.362(3), C1–N2
1.380(3), N3–Mg–N4 76.74(4), N3–Mg–N1 118.00(9), N1–Mg–C1A 102.91(10),
N4–Mg–C1A 124.85(9), N3–Mg–C1A 116.18(10), N4–Mg–N1 118.32(9), process is energetically unfavoured, being endothermic by
Si–N4–Mg 89.54(9), MgA–C1–N2 123.14(18), MgA–C1–N1 129.24(19),
almost 30 kcal mol−1 18
However, it should be noted that by
using a stepwise strategy of sequential deprotonation and elec-
trophilic interception, recent studies in d-block chemistry have
.
N1–C1–N2 106.9(2). The suffix A denotes an inversion-related atom.
vertices to a four-membered {MgNSiN} ring, resulting from the revealed the effective transformation of N-alkyl imidazoles
coordination of the bis(amide) ligand to Mg. The distorted tetra- coordinated to transition metal fragments to N-heterocyclic
hedral geometry of Mg [bond angles ranging from 76.74(4) carbenes.19
to 124.85(9)°; mean 109.41°] indicates an approximately
Solubility in deuterated THF enabled 2 to be characterized
orthogonal disposition of this 4-6-4 fused ring system. This in solution using 1H and 13C NMR spectroscopy (see the
structural motif has a strong resemblance with that recently Experimental section for details). The most informative reso-
reported for deprotonation of bImMe using a homometallic Mg nance in the 13C NMR spectrum appears at 194.0 ppm for the
amido base supported by the β-diketiminate ligand nacnac C2 carbon of bImMe* which is significantly downfield com-
(nacnac = Ar*NC(Me)CHC(Me)NAr*, Ar* = 2,6-i-Pr2-C6H3) as pared to the C2 resonance observed in free bImMe (142.7 ppm)
it also exhibits a dimeric arrangement with a central six- and also noticeably different from that reported for the
membered ring,11 although in this case no discussion of its metallated C of an α-magnesiated N-methylindole species
geometrical parameters was possible due to the low precision of (181.1 ppm).20 Supporting the interpretation of the solid-state
the structure. A comparison of Mg–Nbis(amido) bond distances studies, reflecting the markedly carbenic character of the
in 2 shows a noticeable asymmetry in the coordination of this bImMe* ligands in 2, this chemical shift corresponds more
chelating ligand to the Mg centre [Mg–N4, 2.160(2) Å vs. Mg– closely to those previously reported for the Ccarbene in NHC–Mg
N3, 2.028(2) Å], which could be attributed to the steric con- complexes such as adduct nBu8Mg4·2IPr (IPr
= 1,3-bis-
straints imposed by the rigid tricyclic structure of 2. Interest- (2,6-diisopropylphenyl)imidazol-2-ylidene) which resonates at
ingly, it should also be noted that the Mg–NbImMe* bond 197.3 ppm.14f
length in 2 [Mg–N1, 2.091(2) Å] lies in the same range of these
Remarkably both 1H and 13C NMR spectra of 2 displayed
Mg–Namido values, and is considerably shorter than that well resolved signals with no detectable resonances that could
reported in the related Mg complex [(nacnac)Mg(Bu)(bImMe)] be assigned to a ring-open α-(isocyano)methylanilide species,
[Mg–N1, 2.144(2) Å],11 where bImMe coordinates as a neutral resulting from the cleavage of the C–NMe bond of the bImMe*
N-donor to Mg, hinting at the presence of at least some partial groups. This contrasts with previous NMR studies carried out
negative character of the nitrogen atom. Moreover, despite the for 2-lithiated-benzimidazole which show the existence of an
sp2 character of C, the Mg–C bond distance [Mg–C1A, 2.233(3) Å] equilibrium at room temperature between its ring-opened and
is distinctly elongated compared to that of the butyl precur- ring-closed isomers.21
sor 1 which also contains a similar pseudotetrahedral Mg
To probe the energetics involved in the formation of 2,
environment [Mg–CBu, 2.124(3) Å].9 This, coupled with the theoretical calculations at the DFT level employing the B3LYP
subtle but noticeable variation of the N–C bonds of the imid- method22 and the 6-311G** basis set were used to model the
azolyl rings [C1–N1, 1.362(3) Å; C1–N2, 1.380(3) Å] when com- metallation reaction of bImMe by sodium magnesiate 1 (see
pared to those of related 5-amino-1-methyl-benzimidazole ESI† for details), showing that the formation of 2 (modelled as
[1.3785(18) and 1.350(2)]13 suggest that these bridging ligands monomer 2A) is favoured by −13.9 kcal mol−1 23
This energy
.
exhibit a noticeable carbenic character, and therefore 2 can be gain is even more substantial when considering the dimeric
envisaged as a 1-methyl-3-[magnesium bis(amido)]benzimid- structure exhibited by 2 in the solid state, as the dimerization
azol-2-ylidene dimer. Supporting this interpretation, the Mg–C of 2A to give dimer 2B was estimated to be exothermic by
bond distance in 2 is within the range of other values reported −71.4 kcal mol−1 24
.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4361–4369 | 4363

View Article Online
Paper
Dalton Transactions
Comparing the calculated geometrical parameters of 2B neighbouring monomer. Consistent with its carbene reso-
with those determined experimentally from the X-ray crystallo- nance form, the difference between the calculated N–C bond
graphic studies of 2 shows, in general, an excellent agreement distances of the imidazolyl ring in 2A1 is significantly more
(see Table 1 for selected bond distances and angles), with a subtle [Δ(dC–N–dCNMe) = 0.022 Å] than that computed for 2A2
slight overestimation for the calculated Mg–N1 bond [2.144 Å [Δ(dC–N–dCNMe
) = 0.067 Å], supporting a more localised
(calc.) vs. 2.091(2) Å (exp)]. Interestingly, when comparing the N–CvNMe bonding scenario for the latter, similar to that calcu-
relative stabilities of the two possible isomers of monomeric lated for the parent N-methylbenzimidazole (see ESI† for
2A, as an N-metallacarbene (2A1) or its carbanionic resonance details, Δ(dC–N–dCNMe) = 0.073 Å). Furthermore, a comparison
(2A2) (Fig. 2), it was found that the former is negligibly more of these theoretical values with that found experimentally in 2
stable (by 1.6 kcal mol−1), which supports previous experi- [Δ(dC1–N2–dC1N2) = 0.018 Å] offers further support to the carbe-
mental findings on the bonding flexibility of imidazolyl nic character attributed to its bImMe* bridges on the basis of
ligands when coordinated to metal centres. It should be noted the X-ray crystallographic and NMR spectroscopic studies.
that in model 2A1, the imidazolyl ligand coordinates to mag-
The regioselective C2-magnesiation of bImMe by 1 differs
nesium in an η2-fashion through its C and N atoms, forming a significantly from the reactivity previously reported for related
relatively short Mg–N bond with the imidazolyl ligand 1,3-benzoazole, btz, where the three potential basic arms of 1
(2.099 Å), which is comparable with the Mg–N bond distance react with the heterocycle, even when 1 : 1 mixtures of these
found experimentally in 2 (2.091(2) Å) as well as a Mg–C starting materials are employed, to form the complicated acti-
contact (2.163 Å) which is slightly elongated compared to that vation product [Na2Mg2L2(THF)5] depicted in Scheme 1.9 Fur-
calculated for 2A2 (2.133 Å) where Mg binds solely to the thermore, no further metallation was observed when isolated
carbon of the bImMe* ligand. Related η2-(N,C)-coordination has crystals of 2 were confronted with a 2 molar excess of bImMe
,
been experimentally observed for the structures of lithiated even when the mixture was heated at 65 °C over a 5 hour
1-methyl-4-tert-butylimidazole and benzothiazole.15 Dimeriza- period, evidencing the resilience of the amido groups of the
tion of 2A1 to form 2B results in the cleavage of this {Ph2Si(NAr*)2} ligand to promote the deprotonation of bImMe
.
Mg–C bond to generate a new bond with the Mg centre of a Notwithstanding, considering that for the activation of btz
the rate-determining step appears to be its metallation by the
butyl group 1, 2 can be envisaged as an indirect model for the
first intermediate involved in this cascade activation process,
which in turn reacts with two further equivalents of btz to
form the final product (Scheme 1 and S1 in ESI†). Interest-
Table 1 Comparison of selected calculated bond distances (Å) and
angles (°) for model 2B with those from the X-ray crystallographic data
of 2
2
2B
ingly, it should also be noted that the exclusive alkyl basicity
displayed by 1 diverges from the reactivity reported for related
homometallic magnesium β-diketiminate [(nacnac)Mg(Bu)-
Mg–N1
Mg–C1A
Mg–N3
Mg–N4
C1–N1
C1–N2
2.091(2)
2.233(3)
2.028(3)
2.160(2)
1.362(2)
1.380(3)
2.144
2.270
2.082 (THF)], which fails to metallate, even under refluxing con-
2.210
1.357
1.384
ditions, leading to the isolation of a simple coordination
product, resulting from substitution of the THF molecule co-
ordinated to Mg by a molecule of bImMe, acting as a neutral 2
electron-donor.11 The enhanced metallating ability of 1 over
N1–Mg–C1A
N3–C1–N1
N3–Mg–N4
102.9(10)
118.00(9)
76.74(4)
100.8
107.6
76.4
neutral [(nacnac)Mg(Bu)(THF)] can be rationalised in terms of
its bimetallic anionic ate constitution, which enables the acti-
vation of an otherwise kinetically retarded butyl group.25 Illus-
trating this activating effect, Mulvey has reported the high-
yielding room temperature α-magnesiation of N-methylindole
using sodium tetrabutylmagnesiate [(TMEDA)2NaMgBu4],
where each Bu group reacts with one equivalent of the sub-
strate, whereas MgBu2 on its own fails to deprotonate this
same heterocycle.20
Extension to pyrrole and 2,6-diisopropylaniline: polybasic
behaviour of magnesiate base
In order to explore the possibility of 1 acting as a polybasic
reagent, we next turned our attention to the heterocyclic amine
pyrrole. Its N–H group has a pKa value of 23 making it signifi-
cantly more acidic than the C2–H of bImMe 26
Surprisingly, we
.
found that even when equimolar amounts of this substrate
and mixed-metal base 1 were employed, compound [{(Ph2Si-
(NAr*)(NHAr*))Mg(NC4H4)2(THF)Na(THF)2}] (3), resulting from
Fig. 2 Modelled structures and selected calculated bond distances and
angles for the anionic moieties of monomeric 2A1 and 2A2.
4364 | Dalton Trans., 2014, 43, 4361–4369
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Scheme 3 Amination reactions of 1 with pyrrole and 2,6-diisopropylaniline.
Fig. 3 Molecular structure of 3 with displacement ellipsoids drawn at the
50% probability level and hydrogen atoms (except NH) and minor disorder
components in THF ligand are omitted for clarity. Selected bond lengths (Å)
the deprotonation of two molecules of pyrrole, via the butyl
group and amido arm of the bis(amido) ligand present in 1, and angles (°): Na–O3 2.265(2), Na–O2 2.294(16), Na–N3 2.635(2), Na–N4
2.796(2), Na–C37 2.721(3), Na–C38 3.002(3), Na–C39 3.066(3), Na–C40
2.837(3), Na–C41 3.016(3), Na–C42 3.137(3), Na–C43 2.960(3), Na–C44
2.729(3), Mg–O1 2.033(17), Mg–N1 2.015(18), Mg–N3 2.0514(19), Mg–N4
2.060(2), N1–Mg–O1 115.23(7), N1–Mg–N3 114.99(8), O1–Mg–N3 99.54(7),
was obtained in a 17% isolated yield (Scheme 3) as a crystal-
line product. Compound 3 was characterized by multinuclear
1H and 13C NMR spectroscopy and its structure elucidated by
X-ray crystallography, which confirmed its bimetallic consti-
tutions (see the Experimental section and ESI† for details).
Interestingly, when the rational synthesis of 3 was attempted
by reacting butylmagnesiate 1 with two molar equivalents of
pyrrole at room temperature, compound 3 was obtained as a
crystalline solid along with variable amounts of tris(amido)
[(THF)2NaMg(NC4H4)3] (5), with the later resulting from the
complete amination of 1 involving its three potentially basic
arms. The formation of this compound can be significantly
minimised by carrying out the addition of the amine at 0 °C,
which affords 3 in an improved 57% yield (see the Experi-
mental section for details).27 Tris(pyrrolyl)magnesiate 5 was
N1–Mg–N4 123.27(8), O1–Mg–N4 100.61(8), N3–Mg–N4 99.46(8).
1
characterised by H and 13C NMR spectroscopy and could be
prepared and isolated in a 49% yield by treating 1 with
3 equivalents of pyrrole (see Table S2 and ESI† for details).28
Highlighting the complexity of these amination reactions,
extending these studies to the primary amine 2,6-diisopropyl-
aniline, led to the formation of complicated mixtures of
products. Thus, the reaction of equimolar amounts of this
aniline and 1 deposited a few colourless crystals of the double
Fig. 4 Structure of anion moiety in 4 with displacement ellipsoids
drawn at the 50% probability level and hydrogen atoms (except NHs) are
omitted for clarity.
amination product [{Na(THF)6}+{(Ph2Si(NAr*)(NHAr*))Mg- around Mg 108.85° for 3 and 108.88° for 4), being bound by
(NHAr*)2(THF)}−] (4),29 the structure of which was determined in two newly formed amide ligands as well as terminal amido-
the solid state by X-ray crystallography (Fig. 4). However, 1H NMR (silyl)amine {Ph2Si(NAr*)(NHAr*)} and a solvating molecule of
analysis of the solution filtrate showed that the major product THF, 3 displays a contacted-ion pair structure, where Na and
of the reaction was [{Na(THF)6}+{(Ph2Si(NAr*)2)Mg(NHAr*)- Mg are connected by two pyrrolyl bridges, whereas in 4, Na is
(THF)}−] (6), resulting from the reaction of the butyl group of 1 solvated by six molecules of THF, giving rise to a solvent-separ-
with the amine (Scheme 3). Contrastingly, 1H NMR monitoring ated ion pair motif. In both cases the new amine NHAr*
of the reaction of 1 with two molar equivalents of the aniline group, generated by protonation of the chelating bis(amido)
afforded a complex mixture of the three possible amination ligand of 1, does not interact with the Mg centre, which
products 4, 6 and tris(anilido) [{Na(THF)6}+{Mg(NHAr*)3}−] (7) instead prefers coordination by a molecule of the donor
in a 1 : 2.1 : 1.1 ratio,30 whereas when three equivalents of solvent THF. This coordination preference contrasts with that
NH2Ar were employed, compound 1 was quantitatively con- found in the lithium dimer [{Ph2Si(2,6-Et2C6H3NH)(2,6-
verted into tris(anilido) 7 and bis(amine) Ph2Si(NHAr2).31
Et2C6H3NLi)}2] which contains a similar amido(silyl)amine,
Despite their similar tris(amido) constitution, sodium mag- bonding in a chelating fashion via its two N atoms to Li.32
nesiates 3 and 4 exhibit remarkably different structures in the Unfortunately, the structure of 4 was derived from a twinned
solid state (Fig. 3 and 4, respectively). Although in both cases and weakly diffracting crystal sample and also features con-
Mg displays a similar coordination environment, adopting a siderable disorder with respect to the THF groups. Thus,
distorted four-coordinate tetrahedral geometry (mean angle despite several attempts at data collection, the final model is
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4361–4369 | 4365

View Article Online
Paper
Dalton Transactions
of a relatively low precision, which prevents discussion of any under substoichiometric conditions, using its amido arms.
geometrical parameters, although its connectivity is definite.33 Thus rather than act as a spectator, providing a steric shield to
The structure of 3 is notable for having distinct bonding pre- the Mg centre, this bis(amido) ligand can also play a promi-
ferences of Na and Mg to the pyrrolyl anions. Thus, while Mg nent role in facilitating the metallation of more than one mole-
coordinates exclusively in a σ-manner forming relatively short cule of the substrate by the bimetallic base, which in some
(strong) Mg–N bonds [Mg–N3, 2.0514(19) Å and Mg–N4 2.060(2) Å], cases can lead to a Mg centre coordinating simultaneously to
Na engages with the π-systems of both pyrrolyl NC4-rings. several sensitive anions, facilitating activation processes like
The pattern of the bond distances in these Na⋯π-face inter- the one depicted in Scheme 1 for btz.
actions denotes η5-NC4 coordination, with noticeably shorter
Na⋯N contacts [Na–N3, 2.635(2) Na–N4, 2.796(2) Å] than the
remaining Na⋯C [ranging from 2.721(3) to 3.066(3) Å and from
Conclusions
2.729(3) to 3.137(3) Å for each NC4 ring], which can be attribu-
This study demonstrates that sodium magnesiate 1, which is
ted to the more electron-rich character of the amido N atoms.
supported by the bulky chelating bis(amido) ligand {Ph2Si-
Furthermore, in order to maximize this Na⋯π-surface bonding,
(NAr*)2}, can effectively promote direct Mg–H exchange reac-
both pyrrolyl rings are tilted towards the Na cation, as evi-
tions. Thus, the metallation products of its reaction with
denced by the narrow N3–Mg–N4 bond angle [99.46(8)°].
bImMe and the amines pyrrole and 2,6-diisopropylaniline, 2, 3
Reflecting the isoelectronic and close structural relationship
and 4, respectively, have been isolated and structurally eluci-
dated. Contrasting with our previous reactivity studies with the
between pyrrolyl and cyclopentadienyl (C5H5, Cp) rings, the η5-
NC4 π-bonding present in 3 bears a strong resemblance to that
found in other NaCp species which have been structurally eluci-
related 1,3 benzoazole btz, 1 acts exclusively as a butyl base
towards bImMe
,
facilitating its selective C2-metallation
dated, as for example polymeric [{Na(DME)(η5-C5H5}∞] (DME =
1,2-dimethoxyether).34 Interestingly, a similar σ/π bonding dis-
tinction to that observed in 3 has been recently described by
Mulvey et al. for a series of sodium-zincate pyrrolyl complexes.35
Contrastingly, this earlier study reveals that, in the homometal-
lic sodium derivative [{PMDETA)Na(NC4H4)}2], the Na cation
interacts only with the N atoms of the pyrrolyl units, avoiding
any possible π-contact with the aromatic ring,35 which suggests
that for heterobimetallic 3, the metal–N σ bonding preference
of the NC4H4 is dictated by the stronger Lewis acidity of Mg in
comparison to Na. As far as we can ascertain, 3 represents the
first crystallographically characterized magnesium compound
containing an anionic non-substituted form of pyrrole.36
affording 2. X-ray crystallographic studies of 2 established its
dimeric constitution, containing an unusual dinuclear mag-
nesiate dianion, where the benzimidazolyl ligands coordinate
asymmetrically through their metallated C and N atoms to the
Mg centres. Analysis of its crystallographic and NMR spectro-
scopic data coupled with DFT studies revealed that the Mg–C
atoms of the bImMe* fragments of 2 display a significant carbe-
nic character. Reactivity studies show that even under forcing
refluxing conditions and using an excess of bImMe, the poten-
tially basic arms of the bis(amido) ligand in 1 are unable to
deprotonate this N-heterocyclic molecule. Notwithstanding, 2
can be envisaged as a model for the first intermediate pro-
posed to be involved in the cascade activation of btz mediated
by 1, where the rate-determining step appears to be the depro-
tonation of btz by the butyl group of 1. Establishing the ability
of 1 to execute polybasicity, its reactions with equimolar
amounts of the amines pyrrole and 2,6-diisopropylaniline led
to the isolation of the double amination products 3 and 4 as
crystalline solids. Varying the stoichiometry of these reactions
has shown that complete amination of 1, using its butyl group
and two amido arms of the bulky chelating ligand, can be
accomplished using 3 equivalents of the relevant amine, allow-
ing the isolation of homoleptic tris(amido)magnesiates 5 and 7.
Collectively these findings shed new light on the reactivity
of 1, unveiling its potential to execute selective deprotomagne-
siation reaction, as well as providing new mechanistic insights
into its role in the cascade activation of btz, where 1 uses all
three of its basic arms to formally deprotonate three molecules
of this heterocycle.
1
The H NMR spectrum of 3 in deuterated benzene solution
showed a broad singlet at 4.10 ppm for the NH group and two
i
diagnostic multiplets for the CH groups of the Pr substituents
of the amido(silyl)amine {Ph2Si(NAr*)(NHAr*)} ligand at 4.48
and 2.79 ppm which can be assigned to its amido NAr* and
amine NHAr* arms, respectively.37 Reflecting the increased
negative charge of the N centres of the pyrrolyl ligands in 3 in
comparison with the N–H unit in pyrrole, the signals attributed
1
to the α-positions of the heterocycle in both H and 13C NMR
spectra appear significantly more shielded (at 6.50 and
136.6 ppm, respectively) than to those found for free pyrrole (at
6.37 and 117.5 ppm, respectively).38 This effect is even more pro-
nounced for tris(amido) [(THF)2NaMg(NC4H4)3] (5) whose α-H′s
appear at 7.69 ppm in its 1H NMR spectrum (Table S2, ESI†).
Collectively, these findings from the amination reactions of
1 demonstrate the ability of the bis(amido) ligand {Ph2Si-
(NAr)2} present within sodium magnesiate 1 to participate in
Mg–H exchange processes, converting, in this case, the N–H
bonds of either pyrrole or 2,6-diisopropylaniline, into new
Mg–N bonds. Furthermore, the fact that these conversions
Experimental section
take place even with equimolar amounts of 1 and the relevant Full experimental and computational details are included in
amine shows that depending on the activation level of the the ESI.† CCDC 960101–960103 contain the supplementary
substrate employed, 1 can execute polybasic behaviour even crystallographic data of this paper.
4366 | Dalton Trans., 2014, 43, 4361–4369
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Synthesis of [{Na(THF)5}2 {(Ph2Si(NAr*)2)Mg(bImMe*)}2−] (2)
+
Notes and references
1-Methylbenzimidazole (0.13 g, 1 mmol) was added to a solu-
tion of isolated crystals of 1 (1.14 g, 1 mmol) in THF (3 ml)
affording a pale green solution that was stirred at room temp-
erature for 2 hours. Hexane (5 ml) was added and the resulting
solution was transferred to a freezer (−30 °C). After 48 hours, a
1 For selected references see: (a) L. T. Evans, M. P. Coles,
F. G. N. Cloke and P. B. Hitchcock, J. Organomet. Chem.,
2007, 692, 2548; (b) M. Arrowsmith, M. S. Hill, D. MacDougall
and M. F. Mahon, Angew. Chem., Int. Ed., 2009, 48, 4013;
(c) S. J. Bonyhady, D. Collis, G. Frenking, N. Holzmann,
C. Jones and A. Stasch, Nat. Chem., 2010, 2, 865;
(d) B. M. Day, N. E. Mansfield, M. P. Coles
and P. B. Hitchcock, Chem. Commun., 2011, 47, 4995;
(e) M. Arrowsmith, M. R. Crimmin, M. S. Hill, S. L. Lomas,
D. J. MacDougall and M. F. Mahon, Organometallics, 2013,
32, 4961.
2 (a) S. P. Green, C. Jones and A. Stasch, Science, 2007, 318,
1754; (b) A. Stasch and C. Jones, Dalton Trans., 2011, 40,
5659.
3 (a) S. Harder, J. Spielmann, J. Intemann and H. Bandmann,
Angew. Chem., Int. Ed., 2011, 50, 4156; (b) J. Intemann,
J. Spielmann, P. Sirsch and S. Harder, Chem.–Eur. J., 2013,
19, 8478.
4 For selected references see: (a) M. H. Chisholm,
J. C. Huffman and K. Phomphrai, J. Chem. Soc., Dalton
Trans., 2001, 222; (b) B. M. Chamberlain, M. Cheng,
D. R. Moore, T. M. Ovitt, E. B. Lobkovsky and G. W. Coates,
J. Am. Chem. Soc., 2001, 123, 3229; (c) M. H. Chisholm,
J. C. Gallucci and K. Phomphrai, Inorg. Chem., 2002, 41,
2785; (d) A. P. Dove, V. C. Gibson, E. L. Marshall,
A. J. P. White and D. J. Williams, Dalton Trans., 2004, 570;
(e) F. Drouin, T. J. J. Whitehorne and F. Schaper,
Dalton Trans., 2011, 40, 1396.
5 (a) M. R. Crimmin, M. Arrowsmith, A. G. M. Barrett,
I. J. Casely, M. S. Hill and P. A. Procopiou, J. Am. Chem.
Soc., 2009, 131, 9670; (b) A. G. M. Barrett, I. J. Casely,
M. R. Crimmin, M. S. Hill, J. R. Lachs, M. F. Mahon
and P. A. Procopiou, Inorg. Chem., 2009, 48, 4445;
(c) A. G. M. Barrett, M. R. Crimmin, M. S. Hill and
P. A. Procopiou, Proc. R. Soc. London, Ser. A, 2010, 466, 927.
6 For reviews in the area see: (a) R. E. Mulvey, Organo-
metallics, 2006, 25, 1060; (b) R. E. Mulvey, F. Mongin,
M. Uchiyama and Y. Kondo, Angew. Chem., Int. Ed., 2007,
46, 3802; (c) R. E. Mulvey, Acc. Chem. Res., 2009, 42, 743;
(d) B. Haag, M. Mosrin, H. Ila, V. Malakhov and P. Knochel,
Angew. Chem., Int. Ed., 2011, 50, 9794; (e) R. E. Mulvey,
Dalton Trans., 2013, 42, 6676; (f) A. Harrison-Marchand
and F. Mongin, Chem. Rev., 2013, 113, 7470;
(g) A. Harrison-Marchand and F. Mongin, Chem. Rev., 2013,
113, 7563.
1
crop of yellow crystals was isolated (0.76 g, 71%). H NMR (d8-
THF, 298 K) δ 7.76 [d, 4H, Ph], 6.99 [m, 6H, Ph], 6.95 [m, 1H,
MeBIm*], 6.94 [m, 1H, bImMe*], 6.88 [d, 4H, Ar*], 6.59 [t, 2H,
Ar*], 6.33 [m, 1H, bImMe*], 6.12 [t, 1H, bImMe*], 4.10 [m, 4H,
i
CH, Pr, Ar*], 3.61 [m, 20H, OCH2, THF], 3.51 [s, 3H, CH3,
i
bImMe*], 1.76 [m, 20H, CH2, THF], 0.78 [d, 24H, CH3, Pr, Ar*].
13C{1H} NMR (d8-THF, 298 K) δ 194.0 [C–Mg], 155.9, 155.3,
148.5, 145.4, 143.4 [Cquarternary, Ph, Ar* and bImMe*], 136.9
[CH, Ph], 127.6 [CH, Ph], 127.4 [CH, Ph], 123.8 [CH, Ar*],
123.1, 121.0, 120.6 [CH, bImMe*], 117.5 [CH, Ar*], 116.8 [CH,
i
bImMe*], 68.2 [O–CH2, THF], 34.6 [CH3, bImMe*], 29.3 [CH, Pr,
Ar*], 27.3 [CH3, iPr, Ar*], 26.3 [CH2, THF].
Synthesis of [{(Ph2Si(NAr*)(NHAr*))Mg(NC4H4)2(THF)Na-
(THF)2}] (3)
Pyrrole (0.07 ml, 1 mmol) was added to a solution of sodium
magnesiate 1 (1.14 g, 1 mmol) in hexane (5 ml), affording a
suspension which was stirred at room temperature for
30 minutes. THF (4 ml) was introduced and the suspension
gently heated until all the solid was dissolved affording a pale
yellow solution. This solution was transferred to the freezer
(−30 °C) and colourless crystals were obtained after 4 days,
which were isolated by filtration and transferred to the glove
box (isolated yield 0.155 g, 17%, note that employing this
stoichiometry, the maximum possible yield for 3, with respect
to 1 is 50%). This yield could be improved to 57% (0.530 g) when
two equivalents (0.14 ml, 2 mmol) of pyrrole were employed,
carrying out the amination reaction at 0 °C to minimize the
formation of tris(amido)magnesiate [(THF)2NaMg(NC4H4)3}]
(5).34 1H (C6D6, 298 K) δ 8.17 [d, 4H, Ph], 7.27–7.11 [12H, m,
Ph and Ar*], 6.50 [s, α-CH, MgNC4H4], 6.13 [s, 4H, β-CH,
i
MgNC4H4], 4.48 [m, 2H, CH, Pr, NAr*], 4.10 [s, 1H, NH], 3.35
i
[m, 24H, OCH2, THF], 2.79 [m, 2H, CH, Pr, NHAr*], 1.34 [d,
6H, CH3, iPr, NAr*], 1.27 [m, 24H, CH2, THF], 1.06 [d, 6H, CH3,
i
iPr, NAr*], 0.92 [d, 12H, CH3, Pr, NHAr*]. 13C{1H} NMR (C6D6,
298 K) δ149.9, 146.5, 142.9, 142.5, 139.0 [Cquarternary, Ph and
Ar*], 136.6 [CH, Ph], 128.5 [α-CH, MgNC4H4], 127.3 [CH, Ph],
127.1 [CH, Ph], 124.5, 123.6, 121.3, 120.5 [CH, Ar*], 107.0
i
[β-CH, MgNC4H4], 68.2 [O–CH2, THF], 28.2 [CH, Pr, NAr*],
i
i
27.7 [CH, Pr, NHAr*] 25.8 [CH, Pr, NAr*], 25.3 [CH2, THF],
24.23 [CH3, iPr, NHAr*], 23.6 [CH3, iPr, NAr*].
7 A. Krasovskiy and P. Knochel, Angew. Chem., Int. Ed., 2004,
43, 3333.
8 (a) P. C. Andrikopoulos, D. R. Armstrong, W. Clegg,
C. J. Gilfillan, E. Hevia, A. R. Kennedy, R. E. Mulvey,
C. T. O’Hara, J. A. Parkinson and D. M. Tooke, J. Am. Chem.
Soc., 2004, 126, 116121; (b) P. C. Andrikopoulos,
D. R. Armstrong, D. V. Graham, E. Hevia, A. R. Kennedy,
R. E. Mulvey, C. T. O’Hara and C. Talmard, Angew. Chem.,
Int. Ed., 2005, 44, 3459; (c) E. Hevia, G. W. Honeyman,
Acknowledgements
We thank the EPSRC (equipment funding to W. C.), the Uni-
versity of Strathclyde (studentship to Z. L.), the Royal Society
(University Research Fellowship to E. H.), and the European
Research Council (ERC) for the generous sponsorship of this
research.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4361–4369 | 4367

View Article Online
Paper
Dalton Transactions
A. R. Kennedy, R. E. Mulvey and D. C. Sherrington, Angew. 21 C. Hilf, F. Bosold, K. Harms, M. Marsch and G. Boche,
Chem., Int. Ed., 2005, 44, 68; (d) P. C. Andrikopoulos, Chem. Ber./Recl., 1997, 130, 1213.
D. R. Armstrong, E. Hevia, A. R. Kennedy and R. E. Mulvey, 22 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) C. Lee,
Organometallics, 2006, 25, 2415.
W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter, 1988,
37, 785; (c) S. H. Vosko, L. Wilk and M. Nusair,
Can. J. Phys., 1980, 58, 1200; (d) P. J. Stephens, F. J. Devlin,
C. F. Chabalowski and M. J. Frisch, J. Phys. Chem., 1994, 98,
11623.
9 V. L. Blair, W. Clegg, A. R. Kennedy, Z. Livingstone,
L. Russo and E. Hevia, Angew. Chem., Int. Ed., 2011, 50,
9857.
10 (a) M. Shilai, Y. Kondo and T. Sakamoto, J. Chem. Soc.,
Perkin Trans. 1, 2001, 442; (b) A. Krasovskiy, V. Krasovskaya 23 The reaction of 1 and bImMe to give monomeric 2A was
and P. Knochel, Angew. Chem., Int. Ed., 2006, 45, 2958.
modelled considering only the anionic magnesiate moi-
11 S. E. Baillie, V. L. Blair, T. D. Bradley, W. Clegg, J. Cowan,
eties of 1 (1A) and 2A, see ESI† for details.
R. W. Harrington, A. Hernán-Gómez, A. R. Kennedy, 24 This value was estimated using the resulting point energies
Z. Livingstone and E. Hevia, Chem. Sci., 2013, 4,
1895.
12 K. Shen, Y. Fu, J. N. Li, L. Liu and Q. X. Guo, Tetrahedron,
2007, 63, 1568.
of placing the Na cations 15 Å away from their anionic
counterparts, using the Mg atoms as the origin. This
approximation was made in order to simulate the solvent-
separated ion-pair structure found experimentally for 2.
13 J. Lokaj, V. Kettmann, V. Milata and T. Solčan, Acta Crystal- 25 For a related study comparing the kinetic reactivity of
logr., Sect. E: Struct. Rep. Online, 2009, 65, o1788.
Mg–C bonds vs. Mg–N bonds for the deprotonative metala-
tion of anisole using potassium magnesiate bases see:
W. Clegg, B. Conway, P. Garcia-Alvarez, A. R. Kennedy,
R. E. Mulvey, L. Russo, J. Sassmannshausen and T. Tuttle,
Chem.–Eur. J., 2009, 15, 10702.
14 For references of structurally defined magnesium com-
pounds containing NHC ligands see: (a) A. J. Arduengo III,
H. R. V. Dias, F. Davidson and R. L. Harlow, J. Organomet.
Chem., 1993, 462, 13; (b) D. Zhang and H. Kawaguchi,
Organometallics, 2006, 25, 5506; (c) P. L. Arnold, I. J. Casely, 26 pKa value determined in DMSO solution, for details see:
Z. R. Turner, R. Bellabarba and R. B. Tooze, Dalton Trans., F. G. Bordwell, Acc. Chem. Res., 1988, 21, 456.
2009, 7236; (d) M. Arrowsmith, M. S. Hill, D. J. MacDougall 27 See ESI† for synthetic and spectroscopic details of tris
and M. F. Mahon, Angew. Chem., Int. Ed., 2009, 48, 4013;
(e) A. R. Kennedy, R. E. Mulvey and S. D. Robertson, Dalton
Trans., 2010, 39, 9091; (f) A. R. Kennedy, R. E. Mulvey
and S. D. Robertson, Dalton Trans., 2010, 39, 9091;
(amido)magnesiate 5. It should be noted that even under
these reaction conditions, 1H NMR analysis of crystalline
samples of the crystalline product showed the presence of
5 in an approximately 7% ratio.
(g) A. R. Kennedy, J. Klett, R. E. Mulvey and S. D. Robertson, 28 Compound 5 was isolated as a crystalline solid along with
Eur. J. Inorg. Chem., 2011, 4675.
trace amounts of bis(amine) Ph2Si(NHAr*)2 which is the
byproduct resulting from the reaction of 1 with 3 molar
equivalents of pyrrole. 1H NMR integration indicated that
this impurity is present in an approximately 5% ratio.
15 C. Hilf, F. Bosold, K. Harms, J. C. Lohrenz, M. Marsch,
M. Schimeczek and G. Boche, Chem. Ber./Recl., 1997, 130,
1201.
16 It should be noted that, contrasting with the relatively 29 Despite numerous attempts, compound 4 could not be pre-
1
simple dimeric structure of 2, this compound exhibits a
more complex supramolecular assembly, containing 16 Li
atoms and 8 imidazolyl fragments combined with 2 oxo
and 4 alkoxo ligands. These oxygen-based anions have
been formed as a consequence of the degradation of the
ethereal solvents employed by methyl lithium, which is the
organometallic reagent used for the deprotonation of
1-methyl-4-tert-butylimidazole.
pared as a pure compound. H NMR analysis of crystalline
samples isolated from the reaction of 1 with variable
amounts of 2,6-diisopropylaniline shows the presence of 4
as a minor product along with variable amounts of 6, 7,
Ph2Si(NHAr*)2 and in some cases unreacted 1. Although
these spectra are quite complicated, with significant over-
lapping of most of the signals, the resonances for the para-
H of the NHAr* ligands are well resolved (see Fig. S1, ESI†)
and their integration can be used to estimate the ratio of
products 4, 6 and 7 present in solution.
17 M. F. Schettini, G. Wu and T. W. Hayton, Chem. Commun.,
2012, 48, 1484.
18 G. A. McGibbon, C. Heinemann, D. J. Lavorato and 30 Using 1.5 equivalents of NH2Ar* afforded a mixture of pro-
H. Schwarz, Angew. Chem., Int. Ed., 1997, 36, 478.
ducts where the single-amination product 6 was the major
species present in solution (the 4 : 6 : 7 ratio was found to
be 1.1 : 5 : 1, see Fig. S1, ESI†).
19 (a) J. Ruiz and B. F. Perandones, J. Am. Chem. Soc., 2007,
129, 9298; (b) M. A. Huertos, J. Perez, L. Riera, J. Diaz and
R. Lopez, Angew. Chem., Int. Ed., 2010, 49, 6409; (c) M. Brill, 31 [{Na(THF)6}+{Mg(NHAr*)3}−] (7) was isolated as a colourless
J. Diaz, M. A. Huertos, R. Lopez, J. Perez and L. Riera,
oil, see ESI† for synthesis and spectroscopic characteriz-
Chem.–Eur. J., 2011, 17, 8584; (d) C. H. Hsieh, R. Pulukkody
ation details.
and M. Y. Darensbourg, Chem. Commun., 2013, 49, 32 N. Palanisami and R. Murugavel, J. Organomet. Chem., 2006,
9326. 691, 3260.
20 B. Conway, E. Hevia, A. R. Kennedy and R. E. Mulvey, 33 For other related structurally elucidated magnesium amides
Chem. Commun., 2007, 2864.
containing 2,6-diisopropylanilide see: (a) M. M. Olmstead,
4368 | Dalton Trans., 2014, 43, 4361–4369
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
W. J. Grigsby, D. R. Chacon, T. Hascall and P. P. Power,
Inorg. Chim. Acta, 1996, 251, 273; (b) A. Xia, M. J. Heeg and
H. Kaneko, Y. Yang, H. Tsurugi and K. Mashima, Organo-
metallics, 2012, 31, 2268.
C. H. Winter, Organometallics, 2002, 21, 4717; (c) E. Hevia, 37 A comparison of the chemical shifts observed for the CH
A. R. Kennedy, R. E. Mulvey and S. Weatherstone, Angew.
Chem., Int. Ed., 2004, 43, 1709; (d) B. Conway, E. Hevia,
A. R. Kennedy, R. E. Mulvey and S. Weatherstone, Dalton
Trans., 2005, 1532.
resonance of the iPr groups in the 1H NMR spectra in C6D6
of bis(amine) precursor {Ph2Si(NHAr*)2} and the bis-
(amide) ligand in 1 shows a similar trend. Thus, a multi-
plet at 3.38 ppm is observed for the NHAr* fragment,
which is significantly more shielded than that found for
the NAr*amide groups (at 3.98 ppm).
34 M. L. Cole, C. Jones and P. C. Junk, Dalton Trans., 2002, 896.
35 J. A. Garden, A. R. Kennedy, R. E. Mulvey and
S. D. Robertson, Dalton Trans., 2011, 40, 11945.
36 For other examples of substituted variants see: (a) A. Xia,
M. J. Heeg and C. H. Winter, Organometallics, 2002,
21, 4718; (b) T. K. Panda, K. Yamamoto, K. Yamamoto,
38 See the Experimental section and Table S2 in ESI† for
details. It should be noted that this trend in the chemical
shifts of pyrrolyl anions has also been reported in sodium-
zincate chemistry, see ref. 35.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4361–4369 | 4369