Magnesium amido N-heterocyclic carbene complexes

Article abstract of DOI:10.1039/b803253j

Magnesium dications bind strongly to a tridentate anionic dicarbene ligand L = [N{CH₂CH₂(CNCHCHNMes)}₂] forming dinuclear and trinuclear Mg complexes with some particularly short Mg-C bonds. Treatment of the proligand H₄LCl₃ with three equivalents of methyl magnesium chloride or benzyl magnesium chloride affords Mg₃(HL) Cl₆ in high yield. A suspension of 1 in thf was heated to 80 °C for 2 h to afford Mg₂(L)Cl₃, consistent with the loss of one equivalent of MgCl₂, and the deprotonation of the remaining acidic NH, lost as HCl gas. Treatment of Mg₃(HL)Cl₆ with one equivalent of KC₈ results in deprotonation of the ligand amine NH to afford Mg₃(L)Cl₅; treatment with a second equivalent forms the radical anion of the complex, K[Mg₃(L)Cl₅], which decomposes upon storage, precluding its structural characterisation. The acidic NH proton of the ligand in Mg₃(HL)Cl₆ can also be removed by deprotonation with Li{N(SiMe₃)₂}; additional equivalents of which also exchange the magnesium-bound chlorides for silylamido ligands, affording Mg₂(L)Cl₂N″and Mg ₂(L)Cl(N″)₂, which have both been characterised by single-crystal X-ray diffraction studies.

Full text of DOI:10.1039/b803253j

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www.rsc.org/dalton | Dalton Transactions
Magnesium amido N-heterocyclic carbene complexes†
a
a
Polly L. Arnold,*‡a Ian S. Edworthy,§ Christopher D. Carmichael,b Alexander J. Blakea and Claire Wilson¶
Received 25th February 2008, Accepted 24th April 2008
First published as an Advance Article on the web 9th June 2008
DOI: 10.1039/b803253j
Magnesium dications bind strongly to a tridentate anionic dicarbene ligand L =
[N{CH2CH2(CNCHCHNMes)}2] forming dinuclear and trinuclear Mg complexes with some
particularly short Mg–C bonds. Treatment of the proligand H4LCl3 with three equivalents of methyl
magnesium chloride or benzyl magnesium chloride affords Mg3(HL)Cl6 in high yield. A suspension of 1
in thf was heated to 80 ◦C for 2 h to afford Mg2(L)Cl3, consistent with the loss of one equivalent of
MgCl2, and the deprotonation of the remaining acidic NH, lost as HCl gas. Treatment of Mg3(HL)Cl6
with one equivalent of KC8 results in deprotonation of the ligand amine NH to afford Mg3(L)Cl5;
treatment with a second equivalent forms the radical anion of the complex, K[Mg3(L)Cl5], which
decomposes upon storage, precluding its structural characterisation. The acidic NH proton of the
ligand in Mg3(HL)Cl6 can also be removed by deprotonation with Li{N(SiMe3)2}; additional
equivalents of which also exchange the magnesium-bound chlorides for silylamido ligands, affording
Mg2(L)Cl2Nꢀꢀ and Mg2(L)Cl(Nꢀꢀ)2, which have both been characterised by single-crystal X-ray
diffraction studies.
Introduction
N-heterocyclic carbene (NHC) metal complexes are now widely
used as homogeneous catalysts for organic transformations,1 and
recent years have witnessed an increasing demand for function-
alised carbene-based ligands to tune the reactivity of more Lewis
acidic metal catalysts.2 For the more complex functionalised
carbene ligands, the form containing the free carbene is often
insufficiently kinetically inert to allow it to be isolated. The synthe-
sis of the desired metal complex of many of these functionalised
carbene ligands is then usually achieved by a transmetallation
reaction that transfers the carbene ligand [NHC] from a metal salt
adduct, [MNHC]X, M = group 1, 2, 13, Ag, X = halide.3
For the transfer of anionic functionalised carbene ligands to
early d-block and f-block metal cations, and to avoid potential
problems of ‘ate’ complex formation that are often encountered in
the synthesis of such metal complexes from lithium salts, we have
targeted the synthesis of magnesium carbene ligand adducts.
To date, only a handful of Group 2 NHC adducts have been
reported, although the first magnesium–NHC adducts A and B
(Chart 1), were reported by Arduengo et al in 1993.4 The same
workers reported the synthesis of the metallocene NHC complex
aSchool of Chemistry, University of Nottingham, University Park, Notting-
ham, UK NG7 2RD
bSchool of Chemistry, Joseph Black Building, University of Edinburgh, West
Mains Road, Edinburgh, UK EH9 3JJ
† Electronic supplementary information (ESI) available: Details of the
reaction of 1 with pyridine. CCDC reference numbers 679263 and 679264.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b803253j
‡ Present address: School of Chemistry, Joseph Black Building, University
of Edinburgh, West Mains Road, Edinburgh, UK EH9 3JJ. Email:
Polly.Arnold@ed.ac.uk; Fax: +44 131 650 6453; Tel: +44 131 650 6453.
Chart 1
§ Present address: Onyx Scientific Limited, Silverbriar, Enterprise Park
[Cp*2Mg(C{N(Me)CMe}2] in 1998 via a solvent displacement
reaction, C, as well as metallocene NHC complexes of other group
II elements (Ca, Sr, Ba).5 In 2001, Schumann et al reported a
East, Sunderland, UK SR5 2TQ.
¶ Present address: Rigaku Europe, Chaucer Business Park, Watery Lane,
Sevenoaks, Kent, UK TN15 6QY.
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similar set of metallocene complexes containing two magnesium
complexes, D and E.6
Magnesium adducts of functionalised NHC ligands were first
reported by us in 2004, F,1 and more recently by Kawaguchi and
Zhang, G.7
structure in solution, but somewhat surprisingly, approximately
ten to fifteen ppm higher than the reported values for the other
Mg–NHC complexes B–G. Single crystals suitable for an X-ray
diffraction experiment have not been isolated, but by comparison
with other S-block halide clusters, we suggest a possible structure
in eqn (1), with two of the Mg cations more intimately bound to
the ligand framework (see below). The only identifiable fragment
in the mass spectrum (EI) of 1 is that of mesityl imidazole.
Tridentate bis(pyrazolyl) ligands of the form NZN (Z = N,
O, S), e.g. H, have recently been used as supporting ligands
for metal-catalyzed polymerization processes in which MgII,8
and ZnII adducts function as highly controlled and well-defined
initiators for the polymerization of lactide, while CrII,9 and NiII,10
complexes catalyse the selective dimerization or oligomerization of
ethene. Magnesium adducts of tetradentate tris(pyrazolyl)borate
carbene analogues HB(CNCHCHNBut)3MgBr, I, have also been
used as carbene transfer agents in transition-metal chemistry.11
Here we report the synthesis and reactivity of a set of magne-
sium complexes of a tridentate, monoanionic dicarbene ligand
[N{CH2CH2(CNCHCHNMes)}2], L.
A suspension of 1 in thf was heated to 80 ◦C for 2 h, to afford
a brown solution, which upon standing overnight changed to a
strongly coloured dark purple solution (eqn (2)). Upon addition
of toluene to the solution, a dark purple precipitate was obtained.
The mixture was filtered and the solid washed with toluene.
Elemental analysis of the purple solid 2 indicates that it has an
empirical formula of Mg2(L)Cl3, consistent with the loss of one
equivalent of MgCl2, and the deprotonation of the remaining
acidic NH, which is lost as HCl gas during the reaction.
Results and discussion
(2)
1. MgII adducts of HL and L
Treatment of a cold slurry of H4LCl3, [H2N{CH2CH2(CHN-
CHCHNMes)}2]Cl3 in thf with three equivalents of potassium
hexamethyldisilazide results in the formation of a colourless
precipitate and a yellow solution. A yellow solid, formulated as
HL, can be isolated, but decomposes upon standing in the solid
state to a brown–green solid.
However, treatment of H4(L)Cl3 with three equivalents of methyl
magnesium chloride in thf at 0 ◦C results in the immediate, visible
evolution of gas from the colourless slurry; further stirring of
the mixture for 6 h affords a red solution containing a colourless
precipitate, eqn (1). Filtration of the mixture to isolate the filtrate,
followed by washing with thf affords the magnesium compound
Mg3(HL)Cl6, [HN{CH2CH2(MgCNCHCHNMes)}2]MgCl6 1 as
a colourless powder in 71% yield. Treatment of H4(L)Cl3 with
three equivalents of benzyl magnesium chloride under analogous
conditions yields the same product, again with 1 readily isolated
as a colourless solid from a red thf solution.
The 1H NMR spectrum of purple 2 in d5-pyridine contains two
resonances due to the imidazole protons at 6.87 and 6.80 ppm
and four resonances due to the ligand ethyl groups at 4.76, 3.76,
3.51 and 3.27 ppm. The complex retains the intense purple colour
in pyridine and thf solutions, in which it is soluble without
decomposition. To our knowledge, compounds of this colour
have precedence only in the chemistry of Grignard compounds
with non-innocent a-diimine organic compounds with low lying
LUMOs. Diimine ligands such as bipyridine and phenanthroline
have been known since the 1960s to form strongly coloured species
based on [Mg(a-diimine)R] in which the ligand is reduced to
a radical anion.12 The complex [Mg{(2,6-Pri2Ph)BIAN}(l-Me)]2
(BIAN = bis(imino)acenaphthene) is a deep red colour, and has a
well-resolved EPR spectrum at room temperature.13
This hints at a non-innocence of the carbene p-heterocycle in
the same manner.
Fryzuk has also shown that sterically unhindered yttrium alkyls
supported by a dianionic capping macrocycle, [Y[P2N2]CH2SiMe3]
([P2N2] = [PhP(CH2SiMe2NSiMe2CH2)2PPh]) form strongly
coloured blue solutions in benzene. The colour derives from an
intermolecular r-bond metathesis arene CH activation reaction
which occurs with loss of SiMe4. This results in the isolation of
coupled biaryls which remain p-coordinated to the yttrium centres,
(1)
6
6
forming {Y[P2N2]}2(l,g ,g -C12H10).14
The 1H NMR spectrum of a d5-pyridine solution of 1 confirms
the removal of the two imidazolium protons and the ammonium
proton. The remaining imidazole protons are observed at 7.11
and 6.57 ppm, shifted to lower frequency compared to the
corresponding resonances in H4(L)Cl3 (8.65 and 7.12 ppm). The
However, upon exposure of the purple solid 2 to air, the solid
1
changes colour to yellow and H NMR spectroscopy indicates
reprotonation of the ligand back to (H3L)2+; this confirms the
ligand in 2 does not undergo an aryl or heterocycle coupling,
or a similar rearrangement upon heating. Unfortunately, we have
been unable to grow crystals suitable for single-crystal structural
analysis.
It was also considered possible that the purple colour of complex
2 might originate from the formation of a radical anion-type ligand
in which an extra electron resides on the NHC heterocycle. We have
previously demonstrated the stability of the radical anion of the
amino carbene HN(But)CH2CH2(CNCHCHNBut), in which the
1
amino proton could not be located in the H NMR spectrum,
but an NH absorption is present in the IR spectrum. The 13C
NMR spectrum displays a characteristic carbene resonance at
high frequency, 194.0 ppm, compared to 138.2 ppm for the
corresponding CH carbon chemical shift for H4(L)Cl3. This
carbene resonance is almost identical to that in the dimesityl
carbene in the Mg adduct A (194.8 ppm), which has a dimeric
3740 | Dalton Trans., 2008, 3739–3746
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EPR spectrum shows the electron is coupled to the potassium
reductant and the C2H2 group in the backbone of the carbene
heterocycle.15
An X-band EPR spectrum of 2 in fluid solution, recorded in
thf at room temperature, contains no resonances. An attempt to
generate independently the radical anion of 1 by reduction was
instead carried out.
2. Reduction of MgII adducts of HL and L
Treatment of 1 with one equivalent of potassium graphite at room
temperature in thf yields an orange solution and a black solid.
Filtration and removal of volatiles from the solution affords 3
as a yellow solid. However, elemental analysis confirmed that
deprotonation of 1 had occurred instead of reduction, to afford
3 (eqn (3)), which contains formally a potassium amide group, in
addition to the magnesium dichloride equivalents. The 1H NMR
spectrum of 3 in d5-pyridine is almost identical to that of 1.
Resonances due to the imidazole protons are observed at 7.09
and 6.59 ppm in the 1H NMR spectrum. The 13C NMR spectrum
of 3 contains a resonance due to the carbene at 194.1 ppm, almost
identical to 1, 194.0 ppm.
Fig. 1 X-Band EPR spectrum of 3a thf solution, 300 K. Top: Simulated
spectrum. Bottom: Experimental spectrum. Hyperfine coupling: AK
6.750 G, AN = 5.200 G and AH = 5.000 G.
=
with hyperfine coupling to potassium (AK = 3.07 G), nitrogen
(AN = 6.02 G) and hydrogen (AH = 3.72 G).
The lack of strong colour of solutions of 3a provides further
weight to the argument that the colour of 2 derives from a charge
transfer between adjacent p-systems, rather than the presence of a
p-centred radical system.
(3)
3. Deprotonation of MgII adducts of HL
It is, however, possible to reduce the amide 3 with a further
equivalent of potassium graphite, eqn (4), at room temperature
in thf. This yielded an orange solution and a black solid after
48 h. Filtration and removal of volatiles afforded an orange
Treatment of 1 with two equivalents of lithium hexamethyldisi-
lazide, LiNꢀꢀ, in thf at room temperature yields a pink solution
after 12 h. Removal of volatiles from the solution under reduced
pressure yields crude 4 as a pink solid (Scheme 1). The pink solid
was recrystallised from toluene at −30 ◦C to afford pure 4 as a
colourless crystalline solid.
1
solid. The H NMR spectrum of the orange solid in d5-pyridine
contains no resonances for 3. The ES mass spectrum of a sample
of the reduced species 3a contained no peak for the molecular
ion but contained peaks identifiable as [Mg2Cl3(L) + 2H]+ and
[MesIm]+, and although 3a could be isolated as an orange solid, no
satisfactory elemental analysis was obtained. The reduced species
is unstable in solution and decomposes over the course of two days,
possibly by attack on the a-hydrogens of the ligand ethyl groups,
with the resultant formation of free mesityl imidazole.
(4)
An X-band EPR spectrum of the solid in fluid solution, recorded
in thf at room temperature is shown in Fig. 1. The 10-line spectrum
is centred at a g value consistent with an organic radical. It is
apparent that only one of the imidazole rings had been reduced.
The spectrum is simulated by incorporating hyperfine coupling
of the electron to a potassium cation and to two identical nitrogen
atoms and two identical hydrogen atoms, Fig. 1 upper. The
parameters required for simulation are giso = 2.003, AK = 6.750 G,
AN = 5.200 G and AH = 5.000 G. This indicates that the electron
resides in the NHC p-system and is reminiscent of the potassium–
pyrrole and potassium–permethylcyclopentadienide electrostatic
p-bound fragments observed in reduced systems.2,3 We have shown
previously that the organic radical formed from the amino carbene
HN(But)CH2CH2(CNCHCHNBut) exhibits a 14-line spectrum
Scheme 1
1
The H NMR spectrum of 4 in C6D6 contains four imidazole
proton resonances observed at 6.09, 6.04, 5.97 and 5.43 ppm due to
the lowered symmetry imparted by the amide and chloride ligands.
The different chemical environments for the two magnesium
centres also lead to the splitting of ligand ethyl group resonances
into six multiplets between 5.35 and 2.98 ppm. The 13C NMR
spectrum contains two resonances assigned as the carbene carbon
centres at 182.3 and 179.9 ppm.
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Treatment of 1 with three equivalents of lithium hexamethyl-
disilazide in thf at room temperature yields a pink solution
after 12 h. Removal of volatiles from the pink solution under
reduced pressure afforded crude 5 as a pink solid. The material
was recrystallised from toluene at −30 ◦C to afford pure 5 as a
colourless crystalline solid.
The 1HNMRspectrumof 5 inC6D6 contains onlytwo imidazole
CH proton resonances, at 6.07 and 5.96 ppm, confirming the
symmetry of the two halves of the ligand L in this complex. The
1H NMR spectrum of solutions of 5 also contains a relatively high
frequency resonance for one CH2 of the ligand ethyl groups at
5.48 ppm; the other resonances for the ligand ethyl groups are at
more conventional frequencies, resonating at 3.7, 3.3 and 3.2 ppm.
The 13C NMR spectrum now again contains a single resonance
for the two carbene carbon atoms at 182.3 ppm, comparable to
literature values for Mg–NHC complexes.5
Fig. 2 Displacement ellipsoid plot of the molecular structure of 4
(50% probability). Lattice solvent and hydrogen atoms are omitted for
◦
˚
clarity. Selected distances (A) and angles ( ): Mg1–C2 2.180(2), Mg2–C22
2.153(2), Mg1–N40 2.115(2), Mg2–N40 2.093(2), Mg1–Cl1 2.5299(8);
C2–Mg1–N01 125.81(8), C22–Mg2–Cl2 111.65(6).
Although a number of complexes with bridging trimethylsi-
lylamides are known,16 the addition of an excess of lithium
hexamethyldisilazide to solutions of 1 still affords only 5 as the
product.
[Mg(H)(Cl)]2 although the smaller substituents of H allow for
two ligands to bridge the bimetallic core in those systems.8
X-Ray crystallography reveals a large difference in the bond
4. Single-crystal structures of Mg2(L)X3 complexes
˚
lengths to the bridging chloride, Cl(1), 2.5299(8) A from Mg(1) and
˚
2.3767(8) A from Mg(2). The terminal magnesium–chloride bond
Single crystals suitable for X-ray structure analysis were grown for
both 4 and 5 by slow cooling of a saturated toluene solution of
each complex to −30 ◦C.
˚
is shortest at 2.2939(8) A, and lies at the shorter end of the range
of terminal Mg–Cl distances. Asymmetric magnesium–chloride
bond lengths for a Mg2(l-Cl)2 unit have been shown previously
in a zirconium–magnesium complex with a tridentate diamido
ligand.17 The difference between the bridging magnesium–amide
bonds to N40 in 4 is much smaller, with the bond lengths of
4.1 X-Ray crystal structure of Mg2(L)Cl2Nꢀꢀ 4. The molec-
ular structure of 4 is shown in Fig. 2. Both magnesium cations,
Mg(1) and Mg(2), are four coordinate with a distorted tetrahedral
geometry. Selected distances and angles are collated in Table 1.
One chloride ligand bridges both magnesium atoms as does the
central ligand amide nitrogen. The remaining coordination sites
are filled by one NHC and one chloride for Mg(2) and one NHC
and one silylamide for Mg(1), which accounts for asymmetry
of the ligand environment observed in the NMR spectra. The
general M2-bridging coordination geometry of the ligand L is
reminiscent of the structures formed by the amido-bis(pyrazolyl)
ligand complexes reported by Carpentier and co-workers for
˚
˚
2.115(2) A for Mg(1)–N(40) and 2.093(2) A for Mg(2)–N(40).
˚
Also the Mg–NHC bond lengths differ; 2.180(2) A for Mg(1)–
C(2) and 2.153(2) A for Mg(2)–C(22). Both of these are shorter
than any previously reported literature values for Mg–NHC bond
lengths, which fall in the range 2.194–2.279 A, indicating that this
ligand binds strongly to magnesium. The average Mg–C r-bond
length (data from the CSD, sample size 751)18 is 2.181(2) A, with
˚
fewer than ten complexes exhibiting Mg–C distances shorter than
˚
˚
˚
2.1 A.
Table 1 Selected distances and angles for complexes 4 and 5
4
5
Mg(1)–C(2)
Mg(2)–C(22)
Mg(1)–N(40)
Mg(2)–N(40)
Mg(1)–Cl(1)
Mg(2)–Cl(1)
Mg(2)–Cl(2)
Mg(1)–N(01)
N(1)–C(2)
2.180(2)
2.153(2)
2.115(2)
2.093(2)
2.5299(8)
2.3767(9)
2.2939(8)
1.999(2)
1.354(2)
1.361(2)
1.357(2)
1.360(2)
Mg(1)–C(32)
Mg(2)–C(12)
Mg(1)–N(1)
Mg(2)–N(1)
Mg(1)–Cl(1)
Mg(2)–Cl(1)
Mg(2)–N(01)
N(11)–C(12)
N(13)–C(12)
N(31)–C(32)
N(33)–C(32)
2.199(2)
2.193(2)
2.116(2)
2.123(2)
2.4392(9)
2.4384(9)
2.006(2)
1.358(3)
1.360(3)
1.357(3)
1.364(3)
N(3)–C(2)
N(21)–C(22)
N(23)–C(22)
Mg(1)–N(40)–Mg(2)
Mg(1)–Cl(1)–Mg(2)
N(1)–C(2)–N(3)
N(21)–C(22)–N(23)
C(2)–Mg(1)–N(01)
C(22)–Mg(2)–Cl(2)
96.93(7)
79.79(3)
103.78(17)
103.87(17)
125.81(8)
111.65(6)
Mg(1)–N(1)–Mg(2)
Mg(1)–Cl(1)–Mg(2)
N(11)–C(12)–N(13)
N(31)–C(32)–N(33)
C(12)–Mg(2)–N(01)
97.17(8)
81.34(3)
103.18(18)
103.3(2)
124.97(8)
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There is a large range of angles around each magnesium centre,
120.8–93.6◦ for Mg(2) and 125.8–88.8◦ for Mg(1), comparable to
the distortion in the structure seen for F, the previous example of
functionalised-NHC magnesium adduct reported by our group.1
The low coordination number, combined with the steric demands
of the ligands makes it impossible to suggest if there is a secondary
interaction between the p-donor amido or chloride ligand, and
the formally empty carbenic carbon p*, an interaction that is
suggested to enhance the stability of some d0 metal complexes.19
methylmagnesium chloride leads to the formation of a magnesium
chloride adduct, Mg3(HL)Cl6 1, which has been characterised
by multinuclear NMR spectroscopy, EI mass spectrometry and
elemental analysis.
Reduction chemistry has been carried out on 1, with the unex-
pected result of deprotonation of the amine in 1 and the formation
of a potassium salt, Mg3(KL)Cl6 3, which can subsequently be
reduced to form an organic radical, in which the electron is centred
predominantly on the back of the carbene ring.
Upon heating a solution of 1 in thf a purple-coloured compound
is isolated which was characterised as Mg2(L)Cl3, 2, again as a
result of the elimination of the acidic NH ligand proton, and
loss of one of the incorporated equivalents of MgCl2 from the
salt 1. The reaction of 1 with two or three equivalents of lithium
4.2 X-Ray crystal structure of Mg2(L)Cl(Nꢀꢀ)2 5. The molec-
ular structure of 5 is shown in Fig. 3. Both magnesium cations,
Mg(1) and Mg(2), are four coordinate with distorted tetrahedral
geometry. Selected distances and angles are collated in Table 1.
One chloride ligand bridges both magnesium atoms as does the
central amide of L. The remaining coordination sites are filled by
one NHC and one silylamide for both magnesium centres, which
accounts for the single ligand environment observed in the NMR
spectra.
hexamethyldisilazide leads to the formation of Mg2(L)Cl2Nꢀꢀ
4
and Mg2(L)Cl(Nꢀꢀ)2 5, both of which have been structurally
characterised. The first equivalent of lithium amide is used in
each case to deprotonate the ligand NH group once more. All of
the Mg–C distances determined in the molecular structures of 4
and 5 are particularly short; compound 4 contains the shortest
magnesium–carbene bond reported to date.
The robustness of the Mg2L core in these complexes, and the
ready functionalisation of the remaining anionic ligands of the
two metals suggests that adducts of these complexes may be useful
in Lewis acid catalysis, or coordination polymerisation initiation
applications in the future. They may also serve as transmetallation
sources of the ligand for metals that are capable of binding the
NHC groups more strongly than the Mg cations.
Experimental
General experimental details
Fig. 3 Displacement ellipsoid plot of the molecular structure of 5
(50% probability). Lattice solvent and hydrogen atoms are omitted for
All manipulations of oxygen- or moisture-sensitive materials used
standard Schlenk techniques (rotary pump for vacuum 10−4 mbar)
or a glove box (Mbraun Unilab or Mbraun Labstar) under
dry dinitrogen. NMR spectra were recorded on a Bruker DPX
300 spectrometer, operating frequency 300 MHz (1H), 75 MHz
◦
˚
clarity. Selected distances (A) and angles ( ): Mg1–N02 2.0096(18),
Mg1–N1 2.116(2), Mg1–C32 2.199(2), Mg1–Cl1 2.4392(8), Mg2–N01
2.007(2), Mg2–N1 2.122(2), Mg2–C12 2.193(2), Mg2–Cl1 2.4382(8);
N11–C12–N13 103.18(19), Mg2–Cl1–Mg1 81.34(3), Mg1–N1–Mg2
97.17(8), N1–Mg2–Cl1 90.65(5), C12–Mg2–Cl1 100.40(6), N01–Mg2–C12
124.97(8).
(13C{ H}), 116 MHz (7Li), 121.5 Hz (31P{ H}) and 282.4 MHz
(19F), variable temperature unit set to 300 K unless otherwise
stated. Chemical shifts are reported in parts per million, and
referenced internally to residual solvent proton resonance, and
externally to TMS. FTIR spectra (4000–400 cm−1) were recorded
as nujol mull on a JASCO 460 PLUS spectrophotometer. Mass
spectra (EI, ES) were run by Mr. A. Hollingworth and Mr.
G. Coxhill on a VG autospec instrument at the University of
Nottingham. Elemental analyses were determined by S. Boyer
at London Metropolitan University. EPR spectra were recorded
on a Bruker X-band ESP 300E spectrometer with microwave
frequencies calibrated with a Bruker ER035M NMR gaussmeter.
EPR spectra were simulated using WINEPR SimFonia Version
1.25 (Bruker). All solvents used were either degassed and purified
by passage through activated alumina towers prior to use, or
were freshly distilled from the appropriate drying reagent under
dinitrogen, and thoroughly degassed prior to use: DME and
pyridine from potassium; dichloromethane from CaH2. NMR
spectroscopic grade d6-benzene and d5-pyridine were dried over
potassium metal, thoroughly degassed by the freeze–thaw method
and transferred under reduced pressure before use. The synthesis
1
1
In contrast to the structure of 4, the bridging chloride–
magnesium bonds are now symmetrical in 5; as are the Mg–NHC
bond lengths; these are within the reported literature values for
Mg–NHC bond lengths and longer than in 4. As in 4 there is a
large range of angles around each magnesium centre in 5, 125.0–
90.7◦ for Mg(2) and 127.4–90.8◦ for Mg(1) and with smallest angle
between bridging chloride, magnesium centre and bridging amide,
comparable to 4.
The magnesium–amide bonds for the central ligand amide in 5
˚
˚
are 2.116(2) A for Mg(1)–N(1) and 2.122(3) A for Mg(2)–N(1);
both are longer than the Mg–amide bonds for the magnesium–
NHC adduct F reported by our group, but in the normal range of
magnesium amide bonds.20
Concluding remarks
Attempts to isolate the free carbene, HL, from the protonated
proligand have been unsuccessful due to the thermal lability of
the ligand. However, treatment of the proligand H4(L)Cl3 with
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of [H4L]Cl3 has been previously described.21 MesIm = mesityl
(CH2), 50.0 (CH2), 20.8 (p-CH3) and 18.6 (o-CH3). IR (Nujol):
3122 (m), 3098 (m), 1609 (w), 1547 (w), 1489 (s), 1404 (m), 1236
(w), 1101 (m), 1029 (s), 937 (m), 853 (s), 803 (w), 671 (m), 580
(m). MS (EI): m/z = 586 ([M − 3CH3 − MgKCl2]+, 34%), 305
([Mg3Cl2(L)K − CH2]2+, 94%), 280 ([MgCl2(MesIm)]+, 34%), 197
(100%), 186 ([MesIm + H]+, 58%), 159 ([MesIm − 2CH3 + H]+,
47%). Anal. for C28H34Cl6KMg3N5 [found (calc.)]: C, 44.31 (43.94);
H, 4.53 (4.48); N, 8.94 (9.15%).
imidazole, C3N2H2(C6H2Me3-2,4,6).
Synthetic procedures
Synthesis of HL. To a suspension of H4LCl3 (1.00 g, 1.8 mmol)
◦
in thf (20 cm3) at −30 C was added potassium hexamethyldisi-
lazide (1.09 g, 5.4 mmol). The resulting mixture was stirred at
−30 ◦C for 4 h to afford a yellow solution and white precipitate.
The solution was filtered through a bed of Celite and volatiles were
Reduction of 3, 3a. A Schlenk was charged with 3 (100 mg,
0.13 mmol) and KC8 (18 mg, 0.13 mmol). To this was added thf
(10 cm3) and the resulting mixture was stirred at room temperature
for 48 h. After this time a yellow solution and black solid was
afforded. The solution was filtered and the solid was washed with
thf (2 × 5 cm3) and the washings added to the yellow solution.
Removal of the volatiles from the yellow solution under reduced
pressure afforded an orange solid 3a. EPR spectrum of orange
solid (X-band, modulation amplitude 1.0 G): fluid thf, referenced
to dpph, simulated with giso = 2.00300. Spectral features simulated
according to a hyperfine coupling to potassium, two nitrogens and
two hydrogens, resolved as AK = 6.750 G, AN = 5.200 G and AH =
5.000 G, line width 3.20 G. MS (EI): m/z = 502 (8%), 428 ([H2L −
CH3]+, 17%), 354 (45%), 280 ([MgCl2(L)K − CH3 + H]2+, 55%),
186 ([MesIm]+, 100%), 159 ([MesIm − 2CH3 + H]+, 80%), 144
([MesIm − 3CH3 + H]+, 60%).
◦
removed under reduced pressure at low temperature (> −10 C)
to afford a yellow solid of HL. Solutions and solid samples of
HL decomposed over time to give a green–brown solid. 1H NMR
(C5D5N): d 7.07 (s, 2H, CH), 6.86 (s, 6H, CH + Ar), 4.47 (t, 4H,
3
3J = 6.2 Hz, CH2), 3.22 (t, 4H, J = 6.2 Hz, CH2), 2.21 (s, 6H,
p-CH3), 2.07 (s, 12H, o- CH3).
Synthesis of Mg3(HL)Cl6 1. To a Schlenk containing a cooled
(0 ◦C) suspension of H4(L)Cl3 (4.73 g, 8.58 mmol) in thf (30 cm3)
was added methylmagnesium chloride (3 M solution in thf,
8.6 cm3, 25.75 mmol), dropwise over 10 minutes. The mixture
was stirred at 0 ◦C for 4 h. During this time, the solution became
red in colour and gas evolution was observed. The red solution
was stirred at 0 ◦C for a further 16 h. During this time, a
colourless precipitate formed, which was filtered and washed with
thf (2 × 20 cm3) to afford 1 as a colourless, solid, bis (thf) solvate,
1
HL·3MgCl2·2thf (5.30 g, 70.9%). H NMR (C5D5N): d 7.11 (d,
Synthesis of Mg2(L)Cl2Nꢀꢀ, 4. To a suspension of 1 (0.50 g,
0.57 mmol) in thf (10 cm3) at room temperature was added a
solution of lithium hexamethyldisilazide (0.19 g, 1.15 mmol) in
thf (10 cm3). The resultant mixture was stirred for 48 h to yield a
colourless solution. Removal of volatiles from the solution under
reduced pressure afforded a colourless solid. The product was
extracted with toluene (2 × 10 cm3) to afford 4 as a colourless
powder (0.26 g, 64.3%). Single crystals of 4 suitable for X-ray
analysis were obtained by cooling a saturated toluene solution to
−30 ◦C. 1H NMR (C6D6): d 6.78 (s, 3H, Ar), 6.58 (s, 1H, Ar), 6.09
3
3
2H, J = 1.2 Hz, CH), 6.57 (d, 2H, J = 1.2 Hz, CH), 6.24 (s,
4H, Ar), 4.62 (br, 4H, CH2), 3.60 (t, 8H, thf), 3.27 (br, 4H, CH2),
2.08 (s, 12H, o-CH3), 2.03 (s, 6H, p-CH3) and 1.56 (q, 8H, thf);
1
13C{ H} NMR (C5D5N): 194.0 (NCN), 137.4, 137.1, 128.8 (all
Ar), 121.7 (CH), 120.2 (CH), 67.8 (thf), 51.1 (CH2), 50.0 (CH2),
25.8 (thf), 20.8 (p-CH3) and 18.6 (o-CH3). IR (Nujol): 3274 (w),
3156 (w), 3115 (m), 3087 (m), 1608 (m), 1560 (m), 1480 (s), 1405
(m), 1098 (m), 1028 (s), 935 (w), 867 (m), 806 (w), 749 (m), 682
(w), 581 (w), 424 (m). MS (EI): m/z = 186 ([MesIm]+, 100%),
159 ([MesIm − 2CH3 + H]+, 64%), 144 ([PhIm]+, 47%). Anal. for
C36H51Cl6Mg3N5O2 [found (calc.)]: C, 49.54 (49.62); H, 5.99 (5.90);
N, 8.08 (8.04%).
3
3
(d, 1H, J = 1.5 Hz, CH), 6.04 (d, 1H, J = 1.5 Hz, CH), 5.96
3
3
(d, 1H, J = 1.6 Hz, CH), 5.82 (d, 1H, J = 1.6 Hz, CH), 5.32
(br, 1H, CH2), 5.00 (m, 1H, CH2), 3.61 (2 m, 2H, CH2), 3.27 (m,
1H, CH2), 3.14 (m, 2H, CH2), 2.98 (m, 1H, CH2), 2.13 (s, 3H,
CH3), 2.11 (s, 3H, CH3), 2.07 (s, 6H, CH3), 1.97 (s, 3H, CH3),
Reaction of 1 with thf, 2. A suspension of 1 (0.39 g, 0.45 mmol)
in thf (10 cm3) was heated at 70 ◦C for 16 h yielding a purple
solution. Removal of volatiles from the solution under reduced
1.88 (s, 3H, CH3), 0.15 (s, 18H, N(Si(CH3)3)2); 13C{ H} NMR
1
1
(C6D6): d 182.3 (NCN), 179.9 (NCN), 139.1, 138.8, 136.2, 135.9,
135.7, 135.6, 134.9, 133.9, 130.0, 129.4 (All Ar), 122.4, 122.0,
121.0, 120.6 (CH), 56.0, 55.0, 52.7, 52.4 (All CH2), 21.0, 20.8,
18.5, 18.1, 17.9, 17.8 (All CH3), 6.0 (N(Si(CH3)3)2). IR (Nujol):
3398 (w, br), 3145 (w), 3124 (w), 3107 (w), 1653 (w), 1609 (w),
1488 (m), 1404 (w), 1245 (w), 934 (w), 850 (m), 737 (m), 582 (w).
Anal. for C34H52Cl2Mg2N6Si2 [found (calc.)]: C, 56.87 (56.68); H,
6.91 (7.27); N, 12.10 (11.66%).
pressure afforded a dark purple solid of Mg2(L)Cl3, 2 H NMR
(C5D5N): d 7.28 (s, 2H, Ar), 6.87 (s, 2H, CH), 6.80 (s, 2H, CH),
6.78 (s, 2H, Ar), 4.76 (br, 2H, CH2), 3.78 (br, 2H, CH2), 3.51 (br,
2H, CH2), 3.27 (br, 2H, CH2), 2.20 (s, 12H, o-CH3), 2.03 (s, 6H, p-
CH3). Anal. for C28H34Cl3Mg2N5 [found (calc.)]: C, 56.31 (56.47);
H, 5.75 (5.75); N, 11.64 (11.76%).
Synthesis of Mg3(KL)Cl6 3. A Schlenk was charged with 1
(540 mg, 0.62 mmol) and KC8 (84 mg, 0.62 mmol). To this was
added thf (15 ml) and the resultant mixture was stirred at room
temperature for 48 h. After this time the solution had become
orange and the solid black. The mixture was filtered and volatiles
were removed under reduced pressure to afford 3 as a light brown
Synthesis of Mg2(L)ClNꢀꢀ , 5. To a suspension of 1 (0.40 g,
2
0.46 mmol) in thf (10 cm3) at room temperature was added a
solution of lithium hexamethyldisilazide (0.31 g, 1.84 mmol) in
thf (5 cm3). The resultant mixture was stirred overnight to yield a
pale pink solution. Removal of volatiles from the solution under
reduced pressure afforded an off-white solid. The product was
extracted with toluene giving 5 as a colourless solid (0.37 g, 76.8%).
Single crystals of 5 suitable for X-ray analysis were obtained by
cooling a saturated toluene solution to −30 ◦C. 1H NMR (C6D6):
1
3
solid (0.45 g, 94.8%). H NMR (C5D5N): d 7.09 (d, 2H, J =
3
1.4 Hz, CH), 6.59 (d, 2H, J = 1.4 Hz, CH), 6.26 (s, 4H, Ar),
4.64 (br, 4H, CH2), 3.29 (br, 4H, CH2), 2.10 (s, 12H, o-CH3) and
2.01 (s, 6H, p-CH3); 13C{ H} NMR (C5D5N): d 194.1 (NCN),
1
137.4, 137.1, 129.3, 128.8 (all Ar), 121.6 (CH), 120.3 (CH), 51.1
3744 | Dalton Trans., 2008, 3739–3746
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Table 2 Experimental crystallographic data
4
5
Crystal data
Chemical formula
Mr
C34H52Cl2Mg2N6Si2.C7H8
812.65
Monoclinic
P21/c
150(2)
16.2204(12)
9.3907(7)
31.198(2)
100.191(2)
4677.1(10)
4
C40H70ClMg2N7Si4·2C7H8
1029.73
Orthorhombic
Pbca
150(2)
20.303(2)
15.0642(11)
39.978(3)
Cell setting
Space group
T/K
˚
a/A
˚
b/A
˚
c/A
b/◦
V/A
Z
3
˚
12227(2)
8
1.119
Mo-Ka
0.20
Dc/Mg m−3
Radiation type
l/mm−1
1.154
Mo-Ka
0.25
Crystal form
Colour
Crystal size/mm
Tablet
Cuboid
Colourless
0.75 × 0.62 × 0.60
Colourless
0.30 × 0.23 × 0.10
Data collection
Data collection method
Absorption correction
Tmin
x
x
Multi-scan (based on symmetry-related measurements)
0.815
0.932
0.600
0.769
Tmax
No. measured, indep., observed reflections
Criterion for observation
34501, 10661, 7424
I > 2r(I)
0.044
54860, 11314, 8741
I > 2r(I)
0.047
Rint
◦
hmax
/
27.5
26.45
Refinement
R[F2 > 2r(F2)], wR(F2), S
No. reflections
No. parameters
Weighting schemea
(D/r)max
0.048, 0.122, 0.94
10661
0.047, 0.124, 1.04
11314
482
672
w = 1/[r2(Fo2) + (0.066P)2]
w = 1/[r2(Fo2) + (0.0595P)2 + 8.0274P]
0.001
0.006
−3
˚
Dqmax, Dqmin/e A
0.60, −0.37
0.52, −0.40
a P = (Fo + 2Fc2)/3.
2
d 6.27 (d, 4H, 3J = 8.0 Hz, Ar), 6.07 (d, 2H, 3J = 1.7 Hz, CH), 5.96
(s, 2H, 3J = 1.7 Hz, CH), 5.47 (m, 2H, CH2), 3.65 (m, 2H, CH2),
3.31 (m, 2H, CH2), 3.17 (m, 2H, CH2), 2.20 (s, 6H, CH3), 2.10
(s, 6H, CH3), 1.93 (s, 6H, CH3), 0.24 (s, 18H, N(Si(CH3)3)2), 0.16
F2 using SHELXL97, and all fully occupied non-H atoms
refined with anisotropic atomic displacement parameters (adps).
Hydrogen atoms were geometrically placed and included as part
of a riding model, except for methyl hydrogen atoms which were
located from difference Fourier maps and included as part of
a rigid rotor. Two molecules of toluene were each modeled as
disordered over two half-occupied sites, with isotropic adps and
distance restraints applied.
1
(s, 18H, N(Si(CH3)3)2); 13C{ H} NMR (C6D6): d 182.3 (NCN),
138.9, 136.2, 134.2, 130.2, 129.1 (All Ar), 122.2 (CH), 121.0 (CH),
54.5 (CH2), 53.0 (CH2), 20.9 (CH3), 18.4 (CH3), 18.1 (CH3), 6.4
(N(Si(CH3)3)2), 6.0 (N(Si(CH3)3)2). Anal. for C40H70ClMg2N7Si4
[found (calc.)]: C, 56.67 (56.83); H, 8.28 (8.35); N, 11.42 (11.60%).
Acknowledgements
Crystallographic details
We thank Professor Martin Schro¨der of the University of Not-
tingham for helpful discussions. We also thank the Alexander von
Humboldt Foundation, the Royal Society, the EPSRC and the
Universities of Edinburgh and Nottingham for funding.
Single-crystal diffraction data were collected using graphite
monochromated Mo-Ka X-radiation on either a Bruker SMART
APEX (for 4) or a Bruker SMART1000 (for 5) CCD area detector
diffractometer equipped with an Oxford Cryosystems Cryostream
cooling device.22 Details of the individual data collections, solu-
tions and refinements are given in Table 2.
Notes and references
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CCDC reference numbers 679263 and 679264.
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Tetrahedron: Asymmetry, 2005, 16, 921; D. R. Wang, K. Wurst and
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b803253j
Structures were solved by direct methods using SHELXS97.23
All structures were refined by full-matrix least-squares against
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3746 | Dalton Trans., 2008, 3739–3746
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The Royal Society of Chemistry 2008
©