Multigram Syntheses of Magnesium(I) Compounds Using Alkali Metal Halide Supported Alkali Metals as Dispersible Reducing Agents

Article abstract of DOI:10.1021/acs.organomet.8b00803

The alkali metal halide supported alkali metal materials, ca. 5% w/w Na/NaCl and 5% w/w K/KI, are prepared without specialist equipment by rapidly stirring molten alkali metal with finely ground alkali metal halide powder. Scanning electron microscopy rev

Full text of DOI:10.1021/acs.organomet.8b00803

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Multigram Syntheses of Magnesium(I) Compounds Using Alkali
Metal Halide Supported Alkali Metals as Dispersible Reducing
Agents
Jamie Hicks, Martin Juckel, Albert Paparo, Deepak Dange, and Cameron Jones*
School of Chemistry, Monash University, P.O. Box 23, Melbourne, Victoria 3800, Australia
S
* Supporting Information
ABSTRACT: The alkali metal halide supported alkali metal materials, ca. 5%
w/w Na/NaCl and 5% w/w K/KI, are prepared without specialist equipment by
rapidly stirring molten alkali metal with finely ground alkali metal halide powder.
Scanning electron microscopy reveals the particles of 5% w/w Na/NaCl to lie
largely in the 10−100 μM size range. The freely flowing powders are easily
dispersible in organic solvents and are used as reducing agents for the facile
syntheses of three magnesium(I) compounds, [{HC(MeCNAr)2}Mg]2, Ar = mesityl (Mes), 2,6-diethylphenyl (Dep), or 2,6-
diisopropylphenyl (Dip), which can be obtained in yields of up to 12 g. The potential advantages Na/NaCl and K/KI powders
offer to synthetic inorganic/organometallic chemists, relative to currently available alkali metal reducing agents, are discussed.
(II) iodide precursor complexes with alkali metal mirrors. The
relatively low surface areas of such mirrors typically limits the
scale of these reduction reactions to yield less than 2 g of the
magnesium(I) product, though higher weight yields (but lower
percentage yields) can be obtained inconsistently when large
reaction flasks (e.g., 1 L round-bottomed Schlenk flask) are
employed.6 Here we show that alkali metals supported on
finely ground alkali metal halide salts can be easily prepared
without specialist equipment, and can be utilized as dispersible
reducing agents for the facile, high yield synthesis of up to 12 g
of magnesium(I) compounds in a single batch.
INTRODUCTION
■
Developments in the chemistry of the main group elements
over the last several decades have largely centered on
complexes containing those elements in very low oxidation
states. While such complexes have always been of considerable
fundamental interest, due to the unusual structural types and
bonding they exhibit,1 the past decade has seen these reactive
species finding widespread applications in stoichiometric and
catalytic synthetic protocols. Particular interest has focused on
developing cheap, essentially nontoxic main group complexes
for use in processes which normally require expensive and toxic
late transition metal complexes to proceed.2 As low oxidation
state main group complexes are typically prepared by reduction
of common oxidation state precursor complexes, the develop-
ment of new classes of reducing agent can only aid the
expansion of this field.
In 2007, our group reported on the first isolable
magnesium(I) compounds, LMg−MgL, which are kinetically
stabilized by bulky guanidinate or β-diketiminate ligands (L).3
Since that time, β-diketiminate coordinated magnesium(I)
compounds have emerged as soluble, stable, selective, and safe
reducing agents that have been widely utilized by many
research groups around the world.4 These efforts have allowed
entry to numerous low oxidation state p-block complexes (and
related d-block systems) that cannot be accessed, or are
difficult to access, when using more traditional reducing agents
such as insoluble s-block metals, KC8, or alkali metal
naphthalenide solutions.5 The success of magnesium(I)
compounds in this respect is a result of their moderately
reducing nature, their ease of preparation and handling, and
the fact that reduction reactions involving them are carried out
in a controlled fashion with all reactants in the solution phase.
Despite their increasing importance as reducing agents, all
currently available synthetic routes to β-diketiminato
magnesium(I) compounds involve reduction of magnesium-
RESULTS AND DISCUSSION
■
Considering the limitations of the synthesis of magnesium(I)
compounds on multigram scales by reduction of β-
diketiminato magnesium(II) precursor complexes using alkali
metal mirrors as reducing agents, we have explored a number
of reduction methodologies that involve greater alkali metal
surface areas.7 These include liquid Na/K alloys, Na/Hg
amalgam, KC8, lithium powder, sodium dispersions generated
by rapidly stirring molten sodium in toluene, and graphite,
silica gel, or diatomaceous earth supported sodium powders.5,8
However, in all cases the yields of magnesium(I) compounds
obtained were low, and the products were typically
contaminated with large amounts of homoleptic bis(β-
diketiminato) magnesium(II) compounds, β-diketimine, and/
or other byproducts.
Our attention then turned to the possibility of supporting
sodium or potassium metal on finely divided sodium chloride
or potassium iodide powders, respectively. The former has
been previously prepared as a ca. 10% w/w powder by ball-
Received: November 2, 2018
© XXXX American Chemical Society
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milling sodium metal and sodium chloride in a glovebox and
has proved effective as a reducing agent.9 As far as we are
aware, the latter system has not been previously explored. In
order to circumvent the need to use a ball-milling apparatus
and a glovebox, we have developed a related procedure that
can be simply performed in any organometallic research
laboratory. NaCl or KI are ground to fine powders in a
domestic kitchen blender for approximately 30 min. The
powders are then placed in a large round-bottomed Schlenk
flask and heated under a dynamic vacuum at ca. 300 °C using a
heat gun for several hours. Approximately 5% w/w of Na or K
metal is then added to the cooled flask, and the mixture is
heated under a nitrogen atmosphere, or a static vacuum, at just
above the melting point of the alkali metal, while stirring
rapidly with a large magnetic stir bar. After about 30 min, no
bulk alkali metal remains, and the resultant mixture is a freely
flowing, fine blue/gray powder that can be stored indefinitely
in a Schlenk flask or a glovebox and is easily weighed out in
stoichiometric amounts for further reactions. The powder
rapidly decomposes on exposure to the atmosphere, and
should be treated as a fire hazard when manipulated.
mesityl (Mes), 2,6-diethylphenyl (Dep), and 2,6-diisopropyl-
phenyl (Dip); Scheme 1).6 These were dissolved in toluene/
Scheme 1. Multigram Syntheses of Magnesium(I)
Compounds 1−3 Using ca. 5% w/w Na/NaCl Powders As
Reducing Agents
diethyl ether solvent mixtures in large round-bottomed
Schlenk flasks, to which large excesses of Na/NaCl (ca. 5
equiv of Na) had been added. The resultant suspensions were
1
stirred for 2−3 days, and the reactions were monitored by H
NMR spectroscopy to determine when all magnesium(II)
iodide precursor had been consumed. At this stage the
suspensions were filtered, which yielded yellow solutions of the
target magnesium(I) compounds, 1−3. In the cases of
[{HC(MeCNAr)2}Mg]2 (Ar = Mes 1 or Dip 3),6a,c volatiles
could simply be removed and the residues washed with hexane
to give good yields (60 and 83%, respectively) of the
magnesium(I) compounds as yellow powders. 1H NMR
spectroscopic analyses (see Supporting Information) revealed
the powders to be >97% pure, and they could be used in
subsequent reactions without further purification. The
purification of the other magnesium(I) compound, [{HC-
(MeCNDep)2}Mg]2 (2),6b required recrystallization to obtain
it in a moderate isolated yield (53%). The excess Na/NaCl
from the reactions can be safely decomposed by slow addition
of isopropanol, with stirring, to the Schlenk flask containing
this residue. The reactions do give similar yields when only 2
equiv of Na/NaCl are used, but they take considerably longer
(7−8 days) to reach completion.
During one preparation of Na/NaCl powder, samples were
taken before and after addition of sodium metal and analyzed
by scanning electron microscopy. This revealed a wide particle
size range, with the majority of irregularly shaped fractured
crystals lying between 10 and 100 μm across (see Figure 1a).
The main advantage of this synthetic technique over those
previously published is that the magnesium(I) compounds can
be obtained on multigram scales (6−12 g) with minimal
workup and in two cases without recrystallization. The
obtained yields are significantly greater than when a sodium
mirror is used as the reducing agent, and the reaction times
(2−3 days) are considerably shorter than when using sodium
mirrors (4−5 days).6 Moreover, the Na/NaCl powder can be
easily prepared and stored for indefinite periods prior to use.
Given the facility of preparations of [{HC(MeCNAr)2}Mg]2,
it seems likely that Na/NaCl powders could find use as an
alternative to sodium mirrors or sodium dispersions in
synthetic procedures where those reagents are utilized.
For the sake of comparison, the preparation of the bulkiest
magnesium(I) compound, [{HC(MeCNDip)2}Mg]2 (3), was
attempted using K/KI powder in place of Na/NaCl. Although
the isolated yield (57%) was not as high as when Na/NaCl was
used, the reaction was quicker (1 day), and only a slight excess
of K/KI powder (ca. 1.1 equiv of K) was required. In addition,
after decomposing the small amount of residual potassium in
the K/KI powder with isopropanol, the remaining KI can be
washed, dried and used again for the preparation of K/KI
powders.
Figure 1. Scanning electron microscope images of (a) finely ground
NaCl powder, and (b) large fractured NaCl crystals partially coated
with Na metal (dark gray patches).
After the addition of sodium metal, the particles are partially
coated with sodium metal, but this coating is typically not
uniform, as can be seen in Figure 1b. It is clear, however, that
the surface area of sodium has markedly increased relative to
that which would be expected of a mirror of sodium coating
the inside of a Schlenk flask.
Three β-diketiminato magnesium(II) iodide complexes with
varying steric bulk of their N-substituents were chosen for
reduction reactions, [{HC(MeCNAr)2}MgI(OEt2)] (Ar =
With K/KI in hand, it was seen of interest to explore its use
as a reducing agent in the preparation of other low oxidation
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Preparation of 5% w/w K/KI Powder. Potassium iodide (55 g)
was ground to a fine powder over 30 min using a 1.2 L, 1500 W,
domestic kitchen blender. The powder was dried in an oven for 3 days
at 125 °C. It was then further ground with a mortar and pestle before
being placed in a 500 mL Schlenk flask where it was flame-dried three
times under reduced pressure. After cooling, freshly cut pieces of
potassium (2.57 g, 66 mmol) were introduced. The flask was then
placed under a static reduced pressure (ca. 10−2 mm Hg) and heated
with a heat gun to ca. 100 °C (using a heat gun; approximate
temperature determined using a laboratory thermometer). Once the
potassium had melted, rapid stirring and agitation continued until no
bulk metal was visible (ca. 30 min), leaving a freely flowing blue/gray
powder which could be stored in a Schlenk flask or in a glovebox until
needed (yield 57.5 g).
state main group systems that are difficult to prepare using
potassium metal. As an example, we chose the neutral β-
diketiminato aluminum(I) heterocycle, [{HC(MeCNDip)2}-
Al:] (4), the literature preparation of which by reduction of
[{HC(MeCNDip)2}AlI2] with potassium is low yielding
(21%).10 At least in our hands, we have found from numerous
attempts to repeat this procedure that the outcome is difficult
to regularly reproduce, and only rarely can the literature yield
be obtained. In contrast, reductions of [{HC(MeCNDip)2}-
AlI2] with a slight excess of K/KI in toluene were reproducible,
though typically afforded [{HC(MeCNDip)2}Al:] in lower
yields (ca. 14%) than the literature procedure. With that said,
the greater certainty associated with this procedure may well
benefit the increasing number of chemists who utilize
[{HC(MeCNDip)2}Al:] as a reactant in their research.2b,11
Reduction of [{HC(MeCNDip)2}AlI2] with a 5-fold excess of
Na/NaCl led to an intractable mixture of many products after
1 day, while reaction with a 20% excess of Na/NaCl led to only
partial consumption of [{HC(MeCNDip)2}AlI2] and a
complicated product mixture, not including 4, after 2 days.
Preparation of [{HC(MeCNMes)2}Mg]2 (1). A solution of
[{HC(MeCNMes)2}MgI(OEt2)] (20.0 g, 35.8 mmol) in a mixture
of toluene (600 mL) and diethyl ether (50 mL) was added to a 1 L
Schlenk flask containing 84 g of ca. 5% w/w Na/NaCl powder (174
mmol Na). The resultant suspension was stirred vigorously for 2 days
at room temperature. After this time, the mixture was filtered, and
volatiles were removed from the yellow/orange filtrate in vacuo to give
a yellow solid. This was washed with pentane (2 × 50 mL) and
hexane (2 × 50 mL), then dried in vacuo to give 1 as a pale yellow
1
powder with a purity of >97% as determined by
H NMR
CONCLUSIONS
■
spectroscopy (7.70 g, 60%). The characterizing data for the
compound were identical to those reported.6a
In summary, a straightforward method for the preparation of
alkali metal halide supported alkali metals has been developed.
These freely flowing powders are readily dispersible in organic
solvents, and have been used as reducing agents in facile,
multigram (up to 12 g) syntheses of three synthetically
applicable magnesium(I) compounds. It has also been
demonstrated that one of the reagents, K/KI, can be employed
as a reducing agent in reproducible, but low yielding, syntheses
of a known neutral aluminum(I) heterocycle. We continue to
explore applications of Na/NaCl and K/KI in the synthesis of
low oxidation main group compounds, and believe these
materials will find use in the broader inorganic/organometallic
community.
Preparation of [{HC(MeCNDep)2}Mg]2 (2). A solution of
[{HC(MeCNDep)2}MgI(OEt2)] (20.0 g, 34.0 mmol) in a mixture
of toluene (300 mL) and diethyl ether (40 mL) was added to a 500
mL Schlenk flask containing 84 g of ca. 5% w/w Na/NaCl powder
(174 mmol Na). The resultant suspension was stirred vigorously for 3
days at room temperature. After this time, the mixture was filtered and
volatiles were removed from the yellow/orange filtrate in vacuo to give
a yellow/brown oil. This was taken up in ca. 15 mL of hexane, and the
solution was held at −30 °C overnight to give 2 as a dark yellow
crystalline solid (6.95 g, 53%). The characterization data for the
compound were identical to those reported.6b
Preparation of [{HC(MeCNDip)2}Mg]2 (3). Method A. A
solution of [{HC(MeCNDip)2}MgI(OEt2)] (20.0 g, 31.2 mmol) in
a mixture of toluene (500 mL) and diethyl ether (50 mL) was added
to a 1 L Schlenk flask containing 84 g of ca. 5% w/w Na/NaCl
powder (174 mmol Na). The resultant suspension was stirred
vigorously for 2 days at room temperature. After this time, the mixture
was filtered, and volatiles were removed from the yellow/orange
filtrate in vacuo to give a yellow solid. This was washed with hexane (2
× 100 mL), then dried in vacuo to give 3 as a bright yellow powder
with a purity of >97% as determined by 1 H NMR spectroscopy (12.3
g, 83%). The characterization data for the compound were identical to
those reported.6a
Method B. [{HC(MeCNDip)2}MgI(OEt2)] (10.0 g, 15.6 mmol)
and ca. 5% w/w K/KI powder (15.0 g, 17.3 mmol K, 1.11 equiv) were
combined in a 250 mL Schlenk flask and toluene (80 mL) then added
at room temperature. The suspension was stirred for 23 h, whereupon
filtration and concentration under reduced pressure and storage at
−30 °C overnight led to incipient crystallization of yellow crystalline 3
from the filtrate solution. All volatiles were removed from the mother
liquor in vacuo and the residue washed with hexane (3 × 20 mL). The
resultant yellow residue was dried in vacuo and added to the first
crystalline crop (3.90 g, 57%). The characterization data for the
compound were identical to those reported.6a
Preparation of [{HC(MeCNDip)2}Al:] (4). Toluene (100 mL)
was added to a mixture of [{HC(MeCNDip)2}AlI2] (6.51 g, 9.32
mmol) and ca. 5% w/w K/KI powder (17 g, 19.6 mmol K, 2.1 equiv)
in a 250 mL Schlenk flask. The reaction mixture quickly became
darker, then was stirred for 88 h. The suspension was then filtered,
and the dark red filtrate concentrated under reduced pressure to
incipient crystallization. Placement of the filtrate at −30 °C for 5 days
led to the deposition of red-orange crystalline 4, which was isolated
and dried in vacuo (561 mg, 14%). The characterizing data for the
compound were identical to those reported.10 The mother liquor
EXPERIMENTAL SECTION
■
General Methods. All manipulations were carried out using
standard Schlenk and glovebox techniques under an atmosphere of
high-purity dinitrogen. Hexane and toluene were distilled over
potassium, while diethyl ether and pentane were distilled over Na/
K alloy (50:50). 1H NMR spectra were recorded on a Bruker
AvanceIII 400 spectrometer and were referenced to the resonances of
the solvent used. Scanning electron microscope images were acquired
from uncoated NaCl or Na/NaCl samples using the Magellan
FEGSEM at the Monash Centre for Electron Microscopy. The
microscope was operated using a 2 kV landing energy utilizing beam
deceleration to minimize sample charging. The compounds [{HC-
(MeCNAr)2}MgI(OEt2)] (Ar = Mes, Dep, or Dip)6 and [{HC-
(MeCNAr)2}AlI2]10 were prepared by literature procedures. All other
reagents were used as received.
Preparation of 5% w/w Na/NaCl Powder. Sodium chloride (80
g) was dried overnight in an oven at 110 °C. It was then ground to a
fine powder, using a 1.2 L, 1500 W, domestic kitchen blender over
ca.30 min. The resultant powder was subsequently transferred to a 1 L
Schlenk flask containing a large magnetic stirrer bar and dried under
vacuum, with stirring, for 3 h at ca. 300 °C (using a heat gun;
approximate temperature determined using a hand-held infrared
thermometer). After cooling, sodium metal (4.00 g, 174 mmol) was
added, and the flask was gently heated to ca. 110 °C under an N2
atmosphere with constant agitation and/or stirring. Once the sodium
melted, rapid stirring and agitation continued until no bulk metal was
visible (ca. 30 min), leaving a freely flowing blue/gray powder which
could be stored in a Schlenk flask or in a glovebox until needed (yield
84.0 g).
C
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obtained after crystalline 4 was isolated was evaporated to dryness to
leave an orange-brown solid (3.30 g). An H NMR analysis of this
showed it to contain 4 among other products. Attempts to
recrystallize 4 from this mixture were not successful.
Z.; Liu, L.; Huang, Z.; Chen, P. Int. J. Hydrogen Energy 2016, 41,
15471−15476.
1
(10) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.; Hao,
H.; Cimpoesu, F. Angew. Chem., Int. Ed. 2000, 39, 4274−4276.
(11) (a) Liu, Y.; Li, J.; Ma, X.; Yang, Z.; Roesky, H. W. Coord. Chem.
Rev. 2018, 374, 387−415. (b) Jones, C.; Stasch, A. In The Group 13
Metals Aluminium, Gallium, Indium and Thallium. Chemical Patterns
and Peculiarities; Downs, A. J., Aldridge, S., Eds.; Wiley-Blackwell,
2011. (c) Baker, R. J.; Jones, C. Coord. Chem. Rev. 2005, 249, 1857−
1869.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00803.
■
S
1H NMR spectra of compounds 1−4 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: cameron.jones@monash.edu.
■
ORCID
Cameron Jones: 0000-0002-7269-1045
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Australian Research
Council (grant to C.J.) and the U.S. Air Force Asian Office of
Aerospace Research and Development (grant FA2386-18-1-
0125 to C.J.). A.P. acknowledges the Alexander von Humboldt
Foundation for a Feodor-Lynen Fellowship. We acknowledge
use of the facilities and the assistance of Mr. Nicholas
McDougall and Dr. Xiya Fang at the Monash Centre for
Electron Microscopy.
REFERENCES
■
(1) Selected reviews: (a) Fischer, R. C.; Power, P. P. Chem. Rev.
2010, 110, 3877−3923. (b) Schnockel, H. Chem. Rev. 2010, 110,
̈
4125−4163. (c) Lee, V. Y.; Sekiguchi, A. Organometallic Compounds
of Low-Coordinate Si, Ge, Sn and Pb; Wiley: Chichester, U.K., 2010.
(2) Selected reviews: (a) Weetman, C.; Inoue, S. ChemCatChem
2018, 10, 4213−4228. (b) Chu, T.; Nikonov, G. I. Chem. Rev. 2018,
118, 3608−3680. (c) Hadlington, T. J.; Driess, M.; Jones, C. Chem.
Soc. Rev. 2018, 47, 4176−4197. (d) Sen, S. S.; Khan, S.; Samuel, P. P.;
Roesky, H. W. Chem. Sci. 2012, 3, 659−682. (e) Power, P. P. Acc.
Chem. Res. 2011, 44, 627−637. (f) Asay, M.; Jones, C.; Driess, M.
Chem. Rev. 2011, 111, 354−396. (g) Martin, D.; Soleilhavoup, M.;
Bertrand, G. Chem. Sci. 2011, 2, 389−399. (h) Power, P. P. Nature
2010, 463, 171−177.
(3) Green, S. P.; Jones, C.; Stasch, A. Science 2007, 318, 1754−1757.
(4) (a) Jones, C. Nat. Rev. Chem. 2017, 1, 0059. (b) Jones, C.;
Stasch, A. Top. Organomet. Chem. 2013, 45, 73−102.
(5) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877−910.
(6) (a) Peloso, R.; Resa, I.; Rodríguez, A.; Carmona, E.; Freitag, K.;
Jones, C.; Stasch, A.; Boutland, A. J.; Lips, F. Inorg. Synth. 2018, 37,
33−45. (b) Lalrempuia, R.; Kefalidis, C. E.; Bonyhady, S. J.;
Schwarze, B.; Maron, L.; Stasch, A.; Jones, C. J. Am. Chem. Soc.
2015, 137, 8944−8947. (c) Bonyhady, S. J.; Jones, C.; Nembenna, S.;
Stasch, A.; Edwards, A. J.; McIntyre, G. J. Chem. - Eur. J. 2010, 16,
938−955.
(7) Bonyhady, S. J. PhD Thesis, Monash University, Melbourne,
Victoria, Australia, 2013.
(8) See, for example, (a) Dunbar, A. C.; Macor, J. A.; Girolami, G. S.
Inorg. Chem. 2014, 53, 822−826. (b) Dye, J. L.; Cram, K. D.; Urbin,
S. A.; Redko, M. Y.; Jackson, J. E.; Lefenfeld, M. J. Am. Chem. Soc.
2005, 127, 9338−9339. (c) Screttas, C. G.; Micha-Screttas, M. J. Org.
Chem. 1979, 44, 713−719. (d) Fatt, I.; Tashima, M. Alkali Metal
Dispersions; Van Nostrand, 1961; p 64.
(9) (a) Zhang, M.; Pei, Q.; Chen, W.; Liu, L.; He, T.; Chen, P. RSC
Adv. 2017, 7, 4306−4311. (b) Chen, W.; Wu, G.; He, T.; Li, Z.; Guo,
D
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