Donor-stabilised molecular Mg/Al-bimetallic hydrides

Article abstract of DOI:10.1039/C8DT03343A

Treatment of dimethylmagnesium with dimethylaluminum hydride in common ethereal solvents led to the bimetallic monomagnesium addition compounds [(do)_xMg{(μ₂-H)(AlMe₃)}₂] (do = thf, x = 4, do = Et₂O, x = 3; do = dme, x = 2). Utilization of methyl tert-butyl ether (mtbe) resulted in alkyl/hydrido overexchange and formation of an 8-membered ring structure of composition [(mtbe)₂Mg{(μ₂-H)(AlMe₃)(μ₂-H)(μ₂-H)(AlMe₂)}]₂. Salt metathesis of [(thf)₄Mg{(μ₂-H)(AlMe₃)}₂] with potassium 1,2,3,4,5-pentamethyl-cyclopentadienide (KCp*) gave [Cp*Mg{(μ₂-H)(AlMe₃)}(thf)₂] featuring a rare isolable magnesium half-sandwich hydride complex. Furthermore, magnesium hydride-inherent reactivity was proven for [(thf)₄Mg{(μ₂-H)(AlMe₃)}₂] by the selective reduction of pyridine to 1,4-dihydropyridide (1,4-DHP) at ambient temperature and the hydromagnesiation of 1-hexene in the absence of any transition metal catalyst (low conversion).

Full text of DOI:10.1039/C8DT03343A

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Donor-stabilised molecular Mg/Al-bimetallic
hydrides†
Cite this: Dalton Trans., 2018, 47,
15173
Christoph Stuhl,
Reiner Anwander
Markus M. Katzenmayer,
*
Cäcilia Maichle-Mössmer
and
Treatment of dimethylmagnesium with dimethylaluminum hydride in common ethereal solvents led to the
bimetallic monomagnesium addition compounds [(do)xMg{(µ2-H)(AlMe3)}2] (do = thf, x = 4, do = Et2O, x = 3;
do = dme, x = 2). Utilization of methyl tert-butyl ether (mtbe) resulted in alkyl/hydrido overexchange and for-
mation of an 8-membered ring structure of composition [(mtbe)2Mg{(µ2-H)(AlMe3)(µ2-H)(µ2-H)(AlMe2)}]2.
Salt metathesis of [(thf)4Mg{(µ2-H)(AlMe3)}2] with potassium 1,2,3,4,5-pentamethyl-cyclopentadienide (KCp*)
gave [Cp*Mg{(µ2-H)(AlMe3)}(thf)2] featuring a rare isolable magnesium half-sandwich hydride complex.
Furthermore, magnesium hydride-inherent reactivity was proven for [(thf)4Mg{(µ2-H)(AlMe3)}2] by the selec-
tive reduction of pyridine to 1,4-dihydropyridide (1,4-DHP) at ambient temperature and the hydromagnesia-
tion of 1-hexene in the absence of any transition metal catalyst (low conversion).
Received 15th August 2018,
Accepted 4th October 2018
DOI: 10.1039/c8dt03343a
rsc.li/dalton
largest magnesium-hydride cluster known to date, namely
[Mg13(Me3TACD)6(μ2-H12)(μ3-H6)][A]2 (A = AlEt4, AlnBu4, B-{3,5-
Introduction
Molecular early main group metal hydride chemistry is a (CF3)2C6H3}4).36 Further to this, a well-defined neutral mole-
highly topical research area with far-reaching impact on both cular magnesium dihydride of the type [MgH2(donor)x] has
hydrogen storage materials and catalysis.1–9 Saline [MgH2]n not been isolated so far.9 Early examples of molecular hetero-
has been known for over a century, but due to its high lattice bimetallic magnesium hydride complexes were reported by
energy (ΔH = 2709 kJ mol−1) it is poorly soluble in common Lehmkuhl et al., resulting from the reaction of anthracene
organic solvents and therefore unsuitable as
a
starting magnesium with aluminum hydrides,16 as well as by Mulvey
material for accessing molecular magnesium hydride et al., exploiting a stabilising amido environment in dimeric
complexes.10–12 Quantum chemical studies of molecular mag- [Mg2M2(NiPr2)4(μ-H)2(toluene)x] (M = Na, K).37,38 Likewise
nesium hydride fragments showed an unambiguous relation- inspiring were the investigations by Schlesinger et al., who
ship between thermodynamic stability and cluster size.13 reported in 1951 on the reactivity of LiAlH4 toward metal
Hence, combining hydrido transfer reagents such as LiAlH4,14 alkyls of Be, Mg, Zn, and Cd.39 This paper also featured an
15
16–19
PhSiH3 or RxAlH3−x
with magnesium precursors bearing enlightening synthesis protocol, with dimethylberyllium
sterically demanding ancillary ligands was found central to the and dimethyl aluminum hydride (HAlMe2) being mixed
successful synthesis of molecular magnesium hydrides.14,17–26 together, which however, did not result in well-defined
In line with this strategy are the recently described syntheses beryllium hydride species. In contrast, we have shown that
of discrete hydride complexes of calcium, strontium, and HAlMe2 is effective in producing rare-earth metal hydride
barium.27–31 Not surprisingly, terminal Mg–H complexes complexes of the type [TptBu,MeLu{(μ2-H)AlMe3}2] exploiting
are scarce,14,18,22,24–26 and cluster formation exhibits an the stabilising effect of a superbulky tris(pyrazolyl)borato
expected but intriguing reaction pathway.32–35 For example in ligand.40,41
2015, Okuda and coworkers employed the well-established
Considering our interest in tracking parallels between alka-
phenylsilane-based hydrogenolysis36 for the fabrication of the line-earth and rare-earth metal chemistry, we report herein an
adapted route for alkaline-earth metal chemistry. Most reward-
ingly, discrete methylaluminum-supported magnesium
Insitut für Anorganische Chemie, Eberhard Karls Universität Tübingen (EKUT),
hydride complexes of the type [(donor)xMg{(µ2-H)(AlMe3)}2]
Auf der Morgenstelle 18, 72076, Germany.
42
could be obtained by simply treating pure [MgMe2]n with
E-mail: reiner.anwander@uni-tuebingen.de
HAlMe2 in common ethereal solvents like thf, diethyl ether
(Et2O), or dme. The structural elucidation and hydride-
inherent reactivity of these complexes is presented.
†Electronic supplementary information (ESI) available: NMR and crystallo-
graphic data. CCDC 1853436–1853440. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c8dt03343a
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(Mg–H = 1.98(2) Å), but the Al–H bond lengths appear signifi-
cantly elongated (1.67(4) Å and 1.64(4) Å versus av. 1.52 Å).43,44
The Mg–H distances of other previously mentioned Mg–H–Al
Results and discussion
Synthesis and characterization of Mg/Al-bimetallic hydrides
Treatment of [MgMe2]n with HAlMe2 in thf led to the for- linkages range from 1.79(3) to 2.14(3) Å.16,18,43–46 It should be
mation of the bimetallic hydrido-bridged magnesium alumi- noted that [(thf)4Mg(AlH4)2] is hardly soluble in thf.39 Truly
nate complex [(thf)4Mg{(µ2-H)(AlMe3)}2] (1, Scheme 1). terminal Mg–H bond lengths were found in the range of
Complex 1 could be crystallised in 84% yield (triclinic, P1), is 1.71(2) Å (ref. 26) to 1.85(3) Å.24 1H NMR spectroscopic studies
ˉ
soluble in thf and toluene, but insoluble in n-hexane. The mag- of complex
1 in toluene-d8 showed a sharp singlet at
nesium centre in 1 adopts an octahedral coordination geome- −0.52 ppm for the six AlMe groups and a very broad hydrido
try comprising four equatorial thf molecules and two transoid resonance (4.50 ppm to 2.50 ppm) overlapping with the signal
μ2-bridging hydrido ligands (H1–Mg1–H2, 177.5(14)°; Fig. 1, of the O–CH2 groups of the coordinating thf molecules. For
left). Complex 1 exhibits Mg–H bond lengths (1.96(3) Å and further comparison, the proton resonances of terminal Mg–H
1.94(3) Å) matching those in isostructural [(thf)4Mg(AlH4)2] were found in the range of 3.55 ppm (ref. 18) to 6.58 ppm.14
When thf was replaced by diethyl ether, the resulting
complex became more difficult to isolate but could be crystal-
lised as [(Et2O)3Mg{(µ2-H)(AlMe3)}2] (2) at −40 °C from a satu-
rated Et2O solution. The crystals readily melted at ambient
temperature to form a viscous colourless oil in 51% yield. The
solid-state structure of 2 features 5-coordinate magnesium
centres in a slightly distorted trigonal bipyramidal environ-
ment (monoclinic, P21/c, Fig. 1, mid left). Due to the increased
steric demand of Et2O compared to thf, only three ether mole-
cules coordinate to the magnesium centre, and as a conse-
quence the Mg–H bond lengths in 2 are slightly shorter
(1.83(4) Å and 1.87(4) Å), with the Al–H distances being similar
to those observed in 1 (1.69(4) Å and 1.63(4) Å). The reduced
steric pressure also impacts the H1–Mg1–H2 angle which
decreased to 165.4(16)°. Addition of n-pentane to a solution of
2 in Et2O led to the formation of an amorphous white solid
that could not be dried under vacuum. At low n-pentane levels
Scheme 1 Synthesis of discrete complexes [(thf)4Mg{(µ2-H)(AlMe3)}2]
(1), [(Et2O)3Mg{(µ2-H)(AlMe3)}2] (2), and [(dme)2Mg{(µ2-H)(AlMe3)}2] (3)
involving methyl/hydrido redistribution and exchange. Reaction con-
the precipitate turned back into a clear oil, which evaporated
under long periods of drying. The 1H NMR spectrum of 2
showed two broad signals for the Mg–H–Al moiety, one reach-
ditions: ambient temperature, 2 min.
Fig. 1 Solid-state structures of [(thf)4Mg{(µ2-H)(AlMe3)}2] (1) (left), [(Et2O)3Mg{(µ2-H)(AlMe3)}2] (2) (mid left), [(dme)2Mg{(µ2-H)(AlMe3)}2] (3) (mid
right) and [(mtbe)2Mg{(µ2-H)(AlMe3)(µ2-H)(µ2-H)(AlMe2)}]2 (4) (right). Hydrogen atoms, one lattice thf molecule (1), and the disorder in all Et2O (2) are
omitted for clarity. All other atoms are represented by atomic displacement ellipsoids at the 50% probability level. Selected bond lengths [Å] and
angles [°] for 1: Mg1–H1 1.96(4), Mg1–H2 1.94(4), Al1–H1 1.64(4), Al1–H2 1.67(4), Al2–C20 1.941(6), Al2–C21 1.984(4), O1–Mg1–O2 87.97(11),
O2–Mg1–O3 88.79(11), O3–Mg1–O4 92.06(11), O1–Mg1–O3 176.67(11), O2–Mg1–O4 178.60(11), H1–Mg1–H2 177.5(14), Al1–H1–Mg1 166(2),
Al2–H2–Mg1 165(2). Selected bond lengths [Å] and angles [°] for 2: Mg1–H1 1.83(4), Mg1–H2 1.87(4), Al1–H1 1.69(4), Al1–H2 1.63(4), O1–Mg1–O2
104.54(11), O2–Mg1–O3 103.10(11), O1–Mg1–O3 152.28(11), H1–Mg1–H2 165.4(16), Al1–H1–Mg1 172(3), Al2–H2–Mg1 175(2). Selected bond
lengths [Å] and angles [°] for 3: Mg1–H1 1.83(2), Mg1–H2 1.92(2), Al1–H1 1.70(2), Al1–H2 1.61(2), O1–Mg1–O2 76.90(5), O2–Mg1–O3 89.37(5), O3–
Mg1–O4 76.07(5), O1–Mg1–O4 164.24(5), H1–Mg1–H2 100.5(10), Al1–H1–Mg1 150.8(15), Al2–H2–Mg1 148.6(14). Selected bond lengths [Å] and
angles [°] for 4: Mg1–H1 1.942(19), Mg1–H2 1.856(19), Mg1–H3 1.859(19), Mg1–O1 2.0454(13), Mg1–O2 2.0526(12), Al1–H1 1.610(19), Al1’–H2 1.635(19),
Al2–H3 1.688(19), O1–Mg1–O2 111.04(5), O1–Mg1–H2 126.5(6), O2–Mg1–H2 121.8(6), O1–Mg1–H3 89.5(6), O2–Mg1–H3 95.9(6), H1–Mg1–H2 87.4(8),
H1–Mg1–H3 179.5(8), H2–Mg1–H3 93.0(8), Al1–H2–Mg1 165.0(13), Al2–H3–Mg1 169.2(12).
15174 | Dalton Trans., 2018, 47, 15173–15180
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ing from 4.0–3.4 and another reaching from 3.1–2.9 ppm with Reactivity of Mg/Al-bimetallic hydrides
a ratio of 2 : 1 for Et2O/AlMe3, indicating the existences of
The metal/magnesium hydrogen bonding in bimetallic com-
another species in solution, likely [(Et2O)2Mg{(µ2-H)(AlMe3)}2].
Due to difficulties associated with its isolation, complex 2 was
not used further for reactivity studies.
plexes 1–4 can be either interpreted as a Lewis acid-stabilised
“MgH2” species or
a
Mg2+-cation interacting with two
[HAlMe3]− anions (1–3) or with one each [HAlMe3]− and
[H2AlMe2]− anion (4), respectively. Crucially, in contrast to
hydride complexes 1–4 homoleptic [Mg(AlMe4)2] forms a com-
pletely separated ion pair in thf, namely [Mg(thf)6]2+ and
[AlMe4]−.47 This poses the question whether our Mg/Al bi-
metallic hydrides can act as a magnesium hydride source. We
therefore probed the reactivity of these complexes toward
different substrates. Applying a salt metathesis protocol,
complex 1 reacted readily with KCp* to yield the unique half-
sandwich magnesium hydride derivative [Cp*Mg{(µ2-H)
(AlMe3)}(thf)2] (5). X-ray structure analysis of 5 revealed a
monomeric complex with a 6-coordinate magnesium centre
(orthorhombic, P212121) (Fig. 2). The Mg–H bond length
(1.88(3) Å) turned out to be considerably longer than the Al–H
bond length (1.64(3)). When dissolved in [D6]benzene, half-
sandwich complex 5 engages in a Schlenk equilibrium invol-
ving ligand redistribution to [MgCp*2] and [MgH2]n (cf.,
Fig. S10–S12†).48
Such monomeric “CpMgH” species are exceedingly scarce
and according to the Cambrigde Crystallographic Data Base
only the borohydride adduct [(Cp*Mg(H3BNMe2BH3)(thf)] was
characterised by X-ray structure analysis yet.49 Based on X-ray
powder diffraction, NMR and IR spectroscopic studies Ashby
reported the dimeric complex [CpMgH(thf)]2, accessed from
HCp and [MgH2]n in thf.50 Protonolysis of 1 with trimethyl-
silylcyclopentadiene (HCp′) was performed at elevated temp-
erature in toluene and indicated the formation of hydrogen
gas (NMR-scale reaction, Scheme 2). We were not able to
isolate and analyse the respective half-sandwich complex,
which also converted into the metallocene derivative. The
proton shifts for [MgCp*2] and [MgCp′2] were consistent with
those reported in the literature.51,52
Performing the reaction of [MgMe2]n with HAlMe2 in dme,
bis(adduct) [(dme)2Mg{(µ2-H)(AlMe3)}2] (3) formed readily
(Scheme 1). Single-crystalline 3 was obtained in 83% yield
(orthorhombic, Pna21) (Fig. 1, mid right). In the solid state,
complex 3 shows a pseudo-octahedral geometry, coordinated
by two chelating dme molecules and two hydrido ligands. As
found for complexes 1 and 2, the Al–H bond lengths (Al–H1
1.70(2) Å; Al2–H2 1.61(2) Å) in 3 are shorter than the Mg–H
bond lengths (Mg–H1 1.83(2) Å; Mg–H2 1.92(2) Å). Unlike the
transoid hydrido ligands in complexes 1 (177.5(14)°) and
2 (165.4(16)°), the hydrido ligands in 3 are cis-positioned with
an H1–Mg–H2 angle of 100.5(10)°. The Mg–H resonance in the
1H NMR spectrum of 3 appeared as a broad singlet at
2.30 ppm indicative of a significantly slowed down hydrido
ligand exchange compared to 1.
By further increasing the steric demand of the donor
solvent molecules (do = mtbe = methyl tert-butyl ether) crystal-
lisation afforded complex [(mtbe)2Mg{(µ2-H)(AlMe3)(µ2-H)(µ2-H)
(AlMe2)}]2 (4), involving both methyl/hydrido redistribution
and exchange (Scheme 1). An X-ray crystallographic analysis of
4 (monoclinic, P21/n) revealed the formation of an eight-mem-
bered ring of alternating Mg/H/Al atoms (Fig. 1, right). Each
5-coordinate magnesium atom bears three hydrido ligands,
leading to the highest H/Mg ratio of 3 : 1 for all presented com-
pounds. The Mg–H bond lengths range from 1.856(19) Å to
1.942(19) Å, not being noticeably affected by the extended
Mg2Al4H6 motif (Al–H: 1.610(19) Å – 1688(19) Å), which inte-
grates both almost perfect transoid (H1–Mg1–H3 179.5(8)°)
and cisoid (H2–Mg1–H3 93.0(8)°) alignments. The 1H NMR
spectrum of 4 shows a broad signal for the terminal hydrido
ligands at −0.3 ppm, which is an uncommon region for such
hydrido signals. Hydrido exchange between the two different
[HAlMe3]− and [H2AlMe2]− species in 4 was verified by 1H
EXSY NMR spectroscopy (cf., Fig. S9†).
We were also interested in the isolation of donor-free
complex [Mg(HAlMe3)2], but neither treatment of polymeric
[MgMe2]n nor discrete [Mg(AlMe4)2] with HAlMe2 in aliphatic/
aromatic solvents led to any isolable product. In case of
[MgMe2]n, the solution became turbid, indicating a reaction
but most likely formation of [MgH2]n and AlMe3. The
[Mg(AlMe4)2]-reaction led instantly to the formation of a white
precipitate, the characterization of which was not conclusive.
Both experiments underline the importance of the presence of
a donor solvent in these reactions for obtaining isolable pro-
ducts.39 It is noteworthy that reactions of [YMe3]n with organo-
Fig. 2 Solid-state structure of [Cp*Mg{(µ2-H)(AlMe3)}(thf)2] (5).
aluminum hydrides, e.g., HAlMe2, in toluene also produced an Hydrogen atoms are omitted for clarity. All other atoms are represented
by atomic displacement ellipsoids at the 50% probability level. Selected
bond lengths [Å] and angles [°]: Mg1–H1 1.88(3), Mg1–O1 2.0423(15),
Mg1–O2 2.0340(16), Mg1–Ct 2.058, Mg1–C 2.368(2)–2.410(2), Al1–H1
1.64(3), Al1–C19 1.987(2), Al1–C20 1.989(2), Al1–C21 1.981(2),
O1–Mg1–O2 91.29(7), O1–Mg1–H1 94.0(8), O2–Mg1–H1 93.7(8),
Mg1–H1–Al1 177.1(18).
intractable white precipitate from transient clear solutions.
Tentatively assigned heteroaluminate species “Y(AlHMe3)3”
was suggested by in situ 1H NMR spectroscopy (1JY–H
=
20.5 Hz), while the occurrence of an hydrido transfer was
unambiguously evidenced by the formation of Al2Me6.41
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addition of 1, 3, and 4 proceeded at 130 °C under formation of
Mg(n-hex)2. For complex 1, the hydrometallation efficiency was
comparable to previous findings.60–62 One reason for the
observed overall low yields might be that the stabilising effect
of the ethereal donor is lost at elevated temperatures causing
precipitation of [MgH2]n and formation of AlMe3(thf)
(Scheme 3). Therefore, a high donor-to-magnesium ratio is
needed in order to prevent precipitation, ergo increasing the
yield of Mg(n-hex)2.
Scheme 2 Synthesis of monomeric [Cp*Mg{(µ-H)(AlMe3)}(thf)2] (5) via
salt metathesis and [Cp’Mg{(µ-H)(AlMe3)}(thf)2] (not isolated) via proto-
nolysis and further redistribution to the corresponding magnesocene
complexes.
Conclusions
In summary, by re-investigating Schlesinger’s HAlMe2 experi-
ment,39 we gained access to molecular discrete ether-stabilised
aluminum alkyl-supported magnesium hydride complexes.
Depending on the steric bulk of the ethereal donor either
methyl/hydrido redistribution as in [(thf)4Mg[{µ2-H}{AlMe3}]2]
or methyl/hydrido “overexchange” as in the 8-membered ring
structure of [(mtbe)2Mg{(µ2-H)(AlMe3)(µ2-H)(µ2-H)(AlMe2)}]2
was observed. The stabilising effect of the ether molecules is
crucial for preventing the formation of amorphous insoluble
[MgH2]n. Complex [(thf)4Mg[{µ2-H}{AlMe3}]2] features a valu-
able precursor for accessing half-sandwich magnesium
hydride complexes but also for assessing inherent hydride
reactivity as shown for the selective reduction of pyridine
under mild conditions.
Dihydropyridines are valuable components of natural pro-
ducts (e.g., nicotinamide adenine dinucleotide (NADH)) and
perform as metal-free hydride sources. Van der Kerk reported
in 1980 a [MgH2]n-promoted reduction of pyridine to 1,2- and
1,4-dihydropyridide (1,2-DHP, kinetic product; 1,4-DHP,
thermodynamic product) and outlined the reactivity toward
different organic substrates.53,54 At 60 °C, 1,2-DHP undergoes
a selective, nonreversible rearrangement to 1,4-DHP. Harder
described the stoichiometric hydrogenation of pyridines
as well as the catalytic hydroboration of pyridines with
[(NN)(MgH)2]2 (NN = ((2,6-iPr2C6H3)NC(Me)CHC(Me)N-)2).55
Further to unveil the hydride-inherent reactivity of the Mg/Al
bimetallic complexes under study, we examined the reaction of
complex 1 with pyridine as a basic test reaction as demon-
strated previously by others.56–59 The formation of multiple
[Mg(DHP)2] species could be tracked by an NMR-scale experi-
ment, which isomerised selectively in a couple of hours at
ambient temperature to [Mg(1,4-DHP)2] as the only dihydropyr-
idine derivative (Scheme 3). An HAlMe2-promoted hydro-
alumination reaction under the same conditions gave a com-
plicate mixture of products (cf., Fig. S16†), which emphasises
the magnesium hydride character of compound 1.
Experimental
General experimental procedures and instrumentation
All manipulations were performed under rigorous exclusion of
air and moisture, using standard Schlenk, high-vacuum, and
glovebox techniques (MBraun MB200B; <1 ppm O2, <1 ppm
H2O, argon atmosphere). The solvents n-pentane, diethyl ether
(Et2O), and tetrahydrofuran (thf) were purified using Grubbs
columns (MBraun SPS, solvent purification system). 1,2-
Dimethoxyethane (dme) and methyl tert-butyl ether (mtbe)
were refluxed over a sodium/benzophenone mixture until the
solution turned blue and then distilled off under argon.
Benzene-d6 (99.6%, Sigma-Aldrich) and THF-d8 (99.5%, Euriso-
top) were dried by destillation from Na/K-alloy. Toluene-d8
(>99%, Sigma-Aldrich) was stored over sodium metal and fil-
tered prior to use. All solvents were stored inside a glovebox.
1-Hexene (>99%, Sigma Aldrich), HCp′ (Me3SiC5H5, >97%,
TCI) and phenylsilane (PhSiH3, 97%, Alfa-Aesar) were used as
received. Dimethylaluminum hydride HAlMe2,63 dimethyl-
magnesium [MgMe2]n,64 and KCp* were synthesised according
to literature/standard procedures. NMR spectra of air and
moisture sensitive compounds were recorded by using
J. Young valve NMR tubes at variable temperature on a Bruker
AVII+400 (1H: 400.13 MHz; 13C: 100.61 MHz) and a Bruker
AVII+500 (1H: 500.13 MHz; 13C: 125.76 MHz). 1H and 13C shifts
are referenced to internal solvent resonances and reported in
parts per million relative to tetramethylsilane (TMS). Coupling
Finally, hydromagnesiation experiments with 1-hexene were
monitored by 1H NMR spectroscopy, revealing that the
Scheme 3 Pyridine reduction with 1 leading to the selective formation
of the magnesium 1,4-DHP derivative (top). Hydromagnesiation reaction
of 1-hexene promoted by 1 (bottom).
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constants are given in Hertz. IR spectra were recorded on a to dryness yielded additional product. Total yield: 534 mg,
NICOLET 6700 FTIR spectrometer equipped with a DRIFT cell 1.52 mmol, 83%. IR (KBr): 3022w, 2916s, 2882m, 2811m,
(KBr window), and the samples were prepared in a glovebox 1733s, 1716s, 1683s, 1652s, 1635m, 1558w, 1539w, 1505w,
and mixed with KBr powder. Alternatively, the IR spectra were 1471m, 1456m, 1368w, 1278w, 1246w, 1190m, 1163s, 1099s,
recorded as Nujol mulls sandwiched between CsI plates, on 1055s, 868s, 766s, 718s, 694s, 685s, 594m, 534w, 507m cm−1
.
the same instrument. Elemental analyses were performed on 1H NMR (400 MHz, toluene-d8, 26 °C): δ = 3.28 (s, 8H, O–CH2,
an Elementar Vario Micro Cube. The metal content (Mg, Al) dme), 3.11 (s, 12H, O–CH3, dme), 2.30 (s, 2H, MgH),
was determined by means of inductively coupled plasma −0.57 (s, 18H, Al–CH3) ppm. 13C{1H} NMR (101 MHz,
atomic emission spectroscopy on a Thermo Scientific™ iCAP™ toluene-d8, 26 °C): δ = 71.5 (CH2, dme), 58.9 (CH3, dme),
7000 Series ICP-OES.
−6.8 (Al–CH3) ppm. Elemental analysis calcd (%) for
C
C 47.60, H 11.19.
14H40Al2MgO4 (350.74 g mol−1): C 47.94, H 11.50 found:
Preparation and characterisation of compounds 1–5
[(thf)4Mg{(µ2-H)(AlMe3)}2] (1). To a solution of [MgMe2]n
[(mtbe)2Mg{(µ2-H)(AlMe3)(µ2-H)(µ2-H)(AlMe2)}]2 (4). To a
(100 mg, 1.84 mmol) in thf (3 ml) a solution of HAlMe2 solution of [MgMe2]n (100 mg, 1.84 mmol) in mtbe (3 ml) a
(213 mg, 3.68 mmol) in 1 mL thf was added. The reaction solution of HAlMe2 (213 mg, 3.68 mmol) in mtbe (1 ml) was
mixture was stirred at ambient temperature for 2 min until a added. The reaction mixture was stirred for 2 min at ambient
clear solution was obtained. Crystals suitable for X-ray struc- temperature until a clear solution was obtained. Crystals suit-
ture determination were grown at −40 °C. When evaporating able for X-ray structure determination were grown at −40 °C
the solvent under reduced pressure the clear colourless crystals from a pentane/mtbe mixture of this reaction. The clear col-
changed to a white solid due to loss of lattice thf. Yield: ourless crystals were separated from the solution and dried
705 mg, 1.54 mmol, 84%. IR (KBr): 2981s, 2915s, 2812m, under reduced pressure. After filtration and evaporation of the
1651s, 1460m, 1369w, 1344w, 1295w, 1247w, 1166s, 1032s, remaining solution to dryness additional product could be
918m, 882s, 770s, 760s, 689s, 593m, 540m, 524w cm−1 1H collected. Total yield: 302 mg, 454 μmol, 74%. IR (nujol):
.
NMR (400 MHz, toluene-d8, 26 °C): δ = 4.5–2.7 (br, 2H, MgH), 2955s, 2923s, 2853s, 2725w, 2670w, 2360w, 2342w, 1456s,
3.46 (m, 16H, O–CH2, thf), 1.22 (m, 16H, CH2, thf), −0.52 (s, 1377s, 1303m, 1190m, 1169m, 1041w, 995m, 891w, 828w,
18H, Al–CH3) ppm. 13C{1H} NMR (101 MHz, toluene-d8, 26 °C): 721s, 684m, 531w cm−1 1H NMR (400 MHz, toluene-d8,
.
δ = 69.3 (thf), 25.3 (thf), −8.6 (Al–CH3) ppm. Elemental ana- 26 °C): δ = 4.00–3.13 (br, 4H, Mg{(µ2-H)(µ2-H)Al(CH3)2}),
lysis calcd (%) for C22H52Al2MgO4 (458.92 g mol−1): C 57.58, H 2.93 (s, 12H, O–CH3, mtbe), 0.94 (s, 36H, C(CH3)3, mtbe),
11.42; found: C 56.93, H 10.77.
0-(−0.35) (br, 2H, Mg{(µ2-H)Al(CH3)3}, −0.43 (s, 37H,
[(Et2O)3Mg{(µ2-H)(AlMe3)}2] (2). To a solution of [MgMe2]n Al–CH3) ppm. 13C{1H} NMR (101 MHz, toluene-d8, 26 °C):
(100 mg, 1.84 mmol) in Et2O (3 ml) a solution of HAlMe2 δ = 85.0 (C(CH3)3 mtbe), 52.5 (O–CH3 mtbe), 27.0 (C(CH3)3,
(213 mg, 3.68 mmol) in 1 ml Et2O was added. The reaction mtbe), −4.9 (Al–CH3) ppm. Elemental analysis calcd (%) for
mixture was stirred at ambient temperature for 2 min until a
C
30H84Al4Mg2O4 (664.53 g mol−1): C 54.14, H 12.72 found: C
clear solution was obtained. Crystals suitable for X-ray struc- 50.05, H 11.14. A better elemental analysis was hampered
ture determination were grown at −40 °C. When evaporating by the highly air- and moisture-sensitive nature of com-
the solvent under reduced pressure very carefully the clear col- pound 4.
ourless crystals changed to a colourless oil. Longer evaporation
[Cp*Mg{(µ2-H)(AlMe3)}(thf)2] (5). To
a solution of 1
times led to decomposition of 2. Yield: 369 mg, 939 μmol, (100 mg, 218 μmol) in toluene (3 ml) KCp* (38.0 mg,
51%. IR (nujol): 2953s, 2922s, 2853s, 1733w, 1463m, 1389w, 0.22 mmol) in toluene (1 ml) was added. The reaction mixture
1377w, 1287w, 1188m, 1151w, 1090w, 1038s, 997w, 899w, was stirred at 40 °C until a clear solution was obtained. After
834w, 779w, 709w, 687s, 624w cm−1
benzene-d6, 26 °C): δ = 4.0–3.4 (br, 1H, MgH), 3.34 (q, JHH
7.3 Hz, 8H, O–CH2, Et2O), 3.1–2.9 (br, 1H, MgH), 0.74 (t, 3JHH
.
1H NMR (400 MHz, 16 h of stirring a white precipitate had formed. Filtration and
3
=
=
crystallisation from the separated solution at −40 °C produced
single-crystalline 5 suitable for X-ray structure analysis. The
7.2 Hz, 12H, CH3, Et2O), −0.46 (s, 18H, Al–CH3) ppm. 13C{1H} clear colourless crystals were separated from the solution and
NMR (101 MHz, benzene-d6, 26 °C): δ = 66.2 (Et2O), 13.7 dried under reduced pressure. Yield: 39.2 mg, 104 μmol, 48%.
(Et2O), −8.0 (Al–CH3) ppm. An appropriate elemental analysis Over time the product converted into [Cp*2Mg] and [MgH2]n
was not obtained due to the highly air- and moisture-sensitive (Fig. S14 and S15†). IR (KBr): 2954s, 2921s, 2861s, 2728w,
nature of compound 2.
1725m, 1459m, 1378w, 1345w, 1296w, 1247w, 1182m, 1019s,
[(dme)2Mg{(µ2-H)(AlMe3)}2] (3). To a solution of [MgMe2]n 958w, 919m, 869s, 795w, 713vs, 703s, 695s, 681s, 619m, 596m,
(100 mg, 1.84 mmol) in dme (3 ml) a solution of HAlMe2 522m cm−1. 1H NMR (400 MHz, benzene-d6, 26 °C): δ = 3.40 (t,
(213 mg, 3.68 mmol) in dme (1 ml) was added. The reaction 3JHH = 6.8 Hz, 8H, O–CH2, thf), 2.12 (s, 15H, Cp–CH3, 5), 1.94
mixture was stirred for 2 min at ambient temperature (s, 30H, Cp*2Mg), 1.78 (br, 1H, Mg–H–Al), 1.17 (m, 8H, CH2,
until a clear solution was obtained. Crystals suitable for X-ray thf), −0.26 (s, 9H, H–AlCH3), −0.40 (s, 9H, Al–CH3, AlMe3thf)
structure determination were grown at −40 °C. The clear col- ppm. 13C{1H} NMR (101 MHz, benzene-d6, 26 °C): δ = 110.3
ourless crystals were separated from the solution and dried (Cp*2Mg), 109.4 (Cp*–CH3, 5), 69.3 (O–CH2, thf), 25.2
under reduced pressure. Evaporating the remaining solution (CH2, thf), 11.2 (Cp*2Mg), 10.0 (Cp*–CH3, 5), −6.7 (H–AlCH3),
This journal is © The Royal Society of Chemistry 2018
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−8.6 (Al–CH3, AlMe3thf) ppm. Elemental analysis calcd (%) and fixed on a nylon loop/glas fiber. X-ray data for compounds
for C21H41AlMgO2 (376.28 g mol−1): C 66.93, H 10.97 found: 1, 2, 3, 4 and 5 were collected on a Bruker APEX DUO instru-
C 66.33, H 10.10.
ment equipped with an IμS microfocus sealed tube and
Protonolysis of 1 with HCp′. Solutions of 1 (20.0 mg, QUAZAR optics for MoKα radiation (λ = 0.71073 Å). The data
0.04 mmol) in toluene-d8 (0.3 ml) and HCp′ (6.01 mg, collection strategy was determined using COSMO65 employing
0.04 mmol) in toluene-d8 (0.3 ml) were transferred and com- ω- and ϕ scans. Raw data were processed using APEX66,67 and
bined into a J. Young valve NMR sample tube and monitored SAINT,68,69 corrections for absorption effects were applied
by 1H NMR spectroscopy. The reaction mixture was heated using SADABS.70,71 The structures were solved by direct
carefully in steps of 10 °C until a reaction took place at 110 °C. methods and refined against all data by full-matrix least-
After 4 h the reaction was stopped and a white precipitate, squares methods on F2 using SHELXTL72 and Shelxle.73 All
1
possibly [MgH2]n, had formed. H NMR (400 MHz, toluene-d8, Graphics were produced employing ORTEP-374,75 and
26 °C): δ = 6.22 (d, 3JHH = 11.3 Hz, 4H, Cp′–H), 4.50 (s, H2), 3.42 POV-Ray.76 Further details of the refinement and crystallo-
(m, 20H, O-CH2, thf), 1.23 (m, 20H, CH2, thf), 0.26 (s, 9H, Cp′ graphic data are listed in the CIF files. CCDC 1853436 (1),
Si-CH3), −0.44 (s, 1.8H, AlMe3-species not determined), −0.53 1853437 (2), 1853438 (3), 1853439 (4) and 1853440 (5) contain
(s, 18H, Al–CH3, AlMe3(thf)) ppm.
all the supplementary crystallographic data for this paper.†
Reaction of 1 with pyridine, forming [(py)xMg(1,4-
dihydropyridide)2]
Conflicts of interest
(a) NMR-scale reaction. Solutions of 1 (10.0 mg, 21.8 μmol)
in toluene-d8 (0.3 ml) and of pyridine (13.8 mg, 174 μmol) in There are no conflicts of interest.
toluene-d8 (0.3 ml) were transferred and combined into a
J. Young valve NMR sample tube and monitored by 1H NMR
spectroscopy. After 10 min a mixture of [(py)xMg(1,2-DHP)2],
[(py)2Mg(1,2-DHP)(1,4-DHP)], and [(py)2Mg(1,4-DHP)2] had
Acknowledgements
formed which isomerised overnight at ambient temperature to We are grateful to the German Science Foundation DFG for
[(py)2Mg(1,4-DHP)2] as the only product. Solutions of HAlMe2 generous support (Grant AN 238/15-2).
(10.0 mg, 174 μmol) in benzene-d6 (0.3 ml) and of pyridine
(13.6 mg, 174 μmol) in benzene-d6 (0.3 ml) were transferred
and combined into a J. Young valve NMR sample tube and
Notes and references
monitored by 1H NMR spectroscopy revealing a very unselec-
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