Addition of in situ reduced amidinato-methylaluminium chloride to acetylenes

Article abstract of DOI:10.1039/c5dt03128a

Two ethylene-bridged methylaluminium amidinates and one aluminium amidinate containing three terminal trimethylstannyl-ethynyl groups interconnected by π-coordinated potassium ions were prepared in situ. The re-oxidation of the ethylene-bridged compound by iodine followed by further reduction using the same activation procedure demonstrated the versatility of the approach. The reactivity of an ethylene-bridged methylaluminum amidinate towards HCl was examined to demonstrate the building block concept. DFT calculations were performed to gain insight into the mechanism of the in situ activation of diphenylacetylene.

Full text of DOI:10.1039/c5dt03128a

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Addition of in situ reduced amidinato-
methylaluminium chloride to acetylenes†‡
Cite this: DOI: 10.1039/c5dt03128a
T. Chlupatý,a J. Turek,b F. De Proft,b Z. Růžičkováa and A. Růžička*a
Received 13th August 2015,
Accepted 10th September 2015
DOI: 10.1039/c5dt03128a
www.rsc.org/dalton
Two ethylene-bridged methylaluminium amidinates and one alu-
An important part of these synthetic and structural studies
minium amidinate containing three terminal trimethylstannyl- is the activation of the C–C multiple bond via either in situ
ethynyl groups interconnected by π-coordinated potassium ions generated AlI/AlII species or via the stepwise reaction with the
were prepared in situ. The re-oxidation of the ethylene-bridged isolable Al–Al/Al : /Al• intermediate. The in situ reduction of
compound by iodine followed by further reduction using the same diiodoaluminium N,N-diketiminate in the presence of an
activation procedure demonstrated the versatility of the approach. RCuCR moiety (R = Ph or SiMe3)8 and the stepwise activation
The reactivity of an ethylene-bridged methylaluminum amidinate of RCuCR9 (R = H, Ph, Me or SiMe3) by the isolable LAlI inter-
towards HCl was examined to demonstrate the building block mediate both afforded aluminacyclopropenes. However, dialu-
concept. DFT calculations were performed to gain insight into the minacyclobutenes were obtained with organoaluminium
mechanism of the in situ activation of diphenylacetylene.
compounds containing sterically demanding ligands from the
in situ reduction by KC8 and the stepwise reaction via the
10
In the last few decades, the activation of various small mole-
cules and unsaturated systems by low-valent main group metal
complexes and their subsequent chemical transformations
have attracted considerable attention.1 The chemistry of low-
valent aluminium2 compounds, such as AlI and AlII com-
pounds, developed considerably after the first stable species
with an Al–Al arrangement, [(Me3Si)2CH]2Al–Al[CH(SiMe3)2]2,
was prepared and structurally characterized by Uhl3 in 1988.
However, the key milestone was the synthesis of a stable mono-
meric AlI species, an aluminium analogue of carbene deco-
rated with a crowded bidentate diketiminato ligand reported
in 2000 by Roesky and co-workers.4 In addition to synthetic
routes yielding new low-valent aluminium complexes, such as
AlI species, dialumanes1i,2e,5 and some metalloids/clusters,2g,h,6
new reactivity patterns of the compounds towards smaller and
larger molecules7 were reported.
Al–Al fragment7b of Me3SiCuCSiMe3. To the best of our knowl-
edge, only examples of dinuclear aluminium ethylene
bridged5a or double bridged compounds prepared from bis-
amido-dialane and PhCuCH5c followed by heating of the
product in benzene (1,4-dialuminacyclohexadienes11) and
PhCuCPh, respectively, have been described. Pioneering
studies of the reactivity of trialkyl aluminium compounds with
acetylenes activated by UV light or sodium metal have also
been published.5d,e Furthermore, the reaction of dichloro-
aluminium amide with an excess of alkali metal acetylides
(Li,12 Na and K12b) yielded ate complexes consisting of an ionic
aluminium fragment carrying two or three terminal ethynyl
groups involving alkali metal ions in bridging mode.
Herein, we report the synthesis, structural properties and
reactivity of products resulting from the in situ reduction of
chloromethylaluminium species supported by the NCN chelat-
ing amidinato ligand with various acetylenes. Our approach
stemmed from previous work,13 in which the preparation of
the starting LAlMeCl (L
= DippNC(Me)NDipp) and its
aDepartment of General and Inorganic Chemistry, Faculty of Chemical Technology,
University of Pardubice, Studentská 573, CZ-532 10 Pardubice, Czech Republic.
reduction by a potassium mirror to yield LAlMe2 and L2AlMe
were investigated. The probable existence of transient LAlMe
particles offers the possibility that they could be used in
further studies as a trap for various unsaturated systems.
Thus, the reductive coupling (Scheme 1) of [DippNC(Me)-
NDipp]AlMeCl with either PhCCPh or 4-Me3Si-C6H4CCPh and
potassium at room or lower temperatures yielded novel ethyl-
ene-bridged methylaluminium amidinates 1 (31%) and 2
(27%), respectively, along with aluminum amidinates LAlMe2
E-mail: ales.ruzicka@upce.cz
bEenheid Algemene Chemie (ALGC), Member of the QCMM VUB-UGent Alliance
Research Group, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
†Dedicated to Dr Bohumil Štíbr on the occasion of his 75th birthday in reco-
gnition of his outstanding contributions to the area of boron chemistry.
‡Electronic supplementary information (ESI) available: Experimental details,
spectroscopic characterization, computational details, and X-ray crystallographic
data. CCDC 1406406–1406408. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5dt03128a
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nature of the further reactivity of the complex and the struc-
tural design of the products. The diphenylethylene moiety
(CvC found in 1 C55–C56 1.367(3) Å) serves as a linker (Al1–
C55 1.985(2) and C56–Al2 1.987(3) Å in 1) between the two alu-
minium atoms decorated by bidentately bonded amidinates.
Two mechanisms were proposed by Roesky et al.8 for the
reduction of the similar aluminium complex LAlI2 in the pres-
ence of alkynes, via either the formation of the aluminium
centred radical LAlI•, which couples with alkynes, or via the
electron transfer from K to the alkyne and the formation of the
radical anion K+(RCCR)•−, which displaces the iodide in LAlI2.
In both pathways, the same intermediate, LAlI(RCCR)•, is
formed, yielding the desired product via a further electron
transfer reaction. For our system, the latter pathway could be
ruled out because only one electron transfer reaction can take
place; therefore, alkyne coupling would be observed instead of
the formation of 1.
Scheme 1 Synthesis of dinuclear ethylene-bridged methylaluminium
amidinates (1, 2) with the three-component approach and the reactivity
of 1 towards HCl and iodine.
Based on these facts, DFT calculations were performed to
elucidate a plausible reaction mechanism, suggesting one of
the pathways described above and another possible pathway
via an experimentally postulated dialumane intermediate5a
(Fig. 2 and S16 in ESI,‡ Table S2 in ESI‡). Both start with the
reduction of the chloromethylaluminum complex R yielding
radical INT-1. The first pathway comprises the formation of
the dialumane intermediate with an anti (INT-2A) or syn con-
formation (INT-2A′), which subsequently reacts with diphenyl-
acetylene to form the corresponding final product with the
trans (P) or cis (P′) structural arrangement, respectively. The
trans and anti isomers are 7.1 and 6.9 kcal mol−1 lower in
energy than the respective cis and syn isomers, which is in
good agreement with the experimental results. Moreover, the
prolonged reaction time and lower yields are consistent with
the slightly positive ΔG of the second step of the reaction
and L2AlMe (L = DippNC(Me)NDipp) as side-products that
could be removed by crystallization (ESI‡). In addition, the
blank test showed no reaction between both components
without potassium. The in situ activation of acetylenes within
the three-component framework affording aluminacyclopro-
penes or dialuminacyclobutenes has been published by
Roesky and others.8,10
1 (Fig. 1 and S5 in ESI‡) and 2 (Fig. S6 and S7 in ESI‡) were
1
fully characterized by H and 13C NMR spectroscopy in C6D6,
elemental analyses, and XRD. The structures of compounds
1 and 2 both contained four-coordinate aluminium atoms with
a distorted tetrahedral arrangement of the substituents. The
main feature of both dinuclear structures is the presence of an
Al–C(Ph)vC(Ph)–Al chain fragment with twisted phenyl
groups (torsion angles of 50.50 and 49.95°) in the trans con-
figuration. This structural arrangement may predetermine the
Fig. 1 The molecular structure of 1 (ORTEP view, 30% probability level).
Hydrogen atoms are omitted for clarity. Selected interatomic distances
[Å] and angles [°]: N1–C1 1.337(3); N2–C1 1.337(3); N4–C28 1.333(3);
N3–C28 1.333(3); C1–Al1 2.386(2); C28–Al2 2.368(2); Al1–C27 1.971(2);
Al2–C54 1.980(2); N1–Al1 1.937(2); N2–Al1 1.976(2); N3–Al2 1.960(2);
N4–Al2 1.938(2); Al1–C55 1.985(2); C55–C56 1.367(3); C56–Al2 1.987
(3); N1–C1–N2 109.7(2); N3–C28–N4 110.4(2); C1–Al1–C55 129.10(9);
C28–Al2–C56 132.30(9); Al1–C55–C56 120.06(18).
Fig. 2 Energetic profiles (Gibbs free energies in kcal mol−1) for the
in situ interaction of [DippNC(Me)NDipp]AlMeCl (R) with potassium and
diphenylacetylene.
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sequence. The second pathway is similar to the mechanism
proposed by Roesky et al.,8 suggesting the reaction of methyl-
aluminium radical INT-1 with diphenylacetylene, which gene-
rates intermediate INT-2B. The coupling of the aluminium-
diphenylethylene radical INT-2B with methylaluminium
radical INT-1 forms the expected product P (P′). Similar to the
first pathway, the rate-determining step of the reaction mech-
anism is the activation of a CuC triple bond, which has a
slightly negative ΔG. Therefore, the second pathway seems to
be more thermodynamically favourable; however, the negli-
gible differences in ΔG (−0.9 vs. 2.4 kcal mol−1) mean that the
first pathway cannot be excluded.
Scheme 2 Reduction of [DippNC(Me)NDipp]AlMeCl in the presence of
bis(trimethylstannyl)acetylene.
Finally, the two radicals occurring in the proposed reaction
pathways were investigated. INT-1 is an aluminum-centered
radical (Mulliken spin density at Al 82%), whereas for INT-2B
the spin density is more delocalized (Mulliken spin density
at Cethylene 58%) in the π-system of the phenyl ring (Fig. S15
in ESI‡).
The oxidation of 1 by molecular iodine produced a clear
mixture of diphenylacetylene and [DippNC(Me)NDipp]AlMel
(Fig. S1 and S2 in ESI‡). 1H and 13C NMR of the reaction
mixture are shown in Fig. S9 and S10 in the ESI.‡ The reaction
mixture was used without further workup for a re-reduction
using the same method. The reaction proceeded to the same
product (1) in 34% yield. In addition, oxidizing 1 with oxygen
gas afforded a complex mixture of products, mainly consisting
of benzil and DippNC(Me)NHDipp, along with a small amount
of diphenylacetylene and other by-products.
The importance of the structural arrangement of the
diphenylethylene moiety is demonstrated by the reactivity of
complex 1 towards small molecules (Scheme 1). Based on
the 1H (Fig. S11‡) and 13C NMR spectra (Fig. S12‡) and the
EI-MS results (Fig. S14 in ESI‡), the chemical transformation
of the diphenylethylene fragment by two equivalents of HCl
to trans-stilbene was quantitative. This hydrogen substitution
process was completed by the formation of amidine DippNC-
(Me)NHDipp (1H, 13C NMR and EI-MS), and an unidentified
methylaluminium chloride-containing species (Fig. S13 in
ESI‡) as by-products formed due to the decomposition of the
initially formed [DippNC(Me)NDipp]AlMeCl.
for C–Al) and 97.2 ppm (for C–Sn) in the 13C NMR spectrum,
and at −88 ppm in the 119Sn NMR spectrum (116.2 and
−81 ppm, respectively, for Me3SnCCSnMe3). This agrees well
with the data for the previously described ate complexes
[LAl(CuC–Ph)3]M (M = Li, Na or K; broad signal at ∼110 ppm)12b
and [LAl(CuC–SiMe3)3]Li (95.3 ppm for C–Si).12a The chemical
shift of the central carbon atom of the NCN moiety
(165.9 ppm) in the 13C NMR spectrum indicates the presence
of an anisobidentately bonded ligand, which was also sup-
ported by the XRD analyses of 3 (N1–Al1 1.881(4); N2–Al1
2.413(4)). The structure of centrosymmetric dimer 3 (Fig. 3
and S8‡) displays distorted trigonal bipyramidal geometry
(C–Al–C angles 100.46°, 100.65° and 116.04°) around both
five-coordinated aluminium atoms with Al–C distances of
1.970(3), 1.974(3) and 2.041(4) Å. The [(DippNC(Me)NDipp)Al-
(CuC–SnMe3)3]− cores are stabilized by π-coordination
(2.986–3.223 Å, comparable with {[LAl(CuC–Ph)3]M}2 (M =
Na or K)12b) with two potassium atoms, each atom being
connected to four ethynyl groups.
The analogous in situ reduction of [DippNC(Me)NDipp]
AlMeCl in the presence of bis(trimethylstannyl)acetylene
(Scheme 2) resulted in the formation of ca. 15% ate complex 3
identified by NMR and XRD. In the reaction mixture,
aluminium amidinate 3 was accompanied by major by-products
Me3SnSnMe3 (−109 ppm in the 119Sn NMR spectrum)14 and
L2AlMe (L = DippNC(Me)NDipp).13 This structural arrange-
ment on the aluminum atom is not entirely surprising. Some
examples of these rare types of aluminium ate complexes have
been obtained from the reaction of amido-aluminium dichlor-
Fig. 3 The molecular structure of 3 (ORTEP view, 50% probability
level). Hydrogen atoms and methyl groups from the trimethylstannyl
ide with a large excess of alkali metal acetylide.12 Most prob- and 2,6-diisopropylphenyl moiety are omitted for clarity. Selected
interatomic distances [Å] and angles [°]: N1–C1 1.350(5); N2–C1 1.293(4);
N1–Al1 1.881(4); N2–Al1 2.413(4); Al1–C37 1.974(3); Al1–C27 1.970(3);
Al1–C32 2.041(4); C37–C38 1.214(4); K1–C27 3.014(4); K1–C28 3.030(4);
K1–C32 2.986(3); K1–C32a 2.989(3); K1–C33 3.233(4); K1–C33a 3.220(4);
ably, the potassium atom attacks the Sn–C bond in the first
step to form KCCSnMe3,15 which further reacts with the
MeAlCl fragment of the starting component.
The signals corresponding to terminally bonded trimethyl-
K1–C37a 3.036(3); K1–C38a 3.050(4); N1–C1–N2 113.3(4); C1–Al1–C27
stannylethynyl groups in 3 were found at 110.1 (broad signal 106.42(15); C27–Al1–C32 100.46(15).
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The internal ethynyl groups, which are bridged by two pot-
assium atoms, have K–C distances (K1–C32a vs. K1–C33a, see
caption of Fig. 3) that are different by 0.23 Å, whereas the
other K–C distances were similar to K1–C32a. The Al–CuC
fragment was not linear (angles from 170.53° to 176.15°) in 3
or in structures of {[LAl(CuC–Ph)3]M}2 (M = Li, Na or K)12b
and this could be explained by the small energy difference
between the linear and non-linear Al–CuC arrangements.
In conclusion, we described the in situ activation of an
unsaturated CC multiple bond via the reduction of amidi-
nato-methylaluminium chloride in the presence of various
acetylenes. The reaction was partially reversible by the oxi-
dation of iodine with re-reduction. The structure of the pro-
ducts is strongly affected by the nature of the CuC group
substituents (C substituent vs. Sn substituent). Moreover,
we proposed two possible reaction mechanisms for model
compound 1 by using DFT calculations. In addition, the use
of the building block concept was demonstrated by the re-
activity of 1 towards HCl resulting in the formation of trans-
stilbene.
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Republic (grant nr. P207/12/0223) is acknowledged. J.T. and
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