Complexation and Versatile Reactivity of a Highly Lewis Acidic Cationic Mg Complex with Alkynes and Phosphines

Article abstract of DOI:10.1002/chem.201804802

[(BDI)Mg⁺][B(C₆F₅)₄^?] (1; BDI=CH[C(CH₃)NDipp]₂; Dipp=2,6-diisopropylphenyl) was prepared by reaction of (BDI)MgnPr with [Ph₃C⁺][B(C₆F₅)₄^?]. Addition of 3-hexyne gave [(BDI)Mg⁺?(EtC≡CEt)][B(C₆F₅)₄^?]. Single-crystal X-ray analysis, NMR investigations, Raman spectra, and DFT calculations indicate a significant Mg-alkyne interaction. Addition of the terminal alkynes PhC≡CH or Me₃SiC≡CH led to alkyne deprotonation by the BDI ligand to give [(BDI-H)Mg⁺(C≡CPh)]₂?2 [B(C₆F₅)₄^?] (2, 70 %) and [(BDI-H)Mg⁺(C≡CSiMe₃)]₂?2 [B(C₆F₅)₄^?] (3, 63 %). Addition of internal alkynes PhC≡CPh or PhC≡CMe led to [4+2] cycloadditions with the BDI ligand to give {Mg⁺C(Ph)=C(Ph)C[C(Me)=NDipp]₂}₂? 2 [B(C₆F₅)₄^?] (4, 53 %) and {Mg⁺C(Ph)=C(Me)C[C(Me)=NDipp]₂}₂?2 [B(C₆F₅)₄^?] (5, 73 %), in which the Mg center is N,N,C-chelated. The (BDI)Mg⁺ cation can be viewed as an intramolecular frustrated Lewis pair (FLP) with a Lewis acidic site (Mg) and a Lewis (or Br?nsted) basic site (BDI). Reaction of [(BDI)Mg⁺][B(C₆F₅)₄^?] (1) with a range of phosphines varying in bulk and donor strength generated [(BDI)Mg⁺?PPh₃][B(C₆F₅)₄^?] (6), [(BDI)Mg⁺?PCy₃][B(C₆F₅)₄^?] (7), and [(BDI)Mg⁺? PtBu₃][B(C₆F₅)₄^?] (8). The bulkier phosphine PMes₃ (Mes=mesityl) did not show any interaction. Combinations of [(BDI)Mg⁺][B(C₆F₅)₄^?] and phosphines did not result in addition to the triple bond in 3-hexyne, but during the screening process it was discovered that the cationic magnesium complex catalyzes the hydrophosphination of PhC≡CH with HPPh₂, for which an FLP-type mechanism is tentatively proposed.

Full text of DOI:10.1002/chem.201804802

A Journal of
Accepted Article
Title: Complexation and Versatile Reactivity of a Highly Lewis Acidic
Cationic Mg Complex with Alkynes and Phosphines
Authors: Jürgen Pahl, Tom E. Stennett, Michel Volland, Dirk Guldi, and
Sjoerd Harder
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Supported by

Chemistry - A European Journal
10.1002/chem.201804802
Complexation and Versatile Reactivity of a Highly Lewis Acidic
Cationic Mg Complex with Alkynes and Phosphines
a
a
b
b
a
Jürgen Pahl , Tom E. Stennett , Michel Volland , Dirk Guldi , Sjoerd Harder*
a
Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Egerlandstraße 1, 91058 Erlangen, Germany.
b
Lehrstuhl für Physikalische Chemie I, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Egerlandstraße 3, 91058 Erlangen, Germany.
Figure Table of Contents
Keywords Magnesium – Cations – Lewis acidity – Frustrated Lewis Pair
Abstract
+
–] 1 (BDI = CH[C(CH
[
(BDI)Mg ][B(C
6
F
5
)
4
3
)N-Dipp]
2
; Dipp = 2,6-diisopropylphenyl) was
prepared by reaction of (BDI)MgnPr with [Ph
3
C+][B(C –]. Addition of 3-hexyne gave
6
F
)
5 4
+
F )
–]. Single crystal X-ray analysis, NMR investigations, Raman
[
(BDI)Mg ∙(EtC≡CEt)][B(C
6
5
4
spectra and DFT calculations indicate a significant Mg-alkyne interaction. Addition of the
terminal alkynes for PhC≡CH or Me SiC≡CH led to alkyne deprotonation by the BDI ligand to
3
+
–
+
–]
1
give [(BDI-H)Mg (C≡CPh)]
2
2[B(C
6
F
5
)
4
] (2, 70%) and [(BDI–H)Mg (C≡CSi(CH
3
)
3
)]
2
2[B(C
6
F
)
5 4
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Chemistry - A European Journal
10.1002/chem.201804802
(3, 63%). Addition of internal alkynes PhC≡CPh or PhC≡CMe led to a [4+2]-cycloaddition with
+
F )
–] 4 (53%) and
the BDI ligand to give {Mg C(Ph)=C(Ph)C[C(Me)=NDipp]
2
}
2
2[B(C
6
5
4
+
F )
–] 5 (73%) in which the Mg metal is (N,N,C)-
{
Mg C(Ph)=C(Me)C[C(Me)=NDipp]
2
}
2
2[B(C
6
5
4
+
chelated. The (BDI)Mg cation can be seen as an intramolecular Frustrated Lewis Pair (FLP)
with a Lewis acidic site (Mg) and a Lewis (or Brønsted) basic site (BDI). Reaction of
+
F )
–] 1 with a range of phosphines varying in bulk and donor strength led to
[
(BDI)Mg ][B(C
6
5
4
+
–] (6), [(BDI)Mg ∙PCy
+
F )
–] (7) and
formation of [(BDI)Mg ∙PPh
3
][B(C
6
F
5
)
4
3
][B(C
6
5
4
+
–] (8). The bulkier phosphine PMes
[
(BDI)Mg ∙PtBu
3
][B(C
6
F
5
)
4
3
did not show any interaction.
+
F )
–] and phosphines did not result in addition to the triple
Combinations of [(BDI)Mg ][B(C
6
5
4
bond in 3–hexyne but during the screening process it was discovered that the cationic
magnesium complex catalyses the hydrophosphination of PhC≡Ch with HPPh for which
2
tentatively an FLP-type mechanism is proposed.
Introduction
Activation of molecules by interaction with a strong Lewis acid is one of the most
fundamental principles in organic chemistry and has also has been exploited in catalysis.[1]
Whereas most Lewis acids are based on p-block elements (notably B and Al), we recently
introduced a set of highly Lewis acidic cationic alkaline earth metal complexes that are
stabilized by the (N,N)-chelating ß-diketiminate ligand BDI (BDI = CH[C(CH
3
)N-Dipp] ; Dipp =
2
2
,6-diisopropylphenyl). In contrast to earlier reported cationic BDI alkaline earth metal
[
2]
complexes, these cations are free of Lewis bases and consequently are able to form strong
adducts with benzene (I) or with 3-hexyne (II).[3] Competition experiments showed that
+
+
(
BDI)Mg is even more Lewis acidic than the corresponding Al cation, (BDI)AlMe , or the
+
benchmark Lewis acid B(C
6
F
5
)
3
. Notably, the cation (BDI)Mg is able to bind the otherwise
fully inert silyl ether O(SiMe
3
)
(III).[4] The high Lewis acidity of these cations may be
2
exploited in molecule activation and could have applications in the rapidly growing field of
+
Frustrated Lewis Pair (FLP) chemistry. Indeed, a combination of Lewis acidic (BDI)Ca and
I
2-
species (IV).[5]
Lewis basic (BDI)Al strikingly dearomatize C
6
H
6
to give formally a C
6
H
6
+
Herein we report on extensive investigations of (BDI)Mg -alkyne reactivity which depending
on the alkyne can follow the pathways of (i) coordination, (ii) deprotonation or (iii)
+
cycloaddition. Since (BDI)Mg may have potential as a strong Lewis acid in FLP chemistry, the
2
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10.1002/chem.201804802
interaction with a variety of phosphines is studied in detail. Since there are only very few
examples of Mg/P FLPs,[6] we also explore the possibility of alkyne activation by
+
(
BDI)Mg /phosphine combinations.
Results and Discussion
Mg-Alkyne coordination
+
Our previously reported magnesium alkyne complex [(BDI)Mg ∙EtC≡CEt][B(C
6
F ) ˉ] (II) was
5
4
+
prepared by generating the (BDI)Mg cation followed by addition of 3-hexyne (Scheme 1).
Its crystal structure (Figure 1) shows slightly asymmetric Mg-alkyne bonding (Mg-C:
2
.399(2)/2.480(2) Å; Mg-CCcenter: 2.360(1) Å) complemented by bidentate coordination of the
BDI ligand as well as a Mg∙∙∙F contact to the weakly coordinating anion B(C ˉ. Agostic
CH ∙∙∙Dipp π–interactions (Figure 1b) lead to additional stabilization of this Mg-alkyne
complex.
6
F
)
5 4
2
The bonding situation in II can best be understood by a comparison with other 3-hexyne
coordination complexes, e.g. with the coinage metal complexes (Cu, Ag, Au), bearing a
–
[7]
sterically very similar β–diketiminate ligand [N{C(C
3
F
7
)N(Dipp)}
BDI)Fe∙EtC≡CEt. The most important geometric features are summarized in Table 1. The
C≡C bond in the Mg-alkyne complex II is, at 1.193(3) Å, only marginally elongated compared
2
] , or with the Fe(I) complex
[
8]
(
[
9]
to a typical unactivated (C,H)C≡C(C,H) bond (1.181 Å). In comparison, coinage metal-alkyne
3
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complexes show slightly longer C≡C bonds (1.216(3)-1.233(7) Å). Not surprisingly, the longest
C≡C bond (1.263(4) Å) is found in the Fe-alkyne complex in which partially filled d-orbitals
effectively activate the triple bond by strong -backdonation. Although the triple bond in the
Mg-alkyne complex is barely elongated, it is partially activated for nucleophilic attack
through polarization of the -electron density. This is illustrated by a significant deviation of
the alkyne ligand from linearity: the C≡C–C angles in II vary from 170.7(2)° to 172.0(2)°.
Distortion from linearity increases with increasing C≡C bond lengths and is most pronounced
for the Fe-alkyne complex in which C≡C–C angles down to 148.5(3)° have been measured.
Table 1. Comparison of metal-(3-hexyne) complexes with free 3-hexyne; distances in Å and angles in degrees.
3
-hexyne
Mg[3]
1.193(3)
170.7(2)
172.0(2)
Ag[7]
1.216(3)
164.7(2)
165.3(2)
Cu[7]
1.233(3)
156.3(2)
156.5(2)
Au[7]
1.233(7)
155.0
Fe[8]
1.263(4)
148.5(3)
148.9(3)
a
C≡C (Å)
C≡C–C (°)
1.181
b
180
1
80
155.7
2
2
2
228
244
296
-
-
-
Raman
2193
2240
2104
2003
-
1920
-
–
1
(
cm )
–
1
IR (cm )
2120
-
-
1802
a
[9] b
Average value for a (C,H)C≡C(C,H) bond.
Idealized value.
+
–] (H atoms not shown, anion only partially shown).
Figure 1. (a) Crystal structure of [(BDI)Mg ∙EtC≡CEt][B(C
6
5
F )
4
Selected bond distances (Å) and angles (°): Mg–N1 1.978(1), Mg–N2 1.994(1), Mg–F1 2.066(1), Mg–C32
+
–
2
(
.480(2), Mg–C33 2.399(2). (b) Characteristic bond lengths (Å) and angles (°) in [(BDI)Mg ∙EtC≡CEt][B(C
c) QTAIM representation of the Laplacian of the electron density in II in the plane through Mg and C≡C (bond
critical points are indicated by light-blue spheres).
6
F
5
)
4
].
4
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As in the case of the coinage metal-alkyne complexes, attempts to obtain additional
information about the 3–hexyne activation by IR spectroscopy were unsuccessful
+
(
Supporting Information). Whereas the C≡C vibrations in (BDI)Mg ∙EtC≡CEt could not
unequivocally be detected in the IR spectrum, the Raman spectroscopy gave signals at 2193
-
1
and 2240 cm , corresponding to the C≡C unit. The calculated C≡C frequency of 3-hexyne
-
1
(
2287 cm ) is significantly higher than values observed for II. A direct comparison with free
-
1
-1
3
-hexyne is difficult due to the Fermi coupling in this region (bands at 2228 cm , 2244 cm ,
-1 [10]
and 2296 cm ). It is, however, clear that the C≡C frequencies for the coinage metal-(3-
hexyne) complexes are substantially smaller than those for II. The lowest frequency is found
for the Fe-(3-hexyne) complex for which only an IR vibration has been reported.
Previous DFT calculations (B97X/6-311+G**) have shown that the Mg-alkyne interaction in II
is largely electrostatic: the total NPA charge of +0.042 on 3-hexyne suggests negligible alkyne
[3]
+
→
Mg electron transfer. Complexation of 3-hexyne by the "naked" (BDI)Mg cation results
in a total energy gain of E = -43.1 kcal mol–1 and is therefore more exothermic than
–1
benzene coordination: E = -36.1 kcal mol . This may partially originate from agostic
CH ∙∙∙Dipp π–interactions which, like in the crystal structure, are also observed in the
2
calculated structure. QTAIM analysis of the electron density in II clearly shows a strong
polarization of the tubular C≡C -electron density. This is not only evident from the ellipticity
parameter of 0.0822 a.u. which deviates from zero, the expected value for a cylindrical
electron distribution, but most clearly illustrated by the Laplacian of the electron density
(Fig. 1c). This visualization shows a bond path from Mg to the middle of the C≡C bond and a
strong polarization of the C≡C electron density towards the metal.
The Mg-alkyne complex II does not dissolve in benzene or toluene and loses its alkyne ligand
in polar solvents like THF. Dissolved in a polar non-coordinating solvent like chlorobenzene,
however, the Mg-alkyne bond remains intact. This is clear from a considerable upfield shift
1
of the H NMR signals for coordinated 3-hexyne. Especially the CH
2
group signal, which shifts
from 2.07 ppm (free 3-hexyne) to 1.42 ppm, is indicative of strong bonding. This upfield shift
may be due the CH ∙∙∙Dipp π–interactions, originating from ring current effects in the Dipp
2
substituents. Similar upfield shifts have been recorded for the previously discussed coinage
metal complexes.[7] The preference of (BDI)Mg to coordinate 3-hexyne versus benzene
+
(DFT: E = 7.0 kcal/mol) can also be confirmed in solution by a competition experiment.
Addition of two equivalents of benzene to II dissolved in C
6
D
5
Br gave no changes in the 1H
5
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NMR chemical shifts (Supporting Information). Significant broadening of the CH signals,
2
however, indicate an equilibrium between alkyne versus benzene coordination which is
largely at the alkyne side. The quaternary carbon atom of the C≡C unit could not be detected
1
3
by C NMR spectroscopy, precluding further conclusions on the bonding situation.
Alkyne reactivity
In order to increase our understanding of Mg-alkyne complexes we explored complexation
with terminal alkynes or internal alkynes containing Ph substituents. The latter group of
ligands may provide information on the preference for Ph versus alkyne bonding. In all cases,
however, we observed a distinct alkyne transformation instead of simple coordination.
Reaction of the in–situ generated “naked” cationic complex 1 with the terminal alkynes
PhC≡CH and Me SiC≡CH led to alkyne deprotonation by the BDI ligand (Scheme 1). In a
3
similar fashion to the previously reported reaction of 1 with water,[11] the anionic BDI ligand
is converted to a neutral bis-imine ligand and the Mg-alkynide complexes
+
+
[
(BDI-H)Mg CCPh]
2
∙2[B(C
6
F
5
)
4
ˉ] (2) and [(BDI-H)Mg CCSi(CH
3
)
3
]
2
∙2[B(C
6
F ) ˉ] (3) were isolated
5
4
in yields of 70% and 63%, respectively. This cooperative acid-base transformation may be
described as intramolecular Frustrated Lewis Pair (FLP) reactivity and is analogous to
+
.[12] Interestingly, the reaction with
(alkyne)C-H bond cleavage by the FLP Cp
2
Zr(OAr) /PCy
3
PhC≡CH to form complex 2 takes less than 5 minutes at room temperature whereas
deprotonation of this alkyne by (BDI)MgN(SiMe to give (BDI)MgC≡CPh needs forcing
conditions (60°C, 60 h).[13] Given the pK value of 22.6 for PhC≡CH,[14] deprotonation of the
alkyne by an otherwise innocent BDI ligand is unusual. The analogous conversion of
Me SiC≡CH to form complex 3 needs much more forcing conditions (80°C, 1 h). Since the pK
)
3 2
a
3
a
values of both alkynes are expected to be similar, the nature of the Ph group seems to be
essential for fast conversion. We propose a mechanism in which the Ph group coordinates to
the Lewis acidic Mg center. This leads to strong charge polarization and acidification of the
alkyne C-H group which subsequently is easily deprotonated by the BDI backbone. The
synthesis of complexes 2 and 3 can also be accomplished by starting from the arene adduct
+
–] indicating that terminal alkynes may compete with benzene for
[
(BDI)Mg ∙C
6
H
6
] [B(C
6
F
)
5 4
Mg coordination.
6
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+
Since the BDI ligand in (BDI)Mg is too Brønsted basic to tolerate terminal alkynes with pK
values around 23 (pK
PhC≡CH: 22.6),[14] we focussed on coordination of internal alkynes.
Layering an in situ generated solution of [(BDI)Mg ][B(C
a
a
+
F )
–] in chlorobenzene with a
6
5
4
hexane solution of PhC≡CPh or PhC≡CMe led to crystallization of the products 4 and 5 in
yields of 53% and 75%, respectively (Scheme 1). This [4+2]-cycloaddition does not proceed in
+
the presence of benzene; in this case the benzene adduct (BDI)Mg ∙C
6
6
H is formed. This
indicates that benzene coordinates more strongly than Ph-substituted internal alkynes. It is
likely that the observed cycloaddition proceeds by Mg-alkyne coordination followed by
intramolecular nucleophilic attack by the BDI ligand. Since the alkyne EtC≡CEt does not show
cycloaddition, alkyne activation through Ph-Mg coordination and triple bond conjugation
with at least one Ph group is crucial. The herein observed [4+2]-cycloaddition is limited to a
+
+
few metal complexes of which all except one feature cationic metal centers (Pt, Al , Ru ,
Os+).[15] It is of interest to note that reaction of (BDI)Mg with the unsymmetrical internal
alkyne PhC≡CMe selectively gives BDI attack at the Me-substituted carbon atom. This
+
+
contrasts with the reactivity of (BDI)AlMe with PhC≡CH in which attack occurs selectively at
the Ph-substituted carbon atom.[8] Electronic arguments (+I effect of Me) and steric
arguments (small Me substituent facing the bulky Dipp ligands) are in favour of the latter.
+
The unusual selectivity observed in (BDI)Mg /PhC≡CMe cycloaddition may be explained by
initial coordination of the substrate via its aromatic ring. This leads to polarization of the C≡C
bond facilitating BDI attack at the Me-substituted carbon atom.[16]
+
Scheme 1. Reactivity of [(BDI)Mg ][B(C
6
F ) ˉ] with alkynes (R = Dipp).
5
4
7
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Crystal structures and solution investigations of complexes 2 and 3
Crystal structures of complexes 2 and 3 are displayed in Figure 2. Similarly as for
(BDI)Mg(C≡CPh)]
,[13] the complexes are dimeric featuring an asymmetric bridging alkynide.
The latter shows a combination of Mg-C -bonding (along the sp-hybrid orbital) and side-on
-interaction. Whereas the Mg-C -bonds (Mg–C30; 2: 2.154(3) Å; 3: 2.170(2) Å) compare
well to that in [(BDI)Mg(C≡CPh)] (2.174(2) Å), the side-on -bonds are clearly more
asymmetric than in the neutral dimer (2: 2.250(3)/2.868(3), 3: 2.268(2)/2.751(2) Å,
(BDI)Mg(C≡CPh)] : 2.349(2)/2.671(2) Å). This may be explained by the steric repulsion
between the CH group of the neutral BDI ligand, which is in a boat conformation, and the
alkyne. Protonation of the BDI anion is confirmed by typical C=N bond distances (2: C=N
.285 Å, 3: C=N 1.284 Å) as well as by the C-C distances (2: 1.518 Å, 3: 1.517 Å). Description
of the BDI ligand as a diimine is supported by IR spectra in which clear signals for a C=N
[
2

2
[
2
2
1
-
1
-1
vibration can be observed (2: 1617 cm , 3: 1620 cm ). Although for 3 no absorption
associated with a C≡C stretch vibration could be observed, the C≡C vibrational frequency of
complex 2 could be detected at a wavenumber of 2050 cm-1
This compares well with that in
.
-
1
[
(BDI)Mg(C≡CPh)]
2
observed at 2047 cm .
+
Figure 1. (a) Crystal structure of [(BDI-H)Mg CCPh]
2
∙2[B(C
6
F ) ˉ]
5
4
(2) (H atoms and anion not shown). Selected bond distances (Å)
and angles (°) in one of the independent dimers: Mg–N1 2.106(3),
Mg–N2 2.121(3), Mg–C30 2.154(3), Mg–C30’ 2.250(3), Mg–C31’
2
.868(3), C2–N1 1.285(4), C4–N2 1.284(4), N2–Mg–N1 92.12(10).
+
(b) Crystal structure of [(BDI-H)Mg CCSiMe
3
]
2
∙2[B(C
6
F ) ˉ] (3) (H
5
4
atoms and anion not shown). Selected bond distances (Å) and
angles (°): Mg–N1 2.124(1), Mg–N2 2.131(2), Mg–C30 2.170(2),
Mg–C30’ 2.268(2), Mg–C31’ 2.751(2), C2–N1 1.281(2), C4–N2
1
.286(2) N2–Mg–N1 92.17(6).
NMR investigations on the dimeric complexes 2 and 3
were hampered by their very poor solubility in
chlorobenzene which likely originates from their
dicationic nature. Screening various solvent mixtures,
we found that small amounts of complex 3 dissolve in
1
1
,2-difluorobenzene/C
D
6 6
(2/1). The H NMR spectrum
of 3 in this solvent mixture features two doublets at
8
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Chemistry - A European Journal
10.1002/chem.201804802
3
.79 and 3.92 ppm (2JHH = 17.5 Hz) for the diastereotopic protons of the BDI-CH
2
group,
indicating that asymmetric alkynide bridging is retained in solution. The C NMR spectrum
shows a resonance at 45.8 ppm for the BDI-CH group, which is circa 45 ppm upfield with
respect to the backbone CH signal in anionic BDI ligands. Both carbon atoms of the C≡C bond
1
3
2
1
3
can be observed in the C NMR spectrum at 100.3 and 115.0 ppm which is the expected
region for Mg alkynide complexes. The signals are slightly upfield from values reported for
[
(BDI)Mg(C≡CPh)]
(111.2 and 122.0 ppm).[13]
2
Complex 2 is, once crystallized, completely insoluble. However, in situ generation of this
complex in 1,2-difluorobenzene/C allowed for a circa ten hour time slot to study this
D
6 6
complex in solution (longer measurement times are plagued by product crystallization). The
NMR data for 2 are essentially the same as for complex 3 (see Experimental Section).
Crystal structures and solution investigations of complexes 4 and 5
The crystal structures of complexes 4 and 5 are depicted in Figure 3. The [4+2]-cycloaddition
+
of the alkyne to (BDI)Mg created a (N,N,C)-chelating tridentate ligand for Mg. In contrast to
+
(
BDI)Mg , which in exclusion of aromatic solvents shows strong interactions with the
–
B(C
6
F
5
)
4
anion, the Mg centers in 4 and 5 prefer to interact with the Ph ring of a
neighbouring cation: the shortest Mg∙∙∙C contacts measure 2.360(2) Å (4) and 2.341(3) Å (5).
This may be explained by the higher denticity of the tricoordinate ligand, leaving less space
for cation∙∙∙anion interaction, but could also originate from charge delocalization of formally
negatively charged carbon C30 into the Ph ring. This would explain exclusive bridging of
cations by the Ph substituent instead of by Dipp-substituents. The Mg-C bond lengths in 4
[
9]
(
2.122(2) Å) and 5 (2.100(3) Å) lie in the expected range. The C30-C31 bond distances are
in the characteristic range for a double bond (4: 1.344(3) Å, 5: 1.336(4) Å). In contrast, the
newly formed bond between the BDI ligand and the alkyne (C3-C31) is in both complexes
3
2
[9]
significantly longer than an average (sp )C–C(sp ) bond of 1.507 Å (4: 1.574(2) Å, 5:
.566(4) Å). Such elongation is typical for these kind of complexes[15] and is in agreement
with the reversibility of the cycloaddition reaction.
1
9
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Chemistry - A European Journal
10.1002/chem.201804802
+
–] (4) (H atoms, iPr–groups and
Figure 2. (a) Crystal structure of {Mg C(Ph)=C(Ph)C[C(Me)=NDipp]
2 2
} ∙2[B(C
6
F
)
5 4
anion not shown). Selected bond distances (Å) and angles (°): Mg–N1 2.1382(16), Mg–N2 2.1411(16), Mg–C30
.1218(18), Mg–C41’ 2.3604(19), C2–C31 1.574(2), C30–C31 1.344(3) N2–Mg–N1 93.22(6). (b) Crystal structure
2
+
–
of Mg C(Ph)=C(Me)C[C(Me)=NDipp]
bond distances (Å) and angles (°) in one of the independent dimers: Mg–N1 2.148(2), Mg–N2 2.131(2), Mg–C30
.100(3), Mg–C36’ 2.341(3), C2–C31 1.566(4), C30–C31 1.336(4) N2–Mg–N1 93.21(9).
2 2
} ∙2[B(C
6
F
)
5 4
] (5) (H atoms, iPr–groups and anion not shown). Selected
2
IR spectra of 4 and 5 were recorded in the solid state. Both complexes show absorbance at
-
1
1
614 cm , conclusive for formation of a C=N bond. The signals for the C=C double bonds lie
-1
-1
in the same region (4: 1578 cm , 5: 1583 cm ).
NMR studies on complex 4 were precluded by the complete insolubility of this dicationic
dimer in halogenated arenes. Addition of C
6
D as a weakly -coordinating solvent to break
6
the dimer into benzene ligated monomers resulted in the elimination of PhC≡CPh and the
+
formation of (BDI)Mg ∙C
6
6
D . This is consistent with the observation that complex 4 cannot be
synthesized in the presence of benzene. The reversibity of the [4+2]-cycloaddition was also
observed in weakly coordinating solvents like fluorobenzene and 1,2-difluorobenzene.
Complex 5 could be dissolved in a mixture of C
6
D
6
and C
6
D
5
Br at room temperature but
+
heating this solution to 60 °C led to PhC≡CMe elimination and formation of (BDI)Mg ∙C
6
6
D .
1
0
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Chemistry - A European Journal
10.1002/chem.201804802
NMR data at room temperature corroborate the previously presented characterization by X-
1
ray diffraction and IR spectroscopy. The H NMR signal for the proton attached to the now
3
sp hybridized backbone carbon atom is strongly shifted upfield to 4.39 ppm. Moreover, the
quaternary carbon atoms of the C=N moiety (182.5 ppm) as well as the C=C unit (175.1 ppm
1
13
and 139.5 ppm) could be identified and unambiguously assigned by H, C-correlated NMR
measurements (Supporting information).
Mg-phosphine coordination
Evidence for unsupported interactions of magnesium complexes with neutral phosphine
ligands are limited to the small phosphine PMe
3
or P,P-chelating ligands.[17] The recent
illustrates that there is ample space at the
+
– [17c]
isolation of [(BDI)Mg ∙PPh
3
][Al(OC(CF
3
)
3
) ]
4
+
metal atom in (BDI)Mg to complex a larger phosphine. Herein we investigate the interaction
+
of (BDI)Mg with a series of phosphines of increasing bulk and/or donor properties. In order
+
to assess the spatial accessibility of the metal atom in (BDI)Mg , coordination of triaryl- or
trialkyl-phosphines with increasing Tolman angles was attempted: PPh
PtBu (182°) < PMes
(212°).[18] Donor strength is increased by replacing aryl substituents
with alkyl groups. Adduct formation was investigated by addition of these phosphines to a
3
(145°) < PCy (170°)
3
<
3
3
+
–] in C
1
13
31
solution of [(BDI)Mg ∙C
6
D
6
][B(C
6
F
5
)
4
6
D Br and measurement of H, C and P NMR
5
spectra.
1
13
Addition of one equivalent of PPh
3
led to a complex with H and C NMR chemical shifts
that, despite use of
a
different solvent, closely match those for
+
–].[17c] The P signal at -5.0 ppm is basically identical to that
31
[
(BDI)Mg ∙PPh
3
][Al(OC(CF
3
)
3
)
4
reported by Hill and coworkers (-4.9 ppm). Using the sterically more demanding PCy
3
also
(9.6
3
1
gave an adduct, indicated by a large downfield shift of the P NMR signal for free PCy
ppm) to 20.4 ppm. However, addition of PtBu
unidentified species. This is presumably due to reaction with the solvent. Change of the
solvent for a mixture C F/C or C Cl/C prevented decomposition and led to a
double set of BDI signals that likely originate from (BDI)Mg ∙PtBu
ratio in C F/C and 80:20 ratio in C Cl/C ) suggesting that the formation of the Mg-
PtBu adduct is a solvent-dependent equilibrium. This equilibrium is supported by the P
NMR spectrum, which shows two singlets, one of which one can be assigned to free PtBu
3
3
led to slow conversion of the Lewis base into
6
H
5
6
D
6
6
H
5
D
6 6
+
+
3
and (BDI)Mg ∙C
6
6
D (50:50
6
H
5
6
D
6
6
H
5
D
6 6
3
1
3
3
1
1
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Chemistry - A European Journal
10.1002/chem.201804802
+
(
62.1 ppm). The other is shifted downfield (69.7 ppm), typical for (BDI)Mg -phosphine
coordination. By far the bulkiest phosphine, PMes , did not show any sign of interaction with
3
+
(
BDI)Mg in C
6
5
D Br solution.
+
–
+
F )
–] 7 and
Figure 4. Crystal structures of [(BDI)Mg ∙PPh
3
][B(C
6
F )
5 4
] 6, [(BDI)Mg ∙(PCy
3
)(PhF)][B(C
6
5
4
+
t
–
[(BDI)Mg ∙(P Bu
3
)(PhF)][B(C
6
F
)
5 4
] 8. Distances and angles can be found in Table 2.
Table 2. Selected bond distances (Å) and angles (°) for complexes 6, 7, 8 in comparison with those in
+
[17c]
[
(BDI)Mg ∙PPh
3
][Al(OC(CH
3
)
)
3 4
].
+
+
+
+
t
[
[
(BDI)Mg ∙PPh
3
–]
]
[(BDI)Mg ∙PPh
3
]
[(BDI)Mg ∙PCy
3
]
[(BDI)Mg ∙P Bu
3
]
Complex
–
–
–
Al(OC(CH
3
)
3
)
4
[B(C
F
6 5
)
4
] 6
[B(C
F
6 5
)
4
] 7
[B(C
6
F
5
)
4
] 8
Mg∙∙∙P
C–N–C
2.597(1)
120.9(3)
2.6549(5)
117.6(1)
117.3(1)
2.1705(8)
2.005(1)
2.013(1)
2.6551(6)
118.5(1)
120.3(1)
2.1300(9)
2.021(1)
2.029(1)
2.772(1)
119.0(2)
116.3(2)
2.075(2)
2.044(2)
2.060(2)
1
18.9(3)
Mg∙∙∙F
-
1
1
.977(3)
.983(3)
Mg–N
+
Isolation of well-defined (BDI)Mg -phosphine adducts turned out to be troublesome due to
–
difficulties in the crystallization of their B(C
6
F
5
)
4
salts. Only the PCy adduct could be isolated
3
in a very low yield of 5% and was fully characterized. From the other adducts we have
managed to isolate a few crystals that allowed for determination of their crystal structures,
giving insights into structural changes upon variation of phosphine bulk and donor
+
F )
–] (6),
properties. Figure 4 shows the crystal structures of [(BDI)Mg ∙PPh
3
][B(C
6
5
4
+
–
+
t
–] (8); selected
[
(BDI)Mg ∙(PCy
3
)(PhF)][B(C
6
F
5
)
4
] (7) and [(BDI)Mg ∙(P Bu
3
)(PhF)][B(C
6
F
)
5 4
geometric parameters are summarized in Table 2. Whereas the previously reported
+
) )
–] is a truly separated ion pair,[17c] the crystal structure of
[
(BDI)Mg ∙PPh
3
][Al(OC(CF
3
3
4
1
2
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Chemistry - A European Journal
10.1002/chem.201804802
+
–] (6) shows a single Mg∙∙∙F interaction. Consequently, the Mg-P
[
(BDI)Mg ∙PPh
3
][B(C
6
F
)
5 4
bond in 6 (2.6549(5) Å) is slightly longer than that in its aluminate salt (2.597(1) Å).
The PCy and PtBu adducts 7 and 8 crystallize similarly to 6, but with an additional Mg∙∙∙F
3
3
contact, which in this case stems from cocrystallized fluorobenzene. All structures also
display several C-H∙∙∙C() bonding interactions between the phosphine and BDI ligands. Since
the ligand arrangement and Mg coordination spheres in 6-8 are very similar, the influences
of the phosphine ligand on the structure of the cation can be discussed. Changing the
phosphine ligand from PPh
Apparently, the expected elongation of the Mg-P distance by increasing the bulk is
compensated by bond shortening due to the better donor abilities of PCy . The PtBu adduct
shows an exceptionally large Mg-P bond distance of 2.772(1) Å which is in line with the
weaker Mg–P interaction already observed in solution. Complex 8 also features unusually
long Mg-N bond distances. The bulky phosphine PtBu cannot fully enter the free
coordination site at Mg and, being a strong Lewis base, pulls the Mg metal away from the
BDI ligand. The combination of two large ligands, (BDI) and PtBu , therefore results in
3
to PCy gave an adduct with an equally long Mg-P bond length.
3
3
3
8
3
3
inefficient ligand-Mg interaction and may explain why 8 feautures the shortest Mg∙∙∙F
distance.
Towards FLP reactivity
+
Having established the details of interaction of the “naked” (BDI)Mg cation with a range of
alkynes and phosphines, the question arose whether such highly Lewis acidic cations may be
used as components in Frustrated Lewis Pairs. Investigation of five different alkynes led to
the conclusion that only bis-alkyl substituted alkynes, e.g. EtC≡CEt, do not react with the
Lewis acid alone. Investigations of a range of phosphine complexes indicate that the larger
weakly or non-bonding phosphines, PtBu
3
and PMes , are likely the most successful Lewis
3
bases for probing FLP reactivity. Our recent observation of alkyne addition to the adduct
+
,[8a] however, demonstrates that also adducts with smaller phosphines may
(
BDI)AlMe ∙PPh
3
be reactive. Thus, we investigated the FLP reactivity of EtC≡CEt in C
6
D Br using
5
+
–] as the Lewis acid and the complete series of phosphines (PPh
,
[
(BDI)Mg ∙C
6
D
6
][B(C
6
F
5
)
4
3
PCy
3
, PtBu
3
and PMes
3
) as the Lewis base. None of the phosphines displayed any reactivity
+
towards the alkyne. Even the true FLP with the largest phosphine ligand, (BDI)Mg /PMes
3
,
1
3
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Chemistry - A European Journal
10.1002/chem.201804802
1
did not show any reaction with 3-hexyne, as apparent from H NMR monitoring. Contrary to
expectation, we made the observation that a very small phosphine, HPPh , in combination
2
+
F )
–] led to catalytic hydrophospination of PhC≡CH (Scheme 2; 10
with [(BDI)Mg ∙C
6
D
6
][B(C
6
5
4
mol% catalyst, 2:1 chlorobenzene/benzene, 60 °C, 20 hours, 72% conversion). Notably, a
catalyst-free run under the same conditions gave no conversion. The reaction is slow but
showed clean conversion to H
Ph(H)C=C(H)PPh , the anti-Markovnikoff product in a ratio of 60/40. The exclusive formation
of the Z-isomer of the anti-Markovnikoff product indicates that the H-PPh to alkyne addition
proceeds with a trans-selectivity. Although it was recently shown that hydrophosphination
of PhC≡CH with HPPh may proceed without solvent and catalyst (70 °C, 16 h, 80%
2
C=C(Ph)PPh , the Markovnikoff product, and (Z)-
2
2
2
2
conversion),[20] we could not reproduce these results. As we use very dry conditions and
distilled substrates, small amounts of water may be necessary. Also the product distribution
in the reported catalyst-free, thermally-induced, conversion is different (100% anti-
Markovnikoff, Z/E = 95/5).[20]
+
The (BDI)Mg catalyzed alkyne hydrophosphination reported here has a very limited scope.
Internal alkynes did not react and also the terminal alkyne Me
3
SiC≡CH gave very poor
conversion. Exchange of HPPh
hydrophosphination product.
2
for HPCy
2
gave under the same conditions only traces of the
At this very preliminary stage we can only speculate about a possible mechanism. One could
envision an FLP type mechanism in which the alkyne is activated for nucleophilic attack by
Mg coordination. The current work shows that such Mg-alkyne complexes are feasible and
that the alkyne is activated for nucleophilic attack by polarization of the -electron density
(Figure 1). In a subsequent step, the acidic phosphonium unit protonates the Mg-C bond,
+
+
either directly or by H transfer via the BDI backbone or HPPh
processes in FLP chemistry have been shown previously.[21] Although other mechanisms may
be possible, this tentative FLP mechanism is formally a trans-addition of HPPh to the alkyne,
2
. Examples of such H transfer
2
explaining the formation of an anti-Markovnikoff product exclusively as its Z-isomer. We are
currently extending our explorations in this chemistry.
1
4
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10.1002/chem.201804802
+
F )
–]. The exclusive formation of
Scheme 2. Reaction of PhC≡CH with HPPh
2
catalyzed by [(BDI)Mg ∙C
6
D
6
][B(C
6
5
4
the Z-isomer for the anti-Markovnikoff product confirms rear-side attack of the alkyne by HPPh .
2
Conclusions
We have presented a detailed investigation of the interactions and reactivity of alkynes with
the Lewis base-free, highly Lewis acidic, cationic magnesium complex
+
–].
[
[
(BDI)Mg ∙C
6
D
6
][B(C
6
F
5
)
4
Alkyne-Mg
bonding
in
the
previously
reported
+
–] is essentially electrostatic and results in negligible C≡C bond
(BDI)Mg ∙EtC≡CEt][B(C
6
F
)
5 4
lengthening. A significant decrease in the Raman frequency for the C≡C vibration points to
some bond weakening. As the C≡C electron density is strongly polarized towards the metal,
the triple bond should be activated for rear-side nucleophilic attack. Use of terminal alkynes
led to deprotonation of the alkyne by the BDI ligand. Internal alkynes containing at least one
Ph group reacted by [4+2]-cycloaddition with the BDI ligand. In both transformations the
+
(
BDI)Mg cation could be described as an internal FLP in which alkynes are activated by the
combined interaction with Lewis acidic Mg and the Lewis basic BDI ligand. In addition, we
+
have presented complexation of the (BDI)Mg cation with a range of phosphines that vary in
bulk and donor strength. Whereas there is evidence for complex formation with the
phosphines PPh
3
, PCy
3
and PtBu in the solid state as well in solution, the largest phosphine
3
1
5
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Chemistry - A European Journal
10.1002/chem.201804802
PMes
3
does not form a coordination complex on account of its extensive bulk. Although the
+
latter Lewis acid-base combination (BDI)Mg /PMes
3
can be seen as a true FLP, no reaction
+
with EtC≡CEt was observed. It was, however, observed that (BDI)Mg is a catalyst for
hydrophosphination of PhC≡CH with the secondary phosphine HPPh . Based on the exclusive
2
formation of the Z-isomer for the anti-Markovnikoff product, tentatively a FLP-type
mechanism is proposed: the alkyne is activated by formation of a Mg∙∙∙alkyne complex and
attacked from the rear by HPPh .
2
The results presented herein underscore the high Lewis acidity of Lewis base-free cationic
Mg complexes. The results also show that the BDI ligand may not be the best platform for
small molecule activation. Although the BDI ligand is generally a non-innocent spectator
ligand, the high Lewis acidity of the Mg center activates alkynes to such an extent that the
BDI ligand reacts either as a Brønsted base or as a nucleophile. We are currently working on
as set of cationic Mg Lewis acids with more docile spectator ligands.
Experimental section
General Experimental Procedures
All experiments were conducted under an inert nitrogen atmosphere using standard Schlenk
and glovebox techniques (MBraun, Labmaster SP). All solvents were degassed with nitrogen,
dried over activated aluminium oxide (Solvent Purification System: Pure Solv 400−4−MD,
Innovative Technology) and stored over 3Å molecular sieves. Fluorobenzene, chlorobenzene,
bromobenzene, 1,2-difluorobenzene, phenylacetylene (Fluka, 97%), were dried over calcium
hydride, distilled under N
Trimethylsilyacetylene (98%, Sigma Aldrich), Phenylpropyne (98%, abcr), 3–hexyne (99%,
Sigma Aldrich) and diphenylphosphine (99%, abcr) were dried over molecular sieves 3Å. PPh
99%, Alfa Aesar) was dried over P and recrystallized whereas PtBu (99%, abcr) was used
directly as received. C and C Br (99.6% D, Sigma Aldrich) were dried over 3Å molecular
Bˉ] (Boulder Scientific) was used as received. [(BDI)MgnPr]
,[3]
ˉ] were synthesized
2
atmosphere and stored over molecular sieves 3Å.
3
(
2
O
5
3
6
D
6
D
6 5
sieves. [Ph
3
C+][(C
6
F
5
)
4
2
+
[3]
,[22] and [(BDI)Mg ∙C
+
[3]
[
(BDI)Mg ∙EtC≡CEt][B(C
6
F
5
)
4
ˉ], PMes
3
6
D
6
][B(C
6
F
)
5 4
according to literature procedures. NMR spectra were recorded with a Bruker Avance III HD
00 MHz or a Bruker Avance III HD 600 MHz spectrometer. The spectra were referenced to
the respective residual signals of the deuterated solvents. Elemental analysis was performed
4
1
6
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Chemistry - A European Journal
10.1002/chem.201804802
with a Euro EA 3000 (Euro Vector) analyzer or a Vario MICRO Cube (Elementar). IR spectra
were measured on a ATR platinum T (Bruker) spectrometer. Raman measurements were
carried out with a WITec alpha300 R using laser excitations of 532 nm. All crystal structures
have been measured on a SuperNova (Agilent) diffractometer with dual Cu and Mo
microfocus sources and an Atlas S2 detector. Crystallographic data have been deposited with
the Cambridge Crystallographic Data Centre as supplementary publication no. 1867876 (2),
1
867877 (3), 1867878 (4), 1867879 (5), 1867880 (6), 1867881 (7) and 1867882 (8). Copies of
the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (+44)1223-336-033; E-mail: deposit@ccdc.cam.ac.uk).
+
–
+
F )
–] (0.051 g, 0.043
Synthesis of [(BDI-H)Mg (CCPh)]
2
2[B(C
6
F
5
)
4
] (2): [(BDI)Mg ∙C
6
H
6
][B(C
6
5
4
mmol) was dissolved in a mixture of 1,2-difluorobenzene (0.3 ml) and benzene (0.1 ml). After
addition of phenylacetylene (6.0 µl, 0.055 mmol) the colorless solution was left standing for
1
day at room temperature for crystallization. The crystals were washed with chlorobenzene
(2 x 0.1 ml). The product (2) was obtained as colorless crystals (0.037 g, 0.015 mmol) in a
yield of 70%. Due to the insolubility of the product NMR data could only be obtained from an
1
in situ prepared sample using C
, 298K) δ 3.45 (d, 2JHH = 16.4 Hz, 1H, CCH
3JHH = 6.8 Hz, 2H, CHMe ), 2.38 (hept, 3JHH = 6.8 Hz, 2H, CHMe
3JHH = 6.8 Hz, 6H, CHCH ), 0.97 (d, 3JHH = 6.8 Hz, 6H, CHCH ), 0.90 (d, 3JHH = 6.8 Hz, 6H, CHCH
.38 (d, 3JHH = 6.8 Hz, 6H, CHCH
), (all signals in the aromatic region are obscured by C
13C NMR (101 MHz, C
/benzene-d
32.2 (s, ArC), 132.1 (s, ArC), 129.5 (s, ArC), 129.4 (s, ArC), 125.7 (s, ArC), 125.1 (s, ArC), 118.3
6
H
4
F
2
/C
6
D
6
as a solvent. H NMR (400 MHz, C
6
H
4
F
2
/benzene-
C), 2.48 (hept,
), 1.60 (s, 6H, CCH ), 1.19 (d,
d
6
2
C), 3.08 (d, 2JHH = 16.4 Hz, 1H, CCH
2
2
2
3
3
3
3
),
);
0
3
6
4
H F
2
6
H
4
F
2
6
, 298K) δ 186.3 (s, C=N), 139.5 (s, ArC), 138.8 (s, ArC),
1
(
s, MgC≡C) 104.2 (s, MgC≡C), 45.2 (s, CCH
NC(CH )), 24.5 (s, CHCH ), 23.8 (s, CHCH ), 23.0 (s, CHCH
MHz, C /benzene-d
F, p-CF), -167.26 (t, J = 20 Hz, 8F, m-CF); B NMR (128 MHz, C
6.1 (s, B(C ); IR (ATR, pure): ṽ = 2970 (m), 2931 (w), 2874 (w), 2050 (w), 1641 (m), 1617
2
C), 30.1 (s, CHMe
2
), 28.0 (s, CHMe
2
), 26.2 (s,
1
9
3
3
3
3
), 22.5 (s, CHCH
3
); F NMR (376
6
H
4
F
2
6
, 298K) δ -132.13 – (-132.47) (m, 8F, o-CF), -163.46 (t, 3JFF = 20 Hz,
1
1
4
1
6
H
4
F
2
/benzene-d , 298K) δ -
6
6
F
)
5 4
(
w), 1512 (s), 1460 (vs), 1371 (s), 1311 (s), 1266 (s), 1084 (vs), 977 (vs), 933 (m), 843 (m), 798
s), 754 (vs), 682 (s), 660 (s); Elemental analysis calcd (%) for C122 (2446.29): C
9.90, H 3.87, N 2.29; found: C 59.72, H 3.74, N 1.72.
(
H
94
B
2
F
40Mg
N
2 4
5
1
7
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Chemistry - A European Journal
10.1002/chem.201804802
+
–
+
–] (0.101 g,
Synthesis of [(BDI-H)Mg (CCSi(CH
3
)
3
)]
2
2[B(C
6
F
5
)
4
] (3): [(BDI)Mg ∙C
6
H
6
][B(C
6
F
5
)
4
0
.0842 mmol) and trimethylsilylacetylene (0.0120 ml, 0.0843 mmol) were dissolved in 1,2–
difluorobenzene (1.0 ml) and heated to 80°C for 1 hour. The colorless solution was layered
with hexane (0.3 ml) after which immediate crystallization was observed. Washing the
crystals with fluorobenzene (5 x 0.3 ml) and subsequent drying in vacuo gave the desired
product (3) in a yield of 63% (0.061 g, 0.025 mmol). Due to its extremely low solubility NMR
data were recorded in a 2:1 mixture of C
, 298K) δ 3.92 (d, 2JHH = 17.5 Hz, 2H, CCH
3JHH = 6.9 Hz, 4H, CHMe ), 2.20 (hept, 3JHH = 6.9, 4H, CHMe
), 0.96 (d, 3JHH = 6.8 Hz, 12H, CHCH ), 0.92 (d, 3JHH = 6.8 Hz, 12H, CHCH
.13 (d, 3JHH = 6.8 Hz, 12H, CHCH
), 0.11 (s, 18H, SiMe ), (all signals in the aromatic region
); C NMR (151 MHz, C /benzene-d , 298K) δ 186.1 (s, C=N),
6
H
4
F
2
and C
C), 3.79 (d, 2JHH = 17.5 Hz, 2H, CCH
), 1.71 (s, 12H, CCH
6
D
6
. 1H NMR (600 MHz, C
6
H
4
F
2
/benzene-
C), 2.43 (hept,
), 1.31 (d, 3JHH
),
d
6
2
2
2
2
3
=
6.8 Hz, 12H, CHCH
3
3
3
0
3
3
1
3
were obscured by C
6
H
4
F
2
6
H
4
F
2
6
1
39.8 (s, ArC), 139.2 (s, ArC), 138.7 (s, ArC), 129.8 (s, ArC), 126.2 (s, ArC), 125.3 (s, ArC), 115.0
(
s, MgC≡C), 100.3 (s, MgC≡C), 45.8 (s, CCH
NC(CH )), 24.7 (s, CHCH ), 24.6 (s, CHCH ), 23.8 (s, CHCH
NMR (565 MHz, C /benzene-d
Hz, 4F, p-CF), -167.4 (t, J = 19 Hz, 8F, m-CF); B NMR (193 MHz, C
2
C), 29.7 (s, CHMe
2
), 27.9 (s, CHMe
2
), 26.6 (s,
1
9
3
3
3
3
), 23.1 (s, CHCH
3
), 0.2 (s, SiMe ; F
3
6
H
4
F
2
6
, 298K) δ -131.8 – -132.8 (m, 8F, o-CF), -163.6 (t, 3JFF = 20
1
1
6
H
4
F
2
/benzene-d
, 298K) δ -16.1 (s, C≡CSi); IR (ATR,
pure): ṽ = 2970 (m), 1642 (m), 1620 (w), 1511 (s), 1461 (vs), 1372 (s), 1270 (s), 1089 (vs), 978
vs), 841 (m), 796 (s), 754 (vs), 682 (s), 660 (s); Elemental analysis calcd (%) for
6
, 298K) δ -
2
9
1
6.1 (s, B(C
6
F
5
)
4
); Si NMR (119 MHz, C
6
H
4
F
2
/benzene-d
6
(
C
116
H
102
B
2
F
40Mg
2
N
4
2
Si (2438.46): C 57.14, H 4.22, N 2.30; found: C 57.66, H 3.95, N 2.17.
+
–] (4): [(BDI)Mg(nPr)]
(0.085 g,
Synthesis of {Mg C(Ph)=C(Ph)C[C(Me)=NDipp]
2
}
2
2[B(C
–] (0.148 g, 0.160 mmol) were dissolved in chlorobenzene
0.5 ml) and stirred until an almost colorless solution was obtained (1 min). The solution was
6
F
5
)
4
2
0
.088 mmol) and [Ph
3
C+][B(C
6
F
)
5 4
(
layered with a 0.25 M solution of diphenylacetylene in hexane (0.5 ml) and left standing at
room temperature. After 1 day colorless crystals could be obtained which were washed with
C
6
H
5
Br (2 x 0.3 ml) and hexane (4 x 0.3 ml) before they were dried in vacuo. The product (4)
was isolated in a yield of 53% (0.105 g, 0.0408 mmol). One C Cl molecule cocrystallized per
dimer. Due to complete insolubility in C Br and reversibility of the reaction in other
aromatic solvents no NMR data could be acquired. IR (ATR, pure): ṽ = 2970 (m), 2934 (w),
H
6 5
D
6 5
1
1
642 (m), 1613 (w), 1576 (m), 1512 (s), 1460 (vs), 1391 (m) 1369 (s), 1337 (m), 1297 (m),
273 (s), 1194 (m), 1086 (vs), 977 (vs), 835 (s), 797 (s), 769 (s), 754 (s), 719 (s), 682 (s), 659
1
8
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Chemistry - A European Journal
10.1002/chem.201804802
(s); Elemental analysis calcd (%) for C140
H
107
B
2
ClF40Mg
2
4
N (2711.04) including one equivalent
of cocrystallized C Cl: C 62.03, H 3.98, N 2.07; found: C 62.61, H 4.06, N 1.78.
6 5
H
+
–] (5): [(BDI)Mg(nPr)]
(0.082 g,
Synthesis of {Mg C(Ph)=C(Me)C[C(Me)=NDipp]
.085 mmol) and [Ph
C+][B(C –] (0.150g, 0.163 mmol) were dissolved in chlorobenzene
0.5 ml) and stirred until an almost colorless solution was obtained (1 min). The solution was
layered with a 15:1 mixture of hexane/phenylpropyne (0.5 ml). After 6 d colorless crystals
could be obtained which were washed with C Br (2 x 0.3 ml) and hexane (2 x 0.3 ml)
before they were dried in vacuo. The product was isolated in a yield of 75% (0.153 g, 0.0618
mmol). NMR data were obtained from the crystalline material (including one C Cl
molecule per dimer) in a 2:1 mixture of C Br and C Br/C , 600 MHz,
. 1H NMR (C
98K) δ 7.38 (t, 3JHH = 7.6 Hz, 2H,C=CPh), 7.25 (t, 3JHH = 7.7 Hz, 2H, Dipp–ArH), 7.17 – 7.11 (m,
H, ArH), 6.97 – 6.94 (m, 1H, C=CPh), 6.62 (d, 3JHH = 7.3 Hz, 2H, C=CPh), 4.39 (s, 1H,
), 2.48 (hept, 3JHH = 6.8 Hz, 2H, CHMe ), 2.28 (hept, 3JHH = 6.8 Hz, 2H, CHMe
), 1.74
s, 3H, C=CCH ), 1.61 (s, 6H, NCCH
), 1.29 (d, 3JHH = 6.8 Hz, 6H, CHCH
), 0.90 (d, 3JHH = 6.8 Hz, 6H, CHCH
2
}
2
2[B(C
6
F
5
)
4
2
0
3
6
F
)
5 4
(
H
6 5
H
6 5
6
D
5
6
D
6
6
D
5
D
6 6
2
4
CCH(CN)
2
2
2
(
3
3
3
), 1.11 (d, 3JHH = 6.8 Hz,
13
6
H, CHCH
Br/C
), 147.7 (s, C=CPh), 140.0 (s, Dipp-ArC) 139.46 (s, C=CPh), 138.66 (s, Dipp-ArC), 136.91
d, 1JCF = 255 Hz, B(C
), 132.4 (s, C=CCH ), 129.9 (s, C=CPh), 128.8 (s, C=CPh), 128.7 (s,
Dipp-ArC), 125.4 (s, C=CPh), 125.0 (s, Dipp-ArC), 64.5 (s, CCH(CN) ), 29.3 (s, CHMe ), 28.1 (s,
), 24.1 (s, CH(CH ), 23.9 (s,
3
3
), 0.83 (d, 3JHH = 6.8 Hz, 6H, CHCH
3
); C NMR
(C
6
D
5
6
D
, 151 MHz, 298K) δ 182.5 (s, C=N), 175.1 (s, C=CPh), 149.0 (d, 1JCF = 241 Hz,
6
B(C
6
F
5 4
)
(
6
F
5
)
4
3
2
2
CH(CH
3
)
2
), 27.4 (s, NCCH
3
), 24.4 (s, CH(CH
3
)
2
), 24.2 (s, CH(CH
3
)
2
3 2
)
1
9
CH(CH
3
)
2
), 22.1 (s, C=CCH
3
); F NMR (C
6
D
5
6
Br/C D
6
, 565 MHz, 298K) δ -130.5 – -132.5 (m, 8F,
o-CF), -161.9 (t, 3JFF = 21 Hz, 4F, p-CF), -165.8 (t, J = 20 Hz, 8F, m-CF); B NMR (C
11
D
Br/C
6
5
6
6
D ,
1
93 MHz, 298K) δ -15.8 (s, B(C
6
F ) ); IR (ATR, pure): ṽ = 2968 (m), 2932 (w), 2875 (w), 1642
5
4
(m), 1614 (w), 1583 (w), 1512 (s), 1460 (vs), 1369 (s), 1273 (s), 1187 (w), 1087 (vs), 976 (vs),
8
43 (m), 797 (s), 772 (s), 756 (s), 747 (s), 682 (s), 660 (s); Elemental analysis calcd (%) for
C
124
H
98
B
2
F
2
40Mg N
4
(2474.34): C 60.19, H 3.99, N 2.26; found: C 59.64, H 4.03, N 1.89.
+
–] (6): In situ generation of
In situ generation of [(BDI)Mg ∙PPh
3
][B(C
6
F
)
5 4
+
–
+
F )
–] (0.0140
[
(BDI)Mg ∙PPh
3
][B(C
6
F
5
)
4
] was accomplished by dissolving [(BDI)Mg ∙C
6
H
6
][B(C
6
5
4
g, 0,0117 mmol) and PPh
3
(0.0031 g, 0.012 mmol) in C
6
5
D Br (0.5 ml). For crystallization of the
+
–] (0.0412g, 0.0344 mmol) and substoichiometric quantities
product, [(BDI)Mg ∙C
6
H
6
][B(C
6
F
)
5 4
of PPh (0.0042 g, 0.016 mmol) were dissolved in a 2:1 mixture of chlorobenzene (0.4 ml)
3
1
9
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Chemistry - A European Journal
10.1002/chem.201804802
and toluene (0.2 ml) and the resulting solution was layered with hexane (0.5 ml). A few
colorless crystals suitable for X–ray diffraction could be isolated after 30 days. NMR data are
1
given for the in situ generated adduct. H NMR (C
6
D
5
Br, 400 MHz, 298 K) δ 7.26 – 7.16 (m,
), 6.52 (dd, 3JPH = 12.3
5
H, ArH), 7.04 (d, 3JHH = 7.7 Hz, 4H, Dipp–ArH), 7.03 – 6.95 (m, 6H, PPh
Hz, 3JHH = 7.7 Hz, 6H, PCCHC) 4.97 (s, 1H, CCHC), 2.84 (hept, J = 6.8 Hz, 4H, CH(CH
H, NCCH
), 1.01 (d, 3JHH = 6.8 Hz, 12H, CH(CH
NMR (C
Br, 101 MHz, 298 K) δ 173.2 (s, CCHC), 149.1 (d, 1JCF = 233 Hz, B(C
ArC), 142.4 (s, ArC), 138.9 (d, 1JCF = 240 Hz, B(C ), 137.1 (d, 1JCF = 242 Hz, B(C
d, 2JCP = 14 Hz, PPh ), 132.6 (d, 4JCP = 3 Hz, PPh ), 130.6 (d, 3JCP = 11 Hz, PPh
) 127.6 (s, ArC),
25.3 (s, ArC), 97.8 (s, CCHC), 29.1 (s, CH(CH ), 24.8 (s, CH(CH ), 24.7 (s, NCCH ), 24.1 (s,
CH(CH ), (the quaternary carbon in PPh is not observed); P{ H} NMR (C Br, 162 MHz,
3
3
)
2
), 1.57 (s,
), 0.63 (d, 3JHH = 6.8 Hz, 12H, CH(CH
)
2
; C
13
6
3
3
)
2
3
6
D
5
6
F
5
)
4
), 143.0 (s,
), 133.1
6
F
5
)
4
6
F
5 4
)
(
3
3
3
1
3
)
2
3
)
2
3
3
1
1
3
)
2
3
D
6 5
19
2
98 K) δ -5.0 (s, PPh
3
); F NMR (C
6
D
5
Br, 376 MHz, 298 K): δ -129.8 – -132.0 (m, 8F, m-ArF),
161.4 (t, 3JFF = 21 Hz, 4F, p-ArF), −165.6 (t, 3JFF = 20 Hz, 8F, o-ArF) ppm; B NMR (C
11
−
6
5
D Br, 128
MHz, 298 K): δ −15.6 (s, B(C
6
F ) ) ppm.
5
4
+
–
+
F )
–]
Synthesis of [(BDI)Mg ∙PCy
3
][B(C
6
F
5
)
4
] (7): In situ generation of [(BDI)Mg ∙PCy
3
][B(C
6
5
4
+
–] (0.0141 g, 0,0118 mmol) and PCy
was accomplished by dissolving [(BDI)Mg ∙C
6
H
6
][B(C
6
F
5
)
4
3
(
0.0033 g, 0.0118 mmol) in C
6
D
5
Br (0.5 ml). For crystallization of the product,
(0.0427 g, 0.152 mmol) were
+
–] (0.1489 g, 0.1242 mmol) and PCy
[
(BDI)Mg ∙C
6
H
6
][B(C
6
F
5
)
4
3
dissolved in fluorobenzene (1.0 ml) and the resulting solution was layered with hexane (1.0
ml). Colorless crystals could be isolated after 3 days in a yield of 5% (0.010 g, 0.0068 mmol).
1
The complex crystallized with one PhF molecule. H NMR (C
6
D
5
Br, 400 MHz, 298 K) δ 7.13 –
), 1.56 (s, 6H,
), 1.09 (d, 3JHH = 6.8
), 1.00 – 0.76 (m, 12H, PCy ), 0.75 – 0.57 (m,
Br, 101 MHz, 298 K) δ 173.3 (s, CCHC), 149.1 (d, 1JCF = 241 Hz,
), 142.9 (s, ArC), 142.0 (s, ArC), 138.8 (d, 1JCF = 244 Hz, B(C
), 137.0 (d,
1JCF = 247 Hz, B(C
), 127.9 (s, ArC), 125.1 (s, ArC), 98.1 (s, CCHC), 31.1 (s, PCy ) , 30.8 (d,
2JCP = 16 Hz, PCH), 29.4 (s, CH(CH ), 27.3 (d, 3JCP = 11 Hz, o–Cy), 25.4 (s, PCy
), 25.3 (s,
CH(CH ), 25.0 (s, NCCH ), 24.2 (s, CH(CH Br, 162 MHz, 298 K) δ 20.4
s, PCy
Br, 376 MHz, 298 K): δ -130.0 – -132.5 (m, 8F, m-ArF), −161.8 (t, 3JFF
9 Hz, 4F, p-ArF), −165.2 – -165.7 (m, 8F, o-ArF) ppm; B NMR (C
7
.00 (m, 6H, ArH), 5.00 (s, 1H, CCHC), 2.83 (hept, 3JHH = 6.8 Hz, 4H, CH(CH
NCCH ), 1.62 – 1.39 (m, 12H, PCy
), 1.12 (d, 3JHH = 6.8 Hz, 12H, CH(CH
Hz, 12H, CH(CH ), 1.18 – 1.05 (m, 3H, PCHCH
); C NMR (C
)
3 2
3
3
)
3 2
3
)
2
2
3
1
3
6
H, PCy
3
D
6 5
B(C
6
F
5
)
4
6
F
)
5 4
6
F
5
)
4
3
3
)
2
3
3
1
1
3
)
2
3
3
)
2
); P{ H} NMR (C
6 5
D
1
9
(
3
); F NMR (C
6
D
5
=
1
1
1
6
D
5
Br, 128 MHz, 298 K): δ
2
0
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Chemistry - A European Journal
10.1002/chem.201804802
−
15.6 (s, B(C
6
F
5
)
4
) ppm. Elemental analysis calcd (%) for C77
H79BF20MgN
2
P (1497.55): C
6
1.76%, H 5.32%, N 1.87%. Found: 61.83%, H 5.08%, N 1.96%.
+
–] (8): In situ generation of [(BDI)Mg+∙
In situ generation of [(BDI)Mg ∙PtBu
3
][B(C
6
F
)
5 4
–
+
–] (0.0092 g, 0,0077
Cl/C (0.5 ml). NMR
analysis showed that the phosphine adduct (8) is in equilibrium with the benzene adduct
PtBu
3
][B(C
6
F
5
)
4
] was accomplished by dissolving [(BDI)Mg ∙C
6
H
6
][B(C
6
F
)
5 4
mmol) and PtBu
3
(0.0016 g, 0.0080 mmol) in a 2:1 mixture of C
6
H
5
D
6 6
+
+
F )
–] (0.0505 g, 0.0421
(
BDI)Mg ∙C
6
H
6
. For crystallization of the product, [(BDI)Mg ∙C
6
H
6
][B(C
6
5
4
mmol) and PtBu
3
(0.0085 g, 0.0420 mmol) were dissolved in a 2:1 mixture of benzene (0.4
ml) and fluorobenzene (0.2 ml). The solution was allowed to evaporate to dryness at room
temperature. From the residue it was possible to obtain a few colorless crystals suitable for
X-ray diffraction that were embedded in amorphous material. NMR data are given for the in
situ generated adduct. From the two species that are in equilibrium only data for [(BDI)Mg+∙
–
1
PtBu
hept, 3JHH = 6.7 Hz, 4H, CH(CH
.99 (d, 3JHH = 6.7 Hz, 12H, CH(CH
aromatic region are obscured by C
CCHC), 148.7 (d, 1JCF = 243 Hz, B(C
B(C
), 136.6 (d, 1JCF = 247 Hz, B(C
d, 1JCP = 5.8 Hz, PC(CH ) , 31.4 (d, 2JCP = 5.2 Hz, PC(CH
NCCH ), 24.1 (s, CH(CH
3
][B(C
6
F
5
)
4
] are given. H NMR (C
6
D
6
/C
6
H Cl, 600 MHz, 298 K) δ 4.98 (s, 1H, CCHC), 2.71
5
(
3
)
2
), 1.55 (s, 6H, NCCH
3
), 1.07 (d, 3JHH = 6.7 Hz, 12H, CH(CH
) ),
3
2
t
0
3
)
2
), 0.68 (d, 3JPH = 13.8 Hz, 27H, P Bu
3
). (all signals in the
1
3
6
H
4
F
2
); C NMR (C
), 143.6 (s, ArC), 141.5 (s, ArC), 138.4 (d, 1JCF = 244 Hz,
), 128.2 (s, ArC), 124.8 (s, ArC), 97.2 (s, CCHC), 35.7
), 29.3 (s, CH(CH ), 25.3 (s,
/C Cl, 243 MHz, 298 K) δ
6
D
6
/C
6
H
5
Cl, 151 MHz, 298 K) δ 172.9 (s,
6
F
)
5 4
6
F
5
)
4
6
F
)
5 4
(
3
)
3
3
)
3
)
3 2
3
1
1
3
3
)
2
), 23.6 (s, CH(CH
3
)
2
); P{ H} NMR (C
6
D
6
H
6 5
t
19
6
9.7 (s, P Bu
3
); F NMR (C
6
D
6
/C
6
H
5
Cl, 565 MHz, 298 K): δ -130.5 – -132.5 (m, 8F, m-ArF),
162.4 (t, 3JFF = 20 Hz, 4F, p-ArF), −166.5 (t, 3JFF = 20 Hz, 8F, o-ArF) ppm; B NMR (C
11
−
6
5
D Br, 193
MHz, 298 K): δ −16.1 (s, B(C
6
F ) ) ppm.
5
4
Supporting Information
+
F )
–] (IR, Raman), NMR spectra for
Additional data for complex [(BDI)Mg ∙EtCCEt][B(C
6
5
4
complexes 2–3 and 5–8, IR spectra for complexes 2–5 and details for the crystal structure
determinations of complexes 2-8.
2
1
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Chemistry - A European Journal
10.1002/chem.201804802
Author Information
Corresponding author: sjoerd.harder@fau.de
The authors declare no competing financial interest.
Acknowledgements
We acknowledge Mrs. C. Wronna (University of Erlangen-Nürnberg) for numerous CHN
analyses and J. Schmidt and Dr. C. Färber (University of Erlangen-Nürnberg) for assistance
with the NMR analyses. We acknowledge Boulder Scientific for a sample of
[
Ph
3
C+][B(C
6
F ) ˉ].
5
4
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