Synthesis and reactivity of magnesium complexes supported by tris(2-dimethylaminoethyl)amine (Me6tren)

Article abstract of DOI:10.1021/om4002186

The reaction of tris(2-dimethylaminoethyl)amine (Me₆tren) with Grignard reagents and related Mg precursors has been investigated. Treating Me₆tren with 2 equiv of PhMgBr in diethyl ether resulted in the formation of [(Me₆tren)MgBr]Br (1), in which Me₆tren is bound in a κ⁴ fashion. This is the first example of a Mg complex containing Me₆tren or a related tris(aminoethyl)amine ligand. In contrast, when MeMgBr was treated with either 1 or 2 equiv of Me ₆tren, a mixture containing 1 and the alkyl species [(Me ₆tren)MgMe]Br (3) was produced. It was not possible to separate the two compounds to generate a pure sample of 3. Reaction between Me ₆tren and greater than 4 equiv of MeMgBr formed [(Me ₆tren)MgBr]₂[MgBr₄] (4), an analogue of 1 with a different counterion. The highly unusual dialkyl Mg compound (Me ₆tren)MgMe₂ (5), which features a κ³- bound Me₆tren ligand, was synthesized through the reaction of Me ₂Mg with Me₆tren. The reaction of 5 with excess phenylacetylene or carbon dioxide yielded (Me₆tren)Mg(CCPh) ₂ (6) and Mg(OAc)₂, respectively, while treatment with benzylalcohol, benzylamine, 4-tert-butylcatechol, 4-tert-butylphenol, and aniline all resulted in decomposition. The addition of 1 equiv of 2,6-lutidine·HBAr^F (BAr^F = tetrakis(3,5- bis(trifluoromethyl)phenyl)borate) to 5 formed [(Me₆tren)MgMe] BAr^F (7), a rare example of a neutral ancillary ligand supported cationic monoalkyl Mg species. Compounds 1, 4, and 5 have been crystallographically characterized.

Full text of DOI:10.1021/om4002186

Article
pubs.acs.org/Organometallics
Synthesis and Reactivity of Magnesium Complexes Supported by
Tris(2-dimethylaminoethyl)amine (Me6tren)
Louise M. Guard and Nilay Hazari*
†The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
S
* Supporting Information
ABSTRACT: The reaction of tris(2-dimethylaminoethyl)amine (Me6tren) with
Grignard reagents and related Mg precursors has been investigated. Treating
Me6tren with 2 equiv of PhMgBr in diethyl ether resulted in the formation of
[(Me6tren)MgBr]Br (1), in which Me6tren is bound in a κ4 fashion. This is the first
example of a Mg complex containing Me6tren or a related tris(aminoethyl)amine
ligand. In contrast, when MeMgBr was treated with either 1 or 2 equiv of Me6tren,
a mixture containing 1 and the alkyl species [(Me6tren)MgMe]Br (3) was
produced. It was not possible to separate the two compounds to generate a pure
sample of 3. Reaction between Me6tren and greater than 4 equiv of MeMgBr
formed [(Me6tren)MgBr]2[MgBr4] (4), an analogue of 1 with a different
counterion. The highly unusual dialkyl Mg compound (Me6tren)MgMe2 (5),
which features a κ3-bound Me6tren ligand, was synthesized through the reaction of
Me2Mg with Me6tren. The reaction of 5 with excess phenylacetylene or carbon
dioxide yielded (Me6tren)Mg(CCPh)2 (6) and Mg(OAc)2, respectively, while
treatment with benzylalcohol, benzylamine, 4-tert-butylcatechol, 4-tert-butylphenol, and aniline all resulted in decomposition.
The addition of 1 equiv of 2,6-lutidine·HBArF (BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) to 5 formed
[(Me6tren)MgMe]BArF (7), a rare example of a neutral ancillary ligand supported cationic monoalkyl Mg species. Compounds
1, 4, and 5 have been crystallographically characterized.
INTRODUCTION
■
Tren (tris(2-aminoethyl)amine, H6tren) was the first tripodal
tetraamine ligand to be reported,1 and since its initial
preparation in 1896, more than 50 derivatives have been
synthesized.2 It has been demonstrated that tren and related
ligands will coordinate to almost all transition metals,3−5 and a
compounds. The only H6tren compounds were reported by
variety of different properties of H6tren-containing complexes
have been explored. These include in-depth studies of the
thermodynamics and kinetics of H6tren binding,6 magnetism
and conductivity measurements,4 and structural studies on how
the H6tren ligand affects crystal field splitting and geometry.4,5
Furthermore, H6tren-containing compounds have found utility
as catalysts for a number of processes, such as C−O bond
formation,7 the living radical polymerization of vinyl chlorides,8
and the synthesis of thioesters from thiols and aryl halides.9 A
H6tren-containing complex has even been used as a structure-
directing agent in the synthesis of zeolites.10 A common
derivative of H6tren is tris(2-dimethylaminoethyl)amine
(Me6tren), which provides reactive metal centers with increased
steric protection and also results in complexes with greater
solubility in organic solvents. Transition-metal complexes
incorporating the Me6tren ligand have been used for the
reduction of nitrile ions,11 for the modeling of cytochrome c
oxidase,12,13 as catalysts for atom transfer radical addition
reactions,14 and in aliphatic C−H bond activation.15
White et al., who prepared and structurally characterized
complexes of the type [(H6tren)M]+ (M = Li, Na).16 Using the
Me6tren ligand, Davidson and co-workers synthesized the
amido complex (Me6tren)Li(HMDS) (HMDS = hexamethyl-
disilazide) and the alkoxide species (Me6tren)Na(OR)(HOR)
(R = 2,4,6-trimethylphenoxide) and (Me6tren)Na(OR′) (R′ =
2,6-di-tert-butyl-4-methylphenoxide).17 Subsequently, in collab-
oration with Mulvey and Robertson, Davidson reported
(Me6tren)M(PhCH2) (M = Li, Na, K), where the benzyl
ligand was found to bind in a different mode, depending on the
metal. These are the only examples of s-block organometallic
compounds supported by tren or a related ligand.18 Overall,
group 1 compounds supported by tren or its derivatives are far
more prevalent than group 2 species. In fact, to date the only
reported X-ray crystal structure of tren or a derivative bound to
a group 2 element was described by Koo and co-workers, who
prepared (H6tren)Sr(thd)2 (thd = 2,2,6,6-tetramethyl-3,5-
heptanedionate).19
Although very frequently used to support transition metals,2
tren and its derivatives have rarely been used to stabilize s-block
Received: March 14, 2013
Published: April 15, 2013
© 2013 American Chemical Society
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Organometallic s-block compounds (in particular those of Li
and Mg) have been used extensively in organic synthesis in
both carbon−carbon bond forming reactions and a number of
different carbon−heteroatom bond forming processes.20,21
However, at this stage our knowledge about the reactivity of
organometallic Mg containing species is limited, in part due to
the complex speciation of Grignard reagents in solution,22 and
the synthesis of well-defined monomeric organometallic Mg
complexes could assist in increasing our mechanistic under-
standing of organic reaction pathways. As a result, our group
recently attempted to prepare organometallic Mg complexes
supported by the 2,2′:6,2″-terpyridine (terpy) ligand.23
Unfortunately, we were only able to isolate complexes of the
type (terpy)MgX2 (X = Br, Cl), and when we attempted to
explore the additional reactivity of these compounds, we found
ligand decoordination to be a major problem. We postulated
that using a κ4 tripodal ligand instead of the κ3 terpy ligand
could result in more stable complexes, which would be more
amenable to further experimentation and allow the isolation of
organometallic complexes. Here, we report the synthesis of
both coordination and organometallic complexes of Mg,
supported by Me6tren, which we believe are the first Mg
complexes containing any form of the tren ligand. In particular,
using Me6tren, we have isolated rare examples of monomethyl
and dimethyl Mg species supported by the same ligand set.
Preliminary reactivity studies of the dimethyl Mg species are
described.
decompose in dichloromethane and acetonitrile solutions at
room temperature. Our results for the reaction of PhMgX (X =
Cl, Br) with the tetradentate Me6tren ligand are consistent with
those we recently reported for the reaction of PhMgX with
tridentate terpy ligands, where we also only observed dihalide
species.23 To the best of our knowledge 1 and 2 represent the
first time the tren ligand or any other neutral tetradentate
nonplanar nitrogen ligand has been coordinated to Mg.
Compound 1 was characterized by X-ray crystallography
(Figure 1), which clearly indicates that the coordination
RESULTS AND DISCUSSION
■
The reaction of Me6tren with 1 equiv of PhMgBr in diethyl
ether resulted in the instant formation of a precipitate. The
solid was isolated by filtration, and NMR spectroscopy and X-
ray crystallography (vide infra) were used to establish that it
was [(Me6tren)MgBr]Br (1), which had formed in approx-
imately 50% yield based on Me6tren (eq 1). The filtrate from
Figure 1. ORTEP28 drawing of 1 at the 30% probability level
(hydrogen atoms have been omitted for clarity). A C3 axis is present
along the N(1)−Mg(1)−Br(1) bond. Selected bond lengths (Å) and
angles (deg): Mg(1)−N(1) = 2.193(8), Mg(1)−N(2) = 2.193(5),
Mg(1)−Br(1) = 2.503(4); N(1)−Mg(1)−N(2) = 81.75(12), N(1)−
Mg(1)−Br(1) = 180.00(11), N(2)−Mg(1)−Br(1) = 98.25(12),
N(2)−Mg(1)−N(2′) = 117.98(6).
the reaction mixture contained 0.5 equiv of unreacted Me6tren
and presumably 0.5 equiv of Ph2Mg or a related decomposition
product, which had formed through the disproportionation of
PhMgBr. There was no evidence to indicate that the diphenyl
species (Me6tren)MgPh2 or the monophenyl monohalide
species [(Me6tren)MgPh]Br formed in the reaction.
The reaction of Me6tren with 2 equiv of PhMgBr afforded 1
in near-quantitative yield, with no unreacted ligand (eq 2). The
analogous reaction between 2 equiv of PhMgCl and Me6tren
generated [(Me6tren)MgCl]Cl (2) in 84% yield. The parent
ions corresponding to both 1 and 2 were observed using ESI-
MS, although in the case of 1 the parent ion was also
accompanied by a small ion corresponding to 2, which forms
due to the reaction of 1 with dichloromethane (the solvent for
the ESI-MS experiment). Both 1 and 2 are indefinitely stable
when stored as solids in a nitrogen-filled glovebox but slowly
number around Mg is 5, with an outer-sphere bromide
counterion. Presumably, steric factors prevent the outer-sphere
bromide from coordinating and forming a six-coordinate Mg
center. The geometry around Mg is trigonal bipyramidal, with
the tren ligand occupying three equatorial sites and one axial
site and the coordinated bromide occupying the second axial
site. There is a C3 rotation axis along the N(1)−Mg(1)−Br(1)
bond, and the angle formed among any of the equatorial
nitrogens, the Mg center, and the bromide ligand (for example,
N(2)−Mg(1)−Br(1) is 98.25(12)°) is larger than the expected
90° for an idealized trigonal-bipyramidal structure. This
distortion occurs because the Mg atom sits slightly out of the
́
plane (0.316 Å) formed by the three equatorial nitrogen atoms.
As a result, the bond angles among any two of the equatorial
nitrogen atoms and Mg (for example, N(2)−Mg(1)−N(2′) is
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117.98(6)°) are not 120°. The distances between the Mg
center and the axial and equatorial nitrogens are 2.193(8) and
2.193(5) Å, respectively, both unremarkable for Mg−N
bonds.24 Previously, transition-metal compounds of the type
[(Me6tren)MBr]Br (M = Co,25 Cu,26 Ni,26 Mn,27 Fe,27 Zn27)
were prepared and crystallized by Orioli and co-workers. The
bond lengths and angles around the metal centers in those
species are comparable to those observed in 1.
reaction mixture. The relative ratio of 1 to 3 was 1.4:1. The
addition of 2,6-lutidene·HBr to a CD2Cl2 solution of a mixture
of 1 and 3 resulted in the quantitative conversion of 3 to 1.
Furthermore, the evolution of methane (identified by 1H NMR
spectroscopy) in a quantity consistent with the initial amount
of 3 present in solution was observed. This provides further
evidence for the assignment of 3 as the second component of
the mixture. Unfortunately, 3 could not be separated from 1, as
the solubilities of both compounds were similar in solvents in
which they were stable (THF) and 3 was unstable when
dissolved in dichloromethane or acetonitrile for periods of time
greater than 5 min. To the best of our knowledge the only
other examples of Me6tren-supported organometallic com-
plexes were reported by Vacca and co-workers, who prepared
[(Me6tren)HgR][CF3SO3] (R = Me, Ph),29 though only the
latter was characterized by X-ray crystallography, and Mulvey,
Robertson, and Davidson, who synthesized (Me6tren)M-
(PhCH2) (M = Li, Na, K).17
In contrast to the reaction between Me6tren and PhMgBr, a
mixture containing two products was isolated when 1 equiv of
MeMgBr was treated with Me6tren (eq 3). One of these
The relative ratio of 1 and 3 could be changed by varying the
number of equivalents of MeMgBr used, and though neither
could be formed exclusively, 1 equiv of MeMgBr produced a
higher proportion of 3 (1.4:1 ratio of 1 to 3) in comparison
with 2 equiv (3:1 ratio of 1 to 3). A similar trend was observed
when MeMgCl was used as the Grignard reagent. Again, the
two components could not be separated, as their solubilities in
a range of common solvents were similar. The results from the
reaction of Me6tren and MeMgBr or MeMgCl are different
from those observed in reactions between terpy and the same
Grignard reagents. In the terpy systems we did not see any
evidence for the formation of a ligated mixed Mg methyl/halide
species and only observed the ligated Mg dihalide, free Me2Mg,
and an unidentified terpy-containing species after disproportio-
nation of the Grignard reagent.23 We believe that the κ4
Me6tren ligand assists in thermodynamically stabilizing the
mixed methyl/halide species toward disproportionation in
comparison with terpy. Previously, we have demonstrated
that disproportionation is more favorable for phenyl species in
products was compound 1, while the second product contained
a peak integrating to three protons (relative to the Me6tren
ligand) at −1.69 ppm in the 1H NMR spectrum at 25 °C. This
second product was too unstable to record a 13C{1H} NMR
spectrum at room temperature, but at −40 °C, a peak at −19.2
ppm was visible in the 13C{1H} NMR spectrum. On the basis
1
of the H and 13C{1H} NMR data the second product is
assigned as [(Me6tren)MgMe]Br (3). We believe that the
bromide ligand is outer sphere, by analogy to compound 1. In
order to balance the stoichiometry in eq 3, a third product is
required. Although there was no evidence for the formation of
(Me6tren)MgMe2, a significant amount of free Me6tren and a
small Mg side product were observed. These could be separated
from 1 and 3 by filtration, as 1 and 3 precipitated from the
Figure 2. ORTEP28 drawing of 4 at the 30% probability level (hydrogen atoms and toluene of crystallization have been omitted for clarity). Selected
bond lengths (Å) and angles (deg) for molecule A (left): Mg(1)−N(1) = 2.197(2), Mg(1)−N(2) = 2.177(2), Mg(1)−N(3) = 2.182(2), Mg(1)−
N(4) = 2.197(2), Mg(1)−Br(1) = 2.4970(9); N(1)−Mg(1)−N(2) = 117.42(9), N(1)−Mg(1)−N(3) = 118.98(9), N(1)−Mg(1)−Br(1) =
98.55(7), N(2)−Mg(1)−N(3) = 117.11(9), N(2)−Mg(1)−Br(1) = 99.39(7), N(3)−Mg(1)−Br(1) = 97.68(7), N(4)−Mg(1)−Br(1) = 179.06(7).
Selected bond lengths (Å) and angles (deg) for molecule B (right): Mg(2)−N(5) = 2.213(2), Mg(2)−N(6) = 2.193(3), Mg(2)−N(7) = 2.196(2),
Mg(2)−N(8) = 2.205(2), Mg(2)−Br(2) = 2.4787(9); N(5)−Mg(2)−N(6) = 118.72(9), N(6)−Mg(2)−N(7) = 115.89(10), N(5)−Mg(2)−Br(2)
= 97.20(7), N(7)−Mg(2)−N(5) = 118.38(10), N(6)−Mg(2)−Br(2) = 100.28(7), N(7)−Mg(2)−Br(2) = 99.19(7), N(8)−Mg(2)−Br(2) =
178.37(7).
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comparison with methyl species, and our results for the
reactions of MeMgX and PhMgX with Me6tren are consistent
with this trend.23
The reaction of Me6tren with 4 equiv or greater of MeMgBr
resulted in the formation of a new tren-containing product. X-
ray crystallography revealed that the new product was
[(Me6tren)MgBr]2[MgBr4] (4) (Figure 2), an analogue of 1
with a different counterion. This was also confirmed by ESI-
MS, where the single peak observed was attributed to the
cationic fragment of 4. The change in counterion caused a very
slight perturbation in the 1H NMR shifts. In 4 the most
downfield triplet from the methylene protons appears at 3.06
ppm, whereas in 1 it is seen at 3.08 ppm. Similarly, the NMe2
protons are at 2.56 ppm in 1 and 2.53 ppm in 4. X-ray
crystallography demonstrates that both cations in the structure
are slightly distorted trigonal bipyramids with lengths and
angles comparable to those in 1. As in 1, the metal center is
positioned just below the plane formed by the three terminal
nitrogen atoms; N(1)−Mg(1)−Br(1) is 98.55(7)°, and N(5)−
Mg(2)−Br(2) is 97.20(7)°. The ligand is bound in a κ4
manner, which is supported by the distances between the
tripodal nitrogen and Mg (Mg(1)−N(4) is 2.197(2) Å, and
Mg(2)−N(8) is 2.205(2) Å). A product analogous to 4 was
observed from a reaction using MeMgI by 1H NMR
spectroscopy, though the corresponding chloride product has
not been observed, even when 10 equiv of MeMgCl was used.
Crystallization of 4 by layering of toluene onto a saturated
solution of dichloromethane yielded the same cationic unit, but
with an alternative chloride-containing counterion: [(Me6tren)-
MgBr]2[Br2Mg(μ-Cl)]2 (see the Supporting Information for
Figure 3. ORTEP28 drawing of 5 at the 30% probability level
(hydrogen atoms have been omitted for clarity; only one site of
disordered Me6tren arm is shown). Selected bond lengths (Å) and
angles (deg): Mg(1)−N(1) = 2.3148(11), Mg(1)−N(2) =
2.4814(11), Mg(1)−N(3)
= 2.2959(12), Mg(1)−C(1) =
2.2042(14), Mg(1)−C(2) = 2.1710(13); N(1)−Mg(1)−N(2) =
73.98(4), N(1)−Mg(1)−N(3) = 131.76(4), N(1)−Mg(1)−C(1) =
94.14(5), N(1)−Mg(1)−C(2) = 112.64(5), N(2)−Mg(1)−N(3) =
74.27(4), N(2)−Mg(1)−C(1) = 149.33(5), N(2)−Mg(1)−C(2) =
97.84(4), N(3)−Mg(1)−C(1) = 95.09(5), N(3)−Mg(1)−C(2) =
106.87(5), C(1)−Mg(1)−C(2) = 112.82(6).
2−
more details). This is the first time the [Br2Mg(μ-Cl)]2
counterion has been described in the literature, although a
similar compound, [Cd(Me6tren)I]2[Cd2I6], was previously
reported by Ciampolini and co-workers.30
In an attempt to isolate a pure sample of an organometallic
Mg complex, Me6tren was treated with dimethylmagnesium
(Me2Mg) in diethyl ether. Over a period of 24 h, a solid
precipitated from the reaction mixture at −80 °C. The solid
was isolated and characterized as (Me6tren)MgMe2 (5) by
NMR spectroscopy and X-ray crystallography (eq 4 and Figure
Previously, κ3 coordination of Me6tren has been observed in
transition-metal complexes containing Pd,33 Ru,34 and Cu,14
while the only other example for an s-block element was
reported by Davidson et al., who prepared (Me6tren)Li-
(HMDS).17 Though it was not crystallographically confirmed,
Macbeth and co-workers also postulated that [(Me6tren)Cu-
(CO)]PF6 featured a κ3-bound ligand on the basis of IR
spectroscopy.35 More recently, the Me6tren ligand has been
observed to bind in a κ2 fashion to zinc and bridge three gallium
centers, with each metal bound to one arm.36
A comparison of the structure of 5 with that of (PMDTA)-
MgMe2 (PMDTA = N,N,N′,N′,N″-pentamethyldiethylenetri-
amine), one of only three other crystallographically charac-
terized monomers to feature two terminal methyl groups
bound to Mg,37−39 reveals significant lengthening of the
Mg(1)−N(2) bond of 5. In (PMDTA)MgMe2, which features
a κ3 PMDTA ligand, the corresponding Mg−N bond length is
2.424(2) Å.38 This suggests stronger binding for the tridentate
PMDTA ligand, than for Me6tren, once one arm of the Me6tren
is no longer coordinated. The overall geometry around Mg in 5
is square pyramidal. One of the methyl groups is trans to the
apical nitrogen of the Me6tren ligand, and the strong trans
influence of the methyl ligand is presumably partially
responsible for the elongated Mg(1)−N(2) bond distance.
The other methyl ligand is trans to a vacant site, and as a result
this Mg−C bond distance is significantly shorter (the Mg(1)−
3). Compound 5 is thermally unstable and needed to be stored
at −30 °C in a nitrogen-filled glovebox. This thermal instability
is presumably why 5 was not observed as a disproportionation
product in the reaction between MeMgBr and Me6tren (eq 3).
Interestingly, the solid-state structure of 5 displays an
unusual binding mode of Me6tren, where the third arm of
the ligand is free and is not coordinated to Mg. In addition, the
bond length between the axial nitrogen of Me6tren and the Mg
center is extremely long (the Mg(1)−N(2) bond distance is
2.4814(13) Å). A survey of all Mg−N bonds in the Cambridge
Structural Database24,31 revealed that this distance is signifi-
cantly longer than the mean Mg−N distance, which is 2.120 Å.
The sum of van der Waals radii of Mg and N is 3.28 Å,32 and
this suggests that a bonding interaction between the central
nitrogen in Me6tren and the Mg center is present but weak.
́
C(2) bond distance is 2.1710(13) Å compared to the Mg(1)−
́
C(1) bond distance, which is 2.2042(14) Å). The bond
distances between Mg and the Me6tren ligands are significantly
longer in 5 in comparison to those observed in 1. For example,
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Scheme 1
the Mg(1)−N(1) and Mg(1)−N(3) bond distances in 5 are
2.3148(11) and 2.2959(12) Å, respectively, while the
corresponding distance in 1 is 2.193(5) Å (due to symmetry
requirements, the two Mg−N distances are identical). The
longer distances in 5 probably occur because the compound is
neutral, whereas 1 is cationic.
structure, this could indicate that 6 also features a κ3-bound
Me6tren.
The reaction of 5 with carbon dioxide in C6D6 led to the
instant formation of a white precipitate, and free Me6tren was
1
present in the H NMR spectrum. The same white precipitate
was formed when 2 equiv of acetic acid was added to a solution
Although the solid-state structure of 5 shows one of the
ligand arms to be chemically nonequivalent, the H NMR
of 5 in C6D6. In this experiment 2 equiv of methane was also
observed by H NMR spectroscopy. On this basis and by
1
1
spectrum in toluene-d8 displays only one environment for the
ligand methylene protons between −90 and +25 °C. This
NMR behavior is comparable to that observed in transition-
metal complexes containing κ3-coordinated tren, which also
only show one signal for the methylene protons at low
temperature.14,17 Consistent with the observation of only one
methylene environment, only one Mg−Me resonance (at
−0.99 ppm), which integrates to six protons, is observed in the
1H NMR spectrum of 5, even at low temperature. The 13C{1H}
NMR spectrum of 5 features a resonance at −12.53 ppm, which
is assigned to the Mg-bound Me groups.
comparison with an authentic sample, we believe that the white
precipitate is Mg(OAc)2. It is probable that this reaction occurs
by nucleophilic attack of the methyl group on electrophilic
carbon dioxide, but other mechanisms cannot be ruled out at
this stage. Further evidence of the nucleophilic character of the
methyl was provided by the reaction of 5 with 2 equiv of
benzaldehyde, which yielded 2 equiv of 1-phenylethanol, the
product of nucleophilic attack, after an acidic workup.
Compound 5 did not react with carbon monoxide. The
inability of the Me6tren ligand to remain coordinated after
reactions of 5 is in dramatic contrast with the findings of Parkin
and co-workers, who were able to isolate a wide range of ligated
Mg products from the reactions of {tris(pyrazolyl)-
hydroborato}Mg alkyl derivatives with substrates such as
carbon dioxide, alcohols, terminal alkynes, and ketones,
among others.40−42 It appears that the use of an anionic ligand
vastly improves the stability of Mg complexes supported by
nitrogen-based ligands in comparison with our tripodal neutral
nitrogen donor set.
The addition of 2 equiv of 2,6-lutidine·HBr in CD2Cl2 to 5
formed 1, while reaction with 1 equiv generated the
monomethyl species 3, which was observed by 1H NMR
spectroscopy, though the previously mentioned instability led
to swift decomposition. In an attempt to isolate a solution-
stable Me6tren-ligated Mg monomethyl species, 5 was reacted
with 2,6-lutidine·HBArF (BArF = tetrakis(3,5-bis-
(trifluoromethyl)phenyl)borate) in diethyl ether. The resulting
product, 7, is proposed to be [(Me6tren)MgMe]BArF on the
Given the relative paucity of ligated bis(alkyl) Mg species, we
were interested in exploring the reactivity of 5. The reaction of
5 in C6D6 with a variety of substrates with O−H and N−H
bonds such as aniline, benzylamine, 4-tert-butylcatechol,
benzylalcohol, and 4-tert-butylphenol all resulted in the
formation of Me6tren, and no Mg-containing products were
isolated. In all cases a precipitate formed which could not be
easily dissolved. The observation of free Me6tren suggests that
even the tetradentate Me6tren ligand is not tightly bound to
Mg. The addition of 10 equiv of phenylacetylene to a solution
of 5 in benzene formed (Me6tren)Mg(CCPh)2 (6), which was
thermally unstable (Scheme 1). In a fashion analogous to that
1
for 5, 6 displays a signal pattern in its H NMR spectrum
different from that of 1−4. In 6, a resonance associated with
one of the methylene proton triplets appears furthest upfield,
while in 1−4, the signal associated with the nitrogen methyl
groups appears the furthest upfield. In lieu of an X-ray crystal
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solvents were used, traces of all these solvents were in the atmosphere
and could be found intermixed in the solvent bottles.) Moisture- and
air-sensitive liquids were transferred by stainless steel cannula on a
Schlenk line or in a drybox. Solvents were dried by passage through a
column of activated alumina followed by storage under dinitrogen. All
commercial chemicals were used as received, except where noted.
MeMgBr, MeMgCl, PhMgCl (all in THF), MeMgI in diethyl ether,
and PhMgBr in both diethyl ether and THF were purchased from
Acros Organics and titrated using salicylaldehyde phenylhydrazone.48
2,2′,2″-Triaminotriethylamine (tren) was purchased from Strem
Chemicals. Trimethoxybenzene and 1-phenylethanol were purchased
from Sigma Aldrich, as were phenylacetylene, aniline, benzaldehyde,
benzylalcohol, and benzylamine, which were all distilled prior to use. 4-
tert-Butylphenol and 4-tert-butylcatechol were sublimed before use and
were purchased from Sigma Aldrich and Acros Organics, respectively.
Deuterated solvents were obtained from Cambridge Isotope
Laboratories. CD2Cl2 and CD3CN were dried using CaH2 and
C6D6, and toluene-d8 was dried using sodium metal. All deuterated
solvents were vacuum-transferred prior to use. NMR spectra were
recorded on Bruker AMX-400 and -500 and Varian-300 spectrometers
at ambient probe temperatures unless otherwise stated. Chemical shifts
are reported in ppm with respect to residual internal protio solvent for
1H and 13C{1H} NMR spectra and to an external standard for 19F{1H}
spectra (CFCl3 at 0.0 ppm). NMR coupling constants (J) are given in
Hz. IR spectra were measured using a diamond Smart Orbit ATR on a
Nicolet 6700 FT-IR instrument. Elemental analysis was not
performed, due to the extreme instability of almost all compounds
studied in this work; however 1H and 13C{1H} NMR spectra are
provided in the Supporting Information. Literature procedures were
utilized to synthesize Me2Mg,38 2,6-lutidine·HCl,49 and Me6tren,50
while 2,6-lutidine·HBr and 2,6-lutidine·HBArF were prepared via an
adapted literature procedure.49
X-ray Crystallography. X-ray diffraction experiments were carried
out on a Rigaku Mercury 275R CCD (SCX mini) diffractometer using
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at −50
°C, a Rigaku R-AXIS RAPID diffractometer coupled to a R-AXIS
RAPID imaging plate detector with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) at −180 °C, or a Rigaku MicroMax-007HF
diffractometer coupled to a Saturn994+ CCD detector with Cu Kα
radiation (λ = 1.54178 Å) at −180 °C. The crystals were mounted on
MiTeGen polyimide loops with immersion oil. The data frames were
processed using Rigaku CrystalClear and corrected for Lorentz and
polarization effects. Using Olex2,51 the structure was solved with the
XS52 structure solution program by direct methods and refined with
the XL52 refinement package using least-squares minimization. The
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were refined using the riding model. Details of the crystal structure and
refinement data for 1, 4, and 5 are given in the Supporting
Information.
basis of NMR spectroscopy and ESI-MS. The shifts
corresponding to the ligand are nearly identical with those
observed for 3, and Mg−Me resonances were observed at
−1.65 and −19.38 ppm in the 1H and 13C{1H} spectra,
respectively. A single peak at 62.86 ppm in the 19F NMR
spectrum confirms the presence of the BArF counterion in 7.
The compound is stable for at least 1 h in CD2Cl2 at room
temperature, which is in vast contrast to the case for 3, where
decomposition is observed almost instantly. Addition of 2,6-
lutidine·HBr to a solution of 7 in CD2Cl2 results in the
liberation of methane (detected by 1H NMR spectroscopy) and
the formation of a new tren-containing product. The more
1
downfield H NMR shifts associated with the new compound
in comparison to the shifts for 7 and the disappearance of the
Mg−Me peak are consistent with the formation of [(Me6tren)-
MgBr]BArF. Though Mg monomethyls ligated by anionic
nitrogen ligands are comparatively plentiful,43−46 7 is only the
second example of a compound containing a Mg−Me group
supported only by a neutral nitrogen donor set. The other
example, [MeMg(14N4)]Cp (14N4 = 1,4,8,11-tetramethyl-
−
1,4,8,11-tetraazacyclotetradecane; Cp = C5H5 ), features a
planar nitrogen-containing macrocycle.47 Compound 7 is
considerably less stable than either [MeMg(14N4)]Cp or
examples of monomethyls with anionic ligands.
CONCLUSIONS
■
The reaction of various Grignard reagents with Me6tren has
been shown to yield a series of Mg compounds where the type
and quantity of the Grignard has a pronounced affect on the
identity of the product. For RMgX (R = Ph, X = Cl, Br) only
[(Me6tren)MgX]X was isolated, whereas when R = Me, a
mixture containing both [(Me6tren)MgMe]X and [(Me6tren)-
MgX]X was observed. These compounds represent the first
time Mg has been coordinated to any type of tren. The tren
ligand was also able to support the unusual Mg dimethyl
species (Me6tren)MgMe2, which was formed through the
reaction of Me6tren with Me2Mg. (Me6tren)MgMe2 has an
atypical structure in the solid state, where one of the ligand
arms is not coordinated to the Mg center. When (Me6tren)-
MgMe2 and phenylacetylene were mixed, (Me6tren)Mg-
(CCPh)2 formed, but the use of aniline, benzylalcohol,
benzylamine, 4-tert-butylphenol, and 4-tert-butylcatechol all
resulted in decomposition. Insertion into both Mg−Me bonds
was observed when (Me6tren)MgMe2 was placed under 1 atm
of carbon dioxide, though no reaction was observed with CO.
Reaction with benzaldehyde produced 1-phenylethanol after
acidic workup, confirming the nucleophilic nature of the methyl
group in this complex. The reaction of (Me6tren)MgMe2 with 1
equiv of 2,6-lutidine·HBArF forms [(Me6tren)MgMe]BArF,
which is more stable in solution than the analogous compound
with a halide counterion. Our previous work on the binding of
κ3 terpy to Mg was hindered by both ligand dissociation and an
inability to observe any organometallic products. The use of a
κ4 ligand appears to have stabilized organometallic compounds
and allowed us to prepare relatively rare examples of well-
defined Mg methyl species.
ESI-MS. Mass spectra were collected using the home-built
cryogenic ion mass spectrometer of Johnson and co-workers.53,54
Briefly, millimolar solutions of each species were prepared and drawn
into the electrospray syringe under an inert atmosphere. The syringe
was then quickly transported into a nitrogen-purged enclosure
attached to the inlet capillary of the mass spectrometer, and the
solutions were electrosprayed through a 30 μm fused silica capillary
tip. The generated ions were guided through four differentially
pumped stages using two RF-only quadrupole guides and an octupole
guide. The ions were then directed 90° with a DC quadroupole bender
through a second octupole and einzel lens, which guide the ions into a
Paul trap (Jordan) cooled to 10 K with a closed-cycle helium cryostat.
He buffer gas was introduced into the trap with a pulsed valve,
allowing for collisional cooling of the ions. After equilibrating in the
trap for about 90 ms, the ions were extracted by applying 90 V push/
pull to the entrance and exit lenses of the trap, respectively. The
ejected ions next entered the extraction region of a Wiley−McLaren
TOF mass spectrometer, accelerating the ions through a field-free
flight tube, and finally detected with an MCP detector.
EXPERIMENTAL SECTION
■
General Methods. Experiments were performed under a
dinitrogen atmosphere in an M. Braun drybox or using standard
Schlenk techniques, unless otherwise noted. (Under standard glovebox
conditions, purging was not performed between uses of pentane,
diethyl ether, benzene, toluene and THF; thus, when any of these
Synthesis and Characterization of Compounds. [(Me6tren)-
MgBr]Br (1). A 15 mL portion of diethyl ether was transferred by
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cannula to a Schlenk flask containing PhMgBr (2.71 M in diethyl
ether, 0.64 mL, 1.74 mmol) and subsequently added to a solution of
Me6tren in toluene (50 mg/mL, 4.00 mL, 0.87 mmol) diluted with 15
mL of diethyl ether. The immediate formation of a light brown
precipitate was observed, and the mixture was stirred for 1 h at room
temperature. The reaction mixture was filtered and the off-white solid
collected. The crude product was washed with 2 × 15 mL of toluene
and dried under reduced pressure to give 1 as a white powder. Yield:
0.35 g (97%). X-ray diffraction quality crystals were grown by layering
toluene on a saturated acetonitrile solution of 1 at −30 °C.
1H NMR (CD2Cl2, 400.0 MHz): δ 3.08 (6H, t, CH2, J = 5.51 Hz),
2.88 (6H, t, CH2, J = 5.20 Hz), 2.56 (18 H, s, N(CH3)2). 13C{1H}
NMR (CD2Cl2, 100 MHz): δ 56.48, 50.20, 46.53. ESI-MS (CH2Cl2):
334 (M+). IR (ATR, Smart Orbit diamond plate, cm−1): 2979.3,
2874.6, 1642.0, 1590.7, 1484.0, 1474.7, 1457.5, 1293.9, 1170.8, 1096.7,
1018.7, 1000.2, 938.2, 930.7, 904.7, 798.5, 769.3, 698.1.
solvent by filtration. After the precipitate was washed with pentane (2
× 5 mL), 5 was isolated as a thermally sensitive white powder, which
was stored in a −30 °C freezer in a nitrogen-filled glovebox. Yield: 91
mg (33%). Diffraction-quality crystals were grown by layering diethyl
ether and pentane at −30 °C.
1H NMR (C6D6, 400.0 MHz): δ 2.41 (6H, t, NCH2CH2N, J = 5.66
Hz), 2.06 (6H, t, NCH2CH2N, J = 5.67 Hz), 1.98 (18H, s, N(CH3)2),
−0.99 (6H, s, MgCH3). 13C{1H} NMR (C6D6, 100 MHz): δ 55.29,
49.73, 46.88, −12.53.
(Me6tren)Mg(CCPh)2 (6). Compound 5 (20 mg, 0.07 mmol) was
weighed into a vial, and 2 mL of benzene was added. Phenylacetylene
(77 μL, 0.70 mmol) was added to the solution. The mixture was
agitated for 3 min, and all the volatiles were removed under reduced
pressure to give 6 as a white powder. Compound 6 is thermally
unstable and was stored in a −30 °C freezer in a nitrogen-filled
glovebox. Yield: 31 mg (97%).
[(Me6tren)MgCl]Cl (2). Me6tren in toluene (50 mg/mL, 3.00 mL,
0.65 mmol) was diluted with 20 mL of diethyl ether and added to a
Schlenk flask containing PhMgCl (1.73 M in THF, 753 μL, 1.30
mmol) in 20 mL of diethyl ether at room temperature. A white
precipitate formed instantly, and the mixture was stirred for 1 h. The
crude product was isolated by filtration and purified by addition of
pentane to a concentrated THF solution to give 2 as a white powder.
Yield: 0.18 g (84%).
1H NMR (C6D6, 400.0 MHz): δ 7.76 (4H, app. d, ArH, J = 8.28
Hz), 7.14 (4H, t, ArH, J = 7.77 Hz), 7.00 (2H, tt, ArH, J = 7.41, 1.21
Hz), 2.45 (6H, t, NCH2CH2N, J = 5.41 Hz), 2.25 (18H, s, N(CH3)2),
1.97 (6H, t, NCH2CH2N, J = 5.63 Hz). 13C{1H} NMR (C6D6, 125
MHz): δ 131.90, 130.04, 129.83, 128.35, 125.11, 110.10, 54.98, 49.98,
46.15.
Reaction between 5 and CO2 or CO. Compound 5 (5 mg, 0.02
mmol) was dissolved in C6D6 in a J. Young NMR tube. The mixture
was degassed using three freeze−pump−thaw cycles and carbon
dioxide introduced into the tube using a dual-manifold Schlenk line at
1H NMR (CD2Cl2, 300.0 MHz): δ 3.06 (6H, t, CH2, J = 4.92 Hz),
2.82 (6H, t, CH2, J = 4.69 Hz), 2.51 (18 H, s, N(CH3)2).13C{1H}
NMR CD2Cl2, 75 MHz): δ 56.44, 50.15, 46.27. ESI-MS (CH2Cl2):
290 (M+). IR (ATR, Smart Orbit diamond plate, cm−1): 2968.0,
2845.0, 1472.9, 1293.9, 1173.3, 1101.6, 1039.9, 1023.1, 1010.4, 945.0,
933.2, 904.5, 801.9, 771.8.
1
room temperature. A H NMR spectrum recorded less than 10 min
after carbon dioxide addition showed that free Me6tren was present in
solution along with a precipitate. The solvent was removed under
1
vacuum and the resulting white precipitate dissolved in D2O. The H
[(Me6tren)MgMe]Br (3). Me6tren in toluene (50 mg/mL, 3.00 mL,
0.65 mmol) was diluted with 30 mL of diethyl ether, and MeMgBr
(1.24 M in diethyl ether, 525 μL, 0.65 mmol) was added. A white
precipitate formed instantly, and the mixture was stirred for 1 h at
room temperature. Filtration of the reaction mixture yielded a white
powder containing a mixture of 1 and 3 in a ratio of 1:0.7. The
combined yield was 253 mg. Although both crystallization and
extraction were attempted to separate 1 and 3, these attempts were
unsuccessful due to the similar solubilities of the compounds and the
thermal instability of 3 in solution. The 1H and 13C{1H} NMR spectra
of the mixtures are shown in the Supporting Information. The NMR
line listing for 3 is given below.
NMR spectrum of the precipitate was consistent with an authentic
sample of Mg(OAc)2. The reaction with CO was carried out in an
analogous fashion, but only 5 was observed in the 1H NMR spectrum
after mixing.
Reaction between 5 and Acetic Acid. Compound 5 (5 mg, 0.02
mmol) was dissolved in C6D6 in a screw-cap NMR tube. Acetic acid
(0.083 M in THF, 500 μL, 0.042 mmol) was then added via a micro
pipet and the tube quickly capped. Both Me6tren and methane were
1
visible in the H NMR spectrum. The solvent was removed under
1
reduced pressure and the residue dissolved in D2O. The H NMR
spectrum indicated the formation of Mg(OAc)2, which, as above, was
compared with a spectrum of an authentic sample.
1H NMR (CD2Cl2, 400.0 MHz): δ 2.85 (6H, t, CH2, J = 5.51 Hz),
2.67 (6H, t, CH2, J = 4.54 Hz), 2.33 (18 H, s, N(CH3)2), −1.76 (3H, s,
MgCH3). 13C{1H} NMR (CD2Cl2, 125 MHz, 233 K): δ 55.10, 49.48,
47.50, 42.66, −19.19.
Reaction of 5 with 2,6-Lutidine·HBr. 2,6-Lutidine·HBr (3.3 mg,
0.02 mmol) was dissolved in CD2Cl2 in a screw-cap NMR tube
containing 5 (5 mg, 0.02 mmol) and the sample frozen in liquid
nitrogen. A 1H NMR spectrum recorded at room temperature
indicated that the sample contained 92% 3 and 8% 1. The same
procedure was followed for the reaction with 2 equiv of 2,6-
lutidine·HBr (6.6 mg, 0.04 mmol). In this case only 1 was observed in
[(Me6tren)MgBr]2[MgBr4] (4). Me6tren in toluene (50 mg/mL, 2.00
mL, 0.44 mmol) was placed in a Schlenk flask containing 30 mL of
diethyl ether. MeMgBr (1.36 M in diethyl ether, 6.40 mL, 8.72 mmol)
was added, and a white precipitate formed. The mixture was stirred for
80 min and then filtered. The resulting precipitate was purified by
dissolution in THF and precipitation by addition of pentane. The solid
was collected and dried under reduced pressure to give 4 as a white
powder. Yield: 189 mg (86%). Diffraction-quality crystals were grown
by layering toluene onto a saturated acetonitrile solution of 4 at −30
°C.
1
the H NMR spectrum.
Reaction of 5 with Benzaldehyde. Compound 5 (5 mg, 0.02
mmol) was weighed into a vial and dissolved in 1 mL of benzene to
form a colorless solution. Benzaldehyde (3.6 μL, 0.04 mmol) was
added by micropipet, with no visible change in the appearance of the
solution. After the solution was agitated for 2 min, HCl (0.147 M in
diethyl ether, 238 μL, 0.04 mmol) was added, resulting in the
formation of a white precipitate. The reaction mixture was filtered and
the solvent removed from the filtrate by the passage of dinitrogen over
1H NMR (CD2Cl2, 300.0 MHz): δ 3.05 (6H, t, CH2, J = 5.64 Hz),
2.86 (6H, t, CH2, J = 5.86 Hz), 2.53 (18 H, s, N(CH3)2). 13C{1H}
NMR (CD2Cl2, 75 MHz): δ 56.52, 50.26, 46.52. ESI-MS (CH2Cl2):
334 (M+). IR (ATR, Smart Orbit diamond plate, cm−1): 2972.5,
2874.0, 1471.5, 1457.5, 1354.8, 1293.6, 1171.1, 1097.8, 1019.5, 1000.8,
931.4, 903.8, 873.3, 799.4, 770.3.
(Me6tren)MgMe2 (5). Me6tren in toluene (45 mg/mL, 5 mL, 0.98
mmol) was placed in a Schlenk flask and the toluene removed in vacuo.
To the resulting yellow oil were added diethyl ether (30 mL) and
Me2Mg (53 mg, 0.98 mmol), and a cloudy solution with an off-white
precipitate formed. The mixture was stirred for 20 min and then
filtered into a Schlenk flask in a −78 °C bath. The volume of the
filtrate was reduced to ∼4 mL and placed in a −80 °C freezer. After 24
h, a white precipitate was present, which was separated from the
1
the reaction vessel. A H NMR spectrum of the residue in CDCl3
showed 1-phenylethanol to be the major product. This assignment was
confirmed by comparison with an authentic sample.
[(Me6tren)MgMe]BArF (7). 2,6-Lutidine·HBArF (12.2 mg, 0.01
mmol) was dissolved in 2 mL of diethyl ether and added dropwise
to an agitated solution of 5 (4 mg, 0.01 mmol) in 3 mL of diethyl
ether. The colorless solution was stirred for 1 min, and all volatiles
were removed in vacuo. The residue was washed with 2 mL of toluene
and dried to give 7 as a white solid. Yield: 14 mg (98%).
1H NMR (CD2Cl2, 400.0 MHz): δ 7.72 (8H, app t, ArH, J = 2.37),
7.56 (4H, br s, ArH), 2.72 (6H, t, CH2, J = 5.26 Hz), 2.62 (6H, t, CH2,
J = 6.34 Hz), 2.38 (18 H, s, N(CH3)2), −1.65 (3H, s, MgCH3).
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13C{1H} NMR (CD2Cl2, 125 MHz, 233 K): δ 161.73 (q, J = 49.9),
134.62, 128.61 (q, J = 31.1), 124.42 (q, J = 272.6), 117.52, 54.91,
49.50, 47.58, 42.45, −19.38. 19F{1H} NMR (CD2Cl2, 376 MHz): δ
62.86. ESI-MS (CH2Cl2): 270 (M+).
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ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files, tables, and figures giving X-ray information for 1, 4,
1
and 5 and H and 13C{1H} NMR spectra for all compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for N.H.: nilay.hazari@yale.edu.
■
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank Dr. Michael Takase for assistance with X-ray
crystallography and Professor Mark Johnson and Joseph
Fournier for performing the mass spectrometry experiments.
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