Mechanism of alkyl migration in diorganomagnesium 2,6-Bis(imino)pyridine Complexes: Formation of grignard-type complexes with square-planar Mg(II) centers

Article abstract of DOI:10.1021/acs.organomet.6b00528

Dialkylmagnesium compounds [MgR₂L₂] (R = n-Bu, L = none or R = Bn, L = THF) react with 2,6-bis(imino)pyridines (BIP) to afford different types of Mg(II) alkyl complexes, depending on the nature of R. For R = n-Bu, thermally stable products resulting from selective alkyl transfer to the pyridine nitrogen (N1) atom are obtained. However, NMR studies showed that the reaction of [Mg(Bn)₂THF₂] with ^iPrBIP at -65 °C leads to a thermally unstable product arising from benzyl migration to position C2 in the pyridine ring. Above +5 °C, this compound rearranges, cleanly yielding a mixture of two isomeric complexes, in which the benzyl group has migrated to positions C3 or C4 of the central ring, respectively. Similar isomeric mixtures were obtained when [Mg(Bn)₂THF₂] was reacted with ^iPrBIP or ^MesBIP at room temperature. Such mixtures are thermally stable below 80 °C, but at this temperature, the 3-benzyl isomer converts into the thermodynamically favored 4-benzyl product, albeit not quantitatively. An alternate route was devised for the selective syntheses of the latter type of compounds. The X-ray diffraction structure of one of them provided an unusual example of a square-planar alkylmagnesium(II) center.

Full text of DOI:10.1021/acs.organomet.6b00528

Article
pubs.acs.org/Organometallics
Mechanism of Alkyl Migration in Diorganomagnesium 2,6-
Bis(imino)pyridine Complexes: Formation of Grignard-Type
Complexes with Square-Planar Mg(II) Centers
́
John J. Sandoval, Pilar Palma, Eleuterio Alvarez, Juan Campora,* and Antonio Rodríguez-Delgado*
́
́
Instituto de Investigaciones Químicas, CSIC − Universidad de Sevilla, C/Americo Vespucio, 49, 41092, Sevilla, Spain
S
* Supporting Information
ABSTRACT: Dialkylmagnesium compounds [MgR2L2] (R =
n-Bu, L = none or R = Bn, L = THF) react with 2,6-
bis(imino)pyridines (BIP) to afford different types of Mg(II)
alkyl complexes, depending on the nature of R. For R = n-Bu,
thermally stable products resulting from selective alkyl transfer
to the pyridine nitrogen (N1) atom are obtained. However,
NMR studies showed that the reaction of [Mg(Bn)2THF2]
with iPrBIP at −65 °C leads to a thermally unstable product
arising from benzyl migration to position C2 in the pyridine
ring. Above +5 °C, this compound rearranges, cleanly yielding
a mixture of two isomeric complexes, in which the benzyl
group has migrated to positions C3 or C4 of the central ring,
respectively. Similar isomeric mixtures were obtained when
[Mg(Bn)2THF2] was reacted with iPrBIP or MesBIP at room temperature. Such mixtures are thermally stable below 80 °C, but at
this temperature, the 3-benzyl isomer converts into the thermodynamically favored 4-benzyl product, albeit not quantitatively. An
alternate route was devised for the selective syntheses of the latter type of compounds. The X-ray diffraction structure of one of
them provided an unusual example of a square-planar alkylmagnesium(II) center.
Scheme 1. Common Decomposition Routes Experienced by
[MRn(BIP)] Complexes
INTRODUCTION
■
Alkyl complexes supported by 2,6-bis(imino)pyridine (BIP)
ligands attracted much interest over the last quarter-century,1,2
mainly due to their role in olefin polymerization or
oligomerization,2e,3 and other catalytic processes.4 The high-
light has not been limited to transition metal derivatives, as the
chemistry of main group alkyls with BIP ligands has also been
investigated.5,6 BIP-ligated alkyl complexes are also noteworthy
compounds due to the unusual reactivity patterns imparted to
the alkyl-metal fragment by noninnocent BIP ligands. The
interaction of strongly π electron-acceptor BIP ligands with
electron-rich MRn fragments leads to the weakening of M−C
bonds.7 This effect explains why very few polyalkyls [MRn-
(BIP)] have been isolated, mainly dialkyls [M(CH2SiMe3)2-
(BIP)] (M = Fe8 or Mn2f), whose M−C bonds are stabilized by
β-silylated alkyl groups. Similar derivatives containing non-
stabilized, β-H free R groups are much less stable and evolve
spontaneously experiencing M−C fission. Although these
processes can be complicated by unselective reactions at the
BIP ligand,1 under well-controlled conditions, alkyls of type
[MRn(BIP)] evolve through the basic routes shown in Scheme
1, namely, M−R bond homolysis, affording reduced species of
type [MRn−1(BIP)], or alkyl transfer to the pyridine ring, which
leads to a range of products containing modified BIP ligands
([MRn−1(BIP′)]). M−R bond homolysis has been demon-
strated for transition metals such as, e.g., iron,2c,9 cobalt,10 or
manganese,11 while alkyl migration is considerably more
general, having been observed both for transition and main
metal elements.
Although in general, unstable, polyalkyls [MRn(BIP)] can be
readily generated in solution by reacting BIP ligands with
suitable metal precursors, MRn or [MRn(L′)m], where L′
represents labile, easily displaced coligands such as THF or Py.
The evolution of such polyalkyl species and, more specifically,
·R migration to remote positions on the BIP ligand poses an
interesting problem of selectivity control that could be
influenced by steric effects, and the nature of both the migrating
·R group and the metal center. Such control ranges from poor
(e.g., for MRn = aluminum alkyls6b) to excellent. For example,
Received: July 11, 2016
© XXXX American Chemical Society
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Scheme 2. Magnesium Dialkyls React with iPrBIP or MesBIP
Affording Structurally Different Products
Gibson studied the reactions of LiR, MgR2, or ZnR2 (R = Me,
Et, Pr, iPr) with a variety of BIP ligands, finding that, in
favorable cases, they proceed with high selectivity.5b In such
instances, the primary products invariably result from alkyl
transfer from the metal to the pyridine N atom of the BIP
ligand. In contrast, we have shown that dialkylmanganese BIP
complexes (R = benzyl, allyl, neophyl, and trimethylsilylmethyl)
undergo highly regioselective alkyl migration to the position 4
in the pyridine ring.2f,12 Similar selectivity has also been noted
for ·R migration to the pyridine ring in iron alkyl derivatives.9b
Recent studies suggest that such divergent chemistries could be
more influenced by the nature of the migrating R group than by
the different character of the metal center. Thus, we have
recently reported that the behavior of zinc alkyls [ZnR2(BIP)]
with R = benzyl or allyl is similar to that of their Mn(II)
analogues; i.e., they selectively transfer one R group not to the
N atom of the pyridine ring, but to position 4, affording stable
1,4-dihydropyridinate complexes. A comparison between
analogous Mn(II) and Zn(II) chemistries is meaningful,
because these ions have inert 3d5 and 3d10 shells and similar
covalent radii in their molecular compounds (rcov,Mn = 1.19 Å;
r
cov,Zn = 1.18 Å).13 Mg(II) is the closest d0 analogue of Mn(II)
and Zn(II), although its size is somewhat larger (rcov,Mg = 1.39
Å). Thus, comparisons between Mn(II), Zn(II), and Mg(II) are
also pertinent, especially considering the growing attention of
alkaline-earth reagents in catalysis.14 When studying the
reaction of bulky alkaline-earth alkyls [MR2(THF)n] (M =
Ca, Sr, Ba; R = CH(SiMe3)2) with iPrBIP, Hill observed
competitive R migration to positions 3 and 4 of the heterocyclic
ring.5b These products are thermally unstable in solution at
room temperature, and this leads to the elimination of 2 equiv
of CH2(SiMe3)2 by intramolecular H abstraction from the
weakly acidic acetimidoyl (CH3-CN) arms of the BIP ligand.
Interestingly, for M = Mg, the reaction initially leads to a C2-
symmetrical adduct tentatively identified as the corresponding
dialkyl [MgR2(iPrBIP)], which only undergoes intramolecular
deprotonation at 60 °C without forming ring-alkylated
intermediates. In order to further clarify the reactivity of
alkali-earth alkyls with BIP ligands, we decided to revisit the
reaction of BIP ligands with MgR2, specifically with previously
unexplored R = n-Bu (but similar to those studied by Gibson)
and Bn (benzyl), which is the Mg(II) analogue of the Mn and
Zn benzyls that we investigated before.2f,5a,12 For this purpose,
we chose as starting material the THF solvate of
dibenzylmagnesium, [Mg(Bn2)(THF)2], reported by Schrock
in 1976,15 whose crystal structure has now been determined
and is shown in the Supporting Information (Figure S3).
Figure 1. ORTEP representation of the structure of 1b. Selected bond
lengths (Å) and angles (deg): Mg(1)−C(32), 2.171(14); Mg(1)−
N(1), 2.171(11); Mg(1)−N(2), 2.112(13); Mg(1)−N(3), 2.116(13);
N(1)−C(10), 1.518 (14); N(1)−C5, 1.445 (18); N(1)−C1, 1.451
(14); C1−C2, 1.40 (2); C2−C3, 1.44 (2); C1−C6, 1.39 (2); C6−C7,
1.51(2); N(1)−Mg(1)−C(32), 139.5; N(1)−Mg(1)−N(2), 80.7(5);
N(1)−Mg(1)−N(3), 76.8(5); N(2)−Mg(1)−N(3), 129.6(4); N(2)−
Mg(1)−C(32), 109.0(6); N(3)−Mg(1)−C(32), 117.6(6).
structural characterization of 1b, shown in Figure 1. Its general
features and crystal bond lengths and angles show no significant
differences with those of Gibson’s compounds.5c The geometry
of the Mg atom in this kind of complexes could be described as
a flattened tetrahedron, i.e., intermediate between tetrahedral
16
and square planar. The geometric parameter τ4 for the Mg
center in 1b is 0.65. This parameter takes the value 1 for the
tetrahedron and 0 for square-planar geometry; therefore, the
metal center is closer to tetrahedral. Complexes 1 are stable in
solution up to 40 °C but undergo unselective isomerization on
heating at 60 °C (1b: ca. 40% n-Bu migration from N to
position 2 after 24 h).
RESULTS AND DISCUSSION
■
Both the dibutyl and dibenzyl magnesium derivatives react
instantly when treated with stoichiometric amounts of the
bulky ligands iPrBIP or MesBIP, affording structurally different
products, depending on the nature of R (Scheme 2). In the case
of Mg(n-Bu)2, the 1H NMR spectra of the deep purple reaction
mixtures showed the formation of a single type of product, 1a
or 1b. These are characterized by the high field shift of the H4
and 3,3′ signals of the pyridine ring, from the 7.50−9.00 ppm
region where they appear in the spectra of the free ligands, to
6.80−5.50 ppm (see Figure S1, SI). This is reminiscent of the
data reported by Gibson for the N-alkylated products
[Mg(R)(N-R-BIP)] generated from other dialkylmagnesium
reagents (with R = Me, Et, iPr) and various BIP ligands.5c The
identity of the new compounds was confirmed with the
Treatment of [Mg(Bn)2(THF)2] solutions with stoichio-
metric amounts of iPrBIP or MesBIP in C6D6 induces an
instantaneous color change to dark blue. The 1H NMR spectra
of these solutions appear more complex than those of
compounds 1 (see Figure S2 in the SI) due to the formation
of two products, 2a/b or 3a/b, respectively. Otherwise, both
reactions are very clean and no side products are detected in
the spectra. The identity of the products was unambiguously
1
established on the basis of their NMR spectra, H and 13C,
1
1
bidimensional H−1H COSY, and H−13C heterocorrelations.
B
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As shown in Scheme 2, 2 and 3 are isomeric complexes
Scheme 3. Direct and Selective Synthesis of
Dihydropyridinate Magnesium Alkyls
arising from competitive migration of one benzyl group to
1
positions 3 and 4 in the pyridine ring, respectively. The H
NMR spectra of isomers 3 display diagnostic multiplets at δ
4.13 (3a) or 4.16 ppm (3b) corresponding to the aliphatic
methyne 4-C(Bn)H of their 1,4-dihydropyridinate fragments,
which split as multiplets (ideally, triplet of triplets) by coupling
to the equivalent sp2 methynes 3 and 3′-CH (doublets at δ 5.02
for 3a or 5.04 for 3b), and to the benzyl methylene group
(doublets at δ 2.86 and 2.91 for 3a and 3b, respectively). The
spectra of isomers 2a/b are more complex than those of 3a/b,
due to the breakdown of molecular symmetry caused by the Bn
in position 3. The 4-CH methynes of the central ring split as
doublets of doublets placed at δ 5.25 (2a) or 5.29 (2b) due to
differential coupling with the aliphatic 3-C(Bn)H and the sp2 5-
CH (3JHH ≈ 6 and 9 Hz). In addition, the asymmetry of the 3-
C(Bn)H centers causes the benzyl CH2 protons to become
1
diastereotopic. According to H NMR integrals, isomers 3 are
slightly more abundant than 2, even when benzyl migration to
position 3 is twice as likely than migration to 4. Considering
this statistical factor, the selectivity ratios for migration to 3 vs 4
are 1:3 for iPrBIP and 1:1.5 for MesBIP. The similar values
indicate that the steric bulk of the aryl substituents have little
influence on the migration selectivity.
The outcome of the reactions of dibenzylmagnesium with
BIP ligands stands in stark contrast with the selective formation
of N-alkylated species from Mg(n-Bu)2 or other dialkylmag-
nesiums reported by Gibson, but is akin to Hill’s results with
the bulky bis(trimethylsilylmethyl) derivatives of Ca, Sr, and
Ba.5b As mentioned above, the heavier alkali-earth analogues of
2 and 3 eliminate 2 equiv of alkane upon standing in solution at
room temperature. We heated the C6D6 solutions containing
the 2a/3a isomeric mixture, but no significant changes were
Figure 2. ORTEP representation of the structure of compound 4,
showing the disorder of the n-Bu ligand. Selected bond lengths (Å)
and angles (deg): Mg(1)−C(41A), 2.240(12); Mg(1)−N(1),
1.989(4); Mg(1)−N(2), 2.231(5); Mg(1)−N(3), 2.241(5); N(1)−
C(5), 1.369 (6); N(1)−C1, 1.363(6); C1−C2, 1.348 (7); C2−C3,
1.498(8); C1−C6, 1.464 (7); C6−C7, 1.508(8); N(1)−Mg(1)−
C(41A), 154.0(5); N(1)−Mg(1)−C(41B), 157.8(9); N(1)−Mg(1)−
N(2), 74.83(17); N(1)−Mg(1)−N(3), 74.85(17); N(2)−Mg(1)−
C(41A), 101.2(4); N(2)−Mg(1)−N(3), 149.69 (18); N(3)−Mg(1)−
C(41A), 106.1(4); N(1)−Mg(1)−C(41A), 154.0(5).
1
observed in the H spectra after 48 h at 60 °C. However, the
2a:3a isomeric ratio gradually shifts at 80 °C, becoming 1:9
after heating for 14 h. A similar, but slower, isomerization
process was observed in the MesBIP system, which attains a
2b:3b ratio of 1:2 under the same conditions. No further shift
was observed when the heating was continued for 24 h, but
signs of decomposition started to show up in both cases.
Since the reactions of BIP ligands with dibenzylmagnesium
do not afford single products, we envisioned a straightforward
method to prepare derivatives of type 3. This method involves
the reaction of magnesium dialkyls with the benzylated 1,4-
dihydropyridine 4-Bn-iPrH2BIP, as shown in Scheme 3. To this
purpose, we optimized our Mn-based methodology12 to
prepare the dihydropyridine ligand free from the aromatized
4-Bn-iPrBIP (see the Experimental Section). As anticipated,
[Mg(Bn2)(THF)2] reacts cleanly and selectively with 4-
Bn-iPrH2BIP, yielding a dark purple microcrystalline solid
whose NMR spectra were undistinguishable from those of
the 3a component in the 2a/3a mixture. Complex 3a turns out
to be thermally stable and does not isomerize appreciably to 2a
when heated in solution at 80 °C for 48 h. This means that the
isomeric ratio 1:9 attained after heating the 2a/3a mixture (and
most likely 2b/3b as well) is not determined by a chemical
equilibrium, but is a limiting value due to the slowness of the
isomerization process.
arise from selective migration of alkyl to BIP ligands,2f,5b,6b,9b so
far the only structurally characterized example is the Zn(II)
analogue of 3a that we reported a few years ago.5a In the latter
compound, the dihydropyridinate ligand favors a square-planar
geometry at the Zn(II) center (τ4 = 0.32), and a similar
configuration is observed in 4 (as in other structurally
analogous square-planar Al(III) complexes recently re-
ported17), although, in this case, the carbon atom bound to
Mg departs slightly from planarity. This deviation is small, but
gives rise to a crystallographic disorder, as the Mg−C vector
can take two possible orientations, forming angles of 26°
(configuration A) or −32° with the main coordination plane
(configuration B). Both possible values of τ4, 0.40 (A) or 0.44
(B), indicate that the geometry of the Mg(II) center is closer to
square-planar than in the structure of 1b. Several square-planar
Mg(II) coordination complexes containing macrocyclic18 or
polydentate ligands6,19 have been reported, but this is
unprecedented for Grignard-type compounds. Interestingly,
the Mg−C bond is significantly longer in 4 (2.240(12) Å) than
in 1b (2.151(14) Å), although the Mg-bound n-Bu poses no
important steric hindrance. The difference could be attributed
to the different character of the N donors placed in trans, which
is amide, covalent-type in 4 and amino, dative-type, in 1b. If
this interpretation were correct,20 this would be a highly
unusual case of trans influence in a square-planar magnesium
complex.
In spite of preparing 3a selectively, we were unable to grow
X-ray quality crystals of this compound. However, reacting
Mg(n-Bu)2 with 4-Bn-iPrH2BIP leads to the corresponding n-
butylmagnesium-dihydropyridinate 4, whose crystal structure is
shown in Figure 2. Although similar complexes are known to
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In order to gain further insight on the mechanism of the
reaction of MgR2 with BIP ligands, we monitored the
interaction of [Mg(Bn)2(THF)2] with iPrBIP using variable
supported by the observation of four signals between 2.50 and
4.00 ppm corresponding to methyne groups of the four
inequivalent iPr groups of the iPrBIP ligand. The COSY
spectrum provides further valuable indications of coupling
relationships (sketched in the bottom spectrum of Figure 3)
between three poorly resolved multiplets placed at 4.84, 5.72,
and 6.42 ppm, each of them integrating for 1H. These signals
were assigned to the H3, H4, and H5 protons of the
dearomatized pyridine ring arising from benzyl migration to
position 2, as indicated in Scheme 4. The data for similar Mg,
Zn, and Al complexes reported in the literature match well
those of inta (4.7−5.1, 5.4−5.6, and 6.0−6.3 ppm,
respectively).5c,6b In addition, Gambarotta reported a similar
paramagnetic Fe(II) complex arising as a minor byproduct from
the alkylation of [FeCl2BIP] with LiCH2SiMe3.21 All of these
compounds are stable at room temperature. In contrast, the
observed lability of inta at +5 °C indicates a very low energy
barrier for its isomerization to 2a or 3a, estimated in 18−19
kcal/mol. As a comparison, the estimated energy barrier for the
isomerization of 2a to 3a, which takes place over several hours
at 80 °C, is 27−28 kcal/mol.
1
temperature H NMR between −60 and +25 °C (Figure 3).
Scheme 4. Reaction of iPrBIP with Isolated
[Mg(Bn)2(THF)2]
The facile transformation of inta in 2a and 3a, and the
irreversible isomerization of 2a into 3a, indicates that the
thermodynamic preference of the pyridine alkylation increases
in the order 2 < 3 < 4. On the other hand, the significant
difference between the energy barriers for isomerizations inta
→ 2a and 2a → 3a means that 2a cannot be an intermediate in
the route inta → 3a, that is, the benzyl group must “jump”
directly from position 2 to 4. A concerted 2,4-shift cannot be
excluded, but, very likely, R shifts within the BIP ligand involve
transitory formation of free ·R radicals.7a This helps explain why
benzyl is more prone than normal alkyls such as n-Bu to
migrate to remote positions in the pyridine ring, since the
stabilized benzyl free radical is more easily formed.
1
Figure 3. H NMR monitoring of the reaction of equimolar amounts
of iPrBIP with [Mg(Bn)2(THF)2] in toluene-d8. For clarity, only the
central regions of the spectra are shown. Arrows in the bottom
spectrum indicate the most relevant 1H−1H couplings detected in the
COSY spectrum at −50 °C. See-through bands highlight “growing”
signals of the final products 2a and 3a. The asterisk marks the residual
signal of the deuterated solvent.
CONCLUSIONS
■
In summary, we have studied the reactions of magnesium
dialkyls (MgR2) (R = n-Bu, Bn) with iPrBIP or MesBIP ligands
and demonstrated that the result is critically influenced by the
nature of the R group. The size of the aryl substituents of the
BIP ligand has little impact in the process. For R = n-Bu, one
alkyl group is selectively transferred to the central nitrogen in
the pyridine ring, giving rise to remarkably stable monoalkyl
complexes. In contrast, when R = Bn, competitive migration to
positions 3 and 4 is observed at room temperature, affording
mixtures of complexes of types 2 and 3. These species are
formed under kinetic control, but on heating, 3-benzyl-
enaminates 2 isomerize into the thermodynamically more
stable 4-benzyl-1,4-dihydropyridinate complexes 3. Variable
temperature NMR shows that, at −60 °C, the benzyl group
migrates preferentially to position 2, giving rise to the 2-benzyl-
1,2-dihydropyridinate inta. Above 0 °C, this compound readily
rearranges to the mixture of products observed at room
temperature. On the basis of these observations, we established
a qualitative order of thermodynamic stabilities, being the
compound alkylated in position 4 of the central pyridine the
most stable, and the one on position 2 the least. Since the
reactions of BIP ligands with [Mg(Bn)2(THF)2] do not afford
single products, we developed a convenient route for the
selective synthesis of alkylmagnesium dihydropyridinate com-
plexes 3a and 4. The crystal structure of the latter provides an
unusual example of square-planar coordination in a Grignard-
type magnesium complex.
When the ligand is added to a toluene-d8 solution of the Mg
reagent cooled at −65 °C, a rapid color change ensues, from
yellow to green. The H NMR spectrum recorded at −60 °C
1
indicates full conversion of the starting materials. Careful
analysis of the spectrum at −50 °C (whose resolution was
better than at −60 °C) indicates the presence of several species,
although one of them, intermediate inta, is clearly prevalent.
Little change is observed as the sample is warmed from −60 to
0 °C, but at +5 °C, the signals of 2a/3a begin to be noticeable
(transparent highlighted bars in Figure 3). Further warming at
room temperature caused the original spectrum to be fully
replaced by that of 2a/3a, indicating that all species present in
the original mixture share the same fate.
The spectrum of inta is not as simple as it would be
anticipated for a symmetric dialkyl [MgBn2(iPrBIP)], analogous
to those observed in the reactions of iPrBIP with Zn(Bn)2 at low
temperature,5a or with Mg(CH(SiMe3)2)2 at 23 °C.5b A distinct
feature of this spectrum is a pair of doublets at δ 2.24 and 4.27
ppm due to an AX spin system with JAX = 14 Hz (confirmed in
the COSY spectrum). The large JAX constant is consistent with
geminal rather than vicinal coupling; therefore, it was assigned
to a diastereotopic pair of methylene protons belonging to one
of the benzyl groups. This points to a low symmetry species, as
D
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CHMe2), 3.17 (sept, 2H, 3JHH = 6.9 Hz, CHMe2), 5.48 (t, 1H, 3JHH
=
EXPERIMENTAL SECTION
■
3
7.3 Hz, 4-CHPy), 6.78 (d, 2H, JHH = 7.3 Hz, 3,3′-CHPy), 7.08−7.17
(m, 6H CHAr N-Aryl, overlapping with residual signal of C6D6).
13C{1H} NMR (C6D6, 25 °C, 100 MHz), δ 8.8 (α-CH2 MgnBu), 14.8
(δ-CH3 MgnBu), 14.8 (δ-CH3 Mg-nBu), 14.9 (δ-CH3 N-nBu), 17.0
(Me-CN), 21.7 (γ-CH2 N-nBu), 24.4, 24.7, 24.8, 25.0 (CHMeMe),
29.1 (CHMe2), 29.2 (γ-CH2 N-nBu), 29.7 (CHMe2), 29.8 (β-CH2
N-nBu) 32.6 (β-CH2 MgnBu), 53.0 (α-CH2 N-nBu), 104.4 (4-CHPy),
118.1 (2,2′-CPy), 124.0, 124.5 (m-CHN‑Ar), 126.1 (p-CHN‑Ar), 132.0 (3-
CHPy), 140.9, 142.7 (o-CN‑Ar), 145.3 (i-CN‑Ar), 170.7 (Me-CN). Anal.
Calcd for C41H61MgN3: C, 79.39; H, 9.91; N, 6.77. Found: C, 78.89;
H, 10.43; N, 7.24.
General Considerations. All manipulations were carried out
under an oxygen-free argon atmosphere, using conventional Schlenk
techniques or a nitrogen-filled glovebox. Solvents were rigorously
dried and degassed prior use. Methanol was refluxed over sodium
methoxide, distilled, and kept in a Teflon screw-cap glass ampule over
activated molecular sieves. NMR spectra were recorded on Bruker
Avance III-400 and DRX-500 spectrometers (FT 400 and 500 MHz,
1
1H; 100 and 125 MHz, 13C). The H and 13C{1H} resonances of the
solvent were used as the internal standard, but the chemical shifts are
reported with respect to TMS. Spectral assignations were routinely
helped with monodimensional 13C (gated), DEPT 135 and 2D 1H−1H
COSY, and 1H−13C HSQC and HMBC heterocorrelation spectra.
NMR-scale reactions (typically in a 0.02 mmol scale) were conducted
in NMR tubes sealed with Teflon J. Young-type screw-cap valves.
Benzene-d6 and toluene-d8 were dried over sodium benzophenone
ketyl and vacuum-distilled. The Microanalytical Service of the Instituto
Synthesis of [Mg(n-Bu)(N-n-Bu-MesBIP] (1b). The same
experimental procedure described above for the synthesis of
compound 1a was applied to prepare compound 1b, starting from
1030 mg (2.6 mmol) of MesBIP. Crystallization of the crude product
from hexane (10 mL) at −20 °C yielded 794 mg (57%) of a purple
microcrystalline material. Single crystals suitable for X-ray diffraction
studies were obtained from a concentrated hexane solution of 1b at
́
de Investigaciones Quimicas carried out the elemental microanalyses.
Mg(nBu)2 and Mg(Bn)Cl were purchased from Sigma-Aldrich,
being both titrated prior to use. Derivatives 2,6-[2,6-iPr2C6H3N
C(Me)]2C5H3N (iPrBIP) and 2,6-[2,4,6-Me3C6H2NC(Me)]2C5H3N
1
3
−25 °C. H NMR (C6D6, 25 °C, 400 MHz), δ −0.06 (t, 2H, JHH
=
7.6 Hz, α-CH2 MgnBu), 0.76 (t, 3H, 3JHH = 7.2 Hz, CH3 MgnBu), 0.88
(t, 2H, 3JHH = 7.3 Hz, CH3 N-nBu), 1.06 (quint, 2H, 3JHH = 7.2 Hz, γ-
CH2 MgnBu), 1.19 (quint, 2H, JHH = 7.5 Hz, γ-CH2 N-nBu), 1.54
3
(
MesBIP) were prepared according to conventional procedures that
(quint, 2H, 3JHH = 7.5 Hz, β-CH2 MgnBu), 1.60 (s, 6H, Me-CN), 1.90
(overlapping signals, 8H, 2 × o-MeN‑Ar and β-CH2 N-nBu), 2.13 (s, 6H,
o-MeN‑Ar), 2.23 (s, 6H, p-MeN‑Ar), 2.77 (m, 2H, α-CH2 N-nBu), 5.59 (t,
1H, 3JHH = 7.3 Hz, 4-CHPy), 6.76 (s, 2H, m-CHN‑Ar), 6.80 (d, 2H, 3JHH
= 7.3 Hz, 3-CHPy), 6.82 (s, 2H, m′-CHN‑Ar). 13C{1H} NMR (C6D6, 25
°C, 100 MHz), δ 9.4 (α-CH2 Mg-nBu), 14.6 (CH3 Mg-nBu), 14.9 (CH3
N-nBu), 15.4 (Me-CN), 19.2, 19.6 (o-MeN‑Ar), 21.3 (p-MeN‑Ar), 21.5 (γ-
CH2 N-nBu), 29.3 (γ-CH2 Mg-nBu), 32.3 (β-CH2 N-nBu), 32.8 (β-CH2
Mg-nBu), 51.5 (α-CH2 N-nBu), 105.2 (4-CHPy), 116.5 (2-CPy), 129.5
(3-CHPy), 129.7 (o-CN‑Ar), 130.1, 130.7 (m-CHN‑Ar), 132.1 (o′-CN‑Ar),
134.0 (p-CN‑Ar), 144.7 (i-CN‑Ar), 169.9 (Me-CN). Anal. Calcd for
C35H49MgN3: C, 78.42; H, 9.21; N, 7.84. Found: C, 78.36; H, 9.68; N,
7.51.
involve the condensation of 2,6-diacetylpyridine with the correspond-
ing anilines under azeotropic water-removal conditions.
Preparation of [Mg(Bn)2THF2]. This compound was prepared
adapting a method reported by Schrock,15 which is described as
follows. To a stock solution of Mg(Bn)Cl (0.90 M, 48 mL, 36.0
mmol) in Et2O, stirred at −40 °C, 10 mL of a dioxane solution in the
same solvent (3.6 M, 36.0 mmol) was added dropwise. The mixture
was allowed to warm to room temperature, and the stirring was
continued for 5 h. The resultant white suspension was centrifuged.
The supernatant liquid was transferred via cannula to an oxygen- and
moisture-free oven-dried flask, and the colorless solution was
evaporated and dried under vacuum overnight. The resultant white
solid was then redissolved in 10 mL of THF, and pentane (5 mL
approx.) was added until the solution became slightly turbid. Colorless
crystals suitable for X-ray diffraction studies corresponding to
[Mg(Bn)2THF2] appeared after storing the solution for several days
at −25 °C. Upon filtration and drying, 4.42 g (35%) of a crystalline
white solid was obtained. Its crystal structure is shown in the
Supporting Information (Figure S3) together with selected bond
Reaction of [Mg(Bn)2THF2] with iPrBIP. Formation of [Mg-
(Bn)(4-Bn-iPrHBIP)] (2a) and [Mg(Bn)(4-Bn-iPrHBIP)] (3a). In a
nitrogen filled glovebox, a colorless, cooled (5 °C) solution of
[Mg(Bn)2THF2] (19.3 mg; 0.055 mmol) in 0.4 mL of C6D6 was
added to a yellow suspension of iPrBIP (24.0 mg, 0.050 mmol) in the
same solvent (0.4 mL) at 5 °C, placed in a 5 mL scintillation vial. The
color of the reaction mixture changed instantaneously to dark green,
and then, in less than 2 min, the solid had dissolved to give a dark blue
solution. This was transferred to a screw-cap J. Young NMR tube and
1
distances and angles. H NMR (C6D6, 25 °C, 400 MHz), δ 1.06 (m,
8H, CH2 (2,5)-THF), 1.84 (s, 4H, Mg-Bn), 3.17 (m, 8H, CH2 (3,4)-
THF), 6.76 (t, 2H, 3JHH = 7.1 Hz, p-CHar Bn), 7.10 (d, 4H, 3JHH = 7.5
Hz, o-CHar Bn), 7.18 (overlapping signal of residual benzene and 4H,
m-CHar Bn). 13C{1H} NMR (C6D6, 25 °C), δ 22.9 (Mg-Bn), 24.7
(CH2 (3,4)-THF), 68.9 (CH2 (2,5)-THF), 116.1 (p-CHar CH2Ph),
123.4 (o-CHar Bn), 128.1 (m-CHar Bn), 156.9 (i-Car Bn). Anal. Calcd
for C22H30MgO2: C, 75.33; H, 8.62. Found: C, 75.00; H, 8.58.
Synthesis of [Mg(n-Bu)(N-n-Bu-iPrBIP)] (1a). To a cold (−70
°C) suspension of iPrBIP (927.0 mg, 1.92 mmol) in toluene (40 mL), a
solution of Mg(nBu)2 (1.06 M, 2.0 mL, 2.12 mmol) in heptane was
added via syringe. The color of the mixture changed from yellow to
blue-purple. After 10 min, the cooling bath was removed and the
mixture was stirred overnight at room temperature. All volatiles were
1
analyzed by H NMR. The reaction was found to be complete, the
spectrum showing two sets of signals corresponding to a mixture of
complexes 2a and 3a, in a relative ratio of 1:1.5 (2a/3a) (see Figure
S2, SI). The evolution of the mixture was monitored for a period of 7
h. No color changes and no evidence for changing were observed. In
two separated experiments, similar samples were prepared and their
evolution studied at 60 and 80 °C, respectively. Complex 2a: 1H NMR
3
(C6D6, 25 °C, 400 MHz), δ 0.97 (d, JHH = 6.7 Hz, 3H, CHMeMe),
3
3
1.01 (d, JHH = 6.7 Hz, 3H, CHMeMe), 1.05 (d, JHH = 6.8 Hz, 3H,
CHMeMe), 1.06 (d, 3JHH = 6.8 Hz, 3H, CHMeMe), 1.22−1.36 (12H,
CHMeMe, overlapping signal with CHMeMe of complex 3a), 1.28 (s,
3H, Me-CN), 1.69 (s, 3H, Me-CN), 1.83 (s, 2H, Mg-Bn), 2.47 (dd,
1
removed under vacuum, leaving a blue-purple oil, whose H NMR
3
1H, JHH = 12.6, 3.5 Hz, CHH Py-Bn), 2.79 (m, 1H, CHH Py-Bn,
showed a single set of signals that corresponds to compound 1a
(Figure S1, SI). The solid was dissolved in pentane (15 mL),
concentrated until 1/3 of its initial volume, and stored at −25 °C.
After 3 days, compound 1a precipitated as purple solid. Upon filtration
hidden under the signal CHMe2 of complex 3a; identified by 2D
1H−1H COSY), 2.88 (m, 2H, CHMe2), 2.98 (m, 2H, CHMe2), 3.76
3
(m, 1H, 3-CHPy), 5.25 (dd, 1H, JHH = 9.2, 5.8 Hz, 4-CHPy), 6.14 (d,
1
3
3
and drying, 772.0 mg (65%) of 1a was isolated. H NMR (C6D6, 25
2H, JHH = 7.6 Hz, o-CHAr Mg-Bn), 6.30 (d, 1H, JHH = 9.3 Hz, 5-
CHPy), 6.55 (t, 1H, 3JHH = 7.3 Hz, p-CHAr Mg-Bn), 6.88 (t, 2H,3JHH
=
°C, 400 MHz), δ −0.27 (bs, 2H, α-CH2 MgnBu), 0.78 (bs, 3H, CH3
3
3
MgnBu), 0.88 (t, 3H, JHH = 7.3 Hz, CH3 NnBu), 1.02 (d, 6H, JHH
=
7.7 Hz, m-CHAr Mg-Bn), 7.06−7.25 (m, 11H, CHN‑Ar and CHAr Py-
Bn). 13C{1H} NMR (C6D6, 25 °C, 100 MHz), δ 18.0 (Me-CN), 18.4
(Me-CN), 23.5 (CH2, Mg-Bn), 23.8, 24.1, 24.2, 24.2 (CHMeMe), 24.9,
25.0, 25.0, 25.3 (CHMeMe), 28.4, 28.6 (CHMe2 hidden under signals
of 2a), 38.5 (3-CHPy), 44.9 (broad signal, CH2 Py-Bn), 120.9 (5-
CHPy), 123.8, 124.0, 124.6, 125.3, 125.6, 126.6, 128.6, 128.7, 130.1
(CHN‑Ar, CHPy, CHAr, Mg-Bn, Py-Bn), 138.6 (i-CAr, Py-Bn), 141.1,
3
6.7 Hz, CHMe2), 1.12 (d, 6H, JHH = 6.8 Hz, CHMe2), 1.14−1.25
(overlapping signals, 4H, γ-CH2 N-nBu, γ-CH2 Mg-nBu assigned by
COSY 2D-1H−1H), 1.24 (d, 6H, 3JHH = 6.8 Hz, CHMeMe), 1.28−1.39
(m, 2H, β-CH2 Mg-nBu, overlapping with signals of CHMe2), 1.40 (d,
3
6H, JHH = 6.6 Hz, CHMe2), 1.68 (s, 6H, Me-CN), 1.90 (m, 2H, β-
CH2 N-nBu), 2.84−2.89 (overlapping signals, 4H, α-CH2 N-nBu and
E
DOI: 10.1021/acs.organomet.6b00528
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
146.6, 146.7 (CN‑Ar or CPy)), 157.4 (i-Car Mg-Bn), 158.1 (CN‑Ar or CPy),
166.7 (PyC(Me)NAr), 170.77 (broad, PyC(Me)N-Ar). Complex
3a (Signals lists and assignations confirmed with the spectra of isolated
product; see later.): 1H NMR (C6D6, 25 °C, 400 MHz), δ 0.98 (d, 6H,
3JHH = 6.7 Hz, CHMe2), 1.02 (d, 6H, 3JHH = 6.7 Hz, CHMe2), 1.28 (d,
procedure suppresses the aromatization of such dihydropyridine,
yielding the desired compound:
A THF solution of the MnBn2 reagent was generated in the usual
way: A J. Young Teflon-valve ampule with stirring bar was charged
with 200 mg (2.2 mmol) of MnCl2 and 15 mL of THF. The mixture
was sonicated for 5 min, and then stirred magnetically at −60 °C. To
the cool, stirred solution was added 2.3 mL of a 2.0 M solution of
Mg(Cl)(Bn) in THF. The pale pink color of the mixture changed to
light brown. After 10 min at −60 °C, the cooling bath was removed,
and the stirring continued at room temperature for 60 min, during
which time the mixture took a dark green color.
3
12H, JHH = 6.9 Hz, CHMe2), 1.64 (s, 2H, CH2 Mg-Bn), 1.76 (s, 6H,
3
MeCN), 2.74 (broad septet, 4H, JHH = 4.1 Hz, CHMe2), 2.86 (d,
3
2H, JHH = 6.8 Hz, CH2 Py-Bn), 4.13 (m, 1H, 4-CHPy), 5.02 (d, 2H,
3JHH = 3.9 Hz, 3,3′-CHPy), 6.07 (d, 2H, 3JHH = 7.6 Hz, o-CHAr Mg-Bn),
6.53 (t, 1H, 3JHH = 6.8 Hz, p-CHAr Mg-Bn), 6.85 (t, 2H, 3JHH = 7.2 Hz,
m-CHAr Mg-Bn), 6.99−7.19 (m, 11H, CHN‑Ar and CHAr Py-Bn).
13C{1H} NMR (C6D6, 25 °C, 100 MHz), δ 17.4 (Me-CN), 23.6
The manganous reagent solution was transferred to a suspension of
iPrBIP (950 mg, 1.97 mmol) in 20 mL of toluene, stirred at −60 °C.
The mixture takes a dark brown color. After 10 min, it was allowed to
warm at room temperature. The stirring was continued for 70 min,
after which time its color had changed to deep purple. Next, an excess
of dry methanol was added, carefully avoiding any admission of air,
and the resulting red solution was rigorously evaporated to dryness
under vacuum for 4 h. This left a reddish oily residue that was
extracted with 2 × 20 mL of hexane. The extracts were filtered through
a Celite pad, and evaporated to yield 956 mg of the product 4-
(CH2 Mg-Bn), 23.7, 24.7, 24.8 (CHMe2), 28.5, 28.6 (CHMe2), 40.5
(4-CHPy), 49.7 (CH2 Py-Bn), 103.4 (3,3′-CHPy), 115.7 (p-CHAr Mg-
Bn), 124.0, 124.3 (m-CHN‑Ar), 124.5 (o-CHAr Mg-Bn), 125.9 (m-CHAr
Py-Bn), 126.3 (m-CHAr Mg-Bn), 128.5 (o-CHAr Py-Bn), 128.6 (p-
CHAr Py-Bn), 130.0 (p-CHAr), 138.7, 138.9 (o-CN‑Ar), 139.9 (i-CAr Py-
Bn), 145.2 (2-CPy), 145.6 (i-CN‑Aryl), 158.1 (i-CAr Mg-Bn), 173.4 (Me-
CN).
Reaction of [Mg(Bn)2THF2] with MesBIP. Formation of
[Mg(Bn)(4-Bn-MesHBIP)] (2b) and [Mg(Bn)(4-Bn-MesHBIP)] (3b).
The same experimental protocol than the one described for the
formation of 2a and 3a was set to investigate the reaction of
[Mg(Bn)2THF2] with MesBIP. The initial color of the reaction mixture
1
Bn-iPrH2BIP as a yellow solid. H RMN (C6D6, 25 °C, 500 MHz), δ
3
3
1.07 (d, 6H, JHH = 7.1 Hz, CHMeMe), 1.09 (d, 6 H, JHH = 7.4 Hz,
CHMeMe), 1.10 (d, 6H, 3JHH = 7.1 Hz, CHMeMe), 1.14 (d, 6H, 3JHH
3
= 7.0 Hz, CHMeMe), 1.65 (s, 6H, Me-CN), 2.78 (sept, 4H, JHH
=
1
was brown, and a little later it changed to deep blue. The H NMR
3
6.9 Hz, CHMe2), 2.79 (d, JHH = 3.59 Hz, 2H, CH2 Py-Bn), 3.75 (tt,
spectrum showed that the reaction was completed by the presence of
just two set of new signals, which were attributed to compounds 2b
and 3b with a relative of ratio of 1:1.2. The solution was monitored by
1H NMR for 7 h, and no further changes were observed. Separated
experiments were carried out to determine the evolution of the
3
3
4
1H, JHH = 7.4, 3.6 Hz, 4-CHPy), 5.00 (dd, 2H, JHH = 4.2 Hz, JHH
=
1.6 Hz, 3- and 5-CHPy), 7.11−7.14 (m, 8H, CHN‑Ar and Py-Bn), 7.21
(m, 3H, CHN‑Ar and Py-Bn), 8.87 (bs, 1H, NHPy). 13C{1H} RMN
(C6D6, 25 °C, 125 MHz), δ 15.4 (Me-CN), 22.9 (CHMeMe), 23.3
(CHMeMe), 28.7 (CHMe2), 28.8 (CHMe2), 38.5 (4-CHPy), 46.3
(CH2 Py-Bn), 104.6 (3-CHPy), 123.4 (m-CHN‑Ar), 124.2 (p-CHN‑Ar),
126.4 (p-CHAr, Py-Bn), 128.6 (m-CHAr, Py-Bn), 129.7 (o-CHAr, Py-
Bn), 136.2 (o-CN‑Ar), 136.3 (o-CN‑Ar), 137.5 (2-CPy), 139.3 (i-CAr Py-
Bn), 146.5 (i-CN‑Ar), 159.7 (Me-CN).
1
mixture at 60 and 80 °C. Complex 2b: H NMR (C6D6, 25 °C, 400
MHz), δ 1.48 (s, 3H, Me-CN), 1.54 (s, 2H, CH2 Mg-Bn), 1.61 (s, 3H,
Me-CN), 1.96 (s, 3H, o-MeN‑Ar), 2.00 (s, 3H, o-MeN‑Ar), 2.07 (s, 3H, o-
MeN‑Ar), 2.08 (s, 3H, o-MeN‑Ar), 2.42 (s, 6H, p-MeN‑Ar), 2.48 (dd, 1H,
3
3JHH = 12.6, 4.8 Hz, CHH Py-Bn), 2.70 (dd, 1H, JHH = 12.5, 8.3 Hz,
Reaction of [Mg(Bn)2THF2] with 4-Bn-iPrH2BIP. Synthesis of
3a. A cold (−40 °C) toluene solution (5 mL) of the adduct
[Mg(CH2Ph)2THF2] (130 mg, 0.371 mmol) was added slowly to a
cold (−40 °C) pentane (10 mL) yellow solution of the 4-benzyl-
dihydropyridine 4-Bn-iPrH2BIP (206 mg, 0.360 mmol) placed in a
scintillation 20 mL vial. The reaction mixture was stirred vigorously,
while the color of the solution turned from yellow to clear orange.
During a few minutes, the color kept gradually changing then to blue,
ending up as dark red-purple. After 3 h stirring, the solution was
CHH Py-Bn), 3.76 (dd, 1H, 3JHH ≈ 3JHH = 6.0 Hz, 3-CHPy), 5.29 (dd,
3
3
1H, JHH = 8.8, 6.1 Hz, 4-CHPy), 6.06 (d, 2H, JHH = 7.6 Hz, o-CHAr
3
3
Mg-Bn), 6.37 (d, 1H, JHH = 9.1 Hz, 5-CHPy), 6.64 (t, 1H, JHH = 6.8
Hz, p-CHAr Mg-Bn), 7.0 (t, 2H, 3JHH = 6.8 Hz, m-CHAr Mg-Bn), 6.90−
7.35 (m, 9H, m-CHN‑Ar, CHPy Py-Bn). 13C{1H} NMR (C6D6, 25 °C,
100 MHz), δ 16.0 (Me-CN), 16.1 (Me-CN), 18.7, 18.7, 18.8, 18.9 (o-
MeN‑Ar), 21.1, 21.2 (p-MeN‑Ar), 24.5 (CH2 Mg-Bn), 38.5 (3-CHPy), 44.5
(CH2 Py-Bn), 114.0 (4-CHPy), 115.4 (p-CHAr Mg-Bn), 121.4 (p-CHAr
Mg-Bn), 121.4 (5-CHPy), 122.8 (2-CPy), 123.6 (6-CPy), 123.9 (o-CHAr
Mg-Bn), 126.6 (p-CHAr Py-Bn), 128.7 (m-CHAr Mg-Bn), 129.1 (p-
CNH‑Ar C(Me)-NAr), 129.5, 129.6 (m-CHNH‑Ar C(Me)-NAr),
130.1 (p-CN‑Ar C(Me)NAr), 130.1, 130.2 (m-CHN‑Ar, C(Me)
NAr), 130.3 (two overlapping signals, o,o′-CN‑Ar−C(Me)NAr),
133.3, 133.5 (o,o′-CNH‑Ar C(Me)-NAr), 138.7 (i-CNH‑Ar C(Me)-
NAr), 140.0 (i-CN‑Ar −C(Me)NAr), 146.3 (i-CAr Mg-Bn), 158.6 (i-
CAr Py-Bn), 166.1 (C(Me)-NAr), 170.1 (−C(Me)NAr).
Complex 3b: 1H NMR (C6D6, 25 °C 400 MHz), δ 1.47 (s, 2H,
CH2 Mg-Bn), 1.66 (s, 6H, Me-CN), 1.92 (s, 12H, o-MeN‑Ar), 2.19 (s,
1
evaporated to dryness, leaving a purple foamy residue. The H NMR
of this reaction crude showed a single set of signals corresponding to
compound 3a. The solid was redissolved in pentane (10 mL),
concentrated, and stored at −20 °C. After 6 days, compound 3a
precipitated as a purple solid. Upon filtration and drying, 148.2 mg
(60%) was isolated. Anal. Calcd for C47N57N3Mg: C, 82.02; H, 8.35;
N, 6.11. Found: C, 81.93; H, 8.50; N, 5.76.
Reaction of Mg(nBu)2 with 4-Bn-iPrH2BIP. Synthesis of
[Mg(nBu)(4-Bn-iPrHBIP)] (4). To a cold (−60 °C) hexane yellow
solution (15 mL) of the alkyl-dihydropyridine derivative 4-Bn-iPrH2BIP
(246.9 mg, 0.430 mmol) was added a colorless solution of Mg(nBu)2
(1.0 M, 0.45 mL, 0.45 mmol) in heptane. The resultant solution
changed immediately to dark blue. The reaction mixture was kept cold
for 10 min, the bath was then removed, and the mixture was stirred for
80 min at room temperature. The solution was taken to dryness to
3
6H, p-MeN‑Ar), 2.91 (d, 2H, JHH = 6.5 Hz, CH2 Py-Bn), 4.16 (tt, 1H,
3JHH = 6.5, 3.6 Hz, 4-CHPy), 5.04 (d, 2H, 3JHH = 3.6 Hz, 3-CHPy), 5.97
3
(d, 2H, JHH = 7.6 Hz, o-CHAr Mg-Bn), 6.62 (t, 1H, 6.9 Hz, p-CHAr
3
Mg-Bn), 6.97 (t, 2H, JHH = 7.0 Hz, m-CHAr Mg-Bn), 7.00−7.40 (m,
11H, CHN‑Ar, CHAr Py-Bn). 13C{1H} NMR (C6D6, 25 °C, 100 MHz),
δ 15.2 (Me-CN), 18.2 (o-MeN‑Ar), 18.3 (o-MeN‑Ar), 21.0 (p-MeN‑Ar),
24.2 (CH2 Mg-Bn), 40.7 (4-CHPy), 49.9 (CH2 Py-Bn), 103.1 (3-
CHPy), 115.3 (p-CHAr Mg-Bn), 124.1 (o-CHAr Mg-Bn), 126.3 (p-CHAr
Py-Bn), 127.6 (p-CN‑Ar), 128.6 (m-CHAr Mg-Bn), 129.4 (m-CHN‑Ar),
134.0 (o-CN‑Ar), 145.1 (i-CN‑Ar), 145.4 (2-CPy), 146.4 (i-CAr Mg-Bn),
158.8 (i-CAr Py-Bn), 172.9 (Me-CN).
1
obtain a blue oily residue, whose H NMR spectrum showed only
signals corresponding to complex 4. This residue was redissolved in 10
mL of pentane, concentrated to ca. 3 mL, and stored at −20 °C. The
product precipitated as a microcrystalline solid that was filtered off and
dried under vacuum. Yield: 191.2 mg (68%). Blue-purple crystals,
suitable for X-ray diffraction studies, were obtained by careful
1
recrystallization from pentane at −20 °C. H NMR (C6D6, 25 °C
Preparation of 4-Bn-iPrH2BIP. As previously described,12,5a this 4-
Bn-iPrH2BIP is formed together with the corresponding aromatized
pyridine-type product 4-Bn-iPrBIP in the reaction of Mn(Bn)2 with
iPrBIP, followed by controlled methanolysis. The following modified
400 MHz), δ −0.50 (dd, 2H, 3JHH ≈ 8.3 Hz, α-CH2 Mg-nBu), 0.73 (t,
3H, δ-CH3 Mg-nBu), 0.85 (m, 1H, β-CHH Mg-nBu), 0.91 (m, 1H, β-
3
CHH Mg-nBu), 0.98 (d, 6H, JHH = 6.7 Hz, CHMeMe), 1.01 (d, 6H,
3JHH = 6.7 Hz, CHMeMe), 1.10 (m, 2H, γ-CH2 Mg-Bu), 1.21 (d, 6H,
F
DOI: 10.1021/acs.organomet.6b00528
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
3JHH = 6.7 Hz, CHMeMe), 1.22 (d, 6H, JHH = 6.7 Hz, CHMeMe),
AUTHOR INFORMATION
■
3
1.69 (s, 6H, Me-CN), 2.63 (sept, 2H, JHH = 6.8 Hz, CHMe2), 2.70
Corresponding Authors
*E-mail: antonior@iiq.csic.es (A.R.-D.).
*E-mail: campora@iiq.csic.es (J.C.).
3
3
(sept, 2H, JHH = 6.8 Hz, CHMe2), 2.93 (d, 2H, JHH = 6.8 Hz, CH2,
Py-Bn), 4.06 (m, 1H, 4-CHPy), 5.14 (d, 2H, 3,3′-CHPy), 6.97−7.26 (m,
11H, CHN‑Ar, CHAr Py-Bn). 13C{1H}-NMR (C6D6, 25 °C, 100 MHz),
δ 7.2 (α-CH2 Mg-nBu), 14.3 (δ-CH3 Mg-nBu), 16.2 (Me-CN), 23.8
(CHMeMe), 23.8 (CHMeMe), 23.9 (CHMeMe), 24.0 (CHMeMe),
29.0 (CHMe2), 29.1 (CHMe2), 32.0 (β-CH2 Mg-nBu), 32.2 (γ-CH2
Mg-nBu), 40.3 (4-CHPy), 47.7 (CH2 Py-Bn), 105.3 (3-CHPy), 123.9
(m-CHN‑Ar), 124.0 (m-CHN‑Ar), 126.1 (o-CHAr Py-Bn), 126.3 (p-CHAr
Py-Bn), 128.7 (p-CHN‑Ar), 130.0 (m-CHAr Py-Bn), 138.6 (o-CN‑Ar),
138.7 (o-CN‑Ar), 139.9 (i-CAr Py-Bn), 144.3 (2-CPy), 145.0 (i-CN‑Ar),
173.7 (Me-CN). Anal. Calcd for C44H59N3Mg: C, 80.77; H, 9.09; N,
6.42. Found: C, 80.74; H, 9.11; N, 6.14.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Spanish Ministry of Economy and Innovation (MINECO)
and the FEDER funds of the European Union (CTQ2015-
68978-P) supported this work. J.J.S gratefully thanks MICINN
for a PFPI postgraduate studentship.
Monitoring the Reaction of [Mg(Bn)2THF2] with iPrBIP at
Variable Temperature. In a nitrogen-filled glovebox, a colorless
solution (0.5 mL; C7D8) of [Mg(Bn)2THF2] (6.5 mg; 0.018 mmol)
was placed in a standard NMR tube. Another NMR tube was charged
with 8.9 mg (0.018 mmol) of yellow iPrBIP, which was suspended in
0.5 mL of C7D8. Both tubes were sealed with rubber taps, taken out
from the glovebox, interfaced to a vacuum/argon line, and cooled
down to −65 °C. Then, under an argon atmosphere, the solution of
[Mg(Bn)2THF2] was transferred via cannula to the fine yellow
suspension of iPrBIP at the above-mentioned temperature. The NMR
tube was gently shaken, and the color of the mixture turned rapidly to
green. The tube was transferred to the NMR probe, which had been
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00528.
Figures S1 and S2 show the 1H NMR spectra of
compounds 1a and the mixture of compounds 2a and 3a,
respectively. Figures S3−S6 show ORTEP drawings, and
a summary of crystallographic data is included in Table
S1 (PDF)
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