Oxygen-induced dimerization of alkyl-manganese(II) 2,6-bisiminopyridine complexes: Selective synthesis of a new ditopic NNN-pincer ligand

Article abstract of DOI:10.1021/acs.organomet.6b00538

The outcome of the reaction of manganese(II) dialkyls with 2,6-bisiminopyridine (BIP) ligands is dramatically altered by the presence of very small amounts of oxygen (<0.5 mol %), leading to binuclear species. These arise from the dimerization of the initial product, a Mn(II) 4-alkyl-2,6-bisimino-dihydropyridinate alkyl complex. Cleavage of the binuclear Mn products with methanol affords the free dimeric bases, which can be regarded as a special type of ditopic NNN pincer ligand with an unusual tricyclic framework. The coordinative ability of the new ligands has been probed with the syntheses of Zn and Pd organometallic derivatives.

Full text of DOI:10.1021/acs.organomet.6b00538

Article
pubs.acs.org/Organometallics
Oxygen-Induced Dimerization of Alkyl-Manganese(II) 2,6-
Bisiminopyridine Complexes: Selective Synthesis of a New Ditopic
NNN-Pincer Ligand
́
John J. Sandoval, Cristobal Melero, Pilar Palma, Eleuterio Alvarez, Antonio Rodríguez-Delgado,*
́
and Juan Campora*
́
́
Instituto de Investigaciones Químicas, CSIC − Universidad de Sevilla, c/Americo Vespucio, 49, 41092 Sevilla, SPAIN
S
* Supporting Information
ABSTRACT: The outcome of the reaction of manganese(II)
dialkyls with 2,6-bisiminopyridine (BIP) ligands is dramatically
altered by the presence of very small amounts of oxygen (<0.5
mol %), leading to binuclear species. These arise from the
dimerization of the initial product, a Mn(II) 4-alkyl-2,6-
bisimino-dihydropyridinate alkyl complex. Cleavage of the
binuclear Mn products with methanol affords the free dimeric
bases, which can be regarded as a special type of ditopic NNN
pincer ligand with an unusual tricyclic framework. The
coordinative ability of the new ligands has been probed with
the syntheses of Zn and Pd organometallic derivatives.
Scheme 1. Alkyl Migration Followed by Dimerization of 1,4-
Dihydropyridinates
INTRODUCTION
■
In the nearly two decades elapsed since the discovery of Fe and
Co olefin polymerization catalysts with 2,6-bisiminopyridines
(BIP),1,2 these ligands have proved a fertile ground for new
developments in homogenous catalysis3 and fundamental
organometallic chemistry.4 Applications of BIP complexes in
catalysis have expanded from olefin polymerization5 and
oligomerization2,6 into other domains, such as hydrogenation,7
hydrosylilation,8 or C−C coupling reactions,9 among others. A
key factor in the catalytic ability of these complexes is the
noninnocent character of BIP ligands, i.e., their ability to
reversibly accept and donate electrons.10 This feature is also
responsible for the varied and complex reactivity of both BIP
ligands and their complexes, particularly when they are exposed
to strongly alkylating or reducing reagents, such as organo-
lithium, organomagnesium, or organoaluminum compounds.
These rarely react with transition metal BIP−halide complexes
to afford straightforward transmetalation products.4,11 Most
often, reactions of this type lead to reduced complexes that
apparently bear the metal center in unusual low oxidation
states. Analyses of the electronic structure of such compounds
have shown that in most cases it is the BIP ligand, rather than
the metal, which actually undergoes reduction.12 Moreover,
different types of products containing structurally modified BIP
ligands have been isolated as well,13 and similar structure
alterations take place when BIP ligands react directly with
organomain-group reagents.14 One of the most common
transformations of BIPs takes place when an alkyl group is
transferred from a metal center (either main-group or
transition) to the ligand. For example (Scheme 1), 1,4-
dihydropyridinate species of type A are formed when the alkyl
group migrates from the metal to the remote position 4 in the
pyridine ring.15,16 Further changes may also take place. An
example relevant to this work is the coupling of two units of
type A to give a dimer of type B. Gambarotta and co-workers13b
isolated the first example of the latter class of compounds from
the reaction of [(i‑PrBIP)CrCl3] with Mg(CH2Ph)Cl, and a few
years later Budzelaar14b,d observed the formation of similar
dimers when ligand i‑PrBIP reacts with aluminum trialkyls.
These are rather appealing and potentially useful trans-
formations, but their low yields or lack of selectivity render
them unattractive for any practical application.
Harnessing the reactivity of BIP ligands into well-behaved,
selective processes might prove a productive strategy to expand
the range of molecular architectures available from the basic
BIP motif. However, not many efforts have been oriented
toward this purpose. For some years, our group has been
Received: July 8, 2016
© XXXX American Chemical Society
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Scheme 2. Selective Functionalization and Dimerization of BIP Systems by Organomanganese Reagents
involved in the development of the chemistry of BIP complexes
into useful synthetic transformations.15−17 We found that the
reaction of mildly basic organomanganous reagents MnR2 with
BIP ligands proceeds with unusual selectivity, cleanly leading to
products of type A.15,16 These complexes, or the free
dihydropyridines (4-R-H2BIP) formed upon demetalation,
readily undergo dehydrogenation to afford BIP’s alkylated at
position 4 on the pyridine ring (4-R-BIP), providing a
straightforward route to ligands that would be otherwise
difficult to prepare.16 Lately, we have been interested in
optimizing this methodology in order to prepare 4-R-BIP
ligands in multigram scale. One such preparation afforded a
different type of product, arising from the hydrolysis of a Mn
complex of type B. Repetition of the experiment showed that
the formation of the dimer was fortuitous. However, this result
revealed the existence of previously overlooked relationships in
the chemistry of BIP-based metal alkyls. Thus, we decided to
investigate the causes that led to the formation of dimeric
products. Herein we report the results of our investigation, and
we show that these products can be used as a new type of
ditopic pincer ligand in the synthesis of main group or
transition metal alkyl complexes with potential uses in catalysis.
solution afforded a crystalline solid more amenable of NMR
analysis. The 1H spectrum of this material showed that it was a
mixture containing starting material (i‑PrBIP), together with the
expected products 4-allyl-i‑PrH2BIP and 4-allyl-i‑PrBIP and a
fourth, unknown component. The identity of the latter was
established as the dimer (4-allyl-i‑PrH2BIP)2 by X-ray diffraction
analysis of a single crystal, manually selected from the mixture
(Figure 1). As can be seen, its molecule contains a tricyclic core
composed of a central C6 carbocycle and two terminal C5N
heterocycles arising from the fusion of two 4-allyl-i‑PrH2BIP
units through the 3 and 3′ positions of their 1,4-
dihydropyridine rings. This linkage removes the unsaturation
from the bridgehead (3, 3′) carbon atoms, yielding two
independent imine−imine−enamine systems, oriented in
antiparallel fashion. Overall, the molecule exhibits an inversion
1
center, but it has no symmetry planes. Although the H NMR
spectrum of the mixture is complicated, the spectral features of
the new product match the expected for such (4-
allyl-i‑PrH2BIP)2 dimer. Thus, the lack of symmetry planes
results in the observation of two signals for the enamine- and
imine-bound methyl groups (δ 1.99 and 2.33 ppm) and four
multiplets for the methynes of the four nonequivalent i-Pr
1
groups (δ 2.92, 3.00, 3.47, and 3.54 ppm). The H resonances
of the bridgehead methynes were located at δ 3.35 and 3.96
ppm, and a broad but characteristic signal at ca. 7.5 ppm was
identified as the enamine NH. The ESI-MS spectrum of the
mixture showed an intense signal at m/z 1047.8 for the
molecular ion of the protonated dimer.
RESULTS AND DISCUSSION
■
Scheme 2 presents an overview of the reactivity of BIP ligands
toward Mn(II) dialkyls. As mentioned above, in an attempt to
scale-up the selective alkylation of the BIP ligands on position 4
(as shown in the upper part of the Scheme 2) involving the
allylation of i‑PrBIP with in situ generated Mn(allyl)2, we
obtained after demetalation with methanol a yellow-orange oil
whose 1H NMR spectrum was unexpectedly broad and
complex. Crystallization by slow evaporation of a hexane
Consistent with our previous reports,15,16 when we repeated
the reaction of Mn(allyl)2 with i‑PrBIP followed by methanolytic
demetalation, dihydropyridine 4-allyl-i‑PrH2BIP and aromatized
4-allyl-i‑PrBIP were regularly formed as the main products, but
we realized that trace amounts of the dimer were formed as
B
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when a small amount of air (0.2 mL, 2 μmol of O2) was
intentionally injected in a cold mixture of 1.5 mmol of
Mn(allyl)2 and i‑PrBIP (−60 °C), the mixture developed a
somewhat different, more reddish hue. After 90 min at room
temperature, the usual workup with methanol was applied to
produce a mixture containing the dimer (4-allyl-i‑PrBIPH2)2,
along with 4-allyl-i‑PrH2BIP and 4-allyl-i‑PrBIP, as well as some
starting material (i‑PrBIP) in 3.5:3:1:1.5 relative ratio, i.e., nearly
60% dimerization. Independent experiments using dry oxygen
instead of air led to essentially the same result. Therefore, a
very small amount of dry O2 (ca. 0.15 mol %) suffices to trigger
the dimerization of a substantial fraction of Mn intermediate
complex 1a. However, increasing the amount of O2 does not
improve the dimer formation but rather causes extensive
decomposition of the extremely sensitive organomanganese(II)
intermediates. For example, when the oxygen dose was
increased 10-fold (2.0 mL of air, 1.5 mol O2 %), the reaction
mixture evolved into a brown suspension. No indication of the
dimer was detected in the 1H NMR of the crude product, which
showed mainly unaltered starting material (i‑PrBIP). Attempts
to increase the dimer yield by breaking down the O2 addition in
several doses met with reproducibility problems and in general
led to no clear improvement over our previous results.
Separation of pure samples of the dimer (4-allyl-i‑PrH2BIP)2
either by chromatography or crystallization proved a challeng-
ing task. Seeking more selective transformations, we examined
the effect of oxygen on similar reactions of manganese dialkyls
1
with BIP ligands, as shown in Scheme 2. In addition to H
NMR, the samples were analyzed by electrospray MS, which
turned out to be the technique of choice for the detection of
dimeric products in complicated reaction mixtures. The
reaction of in situ generated dibenzylmanganese (MnBn2)
with i‑PrBIP in the presence of 0.15 mol % O2 closely resembles
that of Mn(allyl)2. The electrospray spectrum of the yellow oily
residue obtained after quenching the reaction mixture with
methanol showed a conspicuous signal at m/z = 1147.8,
corresponding to the protonated molecular ion of the dimer (4-
Bn-i‑PrH2BIP)2, as well as other signals for i‑PrBIP, 4-
Bn-i‑PrH2BIP, and 4-Bn-i‑PrBIP. The relative amount of benzyl
dimer (4-Bn-i‑PrH2BIP)2 was estimated to be ca. 34% of the
recovered products on the basis of the integrals of the 1H NMR
spectrum of the mixture. X-ray quality crystals were growth by
slow evaporation of a hexane solution. The crystal structure of
(4-Bn-i‑PrH2BIP)2 (Figure S7) is very similar to that of its allyl
analogue, with the same configuration and very close bond
distances and angles.
Figure 1. Molecular representation and ORTEP plot of the dimer [4-
allyl-i‑PrH2BIP]2. Selected bond distances (Å): N(1)−C(1): 1.460(3);
N(1)−C(5): 1.304(3); N(2)−C(6): 1.387(3); N(3)−C(11):
1.274(3); C(1)−C(6): 1.350(4); C(5)−C(11): 1.476(3); H(N2)···
N(1): 2.16(2); N(2)···N(1): 2.672(3); angle: N(2)−H(N2)···N(1),
116(2)°.
well. We have observed that 4-allyl-i‑PrH2BIP has no tendency
to dimerize on its own, even at temperatures well above the
ambient (ca. 80−90 °C); hence, it can be inferred that the
anomalous dimerization reaction somehow involved the
conversion of the manganese dihydropyridinate complex
[Mn(allyl)(4-allyl-i‑PrHBIP)], 1a, into 3a, as shown in Scheme
2. Therefore, we tried to promote the dimerization of 1a, either
extending the reaction time (up to 24 h) prior to quenching
with methanol or heating the reaction mixture at 90 °C in a
sealed ampule. No substantial dimer formation was observed
under such forcing conditions. Instead, the relative amount of
4-allyl-i‑PrH2BIP shifted to almost zero in favor of the aromatic
4-allyl-i‑PrBIP, due to the thermally induced dehydrogenation of
1a into 2a. Since these observations imply that dimerization of
1a is not spontaneous, it had to be induced by the action of an
exogenous agent. Two likely candidates were light or air, the
latter adventitiously admitted in the reaction flask. The former
possibility was ruled out when we carried out the reaction
under direct illumination from a regular incandescent lamp,
with the same result as in the previous experiments. However,
Similar reactions were carried out using MesBIP instead of
i‑PrBIP. The ESI-MS spectra of the crude extracts showed
intense signals for the M + H+ ions of the corresponding
dimers, (4-allyl-MesH2BIP)2 (m/z = 879.6) and (4-
Bn-MesH2BIP)2 (m/z = 979.6). Although their 1H NMR spectra
were even more complicated than those arising from i‑PrBIP, in
both cases the characteristic NH signals of the dimers were
observed in the low field region of the 1H NMR spectra (at ca.
7.5 ppm) together with those of the bis(imino)-dihydropyr-
idine and -pyridine byproducts. Integration of the dimer NH
1H NMR resonances showed that the amount of monomeric
byproducts accompanying (4-allyl-MesH2BIP)2 was particularly
low. The yield of the latter was further improved when
successive doses of O2 (6 × 0.1 mL of O2) were added to the
reaction mixture at 1 h intervals. In this case, the ESI MS
spectrum of the final product shows a single main signal for (4-
allyl-MesH2BIP)2 and background impurities (Figure S1).
C
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1
Consistent with this, the H NMR spectrum of the organic
extract (after methanolysis) shows signals for neither the usual
monomeric byproducts nor the MesBIP starting material.
Surprisingly, this spectrum remains far more complex than
expected for (4-allyl-MesH2BIP)2 (Figure S2). A possible
explanation is that the dimer (4-allyl-MesH2BIP)2 occurs as a
mixture of isomers arising from tautomerism of the imine−
imine−enamine moieties and/or different Z/E configuration of
CN or CC bonds. Consistent with this interpretation, an
HPLC analysis of a sample with UV detector showed one main
peak and one minor peak characterized by very similar UV
spectra (Figure S3).
the molecule, such as the bridgehead methyne groups, the
imine moieties, or the Pd-bound alkyl groups giving rise to a
single set of signals. This structural symmetry is confirmed by
the crystal structure of the Pd complex (Figure 2), which is very
Dihydropyridine dimers (4-R-H2BIP)2 can be regarded as
conjugate acids of an unusual kind of ditopic pincer ligands,
with two symmetrical, electronically delocalized anionic NNN
donor sets. In order to confirm the identity of the (4-
allyl-MesH2BIP)2 dimer, we prepared two organometallic
derivatives thereof. As depicted in Scheme 3, the dimer was
Scheme 3. Exchange Reaction of (4-Allyl-MesHBIP)2 with Zn
and Pd Alkyls
Figure 2. ORTEP view of compound 5. Selected bond distances (Å)
and angles (deg): Pd(1)−N(1): 2.025(6); Pd(1)−N(2): 2.098(6);
Pd(1)−N(3): 2.077(6); Pd(1)−C(31): 2.060(7); N(1)−C(1):
1.342(9); N(1)−C(5): 1.335(10); N(2)−C(6): 1.321(9); N(3)−
C(11):1.311(9); C(1)−C(6): 1.444(10); C(5)−C(11): 1.441(10).
N(1)−Pd(1)−C(31): 171.6(3); N(2)−Pd(1)−N(3): 157.6(2).
similar to those of the related Cr13b and Al14b complexes
reported in the literature (for a comparison of some metric
parameters, see Table S2). The lengths of the C−N and C−C
bonds within the N3C2 fragments are similar and in between
those of typical single and double bonds, in contrast with those
of free dimers (4-R-i‑PrH2BIP)2, whose alternating long and
short C−C/C−N bonds are as expected for conjugate NC−
CN−CC−N imino−imino−enamines. Significantly, all
three Pd−N bond lengths are also very similar, the central
Pd(1)−N(1) (2.025 Å) being only slightly shorter than the
terminal Pd(1)−N(2) and Pd(1)−N(3) bonds (ca. 2.09 Å).
The requirement of oxygen to trigger the dimerization
reaction marks a qualitative difference with the Cr- and Al-
based literature precedents, which take place directly from the
corresponding dihydropyridinate precursor.13,14b Woclzansky
has described a number of spontaneous dimerizations of 3d
transition metal complexes with polydentate nitrogen-donor
ligands that also proceed with no need of external oxidizers.19
In our system, oxygen can be viewed as a “molecular switch”
that turns on an alternative pathway to dimeric, tridentate
pincer ligands. The need for an external trigger to switch the
reaction outcome indicates that intermediate Mn(II) dihy-
dropyridinate complexes 1 are themselves unable to undergo
dimerization but become activated when they react with
oxygen. Interestigly, the Woclzansy group has reported recently
that C−C coupling mediated by aza-enolate (iminate)
complexes also requires oxidation of the metal center.20
treated with 2 equiv of a suitable Zn or Pd alkyl precursor. In
each case, the enamine N−H functionality proved acidic
enough to selectively cleave one of the metal−carbon bonds to
afford the corresponding binuclear alkylmetal derivative
[MR′(4-allyl-MesHBIP)]2 in good yields. Dibenzylzinc reacts
instantly at −40 °C to afford a deep red-orange solution from
which zinc derivative 4 was isolated as a dark orange
microcrystalline material. The reaction with the metallacycle
[PdCH2CMe2-o-C6H4(cod)], a versatile starting material
frequently used in our laboratory to prepare palladium alkyl
complexes,18 proceeds at the room temperature, selectively
cleaving the Pd-aryl bond to afford complex 5. The latter was
isolated as very dark bluish-green crystals. The 1H NMR spectra
of the crude mixtures showed that both reactions are essentially
quantitative and cleanly afford single organometallic products
without leaving substantial amounts of unreacted material.
Complexes 4 and 5 are the first of their class to be fully
characterized by elemental analyses, 1H and 13C NMR (Figures
S4 and S5), IR, and X-ray diffraction (for 5). Consistent with
the extensive electron delocalization over the N3C2 fragments,
their NMR spectra are highly symmetrical, with each element of
The effect of oxygen in our system can be rationalized on the
basis of the simplified mechanism presented in Scheme 4. We
believe that oxygen initiates the process when it oxidizes a small
amount of Mn(II) dihydropyridinate complex 1 that is
otherwise unable to dimerize, giving rise to intermediate 1ox.
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Scheme 4. Proposed Mechanism for the O2-Induced Dimerization Process
The latter then couples with another molecule of 1, leading to
the dimer in oxidized form, 3ox. The C−C coupling reaction is
probably enabled by the enhanced electrophilicity of 1ox. A
redox step ensues then, which entails reduction of the dimer 3ox
by a second molecule of 1, to afford neutral dimer 3 and
regenerate 1ox. In principle, a condition required for this cycle
to proceed is that the potential of the 3/3ox redox pair should
be more positive than that of 1/1ox, but if some 3 is in
equilibrium with 3ox, then the overall reaction could be driven
by the irreversibility of the C−C bond formation step even if
the potential of the 3/3ox pair is slightly less positive than that
of 1/1ox. A preliminary DFT calculation on a simplified model
(see Supporting Information) suggests that the redox step is
indeed a reversible, nearly thermoneutral process. However,
any competing process that consumes 1ox or 3ox (e.g.,
dehydrogenative oxidation of 1 to 2) would irretrievably stop
the dimerization process. This would explain the need to add
oxygen in successive doses to drive the reaction to completion,
although the very small amount of O2 required indicates that
once started the cycle is very efficient. The more difficult
dimerization of the bulkier i‑PrBIP derivatives suggests that
steric hindrance during the coupling of the monomeric units
could be the main reason limiting the cycle propagation.
There is ample precedent for the oxidation of σ-
organomanganese(II) compounds by atmospheric oxygen
giving rise to Mn(III) organometallic species, often uninten-
tionally.21 In general, stable Mn(III) compounds isolated from
such reactions incorporate halide21a−d or oxo ligands.21e This
could also happen in our system, since our protocol involves
the presence of magnesium halogenides. Therefore, the
representations of 1ox and 3ox in Scheme 4 should be
considered approximations to the actual structures, where
positive charges could be counterbalanced by an additional
anionic ligand (e.g., chloride or bromide). In this case, the
redox step would become an atom transfer process rather than
a pure electron transfer reaction.
CONCLUSIONS
■
We discovered that small amounts of oxygen modify the
outcome of the reaction of manganese dialkyls with BIP ligands,
leading to the formation of tricyclic dimers. At least in one
specific case the O2-induced reaction can be driven to
completion, cleanly switching the selectivity of the overall
process from formation of 4-R-pyridnes to the corresponding
tricyclic dimer. A mild demetalation procedure allows the
isolation of the free tricyclic bases (4-R- H2BIP)2, which can be
transferred to different metal fragments. The ready synthesis of
binuclear alkyl−zinc and −palladium complexes supported by
(4-allyl-MesHBIP)2 confirmed that these dimeric compounds
can have interesting applications as a promising class of ditopic
NNN pincer ligands. We are currently optimizing procedures in
order to improve the production of (4-R-H2BIP)2 dimers and
exploring their potential as ligands in coordination/organo-
metallic chemistry and in catalysis.
EXPERIMENTAL SECTION
■
All manipulations were carried out under oxygen-free argon
atmosphere, using conventional Schlenk techniques or a nitrogen-
filled glovebox. Solvents were rigorously dried and degassed before
using.22 NMR spectra were recorded with Bruker 400 and 500 MHz
1
spectrometers. The H and 13C{1H} resonances of the solvent were
used as internal standards, but the chemical shifts are reported with
respect to TMS. Spectral assignations were routinely helped with
monodimensional 13C (gated), DEPT135 and 2D 1H−1H COSY, and
1
HMBC and H−13C HSQC heterocorrelation spectra. The following
measurements were carried out by the Analytical Services of the
́
Instituto de Investigaciones Quimicas: high-performance liquid
chromatography (HPLC, Waters Alliance 2695 with detector Waters
996), electrospray ionization mass spectrometry (ESI-MS, Bruker
Esquire 6000), and elemental analysis (LECO TruSpec CHN). High-
resolution mass spectra (HR-MS) were performed by the Spectrom-
́
́ ́
etry Service of the Centro de Investigacion, Tecnologia e Innovacion
de la Universidad de Sevilla, CITIUS (Universidad de Sevilla, Spain).
MnCl2, ZnCl2, PdCl2, and benzylmagnesium chloride (2.0 M in
THF or 1.0 M in Et2O) were purchased from Sigma-Aldrich.
Allylmagnesium bromide (typically 1.4 M in Et2O) was prepared
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according to standard literature procedures.23 The titer for Grignard
reagents was assessed by hydrolysis and titration against HCl.
yield). ESI MS m/z = 979.6 ([M + H]+) (Expected for C68H78N6:
978.6).
Optimized Procedure for the Synthesis of (4-Al-
lyl-MesH2BIP)2. A solution of the Mn(II) diallyl complex in THF
was prepared from MnCl2 as follows: MnCl2 (284 mg, 2.4 mmol) was
suspended in 15 mL of THF, and the mixture was stirred for 18 h.
After this time, the suspension was cooled to −70 °C, and 3.5 mL of a
1.4 M solution of Mg(CH2CHCH2)Br in Et2O (4.9 mmol) was
slowly added with stirring. After 10 min, the resultant mixture was
allowed to warm at the room temperature, and the stirring was
continued for 20 min. As noted before, the stirring should not be
prolonged any longer because Mn(allyl)2 is thermally sensitive, the
onset of its decomposition being noticed by the formation of a dark
material in the walls of the flask. The resulting brown suspension was
transferred via cannula to a vigorously stirred solution of 873 mg (2.2
mmol) of MesBIP in 30 mL of THF, at −70 °C. The mixture was
stirred for 10 min, the cooling bath removed, and the stirring
continued for 70 min at the room temperature. At this point, 0.1 mL
of dry O2 was injected through a septum. No color change was
noticed. After 1 h, a second 0.1 mL dose of O2 was injected. The
procedure was repeated at 1 h intervals until a total volume of 0.6 mL
of O2 had been injected. The stirring was then continued for 22 h,
during which time the mixture turned dark purple. Anhydrous
methanol (5 mL) was added, and the volatiles were evaporated under
reduced pressure. The resultant dark orange oil was extracted in
pentane (3 × 20 mL), and the extracts were filtrated through a pad of
Celite. The solution was then evaporated leaving a yellow solid.
Attempts to crystallize this material were unsuccessful. Yield: 552 mg,
24
Zn(CH2Ph)2 and [PdCH2CMe2-o-C6H4(cod)]18 were prepared as
described in the literature. BIP derivatives 2,6-bis(N-(2,6-
diisopropylphenyl)acetimidoyl)pyridine (i‑PrBIP) and 2,6-bis(N-
(2,4,6-trimethylphenyl)acetimidoyl)pyridine (MesBIP) were prepared
by condensation of 2,6-diacetylpyridine with the corresponding
anilines (2,6-diisopropylaniline or mesitidine, respectively) under
azeotropic water-removal conditions.1a Oxygen was dried for several
days over P2O5 in a Fischer−Porter pressure bottle.
General Procedure for the Formation of (4-R-YH2BIP)2
Dimers. The desired manganous alkyl reagent in THF was generated
from MnCl2 and Mg(R)X (R = allyl, X = Br or R = benzyl, X = Cl), as
follows: The required volume of a solution containing 2.4 mmol of the
Grignard reagent in Et2O was slowly added to a stirred suspension of
150 mg (1.2 mmol) of MnCl2 in THF, cooled at −70 °C. After 10
min, the cooling bath was removed. The stirring was continued at
room temperature for ca. 30 min for R = benzyl; for R = allyl, this time
was reduced to that strictly required for the MnCl2 solid to disappear,
ca. 20 min, due to the low thermal stability of the Mn(allyl)2 reagent.
The onset of the thermal decomposition of the Mn(allyl)2 reagent is
marked by the appearance of a dark ring on the walls of the vessel,
near the surface of the stirred solution. At this point, the reagent must
be used at once. The organomanganous reagent was transferred via
cannula to a suspension of 1.2 mmol of the BIP ligand (i‑Pr4BIP or
MesBIP) in 20 mL of THF, vigorously stirred at −60 °C. Then, 0.2 mL
of air was injected with a syringe provided with a long needle, the tip
of the latter being placed close to the stirring surface. Then, 10 min
later, the mixture was allowed to warm at room temperature, and the
stirring was continued for 90 min, after which time the mixture
evolved from dark purple to a somewhat redder color (the hue can
vary from one experiment to other). At this point, the reaction is
quenched with 5 mL of MeOH and the solvent evaporated under
vacuum. The resulting dark brown oil was extracted in pentane (3 ×
30 mL), and the extracts were filtered through a pad of Celite. The
combined organic extracts were evaporated again, leaving an sticky
orange oil. Attempts to separate the components either by
crystallization or column chromatography were unsuccessful. How-
ever, impure crystalline samples of (4-R-i‑PrH2BIP)2 (R = allyl or
benzyl) can be obtained by slow crystallization from hexane, from
which single crystals suitable for X-ray diffraction were manually
selected.
1
80%. H NMR (C6D6, 25 °C, 400 MHz). Due to complexity of the
spectrum (Figure S2), spectral ranges for each type of signal are given:
δ 1.90−2.40 (m, 4H, CH2−CHCH2), 2.00−2.40 (m 2H, 4-CHPy),
1.80−2.50 (m, 48H, Me, mesityl and imine), 3.32, 3.40, 3.56, 3.65,
4.07, 4.28 (s, total intensity 4H, 3-CHPy), 4.30−5.30 (m, 4H, CH2−
CHCH2), 5.50−6.00 (m, 2H, CH2−CHCH2), 6.30−6.92 (several
signals CHAr), 7.59 (broad s, N−H).13C{1H} NMR (C6D6, 25 °C, 100
MHz): Selected ranges: δ 12.0−15.3 (Me−CN), 17.0−21.0 (o-
MeN−Ar), 30.6−32.0 (p-MeN−Ar), 37.2−37.8 (CH2, Py−CH2CH
CH2), 37.9−42.0 (3-CHPy), 114.0−138.0 (CHAr), 145.7−148.7 (i-
CNH‑Ar), 151.9−156.1 (i-CN−Ar), 162.5−168.4 (2-CPy and MeCN).
ESI MS: m/z 879.7 ([M + H]+) (expected for C60H74N6: 878.6). EI
+
HR-MS: m/z 879.6075 ([M + H]+); Calcd for C60H75N6 : 879.6053.
Synthesis of [ZnBn)2-μ-(4-allyl-MesHBIP)2]2 (4). (4-Al-
lyl-MesHBIP)2 (200 mg, 0.22 mmol) diluted in 5 mL of toluene was
dropwise added to a solution of 130 mg (0.52 mmol) of Zn(CH2Ph)2
in 10 mL of toluene magnetically stirred at −40 °C. The resultant
mixture turned immediately from yellow to orange-red. After 10 min,
the cooling bath was removed, and the stirring was continued for 16 h
at room temperature, after which time the solvent was evaporated
under reduced pressure. The oily residue was extracted in 15 mL of
(4-Allyl-i‑PrH2BIP)2. A yellow oil (380 mg) containing (4-
allyl-i‑PrH2BIP)2, 4-allyl-i‑PrH2BIP, 4-allyl-i‑PrBIP, and i‑PrBIP in an
approximately relative ratio of 3.5:1:3:1.5 (56% dimerization, 39%
absolute yield). 1H NMR (C6D6, 25 °C, 400 MHz) δ 1.11−1.21
(broad s, 48H, CHMe2), 1.99 (s, 6H, MeCN), 2.09 (broad s, 4H,
CH2−CHCH2), 2.33 (s, 6H, MeCN), 2.92 (m, 2H CHMe2), 3.00
(m, 2H CHMe2), 3.35 (broad s, 2H, 3−CHPy), 3.47 (m, 2H CHMe2),
3.54 (m, 2H, CHMe2), 3.96 (broad s, 2H, 3−CHPy), 5.04−5.11 (m,
4H, CH2−CHCH2), 5.70−6.00 (m, 2H, CH2−CHCH2), 7.00−
7.17 (several signals CHAr), 7.83 (broad s, 2H, N−H). The signal of
4−CHPy could not be accurately assigned. ESI MS m/z 1047.8 ([M +
H]+) (expected for C72H98N6: 1046.8).
2
hexane, filtered, and evaporated again to /3 of the initial volume.
Then, the remaining orange solution was stored in the absence of light
at −20 °C. Four days later, a microcrystalline solid had formed. After
filtration and drying under vacuum 190 mg (73%) of 4 were isolated.
1H NMR (C6D6, 25 °C, 400 MHz) δ 1.64 (m, 2H, 4-CHPy), 1.72 (s,
16H, Me−CN, CH2−CHCH2), 1.95 (s, 4H, CH2 Zn−Bn), 2.07,
2.08 (s, 24H, o-MeN−Ar), 2.26 (s, 12H, p-MeN−Ar), 2.79 (broad s, 4H, 3-
(4-Bn-i‑PrH2BIP)2. A yellow oil (399 mg) containing (4-
Bn-i‑PrH2BIP)2, 4-Bn-i‑PrH2BIP, 4-Bn-i‑PrBIP, and i‑PrBIP in an
approximately relative ratio of 3:1:4:0.7 (34% dimerization, 24%
3
CHPy), 4.91 (s, 2H, CH2−CHCHH), 4.94 (d, 2H, JHH = 5.0 Hz,
CH2−CHCHH), 5.66 (m, 2H, CH2−CHCH2), 6.17 (d, 4H, 3JHH
1
3
absolute yield). H NMR (CDCl3, 25 °C, 400 MHz) δ 1.12−1.29
= 7.1 Hz, o-CHAr Zn−Bn), 6.79 (t, 2H, JHH = 6.0 Hz, p-CHAr Zn−
3
(broad s, 48H, CHMe2),1.68 (s, 6H, MeCN), 1.90 (s, 6H, MeC
N), 2.46 (m, 2H, CHMe2), 2.58 (m, 2H, CHMe2), 2.72 (broad s, 2H,
3−CHPy), 3.34 (m, 2H, CHMe2), 3.51 (m, 2H, CHMe2), 3.76 (broad
s, 2H, 3−CHPy), 6.95−7.15 (several signals CHAr), 7.56 (broad s, N−
H). Signals for 4−CHPy and PhCH2 could not be assigned with
confidence. ESI MS m/z 1147.8 ([M + H]+) (Expected for C80H102N6:
1146.8).
Bn), 6.86 (t, 4H, JHH = 7.9 Hz, m-CHArZn−Bn), 6.87 (s, 4H, m-
CHN−Ar), 6.88 (s, 4H, m-CHN−Ar). 13C{1H} NMR (C6D6, 25 °C, 100
MHz) δ 15.4 (Me−CN), 18.4, 18.7 (o-MeN−Ar), 19.9 (CH2 Zn−Bn),
21.0 (p-Me N−Aryl), 30.2 (4-CHPy), 37.7 (CH2−CHCH2), 44.0 (3-
CHPy), 116.2 (CH2−CHCH2), 120.4 (p-CHAr Zn−Bn), 124.3 (2-
CPy), 126.9 (m-CHAr Zn−Bn), 127.7 (o-CHAr Zn−Bn, partially
overlapped by C6D6), 129.4 (m-CHN−Ar), 129.7, 131.4 (o,o′-CN−Ar),
133.7 (p-CN−Ar), 137.3 (CH2−CHCH2), 145.5 (i-CN−Ar), 148.4 (i-
CAr Zn−Bn), 164.2 (Me−CN). Anal. Calcd. for C74H86N6Zn2: C,
74.67; H, 7.28; N, 7.06. Found: C, 74.77; H, 7.92; N, 7.33.
(4-Bn-MesH2BIP). A yellow oil containing a mixture of (4-
Bn-MesH2BIP)2, 4-Bn-MesH2BIP, Bn-MesBIP, and MesBIP in an
approximately ratio 11:1:1.4:6.2. (82% dimerization, 56% absolute
F
DOI: 10.1021/acs.organomet.6b00538
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis of [PdCH2CMe2Ph)2-μ-(4-allyl-MesHBIP)2] (5). Solid
(4) (a) Knijnenburg, Q.; Gambarotta, S.; Budzelaar, P. H. M. Dalton
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Meli, A.; Segarra, A. M. Coord. Chem. Rev. 2006, 250, 1391−1418.
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samples of [PdCH2CMe2-o-C6H4(cod)] (279 mg, 0.74 mmol) and (4-
allyl-MesHBIP)2 (320 mg, 0.36 mmol) were mixed and dissolved in 30
mL of Et2O. The resultant solution was magnetically stirred during 4 h
at 23 °C. A 1H NMR spectrum prepared from an aliquot sample of the
solution showed that the reaction was complete. The solution was
evaporated to dryness leaving a green oily solid. This was extracted in
hexane (10 mL), filtrated and the resultant solution was concentrated
́
Campora, J., Eds.; Springer: Heidelberg, 2011.
2
(6) Boudier, A.; Breuil, P.-A. R.; Magna, L.; Olivier-Bourbigou, H.;
Braunstein, P. Chem. Commun. 2014, 50, 1398−1407.
to /3 of its original volume. Compound 5 crystallized as dark bluish-
green blocks when this solution was allowed to rest at −20 °C for 48
h. After filtration, the crystals were washed three times with a mixture
diethyl-ether/hexane (1:10) at −25 °C and dried under vacuum. Yield,
(7) Chirik, P. J. Acc. Chem. Res. 2015, 48, 1687−1695.
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Milsmann, C.; Hoyt, H. M.; Weller, K. J.; Nye, S. A.; Lewis, K. M.;
Boyer, J.; Delis, J. G. P.; Lobkovsky, E.; Chirik, P. J. Organometallics
2012, 31, 4886−4893. (b) Atienza, C. C. H.; Tondreau, A. M.; Weller,
K. J.; Lewis, K. M.; Cruese, R. W.; Nye, S. A.; Boyer, J. L.; Delis, J. G.
P.; Chirik, P. J. ACS Catal. 2012, 2, 2169−2172.
1
280 mg (57%). H NMR (CD2Cl2, 25 °C, 400 MHz) δ 0.32 (s, 4H,
CH2 Pd−R), 0.66 (s, 12H, CMe2 Pd−R), 1.55 (s, 12H, Me−CN),
1.98−2.01 (broad s, 6H, 4-CHPy, CH2−CHCH2), 2.28, 2.29 (s,
24H, o-MeN−Ar), 2.33 (s, 12H, p-MeN−Ar), 2.36 (s, 4H, 3-CHPy), 5.10
(dd, 2H, 3JHH = 10.1, 2JHH = 2.1 Hz, CH2−CHCHH), 5.21 (dd, 2H,
2
3JHH = 16.8, JHH = 2.1 Hz, CH2−CHCHH), 5.95 (m, 2H, CH2−
(9) (a) Karpiniec, S. S.; McGuinness, D. S.; Britovsek, G. J. P.; Patel,
J. Organometallics 2012, 31, 3439−3442. (b) Greenhalgh, M. D.;
Thomas, S. P. J. Am. Chem. Soc. 2012, 134, 11900−11903.
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8772−8774.
3
CHCH2), 6.39 (d, 4H, JHH = 6.9 Hz, o-CHAr Pd−R), 6.83−6.91
(m, 14H, CHN−Ar, CHAr Pd−R).13C{1H} NMR (CD2Cl2, 25 °C, 100
MHz) δ 15.9 (Me−CN), 18.7, 18.9 (o-MeN−Ar), 21.0 (p-MeN−Ar),
29.9 (4-CHPy), 30.2 (CMe2 Pd−R), 37.6 (CH2−CHCH2), 39.4
(CH2 Pd−R), 41.6 (CMe2 Pd−R), 42.9 (3-CHPy), 116.3 (CH2−CH
CH2), 124.0 (p-CHAr Pd−R), 125.2 (m-CHAr Pd−R and 2-CPy), 127.5
(o-CHAr Pd−R), 128.9 (m-CHN−Ar), 131.9, 132.0 (o-CN−Ar), 134.4 (p-
CN−Ar), 137.9 (CH2−CHCH2), 144.3 (i-CN−Ar), 155.0 (i-CAr Pd−
R), 175.1 (Me−CN). Anal. Calcd for C80H98N6Pd2: C, 70.83; H,
7.28; N, 6.28. Found: C, 70.91; H, 7.47; N, 5.92.
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ASSOCIATED CONTENT
* Supporting Information
■
S
(CIF) The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.organomet.6b00538.
(12) (a) Kooistra, T. M.; Knijnenburg, Q.; Smits, J. M. M.; Horton,
A. D.; Budzelaar, P. H. M.; Gal, A. W. Angew. Chem., Int. Ed. 2001, 40,
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(c) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M.
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Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt, K.;
Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901−13912. (e) Tondreau,
A. M.; Milsmann, C.; Patrick, A. D.; Hoyt, H. M.; Lobkovsky, E.;
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(13) (a) Reardon, D.; Conan, F.; Gambarotta, S.; Yap, G.; Wang, Q.
J. Am. Chem. Soc. 1999, 121, 9318−9325. (b) Sugiyama, H.;
Aharonian, G.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. J.
Am. Chem. Soc. 2002, 124, 12268−12269. (c) Scott, J.; Gambarotta, S.;
Korobkov, I. Can. J. Chem. 2005, 83, 279. (d) Scott, J.; Gambarotta, S.;
Korobkov, I.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005, 127, 13019−
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(14) (a) Blackmore, I. J.; Gibson, V. C.; Hitchcock, P. B.; Rees, C.
W.; Williams, D. J.; White, A. J. P. J. Am. Chem. Soc. 2005, 127, 6012−
6020. (b) Knijnenburg, Q.; Smits, J. M. M.; Budzelaar, P. H. M.
Organometallics 2006, 25, 1036−1046. (c) Bruce, M.; Gibson, V. C.;
Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1998, 2523−2525. (d) Knijnenburg, Q.; Smits, J. M. M.;
Budzelaar, P. H. M. C. R. Chim. 2004, 7, 865−869. (e) Tay, B.-Y.;
Wang, C.; Chia, S. C.; Stubbs, L. P.; Wong, P.−K.; van Meurs, M.
Organometallics 2011, 30, 6028−6033. (f) Arrowsmith, M.; Hill, M. S.;
Additional figures showing NMR and mass spectra and
HPLC plots, crystallographic details and ORTEP plots
for [4-allyl-i‑PrH2BIP]2, (4-allyl-MesH2BIP)2, and 5 with
full numbering schemes, and DFT calculation details
(PDF)
Crystallographic data for [4-allyl-i‑PrH2BIP]2, (4-al-
lyl-MesH2BIP)2, and 5 (CIF)
DFT calculations details and coordinates for optimized
structures (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: antonior@iiq.csic.es.
*E-mail: campora@iiq.csic.es.
■
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.
Kociok-Kohn, G. Organometallics 2010, 29, 4203−4206.
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Gutierrez-Puebla, E.; Cam
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ez, C. M.; Rodríguez-Delgado, A.; Palma, P.; Alvarez, E.;
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DOI: 10.1021/acs.organomet.6b00538
Organometallics XXXX, XXX, XXX−XXX