NHO to aNHC Isomerization at a Pd0-Center

Article abstract of DOI:10.1002/ejic.201800329

N-heterocyclic carbenes (NHCs) are a highly important class of ligands in transition metal-mediated catalysis. Recently, a modification of the NHCs has received considerable interest, the N-heterocyclic olefins (NHOs) with an exocyclic methylene appended to the NHC. In here we investigated the propensity of NHOs to act as a ligands for Pd⁰ complexes derived from [Pd₂(dvds)₃]. The reaction of IPrCH₂ and IMesCH₂ with the Pd⁰ precursor [Pd₂(dvds)₃] resulted in the formation of abnormal N-heterocyclic carbene complexes [(aIPrCH₃)Pd(dvds)] (1) and [(aIMesCH₃)Pd(dvds)] (2). DFT calculations were applied to study this isomerization and to reveal the bonding in 1 and 2. In addition, the backbone-methylated NHOs ^MeIPrCH₂ and ^mMeIMesCH₂ (4) were shown not to undergo the isomerization observed in IPrCH₂ and IMesCH₂.

Full text of DOI:10.1002/ejic.201800329

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Accepted Article
Title: NHO to aNHC Isomerization at a Pd(0)-Center
Authors: Christian Hering-Junghans and André Schumann
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10.1002/ejic.201800329
European Journal of Inorganic Chemistry
COMMUNICATION
NHO to aNHC Isomerization at a Pd(0)-Center
André Schumann,[a] Christian Hering-Junghans*[a]
Dedicated to Professor Philip P. Power on the occasion of his 65th birthday.
(ca. 24) and HCbackbone (ca. 33) to be considerably different.[7]
These findings clearly show the influence of kinetic factors in the
formation of aNHCs as thermodynamically deprotonation of C2
should be favored. The role of the counteranion in the
imidazolium salt has also been discussed and small anions
(such as Br−) clearly favor the formation of the normally
coordinated carbene complex, whereas big, weakly polarizable
Abstract: N-heterocyclic carbenes (NHCs) are a highly important
class of ligands in transition metal-mediated catalysis. Recently, a
modification of the NHCs has received considerable interest, the N-
heterocyclic olefins (NHOs) with an exocyclic methylene appended
to the NHC. In here we investigated the propensity of NHOs to act
as a ligands for Pd(0) complexes derived from [Pd2(dvds)3]. The
reaction of IPrCH2 and IMesCH2 with the Pd(0) precursor
[Pd2(dvds)3] resulted in the formation of abnormal N-heterocyclic
carbene
complexes
[(aIPrCH3)Pd(dvds)]
(1)
and
−
anions (such as BPh4 ) favor the formation of the aNHC complex.
[(aIMesCH3)Pd(dvds)] (2). DFT calculations were applied to study
this isomerization and to reveal the bonding in 1 and 2. In addition,
the backbone-methylated NHOs MeIPrCH2 and mMeIMesCH2 (4) were
shown not to undergo the isomerization observed in IPrCH2 and
IMesCH2.
1,2,3-substituted imidazolium salts can be used to circumvent
metalation in the 2-position and several examples have been
reported in the literature.[8]
Introduction
Carbenes are neutral compounds that feature a divalent carbon
atom with six electrons in its valance shell. These species have
been viewed as elusive species for a long time and pioneering
work on their isolation dates back to the early 20th century. N-
heterocyclic carbenes (NHCs) have been in the focus of
chemical research since the pioneering work of Arduengo and
co-workers in 1991, when they reported an isolable and
“bottleable” carbene stabilized by two adjacent N atoms within
an imidazole heterocycle (Scheme 1, B).[1] The molecular design
of NHCs shows structural resembles to landmark studies by
Wanzlick and Öfele concerned with transition metal carbene
complexes.[2] Owing to its simple preparation and robustness
this first NHC 1,3-di(adamantly)imidazole-2-ylidine (IAd) spurned
the rapid development of synthetic as well as theoretical
carbene chemistry.[3] Today carbenes have evaded their status
as lab curiosity and have been integrated into some of the most
important catalytic transformations in chemical industry,
underlining their role as ligands for transition metal complexes.[4]
NHCs coordinate to metal or p-block elements via the C2 atom
with its two adjacent heteroatoms. Imidazolium-based ligands
that coordinate through the C4 or C5, respectively, are termed
“abnormal” carbenes or “mesoionic” carbenes, as in these
species only zwitterionic Lewis formuli can be written. Abnormal
C4-metalation was first observed by Crabtree and co-workers
when treating iridium polyhydride H5Ir(PPh3)2 with a pyridine-
appended carbene, abnormal coordination through Cbackbone (C4)
rather than as expected through the Ccarbene (C2) position was
detected (Scheme 1, C).[5] Abnormal coordination of IMes to
Pd(II) was reported by Nolan et al., in an attempt to prepare
[PdCl2(IMes)2] in situ from [Pd(OAc)2] and [IPrH]Cl (Scheme 1,
D).[6] Acidity studies have determined the pKa values of HCcarbene
Scheme 1. The prototypical NHC IAd as presented by Arduengo et al. (A).
The first reported aNHC-complex (B) and an early report of a Pd(II) aNHC
complex (C).
N-heterocyclic olefins (NHOs) are a class of compounds that
formally contain
a
methylene appended to an NHC.
Consequently, the exocyclic C=C bond is considerably polarized
introducing increased nucleophilicity at the terminal olefin-
position (resembling the situation found in Wittig-reagents), with
considerable p-character.[9] Therefore, it is expected that NHOs
behave as soft Lewis bases, which should be beneficial to
stabilize low-valent metal centers such as Ni(0) or Pd(0) often
present in active catalysts (e.i. Suzuki-Miyaura coupling). Early
work by Kuhn in 1993 on IMe4CH2 uncovered the donor
capacities of NHOs in the form of a BH3-adduct and a Mo(CO)5-
complex.[10] Recent studies by Rivard and co-workers have
determined TEP-values for a variety of NHOs and these studies
classify NHOs as moderate strength ligands with good σ-donor
capacity, however, negligible π-acceptor properties as was
shown
by
combined
theoretical
and
experimental
investigations.[11] These interesting donor properties have
resulted in the application of NHOs as ligands in transition metal
catalyzed reactions,[12] as polymerization catalyst and as
organocatalysts in various transformations.[13] In this contribution
we present the isolation of Pd(0)(aNHC) complexes, derived
directly from NHO precursors. The experimental work is
supported by theoretical investigations.
[a]
A. Schumann, Dr. C. Hering-Junghans
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Straße 29a
Results and Discussion
18059 Rostock
E-mail: christian.hering-junghans@catalysis.de
www.catalysis.de
Palladium complexes of NHOs have not been reported to date
and we sought to explore the reactivity of NHOs, namely IPrCH2
[IPr = (HCNDipp)2C: Dipp = (2,6-iPr2C6H3)] and IMesCH2 [IMes =
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201800329
European Journal of Inorganic Chemistry
COMMUNICATION
(HCNMes)2C: Mes = (2,4,6-Me3C6H2)] as these are easily
accessible and offer varying steric shielding about the NHO-
position, with [CpPd(allyl)] as a source of Pd(0). As a co-ligand
1,3-divinyltetramethylsiloxane was introduced, as we intended to
prepare novel Pd(0) complexes for the telomerization of 1,3-
butadiene with methanol, a reaction that usually employs
[(NHC)Pd(dvds)] complexes as catalyst.[14] Two different
protocols were utilized for this purpose. In situ generation of
[CpPd(allyl)] from [(μ-Cl)Pd(allyl)]2 and NaCp followed by the
addition of an excess of dvds and one molar equivalent IPrCH2
in THF.[15] After stirring the deep red mixture for 24 hrs at room
temperature, removal of the volatiles and extraction of the
residue with n-hexane afforded a reddish material from which
presumably second conformer (resulting reproductively in
crystals that contain a minor component, which couldn’t be
modeled). Consequently, we hypothesize that the abnormal
coordination mode is preferred when a Pd(0) precursor and an
unsaturated NHO with a protonated backbone are combined.
Similar NHO to aNHC isomerizations have been observed by
Iglesias and Oro in the coordination sphere of Iridum; in two
instances the authors observed the NHO to aNHC isomerization
at 70 °C in toluene with an PCP-type NHO ligand (Scheme 3,
A).[17] Liddle et al. recently reported the isomerization of IMe2CH2
to aIMeCH3 with the f-block amides {[(Me3Si)2N]3M} (M = U, Y,
La, Nd; Scheme 3, B).[18] In both cases a reversal of the
isomerization was not detected, pointing again to the fact that
NHCs and aNHCs are stronger ligands than NHOs, as a result
of their enhanced π-backbonding when compared to NHOs.
To experimentally support our hypothesis that isomerization only
occurs with protonated imidazolines and accessible backbone
protons, we turned to backbone-methylated NHO MeIPrCH2,
which was expected to result in the normally coordinated NHO-
Pd complex, as an isomerization is blocked in this case.
Interestingly, upon complete addition of the ligand the color
intensifies briefly, before a black powder precipitates from the
reaction mixture. Removal of solvent, extraction with n-pentane
and subsequent crystallization at −30°C only afforded crystals of
MeIPrCH2. This demonstrates that NHOs are likely not suited to
act as ligands for electron rich metal centers, such as Pd(0).
colorless X-ray quality crystals were grown by placing
a
saturated toluene solution layered with n-hexane at −30°C over
a period of 72 hrs. These crystals were determined to be an
unexpected aNHC-Pd(0) complex [(aIPrCH3)Pd(dvds)] (1)
(Figure 1, left). To the best of our knowledge [Pd(0)(aNHC)]-
complexes have not been prepared to date. However, just
minimal amounts of the material could be isolated and complex
1 was only one among a number of unidentified products. We
therefore turned to an commercially available [Pd2(dvds)3]-
solution (ca. 10.7 wt.% Pd) in dvds as a source of Pd(0).
Addition of one molar equivalent IPrCH2 in thf to the Pd(0)
solution and stirring the yellowish mixture overnight, resulted in
the clean conversion to 1 (Scheme 2). Recrystallization of the
crude material from pentane at −30°C resulted in pure 1 in
1
moderate isolated yields (ca. 70%). In the H NMR spectrum (in
C6D6) of 1 a singlet resonance is observed for the exocyclic
methyl group at 1.53 ppm, which forms upon isomerization, as
well as only one singlet for a single backbone proton at 6.92
ppm (cf. [(aIPrPh)2PdCl2] 6.97 ppm).[16] As expected for 1, two
different Dipp-environments are detected with two sets of
septets (2.81, 2.45 ppm) and four sets of dubletts for the methyl
groups of the iPr-groups, respectively. Through 2D NMR
experiments the two-coordinate carbene center was determined
to resonate at 137.2 ppm (cf. [(aIPrPh)2PdCl2] 126.84 ppm),[16]
which is upfield-shifted compared to the corresponding NHC
complex [(IPr)Pd(dvds)] (cf. 200.8 ppm).[14]
Scheme 3. A: Isomerization of an [(NHO)Ir(COD)Cl] into an
[(aNHC)Ir(COD)Cl] as observed by Oro and Iglesias.[17] B: IMe2CH2 to
aIMeCH3 isomerization in the coordination sphere of f-group elements.[18]
Thus, we conceived that an NHO with a mono-methylated
backbone should function as a potential aNHC ligand precursor
as well. The imidazolium salt [mMeIMesH]Cl (mMe = mono-
methylated) was reported by Glorius et al.,[19] and the
corresponding free carbene mMeIMes (3) was isolated after
deprotonation of [mMeIMesH]BF4 with NaH in the presence of
catalytic amounts of KOtBu. Treatment of 3 with MeI and
subsequent deprotonation with n-BuLi at −78°C in THF resulted
in the clean formation of mMeIMesCH2 (4) with an
unsymmetrically substituted backbone (Scheme 4). Upon mixing
4 with [Pd2(dvds)3] in thf-d8 no NHO to aNHC isomerization was
Scheme 2. Synthetic route towards [(aNHC)Pd(dvds)] complexes 1 (aNHC =
aIPrCH3) and 2 (aNHC = aIMesCH3).
Next we turned to IMesCH2, to determine whether this mode of
reactivity is restricted to IPrCH2, or if this reactivity is more
general for unsaturated imidazolines. IMesCH2 was added to
[Pd2(dvds)3] and after workup and crystallization from n-pentane
an analogous [(aIMesCH3)Pd(dvds)] (2) complex was isolated in
moderate yields (ca. 70 %) (Figure 1, right). However, even
1
detected by H NMR spectroscopy, only the slow formation of a
Pd-mirror and concomitant liberation of dvds was observed. This
isolated crystals always contained minimal amounts of
a
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10.1002/ejic.201800329
European Journal of Inorganic Chemistry
COMMUNICATION
is further evidence that the kinetic C-H acidity of the backbone
protons is the decisive factor for isomerization to occur.
Interestingly, when a sample of 1 is heated to 80°C in C6D6 over
a period of three hours ca. 67% of the starting complex have
decomposed to yield free IPrCH2, dvds and Pd, indicating that
the NHO to aNHC isomerization is reversible and that the NHO
complex is not preferentially formed.
with no major variations. The most striking difference is the
orientation of the dvds unit, which is folded towards one of the
flanking Mes-groups of aIMesCH3. This contrasts the situation in
1, in which the dvds-backbone is orientated away from the Dipp-
group, presumably as a result of steric hindrance.
Scheme 4. Preparation of NHO mMeIMesCH2 (4) with a mono-methylated
backbone. Reaction conditions: (i) 2 eq. NaH, cat. KOtBu, THF, r.t.; (ii) 1) 1 eq.
MeI at r.t., 2) after 1 hr 1 eq. n-BuLi at -78°C, THF, -LiI, -n-BuH.
Scheme 5. Model reaction for the computational study of the NHO to aNHC
isomerization to yield 1 and 2.
To gain further insight into this unusual NHO to aNHC
isomerization,
DFT
calculations
on
the
b3lyp//6-
31+G(d,p)//ECP28MDF//ECP28MDF_VTZ level of theory were
carried out. In a first step we wanted to compare the relative
energies of the aNHC complex 1 and 2 with their respective
NHO counterparts 1NHO and 2NHO (Scheme 5). Additionally,
we optimized the structures of the free aNHCs aPIrCH3 and
aIMesCH3 to effectively compare the coordinated and
hypothetical free ligand.[22] Frequency analyses determined all
structures to be minima on the potential energy surface. A
comparison of the relative free energies (ΔG298K) clearly shows
that the aNHC complex is energetically favored by 4.75 for 1 and
4.13 kcal/mol for 2, respectively, in agreement with the
experimental findings. Moreover, NBO analysis and inspection
of the Kohn-Sham orbitals were employed to inspect the
bonding in complexes 1 and 2. The HOMO in 1 and 2 is best
described as an antibonding interaction between a Pd-centered
d orbital with the carbon based sp2-type lonepair (LP) at the
carbene center (Figure 2, left, shown here for 2), whereas the
LUMO is centered on the flanking Dipp or Mes groups in 1 and 2,
respectively. The corresponding bonding interaction for 2 is
represented by HOMO-17 (Figure 2, right). Metal to ligand π-
Figure 1 POV-ray depiction of the molecular structure of 1 (left) and 2 (right).
Ellipsoids shown at 50% probability. All hydrogen atoms (except those on
C2/C4) omitted and the flanking Dipp-groups rendered as wireframes for
clarity. Selected bond lengths (Å) and angles (°): 1 C1–Pd1 2.0931(18), C1–
C2 1.365(3), C1–N1 1.417(2), C2–N2 1.397(2), N2–C3 1.335(2), N1–C3
1.346(2), C3–C4 1.486(2); N2-C3-N1 106.68(15), C2-C1-N1 101.93(15),
N1−C3−N2 106.68(15); 2 C1−C2 1.362(4), C2−N1 1.416(3), N1−C3 1.323(3),
C3−C4 1.499(4), C3−N2 1.338(3), N2−C1 1.396(3), Pd1−C1 2.095(3);
N2−C1−C2 101.6(2), N1−C3−N2 107.3(2), N1−C2−C1 110.8(2).
1 crystallizes from toluene/n-hexane in the triclinic spacegroup
P-1 with one molecule of 1 and a molecule of toluene in the
asymmetric unit (1∙C7H8).[20] A solvent-free polymorph was
obtained from n-pentane, also in the triclinic spacegroup P-1
(the discussion will be based on this polymorph). The metrical
parameters in both polymorphs are almost identical. Pd displays
backbonding is minimal and
a
second-order-perturbation
analysis revealed only minimal stabilization (ca. 10 kcal/mol)
stemming from interactions of a Pd-centered d-type LP and the
π*-orbital of the C1−C2 bond. This is further supported by
analysis of the NPA charges of Ccarbene in the free and
coordinated ligand, with a depletion of the negative charge in
complexes 1 and 2 by ca. 0.1 e, as expected when σ-electron
density is donated to the metal. The lack of backbonding is also
illustrated by the Wiberg-Bond-Index for Ccarbene−Pd, which is
0.48 in 1 and 0.50 in 2, respectively. Moreover, no significant
changes within the geometry of the free aNHC and the
coordinated framework are evident, pointing to minimal back-
bonding, as significant changes within the geometry of the 5-
membered heterocycle would be expected.
a
distorted trigonal planar coordination (∑(<Pd) = 359.8°)
environment with the two vinyl groups of dvds coordinating in
side-on fashion. The aNHC is bound through the 4-position, with
concomitant protonation of the vinyl-position of the starting
IPrCH2. The change from IPr=CH2 to aIPr−CH3 is evidenced by
an elongation of the C3−C4 distance (1.490(2) Å; cf. IPrCH2
d(C=C) = 1.322(4) Å ),[21] as expected upon reduction of the
bond order. The Pd−C distance (2.0931(18) Å) is in agreement
with [(NHC)Pd(dvds)] complexes reported in the literature
[davg(Pd−C)
= 2 crystallizes in the monoclinic
2.09 Å).[14]
spacegroup P21/c with one molecule in the asymmetric unit.
After refinement a large residual electron density was detected
within 1.27 Å of C4, which could not be modelled. Therefore,
only the differences between 1 and 2 will be highlighted. The
metrical parameters within the aNHC-five-membered are similar,
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COMMUNICATION
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Figure 2. POV-Ray depiction of the Kohn-Sham orbitals, HOMO (left) and
HOMO-17 (right) of 2.
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Conclusions
In conclusion we have presented an unusual NHO to aNHC
isomerization in the coordination sphere of Pd(0), resulting in the
formation of aNHC complexes 1 and 2. On the basis of DFT
studies the abnormal coordination mode was shown to be
thermodynamically favored by ca. 4.5 kcal/mol, which is in
agreement with the experimentally observed isomerization
occurring at room temperature. However, this mode of
coordination is restricted to unsaturated NHOs and mono- or di-
methylation of the backbone suppresses complex formation. A
solution of 1 was found to be stable at room temperature,
however, decomposition occurred at 80°C, resulting in the
formation of Pd black and free IPrCH2. This indicates that
complexes 1 and 2 might be unstable under conditions for the
telomerization of 1,3-butadiene with methanol,
a catalytic
transformation that we seek to explore in the future. Furthermore,
we seek to systematically test various metal centers and NHOs
to further understand NHO to aNHc isomerization in order to
develop a general concept.
[14] R. Jackstell, S. Harkal, H. Jiao, A. Spannenberg, C. Borgmann, D. Röttger,
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R. S. Ghadwal, Organometallics 2016, 35, 3421.
Acknowledgements
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[19] K. Hirano, S. Urban, C. Wang, F. Glorius, Org. Lett. 2009, 11, 1019.
[20] CCDC 1829091 (for 1∙C7H8), 1829092 (for 1) and 1829093 (for 2) contain
the supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic Data
Centre.
C. H.-J. thanks Prof. M. Beller for his support and the Alexander
von Humboldt Foundation for funding (return fellowship). We
thank our technical and analytical staff for assistance, especially
Dr. Anke Spannenberg for her support regarding X-ray analysis.
We thank Umicore for a generous donation of [Pd2(dvds)3].
Support by LIKAT is gratefully acknowledged.
Keywords: N-Heterocyclic Olefins • Abnormal Carbenes •
Palladium • Isomerization • Structure Elucidation
[21] S. M. I. Al-Rafia, A. C.Malcolm, S. K. Liew, M. J. Ferguson, R. McDonald
and E. Rivard, Chem. Commun. 2011, 6987.
[22] Details are depicted in the Supporting Information.
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141; (b) K. Öfele, J. Organomet. Chem. 1968, 12, P42.
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10.1002/ejic.201800329
European Journal of Inorganic Chemistry
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Transformed. N-heterocyclic olefins
(NHO) have been tested as ligands
for electron-rich Pd(0)-fragments.
This resulted in an unusual NHO to
abnormal carbene (aNHC)
isomerization, which afforded unique
aNHC-Pd(0) complexes. Quantum
mechanical calculations support the
observed transformation.
N-Heterocyclic Olefins*
André Schumann, Dr. Christian
Hering-Junghans*
Page No. – Page No.
NHO to aNHC Isomerization at a
Pd(0)-Center
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