Relative Lability and Chemoselective Transmetallation of NHC in Hybrid Phosphine-NHC Ligands: Access to Heterometallic Complexes

Article abstract of DOI:10.1002/anie.201505958

The relative lability and transmetallation aptitude of trialkylphosphine and NHC donors, integrated in semi-rigid hybrid ligands attached to [Ag₄Br₄] pseudo-cubanes, lies in favor of the NHC and is used to selectively access unprecedented NHC complexes with heterobimetallic cores, such as Ag-Cu (4^Cy) and Ag-Ir (5^tBu). These can be viewed as an arrested state before the full transmetallation of both donors, which gives the homodinuclear Cu (3^Cy) and Ir (6^Cy) complexes. The observed NHC transmetallation aptitude and reactivity urges caution in the common notion that views the NHC as a universal spectator.

Full text of DOI:10.1002/anie.201505958

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Chemie
International Edition: DOI: 10.1002/anie.201505958
German Edition: DOI: 10.1002/ange.201505958
Chemoselective NHC Transmetallation
Relative Lability and Chemoselective Transmetallation of NHC in
Hybrid Phosphine-NHC Ligands: Access to Heterometallic Complexes
Thomas Simler, Pierre Braunstein,* and Andreas A. Danopoulos*
Abstract: The relative lability and transmetallation aptitude of
trialkylphosphine and NHC donors, integrated in semi-rigid
hybrid ligands attached to [Ag4Br4] pseudo-cubanes, lies in
favor of the NHC and is used to selectively access unprece-
dented NHC complexes with heterobimetallic cores, such as
Ag–Cu (4Cy) and Ag–Ir (5tBu). These can be viewed as an
arrested state before the full transmetallation of both donors,
which gives the homodinuclear Cu (3Cy) and Ir (6Cy) com-
plexes. The observed NHC transmetallation aptitude and
reactivity urges caution in the common notion that views the
NHC as a universal spectator.
T
he bonding analogy between phosphines and N-heterocy-
clic carbenes (NHCs), and the stronger s-donor ability of the
latter, implies that NHC ligands are less prone to substitution
than PR3 under comparable molecular environments (that is,
nature of metal, co-ligands, and coordination geometry).[1]
Current computational and experimental methods provide
a solid understanding of metal–NHC bonding across the
periodic table.[2] In late transition metals, and in contrast to
M-PR3, strong and irreversible M–NHC binding dictates the
nature of the complexes formed.[3] In numerous cases,
phosphine substitution by NHCs has been used for the
synthesis of NHC-complexes (for example, Ru,[4] Fe,[5] Ni,[6]
Pd,[7] and Pt[8]). In contrast, the substitution of coordinated
NHCs by phosphines is scarce.[9]
Scheme 1. a) Plausible mechanisms for the transmetallation from Ag–
NHC complexes involving dissociation of the NHC or formation of
bridging NHC intermediates; b) chelating or c) bridging coordination
modes of hybrid P-NHC with Ag and their transmetallation; d) hybrid
P–NHC pro-ligands, previously reported by our group (A and B),[14a]
and studied in this work (1Cy and 1tBu).
À
Within the group 11 metals, the M NHC bond strength
follows the order AuI > CuI > AgI,[1a,10] which underlies the
widespread application of Ag–NHC complexes as reagents
for the synthesis of diverse transition metal–NHC complexes
by transmetallation. The detailed mechanisms of transmetal-
lation remain vague:[11] plausible propositions include the
involvement of free NHCs formed in situ after the establish-
ment of equilibria on dissolution of the Ag–NHC species, or
the presence of transient (hetero)di- or poly-nuclear inter-
mediates with bridging NHC ligands (Scheme 1a),[12] by
analogy with the now recognized bridging aptitude of
phosphines.[13] Recent cases of apparently transmetallation-
inert species,[14] anomalous “reverse transmetallations” to
Ag,[15] and facile NHC dissociation from heavier transition
metals are relevant and perplexing.[16] Understanding the
elementary steps of transmetallation and the associated
driving forces constitute important research targets with
implications in homogeneous catalysis, supramolecular self-
assembly, and materials synthesis.
Hybrid ligands comprising NHC and phosphine donors
are of considerable interest for the study of selective metal–
ligand interactions.[17] Whereas Ir pincer complexes are
directly accessible from A (Scheme 1d),[14a] but not by
transmetallation from the corresponding Ag complex, sug-
gesting that the pathways shown in Scheme 1b,c were not
operative, transmetallations of other P-NHC hybrid ligands
from Ag to Ru,[18] Pd[14b,19] and Rh[20] have been reported.
To gain insight into this unexpected behavior, we targeted
the new ligands 1Cy and 1tBu, containing alkyl-phosphine
donors (electronically more similar to NHC) (Scheme 1d). In
principle, they can adopt chelating (b(i)) or bridging (c(i))
[*] T. Simler, Dr. P. Braunstein, Dr. A. A. Danopoulos
Laboratoire de Chimie de Coordination
Institut de Chimie (UMR 7177 CNRS)
UniversitØ de Strasbourg
4 rue Blaise Pascal, 67081 Strasbourg Cedex (France)
E-mail: braunstein@unistra.fr
Dr. A. A. Danopoulos
Institute for Advanced Study (USIAS), UniversitØ de Strasbourg
4 rue Blaise Pascal, 67081 Strasbourg Cedex (France)
E-mail: danopoulos@unistra.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505958.
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coordination modes, display hemilability and, on transmetal-
lation, can lead to the b(ii) and c(iv) species, respectively. We
provide here evidence that heterometallic intermediates, such
as c(ii) and c(iii) involving bridging P–NHC ligands, can
indeed be detected, and we shed light on important trans-
metallation chemoselectivity issues.
solution. While 2tBu was rigid at room temperature, a fluxional
process was observed for 2Cy, where two 31P NMR doublets,
well resolved below À88C, coalesced at ambient temperature
(Supporting Information). A DG° of 57 Æ 2 kJmolÀ1 was
calculated for this process from variable temperature meas-
urements. Furthermore, the 13C NMR singlets for the CNHC
resonances of 2Cy and 2tBu were rationalized by the fast
exchange between the NHC donor arms.[21b,23] The reaction of
Reactions of 1Cy or 1tBu (Supporting Information) with
Ag2O (Scheme 2) gave good yields of 2Cy and 2tBu, respec-
2tBu with S8 resulted in instantaneous P S formation and
=
concomittant AgBr precipitation, again consistent with P
fluxionality. Consequently, we propose that two separate
exchange processes may be taking place (involving the pairs
of AgP and AgC), but under the conditions used, no cross-
exchange between AgP and AgC was observed (see the
Supporting Information, Section I.5.3). Thus, in the present
system, although donor group lability as inferred by the
dynamic behavior of the cubanes in solution appears to be
a prerequisite for the kinetically controlled ligand transfer to
another metal, it is not always sufficient. This is also
demonstrated by the lack of ligand scrambling between 2Cy
and 2tBu, which is attributable to kinetic reasons.
Scheme 2. Synthesis of pseudo-cubane complexes 2Cy and 2tBu. Yields
are based on the ligand.
To correlate the above mentioned dynamic processes with
reactivity, the ligand transmetallation from the cubanes to Cu
and Ir centers was investigated (Scheme 3).
tively, which in the solid state feature a distorted pseudo-
cubane [Ag4Br4] core with bridging hybrid ligands between P-
coordinated (AgP) and NHC-coordinated (AgC) Ag centers
(Figure 1). This arrangement results in chiral molecular
structures and both enantiomers are present in the unit cell.
Scheme 3. Transmetallation from cubanes (2) to the heterometallic Ag-
Cu (4) and Ag-Ir (5), and the homodinuclear Cu (3) and Ir (6)
complexes. Compounds underlined have been structurally character-
ized.
Figure 1. Molecular structures of 2Cy (left) and 2tBu (right) (thermal
ellipsoids at 30% probability). For clarity, Cy, tBu, and nBu groups are
only shown in the ligands at the top. Only one disordered position for
the Cy group in 2Cy and the nBu group in 2tBu is displayed. H atoms
and crystallization solvent are omitted. Metrical data are given in the
Supporting Information.[30]
In the rare past cases where hybrid NHC ligands have
been used in transmetallation from Ag to Cu, concomitant
transfer of both donor groups was facile and kinetically
indistinguishable.[12c,24] Accordingly, the reactions of 2Cy and
2
tBu with 4 equiv CuBr·SMe2 (Ag/Cu = 1:1) in CH2Cl2 resulted
in the complete exchange of Ag by Cu and the isolation of the
ladder-type tetranuclear species 3Cy and 3tBu (Figure 2). The
same complexes were also obtained from 1Cy and 1tBu and
[Cu(mes)]5 (Supporting Information).
While the [Ag4(halide)4(phosphine)4] motif (phosphine =
monodentate tertiary phosphine) is ubiquitous, it is note-
worthy that [Ag4(halide)4(carbene)4] (carbene = monoden-
tate NHC) is unknown. However, [Ag4(halide)4L2] (L = rigid
methylene- or 1,3-phenylene-bridged bis-NHCs),[21] and
[Ag4I4(P-NHC)2] have recently been described.[14,22]
In contrast, the use of a Ag/Cu = 2:1 stoichiometry
(Scheme 3) led to the heterotetranuclear complexes 4Cy and
4
tBu, in which transfer from Ag to Cu of only the NHC donor
Evidence from DOSY NMR spectroscopy supported the
took place, while the phosphine donor remained attached to
Ag (Figure 3).
idea that the cubane integrity of 2Cy and 2tBu was maintained in
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structure of 5tBu established the transfer of the NHC donor
from Ag to Ir, while the phosphine group remained attached
to Ag (Figure 4). The two Ag(m-P-NHC)Ir moieties are
Figure 2. Molecular structure of the centrosymmetric 3Cy (thermal
ellipsoids at 40% probability). Hydrogen atoms are omitted. See the
Supporting Information for metrical data.[30]
Figure 4. Molecular structure of 5tBu (thermal ellipsoids at 40%
probability). Hydrogen atoms are omitted. See the Supporting Infor-
mation for metrical data.[30]
connected by bromide bridges and weak argentophilic
interactions.[25,26] This structure appears to be the first for
a molecular Ir/Ag/NHC complex (CCDC).
Interestingly, the corresponding heterometallic complex
could not be accessed from the reaction with the cyclohexyl
analogue 2Cy, but instead, the diiridium complex 6Cy (Figure 5)
Figure 3. Molecular structure of the centrosymmetric 4Cy (thermal
ellipsoids at 40% probability). Hydrogen atoms are omitted. See the
Supporting Information for metrical data.[30]
Monitoring the transmetallation of 2tBu with excess
CuBr·SMe2 (> 4 equiv) by NMR spectroscopy at low temper-
atures confirmed that the more labile (but stronger s-
donating) NHC donor transmetallated faster, since 4tBu
appeared to be the only detectable intermediate on the way
to the formation of 3tBu (Supporting Information, Section I.8).
Remarkably, the P–Ag couplings were retained during the
NHC transmetallation, pointing to the absence of Cu/Ag
exchange on the P site and showing that the opposite
heteronuclear complex (P–Cu, NHC–Ag) is not formed as
an intermediate. The structure of 4Cy, which appears to be the
first for a mixed Cu/Ag/NHC complex (CCDC), is similar to
that of 3Cy, but displays AgI···AgI (3.006(1) Š) and CuI···AgI
metallophilic interactions (2.838(1) Š).[25]
Figure 5. Molecular structure of 6Cy (thermal ellipsoids at 35% proba-
bility). Hydrogen atoms are omitted. See the Supporting Information
for metrical data.[30]
was isolated in low yields. The latter was obtained more
efficiently when 2 equiv of [Ir(cod)(m-Cl)]2 (Ag/Ir= 1:1
stoichiometry) were used.
Further insight into the transmetallation reactivity was
provided when 2tBu was reacted with 1 equiv [Ir(cod)(m-Cl)]2
(Ag/Ir= 2:1 stoichiometry) for 2 h at room temperature,
leading to the isolation of the new heterotetranuclear Ag–Ir
complex 5tBu (Scheme 3). Two characteristic doublets in the
31P{1H} NMR spectrum confirm the P–Ag coordination. The
A related bridging P–NHC ligand in a diiridium complex
has been recently reported.[14a] Rationalization of the forma-
tion of 5tBu and 6Cy may invoke additional differences in the
lability of the P arm of the ligand, owing to the different
phosphine substituents. Indeed, transfer and strong coordi-
nation of a functionalized di-tert-butyl phosphine to silver[27]
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The USIAS, CNRS, UniversitØ de Strasbourg, RØgion Alsace
and CommunautØ Urbaine de Strasbourg are acknowledged
for the award of fellowships and a Gutenberg Excellence
Chair (2010-11) to A.A.D. We thank the CNRS and the
MESR (Paris) for funding and for a PhD grant to TS. We are
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crystal structures and Johnson Matthey PLC for a generous
loan of Ir precursor complexes.
Keywords: chemoselectivity · hybrid ligands ·
N-heterocyclic carbene · phosphines · transmetallation
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Received: June 29, 2015
Revised: August 25, 2015
Published online: October 14, 2015
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