Tridentate complexes of group 10 bearing bis-aryloxide N-heterocyclic carbene ligands: Synthesis, structural, spectroscopic, and computational characterization

Article abstract of DOI:10.1021/om5003446

A series of group 10 complexes featuring chelating tridentate bis-aryloxide N-heterocyclic carbenes were synthesized and characterized by using different techniques. Ni(II), Pd(II), and Pt(II) complexes were isolated in good yields by straightforward direct metalation of the corresponding benzimidazolium or imidazolium precursors in a one-pot procedure. All of the compounds were fully characterized, including single-crystal X-ray diffractometric determination for three of the derivatives. In the solid state, the complexes adopt a typical square-planar coordination geometry around the platinum atom, sizably distorted in order to comply with the geometrical constraints imposed by the bis-aryloxide N-heterocyclic carbene ligand. For platinum and palladium derivatives, a joint experimental and theoretical characterization was performed in order to study the optical properties of the newly prepared complexes by means of electronic absorption and steady-state and time-resolved photophysical techniques as well as density functional theory (DFT) and time-dependent DFT in both vacuum and solvent. When the temperature was lowered to 77 K in frozen glassy matrix, three platinum complexes showed broad and featureless, yet weak, photoluminescence in the green region of the visible spectrum with excited-state lifetimes on the order of a few microseconds. On the basis of joint experimental and computational findings and literature on platinum complexes, such emission was assigned to a triplet-manifold metal-ligand-to-ligand charge transfer (³MLLCT) transition.

Full text of DOI:10.1021/om5003446

Article
pubs.acs.org/Organometallics
Tridentate Complexes of Group 10 Bearing Bis-Aryloxide
N‑Heterocyclic Carbene Ligands: Synthesis, Structural, Spectroscopic,
and Computational Characterization
Etienne Borre,† Georges Dahm,† Alessandro Aliprandi,‡ Matteo Mauro,*,‡,§ Samuel Dagorne,*,∥
́
and Stephane Bellemin-Laponnaz*,†,§
́
†Institut de Physique et Chimie des Mater
F-67034 Strasbourg Cedex 2, France
‡Laboratoire de Chimie et des Biomater
Universite de Strasbourg UMR 7006, 8 allee
§University of Strasbourg Institute for Advanced Study (USIAS), 5 allee
∥Institut de Chimie de Strasbourg, CNRS-Universite
́
iaux de Strasbourg, Universite
́
de Strasbourg CNRS UMR 7504, 23 rue du Loess, BP 43,
́
iaux Supramolec
Gaspard Monge, F-67083 Strasbourg, France
du General Rouvillois, 67083 Strasbourg, France
de Strasbourg UMR 7177, 1 rue Blaise Pascal, F-67000 Strasbourg, France
́ ́ ́
ulaires, Institut de Science et d’Ingenierie Supramoleculaires (ISIS),
́
́
́
́
́
́
S
* Supporting Information
ABSTRACT: A series of group 10 complexes featuring chelating tridentate bis-aryloxide N-
heterocyclic carbenes were synthesized and characterized by using different techniques. Ni(II),
Pd(II), and Pt(II) complexes were isolated in good yields by straightforward direct metalation of
the corresponding benzimidazolium or imidazolium precursors in a one-pot procedure. All of the
compounds were fully characterized, including single-crystal X-ray diffractometric determination
for three of the derivatives. In the solid state, the complexes adopt a typical square-planar
coordination geometry around the platinum atom, sizably distorted in order to comply with the
geometrical constraints imposed by the bis-aryloxide N-heterocyclic carbene ligand. For platinum
and palladium derivatives, a joint experimental and theoretical characterization was performed in
order to study the optical properties of the newly prepared complexes by means of electronic
absorption and steady-state and time-resolved photophysical techniques as well as density functional theory (DFT) and time-
dependent DFT in both vacuum and solvent. When the temperature was lowered to 77 K in frozen glassy matrix, three platinum
complexes showed broad and featureless, yet weak, photoluminescence in the green region of the visible spectrum with excited-
state lifetimes on the order of a few microseconds. On the basis of joint experimental and computational findings and literature
on platinum complexes, such emission was assigned to a triplet-manifold metal−ligand-to-ligand charge transfer (3MLLCT)
transition.
luminescent complexes9 and may also be of interest for
INTRODUCTION
■
biomedical applications.5,9,11
N-heterocyclic carbene chemistry (NHC) has become a very
prolific field of research over the past 15 years.1,2 Due to their
unique steric and electronic properties, NHCs have emerged as
a powerful class of ligands in organometallic chemistry and
homogeneous catalysis.3 Interestingly, the applications of metal
NHC complexes in other fields remain much less studied,
although some advances have emerged these past few years.4
Very promising results have in particular been reported in
biology, such as anticancer and antimicrobial agents5 and
bioimaging,6 as well as in materials chemistry for the
preparation of liquid crystals,7 low-molecular-weight gelating
materials,8 and luminescent derivatives.9
Group 10 complexes containing N-heterocyclic carbene
ligands are well-established catalysts in a variety of trans-
formations, including cross-coupling, cycloisomerization, poly-
merization, and hydrosilylation reactions.10 Furthermore, group
10 transition metals bearing NHC-based ligands have shown
attractive properties for use in materials science as, for instance,
Over the past few years π-conjugated cyclometalating ligands
containing NHC moieties have increasingly been studied for
the preparation of transition-metal complexes exerting a sizable
spin−orbit coupling effect (SOC), such as those of Ir(III) and
Pt(II), suitable for the preparation of phosphorescent triplet
emitters.12 After seminal studies by Baldo, Thompson, and
Forrest over a decade ago,13 cyclometalated platinum(II)
species and numerous iridium(III) complexes were reported
to exhibit photoluminescence quantum yield (PLQY) in some
cases approaching 100%.14 In addition, the ability of
phosphorescent complexes to act as electro-active phosphors
in optoelectronics devices, such as organic light emitting diodes
(OLEDs) and light emitting electrochemical cells (LEECs) has
been widely demonstrated.12b−e,14d−g,15
Received: March 31, 2014
Published: August 20, 2014
© 2014 American Chemical Society
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Chart 1
In this area, NHC-based supporting ligands currently play a
pivotal role due to their attractive electronic properties such as
(i) the formation of strong M−CNHC coordination bonds, (ii)
strong σ-donation to the metal center, allowing the typically
quenching d−d metal-centered (MC) states to rise up in
energy and thus reduce the nonradiative deactivation pathways,
and (iii) high-energy π* orbitals mostly due to the weak π-
accepting ability, which allows the emissive excited state to lie
in higher energy regions, in particular in the blue to ultraviolet
(UV) portion of the electromagnetic spectrum.12c−e,16
ligand C. The photophysical properties of some of the platinum
complexes were also studied both in solution at room
temperature and in glassy matrix at 77 K.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of Ni, Pd,
and Pt Complexes. The benzimidazolium precursor 1 was
prepared according to our previous report.26 The azolium salt
precursor was thus treated with 1 equiv of MCl2 (M = Ni, Pd,
Pt) and an excess of potassium carbonate in pyridine at 100 °C
for 12 h (Scheme 1).27 Analysis of the crude product by H
1
The molecular rigidity of the emitter, of crucial importance
to access robust phosphorescent complexes suitable for
optoelectronics applications, disfavors nonradiative deactivation
channels by means of, for instance, geometrical distortions.17
One of the most successful ways to increase molecular rigidity
lies in the use of chelating ligands with higher denticity.12c−e,18
Improvements of photoluminescence quantum yield (PLQY),
(photo)stability, and color tuning in platinum(II)-based
Scheme 1. Synthesis of Group 10 Complexes 2
complexes bearing NHC-containing tridentate14e and tetra-
−h
dentate12c,14f,19 chromophoric ligands have been reported,
showing electronic transitions with admixed ligand-centered
(LC) π−π* and metal-to-ligand charge transfer (MLCT)
character.
It is worth noting that square-planar Pt(II) and Pd(II)
complexes typically exhibit a high tendency toward stacking
when in the solid phase or when embedded in matrices, thus
reducing their solubility and solution processability. In the case
of luminescent derivatives, such a tendency often results in a
loss of emission color purity and poorer PLQY, mostly due to
aggregation quenching phenomena and formation of excimers20
or lower-energy excited states with triplet metal−metal-to-
ligand charge transfer (3MMCT) character upon establishment
of closed-shell metallophilic interactions.21 Nevertheless,
enhancement of the photophysical properties upon aggregation
are sometimes observed.22 To overcome detrimental aggrega-
tion, bulky substituents such as tert-butyl and adamantyl groups
are typically introduced on the ligand backbone.15f,23
Seeking for robust NHC-incorporating pincer-type chelating
ligands, we developed a straightforward synthesis of the new
family of tridentate type bis-aryloxide-NHC ligands A (Chart
1), a ligand structure well suited for coordination to V(V),
Mn(III), and group 4 and 13 metals (Ti, Zr, Hf, Al).24 The
coordination of such structures to late-transition-metal centers
has been little explored, prompting us to investigate the
coordination chemistry of the tridentate bis-aryloxide-NHC
ligand with group 10 transition metals.25 It was envisioned that
the introduction of a benzimidazol-2-ylidene core into the
structure (B) might result in a π-conjugated system over the
three six-membered rings able to accommodate heavy-metal
centers. The resulting transition-metal complexes may display
interesting photophysical properties.
NMR spectroscopy confirmed the absence of any residual
azolium and phenol moieties. Purification of the complexes by
flash column chromatography afforded the corresponding
nickel, palladium, and platinum complexes (NHC)M(pyridine)
(2Ni, 2Pd, and 2Pt, respectively) in 66−79% yield. The proposed
formulations were confirmed by mass spectrometry and
elemental analysis. The formation of the carbene complex
was established by the presence of characteristic M−Ccarbene 13C
NMR resonances at δ 162.4, 165.3, and 153.6 ppm for the Ni,
Pd, and Pt complexes, respectively. The nickel complex was
found to be diamagnetic, in line with a square-planar
environment around the metal. Despite numerous attempts,
no suitable X-ray-quality crystals could be grown for species 2Ni,
2Pd, and 2Pt. Preliminary X-ray data collected for compound 2Pt
were of insufficient quality for a refinement of the structure.28
However, the atom connectivity could be unambiguously
established.
Alternatively, the reaction of proligand 1 with PtCl2 may be
conducted in cyclohexylamine (as the solvent) to access the
corresponding cyclohexylamine Pt complex 3Pt. Thus, the
reaction of benzimidazolium 1 with 1 equiv of PtCl2 at 100 °C
overnight quantitatively yielded the (NHC)Pt-
1
(cyclohexylamine) complex 3Pt, as deduced from H and 13C
NMR, mass spectrometry, and elemental analysis (Scheme 2).
The synthesis of the Pt−DMSO adduct was next attempted
following a similar strategy. The benzimidazolium salt cleanly
reacted with (DMSO)2PtCl2 in acetonitrile and in the presence
of NEt3 acting as a base to afford the NHC platinum complex
4Pt as the major product (50% yield, Scheme 3), whose
molecular structure was established through X-ray crystallog-
raphy studies (Figure 1). Unexpectedly, the chelating ligand is
Herein we report on the straightforward and high-yield
synthesis of robust NHC-group 10 complexes supported either
by ligand B or the novel and presently described imidazolidene
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Scheme 2. Synthesis of Platinum Complex 3Pt in
Cyclohexylamine as Solvent
Scheme 4. Synthesis of the Imidazolium Precursor 5
glyoxal in MeOH at room temperature followed by cyclization
with paraformaldehyde and HCl afforded the desired product
in 30% overall yield (not optimized).30,31
The imidazolium salt precursor 5 was then treated with 1
equiv of MCl2 (M = Ni, Pd, Pt) in pyridine at 100 °C for 12 h
to access the corresponding stable metal complexes 6 in 48−
83% yield (Scheme 5). The formation of the corresponding
Scheme 3. Synthesis of Platinum Complex 4Pt and
Conversion into Complex 3Pt
Scheme 5. Synthesis of Group 10 Complexes 6 and the
Phosphine Complex 7Pt
carbene complexes was confirmed by the presence of a carbene
signal at δ 146.8, 149.8, and 153.5 ppm for the Ni, Pd, and Pt
complexes, respectively, in the 13C NMR spectra. X-ray-quality
crystals of the nickel complex 6Ni were grown as dark red
prisms by diffusion of pentane into a dichloromethane solution
of 6Ni and allowed the determination of the molecular structure
of species 6Ni in the solid state. As shown in Figure 2, a square-
planar environment at Ni is observed in complex 6Ni, with an
O−Ni−O bite angle of 177.44(11)°. The Ni−Ccarbene bond
distance (1.794(3) Å) is comparable to those reported for other
structurally related NHC−Ni complexes.32 The {OCO}Ni
chelate is significantly distorted from planarity (|(1)−N(1)−
N(2)−O(2)| = 29.97(4)°). The Ni−Npyridine bond length
(1.953(3) Å) is consistent with a trans influence of the carbene
ligand.
Whereas pyridine exchange did not proceed with benzimi-
dazolylidene complexes, we found that the corresponding
imidazolylidene complexes were more susceptible to exchange
reactions. For example, addition of 1 equiv of triphenylphos-
phine to the platinum compound 6Pt afforded the correspond-
ing phosphine adduct 7Pt in 80% yield. X-ray-quality yellow
prismatic crystals were grown by diffusion of pentane into a
saturated solution of 7Pt in dichloromethane, and X-ray
diffraction studies allowed the structure determination of 7Pt.
As depicted in Figure 3, a distorted-square-planar environment
around the metal is observed in species 7Pt. The Pt−Ccarbene
distance (1.948(3) Å) lies within the range of related
structures.29 Akin to the Ni complex, the {OCO}Pt chelate
in the complex is significantly distorted from planarity (|O(1)−
N(1)−N(2)−O(2)| = 28.60(4)°). The Pt−P bond length
Figure 1. Molecular structure of the complex 4Pt. Selected bond
distances (Å) and angles (deg): C(1)−Pt(1), 1.950(9); O(1)−Pt(1),
2.016(6); Pt(1)−Cl(1), 2.350(3); S(1)−Pt(1), 2.201(2); N(2)···
H(2), 2.310(9); N(2)−C(1)−N(1), 107.5(7); N(2)−C(1)−Pt(1),
127.9(6); N(1)−C(1)−Pt(1), 124.1(6); C(1)−Pt(1)−O(1), 85.0(3);
C(1)−Pt(1)−S(1), 95.4(3); O(1)−Pt(1)−S(1), 174.14(17); C(1)−
Pt(1)−Cl(1), 171.8(3); N(2)−C(1)−Pt(1)−O(1), −138.3(8);
N(1)−C(1)−Pt(1)−O(1), 33.0(7); N(2)−C(1)−Pt(1)−S(1),
47.5(8).
κ2(C,O)-coordinated with one “dangling” phenol forming an
intramolecular hydrogen bond with a nitrogen atom of the N-
heterocyclic ring (N···H = 2.310(9) Å). As a result, the
dangling phenol points perpendicularly relative to the N-
heterocyclic ring. The geometry around the Pt metal is square
planar, with the chloride and DMSO ligands trans to the
carbene and the phenolate, respectively. The carbene−Pt bond
distance (1.950(9) Å) is similar to that in related complexes.29
The M−Ccarbene 13C NMR signal of 3Pt was observed at 141.3
ppm.
Interestingly, a solution of 4Pt in cyclohexylamine/dichloro-
methane as solvent led to the quantitative formation of the
corresponding tridentate (OCO)Pt chelate complex 3Pt, as
deduced by NMR spectroscopy.
For comparison purposes, the coordination chemistry of the
corresponding imidazolylidene type bis-aryloxide-NHC ligand
C (Chart 1) was also studied. The azolium precursor 5 was
easily prepared from the corresponding aminophenol (Scheme
4). Thus, the reaction of 2-amino-4,6-di-tert-butylphenol5 with
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Figure 2. Molecular structure of the complex 6Ni (top) and view along
the C−Ni axis (bottom; t-Bu groups and pyridine are omitted for
clarity). Selected bond distances (Å) and angles (deg): C(1)−Ni(1),
1.794(3); N(3)−Ni(1), 1.953(3); O(1)−Ni(1), 1.847(2); O(2)−
Ni(1), 1.835(2); N(1)−C(1)−N(2), 105.3(3); C(1)−Ni(1)−O(2),
91.48(14); C(1)−Ni(1)−O(1), 90.71(14); O(2)−Ni(1)−O(1),
177.44(11); C(1)−Ni(1)−N(3), 172.86(15); N(1)−C(1)−Ni(1)−
O(2), −152.4(3); N(2)−C(1)−Ni(1)−O(2), 16.4(3); N(1)−C(1)−
Ni(1)−O(1), 28.9(3); N(2)−C(1)−Ni(1)−O(1), −162.3(3);
C(32)−N(3)−Ni(1)−O(1), 46.65(3); N(1)−C(2)−C(3)−N(2),
−0.04(3).
Figure 3. Molecular structure of the complex 7Pt (top) and view along
the C−Pt axis (bottom; t-Bu groups and Ph3P are omitted for clarity).
Selected bond distances (Å) and angles (deg): C(1)−Pt(1), 1.948(3);
O(1)−Pt(1), 2.006(2); O(2)−Pt(1), 2.017(2); P(1)−Pt(1),
2.3272(7); N(2)−C(1)−Pt(1), 126.3(2); N(1)−C(1)−Pt(1),
125.6(2); C(1)−Pt(1)−O(1), 89.27(10); C(1)−Pt(1)−O(2),
88.89(10); O(1)−Pt(1)−O(2), 175.73(8); C(1)−Pt(1)−P(1),
170.80(9); O(1)−Pt(1)−P(1), 85.96(6); O(2)−Pt(1)−P(1),
96.37(6); N(1)−C(1)−Pt(1)−O(1), 28.17(3); P(1)−O(1)−Pt(1)−
C(1), 172.11(3).
(2.3272(7) Å) is consistent with a trans influence of the
carbene ligand.
Ground-State Geometry and Frontier Orbitals. The
minor extent of the SOC effect exerted by first-row transition
metals and the presence of lower-lying energy MC states make
Ni(II) complexes much less appealing for photophysical and
theoretical studies; thus, such characterization has not been
considered for complex 2Ni. On the other hand, the molecular
structures of the platinum(II) and palladium(II) derivatives
distances, respectively; see Figure S1 in the Supporting
Information for the corresponding atom labeling). Likewise,
M−N1 and M−O bond distances are in the ranges 2.134−2.151
and 1.992−2.018 Å (experimental 2.006 Å), respectively. The
M−P1 bond distance for 7Pt was found to be 2.369 Å
(experimental 2.327 Å). Bond angle values are also in good
agreement with crystallographic data and data for related
cyclometalated platinum(II) and palladium(II) complexes
previously reported. In particular, the C1−M−O and N1−M−
O (P1−M−O) angles are in the ranges 91.2−93.0 and 86.6−
88.0° (86.1−91.4°), respectively. These values are very close to
the ideal chelating arrangement for square-planar M(II)
complexes (90°). Also, they are in good agreement with values
for related tetradentate complexes bearing bis[phenolate(N-
heterocyclic carbene)] recently reported by Che and co-
workers12c,19a and Strassner and co-workers,19b confirming the
suitability of the presently used computational model for
describing the geometrical parameters of the complexes. In
contrast with the reported tetradentate complexes,12c,19 it is
worth noting that in the case of 2Pd−Pt and 3Pt the
benzimidazole ring is highly distorted in order to allow
coordination of the bis-aryloxide moieties to the metal center.
Two views of the optimized geometry for complex 2Pt are
shown in Figure 4. Regarding derivatives 2Pd−Pt, 3Pt, and 6Pd−Pt
at their S0 optimized geometry, the ring of the ancillary pyridine
ligand lies on the molecule plane as defined by the Pt−C1, Pt−
O1, Pt−O2, and Pt−N1 coordination motifs (out-of-plane O−
M−N1−C6 dihedral angles lying between 0.9 and 2.8°).
Notably, this is in spite of the presence of bulky tert-butyl
2Pd−Pt, 3Pt, 6Pd−Pt, and 7Pt were optimized at their electronic
ground state (S0) by means of density functional theory (DFT)
at the PBE0/(6-31G(d,p)+SDD) level, where the tert-butyl
groups were replaced by methyl groups for shorter computa-
tional time. The Perdew−Burke−Erzenrhof parameter-free
hybrid functional33 (PBE0)34 is well-known to properly
describe the electronic and optical properties of phosphor-
escent platinum(II) complexes bearing chelating chromophoric
ligands.15f,18g,23a The main computed geometrical parameters
are given in Table S1 and the geometry is shown in Figure S1
(Supporting Information). The calculated S0 structures are in
good agreement with the geometrical parameters experimen-
tally gathered from single-crystal X-ray diffractometric analyses.
For all complexes, the computed geometries display a
distorted-square-planar coordination geometry around the
metal atom, with either Cs (for complexes 2Pd−Pt, 3Pt, and
6
Pd−Pt) or C1 (for complex 7Pt) point group symmetry. The Cs-
symmetric structures exhibit a σ mirror plane perpendicular to
the O−CNHC−O plane, bisecting the NHC moiety and
containing the metal center. In particular, the DFT-optimized
structures nicely reproduce the M−C1 distances, (1.915−1.962
Å vs 1.948−1.950 Å for the computed and experimental
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character for pyridine- and triphenylphosphine-containing
derivatives, respectively. This change in the nature of the
LUMO is ascribed to the presence of low-lying π-antibonding
orbitals of the pyridine with respect to the triphenylphosphine
coordinating ligand, which in turn stabilizes the lowest-lying
virtual orbital by 0.59 eV (6Pt vs 7Pt). Similarly, going from a
cyclohexylamine ligand to pyridine in complex 3Pt yields a
LUMO with d(Pt)π*(benzimidazole) character. Overall, the
LUMO lies at −1.274, −1.395, −0.810, −1.161, and −1.281 eV
for 2Pd, 2Pt, 3Pt, 6Pd, and 6Pt, respectively. When the same
coordination sphere is retained and the metal ion is changed, it
can be noted that platinum complexes show a smaller
HOMO−LUMO gap energy: 3.598 vs 3.372 eV and 3.673 vs
3.438 eV for 2Pd vs 2Pt and 6Pd vs 6Pt, respectively, which is due
to the concomitant presence of the HOMO lying higher in
energy and the more stabilized LUMO in Pt(II) vs Pd(II)
complexes (see Table S2 in the Supporting Information).
Electronic Spectroscopy and TD-DFT. Figure 6 displays
the electronic absorption spectra of complexes 2Pt and 6Pt in
Figure 4. Two views of the S0 optimized geometry of complex 2Pt.
Hydrogen atoms are omitted for clarity.
groups on the phenolate moieties at the ortho positions. Such a
geometrical arrangement thus allows electronic communication
between the d metal atom orbitals and the π electronic cloud of
the pyridine ligand.
Figure 5 depicts the isodensity surface plots for the most
relevant Kohn−Sham molecular orbitals (MOs) closer to the
frontier region for the investigated complexes, and Figure S2
(Supporting Information) features a more complete list. In
Table S2 (Supporting Information) are given the energies of
the corresponding orbitals from HOMO-8 to LUMO+3, along
with the HOMO−LUMO energy gap. The HOMO is an
antibonding combination of the π orbitals of the two phenolate
moieties and the dxz orbitals of the metal atom with minor
contribution of the N atom of the NHC moiety. The HOMO
lies at −4.872, −4.768, −4.838, and −4.718 eV for 2Pd, 2Pt, 6Pd,
and 6Pt, respectively. In particular, when pairs of complexes
with the same ligands are considered, namely 2Pd/2Pt and 6Pd/
6Pt, palladium(II) derivatives show a HOMO stabilized by ca.
110 meV with respect to its platinum(II) counterparts. This
finding can be ascribed to the higher oxidation potential of
Pd(II) with respect to Pt(II).35 As expected, the presence of a
stronger donating ancillary ligand, such as triphenylphosphine,
induces stabilization (of about 50−60 meV) with respect to the
corresponding pyridine-containing complex: i.e., 6Pt vs 7Pt. The
more stable filled orbitals HOMO-1 and HOMO-2 involve
other π orbitals of the two phenolate moieties and the CNHC
(for HOMO-1) and the π orbitals of the two phenolate and
NHC moiety (for HOMO-2), with only minor contribution of
the d metal orbitals.
Figure 6. Comparison between the experimental absorption (solid
line) spectrum in dichloromethane of complexes 2Pt (black trace) and
6Pt (red trace) and computed vertical transitions (vertical bars) with
the corresponding oscillator strengths calculated in dichloromethane.
dichloromethane at a concentration of 5 × 10−5 M, and Figure
S3 (Supporting Information) shows the corresponding
absorption spectra for 2Pd, 3Pt, and 7Pt. The photophysical
data are given in Table 1. In order to shed light on the
properties of the electronic transitions involved in the optical
absorption processes, time-dependent density functional theory
On the other hand, the lowest unoccupied MOs LUMO and
LUMO+1 show a π* (pyridine) and π* (phenyl(PPh3))
Figure 5. Isodensity surface plots of the molecular orbitals closer to the frontier region and mainly involved in the electronic transitions for the
complexes 2Pd, 6Pd, 2Pt, 3Pt, 6Pt, and 7Pt in the gas phase at their S0 optimized geometries (isodensity value 0.035 e bohr−3). Hydrogen atoms are
omitted for clarity.
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0.408) and 284 nm (f = 0.206) and mainly described as metal-
perturbed 1LC/1LLCT d(Pt)π(phenoxy)π(NHC) → π*-
(phenoxy) π*(pyridine) and 1LLCT d(Pt)π(phenoxy)π(NHC)
→ π*(pyridine) excitation processes for 2Pt and 6Pt,
respectively. As also shown in Figure 6, the computed
transitions are in good agreement with the overall experimental
absorption spectra. Furthermore, such assignments are in line
with those in analogous platinum complexes bearing bis-
[phenolate(N-heterocyclic carbene)] ligands.19a
Table 1. Electronic Absorption in Dilute CH2Cl2 Solution at
Room Temperature and Photoluminescence in CH2Cl2/
MeOH 1/3 Glassy Matrix at 77 K Characteristics for
Complexes 2Pt, 2Pd, 3Pt, 6Pt, and 7Pt
77 K
room temp λabs, nm (ε, 104
a
compound
M−1 cm−1
)
λem, nm
τ, μs
2Pt
260 sh (2.71), 300 (1.71),
372 (0.26), 390 (0.23)
507
1.1 (43%),
8.1 (57%)
1
Upon excitation in the MLCT band at wavelengths in the
2Pd
3Pt
6Pt
7Pt
267 sh (2.39), 290 (2.12),
358 (0.43)
b
b
range 350−450 nm, dilute samples (concentration of 5.0 ×
10−5 M) in either CH2Cl2 or CH3CN and spin-coated
poly(methyl methacrylate) (PMMA) thin films at doping
concentrations as high as 10 wt % of the investigated complexes
did not show any detectable emission at room temperature.
However, when the temperature was lowered to 77 K in a
CH2Cl2/MeOH (1/3) glassy matrix and excitation carried out
at 375 nm, complexes 2Pt, 3Pt, and 6Pt displayed a broad and
featureless, yet weak, emission band in the bluish-green region
of the visible spectrum. The corresponding photoluminescence
spectra are displayed in Figure 7. In particular, complexes 2Pt,
263 sh (2.00), 298 (1.34),
374 (0.25), 413 (0.20)
519
488
b
1.4 (75%),
9.3 (25%)
258 sh (2.10), 289 (0.95),
337 (0.34), 387 (0.27)
3.4 (26%),
12.8 (74%)
275 sh (1.89), 365 (0.42),
b
383 (0.40)
a
b
sh denotes a shoulder. Not emissive.
(TD-DFT) was employed for the investigated complexes in
both vacuum and dichloromethane as solvent by means of the
IEFPCM solvation model. The computed vertical transitions
were calculated at the S0-optimized geometry and described in
terms of one-electron excitations of molecular orbitals of the
corresponding S0 geometry. For the investigated complexes, the
most relevant computed transitions involved in the vertical
excitation processes along with their energy, character, and
oscillator strengths are given in Tables S3 and S4 (Supporting
Information) for singlet excitations and Tables S5 and S6
(Supporting Information) for triplet transitions, in both
vacuum and CH2Cl2. For complexes 2Pt and 6Pt the computed
transitions in CH2Cl2 are reported for comparison in Figure 6.
Further computational details are available in the Experimental
Section.
For complexes 2Pt and 6Pt, the experimental electronic
absorption spectra (CH2Cl2) contain a featureless and weak
band (ε ≈ (2−3) × 103 M−1 cm−1) between 350 and 450 nm
that can be ascribed to the lowest-lying singlet-manifold metal-
to-ligand charge transfer (1MLCT) transition. On the basis of
TD-DFT computations in CH2Cl2, it is possible to assign this
weak band to the overlap of (i) an S0 → S1 transition (HOMO
→ LUMO, f = 0.054 and 0.033 for 2Pt and 6Pt, respectively)
which essentially involves the π orbitals located on the phenoxy
moiety, the platinum d orbital, and the π* orbitals of pyridine
(d(Pt) phenoxy → π*(pyridine)) and (ii) transitions involving
virtual orbitals mainly located on the NHC, i.e. d(Pt)π-
(phenoxy) → π*(NHC), and the combination of phenoxy and
NHC, i.e. d(Pt)π(phenoxy) → π*(phenoxy)π*(NHC), for 2Pt
and 6Pt, respectively. The former can indeed be attributed to
the HOMO → LUMO+1 (375 nm, f = 0.025) vertical
transition and the latter to the HOMO → LUMO+2 (350 nm, f
= 0.066) one-electron excitation process. The calculated lowest-
energy values nicely correspond to the onset in the
experimental absorption spectra that extends toward the
lower energy side, confirming the suitability of the employed
TD-DFT approach for the computation of such optically
populated Franck−Condon excitations. At higher energies (λ
<350), much more intense bands (ε = (0.5−3) × 104 M−1
cm−1) can be encountered at around 300 nm and are better
described as an admixture of spin-allowed ligand-to-ligand
charge transfer (1LLCT) and ligand-centered (1LC) transitions
with small to negligible participation of the metal orbitals. In
particular, very intense transitions are computed at 290 (f =
Figure 7. Emission spectra of complexes 2Pt (black trace), 3Pt (blue
trace), and 6Pt (red trace) in CH2Cl2/MeOH 1/3 at 77 K glassy matrix
at a concentration of 1.25 × 10−5 M. The samples were excited in the
MLCT absorption band at λexc 375 nm.
3Pt, and 6Pt show emission maxima centered at 507, 519, and
488 nm, respectively. This finding together with the absence of
photoluminescence upon only matrix rigidification (i.e., in
PMMA thin films) also support the presence of a low-lying
thermally populated quenching excited state close to the
emitting state. Furthermore, the presence of bulky and prone-
to-rotate tert-butyl groups on the phenate moieties might also
be responsible for the fast and efficient radiationless
deactivation of the photoexcited molecules at room temper-
ature.36 As expected for a lower temperature, emission decay
measured under frozen conditions are long and fall in the time
scale of a few microseconds, as typical of phosphorescent
platinum(II) complexes showing charge transfer transitions
with sizable metal participation, being τ1 = 8.1 (57%) and τ2 =
1.1 (43%), τ1 = 1.4 (75%) and τ2 = 9.3 (25%), and τ1 = 12.8
(74%) and τ2 = 3.4 (26%) μs for complexes 2Pt, 3Pt, and 6Pt,
respectively. On the basis of these findings, a ligand-centered
(LC) transition responsible for this emission can be excluded.37
Also, transition originating from a spin-forbidden distorted
ligand field (LF) excited state can be ruled out, due to the fact
that such a transition in platinum complexes generally shows a
Gaussian-shaped broad emission profile with Stokes shift larger
than those observed here.38 Emission originating from excimer
and aggregates can also be ruled out, due to the presence of
bulky tert-butyl substituents. Thus, the radiative process
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responsible for the emission can be ascribed to a triplet-
manifold transition with sizable involvement of the d(Pt)
orbitals and the π(phenoxy) as well as the π* orbitals of either
the pyridine or the NHC moiety for 2Pt/6Pt and 3Pt,
respectively. The nature of such a transition can be tentatively
described as metal−ligand-to-ligand charge transfer
(3MLLCT).
likely due to the distorted-square-planar structure of the
(OCO)M chelate and the free rotation of the monodentate
ancillary ligand. As a result, the excited complexes may readily
undergo a radiationless deactivation after photoexcitation.
Nevertheless, when the temperature was lowered to 77 K,
pyridine- and cyclohexyl-containing platinum derivatives
displayed an emission band in the green region of the visible
spectrum attributable to a long-lived triplet-manifold excited
state with MLLCT character.
In order to simulate the phosphorescence spectra and shed
light on the electronic transition involved in such an emission
process, the vertical (Franck−Condon) excitation energies for
the optimized S0 state to the lowest lying triplet manifold (T1)
were computed by means of TD-DFT at the same level of
theory as for the ground state and the data for the S0 → Tn (n =
1−3) are given in Tables S5 and S6 (Supporting Information).
Such a method has been previously reported to successfully
reproduce phosphorescence transitions in platinum complex-
es.15f,18g,23a The computed transitions (S0 → T1) in dichloro-
methane as solvent medium lie at 2.59, 2.65, and 2.79 eV for
2Pt, 3Pt, and 6Pt, respectively, and agree with the emission
maxima recorded in 77 K glassy matrix, the experimental values
being 2.45, 2.39, and 2.54 eV for 2Pt, 3Pt, and 6Pt, respectively.39
For all three complexes, such transitions can be mainly
described as a one-electron excitation process involving a
HOMO → LUMO transition: i.e., d(Pt)π(phenoxy) →
π*(pyridine) for 2Pt and 6Pt derivatives and d(Pt)π(phenoxy)
→ d*(Pt)π*(NHC) for 3Pt. These findings are in line with the
assignment given on the basis of the experimental photo-
physical data as well as with closely related complexes.12c On
going from complex 2Pt to 6Pt, the 200 meV computed
hypsochromic shift is mostly ascribed to the lower-lying LUMO
in 6Pt vs 2Pt. Such an assignment also rationalizes the absence of
room-temperature luminescence of the investigated complexes.
In all complexes, the (OCO)M chelate indeed adopts a highly
distorted geometry around the metal center. In addition, in the
case of derivatives 2Pt, 2Pd, and 3Pt, the distortion is such that
the benzimidazole moiety is not planar, giving rise to a
significantly bent geometry (see above). Moreover, for species
2Pt and 6Pt, the lowest-lying virtual orbital is located on a
monodentate ancillary ligand relatively free to rotate. In all
cases, upon photoexcitation the geometrical arrangement is
such that the excited complexes are prone to easily deactivate
via radiationless deactivation channels such as vibrational
motion of the ancillary ligands as well as ligand decoordina-
tion.40 However, when the temperature is lowered to 77 K,
such nonradiative channels are partially suppressed, allowing
radiative transitions to occur.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed under an
inert atmosphere of argon or nitrogen using standard Schlenk line
techniques. Solvents were purified and degassed by standard
procedures. All other reagents were used without further purification.
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker AVANCE 300 spectrometer using the residual solvent
peak as reference (CDCl3: δ(H) 7.26 ppm; δ(C) 77.16 ppm) at 298 K.
MS ESI analyses were made on a microTOF instrument from Bruker
Daltonics. Crystal data were collected at 173 K using Mo Kα graphite-
monochromated (λ = 0.71073 Å) radiation on a Nonius KappaCCD
diffractometer. The structures were solved using direct methods with
SHELXS97. Non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were generated according to stereochemistry and
refined using a riding model in SHELXL97 (except for H(2) in
complex 4Pt, which was located in the Fourier map).
Photophysical Characterization. Steady-state emission spectra
at both room temperature in organic solvent and 77 K in 2-MeTHF
glassy matrix were recorded on a HORIBA Jobin-Yvon IBH FL-322
Fluorolog 3 spectrometer equipped with a 450 W xenon arc lamp as
the excitation source, double-grating excitation and emission
monochromators (2.1 nm mm−1 of dispersion; 1200 grooves
mm−1), and a TBX-04 single-photon-counting device as the detector.
Emission and excitation spectra were corrected for source intensity
(lamp and grating) and emission spectral response (detector and
grating) by standard correction curves. Time-resolved measurements
were performed using the multichannel scaling electronics (MCS)
option on the Fluorolog 3, where a pulsed xenon lamp used with a
repetition rate of 30 Hz was used to excite the samples. The excitation
sources were mounted directly on the sample chamber at 90° to a
double-grating emission monochromator (2.1 nm mm−1 of dispersion;
1200 grooves mm−1) and collected by a TBX-04 single-photon-
counting detector. Signals were collected using an IBH Data Station
Hub photon-counting module, and data analysis was performed using
the commercially available DAS6 software (HORIBA Jobin Yvon
IBH). The quality of the fit was assessed by minimizing the reduced χ2
function and by visual inspection of the weighted residuals. For
multiexponential decays, the intensity, namely I(t), has been assumed
to decay as the sum of individual single-exponential decays (eq 1):
n
⎛
⎞
⎟
⎠
t
I(t) =
α exp −
⎜
∑
i
SUMMARY AND CONCLUSION
τ
⎝
■
i
(1)
i=1
In summary, we have reported the direct and good-yielding
synthesis and characterization of a series of group 10 complexes
(Ni, Pd, Pt) bearing tridentate NHC ligands. Introduction of a
benzimidazolylidene or imidazolylidene bis-aryloxide-NHC
allowed their isolation and characterization in good yields.
These compounds were investigated by means of DFT and
TD-DFT computational methods, and the computed and
experimental data match well. Frontier orbital analysis
performed for all Pd and Pt complexes agrees with a HOMO
centered on the metal and the aryloxide part of the ligand, while
the LUMO is mainly located on the ancillary monodentate
ligand, except for complex 3Pt. No luminescence was observed
for these complexes at room temperature (in CH2Cl2 or
where ti values are the decay times and αi values are the amplitudes of
the components at t = 0. In the tables, the percentages for the pre-
exponential factors, αi, are given upon normalization. For multi-
exponential decays, radiative and nonradiative rate constants were
calculated with respect to the longer component. All solvents were
spectrometric grade.
Computational Investigation. Ground-state (S0) geometries
were optimized by means of density functional theory (DFT),
employing the Perdew−Burke−Erzenrhof parameter-free hybrid
functional33 PBE0, called PBE1PBE in Gaussian. The standard valence
double-ζ polarized basis set 6-31G(d,p)41 was used for C, H, N, and O
for optimization. For Pt, the Stuttgart−Dresden (SDD) effective core
potential was employed along with the corresponding valence triple-ζ
basis set. The nature of all the stationary points was checked by
computing vibrational frequencies, and all of the species were found to
be true potential energy minima, as no imaginary frequencies were
1
CH3CN) upon excitation in the MLCT band at wavelengths
in the region 350−450 nm. The absence of luminescence is
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obtained. In order to simulate the absorption electronic spectrum
down to about 250 nm, for all complexes the 30 lowest singlet (S0 →
Sn, n = 1−30) as well as the 3 lowest triplet excitation energies (S0 →
Tn, n = 1−3) were computed on the fully optimized geometry at the S0
state by means of time-dependent density functional theory
calculations (TD-DFT), at the same level of accuracy as for the
ground state.42 TD-DFT energy calculations were performed in both
vacuum and dichloromethane as the solvent. Solvation effects were
taken into account by means of the nonequilibrium IEFPCM model.43
All calculations were performed with the Gaussian09W program
package.44
Experimental Procedures and Product Characterization.
Benzimidazolium 1. This compound was synthesized as reported in
the literature.26 1H NMR (CDCl3, 300 MHz, 20 °C): δ 1.35 (s, 18H,
C(CH3)3), 1.49 (s, 18H, C(CH3)3), 7.17 (d, J = 2.3 Hz, 2H, CHAr),
7.57 (d, J = 2.3 Hz, 2H, CHAr), 7.59−7.63 (m, 2H, CHAr), 7.65−7.69
(m, 2H, CHAr), 9.05 (s, 1H, CHimid), 9.29 (bs, 2H, OH). 13C NMR
(CDCl3, 75 MHz, 20 °C): δ 29.6 (6C, C(CH3)3), 31.4 (6C,
C(CH3)3), 34.5 (2C, C(CH3)3), 35.8 (2C, C(CH3)3), 114.5 (2C,
CHAr), 120.3 (2C, CHAr), 120.9 (2C, CAr), 126.8 (2C, CHAr), 127.7
(2C, CHAr), 132.3 (2C, CAr), 141.4 (1C, Cimid), 141.8 (2C, CAr), 143.0
(2C, CAr), 149.2 (2C, CAr). MS (positive ESI): [M − Cl]+ calculated
for C35H47N2O2 527.36, found 527.36.
General Procedure for the Synthesis of the NHC Complexes 2, 3,
and 6. A mixture of azolium salt (1 equiv), metal dichloride (1 equiv),
and potassium carbonate (30 equiv) was suspended in pyridine or
cyclohexylamine (1 mL/0.02 mmol of MCl2). The mixture was
sonicated for 10 min and stirred at 100 °C over 12 h under an argon
atmosphere. The resulting suspension was concentrated under reduced
pressure and then dissolved in dichloromethane; this solution was
filtered through a Celite plug and concentrated under reduced
pressure. The desired complex was purified by silica gel chromatog-
raphy (dichloromethane/cyclohexane 1/1).
CHbenzimidazole), 7.54 (t, J = 7.6 Hz, 2H, CHPyr), 7.76 (d, J = 1.2 Hz, 2H,
CHphenoxy), 7.95 (tt, J = 7.7 and 1.6 Hz, 1H, CHPyr), 8.13−8.17 (m, 2H,
CHar), 8.99 (d, J = 4.8 Hz, 2H, NCHPyr). 13C NMR (CDCl3, 75 MHz,
20 °C): δ 29.0 (6C, C(CH3)3), 31.7 (6C, C(CH3)3), 34.3 (2C,
C(CH3)3), 35.6 (2C, C(CH3)3), 114.2 (2C, CHAr), 116.1 (2C, CHAr),
121.1 (2C, CHAr), 123.5 (2C, CHAr), 124.6 (2C, CHPyr), 128.0 (2C,
CAr), 132.9 (2C, CAr), 136.6 (1C, CHAr), 138.6 (2C, CHPyr), 140.2
(2C, CAr), 151.0 (2C, CHPyr), 153.6 (1C, Ccarbene), 157.4 (2C, CAr). MS
(positive ESI): [M − e−]+ calculated for C40H49N3O2Pt 798.35, found
798.35. Anal. Calcd for C40H49N3O2Pt: C, 60.13; H, 6.18; N, 5.26.
Found: C, 60.01; H, 6.12; N, 4.98.
Synthesis of 3Pt. Following the general procedure, 1,3-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)-1H-benzo[d]imidazol-3-ium chloride
(106 mg, 0.19 mmol), PtCl2 (30 mg, 0.19 mmol), and K2CO3 (779
mg, 5.64 mmol) afforded complex 3Pt (153 mg, quantitative, yellow
1
solid). H NMR (CDCl3, 300 MHz, 20 °C): δ 1.11−1.37 (m, 26H,
C(CH3)3 + CH2 CHA), 1.49 (s, 18H, C(CH3)3), 1.66−1.70 (m, 2H,
CH2 CHA), 1.80−1.84 (m, 2H, CH2 CHA), 2.42−2.45 (m, 2H CH2 CHA),
3.20−3.22 (m, 2H CH2 CHA), 3.32−3.42 (m, 1H CHCHA), 7.18 (d, J =
2 Hz, 2H, CHphenoxy), 7.38−7.41 (m, 2H, CHbenzimidazole), 7.71 (d, J = 2
Hz, 2H, CHphenoxy), 8.08−8.11 (m, 2H, CHbenzimidazole). 13C NMR
(CDCl3, 75 MHz, 20 °C): δ 25.0 (2C, CH2 CHA), 25.3 (2C, CH2 CHA),
29.1 (6C, C(CH3)3), 31.8 (6C, C(CH3)3), 34.3 (2C, C(CH3)3), 35.6
(2C, CH2 CHA), 35.8 (2C, C(CH3)3), 53.8 (1C, CHCHA), 114.2 (2C,
CHAr), 116.2 (2C, CHAr), 121.0 (2C, CHAr), 123.4 (2C, CHAr), 127.4
(2C, CAr), 133.0 (2C, CAr), 136.3 (2C, CAr), 139.7 (2C, CAr), 152.7
(1C, Ccarbene), 155.9 (2C, CAr). MS (positive-ESI): [M − e−]+
calculated for C41H57N3O2Pt 818.41, found 818.41. Anal. Calcd for
C41H57N3O2Pt: C, 60.13; H, 7.02; N, 5.13. Found: C, 59.78; H, 6.80;
N, 4.94.
Synthesis of 4Pt. 1,3-Bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1H-
benzo[d]imidazol-3-ium chloride (56 mg, 0.1 mmol), PtCl2(DMSO)2
(42 mg, 0.1 mmol), and NEt3 (63 μL, 0.45 mmol) were stirred in
CH3CN under an inert atmosphere at 75 °C over 12 h. The volatiles
were removed under vacuum, and the residue was purified on silica
(DCM/cyclohexane 8/2). A 40 mg portion of 4Pt was obtained (50%,
pale yellow solid). 1H NMR (CDCl3, 300 MHz, 20 °C): δ 1.43 (s, 9H,
C(CH3)3), 1.45 (s, 9H C(CH3)3), 1.52 (s, 9H C(CH3)3), 1.58 (s, 9H,
C(CH3)3), 2.28 (s, 3H, CH3 DMSO), 3.34 (s, 3H, CH3 DMSO), 7.16−
7.19 (m, 1H, CHAr), 7.31−7.44 (m, 3H, CHAr), 7.59 (d, J = 2.3 Hz,
1H, CHAr), 7.64 (d, J = 2.3 Hz, 1H, CHAr), 7.59 (d, J = 2.5 Hz, 1H,
CHAr), 7.92 (s, 1H, CHAr). 13C NMR (CDCl3, 75 MHz, 20 °C): δ
29.9 (3C, C(CH3)3), 30.2 (3C, C(CH3)3), 31.6 (3C, C(CH3)3), 31.8
(3C, C(CH3)3), 34.4 (1C, C(CH3)3), 34.9 (1C, C(CH3)3), 35.5 (1C,
C(CH3)3), 35.7 (1C, C(CH3)3), 42.6 (1C, CH3), 46.9 (1C, CH3),
112.9 (1C, CHAr), 113.3 (1C, CHAr), 116.6 (1C, CHAr), 123.2 (1C,
CHAr), 124.8 (1C, CHAr), 125.0 (1C, CHAr), 125.2 (1C, CHAr), 125.3
(1C, CHAr), 128.2 (1C, CHAr), 128.5 (1C, CHAr), 131.2 (1C, CHAr),
135.8 (1C, CHAr), 138.0 (1C, CHAr), 141.3 (1C, CHAr), 141.6 (1C,
CHAr), 145.2 (1C, CHAr), 148.8 (1C, CHAr), 155.3 (1C, CHAr). MS
(positive ESI): [M + H]+ calculated for C37H52ClN2O3PtS 834.30,
found 834.30. Anal. Calcd for C37H51ClN2O3PtS: C, 53.26; H, 6.16; N,
3.36. Found: C, 52.82; H, 5.88; N, 3.09.
Synthesis of 2Ni. Following the general procedure, 1,3-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)-1H-benzo[d]imidazol-3-ium chloride (61
mg, 0.11 mmol), NiCl2 (14 mg, 0.11 mmol), and K2CO3 (450 mg, 3.3
1
mmol) afforded complex 2Ni (57 mg, 79%, green solid). H NMR
(CDCl3, 300 MHz, 20 °C): δ 1.05 (s, 18H, C(CH3)3), 1.38 (s, 18H,
C(CH3)3), 7.07 (d, J = 2.5 Hz, 2H, CHphenoxy), 7.38−7.44 (m, 4H,
CHAr), 7.76−7.80 (m, 3H, CHAr), 8.14−8.17 (m, 2H, CHbenzimidazole),
8.94 (d, J = 5.0 Hz, 2H, 2 NCHpyr). 13C NMR (CDCl3, 75 MHz, 20
°C): δ 29.1 (6C, C(CH3)3), 31.7 (6C, C(CH3)3), 34.3 (2C, C(CH3)3),
35.1 (2C, C(CH3)3), 113.7 (2C, CHAr), 114.6 (2C, CHAr), 120.7 (2C,
CHAr), 123.3 (2C, CHAr), 123.6 (2C, CHAr), 127.6, 132.8, 135.3,
137.7, 140.4, 150.3, 154.1, 162.4 (1C, Ccarbene). MS (positive ESI): [M
+ H]+ calculated for C40H50N3NiO2 662.33, found 662.32. Anal. Calcd
for C40H49N3NiO2: C, 72.51; H, 7.45; N, 6.34. Found: C, 72.69; H,
7.75; N, 5.94.
Synthesis of 2Pd. Following the general procedure, 1,3-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)-1H-benzo[d]imidazol-3-ium chloride (50
mg, 0.089 mmol), PdCl2 (16 mg, 0.089 mmol), and K2CO3 (368 mg,
2.66 mmol) afforded complex 2Pd (63 mg, 66%, yellow-brown solid).
1H NMR (CDCl3, 300 MHz, 20 °C): δ 1.31 (s, 18H, C(CH3)3), 1.39
(s, 18H, C(CH3)3), 7.20 (d, J = 2.4 Hz, 2H, CHar), 7.40−7.51 (m, 4H,
CHar), 7.73 (d, J = 2.4 Hz, 2H, CHar), 7.88 (tt, J = 7.7 and 1.6 Hz, 1H,
Synthesis of Imidazolium 5. 2-Amino-4,6-di-tert-butylphenol
(1.580 g, 7.14 mmol), glyoxal (518 mg, 3.57 mmol), and formic
acid (4 drops) were stirred at room temperature over 12 h. The
corresponding diimine was filtered off, washed with cold MeOH, and
used without any further purification and characterization. The
resulting yellow solid was then dissolved in AcOEt, and
paraformaldehyde (114 mg, 3.79 mmol) and 4 N HCl/dioxane
(1.166 mL, 4.67 mmol) were introduced to the reaction mixture. After
2 days of stirring the volatiles were removed and the residue was
purified on silica using DCM/MeOH (from 1/0 to 95/5). A 526 mg
CHPyr), 8.15−8.19 (m, 2H, CHar), 8.89 (d, J = 4.8 Hz, 2H, CHPyr). 13
C
NMR (CDCl3, 75 MHz, 20 °C): δ 29.5 (6C, C(CH3)3), 31.8 (6C,
C(CH3)3), 34.3 (2C, C(CH3)3), 35.6 (2C, C(CH3)3), 114.1 (2C, CAr),
116.2 (2C, CAr), 121.5 (2C, CAr), 123.7 (2C, CAr), 124.2 (2C, CAr),
128.5 (2C, CAr), 133.2 (2C, CAr), 135.7 (2C, CAr), 138.4 (2C, CAr),
140.5 (2C, CAr), 150.6 (2C, CAr), 157.1 (2C, CAr), 165.3 (1C, Ccarbene).
MS (positive ESI): [M − e−]+ calculated for C40H49N3O2Pd 709.29,
found 709.29. Anal. Calcd for C40H49N3O2Pd: C, 67.64; H, 6.95; N,
5.92. Found: C, 67.22; H, 6.66; N, 5.64.
1
portion of 5 was obtained (30%, not optimized). H NMR (CDCl3,
Synthesis of 2Pt. Following the general procedure, 1,3-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)-1H-benzo[d]imidazol-3-ium chloride (50
mg, 0.089 mmol), PtCl2 (24 mg, 0.089 mmol), and K2CO3 (368 mg,
2.66 mmol) afforded complex 2Pt (53 mg, 75%, yellow solid). 1H
NMR (CDCl3, 300 MHz, 20 °C): δ 1.35 (s, 18H, C(CH3)3), 1.36 (s,
18H, C(CH3)3), 7.15 (d, J = 1.2 Hz, 2H, CHphenoxy), 7.41−7.44 (m, H,
300 MHz, 20 °C): δ 1.30 (s, 18H, C(CH3)3), 1.42 (s, 18H, C(CH3)3),
7.05 (d, J = 2.3, 2H, CHAr), 7.47 (d, J = 2.3, 2H, CHAr), 7.64 (s, 2H,
CH), 8.40 (bs, 2H, OH), 8.83 (s, 1H, CHimid). 13C NMR (CDCl3, 75
MHz, 20 °C): δ 29.6 (6C, C(CH3)3), 31.3 (6C, C(CH3)3), 34.4 (2C,
C(CH3)3), 35.7 (2C, C(CH3)3), 119.5 (2C, CH), 123.8 (2C, CAr),
124.5 (2C, CHAr), 126.5 (2C, CHAr), 136.7 (1C, CHcarbene), 141.7 (2C,
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CHAr), 143.2 (2C, CHAr), 148.1 (2C, CHAr). MS (positive ESI): [M −
Cl]+ calculated for C31H45N2O2 477.35, found 477.35.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis of 6Ni. Following the general procedure, 1,3-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)-1H-imidazol-3-ium chloride (103 mg, 0.2
mmol), NiCl2 (26 mg, 0.2 mmol), and K2CO3 (829 mg, 6 mmol)
Figures, tables, and CIF and xyz files giving electronic
absorption spectra for complexes 2Pd, 3Pt, and 7Pt, additional
computational data, all computed molecule Cartesian coor-
dinates in a format for convenient visualization, NMR spectra
for all compounds prepared in this paper, and crystallographic
data for 4Pt, 6Ni, and 7Pt. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
afforded complex 6Ni (102 mg, 83%, brown solid). H NMR (CDCl3,
300 MHz, 20 °C): δ 1.07 (s, 18H, C(CH3)3), 1.34 (s, 18H, C(CH3)3),
7.03 (d, J = 2.3 Hz, 2H, CHPhenoxy), 7.19 (d, J = 2.3 Hz, 2H, CHPhenoxy),
7.41 (t, J = 6.4 Hz, 2H, CHPyr), 7.69 (s, 2H, CH), 7.82 (t, J = 7.7 Hz,
1H, CHPyr), 9.07 (d, J = 4.7 Hz, 2H, NCHPyr). 13C NMR (CDCl3, 75
MHz, 20 °C): δ 29.0 (6C, C(CH3)3), 31.7 (6C, C(CH3)3), 34.1 (2C,
C(CH3)3), 35.2 (2C, C(CH3)3), 111.3 (2C, CH), 115.9 (2C, CHAr),
120.9 (2C, CHAr), 123.4 (2C, CHAr), 125.0 (2C, CHAr), 135.5 (2C,
CHAr), 137.5 (1C, CHAr), 140.2 (2C, CHAr), 146.8 (1C, CHcarbene),
150.4 (2C, CHAr), 151.5 (2C, CHAr). MS (positive ESI): [M − e]+
calculated for C36H47N3NiO2 611.3016, found 611.2863. Anal. Calcd
for C36H47N3O2Ni: C, 70.60; H, 7.73; N, 6.86. Found: C, 70.27; H,
7.54; N, 6.65.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.M.: mauro@unistra.fr.
*E-mail for S.D.: dagorne@unistra.fr.
*E-mail for S.B.-L.: bellemin@unistra.fr.
■
Notes
Synthesis of 6Pd. Following the general procedure, 1,3-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)-1H-imidazol-3-ium chloride (145 mg, 0.28
mmol), PdCl2 (50 mg, 0.28 mmol), and K2CO3 (1.169 g, 8.5 mmol)
afforded 6Pd (142 mg, 76%, yellow solid). 1H NMR (CDCl3, 300
MHz, 20 °C): δ 1.38 (s, 18H, C(CH3)3), 1.39 (s, 18H, C(CH3)3), 7.20
(d, J = 2.3, 2H, CHPhenoxy), 7.27 (d, J = 2.3, 2H, CHPhenoxy), 7.46−7.51
(m, 2H, CHPyr), 7.75 (s, 2H, CH), 7.90 (tt, J = 7.5, 1.5 Hz, 1H,
CHPyr), 9.08 (dd, J = 6.4, 1.5 Hz, 2H, NCHPyr). 13C NMR (CDCl3, 75
MHz, 20 °C): δ 29.5 (6C, C(CH3)3), 31.7 (6C, C(CH3)3), 34.2 (2C,
C(CH3)3), 35.7 (2C, C(CH3)3), 113.1 (2C, CH), 116.9 (2C, CHAr),
121.8 (2C, CHAr), 124.0 (2C, CHAr), 126.3 (2C, CHAr), 135.9 (2C,
CHAr), 138.2 (1C, CHPyr), 140.3 (2C, CHAr), 149.8 (1C, CHcarbene),
150.5 (2C, CHAr), 154.3 (2C, CHAr). MS (positive ESI): [M − e]+
calculated for C36H47N3O2Pd 659.2711, found 659.2711. Anal. Calcd
for C36H47N3O2Pd·CH2Cl2: C, 59.64; H, 6.63; N, 5.64. Found: C,
59.62; H, 6.50; N, 5.65.
Synthesis of 6Pt. Following the general procedure, 1,3-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)-1H-imidazol-3-ium chloride (193 mg, 0.38
mmol), PtCl2 (100 mg, 0.38 mmol), and K2CO3 (1.559 g, 11.3 mmol)
afforded 6Pt (134 mg, 48%, yellow solid). 1H NMR (CDCl3, 300 MHz,
20 °C): δ 1.33 (s, 18H, C(CH3)3), 1.34 (s, 18H, C(CH3)3), 7.13 (d, J
= 2.4, 2H, CHPhenoxy), 7.23 (d, J = 2.4, 2H, CHPhenoxy), 7.52 (td, J = 6.5,
1.1 Hz, 2H, CHPyr), 7.67 (s, 2H, CH), 7.95 (tt, J = 7.7, 1.1 Hz, 1H,
CHPyr), 9.08 (d, J = 6.5 Hz, 2H, NCHPyr). 13C NMR (CDCl3, 75 MHz,
20 °C): δ 29.5 (6C, C(CH3)3), 31.6 (6C, C(CH3)3), 34.1 (2C,
C(CH3)3), 35.8 (2C, C(CH3)3), 113.0 (2C, CH), 115.7 (2C, CHAr),
121.3 (2C, CHAr), 124.4 (2C, CHAr), 126.0 (2C, CHAr), 136.7 (2C,
CHAr), 138.3 (1C, CHPyr), 140.2 (2C, CHAr), 150.9 (2C, CHAr), 153.5
(1C, CHcarbene). MS (positive ESI): [M]+ calculated for C36H47N3O2Pt
748.33, found 748.33. Anal. Calcd for C36H47N3O2Pt·0.33CH2Cl2: C,
56.15; H, 6.18; N, 5.41. Found: C, 55.97; H, 6.04; N, 5.37.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the CNRS and the
Minister
̀
e de l’Enseignement Super
́
ieur et de la Recherche
(MESR) for a PhD grant to G.D. The authors also thank Dr. C.
Bailly and Dr. L. Brelot for X-ray diffraction studies
(Strasbourg). M.M. gratefully acknowledges Prof. L. De Cola
for allowing the use of the spectroscopy laboratory facilities and
computational machine time.
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dx.doi.org/10.1021/om5003446 | Organometallics 2014, 33, 4374−4384