Blue phosphorescent nitrile containing C^C* cyclometalated NHC platinum(ii) complexes

Article abstract of DOI:10.1039/c3dt53264j

Since C^C* cyclometalated Pt(ii) complexes with N-heterocyclic carbene (NHC) ligands have been identified as potential emitter materials in organic light-emitting devices (OLEDs), very promising results regarding quantum yields, colour and stability have

Full text of DOI:10.1039/c3dt53264j

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Blue phosphorescent nitrile containing C^C*
cyclometalated NHC platinum(II) complexes†
Cite this: DOI: 10.1039/c3dt53264j
Alexander Tronnier,a Stefan Metz,b Gerhard Wagenblast,b Ingo Muensterb and
Thomas Strassner*a
Since C^C* cyclometalated Pt(II) complexes with N-heterocyclic carbene (NHC) ligands have been
identified as potential emitter materials in organic light-emitting devices (OLEDs), very promising results
regarding quantum yields, colour and stability have been presented. Herein, we report on four nitrile sub-
stituted complexes with a chelating NHC ligand (1-(4-cyanophenyl)-3-isopropyl-1H-benzo[d]imidazole or
4-(tert-butyl)-1-(4-cyanophenyl)-3-methyl-1H-imidazole) and a bidentate monoanionic auxiliary ligand
(acetylacetone or dimesitoylmethane). The complexes have been fully characterized including extensive 2D
NMR studies (COSY, HSQC, HMBC, NOESY, 195Pt NMR), three of them also by solid-state structures. Photo-
physical measurements in amorphous PMMA films and pure emitter films at room temperature reveal the
impact of the mesityl groups in the auxiliary ligand, which led to a significant increase of the quantum yield,
while the decay lifetimes decreased. The electron withdrawing nitrile groups shift the emission towards blue
colour coordinates.
Received 19th November 2013,
Accepted 10th December 2013
DOI: 10.1039/c3dt53264j
www.rsc.org/dalton
together with acetylacetonate auxiliary ligands and derivatives
thereof.9 Based on this class of complexes (type A, Chart 1) a
Introduction
The invention of luminescent organic devices in 19931 had a lot of variations regarding substituents, size of the π systems,
large impact on the field of organic electronics. Many groups heterocycles and auxiliary ligands were investigated to tune the
since then focused their research activities on organic conduc- photophysical properties.
tors, isolators, optical sensors and emissive compounds.2
A new class of complexes (type B) derived from these emit-
Especially the field of organic light-emitting devices (OLEDs) ters was developed and the first complex was presented in a
has seen rapid development. A breakthrough came with the patent in 2006.10 In this system the pyridine ring is replaced
incorporation of transition metal complexes in 19983 which by a 3-methylimidazolylidene. Although similar complexes
enabled quantum yields beyond the fluorescence limit of 25% were prepared much earlier,11 their potential as emitter mole-
leading to the new class of phosphorescent organic light-emit- cules in OLEDs has only recently been shown.12 First device
ting devices (PhOLEDs).4 Therein the heavy atom is respon- tests with a complex of this class showed EQEs as high as 6.2%
sible for effective spin–orbit coupling (SOC) and triplet at 300 cd m−2 and a maximum luminance of 6750 cd m−2
.
harvesting, leading to exceptional photophysical properties and Since this discovery, continuing studies led to a whole
quantum yields as high as 100%.5 Although different tran- new class of C^C* cyclometalated highly emissive complexes.13
sition metals were investigated as emitting compounds,6
It became obvious that extended π systems (type C) have a
neutral iridium(III)7 and platinum(II)8 complexes with cyclo- beneficial effect on the quantum yield14 while sterically
metalated ligands show the most promising results so far.
Prominent motifs in such Pt(II) complexes are bidentate
cyclometalated aryl pyridine ligands which have been tested
demanding groups are necessary to prevent stacking of
aPhysical Organic Chemistry, Technische Universität Dresden, Bergstrasse 66,
01069 Dresden, Germany. E-mail: thomas.strassner@chemie.tu-dresden.de;
Fax: (+49) 351-463-39679; Tel: (+49) 351-463-38571
bBASF SE, 67056 Ludwigshafen, Germany
†Electronic supplementary information (ESI) available: Details of the photo-
physical measurements and solid-state structures, 2D NMR spectra as well as
Cartesian coordinates for the optimized structures. CCDC 972158–972160.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt53264j
Chart 1 Examples of cyclometalated Pt(II) complexes.
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complex molecules15 and the formation of excimers, which fre-
quently results in red-shifted emission colours.16
Herein we report a series of new complexes derived from
type B which incorporate a different size of the π system and
sterically demanding substituents that show NHC centred
emission in the blue to green region of the visible spectrum.
Since electron withdrawing groups like fluoro or CF3 groups,
which are often necessary to achieve blue emission, may cause
problems in terms of stability, the nitrile group was chosen as
an alternative substituent.
The influence of the auxiliary ligand on the steric shielding
of the central Pt(II) atom and the photophysical properties is
investigated by replacing acetylacetonate with 1,3-dimesityl-
propane-1,3-dionate.
Chart 2 Synthesized complexes 3–6.
Results and discussion
Synthesis
presence of potassium tert-butanolate as a base. The com-
plexes were obtained in yields of 25% to 36% after flash
chromatography.
The synthesis of the ligand precursors 1 and 2 has been
reported previously (Scheme 1).10 Compound 1 was prepared
following a multi-step methodology starting with the synthesis
Characterization
of the unsymmetric N-(4-cyanophenyl)-N′-(2-isopropyl)-pheny- All complexes were characterized by standard techniques. The
lene-diamine with subsequent ring closure. Thus, the corres- formation of an NHC bound cyclometalated species was veri-
ponding NHC ligand has an extended
benzimidazole fragment, and a sterically shielding isopropyl formation of a different coupling pattern for the cyclometa-
group.
lated ring in the 1H NMR. 2D NMR measurements (see ESI,
π system, the fied by the disappearance of the imidazolium proton and the
Ligand precursor 2 was prepared by nucleophilic aromatic Fig. S1–S12†) revealed the carbene carbon signal at
substitution to generate the imidazole with the sterically 150.8–151.7 ppm.
demanding tert-butyl group already in the backbone followed
The 195Pt NMR experiments showed signals typical for Pt(II)
by quaternization. The auxiliary ligand acetylacetone (acac) is complexes of this class in the range of −3320 ppm (5) to
commercially available and dimesitoylmethane (mesacac) −3380 ppm (4) in deuterated chloroform with the lowest
could be readily obtained through published literature values for the mesacac substituted complexes. The complexes
procedures.
show high thermal stability in air with melting points as high
The complexes (Chart 2) were prepared in a one-pot syn- as 310 °C. The compounds with 1-(4-cyanophenyl)-3-isopropyl-
thesis using our established route: deprotonation of the imida- 1H-benzo[d]-imidazole as a NHC ligand were found to decom-
zolium salt with silver(I) oxide and subsequent formation of a pose at 265 °C (3) and 309 °C (5). Generally we found higher
silver(I) NHC complex, transmetalation to dichloro(1,5-cyclo- melting points for the complexes with the mesityl group in the
octadiene) platinum(II), and cyclometalation at elevated temp- auxiliary ligand as well as better solubility in methylene chlor-
erature followed by the reaction with the auxiliary ligand in the ide or even diethyl ether.
Scheme 1 Preparation of the ligand precursors 1 and 2.
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Single-crystals suitable for X-ray diffraction measurements
could be obtained for 3, 4 and 5 by slow evaporation of satu-
rated methylene chloride solutions. The solid-state structures
reveal a square-planar coordination of the central Pt(II) atom.
The methyl substituents (of the isopropyl or tert-butyl group)
at the NHC ligands are always rotated out of the complex plane
which is comprised of the two metallacycles at the central
platinum atom. In all complexes, the angles of these two
cycles show some significant differences. While the O1–Pt1–
O2 angle shows the nearly perfect 90° angle of a square-planar
coordination, the C1–Pt1–C9 angles are about 10 degree
smaller (80.26(15)–80.46(18)°). The bonds from the carbon
atoms to the platinum centre are always shorter than the
oxygen platinum contacts. The carbene carbon platinum bond
(1.940(4)–1.957(7) Å) is the shortest of all four platinum–ligand
bonds. Both oxygen platinum bond lengths differ only slightly
(2.030(4)–2.087(3) Å).
Complex 3 as shown in Fig. 1 crystallizes in a monoclinic
crystal system. The molecules form pairs with a Pt⋯Pt distance
of 3.26 Å which is just below the sum of van der Waals radii
and may hint at a weak metal–metal closed-shell interaction
(see ESI, Fig. S13†).17 These pairs are oriented with an angle of
86° to each other. The molecules in each pair are rotated in
such a way that the NHC moieties face the acac group of the
other complex and the nitrile groups are facing towards the
opposite direction.
Fig. 2 Solid-state structure of 4. Thermal ellipsoids are drawn at 50%
probability. Selected bond lengths [Å] and angles [°]: Pt1–C1 1.954(4);
Pt1–C9 1.986(4); Pt1–O1 2.083(3); Pt1–O2 2.047(3); C1–Pt1–C9
80.26(15); O1–Pt1–O2 90.22(11); Pt1–C1–N1–C8 3.5(4); N1–C1–Pt1–O1
171.1(3).
For the other acac complex 4 (Fig. 2) a similar super-
imposed arrangement is found in the solid state (see ESI,
Fig. S14†). The pairs form a zigzag pattern with an angle of
81.2°, a Pt⋯Pt distance of 3.28 Å between them and a similar
orientation as found for 3. Additionally, a Pt–H contact of
2.83 Å is found between two complexes of different pairs.
Fig. 3 shows the solid-state structure of the mesacac substi-
tuted complex 5. It becomes obvious that the bulky mesityl
groups at the auxiliary ligand have a significant impact on the
steric situation around the platinum centre. The complex crys-
tallizes in an orthorhombic system. Due to the shielding effect
Fig. 3 Solid-state structure of 5. Thermal ellipsoids are drawn at 50%
probability. Selected bond lengths [Å] and angles [°]: Pt1–C1 1.957(7);
Pt1–C9 1.999(7); Pt1–O1 2.082(5); Pt1–O2 2.030(4); C1–Pt1–C9 80.4(3);
O1–Pt1–O2 89.26(17); Pt1–C1–N1–C8 4.2(8); N1–C1–Pt1–O1 171.2(5).
of the auxiliary ligand no pairs of complexes can be formed.
Neither the NHC ligand nor the metal centre can interact with
other molecules. Thus, the shortest Pt⋯Pt distance is as long
as 7.88 Å. The only contact from the metal centre to another
molecule is an agostic interaction to an aryl hydrogen atom
(2.89 Å). Interestingly, the metallacycles and the NHC back-
bone form no planar unit any more with parts of the complex
showing significant twisting (see ESI, Fig. S15†).
The bond lengths and angles around the metal centre
obtained from the X-ray diffraction experiments are in good
agreement with previously published structures. Complexes 3
and 4 with the acetylacetonato ligand are able to form pairs of
Fig. 1 Solid-state structure of 3. Thermal ellipsoids are drawn at 50%
probability. Selected bond lengths [Å] and angles [°]: Pt1–C1 1.940(4);
Pt1–C9 1.969(4); Pt1–O1 2.087(3); Pt1–O2 2.059(3); C1–Pt1–C9 80.46(18);
O1–Pt1–O2 89.35(13); Pt1–C1–N1–C8 −1.5(5); N1–C1–Pt1–O1 177.1(3).
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(PMMA) at room temperature with a concentration of 2 wt%
emitter.
All complexes show an absorption between 270 nm and
290 nm with an additional band at 330 nm. While the high
1
energy transitions can be ascribed to spin-allowed π–π* intra-
ligand charge transfer (ILCT) processes mainly located on the
NHC ligand, the weaker absorptions at higher wavelengths
include the metal d-orbitals and are more of metal-to-ligand
charge transfer (MLCT) nature.
All complexes show intense phosphorescence at room
temperature (Table 1). The luminescence measurements
(Fig. 5) show broad spectra ranging from the blue (450 nm) to
green (540 nm) region of the visible spectrum with strong
vibronic coupling. For all four complexes three discernible
bands are found with an additional weak shoulder at 540 nm.
With the exception of complex 3 the second band of the emis-
sion spectrum at around 470 nm is the most intense. The
vibronic progression between the first and second transition
band is about 1400 cm−1 for all compounds, which corres-
ponds to vibrations of the NHC ligand frame (as taken from
the frequency analysis from the density functional theory
(DFT) calculations).
Fig. 4 Absorption spectra of complexes 3–6 at room temperature (2 wt%
in PMMA).
A comparison of the acac substituted complexes with the
ones bearing the dimesitoylmethanato auxiliary ligand reveals
a strong enhancement of the quantum yields (28% from 4 to
6) and significantly lowered excited-state lifetimes while the
emission maxima remain in the same region, retaining the
blue colour coordinates.
The obtained lifetimes and quantum yields are indicative of
a fast radiative relaxation which is nearly twice as fast for the
mesacac substituted complexes 5 and 6.
Fig. 5 Emission spectra of complexes 3–6 at room temperature (2 wt%
in PMMA).
Furthermore, amorphous, pure emitter films were prepared
to probe for changes caused by the steric demand introduced
by the mesityl groups in the auxiliary ligand for complexes 5
and 6. For compound 3 no such films could be prepared due
to early crystallization. All measured complexes show absorp-
tion bands between 240 nm and 330 nm and emission in the
same region as for the measurement with 2% emitter concen-
tration (see ESI, Fig. S16 and S17†) with quantum yields
ranging from 11% (5) to 22% (6).
molecules with Pt⋯Pt distances slightly below the sum of van
der Waals radii. A further similarity for this class of com-
pounds is the arrangement of the complexes in the solid state
where within the pairs each NHC ligand faces an acetylaceto-
nato ligand. The steric shielding induced by the mesityl
groups at the auxiliary ligand in 5 effectively suppresses the
formation of pairs and should thus be useful in more concen-
trated emitter films.
Photophysical properties
DFT calculations
To investigate the photophysical properties of the complexes, The optimized geometries for the singlet and triplet states
the absorption (Fig. 4) and emission spectra (Fig. 5) were have been calculated for the four complexes by DFT methods
measured in amorphous poly(methyl methacrylate) films (B3LYP/6-31G(d) level with a Hay–Wadt-ECP for platinum).
Table 1 Photoluminescence data (2 wt% in PMMA, room temperature) of cyclometalated complexes 3–6
#
λexc a [nm]
CIE x; yb
λem c [nm]
Φd [%]
τo e [µs]
kr f [103 s−1
]
knr g [103 s−1
]
3
4
5
6
330
355
370
370
0.158; 0.161
0.164; 0.216
0.164; 0.212
0.166; 0.229
444, 473
443, 474
442, 471
443, 471
63
57
80
85
15.5
17.0
8.9
64.4
59.1
114.9
118.9
37.8
44.6
28.7
21.0
8.4
a Excitation wavelength. b CIE coordinates at rt. c Emission wavelength. d Quantum yield in % at λexc, N2 atmosphere. e Decay lifetimes (excited by
laser pulses (355 nm, 1 ns)) given as τo = τv/ϕ. f kr = ϕ/τv. g knr = (1 − ϕ)/τv.
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Fig. 6 Frontier molecular orbitals for complexes 3–6 (HOMO bottom, LUMO top; B3LYP/6-31G(d), isovalue = 0.02).
The results obtained from these calculations agree well with analysis of the FMOs reveals only a minor contribution of the
the structural data from the X-ray diffraction experiments (see benzimidazole phenyl ring to the HOMO, which might explain
ESI, Tables S2–S4, Fig. S18†).
the very similar emission wavelengths observed for all four
Upon changing the multiplicity from the singlet to the complexes.
excited triplet state most complexes retain their geometries
Additionally computed spin densities of the lowest triplet
with only slightly altered bond lengths around the platinum state agree well with the findings from the FMO analysis and
centre. For 5 an additional flipping of the auxiliary ligand out support an MLCT/ILCT process (see ESI, Fig. S20†).
of the central complex plane, created by the two metallacycles,
A recently developed method to reliably predict the emis-
was observed (see ESI, Fig. S19†). A rotation of the two aryl sion wavelength of transition-metal complexes helped with the
groups in the diketonato ligand into the central plane, as development of these blue emitting complexes. We a priori
observed for other complexes of this class,15d did not occur. computed the emission maxima of several complexes and syn-
The methyl groups in the ortho position at the aryl moieties thesized only those which were predicted to have a blue emis-
effectively suppress any significant rotations so that the sion colour.18 The values obtained from the calculations are in
mesityl groups retain their function of shielding the metal very good agreement with the ones measured in the experi-
centre from interactions even in the triplet state.
ments (see ESI, Table S5†).
A possible side product in the synthesis of complex 3, a
species where the platinum is not bound to the 4-cyanophenyl
group but rather to a carbon of the isopropyl group, has been
taken into account during the calculations (see ESI, Fig. S19†).
Conclusions
This C(sp3)–Pt cyclometalated compound, which also forms a Four new C^C* cyclometalated Pt(II) NHC complexes with
five-membered metallacycle, was found to be thermodynami- interesting photophysical properties were prepared and fully
cally less favoured by 10.5 kcal mol−1 and could indeed not be characterized. The choice of NHC ligands with nitrile substitu-
observed experimentally.
ents at the cyclometalated ring resulted in compounds with
The frontier molecular orbitals (FMOs, Fig. 6) show a major blue emission colour and wavelength maxima as low as
contribution from the NHC ligand to the HOMO and LUMO, 445 nm. The complexes with acetylacetonato as an auxiliary
which are both of π character and show some contributions ligand showed good quantum yields (60%) and common
from metal d-orbitals and the π system of the auxiliary ligand excited state lifetimes of 15–17 µs in amorphous PMMA films
as well. This is in good agreement with an MLCT/ILCT emis- at room temperature. Based on DFT calculations we propose
3
sion process as expected from the observed photophysical that the emission process is of ILCT/3MLCT character, which
experiments. Since complexes 3 and 5 have a larger conjugated is in good agreement with the strong vibronic coupling
π electron system, a different HOMO–LUMO gap and conse- observed in the emission spectra. Solid-state structure determi-
quently a different emission colour would be expected in com- nation by X-ray diffraction experiments showed the formation
parison to the imidazole complexes 4 and 6. However, an of a superstructure with Pt⋯Pt distances slightly below the
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sum of van der Waals radii. A second auxiliary ligand with δ −148.9, −149.0 ppm. M.p.: 193–194 °C. Anal. calcd for
bulky mesityl substituents was chosen for comparison and we C17H16BF4N3: C 58.48, H 4.62, N 12.04; found: C 58.38, H 4.77,
obtained dimesitoylmethanato substituted complexes with sig- N 12.08%.
nificantly higher quantum yields (>80%), shorter decay life-
1-(4-Cyanophenyl)-3-methyl-4-(1-methylprop-2-yl)-1H-imida-
times (8–9 µs) and a different superstructure in the solid state. zolium iodide (2). The synthesis followed known literature
These results are in agreement with experimental data from procedures.10 1H NMR (300 MHz, DMSO-d6): δ 9.85 (s, 1H,
the measurements of amorphous 100 wt% emitter films. Thus, NCHN), 8.18–8.22 (m, 3H, CHarom), 8.03 (d, J = 8.9 Hz, 2H,
the mesacac ligand is a promising alternative auxiliary ligand CHarom), 4.03 (s, 3H, NCH3), 1.42 (s, 9H, CCH3) ppm. 13C NMR
for this class of emitters compared to the generally used (75 MHz, DMSO-d6): δ 143.4 (Ci), 137.7 (NCHN), 134.3
acetylacetonate.
(CHarom), 122.1 (CHarom), 117.7 (Ci), 116.7 (CHarom), 112.0 (Ci),
36.6 (NCH3), 31.1 (Ci), 28.4 (CCH3) ppm. M.p.: 297–298 °C.
Anal. calcd for C15H18IN3: C 49.06, H 4.94, N 11.44; found:
C 48.75, H 5.04, N 11.47%.
Experimental section
General considerations
Syntheses of complexes
Solvents of at least 99.5% purity were used throughout
this study. 1,4-Dioxane and DMF were dried using standard
techniques and stored under an argon atmosphere over mole-
cular sieve (3 Å). Dichloro(1,5-cyclooctadiene)platinum(II)
(Pt(COD)Cl2)19 was prepared following a modified literature
procedure.12 Dimesitylmethane was prepared according to a
known procedure.20 Potassium tetrachloroplatinate(II) was
obtained from Pressure Chemicals Co. All other chemicals
were obtained from common suppliers and used without
further purification. 1H, 13C and 195Pt NMR spectra were
recorded on a Bruker 300 MHz, 500 MHz and 600 MHz NMR
spectrometer. 1H and 13C NMR spectra were referenced intern-
ally using the resonances of the solvent (1H: 7.26, 13C: 77.0 for
CDCl3; 1H: 2.50, 13C: 39.43 for DMSO-d6). 19F NMR spectra
were referenced externally against trifluoromethylbenzene
(F3C–C6H5). 195Pt NMR spectra were referenced externally
using potassium tetrachloroplatinate(II) in D2O (−1617.2
(PtCl42−), −2654.1 (PtCl2)). Shifts are given in ppm, coupling
constants J in Hz. Elemental analyses were performed by the
microanalytical laboratory of our institute on a Hekatech
elemental analyser. Melting points have been determined
using a Wagner and Munz Poly Therm A system and are not
corrected. The emitter films were prepared by doctor blading a
solution of emitter (2 mg mL−1) in a 10 wt% PMMA solution
in dichloromethane on a substrate with a 60 µm doctor blade.
Quantum yields were determined by an absolute method using
the integrated sphere technique of a Hamamatsu Quantaurus
apparatus.
The established synthetic route for the general synthesis of the
platinum(II) complexes 3–6 uses silver(I) oxide (93 mg,
0.4 mmol) and the NHC ligand precursor (0.8 mmol) in 20 mL
of dry 1,4-dioxane at room temperature under argon. Pt(COD)-
Cl2 (299 mg, 0.8 mmol) and 10 mL of 2-butanone were added
to the reaction mixture. The mixture was then refluxed for 21 h
and all volatile compounds removed in vacuo. Afterwards, pot-
assium tert-butanolate (357 mg, 3.2 mmol), 3.2 mmol of the
corresponding auxiliary ligand and 20 mL of DMF were added
to the crude reaction mixture. In the last reaction step the solu-
tion was stirred for 21 h at room temperature and 6 h at
100 °C. Upon completion all volatiles were removed in vacuo
again, the remaining solid washed with water, dried and the
complexes isolated by flash chromatography with methylene
chloride–isohexane as the eluent.
[1-(4-Cyanophenyl)-3-isopropyl-1H-benzo[d]imidazol-2-ylidene-
κC2,κC2′]platinum(II)acetylacetonate (3). The synthesis followed
the general procedure. 279 mg (0.8 mmol) of 1 and 0.33 mL
(3.2 mmol) acetylacetone were used. Yield: 133 mg, 22%.
1H NMR (300 MHz, CDCl3): δ 8.18 (d, pseudo-t, JH,H = 1.7 Hz,
JH,Pt = 28.6 Hz, 1H, PtCCHarom), 7.95 (d, JH,H = 7.5 Hz, 1H,
CHarom), 7.69 (d, JH,H = 7.5 Hz, 1H, CHarom), 7.54 (d, JH,H
=
8.2 Hz, 1H, CHarom), 7.46 (dd, JH,H = 1.7 Hz, JH,H = 8.2 Hz 1H,
CHarom), 7.40 (dt, JH,H = 1.3 Hz, JH,H = 7.8 Hz 1H, CHarom), 7.34
(dt, JH,H = 1.3 Hz, JH,H = 7.8 Hz 1H, CHarom), 6.47 (m, 1H,
CH3CHCH3), 5.59 (s, 1H, CH), 2.13 (s, 3H, CH3), 2.03 (s, 3H,
CH3), 1.74 (d, JH,H = 7.0 Hz, 6H, CHCH3) ppm. 13C NMR
(125 MHz, CDCl3): δ 185.9 (OCi), 185.3 (OCi), 159.9 (Ci), 151.6
(NCN), 134.2 (CHarom), 133.1 (Ci), 132.2 (Ci), 128.8 (CHarom),
126.7 (Ci), 124.3 (CHarom), 123.1 (CHarom), 120.1 (Ci), 113.7
Syntheses of ligands
1-(4-Cyanophenyl)-3-isopropyl-1H-benzo[d]imidazolium tetra- (CHarom), 111.3 (CHarom), 111.2 (CHarom), 106.2 (Ci), 102.3
fluoroborate (1). The synthesis followed known literature (OCCH), 49.5 (CH3CHCH3), 28.0 (CH3), 27.9 (2 × CH3), 21.1
procedures.10,21 1H NMR (300 MHz, DMSO-d6): δ 10.25 (s, 1H, (CH3) ppm. 195Pt NMR (128 MHz, CDCl3): δ −3365.2 ppm.
NCHN), 8.26–8.30 (m, 3H, CHarom), 8.10 (d, J = 8.6 Hz, 2H, M.p.: dec. >265 °C. Anal. calcd for C22H21N3O2Pt·0.65CH2Cl2:
CHarom), 7.93 (d, J = 8.5 Hz, 1H, CHarom), 7.73–7.82 (m, 2H, C 44.62, H 3.69, N 6.89; found: C 44.63, H 3.77, N 6.92%.
CHarom), 5.18 (td, J = 6.6 Hz, J = 13.3 Hz, 1H, CH3CHCH3), 1.72
[1-(4-Cyanophenyl)-3-methyl-4-(1-methylprop-2-yl)-1H-imida-
(d, J = 6.7 Hz, 6H, CH(CH3)2) ppm. 13C NMR (75 MHz, DMSO- zol-2-ylidene-κC2,κC2′]platinum(II)acetylacetonate (4). The syn-
d6): δ 141.4 (NCHN), 136.9 (Ci), 134.3 (CHarom), 130.8 (Ci), thesis followed the general procedure. 294 mg (0.8 mmol) of 2
130.6 (Ci), 127.6 (CHarom), 127.0 (CHarom), 126.4 (CHarom), and 0.33 mL (3.2 mmol) acetylacetone were used. Yield:
1
117.8 (Ci), 114.3 (CHarom), 113.5 (CHarom), 112.9 (Ci), 51.1 145 mg, 34%. H NMR (300 MHz, CDCl3): δ 8.04 (d, pseudo-t,
(CH3CHCH3), 21.4 (CH3) ppm. 19F NMR (282 MHz, DMSO-d6): JH,H = 1.7 Hz, JH,Pt = 27.9 Hz, 1H, PtCCHarom), 7.33 (dd, JH,H
=
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1.7 Hz, JH,H = 8.0 Hz 1H, CHarom), 6.96 (s, 1H, CHarom), 6.89 the gradient-corrected density functional BP8624 were used
(d, JH,H = 8.0 Hz, 1H, CHarom), 5.53 (s, 1H, CH), 4.23 (s, 3H, together with the 6-31G(d)25 basis set. No symmetry or internal
NCH3), 2.09 (s, 3H, CH3), 1.98 (s, 3H, CH3), 1.43 (s, 9H, CCH3) coordinate constraints were applied during optimizations. All
ppm. 13C NMR (150 MHz, CDCl3): δ 185.4 (OCi), 184.8 (OCi), reported intermediates were verified as true minima by the
151.7 (NCN), 150.8 (CN), 142.2 (Ci), 134.5 (CHarom), 128.4 absence of negative eigenvalues in the vibrational frequency
(CHarom), 126.6 (Ci), 120.2 (Ci), 110.3 (CHarom), 109.7 (CHarom), analysis. In all cases platinum was described using a decon-
106.5 (Ci), 102.1 (CH), 34.6 (NCH3), 31.5 (CiCH3), 29.2 tracted Hay–Wadt (n + 1) ECP and basis set.26 Approximate
(CCH3), 27.9 (2 × CH3) ppm. 195Pt NMR (128 MHz, CDCl3): free energies were obtained through thermochemical analysis,
δ
−3379.8 ppm. M.p.: 292–293 °C. Anal. calcd for using the thermal correction to Gibbs free energy as reported
C20H23N3O2Pt: C 45.11, H 4.35, N 7.89; found: C 45.25, H 4.55, by Gaussian09. This takes into account zero-point effects,
N 7.51%. thermal enthalpy corrections, and entropy. All energies
[1-(4-Cyanophenyl)-3-isopropyl-1H-benzo[d]imidazol-2-ylidene- reported in this paper, unless otherwise noted, are Gibbs
κC2,κC2′]platinum(II)-1,3-dimesitylpropane-1,3-dionate (5). The free energies at standard conditions (T = 298 K, p = 1 atm),
synthesis followed the general procedure. 279 mg (0.8 mmol) using unscaled frequencies. For visualization GaussView27 and
of 1 and 987 mg (3.2 mmol) dimesitoylmethane were used. CYLview28 were used.
Yield: 221 mg, 36%. 1H NMR (600 MHz, CDCl3): δ 8.08 (d,
JH,H = 1.7 Hz, 1H, CHarom), 7.96 (d, JH,H = 8.2 Hz, 1H, CHarom),
X-ray crystallography
7.66 (d, JH,H = 8.2 Hz, 1H, CHarom), 7.57 (d, JH,H = 8.2 Hz, 1H,
Preliminary examination and data collection were carried out
CHarom), 7.47 (dd, JH,H = 1.7 Hz, JH,H = 8.2 Hz, 1H, CHarom),
on an area detecting system (Kappa-CCD; Nonius, FR590)
using graphite monochromated Mo Kα radiation (λ
7.40 (t, JH,H = 7.8 Hz, 1H, CHarom), 7.33 (t, JH,H = 7.8 Hz, 1H,
CHarom), 6.89 (s, 2H, CHarom), 6.84 (s, 2H, CHarom), 6.39
(m, 1H, CH3CHCH3), 5.79 (s, 1H, CH), 2.35 (s, 6H, CH3), 2.32
=
0.71073 Å) with an Oxford Cryosystems cooling system at the
window of a sealed fine-focus X-ray tube. The reflections were
integrated. Raw data were corrected for Lorentz, polarization,
decay and absorption effects. The absorption correction was
applied using SADABS.29 After merging, the independent
reflections were used for all calculations. The structure was
solved by a combination of direct methods30 and difference
Fourier syntheses.31 All non-hydrogen atom positions were
refined with anisotropic displacement parameters. Hydrogen
atoms were placed in ideal positions using the SHELXL riding
model. Full-matrix least-squares refinements were carried out
by minimizing Σw(Fo − Fc ) with the SHELXL-97 weighting
scheme and stopped at shift/err <0.001. Details of the structure
determinations are given in the ESI.† Neutral-atom scattering
factors for all atoms and anomalous dispersion corrections for
the non-hydrogen atoms were taken from International Tables
for Crystallography.32 All calculations were performed with the
programs COLLECT,33 DIRAX,34 EVALCCD,35 SIR92,30
SADABS,29 PLATON36 and the SHELXL-97 package.31,37 For the
visualization Mercury38 and ORTEP-III39 were used.
(s, 3H, CH3), 2.31 (s, 6H, CH3), 2.30 (s, 3H, CH3), 1.55 (d, JH,H
=
7.0 Hz, 6H, CH(CH3)2) ppm. 13C NMR (150 MHz, CDCl3):
δ 185.7 (CO), 185.6 (CO), 159.4 (Ci), 151.6 (NCN), 139.1 (Ci),
138.6 (Ci), 138.0 (Ci), 137.5 (Ci), 134.3 (CHarom), 133.9 (Ci),
133.4 (Ci), 133.0 (Ci), 132.2 (Ci), 129.1 (CHarom), 128.4 (CHarom),
128.0 (CHarom), 124.4 (CHarom), 123.1 (CHarom), 120.1 (Ci),
113.8 (CHarom), 111.4 (CHarom), 111.2 (CHarom), 107.3 (CH),
106.3 (Ci), 49.5 (CH(CH3)2), 21.1 (CH3), 20.9 (CH3), 19.9 (CH3),
19.6 (CH3), 19.4 (CH3) ppm. 195Pt NMR (64 MHz, CDCl3):
δ −3320.0 ppm. M.p.: dec. >309 °C. Anal. calcd for C38H37N3O2Pt:
C 59.83, H 4.89, N 5.51; found: C 59.69, H 4.96, N 5.37%.
[1-(4-Cyanophenyl)-3-methyl-4-(1-methylprop-2-yl)-1H-imida-
zol-2-ylidene-κC2,κC2′]platinum(II)-1,3-dimesitylpropane-1,3-
dionate (6). The synthesis followed the general procedure.
294 mg (0.8 mmol) of 2 and 987 mg (3.2 mmol) dimesitoyl-
2
2 2
1
methane were used. Yield: 147 mg, 25%. H NMR (300 MHz,
CDCl3): δ 7.94 (d, pseudo-t, JH,H = 1.7 Hz, JH,Pt = 27.2 Hz, 1H,
CHarom), 7.33 (dd, JH,H = 1.7 Hz, JH,H = 8.0 Hz, 1H, CHarom),
6.98 (s, 1H, CHarom), 6.92 (d, JH,H = 8.0 Hz, 1H, CHarom), 6.87
(s, 2H, CHarom), 6.83 (s, 2H, CHarom), 5.73 (s, 1H, CH), 4.10
(s, 3H, NCH3), 2.33–2.28 (3 × s, 18H, CH3), 1.39 (s, 9H, CCH3)
ppm. 13C NMR (150 MHz, CDCl3): δ 185.2 (CO), 184.8 (CO),
151.0 (Ci), 150.8 (NCN), 142.4 (Ci), 139.4 (Ci), 138.7 (Ci), 137.9
(Ci), 137.5 (Ci), 134.6 (CHarom), 133.9 (Ci), 133.6 (Ci), 128.8
(CHarom), 128.4 (CHarom), 128.1 (CHarom), 126.1 (Ci), 120.1 (Ci),
110.2 (CHarom), 109.7 (CHarom), 107.2 (CH), 106.7 (Ci), 34.6
(NCH3), 31.4 (C(CH3)3), 29.2 (C (CH3)3), 21.1 (CH3), 21.0 (CH3),
19.8 (CH3), 19.5 (CH3) ppm. 195Pt NMR (128 MHz, CDCl3):
δ −3330.0 ppm. M.p.: 308–310 °C. Anal. calcd for C36H39N3O2Pt:
C 58.37, H 5.31, N 5.67; found: C 58.69, H 5.45, N 5.63%.
Acknowledgements
We are grateful to Dr Fuchs (BASF), Dr Heinemeyer (BASF) and
Dr Lennartz (BASF) for helpful discussions. We also thank the
ZIH for computation time on their high-performance comput-
ing facility and the BMBF (FKZ: 13N10477) for funding.
Notes and references
Computational details
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