Organoplatinum complexes of the N,N-diisopropyl-diazabutadiene ligand: A structural and spectroscopic study

Article abstract of DOI:10.1002/zaac.200500106

Organoplatinum complexes [Pt(R)₂(iPr-DAB)], [PtCl(R)(iPr-DAB)] or [PtCl₂(iPr-DAB)] (R = methyl or 2,4,6-tri-methylphenyl = mesityl, iPr-DAB = N,N′-diisopropyl-1,4-diazabutadiene) have been investigated by a combination of multiple spectroscopy (NMR, UV/VIS), single crystal XRD and quantum chemical calculations. Excellent agreement of calculated and experimental structural data from XRD or NMR provide a good basis for the quantum mechanical calculations. DFT calculations reveal the contributions of the DAB ligand and the co-ligands to the electronic ground state. Methyl acts more or less purely as a σ-donor, Cl or mesityl provide strong contributions (p or π) to the highest occupied molecular orbital (HOMO) whereas the lowest unoccupied molecular orbital (LUMO) is mainly centred to the DAB ligand. The TD-DFT calculated electronic transitions were in excellent agreement with the experimental ones and allow reliable assignment of the absorption bands.

Full text of DOI:10.1002/zaac.200500106

Organoplatinum Complexes of the N,N-Diisopropyl-diazabutadiene Ligand:
A Structural and Spectroscopic Study
,a
a
´ ˇb
Axel Klein* , Thilo Schurr , and Stanislav Zalis
a Köln, Institut für Anorganische Chemie der Universität zu Köln
b
´
Prague, J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic
Received March 4th, 2005.
Professor Herbert W. Roesky zum 70. Geburtstag gewidmet
Abstract.
Organoplatinum
complexes
[Pt(R)2(iPr-DAB)],
Cl or mesityl provide strong contributions (p or π) to the highest
occupied molecular orbital (HOMO) whereas the lowest unoccup-
ied molecular orbital (LUMO) is mainly centred to the DAB li-
gand. The TD-DFT calculated electronic transitions were in excel-
lent agreement with the experimental ones and allow reliable as-
signment of the absorption bands.
[PtCl(R)(iPr-DAB)] or [PtCl2(iPr-DAB)] (R ϭ methyl or 2,4,6-tri-
methylphenyl ϭ mesityl, iPr-DAB ϭ N,NЈ-diisopropyl-1,4-diazabu-
tadiene) have been investigated by a combination of multiple spec-
troscopy (NMR, UV/VIS), single crystal XRD and quantum che-
mical calculations. Excellent agreement of calculated and experi-
mental structural data from XRD or NMR provide a good basis
for the quantum mechanical calculations. DFT calculations reveal
the contributions of the DAB ligand and the co-ligands to the elec-
tronic ground state. Methyl acts more or less purely as a σ-donor,
Keywords: Crystal structure; Platinum; Diimine ligands; NMR;
Quantum chemical calculations
Introduction
In this study we want to extend this methodology to com-
plexes with halogen (X) co-ligands since halogens are well
known to contribute by π donation to excited states, which
are then referred to XLCT instead of LЈLCT. We have
therefore synthesized and investigated a number of com-
plexes using the most simple α-diimine ligand N,NЈ-diiso-
propyl-1,4-diazabutadiene (Scheme 1). The DFT calcu-
lations on the halide containing complexes are underwent
to elucidate the electronic structure variation and conse-
quent changes of physical properties due to the co-ligand
variation.
Since in the above mentioned study [16, 19] the contri-
bution of aryl or alkyl co-ligands was substantially differ-
ent, we decided to examine two series of complexes:
[Pt(R)2(iPr-DAB)]Ϫ[PtCl(R)(iPr-DAB)]Ϫ[PtCl2(iPr-DAB)]
with R ϭ methyl or mesityl (2,4,6-trimethylphenyl) since it
has been shown that the overall donating power of methyl
and mesityl is almost the same although the character of
the contributions differs markedly [19].
Platinum(II) complexes with α-diimine ligands, their photo-
physical properties and the resulting potential applications
have been a subject of much interest in the last three dec-
ades [1Ϫ19]. Generally the low-energy electronic absorption
bands are ascribed to metal-to-ligand charge transfer
(MLCT) absorption bands. Correspondingly the lowest ex-
cited state is usually assumed to have MLCT character as
well. These assignments agree with the pronounced solva-
tochromism and the extinction coefficients of the absorp-
tion bands. However in recent work we have explored the
role of the co-ligands R in complexes of the type [Pt(R)2(α-
diimine)] (R ϭ alkyl, alkynyl or aryl) [16, 19]. By a combi-
nation of detailed spectroscopy (absorption, resonance Ra-
man and emission) and DFT calculations it was shown that
the R co-ligands contribute to the highest occupied molecu-
lar orbitals (HOMOs) and thus the electronic transitions
and the excited states are better assigned to mixed MLCT/
LЈLCT (ligand-to-ligand charge transfer). This is most pro-
nounced in cases where R possesses a π-system to interact
with platinum or diimine π*-orbitals.
* Prof. Dr. A. Klein
Universität zu Köln
Institut für Anorganische Chemie
Greinstraße 6
D-50939 Köln
e-mail: axel.klein@uni-koeln.de
e-mail: zalis@jh-inst.cas.cz
Scheme 1 Schematic representation of iPr-DAB ϭ N,NЈ-diisopro-
pyl-1,4-diazabutadiene in the non-coordinating free form (A) and
in the square planar platinum(II) complex [PtCl(R)(iPr-DAB)] (B)
with numbering of the C and corresponding H atoms.
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/zaac or from the
author
Z. Anorg. Allg. Chem. 2005, 631, 2669Ϫ2676
DOI: 10.1002/zaac.200500106
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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´ ˇ
A. Klein, T. Schurr, S. Zalis
Results and Discussion
range from 76 to 79° [22Ϫ24] indicating that the bite angle
is a crucial (and thus more or less invariable) structural par-
ameter. The PtϪN, PtϪC and PtϪCl bond lengths are also
in the expected range when compared to related structures.
Within the series of complexes the experimentally observed
or calculated distances follow nicely the trends expected
from trans or cis influence [25, 26]: a strong ligand trans to
a regarded bond will be lengthening the bond (trans influ-
ence) while a cis oriented strong ligand has an opposed ef-
fect (cis influence) albeit usually to a much smaller extent.
E.g. the PtϪN distance decreases in the following series
with trans(cis) positioned co-ligands Me(Cl) > Mes(Cl) >
Mes(Mes) > Me(Me) > Cl(Cl). The series reveals a slightly
superior trans influence of the Me co-ligand compared to
Mes and the lengthening effect of the weak cis-oriented Cl
co-ligand compared to the R co-ligands (cis influence). The
Mes co-ligand is not oriented perfectly perpendicular to the
coordination plane (ClnRmPtNN). The dihedral angles were
around 70° for (Mes)2 and 81.2° for Cl(Mes). Very similar
angles were found in numerous aryl platinum complexes
with π-accepting ligands like diimines [19] pointing to an
adoption of the angles to allow optimum electronic interac-
tion of the Mes co-ligand with the platinum centre and the
π-accepting ligand. We can therefore conclude that the Mes
co-ligand is not only restricted to σ interaction but might
exert some π contributions.
Preparation and analysis
The complexes [Pt(R)2(iPr-DAB)] (R ϭ Me or Mes) have
been synthesized as recently reported [16], [PtCl2(iPr-DAB)]
was easily obtained from [PtCl2(dmso)2] and the iPr-DAB
ligand, the mixed compounds [PtCl(R)(iPr-DAB)] (R ϭ Me
or Mes) were prepared from the complexes [Pt(R)2(iPr-
DAB)] using appropriate amounts of HCl generated in situ
from acetyl chloride and methanol as similarly described by
Clark and Manzer for the generation of [PtCl(Me)(COD)]
from [Pt(Me)2(COD)] [20]. For details see the Experimental
Part. Interestingly the reaction for R ϭ Mes tolerates huge
excesses of HCl (up to 5 times) yielding selectively
[PtCl(Mes)(iPr-DAB)], for the preparation of [PtCl(Me)(-
iPr-DAB)] the exact 1:1 ratio is essential. Since also the
“original” compound [Pt(Me)2(COD)] tolerates excesses of
HCl, the trans directing power of the iPr-DAB ligand and
not the bulkiness of the R co-ligands seems to be respon-
sible for this effect.
Crystal and molecular structures
The crystal and molecular structures of [PtCl(Me)(iPr-
DAB)] was reported before (Vitagliano 1988) [21], all other
compounds with the exception of [PtCl2(iPr-DAB)] were
crystallized and successfully submitted to X-ray diffraction
experiments.
The crystal structures of the complexes [Pt(Me)2(iPr-
DAB)], [Pt(Mes)2(iPr-DAB)] and [PtCl(Mes)(iPr-DAB)]
were all solved in the monoclinic P21/n space group which
contrasts to the orthorhombic Pca21 which was reported
for [PtCl(Me)(iPr-DAB)] [21]. For [Pt(Me)2(iPr-DAB)] three
independent molecules were found with almost identical
molecular structure. In none of the structures we found sig-
nificant intermolecular interactions other than van der
Waals contacts. The crystallographic details are summa-
rized in Table 1.
In all compounds the platinum atoms exhibit an almost
perfect planar surrounding as can be seen e. g. from the
dihedral angle NNPt/ClnCmPt. This is not the case for the
reported [PtCl(Me)(iPr-DAB)] in which the chlorine atom
deviates markedly, leading to a NNPt/ClnCmPt dihedral an-
gle of 11.3° [21]. The overall structure of [PtCl(Me)(iPr-
DAB)] seems to be very poor (if not incorrect) and we will
not discuss any structural details from this structure. Fur-
thermore we did not succeed to get a crystal structure for
[PtCl2(iPr-DAB)]. We have therefore calculated all molecu-
lar structures using both Gaussian and ADF methods. The
calculated data are in good agreement with the experimen-
tal ones (see Supporting Information) the results from
ADF/BP calculations were slightly better in that respect.
Table 2 lists combined essential experimental and calculated
structural data of all compounds, Figure 1 shows selected
molecular structures.
NMR Spectroscopy and calculations
All complexes were studied by 1H and 13C NMR spec-
troscopy to probe for the impact of the R or Cl co-ligands.
At the same time essential NMR parameters were calcu-
lated for the model complexes with the Me-DAB ligand.
Table 3 summarizes essential experimental and ADF calcu-
1
lated H or 13C NMR data, full NMR data is deposited in
the Supporting Information.
When comparing the HϪPt or CϪPt coupling constants
of the iPr-DAB ligand which are relevant for discussing the
trans or cis influence of the co-ligands we always find decreas-
ing coupling constants along the series of trans(cis) positioned
co-ligands Cl(Mes) > Cl(Me) > Cl(Cl) > Mes(Mes) >
Me(Me). The series goes nicely along with an decreasing trans
influence Mes > Me > Cl (strengthening the metal ligand
coupling) superimposed with an increasing cis influence Cl <
Me < Mes which also enhances the metal ligand coupling.
1
The calculated values agree reasonably well for the H data,
only the Cl(Mes) and Cl(Me) are changed in sequence. For
the calculated 13C data the largest CϪPt coupling is obtained
for the Cl(Cl) derivative. Generally the coupling constants are
very sensitvie to geometry but also to solvent effects which
were not included here. Obviously this leads to an overestim-
ation of the strengthening influence of the Cl co-ligand on
the coupling constant. The series of co-ligand influence, al-
though with a changed sequence for the Mes and Me, was
also deduced from the crystal structures. However since the
differences found in the structures between Mes and Me were
small and in crystal structures packing forces and related
The bite angles NϪPtϪN are almost identical for all
complexes. For related platinum DAB complexes the values
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Organoplatinum Complexes of the N,N-Diisopropyl-diazabutadiene Ligand
a)
Table 1 Crystal structure and refinement data for iPr-DAB platinum complexes
(Me)2
(Mes)2
Cl(Mes)
Formula
Formula weight
C10H22N2Pt1
365.39
C26H38N2Pt1
573.67
C17H27Cl1N2Pt1
489.95
Space group
P21/n
P21/n
P21/n
˚
unit
cell
a /A
9.6570(19)
32.541(7)
12.656(3)
101.97(3)
3890.6(13) 12
1.871
11.427(2)
15.765(3)
14.699(3)
106.19(3)
2542.9(9)
1.498
9.0739(17)
13.249(3)
15.318(3)
102.483(15)
1798.0(6)
1.810
7.949
952
˚
b /A
˚
c /A
β /°
3
˚
volume /A , Z
4
4
δcalc. /(mg·mϪ3
)
absorption coefficient. /mmϪ1
F(000)
10.785
2088
5.531
1144
crystal size /mm
Colour
θ range /°
limiting indices
0.3 x 0.1 x 0.1
violet
1.76 to 25.00
Ϫ1 < h < 11,
Ϫ3 < k < 38,
Ϫ15 < l < 14
7278
0.4 x 0.2 x 0.1
violet
1.94 to 27.00
Ϫ14 < h < 14,
0 < k < 20,
0 < l < 18
5550
0.35 x 0.25 x 0.15
red
2.05 to 27.50
Ϫ8 < h < 11,
Ϫ14 < k < 17,
Ϫ19 < l < 19
4327
reflect. collected
independent (Rint
)
6848 (0.0796)
6848 / 1 / 370
1.143
5550 (0.0678)
5550 / 0 / 273
1.074
4081 (0.0485)
4081 / 0 / 197
1.022
data/restr./param.
gof on F2
final R R1
0.0659
0.1296
0.1292
0.1557
0.0695
0.1705
0.1069
0.1942
0.0492
0.0859
0.1030
0.1008
indices wR2
R indices R1
(all data) wR2
Ϫ3
˚
largest diff. peak and hole /e A
1.414 and Ϫ1.401
2.176 and Ϫ2.921
1.898 and Ϫ0.988
a)
2
˚
Measurements at 173(2) K, Radiation wavelength λ ϭ 0.71073 A; Refinement method: Full-matrix least-squares on F ; Absorption correction: empirical,
psi-scans.
Figure 1 View of [Pt(Mes)2(iPr-DAB)] (left) and [PtCl(Mes)(iPr-DAB)] from crystal structure determination (with numbering). The atoms
are represented by 30 % thermal ellipsoids.
phenomena could be superimposed we feel a higher degree
of reliance for the concluded series from NMR data. Further-
more it is interesting to note that although the two imine
protons in the unsymmetrical complexes [PtCl(R)(iPr-DAB)]
are clearly non-equivalent, their JHH coupling is too small
to be detected.
shift in unpolar solvents) which is a strong argument for a
charge transfer transition being responsible for the colours.
On first view all spectra consist of three main band systems,
a broad structured band of medium intensity around
500 nm, a surprisingly invariant band at 360 nm of almost
the same intensity and finally sharp bands, also almost in-
variant, at 238 nm. The first two are tentatively assigned to
charge transfer transitions, the latter to πǞπ*DAB tran-
sitions. From the data we can conclude complexes contain-
ing the Mes co-ligand show markedly lower lying electronic
transitions than the corresponding Me derivatives and the
long-wavelength absorption bands are more complex.
In order to assign the observed absorptions we have cal-
culated the electronic ground states by DFT methods and
3
UV/VIS absorption spectroscopy and electronic states
The compounds under investigation vary in their colour
from red (Cl2) to deep violet (Me2) in the solid as well as
in solution. Figure 2 shows representative spectra, the long-
wavelengths absorption maxima are listed in Table 4. All
compounds exhibit strong negative solvatochromism (red
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Table 2 Selected structural data for iPr-DAB platinum complexes [PtCln(R)m(iPr-DAB)]
a)
b)
b)
˚
distances /A
(Me)2
(Mes)2
Cl(Me), calc
Cl(Mes)
Cl2, calc
Pt1ϪN1
Pt1ϪN2
Pt1ϪC11 (Cl2)
Pt1ϪC21 (Cl1)
C3ϪC4
N1ϪC3
N2ϪC4
N1ϪC2
N2ϪC5
2.034(19)
2.085(13)
2.05(2)
2.049(16)
1.46(3)
1.27(2)
1.30(2)
2.099(9)
2.126
1.996
2.051
2.283
1.429
1.303
1.318
1.452
1.451
2.113(7)
1.998(7)
2.010
2.010
2.289
2.289
1.433
1.314
1.314
1.459
1.459
2.110(10)
2.022(11)
2.038(12)
1.468(19)
1.274(16)
1.274(17)
1.489(15)
1.471(16)
2.022(9) (C)
2.286(2) (Cl)
1.438(13)
1.269(11)
1.301(11)
1.469(12)
1.490(12)
1.48(2)
1.47(2)
angles /°
N1ϪPt1ϪN2
N2ϪPt1ϪC11
C11ϪPt1ϪC21
N1ϪPt1ϪC21
N1ϪPt1ϪC11
N2ϪPt1ϪC21
torsion / dihedral
N1ϪC3ϪC4ϪN2
C2ϪN1ϪC3ϪC4
C5ϪN2ϪC4ϪC3
NNPt/ClnCmPt
78.9(6)
96.8(8)
88.1(9)
96.1(8)
175.6(8)
175.0(8)
angles /°
2(3)
178.2(16
177.0(16)
1.1, 2.8, 2.9
Ϫ
77.8(4)
94.9(4)
90.1(5)
97.3(4)
172.4(4)
174.0(4)
78.0
98.0
87.4
96.5
176.0
174.5
78.4(3)
96.5(3)
88.3(3) (Cl)
96.9(2) (Cl)
174.4(3)
79.3
95.4
89.9
95.4
174.7
174.7
175.2(2) (Cl)
0.5(19)
0.0
2.7(13)
178.0(9)
177.3(8)
2.5
0.6
175.4(11)
173.2(11)
4.1
179.8
179.8
0.1
179.7
179.8
1.3
c)
Mes/coord-plane
70.2, 72.3
Ϫ
81.2
Ϫ
a) Values from molecule 1. b) ADF/BP calculated for the corresponding Me-DAB model complex. c) Coordination plane as defined by NNPtClnCm.
1
a)
Table 3 Selected ADF/BP calculated and experimental H and 13C NMR data of iPr-DAB platinum complexes [PtCln(R)m(iPr-DAB)]
1H
13C
H3,4
exp.
H3,4
CH3
exp.
PtCH3
1.47
85.2
PtCH3
1.49
78.5
oCH3
2.33
6.2
C3,4
exp.
C3,4
PtC
exp.
C2,5
exp.
calc.b)
calc.b)
Me2
δ
PtCH3
Ϫ15.1
785
PtCH3
Ϫ13.0
706.7
PtC1
135.6
1174
8.99
34.0
7.39
35.6
158.1
14.6
146.7
44.7
JPt (Hz)
Cl(Me)
δ
JPt (Hz)
(Mes)2
δ
JPt (Hz)
Cl(Mes)
δ
JPt (Hz)
Cl2
8.54, 9.02
36.3, 106.2
7.29, 7.38
36.2, 112.4
160.5 158.1
6.9, 60.0
149.5, 144.3
69.4, 84.8
8.96
36.8
7.40
37.0
157.5
25.4
146.2
45.3
55.9
39.2
OCH3
2.33
8.3
PtC1
135.9
956.1
8.47, 8.98
36.4, 108.6
7.28, 7.38
34.5, 106.9
161.5, 159.8
< 13, 68.8
150.3, 145.3
69.8, 86.2
57.9, 60.3
26.6, 75.9
δ
8.54
98.5
7.20
90.4
Ϫ
Ϫ
162.2
n.d.
148.3
116.7
59.4
n.d.
c)
c)
JPt (Hz)
a) All measured in CDCl3, numbering according to scheme 1. b) Calculated for Me-DAB model complexes. c) Not detectable due to very low solubility.
electronic transitions using TD-DFT for the two model
complexes [PtCl(R)(Me-DAB)]. Here the G03/B3LYP
method was used since it allows excellent incorporation of
solvent effects (G03/B3LYP/CPCM) for the calculation of
electronic transitions. Figure 3 visualizes the qualitative
MO scheme calculated for the ground states of the two
Cl(R) compounds, the full data (including [PtCl2(Me-
DAB)]) can be found in the Supporting Information. Figure
3 reveals the different contributions of the Mes, Me and Cl
co-ligands: The lowest unoccupied molecular orbital
(LUMO) is in each case predominantly formed by the low-
est DAB π* orbital. If Cl is present, high lying occupied
MO are found with very mixed dPt ϩ pCl character. For the
Cl(Me) and the Cl2 derivative such orbitals form the highest
occupied MO (HOMO). Complexes containing the Mes co-
ligand exhibit additional levels which can be described as
pure π(Mes) or mixed dPt ϩ πMes orbitals. Figure 4 shows
some of the above discussed occupied orbitals of
[PtCl(Mes)(Me-DAB)]. The electronic situation for the Me2
derivative which has been reported recently [16] is charac-
terized by a moderate to small contribution (12 %) of the
Me sp3 hybrid orbitals to the mainly metal (dPt) centred
HOMO, a contribution that is entirely missing in the
Cl(Me) derivative. Although NMR and UV/Vis spec-
troscopy have revealed that the Me co-ligand is a very
strong ligand, he provides virtually no other contribution
than to (de)stabilise platinum orbitals as a strong σ donor.
The energies of the calculated electronic transitions for
the two model complexes [PtCl(R)(Me-DAB)] (R ϭ Mes or
Me) agree quite well with the experimental data (Table 5
and Table 6). Only the calculated long-wavelength tran-
sition in [PtCl(Me)(iPr-DAB)] is markedly lower in energy
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Z. Anorg. Allg. Chem. 2005, 631, 2669Ϫ2676

Organoplatinum Complexes of the N,N-Diisopropyl-diazabutadiene Ligand
Figure 2 Absorption spectra of [PtCl(Me)(iPr-DAB)] (top, left), [PtCl(Mes)(iPr-DAB)] (top, right) [PtCl2(iPr-DAB)] (bottom, left) and
[Pt(Mes)2(iPr-DAB)] (bottom, right) all in CH2Cl2 solution.
a)
Table 4 Observed electronic absorption maxima for complexes [PtCln(R)n(iPr-DAB)]
compound
λ6 (ε)
λ5 (ε)
λ4 (ε)
λ3 (ε)
λ2 (ε)
λ1 (ε)
(Me)2
(Mes)2
Cl(Mes)
Cl(Me)
Cl2
234 (17.0)
237 (15.8)
238 (13.5)
238sh (7.6)
238 (3.0)
Ϫ
390 (3.3)
355 (3.6)
359 (3.4)
362 (3.3)
364 (3.0)
543 (2.5)
438 (3.3)
492 (1.9)
498 (1.5)
471 (1.9)
Ϫ
Ϫ
281sh (6.5)
286 (1.8)
265sh (4.2)
258 (3.0)
533 (3.3)
535sh (1.5)
Ϫ
Ϫ
581sh (2.2)
598sh (0.4)
Ϫ
Ϫ
b)
c)
a) Measured in CH2Cl2, absorption maxima in wavelength [in nm] the molar extinction coefficients ε are given in parentheses (in 1000 MϪ1cmϪ1). b) Further
maximum at 276 nm (sh, ε ϭ 2.1). c) Further maximum at 291 nm (sh, ε ϭ 2.9).
than the experimentally found lowest band. The calculated
oscillator strength agree only qualitatively with the ob-
served intensities. However the overall agreement between
calculated and experimental data presented here is far bet-
ter compared to the previous TD-DFT calculations with
solvent cage modelled by the polarizable continuum model
(PCM) [27] for the complexes [Pt(R)2(Me-DAB)] and
[Pt(R)2(iPr-DAB)] (R ϭ methyl or aryl) [16]. In the G03/
B3LYP/CPCM method applied in this work the CPCM is
included in the TD-DFT formalism giving better agreement
between calculated and experimental energies.
with mainly MLCT character. The intense band around
240 nm consists of two individual transitions with mainly
π
DABǞπ*DAB character. For the complex [PtCl(Mes)(iPr-
DAB)] the three long-wavelength absorptions at 530 and
500 nm can be assigned to mixed MLCT/XLCT transitions
and the band at 360 nm to a more or less pure MLCT tran-
sition as discussed above for the Cl(Me) derivative. The fol-
lowing band at 286 nm is markedly weaker than the pre-
vious charge transfer bands. Following our TD-DFT calcu-
lations this band can be assigned to a mixed transition
(e1A) in which the target orbital has a high dPt contribution.
We can therefore assign the long-wavelength absorptions
for [PtCl(Me)(iPr-DAB)] to two electronic transitions of
In a simplified way this corresponds to the ligand field tran-
8
2
2
sition (dxyǞdx Ϫy ) in a d system. The same type of tran-
sition was also calculated for the Cl(Me) derivative (c1AЈ),
however in the corresponding spectrum this band is not re-
solved but appears as a shoulder. The next calculated tran-
sition (267 nm) can be observed as a shoulder at 276 nm.
The corresponding transition is again of mixed MLCT/
mixed metal(dPt)-to-ligand(π*DAB
)
/
chlorine-to-ligand
charge transfer (MLCT/XLCT) character. The starting or-
bitals (HOMO to HOMO-3) are of mixed dPt ϩ pCl charac-
ter and the target obital is the 13aЉ LUMO (π*DAB). The
intense absorption around 360 nm is due to a transition
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A. Klein, T. Schurr, S. Zalis
XLCT character. The UV absorption at 238 nm can be as-
signed to a πMesǞπ*DAB transition. Obviously the two sys-
tems [PtCl(R)(iPr-DAB)] differ markedly in the UV region:
For the Cl(Me) derivative two individual πDABǞπ*DAB
transitions were calculated and in both cases the LUMO
is the main target orbital. For the Cl(Mes) derivative the
transition (g1A) has mainly πMesǞπ*DAB character and the
target orbital is mainly the LUMOϩ2. Our calculated data
thus well explains the obvious differences in the experimen-
tal spectra Ϫ Cl(Me): two bands of medium strength vs.
Cl(Mes): one very strong band.
Experimental Part
Synthesis
All preparations and manipulations were carried out under an inert
atmosphere of argon or nitrogen. All complexes were prepared and
examined under the protection from intense light. The dimethyl
or dimesityl complexes [Pt(R)2(iPr-DAB)] (R ϭ Me or Mes) were
prepared as previously published for related complexes [16Ϫ18]
from [Pt(CH3)2(COD)] (COD
ϭ 1,5-cyclooctadiene [20] or
Figure 3 Calculated electronic levels in [PtCl(Me)(Me-DAB) ] (left)
and [PtCl(Mes)(Me-DAB)] (right).
[Pt(Mes)2(dmso)2] [28] and the iPr-DAB ligand and analyzed cor-
rectly. The halogeno complexes [PtCl(R)(iPr-DAB)] were generated
by addition of an excess of HCl (from acetyl chloride and meth-
anol) in toluene in the same way described by Clark and Manzer
[20] for [PtCl(Me)(COD)]. [PtCl(Me)(iPr-DAB)] was obtained as
deep red microcrystalline material in 95 % yield. Anal. Calcd. for
C9H19N2ClPt: C 28.02, H 4.96, N 7.26 %. Found: C 28.05, H 4.99,
N 7.22 %.
1H NMR (CDCl3): δ: 1.44 (d, 6 H, 3J ϭ 6.59 Hz, H1,7), 1.47 (d, 6 H, 3J ϭ
6.62 Hz, H6,8), 1.49 (s, 3 H, 3JPtH ϭ 78.5 Hz, PtCH3), 4.71 (septet, 1 H,
H5), 4.81 (septet, 1 H, H2), 8.54 (s, 1 H, 3JPtH ϭ 36.3 Hz, H3), 9.02 (s, 1 H,
3JPtH ϭ 106.2 Hz, H4).
[PtCl(Mes)(iPr-DAB)] was obtained as red violet microcrystalline
material in 97 % yield. Anal. Calcd. for C17H27N2ClPt: C 41.67, H
5.55, N 5.72 %. Found: C 41.69, H 5.59, N 5.77 %.
1H NMR (CD2Cl2): δ: 1.19 (d, 6 H, 3J ϭ 6.4 Hz, H1,7), 1.52 (d, 6 H, 3J ϭ
6.5 Hz, H6,8), 2.18 (s, 3 H, p-CH3), 2.33 (s, 6 H, 4J ϭ 6.4 Hz, o-CH3), 4.04
(septet, 1 H, H5), 4.79 (septet, 1 H, H2), 6.57 (s, 2 H, HMes), 8.47 (s, 1 H,
3J ϭ 36.4 Hz, H3), 8.98 (s, 1 H, 3JPtH ϭ 108.6 Hz, H4)
[Pt(Cl)2(iPr-DAB)]. [Pt(Cl)2(dmso)2] (211 mg, 0.5 mmol) and iPr-
DAB (73 mg, 0.52 mmol) were stirred together in 50 mL of CH2Cl2
for 5 hours. After addition of 30 mL of n-heptane the volume is
reduced to 50 mL during which a orange/red precipitate forms. The
collected solid was washed with pentane and recrystallized from
CH2Cl2 to give a red microcrystalline material. Yield: 98 %. Anal.
Calcd. for C8H16N2Cl2Pt: C 23.65, H 3.97, N 6.90 %. Found: C
23.69, H 3.99, N 6.90 %.
Figure 4 The HOMO 73a (πMes), HOMO-2 71a and HOMO-3 70a
(dPt ϩ pCl) orbitals of [PtCl(Mes)(Me-DAB)]
Table 5 TD-DFT G03/B3LYP/CPCM (CH2Cl2) singlet excitation energies(eV) for [PtCl(Me)(Me-DAB)] with oscillator strength larger
than 0.001.
State
Main components (%)
Transition energy
in eV (nm)
Osc. strength
Exper. trans.
in nm
Ext. coeff.
a)
b1AЈ
a1AЉ
b1AЈ
c1AЈ
d1AЈ
e1AЈ
83 (12aЉǞ13aЉ); 16 (11aЉǞ13aЉ)
84 (28aЈǞ 13aЉ); 15 (27aЈǞ13aЉ)
75 (11aЉǞ13aЉ); 14 (12aЉǞ13aЉ)
84 (28aЈǞ29aЈ); 10 (27aЈǞ29aЈ)
71 (10aЉǞ13aЉ); 13 (27aЈǞ29aЈ)
71 (9aЉǞ13aЉ); 13 (10aЉǞ13aЉ);
2.37 (522)
2.38 (521)
3.25 (382)
4.41 (281)
4.63 (267)
5.37 (230)
0.005
0.002
0.137
0.010
0.025
0.136
498
498
362
291sh
265sh
238sh
1.5
1.5
3.3
2.9
4.2
7.6
a) Extinction coefficient in 1000 MϪ1·cmϪ1
.
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Organoplatinum Complexes of the N,N-Diisopropyl-diazabutadiene Ligand
Table 6 TD-DFT G03/B3LYP/CPCM (CH2Cl2) singlet excitation energies(eV) for [PtCl(Mes)(Me-DAB)] with oscillator strength larger
than 0.001.
State
Main components (%)
Transition energy (eV)
Osc. strength
Exper. trans. (eV)
Ext. coeff.
a)
b1A
c1A
d1A
e1A
f1A
g1A
96 (71aǞ74a)
2.27 (546)
2.46 (503)
3.30 (376)
4.35 (285)
4.64 (267)
5.49 (226)
0.002
0.012
0.145
0.023
0.049
0.232
535sh
492
359
286
276sh
238
1.5
1.9
3.4
1.8
2.1
13.5
69 (70aǞ74a); 18 (68aǞ74a)
78 (68aǞ74a); 17 (70aǞ74a)
78 (71aǞ75a); 14 (69aǞ75a)
78 (66aǞ74a)
78 (73aǞ77a)
a) Extinction coefficient in 1000 MϪ1·cmϪ1
.
1H NMR (CD2Cl2): δ: 1.50 (d, 12 H, 3J ϭ 6.78 Hz, H1,6,7,8), 5.14 (septet,
2 H, H2,5), 8.51 (s, 2 H, 3JPtH ϭ 99.5 Hz, H3,4).
MOs and spectra. All results discussed correspond to optimized
geometries.
Crystal Structure Determination of [Pt(Me)2(iPr-DAB)],
[Pt(Mes)2(iPr-DAB)] and [PtCl(Mes)(iPr-DAB)]. Suitable crystals
were grown by slow evaporation of the solvent from saturated solu-
tions of [Pt(Me)2(iPr-DAB)] in diethyl ether, [Pt(Mes)2(iPr-DAB)]
in toluene, or [PtCl(Mes)(iPr-DAB)] in CH2Cl2. The X-ray data
were collected at 173(2) K on a Siemens P4 diffractometer, using
Physical Measurements
NMR spectra were recorded on Bruker AC 250 (1H: 250.13 MHz,
13C: 62.90 MHz) or Bruker Avance 400 spectrometers (1H:
400.13 MHz, 13C: 100.3 MHz) in CDCl3 at 30 °C; chemical shifts
were referenced to ext. TMS. UV/VIS absorption spectra were re-
corded on Bruins Instruments Omega 10 and Hewlett-Packard
8453 Diode Array spectrophotometers.
˚
graphite monochromated Mo-Kα radiation (λ ϭ 0.71073 A) and
employing Wyckoff scans. Further details are given in Table 1. All
structures were solved by direct methods using the SHELXTL
package, while refinement was carried out with SHELXL97 em-
2
ploying full-matrix least-squares methods on F2 with F02 Ն 2σ(F0 )
[41]. All non-hydrogen atoms were refined anisotropically, hydrogen
atoms were introduced using appropriate riding models. Empirical
absorption correction was performed using Ψ-scans. Crystallo-
graphic data have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC-264430 [Pt(Me)2(iPr-DAB)], -264431
[Pt(Mes)2(iPr-DAB)], and -264432 [PtCl(Mes)(iPr-DAB)]. Copies
of the information may be obtained free of charge from: The Direc-
tor, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
(ϩ44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk; www: http://
www.ccdc.cam.ac.uk).
Theoretical Calculations
Ground state electronic structure calculations on model complexes
[PtCln(R)m(Me-DAB)] have been done on the base of density-func-
tional theory (DFT) methods using the ADF2004.01 [29, 30] and
Gaussian 03 [31] program packages. The lowest excited states of
the closed shell complexes were calculated by the time-dependent
DFT (TD-DFT) method (both ADF and G03 programs). The elec-
trostatic solvent effect was modelled by conductor-like polarizable
continuum model CPCM [32] incorporated into G03. Nuclear spin-
spin coupling constants were calculated by the CPL module [33,
34] and NMR shielding tensors by the NMR module [35] within
ADF2004.01.
Supporting Information: Further figures showing the packing of the
molecules in the unit cell and the molecular structure of [Pt(Me)2(-
iPr-DAB)] are provided. Furthermore two tables comparing exper-
imental and calculated structural data, two tables with complete
1H and 13C NMR data and four tables presenting G03/B3LYP cal-
culated one-electron energies and composition of selected MOs of
[PtCl(Me)(Me-DAB)], [PtCl(Mes)(Me-DAB)] and [PtCl2(Me-
DAB)] and TD-DFT (G03/B3LYP/CPCM) calculated excitation
energies for [PtCl2(Me-DAB)].
Within the ADF program, Slater type orbital (STO) basis sets of
triple-ζ quality with polarization functions were employed for geo-
metry optimisation and TD-DFT calculations. The inner shells
were represented by frozen core approximation (1s for C, N, 2p for
Cl and 1s-4d for Pt were kept frozen). Core electrons were included
for calculations of NMR parameters, here quadruple-ζ with four
polarization functions basis was used for Pt. The following density
functionals were used within ADF: the local density approximation
(LDA) with VWN parametrisation of electron gas data or the func-
tional including Becke’s gradient correction [36] to the local ex-
change expression in conjunction with Perdew’s gradient correction
[37] to the LDA expression (ADF/BP). The scalar relativistic (SR)
zero order regular approximation (ZORA) was used within this
study.
Acknowledgement. This work was carried out in the framework of
the COST D14 and D35 actions. A.K. would like to thank the
late Prof. D. J. Stufkens and Dr. Joris van Slageren (University of
Amsterdam) for their hostility and for fruitful discussions. We are
also grateful for a loan of K2PtCl4 by Johnson Matthey (JM). S.Z.
also acknowledges financial support by the Grant Agency of the
Academy of Sciences of the Czech Republic (grant 1ET400400413).
Within Gaussian 03 Dunning’s polarized valence double-ζ basis
sets [38] were used for C, N, Cl and H atoms and the quasirelativis-
tic effective core pseudopotentials and corresponding optimized set
of basis functions [39] for Pt. Hybrid Becke’s three parameter func-
tional with Lee, Yang and Parr correlation functional (B3LYP) [40]
was used in Gaussian calculations (G03/B3LYP).
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