Synthesis, structure, optical properties and theoretical studies of Pt(P-P)(CN)2 with P-P = 1,2-bis(diphenylphosphanyl)benzene and 2,2′-bis(diphenylphosphanyl)-1,1′-binaphthyl - Luminescence from metal-to-ligand charge transfer and intraligand states

Article abstract of DOI:10.1002/ejic.200400235

The complexes Pt^II(P-P)(CN)₂ with P-P = 1,2-bis(diphenylphosphanyl)benzene (dppb) and 2,2′-bis(diphenylphosphanyl) -1,1′-binaphthyl (binap) were synthesised and characterised by elemental analysis, ESI MS and electronic spectroscopy. The structure of Pt(dppb)(CN) ₂ was resolved by single-crystal X-ray diffraction. This compound consists of mononuclear Pt^II complexes with an almost planar PtP ₂C₂ coordination. While both complexes are not luminescent in solution, they show an emission in the solid state under ambient conditions. It is suggested that the luminescence of Pt(dppb)(CN)₂ originates from a (Pt^II→dppb) metal-to-ligand charge transfer (MLCT) triplet with some dppb intraligand (IL) contribution. This assignment is confirmed by calculations which provide further insight into the excited state properties of the complex. In solution the phosphorescence is absent because the MLCT/IL state is deactivated to a non-emissive ligand field (LF) triplet, which is located at rather low energies owing to its distortion towards a tetrahedral structure. In the solid state this distortion is hindered. In the case of solid Pt(binap)(CN)₂, the phosphorescence is apparently of the IL type as the ππ* triplet of the binaphthyl chromophore occurs at low energies. Again, in solution the phosphorescence is absent owing to the interference by a distorted LF state. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400235

FULL PAPER
Synthesis, Structure, Optical Properties and Theoretical Studies of
Pt(P؊P)(CN)2 with P؊P ؍ 1,2-Bis(diphenylphosphanyl)benzene and
2,2Ј-Bis(diphenylphosphanyl)-1,1Ј-binaphthyl ؊ Luminescence from
Metal-to-Ligand Charge Transfer and Intraligand States
Valeri Pawlowski,[a] Horst Kunkely,[a] Christian Lennartz,*[b] Karlheinz Böhn,[b] and
Arnd Vogler*[a]
Keywords: Electronic spectra / Luminescence / Platinum complexes / Phosphane complexes / Quantumchemical calculations
The complexes PtII(P−P)(CN)2 with P−P = 1,2-bis(diphenyl-
tions which provide further insight into the excited state
phosphanyl)benzene (dppb) and 2,2Ј-bis(diphenylphos- properties of the complex. In solution the phosphorescence
phanyl)-1,1Ј-binaphthyl (binap) were synthesised and char- is absent because the MLCT/IL state is deactivated to a non-
acterised by elemental analysis, ESI MS and electronic spec-
emissive ligand field (LF) triplet, which is located at rather
troscopy. The structure of Pt(dppb)(CN)2 was resolved by low energies owing to its distortion towards a tetrahedral
single-crystal X-ray diffraction. This compound consists of
mononuclear PtII complexes with an almost planar PtP2C2 co-
ordination. While both complexes are not luminescent in so-
lution, they show an emission in the solid state under ambi-
ent conditions. It is suggested that the luminescence of
Pt(dppb)(CN)2 originates from a (PtIIǞdppb) metal-to-ligand
charge transfer (MLCT) triplet with some dppb intraligand
(IL) contribution. This assignment is confirmed by calcula-
structure. In the solid state this distortion is hindered. In the
case of solid Pt(binap)(CN)2, the phosphorescence is appar-
ently of the IL type as the ππ* triplet of the binaphthyl chrom-
ophore occurs at low energies. Again, in solution the phos-
phorescence is absent owing to the interference by a dis-
torted LF state.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
1. Introduction
placed by other strong-field ligands with low-energy excited
states. Promising candidates are bidentate phosphanes that
carry aromatic chromophores. We explored this possibility
and selected the complex PtII(PϪP)(CN)2 with PϪP ϭ 1,2-
bis(diphenylphosphanyl)benzene (dppb) and 2,2Ј-bis(di-
phenylphosphanyl)-1,1Ј-binaphthyl (binap) for the present
study (Scheme 1).
A variety of square-planar PtII complexes has been
shown to be luminescent under ambient conditions.[1Ϫ3] Di-
verse potential applications have stimulated interest in these
compounds.[3] Generally, mononuclear PtII complexes emit
from their lowest-energy triplet, which may be of the metal-
to-ligand charge transfer (MLCT), ligand-to-ligand charge
transfer (LLCT) or intraligand (IL) type. PtII complexes
with low-energy ligand field (LF) triplets are not emissive
at room temperature. The complex Pt(1,2-diimine)(CN)2
with 1,2-diimine ϭ 4,7-diphenyl-1,10-phenanthroline is a
typical example of a PtII complex which emits from an IL
triplet.[4] This room temperature phosphorescence is facili-
tated by the low energy of the ππ* diimine IL states as well
as by the strong LF strength of the cyanide ligands which
push interfering LF states to rather high energies.[4] The
number of emissive PtII complexes may be increased con-
siderably if the diimines of Pt(1,2-diimine)(CN)2 are re-
Scheme 1
The aromatic chromophores of these ligands were ex-
pected to serve as luminophores[5] of both complexes. In
this context it should be mentioned that a few PtII phos-
phane complexes have been reported to display lumi-
nescence. However, these emissions do not originate from
IL states. In some cases the phosphanes are only spectator
[a]
Institut für Anorganische Chemie, Universität Regensburg,
93040 Regensburg, Germany
E-mail: arnd.vogler@chemie.uni-regensburg.de,
christian.lennartz@basf-ag.de
[b]
BASF AG, Computational Chemistry and Biology,
GVC/C-A30, 67057 Ludwigshafen, Germany
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400235
Eur. J. Inorg. Chem. 2004, 4242Ϫ4246

Luminescence from Metal-to-Ligand Charge Transfer and Intraligand States
FULL PAPER
ligands and are not directly involved in emission.[6Ϫ8] In of Pt(binap)(CN)2 in CH3CN (Figure 2) contains bands at
other cases the luminescence is associated with λmax ϭ 343 (3500) and shoulders at about 314 (5100), 290
metalϪmetal interactions in binuclear PtII complexes.[9,10] (8500), 262 (33 500) and 220 nm (86 000). Again, in solu-
Our target compounds Pt(PϪP)(CN)2 were expected to be tion the complex does not emit, but the solid compound
accessible by simple synthetic procedures.
exhibits a luminescence at λmax ϭ 513 nm with distinct
structural features at 470, 550, 600 and 640 nm (Figure 2).
2. Results and Discussion
Experimental Results
The complexes Pt(dppb)(CN)2 and Pt(binap)(CN)2 were
obtained as white or slightly yellow solids by the reaction
of Pt(CN)2 with dppb and binap, respectively. The compo-
sition was confirmed by elemental analysis and mass spec-
troscopy. The positive ion electrospray mass spectrum
(ϩESI MS) of [Pt(dppb)(CN)2] in CH3CN (ϩ 1%
CH3COOH) consists of peaks at m/z ϭ 693, 694, 695, 696
and 697. The complex has apparently added one proton at
the cyanide ligands. In addition, peaks at 1358, 1359, 1360,
1361, 1362, 1363 and 1364 indicate the presence of the di-
mer Pt2(dppb)2(CN)4 which has lost one cyanide ligand.
The ϩESI MS spectrum of the binap complex shows an
analogous pattern. The complex Pt(binap)(CN)2 in its mon-
oprotonated form displays peaks at 869, 870, 871, 872 and
873, while peaks of the cation [Pt2(binap)2(CN)4]ϩ appear
at 1710, 1711, 1712, 1713, 1714, 1715, 1716 and 1717. In
all cases there is a good agreement between the calculated
and observed isotopic peak pattern.
Figure 2. Electronic absorption (a) and emission (e) spectrum of
Pt(binap)(CN)2 at room temperature; absorption: 2.06 ϫ 10Ϫ5

in CH3CN, 1-cm cell; emission: solid, λexc ϭ 350 nm, intensity in
arbitrary units
Interpretation
Complexes with the general composition PtII(PϪP)(CN)2
may be monomeric or dimeric[9,10] (Scheme 2).
The electronic spectrum of dppb in CH3CN/CH2Cl2 (1:1)
exhibits an absorption at λmax ϭ 276 nm (ε ϭ 17 920 Ϫ1
cmϪ1). At room temperature, solutions of dppb are not
emissive, but in ethanol glasses at 77 K a strong phosphor-
escence appears at λmax ϭ 502 nm. The absorption spec-
trum of Pt(dppb)(CN)2 in CH3CN (Figure 1) shows bands
at λmax ϭ 295 (1420), 267 (12 940) and 225 nm (sh, 51 570).
Solutions of the complex do not emit. However, at room
temperature the solid compound shows a slightly structured
luminescence (Figure 1) at λmax ϭ 502 nm with shoulders
at approximately 480 and 540 nm. The absorption spectrum
Scheme 2
In the dimers the cyanide ligands occupy trans positions
and both square-planar PtII complex moieties are connec-
ted by bridging phosphanes. With regard to the complexes
Pt(dppb)(CN)2 and Pt(binap)(CN)2, our analytical data do
not provide an unambiguous distinction. The MS spectra
indicate the presence of monomers as well as dimers. How-
ever, the dimers Pt2(PϪP)2(CN)4 with PϪP ϭ bis(diphenyl-
phosphanyl)methane, bis(dimethylphosphanyl)methane and
bis(dicyclohexyl-phosphanyl)methane show an intense ab-
sorption (ε Ͼ 104) around 330 nm, which is associated with
the PtϪPt interaction.[9,10] These absorptions do not appear
in the spectra of Pt(dppb)(CN)2 and Pt(binap)(CN)2. Ac-
cordingly, the latter complexes are either monomeric at least
in solution, or dimeric but with large PtϪPt distances that
prevent any efficient metalϪmetal interaction.
For Pt(dppb)(CN)2, the monomeric structure was con-
firmed by single-crystal X-ray diffraction (Figure 3). The
complex is essentially planar with PϪPtϪP and CϪPtϪC
bond angles of 86.6° and 89.3°, respectively. The average
Figure 1. Electronic absorption (a) and emission (e) spectrum of
Pt(dppb)(CN)2 at room temperature; absorption: 1.54 ϫ 10Ϫ5

in CH3CN, 1-cm cell; emission: solid, λexc ϭ 300 nm, intensity in
arbitrary units
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V. Pawlowski, H. Kunkely, C. Lennartz, K. Böhn, A. Vogler
FULL PAPER
Figure 3. Crystal structure of monomeric Pt(dppb)(CN)2: visualisation of the steric interactions between the Pt(CN)2 fragment and two
phenyl groups from neighbouring molecules
˚
metalϪligand bond lengths are 2.26 A for PtϪP and 1.98
˚
A for PtϪC. Cyanide is coordinated in a linear fashion.
The lowest-energy excited state of Pt(dppb)(CN)2 seems
to be of the IL type since the complex and the free ligand
show this phosphorescence at the same wavelength (λmax ϭ
502 nm). However, the lowest-energy transitions of simple
aromatic phosphanes are associated with the promotion of
an electron from the lone pair (l) at the phosphorus atom
to the π* orbitals of the aromatic substituents.[11] Upon co-
ordination the lone pair is stabilised by σ-bonding. As a
consequence, the lǞπ* transition of the phosphane be-
comes a σǞπ* IL transition that should be shifted to
higher energies.[11] This concept does not seem to apply to
dppb and Pt(dppb)(CN)2. Fortunately, theory provides a
satisfactory explanation. According to density functional
calculations, the emitting state of the complex is a MLCT Figure 4. Frontier orbitals of the electronic transition at planar
geometry: HOMO (left) and LUMO (right)
state that contains a σǞπ* IL contribution. The HOMO is
essentially metal-based but also includes the PtϪP bonds,
while the LUMO is largely restricted to the aromatic sub- the ground state, while the geometry is heavily distorted in
stituents (Figure 4). the triplet state yielding a deformed tetrahedral geometry
To get further insight into the electronic structure and as indicated in Scheme 3.
geometry of the excited state of the Pt(dppb)(CN)2 mol-
The angle between the cyanocarbons and Pt amounts to
ecule, additional density functional calculations were per- 160°, thus resembling more a linear coordination geometry
formed. Optimising the geometries of the singlet-ground regarding platinum and the two cyano groups than a true
state (S0) and the first triplet state (T1) results in a quad- tetrahedron. Due to this distortion, the calculated energy
ratic-planar coordination geometry with respect to PtII for gap between the triplet state and the ground state is reduced
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Luminescence from Metal-to-Ligand Charge Transfer and Intraligand States
FULL PAPER
tronic transition to the ground state is now basically of a
d-d (LF) type.
The crystal structure of Pt(dppb)(CN)2 (Figure 3) shows
that, in contrast to the isolated molecule, the triplet distor-
tion is hindered by intermolecular steric interactions be-
tween the Pt(CN)2 fragment and two phenyl groups from
neighbouring molecules. These groups are oriented perpen-
dicular to the plane spanned by Pt and the two CN groups.
Scheme 3
For the complex Pt(binap)(CN)2, the situation could be
to 0.1 eV at the triplet geometry. Therefore energetic relax- different as the binaphthyl substituent is characterised by
ation of the triplet state should be dominated by nonradiat- an extended π-electron system. Accordingly, the lowest-en-
ive vibrational relaxation, thereby explaining the nonemiss- ergy transition of binap may not be of the lǞπ* type but
ive behaviour of the molecule in solution.
rather a ππ* transition of the binaphthyl moiety. This as-
To understand the origin of this distortion an orbital cor- sumption is supported by a comparison of the absorption
relation diagram was derived from the density functional and emission spectra of Pt(binap)(CN)2 and Au2Cl2(bi-
calculations. A schematic version is shown in Figure 5. nap).[12] For the latter complex it is quite clear that the low-
Starting on the left the excitation process promotes one est-energy transitions are of the binap IL type as gold()
electron from the HOMO to the LUMO and generates the complexes do not have available metal-centred or MLCT
S1-state. The character of this transition is best described as excited states at low energies.[13] Indeed, the long-wave-
MLCT, which can be seen in Figure 4, where the frontier length region of the absorption spectrum of Pt(binap)(CN)2
orbitals are shown. If the geometry of the S1-state is now between 270 and 400 nm (Figure 2) is very similar to that
slightly distorted in a tetrahedral way due to corresponding of Au2Cl2(binap). Moreover, the emission spectra of both
normal modes, then orbital 28aЈЈ will be substantially complexes are also rather similar and resemble the phos-
stabilised, while orbital 30a’ will be destabilised, in such a phorescence of the free binap ligand which, however, is only
way that these two orbitals now become the frontier orbitals observed at low temperatures.[14] The IL assignment for the
and a net stabilisation energy for the whole system is phosphorescence of Pt(binap)(CN)2 is also supported by
gained. Spin-orbit coupling yields the final triplet state. The the fact that the emission shows a pronounced structure
reason for this orbital behaviour can be seen from the sche- which is typical for the phosphorescence from IL states.[1,15]
matic orbital pictures. While an antibonding interaction in The phosphorescence of Pt(binap)(CN)2 appears only in the
orbital 28aЈЈ vanishes during the rotation, an antibonding solid state and is absent in solution. Again, it is suggested
character is introduced into orbital 30aЈ resulting in a com- that a rigid structure of the solid compound prevents a dis-
pletely different frontier orbital structure for the triplet tortion towards a tetrahedral structure of a low-energy LF
state. As can be seen from orbitals 22aЈЈ and 38aЈ, the elec- state that does not emit in solution.
Figure 5. Schematic view of the orbital correlation diagram; only the orbitals responsible for the geometric rearrangement are shown
Eur. J. Inorg. Chem. 2004, 4242Ϫ4246
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V. Pawlowski, H. Kunkely, C. Lennartz, K. Böhn, A. Vogler
FULL PAPER
combination with a split-valence basis set with polarisation func-
tions on all heavy atoms [SV(P)].[17] For platinum, an effective core
potential with relativistic corrections (ecp-60-mwb: derived from a
multielectron fit to the quasi-relativistic WoodϪBoring total val-
ence energies) was employed.[18] The corresponding valence part
was of SVP quality.[19,20] Triplet states were optimised with the un-
restricted KS approach. Resolution of identity techniques[19] was
used throughout. All calculations were carried out with the
TURBOMOLE software package.[21]
In summary, solid compounds of the type Pt(PϪP)(CN)2
with bidentate phosphane ligands such as dppb and binap,
which carry aromatic chromophores, are triplet emitters un-
der ambient conditions. This phosphorescence originates
from MLCT and/or IL states.
Experimental Section
The orbital correlation diagram is based on the assumption that
the relevant excitations are sufficiently described by one-electron
transitions between orbitals. We performed additional TD-DFT
calculations using the same functional and basis set as described
above to check for the single excitation character of the corre-
sponding transitions. The weight of the leading configuration
amounts to more then 80% for all relevant singlet excitations. This
is also true in the case of the lowest triplet excitation based on the
same singlet ground state orbitals for nondistorted geometries. For
the distorted triplet geometry the UKS orbitals closely resemble
the corresponding RKS orbitals. Therefore the one-electron in-
terpretation is justified.
Materials: All solvents used for spectroscopic measurements were
of spectrograde quality. Pt(CN)2, dppb and binap were commer-
cially available (Aldrich and Strem) and used without further puri-
fication. The complexes Pt(PϪP)(CN)2 were obtained by the fol-
lowing procedures.
[Pt(dppb)(CN)2]: A mixture of Pt(CN)2 (0.76 g, 3 mmol) and dppb
(1.38 g, 3 mmol) in dimethylformamide (70 mL) was refluxed for
6 h. A white powder slowly precipitated. It was collected by fil-
tration, washed with diethyl ether and dried over silica gel under
reduced pressure. The resulting white material was purified by
recrystallisation from dichloroethane/ether yielding 0.92 g (43%).
C32H24N2P2Pt (693.58): calcd. C 55.42, H 3.49, N 4.04; found C
55.06, H 3.60, N 3.94.
Acknowledgments
Financial support of the Regensburg group by BASF is gratefully
acknowledged.
[Pt(binap)(CN)2] ؋ H2O: A mixture of Pt(CN)2 (0.25 g, 1 mmol)
and binap (0.63 g, 1 mmol) in dimethylformamide (40 mL) was re-
fluxed for 20 h. After filtration, ether was added to the solution. A
slightly yellow powder precipitated. It was collected by filtration,
washed with diethyl ether and dried over silica gel under reduced
pressure. The resulting slightly yellow material was recrystallised
from dichloromethane/ether yielding 0.58 g (65%). C46H34N2OP2Pt
(887.82): calcd. C 62.23, H 3.86, N 3.16; found C 62.68, H 3.90,
N 3.07.
[1]
A. Vogler, H. Kunkely, Top. Curr. Chem. 2001, 213, 143.
[2]
J. M. Bevilacqua, R. Eisenberg, Inorg. Chem. 1994, 33, 2913.
[3]
J. Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba,
R. Bau, M. E. Thompson, Inorg. Chem. 2002, 41, 3055, and
references cited therein.
[4]
H. Kunkely, A. Vogler, J. Am. Chem. Soc. 1990, 112, 5625.
[5]
A. Vogler, H. Kunkely, Coord. Chem. Rev. 2002, 230, 243.
[6]
V. W.-W. Yam, P. K.-Y. Yeung, K.-K. Cheung, J. Chem. Soc.,
X-ray Crystallographic Data for [Pt(dppb)(CN)2]: C32H24N2P2Pt,
M ϭ 693.6; T ϭ 203(2) K; monoclinic, P21/n (no. 14); a ϭ
Dalton Trans. 1994, 2587.
J. M. Bevilacqua, J. A. Zuleta, R. Eisenberg, Inorg. Chem.
[7]
˚
˚
˚
1994, 33, 258.
10.031(2) A, b ϭ 14.885(7) A, c ϭ 18.812(5) A, β ϭ 98.138 (5);
[8]
3
˚
J. DePriest, G. Y. Zheng, N. Goswami, D. M. Eichhorn, C.
V ϭ 2781(2) A ; Z ϭ 4; Data from 2817 reflections, 2626 unique
[R(int.) ϭ 0.047], were collected with a Siemens P4 diffractometer
by use of Cu-Kα radiation. The structure was refined by full-matrix
least-squares refinement with the SHELXL-97 programmes. All
non-H atoms were refined anisotropically, hydrogen atoms were in-
cluded in riding mode with Uiso(H) ϭ 1.2 Ueq(C) to give R1 ϭ
0.051, wR2 ϭ 0.137 [I Ͼ 2σ(I)] and R1 ϭ 0.067, wR2 ϭ 0.146
(all data).
Woods, D. P. Rillema, Inorg. Chem. 2000, 39, 1955.
C.-M. Che, V. W.-W. Yam, W.-T. Wong, T.-F. Lai, Inorg. Chem.
[9]
1989, 28, 2908.
[10]
B.-H. Xia, C.-M. Che, D. L. Phillips, K.-H. Leung, K.-K.
Cheung, Inorg. Chem. 2002, 41, 3866.
[11]
C. Kutal, Coord. Chem. Rev. 1990, 99, 213.
[12]
V. Pawlowski, H. Kunkely, A. Vogler, Inorg. Chim. Acta 2004,
357, 1309.
[13]
A. Vogler, H. Kunkely, Coord. Chem. Rev. 2001, 219Ϫ221, 489.
CCDC-226785 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 IEZ, UK: Fax: ϩ44-1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk].
[14]
H. Kunkely, A. Vogler, Inorg. Chem. Commun. 1999, 2, 533.
[15]
H. Kunkely, A. Vogler, Chem. Phys. Lett. 2000, 319, 486.
[16]
J. P. Perdew, Phys. Rev. B 1986, 33, 8822; A. D. Becke, Phys.
Rev. A 1988, 36, 3098.
[17]
A. Schäfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 9, 2571.
[18]
D. Andrae, U. Haeussermann, M. Dolg, H. Stoll, H. Preuss,
Theor. Chim. Acta 1990, 77, 123.
Instrumentation: Absorption spectra were measured with a Shim-
adzu 2100 spectrophotometer. Emission spectra were recorded on
a Hitachi 850 spectrofluorometer equipped with a Hamamatsu 928
photomultiplier for measurements up to 900 nm. The mass spectra
were obtained using a TSQ 7000 Finnigan Thermoquest mass spec-
trometer.
[19]
K. Eichkorn, F. Weigend, O. Treutler, R. Ahlrichs, Theor.
Chem. Acc. 1997, 97, 119.
K. Eichkorn, O. Treutler, H. Oehm, M. Häser, R. Ahlrichs,
Chem. Phys. Lett. 1995, 240, 283.
R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Cölmel, Chem.
Phys. Lett. 1989, 162, 165.
Received March 23, 2004
Early View Article
[20]
[21]
Calculations: Geometries and energies were calculated with the use
of density functional theory involving the BP86 functional,[16] in
Published Online September 9, 2004
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Eur. J. Inorg. Chem. 2004, 4242Ϫ4246