Participation of Co-ligands in electronic transitions of platinum(II) diazabutadiene complexes

Article abstract of DOI:10.1021/ic025547o

The low-lying electronic transitions and photochemical reactions of a series of [(ⁱPr-DAB)Pt(R)₂] (where the coligand R = CH₃, CD₃, adme, neop, neoSi, C≡CⁱBu, C≡CPh, Ph, Mes) compounds were studied using both experimental (electronic absorption and resonance Raman spectroscopy) and theoretical (density functional theory, DFT) techniques. The high-lying filled orbitals were revealed to have a significant co-ligand contribution in the case of alkyl complexes, while this contribution is predominant for the complexes with unsaturated co-ligands. Because the electronic transition removes electron density from the σ(Pt-C) bond in the former complexes, it is best described as a metal-to-ligand charge transfer transition (MLCT) with partial sigma-bond-to-ligand charge transfer (SBLCT) character. Because the σ(Pt-C) orbital is not involved in the HOMOs of the latter complexes, the low-lying transitions were characterized as mixed MLCT/L'LCT, where L'LCT stands for ligand-to-ligand charge transfer from the π system of the unsaturated co-ligand to the τ^*(ⁱPr-DAB) orbital. The alkyl complexes are photoreactive on visible light irradiation with Pt-C bond homolysis as the primary step. The efficiency of the photoreaction increases with increasing σ donor strength of the alkyl ligand. The absolute quantum yield is quite low. The other complexes are virtually photostable, except when irradiated at relatively high energies.

Full text of DOI:10.1021/ic025547o

Inorg. Chem. 2002, 41, 5216−5225
Participation of Co-Ligands in Electronic Transitions of Platinum(II)
Diazabutadiene Complexes
,†
Axel Klein,* Joris van Slageren,‡ and Stanislav Za´lisˇ§
Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart,
Germany, Institute of Molecular Chemistry, UniVersiteit Van Amsterdam, Nieuwe Achtergracht
166, NL-1018 WV Amsterdam, The Netherlands, and J. HeyroVsky´ Institute of Physical Chemistry,
Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18 000 Prague 8, Czech Republic
Received February 19, 2002
i
The low-lying electronic transitions and photochemical reactions of a series of [(Pr-DAB)Pt(R)2] (where the co-
t
ligand R ) CH , CD , adme, neop, neoSi, CtCBu, CtCPh, Ph, Mes) compounds were studied using both
3
3
experimental (electronic absorption and resonance Raman spectroscopy) and theoretical (density functional theory,
DFT) techniques. The high-lying filled orbitals were revealed to have a significant co-ligand contribution in the case
of alkyl complexes, while this contribution is predominant for the complexes with unsaturated co-ligands. Because
the electronic transition removes electron density from the σ(Pt−C) bond in the former complexes, it is best described
as a metal-to-ligand charge transfer transition (MLCT) with partial sigma-bond-to-ligand charge transfer (SBLCT)
character. Because the σ(Pt−C) orbital is not involved in the HOMOs of the latter complexes, the low-lying transitions
were characterized as mixed MLCT/L′LCT, where L′LCT stands for ligand-to-ligand charge transfer from the π
i
system of the unsaturated co-ligand to the π*(Pr-DAB) orbital. The alkyl complexes are photoreactive on visible
light irradiation with Pt−C bond homolysis as the primary step. The efficiency of the photoreaction increases with
increasing σ donor strength of the alkyl ligand. The absolute quantum yield is quite low. The other complexes are
virtually photostable, except when irradiated at relatively high energies.
Introduction
have moderately strong tunable absorption bands in the
visible spectrum. Depending on the nature of R, they can
show luminescence, even in fluid solution at ambient
temperatures,3,5,7,9,11,12 which makes them attractive for
various applications in the molecular photonics field.14-18
Generally, low-energy electronic absorption bands are as-
cribed to metal-to-ligand charge transfer (MLCT) absorption
Organoplatinum(II) complexes of the type [(R-diimine)-
Pt(R)2] with R ) alkyl, alkynyl, or aryl are widely
studied.1-13 One of the main reasons is that these complexes
* To whom correspondence should be addressed. E-mail: aklein@iac.
uni-stuttgart.de.
† Universita¨t Stuttgart.
‡ Universiteit van Amsterdam.
§ Academy of Sciences of the Czech Republic. E-mail: zalis@
jh-inst.cas.cz.
(10) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053.
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A.; Stoeckli-Evans, H. Coord. Chem. ReV. 1994, 132, 75.
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(3) Hissler, M.; McGarrah, J. E.; Connick, W. B.; Geiger, D. K.;
Cummings, S. D.; Eisenberg, R. Coord. Chem. ReV. 2000, 208, 115.
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D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447.
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D. K.; Eisenberg, R. Coord. Chem. ReV. 1998, 171, 125.
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Inorg. Chem. 2000, 39, 2585.
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(12) Vogler, C.; Schwederski, B.; Klein, A.; Kaim, W. J. Organomet. Chem.
1992, 436, 367.
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L. L. Inorg. Chem. 2000, 39, 5192.
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Horwood: Chichester, U.K., 1991.
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(17) Scandola, F.; Argazzi, R.; Bignozzi, C. A.; Chiorboli, C.; Indelli, M.
T.; Rampi, M. A. Antenna Effects and Photoinduced Electron Transfer
in Polynuclear Metal Complexes. In Supramolecular Chemistry; de
Cola, L., Balzani, V., Eds.; NATO ASI Series C; Kluwer: Dordrecht,
The Netherlands, 1992; Vol. 371, p 235.
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2001, 40, 4510.
5216 Inorganic Chemistry, Vol. 41, No. 20, 2002
10.1021/ic025547o CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/10/2002

Platinum(II) Diazabutadiene Complexes
i
Scheme 1. Schematic Representation of Pr-DAB )
of the complexes was calculated using density functional
theory (DFT) methods, which were also used to determine
the nature of the low-energy electronic transitions. Addition-
ally, these transitions were studied by resonance Raman (rR)
spectroscopy, this being virtually the only technique that
gives direct experimental information about the nature of
electronic transitions which are responsible for observed
absorption bands. The vibrations which were found in the
rR spectra were assigned by comparison with DFT calcula-
tions of the molecular vibrations.
N,N′-Diisopropyl-1,4-diazabutadiene in the Noncoordinating Free Form
(Left) and in the Square Planar Platinum(II) Complexes
[(iPr-DAB)Pt(R)2] (Right)
Experimental Section
bands. Correspondingly, the lowest excited state is usually
assumed to have MLCT character as well. These assignments
agree with the pronounced solvatochromism and the extinc-
tion coefficients of the absorption bands.
Synthesis. All preparations and manipulations were carried out
under an inert atmosphere of argon or nitrogen. All complexes were
prepared and examined under protection from intense light. The
dimethyl or dimesityl complexes [(R′-DAB)Pt(R)2] (R ) CH3 or
Mes) were prepared as previously published for related com-
plexes,24-26 from [(CH3)2Pt(µ-SMe2)Pt(CH3)2]27 or [(dmso)2Pt-
(Mes)2]28 and the corresponding R′-DAB ligands.20 The organo-
platinum COD (1,5-cyclooctadiene) or NBD (norbornadiene )
bicyclo[2.2.1]hepta-2,5-diene) complexes including the deuterated
derivative [(NBD)Pt(CD3)2] were obtained as previously described
for the COD derivatives.19
In recent studies on a series of [(COD)Pt(R)2] complexes
(COD ) 1,5-cyclooctadiene), we have found that the high
lying occupied orbitals are not exclusively located on the Pt
atom.19 For R ) alkyl, these orbitals show increasing
contributions of the R ligand with increasing σ donor strength
of that ligand. If R possesses π orbitals, as in the case of R
) alkynyl or aryl, it was shown that these form to a large
extent the high lying occupied orbitals of the molecule. One
consequence of this situation is that the complexes are
photoreactive in solution at room temperature. Irradiation
leads to the formation of R radicals in the case of alkyl
complexes, and to R-R coupling products if R is an alkynyl
ligand. The drawback of these systems for applications such
as photoinitiators or sources of predefined organic radicals
is the fact that the absorption bands occur in the near UV
(350-390 nm).
The aim of this paper is therefore twofold. First of all, we
were interested in preparing complexes that are photoreactive
under visible light irradiation. To this end, we synthesized a
series of [(R-diimine)Pt(R)2] complexes with different co-
ligands R. As stated previously, the absorption bands of such
compounds are located well in the visible range. The other
aim was to determine to which extent the R ligands
participate in the low lying electronic transitions of these
complexes and thus assess the validity of the generally
accepted MLCT assignments of the lowest-energy absorption
bands. Among the co-ligands included in this study are alkyl
groups of varying σ donor strength (CH3, CD3, adme )
1-adamantylmethyl, neop ) neopentyl ) 2,2-dimethyl-1-
methyl, neoSi ) neosilyl ) trimethylsilylmethyl). Fur-
thermore, several complexes with unsaturated co-ligands
(CtCtBu, CtCPh, Ph, Mes) were prepared. As the R-di-
imine ligand, iPr-DAB (N,N′-diisopropyl-1,4-diaza-1,3-buta-
diene)20 (see Scheme 1) was used; its simple structure makes
both the interpretation of experimental data as well as
theoretical calculations easier.21-23 The electronic structure
General Procedure for the Synthesis of [(iPr-DAB)Pt(R)2].
The appropriate starting complexes [(diolefin)Pt(R)2] (diolefin )
i
COD, NBD) (1 equiv, usually 500 mg) and 2 equiv of Pr-DAB
were suspended in mixtures of 100 mL of diethyl ether and 100
mL of toluene and stirred at the required conditions (see following
paragraphs, given as T for reaction temperature and time for reaction
time). The products for R ) Ph, Mes, CH3, adme, CtCtBu, or
CtCPh were obtained by evaporation of the reaction mixtures to
dryness, extraction of the residues with CH2Cl2, and crystallization
from CH2Cl2/heptane (1:3) mixtures. The reaction for R ) neop
or neoSi was driven only to conversions of 50%. The products were
isolated by evaporation of the reaction mixtures to dryness and
subsequent extraction of the residues with several portions of
n-pentane. Careful recrystallization from n-pentane gave the pure
microcrystalline materials.
[(iPr-DAB)Pt(CH3)2]. From [(COD)Pt(CH3)2], T ) 343 K, time
20 h. Yield (violet powder): 83%. Anal. Calcd for C10H22N2Pt:
C, 32.87; H, 6.07; N, 7.67%. Found: C, 32.88; H, 6.10; N, 7.68%.
3
1H NMR (CD2Cl2) δ (ppm): 1.43 (d, 12 H, J ) 6.61 Hz, CH-
(CH3)2), 1.45 (s, 6H, 2JPt-H ) 85.05 Hz, PtCH3), 4.60 (septet, 2H,
3
CH(CH3)2), 9.02 (s, 2H, JPt-H ) 34.8 Hz, Himino).
[(iPr-DAB)Pt(CD3)2]. From [(NBD)Pt(CD3)2], T ) 343 K, time
1
14 h. Yield (violet powder): 87%. H NMR (CD2Cl2) δ (ppm):
1.43 (d, 12 H, 3J ) 6.61 Hz, CH(CH3)2), 4.60 (septet, 2H,
CH(CH3)2), 9.02 (s, 2H, 3JPt-H ) 34.8 Hz, Himino), no signal due to
Pt-CH3.
[(iPr-DAB)Pt(neop)2] (neop ) Neopentyl or 2,2-Dimethyl-
propyl). From [(NBD)Pt(neop)2], T ) 333 K, time 120 h. Yield
(blue powder): 66% (conversion 50%). Anal. Calcd for C18H38N2-
Pt: C, 45.27; H, 8.02; N, 5.87%. Found: C, 45.13; H, 8.05; N,
5.84%. 1H NMR (CD2Cl2) δ (ppm): 1.00 (s, 18 H, C(CH3)3), 1.42
(24) Kaim, W.; Klein, A.; Hasenzahl, S.; Stoll, H.; Zalis, S.; Fiedler, J.
Organometallics 1998, 17, 237.
(25) Hasenzahl, S.; Hausen, H.-D.; Kaim, W. Chem.sEur. J. 1995, 1, 95.
(26) Klein, A.; Hausen, H.-D.; Kaim, W. J. Organomet. Chem. 1992, 440,
207.
(27) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643.
(28) Eaborn, C.; Kundu, K.; Pidcock, A. J. Chem. Soc., Dalton Trans. 1981,
933.
(19) Klein, A.; van Slageren, J.; Zalis, S. J. Organomet. Chem. 2001, 620,
202.
(20) Bock, H.; tom Dieck, H. Angew. Chem. 1966, 78, 549.
(21) Stufkens, D. J. Coord. Chem. ReV. 1990, 104, 39.
(22) Balk, R. W.; Snoeck, T. L.; Stufkens, D. J.; Oskam, A. Inorg. Chem.
1980, 19, 3015.
(23) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151.
Inorganic Chemistry, Vol. 41, No. 20, 2002 5217

Klein et al.
3
2
(d, 12 H, J ) 6.59 Hz, CH(CH3)2), 2.34 (s, 4H, JPt-H ) 84 Hz,
Pt-CH2-), 4.95 (septet, 2H, CH(CH3)2), 9.22 (s, 2H, 3JPt-H ) 32.4
Hz, Himino).
Computational Details. The ground-state electronic structure
i
calculations on complexes [(iPr-DAB)Pt(R)2] (R ) CH3, Pr, Ct
CH, or Ph) were performed using density functional theory (DFT)
methods using the ADF2000.229,30 and Gaussian 9831 program
packages. The lowest-energy electronic transitions of (model)
complexes [(iPr-DAB)Pt(CH3)2], [(iPr-DAB)Pt(Ph)2], and [(iPr-
DAB)Pt(CtCH)2] were calculated by the time-dependent DFT (TD
DFT) method (both ADF and Gaussian 98). The electrostatic solvent
effect was simulated by the polarizable continuum model32 incor-
porated into Gaussian 98. This model defines the solvent cavity as
a union of interlocking atomic spheres.
[(iPr-DAB)Pt(neoSi)2] (neoSi ) Neosilyl or Trimethylsilyl-
methyl). From [(NBD)Pt(neoSi)2], T ) 333 K, time 120 h. Yield
(violet powder): 74% (conversion 50%). Anal. Calcd for C16H38N2-
Si2Pt: C, 37.70; H, 7.51; N, 5.50%. Found: C, 37.69; H, 7.49; N,
1
5.47%. H NMR (CD2Cl2) δ (ppm): -0.07 (s, 18 H, Si(CH3)3),
1.34 (s, 4H, 2JPt-H ) 87.6 Hz, Pt-CH2-), 1.43 (d, 12 H, 3J ) 6.6
3
Hz, CH(CH3)2), 4.69 (septet, 2H, CH(CH3)2), 9.13 (s, 2H, JPt-H
) 39.7 Hz, Himino).
[(iPr-DAB)Pt(adme)2] (adme ) 1-Adamantylmethyl). From
[(COD)Pt(adme)2], T ) 363 K, time 18 h. Yield (blue powder):
88%. Anal. Calcd for C30H50N2Pt: C, 56.85; H, 7.95; N, 4.42%.
Gaussian 98 was used for the calculations of the vibrations of
[(iPr-DAB)Pt(CH3)2], [(iPr-DAB)Pt(CD3)2], [(iPr-DAB)Pt(Ph)2], and
[(iPr-DAB)Pt(CtCMe)2].
1
Found: C, 56.71; H, 7.91; N, 4.38%. H NMR (C6D6) δ (ppm):
Within the ADF program, Slater-type orbital (STO) basis sets
of triple ú quality with polarization functions were employed.
The inner shells were represented by the frozen core approxi-
mation (1s for C and N and 1s-4d for Pt were kept frozen). The
following density functionals were used within ADF: the local
density approximation (LDA) with VWN parametrization of
electron gas data or the functional including Becke’s gradient
correction33 to the local exchange expression in conjunction with
Perdew’s gradient correction34 to the LDA expression (ADF/BP).
The scalar relativistic zero order regular approximation (ZORA)35
was used in this study.
3
1.09 (d, 12 H, J ) 6.57 Hz, CH(CH3)2), 1.48 (d, 12 H, J ) 2.68
Hz, AdꢀCH ), 1.68 (m, 12H, Ad
), 1.82 (s, 4H, JPt-H ) 88.76
γCH2
2
2
Hz, Pt-CH2-), 2.11 (m, 6H, AdδCH), 4.99 (septet, 2H, CH(CH3)2),
3
8.32 (s, 2H, JPt-H ) 32 Hz, Himino).
[(iPr-DAB)Pt(Ph)2]. From [(COD)Pt(Ph)2] in toluene, T ) 393
K, time 48 h. Yield (red powder): 91%. Anal. Calcd for C20H26N2-
Pt: C, 49.07; H, 5.35; N, 5.72%. Found: C, 49.01; H, 5.32; N,
1
3
5.70%. H NMR (CDCl3) δ (ppm): 1.19 (d, 12 H, J ) 6.6 Hz,
CH(CH3)2), 4.24 (septet, 2H, CH(CH3)2), 6.75 (t, 2H, 3JpPh-mPh
)
3
7.2 Hz, p-Ph), 6.92 (dd, 4H, JmPh-oPh ) 6.9 Hz, m-Ph), 7.34 (d,
4H, 3JPt-H ) 69.5 Hz, o-Ph), 8.82 (s, 2H, 3JPt-H ) 34.5 Hz, Himino).
[(iPr-DAB)Pt(Mes)2]. From [(dmso)2Pt(Mes)2] in toluene, T )
393 K, time 72 h. Yield (violet powder): 85%. Anal. Calcd for
C26H38N2Pt: C, 54.43; H, 6.68; N, 4.88%. Found: C, 54.35; H,
6.63; N, 4.86%. H NMR (CDCl3) δ (ppm): 1.20 (d, 12 H, J )
6.57 Hz, CH(CH3)2), 2.17 (s, 6H, p-CH3), 2.31 (s, 12H, JPt-H
5.7 Hz, o-CH3) 3.99 (septet, 2H, CH(CH3)2), 6.55 (s, 4H, 4JPt-H
Within Gaussian 98, Dunning’s polarized valence double ú basis
sets36 were used for C, N, and H atoms, and the quasirelativistic
effective core pseudopotentials and corresponding optimized set
of basis functions37 were used for Pt. Becke’s hybrid three parameter
functional with the Lee, Yang, and Parr correlation functional
(B3LYP)38 was used in Gaussian 98 calculations.
The calculations on [(iPr-DAB)Pt(R)2] were performed in C2V
constrained symmetry (C2 for R ) Ph), with the z axis coin-
cident with C2 symmetry axis and the central atoms of the R
substituents in the yz plane. All results discussed correspond to
optimized geometries. For R ) Ph, the tilt angle was calculated to
65.1°. This angle is found in diarylplatinum complexes of diimines
to vary from 65° to 70° in good agreement with the calculated
value.8,26,39,40
1
3
4
)
)
3
15 Hz, H-Mes), 8.94 (s, 2H, JPt-H ) 36.3 Hz, Himino).
[(iPr-DAB)Pt(CtCtBu)2]. From [(COD)Pt(CtCtBu)2] in tolu-
ene, T ) 373 K, time 72 h. Yield (violet powder): 90%. Anal.
Calcd for C20H34N2Pt: C, 48.28; H, 6.89; N, 5.63%. Found: C,
1
48.29; H, 6.90; N, 5.66%. H NMR (CDCl3) δ (ppm): 1.24 (s,
18H, C(CH3)3), 1.55 (d, 12H, 3J ) 6.65 Hz, CH(CH3)2), 4.76 (septet,
3
2H, CH(CH3)2), 8.63 (s, 2H, JPt-H ) 48.2 Hz, Himino).
[(iPr-DAB)Pt(CtCPh)2]. From [(COD)Pt(CtCPh)2] in toluene,
T ) 373 K, time 60 h. Yield (brown powder): 92%. Anal. Calcd
for C24H26N2Pt: C, 53.62; H, 4.88; N, 5.21%. Found: C, 53.87;
(29) Fonseca Guerra, C.; Snijders, J. G.; Te Velde, G.; Baerends, E. J.
Theor. Chim. Acta 1998, 99, 391.
(30) Van Gisbergen, S. J. A.; Snijders, J. G.; Baerends, J. L. Comput. Phys.
Commun. 1999, 118, 119.
1
3
H, 4.90; N, 5.31%. H NMR (CDCl3) δ (ppm): 1.56 (d, 12 H, J
) 6.43 Hz, CH(CH3)2), 4.86 (septet, 2H, CH(CH3)2), 7.08 (dt, 2H,
(31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian
Inc.: Pittsburgh, PA, 1998.
3JpPh-mPh ) 7.35 H, JpPh-oPh ) 1.37 Hz, p-Ph), 7.20 (dd, 4H,
4
3JmPh-oPh ) 7.58 Hz, m-Ph), 7.38 (d, 4H, o-Ph), 8.70 (s, 2H, 3JPt-H
) 50.7 Hz, Himino).
1
Physical Measurements. H NMR spectra were recorded on
Bruker AC 250 or Varian Mercury 300 spectrometers. UV-vis
absorption spectra were recorded on Bruins Instruments Omega
10, Hewlett-Packard 8453 Diode Array, and Varian Cary 4E
spectrophotometers. Resonance Raman spectra of the complexes
dispersed in KNO3 pellets were recorded on a Dilor XY spectrom-
eter equipped with a Wright Instruments CCD detector, using
Spectra Physics 2040E Ar+ and Coherent CR490 and CR590 dye
lasers (with stilbene, Coumarin 6, and Rhodamine 6G dyes) as
excitation sources. The photoreactions were performed by irradiation
of solutions in CD2Cl2 (Cambridge Isotope Laboratories, 99.9%D)
with an Oriel 6137 high pressure Hg lamp with a suitable cutoff
(32) Amovilli, C.; Barone, V.; Cammi, R.; Cances, E.; Cossi, M.; Mennucci,
B.; Pomelli, C. S.; Tomasi, J. AdV. Quantum Chem. 1999, 32, 227.
(33) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(34) Perdew, J. P. Phys. ReV. A 1986, 33, 8822.
(35) van Lenthe, E.; Ehlers, A.; Baerends, E.-J. J. Chem. Phys. 1999, 110,
8943.
1
(36) Woon, D. E.; Dunning, T. H. J. J. Chem. Phys. 1993, 98, 1358.
(37) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.
Chim. Acta 1990, 77, 123.
(38) Stephens, P. J.; Devlin, F. J.; Cabalowski, C. F.; Frisch, M. J. J. Phys.
Chem. 1994, 98, 11623.
filter for product analysis by H NMR, or in degassed CH2Cl2
solutions using a Spectra Physics 2040E Ar+ laser for the
determination of quantum yields. Calibration was affected using
Aberchrome 540.
5218 Inorganic Chemistry, Vol. 41, No. 20, 2002

Platinum(II) Diazabutadiene Complexes
bands in band system I are called λ1-λ3, while the main
maximum of band system II is given under λ4. Furthermore,
the maxima λ1 and λ2 as measured in very polar solvents
are given to demonstrate the negative solvatochromism,
indicating that the corresponding electronic transitions have
charge transfer character. The extent of solvatochromism for
the first two bands (∆ν) varies with the character of the R
group and is also not identical for λ1 and λ2. In previous
work, we have assigned the observed structuring of band
system I in complexes [(R′-DAB)Pt(CH3)2] (R′ ) tBu, Tol,
Xyl) to vibrational coupling.24,25 However, the systems
reported therein showed pronounced structure only in very
nonpolar solvents. The present systems [(iPr-DAB)Pt(R)2]
with R ) alkyl also allow the observation of structure in
polar solvents that is probably due to their structural
simplicity. Thus, these new results indicate a different
electronic transition to be the origin for the two maxima
rather than vibrational coupling. The alkyl substituted
complexes possess only one strong band in the UV, while
those with unsaturated co-ligands show additional bands,
which are attributable to π-π* transitions. The absorption
spectra of these latter complexes are generally more complex
than those of the former, leading to partial overlap of the
two band systems. To assign the observed absorption bands,
DFT calculations on the electronic structure and transitions
were performed which are reported in following paragraphs.
Electronic Structure and Transitions. The influence of
the σ donating abilities of an alkyl substituent on the
electronic structure was investigated by calculating orbital
energies and compositions of [(iPr-DAB)Pt(R)2] with R )
Figure 1. Absorption spectrum of [(iPr-DAB)Pt(CH3)2] in pentane and
spectral deconvolution.
Results and Discussion
After briefly discussing the syntheses of the complexes,
this section continues with the results of calculations of the
electronic structure and transitions. Experimental evidence
for the assignment of the absorption spectra comes from the
rR spectra in combination with DFT calculations. The section
concludes with the photochemical properties of the com-
plexes.
Synthesis and Characterization. All complexes were
prepared from precursors of the type [(L)2Pt(R)2], where L
) dmso, 1/2 COD (COD ) 1,5-cyclooctadiene), or 1/2 NBD
(NBD ) norbornadiene). The R ) Mes complex can only
be prepared from the precursor complex with L ) dmso,
which has been reported for the dimesityl-platinum(II)
complexes of other R-diimine ligands.11,24 The complexes
with R ) neop, neoSi, and CD3 are best prepared from the
NBD precursor, where use of the COD complex leads to
extensive decomposition. The other complexes can be
prepared from both the COD and the NBD precursors. It
was found that often the best results were obtained if the
reactions were only driven to 50% conversion of the diolefin
platinum precursor. Starting compound and product can then
be separated by careful recrystallization. Introduction of very
σ donating substituents such as benzyl or isopropyl failed
using the same procedure as that previously described and
also failed using direct alkylation attempts with [(iPr-DAB)-
Pt(Cl)2] and RMgBr. Similar findings have been reported in
the literature.2 A reasonable explanation is that the strong
trans effect of such strong σ donor R ligands might give
rise to diimine abstraction and decomposition of the complex.
Optical Spectroscopy. Figure 1 shows the absorption
spectrum of [(iPr-DAB)Pt(CH3)2] in n-pentane, as a repre-
sentative example of the whole series of complexes. The
absorption bands of all complexes can be divided into two
distinct sets, one in the visible and one in the (near) UV.
Hereafter, these are called band systems I and II, respectively.
In Table 1, the absorption maxima of all compounds in
nonpolar solvents are given. The maxima of the absorption
i
Me, Pr. As two examples of complexes with unsaturated
co-ligands, the same calculations were performed on the
compounds with R ) CtCH and Ph.
In Table 2, the results of the ADF/BP calculations for the
methyl and isopropyl complexes are reported. The composi-
tions of the frontier orbitals calculated by G98/B3LYP are
similar. It is clear from this table that for both alkyl
complexes the highest occupied four MOs have predomi-
nant platinum character, with significant contributions of the
sp3(R) orbitals, whereas the two lowest unoccupied MOs are
mainly formed by the π*(iPr-DAB) orbital. They have a
significant platinum contribution as in the corresponding
COD complexes, where the LUMO is the π*(COD) level.
Figures 2 and 3 depict the HOMO and LUMO of [(iPr-
DAB)Pt(CH3)2], respectively. From Figure 2, it is clear that
the d(Pt)-sp3(CH3) interaction in the HOMO is bonding.
i
The clearest difference between the Me and Pr complexes
is found in the alkyl group contribution to the HOMO which
changes from 12% to 24% going from the former to the
latter. Concurrently, this orbital rises in energy by about 4
× 103 cm-1 (1 eV ≈ 8 × 103 cm-1). The absolute
contribution of the alkyl group to the HOMO has decreased
roughly twofold for these iPr-DAB complexes compared to
the corresponding COD ones (31% and 43% for R ) Me
i
and Pr, respectively).19
(39) Klein, A.; McInnes, E. J. L.; Kaim, W. J. Chem. Soc., Dalton Trans
2002, 2371.
(40) Klein, A.; Kaim, W.; Waldhoer, E.; Hausen, H.-D. J. Chem. Soc.,
Perkin Trans. 2 1995, 2121.
The TD DFT (Gaussian 98) calculated transition energies
(Table 3) show that indeed the lowest-energy transitions in
[(iPr-DAB)Pt(CH3)2] occur from the HOMOs to the π*(iPr-
Inorganic Chemistry, Vol. 41, No. 20, 2002 5219

Klein et al.
Table 1. Observed Electronic Absorption Maxima for Complexes [(iPr-DAB)Pt(R)2]a
solvent ) pentane or hexane
solvent ) DMF
R
λ4 (ꢀ)
λ3 (ꢀ)
λ2 (ꢀ)
λ1 (ꢀ)
λ2
λ1
∆ν2 [cm-1
]
∆ν1 [cm-1
]
adme
neop
neoSi
CH3
418 (2.77)
418 (3.06)
412 (2.81)
390 (3.34)
367 (3.38)
386 (4.27)c
409 (3.24)d
321 (16.2)
531 (1.14)
530 (1.19)
519 (0.92)
518 (1.02)
484 (3.36)
504 (1.81)
440 (3.31)
398 (8.95)
576 (1.84)
575 (2.02)
561 (1.61)
560 (2.00)
517 (3.46)
549 (2.19)
490 (2.64)
452 (5.30)
619 (2.24)
617 (2.57)
609 (2.10)
606 (2.47)
558 (3.63)
596 (2.50)
528 (3.20)
515 (3.73)
540
529b
501
513
491
457b
426
410
569
567b
529
538
512
495b
468
443
1160
1512
2130
1640
1030
3370
3070
2270
1420
1430
2480
2090
1610
3420
2430
3160
Mes
Ph
CtCtBu
CtCPh
a Absorption maxima wavelengths are given in nm; the molar extinction coefficients ꢀ are given in parentheses (in 1000 M-1cm-1). Solvatochromic shift
∆ν (≡ν(DMF) - ν(alkane)) values are given in cm-1
.
b Measured in MeCN. c Further close lying maxima: 448 nm (ꢀ ) 1980 M-1 cm-1), 372 nm (ꢀ )
4260 M-1 cm-1). d Further maxima: 388 nm (3022 M-1 cm-1) and 369sh nm.
Figure 2. 19a1 HOMO of [(iPr-DAB)Pt(CH3)2].
Table 2. ADF/BP Calculated One-Electron Energies and Percentage
Figure 3. 11b1 LUMO of [(iPr-DAB)Pt(CH3)2].
Composition of Selected Highest Occupied and Lowest Unoccupied
i
Molecular Orbitals of [(iPr-DAB)Pt(R)2] (R ) CH3 or Pr (in
Table 3. Selected G98/B3LYP Calculated Lowest TD DFT Singlet
Excitation Energies (eV) for [(iPr-DAB)Pt(CH3)2] and Experimental
Absorption Maxima
Parentheses)) Expressed in Terms of Composing Fragments
prevailing
MO
16b2
9a2
E (eV)
0.21
-0.30
character
Pt
R
iPr-DAB
G98/B3LYP (CH2Cl2)a
exptl
Unoccupied
dPt + R
trans. energy
eV (nm)
λ
max nmb
2 py;
35 dyz
1 dxy
48
15
98
state
main character
osc str
(ꢀ M-1cm-1
)
1B1
1B2
1B1
1A1
1B2
98% 19a1 f 11b1
99% 8a2 f 11b1
98% 18a1 f 11b1
94% 10b1 f 11b1
86% 7a2 f 11b1
1.99 (625)
2.09 (595)
2.46 (505)
3.12 (398)
4.99 (249)
0.0016
0.0
0.0001
0.113
0.157
543 (2470)
507 (2000)
470 (1020)
390 (3340)
245 (7800)
π* DAB
11b1 (14b1) -3.37 (-3.34) π* DAB + dPt 18 (16) dxz;
2 (2) 78 (81) π*
1 (1) px
Occupied
19a1 (22a1) -4.77 (-4.27) dPt + R
21 (24) s;
12 (24)
2
2
63 (44) dx -y
;
2
3 (8) dz
i
At higher energies, first a pure MLCT and then an Pr-
DAB intraligand (IL) transition are found. The lowest-energy
absorption bands (band system I in the absorption spectra)
are very solvatochromic. To decrease the difference between
calculated and observed transition energies, the effect of the
solvent was simulated by the polarizable continuum model
included in the Gaussian 98 program (see Experimental Sec-
tion). This led to an increase in calculated transition energies.
The fact that for the three lowest transitions they are still
appreciably lower than the observed ones implies that this
simple model does not simulate the solvent effect well. In
DFT calculations on several other complexes, calculated
transition energies that are too low were found as well.43,44
The calculated and observed transition energies for the other
two, less solvatochromic, transitions (Table 3) correspond
much better. The calculated oscillator strengths of the three
8a2 (11a2) -4.79 (-4.60) dPt
18a1 (21a1) -5.21 (-5.16) dPt + R
82 (85) dxy
4 (2) s;
7 (6) 10 (7)
12 (13) 4 (6)
2
72 (67) dz ;
2
2
5 (10) dx -y
10b1 (10b1) -5.43 (-5.43) dPt + DAB
68 (74) dxz;
7 (5) 24 (21)
4 (10) 89 (87)
1 (0) px
7a2 (7a2)
-7.12 (-7.07) DAB
6 (3) dxy
DAB) LUMO. Hence, they are mainly MLCT in character.
Because of the admixture of the sp3(CH3) orbitals to the
HOMOs, the lowest-energy transitions remove electron
density from the σ(Pt-C) bond. Transitions that remove
electron density from bonding metal-ligand orbitals have
been named σ-bond-to-ligand charge transfer (SBLCT)
transitions.19,41,42 As the methyl group contribution to the
i
HOMO diminishes going from the COD to the Pr-DAB
complexes, a relatively lower photoreactivity for the iPr-DAB
complexes is expected (see later).
(41) Niewenhuis, H. A.; Stufkens, D. J.; McNicholl, R. A.; Al-Obaidi, A.
H. R.; Coates, C. G.; Bell, S. E. J.; McGarvey, J. J.; Westwell, J.;
George, M. W.; Turner, J. J. J. Am. Chem. Soc. 1995, 117, 5579.
(42) Djurovich, P. I.; Watts, R. J. Inorg. Chem. 1993, 32, 4681.
(43) Turki, M.; Daniel, C.; Za´lisˇ, S.; Vlcek, A. J.; van Slageren, J.; Stufkens,
D. J. J. Am. Chem. Soc. 2001, 123, 11431.
(44) Van Slageren, J.; Stufkens, D. J.; Za´lisˇ, S.; Klein, A. J. Chem. Soc.,
Dalton Trans. 2002, 218.
5220 Inorganic Chemistry, Vol. 41, No. 20, 2002

Platinum(II) Diazabutadiene Complexes
Table 4. ADF/BP Calculated One-Electron Energies and Percentage
Composition of Selected Highest Occupied and Lowest Unoccupied
Molecular Orbitals of [(iPr-DAB)Pt(CtCH)2] Expressed in Terms of
Composing Fragments
L′LCT character, where L′LCT denotes charge transfer from
the alkynyl ligand to the iPr-DAB one. Although the orbital
8a2 (Table 4) is the HOMO, the lowest-energy transition is
calculated to be the HOMO-1 f LUMO one (Table 5).
This makes the order of the electronic transitions the same
as that of the alkyl complexes. In addition, one extra
transition is calculated, which has partial IL(CtCH) char-
acter but is rather mixed. The calculated energies of the three
lowest-energy transitions are lower than those of the observed
absorption bands, because of reasons similar to those
described previously and, in this case, possibly also because
the calculations were performed on a model complex. In the
alkynyl complex too, the oscillator strengths are lower than
expected when looking at the extinction coefficients. Again,
our assignment seems to be the only reasonable one on the
basis of these calculations. In contrast, in the literature, the
HOMO of alkynyl-platinum(II)-diimine complexes has
been thought to be primarily metal based on the basis of the
observation that the emission energy decreases with increas-
ing donor strength of the alkynyl ligand.3,4,9 DFT and TD
DFT calculations on [(iPr-DAB)Pt(Ph)2] showed that also
the phenyl group participates in the lowest-energy electronic
transitions of the complex, albeit to a lower extent than the
alkynyl ones (for details see Supporting Information).
prevailing
character
MO E (eV)
Pt
CtCH iPr-DAB
Unoccupied
17b2 -0.68 dPt + CCH + iPr-DAB 8 py;
30
10
23
38 dyz
9a2
11b1 -3.91 π* iPr-DAB + dPt
-0.65 π* iPr-DAB
1 dxy
12 dxz
98
78 π*
Occupied
8a2
-5.08 CCH + dPt
32 dxy
65
74
3
2
2
20a1 -5.18 CCH + dPt
15 dz ;
2
2
10 dx -y
10b1 -5.58 CCH + dPt + iPr-DAB 20 dxz;
62
16
2 px
16b2 -5.62 CCH
98
3
2
2
19a1 -5.79 dPt
21 s;
52 dx -y
23 dz
2
2
;
2
7a2
-7.35 dPt + CCH + iPr-DAB 35 dxy
34
31
lowest-energy transitions are much lower than expected on
the basis of the observed extinction coefficients. However,
in view of the transition energies, we feel confident that our
present assignment of the observed absorption bands to these
calculated transitions is the correct one. Hence, this discrep-
ancy must be due to factors that were not taken into account
in the TD DFT calculations, such as spin-orbit coupling
due to the platinum atom, the influence of vibrational motion
of the molecule which can increase orbital overlap, electronic
through-solvent interaction, and so on. In conclusion, the
calculations suggest that the observed structure of the absorp-
tion bands in band system I is due to the presence of different
electronic transitions rather than vibrational fine structure.
This conclusion is different from that given in a previous
paper, as calculation methods have improved greatly over
the past few years, which makes unambiguous assignment
of electronic transitions to absorption bands now possible.
Summarizing, in all the present complexes, the co-ligand
orbitals play a significant role in the HOMOs and therefore
in the low-lying electronic transitions, in contrast to what is
generally assumed for such complexes. A technique to obtain
experimental evidence about the nature of the electronic
transitions responsible for the observed absorption band is
resonance Raman (rR) spectroscopy. The results of an rR
study on these complexes are reported in the following
paragraphs.
Resonance Raman. The rR spectra of the [(R′-DAB)-
i
Pt(R)2] (R ) CH3, R′ ) Pr, cHx, p-Tol; R ) CD3,
CtCtBu, Mes, R′ ) iPr) complexes dispersed in KNO3 were
recorded on irradiation into their lowest-energy absorption
bands. The rR technique is based on the fact that only those
Raman bands which are due to vibrations influenced by the
electronic transition that is excited are resonantly enhanced
in intensity.45 There are several instances in which resonance
Raman was used to distinguish several electronic transitions
within an apparently single absorption band.46-48
The electronic structure of the complexes with unsaturated
co-ligands is different from that of the alkyl ones. Whereas
the co-ligand contribution to the highest filled MOs is
relatively small in the latter complexes, it is the major
contribution in the former ones. The ADF/BP calculated
orbital characters and energies of the [(iPr-DAB)Pt(CtCH)2]
model complex (Table 4) show that the four highest occupied
orbitals all have over 60% alkynyl character, with the
platinum center contributing about 20-30% to the three
highest ones and not at all to the fourth. Recent IEH
calculations (IEH ) iterative extended Hu¨ckel) on [(phen)-
Pt(CtCR)2] (phen ) 1,10-phenanthrolines, R ) H, F, or
Ph) show lower contributions (around 20%) of the alkynyl
groups to high lying filled orbitals.4 The different calculation
approach is proposed to be the reason for this difference,
particularly the fact that the semiempirical extended Hu¨ckel
method does not take into account the electron correlation.
The LUMO is mainly formed by the π*(iPr-DAB) orbital.
These results are very similar to those obtained for the
corresponding COD complex.19
Figure 5 depicts typical resonance Raman spectra for these
complexes. In each case, there is one very strong resonance
around that ranges from 1514 cm-1 for R ) adme to 1559
cm-1 for R ) CtCtBu. This band is assigned to the νs(CN)
vibration of the DAB ligand in agreement with other studies
on such complexes.41,44,49 The energy of the band increases
parallel to the series adme < neop < CD3 < CH3 < Mes <
(45) Clark, R. J. H.; Dines, T. J. Angew. Chem., Int. Ed. Engl. 1986, 25,
131.
(46) Egolf, D. S.; Waterland, M. R.; Kelley, A. M. J. Phys. Chem. B 2000,
104, 10727.
(47) Balk, R. W.; Stufkens, D. J.; Oskam, A. Inorg. Chim. Acta 1979, 34,
267.
(48) Shin, K.-S. K.; Zink, J. I. J. Am. Chem. Soc. 1990, 112, 7148.
(49) van Slageren, J.; Klein, A.; Za´lisˇ, S.; Stufkens, D. J. Coord. Chem.
ReV. 2001, 219-221, 937.
In view of the orbital composition of the HOMOs (Figure
4), the low-lying electronic transitions have mixed MLCT/
Inorganic Chemistry, Vol. 41, No. 20, 2002 5221

Klein et al.
Figure 4. 8a2 HOMO and 20a1 HOMO - 1 of [(Me-DAB)Pt(CtCH)2].
Table 5. Selected Calculated Lowest TD DFT Singlet Excitation
Energies (eV and nm) for [(iPr-DAB)Pt(CtCH)2] Together with
Experimentally Observed Values for [(iPr-DAB)Pt(CtCtBu)2]
G98/B3LYP G98/B3LYP (CH2Cl2)
trans
trans
energy,
eV (nm)
exptl
energy osc.
osc
str
λ
max nma
state main character
eV
str.
(ꢀ M-1 cm-1
)
1B1 98% 20a1 f 11b1 1.58 0.0020 1.95 (638) 0.0032 477 (3250)
1B2 99% 8a2 f 11b1
1.91 0.0001 2.19 (566) 0.0002
438 (880)
410 (750)
393 (3240)
295 (4310)
268 (6580)
1B1 98% 19a1 f 11b1 2.63 0.0002 2.89 (428) 0.0003
1A1 94% 10b1 f 11b1 3.18 0.095 3.29 (376) 0.092
1B2 86% 20a1 f 17b2 4.65 0.079 4.94 (253) 0.036
1A1 94% 8a2 f 9a2
5.02 0.187 4.92 (252) 0.144
a From absorption spectra in CH2Cl2. Maxima and extinction coefficients
were extracted from spectral deconvolution.
Figure 6. Resonance Raman spectra for [(cHx-DAB)Pt(CH3)2] in a KNO3
pellet obtained upon irradiation at two different irradiation wavelengths.
Figure 5. Resonance Raman spectra for [(iPr-DAB)Pt(R)2] (R ) adme,
Me, or CtCtBu) in KNO3 pellets obtained on irradiation at the given
irradiation wavelengths.
Figure 7. Resonance Raman excitation profile for [(cHx-DAB)Pt(CH3)2]
in a KNO3 pellet.
CtCtBu. Decreasing electron donating power as represented
by this series leads to decreased back-bonding to π*(DAB),
which is antibonding with respect to the CdN bond, with
subsequent strengthening of the CdN bond. This resonance
is shifted to lower energies for the complexes with aryl
substituted R′-DAB ligands as shown here for [(p-Tol-DAB)-
Pt(CH3)2] where the band is found at 1500 cm-1 (for full rR
data, see Supporting Information). In this case, the aryl
substituent causes weakening of the CdN bond by with-
drawal of electron density from the (CdN bonding) π level
of the R′-DAB ligand.
wavelengths. Figure 6 shows the rR spectra for [(cHx-DAB)-
Pt(CH3)2] for the excitation wavelengths corresponding to
the first and second absorption maxima. Clearly, the spectra
are different depending on the excitation wavelength. Figure
7 shows the intensity of two typical bands of [(cHx-DAB)-
Pt(CH3)2] as a function of the excitation wavelength (a so-
called excitation profile), superimposed on an absorption
spectrum in KBr, a matrix similar to KNO3. The fact that
Raman bands are enhanced irradiating into different bands
of band system I proves beyond doubt that the absorption
maxima belong to different transitions rather than being due
to vibrational fine structure.
To probe for any difference between the electronic
transitions belonging to the bands in band system I, rR
spectra were obtained on irradiation at different excitation
To assist in the assignment of the rR bands, DFT
vibrational calculations were performed on complexes
5222 Inorganic Chemistry, Vol. 41, No. 20, 2002

Platinum(II) Diazabutadiene Complexes
CD3).53 On the basis of calculations and the large influence
of deuteration, it was assigned to an umbrella-like movement
of the methyl groups. If the concerned methyl group is
involved in the electronic transition to a large extent, this
vibration shows up as a strongly enhanced band, such as in
the case of [(iPr-DAB)Pt(CH3)4]. In the present [(iPr-DAB)-
Pt(R)2] (R ) CH3, CD3) complexes, it is only weakly
enhanced, showing some but not major involvement of the
methyl substituents in the lowest-energy electronic transition.
This is in excellent agreement with the calculated sp3(CH3)
contribution to the HOMO (12%). Furthermore, a Pt-CH3
bending vibration (observed 874 cm-1 for R ) CH3 and at
660 cm-1 for R ) CD3) and a Pt-C stretching vibration
(observed at 564 cm-1 for R ) CH3 and at 508 cm-1 for R
) CD3) are clearly influenced by deuteration. The weak
bands at ca. 945 and 830 cm-1 belong to in-plane symmetric
Table 6. Experimental and Calculated Raman Bands for
[(iPr-DAB)Pt(CH3)2] and [(iPr-DAB)Pt(CD3)2]
R ) CH3
R ) CD3
exptl calcda exptl calcda
assignment
transb
1
2
3
4
5
6
7
8
9
3066
12 × 2 (3062/3056)
2
2
2
2
2
2
2
2
3030
2906
2827
2760
2666
2573
2480
2365
3062 3030 3063 ν(CH) (imine)
3001 2050 2089 ν(CH)/ν(CD) (Me)
12 + 16 (2833)
12 + 17 (2763)
12 + 18 (2670)
12 + 19 (2575)
12 + 21 (2477)
12 + 21 (2364)
2
2
1
10 2131
11 2045
12 1531
13 1400
14 1379
15 1322
16 1302
17 1232
12 + 22 (2130)
12 + 27 (2041)
1531 1528 1532 ν(CN) (DAB)
1402 1400 1403 δ(CH) (imine+iPr CH)
1382 1377 1382 δ(CH) (iPr) + δ(CH) (imine)
1324 1320 1325 δ(CH) (iPr) out of plane
1312 1300 1311 δ(CH) (imine+iPr CH)
1 + 2
1
1
1
1
1
i
deformations of the Pr-DAB ligand. They are resonantly
1248
952
968 δ(CH3)/δ(CD3)(Me) +
δ(CH) (iPrDAB)
enhanced whenever the electronic system becomes delocal-
ized.49 The fact that they are observed as weak bands means
that this is not the case for these complexes, again in
agreement with the DFT MO calculations (see previous
description). Finally, combined symmetric Pt-C and Pt-N
stretching vibrations are observed between 550 and 600 cm-1.
The rR spectra observed on irradiation into the second
absorption band are more particular. Whereas νs(CN) is still
observed, its intensity is much weaker, indicating a smaller
distortion along the CN coordinate in the excited state.48,49,51
Furthermore, a number of bands are observed between 2000
and 3000 cm-1. In this region, no molecular vibrations are
calculated and indeed not expected. Upon closer inspec-
tion, they proved to be assignable to combination bands of
νs(CN) with the various ligand and metal-ligand stretching
and deformation vibrations, with the exception of the band
at 3030 cm-1 that is assigned to the ν(CH) mode of the DAB
ligand, in agreement with the lack of any deuteration effect.
In the rR spectrum of [(iPr-DAB)Pt(CtCtBu)2] upon
irradiation into the second lowest absorption maximum, there
is a strong enhancement of a band at 2132 cm-1 (Figure 8).
We attribute this band to the CtC stretching vibration ν-
(CC). This band is not enhanced when the compound is
irradiated with lower energy, coincident with the lowest
absorption maxima, where the strongest band is clearly the
1559 cm-1 resonance attributed to the νs(CN) vibration. From
the calculations, it was found that the lowest electronic
transition originates from an orbital that has different
symmetry (HOMO - 1, 20a1) than the second transition
(HOMO, 8a2) (Figure 4). This might be a qualitative
explanation for the different rR behavior. In the related
complex trans-[(dppm)2Pt(CtCPh)2] (dppm ) bis-diphen-
ylphosphinomethane), the ν(CC) mode is also strongly
enhanced (2114 cm-1).52 The authors explain this with a high
18 1139
19 1044
1138 1138 1138 ν(CC) + δ(CΗ) (imine)
1034 1042 1035 ν(CC) + δ(CH) (imine)
1
1
1
1
1
1
1
1
1
20 946w
21 874w
22 833w
23 599
24 575w
25 564w
27 510
928
863
805
582
581
571
487
942
660
828
929 δ(iPr-DAB) cycle
628 δ(CH)/δ(CD) (Me) bend
805 δ(iPr-DAB) cycle
ν(PtC) + ν(PtN)
ν(PtC) + ν(PtN)
508
509 ν(PtC)
δ(iPr-DAB) out of plane
a Calculated values were scaled by 0.97 using experimental (1531 cm-1
)
and calculated (1579 cm-1) values for νs(CN) for [(iPr-DAB)Pt(CH3)2].
b Enhanced for first or second transition.
[(iPr-DAB)Pt(R)2] (R ) CH3, CD3). Table 6 shows the
experimental and calculated vibrations in addition to the
assignments for these complexes. The ultimate column states
whether the particular rR band belongs to the first or second
electronic transition.
Starting with the assignment of the first electronic transi-
tion, the major band is clearly the one at 1531 cm-1 (R )
CH3) or 1528 cm-1 (R ) CD3). The observation of this
νs(CN) band is clear evidence that the electronic transitions
are directed toward the R-diimine ligand in all cases. The
calculated value (1579 cm-1) lies about 3% higher than the
observed one. Such differences are quite normal using DFT
techniques with the employed B3LYP functional. Indeed, a
scaling factor of 0.961 was recommended in the literature.50
To make a comparison between experimental and calculated
values, all calculated values have been rescaled by 0.97. On
irradiation into the lowest-energy absorption band, νs(CN)
is the only strongly enhanced absorption band. Several others
bands are visible, though. The ones between 1300 and 1400
cm-1 belong to imine CH and iPr CH deformation vibrations,
because they are observed at the same values for both the
protonated and deuterated complexes and hence do not
involve the platinum bound methyl groups. The band
observed at 1232 cm-1 (R ) CH3) or 952 cm-1 (R ) CD3)
clearly does. It was observed for other transition metal methyl
complexes, such as [(iPr-DAB)Re(CH3)(CO)3], [(iPr-DAB)-
Ru(Cl)(R)(CO)2] (R ) CH3, CD3), [(iPr-DAB)Ru(R)(SnPh3)-
(CO)2] (R ) CH3, CD3), and [(iPr-DAB)Pt(R)4] (R ) CH3,
(51) Zink, J. I.; Shin, K.-S. K. Molecular Distortions in Excited Electronic
States Determined from Electronic and Resonance Raman Spectros-
copy; Volman, D. H., Hammond, G. S., Neckers, D. C., Eds.; Wiley:
New York, 1991.
(52) Kwok, W. M.; Phillips, D. L.; Yeung, P. K.-Y.; Yam, V. W.-W. Chem.
Phys. Lett. 1996, 262, 699.
(53) Weinstein, J.; van Slageren, J.; Stufkens, D. J.; Za´lisˇ, S.; George, M.
W. J. Chem. Soc., Dalton Trans. 2001, 2587.
(50) Wong, M. W. Chem. Phys. Lett. 1996, 256, 391.
Inorganic Chemistry, Vol. 41, No. 20, 2002 5223

Klein et al.
Figure 9. Absorption spectra of [(iPr-DAB)Pt(neop)2] taken in CH2Cl2
during photolysis at λ ) 514.5 nm. The traces are recorded after each 20
s of irradiation. The last spectrum represents [(iPr-DAB)Pt(Cl)(neop)].
Figure 8. Resonance Raman spectra for [(iPr-DAB)Pt(CtCtBu)2] in a
KNO3 pellet obtained by irradiation at two different irradiation wavelengths.
-
Asterisks denote NO3 bands.
contribution of the CtC group to the radiationless decay
from the MLCT excited state. However, for our system, such
an explanation fails, because we found that the enhancement
of the ν(CC) mode is excitation frequency dependent and
does not occur upon irradiation into the lowest absorption
band. Using time-resolved FTIR spectroscopy, Schanze et
al. have found that the frequency of the ν(CC) modes in
complexes [(R′-bpy)Pt(CtCR)2] (R ) Aryl, R′-bpy ) alkyl
substituted bpy) are about 25-35 cm-1 higher in the excited
states of the molecules than in their ground states which
served as indication for an MLCT character of the excited
state.10 However, their investigations focus on the thermally
relaxed excited state whereas rR provides direct information
on the electronic transition. These can be quite different, as
was recently shown by comparison of these two techniques.53
From the striking agreement of our calculations and rR
results, we conclude that the CtCR group is directly
involved in the electronic transition.
For the mesityl complex, the ν(CN) vibration is found at
1547 cm-1 in excellent agreement with the calculated value
for [(iPr-DAB)Pt(Ph)2] of 1541 cm-1 (see Table S3). With
the help of this calculation, we were able to assign a number
of resonantly enhanced bands that are located in the aryl
substituent. These resonances, that were attributed to CH
deformation modes (1462 and 1171 cm-1), ν(CC) modes
(1037, 1010, and 951 cm-1) or to ring deformations (673
and 604 cm-1), do not depend much on the irradiation
wavelength, in agreement with the calculated electronic
structure that reveals contributions of the aryl substituent to
most of the highest occupied MOs.
constants. This suggests that the primary photochemical step
is Pt-R bond homolysis followed by chlorine abstraction
from the solvent. The photoreactions in non-chlorinated
solvents are very unselective, giving rise to uninterpretable
1H NMR spectra. In most cases, these latter photoreactions
are accompanied by the formation of elemental platinum.
To establish a relative series of efficiencies of the photore-
actions depending on R, their photoreactions in CH2Cl2
solutions were followed by UV-vis spectroscopy, using the
same irradiation method as in the NMR experiments. From
the decay of the absorption band with irradiation time, the
relative order of photoreaction efficiencies was determined
to decrease along R ) adme > neop > neoSi > CH3, which
is the same order as that found for the COD complexes.19
The photoreactions of the [(iPr-DAB)Pt(neop)2] complex
were studied in more detail. First of all, it was found that
upon irradiation at λ > 600 nm, to ensure that only the
lowest-energy transition is excited, no photoreaction oc-
curred, which means either that in the relaxed excited state
the charge density decrease in the σ(Pt-R) is too low to
weaken this bond enough to cause its rupture or that the
excited state lifetime is too short. Indeed, the excited state
lifetime was found to be <5 ns by transient absorption
spectroscopy. In contrast, irradiation using 551.4 nm light
from a Coumarine 6 dye laser (corresponding to the second
absorption maximum) gave rise to measurable photoreactivity
(Figure 9). This implies that in this case a prompt photo-
chemical reaction occurs, that is, a photoreaction from a
thermally nonrelaxed excited state which is fast enough to
compete with vibrational relaxation.54 The absolute photo-
chemical quantum yield was determined by Ar+ laser
irradiation at 514.5 nm to be Φ ) 7.3 × 10-4. As in the
case of the COD complexes, a relative increase in reactivity
of a factor of 50 was found going from the Me to the neop
compound; the photochemical quantum yield of [(iPr-DAB)-
Pt(CH3)2] is expected to be on the order of Φ ) 10-5. The
absolute photochemical quantum yield of [(COD)Pt(CH3)2]
was determined to be Φ ) 5 × 10-3,55 indicating that on
Photochemistry. Both the (TD)DFT calculations and the
rR spectra showed that while the low-lying electronic
transitions of [(iPr-DAB)Pt(R)2] (R ) alkyl) complexes do
possess some SBLCT character, the co-ligand contribution
is much less than in the corresponding COD complexes.19
i
This is expected to diminish the photoreactivity of the Pr-
DAB complexes. To investigate this expectation, their
1
photoreactions were followed by H NMR spectroscopy,
irradiating 10-3 M solutions of the complexes in CD2Cl2
using a high pressure Hg lamp with a suitable cutoff filter
to limit irradiation to band system I. The first observable
products were the monochloro complexes [(iPr-DAB)Pt(Cl)-
(R)], characterized by the chemical shift and 195Pt coupling
i
going from the COD to the Pr-DAB complexes the photo-
reactivity decreases 2 orders of magnitude.
(54) Langford, C. H. Acc. Chem. Res. 1984, 17, 96.
(55) Kunkely, H.; Vogler, A. J. Organomet. Chem. 1998, 553, 517.
5224 Inorganic Chemistry, Vol. 41, No. 20, 2002

Platinum(II) Diazabutadiene Complexes
The complexes with an unsaturated co-ligand (R ) Ct
CtBu, CtCPh, Ph, Mes) are not photoreactive at all on
irradiation into either band system I or band system II. This
is not unexpected because the electronic transition does not
remove electron density from the PtsC bonds of these
complexes, and they are therefore not weakened. The alkynyl
derivatives show appreciable photoreactivity at much higher
energy (λ > 320 nm) irradiation. According to NMR
experiments as described previously, the photoproduct is the
R′sCtC-CtCsR′ (R′ ) tBu or Ph) coupling product. In
view of the elemental platinum formation that was observed
in these experiments, this product is probably formed by
reductive elimination as in the case of the corresponding
COD complexes.
low for a reasonable application. The alkynyl complexes were
found to be virtually photostable except when irradiated at
relatively high energies.
Comparing results from (TD)DFT calculations on the
electronic structure and low-lying transitions of the com-
plexes to experimental information from electronic absorption
and resonance Raman (rR) spectroscopies, it was found that
the DFT methods predict the characters of low-lying
electronic transitions well. However, the calculated energies
and oscillator strengths are lower than those found experi-
mentally. One other interesting result from both the calcula-
tions and the rR spectra is that the structuring of the visible
part of the absorption spectra of these complexes is due to
the presence of several electronic transitions and not to
vibrational fine structure as was assumed in earlier work.
Conclusions
Supporting Information Available: Four tables describing
results from DFT calculations on [(iPr-DAB)Pt(Ph)2] and collective
rR data of all complexes. This material is available free of charge
via the Internet at http://pubs.acs.org.
In this paper, a series of [(iPr-DAB)Pt(R)2] (R ) CH3,
CD3, adme, neop, neoSi, CtCtBu, CtCPh, Ph, Mes) com-
plexes was investigated. The aim was to determine to what
extent the co-ligands are involved in low-lying electronic
transitions with the ultimate goal to create complexes that
are photoreactive on visible light irradiation. The co-ligand
contribution proved to be significant in the case of alkyl co-
ligands, while it is predominant when the co-ligands are
unsaturated. This is an interesting conclusion because it is
generally assumed that the co-ligands are not involved in
the low-lying transitions and excited states of such com-
plexes. Although the photoreactivity of the alkyl complexes
can be significantly enhanced, on increasing the σ donor
strength of the alkyl ligand, in an absolute sense it is too
Acknowledgment. This work was undertaken as part of
the European collaborative COST project D14/0001/99. A.K.
would like to thank Prof. D. J. Stufkens (UvA) for the
opportunity to do research in his group and also for fruitful
discussions, and Prof. W. Kaim (IAC, Universita¨t Stuttgart)
is acknowledged for financial support. We are also grateful
for a loan of K2PtCl4 by Johnson Matthey (JM). S.Z. thanks
also the Ministry of Education of the Czech Republic for
the financial support (OC.D14.20).
IC025547O
Inorganic Chemistry, Vol. 41, No. 20, 2002 5225