o-iminobenzosemiquinonato(1-) and o-amidophenolato(2-) complexes of palladium(II) and platinum(II): A combined experimental and density functional theoretical study

Article abstract of DOI:10.1021/ic011297k

From the reaction mixture of [M^II(bpy)Cl₂], the ligand 2-anilino-4,6-di-tert-butylphenol, H[L^AP], and 2 equiv of a base (NaOCH₃) in CH₃CN under anaerobic conditions were obtained the blue-green neutral complexes [M^II(L^AP-H)(bpy)] (M = Pd (1), Pt (2)). (L^AP-H)^2- represents the o-amidophenolato dianion, (L^AP)^1- is the o-aminophenolate(1-), (L^ISQ)^1- is its one-electron-oxidized, π-radical o-iminobenzosemiquinonate(1-), and (L^IBQ)⁰ is the neutral quinone. Complexes 1 and 2 can be oxidized by ferrocenium hexafluorophosphate, yielding the paramagnetic salts [M^II(L^ISQ)(bpy)]PF₆ (S = 1/2) (M = Pd (1a), Pt (2a)). The reaction of PtCl₂, 2 equiv of H[L^AP], and 4 equiv of base in CH₃CN in the presence of air yields diamagnetic [Pt(L^ISQ)₂] (3), which is shown to possess an electronic structure that is best described as a singlet diradical. Complexes 1, 1a, 2, 2a, and 3 have been structurally characterized by X-ray crystallography at 100 K. It is clearly established that O,N-coordinated (L^AP-H)^2- ligands have a distinctly different structure than the corresponding O,N-coordinated (L^ISQ)^1- radicals. It is therefore possible to unambiguously assign the protonation and oxidation level of o-aminophenol derived ligands in coordination compounds. All complexes have been investigated by cyclic voltammetry, spectroelectrochemistry, EPR, and UV-vis spectroscopy. Complexes 1 and 2 can be reversibly oxidized to the [M^II(L^ISQ)(bpy)]⁺ and [M^II(L^IBQ)(pby)]²⁺ mono- and dications, respectively, and reduced to the [M(L^AP-H)(bpy)]^- anion, where (bpy)^1- is the radical anion of 2,2′-bipyridine. Complex 3 exhibits four reversible one-electron-transfer waves (two oxidations and two reductions) which are all shown to be ligand centered. The EPR spectra of the one-electron-reduced species [Pt(L^AP-H)(L^ISQ)]^- (S = 1/2) and of the one-electron-oxidized species [Pt(L^ISQ)(L^IBQ)]⁺ (S = 1/2) in CH₂Cl₂ solutions have been recorded. To gain a better understanding of the electronic structure of 3 and its monooxidized and reduced forms, relativistic DFT calculations have been carried out. Magnetic coupling parameters and hyperfine couplings were calculated and found to be in very good agreement with experiment. It is shown that both the one-electron oxidation and reduction of 3 are ligand centered. A simple MO model is developed in order to understand the EPR properties of the monocation and monoanion of 3.

Full text of DOI:10.1021/ic011297k

Inorg. Chem. 2002, 41, 4295−4303
o-Iminobenzosemiquinonato(1−) and o-Amidophenolato(2−) Complexes
of Palladium(II) and Platinum(II): A Combined Experimental and Density
Functional Theoretical Study
Xianru Sun, Hyungphil Chun, Knut Hildenbrand, Eberhard Bothe, Thomas Weyhermu1ller,
Frank Neese, and Karl Wieghardt*
Max-Planck-Institut fu¨r Strahlenchemie, Stiftstrasse 34-36,
D-45470 Mu¨lheim an der Ruhr, Germany
Received December 20, 2001
II
From the reaction mixture of [M (bpy)Cl2], the ligand 2-anilino-4,6-di-tert-butylphenol, H[LAP], and 2 equiv of a base
II
(NaOCH ) in CH CN under anaerobic conditions were obtained the blue-green neutral complexes [M (LAP-H)(bpy)]
3
3
(M ) Pd (1), Pt (2)). (LAP-H)2- represents the o-amidophenolato dianion, (LAP)1- is the o-aminophenolate(1−),
(L ) is its one-electron-oxidized, π-radical o-iminobenzosemiquinonate(1−), and (L ) is the neutral quinone.
Complexes 1 and 2 can be oxidized by ferrocenium hexafluorophosphate, yielding the paramagnetic salts
[M (L )(bpy)]PF (S ) /2) (M ) Pd (1a), Pt (2a)). The reaction of PtCl2, 2 equiv of H[LAP], and 4 equiv of base
ISQ 1-
IBQ 0
II ISQ
1
6
ISQ
in CH CN in the presence of air yields diamagnetic [Pt(L )2] (3), which is shown to possess an electronic structure
3
that is best described as a singlet diradical. Complexes 1, 1a, 2, 2a, and 3 have been structurally characterized
by X-ray crystallography at 100 K. It is clearly established that O,N-coordinated (LAP-H)2- ligands have a distinctly
different structure than the corresponding O,N-coordinated (LISQ)1- radicals. It is therefore possible to unambiguously
assign the protonation and oxidation level of o-aminophenol derived ligands in coordination compounds. All complexes
have been investigated by cyclic voltammetry, spectroelectrochemistry, EPR, and UV−vis spectroscopy. Complexes
II ISQ
II IBQ
1 and 2 can be reversibly oxidized to the [M (L )(bpy)]+ and [M (L )(pby)]2+ mono- and dications, respectively,
and reduced to the [M(LAP-H)(bpy )]- anion, where (bpy ) is the radical anion of 2,2′-bipyridine. Complex 3
•
• 1-
exhibits four reversible one-electron-transfer waves (two oxidations and two reductions) which are all shown to be
ISQ
1
ligand centered. The EPR spectra of the one-electron-reduced species [Pt(LAP-H)(L )]- (S ) /2) and of the
one-electron-oxidized species [Pt(L )(L )]+ (S ) /2) in CH Cl2 solutions have been recorded. To gain a better
ISQ IBQ
1
2
understanding of the electronic structure of 3 and its monooxidized and reduced forms, relativistic DFT calculations
have been carried out. Magnetic coupling parameters and hyperfine couplings were calculated and found to be in
very good agreement with experiment. It is shown that both the one-electron oxidation and reduction of 3 are
ligand centered. A simple MO model is developed in order to understand the EPR properties of the monocation
and monoanion of 3.
Introduction
2-anilino-4,6-di-tert-butylphenol, H[LAP], can bind to a
transition metal ion in an O,N-fashion as (i) diamagnetic
o-aminophenolato(1-) monoanion, (LAP)1-, or (ii) dia-
magnetic o-amidophenolato(2-) dianion, (LAP-H)2-, or (iii)
In a series of papers we have recently established1-5 that
o-aminophenols are redox noninnocent when O,N-
coordinated to a transition metal ion Ni(II), Cu(II), Pd(II),
Co(III), Fe(III), or Cr(III). We have shown that for example
1
paramagnetic (Srad ) /2) o-iminobenzosemiquinonato(1-)
* Author to whom correspondence should be addressed. E-mail:
wieghardt@mpi-muelheim.mpg.de.
(1) Verani, C. N.; Gallert, S.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K.;
Chaudhuri, P. Chem. Commun. 1999, 1747.
(3) Chun, H.; Verani, C. N.; Chaudhuri, P.; Bothe, E.; Bill, E.; Weyher-
mu¨ller, T.; Wieghardt, K. Inorg. Chem. 2001, 40, 4157.
(4) Chun, H.; Weyhermu¨ller, T.; Bill, E.; Wieghardt, K. Angew. Chem.,
Int. Ed. 2001, 40, 2489.
(2) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
(5) Chun, H.; Chaudhuri, P.; Weyhermu¨ller, T.; Wieghardt, K. Inorg.
Chem. 2002, 41, 790.
10.1021/ic011297k CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/19/2002
Inorganic Chemistry, Vol. 41, No. 16, 2002 4295

Sun et al.
Scheme 1. Average Bond Distances in (LISQ)•- and O,N-Coordinated
(LAP-H)2- Ligandsa
Experimental Section
The ligand 2-anilino-4,6-di-tert-butylphenol, H[LAP],1,2 and the
complexes [Pd(bpy)Cl2] and [Pt(bpy)Cl2]7 have been prepared as
described in the literature.
[Pd(LAP-H)(bpy)] (1). To a solution of the ligand H[LAP] (150
mg; 0.5 mmol) in dry acetonitrile (30 mL) under an Ar blanketing
atmosphere was added solid [Pd(bpy)Cl2] (166 mg; 0.5 mmol) and
Na[OCH3] (0.5 M CH3OH solution; 2 mL )ˆ 1.0 mmol). The
mixture was heated to reflux for 3 h under Ar. After the solution
was cooled to ambient temperature a dark blue crystalline precipitate
formed, which was collected by filtration and washed with diethyl
ether. Yield: 194 mg (70%). Anal. Calcd for C30H33N3OPd: C,
64.63; H, 5.92; N, 7.54. Found: C, 64.4; H, 6.0; N, 7.5. ESI mass
spectrum: m/z 557 [M+] (100%).
[Pd(LISQ)(bpy)]PF6 (1a). To a deep blue solution of 1 (255 mg;
0.46 mmol) in dry CH2Cl2 (50 mL) under an Ar atmosphere was
added ferrocenium hexafluorophosphate, [Fc]PF6 (152 mg; 0.46
mmol). After the solution was stirred at room temperature for 3 h,
the solvent was partly removed (∼20 mL) by evaporation and
n-hexane (10 mL) was added. A brown crystalline material formed,
which was collected by filtration and washed with diethyl ether.
Yield: 287 mg (89%). Anal. Calcd for C30H33N3OPdPF6: C, 51.28;
H, 4.70; N, 5.98. Found: C, 51.1; H, 4.8; N, 6.0. ESI mass
spectrum: m/z 557 [M+] (100%).
[Pt(LAP-H)(bpy)] (2). To a solution of the ligand H[LAP] (150
mg; 0.5 mmol) in dry acetonitrile (30 mL) under Ar was added
solid [Pt(bpy)Cl2] (179 mg; 0.5 mmol) and Na[OCH3] (0.5 M
CH3OH solution; 2 mL; 1.0 mmol). After heating to reflux for 3 h,
a stream of Ar was passed through the resulting green solution to
slowly evaporate the solvent. Within 2 d dark green crystals suitable
for X-ray crystallography were obtained. Yield: 210 mg (65%).
Anal. Calcd for C30H33N3OPt: C, 55.73; H, 5.11; N, 6.50. Found:
C, 56.0; H, 5.2; N, 6.6. ESI mass spectrum: m/z 646 [M+].
[Pt(LISQ)(bpy)]PF6 (2a). This compound has been prepared as
described above for 1a except that 2 (50 mg; 0.08 mmol) was used
as the starting material and an equimolar amount of [Fc]PF6 (25.6
mg; 0.08 mmol). A red crystalline solid was obtained. Yield: 61
mg (82%). Anal. Calcd for C30H33N3OPtPF6: C, 45.51; H, 4.17;
N, 5.31. Found: C, 46.0; H, 4.3; N, 5.4. ESI mass spectrum: m/z
646 [M+].
a From structures in refs 1-5; the estimated error (3σ) is (0.01 Å.
monoanion, (LISQ)1-. High-quality, low-temperature X-ray
crystallography allows the assignment of protonation and
oxidation level of the O,N-coordinated ligand in a given
complex. Scheme 1 shows their metrical details. It is noted
that no complex containing the neutral quinone form, (LIBQ),
has been structurally characterized to date. Complexes
containing o-aminothiophenolato ligands and their benzo-
semiquinonato analogues have also been identified recently.6
The square-planar, diamagnetic compounds [M(LISQ)2] (M
) Ni, Pd) and their paramagnetic Cu(II) (St ) 1/2) analogue
have recently been described by us.2 We have shown that
the electronic structure of the Ni and Pd complexes is best
described as a species containing a diamagnetic divalent
metal ion (d8) in a square-planar environment of two O,N-
coordinated o-iminobenzosemiquinonato(1-) π-radical ligands.
The spins are intramolecularly antiferromagnetically coupled
yielding the observed S ) 0 ground state. Thus these species
are singlet diradicals. Here we report the analogous platinum
species [Pt(LISQ)2] (St ) 0) (3).
We have also prepared and fully characterized the
diamagnetic neutral complexes [M(LAP-H)(bpy)] (M ) Pd
(1), Pt (2)) which can be reversibly oxidized to the corre-
sponding paramagnetic monocations where bpy represents
the neutral ligand 2,2-bipyridine. The following paramagnetic
salts have been isolated: [M(LISQ)(bpy)]PF6 (M ) Pd (1a),
Pt (2a)). Complexes 1a and 2a are ideally suited to establish
the spectroscopic features of a single O,N-coordinated
[Pt(LISQ)2] (3). A solution of PtCl2 (0.30 g; 1.1 mmol), the ligand
H[LAP] (0.45 g; 1.5 mmol), and NEt3 (0.5 mL) in CH3CN (30 mL)
was stirred at 20 °C in the presence of air for 3 h. The dark blue
solution was filtered and the solvent was removed by evaporation.
The dark blue residue was redissolved in a methanol/acetonitrile
mixture (1:1) and the solvent was allowed to slowly evaporate in
air. A dark blue microcrystalline powder precipitated. Yield: 0.20
g (34% based on H[LAP]). Single crystals suitable for X-ray
diffraction were obtained by recrystallization of dark blue powder
from the CH3CN/CH2Cl2 (1:1) mixture. EI mass spectrum: m/z
(L )
ISQ 1- π-radical in a coordination compound. The structures
of the pairs 1/1a and 2/2a allow the geometrical features of
an O,N-coordinated dianion (LAP-H)2- versus its one-electron
oxidized monoanionic radical (LISQ 1-
to be firmly estab-
)
1
785 [M+]. H NMR (CDCl3, 400 MHz): δ 7.65 (d, 4H), 7.52 (t,
lished. This is of importance when one wishes to discern
experimentally between the two forms A and B.
4H), 7.38 (t, 2H), 6.96 (s, 2H), 6.74 (s, 2H), 1.26 (s, 18H), 1.17 (s,
18H) ppm. Anal. Calcd for C40H50N2O2Pt: C, 61.20; H, 6.42; N,
3.57. Found: C, 61.1; H, 6.5; N, 3.6.
(6) Herebian, D.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. J.
Am. Chem. Soc. 2001, 123, 10012.
(7) Fox, S. G.; Gillard, R. D. Polyhedron 1988, 7, 349.
4296 Inorganic Chemistry, Vol. 41, No. 16, 2002

Palladium(II) and Platinum(II) Complexes
Table 1. Crystallographic Data for 1, 1a, 2, 2a, and 3
1
1a
2
2a
3
chem formula
fw
C30H33N3OPd
557.99
C30H33F6N3OPPd
702.96
C30H33N3OPt
646.68
C30H33F6N3OPPt
791.65
C40H50N2O2Pt
785.91
space group
a, Å
b, Å
P1h, no. 2
10.1455(9)
11.0851(9)
13.1358(12)
87.15(1)
72.93(1)
64.82(1)
1273.2(2)
2
C2/c, no. 15
26.3425(12)
5.7281(4)
38.538(2)
90
99.41(1)
90
5736.8(6)
8
P1h, no. 2
10.1892(9)
11.2083(9)
13.0067(12)
86.83(1)
72.45(1)
65.53(1)
1284.9(2)
2
C2/c, no. 15
26.296(2)
5.7372(4)
38.648(3)
90
99.38(1)
90
5752.7(7)
8
P1h, no. 2
5.7790(3)
11.7080(5)
13.6868(6)
108.51(1)
92.94(1)
94.00(1)
873.37(7)
1
c, Å
R, deg
â, deg
γ, deg
V, Å
Z
T, K
100(2)
100(2)
100(2)
100(2)
100(2)
F calcd, g cm-3
diffractometer used
reflns collected/θmax
unique reflns/I > 2σ(I)
no. of params
µ(Mo KR), cm-1
R1a/goodness of fit b
wR2c (I > 2σ(I))
1.456
1.628
1.671
1.828
1.494
Siemens SMART
14217/33.16
8418/6818
322
7.57
0.0342/0.953
0.0659
Nonius Kappa-CCD
20918/30.00
8093/6831
385
7.72
0.0318/1.054
0.0687
Siemens SMART
14304/33.14
8445/7268
322
54.88
0.0327/1.006
0.0657
Nonius Kappa-CCD
18348/30.00
7829/6241
385
50.03
0.0402/1.011
0.0745
Siemens SMART
9672/33.17
5747/5744
211
40.53
0.0227/1.054
0.0540
2
2
2
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|. b GooF ) [∑[w(Fo2 - Fc )2]/(n - p)]1/2
.
c wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2]]1/2, where
w ) 1/σ2(Fo ) + (aP)2 + bP, P ) (Fo + 2Fc )/3.
2
2
2
X-ray Crystallographic Data Collection and Refinement of
the Structures. Black single crystals of 1, 2, 3, an orange brown
crystal of 1a, and a deep red crystal of 2a were coated with
perfluoropolyether, picked up with a glass fiber, and mounted in
the nitrogen cold stream of the diffractometers. Intensity data were
collected at 100 K with use of graphite monochromated Mo KR
radiation (λ ) 0.71073 Å). Final cell constants were obtained from
a least-squares fit of a subset of several thousand strong reflections.
Data collection was performed by hemisphere runs taking frames
at 0.3° (Siemens SMART) and 1.0° (Nonius Kappa-CCD) in ω.
Crystal faces of 1a, 2, 2a, and 3 were determined and the Gaussian
absorption correction routine of XPREP9 was used to account for
absorption effects. A semiempirical absorption correction using the
program SADABS8 was performed on the data set of 1. Crystal-
lographic data of the compounds and diffractometer types used are
listed in Table 1. The Siemens ShelXTL9 software package was
used for solution, refinement, and artwork of the structure.
Structures 1a and 2a were readily solved by direct methods and
difference Fourier techniques whereas the other solutions were
obtained by the Patterson method. All non-hydrogen atoms were
refined anisotropically and hydrogen atoms were placed at calcu-
lated positions and refined as riding atoms with isotropic displace-
ment parameters.
Physical Measurements. UV-vis, EPR, and infrared spectra
(KBr disks) have been recorded with apparatus described in refs
1-5. Similarly, the electrochemical and the magnetochemical
equipment have been described previously1-5 as have the programs
used for the EPR simulations.
Calculations. Relativistic density functional calculations were
carried out with the ADF program system version 2000.02.10 The
geometries of the complexes were optimized in C2h symmetry by
using the BP86 functional11,12 in the spin-polarized scalar relativistic
ZORA option.13 For the geometry optimizations the ZORA basis
set IV was used for Pt, II for hydrogen, and III for the remaining
atoms. For the property calculations the large ZORA basis set V
was used for Pt and basis set IV for all other atoms. The neutral
complex was optimized in the triplet state. Magnetic coupling
parameters were calculated according to the procedures of van
Lenthe et al. for g-tensors14 and hyperfine couplings (hfc’s15). This
formalism allows for the calculation of the g-tensor and orbital
correction to the hfc’s in a spin-restricted formalism while the
Fermi-contact term and orbital dipolar term are calculated in a spin-
unrestricted formalism.
Results and Discussion
Synthesis and Characterization of Complexes. Dark
blue crystals of [Pd(LAP-H)(bpy)] (1) and dark green crystals
of [Pt(LAP-H)(bpy)] (2) were obtained in ∼70% yield from
the reaction of the ligand H[LAP] and [MII(bpy)Cl2] (M )
Pd, Pt) in equimolar amounts in dry acetonitrile and 2 equiv
of the base sodium methoxide under anaerobic conditions
at elevated temperatures. Both 1 and 2 are diamagnetic,
mononuclear species.
The one-electron-oxidized forms [MII(LISQ)(bpy)]PF6 (M
) Pd (1a), Pt (2a)) were obtained in excellent yields from
reaction mixtures of 1 and 2 and an equimolar amount of
ferrocenium hexafluorophosphate as clean one-electron
oxidant in dry CH2Cl2 at ambient temperature. Crystals of
1a are dark brown and those of 2a dark red. Magnetic
susceptibility measurements of solid samples of 1a and 2a
in the temperature range 4-300 K revealed a temperature-
independent magnetic moment (50-300 K) of 1.8 µB,
respectively. Thus, both complexes possess an S ) 1/2 ground
state.
Dark blue microcrystals of [PtII(LISQ)2] (3) have been
obtained in moderate yields from the reaction mixture of
PtCl2, the ligand H[LAP] (1:2), and 4 equiv of NEt3 in CH3CN
solution in the presence of air at 20 °C. The analoguous
(8) Sheldrick, G. M. Universita¨t Go¨ttingen, 1994.
(9) ShelXTL, V.5; Siemens Analytical X-ray Instruments, Inc., 1994.
(10) ADF, V 2000.02; Scientific Computing and Modeling NV; Vrije
Universiteit, Theoretical Chemistry; Amsterdam, The Netherlands.
(11) Becke, A. D. J. Chem. Phys. 1988, 84, 4524.
(12) Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
(14) van Lenthe, E.; Wormer, P. E. S.; van der Avoird, A. J. Chem. Phys.
1997, 107, 2488.
(15) van Lenthe, E.; van der Avoird, A.; Wormer, P. E. S. J. Chem. Phys.
1998, 108, 4783.
(13) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993,
99, 4597.
Inorganic Chemistry, Vol. 41, No. 16, 2002 4297

Sun et al.
Table 2. Selected Bond Distances (Å) of Complexes
1
2
1a
2a
3
M-N1
M-N2
M-N3
M-O1
N1-C7
N1-C6
C1-O1
C1-C6
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
1.974(2)
2.048(2)
2.019(2)
1.968(1)
1.410(2)
1.387(2)
1.348(2)
1.421(3)
1.406(2)
1.405(3)
1.398(3)
1.402(2)
1.395(2)
1.985(3)
2.015(3)
1.997(3)
1.980(2)
1.418(4)
1.390(4)
1.357(4)
1.417(4)
1.397(4)
1.404(5)
1.394(5)
1.405(4)
1.401(4)
2.014(2)
2.028(2)
2.013(2)
1.989(1)
1.433(2)
1.346(2)
1.309(2)
1.450(2)
1.428(3)
1.382(3)
1.437(2)
1.366(3)
1.420(3)
2.011(4)
2.028(4)
2.011(3)
1.996(3)
1.437(6)
1.350(5)
1.317(5)
1.429(6)
1.429(6)
1.394(6)
1.430(6)
1.376(6)
1.416(6)
1.946(2)
1.977(1)
1.429(3)
1.372(2)
1.317(2)
1.419(3)
1.427(3)
1.376(3)
1.430(3)
1.373(3)
1.414(3)
Figure 1. Structure of the monocation [Pt(LISQ)(bpy)]+ in crystals of 2a.
The structures of the neutral species [M(LAP-H)(bpy)] (M ) Pd (1), Pt (2))
and of the monocation in 1a are very similar and are not shown (see
Supporting Information). Small open circles represent H atoms.
compounds [MII(LISQ)2] (M ) Ni, Pd) have been described
previously.2 Complex 3 is diamagnetic (S ) 0) as was judged
1
from its “normal” H NMR spectrum (see Experimental
Section). It displays a typical very intense (7.3 × 104 M-1
cm-1) interligand charge-transfer band at 813 nm (see below
and ref 2).
Crystal Structure Determinations. The crystal structures
1, 1a, 2, 2a, and 3 have been determined by high-quality
X-ray crystallography at 100 K. Table 2 summarizes selected
bond distances and Figure 1 shows the structure of the
monocation in crystals of 2a; the structures of the neutral
molecules in 1 and 2 and that of the monocations in 1a are
very similar and not shown; they are available in the
Supporting Information. Figure 2 shows the structure of the
neutral molecules in crystals of 3.
All complexes are square planar with an MN3O donor set
in 1, 1a, 2, and 2a and a PtN2O2 in 3. In the following we
compare the C-C, C-N, and C-O distances of the N,N-
and N,O-coordinated bipyridine and aminophenolate derived
ligands, respectively, in the neutral complexes 1 and 2 with
the corresponding ones in the monocations of 1a and 2a. It
is important to note that the average error of these bond
lengths is small at (0.01 Å (3σ). It is therefore significant
that the C-C distances in the bpy ligands and in the N-phenyl
group of the aminophenolate ligands are equidistant within
0.01 Å in all four structures. Thus the bpy and the N-phenyl
groups are internal markers for the consistency of the
structure determinations; they do not respond structurally
upon one-electron oxidation of 1 (2) to 1a (2a). In stark
contrast, the C-O, C-N, and C-C distances of the
o-aminophenol derived, O,N-coordinated ligands respond to
this oxidation in a characteristic fashion. These distances are
significantly different in 1 and 1a and, similarly, in 2 and
2a.
Figure 2. Structure of neutral complex [Pt(LISQ)2] in crystals of 3. H atoms
omitted.
(Table 2) are within experimental error equidistant at 1.40
( 0.01 Å; only C1-C6 is slightly longer at 1.42 ( 0.01 Å.
The C-O bond at 1.35 ( 0.01 Å is typical for a single bond
of a phenolate group; the average C6-N1 distance is at 1.39
( 0.01 Å whereas the distance N1-C7 is at 1.41 ( 0.01 Å.
The amido nitrogen N1 is sp2 hybridized and not protonated.
Thus 1 and 2 can safely be described as [MII(LAP-H)(bpy)]
(M ) Pd, Pt).
The same distances in 1a and 2a clearly point to the
presence of a monoanionic o-iminobenzosemiquinonato(1-)
π-radical: average distances C2-C3 and C4-C5 are short
at 1.38 ( 0.01 and 1.37 ( 0.01 Å, respectively, whereas all
other C-C distances in this ring are rather long at 1.43 (
0.01 Å. Thus this ring displays the characteristic features of
a quinoid structure. At the same time, the C1-O1 distance
at 1.315 ( 0.01 Å and the C6-N1 distance at 1.35 ( 0.01
Å are short indicating considerable double bond character.
Thus 1a and 2a must be described as [MII(LISQ)(bpy)]+.
By using the data in Scheme 1 it is immediately possible
to assign the protonation and oxidation levels of the
aminophenol derived ligands as follows: For 1 and 2 it is
clear that this ligand is dianionic (LAP-H)2-. The five C-C
distances C1-C2, C2-C3, C3-C4, C4-C5, and C5-C6
Finally, in 3 there are clearly two of these (LISQ 1-
O,N-coordinated to the central Pt(II) ion: [PtII(LISQ)2].
)
radicals
4298 Inorganic Chemistry, Vol. 41, No. 16, 2002

Palladium(II) and Platinum(II) Complexes
Table 3. Redox Potentials of Complexesa
transition metal ion Pt. The cvs of 1a and 2a are identical
with those shown in Figure 3 for 1 and 2.
complex
E11/2, V
E21/2, V
E31/2, V
E41/2, V
The one-electron oxidation of the monocations [MII(LISQ)-
(bpy)]1+ to the dications [MII(LIBQ)(bpy)]2+ (M ) Pd, Pt) is
a ligand-centered process where the o-iminobenzosemi-
quinonate anion is oxidized to the o-iminobenzoquinone,
(LIBQ)0, as was judged from the changes observed in their
electronic absorption spectra (see below). Similarly, the one-
electron reduction of 1 and 2 is proposed to be a ligand-
centered process where an N,N-coordinated 2,2′-bipyridine
ligand is reversibly reduced to its monoanionic radical
(bpy•)1-.16-19 Thus the species [MII(LAP-H)(bpy•)]1- (M )
Pd, Pt) are proposed to be generated. They are not stable in
solution at ambient temperature and, therefore, have not been
characterized further.
1
2
3
0.352
0.421
0.70
-0.644
-0.585
0.20
-1.890
-1.900
-1.07
-0.99
-1.07
-1.64
-1.40
-1.64
[Pd(LISQ)2]b
[Ni(LISQ)2]b
0.47
0.08
0.042c
a Potentials are referenced vs the Fc+/Fc couple; CH2Cl2 solutions
containing 0.10 M [N(n-Bu)4]PF6; glassy carbon working electrode;
ferrocene internal standard. b Reference 2. c 2e process: [NiII(LISQ)2] f
[NiII(LIBQ)2]2+ + 2e; ref 2.
The cv of 3 (Figure 3, bottom) is very similar to those
reported for [CuII(LISQ)2] and [Pd(LISQ)2].2 In all of these
cases, four reversible one-electron-transfer waves are ob-
served of which two correspond to successive one-electron
reductions and two to successive one-electron oxidations.
This electrochemical behavior is typical for this class of
square-planar complexes containing two o-phenylenediamido
and related ligands.20 Since the redox potentials E1-4 for
all three complexes are very similar irrespective of the nature
of the central metal ion (CuII, PdII, PtII), we assign these
processes as ligand centered, eq 2.
1/2
Figure 3. Cv of 1 (1 × 10-3 M), 2 (1.0 × 10-3 M), and 3 (0.4 × 10-3
M) in CH2Cl2 solution (0.10 M [N(n-Bu)4]PF6) at 20 °C (glassy carbon
working electrode) at scan rates of 25, 50, 100, 200, and 400 mV s-1 for
1 and 2 and 100, 200, 400, and 800 mV s-1 for 3.
Electro- and Spectroelectrochemistry. Cyclic voltammo-
grams of all complexes have been recorded at 20 °C in
CH2Cl2 solutions containing 0.10 M [N(n-Bu)4]PF6 as
supporting electrolyte at a glassy carbon working electrode
and a Ag/AgNO3 reference electrode. Ferrocene was used
as an internal standard, and potentials are referenced versus
the ferrocenium/ferrocene couple (Fc+/Fc). Table 3 sum-
marizes these results.
Figure 3 displays the cvs of 1, 2, and 3. Both 1 and 2
show three reversible one-electron waves in the range +1.0
to -2.5 V, which correspond to the processes in eq 1 (M )
Pd, Pt):
[M(LIBQ)2]2+
{
E1
+-ee } [M(LIBQ)(LISQ)]+ { +-ee } [M(LISQ)2]0 { +e
}
-e
E2
E3
1/2
1/2
1/2
[M(LAP-H)(LISQ)]- { -+ee } [M(LAP-H)2]2- (2)
E4
1/2
Figure 4 shows the electronic spectra in the visible of 1
(top) and 2 (bottom) and of their respective mono- and
dications which were generated by controlled potential
electrolysis in CH2Cl2 solutions containing 0.10 M [N(n-
Bu)4]PF6. Table 4 summarizes the electronic spectra. In the
(16) Tokel-Takvoryan, N. E.; Hemingway, R. E.; Bard, A. J. J. Am. Chem.
Soc. 1973, 95, 6582.
(17) McInnes, E. J. L.; Welch, A. J.; Yellowlees, L. J. Chem. Commun.
1996, 2393.
+e
[MII(LIBQ)(bpy)]2+
{
E1
+-ee } [MII(LISQ)(bpy)]1+
{
}
-e
E2
1/2
1/2
(18) McInnes, E. J. L.; Farley, R. D.; Macgregor, S. A.; Taylor, K. J.;
Yellowlees, L. J.; Rowlands, C. C. J. Chem. Soc., Faraday Trans.
1998, 94, 2985.
[MII(LAP-H)(bpy)]0 { +-ee } [MII(LAP-H)(bpy•)]1- (1)
(19) McInnes, E. J. L.; Faarley, R. D.; Rowlands, C. C.; Welch, A. J.;
Rovatti, L.; Yellowlees, L. J. J. Chem. Soc., Dalton Trans. 1999, 4203.
(20) (a) Balch, A. L.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201. (b)
Holm, R. H.; Balch, A. L.; Davison, A.; Muki, A.; Berry, T. J. Am.
Chem. Soc. 1967, 89, 2866. (c) Forbes, C. E.; Gold, A.; Holm, R. H.
Inorg. Chem. 1971, 10, 2479. (d) Stiefel, E. I.; Waters, J. H.; Billig,
E.; Gray, H. B. J. Am. Chem. Soc. 1965, 87, 3016.
E3
1/2
Interestingly, the redox potentials, E1-31/2, are very similar
for 1 and 2; they do not vary greatly (60-70 mV) with a
change of the second-row divalent Pd ion to the third-row
Inorganic Chemistry, Vol. 41, No. 16, 2002 4299

Sun et al.
Table 4. Electronic Spectra of Complexesa
λ
max, nm (103 ꢀ, L mol-1 cm-1
)
[Pd(LAP-H)(bpy)]
*[Pd(LISQ)(bpy)]+
450 (1.0), 745 (3.2)
b
425 (4.25), 470 (3.9), 510 sh (1.5), 730
sh (0.6), 820 (0.7), 980 sh (0.6)
400 sh (3.0), 495 (4.2)
*[Pd(LIBQ)(bpy)]2+
[Pt(LAP-H)(bpy)]
480 (3.0), 780 (10.5)
*[Pt(LISQ)(bpy)]+
503 (11.0), 535 sh (9.1), 610 sh (2.0), 780 sh
(1.0), 880 sh (1.2), 1000 sh (0.8)
400sh (4.5), 495 (6.3)
427 (33.2), 507 (19.5)
298 (15.0), 488 (15.4), 531 (17.3), 622
(15.1), 681 (21.5)
224 (60), 318 (13), 430 (5.3), 628 (7.3),
813 (73)
305 (15.5), 339 (15.4), 363 (15.5), 577 (3.4),
821 (3.9), 1059 (15.7)
b
*[Pt(LIBQ)(bpy)]2+
*[Pt(LIBQ)2]2+
*[Pt(LISQ)(LIBQ)]+
[Pt(LISQ)2]
*[Pt(LAP-H)(LISQ)]-
*[Pt(LAP-H)2]2-
346 (21.1), 576 (2.2)
a In CH2Cl2 solution; species marked with an asterisk were electrochemi-
cally generated in CH2Cl2 solution containing 0.10 M [N(n-Bu)4]PF6.
b These spectra are identical with those of CH2Cl2 solutions of [M(LISQ)-
(bpy)]PF6 (M ) Pd (1a), Pt (2a)).
Scheme 2. Complexes and Labels
Figure 4. Electronic spectra of 1 (top) and 2 (bottom) and their respective
mono- and dications in CH2Cl2 solutions (0.10 M [N(n-Bu)4]PF6). The
mono- and dications were generated from 1 or 2 by controlled potential
coulometry.
visible, the very intense absorption bands at 745 nm (ꢀ )
3.2 × 103 L mol-1 cm-1) for 1 and at 780 nm (ꢀ ) 1.05 ×
104 L mol-1 cm-1) for 2, as well as the absorption maximum
at 813 nm (ꢀ ) 7.3 × 104 L mol-1 cm-1) for 3 (see below),
are due to a spin- and dipole-allowed charge-transfer
transition between the two bidentate ligands in square-planar
complexes (Scheme 3).21These transitions have been ob-
served in all complexes of the type C; they are nearly
independent of the nature of the central metal ion (CoII, NiII,
CuII, PdII, PtII) as discussed in ref 2.
Scheme 3. Interligand CT Transitions of 1, 2, and 3
[Cu(dmtacn)(LAP-H)]0 containing the O,N-coordinated di-
anion (LAP-H)2- is featureless at >400 nm. Note that no
interligand CT band as in 1 and 2 is observed because dmtacn
is a very poor π-acceptor ligand. Thus, it is clear that the
oxidation levels of O,N-coordinated o-aminophenolate de-
rivatives can be established from their electronic spectra.
The electronic spectra of 1a and 2a allow the characteriza-
tion of a single O,N-coordinated (L )
ISQ 1- radical. As shown
before by us for [CuII(dmtacn)(LISQ)]PF6 where dmtacn is
the spectroscopically innocent amine 1,4-dimethyl-1,4,7-
triazacyclononane, this π-radical ligand displays four bands
in the visible >500 nm (510, 730, 820, 980 nm) with molar
extinction coefficients (0.6-1.5) × 103 L mol-1 cm-1.2 In
contrast, the spectra of the dications containing an O,N-
coordinated quinone, (LIBQ), display no absorptions >600
nm but two strong bands at 400 and 495 nm, which can be
assigned to quinone CT bands since the uncoordinated
quinone in CH2Cl2 solution displays two such absorption
maxima at 396 (ꢀ ) 4.8 × 103 M-1 cm-1) and 488 nm (ꢀ )
2.7 × 103 M-1 cm-1).2 Interestingly, the spectrum of
Figure 5 shows the electronic spectra of 3, and of its two
reduced and two oxidized forms, respectively, which were
generated by controlled-potential electrolysis of CH2Cl2 solu-
tions of 3 (millimolar) containing 0.10 M [N(n-Bu)4]PF6 as
supporting electrolyte at -20 °C. The potential for the
generation of the monocation was fixed at 0.4 V, whereas
that for the monoanion was fixed at -1.4 V. Both the
oxidation and the reduction went smoothly and in each case
one electron per Pt ion was either removed or added (n )
(1.0 ( 0.05 e/Pt). The same holds for the dication and
(21) Vogler, A.; Kunkely, H. Comments Inorg. Chem. 1990, 9, 201.
4300 Inorganic Chemistry, Vol. 41, No. 16, 2002

Palladium(II) and Platinum(II) Complexes
Figure 5. Electronic spectra of 3 and of its mono- and dication, and of its
mono- and dianion. The oxidized and reduced forms of 3 were generated
electrochemically at -20 °C in CH2Cl2 solution (0.10 M [N(n-Bu)4]PF6).
Figure 7. X-band EPR spectra of the electrochemically generated
monocation [Pt(LIBQ)(LISQ)]+ (top) and monoanion [Pt(LISQ)(LAP-H)]-
(bottom) in CH2Cl2 (0.10 M [N(n-Bu)4]PF6) at 50 and 10 K, respectively.
Conditions: frequency 9.45684 and 9.45478 GHz; power 50.7 and 10 µW;
modulation 12 G for both spectra. Simulation parameters for the mono-
cation: gx ) 1.937, gy ) 1.997, gz ) 2.012; a(195Pt, I ) 0.5, 33.7%): (55,
25, 42) G; and for the monoanion: gx ) 1.873, gy ) 2.001, gz ) 2.234;
a(195Pt, I ) 0.5, 33.7%) ) 134, 187, 88 G.
MHz); a(1H) ) 4.6 G (12.9 MHz), a(105Pd, I ) 2.5, 22.2%)
) 3.56 G (10.0 MHz) (line width 2.3 G); and for the
spectrum of 2a, a(14N) ) 7.1 G (19.9 MHz); a(1H) ) 4.67
G (13.3 MHz), a(195Pt, I ) 0.5, 33.7%) ) 50.7 G (142.0
MHz) (line width: 3.4 G). These data clearly show the
semiquinone character of the ligand (LISQ)1-.
Similarly, the one-electron-oxidized form of 3 is a
1
paramagnetic S ) /2 species as is its one-electron-reduced
form. The X-band EPR spectra are shown in Figure 7. From
a simulation of the spectrum of the monocation three g-values
1.937, 1.997, 2.012 and a 195Pt hyperfine splitting of aiso(195Pt)
) 41 G (111-115 MHz) are calculated. This spectrum
indicates the presence of the radical species [PtII(LISQ)-
(LIBQ)]+. Interestingly, the aminothiophenolato derivative
[PtII(LSIBQ)(LSISQ)]+ displays a similar EPR spectrum at
10 K (g ) 1.998, 2.015, 2.009) with a(195Pt) ) 9.5 G (∼26.6
MHz), where H[LSAP] is 2,4-di-tert-butyl-6-aminothiophenol
and (LSISQ)1- and (LSIBQ) are the corresponding oxidized
ligand derivatives.6
Figure 6. X-band EPR spectra of 1a (top) and 2a (bottom) in CH2Cl2
solution at 298 K (frequency 9.4555 Hz; modulation 0.4 G; power 10 mW)
and simulation with parameters given in the text.
dianion which were electrochemically generated from the
monocation and monoanion. To check the chemical integrity
of the thus generated cations and/or anions, cyclic voltam-
mograms were recorded immediately after the corresponding
electrolysis experiment had been performed. In all cases a
cv as shown in Figure 3 (bottom) for 3 was obtained. The
spectra shown in Figure 5 are very similar to those reported
previously for the two oxidized and two reduced forms of
[NiII(LISQ)2] and [PdII(LISQ)2].2 Thus ligand oxidation levels
as in eq 2 can be readily assigned.
The spectrum of the monoanion [PtII(LISQ)(LAP-H)]-
displays gz > gx,y (gx ) 1.873, gy ) 2.001, gz ) 2.234) and
large hfc’s A(195Pt) ) 134, 187, and 88 G (351, 523, and
275 MHz). We conclude that the unpaired electron must
possess considerable metal d orbital character. Again, this
spectrum is very similar to that reported for [Pt(LSISQ)-
X-Band EPR Spectroscopy. The X-band EPR spectra of
CH2Cl2 solutions of 1a and 2a have been recorded at 298 K
and are shown in Figure 6. Both spectra display a hyperfine
1
(LSAP-H)]-, which displays the expected S ) /2 signal in
1
its X-band EPR spectrum with gx ) 1.827, gy ) 2.031, gz )
2.205 and a large hyperfine splitting of aiso(195Pt) ) 109 G
(∼309 MHz).6
split S ) /2 signal at giso ) 2.002 for 1a and giso ) 2.0 for
2a in agreement with the notion that both complexes contain
an O,N-coordinated o-iminobenzosemiquinonato radical.
Satisfactory simulations were obtained by using the following
parameters: for the spectrum of 1a, a(14N) ) 7.71 G (21.6
All square-planar cationic complexes of the type
[MII(LIBQ)(LISQ)]+ (where MII ) NiII, PdII, PtII; and LIBQ
)
Inorganic Chemistry, Vol. 41, No. 16, 2002 4301

Sun et al.
Table 5. Calculated (Spin-Orbit Corrected ZORA) and Experimental
Values (in parentheses) for the g-Tensors of the Monocation and
Monoacion of 3
g1
g2
g3
[PtII(LISQ)(LIBQ)]+
1.999 (2.012)
1.996 (1.997)
1.988 (1.937)
[PtII(LISQ)(LAP-H)]- 2.269(y)a (2.234) 1.971(x)a (2.001) 1.801(z)a (1.873)
a Calculated g-tensor orientation.
Table 6. Calculated and Experimental Values (in parentheses) for 195Pt
Hyperfine Couplings of the Monocation and Monoanion of 3
A1, MHz
-104
A2, MHz
-65
A3, MHz
-27
[PtII(LISQ)(LIBQ)]+
(|aiso| ) 110)
[PtII(LISQ)(LAP-H)]-
-510(y)c
-176(x)c
-93(x)c
(|523| gmid
-185(z)a,c
+185(z)b,c
(|351| gmin
+343(y)c
(|275| gmax
)
)
)
a Spin-polarized scalar relativistic ZORA. b Spin-restricted spin-orbit
corrected ZORA. c Calculated orientation.
Figure 8. Calculated structures (BP86 functional, spin-polarized, scalar
relativistic ZORA formalism) of 3 and of its monocation and monoanion,
respectively. Bond distances are given in Å.
Table 5 compares the calculated EPR g-values with the
experimentally derived parameters. The calculations faithfully
reproduce the experimental finding that the cation has a
g-tensor that resembles an organic radical while the anion
has a much more anisotropic g-tensor. For the anion the
calculated orientation of the g-tensor is such that the largest
component is in the (x,y) plane and perpendicular to the long
axis of the complex (taken as the x-axis). The gmid component
is also in the (x,y) plane and points along x while the smallest
component is along the normal of the complex plane.
The calculated 195Pt hfc’s are presented in Table 6.
Unfortunately, the present stage of development of the ZORA
method does not allow the simultaneous inclusion of spin-
orbit coupling and spin-polarization in the calculation of the
195Pt hyperfine tensor. However, since the second-order
spin-orbit contribution to the hyperfine coupling is es-
sentially proportional to the deviation of the g-values from
the free-electron value,20 it is readily appreciated that the
second-order correction should be negligible for the cation
where the g-shifts are extremely small. The calculated
maximal hfc for the cation of magnitude 104 MHz agrees
well with the experimentally deduced coupling constant with
a magnitude of ∼110 MHz, but the anisotropy could not be
determined from experiment due to the small anisotropy in
the g-tensor. For the anion, the spin-orbit corrected and
scalar relativistic ZORA results are very different, which
indicates a large second-order spin-orbit contribution to the
195Pt hfc. The ADF program currently does not allow for a
combined calculation that allows for the simultaneous
inclusion of all important effects for hfc calculations (i.e.
spin-polarization and spin-orbit coupling). Agreement with
the experimental numbers can therefore not be expected for
the monoanion. However, qualitatively, spin-orbit coupling
strongly counteracts the strongly negative 195Pt hfc in the
direction of gmax, which is in agreement with the experimental
finding that the smallest component of the 195Pt hfc is
oriented along this direction. This leaves the larger compo-
nents of the 195Pt hfc to the gmid components which have a
positive contribution from the spin-dipolar interaction.
In summary, while the results of the EPR experiments
would seem to argue for a metal-centered reduction due to
o-benzoquinone ligand, and (LISQ -
is its o-benzosemi-
)
quinonato π-radical) display X-band EPR signals at g ∼ 2.0
with very small g anisotropy whereas, in stark contrast, the
corresponding monoanions [MII(LAP-H)(LISQ)]- display rhom-
bic EPR signals with a rather large g anisotropy. This
different spectroscopic behavior has been noted and discussed
in the past,20 but no consistent interpretation has been given.
Density Functional Calculations. For heavy transition
metal containing molecules such as Pt it is necessary to
resort to a treatment that explicitly includes relativistic
effects. In this study this was accomplished by using the
scalar relativistic ZORA method of van Lenthe and co-
workers13 for the geometry optimizations and a spin-orbit
corrected, spin-restricted formalism for the property calcula-
tions.14,15
The calculated structures for 3 together with its anion and
its cation are shown in Figure 8. In these calculations the
neutral complex has been calculated in the triplet state in
accord with the proposal that it is best described as a
diradical.2,6 The structures of the triplet and singlet states
should be rather similar and the triplet state is computation-
ally more readily accessible. The comparison with the
experimental data (Table 2) reveals that the spin-polarized
ZORA calculations overestimate the metal-ligand bond
lengths by ∼0.05 Å. This is not a feature of the relativistic
treatment but of the BP86 functional used. The bond lengths
within the ring are reproduced with much better accuracy
and agree with experiment to ∼0.01 Å. Again this is typical
of the BP86 functional. Thus, the calculations support the
idea that the neutral complex 3 does in fact contain two
coordinated radicals. The one-electron-oxidized form shows
a slightly more pronounced distortion toward a quinoidal
structure and also slightly reduced metal-ligand bond
lengths. By contrast, the anionic form shows a less pro-
nounced distortion than the neutral complex. Both results
are consistent with a ligand-centered oxidation or reduction
that brings the rings closer to their quinone form in the case
of the cation and closer to the aromatic form in the case of
the anion.
4302 Inorganic Chemistry, Vol. 41, No. 16, 2002

Palladium(II) and Platinum(II) Complexes
o-iminobenzosemiquinonate(1-) π-radical in the cations
[MII(LISQ)(bpy)]+ (M ) Pd, Pt) and the neutral species
[PtII(LISQ)2] by X-ray crystallography. In contrast, for the
corresponding neutral species [MII(LAP-H)(bpy)] X-ray crys-
tallography confirms the presence of an O,N-coordinated,
closed shell o-amidophenolato(2-) dianion.
The combination of EPR experiments and relativistic DFT
calculations has given insight into the electronic structures
for the monocation and monoanion of 3. Thus, the mono-
cation features a delocalized mixed-valence system where
the unpaired electron is evenly shared by the two noninnocent
ligands. There is no metal character in the delocalized SOMO
because it transforms in an “ungerade” irreducible represen-
tation and no d-orbital is of appropriate symmetry for mixing
with it. Consequently, the whole radical character resides
on the ligands which are bound to a spectroscopically
innocent Pt(II) ion. Each of the ligands has, on average, one-
half electron in excess of its quinone form, which explains
the more pronounced ring distortion relative to the neutral
complex which features two semiquinonates bound to a
central Pt(II). The situation for the monoanion is different.
The monoanion can also be regarded as a delocalized mixed-
valence system where, again, the unpaired electron is mainly
located on the ligands that are, on average, one-half electron
short of an aromatic configuration. Consequently, the ring
distortion is much less pronounced than in either the neutral
complex or the monocation. An important difference from
the monocation is that in the monoanion the delocalized
SOMO transforms in a “gerade” irreducible representation.
Thus, it can acquire some metal d-character. This mixing of
the SOMO with the metal d-orbitals is to be interpreted as
back-bonding of a Pt(II) ion to the π-accepting ligands and
not as Pt(I) character. The metal character in the SOMO gives
rise to much larger 195Pt hfc because here the mechanism
that gives rise to hyperfine coupling of the unpaired electron
with 195Pt is “direct” while in the monocation it is entirely
indirect and due to spin-polarization. In addition, the “gerade”
symmetry of the ground state in the monoanion allows the
ground state to effectively spin-orbit couple with the d-d
excited states (note that even in the presence of spin-orbit
coupling parity is a good quantum number). This introduces
significant orbital angular momentum in the ground state,
which is evident in both the experiment and the calculation
as a significant deviation of the g-tensor components from
the free-electron g-value.
Figure 9. The two redox-active MOs of 3. The au MO is lower in energy
and the SOMO in the monocation of 3, whereas the bg MO is higher in
energy and the SOMO in the monoanion of 3.
the large g-shifts and 195Pt hfc and a ligand-centered oxidation
due to the small g-shifts and small 195Pt hfc, the interpretation
of this result is more complex.
The calculated magnetic parameters can be understood in
terms of a simple model that only involves two molecular
orbitals which are depicted in Figure 9. These two MOs are
basically the symmetric and antisymmetric combination of
the SOMO of the free semiquinonate ligand.
According to our calculations the ground state of the
2
monocation is Au with the au MO in Figure 9 being singly
occupied. Since this MO transforms “ungerade” under
inversion it cannot mix with any metal orbital. Consequently,
a large 195Pt hfc cannot be expected and is also not observed
experimentally. The residual 195Pt hfc is consequently at-
tributed to spin polarization effects which appear to be
reasonably well modeled by the spin-unrestricted ZORA
calculations presented in the previous paragraph. In addition,
the lack of metal character in the SOMO makes the spin-
orbit coupling of excited states with the ground state very
inefficient. Consequently, there is very little angular mo-
mentum in the ground-state wave function and the observed
and calculated g-shifts are very small and reflect the radical
character of the ground state.
For the monoanion the situation is exactly reversed. Since
the orbital transforms “gerade” under inversion it readily
mixes with the out-of-plane dxz orbital of the Pt(II) and
thereby acquires some metal character. This metal character
gives rise to a sizable first-order 195Pt hyperfine coupling
observed experimentally and also enhances the spin-polariza-
tion of the Pt-core that is responsible for the isotropic Fermi
Acknowledgment. We thank the Fonds der Chemischen
Industrie for financial support. X.S. is grateful to the Max-
Planck Society for a stipend and H.C. thanks the Alexander
von Humboldt Foundation for a postdoctoral fellowship.
2
contact contribution to the hfc. Since the ground-state Bg
Supporting Information Available: Figure S1-S3 displaying
the structures of complexes in 1, 1a, and 2 and tables of
crystallographic structure refinement data, atom coordinates, bond
lengths and angles, anisotropic thermal parameters, and calculated
positional parameters of H atoms for complexes 1, 1a, 2, 2a, 3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
readily mixes with relatively low-lying d-d excited states
it also has a sizable orbital angular momentum that manifests
itself in the large g-shifts observed experimentally. The
calculated value for the spin density at the central Pt is
23.7%, which appears to be a reasonable estimate.17-19
Conclusions. The most salient feature of the present study
is the unequivocal identification of an O,N-coordinated
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