Tetradentate Bis(o-iminobenzosemiquinonate(1-)) π Radical Ligands and Their o-Aminophenolate(1-) Derivatives in Complexes of Nickel(II), Palladium(II), and Copper(II)

Article abstract of DOI:10.1021/ic0302480

The coordination chemistries of the potential tetradentate ligands N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)ethylenediamine, H ₄[L¹], the unsaturated analogue glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil), H₂[L²], and N,N′-bis-(2-hydroxy-3,5-di-tert-butylphenyl)-2,2-dimethylpropylenediamine, H₄[L³], have been investigated with nickel(II), palladium(II), and copper(II). The complexes prepared and characterized are [Ni^II(H₃L¹)₂] (1), [Ni ^II(HL²)₂]·5/8CH₂Cl ₂ (2), [Ni^II(L^3··)] (3), [pd ^II(L^3··)][pd^II(H ₂L³) (4), and [Cu^II(H₂O)(L ⁴)] (5), where (L⁴)^2- is the oxidized diimine form of (L³)^4- and (L^3··) ^2- is the bis(o-iminosemiquinonate) diradical form of (L ³)^4-. The structures of compounds 1-5 have been determined by single crystal X-ray crystallography. In complexes 1 and 2, the ligands (H₃L¹)^- and (HL²) ^- are tridentate and the nickel ions are in an octahedral ligand environment. The oxidation level of the ligands is that of an aromatic o-aminophenol. 1 and 2 are paramagnetic (μ_eff ～ 3.2 μ _B at 300 K), indicating an S = 1 ground state. The diamagnetic, square planar, four-coordinate complexes 3 and [pd^II(L ^3··)] in 4 each contain two antiferromagnetically coupled o-iminobenzosemiquinonate(1-) π radicals. Diamagnetic [pd ^II(H₂L³)] in 4 forms an eclipsed dimer via four N-H...O hydrogen bonding contacts which yields a nonbonding Pd...Pd contact of 3.0846(4) A. Complex 5 contains a five-coordinate Cu^II ion and two o-aminophenolate(1-) halves in (L⁴) ^2-. The electrochemistries of complexes 3 and 4a ([Pd ^II(L^3··)] of 4) have been investigated, and the EPR spectra of the monocations and -anions are reported.

Full text of DOI:10.1021/ic0302480

Inorg. Chem. 2004, 43, 2922−2931
Tetradentate Bis(o-iminobenzosemiquinonate(1−)) π Radical Ligands
and Their o-Aminophenolate(1−) Derivatives in Complexes of Nickel(II),
Palladium(II), and Copper(II)
Kil Sik Min, Thomas Weyhermu1ller, Eberhard Bothe, and Karl Wieghardt*
Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36,
D-45470 Mu¨lheim an der Ruhr, Germany
Received August 6, 2003
The coordination chemistries of the potential tetradentate ligands N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-
ethylenediamine, H [L1], the unsaturated analogue glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil), H [L2], and N,N′-bis-
4
2
(2-hydroxy-3,5-di-tert-butylphenyl)-2,2-dimethylpropylenediamine, H [L3], have been investigated with nickel(II),
4
palladium(II), and copper(II). The complexes prepared and characterized are [NiII(H L1)2] (1), [NiII(HL2)2]‚5/8CH Cl2
3
2
II
II
II
(2), [NiII(L3••)] (3), [Pd (L3••)][Pd (H L3) (4), and [Cu (H O)(L4)] (5), where (L4)2- is the oxidized diimine form of
2
2
(L3)4- and (L3••)2- is the bis(o-iminosemiquinonate) diradical form of (L3)4-. The structures of compounds 1−5
have been determined by single crystal X-ray crystallography. In complexes 1 and 2, the ligands (H L1)- and
3
(HL2)- are tridentate and the nickel ions are in an octahedral ligand environment. The oxidation level of the ligands
is that of an aromatic o-aminophenol. 1 and 2 are paramagnetic (µeff ∼ 3.2 µB at 300 K), indicating an S ) 1
II
ground state. The diamagnetic, square planar, four-coordinate complexes 3 and [Pd (L3••)] in 4 each contain two
II
antiferromagnetically coupled o-iminobenzosemiquinonate(1−) π radicals. Diamagnetic [Pd (H L3)] in 4 forms an
2
eclipsed dimer via four N−H‚‚‚O hydrogen bonding contacts which yields a nonbonding Pd‚‚‚Pd contact of 3.0846-
II
(4) Å. Complex 5 contains a five-coordinate Cu ion and two o-aminophenolate(1−) halves in (L4)2-. The
II
electrochemistries of complexes 3 and 4a ([Pd (L3••)] of 4) have been investigated, and the EPR spectra of the
monocations and -anions are reported.
Introduction
different protonation and oxidation levels in coordination
compounds, as shown schematically in Chart 1. These
protonation and oxidation levels can be determined in a given
compound by high quality single crystal X-ray crystal-
lography because the C-C, C-N, and C-O distances vary
in a systematic fashion upon stepwise one-electron oxidation
of the ligands.
In a series of papers,1-9 we have recently unequivocally
established that organic o-aminophenol molecules are redox-
noninnocent O,N-coordinated ligands which can exist in
* 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.
(2) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Wehyermu¨ller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213.
(3) Chun, H.; Verani, C. N.; Chaudhuri, P.; Bothe, E.; Bill, E.; Weyher-
mu¨ller, T.; Wieghardt, K. Inorg. Chem. 2001, 40, 4157.
(4) Herebian, D.; Ghosh, P.; Chun, H.; Bothe, E.; Weyhermu¨ller, T.;
Wieghardt, K. Eur. J. Inorg. Chem. 2002, 1957.
(5) Chun, H.; Chaudhuri, P.; Weyhermu¨ller, T.; Wieghardt, K. Inorg.
Chem. 2002, 41, 790.
(6) Chun, H.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt, K. Inorg.
Chem. 2002, 41, 5091.
(7) Sun, X.; Chun, H.; Hildenbrand, K.; Bothe, E.; Weyhermu¨ller, T.;
Neese, F.; Wieghardt, K. Inorg. Chem. 2002, 41, 4295.
(8) Chun, H.; Weyhermu¨ller, T.; Bill, E.; Wieghardt, K. Angew. Chem.,
Int. Ed. 2001, 40, 2489.
(9) Min, K. S.; Weyhermu¨ller, T.; Wieghardt, K. J. Chem. Soc., Dalton
Trans. 2003, 1126.
The synthesis of the tetradentate ligand N,N′-bis(3,5-di-
tert-butyl-2-hydroxyphenyl)ethylenediamine, H4[L1], and its
unsaturated counterpart glyoxal-bis(2-hydroxy-3,5-di-tert-
butylanil), H2[L2], have been described in refs 10 and 11,
respectively, but to the best of our knowledge their coordina-
tion chemistry has not been explored to date. These ligands
are shown in Chart 2. In addition, we have synthesized for
the first time the analogue N,N′-bis(2-hydroxy-3,5-di-tert-
butylphenyl)-2,2-dimethylpropylenediamine, H4[L3].
(10) Komissarov, V. N.; Ukhin, L. Yu.; Vetoshkina, L. V. Zh. Org. Khim.
1990, 26, 2188.
(11) Haeussler, H.; Jadamus, H. Chem. Ber. 1964, 97, 3051.
2922 Inorganic Chemistry, Vol. 43, No. 9, 2004
10.1021/ic0302480 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/30/2004

Tetradentate π Radical Ligands
Chart 1
Chart 2
The coordination chemistries of these ligands with nickel-
(II), palladium(II), and copper(II) have been investigated in
some detail here where we have placed special emphasis on
O,N-coordinated o-iminobenzosemiquinonate π radical an-
ions and a comparison of the geometrical details of bound
saturated (L1)4- versus unsaturated (L2)2- ligands. The
complexes prepared are listed in Chart 2.
N,N′-Bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2-dimethylpro-
Experimental Section
pylenediamine, H4[L3]. To a solution of 3,5-di-tert-butylcatechol
(3.0 g; 13.5 mmol) in CH3CN (30 mL) was added dropwise a
solution of 2,2-dimethylpropylenediamine (0.69 g; 6.75 mmol) in
MeCN (2 mL) in the presence of air. After heating to reflux for 30
min, the solution was cooled to room temperature. A colorless
precipitate was isolated by filtration and washed with n-hexane.
Yield: 2.1 g (62%). 1H NMR (400 MHz, CD2Cl2, 300 K): δ 1.13
(s, 6H, CH3 (methyl)), 1.29 (s, 18H, CH3 (t-Bu)), 1.39 (s, 18H,
CH3 (t-Bu)), 2.96 (s, 4H, CH2), 6.88 (arom H, 2H), 6.98 (arom H,
2H). EI mass spectrometry: m/z ) 510 (M+). Anal. Calcd for
C33H54N2O2: C, 77.60; H, 10.66; N, 5.48. Found: C, 77.54; H,
10.62; N, 5.56.
The ligands N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)ethyl-
enediamine, H4[L1], and glyoxal-bis(2-hydroxy-3,5-di-tert-butylphe-
nyl)imine, H2[L2], have been prepared by slightly modified
procedures described in refs 10 and 11.
N,N′-Bis(2-hydroxy-3,5-di-tert-butylphenyl)ethylenedi-
amine, H4[L1]. To a solution of 3,5-di-tert-butylcatechol (3.0 g;
13.5 mmol) in acetonitrile (30 mL) was added dropwise a solution
of ethylenediamine (0.41 g; 6.75 mmol) in CH3CN (2 mL) in the
presence of air. After heating to reflux for 1 h, the solution was
cooled to 20 °C and stirred for a further 3 h during which time a
colorless precipitate of H4[L1] formed which was collected by
filtration, washed with n-hexane, and air-dried. Yield: 2.0 g (64%).
1H NMR (500 MHz, DMSO-d6, 300 K): δ 1.23 (s, 18H, CH3 (t-
Bu)), 1.35 (s, 18H, CH3 (t-Bu)), 3.30 (s, 4H, CH2), 4.70 (s, 2H,
NH), 6.55 (d, 2H, CH (bz), J ) 2.29 Hz), 6.56 (d, 2H, CH (bz), J
) 2.29 Hz), 7.38 (s, 2H, OH). 13C NMR (125 MHz, DMSO-d6,
300 K): δ 29.9, 31.5 (CH3 (t-Bu)), 34.0, 34.5 (t-Bu), 43.4
(ethylene), 107.0, 111.2, 136.1, 138.7, 140.4, 141.8 (Carom). EI mass
spectrometry: m/z ) 468 (M+). Anal. Calcd for C30H48N2O2: C,
76.87; H, 10.32; N, 5.98. Found: C, 76.59; H, 10.31; N, 5.75.
Glyoxal-bis(2-hydroxy-3,5-di-tert-butylphenyl)imine, H2[L2].
To a solution of 3,5-di-tert-butylbenzoquinone (3.0 g; 13.6 mmol)
in MeCN (30 mL) was added dropwise a solution of ethylenedi-
amine (0.41 g; 6.80 mmol) in CH3CN (2 mL) at 0 °C in the presence
of air. The resulting mixture was stirred at 0 °C for 1 h after which
time a yellow (brownish) precipitate was collected by filtration,
washed with petroleum ether, and air-dried. Yield: 1.6 g (51%).
1H NMR (400 MHz, CD2Cl2, 300 K): δ 1.35 (s, 18H, CH3 (t-
Bu)), 1.45 (s, 18H, CH3 (t-Bu)), 7.31 (d, 2H, CH (bz)), J ) 2.22
Hz), 7.36 (d, 2H, CH (bz), J ) 2.20 Hz), 7.89 (s, 2H, OH), 8.65
(s, 2H, NCH). 13C NMR (100 MHz, CD2Cl2, 300 K): δ 29.5, 31.7
(CH3 (t-Bu)), 34.9, 35.3 (t-Bu), 110.4, 126.3, 134.0, 136.2, 142.2,
150.37 (Carom), 155.1 (ethylene). EI mass spectrometry: m/z ) 464
(M+). Anal. Calcd for C30H44N2O2: C, 77.54; H, 9.54; N, 6.03.
Found: C, 77.38; H, 9.38; N, 6.14.
[Ni(H3L1)2] (1). To a solution of the ligand H4[L1] (468 mg; 1.0
mmol) in EtOH (30 mL) was added dropwise with stirring a solution
of Ni(NO3)2‚6H2O (146 mg; 0.5 mmol) in EtOH (5 mL) at room
temperature in the presence of air. From this solution, a micro-
crystalline pink precipitate formed within 1 h which was collected
by filtration, washed with EtOH, and air-dried. Single crystals of
1‚4MeOH suitable for X-ray crystallography were obtained by
solvent diffusion of MeOH into a CH3Cl solution of 1. Yield: 470
mg (95%). EI mass spectrometry: m/z ) 992 (M+). Anal. Calcd
for C60H94N4NiO4: C, 72.49; H, 9.53; N, 5.64. Found: C, 72.36;
H, 9.68; N, 5.66.
[Ni(HL2)2]‚5/8CH2Cl2 (2‚5/8CH2Cl2). A solution of triethylamine
(0.14 mL; 1.0 mmol) in CH3CN (5 mL) was added to a slurry of
NiII(NO3)2‚6H2O (73 mg; 0.25 mmol) and the ligand H2[L2] (234
mg; 0.5 mmol) in CH3CN (20 mL) in the presence of air, causing
immediate dissolution of the ligand and formation of a dark green
solution. The mixture was stirred for 3 h during which time a deep
green precipitate formed. The crude product was recrystallized from
CH2Cl2 to afford deep green microcrystals of 2‚5/8CH2Cl2. Single
crystals suitable for X-ray crystallography were obtained by solvent
diffusion of MeOH into a CH2Cl2 solution of 2. Yield: 190 mg
(74%). ESI mass spectrometry in CH2Cl2 (positive ion mode): m/z
) 985 (M + H)+. Anal. Calcd for C60H86N4NiO4 (5/8CH2Cl2): C,
70.07; H, 8.46; N, 5.39. Found: C, 69.98; H, 8.40; N, 5.28.
Inorganic Chemistry, Vol. 43, No. 9, 2004 2923

Min et al.
[Ni(L3••)] (3). To a solution of NiII(NO3)2‚6H2O (0.29 g; 1.0
mmol) in CH3CN (40 mL) was added with stirring solid H4[L3]
(0.51 g; 1.0 mmol) and triethylamine (0.56 mL; 4.0 mmol) at room
temperature in the presence of air. From this solution, a micro-
crystalline dark green precipitate formed within 1 h which was
collected by filtration, washed with cold CH3OH, and air-dried.
Single crystals of 3 suitable for X-ray crystallography were obtained
by solvent diffusion of CH3OH into a CH2Cl2 solution of 3. Yield:
range 190-1100 nm. Cyclic voltammograms, square-wave volta-
mmograms, and coulometric experiments were performed using an
EG&G potentiostat/galvanostat. EI and ESI mass spectra were
recorded on a Finnigan MAT 8200, a Finnigan MAT 95, or a HP
5989 mass spectrometer. 1H NMR spectra were measured on Bruker
DRX 400 or DRX 500 spectrometers using the solvent as internal
standard. Temperature-dependent (2-298 K) magnetization data
were recorded on a SQUID magnetometer (MPMS Quantum
Design) in an external magnetic field of 1.0 T. The experimental
susceptibility data were corrected for underlying diamagnetism by
the use of tabulated Pascal’s constants. X-band EPR spectra were
recorded on a Bruker ESP ELEXSYS E500 spectrometer equipped
with a helium flow cryostat (Oxford Instruments ESR 910). The
spectra were simulated by iteration of the anisotropic g values,
hyperfine coupling constants, and line widths.
1
0.32 g (57%). H NMR (400 MHz, CD2Cl2, 300 K): δ 1.27 (s,
18H, CH3 (t-Bu)), 1.43 (s, 6H, CH3 (methyl)), 1.50 (s, 18H, CH3
(t-Bu)), 3.47 (s, 4H, CH2), 6.83 (arom H, 2H), 7.10 (arom H, 2H).
EI mass spectrometry: m/z ) 564 (M+). Anal. Calcd for C33H50N2-
NiO2: C, 70.10; H, 8.91; N, 4.95. Found: C, 69.85; H, 8.82; N,
5.06.
[Pd(L3••)][Pd(H2L3)] (4). To a solution of PdII(CH3CO2)2 (0.11
g; 0.5 mmol) in CH3OH (15 mL) was added with stirring solid
H4[L3] (0.26 g; 0.5 mmol) and triethylamine (0.28 mL; 2.0 mmol)
at room temperature in the presence of air. From this solution, a
microcrystalline dark green precipitate formed within 3 h which
was collected by filtration, washed with cold CH3OH, and air-dried.
Single crystals of 4 suitable for X-ray crystallography were obtained
by solvent diffusion of CH3OH into a CH2Cl2 solution of 4. Yield:
0.15 g (49%). We then separated 4 into dark green [PdII(L3••)] (4a)
and yellow [PdII(H2L3)]2 (4b) by high-pressure liquid chromatog-
raphy on a Nucleosil-7-C18 column (Gilson m305 pump, UV-vis
detector operating at 300 nm) at a 6 mL/min flow rate with 2%
CH2Cl2/MeCN as eluent. [Pd(L3••)] (4a) 1H NMR (400 MHz, CD2-
Cl2, 300 K): δ 1.10 (s, 18H, CH3 (t-Bu)), 1.28 (s, 18H, CH3 (t-
Bu)), 1.41 (s, 6H, CH3 (methyl)), 2.84 (m, 4H, CH2), 6.63 (arom
H, 2H), 6.71 (arom H, 2H). EI mass spectrometry: m/z ) 612 (M+).
Anal. Calcd for C33H50N2PdO2: C, 64.64; H, 8.22; N, 4.57.
Found: C, 64.65; H, 8.10; N, 4.41. [Pd(H2L3)] (4b) 1H NMR (400
MHz, CD2Cl2, 300 K): δ 1.32 (s, 18H, CH3 (t-Bu)), 1.46 (s, 18H,
CH3 (t-Bu)), 1.54 (s, 6H, CH3 (methyl)), 1.58 (s, 4H, CH2), 7.22
(arom H, 2H), 7.54 (s, 2H, NH). EI mass spectrometry: m/z )
1230 (M+). Anal. Calcd for C33H52N2PdO2: C, 64.43; H, 8.52; N,
4.55. Found: C, 64.34; H, 8.60; N, 4.62.
[Cu(H2O)(L4)] (5). To a solution of CuIICl2‚2H2O (85 mg; 0.5
mmol) in CH3OH (20 mL) was added with stirring solid H4L3 (0.26
g; 0.5 mmol) and triethylamine (0.28 mL; 2.0 mmol) at room
temperature. The mixture was stirred for 30 min in the presence of
air. The dark green solution was allowed to stand under an argon
blanketing atmosphere until dark brown crystals formed which were
collected by filtration, washed with CH3OH, and air-dried. Yield:
0.13 g (45%). EI mass spectrometry: m/z ) 567 (M - H2O)+.
Anal. Calcd for C33H50N2CuO3: C, 67.60; H, 8.59; N, 4.78.
Found: C, 67.91; H, 8.41; N, 4.72.
X-ray Crystallographic Data Collection and Refinement of
the Structures. Dark green single crystals of 2, 3, 4, and 6 (light
green), yellow-brown crystals of 5, and pale pink crystals of 1 were
coated with perfluoropolyether, were picked up with a glass fiber,
and were immediately mounted in the nitrogen cold stream to
prevent loss of solvent. Intensity data were collected at 100 K using
a Nonius Kappa-CCD diffractometer equipped with a Mo-target
rotating-anode X-ray source and a graphite monochromator (Mo
KR, 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 1.0° (1, 3, 4, and 6), 0.7° (2), and 0.5° (5) in angular frequency
(ω). Crystal faces of 2, 4, and 6 were determined, and the
corresponding intensity data were corrected for absorption using
the Gaussian-type routine embedded in XPREP.13 Data sets of the
other compounds were left uncorrected. Crystallographic data of
the compounds are listed in Table 1. The Siemens ShelXTL13
software package was used for the solution and the artwork of the
structure and ShelXL9714 was used for the refinement. The
structures were readily solved by direct and Patterson methods and
subsequent difference Fourier techniques. All non-hydrogen atoms
were refined anisotropically, and hydrogen atoms were placed at
calculated positions and refined as riding atoms with isotropic
displacement parameters.
Results and Discussion
Syntheses and Characterization. The reaction of an
ethanolic solution of the ligand N,N′-bis(2-hydroxy-3,5-di-
tert-butylphenyl)ethylenediamine, H4[L1], with Ni(NO3)2‚
6H2O (ratio 2:1) in ethanol at 20 °C resulted in the formation
of a pink precipitate of [NiII(H3L1)2] (1). From a similar
reaction of the unsaturated analogue glyoxal-bis(2-hydroxy-
3,5-di-tert-butylphenyl)imine, H2[L2], in acetonitrile with
triethylamine and NiII(NO3)2‚6H2O, a dark green precipitate
of [NiII(HL2)2] was obtained which was recrystallized from
CH2Cl2 as [NiII(HL2)2]‚5/8CH2Cl2 (2‚5/8CH2Cl2).
2,2-Bis(5,7-di-tert-butyl-benzoxazol-2-yl)-propane (6). To a
solution of PdIICl2 (89 mg; 0.5 mmol) in CH3OH (20 mL) was
added with stirring solid H4L3 (0.26 g; 0.5 mmol) and triethylamine
(0.28 mL; 2.0 mmol) at room temperature. After being stirred for
30 min in the presence of air, the brown solution was filtered. The
resulting solution was allowed to stand under an argon blanketing
atmosphere until colorless crystals formed which were collected
by filtration, washed with CH3OH, and air-dried. Yield: 0.13 g
(50%). 1H NMR (400 MHz, CD2Cl2, 300 K): δ 1.36 (s, 18H, CH3
(t-Bu)), 1.42 (s, 18H, CH3 (t-Bu)), 2.03 (s, 6H, CH3 (methyl)), 7.28
(arom H, 2H), 7.55 (arom H, 2H). EI mass spectrometry: m/z )
502 (M+). Anal. Calcd for C33H46N2O2: C, 78.84; H, 9.22; N, 5.57.
Found: C, 78.65; H, 9.15; N, 5.45.
In contrast, when NiII(NO3)2‚6H2O reacted with the new
ligand N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2-dim-
ethylpropylenediamine, H4[L3] (1:1), in the presence of air
in acetonitrile and NEt3 at ambient temperature, a dark green
precipitate of [NiII(L3••)] (3) formed in 57% yield. Here,
(L3••)2- represents the two electron oxidized bis(π radical
(12) Ghosh, P.; Bill, E.; Weyhermu¨ller, T.; Neese, F.; Wieghardt, K. J.
Am. Chem. Soc. 2003, 125, 1293.
(13) ShelXTL, version 5; Siemens Analytical X-ray Instruments, Inc.: 1994.
(14) Sheldrick, G. M. ShelXTL97; Universita¨t Go¨ttingen: Germany, 1997.
Physical Measurements. Electronic spectra of the complexes
and spectra of the spectroelectrochemical investigations were
recorded on an HP 8452A diode array spectrophotometer in the
2924 Inorganic Chemistry, Vol. 43, No. 9, 2004

Tetradentate π Radical Ligands
Table 1. Crystallographic Data of Complexes
1‚4MeOH
2‚5/8CH2Cl2
3
4
5
6
chem formula
fw
space group
a, Å
b, Å
c, Å
C64H110N4NiO8
1122.27
C
60.63H87.3N4NiO4Cl1.3
C33H50N2NiO2
565.46
C66H102N4O4Pd2
1228.32
C33H50CuN2O3
586.29
C33H46N2O2
502.72
1039.12
P21/c, No. 14
27.8848(8)
17.7912(6)
26.7871(8)
90
115.52(1)
90
11992.6(6)
8
P1h, No. 2
9.3467(6)
12.7603(9)
15.6090(9)
112.89(1)
95.38(1)
104.83(1)
1618.4(2)
1
P1h, No. 2
10.3164(3)
10.4724(3)
16.1228(6)
103.22(1)
98.95(1)
106.08(1)
1584.11(9)
2
P1h, No. 2
10.2622(4)
17.3141(6)
19.0743(8)
97.118(1)
98.00(1)
107.13(1)
3158.3(2)
2
P21/m, No. 11
5.8251(9)
33.067(5)
8.086(2)
90
92.54(3)
90
1556.0(5)
2
P21/c, No. 14
10.5174(3)
20.0475(6)
29.0130(9)
90
96.79(1)
90
6074.4(3)
8
R, deg
â, deg
γ, deg
V, Å
Z
T, K
100(2)
100(2)
100(2)
100(2)
100(2)
100
F calcd, g cm-3
reflns collected/2Θmax
unique reflns/I > 2σ(I)
no. of params/restraints
µ(Mo KR), cm-1
R1a/ goodness of fitb
wR2c (I > 2σ(I))
1.152
1.151
1.185
1.292
1.251
1.099
20075/66.30
12291/8684
372/0
3.53
0.0578/1.018
0.1214
103713/59.16
33485/26299
1349/7
4.25
0.0682/1.133
0.1425
45276/66.00
11858/8481
357/0
6.42
0.0534/1.018
0.1156
108652/62.00
20077/15177
719/0
6.17
0.0397/1.030
0.0774
9855/42.00
1676/1102
184/0
7.36
0.0951/1.068
0.1714
111282/52.82
12422/9463
722/132
0.67
0.0583/1.047
0.1182
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|. b GOF ) [∑[w(Fo2 - Fc )2]/(n - p)]1/2
.
c wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2]]1/2, where
2
2
2
w ) 1/σ2(Fo ) + (aP)2 + bP and P ) (Fo + 2Fc )/3.
2
2
2
Scheme 1
anion) of the deprotonated ligand (L3)4-; the ligand contains
two O,N-coordinated o-iminobenzosemiquinonate(1-) radi-
cals.
In a similar reaction of H4[L3] and PdII(CH3CO2)2 (1:1)
in methanol in the presence of air and NEt3, a dark green
precipitate of cocrystallizing [PdII(L3••)][PdII(H2L3)] (4) was
obtained. It is possible to separate the two neutral complexes
from a CH2Cl2/CH3CN solution of 4. Two distinct com-
pounds, namely, dark green [PdII(L3••)] (4a) and yellow [PdII-
(H2L3)]2 (4b), were obtained from high-pressure liquid
chromatography. Both complexes are diamagnetic, as was
1
judged from their normal (but differing!) H NMR spectra
(see the Experimental Section). It is interesting to note that
the dimeric nature of the [PdII(H2L3)]2 complex remains intact
in solution. The EI mass spectrum displays a molecular ion
Table 2. Electronic Spectra of Complexes at 22 °C in CH2Cl2 Solution
complex
max, nm (ꢀ, L mol-1 cm-1
+
peak (M2 ) at m/z ) 1230, whereas the corresponding [PdII-
λ
)
(L3••)] complex is mononuclear (m/z ) 612 (M+)).
1
2
3
551 (12), 886 (12)
In an attempt to generate the corresponding [CuII(L3••)]
complex, a methanolic solution of CuIICl2‚2H2O was treated
with an equivalent of H4[L3] and NEt3 in the presence of air
at ambient temperature. Dark brown crystals of [CuII(H2O)-
(L4)] (5) were obtained in 45% yield. (L4)2- is the oxidized
diimine form of H4(L3), as shown in Chart 2. Thus, the
desired product [CuII(L3••)] has not been obtained.
388 (2.6 × 104), 623 (1.7 × 104)
293 (1.4 × 104), 416 (1.6 × 103), 581 (1.7 × 103),
854 (3.0 × 104)
[Pd(L3••)] (4a)
302 (1.9 × 104), 434 (3.1 × 104), 522 (2.4 × 103, sh),
559 (2.9 × 103), 843 (3.1 × 104)
[Pd(H2L3)] (4b) 294 (1.9 × 104), 419 (8.6 × 103)
5
409 (9.6 × 103), 560 (460)
complex 3 is diamagnetic (S ) 0), as is expected for a square
planar NiII ion coordinated to two ligand π radicals.2,15-18
The electronic spectra of complexes 1 and 2 are sum-
marized in Table 2 and shown in Figure 1. The spectrum of
1 displays two spin allowed d-d transitions of low intensities
(ꢀ < 20 M-1 cm-1) in the visible spectrum, which is
characteristic for an octahedral NiII (d8) ion, whereas the
spectrum of 2 appears to be dominated by an intense (ꢀ >
104 M-1 cm-1) metal-to-ligand or ligand-to-ligand (Scheme
When PdIICl2 was used as starting material instead of
PdII(CH3CO2)2 in the above reaction yielding 4 under
otherwise identical reaction conditions, colorless crystals of
2,2-bis(5,7-di-tert-butylbenzooxazol-2-yl)propane (6) were
obtained in 50% yield, according to Scheme 1.
From temperature-dependent magnetic susceptibility mea-
surements at 1.0 T in the range 2-300 K, it was established
that complexes 1 and 2 are paramagnetic. Temperature-
independent (20-290 K) magnetic moments of 3.1 µB for 1
with g ) 2.12 and 3.2 µB for 2 with g ) 2.13 indicate an S
) 1 ground state for both complexes. Zero-field splitting
parameters of 4 cm-1 for 1 and 6 cm-1 for 2 were calculated
from a fitting to the low temperature (<20 K) data. These
values are typical for octahedral NiII complexes. In contrast,
(15) Balch, A. L.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201.
(16) Holm, R. H.; Balch, A. L.; Davison, A.; Maki, A.; Berry, T. J. Am.
Chem. Soc. 1967, 89, 2866.
(17) Forbes, C. E.; Gold, A.; Holm, R. H. Inorg. Chem. 1971, 10, 2479.
(18) Stiefel, E. I.; Waters, J. H.; Billig, E.; Gray, H. B. J. Am. Chem. Soc.
1965, 87, 3016.
Inorganic Chemistry, Vol. 43, No. 9, 2004 2925

Min et al.
Figure 2. Cyclic voltammograms of 4a in CH2Cl2 solution (0.10 M [N(n-
Bu)4]PF6) at 20 °C recorded by a glassy carbon working electrode at scan
rates of 50, 100, 200, 400, and 800 mV s-1. The inset shows the CVs of
4b in CH2Cl2 at scan rates of 100, 200, and 400 mV s-1
.
Figure 1. Electronic spectra of 1 in CH2Cl2 containing 1 mM [BH4]- as
reductant (top) and of 2 in CH2Cl2 at 20 °C (bottom). Note the difference
of scales of the molar extinction coefficients of 1 and 2.
Scheme 2
Figure 3. Cyclic voltammograms of 3 in CH2Cl2 solution (0.10 M [N(n-
Bu)4]PF6) at 20 °C recorded by a glassy carbon working electrode at scan
rates of 25, 50, 100, 200, 400, and 800 mV s-1
.
Figure 2 shows the CV of 4a and as the inset that of the
dimer 4b. The CV of 4a displays four reversible one-electron
transfer waves (E11/2 ) 0.40, E21/2 ) -0.03, E31/2 ) -1.13,
and E4 ) -1.68 V). Coulometric experiments at ap-
1/2
propriately fixed potentials establish that the first two
correspond to two successive one-electron oxidations of 4a,
whereas the latter two are successive one-electron reductions.
The CV of 4b displays two irreversible oxidations at peak
potentials of 0.85 and 0.54 V, which have not been
investigated in further detail. These data correspond exactly
to those reported2 for trans-[PdII(LISQ)2]: E1-41/2 ) 0.47, 0.08,
-0.99, and -1.40. It was shown that these processes are
ligand centered rather than metal centered, as shown in eq
1, where (LIBQ) represents the neutral o-iminobenzoquinone,
(L
)
ISQ -• represents the o-iminobenzosemiquinonate π radical,
and (LAP-H)2- is the o-iminophenolate dianion.
2) charge transfer band in the visible spectrum. The spectra
of 3 and [PdII(L3••)] (4a) are very similar to those reported
for other square planar [MII(LISQ)2] (MII ) Ni, Pd) complexes,
-e
[MII(LIBQ)2]
w\ \ \x
x [MII(LIBQ)(LISQ)]+ w-+eex [MII(LISQ)2] w-e
2+
+e +e
[M(LAP-H)(LISQ)]- w+-ee
x [MII(LAP-H)2]2- (1)
where (LISQ -•
is an o-iminobenzosemiquinonate(1-) π
)
\
radical anion.2 The most characteristic feature of these bis-
(radical) species is the very intense (>104 L mol-1 cm-1)
ligand-to-ligand charge transfer band at 854 nm for 3 and at
843 nm for 4a.
Figure 3 exhibits the corresponding CV of nickel analogue
3 where also four one-electron transfer waves are observed
at 0.26, -0.00, -1.18, and -1.84 V. The latter three waves
are clearly reversible, whereas in the process E11/2 is not well
Electro- and Spectroelectrochemistry. Cyclic voltam-
mograms (CVs) of 3 and 4a have been recorded in CH2Cl2
solutions containing 0.10 M [N(n-Bu)4]PF6 as supporting
electrolyte by a glassy carbon working electrode and a Ag/
AgNO3 reference electrode. Ferrocene was used as internal
standard, and all potentials are referenced versus the ferro-
cenium/ferrocene couple (Fc+/Fc).
separated from E2 and the value 0.26 V corresponds to a
1/2
peak potential. Again, the CV of trans-[NiII(LISQ)2] is very
similar: at 0.042 V, a reversible two-electron transfer
oxidation has been observed, but at -1.07 and -1.64 V,
two reversible one-electron reductions have been reported.
Such complete (or incomplete) electron transfer series have
2926 Inorganic Chemistry, Vol. 43, No. 9, 2004

Tetradentate π Radical Ligands
Figure 4. Electronic spectra of 4a and its electrochemically generated
mono- and dications and mono- and dianions in CH2Cl2 solution (0.10 M
[N(n-Bu)4]PF6).
Figure 6. Structure of a neutral molecule [NiII(H3L1)2] in crystals of 1.
Intramolecular hydrogen bonding: O1‚‚‚O16, 2.538(3) Å.
Figure 5. Electronic spectra of neutral 3 and its electrochemically
generated monoanion and monocation (0.10 M [N(n-Bu)4]PF6).
been reported for many square planar NiII, PdII, and PtII
complexes containing two catecholate, phenylenediamide,
benzodithiolate, or o-iminothiophenolate ligands.15-18
Controlled-potential coulometry allows the selective gen-
eration of the stable mono- and dications as well as the mono-
and dianions of 4a of which the respective electronic spectra
have been recorded. These are shown in Figure 4. Similarly,
the spectra of neutral 3, its monocation, and its monoanion
are shown in Figure 5. These spectra are again very similar
to those reported for the oxidized and reduced forms of [MII-
(LISQ)2] (M ) Ni or Pd).2
Figure 7. Structure of a neutral molecule [NiII(HL2)2] in crystals of 2.
Intramolecular hydrogen bonding: O1‚‚‚O56, 2.697(4) Å; O16‚‚‚O41,
2.764(4) Å.
molecule possesses crystallographic Ci symmetry. Figure 6
shows such a molecule. Each ligand consists of an O,N-
coordinated o-aminophenolate(1-) half and a dangling
o-aminophenol part, both of which are N,N-connected by
an ethyl bridge, and the latter forms an intramolecular
O-H‚‚‚O bond (O1‚‚‚O16, 2.538(3) Å). It is an important
feature of this structure that the 12 C-C bonds in both phenyl
rings of the (H3L1)- ligand are nearly equidistant (av 1.399
Å) and typical for an aromatic six-membered ring. The C-O
and C-N bond lengths of the coordinated and dangling part
of the ligand also indicate the (LAP)- character (Chart 1) of
these ligands.
Figure 7 displays the structure of the neutral molecule [NiII-
(HL2)2] in crystals of 2. There are two crystallographically
independent molecules with nearly identical structures.
Superficially, their structures are very similar to that of 1,
but clearly, the two o-aminophenol(ate) halves are covalently
N,N′-bound to an unsaturated ethylene bridge and both
nitrogen atoms of the (HL2)- ligand are sp2 hybridized.
It is interesting that both the monocations and the
monoanions are paramagnetic (S ) 1/2) mixed valent
compounds which belong to class III according to Robin and
Day19 where the unpaired electron is delocalized over both
ligands (or both halves of the ligand in 3 and 4a). This has
been explicitly shown by DFT calculations and spectroscopy
for the monocations and -anions of bis(3,5-di-tert-butyl-o-
diiminobenzosemiquinonate)nickel(II) and [PtII(LISQ)2] in refs
20 and 7, respectively.
Crystal Structures. Single crystal X-ray structure deter-
minations of complexes 1, 2, 3, 4, 5, and 6 have been carried
out at 100 K. Table 1 summarizes crystallographic data, and
Table 3 lists selected bond distances of the compounds.
Crystals of 1 consist of well-separated neutral molecules
of [NiII(H3L1)2] where an octahedral NiII ion is facially
coordinated to two tridentate monoanions (H3L1)-. The
(19) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(20) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg. Chem. 2002,
41, 4179.
Inorganic Chemistry, Vol. 43, No. 9, 2004 2927

Min et al.
Table 3. Selected Bond Distances (Å)
Complex 1
Ni-O1
Ni-N7
Ni-N10
C4-C5
C5-C6
C6-N7
2.038(1)
2.102(1)
2.137(1)
1.385(2)
1.395(2)
1.461(2)
O1-C1
C1-C6
C1-C2
C2-C3
C3-C4
N7-C8
1.350(2)
1.406(2)
1.419(2)
1.395(2)
1.405(2)
1.487(2)
C9-N10
C11-C12
C12-C13
C14-C15
O16-C16
O1‚‚‚O16
1.484(2)
1.393(2)
1.386(2)
1.395(2)
1.370(2)
N10-C11
C11-C16
C13-C14
C15-C16
C8-C9
1.440(2)
1.400(2)
1.398(2)
1.412(2)
1.514(2)
Complex 2a
Ni-N47
Ni1-N7
Ni1-O1
Ni1-O41
Ni1-N10
Ni1-N50
C6-N7
2.012(2)
2.014(2)
2.056(2)
2.084(2)
2.166(2)
2.193(2)
1.383(3)
O1-C1
C1-C6
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
1.317(3)
1.426(3)
1.436(3)
1.378(3)
1.420(3)
1.378(3)
1.409(3)
N7-C8
1.298(3)
1.286(3)
1.387(3)
1.401(3)
1.402(3)
1.368(3)
C8-C9
1.449(3)
1.429(3)
1.405(3)
1.398(3)
1.402(3)
C9-C10
C11-C12
C12-C13
C14-C15
O16-C16
N10-C11
C11-C16
C13-C14
C15-C16
Complex 3
Complex 4
Ni-N7
Ni-N11
O1-C1
C1-C6
C3-C4
C5-C6
1.828(1)
1.832(1)
1.312(2)
1.434(2)
1.427(2)
1.417(2)
Ni-O18
Ni-O1
C1-C2
C2-C3
C4-C5
C6-N7
1.841(1)
1.845(1)
1.427(2)
1.381(2)
1.371(2)
1.360(2)
N7-C8
1.458(2)
1.360(2)
1.436(2)
1.429(2)
1.429(2)
1.528(2)
C10-N11
C12-C13
C13-C14
C15-C16
O18-C17
C9-C10
1.458(2)
1.415(2)
1.373(2)
1.381(2)
1.310(2)
1.527(2)
N11-C12
C12-C17
C14-C15
C16-C17
C8-C9
Pd(1)-N(7)
Pd(1)-N(11)
Pd(1)-O(17)
Pd(1)-O(1)
O(1)-C(1)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-N(7)
N(7)-C(8)
C(8)-C(9)
C(9)-C(10)
1.943(2)
1.944(2)
1.994(1)
1.997(1)
1.308(2)
1.431(3)
1.440(3)
1.380(3)
1.427(3)
1.366(3)
1.418(3)
1.353(3)
1.450(3)
1.535(3)
1.525(3)
Pd(2)-O(41)
Pd(2)-O(57)
Pd(2)-N(47)
Pd(2)-N(51)
Pd(2)-Pd(2)#1
O(41)-C(41)
C(41)-C(46)
C(41)-C(42)
C(42)-C(43)
C(42)-C(59)
C(43)-C(44)
C(44)-C(45)
C(44)-C(63)
C(45)-C(46)
C(46)-C(47)
1.9955(14)
C(10)-N(11)
1.448(3)
N(47)-C(48)
C(48)-C(49)
C(50)-N(51)
N(51)-C(52)
C(52)-C(53)
C(52)-C(57)
C(53)-C(54)
C(54)-C(55)
C(55)-C(56)
C(56)-C(57)
O(57)-C(57)
C(49)-C(50)
C(49)-C(67)
C(49)-C(68)
1.479(3)
1.524(3)
1.481(3)
1.465(3)
1.386(3)
1.405(3)
1.385(3)
1.397(3)
1.394(3)
1.415(3)
1.344(2)
1.523(3)
1.530(3)
1.549(3)
1.9963(14)
2.0227(18)
2.0250(17)
3.0846(4)
1.341(2)
1.405(3)
1.414(3)
1.397(3)
1.534(3)
1.398(3)
1.386(3)
1.531(3)
1.382(3)
1.473(3)
N(11)-C(12)
C(12)-C(13)
C(12)-C(17)
C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
O(17)-C(17)
1.354(3)
1.419(3)
1.440(3)
1.365(3)
1.427(3)
1.380(3)
1.426(3)
1.310(2)
Complex 5
Cu(1)-O(20)
Cu(1)-O(1)
Cu(1)-N(7)
O(1)-C(1)
2.638(7)
1.920(6)
1.964(8)
1.339(11)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-N(7)
1.415(12)
1.359(13)
1.395(14)
1.417(13)
C(1)-C(6)
C(1)-C(2)
C(2)-C(3)
1.415(12)
1.429(14)
1.384(13)
N(7)-C(8)
C(8)-C(9)
1.288(11)
1.484(12)
a Data for only one crystallographically independent neutral molecule in crystals of 2‚5/8CH2Cl2 are given here.
The dangling o-aminophenol part exhibits the same
geometrical features as those in 1 with the exception of the
sp2 phenyl-nitrogen bond which is slightly shorter in 2 at
1.43 ( 0.01 Å than the corresponding sp3 phenyl-nitrogen
bond in 1 at 1.46 ( 0.01 Å. It is clearly an aromatic
o-aminophenol half which also forms an intramolecular
O-H‚‚‚O bond (O1‚‚‚O56, 2.697(4); O16‚‚‚O41, 2.764(4)
Å).
for an R,R-diimine radical anion.12,21 Therefore, we propose
that the singlet diradical resonance structure B in Scheme 2
plays an important role.
Figure 8 displays the structure of the neutral molecule [Ni-
(L3••)] in crystals of 3. The nickel(II) ions are in a square
planar ligand environment where the two o-iminobenzosemi-
quinonate(1-) π radical anions are cis relative to each other
due to the presence of the saturated, bridging 2,2-dimeth-
ylpropylenediamine moiety. The geometrical details of the
O,N-coordinated radical anions are within experimental error
identical to those reported for [NiII(LISQ)2]2 and [PdII(LISQ)-
(bpy)]PF6.7
The crystal structure of 4 is interesting, since it reveals
the presence of two different neutral molecules, namely,
mononuclear [PdII(L3••)] (4a) shown in Figure 9 and [PdII-
(H2L3)]2 (4b) shown in Figure 10, which is assembled as a
dimer in the solid state via four N-H‚‚‚O hydrogen bonds.
The most significant difference between O,N,N-coordi-
nated (H3L1)- in 1 and (HL2)- in 2 is that the O,N-
coordinated half of (HL2)- in 2 is apparently not aromatic:
the six CsC bond lengths are not equidistant. Significantly,
they apparently exhibit the typical quinoidal distortions
characteristic for an o-iminobenzosemiquinonate(1-) radical,
(LISQ)-•, as shown in Chart 1. Concomitantly, the bridging
ethylenediimine part is also distorted in a characteristic
fashion: the CsC bond is too short for a CsC single bond,
and both CsN bonds are slightly too long for a typical
>CdNs double bond. These bond lengths are more typical
(21) Gardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee, F. C.; Raston,
C. L. Inorg. Chem. 1994, 33, 2456.
2928 Inorganic Chemistry, Vol. 43, No. 9, 2004

Tetradentate π Radical Ligands
Figure 11. Structure of a neutral molecule [CuII(OH2)(L4)] in crystals of
5.
Figure 8. Structure of a neutral molecule [NiII(L3••)] in crystals of 3.
Figure 12. Structure of the oxidized ligand in crystals of 6. Selected bond
distances (Å): N47-C48, 1.283(3); C48-O41, 1.375(2); C41-O41, 1.384-
(2); C41-C46, 1.378(3); C46-N47, 1.405(3); C48-C49, 1.505(3).
Figure 9. Structure of a neutral molecule [PdII(L3••)] (4a) in crystals of 4.
zosemiquinonate(1-) π radical anions in 4a as in 3 and [PdII-
(LISQ)2], but in the dimer 4b, the two ligand halves are clearly
aromatic and the two nitrogen atoms are sp3 hybridized. The
geometrical features are those of O,N-coordinated (LAP)-
ligands (Chart 1) as in [MnIII(LAP)(LISQ)2].5
The crystal structure of 5 shows that neutral molecules of
[CuII(OH2)(L4)] are present, as shown in Figure 11. Interest-
ingly, the O,N-coordinated halves of the ligand (L4)2- consist
of aromatic iminophenolate(1-) moieties. Both nitrogen
atoms are sp2 hybridized and the C8-N7 distance at 1.29-
(1) Å is short and indicative of a carbon-nitrogen double
bond. The crystal structure is of rather low quality with large
estimated standard deviations. Therefore, we refrain from a
more detailed discussion of the structure.
Finally, Figure 12 exhibits the crystal structure of the
oxidized ligand 6, which is a benzoxazol derivative.
Figure 10. Structure of a neutral dimer [PdII(H2L3)]2 (4b) in crystals of
4.
EPR Spectra. The X-band EPR spectra of the electro-
chemically generated monoanions of 3 and 4 in frozen CH2-
Cl2 solutions at 10 K are shown in Figures 13 and 14,
respectively. Both spectra display a rhombic S ) /2 signal
with significant g anisotropy. Similar spectra have been
reported for all square planar monoanionic complexes of the
type [MII(LAP-H)(LISQ)]-, where (LISQ -
) is a benzosemi-
Two N-H groups of half of a dimer form a contact to two
phenolate oxygen atoms of the second half. The two halves
of [Pd(H2L3)]2 adopt an eclipsed configuration and give rise
to an intramolecular Pd‚‚‚Pd distance of 3.0846(4) Å which
1
II
2
is a nonbonding contact, since the dz orbitals of both Pd
ions are filled. Note that similar metal-metal distances have
been identified in refs 22 and 23 where weak direct metal-
metal bonds prevail. The structure determination of 4 clearly
shows the presence of two O,N-coordinated o-iminoben-
quinonate π radical and (LAP-H)2- is its one-electron
reduced aromatic form.2,15-18 In contrast, the EPR spectrum
of the monocation of 4a with a S ) 1/2 ground state, namely
[PdII(LISQ)(LIBQ)]+, where (LIBQ) represents the one-electron-
oxidized, neutral quinone form of half of (L3••)2-, is nearly
isotropic and shown in Figure 14 (top). The giso value 2.0024
is in excellent agreement with the notion that the unpaired
electron resides on the ligand in a π* orbital. Very similar
(22) Herebian, D.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt, K.
J. Am. Chem. Soc. 2003, 125, 9116.
(23) (a) Millar, M.; Holm, R. H. J. Am. Chem. Soc. 1975, 97, 6052. (b)
Reng, S.-M.; Ibers, J. A.; Millar, M.; Holm, R. H. J. Am. Chem. Soc.
1976, 98, 8037. (c) Peng, S.-M.; Goedken, V. L. J. Am. Chem. Soc.
1976, 98, 8500.
Inorganic Chemistry, Vol. 43, No. 9, 2004 2929

Min et al.
Figure 13. X-band EPR spectrum of the electrochemically generated
monoanion of 3 in frozen CH2Cl2 solution (0.10 M [N(n-Bu)4]PF6) at 10
K. Conditions: 9.45257 GHz; modulation, 10 G; power, 0.2 nW. Simula-
tion: gx ) 2.0098, gy ) 2.0384, gz ) 2.1097.
Figure 15. Highest two MOs in 3 and 4a.
quinonato)metal(II) species (3 and 4a), two electrons occupy
the two close lying a2 and b2 orbitals. This situation gives
rise to near-degeneracy effects and a pronounced diradical
character of the neutral complexes. In the monocation, the
b2 orbital contains only a single unpaired electron. Due to
symmetry reasons, the b2 orbital cannot mix with any metal
d orbital. Consequently, metal hyperfine coupling cannot be
expected and is not observed experimentally. The lack of
metal character in the b2 SOMO makes the spin-orbit
coupling of excited states with the ground state very
inefficient, and therefore, the g shifts are expected to be very
small in agreement with experiment.
In contrast, in the monoanions of 3 and 4a, the b2 orbital
is occupied by two electrons and the a2 orbital is the
delocalized SOMO. As shown in Figure 15, this orbital mixes
with a d orbital of the metal ion which acquires thereby some
metal character. This usually gives rise to sizable first-order
hyperfine couplings. Since the ground state (A2) readily
mixes with relatively low lying d-d excited states, it also
has a sizable orbital angular momentum which manifests
itself experimentally in large g shifts. This is observed for
the monoanions of 3 and 4a.
Conclusions
The most salient features of the present study are sum-
marized as follows:
Figure 14. X-band EPR spectra of the electrochemically generated
monocation (top) and of the corresponding monoanion (bottom) of 4a in
frozen CH2Cl2 solution at 10 K (0.10 M [N(n-Bu)4]PF6). Conditions: (top)
frequency, 9.63497 GHz; modulation, 10 G; power, 0.2 nW; (bottom)
frequency, 9.63564 GHz; modulation, 10 G; power, 0.2 nW. Simulation:
(top) gx ) 2.0007, gy ) 2.0007, gz ) 2.0058, giso ) 2.0024; (bottom) gx )
1.9763, gy ) 2.0154, gz ) 2.0645.
(1) Complexes 1 and 2, containing two tridentate saturated
(H3L1)- and two unsaturated (HL2)- ligands, respectively,
both possess an S ) 1 ground state, but their electronic
spectra (Figure 1) differ significantly. From low-temperature
X-ray crystallography of 1 and 2, it is clearly established
that only in 1 the O,N-coordinated o-iminophenolate part of
(H3L1)- is aromatic with six equivalent CsC bonds in each
ring. In contrast, the equivalent bond distances of the O,N-
coordinated part of the unsaturated ligand (HL2)- in 2
displays quinoid-type distortions characteristic of O,N-
coordinated o-iminobenzosemiquinonate(1-) π radicals and,
concomitantly, the bridging ethylenediimine part also shows
a too short CsC single bond and two too long CdN double
bonds. Thus, it is obvious that the singlet diradical resonance
structure B in Scheme 2 contributes significantly to 2. This
spectra have been reported for [MII(LOISQ)(LOIBQ)]+ and [MII-
(LSISQ)(LSIBQ)]+ (M ) Pd or Pt).2,15-18
In previous work,7,24 we have shown from DFT calcula-
tions for [MII(LISQ)2] complexes that the two molecular
orbitals shown in Figure 15 are highest in energy. These two
MOs are basically the symmetric and antisymmetric com-
bination of the singly occupied molecular orbital (SOMO)
of the free semiquinonate ligand. In the neutral bis(semi-
(24) Herebian, D.; Wieghardt, K.; Neese, F. J. Am. Chem. Soc. 2003, 125,
10997.
2930 Inorganic Chemistry, Vol. 43, No. 9, 2004

Tetradentate π Radical Ligands
is supported by intense ligand-to-ligand charge transfer bands
in the visible spectrum of 2.
(3) Finally, air oxidation of H4[L3] in the presence of CuII
ions does not yield [Cu(L3••)] but instead yields [CuII(H2O)-
(L4)], containing two aromatic iminophenolate moieties
(oxidative Schiff base formation).
(2) The properties of 3 and 4a are typical for square planar,
singlet diradicals2 including the observation of a complete
electron transfer series for each species. The EPR spectra of
their corresponding monocations and monoanions have been
interpreted in terms of two differing ligand centered SOMOs
of which the b2 SOMO lacks metal character, whereas the
a2 ligand orbital mixes significantly with metal d orbitals.
This is the origin of the differing g anisotropy of the EPR
spectra of the monoanions and -cations, both of which
Acknowledgment. This work was supported by the Fonds
der Chemischen Industrie. K.S.M. is grateful to the Max-
Planck Society for a postdoctoral fellowship.
Supporting Information Available: Crystallographic data files
for complexes 1, 2, 3, 4, 5, and 6. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
possess S ) /2 ground states.
IC0302480
Inorganic Chemistry, Vol. 43, No. 9, 2004 2931