Synthesis and properties of diphenoxo-bridged CoII, Ni II, CuII, and ZnII complexes of a new tripodal ligand: Generation and properties of MII-coordinated phenoxyl radical species

Article abstract of DOI:10.1021/ic701283b

Four dinuclear complexes of composition [M^II₂(L) ₂]·xS [M = Co, x = 0.5, S = 1,4-dioxane (1·0.5 1,4-dioxane); Ni, x = 0 (2) [single crystals have x = 2, S = diethyl ether (2·2 diethyl ether)]; Cu, x = 0 (3); Zn, x = 0.5, S = 1,4-dioxane (4·0.5 1,4-dioxane)] have been synthesized using a new tripodal ligand [2,4-ditert-butyl-6-{[(2-pyridyl)ethyl](2-hydroxybenzyl)-aminomethyl}-phenol (H₂L)], in its deprotonated form, providing a N₂O ₂ donor set. Crystallographic analyses reveal that the complexes have a similar diphenoxo-bridged structure. Each metal ion is terminally coordinated by 2,4-ditert-butyl-phenolate oxygen, a tertiary amine, and a pyridyl nitrogen. From each ligand, unsubstituted phenolate oxygen provides bridging coordination. Thus, each metal center assumes M^IIN₂O ₃ coordination. Whereas the geometry around the metal ion in 1·0.5 1,4-dioxane, 2·2 diethyl ether and, 4·0.5 1,4-dioxane is distorted trigonal-bipyramidal, in 3 each copper(II) center is in a square-pyramidal environment. Temperature-dependent magnetic behavior has been investigated to reveal intramolecular antiferromagnetic exchange coupling for these compounds (-J = 6.1, 28.6, and 359 cm^-1 for 1·0.5 1,4-dioxane, 2, and 3, respectively). Spectroscopic properties of the complexes have also been investigated. When examined by cyclic voltammetry (CV), all four complexes undergo in CH₂Cl₂ two reversible ligand-based (2,4-ditert-butylphenolate unit) one-electron oxidations [E_1/2 ¹ = 0.50-0.58 and E_1/2² = 0.63-0.75 V vs SCE (saturated calomel electrode)]. The chemically/coulometrically generated two-electron oxidized form of 3 rearranges to a monomeric species with instantaneous abstraction of the hydrogen atom, and for 4·0.5 1,4-dioxane the dimeric unit remains intact, exhibiting an EPR spectrum characteristic of the presence of Zn^II-coordinated phenoxyl radical (UV-vis and EPR spectroscopy). To suggest the site of oxidation (metal or ligand-centered), in each case DFT calculations have been performed at the B3LYP level of theory.

Full text of DOI:10.1021/ic701283b

Inorg. Chem. 2008, 47, 4471-4480
Synthesis and Properties of Diphenoxo-Bridged CoII, NiII, CuII, and ZnII
Complexes of a New Tripodal Ligand: Generation and Properties of
MII-Coordinated Phenoxyl Radical Species
Atasi Mukherjee,† Francesc Lloret,‡ and Rabindranath Mukherjee*,†
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016,
India, and Departament de Qu´ımica Inorga`nica/Instituto de Ciencia Molecular (ICMOL),
UniVersitat de Vale`ncia, Pol´ıgono de la Coma, s/n, 46980-Paterna (Vale`ncia), Spain
Received June 29, 2007
Four dinuclear complexes of composition [MII2(L)2]·xS [M ) Co, x ) 0.5, S ) 1,4-dioxane (1· 0.5 1,4-dioxane);
Ni, x ) 0 (2) [single crystals have x ) 2, S ) diethyl ether (2· 2 diethyl ether)]; Cu, x ) 0 (3); Zn, x ) 0.5, S
) 1,4-dioxane (4 · 0.5 1,4-dioxane)] have been synthesized using a new tripodal ligand [2,4-ditert-butyl-6-{[(2-
pyridyl)ethyl](2-hydroxybenzyl)-aminomethyl}-phenol (H2L)], in its deprotonated form, providing a N2O2 donor set.
Crystallographic analyses reveal that the complexes have a similar diphenoxo-bridged structure. Each metal ion is
terminally coordinated by 2,4-ditert-butyl-phenolate oxygen, a tertiary amine, and a pyridyl nitrogen. From each
ligand, unsubstituted phenolate oxygen provides bridging coordination. Thus, each metal center assumes MIIN2O3
coordination. Whereas the geometry around the metal ion in 1· 0.5 1,4-dioxane, 2· 2 diethyl ether and, 4· 0.5
1,4-dioxane is distorted trigonal-bipyramidal, in 3 each copper(II) center is in a square-pyramidal environment.
Temperature-dependent magnetic behavior has been investigated to reveal intramolecular antiferromagnetic exchange
coupling for these compounds (-J ) 6.1, 28.6, and 359 cm-1 for 1· 0.5 1,4-dioxane, 2, and 3, respectively).
Spectroscopic properties of the complexes have also been investigated. When examined by cyclic voltammetry
(CV), all four complexes undergo in CH2Cl2 two reversible ligand-based (2,4-ditert-butylphenolate unit) one-electron
1
2
oxidations [E1/2 ) 0.50-0.58 and E1/2 ) 0.63-0.75 V vs SCE (saturated calomel electrode)]. The chemically/
coulometrically generated two-electron oxidized form of 3 rearranges to a monomeric species with instantaneous
abstraction of the hydrogen atom, and for 4· 0.5 1,4-dioxane the dimeric unit remains intact, exhibiting an EPR
spectrum characteristic of the presence of ZnII-coordinated phenoxyl radical (UV-vis and EPR spectroscopy). To
suggest the site of oxidation (metal or ligand-centered), in each case DFT calculations have been performed at the
B3LYP level of theory.
Introduction
two phenol groups (one unsubstituted, and the other with
2,4-ditert-butyl substituents), an ethylpyridine unit, and an
aliphatic amine], and their two-electron oxidized counterparts.
Specifically, we present a comprehensive report on the
Formation of the tyrosyl radical in mononuclear copper
enzyme galactose oxidase,1 a prototypical example of a class
of metalloproteins to use free radicals as cofactors to promote
oxidation reactions,1b,2,3 has triggered current research inter-
est in the preparation and studies of metal-coordinated
phenoxyl radicals.4–11 For a better understanding of the MII-
tyrosyl radical species in the formation and its subsequent
stability/reactivity, we describe herein diphenoxo-bridged
complexes of MII ions (cobalt, nickel, copper, and zinc) of
a new phenol-based tetradentate ligand H2L [comprising of
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* To whom correspondence should be addressed. Tel.: +91-512-2597437.
Fax: +91-512-2597436. E-mail: rnm@iitk.ac.in.
†
Indian Institute of Technology Kanpur.
Universitat de Vale`ncia.
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‡
(3) Limberg, C. Angew. Chem., Int. Ed. 2003, 42, 5932–5954.
10.1021/ic701283b CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 11, 2008 4471
Published on Web 05/08/2008

Mukherjee et al.
synthesis, crystal structure, magnetic behavior, redox chem-
istry, and studies on generation and stability/properties of
MII-coordinated phenoxyl radical species. It is appropriate
to mention here a few noteworthy aspects of the present
complexes of phenol-based tetradentate N2O2 donor ligand
with respect to that reported in the literature.1i (i) To the
best of our knowledge, there is no report in the literature on
structurally characterized monomeric/dimeric CuII complexes
that bear a phenol unit without steric bulk at its ortho
position. The present ligand is unique due to its unsym-
metrical nature. (ii) Only few reports concerning the oxida-
tion behavior of the phenolate complexes of tripodal N2O2
donor set exist. (iii) The instability of phenoxyl radical
complexes, as observed in this work, is common to reported
systems with the N2O2 donor set. In fact, compared to N3O
donor ligands, the phenoxyl radical complexes with N2O2
donor ligands are less stable. (iv) The hydrogen atom
abstraction chemistry implied in this work is noteworthy,
considering reported dicopper(II) systems with N2O2 ligands.
To throw light on the site of oxidation (metal or ligand-
centered), in each case DFT calculations have been per-
formed at the B3LYP level of theory.
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Experimental Section
Materials and Reagents. All reagents were obtained from
commercial sources and used as received. Solvents were dried/
purified as reported previously.12 The precursors 2,4-ditert-butyl-
6-(hydroxymethyl)phenol,13 2,4-ditert-butyl-6-(chloromethyl)phe-
nol,14 and 2-((2-(2-pyridinyl)ethylamino)methyl)phenol15 for the
synthesis of H2L, and tetra-n-butylammonium perchlorate
(TBAP)12awere prepared following literature procedures. Zinc(II)
perchlorate hexahydrate was prepared from the reaction between
ZnCO3 and aqueous HClO4.
Ligand Synthesis. 2,4-ditert-butyl-6-{[(2-pyridyl)ethyl](2-hy-
droxybenzyl)amino-methyl}-phenol (H2L·1,4-Dioxane). To a
magnetically stirred mixture of 2-((2-(2-pyridinyl)ethylamino)-
methyl)phenol (1.74 g, 7.65 mmol) and Et3N (3.49 mL, 25.0 mmol)
in 1,4-dioxane (6 mL), a solution of 2,4-ditert-butyl-6-(chloro-
methyl)phenol (1.95 g, 7.65 mmol) in 1,4-dioxane (4 mL) was
added dropwise. The resulting orange reaction mixture was allowed
to stir for 90 min at 298 K. It was then heated (∼60 °C), and an
additional amount of Et3N (5.23 mL, 37.5 mmol) was added
portionwise over a period of 2 h. The pH of the mixture was
maintained below 10 during this addition. The resulting solution
was then cooled to 40 °C, filtered, and the filtrate was evaporated
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C.; Wolowska, J. Dalton Trans. 2004, 3647–3653. (d) Benisvy, L.;
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4472 Inorganic Chemistry, Vol. 47, No. 11, 2008

Diphenoxo-Bridged CoII, NiII, CuII, and ZnII Complexes
under reduced pressure. The crude solid thus obtained was
recrystallized from a mixture of CH2Cl2 and n-hexane (1:2, v/v),
which afforded a white crystalline solid of H2L·1,4-dioxane (yield:
2.16 g, ∼63%). Anal. Calcd for C33H46N2O4: C, 74.15; H, 8.61; N,
5.24. Found: C, 74.01; H, 8.72; N, 5.18. 1H NMR (80 MHz; CDCl3):
δ 8.72 (d, JHH ) 7.80 Hz, 1H, H6′ of py), 6.72-7.82 (m, 9H, H3′,4′,5′
Characterization of 4 · 0.5 1,4-Dioxane. Anal. Calcd for
C60H76N4O6Zn2: C, 67.80; H, 7.16; N, 5.27. Found: C, 67.71; H,
1
7.09; N, 5.12. H NMR (400 MHz, CDCl3): δ 8.81 (s, 1H, H6′ of
py), 7.63 (t, JHH ) 7.56 Hz, 1H, H4′ of py), 7.26 (s, 2H, H3,5 of
t-buPhO), 7.22 (d, JHH ) 7.96 Hz, 1H, H3′ of py), 7.16 (t, JHH
)
7.80 Hz, 1H, H5′ of py), 6.97 (t, JHH ) 7.50 Hz, 1H, H4′′ of PhO),
6.91 (d, JHH ) 7.10 Hz, 1H, H6′′ of PhO), 6.83 (d, JHH ) 7.02 Hz,
1H, H3′′ of PhO), 6.51 (t, JHH ) 7.23 Hz, 1H, H5′′ of PhO), 3.45 (s,
4H, -CH2-), 3.05 (m, 2H, -CH2-CH2-), 2.17 (m, 2H,
-CH2-CH2-), 1.34 (s, 9 H, 2-tert-butyl), 1.18 (s, 9H, 4-tert-butyl).
Physical Measurements. Elemental analyses were obtained
using a Thermo Quest EA 1110 CHNS-O, Italy. Spectroscopic
measurements were made using the following instruments: IR (KBr,
4000-600 cm-1), Bruker Vector 22; electronic, Perkin Elmer
of py and H3,5,3′′,4′′ 5′′,6′′ of PhOH), 3.86 (s, 4H, -CH2-), 3.71 (s,
,
8H, -CH2-CH2- of 1,4-dioxane), 3.16 (t, JHH ) 8.03 Hz, 2H,
-CH2-CH2-), 2.92 (t, JHH ) 8.40 Hz, 2H, -CH2-CH2-), 1.34
(s, 18H, tert-butyl).
Synthesis of Metal Complexes. [CoII2(L)2]·0.5 1,4-Dioxane
(1·0.5 1,4-Dioxane). To a magnetically stirred solution of H2L (0.2
g, 0.448 mmol) in MeOH (8 mL) was added a solution of
Co(OAc)2 ·4H2O (0.112 g, 0.448 mmol) in MeOH (5 mL) dropwise,
and the resulting mixture was refluxed for 1 h. The pink precipitate
that formed was collected by filtration, washed with MeOH, and
dried in vacuum. The complex was recrystallized by diffusion of
diethyl ether into a concentrated solution of the complex in CH2Cl2.
Pink crystals that obtained (yield: 0.135 g, ∼60%) were found to
be suitable for X-ray structural study.
Characterization of 1 · 0.5 1,4-Dioxane. Anal. Calcd for
C60H76Co2N4O5: C, 68.58; H, 7.24; N, 5.33. Found: C, 68.42; H,
7.15; N, 5.21. IR (KBr, cm-1, selected peak): 1275 ν(C-O) of the
phenolate group. Absorption spectrum [λmax, nm (ꢀ, M-1 cm-1)]:
(in CH2Cl2) 290 (21 000), 380 sh (2900), 550 sh (190), 570 (200),
660 sh (20), 740 (30).
1
Lambda 2 and Agilent 8453 diode-array spectrophotometers. H
NMR spectra (CDCl3 solution) were obtained on either a Bruker
WP-80 (80 MHz) or a JEOL JNM LA (400 MHz) spectrophotom-
eter. Chemical shifts are reported in ppm referenced to TMS.
Electrospray ionization LC-MS mass spectra were recorded using
a Waters Q Tof Premier micromass HAB 213 mass spectropho-
tometer. X-band EPR spectra were recorded using a Bruker EMX
1444 EPR spectrometer operating at 9.455 GHz. The EPR spectra
were calibrated with diphenylpicrylhydrazyl, DPPH (g ) 2.0037).
Variable-temperature magnetic susceptibility measurements on
polycrystalline samples of 1·0.5 1,4-dioxane, 2, and 3 were
performed with a Quantum Design (Model MPMSXL-5) SQUID
magnetic susceptometer at Vale`ncia, Spain.
Cyclic voltammetric (CV) experiments were performed at 298
K by using a PAR model 370 electrochemistry system consisting
of M-174A polarographic analyzer, M-175 universal programmer,
and RE 0074 X-Y recorder. The cell contained a Beckman (M
39273) platinum-inlay working electrode, a platinum wire auxiliary
electrode, and a saturated calomel electrode (SCE) as a reference
electrode. Details of the cell configuration are as described before.16
For coulometry, a platinum wire-gauze was used as the working
electrode. The solutions were ∼1.0 mM in complex and 0.1 M in
supporting electrolyte TBAP.
Crystal Structure Determination. Single crystals of suitable
dimensions were used for data collection. Diffraction intensities
were collected on a Bruker SMART APEX CCD diffractometer,
with graphite-monochromated Mo KR (0.71073 Å) radiation at
100(2) K. The data were corrected for absorption. The structures
were solved by SIR-97, expanded by Fourier-difference syntheses,
and refined with the SHELXL-97 package incorporated into the
WinGX 1.64 crystallographic package.17 The position of the
hydrogen atoms were calculated by assuming ideal geometries but
not refined. All non-hydrogen atoms were refined with anisotropic
thermal parameters by full-matrix least-squares procedures on F2.
For 1·0.5 1,4-dioxane, some degree of disorder was observed with
carbon atoms of one of the tert-butyl groups of the ligand. All the
three carbon atoms were displaced over two positions, and they
were refined with a site occupation factor of 0.65/0.35, 0.7/0.3,
and 0.6/0.4. For 4·0.5 1,4-dioxane, a similar kind of disorder was
observed with carbon atoms of one of the tert-butyl groups of the
ligand, and they were each refined with a site occupation factor of
0.6/0.4. For 1·0.5 1,4-dioxane, after anisotropic refinement, unas-
[NiII2(L)2] (2). To a magnetically stirred mixture of H2L (0.3 g,
0.673 mmol) and Et3N (0.136 g, 1.346 mmol) in MeOH (10 mL)
was added dropwise a solution of Ni(OAc)2 ·4H2O (0.167 g, 0.673
mmol) in MeOH (6 mL). The green solid that precipitated was
collected by filtration, washed with MeOH, and dried in vacuum
(yield: 0.217 g, ∼64%). The complex was recrystallized by diffusion
of diethyl ether into a concentrated solution of the complex in
CH2Cl2, affording green crystals with two molecules of diethyl ether
as solvent of crystallization [NiII (L)2]·2 diethyl ether (2·2 diethyl
2
ether). Such crystals were suitable for X-ray structural study.
Characterization of 2. Anal. Calcd for C58H72N4Ni2O4: C, 69.23;
H, 7.16; N, 5.57. Found: C, 69.39; H, 7.25; N, 5.52. IR (KBr, cm-1
selected peak): 1275 ν(C-O) of phenolate group. Absorption
spectrum [λmax, nm (ꢀ, M-1 cm-1)]: (in CH2Cl2) 290 (16 800), 420
(3200), 670 (60), 970 (20).
[CuII2(L)2] (3). To a magnetically stirred solution of H2L (0.3
g, 0.673 mmol) in MeOH (8 mL), a solution of Cu(OAc)2 ·H2O
(0.134 g, 0.673 mmol) in MeOH (5 mL) was added dropwise. The
brown solid that formed was collected by filtration, washed with
MeOH, and dried in vacuum. Brown crystals suitable for X-ray
structural study were achieved by diffusion of diethyl ether into a
concentrated solution of the complex in CH2Cl2 (yield: 0.200 g,
∼58%).
Characterization of 3. Anal. Calcd for C58H72Cu2N4O4: C,
68.57; H, 7.09; N, 5.51. Found: C, 68.66; H, 7.20; N, 5.60. IR
(KBr, cm-1 selected peak): 1271 ν(C-O) of phenolate group.
Absorption spectrum [λmax, nm (ꢀ, M-1 cm-1)]: (in CH2Cl2) 290
(17 400), 340 sh (5300), 390 (3400), 510 sh (2200), 760 (410).
[ZnII2(L)2]·0.5 1,4-Dioxane (4 ·0.5 1,4-Dioxane). To a magneti-
cally stirred mixture of H2L (0.2 g, 0.448 mmol) and Et3N (0.091
g, 0.896 mmol) in MeOH (10 mL), a solution of [Zn(H2O)6][ClO4]2
(0.167 g, 0.448 mmol) in MeOH (7 mL) was added dropwise. The
resulting solution was stirred for an hour. Evaporation of this
solution afforded light-yellowish solid, which was recrystallized
by dissolving in CH2Cl2 followed by addition of n-hexane. After a
few days, white crystals were obtained, which were suitable for
X-ray structural study (yield: 0.139 g, ∼61%).
(16) (a) Patra, A. K.; Mukherjee, R. Inorg. Chem. 1999, 38, 1388–1393.
(b) Patra, A. K.; Ray, M.; Mukherjee, R. Inorg. Chem. 2000, 39, 652–
657. (c) Singh, A. K.; Balamurugan, V.; Mukherjee, R Inorg. Chem.
2003, 42, 6497–6502.
(17) Farrugia, L. J. WinGX Version 1.64, An Integrated System of Windows
Programs for the Solution, Refinement and Analysis of Single-Crystal
X-ray Diffraction Data; Department of Chemistry: University of
Glasgow, 2003.
Inorganic Chemistry, Vol. 47, No. 11, 2008 4473

Mukherjee et al.
Table 1. Data Collection and Structure Refinement Parameters for 1·0.5 1,4-Dioxane, 2·2 Diethyl Ether, 3, and 4·0.5 1,4-Dioxane
1·0.5 1,4-Dioxane
2·2 Diethyl Ether
3
4·0.5 1,4-Dioxane
chem formula
fw
C60H76Co2N4O5
1051.11
0.2 × 0.1 × 0.1
100(2)
C66H92N4Ni2O6
1154.86
0.2 × 0.1 × 0.1
100(2)
C58H72Cu2N4O4
1016.28
0.2 × 0.2 × 0.1
100(2)
C60H76N4O6Zn2
1079.99
0.2 × 0.1 × 0.1
100(2)
cryst size/mm × mm × mm
temp/K
λ/Å
0.710 69
monoclinic
C2/c (15)
40.868(5)
14.076(5)
20.358(5)
90
105.372(5)
90
11 292(5)
8
0.710 73
triclinic
P-1 (2)
0.710 69
orthorhombic
Aba2 (41)
13.677(5)
34.017(5)
10.880(5)
90
90
90
5062(3)
4
1.334
0.710 69
monoclinic
C2/c (15)
40.963(5)
14.032(5)
20.353(5)
90
105.542(5)
90
11 271(5)
8
cryst syst
space group (No.)
a/Å
9.543(5)
17.559(5)
19.506(5)
102.504(5)
103.981(5)
90.164(5)
3091(2)
2
b/Å
c/Å
R/deg
ꢁ/deg
γ/deg
V/Å3
Z
dcalcd/g cm-3
1.237
0.637
4464
1.241
0.662
1240
1.273
0.904
4576
µ/mm-1
0.891
2152
F(000)
no. reflns collcd
no. unique reflns
no. reflns used [I > 2σ(I)]
GOF onF2
final R indices
[I > 2σ(I)]a,b
R indices
(all data)a,b
b
37 477
13986 (Rint ) 0.0766)
8668
1.094
0.0742
0.1911
0.1243
20 791
14707 (Rint ) 0.063)
9090
1.042
0.1033
0.1822
0.1664
16 544
5332 (Rint ) 0.0591)
4470
1.041
0.0510
0.1173
0.0692
37 188
13875 (Rint ) 0.1114)
7718
1.043
0.0842
0.1961
0.1575
(0.2194)
(0.2286)
(0.1513)
(0.2315)
a R1 ) Σ(|Fo| - |Fc|)/Σ|Fo|. wR2 ) {Σ[w(|Fo|2 - |Fc|2)2]/Σ[w(|Fo|2)2]}1/2
signed electron density peaks of 1.22 e Å-3 and 1.14 e Å-3 and for
2·2 diethyl ether a peak of 1.26 e Å-3 were found in the final
difference Fourier map, which may be due to the poor quality of
crystal chosen for data collection. Pertinent crystallographic
parameters are summarized in Table 1.
.
droxybenzyl)aminomethyl}-phenol (H2L) was synthesized in
good yield by condensing 2,4-ditert-butyl-6-(chloromethyl)-
phenol and 2-((2-(2-pyridinyl)ethylamino)methyl)phenol in
the presence of Et3N in 1,4-dioxane. The 1H NMR spectrum
of H2L is displayed in Figure S1 in the Supporting Informa-
tion. Notably, this ligand comprises two kinds of phenol units
(one unsubstituted and the other one 2,4-ditert-butyl-
substituted) and a 2-pyridylethylamine moiety. The syntheses
of metal complexes were achieved in good yields by reacting
appropriate metal salts and H2L with or without the use of
Theoretical Studies. Calculations were performed using Density
Functional Theory (DFT) with Becke’s three-parameter hybrid
exchange functional18 and the Lee-Yang-Parr correlation func-
tional (B3LYP).19 The atomic coordinates of all of the complexes
were taken from their X-ray structures. The double-ꢀ basis set of
Hay and Wadt (LanL2DZ) was used for metal centers. The ligand
hydrogen, carbon, nitrogen, and oxygen atoms were described using
the 6-31G-(d) basis sets. All calculations were performed with
the Gaussian 03 (G03) suite of programs.20 Orbital diagrams were
generated at isosurface of 0.02 using GaussView 3.0.21
Et3N. The complex [CoII (L)2] · 0.5 1,4-dioxane (1 · 0.5 1,4-
2
dioxane) was isolated as a pink powder under refluxing
conditions. The complexes [NiII (L)2] (2), [CuII (L)2] (3), and
2
2
[ZnII (L)2]·0.5 1,4-dioxane (4·0.5 1,4-dioxane) were isolated
2
as green, brown, and white crystalline solid, respectively.
All complexes are readily soluble in CH2Cl2. H NMR
spectrum of diamagnetic 4 · 0.5 1,4-dioxane is displayed in
Figure S2 in the Supporting Information.
Results and Discussion
1
Syntheses of the Ligand and the Complexes. The
tripodal ligand 2,4-ditert-butyl-6-{[(2-pyridyl)ethyl](2-hy-
Description of the Structures. To confirm the structure
of the complexes, and mode of coordination of the ligand
L2-, single crystal X-ray structure determination of the
complexes was carried out. Selected bond length, and bond
angles are listed in Table 2.
(18) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, U.K., 1989.
(19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. ReV. B 1998, 37, 785–789.
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr. T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P. J.; Dannenberg, J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian,
Inc.: Wallingford, CT, 2004.
[CoII (L)2]·0.5 1,4-Dioxane (1 ·0.5 1,4-Dioxane). A per-
2
spective view of the metal coordination environment in 1·0.5
1,4-dioxane is shown in Figure 1. The crystal structure
reveals a dinuclear complex [CoII (L)2], where the two CoII
2
ions are in a similar five-coordination environment bridged
by two phenolate oxygens from two L2- ligands. Each cobalt
ion is terminally coordinated by two nitrogens (pyridine and
tertiary amine) and a phenolate oxygen of the 2,4-ditert-
butyl-phenolate ring. Thus, each CoII ion is in a N2O3
coordination environment. The equatorial plane of each
cobalt(II) ion consists of two phenolate oxygens O(1), O(2)
at Co(1), O(3), O(4) at Co(2), and a pyridine nitrogen at
(21) Gaussian, Inc.: Wallingford, CT, 2004.
4474 Inorganic Chemistry, Vol. 47, No. 11, 2008

Diphenoxo-Bridged CoII, NiII, CuII, and ZnII Complexes
Table 2. Selected Bond Lengths (Angstroms) and Angles (Degrees) in
1·0.5 1,4-Dioxane, 2·2 Diethyl Ether, 3, and 4·0.5 1,4-Dioxane
1·0.5 1,4-Dioxane
Co(1)-O(1)
Co(1)-O(2)
Co(1)-O(3)
Co(1)-N(1)
Co(1)-N(2)
Co(2)-O(1)
Co(2)-O(3)
Co(2)-O(4)
Co(2)-N(3)
Co(2)-N(4)
Co(1)Co(2)
1.991(2)
1.935(3)
2.125(3)
2.183(3)
2.067(3)
2.112(3)
1.969(3)
1.927(3)
2.166(3)
2.118(4)
3.200(7)
Co(1)-O(1)-Co(2)
Co(1)-O(3)-Co(2)
O(1)-Co(1)-O(2)
O(1)-Co(1)-O(3)
O(1)-Co(1)-N(1)
O(1)-Co(1)-N(2)
O(2)-Co(1)-O(3)
O(2)-Co(1)-N(1)
O(2)-Co(1)-N(2)
O(3)-Co(1)-N(1)
O(3)-Co(1)-N(2)
N(1)-Co(1)-N(2)
O(1)-Co(2)-O(3)
O(1)-Co(2)-O(4)
O(1)-Co(2)-N(3)
O(1)-Co(2)-N(4)
O(3)-Co(2)-O(4)
O(3)-Co(2)-N(3)
O(3)-Co(2)-N(4)
O(4)-Co(2)-N(3)
O(4)-Co(2)-N(4)
N(3)-Co(2)-N(4)
102.46(11)
102.75(11)
122.03(11)
76.99(10)
89.14(11)
124.30(12)
103.56(11)
90.18(12)
113.66(12)
164.18(11)
91.98(12)
89.58(13)
77.77(10)
88.13(11)
169.99(11)
95.31(13)
120.59(12)
93.11(11)
101.29(13)
91.33(12)
137.65(13)
92.50(14)
Figure 1. A perspective view of [CoII2(L)2] in the crystal of [CoII2(L)2]·0.5
1,4-Dioxane (1·0.5 1,4-Dioxane). Only donor atoms are labeled. Hydrogen
atoms and uncoordinated solvent molecules are omitted for clarity.
N(2) and N(4). The axial positions are occupied by a
phenolate oxygen O(3) and an amine nitrogen N(1) at Co(1),
O(1), and N(3) at Co(2). Thus, the geometry around the two
cobalt(II) ions is distorted trigonal-bipyramidal [τ ) 0.66
for Co(1) and 0.52 for Co(2)].22 Whereas Co(1) is positioned
out of the equatorial plane toward O(3) by 0.013(3) Å, Co(2)
is positioned out of the equatorial plane toward N(3) by
0.077(4) Å. The Co2O2 core is almost planar [the angle be-
tween the two planes passing through the atoms Co(1)-O(1)-
Co(2), and Co(1)-O(3)-Co(2) is 2.08(15)°]. Because of the
expected difference in bond length between axial and
equatorial coordination, the Co(1)-O(1) and Co(2)-O(3)
bonds are shorter than the Co(1)-O(3) and Co(2)-O(1)
bonds, attesting to the asymmetrical nature of the Co2O2 core.
The two cobalt(II) centers are separated by 3.200(7) Å, which
is similar to the other phenoxo-bridged dinuclear cobalt(II)
complexes.23 Between the two Co-Ophenolate bond distances
in the equatorial plane, the substituted phenolate ring
coordinates more effectively than the unsubstituted phenolate.
2·2 Diethyl Ether
Ni(1)-O(1)
Ni(1)-O(2)
Ni(1)-O(1)a
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)Ni(1)a
2.002(4)
1.937(4)
2.058(4)
2.084(5)
2.035(5)
3.151(8)
Ni(1)-O(1)-Ni(1)a
O(1)-Ni(1)-O(2)
O(1)-Ni(1)-O(1)a
O(1)-Ni(1)-N(1)
O(1)-Ni(1)-N(2)
O(2)-Ni(1)-O(1)a
O(2)-Ni(1)-N(1)
O(2)-Ni(1)-N(2)
O(1)a-Ni(1)-N(1)
O(1)a-Ni(1)-N(2)
N(1)-Ni(1)-N(2)
100.78(17)
130.22(18)
79.22(18)
95.10(19)
94.20(19)
89.78(17)
92.73(19)
135.08(19)
174.09(19)
93.00(18)
89.0(2)
3
Cu(1)-O(1)
Cu(1)-O(2)
Cu(1)-O(1)a
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)Cu(1)a
1.962(3)
1.892(3)
1.992(3)
2.050(4)
2.245(4)
3.053(4)
Cu(1)-O(1)-Cu(1)a
O(1)-Cu(1)-O(2)
O(1)-Cu(1)-O(1)a
O(1)-Cu(1)-N(1)
O(1)-Cu(1)-N(2)
O(2)-Cu(1)-O(1)a
O(2)-Cu(1)-N(1)
O(2)-Cu(1)-N(2)
O(1)a-Cu(1)-N(1)
O(1)a-Cu(1)-N(2)
N(1)-Cu(1)-N(2)
101.06(13)
160.56(14)
74.04(14)
94.77(14)
98.59(14)
91.52(13)
95.01(15)
97.95(15)
160.18(15)
106.36(14)
91.27(15)
[NiII (L)2] · 2 Diethyl Ether (2 · 2 Diethyl Ether). The
2
molecular structure of [NiII (L)2] is shown in Figure S3 in
2
the Supporting Information. The molecule sits on a crystal-
lographically imposed center of inversion, forming the
bridged dinuclear structure with each nickel being five-
coordinate, sharing two phenolate oxygens from two ligands.
The terminal coordination around each nickel(II) ion is
similar to that in 1·0.5 1,4-dioxane. The dispositions of donor
atoms in the equatorial and axial positions are similar to that
in 1 · 0.5 1,4-dioxane. The geometry around each nickel(II)
center is distorted trigonal-bipyramidal (τ value, 0.65).22 The
Ni2O2 core is planar, as dictated by symmetry. However, the
bridge is again slightly asymmetric. The distance between
the two nickel(II) centers is 3.151(8) Å. The nickel(II) ion
is positioned out of the equatorial plane toward the N(1) atom
by 0.078(4) Å. The trends of other metric parameters are
similar to that in 1 · 0.5 1,4-dioxane. Generally, the metric
4·0.5 1,4-Dioxane
Zn(1)-O(1)
Zn(1)-O(2)
Zn(1)-O(3)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(2)-O(1)
Zn(2)-O(3)
Zn(2)-O(4)
Zn(2)-N(3)
Zn(2)-N(4)
Zn(1)Zn(2)
2.019(3)
1.942(3)
2.118(3)
2.187(4)
2.076(4)
2.091(3)
1.994(3)
1.940(3)
2.164(5)
2.133(5)
3.226(7)
Zn(1)-O(1)-Zn(2)
Zn(1)-O(3)-Zn(2)
O(1)-Zn(1)-O(2)
O(1)-Zn(1)-O(3)
O(1)-Zn(1)-N(1)
O(1)-Zn(1)-N(2)
O(2)-Zn(1)-O(3)
O(2)-Zn(1)-N(1)
O(2)-Zn(1)-N(2)
O(3)-Zn(1)-N(1)
O(3)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
O(1)-Zn(2)-O(3)
O(1)-Zn(2)-O(4)
O(1)-Zn(2)-N(3)
O(1)-Zn(2)-N(4)
O(3)-Zn(2)-O(4)
O(3)-Zn(2)-N(3)
O(3)-Zn(2)-N(4)
O(4)-Zn(2)-N(3)
O(4)-Zn(2)-N(4)
N(3)-Zn(2)-N(4)
103.37(14)
103.29(14)
122.06(14)
76.07(13)
88.81(14)
123.13(15)
103.58(14)
91.68(15)
114.81(15)
162.59(14)
90.72(15)
90.39(16)
77.22(13)
89.05(14)
168.63(15)
93.72(17)
123.02(15)
92.70(15)
100.82(17)
92.15(16)
135.46(17)
93.34(19)
(22) Addison, A. W.; Rao, T. N.; Reedjik, J.; van Rijn, J.; Vershcoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
(23) Rodr´ıguez, L.; Labisbal, E.; Sousa-Pedrares, A; Garc´ıa-Va´zquez, J. A.;
Romero, J.; Dura´n, M. L.; Real, J. A.; Sousa, A. Inorg. Chem. 2006,
45, 7903–7914.
a Symmetry operator for the generated atoms: -x + 1, -y, -z + 1 for
2·2 Diethyl Ether and -x + 2, -y, z for 3.
Inorganic Chemistry, Vol. 47, No. 11, 2008 4475

Mukherjee et al.
d-d transitions at 570, and 740 nm with ꢀ ) 200 and 30
M-1 cm-1, respectively,23 and a shoulder at 380 nm (ꢀ )
2900 M-1 cm-1) (Figure S5 in the Supporting Information)
that is attributed to a phenolate-to-cobalt(II) charge-transfer
transition.28 For 2, the phenolate-to-nickel(II) charge-transfer
transition is observed at 420 nm (ꢀ ) 3200 M-1 cm-1), and
d-d bands are at 670 and 970 nm with ꢀ ) 60 and 20 M-1
cm-1, respectively (Figure S6 in the Supporting Information).
Similar features were observed for a closely similar
complex.7e The intense brown color of 3 suggests the
presence of strong phenolate-to-copper(II) charge-transfer
transition at 390 nm (ꢀ ) 3400 M-1 cm-1), along with a
shoulder at 510 nm (ꢀ ) 2200 M-1 cm-1). The weak band
at 760 nm is associated with a d-d transition of a square-
based copper(II) center25 (Figure S7 in the Supporting
Information). Intraligand transitions are displayed at still
higher energies for 1 · 0.5 1,4-dioxane, 2, and 3 at 290 nm.
Magnetism. To extract information about the nature and
extent of magnetic exchange interaction between the two
metal centers, temperature-dependent (2-300 K) magnetic
susceptibility measurements on powder samples of 1·0.5 1,4-
dioxane, 2, and 3 were carried out. The values of MT at
room temperature are 4.80 (1·0.5 1,4-dioxane), 2.08 (2), and
0.43 cm3 mol-1 K (3). Because of their dinuclear nature,
the experimental data were modeled using the Hamiltonian,
with S1 ) S2 ) 3/2 (1 · 0.5 1,4-dioxane), 1 (2), and 1/2 (3). To
reduce the number of variables, D (zero-field splitting
parameter) and g values (gz ) gx ) gy) were considered to
be identical for the two metal ions.29 It should be mentioned,
however, that the local zero-field splitting corresponding to
the Hamiltonian term was not considered in the case of 3.
Satisfactory fits were obtained: g ) 2.27(1) and D ) 7.7(2)
cm-1, and J ) -6.1(2) cm-1 for 1 · 0.5 1,4-dioxane (Figure
S8 in the Supporting Information);29,30a g ) 2.15(1) and D
) 4.5(2) cm-1, and J ) -28.6(3) cm-1 for 2 (Figure S9 in
the Supporting Information);25,29,30b g ) 2.12(1) and J )
-359(2) cm-1 for 3 (Figure 3).23,25,26,29 Thus the dimetal(II)
centers in these complexes are antiferromagnetically coupled.
Redox Properties. To investigate the possibility of
observing metal-coordinated 2,4-ditert-butyl-phenolate ring
oxidation(s), the cyclic voltammograms (CV) of all four
complexes were recorded in CH2Cl2 at a scan rate of 100
mVs-1, using a platinum working electrode. Each complex
displays two quasireversible one-electron oxidative responses
Figure 2. A perspective view of the crystal structure of [CuII2(L)2] (3).
Only donor atoms are labeled. Hydrogen atoms are omitted for clarity.
parameters are comparable with other diphenoxo-bridged
nickel(II) complexes.24
[CuII (L)2] (3). The asymmetric unit contains two inde-
2
pendent molecules, and each molecule sits on a crystallo-
graphically imposed inversion center. The molecular structure
of [CuII (L)2] is shown in Figure 2. In contrast to the
2
structures of 1 · 0.5 1,4-dioxane and 2 · 2 diethyl ether, here
each copper(II) ion is in perfect square-pyramidal geometry
(τ value, 0.006),22 the ethylpyridine nitrogen N(2) acts as
the axial ligand, and the basal plane consists of three
phenolate oxygens O(1), O(2), and O(1)*, and an amine
nitrogen N(1). As a consequence, the Cu2O2 core is twisted
toward a butterfly shape [the dihedral angle between the two
planes passing through the atoms Cu(1)-O(1)-Cu(1)*, and
Cu(1)-O(1)*-Cu(1)* is 37.80(17)°], leading to a Cu(1) · · ·
Cu(1)* distance of 3.053(4) Å. The copper(II)-phenolate
distances are comparable to other diphenoxo-bridged
complexes.6d,f,8a,e,9,11,25,26
[ZnII (L)2] · 0.5 1,4-Dioxane (4 · 0.5 1,4-Dioxane). The
2
molecular structure of [ZnII (L)2] is shown in Figure S4 in
2
the Supporting Information. The structure is closely similar
to those of 1 · 0.5 1,4-dioxane. The Zn2O2 core is almost
planar with a dihedral angle [between the two planes passing
through the atoms Zn(1)-O(1)-Zn(2) and Zn(1)-O(2)-
Zn(2)] of 2.89(15)°. However, the bridge is slightly asym-
metric. The bonding parameters are closely similar to that
observed in 1·0.5 1,4-dioxane. The distance between the two
zinc(II) centers is 3.226(7) Å, which is similar to the other
phenoxo-bridged dinuclear zinc(II) complexes.8f,27 The
geometry around the two zinc(II) ions is distorted trigonal-
bipyramidal, with τ values of 0.66 for Zn(1) and 0.55 for
Zn(2).22
1
at E1/2 ) 0.57 (1 · 0.5 1,4-dioxane), 0.52 (2), 0.50 (3), and
Absorption Spectra. Absorption spectral analyses (below)
of 1 · 0.5 1,4-dioxane, 2, and 3 in CH2Cl2 clearly support the
presence of five-coordinate cobalt(II), nickel(II), and cop-
per(II). The electronic spectrum of 1·0.5 1,4-dioxane exhibits
2
0.58 (4 · 0.5 1,4-dioxane) and E1/2 ) 0.70 (1 · 0.5 1,4-
dioxane), 0.63 (2), 0.64 (3), and 0.75 (4 · 0.5 1,4-dioxane) V
vs SCE. The CV scans of 1·0.5 1,4-dioxane, 2, 3, and 4·0.5
1,4-dioxane are displayed in Figure S10 (Supporting Infor-
mation), Figure S11 (Supporting Information), Figure 4, and
Figure S12 (Supporting Information), respectively. Because
of the invariant nature of the E1/2 values we assign the
(24) Pal Chaudhuri, U.; Varghese, B.; Murthy, N. N. Acta Crystallogr.
2003, E59, m627–m629.
(25) Gupta, R.; Mukherjee, S.; Mukherjee, R J. Chem. Soc., Dalton Trans.
1999, 402, 5–4030.
(26) Mukherjee, R. Chapter on Copper. In ComprehensiVe Coodination
Chemistry-II: From Biology to Nanotechnology; McCleverty, J. A.,
Meyer, T. J., Fenton, D. E., Eds.; Elsevier/Pergamon: Amsterdam,
2004; pp 747-910.
(28) Du, M.; An, D.-L.; Guo, Y.-M.; Bu, X.-H. J. Mol. Struct. 2002, 641,
193–198.
(29) Details of magnetic behavior from the standpoint of magneto-structural
correlation will be published elsewhere.
(27) (a) Reglinski, J.; Morris, S.; Stevenson, D. E. Polyhedron 2002, 21,
2175–2182. (b) Matalobos, J. S.; Garcꢁa-Deibe, A. M.; Fondo, M.;
Navarro, D.; Bermejo, M. R. Inorg. Chem. Commun. 2004, 7, 311–
314.
(30) (a) Mishra, V.; Lloret, F.; Mukherjee, R. Inorg. Chim. Acta 2006,
359, 4053–4062. (b) Mishra, V.; Lloret, F.; Mukherjee, R. Eur.
J. Inorg. Chem. 2007, 2161–2170.
4476 Inorganic Chemistry, Vol. 47, No. 11, 2008

Diphenoxo-Bridged CoII, NiII, CuII, and ZnII Complexes
change of decay behavior was observed even at 223 K.
Notably, the E1/2 values of two consecutive oxidative
responses (above) and the energies of the HOMOs for 2 and
3 (Table 3) are similar, but the stability of their two-electron
oxidized product is different. We believe that comparatively
greater stability in the case of 3 may arise due to spin-pairing
between the unpaired electron on the phenoxyl radical and
that on the d9 metal center.10d Understandably, the presence
of an additional unpaired electron at each metal center in
the case of 2 cannot allow such a spin-pairing, which in turn
might have contributed to its instability.
The electronic spectrum of the oxidized species, generated
by electrolysis of CH2Cl2 solutions (containing 0.1 M TBAP
as supporting electrolyte) at 298 K, of 3 [applied potential:
0.90 V vs SCE, n ) 1.90, color change: reddish brown to
light yellowish brown] exhibits characteristic bands at 420
nm (ꢀ ) 1600 M-1 cm-1), and at 770 nm (ꢀ ) 470 M-1
cm-1) (Figure 5). This species was also generated in CH2Cl2
by chemical oxidation (AgOTf). The CV scan (starting
potential, 1.00 V) of such oxidized solutions display a
reductive response (cathodic peak potential, Epc ) 0.06 V
and a stripping peak at 0.36 V), which is typical for a
monomeric copper(II) species (Figure S15, Supporting
Information). 3 is EPR-silent due to strong antiferromagnetic
exchange coupling between the two copper(II) centers
mediated by the diphenoxo-bridge (above). However, the
X-band EPR spectrum of the product of two-electron
oxidation exhibits an axial signal at g| ) 2.27, g⊥ ) 2.05,
and A| ) 161 × 10-4 cm-1, which is typical for monomeric
copper(II) (Figure 5). No EPR signal centered at g ∼ 4,
associated with ∆Ms ) ( 1 transition typical of an S ) 1
system (ferromagnetic coupling between the phenoxyl radical
and the copper(II) center) was observed.
Figure 3. Plots of
vs T (left scale) and MT vs T (right scale) for a
powdered sample of 3. The solid lines represent the best theoretical fit,
M
described in the text.
Figure 4. Cyclic voltammogram (100 mV/s) of a ∼1.0 mM solution of 3
at a platinum electrode in CH2Cl2 (0.1 M in TBAP). Indicated potentials
(in V) are vs SCE.
oxidative responses presumably due to a successive oxidation
of coordinated 2,4-ditert-butyl-phenolate rings into phenoxyl
radicals (ligand-centered oxidations, below).
On the basis of the above observations, we speculate that
after two-electron oxidation, 3 is converted to a monomeric
CuII-phenolate-phenoxyl radical species, which instanta-
neously abstracts the hydrogen atom8d (we are not in a
position to identify the source; however, a solvent or trace
of moisture present in solvent is a possibility), and forms a
monomeric CuII-phenolate-phenol species.1i,5a,6g It should
be mentioned here that the coordinated ligand remained intact
during the course of this reaction, as judged by mass (Figure
Nature of Two-Electron Oxidized Species. Coulometri-
cally generated two-electron oxidized species of 1 · 0.5 1,4-
dioxane and 2 are not stable enough to allow their redox
(lack of well-defined redox response) and spectroscopic
(featureless UV-vis spectra and lack of EPR signal)
characterization. Whether or not there is a possibility of
formation of monomeric/dimeric CoIII-phenolate/phenoxyl
radical complex,7g we attempted to synthesize such a
compound.31 To extract information about the stability of
oxidized species of 3 and 4 · 0.5 1,4-dioxane in CH2Cl2, we
investigated the time course of spectral change during decay.
The results are displayed in Figure S13 (Supporting Informa-
tion) and Figure S14 (Supporting Information), respectively.
1
S16, Supporting Information) and H NMR (Figure S17,
Supporting Information) spectra of the extracted organic
component after removal of copper by aqueous ammonia
from such solutions. Notably, addition of 2 equiv of Et3N to
chemically/coulometrically oxidized solutions of 3 quanti-
tatively regenerates 3 (below), implying the presence of an
acidic proton in such a copper(II) complex. If our proposition
is right, then addition of 2 equiv of HClO4 to 3 is expected
to generate a CuII-phenolate-phenol species, as before. In
fact, exactly such a reaction sequence was implicated by an
optical titration. Figure 6 displays such an optical titration
at 298 K of dimeric 3 with 2 equiv of HClO4, followed by
the addition of 2 equiv of Et3N and regeneration of dimer 3.
This result strengthens our proposition of hydrogen-atom
abstraction and the existence of a monomeric CuII-phenolate-
phenol complex (Scheme 1). The CuII-phenolate-phenol
Therefore, two-electron oxidized solutions of 3 (kdecay
)
3.6821 × 10-1 min-1; t1/2 ≈ 1.88 min) and 4·0.5 1,4-dioxane
(kdecay ) 3.1042 × 10-1 min-1; t1/2 ≈ 2.23 min) allowed us
to investigate the nature of oxidized species in reasonable
detail (below). Similar values were reported for monomeric
copper(II) complexes with a N2O2 donor set.6d Notably, no
(31) A mixed-ligand complex of composition [CoIII(L)(acac)] (acac )
acetylacetonate anion) has been isolated. This complex allows cou-
lometric generation of a CoIII-phenoxyl radical species. Details of
this chemistry will be published elsewhere.
Inorganic Chemistry, Vol. 47, No. 11, 2008 4477

Mukherjee et al.
Table 3. Results of DFT Calculation
Composition of HOMO
contribution (%)
from 2,4-ditert-butyl
phenolate ring
energy of HOMO
total contribution (%)
from ligand
contribution (%)
from metal ion
complexes
(kcal/mol)
[CoII2(L)2] (1)
[NiII2(L)2] (2)
[CuII2(L)2] (3)
[ZnII2(L)2] (4)
-101.15
-104.16
-105.00
-91.85
48
52
37
56
91
93
90
94
6
3
6
4
complex is expected to exhibit higher6d,e CuII-CuI redox
potential (cathodic peak potential, Epc ) 0.06 V vs SCE;
Figure S15, Supporting Information) than that of the mon-
omeric complex [CuII(L)(DMF)] [Epc ) -1.08 V vs SCE].32
The species responsible for the reductive CV scan (Figure
S15, Supporting Information), and EPR spectra (Figure 5)
of coulometrically oxidized CH2Cl2 solutions is thus of the
CuII-phenolate-phenol complex.
The electronic spectrum of the oxidized species of 4 · 0.5
1,4-dioxane [applied potential, 1.00 V vs SCE, n ) 1.95;
color change, colorless to orangish yellow] exhibits a band
at 430 nm (ꢀ ) 700 M-1 cm-1) attributed to the π-π*
transition of a phenoxyl radical and another absorption at
840 nm (ꢀ ) 60 M-1 cm-1) (Figure 7). The transition at
840 nm could arise from ligand-based transitions and/or a
ligand f ligand charge-transfer (LLCT) transition.10b After
oxidation, its dimeric structure is retained, as such solutions
display two reductive responses at the same potentials as
those observed for 4·0.5 1,4-dioxane (Figure S15, Supporting
Information). Notably, such solutions in its X-band EPR
spectrum exhibit an isotropic signal at g ) 2.006, typical
for phenoxyl radical species (Figure 7). The interaction
between the two phenoxyl moieties is thus very weak,
meaning thereby the localization of the radicals.6h So, from
the spectral feature of two-electron oxidized species of 4·0.5
1,4-dioxane, we confirm that the observed redox responses
are due to ZnII-coordinated ligand-centered oxidations. It is
worth noting here that contrary to that observed for 4 · 0.5
1,4-dioxane, after two-electron oxidation of 3 the generated
copper(II)-phenolate-phenoxyl radical species instanta-
neously rearranges to copper(II)-phenolate-phenol com-
plex. The reason behind differential stability of two oxidized
species generated from 3 and 4·0.5 1,4-dioxane must be due
to the relative stability of one species over another [cf. Figure
S13 and Figure S14, Supporting Information; t1/2 (3) ≈ 1.88
min and t1/2 (4 · 0.5 1,4-dioxane) ≈ 2.23 min].
Figure 5. (a) Absorption spectra (CH2Cl2 solution) of (i) 3 and (ii) its
coulometrically generated two-electron oxidized species. The ε values are
based on dimer composition; (b) X-band EPR spectrum (CH2Cl2, 120 K)
of coulometrically generated two-electron oxidized species of 3.
Computational Results. To get an idea about the site of
oxidation, we performed single-point DFT calculations at
the B3LYP level of theory on all four complexes using their
X-ray crystallographic coordinates. In each case, analysis of
the HOMO reveals that it is predominantly contributed by
ligand orbitals. The maximum contribution comes from the
2,4-ditert-butyl-phenolate part of the ligand, and there is very
little contribution of the metal center. The HOMO of each
complex is delocalized over the tert-butyl phenol ring of the
Figure 6. Titration of 3 (0.4 mM in CH2Cl2) by H+ (HClO4). Arrows
indicate the direction of absorption change with increasing H+ ion
concentration: (i) initial solution of 3 and (ii)-(iv) after the addition of
0.2, 0.5, and 2.0 equiv of HClO4, respectively. Titration of the CuII-
phenolate-phenol species by Et3N is displayed as an inset. Arrows indicate
the direction of absorption change with increasing Et3N concentration: (i)
initial solution and (ii)-(iv) after the addition of 0.2, 0.5, and 2.0 equiv of
Et3N, respectively.
(32) A mononuclear copper(II) complex of composition [CuII(L)(DMF)]
has been isolated. Characterization has been done by mass spectral
analysis at m/z ) 581.09 [Calcd for (C32H43N3CuO3 + 1)+ ) 581.50].
UV-vis [λ, nm (ε, M-1 cm-1), in DMF]: 450 (1650) and 750 (200).
EPR (solid, 298 K): g| ) 2.27, g⊥ ) 2.07, and A| ) 174 × 10-4
cm-1. Details of this chemistry will be published elsewhere.
4478 Inorganic Chemistry, Vol. 47, No. 11, 2008

Diphenoxo-Bridged CoII, NiII, CuII, and ZnII Complexes
Scheme 1. Coulometrically-generated phenoxyl radical-coordinated dicopper(II) complex transforms to a monomeric complex with instantaneous
abstraction of hydrogen atoms. The trace amount of water present in CH2Cl2 might provide fifth coordination to the copper(II) center
ligand, with the contribution from the phenolic oxygen atom.
The results for 1, 2, 3, and 4 are in Figure S18 (Supporting
Information), Figure 8, and Figure S19 (Supporting Informa-
tion), respectively. Quantitative evaluations of the atomic
orbital contributions to the relevant HOMOs and the energy
of the HOMOs are summarized in Table 3.
groups (one unsubstituted and the other with 2,4-ditert-butyl
substituents), an ethylpyridine unit, and an aliphatic amine]
and different metal ions to synthesize four diphenoxo-bridged
dimetal(II) complexes, with each metal center having a N2O3
coordination environment. Comparing the structures of the
complexes, we found that cobalt(II), nickel(II), and zinc(II)
have distorted trigonal-bipyramidal geometry, and copper(II)
has perfect square-pyramidal geometry. Each ligand provides
a phenoxo bridge by its unsubstituted phenolate group. The
successful preparation and systematic investigation of prop-
erties of four closely similar complexes provide a valuable
source of information for the understanding of their struc-
tures, magnetic properties, and redox behavior (each complex
exhibits two successive oxidations at closely similar poten-
tials). Out of the two phenolate groups present in the ligand,
the one with the 2,4-ditert-butyl substituent is oxidized, as
suggested by the composition of HOMOs of the complexes
(DFT calculations at the B3LYP level of theory). In essence,
the present study gives a lot of insights into the properties
Summary and Concluding Remarks
From the perspective of recent emergence of free radical
enzymology in this article, we successfully combined the
merits of a new tripodal ligand [comprising two phenol
Figure 7. (a) Absorption spectra (CH2Cl2 solution) of (i) 4·0.5 1,4-dioxane,
and (ii) its coulometrically generated two-electron oxidized species. The
enlarged feature between 600-1000 nm is shown as an inset. (b) X-band
EPR spectrum (CH2Cl2, 120 K) of coulometrically generated two-electron
oxidized species of 4·0.5 1,4-dioxane.
Figure 8. Contour plots of the HOMO of (a) 2 and (b) 3.
Inorganic Chemistry, Vol. 47, No. 11, 2008 4479

Mukherjee et al.
of metal-coordinated phenoxyl radicals that play an important
role not only in the enzymatic systems but also in the
catalytic processes in several organic reactions. Efforts are
underway to stabilize metal-coordinated phenoxyl radical
complexes in general and a NiII-phenoxyl radical species
in particular.
Kanpur) for her help in DFT calculations. Comments of the
reviewers were very helpful at the revision stage.
1
Supporting Information Available: H NMR spectra of H2L
and 4·0.5 1,4-dioxane in CDCl3; perspective views of 2·2 diethyl
ether and 4·0.5 1,4-dioxane; absorption spectra of 1·0.5 1,4-
dioxane, 2, and 3 in CH2Cl2; plot of MT vs T for 1·0.5 1,4-dioxane,
2; CV scans in CH2Cl2 of 1·0.5 1,4-dioxane, 2, and 4·0.5 1,4-
dioxane; time-course of spectral change of the two-electron oxidized
species of 3, and 4·0.5 1,4-dioxane in CH2Cl2; CV scans of
coulometrically generated two-electron oxidized species of 3 and
4·0.5 1,4-dioxane in CH2Cl2; mass, and 1H NMR in CDCl3 of two-
electron oxidized species of 3, after removal of copper by aqueous
ammonia; HOMO and HOMO-1 contour plots of 1, and 4. CIF
files for 1·0.5 1,4-dioxane, 2·2 diethyl ether, 3, and 4·0.5 1,4-
dioxane. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. This work is supported by grants from
the Council of Scientific and Industerial Research, and
Department of Science and Technology, Government of
India, New Delhi. R.M. sincerely thanks Prof. Dan Stack
(Department of Chemistry, Stanford University) for arousing
his interest in DFT calculation during the author’s stay
(sabbatical leave 2005-2006) in the laboratory of Prof. Stack.
R.M. also wishes to express his thanks to Dr. Tim Storr and
Mr. Pratik Verma (Stanford University). A.M. sincerely thanks
Ms. Nabanita Sadhu Khan (Department of Chemistry, IIT
IC701283B
4480 Inorganic Chemistry, Vol. 47, No. 11, 2008