Phenolate and phenoxyl radical complexes of Co(II) and Co(III)

Article abstract of DOI:10.1039/b410934a

The new phenol-imidazole pro-ligands ^RLH react with Co(BF ₄)₂·6H₂O in the presence of Et ₃N to form the corresponding [Co^II(^RL) ₂] compound (R = Ph (1), PhOMe (2), or

Full text of DOI:10.1039/b410934a

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F U L L P A P E R
Phenolate and phenoxyl radical complexes of Co(II) and Co(III)†
Laurent Benisvy,*a Eckhard Bill,b Alexander J. Blake,a David Collison,c E. Stephen Davies,a
C. David Garner,*a Christina I. Guindy,a Eric J. L. McInnes,c Graeme McArdle,a
Jonathan McMaster,*a Claire Wilsona and Joanna Wolowskac
a
School of Chemistry, The University of Nottingham, University Park, Nottingham,
UK NG7 2RD. E-mail: Dave.Garner@nottingham.ac.uk
Max-Planck-Instut für Bioanorganische Chemie, Stiftstrasse 34-46, Mülheim an der Ruhr,
b
D-45470, Germany
c
Department of Chemistry, The University of Manchester, Oxford Road, Manchester,
UK M13 9PL
Received 19th July 2004, Accepted 14th September 2004
First published as an Advance Article on the web 4th October 2004
The new phenol–imidazole pro-ligands RLH react with Co(BF4)2·6H2O in the presence of Et3N to form the
corresponding [CoII(RL)2] compound (R = Ph (1), PhOMe (2), or Bz (3)). Also, BzLH, reacts with Co(II) in the
presence of Et3N and H2O2 to form [CoIII(BzL)3] (4). The structures of 1·2.5MeCN, 2·2DMF, 3·4MeOH, and
4·4DMF have been determined by X-ray crystallography. 1, 2, and 3 each involve Co(II) bound to two N,O-bidentate
ligands with a distorted tetrahedral coordination sphere; 4 involves Co(III) bound to three N,O-bidentate ligands in a
mer-N3O3 distorted octahedral geometry. [CoII(RL)2] (R = Ph or PhOMe) undergo two, one-electron, oxidations. The
products of the first oxidation, [1]+ and [2]+, have been synthesised by the chemical oxidation of 1 and 2, respectively;
these cations, formulated as [CoII(RL)(RL)2]+, comprise one phenoxyl radical and one phenolate ligand bound to
Co(II) and are the first phenoxyl radical ligand complexes of tetra-coordinated Co(II). 4 undergoes two, one-electron,
ligand-based oxidations, the first of which produces [4]+, [CoIII(BzL)(BzL)2]+. Unlike [1]+ and [2]+, product of the one-
electron oxidation of [CoII(BzL)2], [3]+, is unstable and decomposes to produce [4]+. These studies have demonstrated
that the chemical properties of [MII(RL)(RL)2]+ (M = Co, Cu, Zn) are highly dependent on the nature of both the
ligand and the metal centre.
Introduction
There is much current interest in the nature and proper-
ties of d-transition metal complexes in which one or more
of the ligands is present in a radical state.1 This interest has
been stimulated by spectroscopic studies of enzymes such as
galactose oxidase (GAO),2 glyoxal oxidase (GLO),3 and ribo-
nucleotide reductase (RNR),4 that have shown the catalytic
action to involve formation of a phenoxyl radical, either
bound (GAO, GLO) or immediately adjacent (RNR), to a
redox-active metal centre. Also, metal complexes, in which one
(or more) of the ligands is a radical, possess novel physical
properties.5 Previously, we have shown that representatives of a
family of pro-ligands (RLH), each of which is comprised of an
imidazole covalently bonded to an o,p-di-tert-butyl-substituted
Scheme 1
PhOMeLH. (2-(2-(4,6-Di-tert-butyl-2-hydroxyphenyl)-4,5-
bis-(p-methoxyphenyl)imidazole). A mixture of 3,5-di-tert-
butylhydroxybenzaldehyde (1.0 g, 8.8 mmol), 4,4-dimethoxy-
benzil (1.2 g, 8.6 mmol), and ammonium acetate (2.6 g,
32.0 mmol) were refluxed in glacial acetic acid (15 cm3) until
a precipitate formed (2–3 days). The reaction mixture was
cooled to room temperature and H2O (ca. 30 cm3) was added
to complete the precipitation. The yellow/green solid was
collected by filtration, washed with H2O (3 × 10 cm3), dis-
solved in CH2Cl2 and the solution was dried over anhydrous
MgSO4. The MgSO4 was removed by filtration and the filtrate
evaporated to dryness. The resultant solid was dissolved in the
minimum volume of CH2Cl2 and purified by flash chromato-
graphy on a silica gel column using CH2Cl2–pentane (6:4) as
the eluant. Evaporation of the solvent produced a pale yellow
powder; recrystallisation of this powder by the addition of
hexane to a CH2Cl2 solution gave PhOMeLH as a white crystal-
line solid. Yield 2.0 g (45%). Elemental analysis: Calc. for
C31H36N2O3: C 76.86, H 7.44, N 5.78; Found C 76.69, H 7.75,
N 5.72%. 1H-NMR (300 MHz, CDCl3) d 1.47 (s, 9H, tBu), 1.62
(s, 9H, tBu), 3.83 (s, 3H, OMe), 3.88 (s, 3H, OMe), 6.86 (d, 2H,
J 9.4 Hz, ArOMeH), 6.97 (d, 2H, J 9.4 Hz, ArOMeH), 7.23 (d, 1H,
phenol (Scheme 1), can exist as a phenoxyl radical—as both
[RLH]+ and [MII(RL)(RL)]+ (M = Cu or Zn).6 Herein, we report
new developments of the coordination chemistry of this family
of pro-ligands, RLH, for R = Ph, PhOMe, or Bz, with cobalt,
including the synthesis and characterisation of Co(II) and
Co(III) phenoxyl radical complexes.
Experimental
Unless stated otherwise, all manipulations were carried out at
ambient temperature under an atmosphere of Ar using standard
Schlenk techniques. Anhydrous MeOH, over molecular sieves,
and Co(BF4)2·6H2O were purchased from Aldrich Chemical
Company and used as received. Dry CH2Cl2, for both syntheses
and electrochemical experiments, was freshly distilled under
N2 from CaH2.
R
Syntheses of LH (R = Ph, PhOMe, or Bz)
The synthesis of PhLH has been reported previously.6
† Electronic supplementary information (ESI) available: Figs. SI1–SI4
and Table SI1. See http://www.rsc.org/suppdata/dt/b4/b410934a/
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 3 6 4 7 – 3 6 5 3
3 6 4 7

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J 2.5 Hz, ArOHH), 7.34 (d, 1H, J 2.5 Hz, ArOHH), 7.43 (d, 2H,
J 9.4 Hz, ArOMeH), 7.59 (d, 2H, J 9.4 Hz, ArOMeH), 9.12 (s, 1H,
OH/NH), 13.2 (s, 1H, OH/NH). 13C-NMR (75 MHz, CDCl3): d
29.54, 31.64, 34.32, 35.26, 55.28, 111.68, 113.88, 114.32, 117.30,
123.11, 125.06, 126.39, 128.38, 129.74, 134.54, 137.35, 140.25,
146.27, 154.39, 158.72, 159.47. M.p. 245 °C. Positive FAB-MS:
m/z 485 {(M + 1)+}. UV/vis (CH2Cl2) kmax/nm (e/dm3 mol−1 cm−1):
232 (26180), 285 (27180), 323 (26175).
as described for 1. The resultant red solution was stored
at −30 °C for one week and an orange crystalline powder
precipitated. This solid was collected by filtration, washed
(MeOH), crushed and dried in vacuo to afford 2·2MeOH as an
orange powder. Yield: 0.090 g (80%). Elemental analysis: Calc.
for C62H68N4O6Co·2MeOH: C 70.50, H 7.21, N 5.14; Found:
C 70.11, H 6.86, N 5.20. Positive FAB-MS: m/z 1025 {(M)+}.
UV/vis (CH2Cl2): kmax/nm (e/dm3 mol−1 cm−1): 430–440 sh
(649); 531 (213), ca.586 (136), ca. 900 (40).
BzLH.
(2-(3,5-Di-tert-butyl-2-hydroxyphenyl)-benzim-
Evaporation of a CH2Cl2/DMF (9:1) solution of 2·2MeOH
afforded red, tablet-like, single crystals of 2·2DMF.
idazole). 3,5-Di-tert-butyl-salicylic acid (10.00 g, 40.0 mmol),
phenol (4.14 g, 44.0 mmol), and N,N-dicyclohexylcarbodiimide
(11.87 g, 57.5 mmol) were dissolved in dry THF (240 cm3)
and stirred under N2 for 4 days. The precipitate was removed
from the mixture by filtration and washed with diethyl ether
(3 × 10 cm3) and the washings added to the filtrate. The
volatile components were removed from the filtrate on a
rotary evaporator and then in vacuo, to yield a peach coloured
oil. Flash chromatography on silica gel, using pentane as the
eluant, followed by evaporation of the solvent, produced
phenyl-3,5-di-tert-butyl-2-hydroxybenzoate as a white powder.
Yield 5.39 g (41.3%). Elemental analysis: Calc. for C21H26O3: C
[Co(BzL)2] (3). A solution of BzLH (0.100 g, 0.31 mmol) in
MeOH (10 cm3) was mixed with a solution of Co(BF4)2·6H2O
(0.053 g, 0.16 mmol) in MeOH (2 cm3), Et3N was then added
as described for 1. The resultant red solution was stored
at −30 °C for one week and an orange crystalline powder
precipitated. This solid was collected by filtration, washed
(MeOH), crushed and dried in vacuo to afford 3·2MeOH as a
red–brown powder. Yield: 0.089 g (75%). Elemental analysis:
Calc. for C42H50N4O2Co·2MeOH: C 69.00, H 7.63, N 7.32;
Found: C 69.33, H 7.00, N 7.40. Positive FAB-MS: m/z 702
{(M)+}. UV/vis (CH2Cl2): kmax/nm (e/dm3 mol−1 cm−1): 436 sh
(400), 517 (96), 569 (92), 589 (96), ca. 900(41).
1
77.27, H 8.03; Found C 76.43, H 8.12%. H-NMR (300 MHz,
t
t
CDCl3) d 1.34 (s, 9H, Bu), 1.44 (s, 9H, Bu), 7.20 (d, 2H, J
7.5 Hz, Ar–H), 7.30 (t, 1H, J 7.5 Hz, Ar–H), 7.45 (t, 2H, J
7.5 Hz, Ar–H), 7.61 (d, 1H, J 2.5 Hz, Ar–H), 7.94 (d, 1H,
J 2.5 Hz, Ar–H), 11.11 (s, 1H, –OH). M.p. 84–85 °C. Positive
EI-MS: m/z 326 {M+}.
Purple, tablet-like crystals of 3·4MeOH were obtained
on storing a solution of the compound, saturated at room
temperature in MeOH at −30 °C.
Phenyl-3,5-di-tert-butyl-2-hydroxybenzoate
(4.61 g,
[Co(BzL)3] (4). Addition of a solution of Co(BF4)2·6H2O
(0.171 g, 0.50 mmol) in MeOH (10 cm3) to BzLH (0.500 g,
1.50 mmol) dissolved in MeOH (15 cm3) produced a pink solu-
tion. Addition of a few drops of Et3N and then ca. 5 cm3 of 9%
w/v H2O2 gave a brown solution. This solution was maintained
at room temperature, stirred for 15 min and then allowed to
stand for ca. 2 h, whence brown crystals were formed. The solid
was collected by filtration, crushed, and dried in vacuo to afford
a dark brown powder. Yield: 0.283 g (55%). Elemental analysis:
Calc. for C63H75N6O3Co: C 73.95, H 7.39, N 8.22; Found:
C 74.06, H 7.10, N 8.48. Positive FAB-MS: m/z 1022{M+}.
14.1 mmol) and 1,2-phenylenediamine (1.53 g, 14.2 mmol)
were refluxed for 3 h and the mixture then allowed to cool to
room temperature. The resulting brown/black solid was dis-
solved in the minimum volume of CH2Cl2 and purified by flash
chromatography on a silica gel column, using a CH2Cl2 :hexane
(2:3) as the eluant; evaporation of the solvent yielded a cream
coloured solid. This solid was dissolved in CH2Cl2 and diffusion
of hexane led to the precipitation of a white powder that was
collected by filtration, washed with hexane (3 × 10 cm3), and
dried in vacuo to give BzLH as a fluffy white solid. Yield 3.96 g
(87%). Elemental analysis: Calc. for C21H26N2O: C 78.23, H 8.13,
N 8.69; Found C 77.91, H 8.13, N 8.78%. 1H-NMR (300 MHz,
CDCl3) d 1.30 (s, 9H, tBu), 1.44 (s, 9H, tBu), 7.21 (m, 4H, Bzim),
7.37 (d, 1H, J 2.5 Hz, Ar–H), 7.44 (d, 1H, J 2.5 Hz, Ar–H),
7.66 (s, 1H, OH/NH) and 10.04 (s, 1H, OH/NH). 13C-NMR
(75 MHz, CDCl3) d 29.53, 31.58, 34.36, 35.33, 111.13, 118.67,
123.1, 126.86, 137.83, 140.53, 152.52, 155.90. M.p. 215–219 °C.
Positive EI-MS: m/z 322 {M+}. UV/vis (CH2Cl2) containing
[nBu4N][BF4] (0.4 M)) kmax/nm (e/dm3 mol−1 cm−1): 222 (42300),
242 (17400), 250 (11200), 266 (9100), 278 (12200), 290 (17700),
298 (25400), 325 (21200).
t
1H-NMR (300 MHz, CDCl3): d 0.61 (s, 9H, Bu,), 0.73 (s, 9H,
tBu), 1.03 (s, 9H, tBu), 1.09 (s, 9H, tBu), 1.16 (s, 9H, tBu), 1.23 (s,
9H, tBu), 6.31–7.62 (m, 18H, ArH and BzH), 10.07 (s, 1H, NH),
10.32 (s, 1H, NH), 10.73 (s, 1H, NH). UV/vis (CH2Cl2): kmax/nm
(e/dm3 mol−1 cm−1): 473 sh (1860), 610 (490), 714 nm (310).
Slow evaporation of a DMF:CH2Cl2 (9:1) solution of this
solid yielded brown, block-like crystals of 4·4DMF.
Syntheses of [1][BF4] and [2][PF6]
[1][BF4]. A solution of 1·2MeOH (0.250 g, 0.26 mmol) in dry
CH2Cl2 (40 cm3) was transferred into a pre-cooled (−10 °C)
stirred suspension of Ag[BF4] (0.054 g, 0.28 mmol) in dry
CH2Cl2 (5 cm3) that was protected from light. An intensely
coloured green solution resulted and a silver mirror was depo-
sited on the sides of the reaction vessel. The reaction mixture
was stirred at −10 °C for 1 h, then allowed to warm to room
temperature and stirred for a further 10 min. Filtration, followed
by evaporation of the solvent, gave a dark green solid. This
solid was dissolved in a minimum volume of CH2Cl2; diffusion
of pentane to this solution at −40 °C, followed decantation
of the mother liquor from the precipitate, afforded [1][BF4] as
a dark green solid. Yield: 0.200 g (79%). Elemental analysis:
Calc. for C58H62N4O2CoBF4: C 70.93, H 6.36, N 5.71; Found:
C 70.18, H 6.38, N 5.39. Positive FAB-MS ion at m/z 905{M+}.
UV/vis (CH2Cl2): kmax/nm (e/dm3 mol−1 cm−1): 232(70,800),
291(40,900), 350(30,000), 529(200), 583(100), 450(500).
Syntheses of [Co(RL)2]
[Co(PhL)2] (1). Addition of PhLH (0.100 g, 0.24 mmol) in
MeOH (10 cm3) to a stirred solution of Co(BF4)2·6H2O (0.040 g,
0.12 mmol) in MeOH (2 cm3) produced an orange solution.
Et3N was then added dropwise until a red coloration persisted.
This solution was stored at −30 °C for one week and an orange
crystalline powder precipitated. The product was collected by
filtration, washed (MeOH), and then dried in vacuo to afford
1·2MeOH as an orange solid. Yield: 0.092 g (81%). Elemental
analysis: Calc. for C58H62N4O2Co·2MeOH: C 74.30, H 7.27, N
5.78; Found: C 74.54, H 6.94, N 5.83. Positive FAB-MS: m/z
906 {(M + H)+}. UV/vis (CH2Cl2): kmax/nm (e/dm3 mol−1 cm−1):
420–440 sh (747), 533 (171), 587 sh (103), ca. 900 (29).
Evaporation of a solution of 1·2MeOH in MeCN afforded
pink, rectangular, block-like, single crystals of 1·2.5MeCN.
[2][PF6]. 2·2MeOH (0.555 g, 0.51 mmol) in dry CH2Cl2
(40 cm3) and Ag[PF6] (0.137 g, 0.54 mmol) in dry CH2Cl2 (5 cm3)
and were reacted as described for [1][BF4]. The resultant dark
green solid was washed with Et2O, to remove the unreacted 2
[Co(PhOMeL)2] (2). A solution of PhOMeLH (0.100 g, 0.21 mmol)
in MeOH (10 cm3) was added to a solution of Co(BF4)2·6H2O
(0.035 g, 0.10 mmol) in MeOH (5 cm3), Et3N was then added,
3 6 4 8
D a l t o n T r a n s . , 2 0 0 4 , 3 6 4 7 – 3 6 5 3

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and then recrystallised by the slow diffusion of pentane into a
CH2Cl2 solution at −40 °C; this yielded a dark green crystal-
line powder. The mother liquor was removed and the powder
dried in vacuo to give [2][PF6]. Yield: 0.416 g (70%). Positive
FAB-MS ion at m/z 1026 {(M + H)+}. Elemental analysis: Calc.
for C62H70N4O6CoPF6: C 63.58, H 6.02, N 4.79; Found: C 63.42,
H 5.90, N 4.71. Positive FAB-MS ion at m/z 1026{(M + H)+}.
UV/vis: kmax/nm (e/dm3 mol−1 cm−1): 390(11330), 409 sh (10200),
589 nm (8760), residual absorption 750–900 nm (ca. 4000).
methods using SHELXS 97.8 Each structure was refined against
F2 using SHELXL 97.8 Unless otherwise stated, all non-H
atoms were refined with anisotropic atomic displacement para-
meters (adps). The hydrogen atoms of: CH3CN, CH3OH, and
(CH3)2NCHO, were located from difference Fourier syntheses
and their positions refined as part of a rigid rotor. All other
H atoms were placed in geometrically calculated positions and
refined as part of a riding model, with U(H)iso = 1.2Ueq(C),
for those of C6H5 and U(H)iso = 1.5Ueq(C), for those of CH3.
t
For 1·2.5MeCN, a disordered Bu group was modeled over
two sites with occupancies of 0.7 and 0.3; suitable geometric
restraints were applied and the C atoms were refined isotropi-
cally. Restraints were also applied to each MeCN molecule; the
position of one of which is half-occupied. For 2·2DMF, one
disordered tBu group was modeled over two half-occupied sites,
with suitable restraints applied, and the C-atoms were refined
isotropically. For 3·4MeOH, the disorder of one oxygen atom
of one MeOH molecule was modeled over two sites (O4s and
O4s) with occupancies of 0.4 and 0.6, respectively; C4s, O4s,
and O4s were refined with isotropic adps—their H atoms and
those of C3s could not be located. For 4·4DMF, one disordered
DMF molecule was modeled over two sites with occupancies of
0.7 and 0.3; O1SC and N1SC are common to both components
and the C, N and O atoms were refined isotropically and
restraints applied to their C–O and N–C distances. The hydrogen
atoms of this DMF molecule were not located.
Physical methods
Elemental analyses of the compounds isolated in these studies
were accomplished by the Microanalytical Laboratory of
the School of Chemistry, of the University of Nottingham.
FAB and EI mass spectra were recorded on a Fisons VG Trio
200 or a Fisons VG Autospec spectrometer, NMR spectra
were recorded on a Bruker DPX300 NMR spectrometer, and
UV/vis spectra were recorded on a Perkin Elmer Lambda 5
spectrophotometer.
The cyclic voltammogram of each compound (1 mM) in
CH2Cl2, containing [NBun ][BF4] (0.4 M) as the background
4
electrolyte, was recorded at room temperature using a glassy
carbon or Pt wire working electrode, a Pt wire secondary
electrode, and a saturated calomel reference electrode. The
cyclic voltammogram of each compound was referenced to
the [Fc]+/[Fc] couple that was used as the internal standard.
When necessary, to avoid overlapping redox couples, the
[FeCp*2]+/[FeCp*2] couple was used as the internal reference
and the potentials of the redox process(es) observed were
referenced to that of the [Fc]+/[Fc] couple by an independent
calibration (DE1/2, [Fc]+/[Fc] vs. [FeCp*2]+/[FeCp*2], = 0.526 V).
All coulometric measurements were performed at 273 K in
CCDC reference numbers 227296–227299.
See http://www.rsc.org/suppdata/dt/b4/b410934a/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
Nature of [Co(RL)2] (1, 2, and 3) and [Co(BzL)3] (4)
a CH2Cl2 solution containing [NBun ][BF4] (0.4 M); the cell
4
The pro-ligand PhOMeLH was synthesised in a similar manner
to that6 for PhLH by the condensation of 3,5-di-tert-butyl-
hydroxybenzaldehyde with 4,4-dimethoxybenzil (1:1) in
AcOH in the presence of an excess of [NH4][OAc]. In con-
trast, BzLH, was prepared by the condensation of the phenyl
ester derivative of the 3,5-di-tert-butyl-salicylic acid and 1,2-
phenylenediamine.
consisted of a Pt/Rh gauze basket working electrode, a Pt/Rh
gauze secondary electrode, and a saturated calomel reference
electrode. Both CV and CPE measurements were performed
using an Autolab PGSTAT20 potentiostat.
UV/vis spectroelectrochemistry was accomplished for
CH2Cl2 solutions containing [NBun ][BF4] (0.4 M) in an
4
Co(BF4)2·6H2O reacted with each of the pro-ligands RLH
(1:2) in the presence of Et3N to form the corresponding
[Co(RL)2] compound (Scheme 1; R = Ph, 1; R = PhOMe 2;
R = Bz, 3). Co(BF4)2·6H2O reacts with BzLH (1:3) and H2O2
in the presence of Et3N to form [Co(BzL)3] (4).
1·2.5MeCN, 2·2DMF, 3·4MeOH, and 4·4DMF have been
characterised by X-ray crystallography (Fig. 1, Table 1). In
each compound solvent molecules form hydrogen bonds to
the phenolate and/or imidazole groups of the ligands. The
atom possessing a lone pair of electrons (MeCN, MeOH,
or DMF(O)) forms an O(N)H–N hydrogen bond to the
imidazole N–H group; also, for 3·4MeOH, MeOH forms a
hydrogen bond to one of the coordinated phenolate oxygen
atoms (Table SI1, see ESI†).
In 1·2.5MeCN, 2·2DMF, and 3·4MeOH the two N,O-
bidentate ligands form a distorted tetrahedral N2O2-coordi-
nation sphere about the Co (Fig. 1); the angle between the
Co(1)N(5)O(1) and Co(1)N(5A)O(1A) planes is 90 ± 5°. The
lengths of the C–C, C–N, and C–O bonds in 1·2.5MeCN,
2·2DMF, 3·4MeOH, and 4·4DMF correspond to each ligand
being present as the L− anion.6 In particular, the length of
the C–O bond of each ligand (1.317(5)–1.337(6) Å) is charac-
teristic of a phenolate and is distinct from that (1.264(5) Å)
of the phenoxyl radical PhL in [CuII(PhL)(PhL)][BF4];6 and
those of other structurally characterised phenoxyl radical
complexes.9 The lengths (Table 2) of the Co–O (1.905(3)–
1.938(4) Å) and Co–N (1.960(4)–1.998(2) Å) bonds of
1·2.5MeCN, 2·2DMF, and 3·4MeOH are in agreement with
those of the corresponding bonds of other10,11 [CoII(NO)2]
complexes, e.g. [CoII(L)2] (LH = N-ethyl-2-(2-hydroxy-3-
methylphenyl)–benzimidazole) with Co–O of 1.922(8) Å and
optically transparent electrochemical (OTE) cell (modified
quartz cuvette, optical pathlength 0.5 mm). A three-electrode
configuration was used in the cell, comprising a Pt/Rh gauze
working electrode, a Pt wire secondary electrode contained in
a fritted PTFE sleeve, and a saturated calomel electrode that
was isolated from the test solution by a bridge tube containing
the electrolyte solution retained by a porous frit. The potential
at the working electrode was controlled by a Sycopel Scientific
Ltd. DD10M potentiostat. UV/vis spectra were recorded on
a Perkin Elmer Lambda 16 spectrophotometer; during the
measurements the spectrometer cavity was purged with N2
and temperature control at the sample was achieved by flowing
cooled N2 across the surface of the cell.
EPR spectra were recorded on a Bruker X-band EMX
spectrometer. Magnetic fields and microwave frequencies were
calibrated with a Bruker ER035M NMR gaussmeter and an
EIP microwave counter, respectively.
Magnetic susceptibility data for 1, 2, 3, [1][BF4] and [2][PF6]
were recorded on a Quantum Design MPMS SQUID magneto-
meter at an applied field of 1 T and were corrected for core
diamagnetism and contributions from the gelatine capsule
sample holder.
Diffraction data for a single crystal of each of 1·2.5MeCN,
2·2DMF, 3·4MeOH, and 4·4DMF were collected on a Bruker
SMART 1000 CCD area detector diffractometer equipped
with an Oxford Cryosystem open-flow nitrogen cryostat7
using x-scans and graphite-monochromated Mo–Ka radiation
(0.71073 Å). Data were corrected for Lorentz and polariza-
tion effects and absorption using a multi-scan method. The
structure of 3·4MeOH was solved by direct methods and
those of 1·2.5MeCN, 2·2DMF, and 4·4DMF by Patterson
D a l t o n T r a n s . , 2 0 0 4 , 3 6 4 7 – 3 6 5 3
3 6 4 9

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Fig. 1 ORTEP representations of the molecular structures of (A) 1 in 1·2.5MeCN, (B) 2 in 2·2DMF, (C) 3 in 3·4MeOH and (D) 4 in 4·4DMF.
Only the solvent molecules that are involved in hydrogen bonding interactions with the complex are shown; however, for clarity these are not shown
for 4·4DMF.
Co–N of 1.979(3) Å.10 In 3·4MeOH the Co(1)–O(1A) bond
(1.938(4) Å) is slightly longer than the Co(1)–O(1) (1.905(3) Å)
bond and the Co–O bonds of 1·2.5MeCN and 2·2DMF
(1.910(2)–1.916(2) Å); this difference is attributed to the
presence of the O(1A)H(2SC) hydrogen bond in 3·4MeOH
(Fig. 1C, Table SI1, see ESI†).
The UV/vis spectra of a solution of 1, 2, and 3 in CH2Cl2
are similar and each is typical of a tetrahedral Co(II) (d7, S =
3/2) centre; each involves two/three d → d transitions between
500 and 600 nm (e  100–250 dm3 mol−1 cm−1), plus a shoulder
at 420–440 nm (e  400–800 dm3 mol−1 cm−1) (Fig. SI2 and
Table SI2, see ESI†) that is attributed to a phenolate → Co(II)
charge transfer transition.12
Fig. SI1).‡ However, 1, 2, and 3 are EPR silent in CH2Cl2
solution at 298 K, presumably because of rapid spin–lattice
relaxation of the Co(II) (d7, S = 3/2) centre.
4·4DMF involves three N,O-bidentate ligands that produce
a distorted mer-N3O3–octahedral coordination sphere about
the Co (Fig. 1D). This inner coordination sphere is analogous
to that of [CoIII(L)3]10 and other [CoIII(N,O)3] complexes13 and
involves Co–O and Co–N bonds of length 1.887(3)–1.914(3) Å
and 1.914(4)–1.944(3) Å, respectively (Table 3). The geometry
at the Co centre approximates to that of an octahedron with
cis- and trans-N(O)–Co–N(O) interbond angles that range from
84.16(14) to 93.63(14)° and from 173.06(14) to 175.77(13)°,
respectively (Table 3). Each imidazole N–H group is hydrogen
bonded to the carbonyl group of a DMF molecule (Table SI1,
see ESI†).
1, 2, and 3, as a solid at ca. 298 K, each possesses a magnetic
moment of 4.2–4.6 BM, consistent with a tetrahedral Co(II)
(S = 3/2) centre with contributions from spin–orbit coupling
as indicated by a relatively large zero field splitting (see ESI†,
4 is considered to involve a Co(III), d6 low spin (S = 0)
centre bound to three BzL− ligands. Thus, 4 is diamagnetic in
the solid state and EPR silent in CH2Cl2 solution. In CDCl3
solution, 4 exhibits a well resolved 1H-NMR spectrum
(Fig. SI3, see ESI†) that is consistent with the distorted octa-
hedral geometry observed for 4·4DMF being maintained in
solution. The spectrum comprises six resonances between 0.6
‡ The data obtained for 1, 2, and 3 were analysed on the basis
of a spin-Hamiltonian description of the electronic ground state:
H1 = DCo[Sz2 − S(S + 1)/3 + (ECo/DCo)(SX2 − SY2)] + lBgCoB·S; where
S = 3/2 is the spin of the metal ion and DCo, ECo/DCo, and gCo are the
local axial and rhombic zero-field parameters and g-values, respec-
tively. For all simulations the ECo/DCo value was not significantly
determined and was therefore fixed to zero. The temperature depen-
dence (3–320 K) of the magnetic moments of 2 and 3 were successfully
interpreted on this basis. (Fig. SI1(b), see ESI†). However, in respect of
the temperature dependence (3–320 K) of the magnetic moment of 1,
the consideration of 1 composed of isolated [Co(PhL)2] molecules only
fits the data observed between 200 and 300 K (Fig. SI1(a), see ESI†).
The lack of agreement between the experimental and simulated data
at a lower temperature (the maximum discrepancy is at ca. 50 K) is
characteristic of solid state interactions between the Co(II) centres.
The plot of 1/v vs. temperature (see Fig. SI1(a) inset, see ESI†) is linear
at high temperature (above 100 K), but at lower temperature shows a
deviation from the Curie law with a Weiss temperature of 10 K. This
deviation is typically indicative of the presence of intermolecular
ferromagnetic interactions (see Fig. SI1(a) inset, see ESI†).
t
and 1.2 ppm, assigned to Bu singlets, and three resonances
between 10.0 and 10.7 ppm, assigned to N–H singlets. The
UV/vis spectrum of 4 in CH2Cl2 solution at ca. 298 K manifests
absorptions at kmax/nm (e/dm3 mol−1 cm−1) 473 sh (1860), 610
(490), and 714 (310).
The above observations indicate that the molecular structures
of 1, 2, 3 and 4, identified in the solid state, are retained in
solution.
Redox properties of 1 and 2 and the nature of [1]+ and [2]+
The cyclic voltammograms of 1 and 2, in CH2Cl2 at ca. 298 K
(Fig. 2) each exhibit two, one-electron (as determined by
coulometry) oxidations. The first oxidation process is electro-
chemically reversible (Fig. 2a) whereas the second oxidation
3 6 5 0
D a l t o n T r a n s . , 2 0 0 4 , 3 6 4 7 – 3 6 5 3

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Table 1 Crystallographic data for 1·2.5MeCN, 2·2DMF, 3·4MeOH, 4·4DMF
1·2.5MeCN
2·2DMF
3·4MeOH
4·4DMF
Empirical formula
M
Crystal system
Space group (no.)
CoC63H69.5N6.5O2
1008.68
Monoclinic
P21/n
CoC68H84N6O8
1172.34
Triclinic
P1
CoC46H66N4O6
829.96
Monoclinic
P21/c
CoC75H103N10O7
1315.60
Monoclinic
C2/c
a/Å
b/Å
c/Å
a/°
16.8711(12)
17.8112(13)
19.2944(14)
90
101.626(1)
90
12.0122(8)
13.1107(9)
22.193(2)
103.221(2)
96.280(2)
111.092(2)
3103.0(6)
2
16.725(4)
10.589(3)
25.658(6)
90
93.876(6)
90
25.561(2)
12.9720(10)
44.240(3)
90
100.116(10)
90
b/°
c/°
V/Å3
5678.9(7)
4
4534.0(3)
4
14440.9(19)
8
Z
T/K
150(2)
1.17
0.350
150(2)
1.25
0.337
150(2)
1.22
0.430
150(2)
1.21
0.297
Dc/g cm−3
l/mm−1
Reflections collected
Independent reflections (Rint)
Observed reflections [I > 2r(I)]
37003
13989 (0.092)
5847
26386
13916 (0.02)
10794
24799
10856 (0.132)
3908
59973
16845 (0.072)
10626
R
Rw
0.0623
0.1781
0.0656
0.1353
0.076
0.224
0.0855
0.1312
Table 2 Selected bond lengths (Å) and angles (°) for 1·2.5MeCN and
2·2DMF and 3·4MeOH
1·2.5MeCN
2·2DMF
3·4MeOH
Co(1)–O(1A)
Co(1)–O(1)
Co(1)–N(5A)
Co(1)–N(5)
1.916(2)
1.911(2)
1.985(3)
1.991(3)
1.914(2)
1.910(2)
1.997(2)
1.998 (2)
1.938(4)
1.905(3)
1.994(4)
1.960(4)
O(1A)–Co(1)–O(1)
N(5A)–Co(1)–N(5)
O(1A)–Co(1)–N(5)
O(1)–Co(1)–N(5A)
O(1A)–Co(1)–N(5A)
O(1)–Co(1)–N(5)
109.58(1)
126.10(11)
118.34 (12)
115.63(12)
93.39(11)
94.47(11)
117.19(7)
112.41(7)
122.63(7)
118.61(7)
94.12(6)
93.88(7)
118.2 (2)
126.0(2)
117.6(2)
114.4(2)
90.1 (2)
93.0(2)
process is not. The potential of each oxidation process is very
similar to that of the corresponding oxidation of the analogous
[M(RL)2] (M = Cu or Zn) compound6,14 (Table 4). Therefore, we
consider that each of these oxidations is ligand-based. UV/vis
spectroelectrochemistry for the first oxidation of 1 and 2 shows
that this process is chemically reversible and UV/vis absorp-
tion spectra of [1]+ and [2]+ (Fig. 3) show a band at ca. 410 nm
(kmax/nm (e/M−1cm−1) 408 (9700) [1]+; 409 (10200) [2]+), the wave-
length and intensity of which is typical of a p–p* transition of
a phenoxyl radical (L).6,5a
[1]+ and [2]+ have been synthesised by the oxidation of
the parent compound with Ag+ (1:1); 1 was treated with
Ag[BF4] and 2 with Ag[PF6]. The products of these oxida-
tions, [1][BF4] and [2][PF6], are inert for several days at room
temperature, both as crystalline powders exposed to air and
in CH2Cl2 solution under an Ar or N2 atmosphere. The tem-
perature dependence of the magnetic moments of [1][BF4] and
[2][PF6], from 3 to 320 K (Fig. 4), have been modelled success-
fully by the Hamiltonian H = H1 + lBgradB·Srad − 2JSCoSrad
Fig. 2 Cyclic voltammograms of: (a) 1 (dotted line) and 2 (solid line);
(b) 3 ; (c) one-electron oxidized solution of 3 recorded at a Pt wire;
(d) 4; (e) 3 at 100 mV s−1; (f) 3 performed at scan-rates of 20, 100 and
500 mV s−1
.
2 on oxidation. The failure to detect an EPR signal for [1][BF4]
and [2][PF6] (at 4 and 77 K, at X- and Q-band frequencies,
both as a solid and in CH2Cl2 solution) is consistent with the
cations possessing a S = 1 ground state, in which the DMs = ±1
transitions cannot be observed due to a relatively high zero-
field splitting.
2
(H1 = DCo[Sz2 − S(S + 1)/3 + (ECo/DCo)(SX2 − SY )]+ lBgCoB·S).
In both cases, the local spin of the Co(II) centre (SCo) is 3/2
and that of the radical ligand (Srad) is 1/2; J represents the
exchange coupling constant and is <−250 cm−1 for [1][BF4]
and = −198 cm−1 for [2][PF6] (Fig. 4). Thus, the magnetic data
are consistent with both [1][BF4] and [2][PF6] possessing an
S = 1 ground state arising from a strong antiferromagnetic
coupling between the Co(II) and the phenoxyl radical centres.
The D values for [1][BF4] and [2][PF6] (DCo = −24.5 ± 2.0 and
DCo = −13.7 ± 2.0 cm−1, respectively, Fig. 4) are comparable to
those of 1 and 2 (DCo = −24.5 ± 2.0 and DCo = −12.9 ± 2.0 cm−1,
respectively, Fig. SI1, see ESI†). This suggests that there are no
substantial changes in geometry about the Co centres of 1 and
Oxidation of 4 and 3
The cyclic voltammogram of 4 (Fig. 2d) manifests three
oxidation processes. Coulometric studies show that the first
and second oxidations are each one-electron processes and
UV/vis OTE investigations indicate that these are chemically
reversible. The first and second oxidation (Fig. 5) processes
are accompanied by the growth of a relatively strong absorp-
tion at 503 nm (e = 4420 dm3 mol−1 cm−1) and 530 nm (e =
7200 dm3 mol−1 cm−1) for [4]+ and [4]2+, respectively. These
D a l t o n T r a n s . , 2 0 0 4 , 3 6 4 7 – 3 6 5 3
3 6 5 1

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Table 3 Selected bond lengths (Å) and angles (°) for 4·2DMF
4·4DMF
Co(1)–O(1)
1.894(3)
1.914(3)
1.887(3)
1.944(3)
1.914(4)
1.924(3)
N(5A)–Co(1)–N(5B)
O(1)–Co(1)–N(5)
O(1)–Co(1)–N(5A)
O(1)–Co(1)–N(5B)
O(1A)–Co(1)–N(5)
O(1A)–Co(1)–N(5A)
88.94(15)
91.37(13)
173.06(14)
84.16(14)
90.45(14)
90.83(14)
Co(1)–O(1A)
Co(1)–O(1B)
Co(1)–N(5)
Co(1)–N(5A)
Co(1)–N(5B)
O(1A)–Co(1)–N(5B)
O(1B)–Co(1)–N(5)
O(1B)–Co(1)–N(5A)
O(1B)–Co(1)–N(5B)
93.63(14)
85.32(14)
89.56(14)
90.59(14)
O(1)–Co(1)–O(1A)
O(1)–Co(1)–O(1B)
O(1A)–Co(1)–O(1B)
N(5)–Co(1)–N(5A)
N(5)–Co(1)–N(5B)
88.83(13)
91.29(13)
175.77(13)
−95.56(14)
−173.88(15)
Table 4 Cyclic voltammetric data for the oxidation processes of
[Co(RL)2] (R = Ph, PhOMe or Bz) and [Co(BzL)3] at 100 mV s−1. All
potentials are expressed vs. [Fc]+/[Fc]
First oxidation
E/V(DE/mV)
Second oxidation
E/V(DE/mV)
DE
Complex
[Fc*]+/[Fc*]/mV
1
2
3
4
0.16(80)a
0.12(80)a
0.35b
0.51
0.49
—
0.52(90)a
70
80
80
90
0.08(80)a
a
a Eꢀ for a reversible redox process. b Ep .
Fig. 4 Temperature dependence of the effective magnetic moment
of (top) [2][PF6]; the solid line represents the best fit by use of the
parameters: gCo = 2.360, grad = 2.000 (fixed), DCo = −13.7 ± 2.0 cm−1
(ECo/DCo = 0.08), J = −198 cm−1; and (bottom) [1][BF4]; the solid line
represents the best fit by use of the parameters: gCo = 2.465; grad = 2.000
(fixed); DCo = −24.5 ± 2.0 cm−1 (Eco/Dco = 0.07). For [1][BF4], the strong
antiferromagnetic coupling results in a fit that shows little variation
in quality for J < −250 cm−1
.
Fig. 3 UV/vis spectroelectrochemistry for the one-electron oxida-
tion of (a) 1 and (b) 2 in CH2Cl2 at 273 K.
are similar to the bands observed for [CuII(BzL)(BzL)]+ and
[ZnII(BzL)(BzL)]+.14 Thus, [4]+ is considered to be constituted
as [CoIII(BzL)(BzL)2]+ with a phenoxyl radical (BzL) and two
phenolates (BzL) coordinated to Co(III). The X-band EPR
spectrum of [4]+, generated electrochemically from 4 in fluid
CH2Cl2 solution at room temperature and then cooled to
175 K (Fig. SI4, see ESI†), consists of a broad (ca. 40 G)
signal centred at g = 2.016. The line width of this signal
excludes the possibility of a large hyperfine coupling (e.g. >10 G)
with the 59Co nucleus (I = 7/2). This result is in agreement with
EPR data obtained for other Co(III)–radical ligand complexes,
for which A(59Co) values <12.5 G have been observed.15–18
3 undergoes a facile, one-electron chemically irreversible
oxidation at Epa = 0.34 V (vs. [Fc]+/[Fc]) (Fig. 2b). The profile
of the cyclic voltammogram for this process differs significantly
from that observed for the first oxidation of 1 and 2 (Fig. 2a).
The first oxidation of 3 is irreversible and [4]+ was the only
Fig. 5 UV/vis spectra for the (a) the one-electron oxidation and
(b) the two-electron oxidation of 4 at 273 K.
3 6 5 2
D a l t o n T r a n s . , 2 0 0 4 , 3 6 4 7 – 3 6 5 3

View Article Online
product of the oxidation to be identified by UV/vis, EPR
and cyclic voltammetric (cf. Figs. 2c and 2d) investigations
of the electrolysed solution. At scan rates <100 mV s−1, the
first oxidation of 3, in the reverse sweep, shows a reduction at
a potential consistent with the process [4]+ → [4]. Furthermore,
multiple scan cyclic voltammetric studies of the first oxidation
of 3 (Fig. 2e) indicate that the depletion of 3 is accompanied
by the development of the [4]+/[4] redox couple. However, at
500 mV s−1 (Fig. 4f) a new cathodic process at Epc = 0.26 V
accompanies the anodic process at Epa = 0.35 V, that is assigned
to a ligand-based [3]+/[3] couple, by comparison with the first
oxidation processes for 1, 2 and the first oxidation of [Cu(BzL)2]
(E1/2 = 0.36 V DE = 70 mV).14
The above observations are consistent with a reaction se-
quence in which one-electron oxidation of [3] produces [3]+,
from which [4]+ is produced. This behaviour is in marked
contrast to the inertness of [1]+ and [2]+, each of which have
been isolated and characterised. Also, [CoII(BzL)2]+ is the only
[MII(RL)2]+ complex of those so far investigated (M = Co, Cu,6,14
or Zn;6,14 R = Ph, PhOMe, or Bz) that is reactive. Our interpre-
tations of these observations are: (i) for these complexes, oxida-
tion of a Co(II) centre to produce Co(III) could occur, whereas
this is not possible for Zn(II) and might not be favoured for
Cu(II); (ii) the formation of Co(III) would be greatly encour-
aged by the addition of a (bidentate) ligand, to produce an
octahedral, d6, low-spin centre; (iii) the formation of a [Co(RL)3]
complex appears to be possible for R = Bz (Fig. 1D) but not
for R = Ph or PhOMe under identical reaction conditions;
the only product isolated from the reaction of Co(BF4)2·6H2O
with RLH (R = Ph or PhOMe) (1:3) is the corresponding
[CoIII(RL)2] complex (i.e. 1 or 2, respectively). Thus, the steric
hindrance induced by the 4,5-diphenyl–imidazole unit of
ligand RLH (R = Ph or PhOMe) may explain the differences in
the chemical properties of the radical complexes [1]+ or [2]+ as
compared to [3]+.
Acknowledgements
We thank the EPSRC for financial support.
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Conclusions
Three new [CoIIL2] complexes of the phenol–imidazole pro-
ligands RLH have been synthesised and characterised. Each of
these complexes undergoes a one-electron, ligand-based, oxida-
tion. The products of this oxidation [CoII(RL)(RL)]+ (R = Ph
or PhOMe) are analogous to those reported previously for
Cu and Zn6,14 and represent the first phenoxyl radical ligand
complexes of tetra-coordinated Co(II) presumably stabilized
by the steric hindrance of the ligands. However, of the systems
so far investigated, [CoII(BzL)(BzL)]+ is unique in being reactive
to form [CoIII(BzL)(BzL)2]+, demonstrating that the chemical
properties of such systems are dependent on the nature of both
the phenoxyl radical and the metal centre.
D a l t o n T r a n s . , 2 0 0 4 , 3 6 4 7 – 3 6 5 3
3 6 5 3