A phenol-imidazole pro-ligand that can exist as a phenoxyl radical, alone and when complexed to copper(II) and zinc(II)

Article abstract of DOI:10.1039/b212209j

A new N,O-bidentate, phenol-imidazole pro-ligand 2′-(4′, 6′-di-tert-butylhydroxyphenyl)-4,5-diphenyl imidazole (LH) has been designed, synthesised, and characterised. LH possesses no readily oxidisable position (other than the phenol) and involves o- and p-substituents on the phenol ring that prevent radical coupling reactions. LH undergoes a reversible one-electron oxidation to generate the corresponding [LH]⁺ radical cation that possesses phenoxyl radical character. The unusual reversibility of the [LH]/[LH]⁺ redox couple is attributed, at least in part, to a stabilisation of [LH]⁺ by intramolecular O-H ...N hydrogen bonding. The compounds [CuL₂] (1) and [ZnL ₂ (2) have been synthesised and characterised structurally, spectroscopically, and electrochemically. The crystal structures of 1·4DMF, 1·SMeOH, and 2·2.5MeCN·0.3CH ₂Cl₂ have been determined and each shown to possess an N₂O₂-coordination sphere, the geometry of which varies with the nature of the metal and the nature of the co-crystallised solvent. 1 and 2 each undergo two, reversible, ligand-based, one-electron oxidations, to form, firstly, [M(L)(L)]⁺ and secondly [M(L )₂²⁺. The [M(L)(L)] ⁺ (M = Cu, Zn) cations have been generated by both electrochemical and chemical oxidation and their [ML₂][BF₄] salts isolated as air-stable, dark green, crystalline solids. The UV/vis, EPR, and magnetic characteristics of these compounds are consistent with each cation involving an M^II (M = Cu or Zn) centre bound to a phenoxide (L^-) and a phenoxyl radical (L). The structural information obtained by a determination of the crystal structures of [CuL₂][BF ₄]·2CH₂Cl₂ and [ZnL₂][BF ₄]·2CH₂Cl₂·0.75 pentane fully supports this interpretation. For each of these salts, there is a clear indication that the coordinated phenoxyl radical is involved in intramolecular π-π stacking interactions that parallel those in galactose oxidase. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b212209j

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A phenol–imidazole pro-ligand that can exist as a phenoxyl
radical, alone and when complexed to copper(II) and zinc(II)†
Laurent Benisvy,*a Alexander J. Blake,a David Collison,b E. Stephen Davies,a C. David
Garner,*a Eric J. L. McInnes,b Jonathan McMaster,*a Gavin Whittakerc and Claire Wilsona
a School of Chemistry, The University of Nottingham, University Park, Nottingham,
UK NG7 2RD. E-mail: Dave.Garner@nottingham.ac.uk
b Department of Chemistry, The University of Manchester, Oxford Road, Manchester,
UK M13 9PL
c Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh,
UK EH9 3JJ
Received 9th December 2002, Accepted 22nd January 2003
First published as an Advance Article on the web 23rd April 2003
A new N,O-bidentate, phenol–imidazole pro-ligand 2Ј-(4Ј,6Ј-di-tert-butylhydroxyphenyl)-4,5-diphenyl imidazole
(LH) has been designed, synthesised, and characterised. LH possesses no readily oxidisable position (other than the
phenol) and involves o- and p-substituents on the phenol ring that prevent radical coupling reactions. LH undergoes
a reversible one-electron oxidation to generate the corresponding [LH] ϩ radical cation that possesses phenoxyl
ؒ
radical character. The unusual reversibility of the [LH]/[LH] ϩ redox couple is attributed, at least in part, to a
ؒ
stabilisation of [LH] ϩ by intramolecular O–H ؒ ؒ ؒ N hydrogen bonding. The compounds [CuL2] (1) and [ZnL2] (2)
ؒ
have been synthesised and characterised structurally, spectroscopically, and electrochemically. The crystal structures
of 1ؒ4DMF, 1ؒ3MeOH, and 2ؒ2.5MeCNؒ0.3CH2Cl2 have been determined and each shown to possess an N2O2-
coordination sphere, the geometry of which varies with the nature of the metal and the nature of the co-crystallised
ϩ
ؒ
solvent. 1 and 2 each undergo two, reversible, ligand-based, one-electron oxidations, to form, firstly, [M(L)(L )]
2ϩ
ϩ
ؒ
ؒ
and secondly [M(L )2] . The [M(L)(L )] (M = Cu, Zn) cations have been generated by both electrochemical
and chemical oxidation and their [ML2][BF4] salts isolated as air-stable, dark green, crystalline solids. The UV/vis,
EPR, and magnetic characteristics of these compounds are consistent with each cation involving an MII (M = Cu
or Zn) centre bound to a phenoxide (L ) and a phenoxyl radical (L ). The structural information obtained by a
determination of the crystal structures of [CuL2][BF4]ؒ2CH2Cl2 and [ZnL2][BF4]ؒ2CH2Cl2ؒ0.75pentane fully supports
this interpretation. For each of these salts, there is a clear indication that the coordinated phenoxyl radical is involved
in intramolecular π–π stacking interactions that parallel those in galactose oxidase.
Ϫ
ؒ
Introduction
Experimental
Inspired by a knowledge of the nature of the catalytic centres of
Cu(BF4)2ؒH2O, Zn(BF4)2ؒH2O, Ag[BF4] and anhydrous MeOH
were obtained from Aldrich Chemicals Ltd.; CH2Cl2 was
freshly distilled from calcium hydride under dinitrogen.
galactose oxidase (GAO)1–3 and glyoxal oxidase (GLO)4
a
new N,O-bidentate pro-ligand, 2Ј-(4Ј,6Ј-di-tert-butylhydroxy-
phenyl)-4,5-diphenyl imidazole (LH) has been designed and
synthesised.5 LH involves the prospective donor groups
O-phenol and N-imidazole that, respectively, resemble the tyro-
sine and histidine residues that coordinate Cu in GAO and
GLO. To enhance the possibility of isolating complexes
Syntheses
LH. A mixture of 3,5-di-tert-butylhydroxybenzaldehyde (2 g,
8.8 mmol), benzil (1.8 g, 8.6 mmol), and ammonium acetate
(5.2 g, 64 mmol) were refluxed for 2 h in glacial acetic acid
(30 cm3), during which time a yellow precipitate formed. An
excess of de-ionised water (ca. 30 cm3) was added to complete
the precipitation. The crude product was collected by filtration,
washed with water, and dried by suction. The resulting solid
was dissolved in CH2Cl2 and dried over MgSO4. The solution
was filtered and the solvent was evaporated from the filtrate, to
produce a pale yellow powder that recrystallised from CH2Cl2–
pentane as a white cotton-wool-like solid in 75% yield (2.8 g).
Single crystals of LHؒMe2CO were obtained by allowing a solu-
tion of LH in acetone to stand at Ϫ30 ЊC for one week; these
crystals proved suitable for analysis by X-ray diffraction. Ele-
mental analysis: calc. for C29H32N2O: C 82.08, H 7.55, N 6.60;
Found: C 81.63, H 7.64, N 6.58%. NMR (1H, 300 MHz,
CDCl3): δ 13.2 (s, 1H, N–H), 9.23 (s, 1H, OH), 7.1–7.7 (m, 12H,
ArH), 1.47 (s, 9H, tBu), 1.62 (s, 9H, tBu). NMR (13C, 300 MHz,
CDCl3): δ 29.5, 31.6, 34.3, 35.3, 111.5, 117.3, 125.4, 125.9,
127.2, 128.4, 128.9, 130.7, 133.5, 135.2, 137.5, 140.3, 146.8,
154.5. Mp 283 ЊC. Positive EI-MS: ion at m/z 424 {Mϩ}. UV/vis
ؒ
containing L , HL contains no readily oxidisable function-
ality, other than the phenol group. The ortho- and para-tBu
substituents inhibit pathways for the decomposition of
the phenoxyl radical state and should stabilise this state by a
positive inductive effect.
† Based on the presentation given at Dalton Discussion No. 5, 10–12th
April 2003, Noordwijkerhout, The Netherlands.
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 3
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1975

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(CH2Cl2): λmax/nm (ε/MϪ1 cmϪ1): 228 (26355), 277 (sh), 291
1CH2Cl2: C 65.52, H 5.92, N 5.18; Found: C 65.28, H 5.98, N
(20395), 321 (21480). IR (KBr pellet): 3426(s), 2961(s), 2903(s),
5.01%. Positive FAB-MS: ion at m/z 910 {Mϩ}.
2864(s), 1605(m), 1584(mw), 1529(m), 1500(mw), 1475(s) cmϪ1
.
[2][BF4]. A solution of 2 (108 mg, 0.119 mmol) in CH2Cl2 (15
cm3) and a suspension of Ag[BF4] (23.2 mg, 0.119 mmol) in
CH2Cl2 (5 cm3) were reacted and the product isolated as
described above. The green solid was dried under a reduced
pressure and recrystallised by diffusion of hexane into a CH2Cl2
solution under an argon atmosphere at 4 ЊC. After one week
microcrystalline dark-green, needle-like crystals of [2][BF4]
were obtained in 70% yield (∼83 mg). Dark, block-like, single
crystals of [2][BF4]ؒ2CH2Cl2ؒ0.75pentane were obtained by dif-
fusion of pentane into a CH2Cl2 solution of [2][BF4] under an
argon atmosphere at room temperature. Elemental analysis:
calc. for C58H62N4O2ZnBF4: C 69.71, H 6.21, N 5.60; Found: C
69.81, H 6.34, N 5.51%. Positive FAB-MS: ion at m/z 911
{Mϩ}.
General procedure for the synthesis of [ML2] compounds
(M ؍ Cu, Zn)
A solution containing LH (2 equivalents) dissolved in the mini-
mum volume of MeOH was added to a solution of M(BF4)2ؒ
H2O in MeOH (2–5 cm3), The mixture was stirred at room
temperature for 1 h, after which time an excess (a few drops) of
Et3N was added. After further stirring (1 h) the product was
isolated and purified according to the particular procedure
described below.
[Cu(L)2] (1). Cu(BF4)2ؒH2O (301 mg, 1.18 mmol) in MeOH
(5 cm3) and LH (1 g, 2.36 mmol) in MeOH (200 cm3) were
reacted as described above. The resultant deep green solution
was left to stand in air for 6 d at room temperature and brown–
purple, rectangular, crystals of 1ؒ3MeOH were obtained. The
crystals were collected by filtration, washed with MeOH,
crushed, and dried in vacuo, affording a dark brown powder of
1 in 85% yield. Elemental analysis: calc. for C58H62N4O2Cuؒ
2MeOH: C 73.96, H 7.19, N 5.75; Found: C 74.22, H 6.84,
N 6.06%. Positive FAB-MS: ion at m/z 910 {Mϩ}. UV/vis
(CH2Cl2): λmax/nm (ε/MϪ1 cmϪ1): 499 (670), 692 (550). Slow
evaporation at room temperature of a solution of 1 in CH2Cl2–
DMF (9 : 1) produced, after 3–6 d, orange block-like single
crystals of 1ؒ4DMF.
Physical methods
Elemental analysis of the compounds isolated in these studies
were carried out at the Microanalytical Laboratory of the
School of Chemistry; University of Nottingham. FAB and EI
mass spectra were recorded on a Fisons VG Trio 200 or a
Fisons VG Autospec spectrometer. 300 MHz 1H- and 13C-
NMR were recorded on a Bruker DPX300 NMR spectrometer.
UV/vis spectra were recorded on a Perkin-Elmer Lambda 5
spectrophotometer. IR spectra were recorded on a Nicolet Ava-
tar 360 FTIR spectrometer for samples pressed into a KBr disc.
For all electrochemical experiments, the CH2Cl2 was freshly
distilled under dinitrogen from calcium hydride. The cyclic
voltammogram of each compound (1 mM) in CH2Cl2 contain-
[Zn(L)2] (2). Zn(BF4)2ؒH2O (245 mg, 0.95 mmol) in MeOH
(5 cm3), LH (800 mg, 1.9 mmol) in MeOH (200 cm3) were
reacted as described above. The resultant colourless solution
was allowed to evaporate at room temperature by exposure to
air for 6 d and a white crystalline powder was obtained. This
powder was collected by filtration, washed with MeOH,
crushed and dried in vacuo to yield 2 as a white powder in 80%
yield. Slow evaporation of a solution of 2 in CH2Cl2–MeCN
(1 : 1) in air at room temperature produced colourless, lath-like,
crystals of 2ؒ2.5MeCNؒ0.3CH2Cl2 after 2–4 d. Elemental
analysis (of powdered 2): calc. for C58H62N4O2Znؒ2MeOH:
C 73.82, H 7.17, N 5.74; Found: C 74.04, H 6.66, N 6.08%.
Positive FAB-MS: ion at m/z 911 {Mϩ}.
n
ing [NBu4 ][BF4] (0.4 M) as the background electrolyte was
recorded under dinitrogen at room temperature using a glassy
carbon working electrode, a Pt wire secondary electrode, and a
saturated calomel reference electrode. The cyclic voltam-
mogram of each compound was referenced to the Fcϩ/Fc
couple that was used as internal standard. When necessary, the
[FeCp2*]ϩ/[FeCp2*] couple was used as the internal reference to
avoid overlapping redox couples and the potentials of the redox
process were referenced to that of Fcϩ/Fc (E1/2 = 0.540 V vs.
SCE) couple by an independent calibration. The ∆E1/2 between
Fcϩ/Fc and [FeCp2*]ϩ/[FeCp2*] recorded under identical condi-
tions was 0.526 V. All potentials were measured vs. SCE but are
quoted vs. Fcϩ/Fc. All coulometric measurements were per-
formed at room temperature or 0 ЊC in a CH2Cl2 solution con-
General procedure for the synthesis of [ML2][BF4] compounds
(M ؍ Cu, Zn)
n
The reactions were carried out in pre-dried Schlenk vessels, in
the absence of light, under an argon atmosphere, and using
freshly distilled solvents. A solution of [ML2] in CH2Cl2 was
added to a suspension of Ag[BF4] (1 eq.) in CH2Cl2 at Ϫ10 ЊC
(ice/acetone bath). This caused the rapid formation of a silver
mirror and an intense dark green coloured solution. The solu-
tion was stirred for 1 h at Ϫ10 ЊC, for 10 min at room temper-
ature, and then filtered using a Teflon cannula equipped with
a glass wool filter at one end (it was noted that the use of
a metallic cannula induced a colour change of the solution and,
therefore, was avoided). The solvent was evaporated under a
reduced pressure to yield a dark coloured solid. Each product
was purified according to the particular procedure described
below.
taining [NBu4 ][BF4] (0.4 M); the cell consisted of a Pt/Rh
gauze basket working electrode, a Pt/Rh gauze secondary elec-
trode, and a saturated calomel reference electrode. Both CV and
CPE measurements were made using an Autolab PGSTAT20
potentiostat.
The UV/vis spectroelectrochemical experiments were carried
out with an optically transparent thin-layer electrochemical
(OTTLE) cell (modified quartz cuvette, optical pathlength:
0.5 mm). A three-electrode configuration, consisting of a Pt/Rh
gauze working electrode, a Pt wire secondary electrode (in a
fritted PTFE sleeve) and a saturated calomel electrode, chem-
ically isolated from the test solution via a bridge tube contain-
ing electrolyte solution and terminated in a porous frit, was
used in the cell. The potential at the working electrode was
controlled by a Sycopel Scientific Ltd. DD10M potentiostat.
The UV/vis spectra were recorded on a Perkin-Elmer Lambda
16 spectrophotometer. The spectrometer cavity was purged
with dinitrogen and temperature control at the sample was
achieved by flowing cooled dinitrogen across the surface of the
cell.
[1][BF4]. A solution of 1 (318 mg, 0.348 mmol) in CH2Cl2 (20
cm3) and a suspension of Ag[BF4] (68 mg, 0.348 mmol) in
CH2Cl2 (5 cm3) were reacted and the product isolated as
described above. The resultant dark green solid was washed
with freshly distilled Et2O (2 × 30 mL) to remove any unreacted
1. The solid was then dried under a reduced pressure to afford
[1][BF4] in 85% yield. Dark, block-shaped, single crystals of
[1][BF4]ؒ2CH2Cl2 were grown by diffusion of hexane into a
CH2Cl2 solution of 1[BF4] under an argon atmosphere at room
temperature. Elemental analysis: calc. for C58H62N4O2CuBF4ؒ
X-Ray crystallography
The unit cell, data collection and refinement parameters for
LHؒMe2CO, 1ؒ3MeOH, 2ؒ2.5MeCNؒ0.3CH2Cl2, and [2][BF4]ؒ
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1976

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Table 1 Crystallographic data for compounds LHؒMe2CO, 1ؒ4DMF, 1ؒ3MeOH, 2ؒ2.5MeCNؒ0.3CH2Cl2, [1][BF4]ؒ2CH2Cl2, and [2][BF4]ؒ2CH2Cl2ؒ
0.75pentane
2ؒ2.5MeCNؒ
0.3CH2Cl2
[2][BF4]ؒ2CH2Cl2ؒ
0.75pentane
5
LHؒMe2CO
1ؒ4DMF5
1ؒ3MeOH
[1][BF4]ؒ2CH2Cl2
Empirical formula
C32H38N2O2
CuC70H90N8O6
CuC61H74N4O5
ZnC63.3H70.1N6.5
O2Cl0.6
-
CuC60H66N8-
O2BF4Cl4
1167.32
Monoclinic
P21/n (14)
12.6062 (7)
18.2942(10)
25.5061(14)
90
96.191(1)
90
5847.9(9)
4
ZnC62.5H70.25N4-
O2BF4Cl4
1203.46
Monoclinic
P21/c (14)
13.964(4)
17.079(5)
28.310(9)
90
91.729(6)
90
6749(3)
4
M
482.64
Monoclinic
P21/n (14)
18.059(2)
8.5608(9)
19.135(2)
90
106.567(2)
90
2835.5(5)
4
1203.04
Monoclinic
C2/c (15)
29.090(2)
13.246(9)
18.630(1)
90
112.506(1)
90
6632.0(7)
4
1006.78
Triclinic
1040.60
Monoclinic
P21/n (14)
16.9519(9)
17.8270(9)
19.2760(10)
90
102.088(1)
90
5696.0(9)
4
Crystal system
Space group (no.)
a/Å
b/Å
c/Å
α/Њ
β/Њ
¯
P1 (2)
12.207(1)
13.790(1)
18.805(2)
70.972(1)
74.630(2)
68.125(2)
2739.5(7)
2
γ/Њ
V/Å3
Z
T /K
150
1.131
0.070
150
1.205
0.386
150
1.220
0.450
150
1.213
0.510
150
1.326
0.61
150
1.18
0.574
Dc/g cmϪ3
µ/mmϪ1
Reflections collected
Independent
16919
6512 (0.064)
30269
8328 (0.035)
19728
12281 (0.043)
46084
13891 (0.078)
40636
14159 (0.043)
33249
15494 (0.054)
reflections (Rint
Observed
)
3621
6303
9231
7197
13749
9548
reflections [I > 2σ(I )]
R
0.0481
0.1479
0.0399
0.1124
0.0501
0.1392
0.0525
0.1248
0.0681
0.1888
0.1178
0.3686
Rw
2CH2Cl2ؒ0.75pentane are given in Table 1; the details for 1ؒ
4DMF and [1][BF4]ؒ2CH2Cl2 are included for comparison.5
Diffraction data for all of these compounds were collected on a
Bruker SMART 1000 CCD area detector diffractometer
equipped with an Oxford Cryosystem open-flow nitrogen
cryostat6 using ω-scans and graphite-monochromated Mo-Kα
radiation (0.71073 Å). Data were corrected for Lorentz and
polarization effects. The structures of LHؒMe2CO and [2][BF4]ؒ
2CH2Cl2ؒ0.75pentane were solved by direct methods and
those of 2ؒ2.5MeCNؒ0.3CH2Cl2 and 1ؒ3MeOH by Patterson
methods using SHELXS-97.7a Each structure was refined
against F 2 using SHELXL-97.7a Unless otherwise stated, all
non-H atoms were refined with anisotropic atomic displace-
ment parameters. The hydrogen atoms of: CH3CN, CH3OH,
(CH3)2CO, and PhOH 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 posi-
benzil (1 : 1) in AcOH in the presence of an excess of
[NH4][OAc] (yield 75%). The structure of LH (in LHؒMe2CO,
see Fig. 1, Table 1) involves an essentially co-planar arrange-
ment of the imidazole and phenol rings, the twist angle between
these planes being 7.7Њ; the corresponding angle in the analo-
gous 2-(2Ј-hydroxyphenyl)imidazole (ImPhOH) is 1.2Њ.8 The
angles between the mean plane of the imidazole ring and those
of the C(6)–C(11) and C(12)–C(17) phenyl rings are 43.4 and
35.8Њ, respectively. The length of the C–C, C–N, and C–O
bonds of the imidazole and phenol groups are as expected (see
Fig. 1).8,9 The N–H group of LH is involved in an inter-
molecular N–H ؒ ؒ ؒ O hydrogen bond [N(2) ؒ ؒ ؒ O(1S) 2.852(2)
Å and N(2)–H(2A) ؒ ؒ ؒ O(1S) 168.8Њ] to an adjacent acetone
molecule. Also, and in respect of the chemical properties of
LH, there is a strong intramolecular hydrogen bond (IMHB)
between the phenolic O–H group and N(5). The strength of this
IMHB, as measured by the O(1) ؒ ؒ ؒ N(5) distance of 2.596(2)
Å and the O(1)–H(1) ؒ ؒ ؒ N(5) angle of 150.7Њ, is very similar
to that in ImPhOH8 [2.545(2) Å and 154(3)Њ], 2-(pyrazol-1Ј-
yl)- and 2,5-bis(pyrazol-1Ј-yl)-1,4-dihydroxybenzene [2.558(9)
Å, 138(9)Њ and 2.612(3) Å, 149(4)Њ, respectively],10 and 2-(2-
hydroxyphenyl)pyrimidine [2.511(3) Å, 153(5)Њ].11
tions 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 CH3.
For 2ؒ2.5MeCNؒ0.3CH2Cl2, a region of the occluded solvent
was disordered and modelled as the stated chemical com-
position with restraints applied to the geometry of the solvent
molecules, but their hydrogen atoms were not located. For
[2][BF4]ؒ2CH2Cl2ؒ0.75pentane, disorder was identified in one
CH2Cl2 molecule which was modelled over two sites with occu-
pancies of 0.4 : 0.6 and isotropic atomic displacement param-
eters. Two (partially occupied) pentane molecules were also
included in this structure. One site has an occupancy of 0.5; the
other molecule is highly disordered and occupies two sites, each
of 0.25 occupancy, which overlap (excepting a terminal carbon)
and lie across an inversion centre, such that only half of each
orientation is in the asymmetric unit. The R-factor of [2][BF4]ؒ
2CH2Cl2ؒ0.75pentane reflects the generally weak nature of the
diffraction data observed at higher scattering angles.
CCDC reference numbers 199488–199491.
See http://www.rsc.org/suppdata/dt/b2/b212209j/ for crystal-
lographic data in CIF or other electronic format.
Fig. 1 ORTEP7b representation of the structure of LHؒMe2CO.
Selected bond lengths: C(19)–O(1) 1.368(2), C(18)–C(19) 1.407(2),
C(19)–C(20) 1.402(2), C(20)–C(21) 1.405(2), C(21)–C(22) 1.403(2),
C(22)–C(23) 1.384(2), C(23)–C(18) 1.399(2), C(1)–N(5) 1.339(2), C(1)–
N(2) 1.360(2) C(3)–N(2) 1.380(2), C(3)–C(4) 1.382(2), C(4)–N(5)
1.385(2), C(1)–C(18) 1.462(2) Å.
Results
؉
ؒ
The pro-ligand LH and the radical cation [LH]
Synthesis and structure of LH. LH was synthesised by the
condensation of 3,5-di-tert-butylhydroxybenzaldehyde with
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1977

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Generation and characterisation of [LH] ؉. The cyclic vol-
[CuL2] (1) and [ZnL2] (2)
ؒ
tammogram of LH in CH2Cl2 at room temperature exhibits
a one-electron reversible‡ oxidation at E1/2 = 0.43 V vs. Fcϩ/Fc
(Epa = 488 mV) (Fig. 2) and an irreversible oxidation at 0.94 V.
In contrast, the cyclic voltammogram of o,p-unprotected
analogues of LH,§ recorded under the same conditions,
exhibits one irreversible oxidation process at 0. 61 V vs. Fcϩ/Fc.
Synthesis and structural characterisation. The reaction of
M(BF4)2ؒH2O (M = Cu, Zn) with LH (1 : 2) in the presence of
an excess of Et3N, yielded the corresponding [ML2] compound
(1 M = Cu; 2 M = Zn). The crystal structures of 1ؒ3MeOH,
1ؒ4DMF and 2ؒ2.5MeCNؒ0.3CH2Cl2 have been determined by
X-ray crystallography (Table 1). In each case, the metal centre
possesses an N2O2-coordination sphere with each ligand acting
as an N,O-bidentate chelate (Fig. 4a–c). The average length of
the M–O and M–N bonds (Table 2) are similar to those of
bis(salicylaldiminato)M() complexes of the corresponding
metal,12 consistent with the formulation of each compound as
an M() centre ligated by two monoanionic phenolate N,O-
donor ligands, LϪ. Each compound incorporates some solvent
molecules in the lattice, some of which engage in hydrogen
bonds with the N–H/imidazole and/or O/phenolate groups.
Thus, for MeCN, MeOH, or DMF(O), the atom possessing a
lone pair of electrons is hydrogen bonded to the imidazole N–H
group and MeOH forms a hydrogen bond with the phenolate
oxygen (Fig. 4a–c).
Fig. 2 The cyclic voltammogram of LH (∼1 mM) in CH2Cl2 at 298 K
n
containing [NBu4 ][BF4] (0.4 M) as supporting electrolyte, at a scan rate
of 100 mV sϪ1
.
One-electron oxidation of LH, at 0 ЊC under dinitrogen in
n
CH2Cl2 containing 1 mM of LH and [NBu4 ][BF4] (0.4 M), by
controlled potential electrolysis led to a drastic colour change
of the solution from colourless to an intense dark blue. The
oxidised species thus produced appeared to remain unchanged
for ca. 1 h. The X-band EPR spectrum of the oxidised species in
CH2Cl2 consisted of a single isotropic signal with giso centered
at 2.004 with a line width of ∼10 G and no resolved hyperfine
splitting.
Electrochemical oxidation of LH, at Ϫ40 ЊC under di-
n
nitrogen in CH2Cl2 containing 1 mM of LH and [NBu4 ][BF4]
(0.4 M), was accomplished in an OTTLE cell. A series of
smooth changes in the absorbance profile were observed; isos-
bestic points were observed at 247, 291, and 351 nm and new
absorption bands with λmax of 379 (9700), 401 (9200), 514
(4000) and 715 nm (5600 MϪ1 cmϪ1) (Fig. 3). Electrochemical
re-reduction of the oxidised species produced a spectrum
corresponding to that originally observed for LH.
Fig. 3 UV/vis spectra observed during the one-electron oxidation of
LH achieved in an OTTLE cell at Ϫ40 ЊC in CH2Cl2 containing
n
[NBu4 ][BF4] (0.4 M) with an applied potential of 1100 mV vs. SCE.
‡ For LH: Epa = 488 mV and Epc = 379 mV (vs. Fcϩ/Fc); ∆E = 103–122
c
c
a
mV (scan rate = 20–200 mV sϪ1); Ϫip /ip = 1 0.1; (scan rate)1/2 ∝ ip
a
(and ip ). For Fcϩ/Fc : ∆E = 73–95 mV (scan rate = 20–200 mV sϪ1).
Controlled potential coulometry of LH at 1.175 V vs. SCE indicated the
transfer of 1.10 electrons moleculeϪ1
.
Fig. 4 ORTEP7b representation of: (a) 1ؒ4DMF; (b) 1ؒ3MeOH and (c)
2ؒ2.5MeCNؒ0.3CH2Cl2. Only the solvent molecules that are involved in
hydrogen bonding interactions with the complex are shown.
§ The o,p-unprotected derivative of LH was synthesised using a pro-
cedure reported by Mahesh et al.38
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1978

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Table 2 Selected bond lengths (Å) and angles (Њ) for 1ؒ4DMF and
1ؒ3MeOH (M = Cu) and 2ؒ2.5MeCNؒ0.3CH2Cl2 (M = Zn)
2ؒ2.5MeCNؒ
0.3CH2Cl2
1ؒ4DMF
1ؒ3MeOH
M(1)–O(1)
M(1)–O(1A)
M(1)–N(5)
M(1)–N(5A)
1.899(1)
1.899(1)
1.958(1)
1.958(1)
1.919(2)
1.913(2)
1.952(2)
1.945(2)
1.914(2)
1.907(2)
1.996(2)
1.993(2)
O(1A)–M(1)–O(1)
N(5A)–M(1)–N(5)
O(1A)–M(1)–N(5)
O(1)–M(1)–N(5A)
O(1A)–M(1)–N(5A)
O(1)–M(1)–N(5)
86.23(6)
101.27(7)
154.22(5)
154.22(5)
91.46(5)
91.46(5)
95.33(6)
100.58(7)
148.97(7)
149.28(7)
90.15(7)
90.10(7)
111.21(8)
123.02(9)
115.57(8)
118.07(8)
95.52(8)
94.53(8)
The Cu() centre of 1ؒ4DMF possesses a distorted square
planar coordination geometry; the angle between the Cu(1)-
N(5)O(1) and Cu(1)N(5A)O(1A) planes is 34.5Њ, whereas the
corresponding angle for 1ؒ3MeOH is 44.0Њ, i.e. closer to that of
a distorted tetrahedron. Moreover, there are some signifi-
cant differences in the length of chemically equivalent Cu–O
bonds that appear to be associated with the presence of
intermolecular solvent/complex hydrogen bonding. Thus, in
1ؒ3MeOH, in which each co-ordinated phenolate oxygen is
hydrogen bonded to a MeOH molecule, each Cu–O bond
[Cu(1)–O(1) = 1.919(2) and Cu(1)–O(1A) = 1.913(2) Å] is sig-
nificantly longer than the Cu–O bonds in 1ؒ4DMF [Cu(1)–O(1)
= Cu(1)–O(1A) = 1.899(1) Å], for which there is no corre-
sponding interaction.
The Cu() centres of 1ؒ4DMF and 1ؒ3MeOH each possesses
a cis-CuN2O2 geometry. One side of each of the complexes is
occupied by two o,p-substituted phenolate groups whilst the
other side is occupied by the two bulky 4,5-disubstituted
imidazole groups.
The Zn() centre of 2ؒ2.5MeCNؒ0.3CH2Cl2 possesses a dis-
torted tetrahedral coordination geometry; the angle between
the Zn(1)O(1)N(5) and Zn(1)O(1A)N(5A) planes is 90.3Њ and
the four N–Zn(1)–O interligand angles range from 111 to 118Њ
(Table 2). The lengths of the Zn–O and Zn–N bonds are in
good agreement with those found for bis-salicyldiminato-
Zn()13 complexes and other Zn() complexes containing two
phenolate O-donor and two N-donor ligands.14
Fig. 5 EPR spectra of 1: (a) X-band spectrum of a solution of 1 in
CH2Cl2–toluene (9 : 1) at 77 K; (inset) zoom of one of the peaks in the
gzz region showing superhyperfine structure; (b) K-band spectrum of a
solution of 1 in CH2Cl2–toluene (9 : 1) at 125 K.
assignment is extended to include the 499 nm band of 1. The
band at 692 nm is assigned to a ligand field (d
d) transition
on the basis of comparisons with distorted square planar bis-
(salicylaldiminato)Cu() complexes that typically contain an
absorption at similar wavelength.19 2 does not manifest any
absorptions in the visible region.
EPR and UV/vis spectra. The X-band and K-band EPR spec-
tra of 1 in frozen CH2Cl2–toluene (9 : 1) solution are shown in
Fig. 5. The EPR spectra are typical of a Cu() complex possess-
Redox properties of 1 and 2. The cyclic voltammogram of 1
(Fig. 6a) in CH2Cl2 at room temperature displays two, reversible,
one-electron oxidation processes at 0.16 and 0.50 V vs. Fcϩ/Fc
(Table 3) and an irreversible reduction process in the region
Ϫ1.5 to Ϫ1.8 V vs. Fcϩ/Fc with no apparent associated oxid-
ation. Similarly, the cyclic voltammogram of 2 (Fig. 6b), in
CH2Cl2 at room temperature exhibits a reversible one-electron
oxidation at 0.23 V vs. Fcϩ/Fc and an essentially reversible
couple at 0.49 V vs. Fcϩ/Fc (Table 3).¶ However, no cathodic
peak was observed in the region 0 to Ϫ2.5 V vs. Fcϩ/Fc.
In addition to these electrochemical processes, both 1 and 2
exhibit further oxidation process(es) at 0.962 and 1.130 (1); and
1.034 (2) V vs. Fcϩ/Fc, each of which is irreversible.
The close correspondence between the electrochemical prop-
erties of 1 and 2 indicates that the two reversible oxidation
1
2
1
2
ing a (dx Ϫy ) or (dxy) ground state (S = 1/2), with gzz > gxx
≈
gyy.15 The X-band EPR spectrum of 1 displays hyperfine and
superhyperfine features in the gxx/gyy region, resulting from the
interaction of the unpaired electron with the nuclear spin of
63,65Cu (I = 3/2) and with the nuclear spin of the coordinated
nitrogen atoms (14N; I = 1); 14N superhyperfine coupling is also
apparent in the gzz region of the X-band spectrum (Fig. 5a,
inset). Spin-Hamiltonian parameters for 1 have been extracted
by the simultaneous simulation of the X- and K-band EPR
spectra of 1 using an in-house program.16 The best fit yielded:
gxx = 2.053, gyy = 2.047, gzz = 2.253; Azz{63,65Cu} = 164 G,
Axx{63,65Cu} = Ayy{63,65Cu} = 29 G, and axx = ayy = azz = a{14N} =
12 G. The Azz and gzz values observed are typical of those of a
tetragonal CuII–N2O2 centre17 and indicate that the N2O2-co-
ordination sphere of 1 is retained in solution. As expected, 2 is
EPR-silent.
c
a
¶ For the redox process at 0.49 V (vs. Fcϩ/Fc) of 2, Ϫip /ip = 1 0.1,
c
a
(scan rate)1/2 ∝ ip (and ip ) and ∆E = 148 mV. A slight shoulder was
observed on the second oxidation wave for both 1 and 2 (Fig. 6). The
controlled potential electrolysis of 1 at 0.78 (at rt) and 1.25 V (at 0 ЊC)
The UV/vis absorption spectrum of 1 in CH2Cl2 at room
temperature is characterised by two bands in the visible region
at 499 (670) and 692 nm (550 MϪ1 cmϪ1). The former absorp-
tion resembles that of other Cu()-phenolate systems (λmax 450–
(vs. SCE) indicated the transfer of 0.91 and 1.86 electrons moleculeϪ1
,
respectively. The controlled potential electrolysis of 2 at 0.87 (at rt)
and 1.20 V (at 0 ЊC) (vs. SCE) indicated the transfer of 1.00 and
2.14 electrons moleculeϪ1, respectively.
530 nm; ε 1000–2000 MϪ1 cmϪ1 18 which have been assigned to a
)
phenolate-to-Cu() charge transfer transition (LMCT). This
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1979

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n
Table 3 Cyclic voltammetric data for 1 and 2 recorded in CH2Cl2 at
298 K. The potentials are expressed in V vs. [Fc]ϩ/[Fc]a
CH2Cl2 solution containing [NBu4 ][BF4] (0.4 M) under a di-
nitrogen atmosphere, with an applied potential of 0.78 and 0.87
V vs. SCE, respectively, induced a drastic colour change. Thus,
in each case an intense, dark green coloured solution was pro-
duced, due to the formation of the oxidation product [1]ϩ and
[2]ϩ, respectively. Remarkably, these oxidised species were found
to be stable under these conditions.
First ligand-
based process
Second ligand-
based process
CuII/CuI
c
Ep
Epa (∆E/mV)
E1/2
Epa (∆E/mV)
E1/2
In contrast to the EPR spectrum of 1 which exhibits an S = ½
1
2
Ϫ1.5 to Ϫ1.8
—
0.202(87)
0.269(87)
0.16
0.23
0.540(90)
0.567(148)
0.50
0.49
signal, typical of Cu() in an axial ligand field, [1]ϩ in CH2Cl2
n
∆E = 106 mV for [FeCp*]ϩ/[FeCp*].a These data have been redeter-
mined; as compared to the data reported previously,5 the E1/2 values
differ by ≤0.02 V.
(containing [NBu4 ][BF4] (0.4 M)) at 77 K, was found to be
EPR-silent [NB a weak (<10% of the intensity observed for 1)
Cu() signal was observed]. The lack of an EPR signal for [1]ϩ
is consistent with a S = 0 or 1 ground state, resulting from the
magnetic coupling between the S = ½ Cu() centre and a S = ½
co-ordinated radical ligand, as has been observed for other
Cu()-phenoxyl radical complexes.18
The EPR spectrum of [2]ϩ, in CH2Cl2 at 77 K, consists of a
single isotropic signal with giso = 2.0047. This EPR spectrum
does not exhibit any hyperfine splitting and possesses a
line width of ∼10 G. The S = ½ EPR signal obtained for [2]ϩ
is typical of that for a radical and the g-value is within
the range of those reported for Zn()-phenoxyl radical
compounds.20
The OTTLE spectra recorded for the one-electron oxidation
n
of 1 and 2, in CH2Cl2, containing [NBu4 ][BF4] (0.4 M) at 0 ЊC,
each shows a series of changes in the absorbance profile (Fig. 7).
The observation of isosbestic points indicate that the oxidation
of 1 and 2, produces a single species, [1]ϩ or [2]ϩ, directly and
without the involvement of any detectable intermediate. In each
case, the electrochemical reduction of [1]ϩ and [2]ϩ returned the
UV/vis spectrum to the profile of the corresponding ML2
compound, confirming the chemical reversibility of the one-
Fig. 6 Cyclic voltammogram of (a) 1 and (b) 2, recorded for a CH2Cl2
n
solution at 298 K containing [NBu4 ][BF4] (0.4 M) as the supporting
electrolyte, using a glassy carbon working electrode at a scan rate of 100
mV sϪ1
.
processes observed (Fig. 6) are ligand-based, especially since
Zn() is not redox active. Thus, these processes are attributed to
the successive one-electron oxidation of each coordinated
ϩ
ؒ
ligand, leading to the formation of [M(L )(L)] initially and
then [M(L )]2ϩ, these complexes involve one or two neutral rad-
ical ligands, L ; respectively (Scheme 1). The reversibility
ؒ
ؒ
ؒ
observed for both oxidation processes indicates that [M(L )-
(L)] and [M(L )2]2ϩ do not decompose during the course of the
ϩ
ؒ
cyclic voltammetric experiment. The irreversible reduction pro-
cesses observed in the region Ϫ1.5 to Ϫ1.8 V vs. Fcϩ/Fc for 1,
but not for 2, is attributed to the Cu()/Cu() couple.
Fig. 7 UV/vis spectra recorded for the one-electron oxidation of 1 and
2 in an OTTLE cell with an applied potential of 865 and 845 mV vs.
SCE, respectively. Each experiment was carried out at 0 ЊC for a
n
solution of 1 and 2 (∼1 mM) in CH2Cl2 containing [NBu4 ][BF4] (0.4 M)
as the supporting electrolyte. λmax/nm (ε/MϪ1 cmϪ1) for [1]ϩ: 278 (40900),
sh 299(38000), br 346 (25900), sh 368 (22200), sh 408 (11800), 484
(4900), 580 (3800), 670–900 (1700); λmax/nm (ε/MϪ1 cmϪ1) for [2]ϩ: 225
(63000), br 279 (37240), br 356 (22000), sh 410 (8500), br ca. 560 (2870),
700–850 (1000). Isosbestic points for the oxidation of 1: 262, 291, 351,
391 nm and for 2: 262, 334, 385 nm.
Scheme 1
؉
ؒ
The phenoxyl radical complexes [M(L )(L)]
Electrochemically generated [1]؉ and [2]؉. Electrochemical
one-electron oxidation of 1 and 2, at room temperature in
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1980

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Fig. 8 Selected bond lengths (Å) of (a) 1ؒ4DMF, in which the two ligands are related by a C2 axis and (b) the cation of [1][BF4]ؒ2CH2Cl2 containing
LA and LB (values for the latter are given in parentheses).
electron electrochemical process on the time scale of the
OTTLE experiment.
The absorption spectrum of [1]ϩ and [2]ϩ each exhibits an
intense absorption at ca. 410 nm (ε/MϪ1 cmϪ1: 11800 [1]ϩ;
8500 [2]ϩ) and less intense broad absorptions between 500 and
900 nm (Fig. 7). The absorption at ca. 410 nm is assigned to the
π
π* transition of the phenoxyl radical ligand, as observed in
the UV/vis spectra of several other Cu()- and Zn()-phenoxyl
radical complexes.18,20 The lower energy band could arise from
ligand-based transitions (π
and/or LLCT transitions. Since these absorptions occur for
both [1]ϩ and [2]ϩ, they can not arise from d
d transitions.
π* or n
π*), MLCT, LMCT,
Chemically generated [1]؉ and [2]؉. The one-electron oxid-
ation of 1 and 2 by Ag[BF4] (1 eq.) in CH2Cl2 produced the
dark green compounds [1][BF4] and [2][BF4], respectively. These
compounds were found to be inert in the solid state, even when
exposed to air, and a solution of each of these compounds in
CH2Cl2, at room temperature under an argon atmosphere,
remained unchanged for several days. The remarkable inertness
of these compounds has permitted their structural, spectro-
scopic, and magnetic characterisation.
Fig. 9 An ORTEP7b representation of the structure of [1]ϩ in [1][BF4]ؒ
2CH2Cl2.
one-electron oxidation of 1. The copper centre of [1][BF4]
possesses a cis-arrangement of the phenol O-donor atoms; i.e.
the two 4,5-diphenyl imidazole units are accommodated on the
same side of the complex, as in 1.
Spectroscopic and electrochemical investigations. The UV/vis
spectra of [1][BF4] and [2][BF4], in CH2Cl2 at 0 ЊC, were essen-
tially identical to those of the corresponding species gener-
ated electrochemically. The room temperature X-band EPR
spectrum of [1][BF4], in the solid and CH2Cl2 solution states,
was found to be EPR-silent over the magnetic field range of
0–6000 G. The 77 K X-band EPR-spectrum of [1][BF4] in
CH2Cl2 solution showed a weak signal which was attributed to
the presence of ∼5% of unreacted starting material 1.
The X-band EPR spectrum of [2][BF4] at 77 K in CH2Cl2
solution exhibited a signal at giso = 2.0047 with a line width of
∼10 G and no observable hyperfine splitting. This signal is
essentially identical to that obtained for the corresponding
electrochemically generated species (giso = 2.004).
In comparing the dimensions of [1]ϩ with the correspond-
ing values of 1, attention is confined to 1ؒ4DMF since, in
1ؒ3MeOH, possible perturbations arise due to the presence of
MeO–H ؒ ؒ ؒ O(phenolate) hydrogen bonds. The structural
details of [1]ϩ differ slightly, but significantly, from the corre-
sponding aspects of 1 in 1ؒ4DMF (Fig. 8). The dihedral angle
between the two Cu(1)O(1)N(5) and Cu(1)O(1A)N(5A) intra-
ligand planes is 46.9Њ which is larger than the corresponding
angles in 1ؒ4DMF and 1ؒ3MeOH (34.5 and 44.0Њ, respectively).
The two ligands of 1ؒ4DMF are related by a C2-axis, but the
two ligands of [1]ϩ in [1][BF4]ؒ2CH2Cl2, designated as LA and
LB (Figs. 8 and 9), are inequivalent. For LB, the length of each
C–C, C–O, and C–N bond is not significantly different from the
length of the corresponding bonds in 1ؒ4DMF. However, for
LA, the C–O distance [1.264(5) Å] is significantly shorter than
that in 1 [1.316(2) Å] and that in LB [1.322(5) Å]. Also, the two
C–C bonds adjacent to the C–O group of LA [C(18A)–C(19A)
= 1.449(6) Å and C(19A)–C(20A) = 1.460(6) Å] are each signifi-
cantly longer than the corresponding bonds of both 1 [1.416(2)
and 1.431(2) Å, respectively] and LB [1.425(5) and 1.424(6) Å,
respectively]. Furthermore, the length of the Cu–O bond
involving LA [1.948(3) Å] is significantly longer than that in 1ؒ
4DMF [1.899(1) Å], but the length of each of the other three
The cyclic voltammograms of [1][BF4] and [2][BF4], in
CH2Cl2 at room temperature, were identical to that of the
corresponding parent compound.
Thus, each of the cations [1]ϩ (in [1][BF4]) and [2]ϩ (in
[2][BF4]) is identical to the corresponding electrochemically
generated species.
Structural investigations
[1][BF4]ؒ2CH2Cl2. The X-ray crystallographic characteris-
ation of [1][BF4]ؒ2CH2Cl2 (see Figs. 8 and 9 and Table 1)
confirms the formation of [1]ϩ as the product of the chemical
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1981

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metal–ligand bonds in [1]ϩ [Cu–O(1) = 1.850(3) Å, Cu–N(5) =
1.924(3) Å and Cu–N(5A) = 1.933(4) Å] is each significantly
shorter than that of its counterpart in 1ؒ4DMF [Cu–O =
1.899(1) Å and Cu–N = 1.958(1) Å] (Fig. 8).
Table 4 Selected bond lengths (Å) for the Zn centre in [2]ؒ2.5MeCNؒ
0.3CH2Cl2 and [2][BF4]ؒ2CH2Cl2ؒ0.75pentane
[2][BF4]ؒ2CH2Cl2ؒ
0.75pentane
[2]ؒ2.5MeCNؒ
0.3CH2Cl2
This structural information is taken to indicate that [1]ϩ con-
B
tains both a phenoxyl radical (LA = L ) and a phenolate (L =
ؒ
Zn–O(1)
1.875(5)
1.997(6)
1.950(6)
1.976(5)
1.914(2)
1.907(2)
1.996(2)
1.993(2)
LϪ) ligand coordinated to CuII; i.e. [1]ϩ is constituted as [CuII-
Zn–O(1A)
Zn–N(5)
Zn–N(5A)
Ϫ
ϩ
ؒ
(L )(L )] . Oxidation of a phenolate ligand in 1 to a phenoxyl
radical in [1]ϩ is accompanied by a shortening of the C–O bond
and the lengthening of the two adjacent C–C bonds. The
changes in the length of these C–O and C–C bonds are in
sistent with the presence of [2]ϩ (Fig. 11 and Table 1). The zinc is
bound to two N,O-bidentate ligands, LA and LB, (Fig. 11)
and the complex has a tetrahedral N2O2-coordination sphere;
the dihedral angle between the two Zn(1)O(1)N(5) and
Zn(1)O(1A)N(5A) intraligand planes is 95.5Њ, similar to that
(90.3Њ) of 2. A comparison of the dimensions of [2]ϩ and 2
shows some important structural changes about the Zn()
(Table 4). Zn–O(1A) in [2]ϩ is significantly longer than both
Zn–O bonds in 2, however, each of the other three Zn–O/N
bonds in [2]ϩ is significantly shorter than that of the corre-
sponding bonds in 2 (Table 4). This pattern resembles that of
[1]ϩ and is consistent with the formulation of [2]ϩ as [ZnII-
ؒ
accord with a description of the electronic structure of L by
the two resonance forms shown in Fig. 10, i.e. the unpaired
electron is delocalised over both ortho positions of the coordin-
ؒ
ated phenoxyl ring. The shorter C–O bond length in L , as com-
pared to that in LϪ, is consistent with the structural information
that has been obtained for [CrIIIL]ϩ [H3L = 1,4,7-tris(3-tert-
butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane]
with one phenoxyl radical and two phenolate ligands. In
[CrIIIL]ϩ the length of the C–O bond of the phenoxyl radical
[1.290(4) Å] is significantly less than that of the two phenolate
ligands [1.328(6) and 1.342(4) Å].21 In [1]ϩ the length of the
C–O bond of LA of 1.264(5) Å emphasising a significant
amount of double bond character (see Fig. 8) in this bond. As
compared to 1ؒ4DMF, the shortening of the C–O bond in LA is
accompanied by a lengthening of the corresponding Cu–O
bond and the contraction of the three other Cu–O/N bonds,
Ϫ
ϩ
A
ؒ
ؒ
(L )(L )] , with L being L . However, the large uncertainties in
the C–C, C–N and C–O bond lengths of [2]ϩ preclude any
assessments of the changes in the dimensions within each co-
ordinated ligand.
ؒ
consistent with the oxygen atom of the phenoxyl (L ) donating
relatively less electron density to the Cu() than the oxygen of
the phenoxide (LϪ).
2ϩ
Ϫ
ؒ
ؒ
Fig. 10 Resonance forms of L in [Cu (L )(L )][BF4].
[2][BF4]ؒ2CH2Cl2ؒ0.75pentane. Although single crystals of
[2][BF4]ؒ2CH2Cl2ؒ0.75pentane of a reasonable quality and size
were obtained, the structure of this compound could be only
partially determined by X-ray crystallography. This situation
prevailed despite repeated attempts to collect good quality dif-
fraction data by employing longer scan times, larger crystals,
and a lower sample temperature (85 K). The problems
encountered may arise due to disordered solvent molecules
within the lattice of the crystal. Nevertheless, the structural
information clearly indicates a stoichiometry of [2][BF4], con-
Fig. 12 Variation of χ vs. T for [1][BF4]; the solid line represents a
simulation of the experimental data.
Magnetic properties of [1][BF4]. The temperature dependence
of the magnetic susceptibility of [1][BF4] over the temperature
range 3 to 320 K (Fig. 12) exhibits a maximum (at ca. 12 K)
indicative of an antiferromagnetic interaction. The variation of
χ with T (Fig. 12) has been modelled successfully by the Hamil-
tonian H = Ϫ2JSCuSrad (J = Ϫ6.1 cmϪ1; SCu = Srad = ½; gCu
=
2.12; grad = 2.00).5 Thus, [1][BF4] possesses a diamagnetic
ground state (S = 0) that is considered to arise from an anti-
ferromagnetic coupling between the unpaired electron of the
Cu() and the unpaired electron of the coordinated phenoxyl
5
ؒ
radical, L .
Discussion
This study has accomplished the design, synthesis, and charac-
terisation of the pro-ligand LH and shown that this molecule is
capable of undergoing a reversible, one-electron, oxidation to
ϩ
ؒ
ϩ
ؒ
form the radical cation [LH] . [LH] has been produced by
electrochemical oxidation. The spectroscopic characteristics
ϩ
ؒ
observed are consistent with [LH] being a phenoxyl radical
cation; i.e. the unpaired electron is delocalised, primarily or
exclusively, over the phenol group. Thus, this moiety exhibits
intense absorptions in the visible region, in particular two
bands at ca. 400 nm (ε ca. 10000 MϪ1 cmϪ1), that resemble the
absorptions observed for phenoxyl radicals22a and phenoxyl
Fig. 11 An ORTEP representation of the structure of [2][BF4] in
[2][BF4]ؒ2CH2Cl2ؒ0.75pentane.
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1982

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ϩ
radical cations.22–24 Also, the EPR spectrum of [LH] com-
prises a signal with a g-value (2.004) that is typical of an
organic radical. The lack of any hyperfine structure in the EPR
different metal complex/solvent interactions, indicating that
small distortions of this CuN2O2 coordination sphere can be
achieved relatively easily.
ؒ
ϩ
ؒ
spectrum of [LH] precludes any analysis of the spin density
distribution within this species. However, since the line width of
the signal is relatively narrow (ca. 10 G), any hyperfine splitting
will be relatively small; this may indicate that the unpaired elec-
tron is not (or only to a limited extent) localised on the N-imid-
azole atoms.
1 adopts a cis-arrangement, i.e. the O(1)Cu(1)O(1A) and
N(5)Cu(1)N(5A) angles (ca. 90Њ and ca. 100Њ, respectively) are
smaller than the O(1)Cu(1)N(5A) and O(1A)Cu(1)N(5) angles
(ca. 150Њ), indicating the relative proximity of O(1) to O(1A)
and of N(5) to N(5A). A cis-arrangement is rarely encountered
for a Cu() centre bound to two N,O-bidentate ligands that
involve phenolate groups, e.g. no such example has been
reported for a bis-salicylaldiminato-Cu() complex. Also, the
adoption of the cis-geometry is surprising since the 4,5-diaryl-
imidazole groups are sterically demanding.29 However, the
adoption of this arrangement may be rationalised by consider-
ing interligand π–π interactions (vide infra).
The reversible cyclic voltammogram of LH is remarkable
and unexpected for a phenol-containing compound. Indeed,
the cyclic voltammograms of phenols that are capable of form-
ing a stable phenoxyl radical (e.g. 2,4,6-tri-tert-butylphenol)22a
exhibit, in non-basic media, a two-electron irreversible oxid-
ation.25 One-electron oxidation produces the phenoxyl radical
ϩ
ؒ
cation, [PhOH] , that contains a strongly acidic O–H group
The one-electron oxidation of 1 and 2 forms [1]ϩ and [2]ϩ,
respectively. These cations are remarkably stable, both in solu-
tion and the solid state, at or below room temperature. The
nature of [1]ϩ and [2]ϩ has been confirmed by the X-ray crystal-
lographic investigations of [1][BF4]ؒ2CH2Cl2 and [2][BF4]ؒ
2CH2Cl2ؒ0.75pentane. These studies have shown that the
geometry around the metal centre of each [ML2]ϩ cation is very
similar to its [ML2] counterpart. However, there are small, but
significant variations in the length of the M–O/N bonds and,
for 1 vs. [1]ϩ, in the lengths of the C–O and the adjacent C–C
bonds. These variations are consistent with the formulation of
ؒ
(pKa ca. Ϫ5)22b that is readily deprotonated to form the phen-
ؒ
oxyl radical, [PhO] . The latter is more easily oxidised than the
corresponding PhOH (e.g. in MeCN, the 2,4,6-tri-tert-butyl-
phenoxyl radical is oxidised at a potential ca. 300 mV less than
that required to oxidise the phenol)26 and, consequently, under-
goes a further one-electron oxidation to form the [PhO]ϩ
cation.25
The crystal structure of LH clearly shows the presence of a
strong O–H ؒ ؒ ؒ N intramolecular hydrogen bond between the
phenol and imidazole groups (Fig. 1). The reversibility of the
ϩ
II
Ϫ
ϩ
ؒ
ؒ
electrochemical oxidation of LH indicates that [LH] retains
this phenolic proton, presumably now involved in an O–
these cations as [M (L )(L )] (M = Cu, Zn), containing a
phenoxyl radical and a phenoxide ligand. In respect of this
information, it is interesting to note that Cu K-edge X-absorp-
tion spectroscopic studies have indicated that there are no
major structural differences in the Cu() coordination spheres
of the inactive and the active (Cu()-tyrosinyl) forms of GAO.30
The UV/vis and EPR properties of [1]ϩ and [2]ϩ are also
consistent with each of these species being constituted as a
ؒ
H ؒ ؒ ؒ N intramolecular hydrogen bond (Fig. 13). The results of
other investigations support this postulate. Firstly, apart from
the results reported herein, only three phenolic compounds
have been reported to exhibit an oxidation that is electro-
chemically reversible; in each case, the electrochemistry was
accomplished in an acidic medium.27 Secondly, Maki et al.23
reported the first example of reversible electron transfer
between a phenol and its phenoxyl radical cation and showed
that this was facilitated by intramolecular hydrogen bonding.
This study involved a 2,4-di-tert-butylphenol that was substi-
tuted at the 6-position by an α-alkylamino group, thus allowing
the formation of a six-membered ring involving an O–H ؒ ؒ ؒ N
intramolecular hydrogen bond (cf. that for LH, Fig. 1). The
EPR (giso = 2.0046; line width 10 G, no hyperfine splitting) and
UV/vis (λmax 386 and 404 nm) properties of this phenoxyl radi-
ϩ
ؒ
[M(L )(L)] complex. The potential for the first oxidation of 1
and 2 is less positive than that for the oxidation of LH, by 286
and 219 mV, respectively; i.e. compared to the phenol, the bind-
ing of a phenolate to Cu() or Zn() facilitates its oxidation
to the phenoxyl radical. This effect has been observed previ-
ously for Cu() complexes by Halcrow et al.31 One mechanism
for the stabilisation of the phenoxyl radical could involve back-
ؒ
donation, from metal d-orbitals into an empty π-orbital of L .
Such facilitation of phenoxyl radical formation presumably
assists the formation of the tyrosinyl radical required for the
active form of GAO.
cal cation are very similar to those of [LH] ϩ. Therefore, it
appears that the presence of an O–H ؒ ؒ ؒ N intramolecular
ؒ
ؒ
hydrogen bond, between the phenoxyl radical and the N-imid-
azole group, makes a significant contribution to the stability of
The Cu()-phenoxyl radical complex, [1]ϩ, possesses a dia-
magnetic (S = 0) ground state that is considered to arise from
antiferromagnetic coupling between the unpaired electron of
[LH] ϩ. Such an interaction is strikingly similar to that pro-
ؒ
ؒ
posed between the Tyr160 (YD) radical and the adjacent His189
the Cu() and that of the coordinated phenoxyl radical, L .
residue in PSII.28
However, this coupling (|J| = 6.1 cmϪ1) is weak compared to
that observed for the Cu()-tyrosinyl radical pair of the active
form of GAO (|J| > 200 cmϪ1).32 This strong antiferromagnetic
interaction has been rationalised by a qualitative magnetostruc-
tural model based on the Goodenough–Kananmori rules that
proposes a significant overlap between the two half-occupied
2
2
orbitals, viz. the Cu() dx Ϫy and a π-orbital of the phenoxyl
radical ligand.33 The weak antiferromagnetic coupling in
[1][BF4] may imply that the orbital containing the unpaired
electron on the Cu() centre and the half occupied π-orbital of
the phenoxyl radical do not overlap effectively.
As for 1, the Cu() centre of [1][BF4] possesses a cis-
arrangement of the two phenol O-donor atoms with the two
4,5-di-phenyl imidazole units being accommodated on the same
side of the complex. Associated with this observation, [1]ϩ
involves two intramolecular π–π interactions, between the
phenyl ring of one ligand and the phenol and/or imidazole ring
of the other ligand (Fig. 14). These involve: (i) the phenyl ring
[C(12)–C(17)] of LϪ and the phenol (PhO/Ph interaction) and
ϩ
ؒ
Fig. 13 The intramolecular hydrogen bonding in [LH]
.
The structural details obtained for 1 and 2 have shown that
normal geometrical preferences of these M() centres are
adopted; i.e. the CuN2O2 core of 1 is distorted square planar
and the ZnN2O2 core of 2 is essentially tetrahedral. The small
angular and radial variations observed between the Cu()
centres of 1ؒ4DMF and 1ؒ3MeOH are presumably due to
ؒ
the imidazole (IM/Ph interaction) ring of L and (ii) the phenyl
ؒ
ring [C(12A)–C(17A)] of L and the phenol (PhO/Ph) and the
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1983

View Article Online
Table 5 Geometrical parameters for the π–π stacking interactions in [1]ؒ[BF4]ؒ2CH2Cl2, 1ؒ4DMF, 1ؒ3MeOH and 2ؒ[BF4]ؒ2CH2Cl2ؒ0.75pentane
Angles
between
PhO and
IM/Њ
Angles
Angles
between
Offset
between
Offset
Compound
Interaction PPh and PIM/Њ dcIMcPh/Å dcIMPPh /Å Ph/IM/Å PPh and PIM/Њ dcPhOcPh/Å dcPhOPPh/Å PhO/Ph/Å
L
L(A)
[1]ؒ
Ph/IM(A)
6.4
9.5
3.43
3.51
3.29
3.30
0.99
1.11
[BF4]ؒ2CH2Cl2
Ph(A)/IM
Ph/PhO(A)
Ph(A)/PhO
8.1
9.9
4.89
4.69
3.50
3.60
3.42
3.01
2.3
8.3
1ؒ4DMF
Ph/IM(A)
Ph(A)/IM
Ph/PhO(A)
Ph(A)/PhO
25.0
25.0
3.62
3.62
3.40
3.40
1.26
1.26
12.8
12.8
5.57
5.57
2.50
2.50
4.96
4.96
18.9 18.9
18.9 15.2
15.7 12.2
1ؒ3MeOH
Ph/IM(A)
Ph(A)/IM
Ph/PhO(A)
Ph(A)/PhO
12.5
14.6
3.51
3.69
3.46
3.52
0.58
1.11
17.0
14.0
4.54
4.24
3.65
3.65
2.70
2.14
2ؒ[BF4]ؒ
Ph/IM(A)
9.6
3.78
3.60
1.15
2CH2Cl2ؒ
0.75pentane
Ph/PhO(A)
6.7
4.34
3.24
2.89
hedral centre of 2. However, in [2]ϩ there is π–π interaction
between the phenyl ring of LϪ [C(12)–C(17)] and the phenol/
ϩ
ؒ
imidazole system of L (Fig. 16, Table 5). Thus, in [2] , the twist
angle between the C(12)–C(17) plane and the imidazole plane
(75.5Њ) in the ligand LϪ is significantly larger than that in L
ؒ
(42.0Њ) and that in both of the ligands in 2 [42.7Њ for L and 27.3Њ
for L(A)]. Thus, oxidation of 2 to [2]ϩ leads to a twist of the
C12–C17 ring that orients this ring parallel to the plane of the
ؒ
phenol/imidazole unit of L . The values obtained for the inter-
actions C(12)–C(17)/PhO(A) [Ph/PhO(A)] and [C(12)–C(17)]/
IM(A) [Ph/IM(A)] (Figs. 15 and 16; Table 5) resemble those
obtained for graphitic interactions between two arenes.34
Fig. 14 View of [1]ϩ in [1][BF4]ؒ2CH2Cl2 showing the intramolecular
B
Ϫ
π–π interactions between the two ligands LA (L ) and L (L ). For each
ؒ
t
ligand, one phenyl ring and the two Bu groups have been omitted for
clarity.
imidazole (IM/PhO) ring of LϪ (Fig. 14). Values of the param-
eters (Fig. 15) used to describe these interactions (Table 5) are in
the range observed for graphitic interactions between two arene
rings.34
Fig. 16 View of [2]ϩ in [2][BF4]ؒ2CH2Cl2ؒ0.75 pentane showing the
intramolecular π–π interactions between the C(12)–C(17) ring of
B
Ϫ
A
ؒ
L (L ) and the phenol–imidazole unit of L (L ). For each ligand, one
phenyl ring and the two tBu groups have been omitted for clarity.
Thus, in [1]ϩ and [2]ϩ the phenoxyl radical ligand is involved
in intramolecular π–π stacking interactions. These interactions
mimic the stacking of Tyr272 above the Trp290 indole ring3 in
the active site of GAO that has been suggested to contribute to
the unusual stability of the phenoxyl radical of the catalytically
active form of GAO.35 However, the significance of this
interaction has been debated by Halcrow36 and Liu et al.37
have shown that, for 1-{pyrid-2-yl}-3-{2Ј,5Ј-dimethoxyphenyl}-
pyrazole radical complexes of CuII, intramolecular π–π stack-
ing has no measurable effect on their kinetic stability. The
results reported herein represent the first structural characteris-
Fig. 15 A representation of the parameters involved in the inter-
actions PhO/Ph and IM/Ph in [1]ϩ and [2]ϩ. The letter c is used for
centroid of a ring and P for plane.
Similar π–π interactions are observed for 1 in 1ؒ4DMF and 1ؒ
3MeOH (Table 5). The presence of these π–π interactions are
consistent with the cis-structures observed for the Cu() centres
of 1ؒ4DMF, 1ؒ3MeOH, and [1][BF4]ؒ2CH2Cl2.
Although the distorted square planar geometry of the Cu()
centres of 1 and [1]ϩ allows the intramolecular interligand π–π
interactions described above, this is not the case for the tetra-
D a l t o n T r a n s . , 2 0 0 3 , 1 9 7 5 – 1 9 8 5
1984

View Article Online
16 F. E. Mabbs and D. Collison, Electron Paramagnetic Resonance of
ation of phenoxyl radical complexes involving π–π stacking
interactions that parallel those observed in GAO. As noted
above, these interactions may well lower the oxidation potential
of the ligand and assist the formation of L , however, the
kinetic and/or thermodynamic effects engendered by the π–π
d-Transition Complexes, Elsevier, Amsterdam, 1992, p. 218.
17 J. Peisach and W. R. Blumberg, Arch. Biochem. Biophys., 1974, 165,
691.
ؒ
18 B. A. Jazdzewski and W. B. Tolman, Coord. Chem. Rev., 2000, 200,
633.
stacking interactions have yet to be established.
19 R. H. Holm, G. W. Everett Jr and A. Chakravorty, Prog.
Inorg. Chem., 1966, 7, 83.
20 (a) J. A. Halfen, B. A. Jazdzewski, S. Mahapatra, L. M. Berreau,
E. C. Wilkinson, L. Que Jr and W. B. Tolman, J. Am. Chem. Soc.,
1997, 119, 8217; (b) E. Bill, J. Müller, T. Weyhermülller and
K. Wieghardt, Inorg. Chem., 1999, 38, 5795; (c) A. Sokolowski,
J. Müller, T. Weyhermülller, R. Schnepf, P. Hildebrant,
K. Hildenbrand, E. Bothe and K. Wieghardt, J. Am. Chem. Soc.,
1997, 119, 8889.
Acknowledgements
We thank the EPSRC for provision of financial support, the
University of Nottingham for the provision of a studentship (to
L. B.) and Dr F. E. Mabbs and Professor M. W. George for
valuable discussions.
21 A. Sokolowski, E. Bothe, E. Bill, T. Weyhermülller and
K. Wieghardt, Chem. Commun., 1996, 1671.
22 (a) E. R. Altwicker, Chem. Rev., 1967, 67, 475; (b) E. J. Land,
G. Porter and E. Strachan, Trans. Faraday Soc., 1961, 57, 1885;
(c) E. J. Land and G. Porter, Trans. Faraday. Soc., 1963, 59, 2016.
23 T. Maki, Y. Araki, Y. Ishida, O. Onomura and Y. Matsumura,
J. Am. Chem. Soc., 2001, 123, 3371.
24 T. A. Gadosy, D. Shukla and L. J. Johnston, J. Phys. Chem. A, 1999,
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25 O. Hammerich, in Organic Electrochemistry, M. M. Baizer and
H. Lund, eds., Marcel Dekker, New York, 1983, p. 485.
26 A. B. Suttie, Tetrahedron Lett., 1969, 12, 953.
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1985