A phenoxyl radical complex of copper(II)

Article abstract of DOI:10.1039/b105186p

A new N,O-bidentate pro-ligand (HL), [ML₂] (M = Cu, Zn) and [CuL₂][BF₄] have been synthesised; [CuL₂]·4DMF and [CuL₂][BF₄]·2CH₂Cl₂ have been crystallographically and s

Full text of DOI:10.1039/b105186p

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A phenoxyl radical complex of copper(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 (in Cambridge, UK) 13th June 2001, Accepted 31st July 2001
First published as an Advance Article on the web 3rd September 2001
A new N,O-bidentate pro-ligand (HL), [ML2] (M = Cu, Zn)
and [CuL2][BF4] have been synthesised; [CuL2]·4DMF and
[CuL2][BF4]·2CH2Cl2 have been crystallographically and
spectroscopically characterised; these data indicate that
[CuL2]+ cations are constituted as [Cu2+(L·)(L2)]+ and
involve the phenoxyl radical L·.
p–p* transition of a phenoxyl radical.10 The X-band EPR
spectrum of the species generated by electrochemical oxidation
of HL in CH2Cl2 (recorded at 110 K) consisted of a single
resonance, giso = 2.004 (line width, 10 G).
The reaction of Cu(BF4)2·H2O with HL (1+2) in MeOH
yielded a brown solution that became deep green upon addition
of Et3N. Slow evaporation of this solution in air produced
purple–brown crystals of [CuL2] (1) (yield 85%); orange
crystals of [CuL2]·4DMF (1·4DMF) were obtained by the slow
evaporation of a solution of 1 in CH2Cl2–DMF (9+1 v/v).
[ZnL2] (2) was synthesised in a similar manner, from
Zn(BF4)2·H2O and HL (1+2). Compounds 1 and 2 each exhibit
two reversible, one-electron oxidations at 0.17, 0.51 (1) and
0.23, 0.51 (2) (E1/2/V vs. Fc+/Fc) (Fig. S1, ESI†). The close
correspondence between the electrochemical properties of 1 and
2 implies that each oxidation is ligand-based.
Galactose oxidase (GAO)1–3 and glyoxal oxidase (GLO)4
belong to an emerging and important class of metalloenzymes
that involve an organic radical as an integral component of their
active site.5 The active form of the catalytic site of both GAO
and GLO involves CuII ligated by a thioether-modified tyrosinyl
radical. This centre is responsible for a two-electron oxidation
of the substrate;1–7 e.g. GAO converts a primary alcohol to the
corresponding aldehyde with the concomitant reduction of O2 to
H2O2. Considerable progress has been made in the development
of chemical analogues of the active site of GAO, including
several examples of complexes that contain CuII bound to a
phenoxyl radical.8,9 However, such species have eluded
crystallographic characterisation.
Oxidation of 1 by Ag[BF4] (1+1) in CH2Cl2 produced an
intensely green coloured product, [1][BF4]·2CH2Cl2 (yield
85%), single crystals of which were obtained by recrystallisa-
tion from CH2Cl2–hexane (1+4 v/v).
Inspired by the nature of the catalytic centres of GAO and
GLO, the new N,O-bidentate pro-ligand 2-[2A-(4A,6A-di-tert-
butylhydroxyphenyl)]-4,5-diphenylimidazole (HL) was de-
signed. To enhance the possibility of isolating complexes of L·,
HL contains no readily oxidisable functionality, other than the
phenol group. Also, the 2,4-tBu groups block pathways for
decomposition of the phenoxyl radical state and could stabilise
this state by a +I effect.
HL was synthesised by the condensation of 3,5-di-tert-
butylhydroxybenzaldehyde with benzil (1+1) in AcOH in the
presence of an excess of NH4OAc (yield 75%). The UV–VIS
spectrum of HL in CH2Cl2 at 0 °C contains absorptions at lmax
228 nm (e = 36 500 M21 cm21), 296 (21 800) and 325
(25 200). The cyclic voltammogram of HL, at room temperature
in CH2Cl2, exhibits a reversible one-electron oxidation at 0.43
Fig. 1 ORTEP representation of the cation of [1][BF4]·2CH2Cl2.
The X-ray crystallographic characterisations‡ of 1·4DMF
and [1][BF4]·2CH2Cl2 have shown that each compound con-
tains a copper centre with a distorted tetrahedral N2O2-co-
ordination sphere. The two ligands of 1·4DMF are related by a
C2-axis, but the two ligands of the cation of [1][BF4]·2CH2Cl2
are structurally inequivalent (LA and LB; Figs. 1 and 2). For LB,
the length of each C–C, C–O and C–N bond is similar to that of
the corresponding bond of 1. However, for LA the C–O distance
is significantly shorter and the two adjacent C–C bonds are
significantly longer than the corresponding bonds of both 1 and
LB. Also, the Cu–O bond involving LA is significantly longer
than that in 1, but each of the other three metal–ligand bonds in
[1]+ is significantly shorter than its counterpart in 1. This
+
1
(E ⁄ /V vs. Fc /Fc, Fig. S1, ESI†). Electrochemical oxidation of
2
HL in CH2Cl2 at 240 °C in an OTTLE cell produced new
absorptions at lmax 379 nm (e = 9700 M21 cm21), 401 (9200),
514 (4000) and 715 (5600); the band at 401 nm is typical of a
† Electronic supplementary information (ESI) available: synthesis and
characterisation of HL, 1, [1][BF4] and 2; crystallographic data for 1·4DMF,
and details of electrochemical experiments. See http://www.rsc.org/
suppdata/cc/b1/b105186p/
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Chem. Commun., 2001, 1824–1825
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b105186p

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Fig. 2 Selected bond lengths of (a) 1·4DMF, 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).
structural information is taken to indicate that [1]+ contains both
a phenoxyl radical (LA = L·) and a phenolate (LB = L2) ligand
coordinated to CuII; i.e. [1]+ is constituted as [Cu2+(L·)(L2)]+.
Thus, the oxidation of a phenolate ligand in 1 to a phenoxyl
group in [1]+ is accompanied by a shortening of the C–O bond
and a lengthening of the Cu–O bond. This observation is
consistent with the structural information obtained11 for a
complex of CrIII that contains one phenoxyl radical and two
phenolate ligands, in which the C–O distance in the phenoxyl
radical is 1.290(4) Å. The C–O distance of LA is 1.264(5) Å,
emphasising the CNO character of this bond. The length of the
C–O and C–C bonds of LA, in comparison with those of LB and
the phenolate ligands of 1, are in accord with an electronic
structure described by the two resonance forms of Fig. 3, i.e. the
unpaired electron of the phenoxyl group is delocalised over both
ortho positions.
[1][BF4]·2CH2Cl2 have been crucial to this assignment12 and
the UV–VIS, EPR and magnetic susceptibility data recorded for
[1][BF4] provide additional and independent support for this
proposal. This study represents the first structural character-
isation of a phenoxyl radical bound to CuII, a chemical
combination that is crucial for the catalytic activity of GAO and
GLO.
We thank the EPSRC and The University of Nottingham for
the provision of a studentship (to L. B.) and Dr F. E. Mabbs for
valuable discussions.
Notes and references
‡ Crystal data for [1][BF4]·2CH2Cl2: C60H66N4O2BF4Cl4Cu, dark green
square plate, M = 1167.3, monoclinic, space group P21/n, a = 12.6062(7),
b = 18.2942(10), c = 25.5061(14) Å, b = 96.191(1)°, V = 5847.9(9) Å3,
Z = 4, Dc = 1.326 g cm23, T = 150(2) K, F(000) = 2432, m(Mo-Ka) =
0.61 mm21, 40636 reflections collected, 14159 unique (Rint = 0.053). The
final agreement factors are R1 = 0.068 for 8068 data with F > 4s(F) and R1
= 0.125, wR2 = 0.189 for 13749 data. The CH2Cl2, BF4 and one tBu group
exhibited disorder which, in each case, was modelled over two sites with
suitable geometric restraints applied. CCDC reference numbers 167603 and
167604. See http://www.rsc.org/suppdata/cc/b1/b105186/ for crystallo-
graphic data in CIF or other electronic format.
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1 and [1][BF4] display identical cyclic voltammograms in
CH2Cl2 and the UV–VIS spectrum of [1][BF4] is the same as
that obtained for [1]+ generated electrochemically. The OTTLE
spectra, (Fig. S2, ESI†) recorded for the oxidation of 1 in
CH2Cl2 at 0 °C show that this process produces an absorption at
410 nm that is assigned to a p–p* transition of the phenoxyl
radical (L) bound to CuII.8,9
The EPR spectrum of 1 in CH2Cl2–toluene (9+1) at 116 K,
(Fig. S3, ESI†) is typical of a rhombic Cu(II) (S = 1/2, I = 3/2)
1
2
1
2
complex with a (dx 2y ) (or (dxy) ) ground state: gxx = 2.053,
gyy = 2.047, gzz = 2.253. In contrast, [1][BF4] in CH2Cl2 at
77 K at X-band over the range 0–6000 G was found to be
essentially EPR silent. The temperature dependence (3–320 K)
of the magnetic susceptibility of [1][BF4] (Fig. S4, ESI†) has
been modelled by the Hamiltonian
H
=
22JSCuSrad,
where J = 26.1 cm21 and SCu = Srad = 1/2, gCu = 2.12, grad
= 2.00. [1][BF4] possesses a diamagnetic ground state (S = 0)
that is considered to arise from an antiferromagnetic coupling
between the unpaired electron of the Cu(II) and the unpaired
electron of the coordinated phenoxyl radical, L·.
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Thus, oxidation of [CuL2] is ligand-based and produces a
cation that is constituted as [Cu2+(L·)(L2)]+, involving CuII
bound to both a phenoxyl radical (L·) and a phenolate (L2)
ligand. The crystallographic information obtained for
Chem. Commun., 2001, 1824–1825
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