Phenolate and phenoxyl radical complexes of Cu(ii) and Co(iii), bearing a new redox active N,O-phenol-pyrazole ligand

Article abstract of DOI:10.1039/c1dt10615e

The synthesis and characterisation of the new N,O-phenol-pyrazole pro-ligand, ^pzLH, comprising a pyrazole covalently linked to an o,p-di-tert-butyl-substituted phenol, are herein reported. In CH ₂Cl₂ at room temperature, the cyclic voltammogram (CV) of ^pzLH exhibits a quasi-reversible one-electron oxidation process (at E_1/2 = 0.66 V vs. Fc⁺/Fc) attributed to the formation of the phenoxyl radical cation [^pzLH]⁺. ^pzLH reacts with M^II(BF₄)₂ (M = Cu, Co) in a 2:1 ratio to afford the bis-Cu^pzL₂ (1) and tris-Co ^pzL₃ (2) complexes respectively. The X-ray structure of 1 reveals a Cu(ii) ion in a square-planar trans-Cu^II-N ₂O₂ coordination environment whereas that of 2 consists of a Co(iii) ion with an octahedral mer-N₃O₃ coordination sphere; formed by the chelation of two (in 1) or three (in 2) N,O-bidentate phenolate ligands respectively. Both structures are preserved in CH ₂Cl₂ solution, as revealed by their NMR (for 2) and EPR (for 1) data. The CVs of 1 and 2 consist of two (at E_1/2: 0.43 and 0.58 V vs. Fc⁺/Fc) and three (E_1/2 = 0.12, 0.54 and 0.89 V vs. Fc⁺/Fc) reversible one-electron oxidation processes, respectively. The one-electron electrochemical oxidation of 1 and 2 produces the oxidised species, 1⁺ and 2⁺, which are stable for several hours at room temperature under inert atmosphere in CH₂Cl ₂. The UV/vis and EPR data obtained for 1⁺ and 2 ⁺ are unambiguously consistent with the latter being formulated as Cu(ii)- and Co(iii)-phenoxyl radical complexes, as [Cu^II( ^pzL)(^pzL)]⁺ and [Co^III( ^pzL)(^pzL)₂]⁺ respectively.

Full text of DOI:10.1039/c1dt10615e

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PAPER
Phenolate and phenoxyl radical complexes of Cu(II) and Co(III), bearing a new
redox active N,O-phenol-pyrazole ligand†
Galina M. Zats,a Himanshu Arora,a Ronit Lavi,a Dmitry Yufitb and Laurent Benisvy*a
Received 10th April 2011, Accepted 25th May 2011
DOI: 10.1039/c1dt10615e
The synthesis and characterisation of the new N,O-phenol-pyrazole pro-ligand, pzLH, comprising a
pyrazole covalently linked to an o,p-di-tert-butyl-substituted phenol, are herein reported. In CH2Cl2 at
room temperature, the cyclic voltammogram (CV) of pzLH exhibits a quasi-reversible one-electron
oxidation process (at E1/2 = 0.66 V vs. Fc+/Fc) attributed to the formation of the phenoxyl radical
cation [pzLH]∑+. pzLH reacts with MII(BF4)2 (M = Cu, Co) in a 2 : 1 ratio to afford the bis-CupzL2 (1) and
tris-CopzL3 (2) complexes respectively. The X-ray structure of 1 reveals a Cu(II) ion in a square-planar
trans-CuII-N2O2 coordination environment whereas that of 2 consists of a Co(III) ion with an octahedral
mer-N3O3 coordination sphere; formed by the chelation of two (in 1) or three (in 2) N,O-bidentate
phenolate ligands respectively. Both structures are preserved in CH2Cl2 solution, as revealed by their
NMR (for 2) and EPR (for 1) data. The CVs of 1 and 2 consist of two (at E1/2: 0.43 and 0.58 V vs.
Fc+/Fc) and three (E1/2 = 0.12, 0.54 and 0.89 V vs. Fc+/Fc) reversible one-electron oxidation processes,
respectively. The one-electron electrochemical oxidation of 1 and 2 produces the oxidised species, 1+ and
2+, which are stable for several hours at room temperature under inert atmosphere in CH2Cl2. The
UV/vis and EPR data obtained for 1+ and 2+ are unambiguously consistent with the latter being
formulated as Cu(II)- and Co(III)-phenoxyl radical complexes, as [CuII(pzL∑)(pzL)]+ and
[CoIII(pzL∑)(pzL)2]+ respectively.
In the last decade or so, remarkable synthetic progress has
been made in generating and characterising persistent phenoxyl
Introduction
The tyrosyl radical (Tyr∑) has proved to be an essential component
to the catalytic activity of the emerging new class of so-called
“radical enzymes”.1 At least three different environments have
been established for a tyrosyl radical within a metalloprotein: (i)
“free” as in class I RNR (iron-dependent ribonucleotide reductase)
enzymes that are relevant in DNA replication and DNA repair;1,2
(ii) H-bonded as in photosystem II (PSII), where Tyrz∑ is kinetically
competent in water oxidation to O2 during photosynthesis;1,3
or (iii) coordinated to a transition metal centre as in the case
of galactose oxidase (GO)1,4 or glyoxal oxidase (GLO)5 which
active forms contain a Cu(II)-Tyr∑ moiety that is efficient for
the two-electron oxidation of primary alcohol to aldehyde (GO)
or aldehyde to acid (GLO). Thus, the development of chemical
analogues mimicking the biological phenoxyl radical properties
in various environments has constituted a major challenge for
chemists.
radicals, hydrogen-bonded or coordinated to a metal centre.6 In
particular, we have previously reported7,8 a family of N,O-pro-
ligands (RLH, R = Ph, PhOMe, Bz; see Scheme 1), each comprising
an imidazole covalently bonded to an o,p-di-tert-butyl-substituted
phenol. These pro-ligands can be oxidised to a persistent phenoxyl
radical state, H-bonded to an imidazolium, as in [RLH]∑+,8 and/or
coordinated to a metal centre, as in [MII(RL∑)(RL)]+ (M = Cu, Zn,
Co) or [CoIII(RL∑)(RL)2]+.7
aDepartment of Chemistry, Bar-Ilan University, Ramat Gan, 52 900, Israel.
E-mail: benisvl@mail.biu.ac.il; Fax: 972-3-7384053; Tel: 972-3-7384599
bDepartment of Chemistry, University of Durham, South Road, Durham,
UK, DH1 3LE
† Electronic supplementary information (ESI) available: EPR simulation
spectrum of 1+ in frozen solution at 77 K. CCDC reference numbers
820764–820766. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10615e
Scheme 1
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These studies have provided an enhanced understanding of the
unique spectral properties and structural attributes of the active
form of GO. However, they have failed in reproducing the catalytic
activity of the enzyme. Indeed, although these radical complexes
are able to oxidize stoichiometrically benzyl alcohols to benzalde-
hyde, mimicking the oxidative activity of GO, the resulting reduced
complex formed (i.e. Cu(I) complex analogue to the reduced form
of GO) dissociates, preventing thus the aerobic regeneration of the
metal radical catalyst. In order to prevent the dissociation of the
reduced metal centre, we believe that the ligand must incorporate
a binding pocket (preferentially with N-donors) suitable for the
stabilisation of the Cu(I) form. Since N3-tris-pyrazole methane
scorpionate ligands are known to form stable Cu(I) complexes
and their synthetic procedures are well documented,9 it appeared
judicious to us to “replace” the imidazole ring from the previous
ligand design (RLH, R = Ph, PhOMe, Bz; see Scheme 1) by a
pyrazole ring (PzLH, Scheme 1). The latter will then be used as
precursor for straightforward versatile syntheses of unprecedented
e.g. N3O3-tris-(phenol-pyrazole) methane or unsymmetrical N3O2-
bis-(phenol-pyrazole)-pyrazole methane for the elaboration of
GO-like prototype catalysts.
131.61, 128.23, 124.52, 35.11, 34.72, 31.32, 30.34, 29.16, 21.88. M
S ES(+): m/z 291 (MH+).
1-(3,5-di-tert-butyl-2-hydroxyphenyl)butane-1,3-dione. To
a
solution of 2-acetyl-4,6-di-tert-butylphenyl acetate (0.300 g,
1.034 mmol) in anhydrous THF (30 mL), a solution of KOtBu
(0.116 g, 1.034 mmol) in THF (5 mL) was added. The reaction
mixture was then refluxed for 12 h under inert atmosphere. The
solution was then cooled to room temperature and the pH was
adjusted to 3 using 3 N HCl aqueous solution. The resulting
mixture was concentrated on a rotary evaporator to give a brown
oil that was extracted with CH2Cl2 (75 mL). The organic layer was
isolated and extracted sequentially with 5% sodium bicarbonate
(2 ¥ 75 mL), water (2 ¥ 75 mL) and brine (2 ¥ 75 mL). The organic
layer was dried over MgSO4, and the solvent was evaporated using
a rotary evaporator. The resulting oily residue was recrystallised
from hexane to yield the desired product as a light-yellow powder
in 88% yield (0.26 g). 1H NMR (CDCl3, 300 MHz): d 7.81 (d, 2.4
Hz, 1H, ArH), 7.57 (d, 2.4 Hz, 1H, ArH), 2.91 (d, 0.9 Hz, 2H,
t
t
CH2), 1.79 (s, 3H, CH3), 1.43 (s, 9H, Bu), 1.31 (s, 9H, Bu). M
S ES(+): m/z 291 (MH+). IR (ZnSe): n 3302, 2951, 1678, 1603,
1475, 1388, 1379, 1280 cm-1.
Herein, we report the synthesis and characterisation of the new
N,O-phenol-pyrazole pro-ligand pzLH (Scheme 1), together with
its corresponding bis-CupzL2 (1) and tris-CopzL3 (2) complexes.
We prove that these compounds can exist in a stable phenoxyl
radical state when free or coordinated to Cu(II) or Co(III)
ions. These preliminary results serve as proof-of-principle that
phenol-pyrazole ligand may be used for the elaboration of future
generations of bis/tris-phenol-pyrazole biomimetic ligands.
2,4-di-tert-butyl-6-(5-methyl-1H-pyrazol-3-yl)phenol
(pzLH).
To a solution of 1-(3,5-di-tert-butyl-2-hydroxyphenyl)butane-
1,3-dione (0.50 g, 1.72 mmol) in 20 mL of EtOH, an excess of
hydrazine hydrate (2 mL, 40 eq.) was added. The reaction mixture
was stirred for half an hour at r.t. during which time the yellow
solution turned colourless. EtOAc (20 mL) was added to the
reaction mixture, and the latter was subsequently washed with 1
N HCl (20 mL), water (20 mL), and brine (20 mL). The organic
layer was dried with MgSO4, and the solvent was evaporated to
afford colorless plates of pzLH in 92% yield (0.4 g). Elemental
analysis: Calc. for C18H26N2O: C 75.48, H 9.15, N 9.78; Found:
Experimental
THF was distilled from Na/benzophenone-ketyl. Dry distilled
CH2Cl2, MeCN and MeOH were purchased from Aldrich Chem-
ical Ltd. Commercially available, solid chemicals were usually
used without further purification; liquids were generally freshly
distilled. Organic chemicals were purchased from Fluka, Acros
and Aldrich. Cu(BF4)2·6H2O, Co(BF4)2·H2O, and 3,5-di-tert-
butylsalicylic acid was purchased from Acros organic.
1
C 75.09, H 9.16, N 9.88. H NMR (CDCl3, 300 MHz): d 11.27
(s, 1H, NH or OH), 9.77 (s, 1H, NH or OH),7.41 (d, 2.4 Hz, 1H,
ArH), 7.28 (d, 2.4 Hz, 1H, ArH), 6.45 (d, 0.6 Hz, 1H, pzH), 2.39
t
t
(d, 0.9 Hz, 3H, CH3), 1.47 (s, 9H, Bu), 1.34 (s, 9H, Bu). 13C
NMR (75 MHz, CDCl3, 25 ◦C): d 152.69, 140.44, 139.51, 136.40,
123.88, 121.20, 115.86, 101.70, 35.16, 31.62, 29.61, 10.85. Mp
◦
203 C. M S ES(+): m/z 287 (MH+). IR (ZnSe): n 3369, 2952,
Synthesis of pzLH
1567, 1603, 1463, 1376, 1357 cm-1. UV/vis (CH2Cl2): lmax/nm
(e/M-1 cm-1): 298 (45 830), 264 (49 160), 303 (22 910).
The synthesis of the methyl ketone precursor (Scheme 2) was
achieved as previously reported.10
Syntheses of the [CupzL2] (1) and [CopzL3] (2)
2-acetyl-4,6-di-tert-butylphenyl acetate. In a 100 mL round-
bottom flask fitted with a stirrer bar, 1-(3,5-di-tert-butyl-2-
hydroxyphenyl)ethanone (1.4 g, 5.64 mmol), anhydrous pyri-
dine (10 mL), acetic anhydride (0.7 mL, 5.64 mmol) and 4-
dimethylaminopyridine (0.1 g) were combined. The reaction
mixture was stirred at room temperature for 12 h. The reaction
was quenched with a mixture of ice (15 g) and 1 N HCl (25 mL)
and the organic phase was extracted with ethyl acetate, washed
with brine (2 ¥ 75 mL) and water (2 ¥ 75 mL). The organic layer
was dried with MgSO4, filtered, and the solvent was removed on
a rotary evaporator. The desired product was obtained as white
powder in 93% yield (1.3 g). 1H NMR (CDCl3, 300 MHz): d 7.59
(d, 2.4 Hz, 1H, ArH), 7.57 (d, 2.4 Hz, 1H, ArH), 2.53 (s, 3H, CH3),
[CupzL2] (1). To a solution of pzLH (0.05 g, 0.175 mmol) in
methanol (10 mL), a solution of Cu(BF4)2·6H2O (0.03 g, 0.0875
mmol) in methanol (10 mL) was added. Then, Et3N (24 mL)
was added to the reaction mixture, causing an immediate brown
microcrystalline precipitate to be formed. After a few minutes,
the solid was collected by filtration. The dark-brown powder was
recrystallised by diffusion of hexane onto a CH2Cl2 solution. After
a few days, large brown rectangular crystals of 1 suitable for X-ray
crystallography were collected in 37% yield.
MS ES(+): m/z 635 (M). Elemental analysis: Calc. for
C36H50N4O2Cu: C 68.16, H 7.94, N 8.83; Found: C 67.67, H 7.73,
N 8.79. UV/vis (CH2Cl2): lmax/nm (e/M-1 cm-1): 325 (27 320), 407
(539), 684 (98). IR (ZnSe): n 3337, 2952, 1609, 1566, 1439, 1406,
1361 cm-1.
t
t
2.34 (s, 3H, CH3), 1.37 (s, 9H, Bu), 1.34 (s, 9H, Bu). 13C NMR
(75 MHz, CDCl3, 25 ◦C): d 199.89, 170.18, 147.72, 144.48, 141.74,
10890 | Dalton Trans., 2011, 40, 10889–10896
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The analogue NipzL2 complex was synthesised in the same way
as for 1 using Ni(BF4)2·6H2O. The light-green precipitate obtained
was recrystallised from diffusion of hexane onto a CH2Cl2 solution
in 35% yield. 1H NMR (CDCl3, 300 MHz): d 10.82 (bs, 2H, NH),
7.31 (bs, 2H, Ar), 7.16 (bs, 2H, Ar), 6.38 (bs, 2H, Py), 2.38 (s, 6H,
CH3), 1.40 (s, 18H, Bu), 1.25 (s, 18H, Bu). MS ES(+): m/z 629
(M). UV/vis (CH2Cl2): lmax/nm (e/M-1 cm-1): 214 (40 000), 251
(46 250), 349 (4625), 488 (98), 633 (111). IR (ZnSe): n 3328, 2951,
1610, 1567, 1441, 1405, 1360 cm-1.
couple that was used as an internal standard. When necessary,
to avoid overlapping redox couples, the [Fe(Cp*)2]+/[Fe(Cp*)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.
[Fe(Cp*)2]+/[FeCp*2]) = 0.526 V). Controlled potential electrolysis
measurements were performed using a Bio Logic SAS Sp-150
potentiostat. A three electrodes configuration was used in the cell,
comprising a Pt gauze working electrode, a Pt wire secondary elec-
trode contained in a fritted PTFE sleeve, and a saturated Ag/AgCl
reference electrode. The potential at the working electrode was
controlled by a Bio Logic SAS Sp-150 potentiostat.
t
t
[CopzL3] (2). To a solution of pzLH (0.100 g, 0.37 mmol) in
methanol (10 mL), a solution of Co(BF4)2·H2O (0.018 g, 0.124
mmol) in methanol (10 mL) was added. Then Et3N (24 mL) and a
few drops of H2O2 (1.24 mL, 9% w/v) were added to the reaction
mixture, causing the colour of the solution to turn dark brown.
This solution was left stirring at room temperature for 2 h. The
solvent was then evaporated in vacuo to afford a dark brown
powder. The latter was dissolved in DMF and the solution was
left standing in the open air for a few days, after which time large
brown rectangular crystals of 2 suitable for X-ray crystallography
were obtained in 55% yield (0.283 g). 1H NMR (CDCl3, 300 MHz):
d 11.18 (bs, 1H, NH), 10.29 (bs, 1H, NH), 8.81 (bs, 1H, NH), 7.34
(bs, 2H, ArH), 6.99 (bs, 4H, ArH), 6.56–6.51 (bs, 2H, pzH), 6.26
(bs, 1H, pzH), 2.35 (bs, 3H, CH3), 2.28 (bs, 3H, CH3), 2.22 (bs, 3H,
X-ray crystallography: Single crystal X-ray diffraction data
for all of the compounds were collected at 120 K on a Bruker
SMART 6000 CCD area detector diffractometer equipped with
a Cryostream (Oxford Cryosystems) open-flow nitrogen cryostat
˚
using graphite-monochromated Mo-Ka radiation (l = 0.71073 A).
All structures were solved by direct methods and refined by
full-matrix least-squares on F2 for all data using SHELXTL10
and OLEX211 software. All non-disordered non-H atoms were
refined with anisotropic atomic displacement parameters. H-
atoms in structure pzLH were located on a difference Fourier
map and refined isotropically. H-atoms in structures 1 and 2
were placed in calculated positions and refined in “riding mode”.
Disordered atoms of tBu-group in molecule 1 were refined
isotropically with fixed SOF 0.7 and 0.3. The crystallographic
and refinement parameters for pzLH, 1 and 2 are given in Table
1. CCDC numbers 820764–820766 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
t
t
t
CH3), 1.15 (bs, 18H, Bu), 1.11 (bs, 9H, Bu), 0.93 (bs, 9H, Bu),
t
t
0.90 (bs, 9H, Bu), 0.83 (bs, 9H, Bu). MS ES(+): m/z 916 (M).
Elemental analysis: Calc. for C54H75N6O3Co: C 70.87, H 8.26, N
9.18; Found: C 70.98, H 8.28, N 9.21. UV/vis (CH2Cl2): lmax/nm
(e/M-1 cm-1): 350 (10 000), 450 (3280), 625 (860), 770 (530), 965
(540). IR (ZnSe): n 3467, 3264, 2949, 2866, 1605, 1577, 1562, 1469,
1362, 1359 cm-1.
Physical methods
Results and discussions
Elemental analyses of the compounds isolated in these studies
were accomplished in the University of Bar Ilan. EI mass spectra
were recorded on a Q-Tof micro (UK)-micromass-Waters spec-
The N,O-phenol-pyrazole pro-ligands pzLH
The synthesis of the pro-ligand pzLH was achieved in four
steps starting from commercially available 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (Scheme 2). First, the latter was converted
to the corresponding methyl ketone using CH3MgBr as previously
reported.12 Then, the phenol group was acetylated to give the
corresponding ester in 93% yield without any further purification.
1
trometer. H-spectra were recorded on a Bruker DPX300 NMR
spectrometer, some of the materials were recorded on 13C NMR
75 MHz. EPR spectra were recorded on a Bruker X-band EMX
spectrometer; all spectra were calibrated using DPPH radical as
an internal standard, and g-values were calculated accordingly.
Frozen solution EPR spectra were recorded at 77 K using a
quartz cold-finger dewar (Wilmad WG-816-Q) filled with liquid
nitrogen. All EPR spectra simulations were performed using the
Bruker Simfonia software package. UV/Vis spectra were recorded
on a Varian Cary 5000 UV/Vis/NIR spectrophotometer. The
measurements were carried out using a quartz cuvette with optical
pathlength of 0.1 cm. IR spectra were recorded on a Nicolet iS10
FT-IR spectrometer using an ATR accessory on ZnSe crystal for
powder samples pressed.
Cyclic voltammetry measurements were carried out using a
Bio Logic SAS Sp-150 potentiostat. For all electrochemical
experiments, the CH2Cl2 was freshly distilled under dinitrogen
from calcium hydride. The cyclic voltammogram of each com-
pound (1 mM) in CH2Cl2 containing [NBu4][PF6] (0.2 M) as
the background electrolyte was recorded under dinitrogen at
room temperature using a Pt working electrode, a Pt wire sec-
ondary electrode, and an Ag/AgCl reference electrode. The cyclic
voltammogram of each compound was referenced to the Fc+/Fc
Scheme 2
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Table 1 Crystallographic data for pzLH, 1 and 2
pzLH
1
2
Empirical formula
M
C18H26N2O
286.41
Monoclinic
P21/c
18.0338(6)
10.0401(3)
27.4941(9)
90.00
92.412(10)
90.00
4973.7(3)
12
120
1.147
0.071
C36H50N4O2Cu
634.34
Monoclinic
P21/c
17.3973(5)
10.2763(3)
9.3303(3)
90.00
91.830(10)
90.00
1667.22(9)
2
120
1.264
0.692
C54H75CoN6O3
915.13
Monoclinic
P21/c
12.9851(6)
31.8067(16)
13.1765(7)
90.00
93.63(2)
90.00
5431.1(5)
4
120
1.119
0.360
Crystal system
Space group (no)
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
T/K
Dc/g cm-3
m/mm-1 (Mo-Ka)
Reflections collected
Independent reflections (Rint
59 766
13 229 (0.0848)
5405
0.0498
0.0812
19 992
4422 (0.0766)
3096
0.0479
0.1193
49 622
12 447 (0.1442)
6117
0.0692
0.1642
)
Observed reflections [I > 2s(I)]
R
Rw
Table 2 Intra- and inter-molecular H-bonding parameters in pzLH
Baker–Venkataraman rearrangement13 in presence of tBuOK was
then achieved successfully to provide the corresponding b-di-
ketone in 88% yield. The desired ligand pzLH could be easily
prepared in 92% yield on treatment with hydrazine.
˚
O–H ◊ ◊ ◊ N
d(OH ◊ ◊ ◊ N)/A
<O–H ◊ ◊ ◊ N>/◦
O1H1 ◊ ◊ ◊ N1
2.5669(19)
2.8326(19)
2.5665(19)
2.8194(19)
2.5607(19)
2.8000(19)
149.5(18)
142.7(15)
150.9(16)
141.4(15)
152.3(16)
142.7(16)
Crystallisation of pzLH from methanol affords colourless plate
single crystals suitable for X-ray analyses (Fig. 1, Tables 1–2).
The molecular structure of pzLH has been determined by X-ray
crystallography. Three crystallographically independent molecules
are present in the asymmetric unit. The molecules differ slightly
N2H2A ◊ ◊ ◊ O2
O3H3 ◊ ◊ ◊ N31
N32H32 ◊ ◊ ◊ O3a
O2H2 ◊ ◊ ◊ N21
N22H22 ◊ ◊ ◊ O1
a 1 - x, 1 - y, 1 - z.
t
by different orientation of Bu groups and are quasi-planar with
a dihedral angle between the phenol and pyrazole mean planes
ranging from 11.40◦ to 13.65◦. Each of the molecules displays
a relatively strong intramolecular OH ◊ ◊ ◊ N between the phenol
OH and the adjacent N-pyrazole, as indicated by the O ◊ ◊ ◊ N
˚
distance in the range of 2.56–◦2.57 A and the <O–H ◊ ◊ ◊ N>
angle in the range of 149.5–152 .14 Similar H-bonding features
have been found in analogue phenol-imidazole compounds RLH
(Scheme 1).7,8 In addition, molecules in the crystal are linked
together in centrosymmetrical R4 (10)15 dimers by intermolecular
4
hydrogen bonds between N–H pyrazole and O-phenol atoms
(Fig. 1, Table 2).
Fig. 2 The cyclic voltammogram of pzLH (~1 mM) in CH2Cl2 at 298 K
containing [NBu4][PF6] (0.2 M) as supporting electrolyte, at a scan rate of
100 mV s-1.
reversibility of this process is characteristic of an oxidation of
the phenol to its corresponding phenoxyl radical cation [pzLH]∑+.
By analogy to its analogous phenol imidazole compounds RLH
depicted in Scheme 1 (E1/2 in the 0.35–0.49 V vs. Fc+/Fc potential
range7,8), we propose that the oxidation process occurs via a
PCET mechanism16 leading to the formation of a phenoxyl radical
hydrogen bonded to a pyrazolium.8,17 Though the reversibility
of this oxidation process indicates that the radical formed is
persistent, at least in the timescale of the CV experiment (ca. a
minute), many attempts to generate and characterise the oxidised
Fig. 1 H-bonded dimer in the structure of pzLH (the displacement
ellipsoids are shown at the 30% probability level). Only the O–H and
N–H hydrogen-atoms are shown.
The cyclic voltammogram (CV) of pzLH in dichloromethane
at room temperature exhibits a quasi-reversible one-electron
oxidation process at E1/2 = 0.66 V vs. Fc+/Fc (Fig. 2). The
10892 | Dalton Trans., 2011, 40, 10889–10896
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◦
˚
Table 3 Selected bond lengths/A and angles/ for 1 and 2
species have failed, due to its apparent instability within the time
scale of the electrolysis experiments (2–4 h) even at temperature as
low as -40◦ C under strictly inert atmosphere.
1
Cu1–O1
Cu1–N1
1.9233(14)
1.9092(19)
180
89.89(7)
90.11(7)
179.999(1)
120.64(14)
130.18(15)
130.91(14)
The bis-CupzL2 (1) and tris-CopzL3 (2) complexes
O1–Cu1–O1a
N1–Cu1–O1a
N1–Cu1–O1
N11–Cu1–N1
N2–N1–Cu1
C3–N1–Cu1
C5–O1–Cu1
2
Crystallography studies. pzLH reacts with CuII(BF4)2·6H2O
in a 2 : 1 ratio in the presence of triethylamine in methanol
to afford the brown CuL2 complex (1). The same reaction
using CoII(BF4)2·H2O did not yield the corresponding CoII(pzL2)
compound but instead afforded the oxidized CoIII(pzL3) compound
(2). The latter complex could also be synthesized quantitatively
using a 3 : 1 ligand : metal ratio in the presence of 3 eq. of
triethylamine and 9% solution of H2O2 (see Experimental section).
Single crystals suitable for X-ray crystallography have been
obtained by slow diffusion of hexane onto a dichloromethane
solution (for 1) or by slow evaporation of a DMF solution
(for 2). Molecular structures and geometrical parameters are
shown in Fig. 3 and Table 3 respectively. In the crystal, the
mononuclear complex 1 occupies a special position in the centre of
symmetry. The metal ion possesses a trans-CuII-N2O2 coordination
environment formed by the ligation of two L- ligands in a N,O-
bidentate fashion. The coordination geometry around the CuII
ion is slightly distorted square-planar with N1–Cu1–O1 and N1–
Cu1–N11 angles of ca. 90◦ and ca. 180◦ (Table 3). The Cu–
Co1–O1
Co1–O2
Co1–O3
Co1–N1
Co1–N3
Co1–N5
1.906(3)
1.869(2)
1.913(3)
1.891(3)
1.882(3)
1.904(3)
O1–Co1–O3
O2–Co1–O1
O2–Co1–O3
N1–Co1–O1
N1–Co1–O3
N1–Co1–N5
N3–Co1–N5
N5–Co1–O1
O2–Co1–N1
O2–Co1–N3
O2–Co1–N5
N3–Co1–O1
N3–Co1–O3
N3–Co1–N1
N5–Co1–O3
O2–Co1–N1
173.52(11)
91.78(11)
93.00(11)
87.52(13)
88.39(13)
91.51(12)
91.63(13)
86.29(12)
86.71(12)
90.11(12)
177.42(12)
91.03(12)
93.33(12)
176.46(13)
88.80(12)
86.71(12)
˚
O1 and Cu–N1 bond distances of 1.9233(14) and 1.9092(19) A
respectively are close to expected values for a Cu(II) ion in a
N2O2 environment.7d Interestingly, 1 displays two intramolecular
hydrogen bonds involving the pyrazole N–H of one ligand and the
coordinated O-phenolate atom of the other ligand, as evidenced
a 2 - x, 1 - y, 1 - z.
◦
˚
by the N ◊ ◊ ◊ O distance of 2.717(2) A and N–H ◊ ◊ ◊ O angle of 121
(Fig. 3, Table 3). The relatively small <N–H ◊ ◊ ◊ O> angle indicates
Table 4 Parameters of short intra-molecular contacts in 1 and 2
<O–H ◊ ◊ ◊ N>/◦
˚
O–H ◊ ◊ ◊ N
d(OH ◊ ◊ ◊ N)/A
N2H2 ◊ ◊ ◊ O1a(1)
N2H2 ◊ ◊ ◊ O3 (2)
N6H6 ◊ ◊ ◊ O1 (2)
2.717(2)
2.685(4)
2.623(4)
121.0(2)
110.8(2)
111.3(2)
a 2 - x, 1 - y, 1 - z.
that the formation of these short intramolecular contacts is most
probably induced by steric factors.
The X-ray structure of 2 reveals a Co(III) ion with an octahedral
mer-N3O3 coordination environment formed by the chelation of
three N,O-bidentate L- ligands. The average Co–O and Co–N
˚
˚
bond lengths of 1.896 A and 1.892 A respectively are in accordance
with a Co(III) ion in an octahedral geometry and are similar to the
corresponding bond of other CoIIIN3O3 complexes reported in the
literature.7d Interestingly, two of the three coordinated phenolate
O-donors (O1 and O3) are intramolecularly H-bonded with
adjacent pyrazole N–H donor groups (N6 and N2 respectively,
Fig. 3, Table 4). While the L- ligands in complex 1 remain almost
planar, the ligands in complex 2 are more distorted with biggest
corresponding torsion angle N3C35C22C21 equal to +20.4◦.
Fig. 3 The molecular structures of 1 (top) and 2 (bottom) (the displace-
ment ellipsoids are shown at the 30% probability level). Only the O–H and
N–H hydrogen atoms and only one component of disordered groups are
shown for clarity. Intramolecular N–H ◊ ◊ ◊ O contacts are represented with
a dashed line.
EPR and NMR studies. As expected for a CuII ion (d9, S =
1/2), complex 1 is EPR-active, in solid state and in solution at RT
or at 77 K. The room-temperature X-band EPR spectrum of 1 in
a dichloromethane solution displays a typical isotropic four line
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spectrum centered at giso = 2.12, with an isotropic superhyperfine
Ph), i.e. gzz is lower and Azz is higher. This is consistent with
empirical and theoretical EPR studies20 for tetracoordinated
Cu(II) centres which have shown that the values of gzz and Azz
change systematically, as the dihedral angle, w, (angle between the
two CuNO intraligand planes in bis-CuN2O2 complexes) varies.
Thus, a distortion from a square planar (w = 0◦) to a tetrahedral
(w = 90◦) geometry induces an increase of gzz and a decrease of
Azz. Clearly the difference in EPR parameters in 1 as compared to
[Cu(RL)2]7 reflects on the deviation of the latter from the square
planar geometry of 1. These results also confirm that the square
planar structure of 1 identified by X-ray crystallography is retained
in CH2Cl2 solution.
coupling constant A Cu of 90 G with the 63,65Cu nuclei spin (ICu
=
63,65
3/2), Fig. 4. In addition, further hyperfine splitting to a five-
line pattern was observed, accounting for the hyperfine coupling
of the unpaired electron with the two coordinated 14N nuclei
spins (I = 1). The spectrum was successfully simulated with an
14
isotropic constant (A N) of 13 G, confirming the coordination
of two ligands to the CuII ion in solution (Fig. 4). The X-
band EPR powder spectrum of 1 at room temperature exhibits
a broad unresolved peak at ca. g = 2.1; the lack of resolution
is characteristic of the involvement of intermolecular interaction
between the paramagnetic centres. However, when a magnetically
diluted (2%) powder of 1 with diamagnetic isostructural NipzL2
compound (98%) was examined, the room temperature EPR
spectrum showed a well-resolved anisotropic Cu(II) axial signal.
The latter spectrum is quasi-identical to that of 1 in frozen CH2Cl2
solution at 77 K, as shown in Fig. 5. This observation clearly
indicates that the structure of 1 in solid state, as revealed by its
X-ray structure, is retained in a CH2Cl2 solution. The spectrum
displays an axial signal, that resembles that of reported [Cu(RL)2]
In contrast to 1, 2 is EPR-silent, and exhibits a well resolved
1H NMR spectrum in CDCl3. Thus, 2 is diamagnetic (S = 0) in
solution as well as in solid state, consistent with a low-spin, d6,
1
Co(III) ion in an octahedral geometry.7c The H NMR spectrum
of 2 in CDCl3 displays three sets of peaks corresponding to
three magnetically non-equivalent coordinated ligands. Thus, the
t
spectrum includes: six Bu resonances (i.e. 3 ¥ 2) in the 0.76–
1.4 ppm range, three pyrazole-CH3 singlets at 2.22–2.35 ppm,
three pz-H at 6.26–6.56 pmm, three Ar–H(meta) at 6.99–7.34
ppm and three pyrazole N–H broad singlets at 8.8, 11.2, and
10.9 ppm. Interestingly one can clearly differentiate hydrogen
bonded N–H (at 11.2 and 10.9 ppm) and non H-bonded N–
H (at 8.8 ppm) resonances (Fig. 6). These data are consistent
with the crystallography data of 2 showing that only two N–H
are intramolecularly H-bonded; and most likely indicate that the
structure of 2 is preserved in solution.
(Scheme 1)7,18 and is characteristic of a Cu(II) complex possessing
1
a (dx ) ground state (S = 1/2) with gzz ꢀ gxx ~ gyy > ge.19 The
2
2
–y
spectrum shows superhyperfine splitting in the gzz region with
63,65Cu (I = 3/2) nuclei; hyperfine and/or superhyperfine coupling
to the 63,65Cu and/or 14N nuclei are also manifest in the gxx/gyy
regions. The best simulated fit was obtained with: giso = 2.12, gzz
=
2.21, gxx = 2.075, gyy = 2.075 with Azz{63,65Cu} = 195 G (see Fig. SI1).
Fig. 4 X-band EPR spectra of 1 (ca. 1 mM) in CH2Cl2 recorded at room
temperature and its simulated spectrum. Simulation parameters: gx = gy =
63,65 Cu
14 N
gz = 2.12; A
= 90 G, A = 13 G; linewidth: 10 G.
Fig. 6 Room-temperature 1H NMR (CDCl3, 300 MHz) spectrum of 2.
UV/vis studies. The UV/Vis spectrum of 1 displays visible
absorption bands at 407 nm (e = 539 M-1 cm-1) and 684 nm
(e = 98 M-1 cm-1). The former absorption is attributed to a
phenolate-to-Cu(II) charge transfer transition, on the basis of
comparisons with the UV/vis spectra of other analogous Cu(II)–
phenolate complexes; the lower energy feature is assigned to
ligand-field transitions.7
The UV/vis spectrum of 2 exhibits visible absorptions:
i.e. a band at 450 nm (e = 3280 M-1 cm-1) assigned to a
phenolate-to-Co(III)-charge transfer transition; a band at 625 nm
(e = 860 M-1 cm-1) assigned to d–d transition; and a broad band
with maximum peaks at 775 and 965 nm (e = 530, 540 M-1
cm-1 respectively) assigned to a LLCT (ligand–ligand-charge-
transfer).7c
Fig. 5 X-band EPR spectra of (top) magnetically diluted 1 (2%) in NiL2
(98%) and (bottom) frozen solution of 1 in CH2Cl2 recorded at 77 K.
Whilst the gzz (2.21) and Azz (195 G) values obtained for 1
fall in the range of expected EPR parameters for tetragonal
{CuIIN2O2} complexes, these values are significantly different from
those obtained for their distorted square planar related complexes
[Cu(RL)2]7 (e.g. gzz/Azz (G): 2.252/158 R = Bz; 2.269/147 R =
10894 | Dalton Trans., 2011, 40, 10889–10896
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Redox properties of 1 and 2 and the nature of 1+ and 2+. The
redox behaviour of 1 and 2 was investigated by cyclic voltam-
metry (CV) techniques, in CH2Cl2 solution at room temperature
(Fig. 7). The CV of 1 displays two reversible one-electron oxidation
processes occurring at E1/2: 0.43 and 0.58 V vs. Fc+/Fc respectively,
and that of 2 manifests three reversible one-electron oxidation
processes (at E1/2 = 0.12, 0.54 and 0.89 V vs. Fc+/Fc, respectively)
(Fig. 7). The oxidation processes obtained for 1 and 2 occur at
similar potentials to those obtained for the previously reported
CuRL2 (E1/2/V vs. Fc+/Fc: 0.36, 0.58 (R = Bz); 0.11, 0.44 (R =
PhOMe); 0.16, 0.50 (R = Ph))7a,b,d and CoBzL3 (E1/2/V vs. Fc+/Fc:
0.08, 0.52 and ca. 1.00)7c complexes, respectively; for which, the
first two processes have been attributed to the successive ligand-
based oxidations to its mono-, bis-phenoxyl radical complexes.
Fig. 8 Room-temperature UV/vis spectra of (top) 1 (solid line) and 1+
(dashed line) and (bottom) 2 (solid line) and 2+ (dashed line), ca. 1 mM in
CH2Cl2 containing [NBu4][PF6] (0.2 M).
as observed in a previously reported Co(III)–phenoxyl radical
complex, [CoIII(BzL∑)(RL)2]+.7c
The electrochemical, one-electron, oxidation of 1 to 1+ is
accompanied by a significant reduction in the intensity of the
EPR signal i.e. 1+ in CH2Cl2 containing [NBu4][PF6] (0.2 M)
at 77 K is essentially EPR silent; only a residual Cu(II) signal,
<5% of the intensity of the parent complex, was observed. The
lack of an EPR signal for 1+ is consistent with an S = 0 or
1 (with a very large zero-field splitting) ground state, resulting
1
Fig. 7 Cyclic voltammograms of 1 (top) and 2 (bottom) (~1 mM) in
CH2Cl2 at 298 K containing [NBu4][PF6] (0.2 M) as supporting electrolyte,
at a scan rate of 100 mV s-1.
from magnetic coupling between an S = Cu(II) centre and an
2
S = coordinated radical ligand, as observed for[Cu(RL)(RL∑)]+
1
2
(Scheme 1)7 and other Cu(II)–phenoxyl radical complexes.6
The X-band EPR spectrum of electrochemically generated
2+ displays, in fluid CH2Cl2 solution at room temperature, an
The electrochemical, one-electron, oxidation of 1, 2, at
263 K under N2 in CH2Cl2 containing [NBu4][PF6] (0.2 M) as the
background electrolyte led to a significant color change from pale
green to intense green-brown (1 → 1+) and from red-brown to dark
brown (2 → 2+). These oxidised species were proved to be stable
for several hours at room temperature under inert atmosphere, as
indicated by the retention of their UV/vis spectra. Importantly
the CVs of these oxidised species were identical to those of their
parent complexes, indicating that no major changes have occurred
during the oxidation process, and only a single species has been
produced.
1
isotropic S = signal centered at g = 2.017, and comprises an eight
2
line pattern characteristic of the hyperfine coupling of the electron
spin with the 59Co nuclei spin (I = 7/2) (Fig. 9). The spectrum
The UV/vis spectrum of 1+ displays a strong absorption band
at ca. 425 nm (e = 2087 M-1 cm-1) and a weaker broad NIR band
centered at 969 nm (e = 591 M-1 cm-1); Fig. 8. Both bands are
characteristic of those of phenoxyl radical complexes,6,7 the former
being assigned to p–p* transition of the coordinated phenoxyl
radical. The UV/vis spectrum of 2+ (Fig. 8) exhibits a relatively
intense sharp band at 423 nm (e = 4515 M-1 cm-1) assigned to
p–p* transition of a coordinated phenoxyl radical and a broad
intense NIR band at 816 nm (e = 1723 M-1 cm-1) assigned to
ILCT. The intensity of this NIR transition may indicate that
the phenoxyl radical is involved in intervalence charge transfer
Fig. 9 Room-temperature X-band EPR spectrum of electrochemically
generated 2+ (ca. 1 mM) in CH2Cl2 containing [NBu4][PF6] (0.2 M), and
59
its simulated spectrum. Simulation parameters: gx = gy = gz = 2.017; A
=
Co
9.5 G, linewidth: 10 G.
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has been successfully simulated with the inclusion of a hyperfine
coupling constant of 9.5 G. The g-value (close to that of the free
electron) and the relatively small hyperfine coupling obtained for
2+ are in agreement with EPR data obtained for other Co(III)–
radical ligand complexes, for which A(59Co) values <12.5 G have
been observed.7c,21–25
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Conclusions
Herein, we have reported the synthesis and characterisation of
the new N,O-phenol-pyrazole pro-ligand, pzLH, suitably designed
for the stabilisation of its phenoxyl radical state. pzLH presents
great similarity with its analogous phenol-imidazole compounds
(RLH, Scheme 1):7,8 i.e. (a) it is one-electron redox-active and can
exist as an oxidised radical state, as [LH]∑+ owing to a strong
intramolecular OH ◊ ◊ ◊ N H-bonding. (b) It is a good N,O-chelating
ligand to Cu(II) and Co(III) ions, as evidenced by the synthesis and
characterisation, in solid and solution states, of the bis-CupzL2
7d
(1) and tris-CopzL3 (2) which resemble the reported bis-CuBzL2
and tris-CoBzL3,7c respectively. (c) Electrochemical studies of 1
and 2 exhibit two and three ligand-based reversible one-electron
oxidation processes, respectively, that occur at similar potential
to bis-CuBzL2 and tris-CoBzL3.7c (d) The one-electron oxidation
7d
of 1 and 2 yields persistent mono-phenoxyl radical complexes of
Cu(II), as [CuII(pzL∑)(pzL)]+ (1+), and Co(III), as [CoIII(pzL∑)(pzL)2]+
(2+), respectively; as evidenced by their characterisation by EPR
and UV/vis spectroscopies. We have found the stabilities of
these radical complexes are similar to the analogous complexes
containing imidazole ring instead of pyrazole. However, these
studies represent a significant development in the elaboration of
a biomimetic model of tyrosyl radical biological sites. Not only
can the new ligand exist in a stable phenoxyl radical state when
coordinated to a transition metal but it also possesses a pyrazole
N–H position that can be synthetically exploited further. Indeed,
pzLH represents an ideal precursor towards the construction of
e.g. N3O3-tris-(phenol-pyrazole) methane or unsymmetrical N3O2-
bis-(phenol-pyrazole)-pyrazole methane, both of which contain a
N3-binding pocket suitable for the stabilisation of the Cu(I) form,
crucial for the GO catalytic activity. These new prototype ligands
should allow the stabilisation of both Cu(II)-radical and Cu(I)
forms, crucial for efficient oxidation catalysis. We are currently
working on the preparation of these new generations of biomimetic
complexes.
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Acknowledgements
The authors wish to acknowledge the University of Bar-Ilan for
funding and the Soref-Kolman Foundation for the provision of a
postdoctoral fellowship to H.A.
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10896 | Dalton Trans., 2011, 40, 10889–10896
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