H-bonding and steric effects on the properties of phenolate and phenoxyl radical complexes of Cu(ii)

Article abstract of DOI:10.1039/c1dt11868d

Herein, the N-R substituted N,O-phenol-pyrazole redox-active pro-ligands, ^RLH (R = Me, Pr) are reported together with their corresponding bis-Cu^RL₂ complexes (2 and 3, respectively). The latter are reversibly oxidised to t

Full text of DOI:10.1039/c1dt11868d

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H-bonding and steric effects on the properties of phenolate and phenoxyl
radical complexes of Cu(II)†
Galina M. Zats,a Himanshu Arora,a Ronit Lavi,a Dmitry Yufitb and Laurent Benisvy*a
Received 3rd October 2011, Accepted 24th October 2011
DOI: 10.1039/c1dt11868d
Herein, the N–R substituted N,O-phenol-pyrazole redox-
active pro-ligands, RLH (R = Me, Pr) are reported together
with their corresponding bis-CuRL2 complexes (2 and 3,
respectively). The latter are reversibly oxidised to the corre-
sponding stable Cu(II)-phenoxyl radical complexes 2+ and 3+.
The properties of the tetrahedrally distorted complexes 2 and
3 (and those of 2+ and 3+) are being compared to those of the
square-planar H-bonded complex 1 (bis-CuHL2) and those
of 1+. These studies have permitted H-bonding and steric
Scheme 1
effects on the redox, spectroscopic and chemical properties
of Cu(II)-phenolate and Cu(II)-phenoxyl radical species to be
established.
coordinated phenolate ligands are involved in intramolecular H-
bonding interactions (Scheme 1). Herein, we introduce the new
N–R substituted ligand RLH (R = Me, Pr; see Scheme 1), which
Galactose Oxidase (GO)1 is a unique enzyme amongst the new
are unable to establish H-bonds when coordinated to the Cu(II)
emerging class of so-called “radical enzymes”.2 Its active site
ion. The properties of the corresponding bis-CuRL2 complexes 2
contains a catalytically active Cu(II)-tyrosyl radical moiety, which
and 3 (Scheme 1) are being compared to those of 1; permitting
has triggered current research interest in the preparation and
H-bonding and steric effects to be established.
studies of metal-phenoxyl radical complexes.3 These studies have
HLH is readilydeprotonated usingNaH andfurther reacted with
provided an enhanced understanding on the unique spectral
MeI or PrI to yield MeLH and PrLH respectively (see ESI†). Despite
properties and structural attributes of the active form of GO.
the introduction of N-substituents, the MeLH and PrLH possess
However, the local factors that modulate the redox properties
similar properties to those of their unsubstituted parent HLH in
of the Cu(II)–Tyr/Cu(II)–Tyr∑ redox couple are not yet fully
solution. Firstly, both display a relatively strong intramolecular
understood.
OH ◊ ◊ ◊ N hydrogen bonding between the phenol OH and the
Suitably protected N,O-phenol-imidazole pro-ligands have been
adjacent N-pyrazole as evidenced by a down field phenol-OH 1H
successfully used to model the active form of GO, as they have
NMR resonance at ca. 11 ppm in CDCl3 (see ESI†).4,5 Secondly, as
permitted the isolation and full characterisation of stable Cu(II)-
for other similar intramolecularly H-bonded phenol compounds,6
phenoxyl radical complexes.4 Recently, we have extended this
HLH, MeLH and PrLH each exhibit a quasi-reversible one-electron
chemistry to the phenol-pyrazole analoguous pro-ligand HLH
PCET oxidation process (at E1/2 = 0.660, 0.710 and 0.719 V
(Scheme 1).5 The latter behaves like its imidazole analogue,
vs. Fc/Fc+, respectively, (Fig. S4–S5†)) that is attributed to the
in that its bis-CuHL2 (1; Scheme 1) complex can be reversibly
formation of a phenoxyl radical cation hydrogen bonded to a
oxidised to the corresponding stable Cu(II)-phenoxyl radical.5
pyrazolium.
However, thanks to the pyrazole-NH group, the square planar
MeLH and PrLH react with CuII(BF4)2·6H2O in a 2 : 1 ratio in
copper complex 1 represents a unique example in which the two
the presence of triethylamine in methanol to afford the purple
bis-CuRL2 complexes (2 and 3 respectively). Slow evaporation
of a DMF solution of 3 affords single crystals suitable for X-
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
ray crystallography.‡ The molecular structure of 3 reveals that
bDepartment of Chemistry, University of Durham, South Road, Durham,
the Cu(II) ion possesses a N2O2 coordination environment in
UK, DH1 3LE
a tetrahedrally distorted geometry (the dihedral angle between
† Electronic supplementary information (ESI) available: Experimental
the two CuNO planes is 37.45◦), formed by the ligation of two
L- ligands in a N,O bidentate fashion (Fig. 1 and Table S1†).
The significant distortion from a square planar geometry results
from the presence of sterically demanding propyl groups, which
prevent the two ligands from being on the same plane. This is in
part and Physical methods; 1H NMR spectra for MeLH and PrLH (Figs. S1–
S2); Cyclic voltamograms of MeLH and PrLH (Fig. S3–4); IR spectra of 1,
2 and 3 (Fig. S5); X-band EPR spectra (exp. and sim) of 2 and 3 (Figs. S6–
7); Decay curve of 1+, 2+and 3+ (Figs. S8–10). CCDC reference number
846432. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt11868d
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Fig. 3 UV-vis spectra of 1, 2 and 3 (~1 mM) in CH2Cl2 at 298 K [lmax/nm
(e/M-1 cm-1) recorded using a 0.1 cm cell: 407 nm (539); 684 nm (150) 1,
520 (610), 755 sh (350) 2; 535 (580), 745sh (300) 3].
Fig. 1 The molecular structure of 3 (the displacement ellipsoids are shown
at the 30% probability level).
great contrast with CuHL2 (1), which possesses a perfect trans-
square planar geometry, associated with the establishment of in-
tramolecular inter-ligand O ◊ ◊ ◊ HN H-bonding between phenolate
and pyrazole groups (see Scheme 1). These H-bonds appear to
affect significantly Cu–O and Cu–N bond distances. Thus, the
average Cu–O bond is significantly longer in 1 than in 3 (1.9233(14)
535 nm (e = 610–580 M-1 cm-1) and weaker ligand field transition
at 755–745 nm (e = 350–300 M-1 cm-1); Fig. 3.4 Importantly, the
phenolate to Cu(II) charge transfer transition appears at much
higher energy for 1 (407 nm (e = 539 M-1 cm-1) than for 2 and 3
(Fig. 3). This most likely evidences the effect of the H-bonding
on the coordinated phenolate in 1, particularly in pulling electron
density from the coordinated phenolate ring.
The CV of 1,5 2 and 3 in CH2Cl2 at room temperature, all
display two reversible one-electron oxidation processes at E1/2/V
vs. Fc/Fc+: 0.430/0.580 (1);5 0.317/0.594 (2); 0.267/0.618 (3); Fig
4. These are assigned to the successive ligand-based oxidation
to the corresponding mono-, and bis-phenoxyl radical complexes.
Remarkably, the first oxidation occurs at much lower potential
for the tetrahedrally distorted 2 and 3 than for the H-bonded
square planar 1 (DE1/2 = 110 mV and 160 mV, respectively). Thus,
H-bonding to a coordinated phenolate increases its oxidation
potential.
˚
˚
A vs. 1.8806(16) A) and the average Cu–N bond is significantly
5
˚
˚
shorter in 1 than in 3 (1.9092(19) A vs. 1.9549(19) A). Importantly,
the phenolate C–O bond appears to be slightly lengthened upon
H-bonding as evidenced by the longer average C–O bond in 1 than
˚
˚
in 3 (1.329(3) A vs. 1.317(3) A).
The EPR spectra of 1, 2 and 3 in frozen CH2Cl2 solution at
77 K exhibit an axial signal characteristic of that of CuII–N2O2
complexes possessing a (dx2-y2)1 or (dxy)1 ground state (S = 1/2)
with gzz ꢀ gxx ~ gyy > ge (Fig. 2).7 The parallel region of the
spectra (i.e. the gzz and Azz{63,65Cu}) is known to be sensitive to
the geometry around the Cu(II) ion.8 Thus, a distortion from a
square planar to a tetrahedral geometry induces an increase of gzz
and a decrease of Azz.8 In that respect, the simulated Azz values
for 2 (165 G) and 3 (160 G) are found to be similar but both are
significantly smaller than that of 1 (195 G); whereas the gzz value
for 2 and 3 (2.258) is significantly higher than that of 1 (2.210).
Thus, clearly the difference in EPR parameters for 2 and 3 as
compared to 14 in solution reflects their difference in geometry:
i.e. whilst 1 is square planar, 2 and 3 are significantly distorted.
Fig. 4 The cyclic voltammograms of 1 , 2 and 3 (~1 mM) in CH2Cl2 at
298 K containing [NBu4n][PF6] (0.15 M) as supporting electrolyte, at a
scan rate of 100 mV s-1.
Fig. 2 X-band EPR spectra of 1, 2 and 3 (~1 mM) in CH2Cl2 recorded at
77 K.
The electrochemical, one-electron oxidation of 1, 2 and 3 at
263 K under N2 in CH2Cl2 containing [NBu4 ][PF6] (0.15 M) as
n
the background electrolyte, led to a significant colour change from
purple to dark brown. The corresponding oxidised species 1+, 2+
and 3+ were proved to be relatively stable at room temperature
with half-lives of 255, 71 and 478 min, respectively. The highest
stablity obtained for 3+ suggests that steric hindrance around the
radical prevent decomposition pathways.
Similarly, the UV-vis spectra (CH2Cl2 at 298 K, Fig. 3) of the
purple coloured complexes 2 and 3 are similar but distinct from
that of the brown complex 1. The spectra of 2 and 3 resemble those
of other distorted square planar Cu–N2O2 complexes; displaying a
relatively intense phenolate to Cu(II) charge transfer band at 520–
48 | Dalton Trans., 2012, 41, 47–49
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The UV-vis spectra of 1+, 2+ and 3+ are very similar, and all
exhibit an intense absorption band at ca. 440 nm; together with a
less intense low energy broad NIR band at ca. 950 nm [lmax/nm
(e/M-1cm-1): 440 nm (2420), 970 nm (590) 1+; 438 (3400), 920
(1200) 2+; 442 (3800), 910 (1240) 3+] (Fig. 5). Both these bands are
characteristic of those of phenoxyl radical complexes, the former
being assigned to the p–p* transition of the coordinated phenoxyl
radical.3 The similarity of the spectra of 1+, 2+ and 3+ indicates
that, as opposed to those of their parent complexes, the absorption
spectra of the Cu(II)-phenoxyl radical complexes are less sensitive
to geometrical change and/or H-bonding interaction. Although
one should note that whilst the ca. 440 nm band in 1+ displays a
gaussian shape, those of 2+ and 3+ are unsymmetrical and display
a quasi-plateau in the 400–440 nm region. These subtle changes
may help to differentiate Cu(II)-phenoxyl radicals with varying
local environments.
oxidised Cu(II)-phenoxyl radical species are modulated by steric
and H-bonding effects. These biomimetic studies are relevant to
understand the factors that govern the reactivity of the Cu(II)-
Tyrosyl radical moiety of GO.
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.
Notes and references
‡ Crystal data for 3. C42H62CuN4O2, Mr = 718.50, monoclinic, space group
˚
P21/c (no. 14), a = 25.4688(7), b = 9.5148(3), c = 16.9516(5) A, b =
◦
3
91.368(10) , U = 4106.7(2) A , Z = 4, rcald = 1.162 g cm-3, m(Mo-Ka) =
˚
0.569 mm-1, 46 126 reflections measured, 9913 unique, T = 120 K, final
R1 = 0.0514 (for 7891 reflections with I > 2s(I)), wR2 (all data) = 0.1342.
CCDC number 846432.
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Inset: enlarged 300–700 nm region.
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The electrochemical one-electron oxidations of 2 and 3 are
accompanied by a significant reduction in the intensity of their
EPR signals, i.e. 2+ and 3+ in CH2Cl2 at 77 K are essentially EPR
silent, only a residual Cu(II) signal, ~<5% of the intensity of the
parent complex, was observed for 2 and 3 (see ESI†). The lack
of an EPR signal for 2+ and 3+ is consistent with an S = 0 or 1
(with a very large zero-field splitting) ground state, resulting from
1
2
1
2
magnetic coupling between an S = Cu(II) centre and an S =
coordinated radical ligand, as observed for 1+ and other Cu(II)-
phenoxyl radical complexes [Cu(II)(RL)(RL∑)]+.3
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Conclusions
We herein have demonstrated that the redox, spectroscopic and
chemical properties of Cu(II)-phenolate and its corresponding
This journal is
The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 47–49 | 49
©