Trigonal-prismatic CuII6-pyrazolato cages: Structural and electrochemical study, evidence of charge delocalisation

Article abstract of DOI:10.1039/b615018g

The structural characterization of hexanuclear Cu^II-complexes and the effect of pyrazole substituents on their two consecutive one-electron oxidation process, leading to mixed-valent products was investigated. Redox-active trinuclear copper clusters constitute the active centers of several copper proteins, such as ascorbate oxidase, laccase, and ceruloplasmin. The formation of triangular structures is favored by the pyrazolato ligand's two N-donor orbitals oriented at 60° and the clipping of the two triangular units provide hexanuclear complexes in a triagonal-prismatic motif. The new redox-active motif of triply-bridged trinuclear units may provide the basis for an extensive study. The results show that the preparation of the hexanuclear Cu-complexes has been achieved in a one-pot synthesis by matching the coordination requirements of its various components.

Full text of DOI:10.1039/b615018g

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Trigonal-prismatic CuII -pyrazolato cages: structural and electrochemical
6
study, evidence of charge delocalisation†‡
Gellert Mezei, Marlyn Rivera-Carrillo and Raphael G. Raptis*
Received 16th October 2006, Accepted 30th October 2006
First published as an Advance Article on the web 13th November 2006
DOI: 10.1039/b615018g
Hexanuclear CuII-complexes of formula PPN[Cu6(l3-O)2(l-
4-Rpz)6(l-4-Rꢀ-3,5-Ph2pz)3] have been structurally character-
ized and the effect of pyrazole substituents on their two con-
secutive one-electron oxidation processes, leading to (possibly
delocalised) mixed-valent products, has been probed by cyclic
voltammetry.
in a trigonal-prismatic motif. The syntheses of the Cu6-cages have
been carried out by two methods (Scheme 1).¶ (1) Reaction of
2 equivalents of green (PPN)2[1] with 1 equivalent of colourless
9
(Ag(3,5-Ph2pz))3 in CH2Cl2 yielded, upon AgCl precipitation,
the dark brown complex PPN[Cu6(l3-O)2(l-pz)6(l-3,5-Ph2pz)3]
(PPN[3]) of D3h molecular symmetry. (2) By an alternative self-
assembly route, the reaction of CuCl2·2H2O, pzH, 3,5-Ph2pzH,
PPNCl and NaOH (6 : 6 : 3 : 1 : 13 molar ratio) in CH2Cl2 provided
PPN[3] in good yield. This 15-component self-assembly is entirely
based on the ability of each component to be directed to the site
dictated by its respective coordination and steric requirements
mentioned above. Complexes 4−, 5− and 6− were prepared in a
similar way, using the 4-substituted 4-Cl-pzH and/or 4-Cl-3,5-
Ph2pzH as necessary.
We report that two eclipsed CuII3(l3-O) units supported by
pyrazolate bridges, are electrochemically oxidised in a stepwise
fashion to the corresponding mixed-valent Cu3(l3-O)5+ species,
presenting evidence of valence delocalisation over all six Cu atoms.
Redox-active trinuclear copper clusters constitute the active
centres of several copper proteins, such as ascorbate oxidase,
laccase and ceruloplasmin.1 Triangular CuII-complexes are conve-
nient models for the study of the fundamental chemical properties
of such active sites, as well as for the elucidation of factors
determining the magnetic exchange among three paramagnetic
centres.2,3 As part of similar efforts, we have been investigating
the chemical, structural and magnetic properties of trinuclear
CuII-pyrazolates.4 In an earlier report, we have described the
electrochemical oxidation of [CuII3(l3-O)(l-pz)3X3]2− (pz = pyra-
zolate anion, C3H3N2−; X = Cl (12−) and CF3COO (22−)), and
the structural characterization of the one-electron chemically
oxidized, mixed valent complexes.5 Theoretical calculations have
7+
suggested that the latter species are valence-delocalised Cu3
-
complexes with a 1A1 (S = 0) ground state.5,6 In the present article,
we extend our work to include the synthesis, characterization and
electrochemical study of four hexanuclear cage-like complexes of
formula [Cu6(l3-O)2(l-4-Rpz)6(l-4-Rꢀ-3,5-Ph2pz)3]−, R = Rꢀ = H
(3−), R = Cl, Rꢀ = H (4−), R = H, Rꢀ = Cl (5−), R = Rꢀ = Cl (6−),
as their PPN+ salts.
Scheme 1
The formation of triangular structures is favoured by the
pyrazolato ligand’s two N-donor orbitals oriented at 60◦; several
CuII-complexes of the formula [Cu3(l3-O(H))(l-4-Rpz)3L3]n− are
known.7 However, we have shown that the corresponding 3,5-
The X-ray crystal structure of 3− (Fig. 1)* consists of two
trinuclear [Cu3(l3-O)(l-pz)3]+ units bridged by three 3,5-Ph2pz
ligands, with the six Cu atoms forming a trigonal prism. Each
trinuclear unit accommodates a l3-O at its centre and 0.232–
disubstituted pyrazole-3,5-Rꢀꢀ pz (Rꢀꢀ = Me, Ph) structures would
2
be sterically hindered and none is known to date;§ dinuclear Cu2(l-
3,5-Rꢀꢀ pz) structures form instead.8 Consequently, the clipping
˚
2
0.238 A above the Cu3-plane, with Cu–O bond lengths of
together of two triangular units using three bridging 3,5-Ph2pz
ligands as clips, described here, provides hexanuclear complexes
˚
˚
1.882(10)–1.894(5) A. The two l3-O atoms are 3.461(9) A apart.
Within the trinuclear units, approximately square-planar Cu atoms
˚
are held at distances of 3.206(4)–3.279(5) A (the prism base),
while significantly shorter Cu–Cu distances of 2.975(3)–3.028(3)
Department of Chemistry, University of Puerto Rico, San Juan, PR,
00931-3346, USA. E-mail: Raphael@adam.uprr.pr; Fax: +1-787-7568242;
Tel: +1-787-7640000 ext. 2598
˚
A are associated with the diphenylpyrazolato bridges (the prism
height). The latter Cu–Cu distances are also significantly shorter
than those found in complexes containing unrestricted Cu–(l-
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Experimental
data. ESI-MS Spectra for PPN[3]. Cyclic voltammograms of 4−, 5− and 6−.
ORTEP drawings for compounds 3−, 4− and 6− showing the atom labeling
scheme. See DOI: 10.1039/b615018g
˚
pz)–Cu moieties (e.g., Cu–Cu = 3.8893(6) A in the [Cu(pz)2]n-
polymer).10 The resulting compression of the Cu6-prism along its
threefold axis strains the bridging 3,5-Ph2pz ligands, as evident by
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its molecular symmetry. In the 1H NMR (CD2Cl2) spectrum,
four paramagnetically shifted resonances are observed at 35.50,
27.74, 18.20, 11.82 ppm, besides those in the aromatic region
(7.0–8.0 ppm). Based on the observed 2 : 1 : 4 : 4 ratio of
integrated intensities, the first two resonances are unequivocally
assigned (Scheme 2) to the H4-(pz) and to the H4-(3,5-Ph2pz)
atoms, respectively. The remaining two resonances correspond
to the H3,5-(pz) and the ortho-H of the phenyl substituents, but
cannot be differentiated with certainty. It is speculated that the
less paramagnetically shifted peak (11.82 ppm) corresponds to the
ortho phenyl hydrogens, arising from a through-space interaction
˚
with the paramagnetic centre (Cu–H distance: 2.52–2.82 A). As a
CD2Cl2 solution of 3− ages under atmospheric conditions, its 1H
NMR spectrum collapses and after approximately two weeks a
new spectrum with several resonances in the 20–40 ppm window
arises. This spectrum resembles that of the polynuclear [Cu(l-
pz)(l-OH)]n-ring assemblies, which we have reported previously.12
Scheme 2
Fig. 1 Ball-and-stick diagrams of 3−, parallel (A) and perpendic-
ular (B) to the prism axis; hydrogen atoms not shown. Selected
◦
˚
distances (A) and angles ( ): Cu–N(pz) 1.934(7)–1.964(7), Cu–N(3,5-
Ph2pz) 2.003(7)–2.056(6); N(pz)–Cu–N(pz) 161.9(3)–168.3(3), O–Cu–
N(3,5-Ph2pz) 156.5(2)–168.8(2), N(pz)–Cu–N(3,5-Ph2pz) 89.3(3)–95.6(3),
O–Cu–N(pz) 88.2(2) – 90.7(2).
Negative mode ESI-MS (at 25
V cone voltage) in
CH2Cl2 solution identifies the singly charged molecular peak
[Cu6O2(pz)6(Ph2pz)3]− at 1474 amu, while the base peak at
501.6 amu agrees with the formula [Cu(Ph2pz)2]−. Other peaks with
lower m/z values, corresponding to pyrazole fragments, are also
observed. The molecular ion peak is also observed at higher cone
voltage (75 V), together with peaks at 840.7 and 992.7 amu, as-
signed to [Cu4O2(pz)5(Ph2pz)]− and [Cu4O2(pz)4(Ph2pz)2]− species,
respectively.
the more acute Cu–N–N angles of 111.5(5)–115.0(4)◦ compared to
the those within the triangular units, 118.1(4)–120.6(5)◦. Solid 3−
is quite robust: thermogravimetric analysis shows no weight loss
up to 255 ◦C, while melting is accompanied by decomposition.
No significant structural differences are observed in the crystal
structures of 4−and 6−, compared to that of 3−.*
There is a previous report of a neutral hexanuclear cage quite
similar to 3− but containing a l6-Cl at the centre of the Cu6-prism
and l3-OMe instead of l3-O, Cu6Cl(OMe)2(pz)9 (7), obtained by
the self-assembly of pyrazole and CuII in the presence of Et3N and
excess Et4NCl in methanol, whose formation was attributed to
chloride templation.11 The formation of structures 3−–6− described
here, demonstrate that the hexanuclear CuII-pyrazolato cage does
not require the presence of any l6-ion. While the intratrimer Cu–
The cyclic voltammogram of 3− (0.5 M Bu4NPF6–CH2Cl2, Pt
working electrode, 295 K) in the potential range −1.0 to +1.0 V
(vs. Fc/Fc+) shows two reversible one-electron oxidations centred
at −0.068 and +0.336 V; no other electrode process is observed
up to the solvent oxidation limit (Fig. 2). The first oxidation
process occurs at a potential quite similar to the one observed
(−0.053 V) for the trinuclear complex [Cu3(l3-O)(l-pz)3Cl3] (12−)
6+/7+
and is by analogy assigned to the Cu3
oxidation of one of
the two trinuclear units of 3−.5 The trinuclear complex 12− did
not show a second oxidation process within the potential window
studied.5 Consequently, the second oxidation process observed
˚
Cu distances of 7 (3.209(1), 3.233(1) A) are similar to those of
−
3 (3.206(4)–3.279(5) A), the longer intertrimer Cu–Cu distances
here in the voltammetry of 3− is tentatively assigned to the
˚
−
6+/7+
˚
˚
of 7 (3.621(1), 3.675(1) A) compared to 3 (2.975(3)–3.028(3) A)
indicate that the Cu6-cage is rather expanded in 7 in order to
accommodate the Cl− ion.
While 3− has no crystallographic symmetry, oNMR studies
indicate an average D3h symmetry in solution, consistent with
corresponding Cu3
oxidation of its second trinuclear unit.
The 0.404 V difference between E1/2 values is consistent with
significant electronic communication between the two Cu3(l3-O)
redox centres (although interactions with solvent and/or counter
ions can influence DE1/2 values13). This implies partial charge
38 | Dalton Trans., 2007, 37–40
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the electronic structures of the oxidised complexes 30 and 3+
by theoretical calculations, is currently in progress. Particularly
intriguing is the possibility that the electronic communication
between the two Cu3(l3-O) units might involve direct metal–metal
interactions across the corresponding short Cu–Cu distances,
instead of a through-ligand (e.g., l-3,5-Ph2pz) interaction.
G. M. and M. R. C. acknowledge NSF-EPSCoR (Grant
EPS0223152) and NIH-RISE (Grant 2R25GM061151) doctoral
scholarships.
Notes and references
§ In contrast, triangular CuI-complexes of 3,5-disubstituted pyrazoles are
common: K. Fujisawa, Y. Ishikawa, Y. Miyashita and K. Okamoto, Chem.
Lett., 2004, 33, 66, and references therein.
¶ PPN[3]: Method (1): 100 mg (63 lmol) (PPN)2[Cu3(l3-O)(l-pz)3Cl3] and
31 mg (32 lmol) (Ph2pzAg)3 were stirred in 5 ml CH2Cl2 for one day and
the brown solution filtered. Slow Et2O-vapor diffusion into the CH2Cl2
filtrate provided dark brown crystals the next day, which were filtered
off, washed with Et2O and dried at 90 ◦C (58 mg, 92%). Method (2):
257 mg (1.50 mmol) CuCl2·2H2O, 102 mg (1.50 mmol) pyrazole, 166 mg
(0.75 mmol) 3,5-diphenylpyrazole, 130 mg (3.25 mmol) NaOH and 144 mg
(0.25 mmol) PPNCl were stirred together in 50 ml CH2Cl2 for one day.
Crystals were obtained after filtration and Et2O vapor diffusion (370 mg,
74%). Mp 255 ◦C (decomp.). Anal. Calc. for C99H81Cu6N19O2P2: C, 59.09;
H, 4.07; N, 13.23%. Found: C, 58.57; H, 4.04; N, 13.22%.
Fig. 2 Cyclic voltammogram of 3−; 0.5 M Bu4NPF6–CH2Cl2, Pt working
electrode, vs. ferrocene–ferrocenium.
Table 1 Cyclic voltammetric data and comproportionation constantsa
for 3−, 4−, 5− and 6−
a
E
1/2(1)/V
E
1/2(2)/V
DE1/2/V
Kc
3−
4−
5−
6−
−0.068
+0.253
+0.012
+0.319
+0.336
+0.659
+0.420
+0.738
+0.404
+0.406
+0.408
+0.419
6.6 × 106
* Crystal data: PPN[3]: C99H81Cu6N19O2P2, M = 2012.01, monoclinic,
˚
7.3 × 106
7.9 × 106
12.1 × 106
space group P21/c, a = 17.98(2), b = 25.31(3), c = 22.76(2) A, b =
◦
3
−1
˚
109.88(2) , V = 9741(18) A , Z = 4, T = 303(2) K, l = 1.377 mm
.
41 689 reflections measured, 14 043 unique (Rint = 0.0604), wR2 = 0.152,
for 1153 parameters with no restraints. PPN[4]: C101H79Cl10Cu6N19O2P2,
a RT ln Kc = nFDE1/2
.
M = 2388.51, triclinic, space group P1, a = 11.67(1),◦b = 20.72(2), c =
¯
3
˚
˚
21.23(3) A, a = 97.971(2), b = 90.476(2), c = 90.654(2) , V = 5097(1) A ,
Z = 2, T = 302(2) K, l = 1.582 mm−1. 22 359 reflections measured, 14 602
unique (Rint = 0.0483), wR2 = 0.135, for 1261 parameters with no restraints.
delocalization over both Cu3-units of 3−. In that case, the oxidized
PPN[6]: C105H86Cl13Cu6N19O3P2, M = 2565.96, orthorhombic, space group
3
˚
˚
P212121, a = 19.588(2), b = 20.268(2), c = 27.541(3) A, V = 10934(2) A ,
Z = 4, T = 299(2) K, l = 1.553 mm−1. 48 230 reflections measured,
15 738 unique (Rint = 0.0392), wR2 = 0.0796, for 1335 parameters with no
restraints. CCDC reference numbers 617268–617270. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b615018g
species 30 should be more accurately described as Cu6
.
13+
The two redox processes of 4−, 5− and 6− are shifted to
higher potentials with respect to those of 3− (Table 1). The
six Cl-substituents within the trinuclear ring of 4− shift both
electrochemical processes by approximately +0.320 V with respect
to those of 3−. However, the three Cl-substituents on the 4-Cl-
3,5-Ph2pz of 5− shifts the redox potential for the processes by
only +0.080 V with respect to 3−. In 6−, the introduction of
nine Cl-substituents in all pyrazole ligands of the complex has an
additive effect resulting in positive potential shift of approximately
+0.400 V for both processes.
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In conclusion, by matching the coordination requirements of
its various components, the preparation of the hexanuclear Cu-
complexes 3−–6− has been achieved in a one-pot synthesis. A
new motif – Cu6-prism – that allows electronic communication
among the six Cu-atoms has been structurally characterised. The
6+
metal-based redox processes, leading from the homovalent Cu3
–
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6+
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7+
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O)] – an extension of studies of Creutz–Taube type complexes
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©