The Goldilocks principle in action: Synthesis and structural characterization of a novel {Cu4(μ3-OH)4} cubane stabilized by monodentate ligands

Article abstract of DOI:10.1039/c3dt51017d

A {Cu₄(μ₃-OH)₄} compound, where four copper(ii) and four μ₃-bridging oxygen atoms occupy alternate corners of a slightly distorted cube, has been prepared and structurally characterized. This species, formulated as [Cu₄(μ₃-OH) ₄(Htmpz)₈](ClO₄)₄·1.5Et ₂O (Htmpz = 3,4,5-1H-trimethyl pyrazole), can be classified as belonging to type I Cu₄O₄ cubane complexes, and is better described as two Cu^II-(μ-OH)₂-Cu^II units held together by four long Cu-O bonds. The central distorted cubane core is stabilized by neutral monodentate ligands (Htmpz) and perchlorate anions, as demonstrated by single-crystal X-ray structure analysis. The title compound was obtained by hydrolysis of a dinuclear methoxo-bridged species, [Cu(μ-OCH ₃)(Htmpz)₂]₂(ClO₄)₂, which was prepared by reaction of [Cu(Htmpz)₄(ClO₄) ₂] with methanol. All these reactions represent a nice example of the Goldilocks principle in action in coordination chemistry, since each single actor (solvent, counteranion, and ligand) has the just right electronic, steric or coordinative properties which determine the fate of the final products.

Full text of DOI:10.1039/c3dt51017d

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The Goldilocks principle in action: synthesis and
structural characterization of a novel {Cu4(μ3-OH)4}
cubane stabilized by monodentate ligands†
Cite this: Dalton Trans., 2013, 42, 12265
G. Attilio Ardizzoia,a Stefano Brenna,*a Sara Durini,a Bruno Therrienb and
Ivan Trentinc
A {Cu4(μ3-OH)4} compound, where four copper(II) and four μ3-bridging oxygen atoms occupy alternate
corners of a slightly distorted cube, has been prepared and structurally characterized. This species, formu-
lated as [Cu4(μ3-OH)4(Htmpz)8](ClO4)4·1.5Et2O (Htmpz = 3,4,5-1H-trimethyl pyrazole), can be classified as
belonging to type I Cu4O4 cubane complexes, and is better described as two CuII–(μ-OH)2–CuII units held
together by four long Cu–O bonds. The central distorted cubane core is stabilized by neutral mono-
dentate ligands (Htmpz) and perchlorate anions, as demonstrated by single-crystal X-ray structure analy-
sis. The title compound was obtained by hydrolysis of a dinuclear methoxo-bridged species, [Cu-(μ-OCH3)
(Htmpz)2]2(ClO4)2, which was prepared by reaction of [Cu(Htmpz)4(ClO4)2] with methanol. All these reac-
tions represent a nice example of the Goldilocks principle in action in coordination chemistry, since each
single actor (solvent, counteranion, and ligand) has the “just right” electronic, steric or coordinative pro-
perties which determine the fate of the final products.
Received 17th April 2013,
Accepted 26th June 2013
DOI: 10.1039/c3dt51017d
www.rsc.org/dalton
(O = OR) systems are known,10 fewer examples of cubane com-
pounds having a Cu4(OH)4 core have been structurally charac-
Introduction
Polynuclear transition-metal complexes have received much terized.11 These species have been prepared either starting
attention in the recent past due to their magnetic potential- from a copper(II) salt or by aerobic oxidation of a Cu(I) precur-
ities1 and their ability to mimic metalloprotein active sites.2 In sor and in most cases the coordination sphere of copper(II)
particular, M4X4 cubane clusters (where M = Cu, Ni, Co, Fe, was completed by bi- or tridentate ligands. Actually, there are a
Mn, Mo; X = O, S)3 have been widely studied for their appli- very limited number of examples where the Cu4(OH)4 core is
cation as single molecule magnets (SMMs),4 in bioinorganic stabilized by mondentate ancillary ligands.11a,e,l,n,12
chemistry5 or in catalysis.6 Focusing the discussion on
In the past, we widely explored the coordination chemistry
copper(II) high-nuclearity systems, they showed interesting of nitrogen-based multidentate ligands,13 with special atten-
magnetostructural properties relevant to the field of multinuc- tion to pyrazole and its analogues (imidazole and triazole)
lear copper oxidases7 and especially to molecular magnetism.8 both in their neutral14 and anionic azolate forms.15 The corres-
In the case of Cu4O4 systems, the CuII–O–CuII angle has been ponding transition-metal complexes have found applications
identified as being responsible for magnetic exchange, in catalysis, material science or in the synthesis of polynuclear
together with the out-of-plane angle of the substituents of the systems. Continuing the investigation on pyrazole systems,
bridging oxygens.9 Although numerous alkoxo-bridged Cu4O4 herein we report the preparation of a copper(II) polynuclear
compound with a cubane skeleton bearing 3,4,5-1H-trimethyl
pyrazole (Htmpz) as an ancillary ligand. The latter is
aDepartment of Science and High Technology, University of Insubria, via Valleggio
coordinated to copper in a monodentate fashion via its pyri-
11, 22100 Como, Italy. E-mail: stefano.brenna@uninsubria.it;
dine-like nitrogen atom. The synthesis and structural charac-
Fax: +39-031-2386119; Tel: +39-031-2386476
terization of the title compound, [Cu4(μ3-OH)4(Htmpz)8]-
(ClO4)4·1.5Et2O (3·1.5Et2O) and its precursors, [Cu(Htmpz)4-
(ClO4)2] (1) and [Cu(μ-OCH3)(Htmpz)2(ClO4)]2 (2), are
described. In our opinion, the reactions discussed here rep-
resent a nice example of the Goldilocks principle applied to
coordination chemistry: actually, like in the fairy tale of
Goldilocks and the Three Bears,16 the solvent (methanol), the
bInstitute of Chemistry, University of Neuchatel, Avenue de Bellevaux 51, CH-2000
Neuchâtel, Switzerland
cInstitut für Biochemie, Ernst-Moritz-Arndt-Universität, Felix-Hausdorff Strasse 4,
17487 Greifswald, Germany
†Electronic supplementary information (ESI) available: Experimental details
with synthesis and spectroscopic data; crystallographic data. CCDC 930623,
930624 and 945309. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt51017d
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counteranion (ClO4−) and the ancillary ligand (Htmpz) each CH2Cl2 was then stirred at room temperature for 6 hours. After
possesses “just right” properties (i.e., perchlorate showing a removal of the solvent and recrystallization of the crude
semi-bonding coordination, neither too strongly nor too product, a light blue compound was isolated in high yields. By
poorly coordinated to CuII) to generate complexes 1–3. The slow diffusion of diethylether in a CH2Cl2 solution of the
effects of the Goldilocks principle operating here are detailed complex, it was possible to grow single crystals suitable for
in the text.
X-ray diffraction analysis. Surprisingly the compound deriving
from the hydrolysis of 2 is characterized by a Cu4O4 cubane
core and can be formulated as [Cu4(μ3-ΟH)4(Htmpz)8]-
(ClO4)4·1.5Et2O (3·1.5Et2O). It is supposed that a transient
[CuII–(μ-OH)2–CuII] dihydroxo bridged system, formed by
hydrolysis of the corresponding methoxo complex, self-assem-
bles into a cubane cluster. This is somehow corroborated by
the crystal structure of cubane 3 itself (vide infra) composed of
two dimeric [CuII–(μ-OH)2–CuII] units brought together by four
parallel, long Cu–O bonds. Moreover, when [Cu(μ-OCH3)-
(Htmpz)2(ClO4)]2 is dissolved in dry CH2Cl2 in the absence of
exogenous water and stirred at room temperature for a pro-
longed period, no reaction takes place. Then, the hydrolysis of
μ-OCH3 is the first step of the reaction converting the
methoxo-bridged complex 2 into cubane 3, the self-assembly
process being very fast.
When 3·1.5Et2O is heated at 40 °C under vacuum for three
hours, a change in color of the solid from the original light
blue into green is observed, associated with the loss of diethyl-
ether. The resulting species showed spectroscopic features (IR,
see ESI†) similar to 3·1.5Et2O and analyzed as [Cu4(μ3-ΟH)4-
(Htmpz)8](ClO4)4 (3). It was easily re-converted into 3·1.5Et2O
after short exposure at room temperature to diethylether
vapors; thus the loss of Et2O from 3·1.5Et2O is reversible and
does not affect the stability of the Cu4O4 cubane core.
Results and discussion
Syntheses of complexes
The reaction of [Cu(ClO4)2]·6H2O with 3,4,5-trimethyl-1H-pyr-
azole (Htmpz) in THF, at 20 °C, leads to the formation of
[Cu(Htmpz)4(ClO4)2] (1) (Scheme 1).
The same reaction can be performed in other solvents
besides THF (i.e. H2O, ethanol) and compound 1 is always iso-
lated in high yields. A particular behavior was noticed in
methanol, where the crude product resulted in a mixture of
the expected violet [Cu(Htmpz)4(ClO4)2] compound and a blue
solid. To better understand this behavior, the reaction of 1
with methanol was investigated. [Cu(Htmpz)4(ClO4)2] readily
dissolves in anhydrous methanol at room temperature to give
a dark green solution, from which soon precipitates a blue
solid formulated as [Cu(μ-OCH3)(Htmpz)2(ClO4)]2 (2)
(Scheme 2). Reasonably, but surprisingly enough, in the
course of the reaction, methanol is deprotonated by the per-
chlorate anion and two moles of perchloric acid are formed.
The acids are eliminated as pyrazolium salt [(H2tmpz+)(ClO4−)]
after protonation of free 3,4,5-trimethyl-1H-pyrazole, present in
the bulk as a consequence of dissociation of two molecules of
Htmpz from the CuII centre. Concurrently, the methoxido
anions coordinate to the copper atoms and the stable dinuc-
lear di-μ-methoxo-bridged [Cu(μ-OCH3)(Htmpz)2(ClO4)]2 is
formed.
Infrared and electronic spectra
Together with the expected broad band at about 1100 cm−1
−
due to the ν3 mode of ClO4 anions of the Td symmetry, the
infrared spectrum of 1 recorded in hexachlorobutadiene
(Fig. S1, ESI†) is characterized by intense absorptions at
3330 cm−1 (N–H of Htmpz) and in the region 2870–2927 cm−1
(C–H stretching, methyl substituents in Htmpz). A similar
pattern is observed in the spectrum of 2, which shows the N–H
peak at 3345 cm−1 and the CH3 groups (Htmpz) at
2883–2927 cm−1 (Fig. S2, ESI†). The N–H of the Htmpz mole-
cules in 2 are not involved in H-bonding interactions with
other atoms, thus leading to a slight increase in νN–H of 2 com-
pared to 1 (3345 vs. 3330 cm−1). The additional sharp vibration
at 2809 cm−1 is clearly attributable to C–H stretching of the
μ-OCH3 groups. By treatment of [Cu(Htmpz)4(ClO4)2] with
CD3OD it has been possible to isolate the deuterated
μ-methoxo dinuclear compound [Cu(μ-OCD3)(Dtmpz)2(ClO4)]2
(2-D4). In its infrared spectrum (Fig. S3, ESI†), the isotopic
shifts of N–D and CD3 stretching respectively to 2492 cm−1
and 2051 cm−1 are detected. Finally, species [Cu4(μ3-ΟH)4-
(Htmpz)8](ClO4)4·1.5Et2O (3·1.5Et2O) shows N–H and CH3
peaks of the Htmpz ligand respectively at 3313 and
2866–2927 cm−1 (Fig. S4, ESI†) and the disappearance of the
sharp stretching at 2809 cm−1 (μ-OCH3) due to the hydrolysis
of the methoxo groups. The broad band at 3551 cm−1 is
As is known, methoxo-bridged Cu(II) compounds are often
closely related to hydroxo-bridged Cu(II) species,17 and then
the hydrolysis of compound 2 was investigated, aiming at iso-
lating a CuII–(μ-OH)x–CuII system. A solution of 2 in wet
Scheme 1 Synthesis of [Cu(Htmpz)4(ClO4)2] (1).
Scheme 2 Formation of [Cu(μ-OCH3)(Htmpz)2(ClO4)]2 (2) in the reaction of (1)
with methanol.
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assigned to νOH of the triply-bridged hydroxyl groups. The
latter are involved in weak O–H⋯O(perchlorato) contacts (see the
crystal structure description below) and the νOH recorded for
cubane 3·1.5Et2O appears at higher wavenumbers with respect
to other previously reported systems.8d
The electronic spectra of complexes 2 and 3 recorded in
dichloromethane display a single unstructured absorption
band at 579 (2) and 566 nm (3) typical of copper(II) centers.18
Crystal structures
Selected bond distances and angles for [Cu(Htmpz)4(ClO4)2]
(1), [Cu(μ-OCH3)(Htmpz)2(ClO4)]2 (2) and [Cu4(μ3-ΟH)4-
(Htmpz)8](ClO4)4 (3) are presented in Tables 1 and 2. The
crystal structure of 1 (Fig. 1) shows the copper(II) centre in a
distorted octahedral geometry, with the equatorial positions
occupied by four Npyridine atoms of four monodentate Htmpz
ligands and one oxygen atoms of the two perchlorato anion
at the axial sites. Bond distances found for 1 are in accordance
with the literature data for similar compounds like
Fig. 1 Crystal structure of 1 at 35% probability level ellipsoids.21
20
[Cu(Hdmpz)4(ClO4)2]19
(Hdmpz = 3,5-dimethylpyrazole), Cu–N bond lengths varying
in from 1.978(4) to 2.014(4) Å. Analogous to [Cu-
and
[Cu(Hdmpz)4(OH2)](ClO4)2
1
(Hdmpz)4(ClO4)2], compound 1 shows a Cu–O(perchlorato) dis-
tance (Cu–O = 2.714(3) Å) within the semi-bonding range.
Moreover, intramolecular hydrogen bonds are detected
between uncoordinated N–H of 3,4,5-trimethyl-1H-pyrazole
Table 1 Selected bond distances (Å) and angles (°) in 1 and 2
Distances (Å)
Angles (°)
−
and one oxygen atom of ClO4 (N–H⋯O, 3.042 and 2.991 Å)
and between neighbouring 3,4,5-trimethyl-1H-pyrazole mole-
[Cu(Htmpz)4(ClO4)2] (1)
Cu(1)–N(1)
Cu(1)–N(3)
Cu(1)–O(4)
O(4)–Cl(1)
2.014(4)
N(1)–Cu(1)–N(3) (trans)
N(1)–Cu(1)–N(3) (cis)
N(1)–Cu(1)–N(1) (cis)
N(1)–Cu(1)–O(4)
N(3)–Cu(1)–O(4)
O(4)–Cu(1)–O(4)
176.12(2)
89.31(1)
87.88(1)
91.89(1)
85.24(1)
176.99(1)
cules (N–H⋯N, 2.954 Å) (Fig. S6, ESI†).
1.978(4)
2.714(3)
1.433(4)
Compound [Cu(μ-OCH3)(Htmpz)2(ClO4)]2 (2) crystallized in
ˉ
the centrosymmetric space group P1, and each copper centre is
in a N2O3 environment identified by two nitrogen atoms of
two adjacent Htmpz ligands, two oxygens of the bridging
methoxo groups and one oxygen of the perchlorato counter-
anion, the latter with a Cu–O length in the semi-bonding range
(Cu–O = 2.657(6) Å) (Fig. 2). The geometry around each metal
centre can be classified as a Jahn–Teller-distorted square pyra-
midal geometry, as suggested by their value of the trigonality
index (τ = 0.14).22 Cu–N and Cu–O(CH3) equatorial bond dis-
tances are in the range for similar CuII complexes23 (Cu–N =
1.956(5)–1.964(5) Å; Cu–O = 1.924(5)–1.926(4) Å). Also, the
[Cu(μ-OCH3)(Htmpz)2(ClO4)]2 (2)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(2)
O(2)–Cl(1)
O(1)–C(1)
1.956(5)
1.964(5)
2.657(6)
1.438(6)
1.400(1)
2.941(1)
Cu(1)–O(1)–Cu(1)
O(1)–Cu(1)–O(1)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–O(1) (trans)
N(2)–Cu(1)–O(1) (trans)
N(1)–Cu(1)–O(1) (cis)
N(2)–Cu(1)–O(1) (cis)
N(1)–Cu(1)–O(2)
99.60(2)
80.40(2)
94.76(2)
165.29(2)
173.51(2)
91.51(2)
93.82(2)
102.87(2)
86.49(2)
Cu(1)⋯Cu(1)′
N(2)–Cu(1)–O(2)
Table 2 Selected bond distances (Å) and angles (°) in 3
Distances (Å)
Angles (°)
Cu(1)–N(1)
Cu(1)–N(3)
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–O(3)
Cu(2)–N(5)
Cu(2)–N(7)
Cu(2)–O(1)
Cu(2)–O(3)
Cu(2)–O(4)
Cu(3)–N(9)
Cu(3)–N(11)
Cu(3)–O(1)
1.986(5)
1.959(6)
1.968(4)
1.964(4)
2.296(5)
1.982(7)
1.966(6)
2.346(4)
1.973(4)
1.968(5)
1.987(5)
1.971(5)
1.952(4)
Cu(3)–O(2)
Cu(3)–O(4)
Cu(4)–N(13)
Cu(4)–N(15)
Cu(4)–O(3)
Cu(4)–O(4)
Cu(4)–O(2)
Cu(1)⋯Cu(2)
Cu(1)⋯Cu(3)
Cu(1)⋯Cu(4)
Cu(2)⋯Cu(3)
Cu(2)⋯Cu(4)
Cu(3)⋯Cu(4)
1.983(4)
N(1)–Cu(1)–O(1)
N(3)–Cu(1)–O(2)
Cu(1)–O(1)–Cu(2)
Cu(1)–O(3)–Cu(2)
N(5)–Cu(2)–O(4)
N(7)–Cu(2)–O(3)
O(1)–Cu(1)–O(3)
O(1)–Cu(2)–O(3)
N(9)–Cu(3)–O(2)
N(11)–Cu(3)–O(1)
Cu(3)–O(2)–Cu(4)
Cu(3)–O(4)–Cu(4)
N(13)–Cu(4)–O(3)
N(15)–Cu(4)–O(4)
O(2)–Cu(3)–O(4)
O(2)–Cu(4)–O(4)
172.1(2)
166.6(2)
97.1(2)
98.6(2)
171.5(2)
163.9(2)
81.8(2)
80.4(2)
166.7(2)
170.4(2)
96.5(2)
97.1(2)
170.0(2)
168.4(2)
82.2(2)
82.3(2)
O(1)–Cu(3)–O(4)
O(1)–Cu(3)–O(2)
O(3)–Cu(4)–O(4)
O(2)–Cu(4)–O(4)
O(1)–Cu(1)–O(3)
O(2)–Cu(1)–O(3)
O(1)–Cu(2)–O(4)
O(1)–Cu(2)–O(3)
Cu(1)–O(3)–Cu(2)
Cu(2)–O(1)–Cu(1)
Cu(1)–O(2)–Cu(4)
Cu(2)–O(4)–Cu(3)
Cu(3)–O(2)–Cu(4)
Cu(4)–O(4)–Cu(3)
Cu(3)–O(1)–Cu(2)
Cu(4)–O(3)–Cu(1)
82.5(2)
77.5(2)
78.0(2)
82.3(2)
81.8(2)
82.5(2)
81.3(2)
80.4(2)
98.6(2)
97.1(2)
96.6(2)
97.2(2)
96.5(2)
97.1(2)
96.6(2)
97.1(2)
2.312(4)
1.997(5)
1.965(5)
1.973(4)
1.968(4)
2.319(4)
3.243(1)
3.064(1)
3.205(1)
3.220(1)
3.056(1)
3.216(1)
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0.127),22 the equatorial sites being occupied by one nitrogen
atom of two monodentate Htmpz molecules and two μ3-OH
groups, whereas the third oxygen bound to copper occupies
the apical position. Analysis of bond distances reveals that the
Cu–Oapical distances are elongated (2.296(4) to 2.346(4) Å) com-
pared to the Cu–Oequatorial distances (1.952(4) to 1.983(4) Å).
Thus, [Cu4(μ3-ΟH)4(Htmpz)8](ClO4)4 is better described as
being composed of two dimeric Cu2(μ-OH)2(Htmpz)4 units
which self-assemble in a tetrameric cubane of type I.11b
According to the classification of Cu4O4 based on Cu⋯Cu dis-
tances proposed by Alvarez,25 type I cubane is classified as
(4 + 2), showing four long inter-dimeric distances and two
shorter intra-dimeric distances between the copper centers.
Analysis of the Cu⋯Cu distances in 3 confirms this trend, with
four long inter-dimeric Cu⋯Cu distances (3.205(1), 3.216(1),
3.220(1), 3.243(1) Å) and two shorter intra-dimeric distances
(3.056(1), 3.064(1) Å) (Fig. 4).
Fig. 2 Crystal structure of 2 at 35% probability level ellipsoids.
Cu⋯Cu distance (2.941(1) Å) is in accordance with the litera-
ture data for related CuII–(μ-OCH3)2–CuII systems.7b The values
of O–Cu–O and Cu–O–Cu angles found for [Cu(μ-OCH3)-
(Htmpz)2(ClO4)]2 fall in the ranges expected for dinuclear CuII–
(OMe)–CuII compounds, Cu–O–Cu and O–Cu–O measuring
respectively 99.60(2)° and 80.40(2)°. This value for the Cu–O–
Cu angle suggests an antiferromagnetic behavior for [Cu-
(μ-OCH3)(Htmpz)2(ClO4)]2, in conformity with what is expected
for symmetrically di-bridged dinuclear copper(II) complexes.24
Finally, out-of-plane angles of the methyl in bridging methoxo
groups is 32.54° (see Fig. S7, ESI†).
Compound 3 represents the fifth example of a structurally
characterized copper(II) tetranuclear compound where the
{Cu4(μ3-OH)4} single-core is stabilized by monodentate ancil-
lary ligands (Htmpz).11a,e,l,n The four copper atoms and the
four oxygens of the bridging hydroxo groups in 3 occupy alter-
nate corners of a slightly distorted cube (Fig. 3). Each copper
centre has an N2O3 surrounding in a square-pyramidal geome-
try (Addison parameter values ranging between 0.023 and
Within the dimers, the Cu–O–Cu angles range from
101.5(2)° to 102.8(2)°, whereas the O–Cu–O span ranges from
77.5(2)° to 78.0(2)°. Hatfield described a correlation between
Cu–O–Cu angles and the magnetic properties of μ-hydroxo
bridged dinuclear compounds,26 the ferromagnetic behavior
being associated with species having Cu–(OH)–Cu angles
smaller than 97.6°. For compounds showing bridging
Cu–O–Cu angles of copper centers higher than 97.6°, the sign
of the magnetic interactions among copper(II) is negative, thus
leading to antiferromagnetic behavior. This is also the case
for [Cu4(μ3-ΟH)4(Htmpz)8](ClO4)4, as corroborated by low-
temperature NMR measurements (see below). External
Cu–O–Cu angles in the Cu4O4 core of 3 range from 96.5(2)° to
102.8(2)°, whereas O–Cu–O external angles span between
80.4(2)° and 82.5(2)°, in accordance with the literature data for
the previous {Cu4(μ3-OH)4} species.11 The crystal structure of 3
is completed by four perchlorate anions in the second coordi-
nation sphere of the cubane core. As already observed for
other {Cu4(OH)4} cores,11b the structure of 3 is further stabil-
ized by the presence of H-bonding between the cluster itself
and the four counteranions. Indeed, the oxygen atoms of the
−
ClO4 form hydrogen bonding both with Htmpz ligands and
the bridging OH groups (Fig. S8, ESI†). Generally, O⋯H–Npz
are longer than O⋯H–Ohydroxo interactions, ranging respecti-
vely between 2.820(1) and 3.000(9) Å (O⋯H–Npz) and between
2.819(6) and 2.932(6) Å (O⋯H–Ohydroxo). In [Cu4(μ3-ΟH)4-
−
(Htmpz)8](ClO4)4, three ClO4 are triply-connected to the
cubane cluster, with two O⋯H–Npz and one O⋯H–Ohydroxo
while the fourth ClO4 anion shows only two H-bonding inter-
,
−
actions (one O⋯H–Npz and one O⋯H–Ohydroxo). In particular,
H-bonding interactions in triply-bridged perchlorate ions are
−
longer than the corresponding ones in doubly-bridged ClO4
,
the lengths being respectively 2.890(1)–3.000(9) Å (O⋯H–Npz)
−
and 2.829(7)–2.932(6) (O⋯H–Ohydroxo) in μ3-ClO4 and 2.820(1)
Å (O⋯H–Npz) and 2.819(6) Å (O⋯H–Ohydroxo) for μ2-ClO4−. As
magneto-structural correlations suggest an antiferromagnetic
behaviour for compounds 2 and 3, we attempted to perform
an NMR investigation. Actually, species 2 and 3 show quite
resolved signals in their 1H NMR spectrum (CD2Cl2, 20 °C,
−
Fig. 3 Crystal structure of 3 at 35% probability level ellipsoids with ClO4
anions omitted for clarity.
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Fig. 4 Cu–O intra-dimeric (a), Cu–O inter-dimeric (b) and Cu⋯Cu (c) bond distances in cubane 3 (*: Cu(1)–Cu(2), 3.243(1) Å; **: Cu(2)–Cu(3), 3.220(1) Å; §: Cu(2)–
Cu(4), 3.056(2) Å).
Fig. S10–S12, ESI†), thus confirming somehow the existence of that two neighboring molecules of [Cu(Htmpz)2(μ-OH)(ClO4)]2
antiferromagnetic coupling between the copper(II) centres (4), generated by hydrolysis of 2, rapidly undergo a self-assem-
in these complexes. This coupling is maintained in a wide bly process, yielding the tetranuclear cluster [Cu4(μ3-ΟH)4-
range of temperatures, as illustrated by low temperature NMR (Htmpz)8](ClO4)4 (Scheme 3, step b). This green species, when
experiments performed on the two samples (down to −60 °C, exposed to diethylether vapors, quickly converts into a light
ESI†).
blue compound, showing spectroscopic features identical to
3·1.5Et2O (Fig. S14, ESI†). Unfortunately, various attempts to
intercept pure intermediate 4 were unsuccessful, due to its ten-
Mechanism of hydrolysis of 2
Moreover, the structural analysis reveals a noticeable analogy dency to rapidly self-assemble into the cubane structure, as
of bond distances and angles in the (CuII–O–CuII)x skeleton reported by Cronin et al. for an analogous {Cu4(μ3-OH)4}
of [Cu(μ-OCH3)(Htmpz)2(ClO4)]2 and [Cu4(μ3-ΟH)4(Htmpz)8]- system.11g Noticeably, in that case the hydrolysis of methoxo-
(ClO4)4. Thus, one can assume that the formation of com- bridged dinuclear copper(II) species to give Cu4O4 moieties
pound 2 is crucial for leading to the title cubane via hydrolysis occurred in the presence of a large amount of water (13% to
of the methoxo-bridges (Scheme 3). In Scheme 3, we assumed 20% volume). Such a quantity was postulated to prevent weak
Scheme 3 Pathways leading to the formation of 3 by hydrolysis of 2.
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−
apical ClO4 coordination, thus ultimately favoring the associ-
ation of dimers into the cubane. In the present case, a lesser
amount of water (<2% vol.) was needed to perform hydrolysis of
the methoxo bridges, and the semi-bonding apical coordination
of ClO4− is fundamental to obtain the cubane (vide infra).
The hydrolysis of 2 was also performed in a coordinating
solvent, namely acetonitrile or benzonitrile, aiming at stabiliz-
ing an intermediate species. The methoxo [Cu(Htmpz)2-
(μ-OCH3)(ClO4)]2 complex, when stirred at room temperature
for 6 h in CH3CN or PhCN, in the presence of water converts
into
a dark green compound, postulated as [Cu(μ-OH)-
(Htmpz)2(ClO4)]2·RCN (R = CH3, 5; R = Ph, 6) according to
elemental analyses and infrared evidence (Scheme 3, step c).
In the IR spectrum of compounds 5 and 6 (Fig. S13, ESI†) the
expected νN–H (3303 cm−1), νC–H (2863–2925 cm−1) and νCl–O
(broad band at 1103 cm−1) are observed. Additionally, a small
shoulder at 3550 cm−1 representative of OH groups is detected.
The weak stretching at about 2265 cm−1 indicates the presence
of RCN in the molecular structure of 5 and 6, which stabilizes
Scheme 4 Proposed mechanism for the generation of [Cu(μ-OCH3)-
(Htmpz)2(ClO4)]2 (2).
the dinuclear system. Actually, when compounds 5 or 6 are one molecule of 3,4,5-trimethyl-1H-pyrazole (L), it loosely co-
stirred at room temperature in dry CH2Cl2 for a few minutes, a ordinates to copper(II) via oxygen. The reaction evolves with a
change in the color of the solution from dark green to light four-centered Cu–O(ClO3)–H–O(CH3) concerted hydrogen-
green is observed (Scheme 3, step d). Again, the spectroscopic transfer from methanol to the coordinated ClO4−. The gener-
and analytical features of the product isolated after exposure ated perchloric acid is eliminated as 3,4,5-trimethyl-1H-pyr-
to diethylether vapors were identical to those of cubane azolium salt (HL+ClO4−), while the resulting four-coordinated
3·1.5Et2O. Then, we suppose that the CH3CN (or PhCN) mole- methoxo species dimerize to the more stable complex 2.
−
cule in 5 (or 6) is easily lost in CH2Cl2 solution, leaving the Although it is quite unusual to see methanol protonate ClO4
,
[Cu(μ-OH)(Htmpz)2(ClO4)]2 (4) dinuclear system which readily we believe that this equilibrium is favored by two factors: (i)
undergoes self-assembly into 3. Unfortunately, this prompt the poor solubility of compound 2, which precipitates off, thus
loss of RCN from 5 or 6 in solution prevented the growth of progressively moving the final step forward and consequently
single crystals of [Cu(Htmpz)2(μ-OH)(ClO4)]2·RCN and all the whole system towards the formation of 2, and (ii) the
attempts resulted in the isolation of light-blue crystals of assistance of Htmpz that, after dissociation, is however in
cubane [Cu4(μ3-ΟH)4(Htmpz)8](ClO4)4·1.5Et2O.
close proximity to the coordination sphere of the copper
centre and favors the final proton transfer.
The Goldilocks principle leading to 3
−
In this process, also ClO4 plays a key role. To prove this,
As discussed above, the hydrolysis reaction of complex 2 plays we prepared [Cu(Htmpz)4(NO3)2] (7) and [Cu(Htmpz)4(BF4)2]
a matchless role in the genesis of cubane 3. Hence, we decided (8) by following a similar procedure to that for the synthesis of
to better explore the formation of the crucial methoxo-bridged 1 and then we reacted them with methanol. In the course of
species. Initially, to check the possibility to obtain a dinuclear the reaction, a color change from violet to blue is observed,
CuII–(OR)2–CuII system employing a different alcohol (R = Et, but no precipitation takes place. When the solvent is removed
iPr) we treated compound [Cu(Htmpz)4(ClO4)2] (1) with ethanol under reduced pressure, the starting complexes are always
or propan-2-ol. Even after a prolonged stirring, no reaction takes quantitatively recovered. It should not be excluded that ana-
place and the violet compound 1 is wholly recovered after logous to the reported copper(II) complexes bearing 3,5-
removal of the solvent, without showing formation of an alkoxo- dimethylpyrazole (Hdmpz) as a ligand18 ([Cu(Hdmpz)4(NO3)2]
bridged compound. Yet, as predictable, performing the same and [Cu(Hdmpz)4(BF4)2]), also compounds 7 and 8 can evolve,
reaction in water does not lead to the title cubane, and again in solution, to species with different stoichiometries. Then,
[Cu(Htmpz)4(ClO4)2] is isolated as the only product. Then, we only the octahedral [Cu(Htmpz)4(ClO4)2] (1) bearing perchlor-
first assume that methanol exclusively leads to the formation of ate in a semi-bonding range leads to the dinuclear methoxo-
the desired alkoxo-bridged dinuclear intermediate.
bridged compound 2 by reaction with methanol. This is due to
This may be due to the “just right” acidity of methanol, the “just right” coordinating properties of ClO4−, which is
which is neither “too acid” (as water) nor “too poorly acid” neither too weakly nor too strongly coordinated to CuII: this
(like ethanol or propan-2-ol).27 Moreover, also the right steric allows H⋯O interaction with methanol to take place, with the
−
hindrance of methanol allows it to take part in the pathway consequent elimination of (HL+)ClO4 (Scheme 4) and gene-
proposed in Scheme 4.
ration of complex 2.
We postulate that methanol is first involved in a H⋯O inter-
Finally, also 3,4,5-trimethyl-1H-pyrazole plays a key role in
action with a perchlorate anion, and then, after dissociation of the genesis of cubane [Cu4(μ3-ΟH)4(Htmpz)8](ClO4)4 (3).
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Indeed, when the more sterically crowded 3,5-di-tert-butyl-1H-
pyrazole or the more acidic 3,5-dimethyl-4-nitro-1H-pyrazole
are employed in the reaction, the stability of the methoxo-
bridged compound markedly drops and neither isolation of
this intermediate nor the formation of cubane species
occurred. Additional experiments using 3,5-dimethyl-1H-pyra-
zole (Hdmpz), which has similar steric hindrance and basicity
properties with respect to Htmpz,28 were then performed. The
complex [Cu(Hdmpz)4(ClO4)2]18 was treated with dry methanol,
leading to the compound [Cu(μ-OCH3)(Hdmpz)2(ClO4)]2 (9) in
good yields, as established by analytical and spectroscopic
data. In particular, in the infrared spectrum of 9 (in hexachloro-
butadiene), a sharp band at 2819 cm−1 can be undoubtedly
assigned to the C–H stretching mode of the methoxo groups
(Fig. S14, ESI†). Addition of water to a suspension of species 9
in CH2Cl2 resulted in a mixture of two compounds, a violet
solution containing the octahedral [Cu(Hdmpz)4(ClO4)2] and a
light blue precipitate identified as Cu(OH)2 (see the Experi-
mental section). Then, hydrolysis of μ-OCH3 moieties in 9 did
not allow either to isolate the corresponding hydroxo-bridged
dinuclear [Cu(μ-ΟH)(Hdmpz)2(ClO4)]2 (10) or the generation of
cubane [Cu4(μ3-ΟH)4(Hdmpz)8](ClO4)4 (11) (Scheme 5).
Fig. 5 The main C–H⋯O contacts between Htmpz and ClO4− of two neighbor-
ing molecules of compound 2, responsible for the self-assembly process.
strongly influences the magnetic properties of cobalt(II) dioxol-
ene compounds. In our case, in Hdmpz, the lack of the 4-CH3
substituent hampers these C–H⋯O contacts, ultimately pre-
venting the self-assembly of two dinuclear hydroxo units.
These convert partly into octahedral [Cu(Hdmpz)4(ClO4)2] and
partly into Cu(OH)2.
Such a dissimilar behavior between Htmpz and Hdmpz has
to be searched in the only difference between them: the
methyl substituent in position 4 of the heterocyclic ring. The
single-crystal X-ray structure analysis of [Cu(μ-OCH3)-
(Htmpz)2(ClO4)]2 shows that the hydrogen atoms of the methyl
groups of the pyrazole ligands are involved in numerous
C–H⋯O contacts with neighboring perchlorate anions (Fig. 5).
In solution, these weak (but strong enough) interactions may
persist for a time sufficiently long to maintain two transient
dinuclear {Cu(μ-OH)(Htmpz)2(ClO4)}2 units close enough to
favor the self-assembly to the cubane moiety.
Conclusions
A
novel copper(II) tetranuclear compound, [Cu4(μ3-ΟH)4-
(Htmpz)8](ClO4)4·Et2O, where the Cu(II) centers are stabilized
by monodentate ligands, has been synthesized. This com-
pound was obtained by hydrolysis of a dinuclear methoxo-
bridged species, [Cu(μ-OCH3)(Htmpz)2(ClO4)]2, derived from
the treatment of [Cu(Htmpz)4(ClO4)2] with methanol. The for-
mation of the cubane compound is the effect of a Goldilocks
principle which controls the reactivity of these complexes. As
illustrated in the text, changing one single factor (solvent,
counteranion or ligand) would prevent the obtainment of the
cubane species. This is due to the “just right” properties
A
similar behavior was reported by Shultz and co-
authors,16c who showed how C–H⋯O lattice hydrogen bonding
−
owned by each element: ClO4 shows well-balanced coordinat-
ing properties toward CuII, stabilizing intermediate species
and allowing them to evolve to products; methanol has the
right acidity and steric hindrance to give the dinuclear
methoxo-bridged intermediate; in the latter, the methyl groups
of Htmpz give C–H⋯O(ClO4−) contacts with the right strength
to maintain two transient {Cu(μ-OH)(Htmpz)2(ClO4)}2 units
close enough to assemble into the cubane.
Acknowledgements
Dr V. Colombo is gratefully acknowledged for collecting XRPD
traces reported in Fig. S12.†
Notes and references
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