Spectroscopic evidence in solid and solution of a discrete copper(I) tetrahedral complex dimer supported by supramolecular interactions

Article abstract of DOI:10.1002/ejic.201101356

The first study of the self-association of a copper(I) tetrahedral complex ([Cu(N-{4-nitrophenyl}pyridine-2-yl-methanimine)(PPh₃)Br]) has been performed. The formation of a discrete dimer supported by supramolecular π-π stacking and C-H·Br inte

Full text of DOI:10.1002/ejic.201101356

SHORT COMMUNICATION
DOI: 10.1002/ejic.201101356
Spectroscopic Evidence in Solid and Solution of a Discrete Copper(I)
Tetrahedral Complex Dimer Supported by Supramolecular Interactions
Danilo H. Jara,[a] Luis Lemus,[a] Liliana Farías,[a] Eleonora Freire,[b,c][‡] Ricardo Baggio,[b]
and Juan Guerrero*[a]
Keywords: Copper / Self-assembly / Pi interactions / Supramolecular chemistry / NMR spectroscopy
The first study of the self-association of a copper(I) tetra-
hedral complex ([Cu(N-{4-nitrophenyl}pyridine-2-yl-meth-
animine)(PPh3)Br]) has been performed. The formation of a
discrete dimer supported by supramolecular π–π stacking
and C–H···Br interactions was established by X-ray diffrac-
tion techniques. 1D- and 2D NMR techniques were used for
the structural characterization of this compound in solution.
Unexpected 1H–1H NOE effects are coherent with the pres-
ence of a dimer in solution, whose structure replicates that
found in the crystal. Dimerization constants obtained by VT-
1H NMR spectroscopy allowed the determination of
the thermodynamic parameters for the self-association pro-
cess, namely ΔS
–2.00Ϯ0.05 kcalmol–1 and ΔG(298 K)
Consequently, this association process is enthalpy driven.
=
–0.67Ϯ0.21 calmol–1 K–1
–1.79 kcalmol–1
,
ΔH
=
.
=
the presence of discrete dimers was observed,[10] but to the
best of our knowledge, studies on the dimerization of cop-
per(I) complexes in the solid as well as in solution have not
been reported.
In this work, we report the synthesis and characterization
of a new CuI complex, [Cu{N-(4-nitrophenyl)pyridine-2-yl-
methanimine}(PPh3)Br], where PPh3 is triphenyl phos-
phane. For this complex, we have obtained experimental
evidence of the self-association both in solution and in the
solid phase by NMR spectroscopy and X-ray diffraction
techniques. The dimerization constant (KD) and thermo-
dynamic parameters for the self-association process were
calculated.
Introduction
Coordination CuI complexes with high π-delocalized li-
gands have attracted considerable attention because of their
interesting photophysical and electrochemical behavior[1] as
well as because of their stereochemical versatility in the
construction of supramolecular structures.[2] Furthermore,
the presence of noncovalent interactions in molecules with
extended, fused aromatic systems has also been investigated
in relation to several topics including crystal engineering,[3]
anti-HIV activity,[4] intercalation of metal complexes into
DNA,[5] and structural stabilization in supramolecular
chemistry.[6] The role of π–π stacking in aggregations of
transition-metal complexes with aromatic ligands has been
reported in several papers.[7] However, the simultaneous evi-
dence of a discrete dimer in the solid and in solution is
limited.[8] Complexes with RuII,[8] OsII,[8a] and PtII[9] met-
allic centers are among those that have been studied.
Results and Discussion
The complex was prepared by reaction of equimolar
amounts of CuBr, PPh3, and the N-(4-nitrophenyl)pyridine-
2-yl-methanimine ligand (NNЈ-NO2),[11] and it was ob-
tained as a dark purple solid that was crystallized by slow
ether diffusion into a dichloromethane solution of the com-
Holdt et al. reported a crystal rearrangement of the first
supramolecular column assembly of homoleptic copper(I)
complexes governed by π–π stacking interactions, in which
1
[a] Facultad de Química y Biología, Universidad de Santiago de
Chile,
plex. The H NMR spectrum of the complex is consistent
with the presence of NNЈ-NO2 and PPh3 coordinated to the
metal center. Unequivocal assignment of the proton signals
was carried out by the concerted use of 1D (1H NMR,
{1H}13C-NMR) and 2D (COSY, HSQC, HMBC) NMR
techniques (Table S1, Supporting Information).
The crystallographic structure shows the copper atom to
be dicoordinated by the NNЈ-NO2 ligand, while PPh3 and
Br complete the coordination sites of the copper(I) in a
pseudo tetrahedral arrangement (Figure 1a). Intramolecu-
lar bond lengths and angles do not depart significantly
Av. Libertador Bernardo O’Higgins 3363, Estación Central,
Casilla 40, correo 33, Santiago, Chile
Fax: +56-2-6812108
E-mail: juan.guerrero@usach.cl
[b] Comisión Nacional de Energía Atómica,
Avenida Gral. Paz 1499, 1650 San Martín, Buenos Aires,
Argentina
[c] Escuela de Ciencia y Tecnología, Universidad Nacional de San
Martín,
Buenos Aires, Argentina
[‡] Member of CONICET
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101356.
Eur. J. Inorg. Chem. 2012, 1579–1583
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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D. H. Jara, L. Lemus, L. Farías, E. Freire, R. Baggio, J. Guerrero
SHORT COMMUNICATION
from commonly accepted values.[12] The geometry around distances shorter than the Cu–P and Cu–Br distances
the copper atom corresponds to a distorted tetrahedron, (Table S3).
elongated along the common bisector of the small (because
Intermolecular interactions are also present, the most
of chelation) N–Cu–N angle and the sensibly larger P–Cu– noticeable are the phenyl/pyridine π–π contacts (Table S4),
Br angle, which presents, as expected, N–Cu coordination which define weakly linked centrosymmetric dimeric units.
In this dimer, the NNЈ-NO2 ligands of two different mole-
cules of the complex are placed in an anti-parallel displaced
conformation, with centroid–to-centroid distances of
3.738(3) Å between the phenyl and pyridyl rings (Figure 1b
and c).
The π–π interactions involving other N-phenylpyridin-2-
yl-methanimine ligands have been reported, as in the crystal
structures of tetracoordinate HgI,[13] HgII, ZnII,[14] and
CuI[15] complexes, whose centroid–to-centroid distances be-
tween the phenyl/pyridine rings of neighboring molecules
are 3.42, 3.674, 3.601, and 3.8 Å, respectively, similar to
that found by us.
Another intermolecular contact is a nonconventional C–
H···Br interaction (Table S5), which links the dimeric units
through a centrosymmetric pair of (C–H)···Brneighbor and
Br···(H–C)neighbor interactions, parallel to each other and
that define “strips” along the crystallographic b axis (Fig-
ure 1b).
The behavior of the complex in CDCl3 solution was
1
studied by H NMR spectroscopy, which shows a strong
dependence of the proton chemical shifts of the ligand with
concentration and temperature. Because only narrow sig-
nals for all the protons of the NNЈ-NO2 ligand are observed
over the temperature and concentration ranges studied, this
equilibrium has a very fast exchange rate relative to the
NMR timescale[16] (Figure 2).
1
Figure 2. H NMR spectra for [Cu(NNЈ-NO2)(PPh3)Br] in CDCl3
at 298 K as a function of concentration.
The nature of this process was investigated by UV/Vis
and NMR spectroscopy. The appearance of new bands was
not observed in the UV/Vis spectra when the concentration
of the compound was changed. The MLCT band shows a
good correlation between absorbance and concentration in
chloroform solutions (Figure S1), and λmax values were in-
dependent of the concentration in the measured range.
These results discard ligand dissociation processes; there-
fore, the shifting of the proton chemical shift in the NMR
spectra can be related to the self-association phenomena.
Figure 1. (a) ORTEP view of the [Cu(NNЈ-NO2)(PPh3)Br] com-
plex. Displacement ellipsoids are drawn at a 50% level. (b) Sche-
matic view of the complex, which shows the linkage of dimers
(thick dashed lines) by C–H···Br bonds (thin dashed lines) to form
chains along the b axis. (c) Scheme showing the way dimers are
formed in the solid by π–π interactions around an inversion center. Little changes can be expected in the HOMO and LUMO
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Discrete Copper(I) Tetrahedral Complex Dimer
energies because of noncovalent interactions involved upon
In this way, the correlation between the crystallographic
dimer formation; however, this fact was not observed in the structure and the NMR findings strongly supports the idea
UV/Vis spectra. that [Cu(NNЈ-NO2)(PPh3)Br] dimerizes in solution, in a
On the basis that this process corresponds to a dimeriza- conformation that resembles the dimeric unit in the crystal,
tion supported on π–π stacking interactions, small amounts where the interactions responsible for its self-association
of benzene were added to the complex solutions, which re- mainly result from π–π stacking between NNЈ-NO2 aro-
sulted in significant changes in the proton spectra. Notice- matic rings and (C–H)···Br interactions. Thus, the equation
ably, as the amount of benzene was increased, the resonance describing the self-association equilibrium is:
of all the protons of the NNЈ-NO2 ligand were shifted up-
field. Such behavior suggests an important noncovalent as-
sociation between benzene and the complex that would ex-
Dimerization constants (KD) in CDCl3 were determined
by the curve fitting method described by Horman and
Dreux.[17] Representative KD values at a given temperature
were obtained by averaging the calculated KD value for each
proton of the NNЈ-NO2 ligand[18] (Figure 3, Tables S1 and
S6).
plain the increased shielding experienced by the all protons
(Figure S2). Thus, the greatest changes in δHi and δH3,
which are close to the bromine atoms in the dimer, indicate
a decrease in the dimerization process by competition with
the benzene/complex aggregation process. However, there is
not enough evidence to discard the interaction of benzene
with the dimer complex.
Dimerization was also confirmed by the presence of
NOE correlations, unexpected for a monomolecular species,
in the NOESY spectra measured under different experimen-
tal conditions. Indeed, NOE effects were observed between
the H4 proton of the pyridine fragment and both the H2Ј
and H3Ј protons of the 4-nitrophenyl moiety of the vicinal
NNЈ-NO2 ligands (Figure S3).
Higher order aggregates were discarded on the basis of a
very good least-squares fit of the dimer model of Horman
1
and Dreux with the H NMR spectroscopic data and the
resulting van ’t Hoff plot.
An increase in the concentration of the complex (from 2
to 35 mm) leads to a progressive upfield shift of some pro-
tons of the NNЈ-NO2 ligand in the spectrum (H4, H5, H6,
and H3Ј), which can be accounted for by the shielding of
Figure 3. Concentration dependence of the proton chemical shifts
for NNЈ-NO2 in the complex at 298 K.
The KD values obtained were in the range 70.4 (220 K)
these protons by the aromatic rings of a second NNЈ-NO2 to 21.8 m–1 (298 K). These values are comparable to those
ligand coordinated to another complex molecule that forms obtained for the dimerization constants of octahedral ru-
the dimeric unit in solution, [Cu(NNЈ-NO2)(PPh3)Br]2. On thenium complexes, [Ru(bipy)2(eilatin)][PF6]2 and [Ru-
the contrary, the Hi, H3, and H2Ј protons of NNЈ-NO2 are (bipy)2(isoeilatin)][PF6]2, which are 260 m–1 and 34 m–1,
shifted downfield as the complex concentration increases respectively, in acetonitrile at 298 K, where the largely fused
(more pronounced for Hi), which could be congruent with aromatic rings allow the formation of dimers in solution
the deshielding caused upon metal–ligand coordination. through π–π stacking.[7a]
However, this is discarded by the sensitivity of the chemical
On the other hand, for aqueous solutions of square-
shift of these protons with complex concentration and planar platinum complexes [Pt(N-N)2(L-S,O)]+ (L-S,O = N-
rather points to a deshielding effect caused by their interac- acyl-NЈ,NЈ-dialkylthioureas and N-N = bipy, phen), KD val-
tion with the electronegative bromine of the other complex ues at 298 K have been reported that range from 1.8 to
in the dimer.
114 m–1, depending on the nature of the N-acyl group and
Consequently, we estimate then that the Br···H interac- the N-N ligand. In addition, a much larger equilibrium con-
tions are the consequence of π–π noncovalent associations stant was reported for the platinum complex [Pt(ter-
that lead toward dimerization of the complex by causing an py)(CH3)]Cl (terpy = 2,2Ј:6Ј,2ЈЈ-terpyridine), in the order of
additional stability of the dimer.
26,000 m–1 at 298 K, where instead of dimer formation,
These results show a good analogy between the confor- highly aggregated species were found to be formed through
mations of the dimeric aggregates in the solid and in solu- π–π stacking. In general, dimerization and self-aggregation
tion. In the crystal structure of the dimeric unit, the short trends are more favored in square-planar complexes than in
distances between the bromine atom of one monomer and octahedral complexes because of the reduced steric hin-
the Hi, H3, and H2Ј protons of the other follow the order drance in the apical positions, which allows better interli-
3.020 (Hi), 3.121 (H3), and 3.550 Å (H2Ј), which agrees with gand π–π stacking, cation–π ligand, and metal–metal inter-
the variation in the downfield chemical shift (Δδ) as the actions.[19]
concentration increases, namely Hi Ͼ H3 Ͼ H2Ј, whose de-
Although it is difficult to make a comparison among
shielding origin is due to the proximity of bromine.
complexes of a different nature, the KD value found for our
Eur. J. Inorg. Chem. 2012, 1579–1583
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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D. H. Jara, L. Lemus, L. Farías, E. Freire, R. Baggio, J. Guerrero
SHORT COMMUNICATION
H, and N were performed by using a Fisons element model EA-
1108.
complex is relatively high by considering its tetrahedral ge-
ometry and also that the NNЈ-NO2 ligand does not have an
aromatic surface large enough to allow strong π–π stacking;
furthermore, there should be a rotational movement of the
nitrophenyl moiety to allow the ligand to become flattened
and to be able to dimerize.
Synthesis of N-(4-Nitrophenyl)(pyridine-2-yl)methanimine (NNЈ-
NO2):[22] To a solution of 4-nitroaniline (3.04 g, 22.02 mmol) in
CH2Cl2 (150 mL) and molecular sieves (15 g, 0.4 nm) was added
the pyridine-2-carbaldehyde (2.82 g, 26.28 mmol). The mixture was
stirred at 60 °C for 35 h and at room temperature for 3 d and fil-
tered, and the solution was evaporated to dryness under vacuum.
The crude NNЈ-NO2 ligand was washed with diethyl ether to ex-
tract the excess pyridine-2-carbaldehyde to yield a yellow powder.
Yield: 51%. 1H NMR (CDCl3, 300 K): δ = 8.75 (d, JH6,5 = 4.57 Hz,
1 H, H6), 8.56 (s, 1 H, Hi), 8.30 (d, JH3Ј,2Ј = 8.78 Hz, 2 H, H3Ј),
8.20 (d, JH3,4 = 7.87 Hz, 1 H, H3), 7.86 (t, J = 7.55 Hz, 1 H, H4),
7.44 (dd, J = 5.37, 6.77 Hz, 1 H, H5), 7.31 (d, JH2Ј,3Ј = 8.78 Hz, 2
H, H2Ј) ppm. 13C NMR (CDCl3): δ = 163.33 (Ci), 156.86 (C1Ј),
150.01 (C6), 153.69 (C2), 145.98 (C4Ј), 136.84 (C4), 125.91 (C5),
122.43 (C3), 125.07 (C3Ј), 121.36 (C2Ј) ppm. UV/Vis (CH3Cl): λmax
(nm) = 301, 332.
From the dependence of the KD values on temperature,
the thermodynamic parameters associated with the
dimerization of [Cu(NNЈ-NO2)(PPh3)Br] were deter-
mined by using van ’t Hoff plots (Table S6 and
Figure S4), which correspond to ΔG = –1.79 kcalmol–1,
ΔS
=
–0.67Ϯ0.21 calmol–1 K–1,
and
ΔH
=
–2.00Ϯ0.05 kcalmol–1. The variation in such parameters is
consistent with a spontaneous process tending to dimeriza-
tion, which is enthalpically driven through the formation of
both π–π and C–H···Br interactions, despite the entropic
cost as the system becomes more ordered.
Synthesis of [Cu(NNЈ-NO2)(PPh3)Br]: To a solution of CuBr
(0.23 g, 1.60 mmol) in acetonitrile (10 mL) was added dropwise
PPh3 (0.42 g, 1.60 mmol) dissolved in acetonitrile (20 mL) to form
a white precipitate. The mixture was stirred for 30 min, and the
NNЈ-NO2 ligand (0.37 g, 1.63 mmol) in an acetonitrile/dichloro-
methane (1:1) mixture (40 mL) was then added dropwise. The mix-
ture was stirred for 2 h to form a dark purple solution. The solvent
volume was reduced, and the solid formed was washed with a mix-
ture of diethyl ether/acetonitrile (9:1). The diffusion of ethyl ether
vapor into a concentrated CH2Cl2 solution gave dark purple crys-
tals. Yield: 85%. C30H24BrCuN3O2P (632.96): calcd. C 56.93, H
3.82, N 6.64; found C 57.00, H 3.88, N 6.19. 1H NMR (CDCl3,
10 mm, 298 K): δ = 8.71 (d, JH6,5 = 4.34 Hz, 1 H, H6), 8.56 (s, 1
H, Hi), 8.09 (d, JH3Ј,2Ј = 8.90 Hz, 2 H, H3Ј), 7.89 (ddd, JH4,3 = 7.54,
JH4,5 = 8.15, JH4,6 = 1.34 Hz, 1 H, H4), 7.84 (d, JH3,4 = 7.54 Hz, 1
H, H3), 7.59 (d, J2Ј,3Ј = 8.90 Hz, 2 H, H2Ј), 7.50 (ddd, JH5,4 = 8.15,
JH5,6 = 4.34, JH5,3 = 1.05 Hz, 1 H, H5), 7.36 (6 H, Hα), 7.32 (3 H,
Hγ), 7.24 (6 H, Hβ) ppm. 13C NMR (CDCl3, 20 mm): δ = 158.71
(Ci), 153.63 (C1Ј), 150.41 (C6), 149.39 (C2), 146.84 (C4Ј), 137.54
(C4), 133.68 (Cα), 129.88 (Cγ), 126.67 (Cβ), 127.98 (C5), 127.98 (C3),
124.89 (C3Ј), 123.15 (C2Ј) ppm. 31P{1H} NMR (CDCl3): δ =
–1.30 ppm. UV/Vis (CH3Cl): λmax (nm) = 307, 457.
Conclusions
We carried out the first study of self-association for a
tetrahedral copper(I) complex in solution, where both π-
stacking and C–H···Br interactions jointly act in a locking
way to produce a discrete dimer, whose structure in solution
replicates that found in the solid state. The self-association
process is enthalpically driven through the formation of
both π–π and C–H···Br interactions.
The electronic effect of the p-phenyl substituent on the
self-association thermodynamics for a related copper(I)
complex series is under study in our group, with some inter-
esting results on the effect of intercomplex supramolecular
interactions on their properties.
Experimental Section
Material: Pyridine-2-carbaldehyde and 4-nitroaniline were pur-
chased from Merck (Germany). All reactions were carried out un-
der purified nitrogen (99.9%, AGA-Chile S.A.). The solvents were
synthesis grade and were used as received. CuBr was prepared as
described in the literature.[20]
Calculation of KD: The values of KD were determined by the
method of Horman and Dreux,[17] which relies on the gradual vari-
ation in the 1H NMR chemical shifts as a function of concentration
at constant temperature. This procedure involves an iterative KD,
by fitting the observed chemical shift (δobs) of each proton by using
the mol fraction of dimer (δi) present at each concentration, start-
ing from a reasonable guess of the association constant. The most
accurate value of KD is defined as that which yields the best linear
relationship between δobs and xi (Figure S5). Once KD is deter-
mined, the chemical shift of each proton for the monomer and
dimer can be obtained from the intercept and slope of the plot of
δobs vs. xi[9a] (Table S1).
Instrumentation: 1H, 13C{1H} NMR, 31P{1H} NMR, 1H–1H 2D-
COSY, 1H–1H 2D-NOESY, 1H–13C 2D-HSQC-ed, and 1H–13C 2D-
HMBC spectra and the dimerization studies by proton NMR spec-
troscopy were performed on a Bruker Avance 400 MHz spectrome-
ter (400.133 MHz for H, 100.16 MHz for 13C, and 160.984 MHz
1
for 31P) equipped with a 5-mm multinuclear broad-band dual probe
head incorporating a z-gradient coil. All the measurements were
carried out in CDCl3. Chemical shifts were calibrated with respect
to the solvent signal (δ =7.26 ppm for proton residual solvent and
77.2 ppm for 13C) and referenced to TMS. 31P{1H} spectra were
calibrated with respect to the external pattern H3PO4 10%. The X-
ray diffraction experiments were performed at room temperature
on an Oxford Diffraction Gemini CCD S Ultra diffractometer, with
graphite monochromatized Mo-Kα radiation (λ = 0.7107 Å). The
structure was solved by direct methods (SHELXS97[21] and refined
by least-squares methods on F2 SHELXL97[21]). The UV/Vis spec-
tra were recorded on a Shimadzu mini-UV 1240 spectrophotometer
in chloroform at room temperature. The concentration effect on
the spectra was evaluated in the range 2 to 10 mm. Analysis of C,
VT-NMR Dimerization Experiments: Stock solutions of different
complex concentrations (2, 5, 10, 15, 20, 25, 30, and 35 mm) were
prepared in flasks of 1 and 2 mL with CDCl3. The proton NMR
spectra were recorded each 10 K in the range 220 to 298 K for all
solutions. Each measurement was recorded after thermal equilib-
rium was established (3 min).
CCDC-836091 ([Cu(NNЈ-NO2)(PPh3)Br]) contains the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Eur. J. Inorg. Chem. 2012, 1579–1583

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Supporting Information (see footnote on the first page of this arti-
cle): Chemical shifts and self-association constants, crystallo-
graphic data, KD and thermodynamic data, plots of chemical shifts
as a function of dimer fraction, van ’t Hoff plot, proton and
NOESY NMR spectra, and UV/Vis spectra are presented.
Acknowledgments
We are grateful for support from DICYT-USACH, FONDECYT-
1101029, MECESUP-0007 for the NMR spectrometer, ANPCyT
(project PME 01113) for the purchase of a CCD diffractometer,
and the Spanish Research Council (CSIC) for providing us with a
free-of-charge license to the CSD system.
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[18] The KD values were obtained for H4, H5, H6, H3Ј and Hi, but
KD values for H3 and H2Ј were not calculated because they
showed a very small variation in chemical shift. Furthermore,
at some concentrations and temperatures, the chemical shift for
H3 could not accurately be determined because it appears as
overlapping with that of H4.
[19] G. Bellachioma, G. Ciancaleoni, C. Zuccaccia, D. Zuccaccia,
A. Macchioni, Coord. Chem. Rev. 2008, 252, 2224–2238.
[20] R. N. Keller, H. D. Wycoff, Inorg. Synth. 1946, 2, 1–4.
[21] SHELXL97, SHELXS97: G. M. Sheldrick, Acta Crystallogr.,
Sect. A 2008, 64, 112–122.
ˇ
[22] B. Z. Jovanovic´, M. Misˇic´-Vukovic´, A. D. Marinkovic´, V. Vajs,
[6] L. Lemus, J. Guerrero, J. Costamagna, G. Estiu, G. Ferraudi,
A. G. Lappin, A. Oliver, B. C. Noll, Inorg. Chem. 2010, 49,
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J. Mol. Struct. 2002, 642, 113–118.
Received: December 5, 2011
Published Online: March 1, 2012
Eur. J. Inorg. Chem. 2012, 1579–1583
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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