Conformational isomerism and weak molecular and magnetic interactions in ternary copper(II) complexes of [Cu(AA)L′]ClO4·nH2O, where AA = L-phenylalanine and L-histidine, L′ = 1,10-phenanthroline and 2,2-bipyridine, and n = 1 or 1.5: Synthesis, single-crystal X-ray structures, and magnetic resonance investigations

Article abstract of DOI:10.1021/ic010182d

Weak molecular and magnetic exchange interactions in ternary copper(II) complexes, viz., [Cu(L-phe)(phen)(H₂O)]ClO₄ (1), [Cu(L-phe)(bpy)(H₂O)]ClO₄ (2), and [Cu(L-his)(bpy)]ClO₄·1.5H₂O (3), where L-phe = L-phenylalanine, L-his = L-histidine, phen = 1,10-phenanthroline, and bpy = 2,2′-bipyridine, have been investigated. Single-crystal X-ray structures reveal that complex 2 crystallizes in a monoclinic space group P2₁, with unit cell parameters a = 7.422(7) A, b = 11.397(5) A, c = 12.610(2) A, β = 102.10(5)°, V = 1043.0(11) A³, Z = 2, R = 0.0574, and R_w = 0.1657. Complex 3 crystallizes in a monoclinic space group C2, with a = 18.834(6) A, b = 10.563(4) A, c = 11.039(3) A, β = 115.23(2)°, V = 1986.6(11) A³, Z = 4, R = 0.0466, and R_w = 0.1211. Molecules of 2, in the solid state, are self-assembled via weak intra- and intermolecular π-π stacking and H-bonding interactions. Molecules of 3 exhibit intermolecular dimeric association with the Cu···Cu separation being 3.811 A. X-ray structures and ¹H NMR studies reveal conformational isomerism in both solid and liquid states of complexes 1 and 2. The aromatic side chain of L-phe in 1 and 2 adopts either a folded (A) or an extended (B) conformation. Variable-temperature ¹H NMR and spin lattice relaxation measurements point out interconversion between conformations A and B at temperatures above 323 K. The change in molecular conformation induces a change in the electron density at the site of copper and band gap energy between HOMO and LUMO orbitals. Interestingly, in spite of paramagnetic nature, complexes 1 and 2 are amenable for both EPR and ¹H NMR spectroscopic studies. Single-crystal EPR spectra of 2 in three orthogonal planes are consistent with three-dimensional magnetic behavior. Intramolecular exchange dominates the dipolar interactions. The EPR spectra of 3 correspond to weak magnetic interactions between associated dimeric units. The structural and magnetic resonance investigations together reveal that the weak π-π stacking interactions are the electronic pathways for magnetic interactions in 1-3.

Full text of DOI:10.1021/ic010182d

Inorg. Chem. 2001, 40, 4291-4301
4291
Conformational Isomerism and Weak Molecular and Magnetic Interactions in Ternary
Copper(II) Complexes of [Cu(AA)L′]ClO4‚nH2O, Where AA ) L-Phenylalanine and
L-Histidine, L′ ) 1,10-Phenanthroline and 2,2-Bipyridine, and n ) 1 or 1.5: Synthesis,
Single-Crystal X-ray Structures, and Magnetic Resonance Investigations
P. S. Subramanian,† E. Suresh,† P. Dastidar,† S. Waghmode,‡ and D. Srinivas*,‡
Silicate and Catalysis Division, Central Salt & Marine Chemicals Research Institute,
Gijubhai Badheka Marg, Bhavnagar 364 002, India, and Catalysis Division,
National Chemical Laboratory, Pune 411 008, India
ReceiVed February 12, 2001
Weak molecular and magnetic exchange interactions in ternary copper(II) complexes, viz., [Cu(L-phe)(phen)-
(H2O)]ClO4 (1), [Cu(L-phe)(bpy)(H2O)]ClO4 (2), and [Cu(L-his)(bpy)]ClO4‚1.5H2O (3), where L-phe ) L-
phenylalanine, L-his ) L-histidine, phen ) 1,10-phenanthroline, and bpy ) 2,2′-bipyridine, have been investigated.
Single-crystal X-ray structures reveal that complex 2 crystallizes in a monoclinic space group P21, with unit cell
parameters a ) 7.422(7) Å, b ) 11.397(5) Å, c ) 12.610(2) Å, â ) 102.10(5)°, V ) 1043.0(11) Å3, Z ) 2, R
) 0.0574, and Rw ) 0.1657. Complex 3 crystallizes in a monoclinic space group C2, with a ) 18.834(6) Å, b
) 10.563(4) Å, c ) 11.039(3) Å, â ) 115.23(2)°, V ) 1986.6(11) Å3, Z ) 4, R ) 0.0466, and Rw ) 0.1211.
Molecules of 2, in the solid state, are self-assembled via weak intra- and intermolecular π-π stacking and H-bonding
interactions. Molecules of 3 exhibit intermolecular dimeric association with the Cu‚‚‚Cu separation being 3.811
1
Å. X-ray structures and H NMR studies reveal conformational isomerism in both solid and liquid states of
complexes 1 and 2. The aromatic side chain of L-phe in 1 and 2 adopts either a “folded” (A) or an “extended”
(B) conformation. Variable-temperature 1H NMR and spin lattice relaxation measurements point out interconversion
between conformations A and B at temperatures above 323 K. The change in molecular conformation induces a
change in the electron density at the site of copper and band gap energy between HOMO and LUMO orbitals.
1
Interestingly, in spite of paramagnetic nature, complexes 1 and 2 are amenable for both EPR and H NMR
spectroscopic studies. Single-crystal EPR spectra of 2 in three orthogonal planes are consistent with three-
dimensional magnetic behavior. Intramolecular exchange dominates the dipolar interactions. The EPR spectra of
3 correspond to weak magnetic interactions between associated dimeric units. The structural and magnetic resonance
investigations together reveal that the weak π-π stacking interactions are the electronic pathways for magnetic
interactions in 1-3.
Introduction
mediated via mono- and multiatomic bridged ligands.5-12
However, studies on complexes wherein the interactions are
Ternary complexes of the type M(AA)L′, where AA ) amino
acid, L′ ) some other ligand or a different amino acid, and M
) metal ion, have been receiving considerable interest due to
their biological relevance. Sigel et al.1 have reported that
electronic effects such as favorable interactions between π
systems, between hydrophobic systems, and between oppositely
charged side groups contribute to the stability of the ternary
structure. These weak interactions indeed play a crucial role in
self-organization of molecules, supramolecular chemistry, and
molecular recognition.2-4 There are many papers describing
magnetic interactions in structures of copper(II) complexes
primarily via weak intermolecular in nature are rather scarce.
In such systems EPR spectroscopy is an ideal, complementary
technique to study magnetic properties.13 With this point in view,
we report here synthesis and magnetic resonance (single-crystal
(5) Magneto-Structural Correlations in Exchange Coupled Systems;
Gatteschi, D., Khan, O., Willet, R. D., Eds.; NATO Advanced Study
Institute Series; Reidel Publishing Co.: Dordrecht, 1985. Khan, O.
Angew. Chem., Int. Ed. Engl. 1985, 24, 834.
(6) Bencini, A.; Gatteschi, D. EPR of Exchange Coupled Systems;
Springer-Verlag: Berlin, 1990; Chapter 10.
(7) Strothkamp, K. G.; Lippard, S. J. Acc. Chem. Res. 1982, 15, 318.
Doedens, R. J. Prog. Inorg. Chem. 1976, 21, 209.
(8) Subramanian, P. S.; Dave, P. C.; Boricha, V. P.; Srinivas, D.
Polyhedron 1998, 17, 443. Subramanian, P. S.; Srinivas, D. Polyhedron
1996, 15, 985.
(9) van Albada, G. A.; Quiroz-Castro, M. E.; Mutikainen, I.; Turpeinen,
U.; Reedjik, J. Inorg. Chim. Acta 2000, 298, 221.
(10) Papoutsakis, D.; Kirby, J. P.; Jackson, J. E.; Nocara, D. Chem.sEur.
J. 1999, 5, 1474.
* Corresponding author. E-mail: srinivas@cata.ncl.res.in. Fax: (91)-
20-5893761.
† Central Salt & Marine Chemicals Research Institute.
‡ National Chemical Laboratory.
(1) Sigel, H. Angew. Chem., Int. Ed. Engl. 1975, 14, 394. Sigel, H.;
Narmann, C. F. J. Am. Chem. Soc. 1976, 98, 730. Mitchell, P. R.;
Sigel, H. J. Am. Chem. Soc. 1978, 100, 1564.
(2) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304.
(3) Desiraju, G. R. Acc. Chem. Res. 1996, 29, 441. Stang, P. J.; Olenyuk,
B. Acc. Chem. Res. 1997, 30, 502.
(11) Balagopalakrishna, C.; Rejasekharan, M. V. Phys. ReV. B 1990, 42,
7794.
(12) Subramanian, P. S.; Suresh, E.; Srinivas, D. Inorg. Chem. 2000, 39,
2053.
(4) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
Frieden, E. J. Chem. Educ. 1975, 52, 754.
(13) Bencini, A.; Benelli, C.; Gatteschi, D.; Znachini, C. Proc.sIndian
Acad. Sci., Chem. Sci. 1987, 98, 13.
10.1021/ic010182d CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/10/2001

4292 Inorganic Chemistry, Vol. 40, No. 17, 2001
Subramanian et al.
EPR and 1H NMR) spectroscopic studies of [Cu(L-phe)(phen)-
(H2O)]ClO4 (1), [Cu(L-phe)(bpy)(H2O)]ClO4 (2), and [Cu(L-
his)(bpy)]ClO4‚1.5H2O (3) complexes. Single-crystal X-ray
structures of 2 and 3 are also reported. The L-phe ligand exhibits
conformational isomerism. The aromatic side chain of L-phe
adopts “folded” (A) and “extended” (B) conformations. Single-
crystal X-ray structure reveals that complex 2 in the present
study crystallizes with the former (A) conformation. The crystal
structure of 2 with the latter (B) conformation was reported
earlier by Sugimori et al.14 Such a conformational isomerism
was also observed by Marcelli et al.15 in the structure of bis-
(L-phenylalaninamidato)copper(II) complex.
mmol) was used in place of the L-phe ligand. Good quality, needle-
shaped, blue crystals were obtained by slow evaporation of the reaction
mixture. Anal. Calcd for CuC16N5H19ClO7.5: C, 38.40; H, 3.80; N,
14.00. Found: C, 38.37; H, 3.80; N, 14.15. FT-IR data (cm-1): ν(O-
H/C-H/N-H) 3450-3026; ν(COO-) 1620, 1253; ν(CdN, CdC)
1576, 1525; ν(ClO4-) 1142, 1120, 1109, 1092, 1034.
CAUTION! Perchlorate salts of metal complexes are potentially
explosive and should be handled with great care.
Physical Measurements. Microanalysis of the complexes was done
by using a Perkin-Elmer PE 2400 series II CHNS/O elemental analyzer.
FT-IR spectra for KBr pellets (1% w/w) were recorded on a Shimadzu
FT-COM 1 spectrophotometer. Electronic spectra were recorded in
several solvents using Shimadzu (UV-160A) and HP diode array (model
8452 A) UV-visible spectrometers. EPR investigations were carried
out on a Bruker EMX X-band spectrometer operating at a field
modulation of 100 kHz, modulation amplitude of 2 G, and microwave
radiation power of 2 mW. Measurements at 77 K were done using a
quartz finger Dewar. Only complex 2 yielded suitable quality single
crystals for EPR measurements. Single-crystal EPR measurements were
performed in three orthogonal planes, at 10° intervals, and spectral
simulations were done by using the Bruker Simfonia software package.
1H NMR spectra of the complexes in D2O were recorded on a Bruker
200 MHz DPX Avance FT-NMR spectrometer. TMS was used as an
internal standard. Prior to NMR measurements the dissolved oxygen
was removed by purging the sample solutions with zero-grade nitrogen
Copper(II) complexes are usually not amenable for 1H NMR
study due to paramagnetism and unfavorable relaxation times.
Interestingly, complexes 1 and 2 could be studied by both EPR
1
and H NMR studies. Single-crystal EPR spectra of 2 showed
an exchange-narrowed signal. Comparative structural and EPR
studies indicate that the weak π-π stacking and H-bonding
interactions are the pathways for the intermolecular magnetic
1
exchange interactions. Variable-temperature H NMR studies
1
in D2O solutions and H spin lattice relaxation measurements
revealed conformational dynamics of the aromatic side chain
of the L-phe ligand at higher temperatures. Molecular modeling
studies on 2 complemented the experimental results. The
noteworthy results of the present study are (i) conformational
dynamics of the coordinated L-phe ligand, (ii) feasibility of the
paramagnetic copper(II) complexes for both EPR and NMR
studies, and (iii) magnetism primarily via weak intermolecular
interactions (π-π stacking) since the closest Cu-Cu distance
is 3.81 Å in complex 3. In fact these properties are responsible
for the highly selective and cooperative activity of metalloen-
zymes in redox transformations. The consequences of confor-
mational changes on the spectra and electronic properties are
investigated.
1
gas. H spin lattice relaxation time (T1) measurements were done by
using the Bruker XWIN NMR software package and the inversion
recovery technique consisting of the following train of pulse
sequences:17
(180° - τ - 90° - Aq - D)n
Here Aq is the acquisition time, D is the delay to allow equilibrium to
be reached and 8.9 µs is the calibrated 90° pulse. The values of
magnetization, directly proportional to NMR signal intensity, vary from
-Mz(0) (when τ ) 0) to Mz(∞) (when τ ) 5T1); τ is the variable time
delay between the two pulses. The magnetization relates to T1 by the
following equation:
Experimental Section
Mz(τ) ) Mz(∞)[1 - 2 exp(-τ/T1)]
(1)
Materials. CuSO4‚5H2O, L-phe, L-his, phen, bpy, and NaClO4 were
obtained from Aldrich Co. The solvents were of A.R. grade and purified
further by standard procedures.16
T1 can be calculated by a least-squares analysis of the experimental
magnetization data as a function of τ.
Synthesis of [Cu(L-phe)(phen)(H2O)]ClO4 (1). A mixture of L-phe
(0.826 g, 5 mmol) and NaOH (0.2 g, 5 mmol) dissolved in 10 mL of
distilled water was added to an aqueous solution (25 mL) of CuSO4‚
5H2O (1.248 g, 5 mmol) with stirring for 30 min. Then phen (0.941 g,
5 mmol) dissolved in 10 mL of ethanol was added. The solution was
stirred for another 2 h with heating at 333 K. At the end of the reaction,
NaClO4 (0.7 g) dissolved in distilled water was added. Slow evaporation
of the reaction mixture at 298 K yielded dark blue, needle-shaped
crystals in 8-10 days. Anal. Calcd for CuC21N3H20ClO7: C, 48.00; H,
3.81; N, 8.10. Found: C, 48.81; H, 3.91; N, 7.68. FT-IR data (cm-1):
ν(O-H/C-H/N-H) 3452, 3333, 3028; ν(COO-) 1622, 1238; ν(Cd
N, CdC) 1589, 1519; ν(ClO4-) 1154, 1121, 1109, 1088, 1033.
Synthesis of [Cu(L-phe)(bpy)(H2O)]ClO4 (2). Complex 2 was
prepared as described above except that bpy (0.821 g, 5 mmol) was
used in place of the phen ligand. Slow evaporation of the reaction
mixture yielded good quality dark blue, platelike crystals in about a
week. Anal. Calcd for CuC19N3H20ClO7: C, 45.51; H, 3.99; N, 8.38.
Found: C, 46.72; H, 3.69; N, 8.60. FT-IR data (cm-1): ν(O-H/C-
H/N-H) 3450, 3113, 2891; ν(COO-) 1651, 1267; ν(CdN, CdC) 1605,
1525; ν(ClO4-) 1144, 1121, 1109, 1088, 1033.
X-ray Crystallographic Analysis. Single-crystal X-ray data of
complexes 2 and 3 were collected on an Enraf-Nonius CAD-4
diffractometer with graphite-monochromatized Mo KR radiation (λ )
0.7107 Å). Unit cell parameters and orientation matrices of 2 and 3
were obtained from least squares refinements using 25 reflections within
the θ range 8-12°. The ω-2θ scan mode with maximum θ value being
24.93° was used to collect the intensity data for both of the complexes.
Preliminary data, cell refinement, and intensity data collection were
carried out using the program CAD4-PC.18 The data reduction was done
using NRCVAX.19 The data were corrected for Lorentz and polarization
effects but not for absorption. Structure solution was carried out using
the Patterson method with full-matrix least squares refinement on F2
using SHELXL-97.20 Scattering factors and anomalous dispersion were
taken from the International Tables for X-ray Crystallography.21
In the case of complex 3, the ClO4- ion was found disordered. After
complete convergence of all of the non-hydrogen atoms of the complex
cation, perchlorate ion was located from the difference Fourier map.
The peak height of Cl was ∼16 e Å-3, and eight peaks attached to the
(17) Beckman, E. D. High-Resolution NMR; Academic Press: New York,
1969.
(18) CAD-4 PC Software, Version 5; Enraf-Nonius: Delft, The Netherlands,
1989.
Synthesis of [Cu(L-his)(bpy)]ClO4‚1.5H2O (3). Complex 3 was
prepared in a manner similar to that of 2 except that L-his (0.771 g, 5
(14) Sugimori, T.; Masuda, H.; Ohata, N.; Koiwai, K.; Odani, A.;
Yamauchi, O. Inorg. Chem. 1997, 36, 576.
(15) Marchelli, R.; Dossena, A.; Casnati, G.; Fava, G. G.; Ferrari, M. J.
Chem. Soc., Chem. Commun. 1985, 1672.
(16) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: Oxford, 1980.
(19) Gabe, E. I.; Le Page, Y.; Charland, I. P.; Lee, F. L.; White, P. S. J.
Appl. Crystallogr. 1989, 22, 384.
(20) Sheldrick, G. M. SHELXL-97, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(21) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, 1974; Vol. IV.

Ternary Copper(II) Complexes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4293
Table 1. Summary of Crystallographic Data for
[Cu(L-phe)(bpy)(H2O)]ClO4 (2) and [Cu(L-his)(bpy)]ClO4‚1.5H2O
(3)
complex 2
complex 3
chem formula
fw
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
C19H20N3O7ClCu
500.36
7.422(7)
11.397(5)
12.610(2)
90.00
102.10(5)
90.00
1043.0(11)
2
C16H19N5O7.5ClCu
500.35
18.834(6)
10.563(4)
11.039(3)
90.00
115.23(2)
90.00
1986.6(11)
4
cryst syst
space group
T (K)
monoclinic
P21 (No. 4)
296
0.7107
1.593
8.98
0.0574
0.1657
monoclinic
C2 (No. 5)
296
0.7107
1.673
12.88
0.0466
0.1211
λ (Å)
F
calcd (g cm-3
)
µ (cm-1
)
R(Fo )a
2
2
Rw (Fo )a
a R ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) [∑w(Fo2 - Fc2)2]/∑[w(Fo )2]1/2; w
2
) 1/σ(Fo)2.
Cl atom with peak heights ranging from 1.2 to 3.8 e Å-3 corresponded
to disordered oxygen atoms; the Cl atom of the disordered perchlorate
anion was refined anisotropically; the occupancy factor for the attached
disordered oxygen atoms was assigned depending on the peak heights.
Inclusion of all of these eight atoms, corresponding to two orientations
of the ClO4- ion, improved significantly the refinement parameters (R,
Rw, and goodness of fit) implying the right assignment of anion disorder.
Hydrogen atoms were either located from the difference Fourier map
or kept fixed using the riding model, except for water hydrogens in
complex 2, which were not available from the difference Fourier map.
In the case of complex 3 all of the hydrogen atoms were kept fixed
during refinement. Crystallographic data and refinements for both of
the complexes are presented in Table 1.
Molecular Modeling and DFT Calculations: Methodology.
Different molecular conformations of complex 2 were generated by
systematically varying the dihedral angles C11-C12-C13-C14 and
C12-C13-C14-C15 between 0° and 360°, in steps of 20°. The single-
point strain energies of these clusters were calculated using the
MMFF94 basis set. MO energies and electron density distributions were
evaluated by the DFT method using the STO DN** basis set. All of
the computations were carried out on a Silicon Graphics O2 workstation
using the SPARTAN and MSI software packages.
Figure 1. ORTEP views of (a) [Cu(L-phe)(bpy)(H2O)]ClO4 (2) and
(b) [Cu(L-his)(bpy)]ClO4‚H2O (3).
In related mixed ligand copper complexes the aromatic side
chain of L-phe exhibited different orientations with respect to
the metal coordination plane.14,23 The aromatic side chain adopts
an “extended” conformation and the phenyl group orients
perpendicular to the coordination square plane in structures [Cu-
(L-phe)(phen)Cl]‚3H2O14 and [Cu(L-o-Me-phe)(phen)(H2O)]-
ClO4.23 On the contrary, the side chain takes up a “folded”
geometry with the phenyl group orienting parallel to the
coordination square plane in structures [Cu(L-NH2Phe)(bpy)]-
NO3‚H2O and [Cu(L-Tyr)(phen)(H2O)] ClO4‚1.5H2O.14 L-Phe
in complex 2 adopts the “folded” conformation. The phenyl ring
is almost parallel to the bpy moiety (Figure 1a) with a tilt angle
of 6.82(5)°. Complex 2 exhibits both intra- and intermolecular
π-π stacking interactions between the phenyl and bpy rings
(Figure 2). The intramolecular stacking distance between
centroids C3G of the phenyl ring (C14 to C19) and C2G of
bpy (N2C6 to C10) is 3.84(1) Å. The distances between centroid
C3G and the nearest bpy atoms N2 and C6 are 3.43(4) and
3.54(2) Å, respectively. A significant deviation of the CR carbon
atom (C12) from the coordination plane toward the phenyl ring
Results
X-ray Structure, Conformational Geometry, and Molec-
ular Association: [Cu(L-phe)(bpy)(H2O)]ClO4 (2). An ORTEP22
view of complex cation 2 along with the atom-numbering
scheme is shown in Figure 1a. Selected bond distances and
angles are listed in Table 2. Copper possesses a distorted square
pyramidal geometry, and L-phe coordinates through amino
nitrogen and deprotonated carboxylato oxygen atoms. The
ligands bpy (Cu1-N1 ) 1.987(9) Å, Cu1-N2 ) 2.002(8) Å)
and L-phe (Cu1-N3 ) 2.028(8) Å, Cu-O1 ) 1.946(7) Å) form
the square base, while the water molecule makes a long axial
coordination (Cu1-O1W ) 2.298(7) Å). The square base
formed by the coordinating atoms is almost planar with a
maximum deviation of 0.053 Å, and the Cu1 atom is out of the
best plane of N1N2N3O1 by 0.220 Å toward the axial water
molecule. The dihedral angle between the planes CuN1N2 and
CuN3O1 is 17.40(5)°.
(22) Johnson, C. K. ORTEP. Report ORNL-3794; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
(23) Mizutani, S.; Toviosue, H.; Kiuosluta, K.; Jitsukana, K.; Masuda, H.;
Einega, H. Bull. Chem. Soc. Jpn. 1999, 72, 981.

4294 Inorganic Chemistry, Vol. 40, No. 17, 2001
Subramanian et al.
The complex makes N-H‚‚‚O and C-H‚‚‚O contacts be-
tween the screw-related molecules. Both of the amino hydrogens
(H1N3 and H2N3) of L-phe are involved in intermolecular
N-H‚‚‚O hydrogen bonding with perchlorate oxygen O6 and
uncoordinated carboxylato oxygen O2, respectively. The ligand
bpy makes C-H‚‚‚O interactions with the perchlorate oxygen
O5 and the uncoordinated carboxylato oxygen O2. These
H-bonding interactions run along the b-axis in layers between
the screw-related molecules. The various H-bonding interactions
are as follows:
Figure 2. Pluton diagram showing the intra- and intermolecular π-π
stacking interactions of aromatic groups in [Cu(L-phe)(bpy)(H2O)]ClO4
(2). Perchlorate anions are omitted for clarity.
D-H‚‚‚A*
D-H (Å) H‚‚‚A (Å) D‚‚‚A (Å)
D-H‚‚‚A (deg)
N3-H1‚‚‚O61
N3-H2‚‚‚O22
C7-H7‚‚‚O53
0.90(1)
0.90(1)
0.93(1)
2.48(1)
2.25(1)
2.426(1)
2.317(1)
3.14(1)
3.03(1)
3.321(3)
3.227(3)
131.2(2)
145.0(3)
161.3(7)
166.0(1)
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of
Cu(L-phe)(bpy)(H2O)]ClO4 (2) and [Cu(L-his)(bpy)]ClO4‚1.5H2O (3)
C10-H10‚‚‚O22 0.93(1)
[Cu(L-phe)(bpy)(H2O)]ClO4 2
*Symmetry code (superscript): 1 ) x, y, -1 + z; 2 ) 1 - x, 1/2 +
y, -z; 3 ) x, y, z.
Cu1-O1
Cu1-N2
Cu-O1W
1.946(7)
2.002(8)
2.298(7)
Cu1-N1
Cu1-N3
1.987(9)
2.028(8)
The C-H‚‚‚O interactions are much stronger than the N-H‚‚‚O
interactions. Even though it was not possible to locate the H
atoms of the coordinated water molecule, O1W is making good
short contacts, along the b-axis, with the carboxylato (O1W‚‚
‚O2 ) 2.83(1) Å) and perchlorate oxygen atoms (O1W‚‚‚O6
) 2.88(4) Å).
The nearest intermolecular Cu‚‚‚Cu distance is 7.14(3) Å,
and its direction is along the diagonal of the ac-plane. The same
Cu center is separated by 7.35(2) and 7.42(2) Å, respectively,
from the 21 screw related molecules, and these Cu‚‚‚Cu
directions are aligned along the a-axis in the solid state packing.
The weak intermolecular stacking and H-bonding interactions
are the causes for three-dimensional magnetic behavior of 2 in
the solid state (vide infra).
O1-Cu1-N1
N1-Cu1-N2
N1-Cu1-N3
O1-Cu1-O1W
N2-Cu1-O1W
C1-N1-Cu1
C6-N2-Cu1
C11-O1-Cu1
92.3(3)
80.8(3)
169.2(4)
90.5
105.5(3)
124.3(8)
114.9(7)
114.6(6)
O1-Cu1-N2
O1-Cu1-N3
N2-Cu1-N3
N1-Cu1-O1W
N3-Cu1-O1W
C5-N1-Cu1
C10-N2-Cu1
C12-N3-Cu1
163.3(3)
81.5(2)
102.7(4)
97.8(3)
91.1(3)
114.6(7)
127.5(7)
105.9(6)
[Cu(L-his)(bpy)]ClO4‚1.5H2O (3)
Cu1-O1
Cu1-N2
Cu1-N4
1.995(5)
2.007(6)
2.114(6)
Cu1-N1
Cu1-N3
2.028(6)
1.962(7)
N3-Cu1-O1
O1-Cu1-N2
O1-Cu1-N1
N3-Cu1-N4
N2-Cu1-N4
C1-N1-Cu1
C5-N1-Cu1
C14-N4-Cu1
C6-N2-Cu1
83.0(2)
93.4(2)
153.2(2)
88.5(3)
N3-Cu1-N2
N3-Cu1-N1
N2-Cu1-N1
O1-Cu1-N4
N1-Cu1-N4
C12-N3-Cu1
C16-N4-Cu1
C10-N2-Cu1
C11-O1-Cu1
174.5(3)
100.9(3)
80.5(2)
[Cu(L-his)(bpy)]ClO4‚1.5H2O (3). The perspective view of
complex cation 3 with the atom-numbering scheme is shown
in Figure 1b. The selected bond distances and angles are listed
in Table 2. As in 2, copper(II) ion in complex 3 also possesses
a distorted square pyramidal geometry. The basal plane is
defined by the coordinating nitrogen atoms of bpy ligand (Cu1-
N1 ) 2.028(6) Å and Cu1-N2 ) 2.007(6) Å) and R-amino
nitrogen (N3) and R-carboxylato oxygen (O1) of L-his ligand
(Cu1-N3 ) 1.962(7) Å, Cu1-O1 ) 1.995(5) Å). The fifth
coordination is provided by imidazole ring nitrogen N4 of L-his.
The Cu1-N4 bond is comparatively longer than the other Cu-N
bonds (Table 2). The square base formed by coordinating N1,
N2, N3, and O1 atoms shows a maximum deviation of 0.192
Å from planarity and a tetrahedral bias (Table 2). Copper is
out of the best plane by 0.271 Å toward the axially coordinated
imidazole nitrogen N4 of L-his.
95.7(3)
96.0(3)
110.8(3)
104.2(4)
131.5(6)
124.9(6)
111.0(5)
126.5(5)
115.3(5)
122.7(5)
115.0(5)
(0.85 Å) together with a low angle between the mean planes
N1N2N3O1 and C14 to C19 (17.02(4)°) perhaps makes the
“folded” geometry possible. Cu(II) makes close contacts with
aromatic carbon atoms C14 and C19 of the L-phe ligand
(Cu‚‚‚C14 ) 3.19(6) Å and Cu‚‚‚C19 ) 3.34(8) Å). Complex
2 also exhibits intermolecular stacking interactions through the
phenyl ring of L-phe of one molecule and bpy of the neighboring
molecule (Figure 2). The distance between centroids C3G and
C2GA is 4.03(6) Å. Here, C2GA is the centroid of the bpy
ring of the adjacent molecule, containing atoms N2A and C6A
to C10A. The closest distance between carbon atom C7A, which
is part of the staggered bpy ring of the adjacent molecule, and
C3G is 3.47(6) Å.
Crystal packing reveals that cations of adjacent molecules
are arranged in dimeric association (Figure 3). These pairs are
held together by π-π stacking interactions between the pyridyl
rings of coordinated bpy ligands. The intermolecular Cu1‚‚‚
Cu1A separation in the associated pair is 3.811 Å, and the
stacking distance is 3.66(4) Å. Pyridyl rings of the stacked pairs
are almost planar, with a slight twist of 2.83°. The strong π-π
stacking interaction of bpy ligands in the solid state is the driving
force for closer approach of metal centers.
Earlier Sugumorai et al.14 reported the X-ray structure of
another crystalline form of complex 2. In their structure14 the
asymmetric unit contained two identical molecules and the
phenyl ring of L-phe had an “extended” conformation, different
from that observed in the present structure. The molecule
exhibited intermolecular stacking interactions between the bpy
units of adjacent molecules, but no intramolecular interactions
were noticed. A combination of coordination and weak intra/
intermolecular π-π stacking interactions are responsible for
the supramolecular architecture of complex 2 in the present
structure, in the solid state.
Molecule 3 is involved in extensive hydrogen-bonding (N-
H‚‚‚O, O-H‚‚‚O, C-H‚‚‚O, O-H‚‚‚N) interactions involving
imidazole nitrogen, carboxylato oxygens, distorted perchlorato
oxygens, lattice water molecules, and bpy, which in turn stabilize
the molecule in the crystal lattice. The lattice water molecule
is far away from the metal center with Cu1‚‚‚O1W ) 5.35 Å.

Ternary Copper(II) Complexes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4295
Figure 3. Intermolecular dimeric association π-π stacking interactions
between bpy ligands in [Cu(L-his)(bpy)]ClO4‚1.5 H2O (3). The centroids
of bpy rings C1G, C2G, C1GA and C2GA are indicated.
The various H-bonding interactions with symmetry code are as
follows:
D-H‚‚‚A*
D-H (Å) H‚‚‚A (Å) D‚‚‚A (Å) D-H‚‚‚A (deg)
N5-H5N‚‚‚O1W1 0.68(1)
2.321(1) 2.938(2)
1.722(2) 2.781(2)
1.086(3) 2.006(2) 3.018(3)
152.10(2)
160.86(3)
153.8(2)
O2W-H1O2‚‚‚O22 1.10(2)
N3-H13N‚‚‚O33
Figure 4. X-band EPR spectra at 298 K of polycrystalline samples:
[Cu(L-phe)(phen)(H2O)]ClO4 (1), (b) [Cu(L-phe)(bpy)(H2O)]ClO4 (2),
and (c) [Cu(L-his)(bpy)]ClO4‚1.5H2O (3). (d) The same as for spectrum
c except at higher spectrometer gain. Arrows indicate the signals
corresponding to dimeric molecular association in 3.
O2W-H2O2‚‚‚N34 1.114(3) 2.437(2) 3.5057(3)
159.99(2)
144.98(4)
170.25(2)
159.23(2)
170.88(2)
161.8(3)
N3-H2N3‚‚‚O15
C1-H1‚‚‚O56
0.938(1) 2.219(2) 3.3032(3)
1.003(1) 2.477(3) 3.469(3)
0.930(1) 2.493(4) 3.379(2)
0.930(2) 2.549(2) 3.470(2)
0.930(3) 2.517(2) 3.413(3)
0.930(2) 2.550(2) 3.377(3)
0.970(2) 2.556(2) 3.324(3)
C3-H3‚‚‚O2W7
C4-H4‚‚‚O28
C7-H7‚‚‚O28
4c). Additional, weak signals appeared at high spectrometer gain
(Figure 4c,d) as shoulders (on either side of the strong isotropic
signal) at 2790 and 3515 G and as a well-separated signal at
1625 G. These signals are consistent with weak dimeric
intermolecular association in the solid state.
EPR spectra for the single crystals of 2 were recorded by
rotating the crystals in three orthogonal planes. Although there
are two molecules in a unit cell, these are magnetically
equivalent and showed a single resonance at all the crystal
orientations. The g parameters at different orientations were
fitted to the following g2 tensor expression:
C15-H15‚‚‚O69
C13-H131‚‚‚O33
148.34(2)
136.18(3)
*Symmetry code (superscript): 1 ) 1/2 + x, -1/2 + y, z; 2 ) 1/2 -
x, 1/2 + y, z; 3 ) 1/2 - x, -1/2 + y, 1 - z; 4 ) 1/2 + x, 1/2 + y, z; 5 )
-x, y, -z; 6 ) -1/2 + x, -1/2 + y, z; 7 ) -1/2 + x, 1/2 + y, z; 8 ) x,
1 + y, z; 9 ) x, -1 + y, z.
Electronic Spectra. Complexes 1-3 were soluble in several
solvents and showed moderately intense transitions of ligand
origin in the range 260-318 nm and a broad asymmetric band
corresponding to d-d transitions in the range 580-690 nm.
The d-d band for 1 and 2 undergoes a red shift in different
solvents in the order THF > pyridine > DMF g H2O g DMSO
g CH3OH > (CH3)2CO > CH3CN, attributable to solvent
coordination or solvolysis. This band for 3 is invariant in CH3-
CN, (CH3)2CO, DMF, DMSO, and pyridine solvents, but it
exhibits a red shift in CH3OH and THF from 640 to 696 nm.
The spectral data (Table 3) are consistent with an octahedral
geometry for 3 in CH3OH and THF.24 The geometry of 3, in
the rest of the solvents, is square pyramidal, as observed in the
solid state. Complexes 1 and 2, on the other hand, are square
planar in CH3CN and (CH3)2CO, square pyramidal in CH3OH,
H2O, DMF, DMSO, and pyridine, and octahedral in THF.
Complexes 1 and 2 showed an additional weak shoulder around
337 nm attributable to a charge transfer transition from the
HOMO(bpy/phen) to LUMO(L-phe) π-orbitals.
g2(i) ) Ri + âi cos 2θ + γi sin 2θ
(2)
where g2(i) is the g value in the ith crystal rotation and θ is the
azimuthal angle of the magnetic field. The angular variations
of g in all three planes are shown in Figure 5. The g anisotropy
is larger in plane III than in planes I and II. The components of
the g tensor were estimated as described earlier.12 The g tensor
has rhombic symmetry with gx ) 2.059, gy ) 2.056, and gz )
2.235. Figure 6 shows the variation of line width (∆Hpp) as a
function of crystal orientation in all three orthogonal planes. A
maximum ∆Hpp of 51.6 G and a minimum of 16.4 G was
observed in plane III. Variation of ∆Hpp is smaller in planes I
and II. The line shape is more toward Gaussian with the
Lorentzian/Gaussian ratio being 0.09. In general, in pure
paramagnetic solids, the line width has contributions from both
dipolar and exchange interactions. The former broadens while
the latter narrows the EPR signal. In three-dimensional magnetic
systems the latter interaction dominates and the signals are
exchange narrowed. However, in low-dimensional systems the
exchange effects are less effective and the line widths are
between the exchange-narrowed and dipolar broadened limits.
For a Cu‚‚‚Cu separation of 7-7.5 Å, one should observe a
EPR Spectra. Polycrystals of 1 and 2 showed similar EPR
spectra, at 298 and 77 K, characteristic of axial g tensors (Figure
4a,b). The copper hyperfine features could not be resolved due
to intermolecular interactions in the solid state. Complex 3, on
the other hand, showed an isotropic signal at gav ) 2.122 (Figure
(24) Hathaway, B. J. In ComprehensiVe Coordination Chemistry; Wilkinson,
G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon: Oxford, 1987;
Vol. 5, Chapter 53, p 558.

4296 Inorganic Chemistry, Vol. 40, No. 17, 2001
Subramanian et al.
Table 3. UV-Vis Data for [Cu(L-phe)(phen)(H2O)]ClO4 (1), [Cu(L-phe)(bpy)(H2O)]ClO4 (2), and [Cu(L-his)(bpy)]ClO4‚1.5H2O (3) in
Different Solvents
λ (ꢀ); nm (M-1 cm-1
)
solvent
1
2
3
CH3CN
(CH3)2CO
CH3OH
H2O
DMSO
DMF
299 (3764), 599 (46)
268 (2543), 306 (2850), 582 (112)
330 (1305), 590 (81)
268 (2724), 304 (3004), 610 (62)
260 (2316), 304 (2497), 618 (60)
312 (2641), 618 (75)
278 (2281), 306 (2887), 620 (69)
318 (1539), 640 (88)
312 (2082), 686 (71)
266 (2605), 644 (67)
328 (802), 642 (90)
266 (2670), 304 (2933), 316 (2967), 694 (102)
260 (2873), 308 (3117), 672 (83)
306 (3132), 320 (3179), 640 (112)
316 (2563), 646 (95)
299 (3653), 612 (24)
262 (1005), 304 (1568), 620 (8)
304 (2103), 612 (22)
302 (1791), 610 (22)
305 (2058), 639 (83)
pyridine
THF
316 (1621), 334 (1149), 636 (99)
318 (2476), 696 (63)
Figure 6. Angular variation of peak to peak line width (∆Hpp, in G)
in three orthogonal planes I, II, and III for [Cu(L-phe)(bpy)(H2O)]ClO4
(2).
Figure 5. Angular variation of g in three orthogonal planes I, II, and
III for [Cu(L-phe)(bpy)(H2O)]ClO4 (2).
solutions of 2 and 3 showed seven superhyperfine signals on
the high-field hyperfine resonance. These features arise due to
1
dipolar contribution of about 320 G. A low value of ∆Hpp
(16.4-51.6 G) for 2 suggests that exchange is the more
dominating interaction.
interaction of the electron spin (S ) /2) with the nuclear spin
of three equivalent nitrogen nuclei (I ) 1) of the coordinating
bpy and L-phe ligands. The superhyperfine structure is partly
resolved in CH3OH and pyridine solvents; CH3CN solutions
did not show these features. The isotropic spin Hamiltonian
parameters (giso, Aiso(Cu), and Aiso(N)) obtained after spectral
simulations are listed in Table 4. These parameters are sensitive
to the solvent.
In frozen solutions of 2 three of the four parallel hyperfine
features were well separated while the fourth one overlapped
with the perpendicular features (Figure 7). The superhyperfine
Complexes 2 and 3 in CH3CN, at 298 K, showed four
resolved, equally spaced, copper hyperfine features typical of
non-interacting monomeric Cu(II) species. The resonances
exhibited mI dependent line widths. The low-field hyperfine
resonance was broader (46.7 G) than the high-field resonance
(41.3 G), and the intensity of the hyperfine signals increased
gradually in increasing field direction. In strongly coordinating
solvents this difference in intensity is more visible. DMF

Ternary Copper(II) Complexes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4297
Table 4. EPR Spin Hamiltonian Parameters of [Cu(L-phe)(phen)(H2O)]ClO4 (1), [Cu(L-phe)(bpy)(H2O)]ClO4 (2), and
[Cu(L-his)(bpy)]ClO4‚1.5H2O (3)
complex
statea
temp/K
giso
Aiso/G
AisoN/G
g
g
A /G
||
A /G
A
N/G
||
||
1
2
pc
pc
CH3CN
298/77
298/77
298
77
298
77
298
77
298
77
298/77
298
77
298
77
298
77
298
77
2.238
2.236
2.056
2.056
NR
NR
NR
NR
2.115
2.122
2.120
2.126
78.8
80.0
73.2
73.2
NR
2.235
2.237
2.234
2.058
2.053
2.050
184.9
183.2
184.0
25.7
22.0
17.8
12.9
12.9
12.2
CH3OH
DMF
10.7
10.5
10.5
pyridine
2.240
2.054
176.5
NR
10.9
NR
3
pc
CH3CN
2.122c
2.122c
NR
2.127
2.132
2.126
2.125
63.1
65.8
64.8
66.0
9.5
2.101c
2.234
2.239
2.254
2.101c
2.058
2.062
2.060
NR
NR
NR
NR
9.5
C2H5OH
DMF
9.5
161.2
168.0
175.5
NR
9.5
13.2
11.5
pyridine
10.5
12.8
a Polycrystals. b Not resolved. c Isotropic signal.
temperature range 295-333 K. The complexes exhibited
paramagnetic shifts due to the interaction of Cu(II) electron spin
1
1
1
(S ) /2) with the nuclear spin of protons (I ) /2). The H
NMR signals due to phen/bpy and the phenyl group of L-phe
were broad at 295 K. A shift in the resonance position and a
decrease in line width were observed with increasing temper-
ature. The signals due to inequivalent protons of bpy/phen were
resolved above 323 K, but the proton spin-spin coupling could
not be resolved. The signals in the chemical shift region 7.24-
8.96 δ are due to the protons of phen/bpy. The broad signal at
15 δ corresponds to the phenyl group, and the sharp signal in
the region 2.1-3. 4 δ is due to methylene protons. Signals due
to C12H could not be detected perhaps due to proximity to the
paramagnetic metal center. Representative 1H NMR spectra of
complexes 1 and 2 as a function of temperature are shown in
1
Figure 8, panels a and b, respectively. The H chemical shifts
are listed in Table 5. Complex 3, on the other hand, showed
very broad signals and yielded no further information.
Proton Spin-Lattice Relaxation Studies. The spin-lattice
relaxation studies of paramagnetic molecules provide informa-
tion about structural dynamics. Generally, organic ligands show
spin-lattice relaxation times (T1) of the order of 1-10 s or
longer, but complexes 1 and 2 have, surprisingly, shorter T1
values. The shortest T1 values, in the range 200-624 ms, were
observed at 296 K. The relaxation time increased gradually with
increasing temperature and became of the order of seconds.
These values at different temperatures for 1 and 2 are listed in
Table 5 along with the chemical shift parameters. T1 values
decreased gradually with an increase in electron-nuclear
distance (rIS). Pyridyl protons of phen/bpy have the shortest T1
values, the methylene protons have the intermediate values, and
the phenyl protons of L-phe have the largest T1 value. The
relaxation times of corresponding groups are in general shorter
in 2 than in 1.
Molecular Modeling and Conformational Isomerism.
Molecular modeling studies on 2 revealed minimum energy for
the “folded” (A) and “extended” (B) conformations. Structures
of these minimum energy conformations are shown in Figure
9. Molecules (A or B) can be in either the R or S enantiomorph.
Among these the R-form has minimum energy. Keeping the
geometry as “folded” or “extended”, the phenyl ring was rotated
by varying the dihedral angle containing atoms C12-C13-
C14-C15 and single-point strain energies were calculated. The
Figure 7. EPR spectra for frozen solutions of [Cu(L-phe)(bpy)(H2O)]-
ClO4 (2) at 77 K: (a) CH3OH, (b) DMF, (c) pyridine, and (d) CH3CN.
structure was better resolved in alcohol, DMF, and pyridine
solvents. In CH3CN, the signals were relatively broader (Figure
7d). Except in CH3CN, in the rest of the solvents complex 3
showed spectra similar to that of 2. Interestingly, a broad
isotropic resonance at giso ) 2.10 was observed in CH3CN. This
suggests that the dimeric molecular association is retained in
noncoordinating solvents like CH3CN. However, in coordinating
solvents (alcohol, DMF, and pyridine) the association is dis-
turbed/weakened due to solvolysis or solvent coordination. The
spin parameters for frozen solutions are also listed in Table 4.
1
1
Variable-Temperature H NMR Spectra. The H NMR
spectra of complexes 1 and 2 in D2O were measured in the

4298 Inorganic Chemistry, Vol. 40, No. 17, 2001
Subramanian et al.
1
Figure 8. Variable-temperature H NMR spectra of (a) [Cu(L-phe)(phen)(H2O)]ClO4 (1) and (b) [Cu(L-phe)(bpy)(H2O)]ClO4 (2) in D2O.
Table 5. 1H Chemical Shifts (ppm) and Spin-Lattice Relaxation Times (in Parentheses (s)) of [Cu(L-phe)(phen)(H2O)]ClO4 (1) and
[Cu(L-phe)(bpy)(H2O)]ClO4 (2) in D2O
complex 1
complex 2
L-phe
L-phe
temp (K)
296
phen
7.24, 7.89
(0.204)
7.39, 8.07
(2.3)
7.49, 8.10
CH2
2.11
(0.624)
2.29
(0.729)
2.45
phenyl
15.0
bpy
CH2
phenyl
11.77
7.20
2.58
(0.276)
2.77
(0.369)
2.96
(0.670)
3.11
(1.83)
3.27
(0.679)
3.43
(0.276)
7.99, 8.29
303
313
323
333
343
14.85
14.48
14.70
14.29
14.05
11.67
8.00, 8.46
(2.43)
8.11, 8.51
(0.302)
11.65
(0.664)
11.64
7.63, 8.19, 8.96, 2.62
(2.45)
7.72, 8.29, 9.06
(2.50)
2.62
(1.18)
2.78
(1.59)
2.75
8.22, 8.59, 8.99
11.64
11.59
8.23, 8.96
8.33, 8.69, 8.98
(0.771)
strain energy of each conformer (A or B) as a function of
dihedral angle is shown in Figure 10. The calculations revealed
minimum energy for the structures when the phenyl ring was
parallel to the molecular plane in the case of the “folded” (A)
conformation and perpendicular in the case of the “extended”
(B) conformation. These results are in excellent agreement with
the X-ray crystal structure of complex 2. The difference in the
total energy of folded (A) and extended (B) structures is 10.3
kcal (Table 6). Depending on the preparation conditions the
complex can crystallize with either or both of the conformations.
The R form of “folded” and the S form of “extended”
conformations have minimum energy. But in the solid state, as
revealed by the X-ray structure, the “folded” (A) form of 2 is
an S enantiomorph. The “extended” (B) conformer crystallizes
in the S form as predicted from theoretical study. The difference
in the “folded” structure is perhaps due to intramolecular
stacking and H-bonding interactions. Mullikan population
analysis has revealed a difference in charge density at the Cu

Ternary Copper(II) Complexes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4299
“folded” S conformer than for the “extended” R conformer
(Table 6). In other words, the theoretical studies while comple-
menting the X-ray structural results suggest that complex 2
exhibits conformational isomerism.
Discussion
Single-crystal X-ray structures revealed that copper has a
tetragonally elongated square pyramidal geometry. The coor-
dination base with N3O donor atoms is almost a perfect plane
in complex 2 while it is distorted with a tetrahedral bias in 3.
Copper is out of the best N1N2N3O1 plane by 0.220 Å in 2
and by 0.271 Å in 3. The metal-axial ligand bond is relatively
weaker in 2 (Cu1-O1w ) 2.298(7) Å) than in 3 (Cu1-N4 )
2.114(6) Å). Complex 2 exhibits conformational isomerism. The
aromatic side chain of L-phe in 2 adopts either a “folded” (A)
or an “extended” (B) conformation. In the former the phenyl
group is almost coplanar with the coordination plane
CuN1N2N3O1 while in the latter conformation it is almost
perpendicular to the coordination plane. The crystal structure
of the “folded” conformation (A) of 2 is reported in the present
study while the structure of the “extended” conformation (B)
was reported earlier by Sugimori et al.14 Molecular modeling
and DFT calculations have revealed that the energy difference
between these two conformations is 10.3 kcal. Depending on
the preparation method and conditions either of the conforma-
tional isomers can be crystallized. The “folded” (A) and
“extended” (B) structures are differed in molecular interactions
and association. Both intra- and intermolecular π-π and
H-bonding interactions stabilize the former structure while
intermolecular interactions alone contribute to the stabilization
of the latter structure. Complex 3 which has a side chain
imidazole ring adopts a “folded” conformation, but due to the
Cu-N4 bond the imidazole ring orients perpendicular to the
coordination plane. Otherwise, the theoretical studies (Figure
10) reveal minimum strain energy for a parallel orientation of
the aromatic ring system in the “folded” (A) conformation and
a perpendicular orientation in the “extended” (B) conformation.
In fact, the structures of ternary copper(II) complexes with the
L-phe ligand reported so far possesses only these two conforma-
Figure 9. Minimum energy (A) “folded” and (B) “extended” confor-
mations of [Cu(L-phe)(bpy)(H2O)]ClO4 (2) obtained from theoretical
calculations.
Table 6. Total Energy, Charge Density, and HOMO-LUMO Band
Gap Evaluated from DFT Calculations for
[Cu(L-phe)(bpy)(H2O)]ClO4 (2)
total energy
(au)
charge
density
band gap
(kcal/mol)
conformation
“folded” (A) S form
“folded” (A) R form
“extended” (B) S form
“extended” (B) R form
-2754.50
-2754.52
-2754.49
-2754.51
+1.38
+1.52
+1.42
+1.49
18.81
31.35
31.35
12.50
site in different conformers. The charge density is more for the
“extended” R conformer (+1.49) than for the “folded” S
conformer (+1.38) while the band gap energy between the
HOMO and LUMO orbitals is more (by 6.3 kcal) for the
Figure 10. Strain energy as a function of dihedral angle for extended (top) and folded (bottom) conformations.

4300 Inorganic Chemistry, Vol. 40, No. 17, 2001
Subramanian et al.
tions.14,23 The theoretical calculations have also revealed that
the S enantiomer has lower energy than the R enantiomer.
However, we find some discrepancy in “folded” geometry of
complex 2 wherein the structure is stabilized in an R enantio-
meric form. We rationalize this discrepancy between the
experimental and theoretical results as due to molecular stacking
interactions which could not be considered in theoretical
simulations. The strong inter- and intramolecular interactions
perhaps lowed the energy of the R enantiomer of the “folded”
conformation.
nuclear spin relaxation induced by the paramagnetism.25,26
Generally, those complexes which are amenable for EPR are
no good for NMR studies.12 Recently we have found that Cu-
(salEen)2ClO4 is suitable for both EPR and NMR studies. The
1
line width of the H NMR signals of Cu(salEen)2ClO4 was
sensitive to the solvent. The signals were narrow in noncoor-
dinating or weakly coordinating CDCl3 and CD3CN solvents
and broad in D2O. This was rationalized due to a change in the
molecular geometry of copper from a distorted square planar
to a square pyramidal due to solvent coordination. The present
1
study confirms our earlier observations. The H NMR signals
Interestingly, complexes 1 and 2 are amenable to both EPR
1
of complexes 1 and 2 in D2O were broad while those of 3 were
much broader. This is consistent with the square pyramidal
structure of the copper complexes. The line width of 1H NMR
signals for 1 and 2 decreased with increasing temperature (Figure
8) while that of 3 was invariant. This difference is because of
structural dynamics in complexes 1 and 2 which is absent in 3.
As evident from the single-crystal X-ray structures and molec-
ular modeling studies, complexes 1 and 2 possess either a
“folded” (A) or an “extended” (B) conformation. The difference
in energy between these two conformation is 10.3 kcal. At
higher temperatures a conversion between these two conforma-
tions is possible. The structural dynamics at higher temperatures
is responsible for the resolution and lower line width of the 1H
NMR signals. As the side chain containing the imidazole group
of L-his is involved in coordination with copper, it is not possible
for 3 to exhibit such a structural dynamics. Generally T1 of
protons in paramagnetic complexes is of the order of mil-
liseconds. Complexes 1 and 2 exhibited higher T1 values at
higher temperatures. This is due to the conformational dynamics
in 1 and 2 at higher temperatures.
UV-vis data in different solvents suggest that the coordina-
tion geometry of copper is sensitive to the donor capacity of
the solvent. Copper expands its coordination number from 5 to
6 in strong donor solvents. This can be inferred from the red
shift in the d-d band position from 580 to 690 nm. When copper
becomes hexacoordinated, the molecule changes its conforma-
tion from a “folded” to an “extended” geometry. This facile
structural transformation of copper is responsible for its reactiv-
ity in metalloenzymes. The conformational change (from a
“folded “ to an “extended” conformation) modulates the electron
density, charge distribution, and redox behavior of copper. As
apparent from Table 6 the charge on copper is lower for an
“extended” (S) conformation than for a “folded” (S) conforma-
tion. In otherwords, nucleophillic attack is favored in an
“extended” (S) conformation while electrophillic attack is
favored in a “folded” (S) conformation. These facile structural
and electronic changes have been realized to be responsible for
the reactivity of copper in metalloenzymes.
and H NMR studies. EPR spectra of pure solids showed no
resolution of metal hyperfine coupling features. As described
before, this is mainly due to dipolar and exchange interactions.24
The peak to peak line width of 52 G as compared to an expected
value of 320 G for purely dipolar interaction suggests that
exchange overrides the dipolar interaction in complexes 1 and
2. In an another study, one-dimensional Cu(II)-Schiff base
complexes exhibited line widths of 175 G, and we have found
that both dipolar and exchange interactions contribute to the
line width in those complexes.12 However, in the present
complexes the exchange dominates the dipolar interaction,
resulting in exchange-narrowed EPR signals of line widths in
the range 16-52 G. Complex 3 showed spectra typical of weak
dimeric interactions. The weak signals at 2790 and 3515 G are
the two allowed perpendicular components of ∆mS ) (1
transitions while the signal in the low-field side at 1625 G is
the half-field forbidden ∆mS ) (2 transition. The separation
between the allowed transitions is zero-field splitting (Dtotal).
This, in general, has contribution from exchange (Dex) and
dipole-dipole interaction (Ddip) terms. The latter interaction can
be related to the distance (r) between the interacting Cu(II)
centers as follows:
Ddip ) geff2â2/r3
(3)
Assuming Dex ) 0 and geff ) 2.122, we estimate a value of
3.005 Å for the intermolecular Cu-Cu separation. But, accord-
ing to the X-ray structure, the Cu-Cu separation is 3.811 Å.
Hence, in the present case the exchange interaction term (Dex)
is nonzero and contributes to the zero-field splitting parameter
(Dtotal). The dimeric association of 3 is present also in weakly
coordinating solvents.
Much has been reported on the exchange interactions in
dimeric paramagnetic complexes bridged through mono- and
multiatomic ligands.5-8 However, studies on magnetic exchange
though weak molecular interactions such as π-π stacking and
H bonding are scarce.9-12 The present complexes have the
noteworthy result that the magnetism is primarily via weak
intermolecular interactions. Exchange interaction is stronger in
complex 3 than in 2. This is mainly due to the difference in
intramolecular stacking interaction and Cu-Cu separation. The
π-π stacking interaction is stronger in 3 than in 2 (stacking
distance is only 3.66 Å in 3 while it is 4.03 Å in 2). Additionally,
the molecules are oriented in such a way that intramolecular
Cu-Cu separation is shorter (3.81 Å) in 3 than in 2 (7.14 Å).
The present study therefore reveals that the molecular stacking
and H-bonding interactions not only stabilize the supramolecular
architecture but also are responsible for the cooperative phe-
nomenon. Such a molecular phenomenon is known in biological
systems. This is also a prerequisite for molecular based sensor
materials.
Conclusions
Three ternary copper(II) complexes, [Cu(L-phe)(phen)(H2O)]-
ClO4 (1), [Cu(L-phe)(bpy)(H2O)]ClO4 (2), and [Cu(L-his)(bpy)]-
ClO4‚1.5H2O (3), were synthesized and characterized by single-
crystal X-ray and spectroscopic (FT-IR, UV-vis, EPR, and 1H
NMR) techniques. Complexes 1 and 2, in the solid state, form
supramolecular networks via weak intra- and intermolecular
π-π stacking and H-bonding interactions, while 3 exists as
weak dimers. Complexes 1 and 2 exhibit conformational
(25) . NMR of Paramagnetic Molecules: Principles and Applications; La
Mar, G. N., Horrocks, W. DeW., Jr., Holm, R. H., Eds.; Academic
Press: New York, 1973.
(26) Bertini, I., Luchinat, C. NMR of Paramagnetic Molecules in Biological
Systems; Benjamin & Cummings: Menlo Park, CA, 1986.
NMR of mononuclear copper complexes is limited due to
very broad and poorly resolved signals as a consequence of rapid

Ternary Copper(II) Complexes
Inorganic Chemistry, Vol. 40, No. 17, 2001 4301
isomerism. The phenyl side chain of L-phe adopts either a
“folded” (A) or an “extended” (B) conformation. H NMR
the capacity to expand the coordination number from 5 to 6 in
strong donor solvents (as revealed by UV-vis spectroscopy),
and the self-organization and three-dimensional magnetic
behavior via weak molecular interactions are the salient features
of the present complexes.
1
studies in D2O reveal an interconversion between A and B
conformers at high temperatures (>323 K). Single-crystal EPR
studies reveal that the weak molecular interactions are the
pathways for magnetic exchange. Complexes 1 and 2 belong
to the class of a few paramagnetic copper(II) complexes
amenable to both EPR and 1H NMR studies. These complexes
also belong to the category of limited reported systems
exhibiting magnetism through weak molecular interactions. The
tetragonally elongated square pyramidal geometry of copper,
Supporting Information Available: X-ray crystallographic files
in CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC010182D