4,4′-Bis(tert-butyl)-2,2′-bipyridinedichlorometal(II) -Synthesis, structure and EPR spectroscopy

Article abstract of DOI:10.1016/j.ica.2010.08.035

Due to the better solubility of the 4,4′-substituted bipyridine ligand a series of 4,4′-bis(tert-butyl)-2,2′- bipyridinedichlorometal(II) complexes, [M(tbbpy)Cl₂], with M = Cu, Ni, Zn, Pd, Pt was synthesised and characterised. The blue copper complex 4,4′-bis(tert-butyl)-2,2′-bipyridinedichlorocopper(II) was isolated in two different polymorphic forms, as prisms 1 with a solvent inclusion and solvent-free as needles 2. Both structures were determined by X-ray structure analysis. They crystallise in the monoclinic space group P2₁/c with four molecules in the unit cell, but with different unit cells and packing motifs. Whereas in the prisms 1, with the unit cell parameters a = 12.1613(12), b = 10.6363(7), c = 16.3074(15) ?, β = 94.446(8)°, the packing is dominated by intra-and intermolecular hydrogen bonds, in the needles 2, with a = 7.738(1), b = 18. 333(2), c = 13.291(3) ?, β = 97.512(15)°, only intramolecular hydrogen bonds appear and the complex molecules are arranged in columns which are stabilised by π-π-stacking interactions. In both complexes the copper has a tetrahedrally distorted coordination sphere. These copper complexes were also studied by EPR spectroscopy in solution, as frozen glass and diamagnetically diluted powder with the analogue [Pd(tbbpy)Cl ₂] as host lattice.

Full text of DOI:10.1016/j.ica.2010.08.035

Inorganica Chimica Acta 365 (2011) 127–132
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Inorganica Chimica Acta
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4,40-Bis(tert-butyl)-2,20-bipyridinedichlorometal(II) – Synthesis, structure and EPR
spectroscopy
⇑
Duha Jawad Awad, Uwe Schilde, Peter Strauch
University of Potsdam, Institute of Chemistry, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam/Golm, Germany
a r t i c l e i n f o
a b s t r a c t
Due to the better solubility of the 4,40-substituted bipyridine ligand a series of 4,40-bis(tert-butyl)-2,20-
bipyridinedichlorometal(II) complexes, [M(tbbpy)Cl2], with M = Cu, Ni, Zn, Pd, Pt was synthesised and
characterised. The blue copper complex 4,40-bis(tert-butyl)-2,20-bipyridinedichlorocopper(II) was iso-
lated in two different polymorphic forms, as prisms 1 with a solvent inclusion and solvent-free as needles
2. Both structures were determined by X-ray structure analysis. They crystallise in the monoclinic space
group P21/c with four molecules in the unit cell, but with different unit cells and packing motifs. Whereas
in the prisms 1, with the unit cell parameters a = 12.1613(12), b = 10.6363(7), c = 16.3074(15) Å,
b = 94.446(8)°, the packing is dominated by intra- and intermolecular hydrogen bonds, in the needles
2, with a = 7.738(1), b = 18. 333(2), c = 13.291(3) Å, b = 97.512(15)°, only intramolecular hydrogen bonds
Article history:
Received 22 March 2010
Received in revised form 12 August 2010
Accepted 21 August 2010
Available online 26 August 2010
Keywords:
4,40-Bis(tert-butyl)-2,20-bipyridine
X-ray structure
EPR
appear and the complex molecules are arranged in columns which are stabilised by p–p-stacking inter-
Copper(II)
actions. In both complexes the copper has a tetrahedrally distorted coordination sphere. These copper
complexes were also studied by EPR spectroscopy in solution, as frozen glass and diamagnetically diluted
powder with the analogue [Pd(tbbpy)Cl2] as host lattice.
Transition metals
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
electron configuration (S = ½) and the well separated ground state
and its nuclear spin of I = 3/2 copper is also very well suited to be
Coordination compounds of transition metals with a cis-N2Cl2-
coordination sphere are in the focus of investigations not only of
their interest in medicine but also in photochemistry [1–5] and
catalysis [6]. The easy replacement of the two chloro ligands makes
these complexes to well suited intermediates on the synthetic
pathway to a great variety of mixed ligand complexes with diimine
ligands.
studied by EPR spectroscopy even at room temperature.
2. Experimental
2.1. Synthesis
2,20-Bipyridine belongs to the best studied ligands in coordina-
tion chemistry but often the poor solubility of its complexes is a
limitation for studies. The substitution of the pyridine rings by
tert-butyl substituents helps to overcome this problem and in-
creases the solubility. Interestingly 2,20-bipyridinedichloro- and
2,20-bipyridine dibromocopper(II) [7–9] complexes as well as
the related dichloro(di-2-pyridylamine-N,N0)copper(II) [10] and
dichloro- and dibromo(di-2-pyridylmethane-N,N0)copper(II) [11–
13] exhibit a high degree of structural flexibility with different
geometries and 4-, 5- and 6-fold coordination, forming monomeric
structures as well as dimers and polymer chains. Also for 2,20-
bipyridinedichloroplatinum(II) two polymorphic forms, a red form
[14] and a yellow form [15] are reported. Copper(II) with its d9
The transition metal chlorides were used as purchased without
any further purification. The synthesis of 4,40-bis(tert-butyl)-2,20-
bipyridine (tbbpy) was carried out following the method of McGill
[16]. To 4.4 g (0.338 mmol) of sodium amide under an argon atmo-
sphere in a round bottomed flask 50 cm3 of 4-tert-butylpyridine
were added, and the mixture was then heated to 145 °C (not to re-
flux) for 6 h, during which it became metallic purple. The solution
was allowed to cool to about 40 °C, during which it became dark
green, and then extracted into a separating funnel with approxi-
mately 100 cm3 of xylene. Twenty milliliters of distilled water
were carefully added to hydrolyse the sodium amide, and the or-
ganic layer was separated off and dried over MgSO4. This mixture
was filtered to remove the drying agent, and the liquids (a mixture
of unchanged 4-tert-butylpyridine and xylene) were distilled off at
60 °C in vacuo, leaving the crude product as a yellow solid. This
was recrystallised twice from the minimum volume of hot acetone,
yielding the product as white needles (3.7 g, 13.9 mmol). Melting
point: 159–160 °C.
⇑
Corresponding author. Tel.: +49 331 9775190; fax: +49 331 9775054.
E-mail address: pstrauch@uni-potsdam.de (P. Strauch).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.035

128
D.J. Awad et al. / Inorganica Chimica Acta 365 (2011) 127–132
IR: (KBr, cmꢀ1):
2866; ring.(C@C, C@N) = 1585, 1547;
1075; d out of plane) = 841 748; dring = 604; ring torsion = 415.
m
(C–H)arom = 3065;
m
(C–H)aliph. = 2961, 2903,
The palladium complex was doped with copper by adding a
m
mring + dH = 1465, 1374; dH(in plane)
=
solution of an equivalent of approximately 1–3% of the copper
complex to a concentrated solution of the isostructural palladium
complex and slow crystallisation.
H(
Elemental Anal. Calc.: C, 80.55; H, 9.01; N, 10.44. Found: C,
80.57; H, 10.17; N, 10.62%.
1H NMR: (CDCl3): d 1.39 (s, 18H, tBu), 7.29 (dd, 2H, H5H6, H3H5),
8.41 (d, 2H, H3), 8.59 (d, 2 H, H6).
2.2. Methods
13C NMR: (CDCl3): d: 161.02 (C1), 118.28 (C3), 140.03 (C4),
120.79 (C5), 148.91 (C6), 56.89 (C7/tbu), 34.94 (C8/Me).
Elemental analyses were carried out on an Elementar vario EL III
analyzer.
Bis(4,40-tert-butyl)-2,20-bipyridinedichloro complexes [M(tbbpy)Cl2]
(general procedure)
Infrared spectra were recorded on a Perkin–Elmer type 16PC FT-
IR spectrophotometer in the region between 4000 and 400 cmꢀ1 as
KBr-pellets (reference KBr).
Metal-4,40-bis(tert-butyl)-2,20-bipyridinedichloro
complexes
The 1H NMR and the 13C NMR spectra were recorded on a Bru-
ker Analytik AV300 spectrometer.
were synthesised according to the method described by Jäger
and van Dijk [17]. A solution of 0.1 of 4,40-bis(tert-butyl)-2,20-
bipyridine (tbbpy) in ethanol was added slowly to an equimolar
well stirred warm solution of the appropriate metal chloride
(MCl2ꢁnH2O) (M = Ni, Cu, Zn) in ethanol. For the Pt and Pd
complexes the appropriate bis(acetonitrile)dichloro complex
[(AN)2PdCl2] or tetrachloroplatinate(II) [PtCl4]2ꢀ were used,
respectively. After 1 h the reaction mixture was allowed to cool
to room temperature. The microcrystalline products were filtered
off, washed with ethanol and dried in vacuum. For the platinum
complex the method was slightly modified: water with some drops
of HCl was used as solvent and the mixture was refluxed for 1 day
[18]. These 4,40-bis(tert-butyl)-2,20-bipyridinedichloro metal(II)
complexes were recrystallised from ethanol.
The melting points were determined with a ‘Mikroheiztisch
Boetius’.
The magnetic susceptibilities of the synthesised complexes
were measured with Gouy method using a magnetic balance type
MSB-Auto (Sherwood Scientific Ltd.).
EPR spectra were recorded at 9.4 GHz (X-band) with a Bruker
CW Elexsys E500 spectrometer. The spectra were simulated with
the experimental parameters using the program Bruker XSophie
[19].
IR-Spectra were recorded on a Perkin–Elmer type 16 PC FT-IR
spectrophotometer in the region between 4000 and 400 cmꢀ1 as
KBr-pellets (reference KBr).
The magnetic susceptibilities of the synthesised complexes
were measured with a magnetic balance MSB-Auto (Sherwood
Scientific Ltd.).
[Cu(tbbpy)Cl2] blue, melting point: 320–324 °C; Anal. Calc. for
C18H24N2CuCl2: C, 53.67; H, 6.00; N, 6.95. Found: C, 52.82; H,
6.29; N, 6.69%. Susceptibility: eff = (1.8 0.5) B.M.
l
X-ray diffraction data were collected at 210 K on a STOE Imag-
ing Plate Diffraction System IPDS-II using graphite monochroma-
[Ni(tbbpy)(H2O)2Cl2]: light green, melting point: >350 °C; Anal.
Calc. for C18H28N2O2NiCl2: C, 49.81; H, 6.00; N, 6.45. Found: C,
49.94; H, 6.41; N, 7.14%. [Zn(tbbpy)Cl2] colourless, melting point:
352–356 °C; Anal. Calc. for C18H24N2ZnCl2: C, 53.42; H, 6.50; N,
6.95. Found: C, 53.14; H, 5.34; N, 6.82%.
[Pd(tbbpy)Cl2] yellow, melting point: decomposes >345 °C;
Anal. Calc. for C18H24N2PdCl2: C, 48.50; H, 5.43; N, 6.92. Found: C,
48.66; H, 5.52; N, 6.33%.
tised Mo K radiation (k = 0.71073 Å). The data were corrected by
a
a spherical absorption correction using the program X-AREA [20]
as well as for Lorentz, polarisation and extinction effects. The
solution of the crystal structure was performed using the program
SHELXS-97 by direct methods (Sheldrick, 1997) [21], and refined
using the program SHELXL-97 (Sheldrick, 1997) [22]. All non-
hydrogen atoms were refined anisotropically. The hydrogen
atoms were located from the difference Fourier map and allowed
to ride on their parent atoms with Uiso(H) = 1.2Ueq(C). For the
visualisation of the structures the graphic programs ORTEP-3 [23]
and DIAMOND [24] were used. The crystallographic and structure
refinement data for the two forms of [Cu(tbbpy)Cl2], 1 and 2,
are summarised in Table 2.
[Pt(tbbpy)Cl2] yellow, melting point: >350 °C; Anal. Calc. for
C18H24N2PtCl2: C, 40.46; H, 4.53; N, 5.24. Found: C, 40.80; H,
4.47; N, 5.24%.
1H NMR: (CDCl3): d 1.43 (s, 18H, tBu), 7.43 (dd, 2H, H5H6, H3H5),
7.81 (d, 2H, H3), 9.44 (d, 2 H, H6). For characteristic IR-Absorptions
see Table 1.
Table 1
Characteristic IR absorptions of dichloro-4,40-bis(tert-butyl)-2,20-bipyridine metal(II) complexes.
Assignment
IR-frequency (cmꢀ1
Cu(tbbpy)Cl2
)
Ni(tbbpy) H2O2Cl2
Zn(tbbpy)Cl2
Pd(tbbpy)Cl2
Pt(tbbpy)Cl2
mC–H(arom.)
mC–H(alph.)
3106–3052 m
2962 vs,
2906 w,
2871 wm
1614 vs
1547 s
3065 w
2961 vs,
2904 w,
2868 wm
1614 vs
1549 s
3111–3067 w
2962 vs,
2908 w,
2871 wm
1614 vs
1551 s
3118–3071 w
2965 vs,
2907 w,
2873 wm
1614 vs
1545 ms
1482 s
3116–3071 w
2965 vs,
2906w,
2871 wm
1617 vs
1540 ms
1483 s
mring.(C@C,
C@N)
mring
mring
1484 s
1482 s
1487 s
m
dH(in
ring + dH
1412 vs
1159 wm
1078 wm
1031 s
851 s
738 m
1408 vs
1153 vw
1079 w
1024 ms
847 s
1411 vs
1156 vw
1079 w
1022 ms
849 s
1415 vs
1156 wm
1077 mw
1028 ms
851 s
1419 vs
1157 wm
1076 mw
1034 mw
850 s
plane)
mring + dring
dH(out
dH(out
dH(out
dring
of plane)
of plane)
of plane)
738 mw
606 s
740 mw
607 s
739 mw
600 s
737 mw
598 s
605 s
Ring torsion
423 wm
417 mw
420 mw
424 mw
417 mw
m: medium, w: weak, wm: weak-medium, vs: very sharp, s: sharp, vw: very weak, ms: medium-sharp, mw: medium-weak.

D.J. Awad et al. / Inorganica Chimica Acta 365 (2011) 127–132
129
Table 2
ferent packing motifs. The packing of form 1 is dominated by intra-
and intermolecular hydrogen bonds of the chloro ligands (see
Fig. 2), whereas in the structure of 2 exclusively intramolecular
hydrogen bonds are formed (see Fig. 4). The intramolecular inter-
actions between the chloro ligands and hydrogen atoms might be
one reason for the tetrahedral distortion of the complex. The crys-
tallographic and refinement data are shown in Table 2.
Crystallographic and structure refinement data for [Cu(tbbpy)Cl2]ꢁ0.5CH3OH, 1 and
[Cu(tbbpy)Cl2], 2.
1
2
Empirical formula
C
18H24Cl2CuN2]ꢁ0.5
C18H24Cl2CuN2
CH3OH
412.84
blue/prism
monoclinic/P21/c
Formula weight
Colour/Habit
Crystal system/space group
402.84
blue/needle
monoclinic/P21/c
In the crystal structure of [Cu(tbbpy)Cl2]ꢁ0.5CH3OH, 1, the mol-
ecules form intermolecular as well as intramolecular C–Hꢁ ꢁ ꢁCl con-
tacts. The chlorine atom Cl1 of one molecule forms two weak
intermolecular hydrogen bonds to the hydrogen atoms H9 at C9
(Cꢁ ꢁ ꢁCl 3.686(3) Å) and H18A at C18 (Cꢁ ꢁ ꢁCl 3.855(4) Å) of the
neighbour molecule and an intramolecular hydrogen bond to
H10 at C10 (Cꢁ ꢁ ꢁCl 3.345 Å); the chlorine atom Cl2 forms also one
intramolecular hydrogen bond with the hydrogen atom H1 at C1
(Cꢁ ꢁ ꢁCl 3.314(3) Å) of the same molecule and two weak intermolec-
ular hydrogen bonds to the hydrogen atoms H4 at C4 (Cꢁ ꢁ ꢁCl
3.683(3) Å) and H7 at C7 (Cꢁ ꢁ ꢁCl 3.673(3) Å) of the neighbour
molecule (see Fig. 2 and Table 3). Whereas in the two reported
structures of 2,20-bipyridinedichloroplatinum(II) [14,15] the coor-
dination geometry around the central platinum(II) is strictly
square-planar, the coordination sphere around the copper(II) is
much more flexible and far from planarity. The dihedral angle be-
tween the (tbbpy) plane and the (Cl–Cu–Cl) plane in 1 is 41.40(5)°.
This situation for the central copper(II) ion is related to those re-
ported for dichloro(di-2-pyridylamine-N,N0)copper(II) [10] and di-
chloro(di-2-pyridylmethane-N,N0)copper(II) [11] with dihedral
angles of 29.0° and 61.3°, respectively. The Cu–Cl distances in
the coordination sphere Cu1–Cl1 (2.2151(7) Å) and Cu1–Cl2
(2.2231(8) Å) are only slightly shorter as reported for the compara-
ble complexes dichloro(di-2-pyridylamine-N,N0)copper(II) [10] and
dichloro(di-2-pyridylmethane-N,N0)copper(II) [11]. The Cu–N dis-
tances Cu1–N1 (1.982(2) Å) and Cu1–N2 (1.972(2) Å) are a little
Unit cell dimension
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Z
12.1613(12)
10.6363(7)
16.3074(15)
94.446(8)
2103.0(3)
4
7.7380(14)
18.333(2)
13.291(3)
97.512(15)
1869.3(6)
4
T (K)
210 (2)
1.272
210 (2)
1.431
Dcalc (g cmꢀ3
)
Crystal size (mm)
(mmꢀ1
F(0 0 0)
0.20 ꢂ 0.25 ꢂ 0.40
0.10 ꢂ 0.21 ꢂ 1.00
l
)
1.293
1.454
836
1.70–25.0
13 334
836
1.90–25.0
11 779
h Range for data collection (°)
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
3709 (0.063)
3709/0/281
0.94
R1 = 0.0326
wR2 = 0.0756
R1 = 0.0454
wR2 = 0.0787
0.54 and ꢀ0.41
3288 (0.135)
3288/0/281
0.76
R1 = 0.0455
wR2 = 0.06931
R1 = 0.0823
wR2 = 0.120
0.40 and ꢀ0.53
R indices [I > 2
r(I)]
R indices (all data)
Largest difference in peak/hole
(e Åꢀ3
)
3. Results and discussion
3.1. X-ray structure analyses
shorter than in the catena-di-l
-chloro(2,20-bipyridine)copper(II)
[8]. The bite angles are 98.65(3)° (Cl1–Cu1–Cl2) and 80.87(8)°
(N1–Cu1–N2). Surprisingly also the (tbbpy) ligand is slightly
twisted along the central C5–C6 bond. The twist angle between
the two pyridine rings is about 10°. In contrast a structure of the
[Cu(bpy)Cl2] is reported consisting of single chloro-bridged cop-
per(II) chains, in which the copper surrounding can be described
as distorted square pyramidal [25] (Cu–Cl(bridging) 2.291/
2.674 Å, Cu–Cl (terminal) 2.259 Å. In another reported structure
Blue crystals of 4,40-bis(tert-butyl)-2,20-bipyridinedichlorocop-
per(II) were crystallised from methanol at room temperature after
1 week. Interestingly, two different crystalline forms could be iso-
lated, [Cu(tbbpy)Cl2]ꢁ0.5CH3OH 1 as prisms, and [Cu(tbbpy)Cl2], 2
(solvent-free) as needles. Both complexes crystallise in the mono-
clinic space group P21/c with four molecules in the unit cell.
The ORTEP plot of [Cu(tbbpy)Cl2] 2, shown in Fig. 1, stands for
both modifications because the molecular structures are very close
and differ only in their dihedral angles (41° and 31°) around the
coordination centre, therefore the ORTEP plot of 1 is not shown.
The difference between the two forms is more expressed in the dif-
the two molecules are double-bridged in a l2-chloro mode forming
a dimeric complex. Copper is there in a five-coordinated distorted
square-pyramidal environment with two bridging chlorides and
one terminal chloride [26] (Cu–Cl (bridging) 2.280/2.909 Å, Cu–Cl
(terminal) 2.272 Å. The crystal consists of chains double-bridged
via the chlorine atoms. Finally in two other structures the coordi-
nation geometry around the copper atom is distorted square-
planar. There are weak intermolecular Cuꢁ ꢁ ꢁCl interactions forming
a chain structure in the crystal. Therefore, the coordination polyhe-
dron around copper in these structures can also be described as a
distorted octahedron. A triclinic (with Cu–Cl/square-planar: 2.267
/2.268 Å, Cu–Cl/bridging 3.035/3.106 Å) and a more symmetric
monoclinic form (with Cu–Cl/square-planar: 2.254 Å, Cu–Cl/bridg-
ing) 3.054 Å) are reported [8,27].
C17
C13
C14
C11
C3
C18
C15
C7
C16
C12
C2
C4
C5
C8
C6
C9
B
In the crystal structure of the second form, 2, weak
p–p-
C
stacking interactions of the aromatic bipyridine rings appear along
the crystallographic a-axis forming a columnar molecular packing
motif (see Figs. 3 and 4). Inside these columns the structure is sta-
A
C10
C1
N1
N2
Cu1
bilised by p–p interaction of the (tbbpy) ligands with a mean inter-
planar distance of 3.528 Å. The chlorine atoms Cl1 and Cl2 form
intramolecular hydrogen bonds to the hydrogen atoms H10 at
C10 and H1 at C1 of the same molecule, with the relatively short
distances C10ꢁ ꢁ ꢁCl1: 3.299(6) Å and C1ꢁ ꢁ ꢁCl2: 3.277(6) Å, respec-
tively. Also in this structure the coordination geometry at the
Cl2
Cl1
Fig. 1. ORTEP drawing of the [Cu(tbbpy)Cl2] complex 2 and the ring labelling scheme.

130
D.J. Awad et al. / Inorganica Chimica Acta 365 (2011) 127–132
Fig. 2. Stereo view on a part of the crystal packing of [Cu(tbbpy)Cl2]ꢁ0.5CH3OH, 1, along the crystallographic b-axis (the solvent molecule is omitted).
Table 3
Selected geometric parameters (Å, °) for [Cu(tbbpy)Cl2]ꢁ0.5CH3OH, 1.
Bond lengths and angles
Cu1–Cl1
Cu1–Cl2
Cu1–N1
Cu1–N2
2.2153(7)
2.2233(8)
1.982(2)
1.972(2)
98.65(3)
149.24(6)
96.65(6)
96.88(6)
152.60(6)
80.87(8)
125.0 (2)
115.3(2)
118.9(2)
116.0 (2)
N1–C1
N1–C5
N2–C6
N2–C10
1.336(3)
1.348(3)
1.354(3)
1.330(3)
125.4(2)
118.5(2)
122.1(3)
121.3(2)
113.7(2)
113.2(2)
121.6(2)
122.5(3)
116(2)
Cl1–Cu1–Cl2
Cl1–Cu1–N1
Cl1–Cu1–N2
Cl2–Cu1–N1
Cl2–Cu1–N2
N1–Cu1–N2
Cu1–N1–C1
Cu1–N1–C5
C1–N1–C5
Cu1–N2–C6
Cu1–N2–C10
C6–N2–C10
N1–C1–C2
N1–C5–C4
N1–C5–C6
N2–C6–C5
N2–C6–C7
N2–C10–C9
N1–C1–H1
N2–C10–H10
116(2)
Intra- and intermolecular hydrogen bonds C–H
Cl–H CꢁꢁꢁCl
C–HꢁꢁꢁCl
C10–H10ꢁ ꢁ ꢁCl1
0.989(3) 2.84 3.345(3) 112(2)
0.897(3) 2.78 3.314(3) 120(2)
1.080(4) 2.79 3.855(4) 167(3)
0.901(3) 2.93 3.686 (3) 107(2)
0.975(3) 2.91 3.673(3) 171(2)
0.856(3) 3.07 3.683(3) 154(2)
41.40(5)
C1–H1ꢁ ꢁ ꢁCl2
C18–H18Aꢁ ꢁ ꢁCl1_i
C9–H9ꢁ ꢁ ꢁCl1_i
Fig. 3. Crystal packing of [Cu(tbbpy)Cl2], 2, along the crystallographic a-axis, the
stacking axis of the columns.
C7–H7ꢁ ꢁ ꢁCl2_i
C4–H4ꢁ ꢁ ꢁCl2_i
Dihedral angle (N1–Cu–N2)/Cl1–Cu–Cl2)
Twist angle (tbbpy)
9.91 (8)
3.2. EPR spectroscopy
Symmetry code: (i) x, 1.5 ꢀ y, 0.5 + z.
The paramagnetic copper complex, dichloro-4,40-bis(tert-
butyl)-2,20-bipyridine copper(II), was studied by EPR spectroscopy
in solution at room temperature, in frozen glass in methanol at
150 K as well as in solid powder diamagnetically diluted in the dia-
copper(II) is tetrahedrally distorted but with a smaller dihedral an-
gle of 30.90(1)° between the (N–Cu–N) and (Cl–Cu–Cl) planes. The
distances Cu1–Cl1 (2.212(2) Å) and Cu1–Cl2 (2.213(2) Å) are com-
parable to the Cu–Cl distances in 1, but the Cu1–N1 (2.016(4) Å)
and Cu1–N2 (2.007(4) Å) distances are slightly shorter (see Table
4). The bite angles Cl1–Cu1–Cl2 (97.30(6)°) and N1–Cu1–N2
(80.07(17)°) are not very different as in 1.
The crystal packing contains disordered solvent (methanol)
molecules. In spite of several attempts, no chemically reasonable
solution could be received for the solvent species. Very high dis-
placement parameters, high estimates and partial occupancy due
to the disorder make it impossible to determine accurate atomic
positions for the solvent molecules. Therefore these disordered sol-
vent species was subtracted from the structure factor calculations
with the SQUEEZE instruction of the PLATON program suite [28]. PLATON/
SQUEEZE calculated a solvent-accessible void volume in the unit cell
of 2103.0 Å3 (13.1% of the total cell volume). The deposited data
(cif) of complex 1 reflect only known cell content. In contrast to
1, compound 2 is solvent-free. According to PLATON the unit cell con-
tains no residual solvent-accessible void.
magnetic host lattice [Pd(tbbpy)Cl2]. Fig.
5 shows the EPR
spectrum of [Cu(tbbpy)Cl2] in dichloromethane recorded at
room temperature. In the spectrum the typical hyperfine (hfs)
quartet of Cu(II) (S = ½, I = 3/2) is resolved. For the present N2Cl2-
coordination sphere a more detailed super hyperfine structure
(shfs) could be expected due to the coupling of the spin of the un-
paired electron with the two nitrogen atoms (14N, I = 1) and the
two chlorine atoms (35Cl, I = 3/2; 37Cl, I = 3/2). If we take the two
copper isotopes into consideration (63Cu, I = 3/2; 65Cu, I = 3/2) for
the isotropic spectrum up to 320 lines could be expected but they
are not resolved.
The anisotropic EPR spectrum of [Cu(tbbpy)Cl2] in frozen meth-
anol at 150 K is shown in Fig. 6 and the EPR parameters are listed in
Table 5. The figure shows a typical axial symmetric spectrum for a
copper(II) complex in frozen solution. Again only the Cu(II) hyper-
fine structure in the parallel part is reasonable resolved. In the per-
pendicular part no hfs at all is resolved. The relatively broad signals
are due to the high number of lines lying underneath each signal as
discussed for the isotropic solution spectrum.

D.J. Awad et al. / Inorganica Chimica Acta 365 (2011) 127–132
131
Fig. 4. Stereo view of the crystal packing of [Cu(tbbpy)Cl2], 2, perpendicular to the stacking axis of the columns which coincides with the crystallographic a-axis.
Table 4
Selected geometric parameters (Å, °) for [Cu(tbbpy)Cl2], 2.
Bond lengths and angles
Cu1–Cl1
Cu1–Cl2
Cu1–N1
Cu1–N2
2.212(2)
2.213(2)
2.016(4)
2.007(4)
97.30(6)
157.2(1)
95.2(1)
95.3(1)
156.8(1)
80.1(2)
N1–C1
N1–C5
N2–C6
N2–C10
1.338(6)
1.325(6)
1.358(6)
1.333(6)
127.5(3)
117.2(4)
122.7(5)
121.6(5)
115.1(4)
113.9(4)
121.6(5)
124.3(5)
117(3)
Cl1–Cu1–Cl2
Cl1–Cu1–N1
Cl1–Cu1–N2
Cl2–Cu1–N1
Cl2–Cu1–N2
N1–Cu1–N2
Cu1–N1–C1
Cu1–N1–C5
C1–N1–C5
Cu1–N2–C6
Cu1–N2–C10
C6–N2–C10
N1–C1–C2
N1–C5–C4
N1–C5–C6
N2–C6–C5
N2–C6–C7
N2–C10–C9
N1–C1–H1
N2–C10–H10
125.2 (3)
115.3(3)
119.1(4)
115.1 (3)
111(4)
Intramolecular hydrogen bonds
C–H
Cl–H
CꢁꢁꢁCl
C–HꢁꢁꢁCl
C1–H1ꢁꢁꢁCl2
0.94(5) 2.74(4) 3.277(6) 117(3)
0.86(5) 2.72(5) 3.299(6) 126(4)
30.90(1)
C10–H10ꢁꢁꢁCl1
Dihedral angle (N1–Cu–N2)/Cl1–Cu–Cl2)
Interplanar distance (tbbpy/tbbpy)
3.528(1)
Fig. 5. Experimental and simulated isotropic EPR spectrum of [Cu(tbbpy)Cl2] in
CH2Cl2 at 295 K.
p–p interactions
Cg–Cg (Å)
a
(°)
d (Å)
A–Ci
A–Bii
B–Cii
3.641(3)
3.686(3)
3.840(3)
4.3(2)
5.4(2)
7.9(3)
3.620(2)
3.601(2)
3.565(2)
Symmetry codes: (i) ꢀx, 1 ꢀ y, ꢀz; ii, 1 ꢀ x, 1 ꢀ y, ꢀz.
Cg–Cg, distance between ring centroids; , dihedral angles between the planes; d,
a
interplanar spacing (distance from one plane to the neighbouring centroid).
In order to achieve a higher degree of resolution we have doped
the paramagnetic Cu(II) complex into the structurally related but
diamagnetic [Pd(tbbpy)Cl2] complex. Fig. 7 shows the EPR spec-
trum of the powdered sample of [Cu(tbbpy)Cl2] diamagnetically
diluted in the [Pd(tbbpy)Cl2] host lattice at room temperature.
The super hyperfine structure (shfs) pattern due to the coupling
with the two nitrogen atoms is only very poorly resolved in the
high field part of the spectrum. Therefore, a correct determination
of parameters becomes impossible, namely due to the fact that also
a coupling with the chlorine nuclei is possible. The experimental
EPR parameters for [Cu(tbbpy)Cl2] are collected in Table 5. The
coupling constants in the parallel part (AfCu) can be only roughly
estimated, due to the missing resolution. The parameters for the
parallel part are of better significance. All parameters are in the ex-
pected range for Cu(II). The variation of the parameters in different
solvents is due to solvent effects. The g-values measured for the
diamagnetically diluted powder sample are significant different
from the g-values of the samples in solution. This reflects a small
Fig. 6. Experimental (exp.) and simulated (sim.) EPR spectrum of [Cu(tbbpy)Cl2] in
methanol at 150 K.
change in the coordination geometry at the central copper(II) to-
wards a more planar arrangement, because for the host lattice

132
D.J. Awad et al. / Inorganica Chimica Acta 365 (2011) 127–132
Table 5
Experimental EPR parameters for [Cu(tbbpy)Cl2].
Cu
Complex
g0/gav
g
0.002
g\ 0.002
a0/Aav
ꢃ74
A
(10ꢀ4 cmꢀ1
2
)
(A Cu) (10ꢀ4 cm-1
)
k
k
k
[Cu(bpy)Cl2]
2.145
2.275
2.08 0.02
164
(ꢃ29)
(DMSO, 150 K) [30]
[Cu(tbbpy)Cl2]
(CH2Cl2, 295 K)
[Cu(tbbpy)Cl2]
(MeOH, 150 K)
[Cu(tbbpy)Cl2]
(in [Pd(tbbpy)Cl2])
2.110
2.136
2.094
70 2.0
2.269
2.206
2.07 0.01
2.038
ꢃ67
163
166
2
1
(ꢃ19)
<20
Appendix A. Supplementary material
CCDC 765237 and 765238 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2010.08.035.
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The financial support by a DAAD Ph.D. scholarship A/05/06200
(D. Jawad Awad) is gratefully acknowledged.