Complexes of 4- and 5-bromo derivatives of 2-(hydroxymethyl)pyridine with copper(II) and cobalt(II) salts. Synthesis and X-ray crystal structures

Article abstract of DOI:10.1016/j.poly.2010.03.001

Four copper(II) complexes (1-4) and a cobalt(II) complex (5) derived from 4-bromo-2-(hydroxymethyl)pyridine (L¹) or 5-bromo-2-hydroxymethyl)pyridine (L²) with Cu(NO₃)₂·3H₂O, CuCl₂·2H₂O and CoCl₂·6H₂O have been synthesized and their respective crystal structures studied. They show specific influences owing to the different kind of metal cations and counter anions, the hydration as well as the different position of the bromine substitution on both the coordination of the complex unit and the network structure of the crystal lattice. The Cu(II) complexes of L¹ are five-coordinate [Cu(L¹)₂NO₃]NO₃·H₂O (1) and [Cu(L¹)₂Cl]Cl·H₂O (2) species with distorted quadratic pyramidal and trigonal bipyramidal coordination geometries of the N₂O₃ and N₂O₂Cl donor atoms around the Cu(II), respectively. The Cu(II) complexes of L² are six-coordinate [Cu(L²)₂(NO₃)₂] (3) and [Cu(L²)₂Cl(H₂O)]Cl·H₂O (4) species with distorted octahedral coordination geometries of the N₄O₂ and N₂O₃Cl donor atoms. A distorted octahedral coordination geometry of the N₂O₂Cl₂ donor atoms is also found in the complex unit [Co(L²)₂Cl₂] of the Co(II) complex 5 but showing the oxygen atoms of the chelating ligand as well as the chloride ions in a cis-position. Depending on the complex, water molecules and chloride anions are shown to act as stabilizing components of the crystal structure. The comparative structural investigation includes also known structures of the bromine-free ligand analogue 2-(hydroxymethyl)pyridine, illustrating the basic implication of the bromine substitution, mostly perceptible in the different modes of crystal packing.

Full text of DOI:10.1016/j.poly.2010.03.001

Polyhedron 29 (2010) 1854–1862
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Complexes of 4- and 5-bromo derivatives of 2-(hydroxymethyl)pyridine with
copper(II) and cobalt(II) salts. Synthesis and X-ray crystal structures
*
Michael Hubrich, Max Peukert, Wilhelm Seichter, Edwin Weber
Institut für Organische Chemie, Technische Universität Bergakademie Freiberg, Leipziger Strasse 29, 09596 Freiberg/Sachs., Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Four copper(II) complexes (1–4) and a cobalt(II) complex (5) derived from 4-bromo-2-(hydroxy-
methyl)pyridine (L1) or 5-bromo-2-hydroxymethyl)pyridine (L2) with Cu(NO3)2Á3H2O, CuCl2Á2H2O and
CoCl2Á6H2O have been synthesized and their respective crystal structures studied. They show specific
influences owing to the different kind of metal cations and counter anions, the hydration as well as
the different position of the bromine substitution on both the coordination of the complex unit and
the network structure of the crystal lattice. The Cu(II) complexes of L1 are five-coordinate [Cu(L1)2NO3]-
NO3ÁH2O (1) and [Cu(L1)2Cl]ClÁH2O (2) species with distorted quadratic pyramidal and trigonal bipyrami-
dal coordination geometries of the N2O3 and N2O2Cl donor atoms around the Cu(II), respectively. The
Cu(II) complexes of L2 are six-coordinate [Cu(L2)2(NO3)2] (3) and [Cu(L2)2Cl(H2O)]ClÁH2O (4) species with
distorted octahedral coordination geometries of the N4O2 and N2O3Cl donor atoms. A distorted octahedral
coordination geometry of the N2O2Cl2 donor atoms is also found in the complex unit [Co(L2)2Cl2] of the
Co(II) complex 5 but showing the oxygen atoms of the chelating ligand as well as the chloride ions in a
cis-position. Depending on the complex, water molecules and chloride anions are shown to act as stabi-
lizing components of the crystal structure. The comparative structural investigation includes also known
structures of the bromine-free ligand analogue 2-(hydroxymethyl)pyridine, illustrating the basic implica-
tion of the bromine substitution, mostly perceptible in the different modes of crystal packing.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 5 February 2010
Accepted 2 March 2010
Available online 6 March 2010
Keywords:
Copper
Cobalt
Pyridine derivatives
Coordination compounds
Crystal structures
1. Introduction
called ‘supramolecular synthons’ created by two approaching bro-
mine atoms, showing a linear or perpendicular contact mode [8]. It
Pyridines have been extensively investigated as ligands in coor-
dination compounds [1]. Recently, referring to the important topic
of coordination polymers [2], pyridine fragments that bind to dif-
ferent metal centers are becoming increasingly significant in the
construction of organic linker molecules [3]. Here, pyridine units
frequently benefit from additional donor atoms adjacent to the
pyridine nitrogen, thus showing the ability to act as chelating
ligands [4]. Typical examples of pyridine moieties modified in
this way are 2-(hydroxymethyl)pyridine and 2,6-bis(hydroxy-
methyl)pyridine having the qualifications to bi- and tridentate
complex formation. This has been realized for the two before-men-
tioned (hydroxymethyl)pyridines with corresponding Cu(II), Co(II)
and Ni(II) complexes that depend in their detailed crystal struc-
tures on the given anion, the presence of coordinated water mole-
cules or suggest a proton control of framework complexity [5,6].
Another parameter of influence could be a substituent having
proven a bearing on the structure of crystalline compounds such
as a bromine substituent [7]. Actually, there are two different so-
is also likely that a bromine substitution should exercise an influ-
ence on the stacking behaviour of pyridine units [9]. Hence,
Á Á Á
p
p
the question arises regarding the concrete consequences of a bro-
mine substitution on the crystal structure of corresponding metal
ion complexes, e.g. concerning the plain chelating ligand 2-
(hydroxymethyl)pyridine if bromine substituents are present in
4- or 5-position of the pyridine ring such as in the ligands L1(4-
brhmp) and L2(5-brhmp), respectively (Scheme 1). This has given
rise to the preparation of complexes of L1 and L2 with different me-
tal salts, including Cu(NO3)2Á3H2O, CuCl2Á2H2O and CoCl2Á6H2O,
namely the complexes 1–5 (Scheme 1). We report on the synthesis
of these complexes and discuss their crystal structures in regard of
the posed question.
2. Experimental part
2.1. General
Melting points (uncorrected) were taken on a heating stage
microscope PHMK (Rapido, Dresden). Elemental analyses were
determined with a Heraeus CHN rapid analyzer.
* Corresponding author. Fax: +49 (0)3731 393170.
E-mail address: edwin.weber@chemie.tu-freiberg.de (E. Weber).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.03.001

M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
1855
for C12H14Br2Cl2CuN2O3 (FW = 528.51): C, 27.27; H, 2.67; N, 5.30.
Found: C, 27.03; H, 2.73; N, 5.41%.
Br
N
2.2.3. Bis[5-bromo-2-(hydroxymethyl-kO)-pyridine-kN]dinitrato
copper(II) (3)
L2 and copper(II) nitrate trihydrate were used. Blue crystals
(42%) were collected; m.p. 168 °C (dec.). Anal. Calc. for
C12H12Br2CuN4O8 (FW = 563.60): C, 25.57; H, 2.15; N, 9.94. Found:
C, 25.64; H, 2.16; N, 9.75%.
OH
L1 (4-brhmp)
2.2.4. Bis[5-bromo-2-(hydroxymethyl-kO)-pyridine-kN]aquochloro
copper(II) chloride mono-hydrate (4)
1: [Cu(L1)2NO3]NO3 · H2O
2: [Cu(L1)2Cl]Cl · H2O
L2 and copper(II) chloride dihydrate were used. Green crystals
(25%) were collected; m.p. 210 °C (dec.). Anal. Calc. for
C12H16Br2Cl2CuN2O4 (FW = 546.53): C, 26.37; H, 2.95; N, 5.13.
Found: C, 26.58; H, 3.08; N, 5.06%.
Br
2.2.5. Bis[5-bromo-2-(hydroxymethyl-kO)-pyridine-kN]dichloro
cobalt(II) (5)
L2 and cobalt(II) chloride hexahydrate were used. Violet crystals
(35%) were collected; m.p. 182 °C (dec.). Anal. Calc. for
C12H12Br2Cl2CoN2O2 (FW = 505.88): C, 28.49; H, 2.39; N, 5.54.
Found: C, 28.72; H, 2.20; N, 5.35%.
N
OH
L2 (5-brhmp)
2.3. X-ray crystallography
3: [Cu(L2)2(NO3)2]
4: [Cu(L2)2Cl(H2O)]Cl · H2O
5: [Co(L1)2Cl2]
Crystals of the complexes 1–5 suitable for structure analysis
were grown by slow evaporation of solvent from ethanol solutions.
The intensity data were collected on a Kappa APEX II diffrac-
tometer (Bruker-AXS) with graphite monochromated Mo K
a radi-
ation (k = 0.71073 Å) using and sans. Reflections were
u
x
corrected for background, Lorentz and polarization effects. Preli-
minary structure models were derived by application of direct
methods [14] and were refined by full-matrix least squares calcu-
lation based on F2 values for all unique reflections [15]. Empirical
absorption correction based on multi-scans was applied by using
the SADABS program [16]. All non-hydrogen atoms were refined
anisotropically. The positions of the hydrogens could be derived
from the difference Fourier ma. Their O–H bond lengths were re-
strained to a target value of 0.84 Å. All other hydrogen atoms were
included in the models in calculated positions and were refined as
constrained to bonding atoms. Crystal and refinement data for the
complexes 1–5 are listed in Table 1 and the structural parameters
of selected interactions are shown in Tables 2–4.
Scheme 1. Compounds studied in this paper.
4-Bromo-2-(hydroxymethyl)pyridine (L1) [10] was synthesized
from 2-methylpyridine-N-oxide via 4-nitro-2-methylpyridine-N-
oxide [11] and 4-bromo-2-methylpyridine-N-oxide [10,12] follow-
ing the literature procedures. 5-Bromo-2-(hydroxymethyl)pyridine
(L2) was prepared from 2-methylpyridine using the reaction
sequence bromination with aluminium chloride and bromine,
oxidation with m-chloroperbenzoic acid and treatment with triflu-
oroacetic anhydride, as described in the literature [13]. 2-Methyl-
pyridine, 2-methylpyridine-N-oxide as well as the copper(II) and
cobalt(II) salts were obtained from usual suppliers.
2.2. General procedure for the synthesis of the complexes 1–5
3. Results and discussion
The respective bromo-2-(hydroxymethyl)pyridine (0.70 mmol)
and of the corresponding metal salt (0.35 mmol) were dissolved
in ethanol (30–50 cmÀ3). The mixture was refluxed for 1 h. The sol-
vent was removed under reduced pressure and the residue recrys-
tallized from a small portion of ethanol.
3.1. Synthesis of L1 and the complexes 1–5
The ligand L1 was synthesized using the known route starting
from 2-methylpyridine-N-oxide which was subjected to a nitration
with nitric acid to give 4-nitro-2-methylpyridine-N-oxide [11],
followed by a bromo substitution of the nitro group using acetyl
bromide [10,12]. Reaction of the resulting 4-bromo-2-methylpyri-
dine-N-oxide with trifluoroacetic acid yielded 4-bromo-2-
(hydroxymethyl)pyridine (L1) [10]. The ligand L2 was also prepared
according to a known reaction sequence [13]. This involves
bromination of 2-methylpyridine with aluminium chloride and
bromine, oxidation of the intermediate bromo compound with
m-chloroperbenzoic acid and subsequent reaction with trifluoro-
acetic anhydride to yield L2. All the complexes 1–5 were prepared
by co-crystallization of 2:1 (ligand:salt) stoichiometric amounts of
L1 or L2 and the respective metal salts [Cu(NO3)2Á3H2O, CuCl2Á2H2O
and CoCl2Á6H2O] from ethanolic solution. Crystals of the complexes
2.2.1. Bis[4-bromo-2-(hydroxymethyl-kO)-pyridine-kN]nitrato
copper(II) nitrate mono-hydrate (1)
L1 and copper(II) nitrate trihydrate were used. Turquoise-green
crystals (28%) were collected; m.p. 148 °C (dec.). Anal. Calc. for
C12H14Br2CuN4O9 (FW = 581.62): C, 24.78; H, 2.42; N, 9.63. Found:
C, 24.56; H, 2.37; N, 9.31%.
2.2.2. Bis[4-bromo-2-(hydroxymethyl-kO)-pyridine-kN]chloro
copper(II) chloride mono-hydrate (2)
L1 and copper(II) chloride dihydrate were used. Turquoise-
green crystals (36%) were collected; m.p. 155 °C (dec.). Anal. Calc.

1856
M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
Table 1
Crystallographic and structure refinement data of the compounds studied.
Compound
1
2
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C12H12N4O8Br2CuÁH2O C12H12N2O2Cl2Br2CuÁH2O C12H12N4O8Br2Cu
C12H12N2O2Cl2Br2CuÁH2O C12H12N4O8Br2Co
581.63
monoclinic
P21/n
528.50
563.61
monoclinic
C2/c
546.53
monoclinic
P21/n
505.87
orthorhombic
Pbcn
triclinic
ꢀ
P1
8.2297(3)
13.9737(5)
16.2520(5)
90.0
101.686(1)
90.0
7.9333(3)
11.1019(3)
11.2436(4)
102.670(2)
110.354(2)
101.724(1)
862.52(5)
2
18.2941(4)
7.7840(2)
13.6194(3)
90.0
116.488(1)
90.0
7.5903(2)
33.2879(7)
7.7807(2)
90.0
14.4933(3)
9.5820(2)
12.2473(2)
90.0
b (Å)
c (Å)
a
(°)
b (°)
115.953(1)
90.0
90.0
90.0
c
(°)
V (Å3)
1830.23(11)
4
1735.84(7)
4
1767.66(7)
4
1700.84(6)
4
Z
F(0 0 0)
1140
2.111
5.616
514
2.035
6.219
1100
2.157
5.914
1068
2.054
6.077
980
1.975
6.024
Dcalc (mg mÀ3
)
l
(mmÀ1
)
Data collection
T (K)
153(2)
30311
1.9–32.1
90(2)
42611
2.0–38.6
153(2)
31904
2.5–29.4
173(2)
33668
1.2–35.2
173(2)
19746
2.8–28.4
Number of collected reflections
Within the h-limit (°)
Index ranges h, k,
l
À12/12, À20/20, À24/ À13/13, À19/19, À19/19 À25/25, À10/10, À18/ À12/12, À53/51, À12/12 À19/19, À11/12, À16/
24
18
16
Number of unique reflections
6406
0.0339
9745
0.1035
2407
0.0262
7859
0.0321
2131
0.0346
Rint
Refinement calculations: full-matrix least-squares on all F2 values
Weighting expression wa
[r
2(Fo2) + (0.0707P)2
[r
2(Fo2) + (0.0793P)2
[r
2(Fo2) + (0.0523P)2
[r
2(Fo2) + (0.0429P)2
[r
2(Fo2) + (0.0471P)2
+ 0.7713P)À1
269
+ 0.0000P)À1
215
+ 0.1047P)À1
128
+ 0.8704P)À1
229
+ 1.7805P)À1
100
Number of refined parameters
Number of F values used [I > 2
Final R-indices
r
(I)]
5072
6571
2125
6184
1740
R =
wR on F2
S = Goodness-of-fit on F2
Final
max Dqmin (e ÅÀ3
R
|
D
F|/
R
|Fo|)
0.0287
0.1042
0.961
0.74/À0.57
0.0562
0.1453
1.005
1.99/À1.68
0.0225
0.0746
1.049
0.52/À0.61
0.0385
0.0964
1.061
0.88/À0.89
0.0289
0.0845
0.972
0.80/À0.91
D
q
/
)
a
P = (Fo2 + 2Fc2)/3.
Table 2
Relevant bond distances (Å), bond angles (°) and torsion angles (°) of 1–5.
Compound
1
2
3
4
5
Bond lengths
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–O(4)
Cu(1)–Cl(1)
Cu(1)–O(1W)
Co(1)–N(1)
Co(1)–O(1)
Co(1)–Cl(1)
1.990(2)
1.976(2)
2.197(2)
1.978(2)
1.983(2)
1.972(2)
1.971(2)
2.179(2)
2.029(2)
1.970(2)
2.004(2)
2.010(2)
2.020(2)
2.337(2)
1.975(1)
2.441(2)
2.258(1)
2.620(1)
1.994(2)
2.125(2)
2.146(2)
2.406(1)
Bond angles
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(1)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–O(1)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(1)
O(1)–Cu(1)–Cl(1)
O(2)–Cu(1)–Cl(1)
N(1)–Cu(1)–O(4)
N(2)–Cu(1)–O(4)
O(1)–Cu(1)–O(4)
O(2)–Cu(1)–O(4)
O(1)–Cu(1)–O(1W)
N(1)–Co(1)–N(1)
O(1)–Co(1)–O(1)
Cl(1)–Co(1)–Cl(1)
N(1)–Co(1)–Cl(1)
O(1)–Co(1)–Cl(1)
170.0(1)
101.5(1)
169.2(1)
95.5(1)
166.2(1)
180.0
180.0
94.2(1)
95.6(1)
109.7(1)
154.8(1)
174.7(1)
170.1(1)
93.4(1)
96.5(1)
90.4(1)
168.1(1)
87.7(1)
91.6(2)
155.6(1)
86.1(1)
93.9(4)
97.5(6)
174.3(1)
Torsion angles
N(1)–C(5)–C(6)–O(1)
N(2)–C(11)–C(12)–O(2)
À20.8(3)
À8.7(3)
À15.5(4)
À14.3(3)
11.4(2)
21.8(3)
24.5(4)
À6.5(2)

M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
1857
Table 3
Structural parameters (distances (Å), angles (°)) of hydrogen bond type interactions.
Atoms involved
Symmetry
Distance
Angle
D–HÁ Á ÁA
DÁ Á ÁA
HÁ Á ÁA
D–HÁ Á ÁA
1
O(1)–H(1A)Á Á ÁO(6)
O(1)–H(1A)Á Á ÁO(7)
O(2)–H(2A)Á Á ÁO(1W)
O(1W)–H(1W1)Á Á ÁO(6)
O(1W)–H(1W)Á Á ÁO(8)
O(1W)–H(1W2)Á Á ÁO(3)
C(1)–H(1)Á Á ÁO(7)
C(1)–H(1)Á Á ÁO(8)
C(6)–H(6B)Á Á ÁO(3)
C(12)–H(12A)Á Á ÁO(5)
1.5 À x, À0.5 + y, 0.5 À z
1.5 À x, À0.5 + y, 0.5 À z
0.5 À x, À0.5 + y, 0.5 À z
x, y, z
3.278(3)
2.659(3)
2.501(3)
2.740(3)
2.967(3)
2.951(3)
3.282(3)
3.211(3)
3.280(3)
3.299(3)
2.60
1.83
1.65
1.91
2.41
2.15
2.37
2.57
2.57
2.48
138
165
177
174
125
164
161
125
128
140
x, y, z
À1 + x, y, z
x, y, z
x, y, z
1.5 À x, À0.5 + y, 0.5 À z
À0.5 + x, 0.5 À y, À0.5 + z
2
O(1)–H(1A)Á Á ÁO(1W)
O(2)–H(2A)Á Á ÁCl(2)
O(1W)–H(1W)Á Á ÁCl(2)
O(1W)–H(2W)Á Á ÁCl(2)
C(8)–H(8)Á Á ÁO(2)
C(10)–H(10)Á Á ÁCl(2)
À1 + x, y, z
2.628(4)
2.970(2)
3.182(3)
3.184(3)
3.314(4)
3.563(3)
1.80
2.14
2.40
2.38
2.45
2.69
171
170
156
162
151
154
1 À x, Ày, 1 À z
1 À x, Ày, 1 À z
x, y, À1 + z
À1 + x, y, z
1 À x, Ày, 1 À z
3
O(1)–H(1A)Á Á ÁO(3)
C(3)–H(3)Á Á ÁO(3)
C(3)–H(3)Á Á ÁO(4)
C(4)–H(4)Á Á ÁO(2)
C(4)–H(4)Á Á ÁBr(1)
C(6)–H(6B)Á Á ÁO(3)
1.5 À x, 0.5 + y, 0.5 À z
2 À x, y, 0.5 À z
2 À x, y, 0.5 À z
0.5 + x, 0.5 + y, z
x, 1 + y, z
2.550(2)
3.422(3)
3.329(3)
3.198(2)
3.981(1)
3.374(2)
1.73
2.59
2.47
2.52
3.05
2.55
167
146
150
128
166
141
x, 2 À y, À0.5 + z
4
O(1)–H(1A)Á Á ÁCl(2)
O(2)–H(2A)Á Á ÁCl(1)
O(1W)–H(2WA)Á Á ÁCl(2)
O(1W)–H(1WA)Á Á ÁO(2W)
O(2W)–H(1WB)Á Á ÁCl(2)
O(2W)–H(2WB)Á Á ÁCl(1)
C(6)–H(6A)Á Á ÁBr(1)
C(10)–H(10)Á Á ÁBr(2)
1 + x, y, 1 + z
À1 + x, y, z
1 + x, y, z
x, y, z
x, y, z
À1 + x, y, À1 + z
2 À x, 1 À y, 2 À z
À1 + x, y, z
2.933(2)
3.173(3)
3.096(2)
2.638(3)
3.225(3)
3.149(2)
3.843(3)
3.792(3)
2.11
2.39
2.27
1.80
2.44
2.32
2.98
2.87
163
156
168
171
158
173
146
164
5
O(1)–H(1A)Á Á ÁCl(1)
x, 2 À y, À0.5 + z
3.066(2)
3.587(2)
2.23
2.87
174
135
C(4)–H(4)Á Á ÁCl(1)
À0.5 À x, À0.5 + y, z
3.2.1. Crystal structure of the complex [Cu(L1)2NO3]NO3ÁH2O (1)
The complex 1 crystallizes in the monoclinic space group P21/n.
The asymmetric entity of the unit cell consists of a [Cu(L1)2NO3]+
cation, one nitrate anion and one molecule of water being associated
among each other by weak C–HÁ Á ÁO [17] and conventional O–HÁ Á ÁO
hydrogen bonds [18] (Fig. 1). In the complex unit, the Cu(II) centre
adopts a slightly distorted quadratic-pyramidal coordination envi-
ronment of the structure N2O3 with the hydroxy oxygen O(1) occu-
pying the axial position of the polyhedron. The Cu–Oequatorial and
C–N distances are 1.978(2) [1.983(2)] and 1.976(2) [1.990(2)] Å,
respectively, whereas the Cu–Oaxial distance is 2.197(2) Å. The
O–Cu–O coordination angles range between 93.4(1)° and 101.5(1)°.
The aromatic rings of the complex are arranged at a dihedral angle
of 72.6(1)°. The complex ligands are twisted in different degrees
around their hydroxymethyl substituents, which is evident from
torsion angles of 20.8(3)° and 8.7(3)° for the atomic sequences
N(1)–C(5)–C(6)–O(1) and N(2)–C(11)–C(12)–O(2). According to
the presence of a two-fold symmetry element, helical supramolecu-
lar strands in which [Cu(L1)2NO3]+ cations, water molecules and the
non-complexed nitrate ions are interlinked by O–HÁ Á ÁO hydrogen
bonds [d(HÁ Á ÁO) 1.65–2.60 Å] represent the basic structure elements
of 1. As illustrated in Fig. 2, interstrand association is accomplished
by the formation of weak C–HÁ Á ÁO hydrogen bonds [d(HÁ Á ÁO)
2.37–2.57 Å] and BrÁ Á ÁO contacts [7a,19]. With reference to this lat-
ter contacts, the separation of 3.02 Å between Br(2) and the nitrate
oxygen O(3) is significantly shorter than the sum of the van der
Waals radii of 3.45 Å [20].
Table 4
Structural parameters (distances (Å), angles (°)) of bromine involved non-covalent
interactions.
Atoms involved
Symmetry
Distance
Angle
C–BrÁ Á ÁA (A = O,Cl)
1
CÁ Á ÁA
BrÁ Á ÁA C–BrÁ Á ÁA
C(9)–Br(2)Á Á ÁO(3)
0.5 + x, 0.5 À y, À0.5 + z
4.825(3) 3.02
160
2
C(3)–Br(1)Á Á ÁCl(1)
C(9)–Br(2)Á Á ÁCl(2)
4
C(2)–Br(1)Á Á ÁCl(1)
C(8)–Br(2)Á Á ÁCl(2)
5
1 À x, 1 À y, Àz
À1 + x, y, z
5.353(3) 3.57
5.303(3) 3.52
157
157
2 À x, 1 À y, 1 À z
5.156(2) 3.26
5.149(2) 3.30
177
166
1.5 + x, 0.5 À y, 0.5 + z
C(2)–Br(1)Á Á ÁCl(1)
0.5 À x, 2.5 À y, À0.5 + z 5.417(2) 3.56
167
1–5 suitable for X-ray crystallography were grown by slow evapo-
ration of solvent from solutions in ethanol.
3.2. X-ray structural study
Crystal and refinement data for the complexes 1–5 are listed in
Table 1 and the structural parameters of selected interactions are
shown in Tables 2–4. Perspective views of the structures including
the numbering schemes of atoms are shown in Figs. 1, 3, 5, 7 and 9.
Packing illustrations are presented in Figs. 2, 4, 6, 8 and 10.

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M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
Fig. 1. ORTEP drawing of the complex [Cu(L1)2NO3]NO3ÁH2O) (1). Thermal ellipsoids are drawn at 40% probability level. Broken lines represent hydrogen bonds.
ꢀ
in the space group P1. The asymmetric cell unit contains one com-
plex cation of the structure [Cu(L1)2Cl]+, one separated chloride an-
ion and one molecule of water (Fig. 3). The coordination
environment of the copper ion shows a distorted trigonal–pyrami-
dal geometry of the donor structure N2O2Cl with the nitrogens
occupying the axial positions of the polyhedron. Taking into ac-
count experimental error, the Cu–N distances are identical
[1.971(2), 1.972(2) Å] while the Cu–O bond lengths differ
[2.029(2) and 2.179(2) Å]. The N(1)–Cu(1)–N(2) bond angle is
169.2(1)°, the angles O(1)–Cu(1)–Cl(1) and O(2)–Cu(1)–Cl(1) are
109.7(1)° and 154.8(1)°, respectively. The dihedral angle between
the mean planes of the pyridine rings is 66.6(1)°. A view of the
crystal structure of 2 along the a-axis (Fig. 4) reveals a complex
three-dimensional pattern of non-covalent intermolecular bond-
ing. The chloride ion excluded from complexation acts as an accep-
tor for the formation of O–HÁ Á ÁCl hydrogen bond type interactions
[21] with the hydroxy hydrogen H(2A) of the complex and the
hydrogens H(1W) and H(2W) of two neighbouring water mole-
cules. The hydroxy hydrogen of the second complex ligand takes
part in hydrogen bonding to the water molecule [O(1)–
H(1A)Á Á ÁO(1W) 1.80 Å, 171°]. The distances between the chloride
and the bromine atoms of adjacent complex units [3.52, 3.57 Å]
are less than the sum of their van der Waals radii (3.60 Å) [20] thus
indicating weak interhalogen contacts [7].
3.2.3. Crystal structure of the complex [Cu(L2)2(NO3)2] (3)
The complex 3 crystallizes in the monoclinic space group C2/c.
The asymmetric cell unit contains one half of the complex moiety,
i.e. the Cu(II) cation resides on a symmetry center (Fig. 5). The
Cu(L2)2 part of the complex shows approximate planarity. Only the
five-membered chelate ring adopts a slight envelope conformation
with the oxygen atom located at a distance of 0.08 Å from the
least-squares plane of the ligand molecule. The metal cation forms
a distorted octahedral coordination environment with the nitrate
ions occupying the axial positions. The bond lengths of Cu–N and
Cu–O in the basal plane are 1.970(2) and 1.975(1) Å, while the axial
Cu–O distances are 2.441(2) Å. A view of the crystal structure along
the crystallographic b-axis reveals a layered arrangement of metal
complexes (Fig. 6). Within the layer plane, the molecules are cross-
linked via C–HÁ Á ÁOnitrate [C(3)–H(3)Á Á ÁO(4) 2.47 Å, 150°] and
C–HÁ Á ÁBr hydrogen bonds [C(4)–H(4)Á Á ÁBr(1) 3.05 Å, 166°] [17],
while O–HÁ Á ÁOnitrate [O(1)–H(1A)Á Á ÁO(3) 1.73 Å, 167°] and aromatic
Fig. 2. Packing structure of the complex [Cu(L1)2NO3]NO3ÁH2O (1) viewed down the
a-axis. Metal ions are specified as shaded circles; O, N and Br atoms are represented
as dotted, hatched and cross-hatched circles, respectively. Dashed lines represent
hydrogen bonds.
3.2.2. Crystal structure of the complex [Cu(L1)2Cl]ClÁH2O (2)
Regarding the complex composition, a situation similar to 1 is
found in the solid phase structure of 2 which, however, crystallizes

M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
1859
Fig. 3. ORTEP drawing of the complex [Cu(L1)2Cl]ClÁH2O) (2). Thermal ellipsoids are drawn at 40% probability level. Broken lines represent hydrogen bonds.
Fig. 4. Packing structure of the complex [Cu(L1)2Cl]ClÁH2O (2) viewed down the a-axis. Metal ions are specified as dark shaded circles; O, N, Br and Cl atoms are represented as
dotted, hatched, cross-hatched and light shaded circles, respectively. Dashed lines represent hydrogen bonds.
p
Á Á Á
p
stacking interactions [22] with a distance of 3.63 Å between
tween the atoms in the equatorial plane range between 78.2(2)°
and 94.7(1)°. The trans-angles are 166.2 [N(1)–Cu(1)–N(2)], 170.1
[O(1W)–Cu(1)–O(1)] and 174.7° [O(2)–Cu(1)–Cl(1)]. The dihedral
angle between the pyridine rings is 58.1°.
the centroids of pyridine rings, being in the ordinary range [9], stabi-
lize the packing structure in direction of the c-axis.
3.2.4. Crystal structure of the complex [Cu(L2)2Cl(H2O)]ClÁH2O (4)
The complex crystallizes in the monoclinic space group P21/n
with the asymmetric cell unit containing one cation of the structure
[Cu(L2)2Cl(H2O)]+, one chloride anion and one molecule of water
(Fig. 7). The Cu(II) ion adopts a distorted octahedral coordination
environment of the structure N2O3Cl with the chloride ion Cl(1)
and the hydroxy oxygen O(2) occupying axial positions. Within the
basal plane, the Cu–N and Cu–O bonds lengths are 2.004(2)
[2.010(2)] and 1.994(2) [2.020(2)] Å, respectively. Due to the Jahn–
Teller effect [23], the axial C–O bond is significantly elongated
[2.337(2) Å], while the Cu–Cl distance is 2.620(1) Å. The angles be-
The crystal structure of the complex is stabilized by a variety of
directed non-covalent interactions. As depicted in Fig. 8, hydrogen
bonds of the O–HÁ Á ÁO and O–HÁ Á ÁCl type [17] involving chloride
ions and water molecules as well as hydroxy hydrogens of the
complex ligands primarily contribute to intermolecular cross-link-
ing (cf. Table 3). In this pattern of hydrogen bonding, the water
molecules and those chloride ions excluded from complexation
form infinite supramolecular strands extending in direction of
the crystallographic c-axis. Moreover, the bromine atoms take part
in the formation of BrÁ Á ÁCl interhalogen contacts [7] and C–HÁ Á ÁBr
hydrogen bond type interactions [17].

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M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
Fig. 7. ORTEP drawing of the complex [Cu(L2)2Cl(H2O)]ClÁH2O (4). Thermal ellip-
soids are drawn at 40% probability level. Dashed lines represent hydrogen bonds.
Cl(1) angles are 155.6(1)° and 174.3(1)°. As shown in Fig. 10, the
intermolecular organization is dominated by hydrogen-bond inter-
actions of the O–HÁ Á ÁCl [d(HÁ Á ÁCl) 2.23] and C–HÁ Á ÁCl type
[d(HÁ Á ÁCl) 2.87] [17] as well as BrÁ Á ÁCl interhalogen contacts [7].
Furthermore, the crystal structure is stabilized along the b-axis
Fig. 5. ORTEP drawing of the complex [Cu(L2)2NO3)2] (3). Thermal ellipsoids are
drawn at 40% probability level.
by
p p stacking [22] with a rather close distance of 3.31 Å be-
Á Á Á
tween pyridine rings of consecutive molecules [9].
4. Structural comparisons and conclusions
The structural characterization of the Cu(II) complexes 1–4 and
the Co(II) complex 5 reveals different coordination geometries
around the respective metal centers. Although crystal growing of
the complexes was carried out under identical conditions using
aqueous ethanol as solvent, only three of the complexes (1, 2, 4)
crystallize as mono-hydrates. The crystal structures of 1 and 2
are composed of cationic complex species of the structure
[Cu(L1)2X]+ (X = Cl, NO3) with five-coordinated Cu(II) cations. The
nature of the anion in 1 and 2 affects the molecular structure to
a less extent but considerably influences the packing behaviour
of the molecules, which can be attributed to different acceptor
potentials of the anions. The fact that one of the counter ions is ex-
cluded from complexation but acts as highly coordinating crystal
component obviously requires the presence of a water molecule
to stabilize the crystal structure. As contrasted with 1 and 2, in 4
one of the water molecules participates in complexation resulting
in an octahedral coordination environment of the Cu(II) center.
Nevertheless, similar to the above mentioned cases, the presence
of non-coordinated species with strong donor/acceptor capability,
such as a water molecule and chloride anion, appears to be an
essential point of the crystal structure of the compound 4. They
give rise to the formation of hydrogen bonded zigzag strands in
which the chloride ions allow association of complex units via
O–HÁ Á ÁCl hydrogen bonds [17]. Contrary to all expectations, the
complex 3 obtained from Cu(NO3)2 trihydrate and L2 does not con-
tain crystal water. Consequently, the anions participate in complex
formation leading to an octahedral coordination sphere of the me-
tal center with a nearly coplanar arrangement of trans-positioned
ligand molecules. Also in the complex 5, the Co(II) cation adopts
an octahedral coordination environment. However, the configura-
tion of which is basically different from the complex 4, in that li-
gand oxygens and the chloride ions are arranged in a cis-position
around the metal cation.
Fig. 6. Packing excerpt of the complex [Cu(L2)2(NO3)2] (3) viewed down the b-axis.
For specification of the atom circles see Fig. 2. Dashed lines represent hydrogen
bonds, dashed double lines arene–arene interactions.
3.2.5. Crystal structure of the complex [Co(L2)2Cl2] (5)
Crystals of the complex 5 show the orthorhombic space group
Pbcn with one half of the complex moiety in the asymmetric cell
unit (Fig. 9). The Co(II) centre adopts a distorted octahedral coordi-
nation environment of the donor set N2O2Cl2 with trans-positioned
nitrogen and cis-positioned oxygen atoms of the chelating ligand
and the two chloride ions also being in a cis-position. The Co–N,
Co–O and Co–Cl bond lengths are 2.125(2), 2.146(2) and
2.406(1) Å, respectively. The N(1)–Co(1)–N(1) and O(1)–Co(1)–
A comparative inspection comprising crystal structures of the
present and the corresponding previously studied bromine
free analogous ligand compound 2-(hydroxymethyl)pyridine [6]

M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
1861
Fig. 8. Packing structure of the complex [Cu(L2)2Cl(H2O)]ClÁH2O (4) viewed down the b-axis. For specification of the atom circles see Fig. 4. Dashed lines represent hydrogen
bonds.
ogen substituent, however, cause intensive interstrand association
leading to the loss of layer formation. Also the copper(II) chloride
complex 2 and its bromine free analogue reveal similarities regard-
ing their molecular structures. The crystal of the latter complex is
constructed of supramolecular O–HÁ Á ÁCl bonded strands with an
alternating order of [Cu(L)2Cl]+ complex units and chloride ions,
but shows only poor interstrand association. Introduction of the
bromine substituent in 4-position of the pyridine ring as well as
the presence of crystal water basically influences the packing
behaviour of the molecules. In the structure of 2, the chloride anion
excluded from complexation provides a strong acceptor site for
hydrogen bonding to the crystal water and the complex cations
resulting in
interactions.
a three-dimensional network of non-covalent
Apart from the coordination behaviour of the respective anion
and the presence of crystal water, which both control the molecu-
lar assembly in the crystal structures, the coordination mode of the
bromine in the present structures is different depending on its po-
sition at the pyridine ring. The linear geometry copper(II) com-
plexes 1 and 2 obviously favour formation of BrÁ Á Áanion contacts,
while in complexes 3 and 4 these substituents take part in the for-
mation of C–H(arene)Á Á ÁBr hydrogen bonds. But, surprisingly en-
ough, a pure BrÁ Á ÁBr contact, relevant to the crystal structures, is
not found in either of the complexes. Another point of interest re-
Fig. 9. ORTEP drawing of the complex [Cu(L2)2Cl2] (5). Thermal ellipsoids are drawn
at 40% probability level.
reveals that the introduction of a bromine substituent in 4- or 5-
position of the pyridine ring has a fundamental influence on the
crystal packing. In all structures, the bromine atom participates
to a more or less degree in the molecular association by forming
interhalogen contacts BrÁ Á ÁCl [7], BrÁ Á ÁO interactions [7a] or C–
HÁ Á ÁBr hydrogen bonds [17]. Being more specific, the reported
structure of the bromine free copper(II) nitrate complex [6] is com-
posed of O–HÁ Á ÁO bonded molecular strands which in turn are
lates to the
mine free complexes
p
Á Á Á
p
stacking property of the ligands. While in the bro-
Á Á Á stacking between the pyridine units is a
p
p
dominating parameter [6], corresponding with an established
trend [9], this particular kind of interaction is restraint in the pres-
ent complexes, possibly caused by the steric hindrance of the bulky
bromine substituent. Thus, introduction of a bromine substituent
into the pyridine ring of the ligand 2-(hydroxymethyl)pyridine
does not really affect the coordination behaviour to the present
metal cations Cu(II) and Co(II) but the more the packing structure
of the respective complex units. This offers a promising possibility
to use the bromine substitution as a controlling function in the
associated among each other by C–HÁ Á ÁOnitrate [13] and
pÁ Á Áp arene
[22] interactions, resulting in a layered packing arrangement of the
complex molecules. In this structure, interlayer interactions are re-
duced to weak van der Waals forces. In a similar fashion the crystal
structure of the corresponding bromine substituted analogous
complex 3 consists of supramolecular strands being stabilized by
O–HÁ Á ÁOnitrate and C–HÁ Á ÁOnitrate hydrogen bonds. The enhanced
acceptor behaviour of nitrate ions as well as the coordinating hal-

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M. Hubrich et al. / Polyhedron 29 (2010) 1854–1862
Fig. 10. Packing structure of the complex [Co(L2)2Cl2] (5) viewed down the b-axis. For specification of the atom circles see Fig. 4. Dashed lines represent hydrogen bonds.
[7] (a) P. Metrangolo, F. Meyer, T. Pilati, G. Resnati, G. Terraneo, Angew. Chem. 120
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5. Supplementary data
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We gratefully acknowledge financial support by the Deutsche
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