Isomerism in copper(II) chloride complexes of bis(2-pyridylmethyl)amine and N-substituted derivatives: Synthesis and X-ray structural characterisation

Article abstract of DOI:10.1016/j.poly.2006.12.005

Copper(II) chloride complexes of tridentate N-alkylated bis(2-pyridylmethyl)amine ligands (R-bpa) form coordination oligomers with either terminal or bridging chloride. Two structural types are identified by X-ray diffraction: a mono μ-chloro bridged species and a di-μ-chloro bridged species, both containing five-coordinate copper(II). In each case, the τ parameter indicates only small deviations from regular square-pyramidal coordination geometry. The product structure is dependent on the presence or absence of non-coordinating anions. In the presence of ClO₄^- or PF₆^-, cationic dinuclear complexes are formed, driven by the formation of an ionic lattice. In the absence of non-coordinating anions, neutral mononuclear [CuCl₂(R-bpa)] complexes are formed. Chloride concentration in methanol solution exerts less influence on the reaction outcome, despite differing chloride stoichiometries in the isolated products. In the presence of the coordinating acetate anion, cationic products are formed in which the terminal chloride ligands are completely or partially substituted by acetate.

Full text of DOI:10.1016/j.poly.2006.12.005

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Polyhedron 26 (2007) 1649–1657
www.elsevier.com/locate/poly
Isomerism in copper(II) chloride complexes of
bis(2-pyridylmethyl)amine and N-substituted derivatives:
Synthesis and X-ray structural characterisation
*
´
Anne Nielsen, Sune Veltze, Andrew D. Bond, Christine J. McKenzie
University of Southern Denmark, Department of Physics and Chemistry, Campusvej 55, 5230 Odense M, Denmark
Received 11 September 2006; accepted 1 December 2006
Available online 13 December 2006
Abstract
Copper(II) chloride complexes of tridentate N-alkylated bis(2-pyridylmethyl)amine ligands (R-bpa) form coordination oligomers with
either terminal or bridging chloride. Two structural types are identified by X-ray diffraction: a mono l-chloro bridged species and a di-l-
chloro bridged species, both containing five-coordinate copper(II). In each case, the s parameter indicates only small deviations from reg-
ular square-pyramidal cÀoordination geometry. The product structure is dependent on the presence or absence of non-coordinating anions.
In the presence of ClO4 or PF6À, cationic dinuclear complexes are formed, driven by the formation of an ionic lattice. In the absence of
non-coordinating anions, neutral mononuclear [CuCl2(R-bpa)] complexes are formed. Chloride concentration in methanol solution exerts
less influence on the reaction outcome, despite differing chloride stoichiometries in the isolated products. In the presence of the coordinat-
ing acetate anion, cationic products are formed in which the terminal chloride ligands are completely or partially substituted by acetate.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Copper(II) complexes; Coordination oligomers; Bis(2-pyridylmethyl)amine; Isomerism; X-ray crystal structures
1. Introduction
This paper describes isomerism in a series of copper(II)
chloride complexes of the N3-tridentate ligand bis(2-pyr-
idylmethyl)amine (bpa) and its N-substituted derivatives
bis(2-pyridylmethyl)methylamine (mebpa) and bis(2-pyr-
idylmethyl)aniline (phbpa). Depending on reaction condi-
tions, the combination of copper(II) chloride and these
N3 ligands leads either to neutral mononuclear dichloro
complexes (denoted A in Scheme 1) or cationic chloro-
bridged dinuclear species (denoted B and C in Scheme 1).
Some degree of control can be exerted on the reaction out-
come by variation of the copper(II) chloride concentration,
the presence or absence of non-coordinating or coordinat-
ing counteranions, and reaction time. The series of com-
pounds expands on the previously characterised neutral
species CuCl2(bpa) [13] and several phbpa copper chloride
complexes reported by Ugozzoli et al. [14]. The latter
authors described structural characterisation of arrange-
ments A and B, and proposed a mononuclear cation D
(Scheme 1) for an isolated complex ‘‘[CuCl(phbpa)](PF6)’’
on the basis of elemental analysis. In the presence of the
In copper(II) complexes with organic ligands, the metal
centre can often exhibit a range of different coordination
numbers and geometries [1]. This ‘‘plasticity’’ is important,
for example, in proteins, where copper(II) centres can lie in
distorted coordination environments imposed by the pro-
tein structure, leading to metastable entatic states [2].
Small-molecule copper(II) complexes are of interest as
potential biomimetic models of such entatic states [3–5].
They also receive considerable attention in the literature
on account of their tendency to exhibit static or dynamic
structural disorder associated with Jahn–Teller effects [6–
8]. When combined with the ability of halide anions to
adopt terminal or bridging coordination modes, structural
isomerism can often be exhibited by copper(II) halide com-
plexes with organic ligands [9–12].
*
Corresponding author. Tel.: +4565502518.
E-mail address: chk@chem.sdu.dk (C.J. McKenzie).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.12.005

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A. Nielsen et al. / Polyhedron 26 (2007) 1649–1657
2.1.3. [Cu2Cl3(phbpa)2]2[Cu2Cl2(phbpa)](PF6)4 Æ
2CH3OH (2b)
Method 1. CuCl2 Æ 2H2O (0.0532 g, 0.31 mmol) in 1 ml
methanol and NH4PF6 (0.1019 g, 0.63 mmol) in 0.25 ml
methanol were added to phbpa (0.0856 g, 0.31 mmol) in
2.25 ml methanol. After 5–10 min, dark green needles of
2b were deposited. Yield: 81%.
Method 2. NH4PF6 (0.0735 g, 0.45 mmol) in 0.5 ml
methanol was added to 1 (0.0925 g, 0.23 mmol) in 4 ml
hot methanol. After 5–10 min, dark green crystals of 2b
precipitated. Yield: 41%.
2.1.4. [Cu2Cl3(phbpa)2](PF6) Æ 0.5CH3OH (3)
Turquoise crystals of 3 deposited from the filtrate of 2b
(both Method 1 and Method 2 above) in days to weeks.
The single crystal used to confirm the structure of 3 was
taken from the filtrate of Method 2.
Scheme 1. Structural arrangements observed (A–C) and postulated (D)
[14] for copper(II) complexes of the N3-tridentate ligands bpa (R = H),
mebpa (R = CH3) and phbpa (R = C6H5).
2.1.5. [Cu2Cl2(bpa)2(CH3OH)2](ClO4)2 (4)
This complex was isolated as a minor crystalline by-
product in an attempt to prepare a more complicated cop-
per complex of an amine-functionalized bpa molecule.
Either contaminant bpa was present in the reaction mix-
ture, or it was produced by N-dealkylation. We have not
optimized this synthesis.
non-coordinating phenyl substituent in phbpa, coordina-
tion oligomerisation to give, for example, the dimeric spe-
cies [Cu2Cl2(phbpa)2]2+ (structure C in Scheme 1) seems
more likely for this latter complex. We report here struc-
tural characterisation of the dinuclear cation type C,
together with its bpa and mebpa analogues. For phbpa,
cation C is observed in the solid state in combination with
the dinuclear cation B, suggesting that an equilibrium
exists between the two in solution. In the presence of the
coordinating acetate anion, a further variation of the
cation type B is observed, namely [CuCl2(phbpa)2-
(CH3CO2)]+, in which one terminal chloride is replaced
by acetate.
2.1.6. [Cu2Cl2(mebpa)2](ClO4)2 (5)
Mebpa Æ 2HClO4 (0.0941 g, 0.23 mmol) in 5 ml hot dry
methanol was added to CuCl2 (0.0307 g, 0.23 mmol) in
6 ml dry methanol. Dark blue crystals of 5 precipitated at
low temperature (À20 ꢁC). Further (polycrystalline) prod-
uct was obtained by evaporation of the solvent. Total yield:
43%.
2. Experimental
2.1.7. [Cu2Cl3(phbpa)2 Æ 0.5(CH3OH)]0.85 [Cu2Cl2-
(CH3CO2)(phbpa)2]0.15(PF6) (6) and [Cu2Cl2-
(CH3CO2)(phbpa)2](PF6) Æ 1.5CH3OH (7)
2.1. Synthesis
Crystals of 6 and 7 were obtained from the same syn-
thesis batch: Cu(CH3CO2)2 Æ H2O (0.0131 g, 0.0656 mmol)
in 0.75 ml hot methanol, NH4Cl (0.0035 g, 0.0654 mmol)
and NH4PF6 (0.0107 g, 0.0656 mmol) were added to
phbpa (0.0181 g, 0.0657 mmol) in 0.75 ml methanol.
Medium blue crystals precipitated at low temperature
(À20ꢁC). The two types of crystals could be distinguished
by their habits: crystals of 6 were plates, while those of 7
were blocks.
bis(2-Pyridylmethyl)amine (bpa) was purchased from
Aldrich. Mebpa Æ 2HClO4 [15] and phbpa [16] were pre-
pared as described previously. Perchlorate salts of metal
complexes are potentially explosive and should be handled
with caution in small quantities.
2.1.1. [CuCl2(phbpa)] (1)
CuCl2 Æ 2H2O (0.1865 g, 1.09 mmol) in 3 ml methanol
was added to phbpa (0.3004 g, 1.09 mmol) in 6 ml metha-
nol. After three days, the initial turquoise precipitate redis-
solved and grass-green rods of 1 were deposited. Yield: 76%.
2.2. Single-crystal X-ray diffraction
X-ray diffraction data were collected at 180(2) K using a
Bruker-Nonius X8APEX-II diffractometer (Mo Ka radia-
tion). Selected crystallographic data are summarised in
Table 1. The structures were solved by direct methods and
refined against F2 using SHELXTL [17]. H atoms bound to
C and N atoms were placed geometrically and refined using
a riding model. The H atom of the coordinated methanol
molecule in 4 was also placed geometrically and refined in
2.1.2. [Cu2Cl3(phbpa)2]2[Cu2Cl2(phbpa)](ClO4)4 Æ
2CH3OH (2a)
CuCl2 Æ 2H2O (0.0535 g, 0.31 mmol) in 1 ml methanol
and NaClO4 Æ H2O (0.0882 g, 0.62 mmol) in 0.75 ml meth-
anol were added to phbpa (0.0774 g, 0.28 mmol) in 2 ml
methanol. After 5–10 min, small dark green crystals of 2a
were deposited. Yield: 63%.

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A. Nielsen et al. / Polyhedron 26 (2007) 1649–1657
1651
Table 1
Summary of crystallographic data
2a
2b
4
5
6
7
Formula
C110H110C112
Cu6N18O18
2778.80
-
C110H110Cl8Cu6-
F24N18O2P4
2960.88
C26H34Cl4Cu2-
N6O10
859.47
C26H30Cl4-
Cu2N6O8
823.46
C36.725H36.15Cl2.85
-
C39.5H43Cl2Cu2-
Cu2F6N6O0.725
P
F6N6O3.5
P
Formula weight
Crystal system
Space group
946.25
monoclinic
P21/n
11.5879(2)
14.3103(3)
23.7622(5)
90
101.521(1)
90
1000.75
monoclinic
P21/c
25.500(3)
14.119(2)
24.361(3)
90
91.61(1)
90
triclinic
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
P1
˚
a (A)
12.9998(5)
15.5051(6)
15.6110(7)
92.689(2)
110.585(2)
94.779(2)
2925.6(2)
1
13.1715(6)
15.6230(7)
15.8596(7)
92.125(2)
111.580(2)
94.086(2)
3020.0(2)
1
8.8587(3)
9.2217(4)
10.7369(5)
90.340(2)
95.754(2)
106.160(1)
837.69(6)
1
8.1461(2)
10.0581(3)
10.8909(3)
102.201(1)
101.077(1)
107.780(1)
798.32(4)
1
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
3861.01(13)
4
1.628
8767.3(19)
8
1.516
Z
Dcalc (g cmÀ3
)
1.577
1.628
1.704
1.713
l(Mo Ka) (mmÀ1
Crystal size (mm)
Crystal colour
h Range
Total data
Unique data
Rint
)
1.418
1.362
1.652
0.15 · 0.15 · 0.07
indigo
3.77–23.36
12841
3375
1.725
1.409
1.200
0.10 · 0.10 · 0.06
dark green
3.53–27.00
32221
12504
0.061
0.20 · 0.05 · 0.02
dark green
3.66–25.03
25851
10482
0.072
0.40 · 0.30 · 0.20
dark blue
3.53–32.82
19543
4913
0.020
0.20 · 0.20 · 0.07
pale blue
3.57–28.28
56091
9556
0.059
0.25 · 0.25 · 0.12
pale blue
3.57–25.87
30549
30549a
0.033
R1 [I > 2r(I)]
wR2 (all data)
S
0.064
0.190
1.09
0.053
0.135
0.96
0.029
0.074
1.03
0.025
0.069
1.14
0.046
0.122
1.08
0.099
0.283
0.94
À3
˚
q
max, qmin (e A
)
1.85, À1.24
0.89, À0.50
0.41, À0.32
0.52, À0.56
0.72, À0.60
1.58, À1.38
CCDC No.
287934
287935
287936
287937
287938
287939
a
All data (including Friedel opposites) treated as independent.
the same way. The H atoms of the solvent methanol mole-
[Cu2Cl3(phbpa)2] complexes, and the empirical formula is
therefore best written [Cu2Cl3(phbpa)2 Æ 0.5(CH3OH)]0.85
cules in 2a, 2b, 6 and 7 were placed so as to form a reason-
-
˚
able hydrogen-bond network, at a distance of 0.84 A along
[Cu2Cl2(CH3CO2)(phbpa)2]0.15(PF6). The X-ray analysis
does not distinguish between two discrete crystal domains,
i.e. [Cu2Cl3(phbpa)2 Æ 0.5(CH3OH)](PF6) and [Cu2Cl2-
(CH3CO2)(phbpa)2](PF6), or disorder that is randomly
distributed.
the vectors from the O atom to which they are bound to the
nearest H-bond acceptor. They were then allowed to ride on
the parent O atom during subsequent refinement.
Refinements of structures 2a, 2b, 4 and 5 were straight-
forward, but structures 6 and 7 presented some difficulties.
For 6, refinement of the complex as [Cu2Cl3(phbpa)2], with
the terminal chloride Cl(1) having full site occupancy
resulted in an equivalent isotropic displacement parameter
All crystals examined for 7 were twinned by 2-fold rota-
tion about c . On account of the b angle that differs only
*
minimally from 90ꢁ, the diffraction patterns from the two
domains exhibit partial overlap. The measured diffraction
pattern was indexed (using the program CELL_NOW [18])
on the basis of two domains, each accounting for ca.
8500 unique discrete reflections, with 14815 unique reflec-
tions considered to be overlapped. The frames were inte-
grated using SAINT-PLUS [19] and scaled using TWINABS [19].
The structure was solved with data taken from the major
domain (including overlapped reflections), then refined
against the two-component data via the HKLF 5 format
in SHELXL [17].
2
˚
(Ueq) of 0.0573(3) A for Cl(1), somewhat larger than
2
˚
0.0458(2) A for the other terminal chloride Cl(3). In addi-
À3
˚
tion, peaks in the residual electron density of ca. 1 e A
were observed in positions close to Cl(1), suggestive of an
acetate anion coordinated to Cu(1). Inclusion of this group
and refinement of site occupancies (constrained to sum to
unity) gave 0.84(1) for Cl(1) and 0.16(1) for the acetate
anion. These values were subsequently constrained to be
0.85 and 0.15, respectively. With the chloride/acetate disor-
2
˚
der modelled, Ueq for Cl(1) refined to 0.0440(3) A and the
highest remaining peak in the residual electron density was
3. Results and discussion
0.70 e AÀ3, lying in the vicinity of the PF6À anion. Disorder
˚
of the substituents bound to Cu(1) has some consequences
for the methanol solvent molecule: the intermolecular con-
tact distances suggest that it can only be present when Cl(1)
is present. The O atom of this methanol molecule lies on a
centre of inversion half-way between two Cl(1) atoms.
Thus, there is only one methanol molecule per two
3.1. Syntheses
The syntheses and products are summarised in Scheme
2. Mixtures of copper(II) chloride and phbpa crystallize
from methanol as the neutral mononuclear complex
[CuCl2-(phbpa)] (1), identical to that reported previously

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A. Nielsen et al. / Polyhedron 26 (2007) 1649–1657
[14]. If perchlorate or hexafluorophosphate anions are
present in solution, however, crystallisation of chloro-
bridged dinuclear cationic species is favoured, presumably
driven at least in part by formation of an ionic lattice.
Formation of the dichloro-bridged cation C or the mono-
chloro-bridged cation B is dependent to some degree on
the nature of the group R bound to the amine-N atom.
For phbpa (R = C6H5), the dichloro-bridged cation C is
observed in the double salts, [Cu2Cl3(phbpa)2]2[Cu2Cl2-
(phbpa)](ClO4)4 Æ 2CH3OH (2a) and [Cu2Cl3(phbpa)2]2-
[Cu2Cl2(phbpa)](PF6)4 Æ 2CH3OH (2b), in both cases with
the monochloro-bridged cation B also present in the crys-
tal. The previously characterised B salt [Cu2Cl3(phbpa)2]-
(PF6) Æ 0.5CH3OH (3) [14] can be isolated by fractional
crystallisation after filtration of 2b from the mother
liquor, suggesting that the formation of cation B in pref-
erence to C is favoured by increasing the relative chloride
concentration. Conversely, a larger counteranion excess
pushes the equilibrium back towards the formation of
C. Cation C also appears to be favoured for both bpa
and mebpa, isolated here as the perchlorate salts [Cu2Cl2-
(bpa)2(MeOH)2]-(ClO4)2 (4) and [Cu2Cl2(mebpa)2](ClO4)2
(5). Cation B has not been isolated to date for bpa or
mebpa. When the coordinating acetate anion is present
in the reaction mixture for phbpa, an additional equilib-
rium exists between cation B and the dinuclear cation in
which one terminal chloride is replaced by an acetate
anion. Two types of crystal have been isolated: 6 exhibits
partial replacement of chloride by acetate and 7 exhibits
complete replacement.
Scheme 2. Summary of the synthesis conditions and reaction products.