A new trigonal-bipyramidal [CuII(pytBuN3)Cl 2] complex: Synthesis, structure and ligand substitution behaviour

Article abstract of DOI:10.1002/ejic.200900302

The synthesis, structure and ligand substitution mechanism of a new five-coordinate copper(II) complex with a sterically constrained pytBuN ₃ chelate ligand [pytBuN₃ = 2,6-bis(3,5di-tert- butylphenyliminomethyl)pyridine] are reported

Full text of DOI:10.1002/ejic.200900302

FULL PAPER
DOI: 10.1002/ejic.200900302
A New Trigonal-Bipyramidal [CuII(pytBuN3)Cl2] Complex:
Synthesis, Structure and Ligand Substitution Behaviour
Shaban Y. Shaban,[a] Frank W. Heinemann,[a] and Rudi van Eldik*[a]
Keywords: Copper / Chelates / Copper(II) complexes / Kinetics / UV/Vis spectroscopy / Biphasic reaction
296
The synthesis, structure and ligand substitution mechanism
of a new five-coordinate copper(II) complex with a sterically
constrained pytBuN3 chelate ligand [pytBuN3 = 2,6-bis(3,5-
di-tert-butylphenyliminomethyl)pyridine] are reported. In
the crystal structure of the complex the [Cu(pytBuN3)Cl2]
k2
= 1.62Ϯ0.06 M
–1 s–1. Substitution of chloride by TU is
characterized by the activation parameters: ∆H# = 42Ϯ2 and
58Ϯ2 kJmol–1, ∆S# = –46Ϯ7 and –47Ϯ6 J K–1 mol–1, and ∆V#
= –6.5Ϯ0.2 and –5.3Ϯ0.7 cm3 mol–1, for the first and second
substitution reactions, respectively. It is concluded from the
chromophore possesses a distorted trigonal-bipyramidal co- activation parameters that both reactions follow an associa-
ordination geometry. The kinetics and mechanism of chloride tive interchange (Ia) mechanism. When the substitution reac-
substitution by thiourea (TU) and N,N,NЈ,NЈ-tetramethylthio- tion was carried out with the sterically hindered nucleophile
urea (TMTU) were studied in detail as a function of nucleo-
phile concentration, temperature and pressure in methanol
as solvent. The kinetics showed that the substitution reaction
of [Cu(pytBuN3)Cl2] is a biphasic process that involves the
subsequent displacement of both chloride ligands. The sub-
TMTU, the rate constant for the displacement of the first
chloride, k2
slower than for the reaction with TU, which further supports
the Ia nature of the substitution mechanism.
= 6.9Ϯ0.5 M
–1 s–1, was more than 133 times
296
stitution of the first chloride by TU, k1296 = 918Ϯ30
M
–1 s–1, is
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
570 times faster than the substitution of the second chloride,
chelate that will cause steric crowding on copper(II) and
may help to induce unusual coordination geometries and
hence interesting mechanistic and redox properties (Fig-
ure 1).[4]
Introduction
Studies on the coordination chemistry of copper(II) com-
plexes with chelates incorporating pyridine and amine do-
nors have been of significant interest in recent years. This
is partly because of their relevance to histidine coordinated
copper proteins such as blue copper proteins, hemocyanin,
tyrosinase and cytochrome c oxidase.[1] Most of the func-
tional and structural models for metalloproteins have been
prepared by varying the substituents of the ligands in order
to match the spectral properties of the metalloproteins.[2] In
continuation of our earlier work in the area of copper(II)
chemistry,[3] we describe here the synthesis and crystal
structure of the [CuII(pytBuN3)Cl2] complex, where
pytBuN3 = 2,6-bis(3,5-di-tert-butylphenyliminomethyl)pyr-
idine. The strategies behind the selection of this chelate are:
(i) it will force monomeric trigonal bipyramidal or square-
pyramidal stereochemistry on copper(II), (ii) the donor
properties of the selected N,N,N-chelate are comparable to
that of 2,2Ј,2ЈЈ-terpyridine (1), the most widely studied che-
late of this class of ligand, which has the trimethine struc-
tural unit and it is well established that it normally behaves
as planar, tridentate ligand towards various kinetically la-
bile metal ions such as CuII, ZnII and CdII halides, and (iii)
the incorporation of two bulky tert-butyl moieties on the
Figure 1. Schematic structure and abbreviations for the ligands.
We found that the synthesized [CuII(pytBuN3)Cl2] com-
plex has a distorted trigonal-bipyramidal structure. The
present study also focuses on the mechanism of chloride
substitution by thiourea (TU) and its derivative N,N,NЈ,NЈ-
tetramethylthiourea (TMTU) as entering nucleophiles. As
far as we know only a limited number of studies on the
ligand substitution mechanism of five-coordinate cop-
per(II) complexes have been reported.[3,4a,4b,5] Thiourea and
N,N,NЈ,NЈ-tetramethylthiourea were selected as entering
nucleophiles because of their high nucleophilicity that will
prevent the back reaction with chloride. Furthermore, they
were selected as a neutral entering ligand such that the over-
all reaction is accompanied by charge creation, and the for-
[a] Inorganic Chemistry, Department of Chemistry and Pharmacy,
University of Erlangen-Nürnberg,
Egerlandstr. 1, 91058 Erlangen, Germany
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S. Y. Shaban, F. W. Heinemann, R. van Eldik
FULL PAPER
mation of the transition state may involve changes in dipole
moment which will affect the activation parameters, espe-
cially the entropy and volume of activation. The selected
[CuII(pytBuN3)Cl2] complex contains bis-imine and pyr-
idine in the back-bone of the ligand and is expected to be
a strong π-acceptor ligand that will increase the electrophi-
licity of the metal centre.[6] We report here a detailed mech-
anistic study of the ligand-substitution reactions in meth-
anol as a function of the entering nucleophile concentra-
tion, temperature and pressure.
Results and Discussion
Synthesis and Structure of [CuII(pytBuN3)Cl2] Complex (3)
Figure 2. Thermal ellipsoid plot of the molecular structure of [CuII-
(pytBuN3)Cl2] (3) showing the atom numbering scheme (50% prob-
ability level).
The ligand 2 was prepared in high yield from the conden-
sation of two equivalents of 3,5-di-tert-butylaniline with
one equivalent of pyridine-2,6-dicarbaldehyde (Scheme 1).
Relevant crystallographic data are given in Table 3,
The ligand was characterized by elemental analyses, and 1H whereas selected bond lengths and bond angles are col-
and 13C NMR spectra confirmed its identity. Complex 3 lected in Table 1. Complex 3 crystallizes in the monoclinic
was synthesized in good yield by treating CuCl2 with 2 in space group C2/c. The complex molecule is situated on a
methanol at room temperature. Complex 3 was charac- crystallographic twofold rotation axis running along the
terized by elemental analyses and IR spectroscopy. The ele- atoms Cu1, N1 and C3 (Wyckoff position 4e), thus exhibit-
mental analyses confirmed that the isolated complex was in ing molecular C2 symmetry. The molecular structure of
accord with the formula [CuII(pytBuN3)Cl2]. The IR spec- the complex molecule involves a five-coordinate [CuII-
trum of ligand 2 shows that the C=N stretching frequency (pytBuN3)Cl2] chromophore constituted by two imine (N2,
appears at 1626 cm–1. In complex 3, the C=N stretching N2A) and one pyridine (N1) nitrogen of the tridentate li-
shifts toward lower frequency at 1612 cm–1 and was greatly gand and two chloride ions. The Cl1–Cu1–Cl1A angle is
reduced in intensity, which points to the interaction be- 119.22(2)° and due to the twofold symmetry the two N1–
tween the imino nitrogens and the copper ion. In addition, Cu–Cl angles are identical and amount to 120.39(2)°. In
the molecular and crystal structure of complex 3 was deter- addition, in 3 the two imino C=N bonds have distinctive
mined by a single-crystal X-ray diffraction study. A thermal double-bond character, with C4=N2 distances of
ellipsoid plot of the molecular structure of complex 3 in- 1.287(2) Å. The Cu–N distances are shorter than the values
cluding the atom-numbering scheme is shown in Figure 2.
reported in the literature for closely related complexes,[7] in-
Scheme 1. Synthesis of the new pytBuN3 ligand (2) and its copper(II) complex (3). a) MeOH, 23 °C, 6 h; b) CuCl2, MeOH, 23 °C, 5 h.
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A New Trigonal-Bipyramidal [CuII(pytBuN3)Cl2] Complex
dicating that the ligand is more tightly bound to the metal
centre. The two phenyl rings are almost in the same plane
of the CuN3 backbone as in this arrangement the steric
pressure between the bulky tert-butyl substituents and the
chloride ions is reduced. The question arises as to whether
the coordination polyhedron around the copper centre can
be described as a square-pyramid or a trigonal bipyramid.
Further information can be obtained by determining the
structural index τ,[8] which represents the relative amount
of trigonality [square pyramid, τ = 0; trigonal bipyramid, τ
= 1; τ = (β – α)/60; where α and β being the two largest
angles around the central atom]. The obtained value of 0.60
for the structural index τ reveals that the coordination ge-
ometry around copper(II) is best described as distorted tri-
gonal-bipyramidal with an axis defined by the nitrogens N2
and N2A, and this axis is slightly bent towards the N1 atom
resulting in an N2–Cu1–N2A angle of 156.26(7)°. The crys-
tal packing of 3 is characterized by the formation of corru-
gated sheets perpendicular to the crystallographic ac plane.
Between these sheets weak intermolecular C–H···Cl interac-
tions with (C–)H···Cl distances in the range of 2.71–2.84 Å
(Figure 3) are observed.
Figure 3. Formation of corrugated sheets perpendicular to the crys-
tallographic ac plane in the crystal packing of complex 3 (weak
intermolecular C–H···Cl interactions are indicated by dotted lines).
Electrochemical Studies on Complex 3
The electrochemical behaviour of complex 3 was studied
to provide more information on the nature of the complex
in solution. Cyclic voltammograms were recorded in MeOH
solution vs. Ag/Ag+ reference electrode. Complex 3 exhibits
a one-electron reduction at 0.443 V and the corresponding
oxidation occurs at 0.527 V, which can be assigned to the
[CuII(pytBuN3)Cl2]0/+ species. The complex [CuII(pytBuN3)-
Cl2] that undergoes reduction is completely regenerated fol-
lowing electrochemical oxidation (the ratio of the peak cur-
rents ipa/ipc is about 1.0), suggesting a chemically reversible
CuII/CuI one-electron transfer process. However, the value
of the limiting peak-to-peak separation (∆Ep, 84 mV),
which lies within the normal values for a reversible one-
Table 1. Selected bond lengths [Å] and angles [°] for [Cu(pytBuN3)-
Cl2] (3).
Bond lengths [Å]
Bond angles [°]
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(2A)
Cu(1)–Cl(1)
Cu(1)–Cl(1A)
C(4)–N(2)
1.954(2)
2.123(2)
2.123(2)
2.3049(4)
2.3049(4)
1.287(2)
1.287(2)
N(2)–Cu(1)–N(1)
N(2A)–Cu(1)–N(1)
N(2)–Cu(1)–N(2A)
N(1)–Cu(1)–Cl(1)
N(1)–Cu(1)–Cl(1A)
N(2)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(1A)
N(2A)–Cu(1)–Cl(1)
78.13(3)
78.13(3)
156.26(7)
120.39(2)
120.39(2)
94.22(4)
97.73(3)
97.73(3)
C(4A)–N(2A)
N(2A)–Cu(1)–Cl(1A) 94.22(4)
Cl(1)–Cu(1)–Cl(1A) 119.22(2)
Figure 4. left) Cyclic voltammograms of [CuII(pytBuN3)Cl2] (Ϫ before and - - - after addition of 0.1  LiCL); right) Cyclic voltammog-
rams of [CuII(pytBuN3)Cl2] as a function of scan rate. Measurements were performed on a Pt working electrode vs. a non-aqueous Ag/
Ag+ reference electrode; add 544 mV [300 mV, Ag/Ag+ to SCE; +244 mV, SCE to SHE] to convert to standard hydrogen electrode (SHE);
Fc/Fc+ couple in CH3CN, E1/2, 0.414 V (CV); (supporting electrolyte 10–3 , 10–1 NBu4PF6, scan rate = 50 mV/s, potentials given in V,
complex concentration, 0.2 m).
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S. Y. Shaban, F. W. Heinemann, R. van Eldik
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electron redox process, suggests that the heterogeneous elec-
tron-transfer process in these complexes is easily revers-
ible[9] (∆Ep, 60 mV for a reversible one-electron redox pro-
cess) and not accompanied by stereochemical reorganiza-
tion.[10] An additional one-electron reduction occurs at
0.763 V and the corresponding oxidation at 0.652 V. The
ipa/ipc value is far from unity, with a relatively higher ∆Ep
values (111 mV), suggesting that the complex undergoes
structural reorganization on electron transfer. This peak
may arises from the oxidation of a four-coordinate CuI spe-
cies obtained by loosing one chloride ion. Thus, when the
scan rate is increased from 50 to 1000 mV (Figure 4) the
additional peak grows in height, suggesting that at higher
scan rates the CuI species is preferentially formed immedi-
ately on reducing the trigonal-bipyramidal [CuII(pytBuN3)-
Cl2] species. On addition of Cl– ions (LiCl), the height of
the additional peak decreases accompanied by an increase
in the first peak, illustrating that the four-coordinate CuI
species combines with the Cl– ion to form a pentacoordi-
nate CuI species, which decreases its concentration. Thus,
the redox behaviour of 3 corresponds to the redox of two
related species: four-coordinate [CuII(pytBuN3)Cl]+/2+ spe-
cies, reduced at more positive E1/2 value, and the five-coor-
dinate [CuII(pytBuN3)Cl2]0/+ species, reduced at less posi-
tive E1/2 value (Scheme 2). Similar schemes have been dem-
onstrated to exist for CuII/I complex systems of the triden-
tate bis(benzimidazole-2Ј-yl) ligand.[11]
Figure 5. UV/Vis spectral changes recorded for the reaction of
complex 3 (1ϫ10–4 ) with thiourea (0.125 ) in methanol at
296 K: (a) spectrum before the reaction; (b) spectrum obtained sev-
eral milliseconds after mixing of the reactants in the stopped-flow
apparatus; (c) spectrum obtained after 50 s.
CuII complex. Throughout the nucleophile concentration
range it was possible to fit the absorbance/time traces to a
two-exponential function by using Equation (1). However,
in the case of TMTU the second reaction was too slow to
obtain accurate rate constants and we only analysed the
first reaction step in more detail (see Figure 6).
A = a1[exp(–kobsd.1t)] + a2[exp(–kobsd.2t)] + A0
(1)
This means that the overall reaction is biphasic, as shown
in Figure 6 for typical kinetic traces. An initial fast reaction
(rate constant kobsd.1) is followed by a much slower one (rate
constant kobsd.2). Rate constant kobsd.1 increases linearly
with TU concentration (Figure 7), which leads to second-
order rate constant k1 = 918Ϯ30 –1 s–1. Rate constant
kobsd.2 also depends linearly on the TU concentration and
the second-order rate constant was found to be k2
=
1.62Ϯ0.06 –1 s–1 (Figure 7). The rate constants demon-
strate that the second substitution is slowed down almost
570 times due to the displacement of one chloride ligand
by thiourea.
Scheme 2. Square scheme that accounts for the observed cVs.
A similar concentration dependence was found for the
first reaction step with TMTU as shown in Figure 8, from
which it follows that k1 = 6.9Ϯ0.5 –1 s–1 at 296 K. The
second substitution step was not studied in more detail for
TMTU as entering ligand as mentioned before. A compari-
son of k1 with that found for TU as entering nucleophile
indicates that the introduction of four methyl substituents
slows down the reaction 133 times at 296 K.
The observed kinetic behaviour can be accounted in
terms of two subsequent substitution reactions (2) and (3)
in which the two chloride ligands are subsequently dis-
placed by either TU or TMTU (Nu) characterized by the
second-order rate constants k1 and k2, respectively.
Kinetic Studies
The reactions of [CuII(pytBuN3)Cl2] (3) with thiourea
and N,N,NЈ,NЈ-tetramethylthiourea can be monitored ki-
netically in the range of ca. 360–440 nm. Solutions were
prepared by dissolving known amounts of the chloro com-
plex in methanol in the presence of 0.002  LiCl in order
to prevent the spontaneous solvolysis reactions. The ligand-
substitution reactions were studied as a function of TU and
TMTU concentration, temperature and pressure in meth-
anol. An example of the UV/Vis spectral changes and a
representative kinetic trace are shown in Figure 5.
[CuII(L)Cl2] + Nu Ǟ [CuII(L)(Cl)(Nu)]
k1
k2
(2)
(3)
Rate constants for the reactions were determined by
using total TU and TMTU concentrations in the range of
0.001–0.125 , i.e. always at least in 10-fold excess over the [CuII(L)(Cl)(Nu)] + Nu Ǟ [CuII(L)(Nu)2]
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A New Trigonal-Bipyramidal [CuII(pytBuN3)Cl2] Complex
Figure 6. Absorbance-time traces at 394 nm for the reaction of complex 3 (1ϫ10–4 ) with 0.125  thiourea (left, inset is the fast reaction
measured by stopped-flow instrument) and N,N,NЈ,NЈ-tetramethylthiourea (right) in methanol at 296 K (solid line obtained by fitting the
data to Equation (1)).
Figure 7. Plot of kobsd vs. thiourea concentration for complex 3 in methanol at 296 K (left: first reaction step; right: second reaction step).
Experimental conditions: [3] = 0.1 m, 0.002  LiCl, wavelength 394 nm.
The activation parameters (∆H϶, ∆S϶ and ∆V϶) were
determined from the temperature and pressure dependence
of k1 and k2 for both substitution reactions of complex 3
in methanol, and are summarized in Table 2. Representative
plots that show typical temperature and pressure depen-
dences are reported in Figures 9, 10, and 11, respectively.
Table 2. Summary of rate and activation parameters for the dis-
placement of coordinated chloride on complex 3 by thiourea (TU)
and N,N,NЈ,NЈ-tetramethylthiourea (TMTU) in methanol.
Activation parameters
TU
TMTU
first reaction second reaction first reaction
k296 [M–1 s–1]
918Ϯ30
42Ϯ2
–46Ϯ7
–6.5Ϯ0.2
1.62 Ϯ0.06
58Ϯ2
–47Ϯ6
6.9Ϯ0.5
52Ϯ2
–58Ϯ8
–
∆H# [kJmol–1]
∆S# [JK–1mol–1]
∆V# [cm3mol–1]
Figure 8. Plot of kobsd vs. [TMTU] for the reaction with complex 3
in methanol at 296 K. Experimental conditions: [3] = 0.1 m,
0.002  LiCl, wavelength 394 nm.
–5.3Ϯ0.7
As seen from the summary of the rate constants in
It follows from this reaction Scheme that kobsd.1 and Table 2, the second substitution reaction with thiourea is
kobsd.2 should depend linearly on the entering nucleophile much slower than the first one, which can be ascribed to
concentration in the absence of a back reaction as shown steric hindrance on the CuII center in complex 3 caused by
in Figures 7 and 8, such that kobsd.1 = k1[Nu] and kobsd.2
=
the displacement of chloride by thiourea. Furthermore, the
k2[Nu].
values of k1 for reaction (2) decrease significantly on going
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S. Y. Shaban, F. W. Heinemann, R. van Eldik
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Figure 9. Temperature dependence of the reactions of complex 3 with thiourea in methanol (left, first reaction step, right: second reaction
step). Experimental conditions: [3] = 0.1 m, [TU] = 0.125 , 0.002  LiCl.
between the five-coordinate CuII complex and the entering
nucleophile. The activation parameters reported in Table 2
also support this suggestion. The values of ∆S# and ∆V#
are all significantly negative and support the operation of
an associative mechanism for both reactions (2) and (3).
The magnitude of the activation volumes are such that it
supports the operation of an associative interchange (Ia)
mechanism rather than a limiting associative (A) mecha-
nism.[12,13] Similar results were reported for water exchange
and ligand-substitution reactions on the five-coordinate
[Cu(tren)H2O]2+ and substituted tren complexes.[3c] In this
case however, only one ligand can be replaced in compari-
son to two ligands on the studied complex 3. For instance
the water exchange reaction on [Cu(tren)H2O]2+ is three or-
ders of magnitude slower than on fully aquated [Cu-
Figure 10. Temperature dependence of the first reaction of complex
(H2O)6]2+ and characterized by ∆S# = –34Ϯ5 J K–1 mol–1
3 with TMTU in methanol. Experimental conditions: [3] = 0.1 m,
and ∆V# = –4.4Ϯ0.2 cm3 mol–1, which is clearly in line with
[TMTU] = 0.125 , 0.002  LiCl.
an associative interchange mechanism. Similarly, the revers-
ible substitution reactions of [Cu(tren)H2O]2+ with pyridine
and substituted pyridines are all characterized by negative
∆S# and ∆V# demonstrating that in all cases the transition
states are more compact than either the reactant or product
states.[3c,14] The magnitude of the activation volumes re-
ported in Table 2 and those for the substitution reactions
on [Cu(tren)H2O]2+ is in line with an Ia substitution mecha-
nism for these trigonal bipyramidal complexes.
from TU to TMTU as a result of the steric hindrance
caused by the four methyl substituents. Both these trends
suggest that ligand substitution on complex 3 follows an
associatively activated mechanism that is controlled by
bond formation between the entering nucleophile and the
CuII center. Thus the transition state of the process is ex-
pected to have six-coordinate character on forming a bond
Figure 11. Pressure dependence of the reaction of complex 3 with thiourea in methanol (left: first reaction step, right: second reaction
step). Experimental conditions: [3] = 0.1 m, [TU] = 0.015 , 296 K, 0.002  LiCl.
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A New Trigonal-Bipyramidal [CuII(pytBuN3)Cl2] Complex
(m, 2 H, Harom.), 7.19 (m, 2 H, Harom.), 1.37 {s, 36 H, [C(CH3)3]}
ppm. 13C NMR (100 MHz, CDCl3, 23 °C): δ = 159.2 (CH=N),
154.8, 151.91, 150.08, 137.32, 123.09, 121.22, 115.62, (Carom.), 35.05
Conclusions
A five-coordinate copper(II) complex with a sterically
constrained pytBuN3 chelate ligand [pytBuN3 = 2,6-bis(3,5- [C(CH ) ], 31.48 [C(CH ) ] ppm. IR (KBr): ν, cm–1 3062 (s, C-
˜
3 3
3 3
H
arom.), 2962 (s, C-Haliph), 1626 (m, C=N), 1595, 1585, 1523 (s,
di-tert-butylphenyliminomethyl)pyridine] has been synthe-
sized and characterized by X-ray crystal structure determi-
nation. In the crystal structure of the complex the [Cu-
(pytBuN3)Cl2] chromophore possesses a distorted trigonal-
bipyramidal coordination geometry in which the two imine
C=Carom.). MS (FD+, CHCl3): m/z = 510 [M]+.
Synthesis of the Complex [Cu(tBupyN3)Cl2] (3): CuCl2 (0.403 g,
0.3 mmol) was added to a suspension of 2,6-bis(3,5-di-tert-butyl-
phenyliminomethyl)pyridine (2) (1.53 g, 0.3 mmol) in methanol and
nitrogen atoms occupy the two axial positions. The kinetics the mixture was stirred for 5 h. The resultant orange-brown prod-
uct was filtered off, washed with cold methanol (5 mL) and dried
in vacuo to give 3 in 75% yield. C35H47Cl2CuN3 (644,22): calcd.
for C 65.25, H 7.35, N 6.52; found C 64.98, H 7.42, N 6.65. IR
and mechanism of chloride substitution by thiourea (TU)
and N,N,NЈ,NЈ-tetramethylthiourea (TMTU) were studied
in detail as a function of entering nucleophile concentra-
tion, temperature and pressure in methanol as solvent. The
kinetics showed that the substitution reaction of [Cu-
(KBr): ν, cm–1 3087 (s, C-Harom.), 2958 (s, C-Haliph), 1612 (m,
˜
C=N). MS (FD+, CH3OH): m/z = 610 [M – Cl]+.
(pytBuN3)Cl2] is a biphasic process that involves the subse- Instrumentation and Measurements: Spectra were recorded with the
following instruments: IR (KBr discs, solvent bands were compen-
sated): Mattson Infinity instrument (60 AR) at 4 cm–1 resolution
in the 400–4000 cm–1 range; NMR: Jeol-JNM-GX 270, EX 270,
and Lambda LA 400 with the protio-solvent signal used as an in-
ternal reference. Mass spectra: Jeol MSTATION 700 spectrometer;
elemental analyses: Carlo–Erba EA 1106 or 1108 analyzer. Cy-
clovoltammetric (CV) measurements were performed in a one-com-
partement three-electrode cell using a gold working electrode (Met-
rohm) with a geometrical surface of 0.7 cm2 connected to a silver
wire pseudo-reference electrode and a platinum wire serving as
counter electrode (Metrohm). Measurements were recorded with
an Autolab PGSTAT 30 unit at room temperature. The working
electrode surface was cleaned using 0.05 µm alumina sonicated and
washed with water every time before use. The working volume of
10 mL was deaerated by passing a stream of high purity N2 through
the solution for 15 min prior to the measurements and then main-
taining an inert atmosphere of N2 over the solution during the
measurements. All CVs. were recorded for the reaction mixture
with a sweep rate of 50 mVs–1 at 25 °C. Potentials were measured
in 0.1  TBAP electrolyte solution and are reported vs. an Ag/AgCl
electrode. Kinetic investigations of the substitution of the dichloro
complex by thiourea or tetramethylthiourea were performed either
in tandem cuvettes with a path length of 0.88 cm, thermally equili-
brated at 23Ϯ0.1 °C before mixing, using a Varian Cary 1G spec-
trophotometer, or on an Applied Photophysics SX 18MV stopped-
flow instrument (also thermostatted at 23.0Ϯ0.1 °C) with an op-
tical pathlength of 1 cm at 394 nm. For experiments at elevated
pressure (1–130 MPa), a laboratory-made high-pressure stopped-
flow instrument was used.[17] The temperature of the instruments
was controlled with an accuracy of Ϯ0.1 °C. Thiourea and
N,N,NЈ,NЈ-tetramethylthiourea were selected as entering nucleo-
phile since their high nucleophilicity prevents the back reaction
with chloride. LiCl solutions (0.002 ) were used to avoid sponta-
neous solvolysis of the chloro complexes. The ligand-substitution
reactions were studied under pseudo-first-order conditions by using
at least a ten-fold excess of thiourea or tetramethylthiourea. All
listed rate constants represent an average value of at least three
kinetic runs under each experimental condition.
quent displacement of both chloride ligands. The substitu-
tion of the first chloride by TU is 570 times faster than the
substitution of the second chloride. The activation param-
eters of both reactions support an associative interchange
(Ia) substitution mechanism. When the substitution reac-
tion was carried out with the sterically hindered nucleophile
TMTU, the rate constant for the displacement of the first
chloride was more than 133 times slower than for the reac-
tion with TU, which further supports the Ia nature of the
substitution mechanism.
Experimental Section
General: All chemicals used were of analytical reagent grade and
of the highest purity commercially available. Copper(II) chloride
dihydrate (Aldrich) was used without further purification. Pyr-
idine-2,6-dicarbaldehyde was synthesized as described in the litera-
ture.[15]
Synthesis of 3,5-di-tert-Butylaniline: 3,5-Di-tert-butylaniline was
prepared in a similar way as described in the literature.[16] At 45 °C,
NaN3 (1.84 g, 28.2 mmol) was added slowly during 1 h to a suspen-
sion of 3,5-di-tert-butylbenzoic acid (1) (6.92 g, 30 mmol) in a mix-
ture of concentrated H2SO4 (20 mL) and CHCl3 (20 mL). The mix-
ture was further stirred for another 5 h at 45 °C. CHCl3 was re-
moved under vacuo and H2O (100 mL) was added (Caution!) to
the residue at 0 °C. The resultant solid was isolated by filtration,
washed with H2O (30 mL) to give 3,5-di-tert-butylanilinium sulfate
as a colorless solid. This solid was resuspended in MeOH (20 mL),
neutralized with KOH (10%) and the resulting solid was isolated
by filtration, washed with H2O (100 mL) and dried in vacuo. Yield
1
4.86 g (80%). H NMR (269.6 MHz, CD2Cl2, 23 °C): δ = 1.28 (s,
18 H, 2 tBu), 3.61 (br., 2 H, NH2), 6.53 (d, 1 H, Ar-H), 6.81 (m, 1
H, Ar-H) ppm. IR (KBr): ν = 3380 (w, N–H). MS (FD+, CH Cl ):
˜
2
2
m/z = 205 [M]+. C14H23N (205.34): calcd. C 81.89, H 11.29, N 6.82;
found C 82.79, H 11.97, N 6.31.
Synthesis of 2,6-Bis(3,5-di-tert-butylphenyliminomethyl)pyridine (2):
Pyridine-2,6-dicarbaldehyde (0.135 g, 1.0 mmol) and 3,5-di-tert-bu-
tylaniline (0.410 g, 2.0 mmol) were stirred in methanol (25 mL) for
6 h in the course of which a yellow solid was formed. This solid
was filtered off, washed with cold methanol and dried in vacuo to
give 2·CH3OH in 96% yield. C36H51N3O (541.82): calcd. C 79.80,
H 9.49, N 7.76; found C 79.76, H 9.00, N 7.98. 1H NMR
(400 MHz, CDCl3, 23 °C): δ = 8.76 (s, 2 H, HC=N), 8.31 (d, J =
Data Collection and Structure Refinement: Orange needle-shaped
single crystals of 3 were grown from a saturated CH3OH solution
at room temperature in the course of three days. A suitable single
crystal was mounted on top of a glass capillary using protective
perfluoropolyalkyl ether oil. Intensity data were collected on a
Bruker–Nonius Kappa CCD diffractometer using Mo-Kα radiation
(λ = 0.71073 Å, graphite monochromator). An empirical absorp-
7.4 Hz, 2 H, Hβ pyridine), 7.95 (t, J = 7.5 Hz, 1 H, Hγ pyridine), 7.37 tion correction based on multiple scans using SADABS[18] was per-
Eur. J. Inorg. Chem. 2009, 3111–3118
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3117

S. Y. Shaban, F. W. Heinemann, R. van Eldik
FULL PAPER
Hugh Powell, A. E. Merbach, I. Fabian, S. Schindler, R.
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formed. The structure was solved by direct methods and refined
on F2 using full-matrix least-squares procedures (SHELXTL NT
6.12).[19] Hydrogen atoms were geometrically positioned and al-
lowed to ride on the corresponding carrier atoms with their iso-
tropic displacement parameter fixed at 1.2 or 1.5 times Ueq of the
proceeding C atom. Table 3 summarizes selected crystallographic
data, data collection and structure refinement details.
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Table 3. Crystallographic data and structure refinement for com-
plex [Cu(pytBuN3)Cl2] (3).
Empirical formula
Formula weight
Crystal size [mm]
Crystal system
Space group
a [Å]
C35H47Cl2CuN3
644.20
0.38ϫ0.06ϫ0.05
monoclinic
C2/c
18.050(2)
18.172(2)
b [Å]
c [Å]
α [°]
11.0018(6)
90
β [°]
110.896(5)
90
3371.3(4)
γ [°]
V [Å3]
Z
4
µ [mm–1]
0.834
1.269
1364
Dcalcd. [gcm–3]
F(000)
Reflections collected
Reflections independent
Reflections observed [IϾ2σ(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
∆ρmax/min
54802
4356
3622
4356/0/193
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1.047
R1 = 0.0306, wR2 = 0.0662
R1 = 0.0440, wR2 = 0.0712
0.438/–0.396
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CCDC-724398 contains the supplementary crystallographic data
for complex 3. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Acknowledgments
The authors gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft (DFG) and the Ägyptische Stu-
dienmission.
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Received: April 1, 2009
Published Online: June 17, 2009
3118
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 3111–3118