Multinuclear copper complexes of pyridylmethylamide ligands

Article abstract of DOI:10.1039/b513763b

Copper complexes of a family of pyridylmethylamide ligands HL^Ph, HL^Me3 and HL^Ph3 were synthesized and characterized [HL^Ph = 2-phenyl-N-(2-pyridylmethyl)acetamide; HL^Me3 = 2,2-dimethyl-N-(2-pyridylmeth

Full text of DOI:10.1039/b513763b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Multinuclear copper complexes of pyridylmethylamide ligands
Urmila Pal Chaudhuri, Laura R. Whiteaker, Lei Yang and Robert P. Houser*
Received 28th September 2005, Accepted 15th December 2005
First published as an Advance Article on the web 18th January 2006
DOI: 10.1039/b513763b
Ph
Me3
Ph3
Copper complexes of a family of pyridylmethylamide ligands HL , HL and HL were synthesized
Ph
Me3
and characterized [HL = 2-phenyl-N-(2-pyridylmethyl)acetamide; HL = 2,2-dimethyl-N-
Ph3
(
2-pyridylmethyl)propionamide; HL = 2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide]. The reaction
of copper(II) salts with the HL family and triethylamine in methanol yields copper(II) complexes
Ph
Me3
Ph3
[Cu
4
(L )
4
(OH)
](OTf)
2
](ClO
4
)
2
(1), [Cu
2
(HL
)
2
(OMe)
2
(MeOH)
2
](OTf)
2
(2) and [Cu
2
(HL
)
2
(OMe)
2
-
(MeOH)
2
2
(3). The complexes have different nuclearity owing to varying steric properties of the
ligands used. Complex 1 self-assembles in the presence of excess base to form a tetranuclear complex.
Complexes 2 and 3 are binuclear and are bridged by a pair of methoxide ligands. Steric encumbrance of
the ligands in 2 and 3 prevent cluster formation.
Introduction
(2-pyridylmethyl)acetamide] and their copper complexes [Cu -
4
Ph
Me3
(
L )
4
(OH)
2
](ClO
4
)
2
(1), [Cu
2
(HL
)
2
(OMe)
2
(MeOH)
(3). Complexes 1–3
2
](OTf) (2),
2
Polynuclear copper complexes have been studied extensively due
to their ability to function as new nanometric materials and
single-molecule magnets. In addition, they can model structural
and functional aspects of several multicopper active sites of
metalloproteins. The metal centers are often bridged by hydroxo,
alkoxo or azido groups mediating magnetic exchange and feature
prominently in magnetochemical studies aimed at developing
correlations between molecular structure and magnetic behavior.
Hydroxo- and alkoxo-bridged metal complexes exhibit magnetic
behavior ranging from ferromagnetic to antiferromagnetic de-
Ph3
and [Cu
2
(HL
)
2
(OMe) (MeOH)
2
2
](OTf)
2
represent interesting variations in core structures due to the
structural differences of the ligands.
1
2
Results and discussion
Syntheses and characterization
Our ligand design and synthesis originate from an earlier report
from our laboratory where copper(II) clusters were synthesized
4
pending on the geometry of the Cu
2
(OR)
2
(R = H, alkyl,
using the ligand N-(2-pyridylmethyl)acetamide (HL). HL was
aryl) bridging units. Studies have established that the type and
magnitude of interaction is influenced by the Cu · · · Cu separation,
Cu–O (bridge) distance, the Cu–O–Cu angle (h), the dihedral angle
synthesized via a simple procedure whereby 2-aminomethyl pyri-
5
dine reacts with acetic anhydride. The related ligand 2-methylthio-
SMe
N-(2-pyridylmethyl)acetamide (HL ) was synthesized in our lab
between the planes formed by the two adjoining CuO units in a
via the DCC-mediated coupling of methylthioacetic acid with 2-
3
6
Cu
2
O
2
core, and the Cu(II) center geometry. Interestingly, these
aminomethyl pyridine. In view of the interesting coordination
SMe
polynuclear complexes, generally formed by self-assembly, display
unique structural features that are susceptible to change upon
minor variation of ligand structure and reaction conditions.
Previously, we reported the structural properties of an octa-
chemistry generated by HL and HL with copper, we targeted
other derivatives of HL and obtained a versatile family of pyridyl
amide ligands using either direct coupling methods or activating
agents.
nuclear copper(II) complex [Cu
pyridylmethyl)acetamide] which self-assembles from the reaction
8
L
8
(OH)
4
](ClO
4
)
4
[HL = N-(2-
Ligand synthesis
4
of copper(II) perchlorate, HL and Et
structure of [Cu (OH) ](ClO
3
N in methanol. The
Ph
Me3
Functionalized derivatives of HL, namely HL and HL
Scheme 1), were synthesized using the direct synthetic route
8
L
8
4
4
)
4
consists of two pseudosym-
(
metric tetracopper clusters that are held together by bridging
ligands and hydrogen bonding. Each tetranuclear half of the full
whereby the respective acid chloride was treated with 2-
◦
aminomethyl pyridine in the presence of triethylamine at 0 C. The
cluster contains four copper ions, four ligands, and two l -hydroxo
3
solutions were stirred for 24 h followed by a basic workup
producing the ligands in high yields. The steric bulk and
electron-withdrawing nature of the additional phenyl groups in
ligands. We have probed this chemistry further by synthesizing a
family of ligands derived from the basic N-(2-pyridyl)acetamide
(
HL) backbone, and have observed the formation of different
Ph3
HL deactivates the amine from undergoing condensation when
core structures under varying reaction conditions. Herein, we
Ph
Me3
Ph3
Ph
triphenylacetic acid was reacted with 2-aminomethyl pyridine
as outlined above, so it was necessary to use a coupling agent.
present the syntheses of ligands HL , HL and HL [HL
=
Me3
2
-phenyl-N-(2-pyridylmethyl)acetamide; HL
N-(2-pyridylmethyl)propionamide; HL
= 2,2-dimethyl-
Ph3
Ph3
Therefore, HL was synthesized using triphenylacetic acid in the
= 2,2,2-triphenyl-N-
presence of triethylamine in DMF, and Py-BOP was used as the
coupling reagent to activate the acid to form the corresponding
activated ester (Scheme 1). Trial reactions with DCC/HOBt were
also carried out but did not drive the reaction to completion,
Department of Chemistry and Biochemistry, University of Oklahoma, 620
Parrington Oval, Norman, Oklahoma, 73019, USA. E-mail: houser@
ou.edu; Fax: (405) 325-6111; Tel: (405) 325-3551
1
902 | Dalton Trans., 2006, 1902–1908
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Scheme 1 Synthesis of the ligands. (a) (CH
3
)
3
CCOCl, Et
3
N, THF.
Cl
(
(
b) Ph
d) (CH
3
CCOOH, Et
3
N, Py-BOP, DMF. (c) PhCH
2
COCl, Et N, CH
3
2
2
.
4
6
3
CO) O, pyridine. (e) CH
2
3
SCH
2
COOH, DCC/HOBt, THF.
so Py-BOP was used with maximum efficiency. All ligands were
1
characterized by elemental analysis, FTIR, and H NMR.
Copper complexes
Ph
Me3
Ph3
Scheme 2 Complexes 1–3.
Copper complexes of HL , HL and HL were synthesized in
methanol by reaction of cupric salts with the respective ligands in
the presence of triethylamine. Previously we reported the synthesis
and characterization of octanuclear copper clusters utilizing
Ph−
of these equivalents generate the amidate form of L
remaining two equivalents deprotonate two waters forming the
l -hydroxo ligands. Reactions were also carried out using two
3
and the
4
N-(2-pyridylmethyl)acetamide (HL). The unusual chemistry of
HL lies in the versatility through which it coordinates to the
copper(II) ions. Complexes with HL in the neutral form are simple
mononuclear species where the ligand coordinates in a bidentate
equivalents of triethylamine for every one equivalent of the ligand
used, but in this case 1 did not form and an uncharacterized
insoluble blue copper species resulted.
4
fashion via the pyridyl N atom and the amido O atom. When
Interestingly, when triethylamine was used to deprotonate
Me3
Ph3
the amide nitrogen is deprotonated, the anionic amidate ligand
the amide nitrogen in HL
and HL , it deprotonated
−
forms octanuclear clusters and the anionic amidate ligand (L )
the solvent methanol and formed isostructural methoxide
Me3
coordinates via three atoms in a bridging fashion. The N atoms
from the pyridyl group and the amidate chelate one copper ion
and the amidate O atom coordinates to a second copper ion.
Along with four bridging hydroxo ligands, eight ligands combine
bridged clusters [Cu
2
(HL
)
2
(OMe)
2
(MeOH)
2
](OTf) (2) and
2
Ph3
[Cu
2
(HL
)
2
(OMe)
2
(MeOH)
2
](OTf) (3). When the same reac-
2
tion was performed in acetonitrile with just one equivalent
of triethylamine, the solution turned turbid and deposited an
insoluble precipitate characterized as copper hydroxide. The same
phenomenon was observed when 1.5 or more equivalents of
triethylamine was used. The formation of copper hydroxide was
4
with eight copper ions to form an octanuclear cluster. A similar
Ph
synthetic trend was observed with HL .
In the absence of a base, the pyridylmethylamide ligands remain
in the neutral form and exclusively produce mononuclear copper
−
1
indicated by the presence of the OH band (m = 3438 cm ) and the
absence of ligand peaks in the FTIR of the precipitate. The TLC
of the solute indicated the presence of the ligand. Also, addition
of aqueous ammonia to the blue powder and extraction of the
resulting solution with dichloromethane indicated no ligand as
verified by TLC. This clearly indicates that the copper complexes
4,7
complexes. However, a base is crucial for the generation of
Ph−
the anionic form of the ligand (L ) as observed in the copper
Ph
cluster [Cu
4
(L )
4
(OH)
2
](ClO
4
2
) (1, Scheme 2). Furthermore, a
base is absolutely essential for the formation of the hydroxide
bridging ions by deprotonation of water, presumably from the
Me3
Ph3
Cu(ClO
4
)
2
·6H
2
O starting material. In order to understand the
of HL and HL are not stable in basic conditions unless the
reaction is carried out in methanol.
stoichiometry and the exact role of the base, we carried out the
reaction using different ratios of triethylamine. We observed that
there was no isolable cluster when one equivalent of triethylamine
was used, but an uncharacterized blue oil forms. However, the use
of slight excess of triethylamine resulted in the formation of the
hydroxide bridged cluster, 1. Optimal yields were obtained when
These results imply that the structural differences of the ligands
impact the formation of their copper complexes. Apparently, the
anionic amidate form of this ligand family is stabilized only when
the steric effect of the R group is minimal. Therefore, HL and
Ph
HL with lower steric bulk both get deprotonated and stabilize
Me3
Ph3
1
.5 equivalents of triethylamine was added for every one equivalent
the anionic form of the ligand whereas HL and HL do not.
Aside from crystallographic characterization of the complexes
(vide infra), they were characterized by elemental analysis, UV/Vis
of ligand used. Hence, to synthesize the cluster containing four
ligands, a total of six equivalents of triethylamine was used. Four
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spectroscopy, FTIR, EPR and mass spectrometric techniques. The
electronic spectrum of complex 1 in acetonitrile exhibits a d–d
transition at 649 nm. Complexes 2 and 3 in methanol show similar
electronic spectral patterns exhibiting d–d transitions at 689 and
The electrospray ionization mass spectra (ESI-MS) of 2 and 3
are in agreement with their dinuclear composition. For complex
Me3
2, a peak at 721.1 m/z is assigned to the ion [Cu
2
(HL
2
) -
+
(OMe)
2
(OTf)] where the coordinated methanol molecules and
7
04 nm, respectively, indicative of similar core structures with
one triflate anion are missing from 2. Complex 3 has a peak at
Me3
+
distorted square-pyramidal geometry. The IR spectrum of 1 has
characteristic mOH stretches for the waters and hydroxides centered
at 3432 cm , while 2 and 3 exhibit mOH stretches characteristic of
methanol at 3440 cm . The N–H stretch of the amide group in
the ligands of 2 and 3 occurs at 3347 cm . The perchlorate stretch
for 1 is observed at 1100 cm . Finally, a sharp peak in the spectra
of 2 and 3 is observed at 2807 cm and is assigned to the OMe
anion.
1095.2 m/z that is assigned to the ion [Cu
2
(HL
)
2
(OMe) (OTf)]
2
that likewise has lost the coordinated methanol molecules and one
triflate anion.
−
1
−
1
−
1
−
1
X-Ray crystal structures
−
1
−
Structure of 1. The molecular structure of 1 contains
two slightly different independent tetranuclear motifs of
To probe the solution state magnetic properties of these systems,
H NMR, EPR and magnetic measurements using the Evans
Ph
2+
[Cu
4
(L )
4
(OH) ] , henceforth referred to as cluster A and cluster
2
1
B. A line drawing of the structure is presented in Scheme 2,
thermal ellipsoid representations are shown in Fig. 1, and selected
method were carried out. Complex 1 is structurally similar to
the previously reported cluster complex, [Cu
particular, 1 has four copper ions bridged via two hydroxide ligands
to form a tetracopper cluster (vide infra), and this tetracopper
cluster is isostructural with half of the [Cu
Solutions of [Cu (OH) ](ClO , which were shown to contain
the full [Cu (OH) fragment by electrospray mass spectro-
metry and conductance, are EPR silent. Magnetic susceptibility
measurements for [Cu (OH) ](ClO showed that partially
frustrated interactions among the four Cu ions making up each
half of the cluster gave good agreement with the data, with J
4
8
L
8
(OH)
4
](ClO
4
)
4
. In
bond lengths and angles are listed in Table 1. Each tetranuclear
Ph−
assembly is composed of four L ligands, two l -hydroxo ligands
3
and four copper ions. The l
centers (Cu1A and Cu2A in cluster A; Cu1B and Cu1B in
3
-hydroxo ligands bridge two copper
4
+
8
L
8
(OH)
4
]
cation.
ꢀ
8
L
8
4
)
4 4
cluster B), forming dimeric Cu
2
O
2
units. The l -hydroxo ligands
3
4
+
8
L
8
4
]
also coordinate to two other copper ions in each cluster (Cu3A
ꢀ
and Cu4A in cluster A; Cu 2B and Cu2B in cluster B). The
8
L
8
4
4
)
4
◦
Cu
2
O
2
units deviate from planarity by 12.2 and 13.0 for clusters
A and B, respectively. The Cu–O–Cu angles range from 96.3–
c
=
◦
˚
1
30.0 and the Cu–O distances are in the range of 1.938–2.039 A.
−
1
106 cm and integer-spin ground and excited states, explaining the
These relatively large angles contribute to the Cu · · · Cu distances
4
absence of the EPR signal despite being paramagnetic. Complex
˚
ranging from 2.98–3.58 A. Many examples of hydroxide bridged
1, as expected, exhibits several paramagnetically shifted signals in
copper complexes have been reported, particularly with respect
to the relationship between the degree and type of magnetic
the region from 0–145 ppm. The exchange coupling of unpaired
spins of the copper(II) ions shortens the electronic relaxation
time from 10 s. This results in relatively sharp signals due to
ligand protons spanning a broader spectral window. Signals that
are shifted far downfield may be due to pyridyl protons, signals
between 25–50 ppm could be due to methylene protons and signals
9
exchange.
−
9
8
In cluster A, in addition to the two l -OH bridging groups,
3
Cu1A and Cu2A are coordinated by the chelating pair of N atoms
from the pyridyl and amidate groups of two ligands and a carbonyl
O atom from a separate ligand, making each metal center five-
coordinate. The metal centers Cu1A and Cu2A are about 0.19 and
between 5–20 ppm may be attributed to phenyl ring protons. The
1
H NMR spectrum of [Cu
8
L
8
(OH)
4
](ClO
4
)
4
is also distinguished
4
by relatively sharp signals spread over a broad spectral window.
The observed magnetic moment of 1.64 l
EPR silence are consistent with the magnetic properties of its
structurally related analogue, [Cu (OH) ](ClO
◦
per Cu for 1 and its
Table 1 Selected bond parameters (A
˚
, ) for 1
B
Cluster A
8
L
8
4
1
4
4
) .
Complexes 2 and 3 exhibit broadened H NMR spectra with
signals in a narrow spectral window of 0–10 ppm. Being dinuclear,
complexes 2 and 3 are expected to have signals spanning a broader
spectral window. However both 2 and 3 have broad and featureless
NMR signals, suggesting the formation of mononuclear species in
Cu1A–Cu2A
Cu1A–Cu3A
Cu2A–Cu3A
Cu4A–Cu1A
Cu4A–Cu2A
Cu1A–O2A
Cu2A–O2A
Cu2A–O1A
Cu3A–O1A
Cu4A–O2A
2.9838(7)
3.0792(6)
3.5820(6)
3.4658(6)
3.0587(6)
1.9384(16) Cu3A–N8C
2.0375(16) Cu4A–N1D
1.9664(16) Cu4A–N8D
1.9861(16) Cu1A–O1A–Cu3A
Cu1A–N1A
Cu1A–N8A
Cu2A–N1B
Cu2A–N8B
Cu3A–N1C
1.9884(19)
1.9282(19)
1.986(2)
1.937(2)
2.010(2)
1.967(2)
1.989(2)
1.970(2)
99.80(7)
solution. Complexes 2 and 3 in CH CN and MeOH show axial
3
EPR spectra which are nearly identical and may be attributed
to mononuclear species in solution. Integration of the EPR
signals for 2 and 3 reveals that in fact only a fraction of the
species in solution are paramagnetic. Assuming a dimer–monomer
equilibrium, just 3.2% of 2 and 5.8% of 3 dissociate into monomer,
according to the quantitation of the EPR signals by integration.
The g values clearly indicate typical tetragonal copper geometry
of the paramagnetic species in 2 and 3. The observed magnetic
moments in solution from Evans measurement of 2 and 3 are 1.19
1.9850(16) Cu2A–O1A–Cu3A 129.98(8)
Cu1A–O1A–Cu2A 96.29(7)
Cu1A–O2A–Cu2A 97.23(7)
Cu4A–O2A–Cu1A 124.10(8)
Cu4A–O2A–Cu2A
98.99(7)
Cluster B
Cu1B–Cu2B
Cu1B–Cu1B
Cu2B–O1B
3.0413(6) Cu1B–N1E
2.9856(8) Cu1B–N8E
1.9841(19) Cu2B–N1F
1.992(2)
1.942(2)
1.999(3)
1.994(2)
2.0369(18)
1.9589(18)
ꢀ
Cu1B–O1B–Cu2B
Cu2B–O1B–Cu1B 126.42(9)
Cu1B–O1B–Cu1B
98.28(8)
Cu2B–N8F
Cu1B–O1B
and 1.17 l per Cu, respectively. These values are consistent with
B
ꢀ
an antiferromagnetically coupled dimer with a small fraction of
paramagnetic monomer in solution.
ꢀ
ꢀ
96.67(8)
Cu1B–O1B
1
904 | Dalton Trans., 2006, 1902–1908
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cluster B, all four copper centers are five-coordinate. Cu1B and
ꢀ
˚
Cu1B are distorted square pyramidal (s = 0.15) and are 0.19 A
out of the least squares plane towards the carbonyl oxygens
O10F and O10F . Cu2B and Cu2B are also distorted square
ꢀ
ꢀ
˚
pyramidal (s = 0.15), and are 0.13 A out of the least squares
plane towards the carbonyl oxygens. Within this unit, too, there
are hydrogen bonding interactions between the hydroxides and
perchlorate oxygens.
The structure of 1 is considerably different from our ear-
4
lier report of an octanuclear complex [Cu
Cu (L) (OH) ](ClO
8
(L)
8
(OH)
4
](ClO ) .
4
4
[
8
8
4
4
)
4
is made up of two tetranuclear fragments
which are connected to each other via the carbonyl oxygen of
the HL ligand. Additional structural stability is also provided by
the hydrogen bonding interactions between the carbonyl oxygen
atom and the hydroxide hydrogen atom. These interactions that
hold the pieces of the cluster together are absent in 1, so the
features that are unique to complex 1 are the presence of two
independent, slightly dissimilar tetranuclear units that do not
interact with each other. The closest interaction between cluster
˚
A and B in 1 is 3.55 A, which is the distance between C5B and
ꢀ
C5E . The different structural arrangement of the cluster in 1 is
likely due to the greater steric bulk of HL compared with HL
Ph
in [Cu
between [Cu
ligand chelates and the way the l
8
(L)
8
(OH)
4
](ClO
4
)
4
. There are, however, striking similarities
](ClO and 1 with respect to the way the
-hydroxo binds.
8
(L)
8
(OH)
4
)
4 4
3
Structures of 2 and 3. The crystal structures of 2 and 3
are illustrated in Fig. 2. Bond distances and angles relevant to
the copper coordination sphere of both compounds are listed
in Table 2. The copper complexes are isostructural with slight
variation in bond lengths and angles. The structures of 2 and
3
consist of two Cu(II) ions with each metal bridged by two l -
2
methoxide ligands. Terminal ligation is provided by the N(pyridyl)
Fig. 1 Thermal ellipsoid representation of the X-ray crystal structure
of 1 (cluster A, top; cluster B, bottom). Thermal ellipsoids are drawn at
the 30% level. The phenyl rings of the benzyl groups, perchlorate anions,
solvent molecules, and all hydrogen atoms are omitted for clarity.
Me3
Ph3
and O donors of HL
and HL for 2 and 3, respectively. A
MeOH on each metal ion completes the coordination sphere in
and 3. As noted before, the amide N atoms of the ligands in 2
and 3 are protonated and hence do not coordinate to Cu, making
2
Me3
Ph3
HL and HL bidentate N, O donor ligands.
˚
0.27 A out of the least squares basal plane towards the carbonyl
The Cu(II) centers exhibit distorted square pyramidal coordi-
nation geometries with the solvent MeOH occupying the axial
oxygen atoms O10D and O10C, respectively. Cu1A and Cu2A
are distorted square pyramidal as evidenced from the value of
˚
10
positions [Cu–O(axial) ∼2.25 A]. A pyridyl nitrogen, a carbonyl
the geometric parameter s, which is 0.09 and 0.32, respectively.
Since the ligand chelates to one metal through the pyridyl and
amidate N atoms, it becomes impossible for the carbonyl O atom
to bind to the same metal. Cu3A is four-coordinate and Cu4A
is five-coordinate. Both Cu3A and Cu4A are coordinated by the
chelating pair of N atoms from the pyridyl and amidate groups
of two ligands, a carbonyl O atom from a separate ligand and a
◦
Table 2 Selected bond parameters ( A˚ , ) for 2 and 3
2
3
Cu1. . .Cu1A
Cu1–O2
Cu1–O2A
Cu1–N1
Cu1–O1
2.9916(10)
1.927(3)
1.919(3)
1.992(3)
2.006(3)
2.279(3)
2.9604(3)
1.9213(8)
1.9283(8)
1.9978(9)
1.9982(8)
2.2395(9)
l
3
–OH group. Furthermore, the carbonyl oxygen O10C, which is
coordinated to Cu2A, is also coordinated to Cu4A at a slightly
˚
Cu1–O3
longer Cu–O distance of 2.2781(16) A. Cu4A is distorted square
Cu1–O2–Cu1A
O2–Cu1–O2A
O2–Cu1–N1
O2A–Cu1–N1
O2–Cu1–O1
O2A–Cu1–O1
N1–Cu1–O1
O2–Cu1–O3
O2A–Cu1–O3
N1–Cu1–O3
O1–Cu1–O3
102.15(14)
77.85(14)
97.25(13)
173.09(14)
169.45(13)
93.48(12)
90.85(13)
97.18(12)
93.94(12)
91.50(13)
89.33(12)
100.53(4)
79.47(4)
96.49(4)
175.08(4)
161.66(3)
91.00(3)
92.11(4)
101.37(4)
94.12(3)
89.42(4)
94.86(3)
˚
pyramidal (s = 0.04) and is 0.24 A out of the least squares basal
plane towards the carbonyl oxygen O10C. Within this unit there
are hydrogen bonding interactions between the hydroxides and
perchlorate oxygens.
The copper ions in cluster B are in a slightly different coordi-
nation environment than in cluster A. The center of symmetry in
cluster B causes the carbonyl O atoms to coordinate differently,
but the pyridyl and amidate N atoms, along with the hydroxo
bridges, coordinate almost identically in clusters A and B. In
This journal is © The Royal Society of Chemistry 2006
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1
apparatus (USA). H NMR spectra were measured on a Varian
3
00 MHz spectrometer using solvent as an internal standard
1
1
13
[
H d (CHCl
CHCl
3
) = 7.27 ppm; H d (CH
3
CN) = 1.94 ppm; C d
(
3
) = 77.23 ppm]. Elemental analyses were carried out by
Atlantic Microlabs (Norcross, GA). Mass spectra were recorded
on a Q-TOF quadrupole time-of-flight mass spectrometer (Mi-
cromass, Manchester, U.K.) equipped with a Z-spray electrospray
ionization (ESI) source. Electronic spectra were measured on
a Shimadzu UV2401PC UV-Vis spectrophotometer. IR spectra
were recorded on a Nexus 470 FTIR spectrometer. X-Band EPR
spectra were recorded on a Bruker EMX spectrometer at 100 K as
frozen solutions. Quantitation of signals was performed using the
15
method of Aasa and V a¨ nng a˚ rd with a copper sulfate solution
1.64 mM CuSO , 20% glycerol, 2.0 M NaClO , 0.1 M HCl) as an
integration standard. Solution magnetic moment measurements
(
4
4
16
were measured using the Evans method.
Caution: Perchlorate salts of metal complexes with organic
ligands are potentially explosive. Although no problems were
encountered in this work, only small amounts of material should
be prepared and handled with caution.
Ph
2
-Phenyl-N-(2-pyridylmethyl)acetamide (HL ). The synthesis
Ph
of HL was carried out via a modified procedure based on a
published method. A solution of phenylacetyl chloride (1.55 g,
0 mmol) in anhydrous dichloromethane (10 mL) was added
17
1
Fig. 2 Representation of the X-ray crystal structures of 2 (top) and 3
bottom) as 30% thermal ellipsoids. All hydrogen atoms except for the
amide and methanol hydrogen atoms have been removed for clarity.
to a solution of 2-aminomethyl pyridine (1.08 g, 10 mmol) and
(
triethylamine (1.32 g, 13 mmol) in anhydrous dichloromethane
◦
(
15 mL) at 0 C dropwise over 15 min. The solution became pale
yellow and developed a precipitate. The solution was warmed to
room temperature and after stirring for 24 h, the solvent was
removed under vacuum. The resulting solid was dissolved in
dichloromethane and washed with saturated sodium bicarbonate
solution followed by water. The resulting solution was dried with
anhydrous magnesium sulfate and the solvent was removed under
vacuum. The remaining solid was crystallized from toluene to give
oxygen and two methoxo oxygens form the basal plane. Cu1 and
˚
˚
Cu2 lie 0.09 A out of the least squares basal plane in 2 and 0.16 A
in 3, illustrating that the copper centers adopt distorted square
pyramidal geometry. Estimates of the degree of distortion from
10
ideal square pyramidal geometry obtained by applying s further
indicate geometries close to square pyramidal (s = 0.06 for Cu1
and Cu2 in 2 and 0.23 for the copper centers in 3).
Ph
HL as pale colorless crystals. Yield: 2.20 g (95%). Mp = 105–
For both compounds, the Cu
2
O
2
unit is planar as indicated
◦
1
06 C. Anal. Calcd for C14
H
14
N O: C, 74.31; H, 6.24; N, 12.38.
2
◦
by the Cu–O–Cu–O dihedral angle of 0 . The Cu · · · Cu distance
1
Found: C, 74.13; H, 6.31; N, 12.35. H NMR (300 MHz, CHCl
3
-
˚
in 2 and 3 is 2.99 and 2.96 A, respectively. Similar trends are
d): d 3.60 (d, 2H), 4.48–4.50 (d, 2H), 6.76 (s, 1 H), 7.13–7.33 (m,
11
also observed in hydroxo and alkoxo bridged complexes, like
7
3
1
1
H), 7.59–7.65 (m, 1H), 8.41–8.43 (d, 1H) ppm. FTIR (KBr):
277, 3078, 3032, 2922, 2362, 2337, 1642, 1601, 1588, 1557, 1493,
471, 1455, 1435, 1420, 1344, 1318, 1267, 1199, 1168, 1155, 1098,
12
[
Cu
2
(Me
3
tacn)
2
(l
2
–OH)
2
](ClO
4
)
2
, with planar Cu
˚
2
2
O units where
the Cu · · · Cu separation is 2.971(1) A. However this behavior is in
contrast to certain hydroxo and alkoxo bridged complexes where
−1
048, 1030, 994, 749, 702, 693, 642, 624, 605, 531, 491, 405 cm .
the Cu
Cu · · · Cu separations. The Cu–O–Cu angles are 102.5 and 100.3
for 2 and 3, respectively, and lie in the expected range for dinuclear
2
O units adopt bent shaped conformations with shorter
2
13
◦
Me3
2
,2-Dimethyl-N-(2-pyridylmethyl)propionamide (HL ). The
Me3
synthesis of HL
was carried out via a modified procedure
18
11,14
based on a published method. A solution of trimethylacetyl
chloride (3.69 g, 30 mmol) in anhydrous THF (10 mL) was added
to a solution of 2-aminomethyl pyridine (3.24 g, 30 mmol) and
copper complexes reported in the literature.
The Cu–Omethoxide
distances are similar to those reported in the literature.
triethylamine (3.95 g, 39 mmol) in anhydrous THF (15 mL) at
Experimental
◦
0
C dropwise over 15 min. The reaction was warmed to room
temperature and after stirring for 24 h the solvent was removed
under vacuum. The resulting oil was dissolved in dichloromethane
and washed with saturated sodium bicarbonate solution followed
by water. The resulting yellow solution was dried with anhydrous
magnesium sulfate and the solvent was removed under vacuum to
General
All reagents were purchased from commercial sources and used
without further purification. Benzotriazole-1-yl-oxytripyrroli-
dinophosphoniumhexafluorophosphate (Py-BOP) was obtained
from Novabiochem and used without further purification. Sol-
vents were dried and purified under nitrogen using standard
methods and distilled immediately before use. Melting points
were determined with a Mel-Temp laboratory melting point
Me3
give HL as a yellow oil. Yield: 4.74 g (82%). Anal. Calcd for
C
11
H
16
N
2
O·1/4H
2
1
O: C, 67.15; H, 8.45; N, 14.24. Found: C, 67.20;
H, 8.56; N, 14.43. H NMR (300 MHz, CHCl
3
-d): d 1.24–1.27 (m,
9H), 4.52–4.54 (d, 2H), 7.05 (s, 1H), 7.17–7.27 (m, 2H), 7.63–7.68
1
906 | Dalton Trans., 2006, 1902–1908
This journal is © The Royal Society of Chemistry 2006

View Article Online
Ph3
Ph3
(
t, 1H), 8.55 (d, 1H) ppm. FTIR (neat): 3297, 3063, 2952, 1511,
[Cu
2
(HL
)
2
(OMe)
2
(MeOH)
2
](OTf)
2
(3). A solution of HL
1
366, 1299, 1202, 1149, 1099, 1070, 1049, 1012, 936, 889, 862, 751,
(0.055 g, 0.15 mmol) and triethylamine (0.014 ml, 0.15 mmol) in
dichloromethane was added to a methanolic solution of Cu(OTf)
0.053 g, 0.15 mmol) forming a clear dark green solution. Vapor
−
1
610, 528 cm .
2
(
Ph3
2,2,2-Triphenyl-N-(2-pyridylmethyl)acetamide
(HL ).
A
diffusion of diethyl ether resulted in the formation of green
solution of triethylamine (1.59 g, 15.6 mmol) in anhydrous DMF
5 mL) was added to a suspension of triphenyl acetic acid (1.5 g,
.2 mmol) and 2-aminomethyl pyridine (0.563 g, 5.2 mmol) in
crystals of the product, 3. Yield: 0.100 g (95%). Anal. Calcd for
(
5
C
58
H
58Cu
2
F
6
N
4
O
12
S
2
: C, 53.25; H, 4.47; N, 4.28. Found: C, 52.34;
H, 4.22; N, 4.45. ESI-MS (CH
OH): m/z = 1095.2, [M–OTf–
2MeOH] ; 472.2, [Cu(HL )(MeOH)] ; 441.1, [Cu(HL )] . UV-
3
+
Ph3
−1
+
Ph3
+
anhydrous DMF (10 mL) dropwise over 5 min. The resulting
◦
−1
suspension was cooled to 0 C and Py–BOP (2.7 g, 5.2 mmol)
Vis [CH
3
OH, kmax, nm (e, M , cm )]: 290 (3300), 689 (159). FTIR
in anhydrous DMF (5 mL) was added dropwise. The solution
became yellow and completely clear. After stirring overnight, the
resulting solution was poured into water (60 mL) with stirring.
(KBr): 3400 (mOH), 3349 (mNH), 2807 (mOMe), 1970, 1821, 1611, 1528,
1447, 1384, 1252, 1166, 1032, 906, 844, 774, 701, 672, 638, 641,
−
1
517, 464 cm . EPR (9.44 MHz, mod. freq. 100 kHz, mod. amp. 8
G, MeOH, 100 K): g = 2.05, A = 140 G. Magnetic
= 2.284, g
moment (Evans, CH OH-d ): 1.17 l /Cu(II).
The resultant precipitate was collected, washed with water (2 ×
ꢁ
⊥
ꢁ
Ph3
1
0 mL) and dried under vacuum to give HL as a white solid.
3
4
B
Yield: 1.86 g (94%). Colorless crystals were obtained by dissolving
the powder in a mixture of toluene and dichloromethane. Mp =
◦
X-Ray crystal structure determination
1
61–163 C. Anal. Calcd for C26
H
22
2
N O: C, 82.51; H, 5.86; N,
1
7
.40. Found: C, 82.67; H, 5.84; N, 7.45. H NMR (300 MHz,
-d): d 4.62–4.63 (d, 2H), 7.03 (bs, 1H), 7.10–7.14 (t, 1 H),
Single crystals of 1–3 were obtained by either slow evaporation
of methanol or acetonitrile solutions of the complex or by vapor
diffusion of diethyl ether into solutions of the complex. Intensity
data for 1–3 were collected using an instrument with a Bruker
CHCl
.17–7.29 (m, 16 H), 7.56–7.62 (t, 1H), 8.39 (d, 1H) ppm. C{ H}
NMR (75 MHz, CHCl -d): d 45.51, 67.98, 122.29, 122.42, 127.13,
28.1, 130.8, 143.57, 149.13, 156.66, 173.62 ppm. FTIR (KBr):
3
1
3
1
7
3
1
1
9
4
19
APEX ccd area detector with graphite-monochromated Mo
603, 1525, 1491, 1441, 1352, 1323, 1269, 1254, 1236, 1102, 970,
28, 902, 792, 765, 738, 698, 673, 641, 621, 577, 533, 519, 461,
˚
Ka (k = 0.71073 A) radiation. Cell parameters were determined
from a non-linear least squares fit of the data. The data were
−
1
32 cm .
20
corrected for absorption by the semi-empirical method. The
structures were solved by direct methods and refined by full-matrix
Ph
Ph
[
Cu
0.150 g, 0.663 mmol) and triethylamine (0.147 ml, 1 mmol) was
added to a methanolic solution of Cu(ClO O (0.245 g,
·6H
.663 mmol) with constant stirring. The blue precipitate was
collected by filtration, washed with methanol and dried under vac-
uum. Yield: 0.2058 g (89%). Anal. Calcd for C56 Cu
C, 48.45; H, 3.92; N, 8.07. Found: C, 47.96; H, 3.89; N, 8.04. UV-
4
(L )
4
(OH)
2
](ClO
4
)
2
(1). A methanolic solution of HL
2 21
least squares methods on F . Hydrogen atom positions were
initially determined by geometry and refined by a riding model.
Non-hydrogen atoms were refined with anisotropic displacement
parameters. Crystal data for 1–3 are summarized in Table 3. In
complex 1, the four perchlorate anions and solvent molecules
(
4
)
2
2
0
H
54Cl
2
4
N
8
14
O :
(
acetonitrile and water) were severely disordered. The solvent
22
molecules were best modeled using the Squeeze program. The
atoms C(12E)–C(17E) were disordered and modeled in two
orientations. The occupancies of these atoms refined to 0.532(6)
and 0.468(6) for the unprimed and primed atoms. The occupancies
of the disordered anion Cl(1I)–O(4I) refined to 0.499(5) and
−
1
−1
Vis [CH
3
CN, kmax, nm (e, M cm )]: 330 (sh, 5200), 649 (750).
H NMR (CH CN-d , 300 MHz, RT): d 1.13, 3.43, 4.59, 5.67,
.82, 7.29, 7.44, 7.9, 8.39, 9.33, 11.70, 13.23, 14.56, 24.37, 26.81,
6.08, 40.86, 43.51, 44.06, 93.08, 93.50, 122.19, 144.54 ppm. FTIR
1
3
3
5
3
(
1
KBr): 3432 (mOH), 1610, 1576, 1487, 1427, 1406, 1352, 1284, 1220,
0
.501(5) for the unprimed and primed atoms. Restraints on the
−
−1
095 (mClO4 ), 1028, 927, 846, 764, 722, 709, 656, 624, 563 cm .
positional and displacement parameters of the disordered atoms
were required.
Magnetic moment (Evans, CH
3
CN-d
3
): 1.64 l
B
/Cu(II).
CCDC reference numbers 285146, 285147 and 285410.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513763b
Me3
[
Cu
2
Me3
(HL
)
2
(OMe)
2
(MeOH)
2
](OTf)
2
(2).
A
solution of
HL
(0.144 g, 0.75 mmol) and triethylamine (0.104 ml,
.75 mmol) in dichloromethane was added to a methanolic
0
solution of Cu(OTf) (0.271 g, 0.75 mmol), forming a clear
2
dark green solution. Vapor diffusion of diethyl ether resulted
in the formation of green crystals of the product, 2. Yield:
Conclusions
0
.320 g (91%). Anal. Calcd for C28
H
46Cu
2
F
6
N
4
O
12
S
2
·1.5H
2
O: C,
The synthesis and structural characterization of a family of copper
complexes were carried out using a variety of sterically hindered
pyridylamide ligands synthesized in our laboratory. The copper
complexes that form are sensitive to the steric properties of
the pyridylamide ligand used. We demonstrate the possibility to
obtain selectively mono-, bi-, tetra- and octanuclear complexes
by tuning synthetic parameters (steric effects of ligand, solvent,
and stoichiometry of the base). In the complete absence of a
base, mononuclear complexes form with all of the pyridylamide
3
4.92; H, 5.13; N, 5.82. Found: C, 34.09; H, 4.18; N, 6.58. ESI-
Me3
+
MS (CH
3
Me3
OH): m/z = 721.1, [Cu
2
(HL
)
2
(OMe)
2
OTf] ; 691.1,
+
Me3
+
[Cu
2
(HL
)
2
(MeOH)(OTf)] ; 404.0, [Cu(HL )(OTf)] ; 255.1,
Me3
+
−1
−1
[
(
(
Cu(HL )] . UV-Vis [CH
3
OH, kmax, nm (e, M , cm )]: 292
4300), 350 (sh, 1700), 704 (124). FTIR (KBr): 3400 (mOH), 3347
NH), 2807 (mOMe), 1970, 1821, 1611, 1511, 1447, 1366, 1252, 1166,
m
−
1
1
032, 906, 844, 774, 701, 672, 638, 641, 528, 464 cm . EPR
9.44 MHz, mod. freq. 100 kHz, mod. amp. 6 G, MeOH, 100 K):
= 2.284, g = 2.05, A = 140 G. Magnetic moment (Evans,
CH OH-d ): 1.19 l /Cu(II).
(
4,7
g
ꢁ
⊥
ꢁ
ligands. However, when triethylamine is added to deprotonate
the amide N, the presence of triphenyl and trimethyl substituents
3
4
B
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1902–1908 | 1907

View Article Online
Table 3 Crystallographic data for 1–3
1
·2CH
3
CN·3H
Cu
2
O
2·1.5H
2
O
3
Formula
M
C
60
H
66Cl
2
4
N
10
O
17
[C56
H
98Cu
4
F
12
N
8
O
27
S
4
]
1/2
C
58 2 6 4 12 2
H58Cu F N O S
1524.29
95(2)
C2/c
42.218(8)
17.377(3)
26.792(5)
90
103.224(5)
90
962.91
203(2)
P-1
9.080(2)
10.766(3)
11.438(3)
76.204(3)
80.283(3)
77.456(3)
1
1051.8(4)
1.520
1.198
0.0704
0.1781
1.099
1308.32
110(2)
P-1
11.3265(8)
12.2802(8)
12.5623(9)
83.3530(10)
63.4270(10)
70.6400(10)
1
1473.27(18)
1.475
Temperature/K
Space group
a/A˚
b/A˚
c/A˚
◦
a/
b/
◦
◦
c /
Z
12
V/ A˚ 3
19134(6)
1.587
1.477
0.0351
0.1050
1.013
−
3
q
calcd/g cm
−
1
l/mm
0.876
0.0264
0.0747
1.042
R1 [I > 2r(I)]
wR2 [I > 2r(I)]
2
GOF on F
Ph3
Me3
on the pyridylmethyl ligand (HL and HL ) results in clean
4 A. Mondal, Y. Li, M. A. Khan, J. H. Ross, Jr. and R. P. Houser, Inorg.
Chem., 2004, 43, 7075–7082.
isolation of binuclear complexes 2 and 3. We hypothesize that
5
C. Li, L. S. Rittmann, A. S. Tsiftsoglou, K. K. Bhargava and A. C.
Ph3
Me3
the steric constraints of HL and HL lead to the preferential
formation of the methoxide bridged species rather than clusters.
The ligand binding mode in 2 and 3 is bidentate via the pyridyl N
and amide O donor atoms. This same binding mode was observed
with HL when a mononuclear complex formed in the absence
of base. However, HL , which contains a benzyl substituent
with reduced steric bulk, exclusively forms a hydroxo bridged
tetranuclear complex, 1. The binding mode of the ligand in 1 is
similar to the octanuclear copper cluster with HL, where the amide
nitrogen has been deprotonated to form the amidate ligand. In the
octanuclear cluster and 1, the anionic amidate N atom coordinates
to copper, providing the means by which clusters can form. The less
sterically hindered HL and HL are able to form clusters, while
Sartorelli, J. Med. Chem., 1978, 21, 874–877.
6 E. L. Klein, M. A. Khan and R. P. Houser, Inorg. Chem., 2004, 43,
7272–7274.
7
8
9
U. Pal Chaudhuri, L. R. Whiteaker, L. Yang, D. Powell and R. P.
Houser, manuscript in preparation.
N. N. Murthy, K. D. Karlin, I. Bertini and C. Luchinat, J. Am. Chem.
4
Ph
Soc., 1997, 119, 2156–2162.
E. Ruiz, P. Alemany, S. Alvarez and J. Cano, J. Am. Chem. Soc., 1997,
1
19, 1297–1303; A. Mukherjee, I. Rudra, M. Nethaji, S. Ramasesha
and A. R. Chakravarty, Inorg. Chem., 2003, 42, 463–468; I. A. Koval,
K. van der Schilden, A. M. Schuitema, P. Gamez, C. Belle, J. L. Pierre,
M. Luken, B. Krebs, O. Roubeau and J. Reedijk, Inorg. Chem., 2005, 44,
4
4
372–4382; J. K. Eberhardt, T. Glaser, R. D. Hoffmann, R. Frohlich
and E. U. Wurthwein, Eur. J. Inorg. Chem., 2005, 1175–1181.
0 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C. Verschoor,
1
Ph
J. Chem. Soc., Dalton Trans., 1984, 1349–1356.
Me3
Ph3
HL
and HL , which are much more sterically encumbered,
11 M. Drillon, A. Grand and P. Rey, Inorg. Chem., 1990, 29, 771–774; C.
Fukuhara, K. H. Tsuneyoshi, K. Katsura, N. H. Matsumoto, S. Kida
and M. Y. Mori, Bull. Chem. Soc. Jpn., 1989, 62, 3939–3943; M. G. B.
Drew, P. C. Yates, F. S. Esho, J. Trochagrimshaw, A. Lavery, K. P.
McKillop, S. M. Nelson and J. Nelson, J. Chem. Soc., Dalton Trans.,
cannot.
Acknowledgements
1
988, 2995–3003.
1
1
2 P. Chaudhuri, D. Ventur, K. Wieghardt, E. M. Peters, K. Peters and A.
Simon, Angew. Chem., Int. Ed. Engl., 1985, 24, 57–59.
This work was supported by the National Science Foundation
(
CHE-0094079) and the Herman Frasch Foundation. We thank
3 B. Graham, M. T. W. Hearn, P. C. Junk, C. M. Kepert, F. E. Mabbs,
B. Moubaraki, K. S. Murray and L. Spiccia, Inorg. Chem., 2001, 40,
Dr Masood Khan for X-ray data collection and Dr Douglas Powell
for assistance in the refinement of the X-ray structure of complex 1.
1
536–1543; M. F. Charlot, O. Kahn, S. Jeannin and Y. Jeannin, Inorg.
Chem., 1980, 19, 1410–1411; M. F. Charlot, S. Jeannin, Y. Jeannin,
O. Kahn, J. Lucreceabaul and J. Martinfrere, Inorg. Chem., 1979, 18,
1675–1681.
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1
908 | Dalton Trans., 2006, 1902–1908
This journal is © The Royal Society of Chemistry 2006