Structural variation in copper(I) complexes with pyridylmethylamide ligands: Structural analysis with a new four-coordinate geometry index, τ4

Article abstract of DOI:10.1039/b617136b

Four Cu(i) complexes were synthesized with a family of pyridylmethylamide ligands, HL^R [HL^R = N-(2-pyridylmethyl)acetamide, R = null; 2,2-dimethyl-N-(2-pyridylmethyl)propionamide, R = Me₃; 2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide, R = Ph₃)]. Complexes 1-3 were synthesized from the respective ligand and [Cu(CH₃CN) ₄]PF₆ in a 2: 1 molar ratio: [Cu(HL)₂]PF ₆ (1), [Cu₂(HL^Me3)₄](PF ₆)₂ (2), [Cu(HL^Ph3)₂]PF₆ (3). Complex 4, [Cu(HL)(CH₃CN)(PPh₃)]PF₆, was synthesized from the reaction of HL with [Cu(CH₃CN) ₄]PF₆ and PPh₃ in a 1: 1: 1 molar ratio. X-Ray crystal structures reveal that complexes 1, 3 and 4 are mononuclear Cu(i) species, while complex 2 is a Cu(i) dimer. The copper ions are four-coordinate with geometries ranging from distorted tetrahedral to seesaw in 1, 2, and 4. Complexes 1 and 2 are very air sensitive and they display similar electrochemical properties. The coordination geometry of complex 3 is nearly linear, two-coordinate. Complex 3 is exceptionally stable with respect to oxidation in the air, and its cyclic voltammetry shows no oxidation wave in the range of 0-1.5 V. The unusual inertness of complex 3 towards oxidation is attributed to the protection from bulky triphenyl substituent of the HL ^Ph3 ligand. A new geometric parameter for four-coordinate compounds, τ₄, is proposed as an improved, simple metric for quantitatively evaluating the geometry of four-coordinate complexes and compounds. This journal is The Royal Society of Chemistry.

Full text of DOI:10.1039/b617136b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Structural variation in copper(I) complexes with pyridylmethylamide ligands:
structural analysis with a new four-coordinate geometry index, s4†
Lei Yang, Douglas R. Powell and Robert P. Houser*
Received 23rd November 2006, Accepted 11th January 2007
First published as an Advance Article on the web 29th January 2007
DOI: 10.1039/b617136b
Four Cu(I) complexes were synthesized with a family of pyridylmethylamide ligands, HLR [HLR =
N-(2-pyridylmethyl)acetamide, R = null; 2,2-dimethyl-N-(2-pyridylmethyl)propionamide, R = Me3;
2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide, R = Ph3)]. Complexes 1–3 were synthesized from the
respective ligand and [Cu(CH3CN)4]PF6 in a 2 : 1 molar ratio: [Cu(HL)2]PF6 (1), [Cu2(HLMe3)4](PF6)2
(2), [Cu(HLPh3)2]PF6 (3). Complex 4, [Cu(HL)(CH3CN)(PPh3)]PF6, was synthesized from the reaction
of HL with [Cu(CH3CN)4]PF6 and PPh3 in a 1 : 1 : 1 molar ratio. X-Ray crystal structures reveal that
complexes 1, 3 and 4 are mononuclear Cu(I) species, while complex 2 is a Cu(I) dimer. The copper ions
are four-coordinate with geometries ranging from distorted tetrahedral to seesaw in 1, 2, and 4.
Complexes 1 and 2 are very air sensitive and they display similar electrochemical properties. The
coordination geometry of complex 3 is nearly linear, two-coordinate. Complex 3 is exceptionally stable
with respect to oxidation in the air, and its cyclic voltammetry shows no oxidation wave in the range of
0–1.5 V. The unusual inertness of complex 3 towards oxidation is attributed to the protection from
bulky triphenyl substituent of the HLPh3 ligand. A new geometric parameter for four-coordinate
compounds, s4, is proposed as an improved, simple metric for quantitatively evaluating the geometry of
four-coordinate complexes and compounds.
we have employed a new polydentate pyridylmethylamide ligand
Introduction
family (HLR; R = H, Me3, Ph and Ph3) with different substituents
Control of the coordination environment in transition metal
on the amide, and probed their complexation properties with
Cu(II) and Cu(I).4,6–9 Novel copper clusters—([Cu8L8(OH)4]4+
and [Cu4(LPh)4(OH)2]2+)—and methoxide-bridged dimers—
([Cu2(HLMe3)2(OMe)2]4+ and [Cu2(HLPh3)2(OMe)2]4+)—were syn-
thesized and characterized.6,8 We established that not only can
the protonation or deprotonation of the amide nitrogen control
the metal nuclearity and stability of complexes formed, but the
substituent groups, R, also turn out to be an important factor to
the coordination geometry of the metal center and the chemical
properties of their copper complexes.
The pyridylmethylamide ligand, HLR, has three potential donor
atoms, the N and O atoms from the amide group and a pyridyl
N atom. The coordination modes of copper complexes formed by
these three donor atoms can be modulated by the steric effect of
the substituents, the use of base, or the use of different copper
salt starting materials. When HLR coordinates in its neutral form,
the pyridyl N and O from the amide group will chelate to the
copper atom in a bidentate fashion, and form mononuclear or
dinuclear species.6,8 If CuCl is used, only the pyridyl N donor will
coordinate to the copper center in a monodentate fashion, and
mononuclear, dinuclear or one dimensional polymer complexes
can be synthesized.9 With the application of base, a tridentate
bridging coordination mode can be observed. The pyridyl and
amidate N donors chelate to the same copper atom and the
amidate O atom bridges to a second copper atom. Illustrating
this coordination mode, the reaction of Cu(ClO4)2·6H2O, HL and
Et3N leads to the formation of a novel octanuclear copper cluster,
in which two pseudosymmetric tetranuclear copper clusters are
linked by four bidentate bridging ligands, L−, and two l3-hydroxo
complexes continues to be the pivotal objective in designing
molecules and materials in varied fields of inorganic chemistry,
including bioinorganic modeling, catalysis, supramolecular self-
assembly, and coordination polymers. The coordination environ-
ment provided by ligands can impart unique and useful properties
on their transition metal complexes. The design and synthesis
of new molecules to be used as ligands is and will continue to
be an important part of inorganic chemistry. At the same time,
relatively simple ligands can, under judicious synthetic conditions,
give rise to complexes of import and interest. One such class of
ligands that we have been investigating in our laboratory is the
pyridylmethylamide ligand system.
The ongoing interest in pyridylmethylamide ligands can be
attributed to their ability to pre-organize and tune the structural
properties of their transition metal complexes. Much effort has
been devoted to the investigation of the influence of substituents,
base, and stoichiometry on the coordination modes of the ligand,
the geometric conformation of the metal centers and the different
properties of the complexes.1,2 Some of the transition metal
complexes with pyridylamide ligands have been used specifi-
cally as biomimetic models,3,4 while others exhibited interesting
supramolecular and/or polymeric structural properties.2,5
As a part of our ongoing research on the design and
synthesis of new copper complexes with pyridylamide ligands,
Department of Chemistry and Biochemistry, University of Oklahoma,
620 Parrington Oval, Norman, Oklahoma, 73019, USA. E-mail:
houser@ou.edu; Fax: +1 (405) 325-6111; Tel: +1 (405) 325-3551
† The HTML version of this article has been enhanced with colour images.
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ligands.8 When the methyl group on the amide is replaced by a
benzyl group, the increased steric effect leads to the formation of
tetranuclear cluster [Cu4(LPh)4(OH)2](ClO4)2.6
Results and discussion
The syntheses of complexes 1–4, summarized in Scheme 1, were
all carried out using [Cu(CH3CN)4]PF6 with the appropriate
ligand(s) in dichloromethane under anaerobic conditions. Com-
plex 1, characterized by elemental analysis and X-ray diffrac-
tometry as [Cu(HL)2]PF6, differs from 4, which was synthesized
from [Cu(CH3CN)4]PF6, HL and PPh3 and characterized as
[Cu(HL)(CH3CN)(PPh3)]PF6, only in that 1 contains two ligands
HL, while 4 contains one ligand HL, one PPh3, and one
coordinated acetonitrile molecule. Finally, on the basis of their
elemental analyses and X-ray structural analysis, complexes 2 and
3 were characterized as [Cu2(HLMe3)4](PF6)2 and [Cu(HLPh3)2]PF6,
respectively.
Complexes 1 and 2 are relatively stable in the solid state when
stored under N2, but the yellow crystals of the two complexes
are oxidized to green powder within 24 h when exposed to open
air. The light yellow solutions of complexes 1 and 2 are very air
sensitive, as indicated by the rapid color change to light blue, and
the reaction of complex 1 with the oxidant S8 gave the simple
oxidation product [Cu(HL)2](PF6)2, which was characterized by
X-ray crystallography. Further characterization of the resultant
green Cu(II) species from the oxidation of complex 2 is currently
under investigation.
Because Cu(I) complexes, especially those with coordinatively
unsaturated Cu(I) centers, play an important role in the under-
standing of metal properties in various biosystems, we are very
interested in the complexation properties of ligands HLR with
Cu(I) ions. Herein, we report the synthesis and characterization
of four new Cu(I) complexes with HL, HLMe3, and HLPh3. The
increasing steric effects of substituent groups in these ligands
lead to the different nuclearities of the copper complexes and
different coordination geometries of the copper centers. The in-
teresting electrochemical properties exhibited by these complexes
are consistent with their structural properties. Because of its two-
coordinate, linear coordination geometry and the protective effect
from the bulky triphenyl groups, complex 3 [Cu(HLPh3)2]PF6 is
remarkably stable in open air, even in coordinating solvents.
Finally, in this paper we introduce a new four-coordinate
metrical parameter. The coordination number of all of the
complexes reported in this paper is four or less. Of the four-
coordinate complexes, several display geometries that are difficult
to objectively describe, being neither close to tetrahedral or square
planar. We propose a new parameter, s4, which can be used to
quantify the geometry of four-coordinate species. The formula is
very simple, and can be calculated by hand using only the bond
angles of the metal coordination sphere.
In contrast, a remarkable and interesting feature of complex 3
is its unusually high stability towards oxidation. No oxidations of
Scheme 1 Synthesis and structure representation of complexes 1–4.
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complex 3 were observed even if the colorless solid was exposed
to open air for months. More interestingly, colorless CH2Cl2 or
CH3CN solutions of complex 3 were also very stable in open
air, with no observable color changes within 8 weeks. Although
examples of linear, two-coordinate Cu(I) species have been re-
ported, structurally characterized two-coordinate Cu(I) complexes
with nitrogen donor ligands are relatively rare.10 Furthermore,
only one two-coordinate, linear Cu(I) complex with monoden-
tate ligands having similar stability towards oxidation has been
reported.11
solution, which helps explain the unusual stability of solutions of
3 in the open air.
X-Ray crystal structures
[Cu(HL)2]PF6 (1). Crystallographic analysis of mononuclear
1 indicates that it consists of one Cu(I) ion, two neutral HL ligands,
and a PF6 anion (Fig. 1). The pyridyl N atom and amide O atom
from both ligands chelate to the same copper ion with the pyridyl
groups trans to each other. The dihedral angle formed by the two
pyridyl rings is only 8.9◦, and the copper center lies out of these
˚
two pyridyl ring planes by an average distance of 0.195 A. One
Electrochemistry
interesting structural feature of this complex is that the two amide
groups stay on the same side of two pyridyl rings with an O1–
Cu1–O2 angle of 101.21(5)◦ (see Table 1). The two Cu–O bonds
pull the copper out of the two pyridyl ring planes with a N1–
Cu1–N3 bond angle of 161.96(6)◦. This very distorted tetrahedral
geometry of the Cu(I) center can be better described as a seesaw
structure, with the nearly linear N1–Cu1–N3 forming the plank
and the O1–Cu1–O2 forming the pivot. The N1–Cu–N3 and O1–
Cu1–O2 planes are nearly perpendicular, with a dihedral angle of
about 92.2◦.
Cyclic voltammetry was performed on solutions of 1–3 in CH2Cl2
or CH3CN containing 0.1 M n-Bu4NPF6 as the supporting
electrolyte. Ferrocene was added to the solution as an internal
standard and potentials are reported vs. Ag/AgCl. Complexes 1
and 2 exhibit similar electrochemical properties in CH2Cl2. Both
undergo one partially reversible reduction at −0.3 V, assigned
to the CuI/Cu0 reduction. One irreversible oxidation process can
also be observed in the CVs of both complexes, but the wave
positions are different for 1 and 2, with 0.54 V for complex 1 and
0.69 V for complex 2. The higher stability towards oxidation of
complex 2 with respect to complex 1 can be interpreted in terms
of the solution conformations of the two complexes. On the basis
of the ESI mass spectrometry of 1 and 2 in CH2Cl2, they both
exist in solution as similar mononuclear species, [Cu(HL)2]+ and
[Cu(HLMe3)2]+. The protection to Cu(I) provided by the bulky tert-
butyl groups of ligand HLMe3 may result in the anodic shift of the
oxidation wave for [Cu(HLMe3)2]+.
The cyclic voltammetry of complex 3 in CH2Cl2 or CH3CN only
displays an irreversible reduction at −0.63 V, which we assign to the
CuI/Cu0 couple. This cathodic shift of the reduction wave indicates
that complex 3 is harder to reduce than complexes 1 and 2. More
interestingly, no oxidation was observed in the range of 0 to 1.5 V. It
has been established that reactivity of Cu(I) species possessing
linear geometries with O2 and CO is unfavorable.12,13 However,
two-coordinate Cu(I) complexes are still very air sensitive if the
ligand is sterically unhindered.11,12,14 The complex reported by
Banthia and Samanta11 has a coordination environment similar
to complex 3, and it is also stable in open air. However, the
oxidation of Samanta’s complex can still be observed at ∼1.15 V,
and its inertness is due to the electronic factors from ligand and
perchlorate anions. The cyclic voltammetry of ligand HLPh3 shows
Fig. 1 X-Ray crystal structure (top) and packing diagram (bottom) for
[Cu(HL)2]PF6 (1) with thermal ellipsoids at the 50% probability level.
Very few examples of mononuclear Cu(I) complexes with a
similar N2O2 coordination environment have been reported,15–17
and the examples structurally characterized as seesaw structures
are even more rare.15,17 The average Cu–N distance of 1.931 A
˚
−
no oxidation or reduction within +1.5 V to −1.5 V, and the PF6
anion has no close interactions with the copper center in complex
3. Therefore the steric hindrance provided by the bulky triphenyl
groups to the linear copper center exerts the greatest influence
to the exceptional stability of this complex. To our knowledge,
complex 3 is the most stable Cu(I) complex with a two-coordinate
linear copper center.
˚
and the average Cu–O distance of 2.298 A for complex 1 are
comparable to the reported examples. The seesaw structure is
different from the square planar geometry of the Cu(II) center
in the similar mononuclear Cu(II) complex, [Cu(HL)2](ClO4)2,
reported previously by our group.8
In supramolecular chemistry, weak interactions such as p–p
stacking and hydrogen bonding contribute significantly to the
self-assembly and molecular recognition processes. For aromatic
Our previous results showed that the amide group of the HLR
ligands is quite flexible, and hydrogen bonding involving the
carbonyl and N–H groups can stabilize the complex significantly.9
The crystal structure of complex 3 (vide infra) shows weak
interactions between the Cu atom and the carbonyl O atoms, so
the two triphenyl groups are “pulled” close to the Cu(I) center
and protect the Cu(I) from oxidation. ESI mass spectrometry also
shows that the mononuclear cationic portion of 3 exists in the
nitrogen heterocycles, p–p interactions are commonly defined by
18
˚
interplanar distances of about 3.3–3.8 A. Results of a search
of the Cambridge Structral Database (CSD) indicate that most
p–p interactions of pyridine or quinoline rings exist with a
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◦
˚
with the chelating ligand. The tetrahedral geometry of the copper
centers are compressed from the N–Cu–N side, resulting in an
average N–Cu–N bond angle of 151.5◦ that is greater than the
average O–Cu–O bond angle of 109.75◦ (Table 1). The distorted
tetrahedral structure at each copper in 2 is at least partially due
to the spatial orientation of the bridging ligands, with some steric
interactions between the tert-butyl groups and pyridyl rings.
Table 1 Selected bond distances (A) and bond angles ( ) of complexes
1–4
1
Cu1–N1
1.9308(15)
1.9320(15)
161.96(6)
92.85(6)
Cu1–O2
Cu1–O1
N3–Cu1–O2
N1–Cu1–O1
O1–Cu1–O2
2.2892(15)
2.3077(14)
99.05(6)
95.93(6)
101.21(5)
Cu1–N3
N1–Cu1–N3
N1–Cu1–O2
N3–Cu1–O1
2
Cu1–N1
Cu1–O1
95.01(6)
1.9519(10)
2.2810(9)
1.9442(11)
2.1869(9)
152.47(4)
101.29(4)
95.20(4)
95.40(4)
111.73(4)
98.22(4)
Cu1–N2
Cu1–O4
Cu2–N5
Cu2–O3
N4–Cu2–O2
N4–Cu2–N5
N5–Cu2–O2
N4–Cu2–O3
N5–Cu2–O3
O2–Cu2–O3
1.9470(10)
2.2235(9)
1.9454(11)
2.3169(9)
102.39(4)
150.59(4)
99.63(4)
91.70(4)
99.90(4)
107.77(4)
Cu2–N4
Cu2–O2
N1–Cu1–N2
N2–Cu1–O4
N2–Cu1–O1
N1–Cu1–O1
O4–Cu1–O1
N1–Cu1–O4
3
Cu1–N1
1.935(3)
2.448(25)
170.64(11)
93.21(9)
86.02(9)
Cu1–N3
1.921(3)
2.835(21)
78.45(8)
103.18(10)
157.13(7)
Cu1 · · · O1
N1–Cu1–N3
N1–Cu1–O2
N3–Cu1–O1
4
Cu1 · · · O2
N3–Cu1–O2
N1–Cu1–O1
O1–Cu1–O2
Cu1–N1
2.067(3)
Cu1–O1
2.159(2)
Cu1–N3
2.031(3)
Cu1–P1
2.1975(10)
114.48(8)
128.39(8)
115.30(7)
N3–Cu1–N1
N3–Cu1–O1
N1–Cu1–O1
100.36(10)
102.59(10)
90.99(9)
N3–Cu1–P1
N1–Cu1–P1
O1–Cu1–P1
Fig. 2 X-Ray crystal structure (top) and hydrogen bonding (bottom)
for [Cu2(HLMe3)4](PF6)2 (2) with thermal ellipsoids at the 50% probability
level.
˚
centroid–centroid distance of about 3.80 A and displacement angle
up to 20◦.18 In complex 1, two mononuclear units stack in an
offset face-to-face fashion and the four pyridyl rings are nearly
parallel to each other with an average interplanar angle of only
The two bridging ligands and the two copper atoms form
a fourteen-membered ring, and the Cu · · · Cu distance is about
˚
6.7 A. The hollow cavity in complex 2 clearly demonstrates that
◦
˚
4.45 . Although the mean centroid–centroid distance of 5.30 A
the bulky tert-butyl group of HLMe3 exerts a large influence on
the structure of the complex. So far, the known coordination
modes of HLR ligands to copper include tridentate, bidentate, and
monodentate.4,6,8,9,19 The two HLMe3 bridging ligands in 2 exhibit
a new bridging coordination mode for the pyridylmethylamide
ligand system. The intermolecular interactions of the cationic
portion of 2 are also interesting. The hydrogen bonding between
the N–H groups of HLMe3 and neighboring PF6− anions extend to
form a double stranded polymeric ladder, which helps to stabilize
complex 2 in the solid state (Fig. 2).
and displacement angle of 52.3◦ indicate very weak interactions,
˚
the average interplanar distances of 3.43 A and closest atom–atom
18
˚
contacts of 3.75 A are still acceptable for p–p interactions. The
p–p stacking arrangement in 1 contributes to the large N1–Cu1–
N3 bond angle of 161.96(6)◦, and the coplanar structure of the
pyridyl rings. This is the driving force for the formation of the
seesaw structure of complex 1. A second, weaker interaction in
complex 1 is the intermolecular hydrogen bonding interactions
−
involving amide N–H groups and the neighboring PF6 anions.
These hydrogen-bonding interactions extend to form 1-D chains
of anionic and cationic units, and stabilize the complex in the solid
state (Fig. 1).
[Cu(HLPh3)2]PF6 (3). Changing the tert-butyl substituent to a
triphenyl group can further modulate the copper center geometry
and resulting structure. A 2 : 1 reaction ratio between ligand HLPh3
and [Cu(CH3CN)4]PF6 leads to the formation of mononuclear 3,
which possesses a linear coordination geometry (Fig. 3 and 4).
The copper center is only coordinated by the two pyridyl N atoms
with a N1–Cu1–N3 bond angle of 170.64(11)◦ (Table 1). Although
[Cu2(HLMe3)4](PF6)2 (2). The steric effects of substituents on
the end of the amide group are essential to the structural
conformation of their copper complexes. For complexes 1 and
2, although the reaction stoichiometry is the same 2 : 1 ratio, the
structures are quite different. When the methyl group in HL is
replaced by a tert-butyl group, instead of forming a mononuclear
species similar to 1, the reaction of two equivalents of ligand HLMe3
with one equivalent of [Cu(CH3CN)4]PF6 produces a dinuclear
copper complex, [Cu2(HLMe3)4](PF6)2 (2, Fig. 2). For each copper
ion, one HLMe3 chelates the copper via the pyridyl N and amide
O atoms, and the two copper ions are bridged by two additional
ligands. The copper coordination sphere is made up by the amide
O atom and pyridyl N atom from the bridging ligands combined
the C O groups of the two HLPh3 ligands are oriented toward the
=
metal center, the very bulky groups on the free amide arms prevents
˚
the formation of Cu–O bonds. The Cu · · · O distances of 2.45 A
˚
and 2.84 A in 3 suggest only weak, non-bonding interactions
between the Cu and O atoms. If these long Cu · · · O interactions are
viewed as bonds, the O1–Cu1–O2 bond angle of 157.1◦ suggests
a four-coordinate geometry around Cu1 that is distorted square
planar. The average distance of the two pyridyl N and two amide
˚
O atoms from their least squares plane is about 0.327 A, and
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the shielding effectprovidedbythe twobulkytriphenylgroups. The
copper atom sits in the center of the cavity formed by the pyridyl
aromatic rings and the amide groups, and is further encased by the
six phenyl aromatic rings of HLPh3. The closest distance of a phenyl
ring to the Cu(I) atom is the intermolecular Cu · · · phenyl ring
˚
centroid distance of about 3.34 A. This intermolecular interaction
may further protect the copper center. In the previously reported
two-coordinate Cu(I) complexes with imidazole and pyrazole
ligands, the steric effect of methyl, ethyl and benzyl substituents
towards reactivity of Cu(I) complexes was discussed.13,20 Complex
3 represents the first time that a triphenyl group has been used to
protect the Cu(I) center successfully.
A structural comparison of complexes 1–3 clearly demonstrates
that increasing steric effects can lead to the formation of different
structural motifs. For ligand HL, the small methyl group allows
the formation of a mononuclear Cu(I) complex, 1, in which
the pyridyl N and O atoms of both ligands coordinate to the
same copper center. For ligand HLMe3, instead of forming a
similar mononuclear complex, the tert-butyl groups force the
carbonyl O atom of the second ligand to bridge to another copper
atom and form a dinuclear Cu(I) complex, 2. Further increasing
the steric effect by the use of the triphenyl group changes the
coordination mode of the ligand from bidentate to monodentate,
and the coordination of two pyridyl N atoms leads to the linear
two-coordinate, mononuclear Cu(I) complex, 3, which possesses
extraordinary resistance to oxidation. Cu(I) complexes with 1,2-
bis(2-pyridylethynyl)benzene ligands reported by Kawano also
illustrate the importance of substituent groups on ligands in
the self-organization of supramolecular structures.21,22 Similar to
complex 2, the introduction of a bulky menthyl substituent to the
1,2-bis(2-pyridylethynyl)benzene ligand led to the formation of
binuclear Cu(I) complexes with a distorted tetrahedral geometry
at each copper.21 Surprisingly, Cu(I) complexes with the less bulky,
unsubstituted 1,2-bis(2-pyridylethynyl)benzene ligand produced
mononuclear, linear two-coordinate species much like 3.22 While
the specifics differ in each case, they all point to the significance of
the presence and/or absence of bulky ligand substituent groups in
the modulation of the metal center geometry, the structures of the
complexes, and more importantly, the reactivity of the compounds.
Fig. 3 X-Ray crystal structure (top) and packing structure diagram
(bottom) for [Cu(HLPh3)2]PF6 (3) with thermal ellipsoids at the 50%
probability level.
[Cu(HL)(CH3CN)(PPh3)]PF6 (4). The Cu(I)–phosphine com-
plex, [Cu(HL)(CH3CN)(PPh3)]PF6 (4), displays a distorted tetra-
hedral structure in which the copper atom is coordinated by
the pyridyl N and amide O atoms from HL, the PPh3 and one
acetonitrile (Fig. 5). The steric crowding from the PPh3 results
˚
in the elongation of the N1–Cu1 bond length (2.067 A, Table 1)
relative to complexes 1–3. In addition, the chelate angle of HL
(N1–Cu1–O1 = 90.99◦) is smaller than those in complexes 1 and
2. To our knowledge, only two examples in which a Cu(I) ion
possesses the same N2OP coordination environment have been
Fig. 4 Space-filling representation of [Cu(HLPh3)2]PF6 (3).
˚
the copper atom lies 0.599 A out of this plane. Because of the
spatial orientation of the two ligands and the steric effects of their
bulky triphenyl groups, the dihedral angle between the two pyridyl
rings is about 23.1◦. The Cu1–N1 and Cu1–N3 bond distances of
reported.23,24 The Cu–P bond distance (2.198 A) in complex 4
˚
is quite comparable with these two examples, but is somewhat
less than that in most other previously reported Cu(I)–PPh3
complexes.25
˚
˚
1.935(3) A and 1.921(3) A, respectively, are longer than those
previously reported for Cu(I) complexes with N donor ligand
11–14
˚
systems possessing linear geometry (1.83–1.89 A).
s4—A new four-coordinate geometry index
The HLPh3 N–H groups are involved in intermolecular hydrogen
bonding with the neighboring PF6− anions with a H · · · F distance
Describing the structures of four-coordinate copper(I) and cop-
per(II) species can be a subjective endeavor, particularly when
the structure is intermediate between the two most common
◦
˚
of 2.52 A and an N–H · · · F angle of 122 . The space-filling
diagram of the cationic portion of 3 shown in Fig. 4 clearly shows
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Scheme 2
also been used to analyze tetrahedral/planar distortions in
seminal work by Dunitz and coworkers,28 and more recently by
Keinan and Avnir.29 While all of these approaches provide a
wealth of information that has contributed to our understanding
of correlations between structure and physical, chemical and
spectroscopic properties,30 the calculations are nontrivial at best,
and in many cases quite complex. A simple metric for molecular
shape in four-coordinate complexes would be tremendously
useful.
The utility of a simple metric to assess molecular shape is
exemplified best by the s parameter for five-coordinate transition
metal complexes (henceforth s5), introduced by Addison, Reedijk
and coworkers in 1984 on the pages of this journal.31 The angular
structural parameter, s5 (eqn (1)),
b − a
s5 =
(1)
60
can be easily calculated using the two largest bond angles in
the five-coordinate complex to give an index of the degree of
trigonality. Hence, a trigonal bipyramidal structure with D3h
symmetry has a s5 = 1, while a square pyramidal structure with
C
4v symmetry has s5 = 0. The simplicity and utility of the formula
Fig. 5 X-Ray crystal structure (top) and packing structure diagram
(bottom) for [Cu(HL)(CH3CN)(PPh3)](PF6)2 (4) with thermal ellipsoids
at the 50% probability level.
in its ability to gauge the structures of five-coordinate complexes
with geometries not easily definable, are reflected in the use of the
formula in perhaps thousands of papers.32
Similar formulas for four-coordinate compounds and com-
plexes that are simpler than the aforementioned approaches have
been proposed. Toyota and Oki defined the tetrahedral character
(THC) parameter for boron complexes with coordinative N → B
bonds using the three bond angles that do not include the N →
B bond.33 Later Ho¨pfl generalized the formula to include all six
bond angles (eqn (2)).34
four-coordinate geometries, tetrahedral and square planar. In
copper complexes, the oxidation state plays a leading role in the
geometry formed—d10 copper(I) complexes prefer four-coordinate
tetrahedral geometries due to the lack of any ligand field
stabilization energy, while d9 copper(II) prefers square planar
four-coordinate, or higher coordination numbers (square pyra-
midal, trigonal bipyramidal, or axially elongated six-coordinate).
However, ligand constraints can play an overpowering role and
force unusual geometries onto copper(I) or copper(II) centers. An
excellent example of this effect is seen in the blue copper electron
transfer proteins, where coordination geometry, constrained by the
protein entatic state, has an influence on their redox properties.
Structures of four-coordinate blue copper sites span distorted
tetrahedral to trigonal pyramidal.26
Four-coordinate transition metal complexes and metalloprotein
sites can be described in subjective terms by placing an adjective,
e.g. “distorted”, in front of one of the geometric forms shown
in Scheme 2: distorted tetrahedral, distorted trigonal pyramidal,
distorted seesaw, or distorted square planar. The advantages of
using a more objective method of analyzing geometry have been
apparent for many years. For example, Muetterties and Guggen-
berger proposed a quantitative measure of shape, not only for
four-coordinate systems, but cluster and coordination compound
classes for all x-atom families, that uses the dihedral angles formed
by the normals to the adjacent polytopal faces.27 Symmetry has
ꢁ
ꢀ
ꢂ
n=1−6 |109.5◦ − hn|
THCDA
=
1 −
× 100
(2)
90◦
The THCDA index is useful for the analysis of the tetrahedral
and trigonal pyramidal extremes expected for N → B complexes.
A perfect tetrahedral structure generates THCDA = 100 while
a perfect trigonal pyramidal structure generates THCDA = 0.
However, other four-coordinate geometries relevant to transition
metal chemistry, e.g. square planar and seesaw, produce negative
values (Table 2). A formula proposed by Holland and coworkers
for the analysis of the belt iron atoms of the iron–molybdenum
cofactor of nitrogenase can also be effectively used to analyze
structures between tetrahedral and trigonal pyramidal.35
Since the THCDA formula and Holland’s formula were designed
for species with structures intermediate between Td and C3v,
the utility for the analysis of four-coordinate transition metal
complexes is limited. Furthermore, the formulas require multiple
steps and, while a simple matter for a computer program to
960 | Dalton Trans., 2007, 955–964
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Table 2 Four-coordinate geometry indices for ideal geometrical shapes
as structures change from tetrahedral to square planar, but not
smoothly. There is a sharp decrease going from trigonal pyramidal
to seesaw with h6 = 90◦, followed by an anomalous increase before
decreasing again. This is an inherent flaw in the THCDA formula
when used for transition metal complexes. FCGP starts at 0.00 for
tetrahedral and increases to 1.00 for trigonal pyramidal, but then
decreases again. Several disparate structures in Table 2 have the
same FCGP values: tetrahedral and seesaw with h6 = 154.4◦ have
FCGP = 0.00, and trigonal pyramidal and seesaw with h6 = 90◦
have FCGP = 1.00. Furthermore, FCGP values greater than 1.00
are possible (vide infra, see Table 3).
Four-coordinate structural parameters for complexes 1–4 are
listed in Table 3, along with parameters for other four-coordinate
copper and transition metal complexes. The s4 parameters nicely
compliment and support the structural descriptions made previ-
ously in this paper on the basis of the bond angles and visual
inspection of the structures. Complexes 1 and 2, with structures
described previously in this paper as seesaw, have s4 values of
s4
THCDA /10034
FCGP/10036
Tetrahedral (Td)
1.00
0.85
0.64
0.50
0.18
0.07
0.00
1.00
0.00
0.00
1.00
1.00
0.70
0.00
Trigonal pyramidal (C3v)
Seesaw (C2v, h6 = 90◦)
Seesaw (C2v, h6 = 109.5◦)
Seesaw (C2v, h6 = 154.4◦)
Seesaw (C2v, h6 = 170◦)
Square planar (D4h)
−0.87
−0.65
−1.15
−1.32
−1.43
−0.24
−0.40
calculate, are significantly more complex than the parameter we
propose.
Another formula designed for main group complexes was
proposed by Hutchison and coworkers.36 Their so-called four-
coordinate geometric parameter (FCGP) defines “faces” as the
sum of the angles involved in each polytopal face of the four-
coordinate structure. Inserting the largest face value and the sum
of the remaining angles into the formula produces the FCGP
parameter (eqn (3)).
∼0.70
0.02, which fits with a seesaw description. Complex
ꢁ
3, which was described as linear two-coordinate, has Cu · · · O
interactions that are close to being within bonding range. When the
Cu–O bonds are included, the four-coordinate geometry around
the copper is close to square planar, and the s4 value of 0.23
accurately reflects this description. Complex 4, which is closer
to trigonal pyramidal, has a s4 value of 0.82, supporting an
assignment of distorted trigonal pyramidal geometry. Other four-
coordinate copper complexes that were described earlier in the
paper in comparison to 1–4, as well as a representative sampling
of other four-coordinate transition metal complexes, are also
summarized in Table 3, illustrating the universality of s4. Table 3
also illustrates the limitations inherent to the THCDA and FCGP
parameters.
ꢃ
ꢄ
largest face − ( remaining 3◦) (5.61) + 1515.15
FCGP =
× 100
(3)
360
Serious drawbacks with the FCGP parameter include the need
to visually assign the polytopal faces (e.g. one cannot generate
the FCGP from a table of the six h angles without visually
inspecting the structure), and the illogical range of negative values
for structures between D4h and C2v (see Table 2). The authors
designed the FCGP parameter to deal specifically with four-
coordinate Group 13 compounds ranging from tetrahedral to
trigonal pyramidal, so the nonsensical results for other geometries
make it unacceptable for transition metal four-coordinate species
with a wider range of structural possibilities.
We propose a very simple geometry index for four-coordinate
complexes, s4 (eqn (4)),
Experimental
360◦ − (a + b)
General
s4 =
(4)
141◦
All reagents were purchased from commercial suppliers and used
as received without further purification, unless otherwise stated.
Ligands N-(2-pyridylmethyl)acetamide (HL), 2,2-dimethyl-N-(2-
pyridylmethyl)propionamide (HLMe3), and 2,2,2-triphenyl-N-(2-
pyridylmethyl)acetamide (HLPh3) were synthesized according to
procedures previously reported.4,6 All solvents were dried under
nitrogen using standard methods and distilled before use. FTIR
spectra were collected on a NEXUS 470 FTIR spectrometer
using the KBr pellet technique. Mass spectra were recorded on
a Q-TOF quadrupole time-of flight mass spectrometer (Micro-
mass, Manchester, UK) equipped with a Z-spray electrospray
ionization (ESI) source. 1H NMR spectra were recorded on
a Varian 300 MHz spectrometer using deuterated solvent as
an internal standard. Elemental analyses were carried out by
Atlantic Microlabs, Norcross, GA. Electrochemical measurements
were performed with a C-3 CV-50W instrument (Bioanalytical
Systems Inc.). Ferrocene (Fc) was added to the solution as an
internal standard and potentials are reported vs. Ag/AgCl and
the potentials of complexes (∼+0.46 V vs. Ag/AgCl).42 Cyclic
voltammetry was performed on complexes 1–3 in CH2Cl2 or
CH3CN solutions containing 0.1 M n-Bu4NPF6 as the supporting
electrolyte at a Pt disk (3 mm diameter) working electrode and
Ag/AgCl reference electrode.
inspired by Addison and Reedijk’s five-coordinate s5 index. The
formula for our four-coordinate s4 index is simply the sum of angles
a and b—the two largest h angles in the four-coordinate species—
subtracted from 360◦, all divided by 141◦. The calculation can
be done with pen and paper or on a calculator without visually
analyzing the structure, simply by choosing the two largest h values
from a table of bond angles. The values of s4 will range from
1.00 for a perfect tetrahedral geometry, since 360 − 2(109.5) =
141, to zero for a perfect square planar geometry, since 360 −
2(180) = 0 (see Table 2). Intermediate structures, including trigonal
pyramidal and seesaw, fall within the range of 0 to 1.00.
The s4 parameter is particularly good for four-coordinate
transition metal complexes since square planar and tetrahedral
geometries are the extremes. A comparison of s4, THCDA , and
FCGP are summarized in Tables 2 and 3. Table 2 compares the
values of the three parameters for ideal geometries. The geome-
tries, shown graphically in Scheme 2, proceed from tetrahedral to
square planar. s4 nicely tracks from 1.00 for tetrahedral, to 0.85
for a perfect trigonal pyramid, through a series of values for C2v
seesaw structures that get smaller as the angle h6 (see Scheme 2)
gets larger. When h6 is 180◦, the geometry is square planar, and
s4 is 0.00. From Table 2 it can be seen that THCDA also decreases
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Table 3 Four-coordinate geometry indices for 1–4 and representative examples from the literature
Geometrya
s4
THCDA /10034 FCGP/10036 Reference
1
Seesaw
Seesaw
Seesaw
Dist. square planar
Dist. trigonal pyramidal
Tetrahedral
Psuedotetrahedral
Dist. tetrahedral
Trigonal pyramidal
Trigonally dist. tetrahedral 0.67
None given
Dist. tetrahedral
Dist. tetrahedral
Dist. square planar
0.69
0.68
0.72
0.23
0.82
0.93
0.81, 0.83
0.79
0.73
−0.29
−0.03
0.03
0.64
0.48
0.53
0.09
0.35
0.06
0.36, 0.39
0.39
0.81
0.81
0.73
1.16
0.68
0.58
This work
This work
This work
This work
2 (Cu1)
2 (Cu2)
3
−1.07
4
0.29
This work
(Et4N)[Fe(StBu)3(NO)]
[Cu2(XYL-O–)(PPh3)2]PF6
Psuedoazurin
0.74
0.31, 0.30
37
23
38
39
24
15
17
21
40
b
0.28
−0.03
−0.35
−0.61
−0.90
−0.76
−0.64
Azurin
[Cu(Tpms)(PPh3)]c
[Cu(10-methylisoalloxazine)2]ClO4
[Cu((C5H5)Fe(C5H4CH2N(CH3)(CH2)2–))2]OTf
[Cu2(L2)2](PF6)2
0.60
0.56
0.55
0.47
d
[{[2,6-(i-
=
Pr)2PhN C(Me)]2(C5H3N)}Mn(CH2SiMe3)][Li(OEt2)4]
[AuI(l-{CH2}2PPh2)2AuIII(C≡CPh)2]
Square planar
0.03
−1.39
−0.36
41
a Geometry as described in the text of this paper or the paper from which the structural data was retrieved. b XYL-O– is the deprotonated form of
XYL–OH. XYL–OH is bis(2-(2-pyridyl)ethyl)ethyl)amine). c Tpms is tris(pyrazolyl)methanesulfonate. d L2 is 1,2-bis(2-pyridylethynyl)benzene.
Syntheses
4.6; N, 5.8%. FTIR (KBr): 3418, 3057, 3032, 2940, 1635, 1608,
1571, 1490, 1444, 1356, 1324, 1258, 1243, 1226, 1188, 1161, 1109,
1085, 1066, 1034, 1001, 974, 934, 836, 767, 740, 700, 671, 638, 558,
[Cu(HL)2]PF6 (1). A solution of HL (0.075 g, 0.5 mmol)
in CH2Cl2 (2 mL) was added to
a stirred solution of
1
521, 431 cm−1. H NMR (300 MHz, CD2Cl2): d = 4.51 (d, 2H);
[Cu(CH3CN)4]PF6 (0.093 g, 0.25 mmol) in CH2Cl2 (2 mL). The
reaction was stirred for 20 h, during which the color changed to
light yellow. The solution was filtered and diffusion of diethyl ether
into the filtrate produced crystals suitable for crystallographic
characterization (0.104 g, 82.0%). Found: C, 38.0; H, 3.95; N,
11.0. Calc. for C16H20CuF6N4O2P: C, 37.8; H, 4.0; N, 11.0%. FTIR
(KBr): 3445, 3425, 3285, 3062, 1645, 1603, 1539, 1482, 1440, 1373,
1358, 1292, 1234, 1032, 842, 834, 765, 751, 615, 559, 510 cm−1. 1H
NMR (300 MHz, DMSO): d = 1.94 (s, 3H); 4.49 (s, 2H); 7.47 (m,
3H), 7.95 (t, 1H); 8.63 (s, 1H) ppm. ESI-MS (CH3CN): m/z =
363.1 [Cu(HL)2]+; 254.1 [Cu(HL)CH3CN]+; 213.0 [Cu(HL)]+.
7.05 (s, 1H); 7.08 (s, 1H); 7.20 (s, 1H); 7.31 (m, 15 H); 7.64 (t, 1H);
8.41 (d, 1H) ppm. ESI-MS (CH3CN): m/z = 819.4 [Cu(HLPh3)2]+;
482.2 [Cu(HLPh3)CH3CN]+; 441.2 [Cu(HLPh3)]+; 379.2 [H2LPh3]+.
[Cu(HL)(CH3CN)(PPh3)]PF6 (4). A solution of HL (0.075 g,
0.5 mmol) in CH2Cl2 (1 mL) was added to a stirred solution
of [Cu(CH3CN)4]PF6 (0.186 g, 0.5 mmol) in CH2Cl2 (2 mL). A
solution of PPh3 (0.1311 g, 0.5 mmol) in 1 mL CH2Cl2 (1 mL)
was subsequently added to the mixture. The reaction was stirred
for 14 h, during which the color changed to light yellow. The
solution was filtered and diffusion of diethyl ether into the
filtrate produced colorless crystals suitable for crystallographic
characterization (0.231 g, 83.8%). Found: C, 47.4; H, 4.1; N,
5.7. Calc. for C29H30Cl2CuF6N3OP2: C, 46.6; H, 4.05; N, 5.6%.
FTIR (KBr): 3426, 3056, 1649, 1603, 1539, 1480, 1435, 1373,
1295, 1234, 1183, 1158, 1095, 1056, 1028, 998, 841, 773, 748,
695, 641, 607, 558, 525, 507, 443 cm−1. 1H NMR (300 MHz,
CD2Cl2): d = 1.96 (m, 6H); 4.41 (d, 2H); 6.99 (s, 1H); 7.24 (t,
1H); 7.42 (m, 16 H); 7.73 (t, 1H); 8.53 (s, 1H) ppm. ESI-MS
(CH3CN): m/z = 587.1 [Cu(PPh3)2]+; 475.1 [Cu(HL)(PPh3)]+;
366.0 [Cu(CH3CN)(PPh3)]+; 254.0 [Cu(HL)(CH3CN)]+; 213.0
[Cu(HL]+.
[Cu2(HLMe3)4](PF6)2 (2). A solution of HLMe3 (0.192 g,
1.0 mmol) in CH2Cl2 (1 mL) was added to a stirred solution of
[Cu(CH3CN)4]PF6 (0.186 g, 0.5 mmol) in CH2Cl2 (1 mL). The
reaction was stirred for 4 h, during which the color changes to
light yellow. The solution was filtered and layering of diethyl
ether onto the filtrate produced light yellow crystals suitable for
crystallographic characterization (0.263 g, 89.0%). Found: C, 45.1;
H, 5.55; N, 9.6. Calc. for C44H64Cu2F12N8O4P2: C, 44. 6; H, 5.4;
N, 9.45%. FTIR (KBr): 3447, 2970, 2873, 1628, 1605, 1571, 1525,
1481, 1444, 1408, 1367, 1294, 1227, 1203, 1160, 1108, 1058, 1013,
992, 828, 772, 740, 716, 625, 558, 530, 481, 445, 423 cm−1. H
1
NMR (300 MHz, CD2Cl2): d = 1.16–1.24 (m, 9H); 4.83 (d, 2H);
7.08 (s, 1H); 7.51 (t, 1H); 7.69 (d, 1H); 8.03 (t, 1H); 8.82 (d,
1H) ppm. ESI-MS (CH3CN): m/z = 446.2 [Cu(HLMe3)2]+; 255.1
[Cu(HLMe3)]+; 193.13 [H2LMe3]+.
X-Ray crystal structure determination
Single crystals of 1–4 were obtained by vapor diffusion or
layering of Et2O into solutions of the complex. Data for 1–4 were
collected on a Bruker Apex CCD area detector diffractometer
[Cu(HLPh3)2]PF6 (3).
A
solution of HLPh3 (0.0946 g,
˚
0.25 mmol) in CH2Cl2 (2 mL) was added to a stirred solution
of [Cu(CH3CN)4]PF6 (0.0465 g, 0.125 mmol) in CH2Cl2 (2 mL).
The reaction was stirred for 24 h, during which the color changed
to light yellow. The solution was filtered and layering of diethyl
ether onto the filtrate produced colorless crystals suitable for
crystallographic characterization (0.085 g, 70.4%). Found: C,
64.55; H, 4.7; N, 6.0. Calc. for C52H44CuF6N4O2P: C, 64.7; H,
with graphite-monochromated Mo-Ka (k = 0.71073 A) radiation.
For complexes 1–4, cell parameters were determined from a
non-linear least squares fit of the data. The data of these four
complexes were corrected for absorption by the semi-empirical
method. The structures were solved by direct methods by use of
the SHELXTL program, and refined by full-matrix least-squares
on F2 by use of all reflections.43 Hydrogen atom positions were
962 | Dalton Trans., 2007, 955–964
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Table 4 Summary of crystal data and refinement parameters for complexes 1–4
1
2
3
4
Formula
M
C16H20CuF6N4O2P
508.87
87(2)
C44H64Cu2F12N8O4P2
1186.05
110(2)
C52H44CuF6N4O2P
965.42
C29H30Cl2CuF6N3OP2
746.94
97(2)
¯
P1
T/K
294(2)
¯
Space group
P21/n
P21/c
P1
˚
a/A
9.5739(16)
8.5420(14)
25.145(4)
90
99.355(5)
90
15.2619(8)
19.9179(11)
18.4954(10)
90
110.1750(10)
90
9.336(3)
9.732(3)
25.406(8)
82.221(5)
81.962(5)
74.409(5)
2
2190.0(12)
1.464
0.609
8.008(2)
11.738(4)
17.289(5)
86.917(5)
89.583(5)
84.150(5)
2
1614.3(8)
1.537
1.005
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
Z
4
4
3
˚
V/A
2029.0(6)
1.666
1.230
5277.4(5)
1.493
0.957
qcalcd/g cm3
l/mm−1
R1 [I > 2r(I)]
wR2[I > 2r(I)]
GOF on F2
0.0259
0.0733
1.007
0.0271
0.0766
1.025
0.0527
0.1376
1.009
0.0474
0.1268
1.050
initially determined by geometry and refined by a riding model.
Non-hydrogen atoms were refined with anisotropic displacement
parameters. In complexes 1 and 3, the PF6 anion was disordered
are crucial to the formation of different molecular structures of
these copper complexes. To assist in the structural analysis of 1–4,
the new four-coordinate metrical parameter s4 was proposed. Like
its five-coordinate inspiration,31 s4 is a simple formula that can be
used to gauge the geometries of four-coordinate transition metal
complexes and main group compounds.
−
and was modeled in three and two orientations, respectively, and
restraints on the positional and displacement parameters of the
anion were required. For complex 1, the occupancies of the anion
were refined to 0.403(8), 0.308(4), and 0.289(7) for the unprimed,
primed, and double primed atoms. For complex 3, the occupancies
of the anion were refined 0.882(3) and 0.118(3) for the unprimed
and primed atoms. In addition, for complex 3, a large residual
peak was located near the metal in the final difference map that is
believed to be due to a small contribution of complete translational
disorder of the cation. Crystal data for 1–4 are summarized in
Table 4, and selected bond lengths and angles are listed in Table 2.
CCDC reference numbers 627606 (1), 627605 (2), 627607 (3),
and 627608 (4).
Acknowledgements
The National Science Foundation (CHE-0094079 and CHE-
0616941) and the Herman Frasch Foundation supported this
work. We also thank the NSF (CHE-0130835) and the University
of Oklahoma for the purchase of a CCD equipped X-ray
diffractometer.
References
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b617136b
1 D. S. Cati and H. Stoeckli-Evans, Acta Crystallogr., Sect. E, 2004, 60,
M174–M176; D. Z. Gao, H. K. Lin and S. R. Zhu, Acta Crystallogr.,
Sect. E, 2005, 61, M2359–M2361; B. W. Lee, K. S. Min and M. K. Doh,
Inorg. Chem. Commun., 2002, 5, 163–165; M. Nonoyama, Inorg. Chim.
Acta, 1975, 13, 5–10; M. Nonoyama, S. Tomita and K. Yamasaki,
Inorg. Chim. Acta, 1975, 12, 33–37; J. M. Rowland, M. M. Olmstead
and P. K. Mascharak, Inorg. Chem., 2000, 39, 5326–5332; P. J. Toscano,
K. A. Belsky, L. M. Engelhardt, K. J. Fordon and A. H. White, Inorg.
Chem., 1990, 29, 1357–1359; P. J. Toscano, K. A. Belsky, T.-C. Hsieh, T.
Nicholson and J. Zubieta, Polyhedron, 1991, 10, 977–991; P. J. Toscano,
K. J. Fordon, D. Macherone, S. Liu and J. Zubieta, Polyhedron,
1990, 9, 2375–2383; N. Tounsi, L. Dupont, A. Mohamadou, C.
Cadiou, M. Aplincourt, R. Plantier-Royon, F. Massicot and C. Portella,
New J. Chem., 2004, 28, 785–792.
2 K. Uemura, S. Kitagawa, K. Fukui and K. Saito, J. Am. Chem. Soc.,
2004, 126, 3817–3828; K. Uemura, S. Kitagawa, M. Kondo, K. Fukui,
R. Kitaura, H.-C. Chang and T. Mizutani, Chem.–Eur. J., 2002, 8,
3587–3600.
3 T. C. Harrop, M. M. Olmstead and P. K. Mascharak, Inorg. Chem.,
2005, 44, 9527–9533; S. R. Zhu, W. W. Brennessel, R. G. Harrison and
L. Que, Inorg. Chim. Acta, 2002, 337, 32–38.
Conclusions
Four new Cu(I) complexes with a set of pyridylmethylamide
ligands, HLR, were synthesized and characterized by X-ray
crystallography, NMR, FT-IR, ESI-MS and cyclic voltammetry.
A new bidentate bridging coordination mode for HLR is observed
in complex 2, where two HLMe3 ligands bridge between two copper
centers. The ligand, or more specifically the functional substituent
group R, influences the chemical and electrochemical properties
of 1–4. Complexes 1 and 2 are very reactive with oxygen, and the
cyclic voltammetry shows irreversible oxidation processes at 0.54 V
and 0.69 V, respectively. In contrast, complex 3 is unusually inert
towards oxidation, and no oxidation waves are observed in the
range 0 to +1.5 V. The remarkable stability of 3 is attributed to the
protection provided by the bulky triphenyl group. The substituent
group R on HLR also influences the metal center geometry of these
complexes. The steric effect of the R group, which increases in the
order of H < Me3 < Ph3, contributes to the variety of copper center
geometries, ranging from distorted tetrahedral geometry (4) to
seesaw (1, 2) to linear geometry (3). Consequently, the substituents
4 E. L. Klein, M. A. Khan and R. P. Houser, Inorg. Chem., 2004, 43,
7272–7274.
5 D. S. Cati, J. Ribas, J. Ribas-Arino and H. Stoeckli-Evans, Inorg. Chem.,
2004, 43, 1021–1030; J. Hausmann, G. B. Jameson and S. Brooker,
Chem. Commun., 2003, 2992–2993; Z. Q. Qin, M. C. Jennings, R. J.
Puddephatt and K. W. Muir, Inorg. Chem., 2002, 41, 5174–5186.
6 U. Pal Chaudhuri, L. R. Whiteaker, L. Yang and R. P. Houser, Dalton
Trans., 2006, 1902–1908.
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Dalton Trans., 2007, 955–964 | 963
©

View Online
7 J. T. Michels, B. G. O’Malley, E. L. Klein, L. Yang, A. Grohmann and
R. P. Houser, Acta Crystallogr., Sect. E, 2005, 61, O3834–O3836.
8 A. Mondal, Y. Li, M. A. Khan, J. H. Ross and R. P. Houser, Inorg.
Chem., 2004, 43, 7075–7082.
25 D. G. Cuttell, S. M. Kuang, P. E. Fanwick, D. R. McMillin and
R. A. Walton, J. Am. Chem. Soc., 2002, 124, 6–7; S. L. Howell, B. J.
Matthewson, M. I. J. Polson, A. K. Burrell and K. C. Gordon, Inorg.
Chem., 2004, 43, 2876–2887; W. L. Jia, T. McCormick, Y. Tao, J. P. Lu
and S. N. Wang, Inorg. Chem., 2005, 44, 5706–5712; L. Kovbasyuk, H.
Pritzkow and R. Kramer, Eur. J. Inorg. Chem., 2005, 894–900; S. M.
Kuang, D. G. Cuttell, D. R. McMillin, P. E. Fanwick and R. A. Walton,
Inorg. Chem., 2002, 41, 3313–3322; P. Manikandan, B. Varghese and
P. T. Manoharan, J. Chem. Soc., Dalton Trans., 1996, 371–376; M.
Schwach, H. D. Hausen and W. Kaim, Inorg. Chem., 1999, 38, 2242–
2243.
9 L. Yang and R. P. Houser, Inorg. Chem., 2006, 45, 9416–9422.
10 L. M. Engelhardt, C. Pakawatchai, A. H. White and P. C. Healy,
J. Chem. Soc., Dalton Trans., 1985, 117–123; A. Habiyakare, E. A. C.
Lucken and G. Bernardinelli, J. Chem. Soc., Dalton Trans., 1991, 2269–
2273; D. Hagrman, C. Sangregorio, C. J. O’Connor and J. Zubieta,
J. Chem. Soc., Dalton Trans., 1998, 3707–3709; P. C. Healy, J. D. Kildea,
B. W. Skelton and A. H. White, Aust. J. Chem., 1989, 42, 115–136; T.
Kawano, J. Kuwana and I. Ueda, Bull. Chem. Soc. Jpn., 2003, 76, 789–
797; B. Liu and Q. Yuan, Inorg. Chem. Commun., 2005, 8, 1022–1024;
S. Lopez and S. W. Keller, Inorg. Chem., 1999, 38, 1883–1888; R. E.
Marsh, Acta Crystallogr., Sect. B, 1997, 53, 317–322; M. Munakata, S.
Kitagawa, H. Shimono and H. Masuda, Inorg. Chim. Acta, 1989, 158,
217–220; M. Munakata, M. Maekawa, S. Kitagawa, M. Adachi and H.
Masuda, Inorg. Chim. Acta, 1990, 167, 181–188; Y. P. Ren, X. J. Kong,
L. S. Long, R. B. Huang and L. S. Zheng, Cryst. Growth Des., 2006,
6, 572–576; X.-S. Wang, H. Zhao, Z.-R. Qu, Q. Ye, J. Zhang, R.-G.
Xiong, X.-Z. You and H.-k. Fun, Inorg. Chem., 2003, 42, 5786–5788.
11 S. Banthia and A. Samanta, Inorg. Chem., 2004, 43, 6890–6892.
12 I. Sanyal, K. D. Karlin, R. W. Strange and N. J. Blackburn, J. Am.
Chem. Soc., 1993, 115, 11259–11270.
26 E. I. Solomon, Inorg. Chem., 2006, 45, 8012–8025.
27 E. L. Muetterties and L. J. Guggenberger, J. Am. Chem. Soc., 1974, 96,
1748–1756.
28 P. Murrayrust, H. B. Burgi and J. D. Dunitz, J. Am. Chem. Soc., 1975,
97, 921–922.
29 S. Keinan and D. Avnir, J. Chem. Soc., Dalton Trans., 2001, 941–947;
S. Keinan and D. Avnir, Inorg. Chem., 2001, 40, 318–323.
30 H. B. Burgi and J. D. Dunitz, in Structure Correlation, ed. H. B. Burgi
and J. D. Dunitz, VCH, Weinheim, 1994, vol. 1, pp. 163–204; T. Auf
der Heyde, in Structure Correlation, ed. H. B. Burgi and J. D. Dunitz,
VCH, Wienheim, 1994, vol. 1, pp. 337–368.
31 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C. Verschoor,
J. Chem. Soc., Dalton Trans., 1984, 1349–1356.
13 T. N. Sorrell and D. L. Jameson, J. Am. Chem. Soc., 1983, 105, 6013–
6018.
32 As of November 2006, reference 31 had been cited 1445 times (ISI
Web of Knowledge: Science Citation Index Expanded, http://por-
tal.isiknowledge.com, 15 November 2006). While every citation is not
necessarily associated with the s5 index, it is also likely that many papers
have used the s5 index without citing the paper.
33 S. Toyota and M. Oki, Bull. Chem. Soc. Jpn., 1992, 65, 1832–1840.
34 H. Ho¨pfl, J. Organomet. Chem., 1999, 581, 129–149.
35 J. Vela, S. Stoian, C. J. Flaschenriem, E. Munck and P. L. Holland,
J. Am. Chem. Soc., 2004, 126, 4522–4523; J. Vela, J. Cirera, J. M. Smith,
R. J. Lachiocotte, C. J. Flaschenriem, S. Alvarez and P. L. Holland,
Inorg. Chem., 2007, DOI: 10.1021/ic0609148.
14 L. Le Clainche, M. Giorgi and O. Reinaud, Eur. J. Inorg. Chem., 2000,
1931–1933.
15 M. W. Yu and C. J. Fritchie, J. Biol. Chem., 1975, 250, 946–951.
16 M. A. S. Goher, F. A. Mautner, A. K. Hafez and A. A. Youssef, Poly-
hedron, 2003, 22, 515–520; L. P. Wu, M. Yamamoto, T. KurodaSowa,
M. Maekawa, Y. Suenaga and M. Munakata, J. Chem. Soc., Dalton
Trans., 1996, 2031–2037.
17 B. DelavauxNicot, A. Bigeard, A. Bousseksou, B. Donnadieu and G.
Commenges, Inorg. Chem., 1997, 36, 4789–4797.
18 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885–3896.
19 L. R. Whiteaker, U. Pal Chaudhuri, D. R. Powell and R. P. Houser,
Acta Crystallogr., Sect. E, 2006, 62, o3337–o3338; L. R. Whiteaker, U.
Pal Chaudhuri, D. R. Powell and R. P. Houser, Acta Crystallogr., Sect.
E, 2006, 62, o3339–o3340.
36 A. R. Hutchison, A. Mitra and D. A. Atwood, Main Group Chem.,
2005, 4, 187–200.
37 T. C. Harrop, D. T. Song and S. J. Lippard, J. Am. Chem. Soc., 2006,
128, 3528–3529.
38 K. Petratos, Z. Dauter and K. S. Wilson, Acta Crystallogr., Sect. B,
1988, 44, 628–636.
39 E. N. Baker, J. Mol. Biol., 1988, 203, 1071–1095.
20 F. Khajenhouri, S. Motallebi and E. A. C. Lucken, J. Mol. Struct.,
1995, 345, 277–281.
21 T. Kawano, J. Kuwana, C. X. Du and I. Ueda, Inorg. Chem., 2002, 41,
40 D. Reardon, G. Aharonian, S. Gambarotta and G. P. A. Yap,
Organometallics, 2002, 21, 786–788.
4078–4080.
22 T. Kawano, J. Kuwana, T. Shinomaru, C. X. Du and I. Ueda, Chem.
Lett., 2001, 1230–1231.
23 K. D. Karlin, R. W. Cruse, Y. Gultneh, A. Farooq, J. C. Hayes and J.
Zubieta, J. Am. Chem. Soc., 1987, 109, 2668–2679.
24 C. Santini, M. Pellei, G. G. Lobbia, A. Cingolani, R. Spagna and M.
Camalli, Inorg. Chem. Commun., 2002, 5, 430–433.
41 L. A. Mendez, J. Jimenez, E. Cerrada, F. Mohr and M. Laguna, J. Am.
Chem. Soc., 2005, 127, 852–853.
42 N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877–910.
43 G. M. Sheldrick, SHELXTL Version 6.10 Reference Manual, Bruker
AXS Inc., Madison, Wisconsin, USA, 2000; G. M. Sheldrick, Interna-
tional Tables for Crystallography, Kluwer, Boston, 1995, vol. C.
964 | Dalton Trans., 2007, 955–964
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