Asymmetric dinuclear copper(i) complexes of bis-(2-(2-pyridyl)ethyl)-2-(N- toluenesulfonylamino)ethylamine with short copper-copper distances

Article abstract of DOI:10.1039/b705684b

Addition of two equivalents of CuCl to deprotonated bis-(2-(2-pyridyl) ethyl)-2-(N-toluenesulfonylamino)ethylamine (PETAEA) and its derivatives yielded new types of dinuclear Cu(i) complexes, Cu(-PETAEA)CuCl, Cu(-PEMAEA)CuCl, and Cu(-PENAEA)CuCl (PEMAEA is the 4-methoxyphenyl derivative of PETAEA and PENAEA is the 4-nitrophenyl derivative), exhibiting a four coordinate N₄Cu center, a two coordinate NCuCl center, and a metal-metal distance within the range of 2.6572(8) to 2.6903(3) A. Analysis of the covalent radii for four coordinate and two coordinate copper(i), the acute copper-nitrogen-copper angles, and density functional theory (DFT) calculations suggest a weak attraction between the two copper atoms. The complexes apparently formed in a two-step process with the formation of the tetracoordinate mononuclear complex preceding the coordination of a second equivalent of CuCl to the lone pair of the sulfonamidate ligand. The Royal Society of Chemistry.

Full text of DOI:10.1039/b705684b

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www.rsc.org/dalton | Dalton Transactions
Asymmetric dinuclear copper(I) complexes of bis-(2-(2-pyridyl)ethyl)-2-
(N-toluenesulfonylamino)ethylamine with short copper–copper distances†‡
Jocelyn M. Goodwin, Pin-Chieh Chiang, Marcin Brynda, Katerina Penkina, Marilyn M. Olmstead and
Timothy E. Patten*
Received 16th April 2007, Accepted 9th May 2007
First published as an Advance Article on the web 24th May 2007
DOI: 10.1039/b705684b
Addition of two equivalents of CuCl to deprotonated bis-(2-(2-pyridyl)ethyl)-2-
(N-toluenesulfonylamino)ethylamine (PETAEA) and its derivatives yielded new types of dinuclear
Cu(I) complexes, Cu(l-PETAEA)CuCl, Cu(l-PEMAEA)CuCl, and Cu(l-PENAEA)CuCl (PEMAEA
is the 4-methoxyphenyl derivative of PETAEA and PENAEA is the 4-nitrophenyl derivative),
exhibiting a four coordinate N4Cu center, a two coordinate NCuCl center, and a metal–metal distance
˚
within the range of 2.6572(8) to 2.6903(3) A. Analysis of the covalent radii for four coordinate and two
coordinate copper(I), the acute copper–nitrogen–copper angles, and density functional theory (DFT)
calculations suggest a weak attraction between the two copper atoms. The complexes apparently
formed in a two-step process with the formation of the tetracoordinate mononuclear complex preceding
the coordination of a second equivalent of CuCl to the lone pair of the sulfonamidate ligand.
Additionally, close copper–copper contacts have been observed
Introduction
in copper(I) containing metalloproteins.12–14 There has been much
debate over the existence of “cuprophilic” interactions to account
for such distances or if geometric constraints of the ligand sphere
dictate the copper–copper distance.15–17 These short metal–metal
distances are found in complexes derived from the clustering of
monomeric species14,18–23 or in complexes with bridging ligands
that bring the metal centers in close proximity. In the latter subset,
many of the complexes have symmetric structures in which each
metal center has the same ligand set.17,24–37 The remainder of
the complexes contain copper centers with different coordination
environments, either as a result of additional ligand coordination
breaking the symmetry of a parent complex38–40 or due to the
supporting ligand enforcing different coordination environments
for the metal centers.41 There are also two examples of unsupported
close copper–copper contacts between complexes in solid state
structures.42,43
Of the group of controlled/living radical polymerizations, atom
transfer radical polymerization (ATRP)1–5 is the subcategory
that utilizes transition metal catalyzed halogen atom transfer to
activate the dormant alkyl halide chain ends and to deactivate
the propagating radicals. For practical applications in the lab-
oratory, mixtures of ligand and metal salts are used to form
the catalyst in situ. The solution chemistries of such catalyst
mixtures are seldom well-understood, and it is quite possible
that the compositions and structures of the complexes involved
in the atom transfer chemistry may be different than assumed.
A current line in ATRP research is to prepare coordination
complexes whose solid state and solution structures are known and
then to apply them to studying fundamental aspects of the poly-
merization catalysis.6–10 While some ligand–metal combinations
have yielded mononuclear complexes capable of catalyzing atom
transfer, other combinations have yielded dinuclear complexes
and more complicated structures.6,10 Here, we report the unantic-
ipated coordination of copper(I) chloride to copper(I) complexes
reported in prior studies: ligand = bis-(2-(2-pyridyl)ethyl)-2-(N-
toluenesulfonylamino)ethylamine (PETAEA).8 The resulting new
type of dinuclear Cu(I) complex exhibits linear and tetrahedral
coordination environments and contains a short metal–metal
distance.
Experimental
Materials
CuCl (Acros) was purified until colorless by grinding into a fine
powder using a mortar and pestle, stirring in glacial acetic acid,
consecutive washes of absolute ethanol and diethyl ether, and
removal of volatile materials under vacuum. All other materials
were purchased from commercial sources and purified using
standard techniques. All syntheses, reactions, and polymerizations
were conducted under inert atmospheres using dry box or Schlenk
techniques. PETAEA, Na(PETAEA) and Cu(PETAEA) were
prepared according to the procedures of Goodwin et al.8
Di- and polynuclear copper(I) complexes are interesting in their
own right as they sometimes display remarkable structures with
11
˚
˚
short metal–metal distances in the range of 2.35 A to 2.80 A.
Department of Chemistry, University of California at Davis, One Shields
Avenue, Davis, CA, 95616-5295, USA. E-mail: patten@chem.ucdavis.edu;
Fax: +1 530 752 8995; Tel: +1 530 754 6181
† CCDC reference numbers 644442–644444. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b705684b
Characterizations
‡ Electronic supplementary information (ESI) available: Crystallographic
data for complexes 1 to 3, and molecular orbital plots for the LUMO and
top 11 HOMOs from the DFT calculations performed on complex 1. See
DOI: 10.1039/b705684b
1
13C{ H} and 1H NMR spectra were recorded on a Varian Mercury
300 NMR spectrometer. Chemical shifts were referenced to the
proton or carbon signal of the NMR solvent. FTIR spectra
3086 | Dalton Trans., 2007, 3086–3092
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were obtained with a Mattson Galaxy Series FTIR 3000. UV-
Vis spectra were recorded on an HP 8452A spectrophotometer
in THF solvent. Elemental analyses were performed by Midwest
Microlabs.
Synthesis of Cu(l-PEMAEA)CuCl. Na(PEMAEA) (0.568 g,
1.23 mmol) was dissolved in acetonitrile (50 mL). Next, CuCl
(0.233 g, 2.35 mmol) was added. A yellow solution formed and
was filtered away from the residual white solid. The solution was
partially evaporated under vacuum and yellow crystals formed.
The crystals were isolated by cannula filtration to give 0.622 g
Synthesis of Cu(PETAEA)/Cu(l-PETAEA)CuCl mixture.
Na(PETAEA) (1.23 g, 2.75 mmol) was dissolved in THF (75 mL).
Next, CuCl (0.273 g, 2.80 mmol) was added and the solution
was heated slightly to aid complexation. The orange solution was
cannula filtered away from the white solid and volatile materials
were removed under high vacuum leaving an orange powder. Some
orange powder remained in the original flask indicating very low
solubility in the solvent. The orange powder was recrystallized
from acetonitrile to give an approximately 50 : 50 mixture of red–
orange rhombic crystals and bright yellow needles. No mass was
obtained of the mixture of products. X-Ray analysis showed that
the red–orange crystals were Cu(I)PETAEA and the yellow needles
were Cu(l-PETAEA)CuCl.
1
(88% yield) of yellow needles. H NMR (300 MHz, CD3CN): d
8.71 (d, 2H, J = 4), 7.82 (d, 2H, J = 5 Hz), 7.76 (td, 2H, J =
1, 6 Hz), 7.29 (m, 4H), 6.96 (d, 2H, J = 7 Hz), 2.81, (t, 2H, J =
6 Hz), 2.82 (br s, 12H) ppm. 13C NMR (75 MHz, CD3CN): d 162.4,
160.9, 154.0, 151.4, 138.1, 136.5, 129.7, 126.1, 123.4, 114.5, 56.9,
56.2, 53.876, 46.8, 35.2 ppm. UV-Vis (THF): kmax = 312 nm (e =
3877 M−1 cm−1), kmax = 406 nm (e = 1055 M−1 cm−1, shoulder).
Elemental analysis: Calculated, C, 45.88%; H, 4.52%; N, 9.35%;
Found, C, 45.94%; H, 4.52%; N, 9.27%.
PENAEA (1.71 g, 3.77 mmol) was dissolved in acetonitrile
(60 mL) then NaOH (0.307 g, 7.67 mmol) was added, and the
solution was stirred for 2 h while yellow solid formed. Next, CuCl
(0.357 g, 3.61 mmol) was added, and the solution was heated to
help dissolve the solids. The reaction mixture was stirred overnight,
but the product never fully dissolved. The solution was cannula
filtered from the remaining solids, and volatile materials were
removed under vacuum to give a reddish-brown sticky powder.
The product was very difficult to remove from the sides of the flask,
so the amount of recovered solid amounted to 0.106 g (9.5% yield).
Crystals for X-ray crystallography were grown from a saturated
THF solution. 1H NMR (400 MHz, CD3CN): d 8.65 (d, 2H, J =
4 Hz), 8.21 (d, 2H, J = 9 Hz), 7.99 (d, 2H, J = 9 Hz), 7.77 (td,
2H, J = 2, 8 Hz), 7.30 (d, 2H, J = 8 Hz), 7.29 (d, 2H, J =
8 Hz), 2.92, (br s, 2H), 2.80 (s, 8H), 2.72 (t, 2H, J = 6 Hz). 13C
NMR (Insufficient solubility). Elemental analysis: Calculated, C,
42.82%; H, 3.92%; N, 11.35%; Found, C, 43.39%; H, 4.13%; N,
11.61%.
1
For Cu(l-PETAEA)CuCl: H NMR (400 MHz, CD3CN): d
8.64 (d, 2H, J = 5 Hz), 7.75 (td, 2H, J = 2, 8 Hz), 7.66 (d, 2H,
J = 7 Hz), 7.28 (d, 2H, J = 8 Hz), 7.26 (d, 2H, J = 8 Hz), 2.84,
(t, 2H, J = 5 Hz), 2.78 (br s, 8H), 2.62 (br s, 2H), 2.35 (s, 3H).
13C NMR (100 MHz, CD3CN): d 160.9, 151.3, 138.0, 129.8, 127.7,
126.0, 123.3, 57.1, 54.0, 35.3, 21.3. IR (KBr, cm−1): m 3064 (w),
2951 (m), 2899 (w), 2849 (w), 1596 (m), 1564 (w), 1478 (m), 1440
(m), 1342 (w), 1284 (m), 1282 (m). UV-Vis (THF): kmax = 316 nm
(e = 4770 M−1 cm−1), kmax = 406 nm (e = 533 M−1 cm−1, shoulder).
Elemental analysis: Calculated, C, 47.13%; H, 4.64%; N, 9.56%;
Found, C, 46.57%; H, 4.67%; N, 9.62%.
Addition of CuCl to Cu(PETAEA). Cu(PETAEA) (0.139 g,
0.285 mmol) was dissolved in acetonitrile (25 mL), and then CuCl
(0.0464 g, 0.469 mmol) was added. The orange solution imme-
diately turned yellow. The yellow supernatant was isolated from
excess copper(I) chloride via cannula filtration and solvent was
removed under vacuum until significant precipitation occurred.
The supernatant was removed from the solid by cannula filtration
and the remaining yellow powder was dried under vacuum
to yield Cu(l-PETAEA)CuCl (0.0345 g, 21%). 1H NMR, 13C
NMR, IR, and UV-Vis data all matched that obtained for Cu(l-
PETAEA)CuCl prepared in the preceding section; Elemental
analysis: Calculated, C, 47.13%; H, 4.64%; N, 9.56%; Found, C,
46.85%; H, 4.64%; N, 9.60%.
X-Ray structure determinations
For complexes 1 and 3, diffraction data were collected with a
Bruker SMART 1000 diffractometer, graphite-monochromated
Mo-Karadiation, and a nitrogen cold stream provided by a CRYO
Industries apparatus. Corrections for absorption were applied
using the program SADABS 2.10.44 For complex 2, diffraction
data were collected with a Siemens P4 diffractometer with the
use of a copper rotating anode source and Siemens LT-2 low
temperature apparatus. A correction for absorption was applied
using the program XABS2.45 The structures were solved by direct
methods (SHELXS-9746) and refined by full-matrix least-squares
on F2 (SHELXL-9746). All non-hydrogen atoms were refined
with anisotropic thermal parameters. Hydrogen atoms on water
molecules were located on a difference map and refined using
distance restraints. Other hydrogen atoms were added by geometry
and refined using a riding model. The maximum and minimum
Complexation of Na(PETAEA) with CuCl in different solvents.
Na(PETAEA) was dissolved in the appropriate solvent then CuCl
was added in the quantities shown in Table 1. Because of the
scale of this reaction, qualitative observations were made without
isolating the product. In acetone a red solution was observed; in
THF a yellow solution was formed; and in toluene a very pale
yellow solution formed.
Table 1
Solvent
Mass of Na(PETAEA)/mg
mmoles of Na(PETAEA)
Mass of CuCl/mg
mmoles of CuCl
Observed color
Acetone
THF
Toluene
49.8
50.9
51.4
0.102
0.105
0.106
10.9
11.6
10.3
0.110 mmol
0.117 mmol
0.104 mmol
Red–orange
Yellow
Pale yellow
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Table 2 Summary of crystal structure determinations for complexes 1 to 3
Cu(l-PETAEA)CuCl
Complex 1
Cu(l-PEMAEA)CuCl
Complex 2
Cu(l-PENAEA)CuCl
Complex 3
Formula
Fw
C23H27ClCu2N4O2S
586.08
93(2)
C23H27ClCu2N4O3S
602.08
133(2)
C22H24ClCu2N5O4S
617.05
90(2)
Temp./K
Cryst. system
Space group
Z
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1
P1
P1
2
2
4
˚
a/A
9.1111(6)
10.8627(7)
13.4127(9)
68.776(3)
84.102(3)
87.524(3)
1230.86(14)
1.581
9.5617(12)
9.9875(10)
13.4720(12)
92.187(8)
91.981(9)
108.049(8)
1220.9(2)
1.638
8.9933(9)
10.8437(11)
26.491(3)
94.085(3)
98.630(4)
92.039(3)
2544.9(4)
1.611
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
D
calc/Mg m−3
l/mm−1
1.949 (Mo-Ka)
0.0241
0.0646
4.233 (Cu-Ka)
0.0628
0.1700
1.897 (Mo-Ka)
0.0270
0.0707
R1 (obs data)a
wR2 (all data)b
GOF
1.04
1.11
1.002
a
2
2
R1 = RꢀFo| − |Fcꢀ/R|Fo|.b wR2 = [R[w(Fo − Fc2)2]/R[(wFo2)2]]1/2; w = 1/[r2(Fo2)+(aP)2 + bP], where P = (Fo + 2Fc2)/3.
peaks in the final difference Fourier map for each structure can
be found in the data tables located in the ESI.‡ Crystal data and
refinement details for the complexes are shown in Table 2.†
Density functional theory calculations
All the DFT calculations were performed at B3LYP/6-31g* level
of theory using Gaussian 03 program.47 The overlap populations
and the Wiberg bond orders were computed and analyzed
using AOMIX software.48,49 The plots of molecular orbitals were
generated with MOLEKEL graphical interface.50
Results and discussion
A dinuclear copper(I) complex of bis-(2-(2-pyridyl)ethyl)-2-
(N-toluenesulfonylamino)ethylamine (PETAEA) was discovered
serendipitously during an attempt to synthesize Cu(PETAEA) us-
ing THF solvent rather than CH3CN. The sodium salt of PETAEA
was prepared in THF by treatment of PETAEA with NaH, and
then CuCl was added. After crystallization, a mixture of red–
orange rhombic crystals and bright yellow needles was obtained.
In contrast, when the synthesis was performed using CH3CN
solvent, only the red–orange rhombic crystals of Cu(PETAEA)
were isolated. Despite the distinct crystal appearances, the NMR
spectra of the two materials were very similar. The yellow needles,
complex 1, were suitable for X-ray analysis, so a molecular
structure was obtained (Fig. 1, Table 2). X-Ray analysis also
confirmed that the red–orange rhombic crystals were, in fact,
Cu(PETAEA).
The structure of complex 1 was an unanticipated dinuclear
copper(I) compound with the basic structure of Cu(PETAEA)
and an additional equivalent of CuCl coordinated to the free lone
pair of the sulfonamidate donor nitrogen. Complex 1 has one
copper center with distorted tetrahedral coordination geometry,
Cu1, and another with linear coordination geometry, Cu2. Table 3
compares key structural parameters of complex 1 and related
Fig. 1 Molecular structure of Cu(l-PETAEA)CuCl, complex-1. Dis-
placement ellipsoids are drawn at the 50% probability level.
complexes (2 and 3, Fig. 2 and 3, Table 2) with the parent complex,
Cu(PETAEA). In complex 1 the Cu1–N bond distances for one
pyridine nitrogen, N3, and tertiary amine nitrogen, N2, were
˚
˚
0.035 A and 0.036 A (respectively) shorter than those found for
Cu(PETAEA). The other pyridine nitrogen, N1–Cu1 distance was
˚
0.019 A longer than that found in the parent complex. The Cu1–
N distance for the sulfonamidate nitrogen donor was significantly
˚
longer (by 0.137 A) than that found in Cu(PETAEA). The bond
angles between the neutral amine donors N3–Cu1–N2, N2–Cu1–
N1, and N3–Cu1–N1 were 3.1◦, 0.4◦, and 3.2◦ (respectively) larger
than those found in the parent complex. These bond distance and
bond angle data were consistent with Cu1 being located closer
to the N1–N2–N3 face of the tetrahedral coordination sphere in
complex 1 versus the parent complex. This repositioning of the
copper center would open up extra space to accommodate the
additional steric demands imposed by the CuCl unit.
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◦
˚
The second copper(I) center, Cu2, exhibited a linear coordi-
Table 3 A comparison of key bond lengths (A) and bond angles ( )
between the parent complex, Cu(PETAEA), and complexes 1 through 3
˚
nation geometry with a Cu2–N4 bond distance of 1.9054(15) A
˚
and a Cu2–Cl1 bond distance of 2.1060(5) A. The Cu–Cl bond
Cu(PETAEA) Complex 1 Complex 2 Complex 3
distance is very similar to that found for the dichlorocuprate
anion, 2.107(1) A, and the Cu–N bond distance is similar to
the amidate nitrogen–copper bond distance of 1.875(2) A found
in another heterocuprate anion containing complex.10 The N4–
Cu2–Cl1 bond angle of 174.18(5)◦ falls short of 180◦ for linear
geometry but is well within the range of bond angles found for
dihalocuprate anions, as these anions are known to distort slightly
from linear geometry in order to accommodate crystal packing
forces.51
51
˚
N1–Cu1
N3–Cu1
N2–Cu1
N4–Cu1
N4–Cu2
Cl1–Cu2
Cu1–Cu2
1.957(4)
2.087(4)
2.203(4)
1.979(4)
N/A
N/A
N/A
1.9755(15)
2.0517(15)
2.1671(15)
2.1158(14)
1.9054(15)
2.1060(5)
2.6903(3)
1.990(4)
2.050(4)
2.154(3)
2.126(3)
1.900(3)
2.0960(12)
2.6572(8)
1.9797(17)
2.0264(17)
2.1456(16)
2.1980(16)
1.8988(17)
2.1068(6)
2.6730(4)
˚
Cu1–N4–Cu2 N/A
83.81(5)
99.48(6)
100.75(6)
109.29(6)
174.18(5)
108.60(6)
85.04(5)
82.39(13)
99.39(15)
100.50(14)
109.96(14)
174.12(11)
109.94(14)
85.75(13)
137.90(14)
81.10(6)
102.61(7)
101.77(7)
116.30(7)
176.67(5)
107.91(6)
82.60(6)
N3–Cu1–N2
N2–Cu1–N1
N3–Cu1–N1
N4–Cu2–Cl1
N3–Cu1–N4
N2–Cu1–N4
N1–Cu1–N4
96.43(16)
100.35(17)
106.11(17)
N/A
107.93(17)
84.69(16)
144.77(17)
The bond distances and angles about the copper centers in
complex 1 are most consistent with a structure comprised of a
tetrahedrally coordinated copper(I) cation bridged by a dative
interaction with the sulfonamidate nitrogen lone pair to a two
coordinate heterocuprate anion (Fig. 4). This conclusion is also
supported by the significantly longer N4–Cu1 distance in complex
1 versus Cu(PETAEA), which is most likely due to both the
delocalization of the sulfonamidate negative charge onto the
chloride ligand in the heterocuprate section and the greater steric
bulk of the three-coordinate versus two-coordinate sulfonamidate
nitrogen donor. Two derivatives of complex 1 were prepared: Cu(l-
PEMAEA)CuCl (PEMAEA is the 4-methoxyphenyl derivative
of PETAEA), complex 2, and Cu(l-PENAEA)CuCl (PENAEA
is the 4-nitrophenyl derivative of PETAEA), complex 3. The
acidity of the ligand’s sulfonamide group varied with substitution
as follows: –CH3, pKa = 10.17; –OCH3, pKa = 10.22; –NO2,
pKa = 9.14.52 Assuming that the pKa difference between PETAEA
and PEMAEA is too small to have a significant impact on the
complex’s structure and considering only differences between
PETAEA and PENAEA, the N4–Cu1 distance correlated in-
versely with the sulfonamide acidity, but there was little difference
in N4–Cu2 distances between the three complexes. The data were
consistent with an increase in electron withdrawing ability of the
ligand weakening the dative interaction between the sulfonamide
nitrogen and the tetrahedrally coordinated copper center but not
affecting the covalent bonding between the sulfonamide nitrogen
and the linearly coordinated copper center to a significant degree.
The Cu1–Cu2 distance also did not appear to correlate with the
pKa of the ligand’s sulfonamide group.
140.05(6)
133.03(6)
Fig. 2 Molecular structure of Cu(l-PEMAEA)CuCl, complex 2. Dis-
placement ellipsoids are drawn at the 50% probability level.
Fig. 4 Drawing of complex 1 showing a probable distribution of partial
charges within the complex.
˚
The Cu1–Cu2 distance in complex 1 was 2.6903(3) A. A Cam-
bridge Crystallographic Database search for dinuclear copper(I)
complexes indicated that this distance was well within the range
of Cu–Cu distances for which ligand-supported “cuprophilic”
interactions have been proposed. This distance was also less than
the interatomic distances for unsupported short copper–copper
43
˚
distances observed in solid state structures (2.905(3) A and
Fig. 3 Molecular structure of Cu(l-PENAEA)CuCl, complex 3. Dis-
placement ellipsoids are drawn at the 50% probability level.
42
˚
2.8924(3) A ). Thus, a question raised by the novel structure of
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complex 1 was what kind of interaction exists between the two
copper centers?
The covalent radius of linear copper(I) has been measured at
53
˚
˚
1.13 A. This value (1.13 A) added to the covalent radius of
˚
chlorine (0.995 A) yields a calculated copper(I) chlorine bond dis-
˚
tance of 2.125 A, which is slightly longer than the experimentally
˚
determined distance of 2.1060(5) A in complex 1 and another
10
˚
reported (NCuCl) heterocuprate Cu(I)–Cl distance of 2.103(1) A.
A reasonable model compound to estimate a radius for tetraco-
ordinate Cu(1) in complex 1 would be a homoleptic, tetrahedral
copper(I) ion with amine/pyridyl ligands: Cu(pyridine)4 .54 The
+
˚
copper–nitrogen bond distance in this complex is 2.046(4) A, and
˚
subtraction of the covalent radius of nitrogen (0.75 A) yields a
˚
radius of 1.296 A for the copper(I) ion. The sum of the radii for
˚
Cu(1) and Cu(2), as estimated, is 2.426 A which is less than the
˚
experimental separation of 2.6903(3) A.
While this estimate for the sum of the radii of Cu(1) and Cu(2)
would suggest that a bonding interaction between Cu(1) and Cu(2)
is not necessary to account for the structure of complex 1, the acute
Cu1–N4–Cu2 angle of 83.81(5)◦ is substantially less than would be
expected for pyramidal geometry about N4. The acute angle would
support the existence of an attractive interaction between the metal
centers. Such acute copper–ligand–copper angles have also been
observed in V-geometry ligand bridged dicopper(I) complexes
Fig. 5 Molecular orbital plots for HOMO-4 (top) and HOMO-5
(bottom) for DFT calculations performed on complex 1.
2
2
dx
orbitals but also exhibited some additional s character. The
−y
calculated Wiberg bondorder (BO) for this Cu(I)–Cu(I) interaction
is relatively small (0.18) but not negligible.
such as Cu2(Si(SiMe3)3)2BrLi(thf)3 (60.6◦, dCu–Cu = 2.369(1) A)
22
˚
and calculated for Se(CuPH3)2 (75.4◦, dCu–Cu = 2.695 A) and
In order to compare this interaction to similar Cu–Cu bond-
ing in other known compounds exhibiting short Cu(I)–Cu(I)
distances, we performed analogous DFT calculations on X-
ray extracted coordinates of Cu2R2 (R = 2-C(SiMe3)2C5H4N)56
and [Cu2(dcpm)2](ClO4)2 (dcpm = bis(dicyclohexylphosphanyl)-
methane).59 For the calculation involving the latter compound,
the bulky cyclohexyl groups were replaced by methyl groups
˚
Se(CuPMe3)2 (74.9◦, dCu–Cu = 2.690 A).
55
˚
In order to assess the degree of the bonding interaction between
the two copper centers, we performed density functional theory
(DFT) calculations on complex 1, as well as on two other
molecular systems exhibiting very short Cu(I)–Cu(I) distances. The
details of the quantum mechanical calculations are reported in
the Experimental section. Complexes with Cu(I)–Cu(I) distances
(dmpm). The metal–metal separations in these species are 2.412
56
2+
˚
˚
as short as 2.412 A are well known; however, the existence of
and 2.639 A, respectively. The calculated BO for [Cu2(dmpm)2]
weak bonding between the formally closed shell d10 centers is still
subject to debate. Despite several theoretical studies that addressed
these so called “cuprophilic” interactions, no clear picture of
this unusual metal–metal bonding is yet available. Early extended
Hu¨ckel calculations by Mehrotra and Hoffmann15 showed the
importance of the copper (n − 1)d − ns/np atomic orbital mixing
in stabilizing the Cu–Cu interaction. The electron correlation
effects that contribute to this type of weak bonding were also
pointed out by Pyykko¨11 as a possible contributing factor to the
ability of Cu(I) centers to cluster. The DFT study by Hermann,
Boche and Schwerdtfeger57 showed that the strength of cuprophilic
interactions depends on the nature of the other ligands and that
the strength of the Cu–Cu interaction increases with increasing r-
donor and p-acceptor capability of the ligands. Vega and Saillard58
have studied Cu(I)–Cu(I) interactions in a series of Cu(I) clusters
by means of DFT and concluded that the extent of the metal–
metal interaction mainly depended on the electron donor ability
of the attached ligand. It has also been shown that intermolecular
forces can enhance the metal–metal interaction.37
(which exhibits Cu–Cu distance similar to complex 1) is 0.49,
while for Cu2R2 the calculated Cu–Cu BO is 0.79. In general,
although the Cu–Cu interaction between two d10 closed shell ions
possesses a formal bond order of 0, the non-negligible extent of
the overlap between the hybrid metal 3d/4s orbitals results in
weak bonding with apparent bond orders60 between 0 and 1. Such
bonding should be therefore characterized as being a relatively
weak interaction, somewhere between van der Waals forces and a
real atomic bond in a purely chemical sense.
This structure of complex 1 suggested a synthetic pathway in
which Cu(PETAEA) was formed first, and then an additional
equivalent of CuCl was incorporated from the reaction mixture
(Scheme 1). The starting material, Na(PETAEA), is less soluble in
THF than CH3CN, so the rate of formation of Cu(PETAEA)
should be slower in the former solvent. Thus, in THF the
Cu(PETAEA) formed early in the reaction remained in contact
with CuCl for a significant amount of time. Under these condi-
tions, the kinetics of CuCl complexation with Cu(PETAEA) could
be competitive with its reaction with Na(PETAEA). The incorpo-
ration of additional equivalents of copper(I) halides into a complex
has also been observed for organocopper(I) compounds.21,22
The formation of the CuCl adducts was investigated using
several deliberate routes. First, the role of deprotonated ligand
solubility was investigated. Solvents of several different dielectric
constants were selected (acetonitrile, acetone, THF and toluene),
The DFT calculations performed on complex 1 using the
geometry extracted from the X-ray crystal structure showed non-
2
negligible molecular orbital (MO) overlap between the hybrid dz
2
2
and dx
orbitals of the two copper centers. Analysis of the wave
−y
function coefficients for HOMO-4 and HOMO-5 (Fig. 5) showed
2
that these hybrid MO’s are mainly composed of the Cu dz and
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Foundation for a Graduate Research Fellowship in Functional
Materials.
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In summary, dinuclear copper(I) complexes were prepared using
PETAEA and its derivatives, and these compounds featured a four
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˚
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Acknowledgements
We thank the ACS Petroleum Research Fund (35150-AC7) for
support of this research. JMG acknowledges the Tyco Electronics
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