Pentafluorophenyl copper-pyridine complexes: Synthesis, supramolecular structures via cuprophilic and π-stacking interactions, and solid-state luminescence

Article abstract of DOI:10.1021/om200989b

The effect of the binding of pyridine ligands to pentafluorophenyl copper, [C₆F₅Cu]₄ (1), on structural features and photophysical properties has been investigated through a combined multinuclear NMR, X-ray crystallography, and photoluminescence study. Reaction of 1 with 2 equiv of pyridine yields a novel pyridine complex, 3, in which the tetranuclear framework of 1 is retained. Complex 3 features a rhombus-shaped tetracopper core with a short diagonal Cu???Cu distance of 2.5941(6) A between the dicoordinate copper centers and a longer distance between the pyridine-coordinated copper centers of 4.178(1) A. In contrast, treatment of 1 with 4 equiv of pyridine results in complete breakdown of the tetranuclear aggregate to give the formally dicoordinate species C₆F ₅Cu(py) (4-H). Reaction of 1 with 2,2′-bipyridine results in formation of the tricoordinate complex C₆F₅Cu(2,2′- bipy) (5). Aggregate breakdown in species 4 and 5 is reflected in a significantly reduced chemical shift difference Δδ( ¹⁹F_meta/para) and a strong downfield shift of the copper-bound carbon atoms in the ¹³C NMR spectra in comparison to 3. A dynamic equilibrium is established at ratios of py/C₆F ₅Cu ranging from 0 to 2. The solid-state structures of all compounds have been determined by single-crystal X-ray crystallography. The supramolecular assembly of complex 3 via arene-arene π-interactions leads to a network structure with solvent-filled channels propagating through the lattice along the crystallographic c axis. The 2,2′-bipyridine complex 5 also shows π-stacking as the dominant feature in the extended solid-state structure. In contrast, a different mode of supramolecular assembly is found for 4-H in that cuprophilic interactions lead to assembly into one-dimensional copper chains with equidistant Cu???Cu contacts of 2.8924(3) A. However, the closely related complexes 4-R with methyl or chloro substituents in either the ortho or the para position form supramolecular stacks with structural features that, again, are dominated by offset perfluoroarene-arene interactions with intermolecular plane-to-plane separations of ca. 3.3-3.6 A. The dicoordinate copper atoms are aligned in one-dimensional chains with alternating short and long Cu(I)???Cu(I) distances of 3.531(1)/3.698(1) A in 4-pMe, 3.2454(5)/4.2970(5) A in 4-oMe, 3.521(1)/3.784(1) A in 4-pCl, and 3.4797(6)/3.8363(6) A in 4-oCl. Compound 4-H is strongly blue fluorescent at 460 nm in the solid state, but yellow-green fluorescent at 77 K, resulting in an interesting example of luminescence thermochromism. In contrast, the substituted compounds 4-R display strong luminescence only at liquid nitrogen temperature. In all cases, the fluorescence emission band is in the range of ca. 410-425 nm and thus at significantly different energy from that in 4-H, which strongly suggests that the short Cu???Cu contacts in 4-H give rise to unique luminescence properties.

Full text of DOI:10.1021/om200989b

Article
pubs.acs.org/Organometallics
Pentafluorophenyl Copper−Pyridine Complexes: Synthesis,
Supramolecular Structures via Cuprophilic and π-Stacking
Interactions, and Solid-State Luminescence
Ami Doshi,† Anand Sundararaman,† Krishnan Venkatasubbaiah,†,§ Lev N. Zakharov,‡
Arnold L. Rheingold,‡ Mykhaylo Myahkostupov,† Piotr Piotrowiak,† and Frieder Jakle*,†
̈
†Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102, United States
‡Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States
S
* Supporting Information
ABSTRACT: The effect of the binding of pyridine ligands to pentafluorophenyl
copper, [C6F5Cu]4 (1), on structural features and photophysical properties has been
investigated through a combined multinuclear NMR, X-ray crystallography, and
photoluminescence study. Reaction of 1 with 2 equiv of pyridine yields a novel
pyridine complex, 3, in which the tetranuclear framework of 1 is retained. Complex 3
features a rhombus-shaped tetracopper core with a short diagonal Cu···Cu distance of
2.5941(6) Å between the dicoordinate copper centers and a longer distance between the
pyridine-coordinated copper centers of 4.178(1) Å. In contrast, treatment of 1 with 4
equiv of pyridine results in complete breakdown of the tetranuclear aggregate to give the
formally dicoordinate species C6F5Cu(py) (4-H). Reaction of 1 with 2,2′-bipyridine
results in formation of the tricoordinate complex C6F5Cu(2,2′-bipy) (5). Aggregate
breakdown in species 4 and 5 is reflected in a significantly reduced chemical shift
difference Δδ(19Fmeta/para) and a strong downfield shift of the copper-bound carbon
atoms in the 13C NMR spectra in comparison to 3. A dynamic equilibrium is established at ratios of py/C6F5Cu ranging from 0
to 2. The solid-state structures of all compounds have been determined by single-crystal X-ray crystallography. The
supramolecular assembly of complex 3 via arene−arene π-interactions leads to a network structure with solvent-filled channels
propagating through the lattice along the crystallographic c axis. The 2,2′-bipyridine complex 5 also shows π-stacking as the
dominant feature in the extended solid-state structure. In contrast, a different mode of supramolecular assembly is found for 4-H
in that cuprophilic interactions lead to assembly into one-dimensional copper chains with equidistant Cu···Cu contacts of
2.8924(3) Å. However, the closely related complexes 4-R with methyl or chloro substituents in either the ortho or the para
position form supramolecular stacks with structural features that, again, are dominated by offset perfluoroarene−arene
interactions with intermolecular plane-to-plane separations of ca. 3.3−3.6 Å. The dicoordinate copper atoms are aligned in one-
dimensional chains with alternating short and long Cu(I)···Cu(I) distances of 3.531(1)/3.698(1) Å in 4-pMe, 3.2454(5)/
4.2970(5) Å in 4-oMe, 3.521(1)/3.784(1) Å in 4-pCl, and 3.4797(6)/3.8363(6) Å in 4-oCl. Compound 4-H is strongly blue
fluorescent at 460 nm in the solid state, but yellow-green fluorescent at 77 K, resulting in an interesting example of luminescence
thermochromism. In contrast, the substituted compounds 4-R display strong luminescence only at liquid nitrogen temperature.
In all cases, the fluorescence emission band is in the range of ca. 410−425 nm and thus at significantly different energy from that
in 4-H, which strongly suggests that the short Cu···Cu contacts in 4-H give rise to unique luminescence properties.
bridging of two or more copper centers with an organic
moiety.4,6 The degree of aggregation depends on a number of
factors, including steric and electronic effects, and the presence
of external donor ligands.4,7,8 For example, unsubstituted
phenylcopper adopts a polymeric structure and hence is
insoluble in common organic solvents, whereas the presence
of substituents in the ortho position of the aromatic ring leads
to well-defined soluble oligomers as a result of the added steric
INTRODUCTION
■
Organocopper compounds have emerged as an important class
of organometallic compounds. They have attracted consid-
erable interest from both inorganic and organic chemists
because of the rich structural diversity and unusual bonding
patterns, as well as their important roles as intermediates in
coupling reactions and as mild and selective reagents and
catalysts in organic synthesis.1−3 An interesting aspect from a
structural perspective is that monomeric species RCu remain
elusive, whereas well-defined homoleptic organocopper species
[RCu]n with varying degrees of aggregation (n = 2−8) have
been identified in solution and also structurally characterized in
the solid state.2,4,5 Aggregation typically occurs through
Special Issue: Fluorine in Organometallic Chemistry
Received: October 15, 2011
Published: December 1, 2011
© 2011 American Chemical Society
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Article
bulk.4 Thus, 2,4,6-triisopropylphenylcopper features a tetranu-
clear square-planar aggregate due to the presence of sterically
demanding isopropyl groups in the ortho positions of the
phenyl ring.9 van Koten and co-workers have demonstrated
that smaller arylcopper aggregates can be stabilized by the
intramolecular coordination with chelating amino or alkoxy
groups in the ortho position of the aromatic ring.10
(Innovative Technologies; alumina/copper columns for hydrocarbon
solvents), and the chlorinated solvents were subsequently distilled
from CaH2 and degassed via several freeze−pump−thaw cycles.
Pentafluorophenyl copper toluene solvate (2) was prepared according
to a literature procedure.24 Compound 2 was heated to 80 °C under
vacuum to yield the solvent-free complex 1 as a colorless white
powdery solid. The synthesis of C6F5Cu(py) (4-H) has been reported
previously.29 2-Picoline, 4-picoline, 2-chloropyridine, 4-chloropyridine
hydrochloride, and 2,2′-bipyridine were purchased from ACROS.
Pyridine was degassed and distilled from calcium hydride prior to use.
2-Picoline, 4-picoline, and 2-chloropyridine were used as received, and
4-chloropyridine hydrochloride was treated with 6 M sodium
hydroxide solution and extracted with ether. After removal of ether
at 0 °C under vacuum, 4-chloropyridine was dissolved in toluene, and
the solution was degassed via several freeze−pump−thaw cycles prior
to use.
It is also well known that treatment of organocopper species
with strongly coordinating solvents or external ligands can lead
to the breakdown of the aggregated structure.11−13 Smaller
organocopper aggregates are obtained, for example, by using N,
P, or S compounds as donors. For instance, phenylcopper,
which adopts a polymeric structure in the absence of an
external Lewis base, has been isolated as the dimethylsulfide
solvate [PhCu]4(SMe2)2 with a square-planar tetranuclear
copper(I) core,14 while a monomeric species PhCuL was
obtained in the presence of the sterically demanding tridentate
ligand l,l,l-tris[(diphenylphosphino)methyl]ethane (triphos).15
However, in general, it is difficult to predict whether
monomeric species or partially aggregated complexes will be
formed, and surprisingly, few nonaggregated species RCuLn
have been isolated and structurally characterized. In most
complexes, either bulky groups15−18 and/or chelating
ligands19−22 enforce the monomeric structure.23 Lewis base
additives have been related to a beneficial effect of strong donor
solvents, such as DMF, NMP, or HMPA, on the reactivity of
organocopper compounds in organic synthesis, including their
use in carbocupration, stereoselective conjugate additions, and
allylic substitution reactions.2,4,8
We became interested in the coordination behavior and
photophysical properties of pentafluorophenyl copper (1),
because we reasoned that the unique combination of steric
demand and the electron-withdrawing effect of the penta-
fluorophenyl group should enable us to isolate and structurally
characterize otherwise inaccessible Lewis base adducts.24
Compound 1 was first introduced by Cairncross and Sheppard
in 1968, and they determined the aggregation number in
benzene to be 3.75−3.85 by cryoscopy and to be 3.95 by vapor
pressure osmometry, suggesting that a tetranuclear aggregate is
present in solution.25 Using single-crystal X-ray diffraction, we
confirmed that, in the absence of Lewis bases, 1 adopts a
tetrameric structure also in the solid state.26 More importantly,
we discovered the formation of π-complexes, such as
[C6F5Cu]4(toluene)2) (2), in which the tetranuclear aggregate
structure is maintained and investigated the assembly of 1 into
offset supramolecular stacks via multiple Cu···π interactions in
the presence of extended π-systems.26−28 In stark contrast, the
presence of an excess of pyridine results in the breakdown of
the tetranuclear structure with formation of a strongly blue-
luminescent monomeric complex C6F5Cu(py) (py = pyr-
idine).29
1
The 499.91 MHz H, 125.68 MHz 13C, and 470.2 MHz 19F NMR
spectra were acquired on a Varian INOVA NMR spectrometer
equipped with a 5 mm dual broad-band gradient probe (Nalorac,
Varian Inc., Martinez, CA). The 100.5 MHz 13C NMR spectrum was
recorded on a Varian VXR-S spectrometer. All NMR data were
recorded at ambient temperature. 1H and 13C NMR spectra were
referenced internally to the solvent peaks, and 19F NMR spectra were
referenced externally to α,α′,α″-trifluorotoluene (0.05% in C6D6; δ =
−63.73 ppm). The abbreviations Pf and py are used for
pentafluorophenyl and pyridyl, respectively.
Variable-temperature solid-state luminescence data were measured
on a Varian Cary Eclipse Fluorescence spectrophotometer equipped
with an Oxford Instruments Cryostat, model Optistat DN. For
phosphorescence measurements, a delay time td of 0.1 ms and a gate
time tg of 1.0 ms were used with excitation and emission slit widths of
5/10 nm. For the lifetime measurements, thoroughly degassed samples
were excited with 5 ns ∼80 mJ pulses at 355 nm (third harmonic of a
Q-switched Nd:YAG laser, Quantel, Brilliant). The emission was
dispersed through a monochromator (Oriel M257) and detected with
a Hamamatsu R928 photomultiplier. The transients were recorded
using a Tektronix SCD 1000 digital oscilloscope controlled by a
Labview subroutine. Fluorescence decays were fitted as single
exponentials using the Igor software package by Wavemetrics, Inc.
DFT calculations were performed with the Gaussian 03 program.30
Geometries and electronic properties were calculated by means of the
B3LYP hybrid density functional with the basis set of 6-311++G (d,p).
The input files and orbital representations were generated with
Gaussview (scaling radii of 75%). Excitation data were calculated using
TD-DFT (B3LYP).30 Elemental analyses were obtained from
Quantitative Technologies, Inc., Whitehouse, NJ, and Schwarzkopf,
Woodside, NY. Melting points and decomposition temperatures were
determined in sealed capillary tubes and are not corrected.
Spectral Data for 1 in CDCl3 and d5-Pyridine. 19F NMR
(470.2 MHz, CDCl3, 20 °C): δ = −104.1 (m, 2F, ortho-F), −141.5 (t,
J(19F, 19F) = 20 Hz, 1F, para-F), −158.1 (m, 2F, meta-F). 13C NMR
(100.5 MHz, CDCl3, 20 °C): δ = 154.7 (dd, J(19F, C) = 244/22 Hz,
Pf-C2,6), 145.7 (dm, J(19F, C) = 262 Hz, Pf-C4), 137.2 (dm, J(19F, C)
= 260 Hz, Pf-C3,5), 98.7 (t, J(19F, C) = 52 Hz, Pf-C1). 19F NMR
(470.2 MHz, C5D5N, 20 °C): δ = −110.6 (m, 2F, ortho-F), −163.4 (t,
J(F, F) = 20 Hz, 1F, para-F), −164.1 (m, 2F, meta-F); 13C NMR
(125.7 MHz, C5D5N, 25 °C): δ = 150.2 (dd, J(19F, C) = 219 Hz/33
Hz, Pf-C2,6), 138.0 (dm, J(19F, C) = 241 Hz, Pf-C4), 136.6 (dm,
J(19F, C) = 253 Hz, Pf-C3,5), 130.3 (t, J(19F, C) = 80 Hz, Pf-C1).
Synthesis of [C6F5Cu]4(py)2 (3). Neat pyridine (80 μL, 1.0
mmol) was added dropwise to a solution of 1 (0.46 g, 0.50 mmol) in
CH2Cl2 (10 mL) at room temperature. Upon addition of pyridine, the
solution turned yellow. The solution was layered with 10 mL of
pentane and stored at −38 °C. Colorless needle-like crystals formed
after 24 h. The crystals contain one molecule of pentane per main
molecule, but were dried for elemental analysis and for the
determination of the yield. Yield: 0.44 g (82%). For 3: Tm: 145 °C
The aim of the current work was to further investigate the
factors that determine the aggregation behavior of pentafluor-
ophenyl copper−pyridine complexes in solution and the solid
state. Special emphasis is given to the role of π-stacking and
cuprophilic interactions on the supramolecular assembly and
the photophysical properties.
EXPERIMENTAL SECTION
■
All reactions and manipulations were carried out under an atmosphere
of prepurified nitrogen using either Schlenk techniques or an inert-
atmosphere glovebox (Innovative Technologies). Hydrocarbon and
chlorinated solvents were purified using a solvent purification system
1
(dec.). H NMR (500 MHz, CDCl3, 20 °C): δ = 8.35 (d, J = 5.0 Hz,
4H, Py-H2,6), 7.94 (t, J = 7.5 Hz, 2H, Py-H4), 7.50 (m, 4H, Py-H3,5).
19F NMR (470.2 MHz, CDCl3, 20 °C): δ = −108.2 (br m, 8F, ortho-
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Table 1. Experimental Data for X-ray Diffraction Studies
3
4-pMe
4-oMe
4-pCl
4-oCl
5
empirical formula
formula weight
T, K
C39H22Cu4F20N2
1152.75
C12H7CuF5N
323.73
C12H7CuF5N
323.73
C11H4ClCuF5N
344.15
C11H4ClCuF5N
344.15
C16H8CuF5N2
386.79
218(2)
100(2)
100(2)
100(2)
100(2)
100(2)
wavelength, Å
cryst syst
space group
a, Å
0.71073
0.71073
triclinic
1.54178
0.71073
1.54178
0.71073
monoclinic
C2/c
monoclinic
P21/c
triclinic
monoclinic
P21/c
monoclinic
P21/c
P1
P1
̅
̅
20.1931(13)
12.4763(9)
15.8809(11)
90
5.9779(4)
7.2228(6)
13.4698(10)
91.634(2)
90.298(2)
95.320(2)
578.83(8)
2
6.10850(10)
7.5034(2)
25.0325(5)
90
5.9903(9)
7.2800(10)
13.0816(19)
88.434(2)
86.879(2)
83.596(2)
565.95(14)
2
12.5523(2)
7.1538(2)
24.8910(5)
90
8.7282(9)
24.282(3)
7.0340(7)
90
b, Å
c, Å
α, deg
β, deg
90.6490(10)
90
95.5830(10)
90
98.5210(10)
90
112.065(2)
90
γ, deg
V, Å3
4000.7(5)
4
1141.91(4)
4
2210.46(8)
8
1381.6(2)
4
Z
ρcalcd, g cm−3
μ, mm−1
F(000)
1.914
1.857
1.883
2.019
2.068
1.859
μ(Mo Kα) 2.225
2264
μ(Mo Kα) 1.935
320
μ(Cu Kα) 3.228
640
μ(Mo Kα) 2.214
336
μ(Cu Kα) 5.561
1344
μ(Mo Kα) 1.640
768
cryst size, mm
limiting indices
0.30 × 0.15 × 0.10
−25 ≤ h ≤ 25
−15 ≤ k ≤ 15
−20 ≤ l ≤ 19
1.92−27.00
13688
0.30 × 0.30 × 0.20
−7 ≤ h ≤ 7
−9 ≤ k ≤ 9
−17 ≤ l ≤ 17
1.51−27.49
4730
0.25 × 0.21 × 0.17
−7 ≤ h ≤ 7
−7 ≤ k ≤ 8
−29 ≤ l ≤ 29
3.55−65.01
8950
0.20 × 0.20 × 0.10
−7 ≤ h ≤ 7
−9 ≤ k ≤ 9
−16 ≤ l ≤ 16
2.82−27.53
4782
0.18 × 0.16 × 0.10
−14 ≤ h ≤ 14
−8 ≤ k ≤ 8
−29 ≤ l ≤ 28
5.42−64.44
13171
0.25 × 0.19 × 0.10
−11 ≤ h ≤ 11
−31 ≤ k ≤ 25
−9 ≤ l ≤ 8
1.68−27.50
8470
θ range, deg
reflns collected
independent reflns 4364
2463
1918
2449
3539
3127
absorption
correction
semiempirical from
equivalents
semiempirical from
equivalents
semiempirical from
equivalents
semiempirical from
equivalents
semiempirical from
equivalents
semiempirical from
equivalents
refinement method full-matrix least-
full-matrix least-
full-matrix least-
full-matrix least-
full-matrix least-
full-matrix least-
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
data/restraints/
parameters
4364/4/302
2463/0/172
1918/0/174
2449/0/172
3539/0/344
3127/0/249
goodness-of-fit on 1.048
1.064
1.215
1.074
1.136
0.967
F2
final R indices [I > R1 = 0.0324
R1 = 0.0422
wR2 = 0.1168
R1 = 0.0464
wR2 = 0.1207
R1 = 0.0250
wR2 = 0.0686
R1 = 0.0253
wR2 = 0.0688
R1 = 0.0275
wR2 = 0.0750
wR2 = 0.0296
wR2 = 0.0767
0.769/−0.420
R1 = 0.0338
wR2 = 0.0893
R1 = 0.0370
wR2 = 0.0927
0.575/−0.346
R1 = 0.0272
wR2 = 0.0628
R1 = 0.0368
wR2 = 0.0649
0.384/−0.236
a
2σ(I)]
wR2 = 0.0938
R indices (all
R1 = 0.0410
wR2 = 0.0991
0.612/−0.217
a
data)
peak/hole (e Å−3
)
1.379/−0.721
0.322/−0.255
a
R1 = ∑∥Fo| − |Fc∥/∑|Fo|; wR2 = {∑[w(Fo − Fc2)2]/∑[w(Fo )2]}1/2
.
2
2
F), −151.2 (br, 4F, para-F), −161.2 (br m, 8F, meta-F). 13C NMR
(125.7 MHz, CDCl3, 20 °C): δ = 152.1 (d, J(19F, C) = 232 Hz, Pf-
C2,6), 148.7 (Py-C2,6), 142.4 (dm, J(19F, C) = 251 Hz, Pf-C4), 139.0
(Py-C4), 136.8 (dm, J(19F, C) = 256 Hz, Pf-C3,5), 125.7 (Py-C3,5),
106.4 (t, J(19F, C) = 59 Hz, Pf-C1). Elemental analysis for
C34H10Cu4F20N2 (1080.61), Calcd: C, 37.79; H, 0.93; N, 2.59%.
Found: C, 36.86; H, 0.71; N, 2.68%. The slightly low C value for the
elemental analysis is likely due to incomplete combustion as a result of
the large content of fluorine or the high air sensitivity of the material.
Synthesis of C6F5Cu(4-picoline) (4-pMe). Neat 4-picoline
(0.067 g, 0.72 mmol) was added dropwise to a solution of 2 (0.16
g, 0.14 mmol) in toluene (5 mL) at room temperature. Upon addition
of 4-picoline, an intense yellow color developed. The reaction solution
was kept at −35 °C for 1 day, and the pure product was isolated from
the reaction solution as pale yellow crystals. The product obtained was
washed with hexanes and dried under high vacuum. Yield: 0.13 g
(72%). For 4-pMe: Tm = 130−134 °C. Tdec = 148−155 °C. 1H NMR
(500 MHz, CDCl3, 25 °C): δ = 8.52 (d, J = 6.0 Hz, 2H, Py-H2,6), 7.38
(d, J = 6.0 Hz, 2H, Py-H3,5), 2.50 (s, 3H, Me). 19F NMR (470.2 MHz,
CDCl3, 25 °C): δ = −112.9 (m, 2F, ortho-F), −160.8 (t, J(19F, 19F) =
20 Hz, 1F, para-F), −164.0 (pst, J = 19 Hz, 2F, meta-F). 13C NMR
(125.7 MHz, CDCl3, 25 °C): δ = 152.5 (Py-C4), 149.3 (dd, J(19F, C)
= 224/32 Hz, Pf-C2,6), 149.4 (Py-C2,6), 138.8 (dm, J(19F, C) = 245
Hz, Pf-C4), 136.5 (dm, J(19F, C) = 253 Hz, Pf-C3,5), 126.7 (Py-C3,5),
122.2 (t, J(19F, C) = 72 Hz, Pf-C1), 21.8 (Me). Elemental analysis for
C12H7CuF5N (323.73), Calcd: C, 44.52; H, 2.18; N, 4.33%. Found: C,
43.21; H, 2.12; N, 3.96%. The slightly low C value for the elemental
analysis is likely due to incomplete combustion as a result of the large
content of fluorine or the high air sensitivity of the material.
Synthesis of C6F5Cu(2-picoline) (4-oMe). Neat 2-picoline
(0.050 g, 0.58 mmol) was added dropwise to a solution of 2 (0.13
g, 0.12 mmol) in toluene (5 mL) at room temperature. Upon addition
of 2-picoline, an intense yellow color developed. The crude product
was obtained as a colorless microcrystalline solid, which was washed
with hexanes. Yield: 0.13 g (84%). Slow evaporation of toluene at RT
gave pale yellow “single crystals” of 4-oMe suitable for X-ray
diffraction analysis. For 4-oMe: Tm = 141−144 °C. Tdec = 147−150
°C. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 8.59 (d, J = 5.0 Hz, 1H,
Py-H6), 7.89 (pst, J = 8.0 Hz, 1H, Py-H4), 7.46 (d, J = 8.0 Hz, 1H, Py-
H3), 7.38 (pst, J = 7.0 Hz, 1H, Py-H5), 2.93 (s, 3H, Me). 19F NMR
(470.2 MHz, CDCl3, 25 °C): δ = −113.1 (m, 2F, ortho-F), −160.9 (t,
J(19F, 19F) = 20 Hz, 1F, para-F), −164.0 (m, 2F, meta-F). 13C NMR
(125.7 MHz, CDCl3, 25 °C): δ = 159.5 (Py-C2), 149.7 (dd, J(19F, C)
= 220 Hz/33 Hz, Pf-C2,6), 149.6 (Py-C6), 139.5 (Py-C4), 138.7 (dm,
J(19F, C) = 244 Hz, Pf-C4), 136.4 (dm, J(19F, C) = 253 Hz, Pf-C3,5),
125.9 (Py-C3), 122.7 (Py-C5), 122.8 (t, J(19F, C) = 71 Hz, Pf-C1),
26.0 (Me). Elemental analysis for C12H7CuF5N (323.73), Calcd: C,
44.52; H, 2.18; N, 4.33%. Found: C, 44.38; H, 2.17; N, 4.17%.
Synthesis of C6F5Cu(4-chloropyridine) (4-pCl). A solution of
4-chloropyridine (ca. 0.3 mmol) in toluene (5 mL) was added to a
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Organometallics
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solution of 2 (0.13 g, 0.12 mmol) in toluene, and the resulting dark
yellow mixture was kept at −35 °C for 1 day. Crystallization from
toluene gave pale yellow crystals of 4-pCl, which were also used for X-
ray diffraction analysis. Yield: 0.11 g (68%). For 4-pCl: Tm = 143−145
°C. Tdec = 147−150 °C. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 8.59
(d, J = 6.5 Hz, 2H, Py-H2,6), 7.60 (d, J = 6.5 Hz, 2H, Py-H3,5). 19F
NMR (470.2 MHz, CDCl3, 25 °C): δ = −112.6 (m, 2F, ortho-F),
−159.9 (t, J(19F, 19F) = 18 Hz, 1F, para-F), −163.7 (m, 2F, meta-F).
13C NMR (125.7 MHz, CDCl3, 25 °C): δ = 150.7 (Py-C2,6), 149.9
(dd, J(19F, C) = 222 Hz/27 Hz, Pf-C2,6), 148.5 (Py-C4), 139.0 (dm,
J(19F, C) = 245 Hz, Pf-C4), 136.5 (dm, J(19F, C) = 253 Hz, Pf-C3,5),
126.6 (Py-C3,5), 121.3 (t, J(19F, C) = 75 Hz, Pf-C1). Elemental
analysis for C11H4ClCuF5N (344.15), Calcd: C, 38.39; H, 1.17; N,
4.07%. Found: C, 37.53; H, 1.17; N, 4.01%. The slightly low C value
for the elemental analysis is likely due to incomplete combustion as a
result of the large content of fluorine or the high air sensitivity of the
material.
Synthesis of C6F5Cu(2-chloropyridine) (4-oCl). Neat 2-chlor-
opyridine (0.056 g, 0.49 mmol) was added dropwise to a solution of 2
(0.11 g, 0.099 mmol) in toluene (5 mL) at room temperature. Upon
addition of 2-chloropyridine, an intense yellow color developed. The
solution was kept at −35 °C for a day, which yielded light yellow
feathery crystals. Yield: 0.12 g (88%). Slow evaporation of toluene at
RT gave pale yellow “single crystals” of 4-oCl suitable for X-ray
diffraction analysis. For 4-oCl: Tm = 123−127 °C. Tdec = 130−133 °C.
1H NMR (500 MHz, CDCl3, 25 °C): δ = 8.5 (d, J = 5.0 Hz, 1H, Py-
H6), 7.90 (pst, J = 8.0 Hz, 1H, Py-H4), 7.55 (d, J = 8.5 Hz, 1H, Py-
H3), 7.46 (pst, J = 6.0 Hz, 1H, Py-H5). 19F NMR (470.2 MHz,
CDCl3, 25 °C): δ = −110.5 (br, 2F, ortho-F), −156.3 (br, 1F, para-F),
−162.9 (br, 2F, meta-F). 13C NMR (125.7 MHz, CDCl3, 25 °C): δ =
151.7 (Py-C2), 150.9 (d, J(19F, C) = 228 Hz/28 Hz, Pf-C2,6), 150.4
(Py-C6), 140.8 (Py-C4), 140.4 (dm, J(19F, C) = 249 Hz, Pf-C4), 136.5
(dm, J(19F, C) = 254 Hz, Pf-C3,5), 125.9 (Py-C3), 123.6 (Py-C5),
115.0 (t, J(19F, C) = 67 Hz, Pf-C1). Elemental analysis for
C11H4ClCuF5N (344.15), Calcd: C, 38.39; H, 1.17; N, 4.07%.
Found: C, 38.25; H, 1.07; N, 4.02%.
Table 1. The X-ray structures of 3, 4-pMe, 4-oMe, 4-pCl, 4-oCl, and 5
have been deposited at the Cambridge Crystallographic Data Center
(CCDC) as supplementary publications nos. CCDC-852837 to
852842. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
(Fax: (+44) 1223-336-033. E-mail: deposit@ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
■
Synthesis. [C6F5Cu]4 (1) was prepared according to a
literature procedure by Sheppard and Cairncross25 and
recrystallized from toluene to give the toluene solvate
[C6F5Cu]4(toluene)2 (2). Compound 2 was either used
directly or after drying in high vacuum at 80 °C to remove
the toluene solvent molecules, yielding the base-free, purified
species 1. Addition of 2 equiv of pyridine to 1 in CH2Cl2 at RT
resulted in a light yellow solution, from which colorless needle-
like crystals of 3 were collected in 82% yield after layering with
hexanes at −38 °C (Scheme 1). When 2 was treated in toluene
Scheme 1. Synthesis of Pentafluorophenyl Copper−Pyridine
Complexes
Synthesis of C6F5Cu(2,2′-bipy) (5). A solution of 2,2′-bipyridine
(712 mg, 4.56 mmol) in THF (10 mL) was added dropwise to a
solution of 1 (1.00 g, 1.08 mmol) in THF (20 mL) at room
temperature. Upon addition, the color of the solution became intense
reddish brown. This solution was stored at −38 °C for 24 h. Orange-
red, needle-like crystals formed and were isolated by decantation of the
mother liquor and dried under high vacuum for 30 min. Yield: 1.18 g
1
(70%). For 5: Tm: = 90 °C (dec.). H NMR (500 MHz, CD2Cl2, 20
or CH2Cl2 with an excess of different pyridine derivatives, air-
sensitive colorless-to-pale yellow needle-like crystals of
compounds 4-R (4-H, 4-pMe, 4-oMe, 4-pCl, 4-oCl) were
isolated after recrystallization from toluene at −38 °C in high
yield. The red-colored bipyridine species 5 was obtained upon
crystallization of an equimolar mixture of 1 and 2,2′-bipyridine
in THF solution at −38 °C. All complexes were fully
°C): δ = 8.91 (br, 2H, bipy-H6,6′), 8.16 (d, J = 7.0 Hz, 2H, bipy-
H3,3′), 8.04 (pst, 2H, bipy-H4,4′), 7.59 (br, 2H, bipy-H5,5′). 19F NMR
(470.2 MHz, CD2Cl2, 20 °C): δ = −111.1 (m, 2F, ortho-F), −164.3 (t,
J(19F, 19F) = 20 Hz, 1F, para-F), −165.1 (m, 2F, meta-F). 13C NMR
(100.5 MHz, THF-d8, 20 °C): δ = 153.7 (bipy-C2,2′), 150.7 (bipy-
C6,6′), 150.2 (dd, J(19F, C) = 226/32 Hz, Pf-C2,6), 139.6 (bipy),
138.1 (dm, J(19F, C) = 244 Hz, Pf-C4), 136.7 (dm, J(19F, C) = 255
Hz, Pf-C3,5), 128.9 (t, J(19F, C) = 83 Hz, Pf-C1), 127.1 (bipy), 122.4
(bipy). Elemental analysis for C16H8CuF5N2 (386.79), Calcd: C,
49.68; H, 2.08; N, 7.24%. Found: C, 49.64; H, 1.59; N, 7.21%.
Crystal Structure Determinations. X-ray diffraction intensities
were collected at 100 and 218 K (3) on a Bruker Apex (Kα Mo
radiation, λ = 0.71073 Å) or Apex 2 (Kα Cu radiation, λ = 1.54178 Å)
diffractometers. For all structures, SADABS (Sheldrick, G. M. SADABS
(2.01): Bruker/Siemens Area Detector Absorption Correction Program;
Bruker AXS: Madison, WI, 1998) absorption correction was used.
Non-hydrogen atoms were refined with anisotropic displacement
coefficients, and hydrogen atoms were treated as an idealized
contribution. A solvent pentane molecule in 3 is highly disordered
about a 2-fold axis. This solvent pentane molecule was refined as
disordered over two positions and with restrictions. The standard C−
C bond lengths were used as targets in the refinement for
corresponding C−C distances, and similar carbon atoms were refined
with the same thermal parameters. H atoms in the disordered pentane
molecule were not taken into consideration. Crystallographic data and
details of data collections and refinements of the structures are given in
1
characterized by H, 13C, and 19F NMR spectroscopies and
elemental analysis, and the solid-state structures were
determined by single-crystal X-ray diffraction. The pyridine
complexes 3 and 4-R are thermally stable to about 125−150
°C, whereas compound 5 decomposes at ca. 90 °C. A
comparison with the reported decomposition temperature for
the dioxane complex31 of 1 (200−220 °C) suggests that
coordination of pyridine ligands leads to destabilization of the
pentafluorophenyl copper complex. This is consistent with
early reports that the isolation and crystallographic character-
ization of organocopper pyridine complexes is complicated by
their relatively low thermal stability.32 We found that the
pyridine complexes can be stored at low temperature (−20 °C)
under a nitrogen atmosphere for extended periods of time.
However, upon exposure to air, they quickly decompose, as
evidenced by a rapid color change to greenish-brown.
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Solution Structures. The pyridine complexes 3 and 4-R
are readily soluble in both polar and nonpolar aprotic solvents,
whereas the 2,2′-bipyridine complex 5 shows only moderate
solubility. We have previously demonstrated that dissolution of
1 in strongly coordinating solvents, such as acetonitrile or
DMSO, leads to aggregate breakdown, which is reflected in a
smaller separation of the 19F NMR signals for the para- and
meta-fluorine atoms, Δδm,p, a downfield shift of the copper-
bound carbon atom, δ(13Ci),33 and an increase in the coupling
chloropyridine (partially) dissociates from the complex,
possibly leading to higher aggregates. The latter is also evident
from the chemical shift of the ipso-C atom of the C6F5 group,
which is between those of the other pyridine complexes (δ in
the range of 121.3−122.8) and that of the base-free tetramer 1
(δ 98.7). Further evidence comes from the relatively small
2
coupling constant J(13Ci, 19F) of 67 Hz. Strong broadening of
the 19F NMR signals for compound 4-oCl at room temperature
is also consistent with the presence of a dynamic equilibrium.
Although the observed spectral features could be the result of
hindered rotation due to the sterically demanding chlorine
substituent, a dissociation equilibrium is more likely, given that
Δδ(19Fm,p) of 4-oCl increases with increasing concentration.
Variable-temperature 19F NMR data for compound 4-oCl were
acquired at temperatures ranging from −50 °C to +30 °C.
Further broadening of the signals was observed at low
temperature. In contrast, low-temperature NMR spectroscopy
of complexes 4-H, 4-pMe, 4-oMe, and 4-pCl showed no
evidence of a dynamic process down to −50 °C.
26
2
constant J(13Ci, 19F) to the ortho-fluorine atoms.
Only a moderate downfield shift for the ipso-carbons in 3
(δ(13Ci) 106.4) relative to 1 (δ(13Ci) 98.7), a slight increase in
2
the coupling constant J(C, F) from 52 to 59 Hz, and a
moderate decrease in Δδm,p from 16.6 to 10.0 ppm are
observed. These data are similar to those recorded for the
tetranuclear toluene complex 2 and thus indicative of
coordination of pyridine without breakup of the tetranuclear
structure.26
In contrast to 3, complexes 4-R show a strong decrease of
Δδ(19Fm,p) to ∼3.1−6.6 ppm, which suggests the breakdown of
the tetranuclear aggregate and formation of monomeric
pyridine complexes (Figure 1, Table 2).34 The signal for the
For the 2,2′-bipyridine complex 5, we observed a more
dramatic decrease of Δδm,p to 0.8 ppm, which is accompanied
by a slight further downfield shift of the ipso-C in the 13C NMR
spectrum to δ(13Ci) 128.9 and an increase in the coupling
constant to 2J(13Ci, 19F) = 83 Hz. This trend is likely a result of
the trigonal environment with two nitrogen donors at copper.
Interestingly, similar data were observed when 1 was dissolved
2
in neat d5-pyridine (Δδm,p = 0.7 ppm, (δ(13Ci) 130.3, J(13Ci,
19F) = 80 Hz), which suggests that, under those conditions,
multiple pyridine molecules coordinate to Cu.35
To further probe the dynamic nature of complexation in
solution, we performed a titration study, in which we treated 1
with varying amounts of pyridine in the noncoordinating
solvent CDCl3 (Figure 2). At ratios of Py/CuC6F5 ranging
Figure 1. 19F NMR spectra of compounds 4-R in CDCl3 at 25 °C.
Table 2. Comparison of Selected 19F and 13C NMR Data
Δδ(19Fm,p)/
δ(13Ci )/
2J(13Ci, 19F)/
a
complex
solvent
CDCl3
ppm
ppm
Hz
1
16.6
10.0
3.4
3.2
3.1
3.8
6.6
0.8
0.7
98.7
52
59
70
72
71
75
67
83
3
CDCl3
CDCl3
106.4
121.8
122.2
122.8
121.3
115.0
128.9
130.3
4-H
4-pMe CDCl3
4-oMe CDCl3
Figure 2. NMR data for the titration of a solution of 1 in CDCl3 with
pyridine at RT.
4-pCl
4-oCl
5
CDCl3
CDCl3
1
from 0 to 2, neither the H nor the 19F NMR spectra showed
b
more than one set of signals. However, the 19F NMR signals
experience a gradual shift and are broadened, suggesting that a
fast equilibrium between complexes with varying numbers of
pyridine ligands, as outlined in Scheme 2, is established in
solution.
At −80 °C, both [C6F5Cu]4 (1) and C6F5Cu(py) (4-H) give
rise to only one set of signals, but a number of different species
were observed at ratios intermediate between Py/CuC6F5 = 0:1
and 2:1. Most notably, at a ratio of Py/CuC6F5 = 0.5:1, not
only resonances for [C6F5Cu]4(py)2 (3) but also smaller sets of
signals for 4-H and one additional species were detected. As
expected, the relative amount of the actual 4:2 complex 3 in
CD2Cl2/THF
1
pyridine-d5
80
a
b
Data were acquired at RT, at ca. 2 × 10−1 M concentration. 19F
NMR data were acquired in CD2Cl2 and 13C NMR data in THF-d8.
copper-bound carbon atom in the 13C NMR spectra of species
4-R (115.0−122.8 ppm) is also strongly downfield shifted and
shows an enhanced coupling of J(C, F) = 67−75 Hz in
comparison with the respective resonance for the intact
tetranuclear species 3 (59 Hz). Notably, the 2-chloropyridine
complex 4-oCl shows a slightly larger value for Δδ(19Fm,p) than
the other complexes 4-R, which indicates that, in solution, 2-
2
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Scheme 2. Proposed Equilibria between Oligonuclear and Mononuclear Arylcopper Complexes
solution diminishes as the ratio of Py/CuC6F5 is either
increased or decreased. The amount of the third species,
which shows broad signals even at −80 °C (δ(19F) −105.1,
−147.2, −158.8; Δδm,p = 11.6 ppm), significantly increases at a
smaller ratio of Py/CuC6F5 = 0.25:1. On the basis of these
observations, we tentatively assign this species to a 4:1 complex
[C6F5Cu]4(py).
Solid-State Structures. A single-crystal X-ray diffraction
study of 3 confirms that the tetranuclear copper aggregate
remained intact, while two pyridine molecules are coordinated
to opposite copper centers (Figure 3). Importantly, the
than for Cu1···Cu1A with 4.178(1) Å. This is a feature that is
characteristic of tetranuclear copper species in which two
copper atoms are coordinated by donor ligands.13,14,26,27,36,37
The Cu−C distances at Cu1 and Cu1A (average = 2.061(3) Å)
in 3 are significantly longer than those at Cu2 and Cu3 (average
= 1.974(2) Å), indicating a highly unsymmetrical bridging
mode of the aryl groups similar to that in 2. This type of bond
alternation is less pronounced14,37 or not observed at all13,36 in
organocopper sulfide complexes. As previously suggested by
van Koten and co-workers for other donor-supported
tetranuclear arylcopper species,4 the long copper−carbon
bonds at Cu1 support a description of 3 as a contact ion pair
consisting of two [(C6F5)2Cu]− cuprate anions bridged by two
pyridine-stabilized copper cations via the ipso-carbon atoms of
the pentafluorophenyl rings.
All complexes 4-R were also subjected to X-ray diffraction
analysis. Single crystals of 4-oMe and 4-oCl were grown via
slow solvent evaporation from a mixture of toluene and
hexanes, whereas needle-like crystals of 4-H, 4-pMe, and 4-pCl
were obtained from the reaction mixture at low temperature.
Structure plots are presented in Figure 4, and selected
geometric parameters are listed in Table 3. The crystal
structure of 4-H has been communicated previously, but the
data are provided in Table 3 for comparison purposes.29 The
molecular structures of complexes 4-R confirm the formation of
mononuclear dicoordinated species C6F5Cu(L) in the solid
state with a linear or nearly linear coordination geometry at
copper (N−Cu−C from 174.66(8) to 178.54(6)°).
The Cu−C bonds of all complexes are within a narrow range
from 1.887(2) to 1.901(3) Å, and thus considerably shorter
than those found for the tetranuclear species 3 (1.955(2)−
2.076(2) Å), which shows bridging rather than terminal aryl
groups. The distances are similar to those of other dicoordinate
organocopper−ligand complexes, such as [(C6H2-2,4,6-t-Bu3)-
Cu(Me2S)],17 with Cu−C = 1.886(3) Å, a related dimeric
complex containing a chelating oxazolinyl group (Cu−C =
1.899(5) Å),22 and N-heterocyclic carbene−copper(I) com-
plexes.38 The copper−nitrogen distances (1.897(2)−1.917(2)
Å) are significantly shorter than the ones in 3 (2.023(2) Å), but
comparable to those in the oxazolinyl complex mentioned
above (Cu−N = 1.902(4) Å)22 and a related dimeric
Figure 3. Plot of the molecular structure of 3; a cocrystallized
molecule of pentane is omitted for clarity. Selected interatomic
distances (Å) and angles (°): Cu(1)−N(1) 2.023(2), Cu(1)−Cu(2)
2.4496(4), Cu(1)−Cu(3) 2.4687(4), Cu(2)···Cu(3) 2.5940(6),
Cu(1)···Cu(1A) 4.179(1), Cu(1)−C(1) 2.045(3), Cu(1)−C(7)
2.077(2), Cu(2)−C(1) 1.991(2), Cu(3)−C(7) 1.956(2), Cu(1)−
Cu(2)−Cu(3) 58.528(11), Cu(1)−Cu(3)−Cu(2) 57.812(10),
Cu(2)−Cu(1)−Cu(3) 63.660(15), Cu(1)−Cu(2)−Cu(1A)
117.06(2), Cu(1)−Cu(3)−Cu(1A) 115.62(2), C(1)−Cu(2)−C(1A)
139.40(15), C(7)−Cu(3)−C(7A) 156.64(14).
structural parameters are different from those of 1 and other
homoleptic tetranuclear organocopper complexes that lack
interactions with donor molecules.26 Indeed, the structure is
more similar to that of the toluene complex 2, except for that
the four copper atoms in 3 lie in a plane rather than adopting a
butterfly arrangement as in the structure of 2 (2; interplanar
angle Cu1Cu3Cu2)//Cu1ACu3Cu2 = 37.7°). The pentafluor-
ophenyl groups are positioned alternately above and below the
Cu4 plane, which leads to strong puckering of the eight-
membered Cu4C4 ring in 3. The separation between opposite
Cu centers is much smaller for Cu2···Cu3 with 2.5940(6) Å
19
pyridylalkyl species [2-(SiMe3)2C(Cu)C5H4N]2 (Cu−N =
1.910(3) Å). Copper−nitrogen distances in dicoordinate
copper cations [py2Cu]+ are also found in the same range
(1.86−1.96 Å).39,40 Noteworthy is that the Cu−N bond lengths
for the two independent molecules of 4-oCl (1.909(2),
1.917(2) Å) are slightly longer than those in the other
complexes, suggesting weaker coordination of the pyridine
ligand to Cu. The latter is consistent with the conclusions
drawn from the solution NMR studies of 4-oCl.
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Figure 4. Plots of the molecular structures of (a) 4-pMe, (b) 4-oMe, (c) 4-pCl, and (d) 4-oCl. Only one of two independent molecules of 4-oCl is
shown. Geometric parameters are summarized in Table 3.
a
Table 3. Comparison of Selected Bond Lengths [Å], Interatomic Distances [Å], and Angles [°] for Complexes 4-R
4-oCl
b
4-H
4-pMe
4-oMe
4-pCl
mol A
mol B
Cu1−C1
Cu1−N1
N1−C7
N1−C11
C−X (X = Cl, CH3)
C1−Cu1−N1
Py//Pf
1.8913(17)
1.9022(15)
1.356(2)
1.891(3)
1.900(2)
1.352(3)
1.345(3)
1.501(4)
177.88(8)
2.89
1.8963(19)
1.9072(16)
1.353(3)
1.351(3)
1.498(3)
174.66(8)
5.32
1.8873(19)
1.8972(16)
1.354(2)
1.901(3)
1.917(2)
1.338(4)
1.349(4)
1.727(3)
177.75(11)
44.21
1.898(3)
1.909(2)
1.340(4)
1.349(4)
1.723(3)
176.19(11)
43.12
1.346(2)
1.345(2)
1.7212(18)
176.34(7)
2.34
178.54(6)
0.0
Cu1···Cu1A
Cu1A···Cu1B
2.8924(3)
2.8924(3)
89.900(5)
89.900(5)
90
3.531(1)
3.698(1)
95.89(7)
83.27(7)
96.32(7)
85.24(7)
95.46(7)
84.88(7)
65.79(20)
112.49(19)
−117.29(21)
65.26(20)
175.16(1)
3.2454(5)
4.2970(5)
92.56(7)
89.42(7)
91.44(7)
92.69(5)
85.23(5)
93.89(5)
60.73(14)
−119.27(15)
122.00(18)
−58.19(16)
168.22(1)
3.521(1)
3.4797(6)
3.8363(6)
3.784(1)
C1−Cu1···Cu1A
C1A−Cu1A···Cu1B
C1−Cu1···Cu1B
N1−Cu1···Cu1A
N1−Cu1···Cu1B
N1A−Cu1A···Cu1B
C7−N1−Cu1···Cu1B
C11−N1−Cu1···Cu1B
C2−C1−Cu1···Cu1B
C6−C1−Cu1···Cu1B
Cu1···Cu1A···Cu1B
98.83(5)
100.14(9)/100.01(9)
103.85(8)/103.00(9)
90.40(9)/92.45(8)
80.31(7)/75.55(7)
75.59(7)/80.53(7)
93.08(7)/87.04(7)
77.94(5)
100.54(5)
82.50(4)
90.097(5)
90.097(5)
90
100.21(4)
81.07(7)
90
115.12(14)
−62.23(14)
−119.67(16)
63.10(14)
170.54(1)
−109.82(20)
−109.98(20)
90
72.85(20)
70.33(02)
90
−116.40(23)
63.35(23)
−113.80(22)
68.76(24)
90
179.716(15)
155.80(2)
a
Symmetry operations used to generate equivalent Cu atoms for 4-H: −x, −y, z + 1/2; −x, y + 1/2, −z + 1/2; x, −y + 1/2, −z; −x, −y, −z; x, y, −z
− 1/2; x, −y − 1/2, z − 1/2; −x, y − 1/2, z. For 4-pMe: (i) −x + 1, −y + 1, −z; (ii) −x + 1, −y + 2, −z; (iii) x, 1 + y, z. For 4-pCl: (i) −x + 2, −y, −
z + 1; (ii) −x + 2, −y + 1, −z + 1; (iii) x, 1 + y, z. For 4-oMe: (i) −x + 2, −y + 1, −z + 1; (ii) −x + 2, −y, −z + 1; (iii) x, y − 1, z. For 4-oCl: (i) x −
1, y, z; (ii) x − 1, y − 1, z; (iii) 1 + x, 1 + y, z; (iv) 1 + x, y, z; (v) x, y − 1, z; (vi) x, 1 + y, z. Reference 29.
b
The pentafluorophenyl groups and the pyridine rings are
coplanar for 4-H and nearly so for 4-pMe (2.89°) and 4-pCl
(2.34°); however, large dihedral angles of 43.12° and 44.21°,
respectively, were observed for the two independent molecules
in the complex 4-oCl. The latter is attributed to the high steric
demand of the chloro substituent in the 2-position of the
pyridine ring. Interestingly, the methyl group in the 2-position
of 4-oMe leads to only a slight twisting of the two aromatic
rings of 5.32°.
The crystal structure of 5 also reveals a monomeric species,
but the copper atoms are in an unusual trigonal-planar
coordination geometry3d,18 as a result of chelation of the 2,2′-
bipyridine ligand (Figure 5). The Cu−N bond lengths of
2.018(2) and 2.082(2) Å are not equivalent, and both distances
are slightly longer than those in the monomeric compounds 4-
R, indicating weaker binding in the trigonal geometry in
comparison with the typical linear dicoordinate geometry. The
unsymmetric coordination of the bipyridine in 5 further
manifests itself in an angle C(1)−Cu(1)−N(1) of 148.30(7)
°, which is larger than the C(1)−Cu(1)−N(2) angle of
131.35(7)°. A more symmetric structure has been reported for
the complex 4-MeOC6F4Cu(phen) (phen = phenanthroline)
with Cu−N = 2.0720(18) and 2.0949(19) Å,3d whereas the
gold complex ArAu(2,2′-dmphen) (Ar = 2,4,6-(NO2)3C6H2;
dmphen = 2,9-dimethyl-1,10-phenanthroline)41 shows much
stronger distortions from trigonal-planar geometry with two
very different Au−N bonds of 2.1363(3) and 2.5753(3) Å. The
Cu−C bond of 1.907(2) Å is slightly longer than that in
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Figure 5. Plot of the molecular structure of 5. Selected bond lengths
(Å) and angles (°): Cu(1)−N(1) 2.017(2), Cu(1)−N(2) 2.082(2),
Cu(1)−C(1) 1.907(2), C(11)−C(12) 1.491(3), C(1)−Cu(1)−N(1)
148.30(7), C(1)−Cu(1)−N(2) 131.35(7), N(1)−Cu(1)−N(2)
79.69(6), C(6)−C(1)−Cu(1) 124.2(1), C(2)−C(1)−Cu(1)
123.0(1), (bipy)//(Pf) 46.5, angle sum at Cu 359.34.
Figure 6. View of the extended structure of 3 along the
crystallographic c axis, illustrating the formation of channels that are
occupied by pentane solvent molecules. Only one position of the
disordered pentane molecule is given for clarity.
compounds 4-R, but significantly shorter than that in the
tetranuclear species 3.
Supramolecular Structures via Cuprophilic and π-
Interactions. All the complexes described in here form
extended supramolecular structures in the solid state via either
π-stacking interactions or short Cu···Cu contacts as a result of
so-called cuprophilic interactions. Supramolecular assembly of
molecular materials via π-stacking of arenes and perfluoroar-
enes continues to be a topic of much current interest.42,43
Aurophilic interactions, the attractive forces between closed-
shell d10 metal ions of gold, have also long been recognized and
are now commonly used in supramolecular chemistry.44 They
are generally considered to be similar in strength to hydrogen
bonding interactions and rely, to a large extent, on relativistic
effects. In contrast, attractive interactions between closed-shell
Cu(I)···Cu(I) pairs have only recently been proposed based on
experimental and theoretical studies, and the relevance and
strength of such interactions remain controversial.40,45,46 On
the basis of MP2 calculations, Schwerdtfeger et al. concluded
that cuprophilic interactions between neutral pairs [RCuL]2
should be attractive by up to −4 kcal mol−1 and that the
interaction potential is very shallow in the range from ca. 2.5 to
3.5 Å.47
The complex [C6F5Cu]4(py)2 (3) displays an interesting
extended structure, in which intermolecular π-interactions
(Py//Py, Pf//Pf, and Py-Pf) lead to channels propagating
along the crystallographic c axis (Figure 6), which are occupied
by pentane solvent molecules. Whereas short intramolecular
Cu···Cu contacts (2.5940(6) Å) are observed within the Cu4
aggregates, as discussed above, the intermolecular Cu···Cu
contacts are very long (>7 Å), precluding the presence of any
Cu···Cu interactions.
Figure 7. Two different views of a fragment of the crystal packing of 5
showing offset π-stacking along the crystallographic c axis between
Cu(2,2′-bipy) fragments and between the pentafluorophenyl groups.
Cu···Cu = 5.631 Å.
centroids = 3.382 Å). The shortest intermolecular C···C and
C···N distances fall in the range from 3.3 to 3.6 Å, and
interestingly, they are augmented by a short Cu···C distance of
3.323 Å to the meta-pyridyl position.48 The Cu···Cu separations
of 5.631 Å, on the other hand, are very long, which is insofar
surprising as dimers with short Cu···Cu contacts have been
reported for the closely related phenanthroline (phen) complex
4-MeOC6F4Cu(phen) (Cu···Cu = 2.5770(6) Å).3d
Inspection of the extended structures of the dicoordinate
organocopper pyridine complexes 4-R reveals, in all cases,
chains of Cu atoms that progress throughout the entire crystal
lattice. However, the stacking direction, and thus the
orientation of the chains, as well as the Cu···Cu separations
vary considerably. The parent compound 4-H, which
Extended structures that form as a result of π-interactions are
also observed in the crystal structure of the 2,2′-bipyridine
complex 5 (Figure 7). Both the bipyridyl units and the
pentafluorophenyl groups form offset stacks along the
crystallographic c axis. The orientation of the bipyridyl units
alternates from layer to layer with a 90° offset between pairs of
bipyridyl units. Consequently, only one of the pyridyl rings and
the central five-membered CuN2C2 chelate ring match up with
the respective parts of the adjacent molecule (distance between
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Figure 8. Plots of the extended structures of 4-R (stacking direction for 4-H is the crystallographic c axis; for all substituted derivatives, it is the b
axis). (a) View sideways, from left to right: 4-H, 4-pMe, 4-pCl, 4-oMe, 4-oCl. (b) View along the crystallographic a axis: 4-H, 4-pMe, 4-pCl, 4-oMe.
View along the crystallographic c axis: 4-oCl. For 4-H, the Cu chain is slightly tilted toward the front to show three-dimensionality. (c) Space filling
illustration of the tilting of the stacks.
crystallizes in the Pbcm space group, adopts a very unusual
structure in that the copper atoms in 4-H are arranged in one-
dimensional chains with Cu···Cu distances of 2.8924(3) Å.29
These Cu···Cu distances are among the very shortest reported
for unsupported Cu(I)···Cu(I) contacts (Figure 8).49 All atoms,
including the hydrogen atoms, reside on a crystallographic
mirror plane. Consequently, all copper atoms are equidistant
and the direction of the chain is perpendicular to the plane of
the molecules. Importantly, the ligands in adjacent RCuL
fragments adopt a staggered conformation, thereby avoiding
any π-stacking and allowing the molecules to approach very
closely. Hence, aggregation leads to supramolecular structures
that are unprecedented in organocopper chemistry.50,51
Intrigued by these unusual observations, we set out to further
study the effects of placing electron-donating Me groups and
electron-withdrawing Cl substituents at the ortho and para
positions of the pyridyl ring on the packing of complexes
C6F5Cu(py-R) (4-R). Surprisingly, in all cases, a packing
pattern that is very different from that of 4-H was observed.
Compounds 4-R form offset stacks that progress along the
crystallographic b axis with the RCuL monomers aligned in
such a way that the C6F5 groups undergo π-interactions with
the pyridyl moieties of the adjacent molecules and vice versa
(Figure 8). In each stack, the successive molecules adopt an
eclipsed, rather than a staggered, arrangement and are offset so
that the resulting stacks are tilted by an angle of α = 23.9° for
(4-pMe), 26.8° for (4-oMe), 24.7° for (4-pCl), and 21.2° for
(4-oCl). This tilting is the result of lateral slippage of the
aromatic groups, a phenomenon that is commonly encountered
for stacks involving perfluoroarene−arene interactions (see
Figure S1, Supporting Information).43,46,52 The tilt angle was
measured using the vector perpendicular to the plane
containing the pentafluorophenyl ring. The Cu···Cu distances
between adjacent molecules alternate, thereby leading to
formation of dimers with relatively short Cu···Cu contacts of
3.245−3.531 Å that are, in turn, connected through longer
Cu···Cu contacts ranging from 3.698 to 4.297 Å to form infinite
Cu chains. The alternation is most pronounced for 4-oMe with
Cu···Cu distances of 3.245 and 4.297 Å, respectively.
Noteworthy is also that, for 4-oMe, relatively short contacts
of 3.245 Å coincide with the largest deviation from linear
coordination geometry at copper (C1−Cu1−N1 = 174.66(8)
°).
For 4-oCl, the formation of a zigzag Cu chain is apparent and
reflected in a relatively small angle Cu1···Cu1A···Cu1B of
155.79(2)°. The latter is likely related to the presence of the
sterically demanding 2-chloro substituent, which also leads to
tilting of the pyridyl and C6F5 groups with respect to each
other.
From the crystallographic studies, we conclude that, unlike
the parent complex 4-H, the substituted pyridine complexes 4-
R show multiple face-to-face pentafluoroarene−arene π-
interactions. On the basis of the Cu···Cu contacts of
2.8924(3) Å in 4-H, which are significantly shorter than the
sum of the van der Waals radii of two Cu(I) centers of 3.78 Å
and close to the sum of the covalent radii of 2.64 Å,53 the
presence of cuprophilic interactions is likely to play a significant
role. Indeed, recent theoretical studies on 4-H are in agreement
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with this assessment.54 The Cu···Cu separations for complexes
4-R are comparative much larger (>3.2 Å), and cuprophilic
interactions are expected to be weaker. The supramolecular
assembly in those complexes is, to a large extent, governed by
perfluoroarene−arene π-interactions, which appears to prevent
the Cu centers in neighboring molecules from approaching
more closely.46
Photophysical Properties. Intrigued by the observation
of infinite copper chains from the X-ray data, we decided to
investigate the photoluminescence properties of complexes 4-R
in the solid state. The parent compound 4-H displays strong
blue fluorescence (λmax = 460 nm, τ = 30 ns) in the solid state
at RT upon excitation at λ = 330 nm. This band is also
observed in phosphorescence mode, possibly as a result of a
delayed fluorescence effect. The 460 nm band is accompanied
by a weaker long-lived emission band (τ = 70 μs) that extends
to ca. 700 nm (Figure 9). The emission wavelength was found
that the observed delayed fluorescence is caused by triplet−
triplet annihilation (T1 + T1 → S1 + S0) rather than by
conventional repopulation of the singlet excited state. This
mechanism, which is common for polymers, solids, and
concentrated solutions, would also account, at least in part,
for the increased singlet emission at high temperatures. Triplet
migration or “hopping” across the solid is faster at room
temperature than at 77 K and facilitates the annihilation
process.57
In stark contrast to the observed strong blue fluorescence of
4-H, among the substituted derivatives, only 4-pMe proved to
be (weakly) emissive in the solid state at room temperature.
Upon excitation at 325 nm, 4-pMe displayed a weak yellow-
green luminescence with a maximum at 558 nm. At 77 K, all
substituted compounds exhibited a band in the range of 411−
425 nm (Figure 10a). Again, these bands correspond to decay
Figure 10. (a) Solid-state fluorescence and (b) phosphorescence
spectra of compounds 4-H (λexc = 330 nm) and 4-R (λexc = 325 nm) at
77 K.
Figure 9. (a) Variable-temperature solid-state fluorescence and (b)
phosphorescence spectra of 4-H (λexc = 330 nm).
from singlet excited states as they are absent when the
experiment was performed in phosphorescence mode. How-
ever, they all are significantly higher energy than that of 4-H
(475 nm at 77 K). No clear correlation with the electronic
effects of the pyridine substituent was observed, suggesting that
the transition is most likely not simply due to a pyridine-based
MLCT state, but rather involves orbitals centered at Cu and/or
the C6F5 moiety. In the case of 4-pMe, the band at 411 nm is
accompanied by a broad emission at ca. 600 nm, which is red
shifted from the room-temperature band at 558 nm.
Theoretical calculations by Yang and Su and co-workers
predict an emission at 449 nm for C6F5Cu(py) due to charge
transfer from the pyridine-centered LUMO to the copper-
centered HOMO-2.54 Interestingly, they also calculated the
emission spectra for a dimeric species and found that the
emission maximum shifted from ca. 460 nm at a Cu···Cu
separation of 3.8 Å to 550 nm at a Cu···Cu separation of 2.8 Å.
to be virtually independent of the excitation wavelength (275−
375 nm), indicating that, although the excitation pathway is the
same for the two bands, they decay independently. The
luminescence properties of 4-H proved to be strongly
temperature-dependent, and upon cooling, the emission color
changes gradually from blue to a bright yellow-green color. This
so-called luminescence thermochromism55 was traced back to
an increase in the intensity of the band at 580 nm relative to
that at 460 nm (Figure 9). The latter all but disappears at 77 K,
revealing a weak shoulder band at ca. 500 nm. On the basis of
the luminescence lifetimes, the emission at 460 nm is assigned
to fluorescence from a metal-to-ligand charge transfer (MLCT)
state, whereas the band at 580 nm is likely due to emission
from a lower-lying triplet excited state.56 Because the two
emissive states are separated by more than 4000 cm−1, that is,
20 times the value of kBT at room temperature, it is more likely
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Taking into consideration the theoretical results by Yang and
Su and co-workers,54 our observations might suggest that the
larger Cu···Cu separations in the substituted derivatives 4-R in
comparison with those in 4-H result in a higher energy
emission due to a smaller contribution of intermolecular orbital
mixing between Cu centers in neighboring molecules.
strong luminescence only at liquid nitrogen temperature. In all
cases, the fluorescence emission band is in the range of ca.
410−425 nm and thus at significantly different energy from that
in 4-H. Triplet−triplet annihilation phenomena are observed
for compounds 4-H and 4-oMe, suggesting that the shorter
Cu···Cu contacts in these compounds give rise to unique
luminescence properties.
The fluorescence features discussed above are accompanied
by decay from a triplet excited state that results in emission
maxima in the range from 545 to 565 nm for the substituted
species 4-R and, for 4-oMe, a feature at 425 nm that is
attributed to delayed fluorescence (Figure 10b). Again, we
notice that the phosphorescence for 4-H is significantly more
bathochromic than that for any of the other derivatives,
independent of whether electron-withdrawing or electron-
donating substituents are attached to the pyridine ligand. On
the basis of our observations, it is clear that the luminescence
features for compound 4-H are distinct from those of all other
derivatives, indicating that the unique solid-state packing with
short Cu···Cu separations affects the photophysical properties,
presumably due to mixing of Cu-centered orbitals of
neighboring molecules. There also appears to be a correlation
between short Cu···Cu contacts and the observation of delayed
fluorescence via triplet−triplet annihilation, which is most
prominent for 4-H (Cu polymer with Cu···Cu = 2.892 Å) and
for 4-oMe (Cu dimers with Cu···Cu = 3.245 Å).
ASSOCIATED CONTENT
* Supporting Information
■
S
Illustration of the offset (slippage) of the π-systems in the
stacks of 4-R and results from DFT and TD-DFT calculations
on complexes 4-R. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fjaekle@rutgers.edu.
■
Present Address
§School of Chemical Sciences, National Institute of Science
Education and Research, Institute of Physics Campus,
Bhubaneswar-751005, Orissa, India.
ACKNOWLEDGMENTS
Acknowledgment is made to the National Science Foundation
and the Alfred P. Sloan Foundation for support of this research.
■
CONCLUSIONS
■
We have demonstrated that, depending on the amount of
pyridine as an external ligand added to solutions of [C6F5Cu]4
(1), either a tetranuclear species [C6F5Cu]4(py)2 (3) or a
mononuclear species C6F5Cu(py) (4-H) can be isolated. NMR
data for a complex with 2,2′-bipyridine (5) are strikingly similar
to those recorded for solutions of 1 in d5-pyridine, indicating
that, in neat pyridine, more than one ligand is bound to Cu and
most likely a tricoordinate species of the type C6F5Cu(py)2 is
present.
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