Influence of second coordination sphere hydroxyl groups on the reactivity of copper(I) complexes

Article abstract of DOI:10.1021/ic202207e

We report the enhanced reactivity of hydroxyl substituted CuN ₃⁺ derivatives, where N₃ = tris(picolinyl) methane (tripic) and related derivatives, upon deprotonation of the O-H functionality. The work capitalizes on new methodology for incorporating hydroxyl groups into the second coordination sphere of copper centers. The key synthetic methodology relies on Pd-catalyzed coupling reactions of dilithiated 6-methyl-2-pyridone with bromopyridyl derivatives. These building blocks allow the preparation of tridentate N₃ ligands with OH and OMe substituents flanking the fourth coordination site of a tetrahedral complex. Coupling of these tridendate ligands gives the corresponding hydroxy- and methoxy-functionalized bistripodal ligands. [Cu[bis(2-methylpyrid-6-yl)(2- hydroxypyrid-6-yl)methane](NCMe)]⁺ ([Cu(2H)(NCMe)]⁺) oxidizes readily in air to afford the mixed valence Cu^1.5 dimer ([Cu₂(2)₂]⁺). Formation of [Cu ₂(2)₂]⁺ is accelerated in the presence of base and can be reversed with a combination of decamethylferrocene and acid. The reactivity of [Cu(2H)(NCMe)]⁺ with dioxygen requires deprotonation of the hydroxyl substituent: neither [Cu(tripic)(NCMe)]⁺ nor the methoxy-derivatives displayed comparable reactivity. A related mixed valence dimer formed upon oxidation of the dicopper(I) complex of a tetrahydroxy bis(tridentate) ligand, [Cu₂(6H₄)(NCMe)₂] ²⁺. The dicopper(I) complex of the analogous tetramethoxy N ₆-ligand, [Cu₂(5)(NCMe)₂]²⁺, instead reversibly binds O₂. Deprotonation of [Cu(2H)(CO)]⁺ and [Cu(2H)(NCMe)]⁺ afforded the neutral derivatives Cu(2)(CO) and Cu₂(2)₂, respectively. The dicopper(I) derivative Cu ₂(2)₂ can be reoxidized, reprotonated, and carbonylated. The silver(I) complex, [Ag(2H)(NCMe)]BF₄, forms an analogous neutral dimer (Ag₂(2)₂) upon deprotonation of the hydroxyl group. The structures of ligand 2H, [Cu₂(5)(NCMe)₂]⁺, [Cu₂(2)₂]⁺, [Cu₂(6H ₂)]⁺, [Ag(2H)(NCMe)]BF₄, and Ag ₂(2)₂ were confirmed by single crystal X-ray diffraction.

Full text of DOI:10.1021/ic202207e

Article
pubs.acs.org/IC
Influence of Second Coordination Sphere Hydroxyl Groups on the
Reactivity of Copper(I) Complexes
Christopher S. Letko, Thomas B. Rauchfuss,* Xiaoyuan Zhou, and Danielle L. Gray
School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, United States
S
* Supporting Information
ABSTRACT: We report the enhanced reactivity of hydroxyl
+
substituted CuN3 derivatives, where N3 = tris(picolinyl)-
methane (tripic) and related derivatives, upon deprotonation
of the O−H functionality. The work capitalizes on new meth-
odology for incorporating hydroxyl groups into the second
coordination sphere of copper centers. The key synthetic meth-
odology relies on Pd-catalyzed coupling reactions of dilithiated
6-methyl-2-pyridone with bromopyridyl derivatives. These build-
ing blocks allow the preparation of tridentate N3 ligands with OH
and OMe substituents flanking the fourth coordination site of a
tetrahedral complex. Coupling of these tridendate ligands gives the
corresponding hydroxy- and methoxy-functionalized bistripodal
ligands. [Cu[bis(2-methylpyrid-6-yl)(2-hydroxypyrid-6-yl)methane](NCMe)]+ ([Cu(2H)(NCMe)]+) oxidizes readily in air to afford
the mixed valence Cu1.5 dimer ([Cu2(2)2]+). Formation of [Cu2(2)2]+ is accelerated in the presence of base and can be reversed with a
combination of decamethylferrocene and acid. The reactivity of [Cu(2H)(NCMe)]+ with dioxygen requires deprotonation of the
hydroxyl substituent: neither [Cu(tripic)(NCMe)]+ nor the methoxy-derivatives displayed comparable reactivity. A related mixed valence
dimer formed upon oxidation of the dicopper(I) complex of a tetrahydroxy bis(tridentate) ligand, [Cu2(6H4)(NCMe)2]2+. The
dicopper(I) complex of the analogous tetramethoxy N6-ligand, [Cu2(5)(NCMe)2]2+, instead reversibly binds O2. Deprotonation
of [Cu(2H)(CO)]+ and [Cu(2H)(NCMe)]+ afforded the neutral derivatives Cu(2)(CO) and Cu2(2)2, respectively. The dicopper(I)
derivative Cu2(2)2 can be reoxidized, reprotonated, and carbonylated. The silver(I) complex, [Ag(2H)(NCMe)]BF4, forms an
analogous neutral dimer (Ag2(2)2) upon deprotonation of the hydroxyl group. The structures of ligand 2H, [Cu2(5)(NCMe)2]+,
[Cu2(2)2]+, [Cu2(6H2)]+, [Ag(2H)(NCMe)]BF4, and Ag2(2)2 were confirmed by single crystal X-ray diffraction.
of the copper center.12 CuN2 and CuN3 sites favor μ-κ2:κ2-O2
INTRODUCTION
The metal-catalyzed reduction of O2 is a central reaction in many
■
derivatives, whereas CuN4 sites, which do not occur naturally, give
complexes with μ-κ1:κ1 O2 ligands. The μ-κ1:κ1-peroxide ligand, like
nonbridging η2-peroxides,13 is generally nucleophilic. Thus, pro-
tonation of [Cu2(tpa)2(μ-κ1:κ1-O2)]2+ (tpa = tris(2-pyridylmethyl)-
amine) affords hydrogen peroxide.14 In contrast, we have shown
that μ-κ2:κ2-O2 complexes resist protonation.4
energy harvesting schemes.1 Because aerobic life depends on this
process, nature has evolved elaborate catalysts for this reduction.2
Synthetic catalysts for the catalytic reduction of O2 are few
however.3,4 Catalysts are required components of fuel cells, which
operate optimally when the oxygen reduction reaction (ORR) is
effected at low pH and close to the thermodynamic potential
(1.23 V at pH = 0).5 Since even platinum-based catalysts
require overpotentials of several hundred millivolts, interest in
improved catalysts remains high.6 Modeling enzyme active sites
has been one the most promising approaches to the develop-
ment of replacements for platinum in fuel cells.7
The premier example of a thermally stable Cu2−O2 complex
is [Cu2(bistripic)(O2)]2+ (bistripic = 1,2-bis[2-(bis(6-methyl-
pyrid-2-yl)methyl)pyrid-6-yl]ethane), as described by Kodera
and co-workers (Figure 1).15,16 This dicopper complex mimics
the behavior of the O2-carrier hemocyanin.17 In the bistripic
system, two CuN3 sites cooperate in binding O2. The related
monocopper derivative [Cu(tripic)(NCMe)]+ (tripic = tris(6-
methylpyrid-2-yl)methane) is unreactive toward O2.18
We hypothesized that by augmenting the bistripic system
with proton donors, the corresponding hemocyanin model
might acquire laccase-like properties. Our hypothesis was
informed by the role of hydrogen-bonding in the enzymatic
reduction of O2,19 which is thought to involve proton-coupled
In the quest for biomimetic O2 reduction catalysts, obvious
approaches involve iron and copper centers capitalizing on the
affinity of biological cuprous and ferrous centers for O2.8 Impressive
advances have been made in constructing functional O2 reduction
catalysts based on cytochrome oxidase.9 In some respects laccase, a
multicopper oxidase containing a trinuclear Cu site O2 receptor,10
presents a simpler design than cytochrome oxidase, although less
progress has been reported in developing functional models.11
Dioxygen has been observed to bridge pairs of Cu centers in two
modes: μ-κ1:κ1 and μ-κ2:κ2, depending on the coordination number
Received: October 11, 2011
Published: March 27, 2012
© 2012 American Chemical Society
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Figure 1. Reaction of O2 with [Cu2(bistripic)(NCMe)2]2+.
electron-transfer (PCET).20 Several studies have examined the
role of hydrogen-bonding substituents in the second coordina-
tion sphere of copper complexes, but with variable success.21−23
A Cu-hydroperoxide is stabilized by hydrogen-bonding to a pen-
dant pivalamide N−H group.23 The stability of [Cu2(tpa)2(μ-
κ1:κ1-O2)]2+ complexes is slightly enhanced by introduction of
amine groups flanking the pyridyl nitrogen centers.22
This Article describes the synthesis of a new set of mono-
and dicopper complexes with hydroxy-functionalized tripodal
ligands, along with a description of their reactivity toward
dioxygen and CO. Deprotonation of the auxillairy hydroxyl
groups enhances the reactivity of hemocyanin-inspired models,
not only for the dicopper(I) species but for the related
monocopper(I) complex.
Figure 2. Structure of the pyridone tautomer of 2H with ellipsoids
shown at 50% probability. The hydrogen atom (green) bound to N1
was located crystallographically. Other hydrogen atoms were omitted
for clarity.
a related amide-pyridine-pyrazine ligand (d(Namide−Npyridine
=
2.598(6), d(Namide−Npyrazine = 2.667(5) Å).25 Amine proton N1−H
was located in the difference map at a short distance of 2.41(2) Å
from N2. The presence of hydrogen-bonding in 2H is
1
consistent with its H NMR spectrum, which exhibits a D2O-
exchangeable signal at δ 11.8.26 Thus, ligand 2H adopts a
conformation similar to complexes of its tautomer.
RESULTS AND DISCUSSION
■
Pd-catalyzed cross-coupling of the dilithiated derivative of 1
with 2-bromo-6-methoxypyridine gave 3, a chiral tripodal ligand
where the binding pocket is flanked by methyl, methoxy, and
hydroxy substituents (Scheme 1).
Synthesis of Hydroxy- and Methoxy-Complemented
Tripodal Ligands. The preparation of copper(I) complexes of
hydroxy-complemented tripodal ligands required modifica-
tion of the coupling routes described by Kodera for tris-
(2-picolinyl)methane.15,24 The specific methodological innova-
tion is based on the generation and use of the dilithiated deriv-
ative of 2-hydroxy-6-methylpyridine. This dilithiated derivative
was found to couple with 2-bromo-6-methylpyridine using ZnCl2
as the transmetallating agent and PdCl2(dppf) as catalyst to
afford (2-picolin-6-yl)(2-pyridon-6-yl)methane (1). The coupled
product 1, which exists as the pyridone tautomer, proved to be a
versatile precursor to other tripodal N3-ligands that contain
flanking hydrogen-bonding functionality. For example, 1 was
cross-coupled with an additional equiv of 2-bromo-6-methylpyr-
idine to afford ligand 2H (Scheme 1).
Hexadentate ligands capable of hosting two copper centers
were prepared via the homocoupling of two tripodal ligands
such as 2H. To avoid complications with the coupling of chiral
precursors, we converted 3 into the dimethoxy tripod 4 by
methylation of 3 with Ag2CO3 and MeI. Dilithiation of 4 using
t-BuLi followed by oxidation of this intermediate with 1,2-
dibromoethane afforded the homodimeric ligand 5 (Scheme 2).
Analogous to the behavior of 2-methoxypyridine,27 5 was
efficiently demethylated with HBr to yield the tetrahydroxy
ligand 6H4. Coordination of CuI to hydroxy-decorated tripodal
ligands occurs with tautomerization of pyridone to hydroxypyr-
idine (see below), as commonly observed when combining
pyridone ligands and soft metal cations.28 The abbreviations 2H
and 6H4 are used here to describe all tautomers of these ligands.
Complexes of Methoxy- and Hydroxy-Functionalized
Ligands. The tripodal ligands 2H and 3 readily formed 1:1
Crystallographic analysis confirmed that 2H exists as the
pyridone tautomer. An intramolecular bifurcated hydrogen-
bond exists between the pyridone N−H and the two picolinyl
nitrogen atoms with a N1−N2 distance of 2.902(2) Å (Figure 2).
Even stronger bifurcated hydrogen-bonds have been observed for
Scheme 1. Synthesis of Hydroxyl-Functionalized Tripodal Ligands
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Scheme 2. Synthesis of Methoxy- and Hydroxy-Functionalized Bistripodal Ligands
complexes upon treatment with salts of [Cu(MeCN)4]+.
Spectroscopic analysis indicated that these species are similar
to [Cu(tripic)(MeCN)]+.18 A silver(I) derivative, [Ag(2H)-
1
(MeCN)]BF4, was also prepared, the H NMR spectrum of
which displays a broad singlet at δ 9.1 assigned to the phenolic
OH. X-ray crystallography confirmed the tetrahedral coordina-
tion with an uncoordinated OH group (Figure 3).
Figure 4. Structure of [Cu2(5)(NCMe)2](BArF )2 with ellipsoids shown
4
at 50% probability. Hydrogen atoms and counteranions were omitted for
clarity. Selected distances (Å): Cu(1)−N(1), 2.090(4); Cu(1)−N(2),
2.09(4); Cu(1)−N(3), 2.060(3); Cu(1)−Cu(1a), 6.7784(9).
broad singlets in the range of δ ∼10−15 assigned to the four
nonequivalent OH groups.29 The indicated asymmetric
structure is attributed to intramolecular hydrogen-bonding.
Consistent with this view, CD3CN solutions at 70 °C display a
1
simplified H NMR spectrum, featuring an OH signal at δ 6.7
which decreases in intensity upon addition of D2O. Treatment
of a CH2Cl2 solution of [Cu2(6H4)(NCMe)x](BArF )2 with
4
Figure 3. Structure of [Ag(2H)(MeCN)]BF4 with ellipsoids shown at
50% probability level. Counteranions and solvent molecules were
omitted for clarity. Selected distances (Å): Ag(1)−N(1), 2.090(4);
Ag(1)−N(2), 2.09(4); Ag(1)−N(3), 2.060(3); Ag(1)−N(4),
6.7784(9). The OH group was located in the refinement.
CO afforded the dicarbonyl derivative [Cu2(6H4)(CO)2]-
(BArF )2 (νCO = 2104 cm−1, eq 1).
4
Upon treatment with copper(I), the binucleating ligands 5
−
and 6H4 gave 2:1 derivatives. Although the PF6 salts of these
dicationic complexes proved poorly soluble in CH2Cl2, the
corresponding tetrakis(3,5-trifluoromethylphenyl)borate (BArF −
)
4
and tetrakis(pentafluorophenyl)borate (BArF5 −) salts exhibited
4
satifactory solubility. Crystallographic analysis of [Cu2(5)-
(NCMe)2](BArF ) confirmed the CuN4 environment provided
4 2
by two methoxypyridyl groups, one picolinyl group, and an
acetonitrile ligand (Figure 4).
Unlike [Cu2(5)(NCMe)2](BArF )2, the salt of the tetrahy-
4
droxy complex, tentatively assigned as [Cu2(6H4)(NCMe)x]-
(BArF )2, appears to adopt an asymmetric structure indicated
4
by its complex 1H NMR spectrum. The −55 °C NMR
1
This salt exhibits a simple H NMR spectrum indicative of high
spectrum of a CD2Cl2 solution exhibits four equally intense but
symmetry at room temperature, in contrast to the MeCN adduct.
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Figure 5. Left: Structure of the cation in [Cu2(2)2]BArF4 with ellipsoids shown at 50% probability. The counteranion and hydrogen atoms have been
omitted for clarity. Right: View down the Cu2−Cu1 bond vector with carbon atoms omitted.
Oxygenation of Mononuclear Cu(I) Complexes. As
reported for [Cu(tripic)(NCMe)]+, the methoxylated complex
[Cu(2Me)(NCMe)]+ is unreactive toward O2. In contrast,
solutions of [Cu(2H)(NCMe)]PF6 oxidized upon exposure to
oxygen, a process that was accelerated by bases such as 2,6-
−
lutidine. Although the PF6 salt of the green oxidized product
was insoluble in common solvents, the corresponding BArF −
4
salt was soluble. ESI-MS analysis indicated that this species has
the formula [Cu2(2)2]+. Reversion of [Cu2(2)2]+ to [Cu(2H)-
(NCMe)]+ is effected by H(OEt2)2BArF in the presence of
4
decamethylferrocene (−0.59 V,30 eq 2).
The structure of the mixed valence complex [Cu2(2)2]BArF
4
was determined by X-ray crystallography. Each (Cu1.5)2 center
is enveloped in an N3 pocket of the tripyridine ligand but is
linked to a second complex via the pyridonate oxygen centers
(Figure 5). The cationic complex has C2h symmetry with
a short Cu−Cu distance of 2.458(1) Å, comparable to the
Cu−Cu distance found in the CuA site of cytochrome oxidase
(∼2.5 Å).31 The Cu centers are trigonal bipyramidal, with each
pyridonate ligand occupying an apical position. The Cu(1)−
Cu(2)−N(4) angle is 86.8(2)°, slightly distorted from 90° for a
trigonal bipyramid. The trigonal bipyramidal geometry of Cu
in [Cu2(2)2]+ is reminiscent of the geometry found for the
(Cu1.5)2 center in an octaaza-cryptand complex.32
The magnetic moment for [Cu2(2)2]+ was determined to be
μeff = 1.85 μB, appropriate to S = 1/2. Electron paramagnetic
Figure 6. Experimental (red) and simulated (black) X-band EPR
resonance (EPR) spectra at 77 K display a seven-line pattern, as
expected for a Cu(1.5)Cu(1.5) complex where the unpaired
electron is delocalized over the two I = 3/2 centers (Figure 6).
In fluid solution at 298 K, the seven-line pattern remains discer-
nible and was simulated with giso = 2.137 and ACu = −159 MHz.
EPR parameters for [Cu2(2)2]BArF4 were similar g-factors to those
for other mixed valence Cu dimers.33
spectra (top = 77 K, bottom = 298 K) for [Cu2(2)2]BArF in
4
CH2Cl2:toluene (1:1). Simulation parameters for 77 K: g = 2.002,
2.185, and 2.214, ACu = 56.3, −195, −344, 92.0, 1.0, and −92.0 MHz;
298 K: g = 2.137, ACu = −159 MHz.
In addition to absorptions at 460 and 655 nm, the UV−vis
spectrum of [Cu2(2)2]BArF4 displays a strong feature 1040 nm
(εM = 1160). Such intense bands are characteristic of
intervalence charge transfer and are observed in other Cu1.5
dimers.35
The coordination geometry, short Cu−Cu distance, and
seven-line EPR spectrum all indicate that [Cu2(2)2]BArF is a
4
type III mixed valence dimer.34
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Carbonylation and Deprotonation of Cu(I) Com-
plexes. Although substitution of a methyl for hydroxyl does
not affect the Cu−CO bonding, deprotonation of the hydroxyl
group has dramatic effects. In CH2Cl2 solution, [Cu(2H)-
(CO)]+ exhibits a νCO of 2094 cm−1 (2088 cm−1 in
tetrahydrofuran (THF)), similar to [(κ3-tpa)Cu(CO)]+
MeCN solution, protonation of Cu2(2)2 efficiently gave
[Cu(2H)(NCMe)]+. Deprotonation of [Ag(2H)(MeCN)]BF4
also afforded a similar dimeric species as evidenced by mass
spectrometry. Although we were unable to obtain single crystals
of Cu2(2)2, suitable crystals were obtained for Ag2(2)2 (Figure 8),
analysis of which confirmed the expected bitetrahedral structure.
(νCO = 2091, 2074 cm−1) and [(tripic)Cu(CO)]+ (νCO
=
2090 cm−1).18,36 Deprotonation of [Cu(2H)(CO)]BArF with
4
KN(SiMe3)2 (KHMDS, pKaTHF = 26)37 led to precipitation of
Cu(2)(CO) as an off-white solid (eq 3, Figure 7).
Figure 8. Molecular structure of one of two (similar) independent
molecules of Ag2(2)2 with ellipsoids at 50% probability. Selected
distances (Å): Ag(1)−Ag(2), 2.7847(3); Ag(1)−N(1), 2.253(2);
Ag(1)−N(2), 2.400(2); Ag(1)−N(3), 2.513(2); Ag(1)−O(2),
2.211(2); Ag(2)−N(4), 2.281(2); Ag(2)−N(5), 2.474(2); Ag(2)−
N(6), 2.380(2); Ag(2)−O(1), 2.204(2).
Oxygenation of Dinuclear Cu(I) Complexes. Solutions
of the dicopper tetrahydroxy species [Cu2(6H4)(NCMe)2]-
(BArF )2 instantly turned dark blue upon exposure to O2. The
4
product [Cu2(6H2)]BArF was obtained in analytical purity.
4
The EPR spectra for [Cu2(6H2)]BArF and [Cu2(2)2]BArF
4
4
closely match. The UV−vis spectrum of the intensely blue
solution of [Cu2(6H2)]BArF displays an intervalence charge
4
transfer absorption at 1100 nm (εM = 1810, 1,2-dichloro-
ethane), which is red-shifted in comparison to that of
Figure 7. IR spectra before and after deprotonation of [Cu(2H)-
(CO)]+ with KHMDS (black = [Cu(2H)(CO)]+, red = Cu(2)(CO)).
The lower intensity of the red band is attributed to the lower solubility
of Cu(2)(CO).
[Cu2(2)2]BArF . Crystallographic analysis of the blue oxidation
4
product revealed that [Cu2(6H2)]+ has idealized C2-symmetry
(Figure 9). The structure of [Cu2(6H2)]+ is similar to that of
Deprotonation is reflected by a decrease in νCO of 26 cm−1 to
νCO = 2062 cm−1. For comparison, single deprotonation of
[Rh(CO)2(3,3′-dihydroxy-2,2′-bipy)]+ shifts νCO from 2108 and
2052 to 2072 and 2002 cm−1.38
Deprotonations with tetramethylguanidine (pKaMeCN = 23)39
and NEt3 (pKaMeCN = 18)40 produced species with hydrogen-
bonds to the conjugate acid or to Cu(2)(CO) as indicated by
νCO bands in the region 2084−2076 cm−1. Deprotonations
were reversed by the addition of HBArF ·2Et2O.
Deprotonation of [Cu(2H)(NCMe)]+4also gave encouraging
results relevant to [Cu2(2)2]+. Addition of 1 equiv of KHMDS
to a THF solution of [Cu(2H)(NCMe)]+ resulted in
immediate precipitation of a brick-red solid. Unlike Cu(2)-
(CO), this species was CH2Cl2-soluble. NMR spectra showed
that this red compound was a MeCN-free, symmetrical species.
We propose that this new compound is the dimer Cu2(2)2,
structurally related to [Cu2(2)2]+, but lacking the Cu−Cu
bond. A CuI dimer similarly bridged by two pyridonate ligands
has been reported by Zhang and co-workers.41 In CH2Cl2
solution, Cu2(2)2 reacts with CO to give Cu(2)(CO). In
Figure 9. Left: Structure of the cation in [Cu2(6H2)]BArF with
4
ellipsoids shown at 50% probability. The hydrogen atoms (green)
bound to O2 and O4 were observed crystallographically. Other
hydrogen atoms and the counteranion have been omitted for clarity.
Right: View down the Cu2−Cu1 bond vector with carbon atoms
omitted.
[Cu2(2)2]+, having a Cu−Cu distance of 2.4939(5) Å. The
mixed valence Cu dimer is additionally stabilized by hydrogen-
bonding between a hydroxypyridyl group and the coordinated
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1
Figure 10. 500 MHz H NMR spectrum of [Cu2(5)(MeCN)2](BArF54)2 (CD2Cl2, 293 K) under 0.4 atm of O2. Signals assigned to the dioxygen
adduct are indicated with *.
O-center of the pyridonate ligands (dO1−O4 = 2.591(3) Å).
Because of this intramolecular hydrogen-bonding, the Cu
centers are enveloped in a cryptand-like cage. Structural
perturbations imposed by the ethylene bridge in [Cu2(6H2)]-
BArF4 distort the idealized trigonal bipyramidal geometry at Cu
from that observed in [Cu2(2)2]BArF . The resulting coordination
4
sphere of each Cu center is nearly square pyramidal.
The methoxy-substituted salt [Cu2(5)(MeCN)2](BArF )2
4
was found to reversibly react with dioxygen. Binding is
indicated by a change from the colorless dicuprous species to
purple upon introducing 1 atm of O2. The UV−vis spectrum of
a CH2Cl2 solution of [Cu2(5)(O2)](BArF )2 (298 K) displays
4
bands at 355 and 509 nm (see Supporting Information), blue-
shifted with respect to [Cu2(bistripic)(O2)](PF6)2 (360 and
532 nm). The 1H NMR spectrum is consistent with a
symmetric diamagnetic adduct (Figure 10), consistent with a
μ-κ2:κ2-O2 ligand. At 0.4 atm O2 (293 K), the equilibrium
constant, KO2, for the binding of O2 by [Cu2(5)(MeCN)2]2+
was found to be 0.012 M. For comparison, [Cu2(bistripic)-
(NCMe)2]2+ has a larger binding affinity for O2 indicated by
complete conversion to the μ-κ2:κ2-O2 complex at 298 K. The
oxygenation of [Cu2(5)(MeCN)2]+ is fully reversible in
contrast to the behavior of the dicopper(I) adducts of the
hydroxylated ligand.
Figure 11. UV−vis spectrum of Cu2(2)2 before (red) and after
(green) the addition of 1 equiv of [Fc]BArF (CH2Cl2 solution) to
4
afford the mixed valence salt [Cu2(2)2]BArF .
4
oxidation wave at a more positive potential of 0.65 V. Addition
of strong oxidants, such as [NO]BF4 (E = 1.0 V), to a CH2Cl2
solution of Ag2(2)2 did not appear to give mixed valence
derivatives.
Electrochemical Studies. Cyclic voltammetry revealed
that the mixed valence dimer [Cu2(2)2]BArF4 oxidizes at 0.99 V
(all potentials vs Fc0/+, where Fc = ferrocene). It exhibits a
reversible 1e− reduction at −1.03 V (ipa/ipc = 0.88) in CH2Cl2
solutions. The mixed valence complex of the binucleating
CONCLUSIONS
■
New families of pyridine-based tripodal ligands, developed from
the dilithiated derivative of 6-methyl-2-pyridone, give rise to a
family of mono- and dicopper complexes wherein the fourth
coordination site is flanked by a mix of methyl, methoxy, and
hydroxy groups. The methoxy substituents exert little influence,
but deprotonation of the hydroxy groups profoundly affects the
behavior of the copper centers by facilitating their oxidation to
bimetallic derivatives. The new N3-pyridonate scaffold stabilizes
binuclear derivatives that give mixed valence species.
Pyridonates and structurally related anionic ligands such as
ligand [Cu2(6H2)]+ is reversibly reduced at −0.56 V (ipa/ipc
=
0.99) and irreversibly oxidized at +0.99 V, the latter being
similar to the irreversible oxidation waves observed for both
[Cu2(2)2]+/2+ and [Cu(2H)(NCMe)]+/2+ couples.
Consistent with the electrochemical measurements, treat-
ment of Cu2(2)2 with [Fc]+ gave [Cu2(2)2]+ (Figure 11). The
analogous silver complex, Ag2(2)2, exhibits an irreversible
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quantity of CH2Cl2, followed by the addition of hexanes (ca. 100 mL)
afforded a light tan precipitate. Filtration of the suspension afforded 1 as a
light tan solid. Yield: 3.0 g (41%). Note: Additional 1 can be isolated
through acidification of the aqueous portion of the filtrate during workup;
however, this requires separation from unreacted 2-hydroxy-6-methylpyr-
idine. 1H NMR (500 MHz, CDCl3): δ 2.59 (s, 3H), 3.92 (s, 2H), 6.06 (d,
J = 7.0 Hz, 1H), 6.41 (d, J = 9.2 Hz, 1H), 7.07 (pt, J = 7.3 Hz, 2H), 7.32
(dd, J = 6.8, 9.2 Hz, 1H), 7.54 (pt, J = 7.7 Hz, 1H), 11.20 (br s, 1H).
Anal. Calcd for C12H12N2O (found): C, 71.98 (71.28); H, 6.04 (5.95); N,
13.99 (13.78).
Bis(2-methylpyrid-6-yl)(2-pyridon-6-yl)methane (2H). A sus-
pension of 1 (1.0 g, 5.0 mmol) in 25 mL of THF was cooled in an ice
bath to 0 °C. Addition of a hexanes solution of n-butyllithium (3.1 mL,
1.6 M) afforded a dark red mixture, which was warmed to room
temperature and stirred for 1 h. 2-Bromo-6-methylpyridine (0.63 mL,
5.5 mmol) and Pd(PPh3)4 (58 mg, 0.05 mmol) were added to the
mixture, which was then heated at reflux for 15 h. The dark mixture
was cooled to room temperature, and the solvent was removed under
vacuum. The remaining dark residue was extracted into a mixture of
30 mL of CH2Cl2 and 30 mL of H2O. The layers were separated, and
the aqueous phase was extracted with CH2Cl2 (2 × 30 mL). The
combined organic layers were dried over MgSO4, filtered, and
concentrated under vacuum to afford a light brown solid. Dissolution
of the light brown solid in a minimum portion of CH2Cl2, followed by
the addition of hexanes (ca. 100 mL) precipitated an off-white solid.
The solid can be further purified by column chromatography on silica
gel, eluting with EtOAc/MeOH (10:1). Yield: 0.93 g (64%). 1H NMR
(500 MHz, CDCl3): δ 2.57 (s, 6H), 5.33 (s, 1H), 6.23 (d, J = 6.7 Hz,
1H), 6.43 (d, J = 9.3 Hz, 1H), 7.04 (d, J = 7.8 Hz, 2H), 7.26 (d, J =
7.8 Hz, 2H), 7.30 (ddd, J = 0.5, 6.9, 9.3 Hz, 1H), 7.53 (t, J = 7.8 Hz,
2H), 11.72 (br s, 1H). Anal. Calcd for C18H17N3O (found): C, 74.20
(74.26); H, 5.88 (5.89); N, 14.42 (14.37). ESI-MS (MeOH, 25 °C):
m/z 292 ([M+H]+). mp 201 °C. Single crystals were obtained by
cooling an EtOAc solution to −35 °C.
amide and urea derivatives, are well-known to bridge pairs of
metals.42 For example, Borovik and co-workers have generated
square planar (Cu1.5)2 complexes by oxidation of a (Cu1)2
precursor using O2 and Fc+.43
The highly negative potential for the [Cu2(2)]+/0 couple
(−1.03 V) is indicative of the stability of the mixed valence
derivative. The second oxidation, corresponding to the
[Cu2(2)]2+/+ couple, is not observed until 0.99 V. The milder
redox couple (−0.56 V) for [Cu2(6H2)]+/0 more closely
matches that for the binuclear CuA site (−0.39 V vs Fc0/+, pH = 8).44
The 0.41 V difference in E(Cu22+/3+) for these two
complexes illustrates the sensitivity of this redox couple to
relatively subtle steric constraints imposed by the ligand. A
reversible one-electron reduction observed for [Cu2(2)2]+
suggests that the conversion of [Cu(2H)(MeCN)]+ to
[Cu2(2)2]+ proceeds via Cu2(2)2. The modulation of a
complex’s oxidation potential via deprotonation of a coordi-
nated ligand has also been established for some Fe and Ru
complexes of N-heterocyclic ligands.45,46 For example, Carina
and co-workers have demonstrated that deprotonation of a
FeII−tetraimidazolyl dication with four equiv of base shifts the
oxidation potential of the FeII/III couple negative by 1.38 V.46
Despite its highly negative reduction potential, [Cu2(2)2]+ is
readily reduced by weak reductants in the presence of acids.
EXPERIMENTAL SECTION
■
All manipulations were performed under an Ar atmosphere using
standard Schlenk techniques, unless otherwise noted. Reagents were
purchased from Sigma-Aldrich and Matrix Scientific. Solvents were
HPLC-grade and dried by filtration through activated alumina or
distilled under nitrogen over an appropriate drying agent. ZnCl2 was
dried by refluxing the solid in SOCl2. [Cu(NCMe)4]BArFn4 (n = 20 or
24) was prepared according to literature.47 All other commercial
( )-(2-Methoxypyrid-6-yl)(2-methylpyrid-6-yl)(2-pyridon-6-
yl)methane (3). This compound was synthesized following the
procedures outlined for 2H, using 2-bromo-6-methoxypyridine in
place of 2-bromo-6-methylpyridine. A quantity of 1.35 g of 1 yielded
0.92 g (45%) of 3. 1H NMR (500 MHz, CDCl3): δ 2.57 (s, 6H), 5.34
(s, 1H), 6.23 (d, J = 7.0 Hz, 1H), 6.42 (dd, J = 0.8, 9.2 Hz, 1H), 7.04
(d, 7.8 Hz, 1H), 7.26 (d, 7.7 Hz, 2H), 7.30 (dd, J = 6.7, 9.2 Hz, 2H),
7.53 (t, J = 7.8 Hz, 2H), 11.73 (br s, 1H). Anal. Calcd for C18H17N3O2
(found): C, 70.34 (70.10); H, 5.58 (5.48); N, 13.67 (13.38). ESI-MS
(MeOH, 25 °C): m/z 308 ([M+H]+).
1
reagents were used as received without further purification. H NMR
spectra were acquired using a Varian 500 spectrometer. 1H NMR
signals are quoted in ppm (δ) referenced to the residual solvent
signal.48 FT-IR spectra were recorded on a Perkin-Elmer Spectrum
100 FT-IR spectrometer. Chromatography was conducted with
Siliaflash P60 from Silicycle (230−400 mesh). UV−vis spectra were
acquired on either a Cary Bio 50 UV−vis or Cary 5000 UV−vis-NIR
spectrophotometer. Electrochemical data were collected using either a
BAS CV-50W or CH Instruments 600D potentiostat. Cyclic
voltammetry was conducted under an inert atmosphere in CH2Cl2
[Cu(2H)(NCMe)]PF6. A solution of [Cu(NCMe)4]PF6 (192 mg,
0.5 mmol) in 5 mL of MeCN was treated with a solution of 2H (150 mg,
0.5 mmol) in 5 mL of MeCN. After stirring this yellow solution
for 1 h, it was diluted with 40 mL of Et2O to precipitate a light-yellow
solid, which was washed with two 20-mL portions of Et2O before
solution using [TBA]BArF (0.1 M) as the electrolyte, iR
4
compensation, a glassy carbon electrode (diameter =1 cm), a Ag
wire quasi-reference electrode, and a Pt counter electrode. Redox
1
couples are referenced vs internal Fc+/0
.
storing under vacuum. Yield: 211 mg (78%). H NMR (500 MHz,
CD2Cl2): δ 2.44 (s, 3H), 2.76 (s, 6H), 5.61 (s, 1H), 6.66 (s, 1H), 6.82
(d, J = 8.3 Hz, 1H), 7.24 (d, J = 7.7 Hz, 2H), 7.28 (d, J = 7.4 Hz, 1H),
7.50 (d, J = 7.7 Hz, 2H), 7.68−7.76 (m, 3H). ESI-MS (MeCN,
25 °C): m/z 395 ([M]+). Preliminary crystallographic data for single
crystals of [Cu(2H)(NCMe)]PF6 (grown from layering a MeCN
solution of [Cu(2H)(NCMe)]PF6 with Et2O) indicated coordination
of the Cu center to the picoline and pyridone group N-atoms of 2H
with MeCN occupying the fourth coordination site (see character-
ization of [Ag(2H)(MeCN)]BF4).
(6-Picolinyl)(6-pyridonyl)methane (1). A hexanes solution of
n-BuLi (50.4 mL, 1.6 M) was added to a suspension of 2-hydroxy-6-
methylpyridine (4.0 g, 37 mmol) in 40 mL of THF at 0 °C. The resulting
red-orange solution was stirred at room temperature for 1 h, followed by
the addition of 50 mL of a THF solution of ZnCl2 (11.0 g, 81 mmol)
at 0 °C (An attempt to synthesize 1 in the absence of the transmetalat-
ing agent ZnCl2 resulted in no conversion). The yellow mixture was
allowed to warm to room temperature and then stirred for 1 h. The
yellow mixture was treated with PdCl2(dppf) (0.27 g, 0.37 mmol) (dppf =
1,1′-bis(diphenylphosphino)ferrocene) and 2-bromo-6-methylpyridine
(4.6 mL, 40 mmol). After heating under reflux for 15 h, the reaction
mixture was cooled to room temperature, and the THF was removed
under vacuum. CH2Cl2 (100 mL) was added to the remaining yellow
residue, followed by the addition of 25 mL aqueous solutions of
Na2S·9H2O (9.7 g) and NaOH (1.6 g). After 1 h of stirring, the frothy
mixture was filtered and the remaining solid was washed with 50 mL of
CH2Cl2. The filtrate layers were separated, and the aqueous layer was
extracted with CH2Cl2 (2 × 50 mL). The combined organic layers were
dried over MgSO4 and filtered. Concentration of the filtrate under
vacuum afforded a light brown solid. Dissolution of the solid in a minimal
[Cu(2H)(NCMe)]BArF . A 3-mL CH2Cl2 solution of 2H (100 mg,
4
0.34 mmol) was treated with a 5-mL CH2Cl2 solution of [Cu(NCMe)4]-
BArF (311 mg, 0.34 mmol). After being stirred for 1 h, the light-yellow
4
solution was diluted with 25 mL of hexanes to precipitate a colorless
solid, which was collected by filtration and washed with hexanes before
storage under vacuum. Yield: 310 mg (85%). 1H NMR (500 MHz,
CD2Cl2): δ 2.44 (s, 3H), 2.73, (s, 6H), 5.50 (s, 1H), 6.48 (br s, 1H), 6.81
(d, J = 8.4 Hz, 1H), 7.22 (d, J = 7.6 Hz, 1H), 7.23 (d, J = 8.0 Hz, 2H), 7.42
(d, J = 7.7 Hz, 2H), 7.56 (br s, 4H), 7.67−7.75 (m, 19H). Anal. Calcd for
C52H32N4BCuF24O (found): C, 49.60 (49.35); H, 2.56 (2.57); N, 4.45
(4.34). The salt [Cu(2H)(NCMe)]BArF5 was prepared analogously.
4
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Inorganic Chemistry
Article
[Cu(3)(NCMe)]BArF54. This off-white salt was prepared following
1H NMR (500 MHz, CDCl3): δ 3.17 (s, 4H), 3.79 (s, 12 H), 5.72
(s, 2H), 6.56 (d, J = 8.2 Hz, 4H), 6.79 (dd, J = 0.9, 7.6 Hz, 2H), 6.85
(d, J = 7.4 Hz, 4H), 7.24 (dd, J = 0.9, 7.8 Hz, 2H), 7.34 (t, J = 7.7 Hz,
2H), 7.47 (dd, J = 7.4, 8.2 Hz, 4H).
procedures outlined for [Cu(2H)(NCMe)]BArF , from 3 (52 mg,
4
0.17 mmol) and [Cu(NCMe)4]BArF5 (154 mg, 0.17 mmol). Yield:
4
157 mg (85%). 1H NMR (500 MHz, CD2Cl2): δ 2.42 (br s, 3H), 2.74
(s, 3H), 3.95 (s, 3H), 5.45 (s, 1H), 6.50 (s, 1H), 6.80 (d, J = 8.6 Hz,
1H), 6.82 (d, J = 8.5 Hz, 1H), 7.23 (d, J = 7.8 Hz, 1H), 7.25 (d, J = 8.4 Hz,
1H), 7.41 (d, J = 7.8 Hz, 1H), 7.70 (t, J = 7.8 Hz, 1H), 7.72 (t, J = 7.8 Hz,
1H), 7.81 (dd, J = 7.6, 8.3 Hz, 1H). Anal. Calcd for C44H20BCuF20N4O2
(found): C, 48.44 (48.00); H, 1.85 (1.52); N, 5.14 (5.08).
[Cu2(5)(NCMe)2](PF6)2. Solutions of [Cu(NCMe)4]PF6 (198 mg,
0.53 mmol) and 5 (170 mg, 0.26 mmol), each in 5 mL of MeCN, were
combined to afford a yellow solution. After 1 h of stirring, 30 mL of
Et2O was added to the solution to precipitate a light-yellow solid,
which was washed with two 20-mL portions of Et2O. Yield: 136 mg
1
[Ag(2H)(MeCN)]BF4. A solution of 2H (0.44 g, 1.5 mmol) and
AgBF4 (0.30 g, 1.5 mmol) in 20 mL of MeCN was prepared. After
stirring for 30 min, the solution was evaporated. The white solid
residue was washed with Et2O (30 mL) and hexanes (20 mL). Yield:
0.69 g (87%). Crystals of [Ag(2H)(MeCN)]BF4 suitable for X-ray
diffraction were obtained by vapor diffusion of Et2O into a MeCN
(43%). H NMR (500 MHz, CD3CN): δ 1.96 (s, 6H), 3.45 (s, 4H),
3.90 (s, 12H), 5.73 (s, 2H), 6.85 (d, J = 8.5 Hz, 4H), 7.14 (d, J =
8.0 Hz, 2H), 7.22 (d, J = 6.4 Hz, 4H), 7.42 (d, J = 7.8 Hz, 2H), 7.64
(t, J = 7.8 Hz, 2H), 7.82 (t, J = 8.0 Hz, 4H).
[Cu2(5)(NCMe)2](BArF ) . A suspension of [Cu2(5)(NCMe)2]PF6
4 2
(100 mg, 0.084 mmol) and KBArF (152 mg, 0.17 mmol) in CH2Cl2
4
1
solution of [Ag(2H)(MeCN)]BF4 at −25 °C. H NMR (400 MHz,
(10 mL) was stirred for 1 h and then filtered via cannula.
Concentration of the filtrate under reduced pressure afforded a
white solid. The product was extracted into 5 mL of CH2Cl2 and
reprecipitated with hexanes. Yield: 175 mg (81%). Treatment of 5 with
CD3CN): δ 9.10 (br, 1H), 7.74 (t, 2H), 7.61 (t, 1H), 7.47 (d, 2H),
7.28 (d, 2H), 6.88 (br, 1H), 6.64 (d, 1H), 5.74 (s, 1H), 2.64 (s, 6H),
1.96 (s, 3H). ESI-MS (m/z): calcd for C20H20AgN4O [M]+ 439.07,
found 439.3. Anal. Calcd for C20H20AgN4BF4O (found): C, 45.58
(44.99); H, 3.82 (3.91); N, 10.63 (10.43).
[Cu(NCMe)4]BArF afforded the same salt. 1H NMR (500 MHz,
4
CD2Cl2): δ 2.16 (s, 6H), 3.54 (s, 4H), 3.87 (s, 12 H), 5.46 (s, 2H),
6.74 (d, J = 8.5 Hz, 4H), 7.02 (dd, J = 1.0, 7.8 Hz, 2H), 7.21 (dd, J =
0.6, 7.5 Hz, 4H), 7.37 (dd, J = 1.0, 7.8 Hz, 2H), 7.50 (t, J = 7.8 Hz,
2H), 7.53 (s, 8H), 7.72 (br t, 16H), 7.76 (dd, J = 7.4, 8.4 Hz, 4H).
Anal. Calcd for C106H66N8B2Cu2F40O4 (found): C, 49.42 (49.73); H,
2.58 (2.65); N, 4.35 (4.28).
(2-Methoxypyrid-6-yl)-bis(2-methylpyrid-6-yl)methane
(2Me). This compound, isolated as a yellow oil, was prepared
following the procedures for 4 starting with 3 (500 mg). Yield: 130 mg
1
(25%). H NMR (500 MHz, CDCl3): δ 2.53 (s, 3H), 3.80 (s, 6 H),
5.72 (s, 1H), 6.56 (d, J = 8.3 Hz, 2H), 6.86 (d, J = 7.4 Hz, 2H), 6.99
(d, J = 7.6 Hz, 1H), 7.33 (d, J = 8.0 Hz, 1H), 7.45 (t, J = 7.9 Hz, 2H),
7.49 (t, J = 8.2 Hz, 1H). ESI-MS (MeOH, 25 °C): m/z 306 ([M+H]+).
[Cu(2Me)(NCMe)]BArF54. This light-yellow solid was synthesized
following the procedures outlined for [Cu(2H)(NCMe)]BArF4. Start-
ing with (2-methoxypyrid-6-yl)-bis(2-methylpyrid-6-yl)methane (110 mg,
0.36 mmol). Yield: 325 mg (83%). 1H NMR (500 MHz, CD3CN): δ 2.71
(s, 6H), 3.95 (s, 3H), 5.87 (s, 1H), 6.89 (d, J = 8.5 Hz, 1H), 7.27 (d, J =
7.8 Hz, 2H), 7.36 (d, J = 7.5 Hz, 1H), 7.54 (d, J = 7.7 Hz, 2H), 7.73 (t,
J = 7.7 Hz, 2H), 7.85 (t, J = 7.9 Hz, 1H). Anal. Calcd for C45H22BCuF20-
N4O·0.25CH2Cl2 (found): C, 48.95 (48.88); H, 2.04 (1.66); N, 5.05
(4.87).
Bis[bis(2-pyridon-6-yl)(2-methylpyrid-6-yl)methane] (6H4). A
solution of 5 (420 mg, 0.66 mmol) in 5 mL of HBr (49%) was heated
at reflux for 4 h. After cooling to room temperature, the tan solution
was neutralized by slow addition to a 200 mL saturated aqueous
solution of NaHCO3. The aqueous solution was extracted with
CH2Cl2 (3 × 100 mL). The combined organic extracts were dried over
MgSO4, filtered, and concentrated to afford a tan solid. Yield: 271 mg
1
(71%). H NMR (500 MHz, CDCl3): δ 3.31 (s, 4H), 5.30 (s, 2H),
6.01 (d, J = 6.8 Hz, 4H), 6.39 (d, J = 9.2 Hz, 4H), 7.03 (d, J = 8.0 Hz,
2H), 7.18 (dd, J = 0.9, 7.8 Hz, 2H), 7.33 (dd, J = 6.8, 9.3 Hz, 4H), 7.37
(t, J = 7.7 Hz, 4H), 11.23 (br s, 4H). ESI-MS (MeOH, 25 °C): m/z
585 ([M+H]+), 607 ([M+Na]+). mp 230 °C.
Bis(2-methoxy-6-pyridyl)(2-methyl-6-pyridyl)methane (4). A
slurry of Ag2CO3 (904 mg, 3.3 mol) and 2 (746 mg, 2.4 mmol) in
40 mL of CHCl3 was treated with MeI (1.52 mL, 24 mmol). The
mixture was stirred in the absence of light for 48 h. The resulting tan
slurry was filtered through a ∼6-cm pad of Celite, which was rinsed
with an additional 50 mL of CHCl3. Concentration of the dark filtrate
afforded a blue oil, which was further purified via elution through a
7.5-cm plug of silica using EtOAc as the eluent. Concentration of the
first band to elute from the plug afforded 4 as a blue oil. Yield: 550 mg
(71%). 1H NMR (500 MHz, CDCl3): δ 2.53 (s, 3H), 3.80
(s, 6 H), 5.72 (s, 1H), 6.56 (d, J = 8.3 Hz, 2H), 6.86 (d, J = 7.4 Hz,
2H), 6.99 (d, J = 7.6 Hz, 1H), 7.33 (d, J = 8.0 Hz, 1H), 7.45 (t, J = 7.9
Hz, 2H), 7.49 (t, J = 8.2 Hz, 1H). Chromatographic separation of 4
[Cu2(2)2]PF6. A solution of [Cu(2H)(NCMe)]PF6 (100 mg,
0.18 mmol) in 5 mL of CH2Cl2 under an atmosphere of air was
treated with 2,6-lutidine (21 μL, 0.18 mmol), resulting in the for-
mation of a green precipitate. After 30 min of stirring, the green
suspension was filtered, and the solids were washed with CH2Cl2 (2 ×
5 mL). Yield: 67 mg (85%). Anal. Calcd for C36H32N6Cu2F6O2-
P·0.5CH2Cl2 (found): C, 48.97 (48.58); H, 3.72 (3.39); N, 9.39
(9.26). ESI-MS (MeOH, 25 °C): m/z 706 ([M]+). [Cu2(2)2]BArF
4
could be generated in a similar manner, but separation of the conjugate
acid proved to be difficult. Single crystals were obtained by slow
evaporation of a MeCN solution of [Cu2(2)2]BArF . UV−vis (BArF
4
4
1
salt, 1,2-dichloroethane), λ (εM): 460 (460), 655 (120), 1110 (1810).
from the blue side product was unsuccessful, although the H NMR
[Cu2(6H2)]BArF . A 5 mL CH2Cl2 solution of [Cu(NCMe)4]BArF
4
4
spectrum of the blue oil showed signals only assignable to 4. Blue-
colored samples of 4 were used in the subsequent homocoupling
reactions.
(373 mg, 0.34 mmol) was added to a 5 mL suspension of 6H4
(100 mg, 0.17 mmol) to afford a bright yellow mixture. After stirring
the solution for 10 min, 30 mL of hexanes was added to precipitate a
yellow oil. The oil was isolated via decantation and redissolved in
10 mL of CH2Cl2 to afford a turbid yellow solution, which was filtered.
The filtrate was evaporated to yield 325 mg a yellow-green solid of
Bis[bis(2-methoxy-6-pyridyl)(2-methyl-6-pyridyl)methane]
(5). A flame-dried flask was charged with 4 (550 mg, 1.7 mmol)
followed by 70 mL of THF to afford a blue solution. To the blue
solution was added t-BuLi (2.1 mL, 1.7 M) at −78 °C. Upon
completion of the addition, the resulting dark red solution was warmed
to 0 °C and stirred for 1 h. The red solution was cooled to −78 °C and
slowly treated with 1,2-dibromoethane (0.3 mL, 3.4 mmol). The
solution was allowed to warm to room temperature and stirred for a
further 14 h. Solvent was removed from the reaction mixture under
reduced pressure to afford a dark red residue. A mixture of 40 mL of
CH2Cl2 and 40 mL of H2O was used to dissolve the residue. The
aqueous layer was further extracted with CH2Cl2 (2 × 40 mL). The
combined organic extracts were dried over MgSO4, filtered, and
concentrated to afford a green oil. The oil was purified by silica
chromatography using a gradient eluent of 4:1 EtOAc/hexanes to 2:1
EtOAc/hexanes to afford 5 as a light yellow oil. Yield: 170 mg (31%).
[Cu2(6H4)(NCMe)x](BArF )2. 1H NMR (500 MHz, CD3CN, 70 °C):
4
δ 3.41 (s, 4H), 5.52 (s, 2H), 6.60 (d, J = 8.5 Hz, 4H), 6.75 (br s, 2H),
7.28 (d, J = 7.7 Hz, 2H), 7.43 (d, J = 7.5 Hz, 2H), 7.58 (br t, J = 7.7
Hz, 4H), 7.74 (t, J = 7.7 Hz, 2H). A solution of [Cu2(6H4)-
(NCMe)x](BArF )2 (50 mg, 0.02 mmol) in 3 mL of CH2Cl2 was
4
exposed to air, resulting in an immediate color change to dark blue.
The dark blue solution was layered with 3 mL of hexanes and stored at
room temperature to afford dark blue crystals of [Cu2(6H2)]BArF .
4
EPR (CH2Cl2:Toluene [1:1], 77 K): g = 2.012, 2.165, 2.250; ACu = 30,
−164, −361, 72.1, 3.8, −72.6 MHz. EPR (CH2Cl2:Toluene [1:1], 298 K):
g = 2.144, ACu = −170 MHz. UV−vis, 1,2-dichloroethane, λ nm (εM):
495 (994), 590 (938), 1040 (1510), 1110 (1810). ESI-MS (CH2Cl2,
4518
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Inorganic Chemistry
Article
25 °C): m/z 708 ([M]+). Anal. Calcd for C66H38BCu2F24N6O4 (found):
ACKNOWLEDGMENTS
■
C, 50.40 (50.08); H, 2.43 (2.40); N, 5.34 (5.10).
This research was supported by Department of Energy grant
DEFG 03-93ER45504. We thank Dr. Mark Nilges for recording
and simulating the EPR spectra.
Oxygenation of [Cu2(5)(NCMe)2](BArF ) . A solution of
4 2
[Cu2(5)(NCMe)2](BArF )2 (8.0 mg) in about 0.75 mL of CD2Cl2
4
in a J. Young NMR tube was saturated with O2, resulting in a purple
solution. 1H NMR data were acquired immediately, as the purple
oxygenated species quickly reverted back to [Cu2(5)(NCMe)2](BArF )2.
REFERENCES
4
■
Prolonged oxygenations resulted in the formation of green NMR-silent
products. 1H NMR (500 MHz, CD2Cl2): δ 3.80 (s, 4H), 4.04 (s, 12 H),
5.59 (s, 2H), 6.83 (d, J = 8.5 Hz, 4H), 7.27 (d, J = 7.4 Hz, 2H), 7.42
(d, J = 7.7 Hz, 4H), 7.59 (d, J = 7.6 Hz, 2H), 7.53 (s, 8H), 7.72 (br t,
16H), 7.83−7.89 (m, 6H).
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Deprotonation of [Cu(2H)(CO)]BArF with NEt3 or TMG. A
4
solution of [Cu(2H)(CO)]BArF in 10 mL of CH2Cl2 was generated
4
(3) Thorum, M. S.; Yadav, J.; Gewirth, A. A. Angew. Chem., Int. Ed.
2009, 48, 165−167. Chaudhuri, P.; Hess, M.; Weyhermuller, T.;
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E. D. J. Am. Chem. Soc. 2011, 133, 3696−3699. McCrory, C. C. L.;
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by carbonylation of a solution of 30 mg (0.024 mmol) of
[Cu(2H)(NCMe)]BArF for approximately one min. Varying equiv
4
of NEt3 or TMG were added from stock CH2Cl2 solutions followed
immediately by recording IR spectra.
Cu(2)(CO). A solution of [Cu(2H)(CO)]BArF5 in 5 mL of THF
4
was generated in situ by carbonylation of 100 mg (0.093 mmol) of the
MeCN complex. Formation of [Cu(2H)(CO)]BArF5 was verified by
4
IR spectroscopy (νCO = 2088 cm−1). The solution was titrated with a
0.05 M THF solution of KHMDS, monitoring IR spectra in the CO
region. Upon adding a total of 1 equiv of KHMDS (2 mL, 0.05 M
THF solution) a colorless solid precipitated. The resulting suspension was
filtered, and the isolated colorless solids were washed with THF
(2 × 3 mL) and dried under vacuum. Yield: 26 mg (73%). The addition
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of 1 equiv of HBArF ·2Et2O to a THF suspension of Cu(2)(CO) resulted
4
in the regeneration of [Cu(2H)(CO)]BArF , as indicated by IR spectro-
4
scopy (νCO = 2088 cm−1).
Cu2(2)2. A THF solution of [Cu(2H)(NCMe)]PF6 (5 mL, 0.056 M)
was treated with a THF solution of KHMDS (5.5 mL, 0.05 M),
immediately resulting in the evolution of a red precipitate. After 30 min of
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1
Yield: 42 mg (42%). H NMR (500 MHz, CD2Cl2): δ 2.44 (s, 6H),
5.37 (s, 1H), 6.44 (s, 2H), 7.01 (s, 2H), 7.22 (s, 1H), 7.30 (s, 2H),
7.56 (s, 2H). Anal. Calcd for C36H32N6Cu2O2·0.8CH2Cl2 (found): C,
56.98 (56.89); H, 4.37 (4.14); N, 10.83 (10.90). Cyclic voltammetry
(CH2Cl2, NBu4BArF ): E1/2 = −1.03 V (ipa/ipc = 0.99), +0.97 V
4
(irreversible) at 50 mV/s.
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Ag2(2)2. A flask was charged with [Ag(2H)(MeCN)]BF4 (0.40 g,
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1 h, this solution was evaporated. The dark brown solid residue was
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CH2Cl2 solution of Ag2(2)2 at −25 °C. 1H NMR (400 MHz, CD2Cl2):
δ 2.25 (s, 6H), 5.32 (m, 1H), 6.44 (dd, 2H), 6.92 (d, 2H), 7.23 (m,
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ASSOCIATED CONTENT
■
S
* Supporting Information
Additional experimental details and data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rauchfuz@illinois.edu.
Notes
The authors declare no competing financial interest.
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